Catalytically active nickel-substituted cerium oxide (CeO2) aerogels, engineered to maximize yield in the water-gas shift reaction (WGS: H2O + CO ↔ H2 + CO2), are characterized using scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS), and energy-dispersive spectroscopy (EDS). STEM shows that the architected CeO2 catalysts with Ni substituted at 2.5, 5, and 10 atomic percentage (at.%) contain atomically distributed Ni within the ∼7 nm CeO2 nanocrystalline domains that comprise the covalently bonded oxide network. The aerogels prepared at the two higher Ni-to-Ce compositions also contain >100 nm nickel-oxide particles, which correlates with our prior report that these catalysts generate undesired CH4 as a byproduct during WGS reactions run at mild temperatures. The micrographic absence of large nickel-oxide particles in 2.5 at. % Ni-substituted CeO2 aerogel substantiates why this WGS catalyst is methane-free to the limit of detection. We find that high electron fluence modifies the cerium oxidation state and nickel distribution, resulting in a mix of Ce3+ and Ce4+ and ∼1 nm nickel aggregates. Density functional theory indicates that Ni cations are mobile and tend to aggregate in a CeO2 fluorite lattice, even in the absence of electron flux, particularly when located near oxygen vacancies. The nature of Ni within the CeO2 aerogel, as observed by STEM/EELS/EDS, validates the need to prevent Ni aggregation in WGS catalysts.

GaN-based high-electron-mobility transistors (HEMTs) are well-suited for high-power and RF device applications due to the excellent transport characteristics and high breakdown field of GaN. However, the technology is inherently limited by high and non-uniform peak electric fields at the drain-edge of the gate that causes premature electric field induced breakdown. Recently, electric field profile management strategies have been proposed utilizing high permittivity (εr) dielectric materials, such as BaTiO3 (BTO). These dielectric BTO layers are typically deposited using techniques such as RF magnetron sputtering, which produces disordered films with lower permittivity values and interface quality. Here, we present an approach to create epitaxial BTO films on a GaN HEMT structure by use of oxide molecular beam epitaxy and employing thin SrTiO3 and TiO2 buffer layers to promote BTO crystallinity. We investigate the effect of BTO deposition temperature on both structural and electrical properties and find a correlation between crystal quality and permittivity, with a BTO film deposited at 850 °C exhibiting an extracted effective εr = 228 and loss tangent value of 2.7 × 10−3 at 10 kHz. These results offer an approach to designing a GaN HEMT structure with high-quality integrated high-εr dielectrics capable of enhanced RF and power-switching performance.

ScN alloyed AlN (ScxAl1-xN, ScAlN) is a wurtzite semiconductor with attractive ferroelectric, dielectric, piezoelectric, and optical properties. Here, we show that ScAlN films (with x spanning 0.18 to 0.36) contain nanoscale Sc-rich clusters which maintain the wurtzite crystal structure. While both molecular beam epitaxy (MBE) and sputter deposited Sc0.3Al0.7N films show Sc clustering, the degree of clustering is significantly stronger for the MBE-grown film, offering an explanation for some of the discrepancies between MBE-grown and sputtered films reported in the literature. Moreover, the MBE-grown Sc0.3Al0.7N film exhibits a dispersive and anomalously large dielectric permittivity, roughly double that of sputtered Sc0.3Al0.7N. We attribute this result to the Sc-rich clusters locally reaching x 0.5 and approaching the predicted ferroelectric-to-paraelectric phase transition, resulting in a giant (local) enhancement in permittivity. The Sc-rich clusters should similarly affect the piezoelectric, optical, and ferroelectric responses, suggesting cluster-engineering as a means to tailor ScAlNs functional properties.

