bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of hydrodynamic expansion on specific power deposition by a heavy ion beam in matter
Authors: Tahir N. A. , Kozyreva A. , Spiller P. , Hoffman D. H. H. , Shutov A.
Published: July 2002

In this paper, we show with the help of two-dimensional numerical simulations that the specific power deposition by a heavy ion beam in matter may significantly decrease due to hydrodynamic expansion of the target during irradiation. It has also been shown that in order to maximize the specific energy deposition, one is required to determine an optimum set of beam and target parameters including ion energy, beam radius, and pulse length. Three different values for the beam radius, namely, 0.5, 1.0, and 1.5 mm are considered, respectively. The target is a solid lead cylinder, which is irradiated by a uranium beam that consists of 1012 ions with a particle energy of 400 MeV/u. Such beam parameters will be available at the future heavy ion synchrotron, SIS-200 (with a magnetic rigidity of 200 Tm) at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt. It is also assumed that the beam is incident on one face of the cylinder and the cylinder length is less than the range of the projectile ions. The ions therefore penetrate the target, deposit a fraction of their energy in the target material along their trajectory, and escape through the opposite face of the cylinder with a substantially reduced energy. The Bragg peak therefore lies outside the target and the energy deposition is approximately uniform along the target length. This beam–target configuration generates an extended volume of high-energy–density matter, without any sharp gradients.
Possibility of a direct solution of Vlasov–Maxwell equations in tokamaks in the ion cyclotron range of frequencies
Authors: Svidzinski V. A. , Swanson D. G.
Published: July 2002

The possibility of a solution of the Vlasov–Maxwell equations as a time-dependent problem in a tokamak geometry at ω∼ωci is studied. The major approximations in the analysis are the following: small Larmor radius relative to the wavelength, particle orbits deviate little from a magnetic surface during the time of the dominant contribution to the orbit integral. This direct integration becomes possible due to a rapid growth of the computing power and the development of multiprocessing. A proper description of the plasma dielectric properties in this time-dependent approach is made. Due to a limited computing power and not completely optimized procedure applied in the analysis, the time evolution of Maxwell equations is limited to cases of relatively small perpendicular wave numbers. The results demonstrate that it is computationally feasible to perform a direct integration of the time-dependent Vlasov–Maxwell equations in a tokamak geometry on a substantially long time interval. The directions of the further optimization of this method are discussed and the suggestions for the further analysis are made.
Direct observation of electrical charges at dislocations in GaAs by cross-sectional scanning tunneling microscopy
Authors: Ebert Ph. , Domke C. , Urban K.
Published: July 2002

We demonstrate the possibility of simultaneous determination of the type and electrical charge state of dislocations in GaAs by cross-sectional scanning tunneling microscopy (STM). The methodology is demonstrated for a dissociated perfect dislocation in highly Si-doped GaAs(110) surfaces. The STM images of the dislocation penetrating GaAs cleavage surface show that both partial dislocation cores as well as the stacking fault between the two partial dislocation cores are negatively charged.
Potential phase control of chromium oxide thin films prepared by laser-initiated organometallic chemical vapor deposition
Authors: Cheng Ruihua , Borca C. N. , Dowben P. A. , Stadler Shane , Idzerda Y. U.
Published: July 2002

We have used laser-initiated chemical vapor deposition to grow the chromium oxide thin films through the oxidation of Cr(CO)6 in an oxygen environment. While both Cr2O3 and CrO2 are present in the film, the relative weight of each phase depends on the oxygen partial pressure. The Curie temperature of the film increases and approaches the bulk TC of CrO2 (397 K) as the partial oxygen pressure is increased.
Dielectrophoresis with application to boiling heat transfer in microgravity. I. Numerical analysis
Authors: Snyder Trevor J. , Schneider John B. , Chung Jacob N.
Published: July 2002

This article presents a numerical analysis for the use of a dielectrophoretic (DEP) force on vapor bubbles to sustain nucleate boiling heat transfer in space where the gravity-driven buoyancy force is absent. The analytic and numerical solution procedures for the DEP force are outlined for an infinite-plate and a finite-plate electrode geometry, respectively. A simple analysis is presented that describes the limits on the magnitude of the DEP force. A comparison is made between the DEP forces produced near the edges of the finite-plate electrodes (where there is a relatively high-gradient electric field) to the forces produced between the electrodes and away from the edges (where there is a relatively low-gradient electric field). The predicted DEP forces are then used to study the effectiveness of bubble transport for both high and low electric-field gradients in microgravity. To effectively maintain boiling in space with the DEP force, a knowledge of the electric field is required which addresses both the DEP force strength and the DEP force spatial distribution in order to appropriately balance between the short-distance vapor bubble detachment needs and the long-distance bubble transport requirements.
Stacking fault effects in pure and n-type doped GaAs
Authors: Schmidt T. M. , Justo J. F. , Fazzio A.
Published: July 2002

Using ab initio total-energy calculations, we investigate the effects of stacking faults on the properties of dopants in pure and n-type doped GaAs. We find that the Si impurity segregates towards a GaAs stacking fault. A Si atom at a Ga site in the stacking fault, in either a neutral or a negative charge state, is energetically favorable as compared to a Si atom at a Ga site in a crystalline environment by as much as 0.2 eV. We also find that a Si impurity in the stacking fault cannot occupy metastable positions, as occurs in the formation of DX centers. Thus, stacking faults can prevent the formation of DX-like centers in GaAs.
Mesoscopic counterpart of a hyperchaos
Authors: Wang Hongli
Published: July 2002

