It is generally known that nucleation effects strongly influence the CoSi to CoSi2 phase transition. According to classical nucleation theory, the small difference in Gibbs free energy between the CoSi and CoSi2 phase is responsible for the nucleation barrier. Adding elements that are soluble in CoSi and insoluble in CoSi2 will influence the entropy of mixing, and thus change ΔG. In this way, the height of the nucleation barrier may be controlled.
By depositing Fe or Ge (respectively replacing Co and Si in the CoSi lattice) in between the Co and the Si substrate, we were able to increase the nucleation barrier. In the presence of Ni, the nucleation barrier is lowered, and low-resistive disilicide is formed at lower temperatures.