bluuebunny/crossref_metadata_2025 huggingface dataset:
Čerenkov radiation in chiral media
Authors: Engheta Nader , Bassiri Sassan
Published: July 2002

In this paper, the Čerenkov radiation in an unbounded homogeneous isotropic chiral medium is studied and analyzed classically. Starting from the Maxwell equations and the proposed constitutive relations for isotropic chiral media, we formulate the problem for the electric and magnetic fields emitted from a charged particle moving with a constant speed in a chiral medium, and find a formal solution for the electromagnetic field components and energy spectral density of radiation. Notable features, such as double cone of propagation, and important characteristics of the Čerenkov radiation in such media in terms of the relative velocity of the particle with respect to the two characteristic phase velocities in the medium are discussed.
Simultaneous generation of optical absorption bands at 5.14 and 0.452 eV in 9 SiO2 :GeO2 glasses heated under an H2 atmosphere
Authors: Awazu Koichi , Kawazoe Hiroshi , Yamane Masayuki
Published: July 2002

Nine SiO2 :GeO2 glasses were heated under an H2 or O2 atmosphere. In the H2 atmosphere optical absorption bands were induced at vacuum ultraviolet (VUV) (>6.22 eV), 5.14 eV (242 nm), 3.82 eV (325 nm), and 0.452 eV (2.75 μm). The intensities of all the induced bands are in linear relation with each other. Heat treatment in the O2 ambient reduced the intensities of the optical absorption bands. A model for the reaction of the glasses with H2 molecules and a structural model of the center responsible for the UV-VUV absorption was proposed.
Optical absorption and emission of GaP1−xSbx alloys
Authors: Reihlen E. H. , Jou M. J. , Jaw D. H. , Stringfellow G. B.
Published: July 2002

The first detailed optical investigation of the metastable III/V semiconductor alloy GaP1−xSbx is presented. Epilayers are grown by atmospheric pressure organometallic vapor phase epitaxy throughout the entire compositional range on GaP, GaAs, and GaSb substrates. The approximately 1-μm-thick layers are partially strained with values of lattice mismatch as large as 1.7×10−2. Values of band gap are determined for the first time from absorption spectra measured at 10 and 300 K and corrected for strain-induced energy shifts. The resultant values of bowing parameter for the direct and indirect band gaps are cΓ=3.11±0.18 eV and cx =2.06±0.18 eV, independent of temperature, yielding a value of direct/indirect crossover composition, xc, of 0.32±0.07. Single photoluminescence (PL) peaks are observed between 10 and 300 K for all samples. For samples with 0.37≤x≤0.47, they are assigned to recombination of carriers localized in bandtail states induced by compositional fluctuations. The stretch of the tails into the gap is greatly enhanced over the random alloy limit by the metastability of the Ga1−xSbx alloys. PL peaks for samples with x≤0.32 are assigned to recombination via deep centers in the gap. The PL peak of a sample with x=0.91 is assigned to recombination involving shallow acceptors.
Shape-memory properties in Ni-Ti sputter-deposited film
Authors: Busch J. D. , Johnson A. D. , Lee C. H. , Stevenson D. A.
Published: July 2002

The objective of this research is to establish a shape-memory effect in sputter-deposited films of nickel titanium. The alloy, generically called nitinol, was prepared from sputtering targets in two different compositions. Films were deposited up to 10 μm in thickness on glass substrates using a dc magnetron sputtering source. The as-deposited films were amorphous in structure and did not exhibit a shape memory. The amorphous films were crystallized with a suitable annealing process, and the transformation properties were measured using differential scanning calorimetry. The crystallized films showed transition temperatures that were much lower than those of the parent material. X-ray diffraction patterns indicated that the films were not a single phase but showed evidence of a second phase. However, the annealed films demonstrated a strong shape-memory effect. Stress/strain measurements and physical manipulation were used to evaluate the shape recovery. These tests demonstrated sustained tensile stresses of up to 480 MPa in the high-temperature phase, and a characteristic plastic deformation in the low-temperature phase.
Deuterium effusion measurements in doped crystalline silicon
Authors: Stutzmann Martin , Brandt Martin S.
Published: July 2002

We describe first results for deuterium effusion from undoped and doped crystalline silicon (n- and p-type) treated in a D2 plasma under different conditions. The dependence of the effusion spectra on doping level, passivation temperature, sample bias, and preannealing are presented and the results are discussed on the basis of different D-bonding configurations in the passivated silicon samples.
CoNbFe soft magnetic thin-film backlayers for glass computer disks
Authors: Mapps D. J. , Akhter M. A. , Pan G.
Published: July 2002

Co84Nb12Fe4 films with very low coercivity down to 1 A/m, anisotropy field 1100 A/m, and saturation magnetic induction 1.1 T have been deposited by rf sputtering onto chemically strengthened glass disk substrates as a backlayer for perpendicular recording Winchester disks. Values of coercivity and anisotropy field were studied over a thickness range from 30 to 15 000 Å and found to be a strong function of film thickness. Comparisons between Néel’s prediction for wall motion coercivity mechanism and the experimental data were made. The experimental coercivity fits the Néel formula when films are thicker than 400 Å. The dependence of coercivity and anisotropy field on deposition conditions was also characterized. The thermal stability of the films was studied by differential scanning calorimetry (DSC) and by thermal annealing up to 500 °C. The crystallization temperature of the CoNbFe films is about 450 °C determined from DSC analysis. Thermal annealing revealed that the magnetic properties were very stable when the annealing temperature was below 400 °C.
Optically induced variation of magnetic and photoelectric properties of ferrites
Authors: Katsnelson E.
Published: July 2002

