bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of fronts separating domains with different symmetries in hydrodynamical instabilities
Authors: Herrero Henar , Pérez-García Carlos , Bestehorn Michael
Published: July 2002

A generalization of the Swift–Hohenberg (SH) equation is used to study several stationary patterns that appear in hydrodynamical instabilities. The corresponding amplitude equations allow one to find the stability of planforms with different symmetries. These results are compared with numerical simulations of a generalized SH equation (GSHE). The transition between different symmetries, the hysteretic effects, and the characteristics of the defects observed in experiments are well reproduced in these simulations. The existence of steady fronts between domains with different symmetries is also analyzed. Steady domain boundaries between hexagons and rolls, and between hexagons and squares are possible solutions in the amplitude equation framework and are obtained in numerical simulations for a full range of coefficients in the GSHE.
On the Riemannian description of chaotic instability in Hamiltonian dynamics
Authors: Pettini Marco , Valdettaro Riccardo
Published: July 2002

In this work we investigate Hamiltonian chaos using elementary Riemannian geometry. This is possible because the trajectories of a standard Hamiltonian system (i.e., having a quadratic kinetic energy term) can be seen as geodesics of the configuration space manifold equipped with the standard Jacobi metric. The stability of the dynamics is tackled with the Jacobi–Levi-Civita equation (JLCE) for geodesic spread and is applied to the case of a two degrees of freedom Hamiltonian. A detailed comparison is made among the qualitative informations given by Poincaré sections and the results of the geometric investigation. Complete agreement is found. The solutions of the JLCE are also in quantitative agreement with the solutions of the tangent dynamics equation. The configuration space manifold associated to the Hamiltonian studied here is everywhere of positive curvature. However, curvature is not constant and its fluctuations along the geodesics can yield parametric instability of the trajectories, thus chaos. This mechanism seems to be one of the most effective sources of chaotic instabilities in Hamiltonians of physical interest, and makes a major difference with Anosov flows, and, in general, with abstract geodesic flows of ergodic theory.
Temperature dependence of coercivity of epitaxial magnetic garnet films
Authors: Pardavi-Horváth M. , Vértesy G.
Published: July 2002

The temperature dependence of the coercivity Hc, magnetization, anisotropy, and domain wall energy was measured for SmGaYIG, SmCaGeYIG, and SmLuCaGeYIG epitaxial bubble garnet films from 100 K up to the Néel temperature TN≳400 K. Data were analyzed together with results published on similar samples on the basis of the theory of domain wall pinning by a random array of uniform defects and a good fit was observed in the form of H1/2c=H1/20 −CT2/3, where H0 and C are constants. The maximum average interaction force between defects and walls is 2.9×10−7 dyne, corresponding to the minimum interaction range equal to the wall width. The density of defects is in the range between 1014 and 1016 cm−3. The size and number of defects derived from the temperature dependence of Hc is in agreement with the microstructure of epitaxial garnets revealed by transmission electron microscopy.
On the long-range order in AlGaAs
Authors: Wang W. I.
Published: July 2002

A charge transfer dipole stabilization model is proposed to explain the recently observed long-range order in AlGaAs alloys. The connection between the large valence band offset of AlAs/GaAs and AlGaAs alloy ordering is pointed out.
Kinetics and moving species during Co2Si formation by rapid thermal annealing
Authors: Lim B. S. , Ma E. , Nicolet M-A. , Natan M.
Published: July 2002

We have investigated the growth kinetics and identified the moving species during Co2Si formation by rapid thermal annealing (RTA). For the kinetics study, samples which consisted of a thin Co film on an evaporated Si substrate were used. To study which species moves, samples imbedded with two very thin Ta markers were employed. Upon RTA, only one silicide phase, Co2Si, was observed to grow before all Co was consumed. The square root of time dependence and the activation energy of about 2.1±0.2 eV were observed during the Co2Si formation up to 680 °C. The marker study indicated that Co is the dominant mobile species during Co2Si formation by RTA. We conclude that Co2Si grows by the same mechanisms during RTA and conventional thermal annealing.
Nonlinear intersubband optical absorption in a semiconductor quantum well
Authors: Ahn D. , Chuang S. L.
Published: July 2002

The third-order nonlinear intersubband absorption in a semiconductor quantum well is studied theoretically using the density matrix formalism including intrasubband relaxation. It is shown that the peak absorption is reduced by half for an optical intensity 1 MW/cm2 for the well size L=126.5 Å with 3.0×1016/cm3 electrons.
Thermodynamic simulation of a rotating Ericsson-cycle magnetic refrigerator without a regenerator
Authors: Hakuraku Y.
Published: July 2002

A magnetic Ericsson cycle, which consists of two isothermal stages and two isofield stages, is generally thought to require regenerators. However, a new concept makes it possible to realize magnetic refrigerators capable of executing an Ericsson-cycle without using regenerators. The basic principle lies in directly linking the two isofield changes by transferring heat between the isofield stages through heat paths rather than through regenerators. A fundamental configuration is proposed for a rotating magnetic refrigerator that operates based on this concept. A simulation of the thermodynamic cycle in this simplified refrigerator model shows that the system is theoretically feasible.
Shock waves in water
Authors: Ridah S.
Published: July 2002

