GaN epitaxial layers are grown by a separate-flow metalorganic chemical vapor deposition (MOCVD) reactor with various distances between the substrate and the ceiling of quartz chamber. The spacing of the growth chamber between substrate and ceiling is varied from 18 to 5 mm. This spacing affects the reactant gas flow pattern near the substrate surface and thus influences the quality of the epitaxial layers. In addition, we use an upper-stream H-2 flow to improve the thermal convection of the reactant gases and the formation of laminar flow above the substrate. The surface morphology and the Hall mobility of GaN are found to depend on this Hz flow rate. A mirror-like surface can be grown for both 10 and 18 mm spacing designs, if the H-2 flow rate is adjusted to around the 5000 cm(3)/min range. X-ray diffraction, photoluminescence, and Hall measurement characterized the quality of GaN epitaxial layers. Device-quality epitaxial layers can be obtained, if the flow spacing is tuned to be 10 mm. (C) 1999 Elsevier Science B.V. All rights reserved.