**4. Recent advancements in the MBE grown III-Nitrides-based PDs**

In the preceding sections, we have focused on the important figures of merit of PDs along with the different mechanisms of photodetection, and the properties and growth methods for group III-Nitride-based devices. Numerous reports exist which demonstrate the photodetection properties of III-Nitrides-based PDs. The recent state-of-the-art III-Nitride-based PDs have been discussed wherein a special emphasis on self-powered photodetection has been given.

## **4.1 III-Nitrides-based devices**

Some early accomplishments in the field of GaN-based PDs grown by MBE have been achieved by several researchers such as Van Hove et al. [41], Son et al. [42], Torvik et al. [43], Osinsky et al. [44], Xu et al. [45], and so on. In 2005, Calarco et al. [46] reported the electrical transport of GaN nanowhiskers grown by MBE, in dark and under UV illumination. The photoresponse has been found to be sensitively dependent on the column diameter of the nanowhiskers and this effect has been quantitatively described through a mechanism of size dependent surface recombination. Jain et al. [47] have shown the effect of symmetric and asymmetric contact electrodes on *c*-GaN/sapphire based UV PD. In 2018, Goswami et al*.* [48] reported the growth of self-assembled GaN nanostructures on Si(111) for applications in UV photodetection (**Figure 7**). The device exhibited a responsivity of 5.7 mAW−1 at a bias of 1 V. Numerous other reports exist demonstrating the photodetection properties of GaN-based PDs.

In the meantime, researchers have also started exploring InN-based devices. One such work has been carried out by Shetty et al. [49]. They have grown InN quantum dots of varying densities on Si substrates using MBE. The device shows a strong response towards infrared illumination. The photoresponse studies revealed that

**161**

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

the increasing dot density results in the improvement in the sensitivity of the PD. The increase in the photocurrent with the density of the quantum dots has been attributed to the increase in the number of photogenerated carriers in InN, which add up with the carriers generated in Si upon light illumination. The results have also been validated using simulations and it has been observed that the experimen-

*(a) Schematic of the fabricated MSM device and (b) trend of photocurrent variation with respect to the applied bias. Inset shows the optical image of the actual PD. Figure has been reprinted with permission* 

In 2018, the mechanism of the higher device performance parameters of the non-polar GaN has been explained by Pant et al*.* [18] by performing azimuth angledependent photodetection. They have shown the non-uniformity in the defects present along the different azimuth directions. This is a consequence of the asymmetry in the strain between the substrate and film, as the lattice mismatch is asymmetric along various in-plane crystal directions. The mismatch in the lattice constants along the [0002] direction is ∼1% whereas, along the [1–100] direction, it is ∼13%. This induces a large number of defects along the [1–100] direction as compared to the [0002] azimuth direction. **Figure 8(a)** shows the *a*-GaN-based device used in this study. Furthermore, it has been shown in **Figure 8(b)** that the overall photocurrent in the UV region is also dependent on the different azimuth angles. A maximum responsivity of ~1.9 AW−1 and ~ 13.0 AW−1 have been obtained at a bias of 1 V and 5 V, respectively. These results underlined the importance of aligning the contact electrodes along the favorable azimuth direction in order to restrict the transport of the charge carriers. In a subsequent work, Pant et al*.* [51] have further shown

tal as well as theoretical results have sufficient agreement between them. However, the epitaxial growth of high-quality III-Nitrides has been always hindered by the lack of lattice-matched substrates, that hinders the development of high-performance devices. In all the above-mentioned reports, the growth has been accomplished on substrates (*c*-plane sapphire, Si(111), etc) which promote the *c*-plane oriented growth of the III-Nitrides i.e., in the polar direction. Moreover, the structures grown along the polar *c*-axis exhibit larger internal electric fields at the heterostructure interfaces, affecting the radiative recombination rates. To overcome these issues, non-polar III-Nitrides are being extensively explored now-a-days because of their several benefits over the polar III-Nitrides. Epitaxial growth of GaN in the non-polar (*a*-plane) direction seems to be a feasible way for the growth of high-quality films as the lattice mismatch between *a*-plane [11–20] GaN and the *r*-plane [1–102] sapphire is the least (1.19%) along one of the azimuth directions. Additionally, the absence of internal polarization fields in non-polar structures may enhance the photodetection performance. Mukundan et al*.* [50] in 2015 have shown improvement in the performance of non-polar GaN in comparison to that of the polar GaN in terms of figures of

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

**Figure 7.**

*from Ref. [48].*

merit as well as the device stability.

