bluuebunny/crossref_metadata_2025 huggingface dataset:
Electromechanical properties of 1-3 piezoelectric ceramic/piezoelectric polymer composites
Authors: Taunaumang H. , Guy I. L. , Chan H. L. W.
Published: July 2002

A study of the fabrication and electromechanical properties of piezoelectric 1-3 ceramic/polymer composites is reported in which the polymer phase, as well as the ceramic phase, is piezoelectric. Composites were prepared by both layering and dice-and-fill techniques. In the dice-and-fill technique, the major problem was preventing damage to the ceramic rods during the filling process. Both types of composite were found to present problems in the poling process. The electromechanical properties of the composites were measured and compared with modeling results. In general, it was found that the composite properties agreed with model predictions, with the exception of the piezoelectric coefficients, which were significantly lower than predicted.
Fabrication of nanocrystalline Fe86Zr7B6Cu1 soft-magnetic compacts with high saturation magnetization
Authors: Kawamura Yoshihito , Inoue Akihisa , Kojima Akinori , Masumoto Tsuyoshi
Published: July 2002

Soft-magnetic cores with nanoscale bcc structure were fabricated by consolidation of amorphous or nanocrystalline Fe86Zr7B6Cu1 (at. %) powder using a hot-pressing machine. The most appropriate process to fabricate nanocrystalline Fe86Zr7B6Cu1 soft-magnetic compacts with high density is the direct consolidation of the amorphous powder into nanocrystalline compacts. This process is superior to the annealing method of the amorphous compacts produced from the amorphous powder. The main reason is that the former process fully utilizes the low viscosity of the amorphous alloy before the crystallization temperature. On the other hand, the consolidation of the nanocrystalline powder prepared by annealing of the amorphous powder is an unsuitable process because of the lack of the decrease in viscosity. The nanocrystalline Fe86Zr7B6Cu1 compact obtained by pressing the amorphous powder for 200 s at 853 K and 1.5 GPa has a high density of 99.9% of the theoretical density and exhibits good soft-magnetic properties. The saturation magnetization Bs, coercive force Hc, maximum permeability μmax, effective permeability μe at 1 kHz and 0.8 A/m, and core loss Wh at 10 kHz and 0.1 T are 1.56 T, 33 A/m, 4300, 1300, and 37 W/kg, respectively. It is therefore established that the present compact possesses good soft-magnetic properties combined with high magnetization.
A model for the reversible magnetization in amorphous alloys
Authors: Appino C. , Fiorillo F.
Published: July 2002

A new model is presented which quantitatively describes the reversible magnetization process and its contribution to the magnetization curve in soft amorphous ribbons. It takes into account the role of the coherent moment rotations, through analysis of the orientation distribution of the magnetization vector Is in the sample cross section and its evolution with the applied field. Such analysis relies on the assumption, dictated by magnetic domains observations, of a spatial distribution of local anisotropies, whose intensity and direction are mainly determined by the long-range random stresses quenched in during the process of rapid solidification. The reversible I(H) curve is obtained by integrating the local magnetization contributions with respect to their angular distribution, according to a minimum energy condition involving the Zeeman, the local anisotropy, and the macroscopic anisotropy terms. It is shown that a relatively simple analytical expression for I(H) can be worked out. The model is applied to experimental I(H) recoil curves, which have been determined as a function of the stress-induced anisotropy in positive magnetostriction (Fe81B14Si3C2) and negative magnetostriction (Co80B10Si10) amorphous ribbons. It is shown that the shape of I(H) and its evolution with the applied tensile/compressive stress are correctly predicted by the theory.
Simulations for the transient response of graded AlxGa1−xN submicron photodetectors
Authors: Joshi R. P.
Published: July 2002

We present Monte Carlo simulations for the dynamic response of a submicron linearly graded AlxGa1−xN metal-semiconductor-metal (MSM) structure, and compare it with a uniform GaN MSM device. Both electron and hole transport, as well as circuit elements are comprehensively included. Our results demonstrate enhancement in speed and frequency bandwidth. The improved response, despite additional alloy and multi-mode scattering, arises from the capability of rapid hole removal from the device. The 20% reduction in the turn-off time for a 0.25 μm device could, in principle, be further enhanced through the use of more complex nonuniform grading profiles.
Experimental study of electron generation induced by a XeCl laser
Authors: Nassisi V. , Stagno V.
Published: July 2002

The generation of photoelectrons by an incident XeCl laser beam of 4.02 eV photon energy on three different targets, Y, Zn, and Ge, having work functions of 2.9, 3.9, and 4.8 eV, respectively, has been studied. Considering that the laser pulse has a trapezoidal evolution time and a rectangular spatial dependence, the evolution of the target temperature at the laser spot has been calculated. Using the computer code egun, the output current picked up by the collector resulted only of 44%, 42%, and 37% of the effective current for Y, Zn, and Ge, respectively, at 3 kV charging voltage. By utilizing the generalized Richardson equation which governs the generated current density, the dominant process existing during the laser-matter interaction has been determined. During the electron extraction, plasma was formed on the target surface particularly at high laser energies and high accelerating voltages. This plasma introduces an impedance into the cathode-anode region which shifts the Child–Langmuir law.
Deep traps in molecular-beam-epitaxial GaAs grown at low temperatures
Authors: Look D. C. , Fang Z.-Q. , Yamamoto H. , Sizelove J. R. , Mier M. G. , Stutz C. E.
Published: July 2002

Deep-level transient spectroscopy has been performed on Si-doped GaAs layers grown by molecular-beam epitaxy at substrate temperatures of 400–450 °C. The λ effect is taken into account and overlapping peaks are analyzed numerically. An 0.65 eV electron trap of concentration 2×1016 cm−3 is believed to be related to the AsGa-associated 0.65 eV Hall-effect center, and also to the trap EB4 found in electron-irradiated GaAs.
Nanocomposite R2Fe14B/Fe exchange coupled magnets
Authors: Withanawasam L. , Murphy A. S. , Hadjipanayis G. C. , Krause R. F.
Published: July 2002

We have studied the crystallization, crystal structure, microstructure and magnetic properties of R-Fe-B (R=Nd,Pr,Dy,Tb) based melt-spun ribbons consisting of a mixture of R2Fe14B and α-Fe phases. All the samples crystallize first to α-Fe and a metastable phase (Y3Fe62B14 for R=Nd,Pr,Dy and TbCu7 for R=Tb) before they finally transform to 2:14:1 and α-Fe. The highest values of coercivity and reduced remanence, 4.5 and 0.63 kOe, respectively, were obtained in a Nd3.85Tb2(Fe-Nb-B)94.15 sample. These properties are the result of a fine grain microstructure consisting of a mixture of α-Fe and 2:14:1 having an average grain size of 30 nm.
Anisotropy in twinned terfenol-D crystals
Authors: Lord D. G. , Harvey D.
Published: July 2002

The highly magnetostrictive cubic compound Terfenol-D (Tb0.3Dy0.7Fe2) solidifies via a {211} dendritic growth front when grown by a free-standing zone technique. The resulting material is usually composed of dendritic plates often containing crystallographic twins, the predominant plate and twin plane being the (11̄1) orthogonal to the (2̄1̄1) growth plane. Results of room temperature magnetic torque analysis from (011) disk specimens, having differing twin densities, are presented which yield both the magnetic anisotropy constants, K1=−1.6×105 J/m3 and K2=−0.16×105 J/m3, and the relative parent/twin volume. Magnetic susceptibility data both parallel and transverse to the applied field are presented which, in conjunction with the anisotropy results, emphasize the importance of twin density on magnetoelastic response for typical application geometries.
Magnetic measurements: A powerful tool in magnetic refrigerator design
Authors: Földeàki M. , Chahine R. , Bose T. K.
Published: July 2002

Magnetic refrigeration, an emerging new technology for cooling and gas liquefaction, needs magnetic materials with specific thermomagnetic behavior. Depending on the thermodynamic cycle selected, the isothermal magnetic entropy change or the adiabatic temperature change upon field application needs to be a preselected function of temperature. To obtain these properties, most designers rely on calorimetry, an expensive and time consuming technique. The present article describes that, classical magnetic measurements, when evaluated within the framework of the Landau theory for the second order phase transition or the thermodynamics in magnetic fields, are able to provide the preliminary information needed for the design of magnetic refrigerators. After reviewing the theory, experimental results on ferromagnetic gadolinium (Gd) and helimagnetic dysprosium (Dy) are analyzed and compared to direct experimental results as well as to those obtained using the molecular field model. The results demonstrate the reproducibility of entropy calculations and the good agreement between the experimental and the calculated specific heat anomalies. While the molecular field theory which assumes simple ferromagnetic order clearly fails for helimagnetic dysprosium, an analysis of the experimental data based on the Landau theory gives reliable results. Besides, the field and temperature dependencies of the isothermal magnetic entropy change allows one to characterize the magnetic structure (nature of the magnetic order) of the sample. Furthermore, magnetic measurements define the useful field range and provide information on transitions that influence the thermal behavior and magnetic losses.
Electroluminescence spectroscopy of AlGaAs/InGaAs and AlGaAs/GaAs high-electron-mobility transistors
Authors: Aniel F. , Boucaud P. , Sylvestre A. , Crozat P. , Julien F. H. , Adde R. , Jin Y.
Published: July 2002

Electroluminescence of pseudomorphic InGaAs high-electron-mobility transistors (HEMTs) on GaAs and of standard AlGaAs/GaAs HEMTs with ultrashort gate lengths is investigated at room and cryogenic temperatures. The spectral distribution of the emitted radiation is analyzed in the 0.7–1.6 eV energy range. In the case of pseudomorphic transistors, three different recombination lines originating from distinct layers are observed: a broad luminescence band around 0.8–0.9 eV; a doublet arising from confined subbands in the InGaAs layer; and a single peak coming from the GaAs substrate. The energy position and the temperature dependence of these different lines under various biases give valuable information on the physical mechanisms which occur under high-electric-field nonstationary transport: lattice self-heating (≊150 K at high-bias conditions), origin of the impact ionization in the channel of the quantum-well layer, and perpendicular transfer of hot holes which recombine with the barrier DX centers thus leading to the broad luminescence band. These characteristics are supported by comparison with analog standard HEMTs and with electrical measurements.
Strain-induced effects in (111)-oriented InAsP/InP, InGaAs/InP, and InGaAs/InAlAs quantum wells on InP substrates
Authors: Chen W. Q. , Hark S. K.
Published: July 2002

