bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the elastic constants of a cobalt disilicide intermetallic compound
Authors: Guénin G. , Ignat M. , Thomas O.
Published: July 2002

Ultrasonic velocity measurements have been performed on a single crystal of cobalt disilicide by the pulse echo overlap method. The elastic constants have been calculated from the measurements of the propagation velocities of longitudinal and transverse waves in suitable crystallographic directions of the sample. The macroscopic elastic parameters of the cobalt disilicide were deduced from the elastic constants. They allow an estimation of the magnitude of the induced residual thermal stresses when this compound is present in a microelectronic device.
Photoreflectance characterization of surface Fermi level in as-grown GaAs(100)
Authors: Kanata T. , Matsunaga M. , Takakura H. , Hamakawa Y. , Nishino T.
Published: July 2002

Photoreflectance (PR) spectroscopy has been used to study the surface Fermi level in GaAs by taking account of photovoltaic properties at the semiconductor surface. The PR signal amplitude ‖ΔR/R‖ is proportional to a modulating photovoltage Vm generated by a modulation light irradiation. To modify the surface potential, the sample was irradiated by a third perturbing light, i.e., a continuous bias light Pb, together with the modulation light. The modulation light power dependence of ‖ΔR/R‖ is extremely sensitive to bias light intensity, surface Fermi level, and temperature. From the analysis of the temperature dependence of ‖ΔR/R‖ on modulation-light power, the surface Fermi level of 0.47±0.09 eV below the conduction band was determined for a molecular beam epitaxially-grown GaAs(100).
Minority-carrier lifetime in p-type (111)B HgCdTe grown by molecular-beam epitaxy
Authors: de Souza M. E. , Boukerche M. , Faurie J. P.
Published: July 2002

Measurements of electron lifetime have been carried out on unintentionally doped layers of (111)B Hg1−xCdxTe (0.216<x<0.320), p-type as-grown by molecular-beam epitaxy. The temperature dependence of the bulk lifetime is explained in terms of the Shockley–Read recombination mechanism for the extrinsic region. The samples have shown a deep level close to midgap. The surface recombination seen in two of the samples is accounted for. Some samples show clearly Auger-limited recombination at high temperatures. From the values of τn0 and τp0 of two samples the trap level appears to have a donorlike character.
Structure and energy of general grain boundaries in bcc metals
Authors: Wolf D.
Published: July 2002

The zero-temperature energies and equilibrium volume expansions of point-defect-free asymmetrical grain boundaries (GBs) in bcc metals are determined using both a many-body potential fitted for Mo and a Johnson-type pair potential spline-fitted for α-Fe. The asymmetrical combinations of lattice planes considered involve one of the five densest planes of the bcc lattice on one side of the interface and a commensurate higher-index plane on the other. As in similar recent work on fcc metals, the two asymmetrical pure tilt boundaries obtained for any given combination of lattice planes give rise to pronounced energy cusps. When a twist component is added to the asymmetrical GB, thus forming a general (or ‘‘asymmetrical twist’’) boundary, the energy and planar unit-cell area increase due to the introduction of screw dislocations. A comparison with earlier work on symmetrical GBs in bcc metals suggests that, except for the densest lattice planes, asymmetrical boundaries may actually have lower energies than symmetrical ones. The underlying causes are elucidated via a comparison with recent simulations of free surfaces and with the random grain-boundary limit (in which the interactions of the atoms across the interface are assumed to be entirely random).
Generic scale invariance in classical nonequilibrium systems (invited)
Authors: Grinstein G.
Published: July 2002

Unlike those in equilibrium, dissipative nonequilibrium systems are capable of generic scale invariance−correlations that decay algebraically in space and time for arbitrary parameter values. For one class of such systems, viz., those (such as fluids in a temperature gradient) subjected to external white noise, the existence of a conserved order parameter is believed to be a necessary and almost sufficient condition for generic scale invariance to occur. The evidence for this assertion and the few exceptions, i.e., noisy, conserving nonequilibrium systems with exponential decays of spatial correlations, are discussed using illustrative examples taken from magnetism wherever possible. Simple calculations of exponents characterizing the algebraic decays are shown. A second class of nonequilibrium systems, exemplified by models of sandpiles and earthquakes, which have also been argued to exhibit generic scale invariance, or ‘‘self-organized criticality,’’ are briefly discussed. These systems are either noiseless or are subjected to strongly correlated external noise, and include among them at least one apparent magnetic realization. The conditions under which scale invariance can occur in this class, and in particular whether a conservation law is necessary, are still unclear.
Deep-level characterization of n-type GaAs by photoreflectance spectroscopy
Authors: Kanata T. , Matsunaga M. , Takakura H. , Hamakawa Y. , Nishino T.
Published: July 2002

Photoreflectance (PR) signal of n-type GaAs grown by molecular-beam epitaxy has been systematically studied as a function of modulation frequency in the PR measurements. The trap activation energy obtained from analysis of the frequency response of PR signal with its temperature dependence is 0.34 eV. The dramatic change in the frequency response has been observed as a function of the modulation (ac) and bias (dc) light intensities. The theoretical analysis has been made by assuming a single level electron trapping model. The calculated result shows a good agreement with the experimental data. Furthermore, photoluminescence spectrum associated with the deep level of the molecular-beam-epitaxy-grown GaAs was measured and compared with the PR data.
Breakdown phenomena in microwave transmit-receive switches
Authors: Löfgren M. , Anderson D. , Bonder H. , Hamnén H. , Lisak M.
Published: July 2002

A detailed experimental and theoretical investigation is made of the physical phenomena involved in the generation of a breakdown plasma in a microwave transmit-receive switch and in the subsequent interaction between the plasma and incident microwave. Particular emphasis is given to characteristics like breakdown and sustainment levels, turn-on time, and reflected, transmitted, and absorbed powers.
Upconversion-pumped 2.8–2.9-μm lasing of Er3+ ion in garnets
Authors: Pollack S. A. , Chang D. B. , Birnbaum M. , Kokta M.
Published: July 2002

Upconversion-pumped lasing characteristics near 3 μm on the 4I11/2-4I13/2 transition of the Er3+ ion in Er:YSGG (erbium-yttrium-scandium-gallium garnet) and Er:YAG (erbium-yttrium-aluminum-garnet), laser state spectroscopy, and population kinetics, are the main subjects discussed. The wide difference in lasing patterns of both garnets can be attributed to the difference in population inversion kinetics which was studied by analyzing the rise and decay of fluorescence starting from the laser states. It is shown that the effective decay times, τ1 and τ2 of the lower and upper laser states, are changed during lasing by the nonlinear energy-transfer processes such that the resulting ratio becomes τ1/τ2<1. In Er:YSGG this condition is established during the declining phase of the 4I11/2 population, whereas in Er:YAG it occurs only for a short time during the rising phase. Consequently Er:YAG lases in a self-terminating pulsed mode. The role of various energy-transfer processes in the upconversion-pumped population inversion kinetics is discussed and analyzed. It is experimentally shown that the lower 4I13/2 laser state in both garnets is depopulated by the cooperative energy summing due to the di- and tri-ionic interactions. Because of these interactions, the 4I11/2 and 4S3/2 states are populated with ensuing fluorescence from these states at 980 and 550 nm, respectively. The indication that the latter state is populated directly by the tri-ionic interaction, rather than via the former state by two sequential ion-pair interactions, follows from the experimental fact that the 550-nm fluorescence rises faster and reaches its maximum sooner than the 980-nm fluorescence. The previously proposed kinetic rate equations for Er:YAG and Er:YSGG were modified by adding a cubic term to represent the tri-ionic interaction. The steady-state solutions yielded approximately cubic dependence of green fluorescence intensity on the intensity of pump radiation in agreement with the experiment. These solutions also explained deviations from cubic dependence observed experimentally in several erbium-doped crystals.
Research for room-temperature magnetic refrigerants in R
xCe2−xFe17 series
Authors: Jin Sui-Geng , Liu Lin-Mei , Wang Yun-Lan , Chen Bao-Xin
Published: July 2002

The Curie temperature Tc of some iron-based pseudobinary alloys such as RxCe2−xFe17, can be adjusted to room temperature by changing x in the range from 0 to 2 when R is Y or Pr. Therefore the room-temperature magnetocaloric effects (RTME) can be greatly increased. When R=Pr and x=1.5, the RTME of the alloy reaches the maximum, which is greater than that of Fe by a factor 30 (about one-half that of Gd). The results are explained in terms of the phase diagram and the critical exponent law.
Comparison of single- and double-heterostructure AlGaAs/InGaP red light-emitting diodes prepared by liquid-phase epitaxy
Authors: Lee Chong-Yi , Wu Meng-Chyi , Lu Shoei-Chyuan
Published: July 2002

The performance of the AlGaAs/InGaP single-heterostructure (SH) and AlGaAs/InGaP/AlGaAs double-heterostructure (DH) light-emitting diodes (LEDs) is compared. For these two types of LEDs, they have a very shining surface morphology and flat interface. However, the SH LEDs always exhibit better properties than the DH LEDs in the ideality factor, electroluminescence, light output power, and external quantum efficiency. From the x-ray diffraction measurements, we find that the main problem for the undesirable performance of DH LEDs is attributed to the difficulty of obtaining the high quality InGaP/AlGaAs heterostructure. Furthermore, by the Auger-depth-profile measurements, it is found that the P atoms with a very high diffusivity will diffuse into the as-grown AlGaAs layer during the growth of InGaP and thus destroy the heterostructure quality.
Defect-property correlations in garnet crystals. VI. The electrical conductivity, defect structure, and optical properties of luminescent calcium and cerium-doped yttrium aluminum garnet
Authors: Rotman S. R. , Tuller H. L. , Warde C.
Published: July 2002

The electrical and optical properties of calcium and cerium-doped yttrium aluminum garnet (Ca,Ce:YAG) have been studied. Ca,Ce:YAG is a mixed ionic and electronic conductor with an ionic conductivity activation energy of 4.3 eV. Evidence of cluster formation with a consequent higher-than-expected activation energy is presented. The cerium normally enters the crystal as Ce+4, but it may be converted to Ce+3 under reducing atmospheres at elevated temperatures.
Pore morphology and the mechanism of pore formation in n-type silicon
Authors: Searson Peter C. , Macaulay John M. , Ross Frances M.
Published: July 2002

