Influence of strain on microwave dielectric properties of (Ba,Sr)TiO3 thin films

Epitaxial Ba1−xSrxTiO3 (BST) thin films have been deposited onto (100)MgO and LaAlO3 substrates using pulsed-laser deposition. Thick (>1 μm) Ag interdigitated capacitors capped with a thin protective layer of Au have been deposited on top of the BST films using electron-beam deposition. The capacitance (C) and dielectric quality factor (Q=1/tan δ) of the structure has been measured at microwave frequencies (1–20 GHz) as a function of electric field (E⩽67 kV/cm) at room temperature. In epitaxial BST films, either high dielectric tuning (4:1), which is defined as {[C(0)−C(E)]/C(0)}×100, or high dielectric Q (∼100–250) was observed but not both at the same time. Film strain was observed by x-ray diffraction and is closely related to the dielectric properties as limiting the ability to obtain both high tuning and high dielectric 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.