Results for “Ellipsometry” — Page 19

NASA NTRS2007

Measurement of Damage Profiles from Solar Wind Implantation

NASA's Genesis Mission launched from Cape Canaveral in August of 2001 with the goal of collecting solar wind in ultra-pure materials. The samples were returned to Earth more than three years later for subsequent analysis. Although the solar wind is comprised primarily of protons, it also contains ionized species representing the entire periodic table. The Genesis mission took advantage of the natural momentum of these ionized species to implant themselves in specialized collectors including single crystal Si and SiC. The collectors trapped the solar wind species of interest and sustained significant damage to the surface crystal structure as a result of the ion bombardment. In this work, spectroscopic ellipsometry has been used to evaluate the extent of this damage in Si and SiC samples. These results and models are compared for artificially implanted samples and pristine non-flight material. In addition, the flown samples had accumulated a thin film of molecular contamination as a result of outgassing in flight, and we demonstrate that this layer can be differentiated from the material damage. In addition to collecting bulk solar wind samples (continuous exposure), the Genesis mission actually returned silicon exposed to four different solar wind regimes: bulk, high speed, low speed, and coronal mass ejections. Each of these solar wind regimes varies in energy, but may vary in composition as well. While determining the composition is a primary goal of the mission, we are also interested in the variation in depth and extent of the damage layer as a function of solar wind regime. Here, we examine flight Si from the bulk solar wind regime and compare the results to both pristine and artificially implanted Si. Finally, there were four samples which were mounted in an electrostatic "concentrator" designed to reject a large fraction (>85%) of incoming protons while enhancing the concentration of ions mass 4-28 amu by a factor of at least 20. Two of these samples were single crystal 6H silicon carbide. (The others were polycrystalline CVD diamond and amorphous carbon that were not examined in the work.) The ion damaged SiC samples from the concentrator were studied in comparison to the flight Si from the bulk array to understand differences in the extent of the damage.

McNamara, K. M.↗

NASA NTRS2007

Genesis Concentrator Target Particle Contamination Mapping and Material Identification

The majority of surface particles were found to be < 5 microns in diameter with increasing numbers close to the optical resolution limit of 0.3 microns. Acceleration grid EDS results show that the majority of materials appear to be from the SRC shell and SLA materials which include carbon-carbon fibers and Si-rich microspheres in a possible silicone binder. Other major debris material from the SRC included white paint, kapton, collector array fragments, and Al. Image analysis also revealed that SRC materials were also found mixed with the Utah mud and salt deposits. The EDS analysis of the acceleration grid showed that particles < 1 m where generally carbon based particles. Chemical cleaning techniques with Xylene and HF in an ultrasonic bath are currently being investigated for removal of small particles by the Genesis science team as well as ultra-pure water megasonic cleaning by the JSC team [4]. Removal of organic contamination from target materials is also being investigated by the science team with the use of UV-ozone cleaning devices at JSC and Open University [5]. In preparation for solar wind oxygen analyses at UCLA and Open University [1, 2], surface particle contamination on three Genesis concentrator targets was closely examined to evaluate cleaning strategies. Two silicon carbide (Genesis sample # 60001 and 60003) and one chemical vapor deposited (CVD) 13C concentrator target (60002) were imaged and mosaic mapped with optical microscopes. The resulting full target mosaic images and particle feature maps were subsequently compared with non-flight, but flight-like, concentrator targets and sample return capsule (SRC) materials. Contamination found on the flown concentrator acceleration grid was further examined using a scanning electron microscope (SEM). Energy dispersive X-ray spectroscopy (EDS) for particle identification was subsequently compared with the optical images from the flown targets. Figure 1 show that all three targets imaged in this report are fully intact and do not show any signs of material fractures. However, previous ellipsometry results and overview imaging of both flown SiC targets show a solar wind irradiation gradient from the center focal point to the outer edge [3]. In addition, due to the hard landing, each target has experienced varying degrees of impacts, scratches, and particle debris from the spacecraft and Utah impact site.

