Analysis of external quantum efficiencies of GaN homojunction p-i-n ultraviolet photodetectors

We have fabricated and characterized a series of homojunction p-i-n ultraviolet (UV) photodetectors on GaN grown by metalorganic chemical vapor deposition (MOCVD), They exhibit record low dark current densities (similar to 2 nA/cm(2) at -10-V bias) and high external quantum efficiencies (similar to 45% at lambda = 362 nm). We hale analyzed the spectral external quantum efficiency of these photodiodes using a standard drift-diffusion model and studied its bias dependence. We have found that an optical "dead space" exists underneath the top surface of the p-GaN layer, which mag be attributed to the internal electrical field at the p-GaN surface. The presence of this region prevents a meaningful extraction of the electron diffusion length from the quantum efficiency data.