bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature and injection current dependence of electroluminescence intensity in green and blue InGaN single-quantum-well light-emitting diodes
Authors: Hori A. , Yasunaga D. , Satake A. , Fujiwara K.
Published: March 2003

Temperature and injection current dependence of electroluminescence (EL) spectral intensity of the superbright green and blue InGaN single-quantum-well (SQW) light-emitting diodes has been studied over a wide temperature range (T=15−300 K) and as a function of injection current level (0.1–10 mA). It is found that, when temperature is slightly decreased to 140 K, the EL intensity efficiently increases in both cases, as usually seen due to the improved quantum efficiency. However, with further decrease of temperature down to 15 K, unusual reduction of the EL intensity is commonly observed for both of the two diodes. At low temperatures the integrated EL intensity shows a clear trend of saturation with current, accompanying decreases of the EL differential quantum efficiency. We attribute the EL reduction due to trapping of injected carriers by nonradiative recombination centers. Its dependence on temperature and current shows a striking difference between the green and blue SQW diodes. That is, we find that the blue InGaN SQW diode with a smaller In concentration shows more drastic reduction of the EL intensity at lower temperatures and at higher currents than the green one. This unusual evolution of the EL intensity with temperature and current is due to less efficient carrier capturing by SQW. The carrier capture in the green and blue diodes also shows a keen difference owing to the different In content in the InGaN well. These results are analyzed within a context of rate equation model, assuming a finite number of radiative recombination centers. Importance of the efficient carrier capture processes by localized tail states within SQW at 180–300 K is thus pointed out for explaining the observed enhancement of radiative recombination of injected carriers in the presence of high-density misfit dislocations.
Carrier dynamics in short wavelength self-assembled InAs/Al0.6Ga0.4As quantum dots with indirect barriers
Authors: Monte A. F. G. , Finley J. J. , Ashmore A. D. , Fox A. M. , Mowbray D. J. , Skolnick M. S. , Hopkinson M.
Published: March 2003

We report the results of time-integrated and time-resolved photoluminescence (PL) spectroscopy on red-emitting self-organized InAs/Al0.6Ga0.4As quantum dots with indirect barriers. The time-resolved PL measurements confirm that carriers excited in the Al0.6Ga0.4As barriers are initially scattered to the X states and are then captured by the dots on a timescale of 45–60 ps, leading to efficient red emission. The dynamics of the PL decay time are consistent with a carrier hopping process between dots, a result also supported by time-integrated PL experiments.
Self-diffusion of Si in thermally grown SiO2 under equilibrium conditions
Authors: Takahashi Tomonori , Fukatsu Shigeto , Itoh Kohei M. , Uematsu Masashi , Fujiwara Akira , Kageshima Hiroyuki , Takahashi Yasuo , Shiraishi Kenji
Published: March 2003

Self-diffusion coefficients of Si in thermally grown SiO2 on a semiconductor-grade silicon wafer have been determined at temperatures between 1150 and 1300 °C under equilibrium conditions using isotope heterostructures (natSiO2/28SiO2). Si self-diffusion was induced by appropriate heat treatments, and the diffusion depth profiles of Si30 isotope from SiO2nat to SiO228 layers were determined by secondary ion mass spectrometry (SIMS). The diffusion coefficients found in the present study for 1150–1300 °C are more than two orders of magnitude smaller than the values measured with semiconductor-grade SiO2 in the presence of excess silicon, i.e., in nonequilibrium conditions, and agree very well with previously reported values of Si self-diffusion in fused silica under equilibrium conditions.
Orientation effects in shocked nickel single crystals via molecular dynamics
Authors: Kum Oyeon
Published: March 2003

Shock wave propagation and crystal deformation features were examined in nickel single crystals using nonequilibrium molecular-dynamics simulations for three crystallographic orientations: 〈100〉, 〈110〉, and 〈111〉. Two Morse-type pair potentials and one embedded-atom method (EAM) empirical potential were used to describe bond forces for single crystal nickel atoms. The two Morse-type pair potentials yield more accurate elastic moduli of nickel than does the EAM potential. The dissipation mechanism leading to shear stress relaxation was investigated at the piston/particle speed up=2.5(ε/m)1/2, where ε is an energy parameter and m is the atomic mass. For shock propagation along the 〈100〉 orientation, the plastic waves overtook almost all elastic precursors for all three potentials. However, for wave propagation along both 〈110〉 and 〈111〉 orientations, elastic precursors followed by steady plastic waves were observed. A clearly discernible leading solitary wave train followed by an extended elastically deformed region was observed in the 〈110〉 orientation. The EAM potential and the pair potentials showed some differences in shock wave propagation. In terms of crystal deformation, the EAM potential showed a greater degree of plasticity than the pair potentials. The two Morse-type pair potentials showed comparable results with respect to wave propagation features and crystal deformation. The details of the atomistic changes associated with shock-induced elastic-plastic deformation are presented for all three potentials.
Controlled excitation of electromagnetic band-gap line and point defect modes at microwave frequencies
Authors: Schuster M. , Klein N.
Published: March 2003

