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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 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Real-space Berry phases: Skyrmion soccer (invited)

Berry phases occur when a system adiabatically evolves along a closed curve in parameter space. This tutorial-like article focuses on Berry phases accumulated in real space. In particular, we consider the situation where an electron traverses a smooth magnetic structure, while its magnetic moment adjusts to the local magnetization direction. Mapping the adiabatic physics to an effective problem in terms of emergent fields reveals that certain magnetic textures, skyrmions, are tailormade to study these Berry phase effects.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

On the location and stability of charge in SiO{sub 2}/SiN{sub x} dielectric double layers used for silicon surface passivation

Dielectric double layers of thermal silicon dioxide–chemical vapour deposition (CVD) silicon nitride are found to produce excellent passivation of silicon surfaces by combining a chemical reduction of surface defect states, with a field effect reduction of carriers at the surface due to charge in the dielectrics. The charge present in such double-layers has previously been attributed to be characteristic of the interface between the two. However, experimental evidence shows this is indirect and inconclusive. This manuscript reports direct measurements that show the charge lies within 10 nm of the interface between passivating double layers of thermal silicon dioxide–plasma CVD silicon nitride. In addition, the passivation efficiency of oxide-nitride layers, deposited using optimised conditions, was found to be largely unaffected by extra charge subsequently added to the film. The passivation efficiency of textured surfaces or those produced using non-optimised deposition conditions is found to be highly dependent on the field effect component provided by extra deposited charge. Using such extra field effect component, surface recombination velocities <2 cm/s have been obtained on single oxide and oxide/nitride double layers. The extra deposited charge was found to have good long term stability when the dielectric films are submitted to a chemical treatment. By contrast, poor stability of the deposited charge was observed when subjected to ultraviolet radiation. These results point to the importance of the interface between dielectrics when considering how to optimise the charge present in passivating dielectric films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of combinatorial coevaporated thin film Cu{sub 2}ZnSnS{sub 4}. I. Temperature effect, crystalline phases, morphology, and photoluminescence

Cu{sub 2}ZnSnS{sub 4} is a promising low-cost, nontoxic, earth-abundant absorber material for thin-film solar cell applications. In this study, combinatorial coevaporation was used to synthesize individual thin-film samples spanning a wide range of compositions at low (325 °C) and high (475 °C) temperatures. Film composition, grain morphology, crystalline-phase and photo-excitation information have been characterized by x-ray fluorescence, scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and photoluminescence imaging and mapping. Highly textured columnar grain morphology is observed for film compositions along the ZnS-Cu{sub 2}ZnSnS{sub 4}-Cu{sub 2}SnS{sub 3} tie line in the quasi-ternary Cu{sub 2}S-ZnS-SnS{sub 2} phase system, and this effect is attributed to structural similarity between the Cu{sub 2}ZnSnS{sub 4}, Cu{sub 2}SnS{sub 3}, and ZnS crystalline phases. At 475 °C growth temperature, Sn-S phases cannot condense because of their high vapor pressures. As a result, regions that received excess Sn flux during growth produced compositions falling along the ZnS-Cu{sub 2}ZnSnS{sub 4}-Cu{sub 2}SnS{sub 3} tie line. Room-temperature photoluminescence imaging reveals a strong correlation for these samples between film composition and photoluminescence intensity, where film regions with Cu/Sn ratios greater than ∼2 show strong photoluminescence intensity, in comparison with much weaker photoluminescence in regions that received excess Sn flux during growth or subsequent processing. The observed photoluminescence quenching in regions that received excess Sn flux is attributed to the effects of Sn-related native point defects in Cu{sub 2}ZnSnS{sub 4} on non-radiative recombination processes. Implications for processing and performance of Cu{sub 2}ZnSnS{sub 4} solar cells are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Room temperature ferromagnetism in Co defused CdTe nanocrystalline thin films

Nanocrystalline Co defused CdTe thin films were prepared using electron beam evaporation technique by depositing CdTe/Co/CdTe stacked layers with different Co thickness onto glass substrate at 373 K followed by annealing at 573K for 2 hrs. Structural, morphological and magnetic properties of of all the Co defused CdTe thin films has been investigated. XRD pattern of all the films exhibited zinc blende structure with <111> preferential orientation without changing the crystal structure of the films. The grain size of the films increased from 31.5 nm to 48.1 nm with the increase of Co layer thickness from 25nm to 100nm. The morphological studies showed that uniform texture of the films and the presence of Co was confirmed by EDAX. Room temperature magnetization curves indicated an improved ferromagnetic behavior in the films with increase of the Co thickness.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dispersion properties and low infrared optical losses in epitaxial AlN on sapphire substrate in the visible and infrared range

Optical waveguiding properties of a thick wurtzite aluminum nitride highly [002]-textured hetero-epitaxial film on (001) basal plane of sapphire substrate are studied. The physical properties of the film are determined by X-ray diffraction, atomic force microscopy, microRaman, and photocurrent spectroscopy. The refractive index and the thermo-optic coefficients are determined by m-lines spectroscopy using the classical prism coupling technique. The optical losses of this planar waveguide are also measured in the spectral range of 450–1553 nm. The lower value of optical losses is equal to 0.7 dB/cm at 1553 nm. The optical losses due to the surface scattering are simulated showing that the contribution is the most significant at near infrared wavelength range, whereas the optical losses are due to volume scattering and material absorption in the visible range. The good physical properties and the low optical losses obtained from this planar waveguide are encouraging to achieve a wide bandgap optical guiding platform from these aluminum nitride thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Porous silicon formation during Au-catalyzed etching

The formation of “black” nano-textured Si during the Au-catalyzed wet-chemical etch process was investigated with respect to photovoltaic applications. Cross-sectional scanning electron microscopy (SEM) images recorded at different stages of the etch process exhibit an evolution of a two-layer structure, consisting of cone-like Si hillocks covered with a nano-porous Si (np-Si) layer. Optical measurements confirm the presence of a np-Si phase which appears after the first ∼10 s of the etch process and continuously increases with the etch time. Furthermore, the etch process was investigated on Si substrates with different doping levels (∼0.01–100 Ω cm). SEM images show a transition from the two-layer morphology to a structure consisting entirely of np-Si for higher doping levels (<0.1 Ω cm). The experimental results are discussed on the basis of the model of a local electrochemical etch process. A better understanding of the metal-catalyzed etch process facilitates the fabrication of “black” Si on various Si substrates, which is of significant interest for photovoltaic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Synthesis and characterization of 10 nm thick piezoelectric AlN films with high c-axis orientation for miniaturized nanoelectromechanical devices

The scaling of piezoelectric nanoelectromechanical systems (NEMS) is challenged by the synthesis of ultrathin and high quality piezoelectric films on very thin electrodes. We report the synthesis and characterization of the thinnest piezoelectric aluminum nitride (AlN) films (10 nm) ever deposited on ultrathin platinum layers (2–5 nm) using reactive sputtering. X-ray diffraction, high-resolution transmission electron microscopy, and fast Fourier transform analyses confirmed the proper crystal orientation, fine columnar texture, and the continuous lattice structure within individual grains in the deposited AlN nanometer thick films. The average extracted d{sub 31} piezoelectric coefficient for the synthesized films is −1.73 pC/N, which is comparable to the reported values for micron thick and highly c-axis oriented AlN films. The 10 nm AlN films were employed to demonstrate two different types of optimized piezoelectric nanoactuators. The unimorph actuators exhibit vertical displacements as large as 1.1 μm at 0.7 V for 25 μm long and 30 nm thick beams. These results have a great potential to realize miniaturized NEMS relays with extremely low voltage, high frequency resonators, and ultrasensitive sensors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Experimental and computational studies on stacking faults in zinc titanate

Zinc titanate (ZnTiO{sub 3}) thin films grown by atomic layer deposition with ilmenite structure have recently been identified as an excellent solid lubricant, where low interfacial shear and friction are achieved due to intrafilm shear velocity accommodation in sliding contacts. In this Letter, high resolution transmission electron microscopy with electron diffraction revealed that extensive stacking faults are present on ZnTiO{sub 3} textured (104) planes. These growth stacking faults serve as a pathway for dislocations to glide parallel to the sliding direction and hence achieve low interfacial shear/friction. Generalized stacking fault energy plots also known as γ-surfaces were computed for the (104) surface of ZnTiO{sub 3} using energy minimization method with classical effective partial charge potential and verified by using density functional theory first principles calculations for stacking fault energies along certain directions. These two are in qualitative agreement but classical simulations generally overestimate the energies. In addition, the lowest energy path was determined to be along the [451{sup ¯}] direction and the most favorable glide system is (104) 〈451{sup ¯}〉 that is responsible for the experimentally observed sliding-induced ductility.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of deposition temperature on electron-beam evaporated polycrystalline silicon thin-film and crystallized by diode laser

