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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS2007

Development of a Quantum Dot, 0.6 eV InGaAs Thermophotovoltaic (TPV) Converter

Thermophotovoltaic (TPV) power conversion has to date demonstrated conversion efficiencies exceeding 20% when coupled to a heat source. Current III-V semiconductor TPV technology makes use of planar devices with bandgaps tailored to the heat source. The efficiency can be improved further by increasing the collection efficiency through the incorporation of InAs quantum dots. The use of these dots can provide sub-gap absorption and thus improve the cell short circuit current without the normal increase in dark current associated with lowering the bandgap. We have developed self-assembled InAs quantum dots using the Stranski-Krastanov growth mode on 0.74 eV In0.53GaAs lattice-matched to InP and also on lattice-mismatched 0.6 eV In0.69GaAs grown on InP through the use of a compositionally graded InPAsx buffer structure, by metalorganic vapor phase epitaxy (MOVPE). Atomic force microscopy (AFM) measurements showed that the most reproducible dot pattern was obtained with 5 monolayers of InAs grown at 450 C. The lattice mismatch between InAs and In0.69GaAs is only 2.1%, compared to 3.2% between InAs and In0.53GaAs. The smaller mismatch results in lower strain, making dot formation somewhat more complicated, resulting in quantum dashes, rather than well defined quantum dots in the lattice-mismatched case. We have fabricated 0.6 eV InGaAs planer TPV cells with and without the quantum dashes

Forbes, David↗
NASA NTRS2006

Quantum-Dot Laser for Wavelengths of 1.8 to 2.3 micron

The figure depicts a proposed semiconductor laser, based on In(As)Sb quantum dots on a (001) InP substrate, that would operate in the wavelength range between 1.8 and 2.3 m. InSb and InAsSb are the smallest-bandgap conventional III-V semiconductor materials, and the present proposal is an attempt to exploit the small bandgaps by using InSb and InAsSb nanostructures as midinfrared emitters. The most closely related prior III-V semiconductor lasers are based, variously, on strained InGaAs quantum wells and InAs quantum dots on InP substrates. The emission wavelengths of these prior devices are limited to about 2.1 m because of critical quantum-well thickness limitations for these lattice mismatched material systems. The major obstacle to realizing the proposed laser is the difficulty of fabricating InSb quantum dots in sufficient density on an InP substrate. This difficulty arises partly because of the weakness of the bond between In and Sb and partly because of the high temperature needed to crack metalorganic precursor compounds during the vapor-phase epitaxy used to grow quantum dots: The mobility of the weakly bound In at the high growth temperature is so high that In adatoms migrate easily on the growth surface, resulting in the formation of large InSb islands at a density, usually less than 5 x 10(exp 9) cm(exp -2), that is too low for laser operation. The mobility of the In adatoms could be reduced by introducing As atoms to the growth surface because the In-As bond is about 30 percent stronger than is the In-Sb bond. The fabrication of the proposed laser would include a recently demonstrated process that involves the use of alternative supplies of precursors to separate group-III and group-V species to establish local non-equilibrium process conditions, so that In(As)Sb quantum dots assemble themselves on a (001) InP substrate at a density as high as 4 x 10(exp 10) cm(exp -2). Room-temperature photoluminescence spectra of quantum dots formed by this process indicate that they emit at wavelengths from 1.7 to 2.3 microns.

Qiu, Yueming↗
NASA NTRS2004

Growing High-Quality InAs Quantum Dots for Infrared Lasers

An improved method of growing high-quality InAs quantum dots embedded in lattice-matched InGaAs quantum wells on InP substrates has been developed. InAs/InGaAs/InP quantum dot semiconductor lasers fabricated by this method are capable of operating at room temperature at wavelengths greater than or equal to 1.8 mm. Previously, InAs quantum dot lasers based on InP substrates have been reported only at low temperature of 77 K at a wavelength of 1.9 micrometers. In the present method, as in the prior method, one utilizes metalorganic vapor phase epitaxy to grow the aforementioned semiconductor structures. The development of the present method was prompted in part by the observation that when InAs quantum dots are deposited on an InGaAs layer, some of the InAs in the InGaAs layer becomes segregated from the layer and contributes to the formation of the InAs quantum dots. As a result, the quantum dots become highly nonuniform; some even exceed a critical thickness, beyond which they relax. In the present method, one covers the InGaAs layer with a thin layer of GaAs before depositing the InAs quantum dots. The purpose and effect of this thin GaAs layer is to suppress the segregation of InAs from the InGaAs layer, thereby enabling the InAs quantum dots to become nearly uniform (see figure). Devices fabricated by this method have shown near-room-temperature performance.

Qiu, Yueming↗
NASA NTRS2004

Self-assembled InAsSb quantum dots on (001) InP substrates

Self-assembled InAsSb quantum dots (QD) on (001) InP substrates have been grown using metalorganic vapor phase epitaxy. The dot density and size are found to be strongly dependent on the presence of arsine. Direct deposition of InSb on InP and GaSb substrates formed large islands of InSb with low density of less than 53109/cm², however, InAsSb QDs of density as high as 4 X10¹º/cm² could be self-assembled by alternating group III and group V precursors, and high density almost pure InSb QDs were achieved on In0.53Ga0.47As/InP. The formation of high density InAsSb QDs is a result of a local nonequilibrium process and a reduction in mobility of In adatoms on the growth surface due to the presence of arsenic atoms, and in the case of high density almost pure InSb QDs on InGaAs/InP, the InAs interface layer is believed to be responsible. Photoluminescence shows that InSb QDs emit light at room temperature in the range of 1.7–2.2µm.

Uhl, David↗
DOE OSTI2022

Forming iron nitride hard magnetic materials using chemical vapor deposition or liquid phase epitaxy

The disclosure describes techniques for forming hard magnetic materials including α″-Fe16N2 using chemical vapor deposition or liquid phase epitaxy and hard materials formed according to these techniques. A method comprises heating an iron source to form a vapor comprising an iron-containing compound; depositing iron from the vapor comprising the iron-containing compound and nitrogen from a vapor comprising a nitrogen-containing compound on a substrate to form a layer comprising iron and nitrogen; and annealing the layer comprising iron and nitrogen to form at least some crystals comprising α″-Fe16N2.

Wang, Jian-Ping↗
DOE OSTI2022

Carbon and silicon background impurity control in undoped GaN layers grown with trimethylgallium and triethylgallium via metalorganic chemical vapor deposition

The unintentional impurity incorporation in GaN epitaxial layers impacts the electrical conductivity and optical properties of the films grown by metalorganic chemical vapor deposition (MOCVD). It is critical to control impurity-related states for device structure development. The aim of this work is to contribute to the understanding of the reasons for the presence of certain background impurities. In this paper, the unintentional impurity incorporation of carbon, hydrogen, oxygen, and silicon in undoped (u-) GaN films grown by low-pressure MOCVD were studied. Background impurity concentrations were evaluated by secondary ion mass spectroscopy (SIMS) to optimize growth parameters including V/III ratio, growth temperature (T g ), growth pressure (P g ), and gallium (Ga) precursors of trimethylgallium (TMG) or triethylgallium (TEG). Unintentional [C] and [Si] incorporations are found to be highly dependent on the growth parameters. Further, for the same growth conditions, u-GaN films grown with a TEG precursor exhibited a lower background concentration of C compared to that of GaN grown with TMG. However, the lowest background [C] achieved was grown with TMG using optimized conditions and exhibited [C]<4×10 15 cm -3 which was at the detection limit of the SIMS measurement. These results were measured for a u-GaN layer grown with TMG at 200 Torr, 1030°C, and a V/III of 3700. The lowest background [Si] of 4×10 15 cm -3 was achieved by growing with TMG at 200 Torr, 1000°C, and V/III of 650.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Non-planar growth of high Al-mole-fraction AlGaN on patterned GaN platforms for mitigating strain-induced cracks beyond the critical layer thickness

Non-planar growth of Al x Ga 1-x N epitaxial layers with an average alloy composition up to Al-mole-fraction of x ~ 0.21 was performed on patterned c-plane GaN on (0001) sapphire substrates with stripe-shaped mesa structures. This approach successfully realized the growth of crack-free AlGaN layers on the top of mesas with layer thicknesses greater than the expected critical layer thickness by relaxing the in-plane stress on the top of the mesa. The effectiveness of the relaxation strongly depends on the width and depth of the stripe mesa. In conclusion, the relaxation of in-plane stress and resultant suppression of cracks in AlGaN layer are qualitatively discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nanosecond semiconductor disk laser emitting at 496.5 nm

