bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrical conduction and electronic structures of novel tetrathionaphthalene derivatives
Authors: Mori Y. , Nakanishi H. , Hilti B. , Mayer C. W. , Rihs G.
Published: July 2002

The electrical conduction of pristine (undoped) and iodine doped vacuum deposited films of the novel donors, dibenzodithiotetra-thionaphthalene and dibenzodithiotetraselenonaphthalene were measured. The electronic structures of these donor compounds were examined experimentally by ultraviolet photoelectron spectroscopy and theoretically by semiempirical MO calculations and compared with the corresponding data of tetrathiotetracene.
The effect of hydrogen content on the optoelectronic properties of amorphous silicon-carbide films
Authors: Girginoudi D. , Thanailakis A.
Published: July 2002

The optoelectronic properties of rf sputtered hydrogenated amorphous silicon-carbide thin films were investigated by optical and infrared absorption, photoconductivity, photoluminescence (PL), and electron spin resonance studies as a function of the hydrogen content. The results show that the hydrogen content has a very significant effect on the optical band gap and the photoconductivity of these films. The increased intensity of the PL spectra with an increase in the hydrogen content is very well correlated with the corresponding decrease in the spin density.
Dislocation-accelerated diffusion of Si in delta-doped GaAs grown on silicon substrates by metalorganic chemical vapor deposition
Authors: Kim Yong , Kim Moo Sung , Min Suk-Ki , Lee Choochon
Published: July 2002

Capacitance-voltage (C-V) profiling of delta-doped GaAs layers grown by metalorganic chemical vapor deposition on Si substrates has been employed to demonstrate a dislocation-accelerated diffusion of Si impurities initially confined in the delta-doped sheets. A close correlation between dislocation densities in the epitaxial layers and the diffusion coefficients obtained from C-V analyses is established. After rapid thermal annealing at 800, 900, 950, and 1000 °C for 7 s, the temperature dependence of the diffusion coefficient is found to be D-30 exp(−3.4 eV/kT) for a delta-doped GaAs-on-Si with a relatively thick buffer layer of 3.3 μm. The result shows that the dislocation-accelerated diffusion of Si impurities is considerable and the inclusion of a thick buffer layer (−3 μm) is not sufficient for preventing the diffusion of the impurities into a device-active region near the GaAs surface if high temperature (>800 °C) processing is involved.
Stoichiometry-dependent deep levels in p-GaAs prepared by annealing under excess arsenic vapor pressure
Authors: Nishizawa Jun-ichi , Oyama Yutaka , Dezaki Kazushi
Published: July 2002

In order to obtain the precise energy spectrum of stoichiometry-dependent deep levels, we apply the photocapacitance (PHCAP) method under constant capacitance conditions on the various p-GaAs crystals prepared by 67 h annealing at 900 °C under excess arsenic vapor pressure followed by rapid cooling. The present PHCAP investigation reveals the deep acceptor levels at 0.48, 0.71, 0.92, and 1.02 eV above the valence band, respectively, and shows that these deep acceptor levels could be detected commonly in various kinds of p-GaAs crystals with regardless of the sort of doped acceptor impurities and the concentrations. In addition, the dependencies of the level density mentioned above are also studied as a function of the arsenic vapor pressure being applied during annealing and of the carrier concentration. These dependencies of the level density are specified into three categories in conjunction with the carrier concentration and with the dependencies of the level density on the arsenic vapor pressure. From the experimental results of the present PHCAP investigation on p-GaAs and of the previously reported results on n-GaAs, it is shown that the excess arsenic atoms in GaAs crystal play a major role on the formation of stoichiometry-dependent deep levels in p-type GaAs crystals.
A passive method to trap an electron beam in a stellatron accelerator
Authors: Deniz A. V.
Published: July 2002

An electron beam can be trapped in a stellatron accelerator with a passive technique by dissipating the energy associated with the poloidal displacement of the beam from its equilibrium position. If the net force on the beam is directed toward the minor center of the vacuum chamber, the beam will spiral toward its equilibrium position. The beam dissipates energy by charging and discharging a conductor that is grounded through a resistor. Numerical solutions in the paraxial approximation illustrate this capture effect, with parameters that show a large enough effect to trap a beam.
Determination of electro-optical coefficients of 2-Methyl-4-Nitroaniline
Authors: Ho Edmond S. S. , Iizuka Keigo , Freundorfer Alois P. , Wah Christopher K. L.
Published: July 2002

The electro-optical constants of 2-Methyl-4-Nitroaniline crystals are measured at a wavelength of 632.8 nm. The results confirm the measurements made by Lipscomb, Garito, and R. S. Narang [J. Chem. Phys. 75, 1509 (1981)] for the first time and settle the descrepency between Tokura, Kurita, and Koda experiment [Phys. Rev. B 31, 2588 (1985)] and Lipscomb, Garito, and R. S. Narang.
Unstable breakup of a linearly accelerated, dense plasma
Authors: Davidson P. A.
Published: July 2002

