Christina M. Rost

Entropy-stabilized oxides.

Configurational disorder can be compositionally engineered into mixed oxide by populating a single sublattice with many distinct cations. The formulations promote novel and entropy-stabilized forms of crystalline matter where metal cations are incorporated in new ways. Here, through rigorous experiments, a simple thermodynamic model, and a five-component oxide formulation, we demonstrate beyond reasonable doubt that entropy predominates the thermodynamic landscape, and drives a reversible solid-state transformation between a multiphase and single-phase state. In the latter, cation distributions are proven to be random and homogeneous. The findings validate the hypothesis that deliberate configurational disorder provides an orthogonal strategy to imagine and discover new phases of crystalline matter and untapped opportunities for property engineering.

Charge compensation and electrostatic transferability in three entropy-stabilized oxides: Results from density functional theory calculations

Density functional theory calculations were carried out for three entropic rocksalt oxides, (Mg0.1Co0.1Ni0.1Cu0.1Zn0.1)O0.5, termed J14, and J14 + Li and J14 + Sc, to understand the role of charge neutrality and electronic states on their properties, and to probe whether simple expressions may exist that predict stability. The calculations predict that the average lattice constants of the ternary structures provide good approximations to that of the random structures. For J14, Bader charges are transferable between the binary, ternary, and random structures. For J14 + Sc and J14 + Li, average Bader charges in the entropic structures can be estimated from the ternary compositions. Addition of Sc to J14 reduces the majority of Cu, which show large displacements from ideal lattice sites, along with reduction of a few Co and Ni cations. Addition of Li to J14 reduces the lattice constant, consistent with experiment, and oxidizes some of Co as well as some of Ni and Cu. The Bader charges and spin-resolved densi...

Size dictated thermal conductivity of GaN

The thermal conductivity of n- and p-type doped gallium nitride (GaN) epilayers having thicknesses of 3–4 μm was investigated using time domain thermoreflectance. Despite possessing carrier concentrations ranging across 3 decades (1015–1018 cm–3), n-type layers exhibit a nearly constant thermal conductivity of 180 W/mK. The thermal conductivity of p-type epilayers, in contrast, reduces from 160 to 110 W/mK with increased doping. These trends—and their overall reduction relative to bulk—are explained leveraging established scattering models where it is shown that, while the decrease in p-type layers is partly due to the increased impurity levels evolving from its doping, size effects play a primary role in limiting the thermal conductivity of GaN layers tens of microns thick. Device layers, even of pristine quality, will therefore exhibit thermal conductivities less than the bulk value of 240 W/mK owing to their finite thickness.

Chemical Homogeneity in Entropy-Stabilized Complex Metal Oxides

Innovation in new mixtures of constituents can lead to discover exciting new materials with unexpected properties and revolutionary applications [1,2]. It is known, the Gibbs energy needs to be minimized, as the main requirement, to achieve a stable single phase compound. Conventional approach to minimize the total energy of system is searching for a large and negative enthalpy. However, in this work, we show that the phase stability can be reached where the configurational entropy is maximized with mixing as many diverse elements as possible.

Nitride surface chemistry influence on band offsets at epitaxial oxide/GaN interfaces

GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions.GaN surface and near-surface chemistry influence on band offsets of oxide overlayers is demonstrated through X-ray photoelectron spectroscopy measurements using epitaxial (111)-oriented MgO films on (0001)-oriented Ga-polar GaN as a case study. For identical cleaning and MgO growth conditions, GaN subsurface oxygen impurities influence the GaN bare surface band bending and the ultimate band offset to MgO heterolayers. As the GaN surface oxygen concentration increases from an atomic concentration of 0.9% to 3.4%, the valence band offset to MgO decreases from 1.68 eV to 1.29 eV, respectively. This study highlights the sensitivity of the oxide/nitride interface electronic structure to GaN epilayer preparation and growth conditions.

Charge-Induced Disorder Controls the Thermal Conductivity of Entropy-Stabilized Oxides

Manipulating a crystalline materials configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy-stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single-crystal entropy-stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.

Nanoscale Compositional Analysis of a Thermally Processed Entropy-Stabilized Oxide via Correlative TEM and APT

The recent demonstration of entropy-stabilized oxides (ESOs) [1] suggests new avenues for controlling the microstructures and, thus, properties within this new family of oxide ceramics through a combination of composition control and thermal processing. In order to better understand the possibilities for engineering specific properties, a detailed understanding at the atomic scale is desirable. The exploration of ESO microstructure has been largely focused on that of the quenched, single phase material [1,2]. However, study of the multi-phase material that results from controlled cooling of the single phase material can provide insight into the destabilization mechanisms and methods of manipulating the microstructure. The correlated use of transmission electron microscopy (TEM) and atom probe tomography (APT) is well-suited for such analyses [3,4].

Hafnium nitride films for thermoreflectance transducers at high temperatures: Potential based on heating from laser absorption

Time domain thermoreflectance (TDTR) and frequency domain thermoreflectance (FDTR) are common pump-probe techniques that are used to measure the thermal properties of materials. At elevated temperatures, transducers used in these techniques can become limited by melting or other phase transitions. In this work, time domain thermoreflectance is used to determine the viability of HfN thin film transducers grown on SiO2 through measurements of the SiO2 thermal conductivity up to approximately 1000 K. Further, the reliability of HfN as a transducer is determined by measuring the thermal conductivities of MgO, Al2O3, and diamond at room temperature. The thermoreflectance coefficient of HfN was found to be 1.4 × 10−4 K−1 at 800 nm, one of the highest thermoreflectance coefficients measured at this standard TDTR probe wavelength. Additionally, the high absorption of HfN at 400 nm is shown to enable reliable laser heating to elevate the sample temperature during a measurement, relative to other transducers.