Influence of Substrate Polarity on Thermal Stability, Grain Growth and Atomic Interface Structure of Au Thin Films on ZnO Surfaces

The influence of the polarity of ceramic substrates on the structural evolution of thin metal films during annealing at elevated temperatures is investigated, including the competing processes of solid state dewetting (SSD) and grain growth, as well as the atomic structure of the epitaxial interface. For this purpose, Au thin films on polar O-ZnO$\left(000\overline{1}\right)$$\left(000\overline{1}\right)$ and Zn-ZnO$\left(0001\right)$$\left(0001\right)$ surfaces are annealed at elevated temperatures and times. Whereas SSD dominates on the O-terminated surface, pronounced grain growth is observed on the Zn-terminated surface. The texture analysis revealed that up to 600°C, both samples exhibit a fiber texture with slightly dominating Au(111)[110] || ZnO(0001)[$11\overline{2}0$$11\overline{2}0$] orientation relation (OR 2). At 800°C, Au on Zn-ZnO exhibits a transformation to a mazed bicrystal structure with Au(111)[110] || ZnO(0001)[$10\overline{1}0$$10\overline{1}0$] orientation relation (OR 1). Comparison of various interface structures in density-functional theory (DFT) indicates that atomically sharp interfaces between the Au(111) films and the ideal bulk-truncated polar ZnO surfaces are energetically favored for both substrate polarities in excellent agreement with atomically resolved electron microscopy. Due to the larger period of the coincidence site lattice in OR 2, the corresponding interface can be described as semi-coherent with clearly separated misfit dislocations. In contrast, the much smaller period of the (approximate) coincidence site lattice in OR 1 leads to a largely incoherent interface with local reconstructions. However, in the experimental situation, even a small rotational deviation from the perfect OR 1 can introduce an interfacial screw dislocation network superimposed on the incoherent interface structure, effectively making the interface semi-coherent.