Stabilization mechanism for the polar ZnO(0001̄)-O surface

When wurtzite ZnO is sliced perpendicular to the (0001) axis, two different polar surfaces, the (0001)-Zn and (0001̄)-O terminated surfaces, are formed. In a simple ionic picture, both surfaces are electrostatically unstable due to a diverging electrostatic energy. Although the ionic picture is an oversimplification, the surfaces adopt a modified surface structure to compensate for the polarity. In close collaboration with experiment, a hexagonal honeycomblike reconstruction has been suggested [J. V. Lauritsen et al., ACS Nano 5, 5987 (2011)]. The remarkable observation is that the (0001̄)-O surface behaves very differently than the (0001)-Zn surface. Here, we present a detailed density functional theory investigation of the (0001̄)-O surface, including a systematic investigation of H and Zn coverage as well as an investigation of various surface reconstructions. The difference between the two polar surfaces is explained by the different bonding preferences of Zn and O atoms: as a d element, Zn atoms are more flexible in their bond formation than O atoms.