bluuebunny/crossref_metadata_2025 huggingface dataset:
A random number generator based on the logit transform of the logistic variable
Authors: Collins J. J. , Fanciulli M. , Hohlfeld R. G. , Finch D. C. , Sandri G. v. H. , Shtatland E. S.
Published: July 2002

A nonperiodic random number generator, which is based on the logistic equation, is presented. A simple transformation that operates on the logistic variable and leads to a sequence of random numbers with a near-Gaussian distribution, is described and discussed. The associated algorithm can be easily utilized in laboratory exercises, classroom demonstrations, and software written for stochastic modeling purposes.
Calorimetric studies of crystallization and relaxation of amorphous Si and Ge prepared by ion implantation
Authors: Donovan E. P. , Spaepen F. , Turnbull D. , Poate J. M. , Jacobson D. C.
Published: July 2002

Amorphous Si and Ge layers, produced by noble gas (Ar or Xe) implantation of single crystal substrates, have been crystallized in a differential scanning calorimeter (DSC). The MeV implantation energies resulted in amorphous layers of micron thickness whose areal densities were determined using the Rutherford backscattering and channeling of 1-MeV protons. These techniques allow determination of the amorphous-crystal interface velocity (which is proportional to the rate of heat evolution ΔḢac) and the total enthalpy of crystallization ΔHac. Amorphous Ge was found to relax continuously to an amorphous state of lower free energy, with a total enthalpy of relaxation of 6.0 kJ/mol before the onset of rapid crystallization. The interface velocity for crystallization on (100) substrates, was found to have an Arrhenius form with an activation energy of 2.17 eV. The value of ΔHac was found to be 11.6±0.7 kJ/mol, the same as for samples prepared by deposition. For Si, ΔHac was determined to be 11.9±0.7 kJ/mol without any evidence of heat release due to relaxation. The (100) interface velocity was found to have an activation energy of 2.24 eV. The effects of the implant depth profile on the regrowth velocity could also be observed directly in the DSC signal. From the value of ΔHac and Gibbs free-energy calculations, the melting temperature of amorphous Si has been determined to be Taℓ =1420 K.
Effect of positive and negative ion energies on H− destruction by mutual neutralization in low-pressure plasmas
Authors: Bruneteau A. M. , Bacal M.
Published: July 2002

The effect of plasma potential profile and collisions with neutrals and charged particles upon positive and negative ion energies has been studied in a low-pressure (2–13.6 mTorr) hybrid magnetic multicusp hydrogen discharge. When the plasma density is lower than 1011 cm−3, the negative ion temperature is always close to the gas temperature, while the positive ion temperature goes down from 1 to 0.1 eV when the pressure goes up. The observed reduction of the relative negative ion density when the pressure is increased is shown to be related to the reduction of positive ion velocities and to the associated increase in the mutual neutralization rate coefficient.
Electron and ion kinetics and anode plasma formation in two applied B
r field ion diodes
Authors: Johnson D. J. , Quintenz J. P. , Sweeney M. A.
Published: July 2002

Two magnetically insulated ion diodes that utilize a radial applied-B field are described. Both diodes generate an annular beam that is extracted along the diode axis. The first diode operated at 1.2 MV and 600 kA for 25 ns and generated a 300-kA ion beam. The second operated at 300 kV, 100 kA and generated 15 kA of ion current. The first diode was used to study diode performance as a function of inner and outer anode-cathode gaps, the applied-B field, and transmission line current ratios. The second diode was used to study anode plasma formation. The diodes were operated below Bcrit, resulting in electron leakage to the anode, especially near the outer cathode. A definition of Bcrit applicable to extraction diodes is given and methods of improving ion production efficiency in these diodes are suggested. The strong correlation of ion production with visible light emission suggests, however, that the electron loss played an important role in anode turn-on. The breakdown of neutral gas desorbed by electron impact is thought to be the anode plasma production mechanism. The grazing incidence leakage electrons affect the breakdown by significantly enhancing space-charge-induced electric fields in the dielectric-filled anode grooves.
Grid-controlled extraction of pulsed ion beams
Authors: Humphries S. , Burkhart C. , Coffey S. , Cooper G. , Len L. K. , Savage M. , Woodall D. M. , Rutkowski H. , Oona H. , Shurter R.
Published: July 2002

Experimental results are presented on a method for extracting well-focused ion beams from plasma sources with time-varying properties. An electrostatic grid was used to stop the flow of plasma electrons so that only ions entered the extraction gap. In this case, ion flow in the gap was controlled by space-charge effects as it would be with a thermionic ion source. Constant extracted current was observed even with large variations of source flux. An insulator spark source and a metal-vapor vacuum arc were used to generate pulsed ion beams. With a hydrocarbon spark, current densities of 44 mA/cm2 were achieved at 20-kV extractor voltage for an 8-μs pulse. With an aluminum-vapor arc, a current density of 15 mA/cm2 (0.3 A total) was measured for a 50-μs pulse.
Estimated gains for a Ni-like exploding foil target
Authors: Maxon S. , Hagelstein P. , Scofield J. , Lee Y.
Published: July 2002

