Copper growth on the O-terminated ZnO(0001¯) surface: Structure and morphology

The effects of copper deposition on the ZnO(0001¯) O-terminated surface at 300 K have been studied by surface x-ray diffraction (SXRD). Terraces about 550 Å wide, presenting a (1×1) bulk-derived order, were obtained on the clean surface after repeated cycles of Ar⁺ sputtering-annealing. The SXRD data show that inward relaxation takes place in the two topmost planes (-0.2 Å for the first O plane) while the site occupancy in the second Zn plane is reduced by one quarter. Upon copper adsorption, a full derelaxation of the O surface is observed. In addition, the ordered fraction of atoms in the top O plane is reduced by one quarter, a feature that may be associated to some O-Cu interaction. The Cu adatoms involved in such interaction could be responsible for the surface derelaxation through a charge transfer. No change is detected concerning the CuZnO(0001¯)-(1×1) surface up to an equivalent of several monolayers of deposited copper. Copper aggregates into flat islands, with two orientational relationships, at 180° from each other: Cu(111)//ZnO(0001¯) with Cu[1¯10]//ZnO[100]. The islands are not strained on the substrate, even at very early stages of growth. The rodlike signal from the (111) facet is already measured for an equivalent of 5 Cu ML. The lateral growth proceeds preferentially along the ZnO[210] axis. Additional diffraction signals, attributed to copper islands nucleating at the ZnO bilayer step edges, are detected.