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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
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 OSTI2025

Pulse-length dependence of the laser-induced damage behavior of a fused-silica antireflective metasurface

A broadband, antireflective metasurface optic on a silica substrate is subjected to laser-induced damage-threshold measurements to quantify its performance under exposure to high-intensity/fluence laser pulses in the near-infrared at four pulse durations, ranging from 20 fs to 1.4 ns. The performance of the metasurface is benchmarked against that obtained from an equivalent bare fused-silica substrate that did not receive reactive-ion-etching metasurface treatment. Results showed that the damage threshold of the antireflective metasurface was always lower than the input-surface damage threshold of the untreated substrate. The damage initiations with nanosecond and picosecond pulses resulted in localized modification and removal of the nanostructures, whereas the onset of laser-induced modification with 20-fs pulses in a vacuum environment manifested as changes in the optical and electronic properties without significant material removal. The broader goal of this work is to develop a preliminary understanding of the laser-induced failure mechanisms of silica-based metasurface optics.

antireflection↗
DOE OSTI2024

Tantalum pentoxide: a new material platform for high-performance dielectric metasurface optics in the ultraviolet and visible region

Dielectric metasurfaces, composed of planar arrays of subwavelength dielectric structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems on chip. The performance of a dielectric metasurface is largely determined by its constituent material, which is highly desired to have a high refractive index, low optical loss and wide bandgap, and at the same time, be fabrication friendly. Here, we present a new material platform based on tantalum pentoxide (Ta 2 O 5 ) for implementing high-performance dielectric metasurface optics over the ultraviolet and visible spectral region. This wide-bandgap dielectric, exhibiting a high refractive index exceeding 2.1 and negligible extinction coefficient across a broad spectrum, can be easily deposited over large areas with good quality using straightforward physical vapor deposition, and patterned into high-aspect-ratio subwavelength nanostructures through commonly-available fluorine-gas-based reactive ion etching. We implement a series of high-efficiency ultraviolet and visible metasurfaces with representative light-field modulation functionalities including polarization-independent high-numerical-aperture lensing, spin-selective hologram projection, and vivid structural color generation, and the devices exhibit operational efficiencies up to 80%. Our work overcomes limitations faced by scalability of commonly-employed metasurface dielectrics and their operation into the visible and ultraviolet spectral range, and provides a novel route towards realization of high-performance, robust and foundry-manufacturable metasurface optics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Optically pumped deep-UV multimode lasing in AlGaN double heterostructure grown by molecular beam homoepitaxy

Multimode lasing at sub-300 nm wavelengths is demonstrated by optical pumping in AlGaN heterostructures grown on single-crystal AlN substrates by plasma-assisted molecular beam epitaxy. Edge-emitting ridge-based Fabry–Pérot cavities are fabricated with the epitaxial AlN/AlGaN double heterostructure by a combined inductively coupled plasma reactive ion etch and tetramethylammonium hydroxide etch. The emitters exhibit peak gain at 284 nm and modal linewidths on the order of 0.1 nm at room temperature. The applied growth technique and its chemical and heterostructural design characteristics offer certain unique capabilities toward further development of electrically injected AlGaN laser diodes.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nanoscale control of LaAlO 3 /SrTiO 3 conductance beyond the polar catastrophe

Conductive atomic force microscope (c-AFM) lithography can be utilized to create a wide range of LaAlO 3 /SrTiO 3 (LAO/STO)-based nanoelectronic devices in a reconfigurable manner. Experiments were generally performed with intrinsically insulating LAO/STO heterostructures, with LAO thickness less than the critical value at which a polar catastrophe takes place [<4 unit cell (u.c.)]. Here, we use inductively coupled plasma reactive ion etching (ICPRIE) to fabricate c-AFM “canvases” on intrinsically conducting LAO/STO samples with ≥4 u.c. LAO. We observe that its interfacial two-dimensional electron gas (2DEG) can be pinched off and then switched back on by c-AFM lithography. Nanowires created with initially conductive LAO/STO interfaces have an order-of-magnitude longer lifetime in ambient conditions, when compared to an identically created 3.4 u.c. LAO/STO nanowire. We also demonstrate key nanoscale properties such as ballistic transport in a quasi-one-dimensional electron waveguide at a 5 u.c. LAO/STO interface. This approach frees c-AFM-written nanodevice designs from time constraints in air associated with <4 u.c. LAO/STO heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Modulating surface acoustic wave generation through superconductivity

Surface acoustic waves (SAWs), with their five orders-of-magnitude slower propagation velocity, allow for considerably shorter wavelengths at the same frequency compared to electromagnetic waves. The short wavelengths allow for device miniaturization and on-chip integration. The generic design of these devices involves piezoelectric substrates with comb-like arrays of Al or Au electrodes known as interdigitated transducers (IDTs) deposited on the surface. However, Al and Au both have shortcomings at the cryogenic temperatures required for quantum applications, namely, the formation of two-level systems and the lack of superconductivity perpetuating Ohmic losses, respectively. In this work, SAWs are generated in the high-MHz to low-GHz range using niobium nitride (NbN) interdigitated transducers and Bragg reflectors. We demonstrate the fabrication of acoustic devices through photolithography and reactive ion etching. The sharp transition between superconducting and normal states and the corresponding change in SAW transmission allows for fine control of the “on” (superconducting) and “off” (normal) states of NbN, with a ΔT = 1 K separating the transmission minimum and maximum. We demonstrate a 16× difference in transmission between the “on” and “off” states of the device. The SAW transmission behavior mirrors the change in resistance of NbN at its T c . These findings open up new possibilities for the integration of NbN SAW resonators into existing quantum architectures based on NbN and a method for adjusting transmission properties independent of applied voltage.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Understanding and Mitigating the Contamination of Intrinsic poly-Si Gaps in Passivated IBC Solar Cells

We investigate factors that are critical for the performance of interdigitated back contact (IBC) solar cells based on polycrystalline silicon (poly-Si) passivated contacts. During patterning of doped lines using direct plasma deposition through a shadow mask, we show that the intrinsic poly-Si gap becomes contaminated with dopants, leading to shunting. Possible contamination mechanisms during masked deposition and high- temperature annealing are tested. Strategies developed to mitigate the contamination, such as reactive ion etching after deposition and amorphous to poly-Si crystallization in oxygen, will lead to improved IBC fabrication methods.

14 SOLAR ENERGY↗
DOE OSTI2021

Submicron Thickness Characterization of Poly-Si Thin Films on Textured Surfaces by X-Ray Diffraction for Minimizing Parasitic Absorption in Poly-Si/SiO2 Passivating Contact Cells

Poly-Si/SiO2 passivating contacts have shown great potential for industry-relevant next-generation solar cells. Previous work has demonstrated an improved Jsc from dry reactive ion etching in an SF6 environment, using front metal grids as a self-aligned mask. Here, we show an effective method to measure the front poly-Si thickness on an alkaline textured surface using X-ray diffraction. The calculated thicknesses are compared with cross-section SEM analysis and Quokka simulation. We show that by thinning the front poly-Si from 200 nm to 60 nm, Jsc increases by 2.4 mA/cm2, leading to an absolute efficiency gain of 1.73%. Finally, we discuss possible reasons for the premature loss of passivation before the removal of all poly-Si.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2021

Nanofabrication of silicon surfaces for reduced virus adhesion

Nanofabrication is a remarkably effective technique to create desirable nanoscale patterns. Here, the effect of surface nanofabrication on altering virus adhesion to the substrates was examined. Arrays of nanoholes, 50 nm in diameter, 22 nm deep, and 100 nm in pitch distance, were created on silicon (Si) wafers by electron-beam lithography and reactive ion etching. MS2 coliphage, which is 26 ± 2 nm in diameter and is frequently used as a surrogate for human viruses, was applied to investigate the interaction between the virions and smooth or nanostructured Si surfaces. Scanning electron microscopy and atomic force microscopy along with surface wettability analyses revealed that the nanofabrication had the effect of reducing not only the number of viruses attached but also the strength of virus adhesion. These effects were ascribed to the presence of nanoholes, which were inaccessible to the virions due to the unique surface topographical parameters and the surface chemistry, resulting in the decrease of the overall solid contact area for MS2 attachment. The periodic spacing of the nanoholes also limited the unit landing area for MS2 particles, restricting the formation of MS2 aggregates and leading to the reduced amount of MS2 attachment. We anticipate that smart design of a surface’s chemical composition and nanostructure will offer a feasible solution to improve mitigations for controlling viral adhesion and transmission to and from food contact surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Synthesis and electrical transport properties of superconducting platinum silicide thin films and devices

Here, we evaluate the material characteristics of superconducting platinum silicide (PtSi) thin films as candidate materials for superconducting quantum information devices compatible with silicon technology. These films were synthesized using magnetron sputtering under ultrahigh vacuum conditions, followed by rapid thermal annealing. Polycrystalline PtSi films synthesized by this method have the favorable properties of superconducting critical temperature of 0.95 K and relatively long zero-temperature Ginzburg-Landau coherence length of 76 nm. We further studied coplanar microbridge devices fabricated by electron beam lithography and chlorine-free reactive ion etching, finding that the temperature-dependent critical current density follows the Ginzburg Landau depairing mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2024

High-resolution chemical patterns from negative tone resists for the integration of extreme ultraviolet patterns of metal-oxide resists with directed self-assembly of block copolymers

