bluuebunny/crossref_metadata_2025 huggingface dataset:
Electrophoretic deposition of textured YBa2Cu3O7−x films on silver substrates
Authors: Hein M. , Müller G. , Piel H. , Ponto L. , Becks M. , Klein U. , Peiniger M.
Published: July 2002

We have deposited 10–20-μm-thick YBa2Cu3O7−x films by an electrophoretic process on silver substrates in a magnetic field of 8 T. X-ray diffraction patterns of these 2.4-cm-diameter samples show a high degree of c-axis texturing perpendicular to the surface. Their microwave surface resistance Rs was measured at 21.5 GHz in the temperature range from 4.2 to 300 K. It was found to drop sharply at 92.4 K to reach 18±3 mΩ at 77 K and less than 3 mΩ at 4.2 K. These first results exceed the values obtained from bulk samples and untextured thick films, and are lower than Rs of pure copper at the same frequency and temperature. The electrophoretic deposition technique is applicable to large substrates of nonplanar geometry and is therefore suitable for applications of high-Tc superconductors in the field of microwave technology.
Electron multiplication in ZnS-type electroluminescent devices
Authors: Bringuier E.
Published: July 2002

In this paper the influence of electron multiplication in ZnS-type electroluminescence ac devices is studied in a model where the ZnS layer is free from bulk defects. The holes are assumed to recombine with electrons at the interfaces: The two limiting cases of slow and fast hole-electron recombination rate are treated in some detail. The kinetic equations for the electric field and the filling level of interface electrons are established then the charge-voltage characteristics are obtained numerically. An anomalously steep charge-voltage characteristic may be observed for high multiplication rates and slow recombination. In all cases the field is shown to be delayed relative to the current: This phase relationship is related to bulk ideality and allows for qualitative comparisons with experimental data.
Internal bias in acceptor-doped BaTiO3 ceramics: Numerical evaluation of increase and decrease
Authors: Lohkämper R. , Neumann H. , Arlt G.
Published: July 2002

The buildup of the internal bias in time and the decrease of the internal bias by forced switching of the polarization by an ac field have been described empirically by various authors. Both time dependencies can now be derived from a simple model, proposed recently by G. Arlt and H. Neumann [Ferroelectrics 87, 109 (1988)]. In this model an oxygen vacancy is a mobile particle in the oxygen octaeder which encloses an acceptor ion on the Ti site. The associate acceptor/oxygen vacancy is an electric and elastic dipole which can orient in the polarized ferroelectric material. The differential equation for Ei(t) is solved for the increase and for the forced decrease of the internal bias. The two solutions are completely different time laws. They agree well with former and new observations. Characteristic parameters of the model can be derived from experimentally observed time dependencies.
Gallium arsenide and other compound semiconductors on silicon
Authors: Fang S. F. , Adomi K. , Iyer S. , Morkoç H. , Zabel H. , Choi C. , Otsuka N.
Published: July 2002

The physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed in this paper. The nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail. The properties of devices fabricated in GaAs and other compound semiconductors grown on Si substrates are discussed in comparison with those grown on GaAs substrates. The advantages of GaAs and other compound semiconductors on Si, namely, the low cost, superior mechanical strength, and thermal conductivity, increased wafer area, and the possibility of monolithic integration of electronic and optical devices are also discussed.
A globally convergent algorithm for the solution of the steady-state semiconductor device equations
Authors: Korman Can E. , Mayergoyz Isaak D.
Published: July 2002

An iterative method for solving the discretized steady-state semiconductor device equations is presented. This method uses Gummel’s block iteration technique to decouple the nonlinear Poisson and electron-hole current continuity equations. However, the main feature of this method is that it takes advantage of the diagonal nonlinearity of the discretized equations, and solves each equation iteratively by using the nonlinear Jacobi method. Using the fact that the diagonal nonlinearities are monotonically increasing functions, it is shown that this method has two important advantages. First, it has global convergence, i.e., convergence is guaranteed for any initial guess. Second, the solution of simultaneous algebraic equations is avoided by updating the value of the electrostatic and quasi-Fermi potentials at each mesh point by means of explicit formulae. This allows the implementation of this method on computers with small random access memories, such as personal computers, and also makes it very attractive to use on parallel processor machines. Furthermore, for serial computations, this method is generalized to the faster nonlinear successive overrelaxation method which has global convergence as well. The iterative solution of the nonlinear Poisson equation is formulated with energy- and position-dependent interface traps. It is shown that the iterative method is globally convergent for arbitrary distributions of interface traps. This is an important step in analyzing hot-electron effects in metal-oxide-silicon field-effect transistors (MOSFETs). Various numerical results on two- and three-dimensional MOSFET geometries are presented as well.
Magnetic force microscopy: General principles and application to longitudinal recording media
Authors: Rugar D. , Mamin H. J. , Guethner P. , Lambert S. E. , Stern J. E. , McFadyen I. , Yogi T.
Published: July 2002

