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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 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Nitride passivation reduces interfacial traps in atomic-layer-deposited Al{sub 2}O{sub 3}/GaAs (001) metal-oxide-semiconductor capacitors using atmospheric metal-organic chemical vapor deposition

Using an atmospheric metal-organic chemical vapor deposition system, we passivated GaAs with AlN prior to atomic layer deposition of Al{sub 2}O{sub 3}. This AlN passivation incorporated nitrogen at the Al{sub 2}O{sub 3}/GaAs interface, improving the capacitance-voltage (C–V) characteristics of the resultant metal-oxide-semiconductor capacitors (MOSCAPs). The C–V curves of these devices showed a remarkable reduction in the frequency dispersion of the accumulation capacitance. Using the conductance method at various temperatures, we extracted the interfacial density of states (D{sub it}). The D{sub it} was reduced over the entire GaAs band gap. In particular, these devices exhibited D{sub it} around the midgap of less than 4 × 10{sup 12} cm{sup −2}eV{sup −1}, showing that AlN passivation effectively reduced interfacial traps in the MOS structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Efficient Schottky-like junction GaAs nanowire photodetector with 9 GHz modulation bandwidth with large active area

Efficient, low capacitance density GaAs/Indium-Tin-Oxide Schottky-like junction photodetectors with a 50 μm square active are fabricated for operation in the gigahertz range. Modulation bandwidth is experimentally measured up to 10 GHz at various applied reverse biases and optical intensities to explore the effects of photo-generated carrier screening on modulation bandwidth. Last, the bandwidth dependence on applied reverse bias and optical intensity is simulated as a means to quantify average carrier velocities in nanowire material systems.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Towards a graphene-based quantum impedance standard

Precision measurements of the quantum Hall resistance with alternating current (ac) in the kHz range were performed on epitaxial graphene in order to assess its suitability as a quantum standard of impedance. The quantum Hall plateaus measured with alternating current were found to be flat within one part in 10{sup 7}. This is much better than for plain GaAs quantum Hall devices and shows that the magnetic-flux-dependent capacitive ac losses of the graphene device are less critical. The observed frequency dependence of about −8 × 10{sup −8}/kHz is comparable in absolute value to the positive frequency dependence of plain GaAs devices, but the negative sign is attributed to stray capacitances which we believe can be minimized by a careful design of the graphene device. Further improvements thus may lead to a simpler and more user-friendly quantum standard for both resistance and impedance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Defect density and dielectric constant in perovskite solar cells

We report on measurement of dielectric constant, mid-gap defect density, Urbach energy of tail states in CH{sub 3}NH{sub 3}PbI{sub x}Cl{sub 1−x} perovskite solar cells. Midgap defect densities were estimated by measuring capacitance vs. frequency at different temperatures and show two peaks, one at 0.66 eV below the conduction band and one at 0.24 eV below the conduction band. The attempt to escape frequency is in the range of 2 × 10{sup 11}/s. Quantum efficiency data indicate a bandgap of 1.58 eV. Urbach energies of valence and conduction band are estimated to be ∼16 and ∼18 meV. Measurement of saturation capacitance indicates that the relative dielectric constant is ∼18.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electrical and physical characterization of the Al{sub 2}O{sub 3}/p-GaSb interface for 1%, 5%, 10%, and 22% (NH{sub 4}){sub 2}S surface treatments

In this work, the impact of ammonium sulfide ((NH{sub 4}){sub 2}S) surface treatment on the electrical passivation of the Al{sub 2}O{sub 3}/p-GaSb interface is studied for varying sulfide concentrations. Prior to atomic layer deposition of Al{sub 2}O{sub 3}, GaSb surfaces were treated in 1%, 5%, 10%, and 22% (NH{sub 4}){sub 2}S solutions for 10 min at 295 K. The smallest stretch-out and flatband voltage shifts coupled with the largest capacitance swing, as indicated by capacitance-voltage (CV) measurements, were obtained for the 1% treatment. The resulting interface defect trap density (D{sub it}) distribution showed a minimum value of 4 × 10{sup 12 }cm{sup −2}eV{sup −1} at E{sub v} + 0.27 eV. Transmission electron microscopy and atomic force microscopy examination revealed the formation of interfacial layers and increased roughness at the Al{sub 2}O{sub 3}/p-GaSb interface of samples treated with 10% and 22% (NH{sub 4}){sub 2}S. In combination, these effects degrade the interface quality as reflected in the CV characteristics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Influence of curvature on the device physics of thin film transistors on flexible substrates