The laser irradiation of ferrites in a dc magnetic field at room temperature leads to a significant change of their residual magnetization and photovoltaic response, both in magnitude and the symmetry with respect to a magnetic field. The photovoltage also acquires a nonlinear field dependence. The variation in the properties is retained by the samples for a long time. This lifetime strongly depends on the wavelength of the laser light. Photoinduced effects are connected with photomagnetization (PM) which arises during irradiation. The PM was directly measured with a magnetometer on single crystal Mn1−xZnxFe2O4 ferrites (bulk) with x=0.09 (1) and 0.56 (2) as well as on MeFe2O4 ferrite films with Me=Mn (3), Ni (4), and Co (5). The effect might be useful for optical recording.
Behavior of gas flow under gradient magnetic fields
Authors: Wakayama Nobuko I.
Published: July 2002

The behavior of gas flow in air has been demonstrated visually under gradient magnetic fields in the range 0–40 T/m under 0–1.5 T. The magnetic behavior depended on the concentration of oxygen. Pure oxygen or nitrogen mixed oxygen (≳30%) was observed to be attracted by a magnetic field and act like a magnetic fluid where the magnetic effect on the gas was comparable to that of normal diffusion. In contrast, nitrogen gas escaped from magnetic fields of high intensities, and no magnetic field effect was observed for the flow of air. These phenomena were explained by the difference between the force acting on the gas and that acting on the air surrounding it under gradient magnetic fields. The magnetic force increased with increasing concentration of oxygen. This explanation can be also applicable to the escape of flames from magnetic fields of relatively high intensity or the magnetic quenching of flames inside a pair of columnar poles.
Picosecond pulse response characteristics of GaAs metal-semiconductor-metal photodetectors
Authors: Moglestue C. , Rosenzweig J. , Kuhl J. , Klingenstein M. , Lambsdorff M. , Axmann A. , Schneider Jo. , Hülsmann A.
Published: July 2002

We present a comprehensive theoretical and experimental analysis of the current response of GaAs metal-semiconductor-metal Schottky photodiodes exposed to 70 fs optical pulses. Theoretical simulations of the carrier transport in these structures by a self-consistent two-dimensional Monte Carlo calculation reveal the strong influence of the distance between the finger electrodes, the external voltage, the GaAs layer thickness and the excitation intensity on the response time and the corresponding frequency bandwidth of these photodetectors. For many experimental conditions, the model demonstrates a clear temporal separation of the electron and hole contributions to the output current due to the different mobilities of the two carrier types. For a diode with an electrode separation of 0.5 μm, an electric-field strength above 10 kV/cm and low intensity of the incident light the theory predicts a pulse rise time below 2 ps, an initial rapid decay as short as 5 ps associated with the electron sweep out and a subsequent slower tail attributed to the hole current. For weaker electric fields and/or higher light intensities a significant slowing down of the detector speed is predicted because of effective screening of the electric field by the photoexcited carriers. Heterostructure layer-based devices are shown to provide superior performance compared to diodes manufactured on bulk substrates. Experimental data obtained by photoconductive or electro-optic sampling on diodes with electrode separation between 0.5 and 1.2 μm agree fairly well with the theoretical predictions.
Wettability effects on oil-water-configurations in porous media: A nuclear magnetic resonance relaxation study
Authors: Borgia Giulio C. , Fantazzini Paola , Mesini Ezio
Published: July 2002

1 H spin-lattice relaxation and other physical properties are compared to study the oil-water-surface interplay in suitably chosen and prepared natural porous media of different wettability. Whereas the water-oil arrangement is well established for both strongly water wet and strongly oil wet porous media, the problem is still open for intermediate wettability surfaces. Relaxation behavior reflects the pore space geometry and shows wettability effects on sweeping efficiency and on immiscible liquid arrangement in well-defined equilibrium situations. A simple picture at the pore-length scale is verified for strongly water wet samples, whereas a more complex picture arises for intermediate wettability samples.
Measuring low alternating magnetic fields by means of Bi-containing rare-earth ferrite-garnet films with planar anisotropy
Authors: Vetoshko Peter M. , Volkovoy Vadim B. , Zalogin Vladimir N. , Toporov Andrey Yu.
Published: July 2002

A method of magnetization control through homogeneous rotation has been applied to Bi-containing rare-earth ferrite-garnet films with planar anisotropy. A dynamical range of 160 dB for low alternating magnetic fields detected at f=50 kHz is shown. A minimum detectable field (10−7 Oe) is limited by the sensitivity of polarimeter at the wavelength of the Ar-ion laser λ=514.5 nm, and the output 1 W. The saturation field applied (about 10 Oe) does not result in remanent magnetization in the limits of static orientation vector M resolution. New magneto-optical techniques for investigation with a homogeneous rotation field and for magnetocrystalline anisotropy investigation of in-plane films are described.
Scattering, absorption, and anomalous spectral tuning of 1.3 μm semiconductor diode lasers
Authors: Peters Frank H. , Cassidy Daniel T.
Published: July 2002

