Fluorite silicides

The experimental determination of the atomic structure of an interface is difficult, as the buried interface is not accessible to most structural probes. Cross-section TEM lattice imaging has shown the outstanding perfection of the fluorite silicide (111) interfaces [3]. Combined with image simulation, it gave quantitative information on the displacement of the sihdde < 111 > lattice planes with respect to the substrate [53, 54]. There are two possible orientations (A and B) of the sihddes on top of the Si(lll) surface (corresponding to the two possible different ways of setting the fluorite structure on top of the siHcon lattice), as shown in Figure 14.9. 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

The two most widely studied epitaxial silicides, NiSi2 and GoSij, have the cubic fluorite stracture CaF2 (Figure 14.8). The lattice mismatch to Si is defined as... 

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