The master equation for a chemical hyperchaos model has been studied by stochastic simulations in this paper. It was revealed that intrinsic fluctuations have intensive effect on hyperchaos, having dramatically altered the dynamics and phase space portrait of the deterministic hyperchaos. This is remarkable because the drastic effect displayed by internal noises on hyperchaos is in considerable contrast with the case of simple type chaos where the effect of intrinsic fluctuations is trivial.
Free energy approximations in simple lattice proteins
Authors: Reith Dirk , Huber Thomas , Müller-Plathe Florian , Torda Andrew E.
Published: July 2002

This work addresses the question of whether it is possible to define simple pairwise interaction terms to approximate free energies of proteins or polymers. Rather than ask how reliable a potential of mean force is, one can ask how reliable it could possibly be. In a two-dimensional, infinite lattice model system one can calculate exact free energies by exhaustive enumeration. A series of approximations were fitted to exact results to assess the feasibility and utility of pairwise free energy terms. Approximating the true free energy with pairwise interactions gives a poor fit with little transferability between systems of different size. Adding extra artificial terms to the approximation yields better fits, but does not improve the ability to generalize from one system size to another. Furthermore, one cannot distinguish folding from nonfolding sequences via the approximated free energies. Most usefully, the methodology shows how one can assess the utility of various terms in lattice protein/polymer models.
Photoacoustic study of layer thickness and moisture content from varnished packaging materials
Authors: Walther H. G. , Christ A.
Published: July 2002

By means of a photoacoustic gas-cell technique we estimated the thickness of varnish layers on base paper. Interpretation of the measurements was supported by a theoretical model which describes the packaging material as a multilayer stack with freely eligible thermal and optical properties. The variable moisture content could be taken into account using mixture formulas for the definition of thermal effusivity and thermal diffusivity at different percentages of water. Satisfying correspondence between photoacoustic phase slope measurements and calculations simulating water migration was able to be achieved.
Room-temperature epitaxial growth of CeO2(001) thin films on Si(001) substrates by electron beam evaporation
Authors: Ami T. , Ishida Y. , Nagasawa N. , Machida A. , Suzuki M.
Published: July 2002

Epitaxial growth of CeO2(001) thin films on Si(001) substrates was achieved by electron beam evaporation. Reflection high-energy electron diffraction and cross-sectional high-resolution transmission electron microscopy established the formation of an epitaxial CeO2(001)/Si(001) structure with a cube-on-cube configuration. The epitaxial structure had to be formed at a temperature below 200 °C with a Si(001)-2×1, 1×2 reconstructed surface, and it could be formed even at room temperature. In order to improve the crystallinity, homoepitaxial growth conditions at a higher temperature with a high oxygen flow rate were also investigated. Homoepitaxy of ceria grown on a 5-nm-thick initial layer was demonstrated by θ/2θ-scan and φ-scan of x-ray diffraction.
Nanoscale surface electrical properties of indium–tin–oxide films for organic light emitting diodes investigated by conducting atomic force microscopy
Authors: Lin Heh-Nan , Chen Sy-Hann , Perng Gung-Yeong , Chen Show-An
Published: July 2002

Nanoscale surface electrical properties of indium–tin–oxide films prepared by different cleaning methods for use as anode materials in organic light emitting diodes are studied by conducting atomic force microscopy. It is found that most of the surface area possesses a nonconducting feature, and an ultraviolet-ozone treatment produces the most nonconductive sample. The conducting regions, which distribute randomly and range from 6 to 50 nm in size, are attributed to the existence of Sn-rich oxide by a comparison with reported scanning electron microscopy images. After scanning the tip with a bias of −8 V on the nonconducting regions, oxide decomposition occurs on as-received and wet-cleaning processed samples, whereas no structure change appears on the ozone treated sample. The results indicate that the generation of stable oxide after ozone treatment is one of the origins for improved device performance.
Magneto-optical properties of stacked self-assembled InAs quantum dots
Authors: Gonzalez J. C. , Plentz F. , Köche N. , Rodrigues W. N. , Moreira M. V. B. , de Oliveira A. G. , da Silva M. I. N. , Andrade M. S.
Published: July 2002

In this article, we report magneto-photoluminescence measurements on stacked self-assembled InAs quantum dots. By applying a magnetic field parallel to the growth direction, we determined the exciton reduced mass and exciton radius from the photoluminescence (PL) peak energy. We observed an asymmetric increase of the full width at half maximum of the quantum dots PL peak to the high-energy side that we associate to the size selectivity of the oscillator strength of the ground state transitions. The observed increase of the integrated intensity of the quantum dots line is explained in terms of the reabsorption of the photons emitted by the GaAs substrate and the InAs wetting layer. These effects are related to the multilayer structure of the sample.
Effect of an antiferromagnetic layer on the temperature dependence of coercivity in biased FeMn/Co systems
Authors: Wang J. , Wang W. N. , Chen X. , Zhao H. W. , Zhao J. G. , Zhan W. Sh.
Published: July 2002