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

**Figure 7.**

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

**3.3 Approaches to improve device performance of group III-Nitride** 

Bangalore, India.

**semiconductors-based PDs**

materials like graphene and MoS2.

**4.1 III-Nitrides-based devices**

sources, the synthesis times have been significantly reduced without compromising in the structural quality. One of the earliest works demonstrating growth of GaN by MBE was reported by Yoshida et al. [39] in 1983, wherein they successfully synthesized high electrical and optical quality GaN thin films on sapphire, with an AlN buffer layer via reactive MBE using ammonia as nitrogen source. **Figure 6(c)** shows the MBE setup located in Materials Research Centre, Indian Institute of Science,

Researchers across the world have employed various approaches to improve the performance of the III-Nitrides-based devices. These include improvement in growth quality by the employment of different growth techniques, adopting novel growth methods like epitaxial lateral overgrowth (ELO), using different materials as buffer layers such as AlN, and by fabricating improved structures to reduce defects. Another way to improve the growth quality of III-Nitrides is to find the alternatives to conventional silicon and sapphire substrates. In this regard, transition metal dichalcogenides, having a small lattice mismatch with III-Nitrides, can be used as potential substrates. Engineering the device structures can also result into the improvement of the device performance, and this can be achieved by making quantum confinement architectures, and by making hybrid structures using 2D

**4. Recent advancements in the MBE grown III-Nitrides-based PDs**

emphasis on self-powered photodetection has been given.

In the preceding sections, we have focused on the important figures of merit of PDs along with the different mechanisms of photodetection, and the properties and growth methods for group III-Nitride-based devices. Numerous reports exist which demonstrate the photodetection properties of III-Nitrides-based PDs. The recent state-of-the-art III-Nitride-based PDs have been discussed wherein a special

Some early accomplishments in the field of GaN-based PDs grown by MBE have been achieved by several researchers such as Van Hove et al. [41], Son et al. [42], Torvik et al. [43], Osinsky et al. [44], Xu et al. [45], and so on. In 2005, Calarco et al. [46] reported the electrical transport of GaN nanowhiskers grown by MBE, in dark and under UV illumination. The photoresponse has been found to be sensitively dependent on the column diameter of the nanowhiskers and this effect has been quantitatively described through a mechanism of size dependent surface recombination. Jain et al. [47] have shown the effect of symmetric and asymmetric contact electrodes on *c*-GaN/sapphire based UV PD. In 2018, Goswami et al*.* [48] reported the growth of self-assembled GaN nanostructures on Si(111) for applications in UV photodetection (**Figure 7**). The device exhibited a responsivity of 5.7 mAW−1 at a bias of 1 V. Numerous other reports exist demonstrating the photodetection properties of

In the meantime, researchers have also started exploring InN-based devices. One such work has been carried out by Shetty et al. [49]. They have grown InN quantum dots of varying densities on Si substrates using MBE. The device shows a strong response towards infrared illumination. The photoresponse studies revealed that

**160**

GaN-based PDs.

*(a) Schematic of the fabricated MSM device and (b) trend of photocurrent variation with respect to the applied bias. Inset shows the optical image of the actual PD. Figure has been reprinted with permission from Ref. [48].*

the increasing dot density results in the improvement in the sensitivity of the PD. The increase in the photocurrent with the density of the quantum dots has been attributed to the increase in the number of photogenerated carriers in InN, which add up with the carriers generated in Si upon light illumination. The results have also been validated using simulations and it has been observed that the experimental as well as theoretical results have sufficient agreement between them.

