bluuebunny/crossref_metadata_2025 huggingface dataset:
An ellipsoidal inclusion or crack in orthotropic piezoelectric media
Authors: Huang Jin H.
Published: July 2002

The inclusion method is developed to examine coupled electroelastic fields in an infinitely extended orthotropic piezoelectric material containing an ellipsoidal inclusion. By utilizing this method, unified expressions for both the coupled electroelastic fields within the inclusion and a set of four electroelastic tensors analogous to the Eshelby tensor for elastic inclusion problems are presented. In particular, the tensors for some practical inclusions, such as elliptical cylinder, circular cylinder, and penny shape, are derived analytically. Furthermore, by taking the electroelastic moduli of the inclusion as zero, the resulting expressions are applied to study crack problems. Three loading cases, a simple tension, a pure shear, and a normal electric displacement, are considered to examine the behavior of flat ellipsoidal cracking. Then, the critical loads of the Griffith criterion for fracture are expressed in closed forms for the electromechanical loading cases. These results could provide us with insight into how the orthotropic piezoelectric material consisting of inclusions will perform and would be helpful in understanding the damage behavior.
Mathematical modeling of key-hole laser welding
Authors: Kar A. , Mazumder J.
Published: July 2002

Laser welding is a unique way of joining materials with less thermal distortion and minimum metallurgical damage to the workpiece. The molten pool formed during welding determines the shape of the final welded region. At high laser intensities, the molten material vaporizes and a key hole is formed during the welding process. This vapor and the shape of the molten pool affect the absorption of laser at the liquid surface. The forces generated at the liquid-vapor interface due to surface tension gradient induce thermocapillary convection in the weld pool. This paper presents a mathematical model by considering these surface forces and the energy balance at the liquid-vapor and solid-liquid interfaces. The model is used to predict the surface velocity and temperature distributions, weld pool shape, key-hole depth and diameter. The velocity field is found to be large in the radial and azimuthal directions before the key hole is formed, and it changes to a radially and axially dominant field after the formation of the key hole. The results of this model are also compared with experimental data.
Photoluminescence of defects induced in silicon by SF6/O2 reactive-ion etching
Authors: Buyanova I. A. , Henry A. , Monemar B. , Lindström J. L. , Oehrlein G. S.
Published: July 2002

Photoluminescence (PL) studies of SF6/O2 plasma-induced defect formation in n-type silicon samples are reported. Ion bombardment of the silicon surface during the SF6 reactive-ion etching (RIE) is shown to introduce defects giving rise to a broad PL band in the 0.70–1.00 eV spectral range and to the carbon-related C and G lines. The role of oxygen during SF6/O2 RIE on the photoluminescence observed is analyzed. It is argued that oxygen contamination enhances the formation of PL centers via the creation of extended defects, such as oxygen precipitates. A lattice contraction nearby these extended defects is suggested to be responsible for the observed splitting of the C and G lines as well as the shift of the phosphorous bound exciton line detected after SF6/O2 RIE.
Per-carrier nonlinear optical response of [111]-oriented piezoelectric InGaAs/GaAs multiple quantum wells
Authors: Huang X. R. , Cartwright A. N. , Harken D. R. , McCallum D. S. , Smirl Arthur L. , Sánchez-Rojas J. L. , Sacedón A. , Calleja E. , Muñoz E.
Published: July 2002

We measure the per-carrier nonlinear responses caused by photoexcited carriers in two strained [111]-oriented InGaAs/GaAs multiple-quantum-well structures, and we compare them to that measured in a [100]-oriented structure. Without an external bias, we find that the absorption coefficient changes per photogenerated carrier for the [111]-oriented piezoelectric materials are smaller than for the [100]-oriented materials, not larger, as originally suggested. Subsequently, measurements of the per-carrier nonlinear responses as a function of reverse bias voltage demonstrate that the smaller per-carrier nonlinearities measured for the [111] structures are partially the result of a broadening of the excitons by the huge in-well fields, but can be primarily attributed to the quality of the [111]-grown materials. When corrected for differing in-well fields and for differing excitonic linewidths, the per-carrier responses are similar in magnitude, suggesting that the [111] response may eventually approach that of [100] material, but probably will not significantly exceed it.
Aging of the dielectric and piezoelectric properties of relaxor ferroelectric lead magnesium niobate–lead titanate in the electric field biased state
Authors: Zhang Q. M. , Zhao J. , Cross L. E.
Published: July 2002

The aging characteristics of the electrostrictive lead magnesium niobate–lead titanate [(1−x)PMN–xPT] under dc bias field were investigated. It was observed that the amount of aging increases with the PT content for the compositions investigated (x≤0.3). For a fixed composition, the aging rate rises with temperature. It was shown that the aging follows a stretched exponential time law, which is typical for the time-dependent behavior in polar glass and random field systems. In PMN–PT, the dielectric constant shows a much weaker aging than the piezoelectric coefficient, reflecting the importance of the stress coupling of the defect field to the micropolar region during the aging since the polar vector of the microregions can have both 180° and non-180° reorientations while only the non-180° reorientation contributes to the observed piezoelectric effect. Experimental results also reveal that the defect field developed during the aging in these materials under dc bias field is quite different from those in normal ferroelectrics and in Mn-doped PMN and 0.9 PMN–0.1PT. The direction of the defect electric field is opposite to the direction of the original dc bias field, indicating that the defect field is the result of a direct coupling to the external bias field rather than induced by the polar domains.
Nanocrystalline metals prepared by low energy ball milling
Authors: Oleszak Dariusz , Shingu Paul H.
Published: July 2002