We report on the controlled excitation of line and point defect modes in a two-dimensional hexagonal electromagnetic band-gap structure made of rods of dielectric material (aluminium oxide). We compared simulation performed with a numerical field simulation software and experimental measurements at microwave frequencies with regard to coupling from external waveguides to line defects and subsequent coupling to resonant modes. We observed that for a line defect in the photonic crystal the impedance matching to a waveguide is strongly dependent on the defect width. We furthermore demonstrated that the coupling to a localized defect resonance can be strongly influenced by the variation of certain single lattice elements, affecting transmission behavior and quality factor of the resonant modes.
Solution of Poisson’s equation in a volume conductor using resistor mesh models: Application to event related potential imaging
Authors: Franceries X. , Doyon B. , Chauveau N. , Rigaud B. , Celsis P. , Morucci J-P.
Published: March 2003

In electroencephalography (EEG) and event related potentials (ERP), localizing the electrical sources at the origin of scalp potentials (inverse problem) imposes, in a first step, the computation of scalp potential distribution from the simulation of sources (forward problem). This article proposes an alternative method for mimicing both the electrical and geometrical properties of the head, including brain, skull, and scalp tissue with resistors. Two resistor mesh models have been designed to reproduce the three-sphere reference model (analytical model). The first one (spherical resistor mesh) closely mimics the geometrical and electrical properties of the analytical model. The second one (cubic resistor mesh) is designed to conveniently handle anatomical data from magnetic resonance imaging. Both models have been validated, in reference to the analytical solution calculated on the three-sphere model, by computing the magnification factor and the relative difference measure. Results suggest that the mesh models can be used as robust and user-friendly simulation or exploration tools in EEG/ERP.
Droplet stability in a finite system: Consideration of the solid–vapor interface
Authors: McGaughey A. J. H. , Ward C. A.
Published: March 2003

The thermodynamic stability of a single, one-component droplet in a finite system with adsorbing walls is investigated. The conditions under which a stable equilibrium state of the droplet is predicted to exist depend critically on the adsorption isotherm of the confining walls. If the amount absorbed remains finite when the pressure in the vapor is greater than the fluid’s saturation pressure, then a stable equilibrium state is possible. When the model is extended to a system of multiple droplets, the stable equilibrium state is predicted to always correspond to a single droplet.
X-ray investigations of formation efficiency of buried azobenzene polymer density gratings
Authors: Geue Thomas M. , Saphiannikova Marina G. , Henneberg Oliver , Pietsch Ullrich , Rochon Paul L. , Natansohn Almeria L.
Published: March 2003

We present the results of time-dependent x-ray scattering measurements during the formation of a buried, modulated density structure below polymer film surface created at temperatures above the glass transition temperature TG during the thermal erasure of a holographically written surface relief grating. The surface relief gratings are initially formed on polymer films containing azobenzene side groups by exposure to a polarization holographic pattern made using light at 488 nm. The material responds to the actinic light by forming a sinusoidal surface relief pattern in a one-step process. Theoretical predictions show that the surface relief grating formation is accompanied by a density grating just below the film surface. Upon annealing above polymers glass transition temperature the surface relief grating melts while some of the density modulation in the bulk remains and a nonsinusoidal density grating is rebuilt after long term annealing at a temperature approximately 30 K above the glass transition temperature. The growth of density volume structure is assisted by exposure to red laser light during annealing. The observed homopolymer pDR1M was found to be the most effective material to build the modulated buried density grating. This might be due to the formation of clusters below the former peak positions of the surface relief grating. A quantitative description and preliminary model is given for this behavior.
Unintended filtering in a typical photodiode detection system for optical tweezers
Authors: Berg-Sørensen Kirstine , Oddershede Lene , Florin Ernst-Ludwig , Flyvbjerg Henrik
Published: March 2003

We characterize the frequency-dependent response of a photo detection system based on a Si-PIN photodiode and a laser with wavelength 1064 nm, a system commonly used with optical tweezers. We chopped the laser beam with chopper frequencies from 200 Hz to 14 kHz, and found an exponentially delayed response of the detection system with a characteristic delay time of ∼20 μs. The physical mechanism causing this time delay is silicon’s transparency to 1064 nm light: Photons are absorbed and create charge carriers not only in the diode’s depletion layer, where they are detected within nano-seconds, but predominantly in the n-layer, where they remain undetected till transported out by thermal diffusion. The diode’s response is dominated by this delay which can be characterized as a first-order low-pass filter with a 3dB-frequency of 8–9 kHz, depending on laser intensity. Measurements exploiting frequencies near or above this 3dB-frequency must be corrected for this unintended filter effect. We describe how to do this, and how to diagnose other systems which may or may not have the same problem. Explanations are intended for users of photo detection systems, and present the little semi-conductor physics needed to make sense.
Nanometer-scale metallic grains connected with atomic-scale conductors
Authors: Anaya A. , Korotkov A. L. , Bowman M. , Waddell J. , Davidovic D.
Published: March 2003

