bluuebunny/crossref_metadata_2025 huggingface dataset:
Gaussian random fields with two level-cuts—Model for asymmetric microemulsions with nonzero spontaneous curvature?
Authors: Arleth Lise , Marc̆elja Stjepan , Zemb Thomas
Published: July 2002

The microstructure of a microemulsion is dominated by the thermodynamics of the surfactant interface between the oil and water domains. As the spontaneous curvature of this surfactant interface is strongly temperature dependent the microstructure of microemulsions also becomes temperature dependent. In the present work we have assumed that the thermodynamics of the interface is determined by the Helfrich Hamiltonian and that the interface can be described by two appropriately chosen level-cuts of a Gaussian random field. It is then possible to express the free energy density of the interface as a functional of the spectral distribution of the Gaussian random field so that the microstructure which minimizes the free energy can be determined by performing a functional minimization of the free energy with respect to the spectral distribution of the Gaussian random field. The two level-cuts are an important feature of the model since they allow us to model microemulsions with nonzero spontaneous curvature and with unequal volume fractions of water and oil. This again makes it possible to simulate the temperature driven phase inversion of the microemulsions described above. The model furthermore allows us to predict the microstructure of the microemulsion for a given composition of water, oil and surfactant and input parameters H0, κ and κ̄ as well as to predict direct space structures and scattering structure factors. Microemulsions with bicontinuous structures, droplet structures or swollen sponge-like structures are predicted dependent on the input parameters and represented in direct and inverse space. Dilution plots for scattering peak positions are in good agreement with experimental results.
High energy photon radiation from a Z-pinch plasma
Authors: Velikovich A. L. , Davis J. , Oreshkin V. I. , Apruzese J. P. , Clark R. W. , Thornhill J. W. , Rudakov L. I.
Published: July 2002

A new approach to the generation of kilovolt x ray radiation in Z-pinch plasma radiation sources is proposed. In cases where the pulse power machine has insufficient energy to efficiently produce K-shell emission from the atomic number element that emits in the required kilovolt energy range, it may be advantageous to produce x rays by recombination radiation emitted from a lower atomic number plasma. The optimal load conditions for maximizing the high energy free–bound continuum radiation that can be produced in a given spectral range are analyzed. The largest yield is expected from a highest-atomic-number element that could efficiently produce K-shell yield on a given pulse power machine. Two options available for the choice of a wire array material to generate x rays with photon energies above 7–8 keV are identified and discussed, aluminum and titanium. The analytical estimates and simulation results for “Z” machine implosions show that continuum radiation from an aluminum plasma in this spectral range could exceed the K-shell output from nickel. If a titanium plasma could be ionized to a H-like state on “Z,” then its yield above 7 keV should be higher compared to that of aluminum by a factor of 3 to 4.
Three-dimensional acoustic scattering by vortical flows. I. General theory
Authors: Llewellyn Smith Stefan G. , Ford Rupert
Published: July 2002

When an acoustic wave is incident on a three-dimensional vortical structure, with length scale small compared with the acoustic wavelength, what is the scattered sound field that results? A frequently used approach is to solve a forced wave equation for the acoustic pressure, with nonlinear terms on the right-hand side approximated by the bilinear product of the incident wave and the undisturbed vortex: we refer to this as the “acoustic analogy” approximation. In this paper, we show using matched asymptotic expansions that the acoustic analogy approximation always predicts the leading-order scattered sound field correctly, provided the Mach number of the vortex is small, and the acoustic wavelength is a factor of order M−1 larger than the scale of the vortex. The leading-order scattered field depends only on the vortex dipole moment. Our analysis is valid for acoustic frequencies of the same order or smaller than the vorticity of the vortex. Over long times, the vortex may become significantly disturbed by the incident acoustic wave. Additional conditions are derived to maintain validity of the acoustic analogy approximation over times of order M−1, long enough for motion of the vortex to be significant on the length scale of the acoustic waves.
Uniaxial and biaxial strain field dependence of the thermal oxidation rate of silicon
Authors: Noma Hirokazu , Takahashi Hiroyuki , Fujioka Hiroshi , Oshima Masahara , Baba Yuji , Hirose Kazuyuki , Niwa Masaaki , Usuda Koji , Hirashita Norio
Published: July 2002

We have investigated the effect of homogenous uniaxial and biaxial strains on the thermal oxidation processes of Si with newly developed techniques. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements have revealed that the biaxial compressive strain suppresses the oxidation rate at temperatures lower than 800 °C, while the uniaxial strain does not affect the oxidation rate. It has been also found that stress during the thermal oxidation does not cause any change in the electronic states of the oxide which are characterized by XPS and x-ray absorption near edge structure.
Rotational pathways in electronic energy transfer
Authors: McCaffery Anthony J. , Marsh Richard J.
Published: July 2002

