bluuebunny/crossref_metadata_2025 huggingface dataset:
Structural and optical properties of 1.3 μm wavelength tensile-strained InGaAsP multiquantum wells grown by metalorganic molecular beam epitaxy
Authors: Sugiura Hideo , Mitsuhara Manabu , Ogasawara Matsuyuki , Itoh Masayuki , Kamada Hidehiko
Published: July 2002

Tensile strained 0.5%–1.8% InGaAsP multiquantum wells (MQWs) grown by metalorganic molecular beam epitaxy are comprehensively characterized by using transmission electron microscopy (TEM), x-ray, photoluminescence (PL), lifetime, photocurrent, and absorption measurements. Cross-sectional and plan-view TEM photos reveal that, irrespective of the strain values, the tensile strained well layers have flat surfaces in the growth direction and consist of fine parallelepipeds laterally. Optimizing the strain compensation conditions, i.e., keeping the net strain less than 0.3%, makes it possible to grow high optical quality MQWs with 1.8%-strained well layers. As the well strain increases from 0.5% to 1.45%, however, the PL intensities of the misfit-dislocation-free MQWs monotonically decrease and the minority carrier lifetime decreases from 1.4 to 0.9 nsec. The laser threshold current density decreased with increasing well strain in the 0.5%–1.3% range, decreased to 0.6 kA/cm2 at 1.3% and slightly increased in the 1.45%–1.8% range.
Monte Carlo simulation of high-field electron transport in GaAs using an analytical band-structure model
Authors: Mouton O. , Thobel J. L. , Fauquembergue R.
Published: July 2002

We present a study of high energy electron transport in GaAs using an analytical model of the band structure. This model is based on piecewise polynomial approximation of the dispersion relation in different regions of the Brillouin zone. It accounts for the first two conduction bands and reproduces all important features of the full band structure. We have used this model to set up a Monte Carlo simulation of electron transport accounting for impact ionization. It has been shown that this “extended valley” model yields essentially the same results as the most rigorous full band Monte Carlo calculations. We have found a large influence of high energy anisotropy on electron transport. Another important result is that most impact ionization events are due to electrons in the second conduction band.
Photoconductivity study of crescent-shaped GaAs/GaAlAs quantum wires grown by flow rate modulation epitaxy
Authors: Hamoudi A. , Ogura M. , Wang X. L.
Published: July 2002

We present photoconductivity data on GaAs quantum wires grown on a V-grooved substrate by flow rate modulation epitaxy. They show that a moderate excitation power density, ∼1 W/cm2, allows the observation of the absorption structure of a single GaAs quantum wire embedded in a p-i-n diode. Furthermore, by increasing the number of active wires inside a diode, the photoconductivity signal is enhanced and additional details of the absorption structure are evidenced. And, finally, a rough quantitative agreement is obtained between the experimental absorption transitions and a simple calculation of the one-dimensional excitonic states using the envelope function approximation.
Lateral carrier profile for mesa-structured InGaAs/GaAs lasers
Authors: Torre M. S. , Esquivias I. , Romero B. , Czotscher K. , Weisser S. , Ralston J. D. , Larkins E. , Benz W. , Rosenzweig J.
Published: July 2002

We study the influence of lateral carrier diffusion on the properties of In0.35Ga0.65As/GaAs multiple quantum well lasers by comparing theoretical and experimental results. A model including the carrier diffusion terms into the rate equations has been used to calculate the dc and small-signal lateral profiles for both unconfined and confined carriers in mesa waveguide devices. The theoretical results were compared with experimental results of the frequency dependence of the subthreshold electrical impedance and small-signal spontaneous emission, and with the measured threshold currents for lasers with different mesa widths. The comparison yielded an estimation for the nonradiative and radiative recombination coefficients, the ambipolar diffusion constant, and the external surface recombination velocity.
Characteristics of quantum well infrared photodetectors
Authors: Ryzhii V.
Published: July 2002

A model is presented for the performance of quantum well infrared photodetectors (QWIPs) utilizing intersubband electron transitions and tunneling injection electrons. The dark current and the responsivity are derived as functions of the QWIP parameters, including the number of the QWs, in an analytical form. Nonlinear effects in the QWIP operation at high infrared power are considered and the threshold value of power density is estimated.
Abnormal magnetic transport in Co/Mn/Co sandwiches
Authors: Zhao H. W. , Chen Y. , Dong G. S. , Lu M. , Jin X. F. , Wang Y. J. , Zhai H. R. , Yang D. , Naik R. , Dunifer G. L. , Auner G. W.
Published: July 2002

A series of Co/Mn/Co sandwiches was prepared on GaAs(001) surfaces at room temperature via a molecular beam epitaxy (MBE) technique. Reflection high-energy electron diffraction (RHEED) and x-ray-diffraction (XRD) analysis indicated a polycrystalline structure. By standard magnetoresistance measurements, it was found that with a Mn spacer thickness of 8 Å, a seldom-seen positive magnetoresistance (PMR) as high as 16% was exhibited at room temperature. The measurement under different geometries of H and I indicated that it was not an anisotropic magnetoresistance (AMR) effect. The MR–H curve displayed no hysteresis in contrast to the magnetometry measurement. An interface effect is proposed as a possible origin of this phenomenon.
Giant magnetostrictive spring magnet type multilayers
Authors: Quandt E. , Ludwig A. , Betz J. , Mackay K. , Givord D.
Published: July 2002

