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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 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Temperature dependent deformation mechanisms in pure amorphous silicon

High temperature nanoindentation has been performed on pure ion-implanted amorphous silicon (unrelaxed a-Si) and structurally relaxed a-Si to investigate the temperature dependence of mechanical deformation, including pressure-induced phase transformations. Along with the indentation load-depth curves, ex situ measurements such as Raman micro-spectroscopy and cross-sectional transmission electron microscopy analysis on the residual indents reveal the mode of deformation under the indenter. While unrelaxed a-Si deforms entirely via plastic flow up to 200 °C, a clear transition in the mode of deformation is observed in relaxed a-Si with increasing temperature. Up to 100 °C, pressure-induced phase transformation and the observation of either crystalline (r8/bc8) end phases or pressure-induced a-Si occurs in relaxed a-Si. However, with further increase of temperature, plastic flow rather than phase transformation is the dominant mode of deformation. It is believed that the elevated temperature and pressure together induce bond softening and “defect” formation in structurally relaxed a-Si, leading to the inhibition of phase transformation due to pressure-releasing plastic flow under the indenter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A thermalization energy analysis of the threshold voltage shift in amorphous indium gallium zinc oxide thin film transistors under simultaneous negative gate bias and illumination

It has been previously observed that thin film transistors (TFTs) utilizing an amorphous indium gallium zinc oxide (a-IGZO) semiconducting channel suffer from a threshold voltage shift when subjected to a negative gate bias and light illumination simultaneously. In this work, a thermalization energy analysis has been applied to previously published data on negative bias under illumination stress (NBIS) in a-IGZO TFTs. A barrier to defect conversion of 0.65–0.75 eV is extracted, which is consistent with reported energies of oxygen vacancy migration. The attempt-to-escape frequency is extracted to be 10{sup 6}−10{sup 7} s{sup −1}, which suggests a weak localization of carriers in band tail states over a 20–40 nm distance. Models for the NBIS mechanism based on charge trapping are reviewed and a defect pool model is proposed in which two distinct distributions of defect states exist in the a-IGZO band gap: these are associated with states that are formed as neutrally charged and 2+ charged oxygen vacancies at the time of film formation. In this model, threshold voltage shift is not due to a defect creation process, but to a change in the energy distribution of states in the band gap upon defect migration as this allows a state formed as a neutrally charged vacancy to be converted into one formed as a 2+ charged vacancy and vice versa. Carrier localization close to the defect migration site is necessary for the conversion process to take place, and such defect migration sites are associated with conduction and valence band tail states. Under negative gate bias stressing, the conduction band tail is depleted of carriers, but the bias is insufficient to accumulate holes in the valence band tail states, and so no threshold voltage shift results. It is only under illumination that the quasi Fermi level for holes is sufficiently lowered to allow occupation of valence band tail states. The resulting charge localization then allows a negative threshold voltage shift, but only under conditions of simultaneous negative gate bias and illumination, as observed experimentally as the NBIS effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect generation in amorphous-indium-gallium-zinc-oxide thin-film transistors by positive bias stress at elevated temperature

We report on the generation and characterization of a hump in the transfer characteristics of amorphous indium gallium zinc-oxide thin-film transistors by positive bias temperature stress. The hump depends strongly on the gate bias stress at 100 °C. Due to the hump, the positive shift of the transfer characteristic in deep depletion is always smaller that in accumulation. Since, the latter shift is twice the former, with very good correlation, we conclude that the effect is due to creation of a double acceptor, likely to be a cation vacancy. Our results indicate that these defects are located near the gate insulator/active layer interface, rather than in the bulk. Migration of donor defects from the interface towards the bulk may also occur under PBST at 100 °C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of mixed Ge/Si cross-linking on the physical properties of amorphous Ge-Si-Te networks

Amorphous Ge{sub x}Si{sub x}Te{sub 1−2x} glasses are studied as a function of composition by a combination of experimental and theoretical methods, allowing for a full description of the network structure in relationship with physico-chemical properties. Calorimetric and thermal measurements reveal that such glasses display an anomalous behavior across a range of compositions x{sub c1}=7.5% and <x<x{sub c2}=9%, in which is manifested a deep minimum in molar volume, non-reversing enthalpy, and liquid fragility. These anomalies allow defining an intermediate phase, where network rigidity onsets as the content x of Group IV atoms (Ge, Si) are increased. The structural manifestation of these anomalies is understood from {sup 119}Sn Mössbauer spectroscopy and First Principles Molecular Dynamics at selected compositions (Ge{sub 20}Te{sub 80}, Si{sub 20}Te{sub 80}, and Ge{sub 10}Si{sub 10}Te{sub 80}). The numerical models reveal the quite different roles played by the modifier or network cross-linker Ge or Si atoms, Si being more tetrahedral in sp{sup 3} geometry, whereas Mössbauer spectroscopy shows that the nature of chemical bonding is dramatically changed around x≃ 8%. The precise evolution of the local structure and chemical bonding ultimately allows understanding the origin of the intermediate phase in these complex tellurides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Template confined synthesis of amorphous carbon nanotubes and its confocal Raman microscopy

