bluuebunny/crossref_metadata_2025 huggingface dataset:
Demagnetization factors for general ellipsoids
Authors: Cronemeyer D. C.
Published: July 2002

Convenient and inclusive tables for the demagnetization factors of general ellipsoids are calculated by an APL program using the mathematical framework of Osborn [Phys. Rev. 67, 351 (1945)] along with elliptical integral function tables of Spenceley and Spenceley [Smithsonian Elliptic Function Tables, 109, 3863 (1947)]. The new demagnetization tables have immediate application to magnetic and superconductor studies of present day prominence. For an ellipsoid of major axes a≥ (R18)b≥ (R18)c the range of variables is from 1≥ (R18)(c/a)≥ (R18)0.0001, and 1≥ (R18)(b/a)≥ (R18)0.01; appropriate approximations are indicated for smaller values of these ratios. In addition details of the mathematical calculation are included so that those interested may duplicate the procedure. Methods of applying this ‘‘ellipsoidal’’ approximation to other shapes such as parallelopipeds are also indicated.
Enhanced line emission from laser-produced plasmas
Authors: Timmer C. , Srivastava S. K. , Hall T. E. , Fucaloro A. F.
Published: July 2002

This communication reports the first systematic study on background gas-induced, spectral line emission enhancement from laser-produced plasmas. Line emission from aluminum plasmas was enhanced by factors of up to 35 by the introduction of He, Ne, Xe, or N2. The enhancement has been attributed to three-body recombination.
On the comparison between Josephson-junction array variations
Authors: Tsang Kwok Yeung , Wiesenfeld Kurt
Published: July 2002

Recent work has shown that the expected performance of a given Josephson-junction series array depends greatly on whether the coupling load is a resistor or an inductor-capacitor combination. This raises the question of how arrays using different loads can be compared in a quantitatively meaningful way, for example, with respect to the effects of noise. We show that it is possible to choose the circuit parameters so that the in-phase output for different loads is, though not identical, nearly so. The key lies in the development of a nonlinear generalization of the load impedance.
Intersubband transitions in partially interdiffused GaAs/AlGaAs multiple quantum-well structures
Authors: Ralston J. D. , Ramsteiner M. , Dischler B. , Maier M. , Brandt G. , Koidl P. , As D. J.
Published: July 2002

Continuous tuning over the entire 8–12 μm wavelength range is demonstrated for the intersubband absorption resonance in n-doped GaAs/AlxGa1−xAs multiple quantum-well structures following partial interdiffusion of the well and barrier layers via rapid thermal annealing. The data indicate that redshifting of the intersubband absorption resonance arises both from interdiffusion-induced modification of the confining potential and from a decrease in the depolarization shift. The latter effect is due in part to a decrease in the free-carrier concentration within the Si-doped quantum wells following rapid thermal annealing. Significant diffusion of the localized Si dopant is also observed over the range of annealing temperatures investigated here. Calculated values of the Al-Ga interdiffusion coefficient, as a function of anneal temperature, indicate that Si diffusion through the heterointerfaces contributes substantially to layer intermixing.
Mode-III cracks in piezoelectric materials
Authors: Zhang Tong-Yi , Hack J. E.
Published: July 2002

The stress and electric fields around a mode-III crack containing a dielectric medium are formulated. Mechanical equilibrium requires that the crack surfaces be traction free. Previous solutions have used the electrical boundary condition that the electric displacement component perpendicular to the crack surfaces should be zero. However, cracks that are filled with a dielectric medium, such as vacuum or air, require that the electric displacement be continuous across the crack faces. Using the boundary condition appropriate for an insulating crack, the stress, strain, and electric-field strength are found to exhibit r−1/2 singularities while the electric displacement does not. The singularity in the electric-field strength arises from piezoelectricity. The driving force for crack growth is only related to the effective level of applied stress. Under constant displacement, the applied field may increase or decrease the effective applied stress depending on its direction. As a result the electric field may promote or retard the crack propagation.
The enhancement of solid solubility limits of AlCo intermetallic compound by high-energy ball milling
Authors: Sui H. X. , Zhu M. , Qi M. , Li G. B. , Yang D. Z.
Published: July 2002

In this work the AlCo intermetallic compound has been prepared by high-energy ball milling of Co and Al powder mixtures with the composition of Co-50 at.% Al, Co-71.4 at.% Al, Co-76.5 at.% Al, and Co-81.8 at.% Al, respectively. X-ray diffraction and transmission electron microscopy analysis of powders milled for different times proved that the AlCo phase formed directly during milling of the powder mixture of the former three compositions. In the Co-81.8 at.% Al powder mixture the amorphous phase was first formed and then crystallized to the AlCo phase. The result of energy dispersion x ray showed that the composition of AlCo phase formed during milling of the above four compositions is in agreement with that of the powder mixture, although their crystal structure is that of the AlCo intermetallic compound. The results indicate that the solubility range of intermetallic compound during mechanical alloying processes can be greatly enhanced by high-energy ball milling. The enhancement of solubility is discussed based on the structural features of nanocrystals.
Optical properties of amorphous C/diamond thin films
Authors: Silva S. R. P. , Amaratunga G. A. J. , Constantinou C. P.
Published: July 2002