The present paper gives a comprehensive examination of equations and charts for shock waves in water. The Tait equation of state is used in the analysis to describe the compressibility of water. The results for an oblique shock wave are presented in the hodograph plane where all possible velocity vectors downstream of the shock wave are seen to lay on a single curve (i.e., shock polar). The equation of the shock polar and the corresponding charts up to upstream Mach number of 3 are given, as well as a more convenient way of adapting the shock polar. The oblique shock-wave detachment condition and the sonic flow condition downstream of a shock are also presented.
Memory switching in bismuth-vanadate glasses
Authors: Ghosh Aswini
Published: July 2002

The electrical V-I characteristics of thin blown films of V2O5-Bi2O3 glasses with 95–70 mol % V2O5 were studied in the temperature range 200–400 K. It was observed that at lower fields, the bulk resistance controlled the current. At higher fields, all the glass compositions showed memory switching characteristics. The decrease in the threshold voltage and increase in the threshold current with the increase of V2O5 content in the glasses and also with increasing temperature were observed. The switching action was associated with a phase transition from a disordered glassy state to an ordered devitrified state due to self-heating. The ideal thermal model was shown to be applicable to the present glasses.
Deep electron traps in GaAs/n-AlxGa1−xAs single-quantum wells
Authors: As D. J. , Epperlein P. W. , Mooney P. M.
Published: July 2002

Deep-level-transient spectroscopy on molecular-beam epitaxially grown square GaAs/ n-AlxGa1−x As (x=0.24–0.39) single-quantum wells shows a series of electron traps in the AlGaAs with energies EC−ET at 0.12, 0.22, 0.29, 0.52, and 0.63 eV and with concentrations of about 5×1015 cm−3. The defects are located in the upper AlGaAs layer near the GaAs well layer. The trap concentrations and the widths of the spatial trap distributions (typically 15 nm) are independent of the well width. For all traps, a nonexponential capture process which is logarithmic in time is observed. Time-dependent depth profiling shows a virtual shift of the trap distribution to the surface for shorter filling pulses. Both effects are due to the nonabrupt depletion edge (Debye tail). No direct emission from the quantum wells is observed.
Localized stimulation of neural tissues in the brain by means of a paired configuration of time-varying magnetic fields
Authors: Ueno S. , Tashiro T. , Harada K.
Published: July 2002

A method of localized stimulation of the human brain is proposed. The basic idea is to concentrate induced eddy currents locally in the vicinity of a target in the cortex by a pair of coils which are positioned outside the head so that time-varying magnetic fields pass through the head in the opposite directions around a target. The eddy currents induced at the target are expected to flow together, which results in an increased current flow at the target. Spatial distributions of induced eddy currents are calculated in cubical and spherical volume conductor models by a finite element method. The results show that the current vectors make themselves two vortexes which flow together at the target. The current density at the target makes a peak which is higher by 2–3 times than current densities at nontarget regions. The validity of the proposed method is demonstrated by experiments using frog nerve-muscle preparations.
Initial epitaxial growth of copper silicide on Si{111} studied by low-energy electron microscopy and photoemission electron microscopy
Authors: Mundschau M. , Bauer E. , Telieps W. , Świȩch W.
Published: July 2002

The epitaxial growth, including submonolayer epitaxial growth, of Cu silicide on Si{111} of standard wafer thickness is studied in situ by low-energy electron microscopy and photoemission electron microscopy. At elevated temperatures up to about 850 K copper silicide grows as a two-dimensional (2D) layer with a ‘‘(5×5)’’ structure followed by epitaxy of three-dimensional (3D) Cu3Si crystals. Atomic steps initially present on the wafer surface bunch into groups of 2 and 3 and facet along the Si〈110〉 directions. Except for steps, monolayer silicide films appear defect- and pinhole-free down to the resolution limit. Films grown by depositing Cu onto substrates held at room temperature followed by annealing differ in morphology and have 500 times the number of 3D islands as do films grown at the annealing temperature. Above 850 K the Cu dissolves into the bulk. Upon cooling Cu segregates to the surface as a 2D silicide with a ‘‘(5×5)’’ structure.
Formation of polyhedral N2 bubbles during reactive sputter deposition of epitaxial TiN(100) films
Authors: Hultman L. , Sundgren J.-E. , Greene J. E.
Published: July 2002

Cross-sectional transmission electron microscopy has been used to investigate the formation of polyhedral N2 bubbles in single-crystal TiN(100). Thin TiN films were deposited on MgO(100) substrates by reactive magnetron sputtering in pure N2 atmospheres at growth temperatures Ts between 550 and 800 °C and negative substrate biases Vs between 0 and 500 V resulting in a deposition rate between 0.35 and 0.33 nm s−1. The energy per incident nitrogen was ∼eVs/2 and the irradiation fluxes were ≤2.5×1015 N+2 cm−2 s−1. Nitrogen bubbles were observed in films grown at 550≤Ts<800 °C with Vs>300 V. In as-deposited films grown at Ts=650 °C with Vs=500 V, the bubbles were distributed in two shapes and size distributions. The largest bubbles were orthorhombohedra, i.e., cubes elongated along [100] directions normal to the growth direction, with 5–20 nm faces on {100} planes most of which exhibited {110} and {111} faceting. Small (≤5 nm) octahedral bubbles with {111} faces were also observed. In situ annealing experiments in the transmission electron microscope showed that the bubbles transformed from orthorhombohedra to octahedra at Ts=350 °C and then returned to orthorhombohedral shape with either continued heating to Ts=600 °C or upon cooling to room temperature. Assuming quasiequilibrium conditions, the surface energy γ of N2 bubbles in TiN was found to be anisotropic and to vary as γ100<γ110<γ111. Some of the smaller orthorhombohedral bubbles exhibited Moiré fringes with a spacing indicative of solid-phase precipitation of cubic α-N2.
Development of interfacial microstructure during cooling of a ZnO varistor material
Authors: Olsson E. , Dunlop G. L. , Österlund R.
Published: July 2002

