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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 1,081 records · Page 60
All sourcesNASA NTRSDOE OSTI
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↗
DOE OSTI2015

A comparative study of 30MeV boron{sup 4+} and 60MeV oxygen{sup 8+} ion irradiated Si NPN BJTs

The impact of 30MeV boron{sup 4+} and 60MeV oxygen{sup 8+} ion irradiation on electrical characteristics of 2N3773 Si NPN Bipolar junction transistors (BJTs) is reported in the present study. The transistors were decapped and irradiated at room temperature. Gummel characteristics, DC current gain and Capacitance-voltage (C-V) characteristics were studied before and after irradiation at different fluences. DC current gain has decreased significantly in both boron and oxygen ion irradiation. Also the value of capacitance decreased 3-4 times with increase in fluence. Both 30MeV boron ion and 60MeV oxygen ion induced similar extent of degradation in electrical characteristics of the transistor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Novel synthesis of Ni-ferrite (NiFe{sub 2}O{sub 4}) electrode material for supercapacitor applications

Novel nanocrystalline NiFe{sub 2}O{sub 4} has been synthesized through combustion route using citric acid as a fuel. Phase of the synthesized material was analyzed using powder X-ray diffraction. The XRD study revealed the formation of spinel phase cubic NiFe{sub 2}O{sub 4} with high crystallinity. The average crystallite size of NiFe{sub 2}O{sub 4} nanomaterial was calculated from scherrer equation. The electrochemical properties were realized by cyclic voltammetry, chronopotentiometry and electrochemical impedance spectroscopy. The electrode material shows a maximum specific capacitance of 454 F/g with pseudocapacitive behavior. High capacitance retention of electrode material over 1000 continuous charging-discharging cycles suggests its excellent electrochemical stability. The results revealed that the nickel ferrite electrode is a potential candidate for energy storage applications in supercapacitor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Growth temperature dependence of Si doping efficiency and compensating deep level defect incorporation in Al{sub 0.7}Ga{sub 0.3}N

The growth temperature dependence of Si doping efficiency and deep level defect formation was investigated for n-type Al{sub 0.7}Ga{sub 0.3}N. It was observed that dopant compensation was greatly reduced with reduced growth temperature. Deep level optical spectroscopy and lighted capacitance-voltage were used to understand the role of acceptor-like deep level defects on doping efficiency. Deep level defects were observed at 2.34 eV, 3.56 eV, and 4.74 eV below the conduction band minimum. The latter two deep levels were identified as the major compensators because the reduction in their concentrations at reduced growth temperature correlated closely with the concomitant increase in free electron concentration. Possible mechanisms for the strong growth temperature dependence of deep level formation are considered, including thermodynamically driven compensating defect formation that can arise for a semiconductor with very large band gap energy, such as Al{sub 0.7}Ga{sub 0.3}N.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Design, development, and results from a charge-collector diagnostic for a toroidal electron plasma experiment

A suitable charge-collector has been designed and developed to estimate charge-content of electron plasmas in a Small Aspect Ratio Toroidal Experiment in a C-shaped trap (SMARTEX-C). The electrons are periodically injected and held in the trap with the aid of electrostatic end-fields and a toroidal magnetic field. After a preset “hold” time, the trapped charges are dumped onto a grounded collector (by gating it). As the charges flow along the magnetic field lines onto the collector, the integrated current gives the charge-content of the plasma at the instant of dump. In designing such a charge collector, several challenges peculiar to the geometry of the trap and the nature of the plasma had to be addressed. Instantaneous charge measurements synchronised with the E × B drift of the plasma, along with fast transit times of electrons to the collector (few 100 ns or less) (due to the low aspect ratio of the trap) essentially require fast gating of the collector. The resulting large capacitive transients alongside low charge content (few nC) of such plasmas further lead to increasing demands on response and sensitivity of the collector. Complete cancellation of such transients is shown to be possible, in principle, by including the return path in our measurement circuit but the “non-neutrality” of the plasma acts as a further impediment. Ultimately, appropriate shielding and measurement circuits allow us to (re)distribute the capacitance and delineate the paths of these currents, leading to effective cancellation of transients and marked improvement in sensitivity. Improved charge-collector has thus been used to successfully estimate the time evolution of total charge of the confined electron plasma in SMARTEX-C.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Temperature dependence of frequency dispersion in III–V metal-oxide-semiconductor C-V and the capture/emission process of border traps

This paper presents a detailed investigation of the temperature dependence of frequency dispersion observed in capacitance-voltage (C-V) measurements of III-V metal-oxide-semiconductor (MOS) devices. The dispersion in the accumulation region of the capacitance data is found to change from 4%–9% (per decade frequency) to ∼0% when the temperature is reduced from 300 K to 4 K in a wide range of MOS capacitors with different gate dielectrics and III-V substrates. We show that such significant temperature dependence of C-V frequency dispersion cannot be due to the temperature dependence of channel electrostatics, i.e., carrier density and surface potential. We also show that the temperature dependence of frequency dispersion, and hence, the capture/emission process of border traps can be modeled by a combination of tunneling and a “temperature-activated” process described by a non-radiative multi-phonon model, instead of a widely believed single-step elastic tunneling process.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electrical properties of GaAs metal–oxide–semiconductor structure comprising Al{sub 2}O{sub 3} gate oxide and AlN passivation layer fabricated in situ using a metal–organic vapor deposition/atomic layer deposition hybrid system