However, the epitaxial growth of high-quality III-Nitrides has been always hindered by the lack of lattice-matched substrates, that hinders the development of high-performance devices. In all the above-mentioned reports, the growth has been accomplished on substrates (*c*-plane sapphire, Si(111), etc) which promote the *c*-plane oriented growth of the III-Nitrides i.e., in the polar direction. Moreover, the structures grown along the polar *c*-axis exhibit larger internal electric fields at the heterostructure interfaces, affecting the radiative recombination rates. To overcome these issues, non-polar III-Nitrides are being extensively explored now-a-days because of their several benefits over the polar III-Nitrides. Epitaxial growth of GaN in the non-polar (*a*-plane) direction seems to be a feasible way for the growth of high-quality films as the lattice mismatch between *a*-plane [11–20] GaN and the *r*-plane [1–102] sapphire is the least (1.19%) along one of the azimuth directions. Additionally, the absence of internal polarization fields in non-polar structures may enhance the photodetection performance. Mukundan et al*.* [50] in 2015 have shown improvement in the performance of non-polar GaN in comparison to that of the polar GaN in terms of figures of merit as well as the device stability.

In 2018, the mechanism of the higher device performance parameters of the non-polar GaN has been explained by Pant et al*.* [18] by performing azimuth angledependent photodetection. They have shown the non-uniformity in the defects present along the different azimuth directions. This is a consequence of the asymmetry in the strain between the substrate and film, as the lattice mismatch is asymmetric along various in-plane crystal directions. The mismatch in the lattice constants along the [0002] direction is ∼1% whereas, along the [1–100] direction, it is ∼13%. This induces a large number of defects along the [1–100] direction as compared to the [0002] azimuth direction. **Figure 8(a)** shows the *a*-GaN-based device used in this study. Furthermore, it has been shown in **Figure 8(b)** that the overall photocurrent in the UV region is also dependent on the different azimuth angles. A maximum responsivity of ~1.9 AW−1 and ~ 13.0 AW−1 have been obtained at a bias of 1 V and 5 V, respectively. These results underlined the importance of aligning the contact electrodes along the favorable azimuth direction in order to restrict the transport of the charge carriers. In a subsequent work, Pant et al*.* [51] have further shown

**Figure 8.**

*(a) Fabrication of contact electrodes in different directions and (b)* I–V *characteristics of the device taken along different azimuth directions. Figure has been reprinted with permission from Ref. [18].*

improvements in the photodetection properties of the non-polar GaN by optimizing the growth parameters and therefore, improving the overall quality of the thin film. A maximum responsivity of 25 AW−1has been achieved at a low bias of 1 V and is among the highest reported responsivities at such low voltages.

Another approach to overcome the problem of lattice mismatch is by using AlN as a buffer layer. Wang et al*.* [52] in 2007 exhibited a Schottky-based metal–semiconductor–metal PD, fabricated on 1 μm-thick and crack-free GaN on Si(111), utilizing an optimized AlxGa1–xN/AlN complex buffer layer. The device showed a high photoresponsivity of 4600 AW−1 at 1 V bias (366 nm) and this superior performance has been attributed to both the crack-free GaN film as well as the high internal gain. In another report, the growth of GaN p-n junction on AlN/Si(111) and the effects of thermal annealing of the Ni/Ag contact electrodes on the photodetector applications have been explored by Yusoff et al*.* [14]. Recently, Ravikiran et al. [53] have demonstrated GaN UV PDs grown on AlN/Si(111) which exhibited a peak responsivity of 0.183 AW−1 at 15 V.

### **4.2 III-Nitride heterostructures-based devices**

In the reports discussed above, various methods leading to enhancement in the responsivity have been highlighted. However, the responsivities and the transit times (in the order of a ms) of most of these PDs still remain inferior to that of the state-of-the-art detectors, and therefore, hamper their usage for the design and development of practical devices. The most elegant way to enhance the device performance is by utilizing a heterojunction with high-quality materials. There are many reports demonstrating PDs based on the heterojunctions of III-Nitrides with other III-Nitride semiconductors, ZnO, perovskites, 2D materials, and so forth.

