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 (/spl sim/2 nA/cm/sup 2/ at -10-V bias) and high external quantum efficiencies (/spl sim/45% at /spl lambda/=362 nm). We have 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 may 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.