Dynamics of high-density excitons and electron-hole plasma in ZnO epitaxial thin films

Time-resolved luminescence spectra due to an electron-hole plasma (EHP emission) and an exciton-exciton scattering process (P emission) were measured in ZnO epitaxial thin films under band-to-band excitation by the optical Kerr gate method. We find that both the EHP and P emissions emerge with a long time delay of a few picoseconds. The time delay observed decreases quadratically with the excitation density below the critical Mott density. These results imply that the P (EHP) emission emerges after completing thermal cooling of excitons (electrons and holes) toward the exciton band bottom (renormalized band gap) through multiple LO-phonon emissions. As the excitation density increases, the exciton-exciton scattering process also plays an important role on cooling and relaxation of high-density excitons.