A strong polarizing field in thin-film paraelectrics

TL;DR

This paper reveals that a strong polarizing field can induce a non-centrosymmetric, polar state in thin films of paraelectric materials, demonstrated through density functional theory on KTaO₃, with practical guidelines for device engineering.

Contribution

It introduces the concept of a strong polarizing field in thin-film paraelectrics, enabling the induction of polar states in centrosymmetric materials, supported by a simple electrostatic model and computational evidence.

Findings

A strong polarizing field can induce polarity in paraelectric thin films.

Density functional calculations confirm the effect in KTaO₃.

A simple recipe and electrostatic model predict the critical thickness for polarization.

Abstract

The surface charge associated with the spontaneous polarization in ferroelectrics is well known to cause a depolarizing field that can be particularly detrimental in the thin-film geometry desirable for microelectronic devices. Incomplete screening of the surface charge, for example by metallic electrodes or surface adsorbates, can lead to the formation of domains, suppression or reorientation of the polarization, or even stabilization of a higher energy non-polar phase . A huge amount of research and development effort has been invested in understanding the depolarizing behavior and minimizing its unfavorable effects. Here we demonstrate the opposite behavior: A strong polarizing field that drives thin films of materials that are centrosymmetric and paraelectric in their bulk form into a non-centrosymmetric, polar state. We illustrate the behavior using density functional computations for perovskite-structure potassium tantalate, KTaO$_3$, which is of considerable interest for its high dielectric constant, proximity to a quantum critical point and superconductivity. We then provide a simple recipe to identify whether a particular material and film orientation will exhibit the effect, and develop an electrostatic model to estimate the critical thickness of the induced polarization in terms of well-known material parameters. Our results provide practical guidelines for exploiting the electrostatic properties of thin-film ionic insulators to engineer novel functionalities for nanoscale devices.