**1. Introduction**

Photodetectors (PDs) are the photonic devices that convert an incoming light signal into an output electrical signal. High-performance PDs are crucial for the advancements in the industrial and scientific communities, and these are being extensively used in areas such as video imaging, space and optical communications, flame detection, photovoltaic applications, environmental monitoring, and so on [1–5]. Some of the most extensively employed inorganic semiconductors for the development of PDs are zinc oxide (ZnO) [6], gallium arsenide (GaAs) [7], indium gallium antimonide (InGaSb) [8], germanium silicide (GeSi) [9] and gallium oxide (Ga2O3) [10], due to their excellent electronic properties like high charge carrier mobility and high light absorption coefficients. However, large exciton binding energies, low values of responsivity, slower response and narrow-band detection are some of the major downsides associated with these materials [11]. Moreover, the synthesis as well as post-processing steps in the fabrication of antimony and arsenic-based devices involves toxic precursors as well as products, which are extremely hazardous for human health and for the environment as well. Thus, the quest is on for the development of high-performance PDs consisting of environment-friendly constituents.

The recent developments in the III-Nitride semiconductors-based devices have made a tremendous impact upon a number of technological areas such as information storage, lighting and full color displays, underwater and space communications, high-power and high-frequency electronic devices, photovoltaics, sensors and detectors, and so on [12–15]. The wurtzite polytypes of indium nitride (InN), aluminum nitride (AlN), and gallium nitride (GaN) have proved to be excellent semiconductors for band gap engineering, due to formation of continuous range of alloys with direct and tunable band gaps in the range of 0.7 to 6.2 eV [16]. Therefore, their intrinsic physical and chemical properties along with intense technological efforts have made the realization of versatile and reliable detectors in the entire ultraviolet (UV)-visible–near infrared (NIR) spectrum.

In spite of the significant progresses, the growth of epitaxial group III-Nitride thin films for practical devices, having low defect densities, has always been a challenge due to the lack of availability of lattice matched substrates. As of now, among various epitaxial growth techniques for III-Nitrides, plasma assisted molecular beam epitaxy (PAMBE) has emerged as the most versatile and environmentfriendly synthesis technique, involving low growth temperature, controlled growth rates which result into the formation of heterostructures with sharp interfaces, and non-hazardous precursors and by-products. Since pristine III-Nitride substrates are immensely costly and therefore, not yet available for research and industrial purposes, the development of this family of semiconductors proceeds entirely by the heteroepitaxial growth on various foreign substrates such as sapphire, Si (111) and 6H-SiC [16–18], and one has to often compromise with the device performance. It may be noted here that hybrid structures of III-Nitrides with other semiconductors such as ZnO, perovskites, and two-dimensional (2D) layered materials such as MoS2 are expected to deliver competitive device performance as compared to the commercial PDs ( [19–21]. In the forthcoming sections, we discuss about the figures of merit involved in the evaluation of a PD, followed by a brief overview of III-Nitrides and their properties. Then, a detailed analysis about the recent advancements in the MBE-grown III-Nitrides for photodetection application has been presented and finally, we wind up by discussing the unresolved difficulties and propose an outlook in this developing field of optoelectronics.

#### **2. Basic concepts about photodetection**

A PD is a sensor that detects an incoming electromagnetic radiation. Whenever light waves of energy greater than or equal to the band gap of a semiconductor are absorbed, there is an overall change in the conductivity of the semiconductor. Thus, PD is a device developed on the principle of this photoconducting effect and it quantifies the speed and the amplitude of the change in the conductivity, in respect of incoming electromagnetic radiations.

#### **2.1 Important figures of merit of a PD**

Several figures of merit and parameters are used for the evaluation of a PD, and these performance metrics allow us to compare various devices. Mostly, the output

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*Group III-Nitrides and Their Hybrid Structures for Next-Generation Photodetectors*

output photocurrent. The output photocurrent ( *PI* ) is defined [11] as

enhanced carrier separation as well as lower dark current.

