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

非晶材料缺乏结构有序性，使得它们难以描述，并且与由空间重复的原子组成的晶体材料相比，难以计算和预测它们的性质。然而，这种缺乏了解并不妨碍非晶材料的重要应用或科学影响;塑料、硅酸盐玻璃和非晶硅光致发光材料都是与日常生活、工业和技术相关的例子。非晶超导体是一个显著的例子，说明了一个基本的科学性质如何超越结构的不完美性。 非晶材料的性质很大程度上取决于它是如何产生的，并且存在一些定义明确的已知缺陷，但是如何描述由不同方法产生的不同非晶结构尚不清楚，即使对于单一元素材料，也不清楚完全无序材料中缺陷的性质是什么。无序存在于不同的长度和能量尺度上，从局部的原子大小的无序到更大的尺度。有趣的是，存在“理想玻璃”的概念，它虽然保持完全无序，但在这种无序中缺乏缺陷，因此接近晶体的独特性，包括其性质的可再现性和可预测性。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“理想性”的可调性。 这项工作将使人们更好地理解是什么原因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参与努力，使物理学能够被代表性不足的STEM种族和社会经济少数群体所利用。技术摘要更详细地说，该项目将着眼于理解完全无序（非晶）材料中的有序性，并将深入了解气相沉积如何实现具有低隧道态密度（TLS）的超稳定玻璃的创建，以及该过程适用于哪些材料。最近的实验表明，两种极其不同的气相沉积材料（吲哚美辛和硅）形成超稳定的玻璃，其焓接近相应的晶体，并且具有低密度的隧穿态，强烈暗示这些接近理想的玻璃。拟议的工作将使用两类非晶材料来测试的假设，即超稳定性是通过气相沉积，只有当生长是在Kauzmann温度TK附近，在理论上产生的理想的玻璃，只有当有足够的表面原子的流动性。 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，并深入了解不同材料中难以捉摸的理想玻璃状态的本质。理想的玻璃本身具有强烈抑制TLS的假设也将被测试。许多拟议的表征研究将提供有关缺陷，TLS和非晶材料的稳定性的见解，并有望指向可概括的结构特征，预示着TLS的存在或不存在。 所提出的材料直接影响技术的许多方面，包括用于隧道势垒、相变存储器、（a-Ge和相关合金），超快飞行时间辐射探测（a-Se），闪存，催化，量子计算，以及晶体管和超导量子位（a-Al 2 O 3）中的高k值晶体管该奖项反映了NSF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.