Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit

H Wang,T Venkatesan,H Tian,Z R Liu,P Yang,G M Chow,E Y Tsymbal,R Guo,W N Lin,J X Xiao,H Y Yoong,J S Chen,T R Paudel,J Wang

doi:10.1038/s41467-018-05662-y

H Wang, T Venkatesan + Show 12 more

Open Access

https://doi.org/10.1038/s41467-018-05662-y

Copy DOI

Abstract

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Direct visualization of stable ferroelectric polarization and its switching behavior in atomically thick films is critical for achieving this goal. Here, ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unit-cell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Based on first-principles calculations and Kelvin probe force microscopy measurements, we explain the mechanism of polarization stabilization by the ionic displacements in oxide electrode and the surface charges. Our results indicate that critical thickness for ferroelectricity in the BiFeO3 film is virtually absent, making it a promising candidate for high-density nonvolatile memories.

Highlights

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices
Robust in-plane spontaneous polarization has been experimentally observed in atomic-thick SnTe10, it is considered that there exists a critical thickness for out-of-plane ferroelectricity in a pristine film due to an intrinsic depolarizing field, arising from incomplete screening of polarization charges at the ferroelectric (FE)/metal interface[11], and the extrinsic effects of interfacial strain[12], misfit dislocations[13], and surface reconstruction[14]
The ferroelectric polarization and interfacial chemical environment are directly determined at the atomic level by aberration-corrected STEM12,23

Summary

Introduction

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unitcell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Robust in-plane spontaneous polarization has been experimentally observed in atomic-thick SnTe10, it is considered that there exists a critical thickness for out-of-plane ferroelectricity in a pristine film due to an intrinsic depolarizing field, arising from incomplete screening of polarization charges at the ferroelectric (FE)/metal interface[11], and the extrinsic effects of interfacial strain[12], misfit dislocations[13], and surface reconstruction[14]. A surprisingly high-tunneling electroresistance effect of ~370% is observed in ferroelectric tunnel junctions (FTJ) using a 1-u.c.-thick BFO film as a barrier at room temperature

Methods

Results

Conclusion