Thin film magnetostrictive materials can be extremely useful as the active material in microactuators. In this article, some results on a novel type of multilayer structure which combines exchange coupled giant magnetostrictive materials and materials with large magnetic polarizations are presented for the first time. Giant magnetostrictions are achieved at low fields, due to the polarization enhancement in such multilayers. Therefore such composite materials should be much more appropriate for applications than the simple homogeneous alloy films studied up to now.
Nonlinear diffusion in anisotropic superconductors
Authors: Mayergoyz I. D. , Neely M.
Published: July 2002

This article presents an analytical study of nonlinear diffusion of electromagnetic fields in anisotropic superconducting media. The case of anisotropic media is treated as a perturbation of isotropic media and analytical expressions for nonlinear diffusion of circularly polarized electromagnetic fields are derived.
Quantum transport and magneto-optics in a magnetic two-dimensional electron gas
Authors: Smorchkova I. , Samarth N. , Kikkawa J. M. , Awschalom D. D.
Published: July 2002

A new class of magnetic heterostructures is studied wherein a two-dimensional electron gas (2DEG) is strongly exchange coupled to a systematic distribution of local moments. From the longitudinal magnetoresistance we extract the sheet concentration, single particle scattering time, and carrier effective mass. Magnetophotoluminescence measurements yield an estimate of the spin splitting of the 2DEG states, implying a high degree of spin polarization in modest magnetic fields at cryogenic temperatures.
Hot hole energy relaxation in Si/Si0.8Ge0.2 two dimensional hole gases
Authors: Braithwaite G. , Mattey N. L. , Parker E. H. C. , Whall T. E. , Brunthaler G. , Bauer G.
Published: July 2002

We have measured the energy loss rate as a function of carrier temperature for hot holes in Si/Si0.8Ge0.2 quantum wells with sheet carrier densities in the range (3–7)×1011 cm−2 at lattice temperatures of 0.35 and 2.0 K. Calculations of the energy loss rate for acoustic phonon deformation potential scattering with coupling constant 4.5 eV show good agreement with measurement. The deformation potential is consistent with a linear interpolation between the bulk Si and Ge values and is in agreement with that deduced from measurements of thermopower in similar samples. In contrast to previous work, we find no evidence for hole coupling to acoustic phonons via a piezoelectric interaction.
Temperature dependence of microstructure and magnetic properties of Co/Ti multilayer thin films
Authors: Wu Ping , Jiang E. Y. , Wang C. D. , Bai H. L. , Wang H. Y. , Liu Y. G.
Published: July 2002

The microstructural and magnetic properties of amorphous Co/Ti multilayer films and their variation with temperature are investigated by transmission electron microscopy (TEM) analysis and thermomagnetic measurements. Thermomagnetic curves showed two peaks at about 400 and 520 °C. The evolution of the structure monitored in the hot stage of the TEM was found to be consistent with the magnetic changes. The first peak of the saturation magnetization Ms at 400 °C was associated with the transformation from amorphous ferromagnetism to paramagnetism due to the amorphous Co existing in the film. Ms began to increase corresponding to the crystallization point of the ferromagnetic Co phase, which decreased with increasing amounts of Co in the film. Ms reached its maximum at 520 °C and then decreased because the phase transition occurred at a temperature greater than 520 °C and approached completion at 650 °C. The amorphous phase and crystalline phase formation and phase transition during annealing were observed in Co/Ti multilayer thin films and successfully explained the thermomagnetic properties of the film.
Optical properties of lead barium niobate (Pb1−xBaxNb2O6) crystals
Authors: Lee Myeongkyu , Lee Howard , Route Roger K. , Feigelson Robert S.
Published: July 2002

PBN (Pb1−xBaxNb2O6) polycrystals have been grown by the vertical Bridgman method. Transparent, stoichiometric, striation-free single-crystal specimens of millimeter size were obtained. The ordinary and extraordinary refractive indices, no and ne, of crystals with compositions Pb0.50Ba0.50Nb2O6 and Pb0.57Ba0.43Nb2O6 were measured by the minimum deviation method using optically polished prisms. Neither no nor ne was found to be strongly dependent on crystal composition, in contrast to the case of SBN (Sr1−xBaxNb2O6), in which ne is strongly dependent on composition. Optical transmission spectra showed PBN to be transparent in the range from 0.4 to 5 μm except for the OH− absorption bands around 2.87 μm. The dependence of the integrated OH− absorption intensity on polarization showed that all of the O–H bonds lie either in the a-b plane or along the c axis, with a greater density of O–H bonds in the a-b plane than along the c axis.
Γ-
X
electron transfer in a triple-barrier heterostructure
Authors: Kim Gyungock , Roh Dong-Wan , Paek Seung Won , Lee El-Hang
Published: July 2002

The electron Γ-X intervalley interlayer transfer through the confined X states of the AlAs barrier and the enhanced electron tunneling effect have been observed in the AlAs/GaAs (001) triple-barrier heterostructure. The effects of the lower X-valley confined states, the field-induced interlayer state mixing, and the quantum-well excited energy states are analyzed with the local density of states under the external bias, using the scattering theoretic Green’s function approach. The experimental results show good agreements with theory.
Very strong influence of moisture on pyroelectric and dielectric properties of triglycine sulfate-gelatin films
Authors: Khutorsky V. E. , Lang Sidney B.
Published: July 2002

