Ab initiocalculation of binding and diffusion of a Ga adatom on theGaAs (001)−c(4×4)surface

We have investigated the diffusive behavior of a single Ga adatom on the $\mathrm{GaAs}\left(100\right)-c(4\mathrm{}\times 4)$ surface by means of the local-density approximation and the all-electron projector augmented wave (PAW) method. The ground-state geometry of the $\mathrm{GaAs}\left(100\right)-c(4\mathrm{}\times 4)$ surface is determined using PAW and is found to agree with experiment and previous calculations. The binding energy for a lone Ga adatom on this reconstruction is calculated as a function of surface position. Based on these data we have identified three relatively stable adsorption sites. In order of increasing energy these sites are site 1 at the center of the missing dimer position; site 2 between the dimer rows and adjacent to a center dimer; and site 3 between the dimer rows, adjacent to an edge dimer. The surface diffusion activation energies have also been identified; the smallest is $0.14\hspace{0.5em}\mathrm{eV}$ for the $\overrightarrow{3}2$ transition, and the largest is $0.45\hspace{0.5em}\mathrm{eV}$ for $\overrightarrow{2}3.$ Kinetic Monte Carlo simulations incorporating these data indicate that diffusion on this surface takes place primarily through diffusion pathways that pass through the strongest binding site (site 1). This site effectively controls diffusion in directions both parallel and perpendicular to the dimer rows.