## 3 Results and Discussion
When examining the relationship between the surface morphologies and the change in the relative atomic percentage of the individual elements in the CIGS:Te thin films, an increase in the relative atomic percentage of Cu and a decrease in the relative atomic percentage of Se were noticeable under the condition (6) where the very small grains with indistinct boundaries appeared. According to previous studies [1; 26], changes in the surface properties of CIGS/CIGST thin films have been observed with a variation in the compositional ratios of Ga/(Ga + In) and Te/(Se + Te); however, the changes in the surface properties of Te/(Se + Te) were not significant, as shown in Figure 2a. To examine the contribution of certain compositional ratios in the CIGS:Te thin films to the surface properties more closely, various compositional ratios other than Ga/(Ga + In) are shown in Figure 2c. Because the change in the relative atomic percentage of Te over Te-sputtering time was very small, the change in the relative atomic percentages of Se + Te was determined by that of Se, and the surface morphology showed the most similar tendency to the change in the compositional ratio of (Se + Te)/(Cu + In + Ga). This is a different result from that for Ga/(Ga + In), known as a prevailing influence factor on the surface morphology in Cu-rich thin films [1; 26], and seems not to be applied under Cu-poor conditions with better energy conversion efficiency. A more rigorous relationship must be investigated. Two parameters, \(\Delta m\) and \(\Delta s\), were examined to determine deviations from molecularity and stoichiometry, respectively: \(\Delta m\) = [Cu]/([In] + [Ga]) \(-\) 1 and \(\Delta s\) = 2([Se] + [Te])/([Cu] + 3[In] + 3[Ga]) \(-\) 1 [2; 27; 28; 29]. The parameters \(\Delta m\) and \(\Delta s\) refer to deviations from the molecularity and valance stoichiometry, respectively. For stoichiometric compounds, the parameters exhibited no deviations. Figure 2d shows \(\Delta m\) and \(\Delta s\) in the non-stoichiometric CIGS:Te thin films at different co-sputtering times for the Te target. The chemical compositions of the CIGS:Te thin films were analyzed by extracting their atomic percentage using EDX. \(\Delta m\) < 0 and \(\Delta s\) < 0 ((Se + Te)-poor) compositions were obtained for all the CIGS:Te thin films, regardless of the Te co-sputtering time. While CIGS thin film without Te had \(\Delta m\) = \(-\)0.296 and \(\Delta s\) = \(-\)0.224, the non-stoichiometric CIGS:Te thin films showed \(\Delta m\) = (2) \(-\)0.258, (3) \(-\)0.245, (4) \(-\)0.231, (5) \(-\)0.231, and (6) \(-\)0.134 as the Te co-sputtering time was increased. The \(\Delta s\) values were (2) \(-\)0.080, (3) \(-\)0.067, (4) \(-\)0.128, (5) \(-\)0.116, and (6) \(-\)0.226, respectively. The value in the non-stoichiometric CIGS:Te thin films under the condition (3) was closest to '0', and it was the furthest away under condition (6). Note that \(\Delta s\) in the \(\Delta m\) < 0 CIGS:Te thin films used in this study showed a similar tendency as those of the surface properties. Under the condition that the compositional ratio of chalcogens increased and those of metals decrease at the same time, surface properties, such as grain size and clarity of grain boundaries, were also improved; that is, the \(\Delta s\) approached the stoichiometric composition. In the opposite case, the surface properties degraded. In this study, a normalized deviation from the stoichiometric composition for each element group is proposed and applied to the pentary compound CIGS:Te. The normalized stoichiometric deviation is calculated for each element group by taking the absolute values and is defined as follows: N\({}_{[In+Ga]}\) = 1 - 1 [(In + Ga)exp] - [(In + Ga)ideal] / [(In + Ga)ideal], which converges to '1' when each element group is close to the stoichiometric composition. The deviations from the stoichiometric compositions of Cu, In + Ga, and Se + Te in the CIGS thin films without Te sputtering were \(-\)2.59, 6.83, and \(-\)4.24, respectively.