bluuebunny/crossref_metadata_2025 huggingface dataset:
SPEXAFS at the L2,3 edges of Eu3Fe5O12, Gd and Tb metal and the alloy HoGd (abstract)
Authors: Knülle M. , Fischer P. , Frahm R. , Schütz G. , Stähler S.
Published: July 2002

Systematic studies of the magnetic or spin-dependent EXAFS (SPEXAFS) were performed at the L2/3 edges of Eu3Fe5O12, Gd and Tb metal and the alloy Ho20Gd80. The magnetic absorption exhibits distinct EXAFS structure, more pronounced than the nonmagnetic EXAFS. The Fourier transform of the Eu L2/3-edges SPEXAFS spectra, which are measured with an accuracy of less than 5×10−5, show a complete disappearance of the oxygen peak. This allows an identification of the positions of the magnetic neighbors for a significantly longer range as it is possible by using the conventional EXAFS profile. Studies of the alloy Ho20Gd80 at the L2/3 edges of both components, which show no significant differences in the polarization averaged EXAFS, indicate an essential influence of the spin structure at the absorbing atom on the magnetic EXAFS spectrum. At the L3 edge of Gd and Tb metal (both of hcp structure) we found similar structures in the conventional as well as in the magnetic EXAFS. The data are interpreted in terms of the sensibility of the SPEXAFS on the angular momentum of the absorbing and neighboring atoms.
Study of the hydrogen stability in evaporated amorphous Si1−xSnx:H (0≤x≤0.2) alloys by neutron scattering and exodiffusion measurements
Authors: Houssaïni S. , Vergnat M. , Bruson A. , Marchal G. , Vettier C.
Published: July 2002

Exodiffusion and low-angle neutron scattering experiments are used to investigate the influence of tin on the hydrogen stability in evaporated amorphous Si1−xSnx (0≤x≤0.2) alloys. The neutron diffraction experiments, which are performed on Si1−xSnx:H/Si1−xSnx:D/... multilayers, consist in following the decay of the diffracted intensity during thermal treatments. Both studies give information on the structure of the alloys and show that the hydrogen stability decreases with increasing tin concentration.
Isochronal annealing of local vibrational modes in proton- and deuteron-implanted InP
Authors: Fischer D. W. , Manasreh M. O. , Talwar D. N. , Matous G.
Published: July 2002

We have performed isochronal annealing experiments on the hydrogen-related local vibrational modes (LVMs) created by proton and deuteron implantation of InP. Implanted samples were annealed in 50 °C increments in the 200–600 °C temperature range for 30 min each and then measured by infrared absorption. A group of four different LVMs is observed, each of which arises from the hydrogen-phosphorus stretching vibration with different defects or impurities at nearest-neighbor sites. Each LVM exhibits an annealing behavior that is different than any of the other LVMs. The annealing results are shown and discussed in relation to the possible microscopic structure of the defect responsible for each LVM.
Depth profiles of As and Sb angle-implanted trenches measured by secondary ion mass spectrometry
Authors: Pas Michael F. , Magel Lissa K. , Anthony Mark , Hemming Susan
Published: July 2002

Depth profiles of As and Sb angle-implanted trench pilot wafers and a fully processed 16 MB dynamic random access memory chip have been measured using secondary ion mass spectrometry (SIMS) to determine the doping level in the walls of the trench capacitors. As and Sb exhibit similar behavior throughout the implant and analysis process. The SIMS data correlates well with scanning electron microscopy cross sections of the trenches and simple geometrical arguments as well as with capacitance-voltage measurements of processed wafers. The presence of a gate dielectric does not appear to interfere with SIMS analysis of trench wall doping, and this technique is therefore applicable as a post-process monitor of doping distribution.
Changes in film structure and increase in coercivity for Co/Pd multilayered films with atmospheric annealing
Authors: Yamane H. , Maeno Y. , Kobayashi M.
Published: July 2002

Co/Pd multilayered films exhibit large perpendicular magnetic anisotropy and have attracted attention as new materials for recording media and for other applications. The coercivity (Hc), however, is small in as-sputtered films. We have found that Hc increased markedly with the atmospheric annealing at 300 °C to 2 kOe, and studied the change in film structure and reason of the increase in Hc with the annealing. The rf magnetron sputtering method was employed in fabricating the films. Film structure changes were observed by x-ray diffraction, Auger electron spectroscopy, transmission electron microscopy, energy dispersive x-ray spectrography, and atomic force microscopy. The increase in Hc was observed only in the atmospheric annealing, and not in the vacuum-annealing. The internal structure of films after the atmospheric annealing at 300 °C consisted of columnar parts that had the multilayered structure that undertook vertical magnetization and column boundaries being surrounded by Co oxides. As a consequence, domain wall binding occurs in the segregated areas of the oxides in the column boundaries, thereby increasing Hc.
Verification of the nonlinear constants of crystals with illustrations made for α-quartz
Authors: Hruska Carl K.
Published: July 2002

This article points out weaknesses of several methods used to verify the third-order nonlinear electromechanical constants of crystals. It suggests that the nonlinear constants should be verified by a direct comparison of their values (or the values of their linear combinations) obtained independently from different experiments. This is illustrated by practical examples using the nonlinear constants of quartz. Three fundamental nonlinear constants and twelve linear combinations have been compared; five of them have been found in agreement. Tentative verification criteria for the nonlinear constants have also been formulated.
Propagation model for the dynamics of gain-guided semiconductor laser arrays
Authors: Wang Shawe-Shiuan , Winful Herbert G.
Published: July 2002

