Controlling and quantifying two-level systems, disorder and ideality in vapor deposited amorphous thin films

与由空间重复原子组成的晶体材料相比，非晶材料缺乏结构秩序，使其难以描述，难以计算和预测其性能。然而，这种缺乏理解并不妨碍非晶材料可能的重要应用或科学影响；塑料、硅酸盐玻璃和非晶硅光伏就是与日常生活、工业和技术相关的例子。非晶超导体是一个显著的例子，说明了基本的科学性质是如何超越结构缺陷的。非晶材料的性质在很大程度上取决于它是如何产生的，并且有一些明确定义的已知缺陷，但是如何描述通过不同方法产生的不同非晶结构并不清楚，即使对于单一元素材料，也不清楚完全无序的材料中缺陷的性质。无序存在于不同的长度和能量尺度上，从局部的原子尺度的无序到更大的尺度。有趣的是，存在一种“理想玻璃”的概念，它在保持完全无序的同时，缺乏这种无序的缺陷，从而接近晶体的独特性，包括其性质的可重复性和可预测性。该项目将研究两种类型的材料，一种是经典的半导体合金，另一种是强键合氧化物，并确定不同制备方法和不同原子产生的无序和缺陷类型之间的关系，以及无序材料“理想”的可调性。这项工作将使人们更好地理解是什么导致了技术上重要的非晶材料的机械和介电损耗，以及如何控制这些损耗。更广泛地说，它将提高对具有技术和基础科学意义的非晶材料的理解和控制。该项目还将教育和培训学生，并帮助增加科学的多样性参与；PI和她的研究小组积极致力于让STEM少数族裔和社会经济少数群体能够接触到物理。更详细地说，该项目将着眼于理解完全无序（非晶）材料中的秩序，并将深入了解气相沉积如何能够创建具有低密度隧道态（TLS）的超稳定玻璃，以及该过程适用于哪些材料。最近的实验表明，两种截然不同的气相沉积材料（吲哚美辛和硅）形成了超稳定玻璃，其焓在相应晶体附近，并且具有低密度的隧道态，强烈表明它们接近理想玻璃。提出的工作将使用两类非晶材料来测试一个假设，即只有当生长在考兹曼温度TK附近时，并且只有当有足够的表面原子迁移率时，才能通过气相沉积实现超稳定性，理论上理想的玻璃是在这个温度下生产的。如果这是真的，这将为创造其他接近理想的低损耗玻璃打开大门，并对不同材料中难以捉摸的理想玻璃状态的性质提供见解。理想玻璃本身具有强烈抑制TLS的假设也将得到验证。许多提出的表征研究将为非晶材料的缺陷、TLS和稳定性提供见解，并有望指出预示TLS存在或不存在的可推广的结构特征。提出的材料直接影响了许多方面的技术，包括隧道屏障、相变存储器（a-Ge和相关合金）、超快速飞行时间辐射探测（a-Se）、闪存、催化、量子计算和晶体管和超导量子比特中的高k介电体（a-Al2O3）。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 SummaryAmorphous materials lack structural order, making them difficult to describe and making it difficult to calculate and predict their properties compared to crystalline materials which consist of spatially repeated atoms. This lack of understanding, however, does not prevent the important applications possible or the scientific impact of amorphous materials; plastics, silicate glasses, and amorphous silicon photovoltaics are examples that are pertinent to daily life, industry, and technologies. Amorphous superconductors are a remarkable example of how a fundamental scientific property transcends structural imperfection. The properties of an amorphous material depend strongly on how it was produced, and there are some well-defined known defects, but it is not clear how to describe the different amorphous structures produced by different methods, even for a single element material, nor what the nature of a defect is in a fully disordered material. Disorder exists on different length and energy scales, ranging from local, atomic-sized disorder to larger scales. Intriguingly, there exists the notion of an "ideal glass", which while remaining thoroughly disordered, lacks imperfections in that disorder and thus approaches the uniqueness of a crystal, including reproducibility and predictability of its properties. The project will look at two types of materials, one a classic semiconductor alloy and the other a strongly bonded oxide, and determine the relationship between types of disorder and defects produced by different preparation methods and for different atoms, and the tunability of the "ideality" of disordered materials. The work will create an enhanced understanding of what causes mechanical and dielectric losses in technologically important amorphous materials and how to control these. More broadly, it will yield improved understanding and control of amorphous materials of technological and fundamental scientific significance. The project will also educate and train students and help to increase diversity participation in science; the PI and her research group actively engage in efforts to make physics accessible to underrepresented STEM ethnic and socioeconomic minorities. Technical SummaryIn more detail, this project will look at understanding order within a completely disordered (amorphous) material, and will provide insight into how vapor deposition enables creation of ultrastable glasses with low density of tunneling states (TLS), and for which materials this process works. Recent experiments show that two extremely different vapor deposited materials (indomethacin and silicon) form ultrastable glasses with enthalpy near the corresponding crystal and with a low density of tunneling states, strongly suggestive that these are close to ideal glasses. The proposed work will use two classes of amorphous materials to test the hypothesis that ultrastability is achieved by vapor deposition only when growth is done near the Kauzmann temperature TK, at which the ideal glass is theoretically produced, and only if there is sufficient surface atomic mobility. If true, this would open the door to the creation of other near-ideal low loss glasses and give insight on the nature of the elusive ideal glass state in different materials. The hypothesis that ideal glasses inherently have strongly suppressed TLS will also be tested. The many proposed characterization studies will give insights about defects, TLS, and stability of amorphous materials, and will hopefully point toward generalizable structural features that herald the presence or absence of TLS. The proposed materials directly impact many aspects of technology including dielectrics for tunnel barriers, phase change memories (a-Ge and related alloys), ultra-fast time-of-flight radiation detection (a-Se), and flash memories, catalysis, quantum computing, and high-k dielectrics in transistors and superconducting qubits (a-Al2O3).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.