*2.2.2 Photovoltaic effect*

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

response, through the phenomenon of polarity switching [27, 28].

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

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

*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* 

**2.2 Different mechanisms of photodetection**

*2.2.1 Photoconductive and photogating effects*

operated in the reverse bias condition.

*which eventually modulates the conductivity of the material.*

mechanisms.

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

**154**

**Figure 1.**

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 detector.

**Figure 2.** *Schematic depicting the photovoltaic effect.*

#### *2.2.3 Photothermoelectric effect*

The origin of photothermoelectric (PTE) effect is the temperature gradient (ΔT) developed due to the thermal effects of the light illumination. Subsequently, a potential gradient (ΔVPTE) is created that serves as the driving force for the transport of the photocurrent through the device (**Figure 3**). The thermoelectric voltage generated is given [32] by

$$
\Delta V\_{\rm PTE} = \text{S} \Delta T \tag{10}
$$

where S is Seebeck coefficient of the material [33]. PTE effect-based PDs can operate without external power, i.e. they are self-powered in nature. However, the thermoelectric potential created is very low, generally in the range of millivolts (mV) and microvolts (μV), which seriously limits the popularization and hence, wide scale applications of PTE-based PDs.

#### *2.2.4 Piezophototronic effect*

The devices that make use of the piezo-potential of the constituent materials for controlling the carrier generation and transport, for improving the overall performance of the opto-electronic devices are referred to as piezophototronic [32]. The basic requirement of such devices is a piezo-electric material such as ZnO, GaN, etc. which can produce an electric potential upon variations in the applied stress. The operational mechanism of a piezophototronic device is based upon the fundamental principles of the conventional Schottky contacts and p–n junctions. Ionic charges are introduced by the effect of piezoelectric polarization, which tune the charge transport at the junction. The effect of piezophototronicity on a p-n junction (GaN/MoS2 in this case) under compressive strain is shown in **Figure 4(a).** When the [0001]-oriented GaN film is under a compressive strain, negative piezopotential is produced inside GaN. This results into the lowering of the junction barrier. Therefore, more photogenerated charge carriers can cross the junction [34]. Hence, the photoresponsivity gets enhanced. An opposite effect (positive piezopotetial in GaN film, which increases the junction barrier) is observed in the case of applied tensile stress.

#### *2.2.5 Photobolometric effect*

Photobolometric effect is the alteration in the electrical resistance of a material, which is induced by the heating effect of uniform light illumination [32]. Typically, the active material layer absorbs the incident photons and then converts them into

#### **Figure 3.**

*(a) Schematic of a PTE based device illuminated locally by focused light. An open circuit voltage which is equal to the thermoelectric voltage* Δ*VPTE gets developed across the electrodes. (b) Thermal circuit equivalent to the device depicted in (a). Figure is reproduced with permission from Ref. [33].*

**157**

**Figure 4.**

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

heat energy. The extent of this effect is proportional to the conductance change in the photoactive material with temperature (dG/dT) as well as the homogeneous temperature change (ΔT) induced by light illumination (**Figure 4(b)**). The change in conductance is influenced by change in the charge carrier mobility of the material because of the induced temperature change. Photobolometric effect generally occurs in the wavelength range of mid-infrared to far-infrared. Additionally, similar to PDs based on photoconductive effect, photobolometric PDs also require an external power source, which differentiates them from the PTE-based PDs.

*(a) Piezophototronic enhancement observed in p-GaN/n-MoS2 heterojunction. When the [0001]-oriented GaN film is compressed, negative piezoelectric charges develop in the GaN film near the interface, and the junction barrier gets lowered. This allows a greater number of carriers to pass through the junction. Figure is reproduced with permission from Ref. [34]. (b) Schematic depicting bolometric effect. The red shaded region indicates elevated temperature with the temperature gradient (*Δ*T) and* Δ*R denotes the resistance change across the* 

From the early decades, silicon has been considered as one of the major components in the semiconductor industry because of its unique properties. Later, III-V materials, particularly the arsenic based compounds, gained much attention because of their superior properties such as high electron mobility, direct and tunable band gap, etc. The group III-Nitrides came into the picture around 1960s, and active research on this material system started with the development of blue light emitting diodes. Over the time, much of the attention of the researchers and scientists has been diverted on different classes of materials. However, group III-Nitride semiconductors continue to maintain their stronghold due to the

Group III-Nitride semiconductors, mainly comprising of AlN, GaN and InN, exist in three different crystal structures, namely, wurtzite, zincblende and rocksalt. Among these, the most stable and the lowest energy structure is the wurtzite system [36]. The wurtzite structure has a hexagonal unit cell and contains six atoms of each type, with space group P63mc. The wurtzite polytype is made up of two interpenetrating hexagonal close-packed unit cells, each containing one type

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

**3. Group III-Nitride semiconductors**

*channel. Figure is adapted from Ref. [35].*

exceptional properties and the unique advantages they offer.

