bluuebunny/crossref_metadata_2025 huggingface dataset:
Theoretical study of the low-lying excited singlet states of furan
Authors: Gromov E. V. , Trofimov A. B. , Vitkovskaya N. M. , Schirmer J. , Köppel H.
Published: June 2003

The lowest two Rydberg and two π–π* valence excited singlet states of furan [referred to as A11(3s), B11(3p) and B12(V), A11(V′), respectively, at the C2v ground-state molecular configuration] have been studied using the equation-of-motion coupled-cluster singles and doubles method (EOM-CCSD). Full geometry optimizations with subsequent computation of harmonic vibrational frequencies have been performed in order to locate and characterize stationary points on the potential energy surfaces (PES). The latter optimization work was enabled by the availability of analytic energy gradient techniques for the EOM-CCSD approach. A major new finding is that both the B12(V) and A11(V′) valence states are unstable with respect to non-totally symmetric distortions at the C2v configuration. The symmetry breaking in the B12(V) state involves an in-plane coordinate of b2 symmetry. The relaxation process begins on the S2 adiabatic PES and, after passing through a conical intersection of the S2 and S1 PES, continues on the S1 surface, taking the system finally to the adiabatic minimum of S1 (1A2 state). The A11(V′) valence state is found to be unstable with respect to the out-of-plane bending coordinates of b1 and a2 symmetry. The resulting relaxed molecular structures have Cs and C2 symmetry, respectively. The present findings are analyzed in terms of a linear vibronic coupling model and spectroscopic implications are discussed.
Molecular simulation study of nanoscale friction between alkyl monolayers on Si(111) immersed in solvents
Authors: Zhang Luzheng , Jiang Shaoyi
Published: June 2003

Nonequilibrium molecular dynamics simulations were performed to study nanoscale friction between two Si(111) surfaces covered with alkyl monolayers immersed in liquid solvents. Three pairs of interfaces, ranging from hydrophobic CH3/CH3 to hydrophilic OH/OH, were studied. Three solvents, including water, methanol, and n-decane were used to represent different solvent polarities. It was showed that friction was dependent not only on surface hydrophobicity, but also on solvent polarity. In polar solvents (e.g., water), friction is much larger for hydrophilic than hydrophobic interfaces, while in nonpolar solvents, e.g., n-decane, friction has no significant difference for both hydrophobic and hydrophilic monolayers. A fundamental understanding of friction at different interfaces in various solvents is very important to micro- or nanoelectromechanical systems (MEMS/NEMS), particularly, bio-MEMS/NEMS.
Modeling of dust in a silane/hydrogen plasma
Authors: Akdim M. R. , Goedheer W. J.
Published: June 2003

A dusty radio-frequency silane/hydrogen discharge is simulated, with the use of a one-dimensional fluid model. In the model, discharge quantities like the fluxes, densities, and electric field are calculated self consistently. A radius and an initial density profile for the spherical dust particles are given and the charge and the density of the dust are calculated with an iterative method. During the transport of the dust, its charge is kept constant in time. The dust influences the electric field distribution through its charge and the density of the plasma through recombination of positive ions and electrons at its surface. In the model this process gives an extra production of silane radicals, since the growth of dust is not included. Results are presented for situations in which the dust significantly changes the discharge characteristics, both by a strong reduction of the electron density and by altering the electric field by its charge. Simulations for dust with a radius of 2 μm show that the stationary solution of the dust density and the average electric field depend on the total amount of the dust. The presence of dust enhances the deposition rate of amorphous silicon at the electrodes because of the rise in the average electron energy associated with the decrease of the electron density and the constraint of a constant power input.
Intrinsic stresses and mechanical properties of Ti-containing hydrocarbon coatings
Authors: Shi Bo , Meng W. J.
Published: June 2003

A detailed examination of the intrinsic stress development within and mechanical properties of Ti containing hydrocarbon (Ti–C:H) coatings deposited in an inductively coupled plasma assisted hybrid chemical vapor deposition/physical vapor deposition environment has been carried out, combining in situ substrate curvature measurements with plasma probe measurements, ex situ electrical resistivity measurements, and instrumented nanoindentation measurements. Intrinsic stresses within Ti–C:H have been found to be compressive over wide ranging compositions and plasma parameters. The intrinsic compression within Ti–C:H was found to depend significantly on the Ti composition, and was related to a percolation type transition in the nanoscale structure. The intrinsic compression within Ti–C:H has further been shown to be significantly influenced by the energy of ionic species bombarding the substrate during growth. Measured stress–thickness history was discussed in terms of possible mechanisms contributing to intrinsic stress generation. Although there are likely multiple mechanisms influencing intrinsic stress development, our present results suggest that ion bombardment plays a significant role in intrinsic stress generation within Ti–C:H, and is likely to influence stress development in other low temperature deposited amorphous hydrocarbon based ceramic nanocomposite coatings.
On a reformulation of the theory of Lifshitz–van der Waals interactions in multilayered systems
Authors: Podgornik Rudi , Hansen Per Lyngs , Parsegian V. Adrian
Published: June 2003