Calaway, Michael J.↗

NASA NTRS2006

Genesis Solar Wind Sample Curation: A Progress Report

In the year since the Genesis solar wind collector fragments were returned, early science samples, specimens for cleaning experiments, and science allocations have been distributed. Solar wind samples are stored under nitrogen and handled in an ISO Class 4 (Class 10) laboratory. For array collector fragments, a basic characterization process has been established. This characterization consists of identification of solar wind regime, whole fragment image for identification and surface quality, higher magnification images for contaminant particle density, and assessment of molecular film contaminant thickness via ellipsometry modeling. Compilations of this characterization data for AuOS (gold film on sapphire), and sapphire from the bulk solar wind for fragments greater than 2 cm are available. Removal of contaminant particles using flowing ultrapure water (UPW) energized megasonically is provided as requested.

Allton, Judith H.↗

NASA NTRS2008

NASA Tech Briefs, November 2008

Topics covered include: Digital Phase Meter for a Laser Heterodyne Interferometer; Vision System Measures Motions of Robot and External Objects; Advanced Precipitation Radar Antenna to Measure Rainfall From Space; Wide-Band Radar for Measuring Thickness of Sea Ice; Vertical Isolation for Photodiodes in CMOS Imagers; Wide-Band Microwave Receivers Using Photonic Processing; L-Band Transmit/Receive Module for Phase-Stable Array Antennas; Microwave Power Combiner/Switch Utilizing a Faraday Rotator; Compact Low-Loss Planar Magic-T; Using Pipelined XNOR Logic to Reduce SEU Risks in State Machines; Quasi-Optical Transmission Line for 94-GHz Radar; Next Generation Flight Controller Trainer System; Converting from DDOR SASF to APF; Converting from CVF to AAF; Documenting AUTOGEN and APGEN Model Files; Sequence History Update Tool; Extraction and Analysis of Display Data; MRO DKF Post-Processing Tool; Rig Diagnostic Tools; MRO Sequence Checking Tool; Science Activity Planner for the MER Mission; UAVSAR Flight-Planning System; Templates for Deposition of Microscopic Pointed Structures; Adjustable Membrane Mirrors Incorporating G-Elastomers; Hall-Effect Thruster Utilizing Bismuth as Propellant; High-Temperature Crystal-Growth Cartridge Tubes Made by VPS; Quench Crucibles Reinforced with Metal; Deep-Sea Hydrothermal-Vent Sampler; Mars Rocket Propulsion System; Two-Stage Passive Vibration Isolator; Improved Thermal Design of a Compression Mold; Enhanced Pseudo-Waypoint Guidance for Spacecraft Maneuvers; Altimetry Using GPS-Reflection/Occultation Interferometry; Thermally Driven Josephson Effect; Perturbation Effects on a Supercritical C7H16/N2 Mixing Layer; Gold Nanoparticle Labels Amplify Ellipsometric Signals; Phase Matching of Diverse Modes in a WGM Resonator; WGM Resonators for Terahertz-to-Optical Frequency Conversion; Determining Concentration of Nanoparticles from Ellipsometry; Microwave-to-Optical Conversion in WGM Resonators; Four-Pass Coupler for Laser-Diode-Pumped Solid-State Laser; Low-Resolution Raman-Spectroscopy Combustion Thermometry; Temperature Sensors Based on WGM Optical Resonators; Varying the Divergence of Multiple Parallel Laser Beams; Efficient Algorithm for Rectangular Spiral Search; Algorithm-Based Fault Tolerance Integrated with Replication; Targeting and Localization for Mars Rover Operations; Terrain-Adaptive Navigation Architecture; Self-Adjusting Hash Tables for Embedded Flight Applications; Schema for Spacecraft-Command Dictionary; Combined GMSK Communications and PN Ranging; System-Level Integration of Mass Memory; Network-Attached Solid-State Recorder Architecture; Method of Cross-Linking Aerogels Using a One-Pot Reaction Scheme; An Efficient Reachability Analysis Algorithm.

Source record↗

NASA NTRS2012

Post Flight Analysis of Optical Specimens from MISSE7

More than 100 optical specimens were flown on the MISSE7 platform. These included bare substrates in addition to coatings designed to exhibit clearly defined or enhanced sensitivity to the accumulation of contamination. Measurements were performed using spectrophotometers operating from the UV through the IR as well as ellipsometry. Results will be presented in addition to discussion of the best options for design of samples for future exposure experiments.

