bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetostatic wave propagation in lossy ferrite layers under perpendicular bias
Authors: Buris N. E.
Published: July 2002

The complex wave number of magnetostatic waves propagating in multiple lossy ferrite layers is found by solving the dispersion equation using Davidenko’s method. The real and imaginary parts of the wave number are used to obtain quantities representing the power of modes as function of distance traveled and time spent in the material. The loss does not change significantly the real part of the wave number. When all ferrite layers have the same magnetization, then the loss experienced by a mode after it has spent a given period of time in the film is unique, independent of the order of the mode or the film thickness or the ground plane distances. When the magnetization differs from layer to layer then the loss experienced by a mode after it has spent a given period of time in the film depends on the order of the mode and the loss of each ferrite layers. The loss experienced by a mode after it has traveled a given distance in the ferrite depends on the order of the mode, the film thickness and the ground plane distances.
Photomodulated absorption spectroscopy on AlGaAs-GaAs heterostructures
Authors: Van Hoof C. , Arent D. J. , Deneffe K. , De Boeck J. , Borghs G.
Published: July 2002

Optical transitions in quantum-well heterostructures are very well revealed by photomodulated absorption spectroscopy. With respect to nonmodulated absorption spectroscopy, a strong increase in both room-temperature resolution and signal-to-noise ratio is observed. This new technique is most attractive for the investigation of multilayers grown on transparent substrates. The evolution of the spectral lineshapes of bulk and excitonic transitions as a function of temperature is shown.
Effects of alternating bias current on the low-frequency noise in dc SQUIDs
Authors: Kuriki S. , Matsuda M. , Matachi A.
Published: July 2002

We have analyzed the characteristics of flux modulated voltages of a dc SQUID for both positive and negative bias currents. It is shown that the change in the modulated voltage caused by fluctuations of the junction critical currents is eliminated after subtraction between the modulated voltages for the two biased currents. In the flux feedback mode using an alternating bias current, a large reduction of 1/f noise is observed for dc SQUIDs made of NbN junctions with amorphous Si tunnel barriers.
Impurity ion temperature and toroidal rotation velocity in JET from high-resolution x-ray and XUV spectroscopy
Authors: Mattioli M. , Ramette J. , Saoutic B. , Denne B. , Källne E. , Bombarda F. , Giannella R.
Published: July 2002

Doppler shifts and widths of strong spectral lines have been measured on JET (Joint European Torus) both in the x-ray (resonance line of He-like Ni xxvii at 1.5886 Å) and in the XUV region (several Ni, C, and He ion lines are available between 100 and 330 Å). Time-resolved measurements of impurity ion temperature Tz and plasma toroidal angular velocity ω are presented. Radial profiles of both Tz and ω are inferred in combination with numerical simulations obtained using an impurity transport code, which allows radial localization of the emitting shells.
Sensitivity of diffusion length determination on background current collection efficiency in light beam induced current analysis using the Zook equation
Authors: Field H. , Ast D.
Published: July 2002

The fall-off of the light beam induced current in the vicinity of a grain boundary is frequently used to determine the minority-carrier diffusion length in the vicinity of the boundary and the surface recombination velocity at the boundary. It is shown that the numerical values obtained can be very sensitive to the choice of the background.
The effects of donor dopant concentration on the grain boundary layer characteristics in n-doped BaTiO3 ceramics
Authors: Al-Allak H. M. , Illingsworth J. , Brinkman A. W. , Russell G. J. , Woods J.
Published: July 2002

Positive temperature coefficient of resistance BaTiO3 specimens containing different donor dopant concentrations of Ho ranging from 0.05 to 1.8 at. % were investigated. The intergranular barrier layer capacitance per unit area, C′L, measured at a constant frequency of 30 kHz at both 40 and 160 °C was found to be proportional to the donor concentration up to 0.55 at. %, but then began to decrease as the donor concentration was increased beyond this. This indicated that both the density of acceptor states at the grain surfaces, Ns, and the relative permittivity εL of the material within the barrier layer were not affected by donor impurity concentrations below 0.55 at. % Ho. However, above this level of Ho concentration, the decrease in C′L appears to be related mainly to an increase in the value of Ns although it is possible that there were changes in εL. Initially both the maximum resistance and the room-temperature resistance (normalized per grain boundary per unit area), ρ′max and ρcold, respectively, were found to decrease sharply with donor concentration towards a broad minimum between ∼0.5 and ∼1.5 at. %, followed thereafter by a gradual increase. The temperature Tmax at which ρ′max occurred was also affected by the donor concentration; initially Tmax was found to increase with donor concentration followed by a reduction forming a broad maximum between about the same donor concentration limits corresponding to the minima in ρcold and ρ′max. These results are interpreted in terms of the well-established Heywang model.
Pulsed laser deposition of iron oxide and ferrite films
Authors: Joshi Sushama , Nawathey Rashmi , Koinkar V. N. , Godbole V. P. , Chaudhari S. M. , Ogale S. B. , Date S. K.
Published: July 2002

