Geometry-dependent phase, stress state and electrical properties in nickel-silicide nanowires

We report that the geometry of single-crystalline Si nanowires (NWs) prior to salicidation at 500 °C is the key factor controlling the phase, stress state, and electrical resistivity of the resulting Ni_xSi_y NWs of width less than 100 nm. This is a radical departure from previous observations of a single phase formation for nickel silicides generated from the silicidation of bulk Si substrates. The phase transition from NiSi for large NWs ( W _{Si NW} = 250-450 nm) to Ni₂Si for small NWs ( W _{Si NW} = 70-100 nm) is well correlated with the observed volumetric expansion and electrical resistivity variation with the NW width. For the extremely small dimensions of Ni_xSi_y NWs, we propose that the preeminent, kinetics-based Zhang and d'Heurle model for salicidation be modified to a more thermodynamically-governed, volume-expansion dependent Ni_xSi_y phase formation. A novel, plastic deformation mechanism is proposed to explain the observed, geometry-dependent Ni_xSi_y NW phase formation that also strongly influences the electrical performance of the NWs.