bluuebunny/crossref_metadata_2025 huggingface dataset:
High temperature superconducting Josephson transmission lines for pulse and step sharpening
Authors: Martens J. S. , Wendt J. R. , Hietala V. M. , Ginley D. S. , Ashby C. I. H. , Plut T. A. , Vawter G. A. , Tigges C. P. , Siegal M. P. , Hou S. Y. , Phillips Julia M. , Hohenwarter G. K. G.
Published: July 2002

An increasing number of high speed digital and other circuit applications require very narrow impulses or rapid pulse edge transitions. Shock wave transmission lines using series or shunt Josephson junctions are one way to generate these signals. Using two different high temperature superconducting Josephson junction processes (step-edge and electron beam defined nanobridges), such transmission lines have been constructed and tested at 77 K. Shock wave lines with approximately 60 YBaCuO nanobridges, have generated steps with fall times of about 10 ps. With step-edge junctions (with higher figures of merit but lower uniformity), step transition times have been reduced to an estimated 1 ps.
Temperature effects on the parametric growth of space-charge waves in a relativistic electron beam
Authors: Willett Joseph E. , Aktas Yildirim , Maraghechi Behrouz
Published: July 2002

Parametric excitation of two space-charge waves in a cylindrical metallic waveguide completely or partially filled with a relativistic electron beam is analyzed. The pump wave in the laboratory reference frame consists of a static, spatially periodic axial electric field. A theory is presented that is fully electromagnetic and allows for a radially nonuniform pump wave field and finite electron beam temperature. Effects of temperature on the frequency and growth rate of the space-charge waves are studied.
Investigation of hot-electron injection and ballistic transport using ultrathin planar doped barrier launchers in vertical field effect spectrometer structures
Authors: Daniels-Race T. , Yamasaki K. , Schaff W. J. , Tasker P. J. , Eastman L. F.
Published: July 2002

A study in the optimization of ultrathin planar doped barriers (PDB) for use as hot-electron/ballistic electron launchers in vertical field effect spectrometer structures was conducted. A comparison was made of results for an all-PDB (PDB launcher/PDB analyzer) versus a PDB launcher/graded AlGaAs spectrometer. Previous work involving PDB launchers and vertical field effect structures predicted little to no enhancement of electron current density. This study conclusively refutes these predictions. A new analyzer-up-top design scheme as well as optimization experiments involving varying PDB layer and associated drift region thickness were developed. This work shows previously unobserved hot electron/ballistic spectra for these structures. Moreover, high current densities in excess of 105 A/cm2 were observed for the all-PDB and for the ultra-thin PDB launcher/graded AlGaAs spectrometers under conditions involving both thermionic emission and tunneling.
Photoelectrochemical effects of BaTiO3 thin film electrodes prepared by the sol-gel method
Authors: Yoon Ki Hyun , Chung Ki Sup
Published: July 2002

The photoelectrochemical properties of La3+-doped BaTiO3 thin film electrodes prepared by the sol-gel method have been investigated as a function of the amount of La3+ content, film thickness, heating atmosphere (air/H2), and optical absorption. In the air-heated electrodes doped with 0.1, 0.2, and 0.3 mol % La3+, the maximum quantum efficiencies have been obtained when the film thicknesses were 10 000, 8000, and 3000 Å, respectively. When the film thicknesses are over certain absorption depths, whose calculated values are over 10 000, 8000, and 6500 Å, respectively, their quantum efficiencies decrease. In the hydrogen-heated electrodes, the depletion layer decreases as the donor concentration increases, and the optical absorption edge moves to longer wavelength by 0.07 eV. Also, the quantum efficiency decreases as the film thickness increases due to the bulk recombination.
Nonisothermal drift-diffusion model of avalanche diodes
Authors: Stoiljković V. , Howes M. J. , Postoyalko V.
Published: July 2002

A nonisothermal drift-diffusion model of avalanche diodes is described. The novel feature of the model is the inclusion of the heat conduction equation in the basic set of semiconductor equations. The influence of a nonuniform temperature distribution across the active region on the diode electrical characteristics is investigated and the usual assumption of constant diode junction temperature is evaluated. Both single-drift and double-drift diodes are simulated. The temperature distribution is calculated as a function of the dc bias current density and the amplitude of the rf driving voltage. The diode impedance is also determined and compared with results from an isothermal simulation. Numerical results are presented at J-(10–20 GHz) and W-band frequencies (75–110 GHz).
Effects of BCl3 magnetron ion etching on deep levels in GaAs
Authors: Buchwald W. R. , Zhao J. H. , McLane G. F. , Meyyappan M.
Published: July 2002

Capacitance versus voltage curves and deep-level transient spectroscopy have been used to investigate the effects of BCl3 magnetron ion etching on the shallow donor concentration and deep-level defects in GaAs. Capacitance versus voltage data reveal that the shallow donor concentration is unaffected by the etching process at power densities ranging from 0.08 to 0.4 W/cm2. Capacitance transient measurements reveal thermal emission of electrons from two deep defect sites with activation energies of 0.25 and 0.74 eV which were unaffected by the etching process. A broad, deep-level transient spectroscopy peak, characterized by an activation energy for thermal emission of electrons of 0.37 eV, was also observed in the etched sample, but not in the unetched sample. Defect depth profiling of the 0.37 eV peak indicates the concentration of this defect to increase with increasing etch power.
Characteristics of discharge-excited barium vapor lasers operating in the 1–5 μm spectral band
Authors: Pask H. M. , Piper J. A.
Published: July 2002

Laser action from barium vapor has been investigated using small-scale (20–40 cm−3) high-repetition-rate discharge-heated and low-repetition-rate externally-heated devices. Strongest output occurs at 1.13, 1.50, and 2.55 μm, with weaker emission at 2.92 and 4.72 μm. The range of discharge conditions for which laser action occurs at these wavelengths is described and the temporal occurrence of laser output is presented. The implications of this study to the future development of barium lasers are considered.
Variational theory of the radiant emittance of the mercury argon discharge and the effects of isotopic enrichment
Authors: Richardson R. W. , Berman S. M.
Published: July 2002

A variational theory of the radiant emittance of the mercury-argon discharge is developed and applied to an investigation of the effects of alterations of the isotopic composition of the mercury. The theory includes the effects of transport of resonance radiation, elastic and inelastic collisions, resonant exchange of excitation, diffusion of the mercury atoms, the isotope- and hyperfine-shifted structure of the resonance line, and the surfaces of the discharge on the emittance. Two sensitive parameters—the branching ratio for radiative decay of the resonance state and the rate constant for resonant exchange of excitation—are fitted to data on natural mercury. The remaining insensitive parameters are given values based on microscopic estimates. The resulting theory reproduces the experimentally observed results for mercury—196 enhanced mixtures. We predict small effects for mixtures with other isotopes enhanced. The crucial role played by resonant exchange of excitation is emphasized.
On the angular and field distribution of the magnetization reversal in hard magnets
Authors: Wirth S. , Christoph V.
Published: July 2002

A measuring procedure for the determination of the switching behavior of hard magnetic particles is presented. For this end, the remanence vector is determined for increasing fields. In the first part of the presented analysis, the angular dependence of the switching field is calculated, neglecting the switching field distribution. In the second part, the portion of switching particles depending on the strength of the applied field and its orientation with respect to the particle easy axes is determined. This switching portion includes information on the angular dependence of the switching field and the switching field distribution for each particle orientation. The method is demonstrated on a magnetically diluted barium ferrite sample with noninteracting single-domain particles.
Attempt to correlate threshold voltage scattering with x-ray Lang topographs for semi-insulating GaAs substrates
Authors: Saito Yasuyuki
Published: July 2002

Because the yields of GaAs metal-semiconductor field-effect transistor (MESFET) devices are negatively impacted if ranges of threshold voltages (Vth) occur for sets of devices fabricated over the total area of the substrate, it is desirable to pinpoint the physical origin of this problem before a solution may be implemented. The presence of high dislocation densities encountered in wafers produced from liquid-encapsulated Czochralski-grown boules of GaAs may be responsible for the phenomenon of wide distributions of Vth, although this view is presently controversial. Accordingly, we have implanted silicon as a dopant into commercially supplied semi-insulating GaAs wafers, formed arrays of MESFETs, and attempted to correlate the dislocation densities determined by Lang x-ray topography with the magnitude of the scattering of Vth, including the observation of spikes, across the wafers in the 〈110〉 direction. The present results indicate that there is no clear correlation between either the magnitudes or the spatial distributions of the threshold voltages and the presence of high densities of dislocations. An alternative explanation is presented based on the presence of point defects such as interstitial arsenic in the samples.
Injection-limited trap modulated hopping conduction in carbonyl doped polyethylene
Authors: Perlman M. M. , Kumar A.
Published: July 2002

A recently developed theory of steady-state conduction of space charge limited (SCL) currents in polyethylene doped with carbonyl hopping sites, and including detrapping, is modified to apply to the injection limited case, i.e., to a nonzero field F0 at the injecting electrode. Good agreement with experiment was previously obtained for SCL carriers (F0=0) for applied fields up to 0.33 MV/cm. Here, we assume that for higher fields, F0 is proportional to the difference between the applied field and 0.33 MV/cm. Excellent agreement between theory and experimental current-applied field characteristics is then obtained for fields up to 0.5 MV/cm. The field distributions in doped and ‘‘pure’’ polyethylene samples are calculated, and are found to be independent of temperature in the range 49–82 °C for constant field.
A study of the gain and noise mechanisms in GaAs planar photoconductive detectors
Authors: Papaioannou G. J.
Published: July 2002

The dependence of the photoconductive gain on the photocarrier injection mechanisms from the surface and the substrate is investigated in GaAs photoconductive detectors. The layer structure and its effect on the dependence of the static gain on both the illumination intensity and the temperature is reported. The effect of deep traps on the gain response to illumination modulation intensity and the generation-recombination noise is modeled.
Superconducting properties of Nb3Sn wire fabricated by the clad-chip extrusion method
Authors: Saito Sakae , Hanada Shuji
Published: July 2002

Nb3Sn superconducting wires are successfully fabricated by a newly developed method named the clad-chip extrusion (CCE) process. It is characterized by extruding the thin chips of Nb-Sn clad sheets in which the thickness ratio of Nb layer to Sn-layer corresponds to Nb3Sn. The CCE-processed Nb3Sn wire heat treated under the optimum condition of 950 °C for 4 min followed by 700 °C for 96 h shows the critical transition temperature, Tc of 17.8 K and the critical current density, Jc of 1.2×104 A/cm2 at 20 T and 4.2 K. The upper critical field, Bc2* is extrapolated to 23.4 T at 4.2 K from Kramer plot of Jc1/2B1/4 vs B.
Dependence of the magnetic properties of Co/Pd multilayered films on the structural parameters estimated accurately by x-ray diffraction
Authors: Kim S. K. , Kang J. S. , Jeong J. I. , Hong J. H. , Koo Y. M. , Shin H. J. , Lee Y. P.
Published: July 2002