The effects of scattering and absorbing regions which exist along the stripe of 1.3 μm InGaAsP semiconductor diode lasers are examined. A model which accounts for scattering and absorbing nonuniformities is used to explain correlations between scattering, absorption, and the spectral properties of the lasers. Normally the dominant mode of the spectrum of a semiconductor laser shifts to longer wavelength as current is increased. Occasionally, the dominant mode will shift to shorter wavelength with increasing current for a limited current range. This negative tuning can be explained by considering the effect of multiple scatterers combined with a localized absorber.
Laser-cleaning techniques for removal of surface particulates
Authors: Tam Andrew C. , Leung Wing P. , Zapka Werner , Ziemlich Winfrid
Published: July 2002

Flash laser heating using short-pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm. This is very useful because micron- and submicron-sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal. Several varieties of the new laser-cleaning techniques have been developed by us as well as by others. For example, the pulsed laser irradiation can be used with or without the simultaneous deposition of a thin liquid film on the surface to be laser cleaned. The laser wavelength can also be chosen so that absorption occurs mainly at the sample surface, or in the liquid, or in the particulate, or in a combination of these. In this paper, we discuss and compare examples of these different approaches. We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laser irradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest ultraviolet laser fluence of ∼0.1 J/cm2.
The diamond-anvil cell as a high-pressure viscometer
Authors: King H. E. , Herbolzheimer E. , Cook Richard L.
Published: July 2002

A high-pressure viscometer based on a diamond-anvil pressure cell is described and its use demonstrated. A sphere (typical diameter 50 μm) rolls down a diamond surface under the force of gravity and its position is tracked electronically. The resulting velocity is converted to viscosity through a modified Stokes equation that includes a term describing the increased drag due to the close proximity of the walls within the pressure chamber. A model for this wall correction is described and that the drag coefficient is constant throughout the data collection is demonstrated. The precision of the resulting viscosity data averages 4%. The accessible viscosity range is 0.5–107 cP using a diamond cell that is capable of pressures exceeding 10 GPa. However, most fluids exceed 107 cP at far lower pressures. The accuracy of the device is demonstrated by comparing the presented data on the pressure dependence of the viscosity for five fluids [chloroform, water, octamethyltrisiloxane, 1-pentanol, and bis(2-ethylhexyl)sebacate] with previous measurements.
The influence of neutron irradiation on the soft magnetic and mechanical properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si13.5B9 alloys
Authors: S̆korvánek I. , Gerling R.
Published: July 2002

The changes in the magnetic and mechanical properties of Fe73.5Cu1Nb3Si13.5B9 alloys after various annealing and neutron irradiation treatments were studied by hysteresis loop measurements and by bending tests. It was shown that annealing of the amorphous alloy slightly above its crystallization temperature leads to the formation of an ultrafine grain structure of α-Fe(Si) with an average grain diameter of about 11 nm embedded in an amorphous matrix. The resulting nanocrystalline samples exhibit very good soft magnetic properties but their mechanical properties are deteriorated. In order to evaluate and compare the resistance of amorphous and nanocrystalline samples to radiation damage and the correlated microstructural changes, specimens of the alloys investigated were irradiated in the in-core reactor position. Deterioration of the soft magnetic properties after neutron irradiation was found to be less significant in the case of nanocrystalline samples. The state of embrittlement of the samples investigated is not influenced by neutron irradiation up to a fluence of 1019 nth cm−2. Post-irradiation thermal treatments revealed that irradiation-induced defect structures may recover after annealing below the crystallization temperature.
Composition dependence of lithium diffusivity in lithium niobate at high temperature
Authors: Jundt D. H. , Fejer M. M. , Norwood R. G. , Bordui P. F.
Published: July 2002

An investigation was performed on the diffusivity of lithium in lithium niobate at 1100 °C in the crystallographic z direction over the composition range from 48.38 to 49.85 mol % Li2O. A vapor transport technique was applied to produce a lithium niobate crystal with a widely varying diffusion-limited lithium concentration profile. The profile was measured through spatially resolved measurement of the phase-match temperature for frequency doubling of a Nd:YAG laser. A Boltzmann–Matano analysis was applied to the profile to estimate diffusivity as a function of composition. The validity of the Boltzmann–Matano results was examined through numerical simulation, taking into account finite interfacial mass transport. A dramatic increase of lithium diffusivity with Li/Nb ratio was observed, ranging from roughly 3×10−9 cm2/s at the 48.38 mol % Li2O congruent composition to roughly 50×10−9 cm2/s at 49.85 mol % Li2O.
General performance characteristics of real heat engines
Authors: Gordon J. M. , Huleihil Mahmoud
Published: July 2002

