bluuebunny/crossref_metadata_2025 huggingface dataset:
Schottky barrier with the degenerate dilute magnetic semiconductor Cd1−xMnxSe
Authors: Couturier G. , von Molnár S. , Giriat W.
Published: July 2002

Consequences of the splitting of the electronic levels in a dilute magnetic semiconductor have been observed by measuring the capacitance of a simple Schottky barrier at 4.2 K in the presence of magnetic fields up to 60 kOe. The material used was a single crystal of degenerately Ga-doped Cd1−xMnxSe. The variation of the diffusion potential in the semiconductor as a function of magnetic field was deduced from C(V) measurements at 1 MHz. By assuming the presence of an interfacial layer at the metal/semiconductor interface, the derived band conduction splitting is in relatively good agreement with that calculated by using the magneto-optical properties of the material.
Field annealing effect on magnetoresistance of metallic glasses
Authors: Jen S. U. , Yang S. M.
Published: July 2002

Two kinds of metallic glasses, 2605S2 and VAC6025, were used in the experiments. Anneals in transverse and longitudinal fields have been performed at various annealing temperatures Ta. The objective was to find how ρ∥ and ρ⊥ vary with field anneal. A large current density sent through the sample has been used to reorient the domain walls without rotating the domain magnetization. Both wall displacements and domain rotation processes were found to contribute to ρ∥ and ρ⊥ below saturation. The ‘‘excess resistivity’’ associated with domain walls was used to determine the anomalous Hall coefficient R1 of both materials. Independent Hall resistivity measurements were also performed to make comparisons of R1.
Effect of He addition on fluorine-fueled excimer lasers
Authors: Mandl A. , Litzenberger L.
Published: July 2002

Measurements were made of the effect of the He addition to XeF and KrF excimer lasers using fluorine-containing fuels. A monotonic decrease of laser output with increasing He was observed. The kinetic arguments of why this would be expected in this and other similar excimer laser systems are presented.
The measurement of the trace Li content and distribution in ZnO films and the thickness of these films
Authors: Wen Liang-Bi , Huang Yan-Wen , Li Shang-Bai
Published: July 2002

The content and distribution of trace Li in ZnO films and the thickness of these films have been determined with reaction 7Li (p,α) 4He. An approximate and simple method used to determine the content of the trace element by the yield of thin standard target at an average energy has been established. This average energy can be found from the spectrum of the emerging particle or from the Rutherford backscattering spectrum of incident protons.
Influence of absorber length on inhomogenously pumped, bistable semiconductor lasers
Authors: Öhlander Ulf , Sahlén Olof , Ivarsson Lennart
Published: July 2002

We report on experiments with inhomogenously pumped semiconductor lasers of InGaAsP (1300 nm) having a saturable absorber. Bistability disappears for long relative length of the absorber, and lasing occurs for wavelengths considerably longer than the luminescence peak. Both these results are very important for designing bistable lasers and are consistent with recently published theoretical data.
Rate constants for light-enhanced thermal oxidation of silicon
Authors: Young E. M. , Tiller W. A.
Published: July 2002

In response to a 1-W (56.6 W cm−2) laser beam stimulation at 5145 A, oxidation enhancement is found to occur and data are given for Si(111) and Si(100) from 900 to 750 °C. Using a heat transfer calculation, the thermal contribution and the nonthermal or optical contribution to the total enhancement have been separated. Analysis of the data in terms of linear and parabolic rate constants shows that the major effect resides in B rather than in B/A, with differences existing between the (111) and (100) orientations. A theoretical explanation is suggested to account for this behavior.
Superconducting properties of powder metallurgically processed Ti-alloyed Nb3Sn microcomposites
Authors: Schaper W. , Wecker J. , Mrowiec K. , Bormann R. , Freyhardt H. C.
Published: July 2002

C uNb3Sn microcomposite wires with Ti additions have been produced by the hot-powder-metallurgical preparation method. The influence of the Nb3Sn filament diameter, heat treatment, and Ti addition on the superconducting properties are discussed on the basis of critical current densities (up to fields of 22 T), critical temperatures, as well as x-ray microprobe analyses. Critical current measurements of Nb3Sn multifilamentary tapes indicate that filament/matrix interfaces are dominant flux pinning centers in these composites. When Ti is incorporated into the A15 phase, the upper critical field is enhanced by about 2 T with respect to unalloyed superconductors. A drastic improvement of the superconducting properties can be achieved by the utilization of larger Nb powder particles. In microcomposites prepared from larger Nb powder (100–300 μm), Bc2 is enhanced by about 3–4 T with respect to finer Nb powder (20–45 μm). Critical fields up to 21.4 T and critical current values of 1.2×104 A/cm2 at a magnetic field of 16 T and 4.5×103 A/cm2 at 18 T are obtained for microcomposites prepared from coarser Nb powder with an addition of 2 wt. % Ti.
Analysis of the feedback voltage during constant current avalanche hole injection experiments
Authors: Trombetta L. P. , Zeto R. J. , Feigl F. J.
Published: July 2002

