Incipient ferroelectrics based on hafnium oxide

铁电(FE)材料因其优异的介电、电光、热释电和压电性能而引起人们的高度技术兴趣。它们的应用领域从执行器和传感器到数据和能量存储。传统的铁电体通常具有所谓的钙钛矿结构，并且是相对复杂的氧化物，最重要的例子之一是锆钛酸铅(PbZrxTi1-xO3，PZT)。在第一个资助期之前，申请者首次观察到了10 nm薄的掺硅HfO2薄膜的FE性质。这一发现尤其令人惊讶，因为人们对HfO2作为陶瓷材料以及半导体技术中的超薄高介电常数介质进行了密集的研究。这种铁电性的起源尚不清楚。提出了一种正交Pca21相来解释这一行为。到那时为止，还不能提供实验证据。讨论了掺杂和机械应力对FE相稳定的潜在影响。因此，实验和理论研究都符合这样的工作假设，即纯HfO2是一种初始铁电体，可以克服上述杠杆促进的准铁电态和铁电态之间固有的小势垒。报告部分讨论了i)Pca21相的稳定和实验证明；ii)不同制备方法的适用性，重点是低成本、灵活的化学溶液沉积；iii)不同掺杂的表现形式和能力；IV)以前只考虑HfO2基铁电体的场循环行为。扩展建议的目的是加深对HfO2基薄膜中铁电相的稳定以及场循环行为机制的物理和理论理解。这对基础材料科学和未来的应用都具有突出的意义。拟议研究的起点是第一个资助期的调查结果，这使得能够清楚地确定相关因素，以建立HfO2/ZrO2系统的多尺度物理模型。与经典的钙钛矿型铁电材料相比，本文拟对其特殊方面进行批判性讨论。在这里，类比将是有帮助的，但也应该强调差异。工作包围绕以下子目标安排：i)了解表面能的影响；ii)相变和极化反转的分析；iii)尚未利用的掺杂剂的研究；iv)阐明氧空位的作用；v)找出与经典钙钛矿型铁电材料的区别。

Ferroelectric (FE) materials are of high technological interest because of their extraordinary dielectric, electro-optic, pyro- and piezoelectric properties. Their fields of application range from actuators and sensors through to data and energy storage. Conventional ferroelectrics usually possess a so-called Perovskite structure and are comparably complex oxides as evidenced by lead zirconate titanate (PbZrxTi1-xO3, PZT) as one of the most important examples.Prior to the first funding period, the first observation of FE properties in 10 nm thin Si-doped HfO2 films was published by the applicants. This finding was especially surprising given the intense research devoted to HfO2 as ceramic material as well as for ultrathin high-permittivity dielectrics in semiconductor technology. The origin of this ferroelectricity remained unclear. An orthorhombic Pca21 phase was proposed to explain the behavior. The experimental proof could not be provided up to that point. Doping and mechanical stress were discussed to potentially account for the stabilization of the FE phase. Therefore, the experimental and theoretical investigations were aligned along the working hypothesis that pure HfO2 is an incipient ferroelectric which could overcome the inherently small barrier between the para- and the ferroelectric state promoted by the abovementioned levers. The report section contains a discussion of I) the stabilization and the experimental proof of the Pca21 phase; II) the suitability of different preparation methods with focus on the cost-efficient, flexible chemical solution deposition; III) the manifestation and aptitude of different dopants; IV) the previously only sparsely considered field cycling behavior of HfO2-based ferroelectrics.Goal of the extension proposal is to deepen the physical and theoretic understanding of the stabilization of the ferroelectric phase as well as of the mechanisms that account for the field cycling behavior in HfO2-based thinfilms. This is of outstanding relevance for both fundamental material science and future applications. Starting point for the proposed studies are the findings of the first funding period, which allowed a clear identification of relevant factors toward a physical multiscale model of the system HfO2/ZrO2. Particular aspects are intended to be critically discussed in comparison to the classic Perovskite ferroelectrics. Here, analogies will be helpful but also differences shall be emphasized. The work packages are arranged around the following subgoals: I) an understanding of the influence of surface energy; II) analysis of the phase transitions and the polarization reversal; III) study of dopants not utilized yet; IV) elucidation of the role of oxygen vacancies; V) working out the differences to classic perovskite ferroelectrics.