bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaporated CoPt alloy films with strong perpendicular magnetic anisotropy (abstract)
Authors: Lin C.-J. , Gorman G. L.
Published: July 2002

Co/Pt multilayers with large Kerr rotations at short wavelengths and the magnetic and material properties desired for magneto-optical (MO) recording have been studied extensively as potential future MO materials. Very good recording performance has been demonstrated.1 However, the typical constituent layers in Co/Pt multilayers are very thin, 0.2–0.4 nm Co layers and about 1 nm/Pt layers. To manufacture Co/Pt multilayers consisting of 10–30 periods of such thin Co and Pt layers is certainly quite a challenge. One would prefer to deal with CoPt alloys if the alloys can be made to have large perpendicular magnetic anisotropy and coercivity. Previously no one was able to directly deposit CoPt alloy films with large perpendicular magnetic anisotropy and saturated remanence. Here we will demonstrate for the first time that CoPt alloy films with large perpendicular magnetic anisotropy and coercivity, and saturated remanence can be directly deposited by e-beam evaporation onto heated substrates, at 200 °C or above. Furthermore, we will demonstrate that the perpendicular magnetic anisotropy of CoPt alloy films can be dramatically enhanced by well (111)-textured Pt underlayers. The key to this success appears to be in controlling the crystallographic orientation of these alloy films such that the CoPt(111) lattice plane is parallel to the film surface. CoPt alloys with wide composition range, e.g., 25–57 at. % Co, have been obtained with saturated perpendicular magnetic remanence, indicating that the perpendicular magnetic anisotropy obtained in these alloy films is not related to the ordered tetragonal Co50Pt50 phase. Further work, however, is needed to clarify the anisotropy mechanism in these CoPt alloy films.
Muon spin relaxation study of the (FμF)− ion in magnetic fluorides
Authors: Noakes D. R. , Ansaldo E. J. , Luke G. M.
Published: July 2002

A positive muon implanted in a solid fluoride insulator can form a hydrogen-bonded (FμF)− ion, in which the nuclear magnetic interactions cause a distinctive zero-field (ZF) muon spin relaxation (μSR) signal. In a fluoride material in which the anions have electronic magnetic moments, the muon’s interaction with nearby 19F nuclear moments must compete with its interaction with more distant but much larger anion moments. We report observation of this competition in ZF-μSR in MnF3 (TN=43 K) and CuF2 (TN=69 K). In both MnF3 and CuF2 characteristic ZF FμF oscillations are observed above the Néel temperature. As temperature drops below TN in MnF3 the relaxation rate of the (FμF)− signal increases dramatically. Below TN in CuF2 a small-amplitude high-frequency coherent oscillation appears on top of the low-frequency (FμF)− signal.
High field transitions in (FeMn)3Si alloys
Authors: Al-Kanani H. , Booth J. G. , Yoon S.
Published: July 2002

The magnetic ordering in the series of solid solutions Fe3−xMnxSi having the D03 structure has been extensively studied at low fields over a wide range of temperatures. The present study reports observations of high-field-induced transitions in the low temperature range below the reordering temperature TR for members of the series falling within the concentration range 1.6≤x≤1.8. These alloys appear antiferromagnetic in low fields below TR but application of a field within the range up to 12 T results in a transition to an apparently ferromagnetic state. The transition occurs at all temperatures below TR and occurs reversibly except at the lowest temperatures where the hysteresis may be ∼0.5 T. The variation of the critical field with temperature follows an approximately quadratic form. Above TR Arrott plots suggest for the alloy with x=1.70 that a narrow ferromagnetic regime exists over the range 65≤T≤85 K above which the material is paramagnetic. For the alloy with x=1.75 however the Arrott plots suggest a direct conversion to paramagnetism at the reordering temperature TR indicating a possible phase diagram similar to that of Au2Mn. A discussion is given in terms of a model previously proposed to explain the low-field behavior.
Cooling-induced phase transition in amorphous CoCrZr alloy
Authors: Zhukov Arcady P. , Shtangeev Borys L.
Published: July 2002

In amorphous Co77Cr11.7Zr11.3 ribbons, produced by melt spinning, irreversible changes of hysteresis magnetic properties and Curie temperature were observed after thermal cycling (TC) (300 K–78 K–300 K). After TC the coercive force Hc increased by 35%, magnetization Bs at the magnetic field 4.5 Oe by 6%, and remanent magnetization Br by 3%. Maturing of thermal-cycled samples at 300 K during 10 h led to the further increasing of Hc, Bs, and Br by 60%, 16%, and 10%, respectively, as compared with the as-cast state. Subsequent TC changed neither Hc nor Bs and Br. After maturing of as-cast ribbons at 300 K for 4 weeks Hc became larger by 55%. The observed dependencies are explained as a sequence of phase transitions in the amorphous state connected with metastable high-temperature modification of Co-based local order after quenching.
Magnetization and magnetic susceptibility of the diluted magnetic semiconductor Cd1−xMnxGa2Se4
Authors: Chen Chen-jia , Ma Yong , Li Yu , Wang Xue-zhong , Borghesi A. , Guizzetti G. , Sassella A. , Viticoli S.
Published: July 2002

Cd1−xMnxGa2Se4 crystals were grown by the vapor-phase transport technique. Magnetization measurements have been done at 1.5 K and magnetic fields up to 7 T by using extraction method on the samples with 0.05≤x≤1. A modified Brillouin function fits the data and the fitting parameters T0 (≥0) and Seff are obtained. These results reflect that there exists a weak antiferromagnetic interaction among Mn++ ions. The magnetic susceptibility was measured in the temperature range 1.5 K≤T≤300 K by using a vibrating sample magnetometer. The susceptibility displays a high-temperature Curie–Weiss behavior. From quantative analysis the exchange integral constant (J1+2J2+J3)/kB was obtained to be ∼−1.5 K.
Domain wall pinning in amorphous CoZrNb films
Authors: Guyot M. , Rouabhi M. , Krishnan R. , Porte M. , Niedoba H. , Cagan V.
Published: July 2002

A series of CoZrNb amorphous films, with thickness t ranging from 200 to 630 nm, has been studied by using several techniques: Kerr imaging, dc and audio frequency hysteresis loops, field dependence of the permeability, torque balance, broadband frequency susceptometer (1 kHz≤f≤500 MHz). Only samples with t≤500 nm have simple domain structures typical of an in-plane anisotropy (with a small dispersion of the easy axis). Although domain walls (DW) are visible, they do not contribute to the initial permeability in the observed frequency range (even in the favorable case where the magnetic field is parallel to the easy axis), but they do play a role on the irreversible magnetization processes as shown by the existence of a critical field Hcr, typical of the onset of irreversible DW motion. These observations are consistant with a model which assumes that the DW are pinned at the film surfaces: the calculations give a negligible DW bulging contribution to the initial permeability, and predict the occurence of a DW unpinning field, thickness dependent as observed.
Properties of melt-spun Co15Fe5Cu80, a bulk granular magnetic solid (abstract)
Authors: Rubenstein M. , Das B. N. , Koon N. C.
Published: July 2002

We have investigated the magnetic and transport properties of melt-spun Co15Fe5Cu80 as a function of heat treatment. Recently Xiao, Jiang, and Chien1 reported clustering and giant magnetoresistance in thin films of related alloys prepared by sputtering. We report qualitatively similar phenomena in these rapidly quenched ribbons. The unannealed samples exhibited saturation magnetoresistance of 0.2%. Annealing at 500 °C for 15 min in hydrogen increases the room temperature for magnetoresistance to 3%, and lowering the temperature to 77 K increased the magnetoresistance to 12.6%. Ferromagnetic resonance measurements at 35 GHz were also made. At room temperature the unannealed sample displayed a very broad resonance centered at 10 kG, consistent with a broad distribution of single domain particles of roughly spherical shape and an overall global magnetization 4πM=3.5 kOe. As the annealing temperature increases, the single domain particles with an inhomogeneous linewidth of ∼2 kOe grow into multidomain particles with a linewidth of ∼6 kOe.
Effects of hypophosphite contents and surface treatment on electroplated Co-Ni-P thin films
Authors: Byun C. , Rauch G. C. , Young D. J. , Klepper C. A. , Greggi J.
Published: July 2002

This article describes the effects of sodium hypophosphite (hypo) content in cobalt nickel sulfate bath, and the duration of surface treatment prior to electroplating, on the magnetic, structural, and electrical properties of Co-Ni-P thin films. The coercivity of the Co-Ni-P layer can be controlled independently either by hypo content or by surface treatment. Transmission electron microscopy microstructure indicates that the width of grain boundaries of Co-Ni-P films is influenced by hypo in the magnetic bath. The mechanism of the coercivity increase by hypo in the bath is thought to result from magnetostatic or exchange decoupling at phosphorus-rich grain boundaries. Effects of these factors on media noise can be well-explained by the microstructural change of Co-Ni-P layers.
Enhanced coercivity in CoPtCr thin film media using CrV underlayer (abstract)
Authors: Nguyen T. A. , Howard J. K.
Published: July 2002

As the areal density of thin magnetic media continues to increase1,2 and aims toward 10 gigabits/in2, the future media’s magnetic properties and recording performance such as the coercive force and signal to noise ratio need to increase substantially.3 In this paper we report the use of a chromium vanadium underlayer as a method to control the microstructure and enhance the coercivity of CoPtCr media. To study the effect of the underlayer composition on microstructure and magnetic properties, CrV-alloy/CoPtCr structures were fabricated with CrV-alloy concentration ranging from pure Cr to pure V. The magnetic media target concentrations were Co75Pt12Cr13, Co74Pt9Cr17, and Co80Pt3Cr17. We observed that the coercivity increases as the vanadium concentration in the CrV alloy increases and peaks at 50 at. % V. The Cr50V50/CoPtCr yielded between 30% and 40% higher coercivity values than Cr/CoPtCr for the same thickness of underlayer and magnetic layer. The microstructures (by TEM) of the CrV underlayer and the CrV/CoPtCr show that the underlayer grains are largest at 50 at. % V and that the CoPtCr grains mimic those of the underlayer. There is also considerable amount of epitaxy between the (200) planes of the CrV and (110) planes of the CoPtCr. The increase in grain size and preferred orientation in the polycrystalline magnetic field is believed to affect the coercivity by reducing the magnetostatic coupling and increasing the magnetocrystalline anisotropy.4
The change in the temperature coefficient of resistivity with Mn addition in Fe- and Co-rich amorphous alloys
Authors: Piramanayagam S. N. , Prasad Shiva , Shringi S. N. , Nigam A. K. , Chandra Girish , Venkataramani N. , Krishnan R.
Published: July 2002