A model is presented for the spatiotemporal dynamics of gain-guided semiconductor laser arrays. The model goes beyond coupled mode theory and treats the array as a single entity. Numerical simulations of twin-stripe gain-guided arrays yield stable or pulsing outputs, depending on array parameters.
Theoretical investigation of impurity scattering limited mobility in quantum wells: The influence of wave-function modeling
Authors: Thobel J. L. , Baudry L. , Dessenne F. , Charef M. , Fauquembergue R.
Published: July 2002

A theoretical investigation of the impurity scattering limited mobility in quantum wells is presented. Emphasis is put on the influence of wave-function modeling, since the literature about this topic is contradictory. For an infinite square well, Dirac and sine wave functions yield the same evolutions of the mobility with temperature, carrier density, and well width. These results contradict those published by Lee [J. Appl. Phys. 54, 6995 (1983)], which are shown to be wrong. Self-consistent wave functions have also been used to compute the mobility in finite barrier height quantum wells. A strong influence of the presence of electrons inside the doped barrier has been demonstrated. It is suggested that, although simple models are useful for qualitative discussions, accurate evaluation of mobility requires a reasonably realistic description of wave functions.
Heterojunctions of CuInSe2 with amorphous hydrogenated silicon
Authors: Wu S. , Haneman D.
Published: July 2002

We have fabricated and analyzed heterojunctions of CuInSe2 (CIDS) and amorphous hydrogenated silicon (ASIL), both p:n and n:p. Reasonable rectifying properties (400:1 at 0.5 V) have been obtained, and capacitance-voltage behavior indicates a sharp interface. Optical internal photoemission measurements under various bias conditions show that the band offsets for p:n structures are 0.38 and 0.43 eV for valence and conduction bands, respectively. Excitation can occur at the interface from the CIDS valence band to the ASIL conduction band. The individual band bendings are also obtained from the measurements.
Photodarkening and paramagnetism in ultraviolet exposed lead lanthanum zirconate ceramics
Authors: Seager C. H. , Warren W. L.
Published: July 2002

Electron paramagnetic resonance (EPR) and photothermal deflection spectroscopy (PDS) have been utilized to characterize samples of lead lanthanum zirconate titanate (PLZT) ceramics before and after ultraviolet (uv) irradiation. We find a variety of EPR resonances in the unirradiated samples, including those attributable to Cu+2, Fe+3, Pb+3, and Ti+3 ions. The dark optical absorption spectra show broad, exponential subgap absorption tails which increase in magnitude with decreasing grain size. In addition, some of the larger grain ceramics show a prominent absorption enhancement which seems to correlate well with the density of Ti+3 centers. During and after uv illumination with light near the PLZT band gap, substantial increases are seen in the density of paramagnetic Ti+3 and Pb+3 ions, and a broad absorption peak appears at ∼2.6 eV. The spatial distribution of the induced absorption correlates well with the location of the absorbed uv, suggesting that photoproduced carrier pairs are trapped at Ti+4 and Pb+2 ions producing the observed paramagnetism. The Ti+3 EPR spectra can be successfully fit using the crystal-field-splitting parameters derived from the PDS data. We also observe that both the paramagnetism and the induced absorption are readily bleached by light absorbed in the spectral region where the photoinduced peak is located. We suggest that this effect is due to photoionization of the localized charges.
Interface roughness of short-period AlAs/GaAs superlattices studied by spectroscopic ellipsometry
Authors: Nguyen N. V. , Pellegrino J. G. , Amirtharaj P. M. , Seiler D. G. , Qadri S. B.
Published: July 2002

Spectroscopic ellipsometry (SE) has been used to study the effects of interface roughness on the optical properties of ultrathin short-period 3×3 GaAs/AlAs superlattices grown by molecular-beam epitaxy (MBE). The complex dielectric function and thickness of the whole superlattice and the thickness of the native oxide overlayer were simultaneously determined by an inversion technique from data in the 1.5–5.0 eV region. The main optical critical points E0, E0+Δ0, E1, E1+Δ1, and E2 were deduced by line-shape fitting of the second derivative of the complex dielectric function of the superlattice to the analytical line-shape expression. The interface roughness is found to shift the optical transitions, except E2, to higher energy and broaden their line shapes. A simple interpretation of the shift and broadening is given. The interface roughness and layer thicknesses obtained by SE are found to be consistent with the results of x-ray diffraction and Raman scattering studies previously reported. The results in this study demonstrate the capability of the post-growth nondestructive characterization by SE to provide useful information about the interface quality of superlattice structures, and consequently to optimize the MBE growth conditions in order to achieve the desired structural parameters.
In situ process evaluation during hydrogen plasma etching of a-Si:H films by microwave detected transient photoconductivity measurements
Authors: Neitzert H. C. , Hirsch W. , Kunst M.
Published: July 2002

During removal of amorphous silicon films deposited on crystalline silicon substrates by dry etching in a hydrogen plasma the kinetics of mobile excess charge carriers have been followed by measuring the change of the microwave reflection after laser pulse illumination. Following the amplitude of the measured transient signals the thickness of the remaining amorphous silicon film can be determined and the decay in the nanosecond time range yields information about the defect density of the substrate surface. The impact of the plasma process on the surface recombination is shown and a criterion for endpoint detection is given in the case of a-Si:H removal from crystalline silicon covered with a SiO2 layer.
YBa2Cu3O7/NdGaO3/YBa2Cu3O7 trilayers by modified off-axis sputtering
Authors: Wang S. Z. , Olsson E. , Alarco J. A. , Ivanov Z. G. , Winkler D. , Langer V. , Berastegui P.
Published: July 2002