The influence of low energy ball milling on the crystallite size, lattice strain, and storage of deformation energies of elemental metal powders is studied. The formation of nanosized grains (5–25 nm) and enhancement of lattice strain up to 0.4% is found. Excess enthalpies of up to 25% of the heat of fusion are reached.
Synthesis and magnetic properties of electrodeposited metal particles on anodic alumite film
Authors: Bao Xiaohua , Li Feiyue , Metzger Robert M.
Published: July 2002

Elongated metal particles have been prepared by depositing transition metals into the pore channels of anodic aluminum oxide (alumite), with well-defined pores that build up into uniform and parallel channels. The highest coercivity for freshly prepared α-Fe particles in the alumite substrate reached 2700 Oe in the easy direction. The remanance curves of alumite films containing electrodeposited metals (Fe and Co) were measured to study the magnetostatic interaction of the particles. The possible reversal mechanisms of the metal particles in the alumite substrates were examined by the rotational hysteresis integral and the angular variation of the coercivity.
Time effects in exchange anisotropy-affected metal-evaporated tapes
Authors: Bottoni G. , Candolfo D. , Cecchetti A.
Published: July 2002

The time-dependent magnetic behavior in metal evaporated tapes for high-density magnetic recording is studied at low temperatures, where the coupling between the ferromagnetic Co and the antiferromagnetic CoO, in the obliquely evaporated CoNiO film, induces a unidirectional exchange anisotropy, and it is compared with the effects obtained at room temperature, where no coupling occurs. At low temperature a contraction of the hysteresis loop as a function of the number of repeated cycles N is observed, with a remarkable decrease of the measured coercive field. No contraction occurs at room temperature. The time dependence of the magnetization is larger at low temperatures, with increase of magnetic viscosity and thermal fluctuation field, than at room temperature. The observed anomalous increase of thermal instability of magnetization and coercivity with the decrease of the temperature is imputed to the presence of exchange coupling between the ferromagnetic and antiferromagnetic phases at low temperature and to the effect that such coupling can exert on the reversal of the magnetization in the ferromagnetic grains.
Coordinated RHEED, XRD, and FMR investigations of MBE grown Co–Cu (100) superlattices
Authors: Lukaszew R. A. , Naik R. , Mountfield K. R. , Artman J. O.
Published: July 2002

A sequence of fcc, 〈100〉 oriented, [Co (50 Å)–Cu (X)] superlattices with X=25–200 Å, was grown by molecular beam epitaxy on Cu(100)/Si(100) substrates. In-plane 33.4 GHz ferromagnetic resonance (FMR) data were analyzed to determine the perpendicular uniaxial magnetic anisotropy field (Hu) and cubic anisotropy field contributions. The Hu values were found to be negative, indicating that the film normal is a magnetic hard axis. The magnitude of Hu depended on the Cu layer thickness. Superlattice x-ray-diffraction and reflection high-energy electron-diffraction patterns were analyzed. The measured strain data for Co superlattice layers, in combination with known elastic and magnetostriction constants for bulk fcc Co, yield calculated strain-induced Hu values which agree well with those obtained from the FMR. The observed Hu values, which are affected by the Cu layer thickness, approach a constant for Cu thickness ≥100 Å.
Thermal diffusivity of anisotropic materials by photothermal methods
Authors: Salazar A. , Sánchez-Lavega A. , Ocáriz A. , Guitonny J. , Pandey G. C. , Fournier D. , Boccara A. C.
Published: July 2002

In this paper we analyze the possibility of extending the photothermal methods developed for thermal diffusivity measurements of isotropic materials [A. Salazar and A. Sánchez-Lavega, Rev. Sci. Instrum. 65, 2896 (1994)] to the case of anisotropic specimens. A full theoretical treatment of the photothermal signal generation for the case of an anisotropic sample with its principal axes aligned with the sample surface is presented. Three photothermal detection schemes have been studied: infrared radiometry, photothermal reflectance, and optical beam deflection (mirage effect). The fundamental result we have obtained is that when using infrared radiometry, photothermal reflectance and collinear mirage setups, the thermal magnitude retrieved is the resistivity to heat diffusion (a tensor defined as the inverse of the thermal diffusivity tensor). Only the perpendicular mirage experiment allows one to directly retrieve the thermal diffusivity along any direction of the material. Experimental measurements performed on two highly anisotropic materials confirm the above-mentioned results.
Magnetic properties and the crystallization of amorphous Fe83B9Nb7Cu1
Authors: Kim Chul Sung , Kim Sung Baek , Lee J. S. , Noh T. H.
Published: July 2002

