bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantized Hamilton dynamics for a general potential
Authors: Prezhdo O. V. , Pereverzev Y. V.
Published: July 2002

The quantization of Hamilton dynamics (QHD) [J. Chem. Phys. 113, 6557 (2000)] that efficiently generalizes classical mechanics to include quantum tunneling and zero-point energy effects is extended to a general position dependent potential. A Taylor series expansion of the potential is considered both around a fixed point and around the moving instantaneous value of the position variable. The equations-of-motion obtained for the moving frame are significantly simpler than for the fixed frame, while still satisfying the classical limit. The number of the QHD variables and the order of the Taylor expansion of the potential constitute two independent approximation parameters. Conservation of the total energy and the Heisenberg commutator relationship is established for the second-order QHD that includes linear and quadratic variables. The formal results are illustrated by examples, including the harmonic oscillator, tunneling in a doublewell potential, and energy exchange between coupled Morse oscillators representing the stretching vibrations of the SPC-F water molecule commonly used in molecular dynamics simulations. QHD provides an improvement over classical mechanics in all cases. The QHD data agree with the exact data in the large ℏ quantum limit and in the ℏ=0 classical limit, with deviations observed for the intermediate ℏ values, depending on the system and initial conditions. QHD provides a quantitative short time approximation. The second-order QHD becomes inappropriate when quantum wave packets split. Higher-order QHD approximations are required in such cases. The second-order QHD is particularly suitable for the description of zero-point energy effects and properly treats moderate tunneling events.
Nuclear reactor safeguards and monitoring with antineutrino detectors
Authors: Bernstein A. , Wang Y. , Gratta G. , West T.
Published: July 2002

Cubic-meter-sized antineutrino detectors can be used to non intrusively, robustly, and automatically monitor and safeguard a wide variety of nuclear reactor types, including power reactors, research reactors, and plutonium production reactors. Since the antineutrino spectra and relative yields of fissioning isotopes depend on the isotopic composition of the core, changes in composition can be observed without ever directly accessing the core itself. Information from a modest-sized antineutrino detector, coupled with the well-understood principles that govern the core’s evolution in time, can be used to determine whether the reactor is being operated in an illegitimate way. A group at Sandia is currently constructing a 1 m3 antineutrino detector at the San Onofre reactor site in California to demonstrate these principles.
Specific heat and thermal conductivity of BaTiO3 polycrystalline thin films
Authors: Davitadze S. T. , Kravchun S. N. , Strukov B. A. , Goltzman B. M. , Lemanov V. V. , Shulman S. G.
Published: July 2002

The results of measurements by the ac heater-probe method of the specific heat and thermal conductivity of polycrystalline BaTiO3 films with thickness between 1 and 0.1 μm in a temperature range 120–440 K are presented. In the heating and cooling runs, the phase transition anomalies of the thermal properties in the region of each phase transition in the sequence of symmetry change m3m-4mm-mm2-3m have been observed. A temperature hysteresis is observed, which is the signature of the first order phase transitions. As film thickness decreases, anomalies shift to lower temperatures and became smeared.
Contributions of Q-machine experiments to understanding auroral particle acceleration processes
Authors: Koepke M. E.
Published: July 2002

Experiments performed over the past 40 years on Q-machine plasma devices [N. Rynn and N. D’Angelo, Rev. Sci. Instrum. 31, 1326 (1960)] have contributed significantly to the basic understanding of plasma behavior. Many of these laboratory results are relevant to plasma processes in the Earth’s ionosphere and magnetosphere. Here are reviewed examples in which Q-machine experiments contributed to understanding particle acceleration in the Earth’s auroral energization region by discovering unexpected behavior, developing physical insight, benchmarking theoretical models, and establishing observational signatures relevant to space plasmas. Magnetic-field-aligned (parallel) electric fields, solitary structures, ion–cyclotron waves, ion–acoustic waves, Kelvin–Helmholtz waves, and lower-hybrid waves are discussed. The legacy of these contributions is a tribute to the Q-machine design.
Generalized plasma dispersion function for a plasma with a kappa-Maxwellian velocity distribution
Authors: Hellberg M. A. , Mace R. L.
Published: July 2002

A generalized plasma dispersion function has previously been obtained for waves in plasmas with isotropic kappa distributions for arbitrary real kappa [Mace and Hellberg, Phys. Plasmas 2, 2098 (1995)]. In many instances plasmas are found to have anisotropic power-law distributions, and hence a similar dispersion function for electrostatic waves in plasmas having a one-dimensional kappa distribution along a preferred direction in space, and a Maxwellian distribution perpendicular to it has now been developed. It is used to study the effect of superthermal electrons and ions on ion-acoustic waves propagating at an angle to a magnetic field. This dispersion function should find application to wave studies both in space plasmas, where the magnetic field defines a preferred direction, and in dusty plasma crystal studies, where the ion flow direction is unique.
Measurements and modeling of ion energy distributions in high-density, radio-frequency biased CF4 discharges
Authors: Sobolewski Mark A. , Wang Yicheng , Goyette Amanda
Published: July 2002