Extreme ultraviolet (EUV) lithography faces significant challenges in designing suitable resist materials that can provide adequate precision, while maintaining economically viable throughput. These challenges in resist materials have led to printing failures and high roughness in EUV patterns, compromising the performance of semiconductor devices. Integrating directed self-assembly (DSA) of block copolymers (BCPs) with EUV lithography offers a promising solution because, while the BCPs register to the EUV-defined chemical guiding pattern, the thermodynamically determined structures of the BCPs automatically rectify defects and roughness in the EUV pattern. Despite the superior resolution of metal-oxide EUV resists (MORs), their application to DSA is limited by the difficulty in converting them into chemical patterns that allow effective transfer of the rectified patterns of DSA films into inorganic materials. To address this challenge, this study introduces a novel strategy for fabricating chemical patterns using hydrogen silsesquioxane (HSQ), a high-resolution negative tone inorganic resist, as a model system for MORs. Initially, a sacrificial Cr pattern is generated from HSQ patterns via reactive ion etching. The sacrificial Cr pattern is converted into a chemical pattern by first grafting a water-soluble polyethylene oxide brush onto the substrate, then wet etching the Cr, and finally grafting nonpolar polystyrene brushes. Assembling polystyrene-block-poly(methyl methacrylate) on these patterns results in structures oriented and registered with the underlying pattern, achieving 24 nm full-pitch resolutions. In conclusion, this approach has the potential to integrate MOR patterns into the DSA process, thereby enabling the generation of high-quality sub-10 nm patterns with high-χ BCPs.

Atomic force microscopy↗
DOE OSTI2022

Micro-fabricated mirrors with finesse exceeding one million

The Fabry–Perot resonator is one of the most widely used optical devices, enabling scientific and technological breakthroughs in diverse fields including cavity quantum electrodynamics, optical clocks, precision length metrology, and spectroscopy. Though resonator designs vary widely, all high-end applications benefit from mirrors with the lowest loss and highest finesse possible. Fabrication of the highest-finesse mirrors relies on centuries-old mechanical polishing techniques, which offer losses at the parts-per-million (ppm) level. However, no existing fabrication techniques are able to produce high-finesse resonators with the large range of mirror geometries needed for scalable quantum devices and next-generation compact atomic clocks. In this paper, we introduce a scalable approach to fabricate mirrors with ultrahigh finesse ( ≥ <#comment/> 10 6 ) and user-defined radius of curvature spanning of four orders of magnitude ( 10 − <#comment/> 4 − <#comment/> 10 0 m ). We employ photoresist reflow and reactive ion etching to shape and transfer mirror templates onto a substrate while maintaining sub-Angstrom roughness. This substrate is coated with a dielectric stack and used to create arrays of compact Fabry–Perot resonators with finesse values as high as 1.3 million and measured excess loss < <#comment/> 1 p p m . Optical ringdown measurements of 43 devices across five substrates reveal that the fabricated cavity mirrors—with both small and large radii of curvature—produce an average coating-limited finesse of 1.05 million. This versatile approach opens the door to scalable fabrication of high-finesse miniaturized Fabry–Perot cavities needed for emerging quantum optics and frequency metrology technologies.

42 ENGINEERING↗
DOE OSTI2020

Micromachined Silicon Platform for Precise Assembly of 2D Multilayer Laue Lenses for High-Resolution X-ray Microscopy

We report on a developed micromachined silicon platform for the precise assembly of 2D multilayer Laue lenses (MLLs) for high-resolution X-ray microscopy. The platform is 10 × 10 mm2 and is fabricated on ~500 µm thick silicon wafers through multiple steps of photolithography and deep reactive-ion etching. The platform accommodates two linear MLLs in a pre-defined configuration with precise angular and lateral position control. In this work, we discuss the design and microfabrication of the platform, and characterization regarding MLLs assembly, position control, repeatability, and stability. The results demonstrate that a micromachined platform can be used for the assembly of a variety of MLLs with different dimensions and optical parameters. The angular misalignment of 2D MLLs is well controlled in the range of the designed accuracy, down to a few millidegrees. The separation distance between MLLs is adjustable from hundreds to more than one thousand micrometers. The use of the developed platform greatly simplifies the alignment procedure of the MLL optics and reduces the complexity of the X-ray microscope. It is a significant step forward for the development of monolithic 2D MLL nanofocusing optics for high-resolution X-ray microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Size-Dependent Electrical Transport Properties in Conducting Diamond Nanostripes

With the advances in nanofabrication technology, horizontally aligned and well-defined nitrogen-doped ultrananocrystalline diamond nanostripes can be fabricated with widths in the order of tens of nanometers. The study of the size-dependent electron transport properties of these nanostructures is crucial to novel electronic and electrochemical applications. In this paper, 100 nm thick n-type ultrananocrystalline diamond thin films were synthesized by microwave plasma-enhanced chemical vapor deposition method with 5% N2 gas in the plasma during the growth process. Then the nanostripes were fabricated using standard electron beam lithography and reactive ion etching techniques. The electrical transport properties of the free-standing single nanostripes of different widths from 75 to 150 nm and lengths from 1 to 128 μm were investigated. The study showed that the electrical resistivity of the n-type ultrananocrystalline diamond nanostripes increased dramatically with the decrease in the nanostripe width. The nanostripe resistivity was nearly doubted when the width was reduced from 150 nm to 75 nm. The size-dependent variability in conductivity could originate from the imposed diffusive scattering of the nanostripe surfaces which had a further compounding effect to reinforce the grain boundary scattering.

