bluuebunny/crossref_metadata_2025 huggingface dataset:
Preferential solvation of Ca2+ in aqueous solutions containing ammonia: A molecular dynamics study
Authors: Floris F. M. , Martı́nez José M. , Tomasi J.
Published: July 2002

Ca 2+ aqueous solutions containing different proportions of ammonia have been studied by means of molecular dynamics simulations. Previously developed ab initio effective pair potentials, in the framework of the polarizable continuum model, and only tested at a cluster computation level, have been employed to describe ion–ligand interactions. Structural and dynamic changes present in the neighborhood of the ion as a function of the ammonia concentration have been followed. Results show a preferential solvation for ammonia, even at very low concentrations. For the pure aqueous solution, calcium ion is coordinated by eight water molecules, while the presence of ammonia favors an equilibrium between an octa and enna-coordinated situation when this ligand becomes predominant, confirming the prediction of cluster calculations. However, the increase in the coordination number is followed by an intrinsic loss of stability for the identifiable solvated structures because of the larger tendency of ammonia to participate in solvent exchange phenomena. Solvent exchange events show, for the most simple case (water–water exchange), a marked mechanistic variety.
Spectroscopic properties of Co2+ in related spinels
Authors: Sardar Dhiraj K. , Gruber John B. , Zandi Bahram , Ferry Michael , Kokta Milan R.
Published: July 2002

We report the absorption and fluorescence spectra of divalent cobalt (Co2+) in a series of related spinels Mg(1−x)CoxAlBOC, where B=2, 4, 6, C=4, 7, 10, and x represents the amount of Co2+ that substitutes for Mg2+ in tetrahedral sites of Td symmetry. The spectra support the assumption that both Mg2+ and Co2+ prefer the tetrahedral cation sites during the growth of these crystalline materials. These observations also support earlier conclusions obtained from crystallographic and thermodynamic studies of the MgO and Al2O3 phase diagram. The observed spectra are analyzed and compared to calculated electronic levels derived from lattice-sum models using crystallographic data determined from the crystals grown in the present studies. Comments are made in relevance to the use of these crystals as saturable absorbers for near infrared Q-switched lasers.
Surface Brillouin scattering of cubic boron nitride films
Authors: Zinin P. , Manghnani M. H. , Zhang X. , Feldermann H. , Ronning C. , Hofsäss H.
Published: July 2002

Surface Brillouin scattering has been used to determine the elastic properties of thin hard submicron cubic boron nitride (cBN) films grown on silicon by mass selected ion beam deposition. The elastic properties of the films have been determined by fitting experimental data to theoretical dispersion curves. A Green’s function method was used to predict Brillouin scattering spectra of the acoustic excitation at the free surface. Our results demonstrate that the effect of the thin hexagonal boron nitride interlayer located between cBN film and the Si substrate on the velocity of the surface acoustic wave does not exceed 2% for a thin (16 nm) film and is negligible for cBN films thicker than 100 nm. The elastic properties of the cBN films are not softer than those of bulk cBN.
Characteristics of the temporal pulse response from the forbidden gap of a quasiperiodic Cantor multilayer
Authors: Cojocaru E.
Published: July 2002

A specific behavior of the transmission spectrum from a quasiperiodic Cantor multilayer is the existence of isolated peaks inside the forbidden gap. Thus, the quasiperiodic Cantor multilayer may be an alternative to the periodic structure with defects. It has been shown by other authors that a Gaussian temporal pulse is compressed when transmitted by a Cantor filter if its frequency spectrum centered at the middle of the gap is enough wide to cover the isolated peaks. We analyzed further the compressing capacity of a three-stage Cantor filter in correlation with the magnitude of transmission at the center of the gap, the location and the width of the isolated peaks, for different nondispersive values of refractive indices. Simple relations for the transmittance at the center of the gap are obtained by using the Herpin equivalent. Diagrams are completed that show the pairs of layer refractive indices at which certain levels of the parameters characterizing the compressing capacity are attained.
Control of electrical conduction in DNA using oxygen hole doping
Authors: Lee Hea-Yeon , Tanaka Hidekazu , Otsuka Yoichi , Yoo Kyung-Hwa , Lee Jeong-O , Kawai Tomoji
Published: July 2002

Using oxygen adsorption experiments on poly (dG)-poly (dC) DNA molecules, we found that their conductance can be easily controlled by several orders of magnitudes using oxygen hole doping, which is a characteristic behavior of a p-type semiconductor. It also suggests that the conductance of the DNA under doping results from charge carrier transport, not from an ionic conduction. On the other hand, we will also show that the poly (dA)-poly (dT) DNA molecules behave as an n-type semiconductor. This letter demonstrates that the concentration and the type of carriers in the DNA molecules could be controlled using proper doping methods.
Beyond the Born–Oppenheimer approximation: High-resolution overtone spectroscopy of H2D+ and D2H+
Authors: Fárnı́k Michal , Davis Scott , Kostin Maxim A. , Polyansky Oleg L. , Tennyson Jonathan , Nesbitt David J.
Published: July 2002

