The surface morphology of GaN(0001) grown on Si(111) by molecular beam epitaxy using ammonia has been studied by near-field microscopy techniques. Two distinct morphologies are observed, depending on the growth kinetics. When using Ga-rich growth conditions, the roughness is independent of the epitaxial layer thickness, while it increases with growth time under N-rich growth conditions. Also, the morphological pattern is different for the two regimes of growth and is not related to the crystallographic subgrains delimited by edge dislocations. However, under Ga-rich growth conditions, screw-type dislocations give rise to a spiral growth mode that imposts both the morphological pattern and the surface roughness. (C) 2001 Elsevier Science B.V. All rights reserved.