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 2.7x10(-4) A/cm(2) whereas APDs grown on FS-GaN substrates had a significantly lower dark current density of 2.1x10(-6) A/cm(2). 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-mu m(2)-area APD yielded a SPDE of similar to 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 similar to 30% under back-illumination-the FS-GaN APDs were only tested under front illumination due to the thick absorbing GaN substrate. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3457783]