bluuebunny/crossref_metadata_2025 huggingface dataset:
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noise in positive-negative-positive (PNP) polycrystalline silicon-emitter bipolar transistors
Authors: Hoque Md Mazhar Ul , Celik-Butler Zeynep , Trogolo Joe , Weiser Douglas , Green Keith
Published: March 2005

The origin of 1∕f fluctuations in positive-negative-positive (PNP) polycrystalline silicon-emitter bipolar-junction transistors is described. The interfacial oxide (IFO) at the monosilicon–polycrystalline silicon interface is found to significantly affect the noise behavior. The low-frequency noise originates from two independent fluctuation mechanisms: in the diffusion and tunneling components of the base current noise power spectral density (SIB) and from the diffusion current and carrier number fluctuations in the collector current noise power spectral density (SIC). The Hooge noise parameters for electrons and holes are calculated from the diffusion fluctuation models for SIB and SIC, respectively. Noise measurements on devices with different sizes and different IFO thicknesses indicate that the fluctuations occur in the minority-carrier (electron) tunneling current component of SIB through the IFO. The thickness of the IFO is estimated using this noise model. The tunneling fluctuations dominate over the diffusion fluctuations for the smaller (0.7×0.7μm2) transistors, while the opposite is the case for the larger (0.7×100μm2) ones. The scaling effect on the noise performance of these transistors is discussed. The effect of the IFO on the dc characteristics and the noise behavior of the PNP transistors is compared to that of the negative-positive-negative (NPN) counterparts on the same wafer.
Charge transfer driven by electron correlation: A non-Dyson propagator approach
Authors: Hennig Holger , Breidbach Jörg , Cederbaum Lorenz S.
Published: March 2005

A hole charge created in a molecular system, for instance, by ionization, can migrate through the system solely driven by electron correlation. This charge transfer due to electron correlation is referred to as charge migration. We introduce in this work a new ab initio method analyzing charge migration due to electron correlation in molecules. This method, a third-order “non-Dyson” propagator approach, aims in the long run, in particular, at the calculation of charge migration in relatively large molecules such as oligopeptides. First results of the new non-Dyson method are compared with a previously used propagator approach.
Growth of single quantum dots on preprocessed structures: Single photon emitters on a tip
Authors: Zwiller V. , Aichele T. , Hatami F. , Masselink W. T. , Benson O.
Published: March 2005

We have grown InP quantum dots in GaInP by gas-source molecular-beam epitaxy on sharp tips etched out of a GaAs substrate. The growth parameters were optimized to produce single quantum dots on the tips, predominantly at the edges. Intensity correlation measurements of the fluorescence from single dots on these tips reveal antibunched photon emision. We propose to use these tip structures with stable single photon emitters at their apex as active probes for further controlled experiments in quantum and nano-optics.
Correlation between the crystalline environment and optical property of Mn2+ ions in ZnGa2O4: Mn2+ phosphor
Authors: Kim Jong Su , Kim Jin Su , Kim Tae Wan , Kim Sun Myung , Park Hong Lee
Published: March 2005

Sol-gel-driven ZnGa2O4:Mn2+ phosphors are annealed at various temperatures. As an increase of the annealing temperature of as-grown ZnGa2O4:Mn2+ phosphors, their lattice constants and crystallinities are increased. The resonance signal intensity of Mn2+ ions on Td sites is drastically increased whereas that of Mn2+ ions on Oh sites is decreased as the annealing temperature is increased. The decay curves are well fitted with two simple exponential decay functions indicating the existence of two origins. Liquid nitrogen-temperature emission spectra show two emission peaks suggesting that they originate from Mn2+ ions on Td and on Oh sites. The emission intensity of Mn2+ ions on Td sites is drastically increased whereas that of Mn2+ ions on Oh sites is decreased. The variation of the optical characteristics agrees with the concentration of Mn2+ ions at Td site and its site environment.
Polarization switching kinetics of epitaxial Pb(Zr0.4Ti0.6)O3 thin films
Authors: So Y. W. , Kim D. J. , Noh T. W. , Yoon Jong-Gul , Song T. K.
Published: March 2005

The polarization switching kinetics of epitaxial Pb(Zr0.4Ti0.6)O3 thin films were investigated by measuring write pulse-width dependences of switched polarization ΔP(t) under various applied electric fields. It was found that ΔP(t) follows the predictions of the Kolmogorov–Avrami–Ishibashi model quite well. However, the detailed behaviors of ΔP(t) were different for low and high electric field regions, which separated out around 220kV∕cm. The coercive field also showed different frequency dependences depending on the region. These differences were attributed to changes in the polarization switching kinetics. A possible origin of the switching kinetics changes is also discussed.
Nuclear-magnetic-resonance diffusion simulations in porous media
Authors: Valfouskaya A. , Adler P. M. , Thovert J.-F. , Fleury M.
Published: March 2005

Nuclear-magnetic-resonance (NMR) diffusion simulations were performed numerically in unconsolidated and consolidated porous media. The curve of the apparent diffusion coefficient as a function of time was found to be a poor indicator of the pore structure. Various representations were tried to gather the data. A successful one uses a combination of surface-to-volume ratio, formation factor, and permeability; it can be used in different ways in order to derive these parameters from NMR measurements.
On the generation of a spiral-type vortex breakdown in an enclosed cylindrical container
Authors: Lim T. T. , Cui Y. D.
Published: March 2005