Realistic upper bounds can be placed on the power and efficiency of real heat engines via a relatively simple analytic treatment of primary sources of irreversibility. Generalized curves for heat engine performance, their universal nature, and quantitative evaluation of upper bounds for power and efficiency are derived for several engine types, specifically: Brayton cycle (gas turbines), Rankine cycle (steam turbines), and cycles with sizable heat leaks, such as thermoelectric generators. The key irreversibility sources include fluid friction, the constraint of the equation of state of the engine’s working fluid, and heat leak. It is demonstrated that maximum power and maximum efficiency operating points are usually relatively close, with the associated implications for the selection of optimal heat engine operating conditions. The limitations of past analyses of endoreversible cycles as models for real heat engines will be discussed and the fortuitous nature of agreement between their predictions and actual heat engine performance will be explained.
Characterization of boron carbide thin films fabricated by plasma enhanced chemical vapor deposition from boranes
Authors: Lee Sunwoo , Mazurowski John , Ramseyer G. , Dowben P. A.
Published: July 2002

We have fabricated boron carbide thin films on Si(111) and other substrates by plasma-enhanced chemical-vapor deposition (PECVD). The PECVD of boron carbides from nido-cage boranes, specially nido-pentaborane(9) (B5H9), and methane (CH4) is demonstrated. The band gap is closely correlated with the boron to carbon ratio and can range from 0.77 to 1.80 eV and is consistent with the thermal activation barrier of 1.25 eV for conductivity. We have made boron carbide by PECVD from pentaborane and methane that is sufficiently isotropic to obtain resistivities as large as 1010 Ω cm at room temperature. This material is also shown to be suitable for photoactive p-n heterojunction diode fabrication in combination with Si(111).
Primary study on the irradiation effects of high energy C+ and H+ on diamond-like carbon films
Authors: Wang Weijie , Wang Tianmin , Chen Buliang
Published: July 2002

Diamond-like carbon films, prepared by the dual-ion beam sputtering method on glass substrate, were irradiated by both carbon ions (C+) and hydrogen ions (H+) under the dose of 1×1017 ion/cm2 and the energy of 114 and 112 keV, respectively. The as-deposited and the irradiated films were characterized by electrical resistivity, infrared transmittance, and Raman spectroscopy. It showed that the irradiation of both the C+ and the H+ can lead to the decrease or even the removal of bond-angle disorder and the increase of the sp2 C-C bond dominated crystallite size and/or number. The irradiation can also break the sp3 C-H bond. But the irradiation of C+ can lead to a bigger decrease of sp3 bonds, forming a film containing more sp2 bonds, and the induced film was then more graphite-like. In irradiation of H+, however, the decrease degree of sp3 bonds is smaller and the irradiated film still showed evident diamond-like characteristics in electrical and optical properties. The different irradiation mechanisms of the two ions on the diamond-like carbon films are briefly discussed.
The structure–property relationships in M-type hexaferrites: Hyperfine interactions and bulk magnetic properties
Authors: Thompson G. K. , Evans B. J.
Published: July 2002

57Fe Mössbauer spectroscopic measurements have been made at 296 K on single crystals of MFe12O19 (M=Ba, Sr, Pb) oriented parallel and perpendicular to the c axis, permitting the establishment of the systematics of the static and dynamical aspects of the hyperfine interactions, and their relationship to crystal/chemical structures for the five Fe3+ sublattices, and to the bulk magnetic properties. With the exception of the electric quadrupole interaction at the 2b site, and the dependence of the 2b intensity on the crystal orientation, the magnitude of the hyperfine interactions of a given Fe3+ site exhibits only small variations among the different hexaferrites. The magnitude of the quadrupole interaction at the 2b site varies by more than 10%, with the 2b site in PbFe12O19 exhibiting the smallest value. The relative intensity of the 2b subspectrum varies markedly among the three hexaferrites for observations parallel and perpendicular to the c axis. Although all display the expected anisotropy resulting from the libration of the 2b Fe3+ parallel to the c axis, the anisotropy is considerably larger for PbFe12O19 than for BaFe12O19 or SrFe12O19. It is remarkable that the bulk magnetic anisotropy follows the same order as the anisotropy in the dynamical displacement and crystalline electric field of the 2b site.
Nonlinear piezoresistance effects in silicon
Authors: Matsuda Kazunori , Suzuki Katuhisa , Yamamura Kazuhisa , Kanda Yozo
Published: July 2002

Nonlinearity of the piezoresistance effects in p- and n-type silicon was measured at room temperature for three surface impurity concentrations and for three crystallographic orientations <100≳, <110≳, and <1̄1̄2≳. Piezoresistance coefficients up to third order in stress were obtained experimentally. Uniaxial stresses up to about 174 MPa were applied with currents flowing either parallel (longitudinal configuration) or perpendicular (transverse configuration) to them. A complete set of nine independent second-order tensor components was determined. The second-order piezoresistance coefficients in various configurations of uniaxial stress, current, and crystallographic orientation were calculated by the sixth rank tensor transformation. The nonlinear piezoresistance effect in n-type silicon was discussed with the many-valley model.
Growth mechanism of 6H-SiC in step-controlled epitaxy
Authors: Kimoto Tsunenobu , Nishino Hironori , Yoo Woo Sik , Matsunami Hiroyuki
Published: July 2002

