bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of cross relaxation between Tm3+ ions in PbO−Bi2O3−Ga2O3−GeO2 glass
Authors: Han Yong Seop , Song Jay Hyok , Heo Jong
Published: August 2003

Mechanisms of cross relaxation (3H4, 3H6→3F4, 3F4) in Tm3+ -doped PbO−Bi2O3−Ga2O3−GeO2 glass were analyzed. The energy gap between the absorption (3H6→3F4) and emission (3H4→3F4) spectra was approximately 600 cm−1. A large decrease in cross relaxation rates at low temperatures indicated that cross relaxation is controlled by a phonon-assisted energy transfer. The bending vibration of the Ga–O–Ga bonds between GaO4 tetrahedra (∼550 cm−1) was directly responsible for this energy transfer. Although GeO2 was added to enhance the thermal stability of PbO−Bi2O3−Ga2O3 glass, the phonon mode associated with vibration of GeO2 (∼770 cm−1) was not involved in the cross relaxation process.
Dynamic evolution of the heat fluxes in a collisionless magnetized plasma
Authors: Ramos J. J.
Published: August 2003

The dynamic evolution equations for the parallel heat fluxes in a collisionless magnetized plasma are consistently derived and shown to reduce to very compact forms. This derivation is based on the fast gyromotion ordering applicable under strong magnetization conditions. For consistency in the analysis, the contribution from the gyroviscous components of the stress tensor must be included. A heuristic closure condition that establishes a new collisionless magnetofluid model is proposed.
High-density hydrogen-plasma production in a simple torus using helicon waves
Authors: Sakawa Y. , Ohshima M. , Ohta Y. , Shoji T.
Published: August 2003

High-density hydrogen-plasma production in a simple torus using the m=+1 azimuthal mode of helicon waves is investigated. The measured dispersion relation of helicon waves in the high-density region shows deviation from that of the m=+1 mode derived using a uniform-plasma dispersion relation. The dispersion relation is calculated for the parabolic, modified-parabolic density profiles, and for a uniform plasma with a vacuum region.
Determining the material structure of microcrystalline silicon from Raman spectra
Authors: Smit C. , van Swaaij R. A. C. M. M. , Donker H. , Petit A. M. H. N. , Kessels W. M. M. , van de Sanden M. C. M.
Published: August 2003

An easy and reliable method to extract the crystalline fractions in microcrystalline films is proposed. The method is shown to overcome, in a natural way, the inconsistencies that arise from the regular peak fitting routines. We subtract a scaled Raman spectrum that was obtained from an amorphous silicon film from the Raman spectrum of the microcrystalline silicon film. This subtraction leaves us with the Raman spectrum of the crystalline part of the microcrystalline film and the crystalline fraction can be determined. We apply this method to a series of samples covering the transition regime from amorphous to microcrystalline silicon. The crystalline fractions show good agreement with x-ray diffraction (XRD) results, in contrast to crystalline fractions obtained by the fitting of Gaussian line profiles applied to the same Raman spectra. The spectral line shape of the crystalline contribution to the Raman spectrum shows a clear asymmetry, an observation in agreement with model calculations reported previously. The varying width of this asymmetrical peak is shown to correlate with the mean crystallite size as determined from XRD spectra.
Post-transit photocurrent spectroscopy studies on the gap state distribution in hydrogenated amorphous silicon: Trap occupation problems in leaky junctions and high-defect-density samples
Authors: Sakata I. , Yamanaka M.
Published: August 2003

Post-transit photocurrent spectroscopy (PTPS) has been employed as an effective method to obtain the energy distribution of gap states in undoped hydrogenated amorphous silicon. In this article, we demonstrate that the simplifying assumptions in conventional PTPS related to the trap occupation are not always valid and special care should be taken when we extend PTPS to samples with larger leakage currents and/or high-defect densities. We show test diode structures and experimental conditions to exactly determine the energy distribution of both electron and hole trap states in these cases. The attempt-to-escape frequency, ν, both for electrons and holes has been evaluated from the temperature dependence of the characteristic points in PTPS signal. It has been found that the density of electron and hole trap states in the deeper energy range increases with long exposure to visible light while the density in the shallower energy range decreases, which suggests the occurrence of defect conversion.
Dense medium plasma synthesis of carbon/iron-based magnetic nanoparticle system
Authors: Denes F. S. , Manolache S. , Ma Y. C. , Shamamian V. , Ravel B. , Prokes S.
Published: August 2003

Using dense medium plasma technology, hybrid iron and iron oxide/carbon-based nanoparticle composites were synthesized under room temperature and atmospheric pressure conditions. Based on results from electron spectroscopy for chemical analysis, Fourier transform infrared spectroscopy, Raman microscopy, atomic force microscopy and scanning electron microscopy, we conclude that the material is composed of spherical particles, 40–60 nm in diameter, which are a graphitic carbon host structure embedded with iron and iron oxide. Thermal gravimetry/differential thermal gravimetry analysis indicates that these composites are stable up to temperatures as high as 600 °C. Ferromagnetic resonance spectroscopy (FMR) and extended x-ray absorption fine-structure spectroscopy suggest that the bulk of the FMR signal in question is due to metallic Fe. Magnetite or maghemite is present, and the metallic content of the metal particles is 58(8)% and the remainder is oxidized with a sixfold oxygen coordination shell similar to that of γ-Fe2O3.
Preparation and characterization of atomically clean, stoichiometric surfaces of n- and p-type GaN(0001)
Authors: Tracy K. M. , Mecouch W. J. , Davis R. F. , Nemanich R. J.
Published: August 2003

It is demonstrated that in situ exposure of the (0001) surface of n- and p-type GaN thin films to flowing ammonia at 860 °C and 10−4 Torr removes hydrocarbon and oxygen/hydroxide species below the detectable limits of x-ray and ultraviolet photoelectron spectroscopies (UPS) and decreases the Ga/N ratio from 1.3 to 1.0. Additional indications of the efficacy of this ammonia-based chemical vapor cleaning (CVC) process were the shifts in the Ga 3d and the N 1s core level positions from the as-loaded to the CVC surfaces of the n-type samples from 21.0±0.1 to 20.6±0.1 eV and from 398.3±0.1 to 398.0±0.1 eV, respectively, and the change in the UPS measured low energy spectrum turn on from 3.9±0.1 (as-loaded samples) to 3.0±0.1 eV (cleaned samples) below the Fermi level. Analogous changes in the p-type samples were from 19.6±0.1 to 18.9±0.1 eV and from 397.1±0.1 to 396.3±0.1 eV for the Ga 3d and the N 1s core levels, and from 3.0±0.1 to 1.1±0.1 eV for the UPS valence band maximum (VBM). The VBM values of the CVC samples indicate band bending of ∼0.3 eV upward on n type and ∼0.8 eV downward on p type. Electron affinities of 2.8±0.1 and 2.6±0.1 eV were determined for the clean n-type and p-type surfaces, respectively. Irrespective of doping, the CVC process left unchanged the (1×1) low energy diffraction pattern, the terraced microstructure, and the root mean square roughness observed for the surfaces of the as-loaded samples, i.e., the surface microstructure was not damaged during the high temperature exposure to ammonia at low pressure.
Auroral electron acceleration by dissipative solitary kinetic Alfvén waves
Authors: Wu D. J. , Chao J. K.
Published: August 2003

The acceleration of auroral electrons has long been an open problem. In a recent work [Wu, Phys. Plasmas 10, 1364 (2003)], the model of a dissipative solitary kinetic Alfvén wave (DSKAW) has been proposed to describe the physics of strong electric spikes observed often in the auroral ionosphere and magnetosphere. In this Brief Communication, the DSKAW model is applied to the auroral acceleration region and the possibility of DSKAWs producing auroral electrons is further discussed. The result shows that DSKAWs in the acceleration region can accelerate electrons enough to reach auroral energies and that the most efficient acceleration occurs just at the bottom of the acceleration region where the Alfvén speed also peaks. Moreover, the acceleration mechanism by DSKAWs can plausibly explain the cut-off above 10 keV, which is inferred from the energy distribution of auroral precipitating electrons.
Structure and piezoelectric properties of sol–gel-derived (001)-oriented Pb[Yb1/2Nb1/2]O3–PbTiO3 thin films
Authors: Zhou Q. F. , Zhang Q. Q. , Trolier-McKinstry S.
Published: August 2003

(1−x) Pb[Yb 1/2 Nb 1/2 ] O 3 –x PbTiO 3 (PYbN–PT, x=0.5) (001)-oriented thin films were deposited onto (111)Pt/Ti/SiO2/Si substrates by sol–gel processing. The crystallographic texture of the films was affected by heating rate, annealing temperature, and film thickness. The dielectric permittivity and loss factor varied only slightly with frequency in the range of 100 Hz–100 kHz. At 1 kHz, the dielectric permittivity of (001)-oriented films was 1030 and the dielectric loss was 0.03. Compared with (111)-oriented PYbN–PT films, (001)-oriented PYbN–PT films exhibited better piezoelectric properties. The effective transverse piezoelectric e31,f coefficient of the films were −8.2 C/m2 when films were poled at room temperature. Enhanced piezoelectric properties were observed by poling the films at higher temperatures. The transition temperature of the (001)-oriented PYbN–PT (50/50) films was around 360 °C.
Energy dispersive x-ray reflectivity technique to study thermal properties of polymer films
Authors: Bhattacharya M. , Mukherjee M. , Sanyal M. K. , Geue Th. , Grenzer J. , Pietsch U.
Published: August 2003

A method to study temperature dependent thickness variation of thin films has been developed based on angle and energy dispersive x-ray reflectivity (EDR) techniques. The data analysis scheme developed for this method combines both energy and angle dispersive x-ray reflectivity (ADR) profiles. The ADR technique provides the electron density profile on an absolute scale and the EDR technique facilitates rapid collection of reflectivity data as a function of temperature. We demonstrate the utility of the developed method with results of a study of thermal expansion of single and bilayer polymer films. We could detect a change of the thickness of the film in angstroms and could demonstrate interplay of negative and positive thermal expansions of dissimilar polymer layers in determining density profiles of polymer–polymer interface as a function of temperature.
Electromechanical coupling behavior of ferroelectric ceramics under multiaxial electric switching
Authors: Li Jackie
Published: August 2003

