Optical phonons of hexagonal and cubic GaN studied by infrared transmission and Raman spectroscopy

Abstract

Using infrared transmission and Raman spectroscopy, we have studied the optical phonon modes of GaN layers grown on GaAs(001) substrates by molecular beam epitaxy. The crystal structure of the GaN layers ranges from predominantly wurtzite to predominantly zincblende depending on the growth conditions. The transverse and longitudinal optical phonons in cubic GaN are found to be at 552 and 739 cm⁻¹, respectively. These frequencies are slightly shifted with respect to the corresponding A₁ and E₁ phonon modes in hexagonal GaN. The frequency shifts, together with Raman scattering selection rules, can be used for identifying the phase composition of GaN. A more distinct fingerprint of the hexagonal phase is provided by the occurrence of the E₂ phonon modes that are spectrally separated from optical phonon modes in the cubic phase and thus uniquely related to the hexagonal phase.