It is shown that an exploding Eu foil target irradiated with 7.5×1013 W/cm2 should reach plasma parameters which look optimistic for soft x-ray lasing. Among the lines of interest are two 4p-4d transitions near 100 Å and two near 80 Å with gains estimated at 4–7 cm−1.
I
n s
i
t
u investigation of the growth of rf glow-discharge deposited amorphous germanium and silicon films
Authors: Antoine A. M. , Drevillon B. , Roca i Cabarrocas P.
Published: July 2002

An in situ investigation of the growth of rf glow-discharge amorphous germanium (a-Ge:H) and silicon (a-Si:H) films using fast real-time spectroscopic phase-modulated elipsometry is presented. The influence of the conditions of preparation is studied in both cases. The same behavior is obtained in a-Ge:H and a-Si:H films deposited in similar conditions. In particular, the initial stage of the growth can be described by a nucleation process in both cases, whatever the conditions of preparation. The incomplete coalescence of the nuclei leads to the formation of a surface roughness on a ∼40-Å scale which is observed during film growth. In comparing real-time ellipsometry measurements performed at different wavelengths, a correlation between the internuclei distance and the thickness of the surface roughness is observed. An enhancement of the surface mobility of the reactive species due to an increase of substrate temperature and/or ion-bombardment energy results in an increase in the density of the nucleation centers. The influence of ion bombardment and gas pressure is discussed by comparing the growth of a-Si:H film deposited in a rf discharge to the results of previous studies using a low-pressure multipole plasma.
Pulse shapes of nanosecond photoacoustic signals in liquids detected by piezoelectric foil
Authors: Komorowski Stanislaw J. , Eyring Edward M.
Published: July 2002

A nonringing detection system based on 28-μm-thick piezoelectric, poly(vinylidene difluoride) foil has been constructed for studying photoacoustic signals in liquids. Excellent agreement between theoretical and experimental photoacoustic pulse shapes has been observed. This system is well suited for monitoring radiationless relaxation processes in solutions on a nanosecond time scale.
Surface properties of Cr2O3
Authors: Kirby R. E. , Garwin E. L. , King F. K. , Nyaiesh A. R.
Published: July 2002

Thin (<5 nm) air-oxidized Cr layers are deposited on the alumina output windows of radio-frequency klystron tubes to prevent electron multipactor by reducing the secondary electron emission yield of the alumina surface. The top several nanometers of these layers appear to be Cr2O3. To compare the measured surface properties of these layers with those of clean stoichiometric Cr2O3, quasibulk Cr2O3 layers were produced by wet-H2-firing magnetron-deposited Cr films on Cu substrates and characterized by x-ray photoelectron-, Auger electron-, and electron energy loss spectra and secondary electron emission yield measurements. Other properties measured were x-ray diffraction structure, sheet resistance, and optical reflectivity. In particular, the peak of the secondary electron yield was found to be ∼1.7, which is considerably higher than the <1 yield value reported earlier in the literature. The Cr2O3 Cr 2p x-ray photoelectron core level spectrum was curve-fit using Doniach–Sunjic line shapes and statistical fitting methods. It is shown that each 2p level is composed of three multiplet-split peaks 1.1 eV apart. In addition, the three 2p1/2 peaks are folded with a 2p3/2 3d satellite.
Photoluminescence and energy-loss rates in GaAs quantum wells under high-density excitation
Authors: Uchiki Hisao , Kobayashi Takayoshi , Sakaki Hiroyuki
Published: July 2002

The time-resolved luminescence spectra from excited conduction subbands in three samples of multi-quantum-well GaAs/AlxGa1−xAs (x=0.3 and 1) semiconductors with several well widths and barrier heights were obtained under high-density excitations by a 30-ps pulsed laser at 532 nm, which generated electron–hole pairs to the concentration of 1010–1013/cm2 per well per pulse at 77 K. The temperature and the Fermi energy of electron were determined by fitting best the constructed time-resolved spectrum to the observed, and the time-dependent electron energy was obtained by using these parameters. The energy-loss rates of hot electrons are at least twice smaller than the calculated ones induced by the electron-polar phonon scattering, including the screening effect due to the high carrier density.
Microwave absorption of the high-T
c superconductor YBa2Cu3O7
Authors: Poirier M. , Quirion G. , Poeppelmeier K. R. , Thiel J. P.
Published: July 2002

Using a resonant-cavity perturbation technique, the microwave absorption at 16.8 GHz of the high-Tc compound YBa2Cu3O7 is measured as a function of temperature (4–300 K) and magnetic field (0–7 T). At zero field the microwave loss shows a rapid decrease below 93 K, the onset of the superconducting transition. An important absorption, which is a decreasing function of temperature, is observed in the superconducting state.When a magnetic field is applied, the transition is broadened and shifted toward lower temperatures: an upper-critical field Hc2(T=0)∼ 150 T is obtained via extrapolation of the data. In the superconducting state, the absorption is found to be highly dependent on the magnetic-field intensity. For magnetic fields above 500 Oe, the absorption is a slowly increasing function and it is understood in terms of the temperature behavior of Hc2 ; for H<200 Oe, the absorption increases very rapidly with field and it is believed to be correlated to Hc1 and to the anisotropic character of the structure.
Inhibition of formation of SF6 molecular clusters in a free supersonic expansion
Authors: Trava-Airoldi V. J. , Ghizoni C. C. , Corat E. J. , Sbampato M. Esther , dos Santos A. M.
Published: July 2002

