Negative spin polarization in AlGaAs photoluminescence

Optical orientation of carrier spins has been studied in pure direct-gap Al_xGa_1-xAs alloys (x up to 15%). It is shown that optical excitation with circularly-polarized light with quanta energy in a narrow range (less than 1.6 meV) just above the free exciton transition results in the appearance of negative circular polarization of near-band edge photoluminescence (PL), which indicates a reversal of the spin orientation direction. The PL polarization degree becomes negative on all the excitonic transitions in the spectrum, free as well as bound. Carrier spin kinetics observed using the Hanle technique shows insignificant modification under the resonant excitation. The resonant switching of PL polarization seen in the excitation spectrum points to a change in the dominating carrier generation channel from heavy-hole to light-hole. The change is attributed to the influence of mechanical stress between the ternary epitaxial film and the substrate of GaAs.