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Showing papers in "Journal of Applied Physics in 2002"


Journal ArticleDOI
TL;DR: In this article, a spin-coated polymer gate dielectric layer was used to obtain a polyvinylphenol-based copolymer-based transistor with a carrier mobility as large as 3 cm2/V's and sub-threshold swing as low as 0.5 V/decade.
Abstract: We have fabricated pentacene organic thin film transistors with spin-coated polymer gate dielectric layers, including cross-linked polyvinylphenol and a polyvinylphenol-based copolymer, and obtained devices with excellent electrical characteristics, including carrier mobility as large as 3 cm2/V s, subthreshold swing as low as 1.2 V/decade, and on/off current ratio of 105. For comparison, we have also fabricated pentacene transistors using thermally grown silicon dioxide as the gate dielectric and obtained carrier mobilities as large as 1 cm2/V s and subthreshold swing as low as 0.5 V/decade.

1,225 citations


Journal ArticleDOI
TL;DR: In this article, a transient hot-wire method was used to investigate the thermal conductivity of Al2O3 nanoparticles with specific surface areas in a range of 5 −124 m2 g−1.
Abstract: Various suspensions containing Al2O3 nanoparticles with specific surface areas in a range of 5–124 m2 g−1 have been prepared and their thermal conductivities have been investigated using a transient hot-wire method. Nanoparticle suspensions, containing a small amount of Al2O3, have substantially higher thermal conductivity than the base fluid, with the enhancement increasing with the volume fraction of Al2O3. The enhanced thermal conductivity increases with an increase in the difference between the pH value of aqueous suspension and the isoelectric point of Al2O3 particle. For the suspensions using the same base fluid, the thermal conductivity enhancements are highly dependent on specific surface area (SSA) of nanoparticle, with an optimal SSA for the highest thermal conductivity. For the suspensions containing the same nanoparticles, the enhanced thermal conductivity ratio is reduced with the increasing thermal conductivity of the base fluid. The crystalline phase of the nanoparticles appears to have no ...

939 citations


Journal ArticleDOI
TL;DR: In this paper, a method to realize multiple electron-hole pair generation per incident photon is proposed, and the theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell's band gap using detailed balance calculations.
Abstract: One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, whi...

938 citations


Journal ArticleDOI
TL;DR: In this paper, single-fiber composites were fabricated to examine the influence of local nanotube reinforcement on load transfer at the fiber/matrix interface, and the results indicated that the nanocomposite reinforcement improves interfacial load transfer.
Abstract: Carbon nanotubes were grown directly on carbon fibers using chemical vapor deposition. When embedded in a polymer matrix, the change in length scale of carbon nanotubes relative to carbon fibers results in a multiscale composite, where individual carbon fibers are surrounded by a sheath of nanocomposite reinforcement. Single-fiber composites were fabricated to examine the influence of local nanotube reinforcement on load transfer at the fiber/matrix interface. Results of the single-fiber composite tests indicate that the nanocomposite reinforcement improves interfacial load transfer. Selective reinforcement by nanotubes at the fiber/matrix interface likely results in local stiffening of the polymer matrix near the fiber/matrix interface, thus, improving load transfer.

826 citations


Journal ArticleDOI
TL;DR: In this article, an up-converter is located behind a solar cell and absorbs transmitted sub-band-gap photons via sequential ground state absorption/excited state absorption processes in a three-level system.
Abstract: A system for solar energy conversion using the up-conversion of sub-band-gap photons to increase the maximum efficiency of a single-junction conventional, bifacial solar cell is discussed. An up-converter is located behind a solar cell and absorbs transmitted sub-band-gap photons via sequential ground state absorption/excited state absorption processes in a three-level system. This generates an excited state in the up-converter from which photons are emitted which are subsequently absorbed in the solar cell and generate electron-hole pairs. The solar energy conversion efficiency of this system in the radiative limit is calculated for different cell geometries and different illumination conditions using a detailed balance model. It is shown that in contrast to an impurity photovoltaic solar cell the conditions of photon selectivity and of complete absorption of high-energy photons can be met simultaneously in this system by restricting the widths of the bands in the up-converter. The upper limit of the ene...