Strain-induced effects in quantum wells of InAsxP1−x/InP, InxGa1−xAs/InP, and InxGa1−xAs/ In0.52Al0.48As on (111) InP substrates are investigated. The strain induces a large piezoelectric field in these structures, and causes a large change in their energy band edges. Consequently the band structure of the quantum wells is dramatically modified, and the dependence of interband transition energies on alloy composition is different from that of quantum wells on (100) substrates. Moreover, a larger critical layer thickness is found for the quantum wells studied.
Fluorine-induced suppression of disorder effects of carbon in the hydrogenated amorphous silicon-carbon alloy thin films
Authors: Bhusari D. M. , Kumbhar A. S. , Kshirsagar S. T.
Published: July 2002

We report here the Raman scattering studies of modifications in the structural disorder due to fluorine incorporation in the hydrogenated amorphous silicon-carbon (a-Si1−xCx:H) alloys. The structural disorder in the unfluorinated a-Si1−xCx:H alloys has been observed to increase continuously with the increasing carbon content, whereas the incorporation of fluorine appears to substantially suppress disordering effects of carbon, irrespective of its concentration in the alloy. Additional support for this suppression of disorder is sought from the enhancement of optical band gap after fluorine incorporation in these alloys. The fluorine-induced suppression of disorder is attributed to the increase in the fraction of sp3 bonded carbon in the fluorinated alloys compared to that of the unfluorinated alloys. The fluorine as well as hydrogen appear to have a preferential attachment with the carbon atoms that tend to form higher hydride and fluoride species which in turn constrain the carbon atoms to form sp3 hybridized C-C or Si-C bonding in the alloys.
Photoluminescence thermal-quenching and carrier localization of AlGaAs disordered superlattices
Authors: Uno Kazuyuki , Noda Susumu , Sasaki Akio
Published: July 2002

Thermal quenching properties of disordered superlattices (d-SLs) are experimentally investigated. Three types of d-SLs with (1) various band-gap differences, (2) various unit lengths of disorder, and (3) various appearance probabilities of disordered layers are used for experiments. Thermal quenching properties of photoluminescence intensities are investigated for the estimation of strength of disorder. Consistent results are obtained in that the thermal quenching becomes small with increasing disorder for each type of d-SL. Abrupt changes in the thermal quenching characteristics are observed at the type-I/type-II transitions of AlyGa1−yAs/GaAs d-SLs and o-SLs.
Dielectric spectroscopy of Ba(B1/2′B1/2″)O3 complex perovskite ceramics: Correlations between ionic parameters and microwave dielectric properties. I. Infrared reflectivity study (1012–1014 Hz)
Authors: Zurmühlen Rudolf , Petzelt Jan , Kamba Stanislav , Voitsekhovskii Valentin V. , Colla Enrico , Setter Nava
Published: July 2002

An attempt has been undertaken to find a correlation between ionic parameters of ceramic materials and their complex permittivity at microwave frequencies. Ten Ba(B1/2′B1/2″)O3 complex perovskite compounds (B′=Y3+, In3+, Nd3+, Gd3+; B″=Nb5+, Ta5+ and B′=Mg2+, Cd2+, B″=W6+) are compared in order to study the effect of ionic radii, mass, and valence state on dielectric properties. Fourier transform infrared reflectivity spectra in the 30–4000 cm−1 range were measured and evaluated by means of Kramers–Kronig analysis and classical oscillator fit. The data were extrapolated below the measured frequency range to estimate the intrinsic microwave losses. The correlations between loss, permittivity, ionic size, mass, and effective charge and polar-phonon mode parameters were investigated. Ionic size was revealed to be the most important parameter, determining the tolerance factor of the structure packing and through this controlling the phonon frequencies and dampings as well as the extrapolated low-frequency intrinsic permittivity and loss. Nb5+ ions showed systematically higher dampings and greater tendencies towards disorder in comparison with Ta5+ compounds with the same tolerance factor. A steep increase in extrapolated loss with increasing permittivity was observed.
Magnetic properties and giant magnetoresistance of magnetic granular Co10Cu90 alloys obtained by direct-current joule heating
Authors: Yu R. H. , Zhang X. X. , Tejada J. , Zhu J. , Knobel M. , Tiberto P. , Allia P. , Vinai F.
Published: July 2002

The direct-current (dc) joule heating technique was exploited to fabricate giant magnetoresistance (GMR) Co10Cu90 granular alloys. The Co cluster precipitation process was investigated by calorimetric and x-ray diffraction measurements. At T=10 K, the largest MR change of 25.0% has been observed for the melt-spun Co10Cu90 ribbon annealed at I=5 A. The magnetoresistance scales approximately as the inverse Co particle size. At room temperature, it was found that the dc joule-heated samples show relatively high GMR in comparison with furnace-annealed samples. Based on the phenomenological GMR model, we assumed that it is a consequence of smaller Co particles formed in dc joule-heated samples.
Transient photocurrent in amorphous selenium and nematic liquid crystal double layers
Authors: Murakami Shuichi , Naito Hiroyoshi , Okuda Masahiro , Sugimura Akihiko
Published: July 2002

Transient photocurrents in amorphous selenium (a-Se) and nematic liquid crystal (NLC) double layers have been investigated to elucidate the charge carrier transport process in NLC. It is shown that the transient photocurrents obtained in the experiment are space-charge limited currents induced by the delayed charge injection from the a-Se layer to the NLC layer. The mobile charge carriers are found to be positive ions with the drift mobility of 3.5×10−6 cm2/V s at 303 K and the ionic radius of 0.32 nm in 4-cyano-4′-alkyl-biphenyl. The origin of the ions is briefly discussed.
Diffusion limited oxygen precipitation in silicon: Precipitate growth kinetics and phase formation
Authors: Vanhellemont Jan
Published: July 2002

