bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural characterization of GaN laterally overgrown on a (111)Si substrate
Authors: Tanaka Shigeyasu , Honda Yoshio , Sawaki Nobuhiko , Hibino Michio
Published: July 2002

Using transmission electron microscopy, we have characterized defect structures in laterally overgrown GaN crystals, grown directly on SiO2 stripe-patterned (111)Si substrates by metalorganic vapor phase epitaxy using AlGaN as an intermediate layer. The width and the period of the stripe windows were nominally 1 and 2 μm, respectively. The average threading dislocation density for a completely coalesced 2-μm-thick GaN crystal obtained on the [112̄]-oriented stripe-patterned substrate was ∼2×109 cm−2. The reduction in threading dislocation density is a consequence of the lateral growth and dislocation reactions at the coalesced front of the mask. On the other hand, valleys and pits tend to remain on the mask during the growth on the [11̄0]-oriented stripe-patterned substrate. Cracks were present in both crystals.
Wafer bonding of different III–V compound semiconductors by atomic hydrogen surface cleaning
Authors: Akatsu T. , Plössl A. , Scholz R. , Stenzel H. , Gösele U.
Published: July 2002

Large-area wafer bonding of different III–V compound semiconductors in an ultrahigh vacuum background is demonstrated. The bonding procedure, the microstructure, and the mechanical strength of the bonded GaAs/InP and GaAs/GaP interfaces were studied. The cleaning procedure and the bonding were separated in order to avoid undesired artifacts and thermal stress at the interface. First, thermally generated atomic hydrogen was employed to clean the surfaces. Then, the wafers were brought into contact below 150 °C. At contact, the interface formed spontaneously over the whole wafer area without application of a mechanical load. Transmission electron microscopy showed the formation of atomically direct interfaces and misfit dislocation networks. The fracture surface energy was measured as being comparable to that of respective bulk materials. Heat treatments of the bonded GaAs/InP samples led to relaxation of the interfaces but also to the formation of nanoscopic voids in the interface plane and volume dislocations.
Layer analysis of the structure of water confined in vycor glass
Authors: Gallo P. , Ricci M. A. , Rovere M.
Published: July 2002

A molecular dynamics simulation of the microscopic structure of water confined in a silica pore is presented. A single cavity in the silica glass has been modeled as to reproduce the main features of the pores of real Vycor glass. A layer analysis of the site–site radial distribution functions evidences the presence in the pore of two subsets of water molecules with different microscopic structure. Molecules which reside in the inner layer, close to the center of the pore, have the same structure as bulk water but at a temperature of 30 K higher. On the contrary the structure of the water molecules in the outer layer, close to the substrate, is strongly influenced by the water–substrate hydrophilic interaction and sensible distortions of the H-bond network and of the orientational correlations between neighboring molecules show up. Lowering the hydration has little effect on the structure of water in the outer layer. The consequences on experimental determinations of the structural properties of water in confinement are discussed.
Ion beam mixing of ZnO/SiO2 and Sb/Ni/Si interfaces under swift heavy ion irradiation
Authors: Kraft Saskia , Schattat Beate , Bolse Wolfgang , Klaumünzer Siegfried , Harbsmeier Felix , Kulinska Agnieszka , Löffl Anton
Published: July 2002

We have investigated the irradiation induced interface mixing in ZnO/SiO2 (α-quartz) and Sb/Ni/Si thin layer systems under swift heavy ion irradiation in the electronic stopping power regime. The irradiations were carried out at 77 K using 100 MeV Ar, 260 MeV Kr, and 200 MeV Xe ions. For the ZnO/SiO2 system experiments were also carried out at lower ion energies (300, 600, and 900 keV, respectively) where nuclear stopping dominates. The alterations of the interface concentration profiles were determined by means of Rutherford backscattering spectrometry performed subsequently at the irradiated and the nonirradiated parts of the samples. While for the semimetal/metal Sb/Ni interface almost no mixing could be found after high-energy irradiation (mixing efficiency for Xe ions: k/Se<0.02 nm5/keV) the ceramic system ZnO/SiO2 strongly reacts upon high energy ion irradiation (Xe: k/Se=2.1 nm5/keV). The Ni/Si interface shows an intermediate effect (Xe: k/Se=0.2 nm5/keV). The mixing behavior found at high ion energies is in contrast to that found in the nuclear stopping regime, where Sb/Ni shows very strong mixing and phase formation while ZnO/SiO2 exhibits only weak ballistic mixing. As was previously observed for the formation of ion tracks, interface mixing due to electronic energy deposition also sets in only if the electronic stopping power exceeds a threshold value.
Diffractive optics-based six-wave mixing: Heterodyne detection of the full χ(5) tensor of liquid CS2
Authors: Kubarych K. J. , Milne C. J. , Lin S. , Astinov V. , Miller R. J. D.
Published: July 2002

