Germanium segregation in the Co/SiGe/Si(001) thin film system

Cobalt disilicide contacts to silicon-germanium alloys were formed by direc t deposition of pure cobalt metal onto silicon-germanium films on Si(001) s ubstrates. Segregation of germanium was observed during the reaction of the cobalt with the silicon-germanium alloy. The nature of the Ge segregation was studied by transmission electron microscopy, energy dispersive spectros copy, and x-ray diffraction. In the case of cobalt films deposited onto str ained silicon-germanium films, the Ge segregation was discovered to be in t he form of Ge-enriched Si1-xGex regions found at the surface of the film su rrounding CoSi and CoSi2 grains. In the case of cobalt films deposited onto relaxed silicon-germanium films, the Ge segregation was dependent on forma tion of CoSi2. In samples annealed below 800 degrees C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the stra ined Si1-xGex case. In samples annealed above 800 degrees C, where CoSi2 wa s the dominant silicide phase, the Ge segregation was also in the form of t etrahedron-shaped, Ge-enriched, silicon-germanium precipitates, which forme d at the substrate/silicon-germanium film interface and grew into the Si su bstrate. A possible mechanism for the formation of these precipitates is pr esented based on vacancy generation during the silicidation reaction couple d with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.