bluuebunny/crossref_metadata_2025 huggingface dataset:
Undoped semi-insulating GaAs grown by a vertical Bridgman method: Electrical property analysis using a simple ambipolar correction
Authors: Tang R.-S. , Blakemore J. S. , Kremer R. E. , Burke K. M.
Published: July 2002

A simple procedure is described for numerical correction of conductivity and Hall-effect data in a (GaAs) sample which is near intrinsic or slightly on the n side of intrinsic, such that hole conduction affects the conductivity and Hall coefficient to a modest extent. The numerical procedure is based on the ratio of the measured Hall coefficient to that expected for intrinsic GaAs at the same temperature. One can thereby deduce the electron concentration n0 and the electron mobility, simply, but with reasonable accuracy. The method can be used for any temperature at which transport measurement are normally made with semi-insulating GaAs, and its use is demonstrated with data for three high-resistivity ‘‘undoped’’ samples from crystals grown by a vertical Bridgman method.
Intra-4f-shell luminescence excitation and quenching mechanism of Yb in InP
Authors: Takahei K. , Taguchi A. , Nakagome H. , Uwai K. , Whitney P. S.
Published: July 2002

The excitation and quenching mechanism of intra-4f-shell luminescence of Yb ions incorporated in an InP host is discussed. The discussion is based on experimental results of photoluminescence excitation spectra and photoluminescence time-decay curves for crystals grown by metalorganic chemical vapor deposition and liquid-phase epitaxy. The present as well as previously reported experimental results for samples grown by various methods can be consistently interpreted by considering a recently reported Yb-related acceptorlike electron trap near the conduction-band edge and by taking into account the shallow donor and acceptor as well as Yb concentrations in the samples. A model is proposed which describes the Yb intra-4f-shell excitation and quenching mechanisms for samples with various concentrations of the impurities.
Comparison of the properties of hydrogenated microcrystalline silicon films deposited by photo–chemical-vapor deposition and glow-discharge deposition processes
Authors: Dutta Joydeep , Unaogu A. L. , Ray Swati , Barua A. K.
Published: July 2002

Hydrogenated microcrystalline silicon (μc-Si:H) films were prepared by mercury-sensitized photo–chemical-vapor deposition (photo-CVD) and glow-discharge (GD) processes from a mixture of silane and hydrogen. The variation of the growth rate, crystallinity, conductivity, and absorption coefficient with the substrate temperature, chamber pressure, and power density were studied. Highly conducting films having dark conductivity (σD)∼1×10−3 S cm−1 and ∼8.7×10−3 S cm−1 were obtained by the photo-CVD and GD deposition processes, respectively. The merits of the photo-CVD method over the GD process for the deposition of μc-Si:H films for solar-cell application are highlighted.
A novel processing technique for fabrication of flexible YBa2Cu3O7−x wire
Authors: Samanta Shyam K. , Samajdar S. , Durrant W. , Gupta M.
Published: July 2002

A fabrication technique for producing flexible superconductor wire by extrusion of a composite of Ag2O-YBa2 Cu3 O7−x is reported. During the course of processing, the superconductive transition of the starting powder is maintained, thus obviating the need for subsequent heat treatment. This nonpoisoning behavior of Ag2 O is of significant technical importance. Scanning electron fractography reveals a mixed mode fracture behavior of the material and the evidence of ductility. Magnetic measurements on the extruded wires show an onset temperature of 91 K. Resistivity measurements indicate that the electrical resistance of the as-extruded wire essentially vanishes at liquid-nitrogen temperature.
Molecular-beam epitaxial growth of high-quality InSb on InP and GaAs substrates
Authors: Oh J. E. , Bhattacharya P. K. , Chen Y. C. , Tsukamoto S.
Published: July 2002

Epitaxial layers of InSb have been grown in InP and GaAs substrates by molecular-beam epitaxy. The dependence of the epilayer quality on flux ratio, JSb4/JIn , was studied. Deviation from an optimum value of JSb4/JIn (∼2) during growth led to deterioration in the surface morphology and the electrical and crystalline qualities of the films. Room-temperature electron mobilities as high as 70 000 and 53 000 cm2/V s were measured in InSb layers grown on InP and GaAs substrates, respectively. Unlike the previous results, the conductivity in these films is n type even at T=13 K, and no degradation of the electron mobility due to the high density of dislocations was observed. The measured electron mobilities (and carrier concentrations) at 77 K in InSb layers grown on InP and GaAs substrates are 110 000 cm2/V s (3×1015 cm−3) and 55 000 cm2/V s (4.95×1015 cm−3), respectively, suggesting their application to electronic devices at cryogenic temperatures.
Heat-treatment-induced defects in low-resistivity silicon
Authors: Dannefaer S. , Puff W. , Mascher P. , Kerr D.
Published: July 2002

Czochralski-grown silicon has been investigated with doping levels up to 5×1018 cm−3 of boron or phosphorus by positron annihilation. For the highly boron-doped samples vacancies could be created upon heat treatment in the 700–1000 °C range up to a concentration of 1017 vacancies cm−3. Reducing either the boron concentration or the interstitial oxygen concentration (by prolonged heat treatment at 1200 °C) reduced the amount of created vacancies. No vacancies were detected in highly phosphorus-doped materials. Combined positron-lifetime and Doppler-broadening measurements indicate that the vacancies are essentially of monovacancy nature and have impurities close by which contribute with high-momentum electrons. Isothermal heat treatments at 700, 750, 900, and 950 °C indicate a reaction of the form X+Y⇄Z, where Z contains a vacancy. The forward reaction has an activation enthalpy of 2.7 eV, while the back reaction has the value 3.0 eV. It is suggested that X corresponds to substitutional boron and Y to interstitial oxygen.
Velocity profiles of species ejected in ultraviolet laser ablation of several polymers examined by time-of-flight mass spectroscopy
Authors: Hansen S. G.
Published: July 2002

Velocity distributions of molecular species ejected by ∼80 mJ/cm2, 266-nm laser ablation of polycarbonate, polyimide, poly(ethylene terephthalate), and poly(α-methylstyrene) are presented and discussed. Time-of-flight mass spectroscopy in conjunction with both 248- and 193-nm laser ionization was used to probe the escaping vapor. Up to three distinct waves of material pass through the ionization zone. The fastest wave (6–8×105 cm/s) appears to consist of highly degraded species such as C3; the arrival profiles are well fit by a velocity offset Maxwell–Boltzmann distribution with offsets typically 3–6×105 cm/s and transverse temperatures above 10 000 K. The second wave has a characteristic velocity of 1–2×105 cm/s, and, except with the poly(α-methylstyrene) target, the associated material is not cleanly ionized to parent ions under our typical conditions. It is hypothesized that this wave consists of hot, fairly heavy (up to a few hundred amu) radicals. The slow wave (2–5×104 cm/s) is composed of stable molecules which do not readily condense on the chamber walls. Its arrival profile is too broad to be described by a simple Maxwell–Boltzmann velocity distribution. A mechanism involving a thermal velocity distribution combined with laser-associated background vapor might explain the broad profiles. Problems related to the largely unknown and highly variable ionization cross sections of diverse organic molecules with 193- and 248-nm light are briefly discussed.
Transport properties of iron-bismuthate glassy semiconductors
Authors: Ghosh Aswini
Published: July 2002

Iron-bismuthate glassy semiconductors were prepared in a range of compositions and their electrical transport properties were measured in the temperature range of 100–500 K. The electrical data were analyzed in the light of the models of polaronic hopping conduction. The analysis showed that the high-temperature conductivity was well explained by the polaronic models. But these models failed to account for the decrease of the activation energy with the decrease of temperature. At lower temperatures, a variable range analysis was made. This analysis provided reasonable values of the decay constant for the localized states and also for the density of states at the Fermi level.
The effect of anodization on the electromigration drift velocity in aluminum films
Authors: Ross C. A. , Drewery J. S. , Somekh R. E. , Evetts J. E.
Published: July 2002

The electromigration drift velocity has been measured, using the Blech–Kinsbron [Thin Solid Films 25, 327 (1975)] edge displacement method, for aluminum thin films under different thicknesses of anodization. The drift velocity is found to decrease with increasing anodization thickness. The results are interpreted in terms of a change in the self-diffusivity of aluminum as a result of the compressive stresses imposed by the anodized layer. The effect of the variation of diffusivity with stress on the stress distribution within a drifting thin-film sample is discussed.
Reactive ion etching induced damage in GaAs and AlGaAs using C2H6/H2/Ar or CCl2F2/O2 gas mixtures
Authors: Pearton S. J. , Chakrabarti U. K. , Hobson W. S.
Published: July 2002

Changes in the near-surface electrical properties of n-type (n=1×1017 cm−3) GaAs and AlGaAs after reactive ion etching in C2H6/H2/Ar or CCl2F2/O2 discharges (4 mTorr, 0.85 W cm−2) were investigated by current-voltage (I-V) and capacitance-voltage measurements on Schottky diodes. Carrier reductions of approximately an order of magnitude were observed immediately after etching GaAs and AlGaAs in ethane-hydrogen-argon; much smaller changes (∼20%) were observed using freon-12–oxygen. For both gas chemistries, annealing in the range 200–300 °C produced the most ideal I-V characteristics in GaAs, whereas 300–400 °C was required for AlGaAs. Replacing H2 by D2 allowed high sensitivity atomic profiling using secondary ion mass spectrometry. Permeation of D2 to depths of ∼0.5 μm is observed in both GaAs and AlGaAs after etching—the D2 diffuses rapidly around 400 °C where dopant reactivation occurs.
Transport and interaction of a relativistic electron beam in low pressure neutral gases
Authors: Iyyengar S. K. , Rohatgi V. K.
Published: July 2002

A numerical study of the transport of a 0.27-MeV, 6.6-kA, 40-ns relativistic electron beam in argon and hydrogen in the pressure range of 0.01–1.0 Torr taking into account charge and current neutralization effects is presented. Ionization by avalanching and by beam and plasma electrons is included in the calculation of plasma density buildup. Plasma heating resulting from return current heating and two-stream instability is taken into account. The computed results of charge transport, net current, and breakdown time are compared with experimental results obtained in this laboratory. The results are in reasonable agreement with the experiment and show a maximum charge transport of 75% at the optimum pressure of 0.1 and 0.6 Torr in argon and hydrogen, respectively. The calculations indicate beam-generated plasma parameters of 1019–1020 m−3 density and 1–5 eV electron temperature.
Morphological changes of the surface structure of polymers due to excimer laser radiation: A synergetic effect?
Authors: Bahners Thomas , Schollmeyer Eckhard
Published: July 2002