Porous layers formed in n-type silicon are characterized by pores of square cross section oriented perpendicular to the (100) plane. The pore walls are defined by the {011} planes in the [100] zone with secondary pores propagating from the main pores in the 〈010〉 and 〈001〉 directions. The pores are located in an array with a characteristic spacing. The characteristic pore morphology during stable pore propagation is explained in terms of a tunneling mechanism due to a high-field region created by the curvature at the pore front. The pore spacing is determined by the distance at which the region between the pores becomes fully depleted.
N vacancies in AlxGa1−xN
Authors: Jenkins David W. , Dow John D. , Tsai Min-Hsiung
Published: July 2002

Predictions of deep levels associated with N vacancies in AlxGa1−xN as functions of alloy composition x explain both (i) the dramatic change from naturally n-type to semi-insulating behavior (for x=xc≂0.5) in terms of a shallow-deep transition for the vacancy’s T2 level, and (ii) the major photoluminescence feature in terms of recombination from the vacancy’s A1 deep level. Extrinisic photoluminescence data for Zn-doped AlxGa1−xN are interpreted in terms of a transition from the conduction band to a T2-symmetric deep level in the lower part of the gap. This level is associated with antisite Zn on a N site, ZnN.
Hall effect and infrared absorption measurements on nitrogen donors in 6H-silicon carbide
Authors: Suttrop W. , Pensl G. , Choyke W. J. , Stein R. , Leibenzeder S.
Published: July 2002

Hall effect and infrared absorption measurements of n-type silicon carbide of the 6H polytype are employed to investigate the energy position of the ground state and excited states of the nitrogen donor. A donor model is proposed that assigns four series of absorption lines to electronic transitions of three donor species residing at three inequivalent lattice sites (h,k1,k2). A valley-orbit splitting of 12.6 meV is determined for donors on the hexagonal site h. For 2p0, 2p±, 3p0, and 3p± excited states, the effective-mass approximation is found to hold within experimental errors assuming a transverse and longitudinal effective electron mass of m⊥=(0.24±0.01) m0 and m∥=(0.34±0.02) m0, respectively.
Electronic structure and magnetic properties of R2Fe17Nx (x=0,3,4) and R2Fe17Cx (x=0,3), (R=Sm,Nd)
Authors: Zeng Zhi , Zheng Qingqi , Lai Wuyan , Pan C. Y.
Published: July 2002

The electronic structure and magnetic properties of rare-earth iron compounds R2Fe17Nx (x=0,3,4) and R2Fe17Cx (x=0,3) (R=Sm,Nd) have been studied by the self-consistent, spin-polarized local spin density functional theory calculations. Several cluster models were used to study the electronic structure of these compounds. For the R2Fe17Nx and R2Fe17Cx (x=0,3), the 9(e) sites in a unit cell are considered as occupied by N or C atoms. The additional N atom in the x=4 case is considered as occupying the 3(b) sites. The distribution of electrons and the magnetic moments for four kinds of crystalline inequivalent Fe atoms, Sm (Nd) and N(C) are calculated. The role of N or C atom in above compounds were discussed.
Corrosion behavior of NdFeB with Co and V additions
Authors: Warren G. W. , Chang Kuo-En , Ma Bao-Min , Bounds C. O.
Published: July 2002

Two NdFeB materials were selected for study. The first is referred to as a base or standard alloy with a composition of (NdDy)15FebalAl0.7B6, and the second is similar, Nd13.5Dy1.5FebalCo5V4AlB8, but alloyed with Co and V. The corrosion behavior of magnets made with these two materials is compared by two different techniques, autoclave testing at elevated temperature and pressure for 96 h, and dc electrochemical testing in an aqueous electrolyte. For the former, the base alloy exhibited poor corrosion resistance with a weight loss on the order of 270 mg/cm2. Significantly, the Co-V-treated samples yielded excellent corrosion resistance, showing no change in weight. Two types of electrochemical tests were performed on electrodes of each material, potentiodynamic polarization and constant potential in sodium sulfate solutions at pH from 1.5 to 4.4. Consistent with the autoclave tests, electrochemical experiments show lower currents for the Co-V alloys.
Magnetocrystalline anisotropy of the 3d sublattice in the cubic intermetallic system Zr6Co23−xMx (M=Fe,Ni)
Authors: Pareti L. , Solzi M. , Paoluzi A.
Published: July 2002

The magnetocrystalline anisotropy behavior of the 3d sublattice was studied in the face-centered-cubic system Zr6Co23−xMx (M=Fe,Ni). A remarkably high anisotropy field was required to saturate the magnetization along both the [100] and [110] hard directions. The values found are, respectively, HA=43 kOe and HA=33 kOe at 293 K for Zr6Co23. The partial substitution of Co with Ni results in a substantial increase of the anisotropy while saturation magnetization and Curie temperature decrease linearly. On the contrary, the partial substitution with Fe gives rise to a decrease of HA and to an increase of both σs and Tc. The 10th power law does not apply to the functional temperature dependence of the resulting anisotropy in this system.
Aberrations of emission cathodes: Nanometer diameter field-emission electron sources
Authors: Scheinfein M. R. , Qian W. , Spence J. C. H.
Published: July 2002

