Development of high gain avalanche photodiodes for UV imaging applications
ABSTRACT High-resolution imaging in ultraviolet (UV) bands has many a pplications in defense and commercial systems. The shortest wavelength is desired for increased spatial resolution, which allows for small pixels and large formats. The next frontier is to develop UV avalanche photodiode (UV-APD) arrays with high gain to demonstrate high-resolution imaging. We compare performance characteristics of front-illuminated Al 0.05 Ga 0.95 N UV-APDs grown on a free-standing (FS) GaN substrate and a GaN/sapphire template. UV-APDs grown on a FS-GaN substrate show lower dark current densities for all fabricated mesa sizes than similar UV-APDs grown on a GaN/sapph ire template. In addition, stable avalanche gain higher than 5×10 5 and a significant increase in the responsivity of UV-APDs grown on a FS-GaN substrate are observed as a result of avalanche multiplication at high reverse bias. We believe that the high crystalline quality of Al 0.05 Ga 0.95 N UV-APDs grown on a FS-GaN substrate with low dislocation density is responsible for the observed improvement of low leakage currents, high performance photodetector characteristics, and reliability of the devices. Keywords: GaN, AlGaN, avalanche photodiodes, ultraviolet, high gain