*2.1.1 Responsivity*

Responsivity ( *R*

the metal contacts.

responsivity ( *R*

following relation:

*2.1.2 Specific detectivity*

λ

λ

given [11] by the following mathematical formula

electrical signal is recorded from the device and realized in the form of an output photocurrent, and thus, all the key figures of merit are defined in terms of this

where *Illuminated I* is the output current detected upon light illumination and *Dark I* is the dark current in the device (i.e. without any light illumination). Photocurrent can be enhanced by reducing the probability of recombination of photogenerated electrons and holes, and this can be achieved by fabricating high-quality and defect-free devices, using heterojunctions with type-II band alignment which result into the effective separation of charge carriers. Devices utilizing p-n junctions which operate in reverse bias, usually show a large photocurrent due to the

incident optical power of a specific wavelength on a predefined device area. It is

λ

<sup>=</sup> <sup>×</sup> *PI <sup>R</sup> P A*

where P is the incident power density of light and A is the device area where light is being illuminated. Its units are AW−1. It quantifies the extent of achievable electrical signal in a PD when illuminated by a light of certain power density. Thus, a larger responsivity signifies a larger electrical output signal for a specific optical excitation power. Replacing a single material-based or homojunction-based device with heterojunction-based devices with type-II band alignment is an effective way to enhance the responsivity due to minimized recombination of photogenerated electrons and holes [22]. Another approach is to promote devices which possess a strong built-in potential that supports the external electric field to enhance the carrier separation and transport [15]. Growth of high-quality crystals reduces the defect density present in the system, thereby suppressing the scattering and recombination of the charge carriers which enhances the responsivity of the device. Manohar et al*.* [6] recently demonstrated a highly cost effective and an elegant method to immensely enhance the photoresponsivity of a PD by suppressing the carrier recombination through coatings of different materials which facilitate either electron or hole transfer to

A fundamental performance parameter for a PD is its specific detectivity, that measures the ability of a PD to detect weak light signals. For a PD, this metric can be defined in terms of the noise equivalent power (NEP). The NEP of a PD is proportional to the ratio of the square root of the dark current ( *Dark I* ) to the

) of the PD at a given wavelength. It can be defined by the

√ ∗∗ (<sup>2</sup> ) <sup>=</sup> *Dark e I*

*R*λ (3)

*NEP*

*II I P Illuminated Dark* = − (1)

) is defined as the ratio of the photocurrent generated and the

(2)

*DOI: http://dx.doi.org/10.5772/intechopen.95389*

electrical signal is recorded from the device and realized in the form of an output photocurrent, and thus, all the key figures of merit are defined in terms of this output photocurrent. The output photocurrent ( *PI* ) is defined [11] as

$$I\_p = I\_{\text{Illumination}} - I\_{\text{Dark}} \tag{1}$$

where *Illuminated I* is the output current detected upon light illumination and *Dark I* is the dark current in the device (i.e. without any light illumination). Photocurrent can be enhanced by reducing the probability of recombination of photogenerated electrons and holes, and this can be achieved by fabricating high-quality and defect-free devices, using heterojunctions with type-II band alignment which result into the effective separation of charge carriers. Devices utilizing p-n junctions which operate in reverse bias, usually show a large photocurrent due to the enhanced carrier separation as well as lower dark current.

## *2.1.1 Responsivity*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

the entire ultraviolet (UV)-visible–near infrared (NIR) spectrum.

in this developing field of optoelectronics.

**2. Basic concepts about photodetection**

of incoming electromagnetic radiations.

