Results and Discussions During the operation of APDs, the uniform distribution of the electric field in the depletion region is a key factor affecting the stability and reliability of the APDs. Therefore, the regulation of electric field distribution by the structure of APDs needs to be considered during APD fabrication. The electric field distribution of the APD at avalanche state is simulated by Silvaco to verify the suppression effect of the SiC SACM APD with a beveled partial mesa structure on the edge electric field (Fig. 2). The results show that partial mesa etching can effectively suppress the peak edge electric field of SiC SACM APDs, and the SACM APD fabricated in this study is a reach-through SACM APD as a high electric field punches through all the active layers. The SACM APD with a partial mesa structure achieves a fill factor of about 58%, which is 1.6 times that of the conventional SACM APD. The current-voltage curves of the reach-through SiC SACM APD and the SiC positive-intrinsic-negative (PIN) APD (Fig. 3) show that the avalanche current of the reach-through SiC SACM APD increases slower than that of the SiC PIN APD. The reverse bias voltage applied to the device does not completely act on the multiplication layer. A small change rate of the electric field intensity at the multiplication layer results in a slow increase in the avalanche current with the voltage, which is beneficial to improving the voltage withstanding performance of the device. In addition, the dark count rate of the reach-through SiC SACM APD is only 0.5 Hz/μm² when the over-bias voltage is 4 V, and the single-photon detection efficiency of the device reaches 8.4% when the dark count rate is 1 Hz/μm² (Fig. 5). The low carrier tunneling probability and high photon avalanche probability of the device lead to the low dark count rate and high single-photon detection efficiency of the punch-through SiC SACM APD, and they all contribute to the extension of the electric field to the absorption layer.

As a weak ultraviolet (UV) detector with unique advantages, SiC avalanche photodiodes (APDs) are imperative in many key fields, such as environmental monitoring, corona detection, missile plume detection, deep space detection, and ultraviolet communication. A SiC APD is highly susceptible to irreversible thermal breakdown as its current is extremely sensitive to the bias voltage when it works under the condition of a critical electric field. Therefore, the overbias voltage withstanding capability of a SiC APD is a key issue affecting the working stability of the APD. In addition, the dark count rate is an important parameter that determines the detection sensitivity of the APD in weak UV detection. However, the reported SiC APDs exhibit low overbias voltage withstanding capabilities and high dark count rates. SiC APDs with high overbias voltage withstanding capabilities and low dark count rate have been designed and fabricated in this study.

In this study, SiC separated-absorption-charge-multiplication (SACM) APDs have been designed and fabricated. The SiC APDs are fabricated on n⁺ type 4H-SiC substrates (Fig. 1). The epitaxial structure of the SiC APDs consists of a 10-μm p type contact layer, a 0.65-μm n^- type multiplication layer, a 0.15-μm n type charge control layer, a 0.6-μm n^- type absorption layer, and a 0.2-μm n type contact layer from bottom to top. The fabrication process starts with mesa etching down to the multiplication layer (to an etching depth of 1.05 μm) by inductively coupled plasma etching. The photoresist reflow technique is employed to obtain a positive beveled mesa (with a small slope angle of about 5°) and thereby prevent mesa edge breakdown. Then, the epitaxial wafer is etched to the bottom contact layer. Subsequently, the APD surface is passivated by a thermal oxidation layer and then by a SiO₂ layer deposited by plasma-enhanced chemical vapor deposition. Both the n-type and p-type Ohmic contact electrodes adopt Ni/Ti/Al/Au (35 nm/50 nm/100 nm/100 nm) layers deposited by e-beam evaporation. Finally, the epitaxial structure is annealed by rapid thermal annealing at 850 ℃ for 3 min in N₂ atmosphere.

In this study, a reach-through SiC SACM APD is designed and fabricated. When the device undergoes avalanche breakdown, the electric field extends from the multiplication layer to the absorption layer and the charge control layer. The change rate of the electric field at the multiplication layer decreases, and the avalanche current exhibits a smaller slope accordingly, which is conducive to improving the over-bias voltage withstanding capability of APDs. Moreover, APDs with a small-slope avalanche current can alleviate the breakdown voltage fluctuation among the pixels in the UV imaging array, which is of great significance for high-quality weak UV imaging. In addition, partial mesa etching adopted for the SiC SACM APD designed in this study not only ensures the reliable operation of the device but also increases the fill factor of the device to about 60%, which is beneficial for improving the integration level of imaging array chips.