Models of ion dynamics in radio-frequency (rf) biased, high-density plasma sheaths are needed to predict ion bombardment energies in plasma simulations. To test these models, we have measured ion energy distributions (IEDs) in pure CF4 discharges at 1.33 Pa (10 mTorr) in a high-density, inductively coupled plasma reactor, using a mass spectrometer equipped with an ion energy analyzer. IEDs of CF3+, CF2+, CF+, and F+ ions were measured as a function of bias frequency, bias amplitude, and inductive source power. Simultaneous measurements by a capacitive probe and a Faraday cup provide enough information to determine the input parameters of sheath models and allow direct comparison of calculated and measured IEDs. A rigorous and comprehensive test of one numerical sheath model was performed. The model, which includes a complete treatment of time-dependent ion dynamics in the sheath, was found to predict the behavior of measured IEDs to good accuracy over the entire range of bias frequency, including complicated effects that are observed when the ion transit time is comparable to the rf bias period.
Thin-film transistors based on well-ordered thermally evaporated naphthacene films
Authors: Gundlach D. J. , Nichols J. A. , Zhou L. , Jackson T. N.
Published: July 2002

We report on organic thin-film transistors fabricated using the small-molecule organic semiconductor naphthacene as the active layer material with device performance suitable for several large-area or low-cost electronics applications. We investigated naphthacene thin films deposited by thermal evaporation onto amorphous substrates held near room temperature. Using atomic-force microscopy and x-ray diffraction we find naphthacene films consist of a high density of submicron-sized grains with a surprisingly high degree of molecular order. Thin-film transistors fabricated using evaporated naphthacene films on thermally oxidized silicon substrates have field-effect mobility larger than 0.1 cm2/V s, current on/off ratio greater than 106, negative threshold voltage, and subthreshold slope of 1 V/decade.
Evidence for size-dependent mechanical properties from simulations of nanoscopic polymeric structures
Authors: Böhme Thomas R. , de Pablo Juan J.
Published: July 2002

Discontinuous molecular dynamics simulations of a model polymer have been conducted to investigate the glass transition of ultrathin films and the mechanical properties of nanoscopic structures. Continuum mechanics models have been applied to interpret simulation data and extract apparent Young’s Moduli. Consistent with experiments, the results of simulations indicate that the glass transition temperature of thin films can be higher or lower than that of the bulk, depending on the nature of polymer–substrate interactions. Simulations also indicate that the mechanical properties of nanoscopic structures can be considerably different from those of the bulk. An analysis of molecular strain distributions in nanostructures undergoing a deformation indicate that significant stress relaxation occurs at air–polymer interfaces. A comparison of these distributions to the results of continuum, finite-element calculations reveal pronounced differences between the continuum and molecular approaches.
Direct measurement of the polarization charge in AlGaN/GaN heterostructures using capacitance–voltage carrier profiling
Authors: Miller E. J. , Yu E. T. , Poblenz C. , Elsass C. , Speck J. S.
Published: July 2002

The polarization charge at AlxGa1−xN/GaN heterostructure interfaces arising from differences in spontaneous polarization between AlxGa1−xN and GaN and the presence of piezoelectric polarization in strained layers has been directly measured using capacitance–voltage carrier profiling in GaN/AlxGa1−xN/GaN heterostructures with varying Al composition grown by molecular-beam epitaxy. The measured polarization charge densities (2.36±0.30×1012 e/cm2, 6.79±0.48×1012 e/cm2, and 6.92±0.74×1012 e/cm2 for 5%, 12%, and 16% AlxGa1−xN/GaN interfaces, respectively) reveal substantial bowing in the polarization charge as a function of Al composition, and are in reasonable agreement with those calculated using a model that accounts for the nonlinearity of the spontaneous and piezoelectric polarizations as functions of Al composition. Our results yield an explicit expression for polarization charge as a function of Al composition at an AlxGa1−xN/GaN interface.
Independent optically addressable nanoparticle-polymer optomechanical composites
Authors: Sershen S. R. , Westcott S. L. , Halas N. J. , West J. L.
Published: July 2002

We report the fabrication and characterization of optomechanically active composite materials consisting of a thermally responsive poly(NIPAAm-co-AAm) hydrogel matrix incorporating a dilute concentration of Au or silica-Au core-shell nanoparticles. Under optical illumination at the resonance absorption wavelength of the nanoparticle dopant, a dramatic volume collapse of the composite occurs due to local photothermal heating of the NIPAAm matrix. Nanoparticle dopants were chosen so that each composite was specifically optically addressable, exhibiting optomechanical behavior at independent wavelengths. Such materials can be useful as independently addressable remotely triggerable switches and gates in a wide variety of micromechanical applications.
Microwave magnetic properties of Co50/(SiO2)50 nanoparticles
Authors: Wu Mingzhong , Zhang Y. D. , Hui S. , Xiao T. D. , Ge Shihui , Hines W. A. , Budnick J. I. , Taylor G. W.
Published: July 2002

Co 50 /( SiO 2 ) 50 nanoparticles were synthesized by a wet chemical method, and their microwave permeability was measured in the 0.1–18 GHz range. The synthesized nanoparticles exhibit two loss peaks at microwave frequencies: one appears around 7.0 GHz and is believed to result from the eddy current effect, the other appears around 250 MHz and is probably caused by natural ferromagnetic resonance. Compared with micrometer-size Co particles, the synthesized nanoparticles exhibit high permeability μ′ and low magnetic loss, especially over 10–18 GHz.
Electrical properties of CdTe films and junctions
Authors: Anthony Thomas C. , Fahrenbruch Alan L. , Peters Michael G. , Bube Richard H.
Published: July 2002

Polycrystalline films of CdTe have been deposited on graphite substrates, and homoepitaxial films on single-crystal CdTe:P substrates, with hole densities as high as 1.5×1016 cm−3 without intentional doping of the films, using close-spaced vapor transport. The dependence of hole density in the films on the properties of the source, the properties of the substrate, the growth rate, and the substrate temperature, indicates that the doping of the films is the result of a complex interaction between out-diffusion of mobile impurities (if present) from the substrate and intrinsic defects. The capacitance-versus-voltage-indicated hole density decreases slowly with time under a Schottky barrier, but not at a free film surface or under a CdS/CdTe heterojunction. CdS/CdTe solar cells have been prepared using these films, with solar efficiencies greater than 6%. The junction properties of CdS/CdTe heterojunctions are compared with those of In/CdTe Schottky barriers as a function of temperature; transport is dominated by tunneling below room temperature and by interface recombination above room temperature.
Phase diagrams of pure and diluted low-anisotropy antiferromagnets: Crossover effects (invited)
Authors: Shapira Y.
Published: July 2002

Phase diagrams of 3D low-anisotropy easy-axis antiferromagnets are reviewed. The topics discussed include (1) mean-field theory and early experiments; (2) bicritical points in pure antiferromagnets; (3) effects of random fields on the behaviors near the Néel and bicritical points; and (4) lower critical dimension of the random-field Ising model. Some unsettled questions in the area of random fields are mentioned.
Preparation and characterization of rf sputtered indium tin oxide films
Authors: Sreenivas K. , Sudersena Rao T. , Mansingh Abhai , Chandra Subhash
Published: July 2002

Indium tin oxide films have been grown by rf sputtering at various Ar-O2 mixtures, at low substrate temperatures (200 °C), and deposition rates (25 Å/min), followed by post deposition annealing (at 350 °C) in different ambients (O2, N2, and cracked ammonia). Influence of a reactive gas (oxygen) on the sputtering rate of a metallic (indium/tin) alloy target has been investigated. Growth parameters and annealing conditions have been optimized. The films were characterized by electron and x-ray diffraction, scanning electron microscopy, and transmittance as a function of wavelength. The effect of heat treatment in various environments on the structural, electrical, and optical properties has been investigated. Effect of a new annealing ambient, cracked ammonia (reducing atmosphere), on the reactively sputtered oxide films is being reported for the first time. Cracked ammonia was found to be very effective and cheap and resulted in films of high quality (electrical and optical) with good structural properties. Films with low sheet resistances (Rs=30 Ω/⧠ at film thicknesses of 800 Å and Rs=8.5 Ω/⧠ at film thicknesses of 5000 Å) with high visible transmission (∼95%) have been achieved by annealing in cracked ammonia.
An improved Au/Be contact to p-type InP
Authors: Hasenberg T. C. , Garmire E.
Published: July 2002

An improved Ohmic contact for p-type InP has been demonstrated which is based on theAu/Be-Au material system but includes a chromium-barrier layer. This Au/Be-Cr-Au contact exhibits a lower specific-contact resistivity than that previously reported for the Au/Be-Au contact and does not degrade with time. The chromium barrier prevents indium out-diffusion into the gold (which has been observed even at room temperature) as well as reduces the phosphorus evaporation from the InP surface during the contact anneal. A specific-contact resistivity of 5×10−5 Ω cm2 has been observed for an anneal temperature of 410 °C and anneal times of 3–4 min. This value is believed to be the lowest reported value for an Ohmic contact on p-type InP which is doped in the 1×1018 cm−3 range. This contact has the advantage of not containing zinc which has a rapid diffusion rate and contaminates vacuum systems.
The localization and crystallographic dependence of Si suboxide species at the SiO2/Si interface
Authors: Grunthaner P. J. , Hecht M. H. , Grunthaner F. J. , Johnson N. M.
Published: July 2002

X-ray photoemission spectroscopy has been used to examine the localization and crystallographic dependence of Si+1, Si+2, and Si+3 suboxide states at the SiO2/Si interface for (100)- and (111)-oriented substrates with gate oxide quality thermal oxides. The Si+1 and Si+2 states are localized within 6–10 Å of the interface while the Si+3 state extends ∼30 Å into the bulk SiO2. The distribution of Si+1 and Si+2 states shows a strong crystallographic dependence with Si+2 dominating on (100) substrates and Si+1 dominating on (111) substrates. This crystallographic dependence is anticipated from consideration of ideal unreconstructed (100) and (111) Si surfaces, suggesting that (1) the Si+1 and Si+2 states are localized immediately within the first monolayer at the interface and (2) the first few monolayers of substrate Si atoms are not significantly displaced from the bulk. The total number of suboxide states observed at the SiO2/Si interface corresponds to 94% and 83% of a monolayer for these (100) and (111) substrates, respectively. We speculate that the remaining interfacial substrate Si atoms that are not associated with bonding to oxygen are bonded to impurity species.
Self-shielding superconducting solenoid systems
Authors: Gabrielse G. , Tan J.
Published: July 2002

Superconducting solenoid systems which produce large magnetic fields can be designed to utilize flux conservation to cancel fluctuations in the ambient magnetic field in which they are located. Such self-shielding solenoids could be very useful for mass spectroscopy of trapped particles, nuclear magnetic resonance experiments, and magnetic resonance imaging.
Characterization of Pb(Zr,Ti)O3 thin films deposited from multielement metal targets
Authors: Sreenivas K. , Sayer M.
Published: July 2002

Lead zirconate titanate [Pb(Zr,Ti)O3 or PZT] thin films have been grown by sputtering a multi-element metal target in oxygen using dc planar magnetron sputtering. Growth parameters and annealing conditions have been optimized. The kinetics of reactive sputtering and the implications of sputtering parameters on film composition have been studied. The studies reveal the requirement for operation at low substrate temperatures (200 °C), high sputtering pressures (4–5 Pa), and a large substrate-to-target distance (10 cm) for obtaining good control over composition. The structural and electrical properties of films were found to depend on the compositional ratio of Zr/Ti, similar to that observed in bulk PZT ceramics. Films having a resistivity of 1010 Ω cm and a dielectric constant ε′∼820 at room temperature (300 K) have been achieved. Ferroelectric hysteresis loop measurements indicated a remanent polarization of 30.0 μC/cm2 and coercive field of 25 kV/cm for the rhombohedral phase composition (Zr/Ti=58/42). Piezoelectric activity in the films is reported for the first time, through the fabrication of a surface-acoustic-wave delay line on a poled polycrystalline PZT thin film.
Switching kinetics of lead zirconate titanate submicron thin-film memories
Authors: Scott J. F. , Kammerdiner L. , Parris M. , Traynor S. , Ottenbacher V. , Shawabkeh A. , Oliver W. F.
Published: July 2002

We have measured coercive field and switching voltage versus thickness in PbZr0.54Ti0.46O3 thin (0.15–0.50 μm) films, together with switching times and current transient shapes versus field and temperature. The results show activation fields of order 120 kV/cm at room temperature, threshold voltages below 1.3 V, and switching speeds faster than 100 ns, demonstrating that fast, nonvolatile memories can be constructed that are compatible with standard silicon or GaAs integrated circuit voltage levels, without the need for an internal voltage pump. The displacement current transient data yield 2.5 as the dimensionality of domain growth if one-step intial nucleation rate is assumed, and are compatible with the theory of Ishibashi, yielding imaxtmax/Ps=1.65±0.23, in comparison with the predicted 1.646. The switching time exhibits an activation field dependence upon both voltage and temperature through a single reduced parameter (TC−T)(VTC),−1 in accord with the theory of Orihara and Ishibashi.
Gas-phase kinetics during diamond growth: CH4 as-growth species
Authors: Harris Stephen J.
Published: July 2002

We have used a one-dimensional kinetic analysis to model the gas-phase chemistry that occurred during the diamond growth experiments of Chauhan, Angus, and Gardner [J. Appl. Phys. 47, 4746 (1976)]. In those experiments the weight of diamond seed crystals heated by lamps in a CH4/H2 environment was monitored by a microbalance. No filament or electric discharge was present. Our analysis shows that diamond growth occurred in this system by direct reaction of CH4 on the diamond surface. C2H2 and CH3, which have been proposed as diamond growth species, played no significant role there, although our results do not address their possible contributions in other systems such as filament- or plasma-assisted diamond growth.
Transport critical current density and electrical characterization of patterned high-T
c superconducting thin films prepared by metallo-organic decomposition
Authors: Lubberts G.
Published: July 2002

Resistivity and transport critical current density have been measured as a function of temperature for patterned YBa2Cu3O7−δ and Bi2Sr2CaCu2O8+δ superconducting thin films on single-crystal (100)-oriented MgO substrates. These granular superconducting films were prepared by the metallo-organic decomposition method. Transition temperatures Tc (ρ=0) are around 72 K for YBa2Cu3O7 and 82 K for Bi2Sr2CaCu2O8 films, both 1 μm in thickness. Critical current densities approaching 105 A/cm2 have been measured in the Bi2Sr2CaCu2O8 films at low temperatures. The critical current densities in YBa2Cu3O7 films are about 40 times lower than those in the Bi2Sr2CaCu2O8 films. It was found that near Tc the critical current density in both films is proportional to (Tc−T)2, in agreement with a superconductor–normal-metal– superconductor model developed by DeGennes [P. G. De Gennes, Rev. Mod. Phys. 36, 225 (1964)]. The reason for the much higher current density in the Bi2Sr2CaCu2O8 film is believed to be due to the stronger coupling between the superconducting grains.
Grain size dependence of the thermal conductivity of polycrystalline chemical vapor deposited β-SiC at low temperatures
Authors: Collins A. K. , Pickering M. A. , Taylor R. L.
Published: July 2002

Thermal conductivity data of polycrystalline chemical vapor deposited cubic silicon carbide are calculated from thermal diffusivity and heat capacity data in the temperature region of 80–300 K. Below 200 K, a linear dependence of the thermal conductivity with the product of grain size of the silicon carbide and cubic temperature is observed. This is explained in terms of phonon scattering by the grain boundaries.
rf residual losses, surface impedance, and granularity in superconducting cuprates
Authors: Halbritter J.
Published: July 2002

