bluuebunny/crossref_metadata_2025 huggingface dataset:
3.0-ns surface discharge development
Authors: Hidaka Kunihiko , Murooka Yoshihiro
Published: July 2002

The nanosecond time-scale developing mechanisms of positive and negative corona streamers on the insulation surface are investigated using a square pulse generator and the Lichtenberg figure technique. The developing length of the corona streamers in air is measured at a pulse duration of 3 ns, an applied voltage of 7–20 kV, and a gas pressure of 150–760 Torr. The results are compared with those obtained in the time range of 5–920 ns. The ratio of the developing length obtained at a certain pressure and that obtained at atmospheric pressure becomes a constant value irrespective of the magnitude of the applied voltage, and it increases as the gas pressure is reduced. The time lag of corona streamer inception increases with decreasing pressure. The time lag for the negative corona streamer obtained at a voltage of 20 kV is 0.1 ns at atmospheric pressure and 0.35 ns at a pressure of 150 Torr, while that for the positive corona streamer obtained at the same voltage, is 0.55 ns at the atmospheric pressure and 1.0 ns at the pressure of 150 Torr. From the results of the developing length and the time lag, the velocity of corona streamer development is estimated. The observed maximum velocity of the negative corona streamer is 8.4×108 cm/s at the conditions of 20 kV and 150 Torr and that of the positive corona streamer is 1.5×109 cm/s at the same conditions except for the polarity of the voltage.
Influence of growth parameters on the incorporation of residual impurities in GaAs grown by metalorganic chemical vapor deposition
Authors: van de Ven J. , Schoot H. G. , Giling L. J.
Published: July 2002

The incorporation of impurities in GaAs epitaxial layers grown from trimethyl gallium (TMG) and AsH3 has been studied in detail by varying a large number of growth parameters. These include the V/III ratio, temperature, the axial position in the reactor, gas sources, substrate and susceptor material, carrier gas, substrate misorientation, and the crystallographic orientation of the substrate. As main characterization techniques photoluminescence and Hall–van der Pauw measurements have been used. Donor and acceptor concentrations in the layers have been found to vary not only with temperature and V/III ratio, but also with the axial position in the reactor, giving rise to p/n transitions and maxima in the carrier mobility. The V/III ratio is shown to be effectively constant for larger axial distances in the cell. Highly doped substrates have been found to give rise to outdiffusion of defect complexes into the layers. The main acceptor impurities found in this work are zinc, silicon, and carbon. They are shown to originate from the TMG gas source, the hot quartz parts in the cell, and the TMG growth component, respectively. Incorporation of these elements appears to be orientation, and in the case of carbon also misorientation, dependent. The results for {001} and misoriented {001} crystals are discussed on the basis of two models for impurity incorporation: CAs acceptors are concluded to be incorporated by a trapping process at growth steps, whereas ZnGa, SiAs, and residual donors most probably incorporate via equilibrium processes. The conclusions from a study of the influence of the misorientation are especially important for this interpretation.
Steady-state ion pumping of a potential dip near an electron collecting anode
Authors: Forest Cary , Hershkowitz Noah
Published: July 2002

A negative potential well is found in front of a positively biased electron collecting plate in a low temperature laboratory plasma. The well parameters are governed by the Child–Langmuir condition and a necessary condition for the existence of such potential wells is the presence of an ion pumping mechanism which removes trapped ions from the wells. Here ion pumping was provided by the presence of negative electrically floating ceramic on the back of the plate.
Transfer of photoelectrons and photoholes through a AgBr/AgCl interface, and relative locations of the energy bands
Authors: Kawasaki Mitsuo , Hada Hiroshi , Uchida Hiroyuki
Published: July 2002

The transfer of photoelectrons and photoholes through the interface between AgBr and AgCl thin-sheet crystals was studied by the transient dc pulse method. Photoelectrons were transferred very easily with near perfect freedom in both directions from AgBr to AgCl and from AgCl to AgBr. The photohole transfer, however, was allowed only from AgCl to AgBr, and the transfer from AgBr to AgCl was effectively blocked at the interface. These results were carefully analyzed in terms of the relative locations of the energy bands of AgBr and AgCl. It was concluded that the bottom of conduction band of AgBr was likely to lie very close to that of AgCl, while the top of valence band of AgBr was separated from that of AgCl by an amount nearly equal to the difference in the band-gap energies of AgBr and AgCl.
Electrical resistivity of rf-sputtered iron oxide thin films
Authors: Mei Yu , Zhou Zeng-Jun , Luo H. L.
Published: July 2002

The resistivities of as-sputtered Fe3O4, partially transformed Fe3O4/γ-Fe2O3, and γ-Fe2O3 films were measured in the temperature range of 4.2–300 K. A resistivity anomaly was observed in Fe3O4 films and can be identified with the Verwey transformation. For Fe3O4 as well as γ-Fe2O3 films, the conductivity follows a simple exponential law for 33–72 K and 44–300 K, respectively. The values of the activation energy were determined. For Fe3O4 films this energy is much smaller than that of the single crystals.
Layered magnetic structures: Evidence for antiferromagnetic coupling of Fe layers across Cr interlayers
Authors: Grünberg P. , Schreiber R. , Pang Y. , Walz U. , Brodsky M. B. , Sowers H.
Published: July 2002

Double layers of (100)-oriented Fe with an individual thickness of ≊100 Å, separated by interlayers of Cr or Au with variable thickness were grown epitaxially on Au (100) surfaces. The spin-wave mode spectra of these double layers as detected by means of light scattering were used to measure the effective exchange coupling of the Fe films across the interlayer. For Au interlayers it decreases monotonically with increasing Au thickness dAu and disappears at dAu≳20 Å. For Cr interlayers of proper thickness dCr, i.e., 4 Å≲dCr≲9 Å we obtain antiferromagnetic coupling of the Fe layers. In small external fields such double layers order antiparallel with their magnetization perpendicular to the external field in analogy to the spin-flop phase of antiferromagnets. This is revealed by the behavior of the static magnetization of such samples as a function of external field and the spectrum of spin-wave modes with the wave vector parallel to a small external field.
The effect of nickel content on the ac magnetic properties of 49Fe-49Co-2V alloys
Authors: Novotny P. M.
Published: July 2002

Experimental alloys containing between 0.03 and 0.67 wt. % nickel were produced to determine the effect of low nickel content on the ac magnetic properties of Carpenter’s 49Fe-49Co-2V Hiperco (a registered trademark of Carpenter Technology Corp.) 50A alloy. The alloys were processed into 1.02×10−4 m (0.004 in.) thick strip, wound into tape toroids, annealed then tested for core loss at 60 and 400 Hz for induction levels of 1.0, 1.5, and 2.0 T. Toroids annealed at 885 °C in hydrogen, followed by slow cooling, exhibited a linear increase in core loss as nickel content increased due to the presence of a second phase in the microstructure. When the annealing temperature was decreased to 845 °C the second phase was eliminated and the core loss did not vary with nickel content. STEM examination of the samples determined that the second phase was a heavily dislocated bcc α′1 phase which had martensitically transformed from the fcc γ1 phase despite the slow cooling rate. Thermal expansion testing determined that increasing the Ni content in the range of 0–1 wt. % decreased the α1 +γ1 /α1 transformation temperature of the 49Fe-49Co-2V alloy.
Effect of uniaxial stress on the Raman spectra of graphite fibers
Authors: Sakata H. , Dresselhaus G. , Dresselhaus M. S. , Endo M.
Published: July 2002

Measurements of the effect of uniaxial stress on the frequency of the k≊0 optical phonons in heat-treated benzene-derived graphite fibers have been performed using first-order Raman scattering. Application of uniaxial stress along the fiber axis is found to cause polarization-dependent splittings and shifts of the Raman peaks of the k≊0 doubly degenerate optical phonons. From these observed splittings and shifts, experimental values for the phenomenological coefficients which describe the changes in the elastic constant of these phonons with strain are determined. It is concluded that Raman spectroscopy can be used to characterize the local stress or strain conditions of graphite fibers nondestructively.
Measurements of hydrogen in metal-oxide-semiconductor structures using nuclear reaction profiling
Authors: Marwick A. D. , Young D. R.
Published: July 2002

We report depth profiles of the hydrogen concentrations in metal-oxide-semiconductor structures measured using the nuclear reaction profiling technique with a 6.4-MeV 15N beam. In both conventionally grown and ultra-dry thermal oxide samples with aluminum or gold gate metal, a peak of hydrogen concentration is observed at the metal/SiO2 interface. The amount of hydrogen at this interface varied from sample to sample in the range 2–6×1015 H/cm2, which was at least 20 times as much as in the SiO2 layers. By continued irradiation with the measuring beam, most of this hydrogen was detrapped from the metal/SiO2 interface and diffused into the SiO2. The detrapping occurred much more rapidly in samples made with Al metallization than in Au gate or unmetallized samples. The data can be fitted by a model in which hydrogen is detrapped from the metal/SiO2 interface by the beam, then diffuses into the SiO2. Redistribution of hydrogen was found to continue until it was uniformly distributed throughout the SiO2, with a residual peak of strongly trapped hydrogen remaining at the metal/SiO2 boundary. At the same time the concentration of hydrogen in the SiO2 increased from an initial low level to about 4×1020 cm−3, depending on the amount of hydrogen initially at the Al/SiO2 interface.
Ion beam propagation in a transverse magnetic field and in a magnetized plasma
Authors: Hong R. , Wessel F. J. , Song J. , Fisher A. , Rostoker N.
Published: July 2002

Propagation of a charge-neutralized ion beam, in a transverse magnetic field (Bz <400 G) and in a magnetized plasma, has been studied. Measurements indicate that the beam propagation mechanism is due to the E×B drift in the region of high β (1<β<400), where β is the ratio of beam kinetic energy to transverse magnetic field energy. Diamagnetic measurements, both internal and external to the propagating beam, confirm the fast diffusion of Bz into the beam on a time scale much shorter than the beam rise time of 10−7 s. When the beam is injected into a magnetized plasma the electric field is shorted to a degree that increases with increasing background plasma density. When the plasma density reaches 1013/cm3 (∼200×the beam density) complete shorting occurs and the beam is deflected by the transverse magnetic field.
Imaging and modification of polymers by scanning tunneling and atomic force microscopy
Authors: Albrecht T. R. , Dovek M. M. , Lang C. A. , Grütter P. , Quate C. F. , Kuan S. W. J. , Frank C. W. , Pease R. F. W.
Published: July 2002

