bluuebunny/crossref_metadata_2025 huggingface dataset:
Frequency response of current-driven polarization modulation in vertical-cavity surface-emitting lasers
Authors: Verschaffelt Guy , Albert Jan , Veretennicoff Irina , Danckaert Jan , Barbay Sylvain , Giacomelli Giovanni , Marin Francesco
Published: July 2002

We present an experimental study of the current-driven polarization modulation properties of vertical-cavity surface-emitting Lasers (VCSELs). Some VCSELs exhibit a high-contrast polarization flip for a particular value of pump current. By modulating the current around this value we measure the critical modulation amplitude necessary to force current-driven polarization switching as a function of the modulation frequency. We thus obtain the polarization modulation frequency response, which shows the same cut-off frequency as the thermal frequency response. This indicates the necessity to incorporate a temperature-dependent variable in realistic models that describe the polarization behavior of these VCSELs.
Effective method for stress reduction in thick porous silicon films
Authors: Kim Han-Su , Zouzounis Eric C. , Xie Ya-Hong
Published: July 2002

A promising stress control process is demonstrated to achieve near-zero stress levels in thick porous silicon (PS) films. Stress reduction is necessary for thick PS structures to be used for radio-frequency applications such as on-chip inductors and transmission lines that require very thick (>100 μm) insulating materials. This study employs a standard sample structure with 50-μm-thick PS films formed on p+ substrates through anodization under 50 mA/cm2 and 25% HF concentration. Mass spectroscopy analysis shows that the desorption of hydrogen is correlated with the stress evolution in PS films. As-prepared PS films are under compressive stress. Thermal annealing in N2 ambient turns it into tensile stress. Further annealing in an oxidizing ambient restores the compressive stress. Such stress evolution can be explained by the atomic structure changes on the pore walls of PS films: H2-passivated and oxidized Si surfaces result in compression in PS films, whereas reconstructed state results in tension. These findings are used to fabricate thick PS films with negligible stress (<1 MPa) at room temperature.
Influence of inertia and topography in thin-cavity flow
Authors: Siddique Mizanur R. , Khayat Roger E.
Published: July 2002

The influence of inertia and cavity topography is examined for the steady-state flow inside thin cavities of arbitrary shape. The problem is closely related to the die casting process. The flow is determined by solving the thin-film equations, using a coupled spectral/finite-difference method. The flows inside flat, contracting, and modulated cavities of rectangular cross section are examined. The interplay between inertia and cavity modulation is particularly emphasized. Regions of shear and elongation are identified; their size and location are strongly dependent on inertia and cavity thickness. The flow inside a grooved cavity (of nonrectangular cross section) is also investigated. In this case, the problem reduces to Poisson’s equation for the streamwise velocity component, and an exact solution is obtained, which is compared against the thin-film solution. The range of validity of the thin-film approximation is thus identified, and is influenced by the cross-section aspect ratio, the modulation (groove) amplitude and wavelength.
Relation of the fractal structure of organic pigments to their performance
Authors: Skillas G. , Agashe N. , Kohls D. J. , Ilavsky J. , Jemian P. , Clapp L. , Schwartz R. J. , Beaucage G.
Published: July 2002

Different pigments embedded in polymer matrices were examined by small angle scattering of x- rays over 3 wave number decades. The scattering intensities show differences both in the mass fractal dimension (varying between 1.4 and 2.67) and the size of the particles. The differences are pronounced between dry pigment powders and the same powders in a polymer matrix as well as between the pigments themselves. Further, a correlation of pigment geometrical configuration and pigment performance, as perceived by the human eye, shows how pigments with a maximum color brightness per pigment mass can be created.
Direct simulation Monte Carlo study of rotational nonequilibrium in shock wave and spherical expansion of nitrogen using classical trajectory calculations
Authors: Koura Katsuhisa
Published: July 2002

The rotational nonequilibrium in a normal shock wave and in a steady spherical expansion is studied for nitrogen using the direct simulation Monte Carlo method coupled with classical trajectory calculations. The intermolecular potential for nitrogen is taken as an accurate potential energy surface constructed by van der Avoird et al. [J. Chem. Phys. 84, 1629 (1986)] and readjusted by Cappelletti et al. [Mol. Phys. 93, 485 (1998)]. The shock wave results are compared with the rotational distribution and the number density and rotational temperature profiles measured by Robben and Talbot [Phys. Fluids 9, 653 (1966)]. The agreement is generally good except at extremely low upstream temperatures, where the classical-mechanical treatment may be inadequate. The spherical expansion results are compared with the rotational distribution and the number density and rotational temperature profiles measured by Marrone [Phys. Fluids 10, 521 (1967)] and Coe et al. [Phys. Fluids 23, 706 (1980)] along the axis of a low density free jet expansion. It is shown that the rotational distribution gradually departs from a Boltzmann distribution farther downstream with an overpopulation in the higher rotational levels, in good agreement with the measured spectral data except near the orifice, where the spherical source expansion analogy may be inapplicable. The rotational temperature obtained by a Boltzmann fit at the low rotational levels or defined as the mean rotational energy agrees satisfactorily, except near the orifice, with the experimental rotational temperatures determined from the spectral data.
Conduction-related voltage instabilities in double-layer dielectric films
Authors: Evseev S. , Cacciato A. , van der Pol J.
Published: July 2002

