Periodic, self-consistent, gradient-corrected (PW91-GGA), density functional theory (DFT) calculations are used to investigate the role of Sb as surfactant during GaN growth. The adsorption and diffusion of Ga, Sb, and N on GaN(0 0 0 1) and GaN(1 1 1 0) single crystal surfaces is studied. We find that N has a much higher diffusion barrier than Ga and Sb on these surfaces. An approximate phase diagram for Sb-induced surface reconstructions of GaN(0 0 0 1) is constructed. Our findings suggest that N transport on GaN is facilitated by the formation of an SbN intermediate, which diffuses easier than N on the GaN surface. We show that this SbN formation-mediated surfactant action of Sb is much more pronounced on the Ga-terminated GaN(0 0 0 1) than on the GaN(1 1 2 0) facet. The insights derived from the DFT studies are used to explain the experimentally observed increased diffusion length of N, decreased surface roughness, and the change in the preferential sidewall facet from (1 1 0 1) to (1 12 0) during lateral epitaxial overgrowth in the presence of Sb. (c) 2005 Elsevier B.V. All rights reserved.