Electron Microscopy Center, Empa, Swiss Federal Laboratories for Materials Science and Technology, 8600 Dübendorf, Switzerland.
Department of Materials, Eidgenössische Technische Hochschule Zürich, 8092 Zürich, Switzerland.
ACS Appl Mater Interfaces. 2023 Apr 12;15(14):18482-18492. doi: 10.1021/acsami.3c00412. Epub 2023 Mar 30.
Improper ferroelectrics are expected to be more robust than conventional ferroelectrics against depolarizing field effects and to exhibit a much-desired absence of critical thickness. Recent studies, however, revealed the loss of ferroelectric response in epitaxial improper ferroelectric thin films. Here, we investigate improper ferroelectric hexagonal YMnO thin films and find that the polarization suppression, and hence functionality, in the thinner films is due to oxygen off-stoichiometry. We demonstrate that oxygen vacancies form on the film surfaces to provide the necessary charge to screen the large internal electric field resulting from the positively charged YMnO surface layers. Additionally, we show that by modifying the oxygen concentration of the films, the phase transition temperatures can be substantially tuned. We anticipate that our findings are also valid for other ferroelectric oxide films and emphasize the importance of controlling the oxygen content and cation oxidation states in ferroelectrics for their successful integration in nanoscale applications.
非理想铁电体有望比传统铁电体更能抵抗去极化场效应，并表现出人们所期望的不存在临界厚度的特点。然而，最近的研究表明，外延非理想铁电薄膜中会丧失铁电响应。在这里，我们研究了非理想铁电六方 YMnO 薄膜，发现较薄的薄膜中极化抑制，即功能丧失，是由于氧的非化学计量。我们证明，在薄膜表面形成氧空位，以提供必要的电荷来屏蔽由带正电荷的 YMnO 表面层产生的大内部电场。此外，我们还表明，通过改变薄膜的氧浓度，可以大幅调节相转变温度。我们预计我们的发现也适用于其他铁电氧化物薄膜，并强调了在纳米尺度应用中成功集成铁电体时控制氧含量和阳离子氧化态的重要性。