When a fully poled ferroelectric polycrystal is subjected to an external electric field at an angle θ to the original poling direction, its electromechanical response will also depend on such an angle. In order to evaluate such an angle-dependent nonlinear response, an orientation-dependent dual-phase equivalent system is introduced, and then a micromechanics theory based on irreversible thermodynamics and physics of domain switch is developed. The developed theory results in a kinetic equation that connects the new domain concentration at a given level of electric field and loading angle through an orientation-dependent Eshelby-type of S-tensor in a transversely isotropic piezoelectric matrix. The calculated results show that, as the angle between the initial poling direction and the newly applied field direction increases, the generated electric displacement at the same level of the electric field also increases. While all the responses—except for θ=0°–are all nonlinear, the calculated nonlinearity is most pronounced with the reversed switch θ=180°. These theoretical results are found to be in good qualitative and quantitative agreement with the experimental data of PZT-5H provided by Huber and Fleck [J. E. Huber and N. A. Fleck, J. Mech. Phys. Solids 49, 785 (2001)].
Modulational instability and localized excitations of dust-ion acoustic waves
Authors: Kourakis I. , Shukla P. K.
Published: August 2003

Theoretical and numerical studies are carried out of the nonlinear amplitude modulation of dust-ion acoustic waves propagating in an unmagnetized weakly coupled plasma comprised of electrons, positive ions, and charged dust grains, considering perturbations oblique to the carrier wave propagation direction. The stability analysis, based on a nonlinear Schrödinger-type equation, exhibits a wide instability region, which depends on both the angle θ between the modulation and propagation directions and the dust number density nd. Explicit expressions for the instability increment and threshold are obtained. The possibility and conditions for the existence of different types of localized excitations are also discussed.
Influence of laser bandwidth and transit time on the excitation probability of a rovibrational two-level system
Authors: Marienfeld S. , Pashayan Y. , Vitanov N. V. , Shore B. W. , Hotop H. , Bergmann K.
Published: August 2003

Practical considerations arising from Doppler distributions and beam divergence typically limit the amount of internal excitation that can be produced as a molecule passes across a cw laser beam. For coherent excitation this population transfer is furthermore limited by coherent population return (CPR), in which transiently excited molecules, off resonance, adiabatically evolve so as to return to the initial state at the end of a smooth pulse. We illustrate this effect with simulations, and show that in principle the presence of incoherence in the laser radiation (i.e., a finite bandwidth) can help suppress CPR, thereby substantially improving the averaged population transfer. For cases of practical relevance, the velocity-averaged population transfer can be optimized by a proper choice of the laser beam waist along the molecular beam direction.
Photodissociation of allyl-d2 iodide excited at 193 nm: Stability of highly rotationally excited H2CDCH2 radicals to C–D fission
Authors: Szpunar D. E. , Liu Y. , McCullagh M. J. , Butler L. J. , Shu J.
Published: August 2003

The photodissociation of allyl-d2 iodide (H2C=CDCH2I) and the dynamics of the nascent allyl-d2 radical (H2CCDCH2) were studied using photofragment translational spectroscopy. A previous study found the allyl radical stable at internal energies up to 15 kcal/mol higher than the 60 kcal/mol barrier to allene+H formation as the result of a centrifugal barrier. The deuterated allyl radical should then also show a stability to secondary dissociation at internal energies well above the barrier due to centrifugal effects. A comparison in this paper shows the allyl-d2 radical is stable to allene+D formation at energies of 2–3 kcal/mol higher than that of the nondeuterated allyl radical following photolysis of allyl iodide at 193 nm. This is most likely a result of a combination of the slight raising of the barrier from the difference in zero-point levels and a reduction of the impact parameter of the dissociative fragments due to the decrease in frequency of the C–D bending modes, and therefore allene+D product orbital angular momentum, |L⃗|=μ|v⃗rel|b. The integrated signal taken at m/e=40 (allene) and m/e=41 (allene-d1 and propyne-d3) shows a minor fraction of the allyl-d2 radicals isomerize to the 2-propenyl radical, in qualitative support of earlier conclusions of the domination of direct allene+H formation over isomerization.
On the construction of self-polar and self-polar Hilbertian norms on Minkowski space of dimension 2
Authors: Hofmann Gerald
Published: August 2003

Considering the real Minkowski space M2 endowed with the indefinite inner product (a⃗,b⃗)=xaxb−yayb, a⃗=(xa,ya),  b⃗=(xb,yb)∈M2, we give a general construction for the boundary ∂Up={a⃗∈M2; p(a⃗)=1} of the unit ball Up of some self-polar norm p on M2. We show further that every self-polar norm p on M2 is obtained by our construction. A detailed investigation of the possible choices of the ingredients of our construction (two points Pj, j=1,2, and a convex arc connecting P1 and P2) yields a complete classification of all self-polar norms on M2. Our results are finally applied to the special case of Hilbertian self-polar norms.
Generalized mean spherical approximation for a model of water with dipole, quadrupole, and short-range potential of tetrahedral symmetry
Authors: Carlevaro Carlos Manuel , Blum Lesser , Vericat Fernando
Published: August 2003