In this study, Co/Pd multilayered films with a few atomic layers of Co were prepared by alternating deposition in an ultrahigh-vacuum physical-vapor-deposition system. The structural parameters were estimated accurately making use of only the angular positions of x-ray diffraction peaks. The magnetic properties were found to vary greatly depending on Pd predeposition and Pd-sublayer thicknesses as well as Co-sublayer thickness. The Pd-predeposited films were found to have a remarkably high coercivity of 4723 Oe and a greatly enhanced interfacial magnetic anisotropy of 0.72 mJ/m2, which indicates an excellent potential as a magneto-optical recording medium.
Observations of strong microwave absorption in collisional plasmas with gradual density gradients
Authors: Stalder K. R. , Vidmar R. J. , Eckstrom D. J.
Published: July 2002

Strong microwave absorption was observed when a 10 GHz source illuminated an underdense collisional plasma that had a density gradient scale length several wavelengths long. Significant reductions in angular scattering and cross-polarized components were also observed. These experiments confirm that absorption was the dominant process. The plasma was created by the photoionization of tetrakisdimethylaminoethylene molecules seeded into atmospheric pressure helium. Sparkboard arrays provided the intense vacuum ultraviolet ionizing radiation. Plasma density profiles were measured using transverse scans of 9.7 GHz probe microwaves and were found to approximate an Epstein profile. The absorption at 10 GHz by this plasma was as large as 28 dB in direct backscattering and 15–20 dB when orthogonally polarized microwaves were launched and detected. The peak absorption scales with sparkboard energy in a way that suggests that electron-ion recombination is the dominant electron-loss mechanism at high plasma densities.
The chemisorption and reaction of BCl3 with Si(111)
Authors: Lapiano-Smith D. A. , McFeely F. R.
Published: July 2002

The chemical mechanisms involved in the decomposition of boron trichloride and the concomitant incorporation of elemental boron into Si(111) were elucidated. The reaction between BCl3 and Si(111) is quit complex due to the presence of Si, B, and Cl in a number of chemically distinct environments simultaneously. Annealing the sample to 570 °C effectively desorbs all molecularly adsorbed BCl3. Additional anneals to 710 and 870 °C largely reduce the BCl2 and BCl3 moeities to form the subsurface-boron reconstructed surface.
Anisotropic differential magnetoresistance of Cu/Co(100) multilayers
Authors: Giron F. , Boher P. , Houdy Ph. , Pierre F. , Beauvillain P. , Chappert C. , Le Dang K. , Veillet P.
Published: July 2002

We report on structural and magnetic properties of diode rf-sputtered fcc Cu/Co(100) multilayers. All the samples exhibit a pseudo-epitaxial structure with interface roughness of the order of 6 Å. As the Cu layer thickness is varied, we observe oscillations of magnetoresistance corresponding to oscillations of the interlayer coupling. The maximum of magnetoresistance is found for tCu=18.5 Å (up to ΔR/R≊6%). We furthermore measure a very high differential magnetoresistance (ΔR/RΔH=1.4 kOe−1). A combination of antiferromagnetic coupling and quadratic in-plane anisotropy is responsible for this large value of magnetoresistivity.
Experimental evidence for the annealing of surface defects in a-Si:H during deposition
Authors: Kleider J. P. , Longeaud C. , Roca i Cabarrocas P.
Published: July 2002

The modulated photocurrent technique performed under different illumination conditions is used to derive a spatial spectroscopy of the defect distribution in hydrogenated amorphous silicon. The results show conclusive evidence for a defect density profile extending from the a-Si:H/air interface toward the bulk in samples deposited by rf glow discharge under standard conditions at low substrate temperature (373 K). This spatial inhomogeneity in defect density almost completely disappears after annealing at 473 K and also seems absent if the material is deposited at higher temperature (523 K). These results are supported by a model in which the surface defects, buried during a-Si:H deposition, are annealed out in a thermally activated process.
Conductive ZnS:Zn thin films grown by molecular beam deposition
Authors: McClean I. P. , Thomas C. B.
Published: July 2002

Thin films of ZnS, with varying concentrations of Zn, have been grown onto p−-Si using the molecular beam deposition method. By increasing the Zn content, a rise in the conductivity is seen and a peak is achieved. With too much Zn, however, the conductivity reduces quickly. The most conducting film forms an ohmic contact with the p−-Si substrate. Calculation of the Fermi energy in this device shows that the Fermi level lies at 0.11 eV below the conduction band. The less conducting material is found to form a pseudoSchottky barrier at the p−-Si interface. The conduction process is thought to occur due to an excess of S vacancies (donor sites) compared to Zn vacancies (acceptor sites). The addition of Zn seems to reduce the effect of autocompensation, usually a hindrance in the fabrication of conducting ZnS films.
Subband tuning in semiconductor quantum wells using narrow barriers
Authors: Schmiedel T. , McCombe B. D. , Petrou A. , Dutta M. , Newman P. G.
Published: July 2002