Published experimental data on silicon oxide precipitate growth kinetics are interpreted in the framework of the theory of Ham for diffusion limited precipitation. A growth law for plate-like, octahedral and spherical precipitates is derived showing a size dependence which varies as the square root of time. Using well accepted data for the solubility and the diffusion constant of oxygen in silicon, the calculations suggest that the precipitated phase is closer to SiO than to SiO2.
Interdiffusion behavior in n-doped and undoped GaInAs/AlGaInAs multiple-quantum-well structures grown by molecular-beam epitaxy
Authors: Hofsäss V. , Kuhn J. , Schweizer H. , Hillmer H. , Lösch R. , Schlapp W.
Published: July 2002

We present investigations on the interdiffusion behavior and thermal stability of n-doped and undoped GaInAs/AlGaInAs multiple-quantum-well structures, grown lattice matched on InP by molecular-beam epitaxy. The activation energy of the main interdiffusion process is determined to Ean doped=2.5 eV and Eaundoped=2.9 eV. The different interdiffusion processes are monitored mainly by photoluminescence spectroscopy at T=8 K after rapid thermal annealing of the samples. The influence of doping is studied by comparing the results of n-doped and undoped structures. Additionally photoluminescence excitation spectroscopy at T=2 K was carried out to verify the different interdiffusion processes.
Ion induced stress generation in arc-evaporated TiN films
Authors: Ljungcrantz H. , Hultman L. , Sundgren J.-E. , Karlsson L.
Published: July 2002

The influence of Ti ion bombardment on the intrinsic stress and microstructure of TiN films during deposition by arc evaporation of Ti in pure N2 has been investigated. Ions with an average charge of +1.6 were accelerated from the arc discharge by a negative substrate bias Vs between 5 and 540 V which yielded a steady-state substrate temperature between 300 and 600 °C, respectively. The compressive intrinsic stresses in the films, as determined by the x-ray-diffraction (XRD) sin2 ψ method after subtracting the thermal stress contribution at room temperature, changed abruptly from 1.9 to a maximum of 6.5 GPa as Vs increased from 5 to 100 V. The compressive stress then decreased monotonically to ∼1.6 GPa as Vs increased to 540 V. Broadening of XRD peaks (β) showed accompanying inhomogeneous strain with a maximum values for Vs=100 V. Cross-sectional transmission electron microscopy showed a dense columnar film microstructure. Electron microdiffraction showed a distorted structure within the same columns for Vs=100 V and better defined grains for Vs=500 V. The observed reduction in intrinsic stress with increasing Ti ion energy is in addition to what is normally observed in sputter-deposited TiN films with the same ion energy range of predominantly Ar-ion bombardment. A comparison with published work of magnetron sputtered TiN showed that defects created by Ti ions are more easily annihilated than defects created using Ar ions. Ar atoms tend to be strongly bound to lattice defects and forming thermally stable complexes and prevent stress relief at high Vs as is observed for arc-evaporated films. The obtained results are compared to models for stress generation during steady-state conditions in the competition between collisionally induced point defect formation and defect annihilation during deposition.
Analysis of heat transfer to a spherical particle from continuum plasma
Authors: Jog M. A.
Published: July 2002

A continuum model is developed to analyze the heat transfer to a solid particle from continuum plasma. A spherical particle introduced in a quiescent collision-dominated weakly ionized gas is considered. At steady state, the particle is negatively charged at the floating potential with the flux of ions and the flux of electrons to the particle being equal. A set of continuum conservation equations for ion and electron temperature and densities are solved with Poisson’s equation for the self-consistent electric field. The variation of ion and electron densities and temperatures is numerically calculated in the entire domain. Thermal ionization and recombination is incorporated in the model. The heat transport to the particle due to the flux of ions and electrons is determined. The results indicate that a thin charge sheath is present near the particle surface with steep gradients in electron and ion number densities. Particle charging significantly affects the amount of heat transferred from the plasma to the particle.
Electronic transport studies of bulk zincblende and wurtzite phases of GaN based on an ensemble Monte Carlo calculation including a full zone band structure
Authors: Kolník Ján , Oğuzman İsmail H. , Brennan Kevin F. , Wang Rongping , Ruden P. Paul , Wang Yang
Published: July 2002

The ensemble Monte Carlo technique including the details of the first four conduction bands within the full Brillouin zone is used to calculate the basic electronic transport properties for both zincblende and wurtzite crystal phases of bulk gallium nitride. The band structure throughout the Brillouin zone is determined using the empirical pseudopotential method. Calculations of the electron steady-state drift velocity, average energy, valley occupancy and band occupancy in the range of electric fields up to 500 kV/cm are presented. It is found that the threshold electric field for intervalley transfer is greater and that the second conduction band is more readily occupied in wurtzite than in zincblende GaN over the range of electric fields examined here.
Resolution limits from charge transport in optically addressed spatial light modulators
Authors: Wang Li , Moddel Garret
Published: July 2002

