Influence of Γ-X mixing on optical orientation and alignment of excitons in (In,Al)As/AlAs quantum dots

The effect of $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}\text{X}$ mixing strength on the energy levels fine structure of indirect in k-space excitons in an ensemble of (In,Al)As/AlAs quantum dots with type I band alignment was experimentally studied. Using the methods of optical orientation and optical alignment in a Faraday magnetic field, an increase in the anisotropic exchange splitting of exciton levels (from approximately 0.6 to 5 \textmu{}eV) to the extent of $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}\text{X}$ mixing was revealed. It was shown that the ratio of direct electronic states admixing to indirect ones depends on the size of the quantum dot. Moreover, the photoluminescence polarization properties of excitons change radically with increasing of the $\mathrm{\ensuremath{\Gamma}}$ states admixture to the X states: in the absence of a magnetic field, the optical orientation of excitons decreases from 18 to 3%, while the alignment of excitons is restored from 6 to 53%.