The effect of native and radiation induced defects on the photoconductivity transients and photoluminescence spectra were examined in GaN epitaxial layers of 2.5 and 12μm thickness grown on bulk n-GaN/sapphire substrates by metal-organic chemical vapor deposition. For comparison, free-standing GaN as-grown samples of 500μm thickness, fabricated by hydride vapor phase epitaxy, were investigated. Manifestation of defects induced by 10keV X-ray irradiation with the dose of 600Mrad and 100keV neutrons with fluences of 5 x 1014 and 1016/cm2 as well as of 24GeV/c protons with fluence 1016/cm2 were revealed through contact photoconductivity and microwave absorption transients. The amplitude of the initial photoconductivity decay was significantly reduced by the native and radiation defect density. Synchronous decrease of the steady-state photoluminescence intensity of yellow, blue and ultra-violet bands peaked at 2.18, 2.85, and 3.42eV, respectively, with density of radiation-induced defects was observed. The decrease of the photoluminescence intensity was accompanied by an increase of asymptotic decay lifetime in the photoconductivity transients, which was 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). The dimension of the disordered structure changed from 4.7 to 0.86 for as-grown and irradiated material, respectively, and it implied the percolative carrier motion on an infinite cluster of dislocations net in the as-grown material and cluster fragmentation after irradiation.

Defect Attributed Variations of the Photoconductivity and Photoluminescence in HVPE and MOCVD As-Grown and Irradiated GaN Structures. E.Gaubas, P.Pobedinskas, J.Vaitkus, A.Uleckas, A.Žukauskas, A.Blue, M.Rahman, K.M.Smith, E.Aujol, B.Beaumont, J.P.Faurie, P.Gibart: Nuclear Instruments and Methods in Physics Research Section A, 2005, 552[1-2], 82-7