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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Microstructure and dielectric properties of piezoelectric magnetron sputtered w-Sc{sub x}Al{sub 1-x}N thin films

Piezoelectric wurtzite Sc{sub x}Al{sub 1-x}N (x = 0, 0.1, 0.2, 0.3) thin films were epitaxially grown by reactive magnetron co-sputtering from elemental Sc and Al targets. Al{sub 2}O{sub 3}(0001) wafers with TiN(111) seed and electrode layers were used as substrates. X-ray diffraction shows that an increase in the Sc content results in the degradation of the crystalline quality. Samples grown at 400 deg. C possess true dielectric behavior with quite low dielectric losses and the leakage current is negligible. For ScAlN samples grown at 800 deg. C, the crystal structure is poor and leakage current is high. Transmission electron microscopy with energy dispersive x-ray spectroscopy mapping shows a mass separation into ScN-rich and AlN-rich domains for x {>=} 0.2 when substrate temperature is increased from 400 to 800 deg. C. The piezoelectric response of epitaxial Sc{sub x}Al{sub 1-x}N films measured by piezoresponse force microscopy and double beam interferometry shows up to 180% increase by the addition of Sc up to x = 0.2 independent of substrate temperature, in good agreement with previous theoretical predictions based on density-functional theory.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Studies of thin films of Ti- Zr -V as non-evaporable getter films prepared by RF sputtering

Non-Evaporable Getter (NEG) films of the Ti-Zr-V prepared on stainless steel substrates by Radio Frequency sputtering. To observe its getter behavior at the lowest activation temperature, the sample is heated continuously at different temperatures (100 Degree-Sign C, 150 Degree-Sign C, 200 Degree-Sign C and 250 Degree-Sign C) for 2 hours. The changes of the surface chemical composition at different temperaturesare analyzed by using XPS and SEM (Scanning Electron Microscopy) techniques. The volume elemental composition of the film has been measured by energy dispersive X-ray spectroscopy (EDX). The in-situ XPS measurements of the activated getter films show the disappearance of the superficial oxide layer through the variation in the oxygen stoichiometry during thermal activation. Results of these studies show that the deposited films of Ti-Zr-V could be used as NEG to produce extreme high vacuum.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Tunable giant magnetic anisotropy in amorphous SmCo thin films

SmCo thin films have been grown by magnetron sputtering at room temperature with a composition of 2-35 at. % Sm. Films with 5 at. % or higher Sm are amorphous and smooth. A giant tunable uniaxial in-plane magnetic anisotropy is induced in the films which peaks in the composition range 11-22 at. % Sm. This cross-over behavior is not due to changes in the atomic moments but rather the local configuration changes. The excellent layer perfection combined with highly tunable magnetic properties make these films important for spintronics applications.

36 MATERIALS SCIENCE↗
DOE OSTI2013

In-situ x-ray diffraction study of the growth of highly strained epitaxial BaTiO{sub 3} thin films

In-situ synchrotron x-ray diffraction was performed during the growth of BaTiO{sub 3} thin films on SrTiO{sub 3} substrates using both off-axis RF magnetron sputtering and pulsed laser deposition techniques. It was found that the films were ferroelectric during the growth process, and the presence or absence of a bottom SrRuO{sub 3} electrode played an important role in the growth of the films. Pulsed laser deposited films on SrRuO{sub 3} displayed an anomalously high tetragonality and unit volume, which may be connected to the previously predicted negative pressure phase of BaTiO{sub 3}.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Conductivity behavior of very thin gold films ruptured by mass transport in photosensitive polymer film

We report on conductivity behavior of very thin gold layer deposited on a photosensitive polymer film. Under irradiation with light interference pattern, the azobenzene containing photosensitive polymer film undergoes deformation at which topography follows a distribution of intensity, resulting in the formation of a surface relief grating. This process is accompanied by a change in the shape of the polymer surface from flat to sinusoidal together with a corresponding increase in surface area. The gold layer placed above deforms along with the polymer and ruptures at a strain of 4%. The rupturing is spatially well defined, occurring at the topographic maxima and minima resulting in periodic cracks across the whole irradiated area. We have shown that this periodic micro-rupturing of a thin metal film has no significant impact on the electrical conductivity of the films. We suggest a model to explain this phenomenon and support this by additional experiments where the conductivity is measured in a process when a single nanoscopic scratch is formed with an AFM tip. Our results indicate that in flexible electronic materials consisting of a polymer support and an integrated metal circuit, nano- and micro cracks do not alter significantly the behavior of the conductivity unless the metal is disrupted completely.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of capping material on interfacial ferromagnetism in FeRh thin films

The role of the capping material in stabilizing a thin ferromagnetic layer at the interface between a FeRh film and cap in the nominally antiferromagnetic phase at room temperature was studied by x-ray magnetic circular dichroism in photoemission electron microscopy and polarized neutron reflectivity. These techniques were used to determine the presence or absence of interfacial ferromagnetism (FM) in films capped with different oxides and metals. Chemically stable oxide caps do not generate any interfacial FM while the effect of metallic caps depends on the element, showing that interfacial FM is due to metallic interdiffusion and the formation of a ternary alloy with a modified antiferromagnetic to ferromagnetic transition temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A study on the evolution of dielectric function of ZnO thin films with decreasing film thickness

Dielectric function, band gap, and exciton binding energies of ultrathin ZnO films as a function of film thickness have been obtained with spectroscopic ellipsometry. As the film thickness decreases, both real (ε{sub 1}) and imaginary (ε{sub 2}) parts of the dielectric function decrease significantly, and ε{sub 2} shows a blue shift. The film thickness dependence of the dielectric function is shown related to the changes in the interband absorption, discrete-exciton absorption, and continuum-exciton absorption, which can be attributed to the quantum confinement effect on both the band gap and exciton binding energies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Polarity-inverted ScAlN film growth by ion beam irradiation and application to overtone acoustic wave (000-1)/(0001) film resonators

Polarity inversion in wurtzite film is generally achieved by the epitaxial growth on a specific under-layer. We demonstrate polarity inversion of c-axis oriented ScAlN films by substrate ion beam irradiation without using buffer layer. Substrate ion beam irradiation was induced by either sputtering a small amount of oxide (as a negative ion source) onto the cathode or by applying a RF bias to the substrate. Polarity of the films was determined by a press test and nonlinear dielectric measurement. Second overtone thickness extensional mode acoustic resonance and suppression of fundamental mode resonance, indicating complete polarity inversion, were clearly observed in bilayer highly oriented (000-1)/(0001) ScAlN film.

36 MATERIALS SCIENCE↗
DOE OSTI2014

The importance of spinning speed in fabrication of spin-coated organic thin film transistors: Film morphology and field effect mobility

We have investigated the film morphology and the field effect mobility of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) thin films which were formed by spin coating on the SiO{sub 2} substrate with solution-processed graphene electrodes. The domain size and the density of aggregates in the C8-BTBT film showed the same dependence on the spinning speed. These competitive two factors (domain size and density of aggregates) give an optimum spinning speed, at which the field effect mobility of C8-BTBT transistor showed a maximum (2.6 cm{sup 2}/V s). This result indicates the importance of spinning speed in the fabrication of solution processed organic thin film transistors by spin coating.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Boron Doped diamond films as electron donors in photovoltaics: An X-ray absorption and hard X-ray photoemission study

Highly boron-doped diamond films are investigated for their potential as transparent electron donors in solar cells. Specifically, the valence band offset between a diamond film (as electron donor) and Cu(In,Ga)Se₂ (CIGS) as light absorber is determined by a combination of soft X-ray absorption spectroscopy and hard X-ray photoelectron spectroscopy, which is more depth-penetrating than standard soft X-ray photoelectron spectroscopy. In addition, a theoretical analysis of the valence band is performed, based on GW quasiparticle band calculations. The valence band offset is found to be small: VBO=VBM{sub CIGS} – VBM{sub diamond}=0.3 eV±0.1 eV at the CIGS/Diamond interface and 0.0 eV±0.1 eV from CIGS to bulk diamond. These results provide a promising starting point for optimizing the band offset by choosing absorber materials with a slightly lower valence band maximum.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Controllable film densification and interface flatness for high-performance amorphous indium oxide based thin film transistors