Direct imaging of ultrathin organic films on solid surfaces is important for a variety of reasons; in particular, the use of such films as ultrathin resists for nanometer scale fabrication and information recording requires that we understand their microstrucure. We have used the Langmuir–Blodgett technique to prepare monolayer and submonolayer films of poly(octadecylacrylate) (PODA) and poly(methylmethacrylate) (PMMA) on graphite substrates. Atomic scale images obtained with the scanning tunneling microscope (STM) and the atomic force microscope of the PODA films showed a variety of structures, including isolated narrow fibrils, parallel groups of fibrils, and an ordered structure consistent with the side chain crystallization expected with that material. The fibrils observed are interpreted as individual polymer chains or small bundles of parallel chains. Images of the PMMA samples show no ordered regions. By applying voltage pulses on the STM tip, we were able to locally modify and apparently cut through the PODA fibrils.
The potential distribution in a layered anisotropic spheroidal volume conductor
Authors: de Munck J. C.
Published: July 2002

Formulas are derived that express the potential distribution in a layered spherical and spheroidal anisotropic volume conductor: the number of layers is arbitrary and each layer may be anisotropic. These expressions are a generalization of existing formulas for localization of sources underlying evoked potentials or electro-encephalographic data. The formulas are presented in a convenient form so that if they are applied in practice the likelihood of software errors is minimal.
Preisach modeling of hysteresis in Terfenol
Authors: Restorff J. B. , Savage H. T. , Clark A. E. , Wun-Fogle M.
Published: July 2002

Terfenol (Tb0.3Dy0.7Fe1.9) and related materials have proven useful as electromechanical transducers. As with other transducers, these materials exhibit hysteresis, which must be included in any model of the transducer. Preisach theory is one of the available models that includes the effects of hysteresis. The theory is able to include the hysteresis by maintaining a history of the field reversals in the material. Both the strain and magnetization hysteresis loops in a 3.8-cm-diam Terfenol rod have been measured under 20.7-MPa longitudinal stress. The data has been used to derive a Preisach model of the rod, including hysteresis effects. It is possible to predict the strain and magnetization of the rod, including minor loops, with about 10%–15% accuracy.
Phase modulation in GaAs/AlGaAs double heterostructures. I. Theory
Authors: Faist J. , Reinhart F.-K.
Published: July 2002

This work aims at a systematic study of phase modulation in GaAs/AlGaAs double-heterostructure waveguides with different doping profiles. Both theoretical (part I) and experimental (part II) aspects are investigated, leading to interesting new results. Phase modulation is the sum of a linear electro-optic term, a quadratic electro-optic term, and a free-carrier term. The carrier term is shown to be the sum of a plasma term, an intervalence-band term (for holes only), a band-filling term, and a band-shrinkage term, the latter being due to many-body effects. A new analytic expression for the band-filling term is derived which shows that the band-filling effect does not depend on the carrier effective mass. We prove that the band-shrinkage term is approximately half of the band-filling term, but has opposite sign. The phase modulation is computed using an overlap integral between the optical intensity and the local refractive-index difference. We also report an analytic expression for the modulation efficiency of a p-i-n junction double-heterostructure modulator. This expression is very accurate and only requires knowledge of the depletion width and the guiding parameters.
Methyl radical and H-atom concentrations during diamond growth
Authors: Harris Stephen J. , Weiner Anita M.
Published: July 2002

The gas-phase composition at the surface of a growing diamond film was measured as a function of the initial methane (CH4) fraction and, for a 2% methane fraction, as a function of added oxygen (O2). The results were modeled with a one-dimensional reactor flow code that includes diffusion and detailed chemical kinetics. We found that most of the ethylene (C2H4) and ethane (C2H6) that was detected was actually not present in the growth chamber but was instead formed in the probe by recombination of methyl radicals (CH3) that were present in the gas phase. Thus, C2H4 and C2H6 acted as surrogates for CH3 in our system, and measurement of those two stable species allowed us to estimate the mole fraction of the CH3 radical. We then took advantage of the fact that CH3, CH4, H2, and H were in partial equilibrium in the diamond growth chamber in order to estimate the concentration of H. A comparison between the mole fractions of CH3 and H, as determined from our experiments, and the mole fractions calculated from the model shows very good agreement.
The influence of Mn on the grain-boundary potential barrier characteristics of donor-doped BaTiO3 ceramics
Authors: Illingsworth J. , Al-Allak H. M. , Brinkman A. W. , Woods J.
Published: July 2002

Two compositions of BaTiO3 positive temperature coefficient of resistance ceramics, prepared identically except for the fact that a small addition of Mn (0.04 at. %) was made to one of them, were studied. The samples were sintered simultaneously in air at 1400 °C for 1 h and then annealed at 1200 ° for 5 h, using a muffle furnace. Room-temperature dielectric measurements in the audio- and radio-frequency ranges confirmed that Mn has a negligible effect on the bulk resistance. Arrhenius plots of resistivity vs 1/[Tε′m(T)] were found to give straight lines for Tc<T<Tmax (where ε′m is the relative permittivity of the specimen measured at a constant frequency of 30 kHz, Tc is the ferroelectric transition temperature, Tmax is the temperature corresponding to the maximum in resistivity, and T is the absolute temperature), in accordance with the well-known Heywang model. The height of the potential barriers at different temperatures, as calculated from the slopes of these plots, were found to increase by about 40% (from ∼0.34 to ∼0.50 eV) by the addition of Mn. A small increase in the acceptor-state density at the grain surfaces, which was again obtained from these plots, was observed in Mn-doped specimens (3.9×1013 cm−2 as compared to 2.7×1013 for Mn-free specimens). It was also found that the inclusion of Mn had a negligible effect on ε′m above Tc.
Ferromagnetic resonance of particulate magnetic recording tapes
Authors: Netzelmann U.
Published: July 2002

