Metastable phases in BCC thin films: formation, stability, and properties

BCC 薄膜中的亚稳态相：形成、稳定性和性能

• 批准号: 1810138
• 负责人: Shefford Baker
• 金额: $ 51.91万
• 依托单位: Cornell University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-07-01 至 2024-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1810138&HistoricalAwards=false
• 关键词: Metastable; phases; BCC; thin; films

NON-TECHNICAL SUMMARYAll pure metals are crystalline in their most stable forms and many metals can adopt different crystal structures under different conditions. The properties of each metal depend strongly on its crystal structure, also known as its phase. For example, tantalum (Ta) in its stable cubic phase is ductile and is a good conductor of electricity. It is widely used in integrated circuits and thin film capacitors. In 1965, Mildred Read and Carl Altman discovered that, in certain atom-by-atom thin film deposition processes, Ta can also be made in a metastable tetragonal phase which is brittle and has much lower electrical conductivity. Use of this phase has been limited to thin film resistors. However, in 2012, an electronic effect (giant spin Hall effect) was discovered in the metastable phase, which promises to revolutionize information storage by making significant further miniaturization of computer devices possible. Thus, there is now much interest in the ability to reliably produce the metastable phase. As it turns out, all of the elements from group 5 and 6 in the periodic table, which, in addition to Ta, include tungsten (W), chromium (Cr), molybdenum (Mo), vanadium (V), and niobium (Nb), have the same stable crystal structure at all temperatures and pressures, and, with the exception of Nb, metastable phases have been reported for all of them (a metastable Nb phase has been predicted, but not realized). This leads to the prospect that, if the metastable phases can be reliably produced in all of these metals, this may open up a class of new materials with potentially interesting properties. Indeed, it has recently been shown that the metastable phase in W has an even stronger giant spin Hall effect than Ta. However, with the exception of metastable Ta, and now W, the metastable phases of these elements have primarily been laboratory curiosities. Neither their formation mechanisms nor their properties are known. In this research, detailed analyses of the first few atomic layers that form as thin films of Ta, W, Cr, Mo, V, and Nb are deposited, atom by atom, onto a substrate will be used to determine how and why the metastable phases form. In addition, a unique ultra-high vacuum deposition system that is capable of making very pure films under a wide range of conditions will be used to explore the range of conditions under which metastable phase films of these materials can be made. Finally studies of the atomic arrangements and properties such as hardness, electrical conductivity, and electronic effects in those films will provide information about possible new applications for these new materials. In addition to fundamental understanding about why certain phases form, this work should enable technologists to develop new applications using these materials and to determine how to control process parameters to reliably obtain metastable films with desired properties for those applications. In particular, this work has the potential to revolutionize computer random access memory technology, which would enable the development of several generations of higher performance microelectronic devices. In addition, approximately 16 students, including 2 PhD students, 2 MS students, and 12 undergraduate students (6 from Cornell and 6 from Houghton College, a small undergraduate liberal arts college in upstate New York) will engage in this effort. All students will be mentored to come up to speed on the goals, participate in the research, and to present their work in both talks and papers. The net effect will be very high-level training for these future STEM professionals. Finally, the project will involve numerous outreach activities to local area schools and institutions, including presentations, demonstrations, curricular assistance, tutoring (especially for disadvantaged/underrepresented students), and many others. These activities are intended to inform the public about the practice, value, and accomplishments of science, including this project, and to encourage students to pursue STEM fields as they see fit.TECHNICAL SUMMARYThe group 5 and 6 elements, Ta, W, Cr, Mo, V, and Nb, are well known for having only one equilibrium crystal structure, the body-centered-cubic phase, at all temperatures and pressures. In addition, it has been shown that in certain atom-by-atom fabrication processes such as sputter deposition, metastable phases can be made in all but Nb (metastable Nb has been predicted, but not realized). However, very little is known about the mechanism by which these phases form, and, with the exception of metastable beta-Ta and beta-W, very little is known about their properties. Recently there has been a spike in interest generated by the discovery of the giant spin Hall effect in both beta-Ta and beta-W. In the present program, phase formation mechanisms and the relationships among film deposition parameters, microstructure, and properties, will be studied for the group 5 and 6 elements. Phase formation will be studied using reflection high energy electron diffraction (RHEED) and angle-resolved photoemission spectroscopy (ARPES) to characterize the initial phases of film growth by molecular beam. The extent to which metastable phases can be produced in W, Cr, Mo, V, and Nb will be determined in an ultra-high vacuum (UHV) sputter deposition system that provides a wide range of deposition parameters (temperature, bias, power, sputter mode) and is capable of reducing oxygen and other impurities to extremely low levels. The effect of various deposition parameters on microstructure will be studied using x-ray diffraction (XRD), electron backscattered diffraction (EBSD) and other methods, and the stability of the metastable phase will be studied by determining stress change (as an indication of phase change) in-situ during heating in the UHV system. Finally, hardness and elastic modulus will be determined using nanoindentation and electrical conductivity will be measured with a four-point probe. This work is expected to provide fundamental details regarding the mechanism of metastable phase formation and the crystal structures of the metastable phases in the group 5 and 6 metals, as well as descriptions of the conditions under which those phases can be grown and the relationships among those deposition conditions and the microstructure and properties of the resulting films.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.

