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Ultraviolet (UV) photodetectors (PDs) have drawn great attention in recent years due to their potential application in civil and military fields. Because of its ultrawide bandgap, low cost, strong radiation hardness, and high thermal and chemical stability with high visible-light transparency, Ga2O3 is regarded as the most promising candidate for UV detection. Furthermore, the bandgap of Ga2O3 is as high as 4.7–4.9 eV, directly corresponding to the solar-blind UV detection band with wavelength less than 280 nm. There is no need of doping in Ga2O3 to tune its bandgap, compared to AlGaN, MgZnO, etc, thereby avoiding alloy composition fluctuations and phase separation. At present, solar-blind Ga2O3 photodetectors based on single crystal or amorphous Ga2O3 are mainly focused on metal–semiconductor–metal and Schottky photodiodes. In this work, the recent achievements of Ga2O3 photodetectors are systematically reviewed. The characteristics and performances of different photodetector structures based on single crystal Ga2O3 and amorphous Ga2O3 thin film are analyzed and compared. Finally, the prospects of Ga2O3 UV photodetectors are forecast.
Photodetectors (PD) are a key component in modern optoelectronic devices and are widely used in civil and military fields such as optical imaging, spatial optical communication, missile guidance and positioning navigation, and so on.[1–3] Currently, the main commercial UV photodetectors are Si-based photodiodes and photomultipliers due to the highly mature Si processes and their low cost.[4] However, because of its narrow bandgap (1.1–1.3 eV),[1–3] the detection band is limited from near UV to infrared light. Additionally, the Si based photodetector usually has a low efficiency and needs high vacuum conditions and high voltage (e.g., in photomultiplier).[5]
To avoid these disadvantages, UV photodetectors based on wide bandgap materials such as SiC, GaN, ZnO, Ga2O3, and diamond have gained more and more attention because of the intrinsic solar-blindness. Moreover, the wide bandgap semiconductors exhibit strong radiation hardness, high thermal and chemical stability, making them feasible to be applied in harsh environment.[6] What is more, GaN and ZnO can alloy with Al and MgO, respectively, to increase the bandgap to detect deeper UV light. However, the alloying process and relatively complicated growth technology will cause alloy composition fluctuations and phase separation, thereby introducing a large defect density and limiting its application under high demand conditions.[7]
Among wide bandgap materials, Ga2O3 has many unique characteristics, including high breakdown voltage and high Baligaʼs figure of merit (BFOM). These outstanding properties make Ga2O3 a promising material for high-temperature and high-power applications. In addition, its bandgap is 4.7–4.9 eV, which is intrinsically suitable for deep ultraviolet (DUV) photodetection[8] without any doping or alloying process. Besides, the growth cost of Ga2O3 is relatively low compared to other wide bandgap materials. The basic properties of Ga2O3 and other wide bandgap materials are listed in Table
![]() | Table 1.
The basic properties of Si and wide bandgap materials. . |
![]() | Table 2.
Key figures of merit for photodetectors. . |
In this paper, we review the recent achievements of Ga2O3 photodetectors. We focus on Ga2O3 photodetectors based on single crystal and amorphous Ga2O3 with different device structures, by analyzing its performance and characteristics. We firstly review the recent progress of MSM structure-based Ga2O3 photodetectors. Secondly, the Ga2O3 photodetectors based on Schottky junction are discussed. Finally, we give an outlook on the future direction in the field of Ga2O3 based UV photodetector.
MSM Ga2O3 photodetectors were firstly proposed because of their fundamental advantages: (i) simple structure; (ii) easy fabrication and integration; and (iii) low capacitance per unit area. Usually, an MSM photodetector is composed of two back-to-back Schottky junctions by depositing interdigitated metal electrodes on the surface of the active layer. A schematic of MSM photodetector structure is shown in Fig.
It is well known that the performance of solar-blind photodetectors is strongly affected by the crystalline quality of the Ga2O3 film. Different kinds of methods have been employed to grow Ga2O3 films, such as metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), and magnetron sputtering.
Takayoshi Oshima et al.[9] first reported MSM Ga2O3 photodetectors in 2007, which were fabricated by a depositing β-Ga2O3 thin film on the c-plane sapphire substrates through molecular beam epitaxy. Ti/Au (50 nm/100 nm) were deposited to form ohmic contact electrodes in the metal–semiconductor–metal DUV photodetector. A small dark current of 1.2 nA at a bias voltage of 10 V was obtained (Figs.
![]() | Fig. 2. (a) and (b) Linear and log scale current–voltage characteristics of the Ti/Au/β-Ga2O3 MSM DUV photodetector in the dark, under black light irradiation, and under low-pressure mercury lamp irradiation.[9] (c) Steady-state I–V characteristic under light illumination and I–V characteristics in the dark (log scale), and (d) the time-dependent photoresponse under the illumination of 236 nm at 20 V (log scale) of the Ni/Au/β-Ga2O3 MSM photodetector before Al2O3 passivation. (e) I–V characteristic under light illumination and I–V characteristics in the dark (log scale), and (f) time-dependent photoresponse under the illumination of 236 nm at 20 V (log scale) of the Ni/Au/β-Ga2O3 MSM photodetector after Al2O3 passivation.[16] |
On the other hand, Weihua Tang et al.[17] reported β-Ga2O3 thin film based solar-blind photodetector by LMBE technology. By optimizing the growth parameters,
![]() | Fig. 3. (a) Schematic diagram of the fabricated Ti/Au/β-Ga2O3 thin film MSM structure photodetector. (b) I–V characteristics of the Ti/Au/β-Ga2O3 photodetector in dark, under 365 nm light and under 254 nm light illumination in a logarithmic coordinate. (c) Time-dependent photoresponse of the Ti/Au/β-Ga2O3 photodetector under UV light illumination at bias voltage of 10 V. (d) Experimental current rise and decay process of the photodetector under 254 nm illumination (red hollow points) and its corresponding fitting curve (black solid line).[17] |
In order to improve the response and recovery speed, Weihua Tangʼs group in situ annealed the thin film deposited by LMBE under an oxygen pressure of 100 Pa at 800 °C for 2 h.[18] They found that the Au/Ti electrodes were ohmic contact with the as-grown films and Schottky contact with the annealed films. The Schottky-type β-Ga2O3 photodetector exhibited lower dark current, higher responsivity, and shorter switching time. From the I–V characteristics (Fig.
![]() | Fig. 4. (a) The I–V characteristics of the β-Ga2O3 thin films MSM photodetector without and with annealing treatment in O2 atmosphere in the dark. The inset (bottom right) shows the schematic figure of the MSM structure. The inset (top left) is the plot of (α hν)2 as a function of hν calculating from UV–vis absorbance spectrum of the β-Ga2O3 thin film. (b) Normalized O 1s XPS spectra of the as-grown and post-annealed β-Ga2O3 thin films. (c) Time-dependent photoresponse of the β-Ga2O3 thin films MSM photodetector under 254 nm illumination. (d) Enlarged view of the rise/decay edges of the transient photoresponse curve and its corresponding exponential fitting.[18] |
Chiungyi Huang et al.[19] employed MOCVD to grow single-crystalline β-Ga2O3 film on (0001) sapphire substrate at low temperature and low pressure environment. The MSM β-Ga2O3 solar-blind photodetector was fabricated. The as-grown β-Ga2O3 film was annealed at 800 °C in atmosphere or in a nitrogen environment, and the thermal annealing effect on the material characteristics of β-Ga2O3 film was investigated. Through x-ray diffraction (XRD), cathodoluminescence (CL) measurement, and I–V characterization of the MSM β-Ga2O3 solar-blind photodetector, they concluded that the film annealed at 800 °C for 15 min in a nitrogen atmosphere would result in improved material characteristics and enhanced device performance. After annealing, the device exhibited lower dark current, which suggested the improved crystalline quality of the β-Ga2O3 epilayer. Besides, Dezhen Shen et al.[20] reported their fabrication of MSM β-Ga2O3 solar-blind photodetector on sapphire substrate based on MOCVD deposited β-Ga2O3 film. Au was deposited as the interdigital electrodes. The fabricated photodetectors exhibited a high responsivity of 17 A/W, UV-to-dark rejection ratio of 8.5×106, and EQE of 8228% at 20 V due to the carrier multiplication occurred in the Ga2O3 under Au electrodes.