**2.1 Important figures of merit of a PD**

In spite of the significant progresses, the growth of epitaxial group III-Nitride thin films for practical devices, having low defect densities, has always been a challenge due to the lack of availability of lattice matched substrates. As of now, among various epitaxial growth techniques for III-Nitrides, plasma assisted molecular beam epitaxy (PAMBE) has emerged as the most versatile and environmentfriendly synthesis technique, involving low growth temperature, controlled growth rates which result into the formation of heterostructures with sharp interfaces, and non-hazardous precursors and by-products. Since pristine III-Nitride substrates are immensely costly and therefore, not yet available for research and industrial purposes, the development of this family of semiconductors proceeds entirely by the heteroepitaxial growth on various foreign substrates such as sapphire, Si (111) and 6H-SiC [16–18], and one has to often compromise with the device performance. It may be noted here that hybrid structures of III-Nitrides with other semiconductors such as ZnO, perovskites, and two-dimensional (2D) layered materials such as MoS2 are expected to deliver competitive device performance as compared to the commercial PDs ( [19–21]. In the forthcoming sections, we discuss about the figures of merit involved in the evaluation of a PD, followed by a brief overview of III-Nitrides and their properties. Then, a detailed analysis about the recent advancements in the MBE-grown III-Nitrides for photodetection application has been presented and finally, we wind up by discussing the unresolved difficulties and propose an outlook

A PD is a sensor that detects an incoming electromagnetic radiation. Whenever light waves of energy greater than or equal to the band gap of a semiconductor are absorbed, there is an overall change in the conductivity of the semiconductor. Thus, PD is a device developed on the principle of this photoconducting effect and it quantifies the speed and the amplitude of the change in the conductivity, in respect

Several figures of merit and parameters are used for the evaluation of a PD, and these performance metrics allow us to compare various devices. Mostly, the output

of the major downsides associated with these materials [11]. Moreover, the synthesis as well as post-processing steps in the fabrication of antimony and arsenic-based devices involves toxic precursors as well as products, which are extremely hazardous for human health and for the environment as well. Thus, the quest is on for the development of high-performance PDs consisting of environment-friendly constituents. The recent developments in the III-Nitride semiconductors-based devices have made a tremendous impact upon a number of technological areas such as information storage, lighting and full color displays, underwater and space communications, high-power and high-frequency electronic devices, photovoltaics, sensors and detectors, and so on [12–15]. The wurtzite polytypes of indium nitride (InN), aluminum nitride (AlN), and gallium nitride (GaN) have proved to be excellent semiconductors for band gap engineering, due to formation of continuous range of alloys with direct and tunable band gaps in the range of 0.7 to 6.2 eV [16]. Therefore, their intrinsic physical and chemical properties along with intense technological efforts have made the realization of versatile and reliable detectors in

**150**

Responsivity ( *R*λ ) is defined as the ratio of the photocurrent generated and the incident optical power of a specific wavelength on a predefined device area. It is given [11] by the following mathematical formula

$$R\_{\lambda} = \frac{I\_p}{P \times A} \tag{2}$$

where P is the incident power density of light and A is the device area where light is being illuminated. Its units are AW−1. It quantifies the extent of achievable electrical signal in a PD when illuminated by a light of certain power density. Thus, a larger responsivity signifies a larger electrical output signal for a specific optical excitation power. Replacing a single material-based or homojunction-based device with heterojunction-based devices with type-II band alignment is an effective way to enhance the responsivity due to minimized recombination of photogenerated electrons and holes [22]. Another approach is to promote devices which possess a strong built-in potential that supports the external electric field to enhance the carrier separation and transport [15]. Growth of high-quality crystals reduces the defect density present in the system, thereby suppressing the scattering and recombination of the charge carriers which enhances the responsivity of the device. Manohar et al*.* [6] recently demonstrated a highly cost effective and an elegant method to immensely enhance the photoresponsivity of a PD by suppressing the carrier recombination through coatings of different materials which facilitate either electron or hole transfer to the metal contacts.

