Strain and Doping Study of Lead-free Antiferroelectric Thin Films

近年来，由于在可持续能源供应方面的预期应用，人们对反铁电(AFE)材料的兴趣有所增加。薄膜是研究本征固态性质的理想模型系统，因为它们可以生长为单晶，具有可控的外延应变和代位掺杂。尤其是外加应变与AFE性能之间关系的阐明，有望改善储能能力等相关性能指标。这是由于在(反)铁电材料中应变和极化之间的基本联系。尽管这一问题与此相关，但迄今为止还没有发现关于反铁电体的明确结果，特别是因为薄膜调查的数量很少，这将使控制应变和掺杂的方法能够得到明确和可修改的方法。缺乏研究的一个原因是很难合成出具有合适结晶质量的材料，包括用于电容测量的底部电极。基于首次在纯NaNbO_3薄膜中明确确认了反极性AFE相，我们提出了以下实验：·通过先进的氧化物分子束外延(ADOMBE)生长AFE钙钛矿NaNbO_3和AgNbO_3高质量薄膜，具有结构(通过X射线衍射(XRD))和成分(通过X射线光电子能谱(XPS))表征的材料性质；以及·研究极性和反极性相之间的相互作用，通过使用具有不同晶格常数的结构兼容的衬底的应变工程来优化AFE性能。基于所获得的实验结果，我们将对应变和掺杂对无铅材料的AFE性质的影响有一个清晰的认识。这些结果将是校准模型预测的有价值的指导，也将有助于确定通过掺杂和应变工程来优化能量存储密度的实验路线。

Recently, the interest in antiferroelectric (AFE) materials has increased due to envisaged applications in sustainable energy supply. Thin films serve as ideal model systems to study intrinsic solid-state properties, as they can be grown single-crystalline, with controlled epitaxial strain and substitutional doping. Especially the clarification of the relation between applied strain and the AFE properties holds the promise to improve relevant performance indicators such as energy storage capacity. This is due to the fundamental connection between strain and polarization in (anti)ferroelectrics in general. Despite the relevance of this issue, no clear results on antiferroelectrics have been uncovered so far, notably due the low amount of thin film investigations, which would allow for a well-defined and revisable way to control strain and doping. One reason for this lack of research originates from the difficulty to synthesize these materials with suitable crystalline quality including a bottom electrode for capacitive measurements. Based on the first time achievement of an unambiguously confirmed antipolar AFE phase in a pure NaNbO3 thin film, we propose the following experiments: • Growth of high-quality thin films of the AFE perovskites NaNbO3 and AgNbO3 by advanced oxide molecular beam epitaxy (ADOMBE) with characterization of structural (by X-ray diffraction (XRD)) and compositional (by X-ray photoelectron spectroscopy (XPS)) materials properties with suitable dopants to stabilize the AFE phase; • In-situ top and bottom contact fabrication for electrical characterization via an already established route using highly conducting perovskite oxides; and • Investigation of the interplay between polar and antipolar phases to optimize AFE properties via strain engineering using structurally compatible substrates with varying lattice constants. Based on the obtained experimental results, we will develop a clear understanding of the influence of strain and dopants on the AFE nature of lead-free materials. These results will be a valuable guideline to calibrate modelling predictions and will also help to define experimental routes towards optimization of energy storage density via doping and strain engineering.