Threshold voltage shift induced on parasitic metal–insulator–semiconductor field-effect transistors by bias-temperature stress in the presence of double-layer insulating stacks is studied as a function of the stack composition and measurement conditions. It is shown that the threshold-voltage shift (ΔVT) closely depends on the conductance difference between the two insulating layers. This shift is completely reversible being larger if the conductance difference is higher and more charge is injected into the dielectric stack. In the case of Si3N4/SiO2 insulating stacks, it is further demonstrated that the voltage shift is caused by charges transferred via the nitride and accumulated in the nitride and/or at the nitride/oxide interface. A first-order charge transfer model presented here explains the observed voltage shift.
Rotation angle sensors based on spin valve structures: A modeling approach
Authors: Tietjen D. , Elefant D. , Schneider C. M.
Published: July 2002

Spin valve systems are widely considered in magnetic rotation angle sensors. We present a simple magnetostatic model to simulate the electrical and magnetic signal of typical spin valve systems as a function of external magnetic field and rotation angle. To match simulation and experiment only a few parameters are needed, which can be determined by a single measurement. Two types of systems are treated and discussed: a simple spin valve and a spin valve with an “artificial antiferromagnet,” often used to enhance the exchange bias. The model shows a very good correspondence of the predictions to the experimental results of the systems investigated. In order to quantitatively compare experiment and model, we introduce a useful concept to quantify the quality of the angle rotation curves: the total harmonic distortion.
Dislocation-induced spatial ordering of InAs quantum dots: Effects on optical properties
Authors: Leon R. , Chaparro S. , Johnson S. R. , Navarro C. , Jin X. , Zhang Y. H. , Siegert J. , Marcinkevičius S. , Liao X. Z. , Zou J.
Published: July 2002

Misfit dislocations were used to modify the surface morphology and to attain spatial ordering of quantum dots (QDs) by molecular beam epitaxy. Effects of anneal time and temperature on strain-relaxed InxGa1−xAs/GaAs layers and subsequent spatial ordering of InAs QDs were investigated. Photoluminescence (PL) and time-resolved PL was used to study the effects of increased QD positional ordering, increased QD uniformity, and their proximity to dislocation arrays on their optical properties. Narrower inhomogeneous PL broadening from the QDs ordered on dislocation arrays were observed, and differences in PL dynamics were found.
Recent Trident single hot spot experiments: Evidence for kinetic effects, and observation of Langmuir decay instability cascade
Authors: Montgomery D. S. , Cobble J. A. , Fernández J. C. , Focia R. J. , Johnson R. P. , Renard-LeGalloudec N. , Rose H. A. , Russell D. A.
Published: July 2002

Single hot spot experiments offer several unique opportunities for developing a quantitative understanding of laser-plasma instabilities. These include the ability to perform direct numerical simulations of the experiment due to the finite interaction volume, isolation of instabilities due to the nearly ideal laser intensity distribution, and observation of fine structure due to the homogeneous plasma initial conditions. Experiments performed at Trident in the single hot spot regime have focused on the following issues. First, the intensity scaling of stimulated Raman scattering (SRS) for classically large damping regimes (kλD=0.35) was examined, and compared to classical SRS theory. SRS onset was observed at intensities much lower than expected (2×1015 W/cm2), from which nonclassical damping is inferred. Second, Thomson scattering was used to probe plasma waves driven by SRS, and structure was observed in the scattered spectra consistent with multiple steps of the Langmuir decay instability. Finally, scattering from a plasma wave was observed whose frequency and phase velocity are between an ion acoustic wave and an electron plasma wave. The presence of this wave cannot be explained by linear Landau theory, and it is shown to be consistent with a BGK-type mode due to electron trapping.
Organic light-emitting diodes with improved hole-electron balance by using copper phthalocyanine/aromatic diamine multiple quantum wells
Authors: Qiu Yong , Gao Yudi , Wei Peng , Wang Liduo
Published: July 2002

Organic light-emitting diodes (OLEDs) with multiple-quantum-well structures, which consist of copper phthalocyanine and N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4, 4′-diamine, have been fabricated. The effect of the MQW structure upon the performance of OLEDs with tris(8-hydroxyquinoline) aluminum as the emitting material has been investigated. Compared with the luminous efficiency of the conventional diode with the MQW number of 0, that of the diode with the MQW number of 4 has been sharply increased up to 10.8 cd/A. Such an improvement in the device performance was attributed to the improved hole-electron balance, which can be further attributed to the introduction of the MQW structure.
Longitudinal optical phonon assisted relaxation of excitons due to intersubband transitions in quantum wells
Authors: Oh I.-K. , Singh Jai , Vengurlekar A. S.
Published: July 2002

