arXiv subjects

Ethan A. Scott

Publications and source records attributed to Ethan A. Scott.

6 recordsLinked to original sources

arXiv2026

Enhanced thermal conductivity of (010) (AlxGa1-x)2O3 epitaxial films utilizing indium-catalyzed molecular beam epitaxy

(AlxGa1-x)2O3/beta-Ga2O3 transistors are an emerging candidate for high-power and high-frequency electronic devices. beta-Ga2O3 in particular is notably limited for anisotropic low thermal conductivity, which is further reduced in (AlxGa1-x)2O3 due to alloy and other defect-driven phonon scattering mechanisms. In this work we show that the thermal conductivity of (010) oriented (AlxGa1-x)2O3 thin films for 0.01 < x < 0.20, measured using time-domain thermoreflectance (TDTR), is limited by alloy scattering without observable adverse scattering by additional defects. This is enabled through the synthesis of the (AlxGa1-x)2O3 films using molecular beam epitaxy (MBE) on \b{eta}-Ga2O3 substrates, leveraging indium-catalyzed growth to suppress dislocation formation and phase separation to achieve single-phase pseudomorphic films up to x = 0.2. This growth process improved the thermal conductivity of (AlxGa1-x)2O3 by 2X as compared to previously reported values. For increasing Al composition (x), we observe a steady decline in (AlxGa1-x)2O3 thermal conductivity due to alloy scattering which is validated using virtual crystal approximation (VCA) model. We also show that the thermal boundary conductance across the Al/(AlxGa1-x)2O3 interface is reduced with increasing x, which we posit is due to the stiffening of the (AlxGa1-x)2O3 acoustic modes with increasing x by comparing experimental results with a diffuse mismatch model (DMM). Overall, these thermal characteristics provide valuable insights for designing heterostructures with optimized interfaces and composition.

cond-mat.mtrl-sci↗

arXiv2026

Synthesis and Characterization of Compositionally Complex (Gd/Ho/Er/Dy)2Zr2O7 Thin Film Combinatorial Library

High-throughput synthesis and characterization of novel ceramic materials with improved thermomechanical properties and phase stability are needed to accelerate the discovery of next-generation thermal barrier materials. A combinatorial thin film material library of (GdDyHoEr)2Zr2O7 were created via combinatorial magnetron reactive sputtering with rare-earth/zirconium alloy targets. Structural, chemical, and thermal property characterization mapping across the four component composition space was performed and correlated with thermal transport measurements. Steady state thermoreflectance mapping identifies a pronounced minimum in thermal conductivity within the Dy/Gd-rich quadrant. This minimum does not coincide with either the equiatomic composition or the region predicted to exhibit maximum cation size disorder. Instead, it corresponds to the largest experimentally observed lattice parameter, despite deviating from Vegard-like chemical averaging, and is independent of grain size and whole-pattern microstrain. These observations suggest that the way the fluorite lattice accommodates compositional complexity, rather than cation size disorder alone, provides a more informative descriptor of thermal transport. Overall, this work establishes a high-throughput workflow for combinatorial thin-film synthesis and multimodal characterization, enabling the rapid identification of previously inaccessible structure-property relationships in compositionally complex ceramics.

cond-mat.mtrl-sci↗

arXiv2026

Lock-In Infrared Thermography: Phase Analysis for Rapid, Wide-Range Thermal Conductivity Measurements

We report on a phase-based lock-in thermography approach, combined with a multilayered thermal model (often employed in thermoreflectance analysis), to measure the thermal conductivity of bulk materials and layered structures. The spatial distribution of the material's thermal phase is monitored with an infrared camera, which is locked into the frequency of a modulated laser used to heat the material. This phase distribution is then fit with a thermal model, in which properties such as thermal conductivity are extracted as fit parameters. This approach enables non-contact, front-side measurements, which are insensitive to surface roughness. The technique does not strictly require the application of a transducer layer, but we highlight the practical benefits of applying a removable adhesive layer to serve as a near-surface absorber. We demonstrate the efficacy of the method by measuring materials with thermal conductivities that span over three orders of magnitude (approximately 1 W/m/K to > 2000 W/m/K).

cond-mat.mtrl-sci↗

arXiv2026

Crystal growth and characterization of the ultra-high temperature substrate $\mathrm{Ta_{1-x}Hf_{x}C_{0.5}}$

