# Nb\({}_{3}\)Sn Cavities Coated by Tin Vapor Diffusion Method at Peking University
## 3 Results Analysis
### Roles of Nucleation in Vapor Diffusion Method
The atomic ratio result of the single-tin-source nucleation is displayed in Table 3. To highlight the depth distribution of tin, the relative proportions of tin to niobium were calculated and Sn/(Nb + Sn)% values are shown in Figure 11. The single-tin-source Sn/(Nb + Sn)% value is higher than the other three, probably because the tin was stored in the bottom boat and an amount of tin evaporated during the high-temperature pre-heating period, which proves again that the tin vapor participates in the nucleation sites forming. It can be seen that the Sn/(Nb + Sn)% value is larger with more SnCl\({}_{2}\) applied from the results of 3.0 g and 6.0 g SnCl\({}_{2}\) nucleation. There is no difference between the 3 rd and 5 th Sn/(Nb + Sn)% values which correspond to different nucleation times. There is no significant distinction of Sn/(Nb + Sn)% values among the upside, middle, and downside samples of 3.0 g SnCl\({}_{2}\) 5 h and 8 h nucleation, which means that the nucleation is generally uniform along the cavity axis. \begin{table}
\begin{tabular}{l l l l l l l} \hline
**Location** & & **C\%** & **O\%** & **Nb\%** & **Sn\%** & **Sn\%** \\ \hline up & 0 nm & 51.85 & 32.22 & 8.32 & 7.61 & 48 \\  & 15 nm & 27.74 & 19.32 & 44.93 & 8.01 & 15 \\ middle & 0 nm & 55.35 & 30.08 & 8.19 & 6.38 & 44 \\  & 10 nm & 23.91 & 21.22 & 49.10 & 5.77 & 11 \\ bottom & 0 nm & 44.04 & 37.40 & 10.50 & 8.06 & 43 \\  & 15 nm & 18.33 & 21.78 & 54.69 & 5.21 & 9 \\ \hline \end{tabular}
\end{table} 
Table 3: XPS elements analysis data from 2 nd nucleation experiment (3 g SnCl\({}_{2}\), 1.5 g Sn, 5 h, single tin source). Figure 9: (**a**) Tin particle coverage rates and (**b**) area of nucleation sites from 2 nd–5 th nucleation experiments; the data are calculated from ImageJ-treated SEM figures. The detected carbon may have come from adsorbed carbon in the air or inside the furnace, and the oxygen is proposed to have originated from the niobium substrate. The contents of elemental carbon and oxygen decrease faster with depth than tin, representing that they mainly exist at the surface. It is worth mentioning that in every nucleation experiment, the contents of oxygen, niobium, and tin in the 1-10 nm range satisfy the linear relationship, O% = 2 \(\times\) Nb% + 2 \(\times\) Sn%, indicating that tin and niobium may exist as NbO\({}_{2}\) and SnO\({}_{2}\) at the 1-10 nm surface. Figure 12 shows the linear fit results between O%/Sn% and Nb%/Sn%. Oxygen from the niobium substrate may help the surface capture tin atoms from the vapor, a possible  
Figure 11: Sn/(Sn + Nb)% of samples within 0–10 nm (**a**) and 15–25 nm (**b**) from the nucleation experiments, measured via XPS. Figure 10: XPS spectrum of a nucleated sample after sputtering. C and O are found in the spectrum; data are from 2 nd nucleation experiment (3 g SnCl\({}_{2}\), 1.5 g Sn, 5 h, single tin source). reaction is given in Formula (4), which needs to be further verified. There is no linear relationship between the contents of Nb, Sn, and C.  
\[2\text{Nb}_{2}\text{O}_{5}+\text{SnCl}_{2}=4\text{Nb}\text{O}_{2}+\text{SnO}_{2} +\text{Cl}_{2} \tag{4}\]  
These nucleated samples were sent to the furnace and only dealt with the coating stage to find any difference in the final Nb\({}_{3}\)Sn films. The samples were analyzed via SEM and EDS. Insignificant differences were found in the grain topography and surface tin content.