## 3 Results and Discussion
### Films' Composition and Chemical Bonding State
#### 3.3.2 FTIR Spectroscopy Data  
The FTIR absorption measurements were performed in order to investigate the effect of synthesis conditions on the chemical structure changes of the prepared films. The interpretation of the IR spectra of the films was carried out using the literature data. It is known that single crystal and polycrystalline rhombohedral B\({}_{4}\)C have bands at 1090 cm\({}^{-1}\) and 1580 cm\({}^{-1}\)[65]. The first of them is assigned to B-C vibrations in B\({}_{11}\)C icosahedra, while the second one corresponds to vibrations of C-B-C chains connecting the icosahedra to each other. There is a series of vibration bands of lower intensities in the region of 700-880 cm\({}^{-1}\) in the IR spectrum of a crystalline B\({}_{4}\)C [66]. The IR spectrum of amorphous boron carbide has a band at 1100 cm\({}^{-1}\) that is shifted to 1200 cm\({}^{-1}\) for carbon-rich a-BC\({}_{\rm x}\) films [67]. It is known that in the spectrum of the crystalline h-BN there are bands of in-plane vibrations of boron and nitrogen atoms (B-N) (V\({}_{\rm 1}\) = 1367 cm\({}^{-1}\)), the frequency and shape of which depend mainly on the deformation of the basal planes, and out-of-plane vibrations of atoms in the B-N-B lattice (V\({}_{\rm\perp}\) = 783 cm\({}^{-1}\)), the parameters of which are determined mainly by the nature of the layer packing [68]. In turbostatic and amorphous BN, there is a low-frequency shift of the band corresponding to out-of-plane vibrations [69]. The C-N bands have an absorption in the interval of 1250-1360 cm\({}^{-1}\). There is a band of C=N/C=C bond vibrations near 1600 cm\({}^{-1}\). Besides, the band of N-H deformation vibrations lies at 1630 cm\({}^{-1}\). Since there are hydrogen-containing bonds in the precursor, their presence is also possible in the films. B-H stretching vibrations, due to BH or BH\({}_{2}\) groups, produce a band at about 2530 cm\({}^{-1}\). The bands between 3300-3500 cm\({}^{-1}\) can be attributed to O-H bond stretching vibrations in water and B-OH. N-H stretching band in NH and NH\({}_{2}\) groups is also observed in this area (3200-3500 cm\({}^{-1}\)). The bands in the interval of 2800-3000 cm\({}^{-1}\) are caused by stretching vibrations of C-H bonds in CH\({}_{3}\) and CH\({}_{2}\) groups [70]. The absorbance bands associated with boron oxide appear in the IR-spectra at 1420 cm\({}^{-1}\) (B-O stretching), 1280 cm\({}^{-1}\) (B-O deformation), and 724 cm\({}^{-1}\) (B-O deformation) [71]. Due to reaction with atmospheric moisture, amorphous boron oxide contains B-OH groups, which have a peak around 3230 cm\({}^{-1}\) in the IR spectrum [72]. The evolution of FTIR spectra of the LPCVD samples is shown in Figure 6a. The spectrum of trimethylamine borane is also shown at the bottom of the Figure for comparison. Comparison of the precursor spectrum and the spectra of the films shows that the initial compound almost decomposed forming BC\({}_{\rm x}\)N\({}_{\rm y}\) films. The spectra of these films are represented by a wide band in the region of 660 to 1540 cm\({}^{-1}\), the maximum of which gradually shifts from 1150 to 1250 cm\({}^{-1}\) with increasing T\({}_{\rm dep}\) from 673 to 973 K, respectively. The observed wide absorption band is the typical one for BC\({}_{\rm x}\) films [52]. Considering that  
Figure 6: Evolution of (**a**) FTIR spectra and (**b**) Raman spectra of BC\({}_{\rm x}\)N\({}_{\rm y}\) films obtained by LPCVD using TMAB + N\({}_{2}\) mixture. the composition of the films varies from B\({}_{0.80}\)C\({}_{0.14}\)N\({}_{0.04}\)O\({}_{0.02}\) to B\({}_{0.67}\)C\({}_{0.24}\)N\({}_{0.08}\)O\({}_{0.01}\) the main elements in all LPCVD films are boron and carbon. Based on these data, it can be assumed that the chemical structure of these films is close to that of boron carbide. It should be mentioned that this band can also include B-N and C-N vibrations, taking into account the content of nitrogen (4-8 at.%) in the films.