bluuebunny/crossref_metadata_2025 huggingface dataset:
Treatment of ionization in plasma collisional-radiative calculations
Authors: Foord M. E. , Nardi E.
Published: July 2002

In this paper a method for treating electron collisional ionization in collisional-radiative calculations is presented that includes the effects of direct ionization into multiple excited states. Ionization from a given state proceeds to terms of the next ionization state using appropriate cross sections and branching ratios. Level populations are calculated for the case of a low-temperature (7 eV), moderate-density (ne=5×1015 cm−3) ionizing carbon plasma using a time-dependent collisional-radiative model. These results show appreciable differences when compared with calculations in which ionization is assumed to couple only into the ground state of the next ionization stage. Applications of these results to astrophysical and laboratory plasmas are discussed.
Magnetic field-induced suppression of acoustic phonon emission in a superlattice
Authors: Kastalsky A. , Efros A. L.
Published: July 2002

We analyze theoretically the influence of magnetic field on the electron lifetime in the excited state of a specially designed superstructure allowing electron injection into the excited miniband. We show that the discrete electron energy spectrum arising in the magnetic field dramatically suppresses the acoustic phonon emission rate and gives rise to electron accumulation in the excited subband. This can be used for obtaining stimulated emission between minibands as well as between Landau levels. The latter is of particular interest since the energy of this radiation can be varied by magnetic field. We also obtain an oscillatory dependence of electron lifetime in the excited state on magnetic field which can be verified in a simple photoluminescence experiment. Finally, we extend our considerations to the case of quantum dots, capable of providing complete energy quantization in the absence of magnetic field.
Millimeter wave surface resistance of grain-aligned Y1Ba2Cu3Ox bulk material
Authors: Wosik J. , Kranenburg R. A. , Wolfe J. C. , Selvamanickam V. , Salama K.
Published: July 2002

We report measurements of the millimeter wave surface resistance of grain-aligned YBa2Cu3Ox bulk material grown by a liquid phase process. The measurements were performed by replacing the endplate of a TE011 cylindrical copper cavity with the superconducting sample. Surface resistance was measured for samples with surfaces oriented perpendicular and parallel to the c-axis of the grains. We show that, for the parallel configuration, the surface resistance at 77 K and 80 GHz is typically near 100 mΩ. For a very well-aligned sample with a very low density of Y2BaCuOy precipitates, measured in the perpendicular configuration, the transition width (10%–90%) is about 2 K and the surface resistance is less than 50 mΩ at 88 K. The effect of microstructure on surface resistance is discussed.
Ellipsometric study of a-Si:H thin films deposited by square wave modulated rf glow discharge
Authors: Lloret A. , Bertran E. , Andujar J. L. , Canillas A. , Morenza J. L.
Published: July 2002

Thin films of hydrogenated amorphous silicon (a-Si:H), deposited by square wave modulated (SQWM) rf silane discharges, have been studied through spectroscopic and real time phase modulated ellipsometry. The SQMW films obtained at low mean rf power density (19 mW/cm2) have shown smaller surface roughness than those obtained in standard continuous wave (cw) rf discharges. At higher rf powers (≥56 mW/cm2), different behaviors depending on the modulating frequency have been observed. On the one hand, at low modulating frequencies (<40 Hz), the SQWM films have shown a significant increase of porosity and surface roughness as compared to cw samples. On the other, at higher modulating frequencies, the material density and roughness have been found to be similar in SQWM and cw films. Furthermore, the deposition rate of the films show more pronounced increases with the modulating frequency as the rf power is increased. Experimental results are discussed in terms of plasma negative charged species which can be relatively abundant in high rf power discharges and cause significant effects on the deposited layers through polymers, clusters, and powder formation.
An electron wave directional coupler and its analysis
Authors: Dagli Nadir , Snider Gregory , Waldman Jonathan , Hu Evelyn
Published: July 2002

In this work a specific quantum interference device, which is two coupled-electron waveguides in close proximity, is described and analyzed. After an initial analysis outlining the principle of operation and the constraints on the structure dimensions and material parameters, a specific structure is simulated using a self-consistent Poisson and Schrödinger solver. Results indicate that this device is feasible using current fabrication technology.
Occupation dynamics of trap states in an a-Si:H thin-film transistor
Authors: Bullock J. N. , Wu C. H.
Published: July 2002

We calculate the dynamical behavior of a-Si:H thin-film transistors with an emphasis on the occupation dynamics of trap states. The appropriate rate equation for the occupation function of trap states is included. We show the relations of filling the trap states with the switch-on time and of emptying the trap charges with the switch-off time. The occupation functions in both cases are non-Fermi distribution. The quasi-equilibrium approximation underestimates those two time constants. Thus, transit time theory cannot describe the speeds of transistors made from disordered materials.
Energy levels and crystal quantum states of trivalent holmium in yttrium aluminum garnet
Authors: Gruber John B. , Hills Marian E. , Seltzer Michael D. , Stevens Sally B. , Morrison Clyde A. , Turner Gregory A. , Kokta Milan R.
Published: July 2002