The behavior of the controlling feedback voltage during constant current avalanche hole injection experiments on metal-oxide-semiconductor capacitors has been analyzed in detail. It is concluded that at dc hole current densities greater than 50 nA/cm2, displacement currents generated in the oxide as a result of a change in the feedback voltage can dominate the injected hole current. The effect of this displacement current on the determination of hole trapping kinetics is evaluated quantitatively. In particular, it is shown that the effect of the displacement current is to reduce the hole current density and cause an apparent delay in oxide charging kinetics. In addition, it is shown that the feedback voltage necessary to maintain current densities larger than 50 nA/cm2 induces a measurable electron current from the gate electrode when the oxide field exceeds 6 MV/cm. Analysis of this phenomenon provides evidence for hole trapping at the Al-SiO2 interface as well as an estimate of the capture cross section for this process. The estimated cross section,1×10−14 cm2, is similar in magnitude to that obtained for hole trapping at the Si-SiO2 interface. It is concluded that while displacement currents and gate electron injection may affect the determination of hole trapping kinetics, these effects can be avoided at current densities below 50 nA/cm2. Finally, consideration of the relative changes in the feedback and flat-band voltage data during low-current density experiments suggests that the feedback voltage does not respond to charge in interface traps.
Transport of self-field dominated dc low-energy electron beams through low-pressure gases
Authors: Rand R. E. , Lampel M. C. , Wang D. Y.
Published: July 2002

A high-intensity dc electron beam with energy on the order of 100 keV propagating through a rarefied gas may be space-charge neutralized and made self-focusing by means of the beam-generated plasma. The purpose of this research was to investigate the propagation of partially and fully neutralized electron beams and to identify the ranges of gas pressure and other parameters which would permit high-quality electron beam transport, with linear self-focusing forces. Experiments were performed to measure transverse current density distributions for an electron beam of energy 60 keV, current 270 mA, propagating through nitrogen at pressures from 10−6 to 10−5 Torr. Theories were developed to describe the radial distributions of neutralization in the two distinct cases of incomplete and complete neutralization. The neutralization distributions in each case were calculated numerically and using a beam sheath tracing program, theoretical values of the beam current density distributions were obtained. Good agreement was generally found between experimental data and the calculations. The results confirm that under conditions where complete and uniform neutralization was expected, the beam optics were, in fact, linear, while partial neutralization produced serious nonlinearities.
Sheet-resistance characterization of ion-implanted silicon resistors at cryogenic temperatures
Authors: Sasaki Nobuo
Published: July 2002

Sheet resistance of both n- and p-type silicon resistors fabricated by using ion implantation are studied in a wide temperature range from 300 to 1.1 K. Significant sheet-resistance temperature dependence is achieved if the peak concentration, for the case of complete activation, is less than Nc, where Nc is the impurity concentration of the metal-insulator transition. Sheet-resistance temperature dependence sufficient for temperature sensors can be obtained for peak concentration larger than Nc, provided that post-implant anneal temperatures are kept low enough to realize incomplete activation. Compensation of a phosphorus-implanted layer by boron implantation can increase the temperature sensitivity for the case of complete activation if the phosphorus concentration is lower than Nc and the compensation ratio is small, about 8%.
An x-ray diffraction study of lattice imperfections in cold-worked face-centered-cubic alloys. VI. Copper-aluminum (α phase)
Authors: Pradhan S. K. , Maity A. K. , De M. , Sen Gupta S. P.
Published: July 2002

Following earlier approaches in this series of work, detailed x-ray peak-shift, peak-asymmetry, and peak-broadening analyses have been performed on the x-ray diffraction line profiles obtained from cold-worked filings of five compositions of Cu-Al alloys in the fcc phase, namely, Cu-3.7, 6.7, 12.3, 14.9, and 18.4 at. % Al. The analysis reveals an increased presence of stacking faults of an intrinsic nature, primarily responsible for the observed peak shift and domain size peak broadening. A very small presence of extrinsic stacking faults accounts for the observed asymmetry in the profiles, and the twin faults are nearly absent. The effective particle sizes decrease and microstrains increase with increasing solute concentration, and these are anisotropic in nature. An estimation of dislocation density ρ and stacking fault energy γ for the alloys was also made which tends to predict a reasonably good value of γ0 (27 mJ/m2) for pure copper. Annealing experiments on three alloys with an increased concentration of solute Al do not clearly reveal any detectable evidence of solute segregation at the stacking faults, as in the earlier case with fcc Ag-Al.
Recombination-current suppression in GaAs p-n junctions grown on AlGaAs buffer layers by molecular-beam epitaxy
Authors: Rancour D. P. , Melloch M. R. , Pierret R. F. , Lundstrom M. S. , Klausmeier-Brown M. E. , Kyono C. S.
Published: July 2002

n + p p +GaAs and n+pP+ GaAs/GaAs/Al0.3Ga0.7As mesa diodes have been fabricated from films grown by molecular-beam epitaxy. The diodes made from films employing an AlGaAs buffer layer show marked improvements (a factor of 5 reduction) in recombination current densities. Deep level transient spectroscopy measurements moreover indicate that deep level concentrations are reduced by the AlGaAs buffer.
Novel high-speed transistor using electron-wave diffraction
Authors: Furuya Kazuhito
Published: July 2002

A new class of devices consisting of a heterojunction npn transistor with a wavelength modulator and a transversal potential grating in the base region is proposed. An electron injected at high energy into the base region is modulated in wavelength due to acceleration by a signal voltage and its transmission to the collector is controlled through diffraction caused by the grating. Diffraction characteristics are analyzed to reveal the possibility of high transconductances. Furthermore, a reduced transit time, which is due to the constant flow of high-energy and collision-free electrons and an extremely small capacitance between the modulation electrodes, provides significant potential for high-speed logic applications.
Ellipsometric studies on zinc sulfide thin films grown by atomic layer epitaxy
Authors: Oikkonen M.
Published: July 2002

