We report the structural and surface morphologies of GaN layer directly grown on 6H-SiC (0001) substrates by plasma-assisted molecular beam epitaxy. The structural properties of the GaN layers grown under Ga-rich conditions have been strongly influenced by the surface oxide-desorption through Ga-metal deposition and flash-off at high temperature. The resulting AFM surface morphology exhibits atomic steps with < 5 Angstrom rms surface roughness. The control of Ga-adatom migration governs the surface morphology of the grown layers, the atomic step features are realized with sufficient surface migration of Ga for the available N-radicals. The excellent structural quality of these layers is manifested by the narrow full-width at half-maximum of X-ray rocking curve (< 124 arcsec) measured along the omega (0002) diffraction. (C) 2002 Elsevier Science B.V. All rights reserved.