bluuebunny/crossref_metadata_2025 huggingface dataset:
On the use of ab initio interaction energies for the accurate calculation of thermodynamic properties
Authors: Garrison Stephen L. , Sandler Stanley I.
Published: December 2002

It is of interest to predict the thermodynamic properties and phase behavior of a substance from quantum-chemical calculations of intermolecular interaction energies followed by molecular simulations. However, while quantum-chemical methods can be quite accurate, they do not provide an exact solution to Schrödinger’s equation (excluding full CI) and additional errors arise when fitting energies to analytic potential functions. The purpose of this communication is to provide an understanding and quantification of the sensitivity of the calculated properties to changes (or uncertainties) in different parts of the potential function. For this purpose, Gibbs ensemble Monte Carlo simulations were used to determine the effects on phase behavior of small perturbations to various regions of the model Lennard-Jones 12–6 potential. The results indicate that repulsive energies play a limited role in determining the phase behavior and critical properties, while the attractive energies strongly affect the critical temperature, critical pressure, saturation densities, and vapor pressure. The critical density is most strongly affected by the location at which the potential is zero. However, when the phase behavior and second virial coefficient are scaled by the critical properties calculated for each potential, the results obey a corresponding states relation. These results are used to understand and predict variations in the calculated phase behavior for intermolecular potentials obtained using various strategies to fit ab initio-calculated interaction energies. The knowledge obtained is used to provide accurate predictions for neon based on quantum-chemical energies and a recommended fitting strategy. We also show that three-body nonadditivity effects are largely unimportant for neon.
Electron correlation and noninteracting v-representability in density functional theory: The Be isoelectronic series
Authors: Morrison Robert C.
Published: December 2002

Reference densities from accurate configuration interaction wave functions for the beryllium isoelectronic series were used to solve the Kohn–Sham equations using a constrained search that minimizes the kinetic energy. For Z<25 in the series, a single Kohn–Sham determinant is sufficient to give the minimum kinetic energy. For higher Z a single Kohn–Sham determinant produces 2p eigenvalues that are lower than the 2s eigenvalues, and a kinetic energy that is not the minimum that can be obtained from an antisymmetric wave function that produces the reference density. Fractional occupation numbers are required to obtain the minimum kinetic energy, and at the minimum kinetic energy the 2s and 2p eigenvalues become equal. Values of the optimal 2p occupation numbers approach 0.09 for high Z.
Density functional theory investigation of gold cluster geometry and gas-phase reactivity with O2
Authors: Wells David H. , Delgass W. Nicholas , Thomson Kendall T.
Published: December 2002

We have conducted a density functional theory investigation into the gas-phase reactivity of small gold cluster ions in the interest of understanding gold cluster reactivity in several catalytic systems. Previously unreported geometries for Au9− and Au10− anions are obtained and reported from geometry optimizations. Predicted values of the vertical detachment energy match well with experiment, as does a rough simulation of its ultraviolet photoelectron spectrum—we found that comparison of predicted spectra with experimental data is a more sensitive analysis of geometry differences. Several binding sites for O2 with different energies are identified on Au10−, but we show that the strongest binding site and orientation is predicted by frontier orbital theory. In addition, weakly stable adsorbed states for O2 on the anion clusters Au9−, Au10−, and Au11− are predicted in agreement with frontier orbital theory. The calculated binding energies are consistent with the experimentally observed patterns in adsorption of O2 on anionic Au clusters. The binding energy for O2 to Au10− was calculated to be 19 kcal/mol, higher than for O2 to either Au9− (4 kcal/mol) or Au11− (5 kcal/mol), and the calculated O–O bond length was found to increase from its gas-phase value of 1.27 angstroms to 1.38 angstroms when adsorbed on the Au10− cluster, approaching the calculated bond length of 1.41 angstroms for the gas-phase superoxide ion O2−.
On using potential, gradient, and Hessian data in least squares fits of potentials: Application and tests for H2O
Authors: Xie Tiao , Bowman Joel M.
Published: December 2002

We present a novel, least-squares fitting approach to obtain a representation of a potential energy surface using potential, gradient, and Hessian data. The method is described in detail and then tested for H2O in two ways. In the first test a global, analytical potential is used to generate the data at 7 and 11 configurations. A comparison of the accuracy of the fit against the exact surface is made, as is a comparison of low-lying vibrational states. In the second test, Density Functional theory (DFT) calculations of the potential, gradient, and Hessian are performed at 7 and 11 configurations to obtain fits. The predictions of the fits are compared to 125 new DFT calculations of the energies and a conventional fit to them, both directly and in vibrational calculations.
Bragg magnifier: A detector for submicrometer x-ray computer tomography
Authors: Stampanoni Marco , Borchert Gunther , Abela Rafael , Rüegsegger Peter
Published: December 2002

X-ray computer microtomography is a powerful tool for nondestructive examinations in medicine, biology, and material sciences. The resolution of the presently used detector systems is restricted by scintillator properties, optical light transfer, and charge-coupled-device granularity, which impose a practical limit of about 1 μm spatial resolution at detector efficiencies of a few percent. A recently developed detector, called the Bragg magnifier, achieves a breakthrough in this respect, satisfying the research requirements that ask for an efficient advance towards the submicron range. The Bragg magnifier uses the properties of asymmetric Bragg reflections to increase the cross section of the diffracted x-ray beam. Magnifications of up to 100×100 can be achieved even at hard x-rays energies (>20 keV). In this way, the influence of the detector resolution can be scaled down accordingly and the efficiency increased. Such a device has been developed and successfully integrated into the Microtomography Station of the Materials Science Beamline of the Swiss Light Source. The device is operated at energies ranging from 21.1 to 22.75 keV, reaching spatial resolutions of 140 nm, and is ready for submicrometer microtomography.
High critical current density in Ti-doped MgB2/Ta/Cu tape by powder-in-tube process
Authors: Fu B. Q. , Feng Y. , Yan G. , Zhao Y. , Pradhan A. K. , Cheng C. H. , Ji P. , Liu X. H. , Liu C. F. , Zhou L. , Yau K. F.
Published: December 2002

Ti-doped MgB2/Cu tapes with Ta as a buffer layer were prepared through the in situ powder in tube method by using Mg, Ti, and B powders. The phase compositions, microstructure features, and superconducting properties were investigated by x-ray diffraction, scanning electron microscope, and superconducting quantum interference device magnetometer. It is found that TiB2 phase was formed in Ti-doped MgB2 tape. Magnetization measurement results show that the critical transition temperature of MgB2/Ta/Cu tape with Ti doping is around 38 K. The irreversibility field Hirr and critical current density Jc can be greatly enhanced by Ti doping. Hirr of the Mg0.9Ti0.1B2 tape is as high as 7.4 T at 10 K. The high critical current density Jc of 1.5×106 A/cm2 (10 K, self field) and 9.3×105 A/cm2 (20 K, self field) are obtained in the Mg0.9Ti0.1B2 tape. In addition, a suitable amount of Ti doping can lead to a high density and fine grain size of MgB2, which may be the reason for high Jc in Ti-doped MgB2 tapes.
Low-cost mechanical shutter for light beams
Authors: Singer Kilian , Jochim Selim , Mudrich Marcel , Mosk Allard , Weidemüller Matthias
Published: December 2002

We present a simple design of a fast mechanical shutter for light beams using a low-cost personal computer loudspeaker. The shutter is capable of closing an aperture of 5 mm at a maximum speed of 1.7 mm/ms with a timing jitter of less than 10 μs. When combined with polarization optics, our device can also be used as an alterable switch and adjustable attenuator.
The infrared and ultraviolet spectra of single conformations of methyl-capped dipeptides: N-acetyl tryptophan amide and N-acetyl tryptophan methyl amide
Authors: Dian Brian C. , Longarte Asier , Mercier Sebastien , Evans David A. , Wales David J. , Zwier Timothy S.
Published: December 2002

A combination of methods, including laser-induced fluorescence excitation, fluorescence-dip infrared (FDIR) spectroscopy, and UV-UV hole-burning spectroscopy, have been used to study the infrared and ultraviolet spectra of single conformations of two methyl-capped dipeptides: N-acetyl tryptophan amide (NATA) and N-acetyl tryptophan methyl amide (NATMA). Density functional theory calculations predict that all low-energy conformers of NATA and NATMA belong to one of two conformational families: C5, with its extended dipeptide backbone, or C7eq, in which the dipeptide backbone forms a seven-membered ring joined by a H bond between the ψ-amide NH and the φ-amide carbonyl groups. In NATA (NATMA), the LIF spectrum has contributions from two (three) conformers. FDIR spectroscopy has been used to record infrared spectra of the individual conformers over the 2800–3600 cm−1 region, free from interference from one another. The NH stretch region provides unequivocal evidence that one of the conformers of NATA is C5, while the other is C7eq. Similarly, in NATMA, there are two C5 conformers, and one C7eq structure. Several pieces of evidence are used to assign spectra to particular C5 and C7eq conformers. NATA(A) and NATMA(B) are both assigned as C5(AP) structures, NATA(B) and NATMA(C) are assigned as C7eq (ΦP), and NATMA(A) is assigned as C5(AΦ). In both molecules, the C5 structures have sharp vibronic spectra, while the C7eq conformers are characterized by a dense, highly congested spectrum involving long progressions that extend several hundred wave numbers to the red of the C5 S1–S0 origins. N-acetyl tryptophan ethyl ester (NATE), which can only form C5 conformers, shows only sharp transitions in its LIF spectrum due to four C5 conformers, with no evidence for the broad absorption due to C7eq. This provides direct experimental evidence for the importance of the peptide backbone conformation in controlling the spectroscopic and photophysical properties of tryptophan.
On quantum trajectories and an uncertainty relation
Authors: McLafferty Frank
Published: December 2002

We discuss quantum trajectories from the point of view of Bohm and Wyatt. We find that the theory can be formally written in terms of a field (probability) [P(x,t)], a position [x], and an average momentum [〈p〉] in a density operator which is normalized and Hermitian, but not positive definite. One consequence of this is that the theory obeys, in a natural way, a formal relation ΔxΔp=0 for these averages. We interpret this as a consistency requirement for a trajectory in space–time with a position x and an average momentum which allows a space–time description. We show that a momentum space form can be written in terms of a field (probability) [P(p,t)], a momentum [p], and an average position [〈x〉]. We briefly discuss potential chemical applications of quantum trajectories in the theory of chemical dynamics, kinetics, and local field theory.
Low melting temperature and liquid surface layering for pair potential models
Authors: Velasco E. , Tarazona P. , Reinaldo-Falagán M. , Chacón E.
Published: December 2002

