GROWTH-MODEL FOR METAL-FILMS ON OXIDE SURFACES - CU ON ZNO(0001)-O

The structural and electronic properties of Cu films vapor deposited o n the oxygen terminated ZnO(0001)-O surface at 130 K have been charact erized using x-ray photoemission spectroscopy (XPS), He+-ion-scatterin g spectroscopy, low-energy electron diffraction work-function and band -bending measurements, angular-resolved XPS, and CO and H2O chemisorpt ion. These results show that Cu is cationic at tiny coverages, but bec omes nearly neutral at coverages beyond a few percent. The Cu clusters into two-dimensional (2D) metallic islands at these coverages. Furthe r deposition of Cu leads to spreading of these 2D islands without form ing thicker layers, until about 50% of the surface is covered. Thereaf ter, these Cu islands grow thicker without filling the gaps between th e islands except at a rate much slower than the rate at which Cu is de posited 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 cove r a small fraction of the surface. We present a model here based on th e energetics of the system which readily explains the apparent contrad iction between this tendency for 3D clustering, and the dynamical effe ct which leads to efficient wetting for coverages up to 1/2 monolayer at low temperatures. This model shows that a large fraction of the sur face can first be covered by a 2D film even when the metal's self-adso rption 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 expla in a variety of confusing results in the growth of metal films on oxid e surfaces.