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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 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Voltage-induced electroluminescence characteristics of hybrid light-emitting diodes with CdSe/Cd/ZnS core-shell nanoparticles embedded in a conducting polymer on plastic substrates

We investigate the electroluminescence (EL) characteristics of a hybrid light-emitting diode (HyLED) with an emissive layer comprised of CdSe/Cd/ZnS core-shell nanoparticles (NPs) embedded in poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO) on a plastic substrate. The EL characteristics change dramatically with increasing of the biased voltage. At low voltages, recombination of electrons and holes occurs only in the PFO film because of poor charge transfer in the PFO-CdSe/Cd/ZnS NPs composite film, while the color of the light-emitting from the HyLED changes from blue to red as the biased voltage increases from 7.5 to 17.5 V. We examine and discuss the mechanism of this color tunability.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Breakdown mechanisms and reverse current-voltage characteristics of organic bulk heterojunction solar cells and photodetectors

We investigate the reverse current-voltage characteristics and breakdown mechanisms of organic bulk heterojunction solar cells and photodetectors. Dark current and photo current measurements at different temperatures indicate that tunneling is the dominant mechanism at high reverse voltage. A band-to-band tunneling model that accommodates either Gaussian or exponential-parabolic density of states distributions is developed and used for simulations. At high reverse bias, the model explains the observed breakdown, which differs from the bias independent dark current behavior predicted by the Onsager-Braun model. At low reverse voltage, the incorporation of shunt resistance in the model provides good agreement between the measured reverse bias characteristics and simulations.

14 SOLAR ENERGY↗
DOE OSTI2014

Perfect electrical switching of edge channel transport in HgTe quantum wells controlled by gate voltage

We present a proposal to realize a perfect electrical switching of topological edge-state transport in a HgTe quantum well (QW). In our device design, we place a strip-like top gate voltage in a conventional quantum-point-contact (QPC) region in the HgTe QW. The numerical calculations show that upon increasing the gate voltage, two new conductance channels are developed in the transport direction and just neighbouring the boundaries of the top gate. The quantum states in the new channels can couple with the edge states to open a gap in energy spectrum, and in turn the gap width can be adjusted by the gate voltage, indicating that switch-on/off of the edge channels can be manipulated in a controllable way. Our device can not only be considered as a development of the conventional QPC structure based on the HgTe QW but also provides a new route to realize topological electrical switchers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Thickness independent reduced forming voltage in oxygen engineered HfO{sub 2} based resistive switching memories

The conducting filament forming voltage of stoichiometric hafnium oxide based resistive switching layers increases linearly with layer thickness. Using strongly reduced oxygen deficient hafnium oxide thin films grown on polycrystalline TiN/Si(001) substrates, the thickness dependence of the forming voltage is strongly suppressed. Instead, an almost constant forming voltage of about 3 V is observed up to 200 nm layer thickness. This effect suggests that filament formation and switching occurs for all samples in an oxidized HfO{sub 2} surface layer of a few nanometer thickness while the highly oxygen deficient thin film itself merely serves as a oxygen vacancy reservoir.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Modelling of illuminated current–voltage characteristics to evaluate leakage currents in long wavelength infrared mercury cadmium telluride photovoltaic detectors

The current–voltage characteristics of long wavelength mercury cadmium telluride infrared detectors have been studied using a recently suggested method for modelling of illuminated photovoltaic detectors. Diodes fabricated on in-house grown arsenic and vacancy doped epitaxial layers were evaluated for their leakage currents. The thermal diffusion, generation–recombination (g-r), and ohmic currents were found as principal components of diode current besides a component of photocurrent due to illumination. In addition, both types of diodes exhibited an excess current component whose growth with the applied bias voltage did not match the expected growth of trap-assisted-tunnelling current. Instead, it was found to be the best described by an exponential function of the type, I{sub excess} = I{sub r0} + K{sub 1} exp (K{sub 2} V), where I{sub r0}, K{sub 1}, and K{sub 2} are fitting parameters and V is the applied bias voltage. A study of the temperature dependence of the diode current components and the excess current provided the useful clues about the source of origin of excess current. It was found that the excess current in diodes fabricated on arsenic doped epitaxial layers has its origin in the source of ohmic shunt currents. Whereas, the source of excess current in diodes fabricated on vacancy doped epitaxial layers appeared to be the avalanche multiplication of photocurrent. The difference in the behaviour of two types of diodes has been attributed to the difference in the quality of epitaxial layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Influence of carrier overflow on the forward-voltage characteristics of InGaN-based light-emitting diodes

