High-sensitivity ultraviolet (UV) detectors are required in many critical applications such as corona discharge, missile plume detection, environmental monitoring, and non-line-of-sight communications. As an attractive candidate for weak UV signal detection, avalanche photodiodes (APDs) operating in Geiger mode exhibit promising performance, including small size, low dark current, and high multiplication gain. Wide-bandgap semiconductor materials, such as GaN and SiC, can effectively shield the influence of visible light and infrared light, showing obvious advantages in the field of UV detection. The defect density of GaN is relatively high, which leads to a generally high dark current in GaN APDs. In addition, the photoresponse behavior of GaN APDs under high pressure undergoes a significant red shift, and the cut-off wavelength is extended to 440 nm, indicating the loss of visible light blindness. In comparison, SiC can construct APDs with a much lower dark current than GaN owing to its excellent material epitaxial technology. However, there is still little research on the high-voltage photoresponse characteristics of SiC APDs, which are a key issue related to the background noise of the device. This work discusses the photoresponse behavior of SiC APD under high voltages. Moreover, owing to material defects, the size of the SiC APD is always below 300 μm, but a device with a large photosensitive area is needed to improve the detection sensitivity. Although some studies have reported SiC APDs with a diameter of 800 μm, the key parameter of the single-photon detection efficiency has not been successfully detected. In this study, low-dark-current SiC APDs with a diameter of 500 μm were successfully fabricated, and the devices exhibited single-photon detection performance. This is clearly a breakthrough in terms of the size of SiC APDs.

SiC APDs were fabricated on n-type 4H-SiC substrates (Fig. 1). The epi-structure from bottom to top consists of a 10 μm p+ layer (N_A=3×10¹⁸ cm^-3), a 0.78 μm n- multiplication layer (N_D=1×10¹⁵ cm^-3), a 0.2 μm n layer (N_D=1×10¹⁸ cm^-3), and a 0.15 μm n+ contact layer (N_D=1×10¹⁹ cm^-3). To suppress peak electrical field around device edge, the beveled mesa with a slope angle of 5° was obtained via photoresist reflow technique, and the mesa was etched down to the multiplication layer by inductively coupled plasma. The device surface was then passivated by thermal oxidation at 1050 ℃ in oxygen atmosphere followed by a 1 μm SiO₂ layer deposited by plasma enhanced chemical vapor deposition at 350 ℃. The n and p type metal stacks, both based on Ni/Ti/Al/Au (35 nm/50 nm/100 nm/100 nm), were deposited by electron-beam evaporation. The devices were then annealed at 850 ℃ for 3 min in N₂ ambient by rapid thermal annealing.

To analyze whether the SiC APDs still have visible light blindness in the Geiger mode, the photoresponse characteristics of the SiC APD are measured under high voltages. The results show that the response peak of SiC APD is always maintained at 280 nm when the voltage changes from 0% to 90% breakdown voltage (Fig. 3). It is proved that SiC APDs still exhibit visible-light blindness characteristics under high voltages. Owing to the properties of SiC, SiC APD enables the shielding effect of visible and infrared light, which greatly reduces the complexity, volume, and cost of the device. The activation energy of the 500 μm SiC APD is 0.131 eV (Fig. 5), which indicates that the tunneling effect is the main cause of the dark current. At present, the best SiC epitaxy technology can grow epitaxial wafers with a dislocation density of 1000-2000 cm^-2. This implies that there are at least 2-4 dislocations in SiC APD with a diameter of 500 μm, which exacerbates defect-assisted tunneling and leads to a rapid increase in dark current. Therefore, the material defect density is a key problem that restricts the development of large-sized SiC APD. The dark current of the reported SiC APDs at 95% breakdown voltage has been calculated, and the comparison shows that the 500 μm SiC APDs fabricated in this work have a lower dark current (Fig. 7). Most importantly, the 500 μm SiC APDs in this work still have the single-photon detection capability. At a dark count rate of 1 Hz/μm², the single photon detection efficiency of the device is 0.7%. The most recently reported largest diameter of SiC APD with single-photon detection capability was 300 μm. Although the single-photon detection efficiency of the 500 μm SiC APD reported in this work needs to be improved, a breakthrough in device size has been achieved.

In this work, by studying the photoresponse characteristics of SiC APDs under high voltages, it is proved that SiC APDs still exhibit visible light blindness in the avalanche breakdown state, which is more suitable for weak UV light detection than traditional Si or GaN. In addition, we successfully achieve a breakthrough in the photosensitive area of SiC APD and fabricate a large-sized SiC APD with single-photon detection performance. The dark current of the device is better than the existing level. However, to further improve the single-photon detection efficiency of large-sized SiC APDs, it is necessary to optimize the quality of SiC epitaxial wafers in future work.