Undoped AlN layers have been grown on c-plane sapphire substrates by metal-organic chemical vapor deposition in order to study the effects of ammonia (NH 3) flow rate and layer thickness on the structural quality and surface morphology of AlN layers by high-resolution X-ray diffraction, scanning electron microscopy, and atomic force microscopy. Lower NH 3 flow rate improves crystallinity of the symmetric (0 0 0 2) plane in AlN layers. Ammonia flow rate is also correlated with surface quality; pit-free and smooth AlN surfaces have been obtained at a flow rate of 70 standard cm 3 per minute. Thicker AlN films improve the crystallinity of the asymmetric (1 0 1 2) plane. © 2011 Elsevier Ltd. All rights reserved.