## 3 Results and Discussion
### Silicidation in Nanowires  
After determining an initial silicidation process window from the experiments for silicidation in thin film structures presented in the previous section, experiments for nanowire silicidation were performed. For these experiments, nanowires were fabricated in \(<\)100\(>\) and \(<\)110\(>\) crystallographic orientations using the top-down approach. Ni was sputtered according to the conditions described in the Materials and Methods section and, subsequently, rapid thermal annealing was performed at 450 \({}^{\circ}\)C in forming gas atmosphere for 80 s. The resulting structures are shown in Figure 4.  
As illustrated in Figure 4, the silicide diffusion length is larger in \(<\)100\(>\) NWs compared to \(<\)110\(>\) NWs. [PERSON] et al. [29] have shown that silicidation diffusion length is inversely proportional to square of the diameter of the NW. The widths of \(<\)100\(>\) and \(<\)110\(>\) NWs in Figure 4a are 28 and 24 nm, respectively, while the silicide diffusion lengths are 745 and 628 nm, respectively. This shows a strong dependence of silicide length on the nanowire orientation. To investigate in detail the dependence of Ni diffusion and silicide formation on crystallographic orientations of NWs, a circular array of NWs with five-degree separation was fabricated. It was followed by the aforementioned steps for silicidation. The results are shown in Figure 5.  
Although the silicide diffusion length varies in different crystallographic orientations of the NWs, it was not possible to extract a clear relation since the results vary in different arrays. To further investigate the orientation dependence of the silicide formation, silicidation of nanowire arrays fabricated in \(<\)100\(>\) and \(<\)110\(>\) crystallographic orientations was carried out and the results are presented in Figure 6a-d. The width of the NWs is 20 nm. The average silicide diffusion length in \(<\)100\(>\)- and \(<\)110\(>\)-oriented SiNWs is 812 and 744 nm, respectively (see Figure 6e). The images distinctly show that the diffusion of Ni and the silicide progression into the NWs of the same array is not homogeneous and on average has scattering of up to 300 nm in \(<\)100\(>\) and 400 nm in \(<\)110\(>\) NWs. Silicidation rates are known to vary  
Figure 4: SiNWs in \(<\)110\(>\) and \(<\)100\(>\) crystallographic orientations: the brighter segments of NWs indicate silicide formation. Silicidation is faster in \(<\)100\(>\) NWs. Annealing is performed at 450 \({}^{\circ}\)C for 80 s.  
Figure 5: SEM images showing (**a**) nanowires fabricated in circular array with a five-degree separation to study the dependence of silicide diffusion length on the nanowire orientation; and (**b**) a higher magnification image showing clearly different diffusion lengths in different NWs. Annealing is performed at 450 \({}^{\circ}\)C for 80 s.  
based on different factors, including the quality of interface between the Ni reservoir and the NW [21]. However, our investigation shows this variation also within an array of NWs, which is processed under the same conditions. We attribute this variation in silicide length to different surfaces of NWs, as these NWs are top-down fabricated and their surfaces can be different due to the subtractive nature of the etching process. Because of this uncontrolled silicide formation and scattered positioning of the Schottky junctions, large top gates have to be placed to ensure that they properly cover the junctions at the two sides of the NWs [14; 34]. This limits the possibilities for downscaling the size of devices and compromises their large-scale fabricability. To have better control over the silicidation length, further investigations were carried out with sets consisting of two and three NWs. The results are shown in Figure 7. It was expected that the outer two NWs might consume excess Ni, giving a better control over the silicide progression in the central NW. However, it was still not possible to obtain a proper control over the silicide progression with this technique.