Triglycine sulfate-gelatin (TGS-gelatin) films were grown by evaporation of the water from a solution containing gelatin and TGS. The pyroelectric and dielectric properties of the films were highly dependent on their water content. Moist films had higher pyroelectric coefficients and lower coercive fields than dry films. The dielectric permittivity increased strongly with an increase of water content and had a fractional power frequency dependence. The results are explained using a model of the TGS-gelatin film structure in which the gelatin phase is a superionic conductor or solid electrolyte. Absorption of water causes the formation of electric double layers at the electrode film and TGS crystallite-gelatin phase interfaces.
Analysis of light emitting polymer electrochemical cells
Authors: Riess I. , Cahen D.
Published: July 2002

Polymer electrochemical cells with ion blocking electrodes were reported to emit light under applied voltage. This work analyzes the current-voltage relations, internal electric fields, and point defect distribution in the polymer. The polymer is regarded as a mixed-ionic-electronic conductor. Two relevant defect models are investigated. A good fit is obtained between experimental data and theory which also takes into consideration electrode overpotentials.
Cathodoluminescence, reflectivity changes, and accumulation of graphitic carbon during electron beam aging of phosphors
Authors: Seager C. H. , Tallant D. R. , Warren W. L.
Published: July 2002

We demonstrate that extended e-beam exposure produces a contaminating overlayer on phosphors whose opacity increases roughly linearly with time. Raman scattering data and optical analysis indicate that this layer is graphitic in nature, arising from the electron-beam-stimulated conversion of hydrocarbons adsorbed from the vacuum ambient. The presence of this contamination optically attenuates emitted cathodoluminescence, prevents many low energy electrons from ever reaching the phosphor grains, and exacerbates surface charging which reduces the arrival energy of electrons above 1.5–2 keV. All of these effects are shown to impact cathodoluminescent output in an important way, but an accurate accounting of their total impact will be required to assess the importance of other degradation mechanisms like enhanced nonradiative electron-hole recombination at surfaces, both carbon and noncarbon related.
Electrical behavior of YBa2Cu3O7−x grain boundary junctions under low magnetic field
Authors: Nicoletti S. , Villegier J.-C.
Published: July 2002

The characterization of high temperature superconductor grain boundary junctions under a magnetic field in the mT range is reported. The devices were obtained by patterning narrow stripes in a YBa2Cu3O7−x (YBCO) film deposited on SrTiO3 and MgO symmetric bi-crystal substrates. To allow the investigation of devices having different current density, two grain boundary disorientations were considered: 24° and 45° for SrTiO3 and 24° for MgO. Current-voltage characteristics as a function of the applied field for several temperatures have been collected. The experimental data are discussed on the basis of the electrical parameters obtained for the different substrates. The periodic modulation of the Josephson critical current with the magnetic field indicates that the behavior of YBCO grain boundary junctions can be approximated by the standard overlap junctions model. A comparison between the experimental variation of the critical current with theoretical behavior allows for the determination of the current density distribution in the grain boundary as a function of the ratio of the largest junction dimension L and the Josephson penetration depth λJ. The nature of the barrier and the transport mechanism across the grain boundary is, therefore, investigated; good agreement between the experimental results and the expected behavior of a superconductor-insulator-superconductor structure where the barrier is intrinsically defective is observed.
Relaxation mechanisms in single InxGa1−xAs epilayers grown on misoriented GaAs(111¯)B substrates
Authors: Edirisinghe Saroja P. , Staton-Bevan Anne E. , Grey Robert
Published: July 2002

Transmission electron microscopy (TEM) has been used to investigate the mechanisms of misfit strain relaxation in InxGa1−xAs epilayers grown on GaAs(111¯)B substrates misoriented 2° towards [211¯]. It was found that the relaxation was brought about by a triangular network of misfit dislocations lying along the three 〈11̄0〉 directions near the interface. However, the dislocation distribution was anisotropic with a much higher density of dislocations lying parallel to the [01̄1] direction. A second relaxation mechanism was also observed which involved the formation of deformation twins. These had nucleated at the epilayer surface and grown down into the epilayer, sometimes entering the underlying buffer layer. Twin formation was also anisotropic with twins forming on the (1̄11)[211] system only. The dislocation and twin anisotropy may not be explained using the Schmid Factor considerations but is thought to be associated with heterogeneous nucleation of dislocations at the [01̄1] surface steps caused by the misorientation. The critical layer thickness for the observation of misfit dislocations by TEM in In0.25Ga0.75As (111¯)B epilayers was found to be between 15 and 25 nm. This is the same range as that observed for (001) epilayers of the same composition. This is as expected from theoretical considerations of the effects of orientation on the elastic modulus and the strain relieving component of the misfit dislocation Burgers vector.
Reflective twisted nematic liquid crystal displays. II. Elimination of retardation film and rear polarizer
Authors: Yu F. H. , Chen J. , Tang S. T. , Kwok H. S.
Published: July 2002

Reflective twisted nematic liquid crystal displays consisting of just one input polarizer, a liquid crystal cell, and a rear reflector are discussed. We first show that all such display modes reported in the literature can be depicted systematically in a series of parameter space diagrams. Then by making use of such diagrams as a guide, we show that other new high quality reflective display modes can be obtained. The three parameters that are varied are the input polarizer angle α, the liquid crystal twist angle φ, and the cell gap–liquid crystal birefringence product dΔn. A full numerical simulation which takes into account of the liquid crystal director deformation under the application of an electric field was used to optimize such displays. Experimental results agree well with the theoretical predictions.
Implantation species dependence of transient enhanced diffusion in silicon
Authors: Uematsu Masashi
Published: July 2002