785 citations


Journal ArticleDOI
TL;DR: In this article, alternating current and direct current (DC) conductivities have been measured in polymer-nanotube composite thin films for a range of concentrations of multi-wall nanotubes in two polymer hosts.
Abstract: Alternating current (ac) and direct current (dc) conductivities have been measured in polymer-nanotube composite thin films. This was carried out for a range of concentrations of multiwall nanotubes in two polymer hosts, poly(m-phenylenevinylene-co-2,5-dioctyloxyp-phenylenevinylene) (PmPV) and polyvinylalcohol (PVA). In all cases the dc conductivity σDC was ohmic in the voltage range studied. In general the ac conductivity displayed two distinct regions, a frequency independent region of magnitude σ0 at low frequency and a frequency dependent region at higher frequency. Both σDC and σ0 followed a percolation scaling law of the form σ∝(p−pc)t with pc=0.055% by mass and t=1.36. This extrapolates to a conductivity of 1×10−3 S/m for 100% nanotube content. Such a low value reflects the presence of a thick polymer coating, resulting in poor electrical connection between tubes. This leads to the suggestion that charge transport is controlled by fluctuation induced tunneling. In the high frequency regime the cond...

748 citations


Journal ArticleDOI
TL;DR: In this paper, the authors discussed and experimentally verified how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD).
Abstract: This article discusses and experimentally verifies how to lower the operating voltage that drives liquid droplets by the principle of electrowetting on dielectric (EWOD). A significant contact angle change (120°→80°) is desired to reliably pump the droplet in microchannels for applications such as lab-on-a-chip or micrototal analysis systems. Typically, much higher voltages (>100 V) are used to change the wettability of an electrolyte droplet on a dielectric layer compared with a conductive layer. The required voltage can be reduced by increasing the dielectric constant and decreasing the thickness of the dielectric layer, thus increasing the capacitance of the insulating layer. This dependence of applied voltage on dielectric thickness is confirmed through EWOD experiments for three different dielectric materials of varying thickness: Amorphous fluoropolymer (Teflon® AF, Dupont), silicon dioxide (SiO2) and parylene. The dependence on the dielectric constant is confirmed with two different dielectric mate...

681 citations


Journal ArticleDOI
TL;DR: In this article, ordered microlens arrays with 10 μm diam poly-dimethyl-siloxane lenses attached to glass substrates increase the light output of organic light emitting devices (OLED) by a factor of 1.5 over unlensed substrates.
Abstract: We demonstrate that ordered microlens arrays with 10 μm diam poly-dimethyl-siloxane lenses attached to glass substrates increase the light output of organic light emitting devices (OLED) by a factor of 1.5 over unlensed substrates. The lenses, which are considerably smaller than, and not aligned to the OLEDs, outcouple light that is emitted outside of the escape cone of the substrate. We show that an electrophosphorescent device based on a fac tris(2-phenylpyridine)Iridium (Ir(ppy)3) doped emitting layer has its external quantum efficiency increased from 9.5% using a flat glass substrate, to 14.5% at low current densities using a substrate with microlenses. No change in the emission spectrum is observed for different viewing angles using the lens arrays.

675 citations


Journal ArticleDOI
TL;DR: In this article, a negative refractive index (NRI) metamaterial was proposed to support a backward cone of radiation, analogous to reversed Cherenkov radiation, which was demonstrated at microwave frequencies.
Abstract: A composite medium consisting of an array of fine wires and split-ring resonators has been previously used to experimentally verify a negative index of refraction. We present a negative refractive index (NRI) metamaterial that goes beyond the original split-ring resonator/wire medium and is capable of supporting a backward cone of radiation. We report experimental results at microwave frequencies that demonstrate backward-wave radiation from a NRI metamaterial—a characteristic analogous to reversed Cherenkov radiation. The conception of this metamaterial is based on a fresh perspective regarding the operation of NRI metamaterials.