In order to investigate the form of the van der Waals interaction in different multilayer geometries we reformulate the Lifshitz theory in terms of an algebra of 2×2 matrices. This device allows us to derive a closed form solution for the secular determinant of the modes in terms of simple quadratures with explicit N dependence. We specifically investigate (i) the van der Waals interactions between a substrate and a multilayer system as a function of the separation between the substrate and the multilayer system and (ii) the interaction between two multilayer systems over a medium of variable separation.
A new efficient approach to the direct restricted active space self-consistent field method
Authors: Klene Michael , Robb Michael A. , Blancafort Lluı́s , Frisch Michael J.
Published: June 2003

We present an implicitly parallel method for integral-block driven restricted active space self-consistent field (RASSCF) algorithms. Our algorithm entirely avoids testing the index space for nonzero contributions to the CI vector, by finding entire blocks of contributions through use of simple algebraic rules (propagation rules). The blocks themselves are efficiently identified by introducing a RAS model space. Our algorithm is capable of making efficient use of modern supercomputer hardware, supporting both shared and distributed memory architectures and hybrids. Applicability of our method is demonstrated with a RASSCF investigation of the first two excited states of indole.
Conformation of a diblock or random square-well copolymer in dilute solution
Authors: Cai Jun , Prausnitz John M.
Published: June 2003

The conformation and thermodynamic properties of a diblock square-well copolymer or a random square-well copolymer have been calculated by the Born–Green–Yvon (BGY) theory, making use of an appropriate approximation for the third-order correlation function as well as the translational invariance approximation. The BGY results for the end-to-end distribution of copolymers are in good agreement with Monte Carlo simulations when the attractive interaction between segments is not strong. BGY calculations semiquantitatively predict the conformations of a copolymer when the attractive interaction is strong. At low temperatures, one block comprised of attractive segments of the diblock copolymer collapses while the other block, comprised of repulsive segments, dominates the scaling behavior. The end-to-end-distance distribution function for a random copolymer is similar to that for an AB⋯AB copolymer when the temperature is at least moderate.
Mössbauer study of the magnetism and structure of amorphous and nanocrystalline Fe81−xNixZr7B12 (x=10–40) alloys
Authors: Kopcewicz M. , Idzikowski B. , Kalinowska J.
Published: June 2003

Soft magnetic amorphous and nanocrystalline Fe81−xNixZr7B12 (x=0–40) alloys with very low coercivity and improved mechanical properties, as compared with the NANOPERM alloys, were prepared by a melt-quenching technique. The nanostructure was formed by annealing amorphous precursors in the temperature range TA=440–620 °C. Formation of the nanocrystalline phase was studied by Mössbauer spectroscopy, differential scanning calorimetry, and x-ray diffraction techniques. The composition of the nanocrystalline phase strongly depends on the Ni content in the alloy. For x<30 the dominating nanocrystalline phase is the bcc Fe, similarly to the NANOPERM alloys. However, the alloy with x=40 behaves in a clearly different way. Annealing of the Fe41Ni40Zr7B12 alloy at TA=520–620 °C causes the formation of the nanograins of magnetically ordered cubic (FeNi)23B6 and FeNi phases, as identified by the Mössbauer and x-ray diffraction measurements. Annealing at temperatures exceeding 590 °C leads to the Mössbauer spectra at room temperature dominated by a single-line nonmagnetic component. Mössbauer measurements performed at low temperatures reveal a superparamagnetic origin of this spectral component. The superparamagnetic relaxation at the sample surfaces is restricted by the stress induced surface anisotropy related to the crystallization of the amorphous phase as revealed by the conversion electron Mössbauer results. The conversion electron Mössbauer measurements which allowed the comparison of the surface and bulk crystallization of Fe81−xNixZr7B12 alloys show clear differences between the surface and bulk crystallization for all alloy compositions. Unconventional Mössbauer studies utilizing radio frequency (rf) fields provide information on the soft magnetic nature of the alloys by observing the degree of rf-induced collapse of the hyperfine fields. The rf-Mössbauer technique, being particularly sensitive to magnetic anisotropy, provided information on the anisotropy fields in the alloys. It was found that the nanocrystalline FeNiB phase is magnetically very soft as revealed by the rf-Mössbauer technique. The complete rf collapse of the magnetic hyperfine structure was observed for the nanocrystalline Fe41Ni40Zr7B12 alloy in clear distinction to the FeNiZrB alloys with x⩽30 and to similar earlier studies of NANOPERM alloys. An improved magnetic softness combined with much reduced brittleness of the Fe41Ni40Zr7B12 samples offer attractive possibilities for technical applications of the nanocrystalline Ni-containing alloys.
Photodegradation of selected π-conjugated electro-optic chromophores
Authors: Galvan-Gonzalez A. , Belfield K. D. , Stegeman G. I. , Canva M. , Marder S. R. , Staub K. , Levina G. , Twieg R. J.
Published: June 2003

The photodegradation properties of six electro-optic chromophores in guest-host polymer formats were measured as a function of wavelength throughout the visible and up to 1.3 μm in the near infrared. The chromophore structure was varied to provide insight into its relationship with photostability.
Competitive adsorption in model charged protein mixtures: Equilibrium isotherms and kinetics behavior
Authors: Fang F. , Szleifer I.
Published: June 2003

