bluuebunny/crossref_metadata_2025 huggingface dataset:
Tunable waveguiding in electrically programmable VO2-based photonic crystals
Authors: Xiao D. , Kim K. W. , Lazzi G. , Zavada J. M.
Published: June 2006

The feasibility of electrically programmable waveguiding in a photonic crystal (PC) is explored based on the metal-insulator transition of vanadium dioxide (VO2). Unlike the ordinary PCs, wave propagation in the desired structure may be switched on/off or redirected by applying an electrical bias on the selective electrodes by taking advantage of the electrically induced VO2 phase transition and subsequent modulation of dielectric properties. The characteristics of the two-dimensional VO2-based PCs with line defects are analyzed using the iterative plane wave and finite difference time domain methods. Particularly, the influence of the Drude relaxation on waveguiding is examined as the high rate typical for metallic VO2 can lead to the signal loss. An optimized structure is proposed to minimize the loss and simplify the fabrication.
Effects of rapid thermal annealing conditions on GaInNAs band gap blueshift and photoluminescence intensity
Authors: Liverini V. , Rutz A. , Keller U. , Schön S.
Published: June 2006

We have studied the effects of various conditions of rapid thermal annealing (RTA) on 10nm GaInNAs∕GaAs single quantum wells (SQWs) with fixed indium concentration and increasing nitrogen content to obtain photoluminescence (PL) in the telecom wavelength regime of 1.3 and 1.5μm. Specifically, we analyzed the results of annealing for a fixed short time but at different temperatures and for longer times at a fixed temperature. In all experiments, InGaAs SQWs with the same In concentration were used as references. For both RTA conditions, the well-known blueshift of the band gap energy and the PL intensity improvement show trends that reveal that these are unrelated effects. At high RTA temperatures the PL efficiency reaches a maximum and then drops independently of N content. On the contrary, the blueshift experiences a rapid increase up to 700°C (strong blueshift regime) and it saturates above this temperature (weak blueshift regime). Both these blueshift regimes are related to the nitrogen content in the SQWs but in different ways. In the strong blueshift regime, we could obtain activation energy for the blueshift process in the range of 1.25eV, which increases with N content. Analysis with high-resolution x-ray diffraction (HRXRD) shows that the blueshift experienced in this regime is not due to a stoichiometric change in the QW. In the weak blueshift regime, the blueshift, which is only partly due to In outdiffusion, saturates more slowly the higher the N content. Annealing at the same temperature (600°C) for a longer time shows that the blueshift saturates earlier than the PL intensity and that samples with higher nitrogen experience a larger blueshift. Only a small In outdiffusion for annealing at high temperatures (>650°C) and long duration was observed. However, this modest stoichiometric change does not explain the large blueshift experienced by the GaInNAs SQWs. We conclude that the mechanism responsible for the drastic blueshift after annealing is related to the N content in the QW, while the improvement in PL integrated intensity is uniquely related to the annealing conditions.
Aspects of the internal physics of InGaAs∕InAlAs quantum cascade lasers
Authors: Mc Tavish James , Indjin Dragan , Harrison Paul
Published: June 2006

We report on the results of our simulations of an InGaAs∕InAlAs midinfrared quantum cascade laser (QCL) designed to operate in continuous wave mode at room temperature [Beck et al., Science 295, 301 (2002)]. Our physical model of the device consists of a self-consistent solution of the subband population rate equations and accounts for all electron-longitudinal-optical phonon and electron-electron scattering rates, as well as an evaluation of the temperature of the nonequilibrium electron distribution. We also consider the role of the doping density and its influence on the electron dynamics. We found that the temperature of the nonequilibrium electron distribution differed significantly from the lattice temperature and that this temperature increased with applied electric field and current density, with coupling constants somewhat larger than analogous GaAs based midinfrared QCLs. Our simulations also reveal physical processes of the device that are not apparent from the experimental measurements, such as the role of electron-electron scattering.
High temperature Hall effect sensors based on AlGaN∕GaN heterojunctions
Authors: Lu Hai , Sandvik Peter , Vertiatchikh Alexei , Tucker Jesse , Elasser Ahmed
Published: June 2006

We report on AlGaN∕GaN heterojunction structures for use in Hall effect sensors working over a wide range of temperatures. Room temperature current-related magnetic sensitivity of 55V∕AT at a sheet resistance below 300Ω∕sq and very low temperature cross sensitivity of 103ppm∕°C up to 300°C were obtained for a square-shaped Hall effect sensor. The active layer of the Hall effect sensor is the two-dimensional electron gas formed at the Al0.3Ga0.7N and GaN heterointerface caused by the gradient in the total polarization between the AlGaN barrier and the GaN buffer layer, which results in the positive polarization induced interface charge attracting free electrons. The temperature-dependent transport properties of the heterojunction were analyzed by Hall measurement. The drop of its electron mobility from room temperature to 300°C is mainly due to the enhanced polar optical scattering, while the very stable sheet carrier density contributes to the excellent temperature cross sensitivity of the Hall effect sensor.
Investigation of the indium-boron interaction in silicon
Authors: Scalese S. , Grasso S. , Italia M. , Privitera V. , Christensen J. S. , Svensson B. G.
Published: June 2006

