Theory of band offsets at semiconductor heterojunctions: An ab-initio linear response approach

We propose a novel ab-initio approach to the problem of band offsets at semiconductor heterojunctions. The actual interface is treated as a perturbation with respect to a periodic crystal: e.g. for the paradygmatic case of the (isovalent and lattice-matched) GaAs/AlAs interface, we take the virtual crystal Ga12Al12As as the unperturbed system. The interface electronic charge, which determines the band offsets, is shown to be very well described by linear response theory in this system, and we expect the same to be true in any case where the chemical differences between the constituents on the two sides are small. The numerical value of the valence-band offset obtained in this work is within 0.01 eV from our previous selfconsistent supercell calculations for the (001), (110) and (111) interface, and well within the present experimental error bar. Our main conclusion is that for all those lattice-matched isovalent interfaces where the electronic response to relevant ion substitutions can be described within linear-response theory, the offset is determined by bulk properties only. This means that the offset is independent not only of the growth direction, but even more generally of the abruptness of the interface. For non isovalent interfaces, the linear approach provides a natural way of studying the geometry-dependence of the offsets.