bluuebunny/crossref_metadata_2025 huggingface dataset:
Cathodoluminescence and electrical anisotropy from α and β dislocations in plastically deformed gallium arsenide
Authors: Esquivel A. L. , Sen S. , Lin W. N.
Published: February 2003

Infrared cathodoluminescence (IR-CL) micrographs were taken of surfaces perpendicular (II2) and parallel (021) to the bend axis [II2] of n-type GaAs single crystals bend at 700°C in four-point bending to introduce α (Ga) and β (As) dislocations. At the bend center, light and dark checkered patterns associated with dislocations could be observed only in the IR-CL micrographs but not by optical, standard infrared, or secondary electron emission microscopy. No significant changes related to the type of dislocation (α or β) or dislocation density were observed either in the IR-CL peak half-width or peak position at E=1.411 eV. However, decreases of 40−70% in the relative IR-CL intensity were observed in the bend samples containing predominantly α or β dislocations. To study electrical anisotropy in the deformed samples, Hall measurements as a function of temperature were made of sections cut parallel and perpendicular to the [II2] bend axis to eliminate effects due to point defects and impurities. The results indicate that relative to the unbent control sample, the conductivity and mobility of sampples with dislocations oriented parallel to the current direction were slightly enhanced, while decreases of 50−60% were noted when the dislocations were aligned perpendicular to the current direction. When oriented in the parallel mode, an increase in conductivity and mobility was noted from both types of dislocations, with the samples containing β dislocations showing a larger increase than those with α dislocations. In contrast, when the dislocations were oriented in the mobility than thoise with β dislocations. The observed electrical anisotropy is discussed in the light of current models describing specular (or difuse) electron scattering dislocations.
Modified mean-field model for rare-earth–iron amorphous alloys
Authors: Heiman Neil , Lee Kenneth , Potter Robert I. , Kirkpatrick S.
Published: February 2003

A mean-field model of the magnetic properties of amorphous rare-earth–iron alloys has been developed which incorporates an Fe spin whose dependence on both the concentration and the species of rare earth is based on Mössbauer spectra. Using this model with a single fixed set of exchange constants, we have been able to calculate Curie temperatures which are in reasonable agreement with the data for amorphous rare-earth (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu–iron alloys containing between 50 and 100 at.% iron. Better agreement with the measured Curie temperatures is achieved if the magnitudes of the Fe-Fe and rare-earth–Fe exchange constants are allowed to increase linearly with rare-earth concentration. The latter case has the additional advantage of reproducing the observed shape of the magnetization-vs-temperature curves for wide range of composition and rare-earth species.
Temperature measurement of shocked copper plates and shaped charge jets by two-color ir radiometry
Authors: Von Holle W. G. , Trimble J. J.
Published: February 2003

A two-band ir radiometer was used to measure the residual temperature of thin copper plates shocked with various explosives. Evidence indicates that a large shock-induced emittance has been largely compensated, and the results agree with other experiments but not with hydrodynamic theory. Preliminary results for the temperature of a copper shaped charge jet in flight yield a temperature of 432 °C.
Criteria for jet formation from impinging shells and plates
Authors: Chou Pei Chi , Carleone Joseph , Karpp Robert R.
Published: February 2003

Criteria on jet formation and jet cohesiveness are proposed for collapsing solid plates and shells. These criteria are also applicable to impinging fluid sheets from plane or annular nozzles. Under the high impact speeds treated here, the solid plates or shells behave as compressible fluids; for the impinging fluid sheets compressibility effects will also be assumed important. Jetting will occur if either the collision is subsonic or the impinging angle is large enough such that the shock in the flow is not attached at the collision point. Jets formed from subsonic collisions are coherent; those from supersonic collisions are not coherent. The criteria are shown to be in general agreement with available experimental evidence. Further, the proposed criteria are also verified by two-dimensional unsteady finite-difference computer calculations. In addition, these calculations indicate the mechanical reasons for the coherency or noncoherency of the jet under various impact conditions. The practical applications of these criteria include collapsing shaped-charge liners, the explosive welding of plates, and steady impinging fluid sheets from annular nozzles.
Spectroscopic diagnostics of laser-supported absorption (LSA) waves produced by a HF laser
Authors: Wei P. S. P. , Nichols D. B.
Published: February 2003

LSA waves have been generated by a pulsed chemical HF laser beam (2.8 μ, 45 J, 7 μsec) when focused on solid surfaces in air. The emission spectrum is found to contain mainly N+ and O+ lines broadened by the Stark effect and superimposed on a strong continuum. The LSA wavefront travels at an average speed of 0.72 cm/μsec as measured by the time-of-flight method. Target species such as atomic Al emit light for 600 μsec after the laser pulse. In contrast with the results from a CO2 laser, however, no forbidden transition of AlI is observed.
Variational analysis of the reflection of surface waves by arrays of reflecting strips
Authors: Sinha B. K. , Tiersten H. F.
Published: February 2003