This work exploits the passive phase stabilization of diffractive optics to implement heterodyne detection of the complete χ(5) tensor of liquid CS2 as an example of a simple liquid. This approach permits the use of two different colors for the excitation, probe, and detection beam protocols and enables full optimization of the signal with respect to discrimination against lower order cascaded third-order responses. This work extends the previous study of polarization selectivity, in combination with heterodyne detection, to all independent polarization components to provide further insight into the origins of the fifth-order response and its connection to the multitime correlation of the liquid dynamics. The characteristic feature that clearly distinguishes the direct fifth-order response from lower order cascades is the pronounced ridge along the τ4 axis (probe pulse delay) with very rapid decay along the τ2 axis (excitation pulse delay). This observation is in contrast to recent related work using one-color homodyne detection. With the determination of the direct fifth-order and cascaded third-order signal amplitudes made possible by heterodyne detection, this difference can be attributed to cross terms between the direct fifth-order and cascaded third-order terms inherent to homodyne detection under phase matching conditions used to discriminate against cascades. In support of theoretical treatments, the previously predicted enhancement of rephasing pathways for certain polarization components has been observed. However, even for these tensor elements the remarkable feature is the very rapid decay in the nuclear coherence along τ2. The experiment is predicated on the ability of a 2-quantum transition involving the Raman overtone to rephase the nuclear coherence. These findings indicate that the nuclear motions, in the frequency range accessed, are strongly damped and draw into question the validity of the overtone as a viable pathway for rephasing. With the isolation of the direct fifth-order Raman response, new information regarding relaxation and dephasing pathways in liquids can be determined for the highest frequency modes. The results are in very good agreement with a recent finite field molecular dynamics simulation of liquid CS2 with respect to the polarization dependence of signal magnitudes, relative cascade signal amplitudes, and qualitative agreement with respect to the predicted temporal profiles.
A directional magnetization potential based model of magnetoelastic hysteresis
Authors: Armstrong William D.
Published: July 2002

The theory presented in this article successfully reproduces and explains the experimental magnetostriction and magnetization hysteresis behavior of Tb0.3Dy0.7Fe1.9. It is well known that a very large number of individual domain wall translation events combine to produce each measurable domain transformation in a macroscopic sized sample. Each individual domain wall may be expected to suffer some level of domain wall translation inhibition due to the presence of defects in the material, however, the severity of the inhibition will spatially vary. We therefore assume that the presence of defects in the material increases the directional magnetization potential of subsequent domain states within a process, and distributes nontrivial probabilities of occupation of 〈111〉 type domain states in a parameter selected, inverse exponential form familiar from the study of statistical thermodynamics. The increased magnetization potential of subsequent high magnetostriction and high magnetization states retards their occupation until higher intensity applied magnetic fields are produced, thus shifting the increasing applied magnetic field curve in a positive field direction and shifting the decreasing applied magnetic field curve in a negative field direction in a form consistent with experiment.
Quadratic diffusion Monte Carlo and pure estimators for atoms
Authors: Sarsa A. , Boronat J. , Casulleras J.
Published: July 2002

The implementation and reliability of a quadratic diffusion Monte Carlo method for the study of ground-state properties of atoms are discussed. We show in the simple yet nontrivial calculation of the binding energy of the Li atom that the method presented is effectively second-order in the time step. The fulfillment of the expected quadratic behavior relies on some basic requirements of the trial wave function used for importance sampling, in the context of the fixed-node approximation. Expectation values of radial operators are calculated by means of a pure estimation based on the forward walking methodology. It is shown that accurate results without extrapolation errors can be obtained with a pure algorithm, explicitely reported, that can be easily implemented in any previous diffusion Monte Carlo program.
Infrared predissociation spectroscopy of I−⋅(CH3OH)n, n=1,2: Cooperativity in asymmetric solvation
Authors: Robertson William H. , Karapetian Karen , Ayotte Patrick , Jordan Kenneth D. , Johnson Mark A.
Published: July 2002

Infrared spectra of I−⋅(CH3OH)n⋅Arm, n=1,2 clusters, obtained via argon and methanol predissociation, are interpreted with the aid of ab initio calculations of the OH stretching fundamentals. The spectra of the cold, argon-solvated clusters establish the coexistence of two isomeric forms of the n=2 cluster, with the asymmetric isomer displaying a dramatic (∼150 cm−1) OH red-shift relative to both the symmetric isomer and the n=1 complex. We trace this red-shift to cooperative H-bonding which is only operative in the asymmetric form. At the higher internal energies afforded by the bare (i.e., Ar-free) complexes, the spectra are radically changed. The strongly red-shifted band is suppressed, reflecting the loss of the cooperative effect as the methanol molecules are separated, while the bands assigned to the more open form are enhanced.
Orthogonal fluxgate mechanism operated with dc biased excitation
Authors: Sasada I.
Published: July 2002

A mode of operation is presented for an orthogonal fluxgate built with a thin magnetic wire. By adding a proper dc bias to the wire excitation, the new mode is easily established. In this case, the fundamental component of the induced voltage at the sensing coil (secondary voltage) is made sensitive to the axial magnetic field, compared to the second harmonic in a conventional orthogonal fluxgate. The operating principle is explained using a magnetization rotation model. A method is proposed to cancel the offset that is inevitable when the magnetic anisotropy is present in a magnetic wire at an angle to its circumference. Experimental results are shown for a sensor head consisting of a 2-cm-long Co-based amorphous wire 120 μm in diameter with a 220-turn sensing coil. The sensitivity obtained is higher than that obtained using a conventional type of the orthogonal fluxgate built with the same sensor head. It is also demonstrated that the proposed method for canceling the offset works well.
Consideration on an undulator magnetic structure for polarization control
Authors: Tanaka T. , Shirasawa K. , Kitamura H.
Published: July 2002

A magnetic structure composed of electromagnets and permanent magnets is proposed as an undulator to control the polarization state of synchrotron radiation. The horizontal field is produced by the permanent magnets and tuned by moving them along the undulator axis, while the vertical field is produced by the electromagnets and tuned by changing the coil current. The proposed device has two important degrees of freedom. One is that of the periodic length of the electromagnets and the other is that of the relative phase between the vertical and horizontal fields. Thanks to them, the proposed device works as various types of undulator to realize polarization control.
Potential shielding by the surface water layer in Kelvin probe force microscopy
Authors: Sugimura Hiroyuki , Ishida Yuzuru , Hayashi Kazuyuki , Takai Osamu , Nakagiri Nobuyuki
Published: July 2002

Kelvin probe force microscopy (KFM) was applied to two-dimensional profiling of silicon pn-structures covered with a 2 nm-thick oxide layer. The surface potential contrast between the p- and n-type regions depended on the hydrophobicity of the oxide surface when KFM imaging was conducted in air with a relative humidity of more than 50%. By decreasing the density of surface hydroxyl groups on the oxide layer through thermal annealing, the potential contrast between the p- and n-type regions increased. While there was no detectable contrast on samples covered with hydrophilic oxide with a water contact angle of almost 0°, contrast increased to greater than 50 mV on the samples covered with hydrophobic oxide with a water contact angle of about 80°. However, when KFM imaging was conducted in a dry nitrogen atmosphere with relative humidity less than 0.6%, a clear potential contrast of about 50 mV could be acquired even on samples covered with the hydrophilic oxide layer. Since samples with less adsorbed water on their surface showed greater potential contrast, contrast degradation is attributed to a shielding effect of the adsorbed water layer.
Improved ion detection efficiency of microchannel plate detectors
Authors: Deconihout B. , Vurpillot F. , Bouet M. , Renaud L.
Published: July 2002

The knowledge of the detection efficiency of the microchannel plate is of upmost importance in many applications using this type of particle detector. However, few attempts have been made to measure the absolute value of the detection efficiency of microchannel plates. In this article, a new way of accurately measuring the absolute value of the detection efficiency is described. When particles hit the detector in the interchannel web of the microchannel plate, secondary electrons are generated. These secondary electrons can be returned to neighboring channels by means of an electric field applied at the front face of the detector. The resulting increase in the detection efficiency has been quantified using the absolute measurement of the detection efficiency. It is found that the value of the returning field has to be carefully set in order to prevent detection artifacts that may lead to a wrong estimation of the detection efficiency. In the best conditions, the detection efficiency increase reaches 40%.
Charge transfer in small hydrogen bonded clusters
Authors: van der Vaart Arjan , Merz Kenneth M.
Published: July 2002

High level ab initio and density functional calculations on clusters of water with acetate, methylammonium, and dimethylphosphate show that charge is transferred from the hydrogen bond acceptor to the hydrogen bond donor. The amounts of charge transferred are small, between 0.01 and 0.05 electron per hydrogen bond, but increase nearly linearly with the number of hydrogen bonds. The transfer of charge is not an artifact of the computation, since charge is also transferred in the limit of zero basis set superposition error. Calculations on a number of hydrogen bonded clusters show that the semiempirical AM1 and PM3 methods give excellent agreement with high level MP2 charge transfer effects, especially for AM1. Our results indicate the importance of charge transfer in hydrogen bond interactions.
Ellipsoidal, cylindrical, bipolar and toroidal wormholes in 5D gravity
Authors: Vacaru Sergiu I. , Singleton D.
Published: July 2002

In this article we construct and analyze new classes of wormhole and flux tubelike solutions for the 5D vacuum Einstein equations. These 5D solutions possess generic local anisotropy which gives rise to a gravitational running or scaling of the Kaluza–Klein “electric” and “magnetic” charges of these solutions. It is also shown that it is possible to self-consistently construct these anisotropic solutions with various rotational 3D hypersurface geometries (i.e., ellipsoidal, cylindrical, bipolar and toroidal). The local anisotropy of these solutions is handled using the technique of anholonomic frames with their associated nonlinear connection structures [S. Vacaru, Ann. Phys. (N.Y.) 256, 39 (1997); Nucl. Phys. B 434, 590 (1997); J. Math. Phys. 37, 508 (1996); J. High Energy Phys. 09: 011 (1998); Phys. Lett. B 498, 74 (2001)]. Through the use of the anholonomic frames the metrics are diagonalized, in contrast to holonomic coordinate frames where the metrics would have off-diagonal components. In the local isotropic limit these solutions are shown to be equivalent to spherically symmetric 5D wormhole and flux tube solutions.
Dislocation evolution in 4H-SiC epitaxial layers
Authors: Jacobson H. , Birch J. , Yakimova R. , Syväjärvi M. , Bergman J. P. , Ellison A. , Tuomi T. , Janzén E.
Published: July 2002

