SURFACE ATOMIC-STRUCTURE AND SYMMETRY PROPERTIES OF ERSI1.7 ON SI(111)

We investigate the surface atomic structure of ErSi1.7 layers epitaxia lly grown on Si (111). Two possible positions of the Si vacancy with r espect to the terminal Si layer are considered : vacancy in the Si gra phite-like plane located either below a Si atom of the outermost (M(1) configuration) or inner (M(2) configuration) planes of the buckled Si top layer. Polarization dependent photoemission experiments combined to electronic band structure calculations and symmetry considerations allow us to rule out the M(2) configuration.