The interaction between indium and boron coimplanted in silicon has been investigated. In particular, the effects of the coimplantation on the diffusion and the electrical activation have been studied in comparison with the single B or In implanted samples. It is shown that, by means of coimplantation, it is possible to obtain p-type carrier concentration profiles with a concentration peak higher than the ones achievable by the single In doping, but with the further advantage of a shallower carrier distribution with respect to the single B implant. It is found that this is due to the formation of In–B complexes with an acceptor level deeper than the one related to substitutional B.
Electric response of DNA hairpins to magnetic fields
Authors: Yi Juyeon , Orland Henri
Published: June 2006

We study the electric properties of DNA molecules with hairpinlike shapes in the presence of a magnetic flux. It is shown that the current amplitude can be modulated by the applied field. The details of the electric response strongly depend on the twist angles. For geometries where the flux through the plaquette of the ladder can be canceled pairwise (commensurate twist), the current exhibits periodicity. Further twisting the geometry and changing its length causes complex aperiodic oscillations. We also study persistent currents. They reduce to simple harmonic oscillations if the system is commensurate, otherwise deviations occur due to the existence of closed paths leading to a washboard shape.
Growth, thermal stability, and magnetic properties of Co films on Ni3Al(001)
Authors: Van Eek Stella M. , Costina Ioan , Podgurski Vitali , David Rudolf , Franchy René
Published: June 2006

The growth, morphology, thermal stability, and the magnetic properties of cobalt deposited on a Ni3Al(001) surface were investigated by scanning tunneling microscopy (STM), low energy electron diffraction, Auger electron spectroscopy, and by the magneto-optical Kerr effect (MOKE). Cobalt grows pseudomorph on Ni3Al(001), showing flat islands by STM. Although the lattice mismatch between the substrate and the Co film is small, we observed that the layer by layer growth is only observed when depositing at higher temperatures. In situ MOKE measurements on Co coverages in the range of 0.55–12 monolayers (ML) were performed. Unexpectedly, cobalt films 0.77 ML thick show ferromagnetism already at room temperature. The easy axis of magnetization is oriented in the [110] direction and the hard axis in the [100] direction of the Ni3Al(001) surface, as expected for cubic Co. Out of plane is a hard direction of magnetization. After annealing between 540 and 700K an irreversible increase on the coercivity Hc of the films appears with respect to the as deposited films, and the saturation ellipticity shows no change. The increase of Hc is correlated with the coalescence of the Co islands observed after annealing by STM and the diffusion of impurities from the substrate into the Co film. During stepwise annealing, at about 550K, the room temperature value of Hc increases abruptly, indicating that 550K is the temperature needed to produce the coalescence of the Co islands. The Co film is stable on the Ni3Al(001) surface up to 750K. Beyond that temperature Co atoms start to diffuse into the substrate. At 1100K, Co disappears completely from the surface via diffusion into the Ni3Al substrate. The Curie temperature of the Co films is beyond 750K, the stability limit of the films.
Temperature and depth dependence of order in liquid crystal interfaces
Authors: Martínez-Miranda L. J. , Hu Y.
Published: June 2006

We have studied the depth dependence and temperature behavior of the ordering of smectic-A films close to the smectic A-nematic transition, deposited on grated glass. X-ray grazing incidence geometry in reflection mode through the glass substrate was used to characterize the samples. Our results indicate the presence of a structure similar to the helical twist grain boundary phase. The structure has two maxima, one close to the glass-liquid crystal interface and another about 8μm above the surface. The structure at 8μm is the one that dominates at higher temperatures. In addition, we find that order is preserved to temperatures close to the nematic-isotropic transition temperature for the deeper gratings. We find also a dependence of the orientation of the structure with the depth of the grating and the elastic constant of the liquid crystal.
Magnetic levitation of superconducting bars
Authors: Sanchez Alvaro , Del Valle Nuria , Pardo Enric , Chen Du-Xing , Navau Carles
Published: June 2006

Levitation force is studied in a two-dimensional system consisting of an infinitely long superconductor immersed in the magnetic field created by different arrangements of infinitely long parallel permanent magnets, based on the critical-state model using the magnetic energy minimization procedure. Results on force, stability, and losses in the levitation process in this system are analyzed.
Morphological, electrical, and optical properties of InN grown by hydride vapor phase epitaxy on sapphire and template substrates
Authors: Song D. Y. , Kuryatkov V. , Basavaraj M. , Rosenbladt D. , Nikishin S. A. , Holtz M. , Syrkin A. L. , Usikov A. S. , Ivantsov V. A. , Dmitriev V. A.
Published: June 2006

We report studies of the morphological, electrical, and optical properties of InN grown by hydride vapor phase epitaxy. The layers have been grown on c-plane sapphire substrates and epitaxial GaN, Al0.7Ga0.3N, and AlN templates grown on sapphire. InN properties are found to depend on template type with improvement of crystal structure in the template substrate order AlN→AlGaN→GaN. X-ray studies reveal InN layers grown on template substrates to be relaxed with lattice constants a=3.542Å and c=5.716Å. The Raman spectra and optical gaps of the InN layers, vary with free-carrier concentration in agreement with previous studies. We obtain a value of 2.5±0.2 for the index of refraction of InN.
Structural and optical properties of BaxSr1−xTiO3 thin films on indium tin oxide/quartz substrates prepared by radio-frequency magnetron sputtering
Authors: Leng W. J. , Yang C. R. , Zhang J. H. , Chen H. W. , Ji H. , Fu C. L. , Liao J. X.
Published: June 2006

