Optimization of the tunability of barium strontium titanate films via epitaxial stresses

The tunability of epitaxial barium strontium titanate films is analyzed theoretically using a phenomenological model. The relative dielectric constant of Ba0.5Sr0.5TiO3 (BST 50/50) films as a function of the applied external electric field is calculated and an electric field-misfit strain phase diagram is developed to assist in the interpretation of the behavior. On the basis of these results, the tunability of BST 50/50 films as a function of the misfit strain is provided and compared with the experimental data in the literature. Analysis shows that a high tunability can be achieved by adjusting the misfit strain especially in the vicinity of a structural phase transformation. The misfit strain in epitaxial films can be controlled with the selection of a substrate material or variations in the film thickness. The film thickness dependence is due to misfit dislocation formation at the film growth temperature. A critical thickness to attain the maximum tunability can be defined for BST 50/50 films on MgO (similar to90 nm) and LaAlO3 (similar to120 nm) substrates. It is suggested that the selection of the substrate and/or the film thickness can be chosen as design parameters to manipulate the strain state in the film to achieve optimum tunability. (C) 2003 American Institute of Physics.