Free 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 naturally dominated by excitonic emissions; h-GaN exhibited well-resolved bound and free exciton peaks and c-GaN exhibited a broadened excitonic emission peak, which was dominated by bound and free excitons below and above 80 K, respectively. The PL spectra at RT of both c-GaN and h-GaN exhibited predominant free excitonic emissions due to the small exciton Bohr radius, large exciton binding energy, and small numbers of LO phonons thermally active at RT.