bluuebunny/crossref_metadata_2025 huggingface dataset:
Low friction in mixed-mu superconducting bearings
Authors: Hull J. R. , Hilton E. F. , Mulcahy T. M. , Yang Z. J. , Lockwood A. , Strasik M.
Published: July 2002

Individual magnetic steel rotors were levitated by combining the attractive force between permanent magnets and the steel with the repulsive force between high-temperature superconductors and the steel. The free spindown of several rotors was observed, and the effective coefficient of friction for the bearing was calculated as a function of geometry. Low-speed coefficients of <10−8 were observed, and the velocity dependence of MnZn ferrite rotors suggest that coefficients of <10−6 are attainable at bearing rim velocities of 100 m/s.
Band offsets in Zn1−xCdxSe/ZnSe multiple quantum wells
Authors: Pellegrini Vittorio , Tredicucci Alessandro , Beltram Fabio , Vanzetti Lia , Lazzarino Marco , Franciosi Alfonso
Published: July 2002

The conduction-band discontinuity in Zn1−xCdxSe/ZnSe multiple-quantum-well structures grown by molecular-beam epitaxy and pseudomorphically strained to ZnSe were evaluated via electrical and intersubband absorption measurements for 0.23≤x≤0.33. A variation between 165±14 and 253±14 meV is found in the cadmium concentrations range analyzed. Our results are consistent with a valence-band contribution to the band-gap difference Qv=0.33±0.03 in the alloy series.
Amorphous (Mo, Ta, or W)–Si–N diffusion barriers for Al metallizations
Authors: Reid J. S. , Kolawa E. , Garland C. M. , Nicolet M.-A. , Cardone F. , Gupta D. , Ruiz R. P.
Published: July 2002

M–Si–N and M–Si (M=Mo, Ta, or W) thin films, reactively sputtered from M5Si3 and WSi2 targets, are examined as diffusion barriers for aluminum metallizations of silicon. Methods of analysis include electrical tests of shallow-junction diodes, 4He++ backscattering spectrometry, x-ray diffraction, transmission electron microscopy, scanning electron microscopy, and secondary-ion-mass spectrometry. At the proper compositions, the M–Si–N films prevent Al overlayers from electrically degrading shallow-junction diodes after 10 min anneals above the melting point of aluminum. Secondary-ion-mass spectrometry indicates virtually no diffusivity of Al into the M–Si–N films during a 700 °C/10 h treatment. The stability can be partially attributed to a self-sealing 3-nm-thick AlN layer that grows at the M–Si–N/Al interface, as seen by transmission electron microscopy.
Structural and optical characterization of monolayer interfaces in Ga0.47In0.53As/InP multiple quantum wells grown by chemical beam epitaxy
Authors: Benzaquen R. , Roth A. P. , Leonelli R.
Published: July 2002

We have carried out a detailed structural and optical characterization of Ga0.47In0.53As/InP multiple quantum wells grown by chemical beam epitaxy using a well-defined sequence of growth interruption times between successive layers. These growth interruption times result in the formation of interfacial layers which drastically alter the structural properties of Ga0.47In0.53As/InP multiple quantum wells. An analysis of double-crystal x-ray diffraction data reveals that exposure of InP to arsine for 2 s is sufficient to create approximately 3 monolayers of InAs0.55P0.45 ternary under biaxial compressive strain at the InP/Ga047In0.53As interface. Moreover, exposure of Ga0.47In0.53As to phosphine for 2 s results in the formation of approximately 2 monolayers of Ga0.48In0.52As0.21P0.79 quaternary under biaxial tensile strain at the Ga0.47In0.53As/InP interface. We find that long exposures to hydrides (over 5 s) rather than short ones give rise to interfacial layers with less compositional disorder and/or thickness fluctuation. Moreover, photoluminescence and absorption spectroscopy data reveal the negligible effect of InAsxP1−x and GaxIn1−xAsyP1−y interfacial layers on the emission and optical absorption properties of Ga0.47In0.53As/InP multiple quantum wells with sufficiently thick Ga0.47In0.53As layers.
Photoluminescence study of the 1.047 eV emission in GaN
Authors: Pressel K. , Nilsson S. , Heitz R. , Hoffmann A. , Meyer B. K.
Published: July 2002

We use photoluminescence to study residual transition metal contaminants in GaN layers, which are grown by the sandwich technique either on 6H-SiC substrate or on sapphire substrate. We observe three no-phonon lines in the near infrared optical region at 1.3 eV, 1.19 eV, and 1.047 eV caused by 3d transition metals. The appearance of GaN related host modes in the phonon sideband of these emissions proves that the luminescence centers are incorporated in the hexagonal GaN layers. In this paper we especially focus on the luminescence band with the no-phonon line at 1.047 eV. Temperature dependent photoluminescence measurements reveal an excited state splitting of 8 meV. In photoluminescence excitation spectroscopy we observe a further excited state at 1.6 eV with a fine structure splitting. The appearance of this excited state in the n-type samples gives evidence that the defect must already exist in its luminescent charge state without illumination. The experimental results on the 1.047 eV emission fit to a 4T2(F)→4A2(F) internal electronic transition of a transition metal with a 3d7 electronic configuration.
Grain growth and ordering kinetics in CoPt thin films
Authors: Barmak K. , Ristau R. A. , Coffey K. R. , Parker M. A. , Howard J. K.
Published: July 2002

Grain growth and ordering kinetics have been studied in a 10-nm-thick CoPt alloy film of equiatomic composition annealed in the temperature range 550–700 °C by quantifying the grain size, grain size distribution, ordered fraction and ordered domain size. The mean grain size of the as-deposited films is 5 nm and the film is fully face-centered cubic. Upon annealing in the temperature range 550–600 °C, the mean grain size reaches a stagnation limit of 27 nm and the grain size distribution is lognormal. Grain growth resumes beyond 600 °C and the mean grain size reaches as high as 55 nm at 700 °C. Ordering occurs by nucleation and growth of L10 ordered domains, with a mean size of 3 nm at 550 °C and 19 nm at 700 °C. The ordered fraction shows a dramatic increase from 1% to ∼28% between the two extremes of annealing temperature. The increase in the coercivity of the annealed films follows the increase in the ordered fraction more closely than the increase in grain size. The shape of the M–H loop shows evidence of coupling between the magnetically hard (ordered) and soft (disordered) regions.
Characteristics of a quantum waveguide: Effects of finite confinement potential and geometrical deformations and comparison with simple theoretical approach
Authors: Tachibana Hiroyuki , Totsuji Hiroo
Published: July 2002

Based on numerical solutions for the transmission characteristics of a typical quantum waveguide (the T-structure and its modifications), some effects of practical importance, the finiteness of confinement potential and the geometrical deviations from the ideal shape, are clarified. Numerical results are also compared with those of the simplified S-matrix method and the applicability of the latter is discussed. The results may be useful in applying a quantum waveguide to electronic devices and in analyzing more complex structures by the simplified S-matrix.
Finite temperature magnetization reversal in ultrathin magnetic films
Authors: Chui S. T.
Published: July 2002

We study the magnetization reversal of Heisenberg spins in a two-dimensional plane under an external reversing field at finite temperatures. In the Ising limit dominated by uniaxial anisotropy and exchange, we observe nucleation of domains in Monte Carlo simulations. Analysis of the probability distribution in the energy provides for an estimate of the free energy for domain nucleation that is much smaller than that obtained from the wall energy at zero field because the energy of the nucleus approaches zero at the threshold of coherent rotation. The coercive field depends linearly on the temperature. When the dipolar interaction becomes important, reversed spins domains form quickly but do not grow. There is an intermediate regime when most spins are aligned parallel to the plane.
Functionalization and applications of nanosized γ-Fe2O3 particles
Authors: Liu Qingxia , Xu Zhenghe
Published: July 2002

A novel method to prepare nanosized γ-Fe2O3 particles containing reactive thio and/or disulphide groups using molecular self-assembly method is described. The monolayers are formed from self-assembled bolaamphiphiles, e.g., 16-mercaptohexadecanoic acid (HS–C15H30–COOH), and characterized using x-ray photoelectron spectroscopy. The magnetization of surfactant-coated particles is determined using a vibrating sample magnetometer. The potential uses of functionalized magnetic particles in recovering precious (Ag+) and base (Cu2+) metal ions from industrial effluent are examined. The modification of thiol on coated-magnetic particles by controlled oxidation is also described to extend the application of self-assembled organic monolayers to stabilizing ferrofluids.
Interface alloying at Fe/Cr interfaces and its role in exchange coupling, angular resolved Auger electron, magneto-optic Kerr effect, and Brillouin light scattering studies (invited)
Authors: Heinrich B. , Cochran J. F. , Venus D. , Totland K. , Atlan D. , Govorkov S. , Myrtle K.
Published: July 2002

Angular resolved Auger electron studies were carried out for Fe whisker/Cr(001) interfaces which were prepared at 100, 180, 246, and 296 °C. The Cr atoms penetrate progressively into the second (counting from the surface) atomic layer at 100, 180, and 246 °C. At 296 °C the Cr atoms enter the third atomic layer. No noticeable fraction of the Cr atoms was found in the fourth atomic layer. The exchange coupling was studied in Fe whisker/Cr/Fe(001) films which were grown in a nearly perfect layer by layer mode. Magneto-optic Kerr effect and Brillouin light scattering measurements showed that the measured change in the phase of the short wavelength oscillations, the presence of a slowly varying exchange coupling bias, and the small measured values of exchange coupling are caused by the same mechanism: interface alloying. The exchange coupling in Fe whisker/Cr/nFe specimens, for n=10, 20, 30, and 40 ML, showed no obvious dependence on the Fe layer thickness.
Magnetic profiles of bonded magnets affected by the magnetizing fixtures
Authors: Chiang Donyau , Wang Shyh-Jier , Huang Der-Ray
Published: July 2002

The four-pole magnetic profiles of the bonded magnets can be obtained by using planar and circular magnetizing fixtures with and without iron backyokes. With the aid of an iron fixture and iron backyoke, the bonded magnets are easily saturated with a shallow saddle profile in each pole. The saturation voltage is about 400 V for a 1600 μF magnetizer. From a practical point of view, it is better to use the planar fixture than to use the circular fixture.
Evolution of shallow donors with proton fluence in n-type silicon
Authors: Ntsoenzok E. , Desgardin P. , Saillard M. , Vernois J. , Barbot J. F.
Published: July 2002

Bipolar components that consist of p+n junctions have been irradiated by MeV protons at fluences ranging from 1011 to 1013 particles cm−2. Capacitance-voltage measurements have been used to investigate changes in the carrier concentration profiles. Shallow donors that can induce harmful effects in electronic devices have been studied as a function of fluence, flux, and annealing parameters.
Magnetoresistance and magnetic properties of La1−x□xMnO3−δ  thin films
Authors: McGuire T. R. , Gupta A. , Duncombe P. R. , Rupp M. , Sun J. Z. , Laibowitz R. B. , Gallagher W. J. , Xiao Gang
Published: July 2002

Epitaxial thin films of La1−x⧠xMnO3−δ(0≤x≤0.33) where ⧠ is a vacancy have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The lanthanum deficiency results in self doping of the system without the need for divalent ion substitution. Both the films and the bulk targets used for deposition were ferromagnetic and showed giant magnetoresistance (GMR). The films increased in saturation magnetization Ms and Curie temperature Tc with increasing x. The Tc, however, was about the same as found in Cax substitutions. Bulk samples showed little change in Ms and Tc with vacancy concentration. Ms in bulk samples is about half the value found in the films which have theoretical saturations for x≥0.2. GMR in the films is a few hundred percent but the bulk values are 25% at H=1.8 T.
Junction recombination current in abrupt junction diodes under forward bias
Authors: Corkish Richard , Green Martin A.
Published: July 2002

A new, analytical method is presented for calculating the depletion-region recombination current for abrupt-junction diodes under forward bias. The method is appropriate when the recombination current is dominated by recombination through Shockley–Read–Hall centers at a single energy level whose density does not vary strongly with position through the device. The new model is systematically compared with earlier models and with the results of finite-element analyses using PC-1D. If it is reasonably assumed that PC-1D is the most accurate of the methods considered here, the others may be ranked according to their proximity to the PC-1D result. It is shown that the new method, despite its simplicity, yields results closer to PC-1D than the earlier models for many practical situations. In addition, it is shown that one existing model may be brought into agreement with the finite-element analysis by a simple modification of the limits of integration.
Failure in a shocked high-density glass
Authors: Bourne N. K. , Millett J. C. F. , Rosenberg Z.
Published: July 2002

One of the outstanding questions concerning the compressive behavior of brittle materials concerns the failure wave observed in glasses. While much work has centered around relatively open structure, low density glasses such as borosilicate (pyrex) and soda-lime (float, which is partially filled), none has addressed the response of highly filled lead glasses. This work presents the results of a series of plate impact experiments carried out on the lead glass DEDF.TM This material was shocked in uniaxial strain and the longitudinal and lateral components of stress and strain were measured. The failure wave, observed in lower density glasses, was observed in this material but its velocity reached that of the shock at less than twice the Hugoniot elastic limit (HEL).
Magnetoluminescence study on the effective mass anisotropy of CuPtB-ordered GaInP2 alloys
Authors: Ernst P. , Zhang Yong , Driessen F. A. J. M. , Mascarenhas A. , Jones E. D. , Geng C. , Scholz F. , Schweizer H.
Published: July 2002

Photoluminescence measurements in a magnetic field between 0 and 13.6 T were carried out on CuPtB-ordered GaInP at liquid-helium temperature. Four samples of different degrees of ordering (η, varying from 0 to 0.54) were studied. Experimental results show that the ordering not only induces a band-gap reduction and valence-band splitting, it also causes changes in effective masses and an effective mass anisotropy. By measuring and analyzing the magnetoluminescence with the magnetic field aligned along both ordering (the [1̄11]) and growth (the [001]) directions, we demonstrate that, for the band-edge excitonic state, the reduced mass in the plane perpendicular to the ordering direction is smaller than that in the ordering direction and also smaller than that of a disordered alloy. The exciton binding energy is found nearly independent of the degree of ordering, in agreement with theoretical predictions.
Continuing degradation of the SiO2/Si interface after hot hole stress
Authors: Al-kofahi I. S. , Zhang J. F. , Groeseneken G.
Published: July 2002

This article reports new experimental results on the continuing interface trap generation post-hot hole injection and investigates the generation mechanism. The generation post-hole injection is found to be two orders of magnitude slower than that post-irradiation and cannot be satisfactorily explained by the transportation of hydrogen species across the gate oxide. The role played by the recombination of trapped holes with free electrons is examined. There is a lack of correlation between the trapped hole removal and the interface trap creation, which is against the prediction of the trapped hole conversion model. The results indicate that the interface traps generated during and post-stress originate from two different defects. The defect responsible for post-stress generation is excited by hole injection and then converted into an interface trap if a positive gate bias is applied. It is found that generation in a poly-Si gated metal–oxide–semiconductor field effect transistor behaves differently from that in an Al-gated device. The possible causes for this difference are discussed.
Media noise and signal-to-noise ratio estimates for high areal density recording
Authors: Tarnopolsky Giora J. , Pitts Peter R.
Published: July 2002

Transition jitter, σjitter, and pulse-width fluctuations, σPW50, limit the attainable signal-to-noise ratio (SNR) in a magnetic recording system. We estimate the media noise power for systems with high areal density. The case of a Lorentzian-shaped voltage pulse is worked out in detail. Experiments show that the model reasonably reproduces data. The analysis is applied to areal densities of 10 Gbit/in.2 and beyond. If the ratio of PW50 to bit length B (the so-called channel density U) is kept constant, the ratio σjitter/B is found to depend only on SNR and U, and is about 3% for SNR=26 dB, at any density. Under those assumptions, σjitter≈0.75 nm at 1 Mfci. Otherwise, for constant PW50, σjitter/B decreases monotonically with density, and is about 1% for PW50=120 nm, 1/B=1 Mfci, SNR=27 dB.
Desirable magnetotransport properties in doped Mn-oxide-based superlattices
Authors: Kwon C. , Kim K.-C. , Robson M. C. , Gu J. Y. , Rajeswari M. , Venkatesan T. , Ramesh R.
Published: July 2002

Superlattice structures based on doped Mn-oxides and SrTiO3 exhibit desirable magnetotransport properties, such as an enhanced magnetoresistance (MR) and a broadened MR transition. An La0.7Ca0.3MnO3 (55 Å)/SrTiO3 (160 Å) superlattice shows 61% MR at 0.5 T and 109 K, defined by [R(0 T)−R(0.5 T)]/R(0 T), compared to 38% MR for a single layer La0.7Ca0.3MnO3 film at 125 K. MR of larger than 85% at 5 T has been observed in a broad temperature range (from 150 to 10 K) for this superlattice. A systematic study of La0.7Ba0.3MnO3/SrTiO3 superlattices shows that the decrease of the La0.7Ba0.3MnO3 layer thickness results in the broadening of the MR peak in temperature.
Grain growth in anisotropic bicrystal films
Authors: Kris R. , Vilenkin A. J. , Brokman A.
Published: July 2002

A model of one dimensional grain growth in a bicrystal film, driven by surface anisotropy, is presented. The equation of surface diffusion (Mullins Equation) and the curvature driven boundary equation of motion are simultaneously solved under a set of matching and boundary conditions that includes the mechanical equilibrium at the discontinuous points (surface corners due to anisotropy and grain boundary groove roots), the conservation of surface flux and the continuity of chemical potential at the groove root. An analytical solution is obtained for the steady state motion. This solution is investigated as a function of the degree of anisotropy. It is found that a steady state motion is possible for a limited range of anisotropies. Below a critical degree of anisotropy the film breaks up. It is shown that the steady state velocity decreases with the degree of anisotropy. Numerical analysis of the time dependent problem demonstrates that the steady state solution is stable. It is shown that anisotropy stabilizes the process of grain growth against film breakup.
Electronic states in diffused quantum wells
Authors: Vlaev S. , Contreras-Solorio D. A.
Published: July 2002

In the present study we calculate the energy values and the spatial distributions of the bound electronic states in some diffused quantum wells. The calculations are performed within the virtual crystal approximation, sp3s* spin dependent empirical tight-binding model and the surface Green function matching method. A good agreement is found between our results and experimental data obtained for AlGaAs/GaAs quantum wells with thermally induced changes in the profile at the interfaces. Our calculations show that for diffusion lengths LD=0−20 Å the optical transition between the ground electron and hole states is less sensitive to the LD changes than the optical transitions between the excited electron and hole states. For diffusion lengths LD=20−100 Å, the optical transition between the second excited states is not sensitive to the diffusion length, but the other optical transitions display large “blue shifts” as LD increases. The observed dependence is explained in terms of the bound states spatial distributions.
Simple model for the dielectric constant of nanoscale silicon particle
Authors: Tsu Raphael , Babić Davorin , Ioriatti Liderio
Published: July 2002

As the physical size approaches several nanometers, reduction in the static dielectric constant ε becomes significant. A modified Penn model, taking into account the quantum confinement induced discrete energy states, was applied to a sphere and to a wire. The calculated size dependent ε is consistent with the wave-vector-dependent ε(q). However, this form of ε is more amenable for calculations of donor and exciton binding energies in a finite quantum confined nanoparticle when a full electrostatic boundary value problem must be tackled. The results of our model compare favorably with other, far more sophisticated, calculations.
Effect of electric field on the phase transition in ZrTiO4 ceramic
Authors: Park Yung , Lee Hong Min , Kim Ho-Gi
Published: July 2002

An x-ray study of electric fields on the incommensurate phase transition in ZrTiO4 ceramic was performed in order to obtain information concerning the nature of the competing forces that give rise to structural instability at a general wave vector. It was found that an increase in dc electric field increases the normal incommensurate phase transition temperature [(dTi/dEi)≅+1.7 °C cm/kV]. The electric field dependence of the incommensurate to commensurate transition was also investigated [(dTc/dEc)≅+1.5 °C cm/kV].
Theory on the quantum confinement-luminescence center model for nanocrystalline and porous Si
Authors: Qin G. , Qin G. G.
Published: July 2002

This article demonstrates, from the theoretical point of view, that owing to the phonon-assisted relaxation rate of the excited electron-hole pair’s transiting to lower states decreases as the scale of Si particle is reduced, the optically excited electron-hole pair in the nanometer silicon particle with sufficient small scale generally tunnel into the SiOx layers which enclose the Si particles and recombine radiatively through the luminescence centers there to emit visible light rather than recombine radiatively in the Si particles. It is proved also that when the density of the luminescence center is 2.5×1022/m3, the upper limit of the average scale of the nanometer silicon particles in the ensemble capable of emitting red light in room temperature is around 8.7 nm, which is much bigger than the generally estimated value of 3 nm.
Highly sensitive porous silicon based photodiode structures
Authors: Balagurov L. A. , Yarkin D. G. , Petrovicheva G. A. , Petrova E. A. , Orlov A. F. , Andryushin S. Ya.
Published: July 2002