Thin film transistors (TFTs) on elastomers promise flexible electronics with stretching and bending. Recently, there have been several experimental studies reporting the behavior of TFTs under bending and buckling. In the presence of stress, the insulator capacitance is influenced due to two reasons. The first is the variation in insulator thickness depending on the Poisson ratio and strain. The second is the geometric influence of the curvature of the insulator-semiconductor interface during bending or buckling. This paper models the role of curvature on TFT performance and brings to light an elegant result wherein the TFT characteristics is dependent on the area under the capacitance-distance curve. The paper compares models with simulations and explains several experimental findings reported in literature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Photo-response of a P3HT:PCBM blend in metal-insulator-semiconductor capacitors

Metal-insulator-semiconductor capacitors are investigated, in which the insulator is cross-linked polyvinylphenol and the active layer a blend of poly(3-hexylthiophene), P3HT, and the electron acceptor [6,6]-phenyl-C{sub 61}-butyric acid methyl ester (PCBM). Admittance spectra and capacitance-voltage measurements obtained in the dark both display similar behaviour to those previously observed in P3HT-only devices. However, the photo-capacitance response is significantly enhanced in the P3HT:PCBM case, where exciton dissociation leads to electron transfer into the PCBM component. The results are consistent with a network of PCBM aggregates that is continuous through the film but with no lateral interconnection between the aggregates at or near the blend/insulator interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Combined effects of Bi deficiency and Mn substitution on the structural transformation and functionality of BiFeO{sub 3} films

Mn-doped BiFeO{sub 3} films with Mn contents of 5 and 10 mol. % were prepared via a chemical route. A carefully controlled amount of Bi deficiency was introduced to further tune the lattice structure and the functionality of multiferroic BiFeO{sub 3}. The crystal structure of Bi{sub 1−δ}Fe{sub 1−x}Mn{sub x}O{sub 3} films was investigated by X-ray diffraction and Raman spectra; a rhombohedral-to-orthorhombic phase transition was revealed. The observed double hysteresis loops and two capacitance maxima from polarization vs electric field and capacitance-voltage measurements indicate an antiferroelectric-like behavior. Additionally, the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases in Bi{sub 1−δ}Fe{sub 1−x}Mn{sub x}O{sub 3} films was revealed from the domain structures obtained by piezoelectric force microscopy. The effects of Mn substitution in conjunction with Bi deficiency on the FE-AFE phase transition and electrical behavior of BiFeO{sub 3} films are discussed in detail. Meanwhile, magnetic and photoluminescence measurements on the films illustrate that Mn substitution gives rise to the net magnetic moment and the defects induced by both Bi deficiency and Mn substitution influence the electronic structure of BiFeO{sub 3} films. This study thus shows a simple and effective way to control the functionalities of BiFeO{sub 3} films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Reduced electrical impedance of SiO{sub 2}, deposited through focused ion beam based systems, due to impurity percolation

The electrical impedance (both the resistive and capacitive aspects) of focused ion beam (FIB) deposited SiO{sub 2} has been correlated to the specific composition of the ion beam, in Ga- and Xe-based FIB systems. The presence of electrically percolating Ga in concert with carbon (inevitably found as the product of the hydrocarbon precursor decomposition) has been isolated as a major cause for the observed decrease in the resistivity of the deposited SiO{sub 2}. Concomitant with the decreased resistivity, an increased capacitance and effective dielectric constant was observed. Our study would be useful to understand the constraints to the deposition of high quality insulator films through FIB based methodologies.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Multilevel characteristics and memory mechanisms for nonvolatile memory devices based on CuInS{sub 2} quantum dot-polymethylmethacrylate nanocomposites