A dense plasma, accelerated by magnetic pressure, is used in an electromagnetic launcher to propel small masses. Such plasmas have been observed to disperse or fragment, and this has been related to a loss of projectile acceleration. We are concerned here with the potential mechanisms of plasma breakup, and with the associated limitations on projectile velocity. An unsteady, one-dimensional model of the plasma is described, which incorporates a simple correction for the effect of wall ablation. Two limiting cases are examined, one where ablation is small, and another where it is large. For the first of these cases, we show that a reduction in magnetic pressure will induce a decelerating body force at the tail of the plasma. It is shown that, if this force is generated on a time scale comparable with the dynamic relaxation time of the plasma, it is accompanied by a substantial reduction in plasma density. To the extent that this results in a loss of plasma conductivity, such a force reversal could lead to the formation of multiple current paths. For the second case, we show that a high level of ablation results in the formation of a parasitic current sheet at the tail of the plasma. Again, this is accompanied by a local deceleration of the tail. It is a matter of debate as to whether or not this leads to fragmentation of the plasma. However, irrespective of whether breakup occurs, the formation of such a current sheet imposes an upper limit on the achievable projectile velocity.
Influence of Fermi-level pinning on barrier height inhomogeneity in PtSi/p-Si Schottky contacts
Authors: Tanabe A. , Konuma K. , Teranishi N. , Tohyama S. , Masubuchi K.
Published: July 2002

The barrier height inhomogeneity in PtSi/p-Si and IrSi/p-Si was observed by internal photoemission. New Fowler equations were introduced, to analyze the observed properties. Two regions with different barrier heights were assumed to coexist, and the individual barrier heights were evaluated. One of two barrier heights was consistent with the generally obtained value in individual contacts. The other was 0.39 eV in both contacts. The origin of two regions was explained in terms of Fermi-level pinning.
The study of structural characteristics of YBaCuO superconducting thin films on (001) Zr(Y)O2 by x-ray diffraction
Authors: Guien Zhou , Jiang Lu , Yunbo Jia , Yunlan Huang , Wangzhou Shi , Jianzhong Shi , Weiguo Yao , Zhenzhong Qi , Yuheng Zhang
Published: July 2002

Three kinds of x-ray diffraction experiment of YBa2Cu3O7−x thin film on Zr(Y)O2 have been carried out. It has been proved that YBaCuO superconducting thin film grow epitaxially on (001) Zr(Y)O2. The orientation relationships between YBaCuO thin film and Zr(Y)O2 substrate are (001) YBaCuO∥(001) ZrO2 and [100] YBaCu∥[110] Zr(Y)O2.
Thermal diffusivity of α-mercuric iodide
Authors: Burger A. , Morgan S. H. , Henderson D. O. , Silberman E. , Nason D.
Published: July 2002

The thermal diffusivity and its anisotropy have been measured for α-HgI2 single crystals using the flash pulse method. The results show that α[100]=3.70×10−3 cm2s−1±3% and α[001] =1.03×10−3 cm2s−1±10%. The analysis contains the necessary corrections for heat losses and finite pulse duration.
Growth and characterization of GaAs on Si by vacuum chemical epitaxy
Authors: Cotta M. A. , de Carvalho M. M. G.
Published: July 2002

Undoped GaAs epitaxial layers have been grown on Si substrates by vacuum chemical epitaxy. Triethylgallium and arsine were used as gallium and arsenic sources, respectively. The layers have shown KOH etch pit densities in the range 7×106–2×107 cm−2 and residual hole concentration of 1015 cm−3. The layer crystallinity was found to be strongly influenced by the substrate preparation and also by the water partial vapor pressure in the growth chamber.
Structure of interfaces in a-Si:H/a-SiNx:H superlattices
Authors: Santos P. V. , Hundhausen M. , Ley L. , Viczian C.
Published: July 2002

We present experimental results on the atomic structure of the interfaces between a-Si:H and a-SiNx:H layers obtained by analyzing the intensity of the Raman lines from zone-folded acoustic phonons and of the peaks of x-ray diffraction at grazing angles. We determine the width of these interfaces and their stability under thermal annealing in temperatures below the crystallization temperature.
Associative ionization and dissociative recombination in mercury vapor
Authors: Majetich Sara , Tomczyk Carol A. , Wiesenfeld John R.
Published: July 2002

We report absolute cross sections for Hg(63P1)+Hg(63P0) associative ionization and Hg+2Hg+2+ e− dissociative recombination in mercury vapor, following saturation of the 253.7-nm transition using pulsed laser excitation. The quantitative time dependencies of all relevant species were determined by combining this information with measurements of the total Hg(63P1) density and the total electron current following the laser pulse. The electron temperature associated with our measurement of kDR is 298 K. The measured values of kAIkAI= (2.5 ± 0.6) × 10−8 cm3 s−1 and kDR = (4.4 ± 1.1) × 10−6cm3 s−1 are 3.5 times smaller and 10.5 times larger, respectively, than those previously reported in cw and discharge flow studies.
High-speed photodetectors on InGaAs/GaAs-on-GaAs superlattices
Authors: Zirngibl M. , Ilegems M.
Published: July 2002

We investigate a new type of metal-semiconductor-metal photodetector (MSM) on an InxGa1−xAs/GaAs strained superlattice whose performances compare to state of the art results achieved with MSMs on lattice matched In0.53Ga0.47As-on-InP material. The superlattices consist of 120 periods of 6-nm GaAs and 6–8 nm InxGa1−xAs with InAs content x between 48% and 66%. The structures were grown by molecular-beam epitaxy on GaAs substrates. Smooth morphologies and defect densities below 500 cm−2 were achieved with a growth temperature of 450–470 °C and with a minimum As to Ga pressure ratio. The cutoff wavelength and the dark current of the detectors increase with InAs content. The dark currents range between 100 nA and 3 μA at 10 V for 2500 μm2 devices and the spectral responses show cutoffs between 1.3 and 1.6 μm. The response time to short laser pulse excitation at 1.3 μm is below 30 ps full width at half maximum, and is limited by the time resolution of the measuring system. The internal quantum efficiency under pulsed excitation achieves maximum values near 60%. The very fast response time and relatively low quantum efficiency are indicative of fast carrier recombination processes in the active region of the detectors.
Frequency dependence of drift mobility in a-Si:H measured by traveling-wave method
Authors: Kaneko Y. , Fritzsche H.
Published: July 2002

