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

非技术：氧化铪等铁电体在数据存储和内存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将研究氧化铪的基本性质和氧化铪基器件的性能。最终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。来自弱势群体的学生将获得宝贵的教育经验，丰富他们的专业发展。外展计划旨在通过开放的公共活动，如国家科学奥林匹克竞赛和纳米夏令营，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。技术方面：由于在已知的铁电体中与CMOS技术的最佳兼容性，在HfO2薄膜中发现铁电性最近引起了极大的兴趣。基于hfo2的铁电材料薄膜具有强大的可开关极化和低漏损，因此在非易失性存储器和铁电场效应晶体管中具有巨大的潜力，具有更高的性能。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研究。与传统的钙钛矿铁电性相比，hfo2基薄膜中的铁电性机制还远未被了解。在多晶HfO2薄膜中，微观结构和应变条件可能控制了铁电相的形成，但在单晶薄膜中没有明确的稳定铁电相的方法。本研究旨在全面了解薄膜微观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、极化逆转动力学、电子输运行为和相关器件性能。本项目研究(1)理论驱动制造具有可控微观结构和稳定铁电态机电边界条件的hfo2电容器结构；(2)薄膜微观结构对外延、织构和多晶hfo2基薄膜极化反转机制的影响；(3)铁电势垒微观结构对hfo2基铁电隧道结极化控制电阻开关的影响。基于密度泛函理论的理论建模将探讨外延应变对稳定单晶正交和菱形HfO2薄膜中铁电相的影响，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验证。通过压电响应力显微镜（PFM）直接观察畴结构演变和脉冲测试方法测量器件级集成瞬态电流，研究HfO2薄膜微观结构随时间偏置的开关行为。局部探针显微镜结合结构表征方法，将隧道电阻效应与hfo2基隧道结的极化态稳定性和微观结构联系起来。因此，本课题的智力价值在于阐明极性相稳定性、薄膜微观结构与界面应变之间的关系，阐明极化反转机理，论证和量化hfo2基铁电薄膜和器件结构中的电阻开关。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.