PHOTO-PRODUCED PHONONS IN SEMICONDUCTORS

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 the ory applies to all 3 systems under very weak photoexcitation, signific ant differences in ballistic phonon production occur under strong phot oexcitation. Beginning at moderate excitation density, an extra compon ent of low-mu phonons is observed. This component is a small fraction of the total detected phonon energy (similar to 1% in GaAs, 5-10% in S i and Ge) but is spatially and temporally concentrated due to the phon on focusing effect. Measurements of the photoluminescence of Si and Ge verify the presence of a dense e-h liquid phase at these densities. W e postulate that acoustic phonons are emitted directly by the e-h plas ma in all 3 systems, bypassing the slower production via anharmonic de cay which results in quasi-diffusion. For large populations of low-nu phonons coalescence processes may be initiated. This may explain the r eduction in ballistic phonon production in Ge at the highest excitatio n levels.