bluuebunny/crossref_metadata_2025 huggingface dataset:
Amorphous phase formation and initial interfacial reactions in the platinum/GaAs system
Authors: Ko Dae-Hong , Sinclair Robert
Published: July 2002

We have investigated the amorphous phase formation and initial crystalline reactions at Pt/GaAs interfaces via high-resolution transmission electron microscopy (HRTEM) and in situ HRTEM. A 3-nm-thick amorphous intermixed layer consisting of three elements, platinum, gallium, and arsenic formed at the Pt/GaAs interface during the deposition of a 500-Å-thick Pt film. The interlayer grew in a planar fashion in an amorphous state upon low temperature (e.g., 200 °C) annealing by a solid-state amorphization reaction. This reaction occurs with a driving force of a negative heat of mixing, and by the dominant diffusion of Pt to the GaAs substrate, which was verified by in situ HRTEM. Following the growth of the amorphous interlayer, the Pt3Ga and PtAs2 phases nucleated within the amorphous layer and grew at the Pt and GaAs sides, respectively. The relative mobility of the three constituents at the low temperature, the structure of the crystalline intermetallic compounds, and local thermodynamical equilibrium are responsible for the sequence of the crystalline phase formation. After a complete reaction at 400 °C for 20 min, we observed the formation of a layered structure of PtGa/PtAs2/GaAs as the final structure. In situ HRTEM experiments also demonstrated growth of the amorphous intermixed layer and crystalline reaction between the Pt film and the GaAs, which is consistent with the results from the ex situ annealing treatment.
Spectroscopic characterization of Tm3+:YVO4 crystal as an efficient diode pumped laser source near 2000 nm
Authors: Ohta Kei , Saito Hideaki , Obara Minoru
Published: July 2002

Spectroscopic characterization of the 5 at. % Tm3+:YVO4 crystal has been performed. The peak absorption cross section of the pumping band at 797.5 nm and the peak emission cross section of the laser transition at 1800 nm were determined to be 2.5×10−20 and 1.6×10−20 cm2, respectively. More than 150 nm tunability range is expected for this material. In addition, a possible extension of the tunability range under high-power pulsed excitation is discussed.
Effect of annealing on the elastic properties of Ag/Ni superlattices
Authors: Carlotti G. , Fioretto D. , Socino G. , Verdini L. , Pelosin V.
Published: July 2002

The modifications of the effective elastic constants of sputter-deposited Ag/Ni superlattices under annealing are studied by use of the Brillouin light-scattering technique. At-temperature measurements, taken while annealing the specimens, made it possible to study the dynamical evolution of the structure under investigation, through detection of up to four guided acoustic modes. Measurements taken at room temperature, after isochronal and isothermal cycles, permitted achievement of a deeper understanding of the variations of the elastic properties. It has been found that those elastic constants that are sensitive to interface effects, namely c44 and c33, are also markedly modified during the thermal treatment; no appreciable variation of c11 has been measured. The observed variations of the elastic constants are interpreted on the basis of the structural informations gained by use of x-ray and transmission electron microscopy; a comparison is also made with electrical resistivity results.
Frequency dependence on the structure of radio frequency glow discharges in Ar
Authors: Kakuta Shigeru , Makabe Toshiaki , Tochikubo Fumiyoshi
Published: July 2002

The frequency dependence of the sustaining voltage in a radio frequency discharge has been investigated under minimum sustaining and constant power conditions in Ar. In addition to the well-known feature that the sustaining voltage is high at low frequency and low at high frequency, the difference between monoatomic and polyatomic gases is recognized. The phase shift between voltage and current at 1 Torr decreases with increasing frequency above 500 kHz. This results from the electron density modulation having a delay with respect to the applied voltage. The phase shift at high frequency increases with increasing applied voltage, due to the balance of drift and diffusion fluxes of electron. The spatiotemporal net excitation rate of Ar(3p5) was measured at 100 kHz, and 4 MHz, and 13.56 MHz. It is reconfirmed that the sustaining mechanism of the discharge at low frequency is ionization by secondary electrons from the electrode, while at high frequency it is ionization due to reflected electrons by the oscillating sheath. At middle frequency, 4 MHz, the sustaining mechanism drastically varies with input power density from the high-frequency type to the low-frequency type.
Accurate reconstruction of the density of states in a-Si:H by constant photocurrent method and photothermal deflection spectroscopy
Authors: Amato G. , Giorgis F.
Published: July 2002

Detailed results about the distribution of occupied localized states in a-Si:H have been obtained by applying a new self-consisting deconvolution procedure to the absorption spectra. This deconvolution does not suffer from the limitations of other methods because no simplifying assumption are made on the shape of the localized states below the Fermi level. Numerical simulations confirm the reliability of our procedure. The results show that the main corrections with respect to the results obtained by means of the derivative method are related to the distribution of the deep defects. The difference between the results as obtained by constant photocurrent method and photothermal deflection spectroscopy is explained in terms of different sensitivities of the two techniques to transitions involving unoccupied defects and surface states.
Raman scattering study on the effects of Ga ion implantation and subsequent thermal annealing for AlSb grown by molecular-beam epitaxy
Authors: Kim S. G. , Asahi H. , Seta M. , Emura S. , Watanabe H. , Gonda S. , Tanoue H.
Published: July 2002

