Luminescence of ZnO crystals under surface and bulk excitation regimes

Luminescence properties of vapor phase grown ZnO single crystals with different content of deep traps were explored under one-photon and two-photon excitation. Room-temperature near bandgap emission of highly excited ZnO crystals was originated by inelastic exciton scattering. Under surface excitation regime, luminescence decay profile was governed by exciton diffusion and bimolecular recombination. At higher excitation, exciton recombination transients were significantly altered by stimulated emission. Two-photon bulk-like excitation revealed exciton recombination dynamics related to defect states. Correlation between the luminescence decay rate and the density of deep trap states of variously annealed crystals was observed. Results on crystal optical quality were in line with estimated values of stimulated emission threshold. The largest value of the average luminescence decay time tau(lu) = 1.5 ns and the lowest value of the stimulated emission threshold I(exc) =0.6 mJ/cm(2) were obtained for ZnO crystal annealed in vacuum. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim