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A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films

Abstract

Hafnia-based thin films are a favoured candidate for the integration of robust ferroelectricity at the nanoscale into next-generation memory and logic devices. This is because their ferroelectric polarization becomes more robust as the size is reduced, exposing a type of ferroelectricity whose mechanism still remains to be understood. Thin films with increased crystal quality are therefore needed. We report the epitaxial growth of Hf0.5Zr0.5O2 thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 substrates. The films, which are under epitaxial compressive strain and predominantly (111)-oriented, display large ferroelectric polarization values up to 34 μC cm−2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This finding, in conjunction with density functional theory calculations, allows us to propose a compelling model for the formation of the ferroelectric phase. In addition, these results point towards thin films of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.

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Fig. 1: XRD structural characterization of HZO films on LSMO-buffered 001-oriented STO.
Fig. 2: Domain configuration and symmetry.
Fig. 3: Electron microscopy characterization.
Fig. 4: Ferroelectric characterization.
Fig. 5: Theoretical calculations and proposed structure.

Data availability

The data that support the findings of this study are included in the main text and Supplementary Information.

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Acknowledgements

We are grateful to S. Volkov and F. Bertram for their help at the P08 beamline in Petra III (DESY0-Hamburg), and T. Schenk for insightful discussions about the paper. Y.W. and B.N. are grateful for China Scholarship Council and Van Gogh travel grants. P.N. acknowledges the funding received from the European Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-Curie grant agreement no. 794954. P.N. and B.D. would like to acknowledge a public grant overseen by the French National Research Agency (ANR) as a part of the `Investissements d‘Avenir’ programme (grant no. ANR-10-LABX-0035, Labex Nanoscalay) and through project ANR-17-CE24-0032/EXPAND. B.D. also acknowledges Luxembourg National Research Fund under Project MULTICALOR:INTER/MOBILITY/16/11259210. H.J.Z. and J.Í. acknowledge the support of the Luxembourg National Research Fund through the PEARL (grant no. FNR/P12/4853155/Kreisel COFERMAT) and CORE (grant no. FNR/C15/MS/10458889 NEWALLS) programmes.

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Contributions

B.N. and Y.W. conceived the idea, and the project plan. Y.W. synthesized the films. P.N. and J.M. prepared samples for TEM, and performed the experiments, and P.N. analysed the data, under the supervision of B.J.K. and B.D. Y.W., M.S. and B.N. performed XRD and analysed the data. Y.W., S.M. and G.A. fabricated devices, tested their ferroelectric properties and analysed the data with help from A.S.E. H.J.Z. and J.Í. performed the first-principles calculations. B.J.K. and G.R.B. extensively helped in understanding the structure and symmetry of the films. P.L. and B.D. provided useful insights all along the project. Y.W., P.N., B.N. and J.Í. co-wrote the manuscript with feedback from all of the authors.

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Correspondence to Beatriz Noheda.

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Supplementary Notes 1–6, Supplementary Figures 1–10

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Wei, Y., Nukala, P., Salverda, M. et al. A rhombohedral ferroelectric phase in epitaxially strained Hf0.5Zr0.5O2 thin films. Nature Mater 17, 1095–1100 (2018). https://doi.org/10.1038/s41563-018-0196-0

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