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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 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Defect level distributions and atomic relaxations induced by charge trapping in amorphous silica

We compute the distribution of electronic levels of native defects in amorphous silica from total energy differences of charge-state density functional theory calculations over an ensemble of atomic structures. The predicted distributions reproduce results from trap spectroscopy by charge injection experiments, validating the calculations. Furthermore, our study characterizes the experimentally inaccessible contributions of individual defect types to the overall distribution. Computed electron and hole trapping levels provide insight into the positive charge buildup in bulk silica observed in negative-bias-temperature-instability, an important degradation mechanism of metal-oxide-semiconductor devices.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2012

Crystalline to amorphous phase transition of tin oxide nanocrystals induced by SHI at low temperature

Tin oxide (SnO{sub 2}) thin films were deposited using pulsed laser deposition (PLD) technique on Si substrates. The as-deposited films were irradiated using 100 MeV Ag ions at different fluences ranging from 3x10{sup 13} to 3x10{sup 14} ions/cm{sup 2} at an incidence angle of 75 deg. with respect to surface normal at liquid nitrogen (LN2) temperature. The as-deposited and irradiated films have been characterized using X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques to study the modifications in structural and surface morphological properties. Nanocrystalline film become completely amorphous and nanograins of tin oxide disappeared from the surface as indicated by XRD spectra and AFM micrographs respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Nanoporosity induced by ion implantation in deposited amorphous Ge thin films

The formation of a nano-porous structure in amorphous Ge thin film (sputter-deposited on SiO{sub 2}) during ion irradiation at room temperature with 300 keV Ge{sup +} has been observed. The porous film showed a sponge-like structure substantially different from the columnar structure reported for ion implanted bulk Ge. The voids size and structure resulted to be strongly affected by the material preparation, while the volume expansion turned out to be determined only by the nuclear deposition energy. In SiGe alloys, the swelling occurs only if the Ge concentration is above 90%. These findings rely on peculiar characteristics related to the mechanism of voids nucleation and growth, but they are crucial for future applications of active nanostructured layers such as low cost chemical and biochemical sensing devices or electrodes in batteries.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Dark current in multilayer stabilized amorphous selenium based photoconductive x-ray detectors

We report on experimental results which show that the dark current in n-i-p structured, amorphous selenium films is independent of i-layer thickness in samples with consistently thick blocking layers. We have observed, however, a strong dependence on the n-layer thickness and positive contact metal chosen. These results indicate that the dominant source of the dark current is carrier injection from the contacts and any contribution from carriers thermally generated in the bulk of the photoconductive layer is negligible. This conclusion is supported by a description of the dark current transients at different applied fields by a model which assumes only carrier emission over a Schottky barrier. This model also predicts that while hole injection is initially dominant, some time after the application of the bias, electron injection may become the dominant source of dark current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Synergistic etch rates during low-energetic plasma etching of hydrogenated amorphous carbon

The etch mechanisms of hydrogenated amorphous carbon thin films in low-energetic (<2 eV) high flux plasmas are investigated with spectroscopic ellipsometry. The results indicate a synergistic effect for the etch rate between argon ions and atomic hydrogen, even at these extremely low kinetic energies. Ion-assisted chemical sputtering is the primary etch mechanism in both Ar/H{sub 2} and pure H{sub 2} plasmas, although a contribution of swift chemical sputtering to the total etch rate is not excluded. Furthermore, ions determine to a large extent the surface morphology during plasma etching. A high influx of ions enhances the etch rate and limits the surface roughness, whereas a low ion flux promotes graphitization and leads to a large surface roughness (up to 60 nm).

