bluuebunny/crossref_metadata_2025 huggingface dataset:
Energetic model of ferromagnetic hysteresis 2: Magnetization calculations of (110)[001] FeSi sheets by statistic domain behavior
Authors: Hauser Hans
Published: July 2002

An energetic model of statistic domain behavior is applied on the magnetization process of grain oriented FeSi-steel sheets. The physical constants of the model are derived from anisotropic energy contributions, initial susceptibility, coercivity, and saturation magnetization. Calculated in the [100], [011], and [111] – directions, the hysteretic magnetization curves indicate good agreement with measurements from the demagnetized state up to saturation. The better correspondence between theory and experiments at weak fields is achieved by using realistic initial occupation probabilities for the domain structure. This assumption leads to complex mutual dependencies of the statistic domain’s volume fractions, which have not been considered in previous works.
Photoluminescence study of highly mismatched In0.53Ga0.47As epilayers grown on InP-coated GaAs substrates
Authors: Chen Y. F. , Shen J. L. , Chang I. M. , Chang S. Z. , Lee S. C.
Published: July 2002

Photoluminescence measurement is used to study the properties of In0.53Ga0.47As epilayers grown on InP-coated GaAs substrates. Several structures are designed to test the qualities of the epilayers. It is shown that device-quality In0.53Ga0.47As on a GaAs substrate can be achieved. The effects of hydrogenation using a photochemical-vapor-deposition system are studied, and an enhancement of the band-edge luminescence intensity by a factor of 4 is observed. The resulting intensity even exceeds that of the film grown directly on a lattice-matched InP substrate. In addition, the emission coming from lattice defects is greatly reduced after hydrogenation. These results not only further support the potential application of using binary buffer layer concepts in large lattice-mismatched systems, but also demonstrate that the photochemical-vapor-deposition system is a useful tool for injecting hydrogen into InGaAs epilayers.
Investigation of the high-field conductivity and dielectric strength of nitrogen containing polycrystalline diamond films
Authors: Boettger E. , Bluhm A. , Jiang X. , Schäfer L. , Klages C.-P.
Published: July 2002

The influence of nitrogen doping on the electrical properties of polycrystalline diamond films has been studied. The films were prepared in a microwave plasma chemical vapor deposition process using a H2/CH4/N2 gas mixture. The CH4 concentration was held constant at 0.5% and the nitrogen to carbon atomic ratio was varied between 0.01 and 0.2. The phase purities, surface morphologies, and the nitrogen contents of the films were analyzed by Raman spectroscopy, scanning electron microscopy, and secondary-ion mass spectroscopy, respectively. From current-voltage characteristics at field strengths up to 106 V cm−1 and in the temperature range between 300 and 800 K the conductivity was determined. The dielectric strength was obtained from the breakdown voltage measured using a voltage ramping rate between 50 and 100 V s−1. A minimum in high field and high-temperature conductivity and a maximum in dielectric strength was found for the samples prepared with a nitrogen to carbon atomic ratio of 0.02. Compared with nitrogen free samples the conductivity is lowered by more than three orders of magnitude and the dielectric strength is enhanced by a factor of two. The results will be discussed in terms of compensation of acceptor states by nitrogen donors and structural changes of the films.
Electrostatic deformation in band profiles of InP-based strained-layer quantum-well lasers
Authors: Seki Shunji , Yokoyama Kiyoyuki
Published: July 2002

We study the electrostatic deformation in the conduction-band and valence-band profiles of 1.3-μm InP-based strained-layer (SL) quantum-well (QW) lasers in the temperature range of 273–373 K. Electrostatic deformation is analyzed by the self-consistent numerical solution of the Poisson’s equation, the scalar effective-mass equation for the conduction band, and the multiband effective-mass equation for the valence band. It is shown that, in InP-based QWs, electrostatic band-profile deformation causes a significant change in effective barrier height for the conduction and valence bands, which has a pronounced influence on the electron and hole distribution throughout the whole QW structure in the temperature range studied. We demonstrate that it is necessary to take into account electrostatic deformation in both band profiles for an analysis of the high-temperature characteristics of InP-based SL-QW lasers.
Optical absorption coefficients of semiconductor quantum-well infrared detectors
Authors: Fu Y. , Willander M. , Xu Wenlan
Published: July 2002

The optical absorption coefficients of semiconductor quantum-well infrared detectors have been carefully analyzed in the most general way using quantum perturbation theory. We have calculated the optical transitions between discrete subbands in quantum-well systems and between continuous energy bands of bulk materials as well as mini-subbands formed in semiconductor superlattices. Nonparabolic effects in energy spectra and the effect of different doping concentrations have also been investigated. The optimal directions, the corresponding absorption coefficients, and the quantum-well widths as functions of responding radiation frequency ℏω are listed in a table for the four most commonly used materials AlAs, GaAlSb, Si, and Ge as a guide for device designs.
Cross-sectional atomic force microscopy of semiconductor nanostructures
Authors: Dwir B. , Reinhardt F. , Kapon E.
Published: July 2002

We performed imaging of semiconductor nanostructures (quantum wells and quantum wires) by atomic force microscopy of the cleaved edge of the samples under ambient conditions. Selective etching was used to enhance the composition contrast of the semiconductor heterostructure layers, and a procedure for retrieving the nanostructure dimensions from the scanned image is presented. The simple sample preparation and the relatively large (up to ∼100×100 μm2) imaged areas offer advantages over more conventional nanometer resolution techniques such as transmission electron microscopy.
Measurements of electromagnetic fields in a planar radio-frequency inductively coupled plasma source
Authors: Meyer J. A. , Wendt A. E.
Published: July 2002

