**Introduction**

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Photodetectors**

**Introductory Chapter: Photodetectors**

DOI: 10.5772/intechopen.82045

Modern day electronic communications, industrial electronics, analytical equipment, medicine and healthcare, automotive and transport, etc. widely employ photodetectors, also known as photosensors, primarily as optical receivers to convert light into electrical signals. These devices may receive the transmitted optical pulses, or sense light or other electromagnetic radiation. Nevertheless, the photodetectors may be classified according to their light detection mechanisms, *viz.* the photoemission or photoelectric effect, thermal effect, polarisation effect, photochemical effect, or weak interaction effects. Photodetectors that employ semiconductors operate on the principle of electron-hole pair creation upon light irradiation. When a semiconductor material is illuminated by photons having energies greater than or equal to its bandgap, the absorbed photons promote valence band electrons into the conduction band, thus leaving behind positively charged holes in the valence band. Conduction band electrons (valence band holes) behave as free electrons (holes) that can diffuse in a concentration gradient, or drift under the influence of an intrinsic, or externally applied, electric field. The photogenerated electron-hole pairs due to optical absorption may recombine and re-emit light, unless subjected to an electric field-mediated separation to give rise to a photocurrent, which is a fraction of the photogenerated free charge carriers collected at the electrodes of the photodetector structure. The magnitude of this photocurrent at a given wavelength is directly

In this chapter, we introduce some representative photodetectors, their properties, performance and applications, as applied in the various design configurations. We also address sensing and detection in the electromagnetic spectrum spanning from the ultraviolet and vis-

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.82045

proportional to the incident light intensity.

ible, to infrared and terahertz.

Kuan W.A. Chee

Kuan W.A. Chee

**1. Introduction**

#### **Chapter 1 Provisional chapter**

#### **Introductory Chapter: Photodetectors Introductory Chapter: Photodetectors**

DOI: 10.5772/intechopen.82045

#### Kuan W.A. Chee Kuan W.A. Chee

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.82045

#### **1. Introduction**

Modern day electronic communications, industrial electronics, analytical equipment, medicine and healthcare, automotive and transport, etc. widely employ photodetectors, also known as photosensors, primarily as optical receivers to convert light into electrical signals. These devices may receive the transmitted optical pulses, or sense light or other electromagnetic radiation. Nevertheless, the photodetectors may be classified according to their light detection mechanisms, *viz.* the photoemission or photoelectric effect, thermal effect, polarisation effect, photochemical effect, or weak interaction effects. Photodetectors that employ semiconductors operate on the principle of electron-hole pair creation upon light irradiation. When a semiconductor material is illuminated by photons having energies greater than or equal to its bandgap, the absorbed photons promote valence band electrons into the conduction band, thus leaving behind positively charged holes in the valence band. Conduction band electrons (valence band holes) behave as free electrons (holes) that can diffuse in a concentration gradient, or drift under the influence of an intrinsic, or externally applied, electric field. The photogenerated electron-hole pairs due to optical absorption may recombine and re-emit light, unless subjected to an electric field-mediated separation to give rise to a photocurrent, which is a fraction of the photogenerated free charge carriers collected at the electrodes of the photodetector structure. The magnitude of this photocurrent at a given wavelength is directly proportional to the incident light intensity.

In this chapter, we introduce some representative photodetectors, their properties, performance and applications, as applied in the various design configurations. We also address sensing and detection in the electromagnetic spectrum spanning from the ultraviolet and visible, to infrared and terahertz.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### **2. Photodetection mechanisms**

Heinrich Hertz discovered in 1887 that ultraviolet light illumination of electrodes generates electric sparks more easily. While studying black-body radiation in 1900, Max Planck suggested that energy carried by electromagnetic waves could only be quantised into units of discrete packets known as photons or quanta. Albert Einstein advanced the foregoing light energy packet hypothesis to explain experimental results using the notion of the photoelectric effect. The light beam photons have a characteristic energy proportional to the frequency of the light. When the light beam irradiates a material, the energy of the photon, if sufficiently high, is absorbed to liberate the electron from atomic bonding, and the remaining photon energy contributes to the free electron's kinetic energy. For photon energies too low to be absorbed, they are re-emitted. However, if the electron acquires energy surpassing the work function of the material, it is ejected as a photoelectron. Whilst the maximum kinetic energy of the emitted photoelectron depends on the frequency of the irradiance, the photoelectron ejection rate (or magnitude of the photoelectric current) is directly proportional to the intensity of the incident light.

of an array of light-sensing pixels or active pixel sensors (APS) at the focal plane of a lens, and are most commonly adopted for imaging (photos or videos) or non-imaging (spectrometry, LIDAR and wave-front sensing) purposes. In radio telescopes, the FPA usually refers to 2-D devices that are sensitive in the infrared. Other image sensors, such as charge-coupled device (CCD) or CMOS sensors, operate in the visible regime. An anti-reflective coating or a surface-plasmon antenna is sometimes used on a photodetector, to enhance the optical absorption or photogeneration of charge carriers (or photocurrent response), respectively. By embedding an ultrathin semiconductor absorption layer into a Fabry-Pérot resonant cavity, resonant cavity enhanced photodetectors can be realised, to boost the quantum efficiency or bandwidth-efficiency product, and provide superior wavelength selectivity and high speed photoresponse for wavelength division multiplexing (WDM) systems. Subwavelength microcell gratings affixed in close proximity to the optical absorber can enable near-field enhance-

Introductory Chapter: Photodetectors http://dx.doi.org/10.5772/intechopen.82045 5

ment of optical absorption through strong electromagnetic field confinement [1].

well suited for high-speed IC applications.

cal power levels with quantum efficiencies eclipsing 100%.

Photovoltaic photodetectors resort to the internal electric field of a *p*-*n* or Schottky junction to achieve the charge separation and photocurrent generation. Solar cells are similar to photovoltaic photodetectors, which also absorb light and convert it into electrical energy, through the photovoltaic effect. The *p*-*n* junction photodetectors include designs consisting of a simple *p*-*n* junction, or *p*-*i*-*n* photodetectors incorporating a nominally undoped semiconductor layer between the *p*- and *n*-regions, or phototransistors combining a photodiode and an additional *n*-region. At equilibrium, the presence of the ionised acceptors and donors within the space charge region (SCR) sets up an internal electric field at the junction. Therefore, electron-hole pairs generated inside the SCR, or within the minority carrier diffusion length from the edges of the SCR, will be separated by the built-in electric field and contribute to the photocurrent. The width of the SCR is inversely related to the dopant concentration in the material, but its expansion may be modulated by reverse biasing, which concomitantly increases the internal electric field at the junction so as to enhance the efficiency of electron-hole pair separation. To improve the photoresponse speed, the electrical resistivity of the photodetector material may be reduced through increasing the dopant concentration, but a nominally undoped layer of a thickness largely determining the SCR width may be introduced between the *p*- and *n*-regions to form the *p*-*i*-*n* structure. With a lower resistivity and a wider SCR width (and hence lower capacitance), the *p-i-n* structure is

Avalanche photodiodes are designed with high *p*- and *n*-type doping to intensify the junction electric field. With a reverse bias sufficiently high (100–400 V) such that the internal electric field approximates the critical breakdown field, the acceleration of the photogenerated charge carriers within the SCR is able to ionise the lattice atoms, hence resulting in an avalanche multiplication of charge carriers. The corresponding gain is typically of the order of 10–20 in these cases. Avalanche photodiodes are well suited for fibre optic systems requiring low opti-

Phototransistors are similar to photodiodes, except that an additional *n*-region is included in the photodetector design. The phototransistor comprises a photodiode with an internal gain, and it can be represented as a bipolar junction transistor enclosed in a transparent case

Other than microchannel plate detectors, a range of photodetectors operate on the basis of the photoelectric or photoemission effect. Gaseous ionisation detectors detect photons having sufficient energy to ionise gas atoms or molecules, and the current flow due to the electrons and ions generated by the ionisation can be measured. Photomultiplier tubes or phototubes contain photocathodes that emit electrons when illuminated, thus conducting a current proportional to the light intensity. The thermal effect is realised when the incident photons cause electrons to transition into the mid-gap states, which then relax into the lower bands, thus leading to phonon generation and heat dissipation. The rise in temperature in turn modifies the electrical properties of the device (e.g., thermopile, pyroelectric detector, cryogenic detector, bolometer, etc.) material, such as its electrical conductivity. The polarisation effect is so called when the incident photons alter the polarisation states of appropriate materials, thereby modulating the refractive index (i.e., photorefractive effect); this is exploited in holographic data storage. Photochemical effects in photodetectors occur when chemical changes in the material are induced by the incident photons. Examples include photoreceptor cells in the retina, or photographic plates. Finally, weak interaction effects occur when secondary effects are induced by photons, such as in photon drag detectors or gas pressure changes in opto-acoustic detectors (e.g., Golay cells).

#### **3. Types of photodetectors**

Photodetectors may be configured in unique ways for various applications. For example, single sensors may detect overall light intensities. A 1-D array of photodetectors may be used to measure the distribution of light along a line, such as in a spectrophotometer or a line scanner. Moreover, a 2-D array of photodetectors may be used to derive images from the light intensity profile, when applied as an image sensor. Focal-plane arrays (FPAs) are devices consisting of an array of light-sensing pixels or active pixel sensors (APS) at the focal plane of a lens, and are most commonly adopted for imaging (photos or videos) or non-imaging (spectrometry, LIDAR and wave-front sensing) purposes. In radio telescopes, the FPA usually refers to 2-D devices that are sensitive in the infrared. Other image sensors, such as charge-coupled device (CCD) or CMOS sensors, operate in the visible regime. An anti-reflective coating or a surface-plasmon antenna is sometimes used on a photodetector, to enhance the optical absorption or photogeneration of charge carriers (or photocurrent response), respectively. By embedding an ultrathin semiconductor absorption layer into a Fabry-Pérot resonant cavity, resonant cavity enhanced photodetectors can be realised, to boost the quantum efficiency or bandwidth-efficiency product, and provide superior wavelength selectivity and high speed photoresponse for wavelength division multiplexing (WDM) systems. Subwavelength microcell gratings affixed in close proximity to the optical absorber can enable near-field enhancement of optical absorption through strong electromagnetic field confinement [1].

**2. Photodetection mechanisms**

4 Advances in Photodetectors - Research and Applications

of the incident light.

opto-acoustic detectors (e.g., Golay cells).

**3. Types of photodetectors**

Heinrich Hertz discovered in 1887 that ultraviolet light illumination of electrodes generates electric sparks more easily. While studying black-body radiation in 1900, Max Planck suggested that energy carried by electromagnetic waves could only be quantised into units of discrete packets known as photons or quanta. Albert Einstein advanced the foregoing light energy packet hypothesis to explain experimental results using the notion of the photoelectric effect. The light beam photons have a characteristic energy proportional to the frequency of the light. When the light beam irradiates a material, the energy of the photon, if sufficiently high, is absorbed to liberate the electron from atomic bonding, and the remaining photon energy contributes to the free electron's kinetic energy. For photon energies too low to be absorbed, they are re-emitted. However, if the electron acquires energy surpassing the work function of the material, it is ejected as a photoelectron. Whilst the maximum kinetic energy of the emitted photoelectron depends on the frequency of the irradiance, the photoelectron ejection rate (or magnitude of the photoelectric current) is directly proportional to the intensity

Other than microchannel plate detectors, a range of photodetectors operate on the basis of the photoelectric or photoemission effect. Gaseous ionisation detectors detect photons having sufficient energy to ionise gas atoms or molecules, and the current flow due to the electrons and ions generated by the ionisation can be measured. Photomultiplier tubes or phototubes contain photocathodes that emit electrons when illuminated, thus conducting a current proportional to the light intensity. The thermal effect is realised when the incident photons cause electrons to transition into the mid-gap states, which then relax into the lower bands, thus leading to phonon generation and heat dissipation. The rise in temperature in turn modifies the electrical properties of the device (e.g., thermopile, pyroelectric detector, cryogenic detector, bolometer, etc.) material, such as its electrical conductivity. The polarisation effect is so called when the incident photons alter the polarisation states of appropriate materials, thereby modulating the refractive index (i.e., photorefractive effect); this is exploited in holographic data storage. Photochemical effects in photodetectors occur when chemical changes in the material are induced by the incident photons. Examples include photoreceptor cells in the retina, or photographic plates. Finally, weak interaction effects occur when secondary effects are induced by photons, such as in photon drag detectors or gas pressure changes in

Photodetectors may be configured in unique ways for various applications. For example, single sensors may detect overall light intensities. A 1-D array of photodetectors may be used to measure the distribution of light along a line, such as in a spectrophotometer or a line scanner. Moreover, a 2-D array of photodetectors may be used to derive images from the light intensity profile, when applied as an image sensor. Focal-plane arrays (FPAs) are devices consisting Photovoltaic photodetectors resort to the internal electric field of a *p*-*n* or Schottky junction to achieve the charge separation and photocurrent generation. Solar cells are similar to photovoltaic photodetectors, which also absorb light and convert it into electrical energy, through the photovoltaic effect. The *p*-*n* junction photodetectors include designs consisting of a simple *p*-*n* junction, or *p*-*i*-*n* photodetectors incorporating a nominally undoped semiconductor layer between the *p*- and *n*-regions, or phototransistors combining a photodiode and an additional *n*-region. At equilibrium, the presence of the ionised acceptors and donors within the space charge region (SCR) sets up an internal electric field at the junction. Therefore, electron-hole pairs generated inside the SCR, or within the minority carrier diffusion length from the edges of the SCR, will be separated by the built-in electric field and contribute to the photocurrent. The width of the SCR is inversely related to the dopant concentration in the material, but its expansion may be modulated by reverse biasing, which concomitantly increases the internal electric field at the junction so as to enhance the efficiency of electron-hole pair separation. To improve the photoresponse speed, the electrical resistivity of the photodetector material may be reduced through increasing the dopant concentration, but a nominally undoped layer of a thickness largely determining the SCR width may be introduced between the *p*- and *n*-regions to form the *p*-*i*-*n* structure. With a lower resistivity and a wider SCR width (and hence lower capacitance), the *p-i-n* structure is well suited for high-speed IC applications.

Avalanche photodiodes are designed with high *p*- and *n*-type doping to intensify the junction electric field. With a reverse bias sufficiently high (100–400 V) such that the internal electric field approximates the critical breakdown field, the acceleration of the photogenerated charge carriers within the SCR is able to ionise the lattice atoms, hence resulting in an avalanche multiplication of charge carriers. The corresponding gain is typically of the order of 10–20 in these cases. Avalanche photodiodes are well suited for fibre optic systems requiring low optical power levels with quantum efficiencies eclipsing 100%.

Phototransistors are similar to photodiodes, except that an additional *n*-region is included in the photodetector design. The phototransistor comprises a photodiode with an internal gain, and it can be represented as a bipolar junction transistor enclosed in a transparent case through which photons are allowed to irradiate the base-collector junction. The electrons generated by the absorbed photons in the base-collector junction SCR are injected into the base, and the photocurrent is amplified. Nevertheless, while a phototransistor is generally a few orders of magnitude more sensitive than the photodiode, the photoresponse speed is much slower. Polysilicon- [2], zinc oxide- [3], or organic polymer-based [4] thin film transistors (TFTs) have been adopted as photodetectors for optical interconnects, ultraviolet imaging and large area displays/flexible substrates, respectively.

for technology adoption. State-of-the-art graphene-on-diamond photodetectors have been demonstrated to exhibit superior responsivity and photocurrent, as well as open circuit voltage [7]. Particularly, in high-speed optical data communications, photodetectors must also be highly responsive to photoexcitation, yet immediately/rapidly relax to the ground state after the light source is switched off. However, the excited non-equilibrium state is usually maintained for a finite amount of time through an effect known as persistent photoconductivity, owing to long recombination times that originate from charge carrier trapping by bulk defects (vacancies or impurities) and surface states. The photodetector may be characterised by various figures of merit such as the spectral response, quantum efficiency, responsivity, bandwidth, gain, noise equivalent power (NEP), dark current, response time and detectivity. The spectral response characterises the photodetector response with respect to the photon frequency. The quantum efficiency is the measure of the number of charge carriers generated per photon. The responsivity is the ratio of the output electrical current to the input optical power to the photodetector. The NEP is the minimum amount of optical power required to generate a signal in the presence of noise in the photodetector. The specific detectivity is the reciprocal of NEP normalised to the square root of the photodetector active area-bandwidth product. The gain is the ratio of the output electrical current to the photogenerated current directly generated by the incident photons. The dark current is a measure of charge carrier flow through a photodetector in the absence of an optical input. The response time is the time needed for a photodetector to rise from 10 to 90% of the final output. The noise spectrum is the intrinsic noise voltage/current as a function of frequency, which can be represented as a noise spectral density. The RF output is constrained by the nonlinearity of the photodetector. All in all, having a large angular acceptance, high temporal resolution, as well as high spectral and energy resolution, may also be crucial design considerations for a high-performance photodetector.

Introductory Chapter: Photodetectors http://dx.doi.org/10.5772/intechopen.82045 7

For a comparison of the viability and performance of photodetectors, an in-depth understanding of their figures of merit is essential. The insights underpinning the physics and technology of various photodetector designs and configurations must be conscientiously examined for successful implementation and integration of high-performance photodetection and optoelectronic sensing within the relevant wavelength ranges, on low-cost substrates or CMOS-compatible substrates. New device concepts and techniques to develop monolithic integration of optoelectronic materials on a single substrate may permit revolutionary ultrafast and ultrasensitive

The following grants are acknowledged: project number 61650110517 supported by the National Natural Science Foundation of China and project numbers 2014A610154 and

near-field photodetection at high spatial, temporal and spectral resolution.

2017A610095 supported by the Natural Science Foundation of Ningbo.

**5. Conclusion**

**Acknowledgements**

Schottky junction photodetectors include Schottky barrier photodiodes and metal-semiconductor-metal (MSM) photodiodes. In the former, the Schottky junction is formed between a metal and a doped semiconductor. Analogous to that formed at the *p*-*n* junction, the SCR is comparable, and its width can be modulated in tandem with the built-in electric field proportional to the reverse bias to the Schottky junction photodetector. Typically, an ultrathin, semitransparent metal layer, for example, Au of about 10-nm thick, is used as the Schottky contact, which allows transmissivity up to 95% and around 30% for infrared and ultraviolet, respectively. MSM photodiodes are designed with two Schottky contacts, with one Schottky junction reversed-biased to support an elongated SCR width, and the other, forward biased. Typically, the semiconductor material is nominally undoped, and hence, the SCRs are spatially extended into the device. The reversed-biased Schottky junction generates the photocurrent, whereas the forward-biased Schottky junction acts as a highly efficient charge carrier collector.

In photoconductors, an electric field is applied across a layer of a semiconductor through electrically biased ohmic contacts on either side, leading to the collection of charge carriers. Photoresistors, light-dependent resistors (LDRs) or photoconductive cells change electrical resistivity according to the light intensity, hence exhibiting photoconductivity. Such devices have a higher gain, as the response of photoconductors is typically several orders of magnitude greater than that of the photovoltaic detector counterpart, based on a given material. However, for photoconductors, the bandwidth, infrared sensitivity, ultraviolet-visible contrast and a range of other key performance parameters are inferior to that of other types of photodetectors. Hence, the scope of potential applications is significantly limited.

Rewritable nanoscale photodetectors have been demonstrated based on insulating oxide (LaAlO3 /SrTiO<sup>3</sup> ) interfaces [5], exhibiting electric field-tunable photoconductive response within the electromagnetic spectrum ranging from the visible to near-infrared. The integration of nanoscale photodetectors based on nanodots and nanowires has also benefited from recent innovations in subwavelength imaging beyond the diffraction limit, by adapting (plasmonic) metamaterials for superlenses suitable for superresolution, near-field light focussing [6].

#### **4. Performance figures of merit**

High sensitivity at the operating wavelength, short response times, linear response over a wide range of light intensities, minimum noise contribution, stability of performance characteristics, reliability, low bias voltage and low cost are amongst the photodetector requirements for technology adoption. State-of-the-art graphene-on-diamond photodetectors have been demonstrated to exhibit superior responsivity and photocurrent, as well as open circuit voltage [7]. Particularly, in high-speed optical data communications, photodetectors must also be highly responsive to photoexcitation, yet immediately/rapidly relax to the ground state after the light source is switched off. However, the excited non-equilibrium state is usually maintained for a finite amount of time through an effect known as persistent photoconductivity, owing to long recombination times that originate from charge carrier trapping by bulk defects (vacancies or impurities) and surface states. The photodetector may be characterised by various figures of merit such as the spectral response, quantum efficiency, responsivity, bandwidth, gain, noise equivalent power (NEP), dark current, response time and detectivity. The spectral response characterises the photodetector response with respect to the photon frequency. The quantum efficiency is the measure of the number of charge carriers generated per photon. The responsivity is the ratio of the output electrical current to the input optical power to the photodetector. The NEP is the minimum amount of optical power required to generate a signal in the presence of noise in the photodetector. The specific detectivity is the reciprocal of NEP normalised to the square root of the photodetector active area-bandwidth product. The gain is the ratio of the output electrical current to the photogenerated current directly generated by the incident photons. The dark current is a measure of charge carrier flow through a photodetector in the absence of an optical input. The response time is the time needed for a photodetector to rise from 10 to 90% of the final output. The noise spectrum is the intrinsic noise voltage/current as a function of frequency, which can be represented as a noise spectral density. The RF output is constrained by the nonlinearity of the photodetector. All in all, having a large angular acceptance, high temporal resolution, as well as high spectral and energy resolution, may also be crucial design considerations for a high-performance photodetector.

#### **5. Conclusion**

through which photons are allowed to irradiate the base-collector junction. The electrons generated by the absorbed photons in the base-collector junction SCR are injected into the base, and the photocurrent is amplified. Nevertheless, while a phototransistor is generally a few orders of magnitude more sensitive than the photodiode, the photoresponse speed is much slower. Polysilicon- [2], zinc oxide- [3], or organic polymer-based [4] thin film transistors (TFTs) have been adopted as photodetectors for optical interconnects, ultraviolet imaging

Schottky junction photodetectors include Schottky barrier photodiodes and metal-semiconductor-metal (MSM) photodiodes. In the former, the Schottky junction is formed between a metal and a doped semiconductor. Analogous to that formed at the *p*-*n* junction, the SCR is comparable, and its width can be modulated in tandem with the built-in electric field proportional to the reverse bias to the Schottky junction photodetector. Typically, an ultrathin, semitransparent metal layer, for example, Au of about 10-nm thick, is used as the Schottky contact, which allows transmissivity up to 95% and around 30% for infrared and ultraviolet, respectively. MSM photodiodes are designed with two Schottky contacts, with one Schottky junction reversed-biased to support an elongated SCR width, and the other, forward biased. Typically, the semiconductor material is nominally undoped, and hence, the SCRs are spatially extended into the device. The reversed-biased Schottky junction generates the photocurrent, whereas

the forward-biased Schottky junction acts as a highly efficient charge carrier collector.

photodetectors. Hence, the scope of potential applications is significantly limited.

(LaAlO3

/SrTiO<sup>3</sup>

light focussing [6].

**4. Performance figures of merit**

In photoconductors, an electric field is applied across a layer of a semiconductor through electrically biased ohmic contacts on either side, leading to the collection of charge carriers. Photoresistors, light-dependent resistors (LDRs) or photoconductive cells change electrical resistivity according to the light intensity, hence exhibiting photoconductivity. Such devices have a higher gain, as the response of photoconductors is typically several orders of magnitude greater than that of the photovoltaic detector counterpart, based on a given material. However, for photoconductors, the bandwidth, infrared sensitivity, ultraviolet-visible contrast and a range of other key performance parameters are inferior to that of other types of

Rewritable nanoscale photodetectors have been demonstrated based on insulating oxide

within the electromagnetic spectrum ranging from the visible to near-infrared. The integration of nanoscale photodetectors based on nanodots and nanowires has also benefited from recent innovations in subwavelength imaging beyond the diffraction limit, by adapting (plasmonic) metamaterials for superlenses suitable for superresolution, near-field

High sensitivity at the operating wavelength, short response times, linear response over a wide range of light intensities, minimum noise contribution, stability of performance characteristics, reliability, low bias voltage and low cost are amongst the photodetector requirements

) interfaces [5], exhibiting electric field-tunable photoconductive response

and large area displays/flexible substrates, respectively.

6 Advances in Photodetectors - Research and Applications

For a comparison of the viability and performance of photodetectors, an in-depth understanding of their figures of merit is essential. The insights underpinning the physics and technology of various photodetector designs and configurations must be conscientiously examined for successful implementation and integration of high-performance photodetection and optoelectronic sensing within the relevant wavelength ranges, on low-cost substrates or CMOS-compatible substrates. New device concepts and techniques to develop monolithic integration of optoelectronic materials on a single substrate may permit revolutionary ultrafast and ultrasensitive near-field photodetection at high spatial, temporal and spectral resolution.

#### **Acknowledgements**

The following grants are acknowledged: project number 61650110517 supported by the National Natural Science Foundation of China and project numbers 2014A610154 and 2017A610095 supported by the Natural Science Foundation of Ningbo.

### **Author details**

Kuan W.A. Chee

Address all correspondence to: kuan.chee@nottingham.edu.cn

Department of Electrical and Electronic Engineering, Faculty of Science and Engineering, University of Nottingham Ningbo China, Ningbo, People's Republic of China

**Section 2**

**Materials and Devices**

#### **References**


## **Materials and Devices**

**Author details**

8 Advances in Photodetectors - Research and Applications

Kuan W.A. Chee

**References**

Address all correspondence to: kuan.chee@nottingham.edu.cn

Foundation (STW); 2009. pp. 52-54

[7] Yuan Q, Lin C-T, Chee KWA. Carbon sp<sup>2</sup>

13: 978-620-2-21118-5, ISBN-10: 6202211180

**44**(10):3471-3476

[3] Ku CJ, Reyes P, Duan Z, Hong W-C, Li R, Lu Y. Mgx

oxide photodetector. Nature Photonics. 2010;**4**:849-852

Department of Electrical and Electronic Engineering, Faculty of Science and Engineering,

[1] Zohar M, Auslender M, Hava S. Ultrathin high efficiency photodetectors based on subwavelength grating and near-field enhanced absorption. Nanoscale. 2015;**7**(12):5476-5479

[2] Rangarajan B, Brunets I, Holleman J, Kovalgin AY, Schmitz J. TFTs as photodetectors for optical interconnects. In: Proceedings of the 12th Annual Workshop on Semiconductor Advances for Future Electronics and Sensors. Utrecht, The Netherlands: Technology

UV photodetector with enhanced photoresponse. Journal of Electronic Materials. 2015;

[4] Hamilton MC, Kanicki J. Organic polymer thin-film transistor photosensors. IEEE

[5] Irvin P, Ma Y, Bogorin DF, Cen C, Bark CW, Folkman CM, et al. Rewritable nanoscale

[6] Memarian M, Eleftheriades GV. Light concentration using heterojunctions of anisotropic


based devices. In: Zhu J, Jin A, Zhu D, editors. New Trends in Nanotechnology, Material and Environmental Science. Saarbrücken, Germany: AkademikerVerlag; 2018. ISBN-

Journal of Selected Topics in Quantum Electronics. 2004;**10**(4):840-848

low permittivity metamaterials. Light: Science & Applications. 2013;**2**:e114

Zn<sup>1</sup>−<sup>x</sup>

O thin-film transistor-based

technology: A guide for future carbon

University of Nottingham Ningbo China, Ningbo, People's Republic of China

**Chapter 2**

Provisional chapter

**Photoconductive Interlocked Molecules and**

DOI: 10.5772/intechopen.79798

Organic compounds and materials with photoconductive properties have been studied for many years because of their importance in many technological applications such as dye-sensitized solar cells, photodiodes, photoresistors, electronics, biomolecular sensing, etc. For multiple purposes, such molecules require intense protection from various factors which can decrease their durability and cause fatigue. Interlocked molecules and macromolecules involving photoconductive organic components and various types of macrocycles, such as cyclodextrins, cyclophanes, or macrocyclic ethers, are promising candidates for new photoconductivity-related applications. In this chapter, a review in this emerging research area in materials science and technology is provided. Focus is placed on photoconductive (poly)rotaxanes and (poly)catenanes. Various types of such materials and compounds are reviewed, and recent examples are provided. The relation

Keywords: photoconductivity, interlocked molecules, rotaxanes, catenanes, photocurrent

In recent years, a new class of supramolecular assemblies has gained the attention of the scientific community [1]. Supramolecular chemistry is a rapidly increasing research field which focuses on the study of complex systems that consist of more than one molecule, where order originates from the weak, non-covalent binding interactions between different chemical building blocks [2, 3]. The kinetic and thermodynamic control of covalent bonds has become a challenge for the synthetic community in order to create discrete molecules performing specific functions. This has accelerated chemists to attain precise control over kinetic and thermodynamic courses

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

between their structure and photoconductive behavior is discussed.

Photoconductive Interlocked Molecules and

Raffaello Papadakis, Ioanna Deligkiozi and Hu Li

Raffaello Papadakis, Ioanna Deligkiozi and Hu Li

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79798

responses, photosensitivity

1. Introduction

**Macromolecules**

Macromolecules

Abstract

#### **Photoconductive Interlocked Molecules and Macromolecules** Photoconductive Interlocked Molecules and Macromolecules

DOI: 10.5772/intechopen.79798

Raffaello Papadakis, Ioanna Deligkiozi and Hu Li Raffaello Papadakis, Ioanna Deligkiozi and Hu Li

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79798

#### Abstract

Organic compounds and materials with photoconductive properties have been studied for many years because of their importance in many technological applications such as dye-sensitized solar cells, photodiodes, photoresistors, electronics, biomolecular sensing, etc. For multiple purposes, such molecules require intense protection from various factors which can decrease their durability and cause fatigue. Interlocked molecules and macromolecules involving photoconductive organic components and various types of macrocycles, such as cyclodextrins, cyclophanes, or macrocyclic ethers, are promising candidates for new photoconductivity-related applications. In this chapter, a review in this emerging research area in materials science and technology is provided. Focus is placed on photoconductive (poly)rotaxanes and (poly)catenanes. Various types of such materials and compounds are reviewed, and recent examples are provided. The relation between their structure and photoconductive behavior is discussed.

Keywords: photoconductivity, interlocked molecules, rotaxanes, catenanes, photocurrent responses, photosensitivity

#### 1. Introduction

In recent years, a new class of supramolecular assemblies has gained the attention of the scientific community [1]. Supramolecular chemistry is a rapidly increasing research field which focuses on the study of complex systems that consist of more than one molecule, where order originates from the weak, non-covalent binding interactions between different chemical building blocks [2, 3]. The kinetic and thermodynamic control of covalent bonds has become a challenge for the synthetic community in order to create discrete molecules performing specific functions. This has accelerated chemists to attain precise control over kinetic and thermodynamic courses

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

utilizing weaker inter- and intramolecular interactions, such as hydrogen bonds, van der Waals forces, dipole-dipole interactions, etc. Controlling these weak interactions allows for targeted architectures of new class of molecules containing distinctive kinds of chemical bonds also known as "mechanical bonds." Assemblies derived by the aforementioned forces consist of a distinct number of molecular components that explore mechanical-like movements (output) in response to pre-definite stimulation (input) [4]. The expression is often more generally applied to molecules that modestly mimic functions that occur at the macroscopic level. After organization and assembly, they are capable of linking molecular motions and reactions to complex macroscopic functions including actuation and signal modulation enabling "molecular machines." The combination and coordination of organic, inorganic, and supramolecular chemistry made it possible to build various mechanically interlocked molecular architectures (MIMAs). The field of interlocked molecules is immense, and up to date, research in this field receives high interest and attention. In 2016, Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa were awarded the Nobel Prize in Chemistry for the "design and synthesis of molecular machines." The term has become state of the art in nanotechnology where a number of favorably complex molecular architectures have been investigated intended to construct MIMAs, estimated to fuel the cutting-edge miniaturization of multifunctional devices (electrical, optical, and chemical) in the near future. The profound investigation of these architectures is endeavored to proceed rapidly due to their valuable properties and potential future applications in biomechanics, molecular electronics, catalysis, drug delivery, electronic materials, and sensing including in general the targeted design of smart novel materials. Photoconductive multifunctional materials involving interlocked molecules and macromolecules are of high importance as they might result in novel hi-tech applications spanning from solar cells and molecular photodiodes to sensing biological applications. In this chapter, we provide a review to published photoconductive interlocked molecules and macromolecules, and we indicate the potentials of various classes of interlocked organic photoconductive dyes.

constitute nanomaterials that have been intensively investigated because of their ability to act as molecular machines and/or switches by giving controllable and reversible transformations. The interlocked components can be forced through a combination of chemical, optical, or electrochemical stimuli to change their orientation with respect to one another [7]. These reversible transformations may exhibit high response rates to various highly controllable physical or chemical external stimulations such as pH changes, electricity, light irradiation, heating or cooling, etc. Rotaxanes and catenanes are promising systems for the construction of artificial molecular machines. Catenanes were among the first supramolecular structures that have been reported, in which two or even more cyclic molecules have been mechanically interlocked together and did not disassembly by any external stimuli [4]. In 1964, the first catenane was synthesized by Schill and Lüttringhaus [8]. Synthetic strategies were improved in the late 1980s and beginning of 1990s, in large extent by Stoddart and coworkers [1]. According to the IUPAC nomenclature, [n]catenanes consist of n-interlocked rings. In their simplest form ([2]catenanes), two rings are non-covalently bound forming a structure like the one depicted in Figure 1 [11]. One of the most synthetically challenging examples of catenanes has been reported in 1994 by the group of Stoddart which was composed of five interlocking

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The disassembly of catenanes into its individual chemical components requires the breaking of one or more covalent bonds within the mechanically linked molecule. One of their fascinating functions is their ability to act as molecular machines where within these assemblies one or more of the macrocyclic ring(s) change position with respect to one another [13]. High synthetic challenges surround the synthesis of catenanes since a macrocyclization reaction is required in order to achieve the interlocked architecture with attendant competition between cyclization and oligomerization. In order to overcome this challenge, catenanes are typically formed under highly diluted conditions which lead consequently to prolonged reaction times,

Figure 1. Symbolic representations of a [2]catenane (A) and a [2]rotaxane (B). (C) Various types of polycatenanes. Reprinted with permission from Niu and Gibson [9]. (D) Topological alignment in a polyrotaxane. Reprinted with

permission from Yu et al. [10].

macrocycles representing a [5]catenane also known as olympiadane [12].

#### 2. Rotaxanes and catenanes

Molecular machines can be divided into two main categories: synthetic and biological. Large, synthetic molecular machines refer to molecules that are artificially designed and synthesized, whereas biological molecular machines are going deep back in history and can be found under various forms in the nature (transport proteins such as kinesin, myosin, and dynein) [5]. Following a self-assembly process, the formation of large molecular and macromolecular structures can be achieved. These assemblies are mainly interlocked, and no covalent bond is responsible for their stability. Stabilizing interaction may be (i) donor/acceptor forces, (ii) metal/ligand coordination, (iii) hydrogen bonding interactions, (iv) π–π stacking, (v) solvophobic repulsion, and/or (vi) electrostatic forces. Non-covalent interactions enable new properties and smart functional materials by the emerging synergy between molecular recognition and advanced chemistry. The introduction of a mechanical bond enters within the wellrecognized chemistry of the subcomponents of supramolecular architectures such as catenanes and rotaxanes. Catenanes and rotaxanes are among the simplest examples of mechanically interlocked molecules with nanometer-scale structures [6]. Many of these molecular assemblies constitute nanomaterials that have been intensively investigated because of their ability to act as molecular machines and/or switches by giving controllable and reversible transformations. The interlocked components can be forced through a combination of chemical, optical, or electrochemical stimuli to change their orientation with respect to one another [7]. These reversible transformations may exhibit high response rates to various highly controllable physical or chemical external stimulations such as pH changes, electricity, light irradiation, heating or cooling, etc. Rotaxanes and catenanes are promising systems for the construction of artificial molecular machines. Catenanes were among the first supramolecular structures that have been reported, in which two or even more cyclic molecules have been mechanically interlocked together and did not disassembly by any external stimuli [4]. In 1964, the first catenane was synthesized by Schill and Lüttringhaus [8]. Synthetic strategies were improved in the late 1980s and beginning of 1990s, in large extent by Stoddart and coworkers [1]. According to the IUPAC nomenclature, [n]catenanes consist of n-interlocked rings. In their simplest form ([2]catenanes), two rings are non-covalently bound forming a structure like the one depicted in Figure 1 [11]. One of the most synthetically challenging examples of catenanes has been reported in 1994 by the group of Stoddart which was composed of five interlocking macrocycles representing a [5]catenane also known as olympiadane [12].

utilizing weaker inter- and intramolecular interactions, such as hydrogen bonds, van der Waals forces, dipole-dipole interactions, etc. Controlling these weak interactions allows for targeted architectures of new class of molecules containing distinctive kinds of chemical bonds also known as "mechanical bonds." Assemblies derived by the aforementioned forces consist of a distinct number of molecular components that explore mechanical-like movements (output) in response to pre-definite stimulation (input) [4]. The expression is often more generally applied to molecules that modestly mimic functions that occur at the macroscopic level. After organization and assembly, they are capable of linking molecular motions and reactions to complex macroscopic functions including actuation and signal modulation enabling "molecular machines." The combination and coordination of organic, inorganic, and supramolecular chemistry made it possible to build various mechanically interlocked molecular architectures (MIMAs). The field of interlocked molecules is immense, and up to date, research in this field receives high interest and attention. In 2016, Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa were awarded the Nobel Prize in Chemistry for the "design and synthesis of molecular machines." The term has become state of the art in nanotechnology where a number of favorably complex molecular architectures have been investigated intended to construct MIMAs, estimated to fuel the cutting-edge miniaturization of multifunctional devices (electrical, optical, and chemical) in the near future. The profound investigation of these architectures is endeavored to proceed rapidly due to their valuable properties and potential future applications in biomechanics, molecular electronics, catalysis, drug delivery, electronic materials, and sensing including in general the targeted design of smart novel materials. Photoconductive multifunctional materials involving interlocked molecules and macromolecules are of high importance as they might result in novel hi-tech applications spanning from solar cells and molecular photodiodes to sensing biological applications. In this chapter, we provide a review to published photoconductive interlocked molecules and macromolecules, and we indicate the potentials of various classes of

Molecular machines can be divided into two main categories: synthetic and biological. Large, synthetic molecular machines refer to molecules that are artificially designed and synthesized, whereas biological molecular machines are going deep back in history and can be found under various forms in the nature (transport proteins such as kinesin, myosin, and dynein) [5]. Following a self-assembly process, the formation of large molecular and macromolecular structures can be achieved. These assemblies are mainly interlocked, and no covalent bond is responsible for their stability. Stabilizing interaction may be (i) donor/acceptor forces, (ii) metal/ligand coordination, (iii) hydrogen bonding interactions, (iv) π–π stacking, (v) solvophobic repulsion, and/or (vi) electrostatic forces. Non-covalent interactions enable new properties and smart functional materials by the emerging synergy between molecular recognition and advanced chemistry. The introduction of a mechanical bond enters within the wellrecognized chemistry of the subcomponents of supramolecular architectures such as catenanes and rotaxanes. Catenanes and rotaxanes are among the simplest examples of mechanically interlocked molecules with nanometer-scale structures [6]. Many of these molecular assemblies

interlocked organic photoconductive dyes.

12 Advances in Photodetectors - Research and Applications

2. Rotaxanes and catenanes

The disassembly of catenanes into its individual chemical components requires the breaking of one or more covalent bonds within the mechanically linked molecule. One of their fascinating functions is their ability to act as molecular machines where within these assemblies one or more of the macrocyclic ring(s) change position with respect to one another [13]. High synthetic challenges surround the synthesis of catenanes since a macrocyclization reaction is required in order to achieve the interlocked architecture with attendant competition between cyclization and oligomerization. In order to overcome this challenge, catenanes are typically formed under highly diluted conditions which lead consequently to prolonged reaction times,

Figure 1. Symbolic representations of a [2]catenane (A) and a [2]rotaxane (B). (C) Various types of polycatenanes. Reprinted with permission from Niu and Gibson [9]. (D) Topological alignment in a polyrotaxane. Reprinted with permission from Yu et al. [10].

since the association between the ring and a macrocycle precursor is weak which diminishes yields. Immense amount of work has been published altering the synthetic protocols as well as introducing a variety of macrocyclic molecules, e.g., crown ethers, cyclophanes, cyclodextrins (CDs), cucurbituril, calixarene, etc. In contrast to catenanes, rotaxanes are composed of one or more macrocycles and "dumbbell-shaped" molecule(s) threaded through them. Stoppering bulky end groups also called "stoppers" prevent disassembly [14]. The word rotaxane is derived from the Latin words "rota" meaning wheel and "axis" meaning axle. The formal naming of rotaxanes according to IUPAC rules is [n]rotaxane, where n indicates the number of chemically independent components in a rotaxane assembly. The simplest form is "[2] rotaxane" which consists of one macrocycle and one dumbbell-shaped molecule. There are several interactions that can initiate self-assembly, needed for the formation of these supramolecular structures in a more efficient manner. These interactions may be hydrophobic, hydrogen bonding, or donor-acceptor interactions. The strength of these interactions varies, and this introduces different stability of the formed complexes depending on the nature of interaction, e.g., Van der Waals forces (2–4 kJ/mol), hydrophobic interactions (4–12 kJ/mol), and hydrogen bonds (8–40 kJ/mol). When considering the strategies of chemical synthesis of rotaxanes, one can distinguish three general approaches: Strategy I, threading of a macrocycle onto a rod molecule and subsequent interaction of the complex formed with the blocking reagents; Strategy II, cyclisation in the presence of compounds having a dumbbell-like structure; and Strategy III, temperature-induced "slipping" of the macrocycle onto bulky terminal groups of the dumbbell-shaped molecule. Accordingly, mechanically interlocked rotaxanes constitute some of the most appropriate candidates to serve as molecular switches and machines in the rapidly developing fields of nanoelectronics and nanoelectromechanical systems (NEMS). Numerous organic cyclic host compounds such as donor-acceptor complexes [15], crown ether complexes [16], and hydrogen bonded complexes involving cyclic amides [17] have been used for rotaxane synthesis. Herein, we shall focus on various rotaxanes and catenanes exhibiting photoconductive properties.

4. Photoconductivity measurements

(hν) excites an electron to the valence band, leaving a positively charged hole behind.

The most prominent method to measure the photoelectrical properties is xerography (method shown schematically in Figure 3) [22]. The target sample is mounted and grounded on a sample holder, which can move forward or backward through a driving chain (Figure 3a). When the sample is moved to position (2) where the corotron is just above, the sample can be charged either positively or negatively. When the sample is moved to position (3), its surface potential can be measured by using an electrometer. A typical scheme of a photodischarge curve produced using this method is shown in Figure 3b. When the electronic shutter is closed, the sample is under totally dark conditions, and dark conductivity can be measured. When the shutter is open, the sample can be under exposure of either an intense erase light to measure the residual potential or a monochromatic light with known intensity to measure the photosensitivity.

Figure 2. Excitation process leading to photoconductivity in a condensed matter system, in which incoming laser light

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This technique is simple and allows for the determination of first-order xerographic properties. Therefore, it has been widely used in the study and evaluation of photoconductive organic material properties [22]. Another intensively utilized method to investigate the photoconductivity of the material is graphically shown in Figure 4. By illuminating the sample with light of various wavelengths and plotting the current evolutions as a function of time, the generated photocurrent can be measured as well as different photoeffects [23, 24], e.g., photodoping,

Figure 3. A schematic of the apparatus (a) and photodischarge curves (b). Reprinted with permission from Law [22].