The hysteresis loops of exchange-biased FeMn/Co bilayers have been measured by means of a superconducting quantum interference device magnetometer. The obtained right and left coercivities exhibit different temperature dependences. With temperature increasing, the left coercivity monotonously increases while the right one remains almost constant until a critical temperature Ti. Upon decreasing the thickness of the antiferromagnet (AF) layer, Ti decreases. The asymmetric behavior of the right and left coercivities indicates that the reversal mechanisms are different for increasing and decreasing field. By taking into account the temperature dependence of the spin alignment in the AF layer, the unusual asymmetric behavior is well explained.
Evolution of island–pit surface morphologies of InAs epilayers grown on GaAs (001) substrates
Authors: Li J. H. , Moss S. C. , Han B. S. , Mai Z. H.
Published: July 2002

We report on a study of the morphological evolution of InAs layers grown on GaAs (001) substrates by molecular-beam epitaxy under In-rich conditions. The surface morphology of the InAs layers is characterized by a feature of island–pit combinations. We show that the vertical sizes of the islands and pits can grow simultaneously beyond the average layer thickness, up to several hundred nanometers. The composition of the islands is found to be ternary InxGa1−xAs rather than the expected binary InAs due to intermixing of the layer and substrate materials. We determine that this intermixing is caused by dissociation of the exposed GaAs at the pits, followed by migration of excess Ga atoms and their incorporation into the islands. The density of the island–pit combinations keeps nearly constant for different layer thicknesses. Eventually, as the layer grows beyond a certain thickness, the pits are filled up by the expanding islands, forming a nearly pure island morphology at the growth front.
Ab initio molecular orbital/Rice–Ramsperger–Kassel–Marcus theory study of multichannel rate constants for the unimolecular decomposition of benzene and the H+C6H5 reaction over the ground electronic state
Authors: Mebel A. M. , Lin M. C. , Chakraborty D. , Park J. , Lin S. H. , Lee Y. T.
Published: July 2002

The potential energy surface for the unimolecular decomposition of benzene and H+C6H5 recombination has been studied by the ab initio G2M(cc, MP2) method. The results show that besides direct emission of a hydrogen atom occurring without an exit channel barrier, the benzene molecule can undergo sequential 1,2-hydrogen shifts to o-, m-, and p-C6H6 and then lose a H atom with exit barriers of about 6 kcal/mol. o-C6H6 can eliminate a hydrogen molecule with a barrier of 121.4 kcal/mol relative to benzene. o- and m-C6H6 can also isomerize to acyclic isomers, ac-C6H6, with barriers of 110.7 and 100.6 kcal/mol, respectively, but in order to form m-C6H6 from benzene the system has to overcome a barrier of 108.6 kcal/mol for the 1,2-H migration from o-C6H6 to m-C6H6. The bimolecular H+C6H5 reaction is shown to be more complicated than the unimolecular fragmentation reaction due to the presence of various metathetical processes, such as H-atom disproportionation or addition to different sites of the ring. The addition to the radical site is barrierless, the additions to the o-, m-, and p-positions have entrance barriers of about 6 kcal/mol and the disproportionation channel leading to o-benzyne+H2 has a barrier of 7.6 kcal/mol. The Rice-Ramsperger-Kassel-Marcus and transition-state theory methods were used to compute the total and individual rate constants for various channels of the two title reactions under different temperature/pressure conditions. A fit of the calculated total rates for unimolecular benzene decomposition gives the expression 2.26×1014 exp(−53 300/T)s−1 for T=1000–3000 K and atmospheric pressure. This finding is significantly different from the recommended rate constant, 9.0×1015 exp(−54 060/T) s−1, obtained by kinetic modeling assuming only the H+C6H5 product channel. At T=1000 K, the branching ratios for the formation of H+C6H5 and ac-C6H6 are 29% and 71%, respectively. H+C6H5 becomes the major channel at T⩾1200 K. The total rate for the bimolecular H+C6H5 reaction is predicted to be between 4.5×10−11 and 2.9×10−10cm3 molecule−1 s−1 for the broad range of temperatures (300–3000 K) and pressures (100 Torr–10 atm). The values in the T=1400–1700 K interval, ∼8×10−11 cm3 molecule−1 s−1, are ∼40% lower than the recommended value of 1.3×10−10 cm3 molecule−1 s−1. The recombination reaction leading to direct formation of benzene through H addition to the radical site is more important than H disproportionations at T<2000 K. At higher temperatures the recombination channel leading to o-C6H4+H2 and the hydrogen disproportionation channel become more significant, so o-benzyne+H2 should be the major reaction channel at T>2500 K.
Passivation of oxygen-related donors in microcrystalline silicon by low temperature deposition
Authors: Nasuno Y. , Kondo M. , Matsuda A.
Published: July 2002

Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (μc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 °C is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped μc-Si abruptly decreases for deposition temperatures lower than 180 °C, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc=0.51 V, Jsc=25 mA/cm−1, FF=0.70) has been obtained in spite of an oxygen concentration of 2×1019 cm−3 in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current, and fill factor is also discussed.
Ultrafast deposition of microcrystalline Si by thermal plasma chemical vapor deposition
Authors: Chae Y. K. , Ohno H. , Eguchi K. , Yoshida T.
Published: July 2002

