  We characterize composition and structure of ultrathin nickel silicide during
formation from 3 nm Ni films on Si(100) using in-situ high resolution ion
scattering and high resolution transmission electron microscopy. We show the
transition to occur in discrete steps, in which an intermediate phase is
observed within a narrow range of temperature from 230 oC to 290 oC. The film
composition of this intermediate phase is found to be 50% Ni:50% Si, without
evidence for long-range structure, indicating the film to be a homogeneous
monosilicide NiSi phase. The final phase is resemblant of the cubic disilicide
NiSi2, but with slightly off-stoichiometric composition of 38% Ni and 62% Si.
Along the [100] axis, the lattices of the film and the substrate are found in
perfect alignment. Due to the epitaxial growth of the silicide, a contraction
of the c lattice constant of the film by 0.7-1% is detected.