Longitudinal optical phonon assisted relaxation of excitons involving intersubband transitions is studied in quantum wells. Considering the relaxation of an exciton from a higher subband to a free electron–hole pair in lower subbands by longitudinal optical phonon emission, we calculate the rate of relaxation as a function of exciton density, temperature, and wave vector K∥ of the center-of-mass of exciton in quantum wells. The theory is finally applied to GaAs quantum wells and results are compared with experiment.
Measurements of ultrastrong magnetic fields during relativistic laser–plasma interactions
Authors: Tatarakis M. , Gopal A. , Watts I. , Beg F. N. , Dangor A. E. , Krushelnick K. , Wagner U. , Norreys P. A. , Clark E. L. , Zepf M. , Evans R. G.
Published: July 2002

Measurements of magnetic fields generated during ultrahigh intensity (>1019 W cm−2), short pulse (0.7–1 ps) laser–solid target interaction experiments are reported. An innovative method is used and the results are compared with particle-in-cell simulations. It is shown that polarization measurements of the self-generated harmonics of the laser can provide a convenient method for diagnosing the magnetic field—and that the experimental measurements indicate the existence of peak fields greater than 340 MG and below 460 MG at such high intensities. In particular, the observation of the X-wave cutoffs and the observed induced ellipticity of the harmonics can provide a reliable method for measuring these fields. These observations are important for evaluating the use of intense lasers in various potential applications and perhaps for understanding the complex physics of exotic astrophysical objects such as neutron stars.
A facility for characterizing the dynamic mechanical behavior of thin membranes for microelectromechanical systems
Authors: Hall J. D. , Apperson N. E. , Crozier B. T. , Xu C. , Richards R. F. , Bahr D. F. , Richards C. D.
Published: July 2002

A bulge testing system capable of applying static and dynamic loads to thin film membranes is described. The bulge tester consists of a sealed cavity, filled with a fluid, bounded on the bottom by a circular stainless steel diaphragm and on the top by the thin film membrane of interest. An actuator is used to apply either a static or a periodic force to the stainless steel diaphragm. The force is transmitted through the water to the thin film membrane. This facility provides for both accelerated lifetime testing and simulated service environment testing. The thin film membranes tested are composite stacks consisting of thin films of silicon, glass, metallic electrodes, and lead-zirconate-titanate. Pressure and deflection of a membrane are acquired simultaneously during loading. An image capture system coupled with an interferometer provides the means to capture interferograms of deflected membranes during both static and dynamic testing conditions. Images are then postprocessed to construct deflection versus pressure relationships, which can be used to extract materials’ properties. Accelerated lifetime testing is performed by subjecting the thin film membranes to cyclic loading at strain levels 45%–90% of the static failure strains. In simulated service environment testing thin film membranes are subjected to cyclic loading over a range of frequencies. For a given applied force, as the resonant frequency is approached the dynamic behavior of the thin film structures vary significantly from that observed for static loading. At resonance the deflection of a thin film membrane is almost three times that of a statically deflected membrane subjected to the same applied force.
Mechanisms of stochastic diffusion of energetic ions in spherical tori
Authors: Kolesnichenko Ya. I. , White R. B. , Yakovenko Yu. V.
Published: July 2002

Stochastic diffusion of the energetic ions in spherical tori is considered. The following issues are addressed: (i) Goldston–White–Boozer diffusion in a rippled field; (ii) cyclotron-resonance-induced diffusion caused by the ripple; (iii) effects of nonconservation of the magnetic moment in an axisymmetric field. It is found that the stochastic diffusion in spherical tori with a weak magnetic field has a number of peculiarities in comparison with conventional tokamaks; in particular, it is characterized by an increased role of mechanisms associated with nonconservation of the particle magnetic moment. It is concluded that in current experiments on National Spherical Torus eXperiment [M. Ono et al., Nucl. Fusion 40, 557 (2000)] the stochastic diffusion does not have a considerable influence on the confinement of energetic ions.
Low temperature specific heat of a typical glass forming alloy
Authors: Bai Hai Yang , Luo Jian Lin , Zhang Jie , Chen Zhao Jia
Published: July 2002

The Zr41Ti14Cu12.5Ni10Be22.5 alloy in short-range order (SRO) metallic glassy, medium-range order metallic glassy, crystallized, and equilibrium crystalline states was prepared. The low temperature specific heat Cp of the alloy in the different states has been investigated. The results show that the SRO glass has largest specific heat among the four states, and Cp decreases with the extent of crystallization or degree of structural ordering in the alloy. The densities of states at the Fermi level, N(EF), and Debye temperatures, θD, of the alloy are determined. It is found that N(EF) for the SRO glass is highest among the four states, and it decreases with the structural ordering. The phenomenon is interpreted by localization of electrons in glassy alloys. The value of θD increases with degree of structural ordering, indicative of marked soften transverse phonons in the SRO glass compared with the other states.
Critical properties of nanoporous low dielectric constant films revealed by Brillouin light scattering and surface acoustic wave spectroscopy
Authors: Flannery C. M. , Wittkowski T. , Jung K. , Hillebrands B. , Baklanov M. R.
Published: July 2002

