The effect of stress on the microwave dielectric properties of Ba0.5Sr0.5TiO3 thin films

Single phase, (1 0 0) epitaxial Ba0.5Sr0.5TiO3 (BST) films have been deposited onto (1 0 0) LaAlO3 and MgO substrates by pulsed laser deposition (PLD). The capacitance and dielectric losses of as-deposited and annealed films have been measured from 1-20 GHz as a function of electric field (0-80 kV/cm) at room temperature. The dielectric properties are strongly affected by the substrate type, post-deposition annealing time (less than or equal to 6 h) and temperature (less than or equal to 1200 degrees C). For epitaxial BST films deposited onto MgO, it is observed that, after a post- deposition anneal the dielectric constant and the dielectric loss decreases. For epitaxial BST films deposited onto LAO, a post-deposition anneal (less than or equal to 1000 degrees C) results in an increase in the dielectric constant and an increase in the dielectric loss. The dc electric field induced change in the dielectric constant tends to increase with the dielectric constant and is largest for as-deposited films on MgO and post-deposited annealed films on LAO. In general, for epitaxial BST films, a large electric field effect is observed in films that have a large dielectric loss and a small electric field effect in films that have a low dielectric loss. High resolution X-ray diffraction measurements indicate that deposited film exhibit a significant tetragonal distortion which is strongly affected by a by a post deposition anneal. The observed differences in dielectric properties of the epitaxial BST films on MgO and LAO are attributed to the differences in film stress which arise as a consequence of the lattice mismatch between the film and the substrate and the differences in the thermal coefficient of expansion between the film and the substrate. A thin amorphous buffer layer of BST has been used to relieve stress induced by the lattice mismatch between the film and the substrate. Unlike epitaxial films, stress relieved films do not show an inverse relationship between dielectric tuning and Q (1/tan delta) and may be superior materials for tunable microwave devices.