bluuebunny/crossref_metadata_2025 huggingface dataset:
Computer simulations to study the explosive consolidation of powders into rods
Authors: Reaugh John E.
Published: July 2002

In the direct method of explosive consolidation, metal or ceramic powder to be consolidated is placed in a metal tube, which in turn is surrounded by a concentric cylinder of explosive. The explosive is detonated at one end so that the detonation front moves axially along the powder-filled tube. The desired result is a crack-free, uniformly consolidated rod with the density of the starting material. At present, the wall thickness of the hollow cylinder of explosive that best produces the desired result is determined by experiment. We present the results of computer simulations revealing details of this dynamic process that are difficult to obtain experimentally. The simulations show that the best thickness depends on the equations of state of both explosive and powder. Such simulations can guide changes to the experimental geometry, even when approximate equations of state are used, although the best explosive and its thickness must ultimately be determined by experiment.
Characterization of silicon-on-insulator films recrystallized by an obliquely scanned pseudoline electron beam
Authors: Horita Susumu , Ishiwara Hiroshi
Published: July 2002

Comprehensive and systematic studies on the growth characteristics of SOI (silicon-on-insulator) films recrystallized by an obliquely scanned pseudoline electron beam are presented. The obliquely scanned pseudoline beam was produced by scanning a spot electron beam along a line faster than the thermal response time of the substrate and it was moved as a whole to a different direction from the normal direction of the line. In the experiment of laterally seeded epitaxial growth of SOI films, the pseudoline beam was always scanned along the seed stripes, while the direction of the seed stripes to the Si substrate and an oblique angle between the scanning direction and the normal direction of the line were changed. It was found that the sub-boundary-free area in the SOI film was enhanced either by increasing the oblique angle or by rotating the seed direction from a 〈110〉 to 〈100〉 axis of the substrate. It was also found that the directions of sub-boundaries hardly depended on the oblique angle but they were governed by the crystal orientation of the substrate at a scanning velocity of 10 cm/s, while their directions were perpendicular to the pseudoline at the velocities below 1 cm/s. A simple model assuming formation of {111} facets at the solidification front is set up in order to explain these results.
Epitaxial growth of neodymium sesquioxide films on calcium fluoride by a reactive-evaporation method
Authors: Takagi K. , Fukazawa T. , Susa K.
Published: July 2002

Cubic rare-earth sesquioxides having fluoritelike crystal structures are proposed as epitaxial insulating materials for semiconductor devices. The possibility of epitaxy is examined using neodymium sesquioxide (Nd2O3). The cubic phase is formed in temperatures ranging from 400 to 600 °C. Although epitaxy on Si is not possible due to surface oxidation, epitaxial growth on calcium fluoride is confirmed by an x-ray double-crystal method.
Properties of hydrogenated amorphous silicon prepared by biased activated reactive evaporation
Authors: Anderson J. C. , Biswas S. , Guo H.
Published: July 2002

The deposition of hydrogenated amorphous silicon by biased activated evaporation is described. The films are characterized by measurements of hydrogen content, conductivity and photoconductivity against temperature for intrinsic, n-type and p-type films, optical absorption, field-effect mobility and density of states, space-charge limited currents, and spin density. Silicon nitride and silicon carbide films are also described. It is concluded that the process offers high-rate deposition of films whose quality is comparable with glow-discharge films but with better thermal stability and no illumination-induced conductivity changes.
Frequency dispersion in the admittance of the polycrystalline Cu2S/CdS solar cell
Authors: Hmurcik L. V. , Serway R. A.
Published: July 2002

We measured the admittance versus frequency (f≤100 kHz) for the Cu2S/CdS solar cell. In the dark, the dispersion fits a model of a simple Debye capacitor, with deviation due to grain-boundary scattering at low frequencies. Under illumination, the dispersion becomes a function of surface roughness. Modeled in fractal geometry, the admittance varies as ( jω)m. A second term of this type occurs at high frequencies and at illuminations greater than 0.1% AM1. In this case, the depletion layer extends deep into the CdS due to insufficient charge states at the interface.
Charge distribution and response time for a modulation-doped extrinsic infrared detector
Authors: Hadek Victor
Published: July 2002

The electric charge distribution and response time of a modulation-doped extrinsic infrared detector are determined. First, it is demonstrated theoretically that the photoconductive layer is effectively depleted of ionized majority-impurity charges so that scattering is small and mobility is high for photogenerated carriers. Then, using parameters appropriate to an actual detector, the predicted response time is 10−8 to ∼10−9 s, which is much faster than comparable conventional detectors. Thus, the modulation-doped detector design would be valuable for heterodyne applications.
Fast algorithm for iteratively solving the nonlinear static Poisson equation in semiconductors
Authors: Keller W. W.
Published: July 2002

The method for iteratively solving the nonlinear static Poisson equation in semiconductors is based on a recent paper of Mayergoyz [J. Appl. Phys. 59, 195 (1986)]. It is the most efficient in computer time. All the advantages of his method, especially low memory requirement and applicability to 1-, 2-, and 3- dimensional problems are also valid for that presented here. A decrease of computer time by more than a factor of 2 without loss of accuracy of the final result is achieved mainly by decreasing the accuracy of a single step of iteration.
Eddy current decay method applied to a new geometry
Authors: Jardim R. F. , Laks B. , Santos C.
Published: July 2002

