bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-temperature characteristics of electron ionization rates in (100)- and (111)-oriented InP
Authors: Osaka Fukunobu , Mikawa Takashi
Published: July 2002

Electron ionization rates α in both (100)- and (111)-oriented InP have been derived at a low-temperature range between 77 and 293 K from photomultiplication data measured on a mesa-type (100)-oriented InP diode and a planar type (111)-oriented InP diode. It has been found that there is no difference in α between the two orientations in this temperature range. This indicates that the contribution of ballistic electrons to impact ionization is insignificant not only at room temperature but at low temperatures. A Monte Carlo simulation of α in both (100)- and (111)-oriented InP at low temperatures have been carried out for the analysis of the experimental data. The simulated results have been found to agree with the experimental ones fairly well in the considered electric field and temperature ranges. α has been demonstrated to have no orientation dependence and the probability of ballistic electrons to cause impact ionization has been found to be extremely small (about 0.01% at 300 K and 0.05% even at 77 K). No anisotropy in electron ionization rates in InP has been attributed to this very small probability.
Free-carrier absorption in quasi-two-dimensional semiconducting structures for nonpolar optical phonon scattering
Authors: Kubakaddi S. S. , Mulimani B. G.
Published: July 2002

The theory of free-carrier absorption is given for quasi-two-dimensional semiconducting structures when the carriers are scattered by nonpolar optical phonons and the radiation field is polarized in the plane of the layer. The absorption coefficient is calculated and is found to be an oscillatory function of the thickness of the layer and is enhanced over its bulk value for low values of thickness of the layer. Separate peaks associated with transitions involving phonon emission and absorption are observed. The results are interpreted in terms of phonon-assisted transitions between size quantized subbands.
Shock-induced phase transformation in lithium niobate
Authors: Goto Tsueneaki , Syono Yasuhiko
Published: July 2002

Shock compression measurements of lithium niobate single crystals are carried out up to 121 GPa using the gun method. Hugoniot elastic limits are found to be 2.4 and 6.6 GPa with the shock propagation direction along [0001] and [112̄0], respectively, reflecting a remarkable elastic anisotropy. A phase transition accompanied by a discontinuous volume decrease is induced at the shock pressure of 32.6±0.6 GPa. The zero-pressure volume of the high-pressure phase is estimated to be smaller by 20.5% than that of the low-pressure-phase lithium niobate. The observed large volume decrease strongly suggests considerable change in the bond character with the phase tranformation.
Temperature distribution along the striped active region in high-power GaAlAs visible lasers
Authors: Todoroki Satoru , Sawai Masaaki , Aiki Kunio
Published: July 2002

Catastrophic degradation related to local heating in GaAlAs visible lasers occasionally occurs under relatively low optical output power. To develop highly reliable lasers, we used laser Raman spectroscope with an argon ion laser focused at about 1 μm≂ to evaluate the local operating temperature rise not only at the facet surface, but also along the striped active region. The local operating temperature rise in the vicinity of the facet’s active region increased exponentially up to 200 °C when the optical output power was 30 mW/facet. This high temperature causes the rapid formation of a dark region and final catastrophic degradation. The calculated temperature rise along the striped active region is about one-half of that of the facet. The internal operating temperature is far higher than the average temperature measured by the thermal resistance method, which is considered to be a large influence on the lifetime and activation energy of lasers in practical applications.
The role of cesium-ion bombardment in the formation of negative hydrogen ions on a converter surface
Authors: van Os C. F. A. , Granneman E. H. A. , van Amersfoort P. W.
Published: July 2002

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a porous tungsten button with liquid cesium flowing through it towards the side which is in contact with the plasma. A high cesium coverage, i.e., a small work function, can easily be maintained with this approach. This is related to the high flux of neutral cesium atoms to which the converter is exposed and to the small cesium density in the discharge. Despite the small work function, we obtain negative-ion yields which are an order of magnitude smaller than is usually found in more conventional experiments, in which the converter is cesiated via injection of cesium vapor into the discharge. Furthermore, our energy distributions show that no negative ions are formed via desorption by cesium-ion impact. This gives a strong indication that the extracted negative hydrogen ions are primarily formed via this process in cesium seeded discharges. Our view is confirmed by the observation that the negative-ion yield increases with an order of magnitude when a small amount of argon gas is injected into the discharge.
Magnetic properties of novel epitaxial films (invited)
Authors: Bader S. D. , Moog E. R.
Published: July 2002

The surface magneto-optic Kerr effect (SMOKE) is used to explore the magnetism of ultra thin Fe films extending into the monolayer regime. Both bcc α-Fe and fcc γ-Fe single-crystalline, multilayer films are prepared on the bulk-terminated (1×1) structures of Au(100) and Cu(100), respectively. The characterizations of epitaxy and growth mode are performed using low-energy electron diffraction and Auger electron spectroscopy. Monolayer range Fe/Au(100) is ferromagnetic with a lower Curie temperature than bulk α-Fe. The controversial γ-Fe/Cu(100) system exhibits a striking, metastable, surface magnetic phase at temperatures above room temperature, but does not exhibit bulk ferromagnetism.
High performance isotropic permanent magnet based on Nd-Fe-B
Authors: McCallum R. W. , Kadin A. M. , Clemente G. B. , Keem J. E.
Published: July 2002

