Grain boundaries in barium strontium titanate thin films: Structure, chemistry and influence on electronic properties

In this paper, we investigate the role of grain boundaries in polycrystalline (BaxSr1-x)Ti1+yO3+z films, grown by metal organic vapor deposition, in the accommodation of nonstoichiometry, as well as their role in the strong composition dependence of the electric and dielectric behavior observed in these films. High-spatial resolution electron energy-loss spectroscopy is used for the analysis of composition and structural changes at grain boundaries, as a function of film composition. The existence of amorphous, titanium rich, TiO2-like phases at the grain boundaries of films with large amounts of excess Ti (y greater than or equal to 0.08) may explain the non-monotonic resistance degradation behavior of the films as a function of Ti content. However, we show that a grain boundary phase model fails to explain the strong composition dependence of the dielectric behavior. Electron energy-loss spectra indicate a distortion of the Ti-O octahedra in the grain interiors in samples with increasing Ti excess. The decrease of the dielectric constant with increasing amounts of excess Ti is therefore more likely due to Ti accommodation in the grain interiors.