Femtosecond carrier dynamics in Ge measured by a luminescence up-conversion technique and near-band-edge infrared excitation

The luminescence at the direct band edge of bulk intrinsic and p-type Ge (p∼2×${10}^{19}$ ${\mathrm{cm}}^{\mathrm{-}3}$) samples was measured in a two-wavelength up-conversion experiment with λ=1.25–1.35 μm excitation and at lattice temperatures 20 and 300 K. The near-band-edge carrier dynamics, measured with 100-fs temporal resolution and at carrier densities of 5×${10}^{16}$–2×${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, are dominated by electron-phonon intervalley scattering and electron thermalization. Thermalization governs the initial rise of the luminescence in the first 500 fs and is studied as a function of energy and electron density. With excitation photon energy 100 meV greater than the direct band gap, the thermalization time of electrons was measured to decrease with increasing density as ${\mathit{n}}^{\mathrm{-}0.55\pm 0.1}$. Degeneracy has a strong influence on the thermalization rate with 50 meV excess energy. The L-electron cooling through the electron-hole interaction, as well as hole screening and degeneracy in p-type Ge, is discussed.