We have fabricated a macroscopically isotropic permanent magnet material (Ovonic Hi-RemTM) which significantly exceeds conventionally understood performance limits for a material with saturation magnetization Ms =16 kG. Enhanced magnetic properties, including a remanent magnetization Mr =9–10 kG or more, and a maximum energy product in excess of 20 MGOe, are observed without preferred orientation. This material has been fabricated by rapid solidification of an alloy structurally and compositionally similar to Nd2Fe14B, but which depends on critical alloying additions and process parameters to obtain optimum magnetic performance.
Behavior of ion-implanted junction diodes in 3C SiC
Authors: Avila R. E. , Kopanski J. J. , Fung C. D.
Published: July 2002

p-n junction diodes have been formed by ion implantation of B or Al in 3C SiC and annealing at 1365 °C. The current-voltage characteristics of the Al-implanted structures show little rectification. The B-implanted diodes show rectification, with ideality factors of 2.2 and higher, breakdown voltages between 5 and 10 V, and a series resistance of the order of 20 kΩ. The current-voltage characteristics were measured between room temperature and 270 °C. The extracted ideality factors increase with temperature.
Radiation-induced conductivity of polymers in different gases
Authors: Faria R. M. , Gross B. , Filho R. Gregório
Published: July 2002

The conductivity induced in various polymers by continuous irradiation with x rays was measured under a vacuum and in different gases. Oxygen, nitrogen, and air increased the induced conductivity in polyvinylidene fluoride (PVDF), while argon left it almost unchanged. Argon and air decreased it in Kapton, and air decreased it in Mylar. The saturation current is that stationary value of the current that is obtained if all radiation-generated carriers are removed by the applied field. The measured current amplitude for PVDF in an atmosphere of air of 100% humidity reached almost 50% of that current, while in the other materials it did not exceed 1% of the saturation value. Therefore, carrier removal by the applied field played a negligible role in Kapton and Mylar.
Free-carrier absorption in semiconducting quantum-well wires for nonpolar optical-phonon scattering
Authors: Kubakaddi S. S. , Mulimani B. G.
Published: July 2002

The theory of free-carrier absorption is developed for quantum-well wires for the case where the carriers are scattered by nonpolar optical phonons and the radiation field is polarized along the length of the wire. The absorption is found to be an oscillatory function of the area of cross section of the wire and is enhanced over its bulk value in going to wires of decreasing cross sections. Phonon-assisted transitions between size quantized subbands give separate peaks associated with transition involving phonon emission and absorption.
A remote laser system for ultrasonic velocity measurement at high temperatures
Authors: Dewhurst R. J. , Edwards C. , McKie A. D. W. , Palmer S. B.
Published: July 2002

The temperature dependence of the longitudinal ultrasonic velocity in iron and Dural has been obtained using a totally remote laser technique. The ultrasound is generated by irradiation of one face of a sample with a Q-switched Nd:YAG laser pulse and is detected on the opposite face using a modified Michelson laser interferometer. The system has proved capable of measurements up to temperatures in excess of 1000 °C, with an absolute accuracy in velocity of ±1%, and relative accuracies of better than 0.1%. Anomalies in the data for iron have been discerned at the Curie temperature of ∼768 °C, due to the ferromagnetic to the paramagnetic phase transition, and at 910 °C due to the crystallographic phase transition from ferrite to austenite.
A generalized self-consistent method for the effective conductivity of composites with ellipsoidal inclusions and cracked bodies
Authors: Miloh T. , Benveniste Y.
Published: July 2002

This paper is concerned with the determination of the effective thermal conductivity of particulate composites with ellipsoidal-type inclusions at nondilute concentrations with special emphasis given to the case of cracked bodies with elliptical cracks. The inclusions or cracks are randomly oriented so that the solid is effectively isotropic. The adopted micromechanics model is the ‘‘generalized self-consistent’’ scheme which is based on the concept that a particle surrounded by some matrix material ‘‘sees’’ an effective medium. The method, whose reliability has already been proved in the literature, is applied for the first time in the present paper to composites containing inclusions of ellipsoidal shape. The generality of the presented analysis allows the treatment of cracked bodies with randomly oriented elliptical cracks. New results are given for particulate composites and cracked bodies.
Anomalously high Hugoniot-elastic limit (35–39 GPa) of Y2O3-doped partially stabilized zirconia ceramics
Authors: Mashimo Tsutomu
Published: July 2002

An anomalously high Hugoniot-elastic limit of 35–39 GPa in stress was measured by the inclined-mirror method for the Y2O3-doped partially stabilized zirconia ceramics. This value is much higher than for stabilized zirconia ceramics by over two, and is the second highest in these materials which have ever been measured. As a result, it is suggested that transformation toughening of the partially stabilized zirconia strongly influences the material strength even under shock compression, as under static conditions.
Fluorescence quantum efficiency and optical heating efficiency in laser crystals and glasses by laser calorimetry
Authors: Ramponi Albert J. , Caird John A.
Published: July 2002

