Structural and microwave properties of (Ba, Sr)TiO3 films grown by pulsed laser deposition

The relationship between the structure and the microwave dielectric propert ies of epitaxial Ba0.5Sr0.5TiO3 (BST) films has been investigated. Single-p hase BST films (40-160 nm) have been deposited onto (100) MgO substrates by pulsed laser deposition. As-deposited films show a significant tetragonal distortion. The in-plane lattice parameters (a) are always larger than the surface normal lattice parameters (c). The tetragonal distortion depends on the thickness of the films and the post-deposition annealing conditions. F ilms annealed at 900 degrees C show less tetragonal distortion than the as- deposited film and the films annealed at higher temperatures. The distortio n in the film is due to stress caused by the lattice mismatch and thermal e xpansion coefficient differences between the film and the substrate. The di electric constant and its change with de bias voltage of BST films on MgO a t microwave frequencies increase with increasing annealing temperature from 900 degrees C to 1200 degrees C, which corresponds to an increase in the t etragonal distortion.