The room-temperature ferromagnetic resonance (FMR) spectra of γ-Fe2O3, CrO2, and barium ferrite particulate magnetic recording tapes have been measured at microwave frequencies of 9.35 and 35 GHz for various orientations of the static and high-frequency magnetic fields with respect to the tape. For CrO2 tapes, the influence of the width of the angular distribution of the particle orientations on the FMR spectra has been studied from the nearly isotropic case up to the highly oriented case. Hysteretic behavior for a CrO2 tape as well as the effect of tape calendering for a γ-Fe2O3 tape has been observed by FMR. Experimental results are found to be in reasonable agreement with results of theoretical calculations based on a model of an ellipsoidal single-domain particle with both shape and magnetocrystalline anisotropy. Magnetostatic interaction inside the magnetic film has been introduced by expressing the total magnetostatic energy as a combination of a part dependent on particle shape and a part dependent on the shape of the tape. As a result of a comparison of experimental data with calculated data from the model, the magnetocrystalline easy axis of the CrO2 particles is found to be parallel with the particle axis.
Anticipated improvement in laser beam uniformity using distributed phase plates with quasirandom patterns
Authors: Epstein R. , Skupsky S.
Published: July 2002

The uniformity of focused laser beams, that has been modified with randomly phased distributed phase plates [C. B. Burckhardt, Appl. Opt. 9, 695 (1970); Kato and Mima, Appl. Phys. B 29, 186 (1982); Kato et al., Phys. Rev. Lett. 53, 1057 (1984); LLE Rev. 33, 1 (1987)], can be improved further by constructing patterns of phase elements which minimize phase correlations over small separations. Long-wavelength nonuniformities in the intensity distribution, which are relatively difficult to overcome in the target by thermal smoothing and in the laser by, e.g., spectral dispersion [Skupsky et al., J. Appl. Phys. 66, 3456 (1989); LLE Rev. 36, 158 (1989); 37, 29 (1989); 37, 40 (1989)], result largely from short-range phase correlations between phase plate elements. To reduce the long-wavelength structure, we have constructed phase patterns with smaller short-range correlations than would occur randomly. Calculations show that long-wavelength nonuniformities in single-beam intensity patterns can be reduced with these masks when the intrinsic phase error of the beam falls below certain limits. We show the effect of this improvement on uniformity for spherical irradiation by a multibeam system.
Current-limiting action of mixed-phase BaTiO3 ceramic semiconductors
Authors: Ravi V. , Kutty T. R. N.
Published: July 2002

Stable and highly reproducible current-limiting characteristics are observed for polycrystalline ceramics prepared by sintering mixtures of coarse-grained, donor-doped BaTiO3 (tetragonal) as the major phase and ultrafine, undoped cubic perovskite such as BaSnO3, BaZrO 3, SrTiO3, or BaTiO3 (cubic). The linear current-voltage (I-V) relation changes over to current limiting as the field strength increases, when thermal equilibrium is attained. The grain-boundary layers with low donor and high Sn, Zr, or Sr have depleted charge carrier density as compared to that in the grain bulk. The voltage drop at the grain-boundary layers diminishes the temperature gradient between the interior and surface regions.
The role of space-charge-limited-current conduction in evaluation of the electrical properties of thin Cu2O films
Authors: Rakhshani A. E.
Published: July 2002

Space-charge-limited-current conduction is the dominant charge transport mechanism in electrodeposited films of cuprous oxide; the measurement of current-voltage characteristic and temperature dependence of conductivity, while charge transport is space-charge limited, is used to evaluate the electrical parameters of the films. From these measurements and the measurement of the material band gap and dielectric constant, the energy-band diagram of Cu2O films has been determined for the first time. An optoelectronic band gap of 2.0 eV and a dielectric constant of 8.8 is measured for these films. The hole mobility in a sample with a resistivity of 1.1×1011 Ω cm is measured to be 8.7 cm2/V s. The conductivity of this sample is controlled by an acceptor-type deep level (0.54 eV above valence band) with a concentration of 6.65×1014 cm−3 which is compensated with a donor-type level (0.92 eV below conduction band) having a concentration of 6.64×1014 cm−3.
Atomic short-range order and magnetic-moment distribution in dilute F
eMn and F
eTi alloys
Authors: Parette G.
Published: July 2002

Neutron measurements on FeMn and FeTi alloys with impurity concentration lower than 3 at. % were done at liquid-nitrogen temperature by using polycrystalline samples quenched from the melt. The most concentrated FeMn alloy exhibited an atomic short-range order characterized by a tendency for clustering of the Mn atoms. Nuclear cross-section data of FeTi alloys could be analyzed by taking in account only the lattice distortions. The measured radial displacements were found to be in reasonable agreement with those calculated within the Krivoglaz model. Positive (+0.62 μB) and negative (−0.50μB) values have been obtained for the magnetic moments on Mn and Ti atoms, respectively. A negative value for the moment perturbation on the two first shells was measured in both cases. These results partly agree with some other determinations and recent calculations.
Infrared and Raman spectra of C60 and C70 solid films at room temperature
Authors: Meilunas Ray , Chang R. P. H. , Liu Shengzhong , Jensen Michael , Kappes Manfred M.
Published: July 2002

