Structural changes induced by metal electrode layers on ultrathin BaTiO3 films

We have investigated the effect of evaporated gold electrodes on the structure of thin BaTiO3 films grown epitaxially on a SrTiO3 substrate. Two films, i.e., one five unit cell and the other ten unit cell thick, were studied. X-ray diffraction measurements analyzed by the coherent Bragg rod analysis (COBRA) method and diffuse x-ray scans show that the thinner film is pseudomorphic with respect to the substrate and very highly ordered, while the thicker one exhibits significant disorder and local discommensuration. The results further show that the ten unit cell film has an island morphology. The evaporation of 30 nm of gold had little effect on the thinner film, but it caused interesting and unexpected structural changes in the ten unit cell sample. In that case, the gold film drove the BaTiO3 film closer to a pseudomorphic state and reduced the amount of disorder in the film. We attribute this behavior to surface tension forces applied by the gold to the BaTiO3 structure. These results suggest a rather general conclusion, namely, that if the film and the evaporated electrode are smooth, the evaporated electrode has very little effect on the underlying structure. In contrast, if the film has an island-type morphology, the evaporated electrode exerts compressive stress on the islands.