We describe a technique for connecting a nanometer-scale gold grain to leads by atomic-scale gold point contacts. These devices differ from previous metallic quantum dots in that the conducting channels are relatively well transmitting. We investigate the dependence of the Coulomb blockade on contact resistance. The high-resistance devices display Coulomb blockade and the low-resistance devices display a zero-bias conductance dip, both in quantitative agreement with theory. We find that in the intermediate regime, where the sample resistance is close to h/e2, the I–V curve displays a Coulomb staircase with symmetric contact capacitances.
Optomechanical properties of stretched polymer dispersed liquid crystal films for scattering polarizer applications
Authors: Amimori Ichiro , Priezjev Nikolai V. , Pelcovits Robert A. , Crawford Gregory P.
Published: March 2003

A scattering polarizer is created by subjecting a polymer dispersed liquid crystal film to tensile strain. The optomechanical properties of the film are investigated by simultaneously measuring the stress–strain and polarization dependent optical transmission characteristics. The correlation between transmittances of two orthogonal polarizations and the stress–strain curve reveals that the polymer orientation as well as the droplet shape anisotropy influences the liquid crystal alignment within the droplets. A Monte Carlo simulation based on the P. A. Lebwohl–G. Lasher [Phys. Rev. A 6, 426 (1972)] model is used to explain the subtle influence of polymer orientation on liquid crystal alignment.
Spin-wave modes and line broadening in strongly coupled epitaxial Fe/Al/Fe and Fe/Si/Fe trilayers observed by Brillouin light scattering
Authors: Kuanr B. K. , Buchmeier M. , Gareev R. R. , Bürgler D. E. , Schreiber R. , Grünberg P.
Published: March 2003

We report on the spin-wave excitation frequencies and the broadening of the mode linewidths in Stokes and anti-Stokes Brillouin light scattering spectra of strongly exchange coupled (2–6.5 mJ/m2), epitaxial trilayers with the following structures: Fe(80 Å)/Si wedge/Fe(100 Å) and Fe(50 Å)/Al wedge/Fe(70 Å). Both spacer materials yield qualitatively similar spectra which evolve in the same way when the spacer thickness increases. We determine the type of interlayer coupling and quantify its strength as a function of the spacer thickness by comparing and fitting the mode positions to a model calculation. Furthermore, we observe clearly different behaviors of the mode linewidths as a function of spacer thickness for the optic and acoustic modes. There are also strong differences between the Stokes and anti-Stokes side of the spectra. The largest linewidths of up to 8 GHz occur at spacer thicknesses between 5 and 8 Å for both spacer materials. Lateral averaging and two-magnon scattering are qualitatively discussed as possible sources for the line-broadening.
The metallicity of aluminum and gold in contact with thin films of a urethane-substituted polythiophene
Authors: Ahn H. , Whitten J. E.
Published: March 2003

The metallic natures of aluminum and gold deposited in ultrahigh vacuum on a urethane-substituted polythiophene, poly[2-(3-thienyl)ethanol n-butoxycarbonylmethylurethane], have been studied by x-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) and electron energy loss spectroscopy (ELS). XPS demonstrates that initially deposited aluminum exists as oxide, similar to Al2O3 in character, up to a metal dose of about 4.0×1015 atoms/cm2. Higher metal doses result in metallic aluminum. UPS valence electronic spectra and ELS show, respectively, the existence of a band gap and an absence of plasmons. These indicate that interfacial aluminum is not metallic. In contrast, gold is metallic even for doses as small as 1.0×1015 atoms/cm2. In situ photoluminescence measurements of the polymer as a function of metal dose have also been performed to assess the ability of interfacial metal to quench photoluminescence. Deposited gold results in significantly more quenching than aluminum per deposited metal atom. However, the fact that nonmetallic aluminum facilitates some photoluminescence quenching indicates that factors, in addition to the existence of a nearby metallic layer, may lead to nonradiative decay of electronic excitations.
Angular dependence of magnetization reversal process in exchange coupled ferromagnetic/antiferromagnetic bilayers
Authors: Liu Z. Y. , Adenwalla S.
Published: March 2003

The angular dependence of the magnetization reversal process in the exchange biased ferromagnetic/antiferromagnetic bilayers has been investigated carefully using the Stoner–Wohlfarth rotation model. Depending strongly on the orientation of the applied field and the competition between the unidirectional and uniaxial anisotropic energies, the magnetization rotation can occur at either the same side or the different sides of the field direction for the decreasing and increasing field branches of the hysteresis loop. The calculated results and the magnetooptical Kerr effect have been used to understand the magnetization reversal process in the NiFe/NiO bilayers, which is caused mainly by the magnetization rotation.
Characterization of deep level traps responsible for isolation of proton implanted GaAs
Authors: Boudinov H. , Coelho A. V. P. , Tan H. H. , Jagadish C.
Published: March 2003

Deep level transient spectroscopy was employed to determine the electrical properties of defects induced in metalorganic chemical-vapor deposition grown n-type and p-type GaAs during proton bombardment. Thermal stability of these defects was investigated and correlation with defects responsible for isolation of GaAs by ion bombardment was discussed. The annealing temperature region (220–250 °C) is similar to proton isolated GaAs below the threshold dose for complete isolation. At least four of the five traps observed in n-type GaAs are not simple interstitial-vacancy pairs. For p-type GaAs we have observed an unknown level with apparent energy of ∼0.64 eV.
Growth and characterization of Hf–aluminate high-k gate dielectric ultrathin films with equivalent oxide thickness less than 10 Å
Authors: Lee P. F. , Dai J. Y. , Wong K. H. , Chan H. L. W. , Choy C. L.
Published: March 2003

