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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Review of the impacts of leaking CO 2 gas and brine on groundwater quality

This review paper provides a synthetic view of the existing knowledge and summarizes data and findings of the recent literature on the subject of the potential leaking of CO2 from the deep subsurface storage reservoirs and the effects on aquifer quality. New ideas and concepts are developed and insights are also provided. The objectives of this paper are to: 1) present and discuss potential risks for groundwater degradation due to CO2 gas and brine exposure; 2) identify the set of geochemical data required to assess and predict aquifer responses to CO2 and brine leakage. Specifically, this paper will discuss the following issues: 1) Aquifer responses (such as changes in aqueous phase/groundwater chemical composition; changes in solid phase chemistry and mineralogy; changes in the extent and rate of reactions and processes and possible establishment of a new network of reactions and processes affecting or controlling overall mobility of major, minor, and trace elements; development of conceptual and reduced order models (ROMs) to describe and predict aquifer responses); 2) The degree of impact such as significant or insignificant changes in pH and major, minor, and trace element release that depend on the following controlling variables; the effect of leaking plume characteristics (gas composition, pure CO2 and/or CO2 -CH4 -H2S mixtures and brine concentration and composition (trace metals); aquifer properties [such as initial aqueous phase conditions and mineralogy: minerals controlling sediments’ response (e.g., calcite, Si bearing minerals, etc.)]; overview of relevant hydrogeological and geochemical processes related to the impact of CO2 gas and brine on groundwater quality; the fate of the elements released from sediments or transported with brine (such as precipitation/incorporation into minerals (calcite and other minerals), adsorption, electron transfer reactions, the role of natural attenuation; whether or not the release of metals following exposure to CO2 harmful (risk assessment).

Qafoku, Nikolla P.↗
DOE OSTI2023

Community vulnerability is the key determinant of diverse energy burdens in the United States

Low-income households generally experience a high energy burden; however, the factors influencing energy burdens are beyond socio-economics. This study explores the relationships between the multidimensionality of community vulnerability factors and energy burden across multiple geospatial levels in the United States. Our study found the distribution of energy burden in 2020 showed a great deal of variety, ranging from a minimum of 2.93 % to a maximum of 30.45 % across 3142 counties. The results of non-spatial and spatial regressions showed that the vulnerability ranks of socioeconomic, household composition and disability, minority and language, household type and transportation, and COVID mortality rate are significant predictors of energy burdens at the national level. However, at the regional level, only socioeconomic, minority and language significantly influence energy burdens. Minority and language negatively impact energy burdens except for the South East-Central region. Additionally, our analyses highlight the need to consider community vulnerability indicators' spatial homogeneity and heterogeneity. At the national level, only the epidemiological factors index is a spatially homogeneous predictor; on the regional and state level, the spatially homogeneous predictors such as socioeconomic status, household composition and disability, and household type and transportation vary by region. Such a region-sensitive relationship between energy burden and the predictors indicates spatial heterogeneity. Here this study suggests policy recommendations through the lens of the multidimensionality of community vulnerability factors. Implementing flexible national energy policies while making particular energy assistance policies for the vulnerable population at the regional or state levels is essential.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2013

Comparison of the Effects of High-Energy Photon Beam Irradiation (10 and 18 MV) on 2 Types of Implantable Cardioverter-Defibrillators

Purpose: Radiation therapy for cancer may be required for patients with implantable cardiac devices. However, the influence of secondary neutrons or scattered irradiation from high-energy photons (≥10 MV) on implantable cardioverter-defibrillators (ICDs) is unclear. This study was performed to examine this issue in 2 ICD models. Methods and Materials: ICDs were positioned around a water phantom under conditions simulating clinical radiation therapy. The ICDs were not irradiated directly. A control ICD was positioned 140 cm from the irradiation isocenter. Fractional irradiation was performed with 18-MV and 10-MV photon beams to give cumulative in-field doses of 600 Gy and 1600 Gy, respectively. Errors were checked after each fraction. Soft errors were defined as severe (change to safety back-up mode), moderate (memory interference, no changes in device parameters), and minor (slight memory change, undetectable by computer). Results: Hard errors were not observed. For the older ICD model, the incidences of severe, moderate, and minor soft errors at 18 MV were 0.75, 0.5, and 0.83/50 Gy at the isocenter. The corresponding data for 10 MV were 0.094, 0.063, and 0 /50 Gy. For the newer ICD model at 18 MV, these data were 0.083, 2.3, and 5.8 /50 Gy. Moderate and minor errors occurred at 18 MV in control ICDs placed 140 cm from the isocenter. The error incidences were 0, 1, and 0 /600 Gy at the isocenter for the newer model, and 0, 1, and 6 /600Gy for the older model. At 10 MV, no errors occurred in control ICDs. Conclusions: ICD errors occurred more frequently at 18 MV irradiation, which suggests that the errors were mainly caused by secondary neutrons. Soft errors of ICDs were observed with high energy photon beams, but most were not critical in the newer model. These errors may occur even when the device is far from the irradiation field.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2015

Radiotherapy Quality Assurance Report From Children's Oncology Group AHOD0031

Purpose: A phase 3 trial assessing response-based therapy in intermediate-risk Hodgkin lymphoma mandated real-time central review of involved field radiation therapy (IFRT) and imaging records by a centralized review center to maximize protocol compliance. We report the impact of centralized radiation therapy review on protocol compliance. Methods and Materials: Review of simulation films, port films, and dosimetry records was required before and after treatment. Records were reviewed by study-affiliated or review center–affiliated radiation oncologists. A deviation of 6% to 10% from protocol-specified dose was scored as “minor”; a deviation of >10% was “major.” A volume deviation was scored as “minor” if margins were less than specified or “major” if fields transected disease-bearing areas. Interventional review and final compliance review scores were assigned to each radiation therapy case and compared. Results: Of 1712 patients enrolled, 1173 underwent IFRT at 256 institutions in 7 countries. An interventional review was performed in 88% of patients and a final review in 98%. Overall, minor and major deviations were found in 12% and 6% of patients, respectively. Among the cases for which ≥1 pre-IFRT modification was requested by the Quality Assurance Review Center and subsequently made by the treating institution, 100% were made compliant on final review. By contrast, among the cases for which ≥1 modification was requested but not made by the treating institution, 10% were deemed compliant on final review. Conclusions: In a large trial with complex treatment pathways and heterogeneous radiation therapy fields, central review was performed in a large percentage of cases before IFRT and identified frequent potential deviations in a timely manner. When suggested modifications were performed by the institutions, deviations were almost eliminated.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2017

Modeling thermal and irradiation-induced swelling effects on the integrity of Ti 3 SiC 2 /SiC joints

Previously, results for CVD-SiC joints created using solid state displacement reactions to form a dual-phase SiC/MAX phase irradiated at 800°C and 5 dpa indicated some extent of cracking in the joint and along the CVD-SiC/joint interface. This paper elucidates the origin of cracking by thermomechanical modeling combined with irradiation-induced swelling effects using a continuum damage approach with support of micromechanical modeling. Three irradiation temperatures (400°C, 500°C and 800°C) are considered assuming experimental irradiation doses in a range leading to saturation swelling in SiC. The analyses indicate that a SiC/MAX joint heated to 400°C fails during irradiation-induced swelling at this temperature while it experiences some damage after being heated to 500°C and irradiated at the same temperature. However, it fails during cooling from 500°C to room temperature. The joint experiences minor damage when heated to and irradiated at 800°C but does not fail after cooling. The prediction agrees with the experimental findings available for this case.

damage modeling↗
DOE OSTI2022

Engineering defect energy landscape of CoCrFeNi high-entropy alloys by the introduction of additional dopants

The concept of high-entropy alloys (HEAs) focusing on tuning the overall chemical complexity represents a novel alloy design strategy. In contrast, alloying of a metallic matrix with minor doping elements with limited and localized tunability has been a common practice to improve material performance. Combining the idea of globally engineering defect energy landscape in HEAs and the localized doping strategy in dilute alloys, in this work, we explore doping effects of minor elements in a HEA matrix to further enhance the overall and localized chemical tunability, aiming to improve its irradiation resistance. Specifically, we study the influence of minor Al, Cu, Ti, and Pd substitutional doping elements on defect energetics in a CoCrFeNi model HEA based on density-functional theory (DFT) calculations. The DFT results indicate that the formation and migration energies of vacancies can be strongly influenced when a dopant is introduced at the first nearest neighbor shells around a vacancy. On the other hand, interstitial energetics are only slightly affected. Among the four elements, Ti and Pd generally decrease vacancy formation energies and increase vacancy migration energies more significantly than Al and Cu. The doping effects become more pronounced when the concentration of the substitutional dopants increases. Based on the energy distributions obtained from DFT, we build a kinetic Monte Carlo (kMC) model to assess the impact of dopants on vacancy-mediated diffusivity in the doped HEAs. Our results suggest that Ti and Pd can lower the tracer diffusivity in the considered HEAs and act as trapping sites, whereas Cu may enhance the atomic transport. This work indicates that substitutional doping in HEAs is an effective strategy in metallurgy to further tune the defect and transport properties of complex alloys.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Testing fast reactor fuels in a thermal reactor: Comparison of transmutation metallic fuel alloys behavior by scanning electron microscopy

To optimize nuclear waste repository performance, the destruction of minor actinide elements, particularly Np and Am, in a neutron fast spectrum reactor is possible by incorporating these elements into nuclear fuel. Evaluating the performance of minor actinide containing fuel is of paramount importance to enabling this technology. However, such a task is challenging without an available domestic fast spectrum test reactor. A comparison of fuel performance tested in an available domestic thermal reactor at the Idaho National Laboratory, the Advanced Test Reactor, and in a fast spectrum reactor in France (Phénix) is presented here in this study. This study evaluates the capability of using a cadmium shrouded test position to mimic the power profile along the fuel radius present in fast spectrum reactors so that thermally driven phenomenon (e.g., constituent redistribution) can be evaluated in a thermal reactor and determined to be prototypical of a fast reactor. Thus, optical microscopy and scanning electron microscopy has been performed on irradiated 35U-29Pu-4Am-2Np-30Zr fuel samples (where the number preceding the element is the weight percent concentration) from the two mentioned reactors that present similar irradiation temperatures and power conditions. The results indicate that fuel performance phenomena are reproducible in the two irradiation conditions. The redistribution of Zr occurred in the same manner for the two samples. Similar partitioning of U-Pu-Zr phases was observed, and the behavior of Am was similar in the analyzed specimens. Finally, the overall microstructure evolution seems not to be affected by minor actinides addition compared to expected behavior of conventional U-19Pu-10Zr ternary metal fuels for both specimens. Slight differences in fuel cladding chemical interaction were, however, observed. This difference is likely driven by difference in cladding composition rather than irradiation conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2012

Relaxations in Ba{sub 2}BiTaO{sub 6} ceramics investigated by impedance and electric modulus spectroscopies