Refractive index and thickness measurements of zinc sulfide thin films were carried out using spectroscopic ellipsometry. The films studied were grown by atomic layer epitaxy (ALE). The thicknesses of the films were between 26 and 658 nm. In the analysis of the ellipsometric data a two-layer film model was used. The modeling was performed for all the films in the wavelength range 400–600 nm and for some films in the range 350–700 nm. The top layer thickness increased from about 5 to 20–30 nm as a function of the total film thickness. At λ=500 nm, the real part n of the refractive index in the top layer was 1.30–2.00 and in the lower layer 2.14–2.41, also depending on the total film thickness. The reference bulk value was 2.42. The imaginary part of the refractive index of the layers could be modeled to be 0.01 in the range 400–600 nm. At 350 nm k was of the order of 0.05. From the refractive index results the average volume fraction of zinc sulfide in the films was determined using effective medium approximation (EMA). The volume fraction increased from about 0.6 to 0.95–0.97 in the thickness range 26–400 nm. After 400 nm the fraction was slightly reduced. The EMA results indicate that the first stages of crystallization of zinc sulfide on the amorphous glass substrate are highly nonideal and that the surface roughness increases with the total film thickness. Except for the boundary regions, the ALE-grown zinc sulfide thin films exhibit very good crystalline properties also in terms of an optical analysis.
Phase formation and dissociation in the thin-film Pt/Al system
Authors: Colgan E. G.
Published: July 2002

Phase formation has been investigated in the following thin-film systems: Al/Pt, Pt2Al/Al, Pt8Al21/Pt, Pt/PtAl, and Pt/Pt5Al21/Al, between 225 and 550 °C. The films were prepared by sequential evaporation and coevaporation, annealed under vacuum, and analyzed by Rutherford backscattering and x-ray diffraction. In all cases, except for Pt/PtAl, the initial phase formed was Pt2Al3, followed by the more Pt- or Al-rich phases, depending on the overall stoichiometry. Al is the dominant moving species during the formation of Pt2Al3.
Frequencies of the energy-trapping modes of the thickness-shear and thickness-twist vibrations in unelectroded circular, AT-cut quartz plates
Authors: Slavov S. , Ouroushev D.
Published: July 2002

Analysis of the frequency spectrum of circular elastic-anisotropic and unelectroded AT-cut quartz plates with free edges is performed by using the equation of the coupled thickness-shear and thickness-twist vibrations. It is found that the frequency spectrum of energy-trapped vibrational modes can be obtained from the parametric zero relations of Mathieu functions, which are solutions of this equation. Experimentally measured frequency values of the fundamental and the first few harmonic overtones are in good agreement with theory.
Intrusions in the active layer of channeled-substrate-planar laser diodes
Authors: Slavin S. E. , Hawrylo F. Z. , Hughes J. J.
Published: July 2002

Channeled-substrate-planar laser diodes from various wafers were observed to develop a node or dark spot in their near-field patterns after failure during lifetesting. ‘‘Node’’ devices from each wafer were angle lapped and stained in an attempt to uncover a physical mechanism for this common failure symptom. Optical microscopic examination of the beveled cross sections revealed that an intrusion, in the form of a spike at the tip of the zinc diffusion front, had penetrated into the active layer of these diodes. The node observed in the near-field pattern appears to be in a position corresponding to the location of the spike in the active region. In addition, cathodoluminescence measurements on other ‘‘node’’ diodes revealed a dark line region approximately 2 μm wide running parallel to and in the middle of the 5-μm contact stripe. We believe that this dark line region is representative of an area of nonradiative recombination which occurs in the portion of the active layer containing the intrusion.
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u study of dissolution and swelling behavior of poly-(methyl methacrylate) thin films in solvent/nonsolvent binary mixtures
Authors: Manjkow J. , Papanu J. S. , Soong D. S. , Hess D. W. , Bell A. T.
Published: July 2002

A single-element rotating-polarizer ellipsometer (psi-meter) was used for in situ characterization of the thermodynamic and kinetic behavior of poly-(methyl methacrylate), PMMA, thin films (1.2 μm) in solvent/nonsolvent binary mixtures of methyl ethyl ketone/isopropanol (MEK/IPA) and methyl isobutyl ketone/methanol (MIBK/MeOH). Thermodynamic effects were inferred from equilibrium behavior by the degree of swelling and polymer-solvent solubility. A sharp transition between complete solubility and almost total insolubility was observed in a narrow concentration range near 50:50 (by volume) solvent/nonsolvent for both MEK/IPA and MIBK/MeOH. In the insoluble regime, the polymer was found to swell up to three times the initial thickness. At 50:50 MEK/IPA, a temperature decrease from 24.8 to 18.4 °C caused a change from complete dissolution to combined swelling/dissolution behavior and rendered the PMMA film only 68% soluble. Kinetic effects were determined by dissolution and penetration rate measurements. A constant penetration velocity was observed for almost all compositions for both binary solvent mixtures with Case II transport assumptions providing good agreement with experimental results. For MEK/IPA, penetration rates increased with increasing MEK concentration. For MIBK/MeOH, however, a maximum was observed at 60:40 MIBK/MeOH.
Optics and calculated efficiencies of mechanically stacked two-junction solar cells
Authors: Partain L. D. , Fraas L. M. , McLeod P. S. , Cape J. A. , Kuryla M. S.
Published: July 2002

Mechanically stacked two-junction solar cells avoid the multijunction problems of interfering growth conditions, shorting layers, and current matching. They also allow the use of well-developed Si and GaAs junctions. Doping the top junction substrates at 2(1017) cm−3 reduces measured free-carrier absorption by up to a factor of 5 compared to doping at 8(1017) cm−3. Separately measured quantum-yield spectra, open-circuit voltages, and fill factors provide the basis to calculate stack efficiencies of 24%–28% for Si stacked under GaAsP and GaAs top junctions for a direct air mass 1.5 (AM1.5D) terrestrial sunlight spectra and a 400× light concentration. The GaAs is a preferred top junction because it has a direct gap, operates at near its theoretical limits, and minimizes transmission loss effects by contributing over 70% of the total stack output without compromising potential stack performance. The open-circuit voltages of GaAs and Si cells are measured to vary with light intensity as predicted by the standard model with a junction ideality factor equal to 1.0. This and other experimental junction data provide the basis to calculate 400×, AM1.5D stack efficiencies of 29% for GaAs stacked on Ge and 30% for GaAs stacked on GaSb. Device improvements are suggested to project GaAs/Ge stack efficiencies of 30% and GaAs/GaSb stack efficiencies of 34% for 400×, AM1.5D. The 400×, AM0 efficiencies for space are 0.88 to 0.91 times the corresponding, terrestrial AM1.5D values.
Formation of γ-Fe as the metastable phase of amorphous Fe83B17 powder alloy
Authors: Yamashita Hiromi , Yoshikawa Masahito , Funabiki Takuzo , Yoshida Satohiro , Isozumi Yasuhito
Published: July 2002

The structural change with temperature in the bulk and near-surface layers of a pulverized Fe83B17 amorphous alloy was studied using transmission Mössbauer spectroscopy, conversion electron Mössbauer spectroscopy, and x-ray diffraction. The paramagnetic component, γ-Fe, was formed as a metastable phase in the bulk of the amorphous Fe-B alloy. The neighboring boron atoms might stabilize the γ-Fe phase.
Effects of discharge parameters on deposition rate of hydrogenated amorphous silicon for solar cells from pure SiH4 plasma
Authors: Ishihara Shin-ichiro , Kitagawa Masatoshi , Hirao Takashi , Wasa Kiyotaka , Arita Takashi , Mori Koshiro
Published: July 2002

A systematic deposition of hydrogenated amorphous silicon films from pure SiH4 plasma was made in a capacitively coupled rf glow-discharge system by changing anode–cathode spacing d and chamber pressure p simultaneously. The data of the deposition rate in the p-vs-d space had two boundaries. One was pd=const. The other seems to be pd2=const. The rf plasma can stably sustain between the boundaries. The boundaries are discussed with rf power per SiH4 molecule and with overlapping Paschen’s lines of various fragments, especially H2 due to the SiH4 dissociation. We found the optimum conditions in which the deposition rate was more than 10 Å/s without large photo-induced degradation. 10% efficient p-i-n solar cells were achieved with the intrinsic layer deposition rate of 3.9 Å/s and more than 6% efficiency with 10 Å/s.
Annealing of interface states on oxidized silicon during chip bonding
Authors: Luo Marie Shiang-Chyong , Sah Chih-Tang
Published: July 2002

Interface states, originating from the dangling bonds at the oxide-silicon interface, can be annealed out during chip bonding between 85 and 380 °C in aluminum-gate metal-oxide-silicon capacitors with a thermal activation energy of 0.15±0.02 eV. Annealing is absent in silicon-gate capacitors. These results suggest that the interface states are annealed out by hydrogenation of the silicon and oxygen dangling bonds and that hydrogen diffusion through the oxide is the rate-limiting mechanism.
Phonon energy dependence of scattering in quasi-two-dimensional electron gases at low temperature
Authors: Manion S.J. , Hess K.
Published: July 2002

We investigate the scattering of electrons in a quasi-two-dimensional electron gas by acoustic phonons at low temperatures. In contrast to scattering at high temperatures, we find that at low temperatures the average scattering rate is dominated by phonon modes with energies much larger than kT. An analytic calculation of either the energy-loss rate or the mobility depends on approximations which are valid only for small phonon wave numbers. Because of the contribution of high-energy phonons, these approximations remain inappropriate to much lower temperatures than previously supposed. For most cases, we find that the approximate analytic solution differs significantly from a more accurate solution obtained by numerical integration.
Motion of a plate driven by an explosive
Authors: Fickett W.
Published: July 2002

In many applications it is useful to have an estimate of the velocity of a metal plate driven by an explosive as a function of time. With reasonable approximations, this problem has been solved exactly, but the result takes the form of a parametric solution, not the most convenient for everyday use. We give a simpler explicit solution and graphs plotted in variables suitable for accurate reading.
Modified double-channel planar-buried heterostructure laser with improved high-temperature stability
Authors: Lahtinen J. A. , Temkin H. , Logan R. A.
Published: July 2002