We analyze rotational distributions from collision-induced atom–diatom electronic energy transfer (EET) experiments in terms of the capacity of the diatomic to dispose of the angular momentum (AM) generated in state-to-state change. Two pairs of systems are chosen as representative of processes broadly categorized as “efficient” or “inefficient” in this regard, namely, Na2–Na, Li2–Li in the former category and N2+–He, CN–Ar in the latter. Note that EET involving electron spin change is not considered here. Using velocity-AM diagrams and quantitative calculations we show the factors that govern the probability of state-to-state transfer in EET are the same as those controlling the outcome of rotational and rovibrational transfer within an electronic state. This suggests that requirements of orbital and rotational AM are of critical importance in providing pathways that allow EET to proceed.
Selective doping of 4H–SiC by codiffusion of aluminum and boron
Authors: Gao Y. , Soloviev S. I. , Sudarshan T. S. , Tin Chin-Che
Published: July 2002

A graphite mask was used to realize selective doping of aluminum/boron in 4H–SiC by thermal diffusion at a temperature range of 1800–2100 °C. The doping profiles investigated by secondary ion mass spectrometry show that a high aluminum concentration of 5×1019 cm−3 near the surface and linearly graded boron profile up to several micrometers in depth can be obtained. Hall effect measurement was also employed to obtain the electrical characteristics of the diffused region, from which the carrier concentration (1×1019 cm−3) and hole mobility (7 cm2/V s) at room temperature were extracted. Room temperature photoluminescence indicates that the dominant luminescence is attributed to the donor acceptor pair recombination, in which boron D complex is the prevailing center rather than Al and boron shallow acceptors. Cathodoluminescence micrographs clearly illustrate a pattern with the locally diffused regions. To confirm the viability of the diffusion process, planar p-n diodes with a fairly low forward voltage drop (3.3 V at 100 A/cm2) and high reverse blocking capability (more than 1100 V) were fabricated. Built-in voltage of 2.9 V, which is typical for 4H–SiC p-n diodes, was obtained by capacitance–voltage measurement.
Effect of Sb as a surfactant during the lateral epitaxial overgrowth of GaN by metalorganic vapor phase epitaxy
Authors: Zhang L. , Tang H. F. , Kuech T. F.
Published: July 2002

Antimony (Sb), an isoelectronic impurity, has been studied as a surfactant during the lateral epitaxial overgrowth (LEO) of gallium nitride (GaN) by metalorganic vapor phase epitaxy (MOVPE). The presence of Sb in the gas phase was found to alter both the LEO growth rates and the predominant facet formations. Vertical facets to the LEO growth appear with the addition of Sb under conditions that normally produce triangular or sloped sidewalls over a range of growth temperatures. While Sb alters the growth facets, only a small amount of Sb was incorporated into the GaN, suggesting that Sb acts as a surfactant during the GaN MOVPE growth. Sb addition produces surface conditions characteristic of a Ga-rich surface stoichiometry indicating both a possible change in the reactivity of NH3 and/or enhanced surface diffusion of Ga adatom species.
A simple positron lifetime spectrometer for a magnetically guided low-energy beam
Authors: Szpala Stanislaw , Petkov Mihail P. , Lynn Kelvin G.
Published: July 2002

We present a new, simple, and inexpensive positron lifetime spectrometer intended for the depth-resolved characterization of thin films and buried interfaces. The spectrometer operates on a conventional magnetically guided positron beam with energies ranging from 1 to ∼50 keV. Given is a detailed description of the performance of the apparatus, built on thorough experimental investigations and computer simulations. A timing resolution of 350±13 ps at full-width-at-half-maximum was obtained. The count rate for thin films (low positron energies) was of the order of 1000 s−1. A maximum peak-to-background ratio>105, aiding the measurements of long-living (10–100 ns) positronium in voids, was achieved by constant beam rate reduction and by beam chopping. Examples are presented for measured lifetimes in well-characterized systems.
Revealing the powdering methods of black makeup in Ancient Egypt by fitting microstructure based Fourier coefficients to the whole x-ray diffraction profiles of galena
Authors: Ungár T. , Martinetto P. , Ribárik G. , Dooryhée E. , Walter Ph. , Anne M.
Published: July 2002

Galena (PbS) is a major ingredient in ancient Egyptian eye makeup. The microstructure of PbS in Egyptian cosmetic powders is used as a fingerprint and is matched with the microstructures produced artificially in geological galena minerals. The microstructure of PbS is determined by x-ray diffraction peak profile analysis in terms of dislocation density, crystallite size, and size distribution. High-resolution powder diffractograms were measured at the ESRF Grenoble synchrotron source with high resolution and high peak-to-background ratios. The Fourier coefficients of the first nine measured reflections of galena are fitted using physically based Fourier coefficients of strain and size functions. Strain anisotropy is accounted for by the dislocation model of the mean square strain. The x-ray data are supplemented by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs, and are compared with archæological documents. It enables us to describe the procedures of eye makeup manufacturing in the Middle and New Kingdoms of Egypt some 2000 years before Christ.
Nonequilibrium effects in supersonic induction plasma
Authors: Selezneva S. E. , Boulos M. I.
Published: July 2002

Supersonic plasma jets find applications in plasma chemistry and plasma processing, metallurgy, experimental physics, and space technology. Usually the plasma in these jets deviates from chemical and thermal equilibrium. To optimize the industrial process detailed study of nonequilibrium effects in supersonic flow is required. In the article we apply numerical simulation to study the supersonically accelerated argon plasma flow downstream of the induction plasma torch. We compare the jets exhausting from two different convergent-divergent nozzles by means of a two-temperature model. The results show that the axial electron number density is rather convective flux controlled than recombination-ionization reaction controlled in both cases. However, the recombination resulting in electron gas heating is more essential in the jet flowing from the nozzle with a higher outlet Mach number. The composition of the jet exhausting from the nozzle with a lower outlet Mach number remains almost unchanged (“frozen”) until the end of the first expansion zone. These results confirm that the chamber pressure and the nozzle design changing leads to the induction plasma jets with different chemical conditions. For low-pressure supersonic plasma, these conditions vary from frozen to recombining. The conclusion is that depending on the industrial process, one can choose the proper torch nozzle geometry to have nonequilibrium plasma with the required properties.
Etching of SiO2 features in fluorocarbon plasmas: Explanation and prediction of gas-phase-composition effects on aspect ratio dependent phenomena in trenches
Authors: Kokkoris George , Gogolides Evangelos , Boudouvis Andreas. G.
Published: July 2002

A model to calculate etching rates in SiO2 features in fluorocarbon plasmas is presented. The model can predict several aspect ratio dependent phenomena such as reactive ion etching (RIE) lag, etch stop, inverse RIE lag, and aspect ratio independent etching (ARIE) at least for a limited range of aspect ratio values. The model includes three components: (a) a surface model for open area etching of SiO2 (and Si) [Gogolides et al., J. Appl. Phys. 88, 5570 (2000)]; (b) a flux calculator, which calculates local fluxes on each elementary surface of the feature being etched; and (c) a coupling of the two models (a) and (b), the focal point of coupling being the simultaneous calculation of the neutral species fluxes and the corresponding effective sticking coefficients. The model is applied for trench etching and the gas phase conditions considered correspond to a generic fluorocarbon gas. A different approach is presented by which the gas phase composition is divided (i.e., mapped) into regions leading to (a) deposition, (b) RIE lag with no etch stop, (c) intense RIE lag and etch stop, (d) inverse RIE lag, and (e) ARIE. Based on the proposed model an explanation of the aspect ratio dependent phenomena and ARIE is attempted, and a comparison with experimental data is done. Two parameters were found to be important in this explanation: the polymer surface coverage at the bottom of the etched feature and the effective sticking coefficients of the neutral species on the sidewalls of the etched feature.
Investigation of exchange bias in FeMnC/FeMn bilayers
Authors: Zhao Hong-Wu , Wang W. N. , Wang Y. J. , Zhan W. S. , Xiao J. Q.
Published: July 2002

The exchange bias (EB) effect and magnetic domain structures of FeMnC/FeMn bilayers prepared by a facing target sputtering system were studied. Unusual doubly shifted hysteresis loops were observed in a series of FeMnC/FeMn bilayers when different magnetic fields were applied to induce the exchange bias. The temperature dependences of the double shifted loops were measured and the ferromagnetic resonance measurement reveals the existence of EB. The correlation between the microscopic domain structures and the magnetization reversal processes was discussed, which suggests that the domain structure distribution of the antiferromagnetic layer is responsible for the loop shift.
Noninvasive imaging of signals in digital circuits
Authors: Gebrial W. , Prance R. J. , Clark T. D. , Harland C. J. , Prance H. , Everitt M.
Published: July 2002

In this article we describe the construction and use of a noninvasive (noncontact) electric potential probe to measure time delays of signals propagating through digital circuits. As we show, by incorporating such probes into a scanning microscope system we have been able to create time delay images of these signals. We suggest that future developments of this technique may lead to real time, high resolution imaging of digital pulses across complex very large scale integrated circuits.
Characterization of the hot electron distribution function using six moments
Authors: Grasser T. , Kosina H. , Heitzinger C. , Selberherr S.
Published: July 2002

The shape of the hot electron distribution function in semiconductor devices is insufficiently described using only the first four moments. We propose using six moments of the distribution function to obtain a more accurate description of hot carrier phenomena. An analytic expression for the symmetric part of the distribution function as a function of the even moments is given which shows good agreement with Monte Carlo data for both the bulk case and inside n+−n−n+ test structures. The influence of the band structure on the parameters of the distribution function is studied and proven to be of importance for an accurate description.
Magnetic properties of chromium oxide and iron oxide films produced by pulsed laser deposition
Authors: Shima M. , Tepper T. , Ross C. A.
Published: July 2002

Chromium oxide (CrOX) and iron oxide (FeOX) films have been grown onto various substrates using pulsed laser deposition. CrOX films grown onto Si and LaAlO3 substrates at 390 °C with a laser beam of 450 mJ/pulse at 20 Hz are found to exhibit a (200) peak in the x-ray diffraction spectra corresponding to the rutile structure of CrO2. X-ray and scanning electron microscopy data indicate that the films are polycrystalline with an average grain size of 20 nm. Films grown at higher laser energies show the (006) peak of the antiferromagnetic Cr2O3 phase, and no strong x-ray peaks were observed from the films made at lower laser energies or lower growth temperatures. FeOX films deposited in vacuum at 400 °C with a laser beam of 500 mJ/pulse at 25 Hz are in the ferrimagnetic γ-Fe2O3 phase with a saturation magnetization of 371 kA/m, while the films deposited with O2 gas consist of the antiferromagnetic α-Fe2O3 phase. The γ-Fe2O3 phase is retained on Si substrates even at film thicknesses of 520 nm.
Granular composites containing “micro-onions,” permeability, and permittivity calculated for application to microwave absorbers
Authors: Abe M. , Kuroda J. , Matsumoto M.
Published: July 2002