The development of interfacial microstructure during cooling of a ZnO varistor material from the sintering temperature has been studied by a combination of analytical electron microscopy and x-ray diffractometry. It was found that pyrochlore is the first Bi-rich phase to form from the liquid-phase sintering medium during cooling from the sintering temperature. When cooling at normal rates, pyrochlore forms between 940 and 740 °C, and γ-Bi2O3 forms at about 740 °C. Quenching from higher temperatures results in the formation of δ-Bi2O3 from the Bi-rich intergranular liquid. The formation of barriers to electrical conduction at individual ZnO junctions was studied by use of microelectrodes. The barriers started to form while cooling between the temperatures of 1000 and 800 °C. Quenching from higher temperatures did not result in varistor behavior, but barriers to electrical conduction could be created in such specimens by post-sintering heat treatments.
Transmittance and reflectance i
n s
i
t
u measurements of semicontinuous gold films during deposition
Authors: Gadenne P. , Yagil Y. , Deutscher G.
Published: July 2002

We have continuously measured in situ reflectance and transmittance of percolating gold films over the entire range of surface coverage P, in the IR regime (1.7 and 2.2 μm). The samples present similar optical and electrical behavior, when compared on a normalized thickness scale. In the fractal region, close to the percolation threshold, the optical properties show a linear dependence on the surface coverage parameter, in agreement with a renormalization argument previously suggested. A strong absorption (about 40%), whose origin is not well understood, is found in the fractal region. The interpretations previously proposed by other authors are not applicable. We conclude that geometrical effects must dominate the optical properties over a large range of surface coverage.
Theory of Faraday rotation in granular magnetic materials
Authors: Xia T. K. , Hui P. M. , Stroud D.
Published: July 2002

We present a formalism to treat Faraday rotation (FR) in composite media. The method is not limited to low concentrations of magnetically active inhomogeneities and is thus useful for understanding the magneto-optical properties of such inhomogeneous media over the whole range of concentrations. It involves the calculation of an effective magnetic-field-dependent dielectric tensor εe using a tensor effective-medium approximation. To illustrate the technique, we present the results of several model calculations involving both paramagnetic and ferromagnetic metals. We find that such materials have strongly enhanced FR over a wide range of frequencies. The effects of particle shapes on FR are discussed.
The effect of annealing on magnetic properties of iron-base amorphous alloy ribbons
Authors: Tsai C. S. , Li B. J. , Jean K. L. , Lin C. S.
Published: July 2002

It is found that there are two minima, for both coercivity and core loss as a function of annealing temperature, when Fe78 B13 Si9 amorphous alloy ribbon is magnetically annealed. The results show that besides stress relaxation and crystallization, alignment of magnetic easy axis also participates in the enhancement of soft magnetic properties, and that the influence of crystallization begins at 623 K. A four-stage model is proposed to interpret the double-minimum behavior. In the first stage, core loss decreases as annealing temperature increases, and magnetic alignment is the dominant mechanism. In the second stage, core loss increases slightly as temperature rises. In this region, the magnetic alignment process is almost completed and surface crystallization begins to occur. In the third stage, the stress relaxation effects overwhelm the crystallization effects and dominate, thus causing the core loss to decrease. In the fourth stage, stress relaxation effects are completed and cannot further reduce the core loss. Meanwhile, crystals have grown up to 200 nm in size, comparable to the size of domain walls, and hence pinning effects dominate the magnetic behavior resulting in the increase of core loss.
Crack formation during laser cutting of silicon
Authors: Gross T. S. , Hening S. D. , Watt D. W.
Published: July 2002

A proposed semiquantitative model for crack formation during laser cutting of single holes in silicon wafers and ribbons predicts that compressive plastic deformation should occur in an annular region surrounding the hole during cutting. When the annular region cools to room temperature, the residual circumferential stresses are predicted to be tensile in the deformed zone and compressive outside. The predicted residual radial stresses are tensile everywhere and are maximum at the deformed zone boundary. The presence of a plastically deformed zone was confirmed by etch pit studies in originally dislocation-free Czochralski silicon. Radial cracks were observed to terminate at the deformed zone boundary and circumferential cracks were observed to follow the deformed zone boundary. Both of these observations are qualitative confirmation of the proposed model of crack formation. Holographic interferometry was used to measure the residual displacement field relative to the uncut state but no residual displacements were observed. The lack of observable residual displacements was attributed to the relaxation of residual stresses by crack growth.
Interaction of gold-related and irradiation-induced defects in silicon
Authors: Koteswara Rao K. S. R. , Kumar V. , Premachandran S. K. , Raghunath K. P.
Published: July 2002