#### *2.1.2 Specific detectivity*

A fundamental performance parameter for a PD is its specific detectivity, that measures the ability of a PD to detect weak light signals. For a PD, this metric can be defined in terms of the noise equivalent power (NEP). The NEP of a PD is proportional to the ratio of the square root of the dark current ( *Dark I* ) to the responsivity ( *R*λ ) of the PD at a given wavelength. It can be defined by the following relation:

$$NEP = \frac{\sqrt{\left(2 \ast e \ast I\_{Dark}\right)}}{R\_z} \tag{3}$$

where *e* is the electronic charge. The specific detectivity ( <sup>∗</sup> *D* ) of a PD is defined as the ratio of the square root of the active area (A) to its NEP [4], and is given by the following equation

$$D^\* = \frac{\sqrt{A}}{NEP} \tag{4}$$

It is measured in Jones. Higher detectivity of a PD indicates that even a very weak signal can be detected. It may be noted that lower is the dark current, higher is the detectivity. PDs based on p-n junctions operated in the reverse bias generally exhibit very high specific detectivities because of low values of dark current.

#### *2.1.3 Internal gain*

The internal gain ( *G* ) of a PD refers to the number of electrons collected at the electrodes per incident photon. It can be determined [4] by the following relation

$$\mathbf{G} = \frac{hc \ast R\_{\lambda}}{\eta e \lambda} \tag{5}$$

where *h* , *c* , η and λ are the Planck's constant, the velocity of light in vacuum, the EQE of the device and the illumination wavelength, respectively.

In other words, internal gain is the ratio of the hole carrier lifetime to the electron transit time and is given [4, 10] by

$$\mathbf{G} = \mathbf{\pi}/\mathbf{t} \tag{6}$$

where τ is the mean hole carrier lifetime and t is the electron transit time. So, high internal gains can be achieved by fabricating devices which exhibit high responsivities. Another approach to enhance gain is by trapping one kind of charge carrier (generally holes), so as to prevent it from recombination. Thus, the carrier lifetime of the holes increases and in turn gain increases.

#### *2.1.4 Sensitivity*

Sensitivity ( *S* ) of a PD is defined [23] as the ratio of the photocurrent to the dark current.

$$\mathbf{S} = \frac{I\_P}{I\_{Dark}} \tag{7}$$

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*Group III-Nitrides and Their Hybrid Structures for Next-Generation Photodetectors*

ties of the constituent semiconductors of the PD. The transit time ( ) *transit*

τ

which in turn decreases the transit time in accordance with Eq. (8).

τ

tion processes, and result into the lowering of the carrier transit time.

constant. The rise time of a PD with an RC circuit is given by:

these time constants, faster is the detection process. Mukhokosi et al*.* [23] have demonstrated that the response times of a PD are controlled by the carrier mobili-

*transit* = =

*drift drift o W W v E*

µ

Other parameters that affect the response speed of a PD are the junction capacitance (C) and the series load resistance (R), and thus, the corresponding RC time

Therefore, decreasing the junction capacitance can further improve the response

In addition, defect-free semiconductors exhibit faster response times, because of reduced recombination of the photogenerated carriers. For the case of linear devices, high-quality photosensitive materials as well as defect-free semiconductor/ electrode interfaces are required to minimize the carrier scattering and recombina-

One of the most critical parameters associated with a PD is its power consump-

Another crucial aspect of PDs is their detection range. PDs are generally classified into two categories based on their spectral range of detection: broad-band (which shows a considerable detection to a wide range of wavelengths) and narrowband or wavelength-selective (whose detection range is very limited) PDs. The most common way to fabricate a wavelength-selective PD is by using a single semiconductor (of desired band gap) as the active material in the device. Another way to achieve wavelength selectivity is by integrating an optical microcavity within the PD. This optical microcavity consists of distributed Bragg reflectors (DBR), which allow multiple reflections of a specific wavelength. These reflections interfere

tion. Generally, PDs require an external power source as the driving force to separate the photogenerated electrons and holes efficiently. Thus, a lot of external energy is required in a system that consists of several such detectors. Therefore, it is a big concern in the present energy scenario. Hence, a lot of research is being focussed these days on achieving self-powered or zero-biased PDs [24, 25], which utilize a built-in electric potential for the effective charge carrier separation. The extensively used technique for fabricating self-powered PDs is by utilizing a p-n junction, where a strong electric field is created at the interface, and therefore, the

time. The junction capacitance depends on the width of the depletion layer, and smaller depletion widths increase the capacitance. Therefore, the width of the depletion region should be optimum to minimize the carrier transit time.