A system of approximate surface wave equations and edge conditions in one scalar variable is derived from Hamilton’s principle for linear piezoelectric media by assuming suitable depth behavior and integrating with respect to depth. The assumed behavior with depth is determined from the known surface-wave solutions of the three-dimensional equations for both the plated and unplated substrate. The influence of the inertia, stiffness, and electrical shorting of the film is included in the analysis. The approximate equations are expressed in terms of the known fundamental material constants and no measurement of model parameters is required. Bulk-wave scattering is not considered. The approximate equations, which admit of a transmission-line representation, are applied in the analysis of surface-wave reflection by both uniformly and nonuniformly spaced arrays of reflecting strips plated on various substrates. Among other things, the calculated reflection curves indicate a slight asymmetry for heavier film materials on account of the dispersion caused by the strips. Although this effect has been observed experimentally, it has not been reproduced by other analytical models.
Phase transformations in nearly stoichiometric NbNx
Authors: Oya Gin-ichiro , Onodera Yutaka
Published: February 2003

Processes of the phase transformation in nearly stoichiometric niobium nitride, from cubic δ-NbN to hexagonal ε-NbN, are studied by selected-area electron diffraction, by using NbNx films of the transition state prepared by annealing δ-NbNx films at about 1000 °C in an atmosphere of H2 gas. The diffraction patterns taken from the annealed NbNx films suggest that the phase transformations (vacancy-poor-) δ-NbNx (cub.) → (N-vacancy-) ordered δ-NbN (tet.)  → ordered ζ-NbN (hex.)  → ordered γ′-NbN (mon.)  → δ′-NbN (hex.)  → ε-NbN (hex.) occur. Ordered δ-NbN, ordered ζ-NbN, ordered γ′-NbN, and δ′-NbN are transient phases which occur during the transformation δ-NbNx → ε-NbN, and the superlattice structures of the three ordered phases are described by the ordering of N vacancies with random Nb vacancies in their fundamental lattices. Mechanisms of the transformations from δ-NbNx to ε-NbN at about 1000 °C are discussed on the basis of the transient phases observed during the phase transformations.
Pressure fixed points between 100 and 200 kbar based on the compression of NaCl
Authors: Yagi Takehiko , Akimoto Syun-iti
Published: February 2003

Transformation pressures in three materials, Pb, ZnS, and GaAs, were measured by means of an x-ray diffraction technique using NaCl as an internal pressure standard. A cubic-anvil type of high-pressure apparatus and a high-power x-ray diffraction system were used for this study. Based on Decker′s equation of state for NaCl, transition pressures in the increasing and decreasing pressure cycle were determined as follows: for Pb 142±3 and 111±2 kbar, for ZnS 162±4 and 110–100 kbar, and for GaAs 193±5 and 115–100 kbar. These values and the high Ba transformation pressure reported from this laboratory (126±2 and 111±2 kbar in the increasing and decreasing pressure cycle) form a set of pressure fixed points between 100 and 200 kbar at room temperature.
Magnetoelasticity and internal friction of an amorphous ferromagnetic alloy
Authors: Berry B. S. , Pritchet W. C.
Published: February 2003

The magnetomechanical behavior of the splat-cooled amorphous alloy Fe75P15C10 has been studied at audio frequencies by a vibrating-reed technique. The ΔE effect exhibits a maximum value at a magnetization near 0.5MS. Such a maximum has only rarely been observed in the past, but now appears to be a general feature of the complete or fully relaxed ΔE effect exhibited in the absence of eddy-current shielding. Attention is drawn to the separation of the ΔE effect into macroscopic, microscopic, and hysteretic contributions, to complement a similar categorization of the internal friction behavior. The ΔE effect of the amorphous alloy can be enhanced to large values (ΔE/E=0.4) by thermal annealing, a behavior attributed to the relief of internal stress and the absence of a strong structural anisotropy. Internal friction measurements made as a function of magnetization, strain amplitude, and frequency reveal the existence of an extra magnetomechanical loss beyond that accounted for by eddy-current damping and Rayleigh-type magnetomechanical hysteresis.
Detection of H2S with Pd-gate MOS field-effect transistors
Authors: Shivaraman M. S.
Published: February 2003

H2S gas sensitivity of Pd-gate MOS field-effect transistors has been studied in air for different temperatures. The possible chemical reactions of H2S and O2 on palladium are discussed.
Outline and comparison of the possible effects present in a metal–thin–film–insulator–semiconductor
Authors: Fonash S. J.
Published: February 2003

The advantages possible with the insertion of a thin-film insulating or semi-insulating layer between a metal and a semiconductor to form MIS photovoltaic device have been presented previously in the literatue. This MIS configuration may be considered as a specific example of a more general class of photovoltaic devices: electrode–thin-film-insulator–semiconductor devices. Since the advantages of the configuration experimental interest in these photovoltaic devices. Because the previous analysis showed that the introduction of the insulator layer could produce several different but advantageous effects, this paper presents a further outline giving a comparison of these effects together with their ramifications.
Control of the hydrogen absorption and desorption of rare-earth intermetallic compounds
Authors: Gualtieri D. M. , Narasimhan K. S. V. L. , Takeshita T.
Published: February 2003

The rate at which rare-earth intermetallic compounds absorb and desorb hydrogen can be controlled by ’’poisoning’’ the catalytic reaction, H2=2H. Specimens treated with SO2, a well-known catalytic ’’poison’’, are prevented from absorbing hydrogen for extended periods. More surprisingly, SO2 treatment after hydriding prevents hydrogen desorption by specimens for weeks. The applications of this process to energy storage are numerous.
Measurement of CO2-laser-induced shock pressures above and below LSD-wave thresholds
Authors: Beverly III R. E. , Walters C. T.
Published: February 2003

TEA-CO2-laser-induced shock pressures in aluminum, cellulose acetate, and polymethyl methacrylate (PMMA) targets were measured by both backsurface and in-material techniques as functions of peak laser power density and ambient air pressure. We report the first experimental observation of shock-pressure profiles in metallic and nonmetallic targets above and below the thresholds for laser-supported-detonation-wave (LSD-wave) initiation. For aluminum irradiations above threshold conditions, the measured peak pressure is consistently below that predicted by cylindrical blast-wave theory, which can be explained in terms of LSD initiation at local surface defect sites and imperfections. Surface-vapor blow-off dominates the shock-pressure response below threshold conditions for both aluminum and the plastics. LSD-wave blockage of the surface vaporization process leads to a decrease in the peak pressure above the initiation threshold for cellulose acetate. The largest peak pressures (accompanied by severe backsurface spallation) are thus recorded just below threshold irradiance conditions for LSD-wave initiation in plastics. LSD initiation times inferred from these shock-pressure measurements are in reasonable agreement with values determined by other methods and support the contention that initiation on practical metallic and nonmetallic surfaces is a highly local phenomenon.
Optical frequency dependence of the photoelastic coefficients of fused silica
Authors: Biegelsen David K. , Zesch James C.
Published: February 2003

Measurements of the dispersion of the photoelastic coefficients and acousto-optic figures of merit of fused silica are presented. Values of p11 and p12 decrease by 10 and 4%, respectively, between 676 and 458 nm. Independent measurements of p44 and (p11 −p12)/2 agree within 1% and are nondispersive.
Control of HF laser output spectrum using selective intracavity loss
Authors: Fisk G. A. , Hays G. N.
Published: February 2003

It is shown experimentally that the addition of gas-phase absorbers into the cavity of a small pulsed HF laser reduces the number of lasing lines, without seriously reducing output energy, for 200-ns FWHM laser pulses. This effect is discussed in terms of the rapid rotational relaxation characteristic of gas-phase HF and the fact that the losses introduced are frequency selective.
Space charge, thermal expansion, and pyroelectricity in polymers
Authors: Salomon R. E. , Lee H. , Bak C. S. , Labes M. M.
Published: February 2003

The nonlinear thermal expansion of an insulating film containing trapped space charge leads to a reversible pyroelectric current. A general expression for the pyroelectric coefficient of such a system which also includes a contribution from a nonuniform dielectric constant is derived. A simplified expression applicable to insulators with small gradients is also derived and used to make an order-of-magnitude estimate of the pyroelectric coefficient of PVF2.
Grain-boundary electromigration in thin films II. Tracer measurements in pure Au
Authors: Tai K. L. , Ohring M.
Published: February 2003

The first direct measurement of the grain-boundary ion drift velocity in thin Au films over the temperature range 120–250 °C is reported. Central regions on the narrow stripe conductors were selectively embedded with 195Au tracer atoms and the extent of the subsequent transport was evaluated by a high-resolution autoradiography technique employing the scanning electron microscope. In one operating mode, the secondary electron image of the exposed Ag grains in the photographic emulsion was revealed. The second mode of operation involved detection of fluorescent Ag-Lα x rays, generated upon impingement of the finely focused electron beam on the Ag grains. The resultant spatial dependence of the x-ray intensity was ultimately converted into tracer activity profiles. It was found (1) that Ion transport in grain boundaries of Au is anode directed (this is the case whether or not a Mo glue layer is present), (2) that the ion drift velocity in grain boundaries can be expressed as V=[(5.7±0.6) ×10−2j/T] exp [−(0.80±0.03 eV)/kT] (cm/sec), with j given in units of A/cm2, (3) that the effective valence of ions migrating in grain boundaries ranged from −89 at 120 °C to −20 at 250 °C (for a diffusion correlation factor of unity), and (4) that a plateau was observed in the low temperature (<180 °C) profiles. (At higher temperatures, the plateau was less apparent and the integrated intensity was higher). This last result is in semiquantitative agreement with the theory developed by the authors to account for grain-boundary diffusion and electromigration, in the presence of simultaneous atom leakage into the lattice.
Emission probability of hot electrons from silicon into silicon dioxide
Authors: Ning T. H. , Osburn C.M. , Yu H. N.
Published: February 2003

An experimental method is described for directly measuring the probability of electron emission from the silicon substrate into the SiO2 layer after the electron has fallen through a certain potential drop in traversing the depletion layer and reached the Si-SiO2 interface. The method is based on optically induced hot-electron injection in polysilicon-SiO2-silicon field-effect-transistor structures of reentrant geometry. The emission probability was studied as a function of substrate doping profile, substrate voltage, gate voltage, and lattice temperature. It was found that the hot electrons could be emitted by tunneling as well as by surmounting the Schottky-lowered barrier. Over-the-barrier emission dominates at large substrate voltages, where the emission probability is high, and tunnel emission becomes appreciable and may even dominate at small substrate voltages where the emission probability is low. A simple model was developed based on the assumption that only those hot electrons lucky enough to escape collision with optical phonons were emitted. Using this model, we found that the expression P=A exp(−d/λ) described very well the dependence of the emission probability on doping profile, substrate voltage, and gate voltage. Here A=2.9 is a constant, λ is the optical-phonon-electron collision mean free path, d is the distance from the Si-SiO2 interface where the potential energy is equal to the ’’corrected’’ barrier of (3.1 eV−βEOX1/2 −αEOX2/3ox), βEOX1/2 is the Schottky lowering of the barrier, and αEOX2/3 is a ’’barrier-lowering’’ term introduced to account for the probability of tunneling. The temperature dependence of the collision mean free path was found to follow the theoretical relationship λ=λo tanh(ER/2kbT), with λo=108 Å and ER=0.63 eV. This model is useful for evaluating potential hot-electron-related instability problems in IGFET and similar structures.
Effects of ion implantation on charges in the silicon–silicon dioxide system
Authors: Learn A. J. , Hess D. W.
Published: February 2003