Al/PS/c-Si photodiode structures were fabricated with a porous silicon (PS) layer of high porosity. Photosensitivity spectra, current–voltage (I–V) dependences at different temperatures, and electron beam induced current profiles were analyzed. Effects of annealing on the device characteristics were studied. The photosensitivity spectrum of the Al/PS/c-Si structures was found to be analogous to that of Al/c-Si structures. The photosensitivity value of as-prepared Al/PS/c-Si structures is 1.3 times that of an Al/c-Si Schottky diode in the wavelength range of 0.5–1.0 μm. The photosensitivity of the annealed structure strongly depends on the reverse bias; it increases by more than two orders of magnitude (up to 10 A/W) when the reverse bias increases from 0 to 5 V. The I–V dependences indicate that band bending on the sides of the PS/c-Si heterointerface are in opposite directions.
Kinetics of ion-beam-induced interfacial amorphization in silicon
Authors: Henkel T. , Heera V. , Kögler R. , Skorupa W. , Seibt M.
Published: July 2002

This article reviews modeling and experimental results of ion-beam-induced interfacial amorphization (IBIIA) in silicon. It is shown that this process differs from the well-known bulk amorphization with regard to the critical energy density approach and the evolution of the roughness of the amorphous/crystalline interface during ion irradiation. IBIIA depends on the substrate temperature, ion flux, and nuclear energy deposition at the amorphous/crystalline interface, which is discussed in detail. Within this scope, new results about the temperature and ion flux dependence of IBIIA are presented that cannot be explained by previous models. Therefore, a new model based on ballistic transport effects that allows one to understand experimental results at low temperatures is proposed. According to this concept IBIIA is controlled by three processes interacting at the amorphous/crystalline interface: an athermal ion-beam-induced defect generation, a thermally activated recombination of defects, and an athermal transport of defects towards the amorphous/crystalline interface as a result of ballistic processes. It is speculated that these defects are mainly interstitials and vacancies involved in those processes.
Conduction-band offset in a pseudomorphic GaAs/In0.2Ga0.8As quantum well determined by capacitance–voltage profiling and deep-level transient spectroscopy techniques
Authors: Lu Liwu , Wang J. , Wang Y. , Ge Weikun , Yang Guowen , Wang Zhanguo
Published: July 2002

The conduction-band offset ΔEC has been determined for a molecular beam epitaxy grown GaAs/In0.2Ga0.8As single quantum-well structure, by measuring the capacitance–voltage (C–V) profiling, taking into account a correction for the interface charge density, and the capacitance transient resulting from thermal emission of carriers from the quantum well, respectively. We found that ΔEC=0.227 eV, corresponding to about 89% ΔEg, from the C–V profiling; and ΔEC=0.229 eV, corresponding to about 89.9% ΔEg, from the deep-level transient spectroscopy (DLTS) technique. The results suggest that the conduction-band discontinuity ΔEC obtained from the C–V profiling is in good agreement with that obtained from the DLTS technique.
Enhancement effect of photoluminescence in assemblies of nano-ZnO particles/silica aerogels
Authors: Mo C. M. , Li Y. H. , Liu Y. S. , Zhang Y. , Zhang L. D.
Published: July 2002

SiO 2 aerogels were prepared by a sol-gel method and supercritical drying. Introduction of nano-ZnO into nanopores of the SiO2 aerogels is performed by immersion, chemical reaction, and hydrolysis. Photoluminescence of these assemblies of nano-ZnO particles/SiO2 aerogels were systematically measured. For each assembly, a very strong photoluminescence (PL) band with the peak position of about 500 nm was observed. The luminescence intensities of the assemblies of nano-ZnO particles/SiO2 aerogels are 10–50 times higher than that of nanostructured ZnO bulks. The quantum efficiency of the PL band for the assemblies is in the range of 0.2%–1%. This luminescence enhancement effect is caused by the increase of the singly ionized oxygen vacancies in nano-ZnO particles, which are located in nanopores of the SiO2 aerogel.
Conduction mechanisms and interface property of silicon oxide films sputtered under different oxygen concentrations
Authors: Choi W. K. , Han K. K. , Choo C. K. , Chim W. K. , Lu Y. F.
Published: July 2002

An investigation on the effects of oxygen mixing in the sputtering gas and rapid thermal annealing on the electrical properties of radio frequency sputtered silicon oxide films has been carried out. The conductivity of the oxide film decreases as the concentration of oxygen in the sputtering gas increases, and also for rapid thermal anneal temperatures higher than 900 °C. Transport mechanisms for films annealed for 50 and 200 s were found to be that of Schottky emission and space charge limited conduction, respectively. From the capacitance versus voltage measurements, it was concluded that the improvement in the oxide quality of the annealed films was not related to the reduction of the interface trapped charge density.
Phase boundary between ripple and hut in the initial roughening stage in heteroepitaxy
Authors: Suh K. Y. , Lee Hong H.
Published: July 2002

A regularly undulating surface topography has been observed during growth of heteroepitaxial layers such as Si1−xGex/Si2 and InxGa1−xAs/GaAs5. We present a modified evolution mechanism of this ripple structure, which consists of initial roughening and evolving stages. A theoretical relationship is derived through energy minimization, which indicates that the ratio of the amplitude to the square of the period of the ripple structure is constant in the evolving stage. Also derived is a criterion for determining the phase boundary between the ripple and hut phases in the Stranski–Krastanov growth.
Modeling and numerical simulations of microwave-induced ionic transport
Authors: Freeman S. A. , Booske J. H. , Cooper R. F.
Published: July 2002

A numerical model was developed to simulate and study microwave-induced transport in ionic solids. The model is based on continuum equations, is very general, and could be applied to many materials. The assumptions, boundary conditions, initial conditions, and numerical techniques used in the model are described. Results are presented from a study of microwave driven defect transport in sodium chloride. Static, high-frequency, and quasistatic results show that ponderomotive rectification of vacancy fluxes will act to deplete the vacancies in a near-surface region and will continue to pull vacancies to the surface through diffusion kinetics. The ponderomotive driving force for this transport is characterized over a wide range of variable space. The magnitude of the driving force falls right in the range such that it can explain why microwave-enhanced mass transport is observed in some experimental cases but not in others.
Low field magnetotransport in La0.7Sr0.3MnO3 films
Authors: Teo B. S. , Mathur N. D. , Isaac S. P. , Evetts J. E. , Blamire M. G.
Published: July 2002

We have observed a strong correlation between the degree of epitaxy and the low field resistance versus field R(H) behavior in highly textured La0.7Sr0.3MnO3 films grown by pulsed laser deposition on SrTiO3 substrates. Highly epitaxial films show a small temperature dependent low field hysteretic R(H) peak near the magnetic ordering temperature (Tc), when the applied field is parallel to the transport current (J∥H). In contrast, films with reduced epitaxy show a much larger low field hysteretic R(H) behavior with a pronounced in-plane angular dependence. The magnitude of this response grows with decreasing temperature below Tc. Our data and analysis suggest that even low angle grain boundaries play a fundamental role in transport properties in colossal magnetoresistive materials.
Electron spin resonance and magnetization in perovskite and pyrochlore manganites
Authors: Tovar M. , Causa M. T. , Ibañez G. , Ramos C. A. , Butera A. , Rivadulla F. , Alascio B. , Oseroff S. B. , Cheong S-W. , Obradors X. , Piñol S.
Published: July 2002

We have performed electron spin resonance (ESR) and dc susceptibility, χdc(T), measurements in La0.7Ca0.3MnO3 perovskite up to 1000 K. This is adequately described in the paramagnetic regime in the constant coupling approximation assuming an effective Heisenberg-like pair interaction. The ESR linewidth is given by ΔHpp(T)=ΔHpp(∞)[C/Tχ(T)]. The ESR intensity follows the T dependence of χdc(T). We conclude that the relaxation mechanism is essentially the same for the whole studied paramagnetic range. The observed behavior reflects the progressive importance of magnetic clustering on approaching Tc. We have found similar T dependence for La0.7Sr0.3MnO3 and La0.7Pb0.3MnO3 perovskites, and also for the pyrochlore In2Mn2O7.
Micromagnetic simulation of magnetizability of nanocomposite Nd–Fe–B magnets
Authors: Schrefl Thomas , Fidler Josef
Published: July 2002

Micromagnetic finite element calculations clearly show that the magnetizability of nanocomposite Nd2Fe14B magnets improves with increasing α-Fe content. The magnetization curves show a steep increase at the domain propagation field in dc demagnetized samples. Thermally demagnetized states store a higher amount of exchange energy, leading to an increase of the initial susceptibility. An applied field of 960 kA/m leads to a saturation of 85% for a two-phase α-Fe/Nd2Fe14B magnet, whereas the Fe3B/Nd2Fe14B magnet and the single-phase Nd2Fe14B magnets reach a saturation of only 70% and 50%, respectively. The improved saturation behavior of two-phase α-Fe/Nd2Fe14B magnets has to be attributed to the exchange field which is provided by α-Fe grains that are already oriented parallel to the field direction. Hard magnetic grains that remain oppositely magnetized after applying the maximum magnetizing field deteriorate the coercive squareness in single-phase Nd2Fe14B magnets and two-phase Fe3B/Nd2Fe14B magnets.
Evaluation of precipitate densities and capture radii from the analysis of precipitation kinetics
Authors: Hieslmair H. , Istratov A. A. , Heiser T. , Weber E. R.
Published: July 2002

The use of Ham’s law in various forms to analyze precipitate site density is discussed. Ham’s law is also applied to situations where the site density is of the same order as the solute concentration. While the results are general, this analysis uses iron precipitation and iron-boron pairing in silicon as example systems. The results of Ham’s paper are reviewed and several implementations of Ham’s Law are compared and fitted to experimental data. Iron-boron pairing is described using Ham’s law and a capture radius of the boron of 6 nm is obtained.
Hot-optical-phonon effects on electron relaxation in an AlGaAs/GaAs quantum cascade laser structure
Authors: Paulavičius G. , Mitin V. , Stroscio M. A.
Published: July 2002

The influence of hot-phonon effects on coupled electron-phonon system relaxation dynamics in an AlGaAs/GaAs quantum cascade laser structure at 10 K has been investigated by the ensemble Monte Carlo technique. The GaAs quantum well laser system considered herein supports lasing between two electron subbands separated by 295 meV. After injection into the upper energy level, electrons transit to the lower subband by means of light emission or phonon-assisted scattering processes. Optical-phonon emission dominates among the latter radiationless electron relaxation channels making the carrier lifetime in the upper subband very short. Therefore, large threshold injection currents are required to create the electron population inversion necessary for lasing; this is one of the most significant shortcomings of quantum cascade lasers. The possibility of increasing the effective lifetime of carriers in the upper laser subband as a result of their return there from the lower subband by means of induced hot-optical-phonon reabsorption was proposed in the literature. However, our simulation results demonstrate that under realistic conditions the role of hot phonons is the opposite: substantial electron heating in the subbands and significant induced optical-phonon emission lead to a reduction in the electron population inversion causing an additional increase in the threshold currents.
Transport properties of boron-doped crystallized amorphous Si1−xGex films
Authors: Raz T. , Edelman F. , Komem Y. , Stölzer M. , Zaumseil P.
Published: July 2002

