bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonlinear interactions of particles in chains
Authors: Jones T. B. , Saha Bijay
Published: July 2002

Measurements of the effective electric dipole moments of conducting particle chains suspended in insulating dielectric liquid reveal the influence of dielectric breakdown between particles. This breakdown can lead to reversal of the sign of the interparticle force from attractive to repulsive. Other measurements with a vibrating sample magnetometer of the effective magnetic dipole moments of ferromagnetic particle chains provide clear evidence that chaining enhances nonlinear magnetization at a field intensity about a factor of 10 below the value at which saturation is observed for single particles. Both of these physical situations are examples of how nonlinear effects can influence the electromechanical interactions of closely spaced particles. The nonlinearity is caused by the very strong field intensification that occurs in the gaps between particles for chains parallel to an applied electric or magnetic field. A correspondingly strong nonlinear effect is predicted for the forces between particles in chains.
Shock wave decay and spall strength in laser-matter interaction
Authors: Eliezer S. , Gazit Y. , Gilath I.
Published: July 2002

A new experimental method was developed in order to estimate the decay of the laser-generated shock waves and the dynamic spall pressure. Experiments were performed on aluminum, copper, and unidirectional carbon fiber epoxy composites with impact strain rates of the order of 107 s−1. The following values for dynamic spall pressure and pressure gradient were obtained (to an accuracy of a factor of two): aluminum [25 kb (kb=kilobars), 60 kb/mm]; copper (20 kb, 180 kb/mm); carbon fiber epoxy composite (0.3 kb, 15 kb/mm) perpendicular to the fiber direction; and (7 kb, 100 kb/mm) when the impact is parallel to the fiber direction.
Hexagonal cobalt carbide formed by carbon ion implantation
Authors: Liu B. X. , Wang J. , Fang Z. Z.
Published: July 2002

Thin films of ferromagnetic metals, i.e., bcc Fe, hcp Co, and fcc Ni, were subjected to 50-keV carbon ion implantation at room temperature. At the dose of 2.5×1017 ions/cm2, the formation of hexagonal Fe3C and Ni3C phases was confirmed by transmission electron microscopy selected area electron diffraction patterns; and more interestingly a similar pattern for Co was also observed for the first time. The phase was identified as hexagonal Co3C with a=2.685 Å and c=4.335 Å based on the spacings and intensities of the diffraction rings. The carbide formation was also confirmed by Auger electron spectra. The stoichiometry of the hexagonal structure may be extended in the range of Co3–2C as estimated from the experiments performed up to the dose of 9×1017 ions/cm2.
A Mössbauer-effect study of a series of R2Fe14C hard magnetic materials
Authors: Long Gary J. , Pringle O. A. , Marasinghe G. K. , Grandjean F. , Buschow K. H. J.
Published: July 2002

The 57Fe Mössbauer-effect spectra of the series of hard magnetic materials, R2Fe14C, where R is Nd, Gd, Tb, Dy, Ho, and Lu, have been measured at 295 K. All of these carbides exhibit uniaxial magnetic anisotropy. The spectra resemble those obtained for the related R2Fe14B compounds and have been fit with the model used earlier for Nd2Fe14B. The magnitude of the hyperfine field on each site, as a function of rare earth, parallels the Curie temperature; the maximum hyperfine fields and the maximum Curie temperature are observed for Gd2Fe14C. A linear correlation is observed between the hyperfine fields on the six sites in the analogous borides and carbides; however, these fields are systematically smaller in the carbides. The decrease in the tetragonal unit cell c-axis length in the carbides apparently reduces the exchange interactions between the 8j and 16k iron layers and hence reduces the moments. The isomer shift on each site decreases as the atomic number of the rare earth increases, whereas the quadrupole interactions are independent of rare earth.
Effective conductivity of suspensions of hard spheres by Brownian motion simulation
Authors: Chan Kim In , Torquato S.
Published: July 2002

A generalized Brownian motion simulation technique developed by Kim and Torquato [J. Appl. Phys. 68, 3892 (1990)] is applied to compute ‘‘exactly’’ the effective conductivity σe of heterogeneous media composed of regular and random distributions of hard spheres of conductivity σ2 in a matrix of conductivity σ1 for virtually the entire volume fraction range and for several values of the conductivity ratio α=σ2/σ1, including superconducting spheres (α=∞) and perfectly insulating spheres (α=0). A key feature of the procedure is the use of first-passage-time equations in the two homogeneous phases and at the two-phase interface. The method is shown to yield σe accurately with a comparatively fast execution time. The microstructure-sensitive analytical approximation of σe for dispersions derived by Torquato [J. Appl. Phys. 58, 3790 (1985)] is shown to be in excellent agreement with our data for random suspensions for the wide range of conditions reported here.
Granular magnetic solids (invited)
Authors: Chien C. L.
Published: July 2002

Granular metals consist of nanometer size metal granules embedded in an immiscible medium. They display a rich variety of physical properties as a result of their unique nanostructure and extra degrees of freedom. They are also suitable for the exploration of finite-size effects, enhanced and tailored properties of fundamental interest, and for technological applications. Granular magnetic solids of elemental metals and alloys will be discussed. Single-domain characteristics, superparamagnetic relaxation, enhanced ferromagnetic properties, granular alloys, and spin glasses are some of the topics covered.
The behavior of silicon p-n junction-based devices at liquid helium temperatures
Authors: Simoen E. , Dierickx B. , Deferm L. , Claeys C.
Published: July 2002

