Al0.1Ga0.9N 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.