The effects of the deposition temperature on the microstructure, crystallographic orientation, and electrical properties of a 10-μm thick evaporated Si thin-film deposited on glass and crystallized using a diode laser, are investigated. The crystallization of the Si thin-film is initiated at a deposition temperature between 450 and 550 °C, and the predominant (110) orientation in the normal direction is found. Pole figure maps confirm that all films have a fiber texture and that it becomes stronger with increasing deposition temperature. Diode laser crystallization is performed, resulting in the formation of lateral grains along the laser scan direction. The laser power required to form lateral grains is higher in case of films deposited below 450 °C for all scan speeds. Pole figure maps show 75% occupancies of the (110) orientation in the normal direction when the laser crystallized film is deposited above 550 °C. A higher density of grain boundaries is obtained when the laser crystallized film is deposited below 450 °C, which limits the solar cell performance by n = 2 recombination, and a performance degradation is expected due to severe shunting.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

(001) Oriented piezoelectric films prepared by chemical solution deposition on Ni foils

Flexible metal foil substrates are useful in some microelectromechanical systems applications including wearable piezoelectric sensors or energy harvesters based on Pb(Zr,Ti)O₃ (PZT) thin films. Full utilization of the potential of piezoelectrics on metal foils requires control of the film crystallographic texture. In this study, (001) oriented PZT thin films were grown by chemical solution deposition (CSD) on Ni foil and Si substrates. Ni foils were passivated using HfO₂ grown by atomic layer deposition in order to suppress substrate oxidation during subsequent thermal treatment. To obtain the desired orientation of PZT film, strongly (100) oriented LaNiO₃ films were integrated by CSD on the HfO₂ coated substrates. A high level of (001) LaNiO₃ and PZT film orientation were confirmed by X-ray diffraction patterns. Before poling, the low field dielectric permittivity and loss tangents of (001) oriented PZT films on Ni are near 780 and 0.04 at 1 kHz; the permittivity drops significantly on poling due to in-plane to out-of-plane domain switching. (001) oriented PZT film on Ni displayed a well-saturated hysteresis loop with a large remanent polarization ~36 μC/cm², while (100) oriented PZT on Si showed slanted P-E hysteresis loops with much lower remanent polarizations. The |e{sub 31,f}| piezoelectric coefficient was around 10.6 C/m² for hot-poled (001) oriented PZT film on Ni.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Bifacial solar cell with SnS absorber by vapor transport deposition

The SnS absorber layer in solar cell devices was produced by vapor transport deposition (VTD), which is a low-cost manufacturing method for solar modules. The performance of solar cells consisting of Si/Mo/SnS/ZnO/indium tin oxide (ITO) was limited by the SnS layer's surface texture and field-dependent carrier collection. For improved performance, a fluorine doped tin oxide (FTO) substrate was used in place of the Mo to smooth the topography of the VTD SnS and to make bifacial solar cells, which are potentially useful for multijunction applications. A bifacial SnS solar cell consisting of glass/FTO/SnS/CdS/ZnO/ITO demonstrated front- and back-side power conversion efficiencies of 1.2% and 0.2%, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Physical properties of Al{sub x}In{sub 1–x}N thin film alloys sputtered at low temperature

In this paper, we report on the structural, optical, and electrical properties of a wide compositional range of Al{sub x}In{sub 1–x}N thin layers deposited on glass and polyethylene terephthalate substrates. Al{sub x}In{sub 1–x}N layers of controlled composition were obtained by a simple reactive magnetron co-sputtering protocol, using a single aluminium target with indium insets, by varying the Al/In target surface area ratio, and the composition of the deposition atmosphere. The relevant physical properties were investigated and discussed. It is shown that the texture of the thin films is dependent on the cation ratio, while the bowing parameters of lattice constants and band gap values are larger than those of epitaxial layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Plastic strain arrangement in copper single crystals in sliding