We have recently proposed [Phys. Rev. Lett. 87, 166101 (2001)] that any isotropic fluid should exhibit surface layering at its liquid–vapor interface above the triple temperature provided that the system has a low triple temperature. In this article we present an extensive study of systems with different isotropic pair interactions, some of which present a very low triple temperature. We have confirmed that surface layering is a general characteristic of very cold liquids, independent of the specific shape of the potential, and that only pair potentials presenting a low triple-point temperature do exhibit surface oscillations; in other cases layering is preempted by solidification. Finally, we study the damping of surface oscillations due to capillary waves and conclude that for any model pair potential the temperature threshold below which layering would be observed for the typical experimental transverse sampling sizes is 15% of the critical temperature.
Ab initio study of C60–silicon clusters
Authors: Masenelli B. , Tournus F. , Mélinon P. , Pérez A. , Blase X.
Published: December 2002

We report on the energetics of C60–Si clusters. By means of ab initio calculations based on the local density approximation to the density functional theory, we have investigated stable and metastable structures of C60–Si, C60–Si–C60, and (C60–Si)2 clusters. In each case, we show that silicon preferentially binds to C60 over a carbon–carbon double bond, in accordance with calculations on the interaction of C60 with silicon surfaces. This bonding is characterized by a partial charge transfer from silicon to C60. We show that the interaction between C60 and silicon is local and not perturbed by the addition of more C60–Si clusters or C60 molecules. The binding energy for stable and metastable (C60–Si)n⩽2 systems is high enough (several eV) to open the prospect of synthesizing nanostructured films from the C60–Si unit. Furthermore, in all three cases, the silicon position on a fivefold symmetry axis is found to be a metastable position. The nature and structure of nanostructured films resulting from the deposition of these clusters is qualitatively discussed.
Validation and assessment of an accurate approach to the correlation problem in density functional theory: The Kriger–Chen–Iafrate–Savin model
Authors: Toulouse Julien , Savin Andreas , Adamo Carlo
Published: December 2002

In the present paper, we validate and assess a correlation functional based on the so-called meta generalized gradient approximation, whose form and parameters are entirely derived only from first-principles criteria. In particular, we have carried out a detailed comparison with the most common, parametrized correlation functionals. Next, we propose a new model in which the correlation functional proposed by Kriger, Chen, Iafrate, and Savin is integrated in a hybrid Hartree–Fock/density functional theory scheme. In such approach only one, or two in the G2-optimized version, parameters are adjusted on experimental data, all the others being derived from purely theoretical considerations. The results obtained for a set of molecular properties, including H-bonded complexes, proton transfer model, SN2 reaction and magnetic properties, are satisfactory and comparable, if not better, with those delivered by the most common functionals including heavy parametrization. The way in which the whole functional is derived and the few empirical parameters used make the new exchange–correlation functional widely applicable.
Interelectronic angle densities of atoms
Authors: Koga Toshikatsu
Published: December 2002

In many-electron systems, the interelectronic angle density A(θ12) represents the probability density function for the interelectronic angle θij spanned by position vectors ri and rj of any two electrons to be θ12. It is shown that in general the interelectronic angle density A(θ12) is expressed by a linear combination of Legendre polynomials Pn(cos θ12). Explicit formulas for the expansion coefficients are presented for atoms described by determinantal wave functions. The results are applied to the 102 atoms He through Lr in their ground states within the Hartree–Fock framework, and the expansion coefficients as well as the average interelectronic angles 〈θ12〉 are reported.
Perdeuterated cyanobiphenyl liquid crystals for infrared applications
Authors: Wu Shin-Tson , Wang Qiong-Hua , Kempe Michael D. , Kornfield Julia A.
Published: December 2002

Perdeuterated 4’-pentyl-4-cyanobiphenyl (D5CB) was synthesized and its physical properties evaluated and compared to those of 5CB. D5CB retains physical properties similar to those of 5CB, such as phase transition temperatures, dielectric constants, and refractive indices. An outstanding feature of D5CB is that it exhibits a much cleaner and reduced infrared absorption. Perdeuteration, therefore, extends the usable range of liquid crystals to the mid infrared by significantly reducing the absorption in the near infrared, which is essential for telecom applications.
Influences of vanadium on magnetocrystalline anisotropy and magnetic properties of Gd2Co17−xVx
Authors: Chu W. G. , Rao G. H. , Liu G. Y. , Yang H. F. , Liu W. F. , Ouyang Z. W. , Feng X. M. , Liang J. K.
Published: December 2002

Single-phase Gd2Co17−xVx compounds (x=0.0–1.5) crystallizing in the rhombohedral Th2Zn17 structure have been synthesized. The lattice parameters a and c of the compounds increase linearly with increasing V content, and the rate of increase of c is about 2.5 times as large as that of a. Substitution of a small amount of V atoms (x=0.3) for Co atoms leads to the occurrence of uniaxial magnetocrystalline anisotropy of Gd2Co17−xVx. The anisotropy field HA increases drastically with increasing V content. The variations of both the lattice parameters and the magnetocrystalline anisotropy with V content suggest a preferential occupation of the V atoms at the 6c dumbbell site. The Curie temperature TC, saturation moment MS, and average Co moment 〈μCo〉 of the compounds decrease greatly as the V content increases. The rapid decrease of TC is essentially attributed to a serious weakening of the Co-Co interactions due to the preferential occupation of the V atoms at the 6c site. The effect of a strong hybridization between the V and Co atoms is plausibly responsible for the decreases of the MS and 〈μCo〉.
Pulse response of thin III/V semiconductor photocathodes
Authors: Aulenbacher K. , Schuler J. , Harrach D. v. , Reichert E. , Röthgen J. , Subashev A. , Tioukine V. , Yashin Y.
Published: December 2002