Ultrathin amorphous Hf–aluminate (Hf–Al–O) films have been deposited on p-type (100) Si substrates by pulsed-laser deposition using a composite target containing HfO2 and Al2O3 plates. Transmission electron microscopy observation of Hf–Al–O films showed that the amorphous structure of Hf–Al–O films was stable under rapid thermal annealing at temperatures up to at least 1000 °C. Capacitance–voltage measurement of a 38 Å Hf–Al–O film revealed that the relative permittivity of the film was about 16. Such a film showed very low leakage current density of 4.6×10−3 A/cm2 at 1 V gate bias. The Hf–Al–O film under optimized condition did not show any significant interfacial layer at the interface and an equivalent oxide thickness of less than 10 Å has been achieved. The formation of Hf–O and Al–O bonds in the film was revealed by x-ray photoelectron spectroscopy.
Simulated causal subwavelength focusing by a negative refractive index slab
Authors: Cummer Steven A.
Published: March 2003

Subwavelength electromagnetic focusing by a negative refractive index slab is shown to occur in a full wave numerical simulation of a causal, physically realizable negative index material. Limitations on the observability of this effect in simulations and experiments are discussed.
Limitations on subdiffraction imaging with a negative refractive index slab
Authors: Smith David R. , Schurig David , Rosenbluth Marshall , Schultz Sheldon , Ramakrishna S. Anantha , Pendry John B.
Published: March 2003

A planar slab of material, for which both the permittivity and permeability have the values of −1, can bring not only the propagating fields associated with a source to a focus, but can also refocus the nonpropagating near fields, thereby achieving resolution beyond the diffraction limit. We study the sensitivity of this subwavelength focus to the slab material properties and periodicity, and note the connection to slab surface plasmon modes. We conclude that significant subwavelength resolution is achievable with a single negative index slab, but only over a restrictive range of parameters.
Bimaterial assembly with a low modulus bonding layer at the ends
Authors: Suhir E.
Published: March 2003

We examine an elongated bimaterial assembly with a nonhomogeneous bonding layer: the Young’s modulus of the peripheral (outer) portions of this layer is lower than the Young’s modulus of the bonding material in the midportion (inner portion) of the assembly. The objective of the analysis is to develop a simple easy-to-apply predictive model for the assessment of the thermally induced interfacial shearing stress, when the assembly experiences a change in temperature. The model is based on a simplified strength-of-materials (structural analysis) approach, rather than on rigorous theory-of-elasticity treatment of the problem. We use the developed model to find out if there is sufficient incentive to use a nonhomogeneous bonding layer of the type in question for lower interfacial stresses. We found that the highest interfacial shearing stress occurred at the ends of the “high modulus” midportion of the assembly and at the assembly edges (i.e., at the free ends of the peripheral portions). We suggest that, for lower maximum shearing stress in the assembly, the Young’s moduli of sufficiently long peripheral portions of the bonding layer be established in such a way that the shearing stress at the ends of the midportion be equal to the shearing stress at the assembly ends. We compare the maximum shearing stress in a “conventional” assembly (i.e., in an assembly with a homogeneous bonding layer) with the maximum stress in an assembly with an “optimized” nonhomogeneous adhesive. The obtained numerical data indicated that a 60%−70% relief in maximum shear stress could be expected as a result of such “optimization.” We conclude, therefore, that there is a definite incentive for implementing a design with a nonhomogeneous bonding layer of the type in question for lower interfacial shearing stress in adhesively bonded or soldered assemblies. Such assemblies are widely used in micro- and optoelectronics.
Yield and fracture properties of the wide band-gap semiconductor 4H-SiC
Authors: Pirouz P. , Zhang M. , Demenet J.-L. , Hobgood H. M.
Published: March 2003

The flow stress of the wide band-gap semiconductor, 4H-SiC, has been measured by uniaxial compression tests over the temperature range 500–1400 °C at different strain rates. At low strain rates, 4H-SiC shows a transition in the yield stress at a temperature Tc. In addition, the brittle-to-ductile transition (BDT) temperature TBDT of the same material has been determined on precracked samples at different values of strain rate ε̇ by 4-point bend tests. Intriguingly, the transition temperature Tc in yielding is very close to the brittle-to-ductile transition temperature TBDT in the fracture behavior. In previous transmission electron microscopy (TEM) investigations, significant microstructural differences were found between low-temperature (TTc) deformed crystals. There, the results showed that in the samples deformed below the transition temperature Tc, deformation proceeds by the generation and motion of single leading partial dislocations on different (0001) planes. Moreover, all the partials appeared to have the same core, silicon. On the other hand, at temperatures above Tc, the samples deformed by the generation and motion of perfect dislocations dissociated in the form of leading/trailing partial pairs separated by a ribbon of stacking fault. Based on the present mechanical tests and previous TEM results—together with experimental evidence from other semiconductors—a model is presented in which Tc and TBDT are identified and correspond to the temperature at which crystal shear takes place by different dislocation types. Below Tc, single leading partials are responsible for crystal shear, whereas above Tc, perfect dislocations (i.e., leading/trailing pairs) accomplish the slip. Since generation of a leading partial from crack tip sources basically shuts the sources off, the crystal remains brittle below this transition temperature. At Tc=TBDT, trailing partials are nucleated from the same sources to clean up the stacking fault and allow multiplicative generation of dislocation avalanches resulting in transition to the ductile mode.
Strain and size effects on heat transport in nanostructures
Authors: Picu R. C. , Borca-Tasciuc T. , Pavel M. C.
Published: March 2003