The formula for the effective permeability 〈μ〉 (or permittivity 〈ε〉) is derived, based on the Maxwell Garnett approximation, for the granular composites having “micro-onions” (micron-meter-sized, multifold core-shell structured particles) dispersed in matrices. We calculated 〈μ〉 and 〈ε〉 up to 3 GHz for the composites in which micro-onions of threefold [i.e., air/(silica+air)/NiZn-ferrite or water/(silica+water)/NiZn-ferrite] structure are dispersed in a Fe/silicon-rubber matrix. Introducing the micro-onions into the Fe/silicon-rubber matrix improves the wave absorber characteristics, increasing absorption and decreasing reflection, at 0.5–3 GHz. Confining the magnetic substance to the outermost shell layers increases the natural ferromagnetic resonance frequency, thus enhancing permeability at high frequencies. It also facilitates fabrication of lightweight electromagnetic wave absorbers, and suppresses eddy current loss in case the magnetic substance is a metal.
Transport-to-quantum lifetime ratios in AlGaN/GaN heterostructures
Authors: Hsu L. , Walukiewicz W.
Published: July 2002

We have calculated ratios of the transport-to-quantum lifetimes τt/τq of the two-dimensional electron gas in AlGaN/GaN heterostructures at low temperatures. In contrast to conventional interpretations, we show that large values of this ratio do not necessarily indicate that long-range scattering mechanisms such as Coulomb scattering are the dominant carrier scattering mechanisms and that large ratios (>20) can be obtained even when short-range scattering mechanisms are dominant.
Solvation forces and liquid–solid phase equilibria for water confined between hydrophobic surfaces
Authors: Koga Kenichiro
Published: July 2002

Solvation force and phase behavior of water confined between hydrophobic surfaces at nanoscale distances have been studied by molecular dynamics simulation of the TIP4P model water. Freezing and melting of confined water are observed at certain intersurface separations in bringing one surface to the other at a fixed temperature and a fixed lateral or bulk pressure. Solvation force curves are found to be discontinuous upon freezing and melting of confined water and exhibit strong hystereses, implying a peculiar manifestation of the hydrophobic effect. The thermodynamics for a confined system at fixed surface separation, temperature, and lateral or bulk pressure is applied for examining the liquid–solid equilibria of confined water.
Mechanics of nanosprings: Stiffness and Young’s modulus of molybdenum-based nanocrystals
Authors: Durkan C. , Ilie A. , Saifullah M. S. M. , Welland M. E.
Published: July 2002

We describe measurements of the stiffness and Young’s modulus, Y of single crystals of molybdenum-based compounds. Atomic force microscopy is used first to image, and then to perform stiffness measurements on crystals which are growing up out of a substrate. Y is extracted by comparing the measured stiffness with that calculated from a continuum elasticity model of the crystals, whose experimentally indeterminate parameters are the Young’s modulus and the geometry. We find a value for Y in the range 0.8–1.4 TPa, depending on the assumed geometry of the nanocrystal. As these crystals are essentially perfectly ordered on the micron scale, this opens the possibility of forming composite materials of immense strength.
Chaos suppression in gas-solid fluidization
Authors: Pence Deborah V. , Beasley Donald E.
Published: July 2002

Fluidization in granular materials occurs primarily as a result of a dynamic balance between gravitational forces and forces resulting from the flow of a fluid through a bed of discrete particles. For systems where the fluidizing medium and the particles have significantly different densities, density wave instabilities create local pockets of very high void fraction termed bubbles. The fluidization regime is termed the bubbling regime. Such a system is appropriately termed a self-excited nonlinear system. The present study examines chaos suppression resulting from an opposing oscillatory flow in gas-solid fluidization. Time series data representing local, instantaneous pressure were acquired at the surface of a horizontal cylinder submerged in a bubbling fluidized bed. The particles had a weight mean diameter of 345 μm and a narrow size distribution. The state of fluidization corresponded to the bubbling regime and total air flow rates employed in the present study ranged from 10% to 40% greater than that required for minimum fluidization. The behavior of time-varying local pressure in fluidized beds in the absence of a secondary flow is consistent with deterministic chaos. Kolmogorov entropy estimates from local, instantaneous pressure suggest that the degree of chaotic behavior can be substantially suppressed by the presence of an opposing, oscillatory secondary flow. Pressure signals clearly show a “phase-locking” phenomenon coincident with the imposed frequency. In the present study, the greatest degree of suppression occurred for operating conditions with low primary and secondary flow rates, and a secondary flow oscillation frequency of 15 Hz.
Confronting students’ alternative conceptions in mechanics with the Force and Motion Microworld
Authors: Tao Ping-Kee
Published: July 2002

