Silicide formation was studied for Co/C/Si and C/Co/Si bilayers. Phase formation and preferential orientation of the CoSi2 phase is strongly dependent on the C layer thickness. For thin interlayers and capping layers, CoSi2 nucleation is delayed to higher temperature and epitaxially aligned nuclei are formed preferentially. To explain these findings, we propose a model based on heterogeneous nucleation theory: C is present as a contaminant in the CoSi and is able to influence CoSi2 nucleation by enhancing the CoSi grain-boundary cohesion and reducing grain-boundary diffusion. For thicker interlayers (≫1 nm), the carbon acts as a diffusion mediating interlayer, and epitaxial CoSi2 is formed by slow outdiffusion of Co from an amorphous Co–C alloy. © 2002 American Institute of Physics.