The evidence of interaction of gold-related defects with irradiation-induced defects in silicon is presented. It is observed that the concentrations of both the gold-related levels increase with increasing dose of irradiation. The linear relation between gold-related acceptor concentration NAu and the divacancy NV-V suggests that the gold-related level is a gold-divacancy complex rather than gold-vacancy complex.
The growth kinetics of diamond films deposited by hot-filament chemical vapor deposition
Authors: Kweon Dae-Weon , Lee Jai-Young , Kim Dongho
Published: July 2002

The growth kinetics of diamond film has been studied, grown by a hot-filament chemical vapor deposition method, to obtain a fundamental information for the growth process of diamond from the gas phase. In order to study the growth kinetics of a pure diamond, the deposition conditions for high-quality diamond are first determined and the kinetics study is carried out under these deposition conditions. Diamond film thickness is directly proportional to the deposition time. This indicates that diamond films are deposited in the steady state under these experimental conditions. The variation of growth rate with substrate temperature shows a distinct maximum. The maximum temperature is shifted somewhat to the higher temperature side with increasing methane concentrations. It is found that this behavior is due to the decrease of supersaturation with increasing substrate temperature. However, the growth rate of diamond film increases linearly with methane concentrations. Therefore, the growth rate of diamond film can be described by the rate constant for a first-order reaction. From the kinetic data of diamond deposition, the deposition rate is controlled by the surface reaction (activation energy of 11 kcal/mol) at the lower temperature range, while the deposition rate is gas phase diffusion controlled (activation energy of 3.8 kcal/mol) at the higher temperature range.
Magnetic and structural properties of electrodeposited Co1−xPx amorphous ribbons
Authors: Rivero G. , Navarro I. , Crespo P. , Pulido E. , Garcia-Escorial A. , Hernando A. , Vazquez M. , Vallet M. , Gonzalez-Calbet J.
Published: July 2002

The specific magnetic moment, coercive force, anisotropy field, and saturation magnetostriction constant have been measured in Co1−xPx amorphous ribbons with 0.04≤x≤0.27. Differential scanning calorimetry, x-ray diffraction, and transmission electron microscopy analysis have been made in order to study the transition from the amorphous state to the crystalline one. Results suggest that transition takes place when x decreases from 0.19.
Magnetic properties and microstructure of light rare earth substituted 2:17 magnets
Authors: Liu S. , Ray A. E. , Chen C. H. , Mildrum H. F.
Published: July 2002

Partial substitution of light rare earths (LRE) for Sm in 2:17 magnets improves the remanence, maximum energy product and the loop squareness without affecting their Curie temperature and the temperature dependence of 4πMs. Uniform, single-phased microstructures were obtained for LRE substituted magnets after heat treatment. Heat treated metallographic specimens revealed striations on the etched surfaces parallel within each grain and perpendicular to the c axis of the magnet alloy.
Diamond film growth in a flowtube: A temperature dependence study
Authors: Martin L. Robbin
Published: July 2002

Data are presented on the rate of diamond film growth in a flowtube at uniform temperature and pressure, and on the rate of H-atom decay in the tube during growth. Diamonds are grown by adding methane to a flow of atomic hydrogen in a carrier gas. The study covers temperatures from 600 to 900 °C, and a range of methane and H-atom concentrations. The profile of diamond growth rate in the tube is a measure of the gas-phase rise and decay of chemical species required for growth. A rudimentary computer model, fitted to the measured H-atom loss rates, is used to compare the gas-phase chemistry with the profiles. Using an assumed rate law for diamond growth, reasonable agreement with the data can only be obtained by including rapid, nonproductive wall loss of more than 95% of the methyl radicals. The data also suggest that methane (or methyl) increases the loss rate of H atoms, and that the converse is true: H atoms increase the loss rate of methyl radicals. Both processes are too fast to be accounted for by gas-phase chemistry. The model comparison yields an estimate for an activation energy of 19±10 kcal for the heterogeneous growth process. This number is meaningful only if the assumed rate law is correct.
Pressure-enhanced crystallization kinetics of amorphous Si and Ge: Implications for point-defect mechanisms
Authors: Lu Guo-Quan , Nygren Eric , Aziz Michael J.
Published: July 2002

The effects of hydrostatic pressure on the solid-phase epitaxial growth (SPEG) rate v of intrinsic Ge(100) and undoped and doped Si(100) into their respective self-implanted amorphous phases are reported. Samples were annealed in a high-temperature, high-pressure diamond anvil cell. Cryogenically loaded fluid Ar, used as the pressure transmission medium, ensured a clean and hydrostatic environment. v was determined by in situ time-resolved visible (for Si) or infrared (for Ge) interferometry. v increased exponentially with pressure, characterized by a negative activation volume of −0.46Ω in Ge, where Ω is the atomic volume, and −0.28Ω in Si. The activation volume in Si is independent of both dopant concentration and dopant type. Structural relaxation of the amorphous phases has no significant effect on v. These and other results are inconsistent with all bulk point-defect mechanisms, but consistent with all interface point-defect mechanisms, proposed to date.

