Film thickness effects in the Co-Si₁-Si_1-xGe_x solid phase reaction

The thickness dependence of the reaction of cobalt with epitaxial silicon-germanium alloys (Si_1-xGe_x) has been studied. The reaction products of Co with (100)-oriented Si_0.79Ge_0.21 after annealing at 800 °C depended on the thickness of the Co film. Complete conversion to CoSi₂ occurred only when the thickness of the Co layer exceeded 350 Å. Interface reactions with Co layers thinner than 50 Å resulted in CoSi formation, while a mixture of CoSi and CoSi₂ was formed at intermediate thicknesses. X-ray diffraction and extended x-ray absorption fine structure measurements indicated no measurable incorporation of Ge had occurred in either the CoSi or CoSi₂. The threshold thickness for nucleation of CoSi₂ on (100)-oriented Si_1-xGe_x was determined in the range 0≤x≤0.25. The threshold thickness increased superlinearly with the Ge concentration x, and did not depend on the doping of the Si(100) substrate or the strain state of the Si_1-xGe_x film. The observed thickness effect was attributed to preferential Co-Si bonding in the reaction zone and the energy cost of Ge segregation, which accompanies the formation of CoSi and CoSi₂ during the reaction of Co with Si_1-xGe_x.