A simplified version of the so-called Yukagua model of water, proposed and studied via Monte Carlo simulations elsewhere by two of the authors [Physica A 265, 396 (1999)], is considered. The model includes dipolar, quadrupolar, and octupolar short-range interactions of tetrahedral symmetry that mimic hydrogen bonds, but it differs from the original Yukagua model in that dipolar and quadrupolar terms are not coupled. For this model we solve, using Baxter–Wertheim factorization technique, the Ornstein–Zernike relation for the molecular pair correlation functions closed by generalized mean spherical closures. From the molecular correlations we derive the atom–atom (oxygen–oxygen, oxygen–hydrogen, and hydrogen–hydrogen) pair correlations. We compare our results with those obtained from diffraction experiments and also from other theoretical treatments available in the recent literature including a modified version of the hypernetted chain integral equation for the SPC/E water model and also ab initio simulations of Car–Parrinello type.
Diffusion in inhomogeneous and anisotropic media
Authors: Christensen Michael , Pedersen J. Boiden
Published: August 2003

Despite common practice, inhomogeneous and/or anisotropic diffusion cannot be considered without regarding the microscopic details breaking the translational and/or angular symmetry. The macroscopic diffusion equation and the stationary solution are determined by the microscopic model and depend in general on all the microscopic parameters and not simply on the combination in the diffusion tensor. The traditional diffusion equation is only valid under special conditions and it cannot, in general, be used for anisotropic diffusion. An alternative form of the diffusion equation has a wider range of applicability. It is shown that for isotropic diffusion all variants of the diffusion equation are mathematically (but not physically) equivalent and can be transformed into each other by introduction of effective potentials. This is not the case for anisotropic diffusion where the traditional diffusion equation in most cases will give incorrect results. Two examples illustrate the differences between the two dynamic equations with respect to stationary solutions and detailed balance.
Infrared Q-branch absorption and rotationally-hindered species in liquids
Authors: Medina A. , Roco J. M. M. , Calvo Hernández A. , Velasco S.
Published: August 2003

We show the interesting sensitivity of infrared Q-branch absorption of HCl in liquid Ar to anisotropic solute–solvent interaction potentials. Comparing the differences among potentials with the different simulated absorption profiles they yield to, it is feasible to extract well-founded conclusions on the main mechanisms contributing to absorption on the Q-branch region. Moreover, it is shown that the well-known Ar–HCl stable quasilinear configuration at low densities is to some extent preserved in the liquid phase. The sizes and orientations of these traces of complexes in the liquid resemble those of van der Waals complexes in low-density gases. Finally, we analyze the meaning of the observed rotational hindering of the molecular probe and its influence on the shape of near-infrared spectra.
Electromagnetic contribution to surface enhanced Raman scattering revisited
Authors: Etchegoin P. , Cohen L. F. , Hartigan H. , Brown R. J. C. , Milton M. J. T. , Gallop J. C.
Published: August 2003

Several aspects of the electromagnetic contribution to surface enhanced Raman scattering (SERS) are revisited and analyzed in terms of known concepts and ideas in the field of photonic crystals. A qualitative understanding of some reported SERS phenomena can be achieved by this analysis of the complex energy distribution and spatial localization of surface plasmon resonances in SERS active media. A few of the outstanding issues in the electromagnetic contribution to SERS (like the overwhelming success of nonresonant excitation) can be explained within this framework and, in addition, new physical phenomena utilizing the stimulation or inhibition of the Stokes/anti-Stokes fields are proposed.
Photochemistry of phenol–(NH3)n clusters: Solvent effect on a radical cleavage of an OH bond in an electronically excited state and intracluster reactions in the product NH4(NH3)n−1 (n⩽5)
Authors: Daigoku Kota , Ishiuchi Shun-ichi , Sakai Makoto , Fujii Masaaki , Hashimoto Kenro
Published: August 2003

The potential energy surfaces of PhOH–(NH3)0,1 and NH4(NH3)1–4 have been investigated theoretically by ab initio methods. Intermolecular stretching in PhOH–NH3 assists in the radical cleavage of an OH bond occurring through a ππ*/πσ* potential crossing. Thus, excited state hydrogen transfer (ESHT) is expected to take place by a solvent-assisted mechanism even in the larger PhOH–(NH3)n. Because sufficient energy is obtained by ESHT from PhOH–(NH3)n (ππ*) to PhO–NH4(NH3)n−1 (πσ*) (n⩽5), hydrogen relocation and/or ammonia migration in the product NH4(NH3)n−1 can readily follow ESHT, which is responsible for observing isomer bands in the absorption spectra of the photoinduced reaction products of PhOH–(NH3)n.
Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of vinyl bromide: Franck–Condon analysis and vibrational assignment
Authors: Lee Mina , Kim Myung Soo
Published: August 2003

Vibrational spectrum of vinyl bromide cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From MATI spectrum, ionization energy to the ground state of the cation was determined to be 9.8171±0.0006 eV (79 180±5 cm−1). Almost complete vibrational assignments for the peaks in the MATI spectrum were possible by utilizing vibrational frequencies and Franck–Condon factors calculated at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-311++G(df,pd) level. Franck–Condon analysis for one-photon MATI spectra is especially useful because calculations of only the ground electronic states are involved while that for two-photon MATI spectra requires excited state calculations.
Onset of collisionless magnetic reconnection in thin current sheets: Three-dimensional particle simulations
Authors: Scholer M. , Sidorenko I. , Jaroschek C. H. , Treumann R. A. , Zeiler A.
Published: August 2003

