Ultrafast Dynamics of Exciton–Exciton and Exciton–Longitudinal Optical-Phonon Scattering Processes in ZnO Epitaxial Thin Films

Time-resolved luminescence spectra due to the exciton–exciton collision process were measured in ZnO thin films under excitonic resonant and band-to-band excitations by the optical Kerr gate technique. The luminescence due to the exciton–exciton collision process instantaneously emerges under excitonic resonant excitation, while it emerges with a delay of a few picoseconds under band-to-band excitation. The time delay observed indicates that, first, exciton–longitudinal optical (LO)-phonon scattering occurs and, then, the exciton–exciton collision process takes place after the completion of the thermal cooling of excitons through the cascade emission of multiple LO-phonons. The time delay is quantitatively evaluated by a simple calculation based on the exciton–LO-phonon scattering process.