4H-SiC commercial wafers and sublimation grown epitaxial layers with a thickness of 100 μm have been studied concerning crystalline structure. The substrates and the epitaxial layers have been separately investigated by high-resolution x-ray diffraction and synchrotron white beam x-ray topography. The results show that the structural quality was improved in the epitaxial layers in the [112̄0] and [1̄100] directions, concerning domain distribution, lattice plane misorientation, mosaicity, and strain, compared with the substrates. Misoriented domains have merged together to form larger domains while the tilt between the domains was reduced, which resulted in nonsplitting in diffraction curves. If the misorientation in the substrate is large, we can only see a slight decrease in the misorientation in the epilayer. At some positions on the substrates block structures (mosaicity) were observed. ω-rocking curves showed smaller full width at half maximum values and more uniform and narrow peaks, while the curvature was almost the same in grown epilayers compared with the corresponding substrates. We show that threading edge dislocations along the c axis in silicon carbide grown crystals transform to deflected dislocations in the epilayer. A formation mechanism for deflected dislocations and supporting facts are presented. We further show that these deflected dislocations are one possible source for the creation of stacking faults that recently has been reported to cause degradation in processed SiC bipolar diodes.
60% magnetoresistance at room temperature in Co–Fe/Al–O/Co–Fe tunnel junctions oxidized with Kr–O2 plasma
Authors: Tsunoda Masakiyo , Nishikawa Kazuhiro , Ogata Satoshi , Takahashi Migaku
Published: July 2002

The influence of the mixed inert gas species for plasma oxidization process of a metallic Al layer on the tunnel magnetoresistance (TMR) was investigated for a magnetic tunnel junction (MTJ), Ta 50 Å/Cu 200 Å/Ta 200 Å/Ni–Fe 50 Å/Cu 50 Å/Mn75Ir25 100 Å/Co70Fe30 25 Å/Al–O/Co70Fe30 25 Å/Ni–Fe 100 Å/Cu 200 Å/Ta 50 Å. Using Kr–O2 plasma, a 58.8% of TMR ratio was obtained at room temperature after annealing the junction at 300 °C, while the achieved TMR ratio of the MTJ fabricated with usual Ar–O2 plasma remained 48.6%. A faster oxidization rate of the Al layer by using Kr–O2 plasma is a possible cause to prevent the over oxidization of the Al layer, which depolarizes the surface of the underlaid ferromagnetic electrode, and to realize a large magnetoresistance.
Quasisaddles as relevant points of the potential energy surface in the dynamics of supercooled liquids
Authors: Angelani L. , Di Leonardo R. , Ruocco G. , Scala A. , Sciortino F.
Published: July 2002

The supercooled dynamics of a Lennard-Jones model liquid is numerically investigated studying relevant points of the potential energy surface, i.e., the minima of the square gradient of total potential energy V. The main findings are (i) the number of negative curvatures n of these sampled points appears to extrapolate to zero at the mode coupling critical temperature Tc; (ii) the temperature behavior of n(T) has a close relationship with the temperature behavior of the diffusivity; (iii) the potential energy landscape shows a high regularity in the distances among the relevant points and in their energy location. Finally we discuss a model of the landscape, previously introduced by Madan and Keyes [J. Chem. Phys. 98, 3342 (1993)], able to reproduce the previous findings.
Water-induced degradation in lead zirconate titanate piezoelectric ceramics
Authors: Chen W. P. , Chan H. L. W. , Yiu F. C. H. , Ng K. M. W. , Liu P. C. K.
Published: July 2002

Water-induced degradation in lead zirconate titanate (PZT) piezoelectric ceramics was studied by a comparison experiment. In one treatment, PZT rings were immersed in a 0.01 M NaOH solution for long periods of time; in the other treatment, PZT rings were placed in a 0.01 M NaOH solution and hydrogen was evolved onto their silver electrodes by the electrolysis of water. No degradation occurred to samples immersed in the NaOH solution; but for the samples that were also treated by electrolysis of water, serious degradation occurred as their mechanical quality factor decreased dramatically and their dielectric loss increased considerably. I–V measurement showed that the resistance decreased greatly in the degraded samples, which could be explained by the formation of charge carriers due to the reduction reaction of atomic hydrogen from the electrolysis of water. It was proposed that the reduction reaction of atomic hydrogen at ambient temperature is an important origin for the degradation of piezoelectric ceramic devices.
Vulnerability to ventricular fibrillation
Authors: Janse Michiel J.
Published: July 2002

One of the factors that favors the development of ventricular fibrillation is an increase in the dispersion of refractoriness. Experiments will be described in which an increase in dispersion in the recovery of excitability was determined during brief episodes of enhanced sympathetic nerve activity, known to increase the risk of fibrillation. Whereas in the normal heart ventricular fibrillation can be induced by a strong electrical shock, a premature stimulus of moderate intensity only induces fibrillation in the presence of regional ischemia, which greatly increases the dispersion of refractoriness. One factor that is of importance for the transition of reentrant ventricular tachycardia to ventricular fibrillation during acute regional ischemia is the subendocardial Purkinje system. After selective destruction of the Purkinje network by lugol, reentrant tachycardias still develop in the ischemic region, but they do not degenerate into fibrillation. Finally, attempts were made to determine the minimal mass of thin ventricular myocardium required to sustain fibrillation induced by burst pacing. This was done by freezing of subendocardial and midmural layers. The rim of surviving epicardial muscle had to be larger than 20 g. Extracellular electrograms during fibrillation in both the intact and the “frozen” left ventricle were indistinguishable, but activation patterns were markedly different. In the intact ventricle epicardial activation was compatible with multiple wavelet reentry, in the “frozen” heart a single, or at most two wandering reentrant waves were seen.
Computation of the Whittaker function of the second kind by summing its divergent asymptotic series with the help of nonlinear sequence transformations
Authors: Weniger Ernst Joachim
Published: July 2002

