Zn Vacancies as Hydrogen Trap Sites in Polar Surfaces: A New Stabilization Mechanism for the ZnO(0001)-(2×2) Surface Reconstruction

Abstract

The (2×2) reconstruction of the ZnO(0001) surface has been investigated by X-ray photoelectron diffraction (XPD). Comparing the XPD measurements with multiple-scattering simulations, the single Zn vacancy per (2×2) surface unit cell model is confirmed, and structures with O adatoms are ruled out. The analysis indicates an outward relaxation of the topmost Zn layer, in contrast to the usually reported results by density-functional theory (DFT) calculations. On the basis of DFT, we describe a new stabilizing mechanism of the polar ZnO surface through surface reconstruction where the Zn vacancies are occupied by three hydrogens atoms. The DFT surface relaxation of the proposed model is in excellent agreement with the XPD findings. Our DFT simulations also strongly indicate that the migration of hydrogens atoms to the surface, coming from the bulk, may influence the desorption of the surface Zn atom to create the vacancy.