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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Sub‐0.6 eV Inverted Metamorphic GaInAs Cells Grown on Inp and GaAs Substrates for Thermophotovoltaics and Laser Power Conversion

Abstract Inverted metamorphic Ga 0.3 In 0.7 As photovoltaic converters with sub‐0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 ± 0.6 × 10 6 and 8.9 ± 1.7 × 10 6 cm −2 on InP and GaAs, respectively. The devices generate open‐circuit voltages of 0.386 and 0.383 V, respectively, under irradiance producing a short‐circuit current density of ≈10 A cm −2 , yielding bandgap‐voltage offsets of 0.20 and 0.21 V. Power and broadband reflectance measurements are used to estimate thermophotovoltaic (TPV) efficiency. The InP‐based cell is estimated to yield 1.09 W cm −2 at 1100 °C versus 0.92 W cm −2 for the GaAs‐based cell, with efficiencies of 16.8 versus 9.2%. The efficiencies of both devices are limited by sub‐bandgap absorption, with power weighted sub‐bandgap reflectances of 81% and 58%, respectively, the majority of which is assumed to occur in the graded buffers. The 1100 °C TPV efficiencies are estimated to increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0–2.3 µm atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 µm irradiances of 1.86 and 2.81 W cm −2 , respectively.

14 SOLAR ENERGY↗
DOE OSTI2020

Dirac energy spectrum and inverted bandgap in metamorphic InAsSb/InSb superlattices

A Dirac-type energy spectrum was shown in gapless ultra-short-period metamorphic InAsSb/InSb superlattices by angle-resolved photoemission spectroscopy (ARPES) measurements. The Fermi velocity value 7.4x10 5 m/s in a gapless superlattice with a period of 6.2nm is in a good agreement with the results of magneto-absorption experiments. An “inverted” bandgap opens in the center of the Brillouin zone at higher temperatures and in the superlattice with a larger period. The ARPES data suggest the presence of a surface electron accumulation layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Effect of Doping Density on the Performance of Metamorphic GaInAs Solar Cells Grown by Dynamic Hydride Vapor Phase Epitaxy

We study the effect of doping density on the performance of inverted metamorphic ~1 eV GaInAs solar cells grown by dynamic hydride vapor phase epitaxy. We find that the doping efficiency of Se in this material is extremely high, with 5x10 17 cm -3 being the lowest controlled value we could achieve with our present Se source. We compared rear heterojunction devices grown with intentional n-doping of 5x10 17 cm -3 and unintentional n-doping of 5x10 15 cm -3 . The intentionally doped sample exhibited an open circuit voltage ~80 mV higher than the unintentionally doped sample due to reduced dark current, and a bandgap voltage offset, or W OC , of 0.407 V. We believe that future doping optimization will improve carrier collection and W OC in these devices.

GaAs↗
DOE OSTI2022

Oxygen diffusion in garnet: Experimental calibration and implications for timescales of metamorphic processes and retention of primary O isotopic signatures

Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotope signatures in natural samples. A series of experiments was undertaken to determine the diffusivity of oxygen in garnet, which remains poorly constrained. The first suite included high-pressure (HP), nominally dry experiments performed in piston-cylinder apparatus at: (1) T = 1050–1600 °C and P = 1.5 GPa and (2) T = 1500 °C and P = 2.5 GPa using yttrium aluminum garnet (YAG; Y3Al5O12) cubes. Second, HP H2O-saturated experiments were conducted at T = 900 °C and P = 1.0–1.5 GPa, wherein YAG crystals were packed into a YAG + Corundum powder, along with 18O-enriched H2O. Third, 1 atm experiments with YAG cubes were performed in a gas-mixing furnace at T = 1500–1600 °C under Ar flux. Finally, an experiment at T = 900 °C and P = 1.0 GPa was done using a pyrope cube embedded into pyrope powder and 18O-enriched H2O. Experiments using grossular were not successful. Profiles of 18O/(18O+16O) in the experimental charges were analyzed with three different secondary ion mass spectrometers (SIMS): sensitive high-resolution ion microprobe (SHRIMP II and SI), CAMECA IMS-1280, and NanoSIMS. Considering only the measured length of 18O diffusion profiles, similar results were obtained for YAG and pyrope annealed at 900 °C, suggesting limited effects of chemical composition on oxygen diffusivity. However, in both garnet types, several profiles deviate from the error function geometry, suggesting that the behavior of O in garnet cannot be fully described as simple concentration-independent diffusion, certainly in YAG and likely in natural pyrope as well. The experimental results are better described by invoking O diffusion via two distinct pathways with an inter-site reaction allowing O to move between these pathways. Modeling this process yields two diffusion coefficients (D values) for O, one of which is approximately two orders of magnitude higher than the other. Taken together, Arrhenius relationships are:logDm2s-1=-7.2(±1.3)+(-321(±32)kJmol-12.303RT) for the slow pathway, andlogDm2s-1=-5.4(±0.7)+(-321(±20)kJmol-12.303RT) for the fast pathway. We interpret the two pathways as representing diffusion following vacancy and inter-stitial mechanisms, respectively. Regardless, our new data suggest that the slow mechanism is prevalent in garnet with natural compositions, and thus is likely to control the retentivity of oxygen isotopic signatures in natural samples. The diffusivity of oxygen is similar to Fe-Mn diffusivity in garnet at 1000–1100 °C and Ca diffusivity at 850 °C. However, the activation energy for O diffusion is larger, leading to lower diffusivities at P-T conditions characterizing crustal metamorphism. Therefore, original O isotopic signatures can be retained in garnets showing major element zoning partially re-equilibrated by diffusion, with the uncertainty caveat of extrapolating the experimental data to lower temperature conditions.

Geochemistry & Geophysics↗
DOE OSTI2021

Towards a III-V Solar Cell with a Metamorphic Graded Buffer Directly Grown on V-Groove Si Substrates

V-groove Si substrates offer a promising route for the direct growth of III-V solar cells on low-cost, photovoltaic-grade Si. In this work, we develop a method for coalescing GaP nucleated on V-groove substrates into thin films suitable for the growth of III-V solar cells. We demonstrate the ability to suppress nucleation-related defects by modifying the V-groove substrate to cover the (0 0 1)-oriented Si V-groove tops with SiN x . A threading dislocation density (TDD) of 5 × 10 7 cm -2 was measured with electron channeling contrast imaging. Continuing work will focus on further reducing the TDD and the growth of a GaAs solar cell on a GaP on V-groove Si template.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2020

Water and heat: New constraints on the evolution of the CV chondrite parent body

This paper focuses on the post-accretion history of CV3 chondrites, through a combination of petrographic and mineralogical characterization, magnetic measurements, spectral (Raman and Infrared) and thermo-gravimetric analysis of 31 meteorites (including 7 falls, 21 Antarctic and 3 non-Antarctic finds) spanning a wide metamorphic range. We classify the 21 Antarctic chondrites and the Bukhara fall into the CV Red , CV OxA , and CV OxB subgroups. We establish quantitative parameters relevant for this sub-classification. In comparison to CV Ox , CV Red chondrites are characterized by (i) a lower abundance of matrix, (ii) a higher abundance of metal, (iii) the presence of Ni-poor sulfides. In comparison to CV OxB , CV OxA are characterized by (i) similar matrix abundance, (ii) a higher abundance of metal, (iii) the presence of metal almost exclusively under the form of awaruite, (iv) lower Ni content of sulfides, (v) lower magnetic susceptibility and saturation remanence. In this work, both CV Ox (CV OxA and CV OxB ) and CV Red experienced aqueous alteration, and contain oxyhydroxides and phyllosilicates. We show that the abundance of these hydrated secondary minerals observed today in individual CV chondrites decreases with their peak metamorphic temperature. This is interpreted either as partial dehydration of these secondary minerals or limited hydration due to the rapid exhaustion of the water reservoir during parent body thermal metamorphism. Moreover, the lower abundance of oxyhydroxides (that have a lower thermal stability than phyllosilicates and may in large part postdate the peak of thermal metamorphism) in more metamorphosed CV chondrites is interpreted as lower availability of aqueous fluids during retrograde metamorphism in these meteorites. Lastly, we show that in comparison to CV OxB , CV OxA are systematically (i) more metamorphosed, (ii) less hydrated, (iii) depleted in ferromagnetic minerals, (iv) but enriched in metal in the form of secondary awaruite. CV OxA may be thermally metamorphosed CV OxB . CV Red are significantly different from CV Ox (matrix abundances, alteration products, opaque minerals), but span the same wide metamorphic range. This could be indicative of a laterally heterogeneous CV parent body, or suggest the existence of distinct parent bodies for CV Ox and CV Red chondrites.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Rubidium and potassium isotopic variations in chondrites and Mars: Accretion signatures and planetary overprints

As moderately volatile elements, isotopes of Rb and K can trace volatilization processes in planetary bodies. Rubidium isotopic data are however very scarce, especially for non-carbonaceous meteorites. Here, in this study, we report combined Rb and K isotopic data (δ 87/85 Rb and δ 41/39 Κ) for 7 ordinary, 6 enstatite, and 4 Martian meteorite falls to understand the causes for the variations in volatile abundances and isotopic compositions. Bulk Rb and K isotopic compositions of planetary bodies are estimated to be (Table 1): Mars +0.10 ± 0.03 ‰ for Rb and -0.26 ± 0.05 ‰ for K, bulk OCs $-0.12^{+0.15}_{-0.24}$ ‰ for Rb and $-0.72^{+0.28}_{-0.41}$ ‰ for K, bulk ECs $-0.02^{+0.29}_{-0.26}$ ‰ for Rb and $-0.33^{+0.37}_{-0.23}$ ‰ for K. The bulk K isotopic compositions of subgroup OCs are estimated to be $-0.72^{+0.26}_{-0.55}$ ‰ for H chondrites, $-0.71^{+0.23}_{-0.39}$ ‰ for L chondrites, and $-0.77^{+0.63}_{-0.30}$ ‰ for LL chondrites. A broad correlation between the Rb and K isotopic compositions of planetary bodies is observed. The correlation follows a slope that is consistent with kinetic evaporation and condensation processes, suggesting volatility-controlled mass-dependent isotope fractionation (as opposed to nucleosynthetic anomalies). Individual ordinary and enstatite chondrites show large Rb and K isotopic variations (-1.02 to +0.29 ‰ for Rb and -0.91 to -0.15 ‰ for K). Samples of lower metamorphic grades display correlated elemental and isotopic fractionations between Rb and K, while samples of higher metamorphic grades show great scatter, suggesting that chondrite parent-body processes have decoupled the two elements and their isotopes at the sample scale. Several processes could have contributed to the observed isotopic variations of Rb and K, including (i) chondrule “nugget effect”, (ii) volatilization during parent-body thermal metamorphism (heat-induced vaporization and gas transport within parent bodies), (iii) thermal diffusion during parent-body metamorphism, and (iv) impact/shock heating. Quantitative modeling of the first two processes suggests that neither of them could produce isotopic variations large enough to explain the observed isotopic variations. Volatilization during parent-body thermal metamorphism [the scenario (ii)], which has been commonly invoked to explain the isotopic variations of volatile elements, is gas transport-limited and its effect on isotopic fractionations of moderately volatile elements should be negligible. Modeling of diffusion processes suggests that (iii) could produce K isotopic variation comparable to the observed variation. The large isotopic variations in non-carbonaceous meteorites are thus most likely due to diffusive redistribution of K and Rb during metamorphism and/or shock-induced heating and vaporization.

58 GEOSCIENCES↗
DOE OSTI2020

Presentation of the Dana Medal of the Mineralogical Society of America for 2019 to Matthew J. Kohn

If there’s one word that describes Matt Kohn’s career, it’s eclectic. What is possibly most astounding is that Matt’s contributions have not been in a single, narrow field, but in two distinct fields that, at least at first glance, are not even that closely related. While Matt is a metamorphic petrologist par excellence making significant contributions to the fields of geo-thermobarometry, metamorphic phase equilibria, metamorphic fluid flow, monazite geochronology, argon geochronology, and metamorphic P-T-t studies and their tectonic implications, he has also pioneered the use of stable isotopes to unravel vertebrate paleodietary patterns and paleoclimate.

58 GEOSCIENCES↗
DOE OSTI2020

InGaSb Defect Filter Layer to Improve Performance of GaSb Solar Cells Grown on GaAs Substrates

The reduction of the threading dislocation density in metamorphic GaSb grown on GaAs substrates through the use of InGaSb defect filter layers has been investigated. More specifically, we study the effects of strain and thickness on the ability of a InGaSb defect filter layer to reduce threading dislocations in GaSb solar cells grown on GaAs substrates. The strain between the GaSb metamorphic layer on GaAs substrate (99.5% relaxed) and the InGaSb defect filter layer is varied by changing the indium composition in the InGaSb layer. Here, it is demonstrated that an InGaSb defect filter layer with 0.6% strain is more effective for blocking threading dislocations compared with higher-strain layers, resulting in improved short-circuit current (J sc ) and open-circuit voltage (V oc ) for the metamorphic GaSb solar cell. The optimization of the defect filter layer involves varying the thickness of the layer to achieve the lowest possible threading dislocation density. This also takes into account the critical thickness of the InGaSb layer on GaSb to avoid generation of threading dislocations from the InGaSb layer itself. It is shown that adding an In 0.11 Ga 0.89 Sb defect filter layer with thickness of 250 nm and 0.6% strain beneath a GaSb solar cell grown on a GaAs substrate improves V oc from 0.1 V to 0.16 V and J sc from 19.7 mA/cm 2 to 24.7 mA/cm 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2021

Magmatic, hydrothermal and ore element transfer processes of the southeastern Archean Superior Province implied from electrical resistivity structure

Although magmatic- and metamorphic- derived fluids are widely recognized mineralizing agents, the role of crustal architecture in defining source and sink zones within the middle to lower crust and upper mantle of ancient orogens remains enigmatic. The globally largest and best-preserved Archean greenstone belts lie in the Superior Province, Canada. They provide an ideal location to investigate the influence of igneous construction and subsequent syn-deformational plutonism and metamorphism on the localization of metal-rich melts and fluids throughout the crustal column. Integration of three-dimensional magnetotelluric modelling and seismic reflection sections across the Abitibi subprovince reveals details of a 'whole-of-crust' magmatic and hydrothermal system. East-west low resistivity structures broadly underlie the surface traces of the major deformation zones that are host to significant gold endowment (>200 Moz), while mid-crustal cross trends suggest mineralized fluids flowed along 'pipes' within fault planes. Most low resistivity structures are inferred to represent domains containing interconnected zones of graphite and/or sulfide. Furthermore, these delineate relict mantle source/transit domains and crustal pathways enriched by the flow of magmas or metamorphic fluids genetically related to a late-stage pulse of ore-bearing magmatism, possibly as a result of slab break-off or delamination. Using the combined MT and seismic data, we develop a 3-D crustal-scale model which highlights how evolving orogenic architecture-controlled mass transfer and metallogenic processes developed.

58 GEOSCIENCES↗
DOE OSTI2022

Subduction, Underplating, and Return Flow Recorded in the Cycladic Blueschist Unit Exposed on Syros, Greece

Exhumed high-pressure/low-temperature (HP/LT) metamorphic rocks provide insights into deep (~20–70 km) subduction interface dynamics. On Syros Island (Cyclades, Greece), the Cycladic Blueschist Unit preserves blueschist-to-eclogite facies oceanic- and continental-affinity rocks that record the structural and thermal evolution linked to Eocene subduction. Despite decades of research, the metamorphic and deformation history (P-T-D) and timing of subduction and exhumation are matters of ongoing discussion. We suggest that Syros comprises three coherent tectonic slices and that each slice underwent subduction, underplating, and syn-subduction return flow along similar P-T trajectories, but at progressively younger times. Subduction and exhumation are distinguished by lineations and ductile fold axis orientations, and are kinematically consistent with previous studies that document top-to-the-S-SW shear (prograde-to-peak subduction), top-to-the-NE shear (blueschist facies exhumation), and then E-W coaxial stretching (greenschist facies exhumation). Amphibole zonations record cooling during decompression, indicating return flow above a cold slab. Multi-mineral Rb-Sr isochrons and compiled metamorphic geochronology show that the three slices record distinct stages of peak subduction (53–52, ~50, and 45 Ma) that young with structural depth. Retrograde blueschist and greenschist facies fabrics span ~50–40 and ~43–20 Ma, respectively, and also young with structural depth. Synthesized data sets support a revised tectonic framework for Syros, involving subduction of structurally distinct coherent slices and simultaneous return flow of previously accreted tectonic slices in the subduction channel shear zone. Distributed, ductile, dominantly coaxial return flow in an Eocene-Oligocene subduction channel proceeded at rates of ~1.5–5 mm/yr and accommodated ~80% of the total exhumation of this HP/LT complex.

58 GEOSCIENCES↗
DOE OSTI2021

Distribution of rare earth elements and other critical elements in beneficiated Pennsylvania anthracites

The Pennsylvania Anthracite Fields are in a complex tectonic and metamorphic terrain, historically hosting one of the largest concentrations of coal mining in the USA. Anthracite mining now largely consists of the surface mining of the pillars remaining from the prime years of underground mining. The geochemical study of the sized coal products and the refuse (largely rock) from three preparation plants (breakers) demonstrates that Principal components analysis (PCA) of select major oxide, minor element, and rare earth elements illustrates some differentiation among the products from the individual plants. The rock samples, with abundant quartz and metamorphic Al-Si minerals and with a lower ash-basis REE concentration than the coals, were distinctly separated from the coal samples on the PCA plots. Plots of Gd N /Gd N * vs. Eu N /Eu N * and Ce N /Ce N * vs. Eu N / Eu N * showed differentiation between the individual suites of coals showed that the refuse samples had distinct REE distributions compared to the associated coals. Further, several minor and trace elements show enrichments in the coal samples. Lithium, with concentrations of up to 314 ppm on an ash basis, is among the most promising of the critical elements, exceeding the enrichment of the REY and Sc.