Nonvolatile memory devices based on CuInS{sub 2} (CIS) quantum dots (QDs) embedded in a polymethylmethacrylate (PMMA) layer were fabricated using spin-coating method. The memory window widths of the capacitance-voltage (C-V) curves for the Al/CIS QDs embedded in PMMA layer/p-Si devices were 0.3, 0.6, and 1.0 V for sweep voltages of ±3, ±5, and ±7 V, respectively. Capacitance-cycle data demonstrated that the charge-trapping capability of the devices with an ON/OFF ratio value of 2.81 × 10{sup −10} was maintained for 8 × 10{sup 3} cycles without significant degradation and that the extrapolation of the ON/OFF ratio value to 1 × 10{sup 6} cycles converged to 2.40 × 10{sup −10}, indicative of the good stability of the devices. The memory mechanisms for the devices are described on the basis of the C-V curves and the energy-band diagrams.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Modeling of n-InAs metal oxide semiconductor capacitors with high-κ gate dielectric

A qualitative analysis on capacitance-voltage and conductance data for high-κ/InAs capacitors is presented. Our measured data were evaluated with a full equivalent circuit model, including both majority and minority carriers, as well as interface and border traps, formulated for narrow band gap metal-oxide-semiconductor capacitors. By careful determination of interface trap densities, distribution of border traps across the oxide thickness, and taking into account the bulk semiconductor response, it is shown that the trap response has a strong effect on the measured capacitances. Due to the narrow bandgap of InAs, there can be a large surface concentration of electrons and holes even in depletion, so a full charge treatment is necessary.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electrostatic force microscopy and electrical isolation of etched few-layer graphene nano-domains

Nanostructured bi-layer graphene samples formed through catalytic etching are investigated with electrostatic force microscopy. The measurements and supporting computations show a variation in the microscopy signal for different nano-domains that are indicative of changes in capacitive coupling related to their small sizes. Abrupt capacitance variations detected across etch tracks indicates that the nano-domains have strong electrical isolation between them. Comparison of the measurements to a resistor-capacitor model indicates that the resistance between two bi-layer graphene regions separated by an approximately 10 nm wide etch track is greater than about 1×10{sup 12} Ω with a corresponding gap resistivity greater than about 3×10{sup 14} Ω⋅nm. This extremely large gap resistivity suggests that catalytic etch tracks within few-layer graphene samples are sufficient for providing electrical isolation between separate nano-domains that could permit their use in constructing atomically thin nanogap electrodes, interconnects, and nanoribbons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Side-gate modulation effects on high-quality BN-Graphene-BN nanoribbon capacitors

High-quality BN-Graphene-BN nanoribbon capacitors with double side-gates of graphene have been experimentally realized. The double side-gates can effectively modulate the electronic properties of graphene nanoribbon capacitors. By applying anti-symmetric side-gate voltages, we observed significant upward shifting and flattening of the V-shaped capacitance curve near the charge neutrality point. Symmetric side-gate voltages, however, only resulted in tilted upward shifting along the opposite direction of applied gate voltages. These modulation effects followed the behavior of graphene nanoribbons predicted theoretically for metallic side-gate modulation. The negative quantum capacitance phenomenon predicted by numerical simulations for graphene nanoribbons modulated by graphene side-gates was not observed, possibly due to the weakened interactions between the graphene nanoribbon and side-gate electrodes caused by the Ga{sup +} beam etching process.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Tuning the polarization-induced free hole density in nanowires graded from GaN to AlN

We report a systematic study of p-type polarization-induced doping in graded AlGaN nanowire light emitting diodes grown on silicon wafers by plasma-assisted molecular beam epitaxy. The composition gradient in the p-type base is varied in a set of samples from 0.7%Al/nm to 4.95%Al/nm corresponding to negative bound polarization charge densities of 2.2 × 10{sup 18 }cm{sup −3} to 1.6 × 10{sup 19 }cm{sup −3}. Capacitance measurements and energy band modeling reveal that for gradients greater than or equal to 1.30%Al/nm, the deep donor concentration is negligible and free hole concentrations roughly equal to the bound polarization charge density are achieved up to 1.6 × 10{sup 19 }cm{sup −3} at a gradient of 4.95%Al/nm. Accurate grading lengths in the p- and n-side of the pn-junction are extracted from scanning transmission electron microscopy images and are used to support energy band calculation and capacitance modeling. These results demonstrate the robust nature of p-type polarization doping in nanowires and put an upper bound on the magnitude of deep donor compensation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Full-range electrical characteristics of WS{sub 2} transistors

We fabricated transistors formed by few layers to bulk single crystal WS{sub 2} to quantify the factors governing charge transport. We established a capacitor network to analyze the full-range electrical characteristics of the channel, highlighting the role of quantum capacitance and interface trap density. We find that the transfer characteristics are mainly determined by the interplay between quantum and oxide capacitances. In the OFF-state, the interface trap density (<10{sup 12} cm{sup –2}) is a limiting factor for the subthreshold swing. Furthermore, the superior crystalline quality and the low interface trap density enabled the subthreshold swing to approach the theoretical limit on a back-gated device on SiO{sub 2}/Si substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Defect-reduction mechanism for improving radiative efficiency in InGaN/GaN light-emitting diodes using InGaN underlayers

The influence of a dilute In{sub x}Ga{sub 1-x}N (x ∼ 0.03) underlayer (UL) grown below a single In{sub 0.16}Ga{sub 0.84}N quantum well (SQW), within a light-emitting diode (LED), on the radiative efficiency and deep level defect properties was studied using differential carrier lifetime (DCL) measurements and deep level optical spectroscopy (DLOS). DCL measurements found that inclusion of the UL significantly improved LED radiative efficiency. At low current densities, the non-radiative recombination rate of the LED with an UL was found to be 3.9 times lower than the LED without an UL, while the radiative recombination rates were nearly identical. This suggests that the improved radiative efficiency resulted from reduced non-radiative defect concentration within the SQW. DLOS measurement found the same type of defects in the InGaN SQWs with and without ULs. However, lighted capacitance-voltage measurements of the LEDs revealed a 3.4 times reduction in a SQW-related near-mid-gap defect state for the LED with an UL. Quantitative agreement in the reduction of both the non-radiative recombination rate (3.9×) and deep level density (3.4×) upon insertion of an UL corroborates deep level defect reduction as the mechanism for improved LED efficiency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Rapid synthesis of monodispersed highly porous spinel nickel cobaltite (NiCo{sub 2}O{sub 4}) electrode material for supercapacitors

Monodispersed highly porous spinel nickel cobaltite electrode material was successfully synthesized in a short time using combustion technique. Single phase cubic nature of the spinel nickel cobaltite with average crystallite size of 24 nm was determined from X-ray diffraction study. Functional groups present in the compound were determined from FTIR study and it further confirms the spinel formation. FESEM images reveal the porous nature of the prepared material and uniform size distribution of the particles. Electrochemical evaluation was performed using Cyclic Voltammetry (CV) technique, Chronopotentiometry (CP) and Electrochemical Impedance Spectroscopy (EIS). Results reveal the typical pseudocapacitive behaviour of the material. Maximum capacitance of 754 F/g was calculated at the scan rate of 5 mV/s, high capacitance was due to the unique porous morphology of the electrode. Nyquist plot depicts the low resistance and good electrical conductivity of nickel cobaltite. It has been found that nickel cobaltite prepared by this typical method will be a potential electrode material for supercapcitor application.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Optical and electrical properties of TiOPc doped Alq{sub 3} thin films

The Titanyl phthalocyanine (TiOPc) was doped in Tris (8-hydroxyquinolinato) aluminum (Alq3) with different concentration. The thin film of optimized doping concentration was studied extensively for optical and electrical properties. The optical properties, studied using ellipsometry, absorption and photoluminescence. The absorption peak of Alq{sub 3} and TiOPc was observed at 387 nm and 707 nm and the photo-luminescence intensity (PL) peak of doped thin film was observed at 517 nm. The DC and AC electrical properties of the thin film were studied by current density-voltage (J-V) characteristics and impedance over a frequency range of 100 Hz - 1 MHz. The electron mobility calculated from trap-free space-charge limited region (SCLC) is 0.17×10{sup −5} cm{sup 2}/Vs. The Cole-Cole plots shows that the TiOPc doped Alq{sub 3} thin film can be represented by a single parallel resistance R{sub P} and capacitance C{sub P} network with a series resistance R{sub S} (10 Ω). The value of R{sub P} and C{sub P} at zero bias was 1587 Ω and 2.568 nF respectively. The resistance R{sub P} decreases with applied bias whereas the capacitance C{sub P} remains almost constant.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