Highlights: Black-Right-Pointing-Pointer We have confirmed that the relaxation observed in Ba{sub 2}BiTaO{sub 6} is due to the conduction mechanism. Black-Right-Pointing-Pointer The conduction mechanism is the oxygen vacancies hopping. Black-Right-Pointing-Pointer We have explained because the activation energy for the Ba{sub 2}BiTaO{sub 6} is lower than observed for Ba{sub 2}BiSbO{sub 6} with basis in zero-point energy of both materials. Black-Right-Pointing-Pointer We have showed that a minor secondary phase is a minor secondary phase, which is common when the BBTO is obtained by ceramic method under air, does not change significantly the electrical properties of BBTO. -- Abstract: Impedance spectroscopy analysis of the dielectric properties of a Ba{sub 2}BiTaO{sub 6} ceramic was performed in the temperature range from room temperature to 500 K. The sample was prepared using conventional solid state synthesis under air and the X-ray diffraction shows the presence of Ba{sub 5}Ta{sub 4}O{sub 15} as a minor secondary phase (0.09%). The impedance data clearly show contributions of the grain and grain boundary. The results indicate that the conduction in Ba{sub 2}BiTaO{sub 6} is due to hopping of oxygen vacancies and that the impurities not influence the conduction mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Microstructures, magnetic and electric properties of diluted magnetic semiconductors InTe{sub 1−x} Fe{sub x} (Co{sub x})

Highlights: ► The prepared InTe{sub 0.9}Fe{sub 0.1} was found to be ferromagnetic at room temperature and can be characterized as diluted magnetic semiconductors. ► The presence of staking faults, various types of defects, strained lattice, grain boundaries and the impurity of minor non-magnetic phase were suggested to participate in high temperature ferromagnetism. - Abstract: InTe compound doped by 10% of Fe or Co respectively was synthesized. X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), energy dispersive X-ray (EDX), vibrating sample magnetometer (VSM) and Kiethley electrometer were used for characterizing the prepared samples. XRD show the presence of InTe{sub 0.9}Fe{sub 0.1} or InTe{sub 0.9}Co{sub 0.1} together with minor In{sub 4}Te{sub 3} phase. InTe{sub 0.9}Fe{sub 0.1} is ferromagnetic with high Curie and high blocking temperature, while InTe{sub 0.9}Co{sub 0.1} is antiferromagnetic with two high Neels temperatures. σRT of InTe{sub 0.9}Fe{sub 0.1} and InTe{sub 0.9}Co{sub 0.1} are greater than those of InTe. The higher conductivity is due to the higher carrier's density obtained from the interaction of the sp-d orbitals, of the electric and magnetic system. The presence of In{sub 4}Te{sub 3} minor phase and different kinds of defects are taking major roles in the formation of high Tc ferromagnetism and antiferromagnetism.

36 MATERIALS SCIENCE↗
DOE OSTI2025

RSPECT: A PROSPECT-based model incorporating the real structure of rice leaves

Radiative transfer models (RTMs) describe how light is absorbed, scattered, and transmitted within leaves by simulating mechanistic light propagation processes. The PROSPECT model is based on measurable parameters (the leaf biochemical content) and a non-measurable parameter (the leaf anatomical structure represented by the leaf structure parameter (N)). The effect of N on the optical properties of leaves has been investigated through a number of local and global sensitivity analyses. Other studies have directly evaluated the effect of the leaf anatomical structure on spectral reflectance, particularly in the near infrared region. However, the relationship between N and the anatomical structure is unclear. Here, in this study, we leveraged eLeaf, a ray tracing-based 3D rice leaf simulator, to establish relationships between leaf anatomical features and spectral properties, enabling us to replace N in the PROSPECT-4 model with measurable leaf anatomical parameters and develop the RSPECT model. The leaf thickness at minor vein, leaf thickness at bulliform cells, mesophyll thickness at minor vein, and distance between two minor veins could be used to predict N effectively. The RSPECT model achieved spectral simulation accuracy comparable to PROSPECT-4 and was more suitable for parameter inversion of the physical and chemical properties of rice leaves, with relative root mean square errors of 7.4% for chlorophyll content, 5.6% for equivalent water thickness, and 7.5% for dry matter content. In conclusion, RSPECT improves radiative transfer modeling by integrating measurable anatomical features and provides a framework for extending this approach to other vegetation types.

hyperspectral↗
DOE OSTI2011

Mouse strain-dependent caspase activation during acetaminophen hepatotoxicity does not result in apoptosis or modulation of inflammation