Structures consisting of thermally grown oxide on silicon were implanted with boron, arsenic, or argon ions. For argon implantation through oxides, an increased fixed oxide charge (Qss) was observed with the increase being greater for 〈111〉 than for 〈100〉 silicon. This effect is attributed to oxygen recoil which produces additional excess ionized silicon in the oxide of a type similar to that arising in thermal oxidation. Fast surface state (Nst) generation was also noted which in most cases obscured the Qss increase. Of various heat treatments tested, only a 900 °C anneal in hydrogen annihilated Nst and allowed Qss measurement. Such Nst apparently arises as a consequence of implantation damage at the silicon–silicon dioxide interface. With the exception of boron implantations into thick oxides or through aluminum electrodes, reduction of the mobile ionic charge (Qo) was achieved by implantation. The reduction again is presumably damage related and is not negated by high-temperature annealing but may be counterbalanced by aluminum incorporation in the oxide.
The recombination properties of lattice-mismatched InxGa1−xP/GaAs heterojunctions
Authors: Ettenberg M. , Olsen G. H.
Published: February 2003

The interfacial recombination of InxGa1−xP/GaAs heterojunctions is measured as a function of lattice-parameter mismatch for values of strain between 3×10−4 and 2.5×10−2. It is found that when InxGa1−xP is epitaxially deposited on GaAs such that the lattice parameter is larger than the underlying substrate, the interfacial recombination velocity s varies linearly with mismatch according to the expression s?2.5×107ε cm sec−1. This behavior can be quantitatively understood in terms of an increasing density of dislocations acting as recombination centers at the interface. When the InxGa1−xP epitaxial layers have smaller lattice parameters than the underlying GaAs, such that ε≳3×10−3, there is a very high recombination velocity and degradation of the electronic properties of GaAs. This is due to the formation of cracks in the InxGa1−xP which form dislocations in the underlying GaAs. For small lattice mismatch, ε<1×10−3, there is an appreciable recombination velocity which cannot be explained in terms of dislocations.
A high-pressure x-ray study of Nylon 11
Authors: Newman B. A. , Sham T. P. , Pae K. D.
Published: February 2003

An x-ray diffraction study of Nylon 11 was carried out at high pressures and high temperatures. Careful measurements made on both wet and dry samples at atmospheric pressure indicated that the structure of Nylon 11 suggested by Slichter was not quite correct. Some distortion of the planar conformation causes a shortening of the c repeat distance from the value required for a fully extended conformation. The compressibility of the crystal lattice at higher pressures up to 20 kbar was studied and related to the anisotropic bonding present in the structure. A crystal transition at atmospheric pressure previously unreported was observed to occur at 95 °C, the triclinic structure (α phase) changing to a pseudohexagonal structure. The influence of pressure on this transition was studied. It was found that the increase in transition temperature with pressure dTt/dP (∼15 K/kbar) was greater than the melting temperature increase with pressure dTm/dP (∼9 K/kbar). A pressure-temperature phase diagram for Nylon 11 was obtained and a triple point was observed at 310 °C and 14.5 kbar. At pressures above 14.5 kbar the α phase is stable until melting.
Excitation of magnetoacoustic surface waves by meander lines
Authors: Tsai Tung-lin , Wu S. D. , Thomas G. , Tuan H. S.
Published: February 2003

The theory of the excitation of magnetoacoustic surface waves (MASW) by meander lines is presented. It is assumed that the magnetoacoustic coupling coefficients are small. The homogeneous equations of motion for the magnetic system are solved and the results are used as forcing functions in the equations of motion for the elastic motion. Numerical calculations of the radiation resistance and the power for various values of magnetic field, number of meander-line pairs, and linewidth-to-spacing ratios are given. It is found that the acoustic power radiated increases with frequency, linewidth-to-space ratio, and the magnetic field up to the gyrofrequency.
Spontaneous emission of charged particles and photons during tensile deformation of oxide-covered metals under ultrahigh-vacuum conditions
Authors: Rosenblum B. , Bräunlich P. , Himmel L.
Published: February 2003

Using Channeltron-type detectors and magnetic discrimination techniques, it has been shown that electrons, positive and negative ions, and photons are all emitted spontaneously while oxide-covered metals such as Al and Ni are being deformed in a tensile mode in a vacuum of approximately 10−10 Torr in total darkness. For dense anodically grown Al2O3 films of a given thickness on high-purity Al substrates, the emission yields of all charged particles and photons are found to exhibit identically the same strain dependence. The same applies to compact thermally grown NiO films on Ni. It is concluded from these and other observations that the entire emission spectrum is generated simultaneously as a result of the formation and propagation of cracks in the oxide film during the deformation process. The shortcomings of previously proposed models for this type of triboinduced emission are pointed out, and some alternative suggestions regarding the emission mechanism are offered.
Effect of crystal orientation on dynamic strength of LiF
Authors: Gupta Yogendra M.
Published: February 2003

