Possible absence of critical thickness and size effect in ultrathin perovskite ferroelectric films

Peng Gao,Jingmin Zhang,Dapeng Yu,Ryo Ishikawa,Ying-Hao Chu,Lei Liao,Yuichi Ikuhara,Naoya Shibata,Kaihui Liu,Bin Feng,Heng-Jui Liu,Mingqiang Li,Yen-Lin Huang,Shulin Chen,En-Ge Wang,Zhangyuan Zhang,Xiumei Ma

doi:10.1038/ncomms15549

Abstract

Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr0.2Ti0.8O3 films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films <10-unit cells thick (∼4 nm). However, approximately the polarization never vanishes. The residual polarization is ∼16 μCcm−2 (∼17%) at 1.5-unit cells (∼0.6 nm) thick film on bare SrTiO3 and ∼22 μCcm−2 at 2-unit cells thick film on SrTiO3 with SrRuO3 electrode. The residual polarization in these ultrathin films is mainly attributed to the robust covalent Pb–O bond. Our atomic study provides new insights into mechanistic understanding of nanoscale ferroelectricity and the size effects.

Highlights

The size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate
For BiFeO3 (BFO), the ferroelectricity has been detected in 4 and 5-unit cells thick films[9,10]. Some of these values are in good agreement with the theoretical work, for example, the critical thickness has been calculated to be 3-unit cells for PTO and 6-unit cells for BTO, while other calculations suggested that the ferroelectricity might even exist in a single-unit cell thick pervoskite film with proper electrodes[13], indicating the possible absence of a critical thickness
The ferroelectricity does not disappear in the ultrathin films, the magnitude of polarization should be progressively decreased[14,15,16] as the critical thickness is approached

Summary

Introduction

The size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. 12), while other calculations suggested that the ferroelectricity might even exist in a single-unit cell thick pervoskite film with proper electrodes[13], indicating the possible absence of a critical thickness. The atomic structure suggests that the residual polarization in the films below 3-unit cells is mainly attributed to the robust covalent Pb–O bond. These results provide new insights into mechanistic understanding of the nanoscale polarization and size effect in ferroelectric thin films

Methods

Results

Conclusion