Amorphous carbon nanotubes (aCNTs) were synthesized by AAO (anodic aluminum oxide) template at a temperature 500 °C in nitrogen atmosphere using the citric acid as a carbon source without the help of any catalyst particles. Morphological analysis of the as prepared samples was carried out by field emission scanning electron microscopy (FESEM). Confocal Raman imaging has been studied and an attempt has been made to find out the graphitic (sp{sup 2}) and disordered phase of the CNTs.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Pressure-induced amorphization of charge ordered spinel AlV{sub 2}O{sub 4} at low temperature

Structural properties of charge ordered spinel AlV{sub 2}O{sub 4} have been investigated under high pressure at low temperature (80K) by synchrotron based x-ray diffraction measurements. It is observed that upon increasing pressure the structure becomes progressively disordered due to the distortion of the AlO{sub 4} tetrahedral unit and undergoes amorphization above ∼12 GPa. While releasing pressure, the rhombohedral phase is only partially recovered at a much lower pressure (below 5 GPa). Within the stability of the rhombohedral phase, the distortion in the vanadium heptamer increases monotonically with pressure, suggesting enhanced charge ordering. This result is in sharp contrast with the recent observation of pressure-induced frustration in the charge ordered state leading to structural transition to the cubic phase at room temperature [JPCM 25, 292201, 2013].

36 MATERIALS SCIENCE↗
DOE OSTI2014

Enhanced electrochemical etching of ion irradiated silicon by localized amorphization

A tailored distribution of ion induced defects in p-type silicon allows subsequent electrochemical anodization to be modified in various ways. Here we describe how a low level of lattice amorphization induced by ion irradiation influences anodization. First, it superposes a chemical etching effect, which is observable at high fluences as a reduced height of a micromachined component. Second, at lower fluences, it greatly enhances electrochemical anodization by allowing a hole diffusion current to flow to the exposed surface. We present an anodization model, which explains all observed effects produced by light ions such as helium and heavy ions such as cesium over a wide range of fluences and irradiation geometries.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Band alignment at interfaces of amorphous Al{sub 2}O{sub 3} with Ge{sub 1−x}Sn{sub x}- and strained Ge-based channels

Spectroscopy of internal photoemission of electrons from Ge and Ge{sub 1−x}Sn{sub x} (x ≤ 0.08) alloys into amorphous Al{sub 2}O{sub 3} is used to evaluate the energy of the semiconductor valence band top. It is found that in Ge and Ge{sub 1−x}Sn{sub x} the valence bands are aligned within the measurement accuracy (±0.05 eV) irrespective of the strain imposed on the semiconductor or by the kind of passivating inter-layer applied between the semiconductor and alumina. This indicates that the Ge{sub 1−x}Sn{sub x}-stressor approach may be useful for strain engineering in p-channel Ge metal-oxide-semiconductor transistors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Direct determination of the band offset in atomic layer deposited ZnO/hydrogenated amorphous silicon heterojunctions from X-ray photoelectron spectroscopy valence band spectra

The chemical composition and band alignment at the heterointerface between atomic layer deposition-grown zinc oxide (ZnO) and hydrogenated amorphous silicon (a-Si:H) is investigated using monochromatized X-ray photoelectron spectroscopy. A new approach for obtaining the valence band offset ΔE{sub V} is developed, which consists in fitting the valence band (VB) spectrum obtained for a-Si:H with a thin ZnO overlayer as the sum of experimentally obtained VB spectra of a bulk a-Si:H film and a thick ZnO film. This approach allows obtaining ΔE{sub V} = 2.71 ± 0.15 eV with a minimum of assumptions, and also yields information on the change in band bending of both substrate and ZnO film. The band offset results are compared to values obtained using the usual approach of comparing valence band edge-to-core level energy differences, ΔE{sub B,CL} − ΔE{sub B,VB}. Furthermore, a theoretical value for the VB offset is calculated from the concept of charge neutrality level line-up, using literature data for the charge neutrality levels and the experimentally determined ZnO/a-Si:H interface dipole. The thus obtained value of ΔE{sub V}{sup CNL} = 2.65 ± 0.3 eV agrees well with the experimental ΔE{sub V}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