Transitions to overtone 2ν2 and 2ν3, and combination ν2+ν3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer “breakdown” effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (<0.1 cm−1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2ν2, ν2+ν3, and 2ν3 excited states indicating directions for further improvement in such treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2ν3 and ν2+ν3 vibrations in D2H+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D2H+, but with statistically significant discrepancies noted for H2D+. Finally, H2D+ versus D2H+ isotopomer populations are analyzed as a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion.
Comment on “Self-diffusion near the liquid–vapor critical point” [J. Chem. Phys. 114, 4912 (2001)]
Authors: Harris Kenneth R.
Published: July 2002

Drozdov and Tucker [J. Chem. Phys. 114, 4912 (2001)] have computed the self-diffusion coefficient along a near-critical isotherm showing anomalous slowing of molecular motion near the critical density and cite some experiments in support. A considered examination of the best literature data shows no such anomaly near neat liquid critical or mixture consolute points.
Infrared absorption study of rapid thermal oxidation and in situ steam generation of thin SiO2 films by gradient etching preparation
Authors: Nagai Naoto , Terada K. , Muraji Y. , Hashimoto H. , Maeda T. , Maeda Y. , Tahara E. , Tokai N. , Hatta A.
Published: July 2002

SiO 2 thin films prepared by rapid thermal oxidation (RTO) and in situ steam generation (ISSG) were characterized by infrared spectroscopy (IR) with gradient etching preparation and grazing incidence x-ray reflectometry. The IR spectra of the RTO films show a lower wave number shift as the film thickness decreases. On the other hand, the IR spectra of the ISSG film produced by the addition of 5% hydrogen did not show such large peak shifts. This means that the 5% hydrogen ISSG film has a very low concentration of lower wave number components that exhibit Si–O stretching mode. These are responsible for defect structures including dangling bonds and/or oxygen deficient defects near the SiO2/Si interface.
Effect of an Fe–Si buffer layer on the magnetoresistance of a Co/Cu multilayer
Authors: Miura Satoshi , Takahashi Daisuke , Tsunoda Masakiyo , Takahashi Migaku
Published: July 2002

Co/Cu multilayers were fabricated on a thermally oxidized Si wafer (SiO2 substrate) as well as a bare Si wafer (Si substrate). The multilayer had an Fe buffer layer and was produced in a sputtering atmosphere into which oxygen was introduced. In the case of the Si substrate, the magnetoresistance (MR) ratio increased as the partial oxygen pressure decreased below 3×10−8 Torr, whereas it steeply decreased in the case of the SiO2 substrate. The increase in the MR ratio in the case of the Si substrate was due to an enlargement of the lateral grain size of the multilayers, which reduced the interfacial roughness of the multilayer. When Fe–Si was used as the buffer layer, the MR ratio of the multilayer on the SiO2 substrate drastically changed in relation to the buffer layer’s Si content. A maximum MR ratio of 40% was obtained at 16% Si, corresponding to the enlargement in the lateral grain size. The MR ratio of the multilayer fabricated on the Fe82Si18 buffer layer remained 28% after annealing at 350 °C. We therefore conclude that the Fe–Si buffer layer is effective in facilitating the lateral grain growth of Co/Cu multilayers and in attaining high thermal stability of the MR ratio.
Nonlinear study of the curvature-driven parallel velocity shear–tearing instability
Authors: Chacón L. , Finn J. M. , Knoll D. A.
Published: July 2002

The nonlinear regime of the parallel velocity shear–tearing instability is studied numerically using a two-dimensional reduced, resistive magnetohydrodynamics model. In this instability, a sheared parallel velocity profile interacts with the perpendicular dynamics via the magnetic field curvature. Linearly, it has been shown [J. M. Finn, Phys. Plasmas 2, 4400 (1995)] that, in the inviscid limit, such interaction alters the classical behavior of the tearing instability, resulting in increased growth rates for classically tearing-unstable regimes (Δ′>0), and destabilizing classically tearing-stable regimes, leading to an electrostatic mode as Δ′→−∞. These trends are seen to hold with finite viscosity as long as the perpendicular plasma viscosity is of the order or smaller than the plasma resistivity. Nonlinearly, it is found that a self-consistent perpendicular shear flow and a reversed (stabilizing) density gradient develop. For favorable curvature, the latter implies an anomalous pinch effect. The shear flow generation mechanism is found, using quasilinear theory, to be related to asymmetries (tilting) introduced by the magnetic curvature coupling term. In the classically tearing-stable regime, the nonlinear behavior is very rich, including relaxation-oscillation phenomena and chaotic behavior. The potential implications of the nonlinear regime of this instability for the plasma confinement are also discussed.
Concentration dependence of the attenuation of first sound in supersaturated superfluid He3–4He solutions under pressure
Authors: Zadorozhko A. A. , Kalko T. V. , Rudavskiı̆ É. Ya. , Usherov-Marshak I. A. , Chagovets V. K. , Sheshin G. A.
Published: July 2002

The concentration dependence of the attenuation coefficient of first sound in superfluid He3–4He solutions in the saturation and supersaturation regions is investigated experimentally at pressures of 0–10 atm. An original technique of continuous variation of the concentration in situ by variation of the osmotic and thermomechanical pressures is used, permitting measurements to be made in the long-lived metastable phase of the superfluid solutions. It is shown that the data obtained are described well in terms of the theory of sound propagation in a gas of Fermi excitations without taking the Fermi-liquid corrections into account. The corresponding values of the effective mass and relaxation time of He3 quasiparticles are taken from an analysis of the existing experimental data. Within the experimental error, no excess sound attenuation was found in the region of supersaturated solutions.
Superconductivity in impurity systems with a lower density of charge carriers and with strong electron correlations
Authors: Palistrant M. E.
Published: July 2002