Excimer-laser-induced ablation of polymers is often accompanied by a modification of the surface morphology at the irradiated regions. Only very recently models have been proposed for this effect, the origin being searched in different absorption properties of crystalline and amorphous material. A study by the authors on synthetic fibers irradiated by the UV laser is in contrast with that assumption and gave evidence of a thermal contribution to laser-polymer interaction. Furthermore, a strong influence of internal or external stress fields is found. On this basis a synergetic understanding of the surface structures is proposed.
High T
c Y-Ba-Cu-O thin films on Si substrates
Authors: Chromik Š. , Sith J. , Štrbík V. , Schilder J. , Šmatko V. , Beňačka Š. , Kliment V. , Levársky J.
Published: July 2002

A low temperature process of vacuum codeposition at 550 °C was used to prepare superconducting Y-Ba-Cu-O thin films on Si substrates without a buffer layer. A zero resistance critical temperature Tce as high as 81.5 K was reached without further high-temperature post-annealing treatment. The surface morphology of films has a granular or porous character dependent on substrate temperature. The atomic emission spectroscopy shows the diffusion of Si from the substrate into the films at the level of several atomic percent.
Ultraviolet laser-assisted metalorganic chemical vapor deposition of GaAs
Authors: York P. K. , Eden J. G. , Coleman J. J. , Fernández G. E. , Beernink K. J.
Published: July 2002

The growth of GaAs irradiated with ultraviolet laser light in a metalorganic chemical vapor deposition reactor has been investigated. Growth rate enhancements of up to 15% were observed at 450 °C by illuminating the substrate with no more than 13 mJ/cm2 of KrF laser (248 nm) radiation. For 5-eV photons, arsine is virtually transparent, while the trimethylgallium (TMG) photoabsorption cross section is approximately 10−19 cm2. Data acquired with and without the optical beam impinging on the substrate are well described by the Langmuir–Hinshelwood model, and the results point to photodissociation of adsorbed TMG as the origin of the growth rate enhancement. Nearly identical experiments carried out at 351 nm (XeF) corroborate this conclusion since no measurable increase in growth rate was observed at this wavelength, where both arsine and TMG photoabsorption is negligible. Conversely, significant improvement in surface morphology for samples grown below 700 °C is observed with ultraviolet laser irradiation of the substrate at each of the wavelengths investigated (193, 248, and 351 nm). Smooth and specular surfaces are obtained with substrate temperatures as low as 550 °C and at fluences well below those which induce a significant rise in the surface temperature.
A note on the Ericsson refrigeration cycle of paramagnetic salt
Authors: Yan Zijun , Chen Jincan
Published: July 2002

It is pointed out that the Ericsson refrigeration cycle of a simple paramagnetic salt cannot possess the condition of perfect regeneration.
Sum rule for multiple scattering of light by neighboring dielectric spheres
Authors: Chen Tuan W.
Published: July 2002

It is pointed out that the eikonal scheme suggests a powerful sum rule which relates the end-on light scattering by a system of N dielectric spheres to scattering by a single sphere. The sum rule is tested numerically for two dielectric spheres and shown to work for wide ranges of size parameter and refractive index.
Magnetovolume effects and structural transformations in CoMn alloys
Authors: John C. , Zähres H. , Acet M. , Stamm W. , Wassermann E. F. , Pepperhoff W.
Published: July 2002

Co100−xMnx alloys form a system with competing magnetic interactions in the concentration range 25≤x≤42 at. %. They have ferromagnetic ground states in the fcc and hcp structures for 0≤x≤25 at. % and antiferromagnetic ground states in the fcc structure for 42≤x≤52 at. %. The thermal expansion of this alloy system has been measured between 4 and 1200 K and positive magnetovolume effects have been observed in samples that have both ferromagnetic and antiferromagnetic ground states.
Magnetic-field-induced alignment in melt-spun Pr2Co14B
Authors: Otani Yoshichika , Sun Hong , Coey J. M. D. , Davis H. A. , Manaf A. , Buckley R. A.
Published: July 2002

Amorphous melt-spun Pr2(Fe1−xCox)14B has been prepared to test the concept of magnetic-field-induced crystallite alignment. Crystallization temperatures (Tx) of alloys with x=0.5, 0.6, and 1.0 were determined to be 622, 646, and 507 °C, respectively. These temperatures are all lower than the Curie temperatures (Tc) of the crystallized 2:14:1 phase. The glasses were then heat treated at a temperature Tt(Tx<Tt<Tc), either in no field or in a magnetic field of 0.3 T applied parallel to the long edge of the ribbon. The degree of alignment was investigated by means of x-ray diffraction and magnetization measurements. A texture was found where the c axes tend to lie in the ribbon plane in zero field, which is slightly modified when a magnetic field is applied during the heat treatment.
The hydrogen decrepitation behavior of alloys and magnets based on Nd16Fe76B8
Authors: McGuiness P. J. , Ahmed A. , Jones D. G. R. , Harris I. R. , Burns S. , Rozendaal E.
Published: July 2002