The crystalline quality of β-Ga2O3 film is readily affected by various factors, one of which is the thermal-expansion or lattice match with the substrate. Qian Feng et al.[21] made a comparison study of the performance of the β-Ga2O3 photodetectors on bulk β-Ga2O3 substrate and sapphire. LMBE technology was used to grow epitaxial film on bulk β-Ga2O3 substrate and c-sapphire substrate. Ti/Au with 10 nm/40 nm thickness was used as the interdigital Schottky contacts. The schematic of the Ga2O3 photodetector is shown in Figs.
![]() | Fig. 5. (a) Top view and (b) cross-sectional schematics of the Ti/Au/Ga2O3 MSM Ga2O3 photodetector. (c) HRTEM image of Ga2O3 grown on bulk Ga2O3 substrate. (d) XRD curves for Ga2O3 device samples on sapphire and bulk. AFM images of (e) Ga2O3 grown on sapphire and (f) bulk Ga2O3.[21] |
The optical-electrical characteristics of the above photodetectors are presented in Fig.
![]() | Fig. 6. (a) I–V characteristics for the MSM photodetectors on bulk Ga2O3 and sapphire in the dark and under light illumination of 254 nm. (b) PDCR for devices on bulk Ga2O3 and sapphire. Responsivity as a function of illumination optical wavelength λ for the Ga2O3 MSM photodetectors on (c) bulk Ga2O3 and (d) sapphire substrate at various Vbias, respectively. (e) Time-dependent photoresponse of the MSM photodetectors on bulk Ga2O3 and on sapphire under a bias voltage of 20 V. The inset lists the response and decay time of the devices.[21] |
To the best of our knowledge, almost all the reported MSM β-Ga2O3 photodetectors were based on sapphire substrate. However, there was no report of MSM β-Ga2O3 photodetectors on the cost-effective Si substrate due to its large lattice and thermal expansion coefficient mismatch with β-Ga2O3. Until recently, Kanika Arora et al.[10] reported an ultrahigh performance of self-powered β-Ga2O3 thin film solar-blind photodetector on Si substrate through magnetron sputtering method. By using high temperature seed layer (HSL), the improved crystalline quality was achieved in comparison to the Ga2O3 thin film grown without a high-temperature seed layer. The device exhibited a high on/off (I254 nm/Idark) ratio of
![]() | Fig. 7. (a) Schematic representation of the fabrication processes of MSM β-Ga2O3 solar-blind photodetectors. (b) Temperature profile for the growth process of the seed-layer-assisted β-Ga2O3 thin film. (c) I–V characteristics of MSM β-Ga2O3 solar-blind photodetectors based on thin films deposited without and with HSL conditions in the dark. Inset shows the I–V curves of MSM β-Ga2O3 solar-blind photodetectors without and with HSL conditions at 250 nm and in the dark in a semilogarithmic coordinate. (d) Photoresponsivity as a function of wavelength of the MSM β-Ga2O3 solar-blind photodetectors grown without and with HSL conditions. (e) Time-dependent photoresponse of MSM β-Ga2O3 solar-blind photodetectors deposited without and with HSL, respectively.[10] |
Recently, more and more works were reported in the fabrication of MSM GaOx photodetectors based on the cost-effective radio frequency (RF) magnetron sputtered GaOx film instead of expensive MOCVD, MBE, or LMBE technology. Among them, some photodetectors exhibited surprisingly high performances, even exceeding those of the majority of Ga2O3 photodetectors based on β-Ga2O3 thin film deposited by MOCVD, MBE, or LMBE technology. Ling-Xuan Qian et al.[11] demonstrated an ultrahigh responsivity and rapid recovery MSM photodetector based on magnetron sputtered amorphous gallium oxide thin film. Another MSM photodetector based on β-Ga2O3 thin film deposited by MBE was fabricated as the control sample. Ti/Al was used as the interdigital electrodes. It can be found that the MBE deposited film was single crystal
![]() | Fig. 8. (a) XRD patterns and three-dimensional AFM surface images of the gallium oxide films deposited by (b) plasma-assisted MBE and (c) magnetron sputtering. (d) Normalized XPS O 1s core-level spectra of the gallium oxide films deposited by magnetron sputtering (red) and plasma-assisted MBE (blue), respectively.[11] |
![]() | Fig. 9. (a) The I–V characteristics of the MSM gallium oxide photodetectors in the dark and under illumination of 254 nm light in a semilogarithmic coordinate. (b) Dark current of the photodetectors on a linear scale. (c) Responsivity vs. illumination optical wavelength on a semilogarithmic scale and (d) normalized spectral response of the MSM gallium oxide photodetectors based on a-GaOx (red) and β-Ga2O3 (blue) thin films. The inset in (d) shows the cutoff wavelength of the photodetectors.[11] |
Shujuan Cui et al.[22] also fabricated an amorphous Ga2O3 solar-blind photodetector with high response speed (
O 1s and Ga 2p peaks (SO1s/SGa2p) are 0.61, 0.64, and 0.65 for S0, S1, and S4, respectively. The increasing SO1s/SGa2p shows an increase of oxygen content in the film and thus a reduction of oxygen vacancies. All the results suggest that the reduction of oxygen vacancies and the increase of Schottky barrier height can significantly promote the response speed. Besides, no obvious degradation of the photoelectric performance in bending states and fatigue tests was observed on the flexible device, suggesting the stability and applicability of the amorphous Ga2O3 thin film in the flexible solar-blind photodetectors. These results indicate that RF-magnetron sputtering can be employed to deposit Ga2Ox film on various substrates and high performance MSM photodetectors can be achieved.