The eddy current decay method for determination of metallic samples resistivities is presented for the case of a hollow circular cylindrical geometry. The theory is developed for two kinds of experimental arrangements: the primary coil is driven by a voltage source or by a current source. The measurements made for several metals (Al, Cu, Nb, brass, and bronze) at room temperature and at the temperature of liquid nitrogen analyzed by the theory give accurate results compared with the four terminal method.
A unified theory of noise in nondegenerate semiconductors
Authors: Min H. S.
Published: July 2002

Explicit forms of the Langevin noise sources due to both intraband and interband scattering are given for the Boltzmann transport equations with the relaxation-time approximation in nondegenerate semiconductors. A generalized formula for the short-circuited noise currents, which holds in both homogeneous and nonhomogeneous one-dimensional nondegenerate semiconductors, is derived in a unified manner by solving the Langevin-type Boltzmann transport equations. From this generalized noise formula the short-circuited noise currents for thermal noise, generation-recombination noise, and 1/f noise in a homogeneous nondegenerate semiconductor bar are calculated. It is shown that for homogeneous nondegenerate semiconductors, Nyquist’s theorem always holds both at thermal equilibrium and under an applied dc electric field with either intraband scattering or interband scattering dominant. Generation-recombination noise is shown to be caused by the electrons or holes whose scattering mechanism is dominated by intraband scattering, and the derived generation-recombination noise formula is the same as the one derived by the conventional methods. It is also shown that 1/f noise may be caused by a very small number of electrons (or holes) lying just around the bottom of the conduction band (or the top of the valence band) when interband scattering is dominant over intraband scattering for these electrons (or holes). The contribution of these small number of electrons (or holes) to the electrical conductivity is negligible because of the smallness of the number of these carriers. The derived 1/f noise formula has the same functional form as the Hooge’s empirical relation [Physica 83, 14 (1976)].
Zero-field NMR studies of physically admixed Co/Co-ThO2 ZSM-5 catalysts
Authors: Murty A. N. , Williams A. A. , Obermyer R. T. , Rao V. U. S.
Published: July 2002

The Co-ZSM-5 and Co-ThO2-ZSM-5 catalysts are a particularly promising group of bifunctional zeolite catalysts used for the conversion of synthesis gas to gasoline-range hydrocarbons. Catalytic properties of these materials, such as activity and selectivity, depend upon the amount of the cobalt on the medium-pore zeolite ZSM-5, as well as upon the presence of promoters such as ThO2. These studies were undertaken to ascertain the effect of thoria upon the magnetic and structural nature of these catalysts. Zero-field nuclear magnetic resonance measurements have been made on a series of zeolite cobalt and cobalt-thoria catalysts with three different concentrations of Co (3, 6, and 9 wt. %). The catalysts were prepared by making physical admixtures of precipitated cobalt, or cobalt-thoria, and ZSM-5. The amount of thoria introduced ranged from 0.5 to 1.5 wt. %. After reduction of the catalyst samples, the normal metallic cobalt resonance line at 213.0 MHz (fcc) and the fault lines at 215.5 and 218.6 MHz were observed. In the unpromoted catalysts, a line at 210.7 MHz, which is lower than the fcc line frequency, and another line at 221.6 MHz, which is higher than the hcp line frequency, were observed. When the promoter thoria was added, there was a systematic absence of the hcp line in the unused catalyst. Previous catalytic tests had shown that small amounts of the promoter thoria (0.4 wt. %) significantly increase the liquid hydrocarbon yields. The change in catalytic behavior can be related to the changes in crystallite species and crystallite size.
Structure and ferromagnetic resonance of Tb/FeCo multilayer thin films
Authors: Shin S. C. , Cofield M. L. , Nuttall R. H. D.
Published: July 2002

Multilayer thin films of Tb and Fe95Co5 were prepared by thermal evaporation from two sources. The films were studied by x-ray diffraction, Auger electron spectroscopy, and ferromagnetic resonance. The low-angle x-ray diffraction and Auger analysis showed that a multilayer structure was achieved for films with a modulation wavelength Λ≥31.5Å. FMR results were consistent with the structural data. The films with Λ≤31.5Å had similar HKeff, line width, and line shape to those of a coevaporated alloy with the same composition. The films with Λ>31.5 Å had an HKeff greater than or equal to that of an Fe95Co5 film. A film with HKeff greater than that of the Fe95Co5 film had multiple resonances. Interfacial stress due to incoherent matching of the Tb and Fe95Co5 layers is thought to give rise to differences in HKeff in the various layers.
The pseudo-brookite spin-glass system studied by means of muon spin relaxation
Authors: Brabers V. A. M. , Boekema C. , Lichti R. L. , Denison A. B. , Cooke D. W. , Heffner R. H. , Hutson R. L. , Schillaci M. E. , MacLaughlin D. E.
Published: July 2002

Zero-field muon spin relaxation (μSR) experiments have been performed on the spin glass Fe1.75Ti1.25O5. Above the spin-glass temperature of 44 K a distinct exponential μSR rate (λ) is observed, while below Tg a square-root exponential decay occurs, indicating fast spin fluctuations. Near 8 K, a maximum in λ is indicative of transverse spin ordering. The low λ values and the sharp λ peak at Tg are very promising for the study of spin freezing models like the Vogel–Fulcher law or the power law.
Microwave/mm wave magnetics and MMIC compatibility (invited) (abstract)
Authors: Adam J. D.
Published: July 2002

