The growth of GaN nanowires from Ga and NH3 sources in the flow of Ar carrier gas using a chemical vapor deposition (CVD) system was systematically studied. The substrates used were Si(111) and Si(100). Fabricated nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). We investigated the influence of growth temperature, catalyst used, Ga amount, and the ratio of Ar and NH3 flow rates on the morphology and properties of GaN nanowires. We found that the best results were obtained for a growth temperature of 950 degrees C. Optimal catalysts were Au and metallic Ni, while the use of nickel nitrate was found to lead to formation of SiOx nanowire bunches in addition to GaN nanowires. For the optimal temperature and catalyst used, the influence of the Ga to N ratio on the nanowire growth was studied. It was found that different types of nanostructures are observed in relatively Ga-rich and in relatively N-rich conditions. Growth mechanisms of different types of nanowires, including the stacked-cone nanowires and the microscale structures formed by lateral growth under N-rich conditions, are discussed. (c) 2006 Elsevier B.V. All rights reserved.