In this paper, the forward current-voltage (I-V) characteristics of Si p-n junction diodes, fabricated in different state-of-the-art complementary-metal-oxide-semiconductor (CMOS) technologies, are investigated at liquid helium temperatures. As will be shown, three different I-V regimes can exist: a forward breakdown/hysteresis regime at a turn-on voltage which may be larger than the built-in potential of the junction; a thermionic emission regime, corresponding to a I=A exp(qV/kT) relation and a high-injection space-charge-limited regime, giving rise to a current which is proportional to Vn. The anomalous turn-on behavior can be explained by considering the small barrier for carrier injection, which exists at the ‘‘ohmic’’ contact. It will be demonstrated that the presence of this forward breakdown is strongly determined by the fabrication technology used. In extreme cases (large-area well diodes) multiple breakdown is observed, indicating an inhomogeneous, filamentary current flow. As will be shown, similar breakdown behavior is observed in more complex junction-based Si devices at 4.2 K; the consequences for deep-cryogenic CMOS circuitry will be briefly addressed.
Magnetostriction and magnetomechanical coupling in amorphous rare earth-iron compounds
Authors: Greenough R. D. , Gregory T. J. , Clegg S. J. , Purdy J. H.
Published: July 2002

The effects of additions of the rare earths (RE) terbium and samarium on the room temperature magnetoelastic properties of amorphous Fe0.82B0.18 and Fe0.90Zr0.10 have been studied for up to 50 and 10 at. % RE, respectively. As the RE content in the Fe–Zr compounds is raised, the relative permeability increases, as do the Curie temperature, magnetostriction and magnetomechanical coupling. The magnetic phase diagram forFe–Zr–RE derived from magnetization data indicates the transition to a random anisotropy phase, evident at low temperatures in Fe0.90Zr0.10, rapidly rises to near room temperature with the addition of Tb and approaches the Curie temperature near ∼9 at. % Tb. This pattern of behavior is not seen in the Fe–B–RE compounds in which the exchange dominated behavior at room temperature remains essentially unchanged and is accompanied by reductions in permeability and magnetostriction until more than ∼18 at. % RE is included.
Magnetic properties of Ba2Me2Fe28O46 (Me2-X, Me=Ni, Cu, Mg, and Zn) hexaferrites
Authors: Gu B. X.
Published: July 2002

Polycrystalline samples of Ba2Me2Fe28O46 (Me2-X, Me=Ni, Cu, Mg, and Zn) hexagonal ferrite have been prepared by the classical ceramic method. Low-temperature magnetic properties and Curie temperature were investigated. The substitution of Fe2+ ion with Ni2+, Cu2+, and Mg2+ ions decreases spontaneous moment of Me2-X due to the preference of these ions for the octahedral sites of spinel S blocks. The substitution of Fe2+ ions with Zn2+ ions increases spontaneous moment due to the preference of Zn2+ ions for the tetrahedral sites. Lower Curie temperatures of Zn2-X, Cu2-X and Mg2-X compounds originate from the much small magnetic moment of these ions. Ni2+ ion has a magnetic moment smaller than Fe2+ ion, while Ni2-X compound has a Curie temperature as high as Fe2-X. It may be due to different space distribution of 3d electrons of Ni from Fe.
Self-consistent dc glow-discharge simulations applied to diamond film deposition reactors
Authors: Surendra M. , Graves D. B. , Plano L. S.
Published: July 2002

Self-consistent particle-fluid hybrid simulations have been used to study the structure of hydrogen dc discharges between parallel plates. A Monte Carlo simulation is used to describe individual energetic electrons in the cathode sheath, while the electrons and ions in the low-field region of the discharge are modeled as a fluid. Simulation results for pure H2 discharges at conditions typical in diamond growth (approximately 20–30 Torr at 1000 K, 100–200 A m−2) are in reasonable agreement with optical emission and Langmuir probe measurements. The space potential in these discharges is negative with respect to the anode, unlike comparable discharges at lower pressures. Anode glows are predicted by the simulation and are observed experimentally. Dissociation of H2 in the anode region contributes significantly to the flux of atomic hydrogen to the anode, where diamond is typically grown.
On the Dodson–Tsao excess stress for glide of a threading dislocation in a strained epitaxial layer
Authors: Freund L. B. , Hull R.
Published: July 2002

The concept of excess stress was originally introduced in a study of elastic strain relaxation in an epitaxial layer grown beyond its critical thickness, where it was proposed as the relevant stress in a kinetic law for dislocation motion. The concept has subsequently been applied in a variety of studies of strain relaxation, but without a standard definition as a basis for quantitative comparisons. The purpose here is to propose a fundamental definition of excess stress as the particular stress measure which is work-conjugate to the Burgers displacement during glide of a threading dislocation in a strained layer. This definition also has the feature of being consistent with definitions of effective stress in kinetic laws of glide in bulk materials.
The inflow moments method for the description of electron transport at material interfaces
Authors: Schroeder Dietmar
Published: July 2002

This paper introduces the inflow moments method as a general procedure for the derivation of interface or boundary conditions for advanced models of carrier transport in semiconductor devices. It is based on a general interface condition for the Boltzmann equation accounting for particle and energy balance at material interfaces, as metal-semiconductor contacts or semiconductor heterojunctions. Interface conditions for transport models based on integrations of the Boltzmann equation are consistently derived by the corresponding integrations of the Boltzmann interface condition. The method is illustrated by a treatment of thermionic emission of hot electrons at a semiconductor heterojunction, resulting in interface conditions for the particle as well as the energy balance equation.
Investigations on nanocrystalline Fe78B13Si9 alloys by positron annihilation spectroscopy
Authors: Tong H. Y. , Ding B. Z. , Wang J. T. , Lu K. , Jiang J. , Zhu J.
Published: July 2002

