From the chemisorption of Co on Si(111)7×7 to the formation of epitaxial A and B-type CoSi2

We present the SEXAFS determination of the room temperature chemisorption configuration for Co on Si(111)7×7, and of the local geometry of the annealed epitaxial CoSi2/Si(111)7×7 interface. The chemisorption is fully determined by the geometry of the 7×7 substrate, both on the unfaulted and stacking-faulted halves of the surface unit cell and the Co sites are not directly related to the epitaxial positions. We propose then a chemically-driven-concerted-atom-exchange (CDCE) mechanism for the transition from chemisorption of Co to epitaxy of CoSi2on Si(111) which explains the coexistence of type A and B epitaxy.