Role of radiation defects in photoconductivity transients and photoluminescence spectra of epitaxial GaN layers

Effect of radiation defects on photoconductivity transients and photoluminescence (PL) spectra has been examined in semi-insulating GaN epitaxial layers. Manifestation of defects induced by x-ray irradiation with the dose of 600 Mrad and neutrons with the fluence of 5 x 1014 cm -2 has been revealed through steady-state and pulsed PL as well as through contact photoconductivity (CPC) and microwave absorption (MWA) transients. Synchronous decrease in the PL intensity of yellow (Y), blue (B) and ultraviolet (UV) bands peaked at 2.19 eV, 2.85 eV and 3.42 eV, respectively, with increasing the density of radiation-induced defects was observed. The observed effect of radiation on the PL characteristics has been explained by interaction of the radiation defects with the native traps, responsible for Y-band PL or by structural modification of the ''yellow'' centers. The decrease in the PL intensity with concentration of radiation-induced defects is accompanied by an increase of the asymptotic decay lifetimes in time-stretched multi-trapping processes. The CPC and MWA decays, reflecting these processes, fit well a stretched-exponent approximation exp[-(t/τ)α] with the time-stretching factor α that transforms from α=0.7 for as-grown material to α=0.3 for irradiated one. (copyright 2005 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)