The hydrogen decrepitation (HD) behaviors of various forms of the permanent magnet alloy Nd16Fe76B8 and stoichiometric composition Nd11.8Fe82.3B5.9 have been investigated to provide background information on the production of sintered magnets by the HD process. The influence of the initial microstructures of the alloys on the subsequent HD behavior has been examined. Thus the as-cast ingots, homogenized ingots, and sintered magnets have been exposed to hydrogen at 2 bars of pressure at room temperature and the hydrogen absorption behaviors investigated by differential thermal analysis. The morphologies of the HD powders have been studied by scanning electron microscopy and have been related to the initial microstructures.
Flux lattice behavior in high-T
c materials studied by neutron depolarization
Authors: Crow M. L. , Goyette R. J. , Nunes A. C. , Pickart S. J. , McGuire T. R. , Shinde S. , Shaw T. M.
Published: July 2002

The depolarization of a neutron beam passing through a sample of the high-Tc superconductor YBa2Cu3O7−δ has been measured as a function of temperature and applied field. The difference in behavior between field-cooled and zero-field-cooled states, the observation of hysteresis correlated with Hc1, and the disappearance of the effect near 55 K (below Tc) suggest an explanation in terms of vortex line lattice formation with possible connection to recently proposed flux line entanglement and melting.
Conduction-band discontinuities of InxAl1−xAs/In0.53Ga0.47As n-isotype heterojunctions
Authors: Lee P. Z. , Lin C. L. , Ho J. C. , Meiners L. G. , Wieder H. H.
Published: July 2002

The conduction-band offsets ΔEc of n-isotype InxAl1−xAs/In0.53Ga0.47As heterojunctions, with 0.47<x≤0.52, grown by molecular-beam epitaxy, lattice matched and pseudomorphically strained on n+-InP substrates, were determined from the capacitance-voltage profiling technique. A more accurate ΔEc value was obtained after the correction for fixed interface charge perturbation. The ΔEc related to the band-gap difference ΔEgs of the heterojunction are found to be ΔEc=0.70ΔEgs. The fixed interface charges are acceptorlike with a density of the order 1010/cm2.
Photoluminescence and laser action of Hg1−xCdxTe (x≊0.5) layer grown by liquid-phase epitaxy
Authors: Ravid A. , Zussman A.
Published: July 2002

Photoluminescence spectra, carrier lifetime, and laser action of Hg1−xCdxTe (x=0.5) layers grown by liquid-phase epitaxy on a CdZnTe substrate were studied as a function of temperature. The minority-carrier lifetime was measured from the recovery of the luminescence signal following pulsed photoexcitation. Photoluminescence spectra of the annealed sample exhibited two lines, 15 meV apart, whereas in an as-grown film one broad line was observed. The transition mechanisms determining the temperature variation of the peak energy and the linewidth are discussed. The luminescence intensity increased rapidly from 300 to 50 K, but leveled off at low temperatures suggesting carrier freezeout in accord with resistivity measurements. Lasing was observed from a cleaved stripe of the wafer using photopumping with a pulsed GaAs laser. The nominal threshold power density increased from 37 W/cm2 μm at 12 K to 380 W/cm2 μm at 130 K, in agreement with threshold calculations. Far-field pattern widths perpendicular and parallel to the film plane of 3.2° and 2.8°, respectively, were measured. Lateral guiding in this device is discussed. The laser wavelength decreased slowly with the photopumping power, suggesting gain pinning.
High-speed photography of impact effects in three-point bend testing of polymers
Authors: Dear J. P.
Published: July 2002

The study presented in this paper relates mostly to impact testing of polymers and similar materials to measure their crack resistance and dynamic properties. For some impact testers, then, at low impact velocities the initial transient forces can be very small and short lived so as to be insignificant. This is so that the specimen mostly experiences a steadily increasing strain until it fails. At higher impact velocities both transient and generally increasing strain are both significant in taking the specimen to its failure point. With very high impact velocities, the specimen can fail mostly due to the initial impact forces. It is possible in some cases to cushion the specimen from the initial higher impact transients and related secondary effects such as bounce at the contact points. This is if a steadily increasing strain to fail the specimen is the main requirement. Of course, this is only feasible when the impact forces are not too dominant. However, as material technology has rapidly advanced so dynamic as well as toughness properties of materials have much improved. Also, the variety and mix of properties now available in different materials has greatly increased. With these trends, testing that can combine well impact, strong dynamic forces and steadily increasing strain loading of the specimen is of increasing interest. This can be so that the force-time curve of such testing to failure of a specimen can also be regarded as its dynamic performance signature. A problem is in arranging and monitoring the dynamic forces of such tests so as to be able to analyze effectively test results. These are the factors examined in this paper.
Effect of carbon on oxygen precipitation in silicon
Authors: Sun Q. , Yao K. H. , Lagowski J. , Gatos H. C.
Published: July 2002

A systematic Fourier-transformed infrared-spectroscopy study of oxygen and carbon in isolated form and as complexes in the silicon lattice has revealed a direct correlation between the decrease of substitutional carbon concentration and the decrease of interstitial oxygen concentration during 750 °C annealing. At a concentration exceeding 2 ppma, carbon was also found to enhance oxide precipitate growth. After completing a three-step annealing (1100 °C+750 °C+1000 °C), an oxide-precipitate-related IR-absorption band was observed. The changes of the IR-absorption band were correlated with annealing-induced changes in the state of carbon. A direct incorporation of carbon into oxide precipitates, and/or carbon interaction with silicon self-interstitials generated during oxygen precipitation, are suggested to have an effect on reducing lattice strain associated with the oxygen-precipitation process.
Interactions of deuterium with ion-irradiated SiO2 on Si
Authors: Myers S. M. , Richards P. M.
Published: July 2002

