bluuebunny/crossref_metadata_2025 huggingface dataset:
Templating of cylindrical and spherical block copolymer microdomains by layered silicates
Authors: Silva Adriana S. , Mitchell Cynthia A. , Fu Tse Mun , Wang Hsien-C. , Krishnamoorti Ramanan
Published: July 2002

The influence of a highly anisotropic layered silicate (organically modified montmorillonite) in directing the mesoscopic self-assembly of a block copolymer blend is studied as a model for the development and tailoring of templated inorganic–organic hybrid materials. The potential for nanometer thick layers to induce large-scale mesoscopic ordering of cylindrical and spherical microdomains in asymmetric block copolymers is studied using a combination of rheology, electron microscopy, and small angle neutron scattering. Spherical microdomains arranged on a bcc lattice are templated by the anisotropic layered silicate and the kinetics of their growth are dramatically accelerated by the presence of even 0.1 wt.% (0.04 vol.%) of the filler. However, for cylindrical microdomain ordering, the kinetics are essentially unaffected by the addition of layered silicates and the development of three-dimensional mesoscopic order is possibly even disrupted. These results suggest that for the development of three-dimensional well-ordered nanostructures, the surface defining the pattern has to be significantly larger than the leading dimension of the structure being templated.
Large magnetostriction in directionally solidified FeGa and FeGaAl alloys
Authors: Srisukhumbowornchai N. , Guruswamy S.
Published: July 2002

The magnetostriction of Fe-x at. % Ga (x=15, 20, and 27.5) was measured, for alloys processed under different directional solidification conditions, and the effect of partial substitution of Ga with Al on the magnetostriction of the alloys was examined. Magnetostriction measurements were carried out at different prestress levels varying from 0 to 55 MPa. Ga additions in the range of 15–27.5 at. % Ga in Fe were found to improve the magnetostriction of the disordered bcc phase of Fe by as much as 1 order of magnitude. The applied fields for saturation magnetostriction and the hysteresis observed were small. Magnetostriction values as high as 271×10−6 were obtained in polycrystalline Fe–27.5 at. % Ga rods prepared using a directional growth (DG) process at a growth rate of 22.5 mm/h. This process, which is essentially a seedless vertical Bridgman technique, resulted in near [001] textured polycrystalline Fe–Ga alloys. The preferred [001] crystallographic orientation of the DG alloys was approximately 14° away from the rod direction. For Ga contents between 15 and 27.5 at. % in Fe, the Ga atoms increase the Fe–Fe spacing in the disordered bcc (A2) phase and reduce the magnetic moment of Fe. Substitution of Ga with Al has a significant effect on the magnetostriction of the Fe–Ga alloys. Small substitution of 5 at. % Al for Ga in the Fe–20 at. % Ga alloy increases the magnetostriction in Fe, and the value is slightly larger than that of the Fe–20 at. % Ga alloy. A higher substitution amount of Al tends to decrease the magnetostriction.
Molecular dynamics simulations of Ca2+ in water: Comparison of a classical simulation including three-body corrections and Born–Oppenheimer ab initio and density functional theory quantum mechanical/molecular mechanics simulations
Authors: Schwenk Christian F. , Loeffler Hannes H. , Rode Bernd M.
Published: July 2002

A classical molecular dynamics simulation including three-body corrections was compared with combined ab initio quantum mechanics/molecular mechanics molecular dynamics simulations (QM/MM–MD), which were carried out at Hartree–Fock (HF) and density functional theory (DFT) level for Ca2+ in water. In the QM approach the region of primary interest—the first hydration sphere of the calcium ion—was treated by Born–Oppenheimer quantum mechanics, while the rest of the system was described by classical pair potentials. Coordination numbers of 7.1, 7.6, and 8.1 were found in the classical, the HF, and the DFT simulation, respectively, using the same double-ζ basis set in both QM methods. The CPU time for one DFT step was about 50% above the time for a HF step, but due to a smaller number of steps needed for equilibration in the DFT case, there was no significant difference in the overall simulation time.
Rotationally resolved A2Πu←X 2Πg electronic transition of NC6N+
Authors: Linnartz H. , Pfluger D. , Vaizert O. , Cias P. , Birza P. , Khoroshev D. , Maier J. P.
Published: July 2002

The rotationally resolved A2Πu←X2Πg electronic origin band spectrum of dicyanodiacetylene cation, NC6N+, has been recorded in the gas phase using frequency-production double modulation spectroscopy in a liquid nitrogen cooled hollow cathode discharge and cavity ring down spectroscopy in a supersonic plasma. The analysis of the complementary results provides accurate molecular parameters for the two spin-orbit components in both electronic states.
A note on discretization of nonlinear differential equations
Authors: Mendes Eduardo M. A. M. , Billings S. A.
Published: July 2002

An important issue when integrating nonlinear differential equations on a digital computer is the choice of the time increment or step size. For example, it is known that if this quantity is not sufficiently short, spurious chaotic motions may be induced when integrating a system using several of the well-known methods available in the literature. In this paper, a new approach to discretize differential equations is analyzed in light of computational chaos. It will be shown that the fixed points of the continuous system are preserved under the new discretization approach and that the spurious fixed points generated by higher order approximations depend upon the increment parameter.
NiFe
2
O
4
nanoparticles formed in situ in silica matrix by mechanical activation
Authors: Zhou Z. H. , Xue J. M. , Wang J. , Chan H. S. O. , Yu T. , Shen Z. X.
Published: July 2002