The response time and spin relaxation time of thin unstrained and strained III/V-semiconductor photocathodes installed in sources of polarized electrons have been investigated. Cathodes of various active layer thicknesses have been studied. An upper limit for the response time of a 150 nm thick strained layer photocathode has been found to be 2.5 ps. As a consequence, the average depolarization during transport in the conduction band to the surface is estimated to be lower than 3% and does not contribute substantially to the upper limit of about 80% for the spin polarization of the emitted electrons. The results indicate a high surface recombination velocity of S>1.2×107 cm/s at the surface band bending region.
On the frequency response of a resonant-cavity-enhanced separate absorption, grading, charge, and multiplication avalanche photodiode
Authors: Das N. R. , Deen M. J.
Published: December 2002

In this article the frequency response of a resonant-cavity-enhanced (RCE) separate absorption, grading, charge, and multiplication (SAGCM) avalanche photodiode (APD) has been investigated. This study on APDs includes the development of a model for the avalanche buildup of carriers including the dead-space effect. In this model, the carriers are characterized by their energy and position in the region of multiplication. The excess energy of the carriers above threshold are assumed to be equally distributed among the carriers after impact ionization. The analysis shown in this article can be used to study the frequency response of the APD for a wide range of reverse bias voltages. For low bias voltages, an analytical expression has been derived to study the emission of carriers from the interface trap. The delay due to diffusion of photogenerated carriers from the undepleted region of the absorption layer and the low velocity of carriers at low field have also been considered. The model is used to calculate gains at different biases and bandwidth at different dc gains. The results show good agreement with published experimental data on a RCE SA(G)CM APD.
Analysis of integrated single-electron memory operation
Authors: Korotkov Alexander N.
Published: December 2002

Various aspects of single-electron memory are discussed. In particular, we analyze single-electron charging by Fowler–Nordheim tunneling, propose the idea of background charge compensation, and discuss a defect-tolerant architecture based on nanofuses.
Formation of misfit dislocations in thin film heterostructures
Authors: Narayan J. , Oktyabrsky S.
Published: December 2002

We have studied characteristics of 60° and 90° dislocations in GaAs/Si(100) thin-film heterostructures grown by metal-organic chemical-vapor deposition at 650 °C. The misfit dislocation network consists of approximately 60% of 90° dislocations, and 40% of the closely spaced pairs of 60° dislocations with intersecting glide planes. This ratio has remained essentially constant after rapid thermal annealing at 800 °C for 90 sec. It is envisaged that these 60° dislocation pairs have parallel screw components and as a result they cannot combine to form a 90° dislocation. Upon annealing, some of the 60° dislocation pairs split to form stacking faults in agreement with our earlier model. Based upon these observations, we propose a model where a first set of 60° dislocations is generated from the undulated surface above a critical thickness. The second set of 60° dislocations is nucleated at a larger thickness and at the smoother surface. The Burgers vectors of these dislocations are controlled by the dislocations from the first set, and only low-energy dislocation pairs are formed through glide towards the interface and later through short glide and climb along the interface plane. We have used a numerical analysis based on elasticity theory to evaluate the changes in the nucleation barrier for the 60° dislocations caused by the interaction with the existing misfit dislocations.
On the enhanced electron mobility in strained-silicon inversion layers
Authors: Fischetti M. V. , Gámiz F. , Hänsch W.
Published: December 2002

The recently reported large enhancement of the electron mobility in strained-Si inversion layers at large carrier concentrations cannot be easily explained: The strong carrier confinement in inversion layers removes the sixfold degeneracy of the conduction-band minima, much as tensile in-plane strain does, so that the effect of strain should become irrelevant at large sheet carrier densities. The problem is studied by calculating the electron mobility accounting for scattering with phonons and interface roughness. Surprisingly, the latter process is found to be significantly stronger in strained layers for a given interface roughness. Only the ad hoc assumption of increasingly smoother interfaces with increasing strain seems to explain the data.
Chlorine mobility during annealing in N2 in ZrO2 and HfO2 films grown by atomic layer deposition
Authors: Ferrari S. , Scarel G. , Wiemer C. , Fanciulli M.
Published: December 2002

Atomic layer deposition (ALD) growth of high-κ dielectric films (ZrO2 and HfO2) was performed using ZrCl4, HfCl4, and H2O as precursors. In this work, we use time of flight secondary ion mass spectrometry to investigate the chlorine distribution in ALD grown ZrO2 and HfO2 films, and its evolution during rapid thermal processes in nitrogen atmosphere. Chlorine outdiffusion is found to depend strongly upon annealing temperature and weakly upon the annealing time. While in ZrO2 chlorine concentration is significantly decreased already at 900 °C, in HfO2 it is extremely stable, even at temperatures as high as 1050 °C.
Thermodynamic consistency and integral equations for the liquid structure
Authors: Leys F. E. , March N. H. , Lamoen D.
Published: December 2002

