Ultra-Robust Deep-UV Photovoltaic Detector Based on Graphene/(AlGa)2O3/GaN with High-Performance in Temperature Fluctuations

2D Embedded Ultrawide Bandgap Devices for Extreme Environment Applications

Ultrawide bandgap semiconductors such as AlGaN, AlN, diamond, and β-Ga2O3 have significantly enhanced the functionality of electronic and optoelectronic devices, particularly in harsh environment conditions. However, some of these materials face challenges such as low thermal conductivity, limited P-type conductivity, and scalability issues, which can hinder device performance under extreme conditions like high temperature and irradiation. In this review paper, we explore the integration of various two-dimensional materials (2DMs) to address these challenges. These materials offer excellent properties such as high thermal conductivity, mechanical strength, and electrical properties. Notably, graphene, hexagonal boron nitride, transition metal dichalcogenides, 2D and quasi-2D Ga2O3, TeO2, and others are investigated for their potential in improving ultrawide bandgap semiconductor-based devices. We highlight the significant improvement observed in the device performance after the incorporation of 2D materials. By leveraging the properties of these materials, ultrawide bandgap semiconductor devices demonstrate enhanced functionality and resilience in harsh environmental conditions. This review provides valuable insights into the role of 2D materials in advancing the field of ultrawide bandgap semiconductors and highlights opportunities for further research and development in this area.

Hydrothermal Growth of an Al-Doped α-Ga2O3 Nanorod Array and Its Application in Self-Powered Solar-Blind UV Photodetection Based on a Photoelectrochemical Cell

Herein, we successfully fabricated an Al-doped α-Ga₂O₃ nanorod array on FTO using the hydrothermal and post-annealing processes. To the best of our knowledge, it is the first time that an Al-doped α-Ga₂O₃ nanorod array on FTO has been realized via a much simpler and cheaper way than that based on metal-organic chemical vapor deposition, magnetron sputtering, molecular beam epitaxy, and pulsed laser deposition. And, a self-powered Al-doped α-Ga₂O₃ nanorod array/FTO photodetector was also realized as a photoanode at 0 V (vs. Ag/AgCl) in a photoelectrochemical (PEC) cell, showing a peak responsivity of 1.46 mA/W at 260 nm. The response speed of the Al-doped device was 0.421 s for rise time, and 0.139 s for decay time under solar-blind UV (260 nm) illumination. Compared with the undoped device, the responsivity of the Al-doped device was ~5.84 times larger, and the response speed was relatively faster. When increasing the biases from 0 V to 1 V, the responsivity, quantum efficiency, and detectivity of the Al-doped device were enhanced from 1.46 mA/W to 2.02 mA/W, from ~0.7% to ~0.96%, and from ~6 × 10⁹ Jones to ~1 × 10¹⁰ Jones, respectively, due to the enlarged depletion region. Therefore, Al doping may provide a route to enhance the self-powered photodetection performance of α-Ga₂O₃ nanorod arrays.

Rapid Response Solar Blind Deep UV Photodetector with High Detectivity Based On Graphene:N/βGa2 O3 :N/GaN p-i-n Heterojunction Fabricated by a Reversed Substitution Growth Method

This work reports a high-detectivity solar-blind deep ultraviolet photodetector with a fast response speed, based on a nitrogen-doped graphene/βGa₂ O₃ /GaN p-i-n heterojunction. The i layer of βGa₂ O₃ with a Fermi level lower than the central level of the forbidden band of 0.2 eV is obtained by reversed substitution growth with oxygen replacing nitrogen in the GaN matrix, indicating the majority carrier is hole. X-ray diffractometershows that the transformation of GaN into βGa₂ O₃ with (-201) preferred orientation at temperature above 900 °C in an oxygen ambient. The heterojunction shows enhanced self-powered solar blind detection ability with a response time of 3.2 µs (rise)/0.02 ms (delay) and a detectivity exceeding 10¹² Jones. Under a reverse bias of -5 V, the photoresponsivity is 8.3 A W^-1 with a high I_light /I_dark ratio of over 10⁶ and a detectivity of ≈9 × 10¹⁴ Jones. The excellent performance of the device is attributed to 1) the continuous conduction band without a potential energy barrier, 2) the larger built-in potential in the heterojunction because of the downward shift of Fermi energy level in β-Ga₂ O₃ , and 3) an enhanced built-in electric field in the βGa₂ O₃ due to introducing p-type graphene with a high hole concentration of up to ≈10²⁰ cm^-3 .