Si p–i–n diode with high performance is a common commercial photodetector. For the practical application of Ga2O3 p–i–n diode, p-type β-Ga2O3 thin film is essential. However, the achievement of p-type β-Ga2O3 thin film still remains a challenge because of lacking of suitable dopants, just like other oxide semiconductors, overcompensated by oxygen vacancy.[23,24] Weihua Tang et al.[25] tried to dope Mg into β-Ga2O3 thin film, and a weak p-type β-Ga2O3 thin film was obtained by RF magnetron sputtering for the first time. The Mg-doped β-Ga2O3 thin film was used to fabricate the MSM solar-blind photodetector. The as-fabricated photodetector exhibited a lower dark current, a higher sensitivity, and a relatively faster decay time. This is probably due to the high insulativity and low defect concentration in the p-type Mg-doped Ga2O3 thin film. This work suggests that doping other elements into Ga2O3 thin film might be a new way for the development of Ga2O3 solar-blind photodetector with high performance in the future.
Almost at the same time, Fikadu Alema et al.[23] reported the fabrication and characterization of MSM solar-blind photodetector based on Zn doped β-Ga2O3 epitaxial thin film. The epitaxial ZnGaO thin film was grown on c-sapphire substrate by MOCVD technology. High responsivity
Qian Feng et al.[26] tried to dope Al into the β-Ga2O3 film and the single crystallinity (AlGa)2O3 epitaxial film was obtained on sapphire substrate. The (AlGa)2O3 epitaxial film exhibited wider bandgap compared to β-Ga2O3. The (AlGa)2O3 solar-blind photodetector showed higher photocurrent and enhanced responsivity, which was due to the enhanced conductivity of the material caused by shallow defects in the (AlGa)2O3 epitaxial film. However, when the Al composition increased up to a specific value, significant performance degradation of the (AlGa)2O3 solar-blind photodetector was observed, which was probably associated with the deep defects. It can be concluded that suitable dopants can modify the material characteristics: change the bandgap, improve the crystalline quality and electrical characteristics, and so on. Doping with optimized composition is a good way to fabricate high performance solar-blind Ga2O3 photodetectors in the future. Recently, Shuo-Huang Yuan et al.[27] reported an MSM aluminum–gallium oxide (AGO) photodetector with improved responsivity drop from 250 nm to 200 nm by incorporating trace aluminum into gallium oxide through magnetron sputtering. By optimizing the Al content, the dark current, photocurrent, responsivity, and detectivity of AGO PD were enhanced by 0.83, 46.4, 53.61, and 96.5 times, respectively, greater than those of the GO one. What is more, the solar-blind AGO photodetector based on magnetron sputtered AGO film showed higher performance than that deposited by MBE techonology,[26] which suggested that the magnetron sputtered AGO film has great potential for deep UV photodetection.