Iron oxide and ferrite films were prepared by pulsed ruby laser evaporation from the respective bulk materials on alumina substrates. The variation in the film properties as a function of the substrate temperature and oxygen partial pressure during deposition was studied. Conversion electron Mössbauser spectroscopic analysis showed that the stoichiometry and microstructure of such films depend on these deposition conditions, especially the oxygen partial pressure in the system. Typically it was observed that when laser deposition is performed by vaporizing α-Fe2O3 in the oxygen partial pressure of 10−4 Torr, Fe3O4 thin films were formed while deposition at a background pressure of 10−6 Torr led to formation of FeO films. Similar experiments were also performed on zinc ferrite and it was observed that the nanosecond pulsed evaporation process transports the stoichiometry of metal constituents from the bulk to the film with a small degree of zinc enrichment. In addition to Mössbauer spectroscopy, other techniques such as scanning electron microscopy and x-ray diffraction were also used to characterize the laser-deposited films.
Magnetic and crystallographic properties of the permanent magnet systems (R1−xScx)2Fe14B (R=Y, Nd, Dy, Er, Lu)
Authors: Chen S. K. , Duh J. G. , Ku H. C.
Published: July 2002

Magnetic and crystallographic measurements have been made on the rare-earth iron boride systems (R1−xScx)2Fe14B (R=Y, Nd, Dy, Er, Lu). Powder x-ray diffraction data indicate that a Nd2Fe14B-type single phase persists up to x=0.2 with lattice parameters decreasing slightly due to the smaller metallic radius of the Sc3+ ions. The variation of magnetic moments per formula unit with nonmagnetic Sc substitution depends on the smaller Sc size and the nature of the magnetic alignment of the rare-earth sublattice (nonmagnetic in Y and Lu, ferromagnetic axial in Nd, ferromagnetic planar in Er, and antiferromagnetic axial in Dy) with respect to the iron sublattice. The Curie temperature TC decreases monotonically with progressive addition of Sc atoms.
Strength of Terfenol-D
Authors: Peterson D. T. , Verhoeven J. D. , McMasters O. D. , Spitzig W. A.
Published: July 2002

The strength of the magnetostrictive material, Terfenol-D, Tb0.27Dy0.73Fe2.0, has been evaluated by measuring the modulus of rupture as a function of the Fe stoichiometry of the compound. It is shown that the strength improves markedly as the Fe concentration decreases from the stoichiometric value of 2.0 in the compound. The increased strength results from the increasing volume fraction of the rare-earth metal phase produced in the Terfenol-D matrix by the solidification process at lower Fe concentrations.
The effects of long-term annealing on superconducting properties in the Bi-Sr-Ca-Cu-O system
Authors: Sarkar Asok K. , Kumar Binod , Maartense I. , Peterson T. L.
Published: July 2002

The effect of long-term annealing on the relative proportions of the low- and high-Tc phases present in the Bi-Sr-Ca-Cu-O system was examined by resistivity and ac susceptibility measurements. The zero-resistivity offset was found to increase from 50 to 68 K and the susceptibility measurements showed the disappearance of the low-Tc , 63-K phase with the enrichment of the high-Tc , 110-K phase as a result of the annealing treatment. Also, the onset of diamagnetism of the high-Tc phase was enhanced by 3 K due to prolonged annealing. The high-Tc phase thus formed is poorly crystallized as determined by the x-ray diffraction patterns. Some impurities are observed in both x-ray diffraction patterns and scanning electron microscopy pictures and are assigned to the low-Tc , Sr, and Bi-doped Ca2CuO3 phases. This study shows that a complex ac susceptibility measurement technique can be a very powerful tool for studying the equilibrium superconducting phases present at a particular temperature in a multicomponent system such as Bi-Sr-Ca-Cu-O.
Electron concentrations and mobilities in AlSb/InAs/AlSb quantum wells
Authors: Tuttle Gary , Kroemer Herbert , English John H.
Published: July 2002

We present data on the electron concentrations and mobilities in deep (≊1.3 eV) AlSb/InAs/AlSb quantum wells grown by molecular-beam epitaxy. High electron sheet concentrations of the order 1012 cm−2, found in the not-intentionally doped wells, indicate the presence of a deep donor in the AlSb barriers. Typical mobilities are between 22 000 and 28 000 cm2/V s at room temperature, increasing with decreasing temperature, and leveling out below 50 K at values between 175 000 and 330 000 cm2/V s. The temperature-independent low-temperature mobilities indicate a nonthermal scattering mechanism, possibly interface roughness scattering. Under illumination the wells exhibit a strong negative photoconductivity, which is explained as a natural consequence of the band structure of the wells.
Influence of coercivity squareness on media noise in thin-film recording media
Authors: Sanders I. L. , Howard J. K. , Lambert S. E. , Yogi T.
Published: July 2002

Signal and media noise measurements have been made for a wide variety of Co-alloy longitudinal magnetic recording media. A strong correlation between media noise and the coercivity squareness, S*, is observed at high transition density. The optimum ratio of isolated pulse amplitude to media noise is found for S*≂0.75. This behavior arises from reduced ferromagnetic exchange coupling between neighboring grains of the film which influence both the coercivity squareness and also the uniformity of the domain boundary at the recorded transitions. The low-noise characteristics of sputtered γ-Fe2O3 also appear to correlate with low values of coercivity squareness. This work confirms that for systems in which media noise is a limiting factor, optimum performance may require considerably lower values of coercivity squareness than commonly used, determined by the relative contribution of media noise to the overall system noise.
Avalanches near a solid insulator in nitrogen gas at atmospheric pressure
Authors: Mahajan S. M. , Sudarshan T. S.
Published: July 2002