The drift mobility was measured in phosphorus-doped hydrogenated amorphous silicon films using the traveling-wave technique as a function of surface acoustic Rayleigh wave frequency and temperature. In the high-temperature range in which the Fermi energy lies between the demarcation energy Ed = kBTln(v0/v) and the mobility edge, the drift mobility was found to be independent of the traveling wave frequency v in agreement with the multiple trapping theory. The frequency dependence of the acousto-electric voltage agrees with the predictions of the theory describing the interaction of the charge carriers with the traveling electric wave.
Very high critical current in a long YBa2Cu3O7−δ single crystalline rod
Authors: Aguillon C. , McCartney D. G. , Regnier P. , Senoussi S. , Tatlock G. J.
Published: July 2002

We studied by magnetic and microscopic techniques a disk (2.0 mm diam and 1.0 mm thick) sliced from a quasi single-crystalline rod prepared by very slow solidification in a small temperature gradient. The sample presents features very similar to a typical millimetric single crystal obtained by the standard flux methods. We deduced a high critical current density Jc from magnetic measurements: 2×106 A/cm2 at 4.2 K in a 3T field parallel to the c axis. The transport critical current density along the rod axis (i.e., ∼45° off the c axis) was found to be about 7000 A/cm2 at 77 K in zero applied field. It is comparable to that deduced from the hysteresis cycle at the same temperature for the same current direction.
Characterization of epitaxial thin GaP films on GaAs by Raman scattering
Authors: Mizoguchi Kohji , Nakashima Shin-ichi , Takamori Akira
Published: July 2002

Accommodation of misfit in the earlier stage of heteroepitaxial growth has been studied for GaP films on GaAs. Raman spectroscopy has been applied to the evaluation of the misfit strain and the distribution of dislocations in GaP films thinner than 1000 Å. The crystallographic orientation of the films has been also evaluated. The misfit strain varies with thickness in the range of 20–800 Å and is partially relaxed even at a thickness of 20 Å. The variation of the misfit strain with thickness has been examined with several models of dislocation nucleation. The analysis has shown that the misfit is accommodated by elastic strains and dislocation introduced in the region near the interface.
Defect-free growth of AlxGa1−xAs by liquid-phase epitaxy on V-grooved (001) GaAs substrates
Authors: Rechenberg I. , Stoeff S. , Krahl M. , Bimberg D. , Höpner A.
Published: July 2002

The crystalline quality of AlxGa1−xAs grown on a V-grooved (001) GaAs substrate is characterized by low-temperature cathodoluminescence and transmission electron microscopy. High-crystalline perfection visualized by narrow neutral donor bound exciton luminescence is attained above the groove. Growth of (Al, Ga) As in the groove takes place without significant change of the aluminium distribution. An order of magnitude increase of silicon acceptor concentration induced by As-vacancy creation is detected at the interface n-GaAs substrate/V groove. Transmission electron microscopy pictures taken from sample cross sections do not show any kind of defects in the groove.
Ion beam controllable modification of YBa2Cu3O7−x superconducting thin films
Authors: Li Yijie , Ren Congxin , Chen Guoliang , Chen Jianmin , Zou Shichang
Published: July 2002

The superconducting transport properties of high-quality epitaxial YBa2Cu3O7−x thin films have been controllably modified using Ar+ ion implantation. It was found that both the critical current density Jc and the superconducting transition temperature Tc significantly decreased with fluence. However, when Jc (at 77 K, H=0) decreased by six orders of magnitude, Tc only decreased from 89 to 77.5 K. A fluence of 5×1012 Ar/cm2 was sufficient to reduce Jc near zero, but Tc still remained above 77 K. We have observed the structure and the morphology of the sample using x-ray diffraction and high-resolution transmission electron microscopy. A film irradiated with Ar+ ions became semiconductor at a bombardment fluence of about 1.2×1013 Ar/cm2, while its superconducting state completely disappeared for an Ar+ dose of about 2.2×1013 Ar/cm2. An Ar+ fluence of 5×1014 Ar/cm2 made the sample amorphous with so high resistivity that it could be treated as insulator. The experimental results showed the possibility of fabricating Josephson junction devices from YBa2Cu3O7−x superconducting thin films and patterning this kind of films by ion implantation technique.
Electromagnetic fields for an irregularly shaped, near-spherical particle illuminated by a focused laser beam
Authors: Barton J. P. , Alexander D. R.
Published: July 2002

A theoretical procedure is derived for the determination of the internal and external electromagnetic fields for a monochromatic beam incident upon a homogeneous particle of arbitrarily-defined shape. The procedure is best suited for the analysis of near-spherical particles (i.e., particles which deviate from a sphere by plus or minus a few tenths of a radii). Verification and demonstration calculations are presented. Also presented are calculations investigating the effect of a surface deformation on resonance excitation within an otherwise spherical particle.
Light-ion bombarded p-type In0.53Ga0.47As photoconductive detectors
Authors: Rao Mulpuri V. , Hong W-P. , Chang G-K. , Papanicolaou N. , Dietrich Harry B.
Published: July 2002