A kinetic analysis of the Spaepen–Turnbull interfacial dangling bond mechanism is presented, assuming thermal generation of dangling bonds at ledges along the interface, independent migration of the dangling bonds along the ledges to reconstruct the network from the amorphous to the crystalline structure, and unimolecular annihilation kinetics at dangling bond ‘‘traps.’’ The model yields v = 2 sin(θ)vsnr exp[(ΔSf + ΔSm)/k] exp− [(ΔHf + ΔHm)/kT], where ΔSf and ΔHf are the standard entropy and enthalpy of formation of a pair of dangling bonds, ΔSm and ΔHm are the entropy and enthalpy of motion of a dangling bond at the interface, vs is the speed of sound, θ is the misorientation from {111}, and nr is the net number of hops made by a dangling bond before it is annihilated. It accounts semiquantitatively for the measured prefactor, orientation dependence, activation energy, and activation volume of v, and the pressure of a ‘‘free-energy catastrophe’’ beyond which the exponential pressure enhancement of SPEG cannot continue uninterrupted due to a vanishing barrier to dangling bond migration. The enhancement of v by doping can be accounted for by an increased number of charged dangling bonds, with no change in the number of neutrals, at the interface. Quantitative models for the doping dependence of v are critically reviewed. At low concentrations the data can be accounted for by either the fractional ionization or the generalized Fermi-level-shifting models; methods to further test these models are enumerated. Ion irradiation may affect v by altering the populatio
Properties of proton exchange waveguides in lithium tantalate
Authors: Matthews Paul J. , Mickelson Alan R. , Novak Steven W.
Published: July 2002

The characteristics and properties of proton exchange and annealed proton exchange waveguides in lithium tantalate are investigated. Planar waveguides are first studied using secondary-ion mass spectrometry (SIMS) to obtain the hydrogen- and lithium-ion concentration profiles. From this data, accurate exchange and anneal diffusion coefficients are calculated. Optical propagation constants are also measured and used in conjunction with the SIMS results to calculate the change in the extraordinary refractive index at λ=0.6328 and 0.829 μm. Infrared-absorption measurements are also taken to monitor the exchange process. In addition, the surface damage due to proton exchange of the x facet of lithium tantalate is qualitatively investigated. Channel waveguides are also characterized. A parameter space study to determine the fabrication conditions necessary to achieve low-loss, single-mode waveguides at λ=0.829 μm is carried out. Mode near-field sizes and the fiber-to-output insertion losses are characterized as a function of anneal time. The polarization extinction ratio and optical damage threshold for typical waveguides are also measured.
Current-voltage curves of bipolar membranes
Authors: Ramírez P. , Rapp H. J. , Reichle S. , Strathmann H. , Mafé S.
Published: July 2002

Bipolar membranes consist of a layered ion-exchange structure composed of a cation selective membrane joined to an anion selective membrane. They are analogous to semiconductor p-n devices as both of them present current-voltage curves exhibiting similar rectification properties. In this article, we present some current-voltage curves obtained for different bipolar membranes at several temperatures. The results can be interpreted in terms of a simple model for ion transport and field-enhanced water dissociation previously developed. The mechanism responsible for water splitting is assumed to be a catalytic proton transfer reaction between the charged groups and the water at the membrane interface. The effects of temperature are taken into account by introducing an Arrhenius-type relationship for the dependence of the forward rate constant of the reaction on temperature. Finally, comparison between theory and experiments provides reasonable values for the parameters introduced in the theoretical model. The analysis aims at developing a better physical understanding of a process in which chemical reactions and transport phenomena are coupled in such a way that the potential technological applications depend strongly on this coupling.
Optimization of a solar-driven heat engine
Authors: Chen Jincan
Published: July 2002

The optimum performance of the solar-driven heat engine consisting of a solar collector and a heat engine is investigated, based on the linear heat-loss model of solar collectors and the endoreversible Carnot cycle model of heat engines. Some new results such as the optimum operating temperature of the solar collector, the maximum overall efficiency of the system, the optimum operating temperatures of the working fluid in two isothermal heat exchange processes of the heat engine, and so on, are derived. It is expounded that these results have more realistic meaning than the previous relative theoretics for the optimum design of practical solar-driven heat engine systems.
In situ infrared reflection and transmission absorption spectroscopy study of surface reactions in selective chemical-vapor deposition of tungsten using WF6 and SiH4
Authors: Kobayashi Nobuyoshi , Nakamura Yoshitaka , Goto Hidekazu , Homma Yoshio
Published: July 2002

The adsorption of tungsten hexafluoride (WF6) and monosilane (SiH4) in selective chemical-vapor deposition (CVD) of tungsten (W) is investigated in situ using Fourier-transform infrared reflection and transmission absorption spectroscopy (FTIR RAS and TMS). The selectivity for W growth is found to originate from the dissociation of SiH4 on a W surface. That is, SiH4 dissociates on a W surface to form Si-containing adsorbed species when the W surface is exposed to SiH4 at temperatures higher than 110 °C, whereas SiH4 does not dissociate on a SiO2 surface. On the other hand, when W and SiO2 surfaces are exposed to WF6 at temperatures from 20 to 300 °C, no adsorbed species are observed by FTIR RAS. However, WF6 can easily react with the Si-containing adsorbed species on a W surface to form W and byproduct gases of SiHF3 and SiF4. The main surface reaction of selective W CVD can be expressed as WF6+2SiH4=W+2SiHF3+3H2. These experimental results support the selective W-CVD mechanism previously proposed, which shows that dissociation of SiH4 and not the dissociation of WF6 has a central role in this process.
Shock response of snow
Authors: Johnson Jerome B. , Solie Daniel J. , Brown Joseph. A. , Gaffney Edward S.
Published: July 2002

