Screening of hot-carrier relaxation in highly photoexcited semiconductors

The carrier density dependence of the hot-carrier energy relaxation rate in highly photoexcited semiconductors is investigated. Results of these calculations indicate important differences between polar direct-gap and nonpolar indirect-gap materials. The critical carrier density (Nc) for the onset of screening in polar semiconductors is found to increase with both effective mass and phonon energy. A method for predicting trends among these materials with respect to Nc is briefly described. Calculations for GaAs predict that the hot-carrier cooling rate begins to decrease at Nc≈6×10¹⁶ cm^-3. Above this density the phonon emission frequency falls rapidly. In contrast, the effects of screening in Si are shown to be negligible for N10¹⁹ cm^-3. In this case, a significant reduction in the energy relaxation rate does not occur until N~10²¹ cm^-3.