The influence of a paramagnetic impurity on the superconducting transition temperature Tc, energy gap Ωg, and order parameter Δ at T=0 is investigated in systems with a lower density of charge carriers and with strong electron correlations. It is shown that, since the Migdal theorem is violated in these systems, taking the vertex and “crossing” functions into account in the weak-coupling approximation explains the substantial change in the calculated quantities, in particular, the increase of Tc and of the critical impurity concentrations. The decrease of Tc, Δ, and Ωg with increasing impurity concentration is slowed considerably in comparison with the case of conventional superconductors on account of nonadiabaticity effects.
Vibrational energy pooling in CO on NaCl(100): Methods
Authors: Corcelli S. A. , Tully J. C.
Published: July 2002

Vibrational states as high as n=15 have been experimentally observed in CO molecules adsorbed in a monolayer on the NaCl(100) surface after pumping the n=0→1 vibrational transition with a short (5 μs) infrared laser pulse. These high states become populated from successive single vibrational quantum exchanges between CO molecules on the surface, CO(m)+CO(n)→CO(m−1)+CO(n+1), mediated by dipole–dipole interactions and driven by the anharmonicity of the CO bond vibration. The rates for all of the possible channels of vibrational energy flow in the CO/NaCl(100) system, exchange, relaxation, and fluorescence, were calculated using perturbation theory for a model in which the CO bond vibration is treated as a Morse oscillator and is coupled to a bath of harmonic oscillators with a Debye density of states representing the underlying NaCl substrate. These rates form a Master equation that governs the overall vibrational population dynamics of CO molecules in the monolayer, and was solved using kinetic Monte Carlo (KMC) techniques. Time-dependent vibrational population distributions, Pn(t), representing the probability of finding a CO molecule in the monolayer in vibrational state n at time t, were obtained from the KMC simulations. The results are in good accord with experiment. The maximum achievable excitation is found to be limited by a crossover in the vibrational relaxation and excitation transfer rates with increasing quantum number.
Integrals of monomials over the orthogonal group
Authors: Gorin T.
Published: July 2002

A recursion formula is derived which allows one to evaluate invariant integrals over the orthogonal group O(N), where the integrand is an arbitrary finite monomial in the matrix elements of the group. The value of such an integral is expressible as a finite sum of partial fractions in N. The recursion formula largely extends presently available integration formulas for the orthogonal group.
Controlled barrier modification in Nb/NbOx/Ag metal insulator metal tunnel diodes
Authors: Grossman E. N. , Harvey T. E. , Reintsema C. D.
Published: July 2002

The nonlinear electrical transport properties of metal-insulator-metal tunnel diodes based on a barrier of naturally grown niobium oxide have been measured at room temperature and analyzed. In most cases excellent agreement is found between the measured current-voltage characteristics and fits to the trapezoidal barrier model, over large ranges in conductance (up to several times the zero-bias value), and including the asymmetry induced by the differing electrode materials, niobium and silver. Moreover, we find that an in situ, Ar plasma etch may be used to modify the tunnel barrier in a controlled fashion. Specifically, as the etch time is increased from 0 to 120 s, the barrier thickness is continuously reduced from ∼2.8 to 1.1 nm, while the barrier height at the base (Nb) electrode remains roughly constant at 300±40 mV. Simultaneously, the barrier height at the Ag counterelectrode is lowered from an initial value of 800 mV to the asymptotic value implied by the difference in work functions of the electrode materials, namely, 230±40 mV.
In search of nanoperfection: Experiment and Monte Carlo simulation of nucleation-controlled step doubling
Authors: Wang Yi , Pearl T. P. , Darling S. B. , Gimmell J. L. , Sibener S. J.
Published: July 2002

In order to make effective use of the extreme density of nanoscale elements that form spontaneously in self-assembling architectures, one must address the associated issue of minimizing defect creation during the formation of such structures. In this article we examine the competing roles that nucleation kinetics and two-dimensional growth processes play in nanostructure formation and defect minimization. We employ oxygen-induced step doubling of vicinal Ni(977) surfaces as our physical system, using elevated temperature scanning tunneling microscopy and Monte Carlo simulations to extract the desired details of interface evolution. Two interesting topological defect features are observed on the surface after doubling reaches its asymptotic limit: (i) “frustrated ends,” which form when two counter-propagating step-doubling events having a single step in common intersect, leaving a stable topological defect, and (ii) residual “isolated single steps,” which form when a single step is unable to partner with an adjacent step. This latter defect occurs when a single step is surrounded on both sides by previously doubled structures. In an attempt to understand and control these results, Monte Carlo simulations indicate that experimental control of the delicate and competing interplay of nucleation kinetics and two-dimensional growth kinetics is the key to the formation of more perfect interfaces. In this instance this corresponds to using a small initial oxygen exposure and reduced substrate temperature to achieve a doubled surface of higher perfection. Such optimized interfaces can act as templates for guiding the hierarchical assembly of nanowires and other nanoscale molecular assemblies.
Spin splitting in modulation-doped AlGaN/GaN two-dimensional electron gas
Authors: Tsubaki K. , Maeda N. , Saitoh T. , Kobayashi N.
Published: July 2002

