FREE TO BOUND EXCITON RELAXATION IN [001] AND [111] GAAS GAALAS QUANTUM-WELLS/

We have investigated the relaxation from free to bound excitons in [00 1] and [111] GaAs/Ga0.7Al0.3As quantum wells by analyzing the photolum inescence polarization in the presence of a magnetic field applied in the Faraday configuration. The Stokes shifts of our samples, which amo unt to approximately 3 meV, indicate that the photoluminescence is due to excitons bound to some kind of defects in the quantum wells. We ha ve obtained a systematic decrease of the degree of polarization with i ncreasing magnetic field for resonant excitation of heavy-hole exciton s. We interpret this fact as a manifestation of the decrease of correl ation between momenta and spins of the excitons as they decay from fre e to bound excitons. We obtain an effective out-scattering time due to the processes of energy relaxation from free to bound excitons of the order of 50 fs, which is independent of the crystallographic orientat ion.