The effect of H+ and B+ bombardment on the dark current and the impulse response speed of p-type InGaAs photoconductive detectors was studied. The dark current of the detectors decreased with light ion bombardment before the material turned n-type. The lowest dark current of H+ bombarded detectors was 10 μA at 6 V bias. The impulse response speed improved with H+ bombardment but did not change with B+ bombardment, as long as the material remained p-type. However the decay time of the detector response increased when the material turned n-type. The photoresponsivity in H+ bombarded detectors at 1.3 μm was 1 A/W, the bandwidth was 1.8 GHz, and the dark noise power into a 50 Ω load was −110 dBm with a source-drain bias of 6 V. The test structures had 5 μm contact spacing.
Deep-level transient spectroscopy characterization of silicon-silicon interfaces
Authors: Stievenard D. , Wallart X. , Mathiot D.
Published: July 2002

Using the capacitance-voltage technique and mainly deep-level transient spectroscopy (DLTS), we studied silicon-silicon epitaxial layer structures grown by the limited reaction process using silane diluted in hydrogen. We develop a simple but original model to interpret the DLTS data. The use of the two techniques allows the determination of the physical parameters of the structure, i.e., the doping level, the thickness of the layers, and the density and capture cross section of the states localized at the different interfaces of the structure.
Rapid thermal processing of WSix contacts to InP in low-pressure N2:H2 and tertiarybutylphosphine ambients
Authors: Katz A. , Feingold A. , Nakahara S. , Pearton S. J. , Geva M. , Lane E. , Jones K. S.
Published: July 2002

WSix thin films deposited on InP substrates have been investigated for possible use as refractory ohmic contact materials for self-aligned laser devices. The films have been rf diode sputtered using various Ar gas pressures from a single commercial target composed of W and Si with an atomic ratio of 1:1. Following the deposition, the WSix/InP samples were rapid thermal processed using a rapid thermal metalorganic chemical vapor deposition system in a controlled low-pressure ambient of N2:H2 (9:1) and tertiarybutylphosphine. The as-deposited films (∼100 nm thick) were amorphous but crystallized in the temperature range of 600–650 °C. The WSi2 phase forms first at 600 °C and then the W5Si3 nucleate with further heating at 650 °C. As a result of the crystallization, a reduction in the specific contact resistance to a value of 7.5×10−6 Ω cm2 and a decrease in the sheet resistance to values lower than 2 Ω/⧠ were observed. In addition, a significant reduction in the internal stress and an improvement in the WSix-to-InP adhesion were found, demonstrating the potential of WSix as a contact material for InP-based self-aligned devices.
Study of superconducting proximity effect by measuring interference effect of Andreev-reflected quasiparticles
Authors: Hatano Mutsuko , Nishino Toshikazu , Hasegawa Haruhiro , Murai Fumio , Kure Tokuo , Nakane Hideaki
Published: July 2002

The change in pair potential for the Al-Nb proximity system is reconstructed by measuring the dependence of differential resistance on incident energy using a lithographic point contact on the Al-Nb bilayer. The bound states due to the interference effect of Andreev-reflected quasiparticles, shift to higher-energy levels. This can be explained by the position dependence of the pair potential near the Al-Nb interface.
The concept of effective film thickness for the determination of bond concentrations from IR spectra of weakly absorbing thin films on silicon
Authors: Brendel R.
Published: July 2002

Thin-film bond concentrations can be determined from infrared transmission and reflection spectra. Simply using the Lambert–Beer law can lead to errors of 80%. The concept of effective film thickness presented here avoids these errors and still allows a quick determination of the absorption coefficient without using a computer. It fully takes into account the effects of multiple reflections. For weakly absorbing thin films on silicon the effective film thickness was calculated numerically (i) for normal incidence transmission spectra and (ii) for 20° reflection spectra with a mirror behind the sample. The latter method is of general interest for the investigation of weak absorptions as it avoids the disturbing interference oscillations and its sensitivity is about twice that of a transmission measurement. The numerical results are verified experimentally for Si-H and N-H vibrations in plasma silicon nitride and analytically for the thin-film limit.
Simulation and observation of the images of dislocations in (100) silicon using infrared piezobirefringence
Authors: Dutta Aloke K. , Ajmera Pratul K.
Published: July 2002

A general algorithm for the simulation of dislocation images in cubic single-crystal infrared transparent semiconductor samples using infrared piezobirefringence is presented in this paper. A data bank has been created for the simulated images of arbitrarily oriented dislocations. These images are functions of the sample orientation, the orientation of the dislocation line with respect to the crystal 〈100〉 axes, the orientation of the Burgers vector with respect to the dislocation line, and the polarization angle of the incident light. A dark-field plane polariscope has been constructed to observe the dislocation images experimentally. In principle, a sample having any arbitrary dislocation can be placed under the polariscope and the experimental images obtained for varying polarization angles of the incident light. Then an image matching procedure between the experimentally observed and the simulated images stored in the data bank can yield the information about the dislocations present in the sample. This technique offers a superior speed advantage over the more conventional defect characterization technique of x-ray topography. Some experimental results and their match with the theoretical ones are also presented.
Implosions of NaF plasmas for photopumping a Na-Ne x-ray laser
Authors: Young F. C. , Hinshelwood D. D. , Apruzese J. P. , Deeney C. , Nash T. , Prasad R. R.
Published: July 2002

