bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of vanadium as diffusion barrier between aluminum and gadolinium silicide contacts
Authors: Eizenberg M. , Thompson R. D. , Tu K. N.
Published: February 2003

Implementation of GdSi2 as low-barrier (0.4 eV) Schottky contacts in a Si device requires a diffusion barrier between the GdSi2 and Al interconnects to prevent Al penetration. In this work, the effectiveness of vanadium films as such a diffusion barrier has been studied by using a set of complimentary experimental techniques: nuclear backscattering, x-ray diffraction, and current-voltage measurement of Schottky barrier height on both n-type and p-type Si substrates. We found that the effectiveness depends strongly on how the thin-film structure of Al/V/GdSi2/Si was made, and especially on the formation process of GdSi2. The GdSi2, which was formed upon deposition onto a high-temperature substrate, is superior in stability by showing no Al penetration to other processing schemes investigated. These results are explained by structural and compositional changes occurring in the layered films of Al/V/Gd/Si as a function of heat treatment. In order to understand the complex interdiffusion and compound formation in the layered structure, the reactions between neighboring layers have been studied: Al interacts with V at ∼450 °C, forming Al3V and possibly Al10V; V does not react with Gd, even at 700 °C; and Gd is known to react with Si to form GdSi2 at∼350 °C. On the other hand, Al reacts readily with Gd at 250 °C to form a metastable compound of Al4Gd, which then transforms to Al3Gd at 400 °C, and the formation of these compounds has been found to retard the reaction between Gd and Si.
Titanium-tungsten contacts to Si: The effects of alloying on Schottky contact and on silicide formation
Authors: Babcock S. E. , Tu K. N.
Published: February 2003

Contact reaction and Schottky barrier heights on Si were studied using Ti-W alloys of increasing W concentration (Ti, Ti8W2, Ti4W6, Ti2W8) in order to determine the effect of alloy composition on Schottky contact behavior. Glancing angle x-ray diffraction, Rutherford backscattering spectroscopy, and scanning electron microscopy were used to analyze the contact reaction. Schottky barrier heights were determined from the I-V behavior of circular diodes. Four-point probe measurements were used to compute the film electrical resistivity. Our experiments show that the addition of small amounts of W has raised the formation temperature of Ti silicides and maintained a low barrier height (∼0.55 eV) Schottky contact on n-type Si up to 550 °C. A bilayer shallow contact metallurgy Si/Ti8W2/Ti3 W7/Al which provides a low barrier metal contact to n-type Si as well as an effective diffusion barrier between Al and Si is proposed as a result of this study.
Superconductive behavior of a multiconnected Josephson network in a periodically varying magnetic field of two frequency components
Authors: Ishida T. , Mazaki H.
Published: February 2003

In a mixing magnetic field of h1 sin 2πf1t and h2 sin 2πf2t, the ac fundamental susceptibility χ1=χ′1−iχ″1 of a microbridge-coupled Tc specimen has been studied through frequency channels of f1 and f2. It has been revealed that h1 sin 2πf1t and h2 sin 2πf2t do not independently contribute to χ1, where f1 is 35 Hz; f2 is 45 Hz, and h1 fixed at 14.6 mOe. As expected, the f2-channel response χ1(f2) is sensitive to h2. The f1-channel response χ1(f1) also depends on h2, but its behavior is unusual compared to χ1(f2). We develop the numerical calculation of χ′1(f1), χ″1(f1), χ′1(f2), and χ″1(f2) by using our phenomenological model. Generated profiles give a fine explanation to observed results.
Melt-spun yttrium-iron alloys: Magnetic properties and crystallization kinetics
Authors: Croat J. J.
Published: February 2003

Magnetic and crystallization data are presented for a series of X-ray amorphous Y1−xFex (0.4≤x≤0.79) alloys prepared by melt-spinning. Magnetization measurements, made from 4.2 to 600 K in fields up to 100 kOe, are interpreted on the basis of asperomagnetic structures in which the Fe moment directions are distributed over a decreasing portion of the unit sphere with increasing Fe concentration. In general, the ratio of spontaneous to saturation magnetization, and ordering temperatures, are higher than found for sputtered Y-Fe alloys, which may indicate a higher degree of compositional short-range order in these melt-spun alloys. The thermal stability of the alloys was investigated by differential scanning calorimetry and X-ray diffraction. Crystallization temperatures varied from 640 K at x=0.4 to 945 K at x=0.79. For the Y-rich (0.4≤x≤0.5) alloys, crystallization occurs by a multiple stage process, which includes the formation of a metastable Y-Fe intermetallic phase. Apparent activation energies for the transformations averaged 2.4–3.6 eV, in the range of moderate to high stability glasses. The kinetics of the x=0.67 and 0.75 compositions fit the Johnson-Mehl-Avrami relationship with a time dependence of t3–4 up to 65% conversion and t1.7–2.3 thereafter, indicative of a two-step crystallization mechanism. The coercive force of these melt-spun alloys was low (200–400 Oe) at all temperatures and, moreover, remained so during crystallization.
Effect of nitrogen on the high coercivity and microstructures of Co-Ni alloy films
Authors: Maeda H.
Published: February 2003

Co-30 at. % Ni alloy films containing N up to 18 at. % were deposited to thicknesses of 100 and 400 nm by rf sputtering in the atmosphere of Ar-N2 mixture and they were annealed. Most of N included diffused out from the film surface after annealing. The concentration of N, (Cnf) before annealing, however, has a significant effect on the coercive force, Hc, and the formation of the microstructures such as hollow channels and c axis preferred orientation parallel to the film plane in the annealed films. The preferred orientation factor, R, has a minimum, and Hc has a maximum in a film with Cnf of 13–15 at. %. There is a linear relation between Hc and R; Hc=Hc0(1−R/R0). The values of Hc0 tend to decrease with the film thickness, and they slightly depend on the microstructures, such as the hollow channels and grain boundaries.
Comparative studies of tunnel injection and irradiation on metal oxide semiconductor structures
Authors: Knoll M. , Bräunig D. , Fahrner W. R.
Published: February 2003

Tunnel injection and irradiation experiments on metal oxide semiconductor (MOS) structures are performed in order to compare the results of both experiments and to check the feasibility of radiation hardness prediction of MOS devices. The comparison is based on the fact that in both hot electron and ionizing irradiation experiments electron-hole pairs are generated in the SiO2. Due to an applied electrical field, these pairs are separated. The fraction of holes, trapped by neutral centers and the number of subsequently captured electrons by these now positively charged traps depend on the amount of available carriers, the magnitude of the respective capture cross sections for electrons and holes, and the number of hole traps. In the case of the tunnel injection experiment the number of the available carriers is a strong function of the field dependent ionization coefficient α. Up to now, its magnitude has not been accurately known. For this reason, a new method is presented which yields additional and reliable figures of α. When comparing the charge accumulation in the SiO2 by means of the flatband voltage shift as a consequence of the tunnel injection and of the irradiation, we determine α for various fields, α=3.3×10−6 exp [78/E(MV/cm)] (cm−1). For this determination we show that it is mandatory to correct for the steady state compensation of positively charged centers by electrons. From the saturation of the flatband voltage shift, caused by these trapped electrons, the field dependent capture cross sections for electrons are deduced. Their values are 2.3×10−14 and 3.5×10−15 cm2 for electrical fields of 7.8 and 8.6 MV/cm, respectively. The feasibility and limitations to predict the radiation hardness of MOS devices are discussed.
Dielectric relaxation of a polycarbonate: Polypropylene blend
Authors: Pillai P. K. C. , Narula G. K. , Tripathi A. K. , Mendiratta R. G.
Published: February 2003

Thermally stimulated current studies on compression molded thin films of polycarbonate: polypropylene blends mixed in a 40:60 weight ratio, respectively, indicate that the polycarbonate dipoles are so constrained in the blend that they do not contribute much to the polarization. The observed polarization is mainly due to induced dipoles. The effect of the variation of field and temperature on the polarization has also been studied. The activation energy is found to be 0.78±0.02 eV.
Silicon carbide blue light emitting diodes with improved external quantum efficiency
Authors: Hoffmann L. , Ziegler G. , Theis D. , Weyrich C.
Published: February 2003

The most relevant features in the fabrication and room temperature operation of blue light emitting 6H-SiC:Al,N diodes are reported. Epilayers are grown either on opaque Acheson substrates or on semi- to fully transparent substrate platelets obtained from larger ingots grown in our laboratory. The p-n junction is obtained by overcompensating Al acceptors with N donors. Electroluminescent light, which is mainly due to N-donor-Al-acceptor pair recombination is generated in the n region of the diode. The emission spectrum is saturated to about 90% at normal driving conditions, the hue of the diodes being slightly dependent on the current density. Optimization of diode preparation and design, especially the use of transparent substrates, yields external quantum efficiencies of 1.1×10−4 for unencapsulated samples, nearly triplicating the best quantum efficiency values for SiC:Al,N diodes reported so far.
Transmission sputtering of gold thin films by low-energy (<1 keV) xenon ions. I. The system and the measurement
Authors: Ayrault Guy , Seidman David N.
Published: February 2003

A novel system for direct measurement of the transmission-sputtering yields of ion-irradiated thin films is described. The system was specifically designed for the study of the transmission sputtering caused by low-energy (<1 keV) xenon ions. The xenon ion beam employed is first mass-analyzed in a specially constructed corssed magnetic- and electric-field mass spectrometer; this analyzer eliminates all energetic neutral and singly charged ions of mass less than 40 amu; it is also expected that ≤2% of the xenon ions which actually reach a specimen are doubly charged. The analyzed xenon ion beam is made to impinge on a gold thin film (∼100–500 Å thick) which is mounted in a JEM 200 transmission electron-microscope holder. The temperature of the specimen can be varied between ∼25 and 300 K employing a continuous transfer liquid-helium cryostat. The particles (atoms or ions) ejected from the unirradiated surface of the gold thin film are detected by two channeltron electron-multiplier arrays (CEMA) in the Chevron configuration; the Chevron detector is able to detect individual transmission-sputtered particles when operated in the saturated mode. To further enhance resolution, the electron cascades (produced by the CEMA), are amplified and shaped electronically into uniform square pulses. The shaped signals are detected with an Ithaco 391A lock-in amplifier (LIA). With the aid of a ratiometer feature in the LIA, we are able to measure directly the ratio of the transmission-sputtered current It to the incident ion current Ib; for Ibn=1 μA cm−2, a ratio of It/Ib as small as 1×10−9 has been measured. A detailed discussion of the calibration procedure and the experimental errors, involved in this technique, are also presented.
Bending gettering of Si crystalline defects
Authors: Sawada R. , Karaki T. , Watanabe J.
Published: February 2003

