Study of barium strontium titanate (BST) thin films and BST/P(VDF-TrFE) 0--3 composites

Fabrication, characterization and modification of barium strontium titanate (BST) thin films and BST/poly(vinylidene fluoride-trifluoroethylene) {lcub}P(VDF-TrFE){rcub} composites were studied. BST thin films possess high dielectric permittivity, low dielectric loss and field dependent dielectric permittivity which can be used for DRAM capacitor and tunable high frequency applications. BST/P(VDF-TrFE) composites with high dielectric permittivity and high breakdown strength are ideal for capacitor and energy storage applications. Fabrication and characterization of Ba_xSr_1−xTiO₃ thin films of compositions x = 0.5, 0.6, 0.7, 0.8, 0.9 and 1 deposited on Pt/Ti/SiO₂/Si substrates were studied. A modified sol-gel technique based on the low concentration precursor solution and layer by layer annealing was used for the thin film fabrication. Thin films were fabricated by spin coating on clean Pt/Ti/SiO ₂/Si substrates. A clear improvement of grain growth and crystallinity in BST thin films were achieved by the use of modified sol-gel method. The crystalline structure, microstructure, dielectric and ferroelectric properties of thin films of different compositions were analysed. Highest relative permittivity and dielectric tunability was observed in the Ba_0.7Sr_0.3TiO ₃ thin film. The behaviours of dielectric permittivity and tunability as a function of composition were analysed using Devonshire phenomenological theory. Only four compositions, namely Ba_0.7Sr_0.3TiO ₃ Ba_0.8Sr_0.2TiO₃, Ba_0.9Sr _0.1TiO₃ and BaTiO₃ showed polarization-electric field (P-E) hysteresis loops at room temperature. BST compositions, which are paraelectric at room temperature, were studied for high frequency dielectric relaxation and tunability. Parallel plate thin film capacitor structure with Au/Cr top electrodes and Pt/Ti/SiO₂/Si substrates were subjected to high frequency measurements. The extrinsic effects originated due to the contact resistance and finite sheet resistance of electrodes were modelled using an equivalent circuit. Effective dielectric permittivity of Ba _0.5Sr_0.5TiO₃ and Ba_0.6Sr_0.4TiO ₃ bulk ceramics at microwave frequencies were determined by the microstrip ring resonator method. Modifications of the structure and properties of BST thin films were investigated by fabricating thin films with different compositional gradients. Rutherford backscattering spectroscopy (RBS) was used to characterize the compositional gradient of the thin films fabricated on Si and Pt/Ti/SiO ₂/Si substrates. The graded structure can be considered as a stack of Ba_xSr_1−xTiO₃ capacitors connected in series. Therefore, the effects of Curie transition peaks on dielectric properties are removed. (Abstract shortened by UMI.)...