bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism and crystal chemistry in REFe12−xGax (RE=Y,Ce,Pr,Nd,Sm,Gd,Tb,Dy,Ho,Er,Tm,Yb,Lu and MM=mischmetal) and (Zr,Pr) (Fe1−xCox)12−yGay
Authors: Weitzer F. , Hiebl K. , Rogl P. , Grin Yu. N.
Published: July 2002

The existence of ternary compounds according to the formula REFe12−x Gax which for x ∼6 represents the iron-rich end of a homogeneous range has been confirmed. X-ray powder analysis of alloys annealed at 800 °C or above generally reveal isotypism with the body- centered tetragonal ThMn12 -type structure. For the alloys with heavier rare-earth elements from Gd to Lu a phase transition to a body-centered orthorhombic structure type (ScFe6 Ga6 type) is observed, which has not been reported before. The transition corresponds to a crystallographic group-subgroup relation (I4/mmm→t2 →Immm), and the transition temperature increases with the ordinal number of the rare earth, indicating the higher the stability of the ScFe6 Ga6 -type structure, the smaller the radius of the rare-earth element. Accordingly, the ThMn12 -type structure is stable for the early rare-earth members and no transition was observed as low as 400 °C. From magnetization curves it is shown that for REFe12−x Gax (RE=rare earth, Y) all magnetic sublattices order simultaneously at temperatures above Tc ∼400 K. For Y, Lu, and light rare-earth-containing alloys collinear or canted ferromagnetism is observed. The vector of magnetization was found to be close to the a,b plane. Strong hysteresis effects are revealed in all alloys. Energy products are highest for (Pr,Sm)Fe∼6Ga∼6. For the compounds with the heavy rare-earth elements a ferrimagnetic behavior is encountered. Both magnetic sublattices, i.e., Fe and RE, couple antiparallel, exhibiting easy plane anisotropy. The crystallographic transformation, tetragonal-orthorhombic, has little effect on the magnetic behavior of these alloys. (Zr,Hf)(Fe1−x Cox )12−y Gay remain temperature-independent paramagnets.
Room-temperature continuous operation of GaAs/AlGaAs lasers grown on Si by organometallic vapor-phase epitaxy
Authors: Choi H. K. , Wang C. A. , Fan John C. C.
Published: July 2002

Graded-index separate-confinement heterostructure single-quantum-well GaAs/AlGaAs diode lasers exhibiting continuous (cw) operation at room temperature have been grown on a Si substrate by organometallic vapor-phase epitaxy, without the use of molecular-beam epitaxy. To improve the quality of the laser structure, a defect-filtering layer was incorporated between this structure and a GaAs buffer layer about 1.5 μm thick grown on the substrate. Of four types of defect-filtering layers investigated, the most effective was one grown with thermal cycling, which made it possible to obtain pulsed threshold current densities as low as 350 A/cm2 for broad-stripe lasers with a cavity length of 500 μm. Ridge-waveguide lasers with this type of defect-filtering layer have exhibited cw threshold currents as low as 25 mA and a differential quantum efficiency of 55%.
Downstream microwave plasma-enhanced chemical vapor deposition of oxide using tetraethoxysilane
Authors: Pai C. S. , Chang C.-P.
Published: July 2002

In this paper, results for dielectric oxide films deposited using downstream microwave plasma-enhanced chemical vapor deposition in the temperature range between 250 and 400 °C are presented. The deposition of oxide using TEOS (tetraethoxysilane)+O2 and TEOS+N2O chemistries are studied. In the reactor, the TEOS is injected directly into the deposition chamber without passing through the discharge. Only He, O2, or N2O are fed through the microwave cavity where the discharge is generated. In addition, no ions but chemically active species are present in the deposition chamber during the deposition. The deposition rate is found to decrease with increasing temperature. In addition, it appears that the deposition rate increases with increasing concentration of active oxygen species in the deposition chamber. These suggest that the generation of intermediate species of TEOS and adsorption/desorption of the reactant on the surface are the key steps that determine the deposition rate. The stress of the deposited oxide films is found to be tensile and less than 2×109 dyn/cm2. The Si-OH concentration in the films is found to be low and can be below the detection limit of infrared spectrometry by increasing the flow ratio of O2/TEOS during the deposition. The step coverage of the oxide films over the Al runners is found to be excellent due to the long diffusion time available for TEOS surface species before forming SiO2. The mechanisms of oxide deposition using TEOS+O2 and SiH4+N2O chemistries are studied and compared. The details of oxide step coverage versus different deposition processes are also discussed.
Dynamics of charge storage in acoustic charge transport devices on GaAs
Authors: Schmukler Bruce C.
Published: July 2002

Acoustic charge transport devices in GaAs are charge-coupled devices that typically transport charge at the velocity of sound. However, charge transport can be temporarily inhibited by electrical barriers in the transport channel, a process known as charge storage. In this paper the fundamental properties of two methods of charge storage, single-packet and double-packet, are investigated and compared using theory and experiments. A 1-D storage model is developed which provides insight into the dynamics of the charge storage process. The impact of charge storage on device frequency response is analyzed. Storage capacity is experimentally characterized as a function of storage voltage. The key factors that limit storage capacity are found to be intercell diffusion, surface charge extraction, barrier transition times, and delay-line charge capacity. Furthermore, double-packet storage has been found to provide higher storage capacity with less storage voltage than single-packet storage.
Exfoliated MoS2 monolayers as substrates for magnetic materials
Authors: Frindt R. F. , Arrott A. S. , Curzon A. E. , Heinrich B. , Morrison S. R. , Templeton T. L. , Divigalpitiya R. , Gee M. A. , Joensen P. , Schurer P. J. , LaCombe J. L.
Published: July 2002

