Asymmetry of adsorption of oxygen at wurtzite AlN (0001) and (000(1)over-bar) surfaces: First-principles calculations

First-principles calculations are performed to study the adsorption of oxygen at wurtzite AlN (0001) and (000 (1) over bar) surfaces as a function of oxygen coverage. We find that the adsorption of oxygen at the AlN (000 (1) over bar) surface has a larger binding energy than at the AlN (0001) surface. The hollow site (H3) is preferred for the (0001) surface, whereas the site directly above the Al sublayer and the H3 site are almost degenerate in energy for the (000 (1) over bar) surface. The trend of the adsorption energy as a function of the oxygen coverage for the AlN (0001) surface is similar to that of GaN, whereas for the AlN (000 (1) over bar) surface it is different from GaN. The asymmetry of the oxygen adsorption at the two surfaces is explained using the electron counting rule and the resulting surface electronic states.