Dielectric properties of (Ba,Sr)TiO3 thin films for tunable microwave applications

Epitaxial Ba1-xSrxTiO3 (BST) thin films have been deposited onto (100) MgO and LaAlO3 substrates using pulsed laser deposition. Thick (>1 mu m) interdigitated capacitors have been deposited on top of the BST films. The capacitance and dielectric Q (1/tan delta) of the structure has been measured at microwave frequencies (1-20 GHz) as a function of electric field (0-67 kV/cm) at room temperature. In epitaxial BST films, either high dielectric tuning (4:1) or high dielectric Q (similar to 100-250) was observed but not both at the same time. Film strain was measured using X-ray diffraction and is closely related to the dielectric properties as limiting the ability to obtain both high tuning and high Q in epitaxial BST thin films. A thin BST buffer layer was used to relieve the strain in the films. In strain-relieved films, both dielectric tuning and dielectric Q were increased after annealing. A theoretical analysis of the strain effect of the films is presented based on Devonshire thermodynamic theory.