The role of Sb and Bi as surfactants in GaN growth is investigated using first-principles, periodic, self-consistent, density functional theory calculations. It is shown that N diffuses much slower than Ga on the Ga-rich GaN(0 0 0 1) surface. Surfactants such as Sb and Bi are considerably more mobile on this surface and they react with N to produce SbN and BiN intermediates. The diffusion of these intermediates on the Ga-rich GaN(0 0 0 1) surface is more facile than that of atomic N. Therefore, this intermediate-mediated transport mechanism would increase the effective diffusion length for N. As a result, Sb and Bi would improve step edge incorporation of N, leading to a reduction in the average surface roughness of the GaN samples. While the barrier for BiN diffusion on GaN(0 0 0 1) is only slightly lower than that of N, the calculated difference in the diffusion barriers of SbN and N on that surface is significant and this would cause the preferential sidewall facets to change from (1 1 0 1) and (1 1 1 2) to the vertical (1 1 2 0) facets during lateral epitaxial overgrowth (LEO). Additional calculations show that Sb and Bi can act as surfactants on the GaN(I 110) surface too. However, the adsorption of all the species on GaN(1 1 2 0) is significantly weaker and the diffusion barriers of SbN and BiN are considerably higher compared to the GaN(0 0 0 1) surface. Consequently, the surfactant effect of Sb and Bi on the GaN(1 1 2 0) surface should be less pronounced compared to that on the GaN(0 0 0 1) surface. (C) 2007 Elsevier B.V. All rights reserved.