Predicting dielectric tunability of compositionally-graded barium strontium titanate thin films on metal substrates by thermodynamic modeling

Abstract Dielectric properties of compositionally-graded barium strontium titanate (BST) thin films respectively on Ti and stainless steel (SS) plates were computed using a modified thermodynamic model. Calculated results predict that higher dielectric constant and tunability can be obtained when the films are onto Ti substrates, which have smaller thermal expansion coefficient (TEC). For Ti substrates, “up-graded” films (Ba/Sr ratio increases from substrate to surface) exhibit higher tunability than “down-graded” films; whereas “up-graded” films on SS plates have relatively lower tunability. The received larger dielectric constant and tunability are because of smaller total out-of-plane polarization, which is resulting from smaller total compressive strain. The calculated dielectric constant and tunability of such graded films are in accordance with those of the films fabricated via sol-gel approaches, indicating that the dielectric behavior of ferroelectric films can be adjusted by designing compositional gradient.