PICOSECOND PHONON DYNAMICS AND SELF-ENERGY EFFECTS IN HIGHLY PHOTOEXCITED GERMANIUM

Self-energies and population kinetics of optical phonons in strongly p hoto-excited intrinsic and p-type doped germanium have been studied us ing picosecond Raman scattering measurements at 295 K for induced carr ier densities up to 2 x 10(20) cm(-3). Time-integrated and time-resolv ed measurements indicate that the nonequilibrium phonon occupation num ber increases sublinearly and its temporal peak shifts as the photoexc ited carrier density is increased above 10(19) cm(-3). A theoretical m odel of coupled carrier and phonon dynamics indicates that this can be attributed to nonequilibrium phonon reabsorption by holes undergoing intra-heavy-hole valence-band transitions. The time-integrated measure ments also reveal broadening and shifting of the Raman lines: for a ph otoexcited carrier density of 2 x 10(20) cm(-3), the line broadening i ndicates that the phonon lifetime is reduced from its quiescent value of 4 ps to similar to 0.5 ps and the phonon frequency is reduced by si milar to 8 cm(-1). We present a microscopic model to describe the phon on self-energy effects that are caused by carrier-phonon interactions. The model indicates that the phonon broadening is consistent with pri marily intra-heavy-hole valence-band transitions, while the phonon fre quency renormalization is consistent with primarily inter-heavy-hole<- >light-hole valence-band transitions.