The competitive adsorption of proteins of different sizes and charges is studied using a molecular theory. The theory enables the study of charged systems explicitly including the size, shape, and charge distributions in all the molecular species in the mixture. Thus, this approach goes beyond the commonly used Poisson–Boltzmann approximation. The adsorption isotherms of the protein mixtures are studied for mixtures of two proteins of different size and charge. The amount of proteins adsorbed and the fraction of each protein is calculated as a function of the bulk composition of the solution and the amount of salt in the system. It is found that the total amount of proteins adsorbed is a monotonically decreasing function of the fraction of large proteins on the bulk solution and for fixed protein composition of the salt concentration. However, the composition of the adsorbed layer is a complicated function of the bulk composition and solution ionic strength. The structure of the adsorb layer depends upon the bulk composition and salt concentration. In general, there are multilayers adsorbed due to the long-range character of the electrostatic interactions. When the composition of large proteins in bulk is in very large excess it is found that the structure of the adsorb multilayer is such that the layer in contact with the surface is composed by a mixture of large and small proteins. However, the second and third layers are almost exclusively composed of large proteins. The theory is also generalized to study the time-dependent adsorption. The approach is based on separation of time scales into fast modes for the ions from the salt and the solvent and slow for the proteins. The dynamic equations are written for the slow modes, while the fast ones are obtained from the condition of equilibrium constrained to the distribution of proteins given by the slow modes. Two different processes are presented: the adsorption from a homogeneous solution to a charged surface at low salt concentration, and large excess of the large proteins in bulk. The second process is the kinetics of structural and adsorption change by changing the salt concentration of the bulk solution from low to high. The first process shows a large overshoot of the large proteins on the surface due to their excess in solution, followed by a surface replacement by the smaller molecules. The second process shows a very fast desorption of the large proteins followed by adsorption at latter stages. This process is found to be driven by large electrostatic repulsions induced by the fast ions from the salt approaching the surface. The relevance of the theoretical predictions to experimental system and possible directions for improvements of the theory are discussed.
Dissociative recombination of dications
Authors: Seiersen K. , Heber O. , Jensen M. J. , Safvan C. P. , Andersen L. H.
Published: June 2003

Dissociative recombination (DR) of doubly-charged positive ions has been studied at the heavy ion storage ring ASTRID. Low-energy electrons were scattered on the dication of the N2 molecule, and the absolute cross section was measured in the energy range of 10−4–50 eV. From the measured cross section, a thermal rate coefficient of 5.8×10−7 cm3 s−1 at 300 K was extracted. Furthermore, we present new results on the CO2+ DR rate, and a summary and comparison of measured DR rate coefficients for both the singly and doubly-charged ions of CO, CO2, and N2 is presented.
USHER: An algorithm for particle insertion in dense fluids
Authors: Delgado-Buscalioni R. , Coveney P. V.
Published: June 2003

The insertion of solvent particles in molecular dynamics simulations of complex fluids is required in many situations involving open systems, but this challenging task has been scarcely explored in the literature. We propose a simple and fast algorithm (USHER) that inserts the new solvent particles at locations where the potential energy has the desired prespecified value. For instance, this value may be set equal to the system’s excess energy per particle in such a way that the inserted particles are energetically indistinguishable from the other particles present. During the search for the insertion site, the USHER algorithm uses a steepest-descent iterator with a displacement whose magnitude is adapted to the local features of the energy landscape. The only adjustable parameter in the algorithm is the maximum displacement, and we show that its optimal value can be extracted from an analysis of the structure of the potential energy landscape. We present insertion tests in periodic and nonperiodic systems filled with a Lennard-Jones fluid whose density ranges from moderate to high values.
Aggregation and counterion condensation in solution of charged proteinlike copolymers: A molecular-dynamics study
Authors: Khalatur Pavel G. , Khokhlov Alexei R. , Mologin Dmirtii A. , Reineker Peter
Published: June 2003

We present the results of molecular dynamics simulations of charged proteinlike hydrophobic–hydrophilic (ℋ𝒫) copolymers with a fixed charge distribution under pure solvent conditions. The processes of coil-to-globule transition, aggregation of polymer globules, and counterion condensation are studied in detail as a function of temperature. Various static structure factors and pair correlation functions, that occur in polyelectrolyte solutions, are also analyzed. Our simulations show that the chains pass through strong conformational changes while changing temperature. We find three different temperature regimes which are characterized by a different behavior of Coulomb energy, chain sizes, and pair correlation functions. In the high-temperature regime, at the reduced temperature T>3.0, the chains have an extended conformation with many hydrophobic blobs. As temperature is decreased, one observes a counterion condensation and sharp decrease in chain size. In this regime, we observe a solution of nonaggregating polymer globules which form a stable array resembling a charge-stabilized (micro)suspension or microgel phase. A further decrease in temperature (at T<0.5) leads to an aggregation of the isolated globules which form aggregates built up from individual globules connected by a layer of counterions condensed on the globular surface. It is found that the transitions between the temperature regimes are thermodynamically reversible.
Distribution of gas temperature in an unconfined glow discharge plasma
Authors: Soukhomlinov Vladimir S. , Sheverev Valery A. , Ötügen M. Volkan
Published: June 2003

