## 5 Results and Discussion
Note that the value of the TO phonon damping factor indicates the GaN epitaxial layers with quality similar to those grown on native and foreign substrates [21; 22]. Next, we investigated the relative spectrum of UID GaN, GaN:C buffer and AlN nucleation layers measuring the intensities \(I_{\text{QWR}}\) and \(I_{\text{SUB}}\) at the output of the QWR and SUB samples, respectively. The \(I_{\text{QWR}}/I_{\text{SUB}}\) ratio spectrum is shown in Figure 4. It was considered as a characteristic transmittance of the stack of all three layers. The theoretical spectrum was also found modeling the ratio of \(T_{\text{QWR}}/T_{\text{SUB}}\), here \(T_{\text{QWR}}\) is the transmittance of the QWR sample calculated using the best-fit parameters modeling the reflectance of the HBR and QWR samples. The model for the transmittance \(T_{\text{SUB}}\) of the 6H-SiC substrate (sample SUB) was described in Section 4. As it is seen from Figure 4, the simulated relative spectrum of the stack of UID GaN, GaN:C buffer and AlN nucleation layers is in a good agreement with the experimental one at wavenumbers \(k<250\) cm\({}^{-1}\), but at higher wavenumbers, there is a noticeable discrepancy. Namely, the experimental relative spectrum demonstrates a negative deviation with values up to about 7% at \(k=400\) cm\({}^{-1}\). Similar negative deviation (of about 6% at \(k=400\) cm\({}^{-1}\)) can be detected for the experimental reflectance spectrum of the QWR sample in the spectral range of 380-450 cm\({}^{-1}\) (see also Figure 3b). Obviously, the simultaneous signal attenuation in both spectra at the same spectral range can be associated with scattering of incident radiation that was not considered in the model used. It might be that the radiation was scattered by the rougher surfaces of the QWR sample caused by reactive ion etching. The transmittance spectra of the whole heterostructure obtained by the FTIR method at two temperatures are shown in Figure 5 by thick solid lines. A significant peculiarity of both spectra is a sharp dip around wavenumber of 240 cm\({}^{-1}\). The dip is not associated with the epilayers of the heterostructure: it is due to folded transverse acoustic phonon modes in the 6H-SiC substrate [23; 24]. The spectra obtained by the TDS method (thin solid lines) demonstrate smaller transmittance values in comparison to the FTIR data. In particular, the transmittance at wavenumber of 80 cm\({}^{-1}\) is smaller by about 5%. The reduction was associated with a single pass of THz pulse through the sample due to a rather short time window taken for analysis of TDS measurements in order to avoid the appearance of F-P oscillations in the spectra. THz performance of 2 DEG was experimentally examined comparing the intensities \(I_{\text{WS}}\) and \(I_{\text{QWR}}\) recorded for the WS and QWR samples, respectively. The \(I_{\text{WS}}/I_{\text{QWR}}\) ratio spectrum is shown in Figure 6. It was attributed to the high-frequency characteristic of 2 DEG layer as the radiation absorption in all epilayers above the QW with a total thickness of 22.8 nm was found to be negligibly small in the whole spectrum under consideration. The simulations demonstrated that the change of the \(I_{\text{WS}}/I_{\text{QWR}}\) ratio should not exceed 0.1% going from the FTIR method to the TDS method and this is much smaller than the experimental accuracy of both methods used. Although, a single pass and multi pass transmission through the substrate result in different absolute values obtained by the two methods, it proportionally modifies both intensities in the numerator and in the denominator of the ratio \(I_{\text{WS}}/I_{\text{QWR}}\). Similarly, the \(I_{\text{QWR}}/I_{\text{SUB}}\) ratio is convenient for the characterization of the stack of UID GaN, GaN:C buffer and AlN nucleation layers (see also Figure 4). Modelled relative spectrum of \(T_{\text{WS}}/T_{\text{QWR}}\) ratio was also calculated.