In 2010, Rigutti et al*.* [54] have shown a single-nanowire PD relying on the charge carrier generation in the GaN/AlN quantum discs (QDs). The photoluminescence studies have shown that the emission energy of the QDs is lesser than the band gap of GaN, which is a consequence of the quantum confined Stark effect. The QD-based PDs exhibited a strong reduction in the dark current with responsivity (300 nm, −1 V) as high as 2 × 103 AW−1. Yusoff et al*.* [55] have demonstrated AlN/ GaN/AlN heterostructures grown via PAMBE on Si substrates. The photoresponse shows promising results towards applications in UV detection. Pandey et al*.* [21] have reported the fabrication of a BaTiO3/GaN (BTO/GaN)-based Schottky junction PD on *c*-plane sapphire and its selective UV photodetection in temperature range of 313–423 K. The responsivity increased with increase in the temperature till

**163**

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

393 K and then it decreased. Such behavior has been explained by the enhancement in the device's dark current with increase in the temperature, which is also evident from the ideal diode equation. The device shows potential to be used as an UV PD in

In another report, Roul et al*.* [13] have demonstrated hybrid ZnO/AlN/Si-based UV PDs with infrared- and visible-blind photoresponse. The heterostructures have been formed by depositing ZnO films on Si(111) substrate with an introduction of AlN as an intermediate layer. The AlN layer helps in improving the crystallinity of the ZnO films and results in excellent optical properties. The vertical transport characteristics of the ZnO/AlN/Si heterojunction-based device under light illumination and in the dark demonstrate an intrinsic infrared- and visible-blind response, with excellent UV responsivity of 14.5 AW−1. The AlN layer acts as an electron blocking layer and allows the holes to get transported across the heterojunction in

In the past few years, loads of efforts have been made in the field of III-Nitrides/2D materials-based heterostructures for high-performance optoelectronic devices. These 2D materials such as graphene, MoS2, etc. are characterized by weak inter-layer van der Waals (vdW) forces, which lead to exceptional electronic properties, and can offer an open platform to design high-performance electronic devices. Moreover, the absence of surface dangling bonds in 2D materials results in high-quality heterointerfaces. Such an integration has been recently demonstrated by Goel et al. [20], wherein they have shown a high responsivity UV PD based on 2D/3D heterojunction, which has been formed by depositing few-layer of MoS2 on GaN thin film. The superior light absorption properties of MoS2 resulted in high performance MoS2/GaN-based PD. The device shows a high responsivity of 3 × 103 AW−1 and detectivity of ~1011 Jones (at a wavelength of 365 nm) at an applied reverse bias of 1 V under a light intensity of 12 mWcm−2. The rise and the decay

Until now, all the reported devices discussed above require an external applied bias for achieving significant photodetection. In recent times, a lot of efforts are being made towards energy storage and energy producing devices due to the current situation of energy crisis [56–61]. Therefore, PDs that do not consume any external power are gaining a lot of attention. These self-powered devices depend upon the built-in potential at the interface, which enables the effective separation of the photogenerated charge carriers. Additionally, the built-in electric potential lowers the dark current, which is another advantage of such PDs. Thus, these self-powered nanodevices have a great outlook for the next-generation optoelectronic devices.

**4.3 III-Nitrides and their heterostructures for self-driven photodetection**

achieve self-driven photodetection with III-Nitrides-based PDs.

Off late, as mentioned above, there has been a tremendous focus on the selfpowered PDs. In this section, we emphasize on the various methods unveiled to

Prakash et al*.* [62] in 2016 have demonstrated a simple approach to fabricate a self-powered PD utilizing reduced graphene oxide (rGO) asymmetrical electrodes on MBE-grown GaN thin film as shown in **Figure 9(a)**. This integration of the transparent rGO contact electrodes on GaN has been realized through a simple drop-casting method, leading to a simple fabrication process as well as reduced processing time and cost. The hybrid shows a low photoresponsivity of 1.5 μAW−1 towards UV light at zero bias (**Figure 9(b)**), with fast response and recovery times of ~60 and ~267 ms, respectively. The difference in the work functions of rGO and GaN leads to formation of depletion regions at the two rGO/GaN interfaces. The drop casted contact electrodes are inhomogeneous in nature, which results into two

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

high-temperature applications.

the reverse biasing condition.

times of the PD were 5.3 and 5.6 ms, respectively.

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

393 K and then it decreased. Such behavior has been explained by the enhancement in the device's dark current with increase in the temperature, which is also evident from the ideal diode equation. The device shows potential to be used as an UV PD in high-temperature applications.