Three-dimensional (3D) particle-in-cell simulations of collisionless magnetic reconnection are presented. The initial equilibrium is a double Harris-sheet equilibrium and periodic boundary conditions are assumed in all three directions. No magnetic seed island is imposed initially, and no flow conditions are imposed. The current sheet width is assumed to be one ion inertial length calculated with the density in the center of the current sheet. The ion to electron mass ratio is mi/me=150, which suppresses the growth of the drift kink instability. Two different runs have been performed: a simulation with exactly antiparallel magnetic field and a simulation with a constant guide field of the same magnitude as the antiparallel field superimposed. In the antiparallel case the inductive field of the waves excited by the lower hybrid drift instability (LHDI) leads to rapid acceleration of the electrons in the center of the current sheet and subsequently to a current sheet thinning. The current increase in the center is balanced by reverse currents in the gradient region. In the thin current sheet rapid reconnection sets in which self-organizes into a two-dimensional structure with a single X line. However, ∼15% of the total flux is reconnected while reconnection is still patchy and 3D. In the guide field case the growth rate of the LHDI is reduced, but leads nevertheless after a considerably longer time to electron acceleration in the current sheet center and to a thinning of the current layer, followed by single X line reconnection. It is suggested that electron acceleration due to LHDI in current sheets of the order of the ion scale results in rapid onset of reconnection.
Theory of Alfvén eigenmodes in shear reversed plasmas
Authors: Breizman B. N. , Berk H. L. , Pekker M. S. , Pinches S. D. , Sharapov S. E.
Published: August 2003

Plasma configurations with shear reversal are prone to the excitation of unusual Alfvén eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping (Alfvén cascades) as the safety factor q changes in time. This work presents a theory that employs two complementary mechanisms for establishing Alfvén cascades: (1) a nonstandard adiabatic response of energetic particles with large orbits and (2) toroidal magnetohydrodynamic effects that are second-order in inverse aspect ratio. The developed theory explains the transition from Alfvén cascades to the toroidicity induced Alfvén eigenmodes (TAEs), including modifications of the TAEs themselves near the shear reversal point.
Nitrobenzene rotational energy barrier: A survey of several ab initio methods
Authors: Sancho-Garcı́a J. C. , Pérez-Jiménez A. J.
Published: August 2003

A detailed study on the torsional potential of nitrobenzene is performed by using state-of-the-art ab initio methods, including density functional theory (DFT) ones. Special emphasis is given to basis set incompleteness in order to get complete agreement with experimental results. On the other hand, the DFT derived the energy difference between planar and perpendicular conformers is mostly dominated by the proportion of the HF exchange contribution to the exchange-correlation functional. A systematic application of hybrid schemes, energy decomposition analysis, and scan of the conjugative, steric, and weak interactions effects along the torsion will be used to rationalize the torsional profiles.
Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation
Authors: Köhn Andreas , Hättig Christof
Published: August 2003

The derivation and implementation of excited state gradients is reported for the approximate coupled-cluster singles and doubles model CC2 employing the resolution-of-the-identity approximation for electron repulsion integrals. The implementation is profiled for a set of examples with up to 1348 basis functions and exhibits no I/O bottlenecks. A test set of sample molecules is used to assess the performance of the CC2 model for adiabatic excitation energies, excited state structure constants and vibrational frequencies. We find very promising results, especially for adiabatic excitation energies, though the need of a single-reference ground state and a single-replacement dominated excited state puts some limits on the applicability of the method. Its reliability, however, can always be tested on grounds of diagnostic measures. As an example application, we present calculations on the π*←π excited state of trans-azobenzene.
Topography of molecular scalar fields. I. Algorithm and Poincaré–Hopf relation
Authors: Balanarayan P. , Gadre Shridhar R.
Published: August 2003

A new algorithm for locating the critical points (CP’s) of a three-dimensional molecular scalar field is discussed. The algorithm is based on a ray search from the surface extrema of appropriately defined atom-centered spheres. The algorithm is tested for molecular electrostatic potentials and electron densities of a few test molecules such as tetrahedrane, cubane, anthracene, diborane, etc. Furthermore, the Poincaré–Hopf relationship is examined for the set of CP’s thus obtained. A topological interpretation of the Euler characteristic of a given isosurface is employed for a stronger regional check on the number of CP’s enclosed in the isosurface.
Analytical energy gradients for internally contracted second-order multireference perturbation theory
Authors: Celani Paolo , Werner Hans-Joachim
Published: August 2003

