ENERGETICS OF GAAS(100)-(2X4) AND GAAS(100)-(4X2) RECONSTRUCTIONS

Formation energies for a variety of GaAs(100) surface structures have been calculated as a function of the atomic chemical potentials using the first-principles pseudopotential density-functional approach. We find that the surface transforms through four phases as the chemical potential varies across its accessible range. As the Ga chemical potential increases the surface transforms from an As-rich c(4x4) through two distinct (2x4) structures and finally to a Ga-rich (4x2) phase. The predicted structures account for most scanning tunneling microscopy observations for the c(4x4), (2x4), and (4x2) phases.