Deformation of tribologically loaded contact zone is one of the wear mechanisms in spite of the fact that no mass loss may occur during this process. Generation of optimal crystallographic orientations of the grains in a polycrystalline materials (texturing) may cause hardening and reducing the deformation wear. To reveal the orientation dependence of an individual gain and simplify the task we use copper single crystals with the orientations of the compression axis along [111] and [110]. The plastic deformation was investigated by means of optical, scanning electron microscopy and EBSD techniques. It was established that at least four different zones were generated in the course of sliding test, such as non-deformed base metal, plastic deformation layer sliding, crystalline lattice reorientation layer and subsurface grain structure layer. The maximum plastic strain penetration depth was observed on [110]-single crystals. The minimum stability of [111]-crystals with respect to rotation deformation mode as well as activation of shear in the sliding contact plane provide for rotation deformation localization below the worn surface. The high-rate accumulation of misorientations and less strain penetration depth was observed on [111]-crystals as compared to those of [110]-oriented ones.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Multilevel approach to the experimental study of plastic deformation of HCP-zirconium alloys

A multilevel approach was developed to the experimental study of plastic deformation in anisotropic polycrystalline materials, using by way of an example HCP-Zr alloys. This allowed establishing the relationship and hierarchy of processes occurring at macro-, meso- and microscale levels in conditions of significant heterogeneity of plastic flow and of texture formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Does surface roughness amplify wetting?

Any solid surface is intrinsically rough on the microscopic scale. In this paper, we study the effect of this roughness on the wetting properties of hydrophilic substrates. Macroscopic arguments, such as those leading to the well-known Wenzel's law, predict that surface roughness should amplify the wetting properties of such adsorbents. We use a fundamental measure density functional theory to demonstrate the opposite effect from roughness for microscopically corrugated surfaces, i.e., wetting is hindered. Based on three independent analyses we show that microscopic surface corrugation increases the wetting temperature or even makes the surface hydrophobic. Since for macroscopically corrugated surfaces the solid texture does indeed amplify wetting there must exist a crossover between two length-scale regimes that are distinguished by opposite response on surface roughening. This demonstrates how deceptive can be efforts to extend the thermodynamical laws beyond their macroscopic territory.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Minimal model for spoof acoustoelastic surface states

Similar to textured perfect electric conductors for electromagnetic waves sustaining artificial or spoof surface plasmons we present an equivalent phenomena for the case of sound. Aided by a minimal model that is able to capture the complex wave interaction of elastic cavity modes and airborne sound radiation in perfect rigid panels, we construct designer acoustoelastic surface waves that are entirely controlled by the geometrical environment. Comparisons to results obtained by full-wave simulations confirm the feasibility of the model and we demonstrate illustrative examples such as resonant transmissions and waveguiding to show a few examples of many where spoof elastic surface waves are useful.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Structural characterization of impurified zinc oxide thin films

Europium doped zinc oxide (Eu:ZnO) thin films have been obtained by pulsed laser deposition (PLD). 002 textured thin films were achieved on glass and silicon substrates, while hetero-epilayers and homo-epilayers have been attained on single crystal SrTiO{sub 3} and ZnO, respectively. X-ray Diffraction (XRD) was employed to characterize the Eu:ZnO thin films. Extended XRD studies confirmed the different thin film structural properties as function of chosen substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Structural study of growth, orientation and defects characteristics in the functional microelectromechanical system material aluminium nitride

The real structure and morphology of piezoelectric aluminum nitride (AlN) thin films as essential components of magnetoelectric sensors are investigated via advanced transmission electron microscopy methods. State of the art electron diffraction techniques, including precession electron diffraction and automated crystal orientation mapping (ACOM), indicate a columnar growth of the AlN grains optimized for piezoelectric application with a (0 0 0 1) texture. Comparing ACOM with piezoresponse force microscopy measurements, a visual correlation of the structure and the piezoelectric properties is enabled. With a quantitative analysis of the ACOM measurements, a statistical evaluation of grain rotations is performed, indicating the presence of coincidence site lattices with Σ7, Σ13a, Σ13b, Σ25. Using a geometric phase analysis on high resolution micrographs, the occurrence of strain is detected almost exclusively at the grain boundaries. Moreover, high resolution imaging was applied for solving the atomic structure at stacking mismatch boundaries with a displacement vector of 1/2 〈1 0 -1 1〉. All real structural features can be interpreted via simulations based on crystallographic computing in terms of a supercell approach.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