MICROWAVE MODULATION OF CIRCULARLY-POLARIZED EXCITON PHOTONLUMINESCENCE IN GAAS ALAS MULTIPLE-QUANTUM WELLS/

This study describes microwave (mw) modulated circularly polarized pho toluminescence (MPL) of the inhomogeneously broadened (e1:hh1)1S excit on, in undoped GaAs/AlAs multiple quantum wells, at T = 1.4 K. A circu larly polarized excitation creates spin-oriented excitons and free car riers, and the latter are heated by the absorption of mw radiation. Th e degree of PL circular polarization (P-cir) and of the MPL signal (De lta P-cir) is measured under a magnetic field, applied normal to the q uantum well plane. This experimental procedure enables us to investiga te the influence of the spin-oriented mw-heated free electrons on the exciton transfer among the localized exciton sites. The modulated spec trum resolves the inhomogeneously broadened PL band into a fine struct ure corresponding to different exciton localizations: delocalized, loc alized, and impurity-bound excitons. Each resolved MPL band has a dist inct intensity dependence on the mw power and on the magnetic-field st rength, which is due to the different exciton trapping rate and the re stricted electron motion (induced by a magnetic field). We studied the effect of the magnetic field on the degree of MPL polarization (Delta P-cir) of the various excitons, and found the MPL of delocalized exci tons to be completely unpolarized, that of localized excitons partiall y polarized, and of impurity-bound excitons completely polarized. The observation of Delta P-cir similar to 1 for strongly localized exciton s is explained by a preferred exciton localization on accepters that w ere neutralized by spin-oriented holes. The other observed Delta P-cir values indicate that the change in the exciton spin orientation, indu ced by the hot-carrier-exciton interaction, depends on their mutual sp in orientation.