Radiation damage and its recovery process of molecular-beam epitaxy grown AlSb implanted with Ga ions is studied using Raman scattering for Ga ion fluences ranging from 1×1013 to 5×1014 cm−2 and for annealing temperatures ranging from 300 to 600 °C. With increasing fluences the AlSb longitudinal optical (LO) phonon mode shifts to lower frequency and exhibits an asymmetric broadening. Recovery of radiation damages in the ion implanted AlSb is observed after annealing at as low as 300 °C. On the other hand, after annealing at above 500 °C, disordering of the crystalline structure due to the outdiffusion of Sb is observed. The damage states and the recovering behaviors are quantitatively estimated using the spatial correlation model on the LO mode.
Interactions of wide band-gap single crystals with 248 nm excimer laser radiation. II. NaCl
Authors: Webb R. L. , Jensen L. C. , Langford S. C. , Dickinson J. T.
Published: July 2002

The emission of charged and neutral particles from single-crystal NaCl irradiated with 248 nm excimer laser light is studied by means of time-resolved photon spectroscopy and quadrupole mass spectrometry. Cathodo- and photoluminescence spectra and scanning electron microscope (SEM) observations indicate the progressive growth of surface damage during pulsed laser irradiation. SEM observations indicate that substantial fracture occurs prior to the onset of rapid vaporization. The plume is composed of charged particles as well as a high density of excited neutrals, including a number of energetic cluster ions. Fracture and subsequent plastic deformation contribute significantly to defect production and processes of laser desorption and vaporization.
High mobility n-type charge carriers in large single crystals of anatase (TiO2)
Authors: Forro L. , Chauvet O. , Emin D. , Zuppiroli L. , Berger H. , Lévy F.
Published: July 2002

Resistivity, thermopower, and Hall-effect measurements on large single crystals of the anatase form of TiO2 all indicate high mobility n-type carriers that are produced by thermal excitation from a density of ∼1018 cm−3 putatively present shallow donor states. The decrease of the mobility with increasing temperature is consistent with the scattering of carriers by the optical phonons of TiO2.
An array of dislocations in a strained epitaxial layer. I. Elastic energy
Authors: Zhang Tong-Yi , Hack J. E. , Guido L. J.
Published: July 2002

The total energy of an array of dislocations in a strained epitaxial layer is composed of the self energy of the dislocations, the strain energy which arises from the lattice mismatch between the layer and its substrate and the interaction energy between the dislocations and the mismatch strains. The sum of the self energy and the interaction energy represents the formation energy of the dislocations. In this study, the self energy is formulated using complex potentials. Two limiting conditions are used to check the solution. The first is that the self energy of the array reduces to that for an isolated single dislocation as the dislocation spacing in the array approaches infinity. Secondly, as the layer thickness approaches infinity, the self energy reduces to that for a dislocation wall. A negative formation energy promotes dislocation generation while a positive formation energy implies a suppression of dislocation generation. A critical thickness required for the generation of an isolated dislocation is found by locating the layer thickness which corresponds to a zero value of the formation energy. The critical dislocation density at a given thickness is also determined.
Coulomb enhancement of optical absorption in narrow-gap semiconductors
Authors: Paskov P. P.
Published: July 2002

The Coulomb enhancement factors of the optical absorption in semiconductors with a nonparabolic band structure are obtained. It is shown that the nonparabolicity increases the role of the residual electron-hole interaction in both the intrinsic and the laser-excited material. A necessity for correcting the calculated nonlinear refractive index due to excitonic effects is found even in the smallest band gap III-V semiconductor InSb.
Forward elastic recoil measurements using heavy ions
Authors: Siegele R. , Haugen H. K. , Davies J. A. , Forster J. S. , Andrews H. R.
Published: July 2002

The application of the elastic recoil detection technique utilizing heavy ions for the analysis of semiconductor samples is demonstrated. With this technique the depth profiles of the primary constituents as well as profiles of all impurities can be measured in one spectrum. Depending on the target material, a depth resolution down to 20 nm can be achieved. All elements except hydrogen can be detected with almost the same sensitivity, namely ∼1×1015 at/cm2 with 136 MeV I in a 30° recoil geometry. For hydrogen, the sensitivity is about four times better.
Sensitive field- and frequency-dependent impedance spectra of amorphous FeCoSiB wire and ribbon (invited)
Authors: Beach R. S. , Berkowitz A. E.
Published: July 2002

Conflicting reports of large magnetoresistive and magnetoinductive effects in amorphous FeCoSiB wires and ribbons prompted the impedance measurements reported here. The spectra (0≤f≤3.2 MHz) were obtained at room temperature using a commercial impedance analyzer both as functions of axial magnetic field (−140<HA<140 Oe) and sense current (1≤Irms≤60 mA). The phase shift due to the test leads was carefully measured and subtracted from the raw data to resolve the spectra into resistive R(f ) and reactive X(f ) components. We find for the Fe4.3Co68.2Si12.5B15 wire (120 μm diameter) and ribbon (20 μm thick) that both R(f ) and X(f ) depend strongly on frequency and magnetic field. For HA=0, each component increases monotonically with frequency, with R(f=0)≊1 Ω/cm and X(f=0)=0. In high fields (HA=140 Oe), R(f ) and X(f ) are nearly frequency independent. The field-dependent response is sharply peaked about HA=0; the full width at half maximum is FWHM≤20 Oe, typically. The change in R(f ) and X(f ) between these two extremes is extraordinarily large; 4.5 Ω/cm at f=1 MHz is a typical value for the wire. The sensitivity of the magnetoresistive response is 44% of the dc resistance per Oe for f=1 MHz. Qualitatively similar phenomena were observed for the Fe7.5Co67.5Si15B10 ribbon, although the field and frequency dependences of the spectra are less pronounced than for the wire. We discuss a model which describes the spectra quantitatively, using classical electrodynamics.
High electron mobility in pseudomorphic modulation-doped In0.75Ga0.25As/InAlAs heterostructures achieved with growth interruptions
Authors: Drouot V. , Gendry M. , Santinelli C. , Viktorovitch P. , Hollinger G. , Elleuch S. , Pelouard J.-L.
Published: July 2002