Ferrite devices can be loosely classified into three different categories, namely: control components using polycrystalline ferrites, tunable filters and oscillators using YIG spheres, and devices based on epitaxial YIG or ferrite films. Ferrite control components such as circulators, isolators, and switches are used in almost all microwave and millimeter wave systems. Tunable YIG sphere devices see more limited use in radar and EW systems, and microwave test equipment while epitaxial YIG devices have yet to make a significant systems impact. GaAs chips for phased array modules are under development by several companies for both radar and EW applications. The GaAs chips can contain small signal and power gain, phase shifters, filters, mixers, and switches. The modules are usually designed, however, with discrete circulators or isolators which are often significantly larger than the MMIC chips. Further reduction in module size and cost will require the design of the module without nonreciprocal components, or the development of ferrite devices which are more compatible with the size, bandwidth, and fabrication of the GaAs device. Integration of nonreciprocal ferrite components on the GaAs chip could have a large impact but presents a significant challenge both in terms of processing compatibility between the ferrite and the GaAs and in terms of cost. The impact in the areas of tunable YIG filters and oscillators and MSW devices are smaller but, fortunately, so are the difficulties. Here the YIG films or spheres, or hexagonal ferrite films can be laid on the GaAs substrate thus forming a hybrid device. Having integrated the ferrite with the GaAs it is necessary to consider the magnetic bias field requirement. Bias fields are not required in latching devices and can be minimized in other devices by use of hexagonal ferrite films with their large anisotropy fields. It may even be possible to integrate a permanent magnet film onto the GaAs chip.
Effect of thorium substitution on magnetism of UCu2Si2 (abstract)
Authors: Giorgi A. L.
Published: July 2002

The compound UCu2Si2 has been found to be ferromagnetic at low temperatures with a Curie temperature (TC) of 101 K. The magnetization follows a Curie–Weiss law in the paramagnetic region with an effective magneton number of 2.12 μB per uranium atom. Partial substitution of Th atoms for the U atoms results in an expansion of the lattice, a rapid drop in TC, and a sizable increase in the coercive field. Magnetization measurements have been conducted on various compositions of U1−xThxCu2Si2 over the range of 0<x<1.0 and the variation of the lattice parameters and the magnetic properties determined. A correlation between the change in the magnetic properties and the U−U separation due to the lattice expansion and the statistical distribution separation effects will be presented.
Generalized Heisenberg quantum spin chains (invited)
Authors: Bonner Jill C.
Published: July 2002

Since the Heisenberg spin chain can be considered the simplest realistic model of magnetism, surprise and some degree of controversy have resulted from recent work of Haldane. The prediction is that quantum spin chains with half-integer spin should all display T=0 phase behavior equivalent to that of the Bethe Ansatz integrable (solvable) spin-1/2 quantum chain. More remarkably, the class of integer spin chains is predicted to show very different phase behavior. In particular, a gap should be present in the spectrum of a Heisenberg antiferromagnetic chain. This remarkable feature is counterintuitive in terms of accepted wisdom in magnetism (spin-wave theory, spin-Peierls theory) and critical phenomena. Consequently the vertification of the prediction is of great interest. A considerable amount of numerical work has been done, involving finite-chain, finite-size scaling, variational, Monte Carlo and other calculations, which will be reviewed here. The present consensus is that the weight of numerical evidence supports the prediction, although puzzling features still remain. Adding additional interactions to the basic Heisenberg Hamiltonian such as spin (XXZ) anisotropy, single-ion anisotropy, biquadratic exchange, and an applied magnetic field, generates a rich and complicated phase diagram for chains with spin >1/2, particularly for the case of integer spin. The s=1 phase diagram seems to display critical behavior of a type not previously encountered. A theoretical appraisal of the Haldane phenomenon will include a discussion of the possible role of nonintegrability. Mention will also be made of current progress in experimental investigation of the phenomenon, including problems that might be encountered. More recent work of Affleck has greatly generalized the field-theoretic mappings which underlay the original work of Haldane. A number of interesting problems have been mapped into quantum spin chains of various types, including field theoretic phenomena and the localization problem of the quantum Hall effect.
Structure and properties of oblique-deposited magnetic thin films
Authors: Gau J.-S. , Yetter W. E.
Published: July 2002

Structural characterizations of angle-of-incidence CoNi films using electron microscopy are reported. Both fixed-angle-of-incidence and continuous-variation-of-incidence films are included. The tangent rule is found to be valid at high oblique angle. The easy axis as measured from the torque curve and angular variation of coercivity in a 170-nm film deposited at 75° is about 15° inclined to the film plane, whereas the columnar axis is about 30°. A weak c-axis texture exists as revealed by the electron diffraction study. The c-axis texture in the continuous-variation-of-incidence films with and without oxygen is not detected by electron diffraction.
Preparation of amorphous metallic glass transducers for use in fiber optic magnetic sensors
Authors: Bucholtz F. , Koo K. P. , Yurek A. M. , McVicker J. A. , Dandridge A.
Published: July 2002