36 MATERIALS SCIENCE↗
DOE OSTI2012

Recrystallization of an amorphized epitaxial phase-change alloy: A phoenix arising from the ashes

Epitaxial Ge{sub 2}Sb{sub 2}Te{sub 5} films grown on Si(111) by molecular beam epitaxy were reversibly switched between crystalline and amorphous states over a large area using femtosecond laser pulses. The structural and spatial homogeneity of the as-grown epitaxial and laser-switched areas on the sample were investigated by synchrotron nanofocus high resolution x-ray diffraction. The investigation clearly demonstrated that the single crystalline metastable cubic phase of Ge{sub 2}Sb{sub 2}Te{sub 5} is restored after switching. No polycrystalline features, not only on the average but even on the nanometer scale of the x-ray beam, were observed.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Highly conductive p-type amorphous oxides from low-temperature solution processing

We report solution-processed, highly conductive (resistivity 1.3-3.8 m{Omega} cm), p-type amorphous A-B-O (A = Bi, Pb; B = Ru, Ir), processable at temperatures (down to 240 Degree-Sign C) that are compatible with plastic substrates. The film surfaces are smooth on the atomic scale. Bi-Ru-O was analyzed in detail. A small optical bandgap (0.2 eV) with a valence band maximum (VBM) below but very close to the Fermi level (binding energy E{sub VBM} = 0.04 eV) explains the high conductivity and suggests that they are degenerated semiconductors. The conductivity changes from three-dimensional to two-dimensional with decreasing temperature across 25 K.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Conditions for a carrier multiplication in amorphous-selenium based photodetector

Amorphous selenium is a promising candidate for high sensitivity photodetector due to its unique carrier multiplication phenomenon. More than 10 carriers can be generated per incident photon, which leads to high photo-conversion efficiency of 1000% that allows real-time imaging in dark ambient. However, application of this effect has been limited to specific devices due to the lack in material characterization. In this article, mechanism of carrier multiplication has been clarified using time-of-flight secondary ion mass spectroscopy and Raman spectroscopy. A prototype photodetector achieved photo conversion efficiency of 4000%, which explains the signal enhancement mechanism in a-Se based photodetector.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2013

The transition from amorphous to crystalline in Al/Zr multilayers

The amorphous-to-crystalline transition in Al(1.0%wtSi)/Zr and Al(Pure)/Zr multilayers grown by direct-current magnetron sputtering system has been characterized over a range of Al layer thicknesses (1.0-5.0 nm) by using a series of complementary measurements including grazing incidence X-ray reflectometry, atomic force microscopy, X-ray diffraction, and high-resolution transmission electron microscopy. The Al layer thickness transition exhibits the Si doped in Al could not only disfavor the crystallization of Al but also influence the changing trends of surface roughness and diffraction peak position of phase Al<111>. An interesting feature of the presence of Si in Al layer is that Si could influence the transition process in Al(1%wtSi) layer, in which the critical thickness (1.6 nm) of Al(Pure) layer in Al(Pure)/Zr shifts to 1.8 nm of Al(1.0%wtSi) layer in Al(1.0%wtSi)/Zr multilayer. We also found that the Zr-on-Al interlayer is wider than the Al-on-Zr interlayer in both systems, and the Al layers do not have specific crystal orientation in the directions vertical to the layer from selected area electron diffraction patterns below the thickness (3.0 nm) of Al layers. Above the thickness (3.0 nm) of Al layers, the Al layers are highly oriented in Al<111>, so that the transformation from asymmetrical to symmetrical interlayers can be observed. Based on the analysis of all measurements, we build up a model with four steps, which could explain the Al layer thickness transition process in terms of a critical thickness for the nucleation of Al(Pure) and Al(1%wtSi) crystallites.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Transient and steady-state dark current mechanisms in amorphous selenium avalanche radiation detectors

A theoretical model for describing bias-dependent transient and steady-state behaviors of dark current in amorphous selenium (a-Se) avalanche detector structures has been developed. The analytical model considers bulk thermal generation current from mid-gap sates, transient carrier depletion, and carrier injection from the electrodes incorporating avalanche multiplication. The proposed physics-based dark current model is compared with the published experimental results on three potential a-Se avalanche detector structures. The steady-state dark current is the minimum for the structures that have effective blocking layers for both holes and electrons. The transient decay time to reach a plateau decreases considerably with increasing electric field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Mechanisms of H{sub 2}O desorption from amorphous solid water by 157-nm irradiation: An experimental and theoretical study

