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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 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Low-loss Si-based Dielectrics for High Frequency Components of Superconducting Detectors

Silicon-based dielectric is crucial for many superconducting devices, including high-frequency transmission lines, filters, and resonators. Defects and contaminants in the amorphous dielectric and at the interfaces between the dielectric and metal layers can cause microwave losses and degrade device performance. Optimization of the dielectric fabrication, device structure, and surface morphology can help mitigate this problem. We present the fabrication of silicon oxide and nitride thin film dielectrics. We then characterized them using Scanning Electron Microscopy, Atomic Force Microscopy, and spectrophotometry techniques. The samples were synthesized using various deposition methods, including Plasma-Enhanced Chemical Vapor Deposition and magnetron sputtering. The film's morphology and structure were modified by adjusting the deposition pressure and gas flow. The resulting films were used in superconducting resonant systems consisting of planar inductors and capacitors. Measurements of the resonator properties, including their quality factor, were performed.

low-loss dielectrics↗
DOE OSTI2016

RBS-channeling study of radiation damage in Ar{sup +} implanted CuInSe{sub 2} crystals

Chalcopyrite solar cells are reported to have a high tolerance to irradiation by high energy electrons or ions, but the origin of this is not well understood. This work studies the evolution of damage in Ar{sup +}-bombarded CuInSe{sub 2} single crystal using Rutherford backscattering/channeling analysis. Ar{sup +} ions of 30 keV were implanted with doses in the range from 10{sup 12} to 3 × 10{sup 16} cm{sup −2} at room temperature. Implantation was found to create two layers of damage: (1) on the surface, caused by preferential sputtering of Se and Cu atoms; (2) at the layer of implanted Ar, possibly consisting of stacking faults and dislocation loops. The damage in the second layer was estimated to be less than 2% of the theoretical prediction suggesting efficient healing of primary implantation defects.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Influence of Fe underlayers on stress evolution of Ti in Ti/Fe multilayers

A series of 40–2 nm bilayer spacing Ti/Fe multilayers were sputter-deposited. As the length scale of individual Ti layers equaled to 2 nm, Ti phase transforms from a hexagonal close packed (hcp)-to-body centered cubic (bcc) crystal structures for equal layer thicknesses in Ti/Fe multilayers. Further equal reductions in bilayer spacing to less than 1 nm resulted in an additional transformation from a crystalline to amorphous structure. Atom probe tomography reveals significant intermixing between layers which contributes to the observed phase transformations. Real-time, intrinsic growth stress measurements were also performed to relate the adatom mobility to these phase transformations. For the hcp Ti/bcc Fe multilayers of equivalent volume fractions, the multilayers undergo an overall tensile stress state to a compressive stress state with decreasing bilayer thickness for the multilayers. When the above phase transformations occurred, a modest reduction in the overall compressive stress of the multilayer was noted. Depending on the Fe thickness, the Ti growth was observed to be a tensile to compressive growth change to a purely compressive growth for thinner bilayer spacing. Fe retained a tensile growth stress regardless of the bilayer spacing studied.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Atomic step disorder on polycrystalline surfaces leads to spatially inhomogeneous work functions

Structural disorder causes materials’ surface electronic properties, e.g., work function (ϕ), to vary spatially, yet it is challenging to prove exact causal relationships to underlying ensemble disorder, e.g., roughness or granularity. For polycrystalline Pt, nanoscale resolution photoemission threshold mapping reveals a spatially varying ϕ=5.70±0.03 eV over a distribution of (111) vicinal grain surfaces prepared by sputter deposition and annealing. With regard to field emission and related phenomena, e.g., vacuum arc initiation, a salient feature of the ϕ distribution is that it is skewed with a long tail to values down to 5.4 eV, i.e., far below the mean, which is exponentially impactful to field emission via the Fowler–Nordheim relation. We show that the ϕ spatial variation and distribution can be explained by ensemble variations of granular tilts and surface slopes via a Smoluchowski smoothing model wherein local ϕ variations result from spatially varying densities of electric dipole moments, intrinsic to atomic steps, that locally modify ϕ. Atomic step-terrace structure is confirmed with scanning tunneling microscopy (STM) at several locations on our surfaces, and prior works showed STM evidence for atomic step dipoles at various metal surfaces. From our model, we find an atomic step edge dipole μ=0.12 D/edge atom, which is comparable to values reported in studies that utilized other methods and materials. Our results elucidate a connection between macroscopic ϕ and the nanostructure that may contribute to the spread of reported ϕ for Pt and other surfaces and may be useful toward more complete descriptions of polycrystalline metals in the models of field emission and other related vacuum electronics phenomena, e.g., arc initiation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Nature of the Nonsingularity of Inner Interfaces in Hydroxyapatite Ceramics

The morphology of inner interfaces in hydroxyapatite (HA) based calcium phosphate ceramics has been studied by transmission electron microscopy. Grain boundaries in the ceramics have been shown to have a vicinal character, which is related to the mechanism of secondary recrystallization in the material: layer growth of grains via sequential motion of elementary steps on planes corresponding to the {11̄00} HA prism faces, which grow through transitions of atoms from adjacent grains that are in contact through their planes with large Miller indices. The recrystallization process may be accompanied by a “collision” of vicinal growth surfaces of grains with relatively large misalignment angles and the formation of grain boundaries nanofaceted by prism planes of adjacent grains. The recrystallization process in such a case should be expected to continue in the grain with a smaller nonsingularity of the growth front. Grain boundaries may allow for a match between planes differing in Miller indices, nd{sub h1k1l1} ≈ md{sub h2k2l2}, and the formation of grain-boundary Pumphrey dislocations, which compensate for the size mismatch between interplanar spacings and/or misalignment of the planes. The observed characteristic grain match configurations are typical of both ceramics produced by sintering HA powders and HA films produced by ion sputtering.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Effect of Heat Treatment on the Electrical Properties of Thin Yttrium-Doped In{sub 2}O{sub 3} Films

Thin indium oxide films and In–Y–O films containing 0.7 to 3.6 at % Y have been grown by ionbeam sputtering of an indium target and a composite (indium + weighed amounts of yttrium) target in a mixture of argon and oxygen. The thin indium oxide films have a cubic crystal structure (sp. gr. Ia3̄). The incorporation of yttrium atoms into indium oxide leads to the formation of an amorphous structure in the as-grown films and an increase in their room-temperature electrical resistance by several orders of magnitude. Lowtemperature electrical resistance data indicate a change in conduction mechanism. High-temperature heat treatment of the thin In–Y–O films leads to the crystallization of their amorphous structure and an increase in their electrical resistance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Efficiency of Tungsten Dust Collection of Different Types of Dust Particles by Electrostatic Probe

Formation of dust particles and clusters is observed in almost every modern thermonuclear facility. Accumulation of dust in the next generation thermonuclear installations can dramatically affect the plasma parameters and lead to the accumulation of unacceptably large amounts of tritium. Experiments on collection of dust particles by a model of electrostatic probe developed for collection of metallic dust at ITER are described in the article. Experiments on the generation of tungsten dust consisting of flakes formed during the destruction of tungsten layers formed on the walls of the plasma chamber sputtered from the surface of the tungsten target by plasma ions were conducted. The nature of dust degassing at elevated temperatures and the behavior of dust in an electric field were studied. The results obtained are compared with the results of the experiments with dust consisting of crystal particles of simple geometric shapes. The effectiveness of collection of both types of dust using the model of an electrostatic probe is determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Growth features and spectroscopic structure investigations of nanoprofiled AlN films formed on misoriented GaAs substrates

Nanostructured aluminum-nitride films are formed by reactive ion-plasma sputtering onto GaAs substrates with different orientations. The properties of the films are studied via structural analysis, atomic force microscopy, and infrared and visible–ultraviolet spectroscopy. The aluminum-nitride films can have a refractive index in the range of 1.6–4.0 at a wavelength of ~250 nm and an optical band gap of ~5 eV. It is shown that the morphology, surface composition, and optical characteristics of AlN/GaAs heterophase systems can be controlled using misoriented GaAs substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Fabrication of a terahertz quantum-cascade laser with a double metal waveguide based on multilayer GaAs/AlGaAs heterostructures

