Geiger-mode operation of ultraviolet avalanche photodiodes grown on sapphire and free-standing GaN substrates

GaN avalanche photodiodes (APDs) were grown on both conventional sapphire and low dislocation density free-standing (FS) c-plane GaN substrates. Leakage current, gain, and single photon detection efficiency (SPDE) of these APDs were compared. At a reverse-bias of 70 V, APDs grown on sapphire substrates exhibited a dark current density of 2.7×10−4 A/cm2 whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1×10−6 A/cm2. Under linear-mode operation, APDs grown on FS-GaN achieved avalanche gain as high as 14 000. Geiger-mode operation conditions were studied for enhanced SPDE. Under front-illumination the 625-μm2-area APD yielded a SPDE of ∼13% when grown on sapphire substrates compared to more than 24% when grown on FS-GaN. The SPDE of the same APD on sapphire substrate increased to ∼30% under back-illumination—the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate.