bluuebunny/crossref_metadata_2025 huggingface dataset:
Long-range propagation of high-power, pulsed electron beams for welding and materials processing applications
Authors: Fernsler Richard F. , Hubbard Richard F. , Lampe Martin
Published: July 2002

The electron beams used commercially for welding and materials processing operate either in vacuum or at very short range in air, which limits system productivity and the types of applications allowed. We show here that these restrictions can be relaxed considerably by raising both the beam energy and current, from under 200 keV to a few MeV and from under 1 A to a few kA, respectively, with the beam chopped into short pulses to preserve average beam power. This technology is made possible by the recent development of high-power generators that produce low-emittance beams with radii as small as 1 mm, pulse lengths of tens of ns, and pulse repetition rates up to several kHz. After studying the constraints imposed by gas scattering and the resistive hose instability, we conclude that high-power beams can propagate and remain tightly pinched over tens of cm in ambient air, without need of external guide fields. Equally important, the propagation requirements can be significantly relaxed by using a narrow conducting pipe filled with air or another gas to guide the beam to the workpiece.
InAsSbBi alloys grown by organometallic vapor-phase epitaxy
Authors: Huang K. T. , Chiu C. T. , Cohen R. M. , Stringfellow G. B.
Published: July 2002

A major remaining challenge for III/V semiconductor materials is the development of materials for photonic devices operating in the infrared region of the spectrum. Atmospheric transmission windows exist in the wavelength ranges from 2 to 4.5 and from 8.5 to 12 μm. Thus, emitters and, particularly, detectors operating in these wavelength ranges are important for many applications. Materials for devices operating in the longer-wavelength 8–12 μm region have typically not been III/V semiconductors because the lowest-band-gap conventional III/V alloy is InAsSb, with a 77 K band gap of 0.145 eV, corresponding to a wavelength of 8.5 μm. Previous work has shown that the addition of Bi to InAsSb alloys grown by organometallic vapor-phase epitaxy results in a rapid reduction in the band-gap energy. However, very low temperatures were required to obtained significant levels of Bi incorporation into the solid, due to the immiscibility of Bi in InAsSb. The low growth temperatures result in high carbon contamination levels using conventional precursors. Clearly, new precursors are required for low-temperature growth of these alloys without excessive levels of carbon contamination. New results for the organometallic vapor-phase-epitaxy growth of InAs1−x−ySbxBiy alloys are presented using the novel precursors tertiarybutylarsine, tertiarybutyldimethyl-antimony, and ethyldimethylindium. Alloys have been studied over the entire range of Sb/As ratios in the solid. For growth at 350 °C, the maximum Bi concentration yielding layers without the presence of a liquid second phase was found to be highest for x=0 (y=0.045) and lowest for x=0.7 (y=0.015). These levels of Bi incorporation yield calculated 77 K band gaps of 0.08 eV for the alloy with x=0.5 and y=0.015. These layers have several orders of magnitude lower levels of carbon contamination than reported previously.
Postfabrication changes and dependence on hydrogen concentration of the refractive index of proton-exchanged lithium tantalate waveguides
Authors: Åhlfeldt Henrik , Webjörn Jonas , Laurell Fredrik , Arvidsson Gunnar
Published: July 2002

Experimental results on proton-exchanged waveguides in lithium tantalate are reported. Proton-exchanged z-cut samples are characterized optically and using secondary-ion-mass spectroscopy (SIMS). The SIMS profiles are used to obtain the temperature-dependent diffusion coefficient. The increase of the extraordinary index is obtained using the prism-coupling technique and the decrease of the ordinary index is deduced from the spectral modulation of the transmission at normal incidence. A series of proton-exchanged samples are annealed and the postannealing changes are followed over time. It is found that for certain parameters the refractive index decreases rapidly the first days after the annealing. There is also a smaller long-term change. Further, on repeated heat treatment this aging process will recommence. A second series of samples are fabricated by proton exchange in sulphate-salt melts. It is found that above a certain proton concentration in the melt, the refractive index decreases when the proton concentration is increased. This has the impact that certain samples with a monotonously decreasing proton concentration will have their maximum refractive index below the surface. This increases the difficulty in accurately reconstructing the index profile from measured effective indices.
Corona-treated isotactic polypropylene films investigated by friction force microscopy
Authors: Overney R. M. , Güntherodt H.-J. , Hild S.
Published: July 2002

The effect of an excessive corona discharge treatment on polypropylene films has been investigated by friction force microscopy, an extension of atomic force microscopy. Friction force microscopy has been applied as a material specific analytical tool resulting in increased friction of morphological changes on the polymer surface, such as the formation of dropletlike protrusions. Shear experiments on the submicrometer scale provide information about the adhesive strength between the dropletlike protrusions and the bulk polymer surface. By applying lateral forces greater than 10−5 N, single protrusions, several hundred nanometers in size, have been tracked and sheared over the polymer bulk surface. This threshold has been quantified by a brief calculation of surface energies. Based on various information during measurements of the protrusion shape and properties, the droplets are assumed to be a product of frozen degradation products.
Photoluminescence and p-type conductivity in CdTe:N grown by molecular beam epitaxy
Authors: Dhese K. A. , Devine P. , Ashenford D. E. , Nicholls J. E. , Scott C. G. , Sands D. , Lunn B.
Published: July 2002

