Ab-initio studies of the adsorption of a B ad-atom on GaN surfaces

The energetics and the structural properties of a B ad-atom adsorbed on the GaN (0001) and GaN(0001¯) surfaces have been investigated using ab-initio total energy calculations. In all cases, the boron ad-atom does not want to occupy positions above Ga atoms, but it prefers to replace a top layer Ga atom. In an ideally GaN (0001) bulk terminated surface, and when the B atom is constrained to remain on top of the surface, the T4 site configuration is the most favorable. However a structure in which the B atom replaces a Ga atom of the first monolayer and the Ga atom occupy a H3-2 site has the lowest energy, and therefore it is the ground state. The GaN (0001) pseudo-(1×1) surface have been modeled with a √3×√3-R30° unit cell with 4 Ga atoms on the top layer. In the most stable configuration, the boron atom substitutes a top layer Ga atom, but because of its smaller size its vertical position is between the top most and the second gallium layers. As in the ideally GaN (0001) bulk terminated surface, in the GaN(0001¯) surface, the boron atom replaces a topmost gallium atom, but now it forms a bond with a second layer N atom. In the most stable configuration there is a rearrangement of the top layer Ga atoms, and the final configuration cannot be considered an ad-atom structure. These results show the same trends of other metal ad-atoms adsorbed on GaN surfaces, but the specific adsorption geometries change due to the small size of the B atom.

► We study the B adsorption on GaN surfaces using the periodic density functional theory.
► In an ideally GaN (0001) surface with B constrained on top of it the T4 is the most favorable site.
► When a B replaces a Ga of the first monolayer and the Ga occupies a H3-2 it forms the ground state.
► In the GaN (0001̄) surface the B replaces the topmost Ga forming bonds with a second layer Ga.
► Results show some similar trends of other metals adsorbed on GaN surfaces.