Spatial resolution of optically addressed spatial light modulators (OASLMs) is degraded by several different transfer processes in these devices. We have developed a general transient charge transport model to calculate and simulate the resolution limits of OASLMs due to the following charge spreading mechanisms during the transfer process in which the input image is converted into a particular charge distribution in the photosensor layer. (i) The effect of charge drift in the photosensor bulk on resolution increases with the thickness of the photosensor and the light-modulating layers. It also increases with the total amount of photogenerated charge collected at the interface. (ii) The effect of charge diffusion in the photosensor bulk on resolution is largely independent of the carrier mobility in the semiconductor photosensor. In most cases the corresponding spatial frequency f50% is proportional to √Vsc/dsc, where dsc is the photosensor thickness and Vsc is the voltage drop in that layer. To have high-sensitivity OASLMs the transit time of charge carriers from the photosensor bulk to the interface must be much shorter than the recombination lifetime. (iii) The effects that charge drift, diffusion, and trapping at the photosensor-light-modulating layer interface have on resolution depend strongly on the interface properties. Decreasing the mobility or the trapping time of charge carriers at the interface can dramatically improve the resolution of OASLMs. The resolution ranges from 3 to 875 line pairs/mm for respective diffusion lengths of 10 to 0.1 μm at the interface. The combined effect on resolution from each of the charge spreading and other resolution-degrading mechanisms is also discussed.
Diffusion coefficients of conductive ions in a copolymer of vinylidene cyanide and vinyl acetate obtained from dielectric measurements using the model of Trukhan
Authors: Compañ Vincente , Smith So/rensen Torben , Diaz-Calleja Ricardo , Riande Evaristo
Published: July 2002

The dielectric dispersion measurements of Furukawa et al. are treated in the light of a previously proposed model of Trukhan. The latter describes the influence of mobile ions on the dielectric dispersion in a slab of material placed between polarizable electrodes. It is shown that the so-called ‘‘constant phase element’’ is just a crude approximation to the predictions of the theory of Trukhan, an approximation not valid at very low frequencies. At low frequencies macropolarizations appear analogous to the ones observed in asymmetric cellulose acetate membranes by Malmgren-Hansen et al. The polarizations are much larger in the present case, and this indicates that there are no microheterogeneities in the polymeric film of Furukawa et al. The diffusion coefficient of the most rapidly diffusing ion (presumably H+) may be found as a function of temperature within some uncertainty. The Arrhenius plot shows clearly the change in activation energy around the glass transition temperature (182 °C). Below the glass transition the activation energy for diffusion is much larger (∼50000 K) than above. The diffusion coefficients increase from ∼10−17 m2/s at 170 °C to ∼5.10−16 m2/s at 195 °C. The concentration of electrolyte present in the polymer increases from ∼15 to ∼200 mol m−3 in the same temperature interval.
Initial stage of native oxide growth on hydrogen terminated silicon (111) surfaces
Authors: Ogawa Hiroki , Ishikawa Kenji , Inomata Carlos , Fujimura Shuzo
Published: July 2002

We investigated the initial stage of native oxide growth on an atomically flat hydrogen terminated silicon (Si) (111) surface by immersion in pure water using Fourier transformed infrared attenuated total reflection spectroscopy and reflection absorption spectroscopy. There is a sharp absorption peak at 2083 cm−1 arising from silicon monohydrides at surface terraces, this peak decreases and broadens with immersion time and seems to separate into two components. This broadening is considered to reflect the progress of very early native oxide growth. The experiment on a vicinal Si(111) surface showed that the absorption peaks arising from monohydrides at the terraces and at the steps on the surface decrease at approximately the same rate with immersion time. This indicates that the native oxide growth arises in spite of terraces and steps on the silicon surface. Furthermore, the large amount of silicon-hydrogen (Si–H) bonds not related with oxygen atoms suggests that the native oxide growth mechanism is not layer-by-layer oxidation.
Field-induced strains and polarization switching mechanisms in La-modified lead zirconate titanate ceramics
Authors: Dai Xunhu , Xu Z. , Li Jie-Fang , Viehland Dwight
Published: July 2002

The field-induced strain characteristics have been investigated for a sequence of La-modified lead zirconate titanate (PLZT) compositions. It was shown that the increase in the ease of polarization switching is essential for maximum electrically induced strains in PLZT ceramics. For PLZT compositions close to the morphotropic phase boundary this was realized by adjusting the base composition to be close to the boundary between tetragonal and rhombohedral states, where the polarization has a maximum number of equivalent orientations (6 for tetragonal plus 8 for rhombohedral) by which to switch. For PLZT compositions located in either the rhombohedral or tetragonal states, the ease of polarization switching was facilitated by La doping until the point where the normal micron-sized ferroelectric domains began to break up into tweedlike subdomain structures. Polarization switching may then occur through the subdomain state, rather than by the creation and subsequent motion of normal micron-sized domain boundaries.
Measurement of interdiffusion in II–VI quantum-well structures using optical methods
Authors: Karla I. , Shaw D. , Hagston W. E. , Hogg J. H. C. , Chalk S. , Nicholls J. E. , Peili C.
Published: July 2002

The extent of interdiffusion in Cd1−xMnxTe/CdTe quantum-well structures grown by molecular-beam epitaxy was monitored by photoluminescence and photoluminescence excitation spectra. Thermal annealing of as-grown and ion-implanted structures in over pressures of cadmium (or tellurium) provide clear evidence that diffusion is controlled by cation vacancies and are consistent with a strong dependence of the interdiffusion coefficient on the vacancy concentration.
Influence of rapid thermal annealing and internal gettering on Czochralski-grown silicon. I. Oxygen precipitation
Authors: Maddalon-Vinante C. , Ehret E. , Barbier D.
Published: July 2002