High-quality epitaxial YBa2Cu3O7−δ (YBCO) thin films were achieved by a modified off-axis sputtering technique with high deposition rates (3.3 nm/min). The film quality and the deposition rate depended crucially on the target-to-substrate separation. Epitaxial YBCO/NdGaO3(NGO)/YBCO trilayers were successfully grown onto SrTiO3, Y-ZrO2, and LaAlO3 substrates by dc and rf sputtering. The epitaxial relations were found to be [001]YBCO//[001]NGO, [100]YBCO, or [010]YBCO//[110]NGO and [001]YBCO//[110]NGO, [100]YBCO, or [010]YBCO//[001]NGO, where the latter orientation relationship was dominating. Subsequent top YBCO layers grew c axis oriented independently of the two epitaxial orientations of the NGO. The orientation relationships between YBCO and NGO were the same. Auger electron depth profiles and transmission electron microscopy indicated that the interdiffusion at the interface between the YBCO and NGO layers was not strong even at 740 °C. The superconducting transition temperatures of the top and bottom YBCO layers were about the same as that of YBCO single layers, i.e., 87–90 K. Scanning electron microscopy of the surface morphologies of the YBCO and the NGO showed that a smaller substrate-target distance resulted in smoother films.
Determination of critical layer thickness and strain tensor in InxGa1−xAs/GaAs quantum-well structures by x-ray diffraction
Authors: Chen Y. C. , Bhattacharya P. K.
Published: July 2002

We have used the double-crystal x-ray rocking curve technique to determine lattice constant, strain relaxation, thickness, and critical thickness of a thin InxGa1−xAs layer embedded in GaAs. In this work we have measured and analyzed x-ray data over a wide scan angle (∼2.0°). This allows the simultaneous determination of buried layer thickness and strain. The measurement results were analyzed by the dynamical diffraction theory. The critical thickness for an InGaAs layer embedded in GaAs obtained from x-ray data is shown to be larger than that predicted by the force balance model. The strain tensors as a function of layer thickness are also analyzed for the buried InxGa1−xAs of different x values.
Comment on ‘‘Application of low-temperature ions in the nonequilibrium state plasma of a Penning source’’ [J. Appl. Phys. 69, 7933 (1991)]
Authors: Hopkins M. B.
Published: July 2002

In a recent communication Sato et al. [J. Appl. Phys. 69, 7933 (1991)], it states that the high current output of multiply charged ions, Ii, where i is the charge state, during the early part of the discharge in a pulsed Penning source is due to the long ion confinement time. They argue that Ii is proportional to the ion confinement time, τ, however, it is shown here that Ii is proportional to τ i−1 and not τ. The power dependence better explains the experimental results but does not conclusively prove the importance of τ.
Combined 2D analytical and finite element solutions for the eddy current computation in 3D induction plasma devices (abstract)
Authors: Féllachi M. , Mékidéche M. R. , Develey G.
Published: July 2002

The proposed work presents an economic and accurate model using a 2D combined analytical and Finite Element (FEM) solutions, to compute the eddy current, in the different parts of a 3D induction plasma device (Fig. 1). In such a device, 3 MHz frequency, the main problem is the representation of the skin effect characterizing the excitation coil and the metallic part of the plasma device cooling system. In some existing eddy current models, the thin skin depth is generally represented when using boundary element method and coupling it to the FEM to discretize the nonlinear domains. The resolution of the obtained algebraic system requires a large computer memory and a long CPU time. The characteristics of the proposed model are as follows: 1. using an x-y analytical solution in the cooling conducting regions (Fig. 2); 2. determining the Neumann boundary conditions, for the domains in which the FEM is required, from the solution obtained in 1; 3. deducing an equivalent electric conductivity that permits one to represent the 3D plasma device as an axisymmetrical system; 4. representing the excitation coil by a line conducting region; 5. finite element discretizing the plasma domain, in which the electric conductivity is strongly dependent on temperature.1 The full paper will present the details of the coupled analytical and numerical formulations and results for the proposed model validity.
Second harmonic generation as a sensitive technique for estimating H
c1 of high T
c materials
Authors: Kumar Shailendra , Roy S. B. , Pradhan A. K. , Chaddah P. , Prasad Ram , Soni N. C.
Published: July 2002

A simple and interesting technique for measuring the Hc1 of the superconducting grains in bulk high Tc superconductor (HTSC) samples is presented herein. This technique takes advantage of interesting history effects observed in the nonlinear magnetic response of HTSC samples. The hysteretic second harmonic magnetization shows sharp structure at low applied fields due to the magnetic flux trapped inside the grains. This feature is used to track the penetration of magnetic field inside the superconducting granular regions.
Search for the breakdown of the spin-wave approximation in EuO (abstract)
Authors: Fernandez-Baca J. A. , Mook H. A.
Published: July 2002

