## 3 Results and Discussion
### Development of a Process Proper for Obtaining a Thick SnS Film Using ALD
In particular, the multidosing method could support the ideal layer-by-layer adsorption in crystalline thin-film deposition but would cause a contribution of chemical vapor deposition (CVD) reaction in the deposition of an amorphous phase film at low temperature. Therefore, CVD contribution could not be completely excluded in the deposition process of a-SnS film in this study. However, this process has a sufficiently high film quality compared to other deposition methods through the use of ALD recipe. Therefore, this process of a-SnS\({}_{2}\) could have advantages as an intermediate method between ALD and CVD. As observed in the XRD results in Figure 1b,c, an increase of the temperature or time from the annealing condition of 400 \({}^{\circ}\)C and 10 min reduced the SnS peak. Specifically, bulk SnS evaporates at a temperature above 485 \({}^{\circ}\)C in a vacuum of 10\({}^{-3}\) mbar (= 1.33 \(\times\) 10\({}^{-3}\) Torr), but the raw material used in this study was an amorphous and directly deposited film [32]. Therefore, SnS crystals formed by vacuum annealing could also evaporate at temperatures below 485 \({}^{\circ}\)C. XRD results showed that the SnS crystals evaporated above 400 \({}^{\circ}\)C. Excessive annealing temperature or time increase at conditions of 400 \({}^{\circ}\)C and 10 min could result in crystallite size reduction and defect formation in the SnS film. On the contrary, when the annealing temperature and time were less than 400 \({}^{\circ}\)C or less than 10 min, the peaks of hexagonal SnS\({}_{2}\) and orthorhombic SnS\({}_{2}\)S\({}_{3}\) appeared. The SnS\({}_{2}\) and SnS\({}_{2}\)S\({}_{3}\) peaks indicate that other tin sulfide crystal phases are mixed in the SnS film when the thermal energy for phase transition is insufficient. The reason for the noticeable phase mixing during the tin sulfide process is the small chemical stability difference between SnS and SnS\({}_{2}\). In group 14, valence s electrons tend to be unionized or unshared according to the atomic weight due to the weak shielding effect of d and f orbitals in the inner cells. These characteristics correspond to the inert pair effect of the post transition metals. For example, Pb has atomic number of 82 and constitutes PbS as a stable phase. In the case of Sn, located directly above Pb in the periodic table, SnS\({}_{2}\) is a stable phase, but the difference of stability between SnS and SnS\({}_{2}\) is small. Therefore, for the process of tin sulfide film, it is important to optimize the process to obtain a pure phase. Additionally, a-SnS\({}_{2}\) film crystallized first during the vacuum annealing process rather than the phase transition to SnS. Thus, untransformed SnS\({}_{2}\) grains were detected as a crystallized phase. Considering the evaporation of the SnS film and the required energy for phase transition from SnS\({}_{2}\) to SnS, further study can be performed using a vacuum annealing time longer than 10 min at a temperature below 400 \({}^{\circ}\)C. In this paper, the annealing condition of 400 \({}^{\circ}\)C and 10 min was considered as an optimal process condition, and subsequent analyses were performed using samples formed by the annealing condition. Figure 2a,b shows the XPS result of ALD a-SnS\({}_{2}\) film and the SnS film formed by vacuum annealing. To eliminate surface contamination, the films were etched 3 nm from the surface by argon sputtering. The derived data were calibrated based on carbon 1s peak (284.5 eV). The Sn 3d\({}_{\text{S/2}}\) peaks of a-SnS\({}_{2}\) and SnS were positioned at 486.4 and 485.8 eV, respectively. Because Sn\({}^{4+}\) has a higher positive oxidation state than Sn\({}^{2+}\), SnS\({}_{2}\) exhibits a higher binding energy due to extra Coulombic interactions. The S 2p\({}_{\text{S/2}}\) peak position of a-SnS\({}_{2}\) was 161.6 eV and shifted to 161.2 eV after transformation to SnS. The distance between Sn 3d\({}_{\text{S/2}}\) and S 2p\({}_{\text{S/2}}\) was 324.8 eV for a-SnS\({}_{2}\) and 324.6 for SnS.