bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of nonreacting solid particle and liquid droplet loading on an exothermic reacting mixing layer
Authors: Miller Richard S.
Published: July 2002

Numerical simulations are conducted of two-dimensional (2D) exothermic reacting mixing layers laden with either solid particles or evaporating liquid droplets. An irreversible reaction of the form fuel+r Oxidizer→(1+r) Products with exothermic Arrhenius kinetics is considered. The temporally developing mixing layers are formed by the merging of parallel flowing oxidizer and fuel streams, each uniformly laden with nonreacting particles or droplets. The gaseous phase is governed by the compressible form of the Navier–Stokes equations together with transport equations for the fuel, oxidizer, product, and evaporated vapor species concentrations. Particles and droplets are assumed smaller than the gas-phase length scales and are tracked individually in the Lagrangian reference frame. Complete “two-way” couplings of mass, momentum, and energy between phases are included in the formulation. The simulation parameters are chosen to study the effects of the mass loading ratio, particle Stokes number, vaporization, flow forcing, and reaction Zeldovich number on the flame evolutions. Quasi-one-dimensional simulations reveal that the asymptotic state of the laminar flames is independent of the particle or droplet loading. For forced 2D simulations, both particles and droplets are preferentially concentrated into the high-strain braid regions of the mixing layer. Cold solid particles entrained into the mixing zone cool the flame in the braid regions due to their finite thermal inertia. This results in flame suppression and, under certain conditions, local flame extinction in the braids. The potential for flame extinction is substantially enhanced by evaporating droplets through the latent heat, and also by the addition of nonreacting evaporated vapor which locally dilutes the reactant concentrations. In contrast, combustion proceeds robustly within vortex cores which have relatively dilute droplet distributions due to preferential concentration; particularly at moderate Stokes numbers. The extent of flame suppression and local extinction are increased with increasing reaction activation energy, dispersed phase mass loading, and also by decreasing particle or droplet Stokes number.
Glyoxal photodissociation. II. An ab initio direct classical trajectory study of C2H2O2→CO+H2CO
Authors: Li Xiaosong , Millam John M. , Schlegel H. Bernhard
Published: July 2002

Photodissociation of glyoxal via the H2CO+CO channel has been investigated by ab initio classical trajectory calculations using Becke’s three-parameter hybrid functional method with split valence and polarized basis set [B3LYP/6-311G(d,p)]. To model the experimental conditions, trajectories were started from a microcanonical ensemble at the transition state with 8.5 kcal/mol excess energy distributed among the vibrational modes and the transition vector. The CO product was produced with a broad rotational distribution but with almost no vibration excitation. When combined with the results from the H2+2CO channel, the calculated vibrational and rotational distributions of CO are in excellent agreement with the experimental observations. The rotational distribution of H2CO was very broad ranging up to J=85. The H2CO product has significant vibrational excitation in the out-of-plane bending, CH2 rocking, CH2 scissoring, and CO stretching modes. For both the H2+2CO and the CO+H2CO channels, the majority of available energy was partitioned into translations.
Simple analytical description for the cross-tie domain wall structure
Authors: Metlov Konstantin L.
Published: July 2002

A closed form analytical expression for the magnetization vector distribution within the cross-tie domain wall in an isotropic ferromagnetic thin film is given. The expression minimizes the exchange energy functional exactly, and the magnetostatic energy by means of an adjustable parameter (wall width). The equilibrium value of the wall width and the film thickness corresponding to the transition between the Néel and the cross-tie walls are calculated. The results are compared with the recent experiments and are in good qualitative agreement.
Fabrication of bulk heterojunction plastic solar cells by screen printing
Authors: Shaheen Sean E. , Radspinner Rachel , Peyghambarian Nasser , Jabbour Ghassan E.
Published: July 2002

We demonstrate the use of screen printing in the fabrication of ultrasmooth organic-based solar cells. Organic films on the order of several tens of nanometers in thickness and 2.6 nm surface roughness were made. The first-generation screen-printed plastic solar cells demonstrated 4.3% in power conversion efficiency when using an aluminum electrode and 488 nm illumination.
Site–site mode-coupling theory for the shear viscosity of molecular liquids
Authors: Yamaguchi T. , Hirata F.
Published: July 2002

A mode-coupling expression for the shear viscosity coefficient of dense molecular liquids based on the interaction site model is presented. It is a natural extension of the corresponding theory for simple liquids, in which the shear stress auto correlation function is described as the linear combination of the bilinear products of the site–site dynamic structure factor. The theory is applied to water at the ambient condition. The agreement between the theory and the computer experiment is fairly good for the simplicity of the theory.
Modeling extinction and reignition in turbulent nonpremixed combustion using a doubly-conditional moment closure approach
Authors: Cha Chong M. , Kosály George , Pitsch Heinz
Published: July 2002

The scalar dissipation rate is introduced as a second conditioning variable into the first-moment, singly conditional moment closure model to describe extinction and reignition effects in turbulent, nonpremixed combustion. A priori testing of the combustion model using direct numerical simulation experiments exhibiting local extinction/reignition events is described. The singly conditional moment closure model is either unable to describe the extinction seen in the numerical experiments or predicts global extinction when it does not occur. The new doubly conditional moment closure approach is able to describe the extinction seen on average, but predicts the onset of reignition too early.
On the gravity-driven draining of a rivulet of a viscoplastic material down a slowly varying substrate
Authors: Wilson S. K. , Duffy B. R. , Ross A. B.
Published: July 2002