The spin-wave approximation in a Heisenberg ferromagnet is valid at low temperatures where the spin-wave interactions are negligible. At higher temperatures these interactions become highly nonlinear and the spin-wave approximation should break down. In a recent paper Fishman and Vignale1 predicted that this breakdown should occur near the temperature T̄=0.4zJs, where z is the number of nearest neighbors, J is the ferromagnetic coupling constant, and s is the spin of the ferromagnet. In another related paper Fishman and Liu2 have calculated that above this temperature T̄ a second pole appears in the correlation function at an energy close to 2zJs. The origin of this new mode is the strong coupling of transverse and longitudinal fluctuations that forces the spins to precess about their local mean field. When the ‘‘spin-wave’’ energies are close to the precessional mode energy 2zJs a splitting of the ‘‘spin-wave’’ branch is predicted. In order to test this prediction we have performed a high-resolution inelastic neutron scattering experiment to measure the spin waves in EuO, a prototypical Heisenberg ferromagnet, in the vicinity of the q-E region where this splitting should occur. Although our experiment was performed at T=20 K, which is above T̄≊12 K, our measurements failed to observe the predicted splitting of the ‘‘spin-wave’’ branch. Only a single resolution-limited spin-wave branch was observed, in agreement with earlier measurements by Passell et al.3
Interface-phonon-assisted Γ–X transitions in short-period superlattices
Authors: Dutta Mitra , Stroscio Michael A.
Published: July 2002

The dielectric continuum model of longitudinal-optical phonons in polar semiconductors is used to define the role of interface longitudinal-optical phonons in affecting phonon-assisted Γ–X transitions in GaAs–AlAs and GaAs–GaP superlattices. In particular, the dielectric continuum model for interface optical phonons is used in conjunction with a Kronig–Penney model of the superlattice electronic properties for two purposes: to specify superlattice parameters where interface-phonon-assisted Γ–X transitions are expected and to estimate relative transition probability amplitudes for interface-phonon-assisted Γ–X transitions in selected short-period superlattices.
Recombination process and its effect on the dc performance of InP/InGaAs single-heterojunction bipolar transistors
Authors: Ouacha A. , Chen Q. , Willander M. , Logan R. A. , Tanbun-Ek T.
Published: July 2002

A simple fitting technique based on the current balancing concept has been used to calculate the minority-carrier diffusion length Ln in the base layer of an InP/InGaAs single-heterojunction bipolar transistor grown by metalorganic chemical-vapor deposition. Due to the emitter-base surface-related problem, the minority-carrier lifetime was shown to be dependent on the collector current. This surface-related problem could be a result of the high SiN deposition temperature used in the processing steps. The method predicts value for Ln of 0.97 μm at low collector current and 3.4 μm at high current.
Aging of glassy transformer core alloys and the activation energy spectrum model
Authors: Ramanan V. R. V. , Liebermann H. H.
Published: July 2002

Aging was characterized through measurement of core loss in small toroidal cores of metallic glass ribbons at various temperatures. The increase in core loss follows log-time kinetics. It is shown that the activation energy spectrum model fits the data very well. Estimates of time for serious degradation of the properties of a commercial Fe-B-Si glass at device temperatures will be provided. In contrast to silicon steels, the addition of carbon to Fe-B-Si glasses is shown to have no adverse effects on aging behavior. The effects of surface crystallinity on the aging behavior are also discussed.
Evidence of an interfacial layer formation during rare-earth deposition onto CdTe: The case of Sm
Authors: Chami A. C. , Daudin B. , Fontenille J. , Gros P. , Ligeon E.
Published: July 2002

An interfacial layer (IL), about a nanometer thick, is produced by Sm deposition onto (001) CdTe grown by molecular-beam epitaxy. This IL is studied by reflection high-energy electron diffraction (RHEED), ion channeling, and x-ray diffraction. A correlation between the (1×2) RHEED pattern and the {111} CdTe stacking sequence is demonstrated, and is in favor of the surface Te dimerization hypothesis. Lattice location by ion channeling has revealed that incorporated Sm mainly occupies the octahedral site in the Te sublattice. A CdTe overlayer grown onto IL presents a remarkable feature, that is, a 90° rotation around the [001] direction with respect to the CdTe buffer. It was checked by ion channeling that the overlayer exhibited a bulklike crystallinity. Superlattices composed of many of these IL/CdTe basic periods have been grown at a temperature of 270 °C. The different experimental features are tentatively interpreted assuming the formation of an IL with the spinel structure.
Simulation of magnetization distribution in magnetoresistive film under a longitudinal bias field
Authors: Ishikawa Chiaki , Suzuki Kaori , Koyama Naoki , Yoshida Kazuetsu , Sugita Yutaka , Shinagawa Kiminari , Nakatani Yoshinobu , Hayashi Nobuo
Published: July 2002

The effects of longitudinal bias field, used for domain control on the magnetization distribution in a magnetoresistive (MR) film, have been investigated by computer simulation. The longitudinal bias field was generated by an exchange-coupled antiferromagnetic or permanent magnetic film formed on the MR film outside the sensing region. It was assumed that the magnetization in the part of the MR film on which the bias-generating films were formed was fixed along the easy axis. The spatial sensitivity of the MR film along the track width was evaluated by calculating the dependence of the resistance change on the position of a narrow track recording medium. It was found that the resistance change in the MR film with the anti-ferromagnetic film was roughly twice as large as the change in the film with the permanent magnetic film. The asymmetric sensitivity profile with respect to reflection about the track width mid-plane was also obtained. The asymmetry in the track sensitivity profile was found to be caused by three factors: asymmetric magnetization distribution about the track width mid-plane due to the transverse bias field, the difference in angular changes in the magnetization direction in the left and right regions facing the recording medium, and anisotropic flux propagation in the MR film.
On the microstructural, optical, and thermal properties of hydrogenated amorphous carbon films prepared by plasma enhanced chemical vapor deposition
Authors: Chou L. H. , Wang H. W.
Published: July 2002

Hydrogenated amorphous carbon films were prepared from CH4, H2, and Ar mixtures by plasma enhanced chemical vapor deposition. Films with various physical properties resulting from various deposition conditions were utilized for this study. The varying deposition parameters included H2 flow rates, Ar flow rates, total pressures, substrate temperatures, and power densities. A systematic study regarding the relationship between deposition conditions and the microstructures, optical, and thermal properties was conducted. Furthermore, how the optical and thermal properties related to the microstructures was analyzed. Fourier transform infrared spectroscopy was employed in this paper for determining the hydrogen concentration and the amounts of tetrahedral and trigonal bondings associated with C—H bond and their relative ratio while the optical properties were measured by optical spectrophotometer. Additionally, photothermal deflection spectroscopy was applied for the measurements of thermal diffusion length.
Compositional instability and the resultant charge variations in mixed B-site cation relaxer ferroelectrics
Authors: Viehland Dwight , Li Jie-Fang
Published: July 2002

A phenomenological model has been applied in an attempt to explain the inability of the ordered regions in the lead magnesium niobate family of relaxor ferroelectrics to coarsen. This approach is based on the concept that the free energy is lowered by an embryonic decomposition along a non-neutral direction. It is proposed that the excess free energy associated with the formation of the non-neutral phase is offset by the distortability of the perovskite structure toward the pyrochlore. The lack of coarsening is then explained as a balance of the electrostatic energy and gradient energy terms, following an earlier published report. This model is then applied to the La-modified (donor-doped) lead magnesium niobate, to explain the dependence of the size of the ordered regions on the degree of doping as observed by other workers.
Effect of rubbing on the molecular orientation within polyimide orienting layers of liquid-crystal displays
Authors: van Aerle N. A. J. M. , Barmentlo M. , Hollering R. W. J.
Published: July 2002

The influence of various rubbing parameters on the molecular reorientation of thin polyimide orienting layers, used to align liquid-crystal (LC) molecules within liquid-crystal displays, has been studied. For this purpose the optical phase retardation in the polymer layer, explicitly induced during the rubbing treatment, was determined. The observed rubbing-induced phase retardation can directly be related to a molecular orientation within the polymer orienting layer, as could be shown by infrared dichroism studies. Furthermore, it is found that the top of the polymer layer, directly contacting the rubbing cloth during the actual rubbing process, is almost instantaneously oriented to a certain maximum value as soon as the rubbing is started. Additional or stronger rubbing has no detectable influence on the orientation within the top layer. Increasing the rubbing density or the rubbing pressure only results in an increase of the penetration depth of the rubbing process, i.e., molecular reorientation occurs deeper within the layer. Experiments show that the penetration depth can be varied from less than 10 nm to more than 60 nm by variation in rubbing conditions. These findings are supported by surface second-harmonic-generation studies of LC monolayers deposited onto rubbed orienting layers and by infrared dichroism studies.
The nature of the hole traps in reoxidized nitrided oxide gate dielectrics
Authors: Mallik A. , Vasi J. , Chandorkar A. N.
Published: July 2002

To find out the nature of hole traps in reoxidized nitrided oxide (RNO), a series of field and thermal detrapping experiments were performed following irradiation. It has been found that not only are the hole traps in RNO located near the gate-SiO2 interface instead of near the Si-SiO2 interface as in conventional dry oxides, but that the energy distribution of these traps is also quite different from that in conventional oxides. This indicates that the origin of the traps in RNO is different from that normally found in conventional oxides.
Dynamic pore collapse in viscoplastic materials
Authors: Tong W. , Ravichandran G.
Published: July 2002

Dynamic pore collapse in porous materials is studied by analyzing the finite deformation of an elastic/viscoplastic spherical shell under impulsive pressure loading. Effects of dynamic loading rate, pore size, initial porosity, strain-rate sensitivity, strain hardening, thermal softening, and mass density of the matrix material on the pore collapse process are examined and results are compared with those from quasistatic analyses of both rate-independent and rate-dependent matrix materials. Dynamic (inertia) effects are found to be significant or even dominant in certain shock wave consolidation conditions. An approximate method is proposed to incorporate dynamic effects into quasistatic pore-collapse relations of viscoplastic matrix materials. Implications of results of current study are discussed in terms of understanding the processes of shock wave consolidation of powders.
Chemical sputtering of Si related to roughness formation of a Cl-passivated Si surface
Authors: Feil H. , Dieleman J. , Garrison B. J.
Published: July 2002

Chemical sputtering of Si in a chlorine environment has been examined with molecular dynamics simulations. It is found that chemical sputtering correlates with the roughness formation of Cl-passivated Si surfaces during low-energy ion bombardment. The chlorine passivation of the Si surface prevents the flattening of the surface due to the high activation barrier for surface diffusion. The rough surface contains reactive intermediates that can be desorbed into the gas phase when, after an ion impact, the region has a large energy content. The observed products and the increase of the sputtering yield are in agreement with experimental observations.
High-efficiency infrared-to-visible upconversion of Er3+ in BaCl2
Authors: Wang Yuhu , Ohwaki Junichi
Published: July 2002