Electromagnetic fields in a planar radio-frequency inductively coupled plasma source were measured using an inductive loop (B-dot) probe. The probe was oriented to measure the time derivative of the axial component of the magnetic field (Ḃz). Using these measurements and Faraday’s law, taking advantage of cylindrical symmetry, the time varying azimuthal electric field (Eφ) was calculated directly. Contour plots of Ḃz and Eφ in the r-z plane show that the radio-frequency electromagnetic fields penetrate further into the plasma at lower gas pressure and lower rf power, corresponding to less effective shielding of the fields at lower plasma density. Estimates of skin depth from the axial decay of the field amplitudes near the axis of the discharge are consistent with values calculated from plasma parameters measured with Langmuir probes, confirming that near the axis the degree of shielding is most strongly dependent on the local plasma density. Near the conducting walls of the chamber, the skin depth calculations from the Langmuir probe data diverge from the B-dot probe data. B-dot probe measurements taken in the absence of plasma show that near the walls of the chamber the axial decay of the field amplitude is partly a geometrical effect in addition to a plasma shielding effect.
Structural and magnetic investigations of epitaxial ferromagnetic τ MnAl films grown on GaAs/AlAs by molecular-beam epitaxy
Authors: Van Roy W. , De Boeck J. , Bender H. , Bruynseraede C. , Van Esch A. , Borghs G.
Published: July 2002

The crystalline quality and magnetic properties of epitaxial ferromagnetic τ MnxAl100−x (x=50–70) films with perpendicular anisotropy grown on AlAs/GaAs by molecular-beam epitaxy are improved by ex situ rapid thermal annealing compared to as-grown thin films. An increase in magnetization of up to 230% is observed for moderate annealing temperatures (≊450 °C). This is strongly related to an improved ordering in the occupation of the two sublattices, which are antiferromagnetically coupled. At the same time a strong reduction in coercive field (up to a factor of 4) upon annealing is attributed to a decreased density of antiphase boundaries in the metal film. Annealing at higher temperatures (≊550 °C and above) results in the partial relaxation of the τ phase, and eventually in the transformation of the entire film to the nonmagnetic ε phase. The composition has a strong influence since the presence of excess Mn (x≳50) reduces the magnetization. The remanent magnetization Mr=465 kA/m for τ  Mn50Al50 is close to the bulk value of 490 kA/m. The thickness of the film has little influence on the annealing behavior.
Investigation of microwave π transitions in cesium beam clocks operated with U-shaped H plane waveguide cavities
Authors: Giordano V. , Pichon L. , Cérez P. , Théobald G.
Published: July 2002

In this article, we present the characteristics of the ΔF=±1, ΔmF=±1, π microwave transitions which appear in the response of cesium beam resonators operating with U-shaped waveguide Ramsey cavities bent in the plane of the H field. Such resonances are due to the presence in the cavity of microwave magnetic induction perpendicular to the static field direction. It is shown that the π resonance feature is derived from a pure two-level atomic system interacting with four spatially separated oscillating fields. Good agreement is found between experimental data and theoretical predictions when we take into account the actual transverse microwave field profile in the cavity obtained with tridimensional electromagnetic field computations.
Design of computationally useful single-electron digital circuits
Authors: Ancona M. G.
Published: July 2002

We formulate and discuss principles for designing computationally useful networks of Coulomb-blockade devices. Our particular focus is on locally interconnected synchronous networks in which the numerical discreteness of a computation is represented directly by the quantization of electron charge, i.e., electrons represent bits. To highlight our emphasis on circuits and architectural issues, and on performing locally interconnected computation rather than traditional logic as has been the interest heretofore (single-electron logic), we refer to our networks as single-electron digital circuits (SEDCs). In addition to being single-electron and locally interconnected, the SEDCs we propose have a regular ‘‘cellular’’ structure with occupancy-independent biasings and with electron-electron interactions carefully controlled. The chief virtue of SEDCs is their scalability, both as devices (because of their Coulomb blockade basis) and as circuits (because of their local interconnectivity), perhaps even to molecular dimensions. We illustrate our approach with a number of new ‘‘device’’ and network designs based on electron-pump-like structures and mostly directed at performing lattice-gas simulation. For this application we effectively create an electron gas within a SEDC which precisely mimics the lattice gas. Finally, we have validated our designs using numerical simulation and expect that at least some of them should be realizable in current technology. However, their promise of enormous levels of integration and performance should be tempered with a clear awareness of the many obstacles associated with fabrication and economics which must be overcome if they are ever to be the foundation for a practical computer technology.
Detection of electric-field-ionized Rydberg atoms originating from laser ablation of Si
Authors: van Ingen R. P.
Published: July 2002

Si* Rydberg atoms in plasmas originating from laser ablation of Si were identified, employing deflection plates for electric-field ionization and a quadrupole mass spectrometer. The angle-resolved time-of-flight and yield distributions of these species showed that both the most probable time-of-flight and the yield of the Si* atoms decreased with an increasing detection angle, as in the case of laser ablated neutrals.
A theoretical description of magnetic switching experiments in picosecond field pulses
Authors: He L. , Doyle W. D.
Published: July 2002

Siegmann et al. have reported that the magnetization in a perpendicularly oriented CoPt film with a uniaxial anisotropy field of 20 000 Oe was reversed when the film was exposed to a 20 000 Oe in-plane pulsed field lasting only 6 ps. From a calculation based on the Gilbert form of the Landau–Lifshitz equation (LLG), it is shown that the experimental result is consistent with the LLG model even for values of the damping constant α as larger as 0.5. The dependence of the switching time on the anisotropy field and the applied field are presented as a function of α and the angle β between the applied field and the easy axis. Experiments are suggested which could illuminate the damping mechanism at these very short times.
Plasma deposition of amorphous SiC:H,F alloys from SiF4-CH4-H2 mixtures under modulated conditions
Authors: Cicala G. , Capezzuto P. , Bruno G. , Schiavulli L. , Amato G.
Published: July 2002

Fluorinated and hydrogenated amorphous silicon-carbon alloys (a-SiC:H,F) are produced by glow discharge decomposition of SiF4-CH4-H2 mixture. Small amount of CH4 in SiF4-H2 mixture are enough to produce silicon carbon alloys having optical gap ranging between 1.8 and 2.6 eV. Materials, having 1.95 eV band gap and exhibiting optoelectronic properties typical of state of art a-SiC:H, are deposited under plasma modulation conditions.
Magnetic properties of nested carbon nanostructures studied by electron spin resonance and magnetic susceptibility measurements
Authors: Bandow Shunji
Published: July 2002

Nested carbon nanostructures, e.g., nanotubes and nanoballs, are separated from the coexisting materials of carbon flakes and needle-like fragments by sonication, centrifugalization and low-temperature combustion. Content of nanotubes at the final stage separation is in excess of 85% by weight. The nested carbon nanostructures (≳85 wt % tubules) are studied by electron spin resonance (ESR) and magnetic susceptibility measurements. The temperature dependence of the conduction-ESR intensity for the nested carbons is similar to that for graphite. On the other hand, the g value is almost constant (g=2.0096±0.0004 at room temperature) between 40 and 300 K, in contrast to that of graphite. These ESR features are discussed in terms of the electronic structure of carbon nanotubes predicted by theoretical calculation. The magnetic field dependence of differential magnetic susceptibility (χdiff) indicates a logarithmic divergence in the magnetic field H≤2 kG and the χdiff is a positive value at H≊0.8 kG, which is qualitatively consistent with the magnetic properties of metallic carbon nanotube enunciated by Ajiki and Ando.
Patterned growth on high-index GaAs (n11) substrates: Application to sidewall quantum wires
Authors: Nötzel Richard , Ramsteiner Manfred , Menniger Johann , Trampert Achim , Schönherr Hans-Peter , Däweritz Lutz , Ploog Klaus H.
Published: July 2002

We have recently found a new phenomenon in the selectivity of growth by molecular-beam epitaxy on patterned GaAs (311)A substrates to form a fast growing sidewall on one side of mesa stripes oriented along the [01−1] direction. Preferential migration of Ga atoms from the mesa top and bottom toward the sidewall forms a smooth convex curved surface profile without facets. Comparison of patterned growth on other high-index (n11)A&B surfaces shows this growth mode to be unique for GaAs (311)A substrates. Lateral quantum wires are realized for step heights in the quantum-size regime. Quantum confinement of excitons in the wires is demonstrated by the transition from two-dimensional to magnetic confinement with increasing magnetic field. For device applications it is important that the wires can be vertically stacked in the growth direction without increase in interface roughness and wire size fluctuations.
Crystal structure and domain-wall contributions to the piezoelectric properties of strontium bismuth titanate ceramics
Authors: Reaney Ian M. , Damjanovic Dragan
Published: July 2002

The piezoelectric properties of SrBi4Ti4O15 and Bi4Ti2.95Nb0.04O12 have been compared as a function of applied ac stress. Evidence of domain-wall motion was observed in Bi4Ti2.95Nb0.04O12 but not in SrBi4Ti4O15. Examination of SrBi4Ti4O15 using transmission electron microscopy revealed that the material had undergone an orthorhombic–orthorhombic, paraelectric–ferroelectric, phase transition in which no piezoelectrically active, non-180°, domain walls could be created. However, for the orthorhombic–monoclinic transformation in Bi4Ti2.95Nb0.04O12 new piezoelectrically active domain walls are allowed and were considered to move under applied stress.
Adhesion in growth of defect-free silicon over silicon oxide
Authors: Raidt H. , Köhler R. , Banhart F. , Jenichen B. , Gutjahr A. , Konuma M. , Silier I. , Bauser E.
Published: July 2002

By applying liquid phase epitaxy, we have grown defect-free silicon and silicon–germanium layers on partially oxide-masked Si wafers. The growth of the layers started epitaxially in oxide-free seeding areas and proceeded laterally over the thermal oxide film. Detailed studies by x-ray topography and electron microscopy show that the obtained thin semiconductor-on-insulator layers bend towards the oxide during lateral growth. The bending of the layers can be ascribed to adhesion and interfacial forces. Adhesion operates across a gap between the closely spaced surfaces of the oxide and the epitaxial Si and facilitates lateral growth of high-quality semiconductor layers on dissimilar layers or substrates. The technical potential of adhesion-dependent solution growth on dissimilar substrates is discussed.
Non-Gaussian noise in a colossal magnetoresistive film
Authors: Hardner H. T. , Weissman M. B. , Jaime M. , Treece, R. E. , Dorsey P. C. , Horwitz J. S. , Chrisey D. B.
Published: July 2002