The pulsed Townsend (PT) technique was used to record the growth of avalanches near a solid insulator in nitrogen gas at 0.1 MPa. Several other nonconventional techniques for releasing initiatory electrons at the cathode are discussed. In this paper, experimental results of avalanches initiated by illuminating a fast (0.6-ns) nitrogen laser onto the cathode triple junction are presented. Data were recorded with plexiglas, Teflon, high-density polyethylene, low-density polyethylene, Delrin, etc. Effect of surface condition, variation of the distance between insulator surface and the avalanche initiation region, and the effect of a large number of previous avalanches on the avalanche characteristics at a particular voltage were studied. The Townsend primary ionization coefficient, hereafter referred to as growth coefficient (α), and drift velocity (Ve) were evaluated through the PT technique. Results indicate that the avalanche growth in the vicinity of a solid insulator is less than that in an identical plain gas gap. Existence of a nonuniform field as a result of surface charges on the insulator and/or field modifications due to the avalanche space charge are believed to be responsible for this behavior.
Spectroscopic plasma tomography on field-reversed configuration
Authors: Sugimoto Satoshi , Niina Torao , Goto Seiich
Published: July 2002

A time-resolved and two-dimensional (2D) measurement system for high-temperature plasma based on spectroscopy and computed tomography has been developed. The projections of plasma emission were collected using a ten-channel fiber optic in the form of fan beam. A visible grating monochromator was employed as a narrow bandpass filter to extract a specific wavelength component from the collected light signals. The wavelength was tunable over visible and ultraviolet regions. The first result of an experiment to measure a 2D profile of a field-reversed configuration plasma is presented. The measurement wavelength (497.6 nm) was appropriate to the detection of bremsstrahlung. The reconstruction results had a spacial resolution of 15×15 and a time resolution of up to 0.5 μs. Although a plasma rotation was assigned in this experiment, the result shows the potential of the developed system for plasma diagnostics.
Direct evidence of porosity in carbon-rich hydrogenated amorphous silicon carbide films
Authors: Arce R. , Koropecki R. R. , Buitrago R. H. , Alvarez F. , Chambouleyron I.
Published: July 2002

Infrared absorption spectroscopy was used to study the oxidation of hydrogenated amorphous silicon carbide (a-Si:C:H) films prepared by the glow-discharge decomposition of gaseous mixtures of silane and methane. It has been found that carbon-rich samples incorporate oxygen when exposed to air, as detected by an increased absorption of the Si-O-Si stretching vibration band. The analysis of the infrared spectra of samples annealed in air at room temperature and at 200 °C indicates that, except for their oxidation rate, no appreciable difference exists in the mechanisms of oxygen incorporation in the films at the two temperatures. The oxidation kinetics suggests an open porous structure for these carbon-rich films. On the contrary, samples having a low carbon content appear to oxidize on the surface only, in a way similar to amorphous silicon.
Stabilization and removal of the native oxides at the surface of (100)InP by low-pressure exposure to NH3
Authors: Moison J. M. , Nissim Y. I. , Licoppe C.
Published: July 2002

The complete reduction of oxides on the chemically polished surface of (100)InP has been obtained by exposure at 300 °C to low-pressure ammonia. The whole process was monitored by various surface techniques. In a first step, exposure to ammonia removes the weakly bonded oxygen atoms and stabilizes the one-monolayer oxide, i.e., turns the initial wet oxide to a thinner, well-defined, and more stable oxide, which should prove to be a better base for further processing. Excitation of the ammonia gas by an ion gauge, i.e., use of highly active radicals, is necessary for the second step, where the oxygen atoms strongly bonded to InP are finally completely removed. The final surface is free of all contaminants and of nitrogen, its crystal structure displays a 4×1 reconstruction, and its Fermi level is pinned at 0.8 eV above the top of the valence band, i.e., 0.2 eV lower than on the clean InP surface. It is mostly InP covered by adsorbed hydrogen, with a small coverage of indium atoms liberated by the reduction of the native oxides and grouped as metallic clusters.
Computer simulation of magnetization reversal in fine hexaferrite particles
Authors: Nakatani Yoshinobu , Uesaka Yasutaro , Hayashi Nobuo
Published: July 2002

Magnetization reversal mechanisms in hexaferrite particles were investigated by computer simulation. The Landau–Lifshitz–Gilbert equation discretized according to the Crank–Nicolson method was solved numerically. The computing region was divided into triangular prisms of equal size which are accommodated to the contour surfaces of the hexagonal particle. The demagnetizing field was calculated using a finite-grid method based on the triangular prisms. The mode of reversal was found to be extremely dependent on the thickness of the particle. In the case of a thin particle, the reversal started at the particle center and then spread to the edges of the particle almost coherently at the same radius. In a thick particle, vortexes of magnetization with opposite polarities of rotation were generated at the top and bottom surfaces, and the switching was incoherent over the particle thickness. In the case of an intermediate thickness, the reversal began at the particle center, but it spread to the particle edges incoherently. The formulation of the calculation and the results of simulation are given.
Microwave and magneto-optic properties of bismuth-substituted yttrium iron garnet thin films
Authors: Butler J. C. , Kramer J. J. , Esman R. D. , Craig A. E. , Lee J. N. , Ryuo T.
Published: July 2002