Transport properties of crystallized amorphous Si1−xGex films, having different Ge content (x) and highly doped with boron were studied. The films were deposited by molecular beam at room temperature and subsequently annealed in vacuum at different temperatures between 500 and 900 °C for 1 h. The microstructure of the crystallized Si1−xGex films was characterized by means of transmission electron microscopy, x-ray diffraction, and scanning electron microscopy. Measured transport properties included Hall hole concentration (pH), Hall mobility (μH), electrical conductivity (σ), and the Seebeck coefficient (S), from which the “power factor” (S2σ) was evaluated. The results obtained for the Hall mobility of the Si1−xGex films are discussed on the basis of the carrier trapping model. The trapping state density at the grain boundaries increases with increasing B concentration, although it is not significantly dependent on Ge content. Consequently, the mobility energy barrier decreases with increasing B concentration and increasing Ge content. It was found that in all the studied Si1−xGex films, independent of x, the predominant scattering mechanism changes from acoustic phonon scattering to ionized impurity scattering with increasing the boron concentration from 5×1018 to 5×1020 cm−3. In addition, the Si1−xGex films demonstrate high electrical conductivity as well as a high Seebeck coefficient, after 1 h annealing at 600–800 °C, and thus exhibit a high “power factor” of the order of 6 μW/cm K2. Thus, these films have potential applications in thin-film thermoelectric devices.
Method for measurement of the thermal diffusivity in solids: Application to metals, semiconductors, and thin materials
Authors: Calderón A. , Muñoz Hernández R. A. , Tomás S. A. , Cruz-Orea A. , Sánchez Sinencio F.
Published: July 2002

We present a method for determining the thermal diffusivity in opaque solids by means of an analysis of the photoacoustic phase signal at low modulation frequencies using the open-cell photoacoustic technique. We show for f⩽(π/2)2fc, where fc is the modulation frequency at which the thermal diffusion length matches the sample thickness, the photoacoustic phase signal can be written in linear form with the modulation frequency f. Then, obtaining the proportionality coefficient by fitting the experimental data, the thermal diffusivity of the sample can be determined. The advantage of this method is that it is realized in a range of modulation frequencies below those normally used, hence, the photoacoustic signal should be alone attributed to the mechanism of thermal diffusion. Moreover, the signal-to-noise ratio will be more reliable. This method was tested in some samples and it is also shown to be important in solids with high diffusivity values and thin materials.
Study on surface and domain structures of PbTiO3 crystals by atomic force microscopy
Authors: Wang Y. G. , Dec J. , Kleemann W.
Published: July 2002

The surface morphology of flux-grown PbTiO3 crystals is examined by atomic force microscopy (AFM) at room temperature. Surface undulations due to a and c domains are observed on as-grown and heated crystals. The surface bending angle at 90° a-c domain walls is measured to be (3.58°±0.05°) in good agreement with the theoretical value, 3.6°. Footprints of ancient domains are found to be overlapped by surface undulations of the actual domain after polishing and heating process. Reciprocal 180° domains embedded in a and c domains are observed by both AFM and by polarizing optical microscopy on etched crystals. Details of the etched pattern are explored. Contrary to abrupt changes of height at 180° walls in c domains, only very small grooves are detected at 180° walls in a domains.
Crystal field in the heavy fermion compound CeAl3
Authors: Goremychkin Eugene A. , Osborn Raymond , Sashin Igor L.
Published: July 2002

We present the results of an analysis of inelastic neutron scattering data on CeAl3. By combining the neutron scattering results with measurements of the single-crystal magnetic susceptibility, we have obtained an unambiguous set of crystal field parameters in CeAl3. A comparison of the crystal field parameters along the RAl3 (R=Ce, Pr, and Nd) series of isostructural compounds shows that in CeAl3, A20〈r2〉 is an order of magnitude larger, and A40〈r4〉 is three times larger, than in PrAl3 and NdAl3. These increases cannot be explained by changes in lattice parameters but are more likely to result from the enhancement of the hybridization of the cerium 4f electrons with the conduction electrons in CeAl3. This conclusion is in agreement with the results of our study of the RCu2Si2 series.
Analysis of the magnetic relaxation in BiPbSrCaCuO high-TC superconductors by means of an interaction model
Authors: Raposo V. , Zazo M. , Flores A. G. , López Diaz L. , Iñiguez J.
Published: July 2002

The magnetic relaxation of cylindrical polycrystalline samples of BiPbSrCaCuO high-TC superconductors has been measured using an inductive technique near the transition temperature. The relaxation data are analyzed by means of an interaction model based on the general theory of magnetic relaxation. This model assumes the existence of an effective energy barrier (Ueff) and considers thermal activation of the vortices over it. The pinning energy (U0) is reduced to the effective barrier Ueff=U0−Uint by the energy associated to the force of repulsion between vortices (Uint). In this work a linear dependence between the interaction energy and the susceptibility has been used, but other dependencies can easily be introduced to account for the observed nonlinearities of the U(J) curve. The model has been used to extract information of the relaxation of BiPbSrCaCuO samples at several magnetic fields and temperatures after switching off the field. A comparison with the results obtained by the Anderson–Kim model is also provided.
Evidence of tetragonal to orthorhombic distortion of HoNi2B2C in the magnetically ordered state
Authors: Kreyssig A. , Loewenhaupt M. , Freudenberger J. , Müller K.-H. , Ritter C.
Published: July 2002

