bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic susceptibility measurements at high pressure using designer diamond anvils
Authors: Jackson D. D. , Aracne-Ruddle C. , Malba V. , Weir S. T. , Catledge S. A. , Vohra Y. K.
Published: March 2003

High pressure magnetic susceptibility experiments can yield valuable insights into the changes in magnetic behavior and electron correlation properties which can accompany extreme compressions of matter. However, magnetic susceptibility experiments with ultrahigh pressure diamond anvil cells are extremely challenging due to the very small size of the high-pressure sample (≈75 μm diameter) and the difficulty of obtaining good coupling between the sample and the sensing coil. As a result, measurement sensitivity and poor signal-to-background ratios tend to be serious concerns which limit the applicability of these experiments. We present here a new approach to high-pressure ac magnetic susceptibility experiments that involve specially fabricated diamond anvils with diamond encapsulated sensing microcoils which are located just 10–20 μm from the high-pressure sample. We also present some test results taken with a gadolinium sample in order to demonstrate the viability of this high-pressure ac susceptibility technique.
The Stark ball: A programmable multipolar environment
Authors: MacAdam K. B. , Hwang C. S.
Published: March 2003

A device is described and mathematically analyzed which creates electrostatic multipole potentials at least up to rank 3 in a 2 cm diam spherical region. The fields in the Stark ball are produced by the application of programmed voltages to 24 conducting rods that are inserted symmetrically in an empty 5 cm conducting cavity. Three mutually orthogonal unobstructed paths through the center of the device between the rods are provided for access of light and particle beams, and four other tetrahedrally located openings can be added for beams, detectors, or imaging. The device is intended for application in atom or molecule trapping and cooling, recoil-ion momentum spectroscopy, particulate analysis, reactive scattering, and coherent control. Potential distributions can be established, altered, and rotated on subnanosecond time scales, and frequencies higher than 20 GHz can be introduced via the same electrodes. Uniform fields, i.e., first-rank multipoles, quadrupole, and octupole potentials can be programmed and superposed with arbitrary and independent time profiles, so that the device may be applied to the study of orientational properties of complex anisotropic molecules, pollens, grains of dust, and other particulates. The device is nonmagnetic. An extensive analysis of fields in the center region, fringing fields, and multipole purity up to rank 7 has been carried out, and recipes are given for pure (l,m)=(1,0), (2,0), and (3,0) multipoles. The principles and methods laid out here and applied to a “truncated octahedron” can be applied to the design of electrostatic environments having fewer or more independent electrodes in any polyhedral geometry.
Spark combustion reactor for 13-carbon isotope enrichment analysis of gases
Authors: May Michael , Gray Michael , Kuo John E. , Tan C. T.
Published: March 2003

A novel spark combustion reactor was designed, built, and utilized for quantitative 13-carbon isotope determination of highly enriched permanent gases. The analytical methodology developed is straightforward and begins by loading the reactor with hydrocarbon and oxygen using a steel gas manifold. High voltage is applied to the platinum electrode spark plug incorporated into the reactor, which rapidly converts hydrocarbon to carbon dioxide (and other products). Carbon dioxide species are then quantitated by quadrupole mass spectrometry. The primary reactor consists of a three-way stainless steel tee, plus the following components connected to threaded ports: (1) a platinum electrode spark plug sealed with a Viton O ring, (2) a steel gas storage cylinder, and (3) a manual bellows valve terminated with a VCO type connector. Making use of the spark combustion reactor, the 13-carbon fraction of highly enriched CH413 was measured to be ⩾99.5 at. % 13C. This portable, static reactor permits determination of the C13/C12 isotope fraction for permanent gases utilizing mass spectrometric detection. The analytical system presented is relatively rapid (due to spark ignition), straightforward, and flexible (applicable to hydrocarbon gases using various gas detectors). Its limitation in performance for C13 isotope work is probably due to carbon embedded in the reactor interior.
Short-time piezoelectric measurements in ferroelectric thin films using a double-beam laser interferometer
Authors: Gerber P. , Roelofs A. , Lohse O. , Kügeler C. , Tiedke S. , Böttger U. , Waser R.
Published: March 2003

An evolution of the double-beam laser interferometer used for piezoelectric measurements in ferroelectric thin films is reported. Measuring the d33 hysteresis of a ferroelectric material using lock-in technique with large time constants requires a varying bias field to be applied to the sample over a long period of time. This long-term application leads to electrical stress during the measurement. We present a measurement technique using a different source for the applied field and a varied method for averaging the interferometric response. The measurement time for a complete d33 hysteresis will be shortened down to several seconds. Also, the cycle frequency becomes comparable to electrical hysteresis measurements. Experimental results on quartz and Pb(Zr(X),Ti(1−X))O3 are given to demonstrate the capabilities of the interferometer and the new measurement method.
Double-cantilever mount for angle-resolving particle detectors
Authors: Waterhouse D. K. , Devlin J. K. , Williams J. F.
Published: March 2003

A double cantilever detector mount, constructed from aluminum, was designed to support an angle-resolving particle detector mounted on a rotary table. The mount allows precise and accurate alignment of the detector’s field of view in the Cartesian x, y, and z plane, and allows precise control of the azimuthal angle of the detector. The mount is simple to adjust, provides high precision and stability, and can be used in any situation where precise and accurate detector alignment is required.
Rapid multiplexed data acquisition: Application to three-dimensional magnetic field measurements in a turbulent laboratory plasma
Authors: Landreman M. , Cothran C. D. , Brown M. R. , Kostora M. , Slough J. T.
Published: March 2003

Multiplexing electronics have been constructed to reduce the cost of high-speed data acquisition at the Swarthmore Spheromak Experiment (SSX) and Redmond Plasma Physics Laboratory. An application of the system is described for a three-dimensional magnetic probe array designed to resolve magnetohydrodynamic time scale and ion inertial spatial scale structure of magnetic reconnection in a laboratory plasma at SSX. Multiplexing at 10 MHz compresses 600 pick-up coil signals in the magnetic probe array into 75 digitizer channels. An external master timing system maintains synchronization of the multiplexers and digitizers. The complete system, calibrated and tested with Helmholtz, line current, and magnetofluid fields, reads out the entire 5×5×8 probe array every 800 ns with an absolute accuracy of approximately 20 G, limited mainly by bit error.
An ion beam reflectron/single-photon infrared emission spectrometer for the study of gas-phase polycyclic aromatic hydrocarbon ions: Testing proposed carriers of the unidentified infrared emission bands
Authors: Kim Hack-Sung , Saykally Richard J.
Published: March 2003

We describe the design and performance of an electron-impact ion beam reflectron system coupled to our “single-photon infrared emission spectrometer” for measurement of infrared emission from excited gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This experiment provides for direct comparison of laboratory infrared emission spectra of gaseous ionized PAHs with the “unidentified infrared emission bands,” the origin of which is still debated.
Development of an interactive molecular dynamics simulation software package
Authors: Knoll P. , Mirzaei S.
Published: March 2003

Computer experiments play a significant role in science today. Molecular dynamic (MD) simulations are extremely powerful tools not only to understand and interpret the experiments at the microscopic level, but also to study regions that are not accessible experimentally. In the last few years interactive visualization of MD simulations has been proven invaluable while developing models and computational techniques. In this work a platform independent code for interactive MD simulations, which can be run either as a stand alone application or can be distributed by a master process among a number of workers by means of JavaSpaces, is presented. This approach is of interest because the parallel mode enables the user to perform serious research work, whereas the stand alone application can be used for educational purposes. The dynamical representation of the simulation is done with the Java3D application-programming interface, which makes it possible for the user to immediately visualize phenomena under study and therefore brings a new level of understanding to the work.
Design of an experimental setup to achieve sinusoidal temperature oscillation in ultrahigh vacuum
Authors: Pesty F. , Garoche P.
Published: March 2003

We designed an experimental setup to produce a temperature oscillation at the surface of a sample, in ultrahigh vacuum. The heating device, a tungsten wire, uses infrared radiation to heat the sample. A regular sine wave cannot produce a harmonic power oscillation, due to the nonlinear character of the Stefan–Boltzmann law of radiation. To achieve it we generate a complex wave form to take into account the thermal behavior of the heating filament as well as its electronic transport properties. An example of temperature oscillation is shown in the case of a tantalum sample, at a 0.12 Hz frequency. It exhibits harmonic behavior with an oscillation amplitude of about 1 K. This method opens the field to new experiments in surface science, to study reversible surface phase transitions.
Susceptibility of atomic force microscope cantilevers to lateral forces
Authors: Sader John Elie
Published: March 2003

V-shaped cantilevers are used widely in the atomic force microscope (AFM) due to their perceived enhanced resistance to lateral forces in comparison to rectangular cantilevers. In this article, we rigorously investigate this premise, and in so doing establish that, contrary to established operating principles and intuition, V-shaped AFM cantilevers are generally more prone to the effects of lateral forces than rectangular AFM cantilevers. This finding suggests that rectangular cantilevers should be used in place of V-shaped cantilevers in applications where the effects of lateral forces are to be minimized.
Accurate Monte Carlo calculation of Boersch energy and angle spreading
Authors: Read F. H. , Bowring N. J.
Published: March 2003

The effects of stochastic electron–electron interactions have been studied by a computational technique that attempts to avoid systematic errors, leaving only the random errors that are inherent in a Monte Carlo calculation. The technique has been applied to the energy and angle spreading in a converging round beam. The results are compared with previous analytical results.
Two linear beetle-type scanning tunneling microscopes
Authors: MacLeod J. M. , Moffat Antje , Miwa J. A. , Mark A. G. , Mullins G. K. , Dumont R. H. J. , Constant G. E. , McLean A. B.
Published: March 2003

Two beetle-type scanning tunneling microscopes are described. Both designs have the thermal stability of the Besocke beetle and the simplicity of the Wilms beetle. Moreover, sample holders were designed that also allow both semiconductor wafers and metal single crystals to be studied. The coarse approach is a linear motion of the beetle towards the sample using inertial slip–stick motion. Ten wires are required to control the position of the beetle and scanner and measure the tunneling current. The two beetles were built with different sized piezolegs, and the vibrational properties of both beetles were studied in detail. It was found, in agreement with previous work, that the beetle bending mode is the lowest principal eigenmode. However, in contrast to previous vibrational studies of beetle-type scanning tunneling microscopes, we found that the beetles did not have the “rattling” modes that are thought to arise from the beetle sliding or rocking between surface asperities on the raceway. The mass of our beetles is 3–4 times larger than the mass of beetles where rattling modes have been observed. We conjecture that the mass of our beetles is above a “critical beetle mass.” This is defined to be the beetle mass that attenuates the rattling modes by elastically deforming the contact region to the extent that the rattling modes cannot be identified as distinct modes in cross-coupling measurements.
Monte Carlo simulation of x-ray spectra generated by kilo-electron-volt electrons
Authors: Llovet X. , Sorbier L. , Campos C. S. , Acosta E. , Salvat F.
Published: March 2003

We present a general algorithm for the simulation of x-ray spectra emitted from targets of arbitrary composition bombarded with kilovolt electron beams. Electron and photon transport is simulated by means of the general-purpose Monte Carlo code PENELOPE, using the standard, detailed simulation scheme. Bremsstrahlung emission is described by using a recently proposed algorithm, in which the energy of emitted photons is sampled from numerical cross-section tables, while the angular distribution of the photons is represented by an analytical expression with parameters determined by fitting benchmark shape functions obtained from partial-wave calculations. Ionization of K and L shells by electron impact is accounted for by means of ionization cross sections calculated from the distorted-wave Born approximation. The relaxation of the excited atoms following the ionization of an inner shell, which proceeds through emission of characteristic x rays and Auger electrons, is simulated until all vacancies have migrated to M and outer shells. For comparison, measurements of x-ray emission spectra generated by 20 keV electrons impinging normally on multiple bulk targets of pure elements, which span the periodic system, have been performed using an electron microprobe. Simulation results are shown to be in close agreement with these measurements.
Dipole moment of a microdroplet containing a macroion
Authors: Tsao Heng-Kwong , Ho Ching-Hung , Tseng Chin-Yao , Sheng Yu-Jane
Published: March 2003

The influence of a macroion contained in a microdroplet on the mean square dipole moment 〈p2〉, which manifests counterion fluctuation and correlation, is investigated through Monte Carlo (MC) simulations. This system models the protein-filled water-in-oil microemulsion droplet. The electrostatic interactions among ions are calculated directly by solving the Poisson equation based on the primitive model. 〈p2〉 obtained from MC is much larger than that calculated by solving the Poisson–Boltzmann equation. The probability of the macroion location is determined and the average macroion location 〈rp〉 is always off-center. In comparison with 〈rp〉 of a hard sphere in a microdroplet without counterions, the neutral particle is repelled from the interface due to electrostatic repulsions among ions. When the interface and the macroion possess charges of similar sign, the macroion is pushed further away from the interface. 〈p2〉 is a function of counterion concentration, macroion size, and macroion charge Zp. When the counterion concentration is increased, 〈p2〉 rises and becomes saturated. At low counterion concentrations, 〈p2〉 rises significantly as the macroion size approaches the droplet size or as |Zp| is increased. When counterion concentrations are high enough, however, 〈p2〉 is essentially independent of size and charge of the macroion.
Averaged kinetic temperature controlling algorithm: Application to spontaneous alloying in microclusters
Authors: Kobayashi Taizo R. , Ikeda Kensuke S. , Shimizu Yasushi , Sawada Shin-ichi
Published: March 2003

A simple algorithm of velocity scaling is proposed for the isothermal simulation of nonequilibrium relaxation processes accompanied with heat generation or absorption. The algorithm controls the kinetic temperature averaged over an arbitrary time interval at an arbitrary relaxation rate and at an arbitrary velocity scaling interval. The general conditions of controlling temperature are derived analytically and criteria for stable control are established. Our algorithm is applied to simulating the effect of substrate on the “spontaneous alloying” process of metal microclusters [H. Yasuda, H. Mori, M. Komatsu, K. Takeda, and H. Fujita, J. Electron Microsc. 41, 267 (1992)]. The results are compared with the results obtained by the Langevin algorithm in which the kinetic energy of every atom is controlled by respective stochastic heat reservoir. In spite of the marked difference between the two algorithms the relaxation dynamics agree very well in quantity over a sufficient wide range of control parameters.
Electromagnetic enhancement effect in scanning tunneling microscope light emission from GaAs
Authors: Uehara Y. , Gotoh H. , Arafune R. , Ushioda S.
Published: March 2003

The electromagnetic enhancement effect in scanning tunneling microscope (STM) light emission from GaAs has been investigated by the finite difference time domain method. We have calculated the intensity of light emitted by the recombination of minority carriers injected from the tip and majority carriers in the sample. The results depend not only on the material and the shape of the tip but also on light polarization. When the tip is tungsten whose dielectric function has a positive real part at 1.5 eV, the p-polarized light intensity depends strongly on the size of the tip and the location of recombination. However, the s-polarized light emission depends only weakly on these parameters. If the tip is a perfect metal, the p-polarized light intensity becomes a few times stronger than that for the W tip of the same shape. On the other hand, the s-polarized light intensity becomes weaker than that for the W tip due to the electromagnetic screening effect of a perfect metal. We conclude that the combination of the W tip and s-polarized light detection is suitable for precise measurement of the tip-position dependence of STM light emission properties. A tip with a negative value of the real part of dielectric function is preferable to the W tip, if one does not analyze the tip-position dependence of STM light emission, because STM light emission is enhanced relative to the case of the W tip.
Dielectric and piezoelectric properties of 〈001〉 fiber-textured 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 ceramics
Authors: Sabolsky Edward M. , Trolier-McKinstry Susan , Messing Gary L.
Published: March 2003

The 0.675Pb(Mg1/3Nb2/3)O3–0.325PbTiO3 (PMN–32.5PT) ceramic composition (with 1 wt. % excess PbO) was fiber textured in the 〈001〉 direction by the templated grain growth process using 5 vol % oriented {001}-BaTiO3 platelet crystals as the templates. The templated ceramics annealed at 1150 °C for 5 h attained texture fractions as high as 0.9. The fiber-textured samples showed an increase in the piezoelectric, electromechanical coupling, and compliance coefficients when poled and measured in the 〈001〉-textured direction. The low drive field (<5 kV/cm) d33 coefficients in the 〈001〉, measured directly from unipolar strain-field measurements, were ∼1150 pC/N. This d33 coefficient is 1.2–1.5 times greater than randomly oriented samples. The poled εmax and εrt for a 0.9-textured PMN–32.5PT ceramic were 21 500 and 2450, respectively. Factors limiting further property improvements are discussed.
Electrical transport through crossed carbon nanotube junctions
Authors: Park J. W. , Kim Jinhee , Yoo K.-H.
Published: March 2003

We have investigated the electrical transport properties through crossed nanotube junction between two metallic single-walled carbon nanotubes. Depending on the temperature range, the crossed junction exhibits different behaviors. At high temperatures, the crossed junction acts as a tunneling junction and its conductance shows a power-law dependence on the temperature, G∝Tα, with α≈1. At low temperatures, however, it exhibits different behaviors from the tunnel junction, suggesting the formation of localized states at the crossing. We have also measured the gate-voltage dependence for the crossed junctions and observed quasiperiodic oscillations supporting the existence of localized states formed at the junction.
I–V
characteristics of the Langmuir probe in flowing afterglow plasmas
Authors: Shun’ko E. V.
Published: March 2003

The specific features of the probe I–V characteristics in flowing-afterglow plasmas are studied experimentally and in theory. As it was found at a probe potential equal to the plasma one, V=0, an electron concentration in a probe vicinity (and a probe current) is decreased due to a predominant outflow of the electrons into an electrical circuit of the probe from the probe vicinity. The expression allowing one to reconstruct the undisturbed-by-probe electron concentration from only experimental data is derived. The reconstructed values of the electron concentration enable one to find from the experiments the semiempiric expressions allowing to describe quantitatively the behavior of the probe I–V characteristics at the electron-attracting as well as at the ion-attracting potential, respectively. The expressions found (both for electron-attracting and ion-attracting potential) include the “separating length,” which merely is the Langmuir length with a factor equal to the square root of the electron mass over the ion mass ratio for two-component plasma. The intermediate part of the probe I–V characteristics is discovered for probes operating with afterglow plasmas. This intermediate part is described in terms of the experimental parameter L0 having a dimension of the length (presumably electron-orbital length). The value of the parameter L0 does not depend on plasma parameters to within the ranges of plasma parameter variations for experimentally investigated plasmas as it was found. The experiments were performed with two cylindrical probes of 10 and 25 μm diam and ∼3 mm lengths in the experimentally investigated ranges of the afterglow plasma parameters: 105 cm−3<n<1010 cm−3, 0.017 eV<kT<0.3 eV, and 0.5 Torr⩽p⩽2 Torr of He gas pressure (corresponding to 950 μm⩾λTe⩾240 μm electron mean-free path) with (or without) Ar addition.
Atomic layer chemical vapor deposition of ZrO2-based dielectric films: Nanostructure and nanochemistry
Authors: Dey S. K. , Wang C.-G. , Tang D. , Kim M. J. , Carpenter R. W. , Werkhoven C. , Shero E.
Published: March 2003

A 4 nm layer of ZrOx (targeted x∼2) was deposited on an interfacial layer (IL) of native oxide (SiO, t∼1.2 nm) surface on 200 mm Si wafers by a manufacturable atomic layer chemical vapor deposition technique at 300 °C. Some as-deposited layers were subjected to a postdeposition, rapid thermal annealing at 700 °C for 5 min in flowing oxygen at atmospheric pressure. The experimental x-ray diffraction, x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and high-resolution parallel electron energy loss spectroscopy results showed that a multiphase and heterogeneous structure evolved, which we call the Zr–O/IL/Si stack. The as-deposited Zr-O layer was amorphous ZrO2-rich Zr silicate containing about 15% by volume of embedded ZrO2 nanocrystals, which transformed to a glass nanoceramic (with over 90% by volume of predominantly tetragonal-ZrO2 (t-ZrO2) and monoclinic-ZrO2 (m−ZrO2) nanocrystals) upon annealing. The formation of disordered amorphous regions within some of the nanocrystals, as well as crystalline regions with defects, probably gave rise to lattice strains and deformations. The interfacial layer (IL) was partitioned into an upper SiO2-rich Zr silicate and the lower SiOx. The latter was substoichiometric and the average oxidation state increased from Si0.86+ in SiO0.43 (as-deposited) to Si1.32+ in SiO0.66 (annealed). This high oxygen deficiency in SiOx was indicative of the low mobility of oxidizing specie in the Zr–O layer. The stacks were characterized for their dielectric properties in the Pt/{Zr–O/IL}/Si metal oxide-semiconductor capacitor (MOSCAP) configuration. The measured equivalent oxide thickness (EOT) was not consistent with the calculated EOT using a bilayer model of ZrO2 and SiO2, and the capacitance in accumulation (and therefore, EOT and kZr−O) was frequency dispersive, trends well documented in literature. This behavior is qualitatively explained in terms of the multilayer nanostructure and nanochemistry that evolves.
Properties of amorphous GaNx prepared by ion beam assisted deposition at room temperature
Authors: Kang Yixiu , Ingram David C.
Published: March 2003

A theoretical study [P. Stumm and D. A. Drabold, Phys. Rev. Lett. 79, 677 (1997)] has shown that amorphous GaN is a promising wide band gap semiconductor material for electronic and optoelectronic device applications. In this work, a-GaNx and a-GaNx:Er thin films were prepared by means of ion beam assisted evaporation at room temperature in an ultrahigh vacuum chamber. The properties of as-deposited and thermally annealed films were studied by a variety of characterization methods. X-ray diffraction, electron diffraction, and Raman spectroscopy results confirmed that the films are amorphous. Films with gallium to nitrogen ratios of 1.0:0.5–1.0:1.4 were obtained. Films with more nitrogen than gallium were highly transparent to light with energy less than that of the band gap, whereas films with less nitrogen than gallium showed much higher absorption. This indicates that a clean gap can be obtained for nitrogen rich films. Electron spectroscopy results showed that the binding energies for nitrogen and gallium are consistent with only Ga–N bonding with the highest valence band state about 1.5 eV below the Fermi level. Both as-deposited and annealed a-GaNx:Er film showed IR emissions in the range of 950–1000 nm by photoluminescent measurement. The annealed film also showed visible emissions near 700 nm.
Optical transmission measurements of explosive boiling and liftoff of a layer of micron-scale water droplets from a KrF laser-heated Si substrate
Authors: Kudryashov Sergey I. , Allen Susan D.
Published: March 2003

Water plume velocities were measured in air by optical transmission as a function of laser fluence using a KrF laser for explosive boiling and liftoff of a layer of micron-scale water droplets from a laser-heated Si substrate of interest for laser particle removal. The thickness of the superheated water layer near the water/Si interface determines acceleration and removal of the water droplets from the Si substrate.
Correlation of stress and texture evolution during self- and thermal annealing of electroplated Cu films
Authors: Lee Haebum , Wong S. Simon , Lopatin Sergey D.
Published: March 2003

Electroplated Cu films with different plating conditions and thicknesses are characterized to study the stress and texture evolution involved in the transition of the microstructure during self-annealing. Grain growth during the process induces stress in the film in the tensile direction. Degradation of the (111) texture and enhancement of the (200) texture are observed in the films during the microstructural transition. The rate of self-annealing increases dramatically as the plating current density increases. Self-annealing is significantly retarded in thinner films and in films plated with pulse reverse current. A strong correlation between stress and texture development is demonstrated for all electroplated Cu films under self- and thermal annealing conditions. This correlation is explained by the surface/interface energy and strain energy in anisotropic metal films. Due to the mechanical anisotropy of Cu, the orientation with the lowest total energy changes from (111) to (200) as the dominant energy that determines the film texture changes from surface/interface related to strain related. The texture remains unchanged until the biaxial stress in the film exceeds a critical value. Above the level of critical stress, the (111) texture starts to degrade and the (200) starts to become enhanced, which reduces the strain energy in the film. Different amounts of stress develop in Cu films during thermal annealing depending on the barrier layers; the stress is largest with TaSiN, medium with Ta and smallest with TaN. This suggests that film stress and texture are also affected by the Cu/barrier interface bond strength.
Dissociative excitation of acetyl cyanide by ultraviolet multiphoton absorption
Authors: Aoyama Jun-ichi , Sugihara Takashi , Tabayashi Kiyohiko , Saito Ko
Published: March 2003

Dissociative excitation of CH3COCN to produce CN(B-X) and CN(A-X) fluorescence was studied by resonance enhanced multiphoton excitation at 292 nm. The laser power dependence of the CN(B-X) fluorescence intensity and the lifetime of the one-photon excited S1 state showed that CN(B) formation takes place in the direct two-photon and two-body dissociation mechanism, CH3COCN+2hν →CH3CO(X̃)+CN(B). Vibrational and rotational energy distributions of the nascent CN(B) fragment were determined by a simulation analysis of the dispersed fluorescence spectrum. The vibrational distribution was found to be of the relaxed type and rotational distribution in each vibrational state could be approximated by a Boltzmann distribution. The best-fit vibrational distribution of CN(B) was Nv′=0: Nv′=1:Nv′=2=1.00: 0.25: 0.07 with the respective rotational temperatures of Tr(v′=0)=2600 K, Tr(v′=1)=1000 K, and Tr(v′=2)=900 K. The internal state distributions were found to be hotter than those predicted by the statistical model with complete energy randomization within the excited molecule. The results indicate a dissociation mechanism where both the vibrational energy deposition in the photoexcitation and available energy redistribution before the bond breakage are limited within the modes of the skeletal CCOCN structure. Possible decay channels other than the CN(B) production, upon two-photon excitation at 292 nm, are also discussed based on the potential surfaces previously predicted. The formation of CN(A) presently observed in the direct two-photon excitation can be interpreted as the dissociation of the electronic excited intermediate states, populated competitively via internal conversion(s) from the upper electronic states. To obtain a deeper understanding of higher excited states of acetyl cyanide, the vacuum UV absorption cross section was also determined in the 110–200 nm region, using a synchrotron radiation source.
The conformational distribution in diphenylmethane determined by nuclear magnetic resonance spectroscopy of a sample dissolved in a nematic liquid crystalline solvent
Authors: Celebre G. , De Luca G. , Emsley J. W. , Foord E. K. , Longeri M. , Lucchesini F. , Pileio G.
Published: March 2003

The deuterium decoupled, proton nuclear magnetic resonance spectrum of a sample of diphenylmethane-d3 dissolved in a nematic liquid crystalline solvent has been analyzed to yield a set of dipolar couplings, Dij. These have been used to test models for the conformational distribution generated by rotation about the two ring-CH2 bonds through angles τ1 and τ2. Conformational distributions, particularly when obtained from a quantum chemistry calculation, are usually described in terms of the potential energy surface, V(τ1,τ2), which is then used to define a probability density distribution, P(τ1,τ2). It is shown here that when attempting to obtain P(τ1,τ2) from experimental data it can be an advantage to do this directly without going through the intermediate step of trying to characterize V(τ1,τ2). When applied to diphenylmethane this method shows that the dipolar couplings are consistent with a conformational distribution centered on τ1=τ2=56.5±0.5°, which is close to the values calculated for an isolated molecule of 57.0°, and significantly different from the asymmetric structure found in the crystalline state.
Local structures and damage processes of electron irradiated α-SiC studied with transmission electron microscopy and electron energy-loss spectroscopy
Authors: Muto Shunsuke , Tanabe Tetsuo
Published: March 2003

Damaged structures of α-SiC below and above the critical temperature of amorphization (Tc) under high-energy electron irradiation were studied by means of transmission electron microscopy and electron energy-loss spectroscopy. Above Tc, crystal fragmentation takes place due to local lattice strains caused by preferential displacements, subsequent outward diffusion of carbon atoms and formation of silicon nano-clusters. On the other hand, the amorphous structure formed below Tc can be well characterized by the formation of Si–Si, Si–C, and sp3 C–C covalent bonds with the tetrahedral coordination locally retained and uniformly distributed. The primary amorphization process under electron irradiation can be interpreted by the defect-accumulation model, in which displaced atoms are frozen at interstitial sites before long-distance diffusion by reconstructing the surrounding structure to relax the local strains. Accordingly the amorphization process is controlled essentially by the mobility of displaced carbon and silicon atoms, and chemical disordering seems to play a minor role in triggering the amorphization. A key issue for irradiation induced volume swelling of amorphous SiC is also presented.
Dielectric properties of amorphous hydrogenated silicon carbide thin films grown by plasma-enhanced chemical vapor deposition
Authors: Brassard D. , El Khakani M. A.
Published: March 2003

The dielectric properties have been determined for stoichiometric amorphous hydrogenated silicon carbide (a-SiC:H) films grown by means of the plasma-enhanced chemical vapor deposition (PECVD) technique. The dielectric constant, dielectric loss, breakdown voltage, and current–voltage (I–V) characteristics of the a-SiC:H PECVD films were systematically determined for various film thicknesses in the 90–1400 nm range. The PECVD a-SiC:H films exhibit not only a dielectric constant as high as 14 but also relatively high breakdown field values around 3 MV/cm. The dielectric constant of the a-SiC:H films was found to remain almost constant over all the investigated frequency range of 1 kHz to 13 MHz, while it decreases as the film thickness is diminished. The analysis of the I–V characteristics of the a-SiC:H films has revealed the existence of two different conduction mechanisms depending on the applied voltages. While exhibiting an ohmic conduction in the low-field region (<0.05 MV/cm), the film conduction at high fields (>0.25 MV/cm) is dominated by the Poole–Frenkel effect. In the low-field region, the conductivity of the a-SiC:H films was found to decrease exponentially from 10−10 S/cm to about 10−12 S/cm as the film thickness is increased from 90 to 1400 nm. At high fields, a thermal breakdown of the a-SiC:H films occurred at about 3 MV/cm as a consequence of thermal instabilities induced by Joule heating. The excellent dielectric properties of the PECVD a-SiC:H films are seen to be a consequence of their high density and low defect concentration.
Mechanism of the Yb–Er energy transfer in fluorozirconate glass
Authors: da Vila L. D. , Gomes L. , Tarelho L. V. G. , Ribeiro S. J. L. , Messadeq Y.
Published: March 2003

The mechanism of the Yb3+→Er3+ energy transfer as a function of the donor and the acceptor concentration was investigated in Yb3+–Er3+ codoped fluorozirconate glass. The luminescence decay curves were measured and analyzed by monitoring the Er3+(4I11/2) fluorescence induced by the Yb3+(2F5/2) excitation. The energy transfer microparameters were determined and used to estimate the Yb–Er transfer rate of an energy transfer process assisted by excitation migration among donors state (diffusion model). The experimental transfer rates were determined from the best fitting of the acceptor luminescence decay obtained using a theoretical approach analog to that one used in the Inokuti–Hirayama model for the donor luminescence decay. The obtained values of transfer parameter gamma [γ(exp)] were always higher than that predicted by the Inokuti–Hirayama model. Also, the experimental transfer rate, γ2(exp), was observed to be higher than the transfer rate predicted by the migration model. Assuming a random distribution among excited donors at the initial time (t=0) and that a fast excitation migration, which occurs in a very short time (t≪γ−2), reducing the mean distance between donor (excited) and acceptor, all the observed results could be explained.
Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition
Authors: Han Daxing , Wang Keda , Owens Jessica M. , Gedvilas Lynn , Nelson Brent , Habuchi Hitoe , Tanaka Masako
Published: March 2003

Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630−640 cm−1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm−1 and 1050−1200 cm−1 appeared, (b) the relative intensity of the 2090 cm−1 absorption increased, (c) the low-energy optical absorption at photon energy <1.4 eV increased one to two orders of magnitude, (d) the low-energy PL band at ∼1.0 eV emerged with a decrease of total PL intensity, and (e) the conductivity activation energy decreased. The aforementioned changes correlated well with the crystallinity of the material. We attribute the observations mainly to the formation of the c-Si gain boundaries during crystallization.
Effects of substrate temperature on nanostructure and band structure of sputtered Co3O4 thin films
Authors: Yamamoto Hiroki , Tanaka Shuhei , Hirao Kazuyuki
Published: March 2003

Effects of substrate temperature (Ts) on nanostructure and band structure of sputtered Co3O4 thin films were investigated. The lattice constant of Co3O4 sputtered at room temperature was longer than the reported bulk Co3O4 value. It decreased with increasing Ts and approached the reported value. Average grain size of the film formed at Ts=918 K was 13.6 nm which was twice as big as that of the film obtained at Ts=293 K. Intensities of optical absorption peaks increased with increasing Ts. Band gaps corresponding to the charge transfer from Co2+(π*e) to Co2+(π*t2)(0.8 eV), from Co3+(π2t2) to Co2+(σ*t2)(1.3 eV), and from O2−(π*Γ) to Co2+(σ*t2) (2.1 eV) increased as a function of Ts. Detailed analysis of the optical absorption spectra in the infrared region indicated that there were many kinds of defects in the Co3O4 thin film formed at Ts=293 K and there were fewer defects in the film formed at Ts=918 K. The defects, low density, and increasing interface area at intergrains of nanoparticles caused a lowering of Eg at lower substrate temperature.
Photoionization cross section of hydrogenic impurities in spherical quantum dots
Authors: Ham Heon , Spector Harold N.
Published: March 2003

We have used a variational wave function for hydrogenic impurities in spherical quantum dots to calculate the dependence of the photoionization cross section of such impurities on the energy and polarization of the photon. The calculation has been performed using both the infinite and finite confining potential well models. The results are presented for various dot radii and for different locations of the impurity in the dot. We find that for hydrogenic impurities at the center of a spherical quantum dot, the transitions take place between the impurity level associated with the ground subband and the free particle state in the second subband and the cross section is independent of the polarization of the photons. For off center impurities, the transitions take place between the impurity level associated with the ground subband and the free particle state in the ground subband when the photons are polarized along the direction connecting the impurity with the center of the dot while for photons polarized perpendicular to this direction, we find that the transitions take place between the impurity level associated with the ground subband and the free particle state in the second subband. Therefore the threshold energy for the photoionization cross section is lower for off center impurities when the photons are polarized along the direction connecting the impurity with the center of the dot than it is for on center impurities or for off center impurities for photons polarized perpendicular to this direction.
Physical and electrical properties of reactive molecular-beam-deposited aluminum nitride in metal-oxide-silicon structures
Authors: Ragnarsson L.-Å. , Bojarczuk N. A. , Copel M. , Gusev E. P. , Karasinski J. , Guha S.
Published: March 2003

We report on the physical and electrical properties of AlN as grown by reactive-atomic-beam deposition under various growth and anneal conditions. The physical characterization shows that AlN grown on hydrogen terminated (HF-last) Si has a thin layer of Si3N4 at the interface while growth of AlN on SiO2 or Al2O3 suffers from nucleation problems. The AlN on HF-last Si, grown and annealed at 650 °C, shows high interfacial quality and four to five orders of magnitude lower leakage current as compared to SiO2. In contrast, poor interfacial properties and leakage current are reported for the AlN grown on SiO2 or Al2O3. The relative permittivity of the AlN is found to be close to 18 and the equivalent thickness of the interfacial layer is approximately 6 Å. The effective electron mobility in Al-gated N-channel metal-oxide-silicon field-effect transistors with a 22 Å thick AlN layer peaks at ∼130 cm2/V s. The relatively low mobility is attributed to a combination of high concentration of fixed charge (>5×1012 cm−2) and charge trapping.
Imaging method for laterally resolved measurement of minority carrier densities and lifetimes: Measurement principle and first applications
Authors: Isenberg Joerg , Riepe Stephan , Glunz Stefan W. , Warta Wilhelm
Published: March 2003

Carrier density imaging, a further development of the infrared lifetime mapping technique [Bail et al., Proceedings 28th IEEE-PVSC (2000)], is presented as an extremely fast, spatially resolved lifetime measurement method. It is based on the principle of absorption of infrared radiation by free carriers. The physical principles and necessary calibration procedures are discussed, showing that the steady state carrier density and thus actual carrier lifetime at low-level injection is measured, whereas many standard lifetime measurement techniques measure differential lifetimes only. A charge coupled device camera operating in the infrared is applied for measuring the infrared transmissivity of the sample. This overcomes the necessity of scanning the sample in order to receive laterally resolved information. Thus measurement times can be cut from hours to minutes or even seconds. An additional advantage is the absence of any moving parts which may cause spatial errors or even failures in the measurement. As is shown in the article the use of lock in technique and an appropriate source of illumination is essential to receive good results on production type materials. The quantitative comparison to other lifetime measurement techniques and the application to silicon samples at different process stages is discussed. Finally methods for further reduction of measurement time are discussed, showing that this method may well be suitable for in-line process control.
Ferromagnetic properties of IV–VI diluted magnetic semiconductor Ge1−xMnxTe films prepared by radio frequency sputtering
Authors: Fukuma Y. , Asada H. , Nishimura N. , Koyanagi T.
Published: March 2003

IV–VI diluted magnetic semiconductor Ge1−xMnxTe films with various Mn compositions up to x=0.53 were prepared on glass substrates by a rf sputtering method. Magnetic measurements indicate that all the Ge1−xMnxTe films (0.07⩽x⩽053) show ferromagnetic order at low temperatures since hysteretic behavior is clearly observed. The effective spin Seff’s of Mn ions deduced from the spontaneous magnetization at 4.2 K are less than 1.0, indicating that Mn ions are partially aligned ferromagnetically. The value of the Seff depends on the carrier concentration as well as the Mn composition. The magnetic order of the Ge1−xMnxTe films is thus caused by the competition between the ferromagnetic Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the antiferromagnetic superexchange mechanism. The Curie temperature based on the mean field approximation increases linearly with increasing Mn composition up to x=0.3 and above this point tends to saturate around 80 K. On the basis of a simple RKKY mechanism, the spin exchange integral Jpd between holes and Mn ions has also been investigated. Compared to the experimental data, the Jpd shows a clear change from 0.62 eV (x⩽0.3) to 0.59 eV (x⩾0.4). This would be attributed to inhomogeneities in Mn distribution at high Mn compositions.
Linear and nonlinear optical properties of cobalt-doped zinc aluminum glass ceramics
Authors: Denisov I. A. , Volk Yu. V. , Malyarevich A. M. , Yumashev K. V. , Dymshits O. S. , Zhilin A. A. , Kang Uk , Lee Kyeong-Hee
Published: March 2003

Optical properties of cobalt-doped zinc aluminosilicate transparent glass ceramics in the visible – near-infrared (IR) spectral range have been studied. It was shown that the absorption and luminescence properties of these glass ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals formed in the glass ceramic matrix. The luminescence in the visible and near IR was assigned to the transitions from the T14(4P) to the A24 and T24 energy levels of tetrahedrally coordinated Co2+ ion. Absorption saturation and bleaching relaxation under excitation of the A24→4T1(4F) transition were studied. Luminescence decay and bleaching relaxation times lie in the ranges 25–200 and 500–800 ns, respectively, depending on the Co concentration. The value of the ground-state absorption cross section of tetrahedrally coordinated Co2+ ions in zinc aluminosilicate transparent glass ceramics was estimated to be 2.0×10−19 cm2 at the wavelength of 1.54 μm.