Thin porous films with nanometer pore sizes are the subject of intense interest, primarily because of their reduced dielectric constant k. The lack of useful characterization tools and the reduction in film mechanical properties with increasing porosity have severely hindered their development and application. We show that both Brillouin light scattering and surface acoustic wave spectroscopy allow one to measure density, porosity and stiffness properties of nanoporous methylsilsesquioxane films of low-k value. Excellent correlations are observed among independent measurements of density, porosity and the Young’s modulus which show that the results obtained are reliable and reveal properties of the films which are difficult or impossible to obtain using other techniques.
Cluster structure and dynamics of liquid aluminum under cooling conditions
Authors: Hui Li , Guanghou Wang , Jijun Zhao , Xiufang Bian
Published: July 2002

The thermal evolution and dynamics properties of small clusters in melt are investigated by molecular dynamics simulation with 500 particles. The interatomic interaction is described by a well-fitted semiempirical tight binding potential. We have studied some solidlike clusters and structural properties of liquid aluminum. The atomic volume, internal energy, pair correlation function, and bonded pairs in liquid are obtained from quick cooling and slow cooling simulation. A comparison between the slow and quick cooling process reveals that the cooling rate significantly affects the evolution route of various clusters in liquid aluminum. Our results show that glass contains a lot of 1551 and 1541 five-fold symmetry clusters. On the contrary, the crystal has some 1421 and 1422 clusters corresponding to a face-centered cubic structure and hexagonal closing-packed structure. This work gives clear physical insight of the structure evolution from liquid to solid.
Self-organized growth of three-dimensional quantum-dot superlattices
Authors: Liu P. , Zhang Y. W. , Lu C.
Published: July 2002

The self-organization of three-dimensional quantum-dot superlattices that spontaneously form during heteroepitaxial growth is investigated by using a three-dimensional kinetic model. The model predictions show that the multilayer arrays of quantum dots can be fully or partially aligned vertically, or completely misaligned with respect to the buried islands, depending on the thickness of the spacer layer and the growth interruption time. In particular, there is a growth window in which a nearly uniform and regular array of dots is arranged in a squared lattice with an alternating stacking sequence; namely, the A–B–A–B stacking sequence.
Spatially-resolved valence-electron energy-loss spectroscopy of Zr-oxide and Zr-silicate films
Authors: Ikarashi Nobuyuki , Manabe Kenzo
Published: July 2002

We examined electronic structures in Zr-oxide (ZrO2) and Zr-silicate (ZrxSi1−xO2) films deposited on Si substrates by using valence-electron energy-loss spectroscopy combined with scanning transmission electron microscopy (the electron probe diameter was about 0.3 nm). Our analysis indicated that both valence-electron excitations in ZrO2 and in SiO2 occurred in the ZrxSi1−xO2 films. Therefore, the band gaps in the ZrxSi1−xO2 films should be dominated by an energy gap between O 2p and Zr 4d states.
ℏ expansion for the periodic orbit quantization of chaotic systems
Authors: Alonso D. , Gaspard P.
Published: July 2002

We report the results of a periodic orbit quantization of classically chaotic billiards beyond Gutzwiller approximation in terms of asymptotic series in powers of the Planck constant (or in powers of the inverse of the wave number κ in billiards). We derive explicit formulas for the κ−1 approximation of our semiclassical expansion. We illustrate our theory with the classically chaotic scattering of a wave on three disks. The accuracy on the real parts of the scattering resonances is improved by one order of magnitude.
Vortex dynamics in three-dimensional continuous myocardium with fiber rotation: Filament instability and fibrillation
Authors: Fenton Flavio , Karma Alain
Published: July 2002

Wave propagation in ventricular muscle is rendered highly anisotropic by the intramural rotation of the fiber. This rotational anisotropy is especially important because it can produce a twist of electrical vortices, which measures the rate of rotation (in degree/mm) of activation wavefronts in successive planes perpendicular to a line of phase singularity, or filament. This twist can then significantly alter the dynamics of the filament. This paper explores this dynamics via numerical simulation. After a review of the literature, we present modeling tools that include: (i) a simplified ionic model with three membrane currents that approximates well the restitution properties and spiral wave behavior of more complex ionic models of cardiac action potential (Beeler-Reuter and others), and (ii) a semi-implicit algorithm for the fast solution of monodomain cable equations with rotational anisotropy. We then discuss selected results of a simulation study of vortex dynamics in a parallelepipedal slab of ventricular muscle of varying wall thickness (S) and fiber rotation rate (θz). The main finding is that rotational anisotropy generates a sufficiently large twist to destabilize a single transmural filament and cause a transition to a wave turbulent state characterized by a high density of chaotically moving filaments. This instability is manifested by the propagation of localized disturbances along the filament and has no previously known analog in isotropic excitable media. These disturbances correspond to highly twisted and distorted regions of filament, or “twistons,” that create vortex rings when colliding with the natural boundaries of the ventricle. Moreover, when sufficiently twisted, these rings expand and create additional filaments by further colliding with boundaries. This instability mechanism is distinct from the commonly invoked patchy failure or wave breakup that is not observed here during the initial instability. For modified Beeler-Reuter-like kinetics with stable reentry in two dimensions, decay into turbulence occurs in the left ventricle in about one second above a critical wall thickness in the range of 4–6 mm that matches experiment. However this decay is suppressed by uniformly decreasing excitability. Specific experiments to test these results, and a method to characterize the filament density during fibrillation are discussed. Results are contrasted with other mechanisms of fibrillation and future prospects are summarized.
A coupled ordinary differential equation lattice model for the simulation of epileptic seizures
Authors: Larter Raima , Speelman Brent , Worth Robert M.
Published: July 2002