Nanocrystalline nickel ferrite (NiFe2O4) particles were successfully synthesized in situ in an amorphous silica matrix by mechanical activation at room temperature. Phase development in the amorphous precursors, derived via a modified sol–gel synthesis route, with increasing mechanical activation time was studied in detail by employing transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. NiFe2O4 nanoparticles of 8.05 nm in mean particle size with a standard deviation of 1.24 nm, which were well dispersed in the silica matrix, were realized by 30 h of mechanical activation. The phase formation of nanocrystalline NiFe2O4 particles involves the nucleation of Fe3O4 in amorphous silica at the initial stage of mechanical activation, followed by the growth of nickel ferrite by incorporation of Ni2+ caions into Fe3O4. Their magnetic anisotropy, surface spin disorder, and cation distribution are investigated by considering both the strain imposed by silica matrix and the buffer effect during mechanical activation.
Measurement of laser-induced refractive index changes of Mg-doped congruent and stoichiometric LiNbO3
Authors: Pálfalvi László , Almási Gábor , Hebling János , Péter Ágnes , Polgár Katalin
Published: July 2002

Light-induced refractive index changes and nonlinear absorption of undoped and Mg-doped LiNbO3 with congruent and stoichiometric composition were investigated by the Z-scan method using an all-visible argon-ion laser with intensities up to the MW/cm2 level. Charge coupled device recordings demonstrate that the Z-scan method is suitable for quantitative study of Mg-doped samples, while it can be used only for qualitative measurements of undoped samples because of beam fanning in the latter case. Stoichiometric LiNbO3 doped with 5 mol % Mg shows positive refractive index changes opposite to those in cases of undoped congruent, undoped stoichiometric and 5 mol % Mg-doped congruent LiNbO3. The refractive index change of Δne=2.5×10−5 measured at 0.37 MW/cm2 light intensity for stoichiometric LiNbO3:Mg is six times smaller than that of congruent LiNbO3:Mg at 0.1 MW/cm2.
Magnetic-field and temperature dependence of the critical current in thin epitaxial films of the high-temperature superconductor YBa2Cu3O7−δ
Authors: Fedotov Yu. V. , Ryabchenko S. M. , Pashitskiı̆ É. A. , Semenov A. V. , Vakaryuk V. I. , Pan V. M. , Flis V. S.
Published: July 2002

The magnetic-field dependence (in a field H applied along the normal to the film plane z) and temperature dependence of the critical current density jc are investigated in high-quality biaxially oriented epitaxial films of YBa2Cu3O7−δ, with maximum values of jc reaching 2×106 A/cm2 in zero magnetic field at T=77 K. It is found that the value of jc(Hz,T), which is independent of Hz in the low-field region HzHm by the function jc(Hz,T)/jc(0,T)=α ln(H*/Hz) over a rather wide region 0.95>jc(Hz,T)/jc(0,T)>0.3. Here H*=Hme1/α is proportional to τ=1−T/Tc, at least in the temperature region where the measurements are made, and the parameter α is nearly independent of temperature. For the sample with the highest value of the critical current density for Hz→0 an anomalously sharp transition from the low-field plateau to the logarithmic behavior is observed. Some published data on jc(Hz,T) in epitaxial thin films of HTSCs are analyzed, and it is shown that an approximation of the given form is also applicable to the results of other authors. Although the value of jc(H=0,τ) itself, like the values of H*, are substantially different for films investigated by different authors, the values of α obtained when their results are approximated by a logarithmic dependence are extremely close to one another and to the values found in our studies. A model is proposed which qualitatively explains the nature of the observed behavior of jc(Hz,T), and the basic properties of this model are discussed. It is shown that the approximately logarithmic dependence is due to a mechanism involving the depinning of the ensemble of Abrikosov vortices pinned at edge dislocations in the low-angle grain boundaries that exist in epitaxial thin films of HTSCs. A sharp transition from the plateau to the logarithmic segment is realized if at H≈Hm there is a change of the mechanism governing the critical current density, specifically, in the case when for H<Hm the value of jc is limited more strongly by the transparency of the grain boundaries to the superconducting current than by the depinning of vortices in low fields.
CO2 laser with modulated losses: Theoretical models and experiments in the chaotic regime
Authors: Pando L. C. L. , Meucci R. , Ciofini M. , Arecchi F. T.
Published: July 2002

We compare two different theoretical models for a CO2 laser, namely, the two- and four-level model, and show that the second one traces with much better accuracy the experimentally observed chaotic dynamics when the cavity losses are sinusoidally modulated. Even though the two-level model provides a qualitative explanation of the chaotic dynamics, only the four-level one assures a quantitative fitting. We also show that, at the onset of chaos, the chaotic dynamics is low dimensional and can be described in terms of a noninvertible one-dimensional map.
A general purpose interactive programmable laboratory interface system using the IEEE-488 Bus
Authors: Hall B. D.
Published: July 2002