Suppression of oxidation-induced stacking faults in a Si wafer, to which bending stress was applied during thermal oxidation, was investigated. Straight dislocations were formed only in the neutral plane of the wafer and were absent at the surfaces where bending stress is greatest. The dislocations were introduced by application of lower bending stress, for a wafer with a great amount of SiO2 precipitates in bulk, and continued to function as sinks for oxidation-induced stacking fault nucleation sites, not only during the first oxidation but also through all subsequent high-temperature treatments. An origin of the dislocations was found based on the SiO2 precipitates in Si bulk. Gettering through bending is described together with the dislocation formation mechanism.
Electron diffusion lengths in p-type InP involved in indium tin oxide/p-InP solar cells
Authors: Gouskov L. , Luquet H. , Soonckindt L. , Oemry A. , Boustani M. , Nguyen P. H.
Published: February 2003

Electron diffusion length measurements have been performed in p-type InP by different methods: Surface photovoltage, spectrum analysis of the photocurrent in an Indium Tin Oxide/p-InP diode, variation of the photovoltage in this diode versus reverse voltage, Electron Beam Induced Current measurements. The results are compared and discussed in detail. A double p-type region model with different electron diffusion lengths has been proposed to explain the experimental results.
Perfect mode locking of solid-state lasers by a double passive modulation
Authors: Martinez O. E. , Marconi M. C. , Dicdati F. P.
Published: February 2003

A new configuration, which incorporates a second passive modulation, has permitted complete selectivity in passive mode locking of solid-state lasers. A theoretical study is presented showing the existence of qualitatively different types of emission depending on the parameters of the system. Computed simulations are made to show the dependence of the emission on such parameters, and are used to determine their optimum range. Experimental results are presented confirming computed predictions.
Meander-line slow-wave antenna for launching lower hybrid waves in a plasma
Authors: Takahashi T. , Takamura S. , Okuda T.
Published: February 2003

A meander-line slow-wave structure was found to be a good launcher of a traveling lower hybrid wave with a large longitudinal retardation. The antenna analysis without plasma loading gives good approximations for the characteristic impedance and the longitudinal retardation even if the plasma is loaded. The space harmonics wave, which characterizes a periodic slow-wave structure, was directly observed.
Effect of refined shape of input atomic beam on plasma density distribution in a double-ended Q machine
Authors: Chung Kyu Sun , Hong Sang Hee
Published: February 2003

The density distributions of equilibrium plasma in a double-ended Q machine are analyzed by means of fluid theory in connection with various shapes of input atomic beam. An inhomogeneous ordinary differential equation for plasma density is analytically solved in terms of modified Bessel functions, and modified Lommel function exhibiting the effect of refined atomic beam impingement. The radial density profiles are discussed in various cases of the atomic beam to find the optimum input-flux shape which results in the improved plasma confinement. As the slope of input distribution becomes steeper, it turns out to have better confinement effect on plasma distribution.
Static and dynamic characteristics of multichannel discharge in rare-gas-mercury mixtures
Authors: Watanabe Yoshio , Yamane Mikiya
Published: February 2003

Analysis of multiarc formation in a concentric discharge tube is described. The effect of diffusion on static characteristics is also examined. The mechanism for current modulation by anode oscillation in a multianode tube is then simulated by an electric switching circuit. The simulated results agree well with the measurement results. It is found that the effect of diffusion on dynamic multiarc characteristics plays an important role in determining excess ionization patterns. When excess ionization takes place on one anode, discharge current increases, plasma density increases, and charged particles diffuse to adjacent channels. Consequently, the plasma density, hence the discharge current, in the adjacent channel also increases. This increased current causes excess ionization on the adjacent channel anode. The peculiar sequence patterns of excess ionization are explained by this effect of diffusion on dynamic characteristics.
Probability distributions for the breakdown voltage between closely spaced electrodes on insulating surfaces
Authors: Kushner M. J.
Published: February 2003

Probability distributions for the breakdown voltage between closely spaced electrodes (≊10 mils) on insulating surfaces are studied with results from a Monte-Carlo simulation. The probability distributions, experimentally measured to be bimodal under certain conditions, are found to be characterized by the number of ionizations per primary electron emitted at the triple junction that is required to initiate the electron avalanche. Bimodal distributions represent a transition region between low variance and high variance normal distributions requiring low and high multiplication coefficients, respectively, for the avalanche to occur. Conditions of moderate preionization and low electron loss rates to the insulating surface (e.g., large secondary electron coefficient) are found to lower the breakdown voltage and reduce bimodal distributions to single normal distributions.
Depletion of various ions from glass surfaces during Auger analysis
Authors: Whitkop Peter G.
Published: February 2003

The kinetics of aluminum and calcium ion depletion from glass surfaces during Auger electron spectroscopic analysis can be represented by a sum of two decaying exponential functions. Iron and titanium decay can be qualitatively described by a sum of exponential decay and rise-and-fall-type functions. These results are explained by a mechanism that includes an electron beam induced interconversion of oxidation states.
Crystal structure and thermal expansion of α-quartz SiO2 at low temperatures
Authors: Lager G. A. , Jorgensen J. D. , Rotella F. J.
Published: February 2003

