## 3 Results and Discussion
(**D**) Analysis of (**i**) Integrated Si–C bond area (700–815 cm\({}^{-1}\)), (**ii**) Si–C stretching mode peak position, (**iii**) full width at half maximum (FWHM), and (**iv**) percentage of crystallinity for as-deposited and annealed SiC thin films as a function of deposition and annealing conditions. Following the in situ post-deposition annealing processes, the Si-C stretching mode peak blue-shifted, while the Si-C bond area increased, as seen in Figure 3(Di,Di). Our annealing studies revealed three significant changes in the FTIR absorption spectra that occur with increasing in situ annealing temperature: (1) a shift in peak position of the Si-C stretching mode toward ~790 cm\({}^{-1}\) (Figure 3(Di)); (2) a significant narrowing of the full width at half maximum (FWHM) of this Si-C mode (Figure 3(Diii)); and (3) an increase in film crystallinity (Figure 3(Div)). Specifically, A3 annealing led to complete crystallization, as indicated by an FWHM narrowing to ~46 cm\({}^{-1}\)--comparable to high-quality crystalline c-SiC [6; 43]. These findings were further supported by HRTEM analysis of the A3-annealed films (Figure S1(Ai)), which revealed ring patterns with radii corresponding to the (111), (220), and (311) planes of the 3C-SiC phase [44]. The observed \(d\)-spacing value of ~2.56 A, derived from HRTEM images of a single grain (Figure S1(Aii)), is also consistent with the (111) plane of 3C-SiC [6]. Considering the morphological requirements for photonics devices and the increased surface roughness of the thin films grown at T\({}_{\text{D}}\geq\)750 \({}^{\circ}\)C, we have primarily studied the optical properties of the 650 \({}^{\circ}\)C- and 700 \({}^{\circ}\)C-grown SiC thin films. SE was employed to extract the primary optical constants of the films, \(n\) and \(k\). This technique is critical for evaluating the optical bandgap (\(E_{g}\)), and \(E_{04}\) gap, Urbach energy (\(E_{u}\)) associated with sub-bandgap defects and/or band tail states, and the absorption, \(\alpha(\lambda)=4\pi k/\lambda\), in the NIR range. The \(E_{04}\) gap corresponds to the energy at which \(\alpha\) is equal to 10\({}^{4}\) cm\({}^{-1}\). Three fitting models have been employed: the Tauc-Lorentz (TL), the Cody-Lorentz (CL), and the Cauchy-Urbach (CU) model, focusing on evaluating and determining their optical properties in the near-infrared (NIR)--telecom range, as presented in Figure 4A. The Tauc-Lorentz model [45] combines [PERSON]'s formula \(T(E)\)[46] with a Lorentz oscillator function \(L(E)\) to describe the imaginary part of the dielectric function \(\varepsilon_{\text{i}}\) near and above the optical band (\(E>E_{g}\)):  
\[\varepsilon_{i}\left(E\right)=A_{T}\bigg{(}\frac{E-E_{g}}{E}\bigg{)}^{2} \Bigg{(}\frac{A_{0}E_{0}BE}{\left(E^{2}-E_{0}^{2}\right)^{2}+\left(B^{2}E^{2 }\right)}\bigg{)}\qquad E>E_{g} \tag{1a}\] \[\varepsilon_{i}\left(E\right)=T(E)L(E),\qquad E>E_{g} \tag{1b}\]  
where \(A_{T}\) is the Tauc's coefficient, \(E_{g}\) is the optical bandgap; \(A_{0}\), \(E_{0}\), and \(B\) are the amplitude, resonance energy, and broadening factor of the Lorentz oscillator, respectively. The real part of the dielectric function \(\varepsilon_{r}\), and thus the index of refraction, \(n\), is obtained via Kramers-Kronig (K-K) transformation [47]. However, this model assumes zero absorption (\(\varepsilon_{i}=0\)) for energies below the bandgap (\(E<E_{g}\)), which limits its accuracy in describing potential sub-bandgap absorption due to defects or band-tail states (blue line in Figure 4A) [48]. To address this, the Cody-Lorentz (CL) model [49; 50] is an extension of the TL model developed to more accurately describe materials that exhibit sub-bandgap absorption (orange line in Figure 4A).