The rf residual surface resistance Rres is an important figure of merit for superconducting microwave devices and for establishing the homogeneity of superconducting surfaces. In granular superconductors not only are high residual rf surface resistance values Rres observed but also there is strong field dependence of Rres(T,ω,Hrf) and the penetration depth λres(T,ω,Hrf), even at very low field levels (of order 1 Oe). These observations can be explained in terms of weak links and insulating interfaces, which provide additional degrees of freedom due to fluxoid motion and electromagnetic strip-line modes. These degrees of freedom cause additional interaction mechanisms with applied rf fields. These mechanisms allow a coherent description of the observed T, ω, Hrf, and Hdc dependence of Rres and λres for the granular cuprates and for similar effects in Pb, Nb, Nb3Sn, and NbN. In addition, on the basis of such rf data, it is possible to quantify the density of weak links and their critical fields and currents.
Growth of native oxide on a silicon surface
Authors: Morita M. , Ohmi T. , Hasegawa E. , Kawakami M. , Ohwada M.
Published: July 2002

The control factors controlling the growth of native silicon oxide on silicon (Si) surfaces have been identified. The coexistence of oxygen and water or moisture is required for growth of native oxide both in air and in ultrapure water at room temperature. Layer-by-layer growth of native oxide films occurs on Si surfaces exposed to air. Growth of native oxides on n-Si in ultrapure water is described by a parabolic law, while the native oxide film thickness on n+-Si in ultrapure water saturates at 10 Å. The native oxide growth on n-Si in ultrapure water is continuously accompanied by a dissolution of Si into the water and degrades the atomic flatness at the oxide-Si interface, producing a rough oxide surface. A dissolution of Si into the water has not been observed for the Si wafer having surface covered by the native oxide grown in air. Native oxides grown in air and in ultrapure de-ionized water have been demonstrated experimentally to exhibit remarkable differences such as contact angles of ultrapure water drops and chemical binding energy. These chemical bond structures for native oxide films grown in air and in ultrapure water are also discussed.
Antiferromagnetism in face-centered-tetragonal iron
Authors: Peng S. S. , Jansen H. J. F.
Published: July 2002

The total energy of face-centered-tetragonal iron is calculated within density- functional theory. We have obtained results for nonmagnetic, ferromagnetic, and antiferromagnetic iron. In the range of tetragonal structures we have studied, our total- energy calculations for the ferromagnetic phase give just two minima: One is nearly bcc (c/a=0.71) and one is nearly fcc (c/a=1). The antiferromagnetic phase yields only one minimum near the fcc structure, but is unstable near the bcc structure. The global minimum in total energy is antiferromagnetic. The difference in total energy between ferromagnetic and antiferromagnetic iron shows an oscillatory behavior as a function of c/a. Our results show that it might be possible to grow iron films with a large in-plane lattice constant that have an antiferromagnetic ordering.
Flashover in wide-band-gap high-purity insulators: Methodology and mechanisms
Authors: Le Gressus C. , Valin F. , Henriot M. , Gautier M. , Duraud J. P. , Sudarshan T. S. , Bommakanti R. G. , Blaise G.
Published: July 2002

The electron beam of a scanning electron microscope (SEM) is used to charge an unmetallized insulator (Al2O3, Y2O3, SiO2) in vacuum. The charging is found to be stable in time after the e-beam is switched off. The SEM is also used to measure the implanted charge by measuring the resulting electrostatic potential. The distribution of potential around the trapped charges is determined by the classical laws of electrostatics. The electrostatic energy stored in the polarized dielectric can thus be determined. The implanted charge can be removed by the introduction of carriers into the polarized sample by flooding the surface with electron beams of varying energies. Slow or rapid relaxation occurs depending on the operating conditions of the flood gun (energy, intensity, etc.) which introduces the electrons. When the relaxation kinetics are slow, the electrostatic charge decreases slowly as a function of time. On the other hand, a rapid relaxation of the dielectric leads to the appearance of a high-density plasma which spreads over the insulator surface, resulting in treeing on the insulator surface. Along the path of the arc, the transfer of energy to the lattice triggers the sublimation of the ceramic and its mechanical fracture through thermal shock. At particular flood gun setup conditions, we have observed the formation of parallel fracture lines along the ceramic surface, away from the treeing region. These results constitute the basis for a new approach to understanding flashover along ceramics-vacuum interfaces. The important step is the plasma initiation, which we interpret on the basis of dielectric relaxation mechanisms. The parameter of the insulator that determines its breakdown initiation is its complex dielectric constant. It is concluded that the insulator’s band gap, the nature and density of defects localized in the band gap, and the dipolar relaxation induced by a variation of the electric field and connected with the presence of defects determine the holdoff level of insulators.
Electron holography of long-range electric and magnetic fields
Authors: Matteucci Giorgio , Missiroli GianFranco , Nichelatti Enrico , Migliori Andrea , Vanzi Massimo , Pozzi Giulio
Published: July 2002

The influence of the perturbed reference wave in electron holography is considered for the case of static electromagnetic microfields, whose extension around the observed specimen cannot be neglected. These microfields are called ‘‘long-range’’ to distinguish them from the ‘‘short-range’’ ones, whose extension is strictly limited within the object wave and hence do not perturb the reference wave. Optical reconstructions of experimental holograms of simple electrostatic or magnetic long-range fields have been modeled and simulated. The results indicate that perturbation effects must be taken into account when long-range microfields are investigated by electron holography; it is also shown that their influence can be minimized by increasing the interference distance between the object and reference wave.
An electromigration model that includes the effects of microstructure and temperature on mass transport
Authors: Dreyer M. L. , Fu K. Y. , Varker C. J.
Published: July 2002

