Surface crystallography ofYSi2−xfilms epitaxially grown on Si(111): An x-ray photoelectron diffraction study

The surface crystallography of epitaxially grown ${\mathrm{YSi}}_{2\mathrm{-}\mathrm{x}}$ (x=0.3 and x=0) films on Si(111) has been investigated using x-ray photoelectron diffraction. For the ${\mathrm{YSi}}_{1.7}$ films, vacancies form an ordered √3 × √3 R30° superlattice within each Si plane. These vacancies are filled for ${\mathrm{YSi}}^2$. The analysis reveals that their surfaces are Si terminated with a displacement upward (0.8 Å) of one Si atom out of two so that they exhibit the same geometry as a 1×1 Si(111) surface.