The dependence of transient enhanced diffusion (TED) on implantation species has been investigated by the simulation of TED induced by P, As, and Si implantation using a unified set of parameters. The TED enhancement at short annealing times critically depends on the effective diffusivities of implanted species. Fast P diffusion induces a self-interstitial supersaturation with respect to the pseudo-equilibrium concentration, which further enhances TED. Therefore, TED induced by P implantation is faster than that by As and Si, where self-interstitials are in thermal equilibrium. At longer annealing times, the enhancement is primarily governed by self-interstitial diffusion, which depends on the carrier concentration and the implanted range, or the proximity of the damage to the surface, and faster self-interstitial diffusion leads to smaller enhancement. Furthermore, the carrier concentration, which also affects the dopant effective diffusivities and the duration of TED, varies with dose and annealing time. Therefore, the dependence on implantation species is closely related to the variations of dose, energy, and annealing time, which leads to complex TED characteristics.
Morphology and microstructure of tensile-strained SiGe(001) thin epitaxial films
Authors: Van Nostrand J. E. , Cahill David G. , Petrov I. , Greene J. E.
Published: July 2002

Tensile-strained Si0.6Ge0.4 alloys are deposited on Ge(001) by molecular beam epitaxy. The morphology and microstructure of as-deposited (6.5–130 nm thick deposited at 325–500 °C) and annealed (2 min at 625 or 700 °C) epitaxial films are studied by in situ scanning tunneling microscopy, ex situ atomic force microscopy, and transmission electron microscopy. 6.5 nm thick films deposited at 325 and 410 °C are atomically flat with low densities of partial dislocations; stress relaxation of 6.5 nm thick films is limited by dislocation blocking. The surface morphology of thicker films grown at 410 °C is strongly influenced by the interactions of stacking faults and surface steps. Annealing of 13 nm thick films at 700 °C produces a severe roughening with the formation of a regular pattern of 50 nm deep surface pits bounded by {113} facets.
Electrical harmonic generation from notched narrow metal lines
Authors: Wen Qingzhe , Clarke D. R.
Published: July 2002

A model for electrical harmonic generation in metal thin films has been developed based on the solution to the thermal diffusion equation using a Green’s function method and a number of simple assumptions. The validity of the model is demonstrated by measurements of the amplitude of the second and third harmonic signals using uniform aluminum lines on sapphire. By introducing damage in the form of narrow notches of prescribed size into the aluminum lines, we have been able to calibrate, and compare, the notch sensitivities of the dc resistance and the second and third harmonic signals. Predictions based on a simple approximate treatment of the notches yields good agreement with damage sensitivity of both the resistance and harmonic signals. It was found that the second and the third harmonic signals are equally sensitive to damage in the aluminum lines. Furthermore, the harmonic signals are considerably more sensitive to the presence of damage than is the dc resistance. Also, the effect of a number of distinct damage sites in a line is shown to be a linear superposition of each for both the harmonic signals and the resistance.
A transverse shear model of a piezoelectric chemical sensor
Authors: Hayward G. L. , Thompson M.
Published: July 2002

This model of a transverse shear-mode piezoelectric quartz-crystal chemical sensor is based on the fundamental equations of motion and piezoelectricity. The geometry includes the crystal as well as a metal electrode and a polymeric sensing layer immersed in a fluid medium. All of these are considered to be viscoelastic materials, and slip at the three interfaces is included. The model is shown to agree well with both experimental data and published correlations of the series resonant frequency and equivalent series resistance under both mass and liquid loading. In addition, the mechanical resonant behavior is also calculated. The interfacial slip parameter and the complex shear viscosity are shown to be mathematically interchangeable but not physically equivalent. Negative elastic coefficients obtained from the slip parameters for less viscous fluids suggests that interfacial slip is an important process, which dominates viscoelastic effects for these fluids.
Growth kinetics of a displacement field in hydrogen implanted single crystalline silicon
Authors: Bisero D. , Corni F. , Frabboni S. , Tonini R. , Ottaviani G. , Balboni R.
Published: July 2002

The growth of a displacement field in single crystal silicon resulting from high dose hydrogen implantation and subsequent heat treatments has been investigated by MeV 4He+ Rutherford backscattering in channeling conditions, double crystal x-ray diffraction, and transmission electron microscopy. The results obtained in samples annealed for various times in the temperature range 220–350 °C have been explained in terms of a kinetic model which assumes the formation of clusters of hydrogen molecules. The growth of the displacement field is thermally activated with an activation energy of 0.50±0.05 eV, suggesting that the limiting process could be the release of hydrogen atoms bounded to defects created by ion implantation.
Tin-related double acceptors in gallium selenide single crystals
Authors: Sánchez-Royo J. F. , Errandonea D. , Segura A. , Roa L. , Chevy A.
Published: July 2002

Gallium selenide single crystals doped with different amounts of tin are studied through resistivity and Hall effect measurements in the temperature range from 30 to 700 K. At low doping concentration tin is shown to behave as a double acceptor impurity in gallium selenide with ionization energies of 155 and 310 meV. At higher doping concentration tin also introduces deep donor levels, but the material remains p-type in the whole studied range of tin doping concentrations. The deep character of donors in gallium selenide is discussed by comparison of its conduction band structure to that of indium selenide under pressure. The double acceptor center is proposed to be a tin atom in interlayer position, with a local configuration that is similar to that of tin diselenide. The hole mobility exhibits an anomalous dependence on the tin content, attaining its maximum value in the ingot with 0.2% nominal tin content. This is proposed to be related to impurity pairing effects giving rise to thermal shallow acceptors with low ionization energy and low carrier scattering cross section, making the hole mobility to be controlled by phonon scattering mechanisms even for relatively high impurity content.
Synergetic effects in annealing and low energy ion bombardment of Si(100) surfaces
Authors: Lee S. M. , Fell C. J. , Marton D. , Rabalais J. W.
Published: July 2002

Synergetic effects have been observed for low energy Ar+ ion bombardment combined with low temperature annealing for producing clean and well-ordered Si(100) surfaces. Low energy, mass-selected Ar+ bombardment of Si(100) surfaces in the range of 50–300 eV was performed over the temperature range 25–500 °C. The surfaces were characterized by in situ Auger electron spectroscopy and reflection high energy electron diffraction and ex situ atomic force microscopy. It is found that a clean and well-ordered Si surface can be obtained at a lower temperature when ion bombardment and annealing are applied simultaneously than if they are applied separately or in sequence. Nearly ideal surfaces can be obtained for the conditions of 100–200 eV Ar+ bombardment at 400–500 °C. This synergy between ion bombardment and annealing is discussed in terms of a subplantation model that includes Ar-defect complexes. Additionally, formation of SiC by 100 eV Ar+ ion bombardment of the carbon contaminated silicon surface was observed.
Effects of the surface deposition of nitrogen on the thermal oxidation of silicon in O2
Authors: Baumvol I. J. R. , Salgado T. D. M. , Stedile F. C. , Radtke C. , Krug C.
Published: July 2002

Nitrogen was deposited on the surface layers of Si(100) by ion implantation at a very low energy (approximately 20 eV), at fluences between 1 and 10×1014 cm−2. The samples were thermally oxidized in dry O2 at 1050 °C, and the areal densities and profiles of N and O were determined by nuclear reaction analysis and narrow nuclear resonance profiling, evidencing that: (i) the retained amounts of N just after ion beam deposition stayed in the range between 0.3 and 7×1014 cm−2; (ii) the oxide growth is influenced strongly by the presence of nitrogen, the thickness of the oxide films (which remained between 4 and 30 nm) decreased with the increase of the areal density of nitrogen; (iii) N is partially removed from the system as oxidation proceeds. These observations are discussed in terms of current models for the thermal growth of silicon oxide in the presence of N.
Evidence for oxide formation from the single and multiphoton excitation of a porous silicon surface or silicon “nanoparticles”
Authors: Gole James L. , Dixon David A.
Published: July 2002

Potential oxidation as monitored by single and multiphoton excitation and associated with the laser induced photoluminescence (PL) from silicon nanoparticles and a porous silicon (PS) surface is considered in the light of recent in situ observations of the photoluminescence process coupled with detailed quantum chemical modeling of the silanone based oxyhydrides. The enhanced oxidation of PS surface bound SiHx moieties as they undergo multiphoton excitation through the SiHx stretch vibrational ladder to the quasicontinuum is suggested as a source of the visible PL produced in the 600–800 nm region. Vibrational-to-electronic energy transfer within the SiHx manifold, as previously suggested, is thought to be unlikely. However, the formation of OSiHx constituencies on the PS surface as these are associated with the silanone-based silicon oxyhydrides would appear to be significant. Evidence for these oxyhydrides may also be apparent in the laser induced PL from silicon nanoparticles. All experimental observations are explained without envoking quantum confinement.
Multigrid methods for computation of magnetostatic fields in magnetic recording problems
Authors: Tsukerman Igor , Plaks Alexander , Bertram H. Neal
Published: July 2002

Calculation of magnetostatic fields is usually the most time-consuming stage of micromagnetic simulations. In this article, the Bramble-Pasciak-Xu multilevel preconditioners (MP) are applied to the computation of magnetostatic fields in media such as magnetic tapes or thin films. The asymptotic number of arithmetic operations is optimal and proportional to the number of nodes in the finite element mesh. Unlike the fast Fourier transform, MP are applicable to irregular meshes. Local (adaptive) mesh refinement can be implemented by switching to the hierarchical basis.
The complex magnetic behavior and the role of dynamic structural fluctuations in La1.2Sr1.8Mn2O7 crystals
Authors: Sharma R. P. , Fournier P. , Greene R. L. , Venkatesan T. , Mitchell J. F. , Miller D.
Published: July 2002

The presence of dynamic structural distortions at the magnetic phase transitions in layered perovskite La1.2Sr1.8Mn2O7 crystals is found by ion channeling. Their amplitude is ∼1.5 picometer between 70 and 150 K, twice as that expected due to the thermal vibrations and static changes seen by neutron diffraction. Over the range 150 to 320 K a total change of ∼2.3 picometer is observed with discontinuities, which correlate well with the anomalous features seen in magnetic studies. In this material a small ferromagnetic component persists in the region 120 to 320 K, and it is fully paramagnetic above 320 K.
The spall strength of alumina ceramics
Authors: Murray N. H. , Bourne N. K. , Rosenberg Z. , Field J. E.
Published: July 2002