#### 3. Photoconductivity

Photoconductivity is the phenomenon in which electrical current is generated in materials under light radiation. When a material such as a semiconductor absorbs photons of sufficient energy, the electrons in the valence band can be excited, cross the bandgap, and lead to the formation of electron-hole pairs resulting in increased conductivity (Figure 2). In principle, photoconductivity is a common physical phenomenon of a material, and it is particularly prominent in semiconductors due to their small bandgaps. Thus, photoconductivity generates great interest for the investigation of the electronic structure, transportation properties of materials, electron-hole pair dynamics, as well as practical applications such as photodetectors, photoresistors, and charge-coupled devices. The classic photoconductive materials consist of doped semiconductors, e.g., Si, Ge and Se [18–20], metal oxides, and sulfides as well as conductive polymers. Apart from these classic materials, the photoconductivity is also observed in an ultracold fermionic gas that is trapped in an optical lattice [21] as well as various organic compounds [22].

Figure 2. Excitation process leading to photoconductivity in a condensed matter system, in which incoming laser light (hν) excites an electron to the valence band, leaving a positively charged hole behind.

#### 4. Photoconductivity measurements

since the association between the ring and a macrocycle precursor is weak which diminishes yields. Immense amount of work has been published altering the synthetic protocols as well as introducing a variety of macrocyclic molecules, e.g., crown ethers, cyclophanes, cyclodextrins (CDs), cucurbituril, calixarene, etc. In contrast to catenanes, rotaxanes are composed of one or more macrocycles and "dumbbell-shaped" molecule(s) threaded through them. Stoppering bulky end groups also called "stoppers" prevent disassembly [14]. The word rotaxane is derived from the Latin words "rota" meaning wheel and "axis" meaning axle. The formal naming of rotaxanes according to IUPAC rules is [n]rotaxane, where n indicates the number of chemically independent components in a rotaxane assembly. The simplest form is "[2] rotaxane" which consists of one macrocycle and one dumbbell-shaped molecule. There are several interactions that can initiate self-assembly, needed for the formation of these supramolecular structures in a more efficient manner. These interactions may be hydrophobic, hydrogen bonding, or donor-acceptor interactions. The strength of these interactions varies, and this introduces different stability of the formed complexes depending on the nature of interaction, e.g., Van der Waals forces (2–4 kJ/mol), hydrophobic interactions (4–12 kJ/mol), and hydrogen bonds (8–40 kJ/mol). When considering the strategies of chemical synthesis of rotaxanes, one can distinguish three general approaches: Strategy I, threading of a macrocycle onto a rod molecule and subsequent interaction of the complex formed with the blocking reagents; Strategy II, cyclisation in the presence of compounds having a dumbbell-like structure; and Strategy III, temperature-induced "slipping" of the macrocycle onto bulky terminal groups of the dumbbell-shaped molecule. Accordingly, mechanically interlocked rotaxanes constitute some of the most appropriate candidates to serve as molecular switches and machines in the rapidly developing fields of nanoelectronics and nanoelectromechanical systems (NEMS). Numerous organic cyclic host compounds such as donor-acceptor complexes [15], crown ether complexes [16], and hydrogen bonded complexes involving cyclic amides [17] have been used for rotaxane synthesis. Herein, we shall focus on various rotaxanes and catenanes exhibiting

Photoconductivity is the phenomenon in which electrical current is generated in materials under light radiation. When a material such as a semiconductor absorbs photons of sufficient energy, the electrons in the valence band can be excited, cross the bandgap, and lead to the formation of electron-hole pairs resulting in increased conductivity (Figure 2). In principle, photoconductivity is a common physical phenomenon of a material, and it is particularly prominent in semiconductors due to their small bandgaps. Thus, photoconductivity generates great interest for the investigation of the electronic structure, transportation properties of materials, electron-hole pair dynamics, as well as practical applications such as photodetectors, photoresistors, and charge-coupled devices. The classic photoconductive materials consist of doped semiconductors, e.g., Si, Ge and Se [18–20], metal oxides, and sulfides as well as conductive polymers. Apart from these classic materials, the photoconductivity is also observed in an ultracold fermionic gas that is trapped in an optical lattice [21] as well as

photoconductive properties.

14 Advances in Photodetectors - Research and Applications

3. Photoconductivity

various organic compounds [22].

The most prominent method to measure the photoelectrical properties is xerography (method shown schematically in Figure 3) [22]. The target sample is mounted and grounded on a sample holder, which can move forward or backward through a driving chain (Figure 3a). When the sample is moved to position (2) where the corotron is just above, the sample can be charged either positively or negatively. When the sample is moved to position (3), its surface potential can be measured by using an electrometer. A typical scheme of a photodischarge curve produced using this method is shown in Figure 3b. When the electronic shutter is closed, the sample is under totally dark conditions, and dark conductivity can be measured. When the shutter is open, the sample can be under exposure of either an intense erase light to measure the residual potential or a monochromatic light with known intensity to measure the photosensitivity.

This technique is simple and allows for the determination of first-order xerographic properties. Therefore, it has been widely used in the study and evaluation of photoconductive organic material properties [22]. Another intensively utilized method to investigate the photoconductivity of the material is graphically shown in Figure 4. By illuminating the sample with light of various wavelengths and plotting the current evolutions as a function of time, the generated photocurrent can be measured as well as different photoeffects [23, 24], e.g., photodoping,

Figure 3. A schematic of the apparatus (a) and photodischarge curves (b). Reprinted with permission from Law [22].

encapsulation of long conductive/photoconductive macromolecules within macrocycles such as CDs could result in the development of polyrotaxane molecular wires which, in a similar fashion to their macroscopic wire analogues, possess a conducting internal and insulating external part. The insulating part could prevent short-circuit problems in future molecular circuits involving these wires. The role of the insulating macrocycles in the photoconductivity of polyrotaxanes is also reviewed in this chapter. The last but substantially beneficiary feature of the development of photoconductive rotaxanes and polyrotaxanes is multifunctionality. Up to date a vast number of rotaxanes have been reported undergoing fully controlled shuttle motions, exhibiting switchability, photo- and electro-chromic, and photoluminescent properties. Combining one or more of these promising properties with photoconductivity could

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result in novel types of materials able to perform multiple functions upon demand.

Azo dyes constitute a widely known class of organic pigments with significant industrial interest. These dyes exist in numerous products of everyday use, ranging from textile and leather dyeing agents to food colorants and DVDR/+R disc recording layer materials. All azo compounds contain one or more units of the azo (–N]N–) chromophore connected to carbon atoms in both sides. The vast majority of azo dyes bear an azo group coupled to aromatic substituents such as benzene or naphthalene rings. To date a remarkable number of azo dyes have been synthesized and characterized [26]. This large number comes as a consequence of the ease of synthesis of azo compounds mainly relying on azo coupling, which involves an electrophilic substitution reaction between an aryl diazonium cation and a coupling partner. Typical coupling partners can be various aromatic compounds possessing electron-donating groups such as –OH, –OR, or –NR2 functional groups [27]. Numerous rotaxanes comprising

The properties of azo dyes are not merely related to their color/light absorption properties but also to their vivid photochemistry, as they readily undergo reversible E/Z isomerizations (see Figure 5) in most of the cases via excitation with near UV or even visible light [28]. The reversibility as well as the low light energy demand for the accomplishment of this photochemical process renders azo compounds and materials thereof even more attractive for a number of optoelectronic applications [26–28]. Indeed, this photoreaction has been exploited in plenty azo-bearing materials including rotaxanes. Murakami et al. described 20 years ago the first light-driven molecular shuttle based on an azo rotaxane [29]. Deligkiozi et al. have reported controllable shuttling motions of α-cyclodextrin in [2]rotaxanes bearing a fully conjugated arylazo-based linear part [30]. Tian and coworkers reported on an azo-involving lightdriven rotaxane molecular shuttle with dual fluorescence addresses comprising two different fluorescent naphthalimides and α-CD [31]. Indeed to date numerous other examples of photosensitive azo-involving rotaxanes have been reported [32]. Importantly, many azo compounds are known to exhibit photoconductive behavior. In 1969, Rau was the first to report the photoconductive behavior of azo dyes and specifically observed the photocurrents that form thin layers of a simple azo compound: l-(phenylazo)-2-naphthol (Figure 5) [33]. Six years after the pioneering work of Rau, Champ and Shattuck reported the use of chlorodiane blue, a bisazo compound (a derivative of 1-(phenylazo)-2-naphthol) as a photogenerating pigment in xerographic devices [34]. These two early scientific reports initiated a huge endeavor for the

5.1.2. Azo dyes and rotaxanes thereof

the arylazo units have been also reported [26].

Figure 4. A schematic of photoconductance experimental setup.

photogating, etc. can be well studied. In addition, by tuning the back-gate voltage of the device, more phenomena such as photoconductive gain effect can also be studied [25].

#### 5. Photoconductive interlocked molecules

#### 5.1. Photoconductive rotaxanes and polyrotaxanes

#### 5.1.1. Why rotaxanes?

Rotaxanation, i.e., the inclusion of an axial molecule in the cavity of a macrocyclic molecule, is an interesting approach for the design of novel photoconductive materials which can efficiently introduce a number of beneficiary characteristics to these materials. There are different reasons which could justify why designing rotaxane photoconductive structures can lead to promising new materials. First of all, the moieties or functional groups which introduce photoconductivity to a compound are often unstable and chemically labile. The cavities of suitable macrocycles could offer protection to such entities, and this is vital for the durability and proper function of a photoconductive compound or material. That is, for instance, the case of azo dyes and squaraines, the rotaxanes of which will be examined in this chapter. Moreover, in many cases of photoconductive materials, prevention of intermolecular interactions is sought after. Encapsulation of photoconductive axial molecules in molecular rings often reduces the intermolecular interactions such as π–π stacking without hampering the charge transport. This is especially important in photoluminescent compounds where parallel alignment and interaction of π-conjugated molecules are obstacles. In addition to that, the noncovalent interactions developed between the axial and macrocyclic components in a rotaxane are overly important as they provide geometry stabilization and optimal orientation of these components so that charge transfer and transport are efficient. Such interactions are essential for the photoconductive behavior of a number of viologen-involving rotaxanes bearing electron-donating counterparts; a few such systems are reviewed herein. Furthermore, the encapsulation of long conductive/photoconductive macromolecules within macrocycles such as CDs could result in the development of polyrotaxane molecular wires which, in a similar fashion to their macroscopic wire analogues, possess a conducting internal and insulating external part. The insulating part could prevent short-circuit problems in future molecular circuits involving these wires. The role of the insulating macrocycles in the photoconductivity of polyrotaxanes is also reviewed in this chapter. The last but substantially beneficiary feature of the development of photoconductive rotaxanes and polyrotaxanes is multifunctionality. Up to date a vast number of rotaxanes have been reported undergoing fully controlled shuttle motions, exhibiting switchability, photo- and electro-chromic, and photoluminescent properties. Combining one or more of these promising properties with photoconductivity could result in novel types of materials able to perform multiple functions upon demand.

#### 5.1.2. Azo dyes and rotaxanes thereof

photogating, etc. can be well studied. In addition, by tuning the back-gate voltage of the

Rotaxanation, i.e., the inclusion of an axial molecule in the cavity of a macrocyclic molecule, is an interesting approach for the design of novel photoconductive materials which can efficiently introduce a number of beneficiary characteristics to these materials. There are different reasons which could justify why designing rotaxane photoconductive structures can lead to promising new materials. First of all, the moieties or functional groups which introduce photoconductivity to a compound are often unstable and chemically labile. The cavities of suitable macrocycles could offer protection to such entities, and this is vital for the durability and proper function of a photoconductive compound or material. That is, for instance, the case of azo dyes and squaraines, the rotaxanes of which will be examined in this chapter. Moreover, in many cases of photoconductive materials, prevention of intermolecular interactions is sought after. Encapsulation of photoconductive axial molecules in molecular rings often reduces the intermolecular interactions such as π–π stacking without hampering the charge transport. This is especially important in photoluminescent compounds where parallel alignment and interaction of π-conjugated molecules are obstacles. In addition to that, the noncovalent interactions developed between the axial and macrocyclic components in a rotaxane are overly important as they provide geometry stabilization and optimal orientation of these components so that charge transfer and transport are efficient. Such interactions are essential for the photoconductive behavior of a number of viologen-involving rotaxanes bearing electron-donating counterparts; a few such systems are reviewed herein. Furthermore, the

device, more phenomena such as photoconductive gain effect can also be studied [25].

5. Photoconductive interlocked molecules

Figure 4. A schematic of photoconductance experimental setup.

16 Advances in Photodetectors - Research and Applications

5.1. Photoconductive rotaxanes and polyrotaxanes

5.1.1. Why rotaxanes?

Azo dyes constitute a widely known class of organic pigments with significant industrial interest. These dyes exist in numerous products of everyday use, ranging from textile and leather dyeing agents to food colorants and DVDR/+R disc recording layer materials. All azo compounds contain one or more units of the azo (–N]N–) chromophore connected to carbon atoms in both sides. The vast majority of azo dyes bear an azo group coupled to aromatic substituents such as benzene or naphthalene rings. To date a remarkable number of azo dyes have been synthesized and characterized [26]. This large number comes as a consequence of the ease of synthesis of azo compounds mainly relying on azo coupling, which involves an electrophilic substitution reaction between an aryl diazonium cation and a coupling partner. Typical coupling partners can be various aromatic compounds possessing electron-donating groups such as –OH, –OR, or –NR2 functional groups [27]. Numerous rotaxanes comprising the arylazo units have been also reported [26].

The properties of azo dyes are not merely related to their color/light absorption properties but also to their vivid photochemistry, as they readily undergo reversible E/Z isomerizations (see Figure 5) in most of the cases via excitation with near UV or even visible light [28]. The reversibility as well as the low light energy demand for the accomplishment of this photochemical process renders azo compounds and materials thereof even more attractive for a number of optoelectronic applications [26–28]. Indeed, this photoreaction has been exploited in plenty azo-bearing materials including rotaxanes. Murakami et al. described 20 years ago the first light-driven molecular shuttle based on an azo rotaxane [29]. Deligkiozi et al. have reported controllable shuttling motions of α-cyclodextrin in [2]rotaxanes bearing a fully conjugated arylazo-based linear part [30]. Tian and coworkers reported on an azo-involving lightdriven rotaxane molecular shuttle with dual fluorescence addresses comprising two different fluorescent naphthalimides and α-CD [31]. Indeed to date numerous other examples of photosensitive azo-involving rotaxanes have been reported [32]. Importantly, many azo compounds are known to exhibit photoconductive behavior. In 1969, Rau was the first to report the photoconductive behavior of azo dyes and specifically observed the photocurrents that form thin layers of a simple azo compound: l-(phenylazo)-2-naphthol (Figure 5) [33]. Six years after the pioneering work of Rau, Champ and Shattuck reported the use of chlorodiane blue, a bisazo compound (a derivative of 1-(phenylazo)-2-naphthol) as a photogenerating pigment in xerographic devices [34]. These two early scientific reports initiated a huge endeavor for the

Figure 5. Scheme depicting the reversible E/Z isomerization of azobenzene (upper panel). The hydroxyl azo/ketohydrazone tautomerism of l-(phenylazo)-2-naphthol (lower panel).

development of novel azo pigments with photoconductive properties, an endeavor which continues to date. Many research groups have come out with various photoconductive azo compounds mostly with structures relative to the parent l-(phenylazo)-2-naphthol, over the years [22]. The photoconductive behavior of this parent azo pigment is narrowly connected to its structure and specifically to the hydroxyl azo/ketohydrazone tautomerism that this molecule and its derivatives exhibit (Figure 5) [22].

The supramolecular insulation provided by encapsulation of an azo dye in α-CD has been earlier utilized by Haque et al. [38]. In their work they managed to thread π-conjugated tri-azo dye molecules through α-CD and then immobilize the resulting [2]rotaxanes onto nanocrystalline TiO2 films (Figure 7). Transient absorption spectroscopy experiments supported that charge recombination was considerably retarded in the case of the as formed TiO2 films when compared to non encapsulated dyes. This finding is very stimulating as it indicates that photocurrents are still generated by the conjugated encapsulated molecules, while the insulating α-CD part maintains a slow charge recombination. In the light of that, these photoconductive interlocked azo compounds are considered as promising for dye-sensitized solar cell

Figure 6. The chemical structure of the tetracationic part of the [2]rotaxane by Deligkiozi et al. [35] (lower panel) and that

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Viologens constitute a class of heterocyclic compounds with remarkable properties [40]. They

substituted by a chemical group which is often an aliphatic chain or an aryl group (see Figure 8) [40]. Due to their intense electron withdrawing (EW) character, aromaticity, as well as photo- and electro-active nature, they have been utilized as key components in a vast number of new materials [40, 42]. Viologens are also well known for their intense electrochromism which is attributed to the reversible one-electron reduction they readily undergo electrochemically or by means of reducing agents. They readily form charge transfer complexes (CTCs) with a variety of electron-donating species, e.g., ferrocyanides [43], tetrathiafulvalene (TTF) derivatives [41], as well as phenols [44]. In these complexes charge is reversibly transferred from the electron-donating part to the viologen upon absorption of visible light. Because of that, CTCs are colorful compounds and very photosensitive. Today, there is clear evidence that CTCs involving viologens exhibit photoconductive properties. This is, for instance, the case in some recent reported viologen/TTF CTCs (see Figure 8). Huo et al. observed marked photocurrent responses directly from such crystalline CTCs or from


(DSSC) applications [39].

are 4,4<sup>0</sup>

5.1.3. Viologen-involving rotaxanes

of the tetracationic linear α-CD-free precursor (upper panel).

Nonetheless, there are also recent reports of photoconductive azo dyes with structure different from the "inspiring" structure of l-(phenylazo)-2-naphthol. Recently, Deligkiozi et al. observed photocurrents from a [2]rotaxane of an azobenzene-based dye encapsulated in α-cyclodextrin (α-CD) as shown in Figure 6 [35]. The photoconductivity of this interlocked azo dye measured using a wet method [35] was proved to be significantly higher than its α-CD-free precursor. The aforementioned [2]rotaxane was one of the first examples of rotaxanes involving an axial linear part with full π-conjugation [30, 36]. This robust aromatic skeleton provides the α-CDfree precursor some conductivity even in the dark which is reinforced when the dye is irradiated with white light. Remarkably though, the corresponding [2]rotaxane with α-CD appears to exhibit a significantly higher photoconductivity than the α-CD-free dye. Here, it is important to note that supramolecular insulation provided by α-CD (an insulating compound) is expected to result in a reduced conductivity of the [2]rotaxane when compared to its α-CD-free analogue. Yet, Cacialli et al. have shown that CD-encapsulated conductive polyrotaxanes with poly(para-phenylene) and poly(4,4<sup>0</sup> -diphenylene vinylene) continue to exhibit high conductivity despite the cyclodextrin insulating impact. It was concluded that cyclodextrin encapsulation inhibits parallel arrangement of the molecules without causing elimination of charge transport [37].

In the case of [2]rotaxane by Deligkiozi et al., photoconductivity was rationalized in terms of the non-covalent interactions of the cavity of α-CD and the encapsulated part of the azobenzene unit of this compound. These interactions result in stabilization of the geometry of the azobenzene part of the molecule [35]. Presumably, this stabilizing geometry effect resembles the corresponding effect observed in the case of the tautomeric l-(phenylazo)-2-naphthol derivatives (Figure 5).

Figure 6. The chemical structure of the tetracationic part of the [2]rotaxane by Deligkiozi et al. [35] (lower panel) and that of the tetracationic linear α-CD-free precursor (upper panel).

The supramolecular insulation provided by encapsulation of an azo dye in α-CD has been earlier utilized by Haque et al. [38]. In their work they managed to thread π-conjugated tri-azo dye molecules through α-CD and then immobilize the resulting [2]rotaxanes onto nanocrystalline TiO2 films (Figure 7). Transient absorption spectroscopy experiments supported that charge recombination was considerably retarded in the case of the as formed TiO2 films when compared to non encapsulated dyes. This finding is very stimulating as it indicates that photocurrents are still generated by the conjugated encapsulated molecules, while the insulating α-CD part maintains a slow charge recombination. In the light of that, these photoconductive interlocked azo compounds are considered as promising for dye-sensitized solar cell (DSSC) applications [39].

#### 5.1.3. Viologen-involving rotaxanes

development of novel azo pigments with photoconductive properties, an endeavor which continues to date. Many research groups have come out with various photoconductive azo compounds mostly with structures relative to the parent l-(phenylazo)-2-naphthol, over the years [22]. The photoconductive behavior of this parent azo pigment is narrowly connected to its structure and specifically to the hydroxyl azo/ketohydrazone tautomerism that this mole-

Figure 5. Scheme depicting the reversible E/Z isomerization of azobenzene (upper panel). The hydroxyl azo/ketohydrazone

Nonetheless, there are also recent reports of photoconductive azo dyes with structure different from the "inspiring" structure of l-(phenylazo)-2-naphthol. Recently, Deligkiozi et al. observed photocurrents from a [2]rotaxane of an azobenzene-based dye encapsulated in α-cyclodextrin (α-CD) as shown in Figure 6 [35]. The photoconductivity of this interlocked azo dye measured using a wet method [35] was proved to be significantly higher than its α-CD-free precursor. The aforementioned [2]rotaxane was one of the first examples of rotaxanes involving an axial linear part with full π-conjugation [30, 36]. This robust aromatic skeleton provides the α-CDfree precursor some conductivity even in the dark which is reinforced when the dye is irradiated with white light. Remarkably though, the corresponding [2]rotaxane with α-CD appears to exhibit a significantly higher photoconductivity than the α-CD-free dye. Here, it is important to note that supramolecular insulation provided by α-CD (an insulating compound) is expected to result in a reduced conductivity of the [2]rotaxane when compared to its α-CD-free analogue. Yet, Cacialli et al. have shown that CD-encapsulated conductive polyrotaxanes with

despite the cyclodextrin insulating impact. It was concluded that cyclodextrin encapsulation inhibits parallel arrangement of the molecules without causing elimination of charge

In the case of [2]rotaxane by Deligkiozi et al., photoconductivity was rationalized in terms of the non-covalent interactions of the cavity of α-CD and the encapsulated part of the azobenzene unit of this compound. These interactions result in stabilization of the geometry of the azobenzene part of the molecule [35]. Presumably, this stabilizing geometry effect resembles the corresponding effect observed in the case of the tautomeric l-(phenylazo)-2-naphthol derivatives (Figure 5).


cule and its derivatives exhibit (Figure 5) [22].

tautomerism of l-(phenylazo)-2-naphthol (lower panel).

18 Advances in Photodetectors - Research and Applications

poly(para-phenylene) and poly(4,4<sup>0</sup>

transport [37].

Viologens constitute a class of heterocyclic compounds with remarkable properties [40]. They are 4,4<sup>0</sup> -bipyridine derivatives having both their nitrogen atoms quaternized, i.e., they are substituted by a chemical group which is often an aliphatic chain or an aryl group (see Figure 8) [40]. Due to their intense electron withdrawing (EW) character, aromaticity, as well as photo- and electro-active nature, they have been utilized as key components in a vast number of new materials [40, 42]. Viologens are also well known for their intense electrochromism which is attributed to the reversible one-electron reduction they readily undergo electrochemically or by means of reducing agents. They readily form charge transfer complexes (CTCs) with a variety of electron-donating species, e.g., ferrocyanides [43], tetrathiafulvalene (TTF) derivatives [41], as well as phenols [44]. In these complexes charge is reversibly transferred from the electron-donating part to the viologen upon absorption of visible light. Because of that, CTCs are colorful compounds and very photosensitive. Today, there is clear evidence that CTCs involving viologens exhibit photoconductive properties. This is, for instance, the case in some recent reported viologen/TTF CTCs (see Figure 8). Huo et al. observed marked photocurrent responses directly from such crystalline CTCs or from

Figure 7. Schematic representation of the α-CD-encapsulated tri-azo dye onto TiO2 nanocrystalline films utilized by Haque et al. (see Ref. [38]).

prepared film electrodes involving the CTCs depicted in Figure 8. A large number of rotaxanes involving viologens have also been reported exhibiting donor-acceptor interactions in which viologens play an important role as strong EW species [32, 45]. In these rotaxanes, viologen units are encountered either as parts of the axial or as parts of the macrocyclic components. In the latter class of rotaxanes, they are often used in cyclophane structures (see Figure 9). Such rotaxanes are considered for high-tech applications due to the ease of control of their function through electrical or light triggering. Feng et al. have achieved reproducible nanorecording on rotaxane thin films comprising TTF-involving axial and viologen cyclophane components [47]. A few years ago, Sheeney-Haj-Ichia and Willner reported that pseudorotaxane monolayers comprising viologen cyclophane units exhibit photocurrents eightfold higher than the ones observed in the case of the control monolayers lacking the viologen component (Figure 9) [46].

These significantly amplified photocurrents observed in the pseudorotaxane assembly were attributed to vectorial electron transfer of photoexcited conduction-band electrons to the strong electron accepting component. According to the authors [46], this fact leads to charge separation and retardation of electron-hole recombination. This finding is also in line with the photoconductive character of viologen CTCs and indicates that interlocked molecules and macromolecules comprising viologen CTC entities are promising materials with potential photoconductive properties.

5.1.4. Squaraine rotaxanes

permission from Huo et al. [41].

substituents (R and R<sup>0</sup>

Squaraine compounds constitute a widely known class of organic photoconductive compounds [22]. It was as early as 1966 when Sprenger and Ziegenbein reported the synthesis of intensely colored compounds derived from squaric acid (see Structure I in Figure 10) [49]. It was observed that the compound produced is characterized by a unique electronic structure resulting in interesting properties. Many relative compounds were subsequently synthesized. These fascinating compounds bear an internal donor-acceptor-donor (D-A-D) structure which can be represented through the resonance structures depicted in Figure 10 [22, 50]. Around 40 years ago, Schmidt proposed the name squaraine for these compounds [22]. In 1974 Champ and Shattuck were the first to report the photoconductive properties of squaraine dyes [51]. They revealed that squaraines are able to generate electron-hole pairs in bilayer xerographic devices through light irradiation [51]. Awhile before this report, squaraines had already been proposed as sensitizers for ZnO photoconductors [52]. As mentioned squaraines are deeply colored compounds, and their absorption and emission are situated in the deep-red and near-infrared (NIR)

Figure 8. (A) Scheme representing the reversible one-electron reduction of a viologen dication comprising two different

and a dianionic TTF derivative according to Huo et al. [41]. (C) The photocurrent responses observed from the CTCs of middle panel, measured in crystals of the CTCs (left) and in thin-film electrodes (right). Plots of panel (C) reprinted with

). (B) Scheme depicting the formation of photoconductive CTC complexes of a group of viologens

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In 2007 Saha et al. [48] reported on a redox-driven multicomponent rotaxane shuttle comprising a linear component which involved TTF, a naphthyl ether, and a porphyrin. The macrocyclic component employed was the same bis-viologen cyclophane utilized by Sheeney-Haj-Ichia and Willner [46]. C60 was utilized as a strong electron acceptor/bulk ending group. The authors emphasized that such donor-chromophore-acceptor system could generate photocurrents. This example constitutes one interesting case of a multifunctional material with potential photoconductive properties being able to also function as a molecular shuttle.

Figure 8. (A) Scheme representing the reversible one-electron reduction of a viologen dication comprising two different substituents (R and R<sup>0</sup> ). (B) Scheme depicting the formation of photoconductive CTC complexes of a group of viologens and a dianionic TTF derivative according to Huo et al. [41]. (C) The photocurrent responses observed from the CTCs of middle panel, measured in crystals of the CTCs (left) and in thin-film electrodes (right). Plots of panel (C) reprinted with permission from Huo et al. [41].

#### 5.1.4. Squaraine rotaxanes

prepared film electrodes involving the CTCs depicted in Figure 8. A large number of rotaxanes involving viologens have also been reported exhibiting donor-acceptor interactions in which viologens play an important role as strong EW species [32, 45]. In these rotaxanes, viologen units are encountered either as parts of the axial or as parts of the macrocyclic components. In the latter class of rotaxanes, they are often used in cyclophane structures (see Figure 9). Such rotaxanes are considered for high-tech applications due to the ease of control of their function through electrical or light triggering. Feng et al. have achieved reproducible nanorecording on rotaxane thin films comprising TTF-involving axial and viologen cyclophane components [47]. A few years ago, Sheeney-Haj-Ichia and Willner reported that pseudorotaxane monolayers comprising viologen cyclophane units exhibit photocurrents eightfold higher than the ones observed in the case of the control monolayers lacking the viologen component (Figure 9) [46]. These significantly amplified photocurrents observed in the pseudorotaxane assembly were attributed to vectorial electron transfer of photoexcited conduction-band electrons to the strong electron accepting component. According to the authors [46], this fact leads to charge separation and retardation of electron-hole recombination. This finding is also in line with the photoconductive character of viologen CTCs and indicates that interlocked molecules and macromolecules comprising viologen CTC entities are promising materials with potential photoconductive

Figure 7. Schematic representation of the α-CD-encapsulated tri-azo dye onto TiO2 nanocrystalline films utilized by

In 2007 Saha et al. [48] reported on a redox-driven multicomponent rotaxane shuttle comprising a linear component which involved TTF, a naphthyl ether, and a porphyrin. The macrocyclic component employed was the same bis-viologen cyclophane utilized by Sheeney-Haj-Ichia and Willner [46]. C60 was utilized as a strong electron acceptor/bulk ending group. The authors emphasized that such donor-chromophore-acceptor system could generate photocurrents. This example constitutes one interesting case of a multifunctional material with potential

photoconductive properties being able to also function as a molecular shuttle.

properties.

Haque et al. (see Ref. [38]).

20 Advances in Photodetectors - Research and Applications

Squaraine compounds constitute a widely known class of organic photoconductive compounds [22]. It was as early as 1966 when Sprenger and Ziegenbein reported the synthesis of intensely colored compounds derived from squaric acid (see Structure I in Figure 10) [49]. It was observed that the compound produced is characterized by a unique electronic structure resulting in interesting properties. Many relative compounds were subsequently synthesized. These fascinating compounds bear an internal donor-acceptor-donor (D-A-D) structure which can be represented through the resonance structures depicted in Figure 10 [22, 50]. Around 40 years ago, Schmidt proposed the name squaraine for these compounds [22]. In 1974 Champ and Shattuck were the first to report the photoconductive properties of squaraine dyes [51]. They revealed that squaraines are able to generate electron-hole pairs in bilayer xerographic devices through light irradiation [51]. Awhile before this report, squaraines had already been proposed as sensitizers for ZnO photoconductors [52]. As mentioned squaraines are deeply colored compounds, and their absorption and emission are situated in the deep-red and near-infrared (NIR)

Figure 9. (A) Illustration of the setup used by Sheeney-Haj-Ichia and Willner without cyclophane. (B) The setup after inclusion in a tetracationic cyclophane. (C) Plot depicting the photocurrent response observed for the system in panel A (solid line) and that in panel B (dashed line) vs. the irradiation wavelength. Plot of panel (C) reprinted with permission from Sheeney-Haj-Ichia and Willner [46].

All these features are narrowly connected to their electronic structure, and they are essential for a vast number of imaging applications [50]. A significant drawback of squaraines is their instability against strong nucleophiles as well as their aggregation propensity which pulls down their fluorescence and potentially photoconductivity. These problems can be solved by the use of protecting threading macrocycles, i.e., through rotaxanation of the sensitive core. This approach was first employed by Leigh and coworkers who managed to synthesize [2] rotaxanes utilizing normal squaraine structures and suitable amide-macrocyclic compounds (Figure 10B) [55, 56]. The as structured rotaxanes are characterized by significantly higher chemical and photophysical stabilities than the non encapsulated squaraines. This revolutionary study inspired a lot of other research groups to design and synthesize a wide variety of squaraine-based rotaxanes with potentials in a number of applications [50]. The corresponding rotaxanes do not hamper the properties of squaraines, but instead the properties are retained or even improved. Due to their high photoconductivity, promising performance in DSSC applications as well as other biologically relevant applications of rotaxanes of squaraines, their

Figure 11. (A) Current-voltage characteristics for a squaraine dye (type (II) with R = C8H17 and R<sup>0</sup> = Et) and photocurrent action spectrum (inset). (B) Chemical structure of squaraine III, (C) frontier orbitals, and (D) charge density of dye III.

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In recent years there is an increasing interest in the design and synthesis/fabrication of molecular wires, i.e., conductive conjugated polymers of high conductivity. Even though the research endeavors to develop molecular wires were initiated theoretically already in the 1940s using quantum mechanics [57], there is today a tremendous interest in this type of nanosized wires for a range of high-tech applications. In such systems prevention of short circuits could be achieved through threading of a conductive polymer within the

use is currently seriously considered.

Figures in Panels A, C, and D reprinted with permission from [53].

5.1.5. Photoconductive polyrotaxanes

Figure 10. (A) The resonance structures of squaraines (Ia–c). (B) A squaraine rotaxane involving an amide macrocycle.

region [50]. These features along with their photoconductivity render squaraines important candidates for DSSC applications. In these technologies novel sensitizers absorbing in NIR wavelength region are required in order to boost the photoconversion efficiency. Indeed Yum et al. reported a photoconversion efficiency as high as 4.5% when using an unsymmetric squaraine dye (structure III depicted in Figure 11B) [54]. This work essentially indicated that squaraines are useful candidates for DSSC (details in Figure 11). Apart from marked photoconductive compounds, squaraines are generally very photosensitive and fluorescent [50].

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Figure 11. (A) Current-voltage characteristics for a squaraine dye (type (II) with R = C8H17 and R<sup>0</sup> = Et) and photocurrent action spectrum (inset). (B) Chemical structure of squaraine III, (C) frontier orbitals, and (D) charge density of dye III. Figures in Panels A, C, and D reprinted with permission from [53].

All these features are narrowly connected to their electronic structure, and they are essential for a vast number of imaging applications [50]. A significant drawback of squaraines is their instability against strong nucleophiles as well as their aggregation propensity which pulls down their fluorescence and potentially photoconductivity. These problems can be solved by the use of protecting threading macrocycles, i.e., through rotaxanation of the sensitive core. This approach was first employed by Leigh and coworkers who managed to synthesize [2] rotaxanes utilizing normal squaraine structures and suitable amide-macrocyclic compounds (Figure 10B) [55, 56]. The as structured rotaxanes are characterized by significantly higher chemical and photophysical stabilities than the non encapsulated squaraines. This revolutionary study inspired a lot of other research groups to design and synthesize a wide variety of squaraine-based rotaxanes with potentials in a number of applications [50]. The corresponding rotaxanes do not hamper the properties of squaraines, but instead the properties are retained or even improved. Due to their high photoconductivity, promising performance in DSSC applications as well as other biologically relevant applications of rotaxanes of squaraines, their use is currently seriously considered.

#### 5.1.5. Photoconductive polyrotaxanes

region [50]. These features along with their photoconductivity render squaraines important candidates for DSSC applications. In these technologies novel sensitizers absorbing in NIR wavelength region are required in order to boost the photoconversion efficiency. Indeed Yum et al. reported a photoconversion efficiency as high as 4.5% when using an unsymmetric squaraine dye (structure III depicted in Figure 11B) [54]. This work essentially indicated that squaraines are useful candidates for DSSC (details in Figure 11). Apart from marked photocon-

Figure 10. (A) The resonance structures of squaraines (Ia–c). (B) A squaraine rotaxane involving an amide macrocycle.

Figure 9. (A) Illustration of the setup used by Sheeney-Haj-Ichia and Willner without cyclophane. (B) The setup after inclusion in a tetracationic cyclophane. (C) Plot depicting the photocurrent response observed for the system in panel A (solid line) and that in panel B (dashed line) vs. the irradiation wavelength. Plot of panel (C) reprinted with permission

from Sheeney-Haj-Ichia and Willner [46].

22 Advances in Photodetectors - Research and Applications

ductive compounds, squaraines are generally very photosensitive and fluorescent [50].

In recent years there is an increasing interest in the design and synthesis/fabrication of molecular wires, i.e., conductive conjugated polymers of high conductivity. Even though the research endeavors to develop molecular wires were initiated theoretically already in the 1940s using quantum mechanics [57], there is today a tremendous interest in this type of nanosized wires for a range of high-tech applications. In such systems prevention of short circuits could be achieved through threading of a conductive polymer within the cavities of insulating (protecting) macrocycles [58]. These polyrotaxane-structured wires also called insulated molecular wires (IMWs), with nanometer dimensions, could be used in nanosized circuits [53]. The role of the insulating components (usually α- and β-CDs) is an important research subject as it clearly affects the conductivity and photoconductivity of polyrotaxane wires. In 2009 Terao et al. [59] studied a permethylated α-CD (PM-α-CD) polyrotaxane of a poly(phenylene ethynylene)-based polymer (Figure 12A) and reported the formation of a prominently insulating organic semiconductor wire exhibiting remarkably high hole mobility along the core π-conjugated polymer. They also reported lightinduced currents observed upon excitation at λ = 355 nm (Figure 12A). Terao et al. some years later [61] based on previous theoretical publications compared experimentally the charge mobilities of linear and zig-zag polyrotaxanes involving conjugated polymers and permethylated α-CD.

thiophene fragments of the macromolecule, and this lead to hampering the transport of

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Catenanes constitute another important class of interlocked molecules. Just like rotaxanes they are stabilized through mechanical bonds [9, 32]. Yet, they consist of two or more macrocycles interlocked in a way that resembles the connectivity of rings in a chain (Latin: catena = chain). There is a large variety of catenated structures reported to date with numerous applications. There are various reasons why catenanes could become important candidates for new photoconductive materials. As also mentioned for rotaxanes, encapsulation of a sensitive photoconductive moiety or functionality in a molecule could significantly increase the durability of the material and protect the desired photoconductive properties. Thus, interlocking photoconductive (or more generally photosensitive) macrocycles could potentially lead to promising stable catenated materials with optoelectronic applications. Moreover, geometry fixation and proper orientation in catenanes can give rise to intermolecular interactions (e.g., π–π stacking, etc.) facilitating efficient charge transfer in such materials. This is a key property which is discussed in more detail in this chapter. Finally, polycatenanes involving photoconductive parts could be perfect candidate multifunctional materials, as in such structures one can introduce photoconductivity via embedding repeated photoconductive catenane units in macromolecules with

special properties, e.g., electrical or thermal conductivity, mechanical strength, etc.

Even though numerous examples of catenanes and polycatenanes have been reported, there is a limited number photoconductive catenanes and polycatenanes. However, there is strong indication that such materials could also exhibit promising photoconductive behavior. The main types of organic photoconductive molecules utilized in rotaxanes and polyrotaxanes

About 15 years ago, Simone [63] reported on the synthesis and characterization of some polycatenane repeated units of cyclophane connected to thiophene rings (red-colored part in Figure 13) interlocked with a bis-viologen tetracationic cyclophane (blue-colored species in Figure 13). This approach involving the aforementioned two cyclophanes was initially employed by Stoddart and coworkers [15] and is a very popular combination for numerous rotaxanes and catenanes (see, for instance, the pseudorotaxane in Figure 9). The resulting polycatenane of Simone and Swager is stabilized through π-stacking between the aromatic bipyridinium and benzene-1,4-diether rings of the interlocked macrocycles. The catenane in Figure 13 which is colored green exhibits a charge transfer visible band situated at λ = 626 nm. This polycatenane as well as another variant was reported to be conductive (linear part is a π-conjugated polymer) [64] but also to exhibit significant photocurrent responses [63]. This example constitutes an important case enabling the design of novel

carriers which in turn yielded in a limited photovoltaic effect [60].

5.2. Catenanes and polycatenanes

5.2.2. Photoconductive catenanes

photoconductive polycatenanes.

can be also utilized in catenanes and polycatenanes.

5.2.1. The structure of catenanes and its benefits

They reported increased charge mobilities for the zig-zag polymer and confirmed the lightinduced formation of charge carriers in the case of the linear polyrotaxane. However, they observed that rapid free carrier-formation processes were overlapped in the zig-zag polyrotaxanes. These stimulating findings indicate that IMWs do exhibit photoconductivity, but clearly the geometry of the macromolecules affects their photoconductive behavior. Encapsulation of a conjugated polymer such as the aforementioned π-conjugated polymer in the insulating cavities of PM-α-CD leads to increased lifetimes of charged radicals on the conjugated core via hindering charge recombination processes [62]. Moreover encapsulation results in marked fluorescence enhancement in this kind of polyrotaxanes, particularly in the solid state, suggesting that encapsulation is crucial for the achievement of efficient fluorescence properties [62]. More recently, Kostromin et al. [60] studied the photovoltaic effect and charge carrier mobility of some bithiophene conducting polymers, both "bear" and encapsulated in β-CD units (see Figure 12B). They concluded that the β-CD introduced marked insulation of

Figure 12. (A) Structure of the IMWs studied by Tarao et al. [59] along with the transient absorption spectrum of IMW (I) after pulse exposure and conductivity transients observed for (I) (blue) and (II) (red) upon 355 nm excitation. Figures reprinted with permission from [59]. (B) Structure of the conjugated polymer and IMW investigated by Kostromin et al. [60].

thiophene fragments of the macromolecule, and this lead to hampering the transport of carriers which in turn yielded in a limited photovoltaic effect [60].

#### 5.2. Catenanes and polycatenanes

cavities of insulating (protecting) macrocycles [58]. These polyrotaxane-structured wires also called insulated molecular wires (IMWs), with nanometer dimensions, could be used in nanosized circuits [53]. The role of the insulating components (usually α- and β-CDs) is an important research subject as it clearly affects the conductivity and photoconductivity of polyrotaxane wires. In 2009 Terao et al. [59] studied a permethylated α-CD (PM-α-CD) polyrotaxane of a poly(phenylene ethynylene)-based polymer (Figure 12A) and reported the formation of a prominently insulating organic semiconductor wire exhibiting remarkably high hole mobility along the core π-conjugated polymer. They also reported lightinduced currents observed upon excitation at λ = 355 nm (Figure 12A). Terao et al. some years later [61] based on previous theoretical publications compared experimentally the charge mobilities of linear and zig-zag polyrotaxanes involving conjugated polymers and

They reported increased charge mobilities for the zig-zag polymer and confirmed the lightinduced formation of charge carriers in the case of the linear polyrotaxane. However, they observed that rapid free carrier-formation processes were overlapped in the zig-zag polyrotaxanes. These stimulating findings indicate that IMWs do exhibit photoconductivity, but clearly the geometry of the macromolecules affects their photoconductive behavior. Encapsulation of a conjugated polymer such as the aforementioned π-conjugated polymer in the insulating cavities of PM-α-CD leads to increased lifetimes of charged radicals on the conjugated core via hindering charge recombination processes [62]. Moreover encapsulation results in marked fluorescence enhancement in this kind of polyrotaxanes, particularly in the solid state, suggesting that encapsulation is crucial for the achievement of efficient fluorescence properties [62]. More recently, Kostromin et al. [60] studied the photovoltaic effect and charge carrier mobility of some bithiophene conducting polymers, both "bear" and encapsulated in β-CD units (see Figure 12B). They concluded that the β-CD introduced marked insulation of

Figure 12. (A) Structure of the IMWs studied by Tarao et al. [59] along with the transient absorption spectrum of IMW (I) after pulse exposure and conductivity transients observed for (I) (blue) and (II) (red) upon 355 nm excitation. Figures reprinted with permission from [59]. (B) Structure of the conjugated polymer and IMW investigated by Kostromin

permethylated α-CD.

24 Advances in Photodetectors - Research and Applications

et al. [60].

#### 5.2.1. The structure of catenanes and its benefits

Catenanes constitute another important class of interlocked molecules. Just like rotaxanes they are stabilized through mechanical bonds [9, 32]. Yet, they consist of two or more macrocycles interlocked in a way that resembles the connectivity of rings in a chain (Latin: catena = chain). There is a large variety of catenated structures reported to date with numerous applications. There are various reasons why catenanes could become important candidates for new photoconductive materials. As also mentioned for rotaxanes, encapsulation of a sensitive photoconductive moiety or functionality in a molecule could significantly increase the durability of the material and protect the desired photoconductive properties. Thus, interlocking photoconductive (or more generally photosensitive) macrocycles could potentially lead to promising stable catenated materials with optoelectronic applications. Moreover, geometry fixation and proper orientation in catenanes can give rise to intermolecular interactions (e.g., π–π stacking, etc.) facilitating efficient charge transfer in such materials. This is a key property which is discussed in more detail in this chapter. Finally, polycatenanes involving photoconductive parts could be perfect candidate multifunctional materials, as in such structures one can introduce photoconductivity via embedding repeated photoconductive catenane units in macromolecules with special properties, e.g., electrical or thermal conductivity, mechanical strength, etc.