Microwave and magneto-optic measurements have been made on bismuth-substituted yttrium iron garnet (BiYIG) films. Forward-volume (FV) magnetostatic-wave (MSW) attenuation has been measured from ferrimagnetic resonance and from pulse delay data. We report the indirect observation of FV MSW in BiYIG using two independent techniques: a pulse transmission technique and a passband measurement technique. Faraday rotation in the films was also recorded at a wavelength of 1.3 μm. The bismuth-substituted films are grown on carefully cleaned substrates and have yttrium:bismuth ratios of 1:1. The composition of the bismuth substituted films is Y1.5Bi1.5Fe5O12 deduced from lattice parameters and absolute Faraday rotation. These films show particular promise for use in waveguide-type high-speed MSW-optical devices where low MSW attenuation and high Faraday rotation are among the necessary criteria for successful operation.
Photoreflectance measurement of strain in epitaxial GaAs on silicon
Authors: Dimoulas A. , Tzanetakis P. , Georgakilas A. , Glembocki O. J. , Christou A.
Published: July 2002

The valence-band splitting due to strain in molecular-beam epitaxially grown GaAs on Si has been observed by photoreflectance. The strain has been obtained from the valence-band splitting and was found to be in agreement with results obtained by x-ray rocking curve measurements, photoluminescence, and Raman spectroscopy. The temperature dependence of the strain has also been measured and found to be in agreement with thermal expansion effects.
A fast, preparation-free method to detect iron in silicon
Authors: Zoth G. , Bergholz W.
Published: July 2002

Iron is one of the most important impurities in silicon integrated-circuit technology. We present a fast (5 min), essentially preparation-free large-area (25 cm2 ) technique to determine the Fe concentration in boron-doped silicon with a sensitivity of 2–5×1011 cm−3. The principle of the method is based on the fact that interstitially dissolved Fe undergoes a reversible pairing reaction with boron and that the minority-carrier diffusion length—as measured by the surface photovoltage method—is modified by this reaction. The method has been calibrated by deep-level transient spectroscopy and is also suitable to measure a surface Fe contamination in combination with a rapid thermal annealing diffusion step.
Structural characterization of very thin GaInAs/InP quantum wells grown by atmospheric pressure organometallic vapor-phase epitaxy
Authors: Wang T. Y. , Jen H. R. , Chen G. S. , Stringfellow G. B.
Published: July 2002

GaInAs/InP quantum wells have been grown on (001) InP substrates by atmospheric pressure organometallic vapor-phase epitaxy with well widths ranging from a few monolayers to 100 Å. The interface quality and the epilayer thickness were examined using x-ray diffraction spectroscopy and transmission electron microscopy. The layers were found to be very uniform with both interfaces apparently free of defects. In addition, for a nominal 8 Å quantum well, a (110) high-resolution cross-sectional lattice image clearly shows the well thickness to be 2–3 monolayers, confirming that the growth rate obtained from thick layers is accurate for very short growth times. These results demonstrate that atmospheric pressure organometallic vapor-phase epitaxy can produce extremely thin GaInAs/InP quantum wells of a few monolayers.
Electrical resistivity of copper and nickel thin-film interconnections
Authors: Johnson B. C.
Published: July 2002

The resistivity and temperature coefficient of resistivity of copper and nickel thin films used as high-density interconnections in hybrid packages have been measured in the temperature interval 298–723 K. The effects of film thickness, deposition conditions, and postdeposition anneal were analyzed in detail. Minimum as-deposited resistivities approximately 18% and 80% above bulk values could be achieved under optimized deposition conditions for 1-μm-thick copper and nickel films, respectively. Contributions to the resistivity values attributable to electron scattering at phonons, surfaces, impurities, grain boundaries, and intragranular defects were also established. Although subsequent annealing had little impact on the resistivity of copper, a significant reduction in the resistivity of nickel was observed, due primarily to dislocation sinking. Lastly, the relationship of resistivity to the temperature coefficient of resistivity for both metal films was found to be in agreement with Matthiessen’s rule.
Measurement of electro-optic and electrogyratory effects in Bi12TiO20
Authors: Wilde J. P. , Hesselink L. , McCahon S. W. , Klein M. B. , Rytz D. , Wechsler B. A.
Published: July 2002

We have conducted measurements of the linear electro-optic coefficient (r41) and electrogyratory coefficient (η41) in Bi12TiO20 at 633 nm. A precise knowledge of these coefficients is important for evaluating the holographic recording properties of this photorefractive material. We have employed both transverse and longitudinal electric field geometries. For each geometry, we present the results of dc as well as ac techniques. Based on these results we find r41=5.75±0.10 pm/V and ‖η41‖≤0.30±0.05 pm/V. We critically examine our measurements and discuss their relative accuracy. Our findings are compared to those previously reported by other investigators.
Parametric study of a radio-frequency glow discharge using a continuum model
Authors: Park Sang-Kyu , Economou Demetre J.
Published: July 2002

An efficient numerical algorithm was developed to solve the continuity equations describing charged particle transport and potential distribution in a low-pressure glow discharge in an argonlike gas. A parametric investigation of the effect of gas pressure and electrode spacing on the discharge properties was conducted.
Growth and characterization of In1−xGaxSb by metalorganic magnetron sputtering
Authors: Rousina R. , Halpin C. , Webb J. B.
Published: July 2002

The growth and optical band gaps of heteroepitaxial layers of the ternary In1−xGaxSb [0≤x≤1] on (100)GaAs is reported. The epilayers, prepared by metalorganic magnetron sputtering using trimethyindium, trimethylgallium, and a sputtered antimony beam, showed good structural and surface morphologies despite a lattice mismatch between substrate and epilayer of 9%–14%. Secondary ion mass spectrometry analysis indicated a background carbon level in proportion to the gallium concentration. The high levels of carbon were not present in the InSb layers prepared using TMI. All films showed optical absorptions characteristic of direct gap semiconductors. The bowing parameter for the system is somewhat lower than that observed for the corresponding bulk material and may be related to compressive stress in the layers.
Desorption studies using Langmuir recoil force measurements
Authors: Peters Randall D. , Kwon M. H.
Published: July 2002

