Radiation-defect-dependent photoconductivity transients and photoluminescence in semi-insulating GaN

Effect of radiation defects on the photoconductivity transients and photoluminescence (PL) spectra have been examined in semi-insulating GaN epitaxial layers grown on bulk n-GaN∕sapphire substrates. Manifestation of defects induced by 10keV x-ray irradiation with the dose of 600Mrad and 100keV neutrons with the fluence of 5×1014cm−2 have been revealed through steady-state and pulsed PL as well as through contact photoconductivity and microwave absorption transients. Synchronous decrease of the PL intensity of yellow, blue, and ultraviolet bands peaked at 2.19, 2.85, and 3.42eV, respectively, with density of radiation-induced defects is observed. The decrease of the PL intensity is accompanied by an increase of asymptotic decay lifetime, which is due to excess-carrier multi-trapping. The decay fits the stretched exponent approximation exp[−(t∕τ)α] with the different factors α in as-grown material (α≈0.7) and irradiated samples (α≈0.3).