541 citations


Journal ArticleDOI
TL;DR: In this paper, a Kramers-Kronig consistent analytical expression was developed to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) versus photon energy E for the alloys.
Abstract: We have developed a Kramers–Kronig consistent analytical expression to fit the measured optical functions of hydrogenated amorphous silicon (a-Si:H) based alloys, i.e., the real and imaginary parts of the dielectric function (e1,e2) (or the index of refraction n and absorption coefficient α) versus photon energy E for the alloys. The alloys of interest include amorphous silicon–germanium (a-Si1−xGex:H) and silicon–carbon (a-Si1−xCx:H), with band gaps ranging continuously from ∼1.30 to 1.95 eV. The analytical expression incorporates the minimum number of physically meaningful, E independent parameters required to fit (e1,e2) versus E. The fit is performed simultaneously throughout the following three regions: (i) the below-band gap (or Urbach tail) region where α increases exponentially with E, (ii) the near-band gap region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-band gap region where (e1,e2) can be simulated assuming a single ...

511 citations


Journal ArticleDOI
TL;DR: In this article, a comparison of metal-plated and bare Nafion and Flemion in various ion forms and various water saturation levels has been performed in the author's laboratories at the University of California, San Diego.
Abstract: Ionic polymer-metal composites (IPMCs) consist of a polyelectrolyte membrane (usually, Nafion or Flemion) plated on both faces by a noble metal, and is neutralized with certain counter ions that balance the electrical charge of the anions covalently fixed to the backbone membrane. In the hydrated state (or in the presence of other suitable solvents), the composite is a soft actuator and sensor. Its coupled electrical-chemical-mechanical response depends on: (1) the chemical composition and structure of the backbone ionic polymer; (2) the morphology of the metal electrodes; (3) the nature of the cations; and (4) the level of hydration (solvent saturation). A systematic experimental evaluation of the mechanical response of both metal-plated and bare Nafion and Flemion in various cation forms and various water saturation levels has been performed in the author’s laboratories at the University of California, San Diego. By examining the measured stiffness of the Nafion-based composites and the corresponding bare Nafion, under a variety of conditions, I have sought to develop relations between internal forces and the resulting stiffness and deformation of this class of IPMCs. Based on these and through a comparative study of the effects of various cations on the material’s stiffness and response, I have attempted to identify potential micromechanisms responsible for the observed electromechanical behavior of these materials, model them, and compare the model results with experimental data. A summary of these developments is given in the present work. First, a micromechanical model for the calculation of the Young modulus of the bare Nafion or Flemion in various ion forms and water saturation levels is given. Second, the bare-polymer model is modified to include the effect of the metal plating, and the results are applied to calculate the stiffness of the corresponding IPMCs, as a function of the solvent uptake. Finally, guided by the stiffness modeling and data, the actuation of the Nafion-based IPMCs is micromechanically modeled. Examples of the model results are presented and compared with the measured data.

Journal ArticleDOI
TL;DR: In this paper, the authors theoretically and numerically studied the dynamic properties of the vortex magnetic state in soft submicron ferromagnetic dots with variable thickness and diameter, and applied the equation of motion for the vortex collective coordinates.
Abstract: We have theoretically and numerically studied the dynamic properties of the vortex magnetic state in soft submicron ferromagnetic dots with variable thickness and diameter. To describe the vortex translation mode eigenfrequencies, we applied the equation of motion for the vortex collective coordinates. We calculated the vortex restoring force with an explicit account of the magnetostatic interaction on the bases of the “rigid” vortex and two-vortices “side charges free” models. The latter model well explains the results of our micromagnetic numerical calculations. The translation mode eigenfrequency is inversely proportional to the vortex static initial susceptibility and lies in GHz range for submicron in-plane dot sizes.