An optically pumped semiconductor disk laser based on a heterostructure containing ten CdS/ZnSe coupled quantum wells with type-II band offsets is studied. The structure was grown by metalorganic vapour phase epitaxy (MOVPE) on a GaAs substrate. The peak power of the semiconductor disk laser achieved at room temperature under longitudinal pumping by a repetitively pulsed N{sub 2} laser was 0.75 W at a wavelength of 496.5 nm, a pulse duration of 3 ns, and a pulse repetition rate of 100 Hz. The slope efficiency of the disk laser was 2.7 %. The total divergence angle at a cavity length of 1.1 mm varied from 5 mrad near the lasing threshold to 15 mrad at the maximum pump power. (lasers)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Optically pumped semiconductor laser based on a type-II CdS/ZnSe heterostructure

An optically pumped semiconductor laser based on a type-II CdS/ZnSe nanoheterostructure containing 10 quantum wells (QWs) was studied. The structure was grown by metal-organic vapour phase epitaxy on a GaAs substrate. The lifetime of electron-hole pairs at a low pump level was measured by luminescence decay to be ∼0 ns. The peak power of the microcavity semiconductor laser at room temperature and longitudinal pumping by a repetitively pulsed N{sub 2} laser was 7.2 W at a wavelength of 514 nm. The relatively low laser slope efficiency (0.35 %) is explained by amplified spontaneous emission propagating along the structure. The peak power and efficiency of the laser in the case of transverse pumping increase to 70 W and 3.5 %, respectively. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Using electron channeling contrast imaging to inform and improve the growth of high-efficiency GaAs solar cells on nanopatterned GaAs substrates

Patterned substrates provide opportunities for reducing the cost of high-efficiency III-V devices by incorporating mechanically weak layers beneficial for substrate reuse (e.g. by spalling). In this work, the functionality of electron channeling contrast imaging (ECCI) as a tool to efficiently understand and mitigate defect formation is exemplified by developing a process in which high-quality III-V material can be grown on nanopatterned GaAs substrates. Reactive ion etching used in the patterning process was found to damage the GaAs substrate surface, leading to the formation of stacking faults in the epitaxial material as observed by ECCI. Etching the patterned substrates in a 1 NH4OH: 1 H2O2: 50 DI H2O solution for 10 s prior to growth removed the substrate surface damage and stacking faults were no longer present. Growth of solar cell device structures initially produced samples with many macroscale flaws creating shunts in the devices, which complicated the assessment of material quality by device measurements. However, ECCI revealed that the epitaxial material surrounding macroscale flaws was free from any crystallographic defects such as stacking faults and threading dislocations. With this knowledge, we focused on refining the patterning process to eliminate the macroscale flaws. Solar cells were then grown on the improved nanopatterned substrates and exhibited device structures with defect densities less than 5 x 105 cm -2 and average conversion efficiency of 24.8%, nearly identical to devices grown on unpatterned epi-ready substrates (25.0%).

14 SOLAR ENERGY↗
DOE OSTI2023

GaAs solar cells grown on acoustically spalled GaAs substrates with 27% efficiency

Acoustic spalling presents a potentially low-cost reuse pathway for III–V epitaxial growth substrates via exfoliation of device layers with recovery and reuse of the substrate. However, surface features formed during spalling can reduce the performance of subsequently grown devices. Here, we develop an understanding of how the surface morphology of acoustically spalled substrates affects GaAs solar cell performance and develop strategies to mitigate these impacts. We demonstrate that minor planarization of the surface by wet chemical etching and/or epitaxial growth, or the redesign of the device structure to thicken critical layers, prevents performance degradation. Using these strategies, we demonstrate a 0.25 cm 2 single-junction GaAs device with 26.9% ± 0.2% photovoltaic conversion efficiency under the AM1.5G spectrum grown on an acoustically spalled substrate. These results enable the growth of high-performance III–V devices on non-traditional substrates with the potential for significantly reduced device costs.