This article describes the development of a suite of four computer simulation programs, collectively called the Force and Motion Microworld (FMM), that was designed to confront some high school students’ alternative conceptions in mechanics. The first program, Motion Graphs, aims at facilitating students’ understanding of velocity-time graphs as representations of motion. The other programs, Spaceship, Model Car, and Skydiver, provide three different contexts in which the effects of force on motion can be explored. Students work in pairs or small groups on the programs carrying out predict-observe-explain (POE) tasks from a set of worksheets. The POE tasks provide discrepant events that contradict students’ alternative conceptions. FMM was originally used as a tool for a classroom study into the process of students’ conceptual change. The findings show that FMM may also be offered as a supplement or alternative to other instructional tools for facilitating students’ understanding in force and motion. © 1997 American Institute of Physics.
Low field magnetic anisotropy in Metglas 2605 CO ribbons
Authors: Hayes Guy H. , Hines William A. , Yang De Ping , Budnick Joseph I.
Published: July 2002

In order to investigate the magnetic anisotropy, low field magnetization measurements have been carried out on various samples of magnetically annealed Metglas 2605 CO. The as-fabricated material exhibits a magnetic anisotropy with the easy axis along the ribbon length. By using the magnetic annealing treatment which yields an enhanced value for the magnetomechanical coupling factor, a magnetic anisotropy is induced with the easy axis in the plane of the ribbon and perpendicular to the length. A value of K=4.3×103 erg/cm3 is calculated for the induced anisotropy energy constant at room temperature. Using a model based on simple domain rotation, a value of λs=25×10−6 is estimated for the corresponding saturation magnetostriction. The anisotropy increases 37% when the temperature is reduced to 77 °K. Values are presented for the saturation magnetization, domain magnetization, coercive field, and initial permeability.
Aspects of electrode erosion in high-power vortex stabilized arc lamps
Authors: Walker I. R. , Curzon F. L. , Dindo S. F.
Published: July 2002

Electrode erosion (especially anode erosion) is still an important problem for high-power vortex stabilized arc lamps. A study of the electrode erosion mechanisms has therefore been carried out in vortex stabilized arcs operating at a maximum input power of 40 kW. It was found that three factors are important in the removal of electrode material: evaporation caused by thermal loading, ablation due to time varying stresses, and corrosion resulting from the presence of impurities in the gas. It was also found that the first two factors, and hence the erosion rates, can be affected significantly by the orientation of the electrodes and the presence of an external magnetic field.
Silicon-to-silicon direct bonding method
Authors: Shimbo M. , Furukawa K. , Fukuda K. , Tanzawa K.
Published: July 2002

It was found that strong bonding takes place when a pair of clean, mirror-polished silicon surfaces are contacted at room temperature after hydrophilic surface formation. Bonding strength reaches the fracture strength of silicon bulk after heating above 1000 °C. Electric resistivity at the interface is less than 10−6 Ω/cm2. Bonding p-type silicon to n-type silicon forms a diode. The reaction between silanol groups formed on the surface may cause the bonding force. Heating above 1000 °C was thought to diffuse oxygen to inside the silicon bulk, forming an epitaxial-like lattice continuity at the interface.
Thermomechanical properties of amorphous silicon and nonstoichiometric silicon oxide films
Authors: Jansen F. , Machonkin M. A. , Palmieri N. , Kuhman D.
Published: July 2002

The thermomechanical properties of glow-discharge-deposited silicon and silicon oxide films were measured between room temperature and 100 °C as a function of composition and substrate temperature during deposition. A cantilevered beam measurement technique, allowing the simultaneous measurement of the linear thermal expansion coefficient of thin films and their mechanical compliance, was used and verified. Hydrogenated amorphous silicon films, deposited at 250 °C with a density of 2.0 g/cm3 and 20 at. % of hydrogen, exhibit a linear thermal expansion coefficient of about 4.4×10−6/°C and a biaxial elastic modulus of 150 GPa. The expansion coefficient of silicon oxide films deposited at 250 °C shows a systematic dependence on the fabrication conditions and ranges from about 10−5/°C to negative values. Strong correlations between the hydrogen concentration of the films, the film density, and thermal expansion coefficient were observed and are discussed. The biaxial elastic modulus of the oxide films is not strongly dependent on the gas ratio and is about 40 GPa, substantially smaller than the values obtained for thermally grown oxides. These differences are attributed to the effect of network terminations by hydrogen and –OH groups. Films of fixed composition (nitrous oxide/silane=12), deposited at substrate temperatures below 200 °C, exhibit an apparent negative coefficient of thermal expansion when first heated. However, this contraction has been determined to be due to the expulsion of water vapor, causing these films to densify. The true thermal expansion coefficients, measured after annealing at 100 °C for extended periods, are positive and do not change within experimental error for this fixed composition over the range of deposition temperatures.
Fluid model simulations of a 13.56-MHz rf discharge: Time and space dependence of rates of electron impact excitation
Authors: Graves David B.
Published: July 2002