A programmable interface system, designed for general use in the laboratory, is described. The system is designed to be used as a peripheral on the IEEE-488 Instrumentation Bus and features an interactive Forth interpreter that allows for considerable flexibility in its applications. The system simplifies both hardware and software problems commonly encountered, in the design of data acquisition and control systems and makes them less dependent on specific features of the controlling computer. The interface system provides simple electrical connections that can be adapted to suit a wide range of instruments, and, through programming, the functionality of an interface system and its associated instruments can be tailored to particular situations. Several interface systems can operate simultaneously on the same bus, in which case a number of data acquisition or control tasks can be executed in parallel, each on a separate interface system CPU. The software implementation of the system is described in detail.
Using multiple RISC CPUs in parallel to study charm quarks
Authors: Stoughton C. , Summers D. J.
Published: July 2002

A system of 16 RISC CPUs has been integrated to help reconstruct and analyze a 1.3 Terabyte data set of 400 million high-energy physics interactions. These new CPUs provided an affordable means of processing a very large data set. The data were generated using a hadron beam and a fixed target at Fermilab Experiment 769. Signals were recorded on tape from particles created in or decaying near the target and passing through a magnetic spectrometer. Because all the interactions were independent, each CPU could completely reconstruct any interaction without reference to other CPUs. Problems of this sort are ideal for multiple processors. In the off-line reconstruction system, we used Exabyte 8 mm video tape drives with an I/O capacity of 7 Terabytes per year and a storage capacity of 2.3 Gigabytes per tape. This reduced tape mounts to one or two per day rather than one or two per hour as would be the case with 9-track tapes. The ETHERNETTM (a trademark of the Xerox Corporation) network used to link the CPUs and has an I/O capacity of 15 Terabytes per year. The RISC CPUs came in the form of commercially supported workstations with little memory and no graphics to minimize cost. Each 25 MHz MIPS R3000 RISC CPU processed data 20 times faster than 16 MHz Motorola 68020 CPUs that were also used. About 8000 h of processing was needed to reconstruct the data set. A sample of thousands of fully reconstructed particles containing a charm quark has been produced.
Pulsed laser codes for use as instructional tools
Authors: Stone David H.
Published: July 2002

Introductory laser courses often avoid much discussion of pulsed laser physics due to the necessity of solving coupled nonlinear rate equations. Several menu-driven PC-based codes have been prepared for student use at the Air Force Institute of Technology. The codes include point and traveling wave models of a generic Q-switched laser, and point models of a repetitively pulsed Q-switched Nd:YAG laser, a TEA CO2 laser, and a TEA CO2 amplifier. The codes use a predictor–corrector integration scheme to enhance numerical accuracy and minimize run time.
Symbolic and numerical computing for chemical kinetic reaction schemes
Authors: Holmes Mark H. , Au Yuklun , Stayman J. W.
Published: July 2002

The idea of using a symbolic manipulator as the engine for analyzing chemical reactions is developed. With the reactions as the input, it is shown how to symbolically derive the governing differential equations for the concentration of each species, the conservation laws for the scheme, and certain steady states. It is also shown how to use the information derived symbolically to construct, and then execute, a numerical scheme for solving the problem. This is done by linking the symbolic manipulator with an X-window display program that produces a graphical interface relating the data, the numerical solution, and the plotting routines. The routines are demonstrated using the Oregonator model that arises in the study of excitable systems. © 1995 American Institute of Physics.
Using symbolic manipulation to evaluate Feynman’s path integral for an interaction of the form F(t)⋅r
Authors: Bao Min-Qi , Starace Anthony F.
Published: July 2002

Obtaining Feynman’s path integral, which is a representation for the system’s Green’s function, is essential for many physical problems. In this paper, a symbolic algebra program is presented to evaluate analytically Feynman’s path integral for an interaction of the form F(t)⋅r. Such an interaction governs the motion of an electron in the field of one or more lasers as well as in a static electric field. © 1996 American Institute of Physics.
A new scheme for calculating weights and describing correlations in nonlinear least-squares fits
Authors: Hessler Jan P. , Current David H. , Ogren Paul J.
Published: July 2002

The equations for nonlinear least-squares analysis are reformulated in terms of dimensionless vectors and matrices. The diagonal elements of a dimensionless curvature matrix give the relative weights of the fit variables. Eigenvectors and eigenvalues of this matrix are used to describe the correlations between all of the parameters, and bivariant correlation coefficients may be calculated directly from its matrix elements. With the dimensionless formulation it is easy to compare confidence limits, correlations, and predictions based on the curvature matrix with results of Monte Carlo simulations. This provides a direct test of the parabolic approximation. Examples from a linear and biexponential model are presented to demonstrate these ideas. © 1996 American Institute of Physics.
Note on using computer simulations in teaching introductory digital electronics courses
Authors: Stafford Baines C. G. , Bacon Michael E.
Published: July 2002

In teaching modern digital electronics to physics students it is becoming increasingly important that both student and instructor learn to use simulation programs. We have found that many of the exercises that recent textbooks recommend for simulation including those of an elementary nature do not work. Using the SR flip-flop as an example, we indicate some of the pitfalls encountered. In an attempt to shorten the learning curve, we highlight the source of the problem and show how the problem can be solved. © 1997 American Institute of Physics.
Accurate and efficient evaluation of the Bose–Einstein functions g3/2 and g5/2
Authors: MacLeod Allan J.
Published: July 2002

