Atomic structure and stability of AlN(0001) and (0001) surfaces

We report first-principles calculations of the relative formation energies for possible reconstructions of the AlN(0001) and AlN(000 (1) under bar) surfaces. Structural models with 2 x 2 symmetry and satisfying the electron counting rule, as well as metallic surfaces with 1 x 1 symmetry, have been considered. For AlN(0001) both A1-T4 and N-H3 adatom models are stable within the allowed range of the Al and N chemical potential: the N-adatom structure is stable in N-rich conditions and the Al-adatom structure is most stable in Al-rich conditions. For the AlN(0001) surface the 2 x 2 A1-H3 adatom model is stable in N-rich conditions, while under Al-rich conditions a one-monolayer adlayer of Al is favored.