Results from fluid model simulations of a 13.56-MHz rf discharge qualitatively reproduce many of the experimentally observed features of time and space resolved electron impact excitation in several gases (oxygen, nitrogen, and silane). The shape of the excitation rate waveform, its direction of propagation, time of occurrence in the rf period, and the initial increase in intensity followed by attenuation are all observed experimentally as well as in the simulation. Secondary electrons play no direct role in the excitation waveform in the simulation, although secondary electron creation is included in the model. The excitation waveform is the result of the combination of electron motion, electric field profiles, and the electron energy balance in the discharge. Electron heating by the rf field peaks at the plasma-sheath boundary, resulting in a local rise in electron mean energy there. It is suggested that this electron heating mechanism is common to all high-frequency rf discharges in electropositive or weakly electronegative gases, and that this mechanism is responsible for increased rates of electron impact molecular dissociation around the plasma-sheath boundary. The qualitative agreement between simulation results and experimental measurements implies that the fluid model captures essential elements of rf discharge physics, and that fluid models are useful for the interpretation of discharge diagnostics.
Silicon-silicon dioxide interface: An infrared study
Authors: Boyd Ian W. , Wilson John I. B.
Published: July 2002

We have investigated, using differential infrared spectroscopy, the transition region between single-crystal silicon and its natural thermally grown oxide. By monitoring the thickness-dependent behavior of the stretching mode of the Si–O bond near 1075 cm−1, we isolate bulk oxide characteristics and features arising from interface constraints, finding that films up to 100 Å are still affected by interface effects. For thicker films we observe a consistent degree of asymmetry in the Si-O absorption band of about 9 cm−1, which does not exhibit a strong dependence on thickness. By contrast, the peak position, width, and degree of symmetry are found to be sensitively dependent upon film thickness below 100 Å, providing evidence for structurally distinct phases of silicon dioxide. Our initial interpretation suggests that the infrared spectra of such thin layers may be significantly affected by strain originating at the Si-oxide interface, rather than oxygen deficiency or overstoichiometry.
A novel algorithm for calculating the time evolution of the electron energy distribution function in gaseous discharges
Authors: Drallos P. J. , Wadehra J. M.
Published: July 2002

We are presenting a novel numerical technique for obtaining the time evolution of the electron velocity and electron energy distribution functions in the presence of a uniform electric field. Using this technique, the various swarm parameters can be evolved for sufficiently long times so that equilibrium can be reached without incurring any numerical instabilities. Results are presented for electron swarms in gaseous neon for various values of E/N.
Current transport in strained n-Si1−xGex/p-Si heterojunction diodes
Authors: Willander Magnus , Shen Guang-Di , Xu Dan-Xia , Ni Wei-Xin
Published: July 2002

The measurement and analysis of I-V characteristics from molecular beam epitaxially grown Si0.9Ge0.1/Si p-n diodes are presented. When a diode is forward biased the current transport is mainly injection diffusion of the holes and defect recombination within the depletion region of the n side. At reverse bias, the current comes from tunneling of holes from the top of the p-type valence band to the n-type conduction band, and also to the interface states followed by multistep recombination tunneling via defect states in the depletion region of the n side. The forward voltage drop was found to be much lower than that of Si diodes. Electron irradiation damages have opposite effects on the forward-biased I-V characteristics of Si0.9Ge0.1/Si diodes as compared to the effects observed for Si diodes.
Characterization of tungsten-related deep levels in bulk silicon crystal
Authors: Fujisaki Yoshihisa , Ando Toshio , Kozuka Hirotsugu , Takano Yukio
Published: July 2002

Tungsten-related deep levels in a silicon crystal are investigated using microwave and conventional deep level transient spectroscopy. It is found that tungsten atoms have two diffusion constants. Most tungsten atoms stay near the surface of Si crystal, but the remainder go deeper than 1 μm from the surface. It is also found that tungsten atoms make both a hole trap of 0.41 eV and an electron trap of 0.22 eV. Device characteristics are found to be seriously degraded due to deep levels made of deeply diffused tungsten atoms.
Effective thermal conductivity of a periodic composite with contact resistance
Authors: Gu Guo-Qing , Tao Ruibao
Published: July 2002

Periodic composite materials with contact resistance on the inclusion surface are studied. A generalized Rayleigh identity, which is applicable to such systems, is derived. Calculations are made for composites containing a simple cubic array of spherical inclusions. These solutions are derived by applying the generalized Rayleigh identity. A definition of the effective conductivity suitable for systems with a discontinuity of the temperature field on surface of inclusions is suggested, and it produced reasonable effective conductivities over all the possible range of Biot’s number. It turned out that the contact resistance can change the effective conductivity dramatically and is rich in its effects on the thermal properties of composites.
Numerical analysis of infrared laser heating in thermoluminescent material layers
Authors: Grupen M. , Kearfott K.
Published: July 2002

The response of thermoluminescent (TL) materials to infrared laser stimulation has been observed but cannot be fully characterized experimentally due to the inability of modern measuring techniques to accurately record the rapidly changing temperatures. A two-dimensional, unsteady-state model has been derived to determine radial and axial temperature profiles in a TL layer when a 4 W CO2 Gaussian laser beam is used to heat a single or multiple spots. The numerical model is solved using the alternating direction implicit scheme on a Convex XP2 mini-supercomputer. The results for LiF plates (0.5–0.1 mm thick) demonstrate that detailed temperature profiles and heating rates may be determined for any number of thermally stimulated regions. As a result, the feasibility of a scanning laser heated TL radiographic imaging system may be evaluated and spatial resolution of such a system may be predicted.
The Meyer–Neldel rule and the statistical shift of the Fermi level in amorphous semiconductors
Authors: Kikuchi Minoru
Published: July 2002