Highly efficient infrared-to-visible upconversion has been observed in Er3+-doped BaCl2 phosphors. The composition optimized for maximum green emission was around 25ErCl3 (mol %), which contains 5–15 mol % more active Er3+ ions than those in the conventional Er3+-doped fluoride phosphors. Pumped by a 0.8 μm laser diode with a power density of ∼3 W/cm2, chloride (25ErCl3-75BaCl2) shows a very bright green emission with the intensity being two orders of magnitude larger than that of the commercially available IR sensor card on which an optimized fluoride phosphor Y0.8Er0.2F3 is pasted. A 0.97 μm laser diode excitation on the chloride yielded blue (0.49 μm), green (0.55 μm), and red (0.66 μm) fluorescences, visually exhibited as a bright greenish-white emission. The visible fluorescence excited by 0.8, 0.97, or 1.5 μm laser diodes shows quadratic or cubic dependencies on the excitation power over the entire power range for the chlorides but lower dependences for the fluoride. The differences in the upconversion characteristics between the chlorides and fluoride are discussed in terms of the rate equations and attributed principally to the different Er3+...Er3+ interionic energy transfer probability and the different multiphonon decay rate from the excited states of Er3+ in these matrices.
Growth mechanism of (InAs)m(GaAs)n strained short-period superlattices grown by molecular beam epitaxy
Authors: Kawai T. , Yonezu H. , Ogasawara Y. , Saito D. , Pak K.
Published: July 2002

The growth mechanism of (InAs)m(GaAs)n strained short-period superlattices grown on GaAs(001) and InAs(001) substrates was investigated by reflection high-energy electron diffraction and transmission electron microscopy. In the growth on a GaAs substrate, the two-dimensional layer-by-layer growth mode (Frank–Van der Merwe mode) was successfully realized even over the critical thickness when the lattice mismatch is less than ∼2.4%. In this case, the critical thickness of the grown layer on GaAs was remarkably increased by applying the superlattice structures. However, the strained short-period superlattices on InAs substrate were grown in the Stranski–Krastanov mode. The growth mode of strained short-period superlattices can be explained by the balance of surface and interface free energies, which could relate to the difference of the bonding energy between In-As and Ga-As. The growth mode of strained short-period superlattices depended strongly on the growth temperature. In the relatively high temperature growth, the growth mode of strained short-period superlattice grown on a GaAs substrate was changed from the Frank–Van der Merwe mode to the Stranski–Krastanov mode. It could be attributed to intermixing of superlattice structure due to the surface segregation of In atoms assisted by thermal energy.
Elastic properties of orthorhombic KNbO3 single crystals by Brillouin scattering
Authors: Kalinichev A. G. , Bass J. D. , Zha C. S. , Han P. D. , Payne D. A.
Published: July 2002

Brillouin light scattering was used to obtain elastic and piezoelectric constants for a single domain orthorhombic KNbO3 single crystal at room temperature and pressure. More than 320 measurements of longitudinal and transverse acoustic wave velocities were obtained in 160 different crystallographic directions. An inversion of these data using the literature values for the dielectric permittivity of KNbO3 resulted in the full set of elastic and piezoelectric constants for the material. It is suggested that the difference between piezoelectric constants obtained by high- and low-frequency methods could be explained by the high-frequency relaxation-type dispersion for the dielectric constant ε33 in the GHz region by analogy with BaTiO3. The directional dependence of electromechanical coupling for longitudinal and transverse acoustic waves in KNbO3 was analyzed. The obtained elastic constants were (in GPa): CE11=224(4), CE22=273(5), CE33=245(5), CE44=75(1), CE55=28.5(5), CE66=95(2), CE12=102(5), CE13=182(10), CE23=130(6), where E denotes constant electric field strength.
Change of magneto-optical Kerr rotation due to interlayer thickness in magnetically coupled films with noble-metal wedge
Authors: Katayama T. , Suzuki Y. , Hayashi M. , Geerts W.
Published: July 2002

An intensity oscillation of the saturation polar Kerr rotation φK due to interlayer thickness d was observed in Fe(5 Å)/Au(d Å)/Fe(5 Å)(100) and Fe(6 Å)/Au(d Å)/Fe(6 Å)(100) sandwiched films. The periods of the oscillation are about 7–8 and 5–6 monolayers of Au thickness, respectively. The oscillation of φK can be observed only in the photon energy range 2.5–3.8 eV for Fe(6 Å)/Au(d Å)Fe(6 Å). These phenomena are considered to be closely related to a formation of spin-polarized quantum-well states of the minority Δ1 band in the Au layer sandwiched with two Fe barrier layers. The φK peak positions and oscillation period do not correspond exactly to those of the change in the in-plane saturation field Hs.
Contributions of fine-particle magnetism to reading the global paleoclimate record (invited)
Authors: Banerjee Subir K.
Published: July 2002

Paleoclimate changes are recorded by proxy as variations in concentration, composition, and grain size of magnetic minerals, principally magnetite (Fe3O4), in the sediments deposited in lakes, oceans, and continental eolian deposits. Cross-validated multiple-parameter magnetic measurements of such sediment cores provide global change data of high temporal resolution, useful for constructing a base-line record against which anthropogenic modifications may be discerned. Theories of superparamagnetism and magnetic domains are used to explain the physical basis of magnetic proxy recording. Examples of applications to validation of Milankovitch theory of climate change and delineation of the glacial and interglacial stages of the last 1 000 000 years are provided.
Angular dependence measurement of individual barium ferrite recording particles near the single domain size
Authors: Chang Thomas , Zhu Jian Gang
Published: July 2002

The switching field angular dependence measurement of individual barium ferrite recording particles with sizes ranging from 55 to 75 nm in diameter were measured via magnetic force microscopy technique. These experimental measurements were then used as a basis to investigate their nucleation mechanism and intrinsic switching modes. Our findings indicate that the effects nonuniform doping of Co-Ti can significantly affect the easy axis nucleation field and the switching mode.
Microstructure of porous media and field-cycling nuclear magnetic relaxation spectroscopy
Authors: Stapf S. , Kimmich R. , Niess J.
Published: July 2002

As a special class of nuclear-magnetic-resonance (NMR) relaxation mechanisms, reorientations mediated by translational displacements, is considered. This particularly refers to systems in which molecules are confined to disordered structures defining (a) local preferential orientations and permitting (b) translational degrees of freedom with a certain reduced dimensionality. Examples are molecules adsorbed on surfaces of macromolecules, particle aggregates, and porous media. The dipolar correlation function of molecules diffusing along such confining structures therefore does not only reflect the molecular dynamics but also the structural properties of the confining system. Using field-cycling and other NMR relaxation techniques the frequency dependence of the spin-lattice relaxation times of several porous materials were measured in a range 3×102–3×108 Hz. The data were numerically analyzed using an ‘‘orientational structure factor’’ which was introduced ad hoc and which renders the distribution of wave numbers of the surfaces forming the geometrical restrictions of molecular displacements. This distribution turned out to consist of a power-law and a single-peak term. The results are discussed with respect to the microstructure. Characteristic length scales can be estimated by bringing in data of the translational diffusion coefficient effective in short time intervals.
Interdiffusion and reaction in Cu/PtSi/Si(100) systems
Authors: Hong Stella Q. , Hong Q. Z. , Li Jian , Mayer J. W.
Published: July 2002

Interdiffusion and reaction in Cu/PtSi/Si(100) structures have been investigated. During annealing Cu migrates across the intermediate PtSi layer and reacts with Si to form Cu silicides, with no indication of PtSi decomposition or a Cu-PtSi reaction. The diffusion of Cu through PtSi proceeds nonuniformly when PtSi is exposed to air before Cu deposition. Uniform transport is observed when the sample is prepared in situ without breaking vacuum. Cu silicides grow linearly with time, while the rates are independent of the thickness of the transport medium. The results indicate that Cu silicide formation is not controlled by Cu diffusion through the interposed PtSi layer.
Effect of growth temperature and doping concentration on the distribution of the emitting centers in CaF2:Er molecular beam epitaxial layers
Authors: Daran E. , Legros R. , Muñoz-Yagüe A. , Fontaine C. , Bausá L. E.
Published: July 2002

Molecular beam epitaxy of Er-doped CaF2 layers on (100)oriented CaF2 substrates was performed using CaF2 and ErF3 evaporation cells. The effect of growth temperature and Er concentration on the distribution of the different emission centers observed in these epitaxial Er-doped layers was investigated. Photoluminescence analyses were performed in the 830–860 nm wavelength range in which the 4S3/2→4I13/2 transitions of the Er3+ ions take place. The evolution of the relative emission intensity between single and aggregate Er3+ centers as a function of growth temperature shows that the emission from isolated Er3+ ions is favored in a growth temperature range of 500–520 °C. Emission lines from complex Er3+ centers are found to rise relative to those of isolated sites as Er concentration increases. Finally, no quenching of the integrated luminescence intensity occurs in the concentration range investigated, 0.05–6 mol % (0.1–16 wt%).
Measurement and control of ion-doping-induced defects in cadmium telluride films
Authors: Kim Donghwan , Fahrenbruch Alan L. , Lopez-Otero Adolfo , Bube Richard H. , Jones Kim M.
Published: July 2002

Homoepitaxial p-type CdTe films were grown by coevaporation of CdTe and phosphorus in vacuum, where the phosphorus vapor was ionized and accelerated toward the substrate. Hole densities up to 2×1017 cm−3 were obtained using an ion energy of 60 eV. Effects of residual ion damage were observed using cross-sectional transmission electron microscopy, etch-pit density, and minority-carrier diffusion length measurements. This ion damage is dependent on both the ion dose and the ion energy. Reducing the ion energy below 60 eV results in lower doping densities, but using electron irradiation and Cd overpressure during deposition makes it possible to achieve equivalent doping levels for 20 eV ions while reducing the ion damage. At an ion energy of 20 eV, using electron irradiation of the growing film, and a 0.2% overpressure of Cd, films with hole density of 1×1017 cm−3 and diffusion length of 0.35 μm were obtained. Photovoltaic behavior of the films deposited in different conditions was tested by fabricating n-CdS/p-CdTe heterojunctions.
Strain effects in high-purity InP epilayers grown on slightly mismatched substrates
Authors: Watkins S. P. , Arès R. , Masut R. A. , Tran C. A. , Brebner J. L.
Published: July 2002

The growth of high-purity InP on various As-, S-, and Fe-doped InP substrates has been investigated using high-resolution photoluminescence spectroscopy (PL) and high-resolution x-ray diffractometry. Substrate induced strains of −7×10−5 or less have been observed using low-temperature PL. In this way information about the strain dependence of the electronic excited states of the donor bound excitons in InP was obtained. In addition, it was shown that the assessment of variations in substrate lattice parameter can be determined with a resolution of at least 5×10−6 by PL techniques.
Electro-optic tensor ratio determination of side-chain copolymers with electro-optic interferometry
Authors: Norwood R. A. , Kuzyk M. G. , Keosian R. A.
Published: July 2002

