ATOMIC STRUCTURES OF GAAS(100)-(2X4) RECONSTRUCTIONS

Atomic structures of the As-rich GaAs(100)-(2x4) reconstructions based on converged first-principles total-energy calculations are reported. All geometries are characterized by similar structural elements such as As dimers with a length of about 2.5 Angstrom, dimer vacancies, and a nearly planar configuration of the three-fold coordinated second-la yer Ga atoms leading to a steepening of the dimer block. For an As cov erage of theta=3/4 we find the two-dimer beta 2 phase to be energetica lly preferred over the three-dimer beta phase. A structure with partia l replacement of As by Ga in the uppermost layer corresponding to an A s coverage of theta=1/2 is found to be slightly less favourable than t he alpha phase of GaAs(100). Geometry parameters are given for all str uctures and compared with the available experimental results.