An analysis is carried out to determine radial temperature distributions in the cylindrical positive column of a glow discharge formed in air in free space without confining walls. The analysis considers discharge with current densities lower than 100 mA/cm2 and at gas pressures of several tens of Torr. The plasma is represented by a set of hydrodynamic equations that include the balances for electron number density, translational energy, and the vibrational energy. The equations are solved using an iterative method to obtain gas temperatures for a range of plasma conditions. The results show that increasing discharge current densities lead to higher gas temperatures on plasma axis, however, unlike in the case with glow discharge restricted by dielectric walls, increased current densities also lead to wider radial profiles of temperature. Increased gas pressure, while leading to higher on-axis gas temperatures, results in narrower temperature profiles, mainly due to the reduced diffusion rates and vibrational-translational energy relaxation times. At low gas pressures and current densities, the electron density profiles are found to be significantly narrower than those for temperature while at higher values of these parameters, the width of the two are comparable. The characteristic radius of the predicted gas temperature distribution is in a good agreement with recent experimental findings.
Effects of Mg accumulation on chemical and electronic properties of Mg-doped p-type GaN surface
Authors: Hashizume Tamotsu
Published: June 2003

Chemical and electronic properties of Mg-doped p-GaN surfaces were systematically investigated by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The doping density of Mg ranged from 3×1019 to 9×1019 cm−3. The XPS and AES analyses revealed the accumulation of Mg for all the air-exposed and chemically treated p-GaN surfaces. The apparent density of Mg calculated from the XPS integrated intensity and the AES intensity was more than one order higher than the value in bulk determined by secondary ion mass spectroscopy. Mg accumulation as well as large amounts of oxides made up the disordered region on the p-GaN:Mg surfaces. Large surface band bending of 1.2–1.6 eV was found at the p-GaN surfaces even after treatment in KOH and NH4OH solutions, due to the existence of high-density surface states. It was found that electron cyclotron resonance assisted N2 -plasma treatment at 300 °C for 1 min is very effective in removing such surface disordered regions and reducing surface band bending.
Two-dimensional periodic positioning of self-assembled Ge islands on prepatterned Si (001) substrates
Authors: Zhong Zhenyang , Halilovic A. , Fromherz T. , Schäffler F. , Bauer G.
Published: June 2003

Two-dimensional (2D) periodic arrays of Ge islands were realized on prepatterned Si (001) substrates by solid-source molecular-beam epitaxy. Atomic-force microscopy images demonstrate that the Ge islands are formed in the 2D laterally ordered pits of patterned substrates. The 2D periodicity of the substrate pattern is replicated throughout a stack of Ge island layers by strain-driven vertical ordering. Photoluminescence spectra of the ordered Ge islands show well-resolved peaks of the no-phonon signal and the transverse-optical phonon replica. These peaks are observed at nearly the same energy as those of random Ge islands deposited under the same conditions on unpatterned Si substrates.
Photorefractive liquid crystal-polymer dispersion with different morphology
Authors: Ono Hiroshi , Shimokawa Hirohito , Emoto Akira , Kawatsuki Nobuhiro
Published: June 2003

The orientational photorefractive properties of the liquid crystal/polymer composites with different morphology are studied experimentally. The composites consist of the similar chemical components, but one shows phase-separated structure (polymer dispersed liquid crystal) and another one, miscible structure (polymer-dissolved liquid crystal composite). Particular attention is given to the observation and qualitative and/or quantitative modeling of the resolution, dependence of the applied dc field, dynamics of grating generation, and photocurrents, which is strongly dependent on the morphology of the composites.
Strain and texture analysis of coatings using high-energy x-rays
Authors: Almer J. , Lienert U. , Peng R. L. , Schlauer C. , Odén M.
Published: June 2003

We investigate the internal strain and crystallographic orientation (texture) in physical-vapor deposited metal nitride coatings of TiN and CrN. A high-energy diffraction technique is presented that uses synchrotron x rays and an area detector, and which allows the strain and intensity distributions of multiple crystallographic planes to be measured by a single x-ray exposure. Unique texture states and nonlinear sin2 ψ strain distributions are observed for all coatings investigated. Quantitative analysis indicates that existing micromechanical models can reasonably predict strain and corresponding stress for mixed-hkl reflections but are inadequate for fully describing measured data. Alternative mechanisms involving deposition-induced defects are proposed.
Closely linear temperature dependence of exchange bias and coercivity in out-of-plane exchange-biased [Pt/Co]3/NiO (11 Å) multilayer
Authors: Liu Z. Y. , Adenwalla S.
Published: June 2003

Strong out-of-plane exchange biasing has been observed in out-of-plane exchange biased [Pt (5 Å)/Co (4 Å)]3/NiO (11 Å) multilayer with perpendicular easy axis. Both the exchange field HE and coercivity HC display a closely linear temperature dependence except at very low temperatures. A thin NiO layer coated on the top of a Pt/Co multilayer has a great effect on the domain pattern of the Pt/Co multilayer, which is in a more irregular configuration with much smaller domain sizes than the uncoupled Pt/Co multilayer. A simulation according to Malozemoff’s random field model gives a good agreement to the experimental temperature dependence of HE and HC, suggesting that the closely linear temperature dependences of HE and HC are strongly related to the behavior of the temperature of anisotropies and of the spin rotation inside the domain walls of a thin NiO layer. The blocking temperature of TB=220 K is much higher than that observed in ferromagnetic/thin NiO systems with in-plane anisotropy.
Origin of vapor transport equilibration induced formation of nanocrystalline ErNbO4 in Er:LiNbO3 crystal
Authors: Zhang De-Long , Pun E. Y. B.
Published: June 2003

