Abstract We have studied the relation between MBE growth parameters and film quality after growth of GaN on the c -plane of sapphire using a conventional Ga-source and an RF-source for the excitation of N 2 . The growth rate was varied using Ga-flux, determined by RHEED oscillations on GaAs, r GaAs = 0.1–0.8 μ m/h. The N 2 -flux was 0.5–1.5 sccm. Most layers of GaN were grown at 760°C as measured by a pyrometer, while higher temperatures were utilised to determine the dismissal of growth. The RF-power and AlN buffer layer parameters were kept constant. The GaN-thicknesses were 0.2–1 μm/h. For low growth rates of ∼ 50 nm/h, there was a preferential growth of microcrystals in the growth direction which therefore overemphasised the measured film thickness. No film could be grown above 800°C as the desorption rate was too high. For a GaN growth rate of 300 nm/h the sticking coefficient was ∼ 85%. By varying the Ga- and N 2 -fluxes it was evident that the film quality, as provided by the photoluminescence spectra, strongly depended on the growth parameters. Photoluminescence peak intensities generally improved with film thickness below 1 μm. In 1 μm thick films, we observed excitonic related peaks close to the band gap as well as peaks 50–60 meV below due to the presence of defects or impurities. We found a strong correlation between growth parameters and optimum growth and therefore the highest layer quality could be obtained only when all parameters were carefully optimised.