This paper discusses the principles of magnetic force microscopy (MFM) and its application to magnetic recording studies. We use the ac detection method which senses the force gradient acting on a small magnetic tip due to fields emanating from the domain structure in the sample. Tip fabrication procedures are described for two types of magnetic tips: etched tungsten wires with a sputter-deposited magnetic coating and etched nickel wires. The etched nickel wires are shown to have an apex radius on the order of 30 nm and a taper half-angle of approximately 3°. Lorentz-mode transmission electron microscopy of the nickel tips reveals that the final 20 μm is essentially single domain with magnetization approximately parallel with the tip axis. Images of written bit transitions are presented for several types of magnetic media, including CoPtCr, CoSm, and CoCr thin films, as well as γ-Fe2O3 particulate media. In general, the written magnetization patterns are seen with high contrast and with resolution better than 100 nm. A number of magnetic recording applications are discussed, including the investigation of overwrite behavior and the writing characteristics in CoSm media at high data density. Computer calculations were performed to simulate the MFM response to written magnetic transitions. By including the extended geometry of the tip, the nonparallel orientation of the cantilever, and the finite width of the magnetic transitions, good agreement with experiment was obtained. The model calculations correctly predict the experimentally observed change in image contrast that occurs as a function of tip orientation. Computer calculations showing the dependence of resolution on tip geometry are also presented.
Device modeling of ferroelectric capacitors
Authors: Miller S. L. , Nasby R. D. , Schwank J. R. , Rodgers M. S. , Dressendorfer P. V.
Published: July 2002

A physically based methodology is developed for modeling the behavior of electrical circuits containing nonideal ferroelectric capacitors. The methodology is illustrated by modeling the discrete ferroelectric capacitor as a stacked dielectric structure, with switching ferroelectric and nonswitching dielectric layers. Electrical properties of a modified Sawyer–Tower circuit are predicted by the model. Distortions of hysteresis loops due to resistive losses as a function of input signal frequency are accurately predicted by the model. The effect of signal amplitude variations predicted by the model also agree with experimental data. The model is used as a diagnostic tool to demonstrate that cycling degradation, at least for the sample investigated, cannot be modeled by the formation of nonswitching dielectric layer(s) or the formation of conductive regions near the electrodes, but is consistent with a spatially uniform reduction in the number of switching dipoles.
Exchange resonance modes in a ferromagnetic sphere
Authors: Aharoni Amikam
Published: July 2002

Magnetostatic resonance modes are usually calculated by assuming that the exchange energy is negligible compared to the magnetostatic energy. It is shown that in the other extreme, when the magnetostatic energy is small compared to the exchange energy, the resonance modes are a quadratic function of the reciprocal size.
rf glow discharge of SiF4-H2 mixtures: Diagnostics and modeling of the a-Si plasma deposition process
Authors: Bruno G. , Capezzuto P. , Cicala G.
Published: July 2002

Hydrogenated and fluorinated amorphous silicon films (a-Si:H,F) have been deposited in a parallel-plate rf plasma reactor fed with SiF4-H2 mixtures. The plasma phase characterization has been performed by optical emission spectroscopy for the analysis of the emitting species (SiF*3,SiF*2,SiF*,H*, and H*2), mass spectrometry for the analysis of stable species, and Langmuir electrical probes for the evaluation of electron density (ne) and temperature (kTe). The deposition rate (rD) has been monitored by laser interferometer. The effect of the rf power, gas composition, total pressure, and dopant (B2H6, PH3) addition on the plasma phase composition and on the film growth rate has been studied. The data have been discussed on the basis of a chemical model where the chemisorption of SiF4 and SiF2 and the subsequent interaction with H atoms are the determinant steps also from the kinetic point of view. It has been found that the rate equation rD ∝ [H][SiF2] is able to fit the experimental results.
Ultrafine particles of Fe, Co, and Ni ferromagnetic metals
Authors: Gong Wei , Li Hua , Zhao Zhongren , Chen Jinchang
Published: July 2002

Fe, Co, and Ni ultrafine particles (UFP) were prepared by a gas evaporation method in an inert atmosphere. The different preparing conditions of the Fe, Co, and Ni UFP included inert gas pressures and kinds were investigated by means of x-ray diffraction, transmission electron microscopy (TEM), and magnetic measurements with low or high temperatures. The experimental results show that coercivities Hc of the Fe, Co, and Ni UFP, depending on particle sizes, obey a single-domain theory. The maximum Hc was 450, 1000, and 1500 Oe, respectively, for Ni (310 Å), Fe (210 Å), and Co (200 Å) at room temperature. The coercivities came up to high in the low temperatures. The saturation magnetizations Ms of the Fe, Co, and Ni UFP decreased with small particle sizes. A face-centered structure of Co UFP (200–300 Å) was confirmed by x-ray diffraction. Some Fe UFP in the shape of balls (200–400 Å) observed in the TEM may be an amorphous structure.
Performance of thin hydrogenated amorphous silicon thin-film transistors
Authors: Kanicki J. , Libsch F. R. , Griffith J. , Polastre R.
Published: July 2002

