The optical properties and the surface morphology of molecular beam epitaxy grown GaN on sapphire(0001) have been studied. The samples were grown under various growth conditions, such as Ga-flux, different buffer layer and different degree of oxygen contamination from the nitrogen gas. High-resolution scanning electron microscopy and reflection high-energy electron diffraction revealed information about the surface morphology. Low-temperature photoluminescence was used for the characterisation of the step-by-step improved optical properties to reveal impurity and defect related transitions. The results clearly show that the linewidth of the GaN emission gradually improved by increasing the Ga-flux while keeping all other growth parameters unchanged. We also found that both the oxygen and carbon contaminations from the nitrogen source and the resulted morphology are directly related to the linewidth of the spectra. (C) 2004 Elsevier B.V. All rights reserved.