A method for controlling the intersubband spacing in GaAs/AlGaAs quantum wells by incorporating a thin AlAs barrier at the well center has been investigated using reflectance and photoluminescence spectroscopy. Two structures with 5.7 and 11.3 Å AlAs barriers were studied. The predicted exciton sequence for the first sample (5.7 Å barrier) is E1h≤E1l≤E2h≤E2l, whereas for the second structure (11.3 Å barrier), the predicted sequence is E1h≤E2h≤E1l≤E2l. This work verifies that the sequence reversal, predicted for the 11.3 Å sample, occurs by performing polarization excitation luminescence measurements on these samples.
Crystallinity of ZnS:Tb,F thin films and characteristics of green-color thin-film electroluminescent devices prepared by rf-magnetron sputtering
Authors: Hsu Chin Tsar , Lin Ying Jen , Su Yan Kuin , Yokoyama Meiso
Published: July 2002

In order to fabricate high-brightness thin-film electroluminescent devices, the dependence of crystallinity on deposition conditions of the ZnS:Tb,F thin films deposited by rf-magnetron sputtering system have been studied. The optimal deposition conditions to get the best crystallinity are obtained as rf power density of 4.39 W/cm2, substrate temperature 150 °C, and post-anneal at 550 °C for 1 h. The (111) plane spacing and lattice constant are 3.1238 and 5.411 Å, respectively. The green electroluminescent devices with the structure of glass/indium-tin-oxide/SiO2/HfO2/ZnS:Tb,F/HfO2/SiO2/Al have the highest brightness and luminous efficiency hη of 830 cd/m2 and 0.8 lm/W, respectively, under 1 kHz sinusoidal wave voltage excitation and the Commission Internationale de l’Eclairage chromaticity is x=0.3096, y=0.5998, respectively.
Raman scattering from heavily doped (311) GaAs:Si grown by molecular beam epitaxy
Authors: Kwok S. H. , Merlin R. , Li W. Q. , Bhattacharya P. K.
Published: July 2002

Raman scattering by localized vibrational modes and plasmons has been used to characterize heavily p- and n-type silicon-doped (311)A GaAs layers grown by molecular beam epitaxy. Consistent with the doping character, p-type samples show two modes associated with Si(As) and the complex defect Si-X. Acceptor-related lines were not observed in n-type samples, an indication that compensation levels in the layers are very low. The results are discussed in relation to growth conditions on (311)A surfaces.
Electrical and structural properties of silicon layers heavily damaged by ion implantation
Authors: Boussey-Said J. , Ghibaudo G. , Stoemenos I. , Zaumseil P.
Published: July 2002

The effects of high-dose silicon and arsenic ion implantation on the electrical and structural properties of silicon layers are investigated. Combining electrical, transmission electron microscopy, and triple-crystal x-ray-diffraction measurements made it possible to characterize the effects of thermal annealing both on defect annihilation mechanisms and on electrical doping activation. It is clearly shown that a low-temperature (≤450 °C) electrical activation process is taking place in the amorphous surface layer induced by high-dose ion implantation. This phenomenon is found to be completely independent of the recrystallization regrowth by solid phase epitaxy which occurs at higher temperature. This electrical activation process is found to be well described by a local relaxation model involving point defect migration.
Properties of hydrogenated amorphous silicon films prepared by low-frequency (50 Hz) plasma-enhanced chemical-vapor deposition
Authors: Tochitani G. , Shimozuma M. , Tagashira H.
Published: July 2002

Deposition of hydrogenated amorphous silicon (a-Si:H) films is performed by low-frequency (50 Hz) plasma-enhanced chemical-vapor deposition (PECVD). The results show that the undoped a-Si:H films deposited at a substrate temperature of 200 °C are high-quality films comparable to those deposited by conventional rf PECVD at a substrate temperature of 300 °C. The photoconductivity and dark conductivity of the films are 4 × 10−4 S/cm and 5 × 10−9 S/cm respectively. The activation energy is 0.78 eV and the optical gap is 1.8 eV. Furthermore, it is possible to control the dark conductivity and activation energy by doping with substitutional impurities of boron and phosphorus. These properties are very similar to those of the a-Si:H films deposited by conventional rf PECVD in the optimum substrate temperature range from 250 to 300 °C. These results show that the 50 Hz PECVD can deposit high-quality a-Si:H films.
Theoretical study of time-resolved Raman scattering profiles of hot electrons in semiconductors
Authors: Chia C. , Sankey Otto F. , Tsen K. T.
Published: July 2002

A comprehensive time-resolved electronic Raman scattering theory for nonequilibrium carrier excitations in semiconductors is presented. The following are simultaneously taken into account: (i) the effects of the ultrashort laser pulse for probing the excited carrier distribution function; (ii) the fact that the fluctuation-dissipation theorem is not valid under conditions of nonequilibrium carrier distributions; (iii) the effects of quasiparticle life time via a finite collision time in the Raman scattering cross section; and (iv) the effect of the time-dependent resonant enhancement factor due to the band structure. The single-particle scattering spectra for spin-density fluctuation contribution is found to be significantly broadened by an ultrashort laser pulse, but is substantially narrowed by the finite collision time. The effect of the time-dependent resonant enhancement factor has been demonstrated to broaden the line shape of single-particle scattering spectra for the spin-density fluctuation contribution as the probe photon energy increases.
Magnetron sputtering of Fe onto GaAs substrates: Energetic bombardment effects
Authors: Bernstein S. D. , Wong T. Y. , Tustison R. W.
Published: July 2002

