In this work, GaN nanowires were fabricated on Si substrates coated with gold and nickel catalyst using a thermally assisted chemical vapor deposition method by evaporating mixture of Ga, GaN, MnO2 (only for Mn-doped GaN) and carbon (single wall carbon nanotubes (SWCNT) or graphite) at 1100 degrees C in ammonia gas flow. The fabricated nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffractometry (XRD). We investigated the influence of the source of carbon and substrate temperature on the morphology and composition of the nanowires. We found that nanowire composition is strongly dependent on the substrate temperature, with oxygen incorporation increasing with decreasing substrate temperature, and also on the composition of the source material. Growth mechanism and properties of obtained nanostructures are discussed. (C) 2007 Elsevier B.V. All rights reserved.