58 GEOSCIENCES↗
DOE OSTI2020

Deposition of sulfate aerosols with positive Δ 33 S in the Neoarchean

Anomalous sulfur isotope compositions present in Archean rocks have been intensely scrutinized over the last 20 years because they record key aspects of Earth's atmospheric composition prior to the appearance of free molecular oxygen ca. 2.3 billion years ago. These isotopic compositions can be described as mass anomalous fractionations (MAF) and are produced in the atmosphere as UV light interacts with SO 2 molecules. Most interpretations suggest that atmospheric processes generate a reduced S-phase with a positive ( 33 S-enriched) MAF signature, as measured in pyrites, and an oxidized S-phase with a negative anomaly, as measured in bedded barite deposits. However, recent data for carbonate-associated sulfate (CAS) — a direct proxy for the isotopic composition of sulfur from seawater sulfate — in Neoarchean rocks showed no such negative values, but rather the opposite. To understand if the positive MAF anomalies we measured in Neoarchean CAS reflect secondary processes (diagenetic, metamorphic, handling) instead of original signals of Archean seawater sulfate, we collected additional sample suites with various degrees of preservation and metamorphic alteration across the Campbellrand-Malmani platform in South Africa. Results illustrate that within this comprehensive suite, less-altered samples all contain positive MAF values while secondary processes tend to either remove CAS from the sample and/or decrease the 33 S-enrichment. This positive MAF signal in sulfate is therefore reasonably interpreted as a primary depositional origin, and implies that the assumption that sulfate always carries a negative MAF anomaly throughout the Archean rock record needs to be reconsidered. Our CAS observations suggest that future experiments and calculations should also consider atmospheric and/or sulfur cycling processes that can produce oxidized sulfur with a positive MAF signature.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Investigation of Off-Stoichiometry in Ternary Nitrides by EDS and HRTEM

We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high-temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer which otherwise filters light from the junctions below and absorbs sub-bandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable bandedge spectrum filtering. Three components required development to enable this device: 1) a lattice-mismatched 1.2 eV AlGaInAs junction, 2) a metamorphic contact layer grown after the graded buffer, and 3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high-power densities with more transparency than the GaAsSb:C/GaInP:Se structure used in past IMM cells. We characterize the tandem device under a high-intensity spectrum that approximates the emission from a 2150 °C blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ~30% of the blackbody irradiance, and 36% ideal TPV efficiency under the full 118 W/cm2 irradiance. Improvements to the back-surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Reduced Dislocation Introduction in III–V/Si Heterostructures with Glide-Enhancing Compressively Strained Superlattices

The novel use of a GaAs y P 1-y /GaP compressively-strained superlattice (CSS) to provide enhanced control over misfit dislocation (MD) evolution and threading dislocation density (TDD) during GaP/Si metamorphic heteroepitaxy is demonstrated. Here, the insertion of the CSS just after critical thickness, and thus prior to substantial dislocation introduction, is found to yield significantly reduced TDD in relaxed, 500 nm thick, n-type GaP/Si versus comparable control samples. The impact of CSS period count on average TDD and the overall dislocation network morphology was examined, supported by quantitative microstructural characterization, revealing a nearly 20× relative TDD reduction (to 2.4 ± 0.4×10 6 cm -2 ) with a 3-period CSS structure. A similarly low TDD (3.0 ± 0.6×10 6 cm -2 ) is maintained when the resultant n-GaP/Si virtual substrate is used for the growth of a subsequent n-type GaAs 0.75 P 0.25 -terminal GaAs y P 1-y step-graded metamorphic buffer. Although the physical mechanism for TDD reduction provided by these structures is not yet entirely understood, this initial work suggests that enhanced glide dynamics of MDs at or within the CSS placed early in the growth leads to a reduction in the total number of dislocations introduced overall, as opposed to annihilation-based reduction that occurs in conventional strained-layer superlattice dislocation filter approaches.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