## 3 Results and Discussion  
### Silicidation in Si Thin Film Structures  
To study silicidation in thin films structures, Si pads were patterned in Si substrates in \(<\)110\(>\) and \(<\)100\(>\) orientations using EBL and dry etching processes. Ni sputtering and subsequent annealing for silicidation was carried out according to conditions mentioned in Section 2. The purpose of this first study was to determine an initial process window for the nanowire silicidation. Figure 1 shows top-view SEM images of a sample after silicidation. Annealing was performed at 450 \({}^{\circ}\)C in forming gas atmosphere for 10 min. It is evident from the bright regions of the Si pads in Figure 1 that the Ni diffusion was not fast enough to reach the nanowires and it was confined within the Si pads. Moreover, the silicide-Si interface has different shapes in Si pads with \(<\)110\(>\) and \(<\)100\(>\) orientations. The silicide makes a 90\({}^{\circ}\) angle with Si in \(<\)110\(>\) orientation, whereas in the \(<\)100\(>\) orientation, the silicide has a step-like interface with Si. TEM analysis was performed on lamellas, prepared by FIB, from the selected samples. The cross-section was inspected by high-resolution transmission electron microscopy (HREM) and electron energy loss spectrum (EELS). Figure 2 shows the overview of the cross-section with the different layers included in it. The top most sample layer is covered by a carbon protection layer, which is part of the  
Figure 1: Top-view scanning electron microscopy (SEM) images of silicon nanowires (SiNWs) with Si pads in (**a**) \(<\)110\(>\) and (**b,c**) \(<\)100\(>\) orientations. The silicided regions of the Si pads are brighter. Silicide-Si interface appears to be flat in \(<\)110\(>\) pads while in the case of \(<\)100\(>\) it has a step-like shape, as indicated by yellow arrows. FIB milling process to mitigate the damage incurred by FIB. Below the carbon layer, there are two Ni silicide layers and the Si substrate. The top Ni-containing layer depicts a Ni-rich silicide phase (region I). The following layer has a Si-rich silicide phase (region II). The interface between region I and II is smooth, whereas the interface between regions II and the Si substrate (region III) is rough and jagged. It is evident from Figure 2b-d that the silicide in region II has followed preferential crystal directions. The HRTEM image in Figure 3e shows an abrupt change in the silicide phase from NiS\({}_{2}\) to Ni-rich phase. To further investigate the silicidation process, zero-loss TEM was performed [56]. The images are presented in Figure 3. The Ni map shows higher concentration of Ni in region I compared to region II. Fast Fourier transform (FFT) confirms that the silicide phase in region II has a cubic lattice structure in accordance to NiSi\({}_{2}\). The silicide in region I is Ni-rich and it has a non-cubic structure. The NiSi\({}_{2}\)-Si interface is atomically sharp (Figure 3c). Figure 3: TEM analysis of silicide junctions. (**a**) Overview of inter-phases between Ni-rich phase and Si-rich phase. (**b,c**) Magnified images show atomically sharp NiSi\({}_{2}\)–Si junction. Figure 2: Cross-sectional transmission electron microscopy (TEM) analyses of a lamella taken from the sample shown in Figure 1. (**a**) SEM top-view image of a SiNW with a Si pad. Focused ion beam (FIB) section was made along the white line to get the cross-sectional image of silicidation in the Si pad. (**b**) TEM image of the cross-section showing three different regions. Region I: Ni-rich Ni-silicide phase; Region II: Si-rich Ni-silicide phase; and Region III: pure Si from the Si pads. (**c**) Energy-filtered TEM (EFTEM) image of Ni to confirm its concentration in the three different regions. Higher brightness corresponds to higher concentration of Ni. (**d**) EFTEM image of Si of the same regions. Higher brightness corresponds to higher concentration of Si.