The theory for computing analytical energy gradients for second-order multireference perturbation theory (MRPT2) with arbitrary multiconfiguration self-consistent field (MCSCF) reference functions is derived and implemented. MRPT2 with complete active space reference functions (CASPT2) is a special case. In our method the configurations with two electrons in the external orbital space are internally contracted. This ansatz strongly reduces the length of the configuration expansion as compared to uncontracted wave functions, but avoids bottlenecks occurring when fully contracted first-order wave functions are used. The method, which also allows the use of state-averaged MCSCF reference functions, is applied to optimize the structures of low-lying valence and Rydberg states of Pyrrole. Nonplanar equilibrium structures and large geometry relaxation effects on the excitation energies are found for the valence states. From the results it can be concluded that CASPT2 underestimates the excitation energies of the valence states, but leads to accurate results for the Rydberg states.
Plasma–surface interaction at sharp edges and corners during ion-assisted physical vapor deposition. Part I: Edge-related effects and their influence on coating morphology and composition
Authors: Macak E. B. , Münz W.-D. , Rodenburg J. M.
Published: August 2003

Ion-assisted physical vapor deposition (PVD) is a common industrial method for growing thin coatings of various interstitial nitride alloys. The interaction between the ions and three-dimensional nonflat samples during the deposition can, however, lead to unwanted local changes in the properties of the coating and thus its performance. We analyze the characteristics of the ion bombardment during ion-assisted PVD on sharp convex substrates and their effect on the growing coating. We show that the magnitude and the spatial extent of the edge-related changes are directly related to the characteristics of the plasma sheath around the biased edges. We examine the influence of the edge geometry and the deposition conditions. The edge-related effects are studied on the example of wedge-shaped samples coated with TiAlN/VN by closed-field unbalanced magnetron deposition process using high-flux low-energy Ar+-ion irradiation (Ji/Jme∼4, Ei=75−150 eV). The samples are analyzed by scanning electron microscopy and energy-dispersive x-ray spectroscopy. Significant changes in the morphology, thickness, and composition of the coatings are found in the edge region. In order to account for the changes, we apply a self-consistent model of the plasma sheath around wedge-shaped samples proposed by Watterson [J. Phys. D 22, 1300 (1989)], to our conditions. For a 30° wedge coated at −150 V, the resputtering rate in the edge region is found to be increased by up to ten times as compared to flat substrate areas. The effect is due to the combined action of an increased ion flux and increased sputtering yield as a result of the nonperpendicular angle of incidence of ions in the edge region. The situation at sharp corners, where even more severe effects are observed, is analyzed and modeled in the companion article E. B. Macak et al., J. Appl. Phys. (2003) (Part II).
Characterization of tellurium layers for back contact formation on close to technology treated CdTe surfaces
Authors: Kraft D. , Thissen A. , Broetz J. , Flege S. , Campo M. , Klein A. , Jaegermann W.
Published: August 2003

We have studied the contact formation on CdTe surfaces following the technologically applied procedure. The electronic properties of wet chemically etched CdTe surfaces has been investigated with photoelectron spectroscopy. For the characterization of the morphology, structure, and elemental distribution in the etched layer atomic force microscopy, scanning electron microscopy, grazing incidence x-ray diffraction, and secondary ion mass spectroscopy have been used. Etching of the samples has been performed in air and in an electrochemistry chamber directly attached to the UHV system. In both cases the formation of an elemental polycrystalline Te layer with a thickness of about 80 Å is detected. For comparison, a thin Te layer has been deposited by physical vapor deposition onto a CdTe substrate. We determine a valence-band offset of ΔEVB=0.5±0.1 eV, independent of the preparation of the interface.
Finite element simulation of the film spallation process induced by the pulsed laser peening
Authors: Zhou M. , Zeng D. Y. , Kan J. P. , Zhang Y. K. , Cai L. , Shen Z. H. , Zhang X. R. , Zhang S. Y.
Published: August 2003

The laser spallation technique for measuring the interface strength between a coating and a substrate is similar to laser shock peening, in which the stress wave induced by laser shock cause debond on the interface between a hard coating with micron thickness and a metal substrate. According to the modified experiment setup of the laser spallation technique, finite element analysis simulated the process of the film spallation by taking the laser loading as a direct input. We presented a numerical model of finite element that the laser spallation process includes two related, but uncoupled procedures. One was transient heat transfer in a two-layer medium. The other was the related transient elastic wave propagation in the same two-layer media, which was the result of the thermal misfit by transient heating. Based on the threshold of film spallation, we analyzed the process of laser shocking to study the propagation of stress wave and evaluate the spall resistance of sputtered films. The analysis result showed the dynamic adhesive strength of the interface between the TiN coating and the 304 stainless steel substrate was 193.0 MPa.
Monte Carlo simulations of copolymer adsorption at planar chemically patterned surfaces: Effect of surface domain sizes
Authors: Semler James J. , Genzer Jan
Published: August 2003

We present results of Monte Carlo simulation studies utilizing the bond fluctuation model in conjunction with single and configurational biased Monte Carlo moves to investigate the adsorption of diblock (A–b–B) and alternating (A–alt–B) copolymers at physically flat surfaces made of an equal number of two chemically different sites, C and D. The adsorption of the copolymer to the surface is driven by the repulsion between the A and B segments along the copolymer and the attraction between the B segments and the D sites on the surface. We address the critical role of the commensurability between the copolymer’s monomer sequence distribution and the size and spatial distribution of the surface adsorbing sites on the copolymer adsorption. We show that both copolymer architectures have the ability to recognize the surface motif and transcribe it into the bulk material. Diblock copolymers can transfer the pattern once the heterogeneous domain sizes match the size of the parallel component to the radius of gyration, which is constituted primarily of the adsorbing species. This behavior results from the ability of the diblock copolymer to adopt a brush type conformation. In contrast to the diblocks, copolymers with the alternating sequence distribution are more likely to “zip to” the surface since the adsorbing species are evenly distributed along the copolymer. This chain conformation creates an entropic penalty, which must be alleviated by the formation of loops and tails. These conformational changes endow the alternating copolymer with the ability to recognize patterns with periodicities much less than the parallel component to the radius of gyration, and to invert the pattern as the distance away from the surface is increased.
Techniques for the measurement of trace moisture in high-purity electronic specialty gases
Authors: Funke Hans H. , Grissom Brad L. , McGrew Clark E. , Raynor Mark W.
Published: August 2003