#### 5.2.2. Photoconductive catenanes

Even though numerous examples of catenanes and polycatenanes have been reported, there is a limited number photoconductive catenanes and polycatenanes. However, there is strong indication that such materials could also exhibit promising photoconductive behavior. The main types of organic photoconductive molecules utilized in rotaxanes and polyrotaxanes can be also utilized in catenanes and polycatenanes.

About 15 years ago, Simone [63] reported on the synthesis and characterization of some polycatenane repeated units of cyclophane connected to thiophene rings (red-colored part in Figure 13) interlocked with a bis-viologen tetracationic cyclophane (blue-colored species in Figure 13). This approach involving the aforementioned two cyclophanes was initially employed by Stoddart and coworkers [15] and is a very popular combination for numerous rotaxanes and catenanes (see, for instance, the pseudorotaxane in Figure 9). The resulting polycatenane of Simone and Swager is stabilized through π-stacking between the aromatic bipyridinium and benzene-1,4-diether rings of the interlocked macrocycles. The catenane in Figure 13 which is colored green exhibits a charge transfer visible band situated at λ = 626 nm. This polycatenane as well as another variant was reported to be conductive (linear part is a π-conjugated polymer) [64] but also to exhibit significant photocurrent responses [63]. This example constitutes an important case enabling the design of novel photoconductive polycatenanes.

(poly)catenanes and (poly)rotaxanes encompassing CTC units are of high importance as materials with significant photoconductivity and photosensitivity. It is high time this fascinating class

Photoconductive Interlocked Molecules and Macromolecules

http://dx.doi.org/10.5772/intechopen.79798

27

This chapter has provided a review of the research field of interlocked molecules and macromolecules placing emphasis on rotaxanes, catenanes, and polymeric structures thereof. Various categories of organic photoconductive rotaxanes and catenanes have been reviewed, and the main structural and photoconductive characteristics have been provided. The (photo) conductive properties of the molecules and macromolecules with and without encapsulation

are compared. A range of examples and potential applications has been also provided.

2 School of Chemical Engineering, National Technical University of Athens (NTUA),

3 School of Electrical and Electronic Engineering, University of Manchester, Manchester,

[1] Barnes JC, Mirkin CA. Profile of Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa, 2016 Nobel Laureates in Chemistry. PNAS, 2017;114(4):620-625. DOI: 10.1073/

[2] Lehn J, Mint M. Perspectives in supramolecular chemistry-from molecular recognition towards molecular information processing and self-organization. Angewandte Chemie.

[3] Würthner F. Supramolecular Dye Chemistry. Heidelberg: Springer; 2005. DOI: 10.1007/

[4] Zheng YB, Yang YW, Jensen L, Fang L, Juluri BK, Flood AH, Weiss PS, Stoddart JF, Huang TJ. Active molecular plasmonics: Controlling plasmon resonances with molecular switches.

of interlocked (macro)molecules was given more attention.

Raffaello Papadakis1,2\*, Ioanna Deligkiozi2 and Hu Li<sup>3</sup>

Laboratory of Organic Chemistry, Athens, Greece

\*Address all correspondence to: rafpapadakis@gmail.com

1990;29(11):1304-1319. DOI: 10.1002/anie.199013041

Nano Letters. 2009;9:819. DOI: 10.1021/nl803539g

1 Department of Chemistry—Ångström Laboratory, Uppsala, Sweden

7. Conclusion

Author details

United Kingdom

References

pnas.1619330114

b105136

Figure 13. (A) The polycatenane synthesized by Simone [63]. (B) Symbolic representation of the polycatenane in panel A. (C) Cyclic voltammograms (solid lines) and conductivity profiles (dashed lines) for the polymer lacking the cyclophane units (colored blue) (i) and polycatenane (ii). Panel C plots reprinted with permission from [64].

It is very important to stress that (poly)catenanes do not exhibit disadvantages when compared to (poly)rotaxanes in terms of their photoconductive behavior/properties. The "strategies" for photocurrent generation are essentially the same for both classes of interlocked (macro)molecules. The downside in the case of (poly)catenanes can sometimes be the more tedious synthetic methodology required, when compared to (poly)rotaxanes (see paragraph 2). To some extent, this might explain the limited number of reported photoconductive (poly) catenanes. Nevertheless, catenated structures are certainly capable of introducing stability and shielding of the photoconductive parts. Additionally, catenated structures could potentially maintain efficient photocurrent generation and slow charge recombination in photoconductive materials. Thus, they should be considered as promising photoconductive interlocked materials/compounds, and they should clearly be given more attention.

#### 6. Applications of rotaxanes and catenanes

Rotaxanes and catenanes are gaining more and more attention due to their applicability in modern technologies. They have been proposed for numerous biological applications such as smart drug-delivery systems corresponding to anticancer drugs [65, 66], imaging of biological matter (e.g., mitochondria) [67], or as useful materials for the enhancement of MRI imaging [68]. Especially, the squarain-involving interlocked molecules described are prominent examples of fluorescent bio-imaging agents and chemosensors [50]. Furthermore, both types of interlocked molecules are prominent candidates for new smart future applications acting as (multi)functional materials and undergoing fully controllable switching, shuttle motions, as well as molecular motor functions [69–71]. Medium- and photo-responsive interlocked molecules are also currently considered as new sensing materials with various possible applications [30, 35, 36, 72]. Taking into account the potentials of the photoconductive interlocked molecules described in this chapter, one could foresee a bright future for new optoelectronic materials, molecular wires, photoconductors, photovoltaics, and many other novel applications. Especially, (poly)catenanes and (poly)rotaxanes encompassing CTC units are of high importance as materials with significant photoconductivity and photosensitivity. It is high time this fascinating class of interlocked (macro)molecules was given more attention.

#### 7. Conclusion

This chapter has provided a review of the research field of interlocked molecules and macromolecules placing emphasis on rotaxanes, catenanes, and polymeric structures thereof. Various categories of organic photoconductive rotaxanes and catenanes have been reviewed, and the main structural and photoconductive characteristics have been provided. The (photo) conductive properties of the molecules and macromolecules with and without encapsulation are compared. A range of examples and potential applications has been also provided.

### Author details

It is very important to stress that (poly)catenanes do not exhibit disadvantages when compared to (poly)rotaxanes in terms of their photoconductive behavior/properties. The "strategies" for photocurrent generation are essentially the same for both classes of interlocked (macro)molecules. The downside in the case of (poly)catenanes can sometimes be the more tedious synthetic methodology required, when compared to (poly)rotaxanes (see paragraph 2). To some extent, this might explain the limited number of reported photoconductive (poly) catenanes. Nevertheless, catenated structures are certainly capable of introducing stability and shielding of the photoconductive parts. Additionally, catenated structures could potentially maintain efficient photocurrent generation and slow charge recombination in photoconductive materials. Thus, they should be considered as promising photoconductive interlocked mate-

Figure 13. (A) The polycatenane synthesized by Simone [63]. (B) Symbolic representation of the polycatenane in panel A. (C) Cyclic voltammograms (solid lines) and conductivity profiles (dashed lines) for the polymer lacking the cyclophane

units (colored blue) (i) and polycatenane (ii). Panel C plots reprinted with permission from [64].

Rotaxanes and catenanes are gaining more and more attention due to their applicability in modern technologies. They have been proposed for numerous biological applications such as smart drug-delivery systems corresponding to anticancer drugs [65, 66], imaging of biological matter (e.g., mitochondria) [67], or as useful materials for the enhancement of MRI imaging [68]. Especially, the squarain-involving interlocked molecules described are prominent examples of fluorescent bio-imaging agents and chemosensors [50]. Furthermore, both types of interlocked molecules are prominent candidates for new smart future applications acting as (multi)functional materials and undergoing fully controllable switching, shuttle motions, as well as molecular motor functions [69–71]. Medium- and photo-responsive interlocked molecules are also currently considered as new sensing materials with various possible applications [30, 35, 36, 72]. Taking into account the potentials of the photoconductive interlocked molecules described in this chapter, one could foresee a bright future for new optoelectronic materials, molecular wires, photoconductors, photovoltaics, and many other novel applications. Especially,

rials/compounds, and they should clearly be given more attention.

6. Applications of rotaxanes and catenanes

26 Advances in Photodetectors - Research and Applications

Raffaello Papadakis1,2\*, Ioanna Deligkiozi2 and Hu Li<sup>3</sup>

\*Address all correspondence to: rafpapadakis@gmail.com

1 Department of Chemistry—Ångström Laboratory, Uppsala, Sweden

2 School of Chemical Engineering, National Technical University of Athens (NTUA), Laboratory of Organic Chemistry, Athens, Greece

3 School of Electrical and Electronic Engineering, University of Manchester, Manchester, United Kingdom

#### References


[5] Kay ER, Leigh DA. Rise of the molecular machines. Angewandte Chemie, International Edition. 2015;54:10080-10088. DOI: 10.1002/anie.201503375

[20] Park YS, Reynolds DC. Exciton structure in photoconductivity of CdS, CdSe, and CdS: Se single crystals. Physical Review. 1963;132:2450-2457. DOI: 10.1103/PhysRev.132.2450

Photoconductive Interlocked Molecules and Macromolecules

http://dx.doi.org/10.5772/intechopen.79798

29

[21] Heinze J, Krauser JS, Fläschner N, Hundt B, Götze S, Itin AP, Mathey L, Sengstock K, Becker C. Intrinsic photoconductivity of ultracold fermions in optical lattices. Physical

[22] Law KY. Organic photoconductive materials: Recent trends and developments. Chemical

[23] Rossler C, Hof KD, Manus V, Ludwig S, Kotthaus JP, Simon J, Holleitner AW, Schuh D, Wegscheider W. Optically induced transport properties of freely suspended semiconductor submicron channels. Applied Physics Letters. 2009;93:071107. DOI: 10.1063/1.2970035

[24] Martinez I, Ribeiro M, Andres P, Hueso LE, Casanova F, Aliev FG. Photodoping-driven crossover in the low-frequency noise of MoS2 transistors. Physical Review Applied. 2017;

[25] Hof K D, Rossler C, Manus S, Kotthaus J P, Holleitner A W, Schuh D, Wegscheider W. Dynamic photoconductive gain effect in shallow-etched AlGaAs/GaAs quantum wires. Physical Review B‑Condensed Matter and Materials Physics. 2008;78:2-6. DOI: 10.1103/

[26] Zollinger H. Color chemistry: Syntheses, Properties, and Applications of Organic Dyes and Pigments. 3rd ed. Zürich: Wiley-VCH, Verlag Helvetica Chimica Acta; 2003. ISBN:

[27] Zollinger H. Diazo Chemistry: Aromatic and Heteroaromatic Compounds. Vol. 1. 1st ed.

[28] Mahimwalla Z, Yager KJ, Mamiya J, Shishido A, Priimagi A, Barrett CJ. Azobenzene photomechanics: Prospects and potential applications. Polymer Bulletin. 2012;69:967-

[29] Murakami H, Kawabuchi A, Kotoo K, Kunitake M, Nakashima N. A light-driven molecular shuttle based on a rotaxane. Journal of the American Chemical Society. 1997;119:7605-

[30] Deligkiozi I, Papadakis R, Tsolomitis A. Synthesis, characterisation and photoswitchability of a new [2] rotaxane of α-cyclodextrin with a diazobenzene containing π-conjugated molecular dumbbell. Supramolecular Chemistry. 2012;24:333-343. DOI: 10.1080/10610278.

[31] Qu D-H, Wang Q-C, Tian H. A half adder based on a photochemically driven [2] rotaxane.

[32] Sauvage J-P, Dietrich-Buchecker C, editors. Molecular Catenanes, Rotaxanes and Knots: A Journey Through the World of Molecular Topology. 1st ed. Weinheim: Wiley VCH; 1999.

Angewandte Chemie, International Edition. 2005;44:5296-5299

Review Letters. 2013;110:085302. DOI: 10.1103/PhysRevLett.110.085302

Reviews. 1993;93:449-486. DOI: 10.1021/cr00017a020

7:1-8. DOI :10.1103/PhysRevApplied.7.034034

Weinheim: Wiley-VCH; 1994. ISBN: 3527292136

1006. DOI: 10.1007/s00289-012-0792-0

7606. DOI: 10.1021/ja971438a

PhysRevB.78.115325

3-906390-23-3

2012.660529

ISBN: 3527295720


[20] Park YS, Reynolds DC. Exciton structure in photoconductivity of CdS, CdSe, and CdS: Se single crystals. Physical Review. 1963;132:2450-2457. DOI: 10.1103/PhysRev.132.2450

[5] Kay ER, Leigh DA. Rise of the molecular machines. Angewandte Chemie, International

[6] Browne WR, Feringa BL. Making molecular machines work. Nature Nanotechnology.

[7] Braunschweig AB, Dichtel WR, Miljanić OŠ, Olson MA, Spruell JM, Khan SI, Heath JR, Stoddart JF. Modular synthesis and dynamics of a variety of donor-acceptor interlocked compounds prepared by a click chemistry approach. Chemistry, an Asian Journal. 2007;2:

[8] Schill G, Lüttringhaus A. The preparation of catena compounds by directed synthesis. Angewandte Chemie, International Edition. 1964;3:546-547. DOI: 10.1002/anie.196405461

[9] Niu X, Gibson HG. Polycatenanes. Chemical Reviews. 2009;109:6024-6046. DOI: 10.1021/

[10] Yu L, Li M, Zhou XP, Li D. Hybrid inorganic-organic polyrotaxane, pseudorotaxane, and sandwich. Inorganic Chemistry. 2013;52:10232-10234. DOI: 10.1021/ic401722c

[11] Safarowsky EO, Windisch B, Mohry A, Vögtl F. Nomenclature for catenanes, rotaxanes, molecular knots, and assemblies derived from these structural elements. Journal für Praktische Chemie. 2000;342:437-444. DOI: 10.1002/1521-3897(200006)342:5<437::AID-

[12] Amabilino DB, Ashton PR, Reder AS, Spencer N, Stoddart JF. Olympiadane. Angewandte Chemie, International Edition. 1994;33(12):1286-1290. DOI: 10.1002/anie.199412861 [13] Wilson MR, Solà J, Carlone A, Goldup SM, Lebrasseur N, Leigh DA. An autonomous chemically fuelled small-molecule motor. Nature. 2016;534:235. DOI: 10.1038/nature18013

[14] Ashton PR, Baxter I, Fyfe MCT, Raymo FM, Spencer N, Stoddart JF, White AP, Williams DJ. Rotaxane or pseudorotaxane? That is the question! Journal of the American Chemical

[15] Sauvage JP, Gaspard G, editors. From Non-Covalent Assemblies to Molecular Machines.

[16] Fielden SDP, Leigh DA, McTernan CT, Pérez-Saavedra B, Vitorica-Yrezabal IJ. Spontaneous assembly of rotaxanes from a primary amine, crown ether and electrophile. Journal of the American Chemical Society. 2018;140(19):6049-6052. DOI: 10.1021/jacs.8b03394 [17] Schalley CA, Weilandt T, Bruggemann J, Vogtle F. Templates in Chemistry I. Vol. 248.

[18] Newman R, Tyler WW. Photoconductivity in germanium. Solid State Phys. 1959;8:49-107.

[19] Vavilov VS, Lotkova EN, Plotnikov AF. Photoconductivity and infra-red absorption in silicon irradiated by neutrons. Journal of Physics and Chemistry of Solids. 1961;22:31-38.

Society. 1998;120:2297-2307. DOI: 10.1021/ja9731276

1st ed. Weinheim: Wiley VCH Verlag GmbH; 2011

Edition. 2015;54:10080-10088. DOI: 10.1002/anie.201503375

2006;1:25-35. DOI: 10.1038/nnano.2006.45

28 Advances in Photodetectors - Research and Applications

634-647. DOI: 10.1002/asia.200700035

cr900002h

PRAC437>3.0.CO;2-7

Berlin: Springer; 2004

DOI: 10.1016/S0081-1947(08)60479-8

DOI: 10.1016/0022-3697(61)90239-6


[33] Rau H. Photo- und Halbleitfähigkeit von Festkörpern isomerer und tautomerer Moleküle. II. Photoleitfähigkeit von 1-Benzolazo-2-naphthol. Berichte der Bunsen-Gesellschaft für Physikalische Chemie. 1969;73:810-819. DOI: 10.1002/bbpc.19690730814

[46] Sheeney-Haj-Ichia L, Willner I. Enhanced photoelectrochemistry in supramolecular CdS-Nanoparticle-stoppered pseudorotaxane monolayers assembled on electrodes. The Jour-

Photoconductive Interlocked Molecules and Macromolecules

http://dx.doi.org/10.5772/intechopen.79798

31

[47] Feng M, Guo X, Lin X, He X, Ji W, Du S, Zhang D, Zhu D, Gao H. Stable, reproducible nanorecording on rotaxane thin films. Journal of the American Chemical Society. 2005;127:

[48] Saha S, Flood AH, Stoddart JF, Impellizzeri S, Silvi S, Venturi M, Credi A. A redox-driven multicomponent molecular shuttle. Journal of the American Chemical Society. 2007;129:

[49] Sprenger HE, Ziegenbein W. Condensation products of squaric acid and tertiary aromatic amines. Angewandte Chemie, International Edition in English. 1966;5:894-894.

[50] Gassensmith JJ, Baumes JM, Smith BD. Discovery and early development of squaraine rotaxanes. Chemical Communications. 2009:6329-6338. DOI: 10.1039/b911064j

[53] Yan H, Choe HS, Nam SW, Hu Y, Das S, Klemic JF, Ellenbogen JC, Lieber CM. Programmable nanowire circuits for nanoprocessors. Nature. 2011;470:240-244. DOI: 10.1038/

[54] Yum JH, Walter P, Huber S, Rentsch D, Geiger T, Nüesch F, De Angelis F, Grätzel M, Nazeeruddin MK. Efficient far red sensitization of nanocrystalline TiO2 films by an unsymmetrical squaraine dye. Journal of the American Chemical Society. 2007;129:10320-10321.

[55] Leigh DA, Murphy A, Smart JP, Slawai AMZ. Glycylglycine rotaxanes—The hydrogen bond directed assembly of synthetic peptide rotaxanes. Angewandte Chemie, Interna-

[56] Gatti FG, Leigh DA, Nepogodiev SA, Slawin AMZ, Teat SJ, Wong JKY. Stiff and sticky in the right places: The dramatic influence of preorganizing guest binding sites on the hydrogen bond-directed assembly of rotaxanes. Journal of the American Chemical Society.

[57] Kuhn H. A quantum-mechanical theory of light absorption of organic dyes and similar compounds. The Journal of Chemical Physics. 1949;17:1198. DOI: 10.1063/1.1747143

[58] Frampton MJ, Anderson HL. Insulated molecular wires. Angewandte Chemie, Interna-

[59] Terao J, Tanaka Y, Tsuda S, Kambe N, Taniguchi M, Kawai T, Saeki A, Seki S. Insulated molecular wire with highly conductive π-conjugated polymer core. Journal of the Amer-

tional Edition. 1997;36:728-732. DOI: 10.1002/anie.199707281

tional Edition. 2007;46:1028-1064. DOI: 10.1002/anie.200601780

ican Chemical Society. 2009;131:18046-18047. DOI: 10.1021/ja908783f

2001;123:5983-5989. DOI: 10.1021/ja001697r

nal of Physical Chemistry. B. 2002;106:13094-13097. DOI: 10.1021/jp022102c

15338-15339. DOI: 10.1021/ja054836j

12159-12171. DOI: 10.1021/ja0724590

[51] Champ RB, Shattuck MD. U.S. Patent 3,824,099. 1974

DOI: 10.1002/anie.196608941

[52] Kampfer H. U.S. Patent 3,617,270. 1971

nature09749

DOI: 10.1021/ja0731470


[33] Rau H. Photo- und Halbleitfähigkeit von Festkörpern isomerer und tautomerer Moleküle. II. Photoleitfähigkeit von 1-Benzolazo-2-naphthol. Berichte der Bunsen-Gesellschaft für

[35] Deligkiozi I, Papadakis R, Tsolomitis A. Photoconductive properties of a π-conjugated αcyclodextrin containing [2]rotaxane and its corresponding molecular dumbbell. Physical

[36] Deligkiozi I, Voyiatzis E, Tsolomitis A, Papadakis R. Synthesis and characterization of new azobenzene-containing bis pentacyanoferrate(II) stoppered push-pull [2]rotaxanes, with α- and β-cyclodextrin. Towards highly medium responsive dyes. Dyes Pigment.

[37] Cacialli F, Wilson JS, Michels JJ, Daniel C, Silva C, Friend RH, Severin N, Samorì P, Rabe JP, O'Connell MJ, Taylor PN, Anderson HL. Cyclodextrin-threaded conjugated polyrotaxanes as insulated molecular wires with reduced interstrand interactions. Nature

[38] Haque SA, Park JS, Srinivasarao M, Durrant JR. Molecular-level insulation: An approach to controlling interfacial charge transfer. Advanced Materials. 2004;16:1177-1181. DOI:

[39] Clifford JN, Martínez-Ferrero E, Viterisi A, Palomares E. Sensitizer molecular structuredevice efficiency relationship in dye sensitized solar cells. Chemical Society Reviews.

[40] Monk PMS. The Viologens: Physicochemical Properties, Synthesis and Applications of the Salts of 4,40-Bipyridine. 1st ed. Chichester: John Wiley & Sons Ltd; 1999. ISBN: 978-0-471-

[41] Huo P, Xue LJ, Li YH, Chen T, Yu L, Zhu QY, Dai J. Effects of alkyl chain length on film morphologies and photocurrent responses of tetrathiafulvalenebipyridinium chargetransfer salts: A study in terms of structures. CrystEngComm. 2016;18:2894-2900. DOI:

[42] Striepe L, Baumgartner T. Viologens and their application as functional materials. Chemistry, a European Journal. 2017;23(67):16924-16940. DOI: 10.1002/chem.201703348

[43] Papadakis R, Deligkiozi I, Giorgi M, Faure B, Tsolomitis A. Supramolecular complexes involving non-symmetric viologen cations and hexacyanoferrate (II) anions. A spectroscopic, crystallographic and computational study. RSC Advances. 2016;6:575-585. DOI:

[44] Kinuta T, Sato T, Tajima N, Kuroda R, Matsubara Y, Imai Y. Solid-state thermochromism observed in charge-transfer complex composed of binaphthol and viologen. Journal of

[45] Raymo FM, Stoddart JF. Interlocked macromolecules. Chemical Reviews. 1999;99:1643-1664.

Molecular Structure. 2010;982:45-49. DOI: 10.1016/j.molstruc.2010.07.048

Physikalische Chemie. 1969;73:810-819. DOI: 10.1002/bbpc.19690730814

Chemistry Chemical Physics. 2013;15:3497-3503. DOI: 10.1039/C3CP43794A

[34] Champ RB, Shattuck MD. U.S. Patent 3,898,084. 1975

30 Advances in Photodetectors - Research and Applications

2015;113:709-722. DOI: 10.1016/j.dyepig.2014.10.005

Materials. 2002;1:160-164. DOI: 10.1038/nmat750

2011;40:1635-1646. DOI: 10.1039/b920664g

10.1002/adma.200400327

98603-4

10.1039/c5ce02479j

10.1039/C5RA16732A

DOI: 10.1021/cr970081q


[60] Kostromin SV, Malov VV, Tameev AR, Bronnikov SV, Farcas A. The photovoltaic effect and charge carrier mobility in layered compositions of bithiophene or related rotaxane copolymer with C70 fullerene derivative. Technical Physics Letters. 2017;43:173-176. DOI: 10.1134/S1063785017020079

**Chapter 3**

Provisional chapter

Ga…, etc.) quite few; hence, a

**Ion-Beam Modified Terahertz GaAs Photoconductive**

DOI: 10.5772/intechopen.79693

Ion-implanted photoconductive GaAs terahertz (THz) antennas were demonstrated to deliver both high-efficiency and high-power THz emitters, which are attributed to excellent carrier acceleration and fast carrier trapping for THz generations by analyzing ultrafast carrier dynamics at subpicosecond scale. The implantation distance at over

few with good mobility similar to bare GaAs ensures excellent carrier acceleration in shallow distance <1.0 μm as photo carriers are generated by the pump laser. The implantation dosage is carefully optimized to make carrier trapping very fast, and screen effects by photo-generated carriers are significantly suppressed, which increases the THz radiation power of SI-GaAs antennas by two orders of magnitude. Under the same photoexcitation conditions (pump laser power, bias voltage), photocurrents from GaAs antennas with optimum conditions 300 keV, 5 � <sup>10</sup><sup>14</sup> cm�<sup>2</sup> for H implantation are decreased by two orders of magnitude; meanwhile, the THz radiation is enhanced by over four times, which means that the electrical-to-THz power conversion efficiency is improved by a

Keywords: terahertz antenna, ion-beam technology, electrical-to-THz power conversion

The development of a terahertz (THz) source has obtained much interest over the last three decades due to their widespread scientific and military applications [1–3]. Photoconductive antennas (PCAs) illuminated by a femtosecond (fs) laser have been becoming the dominant methods for intense THz radiations [3] since the pioneering demonstration of picosecond

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

efficiency, photoconductive antenna, semi-insulating GaAs, ion-implantation

Ion-Beam Modified Terahertz GaAs Photoconductive

**Antenna**

Abstract

factor of over 1600.

1. Introduction

Antenna

Caiming Sun and Aidong Zhang

Caiming Sun and Aidong Zhang

http://dx.doi.org/10.5772/intechopen.79693

Additional information is available at the end of the chapter

2.5 μm is deep enough to make defects (Ga vacancies, As<sup>þ</sup>

Additional information is available at the end of the chapter


#### **Ion-Beam Modified Terahertz GaAs Photoconductive Antenna** Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

DOI: 10.5772/intechopen.79693

[60] Kostromin SV, Malov VV, Tameev AR, Bronnikov SV, Farcas A. The photovoltaic effect and charge carrier mobility in layered compositions of bithiophene or related rotaxane copolymer with C70 fullerene derivative. Technical Physics Letters. 2017;43:173-176. DOI:

[61] Terao J, Wadahama A, Matono A, Tada T, Watanabe S, Seki S, Fujihara T, Tsuji Y. Design principle for increasing charge mobility of p-conjugated polymers using regularly localized molecular orbitals. Nature Communications. 2013;4:1691. DOI: 10.1038/ncomms2707

[62] Terao J. Permethylated cyclodextrin-based insulated molecular wires. Polymer Chemistry.

[63] Simone DL. The Synthesis and Investigation of the Electronic Properties of Crown Ether, [2]-Catenane, and [2]-Rotaxane Architectures. Cambridge, Massachusetts: Massachusetts

[64] Simone DL, Swager TM. A conducting poly(cyclophane) and its poly([2]-catenane). Journal of the American Chemical Society. 2000;122:9300-9301. DOI: 10.1021/ja000970m [65] Shi J, Xu Y, Wang X, Zhang L, Zhu J, Pang T, Bao X. Synthesis and evaluation of a novel rhodamine B pyrene [2]rotaxane as an intracellular delivery agent for doxorubicin. Organic & Biomolecular Chemistry. 2015;13:7517-7529. DOI: 10.1039/C5OB00934K [66] Barat R, Legigan T, Tranoy-Opalinski I, Renoux B, Péraudeau E, Clarhaut J, Poinot P, Fernandes AE, Aucagne V, Leigh DA, Papot S. A mechanically interlocked molecular system programmed for the delivery of an anticancer drug. Chemical Science. 2015;6:

[67] Yu G, Wu D, Li Y, Zhang Z, Shao L, Zhou J, Hu Q, Tang G, Huang F. A pillar[5]arenebased [2]rotaxane lights up mitochondria. Chemical Science. 2016;7:3017-3024. DOI:

[68] Fredy JW, Scelle J, Ramniceanu G, Doan BT, Bonnet CS, Tóth É, Ménand M, Sollogoub M, Vives G, Hasenknopf B. Mechanostereoselective one-pot synthesis of functionalized headto-head cyclodextrin [3]rotaxanes and their application as magnetic resonance imaging contrast agents. Organic Letters. 2017;19:1136-1139. DOI: 10.1021/acs.orglett.7b00153 [69] Erbas-Cakmak S, Leigh DA, McTernan CT, Nussbaumer AL. Artificial molecular machines. Chemical Reviews. 2015;115:10081-10206. DOI: 10.1021/acs.chemrev.5b00146

[70] Feringa BL. In control of motion: From molecular switches to molecular motors. Accounts

[71] Kottas GS, Clarke LI, Horinek D, Michl J. Artificial molecular rotors. Chemical Reviews.

[72] Xue M, Yang Y, Chi X, Yan X, Huang F. Development of pseudorotaxanes and rotaxanes: From synthesis to stimuli-responsive motions to applications. Chemical Reviews. 2015;

of Chemical Research. 2001;34:504-513. DOI: 10.1021/ar0001721

2005;105:1281-1376. DOI: 10.1021/cr0300993

115:7398-7501. DOI: 10.1021/cr5005869

10.1134/S1063785017020079

32 Advances in Photodetectors - Research and Applications

Institute of Technology; 2002

2608-2613. DOI: 10.1039/C5SC00648A

10.1039/C6SC00036C

2011;2:2444-2452. DOI: 10.1039/c1py00243k

Caiming Sun and Aidong Zhang Caiming Sun and Aidong Zhang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79693

#### Abstract

Ion-implanted photoconductive GaAs terahertz (THz) antennas were demonstrated to deliver both high-efficiency and high-power THz emitters, which are attributed to excellent carrier acceleration and fast carrier trapping for THz generations by analyzing ultrafast carrier dynamics at subpicosecond scale. The implantation distance at over 2.5 μm is deep enough to make defects (Ga vacancies, As<sup>þ</sup> Ga…, etc.) quite few; hence, a few with good mobility similar to bare GaAs ensures excellent carrier acceleration in shallow distance <1.0 μm as photo carriers are generated by the pump laser. The implantation dosage is carefully optimized to make carrier trapping very fast, and screen effects by photo-generated carriers are significantly suppressed, which increases the THz radiation power of SI-GaAs antennas by two orders of magnitude. Under the same photoexcitation conditions (pump laser power, bias voltage), photocurrents from GaAs antennas with optimum conditions 300 keV, 5 � <sup>10</sup><sup>14</sup> cm�<sup>2</sup> for H implantation are decreased by two orders of magnitude; meanwhile, the THz radiation is enhanced by over four times, which means that the electrical-to-THz power conversion efficiency is improved by a factor of over 1600.

Keywords: terahertz antenna, ion-beam technology, electrical-to-THz power conversion efficiency, photoconductive antenna, semi-insulating GaAs, ion-implantation

#### 1. Introduction

The development of a terahertz (THz) source has obtained much interest over the last three decades due to their widespread scientific and military applications [1–3]. Photoconductive antennas (PCAs) illuminated by a femtosecond (fs) laser have been becoming the dominant methods for intense THz radiations [3] since the pioneering demonstration of picosecond

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

photoconducting Hertzian dipoles in 1984 [4]. Historically, commercial semi-insulating (SI)- GaAs grown by liquid-encapsulated Czochralski has been the cost-effective choice as the substrate for PCAs, due to its high resistivity (>10<sup>7</sup> Ω cm) and high electron mobility (μ > 7000 cm2 /Vs) [5]. Afterward, about 1 μm-thick film of low-temperature (LT) grown GaAs (LT-GaAs) by molecular beam epitaxy (MBE) on the surface of SI-GaAs substrate is extensively used to reduce carrier lifetime to below 1 ps with high resistivity (10<sup>7</sup> Ω cm) and relatively reasonable mobility μ (100–300 cm<sup>2</sup> /Vs) [6, 7], in order to efficiently generate broadband THz radiations of over 1 THz and reduce the carrier lifetime of PCAs on SI-GaAs (τ > 100 ps). An alternative approach for short lifetime is to create point defects in SI-GaAs by ion-implantation technique. Arsenic, oxygen, nitrogen, carbon, hydrogen (proton), etc., have been implanted into SI-GaAs and the obtained GaAs PCAs are similar to those on LT-GaAs [8–11]. However, the process conditions for either LT-GaAs or ion-implanted GaAs are not easy to reproduce in mass production, because of the difficult control of low-temperature process for MBE [12, 13], extremely high implantation energies (MeV) for heavy ions [11] and the challenging control for post annealing at relatively low temperatures [8, 14].

wafer. A 10/200 nm-thick Ti/Au metal layer stack was deposited on GaAs substrate by e-beam evaporation, functioning as metal electrodes for PCAs. The PCA has a bow-tie antenna structure with a photoconductive gap of 0.4 mm, 90 bow angle, and antenna length of 2 mm, as shown in Figure 1(a). Afterward, ion beam for hydrogen, helium, or oxygen was implanted into such SI GaAs PCAs with penetration depth of 2.5 μm and the acceleration energies are 300 keV, 800 keV, 3 MeV for H, He, O respectively. Implantation energies were selected so that the peak of ion concentration profile is situated deeper than the thickness of THz-relevant layer within SI-GaAs antenna, which is <sup>2</sup> <sup>μ</sup>m. The implantation dosage varied from 1 1012 to

higher dose and lower energies were suitable for lighter ions. The process details for all

As discussed in our previous work [25], 300 keV H implantation was an easily reproducible condition for fabricating SI-GaAs PCAs with ion penetration depth of 2.5 μm, effectively defining the active region for THz generations. The implantation dosage of H ions varied from

Density profiles of the generated defects and the implanted ions were optimized by the stopping and range of ions in matter (SRIM) program [26], and the peak distribution situates as deep as 2.5 μm distance from PCA surface. The defects concentration in shallow regions (<1 μm deep) are over three orders of magnitude lower than the peak concentration at 2.5 μm distance, where most photo carriers are generated under femtosecond laser excitation and accelerated at local electrical fields for THz generations. As carriers transit into defects-rich regions underneath the acceleration layer, they will be efficiently trapped and the carrier

Figure 1. (a) Schematic structure for bow-tie photoconductive antenna (the inset is its photograph). (b) Cross-section of

H-implanted GaAs PCAs with acceleration and trapping of photo carriers.

samples with different implantation conditions are shown in Table 1.

, where lower dose and higher energies were used for heavier ions whereas

, to find out optimum conditions for ion-implanted GaAs PCAs.

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35

<sup>1</sup> 1015 cm<sup>2</sup>

<sup>1</sup> 1014 to 1 1015 cm<sup>2</sup>

Fundamentally, the THz radiation power and optical-to-THz conversion efficiency for GaAs PCAs illuminated by femtosecond laser pulses are proportional to the photoconductive material factor μτ<sup>2</sup> of PCAs [15]. The reduced electron mobility and carrier lifetime as mentioned earlier will seriously affect the THz power and conversion efficiency [3, 16, 17]. The radiation mechanism is attributed to a time-varying current, a result of photo carriers accelerating across photoconductive gap in the presence of applied electrical field [18]. The emitted THz pulse energy is derived from that stored in the static bias field [19]. Grischkowsky has reported that an extremely strong field exists near the anode of electrically biased PCAs [20], and Salem also demonstrated that THz amplitude can be multiplied by many times when the focused laser beam moved to the anode of an ion-implanted GaAs antenna at the same bias voltage [9]. Recently, plasmonic contact electrodes were used to enhance light absorption within distances of 100 nm from the anode and 7.5% optical-to-THz conversion efficiency was recorded at very low pump densities of <10 μJ/cm<sup>2</sup> [21]. However, a tightly focused laser beam will cause a high screening effect [22] and the THz power from PCA becomes lower and lower as optical pump saturates [23, 24], which principally sets an upper limit for the conversion efficiency of THz radiations. Thus, it is critical to find out a strategy of creating sufficient defects to reduce the carrier lifetime without affecting mobility detrimentally. High-energy and low-dosage ionimplantation has been verified to be an efficient method of creating proper profiles of defects, in order to obtain both excellent carrier acceleration at the shallow region and fast carrier trapping at the deep layer for THz generations [11, 25]. Also, hydrogen implantation is extensively used to separate high-power active devices (IGBT, laser diodes, LED, etc.) from their mother substrate and get superior performance of high frequency and high efficiency.

#### 2. Experimental results and discussions

In this work, the photoconductive antenna substrate was a commercial high-resistivity (5 107 <sup>Ω</sup> cm), liquid-encapsulated Czochralski-grown, (100)-oriented, semi-insulating (SI)-GaAs wafer. A 10/200 nm-thick Ti/Au metal layer stack was deposited on GaAs substrate by e-beam evaporation, functioning as metal electrodes for PCAs. The PCA has a bow-tie antenna structure with a photoconductive gap of 0.4 mm, 90 bow angle, and antenna length of 2 mm, as shown in Figure 1(a). Afterward, ion beam for hydrogen, helium, or oxygen was implanted into such SI GaAs PCAs with penetration depth of 2.5 μm and the acceleration energies are 300 keV, 800 keV, 3 MeV for H, He, O respectively. Implantation energies were selected so that the peak of ion concentration profile is situated deeper than the thickness of THz-relevant layer within SI-GaAs antenna, which is <sup>2</sup> <sup>μ</sup>m. The implantation dosage varied from 1 1012 to <sup>1</sup> 1015 cm<sup>2</sup> , where lower dose and higher energies were used for heavier ions whereas higher dose and lower energies were suitable for lighter ions. The process details for all samples with different implantation conditions are shown in Table 1.

photoconducting Hertzian dipoles in 1984 [4]. Historically, commercial semi-insulating (SI)- GaAs grown by liquid-encapsulated Czochralski has been the cost-effective choice as the substrate for PCAs, due to its high resistivity (>10<sup>7</sup> Ω cm) and high electron mobility

(LT-GaAs) by molecular beam epitaxy (MBE) on the surface of SI-GaAs substrate is extensively used to reduce carrier lifetime to below 1 ps with high resistivity (10<sup>7</sup> Ω cm) and relatively

radiations of over 1 THz and reduce the carrier lifetime of PCAs on SI-GaAs (τ > 100 ps). An alternative approach for short lifetime is to create point defects in SI-GaAs by ion-implantation technique. Arsenic, oxygen, nitrogen, carbon, hydrogen (proton), etc., have been implanted into SI-GaAs and the obtained GaAs PCAs are similar to those on LT-GaAs [8–11]. However, the process conditions for either LT-GaAs or ion-implanted GaAs are not easy to reproduce in mass production, because of the difficult control of low-temperature process for MBE [12, 13], extremely high implantation energies (MeV) for heavy ions [11] and the challenging control

Fundamentally, the THz radiation power and optical-to-THz conversion efficiency for GaAs PCAs illuminated by femtosecond laser pulses are proportional to the photoconductive material factor μτ<sup>2</sup> of PCAs [15]. The reduced electron mobility and carrier lifetime as mentioned earlier will seriously affect the THz power and conversion efficiency [3, 16, 17]. The radiation mechanism is attributed to a time-varying current, a result of photo carriers accelerating across photoconductive gap in the presence of applied electrical field [18]. The emitted THz pulse energy is derived from that stored in the static bias field [19]. Grischkowsky has reported that an extremely strong field exists near the anode of electrically biased PCAs [20], and Salem also demonstrated that THz amplitude can be multiplied by many times when the focused laser beam moved to the anode of an ion-implanted GaAs antenna at the same bias voltage [9]. Recently, plasmonic contact electrodes were used to enhance light absorption within distances of 100 nm from the anode and 7.5% optical-to-THz conversion efficiency was recorded at very low pump densities of <10 μJ/cm<sup>2</sup> [21]. However, a tightly focused laser beam will cause a high screening effect [22] and the THz power from PCA becomes lower and lower as optical pump saturates [23, 24], which principally sets an upper limit for the conversion efficiency of THz radiations. Thus, it is critical to find out a strategy of creating sufficient defects to reduce the carrier lifetime without affecting mobility detrimentally. High-energy and low-dosage ionimplantation has been verified to be an efficient method of creating proper profiles of defects, in order to obtain both excellent carrier acceleration at the shallow region and fast carrier trapping at the deep layer for THz generations [11, 25]. Also, hydrogen implantation is extensively used to separate high-power active devices (IGBT, laser diodes, LED, etc.) from their mother substrate and get superior performance of high frequency and high efficiency.

In this work, the photoconductive antenna substrate was a commercial high-resistivity (5 107 <sup>Ω</sup> cm), liquid-encapsulated Czochralski-grown, (100)-oriented, semi-insulating (SI)-GaAs

/Vs) [5]. Afterward, about 1 μm-thick film of low-temperature (LT) grown GaAs

/Vs) [6, 7], in order to efficiently generate broadband THz

(μ > 7000 cm2

reasonable mobility μ (100–300 cm<sup>2</sup>

34 Advances in Photodetectors - Research and Applications

for post annealing at relatively low temperatures [8, 14].

2. Experimental results and discussions

As discussed in our previous work [25], 300 keV H implantation was an easily reproducible condition for fabricating SI-GaAs PCAs with ion penetration depth of 2.5 μm, effectively defining the active region for THz generations. The implantation dosage of H ions varied from <sup>1</sup> 1014 to 1 1015 cm<sup>2</sup> , to find out optimum conditions for ion-implanted GaAs PCAs. Density profiles of the generated defects and the implanted ions were optimized by the stopping and range of ions in matter (SRIM) program [26], and the peak distribution situates as deep as 2.5 μm distance from PCA surface. The defects concentration in shallow regions (<1 μm deep) are over three orders of magnitude lower than the peak concentration at 2.5 μm distance, where most photo carriers are generated under femtosecond laser excitation and accelerated at local electrical fields for THz generations. As carriers transit into defects-rich regions underneath the acceleration layer, they will be efficiently trapped and the carrier

Figure 1. (a) Schematic structure for bow-tie photoconductive antenna (the inset is its photograph). (b) Cross-section of H-implanted GaAs PCAs with acceleration and trapping of photo carriers.


<sup>1</sup> 1012 cm<sup>2</sup> respectively. At such implantation energies, most ions for all three kinds of samples are implanted far below a surface layer of about 2.5 μm deep where most laser power is absorbed within 1 μm-deep distance and most terahertz power is generated within 2 μm-deep distance. The ion concentration in this region is over three orders of magnitude lower than the peak concentration, whereas the vacancy density profile for all ion beams (H, He, or O) is nearly the same within THz generation distance (2 μm below the surface). Another reason to use such implantation energies is to reduce the lifetime and therefore strongly reduce the density of photo carriers produced at a depth of 2 μm, which are not

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37

Figure 3 shows the dark currents versus bias voltage (I-V) characteristics for H-implanted and SI-GaAs PCAs without light illumination, measured by a Keysight B1500A semiconductor device analyzer. Currents passing through the SI-GaAs sample quickly exceed the spacecharge limited (SCL) electron flows at low voltage of 20 V, and then significantly increase by a polynomial V<sup>3</sup> response dominated by a double carrier injection current, as demonstrated in Ref. [21]. It is observed that currents in SI-GaAs antenna under 140 V go up to 10 mA even without any light illumination. This means a considerable temperature increase in active region due to huge heat dissipations, which would affect the efficiency of SI-GaAs antenna and result in electrical breakdown of the device. On the contrary, H-implanted samples follow almost linear dependence of currents on bias voltages in broad range of over 100 V and did not show obvious currents increase of V<sup>3</sup> response even as the bias voltages go up over 200 V. It can be seen that H ions in GaAs extend the linear range of currents dependence on voltages, compared to that for bare SI-GaAs samples. The carrier accumulation near the high electrical field region along anode is significantly suppressed by the corresponding trapping sites and

only useless for terahertz generation but also cause the saturation for optical pump.

the double carrier injection is eliminated effectively.

Figure 3. Current-voltage characteristics for SI-GaAs THz antenna and H-implanted one.

Table 1. Process details for all samples with different implantation conditions.