Monomolecular layers of MoS2 are used as substrates for the preparation of fine particles of magnetic materials. Particles of Ni, Co, and Fe and their alloys and various ferrites, hydroxides, and carbides have been produced. Some of these particles have magnetizations and coercivity values suitable for use in information storage technologies. Although attempts to produce multilayers of (MoS2/Fen)m by chemical methods have not succeeded, new pathways to improved magnetic materials have been found.
Polarization forces and conductivity effects in electrorheological fluids
Authors: Davis L. C.
Published: July 2002

Forces between particles aligned into chains by an applied electric field in an electrorheological (ER) fluid are calculated using finite-element techniques and, approximately, using a dipole approximation with local-field effects. Evaluation of the effective dielectric constant is emphasized and the shear modulus is derived from the shear dependence. For high-frequency (f≳0.1–1 kHz) applied electric fields, the forces and the modulus depend upon the dielectric constants of the suspending fluid and the dispersed particles. For low-frequency or dc electric fields, the conductivities of the components are dominant. These effects are treated within a Maxwell–Wagner approach. If the ratio of particle-to-fluid conductivities substantially exceeds the ratio of dielectric constants, a large enhancement of the modulus is found. Implications for the design of ER fluids are discussed briefly.
Interface fracture and viscoelastic deformation in finite size specimens
Authors: Xu Da-Ben , Hui Chung-Yuen , Kramer Edward J.
Published: July 2002

In the small scale yielding limit, the viscoelastic energy dissipation and the predicted fracture toughness of the interface between two viscoelastic polymers are monotonically increasing functions of the crack growth rate. For a finite size specimen however such prediction are no longer strictly valid. We demonstrate here that there is a maximum viscoelastic energy dissipation (and thus fracture toughness) at a characteristic interface crack velocity which depends not only on the micromechanical properties of the interface and the viscoelastic properties of the bulk polymers but also upon the specimen dimensions. Our analytical results allow us to deduce G=Gad[1+φ(aTȧ,c)] which relates the experimental fracture toughness to the intrinsic fracture toughness of the interface Gad, the WLF shift factor aT and c, a dimensionless parameter which is a function of the micromechanical properties of the interface, the mechanical properties of the polymers and the specimen dimensions. This result is similar to an empirical relation developed to fit experimental results; this relation however was assumed to be independent of c (specimen geometry). We demonstrate that the dependence of φ on c is substantially weaker than its dependence on aTȧ, which accounts for the success of the empirical relation in correlating the experimental data.
Laser ignition of pyrotechnic mixtures: Ignition mechanisms
Authors: Östmark Henric , Roman Nils
Published: July 2002

A laser method for studying the ignition processes of a pyrotechnic mixture is presented. The experimental equipment consists mainly of a 250 W CO2 laser, an explosive chamber, a power meter, and a mechanical shutter. Using the laser ignition apparatus a parametrical study of the ignitability of a pyrotechnic mixture (Mg/NaNO3) was conducted. The influence of the following parameters on the ignition energy was studied: pulse width, beam radius, particle size, different sorts of Mg, and gas pressure. These measurements are complemented by some ignition measurements on a Mg/BaO2 mixture where the influence of catalysis is studied. Furthermore some high-speed photography of the ignition process is presented. The thermal explosion theory is then applied to these measurements for a calculation of some of the important parameters followed by a discussion of the ignition mechanisms. The activation energy (Ea=148 kJ/mol), frequency factor (K0=7.1×1017 s−1), and absorption coefficient (α=3.1104 m−1) were calculated from the measurements. Using the experimentally determined parameters a 2D finite-element computer modeling of the ignition was carried out, showing that the ignition energy can be calculated with a fair degree of accuracy but that the time to ignition cannot be modeled with a pure heat transfer model. The study is concluded with a discussion of ignition mechanisms for pyrotechnic mixtures, which especially points out that the ignition mechanism for Mg/NaNO3 is a melting of the oxidizer followed by a decomposition of the oxidizer, diffusion of the produced oxygen through the oxide layer on the magnesium particles with a consecutive reaction of oxygen and magnesium.
Structural properties of BaTiO3 thin films on Si grown by metalorganic chemical vapor deposition
Authors: Yoon Y. S. , Kang W. N. , Shin H. S. , Yom S. S. , Kim T. W. , Lee Jong Yong , Choi D. J. , Baek S-S.
Published: July 2002

Ferroelectric BaTiO3 thin films were grown on Si(100) substrates at a temperature of 600 °C by in situ metalorganic chemical vapor deposition. X-ray diffraction and transmission electron microscopy results suggested that the 〈110〉 direction of the BaTiO3 preferred oriented films is parallel with the (100) direction of the Si substrates. Auger electron spectroscopy measurements showed that the compositions of the as-grown films were with a uniform distribution throughout the thickness of the films and with a sharp interface. These results indicate that the failure to obtain BaTiO3 epitaxial films was due to the formation of an interfacial amorphous layer prior to the creation of the films.
Neutron powder diffraction study of Tb2Fe14B
Authors: Herbst J. F. , Fuerst C. D. , Yelon W. B.
Published: July 2002

We report Rietveld analyses of neutron powder diffraction data on Tb2Fe14B at 8 and 293 K. Our results confirm the inferences from magnetization measurements that (i) all eight magnetic moments are collinear with the c axis, (ii) the Tb and Fe moments are antiparallel, and (iii) the Tb moments approach their free ion value at low temperatures. Systematics of the variation of the lattice constants and nuclear position parameters across the R2Fe14B series are also discussed.
Study of the luminescence mechanism in porous silicon structures
Authors: Prokes S. M.
Published: July 2002

Measurements of n- and p-type porous silicon indicate no direct correlation between particle size and photoluminescence (PL) energy. Controlled continuous removal of silicon does not result in a continuous PL blueshift, which would be expected in the quantum confinement model. Also, high temperature (1200 °C) anneals of porous silicon lead to a material consisting of 100–200 nm silicon spheres, with very low dangling bond densities, similar to crystalline silicon. This material does not exhibit noticeable PL in the visible range but when dipped in hydrofluoric acid (HF) for 1 s, strong visible PL appears with no structural changes noted. Polysilane/hydride complexes appear with the HF treatment, leading to the conclusion that the visible PL may be the result of a surface phenomenon related to the polysilane/hydride complexes, and not to a bulk Si quantum confinement effect. Anneals of porous silicon to 690 °C also show a significant redshifting of the PL, exhibiting identical behavior to measurements of shrinking of the optical bandgap of hydrogenated amorphous silicon (a-Si:H). With these results in mind, a surface related polysilane luminescence mechanism in porous silicon will be discussed, in which the PL energy is a function of H content and the PL intensity is a function of total surface area.
Nitrogen diffusion in Sm2Fe17 and local elastic and magnetic properties
Authors: Skomski R. , Coey J. M. D.
Published: July 2002

The reaction between nitrogen gas and an intermetallic compound is studied, with particular reference to Sm2Fe17 , treating the Sm2Fe17Ny system as a gas-solid solution. A simple lattice gas model is used to describe the reaction of nitrogen atoms with the metal lattice in terms of the net reaction energy U0=−57±5 kJ/mol. The equilibrium nitrogen concentration is calculated as a function of nitrogenation temperature and gas pressure. Refined diffusion parameters D0=1.02 mm2/s and Ea=133 kJ/mol, determined by thermopiezic analysis of the initial stage of nitrogen absorption, are used to calculate nitrogen profiles and the time dependence of the mean nitrogen content during nitrogenation. Assuming mechanically isotropic grains the elastic strain and stress profiles are calculated. Main results are a large uniaxial strain near the surface of nonuniformly nitrided particles, and core expansion even in the absence of any nitrogen there. Curie temperature and K1 profiles are calculated and suggestions are made regarding the influence of stress on coercivity and disproportionation of the material.
On the fundamental role of oxygen for the photochromic effect of WO3
Authors: Bechinger C. , Oefinger G. , Herminghaus S. , Leiderer P.
Published: July 2002

The photochromic effect of thin amorphous WO3 films, i.e., the coloration upon exposing the sample to ultraviolet light, has been investigated by means of optically excited surface plasmons. Due to the high sensitivity of this technique, it was possible to investigate the dynamics of this effect on a scale as short as seconds. Our results demonstrate the dominant role of oxygen during the coloration and bleaching processes. We found clear evidence of an oxygen exchange between the sample and the ambient atmosphere during the photochromic process, which is at variance with the widely accepted double charge injection model. The coloration rate shows the same wavelength dependence as that of the creation rate of electron-hole pairs formed by optical excitation. The results can be explained by the light-induced decomposition of the incorporated water.
Finite element analysis of thermal residual stresses at graded ceramic-metal interfaces. Part I. Model description and geometrical effects
Authors: Williamson R. L. , Rabin B. H. , Drake J. T.
Published: July 2002

An elastic-plastic finite element method numerical model has been formulated to study residual stresses developed at graded ceramic-metal interfaces during cooling. The results were compared with those obtained for sharp (nongraded) interfaces to assess the potential for achieving residual stress reductions. Analyses were conducted for various axisymmetric cylindrical specimen geometries relevant to structural joining, coating, and thick film applications. The graded microstructure was treated as a series of perfectly bonded layers, each having slightly different properties. Constitutive relations for the interlayers were estimated using a modified rule-of-mixtures approximation, and strain and stress distributions were calculated for simulated cooling from an assumed fabrication temperature. The results demonstrate the importance of accounting for plasticity when comparing graded and nongraded interfaces. Significant geometrical effects on peak stresses were observed in the graded materials. It is shown that in some cases, optimization of the microstructure is required to achieve reductions in certain critical stress components believed to be important for controlling interface failure.
Controlling strength and toughness of multilayer films: A new multiscalar approach
Authors: Adams D. P. , Vill M. , Tao J. , Bilello J. C. , Yalisove S. M.
Published: July 2002

Multiscalar films are produced in order to combine both toughness and strength into a multilayer film. These structures incorporate both a strengthening phase and a toughening phase in a compositionally modulated microcomposite. The mechanical properties and microstructure for thick (∼50 μm) Mo/W multiscalar films have been characterized. A detailed microstructural analysis (including transmission electron microscopy, scanning electron microscopy, and x-ray techniques) of Mo/W multiscalar films has shown that large single-crystal columns of Mo interspersed with epitaxial layers of W extend for the entire film thickness. The microstructure is a zone-II-type microstructure, yet the temperatures during deposition are well below the lower limit (0.3 T/Tm) previously reported for such microstructures. Hardness and tensile tests have shown that a multiscalar approach is capable of tailoring a desired strength and toughness into a multilayered film.
The structure and properties of nanosize crystalline silicon films
Authors: He Yuliang , Yin Chenzhong , Cheng Guangxu , Wang Luchun , Liu Xiangna , Hu G. Y.
Published: July 2002

Nanosize crystalline silicon films are fabricated by using highly hydrogen-diluted silane as the reactive gas and activated with rf+dc double-power sources, in a conventional plasma-enhanced chemical-vapor-deposition system. The structure of the deposited films as studied by means of high-resolution electron microscopy, Raman scattering spectra, x-ray-diffraction pattern, IR transmission spectra, and ultraviolet ray analysis. The results show that there are many novel structural features and new physical properties for these nanosize crystalline silicon films. In particular, it is found that the optical-absorption coefficient α is higher than that of a-Si:H and μc-Si:H films, the room-temperature conductivity σd has the value of 10−3–10−1 Ω−1 cm−1, and the hydrogen content CH in nc-Si:H films is higher than 30 at. %. The nc-Si:H films have their peculiar features which are different from both a-Si:H and μc-Si:H films.
Studies of threshold current dependence on compressive and tensile strain of 630 nm GaInP/AlGaInP multi-quantum-well lasers
Authors: Kamiyama Satoshi , Monnoh Masaya , Ohnaka Kiyoshi , Uenoyama Takeshi
Published: July 2002

Compressive- and tensile-strain effect on 630 nm GaInP/AlGaInP multi-quantum-well lasers is theoretically analyzed. The compressive strain can greatly reduce the hole density of states which results in the reduction of the threshold current as reported. On the contrary, in the case of tensile strain, it is shown that it does not reduce the threshold current, because the wider width, which is required for the given wavelength of 630 nm, reduces the quantum size effect.
Study of Lamb acoustic waveguide modes in unsupported polyimide thin films using real-time impulsive stimulated thermal scattering
Authors: Rogers John A. , Nelson Keith A.
Published: July 2002

An extension of the impulsive stimulated thermal scattering method that allows for one-laser-shot characterization of thin film thermal and viscoelastic properties is described. The technique is demonstrated through an extensive study of free-standing thin (∼1 μm) polyimide films. An exact, analytical solution for excitation of the waveguide acoustic modes is given and used to extract the viscoelastic properties of the films. It is also shown how the technique can be used to detect film delaminations by comparing results from free standing films to results from films tightly bound to silicon substrates.
Positron stopping profiles in multilayered systems
Authors: Aers G. C.
Published: July 2002

It is shown that the results of lengthy Monte Carlo simulations for positron stopping profiles in multilayer systems can be reproduced accurately in the incident energy range 1–25 keV using a simple scaling model. This model takes into account the variation of mean implantation depth between layers and the backscattering effects of interfaces and represents a computation time saving of several orders of magnitude. This development is a significant step in the effort to make detailed multilayer defect profiling with positrons a practical possibility.
Characterization of proton exchange lithium niobate waveguides
Authors: Paz-Pujalt Gustavo R. , Tuschel David D. , Braunstein Gabriel , Blanton Thomas , Lee S. Tong , Salter Lillie M.
Published: July 2002

Proton exchanged samples of LiNbO3 have been profiled by micro-Raman spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering channeling, and by x-ray diffraction (XRD). Following proton exchange (PE) there are two different phases in addition to pure LiNbO3 detected by XRD. After successive annealing steps the outermost phase disappears and an interfacial region forms progressively between PE and LiNbO3. Specific vibrational bands are correlated to electro-optic and nonlinear optical properties of the system, and the recovery of these properties upon annealing is correlated to chemical bonding changes.
The ambipolar diffusion coefficient in silicon: Dependence on excess-carrier concentration and temperature
Authors: Rosling M. , Bleichner H. , Jonsson P. , Nordlander E.
Published: July 2002

Recent articles have been published claiming that the ambipolar diffusion coefficient of semiconductors, Da, is basically independent of the excess-carrier concentration, thus contradicting the conventional result of, for example, Fletcher. However, only a few experimentally verified papers on this subject are published. In the present work, a detailed experimental analysis of the ambipolar diffusion coefficient is presented regarding both temperature and injection-level dependence. The ambipolar diffusion coefficient was measured in the low-doped n base of a p-i-n type diode at different excess-carrier concentrations and temperatures using an open-circuit carrier decay (OCCD) method based on the free-carrier absorption (FCA) technique. This investigation was performed in the carrier-concentration range of 1015–2×1017 cm−3 and in the temperature range of 300–420 K, respectively. The ambipolar diffusion coefficient is experimentally found to behave in reasonable agreement with Fletcher’s theory, thus decreasing with increasing excess-carrier concentration.
Electrical transport in (100)CoSi2/Si contacts
Authors: Lauwers A. , Kyllesbech Larsen K. , Van Hove M. , Verbeeck R. , Maex K. , Van Rossum M. , Vercaemst A. , Van Meirhaeghe R. , Cardon F.
Published: July 2002