Absorption spectra of Ho3+ ions in yttrium aluminum garnet (Ho3+:YAG) are reported between 2.16 and 0.23 μm at various temperatures between 4 K and room temperature. Laser-excited excitation spectra and emission spectra from the 5F4 and 5S2 multiplet manifolds to the ground state manifold 5I8 were obtained at 4, 20, and 55 K. The majority of Ho3+ ions substitute for Y3+ ions in sites of D2 point-group symmetry in the lattice. Over 1000 temperature-dependent transitions (hot bands) establish 280 experimental Stark levels of the 4 f 10(2S+1LJ) multiplet manifolds. Symmetry labels Γ1, Γ2, Γ3, or Γ4, appropriate to D2 symmetry, have been assigned to Stark levels up to 43 000 cm−1. Experimental levels are compared with results obtained from a theoretical calculation. The model Hamiltonian includes Coulombic, spin-orbit, and interconfiguration interaction terms for the 4 f 10 atomic configuration of Ho3+ and crystal-field terms in D2 symmetry. The Hamiltonian was diagonalized within the 50 lowest 2S+1LJ manifolds in a LSJMJ basis set that includes 486 calculated levels. Comparison between 151 experimental Stark levels and the corresponding levels of the lowest 199 calculated levels yields an rms deviation of 4.3 cm−1.
Photoreflectance line shapes of semiconductor microstructures
Authors: Tang Y. S.
Published: July 2002

Photoreflectance line shapes of various semiconductor microstructures were discussed in this paper. Formulas suitable for application in low-dimensional structures were given and used to analyze experimental results from various semiconductor microstructures including GaAs nipi structures, GaAs/AlGaAs multiple quantum wells, InGaAs/GaAs strained-layer superlattices, and
X-ray diffuse scattering from a nitrogen-implanted niobium film
Authors: Rao Satish I. , Houska C. R. , Grabowski K. , Ice G. , Sparks C. J.
Published: July 2002

A 2500-Å niobium single-crystal film was deposited onto a sapphire substrate and subsequently implanted with nitrogen to an average concentration of 0.5 at. %. Synchrotron radiation was used to measure the difference between the implanted and an unimplanted film to isolate the diffuse scattering from the implanted film near two Bragg reflections. This diffuse intensity arises mainly from elastic displacement fields about radiation-damage-related loops located on (211) planes. A small contribution of the scattering is calculated from the displacements about single interstitial nitrogen in octahedral sites. The Burgers vector of the loops is along the [111̄] direction and makes an angle of 62° with the loop plane giving a dominant shear component. Vacancy loops have a radius ∼5 Å while interstitials are somewhat larger ranging from 10 to 15 Å. The number of vacancies and interstitials are nearly the same.
Pump polarization effects in cw dye lasers
Authors: Casperson Lee W. , Sandle W. J. , Wilson A. C. , Warrington D. M. , Ballagh R. J.
Published: July 2002

The effects of pump polarization on the performance of cw dye lasers have been investigated. A recently developed semiclassical model for synchronously pumped mode-locked dye lasers is adapted to the problem of cw dye-laser oscillation, and analytic expressions are obtained for the relationship between the pump power, pump polarization, and dye-laser output power. The theoretical results are compared with experimental data obtained using an argon-laser-pumped rhodamine 6G dye laser.
Optical properties of GaAs partially amorphized by ion implantation: Effective-medium-approximation analysis
Authors: Adachi Sadao
Published: July 2002

We have studied the optical properties of GaAs partially amorphized by As+-ion implantation. The model used is based on the two-phase mixture Bruggeman effective-medium approximation in which the partially amorphized layer is assumed as a physical mixture of fully amorphous GaAs and crystalline GaAs. The dielectric functions of the crystalline components deduced from this study differ appreciably from that of the bulk crystalline GaAs, especially in the vicinity of the sharp critical point (CP) features. We find that the change in the optical properties of the crystalline component is due to the increase in the broadening parameters of the CPs caused by implantation-induced damage. The amorphous volume fraction fa is also found to be simply expressed as fa = ([As+]/A)α, where [As+] is the ion fluence, A(=8.5×1013 cm−2) is an amorphization-threshold fluence (i.e., the minimum fluence required to form fully amorphous layer), and α(=0.79) is an amorphization-rate factor of the incident ions.
Interface-state density analysis of thermally and electron-beam-annealed GaAs surfaces
Authors: Prasad K. , Faraone L. , Nassibian A. G.
Published: July 2002

Schottky diodes were formed on n-GaAs surfaces that had undergone furnace and scanned electron beam (SEB) heat treatments that are similar to those typically used for Ohmic contact formation. The diode parameters of heat-treated samples were inferior to those that were not heat treated. The interface-state density Dit, determined from Tseng and Wu’s interfacial layer model, was found to exhibit a strong correlation with the heat treatments, with furnace-annealed samples exhibiting progressively larger Dit values than SEB annealed samples. The presence of a spin-on glass capping layer during heat treatment resulted in reduced Dit values. Photochemical passivation of the GaAs surface before Schottky metal deposition resulted in a reduction in the interface-state density. Controlled etching of a heat-treated GaAs surface revealed that the surface damage caused by heat treatments extends to about 500 nm. Elevated thermal aging studies of Schottky contacts show a monotonic increase in Dit with aging time, irrespective of surface preparation.
Interferometric determination of optical anisotropy in fibers. II. Fibers with a skin-core structure
Authors: Hamza A. A. , Sokkar T. Z. N. , Shahin M. M.
Published: July 2002

The polarizing interference microscope with a modified method is used to determine the refractive indices and birefringence of both layers of fibers having a skin-core structure. This method is applied to nylon 6 and polyester fibers. Two fibers are used for each measurement. The first fiber is of known mean refractive index and dispersion properties. The other fiber is of unknown values of refractive indices of each layer of the fiber. This method can be used for fibers with regular or irregular cross sections. In the latter case the area enclosed under the interference fringe shift represents the optical path difference integrated across the fiber. The obtained results of refractive indices and birefringence are compared with those obtained from the normal method and found to be in good agreement. Illustrations are given by microinterferograms.
The effect of the denudation anneal on the precipitate dissolution in Czochralski silicon wafers during the three-step internal gettering anneal
Authors: Wijaranakula W. , Matlock J. H.
Published: July 2002

The effect of the denudation anneal on the precipitate dissolution and defect-free zone formation in Czochralski silicon wafers after a three-step internal gettering anneal was studied. The results imply that the residual precipitates containing three or four oxygen atoms could be the predominant defects influencing the formation of the defect-free zone. Based upon the results from the process simulation model, it is proposed that some residual precipitates having a size less than the critical size could be in a metastable state at high temperatures. Thus during the denudation anneal, the precipitate dissolution occurs primarily in the subsurface region where the critical size is substantially increased due to the oxygen outdiffusion.
Theory of optical beam deflection for single microparticles
Authors: Wu Jiaqi , Kitamori Takehiko , Sawada Tsuguo
Published: July 2002

A theory was developed for the optical beam deflection (OBD) signal generated from a single microparticle. From the thermal-diffusion equations, the temperature fields inside and outside the microparticle, which has a two-layer structure, was deduced. A three-dimensional theoretical treatment was established for the deflection signal of the probe beam passing through the temperature field formed by photothermal conversion of the excitation beam energy absorbed by the sample. The proprieties of the theoretical model and its results were confirmed by comparing the theoretical values of the frequency characteristics, probe beam offset dependencies, and particle size dependencies of the OBD signal with the experimental ones for 25–300-μm-radius microparticles. From the theory, the unique particle size dependencies and frequency characteristics of the OBD method for the single microparticle, i.e., higher sensitivity for smaller particles and at high frequencies, were identified as due to the microparticle surface curvature. The optimal experimental conditions in the OBD measurement of the single microparticle were also obtained using theoretical analysis.
Characterization of polycrystalline silicon–single-crystal silicon interfaces and correlation to bipolar transistor device data
Authors: Ronsheim Paul A. , Cunningham Brian , Dupuis Mark D.
Published: July 2002

Depth-resolved secondary-ion mass spectrometry (SIMS) and high-resolution transmission electron microscopy were used to study the composition and structure of the arsenic-doped polycrystalline silicon (polysilicon)–single-crystal silicon interface. This interface remains intact during an 880 °C anneal, with no alignment of the polysilicon occurring. When the interfacial oxide varies from a thin, discontinuous film, to a continuous layer, SIMS analysis is able to quantify differences in the interfacial oxygen content. This interfacial oxygen, expressed as oxygen atoms/cm2, is found to correlate to polysilicon emitter bipolar transistor device current gain.
Accelerated particle radiation in chiral media
Authors: Sun Xiaoguang , Jaggard Dwight L.
Published: July 2002

Here, Lienard–Wiechert potentials are derived for accelerated charged particle radiation in chiral media. Radiation from moving charges in chiral media is formulated and discussed. As two examples, the radiation patterns of both linear and circular (synchrotron) acceleration in chiral media are found. We focus on the effects of chirality and examine the presence of two eigenmodes of radiation, each with a different refractive index and beam characteristics. The interplay between relative velocity and relative chirality is shown.
Micromagnetics of two-layer laminated films with superimposed domains
Authors: Niedoba H. , Heyderman L. J. , Gupta H. O. , Puchalska I. B. , Hubert A.
Published: July 2002

The micromagnetic structure of the walls in two-layer laminated films has been investigated using the high-resolution longitudinal Kerr effect. Superimposed Bloch lines of opposite type (cross or circular) in the two magnetic layers are observed and interpreted. The mixed type of walls with twin and superimposed Néel segments is discussed. The magnetization distribution at crossing points between the twin walls nucleated in a hard axis field is deduced from the Kerr effect contrast of the walls and quasiwalls. The appearance of superimposed 360° walls and segments in 90° walls in samples with biaxial anisotropy is shown.
Magnetic anisotropy and structural characterization of Co/Pt superlattices grown along selected orientations by molecular-beam epitaxy
Authors: Hermsmeier B. D. , Farrow R. F. C. , Lee C. H. , Marinero E. E. , Lin C. J. , Marks R. F. , Chien C. J.
Published: July 2002

In this paper we report the molecular-beam epitaxial growth of Co/Pt superlattices on epitaxial Ag films on GaAs substrates. The growth axis of the superlattice was selected by seeded epitaxy to be along either the [001], [110], or [111] axis of Pt. The magnetic properties of the superlattices depend on the orientation of the growth axis. In situ x-ray photoelectron diffraction studies of the Co-Pt interfaces during their formation reveals that they are not atomically abrupt for any of the orientations. We conclude that models for magnetic anisotropy of Co/Pt superlattices should be based on a combination of magnetocrystalline anisotropy and strain in compositionally mixed interfaces.
Effects of sputtering pressure on magnetic and magneto-optical properties in compositionally modulated Co/Pd thin films
Authors: Shin Sung-Chul , Kim Jin-Hong , Ahn Dong-Hoon
Published: July 2002

We have investigated the effects of sputtering Ar gas pressure on magnetic and magneto-optical properties in compositionally modulated Co/Pd thin films. The samples were prepared by dc magnetron sputtering from 2-in.-diam Co and Pd targets by alternately exposing the substrates to targets. Sputtering Ar gas pressure was varied from 2 to 30 mTorr. All samples had same bilayer thicknesses composed of 2-Å-thick Co and 9-Å-thick Pd sublayers. It was observed that the intrinsic uniaxial anisotropy energy, magnetization, and polar Kerr rotation were monotonically decreased with increasing Ar gas pressures more than about 10 mTorr. Interestingly enough, the coercivity showed a thirtyfold enhancement as Ar gas pressure varied from 2 to 30 mTorr. We believe that the results are mainly ascribed to the variation of microstructure with sputtering Ar gas pressure.
Magnetic and structural characteristics of the Pr2(Co,Fe)7B3 system
Authors: Huang M. Q. , Ma B. M. , Cheng S. F. , Wallace W. E.
Published: July 2002

Pr2Co7−xFexB3(0≤x≤7) alloys have been synthesized and studied at temperatures from 4 to 1100 K at fields up to 90 kOe. The structure and magnetic properties vary significantly with increasing Fe content. The material exhibits the Ce2Co7B3 structure for 0≤x≤0.5. For 0.5≤x≤3 the alloy is comprised of a main phase of hexagonal structure. A change in magnetocrystalline anisotropy occurs around x=3.5; all alloys of x≤3 exhibit conical anisotropy at both 295 and 77 K, and axial anisotropy for x≥3.5. Tc increases from 328 to 890 K when x varies from 0 to 3, then decreased to 564 K when x further increased to 7. The saturation magnetization, Ms, increases monotonically with increasing Fe content. A sintered magnet with a Br of 6.1 kG, a Hci of 10.6 kOe, and a BHmax of 9.1 MGOe were obtained using Pr2Co2Fe5B3.
Organizing centers of bifurcations in spin-wave instabilities (invited)
Authors: Rezende S. M. , Azevedo A. , Cascon A. , Koiller J.
Published: July 2002

We present an experimental and theoretical study of bifurcations leading to self-oscillations in spin-wave instabilities pumped by microwave fields. Experiments done with bulk samples of yttrium iron garnet pumped by X-band microwave radiation reveal a variety of bifurcations and routes to chaos, depending on the experimental parameters. Generally, the frequency of the self-oscillation shows a definite dependence on sample size not predicted by the existing models. On the theoretical side we show that the usual equations describing two interacting spin-wave modes predict various types of bifurcations, depending on the set of parameters. In particular, we investigate Hopf bifurcations and homoclinic phenomena. We also indicate how the spin-wave equations can be modified to predict a sample-size dependence for the self-oscillation frequency.
Magnetic properties and structure of DyNi/Mo multilayers
Authors: Perera P. , O’Shea M. J. , Fert A.
Published: July 2002

Dy80Ni20/Mo multilayers have been prepared by sputtering and the effects of the interface on the magnetic anisotropy of these materials are reported. A large interface anisotropy is found but it is somewhat smaller than that estimated from a point-charge model assuming a sharp interface.
Monte Carlo simulations of transverse spin freezing in the three-dimensional frustrated Heisenberg model
Authors: Thomson J. R. , Guo Hong , Ryan D. H. , Zuckermann M. J. , Grant Martin
Published: July 2002

Monte Carlo simulations of the randomly frustrated three-dimensional Heisenberg model with a moderate fraction (≤20%) of antiferromagnetic bonds show evidence of two distinct ordering events. Below Tc the spins align collinearly and exhibit ferromagnetic order, while below Txy the transverse components of the spins freeze leading to a noncollinear spin structure dominated by ferromagnetic correlations. The phase diagram and behavior of the calculated parameters agree qualitatively with those of experimental systems.
Layered magnetic structures: Antiferromagnetic-type interlayer coupling and magnetoresistance due to antiparallel alignment
Authors: Grünberg P. , Demokritov S. , Fuss A. , Vohl M. , Wolf J. A.
Published: July 2002

Layered Fe/Cr structures are known to display antiferromagnetic-type interlayer coupling and a new magnetoresistance (MR) effect due to antiparallel magnetization alignment. The strength of the coupling is found to be similar in multilayered structures and in double layers. The oscillatory behavior of the coupling, previously found by Parkin, More, and Roche [Phys. Rev. Lett. 64, 2304 (1990)] on sputtered polycrystalline samples, is here confirmed for epitaxial samples, obtained by thermal evaporation. The new MR effect is interpreted as due to a spin-dependent scattering of the electrons at the Fe-Cr interfaces. The investigations have been extended to Fe/V, Fe/Mn, Fe/Cu, Co/Au, Co/Cr, and Co/Cu structures where the antiparallel alignment of the ferromagnetic layers is obtained via hysteresis effects. A MR effect due to antiparallel alignment, which is strong for Co/Au and Co/Cu but weak in the other cases, has been found.
Generalized Preisach model for the description of hysteresis and eddy current effects in metallic ferromagnetic materials
Authors: Bertotti G.
Published: July 2002

We discuss a generalization of the Preisach model, based on the assumption that each point (α,β) of the Preisach plane corresponds to a small correlation region of the system cross section, inside which magnetic flux reverses at a rate Φ̇ ∝ Ha − α (or Ha − β) when the applied field Ha exceeds the threshold α (or β). When the magnetizing frequency f is low, the model reduces to the conventional Preisach model. When f is increased, the model predicts that the area of the hysteresis loop should increase with f according to the law C0 + C1f + C2√f, where C1 is related to the so-called classical loss and C2 is expressed directly in terms of the Preisach function, p(α,β). The prediction of the existence of the excess loss term C2√f is in agreement with experiments. The fact that the coefficient C2 is calculated from p(α,β) implies that one can predict dynamic losses directly from quasistatic hysteresis losses, a remarkable conclusion which is also confirmed by recent investigations with electrical steels.
Forward-bias tunneling at defect clusters in silicon emitter junctions
Authors: Andersson Gert I. , Engström Olof
Published: July 2002

High values of the ideality factor n, in the range 1–6, have been found at low temperatures in highly doped n++p+ junctions with phosphorous and gallium or boron diffusion profiles. By studying the thermal activation of the forward current it has been possible to establish that tunneling related to charge carrier transport plays a decisive role for the generation of the high n factors. The tunneling component can be explained by the presence of local electric fields, much higher than the average field in the space-charge region, and the tunneling becomes more evident for p-n junctions with higher concentration of shallow dopants at the interception point between the p+ and the n++ concentration profiles. The use of a specially designed method based on deep-level transient spectroscopy has made it possible to conclude that the recombination centers, contributing to the recombination current, are positioned in the n++ phosphorous emitter and have the same properties for gallium and boron p base profiles. The concentration of isolated recombination centers decreases with increasing phosphorous concentrations due to clustering or precipitation, and increases the electric field constrictions.
Measurement of electron densities in electron cyclotron resonance plasmas for etching of III-V semiconductors
Authors: Pearton S. J. , Nakano T. , Gottscho R. A.
Published: July 2002

The average electron densities in CH4/H2/Ar and CCl2F2/O2 electron cyclotron resonance discharges typical of those used for etching InP, GaAs, and related materials have been measured approximately 4 cm downstream from the multipolar microwave source as a function of microwave power (50–300 W), additional radio-frequency power (10–50 W), pressure (1–20 mTorr), flow rate (30–90 standard cubic centimeters per minute) and gas composition. At 1 mTorr pressure and 10 W rf, the electron densities (and semiconductor etch rates) increase rapidly with microwave power, from 1.3×1011 cm−3 for 5CH4/17H2/8Ar and 6×1010 cm−3 for 28CCl2F2/2O2 discharges at 50-W microwave power, to 9×1011 cm−3 and 3×1011 cm−3, respectively at 300-W microwave power. At the highest microwave power levels (≥200 W) the InP and GaAs etched surface morphologies are rough due to preferential removal of one of the lattice constituents from each material. The electron densities in both types of discharge show moderate increases with increasing rf power level, pressure or higher Ar or O concentrations.
Polarized neutron reflection used to characterize cobalt/copper multilayers
Authors: Schwarzacher W. , Allison W. , Penfold J. , Shackleton C. , England C. D. , Bennett W. R. , Dutcher J. R. , Falco C. M.
Published: July 2002

The polarized neutron reflectivity (PNR) of cobalt/copper multilayer films has been measured close to the critical edge for total reflection. Prominent features in the scattered neutron intensity, such as the superlattice diffraction peaks and the positions of the critical edge for total reflection, are sensitive to both the magnetic and structural properties of films, making PNR a useful tool for the characterization of magnetic metallic superlattices. The films were prepared by sputter deposition and the sample composition was measured by Rutherford backscattering spectroscopy. It has been found that while the density of the sputtered copper/cobalt multilayers is approximately 5% less than the bulk metals, the cobalt magnetic dipole moment per atom is little changed compared to the bulk. Evidence is also found for oxidation of the top cobalt layers.
On the intrinsic bistability in resonant tunneling structures: Observation of area dependence of hysteresis
Authors: Chen J. G. , Yang C. H. , Wilson R. A.
Published: July 2002

We investigated the current-voltage characteristics of double-barrier resonant tunneling diodes with areas ranged from ∼400 μm2 to ∼10 μm2 at a temperature of 300 and 4.2 K. For diodes of large areas, the characteristic negative differential resistance is always accompanied by a hysteresis and oscillation at several tens of MHz. However, for smaller diodes, the hysteresis and oscillation disappear, and the tunneling current shows a smooth transition between peak and valley. Our observation shows that the hysteresis can result from a load line effect. Were the hysteresis an intrinsic behavior, it should persist as the device area is reduced. We therefore conclude that the intrinsic bistability is yet to be experimentally confirmed.
Ferromagnetic amorphouslike oxide films of the Fe2O3-Bi2O3-PbTiO3 system prepared by rf-reactive sputtering
Authors: Kajima A. , Kaneda T. , Ito H. , Fujii T. , Okamoto I. , Kimura T. , Ohdan K.
Published: July 2002

In this paper, the magnetic properties of ferromagnetic oxide films of the ternary Fe2O3-Bi2O3-PbTiO0 system prepared by rf-reactive sputtering are presented. As-sputtered films deposited at a substrate temperature below 400 °C are amorphous and behave paramagnetically. Amorphous films like these whose composition corresponds to the perovskite solid solution of BiFeO3 -PbTiO3 become crystalline with a perovskite structure on annealing in air, but no distinct spontaneous magnetization appears. On the contrary, in certain compositions in the Fe2O3 -Bi2O3 -PbTiO3 system, the samples retain a glassy (or amorphouslike) state even when the samples were annealed at around 600 °C. Also a spontaneous magnetization appears at room temperature. The maximum room-temperature saturation magnetization 4πMs achieved in this system is 1.67 kG with the composition 0.74Fe2O3 -0.16Bi2O3 -0.10PbTiO3. Experiments imply that the amorphous state is essential for establishment of strong ferromagnetism in the sputtered oxide films. Mössbauer spectroscopy and x-ray photoelectron spectroscopy were performed to examine the microscopic properties of the films that lead to ferromagnetism. The optical transmissivity and magneto-optical Faraday effect of the films are also discussed.
Formation of a new high pressure phase: fcc Pd40Ni40P20 solid solution
Authors: Xu Yingfan , Wang Wenkui
Published: July 2002

The crystallization of bulk metallic glass of Pd40Ni40P20 was investigated under pressure of 4 GPa. Compared with the result obtained at ambient pressure, the crystallization temperature was raised with the application of pressure at a rate of 15K/GPa, and the crystallizing sequence of the alloy was also changed. Furthermore, a supersaturated solid solution phase with fcc structure was formed during the compression at higher temperature.
Critical current density, lower critical field, and flux creep in Ag-sheathed high-J
c tape of Bi-Pb-Sr-Ca-Cu-O
Authors: Oota A. , Yata A. , Hayashi K. , Ohba K.
Published: July 2002

Initial magnetization curve M(H) and magnetic relaxation M(t) were studied on the Ag-sheathed and highly c-axis oriented tape of Bi-Pb-Sr-Ca-Cu-O with transport Jct=5400 and 2400 A/cm2 (77 K, H=0). The Jc value, derived from M(H) curve based on Bean’s critical-state model, agrees fairly well with the Jct. The lower critical field Hc1 for H⊥c axis from M(H) curve increases with a slope of −0.2 Oe/K as temperature decreases from Tc, and it approaches the saturated value of 15 Oe at T∼0 K. The magnitude is much lower than that for the high-Tc superconductors La-Sr-Cu-O and Y-Ba-Cu-O. The creep rate and activation energy U0 are evaluated from M(t) based on the flux creep model. The results show a significant dependence on the measuring process and magnetic field H. The U0 at 5 K for H=500 Oe (H⊥c) is 0.4 eV for the in-field magnetization (IN) and 0.1 eV for the remanent magnetization (REM), which are larger than those for Bi-based high-Tc superconductors reported previously. The U0 for the IN shows a peak of 0.8 eV around 30 K and then decreases as temperature increases, while a peak in it is weakened for the REM. The U0 at H=30 Oe (H⊥c) for the IN is one order larger than that for 500 Oe and the peak temperature is shifted to the lower temperature (∼10 K).
Preparation of high-Tc superconducting Bi-Sr-Ca-Cu-O films by organometallic chemical vapor deposition using second-generation fluorocarbon-based precursors
Authors: Zhang J. M. , Wessels B. W. , Richeson D. S. , Marks T. J. , DeGroot D. C. , Kannewurf C. R.
Published: July 2002

High-Tc superconducting Bi-Sr-Ca-Cu-O thin films with good electrical properties and smooth surface morphologies have been prepared by low-pressure organometallic chemical vapor deposition using the new fluorocarbon-based precursors Sr(hexafluoroacetylacetonate)2⋅tetraglyme and Ca(hexafluoroacetylacetonate)2⋅triglyme together with Cu(acetylacetonate)2 and triphenylbismuth [Bi(C6H5)3]. The fluorinated precursors are air-stable and exhibit high, stable volatility even after prolonged heating. X-ray diffraction measurements reveal that the films deposited at 650 °C contain some fluoride phases but no high-Tc phases. However, post-annealing in oxygen produces films composed predominantly of the Bi2Sr2CaCu2Ox phase with high preferential orientation of the crystallite c axes perpendicular to the substrate surface. Four-probe resistivity measurements indicate the onset of film superconductivity at ∼100 K and zero resistivity achieved at 73 K.
Modulation-doped graded structures: Growth and characterization
Authors: Sundaram M. , Gossard A. C. , Holtz P. O.
Published: July 2002

Remotely doped graded parabolic potential well structures have been grown and studied. Electrons distribute themselves uniformly in a parabolic well, the density being proportional to the curvature or quasidoping in the well. Quasidoped semiconductors are synthesized by molecular beam epitaxy in the GaAs/AlXGa1−XAs system through the digital alloy technique. The analog grading produced by the digital alloy is verified by photoluminescence excitation spectroscopy. Low temperature mobility measurements show higher mobility in these quasidoped semiconductors than in similar real-doped semiconductors. Alloy-disorder scattering is suggested to be the mobility-limiting mechanism in this digital alloy system. Capacitance–voltage profiling analysis of quasidoped semiconductors has been developed, and is used to measure carrier profiles in these structures.
Grain collapses in strained aluminum thin films
Authors: Kristensen Nils , Ericson Fredric , Schweitz Jan-Åke , Smith Ulf
Published: July 2002

A special stress relaxation effect in thin aluminum films evaporated onto oxidized silicon wafers is reported. The effect is observed at elevated temperature and under tensional stress. It appears as if certain Al grains in the film surface suddenly ‘‘collapse’’ compared to surrounding grains. The phenomenon is observed during the cooling phase of an annealing cycle, or when imposing external strain on the film by bending of the substrate at elevated temperature. Such external strain was imposed by micromechanical technique in situ in a scanning electron microscopy (SEM), and the collapse phenomenon was monitored as it happened. The phenomenon occurred instantaneously and only in films of thickness 1 μm or more; thinner films relaxed by hole formation. The grain collapse effect is characterized, and a physical explanation is given, supported by SEM and transmission electron microscopy (TEM) investigations and surface profile measurements.
Variation of threshold current with cavity length in strained-layer InGaAs/GaAs quantum well lasers
Authors: Lee Johnson , Shieh C. , Vassell M. O.
Published: July 2002

The variation of the threshold current on the cavity length in strained-layer InGaAs/GaAs quantum well lasers was studied both theoretically and experimentally. The radiative recombination rates were calculated, while the nonradiative recombination rates were described phenomenologically. Broad-area lasers of various cavity lengths were fabricated on the same wafer for use in the experiments. The valence subband structures were calculated from the Kohn–Luttinger Hamiltonian plus the strain Hamiltonian with appropriate boundary conditions. When lasers were forward biased, the quasi-Fermi levels were determined by the charge neutrality. From the dielectric function in the self-consistent-field method including the band-gap shrinkage effect, the gain and the spontaneous emission rate spectra were obtained. At the threshold of lasing, the current was a sum of the radiative and nonradiative components. In the nonradiative component, we consider two mechanisms: the Auger and the interface recombinations. We found that (1) each subband structure possesses a cutoff in k space; (2) the dominant polarization of the emitted light from lasers under investigation is in TE mode; (3) for long cavity lengths, currents originating from the radiative and interface recombinations are dominant, while for short cavity lengths, current originating from the Auger process is dominant; and (4) as the cavity length decreases, the threshold current first decreases and then drastically increases. Therefore there is an optimum cavity length. Theoretical and experimental results were compared and presented.
Nonlinear guided waves in multilayer systems bounded by optically nonlinear media
Authors: Mihalache D. , Wang Ruo-Peng
Published: July 2002

For the first time, we investigate the properties of transverse electric polarized nonlinear guided waves propagating in an asymmetric N-channel waveguide surrounded on both sides by Kerr-like nonlinear media. We use a transfer matrix formalism that allows the exact calculation of the stationary field distribution and the nonlinear dispersion relation. For a specific material system, several numerical examples of the rich structured nonlinear dispersion curve together with detailed plots of the power-dependent field profiles are presented.
On the nature of the phase transition of helimagnets
Authors: Azaria P. , Delamotte B. , Jolicoeur T.
Published: July 2002

There is a controversy about the nature of the phase transition that occurs in Helimagnets. Early renormalization group calculations, on the appropriate Landau–Ginzburg Hamiltonian in D=4−ε led to no fixed point. This fact has been interpreted as the occurrence of a first-order phase transition in these models. However, results on recent Monte Carlo simulations on stacked triangular antiferromagnets are in favor of a continuous transition in three-dimensions with critical exponents different from those of the standard O(N) models. On the basis of these results, it has been conjectured that a stable fixed point should occur in 3D which is unreachable by perturbation theory near 4D. As a consequence, a new universality class for helimagnets and related frustrated spin systems was proposed. Another general strategy available to study the critical behavior of these systems is the perturbative expansion near 2D of a suited nonlinear sigma model. A fixed point that is O(4) symmetric is found by means of a 2+ε expansion. Thus, if a stable fixed point is likely to occur in D=3, it should be of the N=4 universality class. A simple scenario is proposed that is in agreement with both 4−ε and 2+ε results: Helimagnets can undergo, according to their microscopic Hamiltonian, a first-order transition or a second-order one with either N=4 or tricritical mean-field exponents. It is argued that this view is supported by experimental data.
Electron spin resonance of defects in silicon-on-insulator structures formed by oxygen implantation: Influence of γ irradiation
Authors: Stesmans A. , Revesz A. G. , Hughes H. L.
Published: July 2002

Silicon-on-insulator structures obtained by single-step implantation of oxygen followed by high temperature annealing were studied by K-band electron spin resonance (ESR) at 4.3–31 K. The spectrum has a strong line at g=2.0059±0.0001 with a spin density of 7.1×1013 cm−2. Various features indicate that it is very similar to that characteristic of dangling Si-bonds (DBs) in a-Si but different from the ESR signal of the Pb center associated with single-crystal-Si/SiO2 interfaces. Irradiation by γ rays to a dose of 1 Mrad(Si) resulted in a 2.5-fold increase in DB density and in the appearance of a new, anisotropic signal of spin density 1.1×1012 cm−2. The latter signal is similar to that originating from a shallow donor in Si, of axial symmetry and preferentially aligned along [001]. It is tentatively assigned to an oxygen-related double donor in Si regions close to the Si/SiO2 interface and/or in the buried oxide. These donors are not generated by irradiation; rather, their ionization state is altered through band bending tuning resulting from irradiation-induced charges in the oxide.
Magnetic properties of ultrathin epitaxial Co films on a Pd (111) single crystal
Authors: Purcell S. T. , van Kesteren H. W. , Cosman E. C. , Zeper W. B. , Hoving W.
Published: July 2002

Detailed magnetic studies were made of ultrathin epitaxial Pd/Co bilayers grown by evaporation in UHV on a Pd(111) single crystal. The magnetic properties were studied by polar Kerr hysteresis loops, vibrating sample magnetometry, and ferromagnetic resonance (FMR) at 10 and 34 GHz. The 10-GHz measurements were carried out from room temperature to 1.6 K. The effective anisotropy fields were determined from the FMR. All the magnetic measurements showed that Co films thinner than ∼9 monolayers had an easy magnetization axis perpendicular to the film plane. It was deduced that the first-order uniaxial interface and volume anisotropy constants were, respectively, 0.8±0.05 mJ/m2 and (4 ± 1) × 105 J/m3 at room temperature. The dependence of the FMR lineshape on the angles of the applied fields was analyzed by calculating the FMR response. The observed effects could be explained by allowing for a spread in the magnetic anisotropy throughout the sample. 
Results on modeling magnetic hysteresis using the finite-element methoda)
Authors: Ossart F. , Meunier G.
Published: July 2002