Growth interruptions under cation stabilization have been used to smooth the two interfaces defining the pseudomorphic channel layer of InAlAs/In0.75Ga0.25As/InAlAs heterostructures grown on InP. This leads to a 4 K electron mobility as high as 170 000 cm2/V s, due to the reduction of interface roughness scattering. This structure is used as a reference (‘‘zero’’ interface roughness) for a model to determine the influence of interface roughness scattering on the electron mobility as a function of the channel indium fraction (xIn=0.53, 0.65, and 0.75) in heterostructures grown without growth interruption. It is shown that the interface roughness scattering decreases with increasing indium concentration of the channel in the range 0.53–0.65 and saturates beyond xIn=0.65.
An x-ray photoelectron spectroscopy and work-function study of the Er/α-SiC(0001) interface
Authors: Kennou S.
Published: July 2002

The adsorption of erbium on α-SiC(0001) has been investigated by x-ray photoelectron spectroscopy (XPS) and work-function (WF) measurements. Deposition of more than 15 Å of erbium at room temperature gives a Schottky barrier height of 1.40±0.1 eV accompanied by shifts of all substrate XPS peaks. The WF value of clean α-SiC, 4.5 eV, is reduced to ∼2.9 eV after 5 Å of Er deposition beyond which the Schottky barrier begins to develop. Upon annealing of the Er-covered surface above 650 K the Schottky barrier height increases and reaches 1.80±0.1 eV at 900 K due to an interfacial reaction which leads to silicide formation at the interface.
A one-dimensional solution of the Boltzmann transport equation including electron–electron interactions
Authors: Childs P. A. , Leung C. C. C.
Published: July 2002

In this article a technique is described for solving the one-dimensional spatially dependent Boltzmann transport equation with electron–electron interactions included in the scattering model. The analysis is illustrated by solving the Boltzmann transport equation over a potential profile typical of that found in the channel of a metal–oxide–semiconductor field-effect transistor. A comparison is made between the distribution functions obtained when electron–electron interactions are included and excluded from the scattering model. It is found that electron–electron interactions significantly increase the electron population at energies greater than are available from the electric field.
Modeling of high power semiconductor switches operated in the nonlinear mode
Authors: Stout Phillip J. , Kushner Mark J.
Published: July 2002

Although optically activated high power photoconductive semiconductor switches (PCSS) are usually triggered using uniform illumination, under select conditions they can be activated and closed in <1 ns with a spot of light near the contacts. This observation requires free carriers to either travel at speeds faster than their saturation velocity or for there to be a carrier generation mechanism that propagates with similar speeds. A two-dimensional time-dependent computer model of a GaAs high power switch has been employed to investigate these observations, and activation of PCSS by spots of light in particular. Results from the model suggest that the transport of band-to-band recombination radiation plays an important role in propagating electrons across the switch when the switch is closed with a spatially nonuniform laser pulse. Reabsorption of the recombination radiation and photogeneration of carriers is a mechanism which generates free carriers in the gap between the contacts at speeds greater than saturation velocity. The results also indicate that the switch is sensitive to the location of the activating laser pulse. Less laser fluence is required to close the switch if illumination occurs near the cathode rather than near the anode.
Optical and magneto-optical properties of nanocrystalline cobalt ferrite films
Authors: Stichauer L. , Gavoille G. , Simsa Z.
Published: July 2002

The absorption coefficients and the Faraday rotation spectra of nanocrystalline cobalt ferrite thin films were determined in the visible–near infrared range. The films prepared by rf sputtering in an Ar–O2 atmosphere display magnetic and structural properties which differ from those of the bulk material. Optical and magneto-optical transitions observed in the films are identified by comparing our spectra with those obtained with well characterized bulk materials. In the bulk, cations with an octahedral environment (point group 3̄m) locate on a center of symmetry and the corresponding optical transitions have a very small oscillator strength. In the films, these transitions may lead to a large absorption coefficient. One absorption band appears that cannot be identified on the basis of the symmetry properties of the bulk material. The observed absorption spectra may be explained, however, if some of the cations have one of the following symmetry point groups: 4, 422, and 32. The Faraday rotation spectra show that: (1) the magnetic disorder increases when the oxygen to argon ratio is increased, (2) the disorder influences the cations on both tetrahedral and octahedral sites of the spinel structure.
Microstructural characterization of ferrimagnetic substituted iron garnet heterostructures for magneto-optical applications (abstract)
Authors: Simion B. M. , Ramesh R. , Thomas G.
Published: July 2002