Intense heliumlike sodium 11-Å line radiation is required to resonantly photopump a neon plasma in the Na-Ne soft x-ray laser scheme. The implosion of a NaF capillary-discharge plasma with a 3-MA peak current is used to produce a power exceeding 100 GW in this Heα line. The power is optimized by varying both the initial radius of the 3-cm-long NaF plasma column and the time delay between the capillary current and the generator current. Maximum power of 115 GW is obtained for implosions which occur just after peak current. Burn-through of the heliumlike sodium stage is evident in spectroscopic measurements where sodium Lyα line emission is 2–4 times stronger than Heα emission. Mass loadings of 200–400 μg/cm are inferred from measured implosion times and initial plasma diameters. The nearly pure density dependence of the Heα power and the nearly pure temperature dependence of the Lyα/Heα ratio allow the state of the plasma to be determined by measuring both quantities on a single shot. For these implosions, electron temperatures are 350–560 eV and total ion densities approach 1020 cm−3. The mass load inferred from implosion dynamics is consistent with the ion density deduced from spectral measurements.
Oxidation kinetics of YBa2Cu3O7−x thin films in the presence of atomic oxygen and molecular oxygen by in-situ resistivity measurements
Authors: Yamamoto K. , Lairson B. M. , Bravman J. C. , Geballe T. H.
Published: July 2002

The kinetics of oxidation in YBa2Cu3O7−x thin films in the presence of molecular and atomic oxygen ambients have been studied. The resistivity of c-axis-, a-axis-, and mixed a+c-axis-oriented films, deposited in situ by off-axis magnetron sputtering, was measured as a function of time subsequent to a change in the ambient conditions. Atomic oxygen was produced by a well-characterized, compact electron cyclotron resonance source that is compatible with molecular-beam epitaxy deposition. The resistivity of a film in the presence of atomic oxygen is shown to be determined by the flux of atomic oxygen on the film surface. The activation energy for the decomposition of O2 on the surface of the film was determined to be ∼1.3 and ∼2.1 eV for mixed a+c-axis- and c-axis-oriented films, respectively. The oxidation process is shown to be thermally activated and can be characterized by a diffusion model with an activation energy which varies from approximately 1.2 eV in the presence of molecular oxygen to 0.6 eV for a flux of 2×1015 oxygen atoms/cm2 s. In both cases, diffusivity is found to be insensitive to oxygen stoichiometry, but the rate of oxidation is found to be sensitive to the microstructure and orientation of the films. The lower activation energy observed for the atomic oxygen case is postulated to be due to the large oxygen concentration gradient present during the oxidation.
Electric and magnetic properties of chemical-vapor-deposition carbon films
Authors: Araki Hisashi , Matsuoka Ryuichi , Yoshino Katsumi , Fukuda Masahiko , Mizogami Shigeyoshi
Published: July 2002

Carbonaceous films with metallic luster have been prepared by pyrolytic chemical vapor deposition on the inside surface of a quartz tube at 1000 °C using hydrocarbon including oxygen atoms such as phenols as raw material. The x-ray diffraction, electrical conductivity, thermoelectric power, and electron-spin-resonance (ESR) characteristics of the films heat treated to between 1000 and 2800 °C have been examined. These results indicate complete graphitization of the films by the heat treatment. The three-dimensional ordering of crystallites in the films and increase in the electrical conductivity have proceeded in the range 2000–2200 °C. The concomitant fall in the thermoelectric power in the same range suggests the contribution of two carriers (holes and electrons). The increasing rate of g shift has been also enhanced in this range. The unusual broadening in the ESR linewidth has occurred around the temperature where a localized center appears and seems to be caused by scattering of mobile charge carriers at the localized centers.
Semiconducting characteristics of phthalocyanine solid films formed by an electrochemical method
Authors: Harima Yutaka , Yamashita Kazuo
Published: July 2002

Semiconducting properties of phthalocyanine (Pc) solid films prepared on an indium-tin oxide substrate by an electrolytic micelle disruption (EMD) method are investigated in the electrochemical system with and without illumination. Rectifying behaviors of the Pc solid ranging in film thickness from 0.18 to 0.59 μm are featured through low series resistance of 10 to 60 Ω cm2 and through the increase in exchange current density by increasing Pc layer thicknesses. Both characteristics are interpreted in terms of the enhancement of an effective surface area due to the porous structure of the Pc solids to allow electrolyte permeation. The electrolyte permeability and the resulting low series resistance elucidate some unusual aspects of photoelectrochemical responses of the ITO/Pc(EMD) electrodes in I2/KI and hydroquinone/benzoquinone redox solutions. Extremely high photoactivities of the Pc(EMD) solids are also discussed on the bases of these features of a structural origin.
Second harmonic generation in the binary system of π-conjugated compounds
Authors: Wakita Katsuya , Sonoda Nobuo , Shimizu Tokihiko , Kaida Satoshi
Published: July 2002

A large second harmonic generation (SHG) intensity, which was 60 times as large as that of urea was observed in a p-nitroaniline(PNA)/N-(p-nitrophenyl)ethylenediamine (NPEN) system. This SHG intensity was related to the cooling rate for solidification. The optimum cooling rate for SHG intensity was in the range of 2–10 °C/s. X-ray and thermal analysis done for this new system show that the hydrogen bond and the unique nonplanar structure of NPEN play an important role for SHG and its stability.
Antiguiding in narrow stripe gain-guided InGaAs-GaAs strained-layer lasers
Authors: Beernink K. J. , Alwan J. J. , Coleman J. J.
Published: July 2002

We present data on strong carrier-induced antiguiding present in strained-layer InGaAs-GaAs-AlGaAs lasers. We calculate a value of 12.9 for the Petermann K factor, and a corresponding antiguiding factor of b=6.4. The magnitude of the carrier-induced index depression in the well is estimated to range with drive current from 0.032 to 0.41. In all cases, the antiguiding is much stronger than reported for similar GaAs-AlGaAs lasers. In addition, we show that emission from short cavity devices occurs from two distinct transitions, with emission from the GaAs barriers contributing a single peak to the far field, and emission from the InGaAs well contributing two lobes. The result is a three-lobed far-field pattern at laser threshold of the GaAs barrier emission.
Low-temperature operating life of continuous 300-K AlxGa1−xAs-GaAs quantum-well heterostructure lasers grown on Si
Authors: Hall D. C. , Holonyak N. , Deppe D. G. , Ries M. J. , Matyi R. J. , Shichijo H. , Epler J. E.
Published: July 2002