A photocaloric technique is described for determining the fluorescence quantum efficiencies and optical heating efficiencies of optically active ions in laser materials. Optical absorption within the sample results in a temperature increase until the heat produced by the absorbed power is balanced by heat leakage to the surroundings. The fluorescence quantum efficiency and optical heating efficiency are determined from a measure of the absorbed power, the steady-state temperature, and the time constant associated with sample cooling following laser excitation. An alternative analysis utilizing only the absorbed power and the steady-state temperature as a function of excitation frequency is also shown to yield quantum efficiencies consistent with the first method. Theory and experiment are demonstrated by measuring the fluorescence quantum efficiency and optical heating efficiency for trivalent chromium in gadolinium scandium gallium garnet. Measurements are also reported for several neodymium-doped phosphate laser glasses.
Laser interferometer for the study of piezoelectric and electrostrictive strains
Authors: Zhang Q. M. , Pan W. Y. , Cross L. E.
Published: July 2002

A modified Michelson interferometer is used to study the strain properties of piezoelectric and electrostrictive materials. For small displacement, a feedback loop is introduced to stabilize the system against the low-frequency optical path-length drifting and the system is capable of resolving displacements of the order of 10−3 Å. For the strain induced by domain switching, a dual-channel signal detection scheme is used which automatically reads out the displacement of the sample. The effect on the measurement of the sample bonding to a substrate and other related problems are discussed.
Formation of planar defects in the epitaxial growth of GaP on Si substrate by metal organic chemical-vapor deposition
Authors: Ernst F. , Pirouz P.
Published: July 2002

The very small lattice mismatch between GaP and Si implies negligible coherency strains when GaP is grown epitaxially on Si. Moreover, GaP is expected to show little plasticity below ∼700 °C. It is thus unlikely that either the small coherency strains or the thermal mismatch stresses would be sufficient to generate the partial dislocations responsible for planar defects in an epitaxial film grown at temperatures below ∼700 °C. However, high-resolution electron microscopy micrographs of the early stages of epitaxial growth of GaP on Si at 500 °C show a high density of stacking faults and microtwins in the film. It is argued that in epitaxial growth processes, planar faults are not formed as a result of interfacial stresses; instead, a mechanism for the formation of stacking faults and twins is proposed which is based on errors in the stacking of close-packed planes in the early stages of nucleation and growth.
Laser ablation of bulk YBa2Cu3O7−δ and cluster emission
Authors: Becker C. H. , Pallix J. B.
Published: July 2002

Mass spectral investigations of ions produced directly from 1064-nm plus 532-nm laser beam ablation of a bulk high-Tc superconducting YBa2Cu3O7−δ target reveal significant cluster emission as well as very intense low-mass emission up to but below laser beam energy densities required to visually observe a plume of white light; at higher laser intensities, cluster emission essentially disappears while low-mass ion intensities continue to grow. Cluster mass patterns ranging above 104 amu are observed and believed to be products of condensation rather than direct emission. Cluster ions higher in mass although not in distinguishable patterns, ranging beyond 106 amu, are observed, possibly due to direct emission. These results are relevant to high-Tc superconducting thin-film deposition by the laser ablation technique.
Orientation dependence of valence-subband structures in GaAs-Ga1−xAlxAs quantum-well structures
Authors: Houng Mau-Phon , Chang Yia-Chung , Wang W. I.
Published: July 2002

Valence-subband structures of GaAs-Ga1−xAlxAs quantum wells growth in the [001], [111], and [110] directions are calculated based on the bond-orbital model. The effective mass for the first subband in (111)-oriented quantum wells is found to be substantially smaller than that in (001)-oriented quantum wells for well widths narrower than 70 Å. The subband structures of (110)-oriented quantum wells display large anisotropy, with effective masses along two different in-plane directions ([001] and [11̄0]) differing by almost one order of magnitude. It is also found that the Al composition has a strong effect on the hole effective mass. Implications of our results in device applications are also discussed.
Using a magnetometer to image a two-dimensional current distribution
Authors: Roth Bradley J. , Sepulveda Nestor G. , Wikswo John P.
Published: July 2002

We describe a mathematical algorithm to obtain an image of a two-dimensional current distribution from measurements of its magnetic field. The spatial resolution of this image is determined by the signal-to-noise ratio of the magnetometer data and the distance between the magnetometer and the plane of the current distribution. In many cases, the quality of the image can be improved more by decreasing the current-to-magnetometer distance than by decreasing the noise in the magnetometer.
Study of tertiarybutylphosphine pyrolysis using a deuterated source
Authors: Li S. H. , Larsen C. A. , Buchan N. I. , Stringfellow G. B. , Kosar W. P. , Brown D. W.
Published: July 2002

The pyrolysis of tertiarybutylphosphine (TBP) and the reaction mechanism for the organometallic vapor-phase epitaxial growth of GaP using TBP and trimethylgallium (TMGa) in a He ambient have been studied. A deuterated TBP source (C4H9PD2) was used to label the reaction products and to distinguish possible mechanisms. The reaction was found to be independent of the ambient, and C4H9D was found to be a major product species. However, its relative concentration was determined to increase as the input concentration of the reactant increased. The concentration of C4H9D is even higher when the pyrolysis was catalyzed by GaP surfaces. However, the addition of TMGa retarded both the production of C4H9D and the pyrolysis of TBP. Another species, CH3D, becomes the major deuterated product observed under these conditions. The results lead to the following conclusions: (1) an important reaction for TBP pyrolysis is of second order: the tert-butyl radical (C4H9) attacks TBP; (2) the PH and PH2 species on GaP surfaces, products from the scission of TBP, enhance the pyrolysis of TBP; (3) the same species also attack the adsorbed TMGa, leading to the growth of GaP crystals.
Dispensing particles under atmospheric and vacuum conditions using an electrostatic device
Authors: Olansen Jon B. , Dunn Patrick F. , Novick Vincent J.
Published: July 2002