In this paper we have analyzed the influence of the mask channel length (LM) on the performance of the 55-nm-hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs), incorporating nitrogen-rich hydrogenated amorphous silicon nitride gate dielectric and phosphorus-doped microcrystalline silicon (n+μc-Si:H) source/drain (S/D) contacts. In our TFTs the n+μc-Si:H S/D contacts have a specific contact resistance around or below 0.5 Ω cm2. We have shown that in our TFTs a field-effect mobility and threshold voltage are dependent on LM, and this dependence is most likely due to the influence of the S/D contact series resistance on TFTs characteristics. Finally, we have demonstrated that if the mask channel length is extended by a ΔL (which is a distance from the S/D via edge at which the electron injection/collection is taking place) the field-effect mobility and threshold voltage are independent of the channel length. In such a case μFE, VT, and ON/OFF current ratio around 0.76 cm2/V s, 2.5 V, and 107, respectively, has been obtained.
Cation distribution and intrinsic magnetic properties of Co-Ti-doped M-type barium ferrite
Authors: Batlle X. , Obradors X. , Rodríguez-Carvajal J. , Pernet M. , Cabañas M. V. , Vallet M.
Published: July 2002

The structural and magnetic properties of BaFe12−2xCoxTixO19 (0≤x≤1.0) M-type barium ferrite have been investigated by means of neutron powder diffraction and high-field magnetization measurements. The cationic distribution and magnetic moments of the five different metallic sublattices are determined and compared to the experimental saturation magnetization. It is found that about 50% of Co ions occupy tetrahedral sites, thus being ineffective in the reduction of the uniaxial magnetic anisotropy, while Ti ions prefer the 4fVI octahedral sites. All in all, a clear hierarchy of sublattice preferences is defined. Although the collinearity of the magnetic structure is progressively broken, mainly above x≊0.7, the overall behavior remains ferrimagnetic. The strong local spin canting is tentatively attributed to the localization of diamagnetic Ti ions in the 4fVI octahedral sites.
Deactivation of electrically active arsenic in silicon during cooling-down from elevated temperatures
Authors: Larsen A. Nylandsted , Christensen B. , Shiryaev S. Yu.
Published: July 2002

The deactivation of electrically active ion-implanted arsenic in silicon during cooling-down, following rapid thermal annealing, has been studied. Chemical and carrier-density profiles were measured as a function of annealing temperature and cooling rate. It is found that the cooling rate has a pronounced effect on the maximum carrier concentration which can be obtained at a given temperature. It is further demonstrated that the equilibrium carrier concentration at a given temperature is higher than the previously accepted values.
Magnetic properties of CoxFe3−xO4 during conversion from normal to inverse spinel particles
Authors: Templeton T. L. , Arrott A. S. , Curzon A. E. , Gee M. A. , Li X.-Z. , Yoshida Y. , Schurer P. J. , LaCombe J. L.
Published: July 2002

Cobalt ferrite, CoxFe3−xO4 with 0.55≤x≤0.8, is produced by precipitation from aqueous solutions. Drying and annealing at temperatures below 300 °C results in a coercivity that increases with increasing temperature. Mössbauer effect spectroscopy and magnetic measurements are consistent with the interpretation that these Co ferrites have a (mostly) normal spinel structure as made. These convert to a (mostly) inverse spinel structure on thermal aging. No significant morphological changes occur during this transformation. Coercivities as high as μ0Hc=0.2 T are observed.
Temperature dependence of the electrical resistivity of reactively sputtered TiN films
Authors: Tsai W. , Delfino M. , Fair J. A. , Hodul D.
Published: July 2002

The electrical resistivity of reactively sputtered TiN films was measured as a function of film thickness. The effect of directionality of the sputtered atoms, substrate temperature, bias voltage, deposition rate, and film morphology on the electron conductivity in TiN films was studied. The combination of rapid deposition rate and high substrate temperature with bias-collimated sputtering results in TiN films with the lowest resistivity, 45 μΩ cm, the largest temperature coefficient of resistance, 1355 ppm, and the highest superconducting transition temperature, 5.04 K. These films are characterized by small grains with mixed <111≳ and <200≳ orientation and reduced electron scattering with an estimated electron mean-free path of 96 nm.
Magneto-optical anisometry of ultrathin cobalt films
Authors: Grolier V. , Ferré J. , Maziewski A. , Stefanowicz E. , Renard D.
Published: July 2002

The advantage of magneto-optics to measure magnetic anisotropy constants in ultrathin films exhibiting perpendicular spin anisotropy is reported. This is illustrated for Au/Co/Au sandwiches with Co layer thickness ranging from 8 to 15 Å. We stress the importance of preliminary measurements always necessary to define the (H∥,H⊥) region in which a homogeneous spin rotation occurs. These results confirm the existence of interface anisotropy and lead to its estimation Ks=0.56 erg/cm2 whereas the volumic term is shown to be Kv=7.5×106 erg/cm3.
Theory of magnetization in IV–VI based diluted magnetic semiconductors
Authors: Hota R. L. , Tripathi G. S. , Misra P. K.
Published: July 2002

The magnetization (M) in IV–VI based diluted magnetic semiconductors is expressed as a sum of contributions due to isolated impurities, pair interactions, lattice diamagnetism, and spin density due to carriers. We calculate the contribution of each of these mechanisms for p-type Pb1−xMnxTe for different x and magnetic field (H) and compare the results with experiment.
Thermal conductivity of metal-matrix composites
Authors: Davis L. C. , Artz B. E.
Published: July 2002