An rf nitrogen (N) plasma source has been used to achieve p-type conductivity in molecular beam epitaxy CdTe layers grown with a Cd overpressure. Photoluminescence and secondary ion mass spectrometry measurements have confirmed the incorporation of the N species, and evidence for the resulting p-type conductivity has been obtained using capacitance-voltage and current-voltage techniques. Net hole concentrations as high as 2×1017 cm−3 have so far been achieved, which contrasts with the normally n-type nature of our undoped CdTe layers.
Stark effect and excitonic tunneling escape process in semiconductor quantum wells
Authors: Hernández-Cabrera A. , Aceituno P. , Cruz H.
Published: July 2002

In this work, we have numerically integrated in space and time the effective mass Schrödinger equation for an exciton in a semiconductor quantum-well structure. Considering a Coulomb interaction between the electron-hole pair and an external electric field, we have studied the excitonic tunneling escape process from semiconductor quantum wells. Our method of calculation has been applied to types-I, -II, and -III quantum-well superlattices. In addition, we present the calculated excitonic lifetimes for the GaAs/GaAlAs, InAs/GaSb, and HgTe/HgCdTe systems under an external electric field. In the HgTe/CdTe system, the possibility of having similar electron and hole lifetime values is also found if the applied electric field is large enough.
Preparation and critical current density of a Bi-based superconducting whisker composite
Authors: Funahashi Ryoji , Matsubara Ichiro , Kusumi Shinji , Ogura Toru , Yamashita Hiroshi , Dimesso Lucangelo
Published: July 2002

A Bi2Sr2CaCu2Ox (Bi-2212) superconducting whisker composite consisting of Bi-2212 powder and Bi-2212 superconducting whiskers has been prepared. The powder and the whiskers have been prepared separately, mixed at a weight ratio of 1.0/0.05 (powder/whisker), and sintered in air. The ab plane of the whisker tends to be aligned parallel to the pressed plane in the composite. Zero resistivity temperature ( Tc,zero) for the composite is 75 K and critical current density (Jc) is 250 A/cm2 at 71 K under zero magnetic field. This value is about ten times larger than for a sintered sample without the whiskers (Tc,zero = 74 K). The reason for the increase in Jc is that the whiskers act as bypasses for superconducting current.
Relaxation of radiation damage in silicon planar detectors
Authors: Schmidt B. , Eremin V. , Ivanov A. , Strokan N. , Verbitskaya E. , Li Z.
Published: July 2002

The behavior of radiation-induced carbon-related defects in high-resistivity silicon detectors has been investigated. The defects were introduced by α-particle irradiation and investigated by deep-level transient spectroscopy. An unusual defect behavior consists in low-temperature annealing, including self-annealing at room temperature, of the interstitial carbon Ci with a simultaneous increase of the Ci-Oi-complex concentration. The kinetic parameters of the process have been determined from the increase of the Ci-center concentration versus time. Two annealing velocities have been observed, which arise from different heat treatments during the detector fabrication process.
Diffusion of ion beam created vacancies and their effect on intermixing: A gambler’s ruin approach
Authors: Gillin W. P. , Kimber A. C. , Dunstan D. J. , Webb R. P.
Published: July 2002

Ion implantation has frequently been shown to modify the shape of quantum wells following thermal annealing by enhancing the interdiffusion. We have shown that, independent of chemical effects on the interdiffusion, there is a contribution from the vacancies created by the implant. The effects of the diffusion of these vacancies can be modeled using a very simple expression that does not rely on any knowledge of the diffusion coefficients for the vacancies in the material. By comparing the problem to the classic problem of the gambler’s ruin, we have shown that implantation into a surface should produce even intermixing of layers at different depths below it, ignoring the effects of vacancy trapping and other depth dependent diffusion processes.
Strain and mosaic spread of carbon and gallium co-implanted GaAs
Authors: Horng S. T. , Goorsky M. S. , Madok J. H. , Haegel N. M.
Published: July 2002

The strain and crystalline perfection of GaAs implanted with either carbon, gallium, or both carbon and gallium ions (5×1014 cm−2) were investigated using high-resolution triple axis diffractometry. We determined that a significant amount of carbon occupied substitutional sites after rapid thermal annealing only when gallium was co-implanted. The carbon in the carbon-implanted layer remained in nonsubstitutional sites after annealing. In both cases, most of the lattice strain in the implanted layer decreases upon annealing, but a defective crystalline structure with an extensive mosaic spread evolves. Conventional double-axis x-ray measurements were employed for comparison.
Generation of delocalized E′δ defects in buried Si oxide by hole injection
Authors: Stesmans A. , Vanheusden K.
Published: July 2002

The generation of delocalized E′δ defects through hole injection in buried SiO2 (BOX) layers of standard separation-by-implantation-of-oxygen structures has been studied by electron spin resonance. Selective photoinjection of holes was carried out under positive biasing of deposited semitransparent electrodes. This leads to the production of numerous delocalized E′δ centers (density≊1.8×1013 cm−2) in addition to a large density (∼9×1013 cm−2) of the common E′γ defect (O3≡Si⋅+Si≡O3)—a result qualitatively similar to previous glow discharge damage data. Comparison of damaging agents applied to BOX indicates that E′γ and E′δ defects are generally produced in an approximately fixed relative ratio. The comparison further bears out that, while energetic photons (x and γ) and ion bombardment activate similar E′δ defects of axial symmetry, hole injection generates a more isotropic type, termed E′δ2. It reveals the tendency that as the damaging species gets ‘‘softer,’’ the g matrix anisotropy fades together with the disorder-induced g spread.
Redistribution of constituent elements in Pd/Ge contacts to n-type GaAs using rapid thermal annealing
Authors: Lai Jiun Tsuen , Lee Joseph Ya-min
Published: July 2002

Pd/Ge contact to n-type GaAs is performed by using electron-beam evaporation and rapid thermal annealing. The rapid thermal annealing is performed at 400–500 °C for various time durations. Low specific contact resistivity on the order of 10−6 Ω cm2 is obtained from measurements based on the transmission line model method. The contact depth profiles are analyzed by secondary ion mass spectrometry (SIMS). A very shallow ohmic contact is achieved. The redistribution of constituent elements after heat treatment is examined. A gallium SIMS signal bump is detected in the contact layer and is correlated with good ohmic contact behavior. A model based on Ga vacancies is proposed to explain this phenomenon. This shallow ohmic contact technology has been successfully utilized to fabricate GaAs/AlGaAs and GaAs/InGaAs/AlGaAs negative resistance field-effect transistors, for which shallow ohmic contact is critical.
Dependence of the interface sharpness of a Ge single quantum well on molecular-beam-epitaxial growth conditions
Authors: Kissinger W. , Osten H. J. , Lippert G. , Dietrich B. , Bugiel E.
Published: July 2002

The influence of molecular-beam-epitaxy growth conditions on the properties of five monolayers of germanium, embedded in a (001) silicon matrix for a conventional as well as an antimony-mediated growth in the temperature region from 300 to 450 °C, was investigated. The layers were analyzed by electroreflectance (ER), Raman spectroscopy, and transmission electron microscopy; they show compatible results for all three methods of investigation. For growth without antimony, a tendency toward segregation-induced alloying with increasing growth temperatures was observed. Antimony-mediated growth experiments show that the surfactant is able to improve the bulk character of the germanium layer at higher temperatures only, while it does not significantly influence the layer growth at lower temperatures. Among all investigated growth conditions the best sharpness of the germanium layer interface was found for the antimony-mediated growth at 450 °C. An annealing after growth at increasing temperatures increased the alloying by an interdiffusion of Si and Ge as indicated by Raman measurements. In ER a vanishing of the Ge-like transitions was observed after a treatment at temperatures between 600 and 700 °C for 15 min.
Eck peak in underdamped discrete superconducting vortex flow devices
Authors: van der Zant H. S. J. , Orlando T. P.
Published: July 2002

We have measured vortex motion in a long one-dimensional array of small underdamped niobium tunnel junctions which are coupled in parallel by superconducting wires. In the current-voltage characteristic, a resonant step occurs similar to the Eck peak in long continuous junctions. Due to the discrete nature of our samples, the voltage of this resonant step follows a sinusoidal dependence on the magnetic field rather than the linear dependence found in long continuous junctions. With a control current applied parallel to the array of junctions, the device can be operated as a vortex-flow transistor or as a tunable vortex-flow oscillator. Compared to long continuous junctions, the performance of discrete transistors is better with regard to the threshold current and transresistance, but worse with regard to the output resistance. Discrete oscillators have the advantage over their continuous counterparts in that the output impedance is of the order of 10 Ω. Numerical simulations indicate power levels around 0.5 μW and frequencies above 500 GHz.
X-ray structural studies of nitrogen diffusion in Dy2Fe17
Authors: Girt Er. , Altounian Z. , Chen X. , Mao Ming , Ryan D. H. , Sutton M. , Cadogan J. M.
Published: July 2002

The reaction between N2 and Dy2Fe17 has been studied by thermopiezic analysis on 20–25 μm sized powders in the temperature range 400–500 °C. Partially nitrided powders were analyzed using CuKα x-ray diffraction and thermomagnetic techniques. Both high angle x-ray and thermomagnetic data show only the presence of Dy2Fe17 and Dy2Fe17N3−δ (δ<0.3) with no evidence of intermediate compositions. The results of the x-ray diffraction experiments at several Bragg peaks were simulated using a two phase model structure: a Dy2Fe17 core with a Dy2Fe17N3 surface layer. The results show that at low temperatures the nitride layer is too thin to account for all of the nitrogen absorbed by the sample, indicating that a significant amount of the nitrogen diffused into the core of the particles, presumably along grain boundaries.
Improvement of minority carrier diffusion length in Si by Al gettering
Authors: Joshi Subhash M. , Gösele Ulrich M. , Tan Teh Y.
Published: July 2002

Gettering is already an integral part of fabricating integrated circuits using Si substrates. It is anticipated that this will also be true for solar cell fabrication in the near future. A readily available technique compatible with solar cell processing is gettering by the Si wafer back surface Al. Recently, available solar cell efficiency studies have shown the beneficial effects of the wafer backside Al, including that of gettering, a wafer backside field, and grain boundary and dislocation passivation. In this article, we report on experimental results which showed that Czochralski Si wafer bulk minority carrier diffusion lengths can be substantially improved by wafer backside Al treatment, which also provided an effect of protection from environmental contamination. In these experiments, only the effect of gettering is present and therefore the results constitute an unambiguous demonstration of the benefits of gettering by Al.
Dielectric, ferroelectric, and piezoelectric properties of lead zirconate titanate thick films on silicon substrates
Authors: Chen H. D. , Udayakumar K. R. , Cross L. E. , Bernstein J. J. , Niles L. C.
Published: July 2002