The crystal structure of α-SiO2 (low quartz) has been refined at 296, 78, and 13 K from time-of-flight neutron powder diffraction data. The major effect of temperature from 296 to 78 K is a nearly rigid body rotation of the SiO4 tetrahedra. Below 78 K, tetrahedral rotation is substantially reduced giving rise to a much smaller thermal expansion. A comparison of the volume dependence of the Si-O-Si angle, the rotation angle for SiO4 tetrahedra and the c/a ratio suggests that the mechanism for expansion may be changing from tetrahedral rotation to tetrahedral distortion at the lowest temperatures. The inverse linear relation between the mean Si-O bond distance and −sec(Si-O-Si) which has been observed for silica minerals at both ambient and high-temperature conditions appears to be consistent with the structural variations that occur at low temperatures.
Redistribution of As during Pd2Si formation: Ion channeling measurements
Authors: Wittmer M. , Ting C.-Y. , Ohdomari I. , Tu K. N.
Published: February 2003

We have investigated the redistribution of As during Pd2Si formation with ion channeling technique. The results show that both interstitial As and substitutional As in the Si are pushed ahead by the moving silicide-silicon interface during the growth of Pd2Si at 250 °C. This effect increases the As concentration in a region of about 100 Å beyond the interface. A model for the mechanism of As redistribution during silicide formation is suggested and implications of the effect on electrical characteristics of Schottky diodes and shallow junction devices are discussed.
Amorphous to crystalline phase transformation and growth of thin Te-Se films
Authors: Vermaak J. S. , Petruzzello J.
Published: February 2003

The crystallization of amorphous TeSe-alloy thin films on a carbon substrate occurs by the nucleation and subsequent growth of small single crystalline nuclei with hexagonal structure. The main orientation of these nuclei are with their c axis parallel to the substrate. Isothermal growth rates give an activation energy of 0.7 eV for the growth of the crystalline phase. The morphology of the growing crystallites shows some crystallographic faceting but no preferred c-axis growth. It is proposed that crystal growth takes place by the addition of small segments of extended homogeneous chains of either Te or Se atoms onto specific low energy crystallographic planes. This growth mechanism plus the difference in specific volumes of the crystalline phase compared to the amorphous phase give rise to built-in stresses in the thin crystallized film which might be detrimental when used for device applications.
Thermal Grüneisen parameters of CdAl2O4, β–Si3N4, and other phenacite-type compounds
Authors: Slack G. A. , Huseby I. C.
Published: February 2003

The thermal expansion coefficients of a number of phenacites [Be2SiO4, β-Si3N4, β-Sialon, α-LiAlSiO4, Zn2SiO4, CdAl2O4, and Zn2GeO4] at temperatures at and above the Debye temperature are analyzed in terms of the thermal Grüneisen parameter γ. The phenacites have γ values intermediate between adamantine crystals and quartz. Values of γ between 0.3 and 0.7 are found. A useful correlation exists between γ and the average volume per anion bond W. Values of γ approaching zero occur for sufficiently large values of W.
Depth profiles of Fe and Cr implants in InP after annealing
Authors: Gauneau M. , L’Haridon H. , Rupert A. , Salvi M.
Published: February 2003

The post-annealing concentration profiles of Fe and Cr implants in undoped InP substrates are studied by secondary ion mass spectrometry. Annealing temperatures range from 550 to 750 °C. Iron and chromium show similar behavior; the profiles are divided in three regions: the near surface zone where metals pile up, the zone in front of the projected range (Rp), around 0.8 Rp, where Fe and Cr atoms are trapped by residual damage, and a region around Rp+ΔRp, where a well-defined peak, with maximum concentration reaching 3–4×1018 cm−3, is observed. On each side of that peak, solute atoms are depleted down to the limit solubility: 1×1017 cm−3 for iron, 2×1016 cm−3 for chromium. The results are interpreted according to the calculations of L. A. Christel et al. [J. Appl. Phys. 52, 5050 (1981)] on stoichiometric disturbances in ion-implanted compound semiconductors.
Impurity profile broadening and shifts by ion beam mixing
Authors: Paine B. M.
Published: February 2003

The broadening and shifts of the profile of a buried impurity layer, caused by heavy ion irradiation, have been measured by backscattering spectrometry. The matrix was amorphous Si and the impurity was a 4 Å thick layer of Pt buried at a depth of 500 Å. The samples were held at a temperature of 90 K and irradiated with 300-keV Xe+ ions to fluences ranging from 0.4 to 12.7×1015 ions/cm2. The broadening was monitored by finding the irradiation-induced increases in the variances of the impurity signals in the backscattering spectra. This quantity appeared to increase linearly with irradiation fluence up to 1×1016 ions/cm2. Shifts were monitored by measuring the movement of both the mean and the peak of the impurity signal relative to the position of the signal from a second thin Pt layer buried beyond the range of the Xe ions. These were corrected for the shifts caused by the Xe atoms buried between the two layers. The net shifts of both the mean and the peak were away from the sample surface, with the magnitude of the shift of the mean being about 100 Å for an irradiation fluence of 1×1016 ions/cm2. The shifts increased roughly linearly with irradiation fluence.
Dye diffusion in swollen gels by forced Rayleigh scattering
Authors: Wesson Jeffrey A. , Takezoe Hideo , Yu Hyuk , Chen Stephen P.
Published: February 2003