Number densities and energies for desorption of gas molecules on solid surfaces have been determined using Langmuir recoil force measurements and numerical modeling for the first time. Activation energies are computed from the time-dependent relative recoil force and associated temperature history of the sample. Subsequently, the model uses the activation energy and the absolute recoil force to estimate absolute desorption rates and number densities of adsorbed molecules. Desorption is by laser-induced thermal means and is detectable for sample temperature increases smaller than 100 K above room temperature. The resulting recoil forces are measured with a torsion balance, which is mechanically similar to the Cavendish type. Measurements of deflection are different, however, by using a capacitance transducer instead of an optical sensor.
On the saturation of Tb phosphors under cathode-ray excitation. I. Excited-state absorption in Tb-activated phosphor powders
Authors: Nieuwesteeg K. J. B. M. , Raue R. , Busselt W.
Published: July 2002

Optical transitions between the 5D4 excited state of the Tb3+ (4f)8 configuration and the crystal-field split components of the (4f)7(5d) configuration are observed using high-resolution laser excitation of Tb-doped powder samples at room temperature. Excited-state absorption spectra of Tb3+ in YAG, YAGaG, Y2SiO5, and LaOBr are presented. Superimposed on the broadband excitation spectrum of these transitions we find the relatively narrow 4f→4f lines. We present a theoretical model for interpreting the 4f→5d transitions, which predicts a strict proportionality between the (4f)8 5D4→(4f)7(5d) and the (4f)8 7F6→(4f)7(5d) transitions. This model is used to estimate the optical-absorption cross section for the former transitions. Although the terminating 4f levels lie at the same energy as the 5d bands, the 4f→4f transitions can be interpreted on the basis of the Judd-Ofelt theory. We derived a method for calibration of the Tb3+ 4f→5d fluorescence-excitation spectra from powder samples. For YAG:Tb, the resulting optical cross sections were confirmed by direct-absorption measurements on a Tb3+ -doped epitaxial YAG layer. To our knowledge, this is the first time that the probabilities for excited-state absorption to highly excited 5d and 4f states in Tb3+ have been obtained theoretically and experimentally from Tb-doped powder materials.
Ablation of high-temperature superconductor studied by resonance ionization mass spectrometry (RIMS)
Authors: Estler Ron C. , Nogar Nicholas S.
Published: July 2002

Laser ablation of bulk high-temperature superconductor (HTS) material provides a useful means of producing high-quality HTS thin films. Mass spectrometric probes of the ablation plume provide a microscopic understanding of the ablation event and plume development as well as providing a process monitor for the thin-film production. Resonance ionization mass spectrometry (RIMS) detection of the ablated neutral species supplies valuable physical information about the ablation event necessary for developing models of the process. Time-of-flight/RIMS detection of Cu, Y, and BaO ablated from YBa2Cu3O7−x indicates the ablation involves post-desorption gas phase collisions, which influence the ablation chemistry and dynamics (e.g., angular and velocity distributions). Approximately equal velocities are observed for all neutral species at constant ablation laser fluence. In addition, BaO exhibits a rotational temperature far less than the directed translational temperature.
Dynamics of grating formation in photovoltaic media
Authors: Gu Claire , Hong John , Li Hsin-Yu , Psaltis Demetri , Yeh Pochi
Published: July 2002

The Kukhtarev equations are solved taking into account the photovoltaic effect and different boundary conditions. In the case of open circuit, the voltage across the crystal is found to vary with a time scale similar to the photorefractive time constant. This effect explains the dynamic behavior observed experimentally.
An experimental system for studying dynamic behavior of magnetic microparticles
Authors: Helgesen G. , Skjeltorp A. T.
Published: July 2002

Optical microscopy combined with digital image analysis is used to study the properties of magnetically interacting colloidal microparticles in constant and time-varying magnetic fields. This represents model experiments to observe directly cooperative behavior of many-body systems. Some typical results are presented for experiments related to magnetic aggregation, nonlinear microparticle dynamics, and two-dimensional melting.
Infrared photoresponse of carbon films prepared by plasma decomposition of propylene
Authors: Ozaki Jun-ichi , Nishiyama Yoshiyuki
Published: July 2002

The photoconductive effect of carbon films prepared by plasma decomposition of propylene was investigated for infrared radiation between 520 and 4200 cm−1. The carbon films deposited on glass substrates and on n-type silicon substrates showed an increase in electric current, i.e., photocurrent, on infrared irradiation. The increase in current by irradiation depended on the wave numbers of the light: the current was greater for lower wave numbers, and this spectral response suggested that the increase in the current was caused by an absorption of photons and not by the thermal effect of radiation. The dark current-applied voltage characteristics of the junctions between carbon films and n-Si varied with their preparation temperature (TP ) from rectifying behavior through backward diode behavior to ohmic behavior. The increments in current upon the infrared radiation of all films strongly depended on the applied voltage. The constituent that contributes to the photocarrier generation was estimated to be the poorly stacked condensed aromatic portions in the carbon films, using an infrared absorption parameter proposed by us, which represents the density of states of the π electrons in carbons.
Hydrogen ion beam smoothening of Ge(001)
Authors: Horn K. M. , Tsao J. Y. , Chason E. , Brice D. K. , Picraux S. T.
Published: July 2002