The relative role of the residual strain and dimensional scaling on heat transport in nanostructures is investigated by molecular dynamics simulations of a model Lennard-Jones solid. It is observed that tensile (compressive) strains lead to a reduction (enhancement) of the lattice thermal conductivity. A nonhydrostatic strain induces thermal conductivity anisotropy in the material. This effect is due to the variation with strain of the stiffness tensor and lattice anharmonicity, and therefore of the phonon group velocity and phonon mean free path. The effect due to the lattice anharmonicity variation appears to be dominant. The size effect was studied separately in unstrained thin films. Phonon scattering on surfaces leads to a drastic reduction of the thermal conductivity effect which is much more important than that due to strain in the bulk. It is suggested that strain may be used to tailor the phonon mean free path which offers an indirect method to control the size effect.
On erbium lattice location in ion implanted Si0.75Ge0.25 alloy: Computer simulation of Rutherford backscattering/channeling
Authors: Touboltsev V. , Jalkanen P. , Räisänen J. , Smulders P. J. M.
Published: March 2003

A high crystalline quality Si0.75Ge0.25 alloy layer grown by chemical vapor deposition was implanted with 70 keV Er+ ions to a fluence of 1015 cm−2 at temperature of 550 °C. The implantation was found to result in an Er depth distribution with 1 at. % maximum concentration 30 nm beneath the surface. The location of the erbium atoms in the host matrix lattice is derived through computer simulation of experimental axial channeling angular scans measured by in situ Rutherford backscattering/channeling spectrometry. Using computer code FLUX 7.7 it is shown that 60% of the implanted erbium atoms are located at ytterbium sites, 10% at tetrahedral sites, and the remainder are associated with random locations in the host matrix.
Fast gas spectroscopy using pulsed quantum cascade lasers
Authors: Beyer T. , Braun M. , Lambrecht A.
Published: March 2003

Laser spectroscopy has found many industrial applications, e.g., control of automotive exhaust and process monitoring. The midinfrared region is of special interest because it has stronger absorption lines compared to the near infrared (NIR). However, in the NIR high quality reliable laser sources, detectors, and passive optical components are available. A quantum cascade laser could change this situation if fundamental advantages can be exploited with compact and reliable systems. It will be shown that, using pulsed lasers and available fast detectors, lower residual sensitivity levels than in corresponding NIR systems can be achieved. The stability is sufficient for industrial applications.
Interatomic potential for vanadium suitable for radiation damage simulations
Authors: Han Seungwu , Zepeda-Ruiz Luis A. , Ackland Graeme J. , Car Roberto , Srolovitz David J.
Published: March 2003

The ability to predict the behavior of point defects in metals, particularly interstitial defects, is central to accurate modeling of the microstructural evolution in environments with high radiation fluxes. Existing interatomic potentials of embedded atom method type predict disparate stable interstitial defect configurations in vanadium. This is not surprising since accurate first-principles interstitial data were not available when these potentials were fitted. In order to provide the input information required to fit a vanadium potential appropriate for radiation damage studies, we perform a series of first-principles calculations on six different interstitial geometries and vacancies. These calculations identify the 〈111〉 dumbbell as the most stable interstitial with a formation energy of approximately 3.1 eV, at variance with predictions based upon existing potentials. Our potential is of Finnis–Sinclair type and is fitted exactly to the experimental equilibrium lattice parameter, cohesive energy, elastic constants and a calculated unrelaxed vacancy formation energy. Two additional potential parameters were used to obtain the best fit to the set of interstitial formation energies determined from the first-principles calculations. The resulting potential was found to accurately predict both the magnitude and ordering of the formation energies of six interstitial configurations and the unrelaxed vacancy ground state, in addition to accurately describing the migration characteristics of the stable interstitial and vacancy. This vanadium potential is capable of describing the point defect properties appropriate for radiation damage simulations as well as for simulations of more common crystal and simple defect properties.
Green’s function approach for transport calculation in a In0.53Ga0.47As/In0.52Al0.48As modulation-doped heterostructure
Authors: Vasileska D. , Prasad C. , Wieder H. H. , Ferry D. K.
Published: March 2003

The gate voltage dependence of the low-field electron mobility has been investigated in a In0.53Ga0.47As/In0.52Al0.48As modulation-doped heterostructure using a real-time Green’s function formalism. All scattering mechanisms relevant to this material system have been incorporated into a theoretical model, including alloy disorder scattering, Coulomb scattering from ionized impurities in the buffer layer, and acoustic phonon and piezoelectric scattering. The simulation results for the subband structure suggest the occupation of two subbands at VG=0 V. Good agreement is observed between the simulated sheet electron densities and the experimentally extracted ones from Hall and Shubnikov–de Haas oscillatory magnetoresistance measurements. The mobility results for the structure investigated suggest that alloy-disorder scattering is the dominant mobility degradation mechanism.
Control method for the tunable ordered structures in magnetic fluid microstrips
Authors: Yang S. Y. , Horng H. E. , Hong Chin-Yih , Yang H. C. , Chou M. C. , Pan C. T. , Chao Y. H.
Published: March 2003