We use the lubrication approximation to investigate the steady locally unidirectional gravity-driven draining of a thin rivulet of viscoplastic material, modeled as a biviscosity fluid (or, as a special case, as a Bingham material), down a slowly varying substrate. In contrast to the earlier work on viscoplastic rivulets we consider small-scale flows, such as those found in many industrial coating and printing processes, in which surface-tension effects play a significant role. We interpret our results as describing a slowly varying rivulet draining in the azimuthal direction from the top to the bottom of a large horizontal circular cylinder. Provided that the yield stress is nonzero we find that the flow is always unyielded near the top of the cylinder (where the rivulet becomes infinitely wide in the transverse direction), and, except in the special case when the viscosity ratio is zero, near the bottom of the cylinder (where it becomes infinitely deep). For sufficiently small values of the prescribed volume flux the flow is unyielded everywhere, but for larger values of the flux the flow near the substrate in the center of the rivulet is yielded. We obtain numerically calculated values of the semiwidth of the rivulet and of the yielded region as well as of the maximum height of the rivulet and of the yielded region for a range of parameter values, and describe the asymptotic behavior of the solution in the limits of large and small yield stress, large and small flux, and small viscosity ratio. In the special case of a Bingham material the flow near the top of the cylinder consists of an infinitely wide rigid and stationary plug, while elsewhere it consists of two rigid and stationary “levées” at the edges of the rivulet and a central region in which the flow near the free surface is a “pseudoplug” whose velocity does not vary normally to the substrate, separated from the “fully plastic” flow near the substrate by a “pseudoyield surface.”
Thermal resistance of interfaces in AlN–diamond thin film composites
Authors: Jagannadham K. , Wang Hsin
Published: July 2002

The effective thermal conductivity of single and multilayer AlN/diamond composite films deposited on silicon substrate with a planar interface is determined experimentally. As a result of the small thickness and the good crystalline quality of the AlN and diamond films, the contribution to the effective thermal resistivity from the films remained very small and enabled the evaluation of the thermal barrier resistance associated with the interfaces. The interfacial thermal resistance of AlN/Si, diamond/Si, and AlN/diamond interfaces was evaluated from the experimental measurements of the effective thermal conductivity of the layered structures. The results show that amorphous regions formed along the interfaces are responsible for high thermal resistance in the layered structures. Modeling of the interfacial thermal resistance has been carried out to explain the effective thermal conductivity of the single and multilayer AlN/diamond composite films in terms of the microstructure of the interfaces.
Creep deformation and stress-induced structural disorder near Tg in a Zr55Al10Ni5Cu30 glassy alloy
Authors: Kato H. , Inoue A. , Chen H. S.
Published: July 2002

Creep deformation under a constant applied load in a Zr55Al10Ni5Cu30 glassy alloy at the glass transition region is investigated. At an initial stress, σ0, less than a critical stress, σc=80 MPa, the glass shows a Newtonian flow. When σ0⩾σc, the flow viscosity, η, initially decreases and attains a minimum. It then increases as the true stress, σ, decreases with further deformation. The initial decrease in η and the attendance of viscosity minimum are due to the stress-induced structural disorder and the structural equilibration with the applied stress, respectively. For stress, σ, less than the viscosity minimum stress, the stress dependence of viscosity, η(σ), curves all tend to merge together, and is fitted well with a master curve, η(σ), established previously for the steady-state flow under constant strain-rate experiments. These results render further support to the hypothesis of stress-induced structural disorder and the concept of fictive stress.
Constructing microstructures of poly- and nanocrystalline materials for numerical modeling and simulation
Authors: Gross Dirk , Li Mo
Published: July 2002

A systematic method is developed for construction of microstructures of poly- and nanocrystalline materials for various numerical modeling and simulations. The automatic method uses as input the distribution functions of microstructure quantities such as the grain size or texture and, through a sequence of minimization procedures, obtains the desired microstructures. The digital samples generated using this method agree well with experimental samples in their topological, structural, and statistical properties. The application and impact of this method on atomistic and continuum simulation and modeling are discussed.
Photoacoustic characterization of the mechanical properties of thin films
Authors: Hernandez Carmen M. , Murray Todd W. , Krishnaswamy Sridhar
Published: July 2002

Narrow band photoacoustics (laser ultrasonics) are used to characterize the properties of free-standing nanometer-sized thin films. Photoacoustic generation is achieved by use of a microchip laser which deposits pulsed laser energy in the form of a spatially periodic source on the structure. The resulting narrow band ultrasonic modes are monitored using a Michelson interferometer. By varying the geometry of the spatially periodic source, a wide range of acoustic wave numbers is probed. Results are presented for two-layer thin film aluminum/silicon-nitride (Al/Si3N4) membranes. For such thin films, only the two lowest order guided modes are generated and these in turn can be related to sheet and flexural modes in plates. The mechanical properties and residual stress in the thin films are evaluated from measured acoustic dispersion curves for these two lowest order modes.
New model for nearly constant dielectric loss in conductive systems: Temperature and concentration dependencies
Authors: Macdonald J. Ross
Published: July 2002