Journal ArticleDOI
TL;DR: In this paper, the absorption properties of a thermoplastic natural rubber (TPNR), a lithium-nickel-zinc (Li-Ni-Zn) ferrite and a TPNR-ferrite composite were investigated.
Abstract: Electromagnetic properties of a thermoplastic natural rubber (TPNR), a lithium–nickel–zinc (Li–Ni–Zn) ferrite and a TPNR–ferrite composite subjected to transverse electromagnetic (TEM) wave propagation were investigated. The incorporation of the ferrite into the matrix of the TPNR was found to reduce the dielectric loss but the magnetic loss increased. The absorption characteristics of all the samples subjected to a normal incidence of TEM wave were investigated based on a model of a single-layered plane wave absorber backed by a perfect conductor. It is evident from a computer simulation that the ferrite is a narrowband absorber, whereas the polymeric samples show broadband absorption characteristics. Minimal reflection of the microwave power or matching condition occurs when the thickness of the absorbers approximates an odd number multiple of a quarter of the propagating wavelength. This is discussed as due to cancellation of the incident and reflected waves at the surface of the absorbers. The Li–Ni–Z...

Journal ArticleDOI
TL;DR: In this paper, a decoupled mode-space solution was proposed for modeling electron transport in thin body, fully depleted, n-channel, silicon-on-insulator transistors in the ballistic limit.
Abstract: In this article, we present a computationally efficient, two-dimensional quantum mechanical simulation scheme for modeling electron transport in thin body, fully depleted, n-channel, silicon-on-insulator transistors in the ballistic limit. The proposed simulation scheme, which solves the nonequilibrium Green’s function equations self-consistently with Poisson’s equation, is based on an expansion of the active device Hamiltonian in decoupled mode space. Simulation results from this method are benchmarked against solutions from a rigorous two-dimensional discretization of the device Hamiltonian in real space. While doing so, the inherent approximations, regime of validity and the computational efficiency of the mode-space solution are highlighted and discussed. Additionally, quantum boundary conditions are rigorously derived and the effects of strong off-equilibrium transport are examined. This article shows that the decoupled mode-space solution is an efficient and accurate simulation method for modeling e...

Journal ArticleDOI
TL;DR: In this article, a parameterization for band-to-band Auger recombination in silicon at 300 K was proposed, which accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants.
Abstract: A parameterization for band-to-band Auger recombination in silicon at 300 K is proposed. This general parameterization accurately fits the available experimental lifetime data for arbitrary injection level and arbitrary dopant density, for both n-type and p-type dopants. We confirm that Auger recombination is enhanced above the traditional free-particle rate at both low injection and high injection conditions. Further, the rate of enhancement is shown to be less for highly injected intrinsic silicon than for lowly injected doped silicon, consistent with the theory of Coulomb-enhanced Auger recombination. Variations on the parameterization are discussed.

Journal ArticleDOI
TL;DR: In this article, a self-contained formalism for treating thermomechanical noise, Nyquist-Johnson noise, and adsorption-desorption noise was developed to estimate the impact that these noise processes will have on the noise of a model nanoscale resonator.
Abstract: Nanomechanical resonators can be fabricated to achieve high natural resonance frequencies, approaching 1 GHz, with quality factors in excess of 10^(4). These resonators are candidates for use as highly selective rf filters and as precision on-chip clocks. Some fundamental and some nonfundamental noise processes will present limits to the performance of such resonators. These include thermomechanical noise, Nyquist-Johnson noise, and adsorption-desorption noise; other important noise sources include those due to thermal fluctuations and defect motion-induced noise. In this article, we develop a self-contained formalism for treating these noise sources, and use it to estimate the impact that these noise processes will have on the noise of a model nanoscale resonator, consisting of a doubly clamped beam of single-crystal Si with a natural resonance frequency of 1 GHz.

Journal ArticleDOI
TL;DR: In this article, a framework for 2D quantum mechanical simulation of a nanotransistor/metal oxide field effect transistor is presented, which consists of the nonequilibrium Green's function equations solved self-consistently with Poisson's equation.
Abstract: Quantization in the inversion layer and phase coherent transport are anticipated to have significant impact on device performance in “ballistic” nanoscale transistors. While the role of some quantum effects have been analyzed qualitatively using simple one-dimensional ballistic models, two-dimensional (2D) quantum mechanical simulation is important for quantitative results. In this paper, we present a framework for 2D quantum mechanical simulation of a nanotransistor/metal oxide field effect transistor. This framework consists of the nonequilibrium Green’s function equations solved self-consistently with Poisson’s equation. Solution of this set of equations is computationally intensive. An efficient algorithm to calculate the quantum mechanical 2D electron density has been developed. The method presented is comprehensive in that treatment includes the three open boundary conditions, where the narrow channel region opens into physically broad source, drain and gate regions. Results are presented for (i) dr...