In this work we present experimental results on the inhibition of formation of sulfur hexafluoride molecular clusters in a free supersonic expansion, due to resonant absorption of intense radiation from a pulsed CO2 laser. The spatial energy distribution of the molecular beam in the transverse direction was measured with a pyroelectric detector with and without incident radiation. A skimmer placed in the collisional region of the expansion causes a second expansion of the molecular beam which is more divergent when these molecules are vibrationally excited by a CO2 laser beam perpendicularly crossing the molecular beam before the skimmer. This is due to the inhibition of formation of molecular clusters by the CO2 laser beam which leads to a lower average mass for the expanding beam particles. In a molecular beam with isotopic species, a selective inhibition of formation of clusters would lead to different spatial distribution for different isotopes in the molecular beam.
Large Barkhausen discontinuities in Co-based amorphous wires with negative magnetostriction
Authors: Yamasaki J. , Humphrey F. B. , Mohri K. , Kawamura H. , Takamure H. , Mälmhäll R.
Published: July 2002

Magnetic properties, such as domain patterns and anisotropy, were measured for negative magnetostrictive Co-Si-B amorphous wires exhibiting large Barkhausen discontinuities and the results are compared to those of Fe-Si-B wires with positive magnetostriction. The Co-based wire was found to have a bamboolike domain structure at the wire surface. It was also shown that the amorphous wires prepared by the in-water quenching technique store tensile stress in the radial direction. The magnetostrictive anisotropy due to residual stress will produce an axial component of magnetization in conjunction with the two-dimensional geometry of wires making both Co- and Fe-based wires exhibit large Barkhausen discontinuities along the axis of the wire.
The use of a charge-coupled device and position sensitive resistive anode detector for multiorder spontaneous Raman spectroscopy from silicon
Authors: Acker William P. , Yip Brandon , Leach David H. , Chang Richard K.
Published: July 2002

Two state-of-the-art two-dimensional photon detectors, a high quantum efficiency charge-coupled device, and a microchannel plate photomultiplier with a position sensitive resistive anode (Mepsicron) have been used to observe spontaneous Raman scattering from multiorder phonon modes of Si in the backscattering geometry. These two detectors and an intensified linear photodiode array are compared using the multiorder spontaneous Raman signal from Si as a weak optical source. An attempt is made to assign the observed Raman scattering peaks in the first- through fourth-order Raman shift regions to known peaks in the density of states of the appropriate Si phonon branches.
Sputtering in the presence of a rapid gas flow
Authors: Rae S. C. , Tobin R. C.
Published: July 2002

A rapid flow of noble gas through a hollow cathode glow discharge can be used to entrain a proportion of the sputtered metal atoms and form an extended high-concentration jet. This technique has been used previously to demonstrate copper i and gold i laser action at room temperature. We develop a model to predict the dependence of the ground-state metal atom concentration on the current density and the entrainment in such a system. The sputtering process depends on charge transfer collisions between metal atoms and noble gas ions. By fitting experimental results for a copper cathode to the theoretical predictions, values are found for the rate constant of this reaction and for the entrainment efficiency. In an argon/helium gas mixture, the optimum volume fraction of helium is approximately 90%. The main effects of the helium are to decrease the number of ion collisions in the cathode dark space (leading to increased sputtering yields) and to improve the charge transfer process, although there are a number of additional effects. The time and current dependencies of the copper atom concentration in the cathode region are compared with corresponding measurements taken downstream in the jet.
Study on photoluminescence and Raman scattering of GaInP and AlInP grown by organometallic vapor-phase epitaxy
Authors: Kondow Masahiko , Minagawa Shigekazu
Published: July 2002

The photoluminescence and Raman scattering spectra of GaInP and AlInP grown by organometallic vapor-phase epitaxy are measured to investigate the ordered and disordered crystalline states. It is found that ordered structure exists not only in the GaInP but also in the AlInP, and probably in the entire AlGaInP system, and that zinc doping over 1018 cm−3 makes it disordered. This disordering effect is attributed to the Zn diffusion in the GaInP epilayer during crystal growth which puts the once-formed ordered atomic arrangement into a disordered state.
Cylindrical magnetron discharges. I. Current-voltage characteristics for dc- and rf-driven discharge sources
Authors: Yeom G. Y. , Thornton John A. , Kushner Mark J.
Published: July 2002

Direct current magnetron discharge sources are often characterized by current-voltage characteristics of the form I∝V n where n is typically in the range from 4 to 10. Similar I-V characteristics are found for dc hollow cathode discharges where the cathode configuration effectively provides electrostatic confinement of the primary electrons. Therefore, by analogy, it has been suggested that the exponent n provides an index to the effectiveness of the magnetic electron confinement in a magnetron discharge. When magnetron discharge sources are driven at rf frequencies, the I-V characteristics typically yield n values in the range 1–3. We have examined the I-V characteristics of cylindrical-post magnetron discharge sources of various diameters driven dc and at rf frequencies of 1.8 and 13.56 MHz. The rf-driven discharges yielded n values which, in most cases, were less than 2.5. Electrostatic probe measurements of the interelectrode voltage distribution showed that the low n values, that is, poor confinement, could be explained by the effect of the magnetic field on the electron transport during that portion of the rf cycle when the post electrode is serving as the anode.
X-ray small-angle scattering analysis of porous silicon layers
Authors: Goudeau P. , Naudon A. , Bomchil G. , Herino R.
Published: July 2002

