Influence of the Grain Size on the Conduction Mechanism of Barium Strontium Titanate Thin Films

Sol-gel barium strontium titanate (Ba_0.6Sr_0.4TiO₃) thin films with different grain sizes have been successfully fabricated as metal–insulator–metal (MIM) capacitors. The perovskite structure of the material has been confirmed via X-ray diffraction (XRD). In order to correlate the effect of the grain size to the conduction mechanisms of these films, atomic force microscopy (AFM) results are presented. The grain size shows an important effect on the conduction mechanism for the films. The results show that as the grain size increases, both the impedance and the permittivity of the films decrease, whereas the conductivity shows an inverse variation. The Z^* plane for all films shows two regions, corresponding to the bulk mechanism and the distribution of the grain boundaries–electrodes conduction process. M′′ versus frequency plots reveal non-Debye relaxation peaks, which are not able to be observed in the ε′′ plots. Alternating current (AC) conductivity versus frequency plots show three regions of conduction processes, i.e. a low-frequency region due to direct current (DC) conduction, a mid-frequency region due to translational hopping motion, and a high-frequency region due to localized hopping and/or reorientational motion.