Within an assumed pair potential framework, it has been generally accepted for a long time that far from the critical point the asymptotic form of the direct correlation function c(r) at large r is given by [−φ(r)/kBT]. Here φ(r) is the pair potential and kBT the thermal energy. Subsequently, Kumar, March, and Wasserman [Phys. Chem. Liquids 11, 271 (1982)] examined the condition for thermodynamic consistency between virial and compressibility equations of state. Their study, together with later work by Senatore, Rashid, and March [Phys. Chem. Liquids 16, 1 (1986)], resulted in a decomposition of c(r) into a potential part cp(r) given by Kumar et al. for all r and involving the pair function g(r) and its density derivative, plus a “collective” part cc(r), which must obey a simple sum rule to satisfy thermodynamic consistency. The more recent study of B. C. Eu and K. Rah [J. Chem. Phys. 3, 3327 (1999)] prompts us to bring their results into direct contact with the study of Kumar et al. The work of Eu and Rah gives a prominent place to the Mayer function f(r)=e(−[φ(r)/kBT]−1 which tends to −[φ(r)/kBT] as r→∞ for potentials tending to zero at infinity.
Numerical computation of tunneling fluxes
Authors: Galperin Michael , Toledo Sivan , Nitzan Abraham
Published: December 2002

The computation of tunneling probabilities in three dimensions is a numerical challenge, because the small transition probabilities associated with the overlap of exponentially vanishing wave function-tails require large computational accuracy. In scattering situations arising, e.g., in electron tunneling in metal-molecule-metal junctions, this is compounded by the need to provide a proper truncation procedure at the numerical boundaries of the computed system and by the need to account for electrostatic fields and image interactions. This paper describes a numerical methodology to deal with these problems. A pseudopotential that describes the underlying system is assumed given. Electrostatic fields and image interactions are evaluated for the given boundary conditions from numerically solving Laplace and Poisson equations. Tunneling probabilities are computed using a grid-based absorbing boundary conditions Green’s function method. An efficient and exact way to implement the absorbing boundary conditions by using the exact self-energy associated with separating the scattering system from the rest of the infinite space is described. This makes it possible to substantially reduce the size of the grid used in such calculations. Two applications, an examination of the possibility to resolve the spatial structure of an electron wave function in an electron cavity by scanning tunneling microscopy, and a calculation of electron tunneling probabilities through water, are presented.
On the electrostatic potential profile in biased molecular wires
Authors: Nitzan Abraham , Galperin Michael , Ingold Gert-Ludwig , Grabert Hermann
Published: December 2002

The potential profile across a biased molecular junction is calculated within the framework of a simple Thomas–Fermi-type screening model. In particular, the relationship between this profile and the lateral molecular cross section is examined. We find that a transition from a linear potential profile to a potential that drops mainly near the molecule-metal contacts occurs with increasing cross-section width, in agreement with numerical quantum calculations.
Interfacial and microstructural properties of SrTiO3 thin films grown on Si(001) substrates
Authors: He J. Q. , Regnery S. , Jia C. L. , Qin Y. L. , Fitsilis F. , Ehrhart P. , Waser R. , Urban K. , Wang R. H.
Published: December 2002

The microstructure and interfaces of SrTiO3 thin films directly deposited by metalorganic chemical vapor deposition on silicon (001) substrates were investigated by means of Bragg-diffraction contrast and high-resolution transmission electron microscopy. The observation of the plan-view specimens showed that the SrTiO3 films are polycrystalline with randomly oriented grains. An amorphous layer was observed at the interfaces between the films and the substrates. The growth kinetics of this amorphous layer was investigated in detail. The thickness showed a rapid initial increase, which is much faster than the corresponding growth of amorphous SiO2 in the absence of precursors, and apparently approaches saturation after a short time. The thickness of the interfacial layer increases with the oxygen partial pressure during deposition and a reduction to a value acceptable for gate-oxide applications has been achieved for the minimum pressure given by the oxygen content of the present precursors. However, this comes at the cost of a dramatic increase of the carbon content of the film.
Comparison of excessive Balmer α line broadening of glow discharge and microwave hydrogen plasmas with certain catalysts
Authors: Mills R. L. , Ray P. C. , Dhandapani B. , Mayo R. M. , He J.
Published: December 2002

From the width of the 656.3 nm Balmer α line emitted from microwave and glow discharge plasmas, it was found that a strontium–hydrogen microwave plasma showed a broadening similar to that observed in the glow discharge cell of 27–33 eV; whereas, in both sources, no broadening was observed for magnesium–hydrogen. Microwave helium–hydrogen and argon–hydrogen plasmas showed extraordinary broadening corresponding to an average hydrogen atom temperature of 180–210 eV and 110–130 eV, respectively. The corresponding results from the glow discharge plasmas were 33–38 eV and 30–35 eV respectively, compared to ≈4 eV for plasmas of pure hydrogen, neon–hydrogen, krypton–hydrogen, and xenon–hydrogen maintained in either source. Similarly, the average electron temperature Te for helium–hydrogen and argon–hydrogen microwave plasmas were high, 30 500±5% K and 13 700±5% K, respectively; compared to 7400±5% K and 5700±5% K for helium and argon alone, respectively. External Stark broadening or acceleration of charged species due to high fields can not explain the microwave results since no high field was present, and the electron density was orders of magnitude too low for the corresponding Stark effect. Rather, a resonant energy transfer mechanism is proposed.
Growth mechanism and continuity of atomic layer deposited TiN films on thermal SiO2
Authors: Satta A. , Schuhmacher J. , Whelan C. M. , Vandervorst W. , Brongersma S. H. , Beyer G. P. , Maex K. , Vantomme A. , Viitanen M. M. , Brongersma H. H. , Besling W. F. A.
Published: December 2002