The control of water vapor (moisture) contamination in process gases is critical to the successful manufacture of semiconductor devices. As specified moisture levels have become more stringent, there is a growing demand for more sensitive analytical methods. Instrumental methods currently being used or in development for measuring trace moisture at ppbv levels include Fourier transform infrared spectroscopy, tunable diode laser absorption spectroscopy, cavity ringdown spectroscopy, intracavity laser spectroscopy, electron impact ionization mass spectrometry, and atmospheric pressure ionization mass spectrometry. In addition, sensor-based technologies such as oscillating quartz crystal microbalances, and chilled mirror-, capacitor-, and electrolytic-based hygrometers operate in this regime. These approaches are presented and reviewed. As the success of each trace moisture method is dependent on the degree to which the different process gases interfere with the measurement process, important process gas applications of the techniques are highlighted. Advantages and disadvantages as well as future developments and trends are also presented.
Highly insulating ultrathin SiO2 film grown by photooxidation
Authors: Fukano Atsuyuki , Oyanagi Hiroyuki
Published: August 2003

Highly insulating ultrathin SiO2 films (<5 nm) were grown by means of vacuum ultraviolet photooxidation, by use of 126, 172, and 222 nm wavelength photons generated by excimer lamps. The ultrathin silicon dioxide films were grown onto the Si(001) substrate, at low temperature (<500 °C). We found that the densities of photooxidized SiO2 films were 13%–35% higher than that of thermally oxidized SiO2 film. The density profile was obtained by x-ray reflectivity and showed a remarkable decrease in the SiOx (suboxide) layer thickness at the SiO2−Si interface, in comparison with thermally oxidized conventional oxides. The Si 2p photoelectron spectrum confirmed that the SiOx layer was negligibly thin. High insulating performance of the SiO2 film was demonstrated.
Simulation of a two-dimensional sheath over a flat wall with an insulator/conductor interface exposed to a high density plasma
Authors: Kim Doosik , Economou Demetre J.
Published: August 2003

The structure of the two-dimensional (2D) sheath over a flat, electrically inhomogeneous wall exposed to a high density plasma was investigated by a fluid model. The wall consisted of a floating semi-infinite insulator in contact with a semi-infinite conductor biased by a negative dc voltage. The difference in sheath potential over the two materials resulted in a 2D sheath over the insulator/conductor interface. The ion flux was higher on the conductor side of the interface at the expense of the flux on the insulator side. The spatial extend and magnitude of the ion flux disturbance scaled with the difference in the sheath thickness over the two different materials. The ion impact angle along the surface increased progressively as the material interface was approached. Sheath distortion was exacerbated when the electron temperature was decreased or the bias potential was made more negative.
Proton implantation effects on electrical and recombination properties of undoped ZnO
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Kozhukhova E. A. , Vdovin V. I. , Ip K. , Overberg M. E. , Heo Y. W. , Norton D. P. , Pearton S. J. , Zavada J. M. , Dravin V. A.
Published: August 2003

Electrical and optical properties of undoped n-ZnO crystals implanted with 50 keV protons with doses from 5×1013 to 5×1015 cm−2 are reported. Proton implantation leads to a decrease of the carrier concentration in the near-surface region, but at the end of the proton range shallow donors are observed whose concentration tracks the implant dose and that we attribute to hydrogen donors. Three deep electron traps with apparent activation energies of 0.55, 0.75, and 0.9 eV are introduced by proton implantation. The 0.9 eV traps have been observed through the increased thermal stability of the Schottky diodes prepared on heavily implanted n-ZnO compared to unimplanted or lightly implanted samples. In addition, hole traps located 0.16 eV above the valence band edge were introduced by implantation. Proton implantation also led to the formation of persistent-photocapacitance-active defects and to considerable decrease in the intensity of the band edge luminescence and in the value of the photocurrent of the Au/n-ZnO Schottky diodes. When compared with other wide-band gap materials, such as GaN, the recombination properties of n-ZnO are shown to be more resistant to deterioration upon proton implantation.
Asymptotic behavior of the centroids of a family of vortices
Authors: Emamizadeh B. , Mehrabi M. H.
Published: August 2003

In this article we consider a family of minimization problems whose solutions represent the vortices of two dimensional flows. We prove that if we consider an admissible sequence of vortices, then the corresponding centroids converge to the global minimizer of the Routh function associated with an appropriate energy functional.
The three-body problem with an inverse square law potential
Authors: Jiménez-Lara Lidia , Piña Eduardo
Published: August 2003