Multilayered garnet heterostructures of Y3Fe5O12 (YIG) and Eu1Bi2Fe5O12 (EBIG) have been deposited on single crystalline [111] oriented Gd3Ga5O12 (GGG) substrates, pulsed laser deposition (PLD). Magnetic (vibrating sample magnetometer) and magneto-optical (reflective Faraday effect) measurements have indicated out-of-plane magnetization for all YIG/EBIG films, coercivities up to 1.2 kOe, and Faraday rotations as high as 50 000 deg/cm. According to x-ray diffraction experiments, all films are single crystalline, [111] oriented. Initial high resolution transmission electron microscopy studies have shown epitaxial growth of both YIG and EBIG layers. All samples had YIG deposited as the first layer of the heterostructure because of its small lattice mismatch with the substrate. As a consequence, all films showed sharp interfaces between the substrate and the first YIG layer. At the second interface, between the YIG layer and the first EBIG layer, structural defects were noted to form in the EBIG film. The thickness ratios between YIG and EBIG layers were intentionally changed during the processing to study the effect on magneto-optical properties. Transmission electron microscopy characterization indicated changes in the interface morphology, with some films displaying high coherence and sharpness of the interfaces, while others showed wavy and very incoherent interfaces. The films with sharp interfaces showed the highest values of Faraday rotation. Since there were no processing parameter changes from one film to another, the results appear to indicate that the morphology of the interfaces is the result of thickness variations in the heterostructures.
A study on the exchange interaction in R2Fe17 compounds
Authors: Yang Fuming , Wang Jianli , Gao Yihua , Tang N. , Han Xiufeng , Pan Hongge , Qingan Li , Hu Jifan , Liu J. P. , de Boer F. R.
Published: July 2002

A study on the exchange interaction in R2Fe17 compounds (R represents the heavy rare earth elements) has been performed by means of a mean-field analysis of the high-field magnetization curves, which were measured on the powder samples. Measurements have been carried out on R2Fe17-based quasiternary R2Fe17−xMx compounds with R=Dy, Ho, and Er, M=Al, Ga, and Si. It has been found that the value of the exchange coupling constant decreases with increasing atomic number of R ion of the compound. This behavior is explained by the varying 4f–5d hybridization in the compounds.
Bounds upon grain boundary effects in minority carrier semiconductor devices: A rigorous ‘‘perturbation’’ approach with application to silicon solar cells
Authors: Green Martin A.
Published: July 2002

Rigorous bounds upon the effects of grain boundaries upon minority carrier semiconductor devices such as solar cells are calculated. These bounds can be formulated in terms of an effective lifetime parameter as for spatially uniformly distributed defects. The value of this lifetime parameter depends on grain boundary geometry and activity. This concept applies to grain boundary effects in both bulk quasineutral and depletion regions. The electrostatic enhancement of grain boundary recombination in depletion regions is less severe than in bulk regions near thermal equilibrium.
Visible wavelength (6470 Å) GaxIn1−xP/GaAs0.66P0.34 quantum wire heterostructures
Authors: Moy A. M. , Chen A. C. , Cheng K. Y. , Chou L. J. , Hsieh K. C. , Tu C.-W.
Published: July 2002

Utilizing the strain-induced lateral-layer ordering (SILO) process, we have grown GaxIn1−xP multiple quantum wires (MQWR) on ternary GaAs0.66P0.34 substrates using a modified strain-balance mechanism. The resulting [110] lateral modulation occurred with a periodicity of ∼300 Å. Two dimensions of quantum confinement were obtained by surrounding the laterally confined GaxIn1−xP regions by layers of higher-energy-gap Al0.15Ga0.53In0.32P in the growth direction. A redshift in the photoluminescence emission was observed as the growth temperature was increased attributed to a stronger lateral composition modulation at the higher growth temperatures. Based on the modified strain-balance mechanism, light-emitting diodes with the GaxIn1−xP MQWR active region were fabricated using the SILO process. Strongly TE-polarized room-temperature electroluminescence from these devices was observed at 6470 Å.
Self-consistent model of high current density segmented hollow cathode discharges
Authors: Arslanbekov R. R. , Tobin R. C. , Kudryavtsev A. A.
Published: July 2002

A two-dimensional model of the hollow cathode and the high-voltage segmented hollow cathode sputtering discharges at high current densities is presented. The main plasma parameters (e.g., complete electron distribution function, spatial distributions of particle densities, gas temperature, etc.) are determined self-consistently. The most important mechanisms contributing to the operating characteristics of these hollow cathode discharges are identified and described quantitatively. The most representative hollow cathode configurations are analyzed and the results are compared to existing experimental data for the helium-argon-copper system, e.g., voltage-current characteristics, density of copper atoms as a function of the discharge current, and percentage argon. Good agreement is found between model predictions and experimental results. The model can be adapted to other high-voltage variants of the hollow cathode discharges.
Deep levels in p+-n junctions fabricated by rapid thermal annealing of Mg or Mg/P implanted InP
Authors: Quintanilla L. , Dueñas S. , Castán E. , Pinacho R. , Barbolla J. , Martín J. M. , González-Dı́az G.
Published: July 2002

