The mechanism of silicide formation at the apex of Si nanowire is two-stage silicidation. In the initial stage, as shown in Figure 9a, silicide grows in the radial direction, which is similar to the solid state reaction of metal film with a Si layer. The phase selection between metal and Si couples depends strongly on the atomic ratio
of Ni/Si. This dependence is observed not only in the thin film reactions [19] but also in the nanoparticle reactions [20]. In this study, the apex of Si nanowires covered with a considerable number of Ni atoms, which can be regarded as a system with a high Ni/Si atomic ratio, causing the formation of a metal-rich phase (Ni3Si2) at the Ni-coated part of Ni-silicide. Figure 9 Schematic illustrations of the mechanism of two-stage silicidation at the apex of Si nanowire. (a) A schematic illustration of the initial stage of silicidation.
(b) A schematic illustration of the second stage silicidation Selleck Milciclib in the Si nanowire with small diameter. (c) A schematic illustration of the second stage silicidation in the Si nanowire with large diameter. In the second stage, the Ni silicide axially intruded into the Si nanowire from the Ni-coated part located at the front of the nanowire. Such penetration of Ni silicide involves different thermally activated processes, such as the volume, surface, and interface diffusions of Ni. In this study, the phase selection depended on the diameter of the Si nanowires, such that NiSi2 and NiSi were observed in nanowires Farnesyltransferase selleck screening library with large diameters and small diameters, respectively.
The reasons for this phenomenon are discussed as follows. First, the location of silicide nucleation in the Si nanowires in the axial Citarinostat order direction is discussed. Wu et al. [11] studied the formation of Ni silicide in the Si nanowires through point and line contact reaction. By the point contact reaction between Ni nanodots and a Si nanowire, the nucleation and growth of NiSi grains start at the middle of the point contacts. By the line contact reaction between PS nanosphere-mediated Ni nanopatterns and a Si nanowire, silicide growth starts in the contact area. Wu et al. concluded that the mechanism of silicide growth in Si nanowires is based on the basis of flux divergence. Lu et al. [21] obtained the similar results for the formation of Pt silicide in the Si nanowires. They also performed molecular dynamic simulations to support the experimental results: a low atom flux of Pt caused the dissolution and distribution of Pt in the Si nanowire. Then, the nucleation of a silicide can occur between the two contacts where the Pt atoms dissolve, and the most probable site of nucleation is the middle because of the buildup of concentration that occurs in the middle. Second, the position of nucleation of silicide in Si nanowires in the radial direction is discussed. Chou et al. studied the growth of NiSi [22] and NiSi2[23] in Si nanowires by in situ high-resolution TEM.