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

Cobalt disilicide contacts to silicon-germanium alloys were formed by direct deposition of pure cobalt metal onto silicon-germanium films on Si(001) substrates. 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 spectroscopy, and x-ray diffraction. In the case of cobalt films deposited onto strained silicon-germanium films, the Ge segregation was discovered to be in the form of Ge-enriched Si_1-xGe_x regions found at the surface of the film surrounding CoSi and CoSi₂ grains. In the case of cobalt films deposited onto relaxed silicon-germanium films, the Ge segregation was dependent on formation of CoSi₂. In samples annealed below 800 °C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the strained Si_1-xGe_x case. In samples annealed above 800 °C, where CoSi₂ was the dominant silicide phase, the Ge segregation was also in the form of tetrahedron-shaped, Ge-enriched, silicon-germanium precipitates, which formed at the substrate/silicon-germanium film interface and grew into the Si substrate. A possible mechanism for the formation of these precipitates is presented based on vacancy generation during the silicidation reaction coupled with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.