Comparison of Spectral Responses between Front- and Back-Incidence Configurations in GaN Metal-Semiconductor-Metal Photodetector on Sapphire

Front and back illumination of a metal-semiconductor-metal structure on a 2-mu m-thick GaN layer showed obvious differences in the spectral responsivity in the wavelength range of 300-500 nm. Pt/Au interdigitated electrodes on an unintentionally doped n-GaN were confirmed to be of extremely low leakage Schottky type, and simulations of the electrostatic potential distribution have revealed that the depletion regions do not prevail throughout the thick GaN layer even at a bias of 10 V. The difference observed in the wavelength region shorter than the fundamental absorption edge is due to incomplete depletion of the GaN layer off the Schottky contacts in conjunction with short optical penetration depths, while the back-incidence responsivity in the longer wavelength region reflects extrinsic optical absorptions characteristic to the epitaxial crystal.