Cu films on a Zn-terminated ZnO(0001) surface: structure and electronic properties

Abstract

The structure and electronic properties of ultrathin, vapor-deposited Cu films on a Zn-terminated ZnO(0001)-Zn surface have been studied with low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD), low-energy electron diffraction (LEED), work function and band bending measurements. Below ∼5% of a monolayer (ML), the Cu is very weakly cationic, but thereafter, it is practically charge-neutral and clustered into islands with strong Cu–Cu bonding. Up to ∼33% of a ML, these islands are only one atom thick, and the Cu atoms do not sit in preferred substrate lattice sites. Above ∼33% of a ML, additional Cu adds predominantly on top of existing Cu islands, to make three-dimensional (3D) Cu islands. These eventually adopt a Cu(111) structure, rotationally aligned with the ZnO(0001) substrate, when the individual islands are 3 ML thick or thicker. Annealing above 500 K causes the islands to irreversibly thicken, thus uncovering part of the ZnO surface. The results are compared to Cu films on other faces of ZnO, showing that the geometry of the ZnO surface has only minor effects on the Cu film properties, although the conversion from 2D to 3D islands happens at a lower coverage on this Zn face than on the O face.