The Postgrowth processing of GaAs/AlGaAs multilayer heterostructures for terahertz quantumcascade lasers (QCLs) are studied. This procedure includes the thermocompression bonding of In–Au multilayer heterostructures with a doped n{sup +}-GaAs substrate, mechanical grinding, and selective wet etching of the substrate, and dry etching of QCL ridge mesastripes through a Ti/Au metallization mask 50 and 100 μm wide. Reactive-ion-etching modes with an inductively coupled plasma source in a BCl{sub 3}/Ar gas mixture are selected to obtain vertical walls of the QCL ridge mesastripes with minimum Ti/Au mask sputtering.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Silicon nanowire array architecture for heterojunction electronics

Photosensitive nanostructured heterojunctions n-TiN/p-Si were fabricated by means of titanium nitride thin films deposition (n-type conductivity) by the DC reactive magnetron sputtering onto nano structured single crystal substrates of p-type Si (100). The temperature dependencies of the height of the potential barrier and series resistance of the n-TiN/p-Si heterojunctions were investigated. The dominant current transport mechanisms through the heterojunctions under investigation were determined at forward and reverse bias. The heterojunctions under investigation generate open-circuit voltage V{sub oc} = 0.8 V, short-circuit current I{sub sc} = 3.72 mA/cm{sup 2} and fill factor FF = 0.5 under illumination of 100 mW/cm{sup 2}.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Electrical Properties of p-NiO/n-Si Heterostructures Based on Nanostructured Silicon

Silicon nanowires are formed on n-Si substrates by chemical etching. p-NiO/n-Si heterostructures are fabricated by reactive magnetron sputtering. The energy diagram of anisotype p-NiO/n-Si heterostructures is constructed according to the Anderson model. The current–voltage and capacitance–voltage characteristics are measured and analyzed. The main current-transport mechanisms through the p-NiO/n-Si heterojunction under forward and reverse biases are established.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Structural, Thermal and Luminescence Properties of AlN:Tm Thin Films Deposited on Silicon Substrate and Optical Fiber

Thin films of AlN:Tm are deposited on a Si(111) and Si(100) substrates and optical fiber by rf magnetron sputtering method. 200–400 nm thick films are deposited at various temperatures to observe the effect of temperature on their structure and morphology. Liquid nitrogen is used to deposit films at low temperature of –196°C. An electric heater is used to grow films at 250–700°C range. The deposited films are analyzed for their structure and thermal stability using X-ray diffraction and thermogravimetric analysis. Films deposited at liquid nitrogen temperature are amorphous and those deposited at high temperature are crystalline. Average grain size, strain and interatomic distance of the crystalline films are calculated. The grain size and inter-atomic distance increase from 36.54 to 45.12 nm and from 0.2368 to 0.25 nm respectively. Photoluminescence of the films excited by 783 nm crystal laser shows an intense infrared emission from Tm{sup +3} at 802 nm as a result of {sup 3}H{sub 4} → {sup 3}H{sub 6} transition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Technique for the Formation of Antireflection Coatings Based on ITO Films

A technique for forming transparent conducting indium-tin-oxide films for applications, which require high current densities, are considered. These films have to combine a good surface conductivity with an increased transmittance of radiation within a wide wavelength range. A two-stage process is designed to fabricate such coatings: at the first stage, a dense indium-tin-oxide film with high electrical conductivity is formed; then, an additional antireflection layer of the same material is deposited. Possible methods for optimizing these additional indium-tin-oxide films obtained by forming nanostructured self-assembled coatings on the surface of dense films are investigated. It is shown that the best light extraction over a wide wavelength range is provided by the composite film-deposition technique, which consists in the deposition of self-assembled nanostructured coatings by electron-beam evaporation on a hot substrate and the further filling of voids by magnetron sputtering at room temperature. Since the optical properties of the obtained antireflection coatings depend on the mass ratio of materials deposited at each stage, the technique makes it possible to optimize the film properties on the basis of problems specified by the area of their application. Computer simulation of the behavior of the material distribution in the structure of the studied coatings confirms the ability of the method of combined film deposition to form the smoothest character of the change in the effective refractive index of the coating structure, which leads to the most efficient extraction of optical-range light.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Laser Annealing of Thin ITO Films on Flexible Organic Substrates

