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Valley-Mixing Effects in AlAs/GaAs (001) Superlattices

The effects of ΓX-valley mixing in (AlAs)n(GaAs)n (001) superlattices (SLs) on the electronic properties are theoretically investigated versus the SL period (n = 1–25) and the band offsets. The calculations are based on the empirical sp3s* tight-binding model, which includes only nearest-neighbour interactions. The results show that the highest state of the valence band (VB) is always confined to the GaAs slabs, whereas the bottom state of the conduction band (CB) shows different behaviours as it is sensitive to band-mixing effects. It is due to these effects in the ultrathin-layer SLs (n ≤ 8) that the electrons become localized in the AlAs X-valley and the heterostructure becomes of type-II. Whereas, for thick-layer SLs (n ≥ 9), the GaAs wells become completely separated. As a result, the electrons are localized in the GaAs slabs and the SL is of type-I. The estimated critical layer thickness is about 26 Å (nc = 9), which is consistent with the photoreflectance experiments. The valley-mixing effects are shown to be efficient only for small conduction band offset (CBO ≤ 200 meV) and are essential for the existence of type-II behaviour. The relevance of our work to the photonic device applications is furtherly discussed.