## 5 Results and Discussion  
At first we analyzed the reflectance spectra of the samples QWR and HBR. The selected spectral range of 380-720 cm\({}^{-1}\) overlaps with the Reststrahlen region of AlN and GaN providing for the access of resonant features of TO phonons in UID GaN, GaN:C buffer and AlN nucleation layers of the heterostructure. In particular, the shape of the feature is very sensitive to the variations of the layer thickness and the phonon damping factor of each layer, namely, \(\gamma_{2}\), \(\gamma_{3}\) and \(\gamma_{4}\). The latter values were correlated with the crystal quality of the epitaxial layer. The TO phonon damping factors were used as variable parameters fitting simulated data to the experimental reflectance spectra. \begin{table}
\begin{tabular}{c c c c c} \hline
**Compound** & \(\varepsilon_{0}\) & \(\varepsilon_{\infty}\) & \(\hbar\omega_{\text{TO}}\) & \(\gamma\) \\  & & **(meV)** & **(cm\({}^{-1}\))** \\ \hline AlN & 8.5 & 4.68 & 82.3 & 6.6 \\ GaN & 9.5 & 5.4 & 69.3 & 4.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of transfer optical (TO) phonon resonance for AlN and GaN. The reflectance of the sample HBR at room temperature is shown in Figure 3a. The spectrum was modelled assuming that the amplitude of the reflectance peak in the vicinity of TO phonon resonance of AlN layer (at 675 cm\({}^{-1}\)) is controlled by the damping factor \(\gamma_{4}\) while the interference pattern in the wavenumber range of 450-555 cm\({}^{-1}\) is defined by the \(d_{3}^{t}\) and \(\gamma_{3}\) parameter values of the GaN:C layer possessing the TO phonon resonance at 559 cm\({}^{-1}\). There is a strong dispersion here of the refractive index which results in an irregular interference pattern due to multiple radiation reflections inside the GaN:C layer. Namely, one can see the interference maxima at 513 and 552 cm\({}^{-1}\) and the interference minimum between them at 535 cm\({}^{-1}\). By fitting the simulated results to the experimental spectrum (see Figure 3a), the parameters of the AlN nucleation layer and the GaN:C buffer layer were obtained: \(\gamma_{4}=4.02\) cm\({}^{-1}\), \(d_{3}^{t}=1200\) nm and \(\gamma_{3}=8.0\) cm\({}^{-1}\). It is important to note that the obtained value of the phonon damping factor of 8.0 cm\({}^{-1}\) for the GaN buffer is up to 2.5 times smaller than that found in [22], for a similar buffer layer but grown on sapphire substrate and without the usage of an AlN nucleation layer. Therefore, our results demonstrate that the use of an optimal nucleation layer for the growth of commercial HEMT structures on the SiC substrate is beneficial as the crystal quality of the upper GaN layer improves significantly. Moreover, a considerable improvement of low-frequency electrical performance as well as a weaker quenching of reduced magnitude THz electroluminescence from shallow impurities remaining in a commercial HEMT structures were demonstrated over the AlGaN/GaN/sapphire platform [10; 25]. The increase in the value of phonon damping factor in the bottom part of the GaN buffer was also observed and associated with an extrinsic phonon scattering at the lattice imperfections resulting from lattice mismatch as an effect of a foreign substrate [22]. The parameters of the UID GaN layer (layer 2 in the considered model) were determined from the experimental reflectance of the QWR sample. The results are shown in Figure 3b. The spectrum demonstrates an inversed interference pattern in the range of 450-555 cm\({}^{-1}\), namely, there are two interference minima at 493 and 550 cm\({}^{-1}\) and one maximum between them at 532 cm\({}^{-1}\). This transformation of the interference pattern is associated with the increase of total thickness of GaN layers by a factor of 1.7 in comparison to that in the HBR sample. By fitting the simulation data to the experimental spectrum (see (Figure 3b), the thickness and phonon damping factor for the UID GaN layer was found to be of \(d_{2}^{t}=750\) nm and \(\gamma_{2}=4.0\) cm\({}^{-1}\), respectively.