AlGaN/GaN heterostructure devices have recently been attracting much attention because of their potential for high-performance microwave applications. Therefore, the electronic properties of a two-dimensional electron gas (2DEG) in AlGaN/GaN heterostructures have recently been discussed. We studied the magnetoresistance oscillation of the 2DEG at 0.4 K for various backgate voltages, and observed multiple oscillations resulting from spin splitting. The magnetoresistance shows clear beating due to the superposition of three oscillations. The frequency interval between the first and second largest frequencies is proportional to the total electron concentration and the measured spin-orbit interaction parameter agrees with the theoretical one. Therefore, the first and second largest frequencies are found to correspond to spin splitting by the spin-orbit interaction.
Nonideal effects on occurrence scattering time in classical nonideal plasmas
Authors: Jung Young-Dae
Published: July 2002

Nonideal plasma screening effects on the occurrence time advance for the elastic electron–ion collisions in classical nonideal plasmas are investigated using the eikonal method. An effective pseudopotential model taking into account the plasma screening and collective effects is applied to describe the electron–ion interaction potential in nonideal plasmas. It is found that the nonideal effect reduces the occurrence time advance. The scattering occurrence time gets stronger time advances with increasing the scattering angle. For small scattering angles, the domain of the nonideality plasma parameter (γ) for the minimum occurrence time advance is found to be 1<γ<3.5.
A note on the improvement ambiguity of the stress tensor and the critical limits of correlation functions
Authors: Anselmi D.
Published: July 2002

I study various properties of the critical limits of correlators containing insertions of conserved and anomalous currents. In particular, I show that the improvement term of the stress tensor can be fixed unambiguously, studying the RG interpolation between the UV and IR limits. The removal of the improvement ambiguity is encoded in a variational principle, which makes use of sum rules for the trace anomalies a and a′. Compatible results follow from the analysis of the RG equations. I perform a number of self-consistency checks and discuss the issues in a large set of theories.
Spin filtering in ferromagnetic bilayers
Authors: Drouhin Henri-Jean , Rougemaille Nicolas
Published: July 2002

A general analysis of spin-polarized electron transmission through ultrathin ferromagnetic bilayers is presented. The system is analyzed in terms of density operator and a compact description of spin-filtering and spin-precession effects is given. The Sherman function, which characterizes the spin selectivity, is precisely defined. Application to spin detectors is discussed and, in particular, the two important cases where the layer magnetizations are collinear or orthogonal are studied in detail.
Rayleigh–Taylor instability in the deceleration phase of spherical implosion experiments
Authors: Smalyuk V. A. , Delettrez J. A. , Goncharov V. N. , Marshall F. J. , Meyerhofer D. D. , Regan S. P. , Sangster T. C. , Town R. P. J. , Yaakobi B.
Published: July 2002

The temporal evolution of inner-shell modulations, unstable during the deceleration phase of a laser-driven spherical implosion, has been measured through K-edge imaging [B. Yaakobi et al., Phys. Plasmas 7, 3727 (2000)] of shells with titanium-doped layers. The main study was based on the implosions of 1 mm diam, 20 μm thick shells filled with either 18 atm or 4 atm of D3He gas driven with 23 kJ, 1 ns square laser pulses on OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)]. These targets have similar modulation levels at the beginning of the deceleration phase due to similar modulation growths in the acceleration phase, but different modulation growths throughout the deceleration phase due to different fill pressures (convergence ratios). At peak compression, the measured inner surface, areal-density nonuniformity σrms levels were 23±5 % for more-stable 18 atm fill targets and 53±11 % for less-stable 4 atm fill targets. The inner-surface modulations grow throughout the deceleration phase due to Rayleigh–Taylor instability and Bell–Plesset convergence effects. The nonuniformity at peak compression is sensitive to the initial perturbation level as measured in implosions with different laser-smoothing conditions.
Anisotropy of the critical current and the guided motion of vortices in a stochastic model of bianisotropic pinning. I. Theoretical model
Authors: Shklovskij V. A. , Soroka A. A.
Published: July 2002

A planar stochastic model of bianisotropic pinning created by two different mutually orthogonal systems of periodic “washboard” potentials is examined. Possible implementations of this model, both naturally occurring and artificially created, are proposed. Unlike the stochastic model of uniaxial anisotropic pinning by a system of parallel planes, where the critical current density jc is actually equal to zero for all directions because the motion of vortices along the planes is unpinned, in the proposed model an anisotropic critical current exists for all directions. Theoretical formulas for calculating the anisotropic current-and temperature-related depinning of vortices are interpreted in terms of two basic nonlinear temperature-dependent resistive XY responses, having the physical meaning of the probabilities of a vortex overcoming the XY components of the pinning potential.
Optical waveguides formed in Nd:YVO4 by MeV Si+ implantation
Authors: Chen Feng , Wang Xue-Lin , Wang Ke-Ming , Lu Qing-Ming , Shen Ding-Yu
Published: July 2002