Shock propagation data along 〈100〉, 〈110〉, and 〈111〉 directions in LiF crystals are presented. A marked anisotropy in wave profiles and in dynamic compressive strengths is observed. The variation in dynamic compressive strengths is in excellent agreement with predictions of slip systems based on quasistatic and microscopic studies. The 〈110〉 crystals show a two-wave structure only at a large propagation distance. Also, the 〈110〉 results confirm the presence of a shear-stress threshold for rapid elastic wave attenuation, as determined from earlier studies, along the 〈100〉 direction. For the 〈111〉 orientation, an elastic response to 42-kbar compressive stress is observed. This value corresponds to a resolved shear stress of C′44/27 along the {100} planes, where C′44 is the effective shear modulus for the 〈111〉 direction. This shear stress is approaching the theoretical estimates of ideal lattice strength in pure shear. In contrast to the 〈100〉 crystals, the 〈111〉 crystals are not influenced by heat treatment. The upper limit of the elastic response along 〈111〉 cannot be much higher, and measurements of this response may provide a measure of lattice strength. Present results suggest the use of shock experiments to examine mechanical lattice behavior at stresses beyond the range of quasistatic experiments.
Scattering of Rayleigh waves from topographic irregularities at oblique incidence
Authors: Otto Oberdan W.
Published: February 2003

Several theoretical treatments now exist in the literature for scattering of Rayleigh waves from topograhic features which are parallel to the incident wave front. These treatments, however, are not applicable to a large class of signal-processing devices in which the surface waves are scattered by inclined reflectors. A perturbational approach, similar to the earlier treatments but applicable to oblique incidence and to all perturbation orders, is presented. The first-order reflection coefficient of a Rayleigh wave from an inclined groove is computed. It is shown that an angle similar to a ’’Brewster’’ angle exists for which the surface wave reflection coefficient is zero. A prescription for applying the theory to anisotropic media is suggested. A clarification of the applicability of this and prior theories to steep-walled structures is given.
The electrical characterization of semi-insulating GaAs: A correlation with mass-spectrographic analysis
Authors: Look D. C.
Published: February 2003

The room-temperature electrical properties of 28 semi-insulating GaAs crystals have been determined by using a mixed-conductivity analysis. It is shown that for most of these samples, such an analysis gives good accuracy for the electron mobility μn and electron concentration n, but poorer accuracy for the hole mobility μp, hole concentration p, and intrinsic concentration ni. The intrinsic concentration is determined at 296 °K to be ni? (1.7±0.4) ×106 cm−3, which compares favorably with the theoretical value deduced from the band gap and the effective masses. From a Fermi-level analysis, the dominant Cr acceptor is found to lie at 0.69±0.02 eV from the valence band. For many of the samples, the ionized-impurity concentrations NI have been estimated from spark-source mass-spectrographic measurements and are compared with the concentrations predicted from μn. In general, the expected inverse relationship between μn and NI is found to hold, but the scatter in the data is quite large, mainly due to the uncertainties in the mass-spectrographic results.
A more complete interpretation of optoacoustic data taken with fixed-frequency lasers
Authors: Farrow L. A. , Richton R. E.
Published: February 2003

The response of the cylindrical optoacoustic cell when filled with an absorbing gas and irradiated by a laser beam passing along its axis has been studied theoretically and experimentally as a function of the gas pressure and composition. The pressure on the cell microphone has been calculated by solving numerically, for the experimental parameters of interest, the general heat-diffusion equation with a forcing function which is shown to depend upon both space and time for the realistic case of 50% modulation of the laser beam. It is further shown that if the experimental optoacoustic signal at a given pressure is divided by the signal calculated for that pressure, the ratio is expected to be a constant. The results of a series of experiments using a CO laser to irradiate mixtures of nitric oxide (the absorbing gas) and nitrogen are presented and are shown to confirm the theoretical predictions.
Preparation and superconducting properties of ultrafine powders and sintered compacts of NbC and NbN
Authors: Powell R. M. , Skocpol W. J. , Tinkham M.
Published: February 2003

We have produced ultrafine powders of refractory metals with the nominal compositions NbC and NbN by conversion of a Nb-bearing spray-dried powder in hydrogen-methane and ammonia atmospheres at temperatures as low as 800 and 700 °C, respectively. The resulting powders consist of 5–10-μm balls subdivided into 10–100-nm microcrystallites. We have investigated some of the factors which affect the microcrystallite size and the superconducting transition temperatures of the powders and have observed the changes of morphology which occur during two stages of sintering. In the more heavily sintered compacts of NbN, the finer scale of division into microcrystallites was lost, leaving a compact of approximately 70% density with a characteristic scale for crystallites and voids of 1–5 μm. At 4.2 K the best NbN sample had Hc2≳120 kG, and Jc≳103 A/cm2 at all fields up to 110 kG. These Jc values are generally twice those of the best previously reported sintered compacts of niobium carbonitride materials. It is possible that similar techniques can be applied to the production of other refractory metals such as tungsten carbide.
Transient and steady-state electron transport properties of GaAs and InP
Authors: Maloney T. J. , Frey Jeffrey
Published: February 2003