A model for electromigration in thin metal film interconnects is presented that includes two components of diffusion. The grain-boundary and lattice components of mass transport are considered in terms of their temperature dependence and the metallurgical ‘‘structure’’ of patterned planar interconnects. Interconnect structure is defined in terms of single- and polycrystalline line segments, which result from the local grain microstructure for a patterned interconnect line. The dependence of the diffusional flux on the length and type of line segment is included in the model. The results indicate that the grain structure of the film plays an important role in determining the relative contribution of the diffusion components to mass transport. The model assumes that the length and type of interconnect line segment determines the relative contribution of grain boundary and lattice diffusion components, and provides a means for extrapolating accelerated test results for planar interconnects by taking into consideration the temperature dependence of the diffusion mechanisms, and the effect of the local microstructure on diffusion. The model also indicates that extrapolations made using Black’s equation may result in an overestimate of safe operating conditions. Calculations show that the effective activation energy depends on the median grain size and its distribution parameter, D50 and σ, respectively, and the interconnect linewidth W. Model calculations of electromigration lifetime t50 were compared to experimental results obtained on patterned interconnects using sputter-deposited Al-1.5% Cu alloy films. The experimental data support a linewidth-dependent electromigration activation energy and show that the dependence of t50 on linewidth for W≤3D50 results from a change in the dominant diffusion mechanism with temperature, linewidth, and local interconnect ‘‘structure.’’
Raman scattering from PbTiO3 thin films prepared on silicon substrates by radio frequency sputtering and thermal treatment
Authors: Taguchi I. , Pignolet A. , Wang L. , Proctor M. , Lévy F. , Schmid P. E.
Published: July 2002

Raman scattering has been carried out on PbTiO3 thin films prepared on platinum-coated (100) silicon by radio-frequency (rf)-magnetron sputtering without substrate heating and a post-deposition thermal treatment. The Raman spectra obtained from the thin film are characteristic of powder Raman spectra: In comparison with the single crystal spectra, the intensity of the background is relatively high at low frequencies and the Raman lines are broad. The lattice phonon modes corresponding to the observed lines are identified by comparison with the data on single crystals and powder. The Raman frequencies for the thin film remarkably shift to low frequencies compared with single-crystal data. It is shown that the phenomenon of the frequency shifts is similar with the hydrostatic pressure effect on single crystals of PbTiO3. The result indicates that the thin films are composed of grains that are stressed depending on the grain size by neighboring grains of different orientations when they are split up into ferroelectric domains at the paraelectric-to-ferroelectric transition. This stress effect is significant even for a grain size of ∼0.5 μm. It is found that the lowest frequency E transverse optical (TO) mode in the thin film shows softening with increasing temperature as was reported in previous studies on single crystals.
Fabrication and optical properties of lead-germanate glasses and a new class of optical fibers doped with Tm3+
Authors: Wang J. , Lincoln J. R. , Brocklesby W. S. , Deol R. S. , Mackechnie C. J. , Pearson A. , Tropper A. C. , Hanna D. C. , Payne D. N.
Published: July 2002

In this article we present a study of a new class of optical fibers based on lead germanate glass. The maximum vibrational frequency of this glass is intermediate between silica and zirconium barium lanthanum aluminum fluoride glass, causing a beneficial change in nonradiative decay and therefore quantum efficiency for particular laser transitions. Fabrication of high-strength, low-loss fibers of this glass has been acheived by modification of the composition to produce optimal physical properties for fiber drawing, while retaining the useful vibrational properties of the original PbGeO2 glass. Measurements of both the thermal and optical properties are described. The fibers produced are ideal for many applications in fiber devices.
A kinetic model for As and P incorporation behaviors in GaAsP grown by gas-source molecular beam epitaxy
Authors: Liang B. W. , Tu C. W.
Published: July 2002

A kinetic model has been developed to explain As and P incorporation behaviors in GaAs1−xPx epilayers grown on GaAs (001) by gas-source molecular beam epitaxy. The model can predict the P compositions for various substrate temperatures and flow rates. The model shows that an in situ determination of GaP molar fraction in GaAs1−xPx can be performed by group V-induced intensity oscillations of reflection high-energy-electron diffraction at low substrate temperatures where desorption of group V species is negligible. At high substrate temperatures the compositions can be determined from the arsine and phosphine flow rates.
Tribological properties of polished diamond films
Authors: Bhushan Bharat , Subramaniam Vish V. , Malshe Ajay , Gupta B. K. , Ruan Juai
Published: July 2002

Despite the rapid progress being made in the synthesis of diamond films and recent interest in polishing of diamond films, no systematic measurements of friction and wear on polished diamond films have been reported. In the present study, chemomechanical and laser polishing techniques are used, and friction and wear data on the chemomechanically polished diamond films are presented. With the chemomechanical polishing technique used in this study, the rms roughness of hot filament chemical vapor deposited diamond films can be reduced from about 657 to about 170 nm with rounding off of sharp asperities with no change in the diamond structure. The polished films exhibit coefficient of friction (∼0.1) and wear rates much lower than that of unpolished films. Friction and wear properties of the polished films are comparable to that of single-crystal natural diamond. Based on this study, it is concluded that polished films are potential candidates for tribological applications.
Thermal diffusivity measurements on solids using collinear mirage detection
Authors: Salazar A. , Sánchez-Lavega A. , Fernández J.
Published: July 2002