Fe films were deposited on (100) GaAs substrates by magnetron sputtering. Bombardment of the films during growth by energetic particles from the plasma had a significant impact on film properties including resistivity, crystallographic orientation, and stress. Preferred orientation of the (200) Fe planes parallel to the substrate surface was observed over a wide range of deposition conditions. The (200) Fe rocking curve widths of these films compare favorably with those of films deposited using molecular beam epitaxy, and the resistivities were comparable to that of bulk Fe. Positions directly in front of the cathode as well as low cathode power appear to favor the formation of (200) Fe texture, however, the rocking curve width and lattice parameter are relatively independent of substrate position and cathode power.
Magnetic hysteresis of p+ and 3He++ irradiated melt-textured YBa2Cu3O7−δ
Authors: Song S. N. , Liu J. , Chen I. G. , Weinstein Roy
Published: July 2002

We have measured the magnetic hysteresis loops and temperature dependent trapped fields in melt-textured YBa2Cu3O7−δ samples before and after p+ and 3He++ irradiation using a Hall effect magnetometer (HEM) as well as a commercial vibrating sample magnetometer (VSM). For proper 3He++ fluence, the critical current density may be enhanced by a factor of 10. Calculations based on various critical state models show that before the irradiation, the hysteresis loops can be well accounted for by a critical current density of a modified power law field dependence Jc(T,B)=J0(T)/(1+B/B0)n with n=1/2; after the irradiation, the best fit has been achieved by using an exponential form such as Jc(T,B)=J0(T)exp(−B/B0), where B0 is a model dependent parameter. Jc and its field dependence deduced from HEM hysteresis loops are in good agreement with those deduced from the VSM loops, suggesting that the Hall effect magnetometer can be conveniently used to characterize bulk high Tc oxide superconductors.
Swelling and surface forces-induced instabilities in nanoscopic polymeric structures
Authors: Hill Davide A. , Huang Xiaokang , Bazán Greg , Bernstein Gary H.
Published: July 2002

For closely spaced, nanolithographically defined lines, a thin wall of resist remains to act as the metal line spacer. When exposed to a developer, and then rinsing solution, closely spaced resist walls may become unstable as a result of two effects: (1) internal stresses due to swelling, and (2) lateral surface forces between adjacent walls. In this article we perform a linear stability analysis of a thin polymer wall under the simultaneous action of internal stresses and lateral surface forces. We calculate a stability boundary, and show that internal stresses are necessary for the formation of deformation patterns of finite wavelength. We find that, for slightly subcritical swelling stresses a small lateral force can induce buckling, while, for slightly subcritical surface tractions large internal stresses are necessary to induce instability. The theoretical predictions are in good agreement with experimental data on poly(methyl-methacrylate) walls produced by electron beam lithography.
1/f noise from levels in a linear or planar array. III. Trapped carrier fluctuations at dislocations
Authors: Morrison S. Roy
Published: July 2002

Following Hooge’s suggestion [F. N. Hooge, Phys. Lett. A 29, 139 (1969)] that 1/f noise follows a simple empirical law involving a parameter αH, with the noise caused by mobility fluctuations, we examine the mobility fluctuations produced by dislocations in semiconductors, viz. produced by carrier trapping levels in a linear array. In the present contribution we analyze the fluctuations of trapped carriers, where the trapping is dominated by the cylindrical space charge regions around the dislocations. The corresponding mobility fluctuations are evaluated in Part IV of the series.
Molecular diffusion in plasma-polymerized tetrafluoroethylene
Authors: Butler M. A. , Buss R. J.
Published: July 2002

Diffusion of an array of molecules in micrometer-thick films of plasma-polymerized tetrafluoroethylene has been measured using an optical interferometric technique. The diffusivity is approximately independent of molecular size up to a molar volume of about 100 cm3 and drops rapidly for larger molecules. For much larger molecules no penetration of the films is observed. These results suggest that plasma-polymerized tetrafluoroethylene films are heavily cross linked and that this limits the size of the molecules that can penetrate the polymer. The temperature dependence and the molecular size dependence of the diffusivities are discussed in the context of free-volume theory.
Rheomagnetic measurements on suspensions of magnetic recording particles
Authors: Kwon T. M. , Jhon M. S. , Karis T. E.
Published: July 2002

Measurement of magnetic particle flow orientation properties is of considerable practical and fundamental interest as it relates to the particulate recording tape and disk manufacturing industry. For production of high-quality magnetic tapes and disks, the suspension must be well dispersed. Previous study focused on rodlike single-domain magnetic particles for longitudinal magnetic recording. In contrast, platelike particles are considered here for use in perpendicular recording. Both theory and experiment have demonstrated that a magnetic sensing technique employed to measure the hydrodynamic orientation provides a measure of the flocculation state for rodlike magnetic particle suspensions. Here the previous theory is extended and new experimental results are shown to demonstrate that the hydrodynamic orientation measurement apparatus can also be used to measure the flocculation state of platelike particles. The key new experimental result is the contrast between the flow orientation properties of the rod versus the platelike particles. Rodlike single-domain particles orient parallel to the flow direction causing the suspension magnetic permeability parallel to the flow direction to decrease with increasing flow rate. In the case of the platelike Ba-ferrite particles, the opposite behavior is observed. As the flow rate is increased, the particles orient with their flat side parallel to the flow direction causing the suspension magnetic permeability parallel to the flow direction to increase.
The photoluminescence spectra of porous silicon boiled in water
Authors: Li K.-H. , Tsai C. , Shih S. , Hsu T. , Kwong D. L. , Campbell J. C.
Published: July 2002