Figure 2. Ion implantation and corresponding vacancy profiles for samples with H dosage of 1 <sup>10</sup><sup>15</sup> cm<sup>2</sup> , He dosage of <sup>1</sup> 1013 cm<sup>2</sup> , and O dosage of 1 <sup>10</sup><sup>12</sup> cm<sup>2</sup> , calculated by Stopping and Range of Ions in Matter (SRIM), where a Monte-Carlo simulation of 5 105 ions was performed for hydrogen, helium, and oxygen. Also, the implantation angle of 7 was used to avoid possible channeling to the crystal axis.

acceleration for THz generations is successfully separated from defects by implantation without obvious decrease of transient mobility in shallow regions. Moreover, the accumulation of photo carriers against the electrical bias was significantly suppressed, avoiding the screen effects by the pump laser (Figure 1(b)). Two more sets of samples fabricated on a bare SI-GaAs substrate and 1-μm-thick LT-GaAs grown on an SI-GaAs substrate ("LT-GaAs" hereafter) were prepared for reference.

As shown in Figure 2, density profiles of the generated defects and the implanted ions were simulated with the stopping and range of ions in matter (SRIM) program [26]. The implantation conditions for H, He and O are 300 keV, 1 <sup>10</sup><sup>15</sup> cm<sup>2</sup> ; 800 keV, 1 1013 cm<sup>2</sup> ; and 3 MeV, <sup>1</sup> 1012 cm<sup>2</sup> respectively. At such implantation energies, most ions for all three kinds of samples are implanted far below a surface layer of about 2.5 μm deep where most laser power is absorbed within 1 μm-deep distance and most terahertz power is generated within 2 μm-deep distance. The ion concentration in this region is over three orders of magnitude lower than the peak concentration, whereas the vacancy density profile for all ion beams (H, He, or O) is nearly the same within THz generation distance (2 μm below the surface). Another reason to use such implantation energies is to reduce the lifetime and therefore strongly reduce the density of photo carriers produced at a depth of 2 μm, which are not only useless for terahertz generation but also cause the saturation for optical pump.

Figure 3 shows the dark currents versus bias voltage (I-V) characteristics for H-implanted and SI-GaAs PCAs without light illumination, measured by a Keysight B1500A semiconductor device analyzer. Currents passing through the SI-GaAs sample quickly exceed the spacecharge limited (SCL) electron flows at low voltage of 20 V, and then significantly increase by a polynomial V<sup>3</sup> response dominated by a double carrier injection current, as demonstrated in Ref. [21]. It is observed that currents in SI-GaAs antenna under 140 V go up to 10 mA even without any light illumination. This means a considerable temperature increase in active region due to huge heat dissipations, which would affect the efficiency of SI-GaAs antenna and result in electrical breakdown of the device. On the contrary, H-implanted samples follow almost linear dependence of currents on bias voltages in broad range of over 100 V and did not show obvious currents increase of V<sup>3</sup> response even as the bias voltages go up over 200 V. It can be seen that H ions in GaAs extend the linear range of currents dependence on voltages, compared to that for bare SI-GaAs samples. The carrier accumulation near the high electrical field region along anode is significantly suppressed by the corresponding trapping sites and the double carrier injection is eliminated effectively.

Figure 3. Current-voltage characteristics for SI-GaAs THz antenna and H-implanted one.

acceleration for THz generations is successfully separated from defects by implantation without obvious decrease of transient mobility in shallow regions. Moreover, the accumulation of photo carriers against the electrical bias was significantly suppressed, avoiding the screen effects by the pump laser (Figure 1(b)). Two more sets of samples fabricated on a bare SI-GaAs substrate and 1-μm-thick LT-GaAs grown on an SI-GaAs substrate ("LT-GaAs" hereaf-

Monte-Carlo simulation of 5 105 ions was performed for hydrogen, helium, and oxygen. Also, the implantation angle of

Figure 2. Ion implantation and corresponding vacancy profiles for samples with H dosage of 1 <sup>10</sup><sup>15</sup> cm<sup>2</sup>

) Implantation energy Ion beam

 <sup>10</sup><sup>15</sup> 300 KeV Hydrogen <sup>10</sup><sup>14</sup> 300 KeV Hydrogen <sup>10</sup><sup>14</sup> 300 KeV Hydrogen <sup>10</sup><sup>13</sup> 800 KeV Helium <sup>10</sup><sup>12</sup> 800 KeV Helium <sup>10</sup><sup>12</sup> 3 MeV Oxygen <sup>10</sup><sup>12</sup> 3 MeV Oxygen

Table 1. Process details for all samples with different implantation conditions.

As shown in Figure 2, density profiles of the generated defects and the implanted ions were simulated with the stopping and range of ions in matter (SRIM) program [26]. The implanta-

; 800 keV, 1 1013 cm<sup>2</sup>

, calculated by Stopping and Range of Ions in Matter (SRIM), where a

; and 3 MeV,

, He dosage of

ter) were prepared for reference.

<sup>1</sup> 1013 cm<sup>2</sup>

Implantation dose (cm<sup>2</sup>

36 Advances in Photodetectors - Research and Applications

tion conditions for H, He and O are 300 keV, 1 <sup>10</sup><sup>15</sup> cm<sup>2</sup>

, and O dosage of 1 <sup>10</sup><sup>12</sup> cm<sup>2</sup>

7 was used to avoid possible channeling to the crystal axis.

Figure 4. Schematic diagram of THz TDS setup used for measuring electric field of THz pulse.

The setup for characterization of the THz waves is based on a conventional time-domain spectroscopy system (TDS) triggered by a femtosecond laser as shown in Figure 4. A modelocked Ti: Sapphire laser generates 80 fs light pulses at a wavelength of �780 nm and a repetition rate of 80 MHz. The femtosecond pump pulses were focused by an objective lens with 10 μm-diameter illumination spot on the proximity of anode for a biased photoconductive antenna, which was mounted on the flat side of a Si hemispherical lens with a diameter of 15 mm. The emitted THz radiation was collimated and focused by two pairs of gold-coated off-axis parabolic mirrors onto a photoconductive sampling detector, which was also a photoconductive antenna with bow-tie shape and gap of 20 μm mounted on the back of a Si hemispherical lens with the same diameter. The photoconductive detector was gated by femtosecond probe beam pulses that were separated from the pump beam pulses by a beam splitter.

As<sup>þ</sup> GaFigure 5(a) shows the THz waveforms emitted from GaAs PCAs implanted with H dose of 1 � 1014, 5 � 1014, and 1 � 1015 cm�<sup>2</sup> . The applied bias voltage and pumping power were about 260 V and 60 mW, respectively. The SI-GaAs and LT-GaAs samples were measured under the pump power of 60 mW for reference and the bias voltages for SI-GaAs (40 V) and LT-GaAs (100 V) were kept below breakdown voltages. The waveforms for all samples are normalized at the main peak amplitude in order to see their emission mechanisms clearly. We observe the most sharp THz pulse from GaAs antenna implanted with dose of 5 � 1014 cm�<sup>2</sup> , and its full width at half maximum (FWHM) of the main peak amplitude is as narrow as 0.3 ps (solid curve), which indicates short carrier lifetimes in the ion-implanted THz emitter and �0.85 ps lifetime is confirmed by the pump-probe reflectance measurement. The other Himplanted PCAs also demonstrate very short THz pulses with FWHM of 0.31 ps for dose of <sup>1</sup> � 1014 cm�<sup>2</sup> (dotted curve) and 0.33 ps for dose of 1 � 1015 cm�<sup>2</sup> (dashed curve). The minimum peak after the main peak for the 5 � 1014 cm�<sup>2</sup> antenna is sharper than that for all other GaAs PCAs, and its minimum peak before the main peak is also most sharp. The sharp trends of THz signal increase before the maximum THz pulse peak for samples H-1 � 1014 and H-5 � 1014 cm�<sup>2</sup> , accompanying the current surge in photoconductive region, are completely identical to that case for SI-GaAs sample, which indicates that the carrier mobility in the shallow surface layer for H-1 � 1014 and H-5 � 1014 cm�<sup>2</sup> PCAs is very close to the mobility of bare SI-GaAs materials. The quite few and uniform point defects (Ga vacancies, As<sup>þ</sup> Ga, etc.) in shallow layer contribute the excellent mobility of these H-implanted samples, compared to LT-GaAs and the sample of H-1 � 1015 cm�<sup>2</sup> . In order to further identify the uniqueness of these

ion-implanted GaAs antennas, the fast Fourier transformed (FFT) spectra of the waveform for all implanted PCAs and the reference samples of SI-GaAs are shown in Figure 5(b). We find that ion-implanted GaAs antennas generate THz signals with frequency of over 5 THz and the sample with dose of 5 � 1014 cm�<sup>2</sup> demonstrates the strongest signals in the high-frequency range of 1 –5 THz. As expected, SI-GaAs samples produce the weakest signals at high frequen-

, SI-GaAs, and LT-

Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

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39

Figure 5. (a) Normalized THz pulses emitted from GaAs antennas of 1 � 1014, 5 � <sup>10</sup>14, 1 � <sup>10</sup><sup>15</sup> cm�<sup>2</sup>

GaAs. (b) Fast Fourier transformed spectra of THz signals for different GaAs emitters.

In order to interpret the THz radiation waveform, we analyze the THz radiation assuming that the emitted field ETHzð Þt is proportional to the time derivative of the transient current J tð Þ at far

cies of over 1 THz among all samples.

field, as expressed in Eq. (1).

The setup for characterization of the THz waves is based on a conventional time-domain spectroscopy system (TDS) triggered by a femtosecond laser as shown in Figure 4. A modelocked Ti: Sapphire laser generates 80 fs light pulses at a wavelength of �780 nm and a repetition rate of 80 MHz. The femtosecond pump pulses were focused by an objective lens with 10 μm-diameter illumination spot on the proximity of anode for a biased photoconductive antenna, which was mounted on the flat side of a Si hemispherical lens with a diameter of 15 mm. The emitted THz radiation was collimated and focused by two pairs of gold-coated off-axis parabolic mirrors onto a photoconductive sampling detector, which was also a photoconductive antenna with bow-tie shape and gap of 20 μm mounted on the back of a Si hemispherical lens with the same diameter. The photoconductive detector was gated by femtosecond probe beam pulses that were separated from the pump beam pulses by a beam

Figure 4. Schematic diagram of THz TDS setup used for measuring electric field of THz pulse.

GaFigure 5(a) shows the THz waveforms emitted from GaAs PCAs implanted with H dose

about 260 V and 60 mW, respectively. The SI-GaAs and LT-GaAs samples were measured under the pump power of 60 mW for reference and the bias voltages for SI-GaAs (40 V) and LT-GaAs (100 V) were kept below breakdown voltages. The waveforms for all samples are normalized at the main peak amplitude in order to see their emission mechanisms clearly. We observe the most sharp THz pulse from GaAs antenna implanted with dose of 5 � 1014 cm�<sup>2</sup>

and its full width at half maximum (FWHM) of the main peak amplitude is as narrow as 0.3 ps (solid curve), which indicates short carrier lifetimes in the ion-implanted THz emitter and �0.85 ps lifetime is confirmed by the pump-probe reflectance measurement. The other Himplanted PCAs also demonstrate very short THz pulses with FWHM of 0.31 ps for dose of <sup>1</sup> � 1014 cm�<sup>2</sup> (dotted curve) and 0.33 ps for dose of 1 � 1015 cm�<sup>2</sup> (dashed curve). The minimum peak after the main peak for the 5 � 1014 cm�<sup>2</sup> antenna is sharper than that for all other GaAs PCAs, and its minimum peak before the main peak is also most sharp. The sharp trends of THz signal increase before the maximum THz pulse peak for samples H-1 � 1014 and

identical to that case for SI-GaAs sample, which indicates that the carrier mobility in the shallow surface layer for H-1 � 1014 and H-5 � 1014 cm�<sup>2</sup> PCAs is very close to the mobility

shallow layer contribute the excellent mobility of these H-implanted samples, compared to LT-

of bare SI-GaAs materials. The quite few and uniform point defects (Ga vacancies, As<sup>þ</sup>

, accompanying the current surge in photoconductive region, are completely

. The applied bias voltage and pumping power were

. In order to further identify the uniqueness of these

,

Ga, etc.) in

splitter.

H-5 � 1014 cm�<sup>2</sup>

of 1 � 1014, 5 � 1014, and 1 � 1015 cm�<sup>2</sup>

38 Advances in Photodetectors - Research and Applications

GaAs and the sample of H-1 � 1015 cm�<sup>2</sup>

As<sup>þ</sup>

Figure 5. (a) Normalized THz pulses emitted from GaAs antennas of 1 � 1014, 5 � <sup>10</sup>14, 1 � <sup>10</sup><sup>15</sup> cm�<sup>2</sup> , SI-GaAs, and LT-GaAs. (b) Fast Fourier transformed spectra of THz signals for different GaAs emitters.

ion-implanted GaAs antennas, the fast Fourier transformed (FFT) spectra of the waveform for all implanted PCAs and the reference samples of SI-GaAs are shown in Figure 5(b). We find that ion-implanted GaAs antennas generate THz signals with frequency of over 5 THz and the sample with dose of 5 � 1014 cm�<sup>2</sup> demonstrates the strongest signals in the high-frequency range of 1 –5 THz. As expected, SI-GaAs samples produce the weakest signals at high frequencies of over 1 THz among all samples.

In order to interpret the THz radiation waveform, we analyze the THz radiation assuming that the emitted field ETHzð Þt is proportional to the time derivative of the transient current J tð Þ at far field, as expressed in Eq. (1).

$$E\_{\rm THz}(t) \propto \int \frac{\partial J(t)}{\partial t} d\mathbf{x} d\mathbf{y} d\mathbf{z} \tag{1}$$

concentration in this region is several orders of magnitude lower than the peak concentration and the carrier mobility is able to keep very close to that in SI GaAs. However, the defects for LT-GaAs and the sample of H-1 <sup>10</sup><sup>15</sup> cm<sup>2</sup> have decreased the carrier mobility by scattering to some extent that the carrier acceleration turns slow when comparing with SI-GaAs in Figure 5(a), considering the momentum relaxation time (τm) may increase to be comparable with the laser pulse width (δt) and the current surge is affected accordingly. Meanwhile, the current decay is dominated by the trapping effect in the underneath THz generation layer. This structure will form vertical confinement for the distribution of photo carriers and block the carriers in SI GaAs substrate, which makes the trapping layer return the original insulating state after the fast carrier trapping. It is noted that the carrier trapping time (ttrap) for samples of H-5 1014 and H-1 1015 cm<sup>2</sup> is significantly shorter than that of H-1 1014 cm<sup>2</sup>

GaAs, and SI-GaAs samples because the latter did not have efficient structures for vertical confinement of photo carriers. Therefore, we conclude that this confinement structure for photoconductive antennas will relieve the screen effect caused by charges accumulation in

In Figure 6(a), we show variation of the peak of emitted THz field amplitude with the pump laser power for all samples measured under TDS in Figure 4. The bias voltage of 140 V was used. The SI-GaAs sample without H implantation became saturated as the pump laser power exceeded 30 mW, similar to the reports in Refs. [6–10]. Thermal breakdown of SI-GaAs emitters easily occurred as they are saturated by the pump laser and the bias voltage. Normally, SI-GaAs emitters are recommended to operate far enough away from the saturation status. Hydrogen-implanted GaAs emitter with dose of 1 1014 cm<sup>2</sup> showed relatively linear increase of THz amplitude as the pump laser power. We are able to get the maximum THz field 3.5 times bigger than that from SI-GaAs emitter but no obvious saturation is found at the laser power of over 60 mW. The H-5 1014 cm<sup>2</sup> sample provides the best performance that almost linear dependence of THz fields emitted on the pump laser power is demonstrated, and the maximum THz field we could obtain from H-5 1014 cm<sup>2</sup> sample is five times bigger than that from SI-GaAs emitter. It should be emphasized that the H-implanted GaAs with dose of 5 <sup>10</sup><sup>14</sup> cm<sup>2</sup> did not show any saturation property with the pump laser approaching 100 mW in the TDS measurement range and the bias voltage increasing up to 260 V. The sample of H-1 1015 cm<sup>2</sup> showed deteriorated mobility, and THz fields are smaller than

lower dose of 1 1014–<sup>5</sup> 1014 cm<sup>2</sup> are more uniform and fewer, carrier mobility is kept to be very close to that of SI-GaAs with the carrier momentum relaxation time (τm) as short as 10 fs also. In the optimum operation conditions (i.e., 80 mW of the laser power and 260 V of the bias voltage for 5 1014 cm<sup>2</sup> sample versus 30 mW of the laser power and 140 V of the bias voltage for SI-GaAs), THz power emitted from the H-implanted sample was 100 times bigger than

Figure 6(b) presents the photocurrent as a function of the bias voltage (I-V) at a pump laser power of 60 mW for all H-implanted and SI-GaAs samples. Fairly high photocurrents and saturation behavior are observed for the SI-GaAs sample. With increasing the dosage of H implantation, there is continuous decrease in the photocurrents for the ion-implanted GaAs PCAs compared to the conventional SI-GaAs devices, because the presence of a deep trapping layer at 2.5 μm depth blocks the photo carriers in the SI-GaAs substrate. This means that heat

. Because point defects created by H implantation at

Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

http://dx.doi.org/10.5772/intechopen.79693

photoconductive region and reduce the saturation effect by laser excitation.

those emitted from sample 1 <sup>10</sup><sup>14</sup> cm<sup>2</sup>

that of traditional SI-GaAs THz emitter.

, LT-

41

where the integration is taken over the whole device including carrier acceleration layer and carrier trapping layer (Figure 1(b)). The transient current depends on the free-carrier concentration n and on the mean velocity v of the electrons:

$$J(t) = -en(t)\nu(t)\tag{2}$$

the contribution of the holes which have a much smaller mobility is neglected. We analyze the carrier transport based on a set of kinetic equations [27] which can be written as follows:

$$\frac{dn(t)}{dt} = -\frac{n(t)}{\tau\_c} + G(t) \tag{3}$$

$$\frac{d\nu(t)}{dt} = -\frac{\nu(t)}{\tau\_m} + \frac{eE\_{loc}}{m^\*} - \frac{\nu(t)G(t)}{n(t)}\tag{4}$$

where G is a photoinjection rate, Eloc is the local electric field, τ<sup>c</sup> is the free electron lifetime, and τ<sup>m</sup> is the momentum relaxation time, which is connected to the mobility of the free electrons in shallow layer.

Based on this theoretical model, the main positive peak observed in the waveforms of Figure 5 (a) is attributed to the rises of surge current by photo-carrier injection and the subsequent carrier acceleration under bias fields (tacc. in Figure 5(a)), while the second negative peak after the main peak is related to the decay of current governed by the carrier trapping (ttrap in Figure 5(a)). For the ion-implanted GaAs PCAs, we have to consider the carrier dynamics of acceleration process in shallow layer (<2 μm) and trapping process in the underneath layer (�2.5 μm deep), as shown in Figure 1(b). After laser is absorbed within 1 μm depth, photo carriers are created accordingly and accelerated within 2 μm depth for efficient THz generations. The main peak distribution of implanted ions and related defects at �2.5 μm depth enables efficient carrier trapping and significantly reduce carrier concentration in the trapping layer (Figure 1(b)). If the pump pulse laser width (δt) is larger than the carrier momentum relaxation time (τm), and if the carrier lifetime (τc) is larger than the pump laser pulse width (τ<sup>m</sup> < δt < τc), the carrier acceleration and resultant current rise are determined by the pump laser pulse width, which is related to tacc.. This is exactly the situation in the shallow laser absorption layer (1 μm deep) for ion-implanted GaAs PCAs; where the momentum relaxation time was estimated to be about 10 fs; the laser pulse width was 80 fs, and the carrier lifetime was over 10 ps (similar to lifetime in bare SI GaAs), respectively. Assuming the transition time for photo carriers from absorption region (1 μm) to the trapping layer (2.5 μm) is shorter than the carrier trapping time, the carrier trapping and corresponding current decay depends on peak concentrations of H ions and implantation-related defects at 2.5 μm depth; considering that carrier transition time is about 100 fs, and the carrier trapping time (ttrap) is estimated to be �0.8 ps for the sample H-5 � 1014 cm�<sup>2</sup> . In the progress of current rise dominated by laser pump, carrier acceleration should not be affected by implantation defects because the ion concentration in this region is several orders of magnitude lower than the peak concentration and the carrier mobility is able to keep very close to that in SI GaAs. However, the defects for LT-GaAs and the sample of H-1 <sup>10</sup><sup>15</sup> cm<sup>2</sup> have decreased the carrier mobility by scattering to some extent that the carrier acceleration turns slow when comparing with SI-GaAs in Figure 5(a), considering the momentum relaxation time (τm) may increase to be comparable with the laser pulse width (δt) and the current surge is affected accordingly. Meanwhile, the current decay is dominated by the trapping effect in the underneath THz generation layer. This structure will form vertical confinement for the distribution of photo carriers and block the carriers in SI GaAs substrate, which makes the trapping layer return the original insulating state after the fast carrier trapping. It is noted that the carrier trapping time (ttrap) for samples of H-5 1014 and H-1 1015 cm<sup>2</sup> is significantly shorter than that of H-1 1014 cm<sup>2</sup> , LT-GaAs, and SI-GaAs samples because the latter did not have efficient structures for vertical confinement of photo carriers. Therefore, we conclude that this confinement structure for photoconductive antennas will relieve the screen effect caused by charges accumulation in photoconductive region and reduce the saturation effect by laser excitation.

ð1Þ

ð2Þ

ð3Þ

ð4Þ

where the integration is taken over the whole device including carrier acceleration layer and carrier trapping layer (Figure 1(b)). The transient current depends on the free-carrier concen-

the contribution of the holes which have a much smaller mobility is neglected. We analyze the carrier transport based on a set of kinetic equations [27] which can be written as follows:

where G is a photoinjection rate, Eloc is the local electric field, τ<sup>c</sup> is the free electron lifetime, and τ<sup>m</sup> is the momentum relaxation time, which is connected to the mobility of the free electrons in

Based on this theoretical model, the main positive peak observed in the waveforms of Figure 5 (a) is attributed to the rises of surge current by photo-carrier injection and the subsequent carrier acceleration under bias fields (tacc. in Figure 5(a)), while the second negative peak after the main peak is related to the decay of current governed by the carrier trapping (ttrap in Figure 5(a)). For the ion-implanted GaAs PCAs, we have to consider the carrier dynamics of acceleration process in shallow layer (<2 μm) and trapping process in the underneath layer (�2.5 μm deep), as shown in Figure 1(b). After laser is absorbed within 1 μm depth, photo carriers are created accordingly and accelerated within 2 μm depth for efficient THz generations. The main peak distribution of implanted ions and related defects at �2.5 μm depth enables efficient carrier trapping and significantly reduce carrier concentration in the trapping layer (Figure 1(b)). If the pump pulse laser width (δt) is larger than the carrier momentum relaxation time (τm), and if the carrier lifetime (τc) is larger than the pump laser pulse width (τ<sup>m</sup> < δt < τc), the carrier acceleration and resultant current rise are determined by the pump laser pulse width, which is related to tacc.. This is exactly the situation in the shallow laser absorption layer (1 μm deep) for ion-implanted GaAs PCAs; where the momentum relaxation time was estimated to be about 10 fs; the laser pulse width was 80 fs, and the carrier lifetime was over 10 ps (similar to lifetime in bare SI GaAs), respectively. Assuming the transition time for photo carriers from absorption region (1 μm) to the trapping layer (2.5 μm) is shorter than the carrier trapping time, the carrier trapping and corresponding current decay depends on peak concentrations of H ions and implantation-related defects at 2.5 μm depth; considering that carrier transition time is about 100 fs, and the carrier trapping time (ttrap) is estimated to be

pump, carrier acceleration should not be affected by implantation defects because the ion

. In the progress of current rise dominated by laser

tration n and on the mean velocity v of the electrons:

40 Advances in Photodetectors - Research and Applications

shallow layer.

�0.8 ps for the sample H-5 � 1014 cm�<sup>2</sup>

In Figure 6(a), we show variation of the peak of emitted THz field amplitude with the pump laser power for all samples measured under TDS in Figure 4. The bias voltage of 140 V was used. The SI-GaAs sample without H implantation became saturated as the pump laser power exceeded 30 mW, similar to the reports in Refs. [6–10]. Thermal breakdown of SI-GaAs emitters easily occurred as they are saturated by the pump laser and the bias voltage. Normally, SI-GaAs emitters are recommended to operate far enough away from the saturation status. Hydrogen-implanted GaAs emitter with dose of 1 1014 cm<sup>2</sup> showed relatively linear increase of THz amplitude as the pump laser power. We are able to get the maximum THz field 3.5 times bigger than that from SI-GaAs emitter but no obvious saturation is found at the laser power of over 60 mW. The H-5 1014 cm<sup>2</sup> sample provides the best performance that almost linear dependence of THz fields emitted on the pump laser power is demonstrated, and the maximum THz field we could obtain from H-5 1014 cm<sup>2</sup> sample is five times bigger than that from SI-GaAs emitter. It should be emphasized that the H-implanted GaAs with dose of 5 <sup>10</sup><sup>14</sup> cm<sup>2</sup> did not show any saturation property with the pump laser approaching 100 mW in the TDS measurement range and the bias voltage increasing up to 260 V. The sample of H-1 1015 cm<sup>2</sup> showed deteriorated mobility, and THz fields are smaller than those emitted from sample 1 <sup>10</sup><sup>14</sup> cm<sup>2</sup> . Because point defects created by H implantation at lower dose of 1 1014–<sup>5</sup> 1014 cm<sup>2</sup> are more uniform and fewer, carrier mobility is kept to be very close to that of SI-GaAs with the carrier momentum relaxation time (τm) as short as 10 fs also. In the optimum operation conditions (i.e., 80 mW of the laser power and 260 V of the bias voltage for 5 1014 cm<sup>2</sup> sample versus 30 mW of the laser power and 140 V of the bias voltage for SI-GaAs), THz power emitted from the H-implanted sample was 100 times bigger than that of traditional SI-GaAs THz emitter.

Figure 6(b) presents the photocurrent as a function of the bias voltage (I-V) at a pump laser power of 60 mW for all H-implanted and SI-GaAs samples. Fairly high photocurrents and saturation behavior are observed for the SI-GaAs sample. With increasing the dosage of H implantation, there is continuous decrease in the photocurrents for the ion-implanted GaAs PCAs compared to the conventional SI-GaAs devices, because the presence of a deep trapping layer at 2.5 μm depth blocks the photo carriers in the SI-GaAs substrate. This means that heat

THz power conversion efficiency of H-5 � <sup>10</sup><sup>14</sup> cm�<sup>2</sup> emitter is almost �1600 times better than

Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

http://dx.doi.org/10.5772/intechopen.79693

43

Figure 7. The terahertz power as a function of the photocurrent is represented for ion-implanted GaAs PCAs.

To further confirm the mobility in the THz generation layer of ion-implanted PCAs is superior to that of LT-GaAs, Figure 7 relates the THz radiation power to photocurrents for all ionimplanted PCA samples for a constant laser power in a logarithmic scale. The relationships for all samples are curve-fitted to parallel lines with a slope of 2, indicating the quadratic dependence of the radiation power on the induced photocurrents by the pump laser, and the fact that operation conditions (laser alignment, output THz coupling efficiency, antenna structure, etc.) are the same for all PCAs. The ion-implanted GaAs antennas for H-1 � 1014, H-1 � 1015, and O <sup>1</sup> � 1012 cm�<sup>2</sup> generated stronger THz radiation than LT-GaAs PCAs under the excitation of constant laser power 200 mW, however the DC photocurrents are reduced by about 100 times. This enhancement for THz generation may mainly be attributed to the better quality of photoconductive GaAs with higher mobility than that of LT-GaAs since the conversion efficiency is proportional to the carrier mobility, while the carrier concentration is tightly confined

In order to further interpret the THz radiation power and the optical-to-THz conversion efficiency for GaAs PCAs, we deduced the theoretical model accordingly for photoconductive

According to the Ref. [28], transient photocurrent under fs laser excitations can be written as

where e is the electron charge, f <sup>L</sup> is the optical frequency, h is the Planck constant, Vb is the applied bias voltage, μ<sup>e</sup> is the electron mobility, τ is the lifetime for photocarriers, η<sup>L</sup> is the absorption efficiency for laser illumination, PL is the incident laser power on the gap, d is the

ð5Þ

by deep trapping layer at �2.5 μm-distance in ion-implanted PCAs.

SI-GaAs emitter.

antennas.

below Eq. (5).

length of photoconductive gap.

Figure 6. (a) Peak THz field amplitude from GaAs emitters as a function of pump laser power under a bias voltage of 140 V. (b) Photocurrent-voltage (I-V) characteristics under excitation at the laser power of 60 mW for H-implanted and SI-GaAs samples. The photocurrent was calculated from the measured current under illumination by subtracting the dark current.

generation will be efficiently suppressed in ion-implanted samples as H dose is increased, then thermal breakdown voltage of GaAs PCAs will become higher as higher dose is utilized. Himplanted GaAs PCAs become electrically robust and are able to stably operate from �80 to >260 V. To make a rough estimate of improvement in electrical-to-THz power conversion efficiency of the H-implanted GaAs PCAs, we can compare the THz power emission for H-<sup>5</sup> � 1014 cm�<sup>2</sup> sample with conventional SI-GaAs sample without any implantation. As shown in Figure 6(a), the THz field amplitude is about four times more (power will be 16 times) from H-5 � 1014 cm�<sup>2</sup> sample than SI-GaAs sample under the optical power of 60 mW. Moreover, the corresponding photocurrent for H-5 � 1014 cm�<sup>2</sup> sample is about 100 times smaller than that for SI-GaAs sample at the same voltage of 140 V. Then, under the same photo-excitation conditions (pump laser power, bias voltage), photocurrent is 100 times smaller, but emitted THz power is 16 times more for H-5 � 1014 cm�<sup>2</sup> sample than SI-GaAs sample. The electrical to

Figure 7. The terahertz power as a function of the photocurrent is represented for ion-implanted GaAs PCAs.

THz power conversion efficiency of H-5 � <sup>10</sup><sup>14</sup> cm�<sup>2</sup> emitter is almost �1600 times better than SI-GaAs emitter.

To further confirm the mobility in the THz generation layer of ion-implanted PCAs is superior to that of LT-GaAs, Figure 7 relates the THz radiation power to photocurrents for all ionimplanted PCA samples for a constant laser power in a logarithmic scale. The relationships for all samples are curve-fitted to parallel lines with a slope of 2, indicating the quadratic dependence of the radiation power on the induced photocurrents by the pump laser, and the fact that operation conditions (laser alignment, output THz coupling efficiency, antenna structure, etc.) are the same for all PCAs. The ion-implanted GaAs antennas for H-1 � 1014, H-1 � 1015, and O <sup>1</sup> � 1012 cm�<sup>2</sup> generated stronger THz radiation than LT-GaAs PCAs under the excitation of constant laser power 200 mW, however the DC photocurrents are reduced by about 100 times. This enhancement for THz generation may mainly be attributed to the better quality of photoconductive GaAs with higher mobility than that of LT-GaAs since the conversion efficiency is proportional to the carrier mobility, while the carrier concentration is tightly confined by deep trapping layer at �2.5 μm-distance in ion-implanted PCAs.

In order to further interpret the THz radiation power and the optical-to-THz conversion efficiency for GaAs PCAs, we deduced the theoretical model accordingly for photoconductive antennas.

generation will be efficiently suppressed in ion-implanted samples as H dose is increased, then thermal breakdown voltage of GaAs PCAs will become higher as higher dose is utilized. Himplanted GaAs PCAs become electrically robust and are able to stably operate from �80 to >260 V. To make a rough estimate of improvement in electrical-to-THz power conversion efficiency of the H-implanted GaAs PCAs, we can compare the THz power emission for H-<sup>5</sup> � 1014 cm�<sup>2</sup> sample with conventional SI-GaAs sample without any implantation. As shown in Figure 6(a), the THz field amplitude is about four times more (power will be 16 times) from H-5 � 1014 cm�<sup>2</sup> sample than SI-GaAs sample under the optical power of 60 mW. Moreover, the corresponding photocurrent for H-5 � 1014 cm�<sup>2</sup> sample is about 100 times smaller than that for SI-GaAs sample at the same voltage of 140 V. Then, under the same photo-excitation conditions (pump laser power, bias voltage), photocurrent is 100 times smaller, but emitted THz power is 16 times more for H-5 � 1014 cm�<sup>2</sup> sample than SI-GaAs sample. The electrical to

Figure 6. (a) Peak THz field amplitude from GaAs emitters as a function of pump laser power under a bias voltage of 140 V. (b) Photocurrent-voltage (I-V) characteristics under excitation at the laser power of 60 mW for H-implanted and SI-GaAs samples. The photocurrent was calculated from the measured current under illumination by subtracting the dark

current.

42 Advances in Photodetectors - Research and Applications

According to the Ref. [28], transient photocurrent under fs laser excitations can be written as below Eq. (5).

$$I\_{ph} = \frac{eV\_b\mu\_v\tau\eta\_L P\_L}{hf\_ld^2} \tag{5}$$

where e is the electron charge, f <sup>L</sup> is the optical frequency, h is the Planck constant, Vb is the applied bias voltage, μ<sup>e</sup> is the electron mobility, τ is the lifetime for photocarriers, η<sup>L</sup> is the absorption efficiency for laser illumination, PL is the incident laser power on the gap, d is the length of photoconductive gap.

Meanwhile, the transient resistance under laser excitations can be approximately formulated as Eq. (6) [15].

$$R\_{\text{gap}} = \frac{h f\_L f\_k d^2}{e \mu\_e \eta\_L P\_L} \tag{6}$$

shallow layer for THz generation are as high as native SI-GaAs materials, which also means that the photoconductive material factor μτ<sup>2</sup> is very high to guarantee the high optical-to-THz

Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

http://dx.doi.org/10.5772/intechopen.79693

45

In summary, the GaAs PCAs' saturation effect for the excitation of pump laser is efficiently reduced by hydrogen implantation, due to the vertical confinement of photo carriers in Himplanted emitters. THz emitter implanted by H ions of 300 keV and 5 � 1014 cm�<sup>2</sup> has both excellent mobility and short enough carrier lifetime. Thus, the optical-to-THz conversion efficiency is improved 16 times and the electrical-to-THz conversion efficiency is 1600 times compared to conventional GaAs emitters. Electrically robust H-implanted GaAs PCA is able to operate from �80 to >260 V without any thermal breakdown. The emitted THz power from Himplanted GaAs antenna is more than two order of magnitude stronger than that from

This work was supported by the National Natural Science Foundation of China (Grant No. U1613223). The authors acknowledge Ms. Ho Lai Ching, a staff member at the Department of Electronic Engineering, the Chinese University of Hong Kong, Hong Kong, China, for her

1 Institute of Robotics and Intelligent Manufacturing (IRIM), The Chinese University of Hong

2 Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, NT,

[1] Dhillon SS et al. The 2017 terahertz science and technology roadmap. Journal of Physics D:

conversion efficiency for PCAs.

3. Conclusion

traditional GaAs emitter.

Acknowledgements

Author details

References

assistance in equipment maintenance.

Caiming Sun1,2\* and Aidong Zhang1

\*Address all correspondence to: cmsun@cuhk.edu.cn

Kong, Shen Zhen, People's Republic of China

Applied Physics. 2017;50(4):043001

Hong Kong, People's Republic of China

where f <sup>R</sup> is the repetition rate for incident fs laser pulses.

Therefore, using Eqs. (5) and (6), we can obtain the expression for the electric power generated between the photoconductive gap, subjected to the pump laser power PL and the optical-to-THz conversion efficiency for the antenna ηLE.

$$P\_E = I\_{ph}{}^2 R\_{gap} = \left(\frac{eV\_b \mu\_e \pi \eta\_L P\_L}{h f\_L d^2}\right)^2 \frac{h f\_L f\_R d^2}{e \mu\_e \eta\_L P\_L} = \frac{eV\_b \,^2 \mu\_e \pi^2 \eta\_L P\_L f\_R}{h f\_L d^2} = \frac{eE\_b{}^2 \mu\_e \pi^2 \eta\_L P\_L f\_R}{h f\_L} \tag{7}$$

$$\eta\_{LE} = \frac{P\_E}{P\_L} = \frac{eV\_b^2 \mu\_e \tau^2 \eta\_L f\_R}{hf\_L d^2} = \frac{eE\_{loc}^2 \mu\_e \tau^2 \eta\_L f\_R}{hf\_L} \tag{8}$$

where Eloc is the localized electric field around the anode proximity, for separating the electronhole pairs, accelerating photocarriers, and the generation of transient photocurrents.

As seen from Eq. (8), the optical-to-THz conversion efficiency is directly proportional to the square of the bias voltage Vb and to the photoconductive material factor μτ<sup>2</sup> .

According to [28] and the references wherein, the saturation behavior of THz radiation amplitude Er against the pump intensity FL or pump power PL can be expressed as

$$E\_r \approx \frac{F\_L / F\_s}{1 + F\_L / F\_s} = \frac{P\_L / P\_s}{1 + P\_L / P\_s} \tag{9}$$

where Fs, Ps are the characteristic saturation intensity and saturation power for the PC antenna respectively.

$$F\_s = \frac{hf\_L}{e\mu\_e\eta\_L} \tag{10}$$

Eq. (10) shows that the saturation behavior for PC antennas will easily take place when the carrier mobility of the photoconductive material is high, and Fs is normally below 100 μJ/cm<sup>2</sup> for conventional SI-GaAs and LT-GaAs PCAs [21–24, 29]. Comparably, our GaAs PCAs based on high-energy ion-implantation did not show any saturation property even when the pump laser intensity increases as high as 10 mJ/cm<sup>2</sup> , which indicating the carrier mobility and carrier lifetime are quite low in the deep trapping layer. The carrier mobility and carrier lifetime in the shallow layer for THz generation are as high as native SI-GaAs materials, which also means that the photoconductive material factor μτ<sup>2</sup> is very high to guarantee the high optical-to-THz conversion efficiency for PCAs.

#### 3. Conclusion

ð6Þ

ð7Þ

ð8Þ

ð9Þ

ð10Þ

.

, which indicating the carrier mobility and carrier

Meanwhile, the transient resistance under laser excitations can be approximately formulated

Therefore, using Eqs. (5) and (6), we can obtain the expression for the electric power generated between the photoconductive gap, subjected to the pump laser power PL and the optical-to-

where Eloc is the localized electric field around the anode proximity, for separating the electron-

As seen from Eq. (8), the optical-to-THz conversion efficiency is directly proportional to the

According to [28] and the references wherein, the saturation behavior of THz radiation ampli-

where Fs, Ps are the characteristic saturation intensity and saturation power for the PC antenna

Eq. (10) shows that the saturation behavior for PC antennas will easily take place when the carrier mobility of the photoconductive material is high, and Fs is normally below 100 μJ/cm<sup>2</sup> for conventional SI-GaAs and LT-GaAs PCAs [21–24, 29]. Comparably, our GaAs PCAs based on high-energy ion-implantation did not show any saturation property even when the pump

lifetime are quite low in the deep trapping layer. The carrier mobility and carrier lifetime in the

hole pairs, accelerating photocarriers, and the generation of transient photocurrents.

square of the bias voltage Vb and to the photoconductive material factor μτ<sup>2</sup>

tude Er against the pump intensity FL or pump power PL can be expressed as

where f <sup>R</sup> is the repetition rate for incident fs laser pulses.

THz conversion efficiency for the antenna ηLE.

44 Advances in Photodetectors - Research and Applications

laser intensity increases as high as 10 mJ/cm<sup>2</sup>

as Eq. (6) [15].

respectively.

In summary, the GaAs PCAs' saturation effect for the excitation of pump laser is efficiently reduced by hydrogen implantation, due to the vertical confinement of photo carriers in Himplanted emitters. THz emitter implanted by H ions of 300 keV and 5 � 1014 cm�<sup>2</sup> has both excellent mobility and short enough carrier lifetime. Thus, the optical-to-THz conversion efficiency is improved 16 times and the electrical-to-THz conversion efficiency is 1600 times compared to conventional GaAs emitters. Electrically robust H-implanted GaAs PCA is able to operate from �80 to >260 V without any thermal breakdown. The emitted THz power from Himplanted GaAs antenna is more than two order of magnitude stronger than that from traditional GaAs emitter.

#### Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. U1613223). The authors acknowledge Ms. Ho Lai Ching, a staff member at the Department of Electronic Engineering, the Chinese University of Hong Kong, Hong Kong, China, for her assistance in equipment maintenance.

### Author details

Caiming Sun1,2\* and Aidong Zhang1

\*Address all correspondence to: cmsun@cuhk.edu.cn

1 Institute of Robotics and Intelligent Manufacturing (IRIM), The Chinese University of Hong Kong, Shen Zhen, People's Republic of China

2 Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, People's Republic of China

#### References

[1] Dhillon SS et al. The 2017 terahertz science and technology roadmap. Journal of Physics D: Applied Physics. 2017;50(4):043001

[2] Lewis RA. A review of terahertz sources. Journal of Physics D: Applied Physics. 2014;47 (37):374001

[17] Khiabani N, Huang Y, Shen YC, Boyes SJ. Theoretical modeling of a photoconductive antenna in a terahertz pulsed system. IEEE Transactions on Antennas and Propagation.

Ion-Beam Modified Terahertz GaAs Photoconductive Antenna

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47

[18] Taylor AJ, Benicewicz PK, Young SM. Modeling of femtosecond electromagnetic pulses

[19] Darrow JT, Zhang X-C, Auston DH. Power scaling of large‐aperture photoconducting

[20] Ralph SE, Grischkowsky D. Trap-enhanced electric fields in semi-insulators: The role of electrical and optical carrier injection. Applied Physics Letters. 1991;59(16):1972

[21] Yang S-H, Hashemi MR, Berry CW, Jarrahi M. 7.5% Optical-to-Terahertz Conversion Efficiency Offered by Photoconductive Emitters With Three-Dimensional Plasmonic Contact Electrodes. IEEE Transactions on Terahertz Science and Technology. 2014;4(5):575 [22] Kim DS, Citrin DS. Coulomb and radiation screening in photoconductive terahertz

[23] Rodriguez G, Caceres SR, Taylor AJ. Modeling of terahertz radiation from biased

[24] Benicewicz PK, Roberts JP, Taylor AJ. Scaling of terahertz radiation from large-aperture biased photoconductors. Journal of the Optical Society of America B: Optical Physics.

[25] Sun CM, Zhang AD. Efficient terahertz generation from lightly ion-beam-treated semiinsulating GaAs photoconductive antennas. Applied Physics Express. 2017;10(10):102202

[26] Ziegler JF, Biersack JP, Littmark U. The Stopping and Range of Ions in Solids. Vol. 1. New

[27] Jepsen PU, Jacobsen RH, Keiding RH. Generation and detection of terahertz pulses from biased semiconductor antennas. Journal of the Optical Society of America B: Optical

[28] Smith PR, Auston DH, Nuss MC. Subpicosecond Photoconducting Dipole Antennas. IEEE

[29] Chou R-H, Pan C-L. Gap-Dependent Terahertz Pulses from Mid-Size-Gap Multi-Energy Arsenic-Ion-Implanted GaAs Antennas. Japanese Journal of Applied Physics. 2008;47(11):

photoconductors: Transient velocity effects. Optics Letters. 1994;19(23):1994

from large-aperture photoconductors. Optics Letters. 1993;18(16):1340

antennas. Applied Physics Letters. 1991;58(1):25

sources. Applied Physics Letters. 2006;88:161117

York: Pergamon; 1985. See also http://www.srim.org/

Journal of Quantum Electronics. 1988;24(2):255-260

2013;61(4):1538

1994;11(12):2533

Physics. 1996;13(11):2424

8419-8425


[17] Khiabani N, Huang Y, Shen YC, Boyes SJ. Theoretical modeling of a photoconductive antenna in a terahertz pulsed system. IEEE Transactions on Antennas and Propagation. 2013;61(4):1538

[2] Lewis RA. A review of terahertz sources. Journal of Physics D: Applied Physics. 2014;47

[3] Hafez HA, Chai X, Ibrahim A, Mondal S, Ferachou D, Ropagnol X, Ozaki T. Intense

[4] Auston DH, Cheung KP, Smith PR. Picosecond photoconducting Hertzian dipoles.

