Hot electrons and phonons under high intensity photoexcitation of semiconductors

It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities (<10⁵W/cm² for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T_e different from the lattice temperature. T_e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T_e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10⁵W/cm² for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T_e with excitation wavelength and of the transmission spectra of photoexcited GaAs.