The interactions of deuterium with ion-irradiated SiO2 on Si were investigated during annealing in D2 gas and vacuum over the temperature range 373–1273 K. The uptake and release of D by the oxide layer were monitored using the ion-induced nuclear reaction D(3He,p)4He. Results were analyzed in terms of a new mathematical model which describes the diffusion of atomic D and molecular D2 in conjunction with trapping at irradiation defects via SiD and OD bonding. The combined body of experimental data on release is quantitatively described by the model using fitted binding enthalpies of 2.8 eV for the Si-D interaction and 3.2 eV for the O-D coupling. The model predicts a relationship between the activation barriers for trapping and detrapping which is in semiquantitative agreement with the experimental data on uptake. The same relationship is obeyed by independently reported results on the hydrogen passivation and reactivation of Si-dangling-bond defects at the SiO2-Si interface.
Flow and temperature fields in a weld pool formed by a moving laser
Authors: Babu V. , Korpela Seppo A. , Ramanan Natarajan
Published: July 2002

In this paper a two-dimensional model of fluid and heat flow in weld pools is considered and a solution methodology is presented that is based on a perturbation expansion with the Prandtl number as a small parameter. This allowed us to determine the temperature field efficiently and by using it to determine the location of the phase front to map the molten region into a semicircular region in which the Boussinesq form of the Navier–Stokes equations were solved. Doing so allows the domain to be represented with a much smoother phase boundary than is usual in phase change problems. The flow in the weld pool made with a moving laser shows vigorous convection on the solidification side of the weld pool and crowding of the isotherms on the melting side.
I
n s
i
t
u deposition of BaF2 as a buffer layer and the superconducting thin films of Y-Ba-Cu-O on silicon substrates by metalorganic chemical vapor deposition
Authors: Singh R. , Sinha S. , Hsu N. J. , Chou P.
Published: July 2002

High throughput, sharp interfaces, and selective deposition with direct ion-, electron-, and photon-beam-controlled techniques are some of the key driving forces for the development of superconducting thin films by the metalorganic chemical vapor deposition technique. In this paper, we report on the in situ deposition of a buffer layer of BaF2 and high-temperature superconducting thin films of Y-Ba-Cu-O by metalorganic chemical vapor deposition (MOCVD) on silicon substrates. These films have an on-set temperature of 90 K and zero resistance at 73 K. The use of BaF2 as a buffer layer on Si substrates suggests the possibility of three-dimensional integration with high-temperature superconducting thin films, for hybrid superconductor/semiconductor devices as well as superconducting switches and other related devices. To the best of our knowledge, this is the first report of the deposition of high-temperature superconducting thin films on Si by MOCVD.
Some observations of the effects of high pressures and temperatures on the stability of YBa2Cu3O7−x
Authors: Williams R. K. , Alexander K. B. , Brynestad J. , Henson T. J. , Kroeger D. M. , Lindemer T. B. , Marsh G. C. , Scarbrough J. O.
Published: July 2002

Hot isostatic pressing and heat treatments in high-pressure oxygen were used to study the effect of pressure on the stability of YBa2Cu3O7−x. At 875 °C the compound decomposes at an oxygen pressure of about 7 bars. The decomposition involves formation of Y2BaCu3O5 and a Ba-Cu oxide phase. Formation of the latter phase involves additional oxidation because this phase can contain as many as 1.5 oxygen atoms per barium atom. The decomposition can most simply be visualized by considering the chemical reaction 2(YBa2Cu3O7−x)+( (1)/(4) +x−y/2)O2→Y2BaCuO5+Ba3Cu5O9.5−y. Encapsulated samples also decompose during hot isostatic pressing, but the process appears to be more complex. The compound YBa2Cu4O8 was detected in addition to the two products observed in oxygen-treated samples. The reaction above, with YBa2Cu3O7−x serving as an oxygen source, aids in understanding the microstructural observations for encapsulated samples. Decomposition can be completely reversed by annealing in air for 20 h at 875 °C.
Nonrandom ceramic superconductor-metal composites
Authors: Reich S. , Felner I.
Published: July 2002

The preparation of a nonrandom silver YBCO composite is presented. Transport and magnetic properties of these nonrandom metal superconductor composites (NRMSC) were measured. It is shown that these composites exhibit a transition to the superconducting state in transport measurements for sintering temperatures as low as 600 °C, and exhibit critical current values higher than the pure YBCO ceramic material. The new NRMSC materials exhibit hardness properties similar to those of ductile metals.
Pt2Si formation: Diffusion marker and radioactive silicon tracer studies
Authors: Wandt M. A. E. , Comrie C. M. , McLeod J. E. , Pretorius R.
Published: July 2002

