We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga2O3-based solar-blind photodetectors in metal-semiconductor-metal (MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga2O3 interface is realized by tuning the conductivity of amorphous Ga2O3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga2O3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity (PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA, a high detectivity of 9.82×1012 cm·Hz1/2·W-1, a fast response time of 243.9 μs, and a high ultraviolet C (UVC)-to-ultraviolet A (UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the sub-gap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.
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.
Synapse emulation is very important for realizing neuromorphic computing, which could overcome the energy and throughput limitations of today's computing architectures. Memristors have been extensively studied for using in nonvolatile memory storage and neuromorphic computing. In this paper, we report the fabrication of vertical sandwiched memristor device using ultrathin quasi-two-dimensional gallium oxide produced by squeegee method. The as-fabricated two-terminal memristor device exhibited the essential functions of biological synapses, such as depression and potentiation of synaptic weight, transition from short time memory to long time memory, spike-timing-dependent plasticity, and spike-rate-dependent plasticity. The synaptic weight of the memristor could be tuned by the applied voltage pulse, number, width, and frequency. We believe that the injection of the top Ag cations should play a significant role for the memristor phenomenon. The ultrathin of medium layer represents an advance to integration in vertical direction for future applications and our results provide an alternative way to fabricate synaptic devices.
Gallium oxide (Ga2O3), a typical ultra wide bandgap semiconductor, with a bandgap of~4.9 eV, critical breakdown field of 8 MV/cm, and Baliga's figure of merit of 3444, is promising to be used in high-power and high-voltage devices. Recently, a keen interest in employing Ga2O3 in power devices has been aroused. Many researches have verified that Ga2O3 is an ideal candidate for fabricating power devices. In this review, we summarized the recent progress of field-effect transistors (FETs) and Schottky barrier diodes (SBDs) based on Ga2O3, which may provide a guideline for Ga2O3 to be preferably used in power devices fabrication.
High-resistivity β -Ga2O3 thin films were grown on Si-doped n-type conductive β -Ga2O3 single crystals by molecular beam epitaxy (MBE). Vertical-type Schottky diodes were fabricated, and the electrical properties of the Schottky diodes were studied in this letter. The ideality factor and the series resistance of the Schottky diodes were estimated to be about 1.4 and 4.6× 106 Ω . The ionized donor concentration and the spreading voltage in the Schottky diodes region are about 4× 1018 cm-3 and 7.6 V, respectively. The ultra-violet (UV) photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination. A photoresponsivity of 1.8 A/W and an external quantum efficiency of 8.7× 102% were observed at forward bias voltage of 3.8 V, the proper driving voltage of read-out integrated circuit for UV camera. The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive β -Ga2O3 thin films and the n-type conductive β -Ga2O3 single-crystal substrate.
Monoclinic gallium oxide (β-Ga2O3) nanobelts are synthesized from gallium and oxygen by thermal evaporation in an argon atmosphere and their NO2 sensing properties are studied at room temperature. Electron microscopy studies show that the nanobelts have breadths ranging from 30 to 50 nm and lengths up to tens of micrometers. Both the x-ray diffraction (XRD) and the selected are electron diffraction (SAED) examinations indicate that β-Ga2O3 nanobelts have grown into single crystals. Room temperature NO2 sensing tests show that the current of individual β-Ga2O3 nanobelt decreases quickly, and then gently when the NO2 concentration increases from low to high. It is caused by the NO2 molecule chemisorption and desorption processes in the surface of β-Ga2O3 nanobelt.