Evaluation of the Bose–Einstein functions g3/2 and g5/2, where gn(x)=∑i=1∞xi/in over the physically meaningful range 0⩽x⩽1, is considered. We derive expansions in Chebyshev polynomials that are shown to be very efficient and accurate. © 1997 American Institute of Physics.
Principal-value integrals by a simple and accurate finite-interval method
Authors: Thompson William J.
Published: July 2002

A finite-interval method for computing principal-value integrals that is simple, accurate, and fast is described. Rather than using a numerical limit process near the singularity in the integrand, this method uses a power series expansion of the regular part of the integrand around the singularity to approximate the principal value over a finite interval. Outside this interval, where the integrand is well-behaved, conventional methods of integration can be used. © 1998 American Institute of Physics.
Using image processing and computer animation to analyze spatial and temporal dynamics in a pattern-forming system
Authors: Robbins K. A. , Gorman M.
Published: July 2002

The availability of inexpensive computer equipment for video acquisition and storage can improve the way data from systems with complex spatiotemporal dynamics are acquired, analyzed, and disseminated. Cellular flames exhibit ordered patterns that bifurcate to both periodic and chaotic states. While the general characteristics of the motion can be obtained from direct visual observation, the precise specification of the dynamics requires a more sophisticated quantitative analysis. The application of computer image processing and animation techniques can enhance the analysis of such dynamics. The article describes the entire process from experiment to dissemination and discusses the obstacles encountered at each step. © 1998 American Institute of Physics.
Optimal start of a Taylor integrator by control of local error
Authors: Córdoba-Torres Pedro , Enriquez Fernando , Fairén Vı́ctor
Published: July 2002

The aim of this work is to supply the necessary tools for an optimal choice of initial values of the Taylor-integrator parameters, step size, and order. The optimization in the first step is fundamental to the future evolution of both the global error and the complexity of the algorithm. The local error in the Taylor approximation is controlled by two methods for obtaining an accurate analytic expression for the local error as a function of step size and order. © 1998 American Institute of Physics.
The POOMA Framework
Authors: Reynders John V. W. , Cummings Julian , Dubois Paul F.
Published: July 2002

The Parallel Object-Oriented Methods and Applications (POOMA) Framework is described. The POOMA FrameworK is an integrated collection of C++ classes designed to increase simulation lifetime and agility, ease data-parallel interfaces, and improve portability across rapidly evolving high-performance computing architectures. (AIP)
Physical interpretation of eddy current losses in ferromagnetic materials. II. Analysis of experimental results
Authors: Bertotti G.
Published: July 2002

In the preceding paper [J. Appl. Phys. 57, 2110 (1985)], it was shown that the behavior of eddy current losses in ferromagnetic materials can be fully characterized, in general, in terms of two parameters, the effective number ñ0 of active magnetic objects present at low magnetizing frequency fm, and the field V0, determining the ability of the external field to increase the number of active objects with increasing fm. In this paper, the connection of ñ0, V0 with several microstructural and magnetic properties of iron-based ferromagnetic alloys is investigated. It is shown that, in agreement with theoretical predictions, ñ0 is, in general, weakly dependent on other physical properties, attaining values ∼1 in most cases. Only the application of a tensile stress leads to a drastic increase of ñ0 above this limit. V0, on the contrary, turns out to be very sensitive to the type of material. It continuously decreases with decreasing degree of magnetic orientation, changing by about two orders of magnitude when we pass from monocrystalline to nonoriented SiFe alloys. By mapping dynamic losses in the plane (V0, ñ0), it is shown that the loss behavior in the frequency range of applicative interest (fm∼50 Hz) is essentially governed by V0 only, while ñ0 does not play any significant role. The attempt to work out a unified interpretation of the observed properties of V0 leads to a most unexpected direct connection between hysteresis and dynamic losses, which probably underlies some general physical principle, governing the global features of the magnetization dynamics far beyond the details of specific situations.
Photoluminescence and electrical properties of close space vapor transport GaAs epitaxial layers
Authors: Mimila-Arroyo J. , Legros R. , Bourgoin J. C. , Chavez F.
Published: July 2002

Close spaced vapor transport (CSVT) epitaxial layers have been grown under water vapor partial pressure p ranging from 5×10−2 to 5 mm Hg for source temperatures of 800 and 850 °C and a substrate temperature of 730 °C, using undoped high-purity GaAs as source material and 〈100〉 chromium doped high-resistivity GaAs as substrate. From Hall measurements, all layers were found to be n-type with a majority carrier concentration in the range of (2–3) ×1017 cm−3 and a mobility 3100–3600 cm2 V−1 s−1 as p varies from 5.0 to 5×10−2 mm Hg. Photoluminescence measurements show the following dominant recombination processes: an exitonic peak at 1.514 eV, a free band acceptor at 1.498 eV, a donor acceptor at 1.490 eV, and two peaks involving complexes at 1.47 and 1.42 eV. These peaks depend on the water vapor pressure: for low values of p only the exitonic peaks exist; as p increases the photoluminescence becomes less efficient until it disappears for p=5.0 mm Hg. This study shows that CSVT-GaAs epilayers grown under proper conditions have high quality and could be used for producing some electronic devices.
A deep level transient spectroscopy analysis of electron and hole traps in bulk-grown GaAs
Authors: Auret F. D. , Leitch A. W. R. , Vermaak J. S.
Published: July 2002