The amorphous state of ferromagnetic Fe83B9Nb7Cu1 and its nanocrystallization have been studied by x-ray, Mössbauer spectroscopy, and magnetic moment measurements. In the amorphous state at 13 K, the Mössbauer spectrum exhibits an essentially symmetric hyperfine field distribution with a half-width of 72 kOe. The average hyperfine field Hhf(T) of the amorphous state shows a temperature dependence of [Hhf(T)−Hhf(O)]/Hhf(O)=−0.48(T/TC)3/2 u −0.22(T/TC)5/2 for T/TC<0.7, indicative of spin-wave excitation. The quadrupole splitting just above the Curie temperature TC is 0.43 mm/s, whereas the average quadrupole shift below TC is zero. The Curie and crystallization temperatures are determined to be TC=393 K and Tx=775 K, respectively, for a heating rate of 5 K/min. The occupied area of the nanocrystalline phase at the optimum annealing temperature is about 33%–71%. It is notable that the magnetization of the amorphous phase decreases more rapidly with reduced temperature than those of nanocrystalline ferromagnets, suggesting the presence of the distribution of exchange interactions in the amorphous phase or high metalloid contents.
Structural, optical, and electronic properties of magnetron-sputtered platinum oxide films
Authors: Neff H. , Henkel S. , Hartmannsgruber E. , Steinbeiss E. , Michalke W. , Steenbeck K. , Schmidt H. G.
Published: July 2002

Stable platinum oxide films have been prepared through magnetron sputtering and have been analyzed on the bases of energy-sensitive microanalyses, x-ray diffraction, resistivity, and optical reflectance measurements. The complex dielectric function has been determined for various oxygen contents in the film covering the wave-number regime 50 cm−1–λ−1–50 000 cm−1. The vibrational properties are dominated through a strong band, centered at 765 cm−1, associated with a asymmetric stretching mode of the Pt—O bond. The films are amorphous, with chemical composition PtOx, where 1<x<2.1, and are considered as a homogeneous solid solution of PtO and PtO2. The materials system displays a conductor–insulator transition at x≥2, in connection with an optical band gap Eg of ∼1.2 eV in the fully oxidized state. The conduction mechanism over the whole range of compositions is thermally activated and is determined through a large density of localized states extending into the band gap. At x<2 the optical gap disappears, consistent with the semimetallic behavior of the materials system for this range of composition.
Voltage enhancement in quantum well solar cells
Authors: Barnham Keith , Connolly James , Griffin Paul , Haarpaintner Guido , Nelson Jenny , Tsui Ernest , Zachariou Alexander , Osborne Jane , Button Chris , Hill Geoff , Hopkinson Mark , Pate Malcolm , Roberts John , Foxon Tom
Published: July 2002

It is known that quantum well solar cells (QWSCs) can enhance short circuit current and power conversion efficiency in comparison with similar, conventional solar cells made from the quantum well (QW) barrier material alone. In this article we report measurements of the dark-current and open-circuit voltage (Voc) of a number of quantum well cells in three different lattice-matched material systems, namely, Al0.35Ga0.65As/GaAs, GaInP/GaAs, and InP/InGaAs. We also present the results obtained from comparable control cells without wells formed either from the material of the barriers or the well material alone. Our results clearly demonstrate in all three cases that, at fixed voltage, QWSC dark currents are systematically lower than would be expected from control cells with the same effective absorption edge. Measurements of Voc in a white-light source show that the open-circuit voltages of the QWSCs are higher than those of control cells formed from the well material. Furthermore, this enhancement is more than is expected from the shift in the absorption edge due to the effect of confinement in the wells. We discuss these results in the light of recent theoretical speculation about the upper limit to the efficiency of an ideal quantum well solar cell. We report on a 50 well QWSC with open-circuit voltage higher than the world record conventional cell formed from the well material, namely, GaAs.
Optimizing AlxGa1−xN separate confinement heterostructure lasers with large band discontinuities
Authors: Shah Pankaj , Mitin Vladimir
Published: July 2002

Two-dimensional simulations were performed to optimize the waveguiding region’s aluminum composition of an AlGaN/GaN separate confinement heterostructure (SCH) laser with large band discontinuities. Results demonstrate that the active region’s thickness, waveguiding regions’ thickness, and the material composition of the waveguiding region should be optimized, based on losses of the materials in the structure. Results also demonstrate that the threshold current of a SCH laser may be larger than that of a double heterostructure laser. The increase is caused by a competition between the active region and the waveguiding region which has a parasitic effect on the laser’s modal gain because of the waveguiding region’s slightly larger band gap, so that below the lasing threshold, photon emission and population inversion can occur in both regions. At the minimum threshold current the structure is optimized to strongly confine both the guided optical mode and the charge carriers responsible for the gain.
Relaxation behavior of semiflexible polymers at very low frequencies
Authors: Dı́az-Calleja Ricardo , Sanchı́s M. J. , Alvarez Cristina , Riande Evaristo
Published: July 2002