Amorphous C/diamond films have been prepared by rf plasma enhanced vapor deposition from a CH4/Ar gas mixture. Infrared and optical-ultraviolet absorption characteristics are reported and used to characterize the bonding and optical properties of these films. It has been found that the optical band gap is not related to the hydrogen content in the films and varies according to the dc self bias developed during deposition. The IR data show that the hydrogen in the a-C/diamond films is associated with triply bonded sp1 carbon as in acetelyne. The optical band gap of the films can be varied from 1.2–4.0 eV.
Grain boundary diffusion of cobalt in polycrystalline tungsten
Authors: Arkhipova N. K. , Kaygorodov V. N. , Klotsman S. M. , Kozhevina E. V.
Published: July 2002

The radiometric sectioning method was used to study diffusion of 57Co along the grain boundaries of polycrystalline tungsten in the temperature range from 780 to 1980 K. At T−1280 K (−0.35Tm where Tm is the melting point of the matrix element) the temperature dependence of the grain boundary diffusion (GBD) data exhibits a break which is analogous to the one observed for GBD in silver polycrystals. This break is most likely due to the fact that the mechanism of diffusant ‘‘pumping’’ from the grain boundary to the bulk changes at low temperatures.
Band edge optical absorption in intrinsic silicon: Assessment of the indirect transition and disorder models
Authors: Corkish Richard , Green Martin A.
Published: July 2002

Two competing theories of the optical absorption edge of intrinsic crystalline silicon, the indirect transition theory, and the recent disorder (Urbach edge) theory, are assessed critically. The assessment of the indirect transition theory includes an attempt to predict, by the use of the luminescence spectrum and the principle of detailed balance, the optical absorption in the wavelength range where it is mediated by the simultaneous absorption of up to three phonons. The inability of this method to accurately reproduce the experimental data is used to illustrate limitations of the indirect transition theory: its neglect of band degeneracy and its failure to explain the different absorption replica shapes for different phonons. In its published form, the disorder theory does not explicitly address the question of the role of indirect processes or the nature of the strong transitions which trigger the disorder processes. Some of the evidence presented in favor of the theory is shown to be in doubt. The possible synthesis of the two theories is proposed.
Spin-glass behavior in amorphous BiFeO3
Authors: Nakamura Shin , Soeya Susumu , Ikeda Naoshi , Tanaka Midori
Published: July 2002

Short-range antiferromagnetic (speromagnetic) behavior was observed for amorphous BiFeO3 by static magnetic measurements, ac susceptibility (χac) measurement, and 57Fe Mössbauer spectra measurements. The magnetic behavior is classified into three temperature ranges: (i) paramagnet for T≳220 K; (ii) local clustering of spins for 20 K<T<220 K; and (iii) speromagnet in which all spins are frozen into random directions for T<20 K. In the spin freezing state the magnetization shows irreversibility and the χac−T curve has a cusp. The variation of the spin freezing temperature against measuring magnetic field and measuring frequency indicate that the speromagnetic order is very similar to that of spin glass.
Quantum-well lasers for direct solar photopumping
Authors: Unnikrishnan Sreenath , Anderson Neal G.
Published: July 2002

Semiconductor lasers directly photopumped by focused sunlight may be viable sources of coherent light for intersatellite communications and other low-power spaceborne applications. In this work, we theoretically explore the possibility of realizing such devices. We specifically assess solar pumped operation of separate-confinement-quantum-well heterostructure (SCQWH) lasers based on InGaAs, GaAs, and AlGaAs, as fabrication technology for these lasers is mature and they can operate at very low thresholds. We develop a model for step-index single-well SCQWH lasers photopumped by sunlight, examine how threshold solar photoexcitation intensities (or solar magnification requirements) depend upon material and structure parameters, design optimum structures for solar-pumped operation, and identify design trade offs. Our results suggest that laser action should be possible in properly designed structures at readily achievable solar concentrations (103–104 suns under air-mass-zero conditions), and that optimum designs for solar-pumped SCQWH lasers differ significantly from those for analogous current injection devices.
A method to correct for leakage current effects in deep level transient spectroscopy measurements on Schottky diodes
Authors: Dmowski K. , Lepley B. , Losson E. , Bouabdellati M. El
Published: July 2002

An improved procedure is proposed to determine the accurate values of deep level parameters in Schottky diodes with non-negligible majority-carrier capture rate due to a leakage current. Numerical modeling of deep level transient spectroscopy signals from the EL2 center in GaAs demonstrates the degree to which leakage currents of various magnitudes can distort the spectra and without correction lead to misleading deep level parameters. Finally, deep levels in n-type InP Schottky diodes have been examined using a commercially available lock-in spectrometer DLS-82E to demonstrate the practical application of the proposed procedure.
Optical and transport properties of single quantum well infrared photodetectors
Authors: Bandara K. M. S. V. , Levine B. F. , Leibenguth R. E. , Asom M. T.
Published: July 2002

We have performed an extensive series of measurements on symmetrical barrier bound-to-continuum and asymmetrical barrier bound-to-bound quantum well infrared photodetectors consisting of only a single well. We find that the behavior of the optical (e.g., responsivity) and transport properties (e.g., gain) as a function of bias is strikingly different from that of the usual multi-quantum well detectors. The simplicity of the structure has allowed an accurate theoretical calculation of the potential drops across each barrier, the photoinduced carrier depletion in the quantum well and therefore a detailed understanding of the device physics.
Binding energies and density of impurity states in spherical GaAs-(Ga,Al)As quantum dots
Authors: Porras-Montenegro N. , Pérez-Merchancano S. T. , Latgé A.
Published: July 2002

