Photogenerated high-density electron-hole plasma energy relaxation and experimental evidence for rapid expansion of the electron-hole plasma in CdSe

The kinetics of the nonequilibrium photogenerated electron-hole plasma in CdSe is investigated using picosecond time-resolved spectroscopy at room temperature. Based on the fact that the polar optical-phonon emission rate is reduced due to screening by the high density of the e-h plasma, the remaining dominant mechanism for hot-carrier cooling is the nonpolar optical-phonon emission even though CdSe is a highly polar semiconductor. It has been observed that the photogenerated carrier density is much lower than the estimated carrier density using known values of the absorption coefficient, reflectivity, and photon fluence. Rapid plasma expansion has been proposed as a possible explanation on the grounds of the observed larger spatial width of the photoluminescence relative to the laser spatial width, moderate change of Auger recombination rate with the excitation fluence, the absence of an observed change in the Fermi level with increased excitation intensity, and earlier formation of excitons after the picosecond pulse (5 psec) excitation at a low temperature (12 K). The observed carrier density ≊35 psec after excitation is limited to 1×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$ within the excitation photon fluence of 2×${10}^{15}$–7×${10}^{16}$ photons/${\mathrm{cm}}^2$ at room temperature. Large values of the diffusion constant are explained in terms of a screened electron-phonon interaction. The possibility of saturation of the available states in explaining small carrier densities is eliminated by the observed faster cooling rate at low-excitation intensity and the sublinear change in the luminescence spatial width with excitation fluence.