Ba x Sr 1 − x Ti O 3 (x=0.6 and 0.8) thin films have been prepared on indium-doped tin oxide (ITO) coated quartz substrates using radio-frequency magnetron sputtering. Their structural properties and surface morphologies were examined by x-ray diffraction and atomic force microscopy, respectively. The BaxSr1−xTiO3 (BST) thin films with x=0.6 and 0.8 annealed at 650°C for 20min exhibit good surface morphology and well-crystallized perovskite structure. High quality BST ferroelectric thin films were further investigated by electrical measurements, showing the remnant polarization (Pr) of 6.75μC∕cm2 and the coercive field (Ec) of 43.2kV∕cm. Optical transmittance measurement indicated that the Ba concentration has an effect on the band gap energy (Eg) structure of the BaxSr1−xTiO3 thin films. The Eg decreases linearly with the increase of the Ba content. The refractive index (n) and extinction coefficient (k) of the BST films with x=0.6 and 0.8 were obtained by fitting the spectroscopic ellipsometric data using a parametric dielectric function model. The refractive index (n) and extinction coefficient (k) increase with increasing wavelength. All of the results show that the BST/ITO/quartz heterostructure is promising for applications in integrated optical waveguide devices.
Buckling of double-walled carbon nanotubes modeled by solid shell elements
Authors: Wang C. M. , Ma Y. Q. , Zhang Y. Y. , Ang K. K.
Published: June 2006

A solid shell element model is proposed for the elastic bifurcation buckling analysis of double-walled carbon nanotubes (DWCNTs) under axial compression. The solid shell element allows for the effect of transverse shear deformation which becomes significant in a stocky DWCNT with relatively small radius-to-thickness ratio. The van der Waals (vdW) interaction between the adjacent walls is simulated by linear springs. Using this solid shell element model, the critical buckling strains of DWCNTs with various boundary conditions are obtained and compared with molecular dynamics results and those obtained by other existing shell and beam models. The results obtained show that the solid shell element is able to model DWCNTs rather well, with the appropriate choice of Young’s modulus, tube thickness, and spring constant for modeling the vdW forces.
Electrically switched photoinduced waveguide in unpoled strontium barium niobate
Authors: Chauvet Mathieu , Guo Aqiang , Fu Guoyan , Salamo Gregory
Published: June 2006

The unpoled photorefractive strontium barium niobate crystal exhibiting quadratic electro-optic properties is characterized. It is used to form one-dimensional and two-dimensional bright photorefractive spatial solitons for either positive or negative low applied electric field. Solitons induce long-lived index changes in the medium which forms waveguides that can be tuned using an external voltage.
The influence of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures
Authors: Zhou X. H. , Chen Xiaoshuang , Huang Y. , Duan H. , Sun L. Z. , Lu W.
Published: June 2006

The effect of the additional confining potentials on ferromagnetism in III–V digital ferromagnetic heterostructures has been studied by ab initio calculations in combination with pseudopotential plane-wave method. The electronic and magnetic properties are shown as a function of the thickness of AlAs layers in the GaAs∕AlAs digital ferromagnetic heterostructures. It is found that all the structures show ferromagnetic alignment for the most favored configuration and their electronic structures are half metallicity. The exchange coupling constants N0β are estimated by using the spin splitting of the valence band. It is also shown that the charge density and the strong spin polarization are concentrated mostly on the magnetic layers for all structures. Furthermore, the hole distributions are analyzed in terms of their orbital projected density of states. The concentration of confined hole within the magnetic layer increases with increasing the additional potentials, which is responsible for the enhancement of ferromagnetic interaction in III–V digital ferromagnetic heterostructures.
Pinholelike defects in multistack 1.3μm InAs quantum dot laser
Authors: Liu Wei-Sheng , Chang Holin , Liu Yu-Shen , Chyi Jen-Inn
Published: June 2006

The pinholelike defects often observed in multistack InAs quantum dot structures on GaAs have been investigated comprehensively. Due to the high surface stress of InAs quantum dots, the overgrowth of GaAs and InGaAs capping layers on InAs quantum dots is far from conformal and leads to the formation of these defects. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to completely eliminate the pinholelike defects in multistack quantum dot structures. Ridge-waveguide 1.32μm InAs quantum dot lasers prepared by this method exhibit internal quantum efficiency as high as 62%.
Hydrodynamic loading of microcantilevers vibrating in viscous fluids
Authors: Basak Sudipta , Raman Arvind , Garimella Suresh V.
Published: June 2006

