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 angstrom. 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.