This research is an attempt to apply thermal plasma chemical vapor deposition for the ultrafast deposition of Si films for solar cells. The improvement of stability, controllability, and cleanliness of the process enabled the deposition of μc-Si films at the ultrafast rate of over 1000 nm/s. Moreover, a minimum defect density of 7.2×1016 cm−3 was achieved. Monte-Carlo simulation and step coverage analysis suggested that the precursor is an approximately 1 nm cluster with a sticking probability of about 0.6. The success of this research may change the established concepts of Si deposition technology.
Stress effects of the inter-level dielectric layer on the ferroelectric performance of integrated SrBi2Ta2O9 capacitors
Authors: Hong Suk-Kyoung , Yang B. , Oh Sang Hyun , Kang Young Min , Kang Nam Soo , Hwang Cheol Seong , Kwon Oh Seong
Published: July 2002

The thermal stress effects of the inter-level dielectric (ILD) layer on the ferroelectric performance of integrated Pt/SrBi2Ta2O9(SBT)/Pt capacitors were investigated. Two different thin film materials, pure SiO2 grown at 650 °C and B- and P-doped SiO2 grown at 400 °C by chemical vapor deposition techniques, were tested as an ILD layer. The ILD layer encapsulated the SBT capacitor array. During high temperature thermal cycling (up to 800 °C) after ILD deposition, which is used for both densifying the ILD and curing of the various damage imposed on the SBT capacitors, a large thermal stress occurred in the bottom Pt layer due to the thermal expansion mismatch between the various layers. In particular, the pure SiO2 ILD layer between the capacitors did not allow thermal expansion of the Pt layers, which led to a large accumulation of compressive stress in the layer. This resulted in hillock formation in the bottom Pt layer and eventual capacitor failure. However, the B- and P-doped SiO2 ILD layer contracted during thermal cycling by removing residual impurities, which allowed greater expansion of the Pt layer. Therefore, compressive stress accumulation did not occur and excellent ferroelectric properties were thus obtained from the integrated capacitor array.
Magnetotransport properties in thin films of charge-ordered materials
Authors: Venimadhav A. , Hegde M. S. , Rawat R. , Das I.
Published: July 2002

Thin films of Pr0.7Ca0.3MnO3, Nd0.5Ca0.5MnO3, and Nd0.5Sr0.5MnO3 have been fabricated by pulsed-laser deposition, and the magnetic and electrical transport properties of these films were compared with their bulk properties. All the films grow in the (101) direction on LaAlO3(100). Electrical transport measurements carried out with and without magnetic field have shown dρ/dT<0 in these thin films. Application of a magnetic field has produced an increase in the conductivity of the films. Magnetization studies showed that Nd0.5Sr0.5MnO3 exhibits ferromagnetic behavior at 260 K and antiferromagnetic below 130 K. This is close to the bulk magnetic behavior of Nd0.5Sr0.5MnO3. Pr0.7Ca0.3MnO3 showed a ferromagnetic transition below 130 K while Nd0.5Ca0.5MnO3 remained paramagnetic down to low temperatures, showing a deviation from the magnetization exhibited by their respective bulks. Deviation in the properties of the thin films from the bulk solids is attributed to the growth of the films in a more symmetric structure.
Grain growth in Co27Pt73 alloy films with perpendicular magnetic anisotropy
Authors: Xun Kun , Shen Defang
Published: July 2002

The grain size of Co27Pt73 alloy films as-deposited and annealed at 250 °C has been studied by transmission electron microscopy (TEM) and x-ray diffraction. The grain size was found to increase from 4–8 to 20–35 nm when the thickness of the films changed from 7 to 49 nm, while the magnetic anisotropy constants of the films increased from 0.048 to 0.28 MJ/m3. The grain size increase was found to be independent of the growth time, but dependent on the film thickness. Significant stress in the Co27Pt73 alloy films was revealed by the difference between the grain sizes determined by the Scherrer equation and TEM. Evidence and analysis indicate that the local anisotropy structure resulting in perpendicular magnetic anisotropy in CoxPt1−x (x≈0.25) alloy films may be stable at 250 °C.
Domain duplication in ferromagnetic sandwiches
Authors: Lacour D. , Hehn M. , Lenoble O. , Schuhl A. , Tiusan C. , Ounadjela K.
Published: July 2002

In this article, we give an overview of the domain duplication process which can occur in ferromagnetic sandwiches. A brief theoretical description of the process allows us to extract the main parameters governing the effect. It is shown that even if a domain structure exists in the hard electrode, no duplication can occur for a ferromagnetic coupling below a minimum value. Then, we address also the effects of residual domains on the nucleation field of the hard electrode to reconcile theory and experiments.
Optical recording using a photochromic ferroelectric liquid crystal
Authors: Komitov L. , Tsutsumi O. , Ruslim C. , Ikeda T. , Ichimura K. , Yoshino K.
Published: July 2002

The direction of molecular switching, i.e., the sign of the electro-optic response, in ferroelectric liquid crystals depends on the polarity of the applied electric field as well as on the sign of the spontaneous polarization. Light-induced switching of the molecules by ultraviolet light was found in a photochromic ferroelectric liquid crystal at fixed field polarity. A high contrast image was recorded whose contrast could easily be inverted by reversing the field polarity. Moreover, in the presence of bistable boundary conditions, the optically recorded image can be stored after removal of the electric field. The effect of light-assisted switching in the photochromic ferroelectric liquid crystal seems to be promising for applications such as optical recording, all-optical switches, and image storage.
Control of in-plane orientation of phthalocyanine molecular columns using vicinal Si(001)-(2×1)-H
Authors: Nakamura Masakazu , Matsunobe Takeshi , Tokumoto Hiroshi
Published: July 2002

In-plane crystal orientation of copper phthalocyanine (CuPc) films formed by organic molecular-beam epitaxy have been successfully controlled by using vicinal Si(001)-(2×1)-H as a substrate, containing atomic steps of an approximately 4 nm period. A continuous film was grown at 60 °C and the film thickness ranged between 5 and 8 molecular layers. By observing a frictional force image of the film, 90% of the molecular columns were found to align across the substrate step rows. The preferential orientation is considered to be induced by artificial surface lattices, which result from the striped effective contact area between the rigid CuPc crystals and the stair-like surfaces. The anisotropic optical properties of the film have been also confirmed by polarized reflection measurements.
Theoretical study of the surface resistivity of (111) surfaces of NixPt1−x(111) alloys
Authors: Rous P. J.
Published: July 2002

A layer-Korringa–Kohn–Rostoker calculation is used to study the compositional dependence of the surface resistivity of the (111) surface of NixPt1−x(111) alloys. The compositional disorder in the bulk and at the surface is described by the coherent potential approximation. If it is assumed that the atomic planes near the (111) surface NixPt1−x have the same composition as the bulk layers, then a weak Nordheim effect is observed in the compositional dependence of the surface resistivity. However, we show that surface segregation in NixPt1−x(111) causes an inverse Nordheim dependence in the actual surface resistivity as the bulk composition is varied.
Structural, optical, and electrical characterization of hot wall epitaxy grown 1-methoxy-8-hydroxy-9,10-anthraquinone films
Authors: Mahajan Aman , Bedi R. K. , Kumar Subodh
Published: July 2002

1-methoxy-8-hydroxy-9,10-anthraquinone compound has been synthesized and its films are grown by the hot wall epitaxy technique onto the glass substrates kept at different temperatures in a vacuum of 10−5 Torr. The experimental conditions are optimized to obtain better crystallinity of the films. The films so prepared have been studied for their structural, optical, and electrical properties. Observations reveal that the crystallinity of the films increases with an increase in substrate temperature. Crystallites as large as 3.30 μm are observed in the case of films deposited at 348 K. Analysis of optical absorption measurements on the films indicate that the interband transition energies lies in the range 1.87–2.02 eV. The conduction in these films is found to be ohmic in nature and appears to take place by thermally activated hopping above intermolecular barriers. The electrical resistivity of films decreases with the increase in temperature, while carrier concentration increases.
High-quality strain-relaxed SiGe films grown with low temperature Si buffer
Authors: Luo Y. H. , Wan J. , Forrest R. L. , Liu J. L. , Goorsky M. S. , Wang K. L.
Published: July 2002

High-quality strain-relaxed SiGe templates with a low threading dislocation density and smooth surface are critical for device performance. In this work, SiGe films on low temperature Si buffer layers were grown by solid-source molecular beam epitaxy and characterized by atomic force microscope, double-axis x-ray diffraction, photoluminescence spectroscopy, and Raman spectroscopy. Effects of the growth temperature and the thickness of the low temperature Si buffer were studied. It was demonstrated that when using proper growth conditions for the low temperature Si buffer the Si buffer became tensily strained and gave rise to the compliant effect. The lattice mismatch between the SiGe and the Si buffer layer was reduced. A 500 nm Si0.7Ge0.3 film with a low threading dislocation density as well as smooth surface was obtained by this method.
On the link between the two-dimensional hydrodynamics and the three-dimensional (3-D) magnetostatic: A new method for obtaining a 3-D solution of the magnetostatic equilibrium
Authors: Yanovsky V. , Tur A. , Louarn P. , Le Queau D.
Published: July 2002

It is shown that a formal link relates two-dimensional nonstationary hydrodynamic solutions to the three-dimensional solutions of the magnetostatic equilibrium. This method is used for obtaining new exact analytical solutions of the magnetostatic equilibrium (flux rope type) from known hydrodynamics solutions (point vortex type).
An eight-channel pulse-shape comparator to discriminate signals from fast scintillation detectors
Authors: Bezboruah Tulshi
Published: July 2002

An eight-channel pulse-shape comparator board is designed to discriminate the signals from fast scintillation detectors. Each channel of the comparator board has guaranteed output with maximum speed of 20 ns. The simple design and low cost per channel make it preferable for use in cosmic rays as well as in extensive air shower (EAS) experiments. A description of the environmental operation and test of the electronics is presented. This device is presently used in an EAS experiment, Gauhati University, Assam, India. We present here, details about the hardware aspect of the electronics. Results of the comparator electronics are also presented.
Multichannel detection x-ray absorption near edge structures study on the structural characteristics of dendrimer-stabilized CdS quantum dots
Authors: Zhang P. , Naftel S. J. , Sham T. K.
Published: July 2002