The electron optical properties of nanometer sized field-emission cathodes are examined for suitability as electron sources for low-voltage scanning electron microscopy, low-voltage transmission point projection microscopy, and low-voltage transmission and reflection electron holography. First-order electron optical properties, aperture and chromatic aberrations, and source coherence are computed using an all-orders numerical method, and compared with analytically computed properties where possible. The electron optical properties of planar emitters, conventional field-emission tips, and new nanotip structures are compared in the absence of space-charge effects. It is found that the spherical and chromatic aberrations of nanotips are dominated by their base structures and that beams produced by nanotips can be considered as totally coherent.
Transport measurements in p-type CdTe single crystals and ion-beam doped thin films
Authors: Moesslein Jochen , Lopez-Otero Adolfo , Fahrenbruch Alan L. , Kim Donghwan , Bube Richard H.
Published: July 2002

Electrical transport properties of phosphorus-doped p-type CdTe single crystals and phosphorus-ion-beam doped, homoepitaxial thin films have been investigated by means of van der Pauw Hall effect and resistivity measurements as a function of temperature from 8 to 400 K. Analysis of the data indicates a maximum doping level greater than 2×1017 cm−3 in the films, at least as high as in the single crystals. Phosphorus has an ionization energy of about 40 meV, the degree of compensation is smaller in the films, room temperature mobilities of the films are lower than those for single crystals by about 20%, and the temperature dependence of mobility is similar for both crystals and films. Impurity scattering is dominant at lower temperatures and polar mode scattering is dominant at higher temperatures with a maximum mobility at 150–190 K. Both the single crystals and the ion-assisted doped films show a temperature independent resistivity at temperatures below 40 K, indicating the presence of impurity band conduction.
Scanning Lorentz electron microscope with high resolution and observation of bit profiles recorded on sputtered longitudinal media (invited)
Authors: Yajima Y. , Takahashi Y. , Takeshita M. , Kobayashi T. , Ichikawa M. , Hosoe Y. , Shiroishi Y. , Sugita Y.
Published: July 2002

A transmission electron microscope operating with a cold field emission source has been modified to facilitate differential phase contrast mapping and applied to the observation of microscopic magnetic features appearing in recorded longitudinal media. After describing the design and performance of the scanning Lorentz electron microscope, as we call it, results on the observation of bit patterns delineated on Co-based sputtered longitudinal media are presented. Relations of observed bit profiles to macroscopic magnetic properties of media and to device performance are discussed. Then, magnetization fluctuation on a scale of magnetic crystallites constituting the medium is examined. Also given is an account of a stray field effect inherent in hard magnetic materials.
Electromigration in Al(Cu) two-level structures: Effect of Cu and kinetics of damage formation
Authors: Hu C.-K. , Small M. B. , Ho P. S.
Published: July 2002

The electromigration characteristics and kinetics of damage formation for Al(Cu,Si) line segments on a continuous W line and Al(Cu)/W two-level interconnect structures have been investigated. The mass transport as a function of temperature was measured using a drift-velocity technique. The flux divergence at the line/stud contact was found to be responsible for formation of open failure in the interconnect structure, as shown by a direct correlation observed between mass depletion at the contact and resistance increase of the line/stud chain. The depletion of Al at the stud contact is preceded by an incubation period during which Cu is swept out a threshold distance from the cathode of the line. This leads to a damage formation process which is controlled by both Cu electromigration along grain boundaries and dissolution of the Al2Cu precipitates. This is distinctly different from single-level interconnects measured using a conventional electromigration test site. Measurements of the mean failure lifetime in the two-level interconnect yield an activation energy of 0.58 eV for Al, in contrast to 0.78 and 0.83 eV for Al(0.5 wt % Cu) and Al(2 wt % Cu), respectively. The activation energies of the electromigration drift velocity were found to be 0.86 and 0.68 eV for Cu and Al in Al(2 wt % Cu, 3 wt % Si), respectively. These results enable one to infer that the kinetic process is controlled by electromigration of Cu along grain boundaries instead of by dissolution of the Al2Cu precipitates.
Influence of additives on the electrical properties of dense SnO2-based ceramics
Authors: Las W. C. , Dolet N. , Dordor P. , Bonnet J. P.
Published: July 2002

Highly densified 0.99 SnO2-0.01 CuO-based ceramics with antimony, tantalum, or niobium oxides as additives have been prepared. Samples have been characterized by dc conductivity and complex impedance spectroscopy as a function of temperature and by current density as a function of electric field. Microstructures have been analyzed by scanning electron microscopy, electron microprobe, and Auger spectroscopy. The electrical properties of Ta- and Nb-doped ceramics are grain boundary controlled at least at low temperature. Sb-doped materials show a different electrical behavior due to the presence of a segregation layer of antimony which inhibits grain boundary effects.
A phenomenological magnetomechanical hysteresis model
Authors: Bergqvist A. , Engdahl G.
Published: July 2002

To obtain accurate design tools for applications involving giant magnetostrictive materials, magnetomechanical hysteresis effects should be taken into account. The problem consists of determining the magnetization and mechanical strain from the combined past history of magnetic field and mechanical stress. In this work, coupled magnetomechanical hysteresis has been modeled by using simple path-dependent differentials to accumulate the past history in functions related to the magnetization and strain through material-dependent parameters. By using anhysteretic curves and a few additional parameters to characterize a material, major, and minor loops with respect to both field and stress have been calculated for Terfenol-D and have shown good agreement with experiments.
The Fourier series method for the calculation of strain relaxation in strained-layer structures
Authors: Faux David A.
Published: July 2002