The electron and hole traps occurring in melt-grown undoped n-type GaAs have been studied using deep level transient spectroscopy. Several electron traps with energies ranging from 0.81 to 0.14 eV below the conduction band, as well as three hole traps having energies 0.65, 0.56, and 0.43 eV above the valence band, were measured. Isochronal annealing in the temperature range 500–800 °C indicated that, under optimized ‘‘face-to-face’’ annealing conditions, an 800 °C heat treatment for 1 h resulted in the removal of all the electron traps, with the exception of the EL2 (0.81 eV). Near the surface, however, this annealing procedure reduced the EL2 concentration by more than an order of magnitude. It was further observed that, when a sample is annealed ‘‘face-to-face’’ with another sample, the total defect concentration is reduced much more than when a sample is covered with graphite as during a standard liquid-phase epitaxy (LPE) pregrowth anneal cycle at the same temperature. The effect of surface damage during mechanochemical polishing on defect concentration has also been demonstrated.
Minority-carrier lifetime in doped and undoped n-type CdxHg1−xTe
Authors: Pratt R. G. , Hewett J. , Capper P. , Jones C. L. , Judd N.
Published: July 2002

Measurements of minority-carrier lifetime have been carried out on slices of n-type CdxHg1−xTe (0.2<x<0.3) prepared by both normal Bridgman and accelerated crucible rotation (ACRT) Bridgman techniques. Some of these crystals were deliberately doped with a high level of a single impurity. A study of aging effects has been concluded. Lifetime variations with temperature are explained using a combination of band-to-band and Shockley–Read recombination processes. Frequently, a single level 10–30 meV below the conduction-band edge was assumed, but for ACRT crystals it was necessary to postulate a second level. Attempts to identify the origin of the centers, by measurements on doped crystals, were only successful in the case of iron since it acts as a recombination center without providing conduction electrons in CdxHg1−xTe.
Pt4Si9 formation by hot substrate ion beam mixing
Authors: Hung L. S. , Hong Q. Z. , Mayer J. W.
Published: July 2002

Direct, ion induced formation of metastable compounds in the Pt-Si system has been investigated by implantation with Xe ions through the PtSi-Si interface in the temperature range of 35–325 °C. The mixing increased parabolically with dose and exhibited an exponential dependence on reciprocal temperature with an apparent activation energy of about 0.11 eV. The mixed layer became progressively uniform in composition with increasing temperatures and a well-defined surface layer with a composition of Pt4Si9 was obtained at relatively high temperatures. The Pt4Si9 phase was indexed as a hexagonal crystal structure and transformed back into PtSi and Si at temperatures above 550 °C by thermal steady-state annealing.
Annealing effect on grain texture of cold-rolled 4.5% Si-Fe ribbons prepared by a rapid quenching method
Authors: Arai Ken Ichi , Yamashiro Yasumasa
Published: July 2002

4.5% Si-Fe ribbons with 280-μm thickness were cold rolled to 60-μm thickness with high thickness reduction rate about 30% per pass. In the ribbon the secondary recrystallization of (110)[001] orientation was sufficiently promoted and covered the whole ribbon surface after annealing at 1150 °C. In addition, misorientation angles of all grains were under 5° and the average grain misorientation from (110)[001] was 1.5°. The magnetic properties in the grain-oriented 4.5% Si-Fe ribbon were excellent: the maximum B8 was 1.86 T, and the minimum Hc was 2.2 A/m.
Monte Carlo calculation of the thermalization of atoms sputtered from the cathode of a sputtering discharge
Authors: Turner G. M. , Falconer I. S. , James B. W. , McKenzie D. R.
Published: July 2002

The Monte Carlo technique has been used to simulate the thermalization of sputtered atoms in the filling or background gas within a planar sputtering discharge. The model uses Thompson’s theoretical distribution of the energy of atoms sputtered from the cathode surface together with an approximation to the 6–12 Lennard–Jones potential to describe collisions between sputtered atoms and background gas atoms. The diffusion of the thermalized atoms is included explicitly in the Monte Carlo calculations. The velocity distribution of sputtered atoms between the cathode and substrate is calculated, from which their average kinetic energy is determined as a function of the product Pd, where P is the background gas pressure and d is the distance from the cathode. Because of the effect of the finite cathode-substrate separation on back diffusion, it was found that PD, where D is the cathode-to-substrate separation, is an important parameter in describing thermalization and, consequently, the average energy of sputtered atoms throughout the discharge. These calculations are compared with theoretical analyses of other workers and show good agreement with experimental results.
GaSb booster cells for over 30% efficient solar-cell stacks
Authors: Fraas L. M. , Girard G. R. , Avery J. E. , Arau B. A. , Sundaram V. S. , Thompson A. G. , Gee J. M.
Published: July 2002