A report on the adequacy of the collinear mirage technique for thermal diffusivity measurements on bulk homogeneous solids is presented. A 3D theoretical model for collinear deflection has been developed from which two simple linear relations between measurable parameters and the thermal diffusivity have been obtained. Two methods, the so-called zero-crossing and phase methods, are discussed in detail. The second one seems to be a promising tool for thermal diffusivity determination. It has been validated by means of experimental measurements on a set of samples with known thermal diffusivities. The technique is restricted to semitransparent solids but is also valid for materials with either high or low thermal diffusivities, being specially useful for this last group.
A crystallographic model of (00⋅1) aluminum nitride epitaxial thin film growth on (00⋅1) sapphire substrate
Authors: Sun C. J. , Kung P. , Saxler A. , Ohsato H. , Haritos K. , Razeghi M.
Published: July 2002

High-quality thin aluminum nitride films were grown on different orientations of sapphire substrates by metalorganic chemical vapor deposition. (00⋅1) AlN thin film grown on (00⋅1) Al2O3 has better crystallinity than (11⋅0) AlN on (01⋅2) sapphire. Full width at half maximum of a rocking curve is 97.2 arcsec, which is the narrowest value to our knowledge. A crystallographic model between AlN thin films and sapphire substrates was proposed to explain the process of crystal growth. ‘‘Extended atomic distance mismatch’’ which is the mismatch of atomic distance for a longer period was introduced. It is shown that the mismatch is relaxed by edge-type dislocations. Extended atomic distance mismatch was used to interpret the results that (00⋅1) AlN has better crystallinity than (11⋅0) AlN, but (11⋅0) GaN has better crystallinity than (00⋅1) GaN.
Selfconsistent simulation of sputter deposition with the Monte Carlo method
Authors: Kersch A. , Morokoff W. , Werner Chr.
Published: July 2002

Low pressure sputter atom deposition is a commonly used method for coating and trench filling in semiconductor chip manufacturing. In this paper we present a selfconsistent simulation of this process in which both the sputtered atoms and the background gas are computed by means of the Direct Simulation Monte Carlo (DSMC) method. The results allow a detailed exploration of the sputtering wind effect and indicate that sputter atoms approach equilibrium more slowly than previous test particle Monte Carlo simulations have found. A new molecular collision scatter law for DSMC is introduced that simulates particle interactions more accurately with no additional computational cost. Its use for general DSMC calculations is recommended.
Magnetic susceptibility of RCo3B2 (R=Y, Sm, Gd, and Dy)
Authors: Ido H. , Nanjo M. , Yamada M.
Published: July 2002

The temperature dependence of magnetic susceptibility has been measured in a temperature region from about 100 to 500 K for the compounds RCo3B2 with R=Y, Sm, Gd, and Dy. Their magnetization curves at T=5 K have also been measured in a pulsed high field up to 170 kOe. YCo3B2 seems to have a Pauli paramagnetic nature down to T=5 K. In the paramagnetic temperature region, SmCo3B2 exhibits Van Vleck paramagnetism; however, GdCo3B2 and DyCo3B2 show Curie–Weiss character. The paramagnetic susceptibility for all compounds mentioned above is found to include a Pauli paramagnetic contribution from the Co sublattice. In the case of GdCo3B2 at T=5 K, the magnetic moment of 0.2μB/Co, which is antiparallel to the Gd moment, is considered to be induced by the exchange field from the Gd spin. A similar induced moment of 0.14μB/Co is also estimated in DyCo3B2. The Sm- and Dy-sublattice moments at T=5 K are smaller than the free-ion values, which can be explained by the crystal-field theory. A relatively large value of B02=7.0 K for Dy3+ in DyCo3B2 can explain the experimental Dy3+ spontaneous moment and its increase by external field. The Sm moment in SmCo3B2 at T=5 K may be antiparallel to the induced Co-sublattice moment.
Exchange stiffness and magnetic anisotropies in bcc Fe1−xCox alloys
Authors: Liu X. , Sooryakumar R. , Gutierrez C. J. , Prinz G. A.
Published: July 2002

Brillouin scattering measurements from several bcc Fe1−xCox epitaxial films are reported. The magnetic-field dependence of the spin-wave spectrum as well as the in-plane directional dependence of the surface magnon are determined. The saturation magnetization of the alloys was measured independently through superconducting quantum interference device magnetometry. Self-consistent fitting of the magnon dispersions determined from the separate Brillouin experiments provide values for the concentration dependence of the exchange stiffness D, the fourth-order crystalline cubic anisotropy K1, and surface anisotropies Ks and Ks,p. In agreement with ferromagnetic resonance data, K1 is found to change sign for x beyond about 35 at. % while D steadily increases with cobalt concentration up to x∼0.5 and subsequently decreases in two Co-rich samples.
Formation of crystallographic texture in rf sputter-deposited Cr thin films
Authors: Feng Y. C. , Laughlin D. E. , Lambeth D. N.
Published: July 2002