A Monte Carlo computer technique has been used to calculate nonequilibrium electron transport properties of n-type GaAs and InP at 77 and 300 K, with and without impurity scattering. Characteristics of the electron transient, including velocity overshoot, are presented and analyzed in terms of energy and momentum relaxation times. New information about the steady-state transport properties of InP and about the electron temperatures of GaAs and InP as functions of electric field is also presented and discussed.
Positive column of Na-Ne-Ar ac discharges
Authors: Verbeek T. G. , v. Tongeren H.
Published: February 2003

The positive column of 50-Hz ac discharges in a mixture of sodium vapor and noble gas (99 vol% Ne–1 vol% Ar) has been investigated experimentally. The experimental conditions were as follows: discharge tube radius 10 mm, noble-gas pressure 5.5 Torr at 20°C, and sodium density (2.4–5.7)  × 1019 m−3 which corresponds to a tube wall temperature of 519–538 K. The rms values of the current ranged from 0.25 to 1.5 A. The discharge properties, measured with the aid of electrostatic probes, show hysteresis effects especially for high rms current values. This behavior can be explained from the combined effects of depletion of sodium atoms and diffusion. It is found that the experimental data on the time-resolved electron temperature agree with calculated values. Qualitative agreement is obtained between the measured and calculated data on the electron density, electric field strength, and radiated power.
Reflection and bulk-wave conversion of Rayleigh wave at a single shallow groove
Authors: Parekh J. P. , Tuan H-S.
Published: February 2003

The reflection and surface-to-bulk wave conversion of a Rayleigh wave incident at a single shallow groove on an elastically isotropic substrate have been studied for various rectangularlike and troughlike groove profiles. It is found that the maximum value of the amplitude reflection coefficient is insensitive to the groove profile, and that the groove width producing maximum reflection or zero reflection of the incident surface wave is independent of the profile for all rectangularlike grooves. In all cases, both P and SV bulk waves are excited, with the latter carrying most of the bulk-wave power. While the angular distribution of the bulk-wave power patterns is strongly dependent on the groove width for a given profile, it is surprisingly insensitive to the profile for a given width. The fraction of the total incident power which is converted to bulk waves has been calculated.
The thermally saturated SUPARAMP
Authors: Feldman M. J.
Published: February 2003

The superconducting unbiased parametric amplifier can be saturated by low levels of broadband thermal radiation. Saturation increases the fluctuation noise temperature and complicates the determination of the operating parameters of this device. In this paper the theory of the thermally saturated SUPARAMP is presented. It is found that a particular amplifier can be described by a characteristic temperature, its maximum possible output brightness temperature. The implications of this theory for the interpretation of past experiments and for the design of future experiments are discussed.
Long-term degradation of GaAs-Ga1−xAlxAs DH lasers due to facet erosion
Authors: Chinone N. , Nakashima H. , Ito R.
Published: February 2003

Factors contributing to long-term degradation of GaAs-Ga1−xAlxAs double heterostructure lasers were studied. It has been found that the degradation rate strongly depends on the output-power density and that facet erosion plays an important role in the long-term degradation.
Preparation of CdS/InP solar cells by chemical vapor deposition of CdS
Authors: Bettini M. , Bachmann K. J. , Buehler E. , Shay J. L. , Wagner S.
Published: February 2003

CdS/InP heterojunctions are grown by chemical vapor deposition of CdS on InP in an open-tube H2S/H2 flow system. The importance of H2S admixture for the etching and cleaning of the InP surface is demonstrated. The solar cell has an open-circuit voltage of 0.78 V, fill factor of 0.75, and a power conversion efficiency of 13.5% for AM2 conditions.
Vacuum-evaporated films of chromium with the A-15 structure
Authors: Doherty C. J. , Poate J. M. , Voorhoeve R. J. H.
Published: February 2003

Continuous thin films of Cr 200–400 Å thick and predominantly consisting of the A-15 phase were grown by evaporation in high vacuum onto KCl and NaCl substrates held at 300 °C. The films have a chemical purity of better than 98%, determined by 4He Rutherford scattering. The A-15 phase is not stabilized by impurities. The lattice parameter determined by electron diffraction is 4.60 Å. The grain size is about 500 Å. The films did not show superconductivity down to 0.5 °K.
Static contact resistance of usual metal under high compressive stress and for high current densities
Authors: Boissady C. , Postel C. , Rioux-Damidau F.
Published: February 2003

Static contact resistance for all combinations of usual metals (cold-drawn copper, brass, aluminum, duraluminum, mild steel, and stainless steel) are measured for stresses of up to 16 kg/mm2 and current densities of up to 7.5 kA/mm2. A decreasing resistance has been found for increasing compressive stress and increasing current intensity. Very low resistance values have been observed (0.02 mΩ mm2) with cold-drawn copper and aluminum.
On the transient space-charge-limited current problem
Authors: Lonngren K. E.
Published: February 2003