The photoluminescence (PL) of porous Si that has been hydrogenated in boiling water has been investigated. The PL intensity is observed to increase with a concurrent shift of the spectral peak to shorter wavelengths. These effects can be explained in terms of size effects in the microstructures of porous Si. The spectral changes after annealing and after rehydrogenation are similar to the behavior of a-Si:H. We conclude that hydrogen plays an important role in the luminescence of porous Si.
Production of net positive space charge in a rarefied neutral gas for strong focusing of electron beams
Authors: Kuperman V. Yu. , Friedman G.
Published: July 2002

A stationary process of secondary particle production due to impact ionization of neutral gas by an electron beam and their escape in the presence of an axial focusing magnetic field is studied analytically. It is shown that a region of net positive space charge can be created for strong beam focusing within a short biased auxiliary tube placed inside a drift tube. It is also found that there is a threshold of the magnetic field magnitude after which ion focusing of a continuous beam is not possible because no balance between the production and collisionless escape of ions exists. The dynamics of the secondary electrons is investigated to determine the possibility of reaching a stationary state while keeping the secondary electron concentration smaller than the beam’s concentration thereby allowing to avoid various electron–electron instabilities. Axial escape of the secondary electrons due to the influence of the beam’s fields, ions’ electric field and external magnetic field is assumed. An expression for the secondary electron concentration is obtained.
Effect of inelastic scattering on resonant tunneling studied by the optical potential and path integrals
Authors: Zohta Yasuhito , Ezawa Hiroshi
Published: July 2002

An analytical expression is obtained for the peak and valley currents of a resonant tunneling diode by using the optical potential to represent the effect of inelastic scattering. The assumption is made that scattered electrons in the well tunnel through both the left- and right-hand-side barriers in proportion to the transmission coefficient of each barrier. The result for peak current showing that scattering has little influence on it is in good agreement with microscopic theory. In contrast, the valley current increases in proportion to the scattering rate. It is pointed out that the controversy regarding sequential tunneling arises from the ambiguity of its definition. It is also pointed out that the Fabry–Perot effect still plays a significant role in producing resonance even when scattering is strong.
Optical properties of H2S-annealed CuInSe2 films
Authors: Aren G. , Vankar V. D. , Agnihotri O. P. , Singh Chatar
Published: July 2002

Thin polycrystalline CuInSe2 films have been grown in a range of composition and deposited on the glass substrate by two-source vacuum evaporation of CuInSe2 and Se powders. These films were annealed into H2S. The optical study shows an increase in the energy band gap after annealing. The films were examined by x-ray diffraction, energy dispersive x-ray analysis, and scanning electron microscopy. The optical constants of the films were determined by transmission and reflection at normal incidence in the energy range 0.83–1.77 eV.
Posthydrogenation of low-pressure chemical-vapor-deposited amorphous silicon using a novel internal lamp system and its application to thin-film transistor fabrication
Authors: Uchida Yatsutaka , Deane Steven C. , Milne William I.
Published: July 2002

Posthydrogenation of a-Si produced by low-pressure chemical-vapor deposition was investigated using an internal lamp which can dissociate hydrogen molecules directly. After hydrogenation, the photo–to–dark-current ratio was increased to greater than 4.5×104. The hydrogen in the hydrogenated films was mainly bonded as Si-H, and the penetration depth was about 300 nm. The posthydrogenated thin-film transistor had about 7 orders on/off current ratio and its electron and hole mobilities were 0.1 and 0.01 cm2/V s, respectively.
A model for the diffusion and precipitation of antimony in highly doped δ layers in silicon
Authors: van Opdorp C. , van IJzendoorn L. J. , Fredriksz C. W. , Gravesteijn D. J.
Published: July 2002

Antimony δ-doping layers were made by deposition of Sb on monocrystalline Si, followed by the deposition of amorphous Si and a final solid-phase-epitaxy treatment at 620 °C. After post-annealing at temperatures between 625 and 725 °C, Sb precipitates with a diameter of several nm are observed in the δ plane with the aid of transmission electron microscopy. Using channeling Rutherford Backscattering Spectrometry the increase of the precipitated fraction with time was determined from the minimum-yield signal. The results are interpreted using a model for the generation of Sb nuclei which grow subsequently due to lateral diffusion of Sb atoms in the δ plane, followed by incorporation into the nucleus. The generation of the nuclei appears to take place by way of two parallel processes: (i) fast, simultaneous generation of a limited number of nuclei at low-energetic sites in the δ plane, with subsequent diffusion-controlled growth, and (ii) slow, continuous generation of a larger number of nuclei at random sites in the δ plane, with subsequent incorporation-controlled growth. The Sb diffusion at the extremely high concentrations under consideration is very fast and concentration dependent, which can be explained by the model of vacancy-percolation diffusion of Mathiot and Pfister [J. Appl. Phys. 66, 970 (1989)]. The activation energy for incorporation of Sb atoms into liquid precipitates appears to be considerably lower than for incorporation into solid ones.
Synthesis and characterization of Bi2Sr2Can−1CunOy(n=1–7) thin films grown by off-axis, three target magnetron sputtering
Authors: Narita Hideo , Hatano Takeshi , Nakamura Keikichi
Published: July 2002

