Study On Wide Bandgap Semiconductor Ultraviolet Photo Detecting Devices

Wide-bandgap semiconductors, which are defined as the third generation semiconductor materials, have excellent physical properties, such as large bandgap energy, high breakdown electric field, high electron saturation velocity, and high thermal conductivity, making them attractive for fabrication of high power and high temperature devices as well as high efficiency UV sensing devices. Thanks to the rencent ten years of intense R&D investment, the material quality and device fabrication process of group-Ⅲ nitrides and SiC have improved significantly. In this work, several UV sensing and photoconductive switching devices based on (Al)GaN and SiC have been designed, fabricated and characterized. The main experimental work and results are summaried as below:1. Metal-semiconductor-metal UV photodetectors (PDs) are directly fabricated on Fe-doped semi-insulating GaN template grown by hydride vapor phase epitaxy. Under 20 V bias, the PD exhibits a room temperature low dark current of <2.5 pA, a UV-to-visible rejection ratio of up to 1 X 102 and a corresponding maximum quantum efficiency of ～10%. A temporal response and recovery time of less than 2 ms is obtained for the PD, nevertheless its photocurrent is found to decrease continuously under stable UV illumination. There is evidence that the high-density trap states and recombination centers within the GaN:Fe layer have a strong influence on the photo-response characteristics of the PDs. This study provides a possible route to fabricate low cost GaN-based UV PDs with reasonable performance.2. Photoconductive semiconductor switches (PCSSs) are fabricated on semi- insulating Al0.4Ga0.6N layer and HVPE grown GaN:Fe template, respectively. The PCSS based on HVPE GaN:Fe template exhibits a cutoff wavelength of 365 nm and a dark resistivity of ～1010 Ω. cm. A maximum blocking voltage of more than 1100 V is obtained, corresponding to a breakdown electric field higher than 1.57 MV/cm for the GaN:Fe template. The AlGaN PCSS shows a cutoff wavelength of-280 nm, which is intrinsically immune to the solar background noise. When excited by a 266 nm ultraviolet pulsed laser, the AlGaN PCSS under 500 V bias could produce a peak photocurrent density of 11.5 kA/cm2 within a rise time of ～15 ns. The fall time of the photocurrent pulse for both PCSSs is mainly RC time limited.3.4H-SiC avalanche photodiodes (APDs) with beveled mesa structure are designed and fabricated. Since the peak electrical field along the mesa edge is effectively suppressed by the small angle positive bevel, the APD exhibits a low dark current lower than 1 pA, avalanche gain up to 105, dark count rate of 10 kHz and corresponding single photon detection efficiency of 3%. A method based on scanning Kelvin probe microscopy (SKPM) is used for measuring the electric potential distribution along the mesa sidewall. Based on this technique, the surface potential variation of the APD, exact location of the depletion region, and the difference of electric field profiles between positive bevel and negative bevel structures can be determined. Then based on a modified SNOM system, the distribution of photocurrents and photo counts for an operating APD under different over-bias are mapped, which is useful to locate the defective region and for structural optimizations. Finally, the tunneling characteristics of a 4H-SiC APD with thin intrinsic layer are studied. In addition to the negative temperature coefficient of breakdown voltage, the tunneling componment can also be distinguished from the shape of the transient current pulses when measuring the APD in a passive quenching circuit.