Evidence of Eightfold Coordination for Co Atoms at the CoSi2/Si(111) Interface

We have studied two-dimensional and three-dimensional Co${\mathrm{Si}}_2$ epitaxial layers on Si(111) by polarization-dependent surface extended x-ray-absorption fine structure on the Co $K$ edge (7707 eV). The Co interface atoms are coordinated with eight Si atoms, as in bulk Co${\mathrm{Si}}_2$, with an interface bond length of 2.35(0.03) Å. Ultrathin three-dimensional Co${\mathrm{Si}}_2$ layers are 2.5% contracted in the direction perpendicular to the interface.