An important figure of merit for fiber optic magnetometers is the effective C parameter Ceff relating magnetostriction e to applied field H: e=CeffH2. We describe both the annealing and the characterization after annealing of cylindrical metallic glass transducers and present the results of measurements of Ceff for various anneal temperatures, times, and magnetic field strengths for flat Metglas 2605-S2 samples. Ceff values of (2.0–2.5)×10−6 Oe−2 are observed in the annealed flat strip and cylindrical samples studied here.
Strongly coupled ferromagnetic resonances of Fe films
Authors: Pomerantz M. , Slonczewski J. C. , Spiller E.
Published: July 2002

The ferromagnetic resonance (fmr) fields of ultra-thin Fe films depend on the thicknesses of the films. Pairs of unequal films were prepared in which the resonances were resolved when the films were separated by as little as 1 nm of C. If the C spacing was made nominally zero a single fmr was observed. Depending on the background vacuum pressure, this single resonance occurs at two different fields. The low pressure sample seems to have structural coupling of the films to form one thick film. The fmr of the higher pressure sample qualitatively fits a model calculation of structurally separated films that are strongly magnetically coupled.
Magnetic field-induced transition in Y1−xGdx
Authors: Gotaas J. A. , Rhyne J. J. , Wenger L. E. , Mydosh J. A.
Published: July 2002

The magnetic field dependence of the spin structure in single crystals of Y1−xGdx has been studied by neutron diffraction. In zero field, these dilute alloys exhibit long-range helical order with the propagation vector along the c axis and the magnetic moments lying in the basal plane. On applying a magnetic field in the basal plane, the modulation wave vector remains fixed at 0.28c*, and in fields up to 7 T there is no evidence for the development of higher harmonics. The only observed change is a gradual decrease in the magnetic intensity in the satellites. At a higher field Hc which depends on concentration and temperature, there is an abrupt reduction by a factor of 2 in the intensity for reflections along the c* axis, with no corresponding change in the intensity of basal plane reflections. This is consistent with a change in magnetic structure from the helical state to a linear modulated state in which the moments are collinear with the applied field in the basal plane but undergo a sinusoidal modulation along the c axis.
Dependence of microstructure and magneto-optical properties on deposition angle in evaporated TbFe and TbFeCo films
Authors: Shin Sung-Chul
Published: July 2002

We have investigated the effects of deposition angle on growth morphology and magneto-optical properties in TbFe and TbFeCo films, which were prepared by coevaporation of Tb and Fe (or FeCo) on tilted substrates. Cross-sectional scanning electron microscopy revealed that the films had columnar structure and the growth orientations of columns generally followed the ‘‘tangent rule’’ [Dirks and Leamy, Thin Solid Films 47, 219 (1977)]. Measurements of the polar Kerr hysteresis loops revealed that with increasing the angle of incidence of the vapor beam from the substrate normal, the squareness of the hysteresis loop deteriorated, and also the Kerr rotation angle and the coercivity decreased. The results of the present study clearly demonstrate the importance of normal growth of columns with respect to a film plane to achieve perpendicular magnetic anisotropy in evaporated rare-earth transition-metal alloy films.
Thermal renormalization of spin-wave energies
Authors: Riseborough Peter S.
Published: July 2002

In recent electron-spin-resonance experiments, Benner et al. [J. Magn. Magn. Mater. 45, 354 (1984)] have measured the temperature dependence of the q=0 spin-wave energy for CsNiF3. They have interpreted their data in terms of the thermal renormalizations of the fundamental mass of the classical spin system. The fundamental mass is thermally renormalized by inplane fluctuations and out of plane fluctuations. The experimental data of Benner et al. showed a much smaller temperature dependence than that predicted by the classical theory. In this paper we derive a quantum mechanical expression for the temperature dependence of the fundamental mass, to lowest order in the anharmonic interactions. We find that the quantum expression shows a much smaller temperature dependence of the spin-wave gap than that provided by the classical theory. We note that the high-temperature limit of the quantum expression reduces to the classical expression, with some important changes of the parametrization. This has the result that for systems with small values of the spin, the magnitude of the thermal renormalizations derived from the quantum expression are of much smaller magnitude than those derived on the basis of the purely classical theory. These modifications help to reduce the discrepancy between theory and experiment.
Critical magnetic scattering from the Heisenberg ferromagnet EuS
Authors: Böni P. , Shirane G. , Bohn H. G. , Zinn W.
Published: July 2002

The paramagnetic scattering from the insulating, isotropic ferromagnet EuS is investigated at TC along the [111] direction by means of inelastic neutron scattering. The energy width of the quasielastic scattering is proportional to qz with z=2.54±0.10, in good agreement with the predictions of dynamical scaling theory (z=2.5). z is, however, significantly larger than the value deduced from measurements along the [100] direction (z∼2.2). Near the zone boundary the magnetic scattering exhibits shoulders the shapes of which deviate from theoretical predictions based on the Heisenberg model.
Proton-exchanged, lithium niobate planar-optical waveguides: Chemical and optical properties and room-temperature hydrogen isotopic exchange reactions
Authors: Loni A. , De La Rue R. M. , Winfield J. M.
Published: July 2002