Data are presented on the continuous (cw) 77–200 K operational characteristics of cw 300-K AlxGa1−xAs-GaAs quantum-well heterostructure diode lasers grown on Si substrates. Operation is demonstrated for over 500 h with a junction temperature as high as ∼200 K for a diode previously operated cw 300 K for over 10 h with its junction side mounted away from the heat sink. The data indicate that longer cw 300-K lifetimes than previously demonstrated (17 h) may be possible. The effects of the optical power level on the degradation rate are examined, and it is shown that the maximum cw 300-K power output for these devices (∼30 mW/facet) is limited by catastrophic facet degradation. The effects of naturally occurring microcracks on device stability are also considered, and the effect of stress on the output polarization is measured and discussed.
Optimal current distribution for energy storage in superconducting magnets
Authors: Salingaros N. A.
Published: July 2002

The magnetic energy stored in a cylinder or torus is optimized by adopting a special current distribution with helical windings. A cylindrical cable wound following a Bessel-function distribution that reverses on the surface has greatly reduced self-force and self-stresses. This result applies to energy storage in large-scale superconducting magnets, and is also of importance in plasma physics and astrophysics.
Effect of intracavity spatial filtering on the beam characteristics of a copper vapor laser
Authors: Singh B. , Dixit S. K. , Mittal J. K. , Nakhe S. V. , Bhatnagar R.
Published: July 2002

Intracavity spatial filtering in a copper vapor laser is demonstrated to achieve low divergence output beam. The output beam characteristics were monitored. The average beam divergence of 0.45 mrad (∼two times the diffraction limit) was achieved. Temporal pulse width of 3-ns full width at half maximum was obtained.
The effect of radiative cascade on electron excitation temperature measurements
Authors: Marcum S. D. , Myers J. L. , Gieske M. A. , Tackett M. , Ganguly B. N.
Published: July 2002

Electron excitation temperatures have been measured in a low-pressure (0.05-Torr Cs, 2-Torr total pressure) argon-cesium discharge that uses a heated cathode (900–1100 K). The excitation temperature determinations are based upon a model that allows calculation of cesium excited state densities for low electron density (<1011 cm−3). The model assumes that the dominant creation processes for excited states are electron impact excitation from the ground state and radiative cascade from higher levels, while destruction is by spontaneous emission. Maxwellian electron energy distributions were used and the plasmas were considered to be optically thin. The model indicates that cascade contributions to the production of excited states can be as high as 50% for some cesium levels. Predicted emission spectra with cascade contributions to spontaneous emission intensities agree well with measured spectra except for radiation trapped transitions from low nP states to the ground state. Excitation temperatures are determined by fitting measured and calculated spectra.
Electrical characterization of silicon epitaxial layers grown by limited reaction processing
Authors: Mathiot D. , Regolini J. L. , Dutartre D.
Published: July 2002

The capacitance-voltage technique is used to perform a comparison between various silicon epitaxial layer structures grown by the limited reaction process using silane, disilane, or dichlorosilane, diluted in hydrogen. Accurate analysis of the apparent doping profiles allows the determination of the relevant physical parameters, i.e., the residual doping level and the density of trapped charges at the interfaces between the layers. From the electrical point of view the best result is obtained for growth with disilane.
Effect of charge-state fluctuations of ions moving in solids on high-energy ion implantation
Authors: Bausells J.
Published: July 2002

High-energy implantation involves ion velocities such that ions are partially stripped of their electronic charge when moving inside the solid target. Most ion range distribution calculations so far have been using mean charge states as a function of the ion velocity, in order to determine the ion electronic stopping. However, it is known that fluctuations in the ion charge state lead to an increased energy-loss straggling of the ion beam. In this work the effect of charge-state fluctuations on ion range distribution calculations is studied. We show that the ion equilibrium charge-state distributions can be used to account for charge-state fluctuations inside a Monte-Carlo formalism, and that this effect can be important in order to understand implanted ion range distributions in the MeV energy range.
An in situ ellipsometry study of amorphous silicon/amorphous germanium multilayers
Authors: Chu V. , Fang M. , Drevillon B.
Published: July 2002

We present an in-situ ellipsometric study of hydrogenated amorphous silicon(a-Si:H)/hydrogenated amorphous germanium (a-Ge:H) multilayer structures. We deposit multilayers structures with the thickness of the Si layer equal to that of the Ge layer and with thicknesses of 100 and 500 Å. We find that whereas the a-Ge:H grown on top of a-Si:H follows a uniform growth model, the initial stage of growth of a-Si:H on a-Ge:H must be modelled with a hemispherical nucleation model. These growth mechanisms are observed for both the case of the 100 Å layers and the 500 Å layers. We also find that there are no long-term effects of the deposition on the optical properties of the layers. This is observable visually in the cases where the thickness of the layers allow the real-time trajectories to reach a saturated value and thus guaranteeing the same starting ellipsometric point for the next layer. This assures us that each material at the beginning of the superlattice growth is the same as that grown at the end. The implication of the present analysis on quantum size effects of superlattices is discussed.
Analytical electron microscopy of Ti-doped Nd-TM-B magnets
Authors: Kitano Y. , Shimomura J. , Shimotomai M.
Published: July 2002

