High quality, straight GaN nanowires (NWs) with diameters of 50 nm and lengths up to 3 mu m have been grown on Si(0 0 1) using Au as a catalyst and the direct reaction of Ga with NH3 and N-2:H-2 at 900 degrees C. These exhibited intense, near band edge photoluminescence at 3.42 eV in comparison to GaN NWs with non-uniform diameters obtained under a flow of Ar:NH3, which showed much weaker band edge emission due to strong non-radiative recombination. A significantly higher yield of beta-Ga2O3 NWs with diameters of <= 50 nm and lengths up to 10 mu m were obtained, however, via the reaction of Ga with residual O-2 under a flow of Ar alone. The growth of GaN NWs depends critically on the temperature, pressure and flows in decreasing order of importance but also the availability of reactive species of Ga and N. A growth mechanism is proposed whereby H-2 dissociates on the Au nanoparticles and reacts with Ga giving GaxHy, thereby promoting one-dimensional (1D) growth via its reaction with dissociated NH3 near or at the top of the GaN NWs while suppressing at the same time the formation of an underlying amorphous layer. The higher yield and longer beta-Ga2O3 NWs grow by the vapor liquid solid mechanism that occurs much more efficiently than nitridation. (C) 2010 Elsevier B.V. All rights reserved.