Study On 4H-SiC Ultraviolet Single-photon-Counting Avalanche Photodiodes

Ultraviolet (UV) photodetectors (PDs) have important civilian and military applications, including missile flame detection, biological/chemical agent detection, environmental monitoring, and solar physics. In many critical applications, UV PDs with high sensitivity and signal/noise ratio are required for low-level UV light sensing. Avalanche photodiodes (APDs) with high gain and low noise are desirable for these applications, which could even realize single photon detection.4H-SiC is a wide-bandgap semiconductor suitable for fabricating UV APDs, due to its’large bandgap energy, high electron saturation velocity, large hole/electron impact ionization coefficient ratio, superior radiation hardness, high temperature resistance, and relative mature material growth technology.In this work, the fabrication process of 4H-SiC p-i-n APDs is developed. Based on process optimizations, high performance 4H-SiC APDs with single photon counting capability are realized. The main results of this thesis are shown as beow:1. The fabrication process of 4H-SiC APDs is designed and optimized. Based on ICP dry etching technique, vertical mesa is fabricated by using metal layer as a hard mask; while by developing a special photoresist reflow technique, bevel mesa with small slope angle is realized. Ohmic metallization and high temperature passivation process are also optimized, respectively. A solar-blind bandpass filter based on AI/SiO2/AI resonant cavity structure is designed and fabricated for direct on-chip integration with SiC APDs.2.4H-SiC APDs with two different layer structures (n+-n-n--p and p+-p-p--n+ are fabricated based on the optimized fabrication process, which exhibit the maximum room temperature quantum efficiency and avalanche gain up to 53.4%(at 290 nm) and 2×106, respectively. The dark current of the SiC APD with bevel mesa is considerably lower than that of the device with vertical mesa, confirming that a positive bevel could effectively reduce the peak electrical field along the mesa edge. By integrating a solar-blind filter with Al/SiO2/Al structure on the top of the APD surface, SiC APDs with selective solar-blind spectral response are realized.3. A 1×8 SiC APD linear array is designed and fabricated, which exhibits uniform avalanche breakdown voltage among the pixels with a standard deviation of 0.148 V. While all pixels show similar magnitude of photocurrent, the dark currents of different pixels show certain variation, which are nevertheless all below 10-11 A level at 90% breakdown voltage, suggesting that the linear array could meet the basic requirement of UV detection and imaging applications.4. By using a passive quenching circuit, single photon counting based on 4H-SiC APDs is realized. The single photon detection efficiency (SPDE) of the SiC APD is 6.17%(280 nm) with a dark count rate (DCR) of 22 kHz at room temperature. Even at a high temperature of 150℃, the DCR of the APD is still around 80 kHz level at the same SPDE, indicating that the single photon counting APD could work in high temperature harsh environment. In order to obtain a better single/noise ratio, the SPDE and DCR of the APD are measured and compared at different operation bias and threshold voltage of avalanche pulses, the optimum values of operation bias and threshold voltage of avalanche pulses are evaluated.