Growth of SiC on off-oriented 6H-SiC{0001} substrates was performed between 1100 and 1500 °C. Homoepitaxial growth of 6H-SiC was achieved at temperatures as low as 1200 °C, utilizing step-flow growth. Twinned crystalline 3C-SiC was grown at 1100 °C; this result can be ascribed to suppressed surface migration of adsorbed Si species at the lower temperature, and to the occurrence of two-dimensional nucleation on terraces. The C/Si ratio significantly effected the surface morphology and growth rate. The growth rate was limited by the supply of Si species, where the growth rate activation energy was very small (3.0 kcal/mol), and little difference in the growth rates on Si and C faces was observed. The quantitative analysis revealed that growth of 6H-SiC in step-controlled epitaxy is controlled by the diffusion of reactants in a stagnant layer.
Theory of eddy current inversion
Authors: Norton Stephen J. , Bowler John R.
Published: July 2002

The inverse eddy current problem can be described as the task of reconstructing an unknown distribution of electrical conductivity from eddy-current probe impedance measurements recorded as a function of probe position, excitation frequency, or both. In eddy current nondestructive evaluation, this is widely recognized as a central theoretical problem whose solution is likely to have a significant impact on the characterization of flaws in conducting materials. Because the inverse problem is nonlinear, we propose using an iterative least-squares algorithm for recovering the conductivity. In this algorithm, the conductivity distribution sought minimizes the mean-square difference between the predicted and measured impedance values. The gradient of the impedance plays a fundamental role since it tells us how to update the conductivity in such a way as to guarantee a reduction in the mean-square difference. The impedance gradient is obtained in analytic form using function-space methods. The resulting expression is independent of the type of discretization ultimately chosen to approximate the flaw, and thus has greater generality than an approach in which discretization is performed first. The gradient is derived from the solution to two forward problems: an ordinary and an ‘‘adjoint’’ problem. In contrast, a finite difference computation of the gradient requires the solution of multiple forward problems, one for each unknown parameter used in modeling the flaw. Two general types of inverse problems are considered: the reconstruction of a conductivity distribution, and the reconstruction of the shape of an inclusion or crack whose conductivity is known or assumed to be zero. A layered conductor with unknown layer conductivities is treated as an example of the first type of inversion problem. An ellipsoidal crack is presented as an example of the second type of inversion problem.
Longitudinal elastic constants of tellurite glasses
Authors: El-Mallawany Raouf
Published: July 2002

The aim of the present work is to attempt to make a number of different glasses, based on tellurite as the principal former, which contain substantial quantities of zinc oxide up to 40 mol % ZnO. To study the longitudinal elastic properties, measurements have been made of the velocity of the longitudinal ultrasonic waves propagated through the prepared glasses. The results indicate that the longitudinal elastic constants depend strongly upon the composition of the prepared glasses. A detailed qualitative interpretation of the increase in the modulus is made. Information about the structure of the glass can be deduced after calculating the number of network bonds per unit volume and the crosslink density. The role of the halogen inside the glass network is discussed.
Theoretical models of the electrical discharge machining process. III. The variable mass, cylindrical plasma model
Authors: Eubank Philip T. , Patel Mukund R. , Barrufet Maria A. , Bozkurt Bedri
Published: July 2002

A variable mass, cylindrical plasma model (VMCPM) is developed for sparks created by electrical discharge in a liquid media. The model consist of three differential equations—one each from fluid dynamics, an energy balance, and the radiation equation—combined with a plasma equation of state. A thermophysical property subroutine allows realistic estimation of plasma enthalpy, mass density, and particle fractions by inclusion of the heats of dissociation and ionization for a plasma created from deionized water. Problems with the zero-time boundary conditions are overcome by an electron balance procedure. Numerical solution of the model provides plasma radius, temperature, pressure, and mass as a function of pulse time for fixed current, electrode gap, and power fraction remaining in the plasma. Moderately high temperatures (≳5000 K) and pressures (≳4 bar) persist in the sparks even after long pulse times (to ∼500 μs). Quantitative proof that superheating is the dominant mechanism for electrical discharge machining (EDM) erosion is thus provided for the first time. Some quantitative inconsistencies developed between our (1) cathode, (2) anode, and (3) plasma models (this series) are discussed with indication as to how they will be rectified in a fourth article to follow shortly in this journal. While containing oversimplifications, these three models are believed to contain the respective dominant physics of the EDM process but need be brought into numerical consistency for each time increment of the numerical solution.
Transient ballistic and diffusive phonon heat transport in thin films
Authors: Joshi A. A. , Majumdar A.
Published: July 2002

Ballistic and diffusive phonon transport under small time and spatial scales are important in fast-switching electronic devices and pulsed-laser processing of materials. The Fourier law represents only diffusive transport and yields an infinite speed for heat waves. Although the hyperbolic heat equation involves a finite heat wave speed, it cannot model ballistic phonon transport in short spatial scales, which under steady state follows the Casimir limit of phonon radiation. An equation of phonon radiative transfer (EPRT) is developed which shows the correct limiting behavior for both purely ballistic and diffusive transport. The solution of the EPRT for diamond thin films not only produces wall temperature jumps under ballistic transport but shows markedly different transient response from that of the Fourier law and the hyperbolic heat equation even for predominantly diffusive transport. For sudden temperature rise at one film boundary, the results show that the Fourier law and the hyperbolic heat equation can significantly over- or underpredict the boundary heat flux at time scales smaller than the phonon relaxation times.
Ultrasonic reflectivity technique for the characterization of fiber-matrix interface in metal matrix composites
Authors: Matikas Theodore E. , Karpur Prasanna
Published: July 2002

