High-Gain and Low-Dark Current GaN p-i-n Ultraviolet Avalanche Photodiodes Grown by MOCVD Fabricated Using Ion-Implantation Isolation

Front-illuminated GaN-based p-i-n ultraviolet (UV) avalanche photodiodes (APDs) were grown by metalorganic chemical vapor deposition on a free-standing GaN substrate. X-ray diffraction measurements confirm the high crystal quality of the grown structure. Atomic force microscopy measurements show well-developed atomic step-flow morphologies with sub-nanometer root-mean-square surface roughness for scan sizes of 1 x 1 mu m(2) and 5 x 5 mu m(2). Mesa etching is one of the major challenges in the fabrication of these devices. Inductively coupled plasma mesa etching creates damage which leads to an increase of the dark current, premature device breakdown, and noise. Front-illuminated GaN-based UV APDs were fabricated using ion-implantation isolation to reduce the sidewall leakage current and improve reliability. The devices show a very low reverse-bias low leakage current density of below -10-10 A/cm(2) up to - 35 V. A maximum multiplication gain of 1 x 10(6) under lambda = 355 nm illumination was demonstrated (device limited to a current density of 10 A/cm(2)). The spectral responsivity of an 82-mu m-diameter APD shows a peak responsivity of 1.05 A/W under UV illumination at. = 375 nm at the avalanche-break-down voltage.