Earlier experimental studies [see H. U. Vogel, “Experimentelle Ergebnisse über die Laminare Strömung in einem zylindrischen Gehäuse mit darin rotierender Scheibe,” Max-Planck-Inst. Bericht 6 (1968) and M. P. Escudier, “Observations of the flow produced in cylindrical container by a rotating endwall,” Exp. Fluids 2, 189 (1984)] showed that vortex breakdown in an enclosed cylindrical container with one rotating endwall could exhibit either one, two, or three recirculating bubbles depending on the combination of Reynolds number Re and aspect ratio H∕R, at least for H∕R⩽3.5. However, a recent numerical study by Serre and Bontoux [“Vortex breakdown in a three-dimensional swirling flow,” J. Fluid Mech. 459, 347 (2002)] at H∕R=4.0 showed that under some conditions, an S-shape vortex structure follows by a spiral-type vortex breakdown could also be produced. This finding is most interesting because, to the best of our knowledge, a spiral-type vortex breakdown in an enclosed cylindrical container has not been produced previously in experiments. The aim of the present investigation is to verify if these vortex structures can be produced under laboratory conditions. Experiments conducted using dye visualization techniques confirm their existence, even for H∕R as low as 3.65. However, attempts to produce similar vortex structures in low aspect ratio cases (H∕R⩽2.5) by “artificially” introducing three dimensionality into the flow was less successful. Here, the confined environment makes a bubble-type vortex breakdown extremely robust, and the imposed asymmetry merely distorts the bubble geometry. The present study also reveals that the formation of a bubble-type vortex breakdown is associated with helical instability of decreasing wavelength, and the S-shape structure is linked with a similar instability of increasing wavelength.
Size effects in PbTiO3 nanocrystals: Effect of particle size on spontaneous polarization and strains
Authors: Akdogan E. K. , Rawn C. J. , Porter W. D. , Payzant E. A. , Safari A.
Published: March 2005

The spontaneous polarization (Ps) and spontaneous strains (xi) in mechanically unclamped and surface charge compensated PbTiO3 nanocrystals were determined as a function of particle size in the range <150nm by differential scanning calorimetry and x-ray powder diffraction, respectively. Significant deviations from bulk order parameters (P,xi) have been observed as the particle size decreased below ∼100nm. The critical size (rc) below which the ferroelectric tetragonal phase transforms to the paraelectric cubic phase was determined as ∼15nm. The depression in transition temperature with particle size is 14 °C at 28 nm. No change in the order of m3m→4mm ferrodistortive phase transition is observed. A simple analysis showed that ΔHtr∕(kBT)∼103 at 25 °C for r=16nm, indicating that the stabilization of the cubic phase at rc cannot be linked to an instability in dipolar ordering due to thermal agitations. Comparison of the spontaneous volumetric strains with the strain induced by surface stress indicated that the effect of surface stress on ferroelectric phase stability was negligible. Anomalies in electrostrictive properties were determined for r→rc. The observed size dependence of PS is attributed to the reduced extent of long-range dipole–dipole interactions that arise due to the changes in bonding characteristics of ions with decreasing particle size in the perovskite lattice, in conformity with a recent study by Tsunekawa et al. [Phys. Rev. Lett. 85 (16), 4340 (2000)].
Large upper critical field and irreversibility field in MgB2 wires with SiC additions
Authors: Sumption M. D. , Bhatia M. , Rindfleisch M. , Tomsic M. , Soltanian S. , Dou S. X. , Collings E. W.
Published: March 2005

Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.
Photoluminescence of silicon quantum dots in silicon nitride grown by NH3 and SiH4
Authors: Kim Baek-Hyun , Cho Chang-Hee , Kim Tae-Wook , Park Nae-Man , Sung Gun Yong , Park Seong-Ju
Published: March 2005

The photoluminescence (PL) property of crystalline silicon quantum dots (Si QDs) in silicon nitride grown by using ammonia and silane gases is reported. The peak position of PL could be controlled in the wavelength range from 450 to 700 nm by adjusting the flow rates of ammonia and silane gases. The PL intensity of Si QDs grown by ammonia was more intense compared to that of Si QDs grown by nitrogen gas. To investigate the role of hydrogen in the PL enhancement, the Si QDs grown by nitrogen gas were postannealed under hydrogen ambient. The enhancement in PL intensity was attributed to the hydrogen passivation of dangling bonds related to silicon and/or nitrogen at the interface of Si QDs and silicon nitride.
Erbium-doped and Raman microlasers on a silicon chip fabricated by the sol–gel process
Authors: Yang Lan , Carmon Tal , Min Bumki , Spillane Sean M. , Vahala Kerry J.
Published: March 2005

We report high-Q sol–gel microresonators on silicon chips, fabricated directly from a sol–gel layer deposited onto a silicon substrate. Quality factors as high as 2.5×107 at 1561nm were obtained in toroidal microcavities formed of silica sol–gel, which allowed Raman lasing at absorbed pump powers below 1mW. Additionally, Er3+-doped microlasers were fabricated from Er3+-doped sol–gel layers with control of the laser dynamics possible by varying the erbium concentration of the starting sol–gel material. Continuous lasing with a threshold of 660nW for erbium-doped microlaser was also obtained.
Hollow nanostructures based on the Kirkendall effect: Design and stability considerations
Authors: Tu K. N. , Gösele U.
Published: March 2005