A small-angle x-ray scattering study of porous layers prepared on lightly doped and heavily doped silicon wafers reveals strong differences according to the dopant type. The p-type silicon layers show a roughness between matter and voids below a space-correlation length of about 10 nm and a fractal dimension of aggregates for a length scale higher than 25 nm. The lightly doped n-type is different: the scattering curve obeys the Porod law according to the rod-like structure of the pores. However, the structure of the heavily doped p+ sample is more complicated and not yet well understood.
Electroreflectance study of the temperature dependence of the E1 transition of Hg0.65Cd0.35Te
Authors: Ksendzov A. , Pollak Fred. H. , Wilson J. A. , Cotton V. A.
Published: July 2002

We have investigated the temperature dependence of the E1 optical feature (energy and broadening parameter) of Hg0.65Cd0.35Te using electroreflectance in the metal-insulator-semiconductor configuration. Measurements were made in the temperature range 77–293 K. The observed variation can be fit by either a linear dependence or the empirical Varshni relation [Y.P. Varshni, Physica (Utrecht) 39, 149 (1967)] within experimental error. The obtained linear temperature coefficient for E1 is compared with previous results. The temperature dependence of the broadening parameter also is discussed.
Transport of vacuum arc plasma through straight and curved magnetic ducts
Authors: Storer J. , Galvin J. E. , Brown I. G.
Published: July 2002

We report on an experimental investigation of the transport of a metal vapor vacuum arc plasma through straight and curved magnetic solenoids for the case of an iron plasma and for magnetic field strength up to 650 G. We find that the fraction of plasma transported increases with magnetic field strength and saturates at a field at which the ion gyroradius equals the radius of the plasma channel. The magnetic field strength at the cathode location, where the plasma is injected into the solenoid, was found to be important, and for the present work the injection efficiency optimizes at a field of about 150 G.
The effect of Bi2O3 content on the microstructure and electrical properties of ZnO varistor materials
Authors: Olsson E. , Dunlop G. L.
Published: July 2002

The microstructures of three ZnO varistor materials with different Bi2O3 contents have been evaluated by analytical electron microscopy in combination with x-ray diffractometry. The results have been correlated to microelectrode measurements, where breakdown voltages of individual ZnO junctions were measured, and also to current/voltage characteristics of bulk specimens. The volume fraction of the continuous intergranular network of Bi-rich phases, which lies along the triple junctions of the ZnO grains, increases with increasing Bi2O3 content, The conductivity of this network is strongly influenced by its internal microstructure. It was found that increased volume fractions of δ-Bi2O3 and less interpenetration between α-Bi2O and δ-Bi2O3 increases the conductivity of the network. Individual ZnO/ZnO grain boundaries exhibited breakdowns at 3.2 and 3.6 V, depending upon whether they contained segregated Bi atoms or thin Bi-rich amorphous films. The current/voltage characteristics of heterojunctions between ZnO and intergranular Bi2O3 were asymmetrical with respect to the polarity of the applied voltage. It was found that α-Bi2O3 and δ-Bi2O3 give rise to different breakdowns for electrons traveling from the Bi2O3 into an adjacent ZnO grain.
Ion implantation of sputtered Y-Ba-Cu-O films
Authors: Rubin M. , Brown I. G. , Yin E. , Wruck D.
Published: July 2002

Thin films of Y-Ba-Cu-O were deposited by rf magnetron sputtering from a single stoichiometric target and then altered in composition by ion implantation. Under optimum deposition conditions the films are deficient in Ba and Cu. Ion implantation of Cu was performed using a metal-vapor vacuum-arc source having high current and a broad energy spectrum for good depth distribution. Composition was determined by Rutherford backscattering spectrometry. The zero-resistance temperature was greatly increased after implantation and reannealing. This method could be used to write superconducting patterns on insulating material.
Operating characteristics and comparison of photopyroelectric and piezoelectric sensors for trace hydrogen gas detection. I. Development of a new photopyroelectric sensor
Authors: Christofides Constantinos , Mandelis Andreas
Published: July 2002

A new type of solid-state sensor for the detection of minute concentrations of hydrogen gas has been developed. The sensor was made of thin, commercially available polyvinylidene fluoride (PVDF) pyroelectric film, sputter coated with Pd. An infrared laser beam served to produce alternating temperature gradients on the Pd-PVDF and on reference Al-Ni-PVDF films, which, in turn, generated ac voltages due to the photopyroelectric (P2E) effect. Exposure to hydrogen gas was shown to produce an increased differential signal between the Pd and reference electrodes; this was tentatively attributed to the adsorption and dissociation of hydrogen molecules on the Pd surface, which caused a shift on the Pd-PVDF pyroelectric coefficient, due to interactions at the Pd-PVDF interface. The differential signal was found to be proportional to the square root of the hydrogen partial pressure at very low concentrations (<1000 ppm). A semiquantitative interpretation of the differential signal has been achieved using simple gas-solid interaction theory and the combination of the Langmuir isotherm with the photopyroelectric theory in the range of 4–200 Pa. For high pressures (>200 Pa) the paper is limited only to a phenomenological description. The thickness of the palladium layer has been found to play an important role with respect to the signal response. Presently, hydrogen concentrations as small as 40 ppm, in a flowing H2+N2 mixture, have been detected. The influence of gas flow rate has also been studied. Other characteristics such as the response times, the reversibility, and the durability of the Pd-PVDF-P2E hydrogen detector will also be presented.
Optical plasma resonance in semiconductors: Novel concepts for modulating far-infrared light
Authors: Kuijk M. , Vounckx R.
Published: July 2002