The hydrodynamic loading of elastic microcantilevers vibrating in viscous fluids is analyzed computationally using a three-dimensional, finite element fluid-structure interaction model. The quality factors and added mass coefficients of several modes are computed accurately from the transient oscillations of the microcantilever in the fluid. The effects of microcantilever geometry, operation in higher bending modes, and orientation and proximity to a surface are analyzed in detail. The results indicate that in an infinite medium, microcantilever damping arises from localized fluid shear near the edges of the microcantilever. Closer to the surface, however, the damping arises due to a combination of squeeze film effects and viscous shear near the edges. The dependence of these mechanisms on microcantilever geometry and orientation in the proximity of a surface are discussed. The results provide a comprehensive understanding of the hydrodynamic loading of microcantilevers in viscous fluids and are expected to be of immediate interest in atomic force microscopy and microcantilever biosensors.
Thermally activated low frequency noise in carbon nanotubes
Authors: Reza Shahed , Huynh Quyen T. , Bosman Gijs , Sippel-Oakley Jennifer , Rinzler Andrew G.
Published: June 2006

The low frequency noise of single-walled carbon nanotubes is studied over the 77–300K temperature range. Lorentzian shaped spectra along with 1∕f noise spectra have been observed. From the Lorentzian noise components, a range of thermal activation energies from 0.08to0.51eV for the associated fluctuation mechanisms is obtained. From the 1∕f noise spectra, a distribution of activation energies of fluctuation processes ranging from 0.2to0.7eV is derived. These findings indicate that the observed noise spectra are caused by number fluctuations.
Comparison of organic diode structures regarding high-frequency rectification behavior in radio-frequency identification tags
Authors: Steudel Soeren , De Vusser Stijn , Myny Kris , Lenes Martijn , Genoe Jan , Heremans Paul
Published: June 2006

In this article, we compare the direct current (dc) and high-frequency performance of two different organic diode structures, a vertical diode and an organic field effect transistor (OTFT) with shorted drain-gate contact, regarding their application in a rectifying circuit. For this purpose, we fabricated both diode structures using the organic semiconductor pentacene. dc measurements were performed showing a space-charge-limited current mobility of more than 0.1cm2∕Vs for the vertical diode and a field effect mobility of 0.8cm2∕Vs for the OTFT with shorted source-drain. High-frequency measurements of those diode structures in a rectifier configuration show that both types of diodes are able to follow the base-carrier frequency of 13.56MHz which is essential for viable radio-frequency-identification (rf-ID) tags. Based on those results we evaluate the performance limits and advantages of each diode configuration regarding their application in an organic rf-ID tag.
Plasma treatment of porous SiNx:H films for the fabrication of porous-dense multilayer optical filters with tailored interfaces
Authors: Vernhes R. , Amassian A. , Klemberg-Sapieha J. E. , Martinu L.
Published: June 2006

Porous and dense silicon nitride films with low (1.58) and high (1.88) refractive indices were prepared by using successively microwave and radio frequency (rf) plasma-enhanced chemical vapor deposition. Surface treatments were performed on porous layers using argon and nitrogen rf plasmas in order to densify and flatten their surface, and hence to obtain an abrupt transition between porous and dense films. The processes during deposition and interface treatment were studied by in situ real-time spectroscopic ellipsometry as well as by other characterization techniques. We show that besides the densification effect, preferential sputtering and annealing phenomena occur during plasma treatments at high bias (∣VBtreat∣>400V), leading to silicon enrichment at the film surface and chemical stabilization of the film bulk. Using atomic force microscopy, we observed a significant reduction of the thickness of the surface roughness layer after treatment for single layers (≈70% reduction) and multilayer stacks (≈60% reduction). Porous-dense Fabry-Pérot filters were fabricated using this approach that led to both enhanced optical performance due to minimized interface thickness and improved environmental stability.
Critical dimensions in coherently strained coaxial nanowire heterostructures
Authors: Raychaudhuri S. , Yu E. T.
Published: June 2006

We present a methodology to determine critical dimensions for coherently strained coaxial nanowire heterostructures based on a well-known formalism used to determine the critical thickness in planar epitaxial growth. The unique geometry of the nanowire structure along with the volumetric similarity of the core and shell regions give rise to a number of possible stable core-shell configurations for a given choice of materials. We show that a unique critical core radius and critical shell thickness, dependent on core radius, can quantify these configurations. Illustrative calculations are presented for various nitride semiconductor-based core-shell structures. It is anticipated that this model will serve as a guide to determine the feasibility of specific coherently strained nanowire heterostructure designs.
Determination of spatially resolved trapping parameters in silicon with injection dependent carrier density imaging
Authors: Schubert Martin C. , Riepe Stephan , Bermejo Sandra , Warta Wilhelm
Published: June 2006

We present spatially resolved and injection dependent excess carrier lifetime measurements on silicon. At low level injection conditions an anomalous increase often interferes in such measurements. The origin of the anomalous increase is discussed. Assuming trapping as the origin, highly resolved images of trap parameters together with low level injection recombination lifetimes with strongly reduced trapping effects have been obtained. A theoretical model for infrared lifetime imaging based on the Hornbeck and Haynes model [J. A. Hornbeck and J. R. Haynes, Phys. Rev. 97, 311 (1955)] is presented which describes the trapping effect on the measured lifetime. This model is fitted to experimental spatially resolved data to extract trapping parameters, particularly trap density and the trap-escape ratio, i.e., the ratio between escape rate of minority carriers from the trap level into the minority band (detrapping) and trap rate from the minority band into the trap level (trapping). An upper bound for the low level injection recombination lifetimes is determined. Lifetime and trapping parameters are compared with dislocation density maps. A strong correlation is found between the total trap density and the crystal defect density.
Direct growth of nickel disilicide nanocrystals in silicon dioxide films
Authors: Yoon Jong-Hwan , Lee Gyu-Hyun , Elliman Robert G.
Published: June 2006