The fabrication of GaSb infrared-sensitive photovoltaic cells designed to boost the energy-conversion efficiency in tandem solar cell stacks is reported. Located behind GaAs solar-cells in 50× concentrated light configurations, these GaSb cells will boost the stack efficiency by 6.5 percentage points for space (AM0) and 7.0 percentage points for terrestrial (AM1.5D) applications. Assuming a GaAs cell efficiency of 26.7% (AM1.5D, 50×) as recently reported, the GaAs on GaSb stack efficiency will be 33.7%. Reduced series resistance in future GaSb cells will allow tandem-stack energy-conversion efficiencies over 35%.
Interaction of lateral cracks and plastic zones
Authors: Tu S.-H. L. , Scattergood R. O.
Published: July 2002

A computational method is developed for analyzing interactions between near-surface lateral cracks and residual stresses due to localized plastic zones. The latter are approximated as either prismatic or shear dislocation dipole pairs. Two-dimensional plane-strain conditions are assumed. A complete linear elastic solution is obtained using half-space dislocations for the dipole pairs and for the dislocation distribution function that accounts for the stress-free crack surface boundary condition. The crack-tip stress intensity factors show that mode-II driving forces are important for both prismatic and shear zones. The tendency for crack deflection to the free surface derived from energy release rates is complex. The dipole representation results do not provide a simple rationalization for lateral crack chipping.
InAs/AlSb/GaSb single-barrier interband tunneling diodes with high peak-to-valley ratios at room temperature
Authors: Chen J. F. , Wu M. C. , Yang L. , Cho A. Y.
Published: July 2002

We have fabricated an InAs/AlSb/GaSb single-barrier interband tunneling diode by molecular beam epitaxy. In this structure, a large tunneling current can be obtained by taking the advantage of the large heterojunction-conduction band to valence band overlap (0.15 eV) between InAs and GaSb which offers flexible designs of the AlSb barrier thickness and the doping concentrations. We have obtained a negative differential resistance with a peak-to-valley current ratio as high as 4.7 and a peak current density of 3.5 kA/cm2 at room temperature with a 1.5-nm-thick AlSb barrier. The current transport mechanism in this tunneling structure will be discussed according to the I-V characteristics as a function of temperature.
Oxygen in gallium arsenide
Authors: Bourgoin J. C. , Stievenard D. , Deresmes D. , Mimila Arroyo J.
Published: July 2002

The evolution of the deep level transient spectroscopy spectrum associated with the EL2 defect in epitaxial as well as bulk GaAs materials when they are contaminated by oxygen has been examined in detail. The degree of contamination is evaluated by ion mass spectroscopy. It has been observed that, as previously noticed, the EL2 spectrum contains two components. From the variations of the amplitude of these components versus time at different temperatures and versus electric field, it is concluded that, while one of these components is associated with the EL2 defect, the other one originates from the interaction of EL2 with interstitial oxygen. The deformation of the EL2 spectrum which is currently observed when using aluminum Schottky barriers is then understood as an effect of oxygen contamination and not due, as previously argued, to changes in the barrier characteristics.
Low-high homojunction in the stationary state
Authors: Kuznicki Z. T.
Published: July 2002

This work develops an analytical description of the electrostatic (stationary state) properties of an abrupt low-high homojunction within the context of Maxwell–Boltzmann statistics (nondegenerate semiconductors). The considerations are based on an application of the electrostatic law of abrupt junctions and summarized by a generalized approximate solution of Poisson’s equation as an analytical representation combined with adapted numerical procedures. Low-field calculations give a set of universal curves which may be used to find direct relationships between the different distributions at any point within the free carrier space charge. This approach, having a relatively simple behavior as well as very high accuracy, shows especially the macroscopic features of the accumulated space charge on the lightly doped side and is especially useful in junction modelization and numerical simulations.
Two-photon absorption and self-phase modulation in InGaAsP/InP multi-quantum-well waveguides
Authors: Tsang H. K. , Penty R. V. , White I. H. , Grant R. S. , Sibbett W. , Soole J. B. D. , LeBlanc H. P. , Andreadakis N. C. , Bhat R. , Koza M. A.
Published: July 2002

We report the first measurement of two-photon absorption (TPA) and self-phase modulation in an InGaAsP/InP multi-quantum-well waveguide. The TPA coefficient, β2, was found to be 60±10 cm/GW at 1.55 μm. Despite operating at 200 nm from the band edge, self-phase modulation as high as 8±2 rad was observed for 30-ps optical pulses at 3.8-W peak input power. A theoretical calculation indicates that this enhanced phase modulation is primarily due to bandfilling in the quantum wells and the free-carrier plasma effect.
Investigation of interface states in (Sr,Ca)TiO3−x-based ceramics
Authors: Nakano Yoshitaka , Watanabe Masamitsu , Takahashi Tomoharu
Published: July 2002