The moving species during the formation of first-phase platinum silicide, Pt2Si, by thermal annealing is identified with inert markers (Ti, Co, Ge, As) and radioactive 31Si as a tracer. Rutherford backscattering spectrometry is utilized to monitor the flow of atoms past the marker during the silicide forming reaction, while the position of the tracer after the reaction is determined by using sputter depth profiling and radioactivity measurements. Experiments with thin-film structures employing a reference marker at the substrate silicon/amorphous silicon interface and a mobile marker near the amorphous silicon/platinum interface clearly show a shift of the latter marker towards the surface of the sample. The radioactive tracer, initially embedded in nonradioactive silicon and metal, is moved from this position and concentrates at the sample surface. The outcome of both marker and tracer studies is consistent with a picture in which platinum diffuses during the formation of Pt2Si.
Population inversions behind normal shock waves in CO2-N2-He mixtures seeded with solid particles
Authors: Reddy K. P. J.
Published: July 2002

The propagation of a normal shock wave into the CO2-N2-He gas mixture seeded with solid particles is analyzed. The translation-vibration and vibration-vibration molecular energy exchange processes in the gas mixture are included in the analysis. The results indicate the creation of population inversions due to these processes in the relaxation region behind the shock wave for transitions at 50 and 22 μm in CO2 molecules. The presence of solid particles in the flow is found to be advantageous for observing the laser radiation behind the shock wave. Effects of the frozen shock Mach number, the particle size, the particle-to-gas mass loading ratio, and the laser gas mixture composition on the population inversions are discussed.
The effect of background doping and dose on diffusion of ion-implanted tin in gallium arsenide
Authors: Allen E. L. , Deal M. D. , Plummer J. D.
Published: July 2002

The diffusion of ion-implanted tin in gallium arsenide has been investigated as a function of implant dose and background doping. Secondary ion mass spectroscopy and electrochemical etch profiling were used to obtain chemical and carrier profiles. Based on the shapes of the concentration profiles and the background doping dependence, we have concluded that tin diffusivity is dependent on the electron concentration. This implies that Sn diffuses via negatively charged gallium vacancies. Tin diffusivity outside the implanted region is independent of dose, with an activation energy of 1.98 eV.
Formation of periodic structures during excimer laser-assisted heteroepitaxy of GaP
Authors: Sudarsan U. , Solanki R.
Published: July 2002

Periodic structures have been obtained during nominally unpolarized excimer laser (ArF)-assisted organometallic vapor phase epitaxy of GaP on GaAs. Both crystalline properties and chemical composition of these structures have been examined. The chemical analysis showed a variation in composition across a ripple which was attributed to the modulated thermal profile due to interference.
Silicide formation by reaction of Ta-Ti thin films and a Si single crystal
Authors: Dahan R. , Pelleg J. , Zevin L.
Published: July 2002

The formation of a ternary phase (TiTa)Si2 has been observed in a bulk system of Ti-Ta-Si which has been prepared by arc melting and annealing at high temperature for extended time. Before annealing, two separate phases were found by x-ray diffraction to coexist, TaSi2 and TiSi2. This alloy served for comparison of the tendency to form the ternary compound in a binary Ti-Ta layer configuration also. This geometry was achieved by depositing individual Ta and Ti layers consecutively on Si single crystals. A trend to form the ternary silicide was observed at sufficiently high temperatures. This phase is isomorphous with TaSi2. The slow process of its formation, i.e., the high temperature needed for the formation of the ternary phase is explained on the basis of the different diffusion rates of Si through TaSi2 and the interdiffusion in the Ta-Ti system. It seems very probable that the effect of oxygen in slowing down the progress of the ternary formation, at least in our experiments, might be crucial.
Effects of surface preparation on ion implantation in GaAs
Authors: Dobrilla P.
Published: July 2002

The influence of different polishing methods on the atomic profiles obtained by direct ion implantation in semi-insulating GaAs has been investigated, along with the effects of preimplantation etching. The results indicate that chem-mechanical polishing technologies, contrary to chemical only polishes, produce residual damage that results in shallower and more sharply peaked profiles. Polishing related damage, however, may not be the factor preventing a good electrical activation of implanted species. The acidity of the polishing fluid seems more important in determining the activation. Also, the dose implanted in uncapped, damaged wafers seems to be different, and generally lower, than the dose measured by the implanter.
Modal structure of the Čerenkov second-harmonic wave in optical fibers
Authors: Hayata K. , Sugawara T. , Koshiba M.
Published: July 2002

An analytical wave-optics solution is derived for the modal structure of the Čerenkov second-harmonic optical field in weakly guiding cylindrical fiber waveguides. Numerical results for the axially symmetric mode indicate that the second-harmonic power significantly depends on the dimension of the core.
Microstructure of mechanically alloyed Fe78Mo10Sm12 magnets with the ThMn12 structure
Authors: Koestler C. , Schulz L. , Thomas G.
Published: July 2002

The ThMn12-type Fe78Mo10Sm12 magnet has been characterized by high-resolution electron microscopy, x-ray microanalysis, and microdiffraction. In general, the microstructure of the mechanically alloyed and annealed magnets consists of the tetragonal 1:12 matrix phase with an average matrix grain diameter of 200–300 nm and two rare-earth oxide phases, the SmO and the Sm2O3 phases. Microdiffraction data clearly verify the fcc structure of the single and polycrystalline SmO grains, while the Sm2O3 phase was found to exist only in a single-crystal hcp and monoclinic structure. In this material, no grain boundary phases at multigrain junctions were observed. The high defect concentration in the matrix grains seems to be responsible for the pinning of the domain walls and the good magnetic properties of the Fe78Mo10Sm12 magnets. However, it was not possible to resolve magnetic domains by Lorentz imaging methods.
Surface reactivity and interface morphology for Ti growth on YBa2Cu3O7−x, Y2BaCuO5, and CuO
Authors: Meyer H. M. , Weaver J. H. , Goretta K. C.
Published: July 2002