High purity aluminum in its bulk form has intrinsically high reflectance in the far-ultraviolet (FUV) regime and finds utility in astrophysical instrumentation applications. However, bulk Al oxidizes rapidly in the atmosphere, and its native oxide strongly absorbs and severely degrades the observed FUV properties relative to bare Al. Various techniques have been investigated to produce coatings that inhibit aluminum oxide formation and lead to high FUV mirror reflectance. This work examines the development and use of a uniquely modified, hybrid plasma-enhanced atomic layer deposition (PEALD) system to passivate aluminum mirrors with metal fluoride films. This system combines two plasma sources in a commercial atomic layer deposition (ALD) reactor. The first is a conventional inductively coupled plasma (ICP) source operated as a remote plasma, and the second is an electron beam (e-beam) driven plasma near the mirror surface. To establish the operating conditions for the in situ e-beam plasma source, the effects of sample grounding, SF6/Ar flow, and sample temperature on resulting AlF3 films were investigated. Optimal operating conditions produced mirrors with excellent FUV reflectivity, 92% at 121 nm and 42% at 103 nm wavelengths, which is comparable to state-of-the-art AlF3-based passivation coatings and matches that of previously reported ex situ e-beam plasma-processed mirrors. This optimized in situ e-beam process, along with XeF2 passivation, is then explored to produce a clean seed layer (unoxidized Al surface) for subsequent PEALD of AlF3. Both approaches are demonstrated as valid pretreatments before PEALD of AlF3, showing a promising pathway for the deposition of other fluoride-based layers, such as MgF2 or LiF, with ALD or PEALD.

We present a comprehensive study of dielectric properties including complex permittivity, loss, and leakage of high-ScN-fraction ScAlN thin films grown using molecular beam epitaxy (MBE). Dielectric spectroscopy is carried out on high-ScN-fraction (30 that real permittivity ϵ' increases significantly with increasing ScN fraction; a trend confirmed by density functional theory. Further, ϵ' is strongly dispersive with frequency and increasing ScN fraction, with values for Sc0.4Al0.6N varying from 150 down to 60 with increasing frequency. Loss, dispersion, and DC leakage current correspondingly increase with ScN fraction. The high ϵ' and strongly dispersive behavior in MBE ScAlN are not observed in a sputter-deposited ScAlN control with a similar ScN fraction, highlighting fundamental differences between films produced by the two deposition methods. Microscopy and spectroscopy analyses are carried out on MBE- and sputter-deposited samples to compare microstructure, alloy, and dopant concentration.

We report on the growth of epitaxial ScxAl1−xN (x = 0.3–0.4) on (111) Si substrates via molecular beam epitaxy. Growth of an AlN nucleation layer (NL) is sensitive to the III/V flux ratio, with more N-rich growth conditions leading to evidence of grain tilt and a degradation in the structural quality of subsequently grown ScAlN. Utilizing the optimized AlN NL III/V of 0.9, ScxAl1−xN films were grown with x = 0.3–0.4 having an x-ray diffraction 0002 reflection rocking curve full width at half maximum (FWHM) of 0.69°–1.14°. Incorporation of a graded ScAlN initiation layer is shown to reduce the FWHM and tensile stress magnitude, while yielding a film with rms roughness as low as 0.57 nm and no detectable anomalously oriented grains on the sample surface.

Efficient and uniform Aluminum-based broadband mirrors are essential components for far-ultraviolet (FUV) astronomy. Plasma-enhanced atomic layer deposition ( PEALD) is a low temperature, highly conformal coating process that has previously been demonstrated to produce high quality AlF3 films, although little has been reported on their performance in FUV applications. An ongoing collaboration between the US Naval Research Laboratory (NRL) and NASA Goddard Space Flight Center (GSFC) focuses on optimizing material properties of PEALD AlF3 coatings on Al mirrors to enhance FUV optical performance. PEALD AlF3 films were deposited using trimethylaluminum and SF6 plasma precursors in a modified Veeco Fiji G2 reactor. ALD growth windows (the range of process parameters resulting in ideal growth) were established using an in situ ellipsometer to monitor the fluoride growth rate directly on Al substrates and supplemented with post-deposition x-ray photoelectron spectroscopy to elucidate process-structure property relationships. Optimal AlF3 films had a growth rate of 0.75-0.8 angstrom/cycle, F/Al ratio of approximate to 3, < 2 at% O, indicating that PEALD is a beneficial process technique towards achieving optical coatings on a variety of potential mirror materials. The influence of PEALD parameters on the FUV optical performance of Al mirrors overcoated with PEALD-AlF3 will be also discussed.

