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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 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Photo-induced valence change of the sulfur atom in an L-cysteine thin film grown on a silver metal substrate in a saliva-emulated aqueous solution

A thin film of L-cysteine (HSCH{sub 2}CH(NH{sub 2})COOH) is grown on a silver substrate in saliva-emulated aqueous solution. X-ray photoemission spectroscopic measurements have revealed that the sulfur atom shows valence change under IR laser irradiation at 825 nm. The valence change maintains for about a minute at room temperature and more than an hour between 110 and 250 K after stopping the laser irradiation. It is not observed at all at temperatures lower than 110 K. This temperature-dependent behavior indicates that the photo-excited electronic change should be accompanied by a conformational change in the L-cysteine molecule. It is strongly suggested that the reversible valence change of the sulfur atom is applicable to a memory used around room temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Indium segregation during III–V quantum wire and quantum dot formation on patterned substrates

We report a model for metalorganic vapor-phase epitaxy on non-planar substrates, specifically V-grooves and pyramidal recesses, which we apply to the growth of InGaAs nanostructures. This model—based on a set of coupled reaction-diffusion equations, one for each facet in the system—accounts for the facet-dependence of all kinetic processes (e.g., precursor decomposition, adatom diffusion, and adatom lifetimes) and has been previously applied to account for the temperature-, concentration-, and temporal-dependence of AlGaAs nanostructures on GaAs (111)B surfaces with V-grooves and pyramidal recesses. In the present study, the growth of In{sub 0.12}Ga{sub 0.88}As quantum wires at the bottom of V-grooves is used to determine a set of optimized kinetic parameters. Based on these parameters, we have modeled the growth of In{sub 0.25}Ga{sub 0.75}As nanostructures formed in pyramidal site-controlled quantum-dot systems, successfully producing a qualitative explanation for the temperature-dependence of their optical properties, which have been reported in previous studies. Finally, we present scanning electron and cross-sectional atomic force microscopy images which show previously unreported facetting at the bottom of the pyramidal recesses that allow quantum dot formation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Synthesis of graphene-like transparent conductive films on dielectric substrates using a modified filtered vacuum arc system

Here, we present a reliable process to deposit transparent conductive films on silicon oxide, quartz, and sapphire using a solid carbon source. This layer consists of partially ordered graphene flakes with a lateral dimension of about 5 nm. The process does not require any catalytic metal and exploits a high current arc evaporation (Φ-HCA) to homogeneously deposit a layer of carbon on heated substrates. A gas atmosphere consisting of Argon or Argon/Hydrogen blend acting as a buffer influences the morphology of the growing film. scanning tunneling microscopy, transmission electron microscopy, and Raman spectra were used for a thorough characterization of the samples in order to optimize the growth parameters. The best carbon layers have a surface resistance of 5.7 × 10{sup 3} Ω{sub ◻} whereas the optical transparency of the coatings is 88% with an excellent homogeneity over areas of several cm{sup 2}. Such results are compatible with most semiconductor fabrication processes and make this method very promising for various industrial applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

High quality, giant crystalline-Ge stripes on insulating substrate by rapid-thermal-annealing of Sn-doped amorphous-Ge in solid-liquid coexisting region

Formation of large-grain (≥30 μm) Ge crystals on insulating substrates is strongly desired to achieve high-speed thin-film transistors. For this purpose, we propose the methods of Sn-doping into amorphous-Ge combined with rapid-thermal-annealing (RTA) in the solid-liquid coexisting temperature region for the Ge-Sn alloy system. The densities of micro-crystal-nuclei formed in this temperature region become low by tuning the RTA temperature close to the liquidus curve, which enhances the lateral growth of GeSn. Thanks to the very small segregation coefficient of Sn, almost all Sn atoms segregate toward edges of the stripes during growth. Agglomeration of GeSn degrades the surface morphologies; however, it is significantly improved by lowering the initial Sn concentration. As a result, pure Ge with large crystal grains (∼40 μm) with smooth surface are obtained by optimizing the initial Sn concentration as low as 3 ∼ 5%. Lateral growth lengths are further increased through decreasing the number of nuclei in stripes by narrowing stripe width. In this way, high-crystallinity giant Ge crystals (∼200 μm) are obtained for the stripe width of 3 μm. This “Si-seed free” technique for formation of large-grain pure Ge crystals is very useful to realize high-performance thin-film devices on insulator.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Strain dependence on polarization properties of AlGaN and AlGaN-based ultraviolet lasers grown on AlN substrates

Since the band ordering in AlGaN has a profound effect on the performance of UVC light emitting diodes (LEDs) and even determines the feasibility of surface emitting lasers, the polarization properties of emitted light from c-oriented AlGaN and AlGaN-based laser structures were studied over the whole composition range, as well as various strain states, quantum confinements, and carrier densities. A quantitative relationship between the theoretical valence band separation, determined using k•p theory, and the experimentally measured degree of polarization is presented. Next to composition, strain was found to have the largest influence on the degree of polarization while all other factors were practically insignificant. The lowest crossover point from the transverse electric to transverse magnetic polarized emission of 245 nm was found for structures pseudomorphically grown on AlN substrates. This finding has significant implications toward the efficiency and feasibility of surface emitting devices below this wavelength.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Growth and spin-wave properties of thin Y{sub 3}Fe{sub 5}O{sub 12} films on Si substrates

We describe synthesis of submicron Y{sub 3}Fe{sub 5}O{sub 12} (YIG) films sputtered on Si substrates and present results of the investigation of ferromagnetic resonance (FMR) and spin waves in YIG/SiO{sub 2}/Si structures. It is found that decrease of the annealing time leads to essential reduction of the FMR linewidth ΔH and, consequently, to reduction of relaxation losses of spin waves. Spin-wave propagation in in-plane magnetized YIG/SiO{sub 2}/Si structures is studied. We observe the asymmetry of amplitude-frequency characteristics of the Damon-Eshbach spin waves caused by different localizations of spin waves at the free YIG surface and at the YIG/SiO{sub 2} interface. Growth of the generating microwave power leads to spin-wave instability and changes amplitude-frequency characteristics of spin waves.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Longer than 1.9 μm photoluminescence emission from InAs quantum structure on GaAs (001) substrate

We report on photoluminescence (PL) emission with long wavelength for quantum structure by the sub-monolayer (SML) growth technique on GaAs (001) substrate. It is found that the PL emission wavelength can be controlled by controlling the SML InAs deposition amount. At 12 K, the PL peak position of the grown samples changes from about 1.66 to 1.78 μm. At 120 K, the PL emission of a sample reaches 1.91 μm. The physical mechanism responsible for the measured long wavelength PL emission may be related to strong In segregation and intermixing effects occurred in the structure grown by SML growth technique.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Stress-directed compositional patterning of SiGe substrates for lateral quantum barrier manipulation

While vertical stacking of quantum well and dot structures is well established in heteroepitaxial semiconductor materials, manipulation of quantum barriers in the lateral directions poses a significant engineering challenge. Here, we demonstrate lateral quantum barrier manipulation in a crystalline SiGe alloy using structured mechanical fields to drive compositional redistribution. To apply stress, we make use of a nano-indenter array that is pressed against a Si{sub 0.8}Ge{sub 0.2} wafer in a custom-made mechanical press. The entire assembly is then annealed at high temperatures, during which the larger Ge atoms are selectively driven away from areas of compressive stress. Compositional analysis of the SiGe substrates reveals that this approach leads to a transfer of the indenter array pattern to the near-surface elemental composition, resulting in near 100% Si regions underneath each indenter that are separated from each other by the surrounding Si{sub 0.8}Ge{sub 0.2} bulk. The “stress transfer” process is studied in detail using multiscale computer simulations that demonstrate its robustness across a wide range of applied stresses and annealing temperatures. While the “Si nanodot” structures formed here are not intrinsically useful as quantum structures, it is anticipated that the stress transfer process may be modified by judicious control of the SiGe film thickness and indenter array pattern to form more technologically useful structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Effect of the out-of-plane stress on the properties of epitaxial SrTiO{sub 3} films with nano-pillar array on Si-substrate

A nonlinear thermodynamic formalism has been proposed to calculate the physical properties of the epitaxial SrTiO{sub 3} films containing vertical nano-pillar array on Si-substrate. The out-of-plane stress induced by the mismatch between film and nano-pillars provides an effective way to tune the physical properties of ferroelectric SrTiO{sub 3} films. Tensile out-of-plane stress raises the phase transition temperature and increases the out-of-plane polarization, but decreases the out-of-plane dielectric constant below Curie temperature, pyroelectric coefficient, and piezoelectric coefficient. These results showed that by properly controlling the out-of-plane stress, the out-of-plane stress induced paraelectric-ferroelectric phase transformation will appear near room temperature. Excellent dielectric, pyroelectric, piezoelectric properties of these SrTiO{sub 3} films similar to PZT and other lead-based ferroelectrics can be expected.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Microstructural dependency of optical properties of m-plane InGaN multiple quantum wells grown on 2° misoriented bulk GaN substrates