Polycrystalline Fe78B13Si9 alloys with nanoscale grain sizes prepared by the crystallization method have been studied by positron lifetime and Doppler-broadened line-shape measurements. The results obtained are different from those on amorphous or coarse-grained crystalline alloys with the same composition. When the grain sizes are clearly smaller than the mean positron diffusion length (L+≂100 nm), saturation trapping and annihilation of the positrons in the samples should occur at traps within the interfaces. There are two kinds of defects in the interfaces of the nanocrystalline FeBSi alloys, i.e., the free volumes (the size of which is slightly smaller than that of the amorphous counterpart) and the microvoids. The dependence of grain sizes on the type of interface defects and structure-sensitive properties has also been studied by positron spectroscopy. The measurements and comparison of the mean positron lifetime τ̄ and the Doppler parameter S on the nanocrystalline, amorphous, and coarse-grained crystalline alloys have given a satisfactory explanation for the relationship between the structure-sensitive properties, grain sizes, and crystalline states.
Photoemission spectromicroscopy: A new insight in the chemistry of SnOx films for gas sensors
Authors: Sanjinés R. , Coluzza C. , Rosenfeld D. , Gozzo F. , Alméras Ph. , Lévy F. , Margaritondo G.
Published: July 2002

We used the novel experimental technique of photoemission spectromicroscopy to investigate the active area of SnOx thin films for gas sensors, deposited by dc sputtering. First, we analyzed the degree of oxidation and the homogeneity of as-deposited films in a high lateral resolution (30 μm) spectromicroscopy mode with an energy resolution of 0.4 eV. This led to a surprising discovery of a large amount of tin monoxide on the film surface. Then we studied the interaction of H2 and H2O with the surface of polycrystalline SnO2 films used as sensitive layers in actual gas sensor devices. These results have been related to the resistivity changes of the corresponding devices: we found that such changes are primarily due to a reduction process in the film rather than to mere chemisorption.
Structure and magnetic properties of rare-earth iron nitrides, carbides and carbonitrides (invited)
Authors: Altounian Z. , Chen X. , Liao L. X. , Ryan D. H. , Ström-Olsen J. O.
Published: July 2002

Iron-rich rare-earth (R) compounds, such as R2Fe17 do not show great potential for high-performance magnet materials due primarily to their low Curie temperatures (Tc∼300–400 K). However, relatively large quantities of nitrogen or carbon atoms can be introduced into the structure, resulting in a dramatic enhancement of magnetic properties including Tc (≥700 K). The N or C atoms cause a volume expansion of a few percent of the unit cell without changing the crystal structure. The large increase in Tc can be attributed to the volume dependence of the Fe–Fe exchange interactions. A large uniaxial anisotropy field develops for R=Sm upon nitriding/carbiding with an anisotropy field that is almost double the value for Nd2Fe14B at room temperature. Problems including the precipitation of soft magnetic phases (mainly α–Fe) and the limited thermal stability of the nitrides have so far restricted the applications of these compounds. Here data are presented on combined carbide/nitride alloys prepared using a novel technique. These alloys exhibit many of the advantages of the pure compounds but with greater thermal stability and less interference from precipitated phases. A typical material, Sm2Fe17(NxC1−x)3−δ has a Tc of 758 K and an anisotropy field (μ0HA) of 15 T at room temperature.
Formation of amorphous interlayers in ultrahigh vacuum deposited yttrium thin films on (111)Si
Authors: Lee T. L. , Chen L. J.
Published: July 2002

Formation of amorphous interlayers (a interlayers) has been observed in the interfacial reactions of ultrahigh vacuum deposited yttrium thin films on atomically clean (111)Si at low temperatures. The observation of the a interlayer in the Y/Si system represents the first report of solid-state amorphization for rare-earth metal/Si systems. The Y/Si system is also the only system found to date among all metal/Si systems in which the a interlayer can be grown to a thickness exceeding 10 nm during deposition at room temperature. A process involving significant diffusion of both Y and Si atoms is proposed to account for the dependence of amorphization on the thickness of deposited yttrium films.
Monte Carlo simulations of the magnetocaloric effect in superferromagnetic clusters having uniaxial magnetic anisotropy
Authors: Bennett L. H. , McMichael R. D. , Tang H. C. , Watson R. E.
Published: July 2002

The magnetic entropy change, ΔS (hence the heating or cooling) of ferromagnetically coupled magnetic clusters induced by an application of a magnetic field is substantial and occurs over a broad temperature range. This behavior contrasts with a ferromagnetic material, where larger entropy changes can be obtained, but only over small temperature ranges close to the Curie point. Monte Carlo simulations have been carried out to obtain ΔS when the spins are subjected to uniaxial anisotropy fields. While previous calculations for interacting spins showed increases in the Curie temperature and enhancements of ΔS in the vicinity of the Curie point with increasing anisotropy, the present results for interacting clusters show similar increases in the temperature of ΔS peaks accompanied by decreases in the peak values of ΔS. These results show that the flattening of the ΔS peaks, which make superferromagnetic materials valuable for magnetic refrigeration is assisted by the presence of uniaxial anisotropy. Some effects on ΔS of placing the spins on fixed BCC, FCC, and sc lattices are reported.
Polarization dependence of light emitted from GaAs p-n junctions in quantum wire crystals
Authors: Haraguchi K. , Katsuyama T. , Hiruma K.
Published: July 2002

