Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:С buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate.

We report on the study of far-infrared transmission in silicon carbide of 6H polytype in the region of IR active folded transverse acoustic (FTA) phonon doublet. Joint analysis of the transmittance spectra obtained at low and high resolutions is performed to determine spectral dependences of the refractive index and absorption coefficient. A dual Lorentz oscillator model is used to simulate the FTA phonon contributions to the dielectric function. The oscillator strengths and phonon lifetimes for both the components of the FTA phonon doublet in 6H-SiC are determined. At temperature decrease from 300 to 5 K, the FTA phonon lifetimes increase more than six times, while the oscillator strengths remain the same.

The weakest mode of the infrared active folded phonons in 4H–SiC, folded transverse acoustic (FTA) mode, has been experimentally studied. The lifetime of the FTA phonons has been determined. The lifetime is 58 ± 4 ps at room temperature and increases to 140 ± 7 ps when the temperature is decreased to 5 K. These results were obtained by the experimental study of far-infrared transmission of the SiC crystal and its theoretical simulation using an oscillator model for the dielectric function around the phonon resonance. The FTA phonon lifetime was a fitting parameter of the model when analyzing the irregular interference pattern in the transmission spectrum measured with a high resolution (0.2 cm−1). Oscillator strength of the FTA phonon resonance has also been determined. It is equal to (6.0 ± 0.5)×10−4 and does not depend on temperature.

Temperature evolution of the high-resolution far-infrared transmission spectra of 6H-SiC single crystal slab is experimentally examined in a temperature range of 5–320 K. Temperature dependences of the phonon lifetimes for long-lived phonons, namely the IR active folded transverse acoustic (FTA) phonon doublet, are determined. These dependences are interpreted in the framework of a theoretical model developed using first principle anharmonic lattice dynamics. It is shown that three-phonon anharmonic processes dominate in the FTA phonon decay at temperatures above 160 K resulting in a rapid decrease of the phonon lifetimes with increasing temperature. Contribution of four-phonon anharmonic processes is negligibly small at temperatures below 320 K.

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