Electro-optic interferometry is used to measure the ratio of the electro-optic coefficients r13 and r33 for N-methacryloxyethyl-N-methylaminonitrostilbene/methyl methacrylate (DANS/MMA) and dicyanovinyl-hexatriene/methyl methacrylate (DCVHT/MMA) side-chain copolymers. The electro-optic interferometer uses a reflection modified Mach–Zehnder configuration, and results are compared with those obtained by the phase modulated reflection technique [C. C. Teng and H. T. Man, Appl. Phys. Lett. 56, 1734 (1990)]. Tensor ratios r33/r13 of 3.34 and 5.92 were found for DANS/MMA and DCVHT/MMA, respectively, for poling fields of 100 V/μm. Disagreement between the two techniques may derive from a piezoelectric contribution to the measurements or from restricted motion of the chromophores during poling.
Energy and angle distributions of ions striking a spherical target in plasma source ion implantation
Authors: Wang Dezhen , Ma Tengcai , Deng Xinlu
Published: July 2002

Plasma source ion implantation is a process in which a target is immersed in a plasma and a series of large negative-voltage pulses is applied to it to extract ions from the plasma and implant them into the target. A Monte Carlo simulation model is developed to study the energy and angle distributions of ions striking the spherical target for high pressures of the neutral gas. The ion-neutral charge exchange and momentum-transfer cross sections that depend on the ion energy are taken into account precisely. The energy and angle distributions of Ar+ at the spherical target during the sheath edge evolution after the ion matrix sheath for different pressures are investigated in detail.
Plasma ion implantation of nitrogen into silicon: Characterization of the depth profiles of implanted ions
Authors: Vajo John J. , Williams John D. , Wei Ronghua , Wilson Robert G. , Matossian Jesse N.
Published: July 2002

The in-depth concentration distribution or depth profile of nitrogen implanted into silicon wafer substrates using plasma ion implantation (PII) is studied using secondary-ion-mass spectrometry and Auger electron spectroscopy sputtered depth profiling. Plasma ion implants were performed using a low-pressure (5×10−5 Torr) collisionless plasma at voltages of 50 and 100 kV to a fluence of 1.5×1017 cm−2 using voltage pulses 10 μs in duration, with 1 μs rise time, and at a repetition rate of 200 Hz. The measured depth profiles are compared with those from both conventional ion-beam implantation and numerical simulations. The comparisons indicate an incident flux composed of ∼90% N+2 and ∼10% N+ ions. Compared with ion-beam implants, which exhibit a nearly Gaussian-shaped depth profile, the plasma ion implantation profiles are ‘‘filled in’’ with an approximately constant nitrogen concentration for depths less than the predicted ion range. The profiles are modeled assuming that incident ions have a distribution of energies. Thermal diffusion does not seem to affect the nitrogen profile. Energy distributions are determined using a series of simulated implants at different energies as basis functions to numerically fit the measured profiles. The normalized energy distribution is independent of voltage at 50 and 100 kV, increases monotonically with increasing energy, and indicates that ∼50% of the ions are implanted at energies <80% of the nominal value. Comparisons with several developed models of the PII process suggest that displacement current effects within the expanding ion sheath may significantly affect the incident ion energy distribution.
Compositional and spectroscopic study of the growth of diamond films from several gaseous mixtures
Authors: Marinelli M. , Milani E. , Montuori M. , Paoletti A. , Tebano A. , Balestrino G. , Paroli P.
Published: July 2002

A critical experimental test of the empirical unifying scheme recently proposed by Bachmann, Leers, and Lydtyn [Diamond and Related Mater. 1, 1 (1991)] about the gas compositions useful to grow diamond films by plasma-assisted chemical-vapor deposition has been performed. The data confirm the main concept of the Bachmann scheme, namely, the existence of a single compositional ‘‘diamond domain’’ in a C-O-H triangular diagram, in which the overall gas compositions are plotted; however, quantitatively, the shape and the borders of such a diamond domain are rather different from those assumed in the work of Bachmann and co-workers. For all the gas mixtures investigated, the changes in the plasma optical emission spectra consequent to crossing the border from the ‘‘no-growth zone’’ into the ‘‘diamond domain’’ have been studied. These changes show universal features, which point to the presence of the same growth mechanisms over all the diamond domain, independent of the identity of the initial chemical species.
Time-dependent p-n junction characteristics underneath TiN/Ti contact metal
Authors: Yoshida Takehito , Kawahara Hiroyuki , Ogawa Shinichi
Published: July 2002

Time-dependent p-n junction characteristics (junction depth: xj∼0.2 μm) underneath TiN/Ti contact metal have been determined in terms of contact interface crystallinity. Reverse bias (≥6.0 V) soft-breakdown characteristics result in a leakage current increase during reverse bias and temperature aging stress. It has been elucidated that this degradation is dependent on the crystallinity of Ti-Si interdiffused layers at the contact interface and the p-n junction polarity. The degradation is observed only in amorphous Ti-Si contacted n+-p junctions. Nevertheless, in amorphous Ti-Si contacted p+-n junctions and crystalline TiSi2 contacted n+-p and p+-n junctions, the degradation does not appear. In the case of the amorphous Ti-Si contacted n+-p junction, it is concluded that the time-dependent degradation is caused by diffusion of positively ionized Ti enhanced by the applied electric field.