Electrical resistivity studies have been carried out on a-Co80−x−yFexMnyB12Si8 alloys for 10≤x≤70 and y=0,4,8. It has been found that the decrease of temperature coefficients of resistivity (TCR) with Mn substitution is significantly more in Fe-rich alloys than in Co-rich alloys. A T2 term is not found in Co-rich alloys even up to the addition of 8 at. % of Mn. Although Fe-rich alloys show a T2 dependence for 8 at. % of Mn, a second minimum is not observed. In the absence of Mn, the substitution of Fe for Co does not decrease the TCR while in the presence of Mn, the substitution of Fe does decrease the TCR.
Microstructure and magnetic properties of ultrafine grain iron-based soft ribbons
Authors: Shevchenko N. B. , Zhang Y. J. , Hadjipanayis G. C.
Published: July 2002

The magnetic and structural properties of as-spun and crystallized Fe-Zr alloys with Cu and B substitutions have been studied. The Cu and B additions were both observed to affect the crystallization temperature and subsequent microstructure. Boron was observed to have the added effect of stabilizing the amorphous state. Crystallization at low temperatures (540 °C) gave the samples a single bcc phase with ultrafine grain size (20 nm), a low coercivity (about 0.1 Oe), and a high magnetization (185 emu/gm). The grain size and coercivity increased upon heat treatment at higher temperatures (720 °C), where a small amount of a second phase (Fe3Zr) appeared to coexist with the bcc phase.
Ferromagnetic behavior of decamethylchromocenium tetracyanoethanide [CrCp2*][TCNE]
Authors: Zuo F. , Zane S. , Zhou P. , Epstein A. J. , McLean R. S. , Miller Joel S.
Published: July 2002

Studies on the magnetic properties of decamethylchromocenium tetracyanoethanide, [CrCp*2][TCNE] are reported herein. Near-zero field (H=150 mG) data indicates a ferromagnetic transition with Tc=3.65 K. The temperature dependence below Tc can be fit to a mean-field result M=M0(1−T/Tc)1/2, and the data above Tc gives a critical exponent γ=1.2 for the dc magnetic susceptibility. Data at higher temperature indicates a strong 1-D ferromagnetic behavior. Magnetizations at different H and T at low temperature can be scaled to a universal function with M=f(H/T3.2). The reduction in Tc in [CrCp*2][TCNE] compared with [FeCp*2][TCNE] indicates a competition between the coupling constants and the residing spins.
Effect of next-nearest-neighbor interaction in quantum Heisenberg antiferromagnetic triangular lattices (abstract)
Authors: Loison D. , Diep H. T.
Published: July 2002

The effect of quantum fluctuations in magnetic materials has been a subject of an increasing number of investigations. This may be related, on the one hand, to the problem of magnetism in high-temperature superconductivity, and on the other hand, to artificial materials such as magnetic superlattices and thin films. In this report, we are interested to the case of the triangular antiferromagnet with quantum Heisenberg spins of magnitude 1/2 interacting with each other via nearest-neighbor (nn) and next-nearest-neighbor (nnn) bonds. The system with only nn, although simple, has been widely studied because it possesses several interesting properties due to the frustration, in particular the classical ground-state (GS) spin configuration is noncolinear. Two questions that have been widely studied are the energy and the value of local magnetization in the GS. Several methods have been tried: exact diagonalization of small clusters, variational wave functions, and Holstein-Primakoff technique, among others. The results from these methods are not in complete agreement with each other. The system with nn and nnn has been studied by the Holstein–Primakoff technique. In this paper, we calculate by a Green function method both the local magnetization and the energy of the GS. We briefly describe the formalism and show and compare our results with those obtained by other methods.
Breakdown of hydrodynamics in the classical 1D Heisenberg model (abstract)
Authors: de Alcantara Bonfim O. F. , Reiter George
Published: July 2002

Extensive spin-dynamics simulations have been performed to study the dynamical behavior of the classical Heisenberg chain at infinite temperatures and long wavelengths. We find that the energy and spin show distinctly different dynamics in the isotropic system. The energy correlation function follows the classical diffusion theory prediction, namely, it decays exponentially with q2t. In contrast, the spin correlation function is found to decay exponentially as q2.12t ln t implying a logarithmically divergent diffusion constant and the failure of the usual hydrodynamic assumptions.
Optical study of the temperature-induced valence transition in YbCu4In
Authors: Marabelli F. , Bauer E.
Published: July 2002

In the context of investigations on mixed valence and Kondo systems the compound YbCu4In is interesting due to a temperature-induced valence transition occurring at Tv=40 K. Without distorsion in the C156 crystal structure, the Yb state in the system changes from a nearly 2+ state below Tv to a magnetic (3+) state above Tv. We investigated the effects of such a change on the optical reflectivity which has been measured over a large spectral range (1 meV–6 eV) and at different temperatures above and below Tv, both on a polycrystalline sample of YbCu4In, and on a similar sample of the reference compound LuCu4In. A detailed analysis of the spectra in the far infrared is complicated by the presence both in Yb and Lu-based compounds of an optical structure at about 5 meV, which could be due to a phonon mode. Regardless, some differences in the spectra and changes in the plasma frequency have been observed, allowing for considerations on the connections between the valence of Yb and the density and effective mass of the carriers.
On possible observation of nontrivial magnetic order parameter by neutrons (abstract)
Authors: Barzykin Victor , Gor’kov Lev P.
Published: July 2002