A coupled ordinary differential equation lattice model for the CA3 region of the hippocampus (a common location of the epileptic focus) is developed. This model consists of a hexagonal lattice of nodes, each describing a subnetwork consisting of a group of prototypical excitatory pyramidal cells and a group of prototypical inhibitory interneurons connected via on/off excitatory and inhibitory synapses. The nodes communicate using simple rules to simulate the diffusion of extracellular potassium. Both the integration time over which a node’s trajectory is integrated before the diffusional event is allowed to occur and the level of inhibition in each node were found to be important parameters. Shorter integration times lead to total synchronization of the lattice (similar to synchronous neural activity occurring during a seizure) whereas longer times cause more random spatiotemporal behavior. Moderately diminished levels of inhibition lead to simple nodal oscillatory behavior. It is postulated that both the lack of inhibition and an alteration in conduction time may be necessary for the development of a behaviorally manifest seizure.
Investigation of the effects of nanostructure on the observable behavior of magnetic thin film using large-scale computer simulation
Authors: Fu Hong , Giles R. , Mansuripur M. , Patterson G.
Published: July 2002

Large-scale computer simulations of magneto-optical thin films with nanoscale structures are performed on the Connection Machine. The magneto-optical thin films are modeled by a two-dimensional lattice of magnetic dipoles which follow the Landau–Lifshitz–Gilbert dynamic equation. The effective field on each dipole includes the local anisotropy, the nearest neighbor exchange interactions, the magnetic dipole–dipole interactions, and an externally applied field. The film is divided into many patches (regions) and each patch can have different properties. Four different patchy films are studied. (i) The film has different exchange stiffness constant on the patch borders. (ii) The film has large magnetization, but has zero exchange on the patch borders. (iii) The patches have different easy axis orientations. (iv) The patches have different anisotropy constants. The computer simulations show that films with different kinds of patches have different features in their magnetic behavior. Based on these correspondences between the nanostructure and the magnetic properties, certain features recently observed in several magneto-optical thin films are able to be explained.
Some portable very-long-period random number generators
Authors: Marsaglia George , Zaman Arif
Published: July 2002

It is found that a proposed random number generator ran2, recently presented in the Numerical Recipes column [W. H. Press and S. A. Teukolsky, Comput. Phys. 6, 521–524 (1992)], is a good one, but a number of generators are presented that are at least as good and are simpler, much faster, and with periods ‘‘billions and billions’’ of times longer. They are presented not necessarily to supplant ran2, but to put it in perspective. Any serious user of Monte Carlo methods should have a variety of random number generators from which to choose. In addition to two specific programs, one in Fortran and one in C, a framework is offered within which the readers can easily fashion their own generators with periods ranging from 1027–10101.
Numerical analysis for radiation-resistant GaAs heteroface solar cell structures
Authors: Yamaguchi Masafumi , Amano Chikara
Published: July 2002

Experimental studies and numerical analyses are carried out to optimize A1GaAs-GaAs heteroface solar cell structures. Carrier removal rate and damage constant for diffusion length in n-GaAs due to 1-MeV electron irradiation are found to be larger than those in p-GaAs. These results are explained by taking into account deep-level traps associated with radiation-induced defects. Numerical analysis shows that n++-n+-p heteroface cell structure is relatively radiation resistant in a shallow junction solar cell (below 0.2 μm) with a substrate carrier concentration above 3×1015cm−3. In the p++-p+-n heteroface solar cell, optimum junction depth and substrate carrier concentration are 0.2–0.3 μm and 2–5×1015 cm−3, while those in the n++-n+-p heteroface solar cell are less than 0.1 μm and 3–10×1015 cm−3.
Misfit stress in InGaAs/InP heteroepitaxial structures grown by vapor-phase epitaxy
Authors: Chu S. N. G. , Macrander A. T. , Strege K. E. , Johnston W. D.
Published: July 2002

Misfit stress in InGaAs/InP heteroepitaxial structures grown by hydride-transport vapor phase epitaxy has been studied using an x-ray technique to measure the wafer curvature and using an x-ray double crystal diffractometer to measure mismatches. The stress calculated from the measured radius of curvature was, in all cases, found to be smaller than the value predicted from the lattice mismatch using the simple beam theory and assuming a coherent interface. A simple argument is used to show that the parallel mismatch is directly related to the density of misfit dislocations. By taking into account both the presence of misfit dislocations and tetragonal distortion, relationships between the curvature, density of interfacial misfit dislocations, relaxed lattice mismatch, and measured tetragonally distorted vertical and parallel mismatches are derived. Predicted values for the radius of curvature agree very well with measured values. The results also indicate that in the presence of interfacial misfit dislocations the nature of the stress in the epitaxial layer is not uniquely determined by the mismatch. The stress can change sign depending upon the density of the misfit dislocations. This effect has actually been observed. The defect structure of the epitaxial layer is analyzed by Nomarski interference contrast microscopy, transmission x-ray topography, and transmission electron microscopy. We have found that the misfit stress can be greatly relaxed in the presence of lattice defects such as stacking faults. The theoretical calculation is further generalized from the single epilayer/substrate case to an n-layer heterostructure. The determination of the stresses in each layer and the misfit dislocation densities are discussed.
Dielectric isolation of silicon by anodic bonding
Authors: Anthony Thomas R.
Published: July 2002