The thermal conductivity of metal-matrix composites, which are potential electronic packaging materials, is calculated using effective medium theory and finite-element techniques. The thermal boundary resistance, which occurs at the interface between the metal and the included phase (typically ceramic particles), has a large effect for small particle sizes. It is found that SiC particles in Al must have radii in excess of 10 μm to obtain the full benefit of the ceramic phase on the thermal conductivity. Bimodal distributions of particle size are considered, since these are often used to fabricate high-volume fraction composites. It is found that if the small particles (in a bimodal distribution) have a radius less than 2.5 μm in SiC/Al their addition reduces the thermal conductivity of the composite. Diamond-containing composites, which have large thermal boundary resistance effects, are analyzed. Comparison of the effective medium theory results to finite-element calculations for axisymmetric unit-cell models in three dimensions and to simulation results on disordered arrays of particles in two dimensions confirms the validity of the theory.
The ac electrical behavior of polycrystalline yttria
Authors: Patton V. D. , Wang C. C. , Akbar S. A. , Alim M. A.
Published: July 2002

The ac electrical behavior of the polycrystalline yttria was evaluated in the temperature range of 800–1300 °C in air as a function of frequency (5 Hz≤f≤13 MHz). Resistance-temperature and resistance-time (aging) characteristics were examined using immittance measurements and electron microscopy to establish microstructure-property relationships. The ac electrical data indicated two distinct relaxations when analyzed in the impedance plane. These relaxations are attributed to the lumped grain and grain-boundary contributions in conjunction with a polarization effect at the electrode/sample interface. The admittance plane analysis revealed a semicircular relaxation in the low-frequency region, indicative of a trapping effect associated with grain-boundaries and the electrode/sample interface. The variation in the total electrical resistance with time is found to be dependent on the starting microstructure of the sample. A sample with a larger grain size shows a smaller degree of aging at elevated temperatures. Immittance measurements suggest that the major contribution to the aging behavior comes from the evolution in the microstructure.
The effect of mechanical stress on a Barkhausen noise signal integrated across a cycle of ramped magnetic field
Authors: Sablik M. J. , Augustyniak B.
Published: July 2002

Experimental Barkhausen data are presented for XC10 French steel at various stress levels. The experimental data show an apparent almost linear correlation with stress of WH (hysteresis loss) and WBN (Barkhausen noise amplitude integrated over H). A model previously suggested by Sablik for Barkhausen noise stress and field dependence is used for analysis of the experimental data. With slight modifications of the previous model, it has been possible to produce semiquantitative fits to the Barkhausen peaks as a function of H while at the same time obtaining the apparent linear correlation between WH and WBN and stress. The model suggests however that the apparent linear correlation may only apply to certain types of materials. In addition, the fit to experiment suggests that additional refinements to the model may yet be necessary.
Thermomagnetic properties of amorphous rare-earth alloys with Fe, Ni, or Co
Authors: Liu X. Y. , Barclay J. A. , Gopal R. B. , Földeàki M. , Chahine R. , Bose T. K. , Schurer P. J. , LaCombe J. L.
Published: July 2002

The magnetic properties and field-dependent specific heat of melt-spun amorphous RE70TM30 (RE=Gd, Tb, Dy, Ho and Er; TM=Fe and Ni) and Gd65Co35 alloys were investigated as potential magnetic refrigerants. Essentially zero magnetic hysteresis was observed in all the Gd–TM alloys at temperatures from 5 K up to the ordering temperatures. The coercive force of the RE70TM30 alloys depended mainly on the RE species and increased according to the order of RE=Gd<Ho<Er<Dy<Tb. The magnetic susceptibility of most of the alloys showed apparently normal Curie–Weiss behavior above the ordering temperatures. The heat capacity measurements in zero field and applied fields of 4 and 8 T indicated that the magnetic transition in these alloys are significantly broadened. The maximum adiabatic temperature changes for Er70Fe30, Gd70Ni30 and Gd65Co35 amorphous alloys in a field change of 8 T are 4.0, 3.4, and 3.0 K, respectively. Mössbauer spectroscopy revealed that Fe atoms in the amorphous RE70Fe30 alloys carry a small magnetic moment that may complicate the magnetic ordering in the alloys. A simple model assuming a Gaussian distribution of ordering temperatures around the apparent Curie temperature was constructed to attempt to reconcile the differences in the observed magnetic properties of these amorphous alloys. The broad magnetic transition is attributed to the fluctuation of the exchange integral caused by the structural disorder in amorphous alloys. The calculated susceptibility, magnetization, and heat capacity agreed reasonably well with the experimental data and show that the magnetic susceptibility and magnetization are only weakly affected by the distribution of ordering temperatures, but the heat capacity is much more sensitive to such a distribution. To effectively screen out magnetic refrigerants with sharp magnetic transitions and correspondingly large adiabatic temperature changes from those with broadened transitions and small adiabatic temperature changes, the field-dependent heat capacity measurement technique is a powerful tool to use.
Transport and magnetism correlations in thin-film ferromagnetic oxides
Authors: Hundley M. F. , Neumeier J. J. , Heffner R. H. , Jia Q. X. , Wu X. D. , Thompson J. D.
Published: July 2002

