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

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Non-Technical Summary Amorphous 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 Summary In 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.