Ab initio calculation of the stoichiometry and structure of the (0001) surfaces of GaN and AlN

We have investigated the stoichiometry and the atomic and electronic structure of the anion- and cation-terminated (0001) surfaces of wurtzite-phase GaN and AlN, using ab initio local-orbital calculations based on the local-density approximation and the pseudopotential method. All stable surface configurations studied differ in atomic composition and periodicity from the ideal bulklike termination. We compare the total energy computed for various (Formula presented) geometries of GaN and AlN(0001). Vacancy structures are found to be the most stable configurations for the anion- and cation-terminated surfaces. For metal-rich growth conditions, our calculations favor the adsorption of metal atoms on the cation-terminated surface. Anion- and cation-derived dangling-bond states appear in the bulk band gap as a result of the formation of vacancies or the adsorption of group-III atoms. Flat surfaces of both types are found to be stabilized by a (Formula presented) ML adsorption of hydrogen.