In atomic layer deposition (ALD), film thickness control by counting the number of deposition sequences is poor in the initial, nonlinear growth region. We studied the growth of TiN films formed by sequentially controlled reaction of TiCl4 and NH3 on thermal SiO2 during the transient, nonlinear period. Using low-energy ion scattering and Rutherford backscattering spectroscopy analysis, we have found that a three-dimensional growth of islands characterizes the ALD TiN growth on SiO2. Growth at different temperatures (350 °C and 400 °C) affects the extent of the transient region and the rapid closure of the film. At 400 °C, a reduced growth inhibition and an earlier start of three-dimensional growth of islands results in film closure at about 100 cycles, corresponding to a TiN thickness of 24±3 Å. At 350 °C the minimum thickness at which the TiN layer becomes continuous is 34±3 Å, deposited with 150 cycles.
Anelastic spectroscopy as a selective probe to reveal and characterize spurious phases in solid compounds
Authors: Castellano C. , Cordero F. , Cantelli R. , Ferretti M.
Published: December 2002

We show that it is possible, by anelastic spectroscopy measurements, to identify beyond the intrinsic limits of other techniques such as diffraction, resistivity, or extended x-ray absorption fine structure, the nature of spurious and minority phases possibly present in the experimental samples. We will stress how the high selectivity of this spectroscopy can be useful in studying classes of materials such as colossal magnetoresistance manganites or other transition metal oxides where the characterization of the specimens assumes a crucial role.
Electronic traps and percolation paths in electroluminescent polymers
Authors: von Malm N. , Steiger J. , Heil H. , Schmechel R. , von Seggern H.
Published: December 2002

Trap related properties of the electroluminescent polymers poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene (MDMO-PPV, OC1C10–PPV) and poly[(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene]-[(4′-(3″,7″-dimethyloctyloxy)phenyl-5)-1,4-phenylenevinylene] are investigated in organic light emitting diode (OLED) devices. By means of the thermally stimulated currents (TSC) technique the densities of occupied states, trap polarities and charge transport mechanisms are studied. The fractional TSC technique in combination with optical trap filling reveals broad density-of-occupied-states distributions for both polymers, ranging from 200 to 600 meV located between the highest occupied molecular orbital and lowest unoccupied molecular orbital levels. Conventional TSC experiments in combination with electrical trap filling allow to distinguish between electron and hole traps. In OC1C10–PPV both trap polarities are detected whereas in the copolymer only hole traps and no electron traps were found. In addition, a TSC study on optical and electrical trap filling in OLED devices strongly suggests the presence of percolation paths. Despite larger charging currents in the electrical trap filling case no or much smaller TSC signals were detected in comparison with the optical trap filling case.
Determination of the crystal and magnetic structures of Rn+1Co3n+5B2n (n=1, 2, and 3; R=Pr, Nd, and Dy)
Authors: Zlotea Claudia , Chacon Cyril , Isnard Olivier
Published: December 2002

The crystal and magnetic structures of the Rn+1Co3n+5B2n compounds (R=Pr, Nd, and Dy; n=1, 2, and 3) have been studied by high-resolution powder neutron diffraction and magnetization measurements. The RCo4B (n=1) and R3Co11B4 (n=2) samples are found to be single-phase compounds whereas R3Co11B4 is seen as impurity in R2Co7B3 (n=3). The crystal structures show the existence of unexpected very short cobalt–boron interatomic distances in R2Co7B3. This feature indicates a break in the regular stacking of the Rn+1Co3n+5B2n compounds for n=3 and may explain the absence of stable phases with 3<n<∞. The magnetic structures of RCo4B, R3Co11B4 and R2Co7B3 confirm that a large variety of magnetic moments exists for the cobalt atoms. The magnetism of the cobalt atoms situated in the 2c position is not affected by the substitution of boron for cobalt. On the contrary, the cobalt atoms situated on the 3g, 6i1, and 6i2 positions are carrying significantly reduced magnetic moments in comparison with those on the 2c sites. The R magnetic moments are found to be less sensitive to the presence of boron in their local environment. This behavior tends to indicate that due to the strongly localized 4f electron in the R atoms, the magnetic moment carried by the rare-earth atoms is not significantly modified by the presence of B. Finally, the direction of easy magnetization is found to be unchanged upon the substitution of boron for cobalt and is imposed by the R sublattice for the RCo4B, R3Co11B4, and R2Co7B3 compounds.
Development of magnetic domains in hard ferromagnetic thin films of polytwinned microstructure
Authors: Kazaryan A. , Wang Y. , Jin Yongmei M. , Wang Yu U. , Khachaturyan Armen G. , Wang Lisha , Laughlin David E.
Published: December 2002