A first principles analysis of the effect of hydrogen concentration in hydrogenated amorphous silicon on the formation of strained Si-Si bonds and the optical and mobility gaps

In this paper, we use a model of hydrogenated amorphous silicon generated from molecular dynamics with density functional theory calculations to examine how the atomic geometry and the optical and mobility gaps are influenced by mild hydrogen oversaturation. The optical and mobility gaps show a volcano curve as the hydrogen content varies from undersaturation to mild oversaturation, with largest gaps obtained at the saturation hydrogen concentration. At the same time, mid-gap states associated with dangling bonds and strained Si-Si bonds disappear at saturation but reappear at mild oversaturation, which is consistent with the evolution of optical gap. The distribution of Si-Si bond distances provides the key to the change in electronic properties. In the undersaturation regime, the new electronic states in the gap arise from the presence of dangling bonds and strained Si-Si bonds, which are longer than the equilibrium Si-Si distance. Increasing hydrogen concentration up to saturation reduces the strained bonds and removes dangling bonds. In the case of mild oversaturation, the mid-gap states arise exclusively from an increase in the density of strained Si-Si bonds. Analysis of our structure shows that the extra hydrogen atoms form a bridge between neighbouring silicon atoms, thus increasing the Si-Si distance and increasing disorder in the sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

The effect of Ta interface on the crystallization of amorphous phase change material thin films

The crystallization of amorphous GeTe and Ge{sub 2}Sb{sub 2}Te{sub 5} phase change material films, with thickness between 10 and 100 nm, sandwiched between either Ta or SiO{sub 2} layers, was investigated by optical reflectivity. Ta cladding layers were found to increase the crystallization temperature, even for films as thick as 100 nm. X-Ray diffraction investigations of crystallized GeTe films showed a very weak texture in Ta cladded films, in contrast with the strong texture observed for SiO{sub 2} cladding layers. This study shows that crystallization mechanism of phase change materials can be highly impacted by interface effects, even for relatively thick films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Potential-well depth at amorphous-LaAlO{sub 3}/crystalline-SrTiO{sub 3} interfaces measured by optical second harmonic generation

By a combination of optical second harmonic generation and transport measurements, we have investigated interfaces formed by either crystalline or amorphous thin films of LaAlO{sub 3} grown on TiO{sub 2}-terminated SrTiO{sub 3}(001) substrates. Our approach aims at disentangling the relative role of intrinsic and extrinsic doping mechanisms in the formation of the two-dimensional electron gas. The different nature of the two mechanisms is revealed when comparing the sample response variation as a function of temperature during annealing in air. However, before the thermal treatment, the two types of interfaces show almost the same intensity of the second harmonic signal, provided the overlayer thickness is the same. As we will show, the second harmonic signal is proportional to the depth of the potential well confining the charges at the interface. Therefore, our result demonstrates that this depth is about the same for the two different material systems. This conclusion supports the idea that the electronic properties of the two-dimensional electron gas are almost independent of the doping mechanism of the quantum well.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Higher than 60% internal quantum efficiency of photoluminescence from amorphous silicon oxynitride thin films at wavelength of 470 nm

We reported the study on the photoluminescence internal quantum efficiency (PL IQE) and external quantum efficiency (PL EQE) from the amorphous silicon oxynitride (a-SiNO) films, which were fabricated by plasma-enhanced chemical vapor deposition followed by in situ plasma oxidation. We employed the direct measurement of absolute quantum efficiency within a calibrated integration sphere to obtain the PL EQE. Then, we calculated the PL IQE by combing the measured EQE and optical parameters of light extraction factor, reflectivity, and transmittance of the a-SiNO thin films. We also derived the PL QE through investigating the characteristic of the temperature dependent PL. These results show that the PL IQE as high as 60% has been achieved at peak wavelength of about 470 nm, which is much higher than that of Si nanocrystal embedded thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Superconducting nanowire single photon detectors fabricated from an amorphous Mo{sub 0.75}Ge{sub 0.25} thin film

