Structure of Epitaxial CoSi2 Films on Si(111) Studied with Low-Energy Electron Diffraction (LEED)

Expitaxial films of CoSi 2 on Si(111) were investigated by low-energy electron diffraction. Films of approximately 12 Å thickness were prepared by simultaneous deposition of Co and Si and subsequent annealing. The films were found to crystallize in CaF 2 structure in (111) orientation. Two (1×1) phases of different stoichiometry exist. The surface phase that contains more Co is found to be a CoSi 2(111) bulklike structure terminated by a Si–Co–Si trilayer. The Si-rich phase is terminated by an additional nonrotated silicon bilayer with the lower silicon atoms bound to cobalt in the first CoSi 2 layer. Consequently, these cobalt atoms have an eightfold coordination. Due to the lattice mismatch the silicide films are expanded by 0.5% in the lateral direction and contracted by 1.4% in the vertical direction.