built-in electric field at the junction. Therefore, high-quality substrates with high carrier mobilities such as p-Si, usually result into very small response time. The depletion width can be narrowed down by fabricating highly doped p-n junctions,

µ

τ

*drift* is the carrier mobility and *Eo* is the

*<sup>r</sup>* ≅ 2.2*RC* (9)

of a p-n

(8)

*DOI: http://dx.doi.org/10.5772/intechopen.95389*

heterojunction is given [23] by the following relation

where*W* is the depletion region width,

*2.1.6 Power consumption*

*2.1.7 Spectral range*

device can operate in zero-bias mode.

Hence, sensitivity can be improved by either enhancing the value of photocurrent or lowering the dark current. The enhanced photocurrent can be achieved by fabricating high quality and defect-free interfaces and devices, thus, reducing the scattering effects of the photogenerated electrons and holes, and leading to higher photocurrents. Devices consisting of p-n junctions show ultralow values of dark currents exhibiting higher sensitivities.

#### *2.1.5 Response/recovery time*

An important aspect of a PD is how fast it detects the incident light, and how fast it comes back to its initial state once the incident light is removed. This is quantified by estimating the response/recovery times of the PD. Lower the values of *Group III-Nitrides and Their Hybrid Structures for Next-Generation Photodetectors DOI: http://dx.doi.org/10.5772/intechopen.95389*

these time constants, faster is the detection process. Mukhokosi et al*.* [23] have demonstrated that the response times of a PD are controlled by the carrier mobilities of the constituent semiconductors of the PD. The transit time ( ) *transit* τ of a p-n heterojunction is given [23] by the following relation

$$
\sigma\_{\text{manit}} = \frac{\mathcal{W}}{\upsilon\_{\text{dry}t}} = \frac{\mathcal{W}}{\mu\_{\text{dry}t} E\_o} \tag{8}
$$

where*W* is the depletion region width, µ*drift* is the carrier mobility and *Eo* is the built-in electric field at the junction. Therefore, high-quality substrates with high carrier mobilities such as p-Si, usually result into very small response time. The depletion width can be narrowed down by fabricating highly doped p-n junctions, which in turn decreases the transit time in accordance with Eq. (8).

Other parameters that affect the response speed of a PD are the junction capacitance (C) and the series load resistance (R), and thus, the corresponding RC time constant. The rise time of a PD with an RC circuit is given by:

$$
\pi\_r \cong 2.2RC \tag{9}
$$

Therefore, decreasing the junction capacitance can further improve the response time. The junction capacitance depends on the width of the depletion layer, and smaller depletion widths increase the capacitance. Therefore, the width of the depletion region should be optimum to minimize the carrier transit time.

In addition, defect-free semiconductors exhibit faster response times, because of reduced recombination of the photogenerated carriers. For the case of linear devices, high-quality photosensitive materials as well as defect-free semiconductor/ electrode interfaces are required to minimize the carrier scattering and recombination processes, and result into the lowering of the carrier transit time.

#### *2.1.6 Power consumption*

*Light-Emitting Diodes and Photodetectors - Advances and Future Directions*

given by the following equation

*2.1.3 Internal gain*

where *h* , *c* ,

*2.1.4 Sensitivity*

dark current.

η and λ

electron transit time and is given [4, 10] by

currents exhibiting higher sensitivities.

*2.1.5 Response/recovery time*

lifetime of the holes increases and in turn gain increases.

where *e* is the electronic charge. The specific detectivity ( <sup>∗</sup> *D* ) of a PD is defined as the ratio of the square root of the active area (A) to its NEP [4], and is

<sup>∗</sup> <sup>√</sup> <sup>=</sup> *<sup>A</sup> <sup>D</sup>*

It is measured in Jones. Higher detectivity of a PD indicates that even a very weak signal can be detected. It may be noted that lower is the dark current, higher is the detectivity. PDs based on p-n junctions operated in the reverse bias generally exhibit very high specific detectivities because of low values of dark current.