In double-channel planar-buried heterostructure lasers operating at 1.3 μm, T0 as high as 100° has been obtained with the resulting increase of the cw operating temperature up to 85 °C. The improved high-temperature performance is obtained by reducing the thickness of the p-InP cladding layer of the first to grow double heterostructure with the resulting reduction in the current leakage.
Depth profiling of defects in epilayer semiconductor materials by using synchrotron x-radiation topography
Authors: Kuo C. L. , Bilello J. C.
Published: July 2002

Depth profiling of defect structures can be accomplished by taking advantage of the increased penetration of x rays with decreasing wavelength. Topographs can then be taken at various integrated depths giving a nondestructive profile of the microstructure. This is usually accomplished via a monochromator, which is a slow and expensive procedure, even at a synchrotron source. Appropriate filters can be used in white beam synchrotron topography which reduce fluorescence while at the same time giving a limited, though not monochromatic, bandpass. This allows observations of the microstructure over a narrow energy range, diminishing the complications arising from harmonics in interpreting white beam images. In studies of GaAs crystals it was found possible to use GaAs or Cu prefilters in conjunction with deliberate air attenuation of the long wavelength components of the synchrotron spectrum. Studies were made on GaAs crystals that had been processed in a variety of ways. Examples will be shown for GaAs substrates with low grown-in dislocation density and with Zn-doped GaAs epilayers. This selective energy filtering provides a simple, rapid method of recording reflections varying from approximately 0.2 to 1.6 Å. The combination of this technique with a precision goniometer tilt stage permitted observations of the specimen defect structure from a few microns below the surface to 0.3 mm depth, thus presenting a visual profile of the effect of processing on the defect structure of the sample.
Influence of transverse electric field on the photoluminescence linewidth of excitonic transition in quantum wells: Alloy disorder and composition fluctuation contributions
Authors: Ogale S. B. , Madhukar A. , Cho N. M.
Published: July 2002

The influence of transverse electric field on the photoluminescence linewidth of excitonic transition in a semiconductor quantum well structure is examined within a theoretical model which ascribes the origin of the linewidth to band-edge discontinuity fluctuations arising from local compositional fluctuations at the interfaces, and to short-ranged alloy disorder scattering. In particular, the model is applied to the AlxGa1−xAs/GaAs system and the dependence of linewidth on the width of the quantum well, the average composition of the alloy, the amplitude of composition fluctuation, and the value of applied field (0–100 kV/cm) is studied.
Quantum limit of a narrow-channel GaAs metal-semiconductor field-effect transistor
Authors: Roos G. , Berggren K.-F.
Published: July 2002

The operation of a special GaAs metal-semiconductor field-effect transistor (MESFET), originally proposed by Pepper for studies of low-dimensional transport at low temperatures, is analyzed in terms of a variational model designed for the quantum limit (narrow channel, low temperatures). The relation between electron concentration and gate voltage is calculated in the quantum limit and compared with the results of the classical abrupt depletion model. The quantum curves show no structure associated with sublevels and agree well with the predictions of the abrupt depletion model. Subband separations are derived and compared with experimental estimates.
Electron microscopy of electron damage in tantalum carbide
Authors: Allison C. Y. , Stoller R. E. , Kenik E. A.
Published: July 2002

An intermediate voltage (300-keV) transmission electron microscope was used to create carbon defects in situ in nearly stoichiometric tantalum carbide (TaC0.99). At room temperature, these defects appeared as black spot damage. Varying the accelerating voltage allowed us to determine the threshold energy for carbon displacement to be 23.2±1.1 eV. Electron irradiation at liquid-nitrogen temperature did not lead to the formation of observable defects. However, black spot damage became visible as the temperature of the specimen was raised to room temperature. Estimates of the size of defects produced were obtained by weak-beam dark-field imaging. These observations allowed us to assign a tentative physical interpretation to the three recovery stages seen in previous electron irradiation experiments using resistivity measurements.
The atomic scale structure and the electronic properties of a graded composition semiconductor layer
Authors: Long A. P. , Kelly M. J. , Kerr T. M. , Knowles K. M. , Britton E. G. , Stobbs W. M.
Published: July 2002

We report the results of a study on the relationship between the structural and electronic properties of a graded layer of undoped AlxGa1−xAs, where x was specified to increase linearly from 0 to 0.2 over 50 nm. The layer was grown by molecular-beam epitaxy between layers of doped GaAs. A high-resolution dark-field transmission electron microscopy imaging technique of combining information from 002 and 002̄ reflections of AlxGa1−xAs allowed us to image the graded region with near atomic resolution. Differences between the predicted and measured diode performance of the graded layer are discussed in the light of our results.
Acoustic-wave reflection from water/laminated composite interfaces
Authors: Nayfeh Adnan H.
Published: July 2002

Analytical results are presented for the reflection of acoustic waves from liquid-laminated composite interfaces. The composite is oriented such that the laminates are stacked normal to the x3 directions with propagation confined to the x1-x2 plane. The fluid-composite interface is assumed to be normal to x2. Because of the complex microstructure of composites, continuity conditions exist on both the macro (liquid/composite interface) and micro (laminate interface) scales. Because of the number and nature of these conditions, exact solutions are exceedingly difficult, if not impossible, to obtain. As an alternative, we formulate an approximate analysis in which the composite medium is replaced by a homogeneous, yet dispersive, medium. In the process of replacing the composite with such a material model, the microcontinuity conditions are utilized. Numerical results are presented to demonstrate the dispersive behavior of leaky surface waves on the composite.
Formation of paramagnetic defects in high-purity silica by high-energy ions
Authors: Dooryhee E. , Langevin Y. , Borg J. , Duraud J.-P. , Balanzat E.
Published: July 2002

