Reversal of spin polarization direction in excitonic photoluminescence of AlGaAs

An anomalous reversal of the direction of exciton spin polarization is found in an optical-orientation study of Al_x Ga_1-x As alloys. A negative degree of circular polarization of excitonic photoluminescence is observed in direct-gap AlGaAs under excitation with circularly polarized light with the photon energy lying in a narrow range just above the free-exciton transition. The resonant switching of the polarization direction is attributed to the effect of the minor residual strain in the epitaxial film that results in lifting of the degeneracy of the light-and heavy-hole sub-bands. The electron g-factor is estimated from combined time-resolved and Hanle-effect data and amounts to 0.056-0.086 for x=15% alloy.