Correlation of hot-phonon and hot-carrier kinetics in Ge on a picosecond time scale

We report room-temperature measurements of time-resolved reflectivity from intrinsic crystalline Ge induced and probed by λ=0.575 μm, 2-ps pulses and for photogenerated carrier densities up to ∼${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$. These data are used together with our earlier picosecond-time-resolved Raman-scattering data at 77 and 300 K to self-consistently determine transient nonequilibrium carrier and phonon effects. The combined data sets are modeled with a Boltzmann transport description of coupled electron and phonon kinetics to understand the temporal and spatial evolution of carrier density and temperature and the generation and decay of Raman-active phonons. Using known material parameters in the model, we can account for the magnitude and temporal evolution of the electron, hole, and phonon thermodynamic parameters. In particular, the model accounts for the relative delay between the peak of the pulse and the nonequilibrium phonon population, the number of nonequilibrium phonons generated, and the initially rapid diffusion of hot carriers away from the sample surface on a 5-ps time scale.