Strain and Doping Study of Lead-free Antiferroelectric Thin Films

最近，由于在可持续能源供应方面的应用前景，对反铁电（AFE）材料的兴趣有所增加。薄膜是研究固态特性的理想模型系统，因为它们可以生长成单晶，外延应变可控，并且可以替代掺杂。特别是澄清外加应变与AFE性能之间的关系，有望改善储能容量等相关性能指标。这是由于在（反）铁电体中应变和极化之间的基本联系。尽管这个问题具有相关性，但到目前为止还没有发现反铁电体的明确结果，特别是由于薄膜研究的数量很少，这将允许一种定义良好且可修改的方法来控制应变和掺杂。缺乏研究的一个原因是很难合成这些具有合适晶体质量的材料，包括用于电容测量的底电极。基于首次在纯净的NaNbO3薄膜中明确地证实了反极性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NaNbO3和AgNbO3的高质量薄膜，并通过结构（x射线衍射（XRD））和成分（x射线光电子能谱（XPS））表征材料性质，并使用合适的掺杂剂来稳定AFE相；•使用高导电性钙钛矿氧化物，通过已建立的路线进行原位顶部和底部接触制造，以进行电气特性表征；•研究极性相和反极性相之间的相互作用，通过应变工程使用具有不同晶格常数的结构兼容衬底来优化AFE性能。根据得到的实验结果，我们将清楚地了解应变和掺杂剂对无铅材料AFE性质的影响。这些结果将是校准建模预测的有价值的指导方针，也将有助于通过掺杂和应变工程确定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.