The computation of the Whittaker function Wκ,μ(z) via its divergent asymptotic expansion is discussed. This divergent series can be summed with the help of Padé approximants, which convert its partial sums into rational functions [C. de Izarra, O. Vallée, J. Picart, and N. Tran Minh, Comput. Phys. 9, 318 (1995)]. However, this and related divergent series, as they for instance occur in special function theory or in quantum mechanical perturbation theory, can be summed much more efficiently by some alternative rational approximants that use explicit estimates for the truncation error [D. Levin, Int. J. Comput. Math. B 3, 371 (1973); E.J. Weniger, Comput. Phys. Rep. 10, 189 (1989)]. © 1996 American Institute of Physics.
ASECA: A cellular-automata simulation program for a silicon anisotropic super-micro-etching process in aqueous KOH
Authors: Nishidate Kazume , Baba Mamoru , Gaylord Richard J.
Published: July 2002

A three-dimensional anisotropic super micro-etching of crystalline silicon in aqueous KOH is simulated on an atomic level using a new three-dimensional cellular-automata calculational method. The etching simulator ASECA is developed using a rule-based functional Mathematica programming style. The complete program codes and a detailed description of the algorithm are presented. © 1998 American Institute of Physics.
Validity of Matthiessen’s rule for calculating electron mobility in Ga1−xAlxAs alloys
Authors: Saxena A. K. , Mudares Moustafa A. L.
Published: July 2002

Hall electron mobilities in Ga1−xAlxAs alloys for various compositions have been analyzed using the Matthiessen’s rule and Monte Carlo simulation technique. Both these methods provide an excellent fit to the experimental data, providing the usefulness of Matthiessen’s rule. However, the Monte Carlo method gives mobilities at 300 K which are slightly lower (<10%) than those obtained from Matthiessen’s rule. For a typical alloy composition Ga0.70Al0.30As investigated in the temperature interval 80≲T≲300 K, the maximum deviation between the two results is found to be about 14% at 100 K.
Short-range order in III-V ternary alloy semiconductors
Authors: Ichimura Masaya , Sasaki Akio
Published: July 2002

Atom arrangements in III-V ternary alloy semiconductors are discussed using a thermodynamic analysis in which the elastic strain caused by mixing is considered as the mixing enthalpy. In calculating the strain energy, both bond-stretching and bond-angle distortion are taken into account, and Martin’s microscopic elastic constants are used. The results show that there is a preference for ordering but not for clustering in III-V ternary alloy semiconductors. Values of short-range order parameters which represent the degree of ordering are obtained for several alloy semiconductors.
Competing anisotropies and first-order magnetization processes in (Zn,Co)W-type hexaferrite
Authors: Rinaldi S. , Licci F. , Paoluzi A. , Turilli G.
Published: July 2002

First-order magnetization processes (FOMP) and spin reorientation phase transitions are known to occur in hexaferrites as well as in rare-earth transition-metal compounds. This complex phenomenology has been described using high-order terms in the anisotropy energy expansion. In order to understand the microscopic origin of these high-order anisotropy constants in hexaferrites we have made a detailed analysis of the magnetic properties of BaZn1.1Co0.9Fe16O27 which shows, at the same temperature, easy cone anisotropy and a FOMP. We have found good agreement between the experimental data and the predictions of a simple two sublattice model which takes into account the effect of the competition between the exchange and the difference in the anisotropy of the sublattices. This strongly supports the hypothesis that such a competition and the subsequent noncollinear order are the origin of the observed large high-order anisotropy constants in hexaferrites as in intermetallic compounds.
Electrical characterization of cw xenon arcs at moderate currents
Authors: Walsh P. J. , Kermani A.
Published: July 2002

Radiant output, current-voltage characteristics, and heat conduction losses have been measured for water-cooled xenon arc lamps with d=0.7-cm quartz envelopes, driven by 60-Hz currents I ranging from 10 to 90-A rms. Cold-fill pressures varied from 0.5 to 3.0 atm absolute, and interelectrode spacing S from 11.0 to 20.3 cm. The arc voltage drop has been fitted to an equation of the form V=AIbpcaS/d with the results of A, b, and c being 2.51±0.20, 0.40±0.03, and 0.23±0.04, respectively. Here pa is an effective gas-fill pressure which depends on the ratio of hot-arc volume to cold-lamp volume and accounts for the lower atom density in the hot arc. The percentage of lamp power appearing as near-visible continuum radiation varies in the approximate range, 55%–65%. Our voltage data are compared to the high-current empirical Goncz result and with two simple theoretical models: a gray body ion-resistive arc and a recombination dominated atom-resistive arc. We estimate that the plasma has a very small emissivity. The analysis emphasizes the importance of recombination as a source of continuum radiation in these moderate current arcs and that both atoms and ions contribute to plasma resistivity.
Photon energy conversion and storage with a light-driven insertion reaction
Authors: Betz G. , Fiechter S. , Tributsch H.
Published: July 2002

Experiments are presented that demonstrate the possibility of photon energy conversion with a photoelectrochemical insertion reaction. The reaction is performed with the Cu+-conducting semiconductor copper thiophosphate (Cu3PS4; band-gap energy, Eg=2.3 eV) as the photocathode. The Cu3PS4 crystals were grown by chemical vapor transport with halogen as the transport agent. The thermodynamics of the photoinsertion reaction of copper into Cu3PS4 electrodes was investigated, determining a composition-voltage diagram in the dark and under illumination. In photoelectrochemical cells the kinetics of the photoinsertion reaction was studied. Energy-converting photoinsertion cells could be realized with suitable counterelectrodes. The principles of energy-storing photoinsertion cells are discussed together with efficiency limitations and electrode arrangements that may overcome these difficulties.
Determination of electrical transport properties using a novel magnetic field-dependent Hall technique
Authors: Beck W. A. , Anderson J. R.
Published: July 2002