A detailed investigation of the electrical transport in (100) CoSi2/Si contacts is presented. The transport properties of epitaxial CoSi2 films, obtained both by ion-beam synthesis and by solid-state reaction of a Ti/Co bimetallic layer, are compared with the transport properties of conventional polycrystalline CoSi2 layers. The electrical resistivity, the magnetoresistance, and the Hall effect are measured on Hall bars for temperatures ranging from 1.2 to 300 K and magnetic fields up to 5 T. Very high values of the order parameter kFL0 are observed, indicating that the investigated samples are of very high purity and crystalline order. In addition, the electric transport at the CoSi2/Si interface is studied by current-voltage and capacitance-voltage measurements on Schottky diode structures for temperatures ranging from 173 to 333 K. Ideality factors close to unity are observed for the highest temperatures, for the lower temperatures the ideality factors are found to increase with decreasing temperatures. The observed temperature dependence of the ideality factor and the Schottky barrier height can be attributed to Schottky barrier inhomogeneities.
Site symmetry analysis of the 738 nm defect in diamond
Authors: Brown S. W. , Rand S. C.
Published: July 2002

Based on a detailed analysis of polarized Raman and luminescence measurements of a ‘‘mosaic’’ diamond film, symmetry properties of a ubiquitous point defect observed in diamond films are determined. Specifically, the defect, which gives rise to emission at 738 nm, is determined unequivocally to be a 〈110〉-oriented defect with the transition dipole moment of the center oriented along the 〈110〉 symmetry axis. These results represent the first analysis of the symmetry properties of this point defect and aid in the development of structural model of the center.
A new global equation of state model for hot, dense matter
Authors: Young David A. , Corey Ellen M.
Published: July 2002

The Thomas-Fermi-based quotidian equation of state model has been modified to include new options: (1) solid-solid phase transitions; (2) arbitrary Grüneisen gamma function input; (3) soft-sphere model for the critical point and liquid-vapor region; and (4) diatomic molecular dissociation. The new model is fitted to experimental shock Hugoniot, isothermal, and critical point data to illustrate its flexibility. The limitations of the Thomas-Fermi model and the possibility of further physics improvements in global equation of state models are discussed.
Direct measurement of the deep defect density in thin amorphous silicon films with the ‘‘absolute’’ constant photocurrent method
Authors: Vaněček M. , Kočka J. , Poruba A. , Fejfar A.
Published: July 2002

Direct measurement of the deep defect density in thin amorphous silicon films with the help of the ‘‘absolute’’ constant photocurrent method is demonstrated here. We describe in detail how the optical (photocurrent) absorption spectrum can be measured directly in absolute units (cm−1) without additional calibration and undisturbed by interference fringes. Computer simulation was performed to demonstrate absolute precision of the measurement and to explain residual interferences which are sometimes observed. The residual interferences are shown to be direct fingerprints of an inhomogeneous defect distribution.
Effective conductivity of composites with spherical inclusions: Effect of coating and detachment
Authors: Lu Shih-Yuan , Song Jiann-Long
Published: July 2002

An expression for the reduced effective thermal conductivity, keff/k1, of a random array of coated or debonded spherical inclusions with pair interactions rigorously taken into account is derived. Pair interactions are evaluated through solution of a boundary value problem involving two coated or debonded spheres with twin spherical expansions. The resulting keff/k1 is of O(f2) accuracy, where f is the combined volume fraction of the inclusion and interface. The effect of interfacial characteristics manifested as the reduced thermal conductivity, σ3, and relative thickness, δ/a, of the interfacial layer is thoroughly investigated. It is found that keff/k1 can be approximately viewed as a function of f and the dimensionless dipole polarizability, θ1, over a large parameter domain, despite the existence of higher order polarizabilities in the expression of keff/k1. The value of θ1 alone determines whether the effective inclusion is enhancing (θ1≳0), neutral (θ1=0), or impairing (θ1<0) to the matrix. Furthermore, the evaluation of keff/k1 for the present model system can be approximately replaced with that for composites containing inclusions of no interface but possessive of a reduced thermal conductivity of (1+2θ1)/(1−θ1). A contour plot of keff/k1 on the θ1−f domain that is useful in estimating keff/k1 for interfacial properties characterized by an arbitrary combination of σ3 and δ/a, is constructed.
The correlation of dimensionality with emitted wavelength and ordering of freshly produced porous silicon
Authors: Zhang Qi , Bayliss S. C.
Published: July 2002

Freshly produced red, yellow and green emitting porous Si specimens have been studied by NEXAFS and EXAFS (near edge and extended x-ray absorption fine structure). The emission peaks are at 690, 580, and 520 nm, which almost covers the full visible range that direct anodization can achieve. The correlation between the co-ordination numbers of the first, second and third Si neighbor shells from Fourier transform fitting of EXAFS and both emission peak energies and optical band gaps estimated by PLE (photoluminescence excitation dependence) suggests that the nanostructures of the PS are nanowires, rather than nanocrystalline. Two types of quantum nanowire with one and one-plus-a-fraction dimensionality are proposed to interpret the correlation. The order factors of the theoretical fits suggest the nanowires of the freshly produced PS have crystalline cores.
Band-edge photoluminescence of CuGaSe2 films grown by molecular beam epitaxy
Authors: Yamada Akimasa , Makita Yunosuke , Niki Shigeru , Obara Akira , Fons Paul , Shibata Hajime , Kawai Michihiro , Chichibu Shigefusa , Nakanishi Hisayuki
Published: July 2002