The mechanisms of acetaminophen (APAP)-mediated hepatic oncotic necrosis have been extensively characterized. However, it was recently demonstrated that fed CD-1 mice have a transient caspase activation which initiates apoptosis. To evaluate these findings in more detail, outbred (Swiss Webster, SW) and inbred (C57BL/6) mice were treated with APAP with or without pan-caspase inhibitor and compared to the apoptosis model of galactosamine (GalN)/endotoxin (ET). Fasted or fed APAP-treated C57BL/6 mice showed no evidence of caspase-3 processing or activity. Interestingly, a minor, temporary increase in caspase-3 processing and activity (150% above baseline) was observed after APAP treatment only in fed SW mice. The degree of caspase-3 activation in SW mice after APAP was minor compared to that observed in GalN/ET-treated mice (1600% above baseline). The pancaspase inhibitor attenuated caspase activation and resulted in increased APAP-induced injury (plasma ALT, necrosis scoring). The caspase inhibitor did not affect apoptosis because regardless of treatment only < 0.5% of hepatocytes showed consistent apoptotic morphology after APAP. In contrast, > 20% apoptotic cells were observed in GalN/ET-treated mice. Presence of the caspase inhibitor altered hepatic glutathione levels in SW mice, which could explain the exacerbation of injury. Additionally, the infiltration of hepatic neutrophils was not altered by the fed state of either mouse strain. Conclusion: Minor caspase-3 activation without apoptotic cell death can be observed only in fed mice of some outbred strains. These findings suggest that although the severity of APAP-induced liver injury varies between fed and fasted animals, the mechanism of cell death does not fundamentally change. -- Highlights: Black-Right-Pointing-Pointer During acetaminophen overdose caspase-3 can be activated in fed mice of certain outbred strains. Black-Right-Pointing-Pointer Hepatic ATP levels are not the determining factor for caspase activity. Black-Right-Pointing-Pointer Caspase-3 activity does not result in increased hepatocellular apoptotic cell death. Black-Right-Pointing-Pointer Neutrophil recruitment during acetaminophen occurs independently of nutritional status. Black-Right-Pointing-Pointer Fed or fasted state does not alter the mechanisms of acetaminophen-induced cell death.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Genetic variation in metallothionein and metal-regulatory transcription factor 1 in relation to urinary cadmium, copper, and zinc

Background: Metallothionein (MT) proteins play critical roles in the physiological handling of both essential (Cu and Zn) and toxic (Cd) metals. MT expression is regulated by metal-regulatory transcription factor 1 (MTF1). Hence, genetic variation in the MT gene family and MTF1 might influence excretion of these metals. Methods: 321 women were recruited in Seattle, WA and Las Cruces, NM and provided demographic information, urine samples for measurement of metal concentrations by mass spectrometry and creatinine, and blood or saliva for extraction of DNA. Forty-one single nucleotide polymorphisms (SNPs) within the MTF1 gene region and the region of chromosome 16 encoding the MT gene family were selected for genotyping in addition to an ancestry informative marker panel. Linear regression was used to estimate the association of SNPs with urinary Cd, Cu, and Zn, adjusted for age, urinary creatinine, smoking history, study site, and ancestry. Results: Minor alleles of rs28366003 and rs10636 near the MT2A gene were associated with lower urinary Cd, Cu, and Zn. Minor alleles of rs8044719 and rs1599823, near MT1A and MT1B, were associated with lower urinary Cd and Zn, respectively. Minor alleles of rs4653329 in MTF1 were associated with lower urinary Cd. Conclusions: These results suggest that genetic variation in the MT gene region and MTF1 influences urinary Cd, Cu, and Zn excretion. - Highlights: • Genetic variation in metallothionein (MT) genes was assessed in two diverse populations. • Single nucleotide polymorphisms (SNPs) in MT genes were associated with mean urinary Cd, Cu and Zn. • Genetic variation may influence biomarkers of exposure, and associations of exposure with health.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2020