Surface plasmon polariton is an electro-magnetic wave coupled to free electron oscillations near the surface of metal, and has usually been used to improve the photoelectric properties in many optoelectronic devices. In order to enhance the performance of the Ga2O3 photodetector, Yuehua An et al.[28] introduced surface plasmon Au to form Au nanoparticles (NPs)/β-Ga2O3 composite thin film. The Au ultra-thin film was deposited on the β-Ga2O3 thin film followed by post-thermal treatment. The Au NPs/β-Ga2O3 composite thin film presents another significant absorption around 510 nm besides the absorption at the wavelength of less than 250 nm induced by the β-Ga2O3 thin film. The photoelectric performance of the Au NPs/β-Ga2O3 composite thin film-based photodetector is much higher than the pure β-Ga2O3 photodetector because of the assistance of surface plasmon polariton effect. Besides, Shujuan Cui et al.[29] reported a Ga/Ga2O3 nanocomposite solar-blind photodetector. Photoresponse enhancement was achieved by incorporating Ga surface plasmon into the nanocomposite film. By optimizing the thickness of the Ga interlayer, the Ga2O3/Ga/Ga2O3 photodetector exhibited an extremely low dark current of 8.52 pA at 10 V bias, a very high light-to-dark ratio of ∼8×105 and a responsivity of 2.85 A/W at 15 V bias. These results indicate that the plasmonics are important to improve the photoresponse characteristics of the Ga2O3 photodetector and the plasma modified Ga2O3 photodetector is a potential candidate for practical application.
There are a few reported works on the MSM photodetectors based on exfoliated β-Ga2O3 micro-flake.[30,31] Ni/Au was selected to form the Schottky contact and extremely low dark current and high sensitivity (ratio of photocurrent to dark current
![]() | Fig. 10. (a) Schematic diagram of the exfoliated β-Ga2O3 flake based MSM solar-blind photodetector with graphene electrodes. I–V characteristics of the fabricated solar-blind MSM photodetectors in the dark and under light illumination at different conditions (254 nm or 365 nm): (b) with Ni/Au contacts and (c) with graphene contacts. Time-dependent photoresponse of the exfoliated β-Ga2O3 flake based MSM solar-blind photodetectors under 254 nm or 365 nm light illumination (d) with Ni/Au contacts and (e) with graphene contacts.[30] |
In recent years, solar-blind Ga2O3 photodetectors have achieved much progress and the high-performance photodetectors with high responsivity, high detectivity, and high response speed are approaching to practical application. For example, the solar-blind Ga2O3 photodetectors are a potential candidate for DUV imaging. However, the photodetector array is a prerequisite for real-time imaging and can also improve the detectivity sensitivity. Recently, Yangke Peng et al.[32] firstly fabricated the highly integrated MSM Ga2O3 32 × 32, 16 × 16, 8 × 8, and 4 × 4 photodetector arrays. The image of the photodetectors arrays and their microstructure are shown in Fig.
![]() | Fig. 11. (a) The image of photodetectors arrays. (b) Schematic diagram of the photodetectors arrays. (c) The microstructure image of the fabricated devices. (d) Photograph of the photodetectors arrays packaged in ceramic packages.[32] |
![]() | Table 3.
Comparison of key figures of merit of recent Ga2O3 based MSM photodetectors. . |
Schottky photodiodes have many advantages over MSM photodetectors, such as high response speed and possible zero-bias operation due to the built-in electric field, high quantum efficiency, low dark current, and high UV/dark current ratio because of the existence of Schottky barrier. The characteristic of the Schottky photodiodes is the rectifying behavior of the I–V curve. Usually, a metal is deposited on the surface of the semiconductor, and because of the work function difference of the metal and semiconductor, a Schottky barrier will form in the metal/semiconductor junction. Besides, two kinds of semiconductors are usually put together to form a Schottky junction, and the I–V characteristics also exhibit rectifying property.
The first solar-blind β-Ga2O3 photodiode was reported in 2009 by Rikiya Suzuki et al.[33] Au was deposited on single crystal Ga2O3 substrate as the Schottky contact. The responsivity of the photodiode was enhanced dramatically after the diode was annealed at 400 °C, leading to a responsivity as high as 103 A/W. Chao Yang et al.[34] reported a self-powered SBD photodetector based on single crystal Ga2O3 substrate with Cu (20 nm) as the Schottky contact and Ti/Au (20 nm/200 nm) as the ohmic contact. The device was annealed at 150 °C in N2 for 5 min. As shown in Fig.
