Adsorption of gallium (Ga, Ga+) and gallium nitrides (GaN, GaN+, GaN2, GaN2+) on a model Si(111) surface was studied by density functional theory calculations. Significant binding is found for all the species considered, with binding energies up to 6.13 eV. For GaN, GaN2, and GaN2+ the lowest energy structures are those with the adsorbate forming a bridge between the Si adatom and rest atom, while for GaN+ it is a vertical structure connecting to Si via N. Furthermore, it is found that adsorbed GaN and GaN+ are essentially the same species, resembling free GaN-. (c) 2007 Elsevier B.V. All rights reserved.