Micro p-n junctions are formed in GaAs nanowhiskers having diameters of about 100 nm by metalorganic vapor phase epitaxy. The result of photoluminescence measurement shows that nanowhiskers with p-n junctions have the same confinement effect as one-dimensional quantum wires, the average effective width of which is about 30 nm. Infrared light emitted from the whisker as a result of carrier injection is observed at 4.2 and 77 K. The most important feature of this emitted light is the dependence of its intensity on the polarization. The intensity of the emitted light whose polarization is perpendicular to the whisker axis is about 20% less than the intensity of light with parallel polarization when the influence of the electrode structure is eliminated. This value is in agreement with the theoretically predicted value of 25%, confirming that a nanowhisker is a kind of quantum wire.
Laser-induced chemical vapor deposition of aluminum from trimethylamine alane
Authors: Popov C. , Ivanov B. , Shanov V.
Published: July 2002

Maskless patterning of aluminum has been achieved by using visible light from a copper bromide vapor laser for pyrolytic decomposition of trimethylamine alane (TMAA) on silicon monocrystalline wafer. The analysis of the resultant stripes included scanning electron microscopy, Auger electron spectroscopy, Talystep, and electrical resistance measurements. The crystalline structure of the layers showed well-defined grains. The Auger electron spectra indicated pure aluminum layers with small quantities of oxygen and carbon. The low resistivities of aluminum stripes deposited from TMAA (up to 4.0 μΩ cm) make it a promising precursor for metallization with aluminum and especially for chemical vapor deposition enhanced with a pulsed visible laser.
Characterization of reactively evaporated TiN layers for diffusion barrier applications
Authors: Gagnon G. , Currie J. F. , Béïque G. , Brebner J. L. , Gujrathi S. C. , Ouellet L.
Published: July 2002

Thin TiN layers have been successfully produced on Si and SiO2 by reactive evaporation combined with rapid thermal annealing. Results of composition, resistivity, and stress measurements on these layers are reported. The TiN layers have a resistivity around 40 μΩ cm and a high stress of between 1 and 6 GPa. The composition ratio of nitrogen to titanium, measured by elastic recoil detection (ERD), combined with time-of-flight, was found to vary between 0.8 and 1.0 depending on the deposition conditions. In addition to the stoichiometry determination, ERD also clearly shows the presence of a TiSi2 layer between the TiN and the Si substrate. It is also shown that good TiN layers can be produced by reactive evaporation for nitrogen partial pressures between 1.0 and 2.0×10−5 mbar and for titanium evaporation rates between 0.3 and 0.5 nm/s.
Light-emission phenomena from porous silicon: Siloxene compounds and quantum size effect
Authors: Lee H.-J. , Seo Y. H. , Oh D.-H. , Nahm K. S. , Hahn Y. B. , Jeon I. C. , Suh E.-K. , Lee Y. H. , Lee H. J.
Published: July 2002

It has long been argued whether the luminescent mechanism of anodized porous silicon is mainly due to the chemical compounds such as siloxene derivatives, or the quantum size effect. We performed a comprehensive study using atomic force microscope, infrared transmission, Raman scattering, and photoluminescence measurements in terms of various annealing temperatures. Low - temperature photoluminescence spectra have also been observed. This leads us to conclude that not only the siloxene derivatives but also the quantum size effect gives the luminescence in porous silicon. The previous pseudopotential calculations are used for the explanation of our experimental results.
Magnetic ordering and electric polarizability of Zn-doped La2CuO4+δ single crystals
Authors: Cao G. , O’Reilly J. W. , Crow J. E. , Testardi L. R.
Published: July 2002

Magnetization, magnetic susceptibility, and low-frequency dielectric constant of La2Cu1−xZnxO4+δ (x=0, and 0.07) single crystals were measured. Results obtained in this work show that Zn doping eliminates spins from CuO planes, thus, reducing the correlation length, ξ2D. However, unlike Sr doping, magnetic dilution due to Zn doping is a very local effect, thus suppresses TN relatively slowly and shows little impact on metamagnetic behavior. Our data marks a difference from some results previously reported which claimed a stronger effect on Zn doping on the magnetic properties. It is interesting that Zn doping shows a large influence on the dielectric properties, and resulting in a significant change in the dielectric constant Kc. Most remarkably, the simultaneous polarization and magnetization ordering, described in this study, suggests a coupling of magnetic ordering and electric polarizability in the oxides.
New permanent magnetic MnBiDy alloy films
Authors: Ruiyi Fang , Qingqing Fang , Sheng Zhang , Chubing Peng , Daosheng Dai
Published: July 2002

We have studied the magnetic properties of MnBiDy films fabricated by the thermal coevaporation method and annealed subsequently in a vacuum oven at temperatures of 350 °C–425 °C. The thicknesses of these films are in the range from 60 to 1200 nm. X-ray diffraction analyses have confirmed that those samples have an hcp structure. Magnetic measurements revealed that the saturation magnetization σs=46–78 emu/g, remanence ratio R=0.67–0.98, coercive force μHc=2.2–7.3 kOe, and magnetic energy product (BH)m=3.3–14.3 MGOe. These values are sensitive to the film thicknesses. It was found that μHc reaches a maximum at d≊400 nm.
On the sublattice location of GaAs grown on Ge
Authors: Li Yuan , Lazzarini L. , Giling L. J. , Salviati G.
Published: July 2002

