Polycrystalline Ba0.6Sr0.4TiO3 thin films on r-plane sapphire: Effect of film thickness on strain and dielectric properties

Polycrystalline Ba0.6 Sr0.4 Ti O3 (BST) films grown on r -plane sapphire exhibit strong variation of in-plane strain over the thickness range of 25-400 nm. At a critical thickness of ~200 nm, the films are strain relieved; in thinner films, the strain is tensile, while compressive strain was observed in the 400 nm film. Microwave properties of the films were measured from 1 to 20 GHz by the interdigital capacitor method. A capacitance tunability of 64% was observed in the 200 nm film, while thinner films showed improved Q factor. These results demonstrate the possibility of incorporating frequency agile BST-based devices into the silicon on sapphire process.