The response to plate impact loading of three aluminas with varying glass content and porosity has been investigated. Spall strengths have been shown to be dependent on the amplitude and duration of the compression pulse which precedes the tensile loading, but insensitive to the rate of release. Some tensile strength is measured in impacts where the Hugoniot elastic limit has been exceeded. The effect of the material microstructure on the dynamic tensile strength has also been studied. Low porosity aluminas with many microstructural irregularities were found to possess the lowest spall strengths. Experimental values compare well with those predicted by an energy balance theory developed by Grady [J. Mech. Phys. Solids 36, 353 (1988)].
Deep level spectroscopy of high-power laser diode arrays
Authors: Tomm J. W. , Bärwolff A. , Jaeger A. , Elsaesser T. , Bollmann J. , Masselink W. T. , Gerhardt A. , Donecker J.
Published: July 2002

Deep defect centers in high-power AlGaAs/GaAs laser diode arrays emitting at a wavelength of 808 nm are studied by Fourier-transform photocurrent spectroscopy and by electrical deep level transient spectroscopy. Different types of deep centers with binding energies between 0.15 and 0.8 eV are found in graded-index and step-index diode structures. In all structures, the defect concentration increases with operation time. We demonstrate that different operation conditions of the devices, such as regular operation or accelerated aging at increased temperatures, cause different scenarios of deep level creation. In addition to deep centers, aging leads to the formation of shallow defects and a reshaping of the absorption band edge.
Correlation between structure, microstructure, and ferroelectric properties of PbZr0.2Ti0.8O3 integrated film: Influence of the sol-gel process and the substrate
Authors: Floquet Nicole , Hector Jérôme , Gaucher Philippe
Published: July 2002

The properties of a ferroelectric material deposed as a thin film on a substrate depend on the method of preparation and on the nature of the substrate used. The present work reports fine characterization by x-ray diffraction of structural modifications (cell parameters, preferential orientations) and microstructural modifications (ferroelectric domain arrangement) and electric measurements of a ferroelectric film as a function of its method of preparation (sol-gel process, pyrolysis, annealing) and the nature of its substrate (crystalline structure, crystallographic orientation, thermal coefficient of dilation). Due to the elaboration process, lead zirconate titanate (PZT) films have grain size smaller than 0.3 μm and consequently modified cell parameters (higher a parameter, smaller c parameter, and consequently smaller c/a tetragonality). Thermal treatment induces specific stresses between the PZT film and the substrate that modify the ferroelectric domain microstructure of the PZT grains: the microstructure is a-domain-type on SiO2 and Si, normal on Al2O3 and c-domain-type on SrTiO3 and MgO. Epitaxial orientation of the PZT grains is observed on bare SrTiO3 and MgO single crystals due to their oxide nature, cubic structure with cell parameters close to those of the PZT. On the other hand, the expected (111) orientation of PZT grains is not observed on (111) fiber axis Pt electrode, although cubic cell parameters of Pt are close to those of the PZT. The elaboration process (deposit of 0.2 μm in one coating) and the poor chemical affinity between platinum and PZT could be an explanation of the random orientation of the PZT grains. Structural and microstructural characteristics of the PZT grains do not affect drastically the electrical properties of the PZT film. By comparison, the presence of porosity and a pyrochlore phase has disastrous consequences. An improvement of the electrical properties is observed for dense PZT film with (100) oriented grains whose domain microstructure is c-domain-type.
Slanted coupling of one-dimensional arrays of small tunnel junctions
Authors: Hu G. Y. , O’Connell R. F. , Ryu Jai Yon
Published: July 2002

We have studied the electrostatic problem of the slanted coupling of two one-dimensional (1D) arrays with equal junction capacitances C, equal stray capacitances Co, equal coupling capacitances Cc, and with both arrays biased. In the weak coupling limit (Cc/C≪1), we obtain an analytic solution for the potential profile and the corresponding Gibbs free energy, and we derive threshold voltages for various charge transport modes. Our results show that C0, Cc, and the bias voltage V1 all play important roles in determining the threshold voltage of the system. In the small stray capacitance limit (C0/C≪1), the threshold voltage is proportional to 1/C, while in the large stray capacitance limit (C0/C≫1), the threshold voltage becomes independent of C. Also, in the small Cc/C limit, single electron tunneling always has a lower threshold voltage than that of the electron-hole and mixed tunneling. In addition, we find that V1 has a more dramatic effect on the electron-hole tunneling threshold voltage than on that of the single electron tunneling, i.e., at some favored value of Cc/C a small change in V1 can switch the transport of the system from single electron to electron-hole transport.
Characteristics of 360°-domain walls observed by magnetic force microscope in exchange-biased NiFe films
Authors: Cho Hae Seok , Hou Chunhong , Sun Min , Fujiwara Hideo
Published: July 2002

The characteristics of 360° domain walls have been investigated by magnetic force microscopy (MFM) in glass/Ta(10 nm)/NiFe(7.2 nm)/FeMn(8.0 nm)Ta(5 nm). The sample was prepared by radio-frequency-magnetron sputtering and characterized with a vibrating sample magnetometer. Both closed and line types of 360° domain walls were observed. The closed type (loop) walls include circular, ellipsoidal, and irregular shapes of domain wall, the long axis of which is generally perpendicular to the overall easy direction (pinned direction) of the exchange biased NiFe film. 360° wall loops were also classified by their chirality. Many 360° domain walls were observed near defects and the measured wall thickness ranged from 1.5 to 3 μm. The magnetic structure of the 360° loop was verified by simulations using a simplified model of the MFM imaging process. The formation of a 360° wall loops appears to originate from the rotational nature of the magnetization in the exchange-biased film, which comes from the distribution of the unidirectional anisotropy.
Growth/dissolution model for oxygen precipitation based on the kinetics of phase transformations
Authors: Ko Bong Gyun , Kwack Kae Dal
Published: July 2002

