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 [radical]3[times][radical]3[ital R]30[degree] 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 [radical]3[times][radical]3[ital R]30[degree] structure with Bi atoms in the [ital T][sub 1] sites directly above first-layer Si atoms.