Experiments were conducted to determine the operational characteristics of an electrostatic device that dispenses micrometer-size particles continuously without the use of a carrier gas. The dispenser’s operational performance was determined under different operating conditions based upon four independent experimental variables: the environmental pressure in which the dispenser was operated, its interelectrode spacing, the applied voltage, and the type of particle used. Measurements were made of the dispenser’s operating current and voltage, particle mass dispensing efficiency, and dispensed particle mass flow rate, current, and velocity. An increase in the strength of the dispenser’s internal electric field was found to produce an increase in operating current and dispensed particle velocity, current, and mass flow rate. The measured average charge acquired by a particle and the particle average velocity under vacuum conditions were predicted using single particle theory. All temporal measurements of the dispensed particle mass were correlated nondimensionally, independent of the particle type, interelectrode spacing, and applied voltage. The dispensing efficiency under atmospheric conditions, when expressed as a function of the strength of the internal electric field above the minimum field required for particle levitation against gravity, was shown to be independent of particle type.
Photoluminescence study of gallium antimonide grown by liquid-phase epitaxy
Authors: Chen S. C. , Su Y. K.
Published: July 2002

Low carrier concentrations of undoped p-type and Te-doped n-type GaSb epilayers have been grown on (100)-oriented Te-doped GaSb substrates by liquid-phase epitaxy with supersaturation and undersaturation techniques, respectively. The net carrier concentrations of undoped and Te-doped layers obtained by C-V measurement can be as low as NA −ND =8×1015 cm−3 and ND −NA =1016 cm−3 , respectively. These GaSb-grown layers were characterized by photoluminescence (PL) measurement. The native neutral acceptor transitions appear at 778.2 meV for undoped layers and 777.2 meV for Te-doped layers. The ionization energy for the Te donor in GaSb was determined to be 3.6 meV below the bottom of conduction band at 18 K. The relation between the energy gap of GaSb and temperature was obtained by PL measurement at various temperatures. The spin-orbit splitting of the valence band was measured to be 0.798 eV at room temperature.
Subpicosecond sampling using a noncontact electro-optic probe
Authors: Nagatsuma T. , Shibata T. , Sano E. , Iwata A.
Published: July 2002

This paper discusses a theoretical as well as experimental study of the performance of electro-optic sampling using a noncontact electro-optic probe, i.e., external electro-optic sampling. Sensitivity, temporal resolution, and the capacitive loading effect of this system are calculated by analysis of a static electric field coupled to an electro-optic crystal placed in close proximity to transmission lines such as microstrip lines and coplanar strips. Full-wave analysis is also applied to investigate effect of the electro-optic crystal on the transient property of high-speed electrical signals. Based on these analytical considerations, we have developed an external electro-optic sampling system using precise probe-positioning technology, which improves the measurement reproducibility. A temporal resolution of 0.5 ps and a spatial resolution of 1 μm are confirmed with this system.
Photoluminescence of ZnTe homoepitaxial layers grown by metalorganic vapor-phase epitaxy at low pressure
Authors: Ogawa Hiroshi , Nishio Mitsuhiro
Published: July 2002

Photoluminescence (PL) spectra have been investigated at liquid-He temperature in the ZnTe layers grown by metalorganic vapor-phase epitaxy. The epitaxial layer exhibits the spectra characterized by a neutral acceptor bound-exciton line in the near-band edge and the donor-acceptor pair emission in the wavelength range of 550–580 nm. At the low substrate temperature or low transport rate ratio of diethyltelluride to dimethylzinc, the spectrum of layer becomes dominated by this donor-acceptor pair emission. From the results of secondary-ion mass spectrometry analysis and PL property, it is concluded that Na and Cl are important impurities in the layer.
Negative Poisson’s ratio in a transversely isotropic foam structure
Authors: Warren Thomas L.
Published: July 2002

Recent experimental work has shown that foams having a negative Poisson’s ratio may be fabricated by inverting the usual energetically preferred tetrahedral structure to form a reentrant structure. It is shown here that a linear elastic analysis of a two-dimensional reentrant honeycomb provides a theoretical prediction of the negative Poisson’s ratio in the plane for a transversely isotropic material. The value of Poisson’s ratio depends upon the reticulation angle, which is related to the permanent volumetric compression ratio required to form the reentrant structure.
120-K Tl-based high-T
c superconducting thin films on LaAlO3 substrates
Authors: Lee W. Y. , Vazquez J. , Huang T. C. , Savoy R.
Published: July 2002