The binding energies of hydrogenic donor in both finite and infinite GaAs-(Ga,Al)As spherical quantum dots are calculated as a function of the donor position for different radii within the effective-mass approximation. It is observed an enhancement of the binding energy of donors in quantum dots when compared to results in quantum wells and quantum-well wires, which is an expected consequence of the higher geometrical electronic confinement in these systems. The density of impurity states as a function of the donor binding energy was also calculated. As a general feature it presents structures associated with special impurity positions that may be important in the understanding of the absorption and photoluminescence experiments of doped quantum dots.
A new scheme to percolation thresholds
Authors: Galam Serge , Mauger Alain
Published: July 2002

Using a ferromagnetic Ising model, a new lattice homogeneity breaking approach (HBA) for two-dimensional site dilution is presented. The critical temperature is stated to result from an energetic balance between the cluster center and its surface. This definition produces a breaking in the homogeneity of the system between the fluctuating cluster magnetization and the mean bulk counterpart. Site dilution is then found to generate additional breaking of the system homogeneity, with the introduction of a generalized surface effect which makes our procedure qualitatively different from the Bethe one. The HBA results in a simple new analytic expression for site percolation thresholds, which reproduces the exact values or numerical estimates for all lattice topologies in dimension two, within few percent.
Monte Carlo based calculations of hole transport including the hole-plasmon interaction in degenerate bulk GaAs
Authors: Mansour Nabil S. , Wang Yang , Brennan Kevin F.
Published: July 2002

A theoretical study of the effect of the hole-plasmon interaction on the calculated bulk transport properties of degenerate GaAs is presented. The calculations are performed using an ensemble Monte Carlo simulation which includes the full details of the heavy, light, and split-off valence bands derived from a k○p calculation as well as all of the dominant hole-phonon scattering mechanisms. The hole-plasmon interaction is treated as a separate scattering mechanism and is computed to first order in perturbation theory. The plasmon dispersion relation is determined numerically by finding the zeroes of the dielectric function assuming hole occupation within both the heavy- and light-hole bands. The hole-plasmon scattering rate is computed self-consistently during the course of the Monte Carlo simulation as a function of the hole concentration and carrier temperature within the heavy-hole and light-hole bands. It is found that at degenerate hole concentrations, the hole-plasmon scattering rate is much smaller than the dominant phonon scattering rates in GaAs at the range of applied electric field strengths considered. It is also found that the hole-plasmon scattering rate has little appreciable effect on the calculated steady-state velocity-field and energy-field characteristics in degenerate GaAs within the present model and under the prescribed conditions.
Effects of Co and Y substitution on magnetic properties of CeFe2
Authors: Zhang Xianfeng , Ali Naushad
Published: July 2002

CeFe2 orders ferromagnetically (FM) below TC=227 K; however, this FM state is inherently unstable and partial substitution of Fe by Co precipitates the instability of FM state and a ferromagnetic to antiferromagnetic (AFM) phase transition is observed at lower temperature. This second phase transition is a first-order phase transition. The dependence of TC and TN on the concentration of Co X for Ce(Fe1−XCoX)2 is presented. For Ce(Fe0.8Co0.2)2, the FM transition is at TC=160 K, and the AFM transition is at TN=76 K. The entropy change associated with the FM to AFM phase transition has been determined from magnetization measurements and found to be primarily associated with a change in the electronic density of states at EF. If one starts with Ce(Fe0.8Co0.2)2 and partially substitutes Ce by Y, it is found that TN gradually decreases with Y concentration; and for Y concentration greater than 10%, the AFM phase disappears and FM phase is fully restored. The dependence of TC and TN on the concentration of Y Z is presented for Ce1−ZYZ(Fe0.8Co0.2)2. It is suggested that the second transition (FM to AFM) is a result of the competition between exchange energy and magnetic anisotropy energy. The measurements of magnetization, ac susceptibility, resistivity, and thermal expansitivity are presented.
Modification of surface band bending of diamond by low energy argon and carbon ion bombardment
Authors: Lau W. M. , Huang L. J. , Bello I. , Yiu Y. M. , Lee S.-T.
Published: July 2002

Argon and carbon ion bombardment of p-diamond at 500–5000 eV in ultrahigh vacuum were studied by in situ x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction analysis. Both argon and carbon ion bombardment at room temperature in the present energy range created a defective surface layer. The radiation damage was manifested by the introduction of a distinct C 1s peak (referred to as the ‘‘defect’’ peak later) with a binding energy about 1 eV less than that of the bulklike diamond peak, and by the introduction of some additional filled states (referred to as the ‘‘filled states’’) near the valence band edge of diamond. It was found that in comparison to argon bombardment, carbon bombardment was more efficient in producing the filled states but less efficient in raising the C 1s defect peak. While the filled states disappeared by annealing at about 500 °C, the C 1s defect peak did not change much even with a 1000 °C anneal. These results suggest that the C 1s defect peak, which has also been observed on reconstructed diamond surfaces after hydrogen desorption [see, e.g., B. B. Pate, Surf. Sci. 165, 83(1986)], is associated with vacancy formation and aggregation which give some ‘‘internal surfaces’’ with a behavior like a reconstructed atomically clean diamond surface.

