Growth model for metal films on oxide surfaces: Cu on ZnO(0001)-O

Abstract

The structural and electronic properties of Cu films vapor deposited on the oxygen terminated ZnO(0001)-O surface at 130 K have been characterized using x-ray photoemission spectroscopy (XPS), ${\mathrm{He}}^{+}$-ion-scattering spectroscopy, low-energy electron diffraction, work-function, and band-bending measurements, angular-resolved XPS, and CO and ${\mathrm{H}}_{2}$O chemisorption. These results show that Cu is cationic at tiny coverages, but becomes nearly neutral at coverages beyond a few percent. The Cu clusters into two-dimensional (2D) metallic islands at these coverages. Further deposition of Cu leads to spreading of these 2D islands without forming thicker layers, until about 50% of the surface is covered. Thereafter, these Cu islands grow thicker without filling the gaps between the islands except at a rate much slower than the rate at which Cu is deposited into these clean spaces. The annealing behavior of these films has also been studied between 130 and 850 K. These results show that the Cu has a tendency to cluster into thick 3D islands which only cover a small fraction of the surface. We present a model here based on the energetics of the system which readily explains the apparent contradiction between this tendency for 3D clustering, and the dynamical effect which leads to efficient wetting for coverages up to 1/2 monolayer at low temperatures. This model shows that a large fraction of the surface can first be covered by a 2D film even when the metal's self-adsorption energy significantly exceeds its adsorption energy on the oxide, provided the difference in these energies does not exceed the energy of 2D evaporation from kinks onto terraces. This model helps to explain a variety of confusing results in the growth of metal films on oxide surfaces.