The effect of rapid thermal annealing at 1200 °C as a pretreatment on the precipitation of oxygen during a three-step internal gettering process is studied thanks to infrared analyses. The influence of both the duration and the ambient of this pretreatment is considered. We define a limit duration of the rapid thermal annealing from which the further precipitation is greatly enhanced through a precipitation path leading to the formation of quasispherical precipitates different from the platelets. It is also shown that hydrogen introduced during the rapid thermal anneal delays the oxygen precipitation during the internal gettering process. Finally, considering the inefficiency of the internal gettering of chromium after a rapid thermal annealing at 1200 °C under argon/hydrogen, a new interpretation is proposed: a rapid thermal annealing would produce a modification in the oxygen precipitation path, leading to the formation of precipitates which would not be suitable for an efficient gettering effect.
Spectroscopic study of the vacuum ultraviolet windowless photodissociation of silicon hydrides for silicon-based film deposition
Authors: Henck R. , Fuchs C. , Fogarassy E.
Published: July 2002

Optical emission characteristics of vacuum ultraviolet (VUV) light from large area (≊50 cm2) electrical gas discharges were investigated in order to develop a windowless photochemical vapor deposition (photo-CVD) reactor and to prepare silicon-based thin films at low temperature (≊300 °C). The electrodes configuration is such that the ions impact upon the substrate is strongly minimized. Among the four investigated gases (He, Ar, H2, and N2), argon is the best candidate because it provides the more monochromatic VUV light (121.5 nm) which corresponds well to the maximum of the silane and disilane photoabsorption curves. Emission spectra of the photo-CVD induced in argon–silane mixtures clearly give evidence of the formation of molecular hydrogen and of Si* excited atoms and, indirectly, the part played by low hydrogen content radicals (namely, SiH). The deposition rate of hydrogenated amorphous silicon films obtained by windowless photo-CVD using this strong emission band at 121.5 nm is proportional to the SiH4 and Si2H6 contents in argon up to 20% and 6%, respectively. The deposited films present a low average hydrogen concentration value of less than 10% and smooth surfaces (<20 Å) for silicon hydride gas partial pressures lower than 15%. Over this value, the deposits become very rough (≳500 Å), due to SiHn clusters formation in the gas phase. Finally, low temperature growth of silicon nitride films by the VUV dissociation of silane and/or disilane–nitrogen mixtures is demonstrated.
Ionization dynamics of iron plumes generated by laser ablation versus a laser-ablation-assisted-plasma discharge ion source
Authors: Lash J. S. , Gilgenbach R. M. , Spindler H. L.
Published: July 2002

The ionization dynamics (iron ion and neutral atom absolute line densities) produced in the KrF excimer laser ablation of iron and a laser-ablation-assisted plasma discharge (LAAPD) ion source have been characterized by a new dye-laser-based resonant ultraviolet interferometry diagnostic. The ablated material is produced by focusing a KrF excimer laser (248 nm,<1 J, 40 ns) onto a solid iron target. The LAAPD ion source configuration employs an annular electrode in front of the grounded target. Simultaneous to the excimer laser striking the target, a three-element, inductor–capacitor, pulse-forming network is discharged across the electrode–target gap. Peak discharge parameters of 3600 V and 680 A yield a peak discharge power of 1.3 MW through the laser ablation plume. Iron neutral atom line densities are measured by tuning the dye laser near the 271.903 nm (a 5D–y 5P0) ground-state and 273.358 nm (a 5F–w 5D0) excited-state transitions while iron singly ionized line densities are measured using the 263.105 nm (a 6D–z 6D0) and 273.955 nm (a 4D–z 4D0) excited-state transitions. The line density, expansion velocity, temperature, and number of each species have been characterized as a function of time for laser ablation and the LAAPD. Data analysis assuming a Boltzmann distribution yields the ionization ratio (ni/nn) and indicates that the laser ablation plume is substantially ionized. With application of the discharge, neutral iron atoms are depleted from the plume, while iron ions are created, resulting in a factor of ∼5 increase in the plume ionization ratio. Species temperatures range from 0.5 to 1.0 eV while ion line densities in excess of 1×1015 cm−2 have been measured, implying peak ion densities of ∼1×1015 cm−3.
Surface and plasma simulation of deposition processes: CH4 plasmas for the growth of diamondlike carbon
Authors: Mantzaris Nikolaos V. , Gogolides Evangelos , Boudouvis Andreas G. , Rhallabi Ahmed , Turban Guy
Published: July 2002

A surface model was developed for diamondlike-carbon film deposition, and was connected in a self-consistent way with a one-dimensional plasma chemistry and physics model for a CH4 radio-frequency (rf) discharge. The surface model considers the adsorption of multiple species (CH3, CH2, and H), and solves for the surface coverage of each species. Comparison is also done with a one-adsorbed-species model. Deposition is assumed to take place via direct ion incorporation, and ion-induced stitching of adsorbed neutrals; film removal takes place via etching and sputtering. The effects of ion flux/energy and surface temperature are examined in detail: At high ion energies direct ion incorporation dominates, in spite of competition with sputtering; at intermediate energies stitching prevails, while for lower ion energies etching can become largest. Mass balances are written at the surface–gas interface, permitting the determination of the effective sticking coefficients of the reacting neutrals. The sticking coefficients calculated from the surface model are fed back into the gas-phase chemistry model to recalculate the neutral densities. The process is repeated until a self-consistent solution is obtained. It is shown that the effective sticking coefficient of a neutral changes drastically from a low value for the plasma-off (or low ion energy) state, to a high value for the plasma-on and high ion energy state, resulting in higher consumption at the surface. The results show that it is imperative for meaningful results to solve surface and gas-phase chemistry models in a self-consistent way, a fact demonstrated by successful comparison with experimental data for the deposition rate and the gas-phase densities.
Elastic constants of gallium nitride
Authors: Polian A. , Grimsditch M. , Grzegory I.
Published: July 2002