Slightly Cu-rich CuGaSe2 films were grown on [001] oriented GaAs substrates by molecular beam epitaxy. Photoluminescence of the films showed a remarkable emission peaked at 1.71 eV at low temperature, which is attributed to recombination of free excitons and bound excitons. The dissociation energy of free excitons and their localization energy to a center are found to be 16.2 and 3.3 meV, respectively. The band-gap energy Eg is estimated to be 1.7310 eV at low temperature. It is suggested that the temperature variation of Eg is dominated by interaction with phonons of 26 meV which corresponds to the mean energy of the optical phonons in CuGaSe2.
Observation of giant Hall effect in granular magnetic films
Authors: Pakhomov A. B. , Yan X. , Xu Y.
Published: July 2002

The Hall effect in granular co-sputtered ferromagnetic metal–insulator films was found to increase dramatically as the magnetic volume fraction decreases toward the metal–insulator transition. The saturated Hall resistivity is up to 160 μΩ cm at T=5 K, that is almost four orders of magnitude greater than that in a pure magnetic metal sample. Close to the metal–insulator transition, both magnetoresistivity and the saturated Hall resistivity decrease with increasing temperature. Correlations of the Hall resistivity with resistivity and magnetoresistivity are discussed.
The effects of average grain size on the magnetic properties and corrosion resistance of NdFeB sintered magnets
Authors: Scott D. W. , Ma B. M. , Liang Y. L. , Bounds C. O.
Published: July 2002

Magnets with a nominal composition of Nd14.5Fe79.5B6 have been prepared by the conventional powder metallurgy technique. Precursor alloy powders with average particle sizes of 2.5, 3.0, 3.5, and 4.2 μm were included in this study. Average particle size and oxygen content were effectively manipulated to control the average grain size in the sintered magnets. Typically, for NdFeB sintered magnets, the corrosion resistance of these magnets was improved with increasing oxygen content. The corrosion resistance of magnets varied significantly with the average particle size of precursor alloy powders. For a fixed oxygen content, magnets made from powders of larger sizes exhibited a higher weight loss (a poor corrosion resistance) when compared to those made from smaller alloy powders. However, the Hci of magnets made from 2.5, 3.0, and 3.5 μm precursor alloy powders was found to decrease drastically with increasing oxygen content while magnets made from the 4.2 μm powder was found to remain relatively constant with increasing oxygen content. To optimize magnetic performance, one needs to compromise the corrosion resistance and the Hci obtained by balancing the average particle size of the precursor alloy powder for magnet fabrication, as well as the oxygen content and the average grain size in the finished magnet.
Photoinduced changes in the fatigue behavior of SrBi2Ta2O9 and Pb(Zr,Ti)O3 thin films
Authors: Dimos D. , Al-Shareef H. N. , Warren W. L. , Tuttle B. A.
Published: July 2002

It is shown that SrBi2Ta2O9 (SBT) thin films can be made to exhibit significant polarization fatigue by electric-field cycling under broad-band, optical illumination. Photoinduced fatigue is also observed for Pb(Zr,Ti)O3 (PZT) thin-film capacitors with (La,Sr)CoO3 (LSCO) electrodes. These results demonstrate that both the Pt/SBT/Pt and the LSCO/PZT/LSCO systems are susceptible to fatigue effects, which are attributed primarily to pinning of domain walls due to charge trapping. Capacitors that have been fatigued under illumination can be fully rejuvinated by applying a dc saturating bias with light or by electric-field cycling without light, which indicates an intrinsic, field-assisted recovery mechanism. We suggest that fatigue is essentially a competition between domain wall pinning and unpinning and that domain pinning is not necessarily absent in these nominally fatigue-free systems, but rather these systems are ones in which unpinning occurs at least as rapidly as any pinning. In both cases, the extent of photoinduced fatigue decreases with increased cycling voltage, indicating the relative importance of field-assisted unpinning. Finally, the observation of photoinduced fatigue implies that increased injection rates, potentially due to oxygen vacancy accumulation, may account for the electrode dependence on fatigue in PZT thin films.
Effects of mode structure on three-dimensional laser heating due to single or multiple rectangular laser beams
Authors: Kantor A. , Scott J. E. , Latham W. P.
Published: July 2002

The natural shape of a typical chemical oxygen-iodine laser (COIL) beam is rectangular which means that the transverse cross section of the beam is rectangular. Such a rectangular beam can be split into two or more beams to carry out materials processing. The distance between such beams and the mode structure in the beams can affect the distribution of heat in the workpiece. This, in turn, affects the quality and efficiency of materials processing. A three-dimensional quasi-steady-state thermal model is presented in this article, and an expression for the temperature distribution during laser heating with single or multiple COIL beams having rectangular spots is obtained. Calculations are carried out for single or two rectangular COIL beams. The depth of heating is found to increase when the distance between two beams is less than a certain value.
A general property of endoreversible thermal engines
Authors: Arias-Hernández L. A. , Angulo-Brown F.
Published: July 2002

