Structural investigation of the Ca/Si(111)-(3x2) surface using photoelectron diffraction

The atomic structure of the Ca induced Si(111)-(3x2) surface at 300 K has been investigated using X-ray photoelectron diffraction (XPD). After confirming the quality of the single-domain (3x2) surface by LEED, in which weak x2 streaks were observed likewise the patterns reported in the literature, we have measured the Ca 2p core-level spectra within an azimuthal angle range of +/- 66 degrees, where 0 degrees corresponds to the direction perpendicular to the Ca chain, and at takeoff angles from 9 degrees to 15 degrees using the Mg Ka line. The experimental XPD patterns showed good agreement with the simulated XPD patterns of the T-4 site model, while the simulated XPD patterns of the H-3 model do not agree with the experimental results. Taking these results into account, we conclude that Ca atoms are adsorbed on the T-4 site at 300 K and thus that the weak x2 streaks do not result from the presence of two adsorption sites as proposed in the literature.