The elastic constants of GaN have been determined using Brillouin scattering; in GPa they are: C11=390,  C33=398,  C44=105,  C66=123,  C12=145,  and   C13=106. Our values differ substantially from those quoted in the literature which were obtained from the determination of mean square displacement of atoms measured by x-ray diffraction.
Effects of randomness in gapped antiferromagnetic quantum spin chains
Authors: Yang Kun , Hyman R. A. , Bhatt R. N. , Girvin S. M.
Published: July 2002

The effects of bond randomness are studied for spin chains with excitation gaps in the pure case. Systems with enforced topological structure like the spin-1/2 chain with enforced dimerization and spin-1 chain in the Haldane gapped phase appear very stable against randomness. On the other hand, systems with spontaneously generated topological order like the spin-1/2 chain with spontaneous dimerization are found to be unstable even for weak randomness despite the gap.
Polaron-polaron interactions in diluted magnetic semiconductors
Authors: Wolff P. A. , Bhatt R. N. , Durst A. C.
Published: July 2002

The bound magnetic polaron (BMP) is the characteristic collective state of diluted magnetic semiconductors. Isolated BMP are well understood, but their interactions are only beginning to be explored. Recent polaron magnetization experiments on p-ZnMnTe suggest a ferromagnetic polaron-polaron interaction, in contrast to the invariably antiferromagnetic impurity exchange interaction in conventional semiconductors. To investigate this question theoretically, we have developed a simplified model of polaron pairs whose central feature is competition between the usual, antiferromagnetic, virtual-hopping interaction, and the loss of carrier-magnetic ion exchange energy, by intermediate ions, when the polaron moments are antiferromagnetically aligned. The model is sufficiently simple that its partition function can be calculated in detail. With reasonable parameters, it predicts a ferromagnetic polaron-polaron interaction at low temperatures.
Control of structures of deposited polymer films by ablation laser wavelength: Polyacrylonitrile at 308, 248, and 193 nm
Authors: Nishio Satoru , Chiba Tomonori , Matsuzaki Akiyoshi , Sato Hiroyasu
Published: July 2002

Extensive control of structures of deposited polymer films has been realized when the films have been prepared on a quartz or KBr substrate by laser ablation of polyacrylonitrile (PAN) with 308 (XeCl), 248 (KrF), and 193 nm (ArF) excimer laser beams, as revealed by scanning electron microscopy, infrared spectroscopy, x-ray photoelectron spectroscopy, electron spin resonance, and x-ray diffraction measurements. The original structure of PAN is essentially retained for the films deposited by ablation at 308 nm. Deformation and elimination of nitrile groups occur for the films prepared by ablation at 248 and 193 nm, respectively. Furthermore, a remarkable power-dependent structural change has been observed at 248 nm; an increase in the laser power has led to ring closure of nitrile groups, resulting in the development of π-conjugated systems. In addition, a rather high electric conductivity on the order of 10−2S cm−1 has been given to the films deposited by ablation at 248 and 193 nm on reirradiation with lower-power excimer laser beams of 308 nm.
Microstreamer dynamics during plasma remediation of NO using atmospheric pressure dielectric barrier discharges
Authors: Gentile Ann C. , Kushner Mark J.
Published: July 2002

Environmental concerns have resulted in the need for alternative energy efficient methods for the removal of oxides of nitrogen (NxOy) from atmospheric gas streams and among those techniques is plasma remediation. The devices typically used as plasma sources (corona and dielectric barrier discharges) operate in a filamentary mode which produces radicals and gas heating in narrow channels. Transport of radicals and products is therefore an important consideration in optimizing their efficiency. In this article we describe and present results for a radially dependent model of NxOy remediation in dielectric barrier discharges. We examine the consequences of localized energy deposition, and diffusive and advective radial transport, and determine how these spatial dependencies affect the energy efficiencies for NxOy removal. Local energy deposition in the streamer can produce high temperatures initiating advection and facilitating production of NO through N+O2→NO+O. Diffusion of NO into the streamer region from the bulk gas and advective transport of N outward into the bulk gas increase remediation of NxOy by channeling N into remediation, NO+N→N2+O, rather than radical reassociation, N+N+M→N2+M. Remediation efficiency generally decreases with increasing energy deposition due to the reassociation reaction and formation of NO. Transport of N out of and NO into the streamer region reduces this effect.
Supersonic jet epitaxy of aluminum nitride on silicon (100)
Authors: Brown Kyle A. , Ustin S. A. , Lauhon L. , Ho W.
Published: July 2002

Single phase aluminum nitride (0001) has been grown on atomically clean silicon (100) substrates (720 °C≥Ts≥620 °C) with dual supersonic molecular beam gas sources. The precursors used were triethylaluminum [TEA; Al(C2H5)3] and ammonia (NH3). The maximum growth rate obtained was 0.1 μm/h. The growth rate was found to depend strongly on the kinetic energy of the incident precursors. Single phase films were grown 200–400 nm thick. Structural x-ray studies reveal 2θ full widths at half-maxima between 0.20° and 0.35° for the AlN (0002) peak.
Novel type-II quantum cascade lasers
Authors: Yang Rui Q. , Pei S. S.
Published: July 2002