To determine the Tc dependence of the colossal magnetoresistance (CMR) exhibited by the ferromagnetic La0.7A0.3MnO3+δ (A=Ba, Ca, Sr) system, we examine the magnetic-field and temperature-dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (Tc) ranging from 150 to 350 K. All samples display a large negative MR that is largest near Tc, and samples with a low Tc display significantly larger MR values than do samples with large Tc’s. The quantity ρ(Tc)/ρ(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, varies as exp(Ea/Tc) with an activation energy Ea=0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by ρ(Tc), but by Tc via the ratio ρ(Tc)/ρ(4 K). Phenomenological scaling relationships are also reported that link ρ(H,T) to both H and M(H,T).
Defect-dipole alignment and tetragonal strain in ferroelectrics
Authors: Warren W. L. , Pike G. E. , Vanheusden K. , Dimos D. , Tuttle B. A. , Robertson J.
Published: July 2002

We show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline Pb(Zr,Ti)O3 and BaTiO3 ferroelectric ceramics using electron paramagnetic resonance (EPR). The alignment is demonstrated via orientation dependent paramagnetic centers in the polycrystalline materials and computer modeling of the EPR line shapes. It is shown that defect dipoles can become aligned by oxygen vacancy motion in the octahedron about a negatively charged center for the oxygen vacancy-related dipole complexes or by defect displacement and domain realignment in the lattice for isolated defect centers. We find that the alignment is not observed in nonferroelectric materials such as SrTiO3, and is destroyed in ferroelectric materials by heating above the Curie temperature. These observations suggest an interplay between distortion in the unit cell and the ability to align defect dipoles, as is the case more generally for ferroelectric dipole alignment. We also directly observe aligned intrinsic Ti and Pb ion displacements in the ferroelectric Pb(Zr,Ti)O3 perovskite unit cell.
Contactless potentiometer based on giant magnetoresistance sensors
Authors: Clemens W. , van den Berg H. A. M. , Rupp G. , Schelter W. , Vieth M. , Wecker J.
Published: July 2002

We present a magnetic sensor based on the giant magnetoresistance (GMR) effect, which can be used as a contactless potentiometer. The sensor consists of a novel GMR sensor scheme with an artificial antiferromagnetic subsystem. The sensor gives a sinusoidal signal in dependence on the direction of a rotating external magnetic field Hrot. With two sensors in a planar setup, the whole 360° angle range can easily be covered. The amplitude ΔR/R of the signal is about 5%. The signal amplitude runs through a very flat maximum and changes only by 5% within a field range of about 4.4–27.2 kA/m. This large field range, the magnetic window (Hw), is the main advantage of this sensor compared to other magnetic sensor physical principles for contactless potentiometers. Therefore, large mounting tolerances and variations in the field strength of the rotating permanent magnet can be accepted. The temperature dependence of the sensor is linear both for the ground resistance (R0) and the signal amplitude (ΔR), allowing simple and effective electronic temperature compensation. The sensor can be operated up to 150 °C.
Calibration of surface stress measurements with atomic force microscopy
Authors: Miyatani Tatsuya , Fujihira Masamichi
Published: July 2002

An accurate method to calibrate a commercial atomic force microscope (AFM) with commercial cantilevers for surface stress measurements was investigated. The bending of commercial rectangular cantilevers of the AFM was measured by laser beam deflection under a concentrated load mode and a bending moment mode. Good agreement between the experimental results and the theoretical predictions was obtained. The accurate calibration of the deflection angle for Δσ was possible with the rectangular cantilever by taking into account the dependence of the deflection angle on the laser spot position, a, on the cantilever. The cyclic voltammogram and the electrocapillary curve were measured in the double layer region of a gold thin film on the rectangular cantilever. In addition, it was concluded that a fairly good measurement of Δσ by triangular cantilevers was also possible as long as the deflection angle measurement was carried out with the same triangular cantilever at the same laser spot position a. The change in surface stress due to electrochemical oxygen and hydrogen adsorption was also studied on gold and platinum deposited on rectangular cantilevers. In these cases, the change in reflectance of the cantilevers by electrochemical reactions had to be taken into account for accurate calibration. The discussion described here will be also useful for other stimuli, such as pH, that cause the cantilever bending.
Modeling of thermal stresses in metal interconnects: Effects of line aspect ratio
Authors: Shen Y.-L.
Published: July 2002

Numerical results are presented on the evolution of thermal stresses in metal interconnects. Finite element analyses are carried out for a series of parallel aluminum lines embedded within silicon dioxide. The effects of line aspect ratio on the longitudinal and hydrostatic stresses in lines are investigated, for various spaces between lines. Comparisons with the results based on an isolated line under passivation are also made. It is shown that, contrary to what was frequently found by employing the single-line approach, maximum stresses in periodically arranged aluminum lines do not occur when the line width and the line height are approximately equal. The variation of stresses with line aspect ratio is affected by the spacing between lines. Implications of the present findings to the modeling of stresses and to the damage evolution through void formation are discussed.
Electromagnetic field structure in a weakly collisional inductively coupled plasma
Authors: Godyak V. A. , Piejak R. B.
Published: July 2002