Low-energy hydrogen ion beams are shown to clean and rapidly smoothen Ge(001) surfaces that have been subjected to severe oxygen roughening. Characteristic smoothening times of 1000 s are found at 500 °C for 200 to 600 eV hydrogen ion beams at fluxes of 200 nA/cm2. By comparing hydrogen and noble gas ion bombardment at various temperatures, we show that the hydrogen ion smoothening effect consists of both physical and chemical mechanisms which act to free pinned surface sites of contaminants and enable subsequent thermal smoothening of the germanium surface. Such oxygen roughened surfaces can be recovered to a state suitable for epitaxial growth without resorting to high-temperature annealing, keV ion sputtering or additional growth.
ND4FeBr3, a new one-dimensional S=1 antiferromagnet
Authors: Harrison A. , Stager C. V. , Visser D.
Published: July 2002

Compounds of chemical formula AFeX3 (A=Rb,Cs,Tl, and X=Cl, Br) provide an isomorphous series of one-dimensional induced-moment magnets with an effective spin S=1. They display a variety of ordered magnetic structures at low temperatures, depending on the sign and relative magnitudes of the intra- and interchain exchange and any magnetic dipole–dipole interactions. We report the first preparation of the salt NH4FeBr3. Powder neutron diffraction measurements on a perdeuterated sample showed that at room temperature the unit cell is hexagonal with cell parameters a=7.398(3) Å and c=6.331(2) Å. On cooling, structural phase transitions to cells of successively lower symmetry were observed. These are probably caused by cooperative reorientations of the ND+4 tetrahedra. At 4.8(2) K and 2.8(2) K additional Bragg peaks were observed. These additional reflections were attributed to magnetic long-range order with antiferromagnetic exchange both in the basal plane and along the c axis and could be indexed on an orthorhombic unit cell of dimension 2a ×√ 3a × c. Magnetic susceptibility measurements taken with a SQUID magnetometer on a polycrystalline sample at 2–100 K with an applied magnetic field of 0.1 T showed very similar behavior to that for RbFeBr3, with a broad cusp at approximately 8 K and θ≂−43 K.
Magnetic studies of (Gd1−xMx)5Si4 alloys (M=La or Y)
Authors: Elbicki J. M. , Zhang L. Y. , Obermyer R. T. , Wallace W. E. , Sankar S. G.
Published: July 2002

In 1967, Holtzberg, Gambino, and McGuire [J. Phys. Chem. Solids 28, 2283 (1967)] observed that intermetallic Gd5Si4 exhibits TC = 336 K. This is remarkable in that the TC of Gd (∼291 K) is increased by chemical union with a nontransition metal, which is rare, if not unique. In the present study, magnetization-temperature behavior of Gd5Si4 and (Gd1−xMx)5Si4 systems with M=La or Y were studied and compared with the Brillouin function (BF) to see if molecular-field theory is obeyed. Demagnetization near TC is much sharper for the alloys than expected from the BF. As examples, at T/TC = 0.9, the BF for Gd5Si4 gives M/M0 = 0.38, whereas the observed values range from 0.99 [for (Gd0.8Y0.2)5Si4] to 0.5 [for (Gd0.6La0.4)5Si4]. The factors responsible for these strong deviations from the BF and the high-TC values of these alloys are as yet unclear. X-ray patterns for yttrium-doped samples with x=0.1–0.5 and La with x=0.05 and 0.1 appear identical to the pattern for Gd5Si4, indicating no structural change. For La substitution of x=0.2 and above, there is a change from orthorhombic to tetragonal structure. All of the compounds show a decrease in Curie temperature and magnetic moment as Gd is decreased, as expected because of the replacement of Gd by a nonmagnetic species.
Angular dependence of coercivity in sintered RFeB magnets
Authors: Elbaz D. , Givord D. , Hirosawa S. , Missell F. P. , Rossignol M. F. , Villas-Boas V.
Published: July 2002

The angular dependence of the coercive field Hc(ϑ) is discussed as a means of studying magnetization reversal in permanent magnets. We consider Hc(ϑ) for permanent magnets RFeB (R=Pr, Nd) for temperatures between 4.2 and 300 K. At high temperatures, it is found that Hc may increase by a factor 1.5–2 as ϑ increases from 0 to π/2. At low temperatures, in the Pr magnets, Hc decreases as ϑ increases from 0 to π/4 and then increases for larger angles. Different possible mechanisms for magnetization reversal are considered and the associated angular dependences of the coercive field in single crystals are calculated. The corresponding Hc(ϑ) are deduced for permanent magnets from the grain orientation distribution. Better agreement with experimental data is obtained in models involving domain-wall propagation and leading to a 1/cos ϑ dependence, or close to it, than in models involving coherent rotation. The behavior of the PrFeB magnets is not explained.
Structure and magnetic properties of Co(Zr, B)/Cu multilayers
Authors: Gao C. , O’Shea M. J.
Published: July 2002