Single domain GaAs layers have been grown by atmosphere pressure metal-organic vapor phase epitaxy on Ge(100) substrates misoriented to (111) with different angles of 0°–4°, under various growth conditions. Epilayers have been studied by transmission electron microscopy, molten KOH etch and optical interference contrast microscopy. It is found that at an initial growth temperature of 550 °C the sublattice location of the GaAs layers grown on substrates with small misorientation angles (less than 3°) is reversed as compared to that of the layers grown on substrates with larger misorientation angles, independent of the initial growth rates and V/III ratios. When the initial growth temperature is increased the transition from one type of sublattice location to the other occurs at a lower misorientation angle, while at an initial growth temperature of 700 °C the sublattice location of the layers grown on the different substrates becomes the same. These results can hardly be explained by the existing theories and a new model is proposed based on a concept that the sublattice location of GaAs on Ge is defined by the relative intensity of nucleation at steps and on terraces between steps, taking into account the effects of the growth temperature and the step density of the substrate surface on the nucleation mode, and the fact that single domain GaAs can be obtained by the self-annihilation of antiphase boundaries.
Photoluminescence-excitation analysis of Er-doped GaAs grown by metalorganic vapor phase deposition
Authors: Takahei K. , Taguchi A.
Published: July 2002

Er-doped GaAs grown by metalorganic chemical vapor deposition was studied by photoluminescence and photoluminescence excitation (PLE) measurements using a color center laser that provides tunable light in the 1450–1550 nm range. The light in this wavelength region can directly induce the 4I15/2→4I13/2 intra-4f-shell transition of Er3+ ions. The PLE spectra of a specific Er center formed by oxygen codoping under suitable growth conditions were conclusively assigned by site-selective spectroscopy. Non-site-selective PLE spectroscopy, on the other hand, revealed that the atomic configurations of Er3+ ions change according to the Er concentration, growth temperature, and oxygen codoping. Sharp lines related to Er3+ ions with well-defined atomic configurations, and inhomogeneously broadened structures related to aggregates of Er3+ ions, are evident in the PLE spectra. The PLE spectra due to the direct intra-4f-shell excitation also revealed semiquantitatively that there are Er centers that do not show luminescence under above-band gap photoexcitation, because of low energy transfer efficiency from the host.
Raman scattering studies of surface modification in 1.5 MeV Si-implanted silicon
Authors: Huang X. , Ninio F. , Brown L. J. , Prawer S.
Published: July 2002

The surface layer of crystalline silicon implanted with 1.5 MeV Si ions with doses ranging from 1×1011 to 1×1015 Si+/cm2 has been studied by Raman scattering. Raman line intensities, shapes and shifts have been used to investigate the defects in the near-surface layer. Above doses of 1×1013 Si+/cm2, Raman provides evidence for the presence of amorphous silicon islands within the crystalline structure. The phonon-confinement model (PCM) which is based on the breakdown in wavevector selection rules due to scattering from finite domain sizes has been used to estimate the effective average distance between defects. The PCM has also been modified by introducing a term attributable to residual stress in the near-surface layer. By fitting experimental Raman spectra with the modified PCM, the Raman line shifts due to stress effects are decoupled from those due to phonon confinement. The mechanisms for the stress creation are also discussed.
Oxidation of H-terminated Si(100) surfaces studied by high-resolution electron energy loss spectroscopy
Authors: Ikeda H. , Hotta K. , Yamada T. , Zaima S. , Iwano H. , Yasuda Y.
Published: July 2002

The adsorption of oxygen on H-terminated Si(100) surfaces has been investigated by high-resolution electron energy loss spectroscopy (HREELS). Adsorptions of atomic oxygen occur even at room temperature. Si-OH stretching and Si-O-Si (B1) vibrational modes are observed in HREELS spectra, which indicates that atomic oxygen is adsorbed on sites of Si—H bonds and Si—Si back bonds. On the other hand, H-terminated surfaces are very stable for molecular oxygen, which cannot adsorb until 380 °C on the surface. A dissociative adsorption of molecular oxygen is observed above 380 °C and the activation energy of the adsorption is 2.0 eV at 380–450 °C. This value coincides with the desorption energy of hydrogen atoms from a Si(100) surface with the monohydride phase. These results indicate that the dangling bonds are essential to the adsorption of molecular oxygen on Si(100) surfaces.
Quantum confinement effects of CdS nanocrystals in a sodium borosilicate glass prepared by the sol-gel process
Authors: Mathieu H. , Richard T. , Allègre J. , Lefebvre P. , Arnaud G. , Granier W. , Boudes L. , Marc J. L. , Pradel A. , Ribes M.
Published: July 2002

Experimental evidences of both weak and strong confinement regimes are reported on CdS nanocrystals embedded in a sodium borosilicate glass matrix. A method, based on the sol-gel technique, is used for the preparation of CdS-activated glass. This route is capable of providing nanocrystals covering a wide range of radii with small size dispersion. Low-temperature linear-absorption spectra have been analyzed in terms of excitons and electron-hole confinements by fitting the results of a numerical calculation to experimental findings. The model used, in the envelope-function formalism, involves both a Lorentzian broadening of the exciton energy states inside each nanocrystal and a Gaussian size distribution. The improvement of crystal quality and the sharpening of the size distribution by thermal annealing is also studied versus both time and temperature of treatment. It is shown that we can keep a tight control on the crystallinity, average size, and size distribution of the nanocrystals by rather simple adjustments and short treatments.
Laser-induced crystallization phenomena in GeTe-based alloys. I. Characterization of nucleation and growth
Authors: Coombs J. H. , Jongenelis A. P. J. M. , van Es-Spiekman W. , Jacobs B. A. J.
Published: July 2002

