Using the density functional theory, total energy calculations with the inclusion of gradient corrections, we studied the binding and diffusion of Al ad-atoms on GaN(0 0 0 1). It is found that when the Al atom remains on the surface, the most stable configuration corresponds to the adsorption at T4 sites on top of the N atom. Adsorption on the hollow site (H3) results in a slightly higher energy. The transition site corresponds to the bridge position, with an energy barrier of 0.66 eV. However, a configuration with the Al atom replacing a first layer Ga atom and the latter occupying T4 sites is more stable. Surface diffusion of this newly created Ga ad-atom is now more favorable, with an energy barrier as low as 0.43 eV. At full coverage, in the most stable configuration, the Al atoms replace the first Ga layer, with those atoms located almost on top of the Al atoms and forming chains. In this configuration, no dangling bonds are left behind. (C) 2010 Elsevier B.V. All rights reserved.