ATOMIC-STRUCTURE AND STABILITY OF ALN(0001) AND (0001) SURFACES

We report first-principles calculations of the relative formation ener gies for possible reconstructions of the AlN(0001) and AlN(000 (1) und er bar) surfaces. Structural models with 2 x 2 symmetry and satisfying the electron counting rule, as well as metallic surfaces with 1 x 1 s ymmetry, have been considered. For AlN(0001) both A1-T4 and N-H3 adato m 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 t he Al-adatom structure is most stable in Al-rich conditions. For the A lN(0001) surface the 2 x 2 A1-H3 adatom model is stable in N-rich cond itions, while under Al-rich conditions a one-monolayer adlayer of Al i s favored.