Anomalous non-Gaussian effects appear in the resistance fluctuations of a macroscopic sample of the colossal magnetoresistive material La2/3Ca1/3MnO3. Individual resistance switches are frequently resolved. The results indicate the presence of highly inhomogeneous hopping resistance among magnetic domains in this partially disordered material
Equilibrium critical thickness of epitaxial strained layers in the {111} orientations
Authors: Colson H. G. , Dunstan D. J.
Published: July 2002

Critical thickness of epitaxial strained layers is dependent on growth orientation both through crystallographic geometric factors and through elastic anisotropy. In a standard treatment of critical thickness, these two effects reduce critical thickness in all non-{001}-growth orientations except the {111}. Close to (111), where the (111) slip system is inactive, critical thickness is slightly increased, to 1.23 times the {001} value for GaAs and 1.37 times for silicon. The analysis also shows that critical thickness is decreased for any off-cut away from (001)- or (111)-growth orientations, and the off-cut directions for the smallest decreases are 〈100〉 and 〈21̄1̄〉, respectively.
Chaos and energy loss of nonlinear domain wall motion
Authors: Okuno H.
Published: July 2002

A chaotic region of domain wall motion is calculated as a function of the amplitude and frequency of the external magnetic drive field. It shows an intricate pattern composed of regular and chaotic regions. An energy loss and the Lyapunov exponent of domain wall motion are calculated. A frequency at the peak of the energy loss versus frequency curve, namely a nonlinear resonance frequency, is different from a resonance frequency in the linear theory as it shifts toward a lower frequency with increasing amplitude of the external magnetic drive field. This peak shift is explained by the effect of the higher order term in the nonlinear restoring force. The energy loss versus frequency curve becomes irregular and the energy loss decreases in the chaotic region where the Lyapunov exponent is positive.
Preparation and characterization of large scale periodic magnetic nanostructures (abstract)
Authors: Kirsch S. , Pollmann A. , Thielen M. , Weinforth H. , Carl A. , Wassermann E. F.
Published: July 2002

We report on the preparation and characterization of large scale periodic magnetic nanostructures designed as possible candidates for a future application in magnetic data storage technology. The nanostructures are prepared on glass substrates by UHV electron beam evaporation of Fe, Co, or Co/Pt onto periodically patterned photoresist masks followed by a lift-off of the photoresist. The preceding patterning process of the photoresist is achieved by using an interference lithography process in combination with subsequent selective etching. For the exposure of the photoresist, we use two different experimental setups with Ar ion lasers operating at wavelengths of 457 and 244 nm, respectively. This allows us to control diameter as well as distances between magnetic dots ranging between 300 and 3000 nm. The structural characterization of magnetic nanostructures is performed by electron microscopy as well as atomic force microscopy. Magnetic force microscopy along with additional image calculations based on dipole–dipole interaction between a magnetic tip and a magnetic dot allows for an interpretation of the magnetic properties of single magnetic dots. Accompanying work focuses on comparable nanostructures of smaller sized samples prepared by electron-beam lithography.
Effects of preparation conditions on the hysteresis behavior of granular Fe–SiO2
Authors: Christodoulides J. A. , Shevchenko N. B. , Hadjipanayis G. C. , Papaefthymiou V.
Published: July 2002

The hysteresis behavior of Fex(SiO2)1−x granular films was studied as a function of preparation conditions. The films were prepared by dc and rf magnetron sputtering over the composition range of xv=0.1–0.9, where xv is the volume fraction of Fe. The tandem and composite target deposition methods, both with and without titanium presputtering, were used to prepare the films. Typical granular structure was observed in these films, with grain size in the range of 2–20 nm. X-ray diffraction and selected area diffraction showed a bcc α-Fe-type structure. Magnetic measurements showed that by varying the deposition method and some of the sputtering parameters including sputtering rate, argon flow, film thickness, and substrate temperature, it is possible to switch from a relatively magnetically hard sample (Hc∼650 Oe) to a soft sample (Hc∼20 Oe). For the former sample, a dramatic increase in Hc has been observed at cryogenic temperatures. This result, along with the Mössbauer data, suggest a shell/core granule morphology with an Fe and/or Fe–Si core surrounded by an Fe–Si–O phase.
Investigation of the domain contrast in magnetic force microscopy
Authors: Belliard L. , Thiaville A. , Lemerle S. , Lagrange A. , Ferré J. , Miltat J.
Published: July 2002

Domains were imaged by magnetic force microscopy (MFM) on materials where the domain size exceeds the sample thickness by three orders of magnitude. Selected samples are a magnetooptical medium and ultrathin cobalt films, all with perpendicular magnetization. A strong domain contrast is observed in both cases. This fact is confronted to the usual theory of MFM image formation, in which the stray field from one body (tip or sample) is sensed by the other, without altering of the magnetization distributions. It is shown that the domain contrast in such extreme conditions cannot be explained with that theory. On the contrary, Abraham and McDonald’s model, which considers the response of the sample to the tip field, is quantitatively compared to experiment. It is shown to provide a good qualitative description of the results, but not a quantitative one, because of oversimplification.
Strain and defects depth distributions in undoped and boron-doped Si1−xGex layers grown by solid phase epitaxy
Authors: Rodríguez A. , Rodríguez T. , Kling A. , Soares J. C. , da Silva M. F. , Ballesteros C.
Published: July 2002

