Microstructure and strain effects on ferroelectric and transport properties of hafnium oxide thin films

非技术：铁电材料，如氧化钨，在数据存储和存储处理方面有很好的应用前景。这些材料具有自发的电极化，可以通过施加电场来切换，类似于铁磁性。PI将研究氧化铟的基本性质和基于氧化铟的器件的性能。最终的目标是开发性能优越的铁电存储器件。研究生和本科生将受益于铁电材料和器件科学技术的教育和培训。来自代表性不足群体的学生将获得宝贵的教育经验，以丰富他们的专业发展。外展计划旨在通过公开公共活动，如州科学奥林匹克竞赛和Nanocamp，面向K-12学生和内布拉斯加州居民。这些活动将把研究与社区参与结合起来。技术：在HfO2薄膜中发现铁电性最近引起了极大的兴趣，因为它与已知的铁电材料中的CMOS技术具有最好的兼容性。基于HfO2的铁电材料薄膜具有很强的可切换极化和低漏电特性，因此在性能增强的非易失性存储器和铁电场效应晶体管中具有巨大的潜力。为了在实践中实现这一潜力，需要对这些材料的结构-性能-器件性能关系进行系统的研究。与传统的钙钛矿型铁电材料相比，HfO2基薄膜的铁电性机理还远未被了解。微结构和应变条件可能控制着多晶HfO2薄膜中铁电相的形成，但目前还没有明确的方法来稳定单晶薄膜中的铁电相。这一建议旨在全面了解薄膜微结构和界面应力之间的相互作用如何影响铁电相的稳定性、极化反转动力学、电子输运行为以及相关的器件性能。本项目主要研究：(1)理论驱动的HfO2-电容器结构的制备，该结构具有可控微结构和稳定铁电状态的机电边界条件；(2)薄膜微结构对外延、织构和多晶HfO2基薄膜极化反转机制的影响；(3)HfO2基铁电隧道结中极化控制的电阻开关作为铁电势垒微结构的函数。用基于密度泛函理论的理论模型研究了外延应变对正交和菱形单晶HfO2薄膜中铁电相稳定的影响，并通过脉冲激光沉积在衬底和缓冲层上生长的HfO2薄膜的电学和结构表征进行了验证。用压电响应力显微镜(PFM)直接观察HfO2薄膜的磁畴结构演化，并用脉冲测试方法测量器件级的积分暂态电流，从而研究HfO2薄膜的时间偏压依赖的开关行为。局域探针显微镜结合结构表征方法将被用来将隧道电阻效应与HfO2基隧道结的极化状态稳定性和微结构联系起来。因此，该项目的学术价值将是阐明极性相稳定性、薄膜微结构和界面应变之间的关系，澄清极化反转的机制，以及演示和量化HfO2基铁电膜和器件结构中的电阻开关。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical: Ferroelectrics such as hafnium oxide have promising applications in data storage and memory processing. These materials have a spontaneous electrical polarization that can be switched by applying an electric field, akin to ferromagnetism. The PIs will study the fundamental properties of hafnium oxide and the performance of hafnium oxide-based devices. The ultimate aim is to develop ferroelectric memory devices with superior performance. Graduate and undergraduate students will benefit from education and training in the science and technology of ferroelectric materials and devices. Students from underrepresented groups will be provided valuable educational experiences that enrich their professional development. Outreach programs aim at K-12 students and Nebraska residents through open public events, such as the State Science Olympiad and Nanocamp. These activities will integrate research with community engagement.Technical:The discovery of ferroelectricity in HfO2 films has recently attracted enormous interest due to its best compatibility with CMOS technology among known ferroelectrics. Thin films of HfO2-based ferroelectric materials exhibit robust switchable polarization and low leakage, and thus have a huge potential for being used in nonvolatile memories and ferroelectric field-effect transistors with enhanced performance. To realize this potential in practice, systematic studies of the structure-property-device performance relationship in these materials are required. In contrast to conventional perovskite ferroelectrics, the mechanism of ferroelectricity in HfO2-based films is far from being understood. The microstructure and strain conditions likely control the formation of the ferroelectric phase in polycrystalline HfO2 films, but there is no clear method for stabilizing it in a monocrystalline film. This proposal aims at achieving a comprehensive understanding of how the interplay between the film microstructure and interfacial stress affects the stability of the ferroelectric phase, polarization reversal dynamics, electronic transport behavior, and the related device performance. This project studies (1) theory-driven fabrication of the HfO2-capacitor structures with controlled microstructure and electromechanical boundary conditions stabilizing the ferroelectric state; (2) the effect of film microstructure on the polarization reversal mechanism in the epitaxial, textured, and polycrystalline HfO2-based films; and (3) the polarization-controlled resistive switching in the HfO2-based ferroelectric tunnel junctions as a function of the ferroelectric barrier microstructure. The effect of epitaxial strain on stabilizing the ferroelectric phase in single-crystalline orthorhombic and rhombohedral HfO2 films will be explored using theoretical modeling based on density functional theory and verified by electrical and structural characterization of the HfO2 films grown using pulsed laser deposition on the substrates and buffer layers. The time-bias-dependent switching behavior will be investigated as a function of the HfO2 films microstructure via direct observation of the domain structure evolution by piezoresponse force microscopy (PFM) and measurements of the device-level integrated transient currents by pulse testing methods. Local probe microscopy in conjunction with structural characterization methods will be used to relate the tunneling electroresistance effect to the polarization state stability and microstructure of the HfO2-based tunnel junctions. Thus, the intellectual merit of this project will be elucidation of the relationship between the polar phase stability, film microstructure and interface strain, clarification of the mechanism of polarization reversal, and demonstration and quantification of the resistive switching in the HfO2-based ferroelectric films and device structures.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.