By appropriate fitting of conductive-system frequency-response data for two different ionic materials over ranges of temperature and ionic concentration, it is shown how dispersion associated entirely with ionic motion and that leading to nearly constant dielectric loss (NCL) can be unambiguously distinguished and separated. The latter is clearly associated with polarization of the bulk material, and in the limit of zero mobile-ion concentration NCL appears to approach zero, yielding only a bulk dielectric constant, εD∞0, one that is frequency-independent over the usual immittance-spectroscopy experimental range. For nonzero ionic concentration, however, dielectric NCL appears and can be represented by a small-exponent constant phase element (CPE) complex power law in frequency. This part of the full response may be modeled either by a CPE that includes all bulk dielectric dispersion or, more plausibly, by εD∞0 and a CPE representing only incremental bulk dispersion associated with coupling between ionic motion and bulk polarization. In this case, interestingly, precise power-law dependencies of various dielectric parameters on ionic concentration are established but need theoretical explanation. Fitting of the ionic part of the total dispersion with three different Kohlrausch–Williams–Watts models leads to dependencies of their different β-shape parameters and dielectric quantities on temperature and on ionic concentration and strongly suggests that the widely used original-modulus-formalism dispersion fitting model is incorrect and should be replaced by a corrected version.
Vibrational excitation in the transition state: The CH4+H→CH3+H2 reaction rate constant in an extended temperature interval
Authors: Huarte-Larrañaga Fermı́n , Manthe Uwe
Published: July 2002

Accurate quantum mechanical results for the thermal rate constant of the prototypical six atom reaction, CH4+H→CH3+H2, are reported in this article. Previous k(T) results for temperature values below 500 K are extended up to 1000 K. This is achieved employing a combined iterative diagonalization and statistical sampling approach for the evaluation of the flux correlation function. The accurate reaction rate data obtained for the extended temperature range is used to test several approximations related to the transition state theory. The study especially focuses on the contribution of vibrationally excited states of the activated complex to the thermal rate constant.
Strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy
Authors: Einfeldt S. , Roskowski A. M. , Preble E. A. , Davis R. F.
Published: July 2002

The strain in thin GaN layers grown by maskless pendeoepitaxy has been investigated using high-resolution x-ray diffraction and finite-element simulations. The crystallographic tilt of the free-hanging wings was determined to result from the strain relaxation of the seed stripes along [0001]. The impact of the dimensions of the pendeostructure and of the formation of crystal defects on the expected wing tilt is discussed.
Exchange coupling of molecular-beam-epitaxy-grown Fe/Al/Fe trilayers by dynamic techniques
Authors: Kuanr Bijoy K. , Buchmeier M. , Bürgler Daniel E. , Grünberg P.
Published: July 2002

Network analyzer ferromagnetic resonance (NA-FMR) spectroscopy and Brillouin light-scattering (BLS) techniques are used to probe, in frequency domain, the interlayer exchange coupling (bilinear and biquadratic) of ultrathin Fe/Al/Fe trilayer films epitaxially grown by molecular-beam epitaxy (MBE). These two dynamic techniques provide the possibility to determine the magnetic excitation frequency f (acoustic and optic modes) as well as the frequency linewidth Δf without perturbing the magnetization orientation. A large advantage of the NA-FMR technique compared to conventional FMR is that it can detect the complex magnetic signal at a fixed magnetic field so that the magnetic state of the exchange-coupled trilayer is not perturbed during the frequency sweep. As the spin-wave frequencies are very sensitive to the strength of the static magnetic field and the orientation of the magnetizations, their field dependence reflects the reorientation transitions taking place in a coupled trilayer system. The exchange-coupling constants are derived from the magnetic-field variation of mode positions both from NA-FMR and BLS. We report very strong (>2.5 erg/cm2) antiferromagnetic exchange coupling in the Fe/Al/Fe system. The biquadratic coupling of 1.0 erg/cm2 that we observed at 6 Å Al spacer thickness is much stronger than earlier results for the same system.
Exchange bias of patterned NiFe/IrMn film
Authors: Shen Yaotao , Wu Yihong , Xie Hong , Li Kebin , Qiu Jinjun , Guo Zaibing
Published: July 2002

Exchange bias of different types of patterned NiFe/IrMn microstructures have been studied using the magneto-optical Kerr effect. In general, the exchange field was found to decrease when the exchange coupled double layers were patterned into small dimensions. The magnitude of decrease, however, depends on whether only the NiFe film was patterned or both the NiFe and IrMn were patterned, and also in the cases of grating patterns, whether the grating is along the exchange bias direction or perpendicular to it. The fact that the largest decrease has been found in samples in which both the NiFe and IrMn have been patterned into small square shape of dots suggests that the domain structures in the antiferromagnetic layer mainly determine the strength of exchange bias field.
Simultaneous topographical and optical characterization of near-field optical aperture probes by way of imaging fluorescent nanospheres
Authors: Höppener C. , Molenda D. , Fuchs H. , Naber A.
Published: July 2002