The theory of the Fourier series method is presented for 2-dimensional, homogeneous, isotropic structures with rectangular geometry. The theory allows for any normal stress to be specified at any boundary with the shear stresses at all boundaries assumed to be zero. The stress relaxation near the free surface due to a single isolated strained layer is calculated. It is found that the component of the stress relaxation normal to the surface decays as 1/r within the strained layer, where r is the distance from the free surface. A single strained layer with linearly graded composition from 1% to 0.8% is examined and it is found that only small asymmetries are present in the strain relaxation and surface displacement. Finally, the strain relaxation due to a single strained layer during growth is calculated and it is found that the maximum shear strain at the corner of the structure is slightly larger than that due to a single capped strained layer.
Misfit strain, relaxation, and band-gap shift in GaxIn1−xP/InP epitaxial layers
Authors: Bensaada A. , Chennouf A. , Cochrane R. W. , Graham J. T. , Leonelli R. , Masut R. A.
Published: July 2002

A detailed investigation of the structural and optoelectronic properties of thick GaInP epilayers on sulfur-doped InP substrates is reported. Significant variations of the optical absorption and photoluminescence transition energies from light- and heavy-hole states are observed as a function of the epilayer composition as well as of the degree of relaxation of the misfit strain. High-resolution x-ray measurements were used to determine the Ga concentrations and the strains and indicate significant anisotropic relaxation in several films. Even small relaxations result in a significant increase in the optical linewidths and a rapid drop in the transition intensities. A model with no free parameters based on the strain Hamiltonian of Pikus and Bir provides excellent agreement with the transition energies and serves to identify unambiguously the transitions observed in the optical spectra. Within this model, isotropic in-plane relaxation produces a shift of both light- and heavy-hole energies whereas anisotropic in-plane relaxation contributes only negligibly.
Measurement of n-type dry thermally oxidized 6H-SiC metal-oxide-semiconductor diodes by quasistatic and high-frequency capacitance versus voltage and capacitance transient techniques
Authors: Neudeck P. , Kang S. , Petit J. , Tabib-Azar M.
Published: July 2002

Dry-oxidized n-type 6H-SiC metal-oxide-semiconductor capacitors are investigated using quasistatic capacitance versus voltage (C-V), high-frequency C-V, and pulsed high-frequency capacitance transient (C-t) analysis over the temperature range from 297 to 573 K. The quasistatic C-V characteristics presented are the first reported for 6H-SiC MOS capacitors, and exhibit startling nonidealities due to nonequilibrium conditions that arise from the fact that the recombination/generation process in 6H-SiC is extraordinarily slow even at the highest measurement temperature employed. The high-frequency dark C-V characteristics all showed deep depletion with no observable hysteresis. The recovery of the high-frequency capacitance from deep depletion to inversion was used to characterize the minority-carrier generation process as a function of temperature. Zerbst analysis conducted on the resulting C-t transients, which were longer than 1000 s at 573 K, showed a generation lifetime thermal activation energy of 0.49 eV.
Absolute magnetometry at nanometer transverse spatial resolution: Holography of thin cobalt films in a scanning transmission electron microscope
Authors: Mankos Marian , Scheinfein M. R. , Cowley J. M.
Published: July 2002

A method for the absolute measurement of magnetization at nanometer spatial resolution in magnetic thin films has been developed. A biprism placed in the illumination system of a scanning transmission electron microscope allows the operation of two distinct holography modes. The absolute mode displays a linear change in phase difference for regions of constant magnetization and thickness and the slope determines the magnitude of magnetization. The differential mode displays a constant value of phase difference in these regions allowing a simple and straightforward determination of domain wall profiles. Micromagnetic structure extracted from identical areas of thin Co films is compared using the new holography modes, differential phase constrast Lorentz microscopy and conventional Fresnel Lorentz microscopy in the same instrument.
Magnetic flux mapping, magnetization, and current distributions of YBa2Cu3O7 thin films by scanning Hall probe measurements
Authors: Xing W. , Heinrich B. , Zhou Hu , Fife A. A. , Cragg A. R.
Published: July 2002

Mapping of the magnetic flux distribution in a square-shaped superconducting YBa2Cu3O7 thin film was carried out using a scanning micro-Hall probe with a spatial resolution of about 25 μm. Calculation procedures for converting the measured flux map to the film current distribution are discussed. The flux penetration with the applied field perpendicular to the film surface was studied in initially zero-field-cooled samples. With full flux penetration, the film sheet currents follow the sample edges, whereas with partial flux penetration, the sheet currents are more complex and include multiply connected patterns. In both cases the sheet currents change their flow direction abruptly along the sample diagonals where the shielding is more effective, and this results in distinctive minima of the flux penetration. The saturation magnetization for a square-shaped sample of side 2a was found to be identical to that for disc and cylindrical samples of radius a. It is shown that the multiply connected current patterns corresponding to various parts of the hysteresis cycle can be reconstructed from the superposition of two appropriate virgin current distributions. The critical current density was dependent on magnetic flux density, and it was found to follow the Kim model.
Empirical forms for the electron/atom elastic scattering cross sections from 0.1 to 30 keV
Authors: Browning R. , Li T. Z. , Chui B. , Ye Jun , Pease R. F. W. , Czyżewski Z. , Joy D. C.
Published: July 2002

