Ultrafast biexciton dynamics in a ZnO thin film

The emission lines of biexciton and donor-bound biexciton are observed in a high-quality ZnO thin-film sample with time-resolved photoluminescence (TRPL) measurement. The TRPL intensity profiles reveal the formation sequence of various types of exciton. After free excitons are first generated, part of them is trapped by neutral donors to form donor-bound excitons. The other part contributes to the generation of biexcitons through free exciton scattering. Next, a donor-bound biexciton is generated through the trapping of a biexciton or two free excitons by a neutral donor or the trapping of a free exciton by a donor-bound exciton. Except donor-bound exciton, the relaxations of all other exciton states show two decay stages. Either the increasing or decreasing trends of the calibrated decay times in increasing the excitation power are well interpreted with a four-level model.