Thermal Stresses in Ferroelectric Thin Films and Their Role on the Dielectric, Pyroelectric, and Electrocaloric Properties

In-plane strains develop in thin films due to thermal stresses that arise from differences between the thermal expansion coefficients of the film and the substrate upon cooling from a growth temperature ($T_{G})$ to room temperature (RT). In ferroelectric (FE) films, there is a coupling between strain and polarization through electrostriction. Therefore, thermal strains may have profound effects on the dielectric, pyroelectric, and electrocaloric (EC) responses of FE films. We provide here a quantitative thermodynamic model to investigate the role of in-plane thermal strains on these properties. We show that there is a substantial degradation in the dielectric response and tunability of SrTiO$_{3}$ films on IC-friendly substrates such as Si and $c$-sapphire due to tensile thermal in-plane strains. Our analysis on (001)-textured polycrystalline Ba$_{x}$Sr$_{1-x}$O$_{3}$ (BST) films with different compositions indicates that for BST 60/40 and BST 70/30 films, the pyroelectric response does not display a significant dependence on$ T_{G}$ if the FE is in a paraelectric state. Furthermore, we show that for BaTiO$_{3}$ films on Si, the thermal stresses are sufficient to shift the zero-field Curie temperature to RT, resulting in a strong enhancement of the EC properties as compared to the bulk material.