A review of the properties of ferroelectric materials that are relevant to microwave tunable devices is presented: we discuss the theory of dielectric response of tunable bulk materials and thin films; the experimental results from the literature and from own work are reviewed; the correspondence between the theoretical results and the measured properties of tunable materials is critically analyzed; nominally pure, real (defected), and composite bulk materials and thin films are addressed. In addition, techniques for characterization of tunable ferroelectrics and applications of these materials are briefly presented.

Ferroelectric materials for microwave tunable applications

Highly oriented Ba07Sr03TiO3 thin films were grown on MgO (001) single-crystal substrate using pulsed-laser deposition and the in-plane ferroelectric properties of the film were evaluated X-ray diffraction characterization revealed a good crystallinity and tensile in-plane stress in the film A well-defined ferroelectric hysteresis loop with Pr=95μC∕cm2 was observed along the (100) direction, which implied an enhanced in-plane ferroelectricity in the Ba07Sr03TiO3 thin film in comparison with the Ba07Sr03TiO3 ceramics Curie temperature of the film was found to be ∼88°C, which is nearly 50°C higher than that of the Ba07Sr03TiO3 ceramics The butterfly-shaped C‐V characteristic curve also evidenced the enhanced in-plane ferroelectric property in the film, and a large dielectric tunability of 69% was found at 1MHz under a moderate dc bias field The observation of enhanced ferroelectricity along the in-plane direction in the highly oriented Ba07Sr03TiO3 thin film was explained in terms of the inc

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Enhanced in-plane ferroelectricity in Ba0.7Sr0.3TiO3 thin films grown on MgO (001) single-crystal substrate

https://iopscience.iop.org/article/10.1149/2.0151807jes/pdf

A FIB-STEM study of strontium segregation and interface formation of directly assembled La0.6Sr0.4Co0.2Fe0.8O3-δ Cathode on Y2O3-ZrO2 electrolyte of solid oxide fuel cells

Epitaxial Ba0.6Sr0.4TiO3 films were grown onto (001) LaAlO3 by pulsed-laser deposition, and the dislocation structures of the films were investigated using transmission electron microscopy. Misfit dislocations with a periodicity of about 7 nm and Burgers vectors b=a〈100〉 were observed at the interface. A high density of threading dislocations was present the films and these also had b=a〈100〉. The data indicate that the threading dislocations are not generated as the result of half-loop climb from the deposit surface as proposed previously, but are instead formed when misfit dislocations are forced away from the interface during island coalescence.

Threading dislocation generation in epitaxial (Ba,Sr) TiO3 films grown on (001) LaAlO3 by pulsed laser deposition

Interface effects have been found to play a key role in controlling the epitaxial nature and physical properties on the highly epitaxial ferroelectric thin films. Thin film ferroelectrics are dominantly affected by the strains induced by lattice misfits between the films and the substrates, surface step terrace, both step height and terrace dimension, and the surface terminations. The natures of interface induced local strain formations, edge dislocations, and antiphase domain boundaries are reviewed in this article.

Interface effects on highly epitaxial ferroelectric thin films

Transmission electron microscopy (TEM) of a perovskite Ba0.5Sr0.5TiO3 thin film, grown on a (001) LaAlO3 substrate by pulsed-laser ablation, reveals that the film of single-crystal quality has an unusually distorted lattice with lattice parameters a and b (parallel to the interface) larger than c (perpendicular to the interface) by 1.4%. There is evidence that the as-examined Ba0.5Sr0.5TiO3 film is a variant of its high-temperature cubic phase due to its anisotropic thermal contraction during cooling. A large lattice mismatch value of 5.7% (to be compared to the normal value of 4.13%) is observed from selected area electron diffraction patterns and high-resolution TEM images of cross-sectional specimens, which suggests that the growing high-temperature film under the film growth condition may have a larger lattice constant and a different thermal expansion behavior with respect to the bulk material.

C. L. Lia

Papers

Unusual lattice distortion in a Ba0.5Sr0.5TiO3 thin film on a LaAlO3 substrate