A detailed characterization of undoped and heavily boron-doped Si1−xGex layers with x=0.21, 0.26, and 0.34 grown on (001) Si wafers by solid phase epitaxy is presented. The starting material for solid phase epitaxial growth was prepared by amorphization of epitaxial SiGe-Si heterostructures by ion implantation. In order to obtain doped layers, boron was also implanted into some of the amorphous samples. After regrowth, the strain depth distributions of the SiGe layers were measured using axial channeling angular scans and the defect distributions were observed by high-resolution electron microscopy. A defect-free region ranging from 0 nm (undoped layer of x=0.34) to 30 nm (doped layer of x=0.21) in thickness was observed next to the layer-substrate interface. In the upper region of the layers, strain-relieving defects, identified as planar faults, were observed. Some isolated defects were also present at the layer-substrate interfaces of most of the samples. The measured strain depth profiles show that (i) the defect-free regions are not always fully strained; the defects located at the interfaces being responsible for this partial relaxation; (ii) the strain is almost constant throughout the defect-free layers because it cannot be relieved due to the absence of defects; and (iii) the strain progressively decreases towards the sample surfaces in the region of the layer where the strain-relieving defects are located. Comparison between the undoped and boron-doped layers show the consequences of the strain compensation effect due to the incorporation of boron atoms into the lattice. The defect-free regions of the doped layers are thicker and closer to coherency than those in the undoped layers of the same composition and the defect density in the upper region of the layers is significantly reduced. As a result of the strain compensation effect, a 30-nm-thick heavily doped layer with x=0.21 is found to be defect free and fully strained throughout its whole thickness although the corresponding undoped layer was partially relaxed and showed strain-relieving defects.
Reliability analysis for encapsulated interconnect lines under dc and pulsed dc current using a continuum electromigration transport model
Authors: Clement J. J.
Published: July 2002

The origins of two types of electromigration failure models are explored starting with the basic transport differential equations including the effects of electromigration-induced mechanical stress. We compare the results of a nucleation model, in which failure is determined by the time required to build up a critical stress or a critical vacancy concentration at a site of atomic flux divergence, with a void-growth model, in which failure is linked to the growth of a void to a critical size. Two particular applications of the model are investigated, and the results are shown to be in good agreement with available experimental data. In one example, the effect of the presence of a field-free reservoir region extending beyond the current-carrying region of the line is compared to the case without such a field-free extension. In the other, the effects under pulsed dc current stress are examined in comparison to dc current. Possible applications of this model to design verification are also discussed.
Scanning tunneling microscopy and atomic force microscopy study of graphite defects produced by bombarding with highly charged ions
Authors: Mochiji Kozo , Yamamoto Seiji , Shimizu Hiroshi , Ohtani Shunsuke , Seguchi Takashi , Kobayashi Nobuo
Published: July 2002

The defects produced on a graphite surface by single ion impact using highly charged Ar ions (charge state q⩽8) is investigated by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The defect looks like a protrusion in the STM image, while it is flat in the AFM image. From these two contrasting images, the defects are considered to be due to the increase in the local charge density of state at the surface caused by carbon atom sputtering. The average value for the defect size increases remarkably with the charge state of incident Ar ions. This is explained by the enhancement of potential sputtering due to the Coulomb repulsion between surface holes which are generated by the neutralization of highly charged Ar ions.
Densification of radio frequency sputtered silicon oxide films by rapid thermal annealing
Authors: Choi W. K. , Choo C. K. , Han K. K. , Chen J. H. , Loh F. C. , Tan K. L.
Published: July 2002

A study of the densification of radio frequency sputtered silicon oxide films by rapid thermal annealing was undertaken using both physical (etching, refractive index, and density calculation) and structural [infrared and x-ray photoelectron spectroscopy (XPS)] techniques. It was discovered that the etch rate of annealed films is reduced drastically, compared with as-deposited films and that the refractive index increases with increasing annealing temperature (Tp) or annealing time (tp). The film density also increases as Tp or tp increases and we conclude that annealed films have become denser as compared to the as-deposited films. We also suggest that increasing tp would be more efficient than raising Tp for film densification. Infrared spectrum analysis results show that with increasing Tp or tp, strain in the films has become more relaxed, and a significant amount of the surface hydroxyl groups in the annealed films was removed. The XPS analysis results show that a substantial amount of suboxide species exist within 30–40 Å of the oxide at the Si–SiO2 interface and that the Si–SiO2 interface is not abrupt.
Nonequilibrium Green’s function approach to high-temperature quantum transport in nanostructure devices
Authors: Tsuchiya Hideaki , Miyoshi Tanroku
Published: July 2002