Stewart, Alan F.↗

NASA NTRS2013

Thermal Stability Results of a Fischer-Tropsch Fuel With Various Blends of Aromatic Solution

Fischer-Tropsch (F-T) jet fuel composition differs from petroleum-based, conventional commercial jet fuel because of differences in feedstock and production methodology. F-T fuel typically has a lower aromatic and sulfur content and consists primarily of iso and normal paraffins. The ASTM D3241 specification for Jet Fuel Thermal Oxidation Test (JFTOT) break point testing method was used to test the breakpoint of a baseline commercial grade F-T jet fuel, and various blends of this F-T fuel with an aromatic solution. The goal of this research is to determine the effect of aromatic content on the thermal stability of F-T fuel. The testing completed in this report was supported by the NASA Fundamental Aeronautics Subsonic Fixed Wing Project. Two different aromatic content fuels from Rentech, as well as these fuels with added aromatic blend were analyzed for thermal stability using the JFTOT method. Preliminary results indicate a reduction in thermal stability occurs upon increasing the aromatic content to 10% by adding an aromatic blend to the neat fuel. These results do not specify a failure based on pressure drop, but only on tube color. It is unclear whether tube color correlates to more deposition on the tube surface or not. Further research is necessary in order to determine if these failures are true failures based on tube color. Research using ellipsometry to determine tube deposit thickness rather than color will be continued in follow-up of this study.

Lindsey, Jennifer↗

NASA NTRS2014

Development of Genesis Solar Wind Sample Cleanliness Assessment: Initial Report on Sample 60341 Optical Imagery and Elemental Mapping

Since 2005 the Genesis science team has experimented with techniques for removing the contaminant particles and films from the collection surface of the Genesis fragments. A subset of ~40 samples have been designated as "cleaning matrix" samples. These are small samples to which various cleaning approaches are applied and then cleanliness is assessed optically, by TRXRF, SEM, ToF-SIMS, XPS, ellipsometry or other means [1-9]. Most of these sam-ples remain available for allocation, with cleanliness assessment data. This assessment allows evaluation of various cleaning techniques and handling or analytical effects. Cleaning techniques investigated by the Genesis community include acid/base etching, acetate replica peels, ion beam, and CO2 snow jet cleaning [10-16]. JSC provides surface cleaning using UV ozone exposure and ultra-pure water (UPW) [17-20]. The UPW rinse is commonly used to clean samples for handling debris between processing by different researchers. Optical microscopic images of the sample taken before and after UPW cleaning show what has been added or removed during the cleaning process.

Gonzalez, C. P.↗

NASA NTRS2016

Controlled Contamination of Epoxy Composites with PDMS and Removal by Laser Ablation

Surface preparation is critical to the performance of adhesively bonded composites. During manufacturing, minute quantities of mold release compounds are inevitably deposited on faying surfaces and may compromise bond performance. To ensure safety, mechanical fasteners and other crack arrest features must be installed in the bondlines of primary structures, which negates some advantages of adhesively bonded construction. Laser ablation is an automated, repeatable, and scalable process with high potential for the surface preparation of metals and composites in critical applications such as primary airframe structures. In this study, laser ablation is evaluated on composite surfaces for the removal of polydimethylsiloxane (PDMS), a common mold release material. Composite panels were contaminated uniformly with PDMS film thicknesses as low as 6.0 nm as measured by variable angle spectroscopic ellipsometry. Bond performance was assessed by mechanical testing using a 250 F cure, epoxy adhesive and compared with pre-bond surface inspection results. Water contact angle, optically stimulated electron emission, and laser induced breakdown spectroscopy were used to characterize contaminated and laser ablated surfaces. The failure mode obtained from double cantilever beam tests correlated well with surface characterization data. The test results indicated that even low levels of PDMS were not completely removed by laser ablation.