[6] Tani M, Matsuura S, Sakai K, Nakashima S. Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs. Applied

[7] Tani M, Sakai K, Mimura H. Ultrafast Photoconductive Detectors Based on Semi-Insulat-

[8] Liu TA, Tani M, Pan CL. THz radiation emission properties of multienergy arsenic-ionimplanted GaAs and semi-insulating GaAs based photoconductive antennas. Journal of

[9] Salem B, Morris D, Aimez V, Beerens J, Beauvais J, Houde D. Pulsed photoconductive antenna terahertz sources made on ion-implanted GaAs substrates. Journal of Physics.

[10] Salem B, Morris D, Salissou Y, Aimez V, Charlebois S, Chicoine M, Schiettekatte F. Terahertz emission properties of arsenic and oxygen ion-implanted GaAs based photoconductive pulsed sources. Journal of Vacuum Science and Technology A. 2006;24(3):774

[11] Singh A, Pal S, Surdi H, Prabhu SS, Mathimalar S, Nanal V, Pillay RG, Döhler GH. Carbon irradiated semi insulating GaAs for photoconductive terahertz pulse detection. Optics

[12] Ludwig C, Kuhl J. Studies of the temporal and spectral shape of terahertz pulses generated from photoconducting switches. Applied Physics Letters. 1996;69(9):1194

[13] Kono S, Tani M, Sakai K. Ultrabroadband photoconductive detection: Comparison with

[14] Gregory IS, Baker C, Tribe WR, Evans MJ, Beere HE, Linfield EH, Davies AG, Missous M. High resistivity annealed low-temperature GaAs with 100 fs lifetimes. Applied Physics

[15] Glinskiya IA, Khabibullin RA, Ponomarev DS. Total Efficiency of the Optical-to-Terahertz Conversion in Photoconductive Antennas Based on LT-GaAs and In0.38Ga0.62As. Russian

[16] Huang Y, Khiabani N, Shen Y, Li D. Terahertz photoconductive antenna efficiency. In: Proceedings of the International Workshop Antenna Technology (iWAT); Hong Kong,

free-space electro-optic sampling. Applied Physics Letters. 2001;79(7):898

terahertz radiation and their applications. Journal of Optics. 2016;18(9):093004

[5] Makram-Ebeid S, Tuck B. Semi-Insulating III-V Materials. Nantwich: Shiva; 1982

ing GaAs and InP. Japanese Journal of Applied Physics, Part 2. 1997;36:L1175

(37):374001

Optics. 1997;36:7853

Applied Physics. 2003;93(5):2996

Condensed Matter. 2005;17(46):7327

Express. 2017;23(5):6656

Letters. 2003;83(20):4199

China; 2011. pp. 152-156

MicroElectronics. 2017;46(6):408-413

Applied Physics Letters. 1984;45(3):284

46 Advances in Photodetectors - Research and Applications


**Chapter 4**

Provisional chapter

**Energy Bandgap Engineering of Transmission-Mode**

DOI: 10.5772/intechopen.80704

Aiming to enhance the photoemission capability in the waveband region of interest, a graded bandgap structure was applied to the conventional transmission-mode AlGaAs/ GaAs photocathodes based on energy bandgap engineering, wherein the composition in AlxGa1xAs window layer and the doping concentration in GaAs active layer were gradual. According to Spicer's three-step model, a photoemission theoretical model applicable to the novel transmission-mode AlxGa1xAs/GaAs photocathodes was deduced so as to guide the cathode structural design. Then the cathode material was grown by the metalorganic chemical vapor deposition technique, and the epitaxial cathode material quality was evaluated by the means of scanning electron microscope, electrochemical capacitance-voltage, X-ray diffraction and spectrophotometry. Through a series of specific processes, the cathode material was made into the multilayered module, possessing a glass/Si3N4/AlxGa1xAs/GaAs structure. After the surface treatment including heat cleaning and Cs▬O activation for the cathode module, the image intensifier tube comprising the activated cathode module, microchannel plate, and phosphor screen was fabricated by indium sealing. The spectral response test results confirm the validity of the

novel structure for the enhancement of blue-green photoresponse.

Keywords: AlGaAs/GaAs photocathode, graded bandgap, photoemission model,

Since negative-electron-affinity (NEA) GaAs photocathode was proposed as a type of excellent photoemitter by Scheer and Laar [1], GaAs-based photocathodes have found widespread applications in photodetectors, accelerators, electron microscopes, photon-enhanced thermionic emission devices, and other fields [2–5]. In view of the high visible spectral response,

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Energy Bandgap Engineering of Transmission-Mode

**AlGaAs/GaAs Photocathode**

AlGaAs/GaAs Photocathode

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Yijun Zhang and Gangcheng Jiao

Yijun Zhang and Gangcheng Jiao

http://dx.doi.org/10.5772/intechopen.80704

material epitaxy, tube fabrication

Abstract

1. Introduction

#### **Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode** Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

DOI: 10.5772/intechopen.80704

Yijun Zhang and Gangcheng Jiao Yijun Zhang and Gangcheng Jiao

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80704

#### Abstract

Aiming to enhance the photoemission capability in the waveband region of interest, a graded bandgap structure was applied to the conventional transmission-mode AlGaAs/ GaAs photocathodes based on energy bandgap engineering, wherein the composition in AlxGa1xAs window layer and the doping concentration in GaAs active layer were gradual. According to Spicer's three-step model, a photoemission theoretical model applicable to the novel transmission-mode AlxGa1xAs/GaAs photocathodes was deduced so as to guide the cathode structural design. Then the cathode material was grown by the metalorganic chemical vapor deposition technique, and the epitaxial cathode material quality was evaluated by the means of scanning electron microscope, electrochemical capacitance-voltage, X-ray diffraction and spectrophotometry. Through a series of specific processes, the cathode material was made into the multilayered module, possessing a glass/Si3N4/AlxGa1xAs/GaAs structure. After the surface treatment including heat cleaning and Cs▬O activation for the cathode module, the image intensifier tube comprising the activated cathode module, microchannel plate, and phosphor screen was fabricated by indium sealing. The spectral response test results confirm the validity of the novel structure for the enhancement of blue-green photoresponse.

Keywords: AlGaAs/GaAs photocathode, graded bandgap, photoemission model, material epitaxy, tube fabrication

#### 1. Introduction

Since negative-electron-affinity (NEA) GaAs photocathode was proposed as a type of excellent photoemitter by Scheer and Laar [1], GaAs-based photocathodes have found widespread applications in photodetectors, accelerators, electron microscopes, photon-enhanced thermionic emission devices, and other fields [2–5]. In view of the high visible spectral response,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

good spectral extensibility to the near infrared (NIR) region and low dark current, NEA GaAs, GaAsP, and InGaAs photocathodes are important components in the vacuum photodetectors, for example, low-light-level (LLL) image intensifiers, photomultiplier tubes, and streak tubes [6]. In the modern light sources based on free electron lasers or energy recovery linacs, GaAsbased photocathodes serve as high brightness electron sources with the unique virtues of large current density driven by visible lasers, high spin polarization, low thermal emittance, and narrow energy distribution [7]. In recent years, a spin-polarized transmission electron microscope combining electron microscopy and accelerator technology using GaAs-GaAsP strained superlattice photocathodes was developed to observe dynamically a magnetic field images with high spatial and temporal resolutions [8]. Moreover, with the aid of the ultrahigh speed pulse laser, GaAs photocathodes can satisfy the requirements of fast response speed and large emission current density aiming to THz frequency vacuum devices [9].

are more popular than r-mode ones, because the laser spot size can be reduced through the short focus lens placed on the photocathode backside, which would not hinder the path of the electron beam and is more conducive to achieve a super-high-brightness electron beam

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

51

As proposed by Spicer and Herreragomez [15], the photoemission process from photocathodes consists of electron excitation by incident light absorption, electron transport toward surface, and electron escape across the surface barrier into vacuum. For t-mode photocathodes, some important cathode parameters such as electron diffusion length, interface recombination velocity, and surface escape probability are crucial to the photoemission performance, especially the shortwave photosensitivity [16]. Enhancing the blue-green response of t-mode GaAs photocathodes as far as possible, would not only be beneficial to the detection in sandy or desert terrain for image intensifiers [17], but also increase the current density driven by 532 nm laser for electron sources [18]. Although the external electric field biased across the photocathodes can improve the photoemission capability, the limitations of this approach are the difficulty in making thin electrode pattern and the increased dark current with the strong field [19, 20]. In view of this adverse case, internal built-in electric fields through energy band engineering design could be an alternative approach. In our research, a complex structure composed of the composition-graded structure and the doping-graded structure is proposed to prepare high efficient t-mode AlGaAs/GaAs photocathodes. Furthermore, the photoemission model, cathode structure design, cathode material epitaxy, and vacuum tube fabrication are investigated through the integrated analysis of theory and experiments. Finally, the effectivity of the

designed novel structure is verified by comparison with the common photocathodes.

For the t-mode GaAs photocathode, the AlGaAs and GaAs materials are usually used as the window layer and the active layer, which determine the shortwave cutoff and longwave cutoff, respectively. A built-in electric field in the interior of the photocathode material can be realized by the variation of dopant or composition according to energy band engineering design [21, 22]. Based on this concept, a novel structure is proposed to improve photoelectron emission capability, wherein a composition-graded structure and a doping-graded structure are employed to the AlxGa1�xAs window layer and GaAs active layer, respectively [23, 24], as shown in Figure 2. To form a built-in constant electrical field in the GaAs active layer of the photocathode, the p-type dopant concentration can follow the exponential variation, and the doping formula is

where A denotes the exponential-doping (e-doping) factor, N<sup>0</sup> is the doping concentration at the surface of GaAs active layer, y is the distance from the coordinate origin (i.e., the surface of GaAs active layer), and N(y) is the p-type doping concentration in the GaAs active layer. As a result of the variation of dopant concentration, the initial Fermi level is different. In thermal

N yð Þ¼ N0exp ð Þ Ay (1)

2. Graded bandgap structure

expressed by [22]

[13, 14].

As is well known, GaAs photocathodes can operate in the transmission-mode (t-mode) and the reflection-mode (r-mode), respectively, depending on the difference in the direction of the incident light [10, 11], as shown in Figure 1. For the t-mode operation, the incident light is irradiated on the substrate surface, and the photoelectrons are extracted from the opposite surface side, whereas for the r-mode operation, the incident light and photoelectrons are located on the same emission surface side. Due to the difference in absorption length of longwave and shortwave photons, the shapes of spectral response curves for GaAs photocathodes working in the two modes are different [11]. Differing from r-mode GaAs photocathodes, t-mode ones are difficult to achieve high spectral response in a broadband region from ultraviolet to NIR spectrum. Usually in the practical applications, the researches on tmode photocathodes are more concerned. For example, the image intensifiers and related imaging systems, t-mode photocathodes conform to the optical imaging structure [12]. Besides, as polarized electron sources in photoinjector apparatus, t-mode photocathodes

Figure 1. Schematic diagram of thin photocathode operating in the two different modes.

are more popular than r-mode ones, because the laser spot size can be reduced through the short focus lens placed on the photocathode backside, which would not hinder the path of the electron beam and is more conducive to achieve a super-high-brightness electron beam [13, 14].

As proposed by Spicer and Herreragomez [15], the photoemission process from photocathodes consists of electron excitation by incident light absorption, electron transport toward surface, and electron escape across the surface barrier into vacuum. For t-mode photocathodes, some important cathode parameters such as electron diffusion length, interface recombination velocity, and surface escape probability are crucial to the photoemission performance, especially the shortwave photosensitivity [16]. Enhancing the blue-green response of t-mode GaAs photocathodes as far as possible, would not only be beneficial to the detection in sandy or desert terrain for image intensifiers [17], but also increase the current density driven by 532 nm laser for electron sources [18]. Although the external electric field biased across the photocathodes can improve the photoemission capability, the limitations of this approach are the difficulty in making thin electrode pattern and the increased dark current with the strong field [19, 20]. In view of this adverse case, internal built-in electric fields through energy band engineering design could be an alternative approach. In our research, a complex structure composed of the composition-graded structure and the doping-graded structure is proposed to prepare high efficient t-mode AlGaAs/GaAs photocathodes. Furthermore, the photoemission model, cathode structure design, cathode material epitaxy, and vacuum tube fabrication are investigated through the integrated analysis of theory and experiments. Finally, the effectivity of the designed novel structure is verified by comparison with the common photocathodes.

#### 2. Graded bandgap structure

good spectral extensibility to the near infrared (NIR) region and low dark current, NEA GaAs, GaAsP, and InGaAs photocathodes are important components in the vacuum photodetectors, for example, low-light-level (LLL) image intensifiers, photomultiplier tubes, and streak tubes [6]. In the modern light sources based on free electron lasers or energy recovery linacs, GaAsbased photocathodes serve as high brightness electron sources with the unique virtues of large current density driven by visible lasers, high spin polarization, low thermal emittance, and narrow energy distribution [7]. In recent years, a spin-polarized transmission electron microscope combining electron microscopy and accelerator technology using GaAs-GaAsP strained superlattice photocathodes was developed to observe dynamically a magnetic field images with high spatial and temporal resolutions [8]. Moreover, with the aid of the ultrahigh speed pulse laser, GaAs photocathodes can satisfy the requirements of fast response speed and large

As is well known, GaAs photocathodes can operate in the transmission-mode (t-mode) and the reflection-mode (r-mode), respectively, depending on the difference in the direction of the incident light [10, 11], as shown in Figure 1. For the t-mode operation, the incident light is irradiated on the substrate surface, and the photoelectrons are extracted from the opposite surface side, whereas for the r-mode operation, the incident light and photoelectrons are located on the same emission surface side. Due to the difference in absorption length of longwave and shortwave photons, the shapes of spectral response curves for GaAs photocathodes working in the two modes are different [11]. Differing from r-mode GaAs photocathodes, t-mode ones are difficult to achieve high spectral response in a broadband region from ultraviolet to NIR spectrum. Usually in the practical applications, the researches on tmode photocathodes are more concerned. For example, the image intensifiers and related imaging systems, t-mode photocathodes conform to the optical imaging structure [12]. Besides, as polarized electron sources in photoinjector apparatus, t-mode photocathodes

emission current density aiming to THz frequency vacuum devices [9].

50 Advances in Photodetectors - Research and Applications

Figure 1. Schematic diagram of thin photocathode operating in the two different modes.

For the t-mode GaAs photocathode, the AlGaAs and GaAs materials are usually used as the window layer and the active layer, which determine the shortwave cutoff and longwave cutoff, respectively. A built-in electric field in the interior of the photocathode material can be realized by the variation of dopant or composition according to energy band engineering design [21, 22]. Based on this concept, a novel structure is proposed to improve photoelectron emission capability, wherein a composition-graded structure and a doping-graded structure are employed to the AlxGa1�xAs window layer and GaAs active layer, respectively [23, 24], as shown in Figure 2. To form a built-in constant electrical field in the GaAs active layer of the photocathode, the p-type dopant concentration can follow the exponential variation, and the doping formula is expressed by [22]

$$N(y) = N\_0 \exp\left(Ay\right) \tag{1}$$

where A denotes the exponential-doping (e-doping) factor, N<sup>0</sup> is the doping concentration at the surface of GaAs active layer, y is the distance from the coordinate origin (i.e., the surface of GaAs active layer), and N(y) is the p-type doping concentration in the GaAs active layer. As a result of the variation of dopant concentration, the initial Fermi level is different. In thermal

Figure 2. Energy band structure diagram of the t-mode AlxGa1�xAs/GaAs photocathode with the graded bandgap structure. Eg is the bandgap, E<sup>0</sup> is the vacuum level, EF is the Fermi level, EC is the conduction band minimum, and EV is the valence band maximum.

equilibrium, the Fermi level at different positions in the active layer is unified, and the electric potential energy qV(y) is varied as follows:

$$qV(y) = k\_B T \ln \frac{N(y)}{N\_0} = k\_B T Ay \tag{2}$$

distributed in the AlxGa1�xAs window layer. Under the first-stage built-in electric field, the thermalized photoelectrons in the AlxGa1�xAs layer move toward the GaAs interface. After that, the second-stage built-in electric field in the active layer can promote these photoelectrons toward the emission surface. On the other hand, the GaAs active layer can absorb the longwave light, and the excitated photoelectrons are promoted to move toward the surface with the help of the built-in electric field in the active layer. Consequently, there are reasons to believe that, by virtue of this unique graded bandgap structure, the quantum efficiency over the broadband spectrum, especially in the shortwave response region would be enhanced to

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

53

As is well known, the one-dimensional continuity equation can afford a useful avenue to establish the photoemission model of t-mode or r-mode III–V group photocathodes, which takes account of the spatial photon adsorption, spatial carrier distribution, and interface electron recombination [10, 11]. As shown in Figure 2, the photoelectrons generated in the AlxGa1�xAs layer are able to move into the GaAs layer and contribute to the total emitted electrons. For the composition-graded AlxGa1�xAs layer, some physical properties, for example, electron mobility (μ), electron diffusion coefficient (Dn), and electron recombination lifetime (τ) are the functions of

> <sup>μ</sup> <sup>¼</sup> <sup>8000</sup> � <sup>22</sup>, <sup>000</sup><sup>x</sup> <sup>þ</sup> <sup>10</sup>, <sup>000</sup>x<sup>2</sup> cm<sup>2</sup> <sup>V</sup>�<sup>1</sup> <sup>S</sup>�<sup>1</sup> � �, <sup>0</sup> <sup>&</sup>lt; <sup>x</sup> <sup>&</sup>lt; <sup>0</sup>:<sup>45</sup> �<sup>255</sup> <sup>þ</sup> <sup>1160</sup><sup>x</sup> � <sup>720</sup>x<sup>2</sup> cm2 <sup>V</sup>�<sup>1</sup> <sup>S</sup>�<sup>1</sup> � �, <sup>0</sup>:<sup>45</sup> <sup>&</sup>lt; <sup>x</sup> <sup>&</sup>lt; <sup>1</sup>

Dn <sup>¼</sup> <sup>200</sup> � <sup>550</sup><sup>x</sup> <sup>þ</sup> <sup>250</sup>x<sup>2</sup> cm2 <sup>s</sup>�<sup>1</sup> � �, <sup>0</sup> <sup>&</sup>lt; <sup>x</sup> <sup>&</sup>lt; <sup>0</sup>:<sup>45</sup>

Because of the aforesaid variable physical properties regarding to Al composition, the continuity equation of electron transport in the AlxGa1�xAs window layer is quite complex. For simplicity, the AlxGa1�xAs layer is treated to be of a series of sublayers with different Al compositions. As shown in Figure 2, the AlxGa1�xAs window layer can be considered to be of n sublayers, wherein Twn denotes the thickness of nth sublayer, and Tdn denotes the coordinate point along the y-axis. In this case, the transport of photoelectrons in the AlxGa1�xAs window layer follows the one-dimensional continuity equation through diffusion and drift

þ gi

4:444 � 29:142 1 þ e <sup>x</sup>�0:<sup>3443</sup> <sup>0</sup>:<sup>00468</sup>

�6:<sup>4</sup> <sup>þ</sup> <sup>29</sup><sup>x</sup> � <sup>18</sup>x<sup>2</sup> cm<sup>2</sup> <sup>s</sup>�<sup>1</sup> � �, <sup>0</sup>:<sup>45</sup> <sup>&</sup>lt; <sup>x</sup> <sup>&</sup>lt; <sup>1</sup>

(5)

(6)

, 0 < x < 1 (7)

ð Þ¼ y 0, i ¼ 1, 2, 3, ……, n (8)

3. Photoemission model derivation and simulation

the Al composition x, which are expressed as follows [26, 27]:

τ ¼ 29:142 þ

3.1. Photoemission model derivation

(

(

under the built-in electric field, which is as follows:

j j E<sup>1</sup>

dnið Þy dy � nið Þ<sup>y</sup> τi

Dni d2 nið Þy dy<sup>2</sup> <sup>þ</sup> <sup>μ</sup><sup>i</sup>

some extent.

where q is the electron charge, T is the cathode temperature, and kB is the Boltzmann constant. The diagram of band structure with the downward shape in the GaAs active layer is shown in Figure 2, wherein the built-in electric field E<sup>0</sup> in a certain thick (Te) active layer is given by

$$E\_0 = \frac{dV(y)}{dy} = \frac{k\_B T A}{q} = \frac{k\_B T}{qT\_e} \ln \frac{N(y)|\_{y=T\_e}}{N\_0} \tag{3}$$

In the AlxGa1�xAs window layer, the bandgap is decreased from the substrate interface to the GaAs interface due to the composition-graded structure. Because of the high p-type doping concentration, the valence bands of the AlxGa1�xAs/GaAs heterojunction are aligned, as shown in Figure 2. The graded Al composition in the window layer results in a built-in electric field E1, which is treated to be uniform as follows [25]:

$$E\_1 = \frac{\Delta E\_g}{q \Delta d} \tag{4}$$

where ΔE<sup>g</sup> is the energy bandgap difference of AlxGa1�xAs material, and Δd is the overall thickness of AlxGa1�xAs window layer. Figure 2 illustrates the transport process of photoexcited electrons in the t-mode graded bandgap AlxGa1�xAs/GaAs photocathodes. As for the t-mode photocathodes, the photoelectrons generated by shortwave light excitation are distributed in the AlxGa1�xAs window layer. Under the first-stage built-in electric field, the thermalized photoelectrons in the AlxGa1�xAs layer move toward the GaAs interface. After that, the second-stage built-in electric field in the active layer can promote these photoelectrons toward the emission surface. On the other hand, the GaAs active layer can absorb the longwave light, and the excitated photoelectrons are promoted to move toward the surface with the help of the built-in electric field in the active layer. Consequently, there are reasons to believe that, by virtue of this unique graded bandgap structure, the quantum efficiency over the broadband spectrum, especially in the shortwave response region would be enhanced to some extent.

#### 3. Photoemission model derivation and simulation

#### 3.1. Photoemission model derivation

equilibrium, the Fermi level at different positions in the active layer is unified, and the electric

Figure 2. Energy band structure diagram of the t-mode AlxGa1�xAs/GaAs photocathode with the graded bandgap structure. Eg is the bandgap, E<sup>0</sup> is the vacuum level, EF is the Fermi level, EC is the conduction band minimum, and EV is

where q is the electron charge, T is the cathode temperature, and kB is the Boltzmann constant. The diagram of band structure with the downward shape in the GaAs active layer is shown in Figure 2, wherein the built-in electric field E<sup>0</sup> in a certain thick (Te) active layer is given by

> <sup>q</sup> <sup>¼</sup> kBT qTe

In the AlxGa1�xAs window layer, the bandgap is decreased from the substrate interface to the GaAs interface due to the composition-graded structure. Because of the high p-type doping concentration, the valence bands of the AlxGa1�xAs/GaAs heterojunction are aligned, as shown in Figure 2. The graded Al composition in the window layer results in a built-in electric

<sup>E</sup><sup>1</sup> <sup>¼</sup> <sup>Δ</sup>Eg

where ΔE<sup>g</sup> is the energy bandgap difference of AlxGa1�xAs material, and Δd is the overall thickness of AlxGa1�xAs window layer. Figure 2 illustrates the transport process of photoexcited electrons in the t-mode graded bandgap AlxGa1�xAs/GaAs photocathodes. As for the t-mode photocathodes, the photoelectrons generated by shortwave light excitation are

ln N yð Þ <sup>y</sup>¼Te N<sup>0</sup>

N<sup>0</sup>

¼ kBTAy (2)

<sup>q</sup>Δ<sup>d</sup> (4)

(3)

qV yð Þ¼ kBTln N yð Þ

dy <sup>¼</sup> kBTA

<sup>E</sup><sup>0</sup> <sup>¼</sup> dV yð Þ

field E1, which is treated to be uniform as follows [25]:

potential energy qV(y) is varied as follows:

52 Advances in Photodetectors - Research and Applications

the valence band maximum.

As is well known, the one-dimensional continuity equation can afford a useful avenue to establish the photoemission model of t-mode or r-mode III–V group photocathodes, which takes account of the spatial photon adsorption, spatial carrier distribution, and interface electron recombination [10, 11]. As shown in Figure 2, the photoelectrons generated in the AlxGa1�xAs layer are able to move into the GaAs layer and contribute to the total emitted electrons. For the composition-graded AlxGa1�xAs layer, some physical properties, for example, electron mobility (μ), electron diffusion coefficient (Dn), and electron recombination lifetime (τ) are the functions of the Al composition x, which are expressed as follows [26, 27]:

$$\mu = \begin{cases} 8000 - 22,000\mathbf{x} + 10,000\mathbf{x}^2 \text{ (cm}^2 \text{ V}^{-1} \text{ S}^{-1}), & 0 < \mathbf{x} < 0.45\\ -255 + 1160\mathbf{x} - 720\mathbf{x}^2 \text{ (cm}^2 \text{ V}^{-1} \text{ S}^{-1}), & 0.45 < \mathbf{x} < 1 \end{cases} \tag{5}$$

$$D\_n = \begin{cases} 200 - 550x + 250x^2 \text{ (cm}^2 \text{ s}^{-1}), & 0 < x < 0.45\\ -6.4 + 29x - 18x^2 \text{ (cm}^2 \text{ s}^{-1}), & 0.45 < x < 1 \end{cases} \tag{6}$$

$$\pi = 29.142 + \frac{4.444 - 29.142}{1 + \varepsilon^{\frac{x - 0.343}{0.00468}}}, \qquad \qquad 0 < \text{ x } < 1\tag{7}$$

Because of the aforesaid variable physical properties regarding to Al composition, the continuity equation of electron transport in the AlxGa1�xAs window layer is quite complex. For simplicity, the AlxGa1�xAs layer is treated to be of a series of sublayers with different Al compositions. As shown in Figure 2, the AlxGa1�xAs window layer can be considered to be of n sublayers, wherein Twn denotes the thickness of nth sublayer, and Tdn denotes the coordinate point along the y-axis. In this case, the transport of photoelectrons in the AlxGa1�xAs window layer follows the one-dimensional continuity equation through diffusion and drift under the built-in electric field, which is as follows:

$$D\_{ni}\frac{d^2n\_i(y)}{dy^2} + \mu\_i|E\_1|\frac{dn\_i(y)}{dy} - \frac{n\_i(y)}{\tau\_i} + g\_i(y) = 0, \quad i = 1, 2, 3, \dots, n \tag{8}$$

where gi(y) represents the photoelectron generation function in each AlxGa1�xAs sublayer and is expressed as [28, 29]:

respectively. Considering that the electrons from the AlxGa1�xAs window layer can contribute to the GaAs active layer, the boundary conditions adequate for Eq. (13) are given by [28]:

By solving Eq. (13) via the boundary conditions Eq. (14) and the electron concentration n1(Te) from the AlxGa1�xAs window layer, the concentration of electrons n0(y) in the active layer can be figured out. Finally, the quantum efficiency Y(hv), defined as the emitted electron number per incident photon, for the complex AlxGa1�xAs/GaAs photocathode is calculated as follows:

where P is the surface electron escape probability. If E<sup>0</sup> = 0, the quantum efficiency model of AlxGa1�xAs/GaAs photocathode with the graded-composition (g-composition) and uniformdoping (u-doping) structure can be obtained. In the same way, when E<sup>1</sup> = 0 and E<sup>0</sup> = 0, we can also deduce the quantum efficiency model of common t-mode AlGaAs/GaAs photocathodes with the uniform-composition (u-composition) and u-doping structure. In a word, the aforementioned derivation method of photoemission model is applicable to those t-mode photo-

Meanwhile, it is noted that the quantum efficiency has a close relation with the reflectivity R (hv) of photocathode, as shown in Eqs. (9) and (13), thus the optical properties of t-mode graded bandgap AlxGa1�xAs/GaAs photocathodes need to be investigated. In fact, the usual t-mode photocathode can be treated as a multilayer module, which comprises the glass faceplate, the antireflection layer, the window layer, and the GaAs active layer. The typical structure of t-mode AlxGa1�xAs/GaAs photocathodes is shown in Figure 3. The glass substrate with a thickness of several millimeters is much thicker than other thin layers in the order of nanometers or micrometers, so the glass is treated as the incident medium rather than the thin film. The reflectivity of incident light permeating the glass substrate is greatly declined by the silicon nitride (Si3N4) antireflection film, and then the light in the wave range of interest is absorbed by the AlxGa1�xAs window layer and GaAs active layer in succession. The optical

Figure 3. Structural schematic of multilayered t-mode GaAs cathode module, including the glass substrate, the Si3N4

antireflection layer, the AlxGa1�xAs window layer, and the GaAs active layer.

dn0ð Þy

dy <sup>y</sup>¼<sup>0</sup>=I<sup>0</sup> 

Y hv ð Þ¼ PDn<sup>0</sup>

<sup>y</sup>¼Te ¼ �Sv1n0ð Þ<sup>y</sup> <sup>y</sup>¼Te <sup>þ</sup> Sv1n1ð Þ<sup>y</sup> <sup>y</sup>¼Ten0ð Þ<sup>y</sup> <sup>y</sup>¼<sup>0</sup> <sup>¼</sup> <sup>0</sup>

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

(15)

(14)

55

Dn<sup>0</sup>

dn0ð Þy

dy <sup>þ</sup> <sup>μ</sup>0j j <sup>E</sup><sup>0</sup> <sup>n</sup>0ð Þ<sup>y</sup> 

cathodes with a common or complex structure.

$$g\_i(y) = \begin{cases} (1 - R\_{h\upsilon}) I\_0 a\_{h\upsilon} \left[ \prod\_{m=i+1}^n \exp\left(-a\_{h\upsilon\_m} T\_{w\_m}\right) \right] \exp\left[-a\_{h\upsilon\_i} (T\_{d\_i} - y)\right], & i = 1, 2, \dots, n - 1 \\\\ (1 - R\_{h\upsilon}) I\_0 a\_{h\upsilon\_i} \exp\left[-a\_{h\upsilon\_i} (T\_c + \sum\_{i=1}^n T\_{w\_i} - y)\right], & i = n \end{cases} \tag{9}$$

In Eqs. (8) and (9), i represents the AlxGa1�xAs sublayer along the y axis direction, ni(y) and αhvi denote the excess electron concentration and the absorption coefficient in each part of AlxGa1�xAs sublayer, I<sup>0</sup> is the incident light intensity, Rhv is the reflectivity at the light incident surface, and Te is the active layer thickness. Besides, the three physical properties, that is, Dni, μi, and τ<sup>i</sup> in each sublayer are expressed by aforesaid Eqs. (5)–(7).

The excess electron concentration in the former sublayer should contribute to the latter sublayer, accordingly, the boundary conditions adequate for each sublayer are expressed as [28, 29]:

$$\begin{cases} \left[D\_{\text{ri}} \frac{dn\_i(y)}{dy} + \mu\_i |\mathcal{E}\_1| n\_i(y) \right] \Big|\_{y=T\_{d\_i}} = -\mathcal{S}\_{v\_{i+1}} n\_i(y) \Big|\_{y=T\_{d\_i}} + \mathcal{S}\_{v\_{i+1}} n\_{i+1}(y) \Big|\_{y=T\_{d\_i}}\\ \left[D\_{\text{ri}} \frac{dn\_i(y)}{dy} + \mu\_i |\mathcal{E}\_1| n\_i(y) \right] \Big|\_{y=T\_{\epsilon}} = \mathcal{S}\_{vi} n\_i(y) \Big|\_{y=T\_{\epsilon}} \end{cases}, \ i = 1 \tag{10}$$

$$\begin{cases} \left[D\_{ni}\frac{dn\_i(y)}{dy} + \mu\_i|E\_1|n\_i(y)\right]\Big|\_{y=T\_{d\_i}} = -S\_{v\_{i-1}}n\_i(y)\Big|\_{y=T\_{d\_i}} + S\_{v\_{i+1}}n\_{i+1}(y)\Big|\_{y=T\_{d\_i}}\\ \left[D\_{ni}\frac{dn\_i(y)}{dy} + \mu\_i|E\_1|n\_i(y)\right]\Big|\_{y=T\_{d\_{i-1}}} = S\_{vi}n\_i(y)\Big|\_{y=T\_{d\_{i-1}}} \end{cases}, i = 2, \dots, n - 1 \tag{11}$$
 
$$\int n\_i(y)\Big|\_{y=T\_{d\_i}} = 0$$

$$\left. \left\{ \left[ D\_{ni} \frac{d n\_i(y)}{dy} + \mu\_i |\mathcal{E}\_1| n\_i(y) \right] \Big|\_{y = T\_{d\_{i-1}}} = \mathcal{S}\_{vi} n\_i(y) \Big|\_{y = T\_{d\_{i-1}}} \right\} = n \tag{12}$$

where Svi is the electron recombination velocity at each interface. By recursively solving the above continuity equations, the excess electron concentration n1(Te) reaching the AlxGa1�xAs/ GaAs interface can be calculated.

As for the GaAs active layer, the excess electrons consist of electrons contributed from the AlxGa1�xAs window layer and electrons generated in the GaAs active layer. Under the secondstage built-in electric field, the photoelectron transport process in GaAs active layer follows the one-dimensional continuity equation as described by

$$D\_{n0}\frac{d^2n\_0(y)}{dy^2} + \mu\_0|E\_0|\frac{dn\_0(y)}{dy} - \frac{n\_0(y)}{\pi\_0} + (1 - R\_{\text{fr}})I\_0\alpha\_{\text{fr0}} \left[\prod\_{n=1}^{\text{n}} \exp\left(-a\_{\text{fr}\_n}T\_{\text{ru}}\right)\right] \exp\left[-\alpha\_{\text{fr0}}(T\_c - y)\right] = 0,\tag{13}$$
 
$$y \in [0, T\_c]$$

where n0(y) is the excess electron concentration in the GaAs active layer, αhv<sup>0</sup> is the absorption coefficient of the GaAs active layer, and Dn0, μ0, and τ<sup>0</sup> denote the electron diffusion coefficient, the electron mobility, and the electron recombination lifetime in the GaAs active layer, respectively. Considering that the electrons from the AlxGa1�xAs window layer can contribute to the GaAs active layer, the boundary conditions adequate for Eq. (13) are given by [28]:

where gi(y) represents the photoelectron generation function in each AlxGa1�xAs sublayer and

<sup>i</sup>¼<sup>1</sup> Twi � <sup>y</sup> � � � � , i <sup>¼</sup> <sup>n</sup>

In Eqs. (8) and (9), i represents the AlxGa1�xAs sublayer along the y axis direction, ni(y) and αhvi denote the excess electron concentration and the absorption coefficient in each part of AlxGa1�xAs sublayer, I<sup>0</sup> is the incident light intensity, Rhv is the reflectivity at the light incident surface, and Te is the active layer thickness. Besides, the three physical properties, that is, Dni,

The excess electron concentration in the former sublayer should contribute to the latter sublayer, accordingly, the boundary conditions adequate for each sublayer are expressed as [28, 29]:

� ¼ �Sviþ<sup>1</sup>nið Þ<sup>y</sup> <sup>y</sup>¼Tdi

� �

<sup>y</sup>¼Tdi�<sup>1</sup> � �

where Svi is the electron recombination velocity at each interface. By recursively solving the above continuity equations, the excess electron concentration n1(Te) reaching the AlxGa1�xAs/

As for the GaAs active layer, the excess electrons consist of electrons contributed from the AlxGa1�xAs window layer and electrons generated in the GaAs active layer. Under the secondstage built-in electric field, the photoelectron transport process in GaAs active layer follows the

where n0(y) is the excess electron concentration in the GaAs active layer, αhv<sup>0</sup> is the absorption coefficient of the GaAs active layer, and Dn0, μ0, and τ<sup>0</sup> denote the electron diffusion coefficient, the electron mobility, and the electron recombination lifetime in the GaAs active layer,

Yn m¼1

exp �αhvm Twm ð Þ " #

� � � � �

� ¼ Svinið Þy <sup>y</sup>¼Te

� �

� <sup>þ</sup> Sviþ<sup>1</sup>niþ<sup>1</sup>ð Þ<sup>y</sup> <sup>y</sup>¼Tdi

� <sup>¼</sup> Svinið Þ<sup>y</sup> <sup>y</sup>¼Tdi�<sup>1</sup>

� � �

exp �αhvi Tdi � <sup>y</sup> � � � � , i <sup>¼</sup> <sup>1</sup>, <sup>2</sup>, …, n � <sup>1</sup>

� <sup>þ</sup> Sviþ<sup>1</sup>niþ<sup>1</sup>ð Þ<sup>y</sup> <sup>y</sup>¼Tdi

� � � � � �

, i ¼ 1 (10)

, i ¼ 2, …, n � 1 (11)

, i ¼ n (12)

exp ½�αhv0ð Þ Te � y � ¼ 0,

y∈ 0; Te ½ �

(13)

(9)

exp �αhvm Twm ð Þ " #

exp �αhvi Te <sup>þ</sup> <sup>P</sup><sup>n</sup>

μi, and τ<sup>i</sup> in each sublayer are expressed by aforesaid Eqs. (5)–(7).

j j E<sup>1</sup> nið Þy

j j E<sup>1</sup> nið Þy

y¼Tdi � �

<sup>y</sup>¼Tdi�<sup>1</sup> � �

� �

y¼Tdi � �

y¼Te �

� ¼ �Sviþ<sup>1</sup>nið Þ<sup>y</sup> <sup>y</sup>¼Tdi

� <sup>¼</sup> Svinið Þ<sup>y</sup> <sup>y</sup>¼Tdi�<sup>1</sup>

j j E<sup>1</sup> nið Þy

þ ð Þ 1 � Rhv I0αhv<sup>0</sup>

is expressed as [28, 29]:

8 >><

>>:

Dni

8 >>><

>>>:

Dni

8 >>><

>>>:

Dn<sup>0</sup> d2 n0ð Þy dy<sup>2</sup> <sup>þ</sup> <sup>μ</sup>0j j <sup>E</sup><sup>0</sup>

Dni

Dni

dnið Þy dy <sup>þ</sup> <sup>μ</sup><sup>i</sup>

dnið Þy dy <sup>þ</sup> <sup>μ</sup><sup>i</sup>

dnið Þy dy <sup>þ</sup> <sup>μ</sup><sup>i</sup>

dnið Þy dy <sup>þ</sup> <sup>μ</sup><sup>i</sup>

� �

� �

8 >><

>>:

GaAs interface can be calculated.

� �

� �

j j E<sup>1</sup> nið Þy

j j E<sup>1</sup> nið Þy

nið Þy <sup>y</sup>¼Tdi � � � <sup>¼</sup> <sup>0</sup>

> dnið Þy dy <sup>þ</sup> <sup>μ</sup><sup>i</sup>

one-dimensional continuity equation as described by

dn0ð Þy dy � <sup>n</sup>0ð Þ<sup>y</sup> τ0

Dni

ð Þ 1 � Rhv I0αhvi

54 Advances in Photodetectors - Research and Applications

ð Þ 1 � Rhv I0αhvi

Yn m¼iþ1

gi ð Þ¼ y

$$\left\{ \left[ D\_{n0} \frac{dn\_0(y)}{dy} + \mu\_0 |E\_0| n\_0(y) \right] \Big|\_{y=T\_\epsilon} = -\mathcal{S}\_{v1} n\_0(y)|\_{y=T\_\epsilon} + \mathcal{S}\_{v1} n\_1(y)|\_{y=T\_\epsilon} n\_0(y)|\_{y=0} = 0 \right. \tag{14}$$

By solving Eq. (13) via the boundary conditions Eq. (14) and the electron concentration n1(Te) from the AlxGa1�xAs window layer, the concentration of electrons n0(y) in the active layer can be figured out. Finally, the quantum efficiency Y(hv), defined as the emitted electron number per incident photon, for the complex AlxGa1�xAs/GaAs photocathode is calculated as follows:

$$Y(h\upsilon) = PD\_{n0} \frac{dn\_0(y)}{dy}|\_{y=0}/I\_0 \tag{15}$$

where P is the surface electron escape probability. If E<sup>0</sup> = 0, the quantum efficiency model of AlxGa1�xAs/GaAs photocathode with the graded-composition (g-composition) and uniformdoping (u-doping) structure can be obtained. In the same way, when E<sup>1</sup> = 0 and E<sup>0</sup> = 0, we can also deduce the quantum efficiency model of common t-mode AlGaAs/GaAs photocathodes with the uniform-composition (u-composition) and u-doping structure. In a word, the aforementioned derivation method of photoemission model is applicable to those t-mode photocathodes with a common or complex structure.

Meanwhile, it is noted that the quantum efficiency has a close relation with the reflectivity R (hv) of photocathode, as shown in Eqs. (9) and (13), thus the optical properties of t-mode graded bandgap AlxGa1�xAs/GaAs photocathodes need to be investigated. In fact, the usual t-mode photocathode can be treated as a multilayer module, which comprises the glass faceplate, the antireflection layer, the window layer, and the GaAs active layer. The typical structure of t-mode AlxGa1�xAs/GaAs photocathodes is shown in Figure 3. The glass substrate with a thickness of several millimeters is much thicker than other thin layers in the order of nanometers or micrometers, so the glass is treated as the incident medium rather than the thin film. The reflectivity of incident light permeating the glass substrate is greatly declined by the silicon nitride (Si3N4) antireflection film, and then the light in the wave range of interest is absorbed by the AlxGa1�xAs window layer and GaAs active layer in succession. The optical

Figure 3. Structural schematic of multilayered t-mode GaAs cathode module, including the glass substrate, the Si3N4 antireflection layer, the AlxGa1�xAs window layer, and the GaAs active layer.

properties of multilayer module can be calculated based on the transfer matrix of thin-film optics, and the characteristic matrix of the multilayered cathode module is given by [30]:

$$\begin{Bmatrix} B \\ \mathbb{C} \end{Bmatrix} = \left\{ \prod\_{j=1}^{K} \begin{bmatrix} \cos \delta\_{j} & \frac{\mathrm{i}}{\eta\_{j}} \sin \delta\_{j} \\\\ \mathrm{i}\eta\_{j} \sin \delta\_{j} & \cos \delta\_{j} \end{bmatrix} \right\} \begin{bmatrix} 1 \\\\ \eta\_{K+1} \end{bmatrix} \tag{16}$$

$$\delta\_{\dot{\jmath}} = 2\pi \eta\_{\dot{\jmath}} d\_{\dot{\jmath}} \cos \Theta\_{\dot{\jmath}} / \lambda \tag{17}$$

$$
\hbar\_{\dot{\jmath}} = \mathfrak{n}\_{\dot{\jmath}} - i\mathbf{k}\_{\dot{\jmath}} \tag{18}
$$

1.49 and 2.06, respectively, the extinction coefficients of glass and Si3N4 are zero due to no absorption, and the thicknesses of Si3N4, AlGaAs, and GaAs layer are assumed to be 100, 500 nm and 1.0 μm, respectively. Besides, each sublayer in the g-composition AlxGa1xAs window layer is supposed to have the equal thickness of 0.1 μm. The simulated optical property curves between the two t-mode AlGaAs/GaAs photocathode modules with different window layer structures are shown in Figure 4. It is clear to see that the oscillation number in the entire 400–1100 nm region for the g-composition structure is less than those for the ucomposition structure. In other words, compared with the u-composition Al0.7Ga0.3As/GaAs photocathode, the g-composition AlxGa1xAs/GaAs photocathode exhibits the much smoother reflectivity curve in the spectrum region of 400–900 nm, which is the concerned photon absorption waveband for the AlGaAs/GaAs material. Besides, in the 900–1100 nm region, the locations of peaks and valleys of the reflectivity curves for the g-composition AlxGa1xAs/ GaAs photocathode move toward the shortwave direction, nevertheless, this has little effect on the photoemission performance of GaAs photocathodes since these photons with the wave-

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

57

By using the deduced quantum efficiency models which take into account the reflectivity varying with the wavelength, the quantum efficiency curves of the t-mode AlxGa1xAs/GaAs photocathode with those unique graded bandgap structures are simulated, wherein the window layer is of the g- or u-composition structure, and the active layer is of the e- or u-doping structure, respectively. Figure 5 exhibits the superiority of the AlxGa1xAs/GaAs photocathode with g-composition window layer and e-doping active layer. In Figure 5, some structural parameters such as the Al composition in the u-composition window layer, the Al composition

Figure 4. Simulation comparison of optical properties between the two cathode modules with different AlGaAs window

layer structures.

length greater than 900 nm are hardly absorbed by the GaAs material.