We present results obtained with the two-dimensional finite-element method including hysteretic behavior in magnetostatic-field computation. This software is dedicated to simulation of longitudinal magnetic recording on thin-film media, and so a scalar model of hysteresis is enough to model the media. The model used is based on a polynomial approximation of the magnetization curves. The nonlinear system is solved using an iterative process (Gauss–Seidel). Underrelaxation and smoothing are necessary to ensure the convergence of the computation. Results on a simple infinite-pole head show the effect of the demagnetizing field on magnetization, the remanent magnetization when removing the field, and the transition written when reversing it.
Properties of strained In0.2Ga0.8As/GaAs superlattices with various barrier thicknesses
Authors: Hovinen M. , Salokatve A. , Asonen H.
Published: July 2002

In0.2Ga0.8As/GaAs strained layer superlattices were grown by molecular-beam epitaxy with various GaAs barrier thicknesses to study how this affects the properties of nominally 90-Å-thick InGaAs wells. Double-crystal x-ray diffraction, photoconductivity, and photoluminescence gave similar results regarding superlattice degradation with decreasing barrier thickness. The optical measurements showed that at 58-Å-GaAs thickness, the strained layer superlattice was relaxed with concomitant deterioration of its optical properties.
A liquid-metal free surface in a sliding electrical contact with a strong magnetic field
Authors: Walker J. S. , Audet D. M. , Talmage G. , Brown S. H. , Sondergaard N. A.
Published: July 2002

A low-resistance electrical contact is provided by a liquid metal in a small gap between the perimeter of a rotating disk (rotor) and a static surrounding surface (stator). The liquid metal extends radially inward on both sides of the rotor to free surfaces with an inert cover gas, and there is a strong axial magnetic field. This paper presents results for the shape of the free surface and for the liquid-metal velocity and pressure adjacent to the free surface. The results depend on the magnetic-field strength, the surface tension, the wetting angle at the free-surface–solid intersections, and the voltage difference between the stator and rotor. The copper stator and rotor are perfect conductors compared to the liquid metal. Two cases are considered, with and without electrically insulating coatings on the sides of the rotor.
The dielectric constant measurement of CdIn2Te4
Authors: Couturier G. , El Farji A. , Lestournelle F. , Launay J. C.
Published: July 2002

Electronic properties of various single crystals of CdIn2Te4 have been investigated, and more particularly the static dielectric constant εr. This material is usually known to have a very large dielectric constant, values as large as 200–265 are frequently claimed. Two methods were used to study εr. One is based on the direct measurement of the capacitance of a In/CdIn2Te4/In structure and a second one uses a Schottky diode. In this latter case, the εr value is deduced with the help of the Hall measurements. From our measurements the dielectric constant would be much smaller than 200–265, it is estimated to about 11. Finally our results are compared with those obtained by optical measurements.
Electron injection studies on fluorine-implanted oxides
Authors: Xie Dunxian D. , Young Donald R.
Published: July 2002

The effect of fluorine on electron trapping in SiO2 films has been studied by avalanche electron injection. Samples are prepared by 25-keV fluorine implantation into dry oxides followed by a 1000 °C N2 ambient anneal. Bulk electron traps with capture cross sections on the order of 10−17–10−19 cm2, which are not due to implantation damage, are filled by avalanche electron injection. An optimum dosage of fluorine implantation to suppress the so-called turnaround effect during avalanche injection exists. This suggests that fluorine might passivate slow interface donor states or reduce bulk hydrogen diffusion. The observation that high-temperature (120 °C) injection can eliminate most fast and slow interface states for conventional oxides is also true for fluorinated oxides. Our results indicate an enhanced generation of fast donor states for oxides containing fluorine. These states contribute a positive charge when the Fermi level is in the lower portion of the band gap.
Optical rectification of nematics by nonlinear optical Pockel’s effect and its enhancement by dye-doping
Authors: Sugimura Akihiko , Zhong-can Ou-Yang
Published: July 2002

The optical rectification of a nematic liquid crystal by the nonlinear optical Pockel’s effect induced by a Freedericksz transition [Trans. Faraday Soc. 29, 919 (1933)] is presented as a mechanism for producing an anomalous photoinduced current transient. Explicit expressions for the photovoltaic emf and the current transient are derived and shown to be in good agreement with experiment. Motivated by the theory we have succeeded in finding and implementing a method, dye doping, to enhance the effect.
Redistribution of Fe and Ti implanted into InP
Authors: Ullrich H. , Knecht A. , Bimberg D. , Kräutle H. , Schlaak W.
Published: July 2002

The redistribution of Fe and Ti implanted into InP and its recrystallization is studied using various thermal annealing techniques. Fe and Ti profiles are measured by secondary-ion mass spectroscopy and recrystallization by Rutherford backscattering channeling. Ti shows absolutely superior thermal stability under any circumstances as compared to Fe. Iron always accumulates at the surface and at a depth of approximately twice the projected range Rp. After high-dose implantation Fe additionally accumulates in the 0.8Rp region. At similar doses Ti still shows no diffusion and only faint accumulation between the surface and Rp.