The dielectric activity of poly(monocyclohexylmethylene itaconate) (PMCI) and poly(dicyclohexylmethylene itaconate) (PDCI) in the glassy region and in the glass–rubber transition is studied by thermostimulated discharge current (TSDC) techniques. The spectra obtained by global TSDC experiments show a prominent α-glass–rubber relaxation peak with maxima located at 97 and 55 °C for PMCI and PDCI, respectively, following in decreasing order of temperature for a well developed β absorption and a comparatively low intensity relaxation. Better resolution of the relaxation behavior of these polymers in the glassy region is obtained by calculating the components of the complex dielectric permittivity ε* at extremely low frequencies from partial TSDC experiments. The ac spectra thus obtained suggest that the β absorption is composed by two relaxations, each of them presumably associated with the motions of a side group. The differences observed in these spectra with those reported for the β mechanical relaxation reported for these polymers in the literature are interpreted in terms of the restrictions that the side groups produce in the conformational space of phase of the backbone. These restrictions also explain the small changes in enthalpy in the glass–rubber transition which preclude the possibility of obtaining the glass–rubber transition temperature of these polymers by differential scanning calorimetric techniques.
Micromagnetics of spin valve tape heads
Authors: Albuquerque G. B. , Freitas P. P.
Published: July 2002

A micromagnetic model was applied to study the response of large aspect ratio spin valve heads to tape fields. The contribution of a ferrimagnetic TbCo exchange layer is analyzed. The shielded head response to tape fields is then determined by first calculating the average device permeability from a micromagnetic analysis. This permeability is included in a transmission line model to estimate flux decay. The transfer curve for the shielded device (read half-gap g=0.1 μm, head-tape spacing d=0.075 μm) is calculated for an isolated arctangent transition in low remanence Barium Ferrite (BF) tape media. Linear response occurs for |〈cos(θPINN−θFREE)〉|⩽0.4. For a dibit pulse written on the BF tape with a transition length of 0.09 μm, bit length of 2.56 and 0.256 μm, an average PW50 of 0.42 and 0.29 μm is calculated. Pulse asymmetry is −6.6% and −7.1%, respectively. The rolloff for square wave recording indicates a D50 of 96 kfci, with no significant head saturation at low linear densities. For a 1 mA sense current, a zero to peak voltage output of 80 μV/μm trackwidth is predicted. A similar analysis is done for a sinusoidal signal recorded on thick metal particle tape media. Partial head saturation is observed at intermediate linear densities, leading to a second harmonic distortion of −19.9 dB at 30 kfci.
Recording characteristics of rigid disks with Co-based alloy films using durable silicon substrates
Authors: Noda Kohki , Kawanabe Takashi , Naoe Masahiko
Published: July 2002

Co-based alloy films using silicon substrates were investigated for high-density magnetic recording disks. Silicon-disk substrates have several advantages, such as microscopically ultraflat surface roughness in the range of 1 nm or less, which results in a lower than ever flying height, lithographic fabrication of grooves for servo patterns, minimal outgassing from the substrates, higher substrate temperatures than for aluminum or glass substrates, and higher mechanical strength than for glass substrates. Recent Si substrates are mechanically durable because they have fewer microcracks than previous ones as a result of large improvements in machining processes such as chamfering and holing. In this study, Co85Cr13Ta2/Cr films were deposited by facing targets sputtering on 2.5 in. Si substrates at a substrate temperature of 100 °C and an argon gas pressure of 0.2 mTorr. A carbon protective layer and Z-DOL lubricant were applied on top of the magnetic layers. Their magnetic properties and recording characteristics were evaluated and compared with those reported previously at high substrate temperatures in the 350 °C range. It was found that, although the Hc at a temperature of 100 °C with a Co–Cr–Ta film thickness of 100–400 Å at a background pressure in the 1.5×10−6 Torr range was lower than those in the 350 °C range, the frequency response and noise properties obtained when a magnetoresistive head was used were comparable to those of a disk used in current products, which was employed as a reference. These disks, therefore, will be practical for high-density recording.
Electronic defect and trap-related current of (Ba0.4Sr0.6)TiO3 thin films
Authors: Wang Yin-Pin , Tseng Tseung-Yuen
Published: July 2002

By employing a deep level transient spectroscopy (DLTS) technique, this work investigates the deep trap levels of rf-sputtered (Ba0.4Sr0.6)TiO3 (BST) thin films deposited at various temperatures. Arrhenius plots of DLTS spectra detect a single trap located at 0.45 eV in 450 °C deposited films, whereas two traps located at 0.2 and 0.40 eV appear in 550 °C deposited films. On the other hand, examining the I–V characteristics of the films at the temperature range of 298–403 K reveals the presence of two conduction regions in the BST film capacitors, having ohmic behavior at low voltage (<1 V) and Schottky-emission or Poole–Frenkel mechanism at high voltage (>6 V). The barrier height and trapped level are, respectively, estimated to be 0.46 and 0.51 eV, corresponding to the trap activation energy 0.4–0.45 eV obtained from our DLTS measurements. Compared with previous published reports, the trap distributed at 0.4–0.5 eV should be an intrinsic defect of BST and possibly ascribed to be oxygen vacancies. Meanwhile, the trap plays a prominent role in the leakage current of BST films.
Anisotropic interfacial strain in InP/InGaAs/InP quantum wells studied using degree of polarization of photoluminescence
Authors: Lakshmi B. , Robinson B. J. , Cassidy Daniel T. , Thompson D. A.
Published: July 2002