In nanoscale interdiffusion and reaction, a Kirkendall void in the core of a nanocrystal has been proposed to explain the formation of hollow nanosize particles in recent literature. We present here a simple kinetic analysis of the reaction in nanoparticles, including the effect of the Gibbs–Thomson potential. A general discussion on how to form hollow nanostructures utilizing the mechanism of the Kirkendall effect is given. Furthermore, we show that a hollow nanosize particle is thermodynamically unstable; a high-temperature aging will drive thermal vacancies to the outer surface and transform the hollow nanocrystal to a solid nanocrystal.
In situ Raman imaging of interdiffusion in a microchannel
Authors: Salmon Jean-Baptiste , Ajdari Armand , Tabeling Patrick , Servant Laurent , Talaga David , Joanicot Mathieu
Published: March 2005

This letter presents an experimental setup to probe the interdiffusion of various miscible and nonreacting liquids. A Raman confocal microscope allows us to image the local concentrations of two coflowing liquids in a microchannel. These steady-state measurements provide precise quantitative information about the kinetics of the interdiffusion process. For all the pairs of fluids investigated, we found this process to follow a classical diffusive scaling law with a interdiffusion coefficient D that depends on both liquids.
Interference effects in electromodulation spectroscopy applied to GaAs-based structures: A comparison of photoreflectance and contactless electroreflectance
Authors: Kudrawiec R. , Sitarek P. , Misiewicz J. , Bank S. R. , Yuen H. B. , Wistey M. A. , Harris James S.
Published: March 2005

In this letter, we show that the oscillation features (OFs) usually observed in photoreflectance (PR) spectra of GaAs-based structures grown on the n-type GaAs substrate below the GaAs fundamental gap could be eliminated completely by applying the contactless electroreflectance (CER) instead of PR. This finding confirms that the origin of OFs is the modulation of the refractive index in the sample due to the generation of additional carriers by the modulated pump beam. In the case of CER spectroscopy, any additional carriers are not generated during the modulation hence CER spectra are free of OFs. This advantage of CER spectroscopy is very important in investigations of all structures for which OFs are present in PR spectra. In order to illustrate this advantage of CER spectroscopy we show PR and CER spectra measured first for the GaAs epilayer and next for more complicated steplike GaInNAsSb∕GaNAs∕GaAs quantum well structures.
Generation of picosecond acoustic pulses using a p-n junction with piezoelectric effects
Authors: Lin Kung-Hsuan , Yu Cheng-Ta , Wen Yu-Chieh , Sun Chi-Kuang
Published: March 2005

We demonstrate the generation of picosecond acoustic pulses using a piezoelectric-semiconductor-based p-n junction structure. This p-n junction picosecond ultrasonic experiment confirms that the piezoelectric effect dominates the thermal expansion and deformation-potential coupling in the generation of picosecond acoustic pulses. The characteristics of the p-n initiated acoustic pulses are determined by the width and the field strength inside the depletion region. Our study indicates the future possibility to electrically control the acoustic pulse characteristics if we could apply an external bias to modulate the depletion region width.
Unsteady flow evolution in swirl injector with radial entry. I. Stationary conditions
Authors: Wang Shanwu , Hsieh Shih-Yang , Yang Vigor
Published: March 2005

The vortical flow dynamics in a gas-turbine swirl injector were investigated by means of large eddy simulations. The flow enters the injector through three sets of radial-entry, counter-rotating swirl vanes. The formulation treats the Favre-filtered conservation equations in three dimensions along with a subgrid-scale model, and is solved numerically using a density-based, finite-volume approach with explicit time marching. Several methods, including proper orthogonal decomposition, spectral analysis, and flow visualization, are implemented to explore the flow dynamics in the complex three-dimensional flowfields. Various underlying mechanisms dictating the flow evolution, such as vortex breakdown, the Kelvin–Helmholtz instability, and helical instability, as well as their interactions, are studied for different swirl numbers. The flowfield exhibits well-organized motion in a low swirl-number case, in which the vortex shedding arising from shear instabilities downstream of the guide vanes drives acoustic oscillations of the mixed first tangential and first radial mode. The flowfield, however, becomes much more complicated at high swirl numbers, with each sub-regime dominated by different structures and frequency contents.
Unsteady flow evolution in swirl injectors with radial entry. II. External excitations
Authors: Wang Shanwu , Yang Vigor
Published: March 2005

Our previous study on turbulent flows in a gas-turbine swirl injector was extended to explore the effects of externally impressed excitations on the unsteady flow evolution. Three-dimensional large-eddy simulations were conducted to investigate the responses of the injector flowfield by imposing periodical oscillations of the mass flow rate at the entrance over a wide range of frequencies. Results show that the impressed disturbances are decomposed and propagate in two different modes because of their distinct propagating mechanisms in swirl injectors. The flow oscillation in the streamwise direction travels in the form of acoustic wave, whereas the oscillation in the circumferential direction is convected downstream with the local flow velocity. The vortex breakdown is mainly controlled by the dynamics in the core region near the axis, not so much by the excitation in the main flow passage surrounding the central recirculation zone. External excitations only exert minor influences on the mean flow properties due to the broadband characteristics of the injector flow. One exception is in the outer shear-layer region when the forcing frequency matches the intrinsic frequency of vortex shedding, and the mixing process of two counter-rotating swirl flows is considerably enhanced. The dynamic response of the injector flow, however, depends significantly on the forcing frequency in terms of the acoustic admittance and the mass transfer functions. Energy can be transferred among the various structures in the flowfield under external excitations, causing highly nonuniform spatial and temporal distributions of the oscillatory flow properties at the injector exit. The mass transfer function between the injector exit and entrance at the forcing frequency could be substantially greater than unity when the disturbance resonates with the injector flow. The injector essentially acts as an amplifier under this condition.
Structure of concentrated nanoemulsions
Authors: Graves S. , Meleson K. , Wilking J. , Lin M. Y. , Mason T. G.
Published: March 2005

We use extreme shear to create a dispersion of nanoscale droplets of silicone oil in an immiscible water phase containing an anionic surfactant sodium dodecylsulfate. Using centrifugal size fractionation, we obtain nanoemulsions having a well-defined average radius of a=75nm. We measure the structure of concentrated nanoemulsions over a wide range of volume fractions, 0<ϕ<0.6, using small angle neutron scattering, and we determine the structure factor S(q) of disordered glassy dispersions of uniform deformable droplets interacting through screened surface charge repulsions. Although the low-q behavior of S(q,ϕ) resembles that predicted for hard spheres, the height of the primary peak does not. Instead, it exhibits a maximum as ϕ is increased. This difference cannot be explained solely by the droplet size polydispersity and is likely related to the deformability of the droplets that have been locked into a glassy structure.
Surface plasmon resonance in gold nanoparticle infiltrated dielectric opals
Authors: Romanov S. G. , Susha A. S. , Sotomayor Torres C. M. , Liang Z. , Caruso F.
Published: March 2005

Light reflectance in three-dimensional metallo-dielectric photonic crystals of polyelectrolyte-coated latex spheres infiltrated with gold nanoparticles has been studied. Broad directional reflectance bands associated with the surface plasmon resonance in the lattice of the gold nanoparticle shells are observed in a wavelength range well separated from the diffraction resonance of the opal lattice. Dependence of surface plasmon resonance spectra on the Au nanoparticle distribution has been demonstrated.
Magnetic wires with DNA cores: A magnetic force microscopy study
Authors: Nyamjav Dorjderem , Kinsella Joseph M. , Ivanisevic Albena
Published: March 2005

Magnetic force microscopy (MFM) has been employed to study Fe3O4 nanowires containing DNA cores. The MFM experiments confirmed that long DNA molecules templated with Fe3O4 nanoparticles form a magnetic wire. The components of wires containing particles with sizes below 10 nm were recorded to behave as single domain particles with out-of-plane magnetization. The MFM study showed that one can change the magnetization states of the particles using a magnetic tip. The properties of the magnetic wires with DNA cores make them an attractive material for future magnetostatic devices.
Efficient drift dominated photodiodes using defected materials
Authors: Sun Yanning , Yulius Aristo , Woodall Jerry M.
Published: March 2005

Lattice-mismatched crystal growth can result in a high density of defects, which usually degrades the performance of optoelectronic devices. We demonstrate a drift dominated photodiode, which has high responsivity even with defected material. A drift dominated InP grown on GaP substrate photodiode, with the 8% lattice mismatch, has been developed for InP on Si application. Meanwhile, a conventional p-i-n InP on GaP photodiode was also made as a control sample. Results show that with the same defected material, the drift dominated device has much higher quantum efficiencies than those of the p-i-n device, especially at short wavelengths. The internal quantum efficiencies of the drift dominated InP on GaP photodiode are higher than 70% in UV and visible region.
Optimality of the dynamic procedure for large-eddy simulations
Authors: Meyers Johan , Geurts Bernard J. , Baelmans Martine
Published: March 2005

We present a database analysis to obtain a precise evaluation of the accuracy limitations associated with the popular dynamic eddy-viscosity model in large-eddy simulation. We consider decaying homogeneous isotropic turbulence at two different Reynolds numbers, i.e., Reλ=50 and 100. The large-eddy simulation errors associated with the dynamic model are compared with those arising in the “static” Smagorinsky model. A large number of systematically varied simulations using the Smagorinsky model provides a detailed impression of the dependence of the total simulation error on (i) the spatial resolution and (ii) the resolution of the subgrid dissipation length. This error behavior also induces an “optimal refinement trajectory” which specifies the particular Smagorinsky parameter, in terms of the spatial resolution, for which the total error is minimal. In contrast, the dynamic model gives rise to a self-consistently determined “dynamic trajectory” that represents the dependence of the dynamic coefficient on the spatial resolution. This dynamic trajectory is compared with the optimal refinement trajectory as obtained from the full database analysis of the Smagorinsky fluid. It is shown that the dynamic procedure in which the top-hat test filter is adopted, predicts values for the eddy viscosity as function of resolution and Reynolds number, which quite closely follow the main trends established in the optimal refinement trajectory. Furthermore, a sensitivity analysis, including dependency on test-filter width and filter shape, is discussed. Total simulation errors, due to interacting discretization, and modeling errors associated with the dynamic procedure may be a factor 2 higher compared to the optimum; still the dynamic procedure represents one of the very few self-contained and efficient error-reduction strategies when increasing the spatial resolution.
Optical amplification in two-dimensional photonic crystals
Authors: Raineri F. , Vecchi G. , Cojocaru C. , Yacomotti A. M. , Seassal C. , Letartre X. , Viktorovitch P. , Raj R. , Levenson A.
Published: March 2005

