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Ultrafast time-resolved photoluminescence spectroscopy following one- andtwo-photon excitation of ZnO powder is used to gain unprecedented insight intothe surprisingly high external quantum efficiency of its "green" defectemission band. The role of exciton diffusion, the effects of reabsorption, andthe spatial distributions of radiative and nonradiative traps are comparativelyelucidated for the ultraviolet excitonic and "green" defect emission bands inboth unannealed, nanometer-sized ZnO powders and annealed, micrometer-sizedZnO:Zn powders. We find that the primary mechanism limiting quantum efficiencyis surface recombination because of the high density of nonradiative surfacetraps in these powders. It is found that unannealed ZnO has a high density ofbulk nonradiative traps as well, but the annealing process reduces the densityof these bulk traps while simultaneously creating a high density ofgreen-emitting defects near the particle surface. The data are discussed in thecontext of a simple rate equation model that accounts for the quantumefficiencies of both emission bands. The results indicate how defectengineering could improve the efficiency of ultraviolet-excited ZnO:Zn-basedwhite light phosphors.

Effects of reabsorption and spatial trap distributions on the radiative quantum efficiencies of ZnO