We investigated the paramagnetic defects formed by high-energy ions in amorphous SiO2 using electron spin resonance. We show that the variation of paramagnetic defect densities with both the residual range and the atomic number of the ion is not simply correlated with the total energy deposited in the sample, which is the major controlling parameter for other types of irradiation (γ rays, electrons). This specificity of ion irradiation is most likely related to the high local density of deposited energy. The observed populations of paramagnetic defects constitute a small fraction of the formed defects. However, their characteristics can be related to results on ion track structure obtained by small-angle scattering and track etching methods.
Intermediate phases produced during the icosahedral-crystal transition
Authors: Reyes-Gasga J. , Avalos-Borja M. , José-Yacamán M.
Published: July 2002

An investigation of a new quasicrystalline phase obtained in an alloy of Al86Mn14 under slow solidification rate is carried out. It has been named the N phase and seems to correspond to a transition phase between the T phase and the crystalline state. Its diffraction pattern shows an elliptical arrangement of 10 spots in a pseudo-fivefold direction and a threefold diffraction pattern similar to the one reported in the Al4Mn phase. This result indicates that, depending on the solidification rate, different quasicrystalline phases are obtained. Therefore, the transition between the icosahedral and the crystalline states could give rise to different phases as in a continuous transition.
Observations of ultrafine ZnFe2O4 particles with transmission electron microscopy
Authors: Seki M. , Sato T. , Usui S.
Published: July 2002

Ultrafine ZnFe2O4 particles less than 10 nm in diameter formed by the coprecipitation method were observed using transmission electron microscopy. The chemical composition of these particles was consistent with that of the stoichiometric bulk material. The apparent lattice structure could be observed. These observations revealed that particles 5 nm in diameter were fine, defect-free, and cubic single crystals having a spinel structure. The particle surface became more and more circular with decreasing particle size. Even a particle as small as 2 nm was seen to be homogeneous and possess a spinel structure. To determine the particle size it was necessary to disperse particles by covering their surfaces with a double-adsorbed surfactant film. The particle size distribution was seen to be approximately log normal.
Effects of substrate temperature on GaAs tunneling diodes grown by molecular beam epitaxy
Authors: Katsumoto Shingo , Amano Chikara
Published: July 2002

This communication discusses the effects of substrate temperatures on tunneling peak current densities both during and after growth of GaAs tunneling junctions. Molecular beam epitaxy (MBE) is the method used in this process. It also shows how thermal degradation is dependent upon doping concentrations. This degradation occurs because of high-temperature overgrowth in GaAs tunneling diodes grown by the MBE method. This communication also suggests some possible ways to minimize the thermal degradation of tunneling junctions, in order to obtain high conversion efficiency in multiple-junction solar cells.
N-photon interband absorption coefficients of direct-gap magnetic semiconductors
Authors: Nunes O. A. C.
Published: July 2002

The N-photon interband absorption coefficients (α(N)) of direct-gap magnetic semiconductors are calculated on the basis of a nonperturbative approach. Regardless of the value of N, we show that α(N)∝((e2/ℏc)) Nℏ3N−4S(N)(Ẽg)F(N) ((Nℏω/Ẽg)) , where S(N) is a scaling factor for different materials. The dominant material dependence of α(N) is through the energy-gap factor in S(N), which is seen to decrease as Ẽ3N−5/2g, Ẽg being the renormalized energy gap of the magnetic crystal.
Drift velocity comparison between high-electron mobility transistors and doped-channel field-effect transistors at very small dimensions
Authors: Yokoyama Kiyoyuki
Published: July 2002

Drift velocity characteristics for high-electron mobility transistors (HEMT) (nondoped-channel field-effect transistors) and doped-channel metal-insulator-semiconductor-like transistors (DMT) are compared to discuss high-speed operation at very small dimensions using an electronic transport program. This ensemble Monte Carlo simulation program takes into account multisubband conduction based upon the self-consistently determined potential well. Calculated drift velocity responses are compared between the two transistors by changing the dimensions. Consequently, enhanced drift velocity characteristics due to velocity overshoot effect are confirmed for DMT. That is, the calculated saturation velocity value exceeds 2×107 cm/s for a 0.2-μm-length transistor at 77 K. This fortunate result indcates that high-speed DMT is achievable by reducing the device dimensions. The two types of devices are compared by using a two-dimensional simulator together with the obtained drift velocity versus field relationships. As a result, it is demonstrated that much better high-speed performance is expected for HEMT at reduced dimensions, e.g., 340 GHz for a 0.2-μm-gate-length HEMT and 100 GHz for a 0.2-μm-gate-length DMT, at 77 K. The peak velocity exceeds 7.5×107 cm/s for the 0.2-μm-length HEMT. In conclusion, nondoped-channel field-effect transistors, such as HEMT, promise ultra-high-speed operation.
Optogalvanic investigations in the cathodic region of a neon glow discharge
Authors: DeMarinis E. , Sasso A. , Arimondo E.
Published: July 2002