We introduce a method for a simultaneous topographical and optical characterization of aperture probes for scanning near-field optical microscopy which is based on imaging of small sized fluorescent nanospheres (∼20 nm). The near-field optical fluorescence image of a nanosphere maps the intensity distribution of light at the end face of the probe whereas the simultaneously taken height image contains information about the aperture–sample distance. We used this method to control a mechanical modification of a near-field probe. By squeezing a probe repeatedly against a smooth glass substrate and thereby removing obstructing protrusions the aperture was brought as close as possible to the sample surface which resulted in a strongly improved optical resolution.
High speed x-ray radiography diagnostic of current interruption in circuit breakers
Authors: Zolfaghari A. M. , Kellogg E. , Wendt S. , Gray J.
Published: July 2002

High-speed x-ray radiography is used for the imaging of internal components of a low-voltage air circuit breaker during fast current interruption events. A continuous x-ray source was used to irradiate the circuit breaker during interruption, while a fast camera imaging system recorded the output visible light from the x-ray image intensifier up to a maximum rate of 4000 frames per second. Detailed time-resolved images were obtained of the movable contact position and tripping mechanism components of the circuit breaker during the interruption event. Experimental observations were used to validate numerical simulation results obtained using electromagnetics and dynamics/kinematics analysis software. In addition to imaging of the circuit breaker mechanism, molten metal streams and droplets ejected from the electrodes were also observed. Tracking of these metallic particles can provide valuable information about contact phenomena and gas flow fields in circuit breakers.
Ultrahigh power-bandwidth-product performance of low-temperature-grown-GaAs based metal-semiconductor-metal traveling-wave photodetectors
Authors: Gan Kian-Giap , Shi Jin-Wei , Chen Yen-Hung , Sun Chi-Kuang , Chiu Yi-Jen , Bowers John E.
Published: July 2002

High-output-power and high-bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors. In this letter, we report high peak-output-voltage (∼20 V) and peak-output-current (∼400 mA, 50 Ω load) together with ultrahigh-speed performances (1.5 ps, 220 GHz), observed in low-temperature-grown-GaAs (LTG-GaAs) based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak-output-power and ultrahigh-electrical-bandwidth performances were achieved due to the superior MSM microwave guiding structure and a short carrier trapping time in the LTG-GaAs layer, which reduced the space-charge screening effect and increased the photoabsorption volume without sacrificing electrical bandwidth significantly. We also observed different bias-dependent nonlinear behaviors in MSM TWPDs under high and low illuminated optical power excitations, which are possibly dominated by the space-charge screening and the lifetime increasing effects, respectively.
Dielectric and piezoelectric enhancement due to 90° domain rotation in the tetragonal phase of Pb(Mg1/3Nb2/3)O3-PbTiO3
Authors: Koo T. Y. , Cheong S-W.
Published: July 2002

The anomalous enhancement in dielectric (ε>6000) and piezoelectric constant (d33>600 pC/N) in the tetragonal side (x=35%) of morphotropic phase boundary (MPB) has been observed in polycrystalline (1−x)Pg(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT). On the other hand, x-ray peak position shifts under electric field (E) demonstrate that the intrinsic piezoelectric response becomes maximum at MPB (x=33%–34%), which may be associated with the recently discovered monoclinic phase. Intensity change of Bragg peaks under E indicates that 90° domain rotation is the main contribution to additional strain in the tetragonal side of MPB, which dominates the intrinsic contribution of monoclinic phase in polycrystalline PMN-PT.
A laser-triggered spark gap model
Authors: Kushner M. J. , Milroy R. D. , Kimura W. D.
Published: July 2002

Laser preionization is a method whereby the formative lag time and jitter of spark gaps can be reduced, and breakdown can be initiated at voltages significantly below the self-breakdown value. In the spark gaps of interest (100-ns spark duration), the plasma column expands from an initial laser preionized diameter of approximately 50 μm to an arc of diameter in excess of 1 mm, and conducts greater than 12-kA peak current. Since the time required for the resistive collapse of the plasma is comparable to the spark duration, the spark gap represents a non-negligible resistive loss in the circuit. The objective of this study is to understand the basic mechanisms by which laser-triggered spark gaps develop and to provide a basis to optimize their design and minimize their resistive losses. To study the expansion and conduction phases of laser-triggered spark gaps, a first principles model has been developed. This model includes gas dynamics, electron collision kinetics, radiation transport, and external circuitry in a self-consistent formulation. The formulation of the model is discussed and results are compared to experimental data. We find that growth of the spark column is dominated by gas dynamic expansion of the hot ionized core, augmented by photoionization and thermal ionization at the plasma column boundary. The plasma column is confined within a high-density cylindrical shell of neutral gas that traps ionizing radiation in a region of low E/N (electric field divided by gas density), thereby inhibiting expansion by nonhydrodynamic means (electron avalanche).
Chemical beam epitaxy of InGaAs
Authors: Tsang W. T.
Published: July 2002

