Non-equilibrium distribution of hot carriers in a CdSe thin film

Femtosecond pump-and-probe spectroscopy was adopted to study the hot carrier distribution in a CdSe thin film. In the experiment, a nonthermalized hot carrier distribution was observed when the pump was turned on. The nonthermalized distribution was observed at the 200 meV broad energy range extending from the pump energy to the low energy side. The low energy tail of the nonthermalized distribution indicates that the carrier-LO phonon scattering competes with the carrier-carrier scattering in the thermalization process. In this sense, this observation is a unique example of the nonthermalized hot carrier distribution in semiconductors. From the ratio of the nonthermalized distribution to the total one, the thermalization time was estimated to be 16-40 fs. In the wake of the thermalization, cooling of photogenerated carriers was observed. With an increase of the excitation density up to 1019 cm-3, the carrier cooling rate was slower. The observed cooling rate was slower than the theoretical calculation, taking account of the screening of the carrier-phonon interaction. This is ascribed to the hot phonon effects of the LO phonon and the TO phonon.