Starburst poly-(amidoamine) dendrimers were used to obtain dendrimer stabilized CdS nanoparticles, or quantum dots (QDs). The average particle size of the QDs was examined by UV-vis absorption spectra and powder x-ray diffraction measurements. The technique of x-ray absorption near edge structure (XANES) near the S K-edge was employed to study the structural characteristics of a series of QDs. The use of a multichannel detection scheme in XANES, monitoring total electron yield, x-ray fluorescence yield, and photoluminescence yield simultaneously, was demonstrated to be a powerful tool to evaluate the structure of the QDs from a sulfur perspective. The relationship between the luminescence property and the local structure of sulfur in the QDs was also discussed on the basis of the results of x-ray excited optical luminescence and XANES studies.
Photoelectron spectroscopy of bimetallic aluminum cobalt cluster anions: Comparison of electronic structure and hydrogen chemisorption rates
Authors: Pramann Axel , Nakajima Atsushi , Kaya Koji
Published: July 2002

The photoelectron spectra of small mass-selected aluminum-rich AlnCo− (n=8–17) and cobalt-rich ConAlm− clusters (n=6,8,10; m=1,2) are measured at photon energies of 3.49 eV with the aid of a magnetic bottle photoelectron spectrometer. The electronic structures of the bimetallic clusters are compared with those of pure Aln− and Con− clusters, which are measured under the same conditions. The threshold behavior (electron affinities and vertical detachment energies) is analyzed, and the application of the electronic shell model reveals a similar shell structure of Co-doped Al clusters with a single heteroatom. Additional heteroatom doping induces shell perturbations. Compared to pure Aln clusters, single-atom doped clusters show a hybridization of Al s, p and Co d orbitals resulting in a merging of spectral features at low binding energies. The evolution of the electronic structure is compared with reported ionization potentials [Menezes and Knickelbein, Chem. Phys. Lett. 183, 357 (1991); Z. Phys. D 26, 322 (1993)]. The hydrogen chemisorption behavior of neutral aluminum cobalt clusters [Nonose et al., Chem. Phys. Lett. 164, 427 (1989)] shows a clear influence of Co d states at low binding energies in the case of AlnCo, whereas for ConAlm clusters (n>m) the geometric effect becomes more dominant.
Monte Carlo computer simulation of spreading pressure-area isotherms of Langmuir monolayers of fatty-acid molecules
Authors: Polimeno Antonino , Ros J. Marijin , Levine Yehudi K.
Published: July 2002

We describe an off-lattice model with chemical group resolution for investigating the spreading pressure-area isotherms of Langmuir monolayers of fatty-acid molecules at air–water interfaces. It is shown that a balance of the attractive interactions between the methylene chains and longer-range repulsive interactions between the headgroups determines the form of the isotherms. The model reproduces the experimentally observed dependence of the isotherms on the chain length and unsaturation. At 300 K model palmitic acid chains (C16:0) are shown to form liquid-condensed monolayers at all spreading pressures, while the isotherms of monolayers of myristic acid (C14:0) exhibit a liquid-condensed to liquid-expanded transition in agreement with experiments. Moreover, the simulations show that the introduction of cis-unsaturated segments into the 7–8 positions of the C14 chains depresses the phase transition temperature, so that the monolayers undertake a liquid-expanded structure.
Structure of the triplet excited state of bromanil from time-resolved resonance Raman spectra and simulation
Authors: Puranik Mrinalini , Umapathy Siva , Snijders Jaap G. , Chandrasekhar Jayaraman
Published: July 2002

Time-resolved resonance Raman (TR3) spectroscopy has been used to study the structure of the triplet excited state of bromanil. These experimental results were then simulated using parameters from density functional theoretical (DFT) calculations and wave packet dynamics, in order to understand the structure and mode-specific displacements of the resonant excited state. The transition dipole moments and the energy separation of the T1 and Tn states were obtained from time-dependent DFT calculations. We have demonstrated application of the technique to tetrabromo-p-benzoquinone. From our calculations, the observed T1→Tn absorption spectrum has been assigned to the Bg3→3Bu transition. The geometry has been optimized for the resonant higher triplet state, Tn, and is found to be in good agreement with the predictions of the wave packet dynamical simulations. Mode-specific displacements of the triplet state geometry have been obtained from simulations and these have been rationalized with respect to the molecular orbital involved. Thus, we have demonstrated that from the simulations of the experimental TR3 spectral data, valuable additional information can be derived on the structure of the transient states that may then be used for elucidation of structure-reactivity correlation in the future.
Racemic and chiral structures of banana-shaped smectic liquid crystals viewed as one-dimensional piezoelectric crystals
Authors: Wu Chen-Xu , Yamazaki Yoshitake , Zhong-can Ou-Yang
Published: July 2002

A smectic liquid-crystal phase formed by achiral banana-shaped molecules (CB2) is studied as one-dimensional piezoelectric crystals with fluidity of tilted-C2V anisotropy in the other two spatial dimensions. Variation of the elastic-piezoelectric free energy of the system predicts the existence of racemic (R), left-, and right-handed chiral (H) antiferroelectric equilibrium structures, a result in agreement with the observation by Link et al. [Science 278, 1924 (1997)]. The electric-field-induced ferroelectric state is shown to be a special C2 symmetry, which confirms the measurements by optical second harmonic generation in a CB2 by Macdonald et al. [Phys. Rev. Lett. 81, 4408 (1998)]. The hysteresis loop of switching polarization is also discussed, and the molecular tilt angle dependence of spontaneous polarization is obtained as a function of the elastic and the piezoelectric constants.
Chemical vapor deposition of Si on chlorosilane-treated SiO2 surfaces. I. Suppression and enhancement of Si nucleation
Authors: Yasuda T. , Nishizawa M. , Yamasaki S.
Published: July 2002