In0.53Ga0.47As lattice matched to InP substrate has been grown by chemical epitaxy (CBE). Epilayers with lattice-mismatch Δa/a<1×10−3 have been reproducibly obtained with extreme uniformity in both composition and layer thickness over a large area of 3.8 cm diameter (limited by the present substrate holder size) without the need of substrate rotation during growth. Surface morphologies were routinely featureless and mirrorlike as observed by Nomarski phase contrast microscope. Comparing with high-quality In0.53Ga0.47As epilayers grown by molecular beam epitaxy (MBE), the CBE-grown In0.53Ga0.47As epilayers have similar linewidths but can have substantially higher 300-K photoluminescence intensities. The electron concentration and mobilities were found to be related to the source purity of the TMAs used. For epilayers grown with high-purity TMAs source, room-temperature electron mobility as high as 9000 cm2/V s and concentrations of ∼7×1015 cm−3 were produced. Such value represents the highest value obtained by using trimethylarsine as the arsenic source. In general, the electron mobilities were as good as those obtained from low-pressure metalorganic chemical vapor deposition.
Photophysics of a Zn(Cd)S phosphor on an extended time scale
Authors: Sharma D. K. , Palazzotto M. C. , Sahyun M. R. V.
Published: July 2002

We have studied the photophysics of a Zn(Cd)S:Ag phosphor, in powder form under ambient conditions, over a time domain extending from picoseconds to milliseconds. Pulsed laser luminescence techniques and time-resolved dielectric loss spectrometry have been employed. We propose mechanisms to interpret the results and complement the proposals with a computer simulation. We discuss the implications of the results for energy conversion schemes involving photoassisted catalysis.
Effective medium approximation for elastic constants of porous solids with microscopic heterogeneity
Authors: Berryman James G.
Published: July 2002

Formulas for the scattering from an inhomogeneous sphere in a fluid-saturated porous medium are used to construct a self-consistent effective medium approximation for the coefficients in Biot’s equations of poroelasticity [J. Acoust. Soc. Am. 28, 168 (1956)] when the material constituting the porous solid frame is not homogeneous on the microscopic scale. The discussion is restricted to porous materials exhibiting both macroscopic and microscopic isotropy. Brown and Korringa [Geophysics 40, 608 (1975)] have previously found the general form of these coefficients. The present results give explicit estimates of all the coefficients in terms of the moduli of the solid constituents. The results are also shown to be completely consistent with the well-known results of Gassmann and of Biot and Willis, as well as those of Brown and Korringa.
Damage of coherent multilayer structures by injection of dislocations or cracks
Authors: Hirth J. P. , Evans A. G.
Published: July 2002

Experimental evidence indicates that coherent multilayer structures (strained superlattices) can be grown in a damage-free state when the thickness of the layer is less than a critical value. The resistance of such structures to subsequent damage by dislocation or crack injection is examined in the present study. The structures are determined to exhibit damage resistance that decreases as either the layer thickness or the coherency strains increase.
Phase behavior and conductivity in poly(ethylene oxide)–sodium thiocyanate systems
Authors: Lee Y. L. , Crist B.
Published: July 2002

Observed phases and transition temperatures in mixtures of poly(ethylene oxide) and sodium thiocyanate (PEO–NaSCN) are described by a binary phase diagram. The components of this system are semicrystalline PEO which can dissolve salt up to a mole fraction Xs=0.023, and a crystalline complex of composition PEO3.5NaSCN (Xcc=0.22). The salt-deficient PEO component is normally formed by a eutectic reaction with eutectic temperature Te=63 °C. Conductivity of PEO–NaSCN mixtures is calculated assuming that only ions in the amorphous, liquidlike regions contribute to charge transport. This model predicts a positive discontinuity in conductivity on eutectic melting, and a sharp maximum in conductivity at low salt concentrations, in agreement with observed behavior.
High intrinsic coercivities in iron-rare earth-carbon-boron alloys through the carbide or boro-carbide Fe14R2X (X=BxC1−x)
Authors: Liu N. C. , Stadelmaier H. H. , Schneider G.
Published: July 2002

In the phase Fe14R2X, where R is a lanthanide and X is either boron or carbon, or a mixture of the two, the extent of stability of the carbides and their miscibility with the borides is traced for the lighter rare-earth metals. Like the borides, the carbides are magnetically hard, but unlike them, they do not normally crystallize from the melt, and this property is exploited to produce intrinsic coercivities above 12 kOe in cast materials without the added special processing step of sintering or melt spinning. The high coercivity is related to a cellular microstructure of Fe14R2X in which the cell size is approximately 1 μm. The cell structure, which originates in a peritectoidlike transformation from primary Fe17R2, is quite stable and does not change during prolonged annealing. The coercivity is sensitive to variations in composition.
Photoconductivity spectra of CdGa2Se4:Co2+ single crystals
Authors: Kim Chang-Dae , Cho Tong-San , Kim Jae-Kuen , Kim Wha-Tek , Kim Hyun-Nam
Published: July 2002