Diffusion of a small molecule through a swollen polymeric network is studied by two very different experimental techniques: the first is a microscopic one called forced Rayleigh scattering (FRS), and the second is the macroscopic method of time lag. The diffusant is methyl red, an acid-base indicator dye, and the matrices are swollen crosslinked gelatin networks. The two sets of diffusion coefficients deduced by the two techniques are found to be in complete accord with each other. Since the time-lag technique measures diffusion perpendicular to the gelatin slab and FRS measures diffusion in the plane of the gelatin sample, these experiments indicate structural isotropy and homogeneity of the matrix down to the scale of a few microns. It is also noted that the dependence of the dye diffusion coefficient on the solvent volume fraction does not obey free volume theory predictions.
Method of evaluating the thermal stability of the pyroelectric properties of polyvinylidene fluoride: Effects of poling temperature and field
Authors: DeRossi Danilo , DeReggi Aime S. , Broadhurst Martin G. , Roth Steven C. , Davis G. Thomas
Published: February 2003

The ac pyroelectric response of a number of differently poled polyvinylidene fluoride films has been measured while the temperature was varied at a constant rate ∼5 °C/min from room temperature to near the melting temperature. The response first increases with increasing temperature, which is attributed to an increase of the thermal expansion coefficient and eventually decreases due to melting and/or loss of electric dipole orientation. The details of the temperature dependence are influenced in a reproducible manner by the poling variables, especially the poling temperature. The measurement is therefore proposed as a way of evaluating the effect of processing variables on the thermal stability of the piezoelectric and pyroelectric properties.
Solution-grown crystals of model ethylene copolymers
Authors: Marks B. S. , Carr S. H.
Published: February 2003

Studies on the crystal growth habit of model crystallizable polymers, the very narrow molecular weight distribution hydrogenated polybutadienes, are reported. These polymers are analogues for 4 mol % butene-1 copolymers with ethylene. Crystals have been grown from dilute solutions, either as suspensions or as epitaxial overgrowths on KCl cleavage surfaces. The growth faces of these crystals are highly irregular in shape, and the fold surfaces (imaged using dark field electron microscopy) are unusually thick. Both effects appear to be due to the process by which chains attach to the crystal. This process involves chain attachment to the growth face followed by a partitioning of its ethyl branch-rich segments to the fold surfaces and its ethyl branch-lean segments to the crystal cores. Thus, it must build crystal only locally, and at the same time it excludes other arriving chains from a region centered about its attachment site. Continued repetition of this process ultimately leads to irregularly shaped growth faces of the kinds observed.
A time dependent theory of crazing behavior in polymers
Authors: Chern S. S. , Hsiao C. C.
Published: February 2003

The development of crazing is not only a function of stress, but also a function of time. Under a simple state of tension, a craze opening displacement is closely associated with the viscoelastic behavior of the original bulk polymer medium in which individual crazes initiate and develop. Within each craze region, molecular orientation takes place when conditions permit, and a new phase of rearranged molecules governs its local behavior. Based upon a time-dependent viscoelastic two-dimensional model, using a computer program the craze opening displacement field has been calculated, time-dependent craze length was also computed by taking into consideration the molecular orientation mechanism and large deformations in the craze region. Examples are given for simple viscoelastic media with simplified stress distributions. It is interesting to find out that the occurrence of crazing may be interpreted in terms of the stability or instability of the constitutive behavior of the bulk polymer.
Pressure-volume-temperature studies of a polyurethane elastomer
Authors: Questad D. L. , Pae K. D. , Scheinbeim J. I. , Newman B. A.
Published: February 2003

Experimental pressure-volume-temperature data for a polyurethane elastomer above Tg are fitted to equation of state theories. The cell and hole theories, as applied to polymer liquids, both represent the data fairly well for appropriate choices of P*, V*, and T*; the characteristic pressure, volume, and temperature scaling parameters, respectively. P*, V*, and T* are found to obey a semiempirical equation which holds for a large number of polymers, and, by fitting the data to the hole theory it is found that the hole fraction, 1−y, can be used as an ordering parameter to describe the pressure dependence of Tg.
Raman spectroscopic study of ultraoriented solid state extruded polyethylene
Authors: Wang Y. K. , Waldman D. A. , Stein R. S. , Hsu S. L.
Published: February 2003

Raman spectroscopy was used to follow structural changes for polyethylene extruded at 80 and 130 °C by the split billet method. The all-trans chainlength distributions derived from the low frequency Raman longitudinal acoustic mode (LAM) data varied considerably as a function of the draw ratio or processing temperature. Even though the molecular orientation is extremely high for the sample deformed to the highest draw ratio, LAM measurements revealed a broad chainlength distribution, especially for samples prepared at the low temperature. We have interpreted the broad chainlength distribution to arise from conformational defects introduced during mechanical deformation.
Branched polymer models
Authors: Klein D. J. , Seitz W. A. , Kilpatrick J. E.
Published: February 2003

The importance of excluded volume in determinig the sizes of branched polymers is discussed in terms of several models. First, the branched chain analog of the rotational isomeric model is solved and gives improvement over earlier random flight models but, as was the case for linear chains, the asymptotic dependence of polymer size on monomer number (〈size〉≂ANν) remains the same, except that model differences are found in the preexponential factor A. Next, a hierarchy of subclasses of the full class of generally branched self-avoiding random walks is briefly described and compared to our recent Monte Carlo results on the full problem. Finally, simple heuristic arguments for excluded volume corrections are considered.
Pressure concentrations due to plastic deformation of thin films or gaskets between anvils
Authors: Chan K. S. , Huang T. L. , Grzybowski T. A. , Whetten T. J. , Ruoff A. L.
Published: February 2003

Plastic deformation of either a sample or a gasket between diamond anvils leads to large pressure concentrations, i.e., the maximum pressure can be many times the average pressure. This behavior is discussed using elementary plasticity theory for the case where the pressures are sufficiently low that the yield stress can be assumed not to vary with pressure. It is then shown that the pressure concentration factor can be even much larger when the yield stress of the sample (or gasket) at the highest pressure is much greater than the yield stress at the lowest pressure. This is illustrated with solid xenon where it is shown that the assumption that Nelson and Ruoff [Phys. Rev. Lett. 42, 383 (1979)] made about the pressure distribution in their xenon samples is incorrect. The pressure distribution is shown to be much steeper than they assumed. Thus, the pressure they had when they observed electrical conduction in xenon was, so far as the evidence of the present analysis shows, above 1 Mbar.
Calculation of thermal trapping in shock deformation of aluminum
Authors: Grady D. E. , Asay J. R.
Published: February 2003

A precipitation hardened aluminum alloy (6061-T6) has been investigated extensively in terms of shock deformation properties during past years. Recently, advances in time-resolved measurements of stress waves in solids have assisted in revealing complicating features in the shock deformation process which are not easily incorporated into existing theories of plastic deformation. Specifically, significant changes in the dynamic hardness, post shock hardness, steady wave viscosity, and post shock microstructure have been observed over the shock pressure range of 5–20 GPa. Microscopic heterogeneity in the shock deformation process is one possible cause for the observed effects. Deformation heterogeneities can lead to transient temperature gradients. We have performed calculations which indicate that over the range of strain rates occurring in steady wave shocks in aluminum, a transition from thermal equilibrated to thermal trapping deformation can occur. The shock pressure at which the thermal transition is calculated to occur is consistent with onset of the observed anomalous shock effects.
A study on diffuse antiferroelectric-paraelectric phase transition in pseudobinary Pb(Fe1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 solid solution
Authors: Choo Woong Kil , Lee Moon Ho
Published: February 2003

An antiferroelectric-paraelectric diffuse phase transition in pseudobinary (1−x)Pb(Fe1/2Nb1/2)O3-xPb(Mg1/2W1/2)O3 solid solution for 0.60≤x≤1.00 is investigated by its dielectric property and calorimetric measurements. The dielectric constant shows a diffuse phase transition behavior for 0.60 ≤x≤0.88, but shows a sharp phase transition behavior for 0.90≤x≤1.00. The apparent dielectric transition temperature increases with increasing external frequency for 0.60≤x≤0.88, but remains constant with respect to the change in external frequency for 0.90≤x≤1.00. Dielectric curves as a function of temperature show broadened peaks for 0.60≤x≤0.88, but show relatively sharp peaks for 0.90≤x≤1.00. The transition enthalpy change also increases with increasing x. The diffuse phase transition for 0.60≤x≤0.88 is accompanied by a small enthalpy change. The small enthalpy difference between the antiferroelectric and paraelectric phases seems to lower the barrier between the two phases, and in the process, the diffuse phase transition seems to be induced.
A novel method to detect nonexponential transients in deep level transient spectroscopy
Authors: Thurber W. Robert , Forman Richard A. , Phillips Willie E.
Published: February 2003

In conventional Deep Level Transient Spectroscopy (DLTS) measurements, the analysis of the results is based upon the assumption of an exponential current or capacitance transient. We present experimental and computational results on a novel experimental method for determining when the assumption of exponentiality is not satisfied by the sample under study. The measurement may be performed without any changes in the conventional double-boxcar DLTS system.
Surface-charge-limited transient currents on insulating surfaces
Authors: Chapman T. C. , Wintle H. J.
Published: February 2003

We have studied theoretically the flow of current across the surface of an insulator. We made the assumption that free injection occurred at one electrode, resulting in surface-charge-limited currents. We employed two geometries, the first corresponding to surface electrodes on one side of a thick dielectric, the second corresponding to a dielectric slab inside capacitor plates. In both cases, the current is proportional to the square of the applied voltage. The time-dependent currents approach the steady-state current from below with a time constant comparable to the surface transit time, but the detailed form of the current changes markedly with the geometry. Comparison with experiment suggests that the observed charge spreading does not occur by this surface-charge-limited mechanism.
A study of the chemical oxide/InP interface states
Authors: Lim H. , Sagnes G. , Bastide G.
Published: February 2003

Metal-Insulator-Semiconductor (MIS) structures have been produced by chemical oxidation of InP surfaces. In MIS diodes fabricated without thermal annealing of the as-grown surface oxide, a considerable increase of the ideality factor was observed in highly oxidized samples. It is demonstrated that this phenomenon is partly related to the quality of the as-grown oxide. Two interfacial deep levels with activation energies and capture cross sections, E1=390 meV, σ1=2×10−17 cm2 and E2=510 meV, σ2=4×10−12 cm2, respectively, have been observed. After a detailed study, it has been shown that these two interfacial states originate from the transition region between the as-grown oxide and the InP substrate.
Effects of hydrogenation on lightly doped polycrystalline silicon
Authors: Campbell D. R. , Hwang J. C. M. , Ohdomari I. , Frisch M. , Fitzpatrick W.
Published: February 2003