The filled states introduced by ion bombardment are associated with interstitials or interstitial clusters. The amount of residual defects was found to increase with both an increasing bombardment dose and energy. For an argon bombardment at 1000 eV to a dose of 5×1014/cm2, the defective layer was estimated to be about 1.5 nm. Further, it was found that the radiation damage, particularly the ‘‘vacancy defects’’, could only be annealed (at 1000 °C) when the dose was below 5×1014/cm2 at a bombardment energy of 500 eV. XPS band bending analyses also showed that room temperature bombardment induced a small reduction (0.2 eV) of the surface Fermi level position (EFs) on the p-diamond. However, subsequent vacuum annealing caused a rather large increase of EFs. But the EFs data from about 20 bombarded and annealed samples were always less than 2.2 eV. Thus the formation of an n-type diamond was not observed.
Some new features of the photoluminescence of SiC(6H), SiC(4H), and SiC(15R)
Authors: Haberstroh Ch. , Helbig R. , Stein R. A.
Published: July 2002

Low temperature photoluminescence (LTPL) measurements have been performed on 6H, 4H, and 15R polytype SiC crystals. In addition to well-known emission lines close to the band gap a couple of new features could be observed. Emission lines attributed to the recombination of bound excitons at aluminum acceptors have been seen in all three polytypes. A defect interpreted as a divacancy is described for SiC(4H). In SiC(6H) emission lines due to scandium centers have been observed. Furthermore the effect of a uniaxial stress on the LTPL spectrum is investigated.
Hopf bifurcations and hysteresis in resonant tunneling diode circuits
Authors: Wallis C. R. , Teitsworth S. W.
Published: July 2002

Conditions for Hopf bifurcations and associated current oscillations are presented for a standard model of a resonant tunneling diode circuit. We derive a simple analytic formula for the Hopf bifurcation type (subcritical or supercritical) and discuss how this affects circuit operation, e.g., whether or not hysteresis and bistability are observed in current-voltage curves.
Thermal diffusivity measurements in opaque solids by the mirage technique in the temperature range from 300 to 1000 K
Authors: Sánchez-Lavega A. , Salazar A.
Published: July 2002

A method to measure the thermal diffusivity of solid samples as a function of temperature is presented. The measurement technique is based on the mirage effect and in its linear zero-crossing relation for the transverse deflection, whose slope directly gives the diffusivity of the material. A 3D theoretical model has been developed in order to include both the effects of the radiative and convective heat transfers between the sample and its surroundings, and the temperature dependence of the refractive index and thermal diffusivity of the gas. The model also incorporates the effects introduced by the mirage parameters (sizes of the pump and probe beams, and probe beam height). The samples studied are opaque and thermally thick, and the applicability of the method is restricted to materials with diffusivity ≳1 mm2/s. Two experimental mirage setups are presented, one with the sample being heated in an open environment, and the other with the sample heated within a furnace. In the first case the range of measurable temperatures goes from ambient to ∼500 K, whereas in the second the upper limit is ∼1000 K. A comparison of the experimental results obtained with this method with those from the literature on calibrated samples of pure nickel, pure cobalt, and an AISI-302 alloy of low thermal diffusivity, confirm the validity of the model and method proposed.
Relation of remanence and coercivity of Nd,(Dy)-Fe,(Co)-B sintered permanent magnets to crystallite orientation
Authors: Kim A. S. , Camp F. E. , Stadelmaier H. H.
Published: July 2002

For sintered permanent magnets based on Nd2Fe14B having magnetically oriented crystallites, a simple method of measuring the degree of alignment by x-ray diffraction is described, using the inverse pole figure technique. The results are related to a distribution function for the easy axis which is a direct measure of remanence. The fraction of the ideal [001] texture component in both Nd-Fe-B and Nd,Dy-Fe,Co-B magnets is enhanced by doping with oxygen, resulting in a measurable increase in remanence and improved loop squareness. The observed increase of the intrinsic coercivity iHc with better crystallite orientation is also discussed.
Particle sputtering and deposition mechanism for material transfer in breaking arcs
Authors: Chen Zhuan-Ke , Sawa Koichiro
Published: July 2002

A novel mechanism called particle sputtering and deposition is proposed based on the facts that the arced contact morphology, mass transfer, and arc durations in the metallic phase and the gaseous phase undergo transition phenomena with increasing load current. The mechanism is used satisfactorily as an explanation of the material transfer process and the contact resistance degradation of Pd and Ag contacts during the breaking of dc circuits.
Crystallization and growth of Ni-Si alloy thin films on inert and on silicon substrates
Authors: Grimberg I. , Weiss B. Z.
Published: July 2002

