CoSi2/Si(111) interface: Determination of the interfacial metal coordination number

The morphology and the surface and interface structure of flat ${\mathrm{CoSi}}_2$ films (thickness ∼2 Si-Co-Si triple layers) epitaxially grown on Si(111) have been investigated using medium-energy ion scattering with ultrahigh energy resolution. The data show that the silicide is attached to the substrate through Co-Si bonds rather than through Si-Si bonds. At the interface, the silicide is terminated by a full Si-Co-Si triple layer. The interface metal atoms are thus eightfold coordinated. The silicide-substrate distance is expanded by 0.16±0.08 Å. The first Si substrate layer is relaxed outward by 0.08±0.08 Å. The length of the Co-Si interface bond is thus dilated by 0.08 Å. At the silicide surface, the first Co-Si interplanar distance is found to be relaxed inward by 0.14±0.04 Å. Our measurements suggest that the silicide lattice strain is almost fully relaxed. We propose that such relaxation occurs through a high density of microcracks in the film. The average spacing between cracks is then smaller than roughly 50 Å.