On the basis of the data presented above, a reaction pathway is proposed in Figure 3.14 to summarize the TiSi growth. In our CVD system, TiCl4(g) reacts with Ti powders to produce TiClx subhalides at 1173 K in the first heating zone upstream from the Si substrate. Then, as the TiClx molecules reach the substrate surface at 1073 K, they disproportionate into TiCl4 molecules and Ti atoms, which deposit on the substrate surface.62 The Ti atoms could react with the Si substrate directly to form titanium silicides, such as C54-TiSi2 produced in this study. Alternatively, vapors of
Figure 3.14 Proposed growth pathway of the TiSi NWs. (x denotes variables).
TiCl4 and TiClx could react directly with the Si substrate to produce titanium silicides and SiClx byproducts. SiClx may decompose and act as another source of Si atoms for the silicide formation. After the TiSi2 film grows to a certain thickness, this relatively thick and inert layer may act as a barrier to impede diffusion of Si atoms from the substrate.68 This would hamper further interaction between the Si atoms in the substrate and the Ti atoms deposited on the surface. Consequently, the Si concentration on the surface is reduced while formation of a Si-poor a-TiSi2-x
interlayer begins. We want to emphasize that the interlayer, which may be viewed as a quasi-liquid thin film, might be the key to the successful growth of titanium silicide NWs in this study.69 After the amorphous interlayer is formed, nucleation of crystallites of a different titanium silicide phase, such as TiSi in I and III, as well as Ti5Si3 in V may start. Chemical composition of the interlayer, which varies with the reaction condition, affects the phase nucleated.70 As the reaction proceeds, these
nuclei at a relatively low supersaturation condition act as the seeds for further growth of the single crystalline titanium silicides. Since quantity of the seeds is limited, sites suitable for silicide growth are limited as well. Consequently, the crystalline NWs can grow only from selected spots on top of the amorphous layer. Reports on how quasi-liquid layer affected crystal growths are known. Recently, several research groups demonstrated the fabrication of NWs of metal silicide, including NiSix, CrSi2, FiSi, and CoSi, without the use of any metal nanoparticles.41,46,48,58
These may proceed via analogous routes.
Reaction temperature has a great influence on the crystal phase and the morphology of the NWs grown in this study. We suggest that the phase of the obtained NWs is highly dependent on the elemental composition of the interlayer. At a high temperature, the diffusion rate of Si atoms from the substrate is raised.71 This would increase the Si concentration in the interlayer, as verified in the SEM EDX data shown in Figure 3.8. In addition, the disproportionation of TiClx(g) to form TiCl4(g) and Ti(s) is less favored because the estimated Gibbs free energy of reaction is more positive.72 All of these would allow the interlayer formed at 1073 K contain more Si than that deposited at 973 K. Consequently, the NWs grown at 1073 K have more Si atoms to form the TiSi phase, while the NWs obtained at 973 K, with less Si atoms, have the Ti5Si3 phase. The distinct morphologies of the NWs may result from the difference in their diameters, which are 30 – 80 and 20 – 50 nm for TiSi and Ti5Si3, respectively. In addition, the bulk mechanical hardness of TiSi and Ti5Si3, 18.0 and 9.86 GPa, respectively, may affect their morphology too.28 Besides, Ti5Si3 NWs may contain more defects and become thread-like, whereas the TiSi NWs might be better crystallized so that the NWs grow upward on the substrate. All of the factors may explain why the TiSi NWs are straight while the Ti5Si3 NWs are coiled.
As mentioned in the TEM studies, the TiSi NWs show preferred growth orientation in the [010] direction while the Ti5Si3 NWs show that in the [001]
direction. We analyzed their crystal structures to rationalize the observed phenomena.
Figures 3.15 and 3.16 displayed the crystal models of TiSi and Ti5Si3 viewed along a, b, and c axes. In Figure 3.15, the Si-Si distance found in the structure was 0.2171 nm.
Clearly, this is much smaller than the Si-Si distances of many molecular compounds containing Si-Si single bonds, ranging from 0.2363 to 0.2370 nm.73,74 On the other hand, the Ti-Si and the Ti-Ti distances shown in Figure 3.15 were 0.2599 and 0.3227 nm, respectively. They are longer than the Ti-Si and the Ti-Ti bond distances reported in the literature for the corresponding molecular compounds, 0.2594 – 0.2629 nm and 0.2889 – 0.2942 nm for the Ti-Si and the Ti-Ti bonds, respectively.75,76 This suggests that the Si-Si bonds in TiSi are stronger than the Ti-Si and Ti-Ti bonds. Consequently, forming Si-Si bonds stabilizes the structure more than forming Ti-Si and Ti-Ti bonds does. In Figure 3.15, the presence of high density Si-Si bonds along b axis of TiSi was observed. This suggests that the growth of TiSi along the [010] direction should be more favored because the overall energy is decreased. A similar analysis is carried out to account for the preferred growth of hexagonal Ti5Si3 in the [001] direction. Figure 3.16 displayed the crystal models of hexagonal Ti5Si3 viewed along a and c axes.
Formation of Ti chains with extremely short Ti-Ti distance 0.2575 nm was observed along c axis. The distance is shorter than the Ti-Ti distances observed for many molecules containing Ti-Ti bonds. The nearest Si-Si and Si-Ti distances were 0.3024 nm and 0.2797 nm, respectively. These are longer than the molecular Si-Si and Ti-Si bond distances mentioned above. As a result, the growth of Ti5Si3 along the [001]
direction would produce more Ti-Ti bonds and lower the energy. For comparison, the growth directions of hexagonal phase CrSi2 and Fe5Si3 NWs were also found to be along [001].41,47 On the other hand, the reported growth directions of TiSi NPs and
NWs were different from our observation.35 After comparing the XRD results, we discover that the reported nanostructures had a different TiSi phase (JCPDS-65-2585, ICSD 20375).
Figure 3.15 Crystal models of orthorhombic TiSi viewing along (a) a, (b) b, and (c) c axes. The short Si-Si bonds (0.2171 nm) are linked in yellow sticks. Orthorhombic TiSi (JCPDS 17-0424, ICSD 43494): space group Pnma (no. 62), a = 0.6544 nm, b = 0.3638 nm, c = 0.4997 nm.
Figure 3.16 Crystal models of hexagonal Ti5Si3 viewing along (a) a and (b) c axes.
Viewing along b axis is equivalent to viewing along a axis. The short Ti-Ti bonds (0.25754 nm) are linked in yellow sticks. Hexagonal Ti5Si3 (JCPDS 78-1429, ICSD 62591): space group P63/mcm (no. 193), a = 0.7610 nm, c = 0.51508 nm.