Dielectrically isolated silicon was produced by anodically bonding together a pair of silicon wafers whose surfaces were covered with an electrically nonconductive micron layer of thermally grown oxide. Although anodic bonding normally requires a conductive oxide, anodic bonding works with nonconductive silicon oxide if the total layer of silicon oxide is less than ten microns thick. The time needed for the anodic bonding process decreases monotonically with temperature because the increase in the deformability of silicon oxide overcomes the decrease in the maximum permissible anodic bonding voltage with temperature. However, factors such as silicon degradation and electrode reactions at very high temperatures indicate that a compromise temperature range of 850–950 °C is best for the anodic bonding of silicon oxide. Bonding voltages of 30–50 V for times of about an hour produced the best bonding yields at these temperatures. Anodically bonded silicon wafers were examined with infrared and ultrasonic transmission microscopy for bond quality. Small scattered nonbonded zones comprising on the average 5% of the total wafer area were found in all wafers. These nonbonded zones were the result of dust particles, entrapped gas, and dimensional mismatches between multiple bonding fronts.
The constitution of nitrided oxides and reoxidized nitrided oxides on silicon
Authors: Naiman M. L. , Kirk C. T. , Emerson B. L. , Taitel J. B. , Senturia Stephen D.
Published: July 2002

Chemical analysis has been made of 361-Å thermal oxide films nitrided in ammonia and also subsequently reoxidized in oxygen at 1000 °C, using techniques of ellipsometry and infrared spectrometry. The nitrided film is shown to have a three-layer structure consisting of 22 Å of 48% nitrogen, 334 Å of 17% nitrogen, and 7.4 Å of 100% nitrogen, where the fractions refer to % N/(N+O). After oxidation, the interface layer was unchanged and the surface merged into the bulk, the nitrogen content of which was reduced to 11%. The void content has also been determined.
Channeling dependence of ion-beam-induced epitaxial recrystallization in silicon
Authors: Linnros J. , Holmén G.
Published: July 2002

Buried amorphous layers in silicon, formed by self-ion-implantation, have been regrown using 300-keV neon and nitrogen ion beams in the temperature range 200–400 °C. When the beams were aligned with the 〈100〉 axis in the crystal, the regrowth rate at the shallower amorphous/crystalline interface was reduced, while the deeper regrowth front proceeded with the same rate as that for a random beam direction. The observed channeling effect is attributed to a decreased nuclear energy deposition for the channeled ions, which suggests that the annealing mechanism involves point-defect migration in the crystalline material.
Superconducting critical temperatures, critical magnetic fields, lattice parameters, and chemical compositions of ‘‘bulk’’ pure and alloyed Nb3Sn produced by the bronze process
Authors: Suenaga M. , Welch D. O. , Sabatini R. L. , Kammerer O. F. , Okuda S.
Published: July 2002

Superconducting critical temperatures Tc and magnetic fields Hc2, lattice parameters a0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 μm or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values of Tc, a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat-treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 μm. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value of Tc (by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreased Tc. Also, small additions of these elements significantly increased Hc2. Specifically, the largest values of Hc2 (∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with a value for the pure Nb3Sn of 23.5 T at 4.2 K. The value of a0 decreased with all of the alloying additions; these variations can be explained qualitatively by several models for the lattice parameter of A15 compounds, but none of them can quantitatively predict the variations. In one system, (Nb,Ti)3Sn, values of the normal-state resistivity just above the transition temperature were measured: adding 3 at. % Ti raises the value to ∼55 μΩ cm from the value of 10–15 μΩ cm for pure Nb3Sn. This increase in the resistivity is thought to be a primary reason for the increased Hc2 for this system as well as the other types of alloyed Nb3Sn which were studied here.
Noble-metal-based transparent infrared reflectors: Experiments and theoretical analyses for very thin gold films
Authors: Smith G. B. , Niklasson G. A. , Svensson J. S. E. M. , Granqvist C. G.
Published: July 2002

Very thin gold films were prepared on glass by ion plating (IP) and by conventional evaporation (CE). Below a certain thickness—∼9 nm for IP and ∼15 nm for CE—the films comprised a metal network; above this thickness we found uniform films. Optical properties were recorded by spectrophotometry. Conspicuous near-infrared transmittance plateaus were seen in network films. This effect is conducive to high solar transmission. The spectral features were explained from effective medium theories based on the film structure. The uniform films were consistent with the Drude theory, provided that an anomalously large frequency dependence of the relaxation energy was invoked. Significant induced transmission was found in calculations on dielectric/gold/dielectric coatings. Our results lead to improved noble-metal-based transparent infrared reflectors for potential use on energy efficient windows.
Optical and electrical characterizations of laser–chemical-vapor-deposited aluminum oxynitride films
Authors: Demiryont H. , Thompson L. R. , Collins G. J.
Published: July 2002