Single crystals of CdGa2 Se4 :Co2+ were grown by the chemical transport reaction method. We observed the photoconductivity peaks at 2.13, 1.96, and 1.86 eV due to the cobalt impurity. The photoconductivity impurity peaks correspond to electronic transitions of Co2+ ion with S4 symmetry between the valence subband, the lower conduction band, and the split energy levels of Co2+ in the forbidden band gap of the CdGa2 Se4 :Co2+ single crystals. We also observed photoconductivity peaks at 3.09, 3.20, and 3.32 eV due to the optical transitions from the subbands Γ5 +Γ8, Γ6 +Γ7, and Γ6 +Γ7 located below the valence band to the lower conduction band Γ5 +Γ8. The temperature dependence of the energy of the principal photoconductivity peaks P11 and P21 for the undoped CdGa2 Se4 and the CdGa2 Se4 :Co2+ single crystals can be well modeled by the Varshni equation.
Aluminum ion-implantation enhanced intermixing of GaAs-AlGaAs quantum-well structures
Authors: Kash K. , Tell B. , Grabbe P. , Dobisz E. A. , Craighead H. G. , Tamargo M. C.
Published: July 2002

We have studied aluminum-implantation enhanced intermixing of GaAs-AlGaAs quantum-well structures using low-temperature photoluminescence. The energy shift of the heavy-hole exciton was determined for Al doses varying from 2×1013 cm−2 to 1×1015 cm−2 after either furnace annealing at 800 °C or optical rapid thermal annealing at 925 °C. A variational calculation yields the diffusion length from the energy shift of the exciton. This shift is due both to the increase of Al in the center of the well and to the change in electron and heavy-hole confinement energies. The ion-implantation enhancement of the diffusion length depends on Al-ion dose but not significantly on annealing time or temperature. This work indicates that Al-ion implantation should be useful for the fabrication of structures of reduced dimensionality by patterned implantation of AlGaAs-GaAs quantum wells.
Ferromagnetism in ultrathin metastable films of fcc Fe, Co, and Ni (invited)
Authors: Willis R. F. , Bland J. A. C. , Schwarzacher W.
Published: July 2002

We have applied spin-polarized neutron reflection at a grazing angle of incidence to study the ferromagnetic behavior of thin magnetic films down to the monolayer thickness level. The reflected intensities I+ and I− for spin parallel (+) or antiparallel (−) to the sample magnetization are measured near the angle θc for total reflection: deviation of the flipping ratio F=I+/I− from unity occurs only for ferromagnetic ordering. In general, deviations of F from unity are so small that an accurate determination of the magnetic moment per atom μ is not possible. The success of the present experiments relies on the enhancement (∼500×) of the magnetic signal when the film is overcoated with a nonmagnetic layer of thickness sufficient to produce constructive interference in the reflected neutron wave field. With this method, we have determined the magnetic moments in films of the metastable fcc phases of Fe and Co, grown epitaxially on Cu(001) single-crystal substrates. The fcc Fe(001) films have lowered Curie temperatures TC below 400 K for thicknesses less than 5 monolayers, similar to Ni films. In contrast, the Co(001) films exhibit a constant value of μ≊μbulk=1.8μB down to a single monolayer thickness over the same temperature range. The fcc Fe films exhibit unusual magnetic anisotropy properties which are structure and strain related, as determined by low-energy electron diffraction measurements. The results are discussed in the context of three- to two-dimensional magnetic scaling theories and predictions derived from local-spin-density functional calculations of the magnetic moments per atom as a function of volume strain.
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u infrared reflection absorption spectroscopic characterization of plasma enhanced chemical vapor deposited SiO2 films
Authors: Koller Kent B. , Schmidt William A. , Butler James E.
Published: July 2002

The sensitivity and selectivity of double modulation Fourier transform infrared reflection absorption spectroscopy for absorbing species on a reflecting surface has been employed for the in situ analysis of low-temperature plasma enhanced chemical vapor deposition formed SiO2 films deposited on HgCdTe, silicon, and aluminum substrates. An oblique angle of incidence of ∼55° was chosen to yield maximum sensitivity for the longitudinal optical (LO) phonon mode of SiO2 on Si. The peak frequency and shape of the LO mode absorption band varied with the quality of the SiO2 films thus providing a means of in situ assessment of reaction conditions at any stage of film growth. This diagnostic technique can be readily applied to the in situ analysis of dielectric thin films formed under a variety of reaction conditions.
Excitation process of the Tb emission center in a ZnS:Tb,F thin-film electroluminescent device
Authors: Mikami Akiyoshi , Ogura Takashi , Taniguchi Koji , Yoshida Masaru , Nakajima Shigeo
Published: July 2002

The excitation spectrum for terbium (Tb) emission in a ZnS:Tb,F film varies sensitively with Tb concentration, the amount of fluorine (F) relative to Tb, post-deposition annealing, and the temperature of measurement. There are at least three kinds of channels for the excitation of Tb3+ ions; namely, resonant energy transfer from the recombination of an electron-hole pair, direct transitions between the 4f 8 configuration states of a Tb3+ ion, and the indirect excitation via lattice defects. A rapid quenching of photoluminescence (PL) is observed above 0.2 at. % of the Tb concentration in the case of indirect excitation of energy transfer. The PL intensity additionally reduced in the presence of F− ions, even at the same Tb concentration. In contrast, when the Tb3+ ions are directly excited to the 5D3 energy state, the PL intensity increases proportionally up to a maximum of about 2 at. %. The luminance of the electroluminescent device behaves in the same way. Another excitation band is found in an energy region slightly below the band gap of ZnS, and is identified as due to Tb-related complexes associated with sulfur interstitials. The PL characteristics obtained with three excitation channels are also compared with the electroluminescence (EL) characteristics, concerning the effects of post-deposition annealing and measurement temperature. The excitation mechanism of electroluminescence can be explained on the basis of the direct impact model, and the real nature of the energy transfer process is also discussed.
Sequence of phase formation in Nb/Al multilayered samples
Authors: Im Yoonsoon , Morris J. W.
Published: July 2002