We study the motion of three masses in a plane interacting with a central potential proportional to 1/r2 using the coordinates introduced recently by Piña. We show that this problem with four degrees of freedom (three angles and a distance related to the inertia moment of the system in these coordinates) is partially separable, and can be reduced to a problem with two degrees of freedom (two angles) with a new constant of motion. We find a symmetry of reflection (an involution) for this system and we use the symmetry lines to find periodic orbits in the angular coordinates. These orbits will not be periodic in general on the whole phase space because the coordinate of distance type grows as t when t→∞ and it is unbounded. However, if the inertia moment of the system remains constant, they will be periodic on the whole phase space.
Characterization of sheet electron beams from planar crossed-field secondary emission diodes
Authors: Saveliev Yuri M. , Sibbett Wilson , Parkes David M.
Published: August 2003

Sheet electron beams of 9 cm width having linear current densities of 1.5–3.0 A/cm have been generated from a planar crossed-field secondary emission (CFSE) electron source operated at ∼20 kV diode voltage. The output electron beam consists of two parallel closely spaced sheets of electron flow originated from each side of the cathode. The full width at half maximum of each electron sheet is 1–2 mm depending on the magnetic field strength. The output current depends strongly on the diode voltage as I∝Un where n=5.0±0.5. At a given magnetic field strength B, there is a minimal diode voltage Umin at which the diode is able to maintain a self-sustained operation. The values of Umin for a range of magnetic fields B have been linked to the potential of the diode electron flow V0=0.42 kV which is approximately ten times greater than the first cross-over voltage in the secondary emission coefficient function. The planar CFSE electron source is capable of operating in a long ∼2 ms regime. The generation of low-voltage (<10 kV) electron beams at an elevated background gas pressure of 10−2 mbar has been also demonstrated.
Calibration of colloid probe cantilevers using the dynamic viscous response of a confined liquid
Authors: Notley Shannon M. , Biggs Simon , Craig Vincent S. J.
Published: August 2003

A method is described to determine the spring constant of colloid probe cantilevers used in force measurements with the atomic force microscope. An oscillatory drive applied to the substrate is coupled by viscous interactions to the colloid probe. The dynamic response of the probe, which is unaffected by static interactions, is then used to determine the spring constant of the cantilever. Thus an accurate calibration of the spring constant may be performed simultaneously with a normal colloidal probe force measurement in situ.
A novel spectrometer for spin-polarized electron energy-loss spectroscopy
Authors: Ibach H. , Bruchmann D. , Vollmer R. , Etzkorn M. , Anil Kumar P. S. , Kirschner J.
Published: August 2003

By making use of advanced methods for the calculation of electron trajectories in the presence of space charge fields we have designed and built a novel electron energy-loss spectrometer for the purpose of measuring spin flip excitations at surfaces. The measurement of the spin asymmetry requires a deflection angle between the photocathode and the sample position which amounts to 90° in total. That deflection angle is not provided by standard electron deflectors, or by a combination of those. We show that a 90° deflection angle can be realized by a combination of two novel deflectors with 90° and 180° deflection angles each. The performance of the new spectrometer matches the performance of the best electron spectrometers developed for surface vibration spectroscopy.
Infrared emission from zinc sulfide: Rare-earth doped thin films
Authors: Kale Ajay , Shepherd Nigel , Glass William , DeVito David , Davidson Mark , Holloway Paul H.
Published: August 2003

Infrared (IR) electroluminescent (EL) thin film phosphors were radio frequency magnetron sputter deposited by cosputtering of an undoped ZnS target together with ZnS: 1.5 mole % ErF3 or ZnS: 1.5 mole % NdF3 targets. The ZnS:ErF3 and ZnS:NdF3 thin film phosphors were annealed in a N2 ambient at temperatures ranging from 350 to 475 °C for 1 h to increase radiance. The maximum EL radiance observed was 28 μW/cm2 at 1550 nm for ZnS:ErF3, and 26 μW/cm2 at 910 nm and 15 μW/cm2 at 1060 nm for ZnS:NdF3 (at 40 V above the threshold voltage) after a 425 °C, 1 h anneal in nitrogen. For anneals above 425 °C visible emission increased, while near infrared (NIR) emission from both ZnS:ErF3 and ZnS:NdF3 was either constant or decreased. For ZnS:ErF3, the 1550 nm NIR peak decreased, but the 990 nm peak remained constant in intensity. The crystallinity of ZnS was improved by annealing, and these results are consistent with the postulate that residual defects limit the acceleration of “hot” electrons for anneals at ⩽425 °C. Under these conditions, hot electrons only have sufficient energy to excite Er+3 into the lower lying I413/2 excited state which leads to 1550 nm NIR emission. With increasing annealing temperatures, hot electrons can excite from the I415/2 ground state into higher energy excited states (e.g., the F47/2 state for 990 nm emission). The NIR emissions from ZnS:NdF3 at 910 and 1060 nm originate from the same excited state and both peaks exhibited maximum NIR intensities after annealing at 425 °C. While the emission spectra from Er were independent of annealing temperature, peak shifts were observed for Nd. These shifts were discussed in terms of the nephelauxetic effect and hybridization of the 5d–4f orbitals.