Evaluating the Utility of Permethylated Polysaccharide Solution NMR Data for Characterization of Insoluble Plant Cell Wall Polysaccharides

Plant cell wall polysaccharide analysis encompasses the utilization of a variety of analytical tools, including gas and liquid chromatography, mass spectrometry (MS), and nuclear magnetic resonance (NMR) spectroscopy. These methods provide complementary data, which enable confident structural proposals of the many complex polysaccharide structures that exist in the complex matrices of plant cell walls. However, cell walls contain fractions of varying solubilities, and a few techniques are available that can analyze all fractions simultaneously. We have discovered that permethylation affords the complete dissolution of both soluble and insoluble polysaccharide fractions of plant cell walls in organic solvents such as chloroform or acetonitrile, which can then be analyzed by a number of analytical techniques including MS and NMR. In this work, NMR structure analysis of 10 permethylated polysaccharide standards was undertaken to generate chemical shift data providing insights into spectral changes that result from permethylation of polysaccharide residues. This information is of especial relevance to the structure analysis of insoluble polysaccharide materials that otherwise are not easily investigated by solution-state NMR methodologies. The preassigned NMR chemical shift data is shown to be vital for NMR structure analysis of minor polysaccharide components of plant cell walls that are particularly difficult to assign by NMR correlation data alone. With the assigned chemical shift data, we analyzed the permethylated samples of destarched, alcohol-insoluble residues of switchgrass and poplar by two-dimensional NMR spectral profiling. Thus, we identified, in addition to the major polysaccharide components, two minor polysaccharides, namely, <5% 3-linked arabinoxylan (switchgrass) and <2% glucomannan (poplar). In particular, the position of the arabinose residue in the arabinoxylan of the switchgrass sample was confidently assigned based on chemical shift values, which are highly sensitive to local chemical environments. Furthermore, the high resolution afforded by the 1H NMR spectra of the permethylated switchgrass and poplar samples allowed facile relative quantitative analysis of their polysaccharide composition, utilizing only a few milligrams of the cell wall material. As a result, the concepts herein developed will thus facilitate NMR structure analysis of insoluble plant cell wall polysaccharides, more so of minor cell wall components that are especially challenging to analyze with current methods.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Unique Phase Behavior of Inverse Tapered Block Copolymers: Self-Consistent Field Theory and Molecular Dynamics Simulations

Using theory and simulation, we study the microphase separated structures of inverse tapered block copolymers. These are AB block copolymers with a gradient region inserted between the pure A and B blocks that smoothly changes in composition from B to A, with the B-side of the taper bonded to the pure A block. Tapering is a means to increase miscibility between polymer blocks, and by controlling the length of the taper, the phase behavior and domain spacing can be adjusted. As opposed to better known ABAB tetrablock copolymers that also have four alternating A-rich and B-rich regions, here the intermediate region has a partially mixed composition, promoting relatively diblock-like behavior for shorter inverse tapers or low segregation strength but unique behavior as a function of increasing segregation strength for some systems. In particular, for systems in which 50% of the backbone is the taper, the larger of the two pure end blocks - composed of the overall majority component - can phase separate analogously to the minority component of a diblock copolymer, creating cylinder and network phases in which the majority component’s pure block is the minority phase in a matrix of a relatively disordered mixture of the tapered region and the minority component’s pure block.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Interplay Between Strain and Thickness on the Effective Carrier Lifetime of Buffer-Mediated Epitaxial Germanium Probed by the Photoconductance Decay Technique