We investigate the influence of carrier overflow on the forward-voltage characteristics of the InGaN-based blue light-emitting-diode (LED) by comparing the temperature-dependent characteristics of the electroluminescence (EL) efficiency, the EL spectra, and the current-voltage relation over a wide range of temperature (50–300 K). Based on these experimental results, we demonstrate that the simple ohmic potential drop in the Shockley diode equation is not sufficient to explain the experimental data when the severe carrier overflow to the p-(Al)GaN layer induces the efficiency droop in the LED device. The anomalous relation between current and voltage at cryogenic temperatures is explained by the space-charge-limited current formed by the overflown electrons, rather than by the increase of a constant series resistance in the p-(Al)GaN layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Time-resolved photoluminescence measurements for determining voltage-dependent charge-separation efficiencies of subcells in triple-junction solar cells

Conventional external quantum-efficiency measurement of solar cells provides charge-collection efficiency for approximate short-circuit conditions. Because this differs from actual operating voltages, the optimization of high-quality tandem solar cells is especially complicated. Here, we propose a contactless method, which allows for the determination of the voltage dependence of charge-collection efficiency for each subcell independently. By investigating the power dependence of photoluminescence decays, charge-separation and recombination-loss time constants are obtained. The upper limit of the charge-collection efficiencies at the operating points is then obtained by applying the uniform field model. This technique may complement electrical characterization of the voltage dependence of charge collection, since subcells are directly accessible.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

On the difference between breakdown and quench voltages of argon plasma and its relation to 4p–4s atomic state transitions

Using a relaxation oscillator circuit, breakdown (V{sub BD}) and quench (V{sub Q}) voltages of a DC discharge microplasma between two needle probes are measured. High resolution modified Paschen curves are obtained for argon microplasmas including a quench voltage curve representing the voltage at which the plasma turns off. It is shown that for a point to point microgap (e.g., the microgap between two needle probes) which describes many realistic microdevices, neither Paschen's law applies nor field emission is noticeable. Although normally V{sub BD} > V{sub Q,} it is observed that depending on environmental parameters of argon, such as pressure and the driving circuitry, plasma can exist in a different state with equal V{sub BD} and V{sub Q.} Using emission line spectroscopy, it is shown that V{sub BD} and V{sub Q} are equal if the atomic excitation by the electric field dipole moment dominantly leads to one of the argon's metastable states (4P{sub 5} in our study)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Spectral shape deformation in inverse spin Hall voltage in Y{sub 3}Fe{sub 5}O{sub 12}|Pt bilayers at high microwave power levels

We report on the deformation of microwave absorption spectra and of the inverse spin Hall voltage signals in thin film bilayers of yttrium iron garnet (YIG) and platinum at high microwave power levels in a 9.45-GHz TE{sub 011} cavity. As the microwave power increases from 0.15 to 200 mW, the resonance field shifts to higher values, and the initially Lorentzian spectra of the microwave absorption intensity as well as the inverse spin Hall voltage signals become asymmetric. The contributions from opening of the magnetization precession cone and heating of YIG cannot well reproduce the data. Control measurements of inverse spin Hall voltages on thin-film YIG|Pt systems with a range of line widths underscore the role of spin-wave excitations in spectral deformation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Competitive behavior of photons contributing to junction voltage jump in narrow band-gap semiconductor multi-quantum-well laser diodes at lasing threshold

The junction behavior of different narrow band-gap multi-quantum-well (MQW) laser diodes (LDs) confirmed that the jump in the junction voltage in the threshold region is a general characteristic of narrow band-gap LDs. The relative change in the 1310 nm LD is the most obvious. To analyze this sudden voltage change, the threshold region is divided into three stages by I{sub th}{sup l} and I{sub th}{sup u}, as shown in Fig. 2; I{sub th}{sup l} is the conventional threshold, and as long as the current is higher than this threshold, lasing exists and the IdV/dI-I plot drops suddenly; I{sub th}{sup u} is the steady lasing point, at which the separation of the quasi-Fermi levels of electron and holes across the active region (V{sub j}) is suddenly pinned. Based on the evolutionary model of dissipative structure theory, the rate equations of the photons in a single-mode LD were deduced in detail at I{sub th}{sup l} and I{sub th}{sup u}. The results proved that the observed behavior of stimulated emission suddenly substituting for spontaneous emission, in a manner similar to biological evolution, must lead to a sudden increase in the injection carriers in the threshold region, which then causes the sudden increase in the junction voltage in this region.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Double-gate SnO{sub 2} nanowire electric-double-layer transistors with tunable threshold voltage