Palmieri, Frank↗

NASA NTRSDate not supplied

Enhanced Far Ultra-Violet Optical Properties of Physical Vapor Deposited Aluminum Mirrors Through Fluorination

Astronomical instrumentation for measurements in the Far Ultra-Violet (FUV, 90-200 nm) typically use aluminum (Al) thin films due to their high reflectance over this wavelength range. However, the native aluminum oxide layer that forms on Al upon exposure to the atmosphere is strongly absorbing in this wavelength range, requiring that the films be protected with a dielectric that inhibits oxidation. Typically, magnesium fluoride (MgF 2 ) or lithium fluoride (LiF) coatings are used as protective layers, but each has shortcomings. For example, MgF 2 has an absorption cutoff at 115 nm reducing performance in a critical part of the FUV spectrum of observational interest. A viable option to access these lower wavelengths could be the use of the LiF overcoat as it has a lower absorption cutoff at 102.5 nm, but it is hygroscopic and thus susceptible to degradation in humid conditions. An approach to reduce the hygroscopic nature of LiF would be to implement a passivation process during the deposition of both Al+LiF coatings. Our team at GSFC has developed a new reactive Physical Vapor Deposition (rPVD) process that consists of a fluorination process with XeF 2 gas combined with our traditional PVD process. We have found that this new rPVD coating process offers a protected version of Al+LiF with a more environmentally stable and more transparent LiF layer, along with unprecedent reflectivity. The process starts with a bare optically smooth substrate that is coated with Al in an ultra-high vacuum (UHV) chamber by the conventional PVD process. Then, the bare Al mirror is immediately exposed to a reactive XeF 2 gas before and after the application of the flash PVD evaporated LiF layer. We have also been investigating the use of this rPVD coating process for potential efficiency enhancements of Si-based gratings. Since it is known that the XeF 2 vapor is a strong Si etchant, we are investigating if the native SiO 2 layer on Si is sufficient to protect the groove profile of e-beam-ruled Si gratings from degradation. We will report on the characterization of various Al+LiF witness mirror coatings (both on borosilicate and Si) performed under various deposition conditions using the XeF 2 passivation process. These tests include XPS to determine film stoichiometry and AFM/SEM to measure surface roughness and any etching on the Si substrate. We will also report on FUV reflectance and spectroscopic ellipsometry to characterize optical constants of these passivated films in the UV/Visible/NIR spectral ranges.

Engineering (General)↗

NASA NTRSDate not supplied

Enhanced Far Ultra-Violet Optical Properties of Physical Vapor Deposited Aluminum Mirrors Through Fluorination

Astronomical instrumentation for measurements in the Far Ultra-Violet (FUV, 90-200 nm) typically use aluminum (Al) thin films due to their high reflectance over this wavelength range. However, the native aluminum oxide layer that forms on Al upon exposure to the atmosphere is strongly absorbing in this wavelength range, requiring that the films be protected with a dielectric that inhibits oxidation. Typically, magnesium fluoride (MgF 2 ) or lithium fluoride (LiF) coatings are used as protective layers, but each has shortcomings. For example, MgF 2 has an absorption cutoff at 115 nm reducing performance in a critical part of the FUV spectrum of observational interest. A viable option to access these lower wavelengths could be the use of the LiF overcoat as it has a lower absorption cutoff at 102.5 nm, but it is hygroscopic and thus susceptible to degradation in humid conditions. An approach to reduce the hygroscopic nature of LiF would be to implement a passivation process during the deposition of both Al+LiF coatings. Our team at GSFC has developed a new reactive Physical Vapor Deposition (rPVD) process that consists of a fluorination process with XeF 2 gas combined with our traditional PVD process. We have found that this new rPVD coating process offers a protected version of Al+LiF with a more environmentally stable and more transparent LiF layer, along with unprecedent reflectivity. The process starts with a bare optically smooth substrate that is coated with Al in an ultra-high vacuum (UHV) chamber by the conventional PVD process. Then, the bare Al mirror is immediately exposed to a reactive XeF 2 gas before and after the application of the flash PVD evaporated LiF layer. We have also been investigating the use of this rPVD coating process for potential efficiency enhancements of Si-based gratings. Since it is known that the XeF 2 vapor is a strong Si etchant, we are investigating if the native SiO 2 layer on Si is sufficient to protect the groove profile of e-beam-ruled Si gratings from degradation. We will report on the characterization of various Al+LiF witness mirror coatings (both on borosilicate and Si) performed under various deposition conditions using the XeF 2 passivation process. These tests include XPS to determine film stoichiometry and AFM/SEM to measure surface roughness and any etching on the Si substrate. We will also report on FUV reflectance and spectroscopic ellipsometry to characterize optical constants of these passivated films in the UV/Visible/NIR spectral ranges.

Engineering (General)↗

NASA NTRSDate not supplied

Enhanced Far Ultra-Violet Optical Properties of Physical Vapor Deposited Aluminum Mirrors Through Fluorination

Astronomical instrumentation for measurements in the Far Ultra-Violet (FUV, 90-200 nm) typically use aluminum (Al) thin films due to their high reflectance over this wavelength range. However, the native aluminum oxide layer that forms on Al upon exposure to the atmosphere is strongly absorbing in this wavelength range, requiring that the films be protected with a dielectric that inhibits oxidation. Typically, magnesium fluoride (MgF 2 ) or lithium fluoride (LiF) coatings are used as protective layers, but each has shortcomings. For example, MgF 2 has an absorption cutoff at 115 nm reducing performance in a critical part of the FUV spectrum of observational interest. A viable option to access these lower wavelengths could be the use of the LiF overcoat as it has a lower absorption cutoff at 102.5 nm, but it is hygroscopic and thus susceptible to degradation in humid conditions. An approach to reduce the hygroscopic nature of LiF would be to implement a passivation process during the deposition of both Al+LiF coatings. Our team at GSFC has developed a new reactive Physical Vapor Deposition (rPVD) process that consists of a fluorination process with XeF 2 gas combined with our traditional PVD process. We have found that this new rPVD coating process offers a protected version of Al+LiF with a more environmentally stable and more transparent LiF layer, along with unprecedent reflectivity. The process starts with a bare optically smooth substrate that is coated with Al in an ultra-high vacuum (UHV) chamber by the conventional PVD process. Then, the bare Al mirror is immediately exposed to a reactive XeF 2 gas before and after the application of the flash PVD evaporated LiF layer. We have also been investigating the use of this rPVD coating process for potential efficiency enhancements of Si-based gratings. Since it is known that the XeF 2 vapor is a strong Si etchant, we are investigating if the native SiO 2 layer on Si is sufficient to protect the groove profile of e-beam-ruled Si gratings from degradation. We will report on the characterization of various Al+LiF witness mirror coatings (both on borosilicate and Si) performed under various deposition conditions using the XeF 2 passivation process. These tests include XPS to determine film stoichiometry and AFM/SEM to measure surface roughness and any etching on the Si substrate. We will also report on FUV reflectance and spectroscopic ellipsometry to characterize optical constants of these passivated films in the UV/Visible/NIR spectral ranges.

Engineering (General)↗

NASA NTRSDate not supplied

Exposure of Highly Reflective Far-Ultraviolet Coatings to LEO Environment for TRL Advancement for the Habitable Worlds Observatory

Over the past several years, our team at the NASA Goddard Space Flight Center (GSFC), in collaboration with the Jet Propulsion Laboratory (JPL) and the Naval Research Laboratory (NRL), have developed several new protected Al coating technologies with improved Far-Ultraviolet (FUV) reflectance performance and more robust environmental stability. These include Al protected with lithium-based fluoride (LiF) coatings such as hot deposited LiF (eLiF), XeF 2 -passivated LiF coatings (XeLiF), Li 3 AlF 6 overcoats, AlF 3 (plasma passivated), and XeF 2 passivated MgF 2 coatings (XeMgF 2 ). However, despite these developments, relatively little experimental information exists regarding the long-term stability of these coatings after simultaneous exposure to the Low Earth Orbit (LEO) environment. To evaluate their environmental durability and advance their Technology Readiness Level (TRL), representative samples of these coating technologies, Al+XeLiF, Al+eLiF, Al+Li 3 AlF 6 , Al+AlF 3 , Al+XeMgF 2 , and bare Al as reference samples, were flown onboard the International Space Station (ISS) as part of the Materials International Space Station Experiment 20 (MISSE-20). The MISSE platform is operated by Aegis Aerospace for relatively long-duration (e.g. 6 months) external exposure experiments in a space environment. The coatings were deposited on ULE and Zerodur substrates, and witness samples deposited on the same coatings runs, but on glass slides were left on Earth as control. A subset of the flight samples were fitted with MgF 2 windows to reduce direct exposure to atomic oxygen (AO) and charged particles, and still let through UV and other types of radiation on these samples. After approximately 6 months of exposure in the forward-facing (RAM) direction, the samples were returned to Earth and characterized by performing FUV/NUV/VIS/NIR reflectance measurements to quantify degradation, spectroscopic ellipsometry to evaluate coating thickness and optical constants, atomic force microscopy (AFM) to quantify surface roughness evolution during the flight in the ISS. The MgF 2 protective windows were also analyzed through FUV transmission measurements. Comparison with the witness samples left on the ground and with the flown bare Al samples (with just the naturally occurring Al 2 O 3 layer) provided a quantitative assessment of coating thickness variations, optical performance degradation, and morphological changes induced by prolonged exposure to the LEO environment.

