bluuebunny/crossref_metadata_2025 huggingface dataset:
Grain boundaries and defects in superconducting Bi-Sr-Ca-Cu-O ceramics
Authors: Ramesh R. , Bagley B. G. , Tarascon J. M. , Green S. M. , Rudee M. L. , Luo H. L.
Published: July 2002

Defects and structural interfaces in superconducting Bi-Sr-Ca-Cu-O have been characterized by transmission electron microscopy. The superconducting phase exhibits frequent variations in the stacking sequence (polytypoids). Dislocations, observed inside the grains, either introduce or accommodate the shear in the a-b plane and the local composition fluctuations. In general, the grains exhibit a platelike morphology with the a-b plane as the grain boundary plane. Grain boundaries along the short edge are generally disordered, whereas those near the long edge generally have a thin layer of the lower Tc polytypoid. Coherent intragranular boundaries are also observed.
Dynamical response of cholesteric liquid crystals under dilative stresses
Authors: Scaramuzza N. , Carbone V. , Marcerou J. P. , Bartolino R.
Published: July 2002

We present a theoretical treatment and experimental data for the dynamical behavior of a cholesteric liquid crystal submitted to uniaxial dilative strains applied along the helical axis. The results show the importance of the permeative contribution in the transient regime before a new equilibrium state is found where the apparent pitch is varied.
Infrared study of Fe-B-pair behavior in iron-implanted Czochralski silicon
Authors: Geddo M. , Pivac B. , Borghesi A. , Stella A. , Campisano S. U. , Rimini E.
Published: July 2002

A Fourier transform infrared study of Fe-donor B-acceptor complexes formed in iron-implanted B-doped Czochralski-grown silicon crystals is performed by monitoring the behavior of boron-acceptor excitation spectrum lines. The effect of the iron presence on absorption spectra due to B-acceptor hydrogenlike systems, related to electronic transitions from the ground to excited states associated with the silicon P3/2 valence band, was analyzed in a wide range of fluences. Low-temperature optical data are reported and discussed comparing optical results with secondary-ion mass spectroscopy and spreading resistance measurements, supporting the existence of a fluence threshold that controls iron diffusion into the bulk.
Pulsed photoacoustic measurements of large optical absorption coefficients
Authors: Terzić Mira , Sigrist Markus W.
Published: July 2002

A method to determine large optical absorption coefficients of liquids from the amplitude ratio of acoustic waves generated under rigid- and free-boundary conditions is presented. This has been examined both experimentally and theoretically, taking into account the attenuation of the pulse amplitude ratio with the increased acoustic path length due to attenuation effects. Although the proposed method was tested in a narrow wavelength interval and close to the limit of precision of the experimental arrangement, the results show quantitative agreement with the literature values.
Modeling delamination due to thermal stress in optical storage media
Authors: Nkansah M. A. , Evans K. E.
Published: July 2002

Finite element analysis is used to calculate the shape of blisters formed in bilayer optical storage media due to the buildup of thermal stresses during laser writing. It is shown that practically usable blisters may be expected to form in a time period of about 15 ns. Such a thermal stress delamination process may also precede melting in conventional pit formation processes.
Mechanisms of dehydrogenation during ArF excimer laser patterning of plasma-deposited silicon nitride films
Authors: Donnelly V. M. , Mucha J. A. , McCrary V. R.
Published: July 2002

Using Fourier-transformed infrared spectroscopy, we show that pulsed ArF excimer laser irradiation of plasma-deposited silicon nitride films causes a large reduction in NH (∼100%) and SiH (∼50%) in the top ∼500 Å, and further show that this is responsible for the large suppression of the etch rate in buffered HF solution. Temperature rise calculations qualitatively predict the observed maximum falloff in etch rate and in SiH and NH concentration at the surface of the film. The observed reduction of the effect in thin films, which is due to the heat-sink effect of the underlying substrate, is also predicted. Both are strong indications that NH and SiH bonds are broken in a laser heating process. H2 has been detected as a product evolved during laser irradiation; however, most H2 remains trapped in the film.
An analytical charge control model for AlGaAs modulation-doped field effect transistors
Authors: Abdel-Motaleb Ibrahim M. , Syu T.-L.
Published: July 2002

An analytical charge control model for AlGaAs modulation-doped field effect transistors has been developed. The model has two regions: the linear, represented by a linear correspondence, and the saturation, represented by an exponential function. Using two fitting parameters, the slope of the linear curve is modified to reduce the subthreshold current and to determine the point separating the linear and saturation regions. Using this model, the device performance can be predicted to a very high degree of accuracy, since the charge-voltage relation has been accurately modeled using parameters related to device characteristics and dimensions. A dc model for the device current-voltage (I-V) characteristics has been developed. The parasitic current which may flow through the doped layer of the channel has been considered. The velocity saturation of the charges has been accounted for, and the saturation drain-source voltage has been accurately determined. This model can provide accuracy without sacrificing simplicity. The model has been successfully implemented in a simulation program with integrated circuit emphasis.
Surface sensitivity and bias dependence of narrow-gap metal-semiconductor-metal photodetectors
Authors: Darling Robert B.
Published: July 2002

The electrical field distribution in a Schottky contact metal-semiconductor-metal (MSM) photodetector is one-dimensionally approximated and applied to the solution of the illuminated dc characteristics using a bipolar transport model. A solution is produced that is valid for an arbitrary distribution of the incident optical intensity and all ranges of the applied bias voltage. It is shown that the observed dependence of the response on the illumination distribution and the saturation of the photocurrent with increasing applied bias are both determined by the efficiency of the internal field distribution in separating and collecting the generated electron-hole pairs. This description agrees with experimental measurements of Schottky contact MSM photodetectors fabricated on semi-insulating GaAs.
Optical nonlinear effects in GaAs/GaAlAs Bragg reflectors
Authors: Le Saux G. , Pocholle J. P. , Bretenaker F. , Zibell L. , Barbier E. , Salin F. , Papuchon M. , Brun A.
Published: July 2002

Band filling effects are used to modify the absorption and refractive index of bulk GaAs/Ga1−xAlxAs layers in a Bragg reflector in the wavelength region between the band gap of the two materials. Significant reflectivity modifications are observed in a subpicosecond regime.
Hole drift mobility in organopolysilane doped with trap-forming organic compounds
Authors: Yokoyama Kenji , Yokoyama Masaaki
Published: July 2002

The carrier transport in poly(phenylmethylsilane) films doped with trap-forming organic compounds with different ionization potentials has been investigated. The carrier transport in the doped systems is discussed as trapping events in the transport-active host polymer. Zero-field activation energy E0 corresponds well to the ionization potential difference between the additives and host polymer. The field and temperature dependencies of the mobilities have been analyzed in the framework of Gill’s [W. D. Gill, J. Appl. Phys. 43, 5033 (1972)] expression. Its characteristic parameter T0, at which the field dependence disappears, has been found to depend on trap depth and concentration, and an explicit expression interrelating them has been experimentally derived.
Solid phase reactions and change in stress of TiN/Ti/Si for a diffusion barrier
Authors: Fujimura N. , Matsui T. , Ito T. , Nakayama Y.
Published: July 2002

Solid phase reactions and change in stress of TiN/Ti/Si which is expected to have a low contact resistance to both n+ and p+ silicon and good adhesion to the Si substrate has been investigated. The TiN layer is a uniform polycrystal on the Si substrate; however, its texture is caused by the textured Ti layer. By annealing at 600 °C, Si and N atoms diffuse into the Ti layer which causes a distortion of the Ti lattice and an increase in the sheet resistance. By annealing at a higher temperature, as Ti starts to react with Si, the sheet resistance decreases. The silicidation sequences are the same as the previous results and the kinetics slightly differ. The change in film stress is well explained by these solid phase reactions. A TiN layer has excellent thermal stability in an Al/TiN/Ti/Si system, because TiN does not react with Al, Ti, and Si up to 600 °C.
Defect creation by 10-keV electron irradiation in phosphorous-doped a-Si:H
Authors: Babras Suvarna , Bhide V. G. , Rajopadhye N. R. , Bhoraskar S. V.
Published: July 2002

Changes in gap state distribution due to defects induced by different doses of 10-keV electrons in phosphorous-doped a-Si:H are reported. These defects have been observed qualitatively by electron beam writing. Patterns thus generated were read by electron beam induced current (EBIC). The increased local recombination in the area results in the decrease of EBIC current due to the increased defect density. Shift in the Fermi level position after electron irradiation was observed from the measurement of conductivity with temperature. Deep level transient spectroscopy (DLTS) was used to study the changes in gap states for the different electron dose. It was observed that irradiation causes the generation of additional defects which alter the gap state distribution in the n-type material over a broad range of energies. Creation of defects having activation energies around 0.55 eV below the conduction band was prominent although the formation of dangling bonds which results in increase in states around 0.7 eV is observed. These defects act as nonradiative recombination centers as suggested from the photoluminescence results which show a decrease in the 0.85 photoluminescence peak intensity. The contrast produced in the electron beam writing could be annealed out at 150 °C, indicating the metastability of the defects, which was also confirmed by the DLTS, photoluminescence, and dark conductivity measurements.
A new method for reflectivity measurement at the interface between resist and substrate
Authors: Tanaka T. , Hasegawa N. , Okazaki S.
Published: July 2002

A new method for directly measuring reflectivity at the resist/substrate interface is proposed. In this method, a clear antireflective film (ARCOR: antireflective coating on resist) is spun on a resist film, after which the reflectivity is measured directly through an ARCOR film and a resist film. As an ARCOR film, either a single- or a double-layer film can be used. Since multiple interference effects in a resist film are decreased, resist/substrate interface reflectivity can be measured directly. In this paper this new method and its measurement error are reported. Data is also presented on the resist/substrate interface reflectivities of several substrates. These reflectivities were measured using perfluoroalkyl polyether or spin on glass and di-propoxy-bis (acetylacetonato) titanium as ARCOR materials.
Production and characterization of large-grain bulk samples of Y1Ba2Cu3O7−x
Authors: Mahajan Sukesh , Cappelletti R. L. , Butcher J. A. , Omelian Eric
Published: July 2002

A heat-treatment method is described which yields superconducting Y1Ba2Cu3O7−x pellets having grain inclusions of a few millimeters in size. Resistivity, magnetic susceptibility, and magnetization display sharp transitions near 90 K for at least 85% of the volume of the samples. X-ray powder patterns reveal the presence of both the superconducting and ‘‘green’’ phases. Laue pictures show sharp spots. The normal state resistance of these samples is less than pellets prepared in the usual way by a factor of ∼10.
Comment on ‘‘Formation of polyhedral N2 bubbles during reactive sputter deposition of epitaxial TiN(100) films’’ [J. Appl. Phys. 6
6, 536 (1989)]
Authors: Chou T. C.
Published: July 2002

The previous derivations [R. S. Nelson, D. J. Mazey, and R. S. Barnes, Philos. Mag. 11, 91 (1965)], describing the equilibrium relation of thermally equilibrated bubbles bounded by (110) and (100) planes were invalid. The correct equations were derived and formulated in this article. The new equilibrium relation is demonstrated to be equivalent to Wulff’s law. Based upon this new equilibrium relationship, the surface energy anisotropy between the (110) and (100) planes of a few materials, including the recently reported polyhedral N2 bubbles in TiN(100) films, is recalculated. It is noticed that the degrees of surface energy anisotropy between the {100} and {110} planes of these materials are smaller than those that were published previously.
Isolated iron and chromium as recombination centers in ZnS
Authors: Zakrzewski A. , Godlewski M.
Published: July 2002

Free-carrier capture cross sections of isolated iron and chromium centers in ZnS are estimated. It is found that the Fe and Cr electron capture cross sections are comparable to those for shallow donors active in photoluminescence processes. The cross sections for hole capture are up to 103 times larger than those for the main ZnS acceptors. Our results allow us to explain the photoluminescence quantum yield of ZnS:Cu,Fe.
The structure and electrical characteristics of oxidized semi-insulating polycrystalline silicon
Authors: Hseih B.-C. , Greve D. W.
Published: July 2002

The influence of an oxidizing ambient on the structure and electrical characteristics of thin films of semi-insulating polysilicon (SIPOS) has been studied. It is shown that SIPOS films 135 nm thick can be completely oxidized to amorphous silicon dioxide after 24 h at 600 °C in wet oxygen. The midgap interface state density after oxidation and postmetallization anneal is 4×1010 cm−2 eV−1. We also show that this material is suitable for use as a low-temperature deposited gate dielectric for polycrystalline thin-film transistors.
Analysis of optical polarization bistability in transverse-magnetic-wave-injected semiconductor lasers
Authors: Mori Yoshihiro , Shibata Jun , Kajiwara Takao
Published: July 2002

Optical polarization bistability induced by transverse-magnetic- (TM) wave injection is thought from experimental results to originate from simultaneous generation of the oscillation of the transverse-electric wave (TE oscillation) and the amplification of the transverse-magnetic wave (TM amplification). In order to prove authenticity of this assertion, we use rate equations that include a term of the TM mode and an equation of the transmittance of the laser cavity. The calculations show that the carrier density is kept constant in the low-TM-photon-density region while it varies in the high-TM-photon-density region. It is also shown that the constant gain is divided into both TM amplification and TE oscillation in the former region. The transmittance changes only in the latter region since the refractive index varies with the carrier density. The bistability appears between these two regions. Calculated light output versus light input characteristics show good agreement with the experimental results.
Resolution improvement for linear image sensors using indium-tin-oxide/amorphous silicon junctions
Authors: Mishima Y. , Kondo N. , Kimura T. , Soeda S. , Nagahiro Y. , Kurahashi T. , Akamatsu T.
Published: July 2002

This paper describes two types of photodiode arrays using indium-tin-oxide/amorphous silicon (ITO/a-Si:H) junctions for a contact-type linear image sensor. One type is a photodiode array of separated ITO electrodes, an a-Si:H stripe layer, and a common metal (Cr) electrode. The other consists of a common ITO electrode, an a-Si:H stripe layer, and separated Cr electrodes. We investigated the bias dependence of the photocurrent and its spatial distribution for the two types of diode array. Our results indicate that the photocarriers generated around the element for a common ITO electrode type deteriorate the resolving power of the diode array, which can be improved by using separated ITO electrodes. A separated ITO electrode increases the modulation transfer function and the fineness of linear image sensors.
Al-Ga interdiffusion in heavily carbon-doped AlxGa1−xAs-GaAs quantum well heterostructures
Authors: Guido L. J. , Cunningham B. T. , Nam D. W. , Hsieh K. C. , Plano W. E. , Major J. S. , Vesely E. J. , Sugg A. R. , Holonyak N. , Stillman G. E.
Published: July 2002

Impurity-induced layer disordering experiments on AlxGa1−xAs-GaAs quantum well heterostructures (QWHs) that are doped heavily with carbon are described. The data show that carbon doping retards Al-Ga interdiffusion relative to an undoped crystal, and that interdiffusion in C-doped QWHs is not enhanced by a Ga-rich (versus As-rich) annealing ambient. The data are inconsistent with most Fermi-level-effect models for layer disordering that do not include chemical impurity dependence or sublattice dependence, and that do not consider the possibility of inhibited Al-Ga interdiffusion in extrinsic crystals.
Effects of dc transport current on low-field microwave absorption in ceramic superconducting YBCO samples
Authors: How H. , Karim R. , Seed R. , Widom A. , Vittoria C. , Balestrino G. , Paroli P.
Published: July 2002

The effects of dc transport current on low-field microwave absorption have been investigated systematically on bulk ceramic YBa2Cu3O7−δ samples. At a fixed temperature T, the critical current Ic(T) at which the electron paramagnetic resonance (EPR) signal vanishes varies linearly with T. The EPR absorption characteristics obey a scaling rule in accordance with the flux creep model. A revised version of the flux creep model is also presented.
Time-domain scattering parameters for a lossy dielectric
Authors: Hechtman Charles D. , Hsue Ching-Wen
Published: July 2002

Time domain scattering parameters of a two port are found for a ‘‘quasi-Debye’’-like lossy dielectric. Exact solutions are found for the zeroth- and round-trip S11 parameter and the one-way-trip S21 parameter. Numerical examples are presented for a hypothetical material.
Lifetime of the quasi-bound state in the quantum well in a double-barrier structure with a localized imaginary potential
Authors: Kan S. C. , Yariv A.
Published: July 2002

Tunneling in a double-barrier structure with a localized recombination channel is modeled by a spatially dependent imaginary potential. The general properties of nonhermitian Hamiltonians with an imaginary potential are examined. For a localized imaginary potential, the eigenvalues are found to be real for the extended states. The eigenstates with different eigenvalues are found to be nonorthogonal in general and this nonorthogonality is related to the nonconservation of matter. A continuity equation is derived from the Schrödinger equation showing explicitly a sink of carrier density at the imaginary potential. The lifetime of the quasi-bound state in the quantum well in a double-barrier structure is calculated from the width of the resonance peak in the transmission spectrum of the structure and is found to be decreasing linearly with the magnitude of the imaginary potential. The dependence agrees with a relation derived from the rate equation describing the tunneling and recombination process. A numerical simulation of the tunneling escape of a wavepacket localized initially inside the quantum well in this potential illustrates the loss of matter through the imaginary potential. The lifetime of the electron in the quantum well, the recombination time, and the tunneling time obtained from the simulation are in agreement with those calculated from the width of the resonance peak.
Optical properties of aluminum-implanted and annealed silicon
Authors: Brüesch P. , Halder E. , Kluge P. , Roggwiller P. , Stockmeier Th. , Lindner J. K. N.
Published: July 2002

The main problem associated with the use of aluminum as a p-type dopant in silicon power devices is its low electrical activity in silicon after the anneal process. In order to obtain a deeper insight into the possible mechanisms responsible for the loss of electrical activity, it is necessary to study three different states of the p-n junction fabrication: (1) the unimplanted starting or reference material; (2) the aluminum-implanted material; and (3) the implanted and annealed material. In this paper we present a detailed analysis of reflectivity and transmission measurements of the three different states extending from the far-infrared to the UV region, as well as depth profiles of the reflectivity from as-implanted and bevelled samples. From these investigations we have obtained information about two important aspects, namely lattice damage and free-carrier properties. The refractive index across the implanted layer is essentially constant and considerably larger than that of the crystalline state; together with recent transmission electron microscopy studies it is suggested that this change in refractive index is due to the formation of broken or weakened bonds. In the annealed state those defects induced by implantation which produce a change of the optical properties are healed out to a high degree. From the free-carrier absorption observed in the far-infrared direct information is obtained about the electrical properties, i.e., the mean concentration and mobility of the holes associated with the electrically active aluminum atoms in the thin p-type layer produced by annealing. We obtain an electrical activity (percentage of electrically active Al atoms) of 17%. This result is discussed and compared with recent sheet resistance-, spreading resistance-, and secondary-ion mass spectrometry data obtained from the same sample.
Determination of oxygen in silicon by ratio of A center to E center
Authors: Su Z. , Husain A. , Farmer J. W.
Published: July 2002

Oxygen concentration of silicon has been determined by the ratio of the A center to E center in gamma-irradiated n-type silicon. The concentrations of these defects were measured by the deep-level transient spectroscopy technique. It has been found that the ratio of the A center to E center is simply proportional to that of oxygen to phosphorus content by a factor of 0.072. In addition to extending the range of sensitivity to oxygen to levels below that obtainable using infrared absorption, this new method permits easy determination of the distribution of oxygen.
Acoustic microscope using pressurized superfluid 4He
Authors: Karaki Koichi , Suzuki Masaru , Okuda Yuichi
Published: July 2002

An acoustic microscope has been developed which makes use of pressurized superfluid helium as an acoustic coupling medium. Acoustic micrographs were taken using pure superfluid 4He under saturated vapor pressure at 0.1 K and under 25 atm at 0.1 and 1.9 K, for the frequency of 380 MHz. By pressurizing liquid 4He, the acoustic attenuation decreases, the efficiency of the antireflection coating increases, and the power level of saturation caused by nonlinearity of the superfluid 4He is raised. A good signal-to-noise ratio of imaging is thus obtained. The entire imaging instrument set is within a closed cell. The focusing mechanism is composed of two voice coils of superconducting wire. Using a servocontrol mechanism with a sensor and drive coil, Z-direction stability was better than 0.05 μm. The manipulation sensitivity is 0.5 μm/mV, and the whole stroke is about 2 mm.
Amorphous silicon p-i-n solar cell with a two-layer back electrode
Authors: Xu Zhongyang , Zhao Bofang , Chen Wei
Published: July 2002

A hydrogenated amorphous silicon (a-Si:H) p-i-n solar cell with a two-layer back electrode is proposed. The new type of back electrode consists of a conventional Al layer and an amorphous silicide layer. The properties of amorphous Cr-silicide film and thermal stability of a-Si:H p-i-n solar cells with a two-layer back electrode are studied. The experimental results show that an amorphous Cr-silicide layer can prevent the thermal diffusion of Al from going into a-Si:H, and thus can improve the thermal stability of the a-Si:H solar cell.
Photoreflectance measurements of indium content in indium-alloyed semi-insulating GaAs substrates
Authors: Yu P. W. , Ravipati S. , Taylor B. E. , Mitchel W. C.
Published: July 2002

Room-temperature photoreflectance measurements were used to determine the radial and axial distribution of low levels of indium in 3-in.-diameter semi-insulating bulk GaAs materials grown by the liquid-encapsulated Czochralski method. These results were compared with 4.2-K photoluminescence data and found to be accurate and more convenient for this application. Room-temperature photoreflectance allows an accurate determination of the indium content in the range of mole fraction 0.1%–2.0% with standard deviation of 0.03%. Two types of radial inhomogeneity were found in commercially available GaAs wafers. This is discussed in terms of indium segregation and shape of solid and liquid interface during the crystal growth.
A semiempirical nonrelativistic electron straggling distribution function
Authors: Antolak A. J. , Williamson W.
Published: July 2002

A semiempirical function for the energy-loss distribution of nonrelativistic electrons is presented. The form of the distribution function permits analytic sampling of the energy loss and is, therefore, suitable for Monte Carlo electron transport codes developed for small microprocessor computer systems. Comparisons between calculated transmitted energy distributions using a microprocessor and experimental results are presented for 15–20-keV incident electrons on aluminum, copper, and gold foils.
Superconducting thin films of Y-Ba-Cu-O prepared by metalorganic chemical vapor deposition
Authors: Singh R. , Sinha S. , Hsu N. J. , Chou P. , Singh R. K. , Narayan J.
Published: July 2002

High throughput, low-temperature deposition, sharp interfaces, and selective deposition with direct ion-, electron-, and photon-beam-controlled techniques are some of the key driving forces for the development of superconducting thin films by metalorganic chemical vapor deposition (MOCVD) technique. In this paper we report on the electrical and structural properties of Y-Ba-Cu-O (YBCO) films deposited by MOCVD on yttrium-stabilized zirconia (YSZ) and BaF2/YSZ substrates using a single-step in situ processing method which requires no further annealing. YBCO films deposited on BaF2/YSZ substrates have zero resistance at 80 K. The films were characterized by energy dispersive x-ray analysis, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The films on BaF2/YSZ substrates exhibited textured growth having both the c and a axis perpendicular to the substrate. The use of BaF2 as a buffer layer suggests three-dimensional integration of high-temperature superconducting thin film for hybrid superconductor/semiconductor devices as well as superconductor switches and other related devices.
Enhanced photorefractive gain in Cr-doped strontium barium niobate with an external dc electric field
Authors: Sayano Koichi , Yariv Amnon , Neurgaonkar Ratnakar R.
Published: July 2002

We present experimental results of field-enhanced photorefractive two-beam coupling and gain in Cr-doped strontium barium niobate crystals and its dependence on dopant concentrations. Increases in the exponential gain factor by more than a factor of 2 were observed by applying a field of up to 10 kV/cm. This property has potential applications such as switchable optical phase conjugators, limiters, and thresholding devices.
A recirculating thermoelectric generator based on sodium β″ alumina
Authors: Sayer M. , Bell M. F. , Judd B. A. , Sherrit S. , El-Assal K. , Kindl B.
Published: July 2002

A convectively cooled, recirculating thermoelectric generator based on β″ alumina has been designed and operated over the temperature range of 400–600 °C. The design employs a stainless-steel wick and flame sprayed molybdenum electrodes and the unit can be assembled and operated with ease. A full treatment of all thermal and electrical flows is given and theoretical efficiencies of 10%–15% are predicted. Critical parameters are the internal resistance and radiation coefficients for thermal losses, while it is also shown that the flow of sodium within the electrode and the geometry of the evaporating and condensing surfaces is of importance in limiting the measured efficiency to 1%–3%. Units have been operated for up to 3000 h.
Tunneling through asymmetric double-barrier quantum-well heterostructures
Authors: Rossel C. , Guéret P. , Meier H. P.
Published: July 2002

Data obtained from a set of asymmetric GaAs/AlGaAs double-barrier resonant tunneling structures is presented. Low-temperature I(V) characteristics, temperature dependencies, and magnetotunneling have been investigated. The data confirm conclusions drawn in previous studies on symmetric structures, lending further support to the description of tunneling in terms of sequential processes and momentum randomization most likely induced by interface roughness scattering in the well.
Properties of WNx films and WNx/GaAs Schottky diodes prepared by ion beam assisted deposition technique
Authors: Lee J. S. , Park C. S. , Yang J. W. , Kang J. Y. , Ma D. S.
Published: July 2002