An ultrasonic plane wave reflected by a cylindrical fiber embedded in a homogeneous isotropic matrix is modeled. The model calculates the ‘‘back-reflection’’ coefficient by taking in to account the properties of the fiber and the matrix, the ultrasonic wavelength, the angle of incidence, and a coefficient called ‘‘shear stiffness coefficient’’ which characterizes the elastic behavior between the fiber and the matrix. Results obtained from the theoretical analysis for a model metal matrix composite system are shown. The theory developed in this paper and some of the results obtained are equally applicable in ceramic matrix fiber reinforced composites.
Processes in the prebreakdown stage of a low-pressure discharge and the mechanism of discharge initiation in pseudospark switches
Authors: Kozyrev A. V. , Korolev Yu. D. , Rabotkin V. G. , Shemyakin I. A.
Published: July 2002

A time-dependent model for the rapid current rise during initiation of the discharge in a pseudospark switch is presented. The following assumptions are made: (1) the gas is ionized only by electrons that originate at the cathode; (2) the cathode emissivity is infinite; (3) the average ionization cross section is constant; (4) the ion charge density is independent of gap position. The results are in qualitative agreement with a previous experimental study of a nitrogen-filled switch operating between 6 and 20 kV for a range of gas pressures. The model provides a straightforward approach for estimating pseudospark breakdown times.
Direct deposition of highly coercive gamma iron oxide thin films for magnetic recording
Authors: Dhara Sandip , Rastogi A. C. , Das B. K.
Published: July 2002

Direct preparation of high coercivity gamma iron oxide (γ-Fe2O3) thin films by chemical vapor pyrolysis techniques have been described. Films deposited at temperatures above 500 °C have the γ- and those deposited below 500 °C have the α-Fe2O3 phase. Gas phase reactions leading to oxygen deficiency in the vicinity of substrate and consequently controlled in situ oxidation of Fe3O4 appear responsible for direct growth of the γ-Fe2O3 film. Magnetic properties of these films are optimized by adding cobalt. High coercivity values of 3 kOe for directly deposited γ-Fe2O3 films containing 6 at. % of cobalt are obtained along with reasonable values of remanence ≊2.8 kG and squareness ratio of 0.83. These values are considerably superior for magnetic recording in comparison to those for γ-Fe2O3 films obtained by reduction and reoxidation of α-Fe2O3 films.
Effect of oxidation ambient on the dielectric breakdown characteristics of thermal oxide films of silicon
Authors: Murakami Yoshio , Shiota Takaaki , Shingyouji Takayuki , Abe Hidenobu
Published: July 2002

This study investigated the dielectric breakdown characteristics of thermal oxide films grown by dry and wet oxidation. Oxide films grown by wet oxidation have lower B-mode failure rates and higher B-mode breakdown fields. On the other hand, the reliability of C mode of oxide films does not differ consistently between films grown by dry and wet oxidation. These results indicate that as-grown defect causing B-mode failure may shrink or be reduced during wet oxidation.
Enhanced saturation of sputtered amorphous SiN film frameworks using He- and Ne-Penning effects
Authors: Sugimoto Iwao , Nakano Satoko , Kuwano Hiroki
Published: July 2002

Optical emission spectroscopy reveals that helium and neon gases enhance the nitridation reactivity of the nitrogen plasma by Penning effects during magnetron sputtering of the silicon target. These excited nitrogen plasmas promote the saturation of frameworks of the resultant silicon nitride films. X-ray photoelectron spectroscopy, electron spin resonance, and x-ray diffraction analyses provide insight into the structure of these films, and thermal desorption mass spectroscopy reveals the behavior of volatile species in these films.
Morphologies and photoluminescence of porous silicon under different etching and oxidation conditions
Authors: Lin Chi-Huei , Lee Si-Chen , Chen Yang-Fang
Published: July 2002

It is found from scanning electron microscopy that the porous silicon samples fabricated under different conditions exhibit different morphologies. Some of them show islands or cracks, but the rest appear to be smooth. It is proposed that the formation of the islands and the cracks depends on the porosity of the porous silicon samples and result from the shrinkage of the porous structure. The photoluminescence spectra of the porous silicon samples also depend on the etching conditions. The peak position of the photoluminescence shifts to higher energy with increasing the porosity of the samples or by rinsing the samples in deionized water for a long time. However, in certain situations the photoluminescence peak shifts to lower energy after the samples are placed in air for a couple of days. It is proposed that not only the quantum size effect but also the transition between oxide levels contribute to the photoluminescence of the porous silicon.
Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique
Authors: Guo Ruyan , Bhalla A. S. , Cross L. E.
Published: July 2002

The preparation of Ba(Mg1/3Ta2/3)O3 (BMT) ceramics and the first successful growth of BMT single crystals by the laser heated pedestal growth technique (LHPG) is reported. The single crystal has a simple cubic perovskite structure in comparison to the ordered hexagonal structure normally found in ceramics of the same phase. The dielectric properties of single crystals are examined, the dielectric Q value increases with the increase in B-site ordering, while the dielectric constant κ is relatively independent of ordering. Our results also show that BMT is a candidate substrate for high Tc superconductor thin films as it has one of the highest Q values known for the oxide perovskite family, along with its matching thermal expansion coefficient α=9.0×10−6/ °C and twin-free cubic perovskite structure with a=4.0877 Å. BMT single crystals grown by the LHPG technique are probably the highest melting oxide compounds grown to date.
Atomic-scale and microscale friction studies of graphite and diamond using friction force microscopy
Authors: Ruan Ju-Ai , Bhushan Bharat
Published: July 2002

