GaN nanowires were successfully grown on a Ni-coated Si substrate with a direct reaction of gallium and ammonia in a home-made vertical tubular chemical vapor deposition reactor. The grown GaN nanowires were uniformly distributed across the Si substrate surface, but the density and diameter of the nanowires varied along the length of the substrate surface. Moving upward along the substrate surface from the Ga source surface, the density increased, whereas the diameter decreased. At the upper part of the substrate surface (5 cm above the Ga source), however, the growth of the GaN micro-crystal gains was observed in a few nanowires. The length of the nanowires reached several micrometers. The clear lattice fringes in the high-resolution transmission electron microscopy image indicated the growth of good-quality hexagonal single-crystal GaN nanowires. The GaN nanowires produced a strong band edge emission at similar to 3.4 eV with a negligible deep level yellow emission. Field-emission characteristics of the GaN nanowires showed that the turn-on field was similar to 7.4 V/mun with a field enhancing factor beta of similar to 555. The catalytic growth mechanism of the GaN nanowires is discussed on the basis of these experimental results. (C) 2003 Elsevier B.V. All rights reserved.