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 AIN, 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 p(2X2) geometries of GaN and AW(0001). Vacancy structures are found to be the most stable configurations for the anion-and cation-terminated surfaces. For metal-rich growth conditions, bur 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-Ill atoms. Flat surfaces of both types are found to be stabilized by a 3/4 ML adsorption of hydrogen.