In this work, we investigate the deep levels present in ion implanted and rapid thermal annealed (RTA) InP p+-n junctions. The samples were implanted with magnesium or coimplanted with magnesium and phosphorus. These levels were characterized using deep level transient spectroscopy (DLTS) and capacitance–voltage transient technique (CVTT). Seven majority deep levels located in the upper half of the band gap were detected in the junctions by using DLTS measurements, four of which (at 0.6, 0.45, 0.425, and 0.2 eV below the conduction band) result from RTA, while the origin of the other three levels (at 0.46, 0.25, and 0.27 eV below the conduction band) can be ascribed to implantation damage. An RTA-induced origin was assigned to a minority deep level at 1.33 eV above the valence band. From CVTT measurements, several characteristics of each trap were derived. Tentative assignments have been proposed for the physical nature of all deep levels.
Neutron depolarization study of internal stresses in amorphous Fe40Ni40B20
Authors: de Jong M. , Sietsma J. , Rekveldt M. Th. , van den Beukel A.
Published: July 2002

The magnetic domain structure of amorphous ferromagnets with nonzero magnetostriction is mainly determined by the internal stress state because of the magneto-elastic coupling. The stress and field dependence of the domain structure contains important information on the internal stresses in the material. The three-dimensional neutron depolarization technique has been used to study the stress- and field-dependence of the bulk domain structures in both as-quenched and annealed ribbons of the metallic glass Fe40Ni40B20. A three-layer domain structure model corresponding to compressive and tensile internal stresses is presented to explain the measured data. The influence of surface roughness on the interpretation of neutron depolarization measurements in amorphous ribbons is discussed. Finally, the internal stress relaxation due to the annealing is explained in terms of the viscous behaviour of the glass.
Two-dimensional electron gas in δ-doped double quantum wells for photodetector application
Authors: Lo Ikai , Chang Y. C. , Weng H. M. , Chiang J.-C. , Mitchel W. C.
Published: July 2002

The two-dimensional electron gas in δ-doped double quantum wells has been studied by using the Shubnikov–de Haas measurement. It was discovered that the δ-doping layer at the central barrier is able to prevent the electron coupling in the two wells and that the persistent photoconductivity can be reduced due to the thinner barrier. A δ-doped multiple quantum well structure for photodetector application was also proposed.
Nanowear/nanomechanical testing and the role of stress in sputtered CNx overcoats
Authors: Scharf T. W. , Deng H. , Barnard J. A.
Published: July 2002

a: C–N x films were deposited on Si(111) wafers at ambient temperatures by reactive magnetron sputtering of C in a mixed Ar/N2 discharge. The hardness (H) and elastic modulus (E) were assessed via nanoindentation using a Berkovich diamond indenter. The nitrogenated films exhibited increased hardness and elastic moduli values compared to the amorphous carbonated films, i.e., 25 and 240 GPa to 16 and 160 GPa, respectively. A sphere-on-flat (magnetic tape) wear tester was used to assess wear scar volume losses as a function of wear time. All films were in a state of compression (intrinsic and thermal stress). The bonding, composition, and structure of the films for all the deposition conditions were assessed by micro-Raman spectroscopy, energy dispersive x-ray spectroscopy (EDX), and x-ray diffraction (XRD), respectively, and subsequently correlated to overall film properties.
Proton exchange of quasistoichiometric LiNbO3
Authors: Alcázar deV. A. , Rams J. , Cabrera J. M. , Agulló-López F.
Published: July 2002

Quasistoichiometric LiNbO3 was used as substrate for proton exchanged waveguides prepared by immersion in hot benzoic acid for different times and melt compositions. Both α- and β-phase guides were investigated. Their basic properties were studied and compared with those of guides prepared under the same conditions on congruent substrates. For α-phase guides on quasistoichiometric substrates, the values of the index jump and the effective diffusion coefficient are, respectively, 0.53 and 0.54 times those obtained on congruent substrates under the same conditions. Also, their d33 coefficient for second harmonic generation is about 10% greater, while the optical damage resistance is a factor of 5 smaller. On the other hand, the properties exhibited by β-phase guides depend very little on the substrate stoichiometry. The results are discussed in terms of the lithium deficiency generated during the exchange in comparison to that previously existing in the substrate used.
Domain patterns in epitaxial rhombohedral ferroelectric films. I. Geometry and experiments
Authors: Streiffer S. K. , Parker C. B. , Romanov A. E. , Lefevre M. J. , Zhao L. , Speck J. S. , Pompe W. , Foster C. M. , Bai G. R.
Published: July 2002

Possible domain patterns are developed for (001) oriented (pseudocubic indexing) epitaxial rhombohedral perovskite ferroelectric (FR) films. We assume that the films are grown above their Curie temperature (TC) in a cubic paraelectric (PC) state. The rhombohedral distortion consists of a “stretch” along one of the four 〈111〉 crystallographic directions of the cubic perovskite unit cell. Domain pattern formation is concurrent with the PC→FR transformation on cooling from the growth temperature. The domain patterns form to minimize elastic energy in the film, at the energetic expense of both forming domain boundaries and developing local stresses in the substrate. Eight possible domains may form, half of which are related by inversion, thus leading to four mechanically distinct variants. The possible domain walls are determined by mechanical and charge compatibility and follow closely from the analysis of Fousek and Janovec [J. Appl. Phys. 40, 135 (1969)]. Domain patterns may develop with either {100} or {101} boundaries. In both cases, the individual domains in the patterns are energetically degenerate and thus equal width lamellar patterns are predicted. When polarization is included in the analysis, the {100} boundary patterns have no normal component of the net polarization, whereas the {101} boundary patterns correspond to the fully poled state. We report on experimental observation of {100} domain patterns in epitaxial PbZr0.80Ti0.20O3 and PbZr0.65Ti0.35O3 films.
Magnetic and thermoelectric properties of R3Cu3Sb4 (R=La, Ce, Gd, Er)
Authors: Fess K. , Kaefer W. , Thurner Ch. , Friemelt K. , Kloc Ch. , Bucher E.
Published: July 2002

