Electronic structure of ultrathin (GaAs)n(AlAs)n [001] superlattices and the Ga0.5Al0.5As alloy

Abstract

Using self-consistent electronic structure calculations we contrast the energy levels of the ultrathin (GaAs)n(AlAs)n [001] superlattices (n=1,2) with those of the disordered Ga0.5Al0.5As alloy and a long period (n→∞) superlattice. Conventional Kronig–Penney and effective mass models suggest that, because of the relatively light electron effective masses and small barrier heights, only delocalized superlattice conduction states would exist in the n=1 limit. We find a number of such conventional ‘‘averaging states’’ (delocalized on both sublattices). In addition, we also find states localized on a single sublattice. For small n’s, the latter are divided into two classes: (i) ‘‘repelling states’’ (distinct alloy states which fold in the superlattice into states of identical symmetry, which, in turn, repel each other and tend to localize), and (ii) ‘‘segregating states’’ (a pair of localized states Ψα and Ψβ, where symmetry compels Ψα to have a vanishing angular momentum component l on a subset α of unit cell atoms, whereas the complementary state Ψβ is localized on the other atoms β. These states are split by the potential difference Vβl −Vαl). We analyze new luminescence, reflectance, and Raman data in light of our theoretical model. Studies of the II-VI superlattices (CdTe)1(HgTe)1 shows similar behavior.