Polarized infrared reflectance study of free standing cubic GaN grown by molecular beam epitaxy

•First time experimental studies of IR optical phonons in bulk like, cubic GaN layer.•Detection of extra phonon modes of cubic GaN by polarized IR reflectance technique.•Revelation of IR multiphonon modes of cubic GaN by first derivative numerical method.•Observation of multiphonon modes requires very high angle of incidence.•Resonance splitting effect induced by third phonon mode is a qualitative indicator.

Optical properties of free standing cubic gallium nitride grown by molecular beam epitaxy system are investigated by a polarized infrared (IR) reflectance technique. A strong reststrahlen band, which reveals the bulk-like optical phonon frequencies, is observed. Meanwhile, continuous oscillation fringes, which indicate the sample consists of two homogeneous layers with different dielectric constants, are observed in the non-reststrahlen region. By obtaining the first derivative of polarized IR reflectance spectra measured at higher angles of incidence, extra phonon resonances are identified at the edges of the reststrahlen band. The observations are verified with the theoretical results simulated based on a multi-oscillator model.