## 3 Results  
### NRA Measurements  
Figure 3 displays the NRA spectra obtained for a Ni/Si sample at two steps of the process, before and after the \({}^{13}\)C implantation at 600 \({}^{\circ}\)C, respectively. The signals appearing at around 2770 keV and 5850 keV correspond to the \({}^{12}\)C(d, p)\({}^{13}\)C and \({}^{13}\)C(d, p)\({}^{14}\)C nuclear reactions, with corresponding cross-sections 25.9 mb/sr [24] and 2.36 mb/sr [25], respectively. Owing to the low sensitivity of the \({}^{13}\)C(d, p)\({}^{14}\)C reaction, the detection limit for \({}^{13}\)C signal  
Figure 3: (**a**) NRA spectra obtained for Ni/Si samples at two steps of the process: before (black) and after (red) the \({}^{13}\)C implantation at 600 \({}^{\circ}\)C. In (**b**), the signal \({}^{13}\)C(d, p)\({}^{14}\)C around 5825 keV is magnified. is quoted to no more than 1 EGM. In Table 1 the \({}^{12}\)C and \({}^{13}\)C concentrations obtained by NRA are reported at different steps of the process. The presence of a \({}^{12}\)C(d, p)\({}^{13}\)C contribution before implantation reveals some carbon contamination. Due to the low depth resolution, we were not able to differentiate the carbon contributions coming either from the surface nor diluted into the Ni layer. Thus, only the total carbon concentrations obtained by fitting the experimental spectra are reported. This contamination is measured to \((21\pm 3)\times 10^{15}\) at\(\cdot\)cm\({}^{-2}\) corresponding to around 6 EGM. This carbon pollution very probably occurs during the Ni deposition, corresponding to a relative atomic concentration of:  
\[[\mathrm{C}]/[\mathrm{Ni}]=[\mathrm{C}]\times t\times S\times d_{\mathrm{Ni} }\times\mathrm{N_{A}}/\mathrm{M_{Ni}}=2.1\times 10^{16}/1.6\times 10^{18}=1.3\% \tag{1}\]  
where t is the thickness of the Ni layer (200 nm); \(d_{\mathrm{Ni}}\) and \(M_{\mathrm{Ni}}\) are the number density and the atomic mass of Ni (7.81 g\(\cdot\)cm\({}^{-3}\) and 58.7 g\(\cdot\)mol\({}^{-1}\), respectively); \(\mathrm{N_{A}}\) is the Avogadro number and S is the sample surface (1 cm\({}^{2}\)). It is believed that it is an upper limit to the carbon concentration, as it is assumed in Equation (1) that the carbon is located inside the nickel film, although a significant amount of carbon is also present on the surface, as reported in [20, 21]. This measured concentration is, therefore, in line with the thermodynamic phase diagram carbon-nickel [26]. Moreover, the nickel could also be contaminated during different analyses (such as SEM, _etc._) carried out on the sample. More experiments are presently under investigations in order to find out at which steps this unexpected pollution occurs and to remove it. A small decrease of the \({}^{12}\)C concentration, down to \((17\pm 3)\times 10^{15}\) at\(\cdot\)cm\({}^{-2}\), can be observed after the RTA step at 900 \({}^{\circ}\)C. Thus, it is presumed that some carbon has been consumed by the effective hydrogenation with forming gas. After further implantation at 600 \({}^{\circ}\)C, the \({}^{13}\)C concentration roughly corresponds to the expected carbon dose (13 \(\pm\) 3) \(\times\) 10\({}^{15}\) at\(\cdot\)cm\({}^{-2}\) whereas no significant evolution of the \({}^{12}\)C concentration can be observed. Therefore, the sample after implantation contains (33 \(\pm\) 7) \(\times\) 10\({}^{15}\) at\(\cdot\)cm\({}^{-2}\) of overall carbon after implantation, corresponding to around 60% and 40% of the \({}^{12}\)C and \({}^{13}\)C isotopes, respectively. In conclusion, we have shown by NRA some pollution of the sample by \({}^{12}\)C carbon, mainly at the Ni deposition step, and further investigations are needed to limit, if not suppress, these sources of pollution.