Structural determination of the Si(111) 3×3-Bi surface by x-ray standing waves and scanning tunneling microscopy

X-ray standing-wave measurements and tunneling microscopy have been combined to solve the atomic geometry of the 3×3R30°honeycomb phase of Bi on Si(111). The standing-wave measurements utilize three different diffracting planes to triangulate the surface position of Bi atoms. The unoccupied surface sites required to completely determine the structure can be deduced from Rutherford-backscattering coverage and low-energy electron-diffraction symmetry arguments. These arguments are completely confirmed by a tunneling-microscope study, which is free of the ambiguities of previous studies. The final result is a 2/3-ML 3×3R30°structure with Bi atoms in the T1 sites directly above first-layer Si atoms.