## 3 Results and Discussion
### Functional Characteristics of BC\({}_{x}\)N\({}_{y}\) Films
#### 3.4.1 Optical Properties  
Ellipsometry and spectrophotometry were used to study the optical properties of BC\({}_{x}\)N\({}_{y}\) films. As known, the refractive index of h-BN and h-BC\({}_{x}\)N\({}_{y}\) is influenced by several factors, such as the composition of the films, their stoichiometry, the presence of hydrogen and oxygen-containing bonds, the chemical bond structure, stresses in the films, etc. The authors [89] reported that h-BN flakes of about 50 um thickness had \(n_{\,\mid\,1}=2.14\), \(n_{\,\perp}=1.84\). For h-BN films, the value of the refractive index, equal to 1.8, is given in the reference [90]. In [91] the refractive index of CVD BN films was measured as a function of the film composition. Its value ranged in the interval from 1.7 to 2.8 with an increase in the [B]:[N] ratio from 0.7 to 4, respectively. Thus, this indicates that the boron content in the film controls the refractive index, and boron-rich films have higher n values. In another study, a decrease in the refractive index of h-BN films with ratio [B]:[N] = 1:1, but having hydrogen-containing bonds and B-OH groups, is shown [92]. The presence of boron oxide in the films also contributes to a decrease in the value of n, since its refractive index is 1.45 (\(\lambda=589\) nm) [93]. Known from our previous investigation [31], the refractive index of BC\({}_{x}\)N\({}_{y}\) films deposited by the PECVD method using TMAB + NH\({}_{3}\) mixture at substrate temperatures from 573 to 973 K changed from 1.56 to 2.27, respectively. Figure 9: XPS spectra of (**a**) B 1s, (**b**) C 1s, (**c**) N 1s, and (**d**) O 1s core levels for PECVD B\({}_{0.30}\)C\({}_{0.27}\)N\({}_{0.29}\)O\({}_{0.14}\) film deposited at 523 K.  
Figure 10 shows the dependences of the refractive index on the substrate temperature. The \(n\) values for LPCVD films change insignificantly with an increase in the synthesis temperature and lie in the range of 2.41-2.58. Such values are typical for BC\({}_{x}\)N\({}_{y}\) films with very low nitrogen content (Table 1) and BC\({}_{x}\) films [52]. Films obtained in the plasma-stimulated process have much lower values of refractive index. The values of \(n\) increase with increasing T\({}_{\text{dep}}\) from 1.51 to 1.67. It can be seen that for each series, this value varies in a narrow range. A difference in the values of the refractive index for the studied LPCVD and PECVD films can be explained by a significant difference in their chemical composition with dominant bonds of B-C and B-N, B-C-N, respectively. According to data [58], with the deposition temperature increase from 373 to 973 K, \(n\) was between 1.52-1.65 for PECVD films obtained in a similar process using the mixture of N-trimethylborazine and N\({}_{2}\). The refractive indexes equal to 1.58-1.65 were determined for films with a high content of B-N bonds obtained by LPCVD from a mixture of TMAB+NH\({}_{3}\) at the high partial pressures of ammonia [36]. The optical properties of the BC\({}_{x}\)N\({}_{y}\)/SiO\({}_{2}\) structures were studied with UV-Vis-NIR spectroscopy using a double-beam spectrophotometer. Figure 11a shows the transmittance spectra in the wavelength range of 190-2200 nm for the films of LPCVD and PECVD series and the spectrum of a fused silica substrate. The transparency of LPCVD films is 83-85% in the red and IR range. The PECVD process allows depositing films with transparency of 92-95% in the visible and IR regions of the spectrum. The absorption edge of PECVD films is shifted to the short-wave region compared to LPCVD films. Obviously, this difference is due to the predominance of B-N and B-C bonds in the films, respectively. In the work [54], a gradual increase in the transparency of BC\({}_{x}\)N\({}_{y}\) films and shifting of the absorption edge to the short-wave region with an increase in the nitrogen content in these films was demonstrated. On the other hand, the appearance of an additional phase of disordered amorphous carbon in the films leads to a decrease in their transparency [52,61].