In another report, Roul et al*.* [13] have demonstrated hybrid ZnO/AlN/Si-based UV PDs with infrared- and visible-blind photoresponse. The heterostructures have been formed by depositing ZnO films on Si(111) substrate with an introduction of AlN as an intermediate layer. The AlN layer helps in improving the crystallinity of the ZnO films and results in excellent optical properties. The vertical transport characteristics of the ZnO/AlN/Si heterojunction-based device under light illumination and in the dark demonstrate an intrinsic infrared- and visible-blind response, with excellent UV responsivity of 14.5 AW−1. The AlN layer acts as an electron blocking layer and allows the holes to get transported across the heterojunction in the reverse biasing condition.

In the past few years, loads of efforts have been made in the field of III-Nitrides/2D materials-based heterostructures for high-performance optoelectronic devices. These 2D materials such as graphene, MoS2, etc. are characterized by weak inter-layer van der Waals (vdW) forces, which lead to exceptional electronic properties, and can offer an open platform to design high-performance electronic devices. Moreover, the absence of surface dangling bonds in 2D materials results in high-quality heterointerfaces. Such an integration has been recently demonstrated by Goel et al. [20], wherein they have shown a high responsivity UV PD based on 2D/3D heterojunction, which has been formed by depositing few-layer of MoS2 on GaN thin film. The superior light absorption properties of MoS2 resulted in high performance MoS2/GaN-based PD. The device shows a high responsivity of 3 × 103 AW−1 and detectivity of ~1011 Jones (at a wavelength of 365 nm) at an applied reverse bias of 1 V under a light intensity of 12 mWcm−2. The rise and the decay times of the PD were 5.3 and 5.6 ms, respectively.

Until now, all the reported devices discussed above require an external applied bias for achieving significant photodetection. In recent times, a lot of efforts are being made towards energy storage and energy producing devices due to the current situation of energy crisis [56–61]. Therefore, PDs that do not consume any external power are gaining a lot of attention. These self-powered devices depend upon the built-in potential at the interface, which enables the effective separation of the photogenerated charge carriers. Additionally, the built-in electric potential lowers the dark current, which is another advantage of such PDs. Thus, these self-powered nanodevices have a great outlook for the next-generation optoelectronic devices.

#### **4.3 III-Nitrides and their heterostructures for self-driven photodetection**

Off late, as mentioned above, there has been a tremendous focus on the selfpowered PDs. In this section, we emphasize on the various methods unveiled to achieve self-driven photodetection with III-Nitrides-based PDs.

Prakash et al*.* [62] in 2016 have demonstrated a simple approach to fabricate a self-powered PD utilizing reduced graphene oxide (rGO) asymmetrical electrodes on MBE-grown GaN thin film as shown in **Figure 9(a)**. This integration of the transparent rGO contact electrodes on GaN has been realized through a simple drop-casting method, leading to a simple fabrication process as well as reduced processing time and cost. The hybrid shows a low photoresponsivity of 1.5 μAW−1 towards UV light at zero bias (**Figure 9(b)**), with fast response and recovery times of ~60 and ~267 ms, respectively. The difference in the work functions of rGO and GaN leads to formation of depletion regions at the two rGO/GaN interfaces. The drop casted contact electrodes are inhomogeneous in nature, which results into two

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

improvements in the photodetection properties of the non-polar GaN by optimizing the growth parameters and therefore, improving the overall quality of the thin film. A maximum responsivity of 25 AW−1has been achieved at a low bias of 1 V and is among

*(a) Fabrication of contact electrodes in different directions and (b)* I–V *characteristics of the device taken* 

*along different azimuth directions. Figure has been reprinted with permission from Ref. [18].*

Another approach to overcome the problem of lattice mismatch is by using AlN as a buffer layer. Wang et al*.* [52] in 2007 exhibited a Schottky-based metal–semiconductor–metal PD, fabricated on 1 μm-thick and crack-free GaN on Si(111), utilizing an optimized AlxGa1–xN/AlN complex buffer layer. The device showed a high photoresponsivity of 4600 AW−1 at 1 V bias (366 nm) and this superior performance has been attributed to both the crack-free GaN film as well as the high internal gain. In another report, the growth of GaN p-n junction on AlN/Si(111) and the effects of thermal annealing of the Ni/Ag contact electrodes on the photodetector applications have been explored by Yusoff et al*.* [14]. Recently, Ravikiran et al. [53] have demonstrated GaN UV PDs grown on AlN/Si(111) which exhibited a