The preparation and properties of 120-K Tl-based high-temperature superconducting thin films on LaAlO3 substrates are reported. These films are obtained by post-annealing 2Tl: 2Ca: 2Ba: 3Cu: 10O precursor films at ≂890 °C in a sealed quartz tube filled with ≂ 1-atm O2. Results of ac susceptibility measurements show a sharp drop in the real part of the susceptibility (χ′) and a sharp maximum peak in the imaginary part of the susceptibility (χ″) at a temperature close to the observed zero resistance Tc of ≂ 120 K. The broadening of the transition width in χ′ and the decrease in the temperature where χ″ maximum appears are found to occur at a slower rate with increasing ac field for these films than for the films deposited on Y-ZrO2 and MgO and having a similar zero resistance Tc. This weaker field dependence of χ′ and χ″ is shown to lead to a higher critical current density for the films deposited on LaAlO3 substrates. The higher critical current density is attributed to a stronger coupling between the grains arising from the absence of the formation of well-defined grain boundaries during film growth.
The temperature dependence of the ferromagnetic and paramagnetic resonance spectra in thin yttrium-iron-garnet films
Authors: Laulicht I. , Suss J. T. , Barak J.
Published: July 2002

The ferromagnetic resonance (FMR) spectra of an yttrium-iron-garnet (YIG) film were studied from room temperature to 120 K above TC (TC = 553 K). The spectra were measured for a magnetic field parallel to the film as well as perpendicular to it. In the ferromagnetic phase the FMR lines represent the magnetostatic modes whose field positions and spacing are determined by the magnetization of the sample. When anisotropy terms are added to the theoretical expressions for magnetostatic waves of Damon and van de Vaart [J. Appl. Phys. 36, 3453 (1965)] for the perpendicular orientation and of Damon and Eshbach [J. Phys. Chem. Solids 19, 308 (1961)] for the parallel one, a good agreement is obtained between the experimental results and theory. Magnetization values extracted from the FMR spectra are also in good agreement with the published magnetization data for YIG, obtained by a vibrating sample magnetometer. At temperatures higher than 413 K the normalized cubic anisotropy K1/M is found to be practically temperature independent, in contrast with previous studies on YIG spheres which conclude that K1/M → 0 when approaching TC. In the paramagnetic phase we observe an anisotropy in the resonance fields due to the sample shape and the induced magnetization. The linewidths ΔH have also been measured and the product [ΔH − ΔH(TC)] × T is found to be practically linearly dependent on T − TC in a wide range of temperatures. ΔH(TC) is the linewidth at TC. This result is in good agreement with recent experimental studies of paramagnetic YIG linewidths and in fair agreement with Huber’s zero-field theory of paramagnetic linewidths near TC of non-Heisenberg ferromagnets, where the ‘‘critical speeding-up’’ effect is absent [J. Phys. Chem. Solids 36, 723 (1975)].
Giant magnetoresistance of Fe/Cr multilayers: Impurity scattering model of the influence of third elements deposited at the interfaces
Authors: Gurney Bruce A. , Baumgart Peter , Wilhoit Dennis R. , Dieny Bernard , Speriosu Virgil S.
Published: July 2002

We describe the least complex model of giant magnetoresistance that maintains the essential features of spin-dependent impurity scattering; it pictures the multilayer as a homogeneous distribution of scattering centers whose scattering properties depend only on the impurity element and local magnetization direction of the layers into which the centers are imbedded. Model predictions are compared with results of sputtered Fe/Cr multilayer films where small amounts of third elements (V, Cr, Mn, Al, Ir, and Ge), with known spin-dependent scattering properties in dilute alloys, are deposited at the interfaces. The model is in agreement with the observed behavior.
New laser action of optically pumped atomic vanadium vapor
Authors: Yoshida H. , Takashima N. , Ninomiya H.
Published: July 2002

Laser action has been observed on the vanadium 560.4-, 575.3-, 581.7-, and 637.9-nm lines. Vanadium vapor is produced from a vanadium metal plate by YAG laser (2.0 J/pulse) irradiation, and optically pumped by an XeCl excimer laser of 25-mJ energy. The vanadium laser pulse has a duration of 50 ns and peak output power of about 7 W.
In situ monitoring and Hall measurements of GaN grown with GaN buffer layers
Authors: Nakamura S. , Mukai T. , Senoh M.
Published: July 2002

High-quality gallium nitride (GaN) film was obtained using a GaN buffer layer on a sapphire substrate. Using low-temperature Hall measurements, we obtained a maximum mobility about 3000 cm2/V s, at around 70 K. This mobility value is the highest reported, to our knowledge, for GaN films. The infrared radiation transmission intensity oscillations, which were caused by interference effects, were observed by means of an infrared radiation thermometer during GaN growth. The growth process of GaN film with GaN buffer layers was almost the same as that of GaN film with AlN buffer layers except when the thickness of GaN buffer layers was small. When the thickness of GaN buffer layers was small, an additional new growth process, in which the surface of GaN film became rough during the growth, was observed. The GaN growth with GaN buffer layers had a tendency to improve the surface morphology even if it became poor due to excess Si doping or low buffer layer thickness.
Numerical study of transient stimulated Brillouin scattering
Authors: Chu Raijun , Kanefsky Morton , Falk Joel
Published: July 2002

Seeded, transient, stimulated Brillouin scattering (SBS) is studied using a new noniterative, numerical algorithm to solve this two-point boundary value problem. The coupled pump, Stokes, and phonon equations are solved numerically, including the effects of pump depletion and a finite phonon lifetime. The noniterative approach leads to efficient computation (<30 s of cpu time, VAX 8000 series computer for 500 by 5000 points in space and time, respectively). The numerical study predicts transient oscillations in the pump and the Stokes intensities. The frequency of these oscillations depends on the phonon lifetime and the depleted, exponential SBS gain. This work is the first known prediction of these oscillations.
New breakdown modes of the multipacting discharge
Authors: Gilardini Aldo L.
Published: July 2002

