CAREER: Ferroelectric Multilayers, Superlattices, and Compositionally Graded Films

这个职业生涯项目的重点是研究人工层状铁电超晶格和成分梯度铁电薄膜在微电子学中的应用。在顺电状态下，铁电材料具有一些可达到的最大介电常数，这是与需要瞬态电荷存储的应用特别相关的性质，例如动态随机存取存储器（DRAM）中的电容器。通过内应力、薄膜成分和微观结构的空间变化来寻求增强的性能。可以预见的是，通过铁电材料的本征特性和引入组成和内部应力梯度，可以获得特殊的和不寻常的电气和机电性能，这是不可能的大块铁电体和铁电薄膜。拟议的工作是一个实验和理论相结合的努力。理论部分将基于扩展的平均场朗道-金兹伯格-德文郡现象学。弹性和电之间的相互作用将被建模和铁电多层膜和成分梯度膜的物理性质将被确定。开发的模型将作为一个基础，设计铁电堆叠通过人工创建的非均匀性，如成分变化，不同的应力水平，和界面缺陷结构的增强的物理性能。通过射频溅射或脉冲激光沉积技术，将沉积在组成、厚度和相对于下面的衬底的失配方面具有系统变化的铁电和/或顺电层。首先，重点将放在沉积的原型BaTiO 3/SrTiO 3系统。基于收集的信息，将生长其他钙钛矿体系的多层膜，如PbTiO 3/CaTiO 3、PbTiO 3/SrTiO 3和KNb O 3/KTaO 3。铁电堆栈和成分梯度薄膜将通过X射线衍射和透射电子显微镜的晶体学和微观结构的特点。元素的局部成分分布将通过使用电子能量过滤器的能量色散X射线光谱法（EDXS）确定。将测量极化滞后、介电、压电和热释电特性等物理特性。由于化学和结构的不均匀性，这些多层膜的非常规铁电特性将成为设计新型器件和电路的基础。这部分研究将涉及一个既定的工业合作。该项目解决了具有高技术相关性的材料科学专题领域的基础研究问题。这项研究伴随着一项密集的教育计划。为了促进全面和积极的学习，将为本科生和研究生课程开发高级互动课程网站。此外，还将设计一个以一般非技术性读者为对象的关于薄膜科学和技术的“普及科学”互动网站。这个网站的目标受众是高中教师，初中生和高中生。两个新的跨学科的研究生水平班将设计和实施。拟议项目的另一个教育效益是通过工业合作为参与这项研究的研究生和本科生提供暑期实习机会。总体而言，该项目为学生提供了材料合成，加工和表征方面的新挑战和研究方法，并为教育和培训提供了新的工具和方法。因此，该项目的一个重要特点是高度重视教育，并将研究与教育相结合。***

This CAREER project focuses on the study of artificially layered ferroelectric superlattices and compositionally graded ferroelectric films for applications in microelectronics. In the paraelectric state ferroelectric materials possess some of the largest dielectric constants attainable, a property particularly relevant for applications where transient charge storage is required, such as capacitors in dyanmic random access memories(DRAM). Enhanced properties are sought through spatial variations in internal stresses, film composition, and microstructure. It is anticipated that through intrinsic characteristics of ferroelectric materials and the introduction of compositional and internal stress gradients, exceptional and unusual electrical and electromechanical properties can be obtained which are not possible for bulk ferroelectrics and ferroelectric thin films. The proposed work is a combined experimental and theoretical effort. The theoretical part will be based on an extended mean field Landau-Ginzburg-Devonshire phenomenology. Elastic and electrical interactions between individual layers will be modeled and physical properties of ferroelectric multilayers and compositionally graded films will be determined. The model developed will serve as a basis to design ferroelectric stacks with enhanced physical properties via artificially created non-uniformities such as compositional variations, different stress levels, and interfacial defect structures. Ferroelectric and/or paraelectric layers with systematic variations in composition, thickness, and misfit with respect to the underlying substrate will be deposited by rf-sputtering or pulsed lased deposition techniques. Initially, the focus will be on the deposition of the prototypical BaTiO3/SrTiO3 system. Based on the information gathered, multilayers of other perovskite systems such as PbTiO3/CaTiO3, PbTiO3/SrTiO3, and KNbO3/KTaO3, will be grown. Ferroelectric stacks and compositionally graded films will be characterized crystallographically and microstructurally via x-ray diffraction and transmission electron microscopy. Local compositional distribution of elements will be determined by energy-dispersive x-ray spectroscopy (EDXS) using an electron energy filter. Physical properties such as polarization hysteresis, dielectric, piezoelectric, and pyroelectric properties will be measured. The unconventional ferroelectric properties of these multilayers due to chemical and structural non-uniformity will be the basis for designing novel devices and circuits. This part of the research will involve an established industrial collaboration. %%% The project addresses fundamental research issues in a topical area of materials science having high technological relevance. The research is accompanied by an intensive educational plan. To promote comprehensive and active learning, advanced interactive course web sites will be developed for undergraduate and graduate level courses. Also, a "popular science" interactive web site on thin film science and technology aimed at a general, non-technical audience will be designed. The target audience for this web site is high school teachers, juniors, and seniors. Two new interdisciplinary graduate level classes will be designed and implemented. Another educational benefit of the proposed project sought is summer internships for graduate and undergraduate students involved in this research through industrial collaboration. Overall, the project provides students with new challenges and research approaches in materials synthesis, processing, and characterization, and provides new tools and approaches to education and training. Thus, an important feature of the project is the strong emphasis on education, and the integration of research and education. ***