An alternate deposition method using three target magnetron sputtering and off-axis substrate geometry has been applied to control the number of CuO2 planes n in Bi2Sr2Can−1CunOy thin films. During alternate deposition, the substrate stayed sequentially at each target as Bi→Sr5Cu2O7→CaCuO2→Sr5Cu2O7→Bi. When the staying times at the Bi and Sr5Cu2O7 targets were fixed constant but varied for the CaCuO2 target, single phase films with n=1–7 were obtained. The x-ray diffraction patterns showed a systematic change in the relative line intensity with n in accordance with the calculated patterns, and all possible (00l) lines, including the (004_0_) line of the n=7 film, can be clearly observed below 2θ<60° region. The c dimension increases stepwise by 0.63 nm and approaches 6.27 nm for n=7, indicating a perovskite CaCuO2 plane is regularly inserted with n. The resistivity zero temperature, Tc zero, decreases systematically with n. For n=7, Tc decreased to onset 55 K and end 4 K. The systematic decrease in Tc zero with n can be accounted for by a combination effect of the hole concentration decrease and crystal defect increase with n.
Measurements of spatial charge centroid location in a nonmetalized polymer film
Authors: Xie Xishun , Huang Xiaoqin , Wu Zonghan
Published: July 2002

The method of open circuit thermally stimulated current (TSC) measured in open circuit to determine the spatial charge centroid location in a nonmetalized polymer film is presented in this paper. By performing open circuit TSC together with the measurement of the surface potential of the sample, we can calculate the location of charge centroid in dielectrics. The charge centroid in charged polyethyleneterephthalate film (50-μm thick) measured 1.9±0.3 μm from the upper surface.
Ultrasound attenuation in superlattices
Authors: Rodrigues C. , Nunes O. A. C.
Published: July 2002

The ultrasound attenuation coefficient is evaluated for an ideal, periodic array of quantum wells (superlattice) by taking matrix elements of the electron-phonon interaction between basis states that are plane-wave combinations of finite quantum-well wave functions. Contributions due to intrawell electron-phonon scattering as well as due to interwell transfer were found. In addition, a contribution due to the interference of intrawell and interwell electron-phonon scattering process is obtained. The total attenuation coefficient along the superlattice axis is found to be dependent upon the electron wave number (or the density of states) at the Fermi level.
A study of Si outdiffusion from predoped GaAs
Authors: You Horng-Ming , Gösele Ulrich M. , Tan Teh Y.
Published: July 2002

Experiments have been conducted to study the Si outdiffusion behavior in GaAs using predoped samples. The results showed that the Si diffusivity values are dependent on the As4 vapor-phase pressure in the ambient, and on the electron concentration in the crystal. It is concluded from these results that, in GaAs, diffusion of the Si donor species occupying Ga sites SiGa+ is governed by the triply negatively charged Ga vacancies, VGa3−. The present VGa3−-dominated SiGa+ outdiffusion diffusivity values are, however, larger than those obtained under Si indiffusion conditions by many orders of magnitude. A tentative explanation of this large difference is given in terms of an undersaturation of VGa3− in intrinsic GaAs during indiffusion experiments and of a supersaturation of VGa3− developed during the outdiffusion of Si from n-type, Si-doped GaAs.
Nonreciprocal coupling structure of dielectric wave guides
Authors: Kwan P. , Vittoria C. , Proske J.
Published: July 2002

M-type hexagonal ferrites have been incorporated in a coupled dielectric image guides structure for nonreciprocal microwave circuits design. Two different field configurations have been considered in this paper: for case I the c axis of the ferrite materials was aligned along the direction of propagation; for case II the c axis was aligned in the transverse direction. Electromagnetic wave scattering S parameters of the device were measured for an external biasing magnetic field applied parallel and perpendicular to the c axis of the ferrite slab. We have observed nonreciprocal microwave effects in the ferromagnetic resonance regime as a result of a nonuniform external biasing magnetic field along the propagation direction. We believe that ferrite devices operating at millimeter-wavelength frequencies, such as isolators, filters, modulators, switches, phase shifter, etc., can be implemented from our device design.
Magnetic excitations in grain-aligned HoBa2Cu3O7 (abstract)
Authors: Staub U. , Fauth F. , Guillaume M. , Mesot J. , Furrer A. , Dosanjh P. , Zhou H.
Published: July 2002