The planar waveguide has been fabricated in a Nd:YVO4 crystal by 3.0 MeV Si+ ion implantation at a dose of 1×1015 ions/cm2 at room temperature. The waveguide was characterized by the prism-coupling method. The dark modes are measured before and after the annealing at 240 °C for 60 min in air. The refractive index profile is reconstructed using reflectivity calculation method. It is found that relatively large positive change of ordinary refractive index happens in the guiding region, which is quite different from most of the observed ion-implanted waveguides. The TRIM’98 code is carried out to simulate the energy loss during the implantation in order to obtain a better understanding for the waveguide formation.
Efficient light emission at 1.54 μm from Er in Si excited by hot electron injection through thin suboxide layers
Authors: Markmann M. , Sticht A. , Bobe F. , Zandler G. , Brunner K. , Abstreiter G. , Müller E.
Published: July 2002

We studied the electroluminescence of Er:O-doped Si pn diodes and unipolar structures with thin SiO1.6 suboxide barriers, which were deposited by molecular-beam epitaxy. These suboxide layers reveal a barrier height of about 320 meV in the conduction band and therefore raise the average kinetic energy of electrons injected through the barrier into the Er:O doped region. These electrons turn out to be advantageous for impact excitation processes with the erbium ion. Compared to conventional reverse biased pn diodes a ten-times higher στ product for impact excitation (1.2×10−19 cm2 s) can be achieved in pn diodes with a suboxide injector at 10 K. The saturation electroluminescence (EL) intensity is enlarged in reverse bias and suppressed in forward bias compared to a diode without a suboxide layer. These structures exhibit a reduction of the EL intensity by a factor of 3 for increasing temperature from 10 to 300 K and yield a two-times higher EL output at 1.54 μm and 300 K than an optimized reverse biased pn diode without a suboxide layer. At 300 K this results in an absolute output power of 250 nW and an external quantum efficiency of 1.3×10−4 at 1.54 μm. For the unipolar structure with an integrated suboxide barrier the EL output also depends on the current flow direction: Injecting the electrons hot through the suboxide barrier into the Er:O doped region results in a six times higher EL intensity at 1.54 μm than for the opposite biasing condition. The EL is detectable up to 300 K with a reduction of the intensity by a factor of 8 between 10 and 300 K. Monte Carlo simulations were performed on unipolar structures with an incorporated barrier to provide insight into the carrier density and carrier energy distribution after injection through the barrier.
Conditions for the abrupt bifurcation to chaotic scattering
Authors: Tél Tamás , Grebogi Celso , Ott Edward
Published: July 2002

One of the generic ways in which chaotic scattering can come about as a system parameter is varied is the so-called ‘‘abrupt bifurcation’’ in which the scattering is nonchaotic on one side of the bifurcation and is chaotic and hyperbolic on the other side. Previous work demonstrating the abrupt bifurcation [S. Bleher et al., Phys. Rev. Lett. 63, 919 (1989); Physica D 46, 87 (1990)] was primarily for the case where the scattering potential had maxima (‘‘hilltops’’) which had locally circular isopotential contours. Here we extend these considerations to the more general case of locally elliptically shaped isopotential contours at the hilltops. It turns out that the conditions for the abrupt bifurcation change drastically as soon as even a small amount of noncircularity is included (i.e., the circular case is singular). The illustrative case of scattering from three isolated potential hills is dealt with in detail. One interesting result is a simple geometrical sufficient condition for an abrupt bifurcation in the case of large enough ellipticity of the hill with lowest potential at its hilltop.
Recombination along the tracks of heavy charged particles in SiO2 films
Authors: Oldham Timothy R.
Published: July 2002

Measurements of recombination along the tracks of 2-MeV alpha particles and 0.7-MeV protons in thermally grown silicon dioxide are presented. The results agree quite well with the columnar recombination model at applied fields above about 1 MV/cm if one assumes an initial Gaussian column radius b=3.5 nm. The agreement between theory and experiment at high fields is accomplished even if the excess carriers are assumed to reach instantaneous thermal equilibrium with the lattice. At fields below 1 MV/cm, the agreement between the columnar model and the experiment is improved if one assumes that the carriers remain hot for a finite time before reaching thermal equilibrium with the lattice. The discussion of the details of the thermalization process is intended to be only rough and qualitative because the process is very difficult to model. However, the model and the experiment agree fairly well at low fields if one assumes that the carriers have an energy on the order of 1 eV for a time on the order of 10−13 s. The sensitivity of the model calculations to these assumptions is discussed in some detail.
Hydrogen in low-pressure chemical-vapor-deposited silicon (oxy)nitride films
Authors: Habraken F. H. P. M. , Tijhaar R. H. G. , van der Weg W. F. , Kuiper A. E. T. , Willemsen M. F. C.
Published: July 2002

