Epitaxial NiSi 2 thin films are formed by annealing of Ni on sulfur-implanted silicon (1 0 0). The atomic structure and chemistry of the NiSi 2 /Si interface are investigated by aberration-corrected transmission electron microscopy. The interface is atomically sharp and runs mainly along the (1 0 0) plane. {1 1 1} segments of interface are also observed as minor facets. The atomic structure of the (1 0 0) and (1 1 1) interface has been determined. Interfacial dislocations with Burgers vectors a/4<1 1 1> and a/2<1 1 0> are observed near {1 1 1} facets. In particular, these dislocations have extra half atomic planes in the Si substrate. This configuration of dislocation does not agree with the sign of the lattice mismatch between bulk NiSi 2 and Si. This novel phenomenon is understood by the fact that a high concentration of sulfur in the interface area leads to an expansion of the NiSi 2 lattice and thus inverts the sign of the lattice mismatch. It is suggested that the change of the strain status, in addition to the doping effect of S, also plays a role in the tunable Schottky barrier height in this system.