This paper presents a compressive study on the fabrication and optimization of GaAs metal–oxide–semiconductor (MOS) structures comprising a Al{sub 2}O{sub 3} gate oxide, deposited via atomic layer deposition (ALD), with an AlN interfacial passivation layer prepared in situ via metal–organic chemical vapor deposition (MOCVD). The established protocol afforded self-limiting growth of Al{sub 2}O{sub 3} in the atmospheric MOCVD reactor. Consequently, this enabled successive growth of MOCVD-formed AlN and ALD-formed Al{sub 2}O{sub 3} layers on the GaAs substrate. The effects of AlN thickness, post-deposition anneal (PDA) conditions, and crystal orientation of the GaAs substrate on the electrical properties of the resulting MOS capacitors were investigated. Thin AlN passivation layers afforded incorporation of optimum amounts of nitrogen, leading to good capacitance–voltage (C–V) characteristics with reduced frequency dispersion. In contrast, excessively thick AlN passivation layers degraded the interface, thereby increasing the interfacial density of states (D{sub it}) near the midgap and reducing the conduction band offset. To further improve the interface with the thin AlN passivation layers, the PDA conditions were optimized. Using wet nitrogen at 600 °C was effective to reduce D{sub it} to below 2 × 10{sup 12} cm{sup −2} eV{sup −1}. Using a (111)A substrate was also effective in reducing the frequency dispersion of accumulation capacitance, thus suggesting the suppression of traps in GaAs located near the dielectric/GaAs interface. The current findings suggest that using an atmosphere ALD process with in situ AlN passivation using the current MOCVD system could be an efficient solution to improving GaAs MOS interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2015

LaAlO{sub 3} thickness window for electronically controlled magnetism at LaAlO{sub 3}/SrTiO{sub 3} heterointerfaces

Complex-oxide heterostructures exhibit rich physical behavior such as emergent conductivity, superconductivity, and magnetism that are intriguing for scientific reasons as well as for potential technological applications. It was recently discovered that in-plane magnetism at the LaAlO{sub 3}/SrTiO{sub 3} (LAO/STO) interface can be electronically controlled at room temperature. Here, we employ magnetic force microscopy to investigate electronically controlled ferromagnetism at the LAO/STO interface with LAO thickness t varied from 4 unit cell (u.c.) to 40 u.c. Magnetic signatures are observed only within a thickness window 8 u.c. ≤ t ≤ 25 u.c. Within this window, the device capacitance corresponds well to the expected geometric value, while for thicknesses outside this window, the capacitance is strongly suppressed. The ability to modulate electronic and magnetic properties of LAO/STO devices depends on the ability to control carrier density, which is in turn constrained by intrinsic tunneling mechanisms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Giant magnetoelectric coupling interaction in BaTiO{sub 3}/BiFeO{sub 3}/BaTiO{sub 3} trilayer multiferroic heterostructures

Multiferroic trilayer thin films of BaTiO{sub 3}/BiFeO{sub 3}/BaTiO{sub 3} were prepared by RF-magnetron sputtering technique at different thicknesses of BiFeO{sub 3} layer. A pure phase polycrystalline growth of thin films was confirmed from X-ray diffraction results. The film showed maximum remnant electric polarization (2P{sub r}) of 13.5 μC/cm{sup 2} and saturation magnetization (M{sub s}) of 61 emu/cc at room temperature. Thermally activated charge transport dominated via oxygen vacancies as calculated by their activation energy, which was consistent with current–voltage characteristics. Magnetic field induced large change in resistance and capacitance of grain, and grain boundary was modeled by combined impedance and modulus spectroscopy in the presence of varied magnetic fields. Presence of large intrinsic magnetoelectric coupling was established by a maximum 20% increase in grain capacitance (C{sub g}) with applied magnetic field (2 kG) on trilayer having 20 nm BiFeO{sub 3} layer. Substantially higher magnetoelectric coupling in thinner films has been observed due to bonding between Fe and Ti atoms at interface via oxygen atoms. Room temperature magnetoelectric coupling was confirmed by dynamic magnetoelectric coupling, and maximum longitudinal magnetoelectric coupling of 515 mV/cm-Oe was observed at 20 nm thickness of BiFeO{sub 3}. The observed magnetoelectric properties are potentially useful for novel room temperature magnetoelectric and spintronic device applications for obtaining higher voltage at lower applied magnetic field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Development and preliminary results of radio frequency ion source

A radio frequency (RF) ion source was designed and developed for neutral beam injector. A RF driver test bed was used with a RF generator with maximum power of 25 kW with 1 MHz frequency and a matching box. In order to study the characteristic of RF plasma generation, the capacitance in the matching box was adjusted with different cases. The results show that lower capacitance will better the stability of the plasma with higher RF power. In the future, new RF coils and matching box will be developed for plasma generators with higher RF power of 50 kW.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