Comparison of ultrafast carrier thermalization in Ga_xIn_1−xAs and Ge quantum wells

The thermalization of photoexcited carriers is investigated using femtosecond pump-probe spectroscopy in both (GaIn)As and Ge quantum wells. In both materials a nonthermal electron distribution is observed. The continuous relaxation from the point of injection toward the ground state and the thermalization of the carrier distribution are monitored on a time scale of up to 500 fs at room temperature. Carriers in (GaIn)As thermalize within 300 fs when injected with an excess energy of 250 meV. Separate carrier distributions for the heavy-hole and light-hole systems are found since the angular momentum transfer required for a transition is slow. Thermalization in Ge is found to be slightly slower in comparison due to the lack of Fröhlich interaction.