bluuebunny/crossref_metadata_2025 huggingface dataset:
Kinetics of runaway electrons in a stochastic electric field
Authors: Trakhtengerts Victor Y. , Iudin Dmitriy I. , Kulchitsky Anton V. , Hayakawa Masashi
Published: July 2002

The kinetic theory for runaway electrons in a stochastic electric field is developed. The general kinetic equation for the isotropic part of the electron distribution function is derived and is simplified to the differential form for a particular case of electric field spectral intensity. The stationary analytical solution and numerical dynamic solutions are obtained and are discussed in connection with the problem of energetic electrons in a thunderstorm cloud.
Molecular-beam epitaxy growth of high-quality active regions with strained InxGa1−xAs quantum wells and lattice-matched AlxGayIn(1−x−y)As barriers using submonolayer superlattices
Authors: Reddy M. H. M. , Huntington A. , Buell D. , Koda R. , Hall E. , Coldren L. A.
Published: July 2002

Submonolayer superlattices (SMS) of Ga0.47In0.53As/InAs/Ga0.47In0.52As and Ga0.47In0.53As/Al0.48In0.52As were used for the growth of strained quantum wells (QWs) and lattice-matched barriers, respectively, in the 1.55 μm active region. QWs grown with different compressive strains show excellent room-temperature photoluminescence, demonstrating the versatility of this technique. State-of-the-art current thresholds are reported for the broad-area lasers fabricated using the SMS active region.
Strain maps at the atomic scale below Ge pyramids and domes on a Si substrate
Authors: Raiteri P. , Miglio Leo , Valentinotti F. , Celino M.
Published: July 2002

In this letter, the strain field below uncapped Ge islands of a different shape on a Si(001) substrate is estimated by molecular dynamics simulations at a realistic scale. Comparison to the Fourier transform maps of transmission electron micrographs, recently reported in literature, shows a very good agreement. We point out that the complex deformation in silicon, just below the edges of the Ge islands, is far from being uniaxial. The stress distribution generated by such a strain determines the range of interdot repulsion.
Mapping strain fields in ultrathin bonded Si wafers by x-ray scattering
Authors: Nielsen M. , Poulsen M. , Bunk O. , Kumpf C. , Feidenhans’l R. , Johnson R. L. , Jensen F. , Grey F.
Published: July 2002

X-ray scattering reveals the atomic displacements arising from rotational misalignment in ultrathin silicon bonded wafers. For a 4.3 nm top wafer, the strain field penetrates from the bonded interface to the surface and produces distinctive finite-size oscillations in x-ray data. Analytical calculations permit the atomic displacements throughout the thin top wafer to be modeled.
Model for the director and electric field in liquid crystal cells having twist walls or disclination lines
Authors: Panasyuk G. , Allender D. W.
Published: July 2002

Two examples of the director structure and electric field in patterned electrode liquid crystal cells are studied using a recently developed calculational model. First, a display cell that exhibits a homeotropic to multidomainlike transition with twist wall structures has been considered for a liquid crystal with positive dielectric anisotropy. The model elucidates the behavior of the electric field. Calculations show good agreement between the model and direct computer solution of the Euler–Lagrange equations, but the model is at least 30 times faster. Second, the possibility that a cell has ±1/2 disclination lines instead of a wall defect is probed. A temperature dependent estimate for the size of the defect core is given, and the total free energy of the cell with disclination lines was calculated and compared with the corresponding value for the same cell with wall defect structure.
Fluorescence quantum yield of CdSe/ZnS nanocrystals investigated by correlated atomic-force and single-particle fluorescence microscopy
Authors: Ebenstein Y. , Mokari T. , Banin U.
Published: July 2002

Correlated atomic-force and fluorescence microscopy are used to study single-particle versus ensemble fluorescence quantum yields (QY) of semiconductor nanocrystals by measuring a simultaneous map of the topography and the single-particle fluorescence. CdSe/ZnS nanocrystal quantum dots and quantum rods with high QY were investigated. A significant portion of dark particles is detected. Comparison with the ensemble solution QY shows that samples with higher QY have a larger fraction of bright particles accompanied by an increased single-particle QY. Saturated emission from single nanocrystals could not be detected because of particle darkening under high-power excitation.
Effects of amorphizing species’ ion mass on the end-of-range damage formation in silicon
Authors: Clark Mark H. , Jones Kevin S. , Haynes Tony E. , Barbour Charles J. , Minor Kenneth G. , Andideh Ebrahim
Published: July 2002

The effects of preamorphizing ion mass on the end-of-range (EOR) damage and subsequent enhanced diffusivity have been investigated. Amorphizing silicon with implants of 22 keV 28Si+, 32 keV 73Ge+, 40 keV 119Sn+, and 45 keV 207Pb+ provided the mass comparisons. Cross-sectional transmission electron microscopy analysis showed that the amorphous layer depths were approximately 400 Å. After postimplantation annealing at 750 °C for 30 min, plan-view transmission electron microscopy (PTEM) revealed that increasing the ion mass decreased the defect size and density. Quantitative analysis of PTEM results also showed that increasing ion mass decreased the population of interstitials trapped in the EOR. Secondary ion mass spectrometry depth profiles of grown-in boron marker layers showed that increasing the ion mass decreased the time average diffusivity enhancements of boron (〈DB〉/DB*).
Implementing matrix mechanics in M
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a: Determination of Clebsch–Gordan coefficients by matrix diagonalization
Authors: Dill Dan
Published: July 2002