Indium–tin oxide (ITO) thin films on polyethylene-terephthalate film substrates are fabricated at room temperature by reactive magnetron sputtering. The minimum of the resistivity of ITO films formed at room temperature is 4.5 × 10 -4 Ω cm. The laser annealing of ITO films from 140 to 600 nm in thickness increases their conductivity from 10 to 24% depending on the energy density to the film and irradiation dose. It is established that the laser annealing of films up to 250 nm in thickness is efficient at an energy density in the range from 12 to 35 mJ/cm 2 . Films from 390 to 600 nm in thickness should be annealed by laser radiation with an energy density of no lower than 46 mJ/cm 2 . A model problem taking into account the influence of radiative cooling and heat exchange of the film and the substrate on the variation in the film temperature over time during laser annealing is considered. A one-dimensional thermal conductivity equation for a bilayer medium is used. The maximal stresses in the ITO film under various annealing modes are calculated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Structure and Properties of Gallium-Oxide Films Produced by High-Frequency Magnetron-Assisted Deposition

The properties of gallium-oxide films produced by the radio-frequency magnetron-assisted sputtering of a β-Ga{sub 2}O{sub 3} target with deposition onto sapphire substrates are studied. The as-deposited gallium-oxide films are polycrystalline and contain crystallites of the α and β phases. Exposure to oxygen plasma does not bring about the appearance of new crystallites but makes crystallites several times larger in average dimensions in the substrate plane. After annealing at 900°C, the crystallite size becomes twice as large as that in the unannealed film. The films not subjected to thermal annealing exhibit a high resistance at 20°C. In the range of 50–500°C, the conductivity of the samples (G) only slightly depends on temperature (T) and, as T is elevated further, exponentially increases with the activation energy 0.7–1.0 eV. After annealing of the films in argon at 900°C (30 min), the conductivity G starts to sharply increase at T ≈ 350°C. In the dependence of lnG on T {sup –1}, a maximum in the range of 470–520°C and a portion of decreasing conductivity at higher temperatures are observed. The unusual temperature dependence of the conductivity after annealing is attributed to a change in the structure and phase composition of polycrystalline gallium-oxide films and, possibly, to some effects at the surface. The structures produced on insulating substrates are solar blind in the visible wavelength region and sensitive to radiation in the ultraviolet region (222 nm).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Defect Formation under Nitrogen-Ion Implantation and Subsequent Annealing in GaAs Structures with an Uncovered Surface and a Surface Covered with an AlN Film

The concentration profiles of defects produced in structures upon the implantation of nitrogen ions into GaAs epitaxial layers with an uncovered surface and that covered with an AlN film and subsequent annealing are studied. The ion energies and the implantation doses are chosen so that the nitrogen-atom concentration profiles coincided in structures of both types. Rutherford proton backscattering spectra are measured in the random and channeling modes, and the concentration profiles of point defects formed are calculated for the samples under study. It is found that the implantation of nitrogen ions introduces nearly the same number of point defects into structures of both types, and the formation of an AlN film by ion-plasma sputtering is accompanied by the formation of an additional number of defects. However, the annealing of structures of both types leads to nearly the same concentrations of residual defects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Nanostructured ITO/SiO{sub 2} Coatings

The influence of a SiO{sub 2} layer deposited onto nanostructured transparent conductive films of indium and tin oxide (ITO) on their optical characteristics is investigated. For this purpose, SiO{sub 2} films of various thicknesses are deposited by magnetron sputtering on samples with ITO films containing whiskers of preferentially vertical orientation and possessing steadily decreasing reflectance. It is shown that this makes it possible to attain noticeable coating antireflection under the condition of the uniform overgrowth of ITO whiskers by the SiO{sub 2} layer. The influence of the SiO{sub 2} layer on the optical characteristics of a dense unstructured ITO film is also investigated. The results for structured and unstructured ITO/SiO{sub 2} coatings with identical material mass contents are compared. It is noted that due to the liability of ITO material to degradation during operation in the composition of transparent conductive contacts, the results can also be interesting for the formation of coatings more resistant to the effect of the environment.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