A series of Nb/Al multilayered samples made by a sputter-deposition or a cold-rolling technique was used to simulate reactions in powder metallurgy processed Nb/Al superconducting wires. The sequence of phase formation observed in the Nb/Al multilayered samples was explained using knowledge about the crystal structure of each compound. Three different controlling mechanisms were proposed: the activation energy for nucleation, the energy barrier associated with short range order, and the diffusion barrier by the previously formed phase layer.
The response of manganin gauges to multiple-shock loading
Authors: Rosenberg Z.
Published: July 2002

The response of piezoresistance stress gauges to multiple-shock loading is discussed in terms of the elastoplastic model for their dynamic response. It is argued that their response to a second (or third) shock wave should not depend on previous shocks and should only depend on the amplitude of these consecutive shocks. In order to substantiate these arguments we performed double- and triple-impact experiments in which the impactors were composed of two (or three) metallic disks separated by 0.2-mm gaps. Symmetric experiments in which the shock waves were of equal amplitude resulted in an equal response of the gauge for each shock. Another type of impactor, with thin aluminum disks separated by 0.2 mm from thicker copper disks, resulted in higher second shocks. The response of the gauges to these higher shocks were equal to those obtained by single shocks with the same amplitudes.
Drawing-induced residual stress effects on optical characteristics in pure-silica-core single-mode fibers
Authors: Hibino Yoshinori , Hanawa Fumiaki , Horiguchi Masaharu
Published: July 2002

Residual stress and its influence on optical characteristics have been investigated for pure-silica (SiO2)-core/fluorine-doped-silica(F-SiO2)-clad single-mode fibers. Theoretical analysis has shown the following: (1) Very high residual stress concentrated at the small-diameter core region is induced by drawing tension because of the viscosity difference between core and cladding. (2) The refractive index in the core is reduced greatly by the residual stress via the photoeleastic effects. (3) The decrease in the refractive index results in the shift of the cutoff wavelength to shorter wavelengths. These characteristic features have been ascertained experimentally in SiO2-core/F-SiO2-clad single-mode fibers drawn at various conditions, indicating the optimum drawing condition and structure for the single-mode fibers.
Performance and failure mechanisms of TiN diffusion barrier layers in submicron devices
Authors: Kohlhase A. , Mändl M. , Pamler W.
Published: July 2002

Defects in reactively sputtered titanium nitride diffusion barriers in submicron devices were investigated. Four different failure mechanisms could be extracted, two originating from the production of the layers, two related to foregoing process steps. The latter are contact hole overetch and topology inside the contact hole, i.e., edges, created by deviating isotropic etch ratios of different dielectric layers in a post-treatment after the contact etch. The insufficiencies related to the preparation of the TiN films are microcracks due to excessive stress incorporated and encroachment caused by a Ti-rich titanium nitride on the contact hole walls, proved by an Auger electron spectroscopy analysis. Accidental accumulation of two or more of these defects in a production line can cause the barrier to fail. The process of contact degradation was investigated by transmission electron microscopy and energy-dispersive x rays. It was found that after aluminum penetration the destruction of the contact area proceeds via a ternary AlxTiySiz reaction until all titanium is consumed, followed by the growth of a pure Al spike and the fatal breakdown of the contact.
The structure of Cu3Au above the critical temperature
Authors: Butler B. D. , Cohen J. B.
Published: July 2002

The diffuse x-ray scattering intensity from a single crystal of Cu3Au was measured in a volume of reciprocal space at a temperature of 703 K, approximately 35 K above the transition temperature for ordering in this alloy. Intensity was measured in absolute units and was separated into components due to short-range order (SRO) and atomic displacements. The SRO parameters determined agree qualitatively with previous experimenters but agree best in magnitude with the original study by Cowley [J. Appl. Phys. 21, 24 (1950)]. Computer simulations of the structure, based on the SRO parameters, show that atoms with ordered environments tend to cluster into small domains that have dimensions of 1–10 unit cells in length and are surrounded by a disordered matrix. The average displacements of near-neighbor pairs were found to be contrary to what has previously been assumed: Au-Au neighbors were found to have negative displacements on average and Au-Cu pairs positive displacements, compared to the average interatomic displacements.
Formation, oxidation, electronic, and electrical properties of copper silicides
Authors: Cros A. , Aboelfotoh M. O. , Tu K. N.
Published: July 2002

