Al$_{x}$Ga$_{1-x}$N Ultraviolet Avalanche Photodiodes Grown on GaN Substrates

Al$_{x}$Ga$_{1 - x}$N $(x = 0.05)$ ultraviolet (UV) avalanche photodiodes grown on a GaN substrate are reported. The epitaxial structure was grown by metal–organic chemical vapor deposition on a free-standing bulk GaN substrate having low dislocation density. The growth conditions for Al$_{x}$ Ga$_{1-x}$ N epitaxial layers on GaN substrates were optimized to achieve improved crystalline and structural quality. With UV illumination at $\lambda \sim 250$ nm, devices with mesa diameters of $\sim$30 $\mu$m achieve stable maximum optical gains of ${\sim}$ 50 at a reverse bias voltage of ${\sim}$87 V.