![]() | Fig. 12. (a) The I–V characteristics of the solar-blind Cu/Ga2O3 Schottky photodetector in the dark. (b) Photoresponse spectrum of the photodetector at zero bias.[34] (c) The I–V characteristics of the β-Ga2O3/Ga:ZnO heterojunction based photodetector in the dark. The inset shows the schematic figure of the β-Ga2O3/Ga:ZnO photodetector. (d) Time-dependent photoresponse characteristics under illumination of 254 nm at 0 V and its corresponding fitting results.[37] |
In order to improve the responsivity, Daoyou Guo et al.[36] combined Nb:SrTiO3 (NSTO) with β-Ga2O3 and realized zero-bias operation. The β-Ga2O3/NSTO heterojunction based solar-blind photodetector presented a high responsivity of 43.31 A/W and EQE of 2.1×104% at the bias voltage of −10 V and
Zhenping Wu et al.[37] employed lattice compatible semiconductor Ga:ZnO to fabricate β-Ga2O3/Ga:ZnO heterojunction based DUV photodetector and the zero-bias operation function was achieved. Figures
Because of the high thermal and chemical stability of Ga2O3, it is a promising candidate for photodetectors working in high temperature. Shihyun Ahn et al.[38] evaluated the performance degradation of the Si-implanted β-Ga2O3 photodetectors by elevating temperature from 25 °C to 350 °C. The responsivity and EQE were found to increase from 5 A/W to 36 A/W and 2.5×103 to 1.75×104, respectively, over the temperature range, which was assumed to be caused by the carrier multiplication effect. This result suggests the potential of β-Ga2O3 solar-blind photodetectors for high temperature application.
In addition to using metal as the Schottky contact, other semiconductors are also selected to form Schottky junctions. The other wide bandgap materials such as SiC, GaN, ZnO, and diamond were reported to form Schottky heterojunctions with Ga2O3, and the heterojunction-based photodiodes showed relatively high photoelectric performance, but still need to be improved. Shinji Nakagomi et al.[39] demonstrated a β-Ga2O3/SiC heterojunction based deep UV photodiode (Fig.
![]() | Fig. 13. Device structure of (a) SiC/β-Ga2O3 and (d) GaN/β-Ga2O3 heterojunction-based photodiodes. The I–V characteristics of (b) SiC/Ga2O3 and (e) GaN/β-Ga2O3 heterojunction-based photodiodes under illumination with different light intensities. Photoresponse spectrum of photodiodes based on (c) β-Ga2O3/SiC and (f) GaN/β-Ga2O3 heterojunctions.[39,40] (g) Schematic diagram of the SiC/β-Ga2O3 UV photodetector with different top electrodes. (h) Responsivity as a function of bias voltage of the SiC/Ga2O3 UV photodetector with graphene and Au top electrodes, respectively. (i) Time-dependent photoresponse characteristics of the SiC/Ga2O3 UV photodetector with graphene as the top electrode.[15] |
Meilin Ai et al.[41] reported a graphene/β-Ga2O3/graphene sandwich structure UV photodetector with a fast response speed. The fabricated photodetector exhibited good rectifying behavior in the dark and the current increased obviously under 254 nm light illumination (Figs.
![]() | Fig. 14. (a) Schematic of the graphene/β-Ga2O3/graphene hybrid structure-based photodiode.[41] (b) I–V characteristics of the graphene/β-Ga2O3/graphene hybrid structure-based photodiode in the dark and under light illumination of 254 nm.[41] (c) Time-dependent photoresponse of the graphene/β-Ga2O3/graphene hybrid structure-based photodiode.[41] (d) I–V curves of the graphene-β-Ga2O3 heterojunction-based DUV photodiode in the dark and under 254 nm light illumination.[14] (e) Schematic figure of graphene/Ga2O3/p-GaN heterojunction-based photodiode.[43] (f) Time-dependent photoresponse characteristics of the graphene/Ga2O3/p-GaN heterojunction-based photodiode.[43] (g) Detectivity and responsivity as a function of bias voltage of the graphene/Ga2O3/p-GaN heterojunction-based photodiode.[43] (h) Schematic energy diagram of the α-Ga2O3/ZnO heterojunction at high reverse bias voltage under illumination of 254 nm.[12] (i) Photoconductive gain of the α-Ga2O3/ZnO heterojunction-based solar-blind photodetector as a function of bias voltage under 254 nm and 365 nm illumination.[12] |
Schottky photodiodes usually exhibit high response speed but lower responsivity. Daoyou Guo et al.[44] tried to combine ZnO with β-Ga2O3 and fabricated ZnO/β-Ga2O3 heterojunction-based UV photodetector. The zero-bias operation functionality was achieved but the responsivity of 0.35 A/W remained too low. The avalanche photodiode is an ideal candidate to break the trade-off between the responsivity and response speed. Xuanhu Chen et al.[12] reported a method of combining ZnO with α-Ga2O3 to make solar-blind photodetector with high avalanche gain. LMBE was used to deposit the α-Ga2O3 thin film on ZnO. The energy diagram and photoconductive gain of the device are shown in Figs.