Two-dimensional, phase resolved magnetic probe measurements have been performed in a low pressure inductively coupled cylindrical plasma source driven with a planar coil. The rf electric field and current density distributions determined from these measurements exhibit an abnormal nonmonotonic spatial evolution. Formation of a second current layer, phase bifurcation and a reversal of the rf field phase velocity have been found and are attributed to spatial dispersion of the plasma conductivity due to collisionless electron thermal motion typical of anomalous skin effect.
Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties
Authors: Cebulla R. , Wendt R. , Ellmer K.
Published: July 2002

A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio ji/jn on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about Ei≈17 eV, the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio ji/jn on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances (ρ≈6×10−4 Ω cm), due to lower mechanical stress, leading to higher charge carrier concentrations and mobilities. Undoped ZnO films exhibited the largest compressive stress values up to 2.8 GPa. The aluminium-doped films have a better (001) texture and larger grains (dg≈38 nm), which can be attributed to the beneficial role of Al as a surfactant. The better crystalline film quality of the ZnO:Al films is the reason for the much lower compressive stress of <0.5 GPa in these layers.
Model for strain and magnetization in magnetic shape-memory alloys
Authors: O’Handley R. C.
Published: July 2002

The large magnetic-field-induced strains observed in martensitic phases based on Ni2MnGa and in other magnetic shape memory alloys are believed to arise from a process of twin-boundary motion rather than magnetostriction. The dependence of strain on magnetization, e(M), generally shows a large component that is linear (rather than quadratic) in M below saturation (quadratic dependence being typical of magnetostrictive strain). A simple phenomenological model for the magnetization process and field-induced strain by twin-boundary and phase-boundary motion is proposed for both the strong and weak anisotropy cases. The model is shown to account for the nearly linear dependence of strain on magnetization in the martensitic phases of these materials. It shows the field dependence of the magnetization and strain to be functions of an effective stiffness constant, C, the transformation strain, e0, and the magnetic anisotropy of the martensitic phase, Ku, through two reduced field parameters, he=MsH/Ce02 and ha=MsH/2Ku. The model also accounts for the magnetization remanence and the nonlinear field dependence closer to saturation (which produces little strain). The curvature observed in e(H) at very low fields is not described by this two-variant model and may be related to the fact that more variants exist which respond to the field with a distribution of susceptibilities.
A phenomenological model for magnetoresistance in granular polycrystalline colossal magnetoresistive materials: The role of spin polarized tunneling at the grain boundaries
Authors: Raychaudhuri P. , Nath T. K. , Nigam A. K. , Pinto R.
Published: July 2002

It has been observed that in bulk and polycrystalline thin films of colossal magnetoresistive (CMR) materials the magnetoresistance follows a different behavior compared to single crystals or single crystalline films below the ferromagnetic transition temperature Tc. In this paper we develop a phenomenological model to explain the magnetic field dependence of resistance in granular CMR materials taking into account the spin polarized tunneling at the grain boundaries. The model has been fitted to two systems, namely, La0.55Ho0.15Sr0.3MnO3 and La1.8Y0.5Ca0.7Mn2O7. From the fitted result we have separated out, in La0.55Ho0.15Sr0.3MnO3, the intrinsic contribution from the intergranular contribution to the magnetoresistance coming from spin polarized tunneling at the grain boundaries. It is observed that the temperature dependence of the intrinsic contribution to the magnetoresistance in La0.55Ho0.15Sr0.3MnO3 follows the prediction of the double exchange model for all values of field.
Luminescence studies of localized gap states in colloidal ZnS nanocrystals
Authors: Denzler D. , Olschewski M. , Sattler K.
Published: July 2002

ZnS nanoparticles were prepared by chemical precipitation of Zn2+ with sulfur ions in aqueous solution. The ultraviolet-excited samples reveal detailed structure in the luminescence spectra. A doublet pattern observed in the long wavelength region is attributed to the coexistence of the two crystalline forms in ZnS particles. The visible luminescent radiation at 590.1 nm is due to Mn impurities. The dominant emission band at short wavelengths exhibits a quadruple fine structure with peaks located at 416.1, 423.9, 430.1, and 437.8 nm which are identified with optical transitions arising from vacancy and interstitial sites for both Zn and S atoms.
Oxidation kinetics of small nickel particles
Authors: Karmhag Richard , Niklasson Gunnar A. , Nygren Mats
Published: July 2002

The oxidation kinetics of two kinds of polycrystalline nickel particles with an approximate median diameter of 5 μm but with different morphology, has been studied by thermogravimetric measurements in the temperature range 573–973 K. The oxidation was found to be thermally activated with an apparent activation energy of about 1.5 eV. The kinetics was compared with a homogeneous field, coupled-currents theory for oxidation of spherical metal particles. Calculations using a median particle size or a distribution of particle sizes do not give satisfactory fits to the experimental data. Possible explanations of the deviations in terms of space charge, grain boundary diffusion, grain growth, and sintering are discussed.
Thermal stability of the nanocrystalline Fe–Co–Hf–B–Cu alloy
Authors: Iwanabe Hirofumi , Lu Bin , McHenry Michael E. , Laughlin David E.
Published: July 2002

