Carrier Dynamics and Photoexcited Emission Efficiency of ZnO:Zn Phosphor Powders

Nonstoichiometric ZnO with an excess of Zn atoms (ZnO:Zn) has a long history of use as a green/monochrome phosphor in electron-excited vacuum fluorescent and field emission displays. We previously studied the external quantum efficiency of such ZnO:Zn powders under continuous-wave photoexcitation and found that the efficiency depended sensitively on excitation density [\textit{Appl. Phys. Lett. }\textbf{91}, 011902 (2007)]. Here we study experimentally the time-integrated quantum efficiency and the time-resolved photoluminescence decays of both band edge and defect emission of ZnO:Zn powders under femtosecond pulsed excitation of varying intensity. The results are discussed in terms of a rate equation model which describes energy transfer between band edge and radiative defect levels, as well as nonradiative centers.