Photo-produced phonons in semiconductors

Abstract

The techniques of phonon imaging are used to study optically generated phonon sources in Si, Ge and GaAs at 1.7 K. While quasi-diffusive theory applies to all 3 systems under very weak photoexcitation, significant differences in ballistic phonon production occur under strong photoexcitation. Beginning at moderate excitation density, an extra component of low-ν phonons is observed. This component is a small fraction of the total detected phonon energy (∼ 1% in GaAs, 5–10% in Si and Ge) but is spatially and temporally concentrated due to the phonon focusing effect. Measurements of the photoluminescence of Si and Ge verify the presence of a dense e-h liquid phase at these densities. We postulate that acoustic phonons are emitted directly by the e-h plasma in all 3 systems, bypassing the slower production via anharmonic decay which results in quasi-diffusion. For large populations of low-ν phonons coalescence processes may be initiated. This may explain the reduction in ballistic phonon production in Ge at the highest excitation levels.