This paper reports on the vanadium doping of GaN layers grown by metal-organic vapour phase epitaxy (MOVPE). We have used vanadium tetrachloride (VCl4) to intentionally incorporate vanadium (V) during crystal growth of GaN. The films were grown on sapphire substrates by in-situ SiN treatment. We describe the in-situ monitoring of the growth process by laser reflectometry. The V incorporation in GaN was investigated by secondary ion mass spectroscopy (SIMS). The V diffusion coefficient in GaN has been calculated to be 10(-14) cm(2) s(-1) at 1120 degrees C by fitting the V SIMS profile. The high-resolution X-ray diffraction and atomic force microscopy analysis were employed to study the structural and surface morphology of the films. At room temperature, by comparison with un-doped layers grown in the same conditions, photoluminescence spectra of V-doped GaN layers exhibited a strong blue emission band. The intensity of this blue band increases with the increase of VCl4 flow rate. We propose that is associated with intrinsic defects introduced by the growth conditions or V-related intrinsic defects complexes. Upon V-doping, the yellow luminescence band shows a drastic reduction in integrated intensity, which could be due to a decrease in the concentration of gallium vacancy (root(Ga)) or root(Ga)-related defect complexes. (c) 2005 Elsevier B.V. All rights reserved.