We consider neutron scattering in a system with spin nematic or triple spin correlator order parameter. External magnetic field induces different magnetizations on different sublattices in this system. As a result, one can observe purely magnetic Bragg peaks for polarized neutron scattering in an applied external magnetic field. We present estimates of intensities in the Bragg peaks and their dependence on the external magnetic field. This could be a possible test for nontrivial magnetic order in the heavy fermion compounds URu2Si2 and UPt3.1
Magnetic field calculations inside a conducting slub due to dipole excitations
Authors: Antonopoulos C. S. , Kriezis E. E.
Published: July 2002

The propagation of the electromagnetic field inside a conducting slub extended in the infinity is treated by the use of a boundary element technique. The excitation is a current source arbitrarily oriented inside the conducting material. The conducting slub is situated in the air. The magnetic field intensity is expressed in terms of the electric vector potential T and the magnetic scalar potential Ω. The integral equations for these two field quantities are formed by the use of the Green’s function method into the original Helmholtz and Laplace equations for the T and Ω, respectively. Apart from the boundary conditions, the need of imposing at least one more additional condition between the field quantities becomes obvious during the solution procedure, with the help of a boundary element method. Examples are given using two materials with quite different electric properties, aluminum and graphite. The work can be extended in more complicated stratified media to face propagation problems in geometries with complex boundaries.
Thermodynamic and magnetic properties of (Ni1−xMx)3Al with M=Cu and Pd and Ni3(Al1−xSix)
Authors: Fuller C. J. , Lin C. L. , Mihalisin T. , Chu Fuming , Bykovetz N.
Published: July 2002

The ferromagnetic transition temperature Tc of Ni3Al is found to be about 60 K for a polycrystalline sample with 76 at. % Ni content and 81 K for a single crystal with 76.6 at. % Ni. Our magnetization measurements show that when Cu or Pd is substituted for Ni in Ni3Al, Tc decreases with increasing Cu or Pd concentration and goes to zero in the vicinity of the critical concentrations of 2% Cu or 10% Pd. When Si is substituted for Al, Tc also decreases and goes to zero at a critical concentration of about 10% Si. Specific heat studies indicate that the electronic specific heat coefficient γ is slightly enhanced when these systems approach their critical concentrations. This behavior is discussed in terms of the self-consistent renormalization theory for nearly and weakly itinerant magnets.
Synthesis and characterization of Pb (ZrxTi1−x)O3 thin films produced by an automated laser ablation deposition technique
Authors: Auciello O. , Mantese L. , Duarte J. , Chen X. , Rou S. H. , Kingon A. I. , Schreiner A. F. , Krauss A. R.
Published: July 2002

Ferroelectric lead zirconium titanate [Pb (ZrxTi1−x)O3] (PZT) thin films have been synthesized by using an automated laser ablation deposition technique with a capability for layer-by-layer or simultaneous deposition of elemental film constituents. The technique is suitable for producing multicomponent and/or multilayered thin films with controlled stoichiometry, such as high-temperature superconductor, ferroelectric, and electro-optic thin films. PZT films were synthesized on MgO (100) by either sequential deposition of layers of ZrO2, TiO2, and PbO, produced by laser ablation of ZrO2, TiO2, and PbO targets, or by simultaneous deposition of all species from ablation of stoichiometric or PbO-rich PZT targets. Films were deposited at 200 °C and subsequently annealed at 600 °C for different periods of time. The orientation, microstructure, surface topography, and composition of the films were characterized by x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry, respectively. As-deposited layer films consists of highly oriented (001) PbO layers, from which highly oriented (110) PZT films are produced after postdeposition annealing. On the other hand, laser ablation of bulk PZT targets yields amorphous as-deposited films, which evolve into highly oriented (100) PZT films after postdeposition annealing. Preliminary electrical characterization of the PZT films included polarization hysteresis, fatigue, conductivity (ac and dc), and capacitance versus voltage measurements. From the initial electrical measurements, it appears that the remnant polarization of the layered PZT films is similar to that of the films produced by laser ablation of bulk PZT targets.
Surface plasmon-induced luminescence: A new way to investigate space charge dynamics in thin dielectric films
Authors: Laurent C. , Canet P.
Published: July 2002

A new way is presented for deriving space charge dynamics in thin films. The conduction current flowing through the dielectric excites surface plasmon modes on the positively biased metal electrode. The roughness of the interface couples the plasmon modes to the optical photons. The measurement method uses the detection of the electroluminescence excited by square voltage pulses. Thin films of any thicknesses can be investigated. Electroluminescence provides a very sensitive tool for analyzing space charge dynamics, namely its buildup and relaxation times.
Increased thermal stability of Au/GaAs metal-insulator-semiconductor Schottky diodes with silicon nitride interfacial layer deposited by remote plasma-enhanced chemical vapor deposition
Authors: Kolník J. , Ivančo J. , Ožvold M. , Wyczisk F. , Olivier J.
Published: July 2002

The effect of the ultrathin interfacial layer of silicon nitride deposited by remote plasma-enhanced chemical vapor deposition technique on the Schottky barrier characteristics of Au/n-GaAs contacts is investigated. The changes of both capacitance-voltage and current-voltage characteristics, in dependence on the interfacial layer thicknesses are discussed and explained on the basis of the surface passivation. The influence of thermal annealing on the structure and electrical properties is also presented. In contrast to the very poor stability of the electrical characteristics of reference Au/GaAs contacts, structures with silicon nitride interfacial layers show much improved thermal stability with the minimum ideality factor for the silicon nitride interfacial layer being 9 Å thick and annealed at 450 °C. The effect of silicon nitride interlayer as a diffusion barrier is confirmed by Auger electron spectroscopy analysis of both reference and silicon nitride containing structures.
Band gap renormalization and observation of the type I–type II transition in quantum well systems
Authors: Stirner T. , Harrison P. , Hagston W. E. , Goodwin J. P.
Published: July 2002

Quantum wells (or barriers) are of finite extent (≤100 Å) along the growth direction. Hence, for at least one or two monolayers adjacent to the interface, the corresponding one-electron potential in the conduction (or valence) band must differ from that appropriate to the bulk material. The present article demonstrates the pronounced effect such short-range ‘‘interface potentials’’ can have on the transition energies and oscillator strengths in a dilute magnetic semiconductor system undergoing a type I–type II transition in an external magnetic field.
Singular point detection in multidomain samples
Authors: Asti G. , Bolzoni F. , Cabassi R.
Published: July 2002

In the present work the problem of singular point detection (SPD) is examined in the case of multidomain crystallites. As is known, a uniaxial polycrystalline ferromagnet displays a sharp peak in the second derivative of magnetization M with respect to magnetic field H versus H, located at the anisotropy field H=HA. This singularity was explained on the basis of the classical SPD theory which considers only single-domain crystallites. Here this matter is analyzed in the frame of the Néel phase theory and leads to an interpretation of the SPD peaks which seems to be more realistic. A theoretical study of the magnetization curve of a rotational ellipsoid single crystal clearly shows existence of a critical field Hc separating the multiphase from the single phase mode. The analysis of the magnetization curve of a polycrystalline material is based on the assumption that the main features of the SPD peaks are only determined by the discontinuity in the susceptibility at Hc. Contrary to the classical SPD theory, the shape of the grains is of fundamental importance. If the grains all have the same shape, the peak turns out to be a divergence; on the other hand, aggregates of crystallites having different demagnetizing factors show finite peaks closely resembling the experimental ones. The effects on the peak of crystallites’ shapes and angular distribution have been investigated by means of numerical calculations. The validity of the present approach has also been tested by computer simulations on the basis of an extended Stoner–Wohlfarth model including the Néel phase theory.
Light emission from metal-insulator-metal structures
Authors: Canet P. , Laurent C. , Akinnifesi J. , Despax B.
Published: July 2002

The study of electroluminescence from gold-containing plasma polymerized propane thin films was initially our goal. These films consist of small isolated gold clusters, a few tens of angstrom in diameter, dispersed in an otherwise insulating or semi-insulating matrix below the electrical percolation threshold which sets in around 30% gold volume fraction. dc, ac, and impulse luminescence experiments combined with transport analysis are reported for different metallic concentration in the films. The emission is current controlled and consists of broad spectral peaks located in the red part of the optical spectrum. Among the several possible emission processes, we demonstrate that the light comes from the radiative decay of electrode surface plasmons excited by hot electrons produced in the films. The precise nature of the dielectric is unimportant since the emission process involves the electrode parameters. This is confirmed by testing other sandwich structures constituted by different dielectrics. On the one hand, our results confirm the fact that surface plasmons can be created in a metal-insulator-metal structure without tunneling as the driving mechanism. On the other hand, it is shown that coupling between surface plasmons and photons occur at dielectric-metal interfaces.
Auger electron spectroscopy study on the thermal stability of the Al/Al12W/W2N/Si contact system
Authors: Takeyama Mayumi , Sasaki Katsutaka , Noya Atsushi
Published: July 2002

The thermal stability of the Al/Al12W/W2N/Si contact system has been studied using Auger electron spectroscopy analysis. The interposed Al12W layer between the Al overlayer and the W2N barrier layer effectively suppresses the possible spontaneous reaction between Al and W, which can be carried out by bringing about a chemical equilibrium state at each interface of Al/Al12W and Al12W/W2N. This is achieved by the successive combination of the thin-film assembly; the materials bordering each other consist of one common constituent metal and form a compound at least on one side, which leads the interfaces to a lower free-energy state, i.e., an equilibrium. It is shown that the present contact system tolerating heat treatment at temperatures up to 600 °C is one of the most stable multilayered configurations among those of Al metallization technology.
Identification of deep levels in liquid-encapsulation Czochralski-grown Fe- and Zn-doped InP: A proof of the nonexistence of a Fe4+/Fe3+ donor level
Authors: Wolf T. , Drews D. , Scheffler H. , Bimberg D. , Mosel F. , Kipfer P. , Müller G.
Published: July 2002