Silicon (oxy)nitride films (SiOxNy) have been deposited onto silicon by low-pressure chemical vapor deposition using SiH2Cl2, N2O and NH3 or ND3. Nuclear reaction analysis, elastic recoil detection, and Rutherford backscattering spectrometry have been used to determine the elemental composition of the films with emphasis on the hydrogen and deuterium content. In the as-deposited, NH3-grown films the bulk hydrogen concentration is about 3 at. % for an oxygen/nitrogen atomic ratio (O/N) smaller than 0.4, for O/N>0.4 it is lower. In 900 and 1000 °C vacuum annealed films the bulk hydrogen concentration as a function of O/N goes through a maximum at O/N≊0.4. By comparing this observation with the D content in ND3-grown films as a function of O/N, a model is deduced which explains this behavior. This model involves an oxygen induced increase of the electronegativity of the atoms to which hydrogen/deuterium is bound. Annealing at 1000 °C in a H2/N2 gas mixture of NH3-grown films results in bulk hydrogen concentrations ranging between those measured after the 1000 °C vacuum anneal and the values for the as-deposited state. The chlorine concentration increases with increasing oxygen content in the oxynitride films from 0.04 at. % at O/N=0 to 0.65 at. % at O/N=∞. Implications of the data and the proposed model for the electrical performance of the silicon (oxy)nitrides are briefly discussed.
Computer simulation of high-resolution transmission electron microscopy images of Si/SiO2 interfaces
Authors: Ohdomari Iwao , Mihara Tohru , Kai Kazuhiko
Published: July 2002

Cross-sectional high-resolution transmission electron microscopy (HRTEM) images of the crystalline Si/SiO2 interface have been computer simulated on the basis of various interface models in order to facilitate the interpretation of experimental HRTEM images. Continuous random networks of SiO2 have been constructed on crystalline Si substrates with different orientations and surface morphologies using plastic balls and spokes. The distortion energy introduced by artifacts in the model has been relaxed by modifying the atom positions, HRTEM images of the models have been calculated using the multislice dynamical electron scattering method. The apparent shift of the (111)Si/SiO2 interface into the SiO2 region has been observed due to the localization of first Si atoms in SiO2 region. The contrast of the crystalline Si image due to the amorphous SiO2 region in the wedge-shaped bilayer structure of the Si/SiO2 interface has been obscured. The undulated (100)Si/SiO2 interface delineated with {111} facets has given an unexpectedly flat periodic-nonperiodic interface image.
Flat response detectors for the vacuum ultraviolet and soft x-ray region: InP:Fe photoconductors
Authors: Kania D. R. , Iverson A. E. , Smith D. L. , Wagner R. S. , Hammond R. B. , Stetler K. A.
Published: July 2002

We have demonstrated that InP:Fe photoconductive detectors are sensitive to radiation between 20 and 3000 eV. Within experimental error the measured response is flat in this energy range, in agreement with theory. This property, coupled with the simplicity and high speed (full width at one half maximum of less than 200 ps) of these devices, makes them very interesting for the measurement of pulsed vacuum ultraviolet and x-ray radiation and the total instantaneous integrated radiative power emitted from a pulsed source. Low fidelity response can occur in these devices for excitation times that are many times longer than the primary response time of the detector.
Direct measurement of the energy spectrum of an intense proton beam
Authors: Leeper R. J. , Lee J. R. , Kissel L. , Johnson D. J. , Stygar W. A. , Hebron D. E.
Published: July 2002

A time-resolved magnetic spectrograph was used to measure the energy spectrum of an intense (0.7 TW/cm2) proton beam. A thin gold foil placed at the focus of an ion-diode Rutherford-scattered protons by 90° into the spectrograph, reducing the beam intensity to a level suitable for magnetic analysis. The scattered protons were collimated and then deflected in a samarium-cobalt permanent magnet. The deflected protons were recorded simultaneously on CR-39 and eight p-i-n diodes. A Monte Carlo computer code was used to calculate the sensitivity and resolution of the spectrograph. Data taken on the Proto I accelerator shows a 150–250-keV-wide proton-energy spectrum at each instant in time.
Carrier surface generation and recombination effects in photoconduction of HgI2 single crystals
Authors: Burshtein Z. , Akujieze Justin K. , Silberman E.
Published: July 2002

dc photoconductivity characteristics of HgI2 single crystals have been measured in the range of 1.75–2.75 eV, using CuI as a transparent electrical contact. Hole and electron photocurrents could be separately measured by applying different voltage polarities to the illuminated electrode. It is shown that charge carrier generation near the surface is highly extrinsic, as displayed by differences in the hole and electron photogeneration spectra. Analysis of the photoconductivity versus voltage characteristics indicates that the hole and electron bulk trapping times satisfy τp >70 μs and τn >3 μs, respectively. The hole and electron surface recombination velocities were sp =1.0×104 cm/s and sn =8×105 cm/s, respectively.
Stable single-longitudinal-mode operation in visible (AlGa)As semiconductor lasers coupled with a short external cavity
Authors: Yamamoto Osamu , Hayashi Hiroshi , Miyauchi Nobuyuki , Maei Shigeki , Kawanishi Hidenori , Morimoto Taiji , Yamamoto Saburo , Yano Seiki , Hijikata Toshiki
Published: July 2002

High stability of single longitudinal mode has been achieved in visible wavelength regions using V-channeled substrate inner-stripe lasers coupled with a short external cavity. The temperature dependence of the coupled-gain peak coincided entirely with that of one longitudinal mode for lasers with external-cavity lengths of 50–60 μm. As a result, a wide longitudinal-mode locking range of 31 °C was obtained over 8 mW. This result is in good agreement with the theoretical calculation, in which the effect of the thermal expansion of the external-cavity length is taken into account.
Effect of the capture coefficient in deep-level transient spectroscopy measurements
Authors: Landsberg P. T. , Shaban E. H.
Published: July 2002