We have investigated the nucleation processes in chemical vapor deposition (CVD) of Si on SiO2 surfaces that were treated with different chlorosilane gases SiHnCl4−n, where n=0, 1, or 2. These gases chemisorb on the SiO2 surface in a self-limiting manner. At 493 K, the saturation coverage was reached at 4×108 L exposure for the SiH2Cl2 gas, and at 2×109 L for the SiHCl3 and SiCl4 gases. CVD of Si was carried out on the chlorosilane-saturated SiO2 surfaces using Si2H6 as a source gas at 853 K. It has been found that the SiCl4 and SiHCl3 treatments effectively passivate the nucleation centers on the SiO2 surfaces, while the SiH2Cl2 treatment drastically enhances Si nucleation. The reaction mechanisms causing these differences are discussed, considering the structural changes of the adsorbed chlorosilane fragments at the CVD temperature. Application of the SiCl4 treatment to the patterned SiO2 mask for selective-area epitaxial growth of Si is demonstrated.
Near-field ultraviolet photoluminescence spectroscopy for evaluating the crystallinity of polycrystalline zinc oxide
Authors: Yatsui T. , Shimizu T. , Yamamoto Y. , Kourogi M. , Ohtsu M. , Lee G. H.
Published: July 2002

By extending the optical near-field technique to the ultraviolet region, a two-dimensional evaluation of the optical properties and crystallinity of polycrystalline zinc oxide (ZnO) was carried out at room temperature. Using an ultraviolet fiber probe with an aperture diameter of 80 nm, we obtained spatially resolved photoluminescence spectra from individual ZnO nanocrystallites; the emission intensity depended on the topography and on crystal orientation.
On the influence of the external optical radiation upon anode double layers
Authors: Strat M. , Strat Georgeta
Published: July 2002

The obtained results demonstrate that the changes in the parameters (frequency, amplitude) of electrical oscillations generated in an anode double layer can be used as optical detectors when the anode glow is irradiated with an external optical electromagnetic wave. It has been demonstrated that the local electrical properties of an anode glow have changed under the influence of electromagnetic radiation.
Theory of suspension segregation in partially filled horizontal rotating cylinders
Authors: Govindarajan Rama , Nott Prabhu R. , Ramaswamy Sriram
Published: July 2002

It is shown that a suspension of particles in a partially-filled, horizontal, rotating cylinder is linearly unstable towards axial segregation and an undulation of the free surface at large enough particle concentrations. Relying on the shear-induced diffusion of particles, concentration-dependent viscosity, and the existence of a free surface, our theory provides an explanation of the experiments of Tirumkudulu et al. [Phys. Fluids 11, 507 (1999); 12, 1615 (2000)].
Resolution test for apertureless near-field optical microscopy
Authors: Martin Yves C. , Wickramasinghe H. Kumar
Published: July 2002

A test configuration for apertureless near-field optical microscopy permits characterization of near-field signals free from significant topography effects. Using a second tip as the scanned object, a high-contrast dipole–dipole signal is observed when the two tips are closely spaced, by 10 nm or less. A spatial resolution on the order of 5 nm is demonstrated. The measured signal is also material dependent: an inverted contrast is recorded when one of two high-dielectric (silicon) tips is replaced by a metallic (nickel) tip. Measurement results are accounted for by theory, which also confirms the nature of the detected signal: it results from interference between a near-field dipole–dipole field scattered by the end of the tips, and a reference field scattered by the main body of the tips. Different illumination sources are investigated. In all cases, the component of the electric field parallel to the tip axes provides the major source of signal and contrast.
Automated constant voltage anemometer for measurements with fluid temperature drifts
Authors: Sarma Garimella R. , Comte-Bellot Geneviève
Published: July 2002

An automated procedure has been developed for a constant voltage anemometer to obtain in situ heated and cold resistances of a hot wire in each measurement cycle. From these parameters mean velocity can be deduced in the presence of ambient flow temperature variations, using a calibration curve based on, e.g., King’s law with the assumption of constant coefficients over a given temperature range. The method requires only one hot wire. This is a definite advantage over previous methods established for constant current or constant temperature anemometers where an additional temperature sensor is needed, with a separate temperature calibration of the hot wire. Results from a typical test are presented to demonstrate the procedure. The prototype tested was a 400 kHz bandwidth unit and the measurement cycle can be changed through software commands to suit each test requirements.
Generation of high-resolution surface temperature distributions
Authors: Darhuber Anton A. , Troian Sandra M. , Wagner Sigurd
Published: July 2002

We have performed numerical calculations to study the generation of arbitrary temperature profiles with high spatial resolution on the surface of a solid. The characteristics of steady-state distributions and time-dependent heating and cooling cycles are examined, as well as their dependence on material properties and device geometry. Ideally, low-power consumption and fast response times are desirable. The simulations show that the achievable spatial resolution is on the order of the substrate thickness and that the response time t+ depends on the width of the individual heating elements. Moreover, the rise time t+ can be significantly shortened by deposition of a thermal insulation layer, which also reduces the power consumption and increases lateral resolution.
Bubble formation and growth in organic light-emitting diodes composed of a polymeric emitter and a calcium cathode
Authors: Wang Wei , Lim Shuang Fang , Chua Soo Jin
Published: July 2002