The laser-induced crystallization behavior of GeTe-based amorphous alloy thin films has been quantitatively studied by local reflection measurements with a focused 780 nm laser. The use of multiple laser pulse sequences enables the nucleation rate and crystal-growth speed to be separately deduced, allowing the compositional variation of both these processes to be followed. This not only gives detailed information on the crystallization mechanism, but also allows the fine tuning of phase change alloy compositions for use in erasable optical recording. The differences between the as-deposited and melt-quenched amorphous phases are also discussed. In particular, it is shown that the crystallization speed of the as-deposited layer can differ by over an order of magnitude from that of the melt-quenched amorphous layer. The as-deposited state can, however, be transformed into a modified amorphous state equivalent to that obtained by melt quenching a previously crystalline layer. This allows the determination of the optical constants and crystallization speeds of the amorphous state written during optical recording.
Instability of field emission from silicon covered with a thin oxide due to electron trapping
Authors: Huang Qing-An
Published: July 2002

Based on the method for the derivation of the Fowler–Nordheim equation, a relatively simple expression for field emission from silicon covered with a thin oxide is presented. The current flows from silicon through the thin oxide during field emission. Some of the electrons flowing through the thin oxide may be captured by the electron traps within it. A model has been developed on the assumption that only the electron trapping within the oxide is considered. The model is successfully applied to explain the instability of field emission from the oxide-covered silicon. The model is also compared with available experiments, and good qualitative agreement is achieved.
Disorder and noncollinear magnetism in permanent-magnet materials (invited)
Authors: Lorenz R. , Hafner J.
Published: July 2002

We report calculations of the noncollinear magnetic structures and of the magnetic anisotropies of YFe12−xMox permanent magnets with the ThMn12 structure, using a novel variant of a spin-polarized tight-binding-linear-muffin-tin-orbital technique allowing for local spin-quantization axes on each site and considering spin–orbit coupling. The ternary YFe12−xMox compounds crystallize in the tetragonal ThMn12 structure which can be stabilized only by the partial substitution of Fe by an early transition metal like Mo. We show that the substitutional disorder leads to canted spin-structures at low Mo content (x∼1) and to spin-glass-like behavior at higher Mo content (x∼3). We also find that the character of the anisotropy changes from uniaxial to planar with higher Mo content.
Microstructure and composition in rapidly quenched NdFeB-based hard magnet alloys
Authors: Nguyen Tai D. , Krishnan Kannan M. , Lewis Laura Henderson , Zhu Yimei , Welch David O.
Published: July 2002

A detailed study of the microstructure and composition in hot-pressed (MQ-2) and die-upset (MQ-3) magnet alloys based on the Nd2Fe14B composition, utilizing high resolution and analytical transmission electron microscopy, is reported. The initial magnetic properties of the two samples show different behaviors, which are attributed to the difference in the anisotropy of the grain structure and the grain boundaries. The hot-pressed sample shows faceted grains of the 2-14-1 phase, while die-upset sample shows plate-like grains, together with larger equiaxed grains that contain a speckling of precipitates in the grain interior. The grain structure and composition remain rather similar in the two samples. The grain boundary phase averages ∼1–≳10 nm in width. The thicker grain boundaries are Nd-rich, while the thinner grain boundaries in the hot-pressed sample exhibit an Fe-rich composition near that of the NdFe3 phase. Nd-rich phases are found at the grain boundary junctions of both samples, with the Nd:Fe ratio near 7:3 in the die-upset sample, and up to 3:2 in the hot-pressed sample. The significance of the microstructure and the grain boundary phases on the magnetic behavior in the two samples is discussed.
Effect of Fe and/or Cu contents on the intrinsic coercivity of Sm2Co17-type coercive powders for the bonded magnet application
Authors: Liu W. L. , Liang Y. L. , Scott D. W. , Ma B. M. , Bounds C. O.
Published: July 2002

The microstructure of conventionally cast Sm(CobalFe0.28CuvZr0.022)8.0 alloys, where v=0.043–0.092, were examined by optical microscopy and scanning electron microscope with energy dispersed analytical x ray in the as-cast state. In addition to the regular 2:17 matrix phase, the Sm-rich grain boundary phase and the needle-like Zr-rich phases are present in most commercial Sm(Co,Fe,Cu,Zr)z alloys. A grayish Sm- and Cu-rich grain boundary phase was found in alloys with v≥0.07. Attention was focused on the impact of increasing Cu content to the microstructure and process parameters required to produce coercive powder for bonded magnets. An increase of the Cu content from the v value from 0.043 to 0.092 shifts the liquid+2:17→TbCu7-type transformation temperature from slightly above 1180 to approximately 1140 °C. The Hci of optimally prepared magnet was found to increase significantly while the Br remained relative constant when the Cu content is increased. When the Cu content was increased to beyond v=0.07, a slight decrease in the BHmax was noticed. A magnetizing field of 15 kOe and more than 50 kOe were determined to be necessary to charge magnets with a Hci of 12 and 25 kOe, respectively, to about 90% of their full potential. For a composition of Sm(CobalFeuCu0.07Zr0.022)8.0, a slight increase in Fe concentration from u=0.22 to 0.28 was found to increase the Br of an optimally prepared magnet from 8.07 to 8.28 kG and decrease the Hci from 11.8 to 9.7 kOe. A BHmax of 15 MGOe was obtained on a bonded magnet with a composition of Sm(CobalFe0.28Cu0.07Zr0.022)8.0 when the data are normalized to a specific density of 7.0 g/cm3.
Studies of vibrating atomic force microscope cantilevers in liquid
Authors: Schäffer T. E. , Cleveland J. P. , Ohnesorge F. , Walters D. A. , Hansma P. K.
Published: July 2002

