Interface structure of Si(111)-(root 3x root 3)R30 degrees-ErSi2-x

The interface structure of epitaxial ErSi2-x (x approximate to 0.3) on Si(111) has been measured by means of surface X-ray diffraction. The silicide consists of a stack of alternating Si and Er planes. The Si planes resemble compressed substrate bilayers, in which a regular network of vacancies releases the compressive strain. This vacancy network gives rise to a (root 3 x root 3)R30 degrees reconstruction of the silicide, in which Si atoms are displaced towards the vacancies, and Er atoms are displaced away from the vacancies. The Si-Si bond length in the silicide is 2.36+/-0.02 Angstrom, and the nearest-neighbour Er-Si distance is 2.95+/-0.02 Angstrom. The positions of the atomic planes in the interface region are identical to those found for two-dimensional erbium silicide on Si(111), and first-layer Er atoms are located on T-4 sites on the substrate. Our preparation method (reactive deposition epitaxy) leads to rough silicide films. Nevertheless, we are able to extract the atomic occupancies of eight individual layers. All silicide layers are reconstructed, but only the silicide layer at the interface shows long-range order of the vacancy network.