The rare-earth compounds R3Cu3Sb4 (R=La, Ce, Gd, Er) were synthesized in order to evaluate their potential for thermoelectric applications. Melting in closed tantalum tubes was used to get the materials without loss of volatile Sb components. The structure and the thermal expansion coefficients of these compounds were determined by temperature-dependent x-ray diffraction measurements. We also determined the effective magnetic moments of the rare-earth ions and their valence state. The resistivity of the compounds was measured over a large temperature range from 2 to 470 K. Hall-effect measurements were performed in the range from 30 to 470 K. Additionally, the thermopower and the thermal conductivity were determined. On the basis of the measured data, an estimation of the thermoelectric potential of this group of materials will be given.
Defect states of acoustic waves in a two-dimensional lattice of solid cylinders
Authors: Sigalas M. M.
Published: July 2002

Using the plane-wave expansion method, we study the propagation of acoustic waves through two-dimensional (2D) periodic composites consisting of solid cylinders in air. Defect in those structures create localized states inside the band gaps. We study both single and line defects. Line defects can act as a waveguide for acoustic waves while single defects can be used as acoustical filters.
Anisotropic interfacial strain in InP/InGaAs/InP quantum wells
Authors: Lakshmi B. , Cassidy Daniel T. , Robinson B. J.
Published: July 2002

Numerical simulations based on a valence force field model have been performed to explain experimental results on the degree of polarization of photoluminescence from the [001] direction of InP/InGaAs/InP quantum wells grown on (001) substrates by gas-source molecular beam epitaxy. The results of the simulations indicate an anisotropic strain field owing to fundamental, growth-related differences between the interfaces of the quantum well. The anisotropic strain field is associated with strained Ga–P, Ga–As, and In–As bonds at the InP/InGaAs/InP interfaces. The results of the simulations are in agreement with measurements of the degree of polarization of photoluminescence from the [001] direction of the quantum wells.
An efficient technique to calculate the normalized wave functions in arbitrary one-dimensional quantum well structures
Authors: Haque A. , Khondker A. N.
Published: July 2002

We present a simple yet unified technique to calculate: (i) the eigenenergies and the normalized eigenstates in quantum wells, (ii) the energy broadened spatially varying density-of-states in leaky quantum wells where the particle lifetime is finite, and (iii) the energy position dependent density-of-states in quantum wells where phase-breaking and/or inelastic scattering processes are present. The method is based on the Green’s function formalism. The method is particularly attractive in numerical calculations of multibarrier devices in which the estimation of the self-consistent potential is desired.
Theoretical analysis of the sensor effect of cantilever piezoelectric benders
Authors: Wang Qing-Ming , Du Xiao-hong , Xu Baomin , Cross L. Eric
Published: July 2002

Piezoelectric bending mode elements such as bimorph and unimorph benders can be used as both actuation and sensing elements for a wide range of applications. As actuation elements, these devices convert electric input energy into output mechanical energy. As sensing elements, they convert external mechanical stimuli into electrical charge or voltage. In this article, the sensing effect of cantilever mounted piezoelectric bimorph unimorph and triple layer benders subjected to external mechanical excitations are discussed. General analytical expressions relating generated electric voltage (or charge) to the applied mechanical input excitations (moment M, tip force F, and body force p) are derived based on the constitutive equations of these bending devices. It is found that the clamping effect of each component in the bender devices decreases the dielectric constant. The bimorph bender has a higher voltage sensitivity than the unimorph or triple layer bender with the same geometrical dimensions. The dependence of voltage and charge sensitivities on the thickness ratio and the Young’s modulus ratio of the elastic layer and piezoelectric layer under different conditions are discussed and compared for the unimorph and triple layer benders.
Dependence of the ferrimagnetic resonance linewidth on sintering conditions in manganese ferrite
Authors: Flores A. G. , Raposo V. , Torres L. , Iñiguez J.
Published: July 2002

Ferrimagnetic resonance linewidth of manganese ferrites, Mn1.0Fe2.0O4 has been measured at 8.9 GHz from 77 to 320 K. Single crystal was prepared by floating zone technique and their composition confirmed by inductively coupled plasma spectrometry. Polycrystalline samples were sintered under different conditions of temperature, time, and atmosphere. Eddy current and valence exchange contributions according to Sparks’ theory and a new procedure for evaluating superficial and porosity mechanisms are taken into account to analyze the ferrimagnetic resonance linewidth. Different contributions are present depending on sintering conditions: valence exchange mechanism appears only in samples sintered under low temperatures and high pressures; conductivity decreases for samples sintered at high pressures; porosity is highly raised up when sintering time decreases. Resonance fields, obtained from experimental data, are used to calculate anisotropy fields for polycrystalline samples.
Exchange coupling between two magnetic films separated by an antiferromagnetic spacer
Authors: Rubinstein Mark
Published: July 2002

