Wake-up Free Ferroelectric Rhombohedral Phase in Epitaxially Strained ZrO2 Thin Films

Zirconia- and hafnia-based thin films have attracted tremendous attention in the past decade because of their unexpected ferroelectric behavior at the nanoscale, which enables the downscaling of ferroelectric devices. The present work reports an unprecedented ferroelectric rhombohedral phase of ZrO2 that can be achieved in thin films grown directly on (111)-Nb:SrTiO3 substrates by ion-beam sputtering. Structural and ferroelectric characterizations reveal (111)-oriented ZrO2 films under epitaxial compressive strain exhibiting switchable ferroelectric polarization of about 20.2 μC/cm2 with a coercive field of 1.5 MV/cm. Moreover, the time-dependent polarization reversal characteristics of Nb:SrTiO3/ZrO2/Au film capacitors exhibit typical bell-shaped curve features associated with the ferroelectric domain reversal and agree well with the nucleation limited switching (NLS) model. The polarization-electric field hysteresis loops point to an activation field comparable to the coercive field. Interestingly, the studied films show ferroelectric behavior per se, without the need to apply the wake-up cycle found in the orthorhombic phase of ZrO2. Overall, the rhombohedral ferroelectric ZrO2 films present technological advantages over the previously studied zirconia- and hafnia-based thin films and may be attractive for nanoscale ferroelectric devices. © 2021 American Chemical Society.This work was supported by (i) the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding Contract UIDB/04650/2020 (ii) DST-SERB, Government of India, through Grant ECR/2017/00006, (iii) Project NECL - NORTE-01-0145-FEDER-022096 and Project UID/NAN/50024/2019. This work was also developed within the scope of the project CICECO-Aveiro Institute of Materials, refs UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. R.F.N., M.C.I,. and C.G. acknowledge the financial support from the Romanian Ministry of Education and Research within the project PN-III-P4-ID-PCCF2016-0047, contract 16/2018. Work at LIST was supported by the Luxembourg National Research Fund through projects PRIDE/15/10935404 “MASSENA” (S.D.) and INTER/ANR/16/11562984 “EXPAND” (H.A. and J.Í.). The authors acknowledge the CERIC–ERIC Consortium for access to experimental facilities and financial support under proposal 20192055. The authors also thank José Santos for technical support in the Thin Film Laboratory at CF-UM-UP. The equipment of the Ural Center for Shared Use “modern nanotechnology” Ural Federal University (Reg. No. 2968) was used with the financial support of the Ministry of Science and Higher Education of the RF (Project No. 075-15-2021-677)