ELECTRONIC-STRUCTURE AND INTERFACIAL GEOMETRY OF EPITAXIAL 2-DIMENSIONAL ER SILICIDE ON SI(111)

The two-dimensional band structure of a single epitaxial ErSi2 layer o n Si(111) is calculated by means of the crystalline extension of the e xtended Huckel method for various atomic structures and tested against experimental bands determined by angle-resolved photoemission. In par ticular, adopting for the silicide layer the structure proposed in pre vious work, i.e., a hexagonal Er monolayer underneath a buckled Si top layer, various possible interfacial geometries are investigated, name ly with the Er in top, substitutional, T4, and H-3 sites of the Si(111 ) substrate and for the two possible orientations of the latter with r espect to the buckled Si top layer. With the exception of the substitu tional site, all models show two characteristic bands near the Fermi l evel that are essentially full and empty, respectively, as observed ex perimentally. Yet, the topology of these bands is correctly reproduced for only two interfacial geometries, namely Er in H-3 (T4) sites with the buckled Si top layer having an orientation identical (opposite) t o the substrate Si double layers. For both models the overall agreemen t between calculated and experimental bands is quite satisfactory. The prominent almost-filled band observed experimentally in the 0- 1.7-eV binding-energy range mainly derives from the dangling bonds of the bu ckled Si top layer, but shows a strong hybridization with Er 5d states near the center of the surface Brillouin zone.