Controlling and Quantifying Two-Level Systems, Disorder and Ideality in Tetrahedrally Bonded Amorphous Thin Films

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Non-technical AbstractIn the field of Condensed Matter Physics, materials that lack structural order -- deemed amorphous materials or glasses -- are, compared to crystals, relatively unexplored. Crystals consist of spatially repeated atoms, which permit mathematically simple formalisms that can be used to calculate and predict properties of these systems. Amorphous systems, however, have no such structural repeatability, and thus are less understood. This lack of understanding, however, does not preclude the applicability or scientific impact of disordered systems; plastics, silicate glasses, and amorphous silicon photovoltaics are examples that are pertinent to daily life, industry, and technologies, and amorphous superconductors are a remarkable example of how a fundamental scientific property transcends structural imperfection. The properties of a disordered material depend strongly on how the material was produced, 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, unlike crystals. It is clear that 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 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. Such a determination will yield improved understanding and control of disordered 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 graduate student are women, and actively engage in efforts to make physics accessible to underrepresented STEM groups.Technical AbstractA problem of both longstanding and current interest is the thermodynamic nature of the amorphous or glassy state. The lack of structural order makes these systems less mathematically tractable and makes it a challenge to resolve how disorder affects the thermodynamic properties. Local and global minima on a broad scale (kT) in the energy landscape are relevant to the configurational entropy. An ideal glass has low configurational entropy, approaching that of the crystalline counterpart, thus implying the existence of a unique disordered state that lacks defects. Local minima on a much smaller scale (kT) produce anomalous low temperature properties of glasses that are well described by tunneling or two level systems (TLS), which are widely considered universal although disagreement exists as to what causes these small scale minima. Even more controversial is the connection (if any) between the low and high temperature thermodynamic properties. In recent years, significant exceptions to low temperature universal behavior have been found, suggesting that different classes of disorder exist. A related question concerns the nature and possible interdependence of defects in an amorphous system; e.g. in amorphous silicon, both TLS and dangling bonds are known to exist, and are dependent on atomic density, but are not directly correlated. This (and other) tetrahedrally-bonded materials are fundamentally different than the traditionally studied glasses; they cannot be quenched from the liquid state, their tetrahedral bonding leads to an overconstrained continuous random network, and the difficulty in producing large quantities for conventional calorimetry has previously prevented most thermodynamic measurements. Intriguingly, these are precisely the materials most easily made as thin films by vapor deposition processes. Various growth techniques will be used to produce thin films of tetrahedrally bonded materials to study the link between TLS and density/structure and ideality within disorder; the project will test the hypothesis that ideal glasses do not have TLS or defects. Using unique membrane-based nanocalorimeters, heat capacity and thermodynamic properties will be measured over a wide temperature range, 0.1-1000K. This temperature range covers the TLS at low T (1K) and the proposed high T glass transition temperature for amorphous silicon. The enormously fast heating (10^5 K/sec) and cooling (10^4 K/sec) rates of these calorimeters and wide temperature range permit these previously impossible experiments, including determination of zero temperature configurational entropy.