Journal ArticleDOI
TL;DR: In this paper, a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM) was presented, where stable and uniform magnetization states were observed in 0.3 μm×0.3
Abstract: We present here a magnetic tunnel junction device using perpendicular magnetization films designed for magnetic random access memory (MRAM). In order to achieve high-density MRAM, magnetic tunnel junction devices with a small area of low aspect ratio (length/width) is required. However, all MRAMs reported so far consist of in-plane magnetization films, which require an aspect ratio of 2 or more in order to reduce magnetization curling at the edge. Meanwhile, a perpendicular magnetic tunnel junction (pMTJ) can achieve an aspect ratio=1 because the low saturation magnetization does not cause magnetization curling. Magnetic-force microscope shows that stable and uniform magnetization states were observed in 0.3 μm×0.3 μm perpendicular magnetization film fabricated by focused-ion beam. In contrast, in-plane magnetization films clearly show the presence of magnetization vortices at 0.5 μm×0.5 μm, which show the impossibility of information storage. The PMTJ shows a magnetoresistive (MR) ratio larger than 50% w...

Journal ArticleDOI
TL;DR: In this paper, the authors considered a capacitvely coupled radio frequency discharge plasma generator, where the plasma is generated on the surface of a dielectric circuit board with electrode strips on the top and bottom.
Abstract: Modeling of fluid dynamics and the associated heat transfer induced by plasma between two parallel electrodes is investigated. In particular, we consider a capacitvely coupled radio frequency discharge plasma generator, where the plasma is generated on the surface of a dielectric circuit board with electrode strips on the top and bottom. The electrodes have a thickness of 100 μm, which is comparable to the height of the boundary layer. The regime considered is that the electron component is in the non-equilibrium state, and the plasma is nonthermal. Overall, due to the ion and large fluid particle interaction, the pressure is higher in the downstream of the electrode, causing the velocity structure to resemble that of a wall jet. Parameters related to the electrode operation, including the voltage, frequency, and free stream speed are varied to investigate the characteristics of the plasma-induced flow. Consistent with the experimental observation, the model shows a clear dependence of the induced jet velocity on the applied voltage and frequency. The heat flux exhibited a similar dependence on the strength of the plasma. The present plasma-induced flow concept can be useful for thermal management and active flow control.

Journal ArticleDOI
TL;DR: In this article, the electromagnetic resonances in split ring resonators (SRRs) around 1 GHz were investigated and it was shown that both electric and magnetic fields can induce resonances, the magnetic one being the strongest.
Abstract: We study experimentally and numerically the electromagnetic resonances in split ring resonators (SRRs), around 1 GHz. For an individual SRR, we show that both electric and magnetic fields can induce resonances, the magnetic one being the strongest. The utilization of such resonant structures as efficient microwave filter is also demonstrated. The coupling between two or more SRRs can be quite complex and strongly depends on their geometrical arrangement. For small separation distances, very strong coupling, leading to sharp resonances with high quality factors are observed. In that case a magnetic field circulation which connects neighboring elements is established. The practical implications of these results for the fabrication of a left-handed metamaterial are discussed.

Journal ArticleDOI
TL;DR: In this paper, the authors measured the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1×1017 to 3×1019 cm−3 at temperatures between 15 and 300 K.
Abstract: This work measures the thermal conductivities along free-standing silicon layers doped with boron and phosphorus at concentrations ranging from 1×1017 to 3×1019 cm−3 at temperatures between 15 and 300 K. The impurity concentrations are measured using secondary ion mass spectroscopy (SIMS) and the thermal conductivity data are interpreted using phonon transport theory accounting for scattering on impurities, free electrons, and the layer boundaries. Phonon-boundary scattering in the 3-μm-thick layers reduces the thermal conductivity of the layers at low temperatures regardless of the level of impurity concentration. The present data suggest that unintentional impurities may have strongly reduced the conductivities reported previously for bulk samples, for which impurity concentrations were determined from the electrical resistivity rather than from SIMS data. This work illustrates the combined effects of phonon interactions with impurities, free electrons, and material interfaces, which can be particularly...

