The effect of strain and defects on the dielectric properties of barium strontium titanate thin films for tunable microwave applications

Ferroelectric materials enjoy a large nonlinearity in their dielectric response, which gives rise to an electric field dependent permittivity. This feature makes them particularly attractive for use as frequency-agile microwave electronic components, including phase shifters, varactors, tunable filters and antennas. In this area, (Ba,Sr)TiO₃ (BST)-based ceramic thin films are considered by many as the forerunners for room temperature (RT) applications. However, compared to bulk BST samples, the dielectric constant and nonlinearity of epitaxial thin films are markedly worse. Strain and defects in the film are generally recognized as two major reasons for this degradation of dielectric properties. However, their exact role in this degradation is still not clear.; In this study, a series of heteroepitaxial BST thin films of thicknesses varying from 8 nm to 500 nm were prepared on LSAT and MgO substrates to produce films with systematically varying in-plane stresses. The in-plane dielectric constant ϵ₁₁ was found to increase with increasing tensile stress and decrease with compressive stress. The relationship between dielectric properties and stress agrees very well with our theoretical calculations. We also derived the relationship between the dielectric constant and electric field using an extended thermodynamic model. In addition, we define tunability as $63116Emax$ and the experimental data is analyzed using this new definition.; It is observed by many research groups that annealing leads to partial recovery in the dielectric properties of BST films. Therefore, in this study, carefully designed annealing experiments were performed in different types of gas ambient to study the effect of defects on the dielectric properties of BST films. Extensive x-ray and transmission electron microscopy studies were carried out to study the structural evolution of the BST films upon annealing. We report evidence showing that oxygen vacancies (0-D defect) do not play an important role in the recovery of the dielectric properties of BST thin films. In contrast, it is the decrease of defects such as dislocations (1-D defects) as well as antiphase domain boundaries and small angle grain boundaries (2-D defects) and the relief of local strain associated with these defects that dominate the dielectric recovery process.