The solid state reaction between copper and silicon has been studied using Rutherford backscattering, glancing-angle x-ray diffraction, scanning electron microscopy, and x-ray photoemission spectroscopy. Schottky-barrier-height measurements on n-type Si (100) have also been performed in the temperature range of 95–295 K with the use of a current-voltage technique. The results show that a metal-rich compound with a composition in the Cu3Si range forms at low temperatures (473 K). The electronic properties of the compound are dominated by the hybridization between the Cu(d) and Si(p) valence states. A direct consequence of this hybridization is the peculiar oxidation behavior of the compound surface; both Cu and Si have been found to oxidize at room temperature. The oxidation of Si in the silicide is enhanced as compared with the oxidation of the elemental single-crystalline Si surface. Upon annealing the oxidized surface, a solid state reaction takes place: Cu2O disappears and a thicker SiO2 layer grows, owing to the large difference in free energies of formation between SiO2 and Cu2O. The n-type barrier height of 0.79 eV for both the as-deposited metal and the metal-rich silicide phase decreases with increasing temperature with a coefficient close to the temperature coefficient of the indirect energy gap in Si. These results suggest that the Fermi level at the interface is pinned relative to the valence-band edge, independent of temperature.
Thermal model of the catastrophic degradation of high-power stripe-geometry GaAs/(AlGa)As double-heterostructure diode lasers
Authors: Nakwaski Wl/odzimierz
Published: July 2002

In the present work, the three-dimensional thermal analysis of the process of the catastrophic mirror damage in double-heterostructure stripe-geometry GaAs/(AlGa)As diode lasers has been carried out for the case of their high-power operation. In the analysis, the temperature dependencies of both the thermal conductivity and the thermal diffusivity have been taken into account. The analytical solution has been obtained with the aid of the Green’s function method. The catastrophic mirror damage has been explained from the thermal point of view as a result of a local overheating of the active area close to or at the mirror surface. It has been proved that a determination of an exact value of the critical mirror temperature from the catastrophic degradation point of view is of minor importance because the considered temperature increase is of an avalanche type. The analytical solution obtained in the work has enabled us to show a dependence of a catastrophic-degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe diode lasers. Experimentally observed catastrophic mirror damage in diode lasers caused by an increase in their facet reflectivity has been also investigated. It has been confirmed that an increase in the reflectivity of an unprotected mirror may be accompanied by a very rapid facet failure. It has been also shown in which way one can avoid the above phenomenon.
Characterization of sulfuric acid proton-exchanged lithium niobate
Authors: Cargo J. T. , Filo A. J. , Hughes M. C. , Kannan V. C. , Stevie F. A. , Taylor J. A. , Holmes R. J.
Published: July 2002

A series of bulk proton-exchanged lithium niobate waveguides has been prepared by using concentrated sulfuric acid as the proton source and varying the time and temperature of the exchange process. The optical and physical properties of the proton-exchanged lithium niobate layer have been measured and found to be essentially identical to proton-exchanged layers obtained by other workers using the benzoic acid process. Sulfuric acid offers a number of advantages over benzoic acid as a proton source, including a wider temperature liquid range and a greater ease of handling in a clean-room environment.
Shock waves and acceleration of thin foils by laser pulses in confined plasma interaction
Authors: Romain J. P. , Darquey P.
Published: July 2002

The process of shock wave generation and shock evolution in a solid target by confined laser plasma, in the range 108–1010 W/cm2 incident intensity, is analyzed with the use of computer simulations. Predicted variations of plasma pressure are consistent with theoretical results from simple analytical models and with experimental data. The simulations also provide a complete description of various effects such as target or confinement thickness and acceleration of thin foils.
A 1-kpps transversely excited atmospheric CO2 laser excited by an all-solid-state exciter with a magnetic pulse compressor
Authors: Hatanaka Hidekazu , Tanaka Hiroshi , Obara Minoru , Midorikawa Katsumi , Tashiro Hideo
Published: July 2002

The high-repetition-rate operation of a transversely excited atmospheric CO2 laser pumped by an all-solid-state exciter consisting of a two-stage magnetic pulse compressor has been tested. At a repetition-rate frequency of 1 kpps, an average laser power of 250 W was obtained. A maximum repetition rate of 1.1 kpps was limited by either the capability of the switching power supply (20 kW) or the turn-off time of the silicon controlled rectifier used (400 μs). The energy transfer efficiency of the exciter was 75%, and the overall laser efficiency was 3.2%. By changing the amount of the available magnetic flux swing of the cores used, 30% regulation on the input energy was also achieved with a constant energy transfer efficiency of 78%.
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u cleaning of GaAs surfaces using hydrogen dissociated with a remote noble-gas discharge
Authors: Hattangady S. V. , Rudder R. A. , Mantini M. J. , Fountain G. G. , Posthill J. B. , Markunas R. J.
Published: July 2002

I n situ cleaning of GaAs surfaces has been achieved at 350 °C with a novel technique employing hydrogen that is excited and dissociated using a remote Ar discharge. Reconstructed surfaces characteristic of clean, As-stabilized GaAs surfaces have been observed with reflection high-energy electron diffraction following the cleaning treatment. Auger electron spectroscopy analyses confirm that such a treatment removes both carbon and oxygen contamination from the surface. X-ray photoelectron spectroscopy shows the removal of oxygen bonded to both Ga and As on the surface. Emission spectroscopy shows evidence of excited molecular and atomic hydrogen with the downstream-excitation process.