The internal gain ( *G* ) of a PD refers to the number of electrons collected at the electrodes per incident photon. It can be determined [4] by the following relation

> <sup>∗</sup> <sup>=</sup> *hc R <sup>G</sup> e* λ

In other words, internal gain is the ratio of the hole carrier lifetime to the

*G* = τ/t (6)

where τ is the mean hole carrier lifetime and t is the electron transit time. So, high internal gains can be achieved by fabricating devices which exhibit high responsivities. Another approach to enhance gain is by trapping one kind of charge carrier (generally holes), so as to prevent it from recombination. Thus, the carrier

Sensitivity ( *S* ) of a PD is defined [23] as the ratio of the photocurrent to the

= *<sup>P</sup> Dark*

Hence, sensitivity can be improved by either enhancing the value of photocurrent or lowering the dark current. The enhanced photocurrent can be achieved by fabricating high quality and defect-free interfaces and devices, thus, reducing the scattering effects of the photogenerated electrons and holes, and leading to higher photocurrents. Devices consisting of p-n junctions show ultralow values of dark

An important aspect of a PD is how fast it detects the incident light, and how fast it comes back to its initial state once the incident light is removed. This is quantified by estimating the response/recovery times of the PD. Lower the values of

*<sup>I</sup> <sup>S</sup>*

the EQE of the device and the illumination wavelength, respectively.

η λ

*NEP* (4)

(5)

*<sup>I</sup>* (7)

are the Planck's constant, the velocity of light in vacuum,

**152**

One of the most critical parameters associated with a PD is its power consumption. Generally, PDs require an external power source as the driving force to separate the photogenerated electrons and holes efficiently. Thus, a lot of external energy is required in a system that consists of several such detectors. Therefore, it is a big concern in the present energy scenario. Hence, a lot of research is being focussed these days on achieving self-powered or zero-biased PDs [24, 25], which utilize a built-in electric potential for the effective charge carrier separation. The extensively used technique for fabricating self-powered PDs is by utilizing a p-n junction, where a strong electric field is created at the interface, and therefore, the device can operate in zero-bias mode.

#### *2.1.7 Spectral range*

Another crucial aspect of PDs is their detection range. PDs are generally classified into two categories based on their spectral range of detection: broad-band (which shows a considerable detection to a wide range of wavelengths) and narrowband or wavelength-selective (whose detection range is very limited) PDs. The most common way to fabricate a wavelength-selective PD is by using a single semiconductor (of desired band gap) as the active material in the device. Another way to achieve wavelength selectivity is by integrating an optical microcavity within the PD. This optical microcavity consists of distributed Bragg reflectors (DBR), which allow multiple reflections of a specific wavelength. These reflections interfere

constructively, leading to increased light-matter interactions and result into the enhancement of photocurrent. This occurs only at the design wavelength of the cavity, whereas all the off-resonance wavelengths incident on the PD are rejected by the cavity. On the other hand, broad-band photodetection can be realized by making heterojunctions-based devices consisting of different semiconductors of appropriate band gaps, as demonstrated by Singh et al. [26] by making a hybrid device based on SnS2/p-Si heterojunction, which shows a broad-band response in the entire ultraviolet to near infrared range. Usually, conventional PDs suffer from the tradeoff between selective and broad-band detection. Recently, a few devices have been reported to exhibit a unique feature wavelength-selectivity in a broad-band spectral response, through the phenomenon of polarity switching [27, 28].

### **2.2 Different mechanisms of photodetection**

PDs rely on distinct sensing mechanisms depending upon the intrinsic properties of the photosensitive materials used as well as the architecture of the device structures. We now briefly discuss the most commonly adopted sensing mechanisms.