We present the characteristics of superconducting nanowire single photon detectors (SNSPDs) fabricated from amorphous Mo{sub 0.75}Ge{sub 0.25} thin-films. Fabricated devices show a saturation of the internal detection efficiency at temperatures below 1 K, with system dark count rates below 500 cps. Operation in a closed-cycle cryocooler at 2.5 K is possible with system detection efficiencies exceeding 20% for SNSPDs which have not been optimized for high detection efficiency. Jitter is observed to vary between 69 ps at 250 mK and 187 ps at 2.5 K using room temperature amplifiers.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

An efficient light trapping scheme based on textured conductive photonic crystal back reflector for performance improvement of amorphous silicon solar cells

An efficient light trapping scheme named as textured conductive photonic crystal (TCPC) has been proposed and then applied as a back-reflector (BR) in n-i-p hydrogenated amorphous silicon (a-Si:H) solar cell. This TCPC BR combined a flat one-dimensional photonic crystal and a randomly textured surface of chemically etched ZnO:Al. Total efficiency enhancement was obtained thanks to the sufficient conductivity, high reflectivity and strong light scattering of the TCPC BR. Unwanted intrinsic losses of surface plasmon modes are avoided. An initial efficiency of 9.66% for a-Si:H solar cell was obtained with short-circuit current density of 14.74 mA/cm{sup 2}, fill factor of 70.3%, and open-circuit voltage of 0.932 V.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Amorphous FeCoSiB for exchange bias coupled and decoupled magnetoelectric multilayer systems: Real-structure and magnetic properties

The effect of field annealing for exchanged biased multilayer films is studied with respect to the resultant structural and magnetic film properties. The presented multilayer stacks comprise repeating sequences of Ta/Cu/(1 1 1) textured antiferromagnetic Mn₇₀Ir₃₀ /amorphous ferromagnetic Fe₇₀.₂Co₇.₈Si₂B₁₀. Within the ferromagnetic layers crystalline filaments are observed. An additional Ta layer between the antiferromagnet and ferromagnet is used in order to investigate and separate the influence of the common Mn₇₀Ir₃₀/Fe₇₀.₂Co₇.₈Si₁₂B₁₀ interface on the occurring filaments and structural changes. In situ and ex situ transmission electron microscopy is used for a comprehensive structure characterization of multilayer stacks for selected temperature stages. Up to 250 °C, the multilayers are structurally unaltered and preserve the as-deposited condition. A deliberate increase to 350 °C exhibits different crystallization processes for the films, depending on the presence of crystal nuclei within the amorphous ferromagnetic layer. The influence of volume-to-surface ratio of the multilayer stacks to the crystallization process is emphasized by the comparison of in situ and ex situ investigations as the respective specimen thickness is changed. Complementary magnetic studies reveal a defined exchange bias obtained at the first annealing step and a decrease of total anisotropy field with partial crystallization after the subsequent annealing at 350 °C.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Influence of an anomalous dimension effect on thermal instability in amorphous-InGaZnO thin-film transistors

This paper investigates abnormal dimension-dependent thermal instability in amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors. Device dimension should theoretically have no effects on threshold voltage, except for in short channel devices. Unlike short channel drain-induced source barrier lowering effect, threshold voltage increases with increasing drain voltage. Furthermore, for devices with either a relatively large channel width or a short channel length, the output drain current decreases instead of saturating with an increase in drain voltage. Moreover, the wider the channel and the shorter the channel length, the larger the threshold voltage and output on-state current degradation that is observed. Because of the surrounding oxide and other thermal insulating material and the low thermal conductivity of the IGZO layer, the self-heating effect will be pronounced in wider/shorter channel length devices and those with a larger operating drain bias. To further clarify the physical mechanism, fast I{sub D}-V{sub G} and modulated peak/base pulse time I{sub D}-V{sub D} measurements are utilized to demonstrate the self-heating induced anomalous dimension-dependent threshold voltage variation and on-state current degradation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Improvement in gate bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors using microwave irradiation

In this study, we evaluated the effects of microwave irradiation (MWI) post-deposition-annealing (PDA) treatment on the gate bias stress instability of amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) and compared the results with a conventional thermal annealing PDA treatment. The MWI-PDA-treated a-IGZO TFTs exhibited enhanced electrical performance as well as improved long-term stability with increasing microwave power. The positive turn-on voltage shift (ΔV{sub ON}) as a function of stress time with positive bias and varying temperature was precisely modeled on a stretched-exponential equation, suggesting that charge trapping is a dominant mechanism in the instability of MWI-PDA-treated a-IGZO TFTs. The characteristic trapping time and average effective barrier height for electron transport indicate that the MWI-PDA treatment effectively reduces the defects in a-IGZO TFTs, resulting in a superior resistance against gate bias stress.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