A quantum kinetic equation for the Wigner distribution function including collisional broadening effects is formulated based upon nonequilibrium Green’s function technique, and applied to the high-temperature quantum transport in nanostructure devices. The collisional effects due to longitudinal optical phonon scattering are introduced to the transport equation through the electron spectral function. After studying the influence of quantum size confinement on the spectral functions in nanostructures, the nonlinear current–voltage characteristics of quantum wires and the phonon bottleneck phenomena in quantum dot lasers at high temperature are analyzed.
Electrical transport in paratoluene sulfonate doped polypyrrole films at low temperature
Authors: Ghosh M. , Barman A. , Das A. , Meikap A. K. , De S. K. , Chatterjee S.
Published: July 2002

Transport data for paratoluene sulfonate dope polypyrrole films in the insulating regime and near the metal-insulator (M-I) boundary are presented and analyzed. Samples in the insulating region show a crossover from Mott to Efros–Shklovskii variable range hopping conduction at T=5 K and magnetoconductance of these samples is also explained by variable range hopping theory. The power law dependence of conductivity σ(T)∝Tβ is observed for the sample close to metallic side of the M-I transition with β=0.83 for 20 K<T<300 K and β=1/2 for 1.8 K⩽T⩽20 K. The magnetoconductance of this sample is analyzed by three dimensional electron-electron interaction and weak electron localization theory. The inelastic scattering length Lin obeys a power law temperature dependence, Lin∝T−p/2 with p=1.
Comparison of the spin dynamics in different types of molecular magnetic rings from H1 NMR
Authors: Lascialfari A. , Jang Z. H. , Borsa F. , Gatteschi D. , Cornia A.
Published: July 2002

Proton spin-lattice relaxation (NSLR) measurements have been performed in two molecular copper magnetic rings containing 6 and 8 spins (S=1/2), respectively, in an almost coplanar arrangement in order to probe the spin dynamics of the spins in the ring. The NSLR results obtained in the Cu6 and Cu8 rings as a function of temperature and of applied magnetic field are compared with previous results of NSLR of H1 in the iron(III) rings Fe6 and Fe10 (S=5/2). At room temperature, common features are found in the spin dynamics while at low temperature, when correlation effects become important, important differences are observed in the rings depending on the kind of coupling between magnetic spins (ferromagnetic or antiferromagnetic) and on the spin value S.
Spin-dependent tunneling in epitaxial systems: Band dependence of conductance
Authors: MacLaren J. M. , Butler W. H. , Zhang X.-G.
Published: July 2002

We present first principles based calculations of the tunneling conductance between iron electrodes separated by semiconducting ZnSe. We assume that Fe (100) and ZnSe (100) atomic planes are epitaxed. We find that the conductance depends strongly on the relative alignment of the magnetic moments in the two Fe electrodes. The relative change in conductance increases dramatically as the thickness of the semiconductor increases. We show that this effect is due to the fact that electrons from a particular majority spin band are injected efficiently into the ZnSe from the Fe and also that electrons are ejected efficiently from the ZnSe into this band. Our calculations are based upon the Landauer–Büttiker expression for the conductance which is expressed in terms of the transmission matrix elements.
Magnetization reversal of Nd(Dy)–Fe–B thin films on Si(111) or Ta/Si(111)
Authors: Tsai J. L. , Chin T. S. , Huang E. Y. , Chen S. K.
Published: July 2002

Nd(Dy)–Fe–B films were prepared by dc magnetron sputtering on a Si(111) wafer with or without a Ta underlayer. The reversal magnetization process of Nd–Fe–B/Ta bilayer was found to be dominated by nucleation control model with the magnetic inhomogeneity coefficient αk=0.32 defined by Kronmuller’s formulation of micromagnetic theory. But the coercivity mechanism of Nd(Dy)–Fe–B single layer was fitted well to domain wall pinning behavior. The range factor (half width between pinning sites) r0 is equal to 6.9 nm as r0>δB, the width of the domain wall. The magnetization phenomena of the two films are also manifest from initial magnetization curves.
Evidence for a gap in the excitation spectrum of CrO2
Authors: Barry A. , Coey J. M. D. , Ranno L. , Ounadjela K.
Published: July 2002

Magnetization and resistivity of CrO2 powders and oriented CrO2 films on different TiO2 single-crystal substrates are reported, and the low-temperature specific heat is measured on powders. The M(T) data follow a Bloch T3/2 law, with no sign of a spin-wave gap. The residual resistivity depends strongly on film texture, but a T2 temperature dependence of the resistivity is found in all samples above a cut-off temperature of the order of 70 K. The behavior is attributed to electron-magnon spin-flip scattering which is suppressed at low temperature in a half-metallic ferromagnet.
Bias voltage and temperature dependence of magnetotunneling effect
Authors: Lu Yu , Li X. W. , Xiao Gang , Altman R. A. , Gallagher W. J. , Marley A. , Roche K. , Parkin S.
Published: July 2002

We have studied systematically the magnetotunneling properties of several metallic magnetictunnel-junction systems (Ni80Fe20–insulator–Ni80Fe20,Ni80Fe20–I–Co,Co–I–Co, Ni40Fe60–I–Co). The room-temperature magnetoresistance MR value at zero-bias ranges between 16% and 27%, depending on the spin polarization of the electrodes. There seems to be a general bias dependence of MR in all of these systems. In particular, it requires a bias in the range of 0.22–0.23 V to suppress the maximum MR value by half. We have also measured the bias dependence of MR as a function of barrier parameters (thickness and oxidation time). At low temperature, a sharp cusplike feature appears near zero bias. In some cases, low-temperature MR values substantially exceed expectations from established spin-polarization.
Thermal fluctuation aftereffect of exchange coupled films for spin valve devices
Authors: Fujikata Junichi , Hayashi Kazuhiko , Yamamoto Hidefumi , Nakada Masafumi
Published: July 2002