We report contactless effective minority carrier lifetime of epitaxially grown unstrained and in-plane <110> biaxially tensile-strained (001) germanium (..epsilon..-Ge) epilayers measured using microwave-reflectance photoconductance decay measurements. Strained Ge epilayers were grown using InxGa1-xAs linearly graded buffers on (001) GaAs substrates. Using homogeneous excitation of unstrained Ge epilayers, thickness-dependent separation of minority carrier lifetime components under low injection conditions yielded a bulk lifetime of 114 +/- 2 ns and low surface recombination velocity of 21.3 +/- 0.04 cm/s. More notably, an effective minority carrier lifetime of >100 ns obtained from sub-50 nm 1.6% tensile-strained Ge epilayers showed no degradation relative to the unstrained counterpart. Detailed material characterization using X-ray diffractometry revealed successful strain transfer of 0.61 and 0.89% to the Ge epilayers via InxGa1-xAs metamorphic buffers and confirms pseudomorphic growth. Lattice coherence observed at the e-Ge epilayer and InxGa1-xAs buffer heterointerfaces via transmission electron microscopy substantiates the prime material quality achieved. The relatively high carrier lifetimes achieved are an indicator of excellent material quality and provide a path forward to realize low-threshold Ge laser sources.

bulk lifetime↗
DOE OSTI2017

Understanding and reducing deleterious defects in the metastable alloy GaAsBi

Technological applications of novel metastable materials are frequently inhibited by abundant defects residing in these materials. Using first-principles methods, we investigate the defect thermodynamics and phase segregation in the technologically important metastable alloy GaAsBi. Our calculations predict defect energy levels in good agreement with those from numerous previous experiments and clarify the defect structures giving rise to these levels. We find that vacancies in some charge states become metastable or unstable with respect to antisite formation, and this instability is a general characteristic of zincblende semiconductors with small ionicity. The dominant point defects that degrade the electronic and optical performances are predicted to be As Ga , Bi Ga , As Ga +Bi As , Bi Ga +Bi As , V Ga and V Ga +Bi As , of which the first four and last two defects are minority-electron and minority-hole traps, respectively. V Ga is also observed to have a critical role in controlling metastable Bi supersaturation by mediating Bi diffusion and clustering. To reduce the influences of these deleterious defects, we suggest shifting the growth away from an As-rich condition and/or using hydrogen passivation to reduce the minority-carrier traps. In conclusion, we expect this work to aid in the applications of GaAsBi for novel electronic and optoelectronic devices and to illuminate the control of deleterious defects in other metastable materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

An all optical approach for comprehensive in-operando analysis of radiative and nonradiative recombination processes in GaAs double heterostructures

We demonstrate an all optical approach that can surprisingly offer the possibility of yielding much more information than one would expect, pertinent to the carrier recombination dynamics via both radiative and nonradiative processes when only one dominant deep defect level is present in a semiconductor material. By applying a band-defect state coupling model that explicitly treats the inter-band radiative recombination and Shockley–Read–Hall (SRH) recombination via the deep defect states on an equal footing for any defect center occupation fraction, and analyzing photoluminescence (PL) as a function of excitation density over a wide range of the excitation density (e.g., 5–6 orders in magnitude), in conjunction with Raman measurements of the LO-phonon plasmon (LOPP) coupled mode, nearly all of the key parameters relevant to the recombination processes can be obtained. They include internal quantum efficiency (IQE), minority and majority carrier density, inter-band radiative recombination rate (W r ), minority carrier nonradiative recombination rate (W nr ), defect center occupation fraction (f), defect center density (N t ), and minority and majority carrier capture cross-sections (σ t and σ tM ). While some of this information is thought to be obtainable optically, such as IQE and the W r /W nr ratio, most of the other parameters are generally considered to be attainable only through electrical techniques, such as current-voltage (I-V) characteristics and deep level transient spectroscopy (DLTS). Following a procedure developed herein, this approach has been successfully applied to three GaAs double-heterostructures that exhibit two distinctly different nonradiative recombination characteristics. The method greatly enhances the usefulness of the simple PL technique to an unprecedented level, facilitating comprehensive material and device characterization without the need for any device processing.

14 SOLAR ENERGY↗