Planar-optical waveguides fabricated by [1H] and [2H] hydrogen exchange on x- and z-cut lithium niobate are compared with respect to their behavior towards atmospheric [2H] and [1H] water vapor and their optical properties. The extent of [1H]–[2H] hydrogen isotopic exchange was monitored by infrared spectroscopy in the hydroxyl-group stretching region. We show that [1H]- and [2H]-exchange waveguide materials behave differently and therefore cannot be regarded as structurally identical. However, optical mode-angle measurements using prism coupling have established that the waveguide profiles in these materials are almost identical in form, and can be modeled accurately by a step-index change with Δne ≂0.12 at λ=0.6328 μm. Lithium present in molten benzoic acid after its reaction with z-cut LiNbO3 samples has been determined by atomic absorption spectroscopy. The lithium concentration data obtained are in satisfactory agreement with previous estimates of hydrogen concentrations in the exchanged waveguide layer.
A new frequency-domain fluorometer for the rapid determination of picosecond rotational-correlation times
Authors: Bright Frank V. , Monnig Curtis A. , Hieftje Gary M.
Published: July 2002

A new instrument is described which uses the harmonic content of a mode-locked argon-ion laser for the determination of picosecond rotational-correlation times. The new instrument is very rapid, and can detect in about 8 ms all frequencies between dc and 750 MHz. The signal-averaging capabilities of the instrument also make it applicable to weakly fluorescent species. To test the new instrument, the time-dependent fluorescence polarization of rhodamine 6G was studied as a function of solvent viscosity; a second chemical system investigated was fluorescein in 1,3-propanediol. Both these systems were chosen because they have been shown to exhibit a single rotational-correlation time. Results are in good agreement with those previously reported.
Compositional distribution in the cross section of chemical-vapor-deposited Nb3Ge tape conductors and its correlation with superconducting properties
Authors: Nakagawa Y. , Yamasaki H. , Kimura Y.
Published: July 2002

Chemical-vapor-deposited Nb3Ge tapes 2.5 mm wide with Nb3Ge layer thickness of 5–60 μm were prepared under various processing conditions. Though the depth profile of the Nb concentration in the Nb3Ge tapes determined by x-ray microanalysis was fairly uniform, with a variation less than 4 at. % for many Nb3Ge tapes, the distribution of the Nb concentration over the whole cross section of the Nb3Ge layer varied considerably, with a variation of 10–18 at. % for various Nb3Ge tapes. In this distribution, Nb concentrations at the tape edges were found to be higher than those on its sides for all of the Nb3Ge tapes prepared. The origin of this singular distribution of the composition is discussed. The Tc values for narrower tapes cut out lengthwise from the 2.5-mm-wide tapes were measured resistively. They correlated closely with the compositional distribution in the cross section of the Nb3Ge tapes. The Tc values (midpoints) of Nb3Ge tapes incorporating compositions within the Nb concentration range of 70–75 at. % were found to be near or above 20 K.
Radiation hardness of zinc oxide varistors
Authors: Cordaro J. F. , Shipway C. E. , Schott J. T.
Published: July 2002

The current-voltage characteristics of zinc oxide varistors were examined prior to and after exposure of up to 107 rad (Si) of 60Co gamma radiation. Nominal voltage, current-voltage nonlinearity, and leakage currents remain relatively unaffected by large doses of ionizing radiation.
Thermal stability of WSix/GaAs interface
Authors: Takatani S. , Matsuoka N. , Shigeta J. , Hashimoto N. , Nakashima H.
Published: July 2002

Secondary ion mass spectrometry (SIMS), Rutherford backscattering spectrometry (RBS), and measurements of forward current-voltage characteristics at various temperatures were used to study metallurgical reactions in WSix/GaAs formed by sputter deposition. Both SIMS and RBS showed that annealing at 800 °C results in migration of W and Si into GaAs when the atomic ratio x deviates from 0.5. On the other hand, the forward current-voltage characteristics of WSix/GaAs diodes at various temperatures showed that the excess current across the interface induced after annealing is large when x deviates substantially from 0.5, in agreement with the SIMS and RBS results. It is suggested that the migration of W and Si is directly involved in the thermal degradation of the structural and electrical properties of the WSix/GaAs interface and its x dependence.
Negative resistance in a finite superlattice owing to quantum mechanical reflection
Authors: Aishima Asuo , Fukushima Yoshifumi
Published: July 2002

Sasaki [Jpn. Inst. Electron. Commun. Eng. Tech. Rep. ED-84, 27 (1984)] has proposed a new type of superlattice, a modulation mass superlattice where the effective mass of electrons is changed periodically and the conduction-band edge is aligned so as to eliminate potential discontinuity. We investigate the quantum mechanical transmission of electrons for the case where a voltage is applied perpendicularly to the layers. We find that the modulation mass superlattice exhibits a new type of negative resistance owing to quantum mechanical reflection of electrons. The threshold voltage of negative resistance is much lower (around 10 mV than that (above 0.1 V) of compositional superlattice. The current density of the modulation mass superlattice is two orders of magnitude greater than that of a compositional superlattice. Such properties are very desirable for achieving a low-power ultrafast logic device.
Magnetic properties of charge uncompensated CaGe:YIG
Authors: Batra S. , Lehmann-Szweykowski A. , Wigen P. E.
Published: July 2002