The annealing-time-dependent change in the unidirectional anisotropy field for exchange coupled films with FeMn, PdPtMn, NiMn, and NiO, which are generally used for spin valve, was investigated after thermal annealing below their blocking temperatures. It was shown that the distribution of local blocking temperature was similar for each exchange coupled film. The annealing-time-dependent change in the unidirectional anisotropy was explained by the use of thermal fluctuation aftereffect model, and the relaxation time for the anisotropy change was found to be much larger for FeMn than for PdPtMn, NiMn, and NiO, when normalizing annealing temperature by their blocking temperatures. From the viewpoint of device application, however, NiMn was proven to be the most promising antiferromagnet in terms of thermal stability because of its highest blocking temperature.
Giant magneto-impedance effect in nanocrystalline glass-covered wires
Authors: Chiriac H. , Óvári T. A. , Marinescu C. S.
Published: July 2002

We investigate the evolution of the magneto-impedance (MI) response of Fe73.5Cu1Nb3Si13.5B9 glass-covered wires with the annealing temperature, starting from the as-cast amorphous state up to 600 °C. As-cast FeCuNbSiB amorphous glass-covered wires display a MI ratio of about 1%, this value increasing with the annealing temperature to about 5% for 550 °C when the nanocrystalline phase appears. For wires which have had the glass removed, the MI ratio increases from 5% for the as-cast amorphous state up to 28% after the appearance of the nanocrystalline phase. The MI is larger than that found in CoFeSiB amorphous wires after glass removal. Annealing over 550 °C leads to a decrease of the MI ratio for both glass-covered samples and samples with the glass coating removed. The results demonstrate the role of soft magnetic properties on the sensitivity of the MI effect.
Surfactant effect of Sb on the growth of (111) orientated fullerene thin films
Authors: Hou J. G. , Xu Wentao , Haiqian Wang , Li Yang
Published: July 2002

We report a method for the growth of C60 single crystal thin films using Sb as the surfactant. It is found that the growth mechanism of C60 thin films on the substrates of (001) NaCl was significantly affected by the thin layer of Sb when the substrate temperature is 160 °C. (111) orientated C60 single crystal films were prepared by this method, and the average grain size of our films is about one order of magnitude larger than that of the polycrystalline films grown without Sb. The possible mechanism of the metal enhanced C60 single crystal growth has been discussed.
Device characterization for amorphous diamond-like carbon–silicon heterojunctions
Authors: Konofaos N. , Evangelou E. , Thomas C. B.
Published: July 2002

We report here on the electrical characterization of Al/a-C:H/n-Si devices, where the a-C:H films were ion implanted with boron. The current–voltage characteristics versus temperature demonstrated the creation of p-n heterojunctions and Schottky diodes. Maximum current outputs were reached faster for higher temperatures. Lower doses of boron implants produced Schottky diode characteristics, with a current saturation in the forward region due to the existing barrier. The values of the output currents increased with temperature and implanted dose.
Microscopic bending of GaAs layers grown by epitaxial lateral overgrowth
Authors: Zytkiewicz Z. R. , Domagala J. , Dobosz D. , Bak-Misiuk J.
Published: July 2002

X-ray diffraction has been used to study the influence of the mask material on properties of GaAs layers grown by the liquid phase epitaxial lateral overgrowth (ELO) on (100) GaAs substrates. We show that ELO stripes bend towards the SiO2 mask in the direction perpendicular to seeding lines in a similar way to that as studied recently by x-ray topography for Si lamellae [H. Raidt, R. Kohler, F. Banhart, B. Jenichen, A. Gutjahr, M. Konuma, I. Silier, and E. Bauser, J. Appl. Phys. 80, 4101 (1996)]. The bending disappears when the mask is removed by selective etching. This microscopic bending is reduced by nearly 2 orders of magnitude when graphite instead of SiO2 is used to mask the substrate.
Barrier properties and failure mechanism of Ta–Si–N thin films for Cu interconnection
Authors: Lee Yoon-Jik , Suh Bong-Seok , Kwon Myoung Seok , Park Chong-Ook
Published: July 2002

Cosputtered Ta–Si–N amorphous films of ten different compositions were investigated as a barrier material for Cu interconnection. The films of relatively low nitrogen content (<47 at. %) undergo an abrupt failure with the formation of tantalum silicides and copper silicide between Si and Cu during annealing. Ta43Si4N53 thin film is readily crystallized into TaNx in spite of a remarkable chemical stability with Cu. The films containing nitrogen more than 51 at. % are sacrificial barriers which show the formation of Cu3Si phase at Ta–Si–N/Cu interface even before the films crystallize to form tantalum silicide. According to electrical tests, the barriers which show the sacrificial characteristics are most effective and show no electrical degradation even after annealing at 500 °C for an hour in Si/Cu and 525 °C for an hour in SiO2/Cu metallization.
Relaxation kinetics in quantum cascade lasers
Authors: Slivken S. , Litvinov V. I. , Razeghi M. , Meyer J. R.
Published: July 2002

