Surface phase diagram of (2X4) and (4X2) reconstructions of GaAs(001)

Total-energy calculations for a series of (2x4) and (4x2) reconstructed GaA s(001) surfaces not included in previous theoretical studies are presented. A (2x4) surface model containing single anion dimers in the first and thir d atomic layers is predicted for a balanced surface stoichiometry. It is mo re stable than the two-As-dimer a structure assumed previously, due to its lower electrostatic energy. Our results for the (4x2) reconstructed surface confirm the two-Ga-dimer beta 2 structure suggested by Biegelsen and co-wo rkers. Nearly degenerate in energy, however, are mixed Ga-As dimers adsorbe d on a Ga-terminated substrate.