非技术总结纯金属是最稳定的形式的结晶，许多金属在不同条件下可以采用不同的晶体结构。每种金属的性质在很大程度上取决于其晶体结构，也称为其相。例如，其稳定的立方相的塔塔尔（TA）是延展性的，是电力的良好导体。它被广泛用于集成电路和薄膜电容器中。 1965年，米尔德雷德（Mildred Read）和卡尔·奥特曼（Carl Altman）发现，在某些逐个原子薄膜沉积过程中，TA也​​可以在脆性且具有较低的电导率的亚稳态四个相中制成。此阶段的使用仅限于薄膜电阻器。但是，在2012年，在亚稳态阶段发现了电子效应（巨型旋转厅效应），该阶段有望通过对计算机设备进行大量的微型化来彻底改变信息存储。因此，现在对可靠产生亚稳态的能力引起了极大的兴趣。事实证明，元素周期表中第5组和第6组的所有要素除了TA之外，还包括钨（W），铬（CR），钼（Moybdenum（mo），钒（V）和Niobium（nb），在所有温度和压力下都具有相同的稳定晶体结构，并且与NB相同的稳定结构相同（均为nb，均为nb的异常，并且均具有相同的含量（被预测，但未实现）。这导致了这样的前景，即如果在所有这些金属中可以可靠地生产亚稳态，那么这可能会打开一类具有潜在有趣特性的新材料。确实，最近已经证明，W中的亚稳态相比TA具有更强的巨型旋转效果。但是，除了亚稳态的TA和现在，这些元素的亚稳态阶段主要是实验室的好奇心。他们的形成机制均不知道。在这项研究中，对前几个原子层的详细分析将用于TA，W，CR，MO，V和NB的薄膜，将原子沉积到AtoM上，将用于确定亚稳态相形成以及为什么形成亚稳态的原因。此外，能够在各种条件下制作非常纯净的膜的独特的超高真空沉积系统将用于探索可以在这些条件下进行这些材料的可稳态相膜的范围。最后，对这些薄膜中的硬度，电导率和电子效应等原子布置和特性的研究将提供有关这些新材料可能新应用的信息。除了了解某些阶段为什么形成的基本了解外，这项工作还应使技术人员能够使用这些材料开发新的应用程序，并确定如何控制过程参数，以可靠地获得具有所需属性的可靠性膜。特别是，这项工作有可能彻底改变计算机随机访问记忆技术，这将使几代高性能的微电子设备开发。此外，大约有16名学生，包括2名博士学位学生，2名女士和12名本科生（康奈尔大学的6名学生和霍顿学院的6名学生，纽约州北部的一所小型本科自由艺术学院）将从事这一努力。所有学生都将受到指导，以加快目标，参与研究，并在谈判和论文中介绍他们的作品。对于这些未来的STEM专业人员，净效果将是非常高级的培训。最后，该项目将涉及许多向当地学校和机构的外展活动，包括演讲，示范，课程援助，辅导（尤其是针对弱势群体/代表性不足的学生）等。这些活动旨在告知公众包括该项目在内的科学的实践，价值和成就，并鼓励学生在他们认为合适的情况下追求STEM领域。技术总结第5和6组第5和6个要素，TA，W，CR，MO，MO，V和NB，以仅具有一个平衡晶体结构，体内尺寸的尺寸尺寸，并在所有频率的情况下，闻名。此外，已经表明，在某些逐个原子的制造过程中，例如溅射沉积，可以在除NB以外的所有其他原子中（可以预测亚稳态NB，但未实现）。但是，对于这些阶段形成的机制知之甚少，除了亚稳态beta-ta和beta-W外，对它们的性质知之甚少。最近，在Beta-TA和Beta-W中发现了巨型旋转厅效应引起的兴趣激增。在目前的程序中，将研究第5组和6个元素的相位形成机制以及膜沉积参数，微结构和属性之间的关系。将使用反射高能电子衍射（RHEED）和角度分辨光发射光谱（ARPE）研究相形的形成，以表征分子束生长的初始阶段。在W，CR，MO，V和NB中可以产生亚稳态相的程度将在超高真空（UHV）溅射沉积系统中确定，该系统提供了广泛的沉积参数（温度，偏置，功率，溅射模式），并能够将氧气和其他不友好的水平降低到极低的水平。将使用X射线衍射（XRD），电子反向散射衍射（EBSD）和其他方法来研究各种沉积参数对微结构的影响，并通过确定在UHV系统中供热期间在炎热过程中确定应力变化（作为相变的指示），研究稳定相的稳定性。最后，将使用纳米压力确定硬度和弹性模量，并通过四点探针测量电导率。预计这项工作将提供有关第5和6个金属中可稳态阶段的可稳态形成机制的基本细节，以及对这些阶段可以种植这些阶段以及这些沉积条件以及这些阶段之间关系的条件的描述，以及这些阶段之间的关系以及这些阶段之间的关系，以及通过评估nsf的构建范围的构建元素的构建范围。和更广泛的影响审查标准。