An expression for the interaction strength between two magnetic films separated by an insulating antiferromagnet spacer has been derived as a function of temperature and thickness. We consider the mechanism wherein the magnetic interaction between the ferromagnetic layers is mediated by the intervening antiferromagnetic insulator via the Suhl–Nakamura (SN) interaction. The interaction energy per unit area, σSN, is derived as σSN=18(JC2/JAF)(δ/a)exp(−t/δ). Here, JAF is the magnetic coupling constant between nearest-neighbor antiferromagnetic spins in the spacer, JC is the effective coupling constant (which is greatly reduced from the Heisenberg exchange constant), between the spins in the ferromagnetic film and the nearest-neighbor spins in the antiferromagnetic spacer, t is the separation of the two ferromagnetic plates, and δ is the width of an antiferromagnetic domain wall. This mechanism is the antiferromagnetic analog of the Ruderman–Kittel oscillatory coupling between two magnetic films separated by a normal metal.
Influence of microstructure on magnetic properties of nanocomposite RE5.5(Fe,Cr,M)76.5B18 (RE=Nd, Tb; Cr⩾3 at %; M=Co,Si) magnetic materials
Authors: Bernardi Johannes , Soto Giselle F. , Fidler Josef , David Stephane , Givord Dominique
Published: July 2002

The microstructure and the magnetic properties of nanocomposite (Nd,Tb)5.5(Fe,Cr,M)76.5B18 (M=Co,Si) magnetic materials have been investigated. The samples were prepared by splat cooling and subsequent fast annealing treatment between 700 and 800 °C in order to develop a nanocrystalline microstructure. Optimum magnetic properties are found in samples with 3 at % Cr. A higher Cr content results in an increase of the coercive field but also in a decrease of both remanence and saturation magnetization. The magnetic properties of splat cooled materials with 3 at % Cr can be improved by the substitution of Co or (Co,Si). Splat cooled flakes with 2.5 at % Co and 0.5 at % Si show a coercive field Hc of 551 kA/m and a remanence μ0Mr of 0.91 T. After optimized annealing the magnetic materials consist of a homogeneous microstructure of Fe3B and (Nd,Tb)2Fe14B grains with a grain size in the range of 10–30 nm. The substitution of Co leads to a slight coarsening of the grain size.
Magnetic properties of nanocrystalline didymium (Nd–Pr)–Fe–B alloys
Authors: Betancourt R. J. I. , Davies H. A.
Published: July 2002

The initial results of a systematic study of the effects of Nd:Pr ratio and Fe:didymium (where didymium is a mixture of Nd and Pr) ratio on the magnetic properties of nanocrystalline melt spun didymium–Fe–B alloys are presented and discussed. The stoichiometric composition, corresponding to didymium2Fe14B, with Nd:Pr ratios of 1:3, 1:1, and 3:1 and the alloy series didymiumy Fe94−yB6, with y=8, 10, 12, and 14 at % were investigated. Enhancement of remanence and energy product was observed for all compositions studied, arising from intergrain exchange coupling between the RE2Fe14B crystallites (generally having a mean diameter ∼40 nm) and, for the off-stoichiometric alloys, also between the RE2Fe14B and α-Fe grains. An excellent combination of (BH)max(185 kJ m−3) and intrinsic coercivity (810 kA/m) was observed for the (Pr0.75Nd0.25)2Fe14B alloy.
Formation and decomposition of Fe3B/Nd2Fe14B nanocomposite structure in Fe–Nd–B–Cr melt-spun ribbons under isothermal annealing
Authors: Suzuki K. , Cadogan J. M. , Uehara M. , Hirosawa S. , Kanekiyo H.
Published: July 2002

The decomposition process of the precursor amorphous phase in Nd5Fe77−xB18Crx (x=0 and 3) has been studied by means of x-ray diffractometry and Mössbauer spectroscopy with particular attention paid to the formation of Nd2Fe14B induced by Cr. Amorphous Nd5Fe74Cr3B18 was confirmed to decompose to stable α-Fe/Fe2B/NdFe4B4 via metastable Fe3B/Nd2Fe14B during isothermal annealing at 953 K. However, the metastable formation of magnetically hard Fe3B/Nd2Fe14B in the Cr-added system is spoiled by a preannealing treatment at 873 K which alters the metastable decomposition products from Fe3B/Nd2Fe14B to Fe3B/Nd2Fe23B3. We discuss the Cr-induced formation of Nd2Fe14B from the viewpoint of kinetics.
Time-resolved magneto-optical experiments in Cd1−xMnxTe/ZnTe multiple quantum wells
Authors: Pittini R. , Mitsu H. , Takahashi M. , Shen J. X. , Oka Y.
Published: July 2002

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.
X-ray image analysis and ultramicroscope study of magnetic fluids used as working liquid in heat transfer experiments
Authors: Jeyadevan B. , Torigoe T. , Nakatsuka K. , Nakatani I. , Fujita T. , Oka H.
Published: July 2002

