  The formation of Ni(Pt)silicides on a Si(001) surface is investigated using
an ab initio approach. After deposition of a Ni overlayer alloyed with Pt, the
calculations reveal fast diffusion of Ni atoms into the Si lattice, which leads
initially to the formation of Ni2Si. At the same time Si atoms are found to
diffuse into the metallic overlayer. The transformation of Ni2Si into NiSi is
likely to proceed via a vacancy-assisted diffusion mechanism. Silicon atoms are
the main diffusing species in this transformation, migrating from the Si
substrate through the growing NiSi layer into the Ni2Si. Pt atoms have a low
solubility in Ni2Si and prefer Si-sites in the NiSi lattice, thereby
stabilizing the NiSi phase. The diffusivity of Pt is lower than that of Ni.
Furthermore, Pt atoms have a tendency to segregate to interfaces, thereby
acting as diffusion barriers.