Far Ultraviolet (FUV)↗

NASA NTRS2026

Barium Fluoride as a Standard Reference Material for Infrared Refractometry

This work proposes a standard reference material (SRM) for measurements of refractive index in the infrared spectral region (1–12 μm). The purpose of this material is to enable cross-method comparison of refractive index data obtained through minimum deviation, ellipsometry, interferometry, and prism coupling techniques. This will allow vendors and metrology laboratories, regardless of which technique they employ, to certify that their measurements meet required levels of accuracy, precision, and repeatability. Drawing on foundational work by Tilton at the National Bureau of Standards, a figure of merit for candidate SRM materials is outlined, and three materials are selected for further evaluation. Ultimately, barium fluoride is proposed as the most suitable SRM for the infrared spectral region, based on its broad transmission range, chemical and mechanical stability, low temperature dependence of refractive index, and well-characterized dispersion behavior.

Andrew Howe↗

DOE OSTI2020

Preliminary parameter studies to form pellets of hard materials for optical constants derived via single-angle reflectance spectroscopy

Infrared reflectance spectra can be influenced by many factors: the substrate and the thickness of the layer for liquids and the surface micromorphology, the form (powder, crystal) and the particle size for solids. All these parameters can play a role in the appearance of the measured spectrum. To avoid collecting multitudes of spectra to cover all such scenarios, the optical constants n and k, which are intrinsic properties of a material, can instead be used to model the reflectance spectrum. For solids, two techniques are often used to derive optical constants: ellipsometry and single-angle reflectance spectroscopy. For both methods, best results are usually obtained from single crystals. We have recently demonstrated for ammonium sulfate (a relatively soft material) that by optimizing certain conditions, high quality pellets with specularly reflective surfaces can be used in lieu of crystals. This was confirmed by the excellent agreement between the optical constants derived by these two methods. This work focuses on the possible extension of these methods to harder materials, starting with sodium sulfate. The first goal is to see if high quality pellet surfaces can be obtained as for ammonium sulfate. The reflectance values and the associated optical constants can also be obtained.

Diaz, Emmanuela↗

DOE OSTI2023

Atomic Layer Deposition for Spatial Control of Redox Reaction Selectivity

Photocatalytic water splitting holds great potential in the pursuit of the U.S. Department of Energy's Hydrogen Shot initiative to bring the cost of H2 to $1/kg by 2031. A key challenge in the development of photocatalysts is increasing their overall solar-to-hydrogen efficiency by enhancing charge separation yields and redox selectivity. In this work, we use area selective ALD of oxide films to develop tunable interphase layers for selective oxidation and reduction reactions on a single substrate. This presentation details initial synthesis and characterization of Pt- and Au-based planar thin film electrodes in which Au regions were deactivated towards ALD growth through self-assembled thiol monolayers. The suppression of TiO2 ALD selectivity of thiols was assessed through ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry. A patterned planar sample comprised of interdigitated arrays of Au and Pt, used as a surrogate for a photocatalyst particle containing two different co-catalysts, was exposed to ALD growth and removal of the inhibitor species. Scanning electrochemical microscopy (SECM) was used then to probe the local activity of different regions of the patterned surface towards the hydrogen evolution reaction (HER) and iron oxidation and correlated with the ionic and e- blocking effects of the area selective ALD coating. We will also detail the application of these findings to the ongoing development of 3D, particle-based photocatalysts.

atomic layer deposition↗

DOE OSTI2023

Elastomeric reflection suppressor

Various embodiments of the present technology generally relate to reflection suppressors. More specifically, some embodiments use elastomeric materials doped with optical absorbers for temporary suppression of Fresnel reflections for multiple substrates spanning wide spectral and angular bandwidth. The refractive index of the elastomer can be tuned to match a substrate and thereby minimize reflection. Some embodiments can use the addition of different absorptive dopants to allow for either broadband or wavelength-selective reflection suppression. As performance is limited only by index mismatch, both spectral and angular performance significantly exceed that of anti-reflection coatings. After use, these light traps may be removed and reused without damaging the substrate. These films have uses in spectroscopic ellipsometry, holography, and lithography.