Underlying the conventional deep-level transient spectroscopy (DLTS) method is the assumption of an exponential capacitance transient to the equilibrium state as a result of the thermal emission rate of free carriers from a filled trap energy level. This exponential capacitance transient may prove to be a good approximation for specific cases but, in general, the transient capacitance decay to the equilibrium state following a capture pulse is nonexponential. In this study nonexponential capacitance transients are shown to be encouraged by the presence of the free-carrier tail which spills over abundant free mobile carriers into the space-charge region thus negating the abrupt junction depletion approximation and favoring both capture and emission of carriers. An upper limit for this effect is obtained here by assuming the carrier concentration in the relevant part of the space-charge region which one has in the neutral region. This reduces the thermal emission rate by several orders of magnitude from what one would find with the assumption of pure exponential transient and neglecting spillover, as in the normal DLTS method. A particular case is considered, where both capture and thermal emission processes occur simultaneously in the Shockley–Read–Hall kinetic equation for a single-trap energy level. The variation of carrier occupancy with respect to time leads to a nonexponential capacitance transient decay to the equilibrium state.
Nonresonant multiphoton ionization of the neutrals ablated in laser microprobe mass spectrometry analysis of GaAs and Hg0.78Cd0.22Te
Authors: Schueler Bruno , Odom Robert W.
Published: July 2002

A second, quadrupled Nd:YAG laser system has been incorporated into a laser microprobe mass spectrometer in order to ionize the neutral species contained in the vapor plume produced by the ablation laser of this microanalytical materials analysis technique. The irradiance of this focused postablation ionization (PAI) laser beam operating at 266 nm is sufficient to ionize all the neutral species having an ionization potential less than 9.3 eV that are contained within the irradiated volume via two photon nonresonant multiphoton ionization processes. This laser ablation/laser ionization configuration of the laser microprobe technique has been applied to the microanalysis of high-purity GaAs and Hg0.78Cd0.22Te target materials. The elemental ion yields produced in the PAI microanalysis of these materials are generally larger than those produced by the laser ablation only mode of laser microprobe analysis. The elemental ion yields of the constituents in these materials have also been evaluated as a function of the delay time between the firing of the ablation and PAI lasers at several ablation laser irradiances.
Optical measurements of particle orientation in magnetic media
Authors: Nunnelley L. , Fuller G. G.
Published: July 2002

We describe a rapid, nondestructive method of measuring the orientation in particulate magnetic recording media. This technique uses the dichroic properties of the media. The results documented here agree with calculated results derived from electron micrographs of the media.
Thin heteroepitaxial Si-on-sapphire films grown at 600 °C by reactive ion beam deposition
Authors: Yamada Hiroshi , Torii Yasuhiro
Published: July 2002

Thin low-temperature heteroepitaxial film growth of Si on sapphire (SOS) was investigated by using the reactive ion beam deposition method proposed recently. This method uses low-energy controlled ionized species produced from reactive SiH4 gas plasma. By accommodating the ion energy to two film growth stages, i.e., an initial heteroepitaxial growth stage followed by a homoepitaxial growth stage, thin SOS films about 200 nm in thickness with no superficial microtwins can be grown at the low temperature of 600 °C. The energy level is adjusted to 300 eV in the initial heteroepitaxial film growth stage (about 50 nm in thickness), and then to 100 eV until the total film thickness is about 200 nm.
Ion-induced etching of SiO2: The influence of mixing and lattice damage
Authors: Winters Harold F.
Published: July 2002

Modulated-beam mass spectrometry has been used to investigate the ion-induced etching of SiO2 in the presence of a flux of XeF2. The types of desorbed etch product are characterized. It is also demonstrated directly that mixing leads to an enhanced etch rate while the presence of lattice damage does not produce spontaneous etching. It is concluded that all available data for the etching of SiO2 with XeF2 is consistent with the original hypothesis of Y. Y. Tu, T. J. Chuang, and H. F. Winters [Phys. Rev. B 23, 823 (1981)], that the etching reaction is primarily based on a mechanism involving chemical sputtering.
FMR and TEM studies of annealed and magnetically annealed thin bilayer films
Authors: Layadi A. , Lee J.-W. , Artman J. O.
Published: July 2002

The effect of annealing and magnetic annealing on the magnetic and structural properties of NiFe/FeMn has been assessed by, respectively, ferromagnetic resonance (FMR) and transmission electron microscopy (TEM). A straight anneal, at 350 °C for 1 h, led to the disappearance of the interfacial exchange anisotropy, HE, and to the appearance of FMR spin-wave resonance (SWR) modes. Interdiffusion takes place at the interface and the originally columnar NiFe and FeMn grains tend to become equiaxed. In cross-section view, [011] zone axes are prominent in both layers. Although magnetic annealing did not seem to affect the structural properties of the system, features of the SWR spectra and the HE effect were sensitive to the orientation of the magnetic field during anneal.
Growth kinetics of nickel microstructures produced by laser-induced decomposition of nickel tetracarbonyl
Authors: Tonneau D. , Auvert G. , Pauleau Y.
Published: July 2002

