Exciton spectra of cubic and hexagonal GaN epitaxial films

Fundamental exciton structures in both cubic (c) and hexagonal (h) polytypes of GaN epitaxial films were compared. The photoreflectance (PR) spectra of both polytypes exhibited excitonic transitions up to room temperature (RT). The lowest exciton resonance energy of c-GaN (3.267 eV at 10 K) was confirmed to be smaller by 0.21 eV than that of h-GaN. The low-temperature photoluminescence (PL) spectra of both polytypes were dominated by excitonic emissions; h-GaN exhibited well-resolved bound and free exciton peaks and c-GaN exhibited a broadened excitonic emission, which was dominated by bound and free excitons below and above 80 K, respectively. The PL spectra at RT of h-GaN exhibited predominant, A and B free exciton emissions. The E₂ phonon replicas of excitons were found in the PL spectra of h-GaN, indicating the coupling of excitons with nonpolar phonons through the deformation potential.