A novel concept for modulating far-infrared light is presented based on the ‘‘photon-plasmon, plasmon-photon’’ interaction with a thin layer of a free-electron gas in a n-doped semiconductor. For oblique incidence of p-polarized light at the plasma frequency of the electrons, high-reflection properties arise due to plasma resonance. The influence of all the constituent parameters (angle of incidence, electron mobility and density, thickness of the layer, etc.) on the reflectivity is discussed. The plasma frequency, and hence the reflectivity, can be modulated by changing the effective mass or the density of the electrons. This leads to a small, integrable, and fast modulable mirror, which can be used as a switching device (electrically or optically driven) or even as a way of frequency modulation.
Nonlinear optical investigation of the bulk ferroelectric polarization in a vinylidene fluoride/trifluoroethylene copolymer
Authors: Wicker A. , Berge B. , Lajzerowicz J. , Legrand J. F.
Published: July 2002

The ferroelectric polarization of vinylidene fluoride-trifluoroethylene copolymers (70/30 mol %) is investigated using complementary measurements of the surface charge and the second harmonic intensity (at 530 nm) generated in the polymer from a Nd:YAG laser beam (at 1060 nm). Due to the nonzero electrical conductivity of the polymer (especially above room temperature), the nonlineear optical technique provides better measurement of the bulk polarization, its changes with time, the applied electric field, and temperature. First, we present optical results obtained on polarized films after removal of the electrodes: they confirm the proportionality between the second harmonic intensity and the square of the ferroelectric polarization, and the centrosymmetric character of the paraelectric phase; they also show the temperature dependence of the remanent polarization in the crystal phase. Second, we present simultaneous measurements of the surface charge and of the second harmonic intensity under very low-frequency applied voltage through transparent electrodes. The field dependence of the measured polarization is discussed in terms of a microstructural analysis of the dielectric properties in the semicrystalline material. It is also shown that unipolar voltages applied during long periods of time are able to produce charge injection and space charge in the vicinity of the electrodes which can result in screening of the applied electric field.
Ultrasmall iron particles prepared by use of sodium amalgam
Authors: Linderoth S. , Mo/rup S.
Published: July 2002

Ultrasmall magnetic particles containing iron have been prepared by reduction of iron ions by the use of sodium in mercury. Mössbauer studies at 12 K show that the magnetic hyperfine field is significantly larger than in bulk α-Fe, suggesting that an iron mercury alloy rather than α-Fe has been formed. The particles exhibit superparamagnetic relaxation above 120 K.
Effects of substrate temperature on the electrical and physical properties of silicon dioxide films deposited from electron cyclotron resonant microwave plasmas
Authors: Herak T. V. , Thomson D. J.
Published: July 2002

Silicon dioxide films have been fabricated at growth temperatures ranging from 25 to 330 °C from an electron cyclotron resonant microwave plasma. Films were deposited from a SiH4/Ar/N2O reactant gas mixture. The minimum temperature required to fabricate high-electrical-quality silicon dioxide films is between 255 and 290 °C. In metal-oxide-semiconductor devices, electron injection field strengths and breakdown field strengths were as high as 5 and 8 MV/cm, respectively, for oxides grown above this temperature range. Films grown at temperatures slightly below the 255–290 °C range have much poorer electrical integrity. A concomitant increase in the refractive index was observed with the improvement in the electrical integrity of these films. The refractive index increased with increasing growth temperature and was in the range of 1.44–1.47. In the 255–290 °C temperature range, the refractive index of the silicon dioxide films reached approximately 1.46–1.47, and saturates thereafter. Infrared spectroscopy indicated a hydrogen content in these films of approximately 4 at. %. The hydrogen was bonded as SiOH, while no SiH impurity groups were detected. The dependence of the deposition rate on the substrate temperature was dependent on the location of the substrate. For films grown on substrates in contact with the plasma, the deposition rate was found to decrease with increasing substrate temperature. In contrast, for films grown on substrates located in the plasma afterglow, the deposition rate increased with increasing substrate temperature.
High-efficiency operation of a gas discharge XeCl laser using a magnetically induced resonant voltage overshoot circuit
Authors: Gerritsen J. W. , Keet A. L. , Ernst G. J. , Witteman W. J.
Published: July 2002

An electrical to optical efficiency of 5% has been demonstrated in an x-ray preionized XeCl discharge laser using an improved prepulse-mainpulse circuit, based on resonant charging of the peaking capacitor. In contrast to known prepulse-mainpulse circuits, the new scheme has no time delay between prepulse and mainpulse nor a voltage reversal on the electrodes. We present three possible configurations of the scheme and one of them has been realized for demonstrating its high-efficiency capabilities.
A study of gas chemistry during hot-filament vapor deposition of diamond films using methane/hydrogen and acetylene/hydrogen gas mixtures
Authors: Wu Ching-Hsong , Tamor M. A. , Potter T. J. , Kaiser E. W.
Published: July 2002

The composition of the reaction gases in a hot-filament reactor for chemical vapor deposition of diamond films was analyzed using a gas chromatograph coupled with a quartz microprobe. Concentrations of several hydrocarbons were determined as functions of filament temperature (FT) and the position of the probe relative to the filament for two feed gases, methane/hydrogen and acetylene/hydrogen. The diamond growth rate was measured as a function of FT in both feed gases. The major chemical process in these reaction systems is found to be conversion between methane and acetylene with ethane and ethylene as reaction intermediates. For FT≤1800 °C, the chemical reactivity is low, and no diamond deposition is observed. For FT≥1900 °C, nearly identical chemical composition near the filament is obtained from both feed gases (indicating possible attainment of thermodynamic equilibrium in the gas mixtures), and the measured diamond growth rates are similar. A substantial depletion of carbon in the reaction gases near the filament is observed and is shown to be due to thermal diffusion. Downstream of the filament, a nonequilibrium state is observed, possibly caused by slow methane/acetylene conversion and/or fast diffusion. The trend of the diamond growth rate, which increases with increasing FT from 1900 to 2200 °C, correlates well with the concentration profile of acetylene but not that of methane.
Single scan deep-level transient spectroscopy
Authors: Su Z. , Farmer J. W.
Published: July 2002

A new single scan method for obtaining deep level trap parameters is presented. This method represents an improvement over previous single scan approaches because it does not involve numerical analysis and it uses data over the entire temperature range of the deep-level transient spectroscopy peak. Data are presented for an isolated deep level, the E center. The usefulness of this new method is also demonstrated by an example of successful analysis of overlapping deep level signals. The new single scan method maintains the simplicity of the traditional method of data collection. However, the single temperature scan saves time and more important, is often more accurate than the traditional multitemperature scan technique.
GaInAs/InP selective area metalorganic vapor phase epitaxy for one-step-grown buried low-dimensional structures
Authors: Galeuchet Y. D. , Roentgen P. , Graf V.
Published: July 2002

The selective area epitaxy of GaInAs/InP layers grown by low-pressure metalorganic vapor phase epitaxy through SiO2 patterned masks was investigated. The layers are found to develop mesa structures limited by {111} and (100) facets outside of the opened mask, and perfect selective epitaxy is obtained. The absence of GaInAs growth on {111} facets allows the fabrication of very narrow buried GaInAs layers in a single growth step. For both materials, the growth rates are found to depend strongly on the mask geometry owing to surface diffusion of the reactant species from the no- or low-growth SiO2 mask and {111} facets toward (100) surfaces. A detailed quantitative analysis is made to identify the critical parameters that control the growth behavior, and a model is described from which the upper limit of the growth rates for any mask design can be calculated. Low-temperature cathodoluminescence measurements show strong emission of the buried GaxIn1−xAs layers and indicate local stoichiometry variations Δx≂±5% around the x=47% lattice-matched composition that are attributed to different diffusion coefficients of the reactant species on the SiO2 mask and {111} facets. The results show that selective area metalorganic vapor phase epitaxy is a promising technique for the fabrication of one-step-grown buried quantum-well wire arrays and narrow cavity InP-based buried lasers.
Wire-guided transport of intense ion beams
Authors: Watrous J. J. , Mosher D. , Neri J. M. , Ottinger P. F. , Olson C. L. , Crow J. T. , Peterson R. R.
Published: July 2002

Light ion inertial confinement fusion requires beam transport over distances of a few meters for isolation of the diode hardware from the target explosion and for power compression by time-of-flight bunching. This paper evaluates a wire-guided transport system that uses the azimuthal magnetic field, produced by a current driven through a thin wire, to radially confine the ion beam. Ion orbits are studied to determine the injection efficiency (i.e., the fraction of the beam which is transported) under various conditions. Some ions hit the wire because of too small angular momentum at injection; others hit the wire or are lost to large radius during transport because of chaotic orbit behavior induced by a small number of return-current wires close to the beam envelope. For a multimodular scheme (10–30 beams), individual transport system are packed around the target at some standoff distance. The fraction of the beam which is lost in this field-free standoff region is also evaluated under various conditions. The standoff efficiency is then combined with the injection efficiency to give the dependence of the total transport efficiency, ηt, on diode, focusing, transport, and standoff parameters. It is found that ηt can be as large as about 60% for parameter values which appear to be achievable.
Micromagnetic computation for wall coercivity caused by magnetic nonuniformity
Authors: Matsuyama K. , Hirokado Y. , Asada H.
Published: July 2002

The wall coercivity caused by the magnetic nonuniformity has been studied numerically. The Landau-Lifshits-Gilbert (LLG) equation is integrated by an explicit scheme of the modified Dufort-Frankel method. The computation was carried out for a two-dimensional grid system representing the magnetization direction in a magnetic film plane. Typical magnetic parameters for magneto-optic recording media were assumed. Spatial variations of uniaxial anisotropy were treated as the magnetic nonuniformity. The validity of our numerical approach was demonstrated with preliminary one-dimensional computations, compared to analytical solutions. A wall coercive field of 2.5 kOe was observed for the wall coupling with a pinning site (30-Å width and 360-Å spacing along the wall) of 10 times larger anisotropy compared to the ordinary region. A two-dimensional anisotropy variation (K=106–107 erg/cm3) with a wavelength larger than 60 Å also caused a wall coercivity on the order of 1 kOe, compared to those in magneto-optic recording media. It was also found that a fine pinning site on the order of 100 Å caused a notable coercivity for the bubble domain wall surrounding it.
Growth and electronic properties of thin Si3N4 films grown on Si in a nitrogen glow discharge
Authors: Paloura E. C. , Lagowski J. , Gatos H. C.
Published: July 2002