Three-dimensional computer simulations are used to examine the development of magnetic domains and migration of domain walls in hard ferromagnetic thin films of polytwinned microstructure. The polytwinned microstructure, consisting of three different orientation variants of the L10 ordered phase with orthogonal c-axes, is characterized by a multi-component long-range order (LRO) parameter. The coupling between the magnetic domains and the polytwinned microstructure is described by the coupling between magnetization and the LRO parameter in the anisotropy free energy of the system. The magnetodynamics is described by Landau–Lifshitz equations that are solved numerically in the reciprocal space using a parallel Fast Fourier Transform algorithm. It is found that the coupling results in a multilevel hierarchy in the magnetic domain structure, with intracolony boundaries (across multiple variants within a single polytwinned colony) and intercolony boundaries (across multiple colonies). In thin films of high magnetic anisotropy we find the intracolony boundary to be of {100} type and the intercolony one to be on average of {110} type. The migration of the magnetic domain boundaries in external magnetic fields is studied in a number of simple cases in which the fields are applied along different directions. The simulation results are analyzed and compared with existing experimental observations.
Nucleation and growth of atomic layer deposited HfO2 gate dielectric layers on chemical oxide (Si–O–H) and thermal oxide (SiO2 or Si–O–N) underlayers
Authors: Green M. L. , Ho M.-Y. , Busch B. , Wilk G. D. , Sorsch T. , Conard T. , Brijs B. , Vandervorst W. , Räisänen P. I. , Muller D. , Bude M. , Grazul J.
Published: December 2002

A study was undertaken to determine the efficacy of various underlayers for the nucleation and growth of atomic layer deposited HfO2 films. These were compared to films grown on hydrogen terminated Si. The use of a chemical oxide underlayer results in almost no barrier to film nucleation, enables linear and predictable growth at constant film density, and the most two-dimensionally continuous HfO2 films. The ease of nucleation is due to the large concentration of OH groups in the hydrous, chemical oxide. HfO2 grows on chemical oxide at a coverage rate of about 14% of a monolayer per cycle, and films are about 90% of the theoretical density of crystalline HfO2. Growth on hydrogen terminated Si is characterized by a large barrier to nucleation and growth, resulting in three-dimensional, rough, and nonlinear growth. Thermal oxide/oxynitride underlayers result in a small nucleation barrier, and nonlinear growth at low HfO2 coverages. The use of chemical oxide underlayers clearly results in the best HfO2 layers. Further, the potential to minimize the chemical oxide thickness provides an important research opportunity for high-κ gate dielectric scaling below 1.0 nm effective oxide thickness.
Anomalous increase of photocurrent anisotropy in a liquid crystalline binary mixture
Authors: Sandhya K. L. , Nair Geetha G. , Krishna Prasad S. , Hiremath Uma S. , Yelamaggad C. V.
Published: December 2002

We report photoconductivity measurements in a binary system of naphthalene-based liquid crystals. Under ultraviolet (365 nm) illumination we observe an anomalous increase in the photocurrent for the mixtures. For a particular concentration, the photocurrent anisotropy defined as the ratio of photocurrents orthogonal to and along the director in the Crystal E phase reaches a value of 850, the highest achieved under steady-state illumination conditions from a conventional source. We argue that when using such mixtures, it is possible to reach the limiting values predicted by models based on conduction dictated by charge-carrier hopping.
Carbon impurities in MgB2
Authors: Yan Yanfa , Al-Jassim M. M.
Published: December 2002

The structural and electronic properties of carbon impurities in MgB2 are investigated by first-principles total-energy calculations. We find that high concentration of C impurities could be incorporated in MgB2 due to their low-formation energies. Substitutional C impurities have lower formation energies than interstitial C. Thus, the majority of C impurities in MgB2 should be substitutional, which results in a lattice parameter decrease in the a-b plane. C impurities are found to reduce the density of states of MgB2 around the Fermi energy, and therefore, may cause a decrease in critical temperature Tc.
Analytical model for ion angular distribution functions at rf biased surfaces with collisionless plasma sheaths
Authors: Raja Laxminarayan L. , Linne Mark
Published: December 2002

The article presents an analytical model for evaluation of ion angular distribution functions (IADFs) at a radio frequency (rf)-biased surface in a high-density plasma reactor. The model couples a unified rf sheath model to an assumed ion velocity distribution function-based formulation for determining the IADF under any general rf-bias condition. Under direct-current (dc) bias conditions the IADF profile shape shows a strong dependence on the bias voltage and the ion temperature is relatively independent of the plasma electron temperature, ion density, and the ion mass. The model establishes the importance of rf-bias frequency in determining the IADF. For conditions where the sheath current wave form is sinusoidal, low bias frequencies result in a large-angle tail contribution to the IADF which can potentially lead to poor anisotropic plasma etching behavior. The large-angle tail is absent at higher bias frequencies. An increase in bias power leads to a general narrowing of the IADF, but the large-angle tail for the IADF at low frequencies persists despite increasing bias powers. Therefore, plasma etch anisotropy can be improved by increasing bias powers only if the bias frequency is sufficiently high. Tangential ion drift velocities introduce azimuthal angle dependence on the IADF and a shift in the peak IADF to off-normal polar angles. While the location of the peak IADF in the azimuthal direction is dictated purely by the direction of the drift velocity, the shift in peak IADF in the polar angle depends on both the drift velocity as well as the bias frequency.
Biaxial strain-modified valence and conduction band offsets of zinc-blende GaN, GaP, GaAs, InN, InP, and InAs, and optical bowing of strained epitaxial InGaN alloys
Authors: Kent P. R. C. , Hart Gus L. W. , Zunger Alex
Published: December 2002