Nanocrystalline (Fe1−xCox)88Hf7B4Cu1 alloys have been investigated as candidates for soft magnetic materials for high temperature applications. Four samples were obtained in annealed or as-spun ribbons with x=0,0.10,0.30,0.44. Magnetic properties and thermal stability were studied focusing on the x=0.30 alloy by means of different thermal analysis, hysteresisgrapher, impedance analyzer, transmission electron microscope, etc.
Coupling mechanisms in exchange biased films (invited)
Authors: Schulthess T. C. , Butler W. H.
Published: July 2002

We use an atomistic Heisenberg model in conjunction with the classical Landau Lifshitz equation for the spin motion to study coupling mechanisms between ferromagnetic (FM) and antiferromagnetic (AFM) films. Calculations for CoO/FM illustrate that there are two coupling mechanisms at work, the spin–flop coupling and an AFM–FM coupling through uncompensated defects. While the latter accounts for exchange bias and related phenomena, the former gives rise to a large coercivity and perpendicular alignment between FM spins and AFM easy axis. A combination of the two mechanisms explains apparent discrepancies between reversible and irreversible measurements of the AFM–FM coupling.
Dielectric functions of bulk 4H and 6H SiC and spectroscopic ellipsometry studies of thin SiC films on Si
Authors: Zollner Stefan , Chen J. G. , Duda Erika , Wetteroth T. , Wilson S. R. , Hilfiker James N.
Published: July 2002

Spectroscopic rotating-analyzer ellipsometry employing a compensator and optical transmission were used to measure the dielectric functions of bulk 4H and 6H SiC from 0.72 to 6.6 eV for light propagating nearly parallel to the hexagonal axis. The measurements below the band gap show the presence of a thin surface layer, which was modeled as SiO2. The data are similar to results for cubic (3C) and 6H SiC from the literature, but differences are notable, particularly above 4 eV. At 5.56 eV, we observe a critical point in 4H SiC, which is assigned to direct interband transitions along the U=M−L axis in the hexagonal Brillouin zone after comparison with band structure calculations. No evidence for direct transitions below 6.5 eV was found in 6H SiC. We apply our results to the analysis of a 4H SiC film on insulator (SiCOI) produced by high-dose hydrogen implantation and direct wafer bonding on Si. For comparison, we also studied a 1 μm thick epitaxial layer of 3C SiC on Si, where the interference oscillations are influenced by surface and interface roughness.
Thickness dependence of the crystallization of Ba-ferrite films
Authors: Cho T. S. , Doh S. J. , Je J. H. , Noh D. Y.
Published: July 2002

The crystallization of Ba-ferrite/sapphire(001) films of various thicknesses has been studied using synchrotron x-ray scattering, field emission scanning electron microscope, and atomic force microscope. In films thinner than 1000 Å, Ba-ferrite amorphous precursor was crystallized into perpendicular grains keeping the magnetically easy c-axis normal to the film plane during annealing to 750 °C. In films thicker than 1000 Å, however, acicular grains keeping the c-axis parallel to the film plane were grown on top of the perpendicular grains. The behavior of the saturation magnetization and the intrinsic coercivity was consistent with the thickness dependence of the crystallization. We attribute the thickness dependence of the crystallization to the substrate effect, which prefers the growth of the epitaxial, c-axis oriented perpendicular grains near the film/substrate interfacial area.
Mechanism of enhanced quantum efficiency in light-emitting diode based on a poly(p-phenylenevinylene) derivative
Authors: Jang Jae Won , Oh Dong Keun , Lee Chang Hoon , Lee Cheol Eui , Lee Dong Won , Jin Jung-Il
Published: July 2002

Transient electroluminescence (EL) measurements were made for a indium-tin–oxide (ITO)/poly[(2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)-p-phenylenevinylene] (PPPDPV)/Al organic light-emitting diode (OLED), where PPPDPV is a derivative of poly(pphenylenevinylene) (PPV) carrying the electron-transporting 2-(4-biphenyl)-5-(4-t-butylphenyl-1, 3,4-oxadiazole) moieties directly attached to the PPV backbone. As a result, we were able to explain the superior external quantum efficiency of a PPPDPV OLED device by a faster mobility as well as a longer EL lifetime in comparison to those for a PPV device.
Spin-waves in cylindrical magnetic dot arrays with in-plane magnetization
Authors: Guslienko K. Yu. , Slavin A. N.
Published: July 2002

The discrete spectrum of dipole-exchange spin-wave modes of a tangentially magnetized cylindrical magnetic dot is calculated from the solution of the Landau–Lifshitz equation and the magnetostatic Maxwell equations in a cylindrical geometry. The general surface spin-pinning conditions at the radial dot boundary are considered. The main simplifying assumptions are: (i) the dot radius is much larger than the dot height; (ii) the distribution of the variable magnetization along the dot height is uniform. The approximate dispersion equation for spin-wave modes in a dot is obtained in a simple analytical form similar to the form of the dispersion equation in an infinite film. The quantization effect of the spin-wave frequencies appears due to the finite dot radius and is essential for submicron magnetic dots. The discrete spin-wave frequencies are calculated in a practically important case of the square array of permalloy cylindrical dots. The relative intensities of spin-wave modes, when observed by Brillouin light scattering, are considered. The role of interdot dipole–dipole coupling is discussed.
Ultrahigh density vertical magnetoresistive random access memory (invited)
Authors: Zhu Jian-Gang , Zheng Youfeng , Prinz Gary A.
Published: July 2002