The photodesorption of water molecules from amorphous solid water (ASW) by 157-nm irradiation has been examined using resonance-enhanced multiphoton ionization. The rotational temperature has been determined, by comparison with simulations, to be 425 ± 75 K. The time-of-flight spectrum of H{sub 2}O (v= 0) has been fit with a Maxwell-Boltzmann distribution with a translational temperature of 700 ± 200 K (0.12 ± 0.03 eV). H{sup +} and OH{sup +} fragment ions have been detected with non-resonant multiphoton ionization, indicating vibrationally excited parent water molecules with translational energies of 0.24 ± 0.08 eV. The cross section for water removal from ASW by 7.9-eV photons near 100 K is (6.9 ± 1.8) × 10{sup −20} cm{sup 2} for >10 L H{sub 2}O exposure. Electronic structure computations have also probed the excited states of water and the mechanisms of desorption. Calculated electron attachment and detachment densities show that exciton delocalization leads to a dipole reversal state in the first singlet excited state of a model system of hexagonal water ice. Ab Initio Molecular Dynamics simulations show possible desorption of a photo-excited water molecule from this cluster, though the non-hydrogen bonded OH bond is stretched significantly before desorption. Potential energy curves of this OH stretch in the electronic excited state show a barrier to dissociation, lending credence to the dipole reversal mechanism.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Determination of defect density of state distribution of amorphous silicon solar cells by temperature derivative capacitance-frequency measurement

In this contribution, we demonstrate the application temperature dependent capacitance-frequency measurements (C-f) to n-i-p hydrogenated amorphous silicon (a-Si:H) solar cells that are forward-biased. By using a forward bias, the C-f measurement can detect the density of defect states in a particular energy range of the interface region. For this contribution, we have carried out this measurement method on n-i-p a-Si:H solar cells of which the intrinsic layer has been exposed to a H{sub 2}-plasma before p-type layer deposition. After this treatment, the open-circuit voltage and fill factor increased significantly, as well as the blue response of the solar cells as is concluded from external quantum efficiency. For single junction, n-i-p a-Si:H solar cells initial efficiency increased from 6.34% to 8.41%. This performance enhancement is believed to be mainly due to a reduction of the defect density in the i-p interface region after the H{sub 2}-plasma treatment. These results are confirmed by the C-f measurements. After H{sub 2}-plasma treatment, the defect density in the intrinsic layer near the i-p interface region is lower and peaks at an energy level deeper in the band gap. These C-f measurements therefore enable us to monitor changes in the defect density in the interface region as a result of a hydrogen plasma. The lower defect density at the i-p interface as detected by the C-f measurements is supported by dark current-voltage measurements, which indicate a lower carrier recombination rate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Significant electrical control of amorphous oxide thin film transistors by an ultrathin Ti surface polarity modifier

We demonstrate an enhanced electrical stability through a Ti oxide (TiO{sub x}) layer on the amorphous InGaZnO (a-IGZO) back-channel; this layer acts as a surface polarity modifier. Ultrathin Ti deposited on the a-IGZO existed as a TiO{sub x} thin film, resulting in oxygen cross-binding with a-IGZO surface. The electrical properties of a-IGZO thin film transistors (TFTs) with TiO{sub x} depend on the surface polarity change and electronic band structure evolution. This result indicates that TiO{sub x} on the back-channel serves as not only a passivation layer protecting the channel from ambient molecules or process variables but also a control layer of TFT device parameters.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

High-pressure x-ray diffraction study of YBO{sub 3}/Eu{sup 3+}, GdBO{sub 3}, and EuBO{sub 3}: Pressure-induced amorphization in GdBO{sub 3}

