Al_0.1Ga_0.9N p-i-n Ultraviolet Avalanche Photodiodes With Avalanche Gain Over 10⁶

We report Al _0.1 Ga _0.9 N p-i-n ultraviolet (UV) avalanche photodiodes (APDs) based on sapphire substrates with a record-high gain over $2 \times 10^{6}$ . The devices fabricated with various mesa diameters present consistent avalanche behaviors and identical dark current distributions over multiple ${I}-{V}$ scans, which are comparable to the GaN APDs grown on free-standing GaN substrates. The quadratic fitting of the dark currents versus the mesa sizes reveals that the leakage current at the breakdown voltage is a mixture of surface leakage and bulk leakage, and the surface component is comparable to that of the bulk. Additionally, KOH surface treatment and SiO ₂ passivation are proven to be very effective in suppressing the leakage current and achieving robust avalanche performance.