Abstract text (incl. figure legends and references)

Metal/oxide heterointerfaces are omnipresent in functional materials and their microstructure often determines macroscopic properties. For ceramics with a wurtzit lattice like zinc oxide the surface polarity plays a major role, because it can change electronic properties [1] and influence the properties of deposited metal films [2].

In this study, the epitaxial and microstructural evolution of thin gold films on (0001)/(000-1) oriented ZnO substrates is investigated. For this, Au thin films with a thickness of 40 nm are deposited directly onto the (0001) Zn-ZnO and (000-1) O-ZnO surfaces by electron beam evaporation. Annealing of the samples at elevated temperatures was carried out with an rapid thermal annealing (RTA) furnace. During heating a constant nitrogen flow is introduced, whereas a fast cooling rate of ~200 °C/s is achieved by dropping the sample on a water-cooled steel plate. SEM imaging and EBSD measurements reveal that both epitaxial grain growth (Figure 1) and solid-state-dewetting of the film highly depend on the polarity of the ZnO surface.

From the as-deposited state up to temperatures 600°C fine grains with a predominant orientation of Au(111)[110]||ZnO(0001)[11-20] (OR2) are present on both ZnO terminations. EBSD imaging shows, that during heating at 800 °C and above large grains form in the Au film on the Zn terminated substrate. These grains have an Au(111)[110]||ZnO(0001)[10-10] orientation (OR1) and exhibit a higher thermal stability. In contrast, grain growth and solid-state dewetting of the Au film are retarded and accelerated on the oxygen terminated surface, respectively. As a result, a fiber texture with both OR1 and OR2 evolves. The different dewetting rates can be explained by a larger binding energy of Au to the zinc terminated surface [3], which lowers the Au/Zn-ZnO interface energy and makes it more resistant to dewetting than the weakly bound oxygen side. High-resolution STEM imaging was carried out to investigate the Au ZnO interface. For Au on O-ZnO a semi-coherent interface with misfit dislocations is present (Figure 2a). For the epitaxial Au/Zn-ZnO, interface reconstruction (Figure 2c) with a regular spacing is observed. This reconstruction of the Zn and Au atoms close to the interface likely reduces the interface energy of OR1, thus facilitating the transformation from OR2 to OR1.

[1] Rahman, Faiz. "Zinc oxide light-emitting diodes: a review." Optical Engineering 58.1 (2019)

[2] Dulub, Olga, Matthias Batzill, and Ulrike Diebold. "Growth of copper on single crystalline ZnO: surface study of a model catalyst." Topics in Catalysis 36.1 (2005): 65-76.

[3] Shuyi Wei, Zhiguo Wang, Zongxian Yang, "First-principles studies on the Au surfactant on polar ZnO surfaces", Physics Letters A, Volume 363, Issue 4, (2007)