Thin Si3N4 films were grown on Si by direct exposure to a low-temperature (250–650 °C) nitrogen glow discharge at 13.56 MHz. The activation energy for growth is 0.3±0.12 eV. Their chemical and electronic, bulk, and interfacial properties are studied as a function of the film thickness and growth temperature. The ultrathin films (d<100 Å) exhibit an average breakdown field of 10–12 MV/cm, a dielectric constant equal to 5.5, and are effective barriers against oxidation at 1000 °C. Conduction in ultrathin films is via Fowler–Nordheim tunneling over the temperature range 77–420 K. Deep-level transient spectroscopy analysis reveals the presence of two majority-carrier (hole) traps at the interface, with activation energies 0.1 and 0.35 eV, respectively. The trap at EV+0.35 eV is attributed to the Si dangling bond defect while the trap at EV+0.1 eV could be attributed to a structural defect that is in situ annealed at growth temperatures higher than 500 °C. The interface state concentration is a sensitive function of the growth temperature and varies in the range 8×1010–6×1012 cm−2.
Nonintrusive wafer temperature measurement using in situ ellipsometry
Authors: Kroesen G. M. W. , Oehrlein G. S. , Bestwick T. D.
Published: July 2002

It is shown that in situ HeNe laser ellipsometric measurements performed during and after rf plasma exposure of a Si wafer with or without oxide can be used to obtain the wafer temperature during plasma exposure. The method utilizes either the temperature coefficient δn/δT of the refractive index of Si or the linear thermal expansion coefficient δl/lδT of SiO2. The values of these parameters have been redetermined in this work.
p-type doping of GaSb by Ge and Sn grown by molecular beam epitaxy
Authors: Longenbach K. F. , Xin S. , Wang W. I.
Published: July 2002

p-type doping of molecular beam epitaxy grown GaSb by Ge and Sn has been demonstrated. Both impurities are well behaved with demonstrated free acceptor concentrations as high as 2×1019 cm−3 for Ge and 5×1018 cm−3 for Sn. In addition reflection high-energy electron diffraction measurements during growth indicate that Sn segregation which is common in GaAs does not occur in GaSb. The absence of Sn segregation as well as the p-type nature of Ge and Sn dopants is attributed to the large covalent bond radius of Sb. These dopants are important since they provide an excellent alternative to Be for p-type doping of Sb based materials.
Density of states in semiconductor nanostructures
Authors: Shum Kai
Published: July 2002

The density of electronic states is directly counted in energy space for semiconductor nanostructures in which electrons are confined in all three dimensions. The results for different degrees of confinements in three directions are compared with that calculated in momentum space when momenta are assumed to be good quantum numbers.
Room temperature excitonic absorption in CdZnTe/ZnTe quantum wells: Contributions to exciton linewidth
Authors: Lee D. , Johnson A. M. , Zucker J. E. , Feldman R. D. , Austin R. F.
Published: July 2002

We explore the inhomogeneous and homogeneous contributions to the exciton linewidth that have allowed the recent observation of room temperature excitonic absorption in II–VI semiconductor quantum wells. Our measurements of the absorption spectrum in a range of CdxZn1−xTe/ZnTe quantum well heterostructures indicate that temperature-dependent contributions to the exciton linewidth are influenced by alloy composition.
Observations of the 22×√3 reconstruction of Au(111) under aqueous solutions using scanning tunneling microscopy
Authors: Tao N. J. , Lindsay S. M.
Published: July 2002

We have studied the Au(111) surface under potential control under water, 0.1 M HClO4 and 10 mM NaH2PO4 using scanning tunneling microscopy. The 22× ■ reconstruction was observed, demonstrating that the reconstruction is stable in contact with aqueous solutions. The measured periodicity and z corrugation of the reconstruction are in good agreement with studies done in ultrahigh vacuum. The stability of the reconstruction was found to depend on surface potential (vs a silver quasi-reference electrode). We have also observed structures other than the 22× ■ pattern.
Above-band-gap photoluminescence from Si fine particles with oxide shell
Authors: Morisaki H. , Ping F. W. , Ono H. , Yazawa K.
Published: July 2002

Strong photoluminescence with sub-band-gap photon energies has been observed in fine Si particles prepared by the gas-evaporation technique. After surface oxidation, the Si particles show above-band-gap photoluminescence, the band tail covering the visible light region. The amount of the increased apparent band gap (0.3 eV) estimated from this blueshift can be explained by a quantum-size effect expected to be observed in Si quantum dots with a diameter of 50 Å.
Scattering of electromagnetic missiles by convex metal objects
Authors: Brown Michael F. , Myers John M.
Published: July 2002

The scattered energy flux due to an electromagnetic missile incident upon an infinite circular conducting cylinder is analyzed. The flux obtained directly from Maxwell’s equations is compared with that of a geometrical-optics description, generalizable to a broad class of convex objects with nonzero, bounded surface curvature.
Theoretical critical-state susceptibility spectra and their application to high-T
c superconductors
Authors: Chen Du-Xing , Sanchez Alvaro
Published: July 2002

