bluuebunny/crossref_metadata_2025 huggingface dataset:
Pulsed CO2 laser ablation of graphite and polymers
Authors: Wong K. H. , Tou T. Y. , Low K. S.
Published: July 2002

Spectroscopic analysis of the emission plumes of graphite, polyimide, polyethylene terepthalate, and polymethylmethacrylate that have been ablated by using a pulsed CO2 laser operating at 10.6 μm shows the presence of CN and C2, species not previously reported for CO2 laser ablation. The gross dynamics of the luminous plume, which was studied by using a streak camera, compares favorably with predictions from the snowplow model, which also accurately forecasts the time history of the plume expansion for a wide range of background gas pressures and laser fluences. Framing shadowgraphy reveals the onset of laser-supported detonation waves at approximately 50 mbar Ar, thus somewhat limiting the validity of this model.
Ion assisted growth and characterization of polycrystalline silicon and silicon-germanium films
Authors: Saha Chumki , Das S. , Ray S. K. , Lahiri S. K.
Published: July 2002

Polysilicon films have been deposited at a low temperature by single and dual ion beam sputtering. The structural and electrical properties of as-deposited and annealed films have been characterized by x-ray diffraction, secondary ion mass spectroscopy, atomic force microscopy, and Hall mobility measurements. The films are microcrystalline with average grain size ranging from 200 to 400 Å. The films exposed to a low-energy secondary ion beam during sputtering from a silicon target have exhibited smoother surface topography and different electrical behavior than the films deposited without any secondary ion bombardment. Some preliminary studies on ion beam sputtered SiGe films using a compound target are also presented.
Determination of ordinary refractive index profile for a planar waveguide by transmission spectrum analysis
Authors: Chao Shiuh , Chen Yun-Chiao , Chen Huai-Yi
Published: July 2002

We introduce a new method to determine the ordinary refractive index profile of a planar waveguide on a crystal plate. The index profile of the planar waveguide can be numerically divided into multilayers; the transmission spectrum of the multilayer waveguide can be calculated and numerically analyzed to fit the measured transmission spectrum. We demonstrated this method on a proton exchanged planar waveguide in z-cut LiTaO3. We found that the ordinary refractive index profile of this waveguide can very well be described by a Fermi–Dirac function. The index profile evolution with proton exchange time and anneal time were obtained together with the diffusion coefficients for the proton exchange and anneal processes. We discovered that, for the proton exchange process in LiTaO3, there exists a surface diffusion phenomenon which has a much smaller diffusion coefficient than that of the bulk diffusion.
Magneto-optic study of Ni-based diluted magnetic semiconductors
Authors: Ando K. , Chiba A. , Tanoue H.
Published: July 2002

After establishing the usefulness of ion implantation to synthesize diluted magnetic semiconductors (DMSs) for the case of Zn1−xCoxTe, we apply this technique to obtain Ni-based DMS which has not been reported before. Ni ions are implanted into a ZnTe film with acceleration energies from 30 to 390 keV to achieve a Ni density of 3×1020/cm3. Magnetic circular dichroism (MCD) spectra show that the Ni ions substitute Zn ions and induce sizable Zeeman splittings of the optical transitions. Analyses of the MCD spectra show that the p-d exchange in Zn1−xNixTe is antiferromagnetic.
Heat source distribution, vertical structure, and coating influences on the temperature of operating 0.98 μm laser diodes: Photothermal reflectance measurements
Authors: Dacal L. C. O. , Mansanares A. M. , da Silva E. C.
Published: July 2002

In the present work single-quantum-well laser diodes operating at 0.98 μm are investigated by photothermal reflectance microscopy. Temperature maps were obtained for the output facet of all devices studied. Furthermore, the temperature distribution was determined along the cavity (on the ridge) of lasers soldered with the junction side up. Near the facets, the measured temperature was found to be about seven times the bulk’s temperature, indicating the presence of an important surface heat source. The signal phase distribution of the laser facet shows the important role of the vertical structure on the heat confinement. Comparison between experiments and calculations shows that the confinement layers (GaAlAs and GaInP) thermal parameters are the principal responsible for the heat propagation in these structures near the active region. The same calculations show the role of the coating (Al2O3) in the heat propagation, and give a quantitative ratio between surface and bulk heat sources. Measurements made on the facet and on the ridge as a function of injection current were found to present a quite similar behavior, leading to the conclusion that thermal effects are strongly dominant in these measurements, masking any carrier or electroreflectance effects. Finally, measurements made under different light output power conditions and under the same injection current conditions showed that the surface heat source is caused by laser light absorption at the facets.
Numerical model of a transferred plasma arc
Authors: Aithal S. M. , Subramaniam V. V. , Pagan J. , Richardson R. W.
Published: July 2002