Friction between graphite and diamond surfaces against a sharp silicon nitride tip was measured using a friction force microscope (FFM). Atomic-scale friction images of a freshly cleaved highly oriented pyrolytic graphite exhibited the same periodicity as that of the graphite surface; however, the peaks in friction profiles and those in corresponding topography profiles were displaced relative to each other. Using the Fourier expansion of the interaction potential, the conservative interatomic forces between the FFM tip and the graphite surface have been calculated. It is shown that the variations in atomic-scale friction and the observed displacement between the peaks in the frictional (or lateral) force and those in the corresponding topography can be explained by the variations in interatomic forces in the normal and lateral directions. Thus, the observed variation in friction force may not necessarily occur as a result of the commonly believed atomic-scale stick-slip process, but can be due to variation in the intrinsic lateral force between the sample and the FFM tip. At large scan sizes (50×50 nm2 or larger), the variation of friction for graphite and a single-crystal (IIa) diamond was found to correlate with the variations in the local slope of the sample surface, suggesting that a ratchet mechanism is responsible for variations in microscale friction.
Spatial distribution of trapped holes in SiO2
Authors: Khosru Quazi Deen Mohd , Yasuda Naoki , Taniguchi Kenji , Hamaguchi Chihiro
Published: July 2002

Spatial distribution of trapped holes and the position of charge centroid in the insulating layer of a metal-oxide-semiconductor (MOS) structure are determined after substrate hot-hole injection experiments using MOS transistors with very thin oxides. Considerable hole trapping is observed even in samples with extremely thin (∼4.6 nm) oxide film. Most of the trapped holes exist between 2–6 nm from the Si/SiO2 interface. Trapped holes are found to be distributed in the form of an exponentially decaying function of distance from the same interface. Charge centroid is determined to be located at 3.2 nm from the Si/SiO2 interface for the oxide thickness range used in this study.
Hydrogen catalyzed crystallization of strontium titanate
Authors: Simpson T. W. , Mitchell I. V. , McCallum J. C. , Boatner L. A.
Published: July 2002

The crystallization rate of amorphous strontium titanate is enhanced by more than an order of magnitude during thermal annealing in water vapor as compared to a dry ambient. Time resolved optical reflectivity (TRR) has been combined with Rutherford backscattering spectrometry (RBS) and ion channelling to investigate this effect. Thin amorphous films (0.6 μm) were produced on single-crystal substrates of (100) strontium titanate by bombardment with 1.9 or 2.0 MeV Pb ions. Specimens were annealed under controlled ambient conditions (H2O, D2O, vacuum, 265–430 °C) and the solid phase epitaxial crystallization monitored in situ by TRR (633 nm). The TRR data were calibrated ex situ by transmission electron microscopy and RBS measurements. Isotope substitution, nuclear reaction analysis, and secondary-ion-mass spectrometry were utilized to reveal the uptake of hydrogen and oxygen into the implanted layer. Hydrogen is identified as the only species which penetrates to the crystal/amorphous interface. It is shown that the crystallization rate is proportional to the concentration of diffusing hydrogen (H or D) at the interface. The data show that the effect of water vapor, or more precisely, hydrogen, is to reduce the activation energy of crystallization from 2.1 to 1.0 eV. It is concluded that hydrogen, provided by the dissociation of water molecules at the surface, is a catalyst in the crystallization of amorphous strontium titanate.
Transmission of electromagnetic waves in planar, cylindrical, and spherical dielectric layer systems and their applications
Authors: Ping Er-Xuan
Published: July 2002

Transmission of electromagnetic waves propagating in planar, cylindrical, and spherical single dielectric layer, double-barrier resonant tunneling and multiple period Bragg reflector systems have been systematically studied by the transfer matrix method. Interesting features are found in the transmission spectra of the systems with cylindrical and spherical geometries, due to the coherence of the propagation waves in these systems. In particular, an increase of the resolution power (sharper transmission peaks) in the double-barrier resonant tunneling system is demonstrated in the cylindrical and spherical structures compared to that of the planar structure. Universal behavior, that the transmission at zero frequency limit decreases substantially as the propagation wave changes from plane to cylindrical wave and to spherical wave, has been shown in the spectra of these systems. Potential applications in reducing the radiation energy loss for transferring large density alternative current are discussed.
Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys
Authors: Wei Su-Huai , Zunger Alex
Published: July 2002

