Ordering of conduction band states in thin-layer (AlAs)_m(GaAs)_n(001) superlattices

The electronic structures of thin-layer (AlAs)_m(GaAs)_n(001) superlattices (SLs) are investigated versus the SL layer thicknesses (m, n) and the band offsets. The calculations are based on the empirical sp³s* tight-binding model, which includes only nearest-neighbour interactions. Particular attention is given to the effect of the interface parametrization on the SL electronic properties. This is done, mainly, by varying the band offsets over a sufficiently broad range. The results show that the existence of type-II behaviour in the ultrathin-layer SLs necessitates a large valence band offset (VBO≈0.56 eV) and small conduction band offset (CBO≈190 meV). Providing that these offset values are achieved, it is found that the highest state of the valence band is always confined to the GaAs slabs whereas the bottom state of the conduction band shows different behaviours as it is sensitive to band-mixing effects. It is due to these mixing effects that most of the ultrathin-layer SLs (with m, n≤8) behave as type-II heterostructures, where the electrons are localized in the AlAs X_xy valley. The rest of the ultrathin-layer SLs behave as type-I heterostructures with a direct bandgap at the Γ point, whenever the GaAs slabs are thick enough to make the electron confinement energy small in the GaAs wells. For thick-layer SLs, our results suggest the existence of a critical barrier thickness, beyond which the GaAs wells become completely decoupled and the SL behaves as a type I heterostructure. The estimated critical layer thickness, for the crossover from type-I to type-II behaviour, is n_c = 9 for the SLs with m = n when using VBO = 0.56 eV. This n_c-value is consistent with the photoreflectance experiments. The relevance of our work to photonic device applications is discussed further.