Pt/ZnGa2O4/p-Si Back-to-Back Heterojunction for Deep UV Sensitive Photovoltaic Photodetection with Ultralow Dark Current and High Spectral Selectivity

In this work, a strategy of constructing a back-to-back heterojunction is proposed to fabricate Si-based photovoltaic photodetectors with high deep ultraviolet (DUV) spectral selectivity. By combining Pt with a thickness of 4 nm with a ZnGa₂O₄/Si heterojunction, a back-to-back heterojunction is successfully constructed. Based on that, a Pt/ZnGa₂O₄/p-Si DUV photovoltaic detector with a low dark current density (∼9.6 × 10^-5 μA/cm²), a large photo-to-dark current ratio (PDCR, >10⁵), and a fast response speed (decay time <50 ms) is fabricated. At 0 V bias, this device displays a photoresponsivity of about 1.36 mA/W and a high deep ultraviolet-visible (DUV-vis) rejection ratio (R_258 nm/R_420 nm) of ∼1.1 × 10⁵, which are 1-2 orders of magnitude higher than those of most photovoltaic DUV detectors reported currently. Even at a working temperature of 470 K, the detectivity of this device can still reach ∼1.23 × 10¹⁰ Jones. In addition, compared with Au/ZnGa₂O₄/Si devices, the dark current and PDCR of this Pt/ZnGa₂O₄/Si device decrease by 2 orders of magnitude and increase by 1 order of magnitude, respectively. The enhanced performance of this ZnGa₂O₄/Si device can be attributed to the higher Schottky barrier established between Pt with a higher work function and ZnGa₂O₄. This strategy of adopting a back-to-back heterojunction device structure to hinder the visible light photoresponse of Si-based photodetectors and thus to reduce the dark current of a device can provide a reference for preparing photovoltaic DUV detectors with excellent performance.

Pt/(InGa)2O3/n-Si Heterojunction-Based Solar-Blind Ultraviolet Photovoltaic Detectors with an Ideal Absorption Cutoff Edge of 280 nm

Ga₂O₃ is a popular material for research on solar-blind ultraviolet detectors. However, its absorption cutoff edge is 253 nm, which is not an ideal cutoff edge of 280 nm. In this work, by adjusting the ratio of In/Ga elements in the films, a high-quality (In_0.11Ga_0.89)₂O₃ film with an absorption cutoff edge of 280 nm was obtained, which owns a uniform surface and preferred orientation. On this basis, a solar-blind ultraviolet photovoltaic detector was constructed based on the Pt/(In_0.11Ga_0.89)₂O₃/n-Si heterojunction. When the device is exposed to 254 nm UV light, its open-circuit voltage (V_OC) can reach 354 mV. Under 0 V bias, the device has a responsivity of 0.48 mA/W with a rise time of 0.47 s and a decay time of 0.37 s; under -7 V bias, the device achieves a responsivity of 16.96 mA/W with a rise time of 0.17 s and a decay time of 0.33 s. The spectral response characteristics of the device show that it has a selective response to solar-blind ultraviolet light (cutoff wavelength is 280 nm) with a rejection ratio (R_{254 nm}/R_{310 nm}), which is greater by more than two orders of magnitude. This work provides a good reference for adjusting the band gap of Ga₂O₃-based films and broadening their application fields.

Highly Deformable High-Performance Paper-Based Perovskite Photodetector with Improved Stability

Paper-based photodetectors have attracted extensive research interest owing to their environmentally friendly and highly deformable properties. Although perovskite crystals with outstanding optoelectronic properties have proved to be one of the most promising candidates for photodetectors, the development of paper-based photodetectors is hindered by the moisture absorptivity of paper and the instability of perovskite crystals in a humid atmosphere. In this study, we demonstrate a highly deformable and high-performance paper-based perovskite photodetector. The photodetector maintains its excellent performance even after exposure to a relative humidity of 60% for 120 h.

High-Pressure O2 Annealing Enhances the Crystallinity of Ultrawide-Band-Gap Sesquioxides Combined with Graphene for Vacuum-Ultraviolet Photovoltaic Detection

(Al_xGa_1-x)₂O₃ is emerging as a promising wide-band-gap sesquioxide for vacuum-ultraviolet (VUV, 10-200 nm) photodetectors and high-power field-effect transistors. However, how the key parameters such as the band gap and crystalline phase of the (Al_xGa_1-x)₂O₃-based device vary with stoichiometry has not been explicitly defined, which is due to the unclear underlying mechanism of the Al local coordination environment. In this work, a high-pressure O₂ (20 atm) annealing (HPOA) strategy that can significantly improve the crystallinity of β-(Al_xGa_1-x)₂O₃ and achieve a tunable optical band gap was proposed, facilitating the revelation of the local structure of Al³⁺ varying with Al content and the kinetic mechanism of Al³⁺ diffusion. By combining the as-HPOA-treated single-crystalline β-(Al_0.69Ga_0.31)₂O₃ films with p-type graphene (p-Gr), which serves as a transparent conductor, a VUV photovoltaic detector is fabricated, showing an improved photovoltage (0.80 V) and fast temporal response (2.1 μs). All of these findings provide a rewarding and important strategy for enhancing the band-gap tunability of sesquioxides, as well as the flexibility of zero-power-consumption photodetectors.

