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

所有的纯金属都以最稳定的形式结晶，许多金属在不同的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它的晶体结构，也就是它的相。例如，处于稳定立方相的钽（Ta）具有延展性，是良好的导电体。广泛应用于集成电路和薄膜电容器中。1965年，Mildred Read和Carl Altman发现，在某些原子对原子的薄膜沉积过程中，Ta也可以制成亚稳的方形相，这种相很脆，电导率也低得多。这种相位的使用仅限于薄膜电阻器。然而，在2012年，在亚稳态中发现了一种电子效应（巨自旋霍尔效应），它有望通过使计算机设备进一步小型化来彻底改变信息存储。因此，现在人们对可靠地产生亚稳相的能力很感兴趣。事实证明，元素周期表中第5族和第6族的所有元素，除了Ta，包括钨(W)，铬（Cr），钼（Mo），钒(V)和铌（Nb），在所有温度和压力下都具有相同的稳定晶体结构，并且除了Nb之外，所有这些元素都报道了亚稳相（亚稳Nb相已经被预测，但没有实现）。这意味着，如果亚稳相能够可靠地在所有这些金属中产生，这可能会开辟一类具有潜在有趣性质的新材料。事实上，最近的研究表明，W的亚稳相比Ta具有更强的巨自旋霍尔效应。然而，除了亚稳的Ta和现在的W外，这些元素的亚稳相主要是实验室的好奇心。它们的形成机制和性质都不为人所知。在这项研究中，对形成薄膜的最初几个原子层的详细分析，如Ta， W, Cr, Mo， V和Nb，原子一个原子地沉积在衬底上，将用于确定亚稳相是如何以及为什么形成的。此外，将使用独特的超高真空沉积系统，该系统能够在广泛的条件下制作非常纯的薄膜，以探索这些材料的亚稳相薄膜可以制作的条件范围。最后，研究这些薄膜的原子排列和性质，如硬度、电导率和电子效应，将为这些新材料可能的新应用提供信息。除了对为什么某些相形成的基本理解之外，这项工作应该使技术人员能够使用这些材料开发新的应用，并确定如何控制工艺参数，以可靠地获得具有这些应用所需性能的亚稳薄膜。特别是，这项工作有可能彻底改变计算机随机存取存储器技术，这将使几代高性能微电子设备的发展成为可能。此外，大约有16名学生，包括2名博士生，2名硕士生和12名本科生（6名来自康奈尔大学，6名来自霍顿学院，一所位于纽约州北部的小型本科文理学院）将参与这项工作。所有学生都将接受指导，以跟上目标的速度，参与研究，并在讲座和论文中展示他们的工作。最终的结果将是为这些未来的STEM专业人员提供非常高水平的培训。最后，该项目将涉及到当地学校和机构的许多外展活动，包括演讲、演示、课程援助、辅导（特别是针对弱势/弱势学生）等等。这些活动旨在让公众了解科学的实践、价值和成就，包括本项目，并鼓励学生在他们认为合适的领域从事STEM研究。技术概述5族和6族元素，Ta， W, Cr, Mo， V和Nb，在任何温度和压力下都只有一种平衡晶体结构，即体心立方相。此外，研究表明，在某些原子-原子制备工艺中，如溅射沉积，除Nb外，所有材料都可以形成亚稳相（亚稳Nb已被预测，但尚未实现）。然而，人们对这些相形成的机制知之甚少，而且，除了亚稳的β - ta和β - w外，对它们的性质知之甚少。最近，由于在β - ta和β - w中发现了巨大的自旋霍尔效应，引起了人们的极大兴趣。在本项目中，将研究第5族和第6族元素的相形成机制以及膜沉积参数、微观结构和性能之间的关系。利用反射高能电子衍射（RHEED）和角分辨光谱学（ARPES）研究相的形成，以表征分子束生长薄膜的初始阶段。在W、Cr、Mo、V和Nb中产生亚稳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），并且能够将氧和其他杂质降低到极低的水平。利用x射线衍射（XRD）、电子背散射衍射（EBSD）等方法研究不同沉积参数对微观结构的影响，并通过在特高压系统加热过程中原位测定应力变化（作为相变的指示）来研究亚稳相的稳定性。最后，硬度和弹性模量将用纳米压痕测定，电导率将用四点探针测量。这项工作有望提供有关亚稳相形成机制和5族和6族金属亚稳相晶体结构的基本细节，以及这些相可以生长的条件以及这些沉积条件与所得薄膜的微观结构和性能之间的关系的描述。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。

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.