Low energy ion beam assisted deposition (IBAD) of refractory tungsten nitride films onto GaAs is attempted for the first time. This ion beam technique provides lower process pressure, and less ion damage to substrates and films than conventional reactive sputter deposition. Schottky diode characteristics of W/ and WNx/GaAs and their thermal stability were investigated by capping the refractory films with SiO2 films and subsequent annealing at 700–900 °C for 30 min. While both tungsten and tungsten nitride contacts were stable up to 850 °C, the tungsten nitride contact showed better thermal stability and higher Schottky barrier height. The Schottky barrier heights of W/ and WN0.27/GaAs diodes annealed at 850 °C were 0.71 and 0.84 eV, respectively. These preliminary results are comparable to those of the best results reported with the conventional sputtering methods.
Acousto-optic interactions between optical waves and Bleustein–Gulyaev surface acoustic waves in gallium arsenide and other piezoelectric cubic crystals
Authors: Bright V. M. , Hunt W. D.
Published: July 2002

Investigations of acousto-optic interactions between guided optical waves and Bleustein–Gulyaev surface acoustic waves in gallium arsenide and other piezoelectric cubic crystals indicate very low light diffraction efficiency. The diffraction efficiency is controllable, however, by varying the penetration depth of the surface wave. A simple and effective method of detecting Bleustein–Gulyaev waves using a plane optical wave is also presented.
Generation and annihilation of positive and negative ion-depleted region in soda-lime silicate glass
Authors: Doi A. , Menjou Y. , Ishikawa T. , Abe Y.
Published: July 2002

The combination experiments of ac and dc (as the thermally stimulated polarization current) suggest that, by the dc process, there appeared the positive and negative ion-depleted region at the interface between glass bulk and the positive ion-depleted region near the anode. This may be the ionic analog for a p-n junction under reverse bias of a semiconductor. The relaxation in polarized glass would take place by the annihilation of the positive and negative ion-depleted region by back-diffusion of oxygen ions from the positive ion-depleted region and of sodium ions from the bulk.
A numerical study of cluster center formation in neutron-irradiated silicon
Authors: Shi Y. , Shen D. X. , Wu F. M. , Cheng K. J.
Published: July 2002

A numerical study of the formation of a radiation-induced defect cluster center in neutron-irradiated silicon has been performed by solving a set of semilinear parabolic reaction-diffusion-coupled equations. It is found that most of the primary displacement defects [interstitial (I) and vacancy (V)] will be annihilated by I-V direct recombination in an extremely short time. In particular, the formation of four-vacancy defects is independent of the concentration of sinks and impurities in a sample, and of the energy of the recoil particle relatively. The threshold energy for the formation of a vacancy cluster has also been studied. The results are discussed with the experimental observations.
Cyclotron autoresonance maser in the millimeter region
Authors: Nikolov N. A. , Spasovsky I. P. , Kostov K. G. , Velichkov J. N. , Spasov V. A. , Yovchev I. G.
Published: July 2002

Results from a cyclotron autoresonance maser experiment are reported. Two maxima of the output microwave power (8 and 10 MW) at a wavelength of 5 and 5.5 mm, respectively, have been observed. The nonadiabatic beam pumping is achieved by transit of the electrons through an external magnetic-field local inhomogeneity due to the presence of a copper aperture.
Correlation functions of the X
Y model (abstract)
Authors: Bonner J. C. , Müller G. , Parkinson J. B.
Published: July 2002

Recently an elegant and quite powerful finite-system approach to determine the exponents ηx and ηz from simple spectral properties has been proposed. For critical systems, the two exponents can be expressed in terms of finite-size spectral gaps as follows: η(N)x=2ΔE01(N)/ΔE(N), η(N)z=2ΔE00(N)/ΔE(N). Here ΔE(N) is the finite-size gap between the ground state (SzT=0,k=0) and the lowest excitation at k=2π/N; ΔE01(N) is the gap to the lowest ‖SzT‖=1 excitations (at k=π), and ΔE00(N) is the gap to the next lowest SzT=0 excited state. The η(N) sequence is then extrapolated to N→∞. For XY models, differences between s=1/2 and s≥1 appear. For s=1/2, the excitations which determine ΔE00(N) and ΔE(N) are degenerate, which implies that ηz=1/2, in agreement with the exact analytic result. For spin-1, however, the next lowest SzT=0 state is located at k=2π/N instead of k=π, and is therefore identical to the state which determines the gap ΔE.

The resulting equality ΔE=ΔE00 implies ηz=2, as in the spin-1/2 case. In fact, our result corresponds to power-law decay for all s, and hence we differ from Schulz and Ziman, who claim the out-of-plane correlation function decays exponentially for s>1/2. For the in-plane correlation function, the spectral gap method again agrees with the exact result ηx=0.5 for s=1/2. The consensus of this and other numerical methods for s=1 gives a value ηx≂0.20, considerably different from the case of s=1/2. Hence it is tempting to conjecture that ηx is s dependent, implying that XY models belong to different universality classes for different s. However, a finite-size study of the conformal anomaly produces the result that c=1, independent of s. This situation is further discussed.
Magnetization and ac-loss characteristics of YBa2(Cu1−xFex)3O7 superconductors
Authors: Orehotsky J. , Xu Youwen , Wang Y. L. , Garber M. , Suenaga M.
Published: July 2002

The dc hysteresis behavior of the magnetization and the 60-Hz ac loss characteristics were both measured at 4.2 K as a function of applied field to 2000 Oe on Yba2Cu3O7 and on iron-substituted YBa2(Cu1−xFex)3O7 samples with x=0.04 and 0.10. From the dc hysteresis behavior, energy loss per cycle was found to decrease with increasing Fe content. On the other hand, replacing copper with iron up to 4% did not change the 60-Hz loss, but increasing the iron content up to 10% was found to increase the loss by a significant amount.
A vertical force alternating-gradient magnetometer (abstract)
Authors: Flanders P. J.
Published: July 2002

