Copper growth on the O-terminated ZnO(000(1)over-bar) surface: Structure and morphology

The effects of copper deposition or). the ZnO(000 (1) over bar) O-terminated surface at 300 K have been studied by surface x-ray diffraction (SXRD). Terraces about 550 Angstrom wide, presenting a (1 x 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 Angstrom 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(000 (1) over bar)-(1 x 1) surface up to an equivalent of several monolayers of deposited copper. Copper aggregates into flat islands, with two orientational relationships, at 180 degrees from each other: Cu(111)//ZnO(000 (1) over bar) with Cu[(1) over bar 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.