Picosecond phonon dynamics and self-energy effects in highly photoexcited germanium

Abstract

Self-energies and population kinetics of optical phonons in strongly photo-excited intrinsic and p-type doped germanium have been studied using picosecond Raman scattering measurements at 295 K for induced carrier densities up to 2\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. Time-integrated and time-resolved measurements indicate that the nonequilibrium phonon occupation number increases sublinearly and its temporal peak shifts as the photoexcited carrier density is increased above ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. A theoretical model 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 measurements also reveal broadening and shifting of the Raman lines: for a photoexcited carrier density of 2\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$, the line broadening indicates that the phonon lifetime is reduced from its quiescent value of 4 ps to \ensuremath{\sim} 0.5 ps and the phonon frequency is reduced by \ensuremath{\sim}8 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. We present a microscopic model to describe the phonon self-energy effects that are caused by carrier-phonon interactions. The model indicates that the phonon broadening is consistent with primarily intra-heavy-hole valence-band transitions, while the phonon frequency renormalization is consistent with primarily inter-heavy-hole\ensuremath{\leftrightarrows}light-hole valence-band transitions. \textcopyright{} 1996 The American Physical Society.