Phonon production in weakly photoexcited semiconductors: Quasidiffusion in Ge, GaAs, and Si

Nonequilibrium phonon propagation is observed in Ge, Si, and GaAs at $T=2.2\mathrm{}\mathrm{K}$ when these materials are weakly photoexcited with a pulsed ${\mathrm{Ar}}^{+}$ laser (typically a defocused 10 ns, nanojoule pulse). The overall shapes of the experimental heat pulses agree well with computer simulations incorporating anisotropic phonon propagation (phonon focusing effects), isotope scattering, and anharmonic decay. Quantitative comparisons between the experimental and theoretical decay rates lead us to conclude that (1) although surface reflections near the excitation point play a key role in determining the character of the detected phonon energy, reflections from the sample sidewalls appear to have a smaller effect at times of interest in the experiment; (2) the agreement between the experimental data and our theoretical models of quasidiffusion is better for silicon than for germanium or gallium arsenide. These moderate discrepancies (factors of about 2 in decay rates) are attributed to uncertainties in the elastic-scattering rate and/or anharmonic decay rate. Overall, the basic model of quasidiffusive phonon propagation is quite successful in predicting the principle features of nonequilibrium phonon propagation following direct photoexcitation of a semiconductor.