This article presents results of two-dimensional simulations for a transferred plasma arc. Calculations are performed for the internal plasma flow within the torch body, as well as the external plasma jet impinging on a surface. The governing equations describing the plasma flow are self-consistently coupled. These equations are the compressible Navier–Stokes equations for conservation of mass and momentum (in the radial and streamwise directions), conservation of energy, species continuity with finite-rate ionization and recombination kinetics, and the magnetic diffusion equation for the electromagnetics. The unsteady forms of these equations are time marched to steady state using the linearized block implicit method. This model does not employ any adjustable parameters, and therefore enables direct comparison with experiments. Model predictions are compared with experimental measurements of stagnation pressure distributions recorded on a water-cooled copper plate (workpiece), and indicate good agreement, given the lack of adjustable parameters.
Bound exciton luminescence in Te-doped SrS
Authors: Rack Philip D. , Lewis Jay S. , Holloway Paul H. , Park Wounjhang , Wagner Brent K. , Summers Christopher J.
Published: July 2002

This article describes the synthesis and characterization of SrS:Te powders which were found to have two high energy emission bands under photoluminescent excitation. To understand the radiative recombination mechanism, photoluminescence emission spectra were measured as a function of temperature. A simple configuration coordinate model was used to explain the emission band broadening and luminescence quenching that was observed with increasing temperature. Luminescence decay measurements were also performed, and the empirically determined transition rates were compared to theoretical calculations. Finally, SrS:Te ACTFEL devices were fabricated and tested under electroluminescence excitation.
Magnetotransport and magnetocaloric properties of La0.55Er0.05Ca0.4MnO3
Authors: Gu Gang , Cai Jinhua , Yang Wei , Du Youwei
Published: July 2002

La 0.55 Er 0.05 Ca 0.4 MnO 3 in bulk form was prepared both by solid-state reaction and by a sol–gel method. Samples prepared by both methods showed a moderate magnetic entropy change near its Curie temperature, this spread to a large temperature span upon the application of a magnetic field of 1.8 T. The material prepared by the sol–gel method exhibits a large magnetoresistance below its Curie temperature, which is considered to result from the interfacial spin-dependent tunneling.
Selective wet etching of GaAs on AlxGa1−xAs for AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor
Authors: Moon Eun-A , Lee Jong-Lam , Yoo Hyung Mo
Published: July 2002

Selective wet etching of GaAs over AlxGa1−xAs (x=0.2, 0.23, 0.3, and 1.0) for AlGaAs/InGaAs/AlGaAs pseudomorphic high electron mobility transistor (PHEMT) was studied using current–voltage, etched step measurement, atomic force microscopy (AFM), and x-ray photoelectron spectroscopy. The volume ratio of 50% citric acid solution (citric acid:H2O=1:1) to H2O2 was changed from 0.7:1 to 3:1, and the etching selectivity of GaAs over AlxGa1−xAs was examined. In the solution with the ratio of 1.5:1 of the 50% citric acid/H2O2, the best selectivities for GaAs on AlxGa1−xAs with x=0.2, x=0.23, x=0.3, and x=1.0 are measured to be 137, 143, 159, and 2700, respectively. The AFM measurements show that the AlxGa1−xAs surface etched by the selective etching solution is much smoother than that by a nonselective one, consisting of H3PO4:H2O2:H2O=4:1:180. The amount of Al–O bond at the selectively etched surface of AlxGa1−xAs increases with the Al composition in the AlxGa1−xAs. This means that the insoluble Al2O3 mainly plays a role in suppressing the dissolution of the AlGaAs in the etching solution. The selective etching solution is applied to the fabrication of PHEMT. The PHEMT exhibits excellent and reliable performance, demonstrating the applicability of this solution to gate recess process.
Exact electron momentum-relaxation times in GaN associated with scattering by polar-optical phonons
Authors: Ridley B. K.
Published: July 2002