The crystallization kinetics and thermal stability of NiSi2±0.2 alloy thin films coevaporated on two different substrates were studied. The substrates were: silicon single crystal [Si(100)] and thermally oxidized silicon single crystal. In situ resistance measurements, transmission electron microscopy, x-ray diffraction, Auger electron spectroscopy, and Rutherford backscattering spectroscopy were used. The postdeposition microstructure consisted of a mixture of amorphous and crystalline phases. The amorphous phase, independent of the composition, crystallizes homogeneously to NiSi2 at temperatures lower than 200 °C. The activation energy, determined in the range of 1.4–2.54 eV, depends on the type of the substrate and on the composition of the alloyed films. The activation energy for the alloys deposited on the inert substrate was found to be lower than for the alloys deposited on silicon single crystal. The lowest activation energy was obtained for nonstoichiometric NiSi2.2, the highest for NiSi2—on both substrates. The crystallization mode depends on the structure of the as-deposited films, especially the density of the existing crystalline nuclei. Substantial differences were observed in the thermal stability of the NiSi2 compound on both substrates. With the alloy films deposited on the Si substrate, only the NiSi2 phase was identified after annealing to temperatures up to 800 °C. In the films deposited on the inert substrate, NiSi and NiSi2 phases were identified when the Ni content in the alloy exceeded 33 at. %. The effects of composition and the type of substrate on the crystallization kinetics and thermal stability are discussed.
In situ spectroscopic ellipsometry to monitor the process of TiNx thin films deposited by reactive sputtering
Authors: Logothetidis S. , Alexandrou I. , Papadopoulos A.
Published: July 2002

In situ spectroscopic ellipsometry (SE) in the energy region from 1.5 to 5.5 eV was used to monitor the process of TiNx films deposited by dc reactive magnetron sputtering of Ti. The dielectric function ε(ω) measured by SE yields the optical response of TiNxfilms and valuable information on their structural characteristics that are also verified by electron microscopy observations. The plasma energy of TiNx was found to depend strongly on the stoichiometry of the material and therefore is a valuable parameter for in situ monitoring of TiNx stoichiometry. Analysis of the ε(ω) spectra of the TiNx films deposited in sequential layers by an effective medium theory provides precious information about the initial stages of growth, the composition, the deposition rate, and any change in film growth.
Radiation hardening of pure-silica-core optical fibers by ultra-high-dose γ-ray pre-irradiation
Authors: Griscom D. L.
Published: July 2002

Induced optical absorption in three SiO2-core/SiO2:F-clad fibers (two high-OH, one low-OH) and one SiO2:Ge-core fiber was investigated in situ in the range 415–1100 nm during repeated 60Co γ-ray exposures to doses ≳1 MGy(Si). Injected visible light (∼5 μW) was maintained during the first and second irradiations at ∼27 °C and an intervening period out of the source; an additional irradiation and subsequent out-of-source period were monitored with the light off. Bleaching effects due to radiation alone, light alone, and the synergism of light plus irradiation were independently observed. All visible-range damage in the pure-silica-core fibers in excess of a few tens of dB/km was deduced to arise from activation of precursor states including chloride impurities and process-induced defects. These extrinsic color centers were eventually eradicated and the fibers permanently rad hardened after a cumulative radiation dose of 8.3×106 Gy and a visible light fluence of ∼1.4 mW h. The hardening mechanism is here ascribed to radio- or photostimulated complexing of the extrinsic centers with radiolytically derived interstitial oxygens.
Continuum model of dispersion caused by an inherent material characteristic length
Authors: Rubin M. B. , Rosenau P. , Gottlieb O.
Published: July 2002

Modifications of the Helmholtz free energy and the stress associated with general constitutive equations of a simple continuum are proposed to model dispersive effects of an inherent material characteristic length. These modifications do not alter the usual restrictions on the unmodified constitutive equations imposed by the first and second laws of thermodynamics. The special case of a thermoelastic compressible Newtonian viscous fluid is considered with attention focused on uniaxial strain. Within this context, the linearized problems of wave propagation in an infinite media and free vibrations of a finite column are considered for the simple case of elastic response. It is shown that the proposed model predicts the dispersive effects observed in wave propagation through a chain of springs and masses as the wavelength decreases. Also, the nonlinear problems of steady wave propagation of a soliton in the absence of viscosity and of a shock wave in the presence of viscosity are discussed. In particular it is shown that the presence of the dispersive terms can cause the stress in a shock wave to overshoot the Hugoniot stress by as much as 50%. This phenomenon may cause an underprediction of the threshold level for failure determined by analysis of stress in shock experiments.
Tunnel junctions in a polymer composite
Authors: Paschen S. , Bussac M. N. , Zuppiroli L. , Minder E. , Hilti B.
Published: July 2002

A transparent film of polycarbonate is conducting when it contains long and thin needles of the organic conductor (TSeT)2Cl. Film conductivity of the order of 1 S/cm can be achieved with a content as low as 0.5 vol % of the conducting charge-transfer complex, provided that the needles are obtained by in situ complexation and crystallization during the solution casting of the polymer. We have measured the current/voltage characteristics at different temperatures from 2 to 300 K on both the composite films and on model junctions produced between two needles only. All these data can be understood and fitted to a new three parameter model which is an extension of Sheng’s fluctuation-induced tunnelling idea to arbitrary temperatures and fields.
Surface-diffusion and step-bunching mechanisms of metalorganic vapor-phase epitaxy studied by high-vacuum scanning tunneling microscopy
Authors: Kasu Makoto , Kobayashi Naoki
Published: July 2002

