Effects of interfacial atomic segregation and intermixing on the electronic properties of InAs/GaSb superlattices

Abrupt InAs/GaSb superlattices have In-Sb and Ga-As interfacial chemical bonds that are not present in the constituent materials InAs and GaSb. We study the effect of interfacial atomic mixing on the electronic structure of such superlattices, including electron and hole energies and wave function localization, interband transition energies, and dipole matrix elements. We combine an empirical pseudopotential method for describing the electronic structure with two different structural models of interfacial disorder. First, we use the single-layer disorder model and change in a continous way the composition of the interfacial bonds. Second, we study interfacial atomic segregation using a layer-by-layer kinetic model of molecular beam epitaxy growth, fit to the observed scanning tunneling microscopy segregation profiles. The growth model provides a detailed structural model of segregated InAs/GaSb superlattices with atomic resolution. The application of the empirical pseudopotential method to such structures reveals remarkable electronic consequences of segregation, among them a large blueshift of the band gap. This result explains the surprising gap increase with growth temperature observed for similar structures. In particular we find that (i) superlattices with only In-Sb interfacial bonds have lower band gaps (by 50 meV) than superlattices with only Ga-As interfacial bonds. (ii) Heavy-hole-to-electron transition energies increase with the number of Ga-As interfacial bonds more than light-hole-to-electron transition energies. (iii) The heavy-hole hh1 wave functions show a strong localization on the In-Sb interfacial bonds. The heavy-hole wave functions have very different amplitudes on the Ga-As interface and on the In-Sb interface. (iv) Sb segregates at InAs-on-GaSb growth, whereas As and In segregate at GaSb-on-InAs growth, but Ga does not segregate. (v) The segregation of Sb and In induces a blueshift in the band gap. (vi) There is an in-plane polarization anisotropy due to the low symmetry of the no-common-atom InAs/GaSb superlattice. This anisotropy is reduced by interfacial segregation.