The origin of the formation of nanoprecipitates of ErNbO4 induced by vapor transport equilibration (VTE) in Er:LiNbO3 crystals was investigated by comparing x-ray powder diffraction patterns of VTE-treated, annealed and as-grown Er:LiNbO3 crystals. The results indicated that annealing an Er:LiNbO3 crystal in ambient atmosphere, whether the crystal is x or z cut, and whether it is highly or lower doped, does not influence its crystalline structure, and hence cannot cause the formation of the precipitate. The elevated temperature is not a sufficient requirement for the formation of this additional phase, but is only an essential prerequisite, together with the high-doping level of erbium. The incorporation of a certain amount of additional Li+ ions accompanying the VTE procedure is considered as the direct reason for the formation of the precipitate. The mechanism of the formation of the precipitates of ErNbO4 was tentatively suggested from both microscopic and macroscopic points of view. In addition, the origin of VTE-induced cracking, which occurs only in higher-doped x-cut crystals, was also considered based on the experimental results of annealed crystals. The incorporation of a certain amount of additional Li+ ions by the VTE procedure is again correlated with the cracking.
An improved wedge calibration method for lateral force in atomic force microscopy
Authors: Varenberg M. , Etsion I. , Halperin G.
Published: June 2003

An improved wedge calibration method for quantitative lateral force measurement in atomic force microscopy is presented. The improved method differs from the original one in several aspects. It utilizes a much simpler, commercially available, calibration grating and can be performed at any single specified applied load. It enables calibration of all types of probes, both integrated with sharp tips, and colloidal with any radius of curvature up to 2 μm. The improved method also simplifies considerably the calculation of the calibration factor by using flat facets on the calibration grating to cancel out system errors. A scheme for the data processing for on-line calibration of the lateral force is also presented.
Nonlead perovskite materials: Ba(Li1/4Nb3/4)O3 and Ba(Cu1/3Nb2/3)O3
Authors: Priya Shashank , Ando Akira , Sakabe Yukio
Published: June 2003

This study reports the discovery of two perovskite materials Ba(Li1/4Nb3/4)O3 (BLN) and Ba(Cu1/3Nb2/3)O3 (BCN). BLN has a cubic symmetry at room temperature. The magnitude of dielectric constant at room temperature is of the order of 41. The dielectric spectrum of BLN exhibits a U shaped curve with a flat bottom. This flat region occurs in the temperature range of 200 – 300 °C and the magnitude of dielectric constant in this region is 40 with a 0 ppm/°C temperature coefficient of permittivity. These characteristics appear to be very promising for microwave applications. BCN has a tetragonal structure at room temperature with a high tetragonality of 3.5%. The tetragonal symmetry is maintained until 600 °C where the structure changes to cubic phase. Based upon the x-ray diffraction analysis, dielectric and electromechanical measurements of the BCN−PbTiO3 solid solutions it is predicted that BCN is a ferroelectric with high Curie temperature.
Effect of electrical doping on molecular level alignment at organic–organic heterojunctions
Authors: Gao Weiying , Kahn Antoine
Published: June 2003

The effect of electrical doping on the interface molecular level alignment at organic–organic (OO) heterojunctions is studied with ultraviolet photoemission spectroscopy. Interfaces between hole transport layers (HTL) and electron transport layers are investigated as a function of p doping of the HTLs. Doping induces the formation of an interface dipole with corresponding shift in the relative position of molecular levels across the interface. The modification of the OO electronic structure is believed to be due to the presence of doping-induced excess holes at the interface.
Dependence of two-photon-absorption-excited fluorescence on the angle between the linear polarizations of two intersecting beams
Authors: Rapaport Alexandra , Szipöcs Ferenc , Bass Michael
Published: June 2003

We report experimental observation of the dependence of the fluorescence excited by two-photon absorption (TPA/E fluorescence) in dye solutions on the angle between the linear polarizations of two intersecting pump beams. Both degenerate and nondegenerate TPA/E fluorescence were studied with 4 ns pulsed tunable lasers as pump sources. We find that earlier theoretical analyses do not contain all of the observed changes in fluorescence signal with relative polarization angle.
Influence of implantation energy on the electrical properties of ultrathin gate oxides grown on nitrogen implanted Si substrates
Authors: Kapetanakis E. , Skarlatos D. , Tsamis C. , Normand P. , Tsoukalas D.
Published: June 2003

Metal–oxide–semiconductor tunnel diodes with gate oxides, in the range of 2.5–3.5 nm, grown either on 25 or 3 keV nitrogen-implanted Si substrates at (0.3 or 1) ×1015 cm−2 dose, respectively, are investigated. The dependence of N2+ ion implant energy on the electrical quality of the growing oxide layers is studied through capacitance, equivalent parallel conductance, and gate current measurements. Superior electrical characteristics in terms of interface state trap density, leakage current, and breakdown fields are found for oxides obtained through 3 keV nitrogen implants. These findings together with the full absence of any extended defect in the silicon substrate make the low-energy nitrogen implantation technique an attractive option for reproducible low-cost growth of nanometer-thick gate oxides.
Diffusion in Co90Fe10/Ru multilayers
Authors: Svedberg Erik B. , Howard Kent J. , Bønsager Martin C. , Pant Bharat B. , Roy Anup G. , Laughlin David E.
Published: June 2003