Although it is well known that Cr underlayers sputter deposited on glass or NiP/Al substrates have either the (002) or (110) textures, the mechanism of the formation of the crystallographic textures is not clear. A model is proposed for the formation of the crystallographic texture of sputter-deposited Cr thin films. A systematic set of experiments has been carried out to test the model. It was found that the (110) texture, which is usually found in Cr thin films deposited on substrates without preheating, can form at elevated temperatures (250 °C) when deposited at low Ar pressure or by applying substrate bias. The initial stage of the texture formation was also investigated by using very thin Cr films. It was found that the (002) texture can be initiated directly on the substrate surface, while the (110) texture appears not to form directly on substrate surface, but rather as a result of film growth. The proposed model is consistent with the experimental results.
Structural and optical properties of poly lactic acids
Authors: Kobayashi J. , Asahi T. , Ichiki M. , Oikawa A. , Suzuki H. , Watanabe T. , Fukada E. , Shikinami Y.
Published: July 2002

Fibrous and crystal structures of a helical polymer, poly-L-lactic acid (PLLA), were analyzed by using x-ray diffraction experiments. It was confirmed that the molecular residues were arranged on a nonintegral 10/3 helix as De Santis and Kovacs [Biopolymers 6, 299 (1968)] reported. The atomic positions in a monomeric unit, which were proposed by Hoogsteen, Postema, Pennings, ten Brinke, and Zugenmaier [Macromolecules 23, 634 (1990)], were validated. However, the previous reports on the positions of the two helical chains were found to be in error. The correct positions were determined. The second helical chain shifts from the base center by 0.45, 0.25, and 0.61 Å along a, b, and c axes. Besides, the second chain rotates by 2.46° with respect to the first. Distribution function of the crystallites in various drawn fibers were determined as a function of spiral angle. Optical gyrations of PLLA and poly-D-lactic acid fibers were successfully measured by using high accuracy universal polarimeter, as functions of temperature and drawing ratio. By using x-ray data of the change of the fibrous structure by drawing treatments, the gyration tensor components of PLLA could be calculated. It is of great interest that gyration tensor component g33 along the helical axis is extremely large, ∼(3.85±0.69)×10−2, which corresponds to a rotatory power of (9.2±1.7)×103°/mm, about two orders of magnitude larger than those of ordinary crystals. This is the first experimental evidence that helical polymers will produce enormous optical activity in the solid state. Helical polymers will be important for the elucidation of gyro-optical properties of solids and promising for new optical applications utilizing their large optical activity.
Low-temperature formation of metastable cubic tantalum nitride by metal condensation under ion irradiation
Authors: Ensinger W. , Kiuchi M. , Satou M.
Published: July 2002

Nitrogen containing tantalum was grown on silicon by electron-beam evaporation of the metal under simultaneous irradiation with highly energetic nitrogen ions from an ion source. Phase analysis by means of x-ray diffraction revealed that at low irradiation intensities the deposited material is tantalum metal with dissolved nitrogen. At high intensities a phase transformation to tantalum nitride takes place. By contrast to the stable TaN in the phase diagram, which is hexagonal, the material grown under ion bombardment exhibits a metastable cubic structure of the NaCl type. The crystallographic orientation of the grains is directly dependent on the ion irradiation intensity. Initially, the films show a [111] texture. With increasing ion irradiation intensity the [100] orientation appears additionally. This orientation is independent of the substrate. The lattice spacings are also dependent on the irradiation intensity. With increasing ion irradiation the lattice is widened, then the spacings decrease again. The results are discussed in comparison to data from the literature on sputter deposition with low-energy ions.
Two-dimensional argon metastable density measurements in a radio frequency plasma reactor by planar laser-induced fluorescence imaging
Authors: McMillin Brian K. , Zachariah Michael R.
Published: July 2002

Two-dimensional, relative measurements of the argon 1s5 metastable density distribution were obtained in a low-pressure, 13.56 MHz, parallel-plate, Gaseous Electronics Conference reference cell discharge using planar laser-induced fluorescence imaging. For the conditions examined (pure argon, 75–300 V, 13.3–133.3 Pa), the measured density fields show significant radial and axial variations that depend more strongly on pressure than applied voltage. Generally speaking, the metastable density increases radially from the center to the edge of the discharge by ∼10%–30%. As the pressure is increased, the peak metastable density increases by ∼4 times and the axial distribution changes from a center-peaked parabolic-like profile to an asymmetric profile peaked near the powered electrode. Comparisons of centerline metastable and excited-state emission profiles indicate that, while the metastable distribution is largely determined by the spatially dependent electron-impact excitation function, variations in quenching can significantly affect the resulting metastable density profile at some conditions.
Magnetic switching and in-plane uniaxial anisotropy in ultrathin Ag/Fe/Ag(100) epitaxial films
Authors: Cowburn R. P. , Gray S. J. , Ferré J. , Bland J. A. C. , Miltat J.
Published: July 2002

We have studied the process by which the in-plane magnetization of an ultrathin (4–11 ML) epitaxial iron film reverses under the action of an external magnetic field. Kerr effect measurements reveal a small in-plane uniaxial anisotropy superimposed on the cubic magnetocrystalline anisotropy which greatly influences the reversal. In addition, we find that depending upon the field orientation, reversal can proceed either via a ‘‘1-jump’’ mechanism, by the sweeping of 180° domain walls and which gives a classic square hysteresis loop, or by a ‘‘2-jump’’ mechanism, by the sweeping of 90° domain walls at two distinct applied field strengths—this gives a more unusual hysteresis loop with two irreversible transitions. We have developed a simple phenomenological energy model which explains how so small a uniaxial anisotropy can play so significant a role in the reversal process. The model explains the two reversal mechanisms and predicts with good experimental agreement which should be observed for different applied field orientations.