ELECTRON-HOLE PLASMA-DRIVEN PHONON RENORMALIZATION IN HIGHLY PHOTOEXCITED GAAS

Many-body renormalization of phonon frequencies, due to their interaction with a high-density electron-hole plasma, such as the one created by an intense laser pulse applied to GaAs, is calculated. Results for the change in phonon-dispersion curves are obtained, both for a distribution characterized by zero carrier temperature and for the electron-hole plasma equilibrium temperature. We use a many-valley model for the conduction and valence bands of GaAs, and the deformation potentials available from the literature for the carrier-acoustic-phonon interaction. We do not find complete softening of the phonons below the experimental electron-hole pair density (<10(22) cm(-3)), although -zone-boundary phonon softening is observed at (1-5) X 10(22) cm(-3). From the increase in the mean squared amplitude of the atomic vibrations caused by the phonon softening, we obtain an effective lattice melting temperature which decreases with increasing carrier temperature. Thus, a nonthermal electronic mechanism could at best play only a partial role in semiconductor laser annealing experiments even on ultrafast subpicosecond time scales.