Signal degradation in spin-valve structures is today a concern for long-term stability of data storage devices. One of the possible degradation mechanisms of spin-valve structures in disk drive applications could be thermally activated diffusion between constituent layers. In order to predict and control performance degradation, the interdiffusion coefficients for all bilayers in the spin-valve structure will have to be determined. Here we report results from a Co90Fe10/Ru interface, common in many spin-valve structures. The diffusion in (0002) oriented polycrystalline Co90Fe10/Ru multilayers has been measured and quantified by x-ray reflectivity in the temperature range of 450–540 °C. The bulk diffusion in this case is described by an activation energy of Ea=4.95 eV and a prefactor of D0=6.43×10−9 m2/s. No grain boundary diffusion was detected in the large-grain structure dominated by high symmetry grain boundaries at the temperature interval in this study. For a spin-valve structure that contains Co90Fe10/Ru interfaces it is clear that with the absence of grain boundary diffusion and a very high activation energy to bulk diffusion degradation will first take place at another interface, or by another phenomenon.
Interdiffusion in CoFe/Cu multilayers and its application to spin-valve structures for data storage
Authors: Svedberg Erik B. , Howard Kent J. , Bønsager Martin C. , Pant Bharat B. , Roy Anup G. , Laughlin David E.
Published: June 2003

Spin-valve structures might be exposed to higher temperatures in future disk drive applications and might thus degrade faster than it does today if proper materials and methods are not used. In order to determine whether this degradation is due to interdiffusion between constituent layers or is dominated by other phenomena, the interdiffusion coefficients for all layers in the spin valve have to be determined. For diffusion driven degradation it would then be possible to predict lifetimes based on a maximum allowed reduction in ΔR/R where R is the resistivity. Here we report the initial results for a CoFe/Cu interface, common to many spin-valve structures. Interdiffusion in (111) textured polycrystalline CoFe/Cu multilayers has been measured and quantified by x-ray reflectometry. Bulk diffusion is dominant at temperatures above ∼540 °C and is described by an activation energy of Ea=2.41 eV and a prefactor of D0=2.92×10−8 m2/s. Below temperature of 540 °C grain boundary diffusion dominates and is characterized by Ea=0.90 eV and D0=1.91×10−17 m2/s. Prior to stabilization of the diffusion process there is an initial rapid change in the (111) texture. During initial “sharpening” of the CoFe/Cu multilayer interfaces there is shortening of the periodicity as well as a decrease in out-of-plane lattice spacing.
Influence of growth conditions on the lattice constant and composition of (Ga,Mn)As
Authors: Schott G. M. , Schmidt G. , Karczewski G. , Molenkamp L. W. , Jakiela R. , Barcz A. , Karczewski G.
Published: June 2003

The lattice constant and the alloy composition of (Ga,Mn)As are investigated by high-resolution x-ray diffraction and secondary ion mass spectroscopy. The (Ga,Mn)As layers are grown by low-temperature molecular-beam epitaxy under various growth conditions. We find that, while the alloy composition is mainly determined by the Mn cell temperature (TMn), the substrate temperature (Tsub) and the arsenic to gallium flux ratio (As/Ga) strongly influence the lattice constant. In particular, layers which have the same composition but different growth parameters have quite different lattice constants, caused by the amount of excess As incorporation in the (Ga,Mn)As crystal. This implies that the lattice parameter of (Ga,Mn)As cannot even serve as a rough measure of the crystal composition. (Ga,Mn)As is therefore an example of a system which does not obey Vegard’s law.
Dynamics of stimulated emission in silicon nanocrystals
Authors: Dal Negro L. , Cazzanelli M. , Pavesi L. , Ossicini S. , Pacifici D. , Franzò G. , Priolo F. , Iacona F.
Published: June 2003

Time-resolved luminescence measurements on silicon nanocrystal waveguides obtained by thermal annealing of plasma-enhanced chemical-vapor-deposited thin layers of silicon-rich oxide have revealed fast recombination dynamics related to population inversion which leads to net optical gain. Variable stripe length measurements performed on the fast emission signal have shown an exponential growth of the amplified spontaneous emission with net gain values of about 10 cm−1. The fast emission component is strongly dependent on the pumping length for fixed excitation intensity. In addition, both the fast component intensity and its temporal decay revealed threshold behavior as a function of the incident pump intensity.
Strain-driven composition gradients in nanocrystalline SiGe
Authors: Tzoumanekas C. , Kelires P. C.
Published: June 2003

Monte Carlo simulations shed light onto the structure and composition of nanocrystalline SiGe. A strong enhancement of Ge content in nanocrystals is found, driven by the stress gradient between the crystalline and amorphous components of the network. For a stoichiometric amorphous matrix and for small nanocrystals, typically 4 nm in diameter, the Ge content in the nanocrystals reaches ∼80%. The composition gradient declines with increasing size. The nanocrystals consist of a large ordered Ge-rich core and an outer less-ordered region where alloying is enhanced.
Application of phase-retrieval x-ray diffractometry to carbon doped SiGe(C)/Si(C) superlattice structures
Authors: Siu Karen K.-W. , Nikulin Andrei Y. , Zaumseil Peter , Yamazaki Hiroshi , Ishikawa Tetsuya
Published: June 2003

