Ab initiocalculation 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 $p(2\mathrm{}\times 2)$ 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 $\frac{3}{4}$ ML adsorption of hydrogen.