Stability and electronic structure of ultrathin-layer superlattices: (GaAs)n/(AlAs)n

A first-principles total-energy calculation has been performed for ultrathin-layer (001) superlattices, (GaAs${)}_{\mathrm{n}}$/(AlAs${)}_{\mathrm{n}}$ (n layers of GaAs alternating with n layers of AlAs, with n=1 and 2). The total energies for the superlattices are higher than the averaged total energies of GaAs and AlAs by about 5 meV per atom. This small difference in energy results from the efficient screening of the valence electrons at the As interface layers. The calculated total energy of the alloy semiconductor ${\mathrm{Ga}}_{0.5}$ ${\mathrm{Al}}_{0.5}$As, in turn, is higher than that of the superlattices by 88 meV per atom. An indirect gap in (GaAs${)}_1$/(AlAs${)}_1$ and a direct gap in (GaAs${)}_2$/(AlAs${)}_2$ are inferred from the calculated energy bands.