Surface electronic and atomic structure ofErSi1.7on Si(111)

The surface atomic and electronic structure of ${\mathrm{ErSi}}_{1.7}$ layers epitaxially grown on Si(111) is studied by angle-resolved ultraviolet photoemission spectroscopy. The experimental results are compared to electronic band-structure calculations for various reasonable surface atomic configurations. Satisfactory agreement is obtained for two geometries consisting of reconstructed ${\mathrm{ErSi}}_{1.7}$ (0001) surfaces. Both reconstructions involve a buckled Si top layer similar to (111) double layers in bulk Si but differ in their registries with respect to the bulk silicide layer underneath, leading to a silicide surface termination with ${\mathrm{ErSi}}_{1.7}$ stoichiometry. In contrast, the calculations clearly show that a surface termination with ${\mathrm{ErSi}}_{1.7}$ stoichiometry involving an ordered array of vacancies in the buckled Si top layer would result in a quite different surface electronic structure incompatible with the experimental one. This allows us to rule out this model often invoked in previous work. Finally, models exposing a bulklike flat Si graphitelike top layer, with or without vacancies, can also safely be ruled out on the basis of the present data.