Interface states in (Sr,Ca)TiO3−x based ceramics have been investigated in view of the oxygen chemisorption and the oxidation. By means of deep level transient spectroscopy and isothermal capacitance transient spectroscopy techniques, a discrete acceptor type trap level was detected with reoxidation below the oxidation temperature of grain surfaces, whereas two deep levels were detected with reoxidation above this temperature. Electron spin resonance spectra indicate that the chemisorbed oxygen O− and O2− are stable with reoxidizing anneals below and above the oxidation temperature, respectively. The reoxidation induced change in the interface states can be well explained by the degradation of the chemisorbed oxygen using the O-center model. These interface states are considered to originate from the chemisorbed oxygen O− and O2− on grain surfaces. It is concluded that the oxidation of the grain surfaces has a significant role in stabilizing the O2− chemisorption level.
Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly
Authors: Lee C. H. , Wong Y. M. , Doherty C. , Tai K. L. , Lane E. , Bacon D. D. , Baiocchi F. , Katz A.
Published: July 2002

The possibility of replacing Pt in the Ti/Pt/Au base and traditionally used metallurgical structure by Ni, while bonding InP laser chip to a submount with AuSn (80% Au) solder, has been investigated. Various Ni-based metal alloys have been prepared by evaporation. Reflow experiments were conducted in a chamber under forming gas-controlled ambient. The Ti/Ni/AuSn system provided much longer surface local freezing duration compared to the Ti/Pt/AuSn system. Scanning electron microscopy analysis revealed a smoother surface morphology for the Ti/Ni/AuSn system after the metal refroze. Auger electron spectroscopy depth profiles indicated the formation of a Ni-Sn-Au interacted layer. The interaction took place in two steps: the first stage was the dissolution of Ni into the Au-Sn liquid followed by precipitation of a Ni-Sn-Au intermetallic compound; the second stage was a solid-state interdiffusion of Sn, Au, and Ni which occured in the interacted layer and in the original Ni layer. The latter step was a diffusion-controlled process, resulting in a very slow growth rate. Both Au and Sn reacted to form Ni alloy layers of almost equal thickness, regardless of the reaction duration (up to about 5 min). This intensive reaction, however, did not lead to full consumption of the Ti interfacial layer, which provided an excellent adhesion layer between the submount and the metallurgical structure.
Comparative study of thin poly-Si films grown by ion implantation and annealing with spectroscopic ellipsometry, Raman spectroscopy, and electron microscopy
Authors: Boultadakis S. , Logothetidis S. , Ves S.
Published: July 2002

Polycrystalline silicon (poly-Si) thin films prepared by recrystallization of α-Si either with annealing or ion implantation and annealing have been studied with the optical techniques of spectroscopic ellipsometry (SE) and Raman spectroscopy (RS), as well as with transmission electron microscopy (TEM). A detailed analysis of the SE dielectric function ε(ω) with the effective-medium theory (EMT) has demonstrated the formation of a silicon oxinitride layer on the top of the poly-Si films and a rms surface roughness of about 10 Å. The experimental ε(ω) data have been also analyzed with the second-derivative technique in order to study the effect of the different ion fluencies on the films’ properties and the results are compared with those from TEM. Based on the presented experimental findings we propose the following: (a) The ion-implanted and annealed samples exhibit grain sizes several times larger than those of the only annealed samples; (b) implantation with low ion energies and concentrations induces a disordered material similar to that obtained by annealing; and (c) the mechanism that is responsible for the decrease of the lifetime of the excited carrier is mainly the scattering at the microcrystallite boundaries. Furthermore, the study of the Raman TO vibrational modes reveals the existence of compressive stress in the Si substrate and tensile stress in the poly-Si films. For the latter, results from both RS and SE suggest that the average stress amounts to 2–3 kbar. Finally, the use of a broadening-dependent dielectric function as reference instead of that of c-Si in the EMT analysis, as a consequence of the finite grain size, leads to acceptable structural characteristics similar to those obtained by electron microscopy and RS.
Principles for controlling the optical and electrical properties of hydrogenated amorphous silicon deposited from a silane plasma
Authors: Hishikawa Yoshihiro , Tsuda Shinya , Wakisaka Kenichiro , Kuwano Yukinori
Published: July 2002

The optical, electrical, and structural properties of hydrogenated amorphous silicon (a-Si:H) films are systematically investigated as functions of the substrate temperature (Ts) and plasma parameters, such as the rf power, gas pressure, and electrode dimensions. The films are deposited by the plasma chemical vapor deposition method. The properties of a-Si:H can be controlled over a wide range by varying the plasma parameters at fixed Ts. Reducing the film deposition rate and raising Ts have the same effect on the properties of a-Si:H. A unified relationship is found to exist among those properties of a-Si:H in the range of deposition conditions in this study, which includes ‘‘device-quality’’ conditions. No apparent effects of gas-phase polymerization or ion bombardment are observed. The experimental results suggest that during device-quality a-Si:H film deposition under conventional plasma conditions, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at or near the film-growing surface. The limitations on the controllability of plasma-deposited a-Si:H, especially at low Ts, can be surmounted by adding hydrogen or helium to the plasma, and by treating a-Si:H with the hydrogen plasma.
Luminescence of nanometer-sized amorphous silicon nitride solids
Authors: Mo Chi-mei , Zhang Lide , Xie Cunyi , Wang Tao
Published: July 2002