To avoid the problem of air sensitive and wet-etched Zn and/or Ga contained amorphous oxide transistors, we propose an alternative amorphous semiconductor of indium silicon tungsten oxide as the channel material for thin film transistors. In this study, we employ the material to reveal the relation between the active thin film and the transistor performance with aid of x-ray reflectivity study. By adjusting the pre-annealing temperature, we find that the film densification and interface flatness between the film and gate insulator are crucial for achieving controllable high-performance transistors. The material and findings in the study are believed helpful for realizing controllable high-performance stable transistors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetoresistance of Au films

Classical magnetoresistance (MR) in nonmagnetic metals are conventionally understood in terms of the Kohler rule, with violation usually viewed as anomalous electron transport, in particular, as evidence of non-Fermi liquid behavior. Measurement of the MR of Au films as a function of temperature and film thickness reveals a strong dependence on grain size distribution and clear violation of the Kohler rule. Using a model of random resistor network, we show that this result can be explained if the MR arises entirely from inhomogeneity due to grain boundary scattering and thermal activation of grain boundary atoms. Consequently, the Kohler rule should not be used to distinguish normal and anomalous electron transport in solids.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Sub-nA spatially resolved conductivity profiling of surface and interface defects in ceria films

Spatial variability of conductivity in ceria is explored using scanning probe microscopy with galvanostatic control. Ionically blocking electrodes are used to probe the conductivity under opposite polarities to reveal possible differences in the defect structure across a thin film of CeO2. Data suggest the existence of a large spatial inhomogeneity that could give rise to constant phase elements during standard electrochemical characterization, potentially affecting the overall conductivity of films on the macroscale. The approach discussed here can also be utilized for other mixed ionic electronic conductor systems including memristors and electroresistors, as well as physical systems such as ferroelectric tunneling barriers

36 MATERIALS SCIENCE↗
DOE OSTI2015

First principles-based multiparadigm, multiscale strategy for simulating complex materials processes with applications to amorphous SiC films

Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Spontaneous formation of suboxidic coordination around Co in ferromagnetic rutile Ti 0.95 Co 0.5 O 2 film

To evaluate local atomic structures around Co in high temperature diluted ferromagnetic semiconductor Co-doped TiO 2 , x-ray fluorescence holography and x-ray absorption fine structure experiments were carried out on rutile paramagnetic Ti 0.99 Co 0.01 O 2 and ferromagnetic Ti 0.95 Co 0.05 O 2 films. The Co atoms in the Ti 0.99 Co 0.01 O 2 simply substituted for Ti sites in the rutile structure, whereas a suboxidic arrangement of CoO 2 Ti 4 formed around Co in the Ti 0.95 Co 0.05 O 2 films. A theoretical investigation based on a series of first-principles calculations indicated the stability of the aggregated suboxidic clusters in the rutile TiO 2 , supporting our hypothesis for the formation of suboxidic coordination in the highly Co-doped sample. As a result, the suboxidic coordination may be the source of strong exchange interaction, resulting in the high Curie temperature in Co-doped TiO 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Direct observation of oxygen-vacancy-enhanced polarization in a SrTiO{sub 3}-buffered ferroelectric BaTiO{sub 3} film on GaAs

The integration of functional oxide thin-films on compound semiconductors can lead to a class of reconfigurable spin-based optoelectronic devices if defect-free, fully reversible active layers are stabilized. However, previous first-principles calculations predicted that SrTiO{sub 3} thin films grown on Si exhibit pinned ferroelectric behavior that is not switchable, due to the presence of interfacial vacancies. Meanwhile, piezoresponse force microscopy measurements have demonstrated ferroelectricity in BaTiO{sub 3} grown on semiconductor substrates. The presence of interfacial oxygen vacancies in such complex-oxide/semiconductor systems remains unexplored, and their effect on ferroelectricity is controversial. Here, we use a combination of aberration-corrected scanning transmission electron microscopy and first-principles density functional theory modeling to examine the role of interfacial oxygen vacancies on the ferroelectric polarization of a BaTiO{sub 3} thin film grown on GaAs. We demonstrate that interfacial oxygen vacancies enhance the polar discontinuity (and thus the single domain, out-of-plane polarization pinning in BaTiO{sub 3}), and propose that the presence of surface charge screening allows the formation of switchable domains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