Femtosecond kinetics of photoexcited carriers in germanium

Thermalization, intervalley scattering, and cooling of photoexcited carriers are observed in the indirect-band-gap semiconductor Ge through time-resolved direct-band-gap transmission and luminescence with 100-fs resolution. The transmission experiment, which uses λ=1.53 μm, 120-fs pulses from a 76-MHz optical parametric oscillator takes advantage of the indirect-band-gap character of Ge to unambiguously determine the Γ→L intervalley scattering time at 295 K. The measured Γ→L transfer time for electrons at the Γ valley edge is 230±25 fs yielding 4.2±0.2×${10}^8$ eV/cm for the ${\mathit{D}}_{\mathrm{\Gamma }\mathrm{-}\mathit{L}}$ intervalley deformation potential. The luminescence experiments were performed at 10 K using a standard time and spectrally gated up-conversion technique employing an 82-MHz, 80-fs pulse width Ti:sapphire laser operating at λ=750 nm. From the luminescence experiments conducted with peak carrier densities of ${10}^{18–}$ ${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$, we observe that Γ→L,X scattering competes with carrier-carrier scattering so that nonthermalized carriers are observed for up to several hundred femtoseconds 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 account for a ${10}^8$ eV/cm is obtained for the ${\mathit{D}}_{\mathit{X}\mathrm{-}\mathit{X}}$ deformation potential.