A novel technique is presented for interpreting magnetic field-dependent Hall data at magnetic fields below the range at which Shubnikov–de Haas oscillations occur. The technique generates a ‘‘mobility spectrum’’ in which the maximum carrier density or maximum conductivity is determined as a continuous function of mobility. Examples of the use of the technique with synthetic data as well as data from HgCdTe and GaAs/AlGaAs samples are provided. Other uses of the procedure, including measurement of Fermi surface shapes and direct measurement of the distribution of relaxation times, are discussed.
The deep level transient spectroscopy studies of a ZnO varistor as a function of annealing
Authors: Rohatgi A. , Pang S. K. , Gupta T. K. , Straub W. D.
Published: July 2002

Deep level transient spectroscopy measurements were performed to study the annealing-induced changes in trap centers in ZnO varistors and to shed more light on varistor stability mechanism. Two electron traps, Ec-0.26 eV and Ec-(0.2–0.3) eV, were observed in the unannealed samples in large quantities (7–9×1014 cm−3). The density of the Ec-0.26-eV trap gradually decreases to 4.7×1013 cm−3 at the annealing temperature of 600 °C. Beyond the 600 °C anneal, the Ec-0.26-eV trap begins to grow along with the appearance of another electron trap at Ec-0.17 eV. The minima in the Ec-0.26-eV trap density, coupled with our observation that unannealed devices are unstable whereas 600 °C annealed devices are most stable, suggest that the ZnO varistor instability is related to the Ec-0.26-eV trap. Our data support the ion migration model for device instability where the Ec-0.26-eV defect may be the Zn interstitial or the migrating ion.
An analytic model for calculating trapped charge in amorphous silicon
Authors: Shaw John G. , Hack Michael
Published: July 2002

We present a simple analytic model for calculating the trapped-charge density as a function of free-carrier concentration for realistic density-of-states distributions in hydrogenated amorphous silicon. This is necessary to understand the behavior of amorphous silicon devices as their characteristics are largely determined by the high concentration of charge trapped by the localized stages in the band gap. Such a model is useful as an aid in exploring the nature of the charge-trapping process and for the efficient numerical simulation of amorphous-silicon devices such as thin-film transistors.
Chemical vapor deposition and characterization of 6H-SiC thin films on off-axis 6H-SiC substrates
Authors: Kong H. S. , Glass J. T. , Davis R. F.
Published: July 2002

High-quality, monocrystalline 6H-SiC thin films have been epitaxially grown on 6H-SiC {0001} substrates which were prepared 3° off-axis from 〈0001〉 towards 〈112̄0〉 at 1773 K via chemical vapor deposition (CVD). Essentially, no defects were generated from the epilayer/substrate interface as determined by cross-sectional transmission electron microscopy (XTEM). Double positioning boundaries which were observed in β-SiC grown on 6H-SiC substrates were eliminated as confirmed by plan-view TEM. A strong dependence of the surface morphology of the as-grown thin films on the tilting orientation of the substrates was observed and reasons for this phenomenon are discussed. The unintentionally doped 6H-SiC thin films always exhibit n-type conduction with a carrier concentration on the order of 1016 cm−3. Au-6H-SiC Schottky barrier diodes were fabricated on the CVD 6H-SiC thin films and it was found that the leakage current at a reverse bias of 55 V was only 3.2×10−5 A/cm2. This is compared to SiC films grown on other substrates.
Intercalation of hexagonal boron nitride with potassium
Authors: Doll G. L. , Speck J. S. , Dresselhaus G. , Dresselhaus M. S. , Nakamura K. , Tanuma S.-I.
Published: July 2002

We have performed photoluminescence, photoexcitation, and transmission electron microscopy measurements on boron nitride films grown by chemical vapor deposition and later reacted with potassium. After reaction, the potassium atoms were found to intercalate the BN host and to form a (2×2)R0° in-plane structure which is commensurate with the pristine BN lattice. Optical transitions with ∼2.7 eV onsets were found to occur within the ∼5-eV BN band gap and have been interpreted as Γ-point transitions between the K(4s) band and the BN(2p) bands. The absence of an appreciable shift in the E2g2 phonon frequency of the pristine and reacted films suggests that the charge transfer between the K and BN bands is very small.
Phase equilibria and magnetic properties of iron-rich Fe-Nd-Ti and Fe-Sm-Ti alloys
Authors: Jang T. S. , Stadelmaier H. H.
Published: July 2002