Nickel dots and films were deposited on Si-coated quartz plates by the cw Ar+ laser-induced decomposition of Ni(CO)4 at a temperature in the range of 200–400 °C. The deposited material was characterized by x-ray diffraction, Auger spectroscopy, nuclear reaction analyses, and scanning electron microscopy. The deposition kinetics of Ni dots formed in the laser-heated area of 200 μm in diameter was investigated as a function of irradiation time, output laser power, and Ni(CO)4 pressure. The laser-induced deposition of Ni dots was demonstrated to occur via a purely pyrolytic decomposition of Ni(CO)4. At low Ni(CO)4 pressures (typically below 0.3 Torr) and high output laser powers (above 1 W), the deposition rate of flat-topped Ni dots was found to be independent of the deposition temperature and proportional to Ni(CO)4 pressure. The deposition kinetics of these dots was limited by the number of molecules colliding with the heated area. At reactant pressures ranging from 0.3 to 10 Torr, the deposition rate of Gaussian Ni dots was found to be independent of Ni(CO)4 pressure, and the apparent activation energy was 11.6 kcal mol−1. The deposition kinetics of these Gaussian dots was controlled by the desorption of CO molecules.
Simulation of the bulk and surface properties of amorphous hydrogenated silicon deposited from silane plasmas
Authors: McCaughey Michael J. , Kushner Mark J.
Published: July 2002

A Monte Carlo simulation for the growth of amorphous hydrogenated silicon (a-Si:H) thin films by plasma enhanced chemical vapor deposition is presented. The goal of the model is to predict the bulk and surface properties of films (e.g., hydrogen content, deposition rate, buried hydride/dihydride ratios, porosity, and surface roughness) having thicknesses of ≲2000 Å. The effects on the film properties of the composition of the radical flux incident on the surface are examined. Film properties were found to be critically dependent on the ratio of SiH3/SiH2 in the radical flux. High values for this ratio results in film properties resembling chemical vapor deposition. Film properties obtained with small values resemble physical vapor deposition. Rough films (roughness >10’s of Å) accordingly result from radical fluxes having high SiH2 fractions. We find that surface roughness and hydrogen fraction increase with increasing growth rate and increasing film thickness, though thin films (<10’s of layers) have large hydrogen fractions due to there being a hydrogen rich surface layer with a thickness approximately equal to the surface roughness. We also find a correlation between porosity (subsurface voids) and hydrogen fraction, implying that the inner surfaces of voids are lined with 3/4 SiH configurations. In simulating films grown from Ar/SiH4 gas mixtures, we find that a decrease in the SiH3/SiH2 ratio in the radical flux, and an increase in the ion/radical ratio incident on the surface are largely responsible for the degradation of film properties observed with decreasing silane fraction.
Thermal-field-emission electron optics for nanolithography
Authors: Gesley Mark
Published: July 2002

A new column design for the 25-kV vector-scan Gaussian beam lithography system is described. A field-emission gun, consisting of a three-element electrostatic lens and Zr/O/W〈100〉 cathode operated in a thermal-field mode for current stability, is combined in a demagnifying optics with a magnetic objective lens to focus a high-current-density (1000–3000 A/cm2) electron beam at high resolution (100–300 Å) at the wafer plane without a severe reduction in field size. Optimum beam semiangle, focus mode, and column magnification are determined. The modified system retains the original deflection coils and pattern-generation system which allows immediate implementation of existing subfield stitching, chip registration, and proximity correction software.
Wave-optical effects in semiconductor laser amplifiers
Authors: Chow Weng W. , Depatie David
Published: July 2002

A wave-optical model is used to investigate the power scaling of semiconductor laser amplifiers. The configurations investigated are single-pass amplifiers, double-pass amplifiers, phase-conjugate double-pass amplifiers, regenerative amplifiers, and injection-locked lasers. We are interested in a comparison of conventional double-heterostructure and quantum-well gain media, the factors affecting optical power extraction, and conditions leading to filamentation in the laser beam.
A pile-up phenomenon during arsenic diffusion in silicon-on-insulator structures formed by oxygen implantation
Authors: Normand P. , Tsoukalas D. , Guillemot N. , Chenevier P.
Published: July 2002

Arsenic diffusion in silicon-on-insulator formed by deep oxygen implantation is studied by secondary ion mass spectroscopy and speading resistance measurements. An enhanced diffusivity as well as a pile-up phenomenon are observed in the thin silicon layer. The McNabb and Foster equations [Trans. TMS-AIME 22, 618 (1963)] for diffusion with trapping are solved in order to simulate this last effect.
Thermal stability of the Cu/PtSi metallurgy
Authors: Chang Chin-An
Published: July 2002

Reaction between Cu and PtSi is studied over 200 to 700 °C. Using structures of Cu on a preformed PtSi, and of Cu on Pt/Si, Cu-PtSi reaction is observed below 300 °C for both structures, resulting in increasing sheet resistances above this temperature. The low thermal stability of the Cu/PtSi metallurgy, compared with that of Al/PtSi, is attributed to the ready formation of Cu silicides at low temperatures. Cu3 Si and Cu4 Si are observed for both Cu/PtSi and Cu/Pt/Si structures heated to 300–400 °C. Little improvement is obtained using an 1000 Å barrier layer of Ti, W, or Cr.