## 4 Discussion  
The results presented above as well as those provided in the Supplementary Information show the following:  
* Regardless of the substrate preparation and the synthesis time, a fraction of SiC is present in the thin films. It is most likely formed in its vast part at the Si substrate/thin film interface from gas phase carbon and Si atoms from the substrate. SiC as a separate phase probably forms a continuous film only in samples cleaned according to the protocols mentioned in the experimental section. Continuous films with a well-ordered crystal structure are formed only when the synthesis time equals or exceeds 60 min. The XRD peaks from the XRD patterns and the SAED and HRTEM images taken from these experiments clearly show a well-ordered crystalline phase, and in both cases, the identified crystalline phase is of the polytype 36H-SiC. * The processes of permanent reconstruction of the Si(111) slab at high temperatures determine the deep penetration of C and O atoms into the substrate and the blurring of the thin film/Si-substrate interface. This gives rise to certain difficulties in measuring the thickness of these thin films. The detailed distribution of C- and O-containing species (see Figure 4a-f) unequivocally confirms that oxygen and carbon diffuse into the Si substrate, while the SiC phase predominantly remains within the thin film. This process results in the formation of a region extending from the interface into the substrate depth, where Si is doped with C and O. We assume that this region begins at the interface (or at the point where the carbon concentration decreases by 2-4% from its maximum, while the silicon concentration increases correspondingly), and it extends until the C and O concentrations stabilize at their constant residual values. Based on this, the thickness of these regions ranges from approximately 50-60 nm (forC3 samples) to over 100-110 nm (for C90 samples) (see Figures 5a,b and S5d) in the samples with properly cleaned native oxides and is larger in the uncleaned samples (see Figure S5c). This also explains why no clear results are obtained when measuring the thickness of thin films synthesized in long processes (20 min or more) (see the beginning of the Section 3) using SEM, AFM, and optical profilometry. In these methods, thickness determination relies on mechanical surface scratching; however, the high concentration of C and O near the substrate surface significantly alters the silicon properties and affects thin film adhesion, preventing the scratches from accurately defining the thin film/substrate interface. Similarly, ellipsometric measurements yield slightly increased thickness values for thin films--even those with small thicknesses (up to 15 nm)--because for reasons similar to those mentioned above, the upper part of the substrate exhibits characteristics resembling those of the thin film. Consequently, the ellipsometrically determined thicknesses of samples O3, C3, O5, and C5 were 12, 12, 13.5, and 15 nm, respectively, whereas GDOS measurements determined them to be 4, 5, 7, and 8 nm. This suggests that in ellipsometric measurements, part of the heavily doped region of the Si substrate is perceived as part of the thin film. As mentioned above, our experiments showed that the thin films consist of crystalline/nanocrystalline/amorphous SiC doped with some O, which is on a C and O co- doped Si- substrate. From general considerations, we can suppose that the complex composition of the thin films and highly enriched-with-C-and-O silicon area will more easily compensate for the difference in the lattice parameters of the SiC and Si substrates as well as their coefficients of thermal expansion than a monophase thin film of nanocrystalline/crystalline SiC. From this, we can conclude that the applied method is promising for depositing intermediate thin films of SiC on Si(111) using quartz equipment, i.e., compatible with the Si technology.