Optical amplification is demonstrated in a two-dimensional photonic crystal. Amplification at 1575nm is observed in the direction normal to the plane of periodicity, by pumping the device at 810nm. A gain up to 27dB is measured with moderate pump fluence (50μJ∕cm2). Gain is shown to clamp as laser threshold is attained.
Large-volume and low-frequency atmospheric glow discharges without dielectric barrier
Authors: Shi J. J. , Kong M. G.
Published: March 2005

It is widely believed that, at low frequencies of 1–100kHz, the generation of atmospheric pressure glow discharges (APGD) requires a dielectric barrier added to at least one electrode. This letter reports the experimental observation of a uniform and stable APGD generated between two bare electrodes without a dielectric barrier over a wide frequency range from 20kHz to 260kHz. Below 70kHz, it is shown that preionization in the rising phase of the applied voltage is important and plasma generation occurs in the voltage-falling phase. Mechanism of barrier-free APGD is found to be different from both atmospheric dielectric-barrier discharges and radio-frequency APGD.
Electrical and optical properties of Mg-doped Al0.7Ga0.3N alloys
Authors: Nakarmi M. L. , Kim K. H. , Khizar M. , Fan Z. Y. , Lin J. Y. , Jiang H. X.
Published: March 2005

Mg-doped Al0.7Ga0.3N epilayers (∼1μm) were grown on an AlN/sapphire template by metalorganic chemical vapor deposition and the electrical and optical properties of these epilayers were studied. For optimized Mg-doped Al0.7Ga0.3N epilayers, we have obtained a resistivity around 105Ωcm at room temperature and confirmed p-type conduction at elevated temperatures(>700K) with a resistivity of about 40Ωcm at 800 K. From the temperature dependent Hall effect measurement, the activation energy of Mg acceptor is found to be around 400 meV for Al0.7Ga0.3N alloy. The optimized Mg-doped Al0.7Ga0.3N epilayers have been incorporated into the deep-ultraviolet (UV) (λ<300nm) light-emitting diode (LED) structures as an electron blocking layer. An enhancement in the performance of the UV LEDs was obtained. LEDs with peak emission wavelengths at 280 nm were fabricated with a circular geometry (300 μm disk diameter). Output power reached 0.35 mW at 20 mA and 1.1 mW at 150 mA dc current. The importance of Mg-doped Al0.7Ga0.3N alloys to suppress the long-wavelength emission components in deep-UV LEDs and the fundamental limit for achieving p-type Al-rich AlGaN alloys are also discussed.
Improved performance and stability of organic light-emitting devices with Al–Cu alloy cathode
Authors: Chen I.-Ching , Hwang Shiao-Wen , Chen Chin H.
Published: March 2005

We have developed a stable green organic electroluminescent (EL) device by using an Al–Cu alloy as a cathode, which has better performance and reliability than the device with an Al cathode. The device with an Al–Cu alloy cathode achieved an EL efficiency of 3.78cd∕A(2.18lm∕W) at 20mA∕cm2 and the operating voltage was 5.46 V, while device with Al cathode had an EL efficiency of 3.1cd∕A(1.34lm∕W) and 7.3 V at the same drive condition. The Al–Cu device achieved a 20% decay lifetime (t80) of 1234 h at an initial brightness of 756cd∕m2, which is twice the lifetime of conventional device with the state-of-the-art LiF∕Al cathode.
Instability of 0.4 nm carbon nanotubes in a zeolite template under ultraviolet laser irradiation
Authors: Bai Ming , Li Irene L. , Tang Z. K. , Xiao Xudong
Published: March 2005

The stability of 0.4 nm carbon nanotubes within a zeolite matrix under laser irradiation has been investigated using photoluminescence (PL) spectroscopy. It was found that in the weak excitation power regime the PL spectra have similar line shapes at different excitation wavelengths. When the excitation power is high for ultraviolet light at 325 nm, the PL spectral shape undergoes an irreversible change. This is in contrast to the case with 488 nm laser irradiation and indicates a permanent chemical change in the sample, probably due to photochemical effects.
Adsorption of surfactants onto the surface of a spherical rising bubble and its effect on the terminal velocity of the bubble
Authors: Takemura Fumio
Published: March 2005

Adsorption of surfactants onto the surface of a single rising bubble was estimated by numerically solving the Navier–Stokes equation and the convection-diffusion equations of the surfactants in the bulk and on the surface at steady state. Then, the effects of Reynolds number (Re) and bulk concentration of the surfactants on adsorption and on the terminal velocity of the bubble were also numerically studied. The results revealed that (a) adsorption of surfactants at the front of the bubble with respect to the rise direction is lower than that at the back, and the difference increases as Re increases and∕or the bulk concentration of the surfactants decreases, (b) the critical Re (Rec) at which a bubble behaves as a solid particle can be estimated, and (c) the averaged adsorption of surfactants coincides with the equilibrium adsorption corresponding to a zero net adsorption-desorption flux for ReRec.
Anisotropic thermal properties of nanochanneled alumina templates
Authors: Borca-Tasciuc D.-A. , Chen G.
Published: March 2005

