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

For solar-blind ultraviolet detection, AlGaN avalanche photodiodes (APDs) that 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 the Geiger mode. The APDs were fabricated from high-quality GaN epitaxial layers grown by hydride vapor phase epitaxy. The GaN layer structure consisted of a Zn-doped π 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 sapphire substrate. Capacitance-voltage (C-V) measurements on photodiodes fabricated from some of these layers show that field strengths between 3 and 4 MV/cm are sustainable in the depletion region at voltages slightly below the observed breakdown of ~80 V. Both mesa-etched and planar devices exhibited avalanche gains of 10 in linear-gain mode and ~10⁶ in Geiger mode when top illuminated with a 325 nm HeCd laser. Raster measurements of the photoresponse show highly uniform response in gain mode that becomes slightly more inhomogeneous in Geiger mode