Nickel disilicide (NiS2) nanocrystals (NCs) have been grown in silicon-rich oxide (SiOx) films ion implanted with nickel by annealing at 1100°C. It was found that NiS2 NCs grew into well-defined single crystalline structures embedded in a SiOx matrix and were approximately spherical in shape. The size of NCs can be influenced by limiting either the Ni or excess Si concentration. It was found that the resulting NCs could be produced with diameters in the range from 5to40nm in the SiOx layers with excess Si concentrations of 4–8at.% implanted with Ni concentrations of 0.1–10at.%.
La
(
Fe
1
−
x
Co
x
)
11.44
Al
1.56
: A composite system for Ericsson-cycle-based magnetic refrigerators
Authors: Gomes A. M. , Proveti J. R. , Takeuchi A. Y. , Passamani E. C. , Larica C. , Guimarães A. P.
Published: June 2006

The La(Fe1−xCox)11.44Al1.56 system, with x values of 0, 0.04, 0.08, and 0.12, was investigated for its magnetocaloric potential. For selected values of Co doping, it was possible to cover a wide magnetic ordering temperature range, from 200 up to 370K, and reduce substantially the metamagnetic critical fields. At applied magnetic fields up to 5T, the maximum magnetic entropy change approaches a nearly constant value of about 5J∕kgK for Co-doped compounds, with x varying from 0.04 to 0.12. The magnetic entropy change of a prototype composite was calculated in order to obtain a constant value in a wide temperature span. The results indicate that this material can be a good candidate for magnetic refrigeration using the Ericsson cycle.
Vibrational and defect states in SnOx nanoparticles
Authors: Meier Cedrik , Lüttjohann Stephan , Kravets Vasyl G. , Nienhaus Hermann , Lorke Axel , Ifeacho Pascal , Wiggers Hartmut , Schulz Christof , Kennedy Marcus K. , Kruis F. Einar
Published: June 2006

We have studied SnOx nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the SnOx crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric SnO1.5, leading, together with x-ray diffraction data, to a full characterization of the SnO1.5 lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of SnO1.7.
The role of Joule heating in the formation of nanogaps by electromigration
Authors: Trouwborst M. L. , van der Molen S. J. , van Wees B. J.
Published: June 2006

We investigate the formation of nanogaps in gold wires due to electromigration. We show that the breaking process will not start until a local temperature of typically 400K is reached by Joule heating. This value is rather independent of the temperature of the sample environment (4.2–295K). Furthermore, we demonstrate that the breaking dynamics can be controlled by minimizing the total series resistance of the system. In this way, the local temperature rise just before breakdown is limited and melting effects are prevented. Hence, electrodes with gaps <2nm are easily made, without the need of active feedback. For optimized samples, we observe quantized conductance steps prior to the gap formation.
Mechanism of transition between a-axis and c-axis growth of YBa2Cu3Ox thick films grown on NdGaO3 substrate
Authors: Cai Y. Q. , Yao X. , Lai Y. J.
Published: June 2006

Abrupt transition from a-axis growth to c-axis growth was found in the liquid phase epitaxy (LPE) process of YBa2Cu3Ox (YBCO or Y123) on (110) NdGaO3 substrate. Subsequently, the influence of supersaturation and liquid composition on the orientation of LPE thick films was systematically studied. Dissimilar to the thin film deposition, the a-phase growth was predominant at a low supersaturation (about 17K), especially grown by using a Cu-rich melt. The investigation of the initial stage of YBCO growth was performed, in which we found the protruding epitaxial a-axis grains and preferentially a-phase nucleation on the c-dominated thick film. The energy dispersive x-ray spectrometry attached scanning electron microscope was employed to confirm the composition of the outgrowth of long-axis grains, as well as the microstructure around the dipping boundary line in the initial growth. Based on the “nucleation and growth” mode, a comprehensive description of the competition between a-axis and c-axis growth under various growth environments was given, which further unveiled the origin of a-c transition in YBCO thick films.
The lattice thermal conductivity of a semiconductor nanowire
Authors: Huang Mei-Jiau , Chong Wen-Yen , Chang Tai-Ming
Published: June 2006

It has been found experimentally as well as theoretically that the lattice thermal conductivity can be largely reduced by the size confinement effect. The significant boundary scattering effect is one of the dominant factors. In most existing lattice thermal conductivity models, an empirical relation is used for this scattering rate. An unconfined or confined phonon distribution obtained based on the phonon Boltzmann equation and the relaxation time approximation is then employed to calculate the lattice thermal conductivity. In this work, we first attempt to derive an analytical form of the boundary scattering rate for phonon conduction in a semiconductor nanowire and then claim two reasonable ways to take it into account correctly. Consistent mathematical models in the sense that the effects of the size confinement on (i) the phonon dispersion relation, (ii) the phonon distribution, (iii) the phonon group and phase velocities, and (iv) the Debye temperature are finally proposed.
One-dimensional transient analysis of volumetric heating for laser drilling
Authors: Zhang Chong , Salama Islam A. , Quick Nathaniel R. , Kar Aravinda
Published: June 2006