Wide-Bandgap CaSnO3 Perovskite As an Efficient and Selective Deep-UV Absorber for Self-Powered and High-Performance p-i-n Photodetector

Calcium stannate (CaSnO₃) is an inorganic perovskite material with an ultrawide bandgap (4.2-4.4 eV) that is associated with its unique structural characteristics. Owing to its remarkable optical and electric properties and high physical and chemical stability, it has recently drawn significant interest for various applications such as photocatalysts for the degradation of organic compounds and hydrogen production under UV radiation, gas sensors, and thermally stable capacitors. In this study, we demonstrate a self-powered deep-UV (DUV) p-i-n photodetector consisting of CaSnO₃ thin film as an efficient DUV absorber via a low-temperature solution process. The physical, optical, and electrical properties of the as-synthesized CaSnO₃ are characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, high-resolution transmission electron microscopy, ultraviolet-visible spectroscopy, photoluminescence spectroscopy, space charge limited current, and four-point probe measurements. As a key component in a p-i-n DUV photodetector, the thickness of the CaSnO₃ absorber layer and operating bias are optimized to enhance charge carrier transport, light absorption, and signal-to-noise ratio. As a result, the optimized device shows a high performance at zero bias under 254 nm UV illumination: with a specific detectivity of 1.56 × 10¹⁰ Jones, fast rise/fall time of 80/70 ms, and high 254:365 nm photocurrent rejection ratio of 5.5 along with a stable photoresponse during 100 continuous cycles initially as well as after 1 month of storage. Accordingly, this study suggests that a novel CaSnO₃-based photodiode prepared via a solution process can be employed for many practical DUV-detection applications.

High-Performance X-ray Detector Based on Single-Crystal β-Ga2O3:Mg

X-ray detection plays an important role in medical imaging, scientific research, and security inspection. Recently, the β-Ga₂O₃ single-crystal-based X-ray detector has attracted extensive attention due to its excellent intrinsic properties such as good absorption for X-ray photons, a high breakdown electric field, high stability, and low cost. However, developing a high-performance β-Ga₂O₃-based X-ray detector remains a challenge because of the large dark current and the high oxygen vacancy concentration in the crystals. In this paper, we report a high-performance Mg-doped β-Ga₂O₃ single-crystal-based X-ray detector with a sandwich structure. The reduced dark current enables the detector to have a high sensitivity of 338.9 μC Gy^-1 cm^-2 under 50 keV X-ray irradiation with a dose rate of 69.5 μGy/s. The sensitivity is 16-fold higher than that of the commercial amorphous selenium detector. Furthermore, the reduced oxygen vacancy concentration can improve the response speed (<0.2 s) of the detector. The present studies provide a promising method to obtain the high performances for the X-ray detector based on β-Ga₂O₃ single crystals.

Raman Tensor of Layered SnS2

Recently, tin disulfide (SnS₂) has become a hot research focus in various fields due to its advantages of a high transistor switching ratio, an adjustable band gap in visible light range, excellent Li storage performance, sensitive gas recognition, and efficient photocatalytic capability. However, at present, studies of its basic structure mostly stay on the regulation related to the number of layers. To maximize the value of SnS₂ in the application design, this paper analyzes the angle-resolved polarized Raman spectra of SnS₂ crystals grown under high-temperature sealing systems. Under the parallel scattering configuration test of both the sample basal plane and the cross plane, we observed that how the Raman scattering intensity of the two test planes varies with the polarization angle is different. Combining this experimental result with theory support allows us to reach a conclusion that the differential polarizability of the phonon vibration mode along the z-axis of the cross plane of SnS₂ is proven to be the strongest. This finding is expected to provide favorable support for the application of structural regulation of SnS₂ and work as a reference for studying other van der Waals layered materials with greater potential.

Ultrasensitive Flexible Solar-Blind Photodetectors Based on Graphene/Amorphous Ga2O3 van der Waals Heterojunctions

Flexible photodetectors (PDs) have become the latest research interest owing to their potential applications in future implantable sensors and foldable/wearable optoelectronics. Ga₂O₃, an emerging ultrawide band gap semiconductor, is considered as the native photosensitive material for solar-blind PDs. The reported fabrication temperature of Ga₂O₃ films is usually above 600 °C, which hinders its practical application for flexible devices. In this work, flexible PDs based on graphene/amorphous Ga₂O₃ van der Waals heterojunctions are fabricated, which demonstrate promising photoresponse to solar-blind ultraviolet light. The device yields a high photo-to-dark current ratio (∼10⁵) and large responsivity (22.75 A/W) under 254 nm light illumination, which could be ascribed to the efficient photogenerated electron-hole pair separation by the strong built-in field. Moreover, flexible PDs also show long-term environmental stability and outstanding mechanical flexibility without any encapsulation. Our work provides a new potential candidate for realizing cost-effective high-performance flexible optoelectronic applications.