In order to clarify the role of Ti addition in improving the coercivity of Nd-TM-B magnets, the microstructure was studied extensively using an analytical transmission electron microscope. Electron micrographs revealed several phases in the Ti-doped Nd15(Fe0.585Co0.3Ni0.1Ti0.015)77B8 magnet, namely Nd2(Fe0.6Co0.3Ni0.1)14B, Nd(Fe0.8−xCoxNi0.2)4B (x=0.25–0.4), Nd-TM, and TiB2 phases. Some of the TiB2 precipitates are formed in the Nd-TM phases during the solidification of the magnet alloy, and the others precipitate during the sintering process. Fine TiB2 precipitates restrain grain growth of the magnetic phase during sintering, which leads to a fine-grained structure. It is suggested that the coercivity of Ti-doped Nd-TM-B magnets increases due to the presence of TiB2 precipitates.
NMR study of Nd2(Fe1−xCox)14B for low Co content (abstract)
Authors: Wojcik M. , Budnick J. I. , Panissod P. , Pedziwiatr A. T. , Wallace W. E.
Published: July 2002

In order to improve the understanding of Co site preference and 59Co hyperfine fields in Nd2Fe14B, a NMR study has been carried out on Nd2(Fe1−xCox)14B for low Co contents, 0.0015≤x≤0.15. Using a high sensitivity NMR spectrometer, good quality 59Co spectra for frequencies from 100 to 300 MHz have been recorded at 4.2 K, even for very low Co concentration −0.15%. The identical spectrum shapes for the concentrations 0.0015 and 0.015 show that these spectra are free from the satellite lines due to local environment effects and are representative of the isolated Co atoms in the Nd2Fe14B lattice. A high frequency triplet structure in these spectra, with the maxima of intensity at 249, 260, and 273 MHz, has been identified as arising from a single crystallographic position. With increasing Co content the triplet progressively evolves towards a doublet and shifts rapidly to lower frequencies: 235, 247 MHz for x=0.15. This doublet has been traced through the whole Co concentration range down to pure Nd2Co14B,1 and has been assigned to 16k2 sites based on an NMR spin reorientation study.2 Independent evidence for single site origin of the triplet structure is based on NMR line shape simulation in the special case of an NMR signal, with strong hyperfine field anisotropy, originating from conical domain walls in the canted magnetic structure of Nd2(FeCo)14B. The 16k2 structure, for the lowest Co concentration, contains 60% of the overall spectrum intensity and slowly decreases to around 50% for x=0.15. the rest of the spectral intensity is divided amongst four other lines located at the frequencies 156, 190, 202, and 213 MHz. The ratio of the respective intensities of these lines is close to 1:4:2:1, indicating that they originate from the 4e, 16k1, 8j, and 4c sites, respectively. The results obtained confirm that Co enters preferentially the 16k2 sites and avoids one of 8j sites in Nd2Fe14B structure.
On the magnetic phase diagram of PbSnMnTe; carrier-induced magnetism (abstract)
Authors: Swagten H. J. M. , Story T. , van Kempen R. J. T. , Willekens M. M. H. , de Jonge W. J. M.
Published: July 2002

PbySn1−x−yMnxTe in various compositions, including Sn1−xMnxTe, displays an intriguing phase diagram which includes the carrier concentration p as an important parameter. A ferromagnetic state was found to exist only above a critical concentration pc≊3×1020 cm−3 at which a heavy hole band will be filled and starts to contribute to the RKKY interaction between the Mn ions.1 The present contribution deals with the magnetic phase diagram for low Mn-concentrations (<3%) as a function of the carrier density. The low temperature magnetic properties of PbSn(Mn)Te and Sn(Mn)Te were studied by means of magnetization, susceptibility and specific heat. In analogy with the existing results for other compositions, a ferromagnetic state is observed (Tc≊Θ≊2–3 K), for p exceeding pc. However, for extremely high carrier densities, p≳1021 cm−3, the data show a substantial deviation from what is commonly observed. This is characterized by a significant drop of Tc well below Θ and a rather broad maximum in the specific heat. Tentatively this transformation at higher p might be attributed to a ferromagnetic (F) to spin-glass-like transition, which, qualitatively, is in harmony with the competitive balance between ferro- and antiferromagnetic (AF) exchange characteristic for the simple RKKY interaction: an increasing wave number (∼p1/3) will increase the number of antiferromagnetically long-range coupled ions, at the expense of the strong ferromagnetic coupling between Mn2+ at shorter distances. Moreover, the mean distance between the Mn ions for x=0.02 is such that this delicate interplay between F and AF coupling should be observable.
Mössbauer and magnetic studies of Eu porphyrins (abstract)
Authors: Oliver F. W. , Thomas C. , Hoffman E. , Hambright P. , Sutter T. P. , Thorpe A. N.
Published: July 2002

We report the first measurements on porphyrin compounds using Eu as the Mössbauer nuclide. There have been many Mössbauer studies on iron porphyrins1,2 and a few on tin.3 We have successfully prepared and made Mössbauer and magnetic susceptibility measurements on four Eu based porphyrin compounds to elucidate the relationship between magnetic properties and structure. Absorption spectra using the 21.6 keV gamma ray of 151Eu were obtained at room temperature and 80 K and yielded very broad singlets with room temperature isomer shift values between 0.20±0.02 mm/s and 0.90±0.02 mm/s with respect to EuF3. Experimental results show the Eu ion to be in the trivalent oxidation state. A discussion of the electronic properties and structure will be reported and compared with previous results found on iron and tin based porphyrins.
Collinear interaction of the optical waveguide modes and surface magnetostatic waves in a Lu2.14Bi0.86Fe4.94Mg0.06O12 film
Authors: Matyushev V. V. , Stashkevich A. A. , Desvignes J. M.
Published: July 2002

