bluuebunny/crossref_metadata_2025 huggingface dataset:
Preisach model for the simulation of ferroelectric capacitors
Authors: Bartic Andrei T. , Wouters Dirk J. , Maes Herman E. , Rickes Jürgen T. , Waser Rainer M.
Published: July 2002

The emerging ferroelectric technology needs a reliable model for the simulation of the ferroelectric capacitors. This model would play a crucial role in designing new ferroelectric nonvolatile memories. As a main requirement, such a model must allow the calculation of the polarization variations for an arbitrary voltage applied to the ferroelectric. However, in spite of the large efforts made in modeling, most of the existing solutions fail to satisfy the above requirement or lack a minimal physical background. To address these problems, we developed a model based on a ferroelectric interpretation of the Preisach theory of hysteresis. In this articles, we try to elucidate how this theory, initially developed for ferromagnetic particles, can be adapted to the ferroelectric materials, despite the many differences between the two. Because the Preisach theory assumes a distribution of the coercitive voltages, we try to clarify its physical meaning in the case of the ferroelectric materials and propose a methodology to determine this distribution experimentally. To facilitate the implementation of the model, the experimental results are then fitted by an analytic function and the whole bidimensional distribution is calculated using a linear approximation. To evaluate the validity of the model, we performed simulations using the Spectre® circuit simulator and the results are in very good agreement with the measurements for the saturated hysteresis loops. The differences existing for the partial loops are mainly due to the linear approximation used for the Preisach distribution. This model can be successfully used for the design of the real memories.
Laser heated diamond anvil apparatus at the Photon Factory and SPring-8: Problems and improvements
Authors: Yagi Takehiko , Kondo Tadashi , Watanuki Tetsu , Shimomura Osamu , Kikegawa Takumi
Published: July 2002

Two high pressure high temperature in situ x-ray diffraction systems constructed at the Photon Factory and at SPring-8 are described. Both systems consist of a laser heated diamond anvil cell, CO2 or YAG laser heating systems, and an angle dispersive powder x-ray diffraction system with an imaging plate detector. CO2 laser heating has an advantage in that the sample can be heated stably and uniformly due to stable absorption of the sample. Use of type-IIa diamond, however, caused plastic deformation of the anvils. YAG laser heating, compared to CO2 laser heating, can be used to much higher pressures and temperatures, although a good control system is required for stable heating. Various problems encountered during construction are discussed.
Magnetohydrodynamic scaling: From astrophysics to the laboratory
Authors: Ryutov D. D. , Remington B. A. , Robey H. F. , Drake R. P.
Published: July 2002

During the last few years, considerable progress has been made in simulating astrophysical phenomena in laboratory experiments with high-power lasers. Astrophysical phenomena that have drawn particular interest include supernovae explosions; young supernova remnants; galactic jets; the formation of fine structures in late supernovae remnants by instabilities; and the ablation-driven evolution of molecular clouds. A question may arise as to what extent the laser experiments, which deal with targets of a spatial scale of ∼100 μm and occur at a time scale of a few nanoseconds, can reproduce phenomena occurring at spatial scales of a million or more kilometers and time scales from hours to many years. Quite remarkably, in a number of cases there exists a broad hydrodynamic similarity (sometimes called the “Euler similarity”) that allows a direct scaling of laboratory results to astrophysical phenomena. A discussion is presented of the details of the Euler similarity related to the presence of shocks and to a special case of a strong drive. Constraints stemming from the possible development of small-scale turbulence are analyzed. The case of a gas with a spatially varying polytropic index is discussed. A possibility of scaled simulations of ablation front dynamics is one more topic covered in this paper. It is shown that, with some additional constraints, a simple similarity exists.
Femtosecond-laser-induced spallation in aluminum
Authors: Tamura H. , Kohama T. , Kondo K. , Yoshida M.
Published: July 2002

Ultrashort laser pulses having 50 fs duration were used to cause planer fracture, i.e., spallation, in pure aluminum foils at intensities of (0.07–1)×1015 W/cm2. A linear relationship was obtained between spall depth and foil thickness, approximately corresponding to the proportions that were obtained in other relations for 300 ps and longer laser pulse irradiation. Spall thicknesses less than 5 μm were obtained in foils thinner than 50 μm, and revealed small-scale spallation at a strain rate exceeding 108 s−1. This ultrashort laser pulse evidently produced a shock wave followed by a steep unloading stress profile.
The effect of topographical local charging on the etching of deep-submicron structures in SiO2 as a function of aspect ratio
Authors: Matsui Jun , Nakano Nobuhiko , Petrović Zoran Lj. , Makabe Toshiaki
Published: July 2002

Physical and electrical influences on plasma etching on the inside of a microtrench in SiO2 were numerically investigated using Monte Carlo simulation of ions and electrons with the aid of surface charge continuity and Poisson’s equation. When the aspect ratio is greater than seven, the bottom is charged up to a potential sufficient to prevent the influence of all the incident ions, with a realistic initial energy of 300 eV for SiO2 etching within the period required for monolayer stripping, resulting in etch stop. The cause of etch stop is purely the result of the electrical local charging due to the topography of the trench, and of the initial conditions for incident charged particles. The etch stop caused by a cw plasma will be disorganized or prevented within a short time by the aid of ion–ion plasma in an afterglow phase.
Superconducting quantum interference device as a near-quantum-limited amplifier at 0.5 GHz
Authors: Mück Michael , Kycia J. B. , Clarke John
Published: July 2002

A dc superconducting quantum interference device (SQUID) with a resonant microstrip input is operated as an amplifier at temperatures down to 20 mK. A second SQUID is used as a postamplifier. Below about 100 mK, the noise temperature is 52±20 mK at 538 MHz, estimated from measurements of signal-to-noise ratio, and 47±10 mK at 519 MHz, estimated from the noise generated by a resonant circuit coupled to the input. The quantum-limited noise temperatures are 26 and 25 mK, respectively. The measured noise temperature is limited by hot electrons generated by the bias current.
Characterization of deep levels in InGaP grown by compound-source molecular beam epitaxy
Authors: Kim J. H. , Jo S. J. , Kim J. W. , Song J.-I.
Published: July 2002

Deep levels in Si-doped In0.49Ga0.51P grown by compound-source molecular beam epitaxy (MBE) have been investigated by deep level transient spectroscopy. In0.49Ga0.51P samples were grown by compound-source MBE with V/III ratios of 4, 10, and 17. Depending upon the V/III ratio three major deep levels with activation energies of 0.26±0.02, 0.36±0.02, and 0.82±0.05 eV were observed. The effect of thermal annealing on the behavior of deep levels was also investigated. The deep levels in InGaP grown by compound-source MBE showed behavior of phosphorus antisites and related complexes unlike those found in solid-source MBE-grown InGaP that showed behavior of phosphorus vacancies and related complexes. Si-doped InGaP layers grown with a V/III ratio of 4 showed trap concentration and capture cross section as low as 1.38×1014 cm−3 and 2.9×10−16 cm2, respectively. The results indicate the potential of InGaP grown by compound-source MBE for use in improved low-frequency noise applications.
Removal of small particles on silicon wafer by laser-induced airborne plasma shock waves
Authors: Lee J. M. , Watkins K. G.
Published: July 2002

It has been found that effective removal of small particles from silicon wafer surfaces can be achieved by laser-induced airborne plasma shock waves initiated above the surface. This process has demonstrated successfully the removal of 1 μm sized tungsten particles from the surface. Tungsten is known to be one of the most difficult particles to remove using conventional laser cleaning techniques employing direct laser irradiation onto the surface. The area cleaned by the shock waves was over ten times larger than that achieved by conventional laser cleaning. This provides an advantage in speed for cleaning large areas. The cleaning efficiency was strongly dependent on the gap distance between the laser focus (where laser-induced sparking takes place) and the surface. The particle removal began to occur with a gap of 6 mm and the removal efficiency increased significantly with decreasing the gap resulting in a complete removal at 2 mm. The basic idea behind this new cleaning technique and how to apply it for the removal of small particles from surfaces are described. Theoretical considerations about adhesion forces of small particles on the surface and laser-induced shock wave generation are carried out to understand this process.
Intermolecular exchange-induction and charge transfer: Derivation of approximate formulas using nonorthogonal localized molecular orbitals
Authors: Jensen Jan H.
Published: July 2002

Previous work on the efficient evaluation of the second-order exchange repulsion energy [Mol. Phys. 89, 1313 (1996)] is extended to exchange induction and charge transfer energies. The approximations outlined in the previous work are shown to be sufficient to simplify all new terms in the exchange induction and charge transfer energy terms. Both variational and couple perturbed Hatree–Fock approaches are discussed as is the use of these new energy terms in conjunction with hybrid “quantum mechanical/molecular mechanics” methods [specifically the effective fragment potential method, J. Chem. Phys. 105, 1968 (1996)].
Accuracy of free-energy perturbation calculations in molecular simulation. I. Modeling
Authors: Lu Nandou , Kofke David A.
Published: July 2002

We examine issues involved in applying and interpreting free-energy perturbation (FEP) calculations in molecular simulation. We focus in particular on the accuracy of these calculations, and how the accuracy differs when the FEP is performed in one or the other direction between two systems. We argue that the commonly applied heuristic, indicating a simple average of results taken for the two directions, is poorly conceived. Instead, we argue that the best way to proceed is to conduct the FEP calculation in one direction, namely that in which the entropy of the target is less than the entropy of the reference. We analyze the behavior of FEP calculations in terms of the perturbation-energy distribution functions, and present several routes to characterize the calculations in terms of these distributions. We also provide prescriptions for the selection of an appropriate multistage FEP scheme based on how the important phase-space regions of the target and reference systems overlap one another.
Determination of equilibrium pitch of cholesteric phases by isobaric–isothermal Monte Carlo simulation
Authors: Memmer R.
Published: July 2002

In canonical Monte Carlo simulations of chiral liquid crystals, generally cholesteric phases with nonequilibrium pitch were formed under periodic boundary conditions. In order to overcome system-size effects the phase diagram of chiral calamitic liquid crystals, described here by the chiral Gay–Berne fluid, was now studied by simulations in the isothermal–isobaric ensemble (NpT). In the temperature–chirality parameter plane a rich polymorphism could be proven. In the cholesteric phase the ability of the simulation box to change the dimensions during the simulations enabled the determination of the equilibrium pitch in dependence on the chirality parameter describing the strength of the chiral interaction. Additionally, in dependence on temperature first-order phase transitions characterized by phase transition enthalpy, entropy and relative volume change have been observed. On heating a cholesteric phase (N*), for the first time by computer simulation of a many-particle system based on model intermolecular interactions a temperature-driven phase transition to a blue phase could be proven, a characteristic feature of many experimental and theoretical phase diagrams of chiral liquid crystals. Additionally, on cooling a cholesteric system a phase with a helical superstructure and smectic layers formed perpendicular to the helical axis, both characteristics of a twist grain boundary (TGB) phase, has been observed. This clear evidence shows that a chiral interaction proportional to the first pseudoscalar term of the expansion in rotational invariants is able to yield the observed phase sequence TGB-like phase, cholesteric phase, blue phase, and isotropic phase in dependence on one scalar parameter given here by the temperature, only.
Dielectric properties of strained (Ba, Sr)TiO3 thin films epitaxially grown on Si with thin yttria-stabilized zirconia buffer layer
Authors: Jun Sungjin , Kim Young Sung , Lee Jaichan , Kim Young Woon
Published: July 2002

We have grown epitaxial (Ba0.5, Sr0.5)TiO3 (BST) thin films on Si with very thin yttria-stabilized zirconia (YSZ) buffer layer. The thin YSZ buffer layer affects the stress state of the epitaxial BST layer as well as the growth behavior of the BST layer, i.e., the degree of epitaxy. The epitaxial BST films grown on Si are in two-dimensional tensile stress. We have found that the dielectric constant of the BST films increased with decreasing the lattice distortion in the state of tensile stress. The dielectric constant of the BST films reaches 1300 when the lattice distortion (surface normal lattice constant/in-plane lattice constant) is 0.993.
Ferroelectricity and ferrimagnetism in iron-doped BaTiO3
Authors: Maier R. , Cohn J. L. , Neumeier J. J. , Bendersky L. A.
Published: July 2002

The structural and physical properties of pseudocubic thin-film BaTi1−xFexO3(0.5⩽x⩽0.75) grown by pulsed-laser deposition are reported. This material is of interest because the corresponding bulk compounds have hexagonal structure for comparable x, and because the films are both ferroelectric and ferrimagnetic well above room temperature. A substantial increase of the ferroelectric transition temperature relative to that of bulk BaTiO3 is attributed to lattice expansion induced by Fe doping.
The behavior of diffusion eigenmodes in the presence of internal magnetic field in porous media
Authors: Lisitza N. V. , Song Y.-Q.
Published: July 2002

The eigenmodes of spin diffusion provide a mathematical solution to the diffusion equation and the eigenmode spectrum can be used to characterize the internal space of porous media. The dynamics of these eigenmodes is affected by the presence of an inhomogeneous magnetic field inside the pores. In this paper we examine the behavior of the diffusion eigenmodes as a function of the spatially dependent encoding phase due to the inhomogeneous internal field. We show that in the limit of weak phase encoding, the first diffusion modes are predominantly excited. In this regime the excited signal is found to be linearly proportional to the encoding phase and the eigenmode spectrum reflects the pore size distribution. When the encoding phase is larger than 2π, the spectrum can be dominated by the higher eigenmodes.
Si
29
magic-angle-spinning nuclear-magnetic-resonance study of spinel-type Si3N4
Authors: Sekine T. , Tansho M. , Kanzaki M.
Published: July 2002

Si 29 magic-angle-spinning nuclear-magnetic resonance has been measured on spinel-type cubic silicon nitride (c-Si3N4). c-Si3N4 shows two Si29 resonances at −50.0±0.2 and −225.0±0.2 ppm, corresponding to the tetrahedron SN4 and the octahedron SiN6, respectively. Integration of the spectrum gives SN4/SiN6 about one half that of the spinel structure. Ab initio self-consistent field Hartree–Fock molecular orbital calculations also indicate that the chemical shift for octahedral Si is more negative in nitride than in oxides.
Low-frequency flux noise and visualization of vortices in a YBa2Cu3O7 dc superconducting quantum interference device washer with an integrated input coil
Authors: Straub R. , Keil S. , Kleiner R. , Koelle D.
Published: July 2002

We used low-temperature scanning electron microscopy (LTSEM) for imaging quantized magnetic flux (vortices) in dc superconducting quantum interference devices (SQUIDs) with approximately 1 μm spatial resolution at temperature T=77 K in a controllable magnetic field up to 20 μT. We demonstrate that LTSEM allows to image the spatial distribution of vortices in a YBa2Cu3O7/SrTiO3/YBa2Cu3O7 multilayer thin-film structure consisting of a dc SQUID washer with an integrated input coil on top. Simultaneously, we are able to measure the low-frequency noise of the sample under test, which allows to correlate the spatial distribution of vortices with low-frequency noise in the SQUID.
Effects of material growth technique and Mg doping on Er3+ photoluminescence in Er-implanted GaN
Authors: Kim S. , Henry R. L. , Wickenden A. E. , Koleske D. D. , Rhee S. J. , White J. O. , Myoung J. M. , Kim K. , Li X. , Coleman J. J. , Bishop S. G.
Published: July 2002

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the ∼1540 nm I13/24–I15/24 emissions of Er3+ in Er-implanted and annealed GaN. These studies revealed the existence of multiple Er3+ centers and associated PL spectra in Er-implanted GaN films grown by metalorganic chemical vapor deposition, hydride vapor phase epitaxy, and molecular beam epitaxy. The results demonstrate that the multiple Er3+PL centers and below-gap defect-related absorption bands by which they are selectively excited are universal features of Er-implanted GaN grown by different techniques. It is suggested that implantation-induced defects common to all the GaN samples are responsible for the Er site distortions that give rise to the distinctive, selectively excited Er3+PL spectra. The investigations of selectively excited Er3+PL and PLE spectra have also been extended to Er-implanted samples of Mg-doped GaN grown by various techniques. In each of these samples, the so-called violet-pumped Er3+PL band and its associated broad violet PLE band are significantly enhanced relative to the PL and PLE of the other selectively excited Er3+PL centers. More importantly, the violet-pumped Er3+PL spectrum dominates the above-gap excited Er3+PL spectrum of Er-implanted Mg-doped GaN, whereas it was unobservable under above-gap excitation in Er-implanted undoped GaN. These results confirm the hypothesis that appropriate codopants can increase the efficiency of trap-mediated above-gap excitation of Er3+ emission in Er-implanted GaN.
Transport properties of two-dimensional electron gases containing linear ordering InAs self-assembled quantum dots
Authors: Kim Gil-Ho , Ritchie D. A. , Liang C.-T. , Lian G. D. , Yuan J. , Pepper M. , Brown L. M.
Published: July 2002

We present a study of the anisotropic properties of two-dimensional electron gases formed in GaAs/AlGaAs heterostructures in which InAs self-assembled quantum dots have been inserted into the center of a GaAs quantum well. We observe an anisotropic mobility for the orthogonal [1̄10] and [110] directions. The mobility in the [1̄10] direction was found to be up to approximately twice that in the [110] direction. It is suggested that the interface roughness scattering due to the inserted InAs material could be a cause for the large anisotropies in mobility.
Optical-gain enhancement in two-dimensional active photonic crystals
Authors: Nojima S.
Published: July 2002

Optical-gain-enhancement effects are numerically investigated in detail for two-dimensional photonic crystals with active (gain) lattice points. First, this gain enhancement is shown to occur in the vicinity of every photonic band edge. A thorough examination is carried out to isolate the leading factors that determine this enhancement on the basis of a number of (more than 1000) numerically calculated data points. The gain-enhancement factor is uniquely determined by the product of the time for light to pass a gain rod and the confinement factor of the field energy in the gain rod, irrespective of the choice of dielectric constants, wave numbers, band indices, and polarization directions of light. In designing photonic crystals, therefore, more gentlly sloped bands and a greater contrast in the dielectric constant between gain and background materials are essential in order to intensify optical gain more effectively.
Theory of ultrafast photoinduced heterogeneous electron transfer: Decay of vibrational coherence into a finite electronic–vibrational quasicontinuum
Authors: Ramakrishna S. , Willig F. , May V.
Published: July 2002

Photo-induced electron transfer from a surface attached dye molecule to the band levels of a semiconductor is modeled via an electronic–vibronic quasicontinuum. The description enables one to obtain a fairly accurate expression for the decay of the excited molecular state, including initial vibronic coherences. The model accounts for (a) the effect of a finite band width, (b) variations in reorganization energy and electronic coupling, (c) various energetic positions for the injecting level, (d) different initial vibrational wave packets in the excited state, and (e) two vibrational modes participating in the electron transfer process. Most cases are studied numerically and can be reasonably well understood from the obtained decay expression.
Prediction of effective elements for magnetically induced phase separation in Co–Cr-based magnetic recording media
Authors: Oikawa K. , Qin G. W. , Kitakami O. , Shimada Y. , Fukamichi K. , Ishida K.
Published: July 2002

Thermodynamic calculations of the magnetically induced phase separation of the hcp phase in Co–Cr-based alloy systems, which is the origin of the compositional modulation observed in magnetic recording media, have been carried out. The magnetic and nonmagnetic terms in the Gibbs energy are evaluated from available thermodynamic data and Miedema’s semiempirical values. It is demonstrated that the phase separation in Co–Cr–X ternary alloys can be classified into four types, depending on the values of interaction energies between Co and/or Cr and X atoms. Existing magnetic data on Co–Cr-based recording media are discussed in terms of the phase stability of the hcp phases. The present calculations would be useful for development of high-density magnetic recording media.
Elastic, piezoelectric, and dielectric properties of multidomain 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 single crystals
Authors: Zhang Rui , Jiang Bei , Cao Wenwu
Published: July 2002

The elastic, piezoelectric, and dielectric constants of 0.67Pb(Mg1/3Nb2/3)O3–0.33PbTiO3 domain engineered single crystal were determined experimentally by using ultrasonic and resonance methods. It was confirmed that the single crystal system has large electromechanical coupling coefficient k33 (∼94%) and piezoelectric constant d33 (∼2800 pC/N) if the poling is done along the [001] of pseudocubic directions. A soft shear mode with a velocity of 880 m/s was observed in the [110] direction with displacement in [1̄10]. Using the measured data, the orientation dependence of phase velocities and electromechanical coupling coefficients were calculated. The origin of experimental errors and their influence on measured results are also examined.
Polymer light-emitting devices using ionomers as an electron injecting and hole blocking layer
Authors: Lee Tae-Woo , Park O Ok , Do Lee-Mi , Zyung Taehyoung , Ahn Taek , Shim Hong-Ku
Published: July 2002

The effect of ion concentration, neutralization level and counterions in ionomers was systematically studied to obtain the optimal electroluminescent (EL) characteristics in polymer light-emitting diodes using poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylenevinylene] (MEH-PPV) for the emissive layer and sulfonated polystyrene (SPS) ionomers for the electron-injecting layer. The optimum ion concentration of NaSPS was determined to be at 6.7 mol %. Ionomers with a higher neutralization level make the EL device more efficient, with the highest efficiency being at 200% overneutralization. The ionomer with a smaller metal counter ion greatly enhances the efficiency of EL devices with the indium–tin–oxide/MEH-PPV/LiSPS/Al device having the highest EL quantum efficiency, 1.18% photons/electron. The dominant factor in enhancing the luminance is the number of ionic dipoles near the cathode irrespective of the type of metal counterions, while the hole blocking mostly depends on the restriction of chain segmental motion in ionomers.
Inhomogeneous charge states and electronic transport in manganites
Authors: Kagan M. Yu. , Kugel K. I. , Rakhmanov A. L. , Khomskii D. I.
Published: July 2002

We analyze the interplay between charge ordering, magnetic ordering, and the tendency toward phase separation and its importance for the physics of manganites. A simple model of charge ordering is considered. It takes into account both the Coulomb repulsion at neighboring sites responsible for charge ordering and the essential magnetic interactions. It is shown explicitly that at any deviation from half-filling (n≠1/2) the system is unstable with respect to phase separation into charge-ordered regions with n=1/2 and metallic regions with smaller electron or hole density. A possible structure of this phase-separated state (metallic droplets in a charge-ordered matrix) is discussed. We construct the phase diagram reproducing the main features observed in real manganites. Based on the same physical picture, we also derive explicit expressions for the dc conductivity and noise power in the phase-separated state. It is shown that the noise spectrum has a nearly 1/f form in the low-frequency range.
Classical interatomic potentials for Si–O–F and Si–O–Cl systems
Authors: Ohta H. , Hamaguchi S.
Published: July 2002

Stillinger–Weber (SW)-type potential sets have been developed for Si–O–F and Si–O–Cl systems based on interatomic potential energy data obtained from ab initio quantum-mechanical calculations. We have constructed the new potential sets in such a way that the obtained potentials are supersets of existing well-known SW-type potentials for Si, SiO2, and Si-halogen systems. Our aim of the potential development is to perform molecular dynamics (MD) simulations for both silicon and silicon dioxide etching by F or Cl on the same footing. Presented in this article are details of the potential derivation and some sample MD simulation results.
Influence of disorder on the photoinduced excitations in phenyl substituted polythiophenes
Authors: Brabec Christoph J. , Winder Christoph , Scharber Markus C. , Sariciftci N. Serdar , Hummelen Jan C. , Svensson Mattias , Andersson Mats R.
Published: July 2002

Regioregular poly(3-(4′-(1″,4″,7″-trioxaoctyl)phenyl)thiophenes) (PEOPTs) exhibit interesting properties for the use in polymer electronics. Exposing thin films of the amorphous, disordered phase (orange phase) of the “as prepared” polymer to chloroform vapor or annealing them by heat treatment results in a redshift of the absorption maximum due to the formation of nanocrystals in an ordered phase (blue phase). As such, PEOPT thus is a very interesting conjugated polymeric material, which exhibits two different phases with well-defined order/disorder characters on one-and-the-same material. This property opens up the unique possibility to investigate the role of order/disorder on the photoexcited pattern without being obscured by the differences in chemical structure by using different materials with different crystallinity. The fact, that blue phase PEOPT exhibits absorption edges at relatively low energies around 1.8 eV, thereby demonstrating an enhanced spectral absorption range as compared to the orange phase, makes them attractive for use in photodiodes and solar cells as well. The photoinduced charge generation efficiency in both phases of PEOPT is significantly enhanced by the addition of a strong electron acceptor such as fullerene C60, as observed by quenching of the luminescence and by photoinduced absorption measurements in the infrared and uv–visible regime. The average number and the lifetime of photoinduced carriers in composites of PEOPT with a methanofullerene [6,6]-phenyl C61–butyric acid methyl ester (PCBM) are found to depend on the crystallinity of PEOPT in thin films, which gives rise to charged photoexcitations delocalized between polymer chains. Stronger bimolecular recombination in composites of the blue phase PEOPT with PCBM is observed as compared to the orange phase PEOPT/PCBM films. The origin of this enhanced recombination is found to be related to the hole mobility of the polymer.
Relationship between gain and Yb3+ concentration in Er3+–Yb3+ doped waveguide amplifiers
Authors: Strohhöfer Christof , Polman Albert
Published: July 2002

We have used a rate equation propagation model of an Er3+/Yb3+ doped Al2O3 waveguide amplifier with copropagating pump at 980 nm to investigate the dependence of gain on Yb3+ concentration. The model includes excited state absorption and energy transfer upconversion processes within the Er3+ as well as the relevant energy transfer processes between Yb3+ and Er3+. The results of the calculations indicate a close relationship of the parameters gain, launched pump power, waveguide length, and Yb3+ concentration. Codoping with a well-chosen Yb3+ concentration is shown to increase the gain around 1530 nm for all combinations of these parameters. The gain is improved most by Yb3+ codoping at pump powers around the amplifier threshold. At high pump powers the increase in gain of an Er3+/Yb3+ doped waveguide is insignificant compared to that of its Er3+ doped counterpart. Furthermore for each launched pump power, a nonzero Yb3+ concentration can be determined, which maximizes the gain.
Piezo-driven metrological multiaxis nanopositioner
Authors: Shim Jong-Youp , Gweon Dae-Gab
Published: July 2002

We report on the development of a metrological multiaxis nanopositioning device, which is operated by the piezo-based inertial method, as a sample stage for scanning probe microscopy. It has long moving range, unlimited in principle, and nanometer (microradian) resolution. Two operation methods, inertial sliding and inertial walking, can be applied to the stage and the inertial operating method can make the stage have a simple and compact structure. By the structure and operation method high positioning stability can be obtained which is an important requirement for scanning probe microscopy. For a metrological nanopositioner, a three axes laser interferometric sensing scheme is adopted for planar motion and a 15 channel high voltage amplifier is designed and computer based digital-to-analog conversion is adopted. Therefore the nanopositioner can be feedback controlled with many choices of voltage wave forms and control methods. Design of the nanopositioner and piezo-driver and experimental results are presented. The device provides step sizes of 0.016–10 μm at frequency up to about 7 kHz. The rotational range is limited by the interferometer alignment, 0.2°, and the step size is 0.17–103 arcsec.
Improvement of the optical properties of metalorganic chemical vapor deposition grown GaN on sapphire by an in situ SiN treatment
Authors: Haffouz S. , Kirilyuk V. , Hageman P. R. , Macht L. , Weyher J. L. , Larsen P. K.
Published: July 2002

We have studied the effect of an in situ SiN treatment of sapphire substrates on the optical properties of GaN films grown by metalorganic chemical vapor deposition. The SiN deposition, partially covering the substrate, forms a mask for the formation of nanoscale auto-organized GaN islands. These islands formed upon an increase of the temperature after deposition of a GaN buffer layer on this mask. A photoluminescence study of the GaN epilayers obtained by lateral overgrowth of these islands shows significant enhancement of the luminescence emission intensity of the near band edge peaks and a reduction of the full width at half maximum of the donor bound exciton (D0X) peak by 32% down to 4 meV compared to in our standard process. The GaN films grown using SiN treatment are highly stressed as evidenced by a blueshift of 10 meV in the D0X peak energies. Photoelectrochemical etching in aqueous solution of KOH was applied to reveal the dislocation density. The density of “whisker-like” etch features, which form due to the presence of dislocations, was reduced from 6×109 cm−2 in standard GaN films to 8×108 cm−2 in the GaN layers grown with the optimized SiN treatment.
Nonlinear ground-state absorption observed in a single quantum dot
Authors: Beham Evelin , Zrenner Artur , Findeis Frank , Bichler Max , Abstreiter Gerhard
Published: July 2002

We report level bleaching in the ground state of a single In0.5Ga0.5As quantum dot. This behavior arises from the nonlinear absorption of a single quantum state. The level bleaching is observed in terms of a saturation of the photocurrent with increasing excitation power under the condition of resonant excitation in the quantum dot ground state. Furthermore, the photocurrent saturation is put down to a fundamental rate equation model. The steady-state solutions are in good agreement with the experimentally observed power dependence of the photocurrent.
Complete active space self-consistent field and multireference configuration interaction studies of the low-lying excited states of the ClO–H2O radical complex
Authors: Li Yumin , Francisco Joseph S.
Published: July 2002

The vertical excited states for the three lowest excited states of the ClO–H2O radical complex are calculated with complete active space self-consistent field and multireference configuration interaction and methods. A new state unique to the complex is reported.
Parametric frequency tuning of phase-locked nanoelectromechanical resonators
Authors: Kraus Andreas , Erbe Artur , Blick Robert H. , Corso Gilberto , Richter Klaus
Published: July 2002

We present measurements on nanoelectromechanical systems fabricated in semiconductor materials suited for operating frequencies in the radio band. We demonstrate how to achieve parametric frequency tuning of nanoelectromechanical resonators required for radio frequency sensor and communication applications.
Very-low-energy electron microscopy of doped semiconductors
Authors: El-Gomati M. M. , Wells T. C. R.
Published: July 2002

Imaging of As- and B-doped silicon regions has been performed in a scanning electron microscope operated in the cathode lens mode, with incident electron energies (EP) as low as 15 eV. The doped regions of n+ (As, 2.5×1020 cm−3) and p+ (B, 8×1019 cm−3) on n-type silicon (∼1015 cm−3) show distinct contrast with electron energies of about 3 keV. The brightest region is n+ followed by p+, then the n-type substrate. The highest contrast for the p+ and n+ type regions is reached at about EP=300 and 15 eV, respectively. The contrast mechanisms are explained in terms of metal-semiconductor contact assuming an adventitious carbon film at the surface.
Triplet-triplet annihilation in a poly(fluorene)-derivative
Authors: Hertel Dirk , Bässler Heinz , Guentner Roland , Scherf Ullrich
Published: July 2002

We report on measurements of phosphorescence (Ph) and delayed fluorescence (DF) in poly(2,7-(9,9-bis(2-ethylhexyl)fluorene)) (PF2/6) present as dilute solid solution, as well as in bulk films. From combined experimental investigations of the time and intensity dependence of DF and Ph, as well as the temperature dependence of DF, we are able to show that DF in PF2/6, both in solution and film, is dominated by triplet-triplet annihilation (TTA). As a consequence of the intrachain diffusion and, hence, relaxation of triplets, the kinetics of the bimolecular reaction show a turnover from dispersive to nondispersive regime as borne out by the DF decay. This is in accord with theoretical predictions of TTA in disordered solids. From the transition to equilibrium kinetics, we are able to estimate the width of the density of states of triplets to 40 meV in good agreement with the width of the inhomogeneous broadenend S0←T1 transition of PF2/6.
Effect of oxygen stoichiometry on the electrical properties of zirconia gate dielectrics
Authors: Ramanathan Shriram , Muller David A. , Wilk Glen D. , Park Chang Man , McIntyre Paul C.
Published: July 2002

In this letter, we report on electrical and microstructural properties of ultrathin zirconia dielectrics grown on SiO2 by ultraviolet (UV) ozone oxidation and natural oxidation (no UV light). Capacitance–voltage (C–V) measurements were performed at multiple frequencies on capacitors fabricated from a ZrO2–SiO2 stack. It was found that the C–V curves from samples grown by natural oxidation were distorted and showed severe frequency dependence while samples grown with UV light exposure under otherwise identical conditions had superior electrical behavior. Loss tangent measurements and detailed electron energy loss spectroscopy studies performed on the two samples revealed that the sample grown by natural oxidation was highly oxygen deficient, and this led to its poor electrical properties.
Torsional barrier, ionization potential, and electron affinity of biphenyl—A theoretical study
Authors: Arulmozhiraja Sundaram , Fujii Toshihiro
Published: July 2002

The structure and torsional barriers at 0° and 90° for biphenyl were studied by both ab initio and density functional methods by using various levels of theory with different basis sets. The torsional angle (φ) calculated at the MP2/6-311+G(2d,2p) level was 42.1°, while φ calculated using various density functionals with different basis sets was close to 40°. In contrast with the ab initio results, the torsional barrier at 0° [ΔE0=E(φ=0°)−E(equilibrium)] obtained using various density functionals coincided well with experimental values. The torsional barrier ratio (ΔE90/ΔE0) obtained at the B3LYP/cc-pVTZ level, 1.0988, agreed well with the experimental value, 1.0833, whereas it was 0.416 at the MP2/6-311G(d,p) level. Structural studies on biphenyl ions showed that the biphenyl cation has a nonplanar (φ=19°) structure whereas its anionic counterpart has a planar structure. The ionization potential obtained at the B3LYP/6-311+G(2d,2p) level was 7.86 eV. Contrary to an earlier study, a positive electron affinity (EA) was obtained, in accordance with experimental predictions. EA values of 0.021 and 0.076 eV were obtained at the B3LYP/6-311+G(2d,2p) and B3LYP/aug-cc-pVDZ levels, respectively.
Comparison between dynamic plowing lithography and nanoindentation methods
Authors: Cappella B. , Sturm H.
Published: July 2002

Two different methods of nanolithography, namely dynamic plowing lithography (DPL) and indentation by means of force-displacement curves (FDI), have been compared by performing them on two different polymers, poly (methylmethacrylate) and polystyrene. No fundamental differences can be found out in the efficiency of the two methods, i.e., in the depth of the lithographed structure as a function of the scanning parameters. The main drawback of FDI is that it is much more time consuming than DPL. On the other hand, when the sample is lithographed with DPL, the border walls that surround the lithographed structure are very much bigger than the border walls created through FDI. The physicochemical properties of the border walls created through DPL have been investigated. Several experimental data reveal that such border walls are very much softer and looser than unmodified polymers and suggest that during DPL the fast oscillating tip is able to break polymer chains. This does not happen with FDI, where border walls are simply made up of the material carved out by the tip. A method to eliminate these undesirable border walls is suggested.
Reversible photoinduced spectral change in Eu2O3 at room temperature
Authors: Mochizuki Shosuke , Nakanishi Tauto , Suzuki Yuya , Ishi Kimihiro
Published: July 2002

When Eu2O3 powder compact and film are irradiated with ultraviolet (UV) laser light in a vacuum, their photoluminescence (PL) spectra change from a red sharp-line structure to a white broad band, which can be clearly seen with the naked eye. After removing the UV laser light, the white PL continues for more than several months at room temperature under room light, in spite of any changes of atmosphere. By irradiating with the same UV laser light at room temperature under O2 gas atmosphere, the original red PL state reappears. Such a reversible phenomenon may well yield materials for white-light-emitting devices and erasable optical storage.
High-k titanium silicate dielectric thin films grown by pulsed-laser deposition
Authors: Sarkar D. K. , Desbiens E. , El Khakani M. A.
Published: July 2002

We report the pulsed-laser deposition of high-k titanium silicate thin films. The titanium silicate films were deposited by laser ablating a (Ti, Si) target in an oxygen pressure of 70 mTorr. The deposited films were found to exhibit nanocrystalline structure with a grain size of about 3.5 nm as deduced from x-ray diffraction measurements. Fourier transform infrared spectroscopy confirmed the existence of Ti–O–Si bonds in the films. The binding energies of [Ti 2p3/2 (458.6 eV), Si 2p (102.3 eV), O 1s (531.8 eV)] as measured by means of x-ray photoelectron spectroscopy provided evidence of titanium silicate. On the other hand, the average dielectric constant of the pulse laser deposited titanium silicate thin films were found to be about 11 in the frequency range of 100 kHz to 13 MHz.
A technique and a method for the continuous, simultaneous, and automatic measurement of density and speed of sound in pure liquids and solutions
Authors: Junquera Elena , Ruiz Mariano , López Sergio , Aicart Emilio
Published: July 2002

A highly accurate experimental technique is presented for the continuous, simultaneous, and automatic determination of speed of sound and density in pure liquids and solutions. The digital elements of the equipment are fully computerized by a PC, via IEEE-488 bus and RS232C serial port interfaces. The software, a LabView program developed by us, controls the automatic measuring procedure and obtains, for each concentration, the speed of sound from an average over 150 measurements of the time between two consecutive echoes of a signal digitized at 1 G S/s with 250 000 points, and the density from an average over 90 000 measurements of the period of oscillation of a vibrating tube. The experimental technique and the software have been setup by measuring, at 298.15 K, the speed of sound and density of aqueous solutions of procaine hydrochloride, p-nitrophenyl-α-D-glucopyranoside, and β-cyclodextrin, previously measured by us with a manual method. A comparison between both methods is also included.
Tetrahedral amorphous-carbon thin films for silicon-on-insulator application
Authors: Song Z. R. , Yu Y. H. , Li C. L. , Zou S. C. , Zhang F. M. , Wang X. , Shen D. S. , Luo E. Z. , Sundaravel B. , Wong S. P. , Wilson I. H.
Published: July 2002

The application of a silicon-on-insulator substrate in a high-power integrated circuit is limited by the self-heating effect, caused by the poor thermal conductivity of the buried SiO2. We introduce tetrahedral amorphous-carbon thin films, formed by the filtered arc deposition method, as an alteration. We investigated the surface morphology, microstructure, and electrical properties of these films. The films deposited under a substrate bias of −200 V displayed outstanding surface topography (low surface roughness with the Rrms value under 0.5 nm) and excellent electrical property (breakdown field of 4.7 MV/cm). The film has a high content of sp3 bonds of carbon (87%) and low content of oxygen (<2%).
A simple operation of a plasma-electrode pockel’s cell for the laser megajoules
Authors: Gardelle J. , Pasini E.
Published: July 2002

We report on a simple way to operate plasma-electrode pockel’s cells (PEPC), which are used as large-aperture optical switches in laser drivers for inertial-confinement fusion facilities. In the basic operation of a PEPC, one needs to produce a plasma in each of two gas-filled cells placed on opposite sides of an appropriate crystal plate. Between the resulting highly conductive electrodes, a high-voltage pulse is applied to produce a uniform electric field in the crystal, which then undergoes the Pockel’s effect. We have developed a simple PEPC design in which the high-voltage pulse can be directly applied between the gascells without prior ionization of the gas (by a high-current plasma discharge). With reasonable operating parameters, this regime meets the PEPC optical switching requirements for the laser megajoules. In this paper, we present experimental results of this simple regime that strongly simplifies the PEPC concept and improves its reliability.
Thermal induced reversal in coupled grains and elongated particles
Authors: Wang Xiaobin , Bertram H. Neal , Safonov Vladimir L.
Published: July 2002

A method has been developed to analyze thermal fluctuations in systems with a nonuniform magnetization reversal. The application is to either interacting single domain particles or elongated grains where the magnetization can reverse nonuniformly. In our approach, the stochastic differential equations are obtained by adding damping and thermal fluctuations consistent with the fluctuation-dissipation theorem to each independent mode of the interacting system around equilibrium. The collective thermal reversal mode is obtained from the optimal path of a weakly thermal agitated system. Asymptotic analysis gives the first mean exit time formula for the long time thermal excitation region. Examples of 2:1 and 5:1 aspect ratio particles are given.
Mixed metal films with switchable optical properties
Authors: Richardson T. J. , Slack J. L. , Farangis B. , Rubin M. D.
Published: July 2002

Thin, Pd-capped metallic films containing magnesium and first-row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous hydrogen or following cathodic polarization in an alkaline electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepared by cosputtering from one magnesium target and one manganese, iron, or cobalt target. Both the dynamic optical switching range and the speed of the transition depend on the magnesium-transition metal ratio. Infrared spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg3MnH7, Mg2FeH6, and Mg2CoH5.