Deep levels in liquid-encapsulation Czochralski (LEC) grown p-type InP:Fe codoped with Zn have been investigated by means of temperature-dependent Hall-effect (TDH), deep-level transient spectroscopy (DLTS), calorimetric absorption spectroscopy, and electron spin resonance measurements. Although a dominant deep hole trap is revealed both by DLTS and TDH measurements in the vicinity of the valence band edge at EV+0.2 eV, the spectroscopic analysis unambiguously invalidates previous speculations on the existence of a second energy level of the isolated iron impurity in the band gap of InP, i.e., a Fe4+/Fe3+ donor level. From the axial concentration profile and a comparison with a LEC-grown p-type InP crystal doped with Zn only it seems that the trap is not even iron-related in contrast to tentative assignments often found in the literature. Native or Zn-related defects which depend on the particular growth conditions used are assumed to account for this level.
Evidence for space charge in atomic layer epitaxy ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Douglas A. A. , Wager J. F. , Morton D. C. , Koh J. B. , Hogh C. P.
Published: July 2002

Electrical and optical measurements of ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices grown by atomic layer epitaxy provide evidence for the existence of space charge within the bulk ZnS layer. Blue luminescence is observed during the falling edge of an applied voltage pulse when the ACTFEL device is operated at low temperature. This blue luminescence is attributed to donor-acceptor pair radiative recombination in which chlorine is identified as the donor and a zinc vacancy as the acceptor. This luminescence identification leads to determination of the origin of space charge as arising from impact ionization of the zinc vacancy.
Hydroxyl defects and electrodiffusion (sweeping) in natural quartz crystals
Authors: Bahadur Harish
Published: July 2002

High- and low-H samples of natural crystalline quartz in three different conditions, unswept, Na swept, and H swept, have been examined and analyzed for their low-temperature near-infrared spectra. The observed lines have been attributed to five defect types: Al-OH−, alkali-affected OH−, i.e., Li-H and Na-H; H-sweeping induced; and others. For both high- and low-H samples, the aluminum concentration and the grown-in Al-OH− ratios were found to vary over a limited range while those of alkali-affected OH− were much wider. The variation in Al-Li+ and Al-Na+ defect concentration ratios was also over a moderate range. The very narrow variation in the strength of grown-in Al-Li+ centers in the two types of quartz, high- and low-H, suggests that the Al-OH− centers in natural quartz crystals become stable only after a certain maximum concentration of Al-Li+ centers is reached. This reflects a characteristic feature of the growth mechanism of natural quartz crystals.
Effect of substrate defects on the properties of high T
c superconducting thin films
Authors: Barkay Z. , Racah D. , Grunbaum E. , Deutscher G.
Published: July 2002

We studied the influence of substrate defects on the morphological and transport properties of Y1Ba2Cu3O7−x high Tc superconducting thin films. The substrate defects were characterized by dispersive and nondispersive cathodoluminescence in the scanning electron microscope. The morphology of the films was studied by the secondary electrons and was locally compared with the substrate information. Defects in the substrate induced surface variations in the thin films. Current transport was correlated with the geometry of the substrate defects.
Magnetic studies in some melt spun amorphous Fe-Ho-B alloys
Authors: Krishnan R. , El Marrakchi O. , Lassri H. , Rougier P.
Published: July 2002

Amorphous Fe82−xHoxB18 alloys have been prepared by melt spinning and their magnetic properties have been studied. The Ho moment at 6 K is found to be 10μB which agrees with the theoretical value indicating a collinear spin structure. The magnetic compensation at 6 K occurs for x close to 13. The mean-field theory has been used to explain the temperature dependence of the magnetization.
Velocity-field relation in GaAlAs versus alloy composition
Authors: Hava S. , Auslender M.
Published: July 2002

High electric field transport in n-Ga1−xAlxAs semiconductor alloys for composition x range from 0.0 to 0.5 is studied theoretically. The calculations are based on a shifted Maxwellian approach for the three-valley conduction-band model. An expression for the alloy scattering probability involving no fitting parameters is used in these calculations. The calculations predict a close to linear decrease of peak velocity with an increase in the alloy composition x. The absence of the onset of negative differential mobility at x≥0.35 is found.
A quantitative analysis of the thermoelectric power measurements on composite Al-Ge films
Authors: Hurvits G. , Rosenbaum R. , McLachlan D. S.
Published: July 2002

Thermoelectric power (Seebeck coefficient) measurements at room temperature have been made on composite Al-Ge films for aluminum volume fractions ranging from 20% to 100%, the metal-insulator transition being at about 55%. These measurements were motivated to test theoretical predictions recently suggested by Bergman and Levy. The electrical conductivity, substituted in their equation, was obtained both by direct measurements and an effective media equation while the thermal conductivity was obtained from two different effective media equations. The agreement between the theoretical and experimental results support the correctness both of their theory and the effective media equations used.
Effects of thermal nitridation on the radiation hardness of the SiO2/Si interface
Authors: de Castro A. J. , Fernández M. , Sacedón J. L.
Published: July 2002

Auger electron spectroscopy has been used to obtain both chemical and electrical information about the influence of a high-temperature ammonia anneal on the electrical properties of the SiO2/Si interface. In this study, a relationship between the nitridation and the radiation hardness of the interface is presented. The efficiency of the positive charge generation induced by a high-energy (1–3 kV) electron beam has been clearly reduced in the annealed samples. This behavior is associated with the incorporation of nitrogen at the interface. Moreover, the treatment affects the number of electron traps in the oxide bulk in a selective way: trapping centers with a capture cross section of 10−18 cm2 are significantly reduced, whereas the density of traps with a cross section of 10−17 cm2 remains practically unalterable.
Effect of geometry-induced scattering on the quantum conductance in double-quantum-point constrictions connected in series by a cavity
Authors: Yuan Shi-qi , Gu Ben-yuan
Published: July 2002