X-ray photoemission results make it possible to compare Ti adatom reactivity with clean surfaces of CuO, Y2BaCuO5, and YBa2Cu3O7−x. Changes in the Ti 2p1/2,3/2 and O 1s core level emission during overlayer growth are related to surface reactions between the Ti adatoms and oxygen atoms leached from the substrates. The results show the growth of a TiO2-like reaction species at initial Ti depositions, the formation of Ti suboxides at intermediate coverages, and the onset of metallic behavior at higher Ti coverages. The Ti-O reaction products grow in a layer-by-layer fashion initially, but Ti metal overlayer growth occurs as clusters on this Ti-O reacted region. Substrate modifications associated with Ti-O reactions result in significant changes in the chemical environments of Cu, Ba, and Y for Y2BaCuO5 and YBa2Cu3O7−x.
Conductivity enhancement of poly-ether-ether-ketone by ion implantation
Authors: Bedell C. J. , Sofield C. J. , Bridwell L. B. , Brown I. M.
Published: July 2002

Amorphous poly-ether-ether-ketone (PEEK) films have been implanted with a variety of ions (He, N, F, As, Xe and I) in the energy range 50 keV to 32 MeV. At the lower end of this range, the dependence of the electrical conductivity of the PEEK on the dose and ion species has been explained in terms of a simple model of electronic and nuclear excitation effects. Implantations in the MeV energy range yielded a surface layer on the PEEK with a high conductivity [up to 2.5 (Ω cm)−1] and a moderate hardness (320 knoop, 1-g load). Evidence for diffusion of iodine implanted at the highest energy has been found. The role of the uniform iodine concentration throughout the implanted layer in the prevalent conduction mechanism is not known at present.
Deuterium permeability in gold determined using ion beams
Authors: Myers S. M. , Swansiger W. A.
Published: July 2002

The permeability of deuterium (D) in Au was determined by first ion implanting reactive oxygen sinks and then using the nuclear reaction D(3 He,p)4 He to monitor D accumulation in the sinks during D2 exposure. Results were obtained for temperatures of 573 and 373 K. Previous measurements of the lattice permeability using standard foil-permeation methods extended down only to 523 K, and consequently the present ion-beam study expanded the range of available permeability data by 5 orders of magnitude. We also carried out standard foil-permeation experiments in the temperature range 688–768 K for purposes of comparison with earlier work. When combined with previously published data, our results indicate that the permeability of D in Au has an Arrhenius temperature dependence over 10 orders of magnitude from 373 to 913 K.
Wave reflection at a mixed interface between two solid half-spaces
Authors: Léomy F. , de Billy M. , Quentin G. , Molinero I , Lecuru D.
Published: July 2002

The reflectivity of incident longitudinal and transversal waves at the interface between two solid half-spaces is studied for different degrees of bonding. Numerical calculations based on Murty’s assumptions [G. S. Murty, Geophys. J. R. Astron. Soc. 44, 388 (1976)] are compared with experimental measurements. The agreement between theory and experiments is reasonably good for RHHTT and RLL coefficients. The plots obtained for RVVTT reveal the existence of an interface wave, the theoretical angular position of which coincides with the critical angle calculated by assuming that this interface wave is a type of Rayleigh wave or interface wave, depending on the acoustical parameters.
Properties of semi-insulating GaAs grown by a vertical molten zone method
Authors: Tang R.-S. , Sargent L. , Blakemore J. S. , Swiggard E. M.
Published: July 2002

Electrical and optical measurements are reported for samples from two undoped semi-insulating GaAs crystals grown by a vertical molten zone method. The electrical data, taken over the range 290–420 K, included results for samples from both crystals that were so close to intrinsic as to require an ambipolar correction in determining the electron concentration. The compensation balance in this material is controlled by the EL2 midgap defect, of which the fraction ionized depends on trace presence of CAs shallow acceptors, and of shallow donors, probably including SiGa . An increase of the latter towards the tail (top) end of one crystal led to a reduction of the EL2 ionized fraction, and a lowered resistivity—but one still within the conventional semi-insulating range. Carbon was measured from the strength of its local vibrational mode absorption, while near-infrared measurements showed that EL2 was present in a concentration ∼1016 cm−3, with relatively small variation across a wafer.
Influence of copper doping on the performance of optically controlled GaAs switches
Authors: Ko S. T. , Lakdawala V. K. , Schoenbach K. H. , Mazzola M. S.
Published: July 2002

The influence of the copper concentration in silicon-doped gallium arsenide on the photoionization and photoquenching of charge carriers was studied both experimentally and theoretically. The studies indicate that the compensation ratio (NCu/NSi) is an important parameter for the GaAs:Si:Cu switch systems with regard to the turn-on and turn-off performance. The optimum copper concentration for the use of GaAs:Si:Cu as an optically controlled closing and opening switch is determined.
Random particle packing by reduced dimension algorithms
Authors: Davis I. Lee , Carter Roger G.
Published: July 2002

