Emission-energy dependence of ultrafast P-emission decay in ZnO from bulk to nanofilm

We have performed time-resolved photoluminescence (PL) spectroscopy for ZnO thin films with thicknesses of 90, 460, and 2800nm under intense excitation condition. We clearly observed the P emission due to inelastic exciton–exciton scattering. It was found that, in the 460- and 2800-nm thick samples, the decay time of the P emission considerably depends on the detection energy inversely proportional to the group velocity of the polariton in a bulk crystal with each factor of proportionality. In contrast, the energy dependence is less remarkable in the 90-nm thick sample. The decay times are basically shortened with a decrease in the film thickness. The thickness dependence of the P-emission-decay profiles is explained by considering the crossover from the polariton modes in the 2800-nm thick sample (bulk-like film) to the exciton-/photon-like modes in the 90-nm thick sample (nanofilm). •We clearly observed the P-PL dynamics due to inelastic exciton–exciton scattering.•The P-PL decay times are basically shortened with a decrease in the film thickness.•The P-PL decay time depends on the detection energy in the bulk-like sample.•The energy dependence of the P-PL decay time almost disappears in the 90-nm sample.•The thickness dependence is explained by the crossover between exciton and photon.