The Boltzmann equation was solved exactly for the case of low-field electron transport in bulk GaN at 300 K in the presence of polar-optical phonon scattering only, giving a mobility at low electron densities of 2200 cm2 V−1 s−1 (m*=0.22m0), or 2500 cm2 V−1 s−1 (m*=0.2m0). The effect of increasing the electron concentration to around 1019 cm−3 was found to reduce the mobility to 640 cm2 V−1 s−1. This new result was shown to be a consequence of there being at high densities, a greater proportion of electrons able to emit phonons.
Direct measurement of product of the electron mobility and mean free drift time of CdZnTe semiconductors using position sensitive single polarity charge sensing detectors
Authors: He Z. , Knoll G. F. , Wehe D. K.
Published: July 2002

This article describes novel techniques to directly measure the electron mobility and mean free drift time product μeτe in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy γ rays typical of many applications.
Yield point of as-grown and predeformed GaAs:Zn
Authors: Brion Hans Georg , Siethoff Hans
Published: July 2002

Single crystals of highly Zn-doped GaAs are compressed along 〈123〉 in a constant strain-rate test at temperatures between 330 and 700 °C. The yield point is studied for both the as-grown and the predeformed state, the latter achieved by strain-rate and temperature-change tests. For as-grown material two deformation regimes are established. At low temperatures the deformation is governed by a kink mechanism typical for tetrahedrally coordinated semiconductors. The activation energy and the stress exponent are deduced as U=1.44 eV and n=3.0, respectively. These values are similar to those obtained for undoped GaAs, thus indicating that Zn additions do not appreciably influence the activation energies of kink formation and migration. Nevertheless, a strong efficiency of Zn for locking dislocations in GaAs is observed. At higher temperatures a different regime emerges, which has been also observed in other highly doped semiconductors, the basic mechanism of which, however, has not yet been elucidated. Material predeformed in the temperature-change test exhibits characteristic deviations. In these experiments the crystals behave as if the point-defect concentration or the locking efficiency of obstacles had been appreciably increased on account of predeformation.
Spatially resolved studies of chemical composition, critical temperature, and critical current density of a YBa2Cu3O7−δ thin film
Authors: Gaevski M. E. , Bobyl A. V. , Shantsev D. V. , Suris R. A. , Tret’yakov V. V. , Galperin Y. M. , Johansen T. H.
Published: July 2002

Spatially resolved studies of a YBa2Cu3O7−δ thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 μm resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, ΔTc, were determined with 5 μm resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by the magneto-optical technique. The critical current density jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between jc, Tc, ΔTc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and jc. The profile of jc follows the Tc profile on large length scales and has an additional fine structure on short scales. The profile of jc along the bridge normalized to its value at any point is almost independent of temperature.
Quantum theory for the temperature dependences of magnetic configurations in magnetic ultrathin films
Authors: Hu Liangbin , Tao Ruibao
Published: July 2002

A lattice model and a corresponding quantum mean field approach are proposed to describe the temperature dependences of magnetic configurations in magnetic ultrathin films. This lattice model takes into account the layer-by-layer variations of the local microscopic magnetic parameters such as the exchange coupling constants and the magnetic anisotropy constants. A set of self-consistent equations are derived for the thermal averaged magnitudes and orientation angles of magnetic moments in each layer. The competing effects of the intralayer and interlayer exchange couplings, the different local magnetic anisotropies in different layers, and the long range magnetic dipole–dipole interactions, are studied within the mean field approximation. The theoretical results are in qualitative agreement with experimental observations. The general formulas established can also be applied to magnetic multilayers.
Determination of homogeneity of pure and doped lithium niobate by second harmonic generation temperature phase matching
Authors: Horst Chr. a. d. , Kasemir K.-U. , Betzler K.
Published: July 2002