Empirical forms have been found for the total and differential elastic scattering cross sections for electron/atom scattering. The cross sections are valid over the range 0.1–30 keV and across the periodic table. The empirical forms of the cross sections are derived from trends in tabulated Mott scattering cross sections. The form of the total cross section is similar to a previously published cross section and is based on the screened Rutherford cross section. The fit to the differential Mott cross sections is decomposed into two parts, one part being of the same mathematical form as the screened Rutherford cross section σR, and the second part being an isotropic distribution σI. These two mathematical forms were chosen because they give a straightforward generation of random scattering angles. The screened Rutherford part of the differential scattering cross section is first fitted to the half-angle of the Mott cross sections. This fit of the differential screened Rutherford is in turn reduced to a fit of the screening parameter alone over energy and atomic number. The screened Rutherford part of the cross section is highly peaked in the forward scattering direction and needs to be balanced by the isotropic distribution.

The ratio of the total cross sections (σR/σI) between the screened Rutherford part of the differential scattering cross section and the isotropic part of the distribution is then fitted to give the same ratio of forward to backscattered currents as the tabulated Mott differential cross sections. Using this dual form of the scattering cross section for the differential cross section, and the previously (independently) fitted total cross section, the backscattering coefficients for normal incidence are calculated. The two equations describing the differential cross section, one for the Rutherford screening parameter and one for the ratio σR/σI, are simplified to remove redundant parameters, and then fitted to the backscattering coefficients calculated directly from the tabulated Mott cross sections. A straightforward expression for the differential cross section was found to give backscattering results covering all the major trends with energy and atomic number compared to the backscattering coefficients calculated using tabulated Mott cross sections.
Preparation of high-coercivity fine barium ferrite powder
Authors: Kaczmarek W. A. , Ninham B. W.
Published: July 2002

The structural and magnetic properties of the thermally activated transformation from nanostructural material, mechanically disordered and decomposed BaFe12O19 ferrite powder to pure crystalline phase, have been studied by x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry analysis techniques. All experiments were performed on as-milled (1000 h) in air, and in vacuum and annealed Ba-ferrite samples (4 h at 773 and 1273 K). In parallel with the structure and particle morphology changes, we investigate the influence of heat treatment on the powder M-H hysteresis parameters in relation to the preparation routes, i.e., powder obtained by ball milling in air and vacuum, and annealed in air or vacuum.
Self-consistent quantum-mechanical calculations in ultrathin silicon-on-insulator structures
Authors: Ouisse T.
Published: July 2002

Schrödinger’s and Poisson’s equations have been self-consistently solved in ultrathin silicon-on-insulator (SOI) structures. The interaction between the front and back inversion layers has been studied as a function of the silicon film thickness, electron concentration, and temperature. The splitting of the subband energy levels, due to the possibility of quantum-mechanical interaction between the two inversion layers, strongly depends on the silicon film thickness tSi. The total subband structure can qualitatively vary with the applied gate voltages. For the thinnest devices (tSi<15 nm), volume inversion may occur even for rather large electron concentration and low temperature. For intermediate SOI thicknesses (tSi≂15–50 nm), many more subbands are populated in SOI structures than in their bulk silicon counterparts.
Raman-scattering study on pressure amorphization of LiNbO3 crystal
Authors: Lin Yuankun , Li Yudong , Xu Yongwei , Lan Guoxiang , Wang Huafu
Published: July 2002

A high-pressure Raman-scattering study reveals that crystalline LiNbO2 becomes amorphous at about 31 GPa, and this transition is irreversible above 35.8 GPa. The spectrum of pressure- amorphized LiNbO3 is similar to that of glassy LiNbO3.
Experimental and theoretical study of evaporation of water in a vessel
Authors: Hisatake Kazuo , Fukuda Misako , Kimura Junko , Maeda Mami , Fukuda Yoshiko
Published: July 2002

Using a wind tunnel the evaporation rate of water at normal temperature and pressure was measured as a function of temperature, humidity, air velocity, and diameter of the vessel. In order to measure the temperature of the water surface and the boundary layer on the surface, the temperature above and below the surface was measured as a function of air velocity with a thermocouple of 0.076 mm diameter. The present experimental results are in agreement with the present theory for a laminar flow, which is based on the idea that the evaporation rate is equal to the diffusion velocity of water vapor in the boundary layer. For a turbulent flow the experimental evaporation rate was found to be proportional to U0.86±0.03, where U is air velocity. This is consistent with the velocity dependence of the experimental thickness of the boundary layer d, which was d∝U−0.90. A new semiempirical formula of the evaporation rate is proposed.
A photoemission determination of the band diagram of the Te/CdTe interface
Authors: Niles David W. , Li Xiaonan , Sheldon Peter , Höchst Hartmut
Published: July 2002

Two experiments designed to assist in understanding the physics of certain back contacts on p-type CdTe solar-cell devices are described. In the first experiment, x-ray photoelectron and Auger electron spectroscopies are used to show that etching CdTe in HNO3:H3PO4 results in a Te layer on the CdTe surface. In the second experiment, photoemission spectroscopy is used to explore the electronic properties of evaporated Te deposited on thin-film, polycrystalline p-CdTe in an effort to develop a band diagram for the Te/p-CdTe interface. The motivation for developing the band diagram derives from previous observations that chemically etching polycrystalline p-CdTe solar-cell device material before application of the back contact reduces the series resistance of the device. The key results are that the evaporated Te overlayer is p type and that the valence-band offset between Te and p-CdTe is favorable for low-series-resistance contact, ΔEv=0.26±0.1 eV.
1.3 μm InAsP compressively strained multiple-quantum-well lasers for high-temperature operation
Authors: Oohashi H. , Hirono T. , Seki S. , Sugiura H. , Nakano J. , Yamamoto M. , Tohmori Y. , Yokoyama K.
Published: July 2002

