Valley-mixing effects in AlAs/GaAs (001) superlattices

The effects of Gamma X-valley mixing in (AlAs)(n)(GaAs)(n) (001) superlatti ces (SLs) on the electronic properties are theoretically investigated versu s the SL period (n = 1 - 25) and the band offsets. The calculations are bas ed on the empirical sp(3)s* tight-binding model, which includes only neares t-neighbour interactions. The results show that the highest state of the va lence band (VB) is always confined to the GaAs slabs, whereas the bottom st ate of the conduction band (CB) shows different behaviours as it is sensiti ve to band-mixing effects. It is due to these effects in the ultrathin-laye r SLs (n less than or equal to 8) that the electrons become localized in th e AlAs X-vaIley and the heterostructure becomes of type-II. Whereas, for th ick-layer SLs (n greater than or equal to 9), the GaAs wells become complet ely separated. As a result, the electrons are localized in the GaAs slabs a nd the SL is of type-I. The estimated critical layer thickness is about 26 Angstrom (n(c)=9), which is consistent with the photoreflectance experiment s. The valley-mixing effects are shown to be efficient only for small condu ction band offset (CBO less than or equal to 200 meV) and are essential for the existence of type-II behaviour. The relevance of our work to the photo nic device applications is furtherly discussed.