## 3 Results and Discussion
### Development of a Process Proper for Obtaining a Thick SnS Film Using ALD
The binding energies of both films were similar to the representative reference, as summarized in Table 1. The similar peak positions of a-SnS\({}_{2}\) with reference data indicates that the impurity contents were very small despite the low deposition temperature. The advantage of the ALD method is shown here. As a result, the binding energy of the two films chemically showed that a-SnS\({}_{2}\) films were entirely transformed into the SnS phase. Figure 2: X-ray photoelectron spectroscopy (XPS) spectra of a-SnS\({}_{2}\) film and SnS film transformed from a-SnS\({}_{2}\), indicating (**a**) Sn 3d and (**b**) S 2p peaks. As shown in Figure 3a,b, UV-vis analysis was performed to identify the optical properties of a-SnS\({}_{2}\) and SnS films. The films were deposited on a glass substrate, and the obtained results were calibrated based on absorbance of the glass substrate. Figure 3a exhibits the absorption coefficient of a-SnS\({}_{2}\) and SnS films in visible region. The 500 nm thick a-SnS\({}_{2}\) film had an absorption coefficient of \(3.3\times 10^{3}\) to \(1.3\times 10^{5}\) cm\({}^{-1}\). When the a-SnS\({}_{2}\) film transformed into SnS film, the absorption coefficient was \(3.6\times 10^{4}\) to \(1.1\times 10^{5}\) cm\({}^{-1}\). After the phase transition from a-SnS\({}_{2}\) to SnS, the absorption coefficient of the film increased by 10 times on average. Furthermore, the direct bandgaps of the a-SnS\({}_{2}\) and SnS films were identified in Figure 3b using the following equation:  
\[(\alpha hv)^{n}=(hv-Eg\pm Eph) \tag{1}\]  
where \(E_{g}\), \(E_{ph}\), and \(n\) are optical bandgap, photon energy, and transition probability, respectively. In the case of direct transition, \(n\) has an order of 2 for the allowed transition, and \(E_{ph}\) is 0 eV. Thus, the optical bandgap can be obtained by the plot of \((\alpha hv)^{2}\). Figure 3b exhibits a plot of \((\alpha hv)^{2}\) as a function of photon energy to determine the absorption coefficients of a-SnS\({}_{2}\) and SnS films. When \((\alpha hv)=0\), the photon energy indicates an optical bandgap. The bandgap of the a-SnS\({}_{2}\) film was 2.82 eV, and it decreased to 1.51 eV after transformation from a-SnS\({}_{2}\) into SnS by vacuum annealing. The obtained bandgaps were similar with typical bandgaps of SnS\({}_{2}\) and SnS films [5; 6; 7; 33; 34]. Through UV-vis analysis, we also identified the phase transition from SnS\({}_{2}\) into SnS and the optical properties of both films. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**\textless{Ref *\textgreater{}}**} & \multicolumn{3}{c}{**(eV)**} \\ \hline  & **Sn 3d\textgreater{}2** & **S 2p\textgreater{}2** & **Spacing (Sn 3d\textgreater{}2 - S 2p\textgreater{}32)** \\
**SnS\({}_{2}\)** & 486.5 & 161.6 & 324.9 \\
**SnS** & 485.7 & 161.0 & 324.7 \\ \hline \multicolumn{3}{c}{\textless{}obtained data\textgreater{}} \\ \hline  & **Sn 3d\textgreater{}2** & **S 2p\textgreater{}32** & **Spacing (Sn 3d\textgreater{}2 - S 2p\textgreater{}32)** \\
**SnS\({}_{2}\)** & 486.4 & 161.6 & 324.8 \\
**SnS** & 485.8 & 161.2 & 324.6 \\ \hline \hline \end{tabular}  
* by [PERSON] et al. [1].  
\end{table} 
Table 1: Sn 3d and S 2p peaks of the XPS spectra in comparison with the reference. Figure 3: Optical properties of a-SnS\({}_{2}\) film and the SnS film: (**a**) absorption coefficient in the visible region and (**b**) \((\alpha hv)^{2}\) plot to determine the optical bandgap. Further uniformity testing of the SnS film process was performed. Figure 4a is a schematic image of the showerhead-type ALD chamber used in this study. In this uniformity test, a 500 nm thick a-SnS\({}_{2}\) film was deposited on a four-inch scale thermally oxidized SiO\({}_{2}\)/Si wafer. SnS film was formed by annealing the a-SnS\({}_{2}\) film. The thicknesses of the 16 positions on the wafer were measured by SEM, as shown in Figure 4b.