Incorporation of $\mathrm{Al_{y}Ga_{1-y}N}$ (AGN) semiconductors into high power electronics offers efficiency improvements in power transmission, generation, and use, if approaches to eliminate the defects arising from film-lattice mismatch can be established. Here, we report the optical floating zone crystal growth of $\mathrm{Ta_{1-x}Hf_{x}C_{0.5}}$ (x = 0.2), a new metallic substrate material family lattice matched to the ultra-wide-band-gap, Al-rich side (y = 0.91) of the AGN solid solution. Laue diffraction demonstrates large single crystal domains in the as-grown boule. Single crystal x-ray diffraction at T = 213 K in conjunction with first principles calculations shows that the material adopts a layered crystal structure with AA-type stacking of (Ta/Hf)-C-(Ta/Hf) trilayers described in the trigonal space group P-3m1 (#164), with a = 3.1168(4) Å, c = 4.9644(4) Å, and $β$ = 120.0°. X-ray photoelectron spectroscopy (XPS) measurements show the Hf:Ta ratio to be close to the nominal value of 0.8:0.2 in the grown crystal. Density Functional Theory calculations reveal that this structure is stabilized by the low energy of carbon-vacancy formation of a hypothetical $\mathrm{(Ta/Hf)_{1}C_{1}}$ anti-NiAs structure type, and imply flexibility in interface structure with an overlayer nitride film. A surface preparation/polishing procedure is developed that reduces root mean square (RMS) surface roughness from as-cut 130 nm to 7 nm as measured by atomic force microscopy. Scanning electron microscopy shows the presence of a native surface oxide, removed by polishing, along with carbon-rich pits. Time-domain thermoreflectance measurements show a room temperature thermal conductivity of $κ$ = 18.1(4) W m-1 K-1. These results provide key first steps for utilizing metallic, lattice matched, substrates for the growth of Al-rich AGN semiconductors.

cond-mat.mtrl-sci↗

arXiv2025

Epitaxial growth and semiconductor properties of NiGa2O4 spinel for Ga2O3/NiO interfaces

Unintentionally formed interfacial layers are ubiquitous in semiconductor devices that operate at extreme conditions. However, these layers' structure and properties often remain unknown due to the thinness of these naturally formed interphases. Here, we report on the intentional epitaxial growth and semiconductor properties of NiGa2O4 spinel layers that form at Ga2O3/NiO interfaces used in high-power and high-temperature electronic devices. Cubic spinel NiGa2O4 films of 10-50 nm thicknesses and low surface roughness (~ 2 nm) were grown using pulsed laser deposition at a substrate temperatures in the 300-900 °C range on α-Al2O3 and β-Ga2O3 substrates of different orientation. The optical absorption onset (3.6-3.9 eV) and thermal conductivity (4-9 W m-1 K-1) vary systematically with substrate temperature, consistent with theoretical predictions of varying Ni and Ga cation ordering on the spinel lattice. The valence band offset between NiGa2O4 and β-Ga2O3 is determined to be 1.8 eV. The NiGa2O4-based p-n heterojunction devices on Ga2O3 (001) substrates exhibit a rectification ratio of 10^8 (for +/-2V) and a turn-on voltage of 1.4 V, maintaining diode behavior up to 600 °C. These results highlight the potential of NiGa2O4 as a p-type interlayer in Ga2O3-based devices and shows a new approach to investigate such interfacial layers.

cond-mat.mtrl-sci↗

arXiv2024

Ruddlesden-Popper chalcogenides push the limit of mechanical stiffness and glass-like thermal conductivity in single crystals

Insulating materials featuring ultralow thermal conductivity for diverse applications also require robust mechanical properties. Conventional thinking, however, which correlates strong bonding with high atomic-vibration-mediated heat conduction, led to diverse weakly bonded materials that feature ultralow thermal conductivity and low elastic moduli. One must, therefore, search for strongly-bonded single crystals in which heat transport is impeded by other means. Here, we report intrinsic, glass-like, ultralow thermal conductivity and ultrahigh elastic-modulus/thermal-conductivity ratio in single-crystalline Ruddlesden-Popper Ban+1ZrnS3n+1, n = 2,3, which are derivatives of BaZrS3. Their key features are strong anharmonicity and intra-unit-cell rock-salt blocks. The latter produce strongly bonded intrinsic superlattices, impeding heat conduction by broadband reduction of phonon velocities and mean free paths and concomitant strong phonon localization. The present study initiates a paradigm of mechanically stiff phonon glasses.

cond-mat.mtrl-sci↗