A modification of the alternating-gradient magnetometer1 that is especially suitable for measurements above and below room temperature is described. The gradient of the alternating field, and the resulting force on the sample, are made perpendicular to the magnetization direction and a cantilevered piezoelectric sensing element can then be located 10–15 cm above the sample in a constant temperature environment. Under these conditions, the mechanical resonant frequency and gain of the magnetometer are not influenced by changes in sample temperature. The instrument has high sensitivity for measuring small or weakly magnetic samples: a sensitivity of 10−8 emu/(Hz)1/2 has been obtained over a temperature range from 77 to 700 K. The instrument can also be easily adapted to measure relatively massive or strongly magnetic samples. Samples weighing more than 1 g and magnetic moments above 1 emu have been measured and even higher values could be attained by using heavy piezoelectric elements and extension fibers. Examples of measurements on ferromagnetic, ferrimagnetic, and paramagnetic samples are given, and some practical considerations regarding construction of the instrument and mounting of samples are discussed.
Electrical properties of spinel ferrite films grown by spin-spray plating (abstract)
Authors: Yoo K. C. , Talisa S. H.
Published: July 2002

The spin-spray plating (SSP) is an equilibrium chemical process with which spinel ferrite films can be grown on a variety of substrate materials at temperatures as low as 100 °C. The process is particularly attractive for the monolithic integration of microwave magnetic and electronic GaAs devices. The study of the electrical characteristics of these films is part of an effort aimed at achieving this integration. Low-loss microwave magnetic devices require that the materials used have dielectric loss tangents (tan δ) on the order of 0.001 or lower, and dc resistivities greater than 107 Ω cm. In the SSP process, control of the chemical composition of the films is limited by an incompatibility of desirable transition metal ions (e.g., Fe,Ni,Zn,Mn) with the tetrahedral and octahedral cation sites available in the spinel structure. Therefore, the substitution of a specific cation modifies the site distribution of other ions in the spinel lattice. The overall distribution of the metallic cations determines the electrical and magnetic properties of the ferrite films. Specifically, low resistivity is due to the mixtures of high- and low-valence cations of a metal element in the same lattice site. In this study Ni-Zn ferrite films were produced with resistivities ranging from 10 to 106 Ω cm. Dielectric loss tangents measured, however, were several orders of magnitude higher than desired. Predictably, the loss tangent decreases monotonically with increasing resistivity; however, its values are much higher than the literature values for ceramic ferrites and semiconducting materials for the same range of dc resistivities. The reasons for this are presently being investigated.
Magnetic and microwave properties of ion-beam-sputtered amorphous FexCo80−xB15Si5 films
Authors: Harris V. G. , Oliver S. A. , Nowak W. B. , Vittoria C.
Published: July 2002

Magnetically soft amorphous films of FexCo80−xB15Si5 (x=0, 6, 23, 40, 70, 80) were ion beam sputter deposited onto fused quartz for static and microwave magnetic characterization. Films ranged in thickness from 220 to 260 nm and were deposited at rates of 0.1–0.2 nm/s. Saturation magnetization, coercivity, and loop squareness values were extracted from hysteresis loops generated by a vibrating sample magnetometer. Ferromagnetic resonance measurements were taken using a 9.5-GHz cavity with the applied magnetic field both parallel and perpendicular to the plane of the film, yielding values for the g factor, anisotropy field, effective magnetization, and linewidth. Well resolved quadratic spin-wave resonance spectra allowed for the deduction of exchange stiffness constants. Hysteresis loops showed well-defined uniaxial in-plane anisotropies for Fe-rich films, with easy axis loop squareness decreasing with decreasing Fe content. Saturation magnetization and effective magnetization values were found to reach a maximum at x=70 for those compositions investigated. These films were found to have soft magnetic properties comparable to the FexNi80−xB15Si5 alloy films previously investigated.
Static properties of ferromagnetic quantum chains: Numerical results and experimental data on two S=1/2 systems (invited)
Authors: Kopinga K. , Delica T. , Leschke H.
Published: July 2002

New results of a variant of the numerically exact quantum transfer matrix method have been compared with experimental data on the static properties of [C6H11NH3]CuBr3(CHAB), a ferromagnetic system with about 5% easy-plane anisotropy. Above T=3.5 K, the available data on the zero-field heat capacity, the excess heat capacity ΔC=C(B)−C(B=0), and the magnetization are described with an accuracy comparable to the experimental error. Calculations of the spin-spin correlation functions reveal that the good description of the experimental correlation length in CHAB by a classical spin model is largely accidental. The zero-field susceptibility, which can be deduced from these correlation functions, is in fair agreement with the reported experimental data between 4 and 100 K. The method also seems to yield accurate results for the chlorine isomorph, CHAC, a system with about 2% uniaxial anisotropy.
Chaos in spin clusters: Quantum invariants and level statistics
Authors: Srivastava Niraj , Kaufman Charles , Müller Gerhard
Published: July 2002