Room-temperature, polarization-resolved photoluminescence from a (001) surface has been used to investigate InP/InGaAs/InP quantum wells grown by gas source molecular beam epitaxy. The degree of polarization of photoluminescence from a (001) surface, DOP001, is a direct measure of the anisotropy of polarization of luminescence between [110] and [11̄ 0] directions. DOP001 is observed to be strongly dependent on the quantum well thickness, composition (strain), and the gas switching time at the growth-interrupted interface. Results show that the anisotropy of polarization may be due to an effect of an anisotropic strain field that is associated with strained bonds at the interfaces of the quantum well.
Hybrid organic–inorganic semiconductor-based light-emitting diodes
Authors: Guha S. , Haight R. A. , Bojarczuk N. A. , Kisker D. W.
Published: July 2002

We demonstrate that the ability of GaN-based light-emitting diodes to emit in the short-wavelength regime makes possible “hybrid” organic–inorganic semiconductor light-emitting diodes that consist of two parts: a GaN-based electroluminescent part and an organic thin-film-based fluorescent part that absorbs the electroluminescence and fluoresces at a longer wavelength resulting in color conversion. We also show that organic films with much improved surface smoothness may be obtained by deposition at reduced temperatures.
Solitons on vortex filaments generated by ns laser pulse on metal surface
Authors: Lugomer S. , Maksimović A.
Published: July 2002

Laser-induced vaporization in the nonstationary (oscillatory) regime, on the ns time scale, that generates the nonlinear dynamics of the molten surface layer of tantalum, was studied. Microscale vortex filaments with the cascade of splittings raise up the bushlike structure. The cascade of vortex filament angle-locking splittings was shown to represent the period doubling bifurcation to chaos. Torsion of the filaments generates two-dimensional solitons in the side zones of the bush and three-dimensional solitons in the central zone. Two-dimensional solitons are obtained from a modified Korteweg–de Vries equation, while three-dimensional ones are obtained from nonlinear Schrödinger equation. The surface dynamics associated with metal vaporization appears to be more complex than usually assumed.
Numerical analysis of the transient response in amorphous silicon
Authors: Popović Pavle , Bassanese Elvis , Smole Franc , Furlan Jože , Grebner Stefan , Schwarz Reinhard
Published: July 2002

Using our program, TRansient Amorphous DEvice Simulator (TRADES), for the simulation of transient phenomena in amorphous silicon devices, the transient response after turning the light off is numerically calculated. Parameters for the numerical analysis are obtained by fitting the two families of the measured steady-state characteristics: light-intensity and temperature dependence of the secondary photocurrent. Using these parameters, both temperature and light-intensity dependence of the transients are calculated. It is shown that the transient is faster at higher temperatures and at higher illumination levels. Results are compared with measured transients at different temperatures.
A novel high current density switching effect in electrically conductive polymer composite materials
Authors: Duggal Anil R. , Levinson Lionel M.
Published: July 2002

The electrical switching properties of electrically conductive polymer composites are studied at high current densities. It is shown that the switching properties at high current densities are substantially different from those at low current densities which depend on a positive temperature coefficient of resistance effect. Experiments are described that show that this type of switching appears to be a general feature in conductor-filled polymer composite materials and a qualitative model for the switching phenomenon is proposed. These results suggest that conductor-filled polymer composite materials can provide a new nonmechanical way of rapidly limiting high power short-circuit currents. This should have broad applications in the circuit protection industry.
A carrier escape study from InP/InGaAs single quantum well solar cells
Authors: Zachariou A. , Barnes J. , Barnham K. W. J. , Nelson J. , Tsui E. S. M. , Epler J. , Pate M.
Published: July 2002

Carrier escape from InP/AlGaAs single quantum well structures is studied by means of simultaneous steady state photocurrent and photoluminescence measurements. The activation energy for escape is measured for the first time in this system. The photoluminescence from the InGaAs wells indicates that a significant number of carriers do not escape at room temperature thus affecting the temperature dependence of the cell. An estimate of the nonradiative efficiency of the device studied is given as a function of bias and temperature. The relevance to new applications is discussed.
Dielectric properties of Ta2O5–ZrO2 polycrystalline ceramics
Authors: Cava R. J. , Krajewski J. J.
Published: July 2002

The dielectric properties of (Ta2O5)1−x(ZrO2)x polycrystalline ceramics at 1 MHz and temperatures between 0 and 100 °C are reported for 0.0<x<0.25. The dielectric constant is moderately enhanced in the solid solution as a function of Zr content, and there is a significant decrease in the temperature coefficient of the dielectric constant (TCK). The annealing temperature of the ceramics is found to have a dramatic effect on TCK, associated with the appearance of the low temperature form of tantalum oxide at intermediate Zr concentrations, and Ta4Zr11O32 at high Zr concentrations: a bulk ceramic consisting of a mixture of the latter two phases was found to display a dielectric constant of ≈50 and a temperature coefficient of ≈−40 ppm/°C.
Multitube surface-wave discharges for increased gas throughput at atmospheric pressure
Authors: Moisan M. , Zakrzewski Z. , Etemadi R. , Rostaing J. C.
Published: July 2002

