Initial process of photoluminescence dynamics of self-trapped excitons in a β-Ga2O3 single crystal

We investigate the photoluminescence (PL) dynamics of self-trapped excitons (STEs) in a beta-Ga2O3 single crystal from the viewpoint of the transition process from the free exciton to the STE. We succeed in measuring the PL rise time (similar to 24 ps) at 8 K corresponding to the tunneling time through the barrier between the free exciton and STE states in the adiabatic potential. From the analysis of the PL rise time of the STE based on perturbation theory for the tunneling time considering exciton-phonon interactions, we obtain the following results. Acoustic phonons near the Brillouin zone center contribute to the tunneling process. This suggests that the wave function of the STE is still spatially extended at the final state in the tunneling process. Furthermore, we investigate temperature dependence of the PL rise time of the STE. It is found that the PL rise time decreases with increasing temperature. The PL rise times in the temperature range from 8 to 100 K can be quantitatively explained by an adiabatic theory for the tunneling process. Consequently, the self-trapping process is dominated by the tunneling process at low temperatures.