High-temperature operation of InAsP-based laser diodes at 1.3 μm has been realized according to guidelines of a large conduction-band-offset material (ΔEc) with a large optical confinement factor (Γ). Using photoluminescence excitation spectroscopy measurements, it was found that the conduction-band offset of InAs0.52P0.48P/InGaAsP is the half of the band-gap energy difference (0.5 ΔEg), which is larger than that of conventional quaternary material systems. A strain-compensation growth technique enabled the fabrication of a large number of wells for large Γ. For broad-area laser diodes, the maximum operating temperature increased as the number of wells increased from 4 to 15. In buried heterostructure lasers with ten wells, with high-reflectivity coating on both facets, continuous-wave lasing operation at temperatures up to 150 °C was achieved with a characteristic temperature of 59 K (30<T<70 °C) demonstrating the suitability of InAsP for high-temperature operation.
Photoluminescence of CdTe doped with arsenic and antimony acceptors
Authors: Soltani M. , Certier M. , Evrard R. , Kartheuser E.
Published: July 2002

A detailed characterization of the impurity centers involved in the photoluminescence (PL) of p-type CdTe doped with arsenic (As) and antimony (Sb) has been performed. The PL spectrum has been measured from 1.35 eV up to the band edge and as a function of temperature (4.2 up to 30 K). In addition to the familiar broad PL line centered at 1.45 eV and present in undoped and doped materials, the doped samples exhibit a new band near 1.54 eV showing a fine structure composed of two peaks whose intensities vary with temperature. The observed longitudinal optical (LO) phonon replicas associated with the zero-phonon lines, at 1.45 eV and 1.54 eV, respectively, are characterized by a Huang-Rhys factor S=1.3±0.1 and S=0.30±0.02. The various electron-hole recombination processes are explained by means of a simple analytic model correlating the position of the zero-phonon lines to the relative intensities of the phonon side bands. The model accounts for the chemical shift of the defect centers and describes the effect of the charge carrier LO-phonon interaction in the framework of the adiabatic approximation within the envelope function approach. Comparison between theory and experiment leads to the following values for the effective Bohr radii: aAs=(10.6±0.1) Å, aSb=(10.3±0.1) Å, and ionization energies: EAs=(58±2) meV, ESb=(61±2) meV. It also leads to conclude to the presence of native shallow donors with binding energy ED=(13±2) meV and of deeper native acceptor complexes with effective Bohr radius aA=(6.1±0.1) Å and ionization energy EA=(157±2) meV.
Ferromagnetic–insulator–ferromagnetic tunneling: Spin-dependent tunneling and large magnetoresistance in trilayer junctions (invited)
Authors: Moodera Jagadeesh S. , Kinder Lisa R.
Published: July 2002

Tunneling between ferromagnet–insulator–ferromagnet (FM–I–FM) trilayer thin-film planar junctions has been successfully studied. Tunnel current was observed to be dependent on the relative orientation of the magnetization (M). Co, CoCr, CoFe, Fe0.7Pt0.3, and NiFe were tried as the FM electrodes with Al2O3 or MgO as the barrier layers for the above studies. Large magnetoresistance (MR) was observed as the M alignment of the two ferromagnets changed from being parallel to antiparallel orientation. At room temperature, the highest change in junction MR was 18%, field sensitivity factor reaching 5%/Oe in the best cases. The MR value increased to 25.6% at 4.2 K, and decreased as the dc bias was increased to a fraction of the barrier height. The angular dependence of MR varied nearly as the cosine of the relative angle of M, as predicted by Slonczewski’s theory. The magnitude of MR agrees well with that given by Julliere’s model, which predicts that the MR varies as the product of the conduction electron spin polarization of the FMs. These trilayer junctions can find application as high-density, nonvolatile storage media or as field sensors.
Structural and dielectric studies of Pb(Mg1/3Nb2/3)O3–PbTiO3 ferroelectric solid solutions around the morphotropic boundary
Authors: Noblanc O. , Gaucher P. , Calvarin G.
Published: July 2002

Solid solutions (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 with 0.20≤x≤0.40 were studied using impedance measurements and high resolution x-ray powder diffraction. The dependence of the real and imaginary parts of the dielectric constant with temperature was investigated on unpoled and poled ceramics, showing several transition points. The temperature dependence of the unit-cell parameters showed the onset of two successive spontaneous phase transitions: cubic→tetragonal →rhombohedral. A coexistence of both ferroelectric phases was evidenced in a large temperature and composition range. The dielectric and structural properties were interpreted in terms of relaxor→ferroelectric spontaneous phase transition. An intermediate behavior between a relaxor and a real ferroelectric was evidenced. A partial phase diagram was proposed including both structural and dielectric informations.
Photoluminescence and x-ray-diffraction studies on Sm-doped yttrium aluminum garnet to ultrahigh pressures of 338 GPa
Authors: Liu Jun , Vohra Yogesh K.
Published: July 2002