On a very flat GaAs surface we grow a 1/6 monolayer amount of GaAs by metalorganic vapor-phase epitaxy and observe two-dimensional (2D) nuclei by high-vacuum scanning tunneling microscopy. From the 2D nucleus densities we calculate the surface diffusion coefficient of 2×10−6 cm2/s at 530 °C. During growth, the bunched step (multistep) separation saturates and is independent of the substrate misorientation angle. These results can be explained by a mechanism that takes into account both 2D nuclei formation on a wider terrace and their coalescence on ascending steps. A step-bunching simulation based on our model reveals that the saturated multistep separation is proportional to the 2D nucleus separation, i.e., the inverse of the square root of the density.
Applicability of effective medium theory to ferroelectric/ferrimagnetic composites with composition and frequency-dependent complex permittivities and permeabilities
Authors: Mantese Joseph V. , Micheli Adolph L. , Dungan Dennis F. , Geyer Richard G. , Baker-Jarvis James , Grosvenor John
Published: July 2002

High-frequency (1 MHz–1 GHz) transmission line measurements were used to determine the composition and frequency-dependent complex permittivities and complex permeabilities of ferroelectric/ferrimagnetic (barium titanate and a magnesium-copper-zinc ferrite) composites. The effective medium rules of Maxwell–Garnett give both lower and upper bounds for the effective permittivities and permeabilities and yield accurate estimates of the bulk electric and magnetic properties at low volume fill fraction of either component provided the proper host matrix is chosen. Bruggeman theory yielded the best predictive values for the permittivity and permeability over the entire composition range. In all cases these complex quantities were shown to be constrained by Bergman–Milton bounds.
Nitridation process of sapphire substrate surface and its effect on the growth of GaN
Authors: Uchida K. , Watanabe A. , Yano F. , Kouguchi M. , Tanaka T. , Minagawa S.
Published: July 2002

The nitridated layer formed on a (0001) sapphire (α-Al2O3) substrate surface by heating at 1050 °C in ammonia (NH3) gas was analyzed by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscope (TEM) and energy dispersive x-ray spectrometry (EDX). Their influence on the growth of GaN in the combined usage of initial nitridation and successive deposition of a buffer layer was examined by AFM observations. The intensity of the N1s nitrogen peak in the XPS rapidly increased with nitridation time, reaching saturation in a few minutes, and then continued to increase gradually. This change was found to correspond to morphological change revealed by AFM observations, that is, from a flat nitridated layer to high-density (109–1010 cm−2) nitridated protrusions. TEM observations and EDX measurements showed that the nitridation forms an amorphous layer consisting of AlNxO1−x. The flat nitridated layer, when combined with a buffer layer, favors two-dimensional growth of a thick GaN layer on it, while the layer with protrusions results in three-dimensional growth. Thus, thick GaN layers with smooth surfaces can be grown by controlling the surface of the nitridated layer, where a crystal-amorphous-crystal growth mechanism is successfully operating.
Bulk lifetime and surface recombination velocity measurement method in semiconductor wafers
Authors: Ogita Yoh-Ichiro
Published: July 2002

A noncontact and nondestructive measurement method is proposed to separately determine the bulk lifetime τb, and the front and backsurface recombination velocities S0 and Sw in the semiconductor wafer with different surface recombination velocities in the front and backsurfaces. The method is deduced based on a new analytical solution for photoconductivity decays derived for the wafer with different surface recombination velocities. The solution gives the expressions for a photoconductivity decay and a carrier profile under pulse laser excitation. The analytical relationship between the apparent (or effective) lifetime, S0, Sw, τb and a wafer thickness is given. The photoconductivity decay curve dependencies on various values of S0, Sw, and τb have been demonstrated. The algorithm is shown to separately extract τb, S0, and Sw. The algorithm is characterized by the measurement of two photoconductivity decay curves for pulse laser excitation on both surfaces of the wafer. The influence of the laser pulse width for the photoexcitation on the photoconductivity decays is discussed. The method can be readily applicable to noncontact photoconductivity decay measurement techniques using an electromagnetic wave such as a microwave.
High performance NdFeB magnets (invited)
Authors: Kim A. S. , Camp F. E.
Published: July 2002

Although the production and applications of NdFeB magnets have increased enormously in the last decade because of their outstanding magnetic properties at room temperature, some applications were limited because of poor thermal stability and corrosion resistance. To address these problems, there have been many efforts and much progress. In many instances, the alloy modifications produced an improvement of some characteristics but only at the expense of other characteristics. It is therefore necessary to find a method (or alloy) for improving the thermal stability, coercivity, and corrosion resistance without sacrificing performance. We have found that the proper control of very small amounts of Cu, Co, and O in (Nd, Dy)FeB alloys substantially improves the coercivity, high-temperature capabilities, and corrosion resistance without a reduction of remanence. As a result, a high performance NdFeB magnet with excellent temperature stability and corrosion resistance was developed.
Electronic and magnetic properties of the 4d itinerant ferromagnet SrRuO3
Authors: Singh David J.
Published: July 2002