We performed Hall-effect measurements as a function of temperature on lightly doped polysilicon films. The range of boron concentration encompassed a mobility minimum and a corresponding barrier height maximum. Upon plasma hydrogenation, these two extrema shifted to a lower concentration, clearly indicating a passivating effect. Spin resonance measurements on intrinsic films showed a reduction in unpaired spins after plasma hydrogenation. Desorption experiments confirmed the presence of hydrogen in the plasma treated samples.
Structure of a Nb oxide tunnel barrier in a Josephson junction
Authors: Kuan T. S. , Raider S. I. , Drake R. E.
Published: February 2003

Oxides grown on the surface of a polycrystalline Nb thin film using rf plasma cleaning and plasma oxidation processes were examined by transmission electron microscopy. The oxides were prepared on the Nb surface using the same rf plasma processes developed for fabricating the tunnel barriers of Nb/Nb oxide/Pb alloy Josephson tunnel junctions. Transmission electron diffraction patterns obtained from the tunnel barrier formed on the Nb surface indicated the presence of a crystalline NbC0.25O0.75 layer between the amorphous Nb2O5 tunnel barrier layer and the Nb base electrode. The thickness of this transition layer was found to be sensitive to the rf plasma cleaning conditions prior to the tunnel barrier formation. The formation of this NbC0.25O0.75 layer of proper thickness is correlated to good junction quality. An epitaxial relationship between the NbC0.25O0.75 layer and the underlying Nb crystal was also observed.
Critical current densities of the multifilamentary Nb3Sn superconductors produced by the solid-liquid diffusion method
Authors: Yamasaki H. , Umeda M. , Watanabe M. , Kimura Y.
Published: February 2003

The critical current and critical temperature of the multifilamentary Nb3Sn superconductors produced by the solid-liquid diffusion method were measured for various heat treatments, and were discussed in relation to the process of Nb3Sn formation. The critical current density Jc for the fine-grained Nb3Sn layer, which was formed at the interface of the intermediately formed Nb6Sn5 layer and the Nb matrix, was calculated to be 5–6×105 A/cm2 at 9.5 T and 4.2 K, which is a little higher than the best Jc value for the Nb3Sn layers of the bronze-processed conductors. The Jc value increased during the formation of the fine-grained Nb3Sn layers and reached its maximum value when the reaction process was nearly completed and thick Nb3Sn layers were formed, though Jc decreases during the growth of Nb3Sn layers in the bronze process. By this solid-liquid diffusion method, thick (∼5–20 μm) and high Jc Nb3Sn layers are obtained, so that the overall Jc of the conductor (without a Cu stabilizer area) exceeds 1.5×105 A/cm2 at 9.5 T, and is twice as high as that of the bronze-processed conductors.
High-field magnetization curves of amorphous alloys
Authors: Kohmoto Osamu
Published: February 2003

Effects of Si addition into amorphous (Fe0.064Co0.936)76.5 -B23.5 alloy having zero magnetostriction were investigated by the measurement of high-field magnetization at room temperature in the field range 150≤H≤13 000 Oe. The FeCo-B alloy shows anomalies in both the law of approach to saturation and the para process. The magnetization curve of the FeCo-B alloy was found to be composed of an atypical 1/H1/2 term as well as an ordinary 1/H term and a para process H1/2 term. The 1/H1/2 dependence is observed in the field range 150≤H≤2000 Oe, and the contribution of para process H1/2 term is large in the high-field region (H>2000 Oe). With small addition of Si into the FeCo-B alloy, however, the 1/H1/2 term becomes negligible and the 1/H term increases due to changing defect structure. It was also found that, with the Si addition, the spin wave stiffness constant increases, whereas the saturation magnetization tends to decrease. The Si addition causes a change in amorphous microstructure influencing short-range fluctuation of exchange energy.
Direct measurement of small diffusion coefficients with secondary ion mass spectroscopy
Authors: Macht M.-P. , Naundorf V.
Published: February 2003

Sputter sectioning in combination with secondary ion mass spectroscopy enables the determination of very small diffusion coefficients which are not attainable with classical sectioning techniques. The exceedingly good depth resolution of the sputter sectioning and the high sensitivity of the mass spectroscopy allow to resolve penetration profiles of solutes in the 10-nm range at the ppm level. Two perturbing effects, inherent to the method and limiting its sensitivity are discussed: degradation of depth resolution by surface roughening and atomic mixing, and near surface distortion of profiles by transient erosion effects. Degradation of depth resolution was minimized by use of single crystalline specimens and low energy sputtering with reactive ions. To overcome the near surface distortions a special sample preparation technique has been developed, resulting in single crystalline specimens with one or more inserted layers of the solutes to be diffused. The application of the method is demonstrated by examples of thermal- and irradiation-induced diffusion of nickel in copper, and the main errors are discussed.
Planar multijunction high voltage solar cell chip
Authors: Valco George J. , Kapoor Vikram J. , Evans John C.
Published: February 2003