Relaxation kinetics in a quantum cascade intersubband laser are investigated. Distribution functions and gain spectra of a three-subband double-quantum-well active region are obtained as a function of temperature and injection current. The potentially important role of the nonequilibrium phonons at lasing threshold is shown and discussed in details. It is shown that the threshold current is strongly dependent of the power dissipated in the active region in steady state. The numerical calculations for an 8.5 μm laser illustrate the general issues of relaxation kinetics in quantum cascade lasers. Temperature dependence of the threshold current is obtained in a good agreement with the experiments.
Effects of strain and alloying on the Hall scattering factor for holes in Si and Si1−xGex
Authors: Dijkstra J. E. , Wenckebach W. Th.
Published: July 2002

The Hall factor for holes at T=300 K in strained Si and strained and relaxed Si1−xGex is calculated using two methods, a Monte Carlo method and a method based on the relaxation time approximation. The results of both methods agree very well. The Hall scattering factor is found to depend strongly on strain, varying from a value of ≈3 to ≈0.1. This is explained by the change in anisotropy of the valence band due to strain. Also the effect of alloying and alloy disorder scattering on the Hall factor in Si1−xGex is investigated. Alloying has strong effect on the scattering factor because it changes the anisotropy of the valence band. The effect of alloy disorder scattering is found to be negligible.
Crystallization and surface erosion of SiC by ion irradiation at elevated temperatures
Authors: Heera V. , Stoemenos J. , Kögler R. , Voelskow M. , Skorupa W.
Published: July 2002

The effects of high dose ion irradiation through amorphous surface layers on single crystalline 6H–SiC at elevated temperatures are studied in detail. Material swelling, subsequent densification, and surface erosion are quantified for irradiation at 500 °C. Ion beam induced recrystallization is investigated in the temperature range between 300 and 1300 °C. The results demonstrate that undisturbed epitaxial regrowth of an amorphous surface layer in (0001)-oriented 6H–SiC cannot be achieved by ion irradiation. The shift of the amorphous/crystalline interface observed by Rutherford backscattering spectrometry/channeling analysis is a consequence of columnar growth and surface erosion. The columnar growth starts inside the heavily damaged transition region between the amorphous surface layer and the single crystalline bulk material. It is stopped by random nucleation which is strongly enhanced by ion irradiation in the temperature range between 300 and 1000 °C. Neither the interface roughness nor the kind of impurity atoms influence the thickness of the columnar layer. At higher temperatures, the interface shift is dominated by thermal regrowth and ion beam enhanced surface erosion.
Electrical resistivity of silver–silica nanocomposites
Authors: Banerjee S. , Chakravorty D.
Published: July 2002

Nanoparticles of silver with diameters in the range 10.3–25.7 nm were grown within a silica gel medium by an electrodeposition technique. The dc resistivity of the nanocomposites was measured over the temperature range 100–300 K. The resistivity as a function of inverse temperature shows a maximum at around 175 K. This is explained as arising due to the presence of two conduction mechanisms, viz., an electron tunnelling between metal particles and conduction through a percolated metal structure which is fractal in nature.
Preisach analysis of Nd2Fe14B
Authors: Mitchler P. D. , Roshko R. M. , Dahlberg E. Dan
Published: July 2002

The moment and remanence of a thermally demagnetized sample of sintered, aligned Nd2Fe14B have been measured along the initial magnetizing curve and along the descending branch of the major loop, over an applied field range −12 kOe⩽ha⩽+12 kOe, and over a wide range of temperatures 25 °C⩽T⩽220 °C. Changes in temperature have only a weak influence on the magnetizing process, but have a profound effect on the demagnetizing branch and on the curvature of the Henkel plots, which varies from strongly magnetizing-like at room temperature to strongly demagnetizing-like above 140 °C. We present numerical calculations based on a modified version of the scalar Preisach model, which assumes that moment reversal along the demagnetizing branch is characterized by two distinct sets of energy barriers which must be activated sequentially, so that the availability of one set is conditional upon the activation of the other. The model is able to replicate all of the experimental systematics, and shows that long range, mean field interactions, positive or negative, are absent from this system.
Lorentz microscopy of small magnetic structures (invited)
Authors: Kirk K. J. , Chapman J. N. , Wilkinson C. D. W.
Published: July 2002

Domains and domain walls in micron and submicron sized magnetic elements can be studied at high resolution using Lorentz microscopy in the transmission electron microscope. In situ magnetizing experiments are possible in which magnetization reversal processes can be viewed directly in the presence of varying magnetic fields. These techniques have been used to investigate small magnetic structures fabricated by electron beam lithography on electron transparent membrane substrates. Patterned elements as small as 200 ×40 nm have been imaged magnetically. Detailed studies have been carried out into the properties of high aspect ratio (acicular) elements of Co and a soft NiFe alloy. It has been found that the coercivity increases as the elements become narrower, down to ultrasmall elements with a width of 40 nm. Element length has no effect so long as the aspect ratio is sufficiently high. Magnetization reversal in acicular elements is known to begin from the ends of the elements, therefore the shape of the ends—flat, elliptical, or pointed—has a significant effect on the coercivity. The magnetic environment of an element is also highly important in determining its properties. A one-dimensional array of closely spaced elements has the same average switching field as an isolated element but the spread in values is greatly increased when the gap between elements is made smaller than the width of an element. Adding rows of elements to make a two-dimensional array also has an effect, even if the rows are spaced further apart than the length of the elements.