Ultrafast relaxation of photoexcited carriers in quantum wells and superlattices
The dynamics of photoexcited carriers in GaAs quantum wells and superlattices has been measured with femtosecond resolution using luminescence as well as pump-probe experiments. The electron capture mechanism shows well defined resonances with a time as short as 500 fs at room temperature for a well thickness of 60 AA. This gives strong evidence for the importance of the quantum mechanical, LO phonon-assisted capture mechanism predicted by theoretical calculations. In type-II superlattices, we show that the transfer of electrons from the GaAs layers to the AlAs layer is mediated by zone edge phonons (both LO and TA) in a way similar to intervalley scattering. The observed times range from 140 fs up to 30 ps depending on the thickness of the GaAs layer.