Integrated circuit technology has been successfully developed to design and fabricate a new type of small area planar multijunction high voltage solar cell chip for concentrated sunlight applications. Each of these 1×1 cm solar cell chips was a monolithic device consisting of six internally series interconnected unit cells, fabricated on 75 μm thick p-type single crystal silicon substrate. Each chip consisted of 1.42×9.63 mm n+/p collecting junctions on the back of the wafer. The illuminated front surface area was divided into 0.3 μm deep n+ regions which form front surface field regions corresponding to the n+/p unit cells positioned beneath. A five photomask level photolithographic process together with a standard microelectronics batch process technique was employed to construct the PMJ solar cell chip. The open circuit voltage of the solar cell chip increased rapidly with illumination up to about 4 AMl suns, and then began to saturate at the sum of the individual unit cell voltages of 3.5 V above 4 AMl suns. A short circuit current density per unit cell of 300 mA/cm2 at 20 AMl suns was observed. This low value is attributed to a low minority carrier diffusion length in the base region of the solar cell chip. The suggested process modifications should significantly increase the device efficiency.
An alternate theory for the relativistic magnetron
Authors: Thomas Gary E.
Published: February 2003

An alternate theory describing the operation of the relativistic magnetron is presented. It is postulated that the relativistic magnetron (specifically MIT’s A6 and D6) operates due to a resonance condition (nonsynchronous) rather than due to an rf travelling wave phase velocity equal to the electron drift velocity (synchronous). A nonlinear partial differential equation is derived and reduced by a multiple time scales perturbation technique to a modified nonlinear Schrodinger equation, which has growing soliton solutions. An equation for obtaining the optimum operating voltage Vs is derived, and theoretical predictions are made for MIT’s A6 and D6 magnetrons. These predictions show close agreement with experimental data.
Behavior of a multiloop dc superconducting quantum interference device
Authors: Carelli P. , Foglietti V.
Published: February 2003

Measurements of low inductance dc superconducing quantum interference devices (SQUIDs) are presented. The SQUIDs used for such measurements are made using a photolithographic technique similar to that one developed by IBM for the superconducting computer. On the same chip a coil is evaporated with a coupling coefficient k to the SQUID larger than 0.33. The best measured intrinsic energy sensitivity is 4 h. Possible improvements in the design are proposed.
Diffusion coefficient for the ∼1.35 eV emission center in annealed GaAs
Authors: Nojima Shunji
Published: February 2003

ufonThe ∼1.35 eV emission-intensity profiles are measured for high-temperature annealed GaAs. Diffusion coefficient D for this center is determined by a parametrization using an emission-intensity profile formula. This formula is derived assuming the emission-center profile to be a complementary error function and considering the absorption depth for an excitation light. Temperature dependence of D is expressed by D=3.38×10−4 exp(−2.14/kT), where D is in cm2s−1 and kT in electron volts. This result suggests that the ∼1.35 eV emission may be assigned to the center related to a native defect such as Ga vacancy, rather than to a Cu atom.ufoff
Surface flux pinning in superconducting amorphous (Mo0.6Ru0.4)B18
Authors: Clemens B. M. , Johnson W. L.
Published: February 2003

Superconducting critical current density was measured as a function of a perpendicular applied magnetic field in glassy (Mo0.6Ru0.4)82B18. The pinning force density was observed to depend linearly on 1/w, where w is the sample width measured perpendicular to both the current and field. This dependence is attributed to pinning by the sample edges. The bulk pinning contribution can be separated from the edge pinning contribution by extrapolation of the Fp vs 1/w curve. The edge contribution of the flux pinning was nearly eliminated by electrolytically polishing the sample. The contribution of the flux pinning profile due to edge pinning is analyzed in terms of the dynamic pinning model modified for edge pinning.
Mathematical simulation of a positive contrast guest-host display using nematic-cholesteric phase change
Authors: Gharadjedaghi F. , Voumard R.
Published: February 2003

A mathematical analysis is presented which simulates the electro-optic characteristics of a dichroic display cell containing a mixture of a negative dielectric anisotropy nematic liquid crystal, a dichroic dye, and a chiral component. The numerical results obtained from this analysis are in good agreement with those obtained experimentally. The influence of the elastic, dielectric, and optical properties of the mixture, and the role of the chiral component, are studied for a static system. The conclusions drawn from these studies have enabled the desirable physical properties of the liquid crystal mixture to be determined.
Enhancement in oscillator strength of color centers in electrochromic thin films deposited from WO2 powder
Authors: Yoshimura Tetsuzo , Watanabe Masaki , Koike Yoshio , Kiyota Kohei , Tanaka Masao
Published: February 2003

Color centers in amorphous films evaporated from WO2 powder and WO3 powder were investigated. It was found that the device using ‘‘WO2’’ film, which has a WO3-like structure, exhibits a high-speed response and a high optical density, which is attributed to the enhancement in oscillator strength of the color centers in ‘‘WO2’’ films. The use of a configuration coordinate model revealed that the enhancement in the oscillator strength is caused by the increase in the degree of extension of the polaron wave function in the ‘‘WO2’’ film. The absorption band of the color center in ‘‘WO2’’ exhibits a low-energy shift as compared with that in WO3, which supports the increase in the degree of the small polaron extension in ‘‘WO2.’’ We also studied the electrical conductivity, density, ir absorption spectra, and voltammograms of the ‘‘WO2’’ and WO3 films. These results suggest that the increase in the degree of the extension of the polaron wave function is caused by some change of the lattice condition which is introduced by a lack of oxygen in the ‘‘WO2’’ film.