bluuebunny/crossref_metadata_2025 huggingface dataset:
Temperature dependence of the electrical characteristics of Yb/p-InP tunnel metal-insulator-semiconductor junctions
Authors: Singh A. , Reinhardt K. C. , Anderson W. A.
Published: July 2002

High barrier Yb/p-InP metal-insulator-semiconductor (MIS) and metal-semiconductor (MS) junctions were fabricated by evaporation of Yb on InP:Zn substrates. The capacitance-voltage (C-V) and current-voltage (I-V) characteristics of these devices were measured over a wide range of temperatures. From the room-temperature forward I-V data, the values of 1.06 and 1.30 for the ideality factor (n) were obtained for the MIS and MS diodes, respectively. The higher value of n was attributed to an order of magnitude higher density of interface states in the MS junction than in the MIS diodes. The I-V/T data over the temperature range 190–400 K, indicated that the forward current transport in the Yb/p-InP MIS junction was controlled by the thermionic-field emission (TFE) mechanism. The analysis of the reverse saturation current I0 in terms of the TFE model provided a value of 1.07±0.03 V for the zero bias, zero temperature barrier height (φ0) which was in close agreement with the value of φ0=1.03±0.04 V, provided by the C-V data. For the MS diode, the temperature dependence of the forward I-V characteristics over the temperature range 250–350 K were well described by the thermionic emission process. However, the value of φ0=0.80±0.04 V, determined from the I-V data was much smaller than the value of φ0=0.96±0.04 V, obtained from the C-V data.
Role of heat transfer and fluid flow in the chemical vapor deposition of diamond
Authors: DebRoy T. , Tankala K. , Yarbrough W. A. , Messier R.
Published: July 2002

The role of fluid flow and heat transfer in determining the quality of the diamond films and the rate of their deposition in a hot-filament chemical vapor deposition (HFCVD) reactor was investigated both experimentally and theoretically. The equations of conservation of mass, momentum, and enthalpy were solved numerically to calculate the temperature and fluid flow fields. Experiments were conducted with various flow configurations, and the deposition rates and the spatial variations of film thickness were examined in each case. The films were characterized by Raman spectroscopy, x-ray, and scanning electron microscopy. The influences of free and forced convection, and diffusion due to concentration and temperature gradients (Soret effect) were examined. Comparison of the computed results with the experimental data revealed the importance of thermal diffusion in the HFCVD of diamond.
Modeling AlxGa1−xAs optical constants as functions of composition
Authors: Snyder Paul G. , Woollam John A. , Alterovitz Samuel A. , Johs Blaine
Published: July 2002

Three models for the dielectric function εx(hν) of AlxGa1−xAs are reviewed. All are based on measured optical constants at discrete compositions. The validity of each model near critical point energies, and otherwise, is evaluated. Only the energy-shift model is appropriate over the entire available spectrum (1.5–6.0 eV), including the band-gap (E0) region.
Fatigue and switching in ferroelectric memories: Theory and experiment
Authors: Duiker H. M. , Beale P. D. , Scott J. F. , Paz de Araujo C. A. , Melnick B. M. , Cuchiaro J. D. , McMillan L. D.
Published: July 2002

A theoretical model of fatigue in ferroelectric thin-film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen-deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two-dimensional Ising model and with experimental data on small-grain (40 nm) sol-gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol-gel PZT switching data is obtained.
Characterization of a spheromak plasma gun: The effect of refractory electrode coatings
Authors: Brown M. R. , Bailey III A. D. , Bellan P. M.
Published: July 2002

In order to investigate the proposition that high-Z impurities are responsible for the anomalously short lifetime of the Caltech spheromak, the center electrode of the spheromak plasma gun has been coated with a variety of metals (bare steel, copper, nickel, chromium, rhodium, and tungsten). Visible light (230–890 nm) emitted directly from the plasma in the gun breech was monitored for each of the coated electrodes. Plasma density and temperature and spheromak lifetime were compared for each electrode. Results indicate little difference in gun performance or macroscopic plasma parameters. The chromium and tungsten electrodes performed marginally better in that a previously reported helicity injection effect [Phys. Rev. Lett. 64, 2144 (1990)] is only observed in discharges using these electrode coatings. There are subtle differences in the detailed line emission spectra from the different electrodes, but the spectra are remarkably similar. The fact that (1) contrary to expectations, attempts to reduce high-Z impurities had only marginal effect on the spheromak lifetime coupled with (2) an estimate of Zeff<2 based on a 0-D model suggests that it is not impurities but some other mechanism that limits the lifetime of small, cold spheromaks. We will discuss the general characteristics of the spheromak gun as well as effects due to the coatings.
Deep level transient spectroscopy study of proton irradiated p-type InP
Authors: Walters R. J. , Summers G. P.
Published: July 2002