High ScN fraction ScxAl1−xN has promise in important application areas including wide bandwidth RF resonators and filters, and ferroelectric devices such as non-volatile memory, but demands high crystal quality. In this work, the role of the nucleation layer (NL), ScxAl1−xN growth temperature, and strain management to preserve the wurtzite crystal structure are investigated to maximize both acoustoelectric and ferroelectric material properties for high ScN fraction ScxAl1−xN grown on SiC substrates. A 5 nm AlN nucleation layer reduces the x-ray diffraction 0002 reflection full width at half maximum (FWHM) for a Sc0.32Al0.68N film by almost a factor of 2, and reducing the growth temperature to 430 °C enables a Sc0.40Al0.60N film with a FWHM of 4100 arcsec (1.1°) while being only 150 nm thick. Grading the initial ScxAl1−xN layer from x = 0.32 to 0.40 suppresses the formation of rock-salt grain nucleation at the Sc0.40Al0.60N lower interface and reduces the anomalously oriented grain density by an order of magnitude. Increasing the total ScxAl1−xN growth thickness to 500 nm produces an average x = 0.39 ScxAl1−xN layer with a FWHM of 3190 arcsec (0.89°) and an anomalously oriented grain areal fill factor of 1.0%. These methods enable the lowest heteroepitaxial ScxAl1−xN FWHM reported for x ∼ 0.4, with layer thicknesses and defect densities appropriate for high frequency (>10 GHz) filter applications.

Journal Article The Unique EELS Signature of Point Defects in Cubic Boron Nitride on Diamond Get access Andrew C Lang, Andrew C Lang Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Corresponding author: andrew.lang@nrl.navy.mil Search for other works by this author on: Oxford Academic Google Scholar David F Storm, David F Storm Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Search for other works by this author on: Oxford Academic Google Scholar Sergey I Maximenko, Sergey I Maximenko Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Search for other works by this author on: Oxford Academic Google Scholar Neeraj Nepal, Neeraj Nepal Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Search for other works by this author on: Oxford Academic Google Scholar Virginia D Wheeler, Virginia D Wheeler Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Search for other works by this author on: Oxford Academic Google Scholar David J Meyer David J Meyer Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington DC, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Page 1798, https://doi.org/10.1093/micmic/ozad067.930 Published: 22 July 2023

Tantalum nitride (γ-Ta2N) thin films were grown by radiofrequency plasma molecular beam epitaxy (MBE) on 3 in. diameter 6H- or 4H-SiC substrates. Epitaxial characteristics of these MBE grown layers were determined using high resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM) measurements. HRXRD, TEM, and XRD reciprocal space map measurements show that γ-Ta2N on SiC grows pseudomorphically and stabilizes to a nearly pure γ-Ta2N phase. Structural properties of these layers are uniform across the 3 in. wafer diameter. Measured a and c lattice parameter values of a 43 nm thick γ-Ta2N film on 6H-SiC are 3.079 and 4.898 Å, respectively, and the film has an in-plane tensile strain of 1.03%. MBE growth of AlN/ γ-Ta2N /SiC heterostructures has also been demonstrated. Measured lattice a and c constants of AlN on γ-Ta2N /SiC are 3.120 and 4.974 Å, respectively. TEM and XRD show that SiC substrate and γ-Ta2N films have parallel epitaxial relation.