In this work we propose that endoreversible Carnot–type heat engines have a general property independent of the heat transfer law used to describe heat exchanges between the working fluid and its thermal reservoirs. This property has to do with the so-called ecological function [F. Angulo–Brown, J. Appl. Phys. 69, 7465 (1991)]. According to this property, the efficiency at the maximum of the ecological function is the semisum of the Carnot and the maximum power efficiencies for any heat transfer law. This result is obtained by using the quasiparabolic behavior of power versus efficiency. From this property, we obtain a corollary over a general quantitative relation between the power (and also the entropy production) of both maximum power and maximum ecological regimes. We also discuss a criterion to find the best ecological function.
The van der Waals epitaxial growth of GaSe on Si(111)
Authors: Vinh Le Thanh , Eddrief M. , Mahan John E. , Vantomme André , Song J. H. , Nicolet Marc-A.
Published: July 2002

GaSe, a layered semiconductor, may be grown on the Si(111) surface by molecular beam epitaxy. The crystalline quality is relatively good, in the sense that the MeV He4 ion minimum channeling yield (∼30%) is as low as that of state-of-the-art bulk material, and the interface is atomically abrupt. The initial film deposits are epitaxial islands, and subsequent growth is in the Frank–van der Merwe mode. With the islands already relaxed at the nucleation stage and coalescing to essentially uniform coverage with the first monolayer of deposition, GaSe on Si(111) provides an example of van der Waals epitaxy. However, it is difficult to understand how epitaxy (crystallographic alignment with the substrate) can occur in such a case, where the film is incommensurate starting from the initial nuclei. A mechanism for alignment of the islands is proposed: they are aligned with the silicon substrate through the influence of dangling bonds at their perimeter, being “quasi-commensurate ” by virtue of their small lateral size. Although discommensurate regions are created as the islands grow laterally, there is simply no change in their orientation.
Characterization of photoluminescence intensity and efficiency of free excitons in semiconductor quantum well structures
Authors: Jin Shirong , Zheng Yanlan , Li Aizhen
Published: July 2002

The photoluminescence (PL) intensity of free excitons in a GaInAsSb/GaAlAsSb single quantum well structure is investigated as a function of excitation intensity and lattice temperature. The relationship between PL intensity of free excitons and excitation intensity as well as lattice temperature was developed for the quantitative descriptions of the experimental data.
Selection rules of Raman scattering by optical phonons in strained cubic crystals
Authors: Anastassakis E.
Published: July 2002

Strain characterization of materials through Raman spectroscopy often requires the optical phonon wave vector to be in directions other than those of the strain-modified phonon eigenvectors. As a result, the observed mode frequencies are mixtures of the strain-modified eigenfrequencies. The selection rules for such generalized scattering configurations are derived here for the strained zincblende-diamond family crystals. The formulation is based on the relative magnitude of the LO-TO and the strain-induced splittings. The results are important for the analysis of strain-modified Raman lineshapes and of “forbidden” lines. Specific examples are worked out in detail for externally stressed bulk crystals (90° scattering geometry), and for [001], [111], and [110] strained heterojunctions (backscattering under oblique incidence and/or detection).
Application of a generalized methodology for quantitative thermal diffusivity depth profile reconstruction in manufactured inhomogeneous steel-based materials
Authors: Munidasa Mahendra , Funak Frank , Mandelis Andreas
Published: July 2002

Several applications will be presented using an inversion methodology based on the generalized theoretical formulation [J. Appl. Phys. 80, 5570 (1996)] of the Hamilton-Jacobi formulation of thermal-wave physics to the problem of photothermal radiometric depth profilometry of the thermal diffusivity of inhomogeneous solids. In the depth profile reconstruction algorithm three channels of information, namely, the amplitude, the phase, and the derivative of phase, of the photothermal signal are utilized to reduce multiple solutions to a single consistent and continuous solution. Wherever possible, diffusivity profiles are compared with those resulting from destructive microhardness testing.
Impurity incorporation kinetics during modified-Lely growth of SiC
Authors: Ohtani Noboru , Katsuno Masakazu , Takahashi Jun , Yashiro Hirokatsu , Kanaya Masatoshi
Published: July 2002

The impurity incorporation kinetics during modified-Lely growth of silicon carbide (SiC) have been studied in terms of several growth parameters. It was found that the nitrogen incorporation is well described by a Langmuir isotherm type equation, implying that dynamic equilibrium between the vapor phase and the adsorbed nitrogen is established. The polytype of grown crystal and the seed orientation influence the impurity incorporation. For growth on (0001̄)C, 6H-SiC crystals always incorporate more nitrogen and less boron than 4H-SiC crystals, while no clear polytypic dependence of impurity incorporation is observed for growth on (11̄00) and (112̄0). Atomic force microscope observations revealed that there is a marked difference in the growth morphology between 6H-SiC(0001̄)C and 4H-SiC(0001̄)C. The origin of the polytypic dependence of impurity incorporation during growth on (0001̄)C is discussed with reference to the growth surface morphology.
Determination of Young’s modulus of the reduced layer of a piezoelectric RAINBOW actuator
Authors: Wang Qing-Ming , Cross L. Eric
Published: July 2002