Angle-dispersive synchrotron X-ray diffraction measurements were performed on vaterite-type YBO{sub 3}/Eu{sup 3+}, GdBO{sub 3}, and EuBO{sub 3}, respectively, up to 41 GPa at room temperature using a diamond-anvil cell. Pressure-induced amorphization was observed in hexagonal GdBO{sub 3} with a significant compression along the c-axis. Compared to the ions of the distorted GdBO{sub 3} phase, its anions may lose their long-range order prior to the cations at high pressures. Based on the experimental pressure-volume data, the obtained bulk moduli of YBO{sub 3}/Eu{sup 3+} and GdBO{sub 3} are 329 and 321 GPa, respectively, which are more than 90% larger than that of EuBO{sub 3} (167 GPa) and are presumably attributed to Gd{sup 3+} and Y{sup 3+} with a high density of d valence electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Picosecond and nanosecond laser annealing and simulation of amorphous silicon thin films for solar cell applications

In this work, a picosecond diode pumped solid state laser and a nanosecond Nd:YAG laser have been used for the annealing and the partial nano-crystallization of an amorphous silicon layer. These experiments were conducted as an alternative/complementary to plasma-enhanced chemical vapor deposition method for fabrication of micromorph tandem solar cell. The laser experimental work was combined with simulations of the annealing process, in terms of temperature distribution evolution, in order to predetermine the optimum annealing conditions. The annealed material was studied, as a function of several annealing parameters (wavelength, pulse duration, fluence), as far as it concerns its structural properties, by X-ray diffraction, SEM, and micro-Raman techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Combined current-modulation annealing induced enhancement of giant magnetoimpedance effect of Co-rich amorphous microwires

We report on a combined current-modulation annealing (CCMA) method, which integrates the optimized pulsed current (PC) and DC annealing techniques, for improving the giant magnetoimpedance (GMI) effect and its field sensitivity of Co-rich amorphous microwires. Relative to an as-prepared Co{sub 68.2}Fe{sub 4.3}B{sub 15}Si{sub 12.5} wire, CCMA is shown to remarkably improve the GMI response of the wire. At 10 MHz, the maximum GMI ratio and its field sensitivity of the as-prepared wire were, respectively, increased by 3.5 and 2.28 times when subjected to CCMA. CCMA increased atomic order orientation and circumferential permeability of the wire by the co-action of high-density pulsed magnetic field energy and thermal activation energy at a PC annealing stage, as well as the formation of uniform circular magnetic domains by a stable DC magnetic field at a DC annealing stage. The magnetic moment can overcome eddy-current damping or nail-sticked action in rotational magnetization, giving rise to a double-peak feature and wider working field range (up to ±2 Oe) at relatively higher frequency (f ≥ 1 MHz)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Distribution of nanoscale nuclei in the amorphous dome of a phase change random access memory

The nanoscale crystal nuclei in an amorphous Ge{sub 2}Sb{sub 2}Te{sub 5} bit in a phase change memory device were evaluated by fluctuation transmission electron microscopy. The quench time in the device (∼10 ns) afforded more and larger nuclei in the melt-quenched state than in the as-deposited state. However, nuclei were even more numerous and larger in a test structure with a longer quench time (∼100 ns), verifying the prediction of nucleation theory that slower cooling produces more nuclei. It also demonstrates that the thermal design of devices will strongly influence the population of nuclei, and thus the speed and data retention characteristics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhanced quantum efficiency of amorphous silicon thin film solar cells with the inclusion of a rear-reflector thin film

We investigated the growth mechanism of amorphous silicon thin films by implementing hot-wire chemical vapor deposition and fabricated thin film solar cell devices. The fabricated cells showed efficiencies of 7.5 and 8.6% for the samples without and with the rear-reflector decomposed by sputtering, respectively. The rear-reflector enhances the quantum efficiency in the infrared spectral region from 550 to 750 nm. The more stable quantum efficiency of the sample with the inclusion of a rear-reflector than the sample without the rear-reflector due to the bias effect is related to the enhancement of the short circuit current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