Using density-functional calculations, we obtain the (001) biaxial strain dependence of the valence and conduction band energies of GaN, GaP, GaAs, InN, InP, and InAs. The results are fit to a convenient-to-use polynomial and the fits provided in tabular form. Using the calculated biaxial deformation potentials in large supercell empirical pseudopotential calculations, we demonstrate that epitaxial strain reduces the InGaN alloy bowing coefficient compared to relaxed bulk alloys.
Fabrication and investigation of ultrathin, and smooth Pb(Zr,Ti)O3 films for miniaturization of microelectronic devices
Authors: Hong Jongin , Song Han Wook , Hong Seungbum , Shin Hyunjung , No Kwangsoo
Published: December 2002

Pb ( Zr 0.52 Ti 0.48 ) O 3 (PZT) thin films were fabricated on Pt(111)/TiOx/SiO2/Si substrates at 375 °C by radio frequency magnetron sputtering. A mixture of (110) and (100) orientations was found in all the PZT thin films. However, the in-plane grain size increased with an increase in film thickness, all films had smooth surfaces, and the root mean square roughness of the PZT films was in the range of 1–1.5 nm. As the film thickness increased, a decrease in residual stress and volume density of the PZT films was observed. PZT films become poorly crystallized with a decrease in film thickness. The magnitude of the maximum displacement from atomic force microscopy in local piezoresponse hysteresis mode increased from 187.25±9.363 in 40 nm to 418.5±20.925 in 152 nm. We suggest that the degradation in piezoelectric properties with a decrease in film thickness resulted from degradation of the crystallinity observed using transmission electron microscopy analysis, size effects derived from the grain size, and the residual stress evaluated using a laser reflectance method.
Correlation between infrared phonon modes and dielectric relaxation in Bi2O3–ZnO–Nb2O5 cubic pyrochlore
Authors: Nino Juan C. , Lanagan Michael T. , Randall Clive A. , Kamba Stanislav
Published: December 2002

Analysis of the dielectric relaxation phenomena in Bi2O3–ZnO–Nb2O5 cubic pyrochlore utilizing the Arrhenius equation is revisited to include recent high-frequency measurements. The fitting parameters obtained are further analyzed by comparison with theoretical and experimental activation energies of better understood relaxation systems such as dipolar glasses and relaxor ferroelectrics. The attempt jump frequency obtained from the Arrhenius fit (ν0∼4.385 THz) is found in correspondence with the resonant frequencies of the polar phonon modes previously identified by infrared spectroscopy. This correlation, and in particular the role of the O–A–O and O′–A–O′ bending phonon modes in driving the relaxation, is discussed.
Occupancy of the 3d electron shell of Co and Cr in nanosized CoCrPt magnetic thin films
Authors: Ouyang Hao , Kwan Jen-Tai
Published: December 2002

The effect of adding Pt element in Co–Cr-based alloys was studied in terms of the white-line intensities of the Co and Cr at the onsets of the L2 and L3 absorption edges. These intensities are due to the excitation of 2p1/2 or 2p3/2 core electrons to unoccupied d-like states near the Fermi level. The normalized white-line intensities reflect the filling of the d states. Enhancement and redistribution of the L2,3 white lines in the CoCrPt alloys as compared to pure elements can be correlated with changes in the 3d shell occupancy of Co and Cr atoms upon alloying. Cr and Co were found giving electrons during alloying according to the tendency of electronegativity. The redistribution of the 3d shell in Co was mainly due to the addition of Pt element. The study of d-band variation in the transition metals is helpful to understand the change due to alloying.
Structural evolution and property changes in Nd60Al10Fe20Co10 bulk metallic glass during crystallization
Authors: Zhang Zhi , Xia Lei , Wang Ru Ju , Wei Bing Chen , Pan Ming Xiang , Wang Wei Hua
Published: December 2002

The structural evolution and property changes in Nd60Al10Fe20Co10 bulk metallic glass (BMG) upon crystallization are investigated by the ultrasonic method, x-ray diffraction, density measurement, and differential scanning calorimetry. The elastic constants and Debye temperature of the BMG are obtained as a function of annealing temperature. Anomalous changes in ultrasonic velocities, elastic constants, and density are observed between 600–750 K, corresponding to the formation of metastable phases as an intermediate product in the crystallization process. The changes in acoustic velocities, elastic constants, density, and Debye temperature of the BMG relative to its fully crystallized state are much smaller, compared with those of other known BMGs, the differences being attributed to the microstructural feature of the BMG.
Measurement of three-dimensional force fields with atomic resolution using dynamic force spectroscopy
Authors: Hölscher H. , Langkat S. M. , Schwarz A. , Wiesendanger R.
Published: December 2002

Using dynamic force microscopy and spectroscopy in an ultrahigh vacuum (“noncontact atomic force microscopy”) at low temperatures, we measured three-dimensional force fields with atomic resolution. The method is based on the systematic recording of the frequency shift of a cantilever oscillating near the sample surface. The presented experimental results were obtained on a NiO(001) sample surface with an iron-coated silicon tip, but the measurement principle can be extended to any tip–sample system.