In Fe-Nd-Ti and Fe-Sm-Ti the phase with the hexagonal structure of Co5R is stabilized by the addition of Ti and has a rare-earth content just above 12 at. %, but well below 16.7 at. %, so that it must be classified as a Cu7Tb type. It and Fe17R2 dissolve approximately 3 at. % Ti, accompanied by an increase in Curie temperature in the latter. In Fe-Nd-Ti the phase with the tetragonal Mn12Th structure is found near Fe11TiNd, and its low-anisotropy field is verified. It forms in the cast alloys and is stable only above 1000 °C. At 1000 °C and below it decomposes according to Fe11TiNd→Fe17Nd2 +Fe2Ti+Fe. In Fe-Ti-Sm the high-anisotropy phase Fe11TiSm does not undergo this decomposition down to 700 °C, but it is restricted by the coexistence of Fe17Sm2 and Fe2Ti. As a result, Fe11TiSm is found accompanied by either free Fe or easy-plane Fe17Sm2. A new phase Fe9.5Ti1.5Sm has the structure of tetragonal Ce(Mn,Ni)11 with a=0.8253 and c=0.4825 nm; it has no permanent magnetic moment at room temperature. The high-anisotropy phase near Fe70Ti10Sm20 is not observed between the melting temperature and 700 °C.
Nonlinearity parameter, nonlinearity constant, and frequency dependence of ultrasonic attenuation in GaAs
Authors: Joharapurkar D. N. , Breazeale M. A.
Published: July 2002

Mason’s nonlinearity constants D [W. P. Mason, in Physical Acoustics, edited by W. P. Mason (Academic, New York, 1965), Vol. IIIB, Chap. VI] in GaAs at 298 K have been estimated from second- and third-order elastic constants along the [100], [110], and [111] directions for longitudinal and shear waves. These values are further used to estimate the frequency dependence of the ultrasonic attenuation in the range 40–640 MHz and are found to be in good agreement when compared with experimental values available in the literature. Previously, it was established that phonon-phonon interactions produce a drag on moving dislocations in a solid. In the present work, the drag coefficients for edge and screw dislocations along the [100] and [110] directions in single-crystal GaAs have been estimated from the evaluated nonlinearity constant D values at 298 K. We have also estimated the values of Breazeale’s nonlinearity parameters β [M. A. Breazeale and J. Philip, in Physical Acoustics, edited by W. P. Mason and R. N. Thurston (Academic, New York, 1984), Vol. XVII, Chap. I] along the [100], [110], and [111] directions and found them to be consistent for the three principal orientations, compared with the results for other samples. It is pertinent to note that the mode Grüneisen number γ ji involved in the estimation of Mason’s nonlinearity constant D is deceptively similar to Breazeale’s nonlinearity parameter β. The nonlinearity constant D is evaluated from knowledge of the Grüneisen number γ ji for various modes and directions, whereas Breazeale’s nonlinearity parameter β is the negative ratio of coefficients of nonlinear term to the linear term of the (dissipationless) nonlinear wave equation for pure mode directions. Both D and β, in general, are temperature dependent.
A theoretical study of the hyperbolic electron mirror as a correcting element for spherical and chromatic aberration in electron optics
Authors: Rempfer Gertrude F.
Published: July 2002

The spherical and chromatic aberrations of a converging electron mirror are of opposite sign from those of electron lenses. This important property makes it possible in principle to compensate the aberrations of electron lenses by means of an electron mirror and to design electron microscopes based on a corrected optics system incorporating an electron mirror. In this paper the properties of the hyperbolic electron mirror are calculated, and the conditions for simultaneous correction of spherical and chromatic aberrations are worked out for several types of electron microscopes. The hyperbolic mirror field is a rotationally symmetric potential field between two electrodes. The electrodes are shaped as equipotential surfaces of the hyperbolic field, except for an aperture on the axis of the positive electrode for entrance and exit of electrons. The effect of the aperture is to create a thin diverging aperture lens at the termination of the hyperbolic field. The properties of the mirror are calculated analytically. The problem of separating the electron beam incident on the mirror from the beam returning from the mirror without impairing the image quality is solved by means of magnetic deflecting fields located at image planes. The mirror corrections can be applied to either magnetic or electrostatic lenses. The parameters for correction of aberrations are calculated for systems using electrostatic lenses. With appropriate polarity of the accelerating voltage and the lens and mirror voltages the calculations apply to ion imaging systems as well.
Computer simulation of virtual cathode oscillations
Authors: Lin Tsang-Lang , Chen Wen-Ting , Liu Wen-Chung , Hu Yuan , Wu Mien-Win
Published: July 2002

The dynamics of relativistic electron beam and the oscillations of the virtual cathode in the vircator (virtual cathode oscillator) have been investigated by electrostatic particle-in-cell computer simulations. A specially developed PIC (particle-in-cell) computer code is employed in this study. In the diode region, cathode to anode, the solid electron beam is modeled by one-dimensional sheets for simplicity. In the drift tube region the potential at the center line is obtained by solving the Poisson’s equation analytically in the radial direction and via PIC simulation in the axial direction. The electron beam in the drift tube region is modeled as a solid and uniform beam of constant radius equal to the cathode radius. The simulation results show that the potential minimum in the drift tube region oscillates at broad-band frequencies. The injected electron beam in the diode region is found to be modulated by the bunched reflex electrons. The oscillation frequency of the potential minimum in the drift tube region is found to have strong dependence on the transit time of the electrons between the real cathode and the virtual cathode, and it scales inversely proportional to the cathode to anode distance. The effect of applied voltage, injected current, cathode-to-anode distance, and beam radius on the oscillation frequency has also been studied. The results are compared with predictions of theoretical and empirical formulas. The potential minimum is found to oscillate at a single constant frequency when the reflected electrons are not allowed to enter the diode region.
Surface and bulk leakage currents in transverse junction stripe lasers
Authors: Sin Yongkun , Hsieh K. Y. , Lee J. H. , Kolbas R. M.
Published: July 2002