Using first-principles band-structure theory we have systematically calculated the (i) alloy bowing coefficients, (ii) alloy mixing enthalpies, and (iii) interfacial valence- and conduction-band offsets for three mixed-anion (CuInX2, X=S, Se, Te) and three mixed-cation (CuMSe2, M=Al, Ga, In) chalcopyrite systems. The random chalcopyrite alloys are represented by special quasirandom structures (SQS). The calculated bowing coefficients are in good agreement with the most recent experimental data for stoichiometric alloys. Results for the mixing enthalpies and the band offsets are provided as predictions to be tested experimentally. Comparing our calculated bowing and band offsets for the mixed-anion chalcopyrite alloys with those of the corresponding Zn chalcogenide alloys (ZnX, X=S, Se, Te), we find that the larger p−d coupling in chalcopyrite alloys reduces their band offsets and optical bowing. Bowing parameters for ordered, Zn-based II-VI alloys in the CuAu, CuPt, and chalcopyrite structures are presented: we find that ordered Zn2SeTe has bowing coefficients of 1.44 and 3.15 eV in the CuAu and CuPt structures, while the random ZnSexTe1−x alloy has a bowing of 1.14 eV. The band alignment between CuInSe2 and CuInSe2-derived ordered vacancy compounds are also presented.
Thermodynamic analysis of a solar-driven thermoelectric generator
Authors: Chen Jincan
Published: July 2002

A thermodynamic model including four irreversibilities is used to investigate the optimal performance of a solar-driven thermoelectric generator. The efficiency of the system is derived and taken as an objective function for optimization. Some important curves, such as the efficiency of the system versus the operating temperature of the solar collector, the reduced current, and the load resistance, are obtained. These curves can reveal the performance characteristics of solar-driven thermoelectric generators. The maximum efficiency of the system and the corresponding reduced current are derived for some special cases. The optimal matching of load resistance which is one of the key problems in the optimal design of solar-driven thermoelectric generators is discussed. New bounds of the key performance parameters, such as the efficiency, power output, and load resistance, are determined.
Species and dose dependence of ion implantation damage induced transient enhanced diffusion
Authors: Chao H. S. , Crowder S. W. , Griffin P. B. , Plummer J. D.
Published: July 2002

The implant species and dose effects of ion implantation, including crossing the amorphization threshold, on the transient enhanced diffusion (TED) behavior of a boron marker layer in silicon have been studied. It has been found that for lower implant doses, TED is species independent. However, for higher implanted doses, the dependence of TED on species becomes very significant. It has been found that at these higher doses, including amorphizing doses, P implants cause more TED than either Si or As implants. This result is explained based on the fully coupled diffusion mechanism of the impurity dopants where dopants diffuse by temporarily pairing with point defects. Additionally, both point defect clusters and extended defects such as dislocations significantly affect the dopant profile evolution of both the implanted profile and the buried marker layer. By modeling these effects, the experimental results have been simulated and a consistent parameter set has been found to fit the data to a reasonable extent.
Electroluminescence of the oxide thin film phosphors Zn2SiO4 and Y2SiO5
Authors: Ouyang X. , Kitai A. H. , Xiao T.
Published: July 2002

Mn-activated Zn2SiO4 and Ce-activated Y2SiO5 multilayer thin film electroluminescent (EL) devices were prepared by rf magnetron sputtering. The EL response of the devices under different voltages, frequencies, and pulse widths, as well as the transferred charge and decay characteristics were studied. The EL device using Zn2SiO4:Mn as the phosphor layer is shown to achieve a brightness of over 200 cd/m2 at 400 Hz and a field of 3×106 V/cm (twice the threshold field) and a maximum efficiency of 0.78 lm/W, with a decay time of 0.6 ms. The main characteristics of the oxide phosphors include (a) strong trailing edge excitation in nonsymmetrical EL devices, (b) a narrow transferred charge loop, and (c) a time response sensitive to device structures. Compared with the nonsymmetrical structure, the more symmetrical double-insulated structure is shown to increase the efficiency of the EL device by generating strong EL excitation from both positive and negative voltage pulses.
Plasma immersion ion implantation model including multiple charge state
Authors: Qin Shu , Chan Chung , Jin Zhijiang
Published: July 2002

The multiple charge states of a plasma have been considered to derive an universal dynamic sheath model for the application of plasma immersion ion implantation (PIII) processes. A general form of an effective mass, M, which includes species compositions, charge states, and masses, is used to determine the sheath expansion during the pulse. The implant dose components and the final implant impurity profile can be predicted with this effective mass. This offers a method for easily evaluating PIII processing and obtaining the correct doping results. A plasma simulation code PDP1 is used to compare this model.
Phase modification by instantaneous heat treatment of Sb2S3 films and their potential for photothermal optical recording
Authors: Arun P. , Vedeshwar A. G.
Published: July 2002

The Sb2S3 films subjected to instantaneous heat treatment at various temperatures in air for a short duration of a few seconds show a systematic phase modification from an amorphous to a crystalline state. Structural and morphological analyses show an amorphous to crystalline transformation around 162 °C consistent with the DTA result. The compositional analysis (ESCA) shows an increasing loss of sulphur content in the film as the function of treatment temperatures. The optical properties reveal the changing optical constants and energy gap of the film with the treatment temperatures. The observed degree of optical contrast with heat-treatment temperatures indicates the possibility of photo-thermal optical storage in these films. The results of the real-time photo-thermal optical recording in these films compare well with those of the heat-treatment experiment. The homogeneity of the film over a large area, the stability of the modified phase, use of low laser power for recording, and the formation of a clean boundary of the recorded area, etc., indicate a possible use of Sb2S3 films for write-once-read-many times kind of optical storage applications.