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 f ind that the surface transforms through four phases as the chemical po tential varies across its accessible range. As the Ga chemical potenti al increases the surface transforms from an As-rich c(4x4) through two distinct (2x4) structures and finally to a Ga-rich (4x2) phase. The p redicted structures account for most scanning tunneling microscopy obs ervations for the c(4x4), (2x4), and (4x2) phases.