AlN layers were grown on Si and sapphire substrates in a horizontal and a vertical metalorganic chemical vapour phase epitaxy system. AlN grown in the horizontal reactor is smooth and shows intense band-edge cathodoluminescence but Ga can be found in the layers even after several microns of AlN growth. After cleaning the reactor and using a new Ga-free quartz and graphite system the layer quality is drastically reduced and we observe a rough morphology. In the vertical system pure AlN layers with smooth surfaces can be grown even on a Ga-contaminated susceptor and no Ga is found in the layers. In cathodoluminescence (CL) measurements an enhancement of oxygen or silicon-related luminescence peaks is found for the growth on sapphire or silicon substrates, respectively. A comparison of CL and Raman measurements reveals a strong tensile stress for AlN on Si that corresponds to similar to 1 GPa and relaxes at cracks where a wavelength shift of 4 nm is observed. AlN grown on sapphire is found to be under compressive stress at room temperature. (c) 2006 Elsevier B.V. All rights reserved.