An experimental-analytical technique for the model-independent nondestructive characterization of single crystal alloys is applied to SiGe(C)/Si(C) superlattice structures with different positions of substitutional carbon. The technique is based on the retrieval of the phase of the diffracted x-ray wave via a logarithmic dispersion relation, with subsequent determination of the complex crystal structure factor utilizing limited a priori knowledge of the superlattice structure. High-resolution, x-ray Bragg diffraction profiles were collected using a rotating anode source. The studies have allowed the reconstruction of the complex crystal structure factor as a function of crystal depth, permitting direct observation of strain relaxation in individual layers of the superlattice structure.
Transport and dielectric properties of K doped glassy precursor for high TC superconductor showing ferroelectric behavior due to embedded nanocrystalline ferroelectric phase
Authors: Mukherjee Soma , Sakata H. , Chaudhuri B. K. , Mollah S. , Yang H. D.
Published: June 2003

Partial substitution of Cu by K in multicomponent glassy precursor for high TC superconductor viz. Bi2−xPbxSr2Ca2Cu3Oδ (x=0.1–0.3) is found to destroy superconductivity of the corresponding annealed phase. These glasses viz. Bi2−xPbxSr2Ca2Cu3−yKyOz (x=0.1–0.3, y=0.2), however, show ferroelectric transitions (between 500 and 530 K depending on x). The corresponding K free glasses are not ferroelectric but they are good precursors for high TC superconductor. Nanocrystalline particles/clusters (∼10–50 nm size) embedded in the K doped glasses, observed from transmission electron microscopic studies, are considered to be responsible for the ferroelectric behavior of the bulk glass nanocrystal composite (GNC). Unlike K free glasses, the K-containing glasses do not become superconductor by annealing them at higher temperature. This indicates suppression of superconductivity by the precipitation of ferroelectric nanoparticles in the K doped GNC. Though their dielectric behavior is different, both the K free and K containing glassy systems are semiconductors and follow the similar nonadiabatic small polaron hopping conduction mechanism (obeying Holstein condition). A comparative study of the properties of the two glassy systems showing distinguishing behavior has been made.
Structure and dielectric properties of amorphous LaAlO3 and LaAlOxNy films as alternative gate dielectric materials
Authors: Lu Xu-bing , Liu Zhi-guo , Wang Yi-ping , Yang Ying , Wang Xiao-ping , Zhou Hong-wei , Nguyen Bich-yen
Published: June 2003

Amorphous LaAlO3 (LAO) and LAOxNy (LAON) films have been prepared by pulsed laser deposition technique on Si (100) substrates and Pt coated silicon substrates. X-ray diffraction, transmission electron microscopy and differential thermal analysis investigations showed that both kinds of films remain amorphous up to a high temperature of 860 °C. Atomic force microscopy study indicated that the surface of the deposited films is very smooth with a root mean square roughness of 0.14 nm for 8 nm LAO. LAON films have a smoother surface than that of LAO films. High-resolution transmission electron microscope studies showed there often exists interfacial reaction between LAO and Si. One LAON/Si structure nearly without interfacial layer has been obtained. For LAO films, high bandgap of 6.55 eV and medium dielectric constant of 25–27 have been obtained. The LAON films showed small equivalent oxide thickness of 1.1 nm with a low leakage of 0.074 A/cm2@Vg=+1 V. It is proposed that amorphous LAON films are very promising dielectric materials for high k gate dielectric applications.
Microstructural modifications induced by rapid thermal annealing in plasma deposited SiOxNyHz films
Authors: del Prado A. , San Andrés E. , Mártil I. , González-Dı́az G. , Bravo D. , López F. J. , Fernández M. , Martı́nez F. L.
Published: June 2003

The effect of rapid thermal annealing (RTA) processes on the structural properties of SiOxNyHz films was investigated. The samples were deposited by the electron cyclotron resonance plasma method, using SiH4, O2 and N2 as precursor gases. For SiOxNyHz films with composition close to that of SiO2, which have a very low H content, RTA induces thermal relaxation of the lattice and improvement of the structural order. For films of intermediate composition and of compositions close to SiNyHz, the main effect of RTA is the release of H at high temperatures (T>700 °C). This H release is more significant in films containing both Si–H and N–H bonds, due to cooperative reactions between both kinds of bonds. In these films the degradation of structural order associated to H release prevails over thermal relaxation, while in those films with only N–H bonds, thermal relaxation predominates. For annealing temperatures in the 500–700 °C range, the passivation of dangling bonds by the nonbonded H in the films and the transition from the paramagnetic state to the diamagnetic state of the K center result in a decrease of the density of paramagnetic defects. The H release observed at high annealing temperatures is accompanied by an increase of density of paramagnetic defects.
Theory of semiconductor magnetic bipolar transistors
Authors: Flatté M. E. , Yu Z. G. , Johnston-Halperin E. , Awschalom D. D.
Published: June 2003

