X-ray standing-wave and tunneling-microscope location of gallium atoms on a silicon surface

The position of gallium atoms on a silicon (111) surface has been completely determined using the tunneling microscope and x-ray standing-wave methods. The ({}3 ×{} {}3 )R30^∘{} electron diffraction pattern observed with (1/3-monolayer coverages is shown to result from a simple adatom gallium lattice with the adatoms at a distance 1.49 A{} above the bulk extrapolated surface (111) plane above the filled threefold silicon surface sites. Total-energy calculations correctly predict the binding site with the Ga 1.33 A{} above the bulk (111) plane.