Fast photoluminescence dynamics in ZnO thin films under high-density excitation conditions

We have investigated the photoluminescence (PL) dynamics of crystalline ZnO thin films prepared by an rf-magnetron sputtering method under high-density excitation conditions at 10 K using an optical-Kerr gating (OKG) method. In the time-resolved PL spectra, we clearly observed the change of the PL properties with increasing time delay. In the initial time region up to about 5 ps, a broad PL band appears at the low-energy side of the free exciton band, which is considered to be due to the electron–hole plasma (EHP) accompanied with a band-gap renormalization. After the broad PL band vanishes, a narrow PL band originating from exciton–exciton scattering, the so-called P emission, appears; namely, its peak energy reaches the energy lower than the free exciton energy by the exciton binding energy. Thus, the time-resolved PL spectra clearly demonstrate the dynamical change from the EHP emission to the P emission at the cross-point time of about 5 ps, which corresponds to the dynamical phase transition from the EHP state to the exciton state.