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

The effects of copper deposition on the $\mathrm{ZnO}\left(0001\mathrm{}¯\right)$ O-terminated surface at 300 K have been studied by surface x-ray diffraction (SXRD). Terraces about 550 Å wide, presenting a $(1\mathrm{}\times 1)$ bulk-derived order, were obtained on the clean surface after repeated cycles of ${\mathrm{Ar}}^{+}$ 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 $\mathrm{CuZnO}\left(0001\mathrm{}¯\right)-(1\mathrm{}\times 1)$ 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: $\mathrm{Cu}\left(111\right)//\mathrm{Z}\mathrm{n}\mathrm{O}\left(0001\mathrm{}¯\right)$ with $\mathrm{Cu}\left[1¯10\right]//\mathrm{ZnO}\left[100\right].$ 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.