In this paper the classical Gill–von Engel theory [Proc. R. Soc. London A 192, 446 (1948)], which is used for determining the breakdown voltage of a multipacting discharge, is generalized. Resonance conditions are formulated with reference to the electron two-way transit time, that is, the time required for arriving back at the original electrode after having impacted the opposite one. When this time equals that of an odd number of complete cycles of the applied high-frequency electric field, the Gill–von Engel breakdown modes are found. But breakdown is also possible when the electron two-way transit time equals the time corresponding to an even number of the electric field cycles. In this case the time for crossing of the original electron and that of its secondary are not equal, as in the Gill–von Engel case, but only their sum is important for resonance. The appropriate formulas for evaluating breakdown voltages, cutoff frequencies, and other significant parameters for these new breakdown modes are given, in close analogy to what already is known in the case of the Gill–von Engel modes. Numerical results are also shown for a specific typical case.
New high-contrast developers for poly(methyl methacrylate) resist
Authors: Bernstein Gary H. , Hill Davide A. , Liu Wen-Ping
Published: July 2002

New developers for poly(methyl methacrylate) consisting of mixtures of common developing components have been carefully investigated. It has been found that adding a small percentage of methyl ethyl ketone to methyl isobutyl ketone and Cellosolve results in a significant increase in contrast. Results of contrast experiments as well as improvements in electron-beam lithographic exposures are reported. An explanation of the mechanism of contrast and resolution enhancement is offered.
Pressure-temperature effects in planar Stefan problems with density change
Authors: Charach Ch. , Rubinstein I.
Published: July 2002

An extension of the classical thermal Stefan problem by incorporating the dependence of the phase transition temperature on pressure generated by the flow of the liquid phase due to a density change in the transition process is presented. Two prototypical planar situations are considered. The first is the onset of freezing in an incompressible liquid layer of finite thickness in a gravity field. Asymptotic solutions developed for this problem demonstrate that the initial singularities of the interface velocity and acceleration, typical for the solutions of the classical Stefan problem with an instantaneous temperature drop at the fixed boundary, are regularized by the dynamic pressure effect. The second problem addressed is the freezing or melting of a saturated porous half-space with a flow governed by the Darcy law. Exact similarity solutions (accounting for compressibility of the fluid in the case of freezing) are developed. They indicate that the pressure dependence of the transition temperature may affect significantly the propagation of the phase-change front.
Electron paramagnetic resonance study of gadolinium–gallium–garnet
Authors: Barak J. , Huang M. X. , Bhagat S. M.
Published: July 2002

The electron paramagnetic resonance (EPR) in a single crystal of gadolinium–gallium–garnet has been studied with the aim of understanding the behavior of EPR in systems with large dipolar and small exchange fields, such as encountered in many of the rare-earth based high Tc superconducting compounds. Only one wide line, arising from Gd3+ has been observed. Its shape at room temperature is analyzed using the Kubo–Toyabe [Magnetic Resonance and Relaxation, edited by R. Blinc (North Holland, Amsterdam, 1967), p. 810] stochastic model for the line shape at static modulation. At low T, the anisotropy in the resonance field is a purely shape effect.
Nonresonant third-order optical nonlinearity of all-carbon molecules C60
Authors: Gong Qihuang , Sun Yuxing , Xia Zongju , Zou Y. H. , Gu Zhennan , Zhou Xihuang , Qiang Di
Published: July 2002

The nonresonant third-order hyperpolarizability of all-carbon molecules C60 is measured by nanosecond degenerate four-wave-mixing experiment in C60 toluene solution. The value of γ1111 measured is 1.6×10−31 esu, which corresponds to an estimated value of the third-order nonlinear optical susceptibility χ(3)1111E = 3.3 × 10−9 esu for the solid compound.
Thermal and electrical properties of copper-tin and nickel-tin intermetallics
Authors: Frederikse H. P. R. , Fields R. J. , Feldman A.
Published: July 2002

The thermal diffusivity, electrical resistivity, density, and heat capacity have been measured for Cu6Sn5, Cu3Sn, and Ni3Sn4. These properties were used to evaluate the apparent Lorenz number for comparison with the theoretical value. Acceptable agreement was found.
Chemical vapor deposition and characterization of undoped and nitrogen-doped single crystalline 6H-SiC
Authors: Karmann S. , Suttrop W. , Schöner A. , Schadt M. , Haberstroh C. , Engelbrecht F. , Helbig R. , Pensl G. , Stein R. A. , Leibenzeder S.
Published: July 2002