Generally laser energy is considered to interact only with the substrate surface, as in metals, where the laser beam does not propagate into the substrate beyond a very small absorption depth. There are, however, many instances, particularly for ceramics and polymers, where the laser beam can penetrate into the substrate to substantial depths depending on the laser wavelength and laser-material interaction characteristics. Specifically there are polymeric dielectrics used as multilayer electronic substrates in which a laser beam of wavelength 9.3μm can penetrate into the substrate. The laser energy interacts at the substrate surface as well as inside the substrate. This particular aspect of laser-material interactions is important in laser drilling of blind microvias in polymeric multilayer electronic substrates. A one-dimensional transient heat conduction model including vaporization parameters is constructed to analyze this behavior. The absorption coefficient of the dielectric is also considered in this model and the problem is solved analytically. The microvia drilling speed, temperature distribution in the dielectric, and the thickness of the residue along the microvia walls and at the bottom of the microvia are studied for different laser irradiation conditions. An overheated metastable state of material is found to exist inside the workpiece. The overheating parameters are calculated for various laser drilling parameters and are used to predict the onset of thermal damage and to minimize the residue.
High-temperature polarization and depolarization experiments on poly(ε-caprolactone)
Authors: Suárez N. , Puma M. , Hernández M. C. , Walter J.
Published: June 2006

Thermally stimulated depolarization current (TSDC) and thermally stimulated polarization current (TSPC) techniques are compared, and the results obtained for semicrytalline poly(ε-caprolactone) (PCL) are reported in this work. It is shown that concerning the glass transition relaxation, both techniques give the same information, provided that for the TSDC case the polarization temperature is above the temperature range where the labeled ρ peak occurs. The current reversal observed in TSPC spectrum of this polymer is originally reported here, and its presence predicted by a simple phenomenological model taking into account the T−1 dependence of the equilibrium polarization. The current reversal observed in the TSDC experiments when the sample is polarized at high temperatures is an evidence of the existence of an internal field applied during the heating cycle. Performing two consecutive TSPC experiments (TSPC2) two current reversal temperature zones were observed. The comparison of the dielectric spectra obtained in the same temperature zones by the different techniques envisages the possible presence of a dipolar mechanism contributing to the ρ peak. Further modeling and data treatment need to be done in order to corroborate this hypothesis.
Biofunctionalized magnetic nanoparticles for high-sensitivity immunomagnetic detection of human C-reactive protein
Authors: Horng H. E. , Yang S. Y. , Hong Chin-Yih , Liu C. M. , Tsai P. S. , Yang H. C. , Wu C. C.
Published: June 2006

In this work, we developed immunomagnetic detection techniques for detecting human C-reactive protein (CRP). To enhance the technique’s sensitivity to human CRP, biofunctionalized magnetic nanoparticles were used as markers, and a superconductive quantum interference device gradiometer system was adapted to measure the saturated magnetization of magnetically labeled CRP–anti-CRP immune complexes. Sensitivity for human CRP was found to be as high as 1ng in 0.1ml, i.e., 10ng∕ml in concentration. This sensitivity is much higher than that of conventional enzyme linked immunosorbent assay by one order of magnitude.
Free vibration of periodically poled ferroelectric plate
Authors: Ostrovskii I. V. , Nadtochiy A. B.
Published: June 2006

Free vibration of two dimensional periodically poled ferroelectric resonators is reported. Analytical calculations, finite element method computer simulations, and experimental data from the lithium tantalate samples reveal a domain vibration resonance in this multidomain vibrator. When the acoustic wavelength is equal to the length of two inversely poled domains, a domain vibration resonance takes place, and the highest vibration amplitude is observed. Under this resonance, the domain walls (including plate end faces) vibrate with maximum amplitude. However, the mechanical stresses under domain resonance are exactly zero at the domain wall locations. The acoustic displacements do not depend on the number of domain pairs. At the frequency of the fundamental domain vibration resonance, a distribution of the acoustic displacements over multidomain crystal surface follows a domain periodicity. Distortion of the sample surface is higher in the case of samples with a rotated crystallographic cut in comparison with those having ZY-cut ferroelectric of 3m symmetry group.
Efficient inverted polymer solar cells
Authors: Li G. , Chu C.-W. , Shrotriya V. , Huang J. , Yang Y.
Published: June 2006

