## 3 Results and Discussion
Figure 5D presents the extracted \(E_{u}\) energy values from the CL and CU fittings for the 650 \({}^{\circ}\)C and 700 \({}^{\circ}\)C samples as a function of annealing. Excluding the A3 annealing step, the AD and annealed SiC films grown at 700 \({}^{\circ}\)C generally exhibit lower \(E_{u}\) values than their 650 \({}^{\circ}\)C counterparts within each model. For instance, according to the CL model, the 700 \({}^{\circ}\)C samples exhibit \(E_{u}\) values of \(\sim\)0.19 eV across the annealing sequence up to A2, compared to \(\sim\)0.26 eV for the 650 \({}^{\circ}\)C sample. The CU model yields higher \(E_{u}\) values overall; however, a similar trend is observed, with the 700 \({}^{\circ}\)C consistently exhibiting lower \(E_{u}\) values than the 650 \({}^{\circ}\)C, and thus, lower disorder and/or defect density. In Figure 5E, we plot the extracted \(E_{u}\) values against the corresponding FWHM values of the Si-C stretching mode across the annealing sequence for both 650 \({}^{\circ}\)C and 700 \({}^{\circ}\)C samples. For both samples, the FWHM of the Si-C stretching mode is decreased by \(\sim\)70% upon annealing, from \(\sim\)165 cm\({}^{-1}\) (AD samples) to \(\sim\)48 cm\({}^{-1}\) (A3-annealed samples), resulting in a substantial reduction in bond angle disorder. For the 650 \({}^{\circ}\)C samples, this decrease in FWHM coincides with a \(\sim\)36% reduction in \(E_{u}\), as determined by the CL model and an \(\sim\)10% reduction based on the CU model. In contrast, for the 700 \({}^{\circ}\)C samples, the reduction in FWHM is accompanied by an increase in \(E_{u}\). These results are consistent with previous studies, which have also shown that \(E_{u}\) is not correlated with bond-angle disorder [62]. These trends in optical properties are further supported by vibrational analysis of compositional disorder in our SiC films. The intensity of the Si-H stretching mode at \(\sim\)2080 cm\({}^{-1}\) is approximately 60% less intense in the AD 700 \({}^{\circ}\)C samples compared to their 650 \({}^{\circ}\)C analogs (Figure 3B). Additionally, this Si-H signal is no longer detectable in the A1-annealed 700 \({}^{\circ}\)C samples, whereas it remains present, albeit reduced, in the A1-annealed 650 \({}^{\circ}\)C samples. Furthermore, Raman measurements reveal that C-C bond-related signals near 1400 cm\({}^{-1}\) are observed in the 650 \({}^{\circ}\)C films, a characteristic of compositional disorder, but are absent in the 700 \({}^{\circ}\)C films (Figure S2 ii). These observations, along with the low \(\alpha\) and \(E_{u}\) values in the AD and A2-annealed 700 \({}^{\circ}\)C SiC films, collectively indicate a lower density of sub-band edge states and minimal compositional disorder under our investigated conditions. Figure 5F summarizes the figures of merit (FOMs) from this study with state-of-the-art values reported for SiC-based platforms fabricated using various deposition techniques (e.g., PECVD, ICPCVD) and on different substrates (e.g., Si, SiO\({}_{2}\), LN). Our study demonstrates that, collectively under our investigated conditions, we have achieved improved values across key metrics: hydrogen content, surface roughness, optical absorption, and refractive index. Our A2-annealed 700 \({}^{\circ}\)C SiC thin films exhibit undetectable hydrogen-related bonds and extremely low surface roughness, minimal optical absorption, and refractive indices within the optimal range for photonic integration. This underscores not only the effectiveness of our materials-science engineering approach, but also the impor  
tance of comprehensive material characterization in the development of high-performance photonic devices.