This article reports the fabrication of thick films of lead zirconate titanate (PZT) on platinum-buffered silicon substrates by screen printing. Crack-free films, up to 12 μm on a single pass, show a dielectric permittivity of 200, tangent losses of 0.05, remanent polarization of 2.5 μC/cm2, and coercive field of 40 kV/cm. The field-induced longitudinal piezoelectric coefficient d33 at 40 kV/cm dc bias and 4 kV/cm alternating field corresponded to 50 pC/N. The magnitude of the piezoelectric voltage coefficient g33, computed from the strain coefficient and dielectric permittivity, under the same conditions, was found to be 36×10−3 V m/N, higher than that of a poled PZT bulk ceramic in comparison. These results are promising for a broad variety of sensor applications.
Intense visible light emission from fullerene-doped silica aerogel
Authors: Zhu Lei , Li Yufen , Wang Jue , Shen Jun
Published: July 2002

We report that fullerenes (C60/C70) can be uniformly encased in silica aerogel by sol-gel process together with supercritical drying technique. Intense visible light emission (2.26 eV) is observable with the naked eye from the composite under an Ar+ laser (488 nm) excitation at room temperature. The luminescent intensity is found to increase strongly with low doping of fullerenes (0.05 mol %) and then to decrease while the peak is red shifted, and finally quenched upon heavy doping (up to 2.5 mol %). This effect is consistent with the behavior of the fullerenes entrapped in the micropores linked by the silica networks resulting in the quantum confinement effect which then produce emission far above the interband gap of C60/C70.
Raman spectra in laser ablated lead zirconate titanate thin films near the morphotropic phase boundary
Authors: Zhang Hongxue , Leppävuori Seppo , Karjalainen Pentti
Published: July 2002

The Raman scattering measurements were carried out in laser ablated lead zirconate titanate (PZT) thin films, with a Ti/Zr ratio of 0.45/0.55 near the morphotropic phase boundary (MPB). The thin films were deposited using a Nd:YAG laser and annealed between 500 and 900 °C. In the specimens annealed at lower temperature, the Raman spectrum was found to be identical to that of the ceramic target, except for some extra peaks originating from certain minor phases. Increasing the annealing temperature resulted in the rhombohedral Raman line around 500 cm−1 being broadened and splitting of the peak was eventually observed at 750 °C. Simultaneously, a broad shoulder appeared near the zero frequency. These phenomena indicate the tetragonal Raman mode to be formed during annealing. A contradictory result was obtained from the x-ray diffraction investigation, however, showing the formation of the tetragonal phase only above 800 °C. This discrepancy is thought to be due to a degree of ‘‘mismatch’’ between the structure and polarization state around the MPB, which concerns the non-equilibrium displacement of the central cations in the PZT lattice.
Diffusion length measurements on n-CuInS2 crystals by evaluation of electron-beam-induced current profiles in edge-scan and planar configurations
Authors: Scheer R. , Lewerenz H. J.
Published: July 2002

The diffusion length of minority carriers in n-CuInS2 crystals grown under different values of nonmolecularity Δm is examined by electron-beam-induced current (EBIC) experiments. Current profiles are recorded in edge-scan and planar configurations. The values of the diffusion length are obtained by comparison with modeled profiles showing reasonable agreement between both experimental configurations. The defect-electron diffusion length varies from 0.4±0.15 to 1.25±0.25 μm for samples prepared with Δm=−0.09 to Δm=−0.2, respectively. From EBIC profiles in the edge-scan configuration the surface recombination velocity can be estimated to about S=104 cm s−1 for unpassivated CuInS2 samples.
Distribution of SiO2 precipitates in large, oxygen rich Czochralski-grown silicon single crystals after annealing at 750 °C
Authors: Bouchard R. , Schneider J. R. , Gupta S. , Messoloras S. , Stewart R. J. , Nagasawa H. , Zulehner W.
Published: July 2002

The defects due to oxygen precipitation in Czochralski grown silicon single crystals annealed for 216 h at 750 °C and with oxygen concentration varying between 18 and 12×1017 atoms per cm3 (according to DIN 50438) were studied by means of small angle neutron scattering and γ-ray diffractometry probing the same volume elements in the sample. The size and the shape of the SiO2 precipitates were determined by means of small angle neutron scattering. In the center of the disk-shaped sample of 10 cm diameter one finds spherical precipitates with a radius of ≊20 Å, at the border the precipitates are of plate-like shape, ≊146×146×41 Å in dimension. The k-space distribution of the diffuse scattering caused by the strain field of the SiO2 precipitates has been determined by means of a double-crystal diffractometer and 316 keV γ radiation. Modeling with Huang and Stokes–Wilson theory suggests that the relatively small SiO2 precipitates are loosely bound in larger clusters with a radius of ≊2000 Å in which the crystal matrix is strongly distorted. With the γ-ray diffractometer operated in the single-crystal mode the thickness dependence of the integrated reflecting power was measured from which the static Debye–Waller factor for the 2 2 0 reflection is determined using statistical dynamical theory. The results confirm qualitatively the cluster model. In the center of the sample the size of the precipitates could be calculated by additional measurement of the static Debye–Waller factor of the 4 4 0 reflection. The result is in excellent agreement with the small angle neutron scattering data. Combining all the experimental data taken in the oxygen rich center of the annealed silicon crystal the average distance between the centers of the SiO2 precipitates in the clusters is ≊80 Å and the distance between the large clusters is ≊40 000 Å.
Normal zone propagation in Rutherford-type superconducting cables
Authors: Kovner V. S. , Mints R. G.
Published: July 2002

