## 3 Results and Discussion
### Process to Control the Film Characteristics of SnS Film Using Seed Layer
Furthermore, since the SnS seed layer was in the same phase with the SnS film as a final target, there was no lattice mismatch between crystallized SnS and SnS seed. Thus, only the surface energy of the seed layer was expected to contribute to the crystallization behavior of a-SnS\({}_{2}\) film. The induced strain by surface energy change of the SnS seed should also be considered, but the change in contact angle of the SnS seed by annealing was relatively small. Additionally, the defect reduction of the SnS film was expected due to the increase in film density. The SnS film using the SnS\({}_{2}\) seed layer (sample C) showed peaks of the orthorhombic SnS crystal phase at the same Bragg angles as sample A. In the case of sample C, the main (111) peak at 31.53\({}^{\circ}\) was expected to include the peak of the annealed SnS\({}_{2}\) seed layer as in sample B. Notably, the intensity and sharpness of peaks other than the (111) peak were increased compared to sample A. For simple comparison, the crystallite sizes for each XRD peak of three SnS samples are summarized in Table 2. The crystallite sizes were calculated by the Scherrer formula using full width half maximum (FWHM) of each peak. The calculated crystallite size is not an actual grain size but a numerical value for simple comparison. This is because the crystals assumed in the Scherrer formula are particles having spherical shape. In films, the crystallite size calculated by the Scherrer formula is used only to relatively compare the grain growth according to orientation. FWHMs were determined using a PDXL 2.0 Rigaku, Tokyo, Japan. As shown in the Table 2, the crystallite sizes at the Bragg angle of 27.47\({}^{\circ}\) (021), 39.04\({}^{\circ}\) (111), and 45.49\({}^{\circ}\) (002) of sample C were larger than the crystallite sizes of sample A. The grain growth of the (111) orientation at 31.53\({}^{\circ}\) in sample C cannot be interpreted as increased because it includes the annealing result of the SnS\({}_{2}\) seed layer shown in Figure 5b. As can be seen from the SEM image given in Figure 5f, the SnS grains grew vertically on the substrate and showed a porous film. To reduce the high strain caused by lattice mismatch between the SnS grains and the SnS\({}_{2}\) seed, the SnS grains grew with a distorted direction at the interface of the a-SnS\({}_{2}\) and SnS\({}_{2}\) seeds. In the intermediate step when using the SnS\({}_{2}\) seed layer, shown in Figure 6b, the strain due to lattice mismatch is resolved by reducing the atomic interaction with the SnS\({}_{2}\) seed layer. If the a-SnS\({}_{2}\) comprised a very thin film, then a polycrystalline SnS film would be formed at this step. However, because the a-SnS\({}_{2}\) film was a 500 nm thick film, the strain in the film could not be fully resolved due to the overlaps of grains. Eventually, the grains were aligned vertically to the substrate, exposing edge sites to minimize the strain by the overlap of grains [29]. As described in the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **A (No Seed)** & **B (Using SnS Seed)** & **C (Using SnS\({}_{2}\) Seed)** \\ \hline
2\(\theta\) (deg)/\(\hbar\)\(\hbar\) & \multicolumn{3}{c}{Crystallite size (nm)} \\ \hline
27.47/021 & 35.8 & - & 49.1 \\
30.47/101 & 41.5 & - & 45.2 \\
31.53/111 & 53.2 & 25.0 & 56.4 \\
39.04/131 & - & - & 37.6 \\
45.49/002 & 26.5 & - & 34.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Calculated crystallite size for each \(\{\)\introduction, SnS grains have a higher growth rate in the direction of basal plane than the growth rate in the direction of stacking layers due to the characteristics of 2D structure. Thus, volume expansion of the film occurred by vertical growth of grains. The thickness of sample C was 734.70 nm, which was significantly increased from the thickness of 500 nm before annealing. Due to this grain direction change, sample C showed sharper suborientation peaks than sample A in the XRD results.