A new type of Monte Carlo algorithm to calculate packing fraction and general particle dispersion characteristics for arbitrary random packs of spherical particles is presented. Given arbitrary quantities of arbitrary sizes with arbitrary mass densities, the algorithms calculate the close random packing fraction. If desired, they can return the position and type of each particle in the pack. Since every detail of the positions and types of particles in the pack is known, any pack characteristic can be calculated. The algorithms use a dimension-reducing trick to turn a computationally intractable problem into a tractable one. Planned extensions and improvements of the algorithms are discussed.
Stress from isolation trenches in silicon substrates
Authors: Hu S. M.
Published: July 2002

Stress from isolation trenches in silicon integrated circuits has been studied using an analytical solution that the author recently obtained for the stress problem of a parallelepipedic inclusion in a three-dimensional semispace. The origin of the stress is assumed to be the thermal mismatch between the trench fill and the silicon substrate, or equivalently the intrinsic stress of the trench fill. The effect of various trench parameters such as the trench length, width, and depth, as well as the proximity to other trenches, are investigated for a number of cases. The results are shown for all stress components in order to provide an insight into the characteristics of the trench-induced stress. There are some surprises: Some examples are the existence of a tensile pocket in the stress component normal to the trench side, and that stress components both normal and parallel to the trench increase greatly in intensity as the trench becomes shorter.
Antiferromagnetic coupling between layers in Co/Cr multilayer (invited)
Authors: Stearns M. B. , Cheng Y. , Lee C. H.
Published: July 2002

It has been shown by several groups that Fe layers in epitaxial Fe/Cr sandwiches or multilayers (MLFs) are coupled antiferromagnetically (AF) for very small Cr thicknesses. One way this is manifest in the MLF is that for a fixed Fe thickness (30 Å) the hysteresis curves are progressively flatter as the Cr thickness decreases. This is the opposite behavior to that expected if the Fe layers were uncoupled or ferromagnetically coupled. We have observed the same type of behavior in a series of nominally 21-Å Co/X Cr MLFs where X varies from 14 to 57 Å. These MLFs have been well characterized with both x-ray scattering and cross-sectional high-resolution electron microscopy. They are highly polycrystalline, nonideal MLFs with appreciable mixed interfaces. We have determined that Co/Cr MLF have an interface or exchange anisotropy with an average value of ∼7 kOe in a direction opposite to that of the in-plane magnetization. Such an AF interaction is expected from Cr layers having interface thicknesses comparable to that of the pure Cr regions. It provides a natural mechanism for the Cr layers to align the Co layers either AF or FM depending on the local structural details. Both types of alignment are likely to be present in a MLF and the regions having AF alignment will cause the observed flattening of the hysteresis loops. The magnetoresistance of these MLFs has a behavior similar to, but much smaller than, the ‘‘giant’’ magnetoresistance seen in the epitaxial Fe/Cr MLF and single-crystal Fe whiskers. The magnetoresistance in pure Fe has been attributed to the domain structure and it is likely responsible for this effect in the Co/Cr MLF as well.
Optical studies of germanium implanted with high dose oxygen
Authors: Zhang Qi-Chu , Kelly J. C. , Mills D. R.
Published: July 2002

A model has been developed to explain the greatly reduced reflectivities of Ge samples implanted to doses 1.25–1.5×1018 O/cm2 with 45-keV O+ ions, which have reflectivity values close to zero at 0.7 μm. The model divides the inhomogeneous implanted layer into a series of homogeneous sublayers with different volume fraction and thickness for each sublayer. The complex refractive indexes for every sublayer are calculated using the Maxwell Garnett (MG) and Persson–Liebsch (PL) theories. Using the expressions for the reflectivity of an absorbing multilayer layer systems, the reflectivities have been calculated. The reflectivity curve calculated in the PL approximation is closer to the experimental observations than is the MG approximation over the wavelength range 0.2–3.0 μm.
Role of coherency in the elastic behavior of composition-modulated superlattices
Authors: Jaszczak J. A. , Phillpot S. R. , Wolf D.
Published: July 2002

We investigate the role of coherency in the elastic behavior of composition-modulated superlattices of fcc metals by atomistic computer simulations using Lennard–Jones potentials. Structures, energies, and elastic properties of incoherent superlattices are computed as a function of the compositional modulation wavelength along [001] and compared with those of coherent superlattices. Both superlattice types were taken to have a 10% lattice parameter mismatch between the two materials. The incoherent superlattices, as compared to coherent superlattices, were found to be more structurally disordered and exhibited greater elastic anomalies, which cannot be accounted for by the overall dimensional changes of the superlattices alone. High- and low-frequency elastic constants are briefly compared. It is proposed that increasing the structural disorder in the superlattices by increasing the lattice-parameter mismatch or by introducing a relative rotation between the two materials will enhance all of the elastic anomalies even further.
Electromagnetic energy transport in a dense distribution of conducting spheres
Authors: Ferrari L. A.
Published: July 2002

The scale dependence of transmission of microwave radiation at K-band frequencies (18–26.5 GHz) through a dense distribution of conducting spheres is studied for densities where localization is expected to occur. Scatterer packing fractions ranged from 0.16 to 0.22. Values of the elastic and transport mean free paths, and effective absorption lengths were obtained by fitting the transmission data to classical diffusion models. At the lowest densities the elastic mean free paths are within ∼15% of the value calculated with the single scattering formula. At f=0.22 the difference is about 30%; this result together with an absorption length of ∼6 cm could account for the failure to observe strong coherent effects and associated nonclassical transport.