Large diameter (>10 mm) microwave discharges at atmospheric pressure are often constricted transversely and, when large gas flow rates are used, unstable as far as microwave power coupling is concerned. A group of small bore tubes can be used instead to provide the same gas throughput and ensure a higher probability of interaction of molecules to be processed with the carrier gas. The solution presented provides equal power sharing in these small diameter plasma columns and it employs preferably a single microwave field applicator, therefore enabling one to use only one power generator and yielding a compact system. This scheme is based on the properties of surface-wave sustained plasmas and it calls on basic principles of waveguide circuitry.
Band offsets at the InAlGaAs/InAlAs (001) heterostructures lattice matched to an InP substrate
Authors: Zhang X. H. , Chua S. J. , Xu S. J. , Fan W. J.
Published: July 2002

The first-principles pseudopotential method combined with virtual crystal approximation is used to calculate band offsets at the In0.53(AlzGa1−z)0.47As/In0.52Al0.48As (001) heterostructures lattice matched to an InP substrate. It is found that the valence-band offset (VBO) varies with respect to the aluminum composition as VBO=0.18–0.16z–0.02z2 eV, while the conduction-band offset (CBO) varies as CBO=0.51–0.33z–0.18z2 eV. Our results are in good agreement with the experimental data.
Electron correlation in antiferromagnet and superconductor thiospinel Cu–Co–S4
Authors: Miyatani Kazuo , Tanaka Toshiro , Ishikawa Masayasu
Published: July 2002

The Co-rich system (Co1−xCux)Co2S4 (x=0–1) and the Cu-rich system Cu[CuxCo2−x]S4 (x=0–0.5) were synthesized, and x-ray diffraction, magnetic susceptibility χ, and electrical resistivity ρ were studied. Curie–Weiss-type susceptibility with an antiferromagnetic structure with TN≈20 K was observed for x=0.3–1 in the Co-rich system. This type of antiferromagnetism showed a similar behavior with those obtained previously for an antiferromagnetic and superconductive Cu-rich system. ρ showed a typical metallic-type temperature dependence and is proportional to T for the Co-rich system with antiferromagnetism and showed quadratic temperature dependence with the cross over temperature of 75 K for the Cu-rich system with antiferromagnetism and superconductivity. A 3d hole–electron correlation was observed to be concerned with Cu–S superconductivity.
Voltage dependence of magnetoresistance in spin dependent tunneling junctions
Authors: Zhang J. , White R. M.
Published: July 2002

The voltage dependence of magnetoresistance in spin dependent tunneling (SDT) junctions was studied experimentally and theoretically. Different magnetoresistance (MR)-V dependence in various patterned junctions was observed and correlated with other technologically important parameters, including the magnitude of the MR, linearity of the current–voltage characteristic, temperature dependence of the junction resistance, and the MR. A phenomenological model based on a spin-independent two-step tunneling via defect states in the barrier, in addition to the spin-dependent direct tunneling, is proposed to account for the MR-V dependence. The MR ratio is determined by the ratio of the two currents. The MR-V dependence results from a stronger voltage dependence of the two-step tunneling current compared to that of the direct tunneling current. The same model also satisfactorily predicts other properties of SDT junctions. A high quality barrier is required to minimize the MR-V dependence and improve other junction properties. The approach to achieving desirable junction impedance for data storage applications is discussed.
New bulk amorphous Fe–(Co,Ni)–M–B (M=Zr,Hf,Nb,Ta,Mo,W) alloys with good soft magnetic properties
Authors: Inoue Akihisa , Zhang Tao , Koshiba Hisato , Makino Akihiro
Published: July 2002

We have found that an amorphous phase with a wide supercooled liquid region reaching 85 K before crystallization is formed in Fe–(Co, Ni)–(Zr, Nb, Ta)–B, Fe–Co–(Zr, Nb)–(Mo, W)–B and Co–Fe–Zr–B systems. The high stability of the supercooled liquid enabled the production of bulk amorphous alloys with diameters up to 5 mm by copper mold casting. These amorphous Fe–(Co, Ni)–M–B alloys exhibit good soft magnetic properties, i.e., saturation magnetization of 0.95 to 1.1 T, low coercivity of 1 to 8 A/m, Curie temperature of 560 to 590 K and low magnetostriction of 8–14×10−6. The effective permeability of the Co–based alloys exceeds 25 000 at 1 kHz and keeps high values above 5000 at the high frequency of 1 MHz. The permeability at 1 MHz is much higher than those for any kinds of soft magnetic materials. The frequency at which the imaginary part of permeability shows a maximum is also about 1 MHz. The success of synthesis of new Fe- and Co-based amorphous alloys with good soft magnetic properties and high glass-forming ability is promising for future development of a new type of soft magnetic material.
Magnetocrystalline anisotropy of (Fe1−xCox)3P
Authors: Broddefalk A. , Granberg P. , Nordblad P. , Liu Hui-ping , Andersson Y.
Published: July 2002

The magnetocrystalline anisotropy energy (MAE) of (Fe1−xCox)3P (x=0, 0.1, 0.2, 0.33) has been studied in the temperature range 5 K–300 K. The compound crystallizes in a tetragonal structure with three non-equivalent metal sites and shows an easy planar behavior with the magnetic moments aligned in the ab plane and the c axis as the hard direction. The MAE of Fe3P was investigated using magnetization measurements on a single crystalline sample. The anisotropy constants were found to be K1≈−851 kJ/m3 and K2≈9 kJ/m3 at 5 K and K1≈−648 kJ/m3 and K2≈11 kJ/m3 at 300 K. The MAE of the cobalt substituted systems was investigated on sintered samples formed into spheres. K1 for these samples was calculated from a fit of the magnetization versus internal field strength using the law of approach to saturation.
Negative ion emission from a stainless steel surface due to positive ion collisions
Authors: Walton S. G. , Champion R. L. , Wang Yicheng
Published: July 2002