Employing a comprehensive Sellmeier equation for the refractive indices, we derive calibration curves to be used for the homogeneity characterization of lithium niobate. Applying the calculated results, quantitative homogeneity parameters can be derived from temperature tuning curves for noncritical second harmonic generation, which are usually measured to state the composition of the material. The calibration method is of interest and presently is being used for pure as well as Mg-, Zn-, and In-doped crystals in the whole concentration regime.
Effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.5−xBxCu1Nb3 (x=7,9,16) alloys along nanocrystallization
Authors: Franco V. , Conde C. F. , Conde A.
Published: July 2002

The effect of the Si/B ratio on the magnetic anisotropy distribution of Fe73.5Si22.5−xBxCu1Nb3 (x=7,9,16) alloys has been studied. The influence of isochronal annealing on the hysteresis loop of the three studied alloys has been analyzed. They present two minima in coercivity: the first one can be ascribed to structural relaxation, and the second one is related to the averaging of the magnetocrystalline anisotropy, as predicted by the random anisotropy model. The relative importance of both minima changes with Si content: the lower the Si content, the more effective the structural relaxation and the less important the second minimum are. The mean value of the magnetic anisotropy distribution presents a similar behavior, evidencing the growing importance of the magnetoelastic anisotropy for the relaxed amorphous samples as the Si content is increased. From the evolution of the magnetic anisotropy distribution along nanocrystallization and the microstructural information obtained from transmission electron microscopy images, the behavior of the coercivity minima with changes in Si content can be ascribed to a different degree in compensation of magnetoelastic anisotropy due to the contributions of different signs coming from the nanocrystals and the amorphous matrix.
Application of photoreflectance spectroscopy to the study of interface roughness in InGaAs/InAlAs heterointerfaces
Authors: Bru-Chevallier Catherine , Baltagi Youssef , Guillot Gérard , Hong Kyushik , Pavlidis Dimitris
Published: July 2002

The achievement of high electrical performance InAlAs/InGaAs high-electron-mobility transistors (HEMTs) grown by metalorganic chemical vapor deposition, requires the growth of a good quality InGaAs channel/InAlAs spacer interface, in order to ensure good transport properties in the two-dimensional electron gas channel. In this article, the interface quality is evaluated as a function of growth interruption time, using photoreflectance spectroscopy. The experimental and theoretical approach used for this purpose is described. Layers representative of HEMT designs, namely 250 Å InGaAs single quantum wells between InAlAs layers were used for characterization. The interface roughness is estimated from the broadening of the high order quantum confined transitions. The results obtained suggest that the higher the growth interruption time, the smaller the interface roughness. Electrical characterization results indicate good agreement with the trends observed by photoreflectance.
Effect of ammonia flow rate on impurity incorporation and material properties of Si-doped GaN epitaxial films grown by reactive molecular beam epitaxy
Authors: Kim Wook , Botchkarev A. E. , Morkoç H. , Fang Z.-Q. , Look D. C. , Smith David J.
Published: July 2002

Effect of ammonia flow rate on the impurity incorporation and material properties of Si-doped GaN films grown by reactive molecular beam epitaxy (RMBE) process is discussed. It appears that the ammonia flow rate has a marginal effect on the incorporation of impurities into the Si-doped GaN films except there was a little decrease in O and Si with increasing ammonia flow rate when the Si concentration in the film is higher than 1018 cm−3. Electron Hall mobility of Si-doped GaN films grown by RMBE varies with ammonia flow rate used during film growth. From deep level transient spectroscopy (DLTS) measurements for Schottky diodes grown with different ammonia flow rates, one deep trap (C1) particular to the RMBE films was found. The concentration of C1 trap was found to be the lowest in the sample grown with the condition leading to the highest electron Hall mobility within the scope of this experiment. In addition to the DLTS result, other characterization techniques used (x-ray diffraction, cross-sectional transmission electron microscopy, and low-temperature photoluminescence) also consistently show that the RMBE process requires certain value of ammonia flow rate (or V/III ratio if the Ga flux is fixed) to produce Si-doped GaN films with high quality.
W deposition and titanium fluoride formation during WF6 reduction by Ti: Reaction path and mechanisms
Authors: Ramanath G. , Greene J. E. , Carlsson J. R. A. , Allen L. H. , Hornback V. C. , