Fine structure of charged and neutral excitons in InAs-Al_0.6Ga_0.4As quantum dots

Photoluminescence spectra from single self-assembled InAs quantum dots embedded in an Al_0.6Ga_0.4As matrix are reported. The spectra consist of a higher-energy linearly polarized doublet, with large splitting of the order of 1 meV, and a lower-energy unpolarized line. The polarized lines are explained in terms of exciton recombination at asymmetric dots, with the splitting due to the anisotropic exchange interaction. The lower-energy unsplit, unpolarized line is ascribed to recombination at the same dots but in the presence of an excess charge, which results in zero net electron spin and hence quenching of the exchange interaction. The conclusions are fully supported by magneto-optical investigations.