A theoretical model is proposed to explain the magnetic properties of uncompensated CaGe:YIG with excess calcium. The model proposes that the excess uncompensated charge is localized at low temperatures on the tetrahedral site to produce an Fe4+ ion. At high temperatures, the charge is assumed to be delocalized and the magnetic behavior is similar to that of the charge-compensated YIG. Because of the strong ferromagnetic superexchange between the Fe3+ ion on the octahedral site and the Fe4+ on the tetrahedral site, the ground state of the spin on the Fe4+ site is proposed to be parallel to the octahedral site. Therefore, the theoretical model assumes the presence of three Ising-like magnetic sublattices: (a) a tetrahedral site Fe4+, (b) a tetrahedral site Fe3+, and (c) a octahedral site Fe3+. The magnetization, calculated in the molecular-field theory, shows a compensation point in accordance with the observation. The cubic anisotropy energies are calculated by considering the fourth-order terms in the spin Hamiltonian and using the molecular field on the Fe4+ site. A good agreement is achieved between the theoretical and experimental results.
Appearance-potential spectroscopy and work function
Authors: Gordon R. L.
Published: July 2002

Methods and procedures are described which permit the use of a commercial scanning Auger microprobe (SAM) to obtain work-function measurements at the geometrical resolution routinely achieved in surface composition determinations. The work function is estimated by the appearance potential for secondary electrons. The need for auxiliary circuitry has been eliminated by the development of methods to access the existing interface between the SAM and its computer control. High-level computer languages are used to obtain coincident appearance potential and Auger spectra. A discussion of the relationship between appearance potential and work function is presented. It is shown that the difference between work function and appearance potential depends on the radius R of a work-function patch and the vertical distance to the surface potential barrier maximum, x*. For small x*/R the appearance potential is a good estimate of work function. The technique is illustrated by results obtained from surfaces with nonuniform coverages of residual gases.
Computer-simulation studies of electron-beam propagation through plasma into vacuum
Authors: Chang C. R. , Reiser M.
Published: July 2002

A disk model is developed to simulate the propagation of a cylindrical relativistic electron beam through plasma into vacuum via collective acceleration of positive ions from the plasma. Simulation results show dynamic details of electron and ion flow not noticed previously. The high-energy tail in the ion distribution is attributed to motion of the virtual cathode early in time when the ions enter the system. A comparison between the computer simulation and our analytical model published earlier shows good agreement for the time average behavior of the particles.
Analytical modeling of a-Si:H thin-film transistors
Authors: Chung K. Y. , Neudeck G. W.
Published: July 2002

Accurate analytical expressions have been derived, from basic principles, for the sheet conductance of the n-channel a-Si:H thin-film transistors. The basic principles apply two different energy band models, one for the a-Si bulk and one for the a-Si–SiO2 interface. The physical parameters needed for the model are easily obtained from an experimental sheet conductance versus the gate voltage curve. The basic expressions for the sheet conductance are then simplified and result in two sets of simple formulas which are employed in the calculation of the ID vs VD characteristics.
Strain and pore pressure propagation in a water-saturated porous medium
Authors: van der Grinten Jos G. M. , van Dongen Marinus E. H. , van der Kogel Hans
Published: July 2002

Wave propagation in a water-saturated porous column consisting of fixed sand particles is studied by means of a shock tube technique. Pore pressures and axial strains are recorded simultaneously. The measurements show a coincident compression of pore fluid and porous column during the passage of the first wave. Due to the second wave the pore fluid is compressed while the porous material expands. This observed behavior is in agreement with theoretical predictions. The introduction of a frequency-dependent permeability and an effective pore radius based on a cylindrical duct model yields an improved description of the damping of the second wave.
Growth of undoped ZnSe on (100) GaAs by molecular-beam epitaxy: An investigation of the effects of growth temperature and beam pressure ratio
Authors: DePuydt J. M. , Cheng H. , Potts J. E. , Smith T. L. , Mohapatra S. K.
Published: July 2002

Thin (1.5–2.5 μm thick) ZnSe films have been grown on (100) GaAs substrates by molecular-beam epitaxy to investigate the effects of growth conditions on film properties. A ‘‘growth matrix’’ was generated by systematically adjusting the growth temperature (TG) through 250, 300, 350 and 400 °C while setting the Zn-to-Se beam pressure ratio at 1/4:1, 1/2:1, 1:1, and 2:1. Reflection high-energy electron diffraction (RHEED) patterns monitored during growth showed surface reconstructions which were characteristic of either Zn- or Se-stabilized growth. In addition to the transition between the two surface reconstructions, a transition from streaky to spotty RHEED patterns was discovered. Both transition lines were mapped onto growth parameter space and their intersection provides a unique reference point in growth space. Low-temperature photoluminescence, optical microscopy and Hall measurements were used to characterize the samples in this growth matrix study and these results were found to be related to the RHEED transitions. One narrow region of growth space was found to produce samples with intense, narrow near-band-edge peaks and small deep-level intensity. Although it was still n type, one of the samples with growth conditions in this region was measured to have a carrier concentration less than 5.6×1015 cm−3 and a peak mobility greater than 7150 cm2/V s.
Diffusive and convective vaporization of irradiated droplets
Authors: Armstrong R. L. , Zardecki A.
Published: July 2002

A generalized vaporization model is developed which describes the evaporation of a spherical liquid droplet irradiated by a high-intensity, pulsed laser beam. Diffusive and convective transport processes are included in the hydrodynamic description of a two-component system composed of droplet vapor and ambient gas. Numerical solutions to the two-component hydrodynamic equations are obtained which illustrate the ability of the model to describe both isobaric vaporization, and convective vaporization with the production of shock waves.
Direct measurement of majority-carrier quasi-Fermi levels in Schottky barrier and metal-insulator-semiconductor diodes
Authors: Seager C. H. , Schubert W. K.
Published: July 2002