Journal ArticleDOI
TL;DR: In this article, the dependence of the metal gate work function on the underlying gate dielectric in advanced metal-oxide-semiconductor (MOS) gate stacks was explored.
Abstract: The dependence of the metal gate work function on the underlying gate dielectric in advanced metal-oxide-semiconductor (MOS) gate stacks was explored. Metal work functions on high-κ dielectrics are observed to differ appreciably from their values on SiO2 or in vacuum. We applied the interface dipole theory to the interface between the gate and the gate dielectric of a MOS transistor and obtained excellent agreement with experimental data. Important parameters such as the slope parameters for gate dielectrics like SiO2, Al2O3, Si3N4, ZrO2, and HfO2 were extracted. In addition, we also explain the weaker dependence of n+ and p+ polysilicon gate work functions on the gate dielectric material. Challenges for gate work function engineering are highlighted. This work provides additional guidelines on the choice of gate materials for future MOS technology incorporating high-κ gate dielectrics.

Journal ArticleDOI
TL;DR: In this paper, energy transfer in efficient polymer electrophosphorescent organic light emitting diodes (PHOLEDs) using poly(9-vinylcarbazole) (PVK) host doped with one or more phosphorescent cyclometalated Ir(III) complexes was studied.
Abstract: We study energy transfer in efficient polymer electrophosphorescent organic light emitting diodes (PHOLEDs) using poly(9-vinylcarbazole) (PVK) host doped with one or more phosphorescent cyclometalated Ir(III) complexes. Single dopant double heterostructure PHOLEDs exhibited saturated color luminescence due to emissive triplet metal-to-ligand charge-transfer to ground state transitions of the Ir(III) dopants. Blue PHOLEDs, excited by an endothermic process from the host polymer, exhibited an emission maximum at a wavelength of λmax=474 nm, with an external quantum efficiency of ηext=1.3±0.1% and luminous power efficiency of ηp=0.8±0.1 lm/W. The green PHOLED exhibited ηext=5.1±0.1%, with ηext>2% for both red and yellow emission. Resonant energy transfer between green emitting fac-tris (2-phenylpyridyl)Ir(III) and red emitting bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)Ir(III) was observed to nearly double the efficiency of red emission when both dopants were simultaneously blended in a PVK ho...

Journal ArticleDOI
TL;DR: In this article, the polarization and strain behavior of Ba(Ti1−xZrx)O3 (x=0-03) ceramics are studied and the unipolar strain levels of ∼018% at 40 kV/cm and of ∼025% at ∼120 kv/cm with small hysteresis were obtained for the poramics with 003−xx⩽008.
Abstract: The polarization and strain behavior of Ba(Ti1−xZrx)O3 (x=0–03) ceramics are studied The unipolar strain levels of ∼018% at 40 kV/cm and of ∼025% at ∼120 kV/cm with small hysteresis were obtained for the ceramics with 003⩽x⩽008 The remnant polarization is ∼13–18 μC/cm2 for 003⩽x⩽008 and below 2 μC/cm2 for 015⩽x⩽03 at room temperature The electromechanical coupling coefficient K33=565% and the piezoelectric stain coefficient d33=236 pC/N were obtained for the ceramics with x=005 These results indicated that the Ba(Ti1−xZrx)O3 system is a promising lead-free high strain material for applications

Journal ArticleDOI
TL;DR: In this paper, the photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage, which can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.
Abstract: We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructure field effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.

