Photoluminescence studies of excitonic transitions in GaN epitaxial layers

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni's equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.