A novel point contact technique for directly measuring Fermi-level variations in biased semiconductor diodes is described. This technique is applied to Al/n-Si and Al/SiO2/p-Si structures, and the results are shown to be in good agreement with the drift/diffusion limit of diode transport theory.
Photoelectronic and optical properties of amorphous gallium-selenide thin films
Authors: Di Giulio M. , Micocci G. , Siciliano P. , Tepore A.
Published: July 2002

Optical absorption coefficient, dark conductivity, and photoconductivity properties of amorphous GaSe thin films are reported. The dependence of the absorption coefficient α on the photon energy ℏω at the edge of the absorption band is well described by the relation αℏω=B(ℏω−Eopt)2 with an optical gap Eopt of 1.50–1.55 eV. The dark conductivity σd at low temperature obeys the law ln σd∝T−1/4, indicating a variable range hopping in localized states near the Fermi level. From the analysis, a density of states N(EF)≂1018 cm−3 eV−1 has been estimated. The dependence of the photoconductivity on the temperature and on the light intensity is analyzed by a simple kinetic model which takes into account two Shockley–Read type imperfections.
Formation of buried nitride silicon-on-insulator structures studied by Auger electron spectroscopy and transmission electron microscopy
Authors: Oosting P. H. , Petruzzello J. , McGee T. F.
Published: July 2002

The formation of buried nitride silicon-on-insulator structures is studied by means of Auger electron spectroscopy and transmission electron microscopy. 160-keV N+ ions are implanted to doses of 7.0 and 9.5×1017 atoms cm−2, respectively, at a substrate temperature of 550 °C. Annealing between 1150 and 1200 °C removes most of the damage in the top silicon layer in ≤ 1 h. In ≤ 13 h the buried layer turns into crystalline Si3N4. This crystallization process starts at a depth where the N concentration is well below the value for Si3N4.
Observation of the mechanochemically polished surface of Si wafer by electron microscopy and diffuse x-ray scattering
Authors: Harada J. , Kashiwagura N. , Sakata M. , Miyatake H.
Published: July 2002

From the analysis of rod-shaped diffuse x-ray scattering from a Si wafer it has been suggested that platelike defects are induced by the wafer making process. It was found, however, from the cross-sectional observation of a mechanochemically polished Si wafer surface by a high-resolution transmission electron microscopy that such defects do not exist under the surface. On the basis of this fact and an additional x-ray measurement from the (001) surface of Si wafer the rod-shaped diffuse x-ray scattering is reinterpreted as scattering due to the truncation of the charge density at the crystal surface.
Study of silicon-doped AlxGayIn1−x−yAs quaternary layers by localized cathodoluminescence and electron acoustic microscopy
Authors: Papadopoulo A. C. , Bresse J. F. , Praseuth J. P.
Published: July 2002

Silicon incorporation in Al0.19Ga0.28In0.53As quaternary layers grown by molecular-beam epitaxy has been studied by low-temperature (10-K) cathodoluminescence and scanning electron acoustic microscopy. The maximum attained electron concentration was 6.5×1018 at. cm−3. For heavily doped samples, cathodoluminescence spectra show the presence of several deep levels and a very distinct structure in the near-band-edge transition. Even when a maximum is observed for the near-band-edge cathodoluminescence emission, deep levels and electron acoustic signals increase as function of the silicon concentration.
Magnetoelastic and optoelastic coupling in (111)- and (110)-oriented bismuth-iron garnet films prepared by sputter epitaxy
Authors: Krumme J.-P. , Doormann V. , Strocka B. , Witter K. , Hemme H.
Published: July 2002

High-quality iron-garnet films of composition Gd3−vBivFe5−w−x−yGdw GaxAlyO12−zArz with 1.02≤v≤1.45, 0.01≤w≤0.36, 0.22≤x≤0.24, 0.26≤y≤0.38, and 0.002≤z≤0.014 have been grown epitaxially on (111)- and (110)-oriented gallium garnet substrates with the lattice constant in the range 1.2475≤a0(nm)≤1.2553 by rf magnetron sputtering in argon plasma. For the study of the optoelastic and magnetoelastic properties of these sputter-epitaxial films the lattice mismatch Δa⊥=as−a⊥f has been varied in the range −1.7<Δa⊥(10−2 nm)<0.5 by using substrates with various lattice constants. The near-infrared optical anisotropy normal to (Δnxy) and in the plane (Δnyz) of (110)-oriented films turn out to depend linearly on Δa⊥ over this large range of lattice misfits. No growth-induced contribution has been observed. The same behavior is found for the induced uniaxial (K⊥) and in-plane (K∥) magnetic anisotropy constants at room temperature. The optoelastic coefficients P44 and ΔP, as well as the magnetostriction constants λ111 and λ100 inferred from the measured optical and magnetic anisotropies, respectively, compare well with literature values for the considered film compositions if excess gadolinum is taken as diamagnetic substitution on octahedral lattice sites. The epitaxial structure of the films has been confirmed by the narrow x-ray diffraction linewidth from x-ray double-crystal rocking curves and is supported by the extremely low waveguide losses at 1.33-μm wavelength and the low magnetic wall coercivity, i.e., α=0.15 cm−1 and 60 A m−1, respectively. The structural quality of (110)-oriented films is the same as that of (111)-oriented films.
ArF excimer laser doping of boron into silicon
Authors: Kato S. , Nagahori T. , Matsumoto S.
Published: July 2002

