Effect of the amorphous-to-crystalline transition in Ba0.5Sr0.5TiO3 thin films on optical and microwave dielectric properties

Thin films of Ba0.5Sr0.5TiO3 were deposited at ambient temperature on fused silica substrates by RF magnetron sputtering followed by post-deposition annealing at temperatures between 400 and 1000 degrees C. The post-deposition annealing resulted in an amorphous-crystalline phase transition in the films. The amorphous-crystalline transition is accompanied by an increase in the refractive index, a decrease in the optical band gap and an increase in the microwave dielectric constant. At 800 degrees C the films exhibit a refractive index of 2.21 at 550 nm, an optical band gap of 3.49 eV and a microwave dielectric constant of 265, all of which indicate the crystalline nature of the films. The films lose oxygen when annealed at high temperatures as evidenced by the optical and microwave dielectric behaviour. The observed behaviour is explained in terms of defect centres created in the form of oxygen vacancies due to the annealing process and the structural phase transition.