Effects of the layer thickness on the electronic character in GaAs-AlAs superlattices

Realistic tight-binding calculations for the GaAs-AlAs superlattice clearly demonstrate that if the GaAs layer thickness is less than ∼30 Å, the lowest conduction-band state is confined to the AlAs barrier region instead of the GaAs quantum well region resulting in a spatial separation of electrons and holes. This observation exemplifies a failure of the widely used Kronig–Penney model and emphasizes the importance of the k vector selection rule. This property can be used in photoluminescence spectroscopy to determine the band offset of thin layer superlattices for an essentially arbitrary composition of AlxGa1−xAs. A complete two-dimensional map of the electronic character as a function of the GaAs and the AlAs layer thickness is obtained. The possibility that Xxy states may lie below Xz states for narrow wells is proposed, and implications of the present calculation on interpreting spectroscopic data are discussed.