Structure and thermodynamic stability of GaAs(001) surfaces

The surface energies for a set of structural models for the reconstructions occurring on GaAs(001) were calculated with the first principles pseudopotential method and the local density approximation. On the basis of these calculations we are able to rule out the single-dimer-vacancy model for the 2 × 4 surface as a possible equilibrium structure. In the As-rich limit we find the c(4 × 4) As ad-dimer model to be energetically favorable. As the Ga chemical potential increases the surface is predicted to transform into the β2(2 × 4) structure having two As dimers in the top layer and a third As dimer in the third layer. The α(2 × 4) structure, with two As dimers above a complete layer of Ga is found to be stable in a very narrow range. The β(4 × 2) structure and the model recently proposed by Skala et al. for the (4 × 2) reconstruction were determined to be energetically unfavorable. In the Ga-rich limit the lowest energy structure we have obtained is the β2(4 × 2) model.