We report on an investigation of the optogalvanic effect in the cathodic region of a glow discharge. Analysis of the spatial behavior of the fluorescence intensity enabled us to obtain detailed information on the laser-unperturbed discharge. A description of the discharge maintenance mechanisms through the secondary electron emission from the cathodic surface by the impact of ions, metastable atoms, and VUV photons has been used to describe the current-voltage tube discharge characteristics. The laser-induced perturbation of the secondary emission has been understood as the main mechanism responsible for optogalvanic production in our discharge configuration.
Stability towards photoelectrochemical etching in Ga(As, P) alloys
Authors: Carlsson Per , Holmström Bertil , Samuelson Lars
Published: July 2002

The stability towards photoelectrochemical (PEC) etching in hexacyanoferrate (II) (=R) solution has been determined for a set of single-crystalline ternary alloys in the series GaAs1−yPy (0<y<0.8), using the rotating ring-disk electrode technique. It was found that the stability coefficient S increases monotonicaly but nonlinearly from a low value in the GaAs end to near 1 for y=0.8. The data are consistent with a kinetic model involving competition between decomposition and hole transfer from the valence band edge to R with a probability determined by the thermal fluctuating energy level model. The findings are of potential interest in using selective PEC etching in preparing structures in optoelectronics and multilayer heterostructures for high-speed electronics.
The role of diffusion in long-channel field-effect transistors under condition of the velocity saturation
Authors: Grinberg A. A.
Published: July 2002

An analytical modulation-doped field-effect transistor model including field-dependent mobility and diffusion transport of the electrons in the gradual channel approximation is proposed. The main aim of the model is to clarify the role of diffusion in the condition of the velocity saturation and its influence on the mechanism of the current saturation. Current-voltage characteristics are obtained in the parametric form. An explicit form of the I-V characteristic is deduced for a subthreshold region of the device operation. The influence of the velocity saturation and diffusion on the current flow in different parts of the channel and regions of the I-V characteristic is analyzed. It is shown that diffusion plays an important role in the entire range of the I-V characteristic and cannot be ignored in any realistic model. It follows from the model that in the final stage of current saturation, the latter is governed by the process of diffusion, and change of the current does not depend on the value of the saturated velocity.
A theory of the three-dimensional solenoidal magnetization configurations in ferro- and ferrimagnetic materials
Authors: Vlaming R. , van den Berg H. A. M.
Published: July 2002

A method for constructing three-dimensional solenoidal magnetization distributions, with invariant magnitude of m, in arbitrarily shaped objects is presented. The formalism harks back to the theory developed by van den Berg for two-dimensional m distributions. The space within a general object Ω is partitioned into i subspaces Ωi, described by a family of surfaces to which the magnetization is tangent. A characteristic equation which defines the course of m at each of the surfaces is derived. A boundary condition for m arises naturally, or can be chosen to determine m at the surface. Within the above framework an infinite number of solutions are generated that, in general, exhibit singularities. Special attention, also from the topological point of view, is paid to the m distributions having point defects only.
Effect of preannealing on the kinetics of the reorientation of the field-induced anisotropy in CoZr thin films (abstract)
Authors: Jagielinski Tomasz
Published: July 2002

In this paper, we will analyze the changes in the kinetics of the induced anisotropy due to preannealing in 300-Å and 1-μm-thick Co94Zr6 amorphous films. Samples were prepared by a dc sputtering method. A silicon wafer with SiO2 overcoat was used as a substrate material, and a field of 200 Oe was applied during the deposition to induce uniaxial anisotropy. Based on x-ray data and resistivity measurements, the films were considered amorphous. The samples were annealed in a magnetic field at temperatures up to 500 °C, and the annealing time was varied from 0.5 to 400 h. As previously reported, the anisotropy, Ku, in as-deposited samples is a function of sample thickness and varied from 1.6×104 ergs/cm3 for 300-Å film to ∼1×104 ergs/cm3 for 1-μm films [T. Jagielinski, J. Appl. Phys. 61, 3237 (1987)]. It was found that the Ku in thick films is reversible, and the kinetics can be fully explained by the anelasticity model [T. Egami, Rep. Prog. Phys. 47, 1601 (1984)]. The mean value of the activation energy is ∼2 eV for samples preanneled at 375 °C. However, in the case of thin films, an additional irreversible component of anisotropy, always along the direction of the field applied during the deposition, is observed. The magnitude is a growing function of annealing temperature, and is as large as 3×104 ergs/cm3 for a 300-Å sample preanneled at 400 °C. The origin of the irreversible component of the anisotropy is not well understood. Based on x-ray data, we concluded that this effect is not related to crystallization. However, since anisotropy is thickness dependent, the structure of the film-substrate interface seems to be responsible for the irreversible part. The reversible parts of anisotoropy behave similarly in both thin and thick films and are 1.6×104 ergs/cm3 and 8×103 ergs/cm3 for 300-Å and 1-μm films, respectively.
Transport and magnetic properties of amorphous Fe80−xMxB14Si6 (Mn=Mn, V, Cr)
Authors: Jen S. U. , Yang S. M.
Published: July 2002