A deep level transient spectroscopy study of proton irradiation induced defects in n+p InP mesa diodes grown by metalorganic chemical vapor deposition is reported. In contrast to results reported for InP grown by other methods, 3 MeV proton irradiation produced a DLTS spectrum similar to 1 MeV electron irradiation with the addition of two new peaks. Six majority carrier peaks: HP1(Ea=0.15 eV), H2(Ea=0.20 eV), H3(Ea=0.30 eV), H4(Ea=0.37 eV), H5(Ea=0.54 eV), and H7(Ea=0.61 eV) and three minority carrier peaks: EA(Ea=0.26 eV), EB(Ea=0.74 eV), and EC(Ea=0.16 eV) were detected. The H5 peak displayed a thermally activated capture cross section and a dependence of peak height on injection level. Isothermal annealing at 375 K was performed and thermal annealing rates are presented. Low temperature (200 K), minority carrier injection annealing rates are also presented. For most of the defects, a significant residual concentration remained after injection which could not be annealed further. An equation was developed for the annealing rate of the major defect, H4, as a function of injection level, carrier concentration, and temperature.
Spallation of metals under laser irradiation
Authors: Fortov V. E. , Kostin V. V. , Eliezer S.
Published: July 2002

Laser-driven shock-wave loading enables the study of ultrahigh strain rates (107 s−1). A damage model for the spallation process has been included in a two-dimensional finite-difference hydrodynamic elastoplastic code using Lagrangian coordinates in order to calculate the laser-induced spall phenomena. The damage was investigated using shock waves induced by a short-pulsed laser in copper and aluminum foils. Laser irradiation intensities were in the range of 1010–1012 W/cm2. The foil thickness was in the 100–600-μm range. The ablation pressure attained on the front surface of targets was a few hundred kilobars. The travelling time of the shock wave through the target was of the order of a few tens of nanoseconds. The spall width was calculated for both Al and Cu materials with different thicknesses of foils and various laser-induced shock-wave intensities. The numerical simulations were compared with previously reported experiments. Good agreement was obtained between experimental and simulation results for the spall pressure and the spall widths. The strain rates of materials under laser-induced shock loading were obtained from the computer code.
Investigation of floating body effects in silicon-on-insulator metal-oxide-semiconductor field-effect transistors
Authors: Ouisse T. , Ghibaudo G. , Brini J. , Cristoloveanu S. , Borel G.
Published: July 2002

An analytical model and experimental results are proposed to account for the floating body effects in SOI (silicon-on-insulator) MOSFETs (metal-oxide-semiconductor field-effect transistors). The model enables a quantitative description of hysteresis effects in the static characteristics and shows a strong correlation between the conductance and transconductance. It is demonstrated that an SOI MOSFET may exhibit both a negative conductance and a negative transconductance. A short-channel device may be biased in such a way that a critical phenomenon, comparable to a second-order phase transition, takes place. The major parameters and applications of these critical effects are described in detail.
Cadmium zinc sulfide films and heterojunctions
Authors: Chu T. L. , Chu S. S. , Britt J. , Ferekides C. , Wu C. Q.
Published: July 2002

Cadmium sulfide (CdS) and zinc sulfide (ZnS), direct gap semiconductors with room temperature band-gap energy of 2.42 and 3.66 eV, respectively, form a continuous series of solid solutions (Cd1−xZnxS). The band-gap energy of Cd1−xZnxS can be tailored in the range of the binary band gaps. In this work, polycrystalline films of Cd1−xZnxS have been deposited on glass, SnO2:F/glass, and ZnO:F/glass substrates by the reaction of dimethylcadmium (DMCd), diethlyzinc (DEZn), and propyl mercaptan (PM) in a hydrogen atmosphere. The deposition rate and properties of Cd1−xZnxS films depend on the substrate temperature and the composition and flow rate of the reaction mixture. The deposition rate of Cd1−xZnxS films has been measured at 375 and 425 °C as a function of the DMCd/DEZn molar ratio in the reaction mixture. Without intentional doping, the deposited films are of high lateral resistivity, and the resistivity increases with increasing ZnS concentration. The electrical resistivity of the deposited films can be reduced by using octyl chloride or trimethylaluminum as a dopant. The effects of DMCd/DEZn and (DMCd+DEZn)/PM molar ratios on the optical and electrical properties of Cd1−xZnxTe films have been investigated. Thin film heterojunctions have been prepared by the successive in situ metal organic chemical vapor deposition of Cd0.7Zn0.3S (Eg∼2.8 eV), an absorber, and the ohmic contact on a ZnO:F/glass substrate, and their electrical and photovoltaic properties characterized.
Electrostatic particle-particle interactions in electrorheological fluids
Authors: Chen Y. , Sprecher A. F. , Conrad H.
Published: July 2002

A model for the particle-particle interaction in a chain of spherical particles in an electrorheological (ER) fluid is developed based on an expansion of the potential which satisfies the Laplace equation. The electric-field distribution and force-distance relationship between particles are calculated as a function of Kp/Kf (the dielectric constant of the sphere relative to that of the medium) and particle-particle spacing using this model. The calculations predict electric-field concentrations of about an order of magnitude near particle contacts. This gives a force between particles that is approximately an order of magnitude higher than predicted by the classical point-dipole approximation, and is in reasonable agreement with that measured experimentally. Subtle effects of the carrier fluid on the interaction force and an explanation for the non-Ohmic behavior of ER fluids are deduced in terms of its dielectric constant and dielectric strength.
Preparation of YIG fine particles by mist pyrolysis
Authors: Matsumoto Koji , Yamanobe Yasunori , Sasaki Satoshi , Fujii Toshitaka , Honda Keisuke , Miyamoto Tosiaki
Published: July 2002

Yttrium iron garnet single crystal fine particles were synthesized by mist pyrolysis from an aqueous solution of yttrium and iron nitrates dissolved in distilled water. Mist was generated by an ultrasonic nebulizer operated at 1 MHz. Thermal decomposition was carried out by passing through the mist in a reaction tube kept a temperature at 800 °C. As-collected particles were spherical, but still amorphous. Fine YIG particles with the average diameter of 0.25 μm were obtained by calcination at 1000 °C for 20 min.
Saturation current and excess carrier distribution in exponentially graded p-n junctions
Authors: Schacham S. E. , Finkman E.
Published: July 2002

Excess carrier distribution and saturation current generated by a graded p-n junction are investigated, approximating the dopant profile by an exponential function. Analytical solutions of the steady-state continuity equation are presented for lifetime dominated by Auger, radiative, or Shockley–Read recombination mechanisms, for both ohmic and electrically reflecting boundary conditions. The saturation current generated by the graded region is derived for each of the dominant mechanisms and boundary conditions. Unlike previous published works the result of this analysis is a set of concise analytical expressions that make them a useful tool for simulation and investigation of graded structures such as silicon solar cells. The results are calculated for HgCdTe diodes, demonstrating the effect of the profile slope as determined by surface and bulk concentrations and junction depth. It is shown that the contribution of the graded region to the saturation current, as compared to that of the substrate, is significant, and that by proper selection of the diode parameters it is possible to substantially reduce this contribution.
Two-dimensional state of stress in a silicon wafer
Authors: Liang Hancheng , Pan Yongxiong , Zhao Shounan , Qin Ganming , Chin K. Ken
Published: July 2002

Two-dimensional state of stress in (001) and (111) silicon wafer is studied with infrared photoelasticity. In two widely used groups of coordinate systems, the silicon piezo-optical coefficient tensors due to photoelastic anisotropy of silicon crystal are derived. The relation between stress ellipsoid and refractive index ellipsoid is analyzed with infrared polarized light transmitting through the silicon crystal in certain directions. The applicability of the stress-optical law in (001) and (111) silicon wafers is presented. A three direction observation method is developed to decide the magnitude and direction of principal stresses in silicon wafer. The stress states in (100) and (111) silicon wafers after certain device processes are also measured and calculated. Comparisons of experimental and calculated results are made.
Preisach modeling of hysteresis for a pseudoelastic Cu-Zn-Al single crystal
Authors: Ortín Jordi
Published: July 2002

Stress-strain trajectories associated with pseudoelastic behavior of a Cu-19.4 Zn-13.1 Al (at. %) single crystal at room temperature have been determined experimentally. For a constant cross-head speed the trajectories and the associated hysteresis behavior are perfectly reproducible; the trajectories exhibit memory properties, dependent only on the values of return points, where transformation direction is reverted. An adapted version of the Preisach model for hysteresis has been implemented to predict the observed trajectories, using a set of experimental first-order reversal curves as input data. Explicit formulas have been derived giving all trajectories in terms of this data set, with no adjustable parameters. Comparison between experimental and calculated trajectories shows a much better agreement for descending than for ascending paths, an indication of a dissymmetry between the dissipation mechanisms operative in forward and reverse directions of martensitic transformation.
Electrical isolation of radio-frequency plasma discharges
Authors: Miller Paul A. , Anderson Harold , Splichal Michael P.
Published: July 2002

Plasmas used for processing of microelectronics materials are often powered by radio-frequency (rf) electrical sources. Such plasmas have nonlinear impedance characteristics which cause the plasma state to depend on the nature of the circuitry that supplies electrical power to the plasma. This dependency occurs because the harmonics of the fundamental drive frequency, which are generated by the plasma nonlinearity, interact with the impedance of the external circuitry at the harmonic frequencies. This report describes the successful use of an rf filter in the power feed to the plasma which isolates the plasma electrically and eliminates its sensitivity to changes in the rf generator, cable plant, and matching network.
p-i-n like behavior in three-layered organic solar cells having a co-deposited interlayer of pigments
Authors: Hiramoto Masahiro , Fujiwara Hiroshi , Yokoyama Masaaki
Published: July 2002

Three-layered organic solar cells, having a sandwiched interlayer of co-deposited p-type phthalocyanine (Pc) pigments and n-type perylene derivative (PTC) pigments, exhibited photocurrent enhancement when compared with two-layered cells without the interlayer. A large number of Pc/PTC molecular contacts within the interlayer serve as active sites for effective charge carrier photogeneration. The photovoltaic characteristics of the three-layered configuration suggest the formation of a p-i-n like structure, in which the built-in potential produced by Fermi level differences in the respective pigment layers is mainly distributed across the co-deposited interlayer. Inorganic semiconductor films are also shown to be applicable to the present p-i-n cell structure as an n layer to obtain larger photovoltages.
Comparative study of radiation-induced electrical and spin active defects in buried SiO2 layers
Authors: Hervé D. , Leray J. L. , Devine R. A. B.
Published: July 2002

The results of paramagnetic defect and positive fixed oxide charge creation in x-irradiated buried SiO2 formed by ion implantation and annealing are presented. Charged oxygen-vacancy centers are argued to be the source of spin active defects but not the primary source of fixed oxide charge. Hydrogenation or fluorination of the oxide enhances the radiation sensitivity for creation of spin active defects but not of trapped positive charge. Annealing of the spin active defects may proceed by a mechanism similar to that involved in charged defect annealing or by the trapping of thermally detrapped, diffusing electrons. Annealing does not involve the trapping of diffusing molecular species as is usual in bulk oxides.
Cation distribution and magnetization of BaFe12−2xCoxSnxO19 (x=0.9,1.28) single crystals
Authors: Sandiumenge Felip , Martínez Benjamín , Batlle Xavier , Galí Salvador , Obradors Xavier
Published: July 2002

The distribution of Sn4+ cations within the five crystallographic sites of the magnetoplumbite (M) -like compound BaFe12−2xCoxSnxO19 has been analyzed using single-crystal x-ray-diffraction data. The species Fe3+ and Co2+ cannot be distinguished using x rays because of their very similar atomic numbers; however, the calculation of the apparent valencies for the different sites allows an insight into the Co2+ cation segregation. The use of previous data from neutron powder diffraction allows a precise picture of the cation distribution, which indicates a pronounced site selectivity for both Sn4+ and Co2+ cations. The Sn4+ cations prefer the 4f2 sites and to a much lower extent the 12k sites, while they do not enter the octahedral 2a sites at all. Co2+ cations are distributed among tetrahedral and octahedral sites displaying a clear preference for the tetrahedral 4f1 sites. Magnetic measurements indicate that the compound still exhibits uniaxial anisotropy with the easy direction parallel to the c axis. Nevertheless, the magnetic structure shows a considerable degree of noncolinearity. A strong reduction of the magnetic anisotropy regarding that of the undoped compound is also detected.
Numerical study of relativistic magnetrons
Authors: Chan Hei-Wai , Chen Chiping , Davidson Ronald C.
Published: July 2002

The relativistic multiresonator magnetron is analyzed in full cylindrical geometry, using the two-dimensional particle-in-cell simulation code magic. Detailed comparisons are made between the simulation results and the classic experiments by Palevsky and Bekefi for the relativistic A6 magnetron configuration [Phys. Fluids 22, 986 (1979)]. The computer simulations show the same dependence of microwave power on axial magnetic field as measured in the experiment. It is found that the electron flow in the preoscillation regime differs substantially from ideal Brillouin flow, and that the nonlinear regime is characterized by large-amplitude spoke formation. By analyzing a class of relativistic magnetrons for a wide range of operating voltage and axial magnetic field, it is found that the optimal microwave power exhibits an approximately cubic dependence on the applied diode voltage. Moreover, the simulations indicate that relativistic magnetrons with small aspect ratio oscillate predominantly in the π mode, whereas magnetrons with large aspect ratio can oscillate in various modes which depend upon the system parameters. Finally, it is shown that a finite power-supply impedance can strongly modify the resonance condition, thereby significantly degrading the magnetron performance.
Ion drag on an isolated particulate in a low-pressure discharge
Authors: Kilgore M. D. , Daugherty J. E. , Porteous R. K. , Graves D. B.
Published: July 2002

Classical scattering theory is used to calculate momentum-transfer cross sections between positive ions and isolated particulates in glow discharges for several model particle sheath potentials. Cross sections calculated from a potential profile obtained using self-consistent orbit (Poisson–Vlasov) theory, an attractive screened Coulomb potential, and an attractive cutoff Coulomb potential are compared. At high ion energies, all model potentials result in the same cross-section dependence on ion energy. At low ion energies, however, the cutoff Coulomb potential gives a cross section too low by about a factor of 10. The screened Coulomb potential and Poisson–Vlasov theory potential result in momentum-transfer cross sections equal to within 30% over the entire range of ion energies. An approximate analytic expression for the momentum transfer rate coefficient is given for order of magnitude estimation of the ion drag force.
Evaluation of fatigue damage in steel structural components by magnetoelastic Barkhausen signal analysis
Authors: Govindaraju M. R. , Strom A. , Jiles D. C. , Biner S. B. , Chen Z.-J.
Published: July 2002

This paper is concerned with using a magnetic technique for the evaluation of fatigue damage in steel structural components. It is shown that Barkhausen effect measurements can be used to indicate impending failure due to fatigue under certain conditions. The Barkhausen signal amplitude is known to be highly sensitive to changes in density and distribution of dislocations in materials. The sensitivity of Barkhausen signal amplitude to fatigue damage has been studied in the low-cycle fatigue regime using smooth tensile specimens of a medium strength steel. The Barkhausen measurements were taken at depths of penetration of 0.02, 0.07, and 0.2 mm. It was found that changes in magnetic properties are sensitive to microstructural changes taking place at the surface of the material throughout the fatigue life. The changes in the Barkhausen signals have been attributed to distribution of dislocations in stage I and stage II of fatigue life and the formation of a macrocrack in the final stage of fatigue.
Magnetoelastic effects in rare-earth iron-aluminum compounds
Authors: Prajapati K. , Jenner A. G. , Schulze M. P. , Greenough R. D.
Published: July 2002

Magnetic and magnetoelastic measurements under various conditions of compressive stress and applied field have been carried out on samples with the composition Tb0.3Dy0.7(Fe1−xAlx)1.95, 0≤x≤0.1, prepared by a float-zone growth technique. Their magnetoelastic performances compare favorably with the highly magnetostrictive ‘‘Terfenol-D’’ compounds by having large magnetostrictive strains (λ∼1200 ppm at ∼120 kA/m), good magnetomechanical coupling (k33≊62%) and a large strain coefficient (d33≊80 nm/A). The substitution of aluminum for iron produces a material with potential benefits for use in actuators: higher electrical resistivity and improved ductility, which reduce eddy current effects and increase the mechanical strength.
Thermal and electrical conductivity of monolithic carbon aerogels
Authors: Lu Xianping , Nilsson Ove , Fricke Jochen , Pekala Richard W.
Published: July 2002

The thermal and electrical conductivity of monolithic carbon aerogels was investigated at room temperature. Results showed both the solid thermal conductivity and the electrical conductivity scale with the density in the range between 60 and 650 kg m−3. The scaling exponents for the two conductivities have identical values of 1.5. For a density of 82 kg m−3 a thermal conductivity of 0.029 W m−1 K−1 in air and 0.018 W m−1 K−1 after evacuation was found.
Al-substitution effects on the magnetic properties of RCo5 (R=Y, Pr, Nd, and Sm)
Authors: Konno K. , Ido H. , Cheng S. F. , Sankar S. G. , Wallace W. E.
Published: July 2002

Al substitution effects on the magnetic properties of RCo5 (R=Y, Pr, Nd, and Sm) are studied on the basis of magnetic measurements for RCo5−xAlx with x less than 2. The Al substitution considerably decreases the Curie temperature, the Co-sublattice moment, the magnetocrystalline anisotropy of the R sublattice as well as of the Co sublattice. The Co-sublattice moments in RCo5−xAlx have a different value depending on their directions in the crystal; in addition the rare earth moments are considerably smaller than the values of their free trivalent ions. The decrease of Co-sublattice moment with increasing content of Al can be explained by the rigid band model, and the properties of rare earth sublattices (or atoms) mentioned above seem to be explained by the exchange field at the rare earth site which weakens with increasing Al content.
High resolution x-ray characterization of Co films on Al2O3
Authors: Stierle A. , Abromeit A. , Metoki N. , Zabel H.
Published: July 2002

We have studied the structural properties of Molecular Beam Epitaxy grown thin Co films on α-sapphire (112̄0) substrates by high resolution x-ray scattering measurements in the thickness range of 100 Å to 5000 Å. The electron density profile perpendicular to the film plane was determined by x-ray specular reflectivity measurements. The profile contains information about the total metal film thickness, an oxide layer on top of the film and about the surface and interface roughness. The total metal film thickness was compared with the number of coherently scattering atomic planes by taking radial scans through Bragg peaks parallel to the growth direction. Samples up to 400 Å total thickness (out-of-plane mosaicity 0.02○) show very strong Laue-oscillations about the Co (0002)/(111) Bragg peak. Above a total thickness of 400 Å the oscillations vanish due to a loss in structural coherence. The combination of specular reflectivity and Bragg scans presents a powerful means to analyze the structural coherence of thin films.
The direct observation of cooperative grain-boundary sliding and migration during superplastic deformation of lead-tin eutectic in shear
Authors: Zelin M. G. , Dunlap M. R. , Rosen R. , Mukherjee A. K.
Published: July 2002

Direct observation of superplastic deformation in shear in a scanning electron microscope in Pb-62% Sn eutectic alloy at different microstructural levels were performed. The sliding of grains as an entity was observed. Such cooperative grain-boundary sliding proceeds by means of the sequential shear of grains along shear surfaces, and is accompanied by cooperative grain-boundary migration. The pattern of shear surfaces at the level of the entire deformed volume is close to one that is predicted by slip-lines field theory. The processes to accommodate deformation of grain-boundary sliding include the operation of grain-boundary migration, intragranular deformation, and grain rotation. A critical analysis of the proposed geometrical models for superplastic flow is given. The model of sequential shear of grains as a result of the movement of cellular dislocations is extended to the case of the two-phase material, having structure that is typical for superplastic eutectics.
600-nm wavelength range GaInP/AlInP quasi-quaternary compounds and lasers prepared by gas-source molecular-beam epitaxy
Authors: Kaneko Yawara , Nomura Ichirou , Kishino Katsumi , Kikuchi Akihiko
Published: July 2002

GaInP/AlInP quasi-quaternary (QQ) compounds grown by gas-source molecular-beam epitaxy were systematically investigated. Quasi-quaternary properties were obtained with GaInP/AlInP short-period superlattices when the thickness of the AlInP layers was less than 3 ML. Equivalent band-gap values of QQ compounds were defined in terms of the layer thickness ratios of the GaInP and AlInP layers. Using GaInP/AlInP quasi-quaternary compounds, 607–640-nm wavelength QQ lasers with 20-nm QQ active layers were fabricated. Threshold current densities as low as 1.5 kA/cm2 were attained for QQ lasers emitting at 635 nm without the presence of strain or quantum-well effects.
Transport of YO molecules produced by ArF laser ablation of YBa2Cu3O7−δ in ambient oxygen gas
Authors: Kumuduni Wanniarachchi K. A. , Nakayama Yasushi , Nakata Yoshiki , Okada Tatsuo , Maeda Mitsuo
Published: July 2002

One-dimensional-imaging laser-induced fluorescence spectroscopy (1D-LIF) has been applied to investigate the dynamics of the nonemissive neutral particles (YO molecules) during the ArF excimer laser ablation of YBa2Cu3O7−δ in an ambient oxygen gas. Investigating the 1D-LIF observation, the propagation of particles through the ambient gas at appropriately high pressures is categorized into two phases, the propagation phase and the diffusion phase. In the propagation phase, the point source blast wave model (shock model) describes well the dynamics at high background gas pressures. Particles propagate according to the shock model over a finite distance from the pellet surface after the ablation, and almost stop there. The propagation distance depends on the ambient gas pressure and the ablation fluence. After the propagation ceases, the particles start to diffuse through the background gas; that is the diffusion phase. Rotational temperature variations of YO molecules in the different phases are also measured. Rotational temperatures as high as 1000 K are observed even in the diffusion phase.
Saturation magnetization and anisotropy fields in the Sm(Co1−xCux)5 phases
Authors: Lectard E. , Allibert C. H. , Ballou R.
Published: July 2002

The magnetic characteristics of high-anisotropy Sm(Co1−xCux)5 phases are determined experimentally at 300 K in the range x<0.6. The saturation magnetization Ms decreases and vanishes for SmCo2Cu3. Identical behaviors of Ms and Curie temperature TC versus x are observed in the range x<0.6. For significant Cu substitution rate (x≳0.35), the decrease of the anisotropy field (HA) when x increases is strong. For lower Cu contents, huge values of anisotropy fields make difficult the accurate determination of the substitution effect. The first anisotropy constant K1 is calculated using the available values of HA and Ms. A linear decrease of K1 is observed from SmCo5 to SmCo2Cu3.
Comparative Mössbauer effect study of several R2Fe17 and R2Fe17Nx compounds
Authors: Long Gary J. , Mishra S. , Pringle O. A. , Grandjean F. , Buschow K. H. J.
Published: July 2002

The Mössbauer spectra of Sm2Fe17 and Ho2Fe17 and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R2Fe17 and R2Fe17Nx compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare-earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic lattice, an expansion which is mainly centered on the 9d and 18h iron sites as is indicated by the increase of their Wigner–Seitz cell volumes upon nitrogenation. The 9d and 18h sites show a larger enhancement of their hyperfine fields as compared to the 6c and 18f sites as a result of improved ferromagnetic exchange between these sites and their near neighbors because of the lattice expansion and the consequent reduced iron 3d-iron 3d overlap.
Shock wave equation of state of powder material
Authors: Dijken D. K. , De Hosson J. Th. M.
Published: July 2002

A model is proposed to predict the following quantities for powder materials compacted by shock waves: the pressure, the specific volume, the internal energy behind the shock wave, and the shock-wave velocity Us. They are calculated as a function of flyerplate velocity up and initial powder specific volume V00. The model is tested on Cu, Al2024, and Fe. Calculated Us vs up curves agree well with experiments provided V00 is smaller than about two times the solid specific volume. The model can be used to predict shock-wave state points of powder or solid material with a lower or higher initial temperature than room temperature.
Electron mobilities in gallium, indium, and aluminum nitrides
Authors: Chin V. W. L. , Tansley T. L. , Osotchan T.
Published: July 2002

Electron mobilities in GaN and InN are calculated, by variational principle, as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter. Both GaN and InN have maximum mobilities between 100 and 200 K, depending on the electron density and compensation ratio, with lower electron density peaking at lower temperature. This is due to the interplay of piezoelectric acoustic phonon scattering at low carrier concentrations and ionized impurity scattering at higher carrier concentrations. Above 200 K, polar mode optical phonon scattering is the mobility limiting process. The 300 and 77 K electron and Hall mobilities as functions of carrier concentration in the range of 1016–1020 cm−3 and compensation ratio are also calculated. The theoretical maximum mobilities in GaN and InN at 300 K are about 1000 and 4400 cm2 V−1 s−1, respectively, while at 77 K the limits are beyond 6000 and 30 000 cm2 V−1 s−1, respectively. We compare the results with experimental data and find reasonable correlation, but with evidence that structural imperfection and heavy compensation play important roles in the material presently available. Only phonon limited scattering processes are considered in the calculation of the mobility in AlN since it is an insulator of extremely low carrier concentration. We find a phonon limited electron mobility of about 300 cm2 V−1 s−1 at 300 K.
Modeling of InGaAs/InAlAs coupled double quantum wells
Authors: Chin M. K.
Published: July 2002

A symmetric coupled double-quantum-well (CDQW) structure in InGaAs/InAlAs lattice matched to InP can give enhanced Stark effects for electroabsorption modulation near 1.3 μm. The optical features of this material are the dramatic quenching of the heavy-hole exciton and the simultaneous emergence of the normally forbidden higher-order transitions. An effective-mass model for the double-quantum-well structure yields predictions in good agreement with experimental results. The model can be used to predict the optical properties of not only symmetric CDQWs but also asymmetric structures as well.
Observation of degradation processes of Al electrodes in organic electroluminescence devices by electroluminescence microscopy, atomic force microscopy, scanning electron microscopy, and Auger electron spectroscopy
Authors: Do L. M. , Han E. M. , Niidome Y. , Fujihira M. , Kanno T. , Yoshida S. , Maeda A. , Ikushima A. J.
Published: July 2002

Degradation of top electrodes is one of the most important factors to determine the lifetimes of organic electroluminescence (EL) devices. An organic EL device [indium thin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4, 4′-diamine (TPD)/tris(8-hydroxy- quinoline)aluminum (Alq3)/Al] was prepared and a morphological change of the Al top electrode was observed during and/or after applying voltage by atomic force microscopy and scanning electron microscopy (SEM). The change in the electrode surface, i.e., the increase in surface roughness was observed during the current flow. The degradation process started from faint dark core parts and propagated into disks with different rates depending on the magnitude of applied voltage. Degraded sites of the Al electrode, which were analyzed as aluminum oxide by Auger electron spectroscopy, protruded into the air on the organic layers. In SEM images of a life-end electrode, discontinuities due to crevasse formation in the organic layers sandwiched by the ITO base and the metal top electrodes were observed in many places. These results confirm that one of the most crucial factors of the degradation process was deformation of metal and organic layers due to heat, gas evolution, and oxidation caused by applied voltage.
Electric field-induced antiferroelectric-to-ferroelectric phase transition in lead zirconate titanate stannate ceramics modified with lanthanum
Authors: Shebanov L. , Kusnetsov M. , Sternberg A.
Published: July 2002

Results of measurements in situ of electrostrictive strain, dielectric polarization, dielectric constant, and crystallographic parameters as functions of applied electric field in the temperature range 20–200 °C of Pb0.97La0.02(Zr0.66Ti0.11Sn0.23)O3 composition are reported. The antiferroelectric to ferroelectric phase transition with large volume change ΔV=0.35 Å3 is shown to be the dominant mechanism of the field-induced strain. The microscopic nature of the switching mechanism and the variation of the strain versus polarization squared at various temperatures are discussed.
Spatial variation of the electron distribution function in a rf inductively coupled plasma: Experimental and theoretical study
Authors: Kortshagen U. , Pukropski I. , Zethoff M.
Published: July 2002

The spatial dependence of the electron distribution function (EDF) and of related macroscopic parameters in an inductively coupled argon plasma at 13.56 MHz is studied. The rf induction field is generated using a planar coil. The EDF is determined experimentally with axial and radial resolution via a Langmuir probe technique. The experimental results demonstrate that the EDF can be considered as a spatially homogeneous function of the total energy of electrons. The axial variation of the EDF is described by a one-dimensional, kinetic model, which is based on the nonlocal approach to the solution of the spatially inhomogeneous Boltzmann equation. Good agreement between measured and calculated electron distribution functions is found. Also the axial profiles of the space-charge potential, the electron density, and the mean kinetic energy of the electrons are well reproduced by the theoretical model.
Refractive indices measurement of (GaInP)m/(AlInP)n quasi-quaternaries and GaInP/AlInP multiple quantum wells
Authors: Kaneko Yawara , Kishino Katsumi
Published: July 2002

Measurements of the refractive indices of (GaInP)m/(AlInP)n quasi-quaternaries (QQs), GaInP/AlInP multiple quantum wells (MQWs), and (AlxGa1−x)0.5In0.5P quaternaries were made systematically, using the reflectance method, in photon energy ranges nearly as high as up to the band gap. Data was fitted using the modified single effective oscillator (MSEO) method. A single oscillator energy (Eo) of 4.17+0.49xeq and dispersion energy (Ed) of 35.79–1.16xeq was obtained for (GaInP)m/(AlInP)2 QQs, where the equivalent Al composition xeq is defined by the stacking film thickness ratio xeq=d(AlInP)/[d(GaInP)+d(AlInP)]. Agreement of refractive indices obtained for QQs and quaternary compounds with equivalent xeq has been confirmed. Still, for the GaInP/AlInP MQWs, MSEO fitting was also agreeable, using the same oscillator energy Eo and dispersion energy Ed of the (GaInP)m/(AlInP)2 QQs with the same thickness ratio, and substituting band gap energy EΓ values shifted due to quantum effects.
Electron emission from pseudospark cathodes
Authors: Anders André , Anders Simone , Gundersen Martin A.
Published: July 2002

The pseudospark cathode has the remarkable property of macroscopically homogeneous electron emission at very high current density (≳1 kA/cm2) over a large area (some cm2). The model of electron emission presented here is based on the assumption that the pseudospark microscopically utilizes explosive arc processes, as distinct from earlier models of ‘‘anomalous emission in superdense glow discharges.’’ Explosive emission similar to vacuum are cathode spots occurs rapidly when the field strength is sufficiently high. The plasma remains macroscopically homogeneous since the virtual plasma anode adapts to the cathode morphology so that the current is carried by a large number of homogeneously distributed cathode spots which are similar to ‘‘type 1’’ and ‘‘type 2’’ spots of vacuum arc discharges. The net cathode erosion is greatly reduced relative to ‘‘spark gap-type’’ emission. At very high current levels, a transition to highly erosive spot types occurs, and this ‘‘arcing’’ leads to a significant reduction in device lifetime. Assuming vacuum-arc-like cathode spots, the observed current density and time constants can be easily explained. The observed cathode erosion rate and pattern, recent fast-camera data, laser-induced fluorescence, and spectroscopic measurements support this approach. A new hypothesis is presented explaining current quenching at relatively low currents. From the point of view of electron emission, the ‘‘superdense glow’’ or ‘‘superemissive phase’’ of pseudosparks represents an arc and not a glow discharge even if no filamentation or ‘‘arcing’’ is observed.
Time dependence of switching fields in magnetic recording media (invited)
Authors: Sharrock M. P.
Published: July 2002

Coercivity and other experimental measures of switching field depend upon the time scale of interest. This time-scale dependence has practical importance in magnetic recording, because the effective time scales of writing and storage are very different. A simple model accounts for the time-scale dependence of coercivity in terms of the thermally assisted crossing of an energy barrier whose height is reduced by the applied field. Fitting this barrier-crossing model to data provides an estimate of the volume that must switch magnetization direction in overcoming the barrier. The assumption of Stoner–Wohlfarth reversal is used to obtain an initial estimate of the dependence of the barrier height on the field. With some adjustment of the resulting energy expression, the model gives good agreement between calculated switching volume and actual particle volume for advanced recording particles of three types: acicular oxide, acicular metal, and barium ferrite platelets. The model can be used to estimate minimum practical particle volumes for use in magnetic recording. Switching due to fields nearly perpendicular to the particle’s preferred axis, sometimes used as a measure of the anisotropy field, also shows the effect of thermal assistance. The switching volume determined from such measurements, like that from coercivity, approximates the particle volume.
Linear electro-optic effect based on flexoelectricity in a cholesteric with sign change of dielectric anisotropy
Authors: Rudquist P. , Buivydas M. , Komitov L. , Lagerwall S. T.
Published: July 2002

The linear electro-optic effect in short-pitch cholesterics is based on the linear coupling of the medium with an applied electric field. It has a number of remarkable properties. The electric field causes the optic axis to tilt in a plane parallel to the surfaces of the cell glass plates, giving the same symmetry as the electro-optic effects in the smectic C* phase (surface stabilized ferroelectric liquid crystals and deformed helix mode) and the smectic A* phase (soft mode/electroclinic effect). For shutters and displays this guarantees a very wide viewing angle. The induced tilt is a linear function of the applied field, at least for small fields, which gives a well-controlled continuous grey scale. Furthermore, it is practically independent of temperature. Response times of the order of 100 μs are easily achievable. The most interesting development in this effect would be to extend the linear regime to much larger tilt angles, in particular to 22.5°, where light could be modulated from 100% to zero transmission. In order to do this the perturbation from the quadratic dielectric coupling has to be ruled out or minimized, which requires materials with essentially zero dielectric anisotropy. This has been done, and it has been found that the bare flexoelectric-induced tilt has a surprising range of linearity: The linear response in tilt could be followed up to about 30° after which the high electric field caused breakdown. The response time is typically about 100 μs and below.
Hypersound anomalies and elastic constants in single-crystal PbMg1/3Nb2/3O3 by Brillouin scattering
Authors: Tu C.-S. , Schmidt V. Hugo , Siny I. G.
Published: July 2002

The longitudinal (LA) and transverse (TA) Brillouin spectra along [001] phonon direction have been measured as a function of temperature (50–475 K) in single-crystal PbMg1/3Nb2/3O3 with scattering angles θs=180° and 32.5±0.2°. The Brillouin frequency shift with decreasing temperature shows a broad softening anomaly for both LA and TA phonon modes. For θs=180°, a gradual growth in damping with maximum near 270 K is observed and is attributed to order parameter fluctuations. An additional Landau–Khalatnikov maximum is also observed at Tc∼212 K. This anomaly implies a rapid growth of ferroelectric ordering near Tc and is consistent with the earlier linear birefringence results reported by Westphal et al. The elastic stiffness and compliance constants, CE11, CE44, and sE44 are also determined between 200 and 370 K.
Dynamic process of Vickers indentation made on glass surfaces
Authors: Yoshioka Masato , Yoshioka Naoto
Published: July 2002

Dynamic processes of Vickers indentation on glass surfaces were observed with a newly developed apparatus. It enables us to take successive picture images of a growing indentation from behind a transparent specimen with a CCD camera through an optical microscope with a minimum time interval of 16.7 ms. Growing rates of indentations were measured under several conditions of loading weight and collision velocity of the indenter with the sample surface. An indentation rapidly grows at the initial stage, followed by decrease in growing rate, finally approaching a value specified with the loading weight and the hardness of glass. This behavior can be described by a Voigt model, indicating that ‘‘visco-elastic’’ nature in a microscopic sense is involved in the indentation process of glass. It is also found that the initial growing rate highly depends on the collision velocity; that is, the larger the collision velocity is, the higher the initial growing rate is. The result suggests that local temperature rise in the vicinity of the indentation due to friction or adiabatic compression reduces the viscosity of indented material.
Thermal emission of trapped holes in thin SiO2 films
Authors: Lu Yi , Sah Chih-Tang
Published: July 2002

Distributed (∼1.4 eV) and discrete (<∼0.07 eV) hole traps have been detected in thermally grown, pure, thin SiO2 films using the thermally stimulated charge technique from 77 to 594 K. The distribution of hole traps has a U-shaped density of states with a minimum at about 0.7 eV above the SiO2 valence band edge and is attributed to the amorphous band-edge tail states from the SiO2 valence band. The discrete hole trap is located at 1.44±0.20 eV above the SiO2 valence band edge, which was attributed to the oxygen vacancy center. The experimental energy level is consistent with that calculated by Rudra and Fowler [Phys. Rev. B 35, 8223 (1987)] and by O’Reilly and Robertson [Phys. Rev. B 27, 3780 (1993)].