Boron doping into silicon has been investigated using an ArF excimer laser. Dopant atoms are obtained by pyrolysis of BF3. Two kinds of doping, the doping in BF3 ambients and the doping using adsorbed layers, have been performed. In both cases, the sheet resistance decreases with increasing the number of pulses. The surface concentration and the junction depth increase with the number of pulses. From these results, it is found that dopant atoms are supplied from only adsorbed layers of BF3 formed on the silicon surface. Moreover, the supply of dopant atoms from adsorbed layers limits the doping process.
Characterization of hydrogenation and dehydrogenation of post-plasma treated low-pressure chemical-vapor-deposited amorphous silicon films
Authors: Nakamura Minoru , Ohno Toshiyuki , Konishi Nobutake , Miyata Kenji , Kamezawa Norimasa
Published: July 2002

Utilizing infrared spectroscopy, secondary ion mass spectroscopy, photoluminescence (PL), and electron-spin resonance measurements, we have characterized low-pressure chemical vapor deposition amorphous silicon films (a-Si) post-hydrogenated under various plasma treatment conditions, and annealed ones at various temperatures after plasma treatment. The amount of the total bonded hydrogen (NT,H) in the film brought about by hydrogen plasma varied with radio-frequency power, temperature, and exposure time. The depth of the bonded hydrogen layer increased according to the increase in NT,H, with increasing surface hydrogen concentration in the small NT,H region, and it almost kept the surface saturation of the hydrogen concentration in the larger NT,H region. The PL intensity increased in proportion to the increase of NT,H; it changed sharply at small NT,H, and then slowly at large NT,H. Two peaks appeared in the hydrogenated sample, the origin of which was explained by two differently strained states. The defect number decreased according to the increase of NT,H. However, a considerable amount of Si–Si bonds (300–700 bonds) was cleaved for annihilation of one defect by the hydrogenation. Annealing the post-hydrogenated samples above 400 °C caused a decrease of NT,H, regrowth of the defects, and an increase of Si–Si bond strain.
Resonant tunneling diodes with AlAs barrier: Guides for improving room-temperature operation
Authors: Vakhshoori Daryoosh , Wang Shyh
Published: July 2002

In this communication experimental results of AlAs/GaAs/AlAs resonant tunneling diodes are compared to theory, and the effect of higher resonant states on the room-temperature resonant tunneling current peak-valley ratio is studied. It is shown that for a well thickness of 70 Å, the leakage current through higher resonant states of the well can reduce the peak-to-valley current ratio by more than 400% when compared to a similar diode of 50-Å well thickness. Through these studies a summary of suggestions is listed, which is intended to improve the room-temperature operation of resonant tunneling diodes.
Radiation effects on a low-thermal-expansion glass ceramic
Authors: Tsai T. E. , Higby P. L. , Friebele E. J. , Griscom D. L.
Published: July 2002

Electron-spin-resonance and density gradient column techniques have been used to measure and characterize the effect of radiation on the electronic and mechanical properties of a low-thermal-expansion glass ceramic. Defect centers attributed to Zn+, oxygen hole centers (OHCs), Fe3+, Ti3+, and/or Zr3+, and two types of centers associated with arsenic have been observed; the radiation-induced compaction has been tentatively correlated with the OHC concentration.
Evaluation of insulating property by secondary ion mass spectrometry
Authors: Inoue K. , Isogai A.
Published: July 2002

In secondary ion mass spectroscopy analysis of insulating films, a time dependence of the secondary ion intensity [Is(t)] was measured. When the surface charging was slightly reduced, a minimum intensity (Ib) appeared in Is(t). It was found that Ib was inversely proportional to the thickness and resistivity of the film sample. Consequently, the measurements of Ib revealed that the resistivity of the sputtered silicon-oxide film was higher when the film was grown in lower oxygen pressure.
Photoresponse in amorphous semiconductors as dark conductivity varies
Authors: Smith G. B.
Published: July 2002

The variations in conductivity, under illumination of amorphous semiconductors, is examined as a function of the dark Fermi level (EF) using a simple, generally accurate approach which eliminates the need for a complete numerical solution of equations covering generation, recombination, and trapping. The enhanced photoconduction on doping is readily quantified along with the reduced photosensitivity (total to dark conductivity ratio). Various models can be more readily assessed. Predictions are examined in detail for the often used exponential gap state model. The associated Rose model result for the intensity dependence is seen to be an asymptotic limit which becomes less applicable as dark conductivity increases. Temperature dependence is also examined. Emphasis is on doping, but contrasts are also drawn between the effects of a homogeneous and inhomogeneous shift in dark activation energy. The reduction in the exponent γ for the intensity dependence is far more pronounced when external fields modify the dark conductivity.
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u anneal-induced growth enhancement during rapid isothermal oxidation of silicon
Authors: McGruer N. E. , Singh R. , Weiss J. H. , Rajkanan K.
Published: July 2002

Rapid growth of SiO2 on silicon is observed following annealing in argon. The enhancement may be reset many times, by annealing, for a growing oxide film. Growth kinetics of the enhancement are different than those of the often discussed initial growth regime enhancement. The anneal time required to produce enhanced growth is a factor of 10 less than that required to reduce interface trap density. Study of the phenomenon at time scales not accessible through furnace processing is made possible by the use of a rapid isothermal processor.