14 SOLAR ENERGY↗
DOE OSTI2024

Large Area Near‐Field Thermophotovoltaics for Low Temperature Applications

Abstract Thermophotovoltaics, devices that convert thermal infrared photons to electricity, offer a key pathway for a variety of critical renewable energy technologies including thermal energy storage, waste heat recovery, and direct solar‐thermal power generation. However, conventional far‐field devices struggle to generate reasonable powers at lower temperatures. Near‐field thermophotovoltaics provide a pathway to substantially higher powers by leveraging photon tunneling effects. Here a large area near‐field thermophotovoltaic device is presented, created with an epitaxial co‐fabrication approach, that consists of a self‐supported 0.28 cm 2 emitter‐cell pair with a 150 nm gap. The device generates 1.22 mW at 460 °C, a 25‐fold increase over the same cell measured in a far‐field configuration. Furthermore, the near‐field device demonstrates short circuit current densities greater than the far‐field photocurrent limit at all the temperatures tested, confirming the role of photon tunneling effects in the performance enhancement. Modeling suggests several practical directions for cell improvements and further increases in power density. These results highlight the promise of near‐field thermophotovoltaics, especially for low temperature applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Surface Conversion of Single-Crystal Bi 2 Se 3 to β-In 2 Se 3

In this work, we demonstrate that the surface layers of single-crystal layered-2D Bi 2 Se 3 can be converted to layered-2D rhombohedral ß-In 2 Se 3 by annealing under a trimethylindium (TMIn) flux. Samples were prepared in a metalorganic chemical vapor deposition (MOCVD) chamber, then transferred under vacuum to a surface analysis chamber for analysis with low-energy electron diffraction (LEED) and Auger electron spectroscopy. Additional ex situ characterization included x-ray diffraction, transmission-electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS) elemental mapping, and Raman spectroscopy. The resulting single-crystal ß-In 2 Se 3 adopts the rhombohedral crystal structure (space group R-3m) and orientation of the underlying Bi 2 Se 3 , and the excess Bi atoms generated by this process create an underlying region of Bi-rich Bi x Se y . Due to the difference in bandgap between Bi 2 Se 3 and In 2 Se 3 , this conversion reaction presents a pathway to lateral heterojunctions if only selected regions are converted by masking the surface to spatially define the TMIn exposure. The conversion may also have implications for heteroepitaxy, because the in-plane lattice constants of Bi 2 Se 3 and In 2 Se 3 (0001) surfaces match those of InP and GaAs (111), respectively, and the natural cleavage planes of a layered-2D crystal facilitate substrate removal and reuse.

14 SOLAR ENERGY↗
DOE OSTI2022

Optically thick GaInAs/GaAsP strain-balanced quantum-well tandem solar cells with 29.2% efficiency under the AM0 space spectrum

GaAs is often used as a multijunction subcell due to its high material quality on GaAs substrates, despite having a non-optimal bandgap. The bandgap can be beneficially reduced using many layers of thin, strain-balanced GaInAs in a superlattice or quantum well device, but achieving excellent carrier collection without increased recombination has proven challenging. Here, we develop and demonstrate high performance, optically thick GaInAs/GaAsP strain-balanced solar cells. Excellent material quality is achieved in thick superlattices by using growth conditions that limit progressive thickness and composition fluctuations. Bandgap-voltage offsets as low as 0.31V are shown in superlattice cells using thin, highly strained GaP barriers. Optically thick superlattice cells with over 2500 nm of total GaInAs in the depletion region are developed, enabling 3.8 mA/cm 2 of extra photocurrent beyond the GaAs band edge under the AM0 space spectrum. Optimized superlattice solar cells are incorporated into two-junction devices that achieve 29.2% efficiency under the AM0 space spectrum due to their improved bandgap combination and high subcell voltages.

14 SOLAR ENERGY↗
DOE OSTI2021

Nucleation of High-Quality GaP on Si Through V-Groove Si Substrates

III-V growth on nanopatterned v-groove Si is a promising route to low cost Si-based tandem solar cells. Here, we study a high temperature MOVPE nucleation regime for growing GaP on v-groove Si. It is shown that MOVPE growth conditions can be used to control the facet selectivity of the GaP nucleation onto the v-grooves. The most promising nucleation condition was studied further, and it was demonstrated that it can be coalesced into a smooth thin film with promising material quality.

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