Terahertz Dielectric Response of Nonpolar a-plane GaN Films

Abstract

We report on the dielectric properties of non-polar a-plane (11-20) GaN (a-GaN) films in the terahertz (THz) frequency region, using THz time domain spectroscopy. We measured the transmitted THz time-domain waveforms by rotating the phi angle for 6μm-thick a-GaN films, which were lifted off from sapphire substrates. THz transmittance of a-GaN films clearly showed the angular dependence on azimuth angle, possibly due to the birefringence and optical phonon modes in a-GaN crystal. It is believed that the discrepancy in the refractive index results from the difference of the transverse-optical and the longitudinal optical phonon branches.