The statistical model is used to study the origin of the Meyer–Neldel (MN) rule [σ0∝exp(AEσ)] in a tetrahedral amorphous system. It is shown that a deep minimum in the gap density of states spectrum can lead to the linearity of the Fermi energy F(T) to the derivative (dF/dkT), as required from the rule. An expression is derived which relates the constant A in the rule to the gap density of states spectrum. The dispersion ranges of σ0 and Eσ are found to be related with the constant A. Model calculations show a magnitude of A and a wide dispersion of σ0 and Eσ in fair agreement with the experimental observations. A discussion is given to what extent the MN rule is dependent on the gap density of states spectrum.
Practical approach to ion track energy distribution
Authors: Stapor W. J. , McDonald P. T.
Published: July 2002

A practical approach to the problem of ion track radial energy distribution has been developed in a computer calculation. This approach uses an existing theory that analytically describes the spatial energy deposition by delta rays. The results agree favorably with a Monte Carlo calculation that is more realistic but much less practical. Several cases of incident ions in silicon have been calculated and results presented. The information is useful as input to problems requiring initial track structure for charge transport in semiconductor applications.
High-quality optical and epitaxial Ge films formed by laser evaporation
Authors: Sankur H. , Gunning W. J. , DeNatale J. , Flintoff J. F.
Published: July 2002

High-quality thin films of Ge were deposited by pulsed laser evaporation of molten Ge. Films deposited on 300 °C substrates showed very smooth morphologies and single-crystal grain structures. Energetic ions in the vapor stream, generated by the laser-induced plasma, were observed to affect nucleation and bulk-film growth. Films deposited on ambient temperature substrates by laser evaporation were denser, harder, and exhibited higher values of refractive index and lower levels of intrinsic stress than the films deposited by thermal evaporation.
Computer simulation results for bounds on the effective conductivity of composite media
Authors: Smith P. A. , Torquato S.
Published: July 2002

This paper studies the determination of third- and fourth-order bounds on the effective conductivity σe of a composite material composed of aligned, infinitely long, identical, partially penetrable, circular cylinders of conductivity σ2 randomly distributed throughout a matrix of conductivity σ1. Both bounds involve the microstructural parameter ζ2 which is a multifold integral that depends upon S3, the three-point probability function of the composite. This key integral ζ2 is computed (for the possible range of cylinder volume fraction φ2) using a Monte Carlo simulation technique for the penetrable-concentric-shell model in which cylinders are distributed with an arbitrary degree of impenetrability λ, 0≤λ≤1. Results for the limiting cases λ=0 (‘‘fully penetrable’’ or randomly centered cylinders) and λ=1 (‘‘totally impenetrable’’ cylinders) compare very favorably with theoretical predictions made by Torquato and Beasley [Int. J. Eng. Sci. 24, 415 (1986)] and by Torquato and Lado [Proc. R. Soc. London Ser. A 417, 59 (1988)], respectively. Results are also reported for intermediate values of λ: cases which heretofore have not been examined. For a wide range of α=σ2/σ1 (conductivity ratio) and φ2, the third-order bounds on σe significantly improve upon second-order bounds which just depend upon φ2. The fourth-order bounds are, in turn, narrower than the third-order bounds. Moreover, when the cylinders are highly conducting (α≫1), the fourth-order lower bound provides an excellent estimate of the effective conductivity for a wide range of volume fractions.
The effect of electrode area ratio on low-frequency glow discharges
Authors: Gottscho Richard A. , Scheller Geoffrey R. , Stoneback David , Intrator Thomas
Published: July 2002

In many radio frequency plasma applications, use of unequal area electrodes leads to a dc self-bias voltage that is correlated with sputtering, etching, deposition, and surface damage rates. Using the techniques of laser-induced fluorescence (LIF) and Stark-mixed LIF, we measure (1) ion densities in N2 discharges and (2) electric fields in BCl3 and 5% BCl3/Ar discharges as a function of electrode area ratio (from 1:1 to 7.4:1). We find that the discharge current density is constricted and nonuniform. As a consequence, the dependence of sheath field strengths on electrode area ratio is weaker than expected, ion bombardment of the larger electrode is nonuniform, and large radial electric fields periodically pull ions into the small electrode sheath. These observations are consistent with ballistic (or beam) electron models for low-frequency glow discharges.
Analysis of the dark current in doped-well multiple quantum well AlGaAs infrared photodetectors
Authors: Pelvé E. , Beltram F. , Bethea C. G. , Levine B. F. , Shen V. O. , Hsieh S. J. , Abbott R. R.
Published: July 2002

We have studied the origin of the dark current in λ=8 μm peak wavelength 40-Å GaAs/300-Å Al0.3 Ga0.7 As multiple quantum well photodetectors. By carefully controlling the experimental conditions we have measured the dark current for temperatures in the range 4–120 K. The experimental characteristics are explained by thermionic emission for low biases and tunneling for high biases.
Electrically active defects in silicon after excimer laser processing
Authors: Hartiti B. , Slaoui A. , Muller J. C. , Siffert P.
Published: July 2002