Composition-dependent optical and electrical properties of (Al2O3)1−c-(AlN)c composite films (0≤c≤1) are investigated herein. The films are deposited by the laser-assisted, chemical-vapor-deposition (L-CVD) technique. Film composition is controlled by background vacuum level (e.g., O2) and substrate temperature. Optical parameters of the oxynitride films on quartz substrates are evaluated from spectrophotometric transmittance characteristics. Infrared absorption spectra are used to identify chemical composition of the films. Electrical-field-dependent current flow mechanisms are investigated for Al-insulator-n-Si metal-insulator-semiconductor (MIS) structures. Current-voltage (I-V) characteristics of the MIS structures exhibit two alternative states: the original and the programmed states. The original state I-V characteristics are obtained for virgin films by first application of the tracing voltage. Current density versus electric field follows both exp E and exp (E)1/2 regions. The MIS structure makes a transition from the original to a programmed state at high electric field due to the establishment of a persistent polarization field. Programmed MIS structures exhibit two alternative I-V characteristics corresponding to whether the applied and the polarization field are in the same or opposite directions. Capacitance-voltage (C-V) characteristics of L-CVD (Al2O3)1−c-(AlN)c devices exhibit composition-dependent hysteresis. A large clockwise hysteresis is observed for AlN while smaller size anticlockwise hysteresis is obtained for the Al2O3 case.
Polymer dispersed nematic liquid crystal for large area displays and light valves
Authors: Drzaic Paul S.
Published: July 2002

A new electro-optical material based on nematic liquid crystal dispersed in a polymer matrix has recently been introduced by Fergason. This technology (termed NCAP, for nematic curvilinear aligned phase) is suitable for making very large area (thousands of square centimeter) light valves and displays. The device consists of micron size droplets of liquid crystal dispersed in and surrounded by a polymer film. Light passing through the film in the absence of an applied field is strongly forward scattered, giving a milky, translucent film. Application of an electric field across the liquid crystal/polymer film places the film in a highly transparent state. Pleochroic dyes may be employed in the system in order to achieve controllable light absorption as well as scattering. Microscopically, it is shown that the liquid-crystal director lies preferentially parallel to the polymer wall, leading to a bipolar-like configuration of the liquid-crystal directors within the droplet. The symmetry axes of the droplets are randomly oriented in the unpowered, scattering state, but align parallel to the field in the powered, transparent state. The electric field required to reorient a given droplet varies inversely with the diameter of that droplet, and it is shown that the macroscopic electro-optical properties of the film can be modeled if the distribution of liquid-crystal droplet sizes is known.
Theoretical investigation of laser-sustained argon plasmas
Authors: Jeng San-Mou , Keefer Dennis R.
Published: July 2002

A theoretical model has been developed for laser-sustained argon plasmas in a forced convective axisymmetric pipe flow. The two-dimensional model based on Navier–Stokes equations was solved using a finite difference algorithm, and included geometric optics, temperature-dependent thermodynamic, transport, and optical properties, as well as radiation-induced thermal conductivity. The results showed good agreement with existing experimental data on temperature distribution, shape, and size of the plasma. The calculated velocity distribution revealed the complexity of the flow field and indicated that the constant axial mass flux (product of axial velocity and density) assumption adopted by existing one-dimensional and semi-two-dimensional models is not adequate. The radiation transfer was found to have a significant influence on the predicted temperature distribution and peak temperature of laser-sustained plasmas.
High-frequency noise and current-voltage characteristics of mm-wave platinum n–n+–GaAs Schottky barrier diodes
Authors: Zirath Herbert
Published: July 2002

The noise power spectral density from 100 MHz to 88 GHz has been measured for forward-biased platinum n–n+–GaAs Schottky barrier diodes at room temperature. There appear to be five sources of noise: shot noise, Johnson noise, hot-electron noise, intervalley scattering noise, and trap noise. The magnitudes of these sources are dependent on the bias condition, frequency, and structural parameters as well as the quality of the material. A trap mechanism that may explain the frequency-dependent noise is suggested. The relative importance of each source is investigated for different diodes versus forward bias and frequency. The current-voltage characteristics were also investigated, and show features at high forward bias which are not predicted from simple theories.
Alloy broadening in photoluminescence spectra of Ga0.47In0.53As
Authors: Charreaux C. , Guillot G. , Nouailhat A.
Published: July 2002

