Effects of very thin strain layers on dielectric properties of epitaxial Ba0.6Sr0.4TiO3 films

We have epitaxially grown ${\mathrm{Ba}}_{\mathrm{0.6}}{\mathrm{Sr}}_{\mathrm{0.4}}{\mathrm{TiO}}_{\mathrm{3}}\hspace{0.5em}\mathrm{(BST-0.4)}$ thin films on MgO(001) substrates. By inserting a very thin ${\mathrm{Ba}}_{\text{1−x}}{\mathrm{Sr}}_x{\mathrm{TiO}}_{\mathrm{3}}$ $\text{(x=0.1–0.7)}$ interlayer between the MgO substrate and the main layer of BST-0.4, we are able to manipulate the degree of the stress in BST-0.4 films. We have controlled the stress states, i.e., the lattice distortion ratio $\mathrm{(D=}\mathrm{in-plane}$ lattice constant/out-of-plane lattice constant) of the BST-0.4 films by varying the chemical composition of the interlayers. We have found that small variations of $D$ value can result in significantly large changes of dielectric properties. A BST-0.4 film under small tensile stress, which has a $D$ value of 1.0023, shows the largest dielectric permittivity and tunability.