We study the formation and propagation of a normal zone in Rutherford-type superconducting cables. We use a rectangular conductor model to account for the electric current redistribution process between the multifilamentary strands in the presence of a normal zone. We obtain and integrate numerically the diffusion equations for the temperature and the current density distributions. Our simulations show the formation of stable normal domains propagating along the cable for certain values of the current and parameters characterizing the cable and cooling conditions.
Deep-level transient spectroscopy study in gettered liquid phase epitaxy grown In0.53Ga0.47As
Authors: Pal D. , Bose D. N.
Published: July 2002

Deep-level transient spectroscopy has been carried out using Schottky diodes fabricated on n-In0.53Ga0.47As grown by liquid phase epitaxy (LPE). A defect level corresponding to oxygen having an activation energy Ec−0.32 eV and capture cross section 7×10−17 cm2 was observed. Depth profiling of shallow donors and oxygen showed that concentrations of both these impurities increased towards the surface of the epilayers. Dy gettering during LPE growth significantly reduced the oxygen concentration and also resulted in lower etch-pit density.
Effects of dry etching damage removal on low-temperature silicon selective epitaxial growth
Authors: Tseng H.-C. , Chang C. Y. , Pan F. M. , Chen L. P.
Published: July 2002

The epitaxial silicon layer selectively grown on the reactive ion etched (RIE) silicon substrate using CF4, CHF3 and Ar etching gases has been studied. Defects and contaminants induced by the RIE process result in a rough epilayer, and degrade the current–voltage (I–V) characteristics. An interfacial carbide layer is present between the epilayer and the RIE treated substrate. Using an efficient and convenient after-etching treatment with a CF4/O2 low-energy plasma, we obtain a clean Si surface in the patterned oxide windows for selective epitaxial growth, and the electrical characteristics are significantly improved.
Growth of Zn δ-doped AlxGa1−xAs by low pressure metal organic vapor phase epitaxy
Authors: Li G. , Petravić M. , Jagadish C.
Published: July 2002

A very significant Zn evaporation from nongrowing AlxGa1−xAs surface during a post-δ-doping purge step was observed in growth of Zn δ-doped AlxGa1−xAs (x<0.65) by low pressure metal organic vapor phase epitaxy using dimethylzinc as a doping precursor. A δ-doping sequence different from the normal ‘‘purge-doping-purge’’ is therefore proposed to minimize the Zn evaporation. Using this δ-doping sequence, the dopant memory effect was investigated and the best hole profile of Zn δ-doped GaAs (Al0.35Ga0.65As) was obtained, having a full width at half-maximum of 7.0 nm for a peak concentration of 1.1×1020 cm−3 (13 nm for 4.8×1018 cm−3). It was found that the growth temperature significantly influences the hole concentration of Zn δ-doped GaAs and the hole concentration decreases and the hole profile width increases with increasing Al content of Zn δ-doped AlxGa1−xAs.
Structural characterization of SinC δ layers embedded in a silicon matrix
Authors: Ruvimov S. , Bugiel E. , Osten H. J.
Published: July 2002

Besides a great variety of carbon-containing atomic complexes and precipitates in crystalline silicon, new ordered silicon–carbon structures have recently been predicted theoretically by ab initio calculations [Rücker, Methfessel, Bugiel, and Osten, Phys. Rev. Lett. 72, 3578 (1994)] to be possible in highly C-doped epitaxial layers. These layers are strain stabilized. Due to the high mismatch in lattice parameters, pseudomorphic growth is restricted to layer thicknesses of some monolayers only. Four-hundred kV high resolution electron microscopy (HREM) has been applied to study the microscopic structure of the SinC layers grown by molecular beam epitaxy on (001)-oriented silicon substrate. Local ordering of the carbon with a quasiperiodical variation of its distribution over the layer has been observed on HREM micrographs at certain imaging conditions.
Structure and magnetic properties of NdCo13−xSix
Authors: Weihua Tang , Jingkui Liang , Guanghui Rao , Fuming Yang
Published: July 2002

NdCo13−xSix (2.5≤x≤4.0) compounds have been prepared by arc melting and subsequent annealing. X-ray powder diffraction indicated that they crystallized in a tetragonal NaZn13-derivative structure. Thermomagnetic measurements in an applied field of 1.2 T revealed that the compounds are ferromagnetic. From the magnetization at 1.5 K a value of 2.2μB is obtained for the magnetic moment of Nd3+, and this value is about 1.4μB smaller than the value of a free Nd3+ ion, i.e., the Nd3+ moment in these compounds seems to be significantly quenched.
Epitaxial growth and characterization of Y2Co17(0001) thin films deposited on W(110)
Authors: Robaut F. , Milkulik P. , Cherief N. , McGrath O. F. K. , Givord D. , Baumbach T. , Veuillen J. Y.
Published: July 2002

Y2Co17(0001) was grown on W(110) by pulsed laser deposition. The structural and morphological features of the films as a function of substrate temperature and thickness were studied in situ by means of reflection high-energy electron diffraction, and ex situ by means of scanning electron microscopy, transmission electron microscopy, and x-ray analysis (diffraction and reflection). Different growth modes were observed depending on the experimental conditions. They are discussed by considering the kinetical properties of the deposit and the thermodynamical properties of the system. The magnetic properties of the films have been investigated and related to the structural analysis.
Nonlinear motion of crack tip atoms during dislocation emission processes
Authors: Tan Honglai , Yang Wei
Published: July 2002