Absolute yields of negative ions and secondary electrons resulting from positive ions impacting a stainless steel surface have been determined as a function of the impact energy. The surface conditions range from those commonly found in situ for discharge electrodes to a surface free of any adsorbate. The implications of these findings to discharge modeling are discussed.
Logistic function and evolution of free-electron-laser oscillators
Authors: Dattoli G.
Published: July 2002

In this article we show that the logistic function can be exploited to describe the evolution of free-electron-laser oscillators with the inclusion of lethargy, cavity tuning effects, etc. It is also proved that it offers a useful tool to gain a deeper understanding of the time dependence on the spectral features of the optical packet.
Optical constants and electronic interband transitions of disordered GaAs1−xPx alloys
Authors: Kim Kwang Joo , Lee Myoung Hee , Bahng Jae Ho , Shim Kyurhee , Choe B. D.
Published: July 2002

We report dielectric function spectra of disordered GaAs1−xPx alloys measured using spectroscopic ellipsometry at room temperature in the 1.5–5.5 eV photon energy region. Energies of the interband-transition edges, E0, E1, E0′, and E2, of the alloys were determined by line-shape analyses on their dielectric functions. It is found that the E1, E0′, and E2 energies of the alloys increase linearly with the increase of the P composition between those of GaAs and GaP while the increase of their E0 energy shows finite positive bowing. The energy separation between the E1 and its spin-orbit-split E1+Δ1 edges of the alloys decreases linearly with the increase of the P composition with their lifetime broadenings becoming larger than those of the binary compounds by more than 20%. The effect of compositional disorder on the electronic structure of GaAs1−xPx alloy system was discussed based on the present optical data.
Tip-sample distance regulation for near-field scanning optical microscopy using the bending angle of the tapered fiber probe
Authors: Wei P. K. , Fann W. S.
Published: July 2002

A method to directly measure the bending angle of a vibrating tapered optical fiber probe is demonstrated. This modified optical beam deflection method allows the vibrational amplitude and angle to be measured independently. The results show that there is a bending in the tip shaft. The merits of using this vibrational angle for distance regulation in near-field scanning optical microscopy are discussed.
Comparison between the Monte Carlo method and the drift-diffusion approximation in quantum-well laser simulation
Authors: Güçlü A. D. , Maciejko R. , Champagne A. , Abou-Khalil M. , Makino T.
Published: July 2002

The most widespread approaches to semiconductor device simulation are the drift-diffusion equations, momentum/energy balance equations, and the Monte Carlo method. In this article, the first comparison between results of a Monte Carlo simulation of a multiple-quantum-well structure and those obtained using a classical drift-diffusion simulator is presented. The outcome of the two methods is found to be similar. Still, the Monte Carlo approach offers much more insight into several issues such as the effects of the light holes and the carrier–carrier interactions. The limits and advantages of both methods are discussed.
Influence of the inhomogeneous surface pretilt on zig-zag defects
Authors: Kralj S. , Pirš J. , Žumer S. , Petkovšek R.
Published: July 2002

The influence of the inhomogeneous surface pretilt (slope) on chevron structures and on zig-zag defects in a SmC liquid-crystal cell is studied. The experimentally observed effect of the confining substrate with controlled variation of the surface slope on the formation of zig-zag defects is qualitatively explained with the Landau–Ginzburg-type free-energy expanded in terms of the nematic director field and the smectic complex order parameter. Transitions in the C1 and C2 chevron structures are studied as functions of the surface pretilt angle θt. The threshold conditions are calculated. The width of the straight element of the domain wall (running parallel to the smectic layers) and the costs for its formation are estimated.
Evidence for the influence of thermal spikes on ion induced mixing in Si at energies between 3 and 300 keV
Authors: Cardenas J. , Svensson B. G. , Petravic M.
Published: July 2002

Broadening of Ge and B tracer profiles in Si during sputter depth profiling, using O2+, Ar+, Kr+, and Cs+ ions, has been accurately described using the diffusion approximation of ion induced atomic mixing. The model allows extraction of a parameter yielding the mixing efficiency as a function of deposited energy per unit length (FD) and ion energy. The mixing parameter exhibits a linear dependence on FD3/2 at ion energies between 3 and 300 keV, which provides strong evidence for a dominating influence of thermal spikes on the mixing process.
Elastic-strain tensor by Rietveld refinement of diffraction measurements
Authors: Balzar Davor , Von Dreele Robert B. , Bennett Kristin , Ledbetter Hassel
Published: July 2002