In this paper, we present the vertical magnetoresistive random access memory (VMRAM) design based on micromagnetic simulation analysis. The design utilizes the vertical giant magnetoresistive effect of the magnetic multilayer. By making the memory element into a ring-shaped magnetic multilayer stack with orthogonal paired word lines, magnetic switching of the memory device becomes very robust. The design also adopts the readback scheme in pseudo spin valve MRAM so that only one transistor is needed for each bit line which can connect hundreds of memory elements, yielding a very high area density. It is estimated that the ultimate area density for the VMRAM is 400 Gbits/in.2. It is suggested that this memory design has the potential to not only replace the present semiconductor memory devices, such as FLASH, but also the potential to replace DRAM, SRAM, and even disk drives.
Coatings for optical pumping cells and short-term storage of hyperpolarized xenon
Authors: Breeze Steven R. , Lang Stephen , Moudrakovski Igor , Ratcliffe Chris I. , Ripmeester John A. , Santyr Giles , Simard Benoit , Zuger Irene
Published: July 2002

For a number of years now, siloxanes have been the materials of choice for coating vessels used in the production and short-term storage of hyperpolarized xenon. The methods used to apply this material, however, often vary from one research group to another and it is commonly reported that it is difficult to obtain cells with consistently long spin-lattice relaxation times (T1) and high-polarization levels. In a series of controlled experiments individual production variables were altered and optimized, leading to improved protocols for the reliable production of high-quality siloxane-coated cells. During these studies we discovered that the surface-induced relaxation rates in bare and coated Pyrex cells differ profoundly. This information on Xe relaxation helps to define the limits on the way pumping cells can be improved and suggests the need for further fundamental work on relaxation mechanisms.
Molecular jet study of the solvation of benzene by methane, ethane, and propane
Authors: Schauer Mark , Bernstein E. R.
Published: July 2002

Two color time of flight mass spectroscopy studies of benzene solvated by methane, ethane, and propane in a molecular jet have been carried out. Absorption has been characterized for both the 000 and 610 transitions of benzene (alkane)x (x=1,2,3...). Atom–atom exponential-6 potentials have been employed to calculate cluster geometry and binding energy. Comparisons between calculations and experiments allow the identification of specific configurations for the cluster spectroscopic transitions. Cluster spectral shifts can also be identified and a correlation between the size of the cluster red shift and geometry has been developed. The closer the solvent is to the aromatic π system of the ring, the larger the red shift. Relative intensity data for different clusters has led to a clarification of cluster nucleation. Most clusters are formed by the interaction of solvent dimers or larger species with a solute molecule.
Reentrant phase transition of N-isopropylacrylamide gels in mixed solvents
Authors: Amiya Takayuki , Hirokawa Yoshitsugu , Hirose Yoshiharu , Li Yong , Tanaka Toyoichi
Published: July 2002

Reentrant volume–phase transitions are observed in N-isopropyoacrylamide gels in the methanol–water mixtures. When the solvent composition is varied systematically, the gel undergoes two transitions: a discontinuous collapsing followed by a discontinuous swelling. The reentrant transition defines a closed-loop instability phase boundary having both upper and lower critical points. The closed-loop phase boundary depends on temperature and diminishes to a point at approximately 0 °C. A simple mean field theory is presented to describe the phenomenon, which reveals an alteration of free energy of alcohol–water interaction by presence of polymer network. In the case of ethanol–water mixtures, there appear two closed-loop phase boundaries, whose physico-chemical basis are not yet clear.
Quenching rate coefficients for N2(a′ 1Σ−u)
Authors: Piper Lawrence G.
Published: July 2002

We have studied the kinetics of the lowest excited singlet state of molecular nitrogen, N2(a′ 1Σ−u,v=0), in a discharge flow reactor. The metastables were generated in a hollow cathode dc discharge through molecular nitrogen highly diluted in argon, and detected by VUV flourescence of the forbidden N2(a′ 1Σ−u–X 1Σ+g) band system. Observations of the equilibrium between the N2(a 1Πg) and N2(a′ 1Σ−u) states in our reactor indicate that the metastable has a radiative lifetime ≳23+11−6ms, assuming a radiative lifetime of 80+40−20μs for N2(a 1Πg). Rate coefficients for quenching by NO, CH4, CO, and N2O are approximately gas kinetic, those by H2, O2, and CO2 roughly tenth gas kinetic, and that by molecular nitrogen (1.9±0.5)×10−13 cm3 molecule−1 s−1. The interaction between N2(a′) and CO produced CO(A 1Π–X 1Σ) emission in about 21+10−8% of the quenching events.
The yield of oxygen and hydrogen atoms through dissociative recombination of H2O+ ions with electrons
Authors: Rowe B. R. , Vallée F. , Queffelec J. L. , Gomet J. C. , Morlais M.
Published: July 2002

The branching ratios of the dissociative recombination of excited H2O+ ions with electrons were studied using a plasma flow tube experiment. The total oxygen atom channel accounts for 45% of the recombination with more than 24% in O+H+H. The remaining channel is OH+H for 55%. These results are discussed in comparison with the few available theories and with photodissociation data.