ULTRAFAST RELAXATION OF PHOTOEXCITED CARRIERS IN QUANTUM-WELLS AND SUPERLATTICES

The dynamics of photoexcited carriers in GaAs quantum wells and superl attices has been measured with femtosecond resolution using luminescen ce as well as pump-probe experiments. The electron capture mechanism s hows well defined resonances with a time as short as 500 fs at room te mperature for a well thickness of 60 angstrom. This gives strong evide nce for the importance of the quantum mechanical, LO phonon-assisted c apture mechanism predicted by theoretical calculations. In type-II sup erlattices, we show that the transfer of electrons from the GaAs layer s 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 f rom 140 fs up to 30 ps depending on the thickness of the GaAs layer.