Journal ArticleDOI
TL;DR: In this article, the role of power gain and power dissipation in quantum-dot cellular automata (QCA) was examined. But the authors focused on the energy flow in QCA devices.
Abstract: Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form molecular-scale computing elements. In the QCA paradigm information is represented by the charge configuration of a QCA cell. We develop a theoretical approach, based on the density matrix formalism, which permits examination of energy flow in QCA devices. Using a simple two-state model to describe the cell, and an energy relaxation time to describe the coupling to the environment, we arrive at an equation of motion well suited to the quasi-adiabatically switched regime. We use this to examine the role of power gain and power dissipation in QCA cells. We show that QCA cells can exhibit true signal power gain. The energy lost to dissipative processes is restored by the clock. We calculate the power dissipated to the environment in QCA circuits and show that it is possible to achieve the ultralow levels of power dissipation required at molecular densities.

Journal ArticleDOI
TL;DR: In this paper, the problem of obtaining homogeneously filled pore membranes is discussed in the framework of electrostatic instabilities taking into account the different potential contributions during electrodeposition.
Abstract: Highly ordered silver nanowire arrays have been obtained by pulsed electrodeposition in self-ordered porous alumina templates. Homogeneous filling of all the pores of the alumina template is achieved. The interwire distance is about 110 nm corresponding to a density of silver nanowires of 61×109 in.−2 and the diameter can be varied between 30 and 70 nm. The silver wires are monocrystalline with some twin lamella defects and grow perpendicular to the 〈110〉 direction. The previously encountered difficulty to obtain 100% filling of the alumina pores is discussed in the framework of electrostatic instabilities taking into account the different potential contributions during electrodeposition. To obtain homogeneously filled pore membranes, a highly conductive metal containing electrolyte, a homogeneous aluminum oxide barrier layer, and pulsed electrodeposition are a prerequisite.

Journal ArticleDOI
TL;DR: In this article, the electronic structure and band offsets of the oxides on Si have been calculated for many candidate gate oxides using the local density formalism pseudopotential method, where the predicted conduction band offsets are similar to those found earlier using the tight-binding method, and where available, with experimental values found by photoemission and internal photo emission.
Abstract: Oxides with higher dielectric constants are required to act as gate dielectrics for future generations of electronic devices. The electronic structure and band offsets of the oxides on Si have been calculated for many candidate gate oxides using the local density formalism pseudopotential method. The predicted conduction band offsets are similar to those found earlier using the tight-binding method, and where available, with experimental values found by photoemission and internal photoemission. The oxides which are acceptable as gate oxides in terms of their band offsets are ZrO2, HfO2, La2O3, Y2O3, Al2O3, silicates such as ZrSiO4 and HfSiO4 and aluminates such as LaAlO3.

Journal ArticleDOI
TL;DR: In this paper, a broad classification scheme for half-metallic ferromagnets is proposed, which embraces the possibilities of itinerant and localized electrons, as well as semimetallic and semiconducting electronic structure.
Abstract: A broad classification scheme is proposed for half-metallic ferromagnets which embraces the possibilities of itinerant and localized electrons, as well as semimetallic and semiconducting electronic structure. Examples of each type are given. The problems of defining and measuring spin polarization are discussed and some characteristics of half-metals are reviewed with reference to chromium dioxide.

Journal ArticleDOI
TL;DR: In this article, the structure of activated carbon materials is schematically classified into two parts: sharp G1 and G2 peaks and broad G2 and D2 peaks, sp2 clusters like a-Cs with bond angle disorder.
Abstract: Raman spectra of activated carbon materials have been investigated by a peak-deconvolution technique. It has been found as a result of our fitting trials that four Gaussians and a linear background are necessary and enough to reproduce the spectral data throughout our experiment, with a pair of relatively sharp peaks at about 1600 cm−1 and about 1350 cm−1, namely G1 and D1, and a pair of relatively broad peaks around 1560 cm−1 and 1340 cm−1, namely G2 and D2. From the characteristic behavior of these paired peaks, it has been concluded that the structure of activated carbon materials is schematically classified into two parts. The former part, represented by sharp G1 and G2 peaks, arises from winding short basal planes with bond angle order, while the latter, by broad G2 and D2 peaks, sp2 clusters like a-Cs with bond angle disorder. We have found the peak intensity ratio of G2 to G1, the I(G1)/I(G2) ratio, as a useful parameter expressing the relative content of the disorder part to the order part in the ...