Using a unipolar ion mobility conduction model in planar geometry as suggested by Many and Rakavy, the transient behavior of the bulk electric field and space-charge distribution as a function of space and time is studied. Analytical solutions using the method of self-similar solution of partial differential equations are obtained for both a step voltage and step current excitation.
Magnetic constriction effects in high-current vacuum arcs prior to the release of anode vapor
Authors: Boxman Raymond L.
Published: February 2003

A model is developed describing the interelectrode plasma generated by a multi-cathode-spot cathode as a conducting fluid flowing from the cathode to the anode. The model is analyzed numerically for a sample physical situtation consisting of a 3-kA Cu vapor arc between 25-mm-diam electrodes separated by 9 mm, in which case the mass density and fluid velocity are found to be 5×−5 kg/m3 and 7×103 m/s, respectively. The fluid flow is analyzed and a constriction caused by the magnetic pinch force is found to develop near the anode. A constriction in the current flow is also calculated, caused primarily by the Hall current.
Thermally stimulated currents in polyethylene and ethylene–vinyl-acetate copolymers
Authors: Mizutani T. , Suzuoki Y. , Ieda M.
Published: February 2003

Thermally stimulated currents (TSC) from polyethylene and ethylene–vinyl-acetate copolymers with a variety of vinyl-acetate contents have been investigated using a TSC technique with x-ray irradiation. Five main TSC peaks, which we believe are due to trapped carriers, were observed in these polymers. The detrapping of carriers is closely related to molecular-chain motions, and trapping sites are assigned to physical defects such as cavities formed by local arrangements of molecular chains rather than carbonyl groups. One of the TSC peaks is associated with traps in the amorphous region, one with traps in the boundary region between the amorphous and the crystalline regions, i.e., in the folded region, and another with traps in the crystalline region. The TSC spectrum from the dipole of the vinyl-acetate unit under a bias field shows a TSC peak accompanied by a current inversion. The average effective dipole moment for the vinyl-acetate unit was determined to be about 1.8 D from a TSC analysis.
Nitrogen implantation in GaAs1−xPx. I. Photoluminescence properties
Authors: Wolford D. J. , Anderson R. E. , Streetman B. G.
Published: February 2003

Room-temperature implantation of nitrogen in GaAs1−xPx, following appropriate annealing, has resulted in efficient photoluminescence (PL) involving the N isoelectronic trap. Emission intensities obtained from direct-band-gap materials are comparable to, or greater than, those of unimplanted samples. Similarly, nitrogen-implanted indirect-gap materials give PL intensities which compare with or exceed that of vapor-phase-epitaxial material N doped during growth. Isochronal annealing indicates that most implantation-induced lattice disorder is removed at temperatures below 800 °C; however, promotion of implanted N atoms onto active lattice sites requires temperatures near 950 °C. Both levels of the nitrogen trap, NΓ (or NΓ-X) and NX, are studied by PL over a four order-of-magnitude range in implanted nitrogen fluence. Comparison is made between spectra obtained at 77 and 4.2 °K. Studies suggest association of both N-trap levels with the isolated N center. Apparent increased exciton binding at the NX level for moderate N concentrations and origins of PL degradation at high implant fluences are discussed.
Nitrogen implantation in GaAs1−xPx. II. Annealing properties
Authors: Anderson R. E. , Wolford D. J. , Streetman B. G.
Published: February 2003

Efficient photoluminescence has been obtained from GaAs1−xPx implanted with N at room temperature. With appropriate annealing, emission intensities comparable with conventionally doped material are realized for N-implanted GaAs1−xPx crystals of both direct- and indirect-band-gap compositions. Anneal temperatures of at least 950 °C with dielectric encapsulation are necessary to incorporate the implanted N on optically active sites, although much of the implantation-induced lattice disorder is removed by annealing to 700 °C. Anneal characteristics are investigated by both isochronal and isothermal anneals, and comparison is made between room-temperature and hot implantations. Effects of damage introduced by optically inactive implanted Ne are compared to results for implanted N. The effects of annealing in the presence of arsenic and gallium vapor are studied, and depth profiles of lattice damage and optically active N are obtained.
Phase and amplitude relations between fundamental and second harmonic acoustic surface waves on SiO2 and LiNbO3
Authors: Alippi A. , Palma A. , Palmieri L. , Socino G.
Published: February 2003

The spread of results obtained by different theoretical approaches in studying acoustic surface wave nonlinear interaction and difficulties in treating strongly piezoelectric materials indicate a need for more experimental data. In the present work the spectral asymmetries of the light diffracted by surface waves are analyzed in order to evaluate the complex coupling coefficient in second harmonic surface-wave generation. The experimental conditions are such as to keep the acoustic energy mostly confined to within fundamental and second harmonic waves. Amplitude and phase of nonlinear coupling coefficients were separately determined for LiNbO3 and crystalline SiO2 substrates through different geometries of the acousto-optical interaction, crystal cuts, and propagation directions of the waves.
Hyperfine field in Zn2+0.3 Mn2+0.7 Mn3+y Fe3+2−y O4
Authors: Gupta R. G. , Mendiratta R. G.
Published: February 2003

Mössbauer spectra of Zn2+03. Mn2+0.7 Mn3+y Fe3+2−y O4 with y values ranging from 0.0 to 1.0 in steps of 0.1 have been taken at room temperature. A possible explanation for the observed variations in the internal magnetic field, and the EFG is given.
Theory of the secondary electron avalanche at electrically stressed insulator-vacuum interfaces
Authors: Bergeron K. D.
Published: February 2003

