bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsed field magnetometer for nondestructive monitoring of encapsulated magnetic materials
Authors: Rothwarf F. , Simizu S. , Huang M. Q. , Schaefer R. J.
Published: July 2002

A study was recently undertaken to consolidate Sm2Fe17N3−x-based materials by hot isostatic pressing (HIPing). For HIPing, pressed green samples were placed at the center of evacuated, closely fitting, nonmagnetic, thin-walled, stainless-steel cans. The use of nonmagnetic, high-resistivity containers for the samples offered a unique opportunity to evaluate the magnetic properties of the samples after HIPing, without removing them from the HIP cans, by using a pulsed field magnetometer (PFM). The high electrical resistivity of the samples and their containers permits the slowly varying magnetic field (∼5 ms rise time, 45 ms pulse length) to penetrate a sample fully with no phase lags. Thus, the magnetization of a sample is essentially in equilibrium with the applied, time-varying magnetic field. With this method, a given set of encapsulated samples could be subjected to successively higher temperature heat treatments to determine their effects on the magnetic properties. The PFM allowed the rapid acquisition, storage, and processing of digital data by computer. The design of the PFM system, demagnetization corrections, procedures used, and some results are presented.
First-order transition in antiferromagnetic stacked triangular lattices with vector spins
Authors: Loison D. , Diep H. T.
Published: July 2002

In this article, we show by Monte Carlo simulations that the phase transition is definitely of first order in the antiferromagnetic stacked triangular lattices with both classical XY and Heisenberg spins in a range of the antiferromagnetic next-nearest neighbor interaction J2. The phase diagram in the (J2/J1, T) space (J1: nearest-neighbor interaction,T: temperature) is shown and discussed.
Antiferromagnetic magnons in diluted triangular and Kagomé lattices (abstract)
Authors: Huber D. L. , Ching W. Y.
Published: July 2002

Numerical results are presented for the local field distribution and the distribution of linearized magnon modes in diluted triangular and Kagomé lattices. A nearest-neighbor antiferromagnetic Heisenberg spin Hamiltonian is assumed, and the linearization is carried out with respect to classical ground states obtained by means of an energy minimization algorithm.1 In the case of the triangular lattice, the density of states associated with a 20% vacancy concentration is used to calculate the magnon contribution to the specific heat. With an exchange integral inferred from the Curie–Weiss constant, quantitative agreement is obtained with the experimental results for La0.2Gd0.8CuO2 reported by Ramirez et al.2 over the interval 0.1 K≤T≤0.2 K. The behavior of the diluted Kagomé lattice is compared with that of the triangular array. In contrast to the latter, the local fields in the diluted Kagomé lattice take on the discrete values 2JS, JS, and 0. In the case of a 14% vacancy concentration, the distribution of magnon modes resembles that of the fully occupied array with a noncoplanar ground state. The relevance of these results to the behavior of SrCr8Ga4O19 is discussed.3
Phase transitions and critical behavior of spin 1/2 quasi 2-D systems (abstract)
Authors: Zhou P. , Ravidran K. , Drumheller J. E. , Haines D. N.
Published: July 2002

Zero-field ac susceptibility and isothermal magnetization of the quasi-2D alkanediammonium copper tetrahalide series, [NH3(CH2)nNH3]CuX4, where n=4, 5, 6, and 10 with X=Cl and Br, are reported. The 3D antiferromagnetic ordering temperature Tc for the Cl compounds is determined. It is shown that the critical susceptibilities decay exponentially as the temperature increases (T≳Tc). A power-law divergence at Tc is seen in the Br compounds with n=7 and 10. This behavior is characteristic of 3D ferromagnetic ordering at Tc. The critical exponent γ for the initial susceptibility (T≳Tc) has been obtained for the Br compounds. It is found that there is a second (minor) peak below Tc in the Br compounds with n=5 and 7. The transition associated with this peak may be interpreted as a long range (spontaneous) ordering due to very small spin anisotropies, such as a spin canting between the layers. In the Br compounds the isothermal magnetization is suppressed in low fields, and the value of the critical exponent δ estimated from the isothermal data is considerably smaller than that given by standard models. Spatial- and spin-dimensionality crossovers are apparent in the both initial susceptibility and isothermal magnetization data.
Magnetoresistance in Co/Ag and Fe/Ag nanocomposite films (abstract)
Authors: Liou S. H. , Malhotra S. S. , Yung S. W.
Published: July 2002

Recently a great deal of attention has been focused on giant magnetoresistance (GMR), especially in the coupled multilayer magnetic films. In this report, we show that GMR also exists in nanocomposite films. We have systematically studied the magnetoresistance of Fe/Ag and Co/Ag nanocomposite films with 15 to 55 at. % of Fe and Co, respectively. These films were prepared by magnetron sputtering from composite targets. The crystallite size of the Fe and Co are from 30 to 150 Å which was controlled by the substrate temperature and determined by the analysis of the x-ray diffraction patterns. All samples have a negative magnetoresistance and the change of magnetoresistance varies from a fraction of a percent to more than 25% at 6 K and under 5 Tesla magnetic field. We have investigated the effect of the particle size and particle separation distance on the magnetoresistance and their magnetic properties will be presented.
High-resistive, high-saturation inductive soft magnetic amorphous fluorine modulated (CoFe)BF films
Authors: Sugawara E. , Matsumoto F. , Fujimori H. , Masumoto T.
Published: July 2002

(Co0.5Fe0.5)BF amorphous films with modulated F content were investigated and such multilayered films have ceramic spacers to increase permeability at high frequencies. Modulated films having alternate F-poor layers (15 nm thick) and F-rich layers (5–10 nm thick) within an F content of 2–10 at. % were prepared by a sputtering technique. High-saturation induction (Bs) of 1.6–1.8 T, low coercivity (Hc) less than 1.0 Oe, permeability (μ′) of 800–1000, and enhanced high frequency response were obtained in the case of a modulated film having alternating layers: 15-nm-thick layers, 2 at. % F, ρ=150 μΩ cm and 10-nm-thick layers, 10 at. % F, ρ=800 μΩ cm. The number of periods was variable. Auger-electron spectroscopy and secondary-ion mass spectroscopy depth profiles clarified the periodicity of the modulated films. Multilayered films with four layered F modulated films (0.55 μm/layer) and SiO2 spacers (0.24 μm/layer) exhibited an excellent frequency response of μ′=1500 in the range of 1–100 MHz. The coexistence of high ρ, high Bs, and low Hc in these films may be attributed to the oscillatory resistivity resulting from a compositional modulation in F and B.
The antiferromagnetic Ising model in a magnetic field: Linked-cluster expansion analysis
Authors: Wentworth C. D. , Wang Y. L.
Published: July 2002

The mean-field approximation of the critical behavior of the antiferromagnetic Ising model in a magnetic field shows that the critical field increases first at low temperature. However, the exact solution of a decorated square lattice proves that the critical field rapidly decreases with increasing temperature for the two-dimensional lattice. Three-dimensional lattices of this model have been studied as well using the high temperature expansion, low temperature expansion, and Monte Carlo simulation. The results are different. In this article we have investigated the critical field of the body centered cubic lattice of this model using the linked-cluster expansion technique. An eighth-order perturbation series for the free energy has been calculated. The magnetization and staggered magnetic susceptibility series have been calculated from the free energy. These series have been analyzed using Padé approximants to obtain the phase diagram in the field-temperature plane. A comparison is made with Monte Carlo simulation calculations.
Giant magnetothermopower and giant magnetoresistance in molecular beam epitaxy grown Co/Cu(111) superlattices
Authors: Avdi E. , Hickey B. J. , Greig D. , Howson M. A. , Hall M. J. , Xu J. , Walker M. J. , Wiser N. , de Groot P.
Published: July 2002

We report the observation of a giant magnetothermopower (GMT), giant magnetoresistance (GMR), and on magnetization measurements in Co/Cu[111] superlattices grown by molecular beam epitaxy. The maximum value of the GMT (at room temperature) was 14% for a Cu thickness of 9 Å and the maximum GMR (at 4.2 K) was −26% at 7 Å of Cu. Oscillations in the remnant magnetization and the saturation field as a function of Cu thickness with a period of about 10 Å were observed. However, there were no oscillations in the GMT or the GMR. The maximum values of both the GMT and GMR are associated with saturation fields in excess of 40 kOe and with small remnant magnetizations. These results are consistent with the presence of antiferromagnetic coupling.
Small cluster studies of the lattice Anderson model
Authors: Callaway J. , Kanhere D. G. , Lin H. Q.
Published: July 2002

We report a numerical calculation of the spectral weight function of the lattice Anderson model for a small cluster. Parameters in the weak hybridization-strong interaction sector are considered. The behavior of the f levels is studied as the f occupancy varies from the doubly occupied limit toward the nearly empty case.
Generalized gradient approximation and its effects on the magnetic moments and magneto-optics of f-electron systems
Authors: Lim S. P. , Cooper Bernard R.
Published: July 2002

We have been developing an understanding of the role of correlation (configuration fluctuation) effects on the giant magneto-optic properties of a class of unusual highly anisotropic, magnetically ordered, highly correlated f-electron materials. While orbital polarization in the band calculations improved the ordered moment agreement with experiments for the less correlated materials, the magneto-optic of calculation and experiment remained distinctly different. Moreover, the more highly correlated the material, the worse the agreement of the two quantities. We have now done similar calculations within local-density approximation with gradient (nonlocal) corrections included. While the nonlocal effects introduced by the generalized gradient approximation significantly modify the calculated equilibrium moment and magneto-optic behavior, comparison with experiment reinforces our earlier conclusion that band theory, which treats fluctuations in a time-averaged sense, is inadequate for capturing key effects of interconfigurational fluctuations in correlated-electron systems. Explicit treatment of correlation effects analogous to those we have already reported for the magnetically ordered moments and ordering temperature is necessary for the magneto-optic behavior.
Optimization of multilayered magnetic structures
Authors: Abele M. G. , Rusinek H.
Published: July 2002

This paper describes the categories of permanent magnets built with layers of different magnetic properties and determines the distribution of the remanence in the concentric layers that optimize the figure of merit. As an example, the paper shows the optimization of a two-layered structure composed of two materials with vastly different energy product values, such as a ferrite and a NdFeB alloy. The use of lower remanence material as the outer layer significantly increases the efficiency of the magnetic structure.
The inherent geometry of eddy-current system (abstract)
Authors: Baldomir Daniel
Published: July 2002

The usual way of solving eddy-current equations consists in reducing the associated diffusion equations to a Poisson’s system. This procedure has the mathematical advantage of making us sure of finding a well-defined solution in every point of a material, unless the geometry does not allow one to define properly the differential operators such as in corners, etc. The price to pay for using the static Poisson’s operators consists in avoiding a dynamical understanding of the behavior of the problem.1 Instead of such a transformation we are going to make a transformation into a wave equation. In this form we obtain a complex metric in four dimensions. This analysis enables us to interpret the dynamical behavior of this quasimagnetostatic problem. By considering the interaction of a dissipative eddy-current system with an adjoint system in which energy is being generated it is possible to produce an invariant four-dimensional volume in terms of the separate electric and magnetic energies.2 By means of a variational method, upper and lower bounds can be found for both these energies. Finally, suggestions for efficient computation schemes are given.3
Local structure of Mn atoms and spins in Ag–Mn spin-glass alloys
Authors: Koga K. , Ohshima K. , Niimura N.
Published: July 2002

We studied the atomic and spin correlations in Ag–Mn spin-glass alloys by x-ray and neutron diffuse scattering. The x-ray scattering results from Ag-13.3, Ag-20.8, and Ag-28.1 at.% Mn alloys show diffuse scattering due to atomic short-range order (ASRO) with diffuse maximum at 1,0.5±δ,0 reciprocal-lattice position, where δ depends on composition. The neutron scattering result from Ag-20.8 at.% Mn shows sharp diffuse maxima at 1,0.5±δ′, 0 positions near the atomic ones. We derived values for the magnetic short-range order (MSRO) parameters from both the x-ray and neutron diffuse intensities and have shown that the two types of spin correlations coexisted in the spin-glass state; one is the short-range spin correlation formed on the ASRO structure of Mn atoms and the other is the longer-range spin correlation with about two times longer than atomic one.
Investigation of (Me4N)2[Cu(TIM)(MX4)2] and (Et4N)2[Cu(TIM)(MX4)2] magnetic trimers
Authors: Wang Zhenming , Willett Roger D.
Published: July 2002

Magnetic properties of bimetallic compounds (Me4N)2[Cu(TIM)(MX4)2] and (Et4N)2[Cu(TIM)(MX4)2] (TIM=2,3,9,10-tetramethyl-1,3,8,10-tetraimine-1,4,8,11- tetraazacyclotetradecane; M=Cu(II), Mn(II), Co(II); X=Cl−,Br−) are reported here. These compounds contain linear M-Cu-M trimeric arrangements in which the spin of M assumes values of 1/2, 3/2, or 5/2. Their magnetic susceptibilities are measured and the magnetic susceptibility expressions for these Heisenberg and Ising systems are obtained. The coupling constants between Cu-M sites are determined by fitting the experimental results to the corresponding models. The value of J/k vary from ∼−15 to ∼20 K with the Co(II) and Mn(II) complexes exhibiting antiferromagnetic coupling while the Cu(II) salts have ferromagnetic exchange coupling.
Investigation of Cu(macrocycle)MX4 magnetic chains
Authors: Willett Roger D. , Wang Zhenming
Published: July 2002

Magnetic properties of bimetallic compounds Cu(TIM)MX4 [M=Cu(II), Mn(II), Co(II); X=Cl− or Br−] and Cu(cyclam)MX4 (TIM and cyclam=macrocyclic tetradentate ligands) are reported here. The structures of these magnets consist of infinite chains, built up from the alternating (Cu,M) sites with S(Cu)=1/2 and S(M)=1/2, 3/2, or 5/2. In the case of Cu(TIM)MnCl4, the magnetic behavior exhibits the characteristic feature of a 1D ferrimagnet with a minimum in χMT at intermediate temperature. The coupling constants between Cu-M sites are obtained by fitting the experimental results to appropriate models. The value of J/k range from ∼−7 to ∼6 K with the (Cu,Cu) salts exhibiting ferromagnetic coupling at high temperature. Electron paramagnetic resonance results are also reported and interpreted according to the magnetic models.
Chemical-vapor deposition of metallic copper film in the presence of oxygen
Authors: Hammadi Z. , Lecohier B. , Dallaporta H.
Published: July 2002

Chemical-vapor deposition of copper using copper (II) bis(acetylacetonate) is reported. It is shown that at 0.1 Torr and temperatures in the range of 250–350 °C, the deposition occurs only if oxygen is added in the reactor. Auger spectroscopy, x-ray-diffraction, and resistance measurements as a function of temperature lead to the conclusion that metallic copper is deposited.
Resonant tunneling of plane, cylindrical, and spherical double-barrier quantum wells
Authors: Ping Er-Xuan , Dalal Vikram
Published: July 2002

Resonant tunneling of double-barrier quantum well (DBQW) in plane, cylindrical, and spherical geometry has been studied by a simple model for the first time. Transmission spectrum, current-voltage characteristic, and bias dependent conductance have been calculated for these structures and are found oscillating in the cylindrical and spherical DBQW. Those oscillations are explained by the different coherence of tunneling in these geometries. Dependence of the oscillation on the structures, the materials, and the external fields is discussed.
Giant negative magnetoresistance in granular ferromagnetic systems (invited)
Authors: Chien C. L. , Xiao John Q. , Jiang J. Samuel
Published: July 2002

Giant negative magnetoresistance (GMR) has been observed in a number of granular ferromagnetic systems [Co–Ag, Co–Cu, Fe–Cu, Fe–Ag, and (Fe–Ni)–Ag] with effect sizes as much as 85% at 5 K and 25% at 300 K. It is shown that the GMR is isotropic and is due to magnetic scattering of the conduction electrons by the nonaligned magnetic entities. The essential contribution to the resistivity is ρm[1−F(M/Ms)], where F(M/Ms) measures the spin disorder from ferromagnetic alignment and ρm is the magnetic resistivity that defines the size of the GMR. The magnitude of GMR is affected by the size and density of the magnetic entities which can be controlled by varying the composition and the process conditions. When the composition is varied, the maximum GMR is realized in systems with magnetic constituents of about 25%.
Recombination in semiconductor electrodes: Investigation by the electrical and optoelectrical impedance method
Authors: Vanmaekelbergh D. , de Wit A. R. , Cardon F.
Published: July 2002

The electrical impedance and the optoelectrical impedance due to electron-hole recombination in the depletion layer and at the surface of a semiconductor electrode are calculated. It is shown that both types of impedance follow from a common formula for the ac recombination current density in the external circuit, which is derived from first principles. It is found that both the electrical and optoelectrical impedance methods provide the same information about recombination. Both methods enable one to distinguish surface recombination from recombination in the depletion layer. The theoretical results are compared with the electrical and optoelectrical impedance results measured at GaAs and CdS photoanodes.
Structural and electrical properties of SiNx:H films
Authors: Masaki Yuichi , Gibson Roderick A. G. , LeComber Peter G.
Published: July 2002

The electronic structure in off-stoichiometric SiNx:H films has been investigated using plasma-deposited films prepared from a mixture of ammonia (NH3) and silane (SiH4) for various gas ratio R (=NH3/SiH4). Metal-insulator-metal diodes incorporating SiNx:H films prepared using R values from 0.2 to 5 were fabricated and the electrical characteristics of these diodes were measured. The properties of the SiNx:H films were also evaluated by ultraviolet-visible, infrared, and x-ray photoelectron spectroscopy. For the R range investigated, the optical band gaps and x values in the films varied from 1.6 to 3.6 eV and from 0.19 to 1.09, respectively. The hydrogen bonding configuration also changed from isolated Si—H bonds, with no adjacent Si—N bond, to multi-N-bonded Si—H bonds as R was increased. The anomalous behavior of the dynamic relative permittivity was observed and the existence of two traps with a different energy was deduced. The results of these electrical measurements are discussed and related to those of the compositional investigations.
Infrared reflectivity study of ion-implanted GaAs
Authors: Aizenberg Gustavo E. , Swart Pieter L. , Lacquet Beatrys M.
Published: July 2002

Ion-implanted GaAs is investigated by means of a fast nondestructive optical technique. We analyzed the implanted substrates by performing a simple bilinear transformation of the experimental infrared reflectance followed by a Fourier spectral analysis. Three cases are analyzed in this paper: (i) Cr-doped, semi-insulating <100≳ GaAs implanted at −100 °C and room temperature, respectively, and annealed under different conditions. In this instance the position of the carrier concentration peak is determined. (ii) H+-implanted <100≳ GaAs with a silicon doping concentration of 3.7×1018 cm−3. The thickness of the resulting compensated layer is estimated for various implant doses. (iii) Semi-insulating <100≳ GaAs implanted with protons to a dose of 5×1015 cm−2. In this case we observed damage not detected by cross-sectional transmission electron microscopy. Estimations performed by means of this technique are compared to results obtained by least-square fit techniques, secondary ion mass spectrometry, and cross-sectional transmission electron microscopy. The agreement between results is within 0.4%–12%, depending on the specific case.
Image force and shielding effects of a moving edge dislocation near a semi-infinite crack
Authors: Tsai Yu-Zen , Lee Sanboh
Published: July 2002

The image force and shielding effects of a moving edge dislocation near a semi-infinite crack were investigated. The crack surface was simulated by continuous dislocations. After the dislocation distribution in the crack was obtained, the stress field in the entire space was calculated. Based on the stress field, the stress intensity factor and crack extension force on the crack tip, and the image force on the edge dislocation were computed. The crack shielding or antishielding arising from the edge dislocation slip or climb was studied in detail. The transition velocity of changing behavior of edge dislocation and Lorentz force were discussed. Newton’s third law was invalid in the system of moving edge dislocation and crack.
Prebreakdown and breakdown phenomena in high-field semiconductor-dielectric systems
Authors: Gradinaru G. , Sudarshan T. S.
Published: July 2002

The complexity of the semiconductor-dielectric system behavior under high electric fields is discussed. Time dependence of the voltage drop, electrical current, and light emissions, and the strong influence of the nature and quality of the surrounding dielectric, as well as the quality of the semiconductor processing and contacts, are analyzed for high-purity bulk silicon-dielectric vacuum or gas systems under impulse voltage stress. On the basis of the analysis of the main characteristics of the time response of the system up to breakdown, as well as of the variation of the current components with the applied voltage, a new comprehensive physical model of the prebreakdown and breakdown phenomena in the high-field semiconductor-dielectric systems is proposed. The model points out the active behavior of two parts, the semiconductor and the ambient dielectric, and the main role of the semiconductor in the initiation of the breakdown phenomena in the system. Based on a large number of experimental results, the present model proposes two totally different breakdown processes in semiconductor-dielectric systems: surface flashover and semiconductor bulk breakdown, depending on the defect distribution in the semiconductor. The key to the model is offered by the role of the conduction currents in the semiconductor, especially by the local avalanche current generated by avalanche processes in defect zones of the crystal, for fields much smaller than the theoretical intrinsic breakdown field of the material. The main characteristics of the prebreakdown and breakdown response of the system for different conditions are explained on the basis of the proposed model. A short discussion of the main reported results of other groups is also presented in view of the new physical model. The role of the semiconductor and ambient-dielectric quality, as well as of the system configuration quality in the breakdown phenomena of high-field semiconductor-dielectric systems, are emphasized on the basis of the proposed model.
Physical properties of semi-insulating polycrystalline silicon. II. Optical studies of thin films
Authors: Brüesch P. , Stockmeier Th. , Stucki F. , Buffat P. A. , Lindner J. K. N.
Published: July 2002

The optical properties of semi-insulating polycrystalline silicon (SIPOS), i.e., SiOx with 0≤x≤2 are studied in the spectral range from the infrared to the UV region. The refractive index n(x,E) and the absorption coefficient α(x,E) are evaluated as a function of the oxygen content x and the photon energy E. The actual shape of the n(x) and α(x) curves are determined by two counteracting effects: (1) the increase of n(x) and α(x) caused by the transition from c-Si to a-Si of the Si grains in SIPOS in the range 0.02≤x≤0.2, and (2) the decrease of n(x) and α(x) due to the increasing insulating character of SIPOS with increasing x. The infrared vibrational modes of bulk SIPOS are described on the basis of an oscillator model with effective parameters. Both, the vibrational frequencies ν̃1(x) of the asymmetrical stretching motion and the total oscillator strength S̃(x) increase with increasing x. While ν̃1(x) can be qualitatively described on the basis of a simple mixture model, this is neither the case for the shape of the frequency-dependent conductivity σ(ν̃,x) nor for S̃(x). Both, σ(ν̃,x) and S̃(x) depend sensitively on the real defect structure consisting of Si grains embedded in the oxide matrix consisting of SiO2, SiO, and other suboxides.
Physical properties of semi-insulating polycrystalline silicon. III. Infrared diagnosis of the polycrystalline-Si/c-Si interface
Authors: Brüesch P. , Stockmeier Th. , Stucki F. , Buffat P. A. , Lindner J. K. N.
Published: July 2002

In order to investigate the interface between polycrystalline-silicon (poly-Si) and crystalline silicon (c-Si), which is of crucial importance for the passivation of high-voltage devices, an infrared diagnostic method has been developed which is based on a modified attenuated total reflection configuration. This interface is shown to consist of silicon oxides (mainly SiO2) in the monolayer range with a thickness of 7±2 Å. The interpretation of the experimental results is based on a direct comparison of the infrared reflectivity spectrum of the interface to be studied with that of a reference sample containing a 100-Å thick SiO2 interface layer, as well as on extensive computer calculations. Such calculations have been performed for a three-layer system as well as for a simplified system consisting of a single absorbing layer sandwiched between two transparent half-spaces. The latter system can be solved analytically and provides detailed insight into the physics of the interaction of light with the vibrational excitations of the interface layer. The existence and properties of such a silicon oxide interface layer are compatible with our secondary ion mass spectrometry experiments (oxygen segregation at the interface) and its thickness is in excellent agreement with the thickness of the amorphous interface layer observed by transmission electron microscopy.
Absorption tail of low resistivity CdSexTe1−x: Comparison between absorption and quantum efficiency measurements
Authors: Neumann-Spallart M. , Galun E. , Hodes G. , Lévy-Clément C. , Marfaing Y. , Muranevich E. , Tenne R.
Published: July 2002

Optical-absorption and photocurrent spectra of a Schottky junction under different bias are compared for CdSexTe1−x (0.54≤x≤1), near the energy gap, at room temperature. An exponential dependence (Urbach tail) is established for both kinds of spectra; however the logarithmic slope of the photocurrent tail is shown to be larger than that of the absorption. Furthermore, this slope increases with forward bias applied to the Schottky diode. These phenomena can be accounted for by a model that considers light absorption, tunneling, and transport of minority carriers. Satisfactory agreement with experimental results is obtained.
Optical investigation of the two-dimensional hole energy spectrum in GaAs/AlxGa1−xAs heterojunctions
Authors: Silov A. Yu. , Asnin V. M. , Averkiev N. S. , Haverkort J. E. M. , Weegels L. M. , Weimann G. , Wolter J. H.
Published: July 2002

We present the results of optical studies on the energy spectrum of a two-dimensional hole gas (2DHG) in a GaAs/AlxGa1−xAs structure. The photoluminescence (PL) line shape in the 2DHG is investigated as a function of temperature by heating the holes by a current flow through the 2D hole channel. The line shape of the PL from the 2DHG as a function of temperature is calculated by taking into account the real band structure and the hole–hole final-state interaction. By comparing experiment and theory, it is found that the special features of the band structure predicted theoretically explain the experimental data.
Thermally stimulated exoelectron emission from silicon subjected to argon plasma treatment
Authors: Momose Y. , Yamamoto T. , Takeuchi M. , Sakurai T.
Published: July 2002

We have investigated the thermally stimulated exoelectron emission (TSEE) from Ar plasma-treated Si by x-ray photoelectron spectroscopy and by employing a theoretical equation. The sample surfaces were exposed to ambient air after the plasma treatment. The TSEE glow curve (298–623 K) exhibited broad emission peaks at ∼373 and ∼473 K, and an increase in the intensity above 573 K. A theoretical analysis revealed that the glow curves were composed of four peaks except for the increase above 573 K, where each peak had a different trap depth, but almost the same electron affinity. The resolved two peaks in the lower temperature region increased compared to those in the higher temperature region with an increase in the O1s/Si2p ratio and also in the Si2p (oxide)/Si2p (substrate) ratio. The relationship of the surface chemical structure to the resolved four peaks is considered.
The principle of a new method for determining local trapping states by space-charge-limited current
Authors: Lu Lianggang
Published: July 2002

Unlike the previous space-charge-limited current measurement for the continuous distributions of bulk traps in semiconductors and insulators, a new approach of so-called freezing space-charge-limited current, which extracts the message of the energy profiles by analyzing the J-T curves and its first logarithmic derivatives, is put forward in this article. It is shown theoretically that the expression of density of states in the new method is more sensitive to the logarithmic derivative d ln J/d ln T than to the d ln J/d ln V in the space-charge-limited current test. In the new theory, the effects of the mobility of the free carrier varying with temperature, the limitation of the new method in calculation, and the problem of control for the space-charge-limited condition in experimentation are discussed as well.
Effect of dopant incorporation on structural and electrical properties of sprayed SnO2:Sb films
Authors: Goyal D. J. , Agashe Chitra , Marathe B. R. , Takwale M. G. , Bhide V. G.
Published: July 2002

Antimony-doped tin oxide films were deposited by spray pyrolysis technique. The effect of antimony doping on structural and electrical properties was investigated in detail using the x-ray diffraction technique and room-temperature Hall measurements. Antimony doping did not affect the preferred growth along [200] to a considerable extent. These results were analyzed on the basis of structure factor calculations. From the Hall measurements, the lowest electrical resistivity, i.e., 5.2×10−4 Ω cm was observed for the films with a doping level of 2.3 at. % in the solution. This value of electrical resistivity is the lowest reported so far in the case of spray deposited antimony-doped SnO2 films. The grain boundary and ionized impurity scattering were observed to be prevalent in governing the electronic transport of lightly and heavily doped films, respectively.
Selective plasma oxidation of oxygen-deficient YBa2Cu3O7−x
Authors: Yoshida Akira , Tamura Hirotaka , Gotoh Kohtaroh , Imamura Takeshi , Hasuo Shinya , Akimitsu Jun
Published: July 2002

A selective plasma oxidation method was developed to make superconducting patterns in nonsuperconducting deoxygenated YBa2Cu3O7−x (YBCO) films. Superconductor- nonsuperconductor-superconductor junctions were fabricated in oxygen-deficient c-axis-oriented polycrystalline YBCO films by selective plasma oxidation using Au/BaF2 oxidation masks. These devices displayed SNS-like or SIN-like nonlinear current-voltage relationships, depending on the device size and oxidation time. The oxygen penetration depth beneath the oxidation mask was estimated to be about 2 μm for the polycrystalline films. Oxygen supplied to 300-nm-thick films for 5 min changes its phase, determined from x-ray diffraction patterns, from tetragonal to orthorhombic I having a critical temperature of about 85 K. X-ray diffraction and magnetic susceptibility measurement showed that the plasma oxidation can supply oxygen to a deoxygenated 0.7×0.5×0.05 mm YBCO single crystal, changing the entire crystal from tetragonal to the orthorhombic II phase whose critical temperature is about 50 K. Four-probe resistance measurement of the plasma oxidized single crystal showed that the phase of the surface of the sample became orthorhombic I with the critical temperature of 84 K by plasma oxidation.
Nonlinear electromagnetic field response of high T
c superconducting microparticle composites
Authors: Haus J. W. , Chung-Yau F. , Bowden C. M.
Published: July 2002

We consider a dielectric host medium with spherical high Tc superconducting particles embedded in it. The nonlinear electromagnetic field response of the composite superconductor material is calculated using a time-dependent Landau–Ginzburg model. Specifically, we predict a strong frequency dependence of the nonlinear susceptibility, field intensity dependence of the transition temperature, and third harmonic generation. The mechanism for the nonlinearity is the breaking of quasi-particle bonds by the electromagnetic field.
X-ray photoelectron and Auger electron diffraction probing of Ge heteroepitaxy on Si (001) 2×1
Authors: Diani M. , Bischoff J. L. , Kubler L. , Bolmont D.
Published: July 2002

Epitaxial molecular beam epitaxy growth of nanometric Ge layers on Si (001) 2×1 has been investigated, in situ, by x-ray Si 2p photoelectron diffraction and Auger Ge LMM electron diffraction which consist essentially in preferential scattering of electrons in the direction of interatomic axes. Particular attention was paid to measuring the contrasts of this anisotropic emission in the (11̄0) plane as a function of deposition parameters. It can thus be determined how crystalline material quality and epitaxial perfection are affected by the residual pressure below 5×10−8 mbar, the substrate temperature decrease to room temperature, the deposition rate, and the Ge overlayer thickness.
Thickness dependence of electrodynamic properties of TlCaBaCuO thin films
Authors: Martens J. S. , Hietala V. M. , Venturini E. L. , Lee W. Y.
Published: July 2002

The thickness dependence of electrodynamic properties of Tl2Ca2Ba2Cu3Ox films has been examined after correction for geometrical effects. The surface resistance increases for very thin or thick films as is consistent with the presence of a poorly superconducting layer (phase inclusions or morphology breakdown) as was detected with magnetization measurements. The temperature dependence of surface resistance has been examined at low temperatures and fitted to a multigap model. There was reasonable agreement for moderate thickness films, suggesting why such films have been and will be excellent for applications, while the behavior of other thicknesses may be dominated by grain boundaries.
Thermally stable, buried high-resistance layers in p-type InP obtained by MeV energy Ti implantation
Authors: Martin Jaime M. , Nadella Ravi K. , Vellanki Jayadev , Rao Mulpuri V. , Holland O. W.
Published: July 2002

High-energy Ti+ ions ranging from 1 to 5 MeV were implanted into p-type InP:Zn (for two different zinc concentrations) at both room temperature and 200 °C. The range statistics for Ti implanted at various energies were calculated by analyzing the as-implanted profiles determined by secondary-ion mass spectrometry. Ti did not redistribute during post-implantation annealing except for a slight indiffusion, irrespective of the implant or annealing temperatures used. This behavior is different from the behavior of other implanted transition metals (Fe and Co) in InP, which redistributed highly when the implants were performed at room temperature. In the MeV Ti-implanted InP:Zn the background Zn showed a small degree of redistribution. Rutherford backscattering measurements showed a near virgin lattice perfection for 200 °C implants after annealing. Buried layers with intrinsic resistivity were obtained by MeV Ti implantation in InP:Zn (p=5×1016 cm−3).
Terrace width evolution during step-flow growth with multiterrace adatom migration
Authors: Chalmers S. A. , Tsao J. Y. , Gossard A. C.
Published: July 2002

We have developed a computer-solvable model of step-flow growth that includes both anisotropic multiterrace adatom migration and asymmetric step edge attachment. We find that terrace widths equalize not only when each adatom preferentially attaches to the ascending step edge of the terrace that it lands on, but also when adatoms preferentially migrate over ascending step edges. This latter process can equalize long-range terrace width nonuniformities much more rapidly than can the former process. We also find that a slow lateral movement of terrace width distributions occurs when each adatom adheres to the step edges of the terrace that it lands on. More significantly, we find that a rapid lateral movement of terrace width distributions occurs when adatoms cross multiple step edges. This motion is especially fast when adatoms migrate distances that are comparable to or greater than the terrace width distribution period. We simulated the evolution of an experimentally observed (Al,Ga)Sb lateral superlattice (LSL) terrace width distribution, which led to quantitative estimates of the adatom migration characteristics present during the LSLs growth. At least one type of adatom, probably Ga, migrates nearly isotropically over many terraces. This method of determining adatom migration characteristics can be extended to any material system that allows LSL layers to be grown as terrace width markers.
Role of hydrogen and oxygen in diamond synthesis using carbon-dioxide–methane-gas mixtures
Authors: Chen Chia-Fu , Hong Tsao-Ming , Chen Sheng-Hsiung
Published: July 2002

A negative effect on the quality of diamond film because of the addition of hydrogen to carbon-dioxide–hydrocarbon gases was investigated. To elucidate the role of hydrogen and oxygen in diamond synthesis using microwave plasma chemical-vapor deposition, diamond films were deposited by adding hydrogen and oxygen to carbon-dioxide–methane-gas mixtures. Improvements in the quality of diamond film and extensions of the diamond-forming region were obtained, due to the addition of oxygen to the carbon-dioxide–methane-gas mixtures. The nucleation density of deposits increased when the amount of hydrogen was increased but decreased when the amount of oxygen was increased. The results of optical emission spectroscopy indicate that the amount of atomic hydrogen in the ground state remained nearly the same, respectively, with increasing amounts of hydrogen and oxygen. However, the amount of electronically excited atomic hydrogen and C2 emissions increased with an increase in the amount of added hydrogen; this means that the electronically excited atomic hydrogen did not benefit diamond growth and the C2 radicals facilitated the formation of amorphous or graphitic carbon. In contrast to the addition of hydrogen, when oxygen was added, the electronically excited atomic hydrogen and C2 radicals decreased, and a larger amount of oxygen-containing species such as atomic oxygen and OH radicals was in the plasma, so the quality of the diamond film improved. In summary, adding hydrogen to the carbon-dioxide–methane-gas mixtures had a negative effect on diamond growth, whereas adding oxygen had a beneficial effect.
Degradation of oxides in metal-oxide-semiconductor capacitors under high-field stress
Authors: Patrikar R. M. , Lal R. , Vasi J.
Published: July 2002

High-field effects in metal-oxide-semiconductor capacitors have been studied in detail. A comprehensive set of experiments, stressing identically fabricated capacitors on both P-type and N-type silicon, both in accumulation and in inversion, has been made to study defect generation in the oxide at high fields. There is clear experimental evidence that both bulk hole and electron traps are generated under all stress conditions. High-field prebreakdown properties depend mainly upon the dynamics of generation of traps, trapping and detrapping at these and in previously existing traps. It has been found that of several processes, some dominate, depending upon the type of silicon and polarity during stressing, and this is also true for the final breakdown mechanism. In this paper the dominant mechanisms are identified for each of the field stress conditions that have been studied.
Spectrum of anomalous random telegraph noise
Authors: Wang Yicheng
Published: July 2002

The alternate capture and emission of electrons at an individual defect site generates discrete switching in resistance, referred to as a random telegraph signal (RTS). Recent experiments indicate that some defects might have two mutually exclusive emission modes with distinct emission rates, which result in the anomalous RTS: a rapid-switching RTS modulated in time by a slow-switching RTS of the same amplitude. The spectrum is calculated of the anomalous RTS by assuming that the emission mode for a captured electron is determined at the moment of capture of the electron, and the probability for a given mode is a constant p in each event of capturing. It is shown that a distribution in p might lead to a 1/f spectrum.
Evolution of structural and magnetic properties in Ta/Ni81Fe19 multilayer thin films
Authors: Hashim I. , Atwater H. A. , Kung K. T. Y. , Valletta R. M.
Published: July 2002

The interdiffusion kinetics in short period (12.8 nm) Ta/Ni81Fe19 polycrystalline multilayer films has been investigated and related to the evolution of soft magnetic properties upon thermal annealing in the temperature range 300–600 °C. Small angle x-ray diffraction and transmission electron microscopy were used to estimate the multilayer period. Interdiffusion in the multilayers was directly computed from the decay of the satellites near (000) in a small angle x-ray diffraction spectrum. A kinetic analysis of interdiffusion suggests that grain growth is concurrent with grain boundary diffusion of Ta in Ni81Fe19. The evolution of soft magnetic properties of Ni81Fe19, i.e., lowering of 4πMs and increase in coercivity Hc, also lend support to the above analysis.
Growth and characterization of epitaxial SrTiO3 thin films with prominent polarizability
Authors: Iwabuchi Mamoru , Kobayashi Takeshi
Published: July 2002

Epitaxial SrTiO3 (STO) thin films were grown successfully by rf reactive magnetron sputtering on (111)Pt/(100)MgO substrates. At present, the dielectric constant of the STO film (110 nm thick) has reached 370ε0 at room temperature. Despite the general difficulty of obtaining a high dielectric constant for the ultrathin STO films, a fairly thin STO film of 39 nm showed a large value of 240ε0. The existence of an STO degraded layer at the interface was suggested by the Fourier transform infrared spectrum observed for the ultrathin STO film of 3.5 nm. The interfacial layer with poor quality diminishes the mean dielectric constant of the STO thin films. This interfacial layer effect offers an explanation for the difficulty in obtaining a high dielectric constant of the ultrathin STO films. Moreover, the band diagram of a Cr/STO/Pt metal-insulator-other metal structure is discussed. Compared to the experimental results, a space-charge-free band diagram was concluded to be appropriate and was consistent with the observed Schottky-like leakage current and the inverse proportionality of the capacitance and the STO thickness.
Mechanism of combustion synthesis of silicon carbide
Authors: Narayan J. , Raghunathan R. , Chowdhury R. , Jagannadham K.
Published: July 2002

The mechanism of self-propagating high-temperature synthesis (SHS) or combustion synthesis of SiC has been investigated using pellets consisting of silicon and carbon powders. The combustion reaction was initiated by rapidly heating the pellet on a graphite strip. The reaction products were analyzed using scanning and transmission electron microscopy, x-ray diffraction, and Raman spectroscopy. The results show that it is possible to produce β-SiC without any residual silicon and carbon. Occasionally, a very small number density of α-SiC precipitates embedded in the β-SiC matrix was observed. Based upon the microstructural features, it is proposed that the formation of SiC involves the dissolution of carbon into liquid silicon, diffusion of C into liquid silicon, and subsequent precipitation of SiC. The size of the SiC crystallites is determined by the diffusion coefficient of carbon in liquid silicon and the time available for SiC precipitation. The activation enthalpy for the SHS process is estimated to be 59±3 kcal/mol.
Microstructures and crystallographic orientation of crystalline grains in anisotropic Nd-Fe-Co-B-(Ga or Zr) magnet powders produced by the hydrogenation-decomposition-desorption-recombination process
Authors: Nakayama R. , Takeshita T. , Itakura M. , Kuwano N. , Oki K.
Published: July 2002

The microstructures of anisotropic Nd-Fe-Co-B-(Ga, or Zr) magnet powders produced by the hydrogenation-decomposition-desorption-recombination (HDDR) process and the resulting crystallographic c-axis orientation of Nd2(Fe,Co)14B crystals in the powder particles have been studied. It was found that the powder particles consist of fine Nd2(Fe,Co)14B crystals of the size of about 0.3 μm, and any boundary-layer phase between the Nd2(Fe,Co)14B crystalline grains is almost absent. The morphology of fine Nd2(Fe,Co)14B crystalline grains in anisotropic magnet powders is the same as that of isotropic magnet powders produced by the HDDR process. In anisotropic powders, it was found that there is a strong correlation among the a axes, b axes, and c axes of the fine Nd2(Fe,Co)14B crystal grains. The dispersion in solid angles made by the c-axis direction is less than ±18° in the case of Nd-Fe-Co-B-Ga magnet powder particle. The c-axis direction of fine grains in magnet powders produced by the HDDR process is associated with that of large Nd2(Fe,Co)14B grains in the original cast or the homogenized alloy. The microstructure of anisotropic magnet powders before the hydrogen desorption step in the HDDR process is very complicated. It consists of five distinct regions, and among these, two regions are made of NdH2 and Fe.
Analysis of deep levels in n-type GaN by transient capacitance methods
Authors: Hacke P. , Detchprohm T. , Hiramatsu K. , Sawaki N. , Tadatomo K. , Miyake K.
Published: July 2002

Transient capacitance methods were used to analyze traps occurring in unintentionally doped n-type GaN grown by hydride vapor-phase epitaxy. Studies by deep-level transient spectroscopy (DLTS) and isothermal capacitance transient spectroscopy indicated the presence of three majority-carrier traps occurring at discrete energies below the conduction band with activation energies (eV) ΔE1=0.264±0.01, ΔE2=0.580±0.017, and ΔE3=0.665±0.017. The single-crystal films of GaN were grown on GaN formed by metal-organic chemical-vapor deposition and on sputter-deposited ZnO; a similar deep-level structure was found in both types of samples. Pulse-width modulation tests using DLTS to determine the capture rates of the traps showed that the capture process is nonexponential, perhaps due to the high trap concentration. The origins of the deep levels are discussed in light of secondary-ion-mass-spectroscopy analysis and group theory results in the literature.