Density functional calculations are used to elucidate the electronic and magnetic structure of SrRuO3. Itinerant ferromagnetic behavior is found both in the cubic perovskite and the actual orthorhombic structure. The calculated moment is 1.59 μB. Unusually strong hybridization is evident in both the electronic structure and magnetism. The Fermi velocities indicate significant spin differentiation of the transport properties.
Magnetotransport and hysteretic behavior in epitaxial La0.67Ca0.33MnO3−δ films
Authors: Canedy C. L. , Ibsen K. B. , Xiao Gang , Sun J. Z. , Gupta A. , Gallagher W. J.
Published: July 2002

We have fabricated epitaxial La0.67Ca0.33MnO3−δ thin films using pulsed laser deposition and performed subsequent thermal treatments. Magnetoresistance values as large as 14 000% (Δρ/ρH) were obtained. Above magnetic ordering temperatures, we observed a thermally activated conductivity that also has a quadratic magnetic field dependence. At low temperatures, we found a large hysteresis in the magnetoresistance that correlates with an unusual magnetic hysteresis curve. We interpret the hysteretic behavior as the result of a ‘‘frozen-in’’ canted spin structure. Our results suggest that the electron conduction in this material is most sensitive to the canting angle of the neighboring Mn spins.
Surface anisotropy of a fine γ-Fe2O3 particle
Authors: Zhang Kezhao , Fredkin Donald R.
Published: July 2002

We investigate the effects of surface anisotropy of a single-domain γ-Fe2O3 fine particle (length 3000 Å, aspect ratio 4.6:1) using micromagnetics and the finite element method. We add surface anisotropy energy, axially symmetric about the surface normal, to the total magnetic energy of the particle. We show that surface anisotropy of the appropriate sign enhances the coercivity. Further, the surface magnetization tends to be tangential to the surface and the magnetization reversal starts from the surface. Our simulation suggests that surface anisotropy might be an important mechanism for the coercivity enhancement in the surface-modified iron oxides.
Stress measurements of radio-frequency reactively sputtered RuO2 thin films
Authors: Hong Suk-Kyoung , Kim Hyeong Joon , Yang Hong-Geun
Published: July 2002

The stress-temperature behaviors of RuO2 thin films on (100) Si were investigated by measuring the deflection of film-coated substrates in situ during thermal cycling between 25 and 700 °C. The average biaxial elastic modulus [E/(1−ν)] and thermal expansion coefficient (α) of RuO2 thin film were also determined using the stress-temperature curves of the films on both (100) Si and quartz substrates. For this study, RuO2 thin films were deposited by rf magnetron reactive sputtering at the substrate temperature range of 25–450 °C. As-grown thin films deposited between 300 and 450 °C showed different stress-temperature behavior, which was caused mainly by the microstructure of the thin films. The values of [E/(1−ν)]RuO2 and αRuO2 were calculated to 3.09×1011 Pa and 10.47×10−6/°C, respectively, in the range of 350–600 °C. The αRuO2 obtained from the stress data was compared with the α value calculated from the thermal expansion coefficient, αa and αc, of RuO2 single crystal. Both values were found to be in good agreement.
Electron and phonon thermal waves in semiconductors: An application to photothermal effects
Authors: González de la Cruz G. , Gurevich Yu. G.
Published: July 2002

The electron and phonon temperature distribution functions are calculated in semiconductors. We solved the coupled one-dimensional heat-diffusion equations in the linear approximation in which the physical parameters of the sample are independent of temperature. We also consider the heat flux at the surface of the semiconductor as a boundary condition for each electron and phonon system instead of using a fixed temperature. From this we obtain an expression for electron and phonon temperature, respectively. The characterization of the thermal wave properties is discussed and some practical procedures for this purpose provide us with information about the electron and phonon thermal parameters.
Reduction of silicon dioxide by aluminum in metal–oxide–semiconductor structures
Authors: Dadabhai Ferhad , Gaspari Franco , Zukotynski Stefan , Bland Colby
Published: July 2002

The reduction of SiO2 by Al was studied in Al/SiO2/Si structures above 350 °C. It was found that Al displaces Si in the oxide, forming an Al–O compound with an Al:O concentration ratio between 1:1 and 1.3:1. In the reacted areas, less than 1 at. % Si is left in what was originally a pure SiO2 matrix. The activation energy for the reaction in a pyrogenic oxide is approximately 2 eV. The reaction was found to be responsible for electrical failure of diodes manufactured using Al/SiO2/Si technology.
Cathodoluminescence of AlN–GaN short period superlattices
Authors: MacMillan M. F. , Clemen L. L. , Devaty R. P. , Choyke W. J. , Asif Khan M. , Kuznia J. N. , Krishnankutty S.
Published: July 2002

Cathodoluminescence of AlN–GaN short period superlattice films was measured at 6 K, 77 K, and room temperature. The superlattice films were deposited using a switched atomic layer metalorganic chemical vapor deposition process onto a buffer layer of either AlN or GaN, which was deposited on basal plane sapphire substrates. The individual AlN and GaN layers of the superlattice films ranged in thickness from 2.6 to 20.8 Å. The cathodoluminescence of these samples was measured at several electron acceleration voltages to allow depth profiling of the samples. This allows the region of the sample (superlattice film, buffer layer, and substrate) from which the spectral features originate to be determined. A spectral peak in the ultraviolet region above the 3.5 eV band gap of GaN has been observed in all the superlattice samples studied to date. Our results indicate that the location of this peak is determined by quantum confinement in the GaN layers.
Hydrogen induced anisotropy in magnetic amorphous ribbons
Authors: Alcalá E. M. , López E. , Aroca C. , Sánchez M. C. , Sánchez P.
Published: July 2002