Infrared and high-resolution Raman spectroscopic probes of thin films of C60 and C70 are presented and discussed in terms of previous measurements, semi-empirical calculations, and plausible molecular geometries.
Influence of dc bias voltage on the refractive index and stress of carbon-diamond films deposited from a CH4/Ar rf plasma
Authors: Amaratunga Gehan A. J. , Silva S. Ravi P. , McKenzie David R.
Published: July 2002

The dc self bias voltage developed during CH4/Ar radio frequency plasma-enhanced vapor deposition of thin films containing polycrystalline diamond grains within an a:C matrix (carbon-diamond) is found to influence the optical and mechanical properties of the films. In particular it is shown that there is a simultaneous etch deposition process which takes place, and that the dc bias can be used to control this etch rate, and hence the net film growth rate. When a balance between etching and deposition is achieved, the films show increased residual stress and optical density with exposure to Ar+ bombardment in the plasma. In addition to the measured dc bias the local electric field developed around the substrate is also found to significantly influence the energy with which ions impinge upon the growing film.
Formation of highly uniform and dense diamond microcrystal thin films using a combustion flame surrounded by an inert-gas flow
Authors: Abe Toshimi , Suemitsu Maki , Miyamoto Nobuo
Published: July 2002

An inert-gas curtain method, in which a combustion flame of oxygen/acetylene mixture is protected from the air by a surrounding inert-gas flow, has been developed for the deposition of microcrystalline diamond films. As a result, highly uniform and highly dense microcrystal films of diamonds possessing the quality of natural diamond were successfully prepared. The inert-gas flow allows the hydrocarbon radicals in the flame to be varied for preferential deposition of diamonds.
X-ray optics of diffractors curved to a logarithmic spiral
Authors: Wittry D. B. , Chang W. Z. , Li R. Y.
Published: July 2002

Calculations are given for some properties of x-ray diffractors having the shape of a logarithmic spiral in one direction. The locus of points on a curved diffractor’s surface is calculated for a given deviation from the Bragg angle when the diffractor is used at Bragg angles ranging from 15° to 75°. Four cases are considered, including a cylindrical logarithmic spiral and logarithmic spirals of revolution about an axis lying in the plane of the Rowland circle with the source at the origin of the spiral and with the source displaced from the origin. The resulting collection solid angles are compared with the Johann, the Johansson, the spherical plate, the Wittry, and the point-to-point focusing geometries. It is concluded that the diffractors based on a logarithmic spiral are useful because they provide better performance than some of these other diffractors and may be easier to fabricate.
Plasma-enhanced chemical-vapor-deposited oxide for low surface recombination velocity and high effective lifetime in silicon
Authors: Chen Z. , Pang S. K. , Yasutake K. , Rohatgi A.
Published: July 2002

It is shown that plasma-enhanced chemical-vapor deposition (PECVD) of thin SiO2 on Si wafers followed by rapid thermal annealing (RTA) can result in very high effective carrier lifetime (≳5 ms) and extremely low surface recombination velocity (≤2 cm/s). Thin SiO2 (∼100 Å) layers were prepared by direct PECVD at 250 °C and RTA was performed at 350 °C in forming gas. Detailed metal-oxide-semiconductor analysis and model calculations showed that such a low recombination velocity is the result of moderately high positive oxide charge (5×1011–1×1012 cm−2 ) and relatively low midgap interface-state density (5×1010–1×1011 cm−2 eV−1). RTA was found to be superior to furnace annealing, and a forming gas ambient was better than a nitrogen ambient for achieving a very low surface recombination velocity. Some degradation in the surface recombination velocity or effective lifetime was observed. It is found that a PECVD SiN cap on top of the thin SiO2 not only suppressed this degradation but also enhanced the effective lifetime.
Computations of optical birefringence characteristics of highly eccentric elliptical core fibers under various thermal stress conditions
Authors: Fontaine Marie
Published: July 2002

The birefringence characteristics of optical fibers with a highly eccentric elliptical core are investigated under various thermal stress conditions. A new numerical method for solving the vectorial Maxwell equations is proposed which simultaneously takes into account the effects of ambient temperature and the geometrical distribution of the refractive indices of anisotropic propagation media. To verify the reliability of our numerical procedure, the birefringence characteristics are investigated of a highly birefringent elliptical core fiber used recently in the design of an optical sensor. The birefringence characteristics calculated by this procedure and those obtained from models currently used in the field of fiber-optic sensing devices are compared with experimental data. For highly birefringent fibers, the numerical method proposed is far more accurate.
Crystallography of epitaxial growth of wurtzite-type thin films on sapphire substrates
Authors: Kung P. , Sun C. J. , Saxler A. , Ohsato H. , Razeghi M.
Published: July 2002

In this article, we present a crystallographic model to describe the epitaxial growth of wurtzite-type thin films such as gallium nitride (GaN) on different orientations of sapphire (Al2O3) substrates. Through this model, we demonstrate the thin films grown on (00⋅1)Al2O3 have a better epilayer-substrate interface quality than those grown on (01⋅2)Al2O3. We also show the epilayer grown on (00⋅1)Al2O3 are gallium-terminated, and both (00⋅1) and (01⋅2) surfaces of sapphire crystals are oxygen-terminated.
Scanning tunneling microscopy investigation of graphite surface damage induced by gold-ion bombardment
Authors: Yan Junjue , Li Zhigang , Bai Chuanyong , Yang W. S. , Wang Yugang , Zhao Weijiang , Kang Yixiu , Yu F. C. , Zhai Pongji , Tang Xiaowei
Published: July 2002