This paper presents two ways to reduce leakage currents in transverse junction stripe (TJS) lasers: by reducing surface leakage current and by reducing bulk leakage current. The surface leakage is reduced by treatment with (NH4)2S, while the bulk leakage current is reduced by isoelectronic doping with indium. We report the first detailed experimental investigation on the effect of the chemical treatments on the electrical characteristics and laser thresholds of TJS lasers. Surface treatments of (NH4)2S are demonstrated that reduce surface leakage currents in current injection lasers. After the chemical treatments, a 20-fold reduction in current has been achieved with GaAs/AlGaAs lattice-matched multiple-quantum-well TJS lasers. The laser thresholds of lattice-matched TJS lasers are reduced by 12 mA (or 16%) after the chemical treatments. In addition, InGaAs-GaAs-AlGaAs strained-layer single-quantum-well lasers are treated chemically and a reduction in the laser threshold (10 mA or 14%) is observed. The surface treatments are still effective after 7 days. We also report the first experimental investigation on the effect of isoelectronic In doping on the current-voltage characteristics of Zn-diffused lateral p-n junctions. The trap density in an In-doped AlGaAs layer is reduced by more than one order of magnitude compared to that in an AlGaAs layer without In doping. Bulk leakage currents (shunting currents) in TJS lasers can be reduced by using isoelectronic In doping, which should reduce threshold currents and improve the temperature dependence of TJS lasers.
A model for particulate contaminated glow discharges
Authors: McCaughey Michael J. , Kushner Mark J.
Published: July 2002

Glow discharges are often contaminated by particulates resulting from gas phase nucleation or sputtering of surfaces in contact with the plasma. If these particulates are sufficiently large, they will negatively charge and act as Coulomb-like scattering centers for electrons. In doing so, rate coefficients for high-threshold processes such as ionization may be reduced compared to those in pristine plasmas. If the contamination is nonuniform, then the resulting spatial irregularities in the rates of excitation may lead to plasma properties which are also nonuniform. In this paper, we report on the results of a model for argon glow discharges contaminated by dust. Rate coefficients for this model are generated by a separate Monte Carlo simulation of electron swarms in dusty plasmas. We find that under quasi-steady-state conditions, current flow and subsequent excitation of the gas are channeled into regions of lower dust density, and that these effects depend on the density, size, and distribution of the dust. In low pressure (<10 Torr) glow discharges having similar dimensions, comparable perturbation in plasma properties due to contamination is obtained when NDP is approximately constant (ND is the dust density, P is the gas pressure). The onset of perturbations may occur when NDP≳105 cm−3 Torr.
3d metal-substitution effects on magnetic and crystallographic properties of CrAs
Authors: Suzuki T. , Ido H.
Published: July 2002

Crystal structure parameter of Cr1−xMxAs (M=Ti, Fe, Co, and Ni; x≤0.1) have been precisely determined as a function of temperature from 77 to 320 K. Discontinuous changes of lattice parameters have been observed at a special temperature. By this temperature the Néel temperatures TN of Cr1−xMxAs have been determined because the anomaly of magnetic susceptibility has not been observed for them. The substitutions by the M atoms generally make TN of CrAs decrease remarkably. For the Fe, Co, and Ni substitutions, a nonmagnetic state is expected for a concentration more than 5%, 6%, and 9%, respectively. Dependence of TN on the substituent concentrations has been determined by taking account of the crystal volume effect on TN.
Heavy-ion migration through argon and helium in weak electric fields
Authors: Howard A. J. , Strange W. P.
Published: July 2002

An electrostatic precipitation method is employed to study the collection efficiency for 21884Po+ ions formed by 222Rn atoms immersed in hydrostatic gaseous argon and helium for electric fields E in the range of 0.01–10 V/cm. The method employs a volume of gas wherein the electric field is reasonably uniform over a ∼25-cm long drift path, and so neutralization times above 10 s are observable. Measurements are performed on the ion collection efficiency as functions of radon concentration CRn in the 2–80 pCi/ℓ range at argon and helium pressures p in the 0.03–1.0 atmosphere range. Results indicate that while the collection efficiency associated with saturation values of E is essentially independent of both CRn and p, it is proportional to √CRn and p for values of E below the associated saturation values. The roles of various sources of ionization in the gas, which ultimately lead to neutralization of the heavy radioactive ions, are discussed, as are device-dependent effects generally associated with the finite size (and shape) of the containment vessels employed. Potential implications and applications of the general findings are presented.
Ising spin-glass critical and multicritical fixed distributions from a renormalization-group calculation with quenched randomness
Authors: Hartford Edward J. , McKay Susan R.
Published: July 2002

The Ising model with a quenched random distribution of ferromagnetic and antiferromagnetic interactions has been investigated by tracking the probability distribution of interactions under rescaling. As the initial probability of a ferromagnetic interaction is increased, the phase diagram in three dimensions shows three ordered phases: antiferromagnetic, spin glass and ferromagnetic. We find that, within the spin-glass region, effective interactions between pairs of spins at successive length scales occur in a chaotic sequence, while the average magnitudes of the ferromagnetic and the antiferromagnetic interactions both flow to infinity under iteration. Our calculated spin-glass multicritical points are on the Nishimori line in cases of both bimodal (±J) and diluted (trimodal) distributions, and these points flow under rescaling to a common fixed distribution with a specific heat exponent α of approximately −3.7.