Nonlinear features of dislocation emission processes under mode II loads are explored from an atomic scale. Crack tip atom string models coupling with the continuum mechanics analysis are devised. Dynamic analysis shows that the atom motion at the crack tip changes from periodic to chaotic, as the mode II stress intensity factor increases. The chaotic atom motion dictates the dislocation nucleation process at the crack tip. Study on the dislocation emission band reveals the phenomenon of cloudlike drifting of the dislocation core ahead of the crack tip.
Crosslinked polyimide electro-optic materials
Authors: Kowalczyk T. C. , Kosc T. Z. , Singer K. D. , Beuhler A. J. , Wargowski D. A. , Cahill P. A. , Seager C. H. , Meinhardt M. B. , Ermer S.
Published: July 2002

We report studies of the optical and electro-optic properties of guest–host polymeric nonlinear optical materials based on aromatic, fluorinated, fully imidized, organic soluble, thermally, and photochemically crosslinkable, guest–host polyimides. We have introduced temperature stable nonlinear optical chromophores into these polyimides and studied optical losses, electric field poling, electro-optic properties, and orientational stability. We measured electro-optic coefficients of 5.5 and 12.0 pm/V for ((2,6-Bis(2-(3-(9-(ethyl)carbazolyl))ethenyl)4H-pyran-4-ylidene)propanedinitrile (4-(Dicyanomethylene)-2-methyl-6-(p -dimethylaminostyryl)-4H-pyran) DCM-doped guest–host systems at 800 nm using a poling field of 1.3 MV/cm. Poling induced nonlinearities in single-layer films were in agreement with the oriented gas model, but were lower in three-layer films due to voltage division across the layers.
Heavily doped CdTe films grown by close-spaced vapor transport technique combined with free evaporation
Authors: Castro-Rodríguez R. , Zapata-Torres M. , Zapata-Navarro A. , Oliva A. I. , Peña J. L.
Published: July 2002

Very heavily doped n-type polycrystalline CdTe films doped with metallic cadmium were prepared by using close-spaced vapor transport technique combined with free evaporation and the electrical, structural, and morphological properties were investigated. Cadmium was introduced as a dopant by evaporation during films preparation. The highest dark conductivity at room temperature of the films obtained was 1.18×104 S cm−1. The dark conductivity decreased with the increase of the ambient temperature. The highest dark electron concentration obtained was 1.59×1022 cm−3 and increased with the temperature. The mobility decreased with the temperature. The film was a polycrystalline cubic phase. We show the surface topography of the film using scanning electron microscope and scanning tunneling microscopy techniques, in order to see the growth patterns. The crystallite size was very uniform of a few μm, and the growth patterns on the grain surface were in form of terraces.
Temperature- and carrier-density-dependent electron tunneling kinetics in (Ga,In)As/(Al,In)As asymmetric double quantum wells
Authors: Ten S. , Krol M. F. , McGinnis B. P. , Hayduk M. J. , Khitrova G. , Peyghambarian N.
Published: July 2002

We present a detailed investigation of electron tunneling in (Ga,In)As/(Al,In)As asymmetric double quantum wells as a function of different excitation and temperature conditions. We show that tunneling dynamics depend strongly on the initial carrier temperature and momentum. For example, electron and hole tunneling out of the narrow well is complete at low temperature. However at room temperature carriers do not exhibit any tunneling kinetics. We propose a simple kinetic model which describes the observed population dynamics at different carrier densities, temperatures, and excitation conditions.
Giant magneto-impedance effect in nanostructured magnetic wires
Authors: Knobel M. , Sánchez M. L. , Gómez-Polo C. , Marín P. , Vázquez M. , Hernando A.
Published: July 2002

The giant magneto-impedance effect (GMI) is studied as a function of the structural modification induced in an Fe73.5Si13.5B9Cu1Nb3 amorphous alloy wire by annealing. The values of GMI are correlated to those structural changes and with the corresponding variation of the magnetic properties and intrinsic resistivity. Excellent soft magnetic properties, associated with low resistivity values, make this nanostructured material as one of the most promising for future applications of the GMI effect. The tailoring of the structure which can be induced by adequate thermal treatments easily allows one to obtain excellent combinations of circumferential permeability μφ and resistivity ρ during different devitrification stages, in order to produce materials for specific aims. Maximum GMI ratios of 200% are found after annealing the wires in the range 550–600 °C, where an optimum compromise between μφ and ρ is found. A simple model is developed correlating the fundamental physical properties of the soft magnetic wires with the measured values of both components of the impedance, allowing the prediction of experimental GMI ratios and an easy visualization of the phenomenon.
Fluttering fountains: Annular geometry
Authors: Casperson Lee W.
Published: July 2002

Under certain conditions of flow rate, height, and feedback, periodic or chaotic fluttering oscillations can be observed as the sheet of water from a dam or waterfall fountain descends through the air. Numerical and analytical interpretations of this phenomenon have recently been reported. The extension of these results to other fountain geometries is discussed here together with experimental observations on an annular waterfall fountain.
Electrical and dielectric properties of systems of porous solids and salt-containing nonpolar liquids
Authors: Nettelblad B.
Published: July 2002