Surface damage on highly oriented pyrolytic graphite induced by bombardment of 530 keV Au+ or 4.5 MeV Au++ at a low dose of 1×1012 cm−2 was studied by scanning tunneling microscopy. The most prominent surface features are hillocks with a diameter of about 1.8 nm. The average spatial density of the hillocks is in good agreement with the ion dose used, indicating that they are induced by impact of individual incident gold ions. There also exists another type of hillock (which is called bump in this paper) with a smaller diameter and a frequency of about twice the ion dose. These small hillocks have never been reported before and are likely to be caused by the outward sputtering of secondary particles as products of collision cascades. Beside the various √3×√3 R 30° superstructures of graphite, Friedel oscillations are very often seen surrounding most of the hillocks. The physical mechanism of the formation of both types of the hillocks is also discussed.
High field magnetization measurements of Sm2Fe17, Sm2Fe17Cx, and Sm2Fe17CxH5−x
Authors: Isnard O. , Miraglia S. , Guillot M. , Fruchart D. , Buschow K. H. J.
Published: July 2002

The magnetic properties of ferromagnetic compounds of Sm2Fe17, Sm2Fe17C0.6, and Sm2Fe17C0.6H4.4 were investigated. Sm2Fe17 nitrides, carbides, and carbohydrides are suitable for making high performance permanent magnets. The high field magnetization measurements performed up to 200 kOe in continuous fields are reported. In this article, the isothermal magnetization curves measured between 2 and 300 K on powder samples embedded in a resin and then aligned under a magnetic field of about 10 kOe in order to get oriented samples are presented. The magnetic anisotropy constants K1 and K2 are determined taking into account the angular distribution of the grains axis. The results are discussed in light of previously reported data and then the effects of C or H on the Sm contribution to the anisotropy is compared to that of N. It is shown that carbon and hydrogen have opposite influence on the Sm anisotropy and it is suggested that a wide range of magnetic anisotropy can be obtained depending on the H and C content. The influence of both interstitial elements H and C on magnetic features such as the Curie temperature and the saturation magnetization is analyzed.
Magnetic properties and magnetoresistance in granular Fe-Cu alloys
Authors: Peng Chubing , Dai Daosheng
Published: July 2002

The structural, magnetic, and electrotransport properties of a metallic granular FexCu1−x alloys (x=0–0.35) were studied. These alloys were fabricated by using a coevaporation technique with an extremely high deposition rate. Observations by transmission electron microscope confirmed that these alloys consist of small Fe particles embedded in a Cu matrix. The ferromagnetic-superparamagnetic transition occurs at the blocking temperature. The blocking temperature TB, dependent on the atomic fraction x of Fe, is in a range of 80–300 K as x=0.10–0.22. Above TB, the Fe particles exhibit superparamagnetic relaxation. Their magnetic behavior can be understood based on superparamagnetic theory. At low temperature, the temperature dependence of magnetization has shown that the magnetic behavior of Fe particles in granular Fe-Cu alloys evolves from three- to two-dimensional ferromagnetism as the Fe concentration is reduced. Moreover, large magnetoresistance in the granular Fe-Cu alloys was observed at 77 K. Its magnitude is affected by the size and density of the magnetic entities, which can be controlled by varying the Fe concentration in the Cu matrix.
Theoretical study of hole initiated impact ionization in bulk silicon and GaAs using a wave-vector-dependent numerical transition rate formulation within an ensemble Monte Carlo calculation
Authors: Oğuzman İsmail H. , Wang Yang , Kolník Ján , Brennan Kevin F.
Published: July 2002

In this paper, calculations of the hole initiated interband impact ionization rate in bulk silicon and GaAs are presented based on an ensemble Monte Carlo simulation with the inclusion of a wave-vector-dependent numerical transition rate formulation. The ionization transition rate is determined for each of the three valence bands, heavy, light, and split-off, using Fermi’s golden rule with a two-body, screened Coulomb interaction. The dielectric function used within the calculation is assumed to be wave-vector-dependent. Calculations of the field-dependent impact ionization rate as well as the quantum yield are presented. It is found from both the quantum yield results and examination of the hole distribution function that the effective threshold energy for hole initiated impact ionization is relatively soft, similar to that predicted for the corresponding electron initiated ionization rate threshold in both GaAs and silicon. It is further found that light-hole initiated ionization events occur more frequently than either heavy or split-off initiated ionization events in bulk silicon over the applied electric field strengths examined here, 250–500 kV/cm. Conversely, in GaAs, the vast majority of hole initiated ionization events originate from holes within the split-off band.
Parameter dependencies of characteristics of a high-T
c dc superconducting quantum interference device
Authors: Enpuku K. , Tokita G. , Maruo T. , Minotani T.
Published: July 2002

Comprehensive comparison between theory and experiment has been made on the characteristics of a high-Tc dc superconducting quantum interference device (SQUID). Using the theoretical expressions for the SQUID characteristics, we can quantitatively predict the transfer function Vφ and the magnetic-flux noise SΦ from SQUID parameters without any adjustable parameter. It is shown that the theoretical predictions agree well with the experimental results for a wide range of SQUID parameters. This agreement shows the usefulness of the present theory in the design of the high-Tc SQUID. Exceptional deviations between theory and experiment are the absolute values, i.e., the factor of 0.6 difference in Vφ and the factor of 10 difference in SΦ. Possible reasons for these discrepancies are discussed.
Measurement of the lattice parameters in the individual layers of single-crystal superlattices
Authors: Birch J. , Sundgren J.-E. , Fewster P. F.
Published: July 2002

