GaN avalanche photodiodes operating in linear-gain mode and Geiger mode

For solar-blind ultraviolet detection, AlGaN avalanche photodiodes (APDs) t hat operate in Geiger mode can outperform conventional AlGaN photodiodes in sensitivity and should compare favorably to photomultiplier tubes. Toward this goal, we report GaN APDs that operate in the linear-gain mode and in t he Geiger mode. The APDs were fabricated from high quality GaN epitaxial la yers grown by hydride vapor phase epitaxy. The GaN layer structure consiste d of a Zn-doped pi layer, an unintentionally doped n layer, and a Si-doped n+ layer-all on top of a thick GaN unintentionally doped n layer on a sapph ire substrate, Capacitance-voltage (C-V) measurements on photodiodes fabric ated from some of these layers show that held strengths between 3 and 4 MV/ cm are sustainable in the depletion region at voltages slightly below the o bserved breakdown of similar to 80 V. Both mesa-etched and planar devices e xhibited avalanche gains of 10 in linear-gain mode and similar to 10(6) in Geiger mode n hen top illuminated with a 325 nm HeCd laser. Raster measurem ents of the photoresponse show highly uniform response in gain mode that be comes slightly more inhomogeneous in Geiger mode.