This paper reports the maximum diffraction efficiency per unit microwave power achieved so far in experiments on the interaction of optical waveguide modes with magnetostatic waves (MSW) in ferrimagnetic films. To obtain high efficiency of the interaction a film of Lu2 .14Bi0.86Fe4.94Mg0.06O12 grown on a gadolinium gallium garnet substrate, and collinear propagation of the surface MSW and guided light were used. Measurements of the spectrum of optical modes in the film have shown extremely high values of the birefringence between a TE and a TM mode, induced by the mismatch of the lattice parameters of the film and the substrate (Δnbi = 4.5 × 10−3). A He-Ne laser (λ=1.152 μm) was used as the source of optical radiation. The measurements of the efficiency of the interaction were carried out for two geometries: (i) parallel propagation of the light and MSW, and (ii) antiparallel propagation of the light and MSW. As predicted by theory, the efficiency of the parallel interaction was higher than that of the antiparallel interaction. The efficiency for the case of the parallel interaction was of the order of 0.8% per 1 mW of the MSW microwave power.
Raising the Curie point in the iron-cobalt-(aluminum) system
Authors: Jacobs I. S. , Patchen H. J. , Johnson N. A.
Published: July 2002

The ternary alloy system Fe-Co-Al exhibits a little-noticed anomaly in that some alloys containing Al have higher Curie points than their undiluted counterparts. New results present the evolution of Curie point and saturation magnetization as a function of Al content, especially below 7 wt % (∼13.5 at. %) in the midrange of the Fe-Co composition field. Maxima of Tc in the range of 1000–1025 °C are found for Al contents of about 2–5 wt % (4–10 at. %) for alloy series with 30- , 40- , and 50-wt % Co (∼27- , 37- , and 47-at. % Co). These values are some 25–45 °C higher than the corresponding zero-Al alloys. Correlation of low-field thermal hysteresis and saturation-field thermomagnetic curves indicate first-order Curie point behavior at low-Al content, followed by classical second-order transitions at and above the Al content corresponding to Tc maxima. The former follow the α-bcc to the γ-fcc phase transition. For the latter class the α phase has been stabilized to higher temperatures with a relatively uncommon Curie point enhancement behavior.
Some properties of 2π Néel walls and Néel points in the XY model
Authors: Dieny B. , Jones B. A.
Published: July 2002

The properties of two types of topological defects in the 2D classical XY model, Néel lines and Néel points, have been investigated via numerical simulation. The effect of a symmetry-breaking magnetic field has been considered. For cyclic boundary conditions a uniform array of line defects can be stabilized. Along a line defect, point defects can be introduced as spatial changes of chirality of the wall. These point defects consist of pairs of bound vortices of the same sign. When several such point defects are introduced on a given line defect, they also tend toward uniform spacing.
High-vacancy-content ferrite with fine particles
Authors: Kodama T. , Itoh T. , Tabata M. , Tamaura Y.
Published: July 2002

The high-vacancy-content ferrites represented by x(MFe2O4)⋅y(Fe3O4)⋅z(γ-Fe2O3), where x+y+z=1 (z≳0.50), were obtained in the clear and strongly alkaline solutions of Fe(III) and M(II) tartrate [M(II)=Zn(II), Ni(II), and Cd(II)] or dextrose at 100 °C. The vacancies were replaced with the bivalent metal ions in the reaction solutions, and the replaced number increased with an increase in the concentration of the bivalent metal ions. The ferrite particle size was dependent on the bivalent metal species and the content (x,y). The Fe(II) ions enhanced the crystal-growth rate. The particle size of the magnetites (x=0) increased from 100 to 800 Å with an increase in the Fe(II) ion content (y=0.10–0.35). The high-vacancy-content magnetite was transferred from superparamagnetic to ferrimagnetic particles as the size increased. The Zn(II), Ni(II), and Cd(II) ions did not enhance the growth rate so much as compared to the Fe(II) ions. The particle sizes were less than 200 Å, and most of the particles were superparamagnetic. The saturation magnetizations were lower than the theoretical values, and increased with an increase in the particle size, indicating the surface effect.
Fe enriched Sm-Co based high energy product sputtered films with precise crystal textures
Authors: Hegde H. , Chen K. , Cadieu F. J.
Published: July 2002

Sm-Co based 2-17 type films with the Fe concentrations increased up to 30 at. % have been sputter synthesized from Fe enriched TDK targets through thermalized sputtering onto substrates heated to 400–500 °C. The crystal structure of these Fe enriched films for total Sm concentrations of about 13 at. % was found to be rhombohedral 2-17, Th2Zn17 type. A minor 1-5 phase was also found to be present. For Fe concentrations of around 20 at. %, at these relatively low deposition temperatures, the films could not be formed in the ordered 2-17 phase. Instead, these lower Fe concentration films formed in the disordered TbCu7 type structure. On post-deposition anneal at temperatures in the 400–500 °C range for 1–2 h, the Fe enriched films showed significant improvements in the demagnetization behavior while no improvements were seen for the TbCu7 type films. The Fe enriched rhombohedral 2-17 type films exhibited unusually strong (220) texturing with I(113)/I(220)<1/40, for out of plane c axes to in plane textures, and I(300)/I(220)<1/50, for relative c axes in plane textures.