Silicon donor states in heavily doped thin GaAs-AlAs(001) superlattices

The self-consistent pseudopotential method is used to investigate the electronic structure of silicon donor states at and near the interface of thin GaAs/AlAs(001) superlattices under high doping concentrations. The silicon impurity is substituted in the GaAs and AlAs regions and at different distances from the interface. For all cases, we find that the donor state is highly localized around the silicon donor ion. With the impurity placed at the interface, there is a shift of the donor-state charge density from AlAs to GaAs. The qualitative changes in the valence- and conduction-band discontinuities with doping are discussed. Our results indicate that doping in the GaAs region has a much larger effect on the band-edge discontinuities. In addition, the results for the zone folding of the zinc-blende X-point state onto the superlattice point are reported. For the thin superlattices treated here, (GaAs)n-(AlAs)n (for n=2, 3, and 4), we find that the highest occupied valence-band state is derived from the zinc-blende GaAs point and the lowest unoccupied conduction-band state is derived from the zinc-blende AlAs X-point state. The change in the localization of the conduction-band-edge states with increasing n suggests a transition region, at which the lowest unoccupied conduction-band state becomes the zinc-blende GaAs point. A critical layer thickness is estimated for this transition. © 1988 The American Physical Society.