We have performed dielectric measurements on porous samples, made of sintered polypropylene grains, impregnated with an organic liquid (a mixture of mono- and dibenzyl toluenes) into which we had dissolved a salt (tetraisoamylammonium picrate). This is a system that has not been widely studied before. We find that the formation factor of electrical conduction has a cementation exponent similar to what has been previously reported for sintered glass beads. We also find an increase in the permittivity at very low frequencies. We attribute this to diffusion effects at the solid–liquid boundary. We show how this low-frequency relaxation changes with temperature.
Magnetic properties of (11̄02) Dy/Y superlattices
Authors: Theis-Bröhl K. , Ritley K. A. , Flynn C. P. , Hamacher K. , Kaiser H. , Rhyne J. J.
Published: July 2002

Epitaxial (11̄02) Dy/Y rare earth superlattices and a thick (11̄02) Dy film, grown on sapphire with Y/Ta buffer layers, have been prepared by molecular beam epitaxy. Neutron diffraction and SQUID magnetization measurements on a single 200 nm thick (11̄02) Dy layer showed nearly bulk behavior. However, for the (11̄02) Dy/Y superlattices we found different magnetic behavior depending on the relative Dy to Y-layer thickness. The superlattices exhibit both ferromagnetic and helical phases, but with the Néel and Curie temperatures significantly different from bulk Dy. These results differ from previous findings in Dy/Y superlattices grown along [0001].
Structure and properties of tin-doped indium oxide thin films prepared by reactive electron-beam evaporation with a zone-confining arrangement
Authors: Rauf I. A.
Published: July 2002

The microstructure and electrical properties of tin-doped indium oxide thin films crystallized under the action of a temperature gradient are compared with those of films prepared under uniform temperature. This zone-confining process, coupled with slow deposition rates, causes preferential segregation of dopants from grains growing at lower temperatures toward grain boundaries located at higher temperature. Recrystallization within a chain or zone of grains on a line of equal temperature causes these grains to be oriented in the same direction. The boundaries separating grains are twin boundaries. Electronic mean free paths in the zone-confined specimens are from two to four times the average size of the grains.
Tb impurities in Th: A Kondo system with undercompensated magnetic moment?
Authors: Sereni J. G. , Schlottmann P.
Published: July 2002

Dilute TbxTh1−x alloys have been thoroughly studied in the light of a possible valence instability and Kondo effect in Tb3+ and Tb4+ ions. We review the experimental data for the specific heat, magnetic susceptibility, magnetization, magnetoresistivity, and the change of the superconducting properties of Th. In analogy to Tm (Tm2+ and Tm3+) and Pr (Pr3+ and Pr4+) impurities in metals, Tb involves two electronic configurations each with a magnetic Hund’s rule ground multiplet. Two classes of models for valence admixtures of two magnetic configurations have been investigated in the past. Depending on the form of the hybridization and the jj coupling within the 4f shell, the models either have a singlet or a magnetic ground state. As for Tm impurities the latter class of models, related to the undercompensated Kondo effect, appears to describe the system TbTh more appropriately. A qualitative explanation of the data is attempted within a scheme that combines Kondo screening with crystalline field splitting.
Thermal effects on surface Fermi level for GaAs(001)
Authors: Hirota Y. , Sumitomo K.
Published: July 2002

Thermal effects in a high vacuum on horizontal Bridgman grown GaAs(001) surface prepared by deoxygenated and de-ionized water treatment were investigated by x-ray photoelectron spectroscopy, low-energy electron diffraction, ultraviolet photoelectron spectroscopy, and photoluminescence (PL) measurement. The ultraviolet photoelectron spectra show that, below 450 °C, the surface Fermi level lies at almost 0.85–1.0 and 0.68–0.8 eV above the valence-band maximum, respectively, for lightly and highly Si-doped GaAs surfaces. Above 480 °C, the surface Fermi levels of both the surfaces gradually come close to 0.45–0.54 eV above the valence-band maximum even though the surface keeps the 2×4 structure. PL measurements suggest that the surface Fermi level position is strongly affected by arsenic and gallium vacancies created near the surface during thermal processing.
Ohmic contacts formation on n-InP
Authors: Morais J. , Fazan T. A. , Landers R. , Sato E. A. S.
Published: July 2002

We have investigated the correlation between contact resistance, heat treatment, and micro structure in ohmic contacts on n-InP. The samples consisted of three different structures: 50 nm-Ni/200 nm-AuGe/n-InP, Au/100 nm-Pt/100 nm-Ti/50 nm-Ni/n-InP, and 200 nm-Au/100 nm-Pt/100 nm-Ti/50 nm-Ni/200 nm-AuGe/n-InP. After annealing the samples, depth profiles obtained by Auger electron spectroscopy and ion sputtering showed a tendency of the Ge to migrate from the Au–Ge alloy towards the Ni layer, as well as an accumulation of Ni at the semiconductor interface. A Ni–P phase is identified as being responsible for the ohmic character of the metal/InP interface. Specific contact resistivities were measured for the three different types of metallizations. We observed that the resistivity is sensitive to the annealing temperature and related to the amount of Ni at the metal/InP interface. The use of overlayers is suggested to avoid migration of the semiconductor components towards the contact surface.
Analysis of the dark current in the bulk of InAs diode detectors
Authors: Kuan C. H. , Lin R.-M. , Tang S.-F. , Sun T.-P.
Published: July 2002

