NONEQUILIBRIUM DISTRIBUTION OF HOT CARRIERS IN A CDSE THIN-FILM

Femtosecond pump-and-probe spectroscopy was adopted to study the hot c arrier distribution in a CdSe thin film. In the experiment, a non-ther malized hot carrier distribution was observed when the pump was turned on. The non-thermalized distribution was observed at the 200 meV broa d energy range extending from the pump energy to the low energy side. The low energy tail of the non-thermalized distribution indicates that the carrier-LO phonon scattering competes with the carrier-carrier sc attering in the thermalization process. In this sense, this observatio n is a unique example of the non-thermalized hot carrier distribution in semiconductors. From the ratio of the non-thermalized distribution to the total one, the thermalization time was estimated to be 16-40 fs . In the wake of the thermalization, cooling of photogenerated carrier s was observed. With an increase of the excitation density up to 10(19 ) cm-3, the carrier cooling rate was slower. The observed cooling rate was slower than the theoretical calculation, taking account of the sc reening of the carrier-phonon interaction. This is ascribed to the hot phonon effects of the LO phonon and the TO phonon.