We studied the correlation between the low-temperature optical properties and the surface morphology of molecular beam epitaxy (MBE) grown GaN on sapphire(0001). The samples were grown under Ga-rich conditions, with the Ga-flux as a parameter, and with all other growth parameters constant. High-resolution scanning electron microscopy (SEM) provided a measure of the surface morphology, while the optical properties were characterised by low-temperature photoluminescence. These spectra were dominated by the exciton bound to neutral donor transition, at 3.472-3.474 eV, indicating fairly strain relaxed layers. This peak width is increased when the surface morphology improved. Our results also showed a clear correlation between the optical properties and the purity of the nitrogen source, as improved oxygen purification improved the photoluminescence linewidth from 41 to 20 meV.