The fundamental complex susceptibilities χ=χ′−jχ″ are calculated from the symmetric critical-state hysteresis loops M(H) for an infinitely long hard superconductor with a rectangular cross section 2a×2b (a≤b). For the critical-state model, the Bean, the Kim, the exponential law, and the triangular-pulse local-internal-field-dependent critical-current densities Jc(Hi) are chosen. The results of χ′ and χ″ are given as functions of the field amplitude Hm normalized to the full-penetration field Hp, the sample dimensional ratio a/b, and the p parameter that characterizes the Hi nonuniformity in the sample at H = Hp on the initial M(H) curve. χ″(−χ′) curves are also given for the different functional Jc(Hi) and other conditions. These theoretical critical-state susceptibilities are particularly useful in the study of sintered high-Tc superconductors. For these materials, the procedures to determine the effective grain volume fraction f*g and the averaged and the local intergranular critical-current densities 〈Jc〉acs and Jc(Hi) by means of ac susceptibility measurements using such theoretical critical-state-susceptibility functions are described. Related problems met in the high-Tc superconductor study such as sample performance nonuniformity, frequency dependence, grain clusters, and susceptibilities for the grains are discussed.
Electron-acoustic phonon scattering in SiO2 determined from a pseudo-potential for energies of E≳E
BZ
Authors: Bradford J. N. , Woolf S.
Published: July 2002

The existing description of hot electron transport in silicon dioxide contains the deficiency that the resulting electron inverse mean free paths and loss rates associated with electron-acoustic phonon scattering continue to increase in an unphysical way at energies above Egap. One can remove that discrepancy by introducing a pseudo-potential which reflects the screened atom characteristic of higher energy electron-lattice interactions. The low energy, low q scattering, described in terms of the deformation potential, is then recovered, intact, in the low q limit. The use of the screened Coulomb potential introduces no adjustable parameters and results in an acoustic scattering cross section which approaches the phase shift derived elastic scattering cross section at E ≳ Egap.
Shading losses of solar-cell metal grids
Authors: Blakers A. W.
Published: July 2002

The effective shading of metal-plated solar-cell fingers after cell encapsulation is substantially less than would be expected from simple geometric calculations. The reason for this is that much of the light reflected from the fingers is either directed downwards toward the silicon or is totally internally reflected at the air/glass interface. Experimental data is presented to support calculations of this effect. Calculations of the impact of this effect on solar-cell performance are also included.
Influence of substrate temperature on the properties of oxygen-ion-assisted deposited CeO2 films
Authors: Al-Robaee Mansour S. , Rao K. Narasimha , Mohan S.
Published: July 2002

Substrate temperature and ion bombardment during deposition have been observed to modify significantly the optical and structural properties of dielectric thin films. Single-layer films of CeO2 have been deposited by electron beam evaporation with simultaneous oxygen-ion bombardment using a Kaufman broad beam ion source and maintaining the substrates at elevated temperature. A systematic study has been made on the influence of (a) substrate temperature in the range ambient to 300 °C, (b) ion energy in the range 300–700 eV, and (c) ion current density 100–220 μA/cm2 on optical properties such as refractive index, extinction coefficient, inhomogeneity, packing density, and structural properties. The refractive index increased with in increase in substrate temperature: ion energy up to 600 eV and ion current density. Homogeneous, absorption free and high index (2.48) films have been obtained at 600 eV, 220 μA/cm2 and at substrate temperature of 300 °C. The packing density of the films was observed to be unity for the same deposition conditions. Substrate temperature with simultaneous ion bombardment modified the structure of the films from highly ordered to fine grain structure.
Use of an asymmetric wiggler to study the magnetic circular x-ray dichroism of Ni, Ho, and Tb
Authors: Sainctavit Ph. , Lefebvre D. , Cartier Ch. , Laffon C. , Krill G. , Brouder Ch. , Kappler J. -P. , Schillé J.-Ph. , Goulon J.
Published: July 2002

The asymmetric wiggler, inserted since the end of 1990 on the Super-Aco storage ring in Orsay, has been built to produce a high rate and high flux of circularly polarized soft x rays in the 100–3000 eV range. This insertion device is particularly well suited to measure x-ray absorption spectra corresponding to dipole transitions towards electronic orbitals that are directly responsible for magnetism (3d states of first row transition metals, 4f states of rare earths). In this article we report the first magnetic circular x-ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.
X-ray photoelectron spectroscopy studies of n-type bismuth-modified amorphous thin films of Ge20Se80 and As2Se3
Authors: Kumar Sunil , Kashyap Subhash C. , Chopra K. L.
Published: July 2002

The charged state of bismuth atoms in n-type bismuth-modified amorphous thin films of Ge20Se80 and As2Se3 has been studied by x-ray photoelectron spectroscopy. On the basis of the observed chemical shift, it is concluded that the Bi atoms in the modified films are positively charged. The charged Bi atoms perturb the equilibrium between positively and negatively charged defect centers, thereby causing a shift of the Fermi level towards the conduction band. The observed enhanced electrical conductivity and the conductivity conversion from p to n type in the modified films is the result of such a Fermi level shift.