Double-gate Sb-SnO{sub 2} nanowire electric-double-layer (EDL) transistors with in-plane gates were fabricated using only one shadow mask. The threshold voltage of such devices can be tuned in a wide range from −0.13 V to 0.72 V by the in-plane gate, which allows the device to switch from depletion-mode to enhancement-mode operation. The operation voltage of the double-gate device is 1 V because the EDL gate dielectric can lead to a high gate capacitance (>3.5 μF/cm{sup 2}). Moreover, all double-gate devices show good electrical characteristics with high field-effect mobility (>200 cm{sup 2}/V·s), high drain-current I{sub on/off} ratio (>7 × 10{sup 4}), and small subthreshold slope (<100 mV/dec). These double-gate nanowire EDL transistors can pave the way for an electrically working low-voltage nano-electronic process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Polymer/metal oxide hybrid dielectrics for low voltage field-effect transistors with solution-processed, high-mobility semiconductors

Transistors for future flexible organic light-emitting diode (OLED) display backplanes should operate at low voltages and be able to sustain high currents over long times without degradation. Hence, high capacitance dielectrics with low surface trap densities are required that are compatible with solution-processable high-mobility semiconductors. Here, we combine poly(methyl methacrylate) (PMMA) and atomic layer deposition hafnium oxide (HfO{sub x}) into a bilayer hybrid dielectric for field-effect transistors with a donor-acceptor polymer (DPPT-TT) or single-walled carbon nanotubes (SWNTs) as the semiconductor and demonstrate substantially improved device performances for both. The ultra-thin PMMA layer ensures a low density of trap states at the semiconductor-dielectric interface while the metal oxide layer provides high capacitance, low gate leakage and superior barrier properties. Transistors with these thin (≤70 nm), high capacitance (100–300 nF/cm{sup 2}) hybrid dielectrics enable low operating voltages (<5 V), balanced charge carrier mobilities and low threshold voltages. Moreover, the hybrid layers substantially improve the bias stress stability of the transistors compared to those with pure PMMA and HfO{sub x} dielectrics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Threshold-voltage modulated phase change heterojunction for application of high density memory

Phase change random access memory is one of the most important candidates for the next generation non-volatile memory technology. However, the ability to reduce its memory size is compromised by the fundamental limitations inherent in the CMOS technology. While 0T1R configuration without any additional access transistor shows great advantages in improving the storage density, the leakage current and small operation window limit its application in large-scale arrays. In this work, phase change heterojunction based on GeTe and n-Si is fabricated to address those problems. The relationship between threshold voltage and doping concentration is investigated, and energy band diagrams and X-ray photoelectron spectroscopy measurements are provided to explain the results. The threshold voltage is modulated to provide a large operational window based on this relationship. The switching performance of the heterojunction is also tested, showing a good reverse characteristic, which could effectively decrease the leakage current. Furthermore, a reliable read-write-erase function is achieved during the tests. Phase change heterojunction is proposed for high-density memory, showing some notable advantages, such as modulated threshold voltage, large operational window, and low leakage current.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

MoS{sub 2} oxygen sensor with gate voltage stress induced performance enhancement

Two-dimensional (2D) materials have recently attracted wide attention and rapidly established themselves in various applications. In particular, 2D materials are regarded as promising building blocks for gas sensors due to their high surface-to-volume ratio, ease in miniaturization, and flexibility in enabling wearable electronics. Compared with other 2D materials, MoS{sub 2} is particularly intriguing because it has been widely researched and exhibits semiconducting behavior. Here, we have fabricated MoS{sub 2} resistor based O{sub 2} sensors with a back gate configuration on a 285 nm SiO{sub 2}/Si substrate. The effects of applying back gate voltage stress on O{sub 2} sensing performance have been systematically investigated. With a positive gate voltage stress, the sensor response improves and the response is improved to 29.2% at O{sub 2} partial pressure of 9.9 × 10{sup −5} millibars with a +40 V back-gate bias compared to 21.2% at O{sub 2} partial pressure of 1.4 × 10{sup −4} millibars without back-gate bias; while under a negative gate voltage stress of −40 V, a fast and full recovery can be achieved at room temperature. In addition, a method in determining O{sub 2} partial pressure with a detectability as low as 6.7 × 10{sup −7} millibars at a constant vacuum pressure is presented and its potential as a vacuum gauge is briefly discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Space charge inhibition effect of nano-Fe{sub 3}O{sub 4} on improvement of impulse breakdown voltage of transformer oil based on improved Kerr optic measurements

Transformer oil-based nanofluids (NFs) with 0.03 g/L Fe{sub 3}O{sub 4} nanoparticle content exhibit 11.2% higher positive impulse breakdown voltage levels than pure transformer oils. To study the effects of the Fe{sub 3}O{sub 4} nanoparticles on the space charge in transformer oil and to explain why the nano-modified transformer oil exhibits improved impulse breakdown voltage characteristics, the traditional Kerr electro-optic field mapping technique is improved by increasing the length of the parallel-plate electrodes and by using a photodetector array as a high light sensitivity device. The space charge distributions of pure transformer oil and of NFs containing Fe{sub 3}O{sub 4} nanoparticles can be measured using the improved Kerr electro-optic field mapping technique. Test results indicate a significant reduction in space charge density in the transformer oil-based NFs with the Fe{sub 3}O{sub 4} nanoparticles. The fast electrons are captured by the nanoparticles and are converted into slow-charged particles in the NFs, which then reduce the space charge density and result in a more uniform electric field distribution. Streamer propagation in the NFs is also obstructed, and the breakdown strengths of the NFs under impulse voltage conditions are also improved.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Voltage-controlled magnetic anisotropy in Fe|MgO tunnel junctions studied by x-ray absorption spectroscopy

In this study, voltage-controlled magnetic anisotropy (VCMA) in Fe|MgO tunnel junctions was investigated via the magneto-optical Kerr effect, soft x-ray absorption spectroscopy, and magnetic circular dichroism spectroscopy. The Fe|MgO tunnel junctions showed enhanced perpendicular magnetic anisotropy under external negative voltage, which induced charge depletion at the Fe|MgO interface. Despite the application of voltages of opposite polarity, no trace of chemical reaction such as a redox reaction attributed to O{sup 2−} migration was detected in the x-ray absorption spectra of the Fe. The VCMA reported in the Fe|MgO-based magnetic tunnel junctions must therefore originate from phenomena associated with the purely electric effect, that is, surface electron doping and/or redistribution induced by an external electric field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Radio-frequency sheath voltages and slow wave electric field spatial structure

We investigate theoretically how sheath radio-frequency (RF) oscillations relate to the spatial structure of the RF parallel electric field emitted by Ion Cyclotron (IC) wave launchers, using a simple model of Slow Wave (SW) evanescence coupled with Direct Current (DC) plasma biasing via sheath boundary conditions in a plasma-filled 2-dimensional (parallel, radial) rectangle. Within a “wide sheaths” asymptotic regime, valid for large-amplitude near RF fields, our model becomes partly linear: the sheath oscillating voltage at open field line boundaries is a linear combination of elementary contributions by every source point of the radiated RF field map. These individual contributions are all the more intense as the SW emission point is toroidally nearer to the sheath walls. A limit formula is given for a source infinitely close to the sheaths. The decay of sheath RF voltages with the sheath/source parallel distance is quantified as a function of two characteristic SW evanescence lengths. Decay lengths are smaller than antenna parallel extensions. The sheath RF voltages at an IC antenna side limiter are therefore mainly sensitive to SW emission near this limiter, as recent observations suggest. Toroidal proximity effects could also explain why sheath oscillations persist with antisymmetric strap toroidal phasing, despite the parallel anti-symmetry of the radiated field map. They could also justify current attempts at reducing the RF fields induced near antenna boxes to attenuate sheath oscillations in their vicinity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Dynamic model based on voltage transfer curve for pattern formation in dielectric barrier glow discharge

Simulation work is very important for understanding the formation of self-organized discharge patterns. Previous works have witnessed different models derived from other systems for simulation of discharge pattern, but most of these models are complicated and time-consuming. In this paper, we introduce a convenient phenomenological dynamic model based on the basic dynamic process of glow discharge and the voltage transfer curve (VTC) to study the dielectric barrier glow discharge (DBGD) pattern. VTC is an important characteristic of DBGD, which plots the change of wall voltage after a discharge as a function of the initial total gap voltage. In the modeling, the combined effect of the discharge conditions is included in VTC, and the activation-inhibition effect is expressed by a spatial interaction term. Besides, the model reduces the dimensionality of the system by just considering the integration effect of current flow. All these greatly facilitate the construction of this model. Numerical simulations turn out to be in good accordance with our previous fluid modeling and experimental result.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