Composition-spread epitaxial ferroelectric thin films for temperature-insensitive functional devices

Abstract

Ba<sub><i>x</i></sub>Sr<sub>1–<i>x</i></sub>TiO<sub>3</sub> (BST) ferroelectric thin films are widely used in microwave tunable devices due to their high dielectric constants, strong electric field tunabilities and low microwave losses. However, because of the temperature dependence of dielectric constant in ferroelectric material, the high-tunability for conventional single component ferroelectric thin film can only be achieved in the vicinity of Curie Temperature (<i>T</i><sub>C</sub>) which leads the ferroelectric thin films to be difficult to operate in a wide temperature range. To obtain ferroelectric thin films for temperature stable functional devices, single composition Ba<sub>0.2</sub>Sr<sub>0.8</sub>TiO<sub>3</sub> thin films, Ba<sub>0.5</sub>Sr<sub>0.5</sub>TiO<sub>3</sub> thin films, and Ba<sub>0.2</sub>Sr<sub>0.8</sub>TiO<sub>3</sub>/Ba<sub>0.5</sub>Sr<sub>0.5</sub>TiO<sub>3</sub> heterostructure thin films are deposited by pulsed laser deposition (PLD). By comparing their dielectric properties in a wide temperature range, it is found that the temperature sensitivity of BST film can be effectively reduced by introducing a composition gradient along the epitaxial direction. However, the heterostructure engineering may bring extra troubles caused by interfaces, which may limit the quality factor <i>Q</i>. In this paper, we extend our combinatorial film deposition technique to ferroelectric materials, and we successfully fabricate in-plane composition-spread Ba<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>TiO<sub>3</sub> thin films, which are expected to broaden the phase transition temperature ranges of BST films while avoiding the problem of interface control.