We report on analysis and comparison of the dark current characteristics between InAs p-n and p-i-n diodes at the temperature range from 30 to 300 K. The situation that the dark current is completely due to the bulk instead of the surface leakage is made sure by passivation treatment. The experimental results agree well with a tunnel diode model in which the p-n and p-i-n diodes are described, respectively, with a linear electrical field and a constant electrical field. The diffusion current in our diodes is dominated by the electron diffusion in the p-type material. Through this model and detailed analysis, we conclude that the tunneling current can be suppressed with an intrinsic layer and a low n-type doping density while the diffusion current can be decreased with a high p-type doping density. The advantage of the p-i-n structure is not only to cut down the tunneling current but also to increase the uniformity. In terms of this model, a new p-i-n diode is designed to dramatically increase its zero-bias resistance area product and improve the detector performance.
Laser beam interference effects on the photovoltage of a p-n junction diode
Authors: Weiser K. , Dahan F. , Schacham S. E. , Shur M. , Towe E. , Park H.
Published: July 2002

The photovoltage developed by a p-n junction diode illuminated by an interference pattern through a slit depends both on the distance between fringes and the phase of the interference pattern relative to the position of the slit. For a slit width which accommodates an integer number N of fringes, the voltage is independent of the phase of the pattern but this is no longer the case for a noninteger number of fringes. The maximum dependence is observed for 𝒩+1/2 fringes within the slit but the effect decreases as 𝒩 increases. When the fringe distance is no longer negligible compared to the minority carrier diffusion length the dependence of the photovoltage on the number of fringes becomes more pronounced. A theory is presented which shows how the diffusion length can be obtained from the experimental data. Experiments on GaAs and Si diodes are reported.
Reflection of plane waves at medium–composite interfaces and input acoustic impedance of laminated piezoceramic polymer composites
Authors: Zhang Q. M. , Geng Xuecang , Yuan J.
Published: July 2002

Plane wave propagation at the composite–medium interface is analyzed for the piezoceramic polymer composites with laminated periodic structure. It is found that the input acoustic impedance of the composite depends on the elastic properties of the medium at the interface and in general is a complex number and dispersive. It is also found that the reflected wave from the medium–composite interface suffers a more than 180° phase change, which implies that the thickness of the conventional antireflection ‘‘quarter wave matching layer’’ at the interface should be less than a quarter wave length.
Leakage current in rapid thermally processed lead iron niobate films synthesized by a sol gel method
Authors: Sedlar Miroslav , Sayer Michael , Amm David T.
Published: July 2002

The electrical conductivity of two different types of lead iron niobate thin films prepared by a modified sol gel method have been studied. PEN films rapid thermally processed at 425 °C/30 s and at 650 °C/1 s exhibited a highly nonlinear leakage current behavior. At room temperature space charge limited current conduction was not observed for any of the samples. Due to enhanced thermal detrapping at higher measurement temperatures the conduction became bulk limited and the leakage current was controlled by Poole–Frenkel emission. In samples processed at 650 °C/1 s the injection of electrons occurred through a Schottky barrier near the bottom electrode resulting from a higher processing temperature.
Thermal and electrodynamic effects in melting current-carrying conductors
Authors: Dolinsky Yu. , Elperin T.
Published: July 2002

This work studies dynamics of melting in current-carrying conductors. It is shown that during equilibrium melting, when the dynamics of a phase-transition front are determined by a heat balance, there exists a single-valued correlation between the rate of inductance change and a temperature at the phase-transition front. It is demonstrated that, although surface melting begins at temperatures less than the melting temperature of the current-free conductor, corrugation of a phase-transition front occurs at this temperature. It is shown that the nucleation rate of nuclei flattened in the direction normal to a conductor’s axis is negligibly small. Current dynamics at the stage of melting are analyzed for the case of a fixed external voltage. It is demonstrated that there exists an instability stage at which an excitation of the electric current occurs. Conditions for the experimental observation of the predicted phenomena are discussed.
Lamp-pumped laser performance of Nd3+:Sr5(PO4)3F operating both separately and simultaneously at 1.059 and 1.328 μm
Authors: Zhang X. X. , Bass M. , Chai B. H. T. , Johnson P. J. , Oles J. C.
Published: July 2002

Lamp-pumped laser performance of Nd3+-doped strontium fluorapatite, Sr5(PO4)3F or S-FAP, has been characterized and compared with that of Nd3+-doped yttrium aluminum garnet (YAG) at both 1.06 and 1.3 μm. Nd3+:S-FAP was found to exhibit lower thresholds and lower slope efficiencies than Nd3+:YAG. The former is attributed to the higher emission cross section, and the latter to lower Nd3+ concentration in S-FAP. The 1.3 μm lasing of Nd3+:S-FAP is of particular interest because of its high emission cross section (2.4×10−19 cm2). Q-switched and dual-wavelength lasing operation were also demonstrated in Nd3+:S-FAP.
Vibrational local modes of a-Si1−xCx:H alloys and variation of local modes in different local environments
Authors: Lin Shu-Ya
Published: July 2002

The vibrational density-of-states for various bonding structures in a-Si1−xCx:H are calculated. The local modes induced by the SiHn and CHn groups are identified. The bending modes of SiHn group have large variation between the a-Si1−xCx:H and a-Si:(H,C) alloys. The bending modes induced by the CHn groups are almost the same in the above two different alloys. The C-dangling bond has the effect of shifting the Si–C vibrational mode toward lower frequency.