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Results for “VAPOR PHASE EPITAXY”

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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
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)↗
DOE OSTI2021

The Right Resources to Develop Thermophotovoltaic Technologies

Photovoltaic technologies have come a long way, but new breakthroughs are always around the corner. Thermophotovoltaics (TPV) capture heat and convert it to electricity at high efficiency. NREL is comprehensively situated to advance this new energy pathway with breakthrough potential.

14 SOLAR ENERGY↗
DOE OSTI2020

Multilayer iron nitride hard magnetic materials

The disclosure describes multilayer hard magnetic materials including at least one layer including α″-Fe16N2 and at least one layer including α″-Fe16(NxZ1-x)2 or a mixture of α″-Fe16N2 and α″-Fe16Z2, where Z includes at least one of C, B, or O, and x is a number greater than zero and less than one. The disclosure also describes techniques for forming multilayer hard magnetic materials including at least one layer including α″-Fe16N2 and at least one layer including α″-Fe16(NxZ1-x)2 or a mixture of α″-Fe16N2 and α″-Fe16Z2 using chemical vapor deposition or liquid phase epitaxy.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Adatom-Driven Oxygen Intermixing during the Deposition of Oxide Thin Films by Molecular Beam Epitaxy

Thin film deposition from the vapor phase is a complex process involving adatom adsorption, movement, and incorporation into the growing film. While simulations have been able to capture key aspects of these processes to reproduce film morphology, these models typically ignore any subsurface diffusion that occurs. The results of these diffusion and intermixing processes are often observed experimentally, but cannot be explained by invoking bulk lattice diffusion at the deposition temperature. In this work, we present quantitative experimental data that reveals anion and cation intermixing over long length scales during the deposition of epitaxial Fe 2 O 3 and Cr 2 O 3 films and heterostructures by oxygen-plasma-assisted molecular beam epitaxy. We track this diffusion by incorporating well-defined tracer layers containing 18 O and/or 57 Fe, and measure their redistribution on the nanometer scale with atom probe tomography. Molecular dynamics simulations suggest potential intermixing events, which are then examined via nudged elastic band calculations. These reveal that adatoms on the film surface act to “pull up” subsurface O and Fe. Subsequent ring-like rotation mechanisms involving both adatom and subsurface anions then facilitate their mixing. A simple model incorporating these events qualitatively agrees with the experimental intermixing results. In addition to film deposition, these intermixing mechanisms may to be operant during other surface-mediated processes such as heterogeneous catalysis and corrosion.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Templated Vapor-Liquid-Solid Epitaxy of III-V Semiconductors on Silicon

Epitaxial growth on silicon remains the most promising method to reduce the cost of optoelectronic-quality III-V semiconductors. Several approaches exist to epitaxially grow III-V material from the vapor phase, but control of 3-D structures remains difficult with these methods. In this work, we present a novel epitaxial growth technique in which photolithography and wet etching are used to establish the geometry of group-III metal, which is then converted to III-V semiconductor by annealing with exposure group-V precursor. The resulting material has been shown to align to the substrate lattice and is optically active, making it a promising approach for III-V PV technologies.

government↗
DOE OSTI2021

Epitaxy of LiNbO 3 : Historical Challenges and Recent Success

High-quality epitaxial growth of thin film lithium niobate (LiNbO 3 ) is highly desirable for optical and acoustic device applications. Despite decades of research, current state-of-the-art epitaxial techniques are limited by either the material quality or growth rates needed for practical devices. In this paper, we provide a short summary of the primary challenges of lithium niobate epitaxy followed by a brief historical review of lithium niobate epitaxy for prevalent epitaxial techniques. Available figures of merit for crystalline quality and optical transmission losses are given for each growth method. The highest crystalline quality lithium niobate thin film was recently grown by halide-based molecular beam epitaxy and is comparable to bulk lithium niobate crystals. However, these high-quality crystals are grown at slow rates that limit many practical applications. Given the many challenges that lithium niobate epitaxy imposes and the wide variety of methods that have unsuccessfully attempted to surmount these barriers, new approaches to lithium niobate epitaxy are required to meet the need for simultaneously high crystalline quality and sufficient thickness for devices not currently practical by existing techniques.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Room-Temperature Ferroelectric Epitaxial Nanowire Arrays with Photoluminescence

