Presence of a (1x1) oxygen overlayer on bare ZnO(0001) surfaces and at   Schottky interfaces
  The atomic surface and interface structure of bare and metal-coated ZnO(0001) Zn-polar wafers were investigated via surface x-ray diffraction. All bare samples showed the presence of a (1x1) overlayer of oxygen atoms located at the on-top position above the terminating Zn atom, a structure predicted to be unstable by several density functional theory calculations. The same oxygen overlayer is clearly seen at the interface of ZnO with both elemental and oxidized metal contact layers. No significant atomic relaxations are observed at surfaces and interfaces processed under typical device fabrication conditions. 
Christian M. Schlep\"utz, Yongsoo Yang, Naji S. Husseini, Robert   Heinhold, Hyung-Suk Kim, Martin W. Allen, Steven M. Durbi...
cond-mat.mtrl-sci