We have studied the optical emission properties of a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition. Together with the typical band edge exciton emission, the photoluminescence (PL) spectra reveal three low-energy emissions peaked at 3.42, 3.34 and 3.29eV. which are related to structural defects. Temperature and excitation dependent stationary PL and the time-resolved PL have been employed in order to understand the exact origin of these emissions. The 3.42 and 3.34eV emissions are found to be of an intrinsic origin and are associated with carriers localized at stacking faults. The emission at 3.29eV shows a donor-acceptor pair behavior suggesting that impurities attached to structural defects most likely partial dislocations terminating stacking faults are involved. (c) 2005 Elsevier B.V. All rights reserved.