In the reports discussed above, various methods leading to enhancement in the responsivity have been highlighted. However, the responsivities and the transit times (in the order of a ms) of most of these PDs still remain inferior to that of the state-of-the-art detectors, and therefore, hamper their usage for the design and development of practical devices. The most elegant way to enhance the device performance is by utilizing a heterojunction with high-quality materials. There are many reports demonstrating PDs based on the heterojunctions of III-Nitrides with other III-Nitride semiconductors, ZnO, perovskites, 2D materials, and so forth. In 2010, Rigutti et al*.* [54] have shown a single-nanowire PD relying on the charge carrier generation in the GaN/AlN quantum discs (QDs). The photoluminescence studies have shown that the emission energy of the QDs is lesser than the band gap of GaN, which is a consequence of the quantum confined Stark effect. The QD-based PDs exhibited a strong reduction in the dark current with responsivity

GaN/AlN heterostructures grown via PAMBE on Si substrates. The photoresponse shows promising results towards applications in UV detection. Pandey et al*.* [21] have reported the fabrication of a BaTiO3/GaN (BTO/GaN)-based Schottky junction PD on *c*-plane sapphire and its selective UV photodetection in temperature range of 313–423 K. The responsivity increased with increase in the temperature till

AW−1. Yusoff et al*.* [55] have demonstrated AlN/

the highest reported responsivities at such low voltages.

peak responsivity of 0.183 AW−1 at 15 V.

(300 nm, −1 V) as high as 2 × 103

**4.2 III-Nitride heterostructures-based devices**

**162**

**Figure 8.**

**Figure 9.** *(a) GaN-based device with rGO electrodes, (b)* I-V *characteristics of the PD, (c) energy band diagram and (d) mechanism for self-powered photodetection. Figure has been reproduced with permission from Ref. [62].*

unlike built-in fields at these interfaces. Therefore, a net internal electric field is developed, leading to the self-powered detection (**Figure 9(c, d)**).

Using the same approach as described above, Pant et al*.* [63] have reported a self-driven *a*-GaN-based UV-A PDs showing a responsivity and detectivity of ~4.67 mAW−1 and 3.0 × 1013 Jones, respectively at a wavelength of 364 nm. In another work, Aggarwal et al*.* [64] have shown a UV PD based on GaN nanoflowers grown via MBE on Si(111) substrate. Under self-biased condition, the PD exhibits a very low dark current in the range of ≈nA, with a high responsivity of 132 mAW−1 and fast rise/fall times of 63/27 ms. In another report, Chowdhury et al. [25] have reported self-powered photodetection of an InN/AlN/Si semiconductor–insulator–semiconductor-based PD (λ = 1550 nm), where a photoresponsivity of ~3.36 μAW−1 has been observed with response/recovery times in milliseconds.

The major shortcoming of such devices lies with the obtained responsivities, which is entirely reliant on the degree of inhomogeneity or asymmetry between the electrodes, hence, limiting the PD's performance. One of the most effective ways for realizing self-driven PDs is by fabricating heterojunctions and utilizing the built-in potential at the heterointerface. Heterojunctions of III-Nitrides with several semiconductors such as Si, ZnO, Ga2O3, and so on have been explored and promising results have been achieved. In 2015, the advances in the high growth quality of epitaxial InGaN films on Si substrates synthesized via MBE along with the maturity in the Si-based technology have resulted in the demonstration of highly efficient PDs, as shown by Chandan et al*.* [65]. A self-powered n-InGaN/n-Si isotype heterojunction-based PD has been reported. The device shows a non-linear behavior and a responsivity of 0.094 AW−1, with rise/fall times less than 100 ms. The mechanism of self-powered photodetection has been explained based on the presence of interfacial internal electric field.

