The anisotropy is expected to arise from two factors. GaAs(100), like Si(100), forms reconstructions based on dimer rows. This leads to different activation energies for diffusion in the direction parallel and perpendicular to the rows. Second, an atom sees two types of binding site at an island - one where the step edge runs parallel to the dimer row direction and one where the step edge is perpendicular to the dimer row direction. This leads to different rates of attachment at the two types of step.