Surface Diffusion Controlled Formation of Nickel Silicides in Silicon Nanowires

Nickel silicide/silicon contacts used in field-effect transistors (FET) based on silicon nanowires (SiNWs) can be formed by thermally activated axial intrusion of nickel silicides into the SiNW from prepatterned nickel reservoirs located at both ends of the NWs. This method seems promising for future electronic applications. Transformation of the longitudinal NW segments into single-crystalline nickel silicides throughout the entire NWs bulk has been interpreted as evidence of a volume diffusion control process. However, the volume diffusion coefficients of nickel in Ni(2)Si at 300A degrees C to 400A degrees C are inconsistent with observable nickel silicide intrusion lengths. The experimental results published so far show a distinct dependence of nickel silicide intrusion length on the silicon NW diameter, which is indicative of a surface diffusion or a surface reaction controlled process. In this work, this problem was considered theoretically in the framework of a model of a diffusion-controlled phase formation. Diffusion growth of a wedge-like new phase in a cylindrical NW was described using a quasistationary approximation. The rate of longitudinal growth depends on the NW radius, R, and decreases with the radius increase as similar to R (-0.75). The dependence of R on annealing time, t, is close to t (0.5). The profile of the new phase was described for different combinations of two dimensionless parameters: R/delta and D (gamma)/D (s gamma), where delta is the thickness of the high-diffusivity surface layer with diffusion coefficient D (s gamma), and D (gamma) is the volume diffusion coefficient. After the formation of a continuous layer of a new phase, further growth is controlled exclusively by the interface diffusion of Ni along the nickel silicide surface and Si/Ni(2)Si interface. The growth kinetics depends on the ratio of diffusion coefficients D (s gamma)/D (b), where D (b) is the interface diffusion coefficient, and may be parabolic or linear. The calculated dependencies were compared with the published experimental results for nickel silicide formation in SiNWs. The analysis performed indicates that surface and interface diffusion of nickel play an important role in the formation of nickel silicides in NWs-a critical finding that should be considered in the design of SiNW FETs.