Homoepitaxial growth of single crystalline 6H-SiC layers is performed by chemical vapor deposition (CVD). 6H-SiC substrates are grown by a sublimation technique. They have vicinal surfaces inclined 1.5° to 2° from the (0001) plane towards the [11̄00] direction. We report CVD growth at 1600 °C in the hydrogen-silane-propane gas system with nitrogen as a dopant. High quality films are achieved with growth rates of about 1.8 μm per hour. The layers are examined by optical microscopy, infrared reflection, photoluminescence, and Rutherford backscattering. For electrical characterization capacitance-voltage and Hall measurements are performed. Unintentionally doped layers have donor concentrations in the upper 1015 cm−3 range. Electron mobilities of 370 cm2/V s at room temperature and about 104 cm2/V s at 45 K are observed. To the authors’ knowledge this is the highest mobility so far reported for 6H silicon carbide.
Effect of electron-hole scattering on the current flow in semiconductors
Authors: Kane D. E. , Swanson R. M.
Published: July 2002

The effect of electron-hole scattering on transport in semiconductors has been considered from first principles. We conclude that the conventional equations for electron and hole currents are theoretically incorrect when electron-hole scattering is present. From thermodynamic considerations, we introduce the more general equations. A Boltzmann transport calculation including electron-hole scattering has been performed for Si. The key result is that the impact of electron-hole scattering depends primarily on the relative velocities of electrons and holes and therefore shows up differently in different device situations. Electron-hole scattering has the largest effect during conduction in high-level injection. This is because electrons and holes have net drift velocities in opposite directions. In contrast, electron-hole collisions have almost no effect on ambipolar diffusion because the carriers are moving in the same direction. Intermediate cases are also covered by this treatment. In low injection, electron-hole scattering serves to reduce the effective minority-carrier mobility. The degree of the reduction depends on whether the majority carriers are nearly static, as in diffusion situations, or whether they are flowing by drift, as in the Haynes–Shockley experiment.
Superconductivity in intermetallic compound La3Co
Authors: Sato Kiyoo , Umehara Izuru , Maezawa Kunihiko , Fukuhara Tadashi , Isikawa Yosikazu , Mori Katsunori
Published: July 2002

The electrical resistivity, the magnetization, and the specific heat were measured. The residual resistivity ρ0 and the residual resistivity ratio ρR.T./ρ0 are 31 μΩ cm and 7.1, respectively. It was found that La3Co shows superconductivity below a transition temperature Tc of 4.4 K. The upper critical field Hc2 and lower critical field Hc1 at 0 K are estimated to be about 21 and 1 kOe, respectively, and using the relation of Hc1/Hc2≂ξ2/λ2, the ratio ξ/λ is estimated to be 0.2. The energy gap 2Δ and the Fermi energy are estimated to be 13×10−4 and 0.14 eV, respectively.
Light-emitting diodes from partially conjugated poly(p-phenylene vinylene)
Authors: Zhang C. , Braun D. , Heeger A. J.
Published: July 2002

We report the effect of conversion conditions on the device characteristics of poly(p-phenylene vinylene) (PPV) light-emitting diodes. Both electroluminescence and photoluminescence intensities decrease with increasing degree of conversion. Partial conjugation enhances the electroluminescence intensity and gives an efficiency (with Ca as electron-injecting contact) as high as 0.75% photons per electron, about two orders of magnitude more efficient than from similar devices prepared from fully converted PPV. The results of constant current stress measurements suggest that the partially conjugated PPV diode is relatively stable at room temperature.
Capacitance-voltage profiling through graded heterojunctions: Theory and experiment
Authors: Sundaram M. , Gossard A. C.
Published: July 2002

Starting from the general equations for capacitance-voltage (CV) profiling through a graded heterojunction, we obtain numerical solutions to yield thermal-equilibrium energy-band diagrams, real electron profiles, and apparent electron profiles (i.e., the profile extracted from a CV measurement) for modulation-doped square, triangular, and parabolic potential wells. Room-temperature CV measurements are performed on parabolic potential wells grown by molecular beam epitaxy in the AlxGa1−xAs system, and the measured apparent electron profiles fitted to computer reconstructions whence the real electron distributions are deduced. These measurements reflect a uniform electron distribution in a parabolic well, with 3D electron density determined by well curvature. Data analysis also suggests the presence of a doping asymmetry in the modulation doping of the well. Appropriate corrections to growth conditions remove these asymmetries, as reflected in CV measurements. Besides its importance in the analysis of potential wells of different shapes, the theory presented is applicable to the determination of band offsets by the CV profiling technique where the unintentional grading of the band gap and/or doping in the neighborhood of the isotype abrupt heterojunction is known.
Phase matching retracing behavior for second harmonic generation in LiB3O5 crystal
Authors: Lin Shujie , Wu Baichang , Xie Fali , Chen Chuangtian
Published: July 2002

The nonmonotonic relation between the increasing wavelength and the noncritical phasematching (NCPM) temperature are analyzed in detail for temperature-tuned as well as the phase matching angle for angle-tuned second harmonic generation in a LiB3O5 (LBO) crystal. This relation was called ‘‘the phase matching retracing behavior.’’ Because of the retrace, LBO is the unique nonlinear optical crystal that can produce the spectra through blue to near infrared by NCPM. It has been shown that the retracing wavelengths (∼1.3 μm), and furthermore, the conjugation relation for phase matched wavelengths are approximately independent of the tuning means. There exist at the retracing wavelengths not only the retrace for the tuning curves but also for the beam walkoff, group velocity mismatch, and the angular and spectral acceptances. The spectral noncritical phase matching is achieved and the group velocity mismatch is zero at the retracing points. The temperature acceptances are also very large. These properties of LBO, combining its high damage threshold, high transparency, nonhygroscopicity, and the availability of the large and high quality crystal, make LBO the best choice in doubling, either by type I or type II, by temperature-tuned or angle-tuned, the radiations (including ultrafast pulses) in the vicinity of 1.3 μm, such as those from some diode lasers, optical parametric oscillator/amplifier, and Nd:YAG/YAP/YLF lasers working at 4F3/2–4I13/2 transition.
Quantitative infrared analysis of the stretching peak of SiO2 films deposited from tetraethoxysilane plasmas
Authors: Goullet A. , Charles C. , Garcia P. , Turban G.
Published: July 2002

