## 3 Results and Discussion  
FESEM was used to analyze the surface morphologies of the RTA-treated CIGS:Te thin films with different co-sputtering times for the Te target. The top-view FESEM images of the CIGS/CIGS:Te thin films shown in Figure 1 show no extended cracks after the RTA treatment [23; 24]. The fine grains were tightly connected, with a relatively indistinct boundary in the CIGS thin film, as shown in Figure 1a. No significant difference between the CIGS:Te thin film with a Te co-sputtering time of 270 s (Figure 1b) and the CIGS thin film (Figure 1a) was observed, except for slightly larger grains in the former. For the CIGS:Te thin film with a Te co-sputtering time of 510 s (Figure 1c), the grains accumulated to form larger grains with clear boundaries after RTA treatment despite the same conditions. Large features were observed on the surfaces of the CIGS:Te thin film, as shown in Figure 1c, similar to the larger grains on the smaller grain background in CIGST and Cu(In,Ga)Te\({}_{2}\) (CIGT) thin films formed by compositional variations [1; 25]. Well-defined and dense grains with distinct boundaries were also observed in the CIGS:Te thin films with Te co-sputtering times of 750 s (Figure 1d) and 945 s (Figure 1d). At a Te co-sputtering time of 1140 s (Figure 1f), the grains became very small with a blurred boundary as the thickness ratio of Te during the co-sputtering of CIGST:Te thin films increased, and the surface characteristics changed with no consistent tendency. The compositional characteristics of RTA-treated CIGS:Te thin films fabricated at different Te co-sputtering times were examined using EDX. Considering the different deposition rates of the Te/CIGS targets, thickness ratios for all 400-nm-thick CIGS:Te thin films were estimated. The expected thickness ratios of Te/CIGS were 0/400, 18/382, 34/366, 50/350, 63/337, 76/324, and 88/312 nm at the co-sputtering times for each target shown in Figure 2a, which correspond to the compositional ratios, Te/(Se + Te), of 0.0, 10.3, 15.7, 22.2, 27.2, 31.9, and 36.1%, respectively. However, the atomic percentage of Te/(Se + Te) after the Te co-sputtering was in the range of 4.2-6.0%. The increase was not linear but showed slight ups and downs. Controlling the compositional ratio by controlling the thicknesses of the elemental layers within the precursor stack is reported to be difficult, even with RTA treatment after the evaporation of the stacks [1]; additionally, controlling the compositional ratio by adjusting the deposition rates in the co-sputtering method is also not easy. All the CIGS:Te thin films prepared using the Cu\({}_{0.9}\)In\({}_{0.7}\)Ga\({}_{0.3}\)Se\({}_{2}\) target showed Cu-poor compositions, and the CIGS thin film without Te exhibited a Cu-poor composition of around 22.4%, as shown in condition (1) of Figure 2b, which also clearly  
Figure 1: Top-view field emission scanning electron microscopy (FESEM) surface images of the CIGS:Te thin films with Te co-sputtering times of: (**a**) 0, (**b**) 270, (**c**) 510, (**d**) 750, (**e**) 945, and (**f**) 1140 s.  
shows the lack of Se with a composition of around 45.8%. The atomic percentage of In + Ga remained relatively even, within the range of 28.5-32.0% under all conditions. Once Te was co-sputtered, the relative atomic percentage of Se + Te increased by approximately 4%. After increasing the Te co-sputtering time, the relative atomic percentage of Te in the CIGS:Te thin films tended to be similar or slightly reduced, while the relative atomic percentage of Se was constant overall but partially large, even though they were samples that used the same target. The relative atomic percentage of Cu in the CIGS:Te thin films increased and then decreased under the condition (6) after becoming constant, and the relative atomic percentage of Se tended to be inversely proportional to Cu under these conditions.