The formation and growth of bubbles within dark spots in organic light-emitting diodes comprised of an electroluminescent polymer and a Ca–Ag cathode have been observed and studied. Our studies indicate that electric stress and pinholes are two important parameters affecting the formation of bubbles. More detailed analysis reveals the dependence of bubble area on the pinhole diameter.
Analysis of dual-band quantum well photodetectors
Authors: Ryzhii V. , Ryzhii M. , Liu H. C.
Published: July 2002

We present a simple model for quantum well photodetectors for simultaneous middle infrared and near infrared or visible dual-band detection. We derive analytical formulas for the responsivity and detectivity as functions of the material and structural parameters. It is shown that the characteristics of near infrared and visible radiation detection can be sensitive to parameters of the hole transport, capture into quantum wells, and reflection from the electron emitting contact. We demonstrate that a properly designed photodetector can exhibit comparable performance for both middle infrared and near infrared (or visible) detection. The obtained results can be used to optimize the photodetector design and characteristics in both spectral ranges.
Nucleation of oxide precipitates in vacancy-containing silicon
Authors: Voronkov V. V. , Falster R.
Published: July 2002

Steady-state nucleation of oxide precipitates is described by the distribution function C(m,n) over two basic size variables of an oxygen cluster: the number n of agglomerated oxygen atoms, and the number m of silicon atoms removed from the cluster location by vacancy consumption and self-interstitial emission. The model is used to treat the experimental data of precipitation in wafers with controlled vacancy concentration induced by rapid thermal annealing. The observed dependence of the nucleation rate at 800 °C on vacancy and oxygen concentration is well reproduced by the model, using the specific surface energy σ as a fitting parameter. The best-fit value was found to be approximately 900 erg/cm2.
Field distribution and collection efficiency in an AlGaN metal–semiconductor–metal detector
Authors: Hirsch L. , Moretto P. , Duboz J. Y. , Reverchon J. L. , Damilano B. , Grandjean N. , Semond F. , Massies J.
Published: July 2002

Metal–semiconductor–metal detectors have been fabricated based on AlGaN grown on Si by molecular beam epitaxy. Field distribution and collection efficiency were studied with the ion beam induced charge collection method. The results were explained by numerical two-dimensional calculations of the electric field distribution. The calculated field map and charge buildup at the electrodes are used to explain the bias and position dependence of the ion beam induced charge collection. The similarities and differences with the case of optical detection are discussed.
Extension of the Zimm and Rouse model to a polymer chain confined by a harmonic potential in θ-solvent conditions
Authors: Wenczel Rob , Shew Chwen-Yang
Published: July 2002

We consider a polymer chain confined by a harmonic potential in θ solvents using the Zimm and Rouse model to elucidate the chain relaxation behavior in weak and strong fields, respectively. We investigate a case in which the center of the field is tuned to match the center of mass of the polymer at the instant when the field is switched on. The closed-form expressions are obtained for these models. When the field strength is weak enough so that the chain conformation is close to ideal Gaussian, the Zimm model predicts that the chain molecule would fluctuate within the confined space induced by the applied field. Moreover, the molecular rotation relaxes faster than the translational motion of the center of mass of the polymer molecule. However, under a strong field, the polymer molecule contracts continuously from a random coil to a collapsed conformation after the field is switched on. The Rouse model makes predictions that the center of mass of the confined polymer molecule would achieve its equilibrium state first. After the relaxation of the center of mass, the polymer molecule reaches the equilibrium chain conformation, followed by the molecular rotation. Furthermore, the Rouse model also predicts that in the presence of a strong field, the Rouse time is predominated by the field strength only.
Ambipolar diffusion in an inhomogeneous dusty plasma
Authors: Ma J. X. , Yu M. Y. , Liang X. P. , Zheng J. , Liu W. D. , Yu C. X.
Published: July 2002

The ambipolar diffusion of plasma particles in a column of inhomogeneous plasma containing dust grains is investigated analytically as well as numerically. The grains act as charge sinks and behave like an immobile but charge-varying negative background. They can lead to significant depletion of the electron density and decrease the diffusion scale length. The theoretically obtained election density distribution agrees well with that from experiment.
Chaotic scattering in heavy-ion reactions
Authors: Baldo M. , Lanza E. G. , Rapisarda A.
Published: July 2002

We discuss the relevance of chaotic scattering in heavy-ion reactions at energies around the Coulomb barrier. A model in two and three dimensions which takes into account rotational degrees of freedom is discussed both classically and quantum mechanically. The typical chaotic features found in this description of heavy-ion collisions are connected with the anomalous behavior of several experimental data.
Large-scale Windows 95-based data-acquisition system using LabVIEW
Authors: Mandrake L. , Gekelman W.
Published: July 2002

Hardware and software for a high-speed, pulsed-data-acquisition system are described. Data can be obtained via any device attachable to a VXI crate, GPIB controller, or directly via serial or parallel ports. Stepper motors controlled via serial port automate probe movement. LabVIEW and custom C++ modules are used to handle setup, data gathering and processing, and user interface. Experimental parameters can be controlled at each point via any GPIB-ready device. © 1997 American Institute of Physics.