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We report high-performance Al0. 1Ga0. 9N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with the employment of negative bevel edge terminations. An ultra-low slope angle of 2° was achieved through the optimization of photoresist reflow and inductively coupled plasma (ICP) etching processes. The devices present stable breakdown voltages (V ₁ₑ) around 115. 9 V, low dark current densities (J ₁ₑ) below 5 10^-{4} A/cm2 and a record-high avalanche gain over 7 10^{6}. The electroluminescence (EL) during the avalanche multiplication process assembles in the device center, which is in good agreement with the TCAD simulations, indicating that the electric field crowding effect along the device periphery is significantly suppressed. Additionally, the tapered sidewall profile ameliorates the UV absorption loss in the p-GaN layer and leads to a high external quantum efficiency (EQE) of 29. 74% under zero bias. Finally, through the exponential fitting of the dark current density versus the electric field strength, variable-range hopping (VRH) conduction is found to be the dominant leakage mechanism for Al0. 1Ga0. 9N APDs for the first time.
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