#### *2.2.1 Photoconductive and photogating effects*

The photoconductive effect involves photogeneration of excess free carriers in a semiconductor material, when photons with energy higher than the band gap of the semiconductor are absorbed, which eventually leads to a change in its electrical conductivity. The photoconductive effect is the most primitive form of photodetection, where two Ohmic contacts are deposited on the semiconductor surface to form a metal–semiconductor–metal (MSM) type linear device configuration [29]. The schematic of a PD depicting the process of photoconduction is shown in **Figure 1(a)**. It is important to note that this change in conductivity is a result of the change in the free charge carrier concentration due to the photogeneration process. Thus, the effective spectral range of photoconductive PDs is limited by the band gap of the semiconducting material or the photoactive layer. Additionally, a photoconductive PD usually requires an externally applied voltage for the effective separation and the directional propagation of the photogenerated carriers. This generally results in relatively larger values of the dark current, which leads to a lower on/off ratio and higher energy consumption. Additionally, conventional photodiodes (non-linear devices) also work in the photoconductive mode when operated in the reverse bias condition.

#### **Figure 1.**

*Schematic view of (a) a photoconductor depicting the process of electron–hole pairs generation and their transport and (b) a PD based on the photogating effect under illumination. The defect states capture electrons which eventually modulates the conductivity of the material.*

**155**

**Figure 2.**

*Schematic depicting the photovoltaic effect.*

detector.

*Group III-Nitrides and Their Hybrid Structures for Next-Generation Photodetectors*

Photogating effect is a type of photoconducting effect, wherein certain localized states such as defects or surface states exist within the forbidden gap of the semiconductor. The effect normally originates when the photogenerated charge carriers are captured by the localized trapping states present within the system [30] as depicted in **Figure 1(b)**. This effect is pronounced in low-dimensional material systems such as 2D layered materials and quantum dots, which have very large surface-to-volume ratio and less screening in the z-direction. Since one type of charge carriers are trapped, it leads to prolonged carrier lifetimes, and therefore, PDs which exhibit the photogating effect usually show higher photogains. In addition, faster transit times and enhanced photocurrent is observed due to suppressed

Photovoltaic effect is the phenomenon of spontaneous generation of a photocurrent upon light illumination in a PD. This effect is generally realized in p-n junction-based devices, where a built-in electric potential exists at the interface [31]. Upon light illumination, electrons and holes are created near the semiconductor interface. Under the influence of the existing built-in voltage, these electrons and holes get separated, thereby, causing a photocurrent to flow along the direction of this built-in potential (**Figure 2**). The photovoltaic effect is closely related to the photoelectric effect and therefore, the effective wavelength range is usually limited by the band gap of the constituent photosensitive material. However, in the case of p-n heterojunctions, due to the intimate energy band coupling that enables interband transition between different semiconductors, the effective detection range can be modulated beyond the limitation of the band gaps. The biggest advantage associated with PDs based on the photovoltaic effect is that due to the intrinsic built-in electric potential, the PDs do not require any external power for their operation [11]. Furthermore, the photovoltaic PDs possess low dark current under the zero-biased working mode, which is beneficial for the

*DOI: http://dx.doi.org/10.5772/intechopen.95389*

carrier recombination.

*2.2.2 Photovoltaic effect*

#### *Group III-Nitrides and Their Hybrid Structures for Next-Generation Photodetectors DOI: http://dx.doi.org/10.5772/intechopen.95389*

Photogating effect is a type of photoconducting effect, wherein certain localized states such as defects or surface states exist within the forbidden gap of the semiconductor. The effect normally originates when the photogenerated charge carriers are captured by the localized trapping states present within the system [30] as depicted in **Figure 1(b)**. This effect is pronounced in low-dimensional material systems such as 2D layered materials and quantum dots, which have very large surface-to-volume ratio and less screening in the z-direction. Since one type of charge carriers are trapped, it leads to prolonged carrier lifetimes, and therefore, PDs which exhibit the photogating effect usually show higher photogains. In addition, faster transit times and enhanced photocurrent is observed due to suppressed carrier recombination.