Sputtered Co, CoB and CoZrB alloys in thin-film and multilayer form have been prepared using Cu as a spacer. All of the multilayers show a weak contribution to the magnetic anisotropy from the interface which favors perpendicular magnetic anisotropy and a strong contribution from the bulk which favors in-plane anisotropy. All the multilayers also show a stronger coercivity perpendicular to the film than in the plane of the film. The coercivity perpendicular to the film shows a dependence on magnetic layer thickness with the coercivity decreasing with decreasing thickness and this is discussed in terms of the magnetic domain size which is restricted by the magnetic layer thickness. CoZrB/Cu multilayers show the smallest coercivity of the above multilayer systems.
Magnetic properties and domain structure studies in dc triode-sputtered permalloy/carbon multilayer films
Authors: Gupta H. O. , Niedoba H. , Heyderman L. J. , Tomas I. , Puchalska I. B. , Sella C.
Published: July 2002

Double-layer and multiple-layer thin films of permalloy with carbon as a intermediate nonmagnetic spacer were prepared by low-energy dc triode sputtering. Coupling between the two layers of the double films is investigated on a series of samples with a fixed thickness of permalloy layers (dp = 300 Å) and thickness of the carbon layer dc, varying from 0 to 25 Å. Thickness dc = 10 Å is found to be the limiting value for the carbon layer to be continuous and 20 Å for domain structures in the two magnetic films to decouple. Saturation magnetization Ms, uniaxial anisotropy field, Hk, and coercive force Hc of multilayers with dc = 10 Å and dp from 50 to 600 Å (number of magnetic layers from 12 down to 1) are measured and found consistent with earlier results by other authors. Multiple domain walls in the multilayers are investigated by Lorentz electron microscopy.
The DX center in Si-planar-doped AlxGa1−xAs (x=0.32)
Authors: Mejri H. , Selmi A. , Maaref H. , Bourgoin J. C.
Published: July 2002

We examine in detail the photoluminescence properties of the DX center in periodically δ-doped Si-AlxGa1−xAs (x=0.32). The DX center is shown to be band structure dependent rather than a conduction miniband resonance. New photoluminescence lines at energies below the band gap at 1.878, 1.787, and 1.695 eV have been detected, and ascribed to a radiative recombination of electrons in confined subbands or in minibands with holes located at the top of the valence band between the δ layers.
The electron high-energy distribution function: A comparison of analytical models with Monte Carlo calculations
Authors: Vogelsang Th. , Hänsch W.
Published: July 2002

We compare the high-energy distribution function of the lucky electron model, the Maxwellian distribution, and the asymptotic solution of Boltzmann’s equation with Monte Carlo calculations in the effective-mass approximation. We show that the Monte Carlo calculations are in good agreement with the asymptotic solution of Boltzmann’s equation for constant and for space-dependent electric fields while the other distributions do not agree with the Monte Carlo results.
Model of Γ to X transition in thermally activated tunnel currents across AlxGa1−xAs single barriers
Authors: Rossmanith M. , Leo J. , von Klitzing K.
Published: July 2002

A theoretical study of thermally activated currents across AlxGa1−xAs barriers is presented, where x is varied from zero to one. By increasing the Al content, AlxGa1−xAs changes from a direct to an indirect band-gap semiconductor. Previous experiments on activated transport have shown that this crossover strongly affects the transport properties. However, usually these experiments are analyzed in terms of the classical Richardson law, which completely fails to explain the prefactor for high Al concentrations which drops by three orders of magnitude. A model is presented that describes the transport by considering two competing channels, one via the Γ minimum and the other via the longitudinal X minimum. The current flow through each channel is evaluated by calculating the transmission coefficient separately for the Γ and X barriers. This model gives new insights into the interpretation of the experimentally measured activation energy and the prefactor.
Transport of relativistic electron beams along wire arrays
Authors: Richardson R. A. , Denavit J. , Di Capua M. S. , Rambo P. W.
Published: July 2002

Theoretical prediction and particle simulations indicate that an array of longitudinal wires will allow electron beam propagation, without an external guiding magnetic field and above the drift tube limit, by providing a charge and current neutralizing background. This paper presents experimental tests of this transport concept by injecting a 60 ns, 18 kA pulse of 1.4 MeV electrons into an array of 1-m long wires with a 1 cm spacing, filling a hexagon with 8.7-cm average radius. Arrays were tested with wires of varying resistances and diameters ranging from 12 to 1.3 mil. Transport was tested with wires terminating on a common conducting beam collector and with wires terminating on individually insulated beam collectors. The data show good transport (up to 90% of injected current) without significant energy loss, for wire diameters 3 mil or less, while transport is cutoff after 10 ns when 12 mil wires are used. Particle simulations show that this cutoff is due to an instability fed by excess return current resulting from electron scattering by the wires. The arrays terminated on individually insulated beam collectors showed better transported beam profiles, with less pinching, than the arrays with a common conducting beam collector. Particle simulations also correlate well with this result and provide an understanding of the effect of scattering on transport and beam quality.
A quantitative model for an interaction between extended dislocation loops and impurities in Czochralski silicon based upon the photoluminescence analysis
Authors: Wijaranakula W.
Published: July 2002

D-line luminescence at the photon energies of 0.81 and 0.87 eV in Czochralski silicon containing extended dislocation loops has been characterized. In samples containing a high-oxygen concentration and in tin-doped samples, the D-line luminescence was observed. In contrast, no D-line luminescence was detected in samples containing a low-oxygen concentration, except when the sample was deliberately contaminated with Cu. The results clearly indicate that the optical centers responsible for the D-line luminescence are associated with an interaction between the dislocations and impurities. It is hypothesized that the optical centers responsible for the D1 line at 0.81 eV could be associated with an interaction between the edge dislocation component of the dislocation loop and impurities, while the D2 line at 0.87 eV could be produced as a result of an interaction between the screw dislocation component and impurities.
The electrical and defect properties of erbium-implanted silicon
Authors: Benton J. L. , Michel J. , Kimerling L. C. , Jacobson D. C. , Xie Y.-H. , Eaglesham D. J. , Fitzgerald E. A. , Poate J. M.
Published: July 2002

A detailed study of the electrical and defect properties of ion-implanted erbium in silicon shows that erbium doping introduces donor states. The concentration of erbium related donors as a function of implant dose saturates at 4×1016 cm−3 at a peak implanted Er-ion concentration of (4–7)×1017 cm−3. The defect levels related to erbium in silicon are characterized by deep level transient spectroscopy and identified as E(0.09), E(0.06), E(0.14), E(0.18), E(0.27), E(0.31), E(0.32), and E(0.48). The dependence of the photoluminescence on annealing temperature for float zone and for Czochralski-grown silicon show that oxygen and lattice defects can enhance the luminescence at 1.54 μm from the erbium. Temperature-dependent capacitance-voltage profiling shows donor emission steps when the Fermi level crosses EC − ET = 0.06 eV and EC − ET = 0.16 eV.
Strain gradients in Al-2% Cu thin films
Authors: Shute C. J. , Cohen J. B.
Published: July 2002

The strain gradient through the thickness of 0.5-, 1.0-, and 2.0-mm-thick Al-2% Cu films on oxidized (001) Si wafers has been examined by using glancing-angle x-ray diffraction to measure the d spacing as a function of penetration depth of the incident x-ray beam. Samples with and without a 1-μm sputtered quartz passivation layer were examined. The only gradient observed in these samples corresponded to surface relaxation in the unpassivated samples at depths of less than 50 Å. This result was verified by examining the shape of the diffraction peaks. The dislocation densities of the films were also determined from the peak shape.
Reflectance difference for in situ control of surface V/III ratio during epitaxial growth of GaAs
Authors: Jönsson J. , Paulsson G. , Samuelson L.
Published: July 2002

Optical, in situ, real time control of surface processes during epitaxy is becoming increasingly important for the understanding and control of crystal growth. Here results are presented of in situ studies of the surface V/III balance using the reflectance-difference (RD) method during vacuum chemical epitaxy of (001) GaAs from arsine and triethylgallium. We have found a relationship between the RD signal and the V/III ratio, and we demonstrate the possibility for the use of this relationship for the optimization of growth. We have, by RD, detected a sharply defined, critical V/III ratio, below which the morphology and the photoluminescence intensity deteriorate dramatically, and which can be used to control the conversion from n-type to p-type conductivity. We believe that these observations will be of great importance for the in situ optimization of epitaxial growth, and in eliminating much of the uncertainties involved in reproducing surface V/III ratios.
Investigation of the mechanism of Ar+ ion-assisted Cl2 etching of GaAs{110}: Role of ion-induced charge acceptor states
Authors: DeLouise L. A.
Published: July 2002

An investigation of the Ar+ ion-assisted Cl2/GaAs{110} etching reaction mechanism is presented. Using modulated supersonic beam scattering and time-of-flight techniques we have measured the intensity and kinetic energy of the unreacted Cl2 and the dominant Ga-containing reaction product as a function of ion dose and surface temperature. Ion-enhanced reactivity is demonstrated and evidence that the ion-assisted GaCl2 etch product is desorbed by a physical momentum transfer mechanism is given. Surface stoichiometric and chemical state changes have been recorded as a function of etch parameters using in situ x-ray photoelectron spectroscopy. A significant dependence of the post-etch surface composition on the Ar+/Cl2 flux ratio is found ranging from Ga-rich under high Ar+ flux conditions to As-rich under high Cl2 flux conditions. These observations indicate a strong dependence of the relative As to Ga product desorption rates on the reaction parameters. From core level and valence band binding energy measurements we have identified important ion-induced band bending effects that indicate the formation of midgap charge acceptor states. The implication of these results on the ion-assisted etch mechanism is discussed.
Emission measurements and crystal-field calculations for 4G5/2 to 6H7/2 transitions in Sm3+:YAG
Authors: Stevens Sally B. , Morrison Clyde A. , Seltzer Michael D. , Hills Marian E. , Gruber John B.
Published: July 2002

A previously published analysis of the energy levels of Sm3+ in Y3Al5O12 (YAG) has been extended to include the 4G5/2 multiplet. New absorption and emission data are reported, and some previously reported levels have been reassigned. A theoretical calculation of intensities of emitted radiation originating at the 4G5/2 multiplet predicts that radiation emitted in transitions to the 6H7/2 multiplet is particularly strong. The strongest transitions originate at the lowest of the three levels in the 4G5/2 multiplet.
DX-center-related features by capacitance measurements in AlGaAs
Authors: Ghezzi C. , Gombia E. , Mosca R.
Published: July 2002

The role of quasistatic C-V measurements in investigating DX-center-related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron-capture rate by the DX center, at low temperature, is responsible for a frozen-step-like density profile of positively charged DX centers near the metal-semiconductor interface. This causes a knee-shaped 1/C2-vs-V plot and gives rise to an apparent built-in potential. The low-temperature freezing in of the free-electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low-temperature C-V measurements performed on the sample cooled at different cooling rates.