A study about induced anisotropy by hydrogen charging has been performed in both, positive and negative magnetostrictive amorphous alloys. A strong perpendicular to surface anisotropy at the sample surface in every hydrogenated sample has been observed from Bitter pattern and from hysteresis loops obtained by magnetooptical Kerr effect. This perpendicular anisotropy is only in the volume fraction near to sample surface, as it is inferred from hysteresis loops and magnetization curves measured by induction. A model assuming that the large perpendicular induced anisotropy is due to the mechanical stresses produced by the change of volume of the hydrogenated zones has been developed. From this model, by fitting experimental results, anisotropy energy, internal stresses, and hydrogen charged volume have been calculated.
Investigation of the effect of hydrogen on electrical and optical properties in chemical vapor deposited on homoepitaxial diamond films
Authors: Hayashi Kazushi , Yamanaka Sadanori , Watanabe Hideyuki , Sekiguchi Takashi , Okushi Hideyo , Kajimura Koji
Published: July 2002

We have investigated electrical and optical properties of the high-conductivity layers formed in both undoped and B-doped diamond films prepared by chemical vapor deposition. It is found that both hydrogenated undoped and B-doped diamond films have high-concentration holes of ∼1018 cm−3 at 297 K. These films exhibit little temperature dependence of the hole concentration between 120 and 400 K, while that of the oxidized B-doped film has a strong temperature dependence with an activation energy 0.38 eV. The Hall mobility of all the hydrogenated films of ∼30 cm2/Vs at 297 K is one to two orders of magnitude smaller than that of the oxidized B-doped film and increases with increasing temperature. The I−V characteristics of Al–Schottky contacts to the hydrogenated undoped film show excellent rectification properties and the temperature dependence of their forward characteristics is well explained by a junction theory inclusive of the tunneling process, i.e., thermionic-field emission theory, indicating that the depletion layer becomes thin due to high-density space charge in the depletion layer. We have also found a broad cathodoluminescence peak at around 540 nm in the hydrogenated films which disappears with subsequent oxidation treatment, indicating the existence of hydrogen-related gap states in the subsurface region of as-deposited homoepitaxial diamond films. High density hydrogen is detected in the subsurface region of the hydrogenated films by secondary ion mass spectroscopy. These experimental results suggest the existence of hydrogen-induced shallow acceptors in the surface region of as-deposited (hydrogenated) diamond films and that the difference between the hydrogenated and the oxidized films observed in both electrical and optical properties originates from hydrogen incorporated in the subsurface region.
Phase transformations of silicon caused by contact loading
Authors: Kailer A. , Gogotsi Y. G. , Nickel K. G.
Published: July 2002

Combining hardness indentation tests and micro-Raman spectroscopy it is shown that metallic Si-II is produced near the interface of a diamond indenter and silicon to a depth of about 0.5 μm, where the highest stresses (hydrostatic and deviatoric) exist. At fast unloading rates Si-II transforms to the amorphous state, whereas a mixture of the r8 high pressure polymorph Si-XII and the bc8 phase Si-III forms upon a slow load release. The region of Si-III+Si-XII is surrounded by the wurtzite structured Si-IV, where the stresses during the indentation had not been high enough to cause the transition to the metallic state. Thus, because of shear deformation a direct transformation to Si-IV takes place. Outside the phase-transformed regions the classical aspects of indentation-induced deformation by dislocation glide, twinning and crack formation are observed. Annealing of the high pressure phases leads to the formation of Si-IV at moderate temperatures and to the reversal to the original diamond structure (Si-I) at temperatures above 500 °C. Using the laser beam of the Raman spectrometer to anneal the samples the phase transitions could be monitored directly. The formation of silicon polymorphs other than amorphous or metallic structures during hardness indentation is, to the best of our knowledge, reported here for the first time. The results compare well with the polymorphism in Si that is known from diamond anvil cell experiments.
Optical absorption edge of ZnO thin films: The effect of substrate
Authors: Srikant V. , Clarke D. R.
Published: July 2002

The optical absorption edge and the near-absorption edge characteristics of undoped ZnO films grown by laser ablation on various substrates have been investigated. The band edge of films on C [(0001)] and R-plane [(1102)] sapphire, 3.29 and 3.32 eV, respectively, are found to be very close to the single crystal value of ZnO (3.3 eV) with the differences being accounted for in terms of the thermal mismatch strain using the known deformation potentials of ZnO. In contrast, films grown on fused silica consistently exhibit a band edge ∼0.1 eV lower than that predicted using the known deformation potential and the thermal mismatch strains. This behavior is attributed to the small grain size (50 nm) realized in these films and the effect of electrostatic potentials that exist at the grain boundaries. Additionally, the spread in the tail (E0) of the band edge for the different films is found to be very sensitive to the defect structure in the films. For films grown on sapphire substrates, values of E0 as low as 30 meV can be achieved on annealing in air, whereas films on fused silica always show a value >100 meV. We attribute this difference to the substantially higher density of high-angle grain boundaries in the films on fused silica.