A scheme is presented by which Clebsch–Gordan coefficients can be determined by matrix diagonalization. The method uses Mathematica, a system for doing mathematics by computer, and shows one way matrix mechanics can be implemented in such a system. The result is a Mathematica package for angular momentum matrix mechanics. It is applied to a one-electron atom in an external magnetic field to explore the competition between spin–orbit interaction and the separate interaction of the spin and orbital moments with the external field.
Magnetic viscosity and thermal activation energy
Authors: Gaunt P.
Published: July 2002

Magnetic viscosity arises when the activation energy E required to produce a change in mangetization is provided thermally. Activation energy functions for single domain particles and materials with ‘‘strong’’ and ‘‘weak’’ domain-wall pinning are discussed. The magnetic viscosity parameter Sv, given by kT(∂E/∂H)−1T, where kT is the Boltzmann energy and H is the magnetic field, is shown to be proportional to kT/(v Ms), where Ms is the spontaneous magnetization and v is the activation volume swept out as the energy barrier is overcome. For single domain particles and strong domain-wall pinning the constant of proportionality is one while for weak pinning it is 1/2. The generality of this simple relationship is shown to be independent of the details of the activation model.
Interface properties of plasma-enhanced chemical vapor deposited SiOxNy/n-GaAs metal-insulator-semiconductor system
Authors: Chou T. Y. , Lin M. S.
Published: July 2002

Interface properties of plasma-enhanced chemical vapor deposited SiOxNy/n-GaAs metal-insulator-semiconductor systems are investigated by capacitance-voltage and deep level transient spectroscopy measurements. At room temperature, these metal-insulator-semiconductor devices exhibit deep depletion mode operation which is characteristic of the low thermal generation rate for minority carriers in GaAs and the large number of traps located at or near the interface. For accumulation mode bias voltage, the small signal capacitance does not approach the SiOxNy capacitance and exhibits anomalous frequency dispersion behavior. A simple method is used to correlate the capacitance-voltage and deep level transient spectroscopy data. An interface-state band model best fits the experimental results.
Observation of the Biot slow wave in a plastic foam of high flow resistance at acoustical frequencies
Authors: Allard Jean F. , Depollier Claude , L’Esperance André
Published: July 2002

The surface impedance of a plastic foam with high flow resistance has been measured with a new method at acoustical frequencies. The Biot theory provides a description of the behavior of this surface impedance from the contributions of the fast and the slow compressional waves to pressure and acoustical velocity at the surface of the foam. The contribution of the slow wave can be observed in spite of its strong damping.
Deep-level transient spectroscopy studies of defects in GaAs-AlGaAs superlattices
Authors: Martin P. A. , Hess K. , Emanuel M. , Coleman J. J.
Published: July 2002

We present preliminary results of a study of defects in GaAs-AlGaAs superlattices using deep-level transient spectroscopy (DLTS). A dramatic difference is observed between the DLTS spectra of two superlattices when the superlattice period is doubled. This is explained by the presence of miniband conduction in the case of the smaller period and its absence in the case of the larger, and by the consequent difference in the distribution of electrons in the superlattice. Observed activation energies must be reinterpreted for comparisons to bulk material.
Ion beam crystallography of InGaAs epitaxial layers on InP substrates
Authors: Cole J. M. , Earwaker L. G. , Cullis A. G. , Chew N. G. , Bass S. J.
Published: July 2002

We report the first direct measurements of the strain in a series of In1−xGaxAs/InP structures by ion beam crystallography. A compression strain of 0.0152±0.0008 was obtained for an In0.47Ga0.53As epitaxial layer on an InP substrate compared with an expansion strain of 0.0126±0.0010 for an In0.58Ga0.42As layer structure. The method used allows quantitative strain measurements to be performed without the need for computer simulations.
Some properties of Zn3P2 polycrystalline films prepared by hot-wall deposition
Authors: Fuke S. , Kawarabayashi S. , Kuwahara K. , Imai T.
Published: July 2002

Zinc phosphide (Zn3P2) thin films have been deposited by hot-wall deposition technique on glass substrate, and the deposition behaviors as well as structural properties of deposited films are studied. The source temperature and the temperature difference between source and substrate mainly affect the growth rate. Scanning electron microscope observation indicates the growth of columnar structure perpendicular to the substrate, and highly oriented thin films along the c axis are confirmed by x-ray diffraction analyses. The resistivity of the films is strongly related to the phosphorus composition X in Zn(1−X)PX, and the films having higher X show lower resistivity. The direct absorption edge is obtained as about 1.5 eV from the optical absorption data.
Morphology and kinetics of crystallization of amorphous V75Si25 thin-alloy films
Authors: Nava F. , Weiss B. Z. , Tu K. N. , Smith D. A. , Psaras P. A.
Published: July 2002

Electrical and microstructural changes of coevaporated V75Si25 alloy thin films have been studied as a function of temperature from room temperature to 830 °C. In situ resistivity measurements, hot-stage transmission electron microscopy, Rutherford backscattering spectroscopy and the Seeman–Bohlin glancing angle incidence x-ray diffraction technique were applied. Upon heat treatment at a heating rate of 8 °C/min, a sharp decrease in resistivity occurs at ∼670 °C which results from an amorphous to crystalline phase transformation. The crystallized phase was identified as V3Si. The mechanism of transformation is random nucleation at a rapidly decreasing rate and a fast quasi-isotropic growth. The kinetics of crystallization have been studied by utilizing electrical resistivity measurements during isothermal heat treatment. Six different temperatures between 570 °C and 630 °C were adopted. The apparent activation energy (∼3.6 eV) obtained from isothermal measurements was found to be in agreement with that obtained from nonisothermal treatments at varying rates of heating. The distinct change of the Avrami mode parameter from 4 to 2 at a constant value of t/τ during the process of crystallization is not immediately understood.
Mössbauer effect of dilute Fe in sputtered Co-Cr films
Authors: Parker F. T.
Published: July 2002

Mössbauer effect measurements have been obtained on sputtered Co–Cr films containing a small amount of iron, with compositions Co76Cr20Fe4. Two features are observed: a single line (about 25% of the spectral area) and a broadened, magnetically split spectrum. From the isomer shift and lack of magnetic splitting in the former, the nonmagnetic phase in sputtered Co–Cr alloys is identified as being nearly pure Cr. In particular, the nonmagnetic phase can contain less than a few percent of the initial Co concentration. The magnetic phase has a decreased average magnetic hyperfine field relative to Fe in Co, and an isomer shift consistent with a Cr concentration about one-half that of the initial Cr concentration. A substantial perpendicular polarization of the Fe moments is seen, although magnetic data indicate an easy plane orientation. The phase segregation in Co–Cr films is shown to be similar to the ‘‘475 °C embrittlement’’ long known in Fe–Cr alloys, and to the segregation that occurs in ductile Fe–Co–Cr–Si permanent magnets.
Electron injection in diodes with field emission
Authors: Denavit Jacques , Strobel George L.
Published: July 2002

This paper presents self-consistent steady-state solutions of the space charge, transmitted current, and return currents in diodes with electron injection from the cathode and unlimited field emission of electrons and ions from both electrodes. Time-dependent particle simulations of the diode operation confirm the analytical results and show how these steady states are reached. The results are applicable to thermionic diodes and to photodiodes.
Suppression of prebreakdown current by a cathode coating of silicon
Authors: Smith Joe N.
Published: July 2002

Reduction of prebreakdown current by three orders of magnitude is reported for Si coatings up to 1.1 μm thick on stainless steel in the range of electric field from 0.25 to 1.0 MV/cm, in vacuum gaps of ∼300 μm. These results, as well as earlier results with CaF2 coatings reported by the author, are adequately described by a simple double-barrier high-field tunneling model which then suggests electronic criteria for coatings to be most effective in suppressing prebreakdown current in HV vacuum applications.
Photoluminescence study of ZnSe–ZnTe strained-layer superlattices grown on InP substrates
Authors: Kobayashi Masakazu , Mino Naoki , Katagiri Hironori , Kimura Ryuhei , Konagai Makoto , Takahashi Kiyoshi
Published: July 2002

Optical properties of ZnSe–ZnTe strained-layer superlattice (SLS) grown on InP substrates were evaluated by photoluminescence. The luminescence peak intensity and position were affected by the photoexcitation intensity. The line shape was also affected by the growth temperature. The peak position was shifted by tailoring the structure of the superlattice, but it did not correlate exactly with previously reported theoretical values. We have reconsidered their data, and a better result on theoretical calulation was achieved. The luminescence color changed in the visible region from green to red. This SLS could be a new material for optoelectronic devices.
Fields from a uniform current source: Instantaneous switch-on
Authors: Lee Hung-Mou
Published: July 2002

Electromagnetic fields from switching on and off a constant current distributed over a plate of finite extent are studied. Such current distribution will set up a magnetostatic field and an electric dipole field behind the initial transient pulse. Over an area equal to that of the plate, there is a finite volume in which the strengths of the E and H fields of the transient pulse do not decrease with distance. This finite volume decreases as the inverse of the distance from the source so that the energy contained in this volume also decreases as the inverse of the distance traveled.
Optical and electronic properties of CuInSe2-based photoelectrochemical solar structures
Authors: Szot Jozef , Storr G. J. , Haneman D.
Published: July 2002

Optical and electronic properties of polycrystalline bulk and thin-film CuInSe2-based photoelectrochemical solar cells have been evaluated. The minority-carrier diffusion length was measured using the surface photovoltage technique and polyiodide electrolyte yielding 1 and 0.8 microns for n-type bulk and thin-film material, respectively. The first bulk optical absorption data have been obtained from transmission, using thinned samples and Fourier transform infrared spectroscopy techniques. These confirm high optical-absorption coefficients (4×105 cm−1) away from threshold.
Electrical transport properties in Co-silicides formed by thin-film reactions
Authors: Aprilesi G. , Mazzega E. , Michelini M. , Nava F. , Queirolo G. , Meda L.
Published: July 2002

Four different Co-silicide compounds were obtained by solid-state reaction at 800 °C in thin bilayers of amorphous silicon and cobalt evaporated on SiO2 substrates. Rutherford backscattering spectroscopy (2 MeV 4He+), x-ray diffraction, and Auger electron spectroscopy were used to obtain information about the chemical and crystallographic characteristics of the samples. Results indicate that in each sample only one of the following phases is present: CoSi2, CoSi, Co2Si, and Co4Si, the latter identified on the basis of the stoichiometric ratio only. Electrical resistivity and Hall effect measurements on van der Pauw structures were carried out as a function of the temperature in the intervals: 10–1000 and 10–300 K, respectively. At room temperature the resistivity ranges from the value 19 μΩ cm for CoSi2 to the value 142 μΩ cm for CoSi. There are some analogies with the case of a classical metal, but remarkable differences are also detectable in the resistivity versus temperature behavior and in the order of magnitude of the resistivity and of the Hall coefficient. In particular, at T>300 K the resistivity of the CoSi2 samples linearly depends on temperature and is well fitted by the classical Bloch–Grüneisen expression. The other silicides show, in the same temperature range, a deviation from linearity (d2ρ/dT2<0), while a quasi saturation of the resistivity can be extrapolated at higher temperatures. This saturation phenomenon can be described by the parallel of an ideal conductivity and of a saturation conductivity, and associated with the electron mean free path approaching interatomic distances. A similar model already has been put forth to describe the saturation of the resistivity in systems, such as A-15 superconducting compounds, characterized by a high value of the room-temperature resistivity. The transport parameters, deduced in a free electron framework from the resistivity curves of the Co silicides, show values which are consistent with the proposed model. Hall coefficient versus temperature behavior indicates that between 10 and 300 K a multicarrier effect is present. Conduction is predominantly n type in CoSi and p type in the other silicides.
Analysis of the effects of annealing on resistivity of chemical vapor deposition tungsten–silicide films
Authors: Shioya Yoshimi , Maeda Mamoru
Published: July 2002

Chemical vapor deposition WSix films were formed with varying compositions and film thicknesses. The were films annealed in N2 for 30 min and their resistivities were measured. The maximum resistivity was obtained for annealing temperatures between 500 and 600 °C if the film was comparatively richer in tungsten and was thick. The film was analyzed by x-ray diffraction and secondary ion mass spectrometry. The maximum resistivity is considered to be related to crystallization, which changes amorphous WSix into a mainly hexagonal structure, and to the difference in the conduction mechanism of electrons between amorphous, hexagonal, and tetragonal WSi2. It is clear that impurities in the film do not contribute to the maximum resistivity. There is also no relationship between resistivity and the number of grains of tetragonal WSi2 per unit length for annealing below 600 °C.
The localized density of states in amorphous silicon determined by electrophotography
Authors: Imagawa O. , Iwanishi M. , Yokoyama S. , Shimakawa K.
Published: July 2002

The localized density of states g(E) near midgap for undoped, B2H6-doped, and PH3-doped hydrogenated amorphous silicon is evaluated from the electrophotographic (xerographic) method. The density of hole traps decreases with increasing B2H6 concentration and that of electron traps decreases with increasing PH3. These features are qualitatively similar to those deduced from the deep-level transient spectroscopy, the photothermal deflection spectroscopy, the time-of-flight measurements, and the isothermal capacitance transient spectroscopy. The energy positions of neutral dangling bonds D0 and doubly occupied D− are estimated to be 0.85–0.88 eV from the valence band and 0.62–0.67 eV from the conduction band, respectively. Comparison of the energy peaks of electron and hole traps shows that the positive correlation energy of dangling bonds is about 0.3–0.4 eV.
Fast pulse response of zinc-oxide varistors
Authors: Modine F. A. , Wheeler R. B.
Published: July 2002

The fast pulse response of zinc-oxide varistors is studied for both high-impedance and low-impedance pulse sources. The effects of varistor capacitance and source impedance on the pulse response are examined, and quantitative descriptions of the pulse response are presented. Evidence that the varistor capacitance does not increase near breakdown voltages is found, and the implications of this result with regard to models of varistor conduction are discussed. The relationships between the properties of a varistor and its ability to protect against fast pulses are considered.
Photoconductive behavior of Au/Pb2CrO5/SnO2 devices
Authors: Toda Kohji , Morita Shinichi
Published: July 2002

The photoconductive characteristics of a multilayered structure of Au/Pb2CrO5 thin film/SnO2 have been studied as a function of applied voltage, light intensity, and light wavelength. The Pb2CrO5 thin film was fabricated by an electron-beam evaporation technique. This device has two barriers at the Au/Pb2CrO5 and SnO2/Pb2CrO5 junctions. Holes dominate the photoconductive current in the case of illumination on the Au electrode under a forward voltage bias and on a SnO2 electrode reverse biased. A band model of this device is also proposed from the experimental results of the photoconductive effect.
The effect of strain on the critical-current density of Cu-Nb composites
Authors: Klein J. D. , Rose R. M.
Published: July 2002

Microfilamentary superconducting composites of Nb fibers in Cu matrices prepared by the stack and draw method were tested for tensile critical-current performance at 4.2 K. The superconducting critical-current densities increased exponentially under the influence of an applied mechanical strain until the onset of Nb fiber plastic deformation. In the elastic range, the critical-current densities conformed to log10 Jc =m (strain)+b. In several tests the critical current was increased by more than an order of magnitude by the applied strain. This behavior is consistent with an increase in the upper critical field of the Nb fibers by the applied stress.
Depth distributions and range and shape parameters for 1H and 2H implanted into Si and GaAs in random and channeling orientations
Authors: Wilson R. G.
Published: July 2002

We report the results of a systematic study of the implantation of 1H and 2H ions at 50, 100, and 200 keV energies and 3×1013, 3×1014, and 3×1015 cm−2 fluences into the same Si or the same GaAs crystals in (100) random and 〈100〉 and 〈110〉 channeling orientations measured using secondary ion mass spectrometry. We report ranges and profile shape parameters for the resulting random and channeled depth distributions, and various ratios and comparisons among these. In addition, values of the energy exponent p in the expression for electronic stopping Se =kEp are determined in this energy range, and values of electronic stopping Se and k are calculated for 1H and 2H in Si and GaAs. The 〈100〉 channeling ranges are slightly deeper than the (100) random ranges Rp. The 〈110〉 channeling ranges are 1.4 to 1.7 times as deep (varying with energy) as the random ranges Rp in GaAs, and 1.2 to 1.5 times as deep as Rp in Si. The random and channeling ranges for 2H are slightly greater than those for 1H in this energy range. Values of p are 0.83 for 1H in 〈110〉 Si and 0.76 for 2H in 〈110〉 Si, and 0.85 for 1H in 〈110〉 GaAs, and 0.73 for 2H in 〈110〉 GaAs.
The kinetics of electromigration
Authors: Rodbell K. P. , Rodriguez M. V. , Ficalora P. J.
Published: July 2002

A well-known model for the rate of heterogeneous nucleation and growth of a second-phase precipitate in a solid matrix has successfully been applied to electromigration rate studies in metallic thin-film interconnects. The application of these rate equations is based on the analogy between the nucleation and growth of a second-phase precipitate in a solid solution matrix and the nucleation and growth of a void in a thin-film conductor. In the first case, the driving force, at constant composition, is temperature, whereas the driving force for electromigration is both temperature and the applied electric field. Once developed, the model is applied to the resistance versus time data of a variety of thin-film systems, i.e., Al, Al-0.3% Cu, Ti, Ag, Ti/Au, and Au. The results show that this model can accurately be used to describe the failure mechanism in electromigration. Furthermore, if the growth rate of voids is known for a particular thin film, then it is possible to predict failure times.
Relativistic non-neutral electron flow in a planar triode
Authors: Chan Hei W. , Davidson Ronald C. , Nguyen Khanh T. , Uhm Han S.
Published: July 2002

The steady-state cold-fluid-Poisson equations are used to investigate the influence of space-charge effects on relativistic non-neutral electron flow in a planar triode with cathode located at z=−LCA, anode located at z=0, and a plane conductor located at z=d. An intense relativistic electron beam with current density −J0 (generated in the diode region) is injected through the anode plane. A one-dimensional theoretical model is employed in which the spatial variations are assumed to be perpendicular to the planar conductors. Relativistic effects on the electron motion are included, but self-magnetic fields are neglected. An exact analytical expression for the relativistic current flow is obtained. The steady-state and time-averaged equilibrium properties are investigated analytically. The results are compared with approximate analytical solutions and numerical simulation results using a multiple-sheet model. Based on this investigation, it is found that the formation of a virtual cathode downstream can be suppressed by an applied positive potential. The forward electron current is substantially enhanced, while the stable operating range of the steady-state triode current is reduced substantially.
Experimental investigations and numerical integration of the thermally stimulated current rate equations for defect parameters in CdTe crystals
Authors: Elkomoss S. G. , Samimi M. , Unamuno S. , Hage-Ali M. , Siffert P.
Published: July 2002

The thermally stimulated current (TSC) rate equations have been solved numerically, taking into consideration the T−2 dependence of the cross section on the temperature T. Values of the trap depth E and the capture cross section have been calculated for fourteen defect levels in the traveling heater method grown CdTe crystal. To solve the TSC rate equations numerically, one needs to know the concentrations of carriers in traps at the initial temperature, the concentrations of trapping states, and the recombination lifetime for the carrier. These quantities have also been measured experimentally. For the first eight peaks, the results are almost the same as those for slow retrapping. For the last six defect levels, the values of E are smaller than those of fast retrapping and differ by 100–150 meV from those calculated in the slow-retrapping approximation.
Small-scale laser effects experiments on graphite: Coupling coefficient, lateral loss, and effective heat of ablation
Authors: Klein Claude A. , Menefee Richard F. , Krenek Brendan D. , Berry Michael J.
Published: July 2002

This work concerns laser-interaction experiments performed on a fine-grained, nearly isotropic graphite grade known as GraphNOL. They were carried out with a cw/DF laser of 100-W output power, at peak irradiances varying from 10 to 50 kW/cm2 on targets of, typically, 1.5-mm diameter. One of the major objectives was to accept the difficulties associated with such small sizes and rely on analytical techniques for estimating the lateral heat loss, developing a scaling law, and formulating an improved, parametric representation of the effective heat of ablation (Q*). The procedure rests on Breaux’s formula (Ballistic Research Laboratories report no. 1834) heuristically extended to accommodate the concept of a lateral loss parameter, which relates linearly to a scaling parameter that combines target thickness, target diameter, and spot size, thus specifying how geometrical factors correlate in terms of their impact on radial losses. Our investigation demonstrates that a small-scale, low-cost laser ablation experiment can generate a rich set of well-characterized, highly accurate data, which are amenable to a comprehensive analytical evaluation. The coupling coefficient of GraphNOL to 3.8-μm radiation is 86±1%, averaged over the duration of the burn, in a burnthrough experiment; this result leads to the conclusion that the intrinsic Q* should be close to 38 kJ/g at the DF-laser wavelength.
Onset of runaway nucleation in aerosol reactors
Authors: Wu Jin Jwang , Flagan Richard C.
Published: July 2002

The onset of homogeneous nucleation of new particles from the products of gas phase chemical reactions was explored using an aerosol reactor in which seed particles of silicon were grown by silane pyrolysis. The transition from seed growth by cluster deposition to catastrophic nucleation was extremely abrupt, with as little as a 17% change in the reactant concentration leading to an increase in the concentration of measurable particles of four orders of magnitude. From the structure of the particles grown near this transition, it is apparent that much of the growth occurs by the accumulation of clusters on the growing seed particles. The time scale for cluster diffusion indicates, however, that the clusters responsible for growth must be much smaller than the apparent fine structure of the product particles.
Correlation between solid-state reaction and electrical properties of the Rh/GaAs Schottky contact
Authors: Yu Kin Man , Cheung S. K. , Sands T. , Jaklevic J. M. , Haller E. E.
Published: July 2002

The solid-state reactions between (100) GaAs substrates and Rh films ∼150 Å and ∼600 Å thick, annealed at temperatures between 300 and 800 °C, are investigated with conventional and heavy ion Rutherford backscattering spectrometry, and x-ray diffraction. Initiation of interface reactions between the Rh films and the GaAs substrate is observed at ∼300 °C. Laterally segregated RhGa, RhAs, and RhAs2 phases are detected for the 150-Å Rh/GaAs contact annealed in the temperature range of 300–700 °C. For thick Rh film (∼600 Å) on GaAs, vertical phase separation between the RhGa and the Rh-As phases is observed after annealing. After annealing at 700 °C for 20 min, the reaction between the 600-Å Rh film and GaAs is complete and a layer sequence of RhGa/RhAs2/GaAs results. Electrical properties of Rh/n-GaAs diodes are measured using the current voltage dependence. A correlation between the electrical behavior and the metallurgical reaction is observed.
Dynamical properties of fluxon propagation in a Josephson transmission line with a trigger turning point
Authors: Fujimaki Akira , Nakajima Koji , Sawada Yasuji
Published: July 2002

Dynamical properties of fluxon propagation in a discrete Josephson transmission line are reported. The circuit consists of three Josephson transmission lines and a trigger turning point. By using the Josephson sampler, we confirmed fan-out operation of the phase-mode logic circuit in which single fluxons were employed as information bits. We measured the fluxon velocity propagating in the Josephson transmission line. The measured velocity of 3.38×107 m/s was larger than that of the continuous Josephson transmission line, and it is a manifestation of a remarkable feature of the discrete Josephson transmission line. The experimental results affirm the high-speed operation of the phase-mode circuits constructed by discrete Josephson transmission lines.
Conductivity of polyethylene terephthalate induced by high electron beam current densities
Authors: Johnson D. J. , Kurtz S. R. , Sweeney M. A.
Published: July 2002

The radiation induced conductivity (RIC) of polyethylene terephthalate (Mylar) produced by the electron beam from a pinched electron beam diode has been measured experimentally. Data were obtained for 4–10 ns duration electron beam pulses at radiation dose rates up to 4×1016 rad/s (100 kA/cm2 current density). The conductivity is roughly proportional to radiation dose rate up to about 100 Mrad and at higher doses scales approximately with radiation dose to the (3)/(2) power. The experimental data are compared to the predictions of a model of RIC using dispersive electronic transport and bimolecular recombination. The parameters used in the model were obtained from published low dose (<100 rad) time-resolved photoconductivity measurements on Mylar. The data agreed well with the calculations before the onset of significant sample heating (10 Mrad). The highest reported RIC results are compared to the expected results for a weakly ionized plasma experiencing electron neutral collisions. In this case the experimental conductivity is significantly higher than the calculated value.
Low-temperature heat capacities of silicon carbide
Authors: Lin G. J. , Ho J. C. , Dandekar D. P.
Published: July 2002

Temperature dependence of heat capacity data between 4 and 12 K can be represented by a single term, βT3, associated with lattice vibrations. The coefficient β corresponds to a large Debye temperature of 990 K consistent with the high melting point and hardness of this refractory ceramic material.
C-induced deep levels in crystalline Si
Authors: Endrös A. , Krühler W. , Koch F.
Published: July 2002

A study of deep traps in the depletion regions of Schottky barrier junctions on silicon was made using deep level transient spectroscopy (DLTS). The Si samples, which had different carbon contents (C), were exposed to several annealing processes. The measurements show that depending on the C concentration and the annealing process employed C-induced deep traps with activation energies of 355, 290, and 216 meV appear above the valence band. The density, the trap levels, and the capture cross sections of these C-induced traps were determined from the DLTS data. The deep trap densities are correlated with the behavior of the diffusion length of the minority carriers in the different Si materials.
Mechanism of the negative slope in the current-light characteristics of twin-stripe lasers
Authors: Watanabe Masanobu , Itoh Hideo , Mukai Seiji , Yajima Hiroyoshi , Saito Masami , Uekusa Shin-ichiro
Published: July 2002

The mechanism of the negative slope in the current-light characteristics of twin-stripe lasers is explained by the dependence of a near-field pattern on the current distribution. A guiding principle for designing the laser structure for a more effective negative-slope characteristic is discussed.
Laser-induced chemical vapor deposition of silicon nitride films: Film and process characterizations
Authors: Pan E. T.-S , Flint J. H. , Adler D. , Haggerty J. S.
Published: July 2002

We have deposited hydrogenated amorphous silicon-nitride (a-SixN1−x:H) films from NH3-SiH4-Ar gas mixtures, heated by gas-phase absorption of CO2 laser radiation. For the first time, stoichiometric (Si/N=0.75) a-Si3N4:H films were obtained for NH3/SiH4 flow ratios of the order of 1000 and substrate temperatures Ts of about 500 °C. Growth rates as high as 13 Å/min were observed, depending on the partial pressure of SiH4, P(SiH4), the gas temperature Tg and Ts. Tg was calculated from a derived energy-balance equation, which depends sensitively on process parameters. To model the deposition process, the experimental film growth rate is separated into the Si growth rate G(Si) and the N growth rate G(N). Film stoichiometry is interpreted as the ratio G(Si)/G(N). The rate-limiting steps for Si growth are the gas-phase decomposition of SiH4 for Tg below about 750 °C and the SiH4 flow rate at higher Tg. G(N) is affected by both Ts and Tg. The NH3/SiH4 flow ratio must be kept large to ensure a sufficient concentration of N to attain stoichiometry. The reactions are likely to occur both in the gas phase and on the film surface.
Temperature dependence of the hyperfine field of Ce impurities in Gd metal: A theoretical study
Authors: Leal C. E. , Troper A.
Published: July 2002

An intermediate valence model for Ce impurities diluted in Gd metal is used to describe the formation of the Ce local magnetization. The Ce impurity and the Gd host are described as a strongly d-f correlated and hybridized system. The value obtained for the Ce valence agrees quite well with the experimental results. The temperature dependence of the hyperfine field is discussed. We briefly comment also on the case of Yb impurities diluted in Gd metal.
Transfinite elements: A highly efficient procedure for modeling open field problems
Authors: Lee Jin-Fa , Cendes Zoltan J.
Published: July 2002

This paper presents a new approach for modeling open boundary conditions in electric and magnetic field problems that is considerably simpler and more efficient than the existing alternatives. The procedure is called the transfinite element method because it employs analytic basis functions to transfer the exterior boundary condition over an infinite domain to a finite-sized numerical solution region. In the transfinite element method, the problem region is divided into an interior region Ωi and an exterior region Ωe by a circular boundary Γ. Since the exterior region involves only a circular boundary, it may be solved analytically using the separation of variables procedure. Substituting this solution into the variational principle for the electric or magnetic field, minimizing it, and imposing continuity between the finite element solution in the interior region and the analytical solution in the exterior region results in a real, symmetric matrix equation that is solved for the field.
Anisotropic spin fluctuations in the itinerant-electron ferromagnet MnP
Authors: Yamada K. , Todate Y. , Endoh Y. , Ishikawa Y. , Böni P. , Shirane G.
Published: July 2002

The paramagnetic scattering cross section of the anisotropic metallic ferromagnet MnP has been measured up to the zone boundaries along the [100] and [010] directions perpendicular to the easy magnetization axis. The spectral shape functions in both directions are well fitted to a Lorentzian form at T=TC (290.5 K), 1.03TC, and 1.20TC except those close to the zone boundaries. Near TC the linewidth along [010] follows the dynamical scaling prediction for the isotropic ferromagnet Γ(q)∝q2.5. On the contrary, a unique q dependence, Γ(q)∝q3.2, along [100] was observed. However, these q dependencies deviate at small q(q<0.1 A−1 along [100] and q<0.02 A−1 along [010]), suggesting the existence of a residual linewidth of Γ0∼0.02 meV at q=0.
Magnetism of rare-earth salts R(CF3SO3)3⋅ 9H2O with R=Tb, Dy, and Ce
Authors: Bellesis G. H. , Simizu Satoru , Friedberg S. A.
Published: July 2002

Magnetic susceptibilities have been measured between 0.05 and 20 K for three rare-earth trifluoromethanesulfonate nonahydrates R(CF3SO3)3⋅ 9H2O or RTFMS whose hexagonal crystal structure is very similar to that of the rare-earth ethylsulfate nonahydrates R(C2H5SO4)3⋅ 9H2O or RES. TbTFMS and DyTFMS order ferromagnetically at TC=0.240 K and TC=0.111 K, respectively. CeTFMS remains paramagnetic down to 0.089 K. All three compounds show extreme Ising-type anisotropy with g⊥∼0. g∥ of each RTFMS compound compares favorably with that of the corresponding RES as expected from crystalline field theories based on a point-charge model. In each case, the Weiss constant is close to what is expected for a system with predominantly dipolar interactions. The ferromagnetic ordering in TbTFMS and DyTFMS is essentially that of a dipolar Ising system. The splitting Δ of the ground quasi-doublet in TbTFMS is only ∼0.5 K and the ordering appears to be largely explained by electronic models although substantial ordering of nuclear spins is indicated in our preliminary heat capacity data.
Comparison of thin-film transistors fabricated at low temperatures (≤600 °C) on as-deposited and amorphized-crystallized polycrystalline Si
Authors: Kung K. T.-Y. , Reif R.
Published: July 2002

A comparison of the performances of thin-film transistors fabricated on as-deposited and amorphized-crystallized (low-pressure chemical-vapor deposited) polycrystalline Si films is reported in this paper. The polycrystalline Si films, 3500 Å thick, were grown on SiO2 via the pyrolysis of SiH4 at 625 °C, and the entire amorphization (ion implantation), crystallization, and device fabrication processes that followed were carried out at temperatures ≤600 °C, all compatible with low-cost glass substrates. Both n-channel and p-channel metal-oxide-semiconductor (NMOS and PMOS) transistors with three different geometries (W/L=200/20, 200/10, and 200/5 μm/μm) were fabricated and compared. The results showed that the amorphized-crystallized film was in general a more superior thin-film transistor material than the as-deposited film, although the ‘‘aspects’’ of superiority differed between the NMOS and the PMOS transistors. The NMOS transistors fabricated on the amorphized-crystallized film exhibited a lower threshold voltage (21–25 vs 23–25 V), a higher field-effect mobility (15 vs 3.5 cm2/V s), but the same on–off current ratio (2×104) as those on the as-deposited film. The PMOS transistors fabricated on the amorphized-crystallized film exhibited a higher field-effect mobility (10 vs 3.4 cm2/V s), a higher on–off current ratio (6×104 vs 3×104), but the same threshold voltage (−25 V) as those on the as-deposited film. When fabricated on the same material (as-deposited or amorphized-crystallized), NMOS outperforms PMOS in terms of a lower threshold voltage and a higher field-effect mobility, but PMOS outperforms NMOS in terms of a higher on–off current ratio. All thin-film transistors fabricated in this work qualify as the switching elements for large-area liquid-crystal displays.
An investigation of the voltage-withstanding mechanism of BaTiO3 ceramic semiconductors
Authors: Li Biao-rong , Gong Shu-ping , Zhou Dong-xiang
Published: July 2002

The voltage-withstanding behavior of a BaTiO3 ceramic semiconductor is closely related to the grain boundary structure of this kind of ceramic. This mainly depends on the height of the potential barrier in the grain boundary. A novel concept of the barrier height per unit width is proposed, by which the voltage-withstanding mechanism can be satisfactorily explained. Positive temperature coefficient devices of high performance have been developed, which possess resistivity (<102Ω cm) at room temperature and high withstanding voltage (>300 V/mm).
Ensemble Monte Carlo simulations of femtosecond thermalization of low-energy photoexcited electrons in GaAs quantum wells
Authors: Bailey D. W. , Artaki M. A. , Stanton C. J. , Hess K.
Published: July 2002

We present results of ensemble Monte Carlo simulations of the room-temperature relaxation of nonthermal photoexcited electrons in GaAs quantum-well structures. Electrons are excited at a mean energy of 20 meV above the band edge, similar to the experiments of Knox et al. [Phys. Rev. Lett. 56, 1191 (1986)] and Oudar et al. [Phys. Rev. Lett. 55, 2075 (1985)]. Since this energy is less than the optical-phonon emission threshold, energy relaxation occurs primarily via carrier-carrier scatterings. We find that the excited electrons thermalize with the background electrons within 200 fs, in agreement with experiment.
Contribution of the band-filling effect to the effective refractive-index change in double-heterostructure GaAs/AlGaAs phase modulators
Authors: Mendoza-Alvarez J. G. , Yan R. H. , Coldren L. A.
Published: July 2002

In this work we have theoretically estimated the effective refractive-index variation Δneff due to the removal of carriers in the depletion region of a double-heterostructure (DH) n-AlGaAs/n-GaAs/p-AlGaAs phase modulator when a reverse bias is applied to this structure. Δneff is calculated by a Kramers–Kronig analysis of the absorption spectrum, which changes around the absorption edge because of the band-filling effect associated with the presence of carriers. We also calculated the phase shift Δφ due to Δneff, and we showed that this contribution due to the band-filling effect can account for up to 28% of the total phase shift for a specific DH structure, showing in this way that the band-filling effect can no longer be ignored in the analysis of phase modulators.
On the measurement of excess carrier lifetime in semiconductors by the use of penetrating radiation
Authors: Hlávka J.
Published: July 2002

Evidence is given that erroneous results can be caused in excess carrier lifetime measurements by scattering and internal reflection of penetrating radiation in a semiconductor specimen confined with lapped surfaces or in a specimen with a Schottky contact.
Lateral electromagnetic pulses generated by a vertical dipole on the boundary between two dielectrics
Authors: Wu Tai T. , King Ronold W. P.
Published: July 2002

An exact solution in terms of elementary functions is obtained in the time domain for the vertical electric field Ez(ρ,t) and the transverse magnetic field Bφ(ρ,t) of a dipole located on the plane boundary z=0 between air (region 2, z<0) and a perfect dielectric (region 1, z>0) when the dipole is excited by a single delta-function current pulse. The vertical electric field on the boundary consists of a delta-function pulse that travels in the air with the velocity c, an oppositely directed delta-function pulse that travels in the dielectric with the smaller velocity c/ε1/2, an amplitude structure that varies in time between the two pulses, and a final static electric field due to the charges left on the dipole. The horizontal magnetic field is similar. For comparison, the vertical electric field in the equatorial plane of the same dipole in a homogeneous dielectric is also derived. The comparison indicates that the field along the boundary is a surface-wave or lateral pulse.
Lithographic properties of amorphous WO3 films exposed to photons, electrons, and hydrogen plasma
Authors: Gupta P. K. , Chopra K. L.
Published: July 2002

Lithographic properties of amorphous WO3 films exposed to photons, electrons, and glow-discharge hydrogen plasma have been investigated. It has been observed that the etching rates of the films in an alkaline solution are lower compared to the unexposed ones, giving rise to a negative tone behavior of the material. The etching characteristics as a function of exposure time and their correlation with the optical transmission have been studied. The contrast (γ) values obtained are 2.3 for photons, 4.0 for electrons, and 2.7 for hydrogen plasma. The sensitivity (S) values obtained are∼1021 photons /cm2 for photons and 10−3 C/cm2 for electrons.