A non-polar m-plane structure consisting of five InGaN/GaN quantum wells (QWs) was grown on ammonothermal bulk GaN by metal-organic vapor phase epitaxy. Surface step bunches propagating through the QW stack were found to accommodate the 2° substrate miscut towards the -c direction. Both large steps with heights of a few tens of nanometres and small steps between one and a few atomic layers in height are observed, the former of which exhibit cathodoluminescence at longer wavelengths than the adjacent m-plane terraces. This is attributed to the formation of semi-polar facets at the steps on which the QWs are shown to be thicker and have higher Indium contents than those in the adjacent m-plane regions. Discrete basal-plane stacking faults (BSFs) were occasionally initiated from the QWs on the main m-plane terraces, but groups of BSFs were frequently observed to initiate from those on the large steps, probably related to the increased strain associated with the locally higher indium content and thickness.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Influence of substrate temperature and post annealing of CuGaO{sub 2} thin films on optical and structural properties

A transparent p-type thin film CuGaO{sub 2} was deposited by using RF sputtering deposition method on plastic (PET) and glass substrate. The characteristics of the film is investigated. The thin film was deposited at temperature range from 50-250°C and the pressure inside the chamber is 1.0×10{sup −2} Torr and Argon gas was used as a working gas. The RF power is set to 100 W. The thickness of thin film is 300nm. In this experiment the transparency of the thin film is more than 70% for the visible light region. The band gap obtain is between 3.3 to 3.5 eV. The details of the results will be discussed in the conference.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synthesis of Cu doped ZnS nanostructures on flexible substrate using low cost chemical method

Flexible electronics is one of the emerging area of this era. In this paper we have reported synthesis of Cu doped Zinc sulphide nanostructures on filter paper flexible substrates. Zinc chloride and Thio urea were used as a precursor for Zinc and Sulphur. The structures were characterized by XRD, FE-SEM and UV visible spectrometer. All the peaks identified for cubic structure of ZnS. Appearance of small Cu peaks indicates incorporation of Cu into ZnS lattice. Zns nanostructures assembled as nanobelts and nanofibers as shown in FE-SEM micrographs. Compound Structures provide the reasonable electrical conductivity on filter paper. Absorption in UV region makes them suitable for flexible electronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

LiNbO{sub 3}: A photovoltaic substrate for massive parallel manipulation and patterning of nano-objects

The application of evanescent photovoltaic (PV) fields, generated by visible illumination of Fe:LiNbO{sub 3} substrates, for parallel massive trapping and manipulation of micro- and nano-objects is critically reviewed. The technique has been often referred to as photovoltaic or photorefractive tweezers. The main advantage of the new method is that the involved electrophoretic and/or dielectrophoretic forces do not require any electrodes and large scale manipulation of nano-objects can be easily achieved using the patterning capabilities of light. The paper describes the experimental techniques for particle trapping and the main reported experimental results obtained with a variety of micro- and nano-particles (dielectric and conductive) and different illumination configurations (single beam, holographic geometry, and spatial light modulator projection). The report also pays attention to the physical basis of the method, namely, the coupling of the evanescent photorefractive fields to the dielectric response of the nano-particles. The role of a number of physical parameters such as the contrast and spatial periodicities of the illumination pattern or the particle deposition method is discussed. Moreover, the main properties of the obtained particle patterns in relation to potential applications are summarized, and first demonstrations reviewed. Finally, the PV method is discussed in comparison to other patterning strategies, such as those based on the pyroelectric response and the electric fields associated to domain poling of ferroelectric materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Investigation of single-layer/multilayer self-assembled InAs quantum dots on GaAs{sub 1-x}Sb{sub x}/GaAs composite substrates

The structure-performance properties of single-layered and multi-layered InAs/GaAs{sub 1−x}Sb{sub x} quantum dot (QD) system, grown by molecular beam epitaxy on GaAs (001) substrates, have been investigated as a function of Sb concentration. Electron microscopy observations showed no significant crystalline defects for the single-layered InAs QDs (Sb 20%). X-ray diffraction analysis revealed that the increase of Sb concentration from 7.3% to 10.2% for the multi-layered QDs increased the strain relaxation from 0% to ∼23% and the dislocation density of GaAsSb layers went up to 3.6 × 10{sup 9 }cm{sup −2}. The peak energy of QD luminescence was red-shifted with increasing Sb concentration due to reduced strain inside QDs. Moreover, the carrier lifetime of the QDs was highly improved from 1.7 to 36.7 ns due to weak hole confinement as the Sb concentration was increased from 7.3% to 10.2%. These structures should be highly promising as the basis for photovoltaic solar-cell applications. Finally, the increased Sb concentration increased the thermal activation energy of electrons confined in the QDs from 163.7 to 206.8 meV, which was indicative of the improved thermal stability with Sb concentration.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Room temperature analysis of dielectric function of ZnO-based thin film on fused quartz substrate