**165**

**Figure 10.**

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

Recently, a high-performance PD based on the p-GaN/n-ZnMgO heterojunction has been demonstrated [24]. The PD shows a clear rectifying *I–V* behavior characterized with a turn-on voltage of ~2.5 V. At zero-bias condition, the device exhibits a responsivity of 196 mAW−1 at a wavelength of 362 nm. The rise and the decay times of the PD are as short as 1.7 and 3.3 ms, respectively. This high performance of the device has been attributed to the excellent crystalline quality and electrical proper-

In another work, a simple approach has been reported [66] to fabricate a GaN/ rGO: Ag nanoparticles (NPs) p–n heterojunction-based PD, integrated with a network of transparent Au nanowires (NWs) as the top contact electrode. The device demonstrates an excellent rectification ratio of ~105 with a broad-band photoresponse due to the presence of both the GaN layer (UV region) and the silver-loaded reduced graphene oxide (visible to infrared region). Furthermore, the reducing effect of the Ag NPs to graphene oxide in addition to the localized surface plasmon resonance has been utilized to improve the photoresponse in the NIR and the visible regions. The transparent Au NWs network efficiently collects the charge, ensuing high photoresponsivity and fast switching behavior. The heterojunction exhibits a responsivity of ~266 mAW−1 and detectivity of ~2.62 × 1011 Jones, under illumination of 360 nm light. Owing to the high built-in electric potential at the heterointerface, the self-powered operation is demonstrated under the entire excitation

Exploiting the criterion of the difference between the electron affinities of the constituent semiconductors to create an internal field, an improvisation in the InGaN/Si-based structure has been demonstrated by Chowdhury et al*.* [15], where introduction of an AlN layer in between InGaN and Si leads to the formation of a semiconductor–insulator–semiconductor type structure, resulting in the multi-

obtained AlN/*n*-Si template using MBE. The device exhibits an exceptional selfpowered and broad-band photorespnse under the illumination of UV–visible light (300–800 nm). The self-powered PD exhibits a high responsivity of 9.64 AW−1 at light illumination of 580 nm, with an ultrafast response/recovery time of ~20/21 μs, respectively. The maximum response at 580 nm is believed to be because of the deep

*(a) Spectral response of the MoS2/AlN/Si-based PD. (b) Schematic of the deep defect states-modulated carrier transport in MoS2/AlN/Si-based device. Figures have been reproduced with permission from Ref. [4].*



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

ties of p-GaN epilayer.

wavelength range (360–980 nm).

fold enhancement in the device performance. The *n*<sup>+</sup>

donor defect states present in the InGaN epilayer.

structure has been realized by growing an *n*<sup>+</sup>

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

Recently, a high-performance PD based on the p-GaN/n-ZnMgO heterojunction has been demonstrated [24]. The PD shows a clear rectifying *I–V* behavior characterized with a turn-on voltage of ~2.5 V. At zero-bias condition, the device exhibits a responsivity of 196 mAW−1 at a wavelength of 362 nm. The rise and the decay times of the PD are as short as 1.7 and 3.3 ms, respectively. This high performance of the device has been attributed to the excellent crystalline quality and electrical properties of p-GaN epilayer.

In another work, a simple approach has been reported [66] to fabricate a GaN/ rGO: Ag nanoparticles (NPs) p–n heterojunction-based PD, integrated with a network of transparent Au nanowires (NWs) as the top contact electrode. The device demonstrates an excellent rectification ratio of ~105 with a broad-band photoresponse due to the presence of both the GaN layer (UV region) and the silver-loaded reduced graphene oxide (visible to infrared region). Furthermore, the reducing effect of the Ag NPs to graphene oxide in addition to the localized surface plasmon resonance has been utilized to improve the photoresponse in the NIR and the visible regions. The transparent Au NWs network efficiently collects the charge, ensuing high photoresponsivity and fast switching behavior. The heterojunction exhibits a responsivity of ~266 mAW−1 and detectivity of ~2.62 × 1011 Jones, under illumination of 360 nm light. Owing to the high built-in electric potential at the heterointerface, the self-powered operation is demonstrated under the entire excitation wavelength range (360–980 nm).

