Film thickness effects in the Co-Si1-xGex solid phase reaction

The thickness dependence of the reaction of cobalt with epitaxial silicon-germanium alloys (Si1-xGex) has been studied. The reaction products of Co with (100)-oriented Si0.79Ge0.21 after annealing at 800 degrees C depended on the thickness of the Co film. Complete conversion to CoSi2 occurred only when the thickness of the Co layer exceeded 350 Angstrom. Interface reactions with Co layers thinner than 50 Angstrom 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 Si1-xGex was determined in the range 0 less than or equal to x less than or equal to 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 Si1-xGex 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 Si1-xGex. (C) 1998 American Institute of Physics. [S0021-8979(98)05620-5].