FEMTOSECOND KINETICS OF PHOTOEXCITED CARRIERS IN GERMANIUM

Thermalization, intervalley scattering, and cooling of photoexcited ca rriers are observed in the indirect-band-gap semiconductor Ge through time-resolved direct-band-gap transmission and luminescence with 100-f s resolution. The transmission experiment, which uses lambda '' 1.53 m um, 120-fs pulses from a 76-MHz optical parametric oscillator takes ad vantage of the indirect-band-gap character of Ge to unambiguously dete rmine the GAMMA --> L intervalley scattering time at 295 K. The measur ed GAMMA --> L transfer time for electrons at the GAMMA valley edge is 230+/-25 fs yielding 4.2+/-0.2 x 10(8) eV/cm for the D(GAMMA-L) inter valley deformation potential. The luminescence experiments were perfor med at 10 K using a standard time and spectrally gated up-conversion t echnique employing an 82-MHz, 80-fs pulse width Ti:sapphire laser oper ating at lambda = 750 nm. From the luminescence experiments conducted with peak carrier densities of 10(18)-10(19) cm-3, we observe that GAM MA --> L, X scattering competes with carrier-carrier scattering so tha t nonthermalized carriers are observed for up to several hundred femto seconds after an excitation pulse. Because the initial carrier kinetic energy is quite high we find that in addition to phonon-assisted L-L intervalley scattering, X-L and X-X processes must be included to acco unt for a < 1-ps carrier-cooling time. A value of 5 X 10(8) eV/cm is o btained for the D(X-X) deformation potential.