Exploiting the criterion of the difference between the electron affinities of the constituent semiconductors to create an internal field, an improvisation in the InGaN/Si-based structure has been demonstrated by Chowdhury et al*.* [15], where introduction of an AlN layer in between InGaN and Si leads to the formation of a semiconductor–insulator–semiconductor type structure, resulting in the multifold enhancement in the device performance. The *n*<sup>+</sup> -InGaN/AlN/*n*-Si(111) hybrid structure has been realized by growing an *n*<sup>+</sup> -InGaN thin film on a commercially obtained AlN/*n*-Si template using MBE. The device exhibits an exceptional selfpowered and broad-band photorespnse under the illumination of UV–visible light (300–800 nm). The self-powered PD exhibits a high responsivity of 9.64 AW−1 at light illumination of 580 nm, with an ultrafast response/recovery time of ~20/21 μs, respectively. The maximum response at 580 nm is believed to be because of the deep donor defect states present in the InGaN epilayer.

#### **Figure 10.**

*(a) Spectral response of the MoS2/AlN/Si-based PD. (b) Schematic of the deep defect states-modulated carrier transport in MoS2/AlN/Si-based device. Figures have been reproduced with permission from Ref. [4].*

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

unlike built-in fields at these interfaces. Therefore, a net internal electric field is

*(a) GaN-based device with rGO electrodes, (b)* I-V *characteristics of the PD, (c) energy band diagram and (d) mechanism for self-powered photodetection. Figure has been reproduced with permission from Ref. [62].*

Using the same approach as described above, Pant et al*.* [63] have reported a self-driven *a*-GaN-based UV-A PDs showing a responsivity and detectivity of ~4.67 mAW−1 and 3.0 × 1013 Jones, respectively at a wavelength of 364 nm. In another work, Aggarwal et al*.* [64] have shown a UV PD based on GaN nanoflowers grown via MBE on Si(111) substrate. Under self-biased condition, the PD exhibits a very low dark current in the range of ≈nA, with a high responsivity of 132 mAW−1 and fast rise/fall times of 63/27 ms. In another report, Chowdhury et al. [25] have reported self-powered photodetection of an InN/AlN/Si semiconductor–insulator–semiconductor-based PD (λ = 1550 nm), where a photoresponsivity of ~3.36 μAW−1 has been observed with response/recovery times in milliseconds. The major shortcoming of such devices lies with the obtained responsivities, which is entirely reliant on the degree of inhomogeneity or asymmetry between the electrodes, hence, limiting the PD's performance. One of the most effective ways for realizing self-driven PDs is by fabricating heterojunctions and utilizing the built-in potential at the heterointerface. Heterojunctions of III-Nitrides with several semiconductors such as Si, ZnO, Ga2O3, and so on have been explored and promising results have been achieved. In 2015, the advances in the high growth quality of epitaxial InGaN films on Si substrates synthesized via MBE along with the maturity in the Si-based technology have resulted in the demonstration of highly efficient PDs, as shown by Chandan et al*.* [65]. A self-powered n-InGaN/n-Si isotype heterojunction-based PD has been reported. The device shows a non-linear behavior and a responsivity of 0.094 AW−1, with rise/fall times less than 100 ms. The mechanism of self-powered photodetection has been explained based on the presence of interfacial

developed, leading to the self-powered detection (**Figure 9(c, d)**).

**164**

**Figure 9.**

internal electric field.

In a recent work, Singh et al*.* [4] have fabricated an MoS2/AlN/Si-based PD, combining the mature technologies of III-Nitride and Si with the unique properties of MoS2. Additionally, due to the large difference between the work functions of these semiconducting materials, the band bending at the heterointerfaces resulted into the self-driven behavior. The vertical transport behavior of the device shows a broad-band photoresponse (300–1100 nm) with maximum responsivity of ~10 AW−1 under self-biased condition as shown in **Figure 10(a)**. The device also shows an ultrafast detection speeds (response/recovery times: ~13/15 μs). The importance of sandwiching the AlN layer has been shown as the MoS2/Si-based PD shows a responsivity ~5 times less in the zero-bias mode. The authors have confirmed through transmission electron microscopy and X-ray photoelectron spectroscopy that oxygen defects exist throughout the AlN layer. These impurities form deep donor levels in AlN and moderate the charge transport, which leads to the enhanced device performance (**Figure 10(b)**).