A set of sample consist of pure ZnO and Cu-doped ZnO film were grown on fused-quartz substrates using pulsed laser deposition (PLD) technique. Here, we report room temperature spectroscopic ellipsometry analysis (covering energy range of 0.5 to 6.3 eV) of pure ZnO film and Cu doped ZnO film at 8 in at. %. The thickness of pure ZnO and Cu-doped ZnO film using in this study is about 350 nm. To extract the dielectric function of ZnO thin film, multilayer modeling is performed which takes into account reflections at each interface through Fresnel coefficients. This method based on Drude-Lorentz models that connect with Kramers-Kronig relations. The best fitting of Ψ (amplitude ratio) and Δ (phase difference) taken by SE measurement are obtained reasonably well by mean the universal fitting of three different photon incident angles. The imaginary part of dielectric function (ε{sub 2}) show the broad peak at around 3.3 eV assigned as combination of optical band energy edge with excitonic states. The exitonic states could not be observed clearly in this stage. The evolution of extracted dielectric function is observable by introducing 8% Cu as indicated by decreasing of excitonic intensity. This result indicates the screening of excitonic state. This study will bring us to have a good undestanding for the role of Cu impurities for ZnO thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optical properties analysis of Ta-doped TiO{sub 2} thin films on LaAlO{sub 3} substrates

We study optical properties of Ta-doped TiO{sub 2} thin film on LaAlO{sub 3} substrate using spectroscopy ellipsometry (SE) analysis at energy range of 0.5 – 6.5 eV. Room temperature SE data for Ψ (amplitude ratio) and Δ (phase difference) between p- and s- polarized light waves are taken with multiple incident angles at several spots on the samples. Here, absorption coefficient has been extracted from SE measurements at photon incident angle of 70° for different Ta concentration (0.01, 0.4, and 5 at. %). Multilayer modelling is performed which takes into account reflections at each interface through Fresnel coefficients to obtain reasonably well the fitting of Ψ and Δ data simultaneously. As the results, we estimate that film thickness increases by increasing Ta concentration accompanied by the formation of a new electronic structure. By increasing Ta impurities, the blueshift of absorption coefficient (α) peaks is observable. This result indicates that TiO{sub 2} thin film becomes optically resistive by introducing Ta doping. Schematic model of interband transition inTiO{sub 2}:Ta will be proposed base on obtained optical properties. This study enables us to predict the role of Ta doping on the electronic and optical band structures of TiO{sub 2} thin film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optical response of columnar TiO{sub 2}:Co on Si substrate

Vertically arranged columnar TiO{sub 2}:Co structure has been deposited on Si (100) substrate using the metal organic chemical vapor deposition (MOCVD) technique. SEM images confirm the columnar structure of the TiO{sub 2}:Co layer on Si (100). Concentration of the Co dopant has been defined by means of the energy dispersive x-ray (EDX) spectra. Co concentrations are varied at 0.18, 0.33, and 0.68 at. % using pure TiO{sub 2} as the reference. X-ray diffraction (XRD) spectra show that the TiO{sub 2}:Co layer have a rutile phase. Further analysis of this structure has been performed by using room temperature spectroscopic ellipsometry (SE) measurements in the energy range of 0.5 - 6.5 eV. This optical response shows the unique feature of significantly high density fringe especially below the band gap energy region (lower than 3.3 eV) in contrast to the conventional TiO{sub 2} thin film structure. This unique feature is interpreted as a dominant contribution of the vertical interfaces at columnar boundaries towards reflected light from the sample. The evolution of Ψ and Δ will be discussed as the function of Co concentration and some optical properties will be extracted after reasonably well data fitting by multilayer modelling. Furthermore, by comparing the SE data from different measurement orientations, it has been shown that TiO{sub 2}:Co tends to be anisotropic. This study enables us to further evaluate structural properties through optical responses given by SE measurement.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Observation of reduced 1/f noise in graphene field effect transistors on boron nitride substrates

We have investigated the low frequency (f) flicker (also called 1/f) noise of single-layer graphene devices on h-BN (placed on SiO{sub 2}/Si) along with those on SiO{sub 2}/Si. We observe that the devices fabricated on h-BN have on average one order of magnitude lower noise amplitude compared with devices fabricated on SiO{sub 2}/Si despite having comparable mobilities at room temperature. We associate this noise reduction to the lower densities of impurities and trap sites in h-BN than in SiO{sub 2}. Furthermore, the gate voltage dependent noise amplitude shows a broad maximum at Dirac point for devices on h-BN, in contrast to the M-shaped behavior showing a minimum at Dirac point for devices on SiO{sub 2}, consistent with the reduced charge inhomogeneity (puddles) for graphene on h-BN. This study demonstrates that the use of h-BN as a substrate or dielectric can be a simple and efficient noise reduction technique valuable for electronic applications of graphene and other nanomaterials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