Ultrafast carrier relaxation and diffusion dynamics in ZnO

Abstract

We report on measurements and calculations of the ultrafast exciton relaxation dynamics in ZnO. Time-resolved differential reflectivity measurements of bulk ZnO were performed as a function of excitation wavelength. Bi-exponential decays of the A and B exciton states are observed with a fast (~2-5 ps scale) and a slower (~50-100 ps scale) component, which depend strongly on excitation wavelength. Theoretical calculations based on a multi-state, coupled rate equation model were directly compared with the experiments to account for the rapid scattering between the A and B valence bands. Results show that the inter-valence band scattering is most likely not responsible for the fast initial relaxation. Instead our results show that carrier diffusion can play an important role in explaining the initial fast relaxation.