Microwave dielectric properties of strained (Ba0.4Sr0.6)TiO3 thin films

Abstract

Ferroelectric Ba0.4Sr0.6TiO3 (BST) thin films grown on (001) MgO by pulsed laser deposition show a strong correlation between their structure and their microwave dielectric properties. Epitaxially grown BST films are observed by x-ray diffraction to be tetragonally distorted. The oxygen deposition pressure affects the magnitude of the tetragonal distortion (the ratio of in-plane and surface normal lattice parameters, D=a/c) of the deposited BST films. D varied from 0.996 to 1.004 at oxygen deposition pressure of 10–800 mTorr. The dielectric properties of BST films measured at microwave frequencies (1–20 GHz) exhibit an oxygen deposition pressure dependent dielectric constant (ε=100–600), and quality factor Q (1/tan δ=10–60). The BST film grown at the oxygen deposition pressure of 200 mTorr exhibits the highest figure of merit [% tuning in ε×Q0V, where % tuning is 100×(ε0−εb)/ε0, and ε0 and εb are dielectric constant at 0 and 80 kV/cm]. This corresponds to the film with the lowest distortion (D=1.001). The observed microwave properties of the films are explained by a phenomenological thermodynamic theory based on the strain along in-plane direction of the films.