Pyroelectric Properties of Epitaxial (001) Barium Strontium Titanate as a Function of Space Charge Density

Abstract

A non-linear thermodynamic model was developed to investigate the effect of charge compensation on polarization and pyroelectric properties of graded monodomain ferroelectric multilayers. To represent charge compensation at the interfaces, a dimensionless parameter, "coupling strength," was introduced. Heterostructures in which interfaces are perfectly insulating so that coupling strength is maximized behave as if they are a single ferroelectric material. Upon introduction of localized space charges to the interfaces, the coupling strength is decreased and ferroelectric layers behave independently of each other. Also, there exists an optimum value of the coupling strength at which average polarization of the heterostructure is maximized.

Validity of Nonlinear Thermo dynamic Models in Ferroelectric-Paraelectric Bilayers and Superlattices

“…[12] (b) Evolution of ferroelectricity should follow the variation in coupling strength between different layers. [17,18] (c) The spontaneous polarization in the PE layer derived by thermodynamic theory should be consistent with the electricfield-induced polarization, which can be obtained from the relation between the electric field and dielectric constant. [19] We basically consider an FE-PE bilayer between electrodes on a substrate which is much thicker than the bilayer under short-circuit electrical boundary conditions.…”

“…From a physical point of view, it is necessary to consider their results seriously. In our previous study, we found that treating coupling strength as an adjustable parameter in some phenomenological models [17,18] to describe the electrostatic coupling with interfacial space charge is inappropriate. [19] In this study, we examine the validity of their models [9−15] regardless of the forgoing important physical problems.…”

“…[19] We basically consider an FE-PE bilayer between electrodes on a substrate which is much thicker than the bilayer under short-circuit electrical boundary conditions. Just like other authors, [13][14][15]17,18] we simplify the charged defects and assume a constant interfacial space charge density at the interface of different layers, denoted as 𝜎. We restrict ourselves to the case of a bilayer film with a polar axis perpendicular to the film plane, where 𝑃 1 (𝑃 2 ) is the polarization and 𝐸 1 (𝐸 2 ) is the total electric field along the polar axis in the PE (FE) layer.…”

“…We restrict ourselves to the case of a bilayer film with a polar axis perpendicular to the film plane, where 𝑃 1 (𝑃 2 ) is the polarization and 𝐸 1 (𝐸 2 ) is the total electric field along the polar axis in the PE (FE) layer. The equations of state for 𝐸 𝑖 in a bilayer can be obtained through Maxwell's equations at the interlayer interfaces: [9,10,[13][14][15]17,18]…”

“…( 1) and ( 2) are the strength of 𝐸 ext in layer 𝑖. [15] The second terms correspond to the internal electric field (called the depolarization field [13,14,17,18] ) in each layer. According to Maxwell's equations, the polarization jump at the FE-PE interface is the origin of the bound charge, e.g., 𝑃 2 − 𝑃 1 = −𝜎 𝑃 , where 𝜎 𝑃 is surface bound charge density.…”