Phonon modes of GaN/AlN heterojunction field-effect transistor structures grown on Si(111) substrates

Phonon modes of GaN/AlN heterojunction field-effect transistor (HFET) structures were investigated using Fourier-transform infrared spectroscopy. The HFET structure was grown on Si(111) substrate with AlN-based buffer layers. The phonon modes were also investigated in structure without AlGaN cap layer. The phonon mode spectra were obtained at the normal incident, waveguide, and Brewster’s angle configurations. Several vibrational frequencies were observed and found to be strongly dependent on the angle of the incident light. In particular, a phonon frequency of ∼734 cm−1 was observed only in the spectra when the samples are in the Brewster’s angle configuration. Moreover, a phonon mode was observed at 880 cm−1 in both waveguide and Brewster’s angle configurations. Additionally, a phonon absorption band is observed around 576 cm−1 , which appears to be composed of two modes, was redshifted to ∼550 cm−1 when the spectra were recorded in the waveguide configuration.