The energy levels of defect states in nanometer-sized amorphous silicon nitride solids were systematically studied in terms of ultraviolet emission spectra. Six emission bands were observed, corresponding to 3.2, 2.8, 2.7, 2.4, 2.3, and 2.0 eV, respectively. With increasing the heat-treated temperature from room temperature to 1000 °C in low vacuum, these emission bands became high. For the specimen heated at 1000 °C, a new emission band of 3.0 eV appeared. The appearance of these emission bands is closely related to the formation of the energy levels of the defect states in the energy gap. The origin of these emission bands are discussed in detail.
Determination of the lattice contraction of boron-doped silicon
Authors: Holloway H. , McCarthy S. L.
Published: July 2002

We report a study of the change in lattice constant when single-crystal silicon is substitutionally doped with boron. The measurements were made using 10-μm-thick epilayers with boron concentrations (NB) in the range 1.7×1019–1.2×1020 cm−3. The influence of elastic strain in the epilayers and their substrates was eliminated by including Bragg reflections from planes that were inclined to the (100) surface. We obtained a value for the lattice contraction coefficient β=(5.19±0.09)×10−24 cm3, where the range is ± one standard error. Specimens with NB≊2×1019 cm−3 were strained to give coherent interfaces with their substrates. Evidence for relaxation (presumably by misfit dislocations at the interface) was observed with NB≊5×1019 cm−3 (corresponding to a lattice mismatch of approximately 2.5×10−4), but this occurred in a patchy fashion and remained incomplete while NB was increased to 1.2×1020 cm−3 (corresponding to a mismatch of 6.5×10−4). This partial relaxation occurred at somewhat smaller mismatches than would be expected from reported studies of silicon-germanium alloy epilayers on silicon substrates. Our technique for analyzing a set of Bragg reflections to separate the effects of elastic strain and lattice mismatch appears to be novel and has general applicability to the study of epilayer/substrate combinations.
Magnetoresistance and magnetization in submicron ferromagnetic gratings
Authors: Shearwood C. , Blundell S. J. , Baird M. J. , Bland J. A. C. , Gester M. , Ahmed H. , Hughes H. P.
Published: July 2002

A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron- and ion-beam lithography enabling high-quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto-optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the width of the sample beneath saturation for transverse applied fields. In this case, the detailed form of the field dependence of the MR is strikingly modified from that observed in the continuous film and is consistent with coherent rotation of the magnetization.
Exciton magnetic polarons in quantum wells
Authors: Stirner T. , Hagston W. E. , Harrison P. , Goodwin J. P.
Published: July 2002

Theoretical calculations of free-exciton magnetic polaron energies in semimagnetic quantum-well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated. Polaron energy calculations in diffused semimagnetic quantum wells show a strong dependence of the polaron energy on the amount of diffusion in the quantum well. Consequently, magnetic polaron energies could be used as a means of measuring the diffusion coefficient.
Space-charge physics and the breakdown process
Authors: Blaise G.
Published: July 2002

The trapping of charges in hard dielectric materials has been recently described [G. Blaise, IEEE Trans. Electr. Insul. EI-28, 437 (1993)] on the basis of the polaron concept. A local increase of the internal energy of the medium Up∼5–10 eV results from the polarization around a trapped charge. The relaxation of the lattice after a rapid detrapping of charges from their site releases the local excess of internal energy to the medium, producing breakdown when critical conditions are reached. This scenario is applied to the electric breakdown of a parallel-plate capacitor in which the space charge results from the injection of electrons at the cathode. Breakdown at the cathode is produced by the destabilization of a static space charge when the field in dielectric gap exceeds the detrapping field of charges. This determines the minimum value ‖EM1‖ of the field strength. At the anode breakdown is due to the trapping-detrapping of flowing charges. This corresponds to the maximum value of the field strength ‖EM2‖. The model allows the interpretation of scaling laws observed experimentally: an inverse relationship of the field strength with the dielectric constant ε; a dependence of the field strength on the length l of the gap (size effect). It is demonstrated that, provided the total amount of charges is bounded when l→∞, the field strength ‖EM2‖ is necessarily a decreasing function of l, tending toward a finite value. This interpretation of breakdown applies to bulk breakdown observed in small-gap capacitors and surface flashover observed in large-gap capacitors as well.
Electronic conductivity of hydrogenated nanocrystalline silicon films
Authors: Hu G. Y. , O’Connell R. F. , He Y. L. , Yu M. B.
Published: July 2002

A heteroquantum-dots (HQD) model for hydrogenated nanocrystalline silicon films (nc-Si:H) is proposed. The main features of our model are as follows. (i) the nanocrystalline grains and the amorphous counterparts in which they are embedded have very different band gap and band structures. As a result, they form heterojunctionlike structures in the interface regions, where the band offset effects dramatically reduce the activation energy and the grains act like quantum dots. (ii) In the presence of an external field, the activated electrons in the quantum dots conduct via quantum tunneling through the interface barriers. By means of the HQD model, we have identified the conduction of nc-Si:H as a thermal-assisted tunneling process. Our results show that there are two distinct regimes for the conductivity of nc-Si:H: (i) the low-temperature regime, where there is a simple activation energy ΔE; (ii) the high-temperature regime, where ΔE is effectively enhanced by the temperature effect of the electronic tunneling in the nanoscale particles. The theory is in good agreement with the experiments.