Infrared transmission spectra of silicon dioxide (SiO2) thin films (∼4500 Å) prepared by plasma-enhanced chemical-vapor deposition have been quantitatively analyzed. The films were deposited at different substrate temperatures (30–450 °C) using tetraethoxysilane (TEOS)/He, TEOS/He/O2, and TEOS/O2 gas mixtures in a parallel-plate radio-frequency reactor. The infrared transmission fits prove to be very accurate showing evidence of deconvolution into three separated Gaussian profiles to account for the asymmetric line-shape feature of the infrared stretching peak between 950 and 1300 cm−1. The examination of the Fourier transform infrared spectroscopy spectra in the complete frequency range (400–4000 cm−1) and ex situ x-ray photoelectron spectroscopy spectra indicates that some extra structures originate from the incorporation of carbon and hydrogen impurities in the film. As the substrate deposition temperature is increased, impurities are gradually removed from the growing layer. Films deposited at high substrate temperatures reveal a better stoichiometry and present similar deconvolution bands regardless of the gas-phase composition; the corresponding frequencies are shifted to lower energies compared to thermal oxides. In addition, the intensity of the first Gaussian profile, associated with the low-energy asymmetry of the stretching peak, increases with the substrate deposition temperature while the intensity of the third Gaussian profile associated with the presence of the high-energy peak shoulder decreases. The vibrational properties of the film seem to be strongly related to the deposition conditions.
Investigations of magnetic properties and microstructure of 40Cedidymium–Fe–B based magnets
Authors: Zhou S. X. , Wang Y. G. , Ho/ier R.
Published: July 2002

The influence of Si, Co, and Dy2O3 on magnetic properties of sintered 40Cedidymium–Fe–B magnets was investigated and related to the microstructure through transmission electron microscopy (TEM) investigations. It is found that the decrease in iHc and Tc caused by the presence of Ce in the 40Cedidymium could partly be compensated by additives. Sintered magnets with iHc = 9.2 kOe and (BH)max=28.2 MG Oe were successfully achieved. TEM investigations show an intragranular amorphous phase within the R2Fe14B (R=Nd, Pr, or Ce) grains and three new intergranular phases at grain boundaries and junctions of the R2Fe14B grains. It is suggested that the deterioration in the magnetic hardening is closely related to these new phases as well as the low magnetic anisotropy field of the Ce2Fe14B compound. The intragranular phase is likely to act as nucleation positions for reverse magnetic domains.
Bending stresses and bistable behavior in Fe-rich amorphous wire
Authors: Vázquez M. , Gómez Polo C. , Velázquez J. , Hernando A.
Published: July 2002

The aim of this work is to analyze for the first time the changes in magnetic properties of an Fe-rich amorphous wire (Fe77.5Si7.5B15) when it is submitted to bending stresses. Upon a reduction of the radius of curvature, Rc, of the wire (i.e., increasing bending stresses), the main changes in the magnetic properties are summarized as follows: (a) Bistable behavior disappears when reducing Rc below about 11 cm but it is again observed for Rc less than about 2.5 cm. This latter effect is also obtained for short wires (less than around 7 cm) which do not show spontaneous bistability. (b) For the case when bending stresses make bistability disappear, the susceptibility increases more than one order of magnitude with regards to the case of bistable wire, and parallel to the increase of susceptibility, a reduction of remanent magnetization is observed. The disappearance and later occurrence of the bistable behavior with increasing bending stresses are discussed in terms of the tensile and compressive stresses induced when the sample is bent. The possibility of having bistable wires with toroidal symmetry is also discussed owing to its interest for particular applications as pulse generators with reduced size and magnetic switches.
Magneto-optical Kerr effect study of ac susceptibilities in ultrathin cobalt films
Authors: Berger A. , Knappmann S. , Oepen H. P.
Published: July 2002

The ac susceptibility of ultrathin Co/Cu(100) films has been investigated in situ under ultrahigh vacuum conditions using the magneto-optical Kerr effect. A conventional Kerr effect setup was combined with an ac-field modulation technique to improve the experimental detection limit. A sensitivity in the range of 10−8 rad for the Kerr ellipticity has been achieved in the present study. The ac susceptibility χac of the investigated cobalt films is mainly characterized by a pronounced and sharp maximum as a function of the temperature T. This χac(T) dependency corresponds to the ferromagnetic-paramagnetic phase transition occurring at the Curie temperature Tc of the cobalt films, which is also confirmed by experimental observation of the hysteresis behavior. Besides the measurement of Curie temperatures, the experimental technique presented here allows the determination of magnetic properties for ultrathin films even in the paramagnetic regime far above Tc.