Boiling characteristics of water-based and ionic magnetic fluid have been visualized using x-ray microfocus. During magnetic fluid boiling, large vapor bubbles nucleated around the vicinity of the heating surface and particles that came into contact with the heating element formed agglomerates large enough to be seen in real time x-ray images. This caused irreversible damage to the magnetic fluid heated to temperatures higher than 373 K. However, application of the magnetic field showed improvements in heat transfer characteristics and ionic magnetic fluid performed better over the water-based magnetic fluid. Ultramicroscope observations of ionic and water-based magnetic fluids heated only up to 353 K in vacuum for ten consecutive days and were found stable. However, ionic magnetic fluid heated in a vacuum glass cell at 373 K burst suggesting the accumulation of noncondense gas.
Shannon entropy characteristics of two-phase flow systems
Authors: Zhang Zuoyi , Shi Lei
Published: July 2002

A method to calculate Shannon entropy of two-phase flow systems from the power spectral density of the output signal is introduced in this article. Two series of experiments are made successively. One is used to investigate the density wave instability in a test facility to study the Chinese 200 MW nuclear heating reactor. After studying the Shannon entropy of the inlet pressure drop in all 534 experimental tests under different heating power, inlet subcooling, operating pressure, and inlet orifice resistance, we find that tests with higher negative Shannon entropy (negentropy) are unstable while those with lower negentropy are stable, like energy used in many fields. The other experiment was made to research two-phase flow regimes in vertical pipes. We calculate the time-dependent negentropy of the air–water flow through the power spectral density of the pressure drop. From the results, we find that the negentropy of the bubble flow is the smallest, the slug flow’s negentropy is the largest, and the negentropy of the annular flow is between the bubble flow and annular flow.
Interparticle interactions and time effects in very thin Fe–SiO2 granular films
Authors: Otte D. , Weston J. L. , Zhou J. N. , Barnard J. A.
Published: July 2002

Time decay and remanence measurements have been made on a series of very thin Fe–SiO2 granular thin films of varying composition. The susceptibility and the magnetic viscosity decreases drastically when the film thickness is thin enough to reach a two-dimensional arrangement of the granules. The measured values of the magnetic viscosity are very low (2% loss per decade) in the thinnest films so that further investigations in granular magnetic systems with higher anisotropy promise greater advantages for recording media.
Nonstationary effects in pulsed laser ablation
Authors: Itina T. E. , Marine W. , Autric M.
Published: July 2002

Monte Carlo simulation is used to study the influence of the time evolution of the target temperature on the characteristics of the laser-ablated flow. The time dependencies of the flow parameters at the border of the Knudsen layer are compared with these at the target. Moreover, the mean number of collisions and the backscattered flow are obtained for different desorption fluxes. The effect of the time evolution of the desorption flux on time-of-flight (TOF) distributions is considered, and a possible explanation of the inconsistencies that frequently appear when TOF is fitted by Maxwell–Boltzmann distribution is proposed.
Dependence on the atmosphere of preparation of the luminescence of spark processed porous GaAs
Authors: Rojas-López M. , Vidal M. A. , Navarro-Contreras H. , Gracia-Jiménez J. M. , Gómez E. , Silva-González R.
Published: July 2002

We report on the preparation of photoluminescent porous GaAs by the application of high voltage spark discharges in atmospheres of pure oxygen, pure nitrogen, and in mixed N2:O2 ratios of 4:1 and 1:4. The spark-processed porous (spp) samples were characterized by the observation of their visible photoluminescence (PL) when illuminated with UV monochromatic radiation. Some differences are observed in the initial PL spectra of the spp-GaAs according to the atmosphere of preparation under similar sparking and time conditions. The PL consists of two dominant bands, a yellow-green band between ∼2.2 and 2.6 eV and a blue-violet band, centered at 3.1 eV. Comparison with Raman and PL results from As2O3 and As2O5 indicates that the PL in the spp-GaAs is produced by the formation of these compounds by exposure to oxygen during the preparation. This is reinforced by energy dispersive x-ray spectroscopy measurements that indicate that the spp-GaAs is always oxidized, even when prepared under a nitrogen flow. The blue-UV emission at 3.1 eV suggests that we cannot rule out confinement as a contributing mechanism for this PL. Raman spectra indicate that for samples prepared in pure nitrogen, the resulting material consists of amorphous As and GaAs, and the cubic form of As2O3, arsenolite. PL and Raman indicate that there exists an increasing degree of amorphization of the resultant material with the introduction of nitrogen in the preparation atmosphere.
Fluorescence microscopic imaging through tissue-like turbid media
Authors: Gan X. S. , Gu M.
Published: July 2002

In this article, effective point spread functions for fluorescence microscopic imaging are introduced to investigate the effects of scattering particle size and optical gating on image resolution under single-photon (1p) and two-photon (2p) excitation. The dependence of image resolution on these effects shows a deeper penetration depth under 2p excitation due to the use of a longer illumination wavelength and the nonlinear dependence of the fluorescence on excitation intensity. The fundamental difference between 1p and 2p fluorescence imaging is that 1p fluorescence imaging mainly depends on the fluorescence light excited by scattered photons, in which case the penetration depth is limited by the degradation in image resolution. However, 2p fluorescence imaging is determined by the fluorescence light excited by ballistic photons, in which case the penetration depth is limited by the loss in signal strength. The results also reveal that the pinhole gating method is efficient in 1p fluorescence imaging, but exhibits a limited influence on 2p fluorescence imaging. It is also demonstrated that in 2p fluorescence imaging, a high numerical aperture objective gives a strong signal while retains an image of high resolution if the turbid medium is not so thick.