Anodic porous alumina templates contain highly ordered nanochannels that are frequently used for fabrication of thermoelectric nanowires. Thermal properties of the templates are of high interest in thermoelectric device applications. This paper reports the temperature-dependent, anisotropic thermal conductivity and thermal diffusivity of 55 μm thick anodic template Anodisc from Whatman, Inc. of 0.02 μm pore diameter and ∼30% porosity. The 3ω technique was employed to carry out measurements of the effective thermal conductivity and thermal diffusivity in the direction parallel to the nanochannels axis, while thermal anisotropy of the sample was estimated theoretically using the Maxwell model. Measured effective thermal conductivity is between 0.54 W/mK at 80 K and 1.3 W/mK at 300 K. Measured thermal diffusivity decreases from 2.1×10−6m2s−1 at 80 K to 1.08×10−6m2s−1 at 300 K. A photothermoelectric technique was employed to measure independently the components of the thermal diffusivity along the direction parallel to the nanochannels axis and in the plane perpendicular to this direction. Reasonable agreements were found between the results provided by the two characterization techniques.
Chain of ellipsoids approach to the magnetic nanowire
Authors: Lu Xiancong , Ge Shihui , Jiang Lixian , Wang Xinwei
Published: March 2005

A chain of ellipsoids model based on “chain-of-spheres” is proposed to investigate the reversal mechanism of the magnetic nanowire. When the diameter of the individual nanowire is small enough we assume the nanowire to be a chain of rotational symmetric ellipsoids. We investigate the reversal mechanism by this model and derive the expression of the switching field. By the numerical method, the magnetization loops and energy morphology of an irreversible rotation process are obtained, and the relation between coercivity and shape factor of the ellipsoid is discussed. The angular dependence of the switching field of the Ni nanowire is calculated, and the results accord well with the experiment results of Wernsdorfer et al. [Phys. Rev. Lett. 77, 1873 (1996)] and J.-E. Wegrowe et al. [Phys. Rev. Lett. 82, 3681 (1999)]. Fe1−xCox alloy nanowire arrays over the whole composition range have been produced by the electrochemical method, and the composition dependence of coercivity is calculated by the chain of ellipsoids model. Detailed comparison of calculated curves with experimental results is made, and good agreement is acquired.
Pre-metallization processing effects on Schottky contacts to AlGaN∕GaN heterostructures
Authors: Bradley S. T. , Goss S. H. , Hwang J. , Schaff W. J. , Brillson L. J.
Published: March 2005

Changes in the Schottky barrier height of Ni on AlGaN∕GaN heterostructure field effect transistor structures are characterized by internal photoemission spectroscopy (IPE) as a function of pre-metallization processing conditions and postmetallization ultrahigh vacuum annealing. Low energy electron-excited nanoluminescence spectroscopy and mapping reveal AlGaN near band edge emission variations that correlate with IPE Schottky barrier height. Ni∕AlGaN interface impurities measured by secondary ion mass spectrometry are also correlated with IPE Schottky barrier height. We show that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. Interfacial oxygen and carbon have secondary but systematic effects as well.
Simulation of particle deposition beneath Faraday waves in thin liquid films
Authors: Saylor J. R. , Kinard A. L.
Published: March 2005

Simulations are presented of particle deposition beneath shallow, standing capillary waves. The standing waves occur on the surface of a thin liquid film. The film is sufficiently thin such that the wave motion affects the trajectories of the settling particles. The simulations reveal that particles of all size tend to accumulate at specific nodal regions beneath the waves, and that there is an optimal particle diameter for which this accumulation is maximized. The diameter at which this maximal accumulation occurs is a function of the wave field, the fluid thickness, the particle density, and the fluid properties. The accumulation is quantified by the standard deviation of the final location of the settling particles and plots are presented of this standard deviation as a function of particle diameter. Previous experimental work has shown that this method can be implemented by generating standing Faraday waves on the surface of thin films of a liquid∕particulate mixture. The present work shows how these films can be tailored via the particle diameter, liquid thickness, and wavelength. Some aspects relevant to the practical implementation of the method are also discussed.
Sedimentation of particles of general shape
Authors: Doi Masao , Makino Masato
Published: March 2005

When a set of complex shaped particles settles down under gravity, they will generally spread in vertical and horizontal directions even though there is no thermal motion since the velocity of complex shaped particle is not in the direction of gravity and depends on the initial orientation. We study the relationship between the dispersion behavior and the particle characteristics. It is shown by theory and simulation that the dispersion behavior can have various types depending on whether the particle is skewed or not or whether the gravity gives a torque or not. The conditions for these cases to happen is discussed and its validity is demonstrated by computer simulation.
Characterization of self-assembled InAs quantum dots with InAlAs∕InGaAs strain-reduced layers by photoluminescence spectroscopy
Authors: Chang K. P. , Yang S. L. , Chuu D. S. , Hsiao R. S. , Chen J. F. , Wei L. , Wang J. S. , Chi J. Y.
Published: March 2005

The optoelectronic characteristics of self-assembled InAs quantum dots (QDs) with strain-reduced layers (SRLs) were investigated using photoluminescence (PL) spectroscopy. Various SRLs that combine In0.14Al0.86As and In0.14Ga0.86As with the same total thickness were examined to ascertain their confining effect on carriers in InAs QDs. The emission wavelength is blueshifted as the thickness of InAlAs is increased. The energy separation between the ground state and the first excited state of QDs with InAlAs SRLs greatly exceeds that of QDs with InGaAs SRLs. Atomic force microscopic images and PL spectra of the QD samples demonstrated that high-quality InAs QDs with long emission wavelengths and a large energy separation can be generated by growing a low-temperature, thin InAlAs SRL onto self-assembled QDs.
Design and performance of a high-resolution frictional force microscope with quantitative three-dimensional force sensitivity
Authors: Dienwiebel M. , de Kuyper E. , Crama L. , Frenken J. W. M. , Heimberg J. A. , Spaanderman D.-J. , van Loon D. Glastra , Zijlstra T. , van der Drift E.
Published: March 2005

