Evidence of Eighfold Coordination of Co/Si(111) Interface

We have studied two-dimensional and three-dimensional CoSi2 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 CoSi2, with an interface bond length of 2.35(0.03) A. Ultrathin three-dimensional CoSi2 layers are 2.5% contracted in the direction perpendicular to the interface.