A method for determination of the lattice parameters, parallel to the sample surface as well as normal to the sample surface, in the individual layers of single-crystalline superlattices is derived. The method is based on simulations of low angle reflectivity measurements in combination with x-ray diffraction reciprocal space mapping. The number of unknown simulation parameters is reduced from three to one, namely the layer thickness of one of the layers which constitutes the bilayer period, if compared to techniques based only on simulations of the high angle diffraction pattern. The technique is demonstrated by characterizing a single-crystalline Mo/V(001) superlattice grown by dual-target magnetron sputtering onto MgO(001) substrates. The lattice parameters of the tetragonally distorted layers were aMo=0.309 nm, cMo=0.319 nm, aV=0.305 nm, and cV=0.298 nm which correspond to a misfit dislocation density of ∼0.056 dislocations per nm at each interface and a relaxation of ∼1/3 of the coherency strain. The lattice parameters obtained by the method were confirmed by comparing a kinematical calculation of the high angle Mo/V(002) θ–2θ diffraction pattern (using the obtained values as input parameters) with the experimentally determined pattern.
Hydrogen desorption process of Si(100)/PH3
Authors: Yoo D. S. , Suemitsu M. , Miyamoto N.
Published: July 2002

Adsorption of PH3 onto Si(100) and hydrogen desorption therefrom at various adsorption temperatures Ta have been investigated by the temperature-programmed-desorption (TPD) method, which includes measurements on repeatedly adsorbed surfaces to obtain the surface phosphorus coverage. The TPD peak showed a shift toward higher temperatures for Ta above 400 °C, which can be correlated to the onset of the hydrogen desorption and a resultant concentrated adsorption of the phosphorus atoms during exposure. A support for this correlation is given by further analysis of the TPD line shape, which clarified that surface phosphorus restricts hydrogen desorption both by suppressing hydrogen association and by increasing the desorption energy.
Predictions of ion energy distributions and radical fluxes in radio frequency biased inductively coupled plasma etching reactors
Authors: Hoekstra Robert J. , Kushner Mark J.
Published: July 2002

Inductively coupled plasma (ICP) reactors are being developed for low gas pressure (<10s mTorr) and high plasma density ([e]≳1011 cm−3) microelectronics fabrication. In these reactors, the plasma is generated by the inductively coupled electric field while an additional radio frequency (rf) bias is applied to the substrate. One of the goals of these systems is to independently control the magnitude of the ion flux by the inductively coupled power deposition, and the acceleration of ions into the substrate by the rf bias. In high plasma density reactors the width of the sheath above the wafer may be sufficiently thin that ions are able to traverse it in approximately 1 rf cycle, even at 13.56 MHz. As a consequence, the ion energy distribution (IED) may have a shape typically associated with lower frequency operation in conventional reactive ion etching tools. In this paper, we present results from a computer model for the IED incident on the wafer in ICP etching reactors. We find that in the parameter space of interest, the shape of the IED depends both on the amplitude of the rf bias and on the ICP power. The former quantity determines the average energy of the IED. The latter quantity controls the width of the sheath, the transit time of ions across the sheath and hence the width of the IED. In general, high ICP powers (thinner sheaths) produce wider IEDs.
Loss simulations in magnetostrictive actuators
Authors: Engdahl G. , Bergqvist A.
Published: July 2002

Power losses in a magnetostrictive actuator have been studied. The system consists of a cylindrical rod of Terfenol-D, a surrounding magnetic circuit to get a homogeneous flux, a mechanical load applied to the rod ends, and an electric coil. We consider transient operating conditions with time-varying driving voltage. Nonlinearity and hysteresis are handled by using a magnetomechanical hysteresis model as the constitutive law. Eddy currents and field constriction are taken into account by a finite difference representation of the diffusion equation for the active rod. Coil losses are trivially found through Ohm’s law. The influence of laminations in the magnetostrictive rod on the eddy current loss is examined using finite element methods.
Fabrication and properties of light-emitting diodes based on self-assembled multilayers of poly(phenylene vinylene)
Authors: Fou A. C. , Onitsuka O. , Ferreira M. , Rubner M. F. , Hsieh B. R.
Published: July 2002

Light-emitting diodes have been fabricated from self-assembled multilayers of poly(p-phenylene vinylene) (PPV) and two different polyanions; polystyrene sulfonic acid (SPS) and polymethacrylic acid (PMA). The type of polyanion used to assemble the multilayer thin films was found to dramatically influence the behavior and performance of devices fabricated with indium tin oxide and aluminum electrodes. Light-emitting devices fabricated from PMA/PPV multilayers were found to exhibit luminance levels in the range of 20–60 cd/m2, a thickness dependent turn-on voltage and classical rectifying behavior with rectification ratios greater than 105. In sharp contrast, the devices based on SPS/PPV exhibited near symmetric current–voltage curves, thickness independent turn-on voltages and much lower luminance levels. The significant difference in device behavior observed between these two systems is primarily due to a doping effect induced either chemically or electrochemically by the sulfonic acid groups of SPS. It was also found that the performance of these devices depends on the type of layer that is in contact with the Al top electrode thereby making it possible to manipulate device efficiency at the molecular level.
Active microlevers as miniature torque magnetometers
Authors: Rossel C. , Bauer P. , Zech D. , Hofer J. , Willemin M. , Keller H.
Published: July 2002