A model is presented for the growth and dissolution of oxygen precipitates in Czochralski silicon during heat treatment. Growth and dissolution rates are newly derived and inserted into a set of chemical rate equations and a Fokker–Planck equation. It can calculate the size distribution of the oxygen precipitates and oxygen concentration profile without calculation of the interfacial concentrations at the interface of Si matrix and precipitates. It accounts for the oxidizing ambient effect, the solubility enhancement effect of oxygen, and the surface recombination and generation of point defects. The formation of stacking faults is also taken into account. This approach allows one to calculate more accurately the residual oxygen depth profile and the density distribution of oxygen precipitates which can be measured experimentally. By comparing the simulated results with experimental ones, it is proved that this model can be used to estimate the depth profile and the defect densities under inert conditions and oxidation conditions.
Effects of discrete trap levels on organic light emitting diodes
Authors: Yang Jie , Shen Jun
Published: July 2002

We have studied the effects of discrete trap levels on organic light emitting diodes under the space charge limited conditions. The interplay among the free, discrete, and exponential trap charges in single- and double-carrier devices is discussed. The sharp transitions in the current–voltage characteristics often associated with the discrete trap levels in single-carrier devices may disappear in double-carrier devices such as the organic light emitting diodes. The physics is elucidated using the energy and charge profiles.
Microstructure evolution study of Pd–Ge ohmic contact formation on GaAs
Authors: Radulescu F. , McCarthy J. M.
Published: July 2002

The microstructure evolution and the corresponding solid-state reactions that take place during the formation of the Pd–Ge ohmic contacts on GaAs were studied using constant-heating-rate differential calorimetry (DSC) and cross-sectional transmission electron microscopy (XTEM). DSC analysis at different scan rates was performed on Pd(20 nm)/Ge(150 nm)/Pd(50 nm) thin film stacks that were lifted off the substrate and four solid-state reactions were identified. Specimens heated at temperatures that coincide with the DSC peaks were quenched in a He atmosphere and the resulting microstructure was characterized by XTEM. Variable constant-heating-rate DSC experiments allowed us to determine the activation energy associated with each solid-state reaction by the Kissinger plot method. The results were as follows: for Pd:Ge interdiffuson, the activation energy Q=1.03 eV, for hexagonal Pd2Ge formation Q=1.12 eV, for orthorhombic PdGe formation Q=1.33 eV and for Ge crystallization Q=1.8 eV. Based on these correlations, the mechanisms that contribute to the formation of an optimal ohmic contact microstructure were identified.
Curling reversal mode in nonellipsoidal ferromagnetic particles
Authors: Aharoni Amikam
Published: July 2002

In the appropriate size range it can be argued that the absolute value of a negative curling nucleation field in a prolate spheroid is a good approximation for the switching field in any regularly shaped ferromagnetic body, which has the same volume, provided the correct demagnetizing field is used. An example is given for the case of a square prism. The discrepancy between this theory and some experimental results is claimed to be due to volume and surface anisotropies in the experimental particles.
Impedance of a cylindrical coil over an infinite metallic half-space with shallow surface features
Authors: Uzal Erol , Kaya Metin O. , Özkol Ibrahim
Published: July 2002

An analytical solution is derived for the electric and magnetic fields produced by an air-core cylindrical coil driven by a constant ac current source and placed over an infinite metallic half-space with small surface perturbations. The depth of the features on the half-space is small compared to the mean radius of the coil. The perturbation technique is used to solve the Maxwell equations in the quasistatic approximation. Formulas are given for the electric field and the impedance. Real and imaginary parts of the impedance are computed and plotted for several test cases.
Microcavity organic light-emitting diodes for strongly directed pure red, green, and blue emissions
Authors: Tokito Shizuo , Tsutsui Tetsuo , Taga Yasunori
Published: July 2002

In this article we demonstrate strongly directed pure red, green, and blue emissions in the organic light-emitting diodes (OLEDs) with a planar microcavity defined by a pair of dielectric mirror and a metal mirror. By careful control of the cavity mode and the position of the resonance wavelength, the strong directionality in the forward direction as well as the spectral narrowing and the intensity enhancement are realized in the microcavity OLEDs. The intensity enhancements at the resonance wavelength are 1.5–5 compared to the noncavity OLEDs, and the chromaticity coordinates of the emission colors are the ideal primary colors. The experimental results are compared to theoretically calculated ones.
Annealing effects and radiation damage mechanisms of PbWO4 single crystals
Authors: Han Baoguo , Feng Xiqi , Hu Guanqin , Zhang Yanxing , Yin Zhiwen
Published: July 2002

The annealing treatments of a PbWO4 crystal sample have been sequentially carried out in air from 640 to 1040 °C. After annealing at each temperature, the optical absorption spectra of the crystal have been recorded pre and post UV irradiation exposure. The experimental results show that annealing PbWO4 crystals at a temperature above 740 °C in air can effectively wash out the intrinsic color centers causing a 350 nm optical absorption band and improve the radiation hardness of PbWO4 crystals. In addition, during the radiation procedure, the conversions of the 350 nm intrinsic color centers to the 410 nm temporary color centers are evidenced in the experiments. Based on the earlier results, the annealing effects and the radiation damage mechanisms of PbWO4 crystals are discussed.
La–Zn substituted hexagonal Sr ferrite thin films for high density magnetic recording
Authors: Liu Xiaoxi , Bai Jianmin , Wei Fulin , Xu Hai , Yang Zheng , Piramanayagam S. N. , Takei Shigeto , Morisako Akimitsu , Matsumoto Mitsunori
Published: July 2002

