A novel separate-flow reactor design provides a different stream channel to improve the GaN layer quality and prevent the pre-reaction of Ga(CH3)(3) with NH3 sources from metalorganic chemical-vapor deposition. Before flowing the main reactant gas into the reaction chamber, the gas source is separated into two flows, one that carries a group-III source and the other that carries the group-V source. Therefore, this reactor can effectively control the amount of the main-reactant gas onto the substrate. The surface morphology of GaN epitaxial layers grown directly onto sapphire substrates is investigated for different nucleation layer thicknesses for the separate-flow reactor. The surface displays a mirror-like morphology for a sample with a nucleation layer thickness of around 250 Angstrom. The maximum Hall mobility is 210 cm(2)/V s at 300 K for the undoped GaN layers. The narrowest full width at half-maximum of double crystal X-ray diffraction and 300 K photoluminescence spectrum is around 220 arcsec and 57 meV, respectively. (C) 1999 Elsevier Science B. V. All rights reserved.