We investigate the effect of interfacial buffer layers—vanadium oxide (V2O5) and cesium carbonate (Cs2CO3)—on the performance of polymer solar cells based on regioregular poly-(3-hexylthiophene) and [6,6]-phenyl C60 butyric acid methyl ester blend. The polarity of solar cells can be controlled by the relative positions of these two interfacial layers. Efficient inverted polymer solar cells were fabricated with the structure of indium tin oxide (ITO)/Cs2CO3/polymer blend/vanadium oxide (V2O5)/aluminum (Al). Short-circuit current of 8.42mA∕cm2, open-circuit voltage of 0.56V, and power conversion efficiency of 2.25% under a AM1.5G 130mW∕cm2 condition were achieved. The interfacial layers were also used to fabricate polymer solar cells using ITO and a thin gold (Au) layer as the transparent electrodes. The thickness of V2O5 layer (10nm) makes it an effective protective layer for the active layer so that ITO can be used for both the electrodes, enabling highly efficient transparent polymer solar cells (i.e., polymer solar cells with transparent electrodes). Application of this structure for multiple-stacking polymer solar cells is also discussed.
Parametric studies on traveling compression regions observed in the Earth’s magnetotail
Authors: Ugai M. , Zheng L.
Published: June 2006

On the basis of the spontaneous fast reconnection model, the traveling compression region (TCR) is studied by magnetohydrodynamic simulations for various plasma parameters. Once the fast reconnection mechanism involving slow shocks builds up, the general features of TCR, observed by virtual satellites located in the simulation box, are in good agreement with actual satellite observations. Quantitatively, the TCR signature is not significantly influenced by plasma β, whereas its duration time is proportional to VAe−1, where VAe is the Alfvén velocity in the magnetic field region; also, the compression ratio of TCR is larger for the smaller plasma density ρe in the magnetic field region because of larger compressibility. In the diffusion region, resistive tearing is likely to occur, giving rise to multiple small-scale TCRs following the major TCR. For the uniform resistivity model, where the fast reconnection mechanism is not realized, any TCR signature cannot be observed. Hence, the TCR signature, observed in association with substorms, provides a definite evidence such that the fast reconnection mechanism is realized in the Earth’s magnetotail.
Quantitative one-dimensional thermal-wave cavity measurements of fluid thermophysical properties through equivalence studies with three-dimensional geometries
Authors: Matvienko Anna , Mandelis Andreas
Published: June 2006

The thermal-wave field in a photopyroelectric thermal-wave cavity was calculated with two theoretical approaches: a computationally straightforward, conventional, one-dimensional approach and a three-dimensional experimentally more realistic approach. The calculations show that the dimensionality of the thermal-wave field in the cavity depends on the lateral heat transfer boundary conditions and the relation between the beam size of the laser impinging on the thermal-wave generating metallic film and the diameter of the film itself. The theoretical calculations and the experimental data on the photopyroelectric signal in the cavity were compared. The study resulted in identifying ranges of heat transfer rates, beam sizes, and cavity radii for which accurate quantitative measurements of the thermal diffusivity of intracavity fluids can be made within the far simpler, but only approximate, one-dimensional approach conventionally adopted by users of thermal-wave cavities. It was shown that the major parameters affecting the dimensionality of thermal-wave cavities are the laser beam spot size and the Biot number of the medium comprising the sidewalls of the (cylindrical) cavity.
Construction of super-resolution microscope based on cw laser light source
Authors: Iketaki Yoshinori , Watanabe Takeshi , Bokor Nándor , Fujii Masaaki
Published: June 2006

We constructed a super-resolution microscope system based on a cw laser light source. Electro-optical modulators convert the cw laser light into a light pulse with a width of 15ns and provide a repetition rate of 100kHz. The performance of the microscope was evaluated using 100nmϕ fluorescence microbeads. The fluorescence signal from the beads can be detected with very low statistical fluctuation. The super-resolution image was obtained with better signal to noise ratio compared with that given by a pulse laser light source with a repetition rate of several tens of hertz. The fluorescent size of the beads was 150nm, which is 2.3 times smaller than the diffraction limit. Experimental results confirmed that the two-point resolution also overcomes the diffraction limit. The constructed system is expected to become a commercial microscope.
Noninvasive imaging using an array of electric potential sensors
Authors: Gebrial W. , Prance R. J. , Harland C. J. , Clark T. D.
Published: June 2006

We present a design for a linear array of eight electric potential sensors arranged with 1mm spacing and configured to measure spatially varying potential at the microscopic scale. The array successfully detects a 50μm wide feature associated with one of the samples tested. In a single sensor arrangement we have demonstrated <1μm resolution, but the data acquisition times can become prohibitive. The sensors operate noninvasively by capacitively coupling to the sample. The issues associated with using an array of sensors in close proximity are addressed. Cross coupling and strategies for matching the response of the sensors are described in detail. Results are presented for a range of samples including a resistive potential divider, a ceramic microwave circuit board, and a section taken from an oil drill pipe containing a known fault. The data acquisition times are compared with those of a single sensor system, with improvements of 4.5 times in speed reported. In one case real-time simultaneous data acquisition is demonstrated using all eight sensors. Since these sensors operate via the displacement current they may also be applied to the characterization of material properties, including, for example, insulators, dielectrics, and poorly conducting composite materials. It is concluded that we see significant improvements in the data acquisition times for the linear array over a single sensor as expected and are able to overcome the difficulties associated with operating an array of sensors in close proximity.
Intrinsic Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ
Authors: Hirose Yasushi , Hitosugi Taro , Furubayashi Yutaka , Kinoda Go , Inaba Kazuhisa , Shimada Toshihiro , Hasegawa Tetsuya
Published: June 2006