La–Zn substituted M type Sr-ferrite thin films were prepared by conventional rf diode sputtering. The crystallography, surface morphology, magnetic properties, and their temperature dependence were investigated by x-ray diffractometry, atomic force microscopy, and vibrating sample magnetometry. La–Zn substitution reduces the grain growth rate, so that it was possible to obtain fine grains with mean grain size about 12 nm×12 nm. Suitable amount of La–Zn substitution increases the saturation magnetization Ms of the films without changing the magnetocrystalline anisotropy of the material. As a result, the coercivity Hc increases with the increase of the La–Zn substitution. La–Zn substitution also reduces the values of dHc/dT of the films in the vicinity of room temperature.
Positive magnetoresistance in low-doped La1−xSrxMnO3 (x⩽0.14) perovskites
Authors: Senis R. , Balcells Ll. , Laukhin V. , Martínez B. , Fontcuberta J. , Pinsard L. , Revcolevschi A.
Published: July 2002

The magnetic and transport properties, under magnetic field and hydrostatic pressure, in a series of single crystalline samples of low-doped La1−xSrxMnO3 (0.1⩽x⩽0.14) manganites are studied. A region with positive magnetic resonance around TFI is detected in samples with x=0.1 and 0.125. At this temperature the transition from the ferromagnetic-metallic (FM-M) phase to the ferromagnetic orbital ordered-insulating (FI) low-temperature phase occurs. This transition is accompanied by a substantial upward jump in resistance that appears on entering the FI phase. A jump in the magnetization, on crossing TFI downward, with respect to that measured in the FM-M phase is also detected.
Oxygen-induced positive spin polarization from Fe into the vacuum barrier
Authors: Tsymbal E. Yu. , Oleinik I. I. , Pettifor D. G.
Published: July 2002

Bonding at the ferromagnet–insulator interface is an important factor which influences spin polarization of the tunneling current in ferromagnetic tunnel junctions. In this article we investigate the spin-polarized electronic structure of the (001) surface of body-centered-cubic iron covered by an oxygen overlayer, as this could reflect the mechanism of bonding and spin polarization in iron/oxide tunnel junctions. The Fe/O atomic structure is optimized using the plane-wave code CASTEP within the spin-polarized generalized-gradient approximation. The electronic structure and local densities of states are calculated using the linear muffin-tin orbital method. The results show hybridization of the iron 3d orbitals with the oxygen 2p orbitals, the strong exchange splitting of the former resulting in exchange-split bonding and antibonding oxygen states. These antibonding states are partially occupied for the majority spins but are almost unoccupied for the minority spins, which leads to a positive spin polarization in the density of states of the oxygen atoms at the Fermi energy. This positive spin polarization propagates from oxygen into the vacuum barrier. This is opposite to what is observed for clean Fe films, where the surface Fe layer has a negative spin polarization at the Fermi energy and remains negative into the vacuum. We infer that this p–d bonding mechanism might be responsible for the experimentally observed positive spin polarization of the tunneling current from ferromagnetic metals through alumina.
23.8 Gb/in.2 areal density demonstration
Authors: Gangopadhyay Sunita , Subramanian Krishnan , Ryan Patrick , Mack Anthony , Murdock Edward , Covault Michael L. , Yan Eileen , Champion Eric , Tamopolsky Giora J. , Feng Yong Chang , Zhang Zhenyong , Chen David Q. , Harkness Samuel D. , Ranjan Rajiv Y. , Rauch Gary C. , Richter Hans J.
Published: July 2002

We have demonstrated 23.8 Gb/in.2 areal density using a merged read-write grant magnetoresistive head, with an oriented thin film medium tested with broadband electronics and enhanced EPR4 channels. The medium had high signal to noise ratio metrics that was robust unto temperatures as high as 75 °C. A unique aspect of the head design at such a narrow track width is the simultaneous enhancement of the transducer sensitivity while keeping product and system manufacturability in the forefront. The areal density was demonstrated at a track density of 45.8 k tracks/in., using photolithographically defined poles and linear density of 520 k bits/in.
Writing performance of narrow trackwidth single pole perpendicular heads
Authors: Yamakawa Kiyoshi , Taguchi Kaori , Honda Naoki , Ouchi Kazuhiro , Iwasaki Shun-ichi
Published: July 2002

The writing performance of narrow trackwidth single pole heads was studied on double layer perpendicular media. The main pole of the head was made of high moment Fe–Si–N laminated with Si–N. The narrow poles were made by trimming the wide main pole with an FIB milling system. Saturation recording was achieved by a 0.3 μm trackwidth head for media composed of a Co/Pd or Co–Cr–Nb–Pt recording layer and a Ni–Fe–Nb soft magnetic backlayer. With this head, clear recorded patterns were obtained. It was found that the number of reversed magnetic domains decreased remarkably as the trackwidth was reduced. The MFM images of 0.14 μm trackwidth recording at 400 kFRPI reveals a potential for recording densities exceeding 50 Gbit/in.2