The common practice for nickel silicide formation in silicon nanowires (SiNWs) relies on axial growth of silicide along the wire that is initiated from nickel reservoirs at the source and drain contacts. In the present work the silicide intrusions were studied for various parameters including wire diameter (25-50 nm), annealing time (15-120 s), annealing temperature (300-440 C), and the quality of the initial Ni/Si interface. The silicide formation was investigated by high-resolution scanning electron microscopy, high-resolution transmission electron microscopy (TEM), and atomic force microscopy. The main part of the intrusion formed at 420 C consists of monosilicide NiSi, as was confirmed by energy dispersive spectroscopy STEM, selected area diffraction TEM, and electrical resistance measurements of fully silicided SiNWs. The kinetics of nickel silicide axial growth in the SiNWs was analyzed in the framework of a diffusion model through constrictions. The model calculates the time dependence of the intrusion length, L, and predicts crossover from linear to square root time dependency for different wire parameters, as confirmed by the experimental data.
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 singlecrystalline 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 300°C to 400°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 $R À0.75 . 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/d and D c /D sc , where d is the thickness of the high-diffusivity surface layer with diffusion coefficient D sc , and D c 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 sc /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.
Thermally activated axial intrusion of nickel silicides in silicon nanowires (SiNWs) is utilized to form nickel silicide/silicon contacts in SiNW field effect transistors. The growth of different nickel silicides is often accompanied by local thickening and tapering of the NW, up to its full disintegration. In this paper, this process was investigated in SiNWs of 30-60 nm in diameters with prepatterned Ni electrodes after annealing cycles at different temperatures of 300°C-440°C and times up to 120 s. From the temperature dependence of the intrusion lengths, activation energy of 1.45 eV for the surface diffusion of nickel was extracted. In several cases, periodic thickening of the nickel-rich part is accompanied by tapering of the monosilicide part up to its full dissolution. The kinetics of the nickel silicides growth was described by phenomenological model. For a certain set of parameters, tapering and dissolution of the monosilicide part of the intrusion were predicted, similar to the experimental results.
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