This work is the first which clearly demonstrates, by deep-level transient spectroscopy analysis in a Schottky structure, the presence of excimer laser-related defects in both n- and p-type virgin monocrystalline silicon. The detected point defects are correlated to those observed after solid-state laser (ruby, YAG) annealing in silicon. The main part of these traps could be attributed to the melt-related defects produced by the fast quenching rate in pulsed laser treatment.
Magnetic properties, compositions, and microstructures of high-energy product strontium hexaferrites
Authors: Thompson G. K. , Evans B. J.
Published: July 2002

Commercial, high-energy product strontium hexaferrites have been characterized in terms of their chemical compositions, microstructures, and bulk and submicroscopic magnetic properties. Compared to earlier permanent magnetic hexaferrites, these materials have a unique composition, containing no elements other than Sr and Fe at appreciable concentrations; dopants for nucleation and microstructure control are apparently absent. The microstructures indicate that, in some cases, the materials may be fully reacted prior to forming the final ceramic bodies. The compositions are variable and tend to be nonstoichiometric despite the absence of impurity elements. These deviations from stoichiometry are reflected in the Mössbauer spectra which indicate that further refinements can be made in the definition of the different magnetic sublattices. These refinements may lead to further advances in the magnetic properties.
Gate dielectric-dependent flicker noise in metal-oxide-semiconductor transistors
Authors: Wong H. , Cheng Y. C.
Published: July 2002

The paper presents low-frequency (flicker) noise measurements on n-channel metal-oxide-semiconductor transistors with TCE (trichoroethylene), dry, thermally nitrided, or reoxidized nitrided oxides as gate dielectrics. Results show that the noise level is lowest for conventional silicon oxide with TCE treatment and highest for thermally nitrided oxide. The frequency index (γ), which characterize a least-square fit of the noise spectrum in the frequency range of 10 Hz–5 kHz to the spectrum (∝f−γ), is not a constant, but fluctuates between 0.8 and 1.4 with the gate bias. In addition a trend is found that the higher the interface-state density at the midgap of silicon, the higher the noise level. But these two quantities are not in direct proportion to each other especially for the nitrided oxide device. All these observations lead to a unique implication: in addition to interface states, the oxide traps also contribute to the flicker noise generation. Our new observations also lend support to the past speculation that the carrier number fluctuation in the conduction channel is responsible for the noise generation. However, not all the noise parameters in the present study agree well with the general predictions of the existing flicker noise models.
Investigation of resonance and excimer radiation from a dielectric barrier discharge in mixtures of mercury and the rare gases
Authors: Eliasson B. , Gellert B.
Published: July 2002

The excitation of resonance and excimer radiation in mixtures of mercury and the rare gases has been investigated. The gas mixtures were excited in a dielectric barrier discharge. The emission features of such discharges which are mostly in the UV and VUV range were studied in detail. Whereas resonance radiation dominates in mixtures of mercury with He, Ne, and Ar, efficient excimer formation takes place in mixtures of mercury with Kr and Xe. All the emission features of the HgXe excimer which are expected between and around the resonance lines of mercury were found and measured. A theoretical model of HgXe excimer formation is presented. This model takes both the discharge formation and the charged particle kinetics as well as the excimer and excited species formation into account. Experimentally determined Boltzmann temperatures show a decline with mercury density as is predicted by the theory for the average electron temperature. Furthermore it is, e.g., possible to calculate the efficiency of the generation of the various species observed. Due to the geometrical flexibility and high obtainable power levels, dielectric barrier discharges are very well suited for the generation of UV radiation. Apart from well-established techniques like UV lithography, disinfection, or UV curing, also new techniques in photochemistry such as, e.g., photochemical vapor deposition, profit from these new sources.
A criterion for arrest of a threading dislocation in a strained epitaxial layer due to an interface misfit dislocation in its path
Authors: Freund L. B.
Published: July 2002

In a strained layer grown epitaxially on a substrate, the motion of a dislocation on any particular glide plane in the layer can be influenced by the presence of dislocations on other glide planes. The focus here is on the glide of a dislocation extending from the free surface of the layer to the layer-substrate interface, the so-called threading dislocation. A general definition of driving force for glide of a threading dislocation in a nonuniform stress field is adopted to calculate the driving force on a threading dislocation due to an encounter with an interface misfit dislocation on an intersecting glide plane. The result is examined in detail for the case of cubic materials, taking into account different combinations of Burgers vectors. The analysis makes it clear that the misfit dislocation forces the threading dislocation to glide through a channel of width less than the full layer thickness. A blocking criterion is proposed, based on the presumption that blocking will occur if the channel width is less than the critical thickness for the local reduced strain. The results indicate that this effect can be significant in blocking the glide of a threading dislocation, depending on the mismatch strain magnitude and the layer thickness.
Analysis of current spreading, carrier diffusion, and transverse mode guiding in surface emitting lasers
Authors: Dutta N. K.
Published: July 2002

A model for current spreading and carrier distribution in the active region of a gain-guided or weakly index-guided surface emitting laser is presented. At high currents, current ‘‘crowding’’ takes place which reduces the amount of carrier spreading beyond the contact and hence can help stabilize the fundamental mode oscillation. The fundamental mode is a Gaussian whose width varies approximately as the square root of the contact radius.