Quantum-mechanical calculations of the conductance for model devices, consisting of double-quantum-point constrictions (QPCs) connected in series by a cavity, are carried out with use of the coupled-mode transfer method and wave-function matching technique. The effect of the geometry-induced scattering on the electron transport is in detail studied by varying the shape of the cavity and geometric scale of the devices. The characteristics of the conductance of the device are essentially determined by the quantum interference of the electron waves with different phases arising from multiple scattering and reflections of electrons in the cavity. When electrons are initially injected into the lowest transverse mode in the structure, as the energy of incident electrons increases the longitudinal quantum momentum of electrons becomes large and therefore electrons are substantially collimated along the propagating direction. However, when the energy of the electrons increases continuously the second conducting channel begins to open and the corresponding transverse quantum momentum of incident electrons is larger than that of the ground mode. This leads to the permitted divergent (deflection) angle for the electrons in the cavity against the propagating axis enlarges, thus, the reflection times of electrons by the cavity walls increase. The destructive interference of the multiple reflection waves leads to a conductance value less than the ideal quantization value, exhibiting ohmic-like-type transport with additivity of the series resistances. When the shape of the cavity takes more opening, the geometry-induced scattering is largely suppressed and the conductance in the series dual QPC structures is finally determined by that of the QPC with the lowest conductance.
Study of Co silicide formation by multiple implantation
Authors: Witzmann A. , Schippel S. , Zentgraf A. , Gajduk P. I.
Published: July 2002

The annealing behavior of 〈111〉 Si implanted with cobalt to doses between 2.5×1016 and 2×1017 cm−2 at energies between 30 and 250 keV has been investigated. The silicide formation during postannealing has been found to proceed in two phases characterized by activation energies of 0.8±0.3 and 2.9±0.3 eV. During the first phase a fast Co redistribution and pronounced nucleation and growth of the CoSi2 precipitates at defects has been observed. The investigation of the dose dependence proved that the amount of Co redistributed during this phase depends on the initial Co and defect concentration. However, the processes contributing to this fast Co redistribution require further investigation. The second step is well characterized by Ostwald ripening. Multiple implantations of Co at different energies have been applied to modify the Co concentration profile. In this way, it is possible to form Si/CoSi2/Si/CoSi2-layer systems on Si substrate with different layer thicknesses. All Si layers and the deeper lying CoSi2 layer are A type, whereas the crystalline orientation of the near surface layer depends on the procedure of formation. If the dose of the second implantation is too high CoSi nucleates during implantation. The CoSi precipitates are dissolved during annealing and a CoSi2 surface layer of type A is formed. Additional Si implantations offer the possibility to modify the defect profile without changing the Co distribution. During the subsequent annealing a redistribution of Co into the region of the additional defects was observed. Thus, implantation defects act as sinks for the metal atoms and play an important role during the first phase of the annealing process.
Relation between photoacoustic signal and sample thickness
Authors: Wang Guifen , Chang Yongmao , Ma Genyuan , Guo Qiang , Li Xueqian , Zhang Guangyin
Published: July 2002

The object of this work is to study the relation between the amplitude of photoacoustic signals and the sample thickness when the sample wafer is perpendicular to the piezoelectric ceramics, to which is attached by an edge. Treating the sample as an infinite elastic layer, a three-dimensional temperature function is introduced. Using the Green’s function for the thermal stress, we solve the thermoelectric equation obtaining an expression for the dependence of the photoacoustic signal on the thermal, mechanical, geometric, and optical properties of the sample. The relation between the photoacoustic signal and the sample thickness is applied for a silicon single crystal. The calculated results are in good agreement with the experimental data.
Modeling inactive boron during predeposition processes
Authors: Vandenbossche Eric , Baccus Bruno
Published: July 2002

A new type of boron cluster is presented through the study of boron diffusion at high concentrations under predeposition conditions. Recent experimental results of boron predeposition give the opportunity to analyze, with the simulations, the behavior of the inactive part of the boron profile, found to increase dramatically with surface concentrations. After a rapid thermal annealing, the electrical activation appears to be extremely high due to the dissolution of the inactive part of the profile. The only way to reproduce the experimental observations has been to consider an inactive and mobile 2-atom boron cluster. A complete kinetic model, including the reactions between boron cluster, boron-interstitial pair, substitutional boron and point defects, is used in order to determine all the parameters characterizing this cluster. The diffusivity of the cluster is much less than the one of the boron-interstitial pair (about 40 times). It is suggested that the nature of this cluster may depend on the exact predeposition conditions. Finally, from a general point of view, the behavior of supersaturations of interstitials during predeposition as a function of surface concentration is presented, in order to understand the implication of each basic reaction involved in the kinetic model.
Optimization of a ridged waveguide
Authors: Subramaniam Srisivane , Devendran Vasantha , Hoole S. Ratnajeevan H.
Published: July 2002

In industrial design, synthesizing a guide for a given cut-off frequency or attenuation limit is a challenging problem. In this paper, an algorithm is applied for the quick computation of the gradients of the object function, without using the solution of two finite-element eigenvalue problems to compute the gradient of the object function with respect to every parameter of description. It is shown that because of the presence of deep valleys in the object function, the traditionally inferior zeroth-order pattern search method performs better than the first order conjugate gradient method.
Effective thermal conductivity of a low-density polyethylene and ionic salts composite
Authors: Kutcherov Vladimir , Chernoutsan Alexei
Published: July 2002

The effective thermal conductivity λe of model dispersed composites prepared from well specified powders of NaF and low-density polyethylene (LDPE) was measured at different volume fractions at temperatures in the range 120–320 K and pressures up to 1.73 GPa. Measurements were made using the hot-wire technique. The ratio of thermal conductivities of NaF and LDPE reached values up to 60 at the temperatures and pressures employed. These results together with the previous data on AgCl-LDPE and NaCl-LDPE composites were used for testing the extensive set of theoretical models which permit us to calculate λe in the wide range of volume concentrations and component conductivity ratios. We have come to the conclusion that at low concentrations of the more conductive component the bond lattice model describes the experimental data better then site lattice model, and at high concentrations the situation is the opposite. The best single model for all data is the ‘‘renormalized effective medium approximation’’ model of Sahimi which brings together effective medium and renormalization group approaches.
Magnetic properties of glass-metal nanocomposites prepared by the sol-gel route and hot pressing
Authors: Roy S. , Das D. , Chakravorty D. , Agrawal D. C.
Published: July 2002

Glass-metal nanocomposite powders in the systems Fe/SiO2 and Ni/SiO2 have been prepared by the sol-gel technique followed by reduction treatment. Bulk nanocomposites are then fabricated by hot pressing these powders. The metal particle diameters range from 8.9 to 14.8 nm. The materials show enhanced coercivities, e.g., a maximum of 82 Oe in the case of Ni/SiO2 and a maximum of 474 Oe in the case of Fe/SiO2 systems. The Mössbauer spectra of Fe/SiO2 samples are comprised of a ferromagnetic component superposed on a superparamagnetic doublet.
Heteroepitaxy, polymorphism, and faulting in GaN thin films on silicon and sapphire substrates
Authors: Lei T. , Ludwig K. F. , Moustakas T. D.
Published: July 2002

The structure of GaN films grown by electron-cyclotron-resonance-assisted molecular beam epitaxy on Si(111), Si(001), basal-plane sapphire, a-plane sapphire, and r-plane sapphire substrates was studied with four-circle x-ray diffractometry. Phase content, domain size, inhomogeneous strain, and in-plane and out-of-plane domain misorientations were measured and compared for films grown on each type of substrate. Wurtzite and zinc blende polymorphs were found to coexist in films grown on Si(111). The two structures grow in the (0002) and (111) orientations, respectively, so that they may transform into each other via stacking faults on close-packed planes. Smaller amounts of zinc blende material were also found in predominately (0002) wurtzitic films on a-plane sapphire and (112̄0) wurtzitic films on r-plane sapphire.
Novel applications of photonic band gap materials: Low-loss bends and high Q cavities
Authors: Meade Robert D. , Devenyi A. , Joannopoulos J. D. , Alerhand O. L. , Smith D. A. , Kash K.
Published: July 2002

In this paper we discuss a novel material which has nearly ideal properties at optical frequencies. It combines the low dissipation of a dielectric with the reflectivity of a metal. This material employs a two-dimensional photonic band gap structure to achieve in-plane confinement of light and uses index contrast to achieve vertical confinement. We discuss how this material can be used to create microcavities for the production of low threshold lasers and waveguides capable of low-loss bends.
First phase nucleation of metal-rich silicide in Ta/Si systems
Authors: Noya Atsushi , Takeyama Mayumi , Sasaki Katsutaka , Nakanishi Taichi
Published: July 2002

Solid-phase reactions in the interfacial region of Ta/(100)Si systems have been studied by x-ray diffraction and Auger electron spectroscopy. The metal-rich silicide of Ta5Si3, which has never been observed in Ta/Si systems so far, is first nucleated at annealing temperatures of 600–650 °C in the first stage of the reaction. Subsequent annealing at 700 °C leads to the formation of a TaSi2 phase, and the metal-rich phase is replaced by TaSi2 at annealing temperatures over 700 °C by the out-diffusion of Si.
Growth of SiGe quantum wires and dots on patterned Si substrates
Authors: Hartmann A. , Vescan L. , Dieker C. , Lüth H.
Published: July 2002

SiGe/Si quantum well layers are selectively grown by low pressure chemical vapor deposition on patterned Si substrates. Transmission electron microscopy (TEM) shows that the growth rate of SiGe in convex corners between different surface planes is at least ten times higher than the growth rate observed on (001) planes. This high growth rate leads to the formation of quantum wires and dots between such facets. Photoluminescence (PL) spectra of square and rectangular patterns, bounded by quantum wires, ranging in size from 300μm down to 500nm are taken. The observed energy shifts of the (001) quantum well PL–peaks are explained by surface diffusion of Ge adatoms into the quantum wires. A surface diffusion model is used to obtain a Ge diffusion length of λ=2.5±0.6 μm at 700°C. Thus, a method for the determination of surface diffusion lengths in strained layer epitaxy is introduced. For SiGe layers grown above the Stranski–Krastanow critical thickness for three dimensional (3D) growth, a competition between the SiGe wires in the interfacet corners and the SK islands on the (001) planes is observed. In squares as large as 2×2 μm2 the SiGe wires lead to a suppression of 3D growth on the (001) plane altogether, as observed by TEM and PL.