Asymmetry of adsorption of oxygen at wurtzite AlN (0001) and $\left(000\overline{1}\right)$ surfaces: First-principles calculations

First-principles calculations are performed to study the adsorption of oxygen at wurtzite AlN (0001) and $\left(000\overline{1}\right)$ surfaces as a function of oxygen coverage. We find that the adsorption of oxygen at the AlN $\left(000\overline{1}\right)$ 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 $\left(000\overline{1}\right)$ 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 $\left(000\overline{1}\right)$ 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.

Figure 1

Three dimensional views of the upper part of (a) the ideal (0001) surface, and (b) the Al-adlayer-terminated $\left(000\overline{1}\right)$ surface; (c) the top view of the (0001) surface and the possible adsorption sites are marked by dashed circles. H3 is the hollow site, atop is the site directly above the surface atoms, and T4 is the site directly above the sublayer (second layer) atoms.

Figure 2

Oxygen adsorption energies per O atom at H3 sites of (0001), solid line, and $\left(000\overline{1}\right)$, dashed line, surfaces.

Figure 3

(Color online) Top and side views of relaxed atom configurations with 0.25, 0.5, 0.75, and 1 ML oxygen coverages at H3 sites of the AlN (0001) surface.

Figure 4

Density of states (in arbitrary units) for the surface models: (a) AlN (0001) free surface and surface covered by 1 ML of oxygen; (b) AlN $\left(000\overline{1}\right)$ free surface, free surface with Al adlayer, and 1 ML of oxygen covered on Al adlayer. The dotted lines are VBM and CBM of bulk AlN.