In this article, the construction and initial tests of a frictional force microscope are described. The instrument makes use of a microfabricated cantilever that allows one to independently measure the lateral forces in X and Y directions as well as the normal force. We use four fiber-optic interferometers to detect the motion of the sensor in three dimensions. The properties of our cantilevers allow easy and accurate normal and lateral force calibration, making it possible to measure the lateral force on a fully quantitative basis. First experiments on highly oriented pyrolytic graphite demonstrate that the microscope is capable of measuring lateral forces with a resolution down to 15pN.
Surface roughness in sulfur ion-implanted InP with molecular beam epitaxy regrown double-heterojunction bipolar transistor layers
Authors: Hu Ting-Chen , Chang M. F. , Weimann Nils , Chen Jianxin , Chen Young-Kai
Published: March 2005

We report on deep ion-implantation of sulfur into InP substrates to replace the epitaxial subcollector layer of double-heterojunction bipolar transistors. Using optimized implantation conditions of 350keV energy and 1×1015cm−2 dose, we achieved a subcollector sheet resistance of 15Ω∕square. Under well-controlled regrowth conditions a root-mean-square roughness of 12Å is measured from DHBT epitaxial layers grown on implanted InP substrates, comparable to DHBT epitaxial layers grown on n+ epiready unimplanted substrates. We observe a pronounced increase in surface roughness of epitaxial layer beyond a threshold ion dose, depending on implantation energy. Large-area DHBT devices result with sulfur-ion implanted subcollector shows similar characteristics compared to devices fabricated on n+-doped InP substrates.
Germanium diffusion and nanocrystal formation in silicon oxide on silicon substrate under rapid thermal annealing
Authors: Choi W. K. , Ho V. , Ng V. , Ho Y. W. , Ng S. P. , Chim W. K.
Published: March 2005

The effect of rapid thermal annealing temperature on the diffusion of silicon (Si) and germanium (Ge) and the formation of Ge nanocrystals in a silicon oxide matrix was investigated. The formation of Ge nanocrystals was attributed mainly to the reduction of Ge suboxides by Si diffused from the Si substrate. For samples annealed at 800°C, the nanocrystals were uniform in size and distributed evenly in the bulk of the oxide but became denser nearer to the silicon–silicon oxide (Si–SiO2) interface. When the sample was annealed at 900°C, two regions with different nanocrystal densities and size distributions separated by a region void of nanocrystals were observed. The region of denser nanocrystals was located near the Si–SiO2 interface. For annealing at 1000°C, nanocrystals were only observed at the Si–SiO2 interface and these have significant size variation, with the rest of the oxide being void of nanocrystals. The nanocrystals formed at 900 and 1000°C were generally found to be defective.
Reducing azimuthal domains in epitaxial ferroelectric lanthanum-substituted bismuth titanate films using miscut yttria-stabilized zirconia substrates
Authors: Lee Sung Kyun , Hesse Dietrich , Gösele Ulrich , Lee Ho Nyung
Published: March 2005

We report the effect of a definite miscut of yttria-stabilized zirconia (YSZ) (100) single-crystal substrates onto the number of azimuthal domain variants within epitaxial La-substituted Bi4Ti3O12 (BLT) ferroelectric thin films as well as within SrRuO3 electrode layers, both grown on these substrates. YSZ substrates with a miscut angle of 5° were studied, with two different directions of the miscut, viz., YSZ[001] and YSZ[011]. A reduction of the number of azimuthal domain variants by 50% was attained on substrates with a [011]-directed miscut. Due most probably to the reduced number of azimuthal domain boundaries, larger remanent polarization values were attained in BLT films when grown on miscut substrates.
Temperature dependence of the negative bias temperature instability in the framework of dispersive transport
Authors: Kaczer B. , Arkhipov V. , Degraeve R. , Collaert N. , Groeseneken G. , Goodwin M.
Published: March 2005

Negative bias temperature instability (NBTI) is studied in multiple-gate field-effect transistors with an ultrathin gate oxide. It is observed that the threshold voltage shift in these devices follows a power-law function of time, with the exponent depending linearly on temperature. An analytic model is proposed that explains this temperature dependence by dispersive diffusion of hydrogen in the bulk of the gate oxide. Based on both the experimental data and the model, it is concluded that NBTI is an inherently non-Arrhenius process.
Strain relaxation of epitaxial SrTiO3 thin films on LaAlO3 by two-step growth technique
Authors: Yamada Tomoaki , Astafiev Konstantin F. , Sherman Vladimir O. , Tagantsev Alexander K. , Muralt Paul , Setter Nava
Published: March 2005

Using pulsed-laser deposition, a two-step growth technique was applied to epitaxial SrTiO3 (STO) thin films on LaAlO3 substrates providing a way to obtain an effective strain relaxation in these films otherwise strained due to lattice mismatch between film and substrate. By changing the thickness of a first layer, deposited at a temperature as low as 100°C before the deposition of the main layer at 750°C, different strain relaxation states of the films could be systematically realized. With a 10-nm-thick first layer, an almost full strain relaxation at the deposition temperature of the main layer was achieved, suggesting a strong impact of this method on strain relaxation. The in-plane dielectric measurements displayed that the ferroelectric transition temperature increases with strain relaxation during the growth. This trend is correct and compatible with the theoretical prediction of the behavior of strained STO derived from Landau theory.
Oxygen tracer diffusion in Pb(Zr,Ti)O3 thin film enhanced by catalytic platinum
Authors: Cross J. S. , Kurihara K. , Kamehara N. , Haneda H. , Sakaguchi I.
Published: March 2005