Here, we use the micro-transfer printing technique to demonstrate the device-level heterogeneous integration of two solid-state RF device technologies on the same interposer: GaN and GaAs high-electron-mobility transistors. The devices are released from their growth substrate using an epitaxial sacrificial layer while a thin polymer adhesion layer facilitates a strong bond between the target substrate and the compound semiconductor devices, allowing for post-transfer microfabrication processing. Transmission electron microscopy reveals no voids at the device/interposer interface and a polymer adhesion layer thickness of 5 ± 2 nm. No significant degradation in dc electrical characteristics is observed after device transfer for either device technology. Improvement in thermal performance of GaN devices was demonstrated when transferred to a diamond substrate, even with the thin polymer adhesion layer at the device/interposer interface, illustrating a pathway for enhanced thermal management for GaN and other high-output-power density semiconductor technologies. The ability to combine various solid-state technologies at the device level with high density provides an approach to meet next-generation demands for RF and mixed-signal circuits.

Short range order (SRO) is critical in determining the performance of many important engineering materials. However, accurate characterization of SRO with high spatial resolution -which is needed for the study of individual nanoparticles and at material defects and interfaces -is often experimentally inaccessible. Here, we locally quantify SRO via scanning transmission electron microscopy with extended energy loss fine structure analysis. Specifically, we use novel instrumentation to perform electron energy loss spectroscopy out to 12 key, accessing energies which are conventionally only possible using a synchrotron. Our data is of sufficient energy resolution and signal-to-noise ratio to perform quantitative extended fine structure analysis, which allows determination of local coordination environments. To showcase this technique, we investigate a multicomponent metallic glass nanolaminate and locally quantify the SRO with <10 nm spatial resolution; this measurement would have been impossible with conventional synchrotron or electron microscopy methods. We discuss the nature of SRO within the metallic glass phase, as well as the wider applicability of our approach for determining processing-SRO-property relationships in complex materials.(c) 2022 Elsevier Ltd. All rights reserved.

Heterogeneous integration of functional oxides with ultra-wide bandgap (UWBG) semiconductors is desired for the realization of novel hybrid systems applicable to a wide array of commercial electronics and defense applications. In this work, we demonstrate the growth of crystalline SrTiO3 (STO) thin films on high-electron-mobility transistor (HEMT) heterostructures based on an emergent UWBG semiconductor ScAlN, used as the barrier layer on a GaN channel, and determine the effects of the pre-growth chemical treatments of the ScAlN surface on resultant heterostructure properties. We investigate wet chemical cleans of ScAlN with solvents, piranha solution, UV ozone and hydrofluoric acid, and a sulfuric-phosphoric acid mix prior to STO growth, and show that the commonly used piranha solution degrades the ScAlN surface, thereby reducing the crystal quality of the deposited STO layers and lowering the channel mobility. We determine that among the treatments studied, the solvent and sulfuric-phosphoric acid cleans were the least disruptive to the electrical properties of the GaN channel as evidenced from Hall effect measurements, but the sulfuric-phosphoric acid clean results in best oxide crystallinity, as determined from structural characterizations. We perform transmission electron microscopy imaging on the piranha-treated and sulfuric-phosphoric-treated samples to compare the microstructure and find that while intermixing occurs at the oxide-nitride interfaces for both samples, the interface roughness is lower and the STO grain size is larger in the sample with sulfuric-phosphoric acid treatment. This work demonstrates the first epitaxial growth of STO on an UWBG semiconductor and motivates STO/ScAlN/GaN as material platforms for high-frequency, high-power-density HEMTs.

Journal Article Probing Defects in Epitaxially Grown Cubic Boron Nitride on Diamond Get access Andrew C Lang, Andrew C Lang Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Corresponding author: andrew.lang@nrl.navy.mil Search for other works by this author on: Oxford Academic Google Scholar David F Storm, David F Storm Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Sergey I Maximenko, Sergey I Maximenko Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Neeraj Nepal, Neeraj Nepal Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar David J Meyer, David J Meyer Electronics Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Rhonda M Stroud Rhonda M Stroud Materials Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 2382–2383, https://doi.org/10.1017/S143192762200914X Published: 01 August 2022