The reduced and internally biased oxide wafer (RAINBOW) actuator, a new type of monolithic piezoelectric bending device, is composed of a reduced electromechanically passive layer and an unreduced piezoelectric layer. The determination of the elastic properties of the reduced layer is important for optimizing actuator performance. In this paper, an analytical expression for bending resonant frequency is derived for the cantilever RAINBOW actuator by using composite beam theory. A resonance method is then described for determining Young’s modulus of the reduced layer of RAINBOW actuator. As a piezoelectric bending actuator, RAINBOW can be excited by its electromechanically active PZT layer without using any external excitation method. Young’s modulus can be calculated by measuring its resonant frequency, sample geometry, and densities of component parts. For comparison, the Young’s modulus is also determined by measuring the resonant frequency of a completely reduced plate with an external acoustic excitation method. The results obtained by these two excitation methods show good agreement.
The photoluminescence in Si+-implanted SiO2 films with rapid thermal anneal
Authors: Chou Shu-Tsun , Tsai Jen-Hwan , Sheu Bor-Chiou
Published: July 2002

Two photoluminescence (PL) bands were observed from Si+-implanted SiO2 films after rapid thermal anneal (RTA) at ⩾950 °C. The PL band at 2.2 eV was obtained from the films with RTA in dry nitrogen and the other one at 1.9 eV was obtained from the films with RTA in wet nitrogen. The luminescence at 2.2 eV disappeared after the films were reannealed with an electrical oven at ⩾600 °C, which is similar to the behavior of oxygen- and hydrogen-deficient structures, and therefore, the mechanism of this PL band was attributed to the Eδ′ center. The other one at the 1.9 eV band, being related closely to Si–O–H structures and still appearing after being reannealed to 800 °C, could be ascribed to the effect of nonbridging oxygen hole centers.
Determination of secondary electron yield from insulators due to a low-kV electron beam
Authors: Yong Y. C. , Thong J. T. L. , Phang J. C. H.
Published: July 2002

A technique for the accurate determination of secondary electron (SE) yield of insulators due to low-kV electron beam is presented. It is based on a capacitatively coupled charge measurement by subjecting the insulating film to a controlled pulsed electron beam in a scanning electron microscope. SE emissions from several insulating materials employed in integrated circuit manufacturing including wet and sputtered silicon dioxide (SiO2), polyimide, and AZ1350J photoresist, have been investigated for a range of primary energies between 0.5 and 2.5 keV. Comparisons are made between experimental data for SiO2 and polyimide with previous results. The dependence of SE emission on incidence angle and topography for SiO2 was investigated. Experimental results indicate that the dependence of SE emission on surface tilt for SiO2 is in good agreement with the power law for tilt angles below 70°, while emission saturation is observed at higher tilt angles. The SE yield from sputtered oxide was found to be higher than that of wet oxide, which is related to differences in topography between the two materials.
Charge control and mobility studies for an AlGaN/GaN high electron mobility transistor
Authors: Zhang Yifei , Singh Jasprit
Published: July 2002

A charge control model and a mobility model are developed for the Al–GaN/GaN high electron mobility transistor (HEMT) device. The model addresses issues of how piezoelectric effect and interface roughness influence device properties. We find that even small amount of interface roughness has very strong effect on the two-dimensional electron gas properties. Low-lying electronic states are strongly localized and transport through these states is not described by Born approximation but by phonon-assisted hopping. At low temperature the effects of localization are quite important and we use the Kubo formula to study this transport. Results for charge control and mobility are presented as a function of Al composition in the AlGaN/GaN HEMT.
Growth of self-organized GaSb islands on a GaAs surface by molecular beam epitaxy
Authors: Wang T. , Forchel A.
Published: July 2002

We systematically investigated the growth of self-organized GaSb islands on a GaAs surface by molecular beam epitaxy technique. The lattice mismatch between GaAs and GaSb is 7.8%. At low growth temperature, only one kind of island can be observed, which is thought to be issued from the nucleation cluster. When the growth temperature is increased, another kind of island appears due to coalescence. When the growth temperature is above 535 °C, only the latter can be observed. From the temperature dependence of island density and size, surface diffusion energy and activation energy of GaSb on a GaAs surface have been determined to be 0.85 and 2.15 eV, respectively.
Raman analysis of the E1 and A1 quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN
Authors: Bergman Leah , Dutta Mitra , Balkas Cengiz , Davis Robert F. , Christman James A. , Alexson Dimitri , Nemanich Robert J.
Published: July 2002

This article presents a study of the quasi-longitudinal optical and quasi-transverse optical modes in wurtzite AlN which originate from the interaction of phonons belonging to the A1 and E1 symmetry groups. In order to analyze the allowed quasi as well as pure Raman modes, the modes were observed in a rotating crystallographic coordinate system, and the Raman tensors of the wurtzite crystal structure were calculated as a function of the crystallographic rotation. The frequencies of the quasimodes of wurtzite AlN were also analyzed in terms of the interaction of the polar phonons with the long range electrostatic field model. The experimental values of the Raman frequencies of the quasiphonons concur with these expected from the model, implying that the long range electrostatic field dominates the short range forces for polar phonons in AlN.
Electronic structure of oxygen vacancy in Ta2O5
Authors: Sawada H. , Kawakami K.
Published: July 2002

Ta 2 O 5 is a potential material for high dielectric constant insulators. The leakage current, however, should be reduced for application to 256 megabit dynamic random access memory. A state in the band gap induced by the oxygen vacancy is considered to lead to the leakage current. Ta2O5 was investigated theoretically in order to determine the effect of the oxygen vacancy. The calculated state originating from the oxygen vacancy is deeper than the experimentally obtained value. The strongly distorted local structure around the oxygen vacancy may give a shallow energy level, because the energy level of the vacancy state is shallower before structure optimization than after it.