## 2 Materials and Methods  
BC\({}_{\rm x}\)N\({}_{\rm y}\) films were obtained on polished n-type Si(100) substrates, n-type phosphorus-doped Si(100) substrates with a resistivity of 4.5 \(\Omega\)-cm, with a thickness of 470 \(\upmu\)m and an area of 1.0 \(\times\) 1.0 cm\({}^{2}\), by two methods such as LPCVD and RF PECVD (13.56 MHz, 50 W)using the equipment presented in [44]. Trimethylamine borane (CH\({}_{3}\))\({}_{3}\)N\(\cdot\)BH\({}_{3}\) (TMAB) (97% pure, Sigma-Aldrich, Merck Ltd., Darmstadt, Germany) thermostated at a temperature of 273 K was a source of B, C, and N. Nitrogen gas of high purity was used as a reagent gas. The ratio of partial pressures of TMAB to nitrogen was P\({}_{\text{TMAB}}\):P\({}_{\text{N2}}\) = 1:1. The value of the base pressure in the reactor chamber was 4 \(\times\) 10\({}^{-3}\) Torr. The variable parameter was the temperature in the substrate zone (T\({}_{\text{dep}}\)), which varied in the ranges of 673-973 K and 373-873 K for films obtained by the LPCVD and PECVD methods, respectively. The reproducibility of the deposition processes was confirmed by a series of parallel experiments. Immediately before the synthesis of the films, the substrates were degreased successively in trichloroethylene and acetone, then etched successively in ammonia-peroxide, in hydrochloric acid-peroxide solutions, and in concentrated hydrofluoric acid, according to the standard procedure. Fused silica plates were used as substrates for the UV-Vis transmittance study. For them, only the degreasing procedure was carried out. Degreasing and each stage of substrate etching ended with washing with deionized. Finally, the substrates were dried in a nitrogen flow at 353 K.  
The elemental composition of the films was determined by X-ray energy dispersive spectroscopy (EDX) using a JSM-6700F (Jeol, Tokyo, Japan) scanning electron microscope (SEM), with a Quantax 200 (Bruker, Berlin, Germany) EDX attachment at an accelerating voltage of 3 keV [45]. The surface morphology of the films was studied using SEM. The topology of the samples was studied using a Ntegra Prima II atomic force microscope (AFM) (NT-MDT, Zelenograd, Russia) in the semicontact mode. An HA_NC (A) probe was used with the following parameters: probe length of 123 \(\upmu\)m, width of 34 \(\upmu\)m, thickness of 3 \(\upmu\)m, curvature radius of 10 nm, force constant of 17 N/m, resonant frequency of 230 kHz. The calculation of the root mean square surface roughness (RMS) was carried out using the Nova SPM software according to the standard ISO 4287-1. X-ray diffraction patterns of the samples were recorded on a Bruker D8 Advance diffractometer (Bruker, Berlin, Germany) using CuK\(\alpha\) radiation with LYNXYEYE XE-T linear detector in the range of 20 = 5-65\({}^{\circ}\) with 20 step of 0.03\({}^{\circ}\) at an accumulation of 5 s per point. The thickness \(d\) and refractive index \(n\) of the films were calculated using data of monochromatic ellipsometry using an LEF-3M (Instrument-making plant, Feodosiya, USSR) scanning null laser ellipsometer equipped with a He-Ne laser (\(\lambda\) = 632.8 nm). The measurements were carried out at 5 incidence angles: 50\({}^{\circ}\), 55\({}^{\circ}\), 60\({}^{\circ}\), 65\({}^{\circ}\), and 70\({}^{\circ}\). The thickness of the films obtained by the PECVD method at T\({}_{\text{dep}}\) = 573-873 K was estimated from the SEM micrograph of a cross-section of the BC\({}_{\text{x}}\)N\({}_{\text{y}}\)/Si(100) structure. The error in determining \(n\) was 0.02-0.04. The chemical structure of the films was studied using Fourier transform infrared (FT-IR) spectrometry in absorption mode with a SCIMITAR FTS 2000 spectrometer (Digilab, Hopkinton, MA, USA). The spectral range between 375 and 4000 cm\({}^{-1}\) was probed with a resolution of 2 cm\({}^{-1}\). A Si(100) substrate after standard chemical treatment was used as a reference sample. In order to compare the peaks' intensities, the absorbance values were normalized by each film's thickness.