## 3 Results and Discussion
In fact, the amorphous oxide thin films are nonstoichiometric. Si2p peak was formed at 103.38 eV and the O1s spectra were located at 532.08 eV as shown in Figure 2b,c. [PERSON] et al. reported the different oxygen concentration in SiO\({}_{x}\) as a function of Si2p binding energy. The behavior of the Si2p was shifted from 101.9 eV to 103.8 eV as the oxygen concentration increased from x = 1.05 to 2 in SiO\({}_{x}\)[15]. [PERSON] et al. reported that the binding energies of Si2p and O1s are 103.4 and 533.0 eV, respectively [16]. Based on comparisons with the above references, the current results obtained from Figure 2b,c reflect that the binding energies of Si\({}^{4+}\) and O\({}^{2-}\) are combined in the form of SiO\({}_{2}\). The doublet with binding energies \(\sim\)207.18 (Nb3d\({}_{5/2}\)) and \(\sim\)209.98 eV (Nb3d\({}_{3/2}\)) was associated with photoelectrons emitted from Nb atoms as shown in Figure 2d. Other studies reported that three XPS peaks are assigned to Nb\({}_{2}\)O\({}_{5}\), NbO\({}_{2}\), and NbO, which are located at \(\sim\)208.6 eV, 206.2 eV, and 203.6 eV [17]. [PERSON] et al. reported that the niobium spectra broaden and shift to a lower binding energy, indicating the presence of multiple oxidation states, i.e., Nb\({}^{4+}\) (208.6, 205.9 eV) and Nb\({}^{2+}\) (206.6, 203.9 eV) [18]. [PERSON] et al. reported that the doublet at binding energy 207.1 eV (Nb3d\({}_{5/2}\)) is characteristic of the Nb\({}^{5+}\) state, and the peak at 209.8 eV (Nb3d\({}_{3/2}\)) also confirms the film composition as diniobium(V) pentoxide, Nb\({}_{2}\)O\({}_{5}\). The binding en  
Figure 1: XRD pattern of SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films on three substrates (B33, D263, and D270). ergies \(\sim\)207.18 (Nb3d\({}_{5/2}\)) and \(\sim\)209.98 eV (Nb3d\({}_{3/2}\)) from the current XPS spectrum were very similar to the reported results, i.e., characteristic peaks at 209.8\(-\)209.9 eV (Nb3d\({}_{3/2}\)) and 207.1\(-\)207.2 eV (Nb3d\({}_{5/2}\)) indicating the presence of Nb\({}^{5+}\)[18, 19]. Thus, the above XPS results indicated the major presence of Si\({}^{4+}\) and Nb\({}^{5+}\) in the SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films. Although the ratios are substoichiometric for both silicon and niobium, they are written as simplified SiO\({}_{2}\)/Nb\({}_{2}\)O\({}_{5}\) multilayer thin films in the paper. In order to investigate the optical parameters, single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films on a D263 substrate were prepared. The refractive index was compared using a UV-Vis spectrophotometer and spectroscopic ellipsometer. Figure 3a shows the transmittance of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films by using a UV-Vis spectrophotometer. Essential Macleod was used to calculate the refractive index (\(n\)) from the spectral data of the UV-Vis spectrophotometer. The measured refractive index (\(n\)) of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films obtained were 1.481 and 2.398 at 530 nm, respectively. Figure 3b shows the refractive index (\(n\)) of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films, as well as a D263 substrate measured by using a spectroscopic ellipsometer. The refractive index (\(n\)) and extinction coefficient (\(k\)) were calculated from spectra fits on the basis of a Lorentzian multi-oscillator model [20]. The measured refractive index (\(n\)) of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films at 530 nm were 1.468 and 2.404, respectively. The extinction coefficient (\(k\)) of the single-layer Nb\({}_{2}\)O\({}_{5}\) thin films at 530 nm was 1.0 \(\times\) 10\({}^{-5}\). According to two methods, the difference of refractive index (\(n\)) of the single-layer SiO\({}_{2}\) and Nb\({}_{2}\)O\({}_{5}\) thin films were 0.87% and 0.25%, respectively.