Bipolar transistors with a ferromagnetic base are shown theoretically to have the potential to generate almost 100% spin-polarized current injection into nonmagnetic semiconductors. Optical control of ferromagnetism and spin splitting in the base can lead to either long-lived or ultrafast switching behavior. Fringe field control of the base magnetization could be used for information transfer between metallic magnetoelectronics and conventional semiconducting electronics.
High-efficiency optical transfer of torque to a nematic liquid crystal droplet
Authors: Juodkazis Saulius , Matsuo Shigeki , Murazawa Naoki , Hasegawa Ichiro , Misawa Hiroaki
Published: June 2003

We report on the determination of the difference in the refractive index between the ordinary and extraordinary rays in droplets of nematic liquid crystal E-44 at the wavelength of laser trapping, 1064 nm. This difference was calculated by measuring the ellipticity of laser tweezers for the moment at which rotation of the trapped droplet starts. The precision of this determination of the refractive index difference is approximately 5×10−3. Hydrophobic nematic liquid crystal E-44 formed 0.5–5-μm droplets when cast into D2O. The efficiency of the transfer of light torque to the liquid crystal droplets was determined to be 70% for the no-slip-boundary condition.
Improved AlGaN/GaN high electron mobility transistor using AlN interlayers
Authors: Jiménez A. , Bougrioua Z. , Tirado J. M. , Braña A. F. , Calleja E. , Muñoz E. , Moerman I.
Published: June 2003

This work reports on the effects of AlN interlayers embedded into the GaN semi-insulating buffer of AlGaN/GaN high electron mobility transistors, in comparison with standard heterostructures without AlN interlayers. Detailed optical and structural characterization data are presented, along with computer simulation results. The AlN interlayers generate a compressive strain in the GaN topmost layer, which slightly reduces the total polarization field, but most important, it prevents the AlGaN barrier from plastic relaxation. The final result is an enhanced polarization field with respect to standard heterostructures, providing an increased channel carrier density and pinch-off voltage. Electrical characterization confirms the advantages of using AlN interlayers, reaching maximum drain current density and extrinsic transconductance as high as 1.4 A/mm and 266 mS/mm, respectively, for 0.2-μm gate length.
Anomalous compositional pulling effect in InGaN/GaN multiple quantum wells
Authors: Hao M. , Ishikawa H. , Egawa T. , Shao C. L. , Jimbo T.
Published: June 2003

A series of InGaN/GaN multiple quantum wells (MQWs) was grown by metalorganic chemical vapor deposition with different well thickness. High-resolution x-ray diffraction studies revealed that the In composition is increasing along the growth direction from the bottom to the top of each well layer in these MQWs. While the In composition at the bottom of each well layer almost keeps constant, the increasing rate of In composition becomes obviously larger when the growth temperature is decreased. The important conclusion of this study is that the InGaN/GaN MQWs is shaped like a triangle due to the increasing of In composition from the bottom to the top of the well layer. The emission mechanism of the InGaN/GaN MQWs has to be discussed based on the triangular band gap structure.
Magnetocardiograph based on a 4×4 array of high-temperature superconducting quantum interference devices
Authors: Yokosawa Koichi , Kandori Akihiko , Miyashita Tsuyoshi , Suzuki Daisuke , Tsukada Keiji , Tsukamoto Akira
Published: June 2003

A magnetocardiograph, consisting of superconducting quantum interference device (SQUID) magnetometers made of a high-critical-temperature (Tc) superconductor and a compact magnetically shielding cylinder, has been developed. We assembled a sensor array of 4×4-matrix-arranged magnetometers which are treated as 24 electronic first-order gradiometers by taking the differences between the outputs in every pair of adjacent magnetometers. Field gradients generated by P waves were estimated from the results for 21 subjects of a conventional magnetocardiograph made of low-Tc SQUID gradiometers. Every gradiometer of the assembled sensor array was then designed to be sufficiently sensitive to detect the field gradient corresponding to the mean P wave. The clear magnetocardiograms obtained without averaging indicate the clinical applicability of the developed magnetocardiograph.
Wetting effect and morphological stability in growth of short-period strained multilayers
Authors: Huang Zhi-Feng , Kandel Daniel , Desai Rashmi C.
Published: June 2003

We explore the morphological stability during the growth of strained multilayer structures in a dynamical model which describes the coupling of elastic fields, wetting effect, and deposition process. We quantitatively show the significant influence of the wetting effect on the stability properties, in particular for short-period multilayers. Our results are qualitatively similar to recent experimental observations in AlAs/InAs/InP(001) system. We also give predictions for strain-balanced multilayers.
Direct and subdiffraction-limit laser nanofabrication in silicon
Authors: Huang S. M. , Hong M. H. , Luk’yanchuk B. S. , Chong T. C.
Published: June 2003

We propose a method for nanofabrication at a resolution much below the diffraction limit for projection optical lithography using a backside-irradiation method. Feature sizes below λ/260 have been achieved in silicon. An infrared laser (CO2, λ=10.6 μm) was used to illuminate the backside of a Si substrate with Au particles on its polished surface. The morphologies of created features were characterized by atomic force microscope and field emission scanning electron microscope. The formation mechanisms of these nanofeatures have been discussed and associated with localized optical excitations in Au colloid aggregates with a fractal structure. This backside-irradiation laser-assisted nanofabrication method may also be extended to various metals, particle shapes, particle sizes, substrates, and other lasers. It can be used to investigate optical excitations and near fields in these systems.