Zhou, Andrew F. (ORCID:0000000243050409)↗
DOE OSTI2022

GaN vertical-channel junction field-effect transistors with regrown p-GaN by metal organic chemical vapor deposition (MOCVD)

Fabricating a vertical-channel junction field-effect transistor includes forming an unintentionally doped GaN layer on a bulk GaN layer by metalorganic chemical vapor deposition, forming a Cr/SiO 2 hard mask on the unintentionally doped GaN layer, patterning a fin by electron beam lithography, defining the Cr and SiO 2 hard masks by reactive ion etching, improving a regrowth surface with inductively coupled plasma etching, removing hard mask residuals, regrowing a p-GaN layer, selectively etching the p-GaN layer, forming gate electrodes by electron beam evaporation, and forming source and drain electrodes by electron beam evaporation. The resulting vertical-channel junction field-effect transistor includes a doped GaN layer, an unintentionally doped GaN layer on the doped GaN layer, and a p-GaN regrowth layer on the unintentionally doped GaN layer. Portions of the p-GaN regrowth layer are separated by a vertical channel of the unintentionally doped GaN layer.

Zhao, Yuji↗
DOE OSTI2023

High absorption photovoltaic material and methods of making the same

A high absorption photovoltaic material and method of making the material for use in a solar cell are disclosed. The photovoltaic material includes a surface modified with a layer of repeating photonic crystal structures. The photonic crystal structures are approximately inverse conically shaped and have a curved sidewall that has an approximately Gaussian shape. The photonic crystal structures generally have a high vertical depth and sidewall angle. The structures also have a gradient refractive index profile and exhibit the parallel-to-interface refraction light trapping effect. An anti-reflective coating is disposed over the photonic crystal structure layer. The photovoltaic material exhibits near unity light absorption over a broad range of visible and near infrared wavelengths and incidence angles, even at reduced thicknesses. The photovoltaic structures are formed via a combined photolithography and reactive-ion etching method at low power with a gas mixture having a high ratio of an etchant component to a passivation component.

Kuang, Ping↗
DOE OSTI2020

Infiltration synthesis of hybrid nanocomposite resists for advanced nanolithography

In this work, we demonstrate a simple ex-situ inorganic infiltration route for transforming standard organic resists into highperformance positive tone hybrid resist platform. A model thin film PMMA-AlOx hybrid resist system has been synthesized by hybridization of PMMA with AlOx and investigated for electron beam lithography. The approach possesses full controllability of the resist performance in terms of critical does, patterning contrast reaching up to 30 and etch resistance for plasma-based pattern transfer processes. The high selectivity Si etching capability demonstrated using a lowtemperature cryo-Si etch process, based on the controlled infiltration outperforms commercial resists and typical hard mask material thermal SiO 2 , with estimated achievable selectivity in excess of ~300. Si nanostructures down to ~30 nm with aspect ratio up to ~17 are also transferred into the Si substrate. Easy implementation and adaptability for different inorganic infiltrations, this platform is well capable of potentially delivering the resist performance and throughput necessary for EUV lithography.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Fabrication methods for high reflectance dielectric-metal point contact rear mirror for optoelectronic devices

The patterned dielectric back contact (PDBC) structure can be used to form a point-contact architecture that features a dielectric spacer with spatially distributed, reduced-area metal point contacts between the semiconductor back not recognized contact layer and the metal back contact. In this structure, the dielectric-metal region provides higher reflectance and is electrically insulating. Reduced-area metal point contacts provide electrical conduction for the back contact but typically have lower reflectance. The fabrication methods discussed in this article were developed for thermophotovoltaic cells, but they apply to any III-V optoelectronic device requiring the use of a conductive and highly reflective back contact. Patterned dielectric back contacts may be used for enhanced sub-bandgap reflectance, for enhanced photon recycling near the bandgap energy, or both depending on the optoelectronic application. The following fabrication methods are discussed in the article: PDBC fabrication procedures for spin-on dielectrics and commonly evaporated dielectrics to form the spacer layer; methods to selectively etch a parasitically absorbing back contact layer using metal point contacts as an etch mask; methods incorporating a dielectric etch through different process techniques such as reactive ion and wet etching.

14 SOLAR ENERGY↗