Structural evolution along the edges of magnetic fluid microstrips under external magnetic fields is investigated in this article. It was found that when the external magnetic field increased the structure evolves from an originally monodispersed state to a disordered column state, and then to several levels of ordered structures. For the ordered structure state, a one-dimensional periodic structure resulted under lower field strengths, and with an increase in field strength, more columns formed. Hence, a two-dimensional ordered array of columns was achieved. Further experimental results showed that the ordered structure can be manipulated by adjusting control parameters, such as field strength, sweep rate, fluid concentration, strip width, and strip thickness. These results lay the groundwork for utilizing controllable ordered structures in magnetic fluid microstrips to develop potential applications, such as photonic crystals or waveguides.
Optical properties and frequency upconversion fluorescence in a Tm3+ -doped alkali niobium tellurite glass
Authors: Poirier Gaël , Cassanjes Fabia C. , de Araújo Cid B. , Jerez Vladimir A. , Ribeiro Sidney J. L. , Messaddeq Younes , Poulain Marcel
Published: March 2003

Optical spectroscopic properties of Tm3+-doped 60TeO2−10GeO2−10K2O−10Li2O−10Nb2O5 glass are reported. The absorption spectra were obtained and radiative parameters were determined using the Judd–Ofelt theory. Characteristics of excited states were studied in two sets of experiments. Excitation at 360 nm originates a relatively narrow band emission at 450 nm attributed to transition D21→3F4 of the Tm3+ ion with photon energy larger than the band-gap energy of the glass matrix. Excitation at 655 nm originates a frequency upconverted emission at 450 nm (1D2→3F4) and emission at 790 nm (3H4→3H6). The radiative lifetimes of levels D21 and H43 were measured and the differences between their experimental values and the theoretical predictions are understood as due to the contribution of energy transfer among Tm3+ ions.
Remanence enhancement based on L1 ordering in Fe–Pt permanent magnets
Authors: Tanaka Y. , Hisatsune K.
Published: March 2003

This study investigated the relationship between the hard magnetic properties and microstructure of bulk FePt magnets, which can potentially be used in dental prostheses. A high level of remanence was obtained in a Fe–39.5 mol %Pt alloy aged at 873 K, in which minute FePt ordered domains of about 10 nm in size were uniformly created. The change in the maximum energy product of this alloy was in good agreement with the level of remanence: they both decreased with increasing aging time, and the size of the ordered domains gradually grew. The remanence ratio of as-quenched Fe–40 mol %Pt magnet has been estimated at 0.69, despite the morphological isotropy of the magnet. Remanence enhancement occurred in an FePt single phase without the presence of the magnetically soft disordered phase, since the spring back phenomenon was not observed in the recoil curve measurements, and the entire area was covered with minute FePt ordered domains. The hard magnetic properties were enhanced at the optimum ordered domain size of about 10 nm. This study found that the ordered domains behaved as a single magnetic domain particle and improved remanence through intergranular exchange interaction.
Light-controlled growth of gallium nanoparticles
Authors: Fedotov V. A. , MacDonald K. F. , Zheludev N. I. , Emel’yanov V. I.
Published: March 2003

Optical excitation of covalent bond structures via plasmon-mediated local fields inside elemental gallium nanoparticles can explain the recently observed phenomenon of light-controlled gallium nanoparticle growth on silica substrates under very low intensity laser illumination. Alongside the previously reported mechanism of atomic desorption induced by electronic excitation, we introduce another parallel mechanism based on the suppression of adatom adsorption, and illustrate how both mechanisms lead to a narrowing of the size distribution of the particle during the growth process. © 2003 American Institute of Physics.
Numerical modeling of capacitively coupled hydrogen plasmas: Effects of frequency and pressure
Authors: Novikova T. , Kalache B. , Bulkin P. , Hassouni K. , Morscheidt W. , Roca i Cabarrocas P.
Published: March 2003