The development of large-scale, high-quality ferroelectric semiconductor nanowire arrays with interesting light-emitting properties can address limitations in traditional wide-bandgap ferroelectrics, thus serving as building blocks for innovative device architectures and next-generation high-density optoelectronics. Here, we investigate the optical properties of ferroelectric CsGeX 3 (X = Br, I) halide perovskite nanowires that are epitaxially grown on muscovite mica substrates by vapor phase deposition. Detailed structural characterizations reveal an incommensurate heteroepitaxial relationship with the mica substrate. Furthermore, photoluminescence that can be tuned from yellow-green to red emissions by varying the halide composition demonstrates that these nanowire networks can serve as platforms for future optoelectronic applications. In addition, the room-temperature ferroelectricity and ferroelectric domain structures of these nanowires are characterized using second harmonic generation (SHG) polarimetry. Finally, the combination of room-temperature ferroelectricity with photoluminescence in these nanowire arrays unlocks new avenues for the design of novel multifunctional materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

A Reduced-Temperature Process for Preparing Atomically Clean Si(100) and SiGe(100) Surfaces with Vapor HF

Silicon processing techniques such as atomic precision advanced manufacturing (APAM) and epitaxial growth require surface preparations that activate oxide desorption (typically >1000 °C) and promote surface reconstruction toward atomically clean, flat, and ordered Si(100)-2 × 1. Here, in this study, we compare the aqueous and vapor phase cleaning of Si and Si/SiGe surfaces to prepare APAM-ready and epitaxy-ready surfaces at lower temperatures. Angle resolved X-ray photoelectron spectroscopy (ARXPS) and Fourier transform infrared (FTIR) spectroscopy indicate that vapor hydrogen fluoride (VHF) cleans dramatically reduce carbon surface contamination and allow the chemically prepared surface to reconstruct at lower temperatures, 600 °C for Si and 580 °C for a Si/Si 0.7 Ge 0.3 heterostructure, into an ordered atomic terrace structure indicated by scanning tunneling microscopy (STM). After thermal treatment and vacuum hydrogen termination, we demonstrate STM hydrogen desorption lithography (HDL) on VHF-treated Si samples, creating reactive zones that enable area-selective chemistry by using a thermal budget similar to CMOS process flows. We anticipate that these results will establish new pathways to integrate APAM with Si foundry processing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Realizing crack-free high-aluminum-mole-fraction AlGaN on patterned GaN beyond the critical layer thickness

Wide-bandgap III-nitride heterostructures are required for a variety of device applications. However, this alloy system has a large lattice constant and thermal expansion coefficient mismatch that limits the alloy composition and layer thickness for many heteroepitaxial device structures. Consequently, various methods have been devised to allow the heteroepitaxial growth of AlInGaN heterostructures to accommodate this inherent strain. In this work, we describe a non-planar-growth approach that enables the deposition of crack-free high-Al-mole-fraction Al x Ga 1-x N on patterned GaN/sapphire templates and bulk GaN substrates with large-area mesas. We have studied the effects of the patterned mesa width, the mesa etch depth, and the gap between the mesas on the heteroepitaxy of Al x Ga 1-x N superlattices with an average Al molar fraction 0.11 < x¯ < 0.21 and non-planar overgrowth growth thicknesses up to 3.5 μm. Similar to the planar growth approach, increasing the thickness and Al mole fraction of the Al x Ga 1-x N superlattices leads to surface cracking when exceeding the critical layer thickness. However, limiting the mesa dimension in one direction enables strain mitigation and drastically increases the critical layer thickness. Additionally, larger etch depths of the mesas increase the Al alloy composition and thickness for crack-free AlGaN heteroepitaxy whereas the gap in between the mesas seems to have no crucial influence. We demonstrate that the Al alloy composition and layer thicknesses of such heterostructures can be increased far beyond the critical layer thickness for planar growth and demonstrate the growth of a crack-free full AlxGa 1-x N/GaN quantum-well laser heterostructure designed for operation at ~370 nm.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