The self-diffusion of O18 into Pb(Zr,Ti)O3 [PZT] thin films on Pt∕TiO2 coated Si wafers from an ambient of 99% O218 gas tracer was investigated by secondary ion mass spectroscopy (SIMS) when annealed at 450°C, 550°C, and 650°C for up to 30min. The results show that the O18 profile in PZT changed significantly with anneal temperature and the highest concentration of exchanged oxygen was at the PZT/Pt interface when annealed at 650°C. Modeling of the oxygen tracer diffusion profiles from the 450°C and 550°C data yielded an activation energy of 0.83eV when assuming 1D diffusion. Simulation of the 650°C SIMS data indicated that the oxygen was dissociating on the catalytic Pt film underneath the PZT and then back diffusing into the PZT causing a higher concentration of oxygen at the PZT/Pt interface than at the surface.
Band-gap discontinuity in GaN0.02As0.87Sb0.11∕GaAs single-quantum wells investigated by photoreflectance spectroscopy
Authors: Kudrawiec R. , Ryczko K. , Misiewicz J. , Yuen H. B. , Bank S. R. , Wistey M. A. , Bae H. P. , Harris James S.
Published: March 2005

Ga N 0.02 As 0.87 Sb 0.11 ∕ Ga As single-quantum wells have been investigated by photoreflectance (PR) at room temperature. PR features related to the ground and excited state transitions have been clearly observed. The experimental data have been compared with the calculations in the envelope function formalism taking account the effect of strain. The band gap lowering and the increase in the electron effective mass due to the incorporation of nitrogen atoms into GaAsSb have been included. Excellent agreement between experimental data and calculation results have been found for band structure Type-I with the conduction-band offset ratio of 50%.
Precipitation of Ge nanoparticles from GeO2 glasses in transmission electron microscope
Authors: Jiang Nan , Qiu Jianrong , Spence John C. H.
Published: March 2005

We show, using spatially resolved energy loss spectroscopy in a transmission electron microscopy (TEM), that GeO2 and GeO2–SiO2 glasses are extremely sensitive to high energy electrons. Ge nanoparticles can be precipitated in GeO2 glasses efficiently by the high-energy electron beam of a TEM. This is relevant to TEM characterization of luminescent Ge nanoparticles in silicate glasses, which may produce artificial results.
Field approach to three-dimensional gene expression pattern characterization
Authors: Costa L. da F. , Travençolo B. A. N. , Azeredo A. , Beletti M. E. , Müller G. B. , Rasskin-Gutman D. , Sternik G. , Ibañes M. , Izpisúa-Belmonte J. C.
Published: March 2005

We present a vector field method for obtaining the spatial organization of three-dimensional patterns of gene expression based on gradients and lines of force obtained by numerical integration. The convergence of these lines of force in local maxima are centers of gene expression, providing a natural and powerful framework to characterize the organization and dynamics of biological structures. We apply this methodology to analyze the expression pattern of the enhanced green fluorescent protein (EGFP) driven by the promoter of light chain myosin II during zebrafish heart formation.
Tetracene light-emitting transistors on flexible plastic substrates
Authors: Santato C. , Manunza I. , Bonfiglio A. , Cicoira F. , Cosseddu P. , Zamboni R. , Muccini M.
Published: March 2005

We report on organic light-emitting (field-effect) transistors (LETs) fabricated on a flexible and transparent plastic foil (Mylar), acting both as substrate and gate dielectric. The foil is patterned on one side with bottom-contact gold source and drain electrodes, while a thin film of gold is evaporated on the opposite side of the foil to form the gate electrode. A vacuum sublimed tetracene film is employed as an active layer for charge transport and light emission. Atomic force microscopy shows that tetracene films have a good adhesion on Mylar and exhibit a granular structure. The transistor shows unipolar p-type behavior with mobilities typically of 5×10−4cm2∕Vs. Drain-source current and electroluminescence have been simultaneously measured. Provided a suitable gate bias is applied, light emission occurs at drain-source voltages (Vds) above saturation. LETs on plastic substrates could open the way to flexible devices combining the switching function of a transistor and the light emission.
First-principles studies of the intrinsic effect of nitrogen atoms on reduction in gate leakage current through Hf-based high-k dielectrics
Authors: Umezawa N. , Shiraishi K. , Ohno T. , Watanabe H. , Chikyow T. , Torii K. , Yamabe K. , Yamada K. , Kitajima H. , Arikado T.
Published: March 2005

The atomistic effects of N atoms on the leakage current through HfO2 high-k gate dielectrics have been studied from first-principles calculations within the framework of a generalized gradient approximation (GGA). It has been found that the intrinsic effects of N atoms drastically reduce the electron leakage current. N atoms couple favorably with oxygen vacancies (VO) in HfO2 and extract electrons from VO. As a result, VO energy levels are drastically elevated due to the charged-state change in VO from neutral (VO0) to positively charged (VO2+). Accordingly, N incorporation removes the electron leakage path mediated by VO related gap states.