Using high resolution neutron scattering on a powder sample of HoNi2 11B2C magnetoelastic effects are observed in this compound for the first time. At low temperatures the tetragonal lattice is distorted along the [110] direction, in which the long-range ordered Ho moments are aligned. The length of the unit cell in [110] direction is shortened by about 0.19% compared to its length in [1̄10] direction. This lattice distortion is considered to be related to the existence of the commensurate c-axis modulated antiferromagnetic structure.
Texture of Nd–Fe–B thin films prepared by magnetron sputtering
Authors: Araki Takeshi , Nakanishi Teruo , Umemura Toshio
Published: July 2002

We have investigated the texture and magnetic properties of Nd12Fe88−xBx sputtered films with various boron contents, x. The c-axis orientation of Nd2Fe14B phase in the films was higher and the perpendicular coercive force became greater with increment of x. With more increment of x than 15, however, the coercive force decreased owing to formation of a large amount of amorphous soft magnetic phase. The transmission electron microscope observations revealed that highly c-axis oriented Nd12Fe71B17 film has a unique microstructure; oriented Nd2Fe14B phase and amorphous phase with widths of about 200 and 50 Å, respectively, are alternately developed. This two-phase structure has realized an exchange coupled nanostructured magnet composed of aligned hard magnetic phase and soft magnetic phase. The maximum energy product of Nd12Fe71B17 film was estimated at 270 kJ/m3 with micromagnetic analysis assuming that the film was composed of complete regular structure.
Chemical bonding and electronic properties of SeS2-treated GaAs(100)
Authors: Sun Jingxi , Seo Dong Ju , O’Brien W. L. , Himpsel F. J. , Ellis A. B. , Kuech T. F.
Published: July 2002

SeS 2 -passivated n-type GaAs (100) surfaces, formed by treatment of GaAs in SeS2:CS2 solution at room temperature, were studied with high-resolution core-level photoemission spectroscopy excited with synchrotron radiation source. The SeS2-treated surface consists of a chemically stratified structure of several atomic layers thickness. Arsenic-based sulfides and selenides reside in the outermost surface layer while gallium-based selenides are adjacent to the bulk GaAs substrate. The shift of the surface Fermi level within the band gap was monitored during controlled thermal annealing, allowing for the identification of the specific chemical entities responsible for the reduction in surface band bending. Arsenic-based species are removed at low annealing temperature with little shift of the Fermi level. Gallium-based selenides are shown to be associated with the unpinning of the surface Fermi level.
High-performance photorefractivity in high- and low-molar-mass liquid crystal mixtures
Authors: Ono Hiroshi , Kawatsuki Nobuhiro
Published: July 2002

High-performance photorefractivity in a thick-grating regime was observed in high- (H-LC) and low-molar-mass liquid crystal (L-LC) mixtures (HL-LCMs) doped with fullerene. The photorefractive performance strongly depended on the concentration ratio of H-LC and L-LC. A high net gain coefficient over 600 cm−1 was achieved with low applied dc fields (0.7 V/μm) and a fringe spacing of 2.8 μm. The gain coefficients and the resolution increased with increasing the concentration of the H-LCs. The photorefractive grating formation time was varied between 0.12 and 3.5 s, which depended on the concentration of the H-LCs.
Effects of magnetic interactions on the stability of particulate dispersions
Authors: Mercer T. , Bissell P. R. , Gotaas J. A. , Gilson R. G.
Published: July 2002

The effects of magnetic interactions on the stability of recording media dispersions has been investigated. Variation of the pigment’s magnetic content shows that increasing the interactions results in lower stability. This has been interpreted in terms of hindered settling theory as being caused by clustering of the particles in suspension under the influence of magnetostatic attraction. As dispersion structure is important in the media manufacturing process, these investigations are continuing.
Angular dependence of magnetism of Fe films sputtered with ion beam perpendicular to target
Authors: Iwatsubo S. , Takahashi T. , Naoe M.
Published: July 2002

Fe films were deposited by sputtering with ion beam perpendicular to the target. The substrates were arranged to be in the arc shaped position around the center of the ion bombarded area of the target. The sputtering voltage Vs was 1200 V. The angle θs between the direction of the ion beam and the normal line of the substrate plane was varied in the range of 10°–80°. The energy of the sputtered atoms arriving to the substrate was calculated by Monte Carlo simulation, and the average energy EFe of the sputtered Fe atoms varied drastically with θs. EFe increased with increase of θs. The morphology and the magnetic properties of the Fe films significantly depended on θs. The morphology changed from a columnar structure to a round-grain one by propagation of recrystallization with the increase of θs. The saturation magnetization 4πMs was about 21.5 kG. The coercivity Hc took the minimum value of 5 Oe for θs above 50°. This suggests that the angular relation between the target and the substrate should be one of essential factors for controlling the magnetic properties of the Fe films.
Evaluation of magnetic properties of CoZrCr for high speed write heads
Authors: Hossain S. A. , He C. , Miloslavsky L. , Qian C. , Miller M. S. , Tong H. C.
Published: July 2002

The CoZrCr alloy has been developed for high speed write heads. This sputter deposited alloy exhibits soft magnetic properties and high resistivity which is suitable for high data rate write head applications. Resistivity of these sputtered films is about 90 μΩ cm, saturation magnetization is 13.6 kG, anisotropy field (Hk) is 15 Oe in as deposited state, which could be reduced to 5 Oe by annealing. Easy and hard axis coercivities are <0.2 and <0.3 Oe, respectively. Improved magnetic properties on sloped surfaces are observed for CoZrCr films, when deposited with 75 V substrate bias.