In Eqs. (16)–(18), δ<sup>j</sup> and η<sup>j</sup> are the optical phase difference and complex refractive index of the jth film layer, ηK + <sup>1</sup> is the optical constant of emergent medium, nj and kj constituting the complex refractive index are the refractivity and the extinction coefficient, dj is the thickness of the jth film layer, and θ<sup>j</sup> is the refraction angle of incident light. When light is perpendicularly incident on the surface of the glass substrate, the refraction angle is equal to zero. The reflectivity Rhv and transmittivity Thv of the multilayered photocathode module can be calculated by the following expressions [30]

$$R\_{hv} = \left(\frac{\eta\_{\mathcal{g}}B - \mathbb{C}}{\eta\_{\mathcal{g}}B + \mathbb{C}}\right) \left(\frac{\eta\_{\mathcal{g}}B - \mathbb{C}}{\eta\_{\mathcal{g}}B + \mathbb{C}}\right)^{\*}\tag{19}$$

$$T\_{hv} = \frac{4\eta\_g \eta\_{K+1}}{\left(\eta\_g B + \mathcal{C}\right) \left(\eta\_g B + \mathcal{C}\right)^\*}\tag{20}$$

where η<sup>g</sup> denotes the optical constant of the glass substrate. For the Al composition-varied window layer, the optical parameters, for example, the refractivity and extinction coefficient are different in each AlGaAs sublayer [31]. When the AlxGa1�xAs window layer is composed of n sublayers, the t-mode photocathode module can be treated as the thin film system of n + 2 layers to calculate the optical properties changing with incident photon wavelength, which are used as the necessary supplement to the quantum efficiency model.

#### 3.2. Quantum efficiency simulation

As to the t-mode AlGaAs/GaAs photocathodes, the optical properties between the gcomposition and u-composition structures should be different. For simplified calculation, the composition-graded AlxGa1�xAs window layer is assumed to be of five sublayers with the fixed Al composition in each sublayer. The five Al composition values are assumed to be 0.9, 0.675, 0.45, 0.225, and 0, respectively, distributed from the AlGaAs/Si3N4 interface to AlGaAs/ GaAs interface. For the u-composition AlGaAs/GaAs photocathode, the Al composition in the AlGaAs window layer is assumed to be 0.7. The optical properties including the reflectivity Rhv and transmittivity Thv can be simulated by utilizing Eqs. (16)–(20) by referring to the structure of Figure 3. In the simulations, the refractivity and extinction coefficients of AlxGa1�xAs with different Al compositions are referred to [31], the refractivity coefficients of glass and Si3N4 are 1.49 and 2.06, respectively, the extinction coefficients of glass and Si3N4 are zero due to no absorption, and the thicknesses of Si3N4, AlGaAs, and GaAs layer are assumed to be 100, 500 nm and 1.0 μm, respectively. Besides, each sublayer in the g-composition AlxGa1xAs window layer is supposed to have the equal thickness of 0.1 μm. The simulated optical property curves between the two t-mode AlGaAs/GaAs photocathode modules with different window layer structures are shown in Figure 4. It is clear to see that the oscillation number in the entire 400–1100 nm region for the g-composition structure is less than those for the ucomposition structure. In other words, compared with the u-composition Al0.7Ga0.3As/GaAs photocathode, the g-composition AlxGa1xAs/GaAs photocathode exhibits the much smoother reflectivity curve in the spectrum region of 400–900 nm, which is the concerned photon absorption waveband for the AlGaAs/GaAs material. Besides, in the 900–1100 nm region, the locations of peaks and valleys of the reflectivity curves for the g-composition AlxGa1xAs/ GaAs photocathode move toward the shortwave direction, nevertheless, this has little effect on the photoemission performance of GaAs photocathodes since these photons with the wavelength greater than 900 nm are hardly absorbed by the GaAs material.

properties of multilayer module can be calculated based on the transfer matrix of thin-film optics, and the characteristic matrix of the multilayered cathode module is given by [30]:

iη<sup>j</sup> sin δ<sup>j</sup> cos δ<sup>j</sup>

In Eqs. (16)–(18), δ<sup>j</sup> and η<sup>j</sup> are the optical phase difference and complex refractive index of the jth film layer, ηK + <sup>1</sup> is the optical constant of emergent medium, nj and kj constituting the complex refractive index are the refractivity and the extinction coefficient, dj is the thickness of the jth film layer, and θ<sup>j</sup> is the refraction angle of incident light. When light is perpendicularly incident on the surface of the glass substrate, the refraction angle is equal to zero. The reflectivity Rhv and transmittivity Thv of the multilayered photocathode module can be calculated by

! <sup>η</sup>g<sup>B</sup> � <sup>C</sup>

ηgB þ C !<sup>∗</sup>

ηgB þ C

i ηj sin δ<sup>j</sup> 3 7 5

9 >=

1 <sup>η</sup><sup>K</sup>þ<sup>1</sup>

dj cos θj=λ (17)

� �<sup>∗</sup> (20)

η<sup>j</sup> ¼ nj � ikj (18)

(16)

(19)

" #

>;

cos δ<sup>j</sup>

δ<sup>j</sup> ¼ 2πη<sup>j</sup>

Rhv <sup>¼</sup> <sup>η</sup>g<sup>B</sup> � <sup>C</sup> ηgB þ C

used as the necessary supplement to the quantum efficiency model.

Thv <sup>¼</sup> <sup>4</sup>ηgη<sup>K</sup>þ<sup>1</sup> ηgB þ C � �

where η<sup>g</sup> denotes the optical constant of the glass substrate. For the Al composition-varied window layer, the optical parameters, for example, the refractivity and extinction coefficient are different in each AlGaAs sublayer [31]. When the AlxGa1�xAs window layer is composed of n sublayers, the t-mode photocathode module can be treated as the thin film system of n + 2 layers to calculate the optical properties changing with incident photon wavelength, which are

As to the t-mode AlGaAs/GaAs photocathodes, the optical properties between the gcomposition and u-composition structures should be different. For simplified calculation, the composition-graded AlxGa1�xAs window layer is assumed to be of five sublayers with the fixed Al composition in each sublayer. The five Al composition values are assumed to be 0.9, 0.675, 0.45, 0.225, and 0, respectively, distributed from the AlGaAs/Si3N4 interface to AlGaAs/ GaAs interface. For the u-composition AlGaAs/GaAs photocathode, the Al composition in the AlGaAs window layer is assumed to be 0.7. The optical properties including the reflectivity Rhv and transmittivity Thv can be simulated by utilizing Eqs. (16)–(20) by referring to the structure of Figure 3. In the simulations, the refractivity and extinction coefficients of AlxGa1�xAs with different Al compositions are referred to [31], the refractivity coefficients of glass and Si3N4 are

B C � �

56 Advances in Photodetectors - Research and Applications

the following expressions [30]

3.2. Quantum efficiency simulation

<sup>¼</sup> <sup>Y</sup> K

>:

8 ><

j¼1

2 6 4

> By using the deduced quantum efficiency models which take into account the reflectivity varying with the wavelength, the quantum efficiency curves of the t-mode AlxGa1xAs/GaAs photocathode with those unique graded bandgap structures are simulated, wherein the window layer is of the g- or u-composition structure, and the active layer is of the e- or u-doping structure, respectively. Figure 5 exhibits the superiority of the AlxGa1xAs/GaAs photocathode with g-composition window layer and e-doping active layer. In Figure 5, some structural parameters such as the Al composition in the u-composition window layer, the Al composition

Figure 4. Simulation comparison of optical properties between the two cathode modules with different AlGaAs window layer structures.

distribution in each sublayer of AlxGa1xAs window layer, and the thicknesses of Si3N4, AlGaAs and GaAs layers are identical to those in Figure 4. In the GaAs active layer, the doping concentration for e-doping structure is exponentially varied from 1 1019 to 1 1018 cm<sup>3</sup> , and that for the u-doping structure is 1 1019 cm<sup>3</sup> . In addition, the surface electron escape probability P is assumed to be 0.5. As a result of the reduced lattice mismatch by the seamless AlxGa1xAs/GaAs heterojunction, the interface recombination velocity Svi for g-composition structure cannot exceed 104 cm/s, while Sv for the common u-composition structure is usually 106 cm/s [5].

photocathodes, the case is different. The GaAs active layer just absorbs the longwave photons, and the transport efficiency for these generated photoelectrons can just be improved by the

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To guide the structural design of t-mode graded bandgap AlxGa1xAs/GaAs photocathode, the changes of quantum efficiency with the active layer thickness and the window layer thickness are analyzed, as shown in Figure 6. Figure 6(a) shows the changes of quantum efficiency curves with the active layer thickness Te, assuming Al composition distribution, Svi and Twi are the same as those in Figure 5. As Te increases, more space in the bulk for absorption of the longwave photons in the region of 650–900 nm is provided to generate more electrons to increase the quantum efficiency. If the GaAs active layer is thin, photoelectrons generated by shortwave light in the AlxGa1xAs window layer would easily transport toward the GaAs active layer through diffusion and drift under the two-stage built-in electric field and finally escape into vacuum. In such a case, the quantum efficiency in the shortwave region would remain unchanged. Nevertheless, the thickness of the GaAs active layer must be controlled within a certain range, and the sufficiently thick active layer would decrease the quantum efficiency in the shortwave region, as shown in Figure 6(a). Therefore, the thickness of the active layer should be designed to balance the longwave response and shortwave response. When the AlxGa1xAs window layer is 500 nm in total thickness, the appropriate thickness is

Considering that the built-in electric field in the window layer is inversely proportional to window layer thickness, the effect of the window layer thickness on quantum efficiency in the shortwave region, especially in the blue-green waveband for g-composition photocathodes, is more pronounced than that for the u-composition ones. Figure 6(b) shows the quantum efficiency changing with the window layer thickness Tw, assuming Te = 1.0 μm. As Tw decreases, the quantum efficiency in the waveband region from 400 to 720 nm is greatly enhanced arising from the enhanced g-composition induced electric field. When Tw is thin,

Figure 6. Quantum efficiency simulations with the changes of (a) active layer thickness and (b) window layer thickness

doping-induced electric field.

thought to be in the range of 1.0–1.5 μm.

for the t-mode graded bandgap AlxGa1xAs/GaAs photocathodes.

It is seen clearly from Figure 5 that the t-mode g-composition and e-doping photocathode can obtain the highest quantum efficiency in the spectrum region from 400 to 900 nm in contrast to other photocathodes. The quantum efficiency in the shortwave region, that is, blue-green region are enhanced greatly for the two former photocathodes with the g-composition structure. In the g-composition AlxGa1xAs window layer, the photoelectrons excitated by shortwave light would be promoted toward the GaAs active layer under the g-composition induced electric field. Then, these shortwave photoelectrons are successively boosted toward the emission surface under the built-in electric field formed by the e-doping structure. As shown in Figure 5, the e-doping structure for the g-composition AlxGa1xAs/GaAs photocathodes can slightly enhance the quantum efficiency, which is not like the case for the u-composition AlGaAs/GaAs photocathodes. The possible reason is that the g-composition AlxGa1xAs layer can also absorb some extra longwave photons, which are originally absorbed by the GaAs active layer. In other words, more enough absorption space for longwave photons can be provided by the g-composition structure. While for the u-composition AlGaAs/GaAs

Figure 5. Simulation comparison of quantum efficiency among the t-mode photocathodes with different AlGaAs window layer and GaAs active layer structures.

photocathodes, the case is different. The GaAs active layer just absorbs the longwave photons, and the transport efficiency for these generated photoelectrons can just be improved by the doping-induced electric field.

distribution in each sublayer of AlxGa1xAs window layer, and the thicknesses of Si3N4, AlGaAs and GaAs layers are identical to those in Figure 4. In the GaAs active layer, the doping concentration for e-doping structure is exponentially varied from 1 1019 to 1 1018 cm<sup>3</sup>

probability P is assumed to be 0.5. As a result of the reduced lattice mismatch by the seamless AlxGa1xAs/GaAs heterojunction, the interface recombination velocity Svi for g-composition structure cannot exceed 104 cm/s, while Sv for the common u-composition structure is usually

It is seen clearly from Figure 5 that the t-mode g-composition and e-doping photocathode can obtain the highest quantum efficiency in the spectrum region from 400 to 900 nm in contrast to other photocathodes. The quantum efficiency in the shortwave region, that is, blue-green region are enhanced greatly for the two former photocathodes with the g-composition structure. In the g-composition AlxGa1xAs window layer, the photoelectrons excitated by shortwave light would be promoted toward the GaAs active layer under the g-composition induced electric field. Then, these shortwave photoelectrons are successively boosted toward the emission surface under the built-in electric field formed by the e-doping structure. As shown in Figure 5, the e-doping structure for the g-composition AlxGa1xAs/GaAs photocathodes can slightly enhance the quantum efficiency, which is not like the case for the u-composition AlGaAs/GaAs photocathodes. The possible reason is that the g-composition AlxGa1xAs layer can also absorb some extra longwave photons, which are originally absorbed by the GaAs active layer. In other words, more enough absorption space for longwave photons can be provided by the g-composition structure. While for the u-composition AlGaAs/GaAs

Figure 5. Simulation comparison of quantum efficiency among the t-mode photocathodes with different AlGaAs win-

dow layer and GaAs active layer structures.

and that for the u-doping structure is 1 1019 cm<sup>3</sup>

58 Advances in Photodetectors - Research and Applications

106 cm/s [5].

,

. In addition, the surface electron escape

To guide the structural design of t-mode graded bandgap AlxGa1xAs/GaAs photocathode, the changes of quantum efficiency with the active layer thickness and the window layer thickness are analyzed, as shown in Figure 6. Figure 6(a) shows the changes of quantum efficiency curves with the active layer thickness Te, assuming Al composition distribution, Svi and Twi are the same as those in Figure 5. As Te increases, more space in the bulk for absorption of the longwave photons in the region of 650–900 nm is provided to generate more electrons to increase the quantum efficiency. If the GaAs active layer is thin, photoelectrons generated by shortwave light in the AlxGa1xAs window layer would easily transport toward the GaAs active layer through diffusion and drift under the two-stage built-in electric field and finally escape into vacuum. In such a case, the quantum efficiency in the shortwave region would remain unchanged. Nevertheless, the thickness of the GaAs active layer must be controlled within a certain range, and the sufficiently thick active layer would decrease the quantum efficiency in the shortwave region, as shown in Figure 6(a). Therefore, the thickness of the active layer should be designed to balance the longwave response and shortwave response. When the AlxGa1xAs window layer is 500 nm in total thickness, the appropriate thickness is thought to be in the range of 1.0–1.5 μm.

Considering that the built-in electric field in the window layer is inversely proportional to window layer thickness, the effect of the window layer thickness on quantum efficiency in the shortwave region, especially in the blue-green waveband for g-composition photocathodes, is more pronounced than that for the u-composition ones. Figure 6(b) shows the quantum efficiency changing with the window layer thickness Tw, assuming Te = 1.0 μm. As Tw decreases, the quantum efficiency in the waveband region from 400 to 720 nm is greatly enhanced arising from the enhanced g-composition induced electric field. When Tw is thin,

Figure 6. Quantum efficiency simulations with the changes of (a) active layer thickness and (b) window layer thickness for the t-mode graded bandgap AlxGa1xAs/GaAs photocathodes.

there is not enough space to absorb shortwave photons, and these shortwave photons can be absorbed by GaAs active layer. The quantum efficiency in the shortwave region would get increased as Tw decreases since that the transport capacity of photoelectrons in GaAs layer is better than that in AlGaAs layer. Nevertheless, a passivation layer is necessary to prevent impurities from the substrate into the active layer for the t-mode photocathodes, and thus the AlxGa1xAs window layer also utilized as the passivation layer should not be extremely thin.

#### 4. Epitaxial growth and quality characterization

#### 4.1. Epitaxial growth of photocathode materials

In modern epitaxial growth techniques, the metalorganic chemical vapor deposition (MOCVD) technique is suitable for growing the complex ultrathin multilayer materials with the composition-graded or doping-graded structures. To confirm the actual effect of the gcomposition and e-doping structure on the quantum efficiency of t-mode AlGaAs/GaAs photocathodes, the 2-inch-diameter AlxGa1xAs/GaAs epilayers with two different structures were grown on the low-defect n-type GaAs (100) substrates in the horizontal low-pressure MOCVD reactor from AIXTRON. As shown in Figure 7(a), the multiple epitaxial layers consist of four AlGaAs/GaAs heterostructures, which follow the "inverted structure" technology [32, 33]. In Figure 7(a), the AlGaAs stop layer serves as an etching-resistance layer, and the GaAs cap layer serves as an oxidation-blocking layer. The detailed structures of the two types of cathode materials are shown in Figure 7(b) and (c). The difference between the two samples is the structure of window layer, wherein one is of g-composition AlxGa1xAs layer, and the other is of u-composition Al0.7Ga0.3As layer. Note that, as a result of the current epitaxial limitation, the GaAs active layer exhibits a quasi-exponential doping structure with the p-type dopant concentration varying from 1 1019 to 1 1018 cm<sup>3</sup> .

During the epitaxial growth process of the multiple layers, the group III sources are the trimethylgallium (TMGa) and trimethylaluminum (TMAl), the group V source was the AsH3, the dopant source was the diethylzinc (DEZn), and the carrier gas was the H2 gas. Additionally, the growth process was monitored in situ using the LayTech EpiRAS-200 spectrometer. The parameters of the epitaxial growth process are as follows: the growth rate was about 2.5 μm/h, the V/III flux ratio was adjusted at 10–15, the Al composition was controlled by the flow ratio of TMGa to TMAl, and the growth temperature was set as 680C and 710C for GaAs and AlxGa1xAs, respectively.

noted that many cracks in Figure 8(b) are caused by the inappropriate cleavage, which cannot reflect the true quality of the epitaxy. From the SEM photographs, it is judged that the vertically multilayered constructions of the epitaxial cathode materials agree well with the struc-

Figure 8. Cross-sectional SEM photographs of the cleaved epitaxial cathode samples with (a) g-composition and e-

Figure 7. (a) Schematic diagram of the epitaxial t-mode AlGaAs/GaAs photocathode materials following the "inverted structure" technology, the detailed epitaxial structures of (b) g-composition and e-doping cathode sample, and (c) u-

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tural design.

composition and e-doping cathode sample.

doping structure and (b) u-composition and e-doping structure.

#### 4.2. Quality characterization of photocathode materials

To understand the profile structure of the multilayered photocathode samples, the crosssectional photographs of the multilayered structure for the two cathode material samples were measured by the scanning electron microscope (SEM) from Hitachi. It is clearly seen from Figure 8 that differing from the case for u-composition sample, no sharp borderline exists at the interface of the AlxGa1xAs window layer and GaAs active layer for the g-composition sample. This seamless interface would greatly reduce the interface electron recombination. It is Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode http://dx.doi.org/10.5772/intechopen.80704 61

there is not enough space to absorb shortwave photons, and these shortwave photons can be absorbed by GaAs active layer. The quantum efficiency in the shortwave region would get increased as Tw decreases since that the transport capacity of photoelectrons in GaAs layer is better than that in AlGaAs layer. Nevertheless, a passivation layer is necessary to prevent impurities from the substrate into the active layer for the t-mode photocathodes, and thus the AlxGa1xAs window layer also utilized as the passivation layer should not be extremely thin.

In modern epitaxial growth techniques, the metalorganic chemical vapor deposition (MOCVD) technique is suitable for growing the complex ultrathin multilayer materials with the composition-graded or doping-graded structures. To confirm the actual effect of the gcomposition and e-doping structure on the quantum efficiency of t-mode AlGaAs/GaAs photocathodes, the 2-inch-diameter AlxGa1xAs/GaAs epilayers with two different structures were grown on the low-defect n-type GaAs (100) substrates in the horizontal low-pressure MOCVD reactor from AIXTRON. As shown in Figure 7(a), the multiple epitaxial layers consist of four AlGaAs/GaAs heterostructures, which follow the "inverted structure" technology [32, 33]. In Figure 7(a), the AlGaAs stop layer serves as an etching-resistance layer, and the GaAs cap layer serves as an oxidation-blocking layer. The detailed structures of the two types of cathode materials are shown in Figure 7(b) and (c). The difference between the two samples is the structure of window layer, wherein one is of g-composition AlxGa1xAs layer, and the other is of u-composition Al0.7Ga0.3As layer. Note that, as a result of the current epitaxial limitation, the GaAs active layer exhibits a quasi-exponential doping structure with the p-type dopant con-

.

During the epitaxial growth process of the multiple layers, the group III sources are the trimethylgallium (TMGa) and trimethylaluminum (TMAl), the group V source was the AsH3, the dopant source was the diethylzinc (DEZn), and the carrier gas was the H2 gas. Additionally, the growth process was monitored in situ using the LayTech EpiRAS-200 spectrometer. The parameters of the epitaxial growth process are as follows: the growth rate was about 2.5 μm/h, the V/III flux ratio was adjusted at 10–15, the Al composition was controlled by the flow ratio of TMGa to TMAl, and the growth temperature was set as 680C and 710C for

To understand the profile structure of the multilayered photocathode samples, the crosssectional photographs of the multilayered structure for the two cathode material samples were measured by the scanning electron microscope (SEM) from Hitachi. It is clearly seen from Figure 8 that differing from the case for u-composition sample, no sharp borderline exists at the interface of the AlxGa1xAs window layer and GaAs active layer for the g-composition sample. This seamless interface would greatly reduce the interface electron recombination. It is

4. Epitaxial growth and quality characterization

4.1. Epitaxial growth of photocathode materials

60 Advances in Photodetectors - Research and Applications

centration varying from 1 1019 to 1 1018 cm<sup>3</sup>

4.2. Quality characterization of photocathode materials

GaAs and AlxGa1xAs, respectively.

Figure 7. (a) Schematic diagram of the epitaxial t-mode AlGaAs/GaAs photocathode materials following the "inverted structure" technology, the detailed epitaxial structures of (b) g-composition and e-doping cathode sample, and (c) ucomposition and e-doping cathode sample.

noted that many cracks in Figure 8(b) are caused by the inappropriate cleavage, which cannot reflect the true quality of the epitaxy. From the SEM photographs, it is judged that the vertically multilayered constructions of the epitaxial cathode materials agree well with the structural design.

Figure 8. Cross-sectional SEM photographs of the cleaved epitaxial cathode samples with (a) g-composition and edoping structure and (b) u-composition and e-doping structure.

The depth distribution of carrier concentration in the multilayered p-type AlGaAs/GaAs materials was measured by the electrochemical capacitance-voltage (ECV) system from Bio-Rad. As shown in Figure 9, a series of sublayers forming the graded doping structure can be realized by the MOCVD technique. The carrier concentration of no more than 8 <sup>10</sup><sup>18</sup> cm<sup>3</sup> in the GaAs active layer shows a gradient distribution. For the AlxGa1xAs window layer in Figure 9 (a), the carrier concentration decreases with the increase in Al composition, which exactly reflects the composition-graded structure.

rightmost peak represents the GaAs material, which is the superposition of the diffraction peaks of the GaAs cap layer, active layer, and substrate. The only one diffraction peak indicates that the crystalline perfection of the GaAs epilayers is consistent with the GaAs substrate. The left two diffraction peaks for the u-composition sample represent the AlGaAs window layer and stop layer, respectively. In the g-composition sample, there is no diffraction peak denoting the window layer, and a series of diffraction peaks exist nearby the peak of the GaAs layer, which are caused by the g-composition AlxGa1xAs epilayer. The slightly narrower full width at half maximum of the GaAs diffraction peak indicates that the GaAs active layer in the

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Following the recipe of fabricating glass-sealed t-mode AlGaAs/GaAs photocathodes [32, 33], the epitaxial cathode materials cutted from the 2-inch-diameter epitaxial wafer were fabricated into the multilayered t-mode cathode module. The schematic process flow for fabricating t-mode AlGaAs/GaAs photocathode modules is shown in Figure 11. First, the GaAs cap layer was removed by chemical etching to expose the AlGaAs window layer, and by plasma enhanced chemical vapor deposition (PECVD), a thin antireflective layer of 100 nm-thick Si3N4 was deposited on the exposed window layer surface. Then, the 7056 glass, serving as the incident window and support layer, was bonded on the Si3N4 antireflection layer by thermocompression. Following that, through selective etching process, the GaAs substrate and AlGaAs stop layer were etched away to expose the GaAs active layer to prepare the NEA surface [32]. Finally, the Cr-Ni ring electrode applied to bias on the cathode was prepared by the physical vapor deposition (PVD), such as magnetron sputtering method. After these processing steps, the multilayered cathode module with a glass/Si3N4/AlGaAs/GaAs structure was finished. In addition, to eliminate etching-induced damage at the active layer surface, the polishing treatment was

The optical property curves of the t-mode cathode modules with two different structures were measured by utilizing the Shimadzu UV-3600 spectrophotometer, which possesses three detectors working from ultraviolet to NIR waveband. The optical properties were measured based on the double optical path method, and light was incident on the surface of glass faceplate in a normal direction. Figure 12 shows the experimental reflectivity and transmissivity curves of the two different multilayered module samples. It is found that, just as the simulated results in Figure 4, the reflectivity curve in the region of 400–800 nm for g-composition structure is relatively smoother than that for u-composition structure. In other words, the smooth reflectivity curve verifies the composition-graded structure in the AlxGa1xAs window layer from another aspect. Thereby, the characterization results regarding the cross-sectional photographs, carrier concentration distributions, X-ray diffraction peaks, and optical properties all reflect the special

implemented, which slightly decreased the thickness of the GaAs active layer.

g-composition sample has a better crystalline quality.

5. Device fabrication and spectral response

5.1. Transmission-mode cathode module fabrication

design structure.

To investigate the crystalline quality of the epitaxial cathode materials, the X-ray diffraction (XRD) curves were measured by the X'Pert Pro MRD system. As shown in Figure 10, the

Figure 9. Depth distribution of carrier concentration in the cleaved epitaxial cathode samples with (a) g-composition and e-doping structure and (b) u-composition and e-doping structure.

Figure 10. XRD curves of the cleaved epitaxial cathode samples with two different structures.

rightmost peak represents the GaAs material, which is the superposition of the diffraction peaks of the GaAs cap layer, active layer, and substrate. The only one diffraction peak indicates that the crystalline perfection of the GaAs epilayers is consistent with the GaAs substrate. The left two diffraction peaks for the u-composition sample represent the AlGaAs window layer and stop layer, respectively. In the g-composition sample, there is no diffraction peak denoting the window layer, and a series of diffraction peaks exist nearby the peak of the GaAs layer, which are caused by the g-composition AlxGa1xAs epilayer. The slightly narrower full width at half maximum of the GaAs diffraction peak indicates that the GaAs active layer in the g-composition sample has a better crystalline quality.

#### 5. Device fabrication and spectral response

The depth distribution of carrier concentration in the multilayered p-type AlGaAs/GaAs materials was measured by the electrochemical capacitance-voltage (ECV) system from Bio-Rad. As shown in Figure 9, a series of sublayers forming the graded doping structure can be realized by the MOCVD technique. The carrier concentration of no more than 8 <sup>10</sup><sup>18</sup> cm<sup>3</sup> in the GaAs active layer shows a gradient distribution. For the AlxGa1xAs window layer in Figure 9 (a), the carrier concentration decreases with the increase in Al composition, which exactly

To investigate the crystalline quality of the epitaxial cathode materials, the X-ray diffraction (XRD) curves were measured by the X'Pert Pro MRD system. As shown in Figure 10, the

Figure 9. Depth distribution of carrier concentration in the cleaved epitaxial cathode samples with (a) g-composition and

Figure 10. XRD curves of the cleaved epitaxial cathode samples with two different structures.

reflects the composition-graded structure.

62 Advances in Photodetectors - Research and Applications

e-doping structure and (b) u-composition and e-doping structure.

#### 5.1. Transmission-mode cathode module fabrication

Following the recipe of fabricating glass-sealed t-mode AlGaAs/GaAs photocathodes [32, 33], the epitaxial cathode materials cutted from the 2-inch-diameter epitaxial wafer were fabricated into the multilayered t-mode cathode module. The schematic process flow for fabricating t-mode AlGaAs/GaAs photocathode modules is shown in Figure 11. First, the GaAs cap layer was removed by chemical etching to expose the AlGaAs window layer, and by plasma enhanced chemical vapor deposition (PECVD), a thin antireflective layer of 100 nm-thick Si3N4 was deposited on the exposed window layer surface. Then, the 7056 glass, serving as the incident window and support layer, was bonded on the Si3N4 antireflection layer by thermocompression. Following that, through selective etching process, the GaAs substrate and AlGaAs stop layer were etched away to expose the GaAs active layer to prepare the NEA surface [32]. Finally, the Cr-Ni ring electrode applied to bias on the cathode was prepared by the physical vapor deposition (PVD), such as magnetron sputtering method. After these processing steps, the multilayered cathode module with a glass/Si3N4/AlGaAs/GaAs structure was finished. In addition, to eliminate etching-induced damage at the active layer surface, the polishing treatment was implemented, which slightly decreased the thickness of the GaAs active layer.

The optical property curves of the t-mode cathode modules with two different structures were measured by utilizing the Shimadzu UV-3600 spectrophotometer, which possesses three detectors working from ultraviolet to NIR waveband. The optical properties were measured based on the double optical path method, and light was incident on the surface of glass faceplate in a normal direction. Figure 12 shows the experimental reflectivity and transmissivity curves of the two different multilayered module samples. It is found that, just as the simulated results in Figure 4, the reflectivity curve in the region of 400–800 nm for g-composition structure is relatively smoother than that for u-composition structure. In other words, the smooth reflectivity curve verifies the composition-graded structure in the AlxGa1xAs window layer from another aspect. Thereby, the characterization results regarding the cross-sectional photographs, carrier concentration distributions, X-ray diffraction peaks, and optical properties all reflect the special design structure.

samples. Through detecting the gas presence of the QMS traces at m/e = 18 (H2O), 75 (As), 91 (AsO), 150 (As2), and 156 (Ga2O), it can be judged that whether the oxides on the GaAs surface, such as As2O3 and Ga2O3, are cleared away with the increased temperature or not [34]. It can be inferred from Figure 13 that both the cathode module samples obtained an oxide-free clean

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After the sample cooled to room temperature, the Cs▬O activation to form the NEA state at the cathode surface was performed in the UHV chamber with a base pressure of 10<sup>9</sup> Pa. The Cs and O sources used in the activation are solid dispensers easily controlled by direct current, and the flux is proportional to the operating current [35]. During the activation, the Cs source was on all the time, and the O source was switched on and off [35]. The operating current of Cs and O dispensers was regulated by program control current supply, and the photocurrent induced by a white light source was monitored in real time by the computer-controlled test system [35]. The initial Cs supply caused the gradual increase of the photocurrent. With the continuous Cs flux, when the photocurrent dropped to 80% of its peak, the O source was open. In subsequent alternate activation cycles, the O source was closed when the photocurrent reached its peak and was open again when the photocurrent dropped to 80% of the peak. The operating current ratio of Cs source to O source for both samples was regulated as the same 1.65/1.8. Until the photocurrent peak no longer increased, the O source and Cs source were closed successively, and the activation process was finished. To further improve the photoemission performance, the second heat treatment with a lower temperature was employed to the samples [36]. After that, the samples were activated again using the same co-deposition activation. As seen from Figure 14, the second activation can dramatically enhance the final cathode performance. Meanwhile, the

After the two-step Cs▬O activation process, the cathode module in the UHV activation chamber was transferred to the UHV seal vacuum chamber and indium sealed into an image

Figure 13. Residual gas changes during high-temperature thermal cleaning process for (a) g-composition and (b)

surface after the heat treatment procedure in terms of these obvious QMS trace peaks.

final photocurrent peaks of the two samples are approximately the same.

5.3. Tube package and spectral response test

u-composition AlGaAs/GaAs cathode modules.

Figure 11. Schematic of the process flow for fabricating t-mode AlGaAs/GaAs photocathode modules following the "inverted structure" technology.

Figure 12. Experimental optical property curves of the t-mode AlGaAs/GaAs cathode modules with two different structures.

#### 5.2. Activation of photocathode surface

Prior to activation, the 18-mm-diameter cathode modules experienced the chemical cleaning and vacuum annealing to obtain an atomic level clean surface. The heat treatment with a suitable temperature under ultrahigh vacuum (UHV) condition is particularly important for the activation, and the quadrupole mass spectrometer (QMS) was adopted to monitor the change of residual gas components during the programmed temperature rose and fell. Figure 13 shows the changes of mainly concerned residual gas components for the two t-mode cathode module

samples. Through detecting the gas presence of the QMS traces at m/e = 18 (H2O), 75 (As), 91 (AsO), 150 (As2), and 156 (Ga2O), it can be judged that whether the oxides on the GaAs surface, such as As2O3 and Ga2O3, are cleared away with the increased temperature or not [34]. It can be inferred from Figure 13 that both the cathode module samples obtained an oxide-free clean surface after the heat treatment procedure in terms of these obvious QMS trace peaks.

After the sample cooled to room temperature, the Cs▬O activation to form the NEA state at the cathode surface was performed in the UHV chamber with a base pressure of 10<sup>9</sup> Pa. The Cs and O sources used in the activation are solid dispensers easily controlled by direct current, and the flux is proportional to the operating current [35]. During the activation, the Cs source was on all the time, and the O source was switched on and off [35]. The operating current of Cs and O dispensers was regulated by program control current supply, and the photocurrent induced by a white light source was monitored in real time by the computer-controlled test system [35]. The initial Cs supply caused the gradual increase of the photocurrent. With the continuous Cs flux, when the photocurrent dropped to 80% of its peak, the O source was open. In subsequent alternate activation cycles, the O source was closed when the photocurrent reached its peak and was open again when the photocurrent dropped to 80% of the peak. The operating current ratio of Cs source to O source for both samples was regulated as the same 1.65/1.8. Until the photocurrent peak no longer increased, the O source and Cs source were closed successively, and the activation process was finished. To further improve the photoemission performance, the second heat treatment with a lower temperature was employed to the samples [36]. After that, the samples were activated again using the same co-deposition activation. As seen from Figure 14, the second activation can dramatically enhance the final cathode performance. Meanwhile, the final photocurrent peaks of the two samples are approximately the same.

#### 5.3. Tube package and spectral response test

5.2. Activation of photocathode surface

"inverted structure" technology.

64 Advances in Photodetectors - Research and Applications

Prior to activation, the 18-mm-diameter cathode modules experienced the chemical cleaning and vacuum annealing to obtain an atomic level clean surface. The heat treatment with a suitable temperature under ultrahigh vacuum (UHV) condition is particularly important for the activation, and the quadrupole mass spectrometer (QMS) was adopted to monitor the change of residual gas components during the programmed temperature rose and fell. Figure 13 shows the changes of mainly concerned residual gas components for the two t-mode cathode module

Figure 12. Experimental optical property curves of the t-mode AlGaAs/GaAs cathode modules with two different structures.

Figure 11. Schematic of the process flow for fabricating t-mode AlGaAs/GaAs photocathode modules following the

After the two-step Cs▬O activation process, the cathode module in the UHV activation chamber was transferred to the UHV seal vacuum chamber and indium sealed into an image

Figure 13. Residual gas changes during high-temperature thermal cleaning process for (a) g-composition and (b) u-composition AlGaAs/GaAs cathode modules.

Thus, the input LLL image is intensified and appears as the output image on the phosphor screen. In addition to the function of direct eye observation, the LLL image intensifier can be coupled with CCD/CMOS array by the fiber optic taper to realize video output and remote

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The sealed image intensifiers were extracted from the seal vacuum chamber into ambient air, and the spectral response curves were measured by the spectral response testing instrument [22]. Through the spectral response values corresponding to the wavelength, the quantum efficiency values corresponding to the wavelength for the two different cathode samples were obtained [40]. In the spectral region of 600–750 nm, the quantum efficiency exceeds 40%. As shown in Figure 16, it is found that in contrast to the u-composition structure, the gcomposition structure is especially useful to the enhancement of shortwave quantum efficiency, which conforms to the original intention of our design concept. By fitting the experimental optical property and quantum efficiency data based on the theoretical photoemission model, the internal cathode parameters difficult to be measured directly can be obtained. The thickness values of each layer calculated by fitting the experimental reflectivity and transmittivity curves are listed in Table 1. It is seen that the Al composition in the gcomposition window layer is not distributed uniformly, and the sublayers with low Al composition are relatively thinner compared to those with high Al composition. For the two samples, the thicknesses of the GaAs active layer are smaller than the design values, which

are caused by the polishing treatment after the fabrication of cathode modules.

By means of fitting the experimental quantum efficiency curves, we can obtain some performance parameters, for example, interface recombination velocity Sv and surface escape probability P. It is seen from Table 1 that the two samples have the same P, which means that the

Figure 16. Experimental and fitted quantum efficiency curves for the two different t-mode AlGaAs/GaAs photocathodes

monitoring [38, 39].

samples.

Figure 14. Photocurrent changes during Cs▬O activation for the two t-mode AlGaAs/GaAs cathode modules.

intensifier tube, wherein the t-mode AlGaAs/GaAs cathode module was equipped in association with the filmed microchannel plate (MCP), phosphor screen, output window, ceramics, and Kovar sealing parts [37]. The schematic structure and the photograph of the LLL proximity focused image intensifier are shown in Figure 15. As shown in Figure 15(a), the proximity focused image intensifier is capable of enhancing a LLL image from several thousands to tens of thousands of times. The input LLL image is converted into photoelectrons by the AlGaAs/ GaAs photocathode, and then the number of photoelectrons is multiplied several thousands of times by the MCP coated with a thin ion barrier film which can prevent ion feedback. Lastly, the multiplied photoelectrons bombard the phosphor screen and are converted into photons.

Figure 15. (a) Schematic structure of the low-light-level proximity focused image intensifier and (b) photograph of the sealed proximity focused image intensifier.

Thus, the input LLL image is intensified and appears as the output image on the phosphor screen. In addition to the function of direct eye observation, the LLL image intensifier can be coupled with CCD/CMOS array by the fiber optic taper to realize video output and remote monitoring [38, 39].

The sealed image intensifiers were extracted from the seal vacuum chamber into ambient air, and the spectral response curves were measured by the spectral response testing instrument [22]. Through the spectral response values corresponding to the wavelength, the quantum efficiency values corresponding to the wavelength for the two different cathode samples were obtained [40]. In the spectral region of 600–750 nm, the quantum efficiency exceeds 40%. As shown in Figure 16, it is found that in contrast to the u-composition structure, the gcomposition structure is especially useful to the enhancement of shortwave quantum efficiency, which conforms to the original intention of our design concept. By fitting the experimental optical property and quantum efficiency data based on the theoretical photoemission model, the internal cathode parameters difficult to be measured directly can be obtained. The thickness values of each layer calculated by fitting the experimental reflectivity and transmittivity curves are listed in Table 1. It is seen that the Al composition in the gcomposition window layer is not distributed uniformly, and the sublayers with low Al composition are relatively thinner compared to those with high Al composition. For the two samples, the thicknesses of the GaAs active layer are smaller than the design values, which are caused by the polishing treatment after the fabrication of cathode modules.

By means of fitting the experimental quantum efficiency curves, we can obtain some performance parameters, for example, interface recombination velocity Sv and surface escape probability P. It is seen from Table 1 that the two samples have the same P, which means that the

intensifier tube, wherein the t-mode AlGaAs/GaAs cathode module was equipped in association with the filmed microchannel plate (MCP), phosphor screen, output window, ceramics, and Kovar sealing parts [37]. The schematic structure and the photograph of the LLL proximity focused image intensifier are shown in Figure 15. As shown in Figure 15(a), the proximity focused image intensifier is capable of enhancing a LLL image from several thousands to tens of thousands of times. The input LLL image is converted into photoelectrons by the AlGaAs/ GaAs photocathode, and then the number of photoelectrons is multiplied several thousands of times by the MCP coated with a thin ion barrier film which can prevent ion feedback. Lastly, the multiplied photoelectrons bombard the phosphor screen and are converted into photons.

Figure 15. (a) Schematic structure of the low-light-level proximity focused image intensifier and (b) photograph of the

sealed proximity focused image intensifier.

66 Advances in Photodetectors - Research and Applications

Figure 14. Photocurrent changes during Cs▬O activation for the two t-mode AlGaAs/GaAs cathode modules.

Figure 16. Experimental and fitted quantum efficiency curves for the two different t-mode AlGaAs/GaAs photocathodes samples.


Author details

\* and Gangcheng Jiao2

\*Address all correspondence to: zhangyijun423@njust.edu.cn

1 School of Electronic and Optical Engineering, Nanjing University of Science and Technology,

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

69

[1] Scheer JJ, Laar JV. GaAs-Cs: A new type of photoemitter. Solid State Communications.

[2] Chrzanowski K. Review of night vision technology. Opto-Electronics Review. 2013;21:153-181 [3] Liu W, Chen Y, Lu W, Moy A, Poelker M, Stutzman M, et al. Record-level quantum efficiency from a high polarization strained GaAs/GaAsP superlattice photocathode with

[4] Baum A, Arcuni P, Aebi V, Presley S, Elder M. Prototype negative electron affinity-based multibeam electron gun for lithography and microscopy. Journal of Vacuum Science and

[5] Schwede JW, Sarmiento T, Narasimhan VK, Rosenthal SJ, Riley DC, Schmitt F, et al. Photon-enhanced thermionic emission from heterostructures with low interface recombi-

[6] Williams BF, Tietjen JJ. Current status of negative electron affinity devices. Proceedings of

[7] Karkare S, Boulet L, Cultrera L, Dunham B, Liu X, Schaff W, et al. Ultrabright and ultrafast III–V semiconductor photocathodes. Physical Review Letters. 2014;112:097601

[8] Kuwahara M, Takeda Y, Saitoh K, Ujihara T, Asano H, Nakanishi T, et al. Development of spin-polarized transmission electron microscope. Journal of Physics: Conference Series.

[9] Mitsuno K, Masuzawa T, Hatanaka Y, Neo Y, Mimura H. Activation process of GaAs NEA photocathode and its spectral sensitivity. In: 3rd International Conference on Nano-

technologies and Biomedical Engineering. Singapore: Springer; 2016. pp. 163-166

the semitransparent mode. Journal of Applied Physics. 1970;41:2888-2894

[10] Martinelli RU, Fisher DG. The application of semiconductors with negative electron affinity surfaces to electron emission devices. Proceedings of the IEEE. 1974;62:1339-1360 [11] Antypas GA, James LW, Uebbing JJ. Operation of III-V semiconductor photocathodes in

2 Science and Technology on Low-Light-Level Night Vision Laboratory, Xi'an, China

distributed Bragg reflector. Applied Physics Letters. 2016;109:252104

Yijun Zhang<sup>1</sup>

Nanjing, China

References

1965;3:189-193

Technology B. 1999;17:2819-2822

the IEEE. 1971;59:1489-1497

2011;298:012016

nation. Nature Communications. 2013;4:1576

Table 1. Fitted parameters of the two different t-mode AlGaAs/GaAs photocathode samples.

surface barrier shapes changed by the activation process are the same. Whereas, Sv for the gcomposition sample, it is considerably reduced in contrast to that for the u-composition sample. It is known that the Sv is mainly determined by the crystal quality of photocathode itself, such as misfit dislocations and stacking faults at the AlGaAs/GaAs interface. The gcomposition structure in the window layer can not only mitigate the interface discontinuity caused by the interface lattice mismatch, but also form an internal electric field to facilitate the transport of shortwave photons excitated electrons toward the emitting surface.

#### 6. Conclusions

In this chapter, we have carried out systematically theoretical and experimental researches on t-mode AlGaAs/GaAs photocathodes, with regard to bandgap structure design, photoemission model derivation, epitaxial growth, surface activation, device fabrication, and performance evaluation. Compared with the common t-mode AlGaAs/GaAs photocathode, the graded bandgap t-mode AlxGa1xAs/GaAs photocathode with a g-composition and e-doping structure can achieve higher quantum efficiency in the shortwave response region, particularly the blue-green spectral region of interest. In addition, this g-composition structure is helpful to mitigate the interface recombination and enhance the absorption of the longwave light, which leads to the enhanced photoemission capability. This work has reference significance for the design of other graded bandgap III-V group photocathodes.

#### Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant nos. 61771245 and 61301023) and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation of China (grant no. J20150702). The authors would like to thank Dr. Feng Cheng for her efforts in the theoretical photoemission model, and the staff from Science and Technology on Low-Light-Level Night Vision Laboratory for their assistance in the fabrication of t-mode cathode modules and image intensifiers.

### Author details

Yijun Zhang<sup>1</sup> \* and Gangcheng Jiao2

\*Address all correspondence to: zhangyijun423@njust.edu.cn

1 School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing, China

2 Science and Technology on Low-Light-Level Night Vision Laboratory, Xi'an, China

### References

surface barrier shapes changed by the activation process are the same. Whereas, Sv for the gcomposition sample, it is considerably reduced in contrast to that for the u-composition sample. It is known that the Sv is mainly determined by the crystal quality of photocathode itself, such as misfit dislocations and stacking faults at the AlGaAs/GaAs interface. The gcomposition structure in the window layer can not only mitigate the interface discontinuity caused by the interface lattice mismatch, but also form an internal electric field to facilitate the

u-Composition 107 0.7 485 1256 105 0.52

Cathode sample dSi3N4 (nm) Al composition in each AlGaAs sublayer dAlGaAs (nm) dGaAs (nm) Sv (cm/s) P g-Composition 109 0.9 106 857 103 0.52

> 0.675 117 0.45 125 0.225 63 0 72

In this chapter, we have carried out systematically theoretical and experimental researches on t-mode AlGaAs/GaAs photocathodes, with regard to bandgap structure design, photoemission model derivation, epitaxial growth, surface activation, device fabrication, and performance evaluation. Compared with the common t-mode AlGaAs/GaAs photocathode, the graded bandgap t-mode AlxGa1xAs/GaAs photocathode with a g-composition and e-doping structure can achieve higher quantum efficiency in the shortwave response region, particularly the blue-green spectral region of interest. In addition, this g-composition structure is helpful to mitigate the interface recombination and enhance the absorption of the longwave light, which leads to the enhanced photoemission capability. This work has reference significance for the

This work was supported by the National Natural Science Foundation of China (grant nos. 61771245 and 61301023) and Science and Technology on Low-Light-Level Night Vision Laboratory Foundation of China (grant no. J20150702). The authors would like to thank Dr. Feng Cheng for her efforts in the theoretical photoemission model, and the staff from Science and Technology on Low-Light-Level Night Vision Laboratory for their assistance in the fabrication

transport of shortwave photons excitated electrons toward the emitting surface.

Table 1. Fitted parameters of the two different t-mode AlGaAs/GaAs photocathode samples.

68 Advances in Photodetectors - Research and Applications

design of other graded bandgap III-V group photocathodes.

of t-mode cathode modules and image intensifiers.

6. Conclusions

Acknowledgements


[12] Estrera JP, Ostromek T, Bacarella A, Isbell W, Iosue MJ, Saldana M, et al. Advanced image intensifier night vision system technologies: Status and summary 2002. Proceedings of SPIE. 2003;4796:49-59

[26] Levinshtein M, Shur MS, Rumyanstev S, editors. Handbook Series on Semiconductor

Energy Bandgap Engineering of Transmission-Mode AlGaAs/GaAs Photocathode

http://dx.doi.org/10.5772/intechopen.80704

71

[27] Zarem HA, Lebens JA, Nordstrom KB, Sercel PC, Sanders S, Eng LE, et al. Effect of Al mole fraction on carrier diffusion lengths and lifetimes in AlxGa1xAs. Applied Physics

[28] Feng C, Zhang YJ, Qian YS, Xu Y, Liu XX, Jiao GC. Quantum efficiency of transmissionmode AlxGa1xAs/GaAs photocathodes with graded-composition and exponential-

[29] Feng C, Zhang YJ, Qian YS, Wang ZH, Liu J, Chang BK, et al. High-efficiency AlxGa1xAs/ GaAs cathode for photon-enhanced thermionic emission solar energy converters. Optics

[30] Zhao J, Xiong YJ, Chang BK, Zhang YJ, Zhang JJ. Research on optical properties of transmission-mode GaAs photocathode module. Proceedings of SPIE. 2011;8194:81940J

[31] Aspnes DE, Kelso SM, Logan RA, Bhat R. Optical properties of AlxGa1xAs. Journal of

[32] Antypas GA, Edgecumbe J. Glass-sealed GaAs-AlGaAs transmission photocathode.

[33] André JP, Guittard P, Hallais J, Piaget C. GaAs photocathodes for low light level imaging.

[34] Yamada M, Ide Y. Anomalous behaviors observed in the isothermal desorption of GaAs

[35] Zhang YJ, Qian YS, Feng C, Shi F, Cheng HC, Zou JJ, et al. Improved activation technique for preparing high-efficiency GaAs photocathodes. Optical Materials Express. 2017;7:3456

[36] Rodway DC, Allenson MB. In situ surface study of the activating layer on GaAs (Cs, O)

[37] Thomas N. System performance advances of 18-mm and 16-mm subminiature image

[38] Nützel G. Single-photon imaging using electron multiplication in vacuum. In: Seitz P, Theuwissen AJP, editors. Single-Photon Imaging. Berlin: Springer; 2011. pp. 73-102 [39] Vallerga JV, Siegmund O, Dalcomo J, Jelinsky PN. High-resolution (<10 μm) photon-

[40] Zhang YJ, Zou JJ, Niu J, Chang BK, Xiong YJ. Variation of spectral response for exponential-doped transmission-mode GaAs photocathodes in the preparation process.

photocathodes. Journal of Physics D: Applied Physics. 1986;19:1353-1371

Parameters. Vol. 2. London: World Scientific; 1999

doping structure. Optics Communications. 2016;369:50-55

Letters. 1989;55:2622-2624

Communications. 2018;413:1-7

Applied Physics. 1986;60:754-767

Applied Optics. 2010;49:3935-3940

Applied Physics Letters. 1975;26:371-372

Journal of Crystal Growth. 1981;55:235-245

surface oxides. Surface Science. 1995;339:L914-L918

intensifier sensors. Proceedings of SPIE. 2000;4128:54-64

counting intensified CCD. Proceedings of SPIE. 1997;3019:156-167


[26] Levinshtein M, Shur MS, Rumyanstev S, editors. Handbook Series on Semiconductor Parameters. Vol. 2. London: World Scientific; 1999

[12] Estrera JP, Ostromek T, Bacarella A, Isbell W, Iosue MJ, Saldana M, et al. Advanced image intensifier night vision system technologies: Status and summary 2002. Proceedings of

[13] Jin X, Yamamoto N, Nakagawa Y, Mano A, Kato T, Tanioku M, et al. Super-high brightness and high-spin-polarization photocathode. Applied Physics Express. 2008;1:045002

[14] Pastuszka S, Hoppe M, Kratzmann D, Schwalm D, Wolf A, Jaroshevich AS, et al. Preparation and performance of transmission-mode GaAs photocathodes as sources for cold dc

[15] Spicer WE, Herreragomez A. Modern theory and applications of photocathodes. Proceed-

[16] Fisher DG, Enstrom RE, Escher JS, Gossenberger HF, Appert JR. Photoemission characteristics of transmission-mode negative electron affinity GaAs and (ln,Ga)As vapor-grown

[17] Sinor TW, Estrera JP, Phillips DL, Rector MK. Extended blue GaAs image intensifiers.

[18] Zhang S, Benson SV, H-Garcia C. Observation and measurement of temperature rise and distribution on GaAs photo-cathode wafer with a 532 nm drive laser and a thermal imaging camera. Nuclear Instruments and Methods in Physics Research. 2011;631:22-25

[19] Spicer WE. Negative affinity 3-5 photocathodes: Their physics and technology. Applied

[20] Costello K, Aebi V, Davis G, Rue RL, Weiss R. Transferred electron photocathode with greater than 20% quantum efficiency beyond 1 micron. Proceedings of SPIE. 1995;2550:

[21] Jones LB, Rozhkov SA, Bakin VV, Kosolobov SN, Militsyn BL, Scheibler HE, et al. Cooled transmission-mode NEA-photocathode with a band-graded active layer for high bright-

[22] Zhang YJ, Niu J, Zhao J, Chang BK, Shi F, Cheng HC. Influence of exponential-doping structure on photoemission capability of transmission-mode GaAs photocathodes. Journal

[23] Zhang YJ, Chang BK, Niu J, Zhao J, Zou JJ, Shi F, et al. High-efficiency graded band-gap AlxGa1xAs/GaAs photocathodes grown by metalorganic chemical vapor deposition.

[24] Feng C, Zhang YJ, Qian Y, Chang BK, Shi F, Jiao GC, et al. Photoemission from advanced heterostructured AlxGa1xAs/GaAs photocathodes under multilevel built-in electric field.

[25] Yang Y, Yang W, Sun C. Heterostructured cathode with graded bandgap window-layer for photon-enhanced thermionic emission solar energy converters. Solar Energy Materials

ness electron source. AIP Conference Proceedings. 2008;1149:1057-1061

electron beams. Journal of Applied Physics. 2000;88:6788-6800

structures. IEEE Transactions on Electron Devices. 1974;21:641-649

SPIE. 2003;4796:49-59

70 Advances in Photodetectors - Research and Applications

ings of SPIE. 1993;2022:18-35

Physics. 1977;12:115-130

of Applied Physics. 2010;108:093108

Applied Physics Letters. 2011;99:101104

Optics Express. 2015;23:19478-19488

& Solar Cells. 2015;132:410-417

177-188

Proceedings of SPIE. 1995;2551:130-134


**Chapter 5**

**Provisional chapter**

**a-Si:H p-i-n Photodiode as a Biosensor**

**a-Si:H p-i-n Photodiode as a Biosensor**

DOI: 10.5772/intechopen.80503

The p-i-n a-Si:H photodiode is a promising device as a transducer in biosensors. The native and light-induced localized state density and energy distribution in the energy gap of a-Si:H have a large effect on the photoconductivity of thin-film photodiodes. Depending on their nature, they play a crucial role in trapping and recombination processes and consequently influence the photodiode capacitance. The optical bias dependence of modulated photocurrent, OBMPC, method using the blue LED light is applied to clarify the nature and energy distribution of the energy gap density of states and their influence on the photodiode capacitance, from photodiodes transient response. It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. Also, the capacitance achieves the upper limit around the built-in potential. Based on this method and obtained results, the a-Si:H p-i-n photodiode is used as a bio-

sensor transducer in the detection of mammalian cell chemiluminescence.

**Keywords:** a-Si:H p-i-n photodiode, biosensor, blue light, capacitance, defects, density

The recent advances, miniaturization and integration, in nanotechnology and CMOS technology afforded by photolithographic patterning, have had a transformative impact on the field of single-cell biology and diseases that depend on small collections of cells in their initial stages such as cancer. The microfluidic Lab-on-a-chip technology, still under development, meets point-of-care (POC) requirements for biomolecular analyses. The biosensors consisting of amorphous silicon (a-Si:H) p-i-n photodiode as integrated luminescence sensor in lab-on-achip devices, coupled with a microLED, have progressed rapidly over the last two decades and are still under development [1, 2]. The a-Si:H p-i-n photodiode is widely used as a transducer

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Vera Gradišnik and Darko Gumbarević

Vera Gradišnik and Darko Gumbarević

http://dx.doi.org/10.5772/intechopen.80503

of states, LED, transient response

**Abstract**

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **a-Si:H p-i-n Photodiode as a Biosensor a-Si:H p-i-n Photodiode as a Biosensor**

Vera Gradišnik and Darko Gumbarević Vera Gradišnik and Darko Gumbarević

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80503

#### **Abstract**

The p-i-n a-Si:H photodiode is a promising device as a transducer in biosensors. The native and light-induced localized state density and energy distribution in the energy gap of a-Si:H have a large effect on the photoconductivity of thin-film photodiodes. Depending on their nature, they play a crucial role in trapping and recombination processes and consequently influence the photodiode capacitance. The optical bias dependence of modulated photocurrent, OBMPC, method using the blue LED light is applied to clarify the nature and energy distribution of the energy gap density of states and their influence on the photodiode capacitance, from photodiodes transient response. It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. Also, the capacitance achieves the upper limit around the built-in potential. Based on this method and obtained results, the a-Si:H p-i-n photodiode is used as a biosensor transducer in the detection of mammalian cell chemiluminescence.

DOI: 10.5772/intechopen.80503

**Keywords:** a-Si:H p-i-n photodiode, biosensor, blue light, capacitance, defects, density of states, LED, transient response

#### **1. Introduction**

The recent advances, miniaturization and integration, in nanotechnology and CMOS technology afforded by photolithographic patterning, have had a transformative impact on the field of single-cell biology and diseases that depend on small collections of cells in their initial stages such as cancer. The microfluidic Lab-on-a-chip technology, still under development, meets point-of-care (POC) requirements for biomolecular analyses. The biosensors consisting of amorphous silicon (a-Si:H) p-i-n photodiode as integrated luminescence sensor in lab-on-achip devices, coupled with a microLED, have progressed rapidly over the last two decades and are still under development [1, 2]. The a-Si:H p-i-n photodiode is widely used as a transducer

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

in biosensors for biochemical analysis, where are applied pico- to nano-liters (microliters) of volumes of fluids in channels of tens to hundreds of micrometers. The photodiode array must have very high detection precision and allow conducting parallel experiments for the detection of biomolecules. Biosensor response must satisfy the main performance criteria: selectivity, sensitivity, linearity, and response time. At the same time, the photonic method for measuring the oxygen consumption rate (OCR) of a single cell must be developed.

charge inside i-layer, and capacitance. The concentration of midgap states and their spatial

current and steady-state thermal generation current, as described by Murthy and Dutta, and by Mahmood and Kabir [15, 16]. Models of transport and recombination through localized states have also been well described by Fuhs [14] and Dhariwal et al. [17–19]. Several techniques based on steady-state and transient photocurrent techniques have been developed to determine the nature and role of gap density of states (DOS) in the trapping-detrapping, recombination processes of mobile carriers and gap-state parameters [20]. To estimate the DOS in the lower part of band gap between the Fermi level and valence band edge, methods such as constant photocurrent method (CPM) [21], Fourier transform photocurrent spectroscopy (FTPS) [22, 23], and dual beam photoconductivity (DBP) [24] were used in the past. On the other hand, the multiexponential trapping rate and modulated photocurrent (MPC) technique [25–27] allow determining parameters of localized states throughout the entire energy

The aim of our research is the mammalian cells chemiluminescence detection, which is based on the phenomenon that under illumination of two-beam, low intensity probe beam and simultaneously a higher intensity bias beam, reverse-biased a-Si:H p-i-n photodiode photocurrent

The transient response of a-Si:H p-i-n photodiode to blue LED light pulse superimposed to the blue LED light optical bias (optical bias dependence of modulated photocurrent method— OBMPC [11, 27]) at various reverse bias voltages and one frequency is applied to clarify the nature and energy distribution of energy gap density of state and their influence on the photodiode capacitance [28]. It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. Also, the capacitance achieves the upper limit around

Based on this method and obtained results, we describe our experiment, where the a-Si:H p-i-n photodiode is used as a biosensor transducer in the detection of mammalian cell's

The fundamental structure of a photodiode in amorphous silicon is p-i-n or n-i-p. The a-Si:H p-i-n structure (**Figure 1**) investigated in this work (Sunčane ćelije d.o.o. Split, Croatia) was deposited on a transparent conductive oxide (TCO)-coated glass from undiluted SiH4

plasma-enhanced CVD at 13.56 MHz. The different layers of the p-i-n structure have the parameters of standard solar cell production. The thicknesses of the n-type, i-type, and p-type layers were 5, 300, and 5 nm, from top to the bottom, respectively. The n-type layer

source gas during growth. The back contact was aluminum deposited by evaporation.

and the p-type by adding diborane B2


a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 75

by

into the silane

H6


distribution in the i-layer and at p+

gap by employing frequency and temperature scans.

exceed expected primary photocurrent [26].

the built-in potential.

chemiluminescence.

SiH4

**2. a-Si:H p-i-n photodiode**

**2.1. Device structure and characterization**

was made by adding phosphine PH3

The sensitivity of thin-film a-Si:H p-i-n photodiodes, integrated with microfluidics, allows lowlevel luminescence signal detection from the volume of a microfluidic channel. The thin-film hydrogenated amorphous silicon (a-Si:H) technology [3] allows the custom fit of amorphous silicon photodiode arrays to the geometry of the flow microfluidic channel. The low-temperature (below 200°C) technology plasma-enhanced chemical-vapor deposition (PECVD) [2] or hot wire chemical-vapor deposition (HWCVD) [4] allows deposition of amorphous layers on the glass and polymer substrates, respectively, and on top of crystalline silicon integrated circuits without any damage to the circuits below [5]. At appropriate RF power, gas flows, chamber pressure, and substrate temperature in PECVD, hydrogen atoms are introduced into the thin film to passivate the silicon dangling bonds (DBs) and remove a part of (metastable) defect states from the forbidden band gap. In pure amorphous silicon, unsaturated dangling bonds (DBs) give rise to electronic states inside the band gap. The hydrogen atoms restore the energy gap and semiconductor properties. Due to the disordered structure alloying virtually all optical transitions, the absorption coefficient of a-Si:H is higher than that of c-Si (500–650 nm) [6]. Besides, a much lower dark current of a-Si:H than c-Si at room temperature enables its use as a photodiode material for low-noise detection. The photodiodes, as part of active area in active pixel sensors (APSs) [7–10] and other devices based on amorphous silicon, recently entered the field of microelectronics. The main part of applications was directed toward steady-state illumination of slowly varying light signals. The transient photocurrent was used for the material properties characterization and color detection [11–13].

The amorphous silicon photodiode can operate in integrated and in a reverse-biased photodiode mode. In the latter, they have a high response speed and the photocurrent is only controlled by the light intensity. In amorphous silicon, the transport of free carriers involves trapping, detrapping through a large density of midgap states (DOS), and motion through transport in the extended states—localized band tail state [14]. These native and light-induced densities of state and their energy distribution in the energy gap of a-Si:H have a large effect on the photoconductivity of thin-film photodiodes. Depending on their nature, they play a crucial role in trapping and recombination processes and consequently influence the photodiode capacitance and relaxation time. Furthermore, they lead to a high RC constant of a thin-film a-Si:H photodiode.

The disordered structure of hydrogenated amorphous silicon (a-Si:H) leads to localized states as band tails that extend inside the energy gap. The coordination defects associated with dangling bonds are sources of defect states located around the midgap. The tail states are shallow states, and the dangling bonds, the deep states. Both of them influence the recombination processes, capture and reemission of carriers in semiconductor. The emission of free charge carriers from deep states at the p-i and i-n interfaces influences the dark current, space charge inside i-layer, and capacitance. The concentration of midgap states and their spatial distribution in the i-layer and at p+ -i and n+ -i interfaces can be extracted from transient dark current and steady-state thermal generation current, as described by Murthy and Dutta, and by Mahmood and Kabir [15, 16]. Models of transport and recombination through localized states have also been well described by Fuhs [14] and Dhariwal et al. [17–19]. Several techniques based on steady-state and transient photocurrent techniques have been developed to determine the nature and role of gap density of states (DOS) in the trapping-detrapping, recombination processes of mobile carriers and gap-state parameters [20]. To estimate the DOS in the lower part of band gap between the Fermi level and valence band edge, methods such as constant photocurrent method (CPM) [21], Fourier transform photocurrent spectroscopy (FTPS) [22, 23], and dual beam photoconductivity (DBP) [24] were used in the past. On the other hand, the multiexponential trapping rate and modulated photocurrent (MPC) technique [25–27] allow determining parameters of localized states throughout the entire energy gap by employing frequency and temperature scans.

The aim of our research is the mammalian cells chemiluminescence detection, which is based on the phenomenon that under illumination of two-beam, low intensity probe beam and simultaneously a higher intensity bias beam, reverse-biased a-Si:H p-i-n photodiode photocurrent exceed expected primary photocurrent [26].

The transient response of a-Si:H p-i-n photodiode to blue LED light pulse superimposed to the blue LED light optical bias (optical bias dependence of modulated photocurrent method— OBMPC [11, 27]) at various reverse bias voltages and one frequency is applied to clarify the nature and energy distribution of energy gap density of state and their influence on the photodiode capacitance [28]. It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. Also, the capacitance achieves the upper limit around the built-in potential.

Based on this method and obtained results, we describe our experiment, where the a-Si:H p-i-n photodiode is used as a biosensor transducer in the detection of mammalian cell's chemiluminescence.

### **2. a-Si:H p-i-n photodiode**

in biosensors for biochemical analysis, where are applied pico- to nano-liters (microliters) of volumes of fluids in channels of tens to hundreds of micrometers. The photodiode array must have very high detection precision and allow conducting parallel experiments for the detection of biomolecules. Biosensor response must satisfy the main performance criteria: selectivity, sensitivity, linearity, and response time. At the same time, the photonic method for

The sensitivity of thin-film a-Si:H p-i-n photodiodes, integrated with microfluidics, allows lowlevel luminescence signal detection from the volume of a microfluidic channel. The thin-film hydrogenated amorphous silicon (a-Si:H) technology [3] allows the custom fit of amorphous silicon photodiode arrays to the geometry of the flow microfluidic channel. The low-temperature (below 200°C) technology plasma-enhanced chemical-vapor deposition (PECVD) [2] or hot wire chemical-vapor deposition (HWCVD) [4] allows deposition of amorphous layers on the glass and polymer substrates, respectively, and on top of crystalline silicon integrated circuits without any damage to the circuits below [5]. At appropriate RF power, gas flows, chamber pressure, and substrate temperature in PECVD, hydrogen atoms are introduced into the thin film to passivate the silicon dangling bonds (DBs) and remove a part of (metastable) defect states from the forbidden band gap. In pure amorphous silicon, unsaturated dangling bonds (DBs) give rise to electronic states inside the band gap. The hydrogen atoms restore the energy gap and semiconductor properties. Due to the disordered structure alloying virtually all optical transitions, the absorption coefficient of a-Si:H is higher than that of c-Si (500–650 nm) [6]. Besides, a much lower dark current of a-Si:H than c-Si at room temperature enables its use as a photodiode material for low-noise detection. The photodiodes, as part of active area in active pixel sensors (APSs) [7–10] and other devices based on amorphous silicon, recently entered the field of microelectronics. The main part of applications was directed toward steady-state illumination of slowly varying light signals. The transient photocurrent

measuring the oxygen consumption rate (OCR) of a single cell must be developed.

74 Advances in Photodetectors - Research and Applications

was used for the material properties characterization and color detection [11–13].

thin-film a-Si:H photodiode.

The amorphous silicon photodiode can operate in integrated and in a reverse-biased photodiode mode. In the latter, they have a high response speed and the photocurrent is only controlled by the light intensity. In amorphous silicon, the transport of free carriers involves trapping, detrapping through a large density of midgap states (DOS), and motion through transport in the extended states—localized band tail state [14]. These native and light-induced densities of state and their energy distribution in the energy gap of a-Si:H have a large effect on the photoconductivity of thin-film photodiodes. Depending on their nature, they play a crucial role in trapping and recombination processes and consequently influence the photodiode capacitance and relaxation time. Furthermore, they lead to a high RC constant of a

The disordered structure of hydrogenated amorphous silicon (a-Si:H) leads to localized states as band tails that extend inside the energy gap. The coordination defects associated with dangling bonds are sources of defect states located around the midgap. The tail states are shallow states, and the dangling bonds, the deep states. Both of them influence the recombination processes, capture and reemission of carriers in semiconductor. The emission of free charge carriers from deep states at the p-i and i-n interfaces influences the dark current, space

#### **2.1. Device structure and characterization**

The fundamental structure of a photodiode in amorphous silicon is p-i-n or n-i-p. The a-Si:H p-i-n structure (**Figure 1**) investigated in this work (Sunčane ćelije d.o.o. Split, Croatia) was deposited on a transparent conductive oxide (TCO)-coated glass from undiluted SiH4 by plasma-enhanced CVD at 13.56 MHz. The different layers of the p-i-n structure have the parameters of standard solar cell production. The thicknesses of the n-type, i-type, and p-type layers were 5, 300, and 5 nm, from top to the bottom, respectively. The n-type layer was made by adding phosphine PH3 and the p-type by adding diborane B2 H6 into the silane SiH4 source gas during growth. The back contact was aluminum deposited by evaporation.

The H content in the material influences the band gap values which are typically around 1.7–1.8 eV. These metastable localized states act as defect (D) states at discrete energies and as recombination centers. Dangling bonds are the main defect in a-Si:H and have defect pool model distribution and Gaussian distribution (**Figure 3**) [27]. They can be in neutral *D*<sup>0</sup>

sity and temperature. The transition *D*+/0 follows acceptor statistics and *D*0/− donor statistics.

**Figure 2.** The p-i-n a-Si:H PD current-voltage, I-V, characteristics measured under the dark and blue LED light illumination,

**Figure 3.** Scheme of band tail distribution (Dv, Dc), DOS equilibrium distribution according to defect pool model, and

(*E*) charge states and their distributions depend on light inten-

positive *D+*

*λ* = 430 nm.

*D*<sup>1</sup>

(*E*) acceptor-like Gaussian distribution after [27].

(*E*), and negative *D<sup>−</sup>*

(*E*),

77

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503

**Figure 1.** The a-Si:H p-i-n photodiode structure.

The active area of the pixel was 0.81 cm2 . The basic device characterization and experimental system are described in more detail in [12, 13]. Photoillumination was obtained through the bottom p-type layer.

The doped layers in a-Si:H are nearly transparent to visible light and should be as thin as possible to minimize parasitic absorption. The minority carriers have small diffusion lengths; therefore, n-type and p-type a-Si:H are not photoactive layers. The i-layer is a region with high electric field. The light is mostly absorbed in the intrinsic i-layer, where the photo-generation occurs. The photocarriers at reverse bias voltages are swept away by the electric field in the i-layer, electrons to the n-type and holes to the p-type, and contribute mainly to drift photocurrent. Dark current increases with bias voltage as shown in **Figure 2**. It is very small in a-Si:H devices at low bias voltages and is given by thermal carrier emission from the bulk. With increased bias voltage, the injection from the doped layer increases too [15]. The signal current should be much higher than the leakage (dark) current at applied reverse bias voltage at which the electric filed, necessary to achieve full depletion inside the i-layer, collects all the photo-generated e-h pairs. At the same time, the absorbed light creates additional defects.

Defects in amorphous silicon lead to a low mobility of the charge carriers. The recombination losses of free carriers, trapping-detrapping in midgap states, and band tail states lead to photo-generated space charge in the i-layer. The space charge distribution at the p-i edge and at the n-i edge influences the internal field and screens the applied field. It is associated with the electrons and holes' capacitance in the series. In amorphous silicon, the localized states arise from their disordered nature, bond lengths, and angles between the silicon atoms. The broken or dangling bonds (DBs) arise from not-satisfied Si-Si bonds. To passivate those DBs in material is introduced the hydrogen to form the chemical bonds with the defects. The H content in the material influences the band gap values which are typically around 1.7–1.8 eV. These metastable localized states act as defect (D) states at discrete energies and as recombination centers. Dangling bonds are the main defect in a-Si:H and have defect pool model distribution and Gaussian distribution (**Figure 3**) [27]. They can be in neutral *D*<sup>0</sup> (*E*), positive *D+* (*E*), and negative *D<sup>−</sup>* (*E*) charge states and their distributions depend on light intensity and temperature. The transition *D*+/0 follows acceptor statistics and *D*0/− donor statistics.

The active area of the pixel was 0.81 cm2

**Figure 1.** The a-Si:H p-i-n photodiode structure.

76 Advances in Photodetectors - Research and Applications

bottom p-type layer.

. The basic device characterization and experimental

system are described in more detail in [12, 13]. Photoillumination was obtained through the

The doped layers in a-Si:H are nearly transparent to visible light and should be as thin as possible to minimize parasitic absorption. The minority carriers have small diffusion lengths; therefore, n-type and p-type a-Si:H are not photoactive layers. The i-layer is a region with high electric field. The light is mostly absorbed in the intrinsic i-layer, where the photo-generation occurs. The photocarriers at reverse bias voltages are swept away by the electric field in the i-layer, electrons to the n-type and holes to the p-type, and contribute mainly to drift photocurrent. Dark current increases with bias voltage as shown in **Figure 2**. It is very small in a-Si:H devices at low bias voltages and is given by thermal carrier emission from the bulk. With increased bias voltage, the injection from the doped layer increases too [15]. The signal current should be much higher than the leakage (dark) current at applied reverse bias voltage at which the electric filed, necessary to achieve full depletion inside the i-layer, collects all the photo-generated e-h pairs. At the same time, the absorbed light creates additional defects.

Defects in amorphous silicon lead to a low mobility of the charge carriers. The recombination losses of free carriers, trapping-detrapping in midgap states, and band tail states lead to photo-generated space charge in the i-layer. The space charge distribution at the p-i edge and at the n-i edge influences the internal field and screens the applied field. It is associated with the electrons and holes' capacitance in the series. In amorphous silicon, the localized states arise from their disordered nature, bond lengths, and angles between the silicon atoms. The broken or dangling bonds (DBs) arise from not-satisfied Si-Si bonds. To passivate those DBs in material is introduced the hydrogen to form the chemical bonds with the defects.

**Figure 2.** The p-i-n a-Si:H PD current-voltage, I-V, characteristics measured under the dark and blue LED light illumination, *λ* = 430 nm.

**Figure 3.** Scheme of band tail distribution (Dv, Dc), DOS equilibrium distribution according to defect pool model, and *D*<sup>1</sup> (*E*) acceptor-like Gaussian distribution after [27].

In a-Si:H, electrons occupying the localized states are trapped or immobile, and electrons occupying extended states are assumed to be mobile and are characterized by a "band mobility" (*μn* <sup>≃</sup> <sup>10</sup> cm<sup>2</sup> <sup>s</sup><sup>−</sup><sup>1</sup> ). The localized band tail states are divided from extended electron states by EC conduction mobility edge. There are the valence mobility-edge EV separating delocalized transport states (below EV), where the free holes are characterized by a "band mobility"(*μp* <sup>≃</sup> <sup>1</sup> cm2 <sup>s</sup><sup>−</sup><sup>1</sup> ), and localized traps (above EV). The band tail states have an exponential distribution ("Urbach" tail). The conduction band tail (acceptor type) width is assumed to be <sup>∆</sup>E*<sup>C</sup>* <sup>=</sup> <sup>25</sup> eV and the valence band tail (donor type) width ∆E*<sup>V</sup>* <sup>=</sup> <sup>45</sup> <sup>−</sup> <sup>50</sup> meV, respectively. Hence, in a-Si:H, the mobility gap denotes the switch from small to larger mobility.

is due to the injection of free carriers, electrons and holes, from the n and p contacts, and their recombination through a single defect level. The integration term is due to the number of defect states which act as recombination centers and are located between the quasi-Fermi

> \_\_\_ kT *q* dln(*I <sup>D</sup>*) \_\_\_\_\_\_ dV ]

The shape of the *n*(*V*) curve, shown in **Figure 4**, reflects the energy distribution, as a Gaussian

The total recombination current density conducted through the device expressed by its activation energy of SRH recombination [30] shown in **Figure 5** is calculated by the expression:

**Figure 4.** The voltage-dependent ideality factor, *n*(*V*), as a function of voltage at room temperature for a p-i-n photodiode with an i-layer thickness of 300 nm. Calculation is done using the Deng and Wronski definition of voltage-dependent

\_\_\_\_ qV

−1

. Their concentrations increase with

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 79

nkT) (1)

. (2)

*<sup>R</sup>*(*V*) <sup>=</sup> *Eμ* <sup>−</sup> *<sup>V</sup>* \_\_\_\_\_ <sup>2</sup> <sup>+</sup> <sup>3</sup>*kT* (3)

, and trapped electrons, *E*fn*<sup>t</sup>*

increased applied voltage, as the separation of quasi-Fermi levels increases.

levels for trapped holes, *E*fp*<sup>t</sup>*

From dark current-voltage characteristics

*I*(*V*) = I0 exp(

*n*(*V*) = [

one, of the defect states in the *i-*layer.

*Ea*

ideality factor.

and the ideality factor defined by Deng and Wronski [31] is

The localized state density (DOS) is so large that an electron can move from one localized site to another by hopping and the transport via these gap states is possible, but usually in numerical analysis it is neglected. The DBs act as main recombination centers. The empty gap states (trap) which interact with majority carriers via trapping-detrapping processes can be probed under sufficiently weak bias illumination level and high modulation frequency MPC method [27]. At low frequency regime, the recombination of free electrons through the recombination centers in gap distributions D(E) occupied by holes between the trap quasi-Fermi levels of electrons and holes can be probed depending on the magnitude of the capture coefficients of the recombination centers. The scheme of the DOS distribution in undoped a-Si:H, according to the defect pool model and Gaussian distribution, is shown in **Figure 3**.

Han et al. [29] have reported the most interesting feature of optical bias. Optical bias impedes deep trapping, thus enhancing electron drift. Their photocapacitance and capacitance transient measurement result indicates the band tail transport occurs in time shorter than 10 μs which is not affected by optical bias, electron trapping, and further drift following reemission from the deep trap in time longer than 1 ms.

To use the a-Si:H p-i-n photodiode as a biosensor transducer in detection of mammalian HeLa cells' chemiluminescence in our main experiment, the photodiode characterization is first done. All measurements were performed at the room temperature. LEDs (Kingbright) emitting at 430 nm for blue (B) were used in the experiment and the dc forward current through the LED was *I <sup>F</sup>* = 20 mA. The energy of monochromatic LED light is higher than the band gap energy.

The photodiode current-voltage (I-V) characteristics measured under the dark and blue LED light illumination at *λ* = 430 nm are shown in **Figure 2**. Under low forward voltages, the dark current is dominated by Shockley-Read-Hall (SRH) recombination [30].

In order to obtain the information on the recombination rate in dark, the ideality factor is studied. It is well known that in a-Si:H, the ideality factor is a non-integer and decreases with temperature [30].

The recombination rate depends on the concentration of active recombination centers which include all traps between the quasi-Fermi levels for trapped charges. Second, it depends on the recombination efficacy of each of these recombination centers. These two factors are voltage dependent due to the continuous density of states in the band gap. The dark current has an exponential term and the integration term. The exponential term with ideality factor n = 2 is due to the injection of free carriers, electrons and holes, from the n and p contacts, and their recombination through a single defect level. The integration term is due to the number of defect states which act as recombination centers and are located between the quasi-Fermi levels for trapped holes, *E*fp*<sup>t</sup>* , and trapped electrons, *E*fn*<sup>t</sup>* . Their concentrations increase with increased applied voltage, as the separation of quasi-Fermi levels increases.

From dark current-voltage characteristics

In a-Si:H, electrons occupying the localized states are trapped or immobile, and electrons occupying extended states are assumed to be mobile and are characterized by a "band mobility"

duction mobility edge. There are the valence mobility-edge EV separating delocalized transport states (below EV), where the free holes are characterized by a "band mobility"(*μp* <sup>≃</sup> <sup>1</sup> cm2 <sup>s</sup><sup>−</sup><sup>1</sup>

localized traps (above EV). The band tail states have an exponential distribution ("Urbach" tail). The conduction band tail (acceptor type) width is assumed to be <sup>∆</sup>E*<sup>C</sup>* <sup>=</sup> <sup>25</sup> eV and the valence band tail (donor type) width ∆E*<sup>V</sup>* <sup>=</sup> <sup>45</sup> <sup>−</sup> <sup>50</sup> meV, respectively. Hence, in a-Si:H, the mobility gap denotes

The localized state density (DOS) is so large that an electron can move from one localized site to another by hopping and the transport via these gap states is possible, but usually in numerical analysis it is neglected. The DBs act as main recombination centers. The empty gap states (trap) which interact with majority carriers via trapping-detrapping processes can be probed under sufficiently weak bias illumination level and high modulation frequency MPC method [27]. At low frequency regime, the recombination of free electrons through the recombination centers in gap distributions D(E) occupied by holes between the trap quasi-Fermi levels of electrons and holes can be probed depending on the magnitude of the capture coefficients of the recombination centers. The scheme of the DOS distribution in undoped a-Si:H, according

Han et al. [29] have reported the most interesting feature of optical bias. Optical bias impedes deep trapping, thus enhancing electron drift. Their photocapacitance and capacitance transient measurement result indicates the band tail transport occurs in time shorter than 10 μs which is not affected by optical bias, electron trapping, and further drift following reemission

To use the a-Si:H p-i-n photodiode as a biosensor transducer in detection of mammalian HeLa cells' chemiluminescence in our main experiment, the photodiode characterization is first done. All measurements were performed at the room temperature. LEDs (Kingbright) emitting at 430 nm for blue (B) were used in the experiment and the dc forward current through

The photodiode current-voltage (I-V) characteristics measured under the dark and blue LED light illumination at *λ* = 430 nm are shown in **Figure 2**. Under low forward voltages, the dark

In order to obtain the information on the recombination rate in dark, the ideality factor is studied. It is well known that in a-Si:H, the ideality factor is a non-integer and decreases with

The recombination rate depends on the concentration of active recombination centers which include all traps between the quasi-Fermi levels for trapped charges. Second, it depends on the recombination efficacy of each of these recombination centers. These two factors are voltage dependent due to the continuous density of states in the band gap. The dark current has an exponential term and the integration term. The exponential term with ideality factor n = 2

*<sup>F</sup>* = 20 mA. The energy of monochromatic LED light is higher than the band gap

to the defect pool model and Gaussian distribution, is shown in **Figure 3**.

current is dominated by Shockley-Read-Hall (SRH) recombination [30].

). The localized band tail states are divided from extended electron states by EC con-

), and

(*μn* <sup>≃</sup> <sup>10</sup> cm<sup>2</sup> <sup>s</sup><sup>−</sup><sup>1</sup>

the LED was *I*

temperature [30].

energy.

the switch from small to larger mobility.

78 Advances in Photodetectors - Research and Applications

from the deep trap in time longer than 1 ms.

$$I(V) = I\_0 \exp\left(\frac{\mathbf{q}V}{\mathbf{nkT}}\right) \tag{1}$$

and the ideality factor defined by Deng and Wronski [31] is

$$m(V) = \left[\frac{kT}{q} \frac{d\ln(l\_0)}{dV}\right]^{-1} \,. \tag{2}$$

The shape of the *n*(*V*) curve, shown in **Figure 4**, reflects the energy distribution, as a Gaussian one, of the defect states in the *i-*layer.

The total recombination current density conducted through the device expressed by its activation energy of SRH recombination [30] shown in **Figure 5** is calculated by the expression:

$$E\_s^{\mathbb{R}}(V) = \frac{E\_\mu - V}{2} + 3kT\tag{3}$$

**Figure 4.** The voltage-dependent ideality factor, *n*(*V*), as a function of voltage at room temperature for a p-i-n photodiode with an i-layer thickness of 300 nm. Calculation is done using the Deng and Wronski definition of voltage-dependent ideality factor.

where *Eμ* is mobility gap and *V* the applied voltage. Calculation is done following the Kind et al. expression for the voltage-dependent activation energy of the total recombination at various mobility gap and voltage-dependent ideality factor values shown in **Figure 4**. For comparison is given the activation energy at constant ideality factor n = 2 (the thermal ideality factor defined by Pieters et al. and used in [30]).

#### **2.2. Photodiode capacitance**

The time domain technique at low frequency is used to measure the photodiode's capacitance [32]. The measurements have been carried out on a-Si:H p-i-n cells under forward and reverse bias voltages, in dark and upon blue LED illumination and voltage pulses at 333 Hz [33].

The total charge stored in photodiode capacitor depends on the photodiode voltage as described by

$$\mathbf{Q} = \int\_{0}^{\mathbf{V}\_{\rm proj}} \mathbf{C}\_{\rm proj} \, \mathbf{dV} \tag{4}$$

*Cq* <sup>=</sup> \_\_\_*<sup>Q</sup>*

on the voltage under illumination.

responds to the voltage as a resistor.

bias voltage VPD = −1.5 V.

*V*PD = ∫ 0 *V*PD idt \_\_\_\_\_ *V*PD = ∫ 0 *V*PD *<sup>C</sup>*PD dv \_\_\_\_\_\_\_ *V*PD

(6) and divided with the corresponding photodiode bias voltage.

The photodiode current is measured with digital storage oscilloscope (Keysight InfiniiVision 2000 X-Series Oscilloscopes) by voltage drop across the resistor. The dc bias voltage (−2 to 0.7 V) is applied and measurements are carried out. The characteristic photodiode transient response on voltage pulse is shown in **Figure 6**. The cell capacitance is calculated from total charge obtained by integration of photodiode current transient response on voltage pulse Eq.

The dark capacitance's dependence on photodiode voltage and capacitance under illumination with blue light is shown in **Figure 7**. It shows a quasi-linear dependence of capacitance

It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. It is evident that around the built-in voltage, the injected charge in the dark and photo-generated charge have the same value. At higher voltages prevails the injected charge in dark. Also, the capacitance achieves the upper limit around the built-in potential. The capacitance degradation effect happens at sufficiently high forward voltages around built-in voltage (*Vbi*), where the diode injection capacitance becomes more dominant and the device

The area under the current response curve gives the total charge (**Figure 8**) accumulated in the photodiode. In dark, at reverse bias voltages higher than 0.5 V, the changes in space charge

**Figure 6.** The transient response of a-Si:H photodiode to a square voltage pulse upon blue light illumination and reverse

(6)

81

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503

where *Q* is the total charge stored, *C*PD is the photodiode capacitance as a function of voltage, and *V*PD is the voltage across the photodiode capacitance. The current due to stored charge is

$$\dot{i}(t) = \frac{d\mathbf{Q}}{dt}.\tag{5}$$

The charge equivalent linear capacitor *Cq* , which stores the same amount of charge as a photodiode capacitor at photodiode voltage *VPD,* is defined as

**Figure 5.** The activation energy as a function of voltage for an a-Si:H p-i-n photodiode with an i-layer thickness of 300 nm at room temperature.

#### a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 81

$$\mathbf{C}\_q = \frac{Q}{V\_{\rm p0}} = \frac{\int^{V\_{\rm p0}} \mathrm{idt}}{V\_{\rm p0}} = \frac{\int^{V\_{\rm p0}} \mathbf{C}\_{\rm p0} \, \mathrm{d} \mathbf{v}}{V\_{\rm p0}} \tag{6}$$

The photodiode current is measured with digital storage oscilloscope (Keysight InfiniiVision 2000 X-Series Oscilloscopes) by voltage drop across the resistor. The dc bias voltage (−2 to 0.7 V) is applied and measurements are carried out. The characteristic photodiode transient response on voltage pulse is shown in **Figure 6**. The cell capacitance is calculated from total charge obtained by integration of photodiode current transient response on voltage pulse Eq. (6) and divided with the corresponding photodiode bias voltage.

where *Eμ*

by

factor defined by Pieters et al. and used in [30]).

80 Advances in Photodetectors - Research and Applications

Q = ∫

*i*(*t*) = \_\_\_

diode capacitor at photodiode voltage *VPD,* is defined as

The charge equivalent linear capacitor *Cq*

at room temperature.

**2.2. Photodiode capacitance**

is mobility gap and *V* the applied voltage. Calculation is done following the Kind

et al. expression for the voltage-dependent activation energy of the total recombination at various mobility gap and voltage-dependent ideality factor values shown in **Figure 4**. For comparison is given the activation energy at constant ideality factor n = 2 (the thermal ideality

The time domain technique at low frequency is used to measure the photodiode's capacitance [32]. The measurements have been carried out on a-Si:H p-i-n cells under forward and reverse bias voltages, in dark and upon blue LED illumination and voltage pulses at 333 Hz [33].

The total charge stored in photodiode capacitor depends on the photodiode voltage as described

*C*PD dV (4)

dt . (5)

, which stores the same amount of charge as a photo-

0 *V*PD

where *Q* is the total charge stored, *C*PD is the photodiode capacitance as a function of voltage, and *V*PD is the voltage across the photodiode capacitance. The current due to stored charge is

**Figure 5.** The activation energy as a function of voltage for an a-Si:H p-i-n photodiode with an i-layer thickness of 300 nm

dQ

The dark capacitance's dependence on photodiode voltage and capacitance under illumination with blue light is shown in **Figure 7**. It shows a quasi-linear dependence of capacitance on the voltage under illumination.