**3.1 Crystal structure and optical properties of group III-Nitrides**

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

#### **Figure 4.**

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

The origin of photothermoelectric (PTE) effect is the temperature gradient (ΔT) developed due to the thermal effects of the light illumination. Subsequently, a potential gradient (ΔVPTE) is created that serves as the driving force for the transport of the photocurrent through the device (**Figure 3**). The thermoelectric voltage

where S is Seebeck coefficient of the material [33]. PTE effect-based PDs can operate without external power, i.e. they are self-powered in nature. However, the thermoelectric potential created is very low, generally in the range of millivolts (mV) and microvolts (μV), which seriously limits the popularization and hence,

The devices that make use of the piezo-potential of the constituent materials for controlling the carrier generation and transport, for improving the overall performance of the opto-electronic devices are referred to as piezophototronic [32]. The basic requirement of such devices is a piezo-electric material such as ZnO, GaN, etc. which can produce an electric potential upon variations in the applied stress. The operational mechanism of a piezophototronic device is based upon the fundamental principles of the conventional Schottky contacts and p–n junctions. Ionic charges are introduced by the effect of piezoelectric polarization, which tune the charge transport at the junction. The effect of piezophototronicity on a p-n junction (GaN/MoS2 in this case) under compressive strain is shown in **Figure 4(a).** When the [0001]-oriented GaN film is under a compressive strain, negative piezopotential is produced inside GaN. This results into the lowering of the junction barrier. Therefore, more photogenerated charge carriers can cross the junction [34]. Hence, the photoresponsivity gets enhanced. An opposite effect (positive piezopotetial in GaN film, which increases the junction barrier) is

Photobolometric effect is the alteration in the electrical resistance of a material, which is induced by the heating effect of uniform light illumination [32]. Typically, the active material layer absorbs the incident photons and then converts them into

*(a) Schematic of a PTE based device illuminated locally by focused light. An open circuit voltage which is equal to the thermoelectric voltage* Δ*VPTE gets developed across the electrodes. (b) Thermal circuit equivalent to the* 

*device depicted in (a). Figure is reproduced with permission from Ref. [33].*

∆ =∆ *V ST PTE* (10)

*2.2.3 Photothermoelectric effect*

generated is given [32] by

*2.2.4 Piezophototronic effect*

wide scale applications of PTE-based PDs.

observed in the case of applied tensile stress.

*2.2.5 Photobolometric effect*

**156**

**Figure 3.**

*(a) Piezophototronic enhancement observed in p-GaN/n-MoS2 heterojunction. When the [0001]-oriented GaN film is compressed, negative piezoelectric charges develop in the GaN film near the interface, and the junction barrier gets lowered. This allows a greater number of carriers to pass through the junction. Figure is reproduced with permission from Ref. [34]. (b) Schematic depicting bolometric effect. The red shaded region indicates elevated temperature with the temperature gradient (*Δ*T) and* Δ*R denotes the resistance change across the channel. Figure is adapted from Ref. [35].*

heat energy. The extent of this effect is proportional to the conductance change in the photoactive material with temperature (dG/dT) as well as the homogeneous temperature change (ΔT) induced by light illumination (**Figure 4(b)**). The change in conductance is influenced by change in the charge carrier mobility of the material because of the induced temperature change. Photobolometric effect generally occurs in the wavelength range of mid-infrared to far-infrared. Additionally, similar to PDs based on photoconductive effect, photobolometric PDs also require an external power source, which differentiates them from the PTE-based PDs.

## **3. Group III-Nitride semiconductors**

From the early decades, silicon has been considered as one of the major components in the semiconductor industry because of its unique properties. Later, III-V materials, particularly the arsenic based compounds, gained much attention because of their superior properties such as high electron mobility, direct and tunable band gap, etc. The group III-Nitrides came into the picture around 1960s, and active research on this material system started with the development of blue light emitting diodes. Over the time, much of the attention of the researchers and scientists has been diverted on different classes of materials. However, group III-Nitride semiconductors continue to maintain their stronghold due to the exceptional properties and the unique advantages they offer.

#### **3.1 Crystal structure and optical properties of group III-Nitrides**

Group III-Nitride semiconductors, mainly comprising of AlN, GaN and InN, exist in three different crystal structures, namely, wurtzite, zincblende and rocksalt. Among these, the most stable and the lowest energy structure is the wurtzite system [36]. The wurtzite structure has a hexagonal unit cell and contains six atoms of each type, with space group P63mc. The wurtzite polytype is made up of two interpenetrating hexagonal close-packed unit cells, each containing one type

of atom, and with an offset along the c-axis by 5/8 of the cell height. The stacking sequence in the hexagonal structure consists of alternating hexagonal planes of group III and N atom, with a stacking sequence of ABAB [36]. A stick-and-ball model-based diagram of the hexagonal unit cell of III-Nitride semiconductors is shown in **Figure 5**. The group III semiconductors and the nitrogen atoms have been shown in different colors [37]. The polytypes of the III-Nitrides having wurtzite structure, form a continuous alloy system, with direct band gaps ranging from 0.7 eV for InN, 3.4 eV for GaN, and to 6.2 eV for AIN [16]. Therefore, the III-Nitrides are potential candidates for fabrication of optical devices which are active at wavelengths ranging from the NIR to the UV.