We conducted gas-gun impact experiments on snow, with initial densities of 100–520 kg m−3 and at temperatures from −2 to −23 °C and stress levels of 2–40 MPa. Carbon stress gauges were embedded in the snow to measure shock stress histories and arrival times. The unsteady and complex nature of the shock stresses necessitated the use of finite element and reverberation analysis techniques to determine the shock pressure-density (P-ρ) relationships for snow. Experimental results indicate that variations in initial snow density are reflected in differences in the P-ρ deformation path. The pressure needed to compact snow to a specific final density increases with lower initial density. Snow deformation was not affected by initial temperature, but was found to be strain rate dependent. Estimated release moduli increased nonlinearly from 50 MPa at a peak compression pressure of about 15 up to 2700 MPa at a peak pressure of about 40 MPa. Calculated stress histories and shock arrival times agreed with measured values within 20%.
Destruction mechanisms for formaldehyde in atmospheric pressure low temperature plasmas
Authors: Storch Daniel G. , Kushner Mark J.
Published: July 2002

Formaldehyde (CH2O) is a common pollutant of indoor air in residences and commercial buildings. The removal of CH2O from atmospheric pressure gas streams (N2/O2/H2O/CH2O) using plasmas generated by a dielectric barrier discharge has been theoretically investigated with the goal of cleansing indoor air. The model consists of a full accounting of the electron, ion, and neutral chemical kinetics in contaminated humid air. We find that the destruction of CH2O results dominantly from chemical attack by OH and O radicals, with the primary end products being CO and H2O. The predicted destruction rates for CH2O are typically 2–8 ppm/(mJ cm−3) (parts per million of CH2O in air/energy deposition). The elimination of the unwanted byproducts, CO and NO, using a platinum catalyst is discussed.
Solid phase crystallization of thin films of Si prepared by plasma-enhanced chemical vapor deposition
Authors: Masaki Y. , LeComber P. G. , Fitzgerald A. G.
Published: July 2002

Solid phase crystallization of thin films of undoped amorphous Si prepared by plasma enhanced chemical vapor deposition has been studied by transmission electron microscopy (TEM). From the TEM images, the thermodynamic parameters for the amorphous and crystalline phases were extracted. These parameters were compared with those previously reported for evaporated, chemical vapor deposited, and self-implanted amorphous Si. We conclude that the thermodynamic parameters are very similar for different amorphous Si films, although the initial structure of the films is comparatively different from one to another. To explain this, the existence of an intermediate amorphous state is assumed and discussed.
Process-induced mechanical stress in isolation structures studied by micro-Raman spectroscopy
Authors: De Wolf I. , Norström H. , Maes H. E.
Published: July 2002

Micro-Raman spectroscopy is used to study mechanical stress in local isolation structures on silicon substrates (poly-buffered local oxidation of silicon, called PBLOCOS or LOPOS). The influence of processing parameters such as nitride film thickness and width, pad oxide thickness, and field oxide thickness is studied. Also the change of the local stress during the successive processing steps of the isolation is investigated: deposition of the nitride mask, field oxidation, and removal of the nitride mask. The results are explained using a simple analytical model. It is found that stress varies very much during the different processing stages. After deposition of the nitride film, the stress is compressive under the mask lines and tensile outside the lines, close to the line border. The stress magnitude is highly dependent on the thickness of the nitride film. It can be described by edge forces. During field oxidation, this edge-force-induced stress nearly completely relaxes. In LOPOS structures with field oxide, the magnitude and distribution of the mechanical stress is mainly determined by the compressive forces exerted by the field oxide and the bird’s beak on the silicon, and tensile forces which are related to the bending of the nitride film at the edges. The influence of nitride film thickness on the stress at the center of the lines, ωc, is much smaller after field oxidation. For wide lines ωc is compressive and much higher than before field oxidation. For small widths ωc becomes tensile due to the influence of the vertical forces acting at the bird’s beak edge. The tensile stress due to the bending of the nitride film edge is much larger than the field-oxide-induced compressive stress, and dominates the stress picture at the bird’s beak. Only when very thick field oxides and thin nitride films are used, the field-oxide-induced compressive stress becomes visible. This becomes more clear when, in a further process stage, the nitride film is removed. The stress picture is then completely determined by the field oxide and the stress in the active area is always compressive.
A model for hysteretic magnetic properties under the application of noncoaxial stress and field
Authors: Sablik M. J. , Rubin S. W. , Riley L. A. , Jiles D. C. , Kaminski D. A. , Biner S. B.
Published: July 2002

Although descriptions of the effect of stress on spontaneous magnetization within a single domain already exist, there remains no adequate mathematical model for the effects of noncoaxial magnetic field and stress on bulk magnetization in a multidomained specimen. This article addresses the problem and provides a phenomenological theory that applies to the case of bulk isotropic materials. The magnetomechanical hysteresis model of Sablik and Jiles is thus extended to treat magnetic properties in the case of noncoaxial stress and magnetic field in an isotropic, polycrystalline medium. In the modeling, noncollinearity between magnetization and magnetic field is taken into account. The effect of roll-axis anisotropy is also considered. Both magnetic and magnetostrictive hysteresis are describable by the extended model. Emphasis in this article is on describing properties like coercivity, remanence, hysteresis loss, maximum flux density, and maximum differential permeability as a function of stress for various angular orientations between field and stress axis. The model predictions are compared with experimental results.
A new Faraday rotation glass with a large Verdet constant
Authors: Yuan Sui hua , Shu Xiao Zhou
Published: July 2002

Heavily terbium-doped boron-silicate glasses (Tb3+10.4 at. %) with a Verdet constant (632.8 nm, room temperature) as large as −0.338 min/Oe cm, 38% larger than that of the famous FR-5 glasses, have been made. The wavelength and temperature dependencies of the Verdet constants of samples have been investigated in the ranges of 400 to 800 nm and 100 to 300 K. The results show paramagnetic Faraday rotations. The measured susceptibilities between 77 and 300 K support this point but with an effective Bohr magneton number 7% smaller than the theoretical value for free Tb3+ ions. Susceptibilities are further measured at temperatures from 1.5 to 100 K and magnetic fields from 0.2 to 5.0 T. The results show saturation of magnetization and give a Néel temperature of about −9 K. The effective Bohr magneton number in the state of saturated magnetization is 55% smaller than the free ion value. These results may indicate the single ion anisotropy reduction of the magnetization of the disordered Tb3+ ions at low temperatures. The measured indices, Abel number, and nonlinear-index coefficient are 1.745 92 (Nc), 1.746 70 (N632.8), 1.749 84 (ND), 1.760 31 (NF), 49.92, and 7.638×10−20 m2/W, respectively. The transmittance of the sample (15.2 mm thick) is up to 85% in the range from 400 to 1500 nm. It is known that the lower transmittance is mainly due to Nd3+ and Pr3+ impurities in the raw material Tb4O7 by optical absorption analysis.
Modified Judd–Ofelt technique and application to optical transitions in Pr3+-doped glass
Authors: Quimby R. S. , Miniscalco W. J.
Published: July 2002

An extension of the Judd–Ofelt technique [B. R. Judd, Phys. Rev. 127, 750 (1962)] is introduced in which measured fluorescence branching ratios are incorporated into the calculation of the Judd–Ofelt parameters. It is found that including branching ratio measurements into the analysis allows the Judd–Ofelt parameters to be reliably calculated with fewer ground-state absorption measurements than would otherwise be necessary. The method is applied to Pr3+:ZBLAN glass, an important material for 1.3 μm optical amplifiers. A closed-form expression is also derived for determining the Judd–Ofelt parameters from ground-state absorption data.
Single-crystal elastic constants of fluorapatite, Ca5F (PO4)3
Authors: Sha Michael C. , Li Zhuang , Bradt Richard C.
Published: July 2002

The single-crystal elastic stiffnesses of fluorapatite, Ca5F(PO4)3, have been measured by an ultrasonic technique. The values are: C11=152.0, C12=49.99, C13=63.11, C33=185.7, and C44=42.75 GPa. These results are compared with previously published values to resolve anisotropy contradictions and differences in magnitude. The apatite crystal structure is not very elastically anisotropic. It suggests that the elastic anisotropy of bone derives primarily from the hierarchical microstructure of that biocomposite and not from the anisotropy contribution of the apatite mineral component.
Investigation of the structure of barium titanate thin films by Raman spectroscopy
Authors: Robins Lawrence H. , Kaiser Debra L. , Rotter Lawrence D. , Schenck Peter K. , Stauf Gregory T. , Rytz Daniel
Published: July 2002

Raman spectroscopy was used to examine the structure of barium titanium oxide thin films grown by metal-organic chemical vapor deposition (MOCVD) and laser-assisted deposition. The spectra were compared with the spectra of a ceramic specimen and a single crystal. Raman peaks specific to the tetragonal ferroelectric phase of BaTiO3 were seen in the spectra of several films. Other Raman peaks were ascribed to impurity (non-BaTiO3) phases in the films or to the substrates (fused quartz, MgO). Some of the Raman peaks showed a strong polarization dependence. The MOCVD films were also characterized by x-ray diffraction, energy-dispersive x-ray spectroscopy, and transmission electron microscopy. The film-to-film variation of the strength of BaTiO3 features in the Raman spectrum, relative to impurity-phase features, was qualitatively consistent with the x-ray diffraction and electron microscopy results. Spatially resolved Raman measurements showed that the structure of the laser-deposited film varies significantly over the deposited area. The temperature dependencies of the Raman spectra of two MOCVD films were examined in the 25–175 °C range. Raman peaks due to the tetragonal phase of BaTiO3 were observed at temperatures well above the Curie temperature of bulk single-crystal BaTiO3 (132 °C). This observation suggests that the tetragonal ferroelectric phase is stabilized by an anisotropic film-substrate interaction that gives rise to a two-dimensional stress in the plane of the film.
Dynamic modeling of piezoceramic polymer composite with 2-2 connectivity
Authors: Zhang Q. M. , Geng Xuecang
Published: July 2002

A dynamic theory on the piezoceramic polymer composite with 2-2 connectivity is presented. By adopting an approach similar to the Lamb waves in a plate, the distributions of the elastic and electric fields in a piezocomposite are solved and the dispersion curves are obtained. Based on these results, a method is developed to evaluate the sound velocity, acoustic impedance, and electromechanical coupling factor kt in a composite. The theoretical predictions are in excellent agreement with the experimental results.
Experimental measurements and numerical simulations of the gas composition in a hot-filament-assisted diamond chemical-vapor-deposition reactor
Authors: McMaster Mark C. , Hsu Wen L. , Coltrin Michael E. , Dandy David S.
Published: July 2002