There are some works based on ZnO-Ga2O3 core-shell heterojunction microwire to make solar-blind photodetectors.[45,46] In Ref. [46], the CVD grown ZnO-Ga2O3 core-shell microwire was transferred to PET substrate, and piezo-phototronic effect of ZnO was utilized to enhance the UV photocurrent response by about three times under −0.042% static strain. Bin Zhao et al.[45] also used CVD approach to grow the ZnO-Ga2O3 core-shell microwire and the heterojunction showed good rectifying behavior according to the I–V characteristics. The device exhibited high UV/visible rejection ratio (R251 nm/R400 nm) of 6.9×102 under zero bias. What is more, the self-powered photodetector showed ultrahigh response speed with rise time of
Besides, Yan-Cheng Chen et al.[13] made a self-powered diamond/β-Ga2O3 photodetectors, and for the first time, they reported on solar-blind imaging of Ga2O3 based photodetectors, as shown in Fig.
![]() | Fig. 15. (a) Schematic presentation of the imaging system by using the diamond/β-Ga2O3 heterojunction-based solar-blind photodetector as the sensing pixel at bias voltage of 0 V. (b) The imaged object with letters of “UV” on a black paper. (c) The image obtained from the imaging system.[13] |
![]() | Table 4.
Comparison of key figures-of-merit of recent Ga2O3 based Schottky photodiodes. . |
We have reviewed the research works on Ga2O3 based solar-blind photodetectors reported in recent years. Great achievements were realized. The classification, mechanism, and performance of different photodetectors have been summarized. The Ga2O3 based solar-blind photodetectors are now mainly based on two structures: MSM and Schottky diode. MSM structure based Ga2O3 photodetectors have the advantages of easy-fabrication and integration and their performances are dependent on the size of the interdigital electrodes. The Ga2O3 Schottky diodes exhibit high response speed and high rejection ratio. Different kinds of metals were used to form Schottky contact with Ga2O3, and other wide bandgap materials such as SiC, GaN, ZnO, and diamond were also employed to form heterojunction with Ga2O3. Good rectifying characteristics were achieved in these devices. Besides, graphene was used to form Schottky contact with Ga2O3, and the photodetectors exhibited high performance. The photoelectric performance of the Ga2O3 solar-blind photodetector is not only dependent on the device structure but also dependent on the quality of the Ga2O3. At the beginning, the Ga2O3 solar-blind photodetectors were based on single crystal bulk or thin film β-Ga2O3. Different nano-optoelectronic devices have been reported to detect DUV light using exfoliated Ga2O3 nano-flakes. These devices also exhibited excellent DUV photodetection characteristics, such as fast switching speed and high responsivity. The combination of exfoliated Ga2O3 nano-flakes and 2D materials opens up a way for novel functional nano-devices. Amorphous Ga2O3 based solar-blind photodetectors are being paid more and more attention due to their cost-effective, easy-fabrication process and high performance in certain aspects. By optimizing the growth condition, amorphous Ga2O3 thin film based Ga2O3 photodetectors are potential for the practical application in the future. Although great progress has been made in Ga2O3 photodetectors, the responsivity remains too low to be applicated practically. Much more efforts need to be made to improve the responsivity, EQE, and the response speed at the same time. Besides, p-type doping of Ga2O3 is still a bottleneck problem which severely limits its application in the optoelectronic system. Furthermore, the severe persistent photoconductivity effect, which is common in oxide, needs to be addressed. In addition, for further practical application, the imaging and ultraviolet light communication based on Ga2O3 photodetectors deserve to be more insensitively explored.
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