An atomic force microscope (AFM) design providing a focused spot of order 7 μm in diameter was used to analyze the motion of vibrating cantilevers in liquid. Picking an operating frequency for tapping mode AFM operation in liquid is complex because there is typically a large number of sharp peaks in the response spectrum of cantilever slope amplitude versus drive frequency. The response spectrum was found to be a product of the cantilever’s broad thermal noise spectrum and an underlying fluid drive spectrum containing the sharp peaks. The geometrical shape of transverse cantilever motion was qualitatively independent of the fluid drive spectrum and could be approximately reproduced by a simple theoretical model. The measurements performed give new insights into the behavior of cantilevers during tapping mode AFM operation in liquid.
The metastable changes of the trap spectra of CuInSe2-based photovoltaic devices
Authors: Igalson M. , Schock H. W.
Published: July 2002

Spectra of hole and electron traps of CuInSe2/CdS/ZnO photovoltaic devices have been investigated using deep-level transient spectroscopy. A decrease of the concentration of shallow electron traps and an increase of the hole trap concentration after an injection of electrons has been observed. The effect is metastable below 200 K. A proposed explanation is based on the idea that both levels belong to the same defect in a different charge state. A resemblance of the phenomena related to that defect and to ‘‘dangling bond’’-type centers in amorphous semiconductors has been indicated. Some consequences of defect conversion for current transport and performance of photovoltaic devices have been discussed.
Simulation of the effects of grain structure and grain growth on electromigration and the reliability of interconnects
Authors: Knowlton B. D. , Clement J. J. , Thompson C. V.
Published: July 2002

Electromigration-induced failure of on-chip interconnects continues to be a problem for the microelectronics industry. The electromigration failure rate of a population of similar lines is sensitive to statistical variation in the microstructural characteristics from line to line. These microstructural details depend upon the process and thermal history of the line as well as on the feature size (line length and width). Moreover, as the widths of interconnect lines have fallen below the median grain size of the films from which they are patterned, two distinct types of failure mechanisms have been observed. Determining which failure mechanism(s) will be observed, and in what proportion, is a crucial concern of reliability engineers. This article investigates the complex dependence of the dominant failure mechanism and overall reliability on process history, minimum feature size, line type, line microstructure, and line test conditions. A grain growth simulator has been used to model the microstructural evolution of a film during processing, both prior to and after patterning. In this fashion, large populations of lines with realistic microstructures have been generated in order to observe statistical differences in microstructure and failure rate. An electromigration model is then used to calculate stress evolution. By assuming a critical stress at which the line fails, the failure distributions and overall reliability of the interconnects are obtained, and the conditions under which a transition-in-failure mechanism will occur are predicted.
Systematic study of molecular beam epitaxy growth and magnetic properties of Fe on Au(111)
Authors: Xu J. , Howson M. A. , Hucknall P. , Hickey B. J. , Venkataraman R. , Hammond C. , Walker M. J. , Greig D.
Published: July 2002

We have grown a series of Fe samples on Au(111) by molecular beam epitaxy. Fe was grown on Au at 30 °C and shows clear reflection high energy electron diffraction (RHEED) oscillations up to 8 monolayers, suggesting layer-by-layer growth on Au. A combined study of RHEED, medium energy ion scattering, and x-ray diffraction scans has strongly indicated that Fe initially grew as fcc(111) on Au(111) below the thickness of 3 monolayer and for the subsequent growth the Fe bcc(110) structure begins to form. Magneto-optical Kerr effect measurement has shown that the magnetization easy axis lies in the plane of the film for the thickness range from 3 to 20 Å and there is a perpendicular anisotropy when the thickness of Fe is about one monolayer.
Photorefractive effect in doped Pb5Ge3O11 and in (Pb1−xBax)5Ge3O11
Authors: Yue Xuefeng , Mendricks S. , Hu Yi , Hesse H. , Kip D.
Published: July 2002

The photorefractive effect is studied in ferroelectric lead germanate crystals Pb5Ge3O11, including undoped, Fe- and Rh-doped crystals, as well as (Pb1−xBax)5Ge3O11 solid solutions. Two kinds of processes are involved in photorefractive interactions: a fast response with a time constant generally less than 1 s and the formation of slow gratings with time constants of several hours for intensities in the range of 1–30 kW/m2. Basic photorefractive parameters corresponding to the fast response, such as dark- and photoconductivities, the sign of the main charges involved in the transport process, effective trap densities, and activation energies are determined. Compared to theoretical predictions the measured effects are too small which is attributed mainly to electron-hole competition.
Neutron and x-ray reflectivity studies of self-assembled heterostructures based on conjugated polymers
Authors: Tarabia M. , Hong H. , Davidov D. , Kirstein S. , Steitz R. , Neumann R. , Avny Y.
Published: July 2002