We report photoluminescence and x-ray-diffraction studies on Sm-doped yttrium aluminum garnet (YAG) to ultrahigh pressures of 338 GPa in a diamond-anvil cell at room temperature. Photoluminescence spectra from Sm:YAG could be clearly identified with increasing pressures to 304 GPa. The rapidly shifting fluorescence from diamond anvil obscures the Sm:YAG emission spectra above 300 GPa. We provide the shift of the Y4 emission band of Sm:YAG to 304 GPa and give its pressure coefficients as referenced to the ruby standard to 180 GPa and platinum standard to 304 GPa. We present possible structural disorder mechanisms in Sm:YAG above 100 GPa which give rise to the line broadening and changes in intensities of various photoluminescence bands. The application of Sm:YAG as an optical pressure sensor in the ultrahigh-pressure regime is also discussed.
Polygonization of directionally solidified high critical current YBa2Cu3O6+x
Authors: Sandiumenge F. , Vilalta N. , Obradors X. , Piñol S. , Bassas J. , Maniette Y.
Published: July 2002

The mosaic structure of single domain YBa2Cu3O6+x/Y2BaCuO5 (123/211) directionally solidified composites is investigated by x-ray diffraction and transmission electron microscopy. This mosaic structure results from a polygonization process before the oxygenation step. A topological model is given based on the observed distribution of preferred subgrain boundary planes. This polygonization, characteristic of melt textured 123, is proposed to be an intrinsic critical current density limitation for these materials.
Enhancement of light emitting diodes based on self-assembled heterostructures of poly(p-phenylene vinylene)
Authors: Onitsuka O. , Fou A. C. , Ferreira M. , Hsieh B. R. , Rubner M. F.
Published: July 2002

Thin film light emitting diodes based on multibilayer combinations of the conjugated polymer poly(phenylene vinylene) (PPV) and different polyanions were fabricated via the use of a simple layer-by-layer molecular-level processing scheme. Molecular-level manipulation of the type and sequence of PPV bilayers used to construct the films was found to produce devices with dramatically improved luminance levels and efficiencies. It was also found that device efficiency could be improved by controlling the nature of the thin film/electrode interface.
X-ray photoelectron spectroscopic investigation of carbon incorporation and segregation during pseudomorphic growth of Si1−yCy on Si(001)
Authors: Kim Myeongcheol , Lippert G. , Osten H. J.
Published: July 2002

An important issue for the epitaxial growth of strained Si1−yCy alloy layers is the relation between substitutional and interstitial carbon incorporation, which is strongly influenced by the growth conditions. We use in vacuu x-ray photoelectron spectroscopy (XPS) to investigate Si1−yCy layers (y<2 at. %) grown pseudomorphically on Si(001) with constant carbon and Si fluxes, but at different growth temperatures. The total carbon concentration measured by secondary-ion mass spectroscopy (SIMS) is constant in all the layers, but the amount of substitutionally incorporated carbon measured by x-ray diffraction (XRD) decreases with increasing growth temperature. The transformation of substitutional carbon atoms to carbon in interstitial Si-C defect complexes is indicated by a shift in the binding energy of the C 1s signal from 283.06 to 282.30 eV. The measured binding energy of the defect complexes leads to the suggestion that their chemical structure is very similar to that of silicon carbide. Angle-resolved XPS measurements clearly show that the carbon concentration increases toward the sample surface with increasing growth temperatures and this is interpreted to result from surface segregation of carbon atoms during growth. Interstitial carbon-containing defects are more mobile than substitutional carbon atoms. Therefore, they are probably involved in the supposed segregation. The energy barrier for the segregation process is estimated to be (0.25±0.02) eV.
Observation of microstructure and damage in materials by phase sensitive radiography and tomography
Authors: Cloetens P. , Pateyron-Salomé M. , Buffière J. Y. , Peix G. , Baruchel J. , Peyrin F. , Schlenker M.
Published: July 2002

The novel possibilities of phase feature detection in radiography at a third generation synchrotron radiation source are used to image, both in projection and in computed tomography, a cracked silicon single crystal and metal matrix composites strained in tension. Through an instrumentally very simple technique, based on Fresnel diffraction, phase jumps related to the interface between the matrix and the reinforcing phases of the composites are detected even when these phases show very similar x-ray attenuation. Strain-induced cracks with openings below the micrometer range are also visible through the phase modulation they introduce, illustrating the potential of the technique for assessing damage in materials with improved resolution and sensitivity.
Spin polarized tunneling in reactively sputtered tunnel junctions
Authors: Platt C. L. , Dieny B. , Berkowitz A. E.
Published: July 2002

In order to study spin-polarized electron tunneling, we have grown MOM junctions where M is a magnetic transition metal (Co, Fe or CoFe) and O a reactively sputtered oxide (CoO, NiO, Ta2O5, MgO, HfO2). The oxide thickness ranged between 20 and 300 Å. The structural, magnetic, and transport properties of these junctions have been investigated. Among these various oxides, large magnetoresistance due to spin-polarized tunneling effects have been observed in MgO on the order of 20% at 77 K, and a maximum of 31% has been observed in HfO2 based junctions at 30 K. The other oxides had a much higher tendency to form pinholes and had smaller dielectric breakdown thresholds.