A series of amorphous Fe80−xMxB14Si6 (M=Mn, V, Cr, and x=2, 4, 6, 8, 10, 12, and 14 at. %) alloys have been made by the spinning-wheel method. We have measured the density δ, saturation magnetization Ms , electrical resistivity ρ0, Hall resistivity ρH, and magnetoresistance Δρ/ρ0 of each sample at room temperature. The anomalous Hall coefficient Rs is determined from the relation ρH=R0B+RsMs. Rs is found to vary as Rs=Aρ1.70. The density, magnetization, and magnetoresistance are all decreasing functions of x. The Curie temperature TC is determined from the low-field thermomagnetization data and the Arrott plot. The positive forced magnetoresistivities are discussed from the viewpoint of the current models.
Preparation and properties of sputter-deposited crystalline ultrafine particles
Authors: Ohnuma S. , Kunimoto A. , Masumoto T.
Published: July 2002

The composition and size dependence of the magnetic properties of crystalline Fe-Co and Ni ultrafine particles has been investigated. Samples were prepared by sputter deposition onto sputter-etched polyimide substrates. Little difference between the magnetic properties of crystalline and amorphous particles was found in Fe-rich alloys, but we found a greater influence of magnetocrystalline anisotropy in Co-rich alloys. When the particles are larger than 2500 Å, the samples have negative anisotropy Ku (easy direction parallel to the substrate plane). As the particle diameter decreases, Ku becomes positive, and the coercive field Hc increases from a few hundred oersteds to a maximum of more than 1000 Oe. Below a particle diameter of about 1000 Å, Ms , Ku, and Hc all drop rapidly toward zero, presumably due to the appearance of superparamagnetism. Annealing above about 200 °C leads to significant increases in Hc, Ku, and Ms, and also to significant changes in the line shapes of the x-ray diffraction patterns, as a result of stress relaxation. A low-temperature diffusion process leading to changes in the morphology of the subparticles may also be a cause of the increment of magnetic properties on annealing.
X-ray absorption spectroscopy studies of high-T
c superconductors
Authors: Jeon Y. , Lu F. , Jhans H. , Shaheen S. A. , Croft M. , Ansari P. H.
Published: July 2002

X-ray absorption spectroscopy measurements on rare-earth (R) and Ba sites in the new superconducting oxides and in binary rare-earth oxides are discussed. All of the near-edge spectra appear to manifest a continuum resonance feature whose energy is quantitatively well correlated with the R–O and Ba–O bond lengths. Evidence supporting a second such feature related to the R/Ba interatomic distance is also presented. Finally, simultaneous consideration of both of these features appears consistent with the inverse square-law expectation for the interatomic distance dependence of the energies of these spectra features.
Magnetic ordering and crystal field effects in REBa2Cu3O7−x (RE=Gd, Dy, Ho, Er) (abstract)
Authors: Dunlap B. D. , Slaski M. , Hinks D. G. , Segre C. , Zhang K. , Soderholm L. , Beno M. , Crabtree G. W. , Kwok W. K. , Malik S. K. , Schuller I. K. , Jorgensen J. D. , Sungaila Z.
Published: July 2002

Heat capacity, resistivity and magnetic susceptibility data have been used to study electronic and magnetic properties of REBa2Cu3O7−x compounds. Neutron diffraction on the Ho compound shows identical crystal structure to YBa2Cu3O6.8. All the compounds are superconducting at Tc=(93±2) K. Magnetic transitions are observed in the heat capacity data at Tm=2.2, 0.95, 0.17, and 0.59 K for RE=Gd, Dy, Ho, and Er, respectively, and the magnetically ordered state is found to coexist with superconductivity. Estimates have been obtained for the magnetic moment in the crystal field ground state and the energy splitting of the first excited state crystal field level.
A new method for determining superconducting transition temperature T
c (abstract)
Authors: Xue Ronghua , Lu Quan
Published: July 2002

A new method for measuring superconducting transition temperature Tc is presented. In this method, the initial superconducting transition temperature Tc is determined by the temperature dependence of the voltage which is induced on a M pick-up coil and is proportional to the diamagnetic susceptibility of the superconducting sample. The alternating magnetic field for the measurement may be very low (down to 80 A/m) and only several milligrams of sample are needed. The experimental results show that this method is very sensitive and simple.
Optical absorption of amorphous-Si:H/amorphous-SixN1−x:H multilayers: Effects of interface alloying
Authors: Mui K. , Smith F. W.
Published: July 2002

A model for the optical absorption of a-Si:H/a-SixN1−x :H multilayers has been developed which takes into account the effects of alloying in the interface regions between the a-Si:H and a-SixN1−x :H layers. Four different spatial variations of the film composition at the interfaces have been considered: abrupt, step, linear, and parabolic. The optical response of the interface regions is proposed to arise from Si-centered tetrahedra, containing both Si and N atoms. Calculations of the dielectric function of the multilayer using the appropriate effective medium approximation have indicated that alloying at the interfaces leads to a lower optical absorption and a broadening of the multilayer absorption edge as compared to multilayers in which interface alloying is ignored. However, interface alloying apparently does not contribute appreciably to the observed increase in the optical energy gap of the a-Si:H layer in the limit of decreasing layer thickness.
Relation between variance and sample duration of 1/f noise signals
Authors: Kleinpenning T. G. M. , de Kuijper A. H.
Published: July 2002

A theoretical relation is derived for the variance of a 1/f noise signal as a function of the sample duration T. With the help of this relation the 1/f character of a noise signal in the microhertz range is verified by measuring the variance as a function of T, with T≤3×105 s. At f=3×10−6 Hz the spectral density of the noise signal was still found to be inversely proportional to the frequency.