Neutron and x-ray reflectivity studies of modulated heterostructures consisting of alternate layers of conjugated and nonconjugated polymers is described. Such heterostructures are currently being used to fabricate polymer-based light emitting diodes. The heterostructures were prepared by the layer-by-layer self-assembly technique using the precursor of the conjugated polymer, deuterated poly(phenylenevinylene) (D-PPV), and other polyelectrolyte spacers. Heat treatment after the layer assembly converted the pre-D-PPV to a conjugated semiconducting polymer. For the first time in such heterostructures, we were able to observe quasi-Bragg reflections (up to the third order) due to the formation of ordered modulated structures. Both the neutron and the x-ray data were analyzed using the same layer-by-layer model and the same fitting procedure with consistent results. Most importantly, the model and the fitting procedure yield the buried interlayer roughness at the D-PPV/spacer interface. This roughness parameter, of the order of 12±3 Å, was found to be smaller than the thickness of the D-PPV and the spacer layers, suggesting that the length over which interdigitation between neighboring polymer layers occurs is significantly smaller than the spacer layer. We demonstrate that the conversion to conjugated polymer by the heat treatment leads to ∼7% reduction of the repeat unit and the film thickness without significant changes of other structural properties; in fact, the interfacial roughness was somewhat improved. The fabrication of high quality modulated structures with controlled layer thickness and relatively small interfacial roughness may be a first step towards polymer-based multiquantum wells analogous to such devices in inorganic heterostructures.
Soft magnetic properties of LaCo13 and La(Co, Fe)13 alloys
Authors: Huang M. Q. , Wallace W. E. , McHenry M. E. , Chen Q. , Ma B. M.
Published: July 2002

LaCo 13 and La(Co, Fe)13 alloys have been prepared and studied as high temperature, high performance soft magnetic materials. The dc magnetic properties have been measured over a temperature range of 10–1273 K in fields of 0 to 5 T. Data obtained show that the dc magnetic properties (Hc,K1) of the La(Co, Fe)13 bulk alloys are comparable with Fe bulk alloy and Tc=1021–1297 K are higher than that of Fe and Hiperco; ac magnetic properties are first time reported in a bulk LaCo13 alloy, showing a reasonably higher power loss than that of commercial Hiperco alloy. The ac magnetic properties can be improved by making LaCo13 alloys as laminated thin sheets.
Fatigue of Pb(Zr0.53Ti0.47)O3 ferroelectric thin films
Authors: Du Xiaofeng , Chen I.-Wei
Published: July 2002

Fatigue of Pb(Zr0.53Ti0.47)O3 ferroelectric thin films has been studied with several novel electrode modifications. Doped amorphous silicon is used as a gating layer between the ferroelectric and metal electrode to regulate the type of charge carrier injection during switching of the ferroelectric. This configuration requires a low switching frequency in order to avoid the relaxation effect that arises from the limited charging ability of the semiconductor. In addition, charge depletion in the semiconductor causes a polarization asymmetry that reflects a depolarization field due to incomplete compensation of the polarization charge. Fatigue tests reveal that electron injection is a necessary condition for polarization degradation, whereas hole injection is not. Blocking interfaces by SiO2 or c-oriented Bi4Ti3O12 can also mitigate fatigue, albeit at a reduced polarization level. On the other hand, compensation of Pb loss through PbO undercoat layer has little effect on fatigue. Based on these observations, injected electrons in association with oxygen vacancies generated during cycling are proposed as the defects responsible for fatigue. These defects may impede domain switching by segregation at the electrode interface or at domain boundaries, with an enhanced kinetics because of the lower valence of the partially de-ionized entity.
Magnetic interactions within patterned cobalt nanostructures using off-axis electron holography
Authors: Dunin-Borkowski R. E. , McCartney M. R. , Kardynal B. , Smith David J.
Published: July 2002

The magnetization reversal behavior of submicron-sized patterned Co elements has been studied using off-axis electron holography. Magnetic fields were applied to the samples in situ to determine both the hysteresis loops of individual elements and the interactions between closely spaced elements. The observations show that an understanding of the formation of remanent states is likely to be of particular importance for the design of magnetic elements for device applications.
Role of small atoms in the formation and properties of Zr–Ti–Cu–Ni–Be bulk amorphous alloys
Authors: Wang Wei-Hua , Bai Hai Yang
Published: July 2002

The formation, thermal stability and mechanical properties of ZrTiCuNiBe bulk amorphous alloy were investigated by means of x-ray diffraction, differential scanning calorimetry and microhardness measurements. The amorphous alloys Zr41Ti36.5−xCu12.5Ni10Bex in the composition range 15⩽x⩽32 at. % were prepared to investigate the role of Be in the glass forming ability. It is found that the formation, crystallization, and thermal properties of the amorphous alloy are sensitive to the addition of small sized atoms of Be. The amorphous alloy becomes more stable with increasing Be contents. A small amount of carbon addition can further extend the supercooled liquid range and enhance the strength of the alloy. More carbon additions result in the formation of a crystalline/amorphous mixture accompanied by a pronounced increase in strength. The role of Be and C in the excellent glass former alloys is discussed in terms of the thermodynamic and microstructural aspects. The decomposition phenomenon in the Zr41Ti14Cu12.5Ni10Be22.5 alloy is interpreted based on the experimental results. The effects of carbon on the strength of the alloy are discussed from a structural point of view.