It is observed that the deep defect states of the i-layer contribute to the capacitance at various bias voltages. It is evident that around the built-in voltage, the injected charge in the dark and photo-generated charge have the same value. At higher voltages prevails the injected charge in dark. Also, the capacitance achieves the upper limit around the built-in potential. The capacitance degradation effect happens at sufficiently high forward voltages around built-in voltage (*Vbi*), where the diode injection capacitance becomes more dominant and the device responds to the voltage as a resistor.

The area under the current response curve gives the total charge (**Figure 8**) accumulated in the photodiode. In dark, at reverse bias voltages higher than 0.5 V, the changes in space charge

**Figure 6.** The transient response of a-Si:H photodiode to a square voltage pulse upon blue light illumination and reverse bias voltage VPD = −1.5 V.

**2.3. The blue light-induced defect creation examined with the OBMPC method**

photocurrent degradation and capacitance contribution.

erated near the front surface in the vicinity of the p+

levels (capture and release), as described in [11].

τ = v0

, *Etp* <sup>&</sup>lt; *<sup>E</sup>* <sup>&</sup>lt; *Etn*.

determine the high or low frequency regime of the experiment.

experiment, taking in to account the characteristic capture frequency *ω<sup>c</sup>*

(8) in [36]. At *Etn* and *Etp,* the occupation function, *f*

electrons, *Etn* and holes, *Et*

Using moderated OBMPC [11, 27, 36], we examine the light-induced defects kinetics and nature in the i-layer of a-Si:H p-i-n photodiode. Furthermore, we clarify their influence on

The photodiode was illuminated with two blue LEDs (430 nm), a constant pump (optical bias) light and square pulse (probe) light at frequency of 333 Hz with 50% duty cycle. The intensity of the optical bias light and the pulsed probe beam was adjusted with 20-mA current through the LEDs. The illuminations were from the p-type layer side. The measurements were performed in the range from forward bias voltages of 0.7 V to reverse bias voltages of −2 V. From the measured switch-off transient response to a blue light pulse, we numerically analyze, by the generalized Foss method and general solution developed by Jeričević [33, 34], the trap and recombination localized states' energy distribution in the energy gap. The number of components, not known in advance, in multiexponential decay of measured switch-off transient response is determined by its best fit with numerically modeled transient response. The photo-generated electron-hole pairs upon blue light illumination are nonuniformly gen-

carrier densities, electrons, and holes, have dc and time-dependent pulsed components.

is dependent on the time that an electron spent in discrete localized states *N*(*Ei*

The holes' contribution to the transient photocurrent is small, due to their trapping near the p+

interface where arises the space charge density or their movement into the front contact [11]. We observe a short time delay of transient photocurrent ascribed to trapping and release interaction of free carriers with shallow band gap localized states. The transient photocurrent decay in tail-like form, dependent on applied voltage, often happens due to deep trapping. It

> <sup>−</sup><sup>1</sup> *e*(*Ei* /kT)

Based on the MPC theory described in [36], the band gap energy is divided into three energy ranges. The energies from which electrons (holes) can be trapped and released to the conduction (valence) band, *E,* are above, *E > Etn* (below, *E < Etp*) quasi-Fermi level for trapped electrons, *Etn* (trapped holes, *Etp*), and recombination states between quasi-Fermi level for trapped

The position of quasi-Fermi level for trapped electrons, *Etn* [35, 36], is determined from the measured dc photocurrent at applied bias voltage and under constant illumination from Eq.

two steps. The dc generation rate characteristic time response (Eq. (1) in [36]), extracted from measured photocurrent transient response is compared with the characteristic time of the

From calculated values, in our experiment, the chosen frequency falls in the low-frequency regime. In this regime, only the defects around the Fermi level can be probed. The requirement

/i interface. The photo-generated free

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503

. (7)

*dc,* of gap states changes from 1 to 0 in

) at *Ei*

(Eq. (2) in [36]) to

/i

83

energy

**Figure 7.** The a-Si:H photodiode capacitance versus bias photodiode voltages.

**Figure 8.** The a-Si:H photodiode total accumulated charge at different device voltages in dark and under blue LED light illumination (430 nm).

and local electric field in i-layer around p/i and n/i interfaces lead to the increase in total accumulated charge and consequently the capacitance increases. The increase in photo-generated charge with increased reverse bias voltage is smaller than dark charge. The proposed method can be used for further development of photodiode-integrated system and biosensors.

#### **2.3. The blue light-induced defect creation examined with the OBMPC method**

Using moderated OBMPC [11, 27, 36], we examine the light-induced defects kinetics and nature in the i-layer of a-Si:H p-i-n photodiode. Furthermore, we clarify their influence on photocurrent degradation and capacitance contribution.

The photodiode was illuminated with two blue LEDs (430 nm), a constant pump (optical bias) light and square pulse (probe) light at frequency of 333 Hz with 50% duty cycle. The intensity of the optical bias light and the pulsed probe beam was adjusted with 20-mA current through the LEDs. The illuminations were from the p-type layer side. The measurements were performed in the range from forward bias voltages of 0.7 V to reverse bias voltages of −2 V. From the measured switch-off transient response to a blue light pulse, we numerically analyze, by the generalized Foss method and general solution developed by Jeričević [33, 34], the trap and recombination localized states' energy distribution in the energy gap. The number of components, not known in advance, in multiexponential decay of measured switch-off transient response is determined by its best fit with numerically modeled transient response.

The photo-generated electron-hole pairs upon blue light illumination are nonuniformly generated near the front surface in the vicinity of the p+ /i interface. The photo-generated free carrier densities, electrons, and holes, have dc and time-dependent pulsed components.

The holes' contribution to the transient photocurrent is small, due to their trapping near the p+ /i interface where arises the space charge density or their movement into the front contact [11].

We observe a short time delay of transient photocurrent ascribed to trapping and release interaction of free carriers with shallow band gap localized states. The transient photocurrent decay in tail-like form, dependent on applied voltage, often happens due to deep trapping. It is dependent on the time that an electron spent in discrete localized states *N*(*Ei* ) at *Ei* energy levels (capture and release), as described in [11].

$$
\mathbf{r} = \mathbf{v}\_0^{-1} e^{\left(\mathbf{f}\_0 \mathbf{M} \mathbf{T}\right)}.\tag{7}
$$

Based on the MPC theory described in [36], the band gap energy is divided into three energy ranges. The energies from which electrons (holes) can be trapped and released to the conduction (valence) band, *E,* are above, *E > Etn* (below, *E < Etp*) quasi-Fermi level for trapped electrons, *Etn* (trapped holes, *Etp*), and recombination states between quasi-Fermi level for trapped electrons, *Etn* and holes, *Et* , *Etp* <sup>&</sup>lt; *<sup>E</sup>* <sup>&</sup>lt; *Etn*.

The position of quasi-Fermi level for trapped electrons, *Etn* [35, 36], is determined from the measured dc photocurrent at applied bias voltage and under constant illumination from Eq. (8) in [36]. At *Etn* and *Etp,* the occupation function, *f dc,* of gap states changes from 1 to 0 in two steps. The dc generation rate characteristic time response (Eq. (1) in [36]), extracted from measured photocurrent transient response is compared with the characteristic time of the experiment, taking in to account the characteristic capture frequency *ω<sup>c</sup>* (Eq. (2) in [36]) to determine the high or low frequency regime of the experiment.

**Figure 8.** The a-Si:H photodiode total accumulated charge at different device voltages in dark and under blue LED light

**Figure 7.** The a-Si:H photodiode capacitance versus bias photodiode voltages.

82 Advances in Photodetectors - Research and Applications

and local electric field in i-layer around p/i and n/i interfaces lead to the increase in total accumulated charge and consequently the capacitance increases. The increase in photo-generated charge with increased reverse bias voltage is smaller than dark charge. The proposed method

can be used for further development of photodiode-integrated system and biosensors.

illumination (430 nm).

From calculated values, in our experiment, the chosen frequency falls in the low-frequency regime. In this regime, only the defects around the Fermi level can be probed. The requirement that Fermi level of free electrons coincides with quasi-Fermi level of trapped electron will be satisfied.

The measured a-Si:H p-i-n photodiode switch-off photocurrent transient response on blue probe light at blue bias light and 0 V bias voltage on 10 kΩ load resistor, the calculated transient response, and difference between them are shown in **Figure 9**. The two exponential functions, as in **Figure 9**, are present in all the cases of applied bias voltage.

**Figure 10** shows the numerically extracted energies of localized states from measured photocurrent transient response. The weighting factor (pre-exponential factor) of localized states is shown in **Figure 11**. The weighting factors corresponding to the deeper gap states (*E*<sup>2</sup> ) are higher than those of the shallower (*E*<sup>1</sup> ) states for all voltages below the built-in voltage. With increasing forward bias voltage, there is an increase in weighting factor corresponding to energy *E*<sup>1</sup> and decrease in those of energy *E*<sup>2</sup> . The energy levels *E*<sup>1</sup> and *E*<sup>2</sup> shift toward deeper energy levels for moderate forward voltages below the built-in voltage. At high forward voltages, both shift toward shallower values. This is in agreement with [27], where the capture coefficients of the midgap states were higher than those of the shallow localized states. Also, these results confirm the capacitance upper limit described above (**Figures 7** and **8**).

#### **2.4. a-Si:H p-i-n photodiode as a transducer in biosensors**

By definition of Mehrotra, biosensors are analytical devices that convert a biological response into an electrical signal [37]. They have many applications in medical diagnostics, pharmaceutical, food, beverage, agricultural, environmental, and biotechnological industries. Two main components of biosensors are the bioreceptor and transducer [38, 39]. Bioreceptor is a

**Figure 9.** The measured a-Si:H p-i-n photodiode switch-off photocurrent transient response on blue probe light at blue bias light and 0 V bias voltage on 10 kΩ load resistor, the numerically reconstructed transient response (theory), and difference between them.

part that recognizes the analyte of interest, while biotransducer is a physicochemical detector that converts the bioreceptor-analyte complex into a measurable signal. As the name says, a bioreceptor is a biological molecule like enzymes, antibodies, and nucleic acid, but it can also be a tissue, organelle, or microorganism, while the biotransducer's measurable signal may be viscosity, mass, temperature, electrical current, electrical potential, impedance, conductance,

**Figure 11.** Weighting factor of localized states extracted from measured photocurrent transient response of a-Si:H p-i-n

**Figure 10.** The energies of localized states extracted from measured photocurrent transient response of a-Si:H p-i-n PD

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 85

on blue probe light at blue bias light at applied voltages Vappl. = −2 to 0.7 V.

PD on blue probe light at blue bias light at applied voltages Vappl. = −2 to 0.7 V.

**Figure 10.** The energies of localized states extracted from measured photocurrent transient response of a-Si:H p-i-n PD on blue probe light at blue bias light at applied voltages Vappl. = −2 to 0.7 V.

**Figure 11.** Weighting factor of localized states extracted from measured photocurrent transient response of a-Si:H p-i-n PD on blue probe light at blue bias light at applied voltages Vappl. = −2 to 0.7 V.

part that recognizes the analyte of interest, while biotransducer is a physicochemical detector that converts the bioreceptor-analyte complex into a measurable signal. As the name says, a bioreceptor is a biological molecule like enzymes, antibodies, and nucleic acid, but it can also be a tissue, organelle, or microorganism, while the biotransducer's measurable signal may be viscosity, mass, temperature, electrical current, electrical potential, impedance, conductance,

**Figure 9.** The measured a-Si:H p-i-n photodiode switch-off photocurrent transient response on blue probe light at blue bias light and 0 V bias voltage on 10 kΩ load resistor, the numerically reconstructed transient response (theory), and

that Fermi level of free electrons coincides with quasi-Fermi level of trapped electron will be

The measured a-Si:H p-i-n photodiode switch-off photocurrent transient response on blue probe light at blue bias light and 0 V bias voltage on 10 kΩ load resistor, the calculated transient response, and difference between them are shown in **Figure 9**. The two exponential

**Figure 10** shows the numerically extracted energies of localized states from measured photocurrent transient response. The weighting factor (pre-exponential factor) of localized states is shown in **Figure 11**. The weighting factors corresponding to the deeper gap states (*E*<sup>2</sup>

increasing forward bias voltage, there is an increase in weighting factor corresponding to

energy levels for moderate forward voltages below the built-in voltage. At high forward voltages, both shift toward shallower values. This is in agreement with [27], where the capture coefficients of the midgap states were higher than those of the shallow localized states. Also,

By definition of Mehrotra, biosensors are analytical devices that convert a biological response into an electrical signal [37]. They have many applications in medical diagnostics, pharmaceutical, food, beverage, agricultural, environmental, and biotechnological industries. Two main components of biosensors are the bioreceptor and transducer [38, 39]. Bioreceptor is a

these results confirm the capacitance upper limit described above (**Figures 7** and **8**).

. The energy levels *E*<sup>1</sup>

) states for all voltages below the built-in voltage. With

and *E*<sup>2</sup>

) are

shift toward deeper

functions, as in **Figure 9**, are present in all the cases of applied bias voltage.

difference between them.

satisfied.

energy *E*<sup>1</sup>

higher than those of the shallower (*E*<sup>1</sup>

84 Advances in Photodetectors - Research and Applications

and decrease in those of energy *E*<sup>2</sup>

**2.4. a-Si:H p-i-n photodiode as a transducer in biosensors**

electromagnetic field, electromagnetic radiation, or visible light. Biosensors can be label-free or label based which depends on their detection system [40].

[43–45]. From previous works, it is known that bilirubin and riboflavin decompose under exposure of blue light [43, 46]. There are numerous factors that influence photodegradation, like radiation source, intensity, wavelength, pH, buffer, solvent polarity, and viscosity [43]. The influence of blue light on (a) complemented DMEM medium and (b) HeLa cells can be

The photodiode's (Department of Information Engineering, Electronics and Telecommunications, Sapienza Università di Roma, Italy) p-doped/intrinsic/n-doped junction of a:Si-H

a-Si:H layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) in a three-chamber high-vacuum system. The bottom electrode is a 180–nm-thick indium tin oxide (ITO) layer. The top metal electrode is a three-metal-layer stack (30-nm-thick Cr/150-nm-thick

The a-Si:H photodiode illuminated with blue LED light (RGB LED Lamp Kingbright emitting at 430 nm) is placed in a dark metallic box. The LED current was fixed at 20 mA to provide constant illumination. A reverse bias voltage equal to 2 V was applied to the photodiode. The measurements for calibration were performed at room temperature for 1 h. Before starting the assay, the 3-ml solutions containing the appropriate concentration of HeLa cells in DMEM and DMEM, respectively, are introduced with pipette in a plastic well posted on a photodiode surface. The box is then closed to minimize room light interference. The measurements are performed at 2-V reverse bias. The photodiode current and voltage are monitored for 1 h using the Keysight BenchVue software. The photodiode is connected in series with a load resistor, RL, of 10 kΩ, voltage source (Agilent Technologies E3631A DC voltage source), and digital multimeters, DMMs (Agilent Technologies 34450A meter). Before starting the assay, the a-Si:H photodiode is illuminated with white light to neutralize the defects induced with previous blue light illumination and to reverse the process of decreasing of photoconductivity.

The significant changes in current are observed in first 20 minutes. The current characteristic transients corresponding to blue LED-induced HeLa cells' chemiluminescence detected by

It can be deduced from the **Figure 12** that the photocurrent initially decreases due to creation of two types of defects under blue light illumination. The measured photocurrent (a) when 3 mL of complemented DMEM and trypsin are placed in plastic well has faster exponential decay than the photodiode in first 2 min. The decrease in photocurrent can be attributed to absorption of blue light in the DMEM solution and in the photodiode. After 20 min, the photocurrent decreases which can be attributed to the decomposition of riboflavin (not presented here). It is also known from the literature that bilirubin and riboflavin obey first-order decomposition kinetics when they are exposed to blue light; and although the kinetic coefficient for riboflavin is 10 times greater than for bilirubin, it can be speculated that riboflavin

Effects of visible spectra on live organisms have been studied for different approaches. Light can induce photochemical reactions in living cells and can have benefits in treatment of some

glass substrate and arranged in 5 × 6 array. The

. Further details on the photodi-

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 87

monitored by amorphous silicon (a-Si:H) photodiodes.

Al/30-nm-thick Cr). The area of each photodiode is 2 × 2 mm2

layers were deposited on 50 × 50 × 1.1 mm3

ode array fabrication can be found in [47].

a-Si:H p-i-n photodiode are shown in **Figure 12**.

decomposes in our experiment [46, 48].

Biosensing elements can be described as follows:

Enzymes: protein molecule which acts as a catalyst in chemical reactions. They can be mobilized on transducers by gel entrapment technology, covalent binding, or physical adsorption.

Microbes: they are capable of transforming analytes to specific products which can be monitored by transducer.

Organelle: more specific for analysis.

Antibodies: highly selective to antigens and can be attached to matrix surface of transducer.

Nucleic acids: are DNA and RNA molecules which can be hybridized with other nucleic acids, so it can be a good sensing element for metabolic disorders, infection disease, cancer, and genetic disorders.

Aptamers: those are single-stranded DNA or RNA molecules and can be specific against amino acids, proteins, and other molecules by adopting specific and stable secondary structures against mentioned analytes.

Biosensors can be classified as electrochemical, mass dependent, optical, radiation sensitive [39], or piezoelectric based on their transduction principle. Based on the detected analyte, they can be immunosensors, aptasensors, genosensors, or enzymatic biosensors.

Optical biosensors have light as the output transducer signal. Light is generated by optical diffraction and electrochemiluminescence as main mechanisms for light production [41]. Bioluminescence is a process in which biomolecules absorb light, from the excitation source and enter into excited state, then fall down to the ground state and emit light as fluorescence or phosphorescence. Chemiluminescence is a type of luminescence when the light is emitted by chemical reaction. If the chemical reaction is catalyzed by an enzyme, it is called bioluminescence [42].

Regard, their above described characteristics, the a-Si:H photodiodes have become driving force in the scientific community for detection of tumor cells. For in vitro testing of HeLa cells, it is important to note that:


DMEM (Dulbecco's Modified Eagle Medium) has been proposed for culturing normal and tumor cells. Constituents of the medium are high level of glucose, essential minerals, amino acid, and vitamins. Alone, it does not function for cell culturing; so, it must be complemented with fetal bovine serum, antibiotics, and l-glutamine. The components of the complemented medium DMEM that absorb blue light are riboflavin (vitamin B12), hemoglobin, and bilirubin [43–45]. From previous works, it is known that bilirubin and riboflavin decompose under exposure of blue light [43, 46]. There are numerous factors that influence photodegradation, like radiation source, intensity, wavelength, pH, buffer, solvent polarity, and viscosity [43]. The influence of blue light on (a) complemented DMEM medium and (b) HeLa cells can be monitored by amorphous silicon (a-Si:H) photodiodes.

electromagnetic field, electromagnetic radiation, or visible light. Biosensors can be label-free

Enzymes: protein molecule which acts as a catalyst in chemical reactions. They can be mobilized on transducers by gel entrapment technology, covalent binding, or physical adsorption. Microbes: they are capable of transforming analytes to specific products which can be moni-

Antibodies: highly selective to antigens and can be attached to matrix surface of transducer. Nucleic acids: are DNA and RNA molecules which can be hybridized with other nucleic acids, so it can be a good sensing element for metabolic disorders, infection disease, cancer,

Aptamers: those are single-stranded DNA or RNA molecules and can be specific against amino acids, proteins, and other molecules by adopting specific and stable secondary struc-

Biosensors can be classified as electrochemical, mass dependent, optical, radiation sensitive [39], or piezoelectric based on their transduction principle. Based on the detected analyte,

Optical biosensors have light as the output transducer signal. Light is generated by optical diffraction and electrochemiluminescence as main mechanisms for light production [41]. Bioluminescence is a process in which biomolecules absorb light, from the excitation source and enter into excited state, then fall down to the ground state and emit light as fluorescence or phosphorescence. Chemiluminescence is a type of luminescence when the light is emitted by chemical reaction. If the chemical reaction is catalyzed by an enzyme, it is called bioluminescence [42]. Regard, their above described characteristics, the a-Si:H photodiodes have become driving force in the scientific community for detection of tumor cells. For in vitro testing of HeLa cells,

**1.** Cells are standardly grown in complemented Dulbecco's Modified Eagle Medium (DMEM)

DMEM (Dulbecco's Modified Eagle Medium) has been proposed for culturing normal and tumor cells. Constituents of the medium are high level of glucose, essential minerals, amino acid, and vitamins. Alone, it does not function for cell culturing; so, it must be complemented with fetal bovine serum, antibiotics, and l-glutamine. The components of the complemented medium DMEM that absorb blue light are riboflavin (vitamin B12), hemoglobin, and bilirubin

**2.** For counting, cells are removed from the surface plate by use of enzyme trypsin.

**3.** All the components (cells, DMEM, and trypsin) absorb blue light.

they can be immunosensors, aptasensors, genosensors, or enzymatic biosensors.

or label based which depends on their detection system [40].

Biosensing elements can be described as follows:

86 Advances in Photodetectors - Research and Applications

tored by transducer.

and genetic disorders.

Organelle: more specific for analysis.

tures against mentioned analytes.

it is important to note that:

with fetal bovine serum addition.

The photodiode's (Department of Information Engineering, Electronics and Telecommunications, Sapienza Università di Roma, Italy) p-doped/intrinsic/n-doped junction of a:Si-H layers were deposited on 50 × 50 × 1.1 mm3 glass substrate and arranged in 5 × 6 array. The a-Si:H layers were deposited by plasma-enhanced chemical vapor deposition (PECVD) in a three-chamber high-vacuum system. The bottom electrode is a 180–nm-thick indium tin oxide (ITO) layer. The top metal electrode is a three-metal-layer stack (30-nm-thick Cr/150-nm-thick Al/30-nm-thick Cr). The area of each photodiode is 2 × 2 mm2 . Further details on the photodiode array fabrication can be found in [47].

The a-Si:H photodiode illuminated with blue LED light (RGB LED Lamp Kingbright emitting at 430 nm) is placed in a dark metallic box. The LED current was fixed at 20 mA to provide constant illumination. A reverse bias voltage equal to 2 V was applied to the photodiode. The measurements for calibration were performed at room temperature for 1 h. Before starting the assay, the 3-ml solutions containing the appropriate concentration of HeLa cells in DMEM and DMEM, respectively, are introduced with pipette in a plastic well posted on a photodiode surface. The box is then closed to minimize room light interference. The measurements are performed at 2-V reverse bias. The photodiode current and voltage are monitored for 1 h using the Keysight BenchVue software. The photodiode is connected in series with a load resistor, RL, of 10 kΩ, voltage source (Agilent Technologies E3631A DC voltage source), and digital multimeters, DMMs (Agilent Technologies 34450A meter). Before starting the assay, the a-Si:H photodiode is illuminated with white light to neutralize the defects induced with previous blue light illumination and to reverse the process of decreasing of photoconductivity.

The significant changes in current are observed in first 20 minutes. The current characteristic transients corresponding to blue LED-induced HeLa cells' chemiluminescence detected by a-Si:H p-i-n photodiode are shown in **Figure 12**.

It can be deduced from the **Figure 12** that the photocurrent initially decreases due to creation of two types of defects under blue light illumination. The measured photocurrent (a) when 3 mL of complemented DMEM and trypsin are placed in plastic well has faster exponential decay than the photodiode in first 2 min. The decrease in photocurrent can be attributed to absorption of blue light in the DMEM solution and in the photodiode. After 20 min, the photocurrent decreases which can be attributed to the decomposition of riboflavin (not presented here). It is also known from the literature that bilirubin and riboflavin obey first-order decomposition kinetics when they are exposed to blue light; and although the kinetic coefficient for riboflavin is 10 times greater than for bilirubin, it can be speculated that riboflavin decomposes in our experiment [46, 48].

Effects of visible spectra on live organisms have been studied for different approaches. Light can induce photochemical reactions in living cells and can have benefits in treatment of some

**Figure 12.** Normalized measured a-Si:H p-i-n photodiode, PD photocurrent versus time, with HeLa cells, and DMEM and trypsin, respectively, in plastic well on PD surface.

diseases, that is, psoriasis and neonatal hyperbilirubinemia [49, 50]. It can modulate the endocrine system and accelerate the maturation of ovaries in young rats [51].

concluded that HeLa cells produce chemiluminescence radiation in the visible part of spec-

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 89

We performed an experiment on mammalian cells' chemiluminescence detection based on the phenomenon that under illumination of two-beam, reverse-biased a-Si:H photodiode current exceed expected primary photocurrent. The native and metastable defects in a-si:H p-i-n phtodiodes activated in this phenomenon are first characterized with simultaneous blue light pulse and constant blue light illumination at low frequency. From a transient response, the photocapacitance is analyzed. Finally, the HeLa cells' chemiluminescence reaction measurement method is done. It can be concluded that a-Si:H photodiodes can be good transducers in optical biosensors for detecting tumor cells and chemiluminescence reaction inside cells.

The authors thank Prof. Domenico Caputo and Prof. Giampiero de Cesare at University of Rome "La Sapienza", Italy and group from process line at Solar cells d.o.o. Split Croatia for

trum, while in the DMEM solution, this is not observed.

**Figure 13.** The normalized photocurrent difference of HeLa cells and DMEM and trypsin.

**3. Conclusions**

**Acknowledgements**

provision of samples used in this work.

Blue light can influence skin-keratinocytes exerting antiproliferative effect and inducing differentiation; so, it can have therapeutic effects for hyperproliferative skin conditions [52]. Effects of blue light on human health are very beneficial because it can inhibit the growth of tumors, killing bacterial spores or inactivate microorganisms [53–55]. There are a number of chromophores inside cells that absorb blue light like riboflavin, flavin proteins, iron-sulfur proteins, cytochromes, etc. a-Si:H p-i-n photodiode can be a good detector with high sensitivity, good spectral responsivity, and small reflectance for blue light, for measuring low light intensity in visible spectrum (430–780 nm). So, (b) in experiment with HeLa cells under blue light illumination the low intensity light, which is product of chemiluminescence inside cells, can be detected. According to this experiment, HeLa cells under illumination with blue light exert dramatic changes in their metabolic activity. It is well known that blue light can induce hydrogen peroxide production in mammalian cells, and release nitrogen oxide from nitrosylated proteins [52, 53]. In tumor HeLa cells, nitric oxide modulates a number of biological processes which can be witnessed by increase in NO-synthetase levels [56]. Also, it is well known that nitric oxide and hydrogen peroxide can react and release light from chemiluminescence reaction producing the toxic reactive oxygen species singlet oxygen [57]. Singlet oxygen can induce serious damage in cells and could kill 43% of tumor cells in 1 h in our experiments. So, the blue light has two effects on tumor HeLa cells: inducing chemiluminescence and killing tumor cells. Chemiluminescence can be detected by a-Si:H photodiode and that chemiluminescence reaction rate versus time sequence obeys the exponential decay. **Figure 13** shows the difference in photocurrent of HeLa cells and DMEM and trypsin. It can be deduced that absorption of HeLa cells can be separated from complemented DMEM medium. It can be

**Figure 13.** The normalized photocurrent difference of HeLa cells and DMEM and trypsin.

concluded that HeLa cells produce chemiluminescence radiation in the visible part of spectrum, while in the DMEM solution, this is not observed.

#### **3. Conclusions**

diseases, that is, psoriasis and neonatal hyperbilirubinemia [49, 50]. It can modulate the endo-

**Figure 12.** Normalized measured a-Si:H p-i-n photodiode, PD photocurrent versus time, with HeLa cells, and DMEM

Blue light can influence skin-keratinocytes exerting antiproliferative effect and inducing differentiation; so, it can have therapeutic effects for hyperproliferative skin conditions [52]. Effects of blue light on human health are very beneficial because it can inhibit the growth of tumors, killing bacterial spores or inactivate microorganisms [53–55]. There are a number of chromophores inside cells that absorb blue light like riboflavin, flavin proteins, iron-sulfur proteins, cytochromes, etc. a-Si:H p-i-n photodiode can be a good detector with high sensitivity, good spectral responsivity, and small reflectance for blue light, for measuring low light intensity in visible spectrum (430–780 nm). So, (b) in experiment with HeLa cells under blue light illumination the low intensity light, which is product of chemiluminescence inside cells, can be detected. According to this experiment, HeLa cells under illumination with blue light exert dramatic changes in their metabolic activity. It is well known that blue light can induce hydrogen peroxide production in mammalian cells, and release nitrogen oxide from nitrosylated proteins [52, 53]. In tumor HeLa cells, nitric oxide modulates a number of biological processes which can be witnessed by increase in NO-synthetase levels [56]. Also, it is well known that nitric oxide and hydrogen peroxide can react and release light from chemiluminescence reaction producing the toxic reactive oxygen species singlet oxygen [57]. Singlet oxygen can induce serious damage in cells and could kill 43% of tumor cells in 1 h in our experiments. So, the blue light has two effects on tumor HeLa cells: inducing chemiluminescence and killing tumor cells. Chemiluminescence can be detected by a-Si:H photodiode and that chemiluminescence reaction rate versus time sequence obeys the exponential decay. **Figure 13** shows the difference in photocurrent of HeLa cells and DMEM and trypsin. It can be deduced that absorption of HeLa cells can be separated from complemented DMEM medium. It can be

crine system and accelerate the maturation of ovaries in young rats [51].

and trypsin, respectively, in plastic well on PD surface.

88 Advances in Photodetectors - Research and Applications

We performed an experiment on mammalian cells' chemiluminescence detection based on the phenomenon that under illumination of two-beam, reverse-biased a-Si:H photodiode current exceed expected primary photocurrent. The native and metastable defects in a-si:H p-i-n phtodiodes activated in this phenomenon are first characterized with simultaneous blue light pulse and constant blue light illumination at low frequency. From a transient response, the photocapacitance is analyzed. Finally, the HeLa cells' chemiluminescence reaction measurement method is done. It can be concluded that a-Si:H photodiodes can be good transducers in optical biosensors for detecting tumor cells and chemiluminescence reaction inside cells.

#### **Acknowledgements**

The authors thank Prof. Domenico Caputo and Prof. Giampiero de Cesare at University of Rome "La Sapienza", Italy and group from process line at Solar cells d.o.o. Split Croatia for provision of samples used in this work.

#### **Conflict of interest**

The authors declare that there are no conflict of interests regarding the publication of this paper.

[8] Izadi MH, Tousignant O, Feuto Mokam M, Karim SK. An a-Si active pixel sensor (APS) array for medical X-ray imaging. IEEE Transactions on Electron Devices. 2010;**57**:

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 91

[9] Karim KS, Nathan A. Readout circuit in active pixel sensors in amorphous silicon technology. IEEE Electron Device Letters. 2001;**22**:469-471. DOI: 10.1109/55.954914

[10] Karim KS, Nathan A. Amorphous silicon active pixel sensor readout circuit for digital imaging. IEEE Transactions on Electron Devices. 2003;**50**:200-208. DOI: 10.1109/TED.2002.

[11] Shen DS, Wagner S. Transient photocurrent in hydrogenated amorphous silicon and implications for photodetector devices. Journal of Applied Physics. 1996;**79**:794-801.

[12] Gradišnik V, Pavlović M, Pivac B, Zulim I. Study of the color detection of a-Si:H by transient response in the visible range. IEEE Transactions on Electron Devices. 2002;**49**:

[13] Gradišnik V, Pavlović M, Pivac B, Zulim I. Transient response times of a-Si:H p-i-n color detector. IEEE Transactions on Electron Devices. 2006;**53**:2485-2491. DOI: 10.1109/

[14] Fuhs W. Recombination and transport through localized states in hydrogenated amorpohus and microcrystalline silicon. Journal of Non-Crystalline Solids. 2008;**354**:2067-2078.

[15] Murthy VR, Dutta V. Underlying reverse current mechanisms in a-Si:H p+−i-n+ solar cell and compact SPICE modelling. Journal of Non-Crystalline Solids. 2008;**354**:3780-3784.

[16] Mahmood SA, Kabir MZ. Modeling of transient and steady-state dark current in amorphous silicon p-i-n photodiodes. Current Applied Physics. 2009;**9**:1393-1396. DOI: 10.1016/

[17] Dhariwal SR, Rajvanshi S. Theory of amorphous silicon solar cell (a): Numerical analysis. Solar Energy Materials & Solar Cells. 2003;**79**:199-213. DOI: 10.1016/S0927-0248(02)

[18] Dhariwal SR, Rajvanshi S. Theory of amorphous silicon solar cell (b): A five layer analytical model. Solar Energy Materials & Solar Cells. 2003;**79**:215-233. DOI: 10.1016/S0927-0248

[19] Dhariwal SR, Smirty M. On the sensitivity of open-circuit voltage and fill factor on dangling bond density and Fermi level position in amorphous silicon p-i-n solar cell. Solar Energy Materials & Solar Cells. 2006;**90**:1254-1272. DOI: 10.1016/j.solmat.2005.08.001

[20] Kopprio L, Longeaud C, Schmidt J. Obtainment of the density of states in the band tails of hydrogenated amorphous silicon. Journal of Applied Physics. 2017;**122**:085702. DOI:

3020-3026. DOI: 10.1109/TED.2010.2069010

806968

DOI: 10.1063/1.360827

TED.2006.882265

j.cap.2009.03.011

00414-2

(02)00415-4

10.1063/1.4999626

550-556. DOI: 10.1109/16.992861

DOI: 10.1016/j.jnoncrysol.2007.09.008

DOI: 10.1016/j.jnoncrysol.2008.03.041

#### **Author details**

Vera Gradišnik1 \* and Darko Gumbarević<sup>2</sup>

\*Address all correspondence to: vera.gradisnik@uniri.hr


#### **References**


[8] Izadi MH, Tousignant O, Feuto Mokam M, Karim SK. An a-Si active pixel sensor (APS) array for medical X-ray imaging. IEEE Transactions on Electron Devices. 2010;**57**: 3020-3026. DOI: 10.1109/TED.2010.2069010

**Conflict of interest**

90 Advances in Photodetectors - Research and Applications

**Author details**

Vera Gradišnik1

**References**

10.1109/JSEN. 2017.2751253

10.1016/j.jnoncrysol.2007.10.039

10.1063/1.370202

\* and Darko Gumbarević<sup>2</sup> \*Address all correspondence to: vera.gradisnik@uniri.hr

1 Faculty of Engineering, University of Rijeka, Rijeka, Croatia

Chemistry. 2003;**75**:5300-5305. DOI: 10.1021/ac0301550

2004;**338-340**:729-731. DOI: 10.1109/NSSMIC.2005.1596579

US: SPIE; 1900. pp. 2-14. DOI: 10.1117/12.148585

2 Department of Biotechnology, University of Rijeka, Rijeka, Croatia

paper.

The authors declare that there are no conflict of interests regarding the publication of this

[1] Robbins H, Sumitomo K, Tsujimura N, Kamei T. Integrated thin film Si fluorescence sensor coupled with a GaN microLED for microfluidic point-of-care testing. Journal of Micromechanics and Microengineering. 2018;**28**:024001. DOI: 10.1088/1361-6439/aa9e6d

[2] Santos DR, Soares RRG, Chu V, Conde JP. Performance of hydrogenated amorphous silicon thin film photosensors at Ultra-Low light levels: Towards attomole sensitivities in Lab-on-Chip biosensing applications. IEEE Sensors Journal. 2017;**17**:6895-6903. DOI:

[3] Kamei T, Paegel BM, Scherer JR, Skelley AM, Street RA, Mathies RA. Integrated hydrogenated amorphous Si photodiode detector for microfluidic bioanalytical devices. Analytical

[4] Feenstra KF, Schropp REI, Van der Weg WF. Deposition of amorphous silicon films by hot-wire chemical vapor deposition. Journal of Applied Physics. 1999;**85**:6843-6852. DOI:

[5] Moraes D, Anelli G, Despeisse M, Disertori G, Garrigos A, Jarron P, et al. A novel low noise hydrogenated amorphous silicon pixel detector. Journal of Non-Crystalline Solids.

[6] Yoshida N, Shimizu Y, Honda T, Yokoi T, Nonomura S. A study of absorption coefficient spectra in a-Si:H films near the transition from amorphous to crystalline phase measured by resonant photothermal bending spectroscopy. 2008;**354**:2164-2166. DOI:

[7] Fossum ER. Active pixel sensors: Are CCDs dinosaurs? In: Proceedings of the SPIE 1900 Charge-Coupled Devices and Solid State Optical Sensors III; 12 July 1993. San Jose, CA,


[21] Vanecek M, Kočka J, Poruba A, Fejfar A. Direct measurement of the deep defect density in thin amorphous silicon films with the "absolute" constant photocurrent method. Journal of Applied Physics. 1995;**78**:6203-6210. DOI: 10.1063/1.360566

[33] Čović M, Gradišnik V, Jeričević Ž. The investigation of influence of localized states on a-Si:H p-i-n photodiode transient response to blue light impulse with blue light optical bias. In: Proceedings of the 39th International Convention on Information and Communication Technology, Electronics and Microelectronics (MIPRO 2016); 30 May-3 June 2016; Opatija.

a-Si:H p-i-n Photodiode as a Biosensor http://dx.doi.org/10.5772/intechopen.80503 93

[34] Jericevic Z. Method for Fitting a Sum of Exponentials to Experimental Data by Linearization Using Numerical Integration Approximation, and Its Application to Well Log Data. [US

[35] Kounavis P. Changes in the trapping and recombination process of hydrogenated amorphous silicon in the Staebler–Wronski effect. Journal of Applied Physics. 1995;**77**:

[36] Kleider J-P, Longeaud C, Gueunier M-E. The modulated photocurrent technique: A powerful tool to investigate band gap states in silicon based thin films. Physica Status

[37] Mehrotra P. Biosensors and their applications—A review. Journal of Oral Biology and

[38] Darsanaki RK, Azizzadeh A, Nourbakhsh M, Raeisi G, Aliabadi MA. Biosensors: Functions and applications. Journal of Biology and Today's World. 2013;**2**:53-61. DOI: 10.15412/J.

[39] Ali J, Najeeb J, Ali MA, Aslam MF, Raza A. Biosensors: Their fundamentals, designs, types and most recent impactful applications: A review. Journal of Biosensors and

[40] Vigneshvar S, Sudhakumari CC, Senthilkumaran B, Prakash H. Recent advances in biosensor technology for potential applications—An overview. Frontiers in Biotechnology

[41] Duval D, Lechuga LM. Breakthroughs in Photonics 2012: 2012 Breakthroughs in Lab-on-a-Chip and Optical Biosensors. IEEE Photonics Journal. 2013;**5**. DOI: 10.1109/

[42] Nabi A. Instrumentation for chemiluminescence and bioluminescence assays (Part-I).

[43] Sheraz MA, Kazi SH, Ahmed S, Anwar Z, Ahmad I. Photo, thermal and chemical degradation of riboflavin. Beilstein Journal of Organic Chemistry. 2014;**10**:1999-2012. DOI:

[44] Horecher BL. The absorption spectra of hemoglobin and its derivatives in the visible and

[45] McDonagh AF, Palma LA, Lightner DA. Blue Light and Bilirubine Excretion. Science.

near infra-red regions. Journal of Biological Chemistry. 1943;**148**:173-183

Craniofacial Research. 2016;**6**:153-159. DOI: 10.1016/j.jobcr.2015.12.002

Croatia: IEEE; 2016. pp. 24-27. DOI: 10.1109/MIPRO.2016.7522104

Solidi (C). 2004;**1**:1208-1226. DOI: 10.1002/pssc.200304322

Bioelectronics. 2017;**8**:9. DOI: 10.4172/2155-6210.1000235

Journal Chemical Society of Pakistan. 1991;**13**:90-95

1980;**208**:145-151. DOI: 10.1126/science.7361112

and Bioengenieering. 2016;**4**:11. DOI: 10.3389/fbice.2016.00011

Patent] USP #7,088,097; 2006

JBTW.01020105

JPHOT.2013.2250943

10.3762/bjoc.10.208

3872-3878. DOI: 10.1063/1.358565


[33] Čović M, Gradišnik V, Jeričević Ž. The investigation of influence of localized states on a-Si:H p-i-n photodiode transient response to blue light impulse with blue light optical bias. In: Proceedings of the 39th International Convention on Information and Communication Technology, Electronics and Microelectronics (MIPRO 2016); 30 May-3 June 2016; Opatija. Croatia: IEEE; 2016. pp. 24-27. DOI: 10.1109/MIPRO.2016.7522104

[21] Vanecek M, Kočka J, Poruba A, Fejfar A. Direct measurement of the deep defect density in thin amorphous silicon films with the "absolute" constant photocurrent method.

[22] Melskens J, Smets AHM, Schouten M, Eijt SWH, Schut H, Zeman M. New insights in the nanostructure and defect states of hydrogenated amorphous silicon obtained by annealing. IEEE Journal of Photovoltaics. 2013;**3**:65-71. DOI: 10.1109/JPHOTOV.2012.2226870 [23] Melskens J, Schouten M, Santbergen R, Fischer M, Vasudevan R, vanderVlies DJ, et al. In situ manipulation of the subgap states in hydrogenated amorphous silicon monitored by advanced application of Fourier transform photocurrent spectroscopy. Solar Energy

[24] Jiao L, Liu H, Semoushikina S, Lee Y, Wronski CR. Initial, rapid light-induced changes in hydrogenated amorphous silicon materials and solar cell structures: The effects of

[25] Melskens J, Schouten M, Mannheim A, Vullers AS, Mohammadian Y, Eijt SWH, et al. The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon films and solar cells IEEE. Journal of Photovoltaics. 2014;**6**:1331-1336. DOI:

[26] Zollondz J-H, Reynolds S, Main C, Smirnov V, Zrinscak I. The influence of defects on response speed of high gain two-beam photogating in a-Si:H PIN structures. Journal of Non-Crystalline Solids. 2002;**299-302**:594-598. DOI: 10.1016/S0022-3093(01)01204-2 [27] Pomoni M, Kounavis P. Determination of trapping–detrapping events, recombination processes and gap-state parameters by modulated photocurrent measurements on amorphous silicon. Philosophical Magazine. 2015;**94**:2447-2471. DOI: 10.1080/14786435.2014.

[28] Gradišnik V, Jeričević Ž. The a-Si:H device characteristics degradation upon the light induced defects. In: Book of Abstracts of the 3rd Euroregional Workshop on Photovoltaics & Nanophotonics 2016 (EUROREG PV 2016); 21-23 September 2016; Lubljana. Slovenia:

[29] Han D, Melcher DC, Schiff EA. Optical-bias effects in electron-drift measurements and defect relaxation in a-Si:H. Physical Review B. 1993;**48**:8658-8666. DOI: 10.1103/

[30] Kind R, van Swaaij RAACMM, Rubinelli FA, Solntsev S Zeman M. Thermal ideality factor of hydrogenated amorphous silicon p-i-n solar cells. Journal of Applied Physics.

[31] Deng J, Wronski CR. Carrier recombination and differential diode quality factors in the dark forward bias current-voltage characteristics of aSi:H solar cells. Journal of Applied

[32] Kumar RA, Suresh MS, Nagaraju J. Time domain technique to measure solar cell capacitance. Review of Scientific Instruments. 2003;**74**:3516-3519. DOI: 10.1063/1.1582391

charged defects. Applied Physics Letters. 1996;**69**:3713. DOI: 10.1063/1.117198

Journal of Applied Physics. 1995;**78**:6203-6210. DOI: 10.1063/1.360566

Materials & Solar Cells. 2014;**129**:70-81. DOI: 10.1016/j.solmat.2014.03.022

10.1109/JPHOTOV.2014.2349655

92 Advances in Photodetectors - Research and Applications

LPVO Faculty of Electrical Engineering; 2016. p. 21

2011;**110**:104512. DOI: 10.1063/1.3662924

Physics. 2005;**98**:024509. DOI: 1063/1.1990267

914262

PhysRevB.48.8658


[46] Ahmad HB, Ahmad S, Shad MA, Hussain M. Kinetic measurement of photodecomposition of bilirubin. Asian Journal of Chemistry. 2013;**25**:7945-7948. DOI: 10.14233/ajchem. 2013.14749

**Section 3**

**Technology and Applications**