In the field of plasma deposition of amorphous and microcrystalline silicon, the increase of the excitation frequency has often been considered as a way to enhance the deposition rate. Moreover, the increase of pressure has also been shown to enhance the deposition rate and improve the film properties. We attempt to clarify the effects of frequency in the 13.56–40.68 MHz range and to compare them to those of the pressure in the range of 0.5–1.5 Torr. For that purpose we use a numerical modeling of capacitively coupled hydrogen plasma, particularly relevant for the deposition of microcrystalline silicon. We use a one-dimensional time-dependent fluid model for the description of neutrals, positive and negative ions, and electrons, which involves a chemistry model taking into account 32 reactions in the gas phase and on the surface of the electrodes. The results of the model for the symmetrical system show that both pressure and frequency have pronounced influence on the parameters of the discharge: sheath thickness, ratio between power transferred to ions and electrons, and concentration and flux of atomic hydrogen at the electrode surface. We found that increasing the excitation frequency, while keeping constant the power dissipated in the plasma, leads to a more moderate increase of electron density as compared with the case of constant rf-voltage amplitude. The analysis of this phenomenon reveals that, with increase of frequency, the power coupling to the electrons becomes more efficient due to the decrease of the phase shift between voltage and current for both constant power and constant voltage conditions. There is, in addition, a significant drop of the sheath voltage with frequency when the power dissipated in the plasma is kept constant. This leads to the reduction in the drift loss rate for charged species. The increase of pressure mainly reduces the diffusive component of the loss rate for both charged and neutral species and, as a result, electron density enhancement is less pronounced. The increase of pressure leads to a more uniform spatial dissipation of the power coupled to the plasma, whereas the increase in frequency results in a higher amount of power dissipated on the plasma-sheath boundaries due to the decrease of the sheath width.
Plasma plume characteristics and properties of pulsed laser deposited diamond-like carbon films
Authors: Haverkamp J. , Mayo R. M. , Bourham M. A. , Narayan J. , Jin C. , Duscher G.
Published: March 2003

Pulsed laser deposition is a unique technique for the deposition of hydrogen-free diamond-like carbon films. During deposition, amorphous carbon is evaporated from a solid target by a high-energy KrF laser, ionized, and ejected as a plasma plume. The plume expands outwards and deposits the target material on a substrate. The plasma properties of the plume determine the quality of the thin films deposited on the substrate. These plume properties include ion density, ion flow speed, electron temperature, and plume peaking parameter. In this research, a triple Langmuir probe is used to determine various plasma properties of the plume created from the pulsed laser ablation of amorphous graphite as a function of laser energy density and laser spot size on the target. A thin diamond-like carbon film is deposited and analyzed with electron energy-loss spectroscopy to determine the sp3/sp2 fraction. A special preparation technique was used to prepare the thin film for analysis to prevent the damage that may be caused by conventional ion milling techniques.
Effects of coexistent pores and paramagnetic defects on positron annihilation in silicon oxide thin films
Authors: Yu R. S. , Ito K. , Hirata K. , Zheng W. , Kobayashi Y.
Published: March 2003

We measured the Doppler broadening of positron 2γ annihilation and the probability of positron 3γ annihilation (I3γ) for paramagnetic defects (E′ and Pb) containing SiOx (1.5<x<2) films with different porosities, which were prepared by radio-frequency cosputtering of Si and SiO2 at different Ar pressures. The observed increase of I3γ with increasing Ar pressure was attributed to the increased open porosity of the film. Under a condition of constant paramagnetic defect concentration, Doppler broadening S parameter increases with increasing I3γ and open porosity of the film. This was caused by enhancement of a relative contribution from p-Ps to the 511 keV 2γ annihilation peak as a result of reduced probability of o-Ps pickoff annihilation. The relationship between the S parameter and I3γ can be affected strongly by the paramagnetic defects in the sense that film with a higher open porosity does not necessarily present a higher S parameter. A low porosity film with higher concentration Pb centers exhibited a higher S parameter than did more porous films containing lower concentration Pb centers because in the former film a larger number of positrons annihilate from p-Ps as a consequence of the Ps spin exchange caused by the paramagnetic defects.
First principles studies for the dissociative adsorption of H2 on graphene
Authors: Miura Y. , Kasai H. , Diño W. , Nakanishi H. , Sugimoto T.
Published: March 2003

We investigate and discuss the interaction of H2 with graphene based on density functional (DFT) theory. We calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites on graphene. Our calculation results show that reconstructions of the carbon atoms play an important role in the H2 -graphene interactions. Activation barrier for H2 dissociation on an unrelaxed graphene is considerably high, ∼4.3 eV for a T–H–T geometry and ∼4.7 eV for a T–B–T geometry. The T–H–T(T–B–T) geometry means that the center of mass position of H2 is at the hollow(bridge) site, and the two H atoms are directed towards the top sites on the graphene. On the other hand, when the carbon atoms are allowed to relax, the activation barrier decreases, and becoming 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry. In this case, the two carbon atoms near the hydrogen atoms move 0.33 Å towards the gas phase for the T–H–T geometry and 0.26 Å for the T–B–T geometry.
Growth kinetics of thermal silicon oxynitridation in nitric oxide ambient
Authors: Dasgupta Anindya , Takoudis Christos G.
Published: March 2003

The chemical kinetics and process-structure relationships of silicon oxynitride films grown in nitric oxide (NO) ambient at 900–1000 °C are studied in flow reactor systems. Residual gas analyses at the reactor exit suggest that no detectable gas phase decomposition of NO takes place at these conditions. Two regions, one nitrogen-depleted (near the top dielectric surface) and one nitrogen-rich (near the dielectric/substrate interface), are considered within the dielectric film, based on reported secondary ion mass spectrometric and angled resolved x-ray photoelectron spectroscopic data. Nitrogen removal reactions are accounted for in the nitrogen-rich layer, while nitrogen incorporation reactions are considered at the dielectric/substrate interface. The diffusion-reaction growth rate model makes use of literature values for the solid-state diffusivities, reaction rate constants, and fraction of the nitrogen-rich layer. The model predictions are found to be in good agreement with all trends of the experimental data at the conditions investigated.
Approach for imaging optical super-resolution based on Sb films
Authors: Ou Ding Rong , Zhu Jing , Zhao Jia Hao
Published: March 2003