A new class of quantum cascade lasers based on type-II quantum wells is analyzed. In these novel mid- and long-wavelength IR lasers, not only can a population inversion be easily created with a nearly 100% current injection efficiency, but also the nonradiative loss from the optical phonon scattering can be greatly suppressed. A general description of how the lasing threshold current depends on the injection, radiative, and inversion efficiencies is formulated to illustrate the expected improvements over the recently reported quantum cascade laser. Also, the features that distinguish quantum cascade lasers from traditional bipolar lasers are discussed in the context of the carrier transport in equivalent circuit models to illustrate the advantages of quantum cascade lasers for high power mid- and long-wavelength IR source applications.
Synchrotron-radiation-excited epitaxy of Ge with GeH4
Authors: Akazawa Housei
Published: July 2002

Synchrotron-radiation (SR) -excited chemical-vapor deposition of Ge crystals on Si(100) using GeH4 has been investigated. When SR is incident parallel to the surface, reaction between a photofragment and a surface hydride yields constant deposition rate. Above 280 °C, thermal growth rate limited by H2 desorption at the surface is superimposed. When SR is perpendicularly directed to the surface, the growth is enhanced and the activation barrier is reduced. Electronic excitation of admolecules may open an efficient bond rearrangement pathway to densify the hydrogen-rich Si film. The Ge film is uniform at high pressures due to surfactant effect of hydrogen atoms, but when islanding occurs with decreasing hydrogen coverage, the activation energy further decreases. Isothermal Si/Ge multilayer growth at 300 °C is demonstrated by alternately feeding GeH4 and Si2H6.
Rotor dynamics of superconducting magnetic bearings
Authors: Schmitt U. , Tonoli A. , Bornemann H. J.
Published: July 2002

A two dimensional proximity sensor system was used to study dynamics and magnetic unbalance effects of superconducting magnetic bearings. Both dynamic stiffness and vibration damping were found to increase with decreasing bearing gap. Lateral vibrations of an aluminum disk integrated into a superconducting flywheel system were measured during free rotation. From the measurements, the magnetic unbalance was deduced and it is shown that related effects, such as bearing losses and instability at the critical, can be reduced significantly by shifting the center of gravity of the disk assembly towards the magnetic symmetry axis.
Nanocrystalline FeNdB permanent magnets with enhanced remanence
Authors: Bauer J. , Seeger M. , Zern A. , Kronmüller H.
Published: July 2002

Isotropic nanocrystalline exchange coupled FeNdB magnets with enhanced remanence were produced using the melt spinning procedure. Starting from nearly single phase Fe14Nd2B magnets, the content of additional α-Fe in composite magnets was stepwise increased up to 40 vol % by reducing the Nd and B content. The maximum remanence of JR=1.25 T was found in composite magnets containing 30 vol % α-Fe, the maximum energy product reaches the value (BH)max=185 kJ/m3, whereas the coercive field is μ0HC=0.53 T for this composition. The microstructural investigations of the composite magnets reveal two characteristic maxima in the grain size distribution at about 15 and 25 nm corresponding to the α-Fe phase and to the Fe14Nd2B phase, respectively. The influence of the exchange coupling on the coercive field can be described by a microstructural parameter αex. From the temperature dependence of the coercive field the microstructural parameters αKαex and Neff are determined.
Dose-rate effects in GaAs investigated by discrete pulsed implantation using a focused ion beam
Authors: Musil Christian R. , Melngailis John , Etchin Sergey , Haynes Tony E.
Published: July 2002

The dependence of the retained lattice damage upon dose rate was investigated by focused ion beam (FIB) implantation of 210 keV Si++ into GaAs at room temperature. The as-implanted and postannealed states were characterized by ion channeling and Hall-effect measurements, respectively. Dose-rate effects arise from stabilizing interactions between populations of defects produced by different ions, and these experiments were designed to probe the time constants of those interactions. In the context of dose-rate experiments, direct-write FIB represents a much more controllable means of implantation over conventional broad beams since the exact timing of dose delivery may be precisely defined and varied. In this work, the final implanted dose was achieved by the successive application of individual flux pulses of constant intensity but of varying duration td and repetition period tr. A consistent trend toward a greater concentration of displaced atoms directly after implantation and a higher sheet resistance after annealing was observed for longer td and for shorter tr. This effect did not manifest itself simply in terms of the average current density, Javg∝td/tr. Furthermore, it was observed on all time scales accessible in this experiment, suggesting that the important self-annealing mechanisms have a wide range of time constants, from less than 1 μs to more than 1 s. A heterogeneous model of damage nucleation is discussed whereby the defect track of an individual ion event self-anneals until it is overlapped by a following event.
Hole mass determination in ZnSe by observation of standing polariton waves
Authors: Lankes S. , Meier M. , Reisinger T. , Gebhardt W.
Published: July 2002

Interference pattern due to standing polariton waves have been observed in thin planparallel ZnSe layers. The samples consist of pseudomorphic ZnSe/ZnCdSe quantum well structures grown by molecular beam epitaxy with cap layers of various thicknesses. The interference pattern appearing in reflectance and photoluminescence excitation spectra were fitted to calculated spectra applying a simple theoretical model. This fit results in the determination of the deadlayer thickness and the evaluation of the Luttinger parameter γ1 and γ2.