The energy-level sequence, whose spacings distribution is the most frequently invoked indicator of quantum chaos, can be derived (for systems with two degrees of freedom) from a two-dimensional representation of quantum invariants by projection. In this representation, such properties of level sequences as effective randomness in integrable models and level repulsion in nonintegrable models can be more directly interpreted in terms of physical properties. In integrable models, anharmonicities convert quasiperiodic level sequences into effectively random sequences.
Anomalous low-frequency butterfly curves for subsidiary absorption and ferromagnetic resonance overlap at 3 GHz (abstract)
Authors: Cross R. W. , Patton C. E. , Srinivasan G. , Booth J. G. , Chen M.
Published: July 2002

Subsidiary-absorption butterfly curves of spin-wave-instability threshold versus static in-plane field have been obtained for yttrium-iron-garnet (YIG) thin films at 3 GHz. The butterfly curves have been found to be rather anomalous, typically displaying a pronounced dip and a very low minimum threshold. These anomalous features are attributed to the overlap of the subsidiary-absorption field region with ferromagnetic resonance (FMR). First-order instability theory was extended to include the uniform-mode response near FMR. The extended theory yields good fits to the data for reasonable values of the YIG FMR linewidths. The theoretical analysis also shows a predicted flip in the azimuthal propagation angle φk for the unstable spin waves in the region of FMR overlap. With increasing field, there are predicted discontinuous changes in φk from 90 ° to 0 ° and back to 90 ° in the region of FMR.
Structural inhomogeneity and magnetic properties of amorphous Gd17Co83 compositionally modulated thin films
Authors: Choe G. , Valanju A. P. , Walser R. M.
Published: July 2002

Amorphous Gd17Co83 compositionally modulated films (CMFs) with different bilayer periods were prepared by sequential deposition in a rf diode multitarget sputtering system. Morphological inhomogeneities with variable size, shape, and uniformity on the scale of 20–100 Å were produced by variations in deposition parameters including substrate bias. Data from small-angle electron scattering and Fourier digital image processing of TEM micrographs revealed that the morphological inhomogeneities were due to compositional fluctuations. The −65 V bias sputtered films (bilayer period <10 Å) exhibited uniform small particles, while the unbiased films showed a wide distribution of morphological inhomogeneities. Magnetic domain observations indicated that the perpendicular magnetic anisotropy was determined by the size, shape, and distribution of the film inhomogeneities. The results suggested that the perpendicular magnetic anisotropy was a result of exchange coupling between uniformly shaped inhomogeneities.
Effects of magnetic surface anisotropy near the (110)Fe/MnF2 and (110)Fe/(111)Ag/MnF2 interfaces (abstract)
Authors: Tang H. , Qiu Z. Q. , Du Y. W. , Walker J. C.
Published: July 2002

Two series of samples, Ag/56Fe/57Fe/56Fex/MnF2 and Ag/56Fe/57Fe/Agx/MnF2 with Ag and Fe in (111) and (110) orientations, respectively, have been grown epitaxially in ultrahigh vacuum (∼1×10−9 τ). Several thicknesses are chosen for the 56Fe and Ag intervening layers separating 57Fe and MnF2. The magnetic hyperfine field for the 57Fe probe layer about 3 monoatomic layers (ML) thick is measured with Mössbauer spectroscopy from room temperature down to 4.2 K. For the first series of samples, a quasi-linear temperature dependence of the hyperfine field is observed for the probe layer up to 10 ML away from the (110)Fe/MnF2 interface. The magnitude of the linear slope is smaller for the 10-ML sample than for the 5-ML sample. An ordinary T3/2 dependence is recovered when the 57Fe probe layer is placed 25 ML away from the interface. This result can be qualitatively understood within the framework of a semiclassical spin-wave calculation involving a large positive surface anisotropy constant Kss by Rado. For the second series of samples, significant amount of FeF2 (about 40% of the 57Fe) is found to be present in the 57Fe probe layer. The temperature dependence of the hyperfine field exhibits an initial weak dependence at low temperatures (<76 K) changing over to a fast quasi-linear falloff at higher temperatures for both samples with x(Ag)=9 and 19 ML. A possible explanation for this behavior is that below ∼76 K, the Néel temperature for antiferromagnetic FeF2, the 57Fe probe behaves as a three-dimensional antiferromagnet and thus exhibits a T2 dependence, while for temperatures above ∼76 K it exhibits a quasi-linear temperature dependence presumably due to the same mechanism responsible for the quasi-linear temperature dependence in the first series of samples.
The band structure of metallic superlattices (abstract)
Authors: Fairbairn W. M. , Yip S. Y.
Published: July 2002

The bilayers which form the basic unit in superlattices define a unit cell which normally contains a large number of inequivalent sites. With over 20 such sites being involved it is not at present feasible to use the known sophisticated self-consistent and relativistic computational procedures to obtain the band structure. A simple model is considered in which the lattice structure is represented in the superlattice direction by an appropriate number of square well potentials (different for each of the constituent atoms) and in the other two perpendicular directions by a quasi-free periodic potential. The resulting three-dimensional band structure exhibits the behavior expected for that of a metallic superlattice, and is used to evaluate the RKKY interaction as a function of distance between sites within the structure. When one of the two constituent elements in the superlattice is a metallic rare earth for which the ions possess a magnetic moment then this interaction should be dominant in determining any magnetic ordering which occurs. A comparison is made between the orderings so predicted and those which have been found experimentally.