A combination of defect-sensitive techniques was used to study native defects and impurities in GaN films which had been deposited onto the basal plane of sapphire. The optical and electronic properties of the undoped samples depended upon the degree of compensation between shallow donor and acceptor levels. Paramagnetic studies of undoped films revealed resonances that arose from effective-mass and deep-donor states. Photoluminescence studies of films which were doped with Mg acceptors revealed a possible dependence of the localization of the Mg-related center upon an increasing concentration of activated acceptors. Optically detected magnetic resonance measurements of Mg-doped samples showed that the Mg acceptor was perturbed from its effective-mass state. The photoluminescence spectra of undoped films were characterized by a sharp emission band that was attributed to the recombination of excitons that were bound to an unidentified donor. A broad 2.2eV band was also observed. Optically detected magnetic resonance studies of Mg-doped films confirmed the existence of 2 donors with differing localization levels, and an additional resonance was detected which was associated with Mg acceptors. Spectral analysis of the optically detected magnetic resonance spectra indicated that Mg was not a typical effective-mass acceptor. The observation of both donor states in most samples suggested that they originated from an intrinsic defect which gave rise to both shallow and deep donor states.

J.A.Freitas, T.A.Kennedy, E.R.Glaser, W.E.Carlos: Solid-State Electronics, 1997, 41[2], 185-8