We have investigated the Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ films grown on Al2O3 (0001). The Faraday spectra strongly depended on the film thickness, revealing the significant effect of the multiple reflections of light. The intrinsic Faraday spectra of rutile Ti1−xCoxO2−δ were corrected for this multiple reflection effect on the basis of an optical model and compared with the absorption spectra in detail. The quantitatively good correspondence between intrinsic Faraday ellipticity and optical band edge strongly suggests that rutile Ti1−xCoxO2−δ is an intrinsic ferromagnetic semiconductor. The exchange parameters, N0α and N0β, were estimated as the order of ∼0.1eV.
Efficient white-light-emitting diodes based on poly(N-vinylcarbazole) doped with blue fluorescent and orange phosphorescent materials
Authors: Shih Ping-I , Shu Ching-Fong , Tung Yung-Liang , Chi Yun
Published: June 2006

We have fabricated polymer white-light-emitting devices possessing a single emitting layer containing a hole-transporting host polymer, poly(N-vinylcarbazole), and an electron-transporting auxiliary, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, doped with a blue-light-emitting amino-substituted distyrylarylene fluorescent dye and an orange-light-emitting osmium phosphor. The doubly doped device exhibited an intense white emission having Commission Internationale de l’Eclairage coordinates of (0.33, 0.34), a high external quantum efficiency of 6.12% (13.2cd∕A), and a maximum brightness of 11306cd∕m2. The color coordinates remained unchanged over a range of operating voltages, even at luminance as high as 1×104cd∕m2.
Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si
Authors: Liu B. T. , Cheng C. S. , Li F. , Ma L. , Zhao Q. X. , Yan Z. , Wu D. Q. , Li C. R. , Wang Y. , Li X. H. , Zhang X. Y.
Published: June 2006

We report on the use of amorphous Ni–Al film (a-Ni–Al) as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 capacitors on silicon. Cross-sectional observation by transmission electron microscope reveals clean and sharp interfaces without any discernible interdiffusion/reaction in the sample. The physical properties of the capacitors are vertically characterized as the parameters of memory elements. Excellent ferroelectric properties, e.g., large remnant polarization of ∼22μC∕cm2, small coercive voltage of ∼1.15V, being fatigue-free, good retention characteristic, imply that amorphous Ni–Al is an ideal candidate for diffusion barrier for the high-density ferroelectric random access memories integrated with silicon transistor technology.
Nanotube micro-optomechanical actuators
Authors: Lu Shaoxin , Panchapakesan Balaji
Published: June 2006

In this letter we demonstrate a simple carbon nanotube patterning technique that combines nanotube film bonding, photolithography, and O2 plasma etching. Well defined carbon nanotube film structures with line widths less than ∼1.5μm and thickness ranging from 40to780nm were readily fabricated. A micro-optomechanical actuator based on this process has been demonstrated. This patterning process can be utilized for the integration of nanomaterials for wide variety of devices including microeletromechanical systems, field emission displays, and micro-optomechanical systems (MOMS).
Strain distribution in a transistor using self-assembled SiGe islands in source and drain regions
Authors: Kar G. S. , Kiravittaya S. , Denker U. , Nguyen B.-Y. , Schmidt O. G.
Published: June 2006

We propose to improve a p-channel metal-oxide-semiconductor field-effect transistor using laterally closely spaced double self-assembled SiGe∕Si islands as drain and source to create a high hole mobility channel. The strain distribution in and around the channel is calculated for two realistic island geometries with various distances between the islands. A compressive strain of more than 1% in the channel can be achieved for SiGe islands and small distance between these two islands. We demonstrate that the proposed double SiGe∕Si island structure can be realized by epitaxial growth on patterned substrates designed for static random access memory cell.
Ga-triggered oxide desorption from GaAs(100) and non-(100) substrates
Authors: Lee J. H. , Wang Zh. M. , Salamo G. J.
Published: June 2006

Surface pit formation is observed to occur for conventional thermal oxide desorption, regardless of the GaAs surface index. The resulting surface roughening can be substantially reduced by depositing an appropriate amount of Ga in the absence of As flux. The amount of Ga required to optimize the quality of the oxide-free GaAs surface is found to be different for different indexed substrates and is dependent on the oxide thicknesses. Due to the flexibility of the GaAs surface reconstruction, the Ga-triggered oxide desorption is observed to be much more robust than was previously believed.
Origin of the n-type conductivity of InN: The role of positively charged dislocations
Authors: Piper L. F. J. , Veal T. D. , McConville C. F. , Lu Hai , Schaff W. J.
Published: June 2006

As-grown InN is known to exhibit high unintentional n-type conductivity. Hall measurements from a range of high-quality single-crystalline epitaxially grown InN films reveal a dramatic reduction in the electron density (from low 1019 to low 1017cm−3) with increasing film thickness (from 50to12000nm). The combination of background donors from impurities and the extreme electron accumulation at InN surfaces is shown to be insufficient to reproduce the measured film thickness dependence of the free-electron density. When positively charged nitrogen vacancies (VN+) along dislocations are also included, agreement is obtained between the calculated and experimental thickness dependence of the free-electron concentration.