Super-resolution technology has been applied to overcome the optical diffraction limit in recent years. In this work, optical super resolution based on Sb films has been studied. Experimental results show that the amorphous Sb–SR will considerably concentrate the energy into the center of the laser beam, while the crystal Sb–SR will not. Considering Sb as a semiconductor with a small energy gap, a three-order nonlinear response of surface plasmons is deduced to explain the phenomenon. Estimation is made and the calculated results are in agreement with the experimental results.
Optical manifestation of quantized longitudinal polarization waves of excitons in thin films
Authors: Flores-Desirena B. , Silva-Castillo A. , Pérez-Rodrı́guez F.
Published: March 2003

The optical response of near-surface localized excitons and their corresponding quantized longitudinal polarization waves in thin semiconductor films on substrate is investigated theoretically. The identification of resonances associated with such longitudinal modes in simple spectra of reflectivity for p-polarized incident light (Rp) is rather difficult because of the large damping factor for a thin film near the longitudinal frequency (ωL) and a background contribution to Rp. However, as is shown here, the quantized longitudinal polarization waves manifest themselves more clearly in spectra of 45° reflectometry, which is based upon the measurement of the difference between the reflectivity Rp and the squared reflectivity for s-polarized light (Rs2) with an angle of incidence of 45°.
Magnetic behavior of amorphous CoP cylinder arrays
Authors: Shima M. , Hwang M. , Ross C. A.
Published: March 2003

Arrays of low-aspect-ratio cylindrical amorphous CoP nanomagnets with diameters near 100 nm have been fabricated using electrodeposition. The remanence of individual particles and the easy axis direction are consistent with the predictions of a micromagnetic model, but the behavior of the arrays is dominated by magnetostatic interactions because the switching fields of the particles are low compared to the magnitude of the nearest-neighbor interactions.
Effect of dilute amounts of oxygen solute on nucleation of zirconium
Authors: Wert M. J. , Hofmeister W. H. , Bayuzick R. J.
Published: March 2003

Classical nucleation theory was used as the basis for investigating the effects of dilute oxygen solute on the nucleation behavior of pure zirconium. Undercooling distributions were obtained for zirconium samples via electrostatic levitation experiments and kinetic parameters were determined using statistical analysis. Oxygen and residual impurity content were determined using glow discharge mass spectrometry. The work of formation of a critical nucleus, ΔG*, and the kinetic prefactor, Kv, increased with decreasing oxygen content. To examine the effects of oxygen on ΔG*, a sharp interface model was assumed to allow separation of volume free energy and interfacial free energy components. Dilute oxygen has a minimal effect on the volume free energy. A temperature-dependent reduced interfacial free energy, αm, was used to examine the effect of oxygen on interfacial free energy. Experimentally derived values for αm and the corresponding interface configurational entropy values varied significantly from theoretical predictions for “pure” zirconium. Oxygen compositional effects increased the configurational entropy of the interface atoms, resulting in a decrease in the reduced interfacial free energy.
Ionic conduction due to oxygen diffusion in La0.8Sr0.2GaO3−δ electrolyte
Authors: Iguchi E. , Nakamura S. , Munakata F. , Kurumada M. , Fujie Y.
Published: March 2003

Dielectric and dc measurements have been carried out on La0.8Sr0.2GaO3−δ, which has a high potential for the electrolyte material in a solid oxide fuel cell, so as to elucidate the correlation between O2− diffusion and electric conduction. A dielectric relaxation shows up in loss tangent. The relative relation of activation energies as to dielectric relaxation and dc conduction suggests the ionic conduction due to O2− diffusion assisted by free single oxygen vacancies taking place. The change of dc conductive behavior associated with attenuation of dielectric relaxation around 950 K indicates that another type of O2− diffusion dominates the ionic conduction at high temperature.
Trapped metastable phases in Pb(Zn1/3Nb2/3)O3–(8–9)%PbTiO3 single-crystal wafers
Authors: Lim L. C. , Kumar F. J. , Amin A.
Published: March 2003

In addition to locally stressed rhombohedral phase, evidence for trapped metastable phases in (001)-cut single-crystal wafers of Pb(Zn1/3Nb2/3)O3–(8–9)%PbTiO3 has been obtained by means of the x-ray diffraction technique. The rhombohedral phase is stressed predominantly in compression in the plane of the wafer, both in unpoled and poled conditions, giving rise to the broad shoulder at the lower 2θ end of the rhombohedral peak. Trapped tetragonal phase, with its c axis lying both in the plane and perpendicular to the plane of the wafer, can also be detected in as-sliced, unpoled wafers. Upon poling along the [001] thickness direction, the tetragonal phase with its c axis lying in the plane of the wafer is transformed readily to the rhombohedral phase. In contrast, a minute amount of tetragonal phase with its c axis perpendicular to the plane of the wafer can be stabilized by the stressed rhombohedral matrix in poled state. At sufficiently high poling fields (i.e., ⩾1.2 kV/mm), a trapped pseudomonoclinic phase may be formed among the locally stressed rhombohedral lattice.