Miller, David↗

DOE OSTI2024

Advances in Molecular Beam Epitaxy Growth of Ultra-Wide Bandgap Ga2O3 Based Alloys

Gallium oxide (Ga2O3) is an emerging ultra-wide bandgap semiconductor material that has attracted attention for its potential to outperform existing SiC and GaN based devices operating at high breakdown voltages and high temperature. Isovalent alloying of In and Al in Ga2O3 provides the ability to engineer bandgap energy and strain of the material. Alloying with Al increases the bandgap energy and the theoretically achievable Baliga's figure of merit, a key measure of a material's ultimate performance limits for high power switching devices. Alloying with In introduces compressive strain and can be used to counteract the tensile strain of Al incorporation. The resulting (AlxGa1-x-yIny)2O3 alloy can be lattice-matched to commercially available Ga2O3 wafers and has a tunable bandgap energy greater than that of Ga2O3, 4.76 eV. Such lattice-matched material can be grown arbitrarily thick without the detrimental effects of elastic strain and relaxation, making it suitable for high voltage diodes and transistors. However, efforts to synthesize isovalent alloys are complicated by their tendency to phase separate into corundum Al2O3 or bixbyite In2O3. Literature reports of the quaternary (AlxGa1-x-yIny)2O3 are limited to <1% unintentional indium incorporation in In-catalyzed (AlxGa1-x)2O3. The primary limitation to quaternary growth is the limited incorporation of indium at elevated growth temperatures. This limited incorporation is due to both the volatility of indium oxide and Al and Ga cation exchange reactions which replace indium in In2O3. We report on the development of a novel high-throughput molecular beam epitaxy (MBE) technique to screen the growth conditions for the ternary alloy (InyGa1-y)2O3, and the application of these findings to the first successful synthesis of phase pure monoclinic (AlxGa1-x-yIny)2O3 by MBE. By leveraging the unique sub-oxide chemistry of Ga2O3 and in-situ monitoring of crystal properties by reflection high-energy electron diffraction (RHEED), a cyclical growth and etch-back method is developed and applied to rapidly characterize the (InyGa1-y)2O3 growth space. This cyclical method provides approximately 10x increase in experimental throughput and up to 46x improvement in Ga2O3 substrate utilization. Appropriate growth conditions for monoclinic (InyGa1- y)2O3 are identified by machine learning analysis of RHEED patterns and targeted growths are characterized ex-situ to confirm improved In incorporation. These growth conditions are then combined with established (AlxGa1-x)2O3 growth conditions to grow quaternary (AlxGa1-x-yIny)2O3 with Al mole fractions ranging from 1.4% - 24.4% and In mole fractions ranging from 3.1% to 15.5%. The chemical and optical properties of the alloys are investigated by XRD, XPS, and spectroscopic ellipsometry. A lattice-matched (AlxGa1-x-yIny)2O3 alloy is examined by 4D-STEM and the chemical and physical uniformity of Al and In incorporation are discussed.

alloy↗

DOE OSTI2022

Transport and optical properties of the chiral semiconductor Ag 3 AuSe 2

Previous band structure calculations predicted Ag 3 AuSe 2 to be a semiconductor with a band gap of approximately 1 eV. Here, we report single crystal growth of Ag 3 AuSe 2 and its transport and optical properties. Single crystals of Ag 3 AuSe 2 were synthesized by slow-cooling from the melt, and grain sizes were confirmed to be greater than 2 mm using electron backscatter diffraction. Optical and transport measurements reveal that Ag 3 AuSe 2 is a highly resistive semiconductor with a band gap and activation energy around 0.3 eV. Our first-principles calculations show that the experimentally determined band gap lies between the predicted band gaps from GGA and hybrid functionals. We predict band inversion to be possible by applying tensile strain. The sensitivity of the gap to Ag/Au ordering, chemical substitution, and heat treatment merit further investigation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