An extremely sensitive miniature torque magnetometer using Si p-doped piezoresistive cantilevers is described. The magnetization m↘ of very small magnetic or superconducting samples (≤1 μg) deposited on the cantilever can be measured via the torque τ↘=m↘×B↘ produced on them by an applied field B. The high resolution in the lever deflection of the order of 0.1 Å corresponds to a torque sensitivity of the order of Δτ≂10−14 Nm. In a homogeneous field of 1 T this device allows magnetic moments as small as Δm≂10−14 Am2 to be measured, a value far smaller than that measurable by the best commercial superconducting quantum interference device magnetometers. Measurements performed on microcrystals of the high-Tc superconductors Bi2Sr2Ca1Cu2O8 and Hg1Ba2Ca3Cu4O10 in the static and dynamic modes demonstrate the excellent performance of this device at low temperature and in magnetic fields ranging between a few mT and 5 T.
Optical phonon spectra of GaSb/AlSb superlattices: Influence of strain and interface roughnesses
Authors: da Silva S. W. , Pusep Yu. A. , Galzerani J. C. , Lubyshev D. I. , Milekhin A. G. , Preobrazhenskii V. V. , Putiato M. A. , Semjagin B. R.
Published: July 2002

Raman spectroscopy indicates the formation of extended intermixture alloy regions at the interfaces in the GaSb/AlSb superlatttices, which suppress the optical confined modes. A misfit strain was found to be responsible for the high-frequency shift of the optical phonons confined in the AlSb layers. It turned out that, in the ultrathin layer GaSb/AlSb superlattices with layer thicknesses less than 7 monolayers, the strain was completely relaxed in the alloy interface regions.
Water-induced room-temperature oxidation of Si–H and –Si–Si– bonds in silicon oxide
Authors: Liao Wen-Shiang , Lee Si-Chen
Published: July 2002

Hydrogenated amorphous silicon oxide (a-SiOx:H) films have been fabricated by plasma-enhanced chemical vapor deposition at temperature ranging from 15 to 150 °C. It is found that once these films are taken out from the reaction chamber and immersed into water or exposed to the atmosphere, the Si–H and –Si–Si– bonds in the films start to oxidize. After immersing the films into deionized water, many small gas bubbles are formed upon the films’ surface and these bubbles were collected into sealed tubes and identified as hydrogen gas (H2(g)) using gas chromatography technique. By using infrared absorption spectroscopy and monitoring these oxidation processes at various times, it is clear that at room temperature water (H2O) molecules react with Si–H bonds and –Si–Si– bonds and form more chemically stabilized Si–O–Si, Si–O–H, H–O–H bonds, and H2(g). Study of the Si–H decreasing rates reveals that second-order reaction occurs at the initial stage of oxidation, which is consistent with two adjacent Si–H bonds oxidizing together with one attacking H2O molecule. A model of porous structure, which is probably inherent at low temperature deposition, is proposed to explain why H2O molecules can percolate through the films and eventually fully oxidize these films.
Photoluminescence behavior of hydrogenated amorphous carbon
Authors: Rusli  , Robertson J. , Amaratunga G. A. J.
Published: July 2002

The photoluminescence (PL) of amorphous hydrogenated carbon (a-C:H) has been investigated as a function of optical gap and excitation energy. The PL spectra exhibit a red shift when the excitation energy falls below the optical gap. The bandwidth of the PL spectra increases with the excitation energy but depends very weakly on the gap. The results are discussed in terms of the electronic structure of a-C:H and the configuration of sp2 sites. It is proposed that PL arises by the geminate radiative recombination of the electron–hole pairs confined to the π states of the sp2 bonded clusters. The quantum efficiency is found not to decrease for excitation above the gap, indicating that the electron–hole pair remain bound, and suggesting that any mobility edges lie beyond the optical band edges. The PL efficiency is found to increase roughly exponentially with optical gap, displaying two regimes. At higher gap the efficiency depends just on the gap, while at lower gap the dependence may depend on the capture radius of nonradiative recombination centers.
Auger and x-ray photoemission spectroscopy study on Cs2Te photocathodes
Authors: di Bona A. , Sabary F. , Valeri S. , Michelato P. , Sertore D. , Suberlucq G.
Published: July 2002

Thin films of Cs2Te have been produced and analyzed by Auger depth profiling and x-ray photoemission spectroscopy (XPS). The formation of the photoemissive material passes through different phases, each of them has been characterized by XPS and by its total yield in the spectral region 3.5–5 eV. Copper and molybdenum substrates have been considered. While Mo behaves to all practical purposes like an ideal support for Cs2Te, strong diffusion from the substrate material into the photoemissive film has been observed on photocathodes fabricated on Cu. The ruggedness of the photocathodes has been tested by exposing them to a few hundred Langmuirs of different gases, namely O2, CO2, CO, N2, and CH4. The last three have no effect on the photocathode lifetime, while a substantial reduction of the quantum efficiency has been observed after the exposure to oxygen. The main reason for this is the formation of a thick cesium oxide layer at the surface of the photocathode. However, the oxygen pollution can be partially recovered by the combined effect of heating the photocathode at 230 °C and illuminating the poisoned material with the 4.9 eV radiation. No rejuvenation has been observed under the effect of the temperature or the radiation alone.