Trace amounts of Mg deposited on a diamond (100) substrate surface facilitate the growth of cubic boron nitride (c-BN) by ion beam-assisted molecular beam epitaxy. Fourier transform infrared spectroscopy indicates that films grown with Mg are cubic, while those without Mg are either hexagonal BN or lacking measurable cubic or hexagonal signatures. Initiating the growth with 0.005 monolayer equivalent of Mg is sufficient to yield epitaxial films with >99% c-BN. Reflection high energy electron diffraction, electron energy loss spectroscopy, and X-Ray photoelectron spectroscopy indicate the surface of the film to be sp(2)-bonded BN, consistent with the results of other groups. High-resolution scanning transmission electron microscopy reveals c-BN with a high density of stacking faults and twinning. A model is proposed by which Mg locally diminishes the energy barrier to dissociation of the as-deposited sp(2)-bonded BN, facilitating the nucleation of c-BN.

Epitaxial transition metal nitrides (TMNs) are an emerging class of crystalline thin film metals that can be heteroepitaxially integrated with common group III-nitride semiconductors such as GaN and AlN. Within a binary family of TMN compounds (i.e., TaxNy), several phases typically exist, many with similar crystal structures that are difficult to distinguish by conventional X-ray diffraction or other bulk characterization means. In this work, we demonstrate the combined power of high-resolution transmission and aberration-corrected scanning transmission electron microscopy for definitive phase identification of tantalum nitrides with different N-sublattice ordering. Analysis of molecular beam epitaxy-grown γ-Ta2N films on SiC substrates shows that the films are γ phase, threading dislocation-free, and Ta-deficient. The lack of Ta manifests as ordered Ta vacancy planar defects oriented in the plane perpendicular to the [0001] growth direction and accounts for the substoichiometry. Optimization of the growth parameters should reduce the Ta vacancy concentration, and alternatively, exploitation of the attractive nature of the Ta vacancies may enable novel planar structures. These findings serve as an important first step in applying this epitaxial TMN material for new electronic and superconducting device structures.

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Control of surface functionalization of MXenes holds great potential, and in particular, may lead to tuning of magnetic and electronic order in the recently reported magnetic Cr2TiC2Tx. Here, vacuum annealing experiments of Cr2TiC2Tx are reported with in situ electron energy loss spectroscopy and novel in situ Cr K-edge extended energy loss fine structure analysis, which directly tracks the evolution of the MXene surface coordination environment. These in situ probes are accompanied by benchmarking synchrotron X-ray absorption fine structure measurements and density functional theory calculations. With the etching method used here, the MXene has an initial termination chemistry of Cr2TiC2O1.3F0.8. Annealing to 600 C results in the complete loss of -F, but -O termination is thermally stable up to (at least) 700 C. These findings demonstrate thermal control of -F termination in Cr2TiC2Tx and offer a first step towards termination engineering this MXene for magnetic applications. Moreover, this work demonstrates high energy electron spectroscopy as a powerful approach for surface characterization in 2D materials.

High entropy alloys (HEAs) have been the subject of significant research in recent years due in part to their excellent mechanical properties and unique microstructure. Chemical disorder in these alloys is thought to lead to a complex energetic environment that affects the nucleation and movement of dislocations. In this study the development of deformation substructures is assessed in the Cantor HEA (CoCrFeMnNi) due to quasistatic and dynamic compression. Scanning and transmission electron microscopy (SEM/TEM) techniques are used to image and quantify dislocation density. When increasing the strain rate from 10−3 s−1 to 5000 s−1 we observe an increase in the overall dislocation density which leads to the formation of deformation twins. Further at a rate of 8000 s−1, both deformation twins and microbands become operative plasticity modes, which are usually associated with different extremes of stacking fault energy. To relate this plastic response to chemical disorder, atomistic calculations are used to compute the generalized stacking fault energy curve and approximate the temperature dependence of the intrinsic stacking fault energy for the Cantor alloy, which reveals significant local variation in these critical energetic parameters associated with dislocation and twinning behaviors.