A procedure to obtain all components of the elastic-strain tensor by simultaneous Rietveld refinement of diffraction patterns collected at different specimen orientations is described. The refined lattice parameters yield the hydrostatic strain component. If the lattice constants of the unstrained reference specimen are not known, the deviatoric strain tensor can still be determined. An anisotropic strain component may be refined for cubic and hexagonal crystal structures along with the isotropic. The method is applied to Al/SiC (short whisker) composites and systematic errors are assessed.
Effect of interphase layers on thermal stresses within an elliptical inclusion
Authors: Ru C. Q.
Published: July 2002

Thermal mismatch induced stresses are identified as the major cause of failure in a wide variety of materials and devices, ranging from metal–ceramic composites to passivated interconnect lines in integrated circuits. To reduce thermal stresses, an effective method is to add an intermediate layer, with appropriate thermal expansion coefficient, between the components of dissimilar materials. This paper gives a general analysis of the effects of the intermediate layers on thermal stresses within an elliptical inclusion, with particular emphasis on the role of thermal mismatch. The exact closed-form solution is obtained for stress field. One of the unique features of the present model is that the thermal stresses within the elliptical inclusion are uniform, making simple formulas available for quantitative analysis of the effects of the interphase on thermal stresses within the inclusion. It is found that the interphase layers have a strong effect on the deviatoric stress and a moderate effect on the mean stress within the inclusion. In particular, the effect of the interphase on the mean stress is sensitive to both the aspect ratio of the elliptical inclusion and the elastic mismatch between the inclusion and the surrounding materials, but not for the deviatoric stress within the inclusion. To reduce the thermal stresses within the inclusion, the optimum thermal expansion coefficient of the interphase is not necessarily between those of the inclusion and the matrix. However, if the design goal is to reduce the thermal stresses within both the inclusion and the interphase layer, the optimum interphase should have an intermediate thermal expansion coefficient between those of the matrix and the inclusion.
Carbon and group II acceptor coimplantation in GaAs
Authors: Morton R. , Lau S. S. , Poker D. B. , Chu P. K. , Fung K. K. , Wang N.
Published: July 2002

Coimplantations of carbon and one of the group II acceptors, Mg, Zn, or Cd, were performed and compared to implantations involving only a single element (Mg, Zn, or Cd) or Ga and C coimplanted into GaAs substrates. The group II and C (II/C) coimplantations act to balance the crystal stoichiometry since group II atoms prefer to reside in the Ga sublattice and C prefers to reside in the As sublattice. The electrical characteristics of the various implantations were obtained from sheet and differential Hall measurements. Rutherford backscattering spectrometry was employed to determine the amount of implantation-induced damage which was then correlated to the amount of C activation in the group II/C coimplanted samples. It was found that coimplantation of the heavier group II acceptors, Zn and Cd, resulted in layers with larger peak hole concentrations. This is a result of the large amount of lattice damage created by these elements which is thought to provide the necessary abundance of As vacancies for C activation. Secondary ion mass spectroscopy measurements of the samples after implant activation indicate that C coimplantation significantly reduces the diffusivity of the group II acceptors. Cross-sectional transmission electron microscopy indicated a unique defect structure (extrinsic dislocation loops) for the cases of group II/C coimplantation. These dislocation loops are located at the diffusion front of the group II element in the samples and therefore have a rather profound influence on the diffusion of the group II elements. A rationalization of the defect structure and the effect it has on the diffusion of group II elements is given.
Enhanced anticrossing in an asymmetric double-well GaAs/AlAs superlattice with two kinds of Stark-ladder states
Authors: Kawashima Kenji , Fujiwara Kenzo , Yamamoto Teiji
Published: July 2002

We have experimentally and theoretically studied electroabsorption properties of a novel superlattice having well width asymmetry and barrier sequence bi periodicity. Electric field-induced energy variations of the optical transitions observed by room temperature photocurrent spectroscopy show that several Stark-ladder branches in the fan diagram converge to different points. These “staggered” Stark ladders undergo large level anticrossings and result in a distinct blueshift of the optical absorption edge. Theoretical analyses on the variations of eigenenergies and the localization of wave functions of the Stark-ladder states clearly explain the experimental data. An advantage of the novel structure to optoelectronic device application is also shown in comparison with a conventional mono-periodic superlattice.
Low temperature irreversibilities in the magnetothermal behavior of Gd2CuO4
Authors: Klamut P. W. , Rogacki K. , Sikora A. , Dabrowski B.
Published: July 2002

The detailed investigation of the low temperature magnetic properties of Gd2CuO4 reveals the irreversible temperature behavior of the dc magnetization and ac susceptibility that is associated with the presence of magnetic anomalies inbetween TNGd=6.6 K and T1≈20 K. Observed hysteresis is centered around 8 K—the temperature of the sudden drop in the magnetization that corresponds to the temperature of reorientation-type magnetic phase transition in the Cu sublattice. The significant irreversibility has also been found in the temperature range of the magnetic anomaly at T1≈20 K. Following the theoretical model of the low temperature magnetic interactions proposed in Vitebsky et al. [Fiz. Niz. Temp. 20, 25 (1994)], the irreversible behavior around T1 can be qualitatively understood assuming the existence of the magnetic clusters in the form of groups of Gd–Cu–Gd sandwiches. Comparing the results obtained for quenched and slowly cooled samples it can be concluded that the long distance coherence, presumably of the oxygen displacements in the crystal structure, is the predominant factor determining the low temperature magnetic behavior of the system.