Inelastic neutron scattering has been employed to study the magnetic excitations of Ho3+ in a grain-aligned sample of the high-Tc compound HoBa2Cu3O7. From these measurements, we derive direct information on the anisotropic nature of the pair coupling within the rare-earth sublattice. The HoBa2Cu3O7 sample was grain-aligned at high temperatures under the action of an external field. This procedure resulted in an alignment of the grains along the z axis, whereas the x and y axes remained randomly oriented. We have particularly chosen HoBa2Cu3O7, since its crystal-field level scheme is well-established and characterized by a series of low-lying states1 which can be sufficiently resolved to detect even very small dispersion effects. The dispersive behavior of the Γ3→Γ4 crystal-field excitation is exemplified in Fig. 1 for various values of Q=(x′,z), x′=√x2+y2, at T=1.5 K. The absence of energy dispersion along the z axis proves the predominantly 2D nature of the magnetic coupling between the Ho3+ ions, as actually expected from their large separation along the z axis. The observed energy spectra correspond to a superposition of transverse and longitudinal components which we have been able to separate through selected scans in Q space. Nearest-neighbor exchange parameters are estimated on the basis of an effective two-level model. The coupling of the Ho spins turns out to be extremely anisotropic.2
X-ray magnetic circular dichroism studies of magnetic alloys and multilayers (invited) (abstract)
Authors: Wu Y.
Published: July 2002

X-ray magnetic circular dichroism (XMCD) is a newly developed technique that measures local magnetic properties with element specificity. The element specificity makes it possible to probe both the magnetic and the ‘‘nonmagnetic’’ elements in alloys and multilayers. When XMCD is combined with a suitable electron or x-ray microscopy technique, it can be used on multidomain samples without any external magnetic field and the samples can be studied with high spatial resolution. We have carried out a series of XMCD studies on magnetic thin films and multilayers that clearly demonstrated all of these aspects. Large XMCD signals have been observed near the Co L3 and L2 edges in pure Co thin film, Co0.2Pd0.8 alloy and Co/Pd multilayers. Application of a recently proposed sum rule to our data indicates that the orbital magnetic moments at the Co site in both the alloy and multilayer samples are greatly enhanced compared to pure Co metal. This gives strong support to recent theoretical predictions and has implications for the strong perpendicular anisotropy observed in the alloy and multilayer samples. XMCD results near the Pd L3 and L2 edges on Co/Pd multilayers showed that Pd atoms near the interface have nonvanishing magnetic moments with its direction parallel to that of Co atom. We have also used the XMCD technique along with an electrostatic imaging lens system to examine a CoPtCr magnetic recording disk which had been patterned with a recording signal. The magnetic contrast arises from the fact that x-ray absorption depends on the relative orientation of the photon spin and the local magnetization direction. We have obtained images with good contrast (up to 20%) and a spatial resolution of 1 μm. With improved electron optics and x-ray source, this new microscopy technique should provide a much higher spatial resolution. It appears particularly valuable for the investigation of magnetism at interfaces and in complex materials where the magnetic contribution from different elements needs to be distinguished.
Structural and magnetic ordering in iron oxide/nickel oxide multilayers by x-ray and neutron diffraction (invited)
Authors: Lind D. M. , Tay S.-P. , Berry S. D. , Borchers J. A. , Erwin R. W.
Published: July 2002

Presented are studies of the magnetic and structural ordering in superlattices composed of Fe3O4 and NiO, and their study by a variety of techniques including x-ray and neutron diffraction, and SQUID magnetometry. X-ray diffraction indicates that structures with individual layer thicknesses down to 8.5 Å grow as single crystals in registry with the substrate lattice with a layer-thickness-dependent tetragonal lattice distortion due to epitaxial and interfacial lattice mismatch. The lattice coherence of the Fe3O4 layers, however, is degraded by stacking faults between adjacent spinel layers. Neutron diffraction indicates that the NiO orders antiferromagnetically along the [111] direction with a magnetic coherence that extends over several superlattice bilayers, and the presence of an enhancement in the NiO Néel temperature in thin layered superlattices. These results are compared with SQUID magnetometry, which shows large anisotropy energies, but a lack of favored magnetization direction, indicating that the magnetic ordering in these systems is dominated by the exchange coupling across the interfaces.
Magnetism and epitaxy in Lu/Dy/Lu trilayers
Authors: Beach R. S. , Matheny A. , Salamon M. B. , Flynn C. P. , Borchers J. A. , Erwin R. W. , Rhyne J. J.
Published: July 2002

Thin dysprosium c-axis films (40–400 Å) were grown coherently between 500-Å lutetium layers by molecular beam epitaxy. Bulk magnetization measurements show that these sandwich structures order magnetically at TN≂178 K (=TN of elemental Dy) and undergo ferromagnetic transitions at temperatures which range from 100 K (400 Å Dy) to 175 K (40 Å Dy), significantly enhanced from the bulk TC=85 K. The Dy basal plane lattice parameters in the films were determined by room-temperature x-ray diffraction. We observe a change in these values that correlates with the rise in TC, which suggests that this rise is due to epitaxial strain. The relatively small low-temperature magnetic susceptibility displayed by these samples indicates the presence of a large anisotropy in the basal plane. We address both the issues of the susceptibility and the high ferromagnetic transition temperature.
Composite superconductor-polymer material preparation for low-magnetic-field shielding
Authors: Valko Pavol , Grusková Anna , Michalíková Marianna
Published: July 2002

Composite superconductor/polymer foils with up to 80% superconductor mass fraction were prepared. The standard YBa2Cu3O7 system and polyvinylchloride were chosen as superconducting and plastic constituents, respectively. Density measurements showed close to ideal volume filling in samples. Full plasticity of samples at room temperature was conserved after numerous thermal cycles. Magnetic measurements showed maximum magnetization of −800 A/m at liquid-nitrogen temperature using foils with 80% superconductor mass fraction. No clear signs of possible collective (percolative) effects were seen.