The low-temperature photoluminescence of undoped GaxIn1−xAs layers, lattice matched to InP, grown by liquid-phase epitaxy and molecular-beam epitaxy has been studied. In this work, we focus mainly on the origin of the line broadening of the two main emissions (excitonic and donor-acceptor pair transitions) observed. We find that, as it has been recently shown in GaxAl1−xAs, the dominant broadening mechanism is alloy broadening, due to random cation distribution. This model gives linewidths of the bound exciton and the donor-acceptor pair transitions based on compositional fluctuations within the crystal volumes which are characteristic of the two transitions. Calculated linewidths agree rather well with experimental results, thus demonstrating that alloy broadening leads also in GaxIn1−xAs to a quantitative understanding of the low-temperature photoluminescence spectra. Careful analysis of the donor-acceptor pair band yields acceptor activation energies of 15, 22, and 25 meV which are attributed to C, Zn, and Si, respectively.
Synchrotron radiation-excited chemical vapor deposition of SixNyHz film
Authors: Kyuragi Hakaru , Urisu Tsuneo
Published: July 2002

The photochemical vapor deposition of SixNyHz films (y/x≤1.1) is demonstrated, using the vacuum UV (VUV) from synchrotron radiation (SR) for the first time. In the configuration with the beam axis parallel to the substrate surface, the deposition rate achieved is about 0.05 nm/min for a 100 mA ring current and a 2.7/13.3 Pa SiH4/N2 pressure. Values of about nine times larger are obtained in the perpendicular configuration. It is shown that decomposition of adsorbed molecules by SR irradiation is the primary process involved in deposition in the perpendicular configuration.
Carrier concentration and activation energy in heavily donor-doped silicon
Authors: Schechter Daniel
Published: July 2002

The Lee–McGill [J. Appl. Phys. 46, 373 (1975)] model for computing the populations in heavily doped n-type silicon is applied with several objectives. First, it is used to evaluate Neumark’s [Phys. Rev. B 5, 408 (1972(; J. Appl. Phys. 48, 3618 (1977)] expressions for screening and for the decrease of the activation energy in such systems. Next, it is used to systematically investigate the decrease in activation energy as doping density increases. Lastly, it is used to evaluate the adequacy of using dilute concentration statistics to fit experimental data.
Ultrasonic attenuation in nickel and iron at 9.4 GHz
Authors: Homer R. , Alexandrakis G. C. , Dewar G.
Published: July 2002

Sound transmission experiments on single-crystal and polycrystalline nickel and iron samples 10–20 μm thick demonstrate an amplitude dependence on conductivity indicative of an electron damping effect. The samples form part of the common wall between two microwave cavities. Transverse sound waves were generated via magnetostriction with a static magnetic field perpendicular to the sample surface and perpendicular to the incident microwave magnetic field. Longitudinal sound waves were generated by direct electromagnetic excitation with a static magnetic field parallel to the sample surface and parallel to the incident microwave magnetic field. An exponential dependence of transmitted sound amplitude versus sample conductivity was observed as the temperature was varied. This exponential amplitude dependence on conductivity is what is expected from theories for sound wave attenuation in metals. For our samples this attenuation coefficient is of the order of 0.5 dB/μm. At low temperatures, where the product of the sound propagation constant and the electron mean free path approaches and exceeds unity, a decrease in the sound amplitude dependence on conductivity was observed, also in agreement with theory.
Plasma source ion-implantation technique for surface modification of materials
Authors: Conrad J. R. , Radtke J. L. , Dodd R. A. , Worzala Frank J. , Tran Ngoc C.
Published: July 2002

Plasma source ion-implantation (PSII) is a new ion-implantation technique which has been optimized for surface modification of materials such as metals, plastics, and ceramics. PSII departs radically from conventional implantation technology by circumventing the line-of-sight restriction inherent in conventional ion implantation. In PSII, targets to be implanted are placed directly in a plasma source and then pulse biased to a high negative potential. A plasma sheath forms around the target and ions bombard the entire target simultaneously. Preliminary experiments have demonstrated that PSII: (1) efficiently implants ions to concentrations and depths required for surface modification, (2) produces material with improved microhardness and wear properties, and (3) dramatically improves the life of manufacturing tools in actual industrial applications. For example, the tool life of M-2 pierce punches used to produce holes in mild steel plate has been increased by a factor of 80.
Explosive vaporization of small droplets by a high-energy laser beam
Authors: Chitanvis Shirish M.
Published: July 2002

We have constructed a model of the explosive vaporization of small droplets caused by the absorption of energy from a high-energy laser beam. The model consists of polarizable fluid streaming from the exploded droplet and interacting with laser radiation. A novel criterion for the explosive vaporization in the droplet is introduced, in which the absorbed energy density has to be greater than the restrictive pressure due to surface tension. Self-similarity is invoked to reduce the spherically symmetric problem involving hydrodynamics and Maxwell’s equations to simple quadrature. We have compared our model with experiments. An analytic solution has been obtained in a limiting case.
Thermal analysis of optical elements and arrays on thick substrates with convection cooling
Authors: Halley John M. , Midwinter John E.
Published: July 2002

By solving the heat equation for a Gaussian heating source on the surface of a thick substrate with forced cooling, the relative desirability of conductive and convective cooling strategies for removing heat from single elements and arrays has been investigated. Small sources or arrays are most efficiently cooled by diffusion into a thick heat sink and here temperature is not critically dependent on cooling strength. As the arrays get larger, however, the relative efficiency of conductive heat removal is less and it is desirable to maximize convective heat transfer instead. So large arrays require very thin substrates and strong convective cooling.