On the persistence of polar domains in ultrathin ferroelectric capacitors

TL;DR

This study demonstrates that ultrathin BaTiO₃ ferroelectric capacitors maintain ferroelectric polarization at nanometer scales, but domain wall dynamics significantly influence their electrical properties and stability, even with high screening electrodes.

Contribution

The paper provides conclusive experimental evidence of ferroelectricity in ultrathin BaTiO₃ capacitors and highlights the critical role of domain wall motion in their electrical behavior.

Findings

Ferroelectric polarization persists in ultrathin BaTiO₃ films.

High screening electrodes are insufficient to stabilize polarization at room temperature.

Domain wall motion impacts the dielectric response and stability.

Abstract

The instability of ferroelectric ordering in ultra-thin films is one of the most important fundamental issues pertaining realization of a number of electronic devices with enhanced functionality, such as ferroelectric and multiferroic tunnel junctions or ferroelectric field effect transistors. In this paper, we investigate the polarization state of archetypal ultrathin (several nanometres) ferroelectric heterostructures: epitaxial single-crystalline BaTiO$_3$ films sandwiched between the most habitual perovskite electrodes, SrRuO$_3$, on top of the most used perovskite substrate, SrTiO$_3$. We use a combination of piezoresponse force microscopy, dielectric measurements and structural characterization to provide conclusive evidence for the ferroelectric nature of the relaxed polarization state in ultrathin BaTiO$_3$ capacitors. We show that even the high screening efficiency of SrRuO$_3$ electrodes is still insufficient to stabilize polarization in SrRuO$_3$/BaTiO$_3$/SrRuO$_3$ heterostructures at room temperature. We identify the key role of domain wall motion in determining the macroscopic electrical properties of ultrathin capacitors and discuss their dielectric response in the light of the recent interest in negative capacitance behaviour.