Molecular-beam mass spectroscopy was used to measure the gas composition near a growing diamond surface in a hot-filament-assisted chemical-vapor-deposition reactor. The dependencies of the gas composition on changes in (1) the carbon mole fraction in the reactor feed XC, (2) the identity of the inlet carbon source (CH4 versus C2H2), and (3) the surface temperature TS, were studied. For XC≤0.02, the gas composition appeared to be nearly independent of the identity of the inlet hydrocarbon source and depended only on the C/H ratio in the feed gas. At higher values of XC, catalytic poisoning of the hot filament resulted in different product distributions in these two systems. Increasing the surface temperature affected changes in the hydrocarbon composition; the dependencies of the CH3 and C2H2 mole fractions on TS can each be characterized as having an activation energy of 3±1 kcal/mol. Surprisingly, the H-atom mole fraction was independent of TS. These results suggest that reported temperature sensitivities of film growth properties are primarily due to changes in the kinetics of surface processes rather than changes in the gas composition near the surface. A numerical model of the process is presented. In the study of the compositional change as a function of XC, the code gives good prediction for the methane case but grossly underestimates the methane and methyl concentrations for the acetylene case. The H-atom mole fraction is predicted to increase by ×7 if the H destruction probability on the diamond surface is expected to have an activation energy of 7.3 kcal/mol. Good agreement with experimental data can be obtained, however, if H loss by lateral transport to the walls is taken into account.
Solid phase crystallization of amorphous Si1−xGex films deposited on SiO2 by molecular beam epitaxy
Authors: Hwang Chang-Won , Ryu Myung-Kwan , Kim Ki-Bum , Lee Seung-Chang , Kim Chang-Soo
Published: July 2002

We have investigated solid phase crystallization behavior of the molecular beam epitaxy grown amorphous Si1−xGex (x=0 to 0.53) alloy layers using x-ray diffractometry and transmission electron microscopy (TEM). Our results show that the thermal budget for the full crystallization of the film is significantly reduced as the Ge concentration in the film is increased. In addition, we find that a pure amorphous Si film crystallizes with a strong (111) texture while that of the Si1−xGex alloy film crystallizes with a (311) texture suggesting that the solid phase crystallization mechanism is changed by the incorporation of Ge. TEM analysis of the crystallized film shows that the grain morphology of the pure Si is an elliptical and/or a dendrite shape with a high density of microtwins in the grains while that of the Si0.47Ge0.53 alloy is more or less equiaxed shape with a much lower density of defects. From these results, we conclude that the crystallization mechanism changes from a twin-assisted growth mode to a random growth mode as the Ge concentration in the film is increased.
New development of nonlinear optical crystals for the ultraviolet region with molecular engineering approach
Authors: Chen Chuangtian , Wang Yebin , Xia Younan , Wu Baichang , Tang Dingyan , Wu Kechen , Wenrong Zeng , Yu Linhua , Mei Linfeng
Published: July 2002

Potassium fluoroboratoberyllate KBe2BO3F2 (KBBF) has been revealed theoretically and experimentally as a novel ultraviolet nonlinear optical crystal, but it is found to be very difficult to grow in a large size, because of the weak binding interaction between the (Be2BO3)∞ units, which leads to an apparent layer habit in the growth. By using a molecular engineering approach, oxygen bridges when brought in to strengthen the binding between the infinite units are found to be useful to overcome the above shortcoming of KBBF, and in the light of it another new ultraviolet nonlinear optical crystal—strontium boratoberyllate Sr2Be2B2O7 (SBBO) has been discovered. The linear optical properties of SBBO are similar to KBBF’s, but its nonlinear optical properties are better than that of the latter. d22(SBBO)≂d22(β-BaB2O4), which is two times higher than d11 of KBBF. SBBO has very good mechanical properties, and it is also not deliquescent. So SBBO is expected to have great potential for the application in ultraviolet nonlinear optical devices.
Photoluminescence of radiation induced defects in 3C-SiC epitaxially grown on Si
Authors: Itoh Hisayoshi , Yoshikawa Masahito , Nashiyama Isamu , Okumura Hajime , Misawa Shunji , Yoshida Sadafumi
Published: July 2002

Photoluminescence (PL) has been used to study defects introduced by 1-MeV-electron irradiation in cubic silicon carbide (3C-SiC) films epitaxially grown on Si substrates by means of chemical vapor deposition. A dominant PL line of 1.913 eV observed in 3C-SiC irradiated with electrons was found to disappear at annealing stages of ≊100 and 700 °C. The annealing stages of the 1.913 eV PL center and its fraction annealed at each annealing stage were in good agreement with those obtained for the T1 electron spin resonance center, which is attributed to isolated vacancies at silicon sublattice sites [H. Itoh, M. Yoshikawa, I. Nashiyama, S. Misawa, H. Okumura, and S. Yoshida, IEEE Trans. Nucl. Sci. NS-37, 1732 (1990)]. This result indicates that the 1.913 eV PL line arises from silicon vacancies in 3C-SiC. The characteristics of other PL lines induced in 3C-SiC epilayers by irradiation are also discussed.