Several aspects of the secondary emission avalanche along an insulator surface (which is believed to play a role in insulator flashover) are addressed theoretically. The saturation condition for the two extremes of supply-limited and space-charge-limited current are obtained in analytic form, and comparison with the computer simulation of Anderson is made. Also, the effect of a strong magnetic field parallel to the insulator surface and perpendicular to the electric field is analyzed, taking account of the distribution of emission angles. It is found that the critical magnetic field for inhibition of flashover is reduced by a factor of about 2 when the distribution of angles is included in the calculation.
Transient heat transfer into liquid helium I
Authors: Brodie L. C. , Sinha Dipen N. , Semura J. S. , Sanford C. E.
Published: February 2003

We have investigated the transient transfer of heat from a crystal of bismuth into a bath of liquid helium I in response to a step-function current applied to the crystal. The superheat temperature ΔT of the helium in contact with the crystal was monitored through the magnetoresistance of the bismuth which is strongly temperature dependent at liquid-helium temperatures. It is found that as a function of time, the transient superheat temperature can rise above (i.e., ’’overshoot’’) its steady-state value. Of particular interest is the fact that, within a certain range of bath temperatures and heat fluxes, these transient overshoots develop quite sharp peaks at a superheat temperature ΔTmc which is independent of the applied heat flux. This overshooting of the transient superheat temperature is discussed in terms of the delayed onset of nucleation. In particular, it is suggested that the sharp overshoots result from the rapid onset of homogeneous nucleation. The temperatures associated with the sharp overshoots are compared with theoretical estimates of the homogeneous nucleation temperature for liquid helium.
Solid-phase crystallization of Si films in contact with Al layers
Authors: Harris J. M. , Blattner R. J. , Ward I. D. , Evans C. A. , Fraser H. L. , Nicolet M. A. , Ramiller C. L.
Published: February 2003

Low-temperature (400–540 °C) crystallization of amorphous and polycrystalline Si films deposited on SiO2 and covered with an evaporated Al layer has been studied using SEM, TEM, electron diffraction, electron channeling, and MeV 4He+ backscattering. Silicon deposited by evaporation and chemical vapor deposition (CVD) at 640 °C (both amorphous) was found to crystallize into islands of polycrystalline aggregates. Silicon deposited by CVD at 900 °C (polycrystalline with ∼2000-Å grains) produced relatively large (∼10 μm) single-crystal islands. In both cases island size increased with annealing time, and the rate of crystallization increased with temperature. Crystallization rates were observed to be the same for both sources of amorphous Si, while 900 °C CVD Si was noticeably slower, consistent with the postulate that the driving force for the reaction is the free-energy difference between initial and final states. The crystallization rate for 900 °C CVD Si decreased when the Al layer thickness was reduced to a value less than the initial Si grain size. The inclusion of a native oxide layer between the deposited Si and Al layers greatly retarded the crystallization process.
Space-charge-limited currents in insulators with two sets of traps distributed in energy: Theory and experiment
Authors: Rizzo A. , Micocci G. , Tepore A.
Published: February 2003

The theory of space-charge-limited current (SCLC) flow in insulators has been extended to include the case of materials with two sets of traps. By using the regional approximation method, two theoretical current-voltage characteristics have been derived for this situation, referring to different starting positions of the thermal equilibrium Fermi level in the forbidden gap. The general case of traps not localized at a single energy, but rather distributed in energy, has been taken into account for the first time. To account for the effect of the distribution of traps around a single energy, say Et, on SCLC, the following distribution function has been used: H (E) = (Nt/kTt)  exp[(E−Et)/kTt] {exp[(E−Et)/kTt]+1}−2. The theory has been compared with the experimental results obtained in n-type GaSe(I2) single crystals, which show current-voltage characteristics exceptionally similar to the theoretical ones. From the analysis, two electron traps have been found to be centered at 0.63 and 0.42 eV from the conduction band. These results have been confirmed by a different method of analysis of SCLC curves and by other measurements. Finally, two particular cases are analyzed, already experimentally evidenced in GaS and nominally undoped p-type GaSe.
Magnetic fields produced by models of biological current sources
Authors: Cuffin B. Neil , Cohen David
Published: February 2003

As a means to understand the magnetic fields produced by electrical sources in biological organisms, the fields produced by special sources which approximate the biological sources are calculated. Two categories of sources are considered. The first consists of current dipoles which are aligned in a particular direction; the second consists of randomly oriented dipoles. Although the field is produced by both the source directly and the current generated in the volume conductor by the source, only expressions for the field produced by the source alone are developed. This is done because in certain special volume conductor shapes, such as a semi-infinite volume, an infinite slab, and a sphere, the current flowing in the volume conductor gives zero contribution to the field component perpendicular to the surface of the conductor and greatly simplifies the expressions for this component. Approximations to these special shapes exist on the human body and allow development of simple expressions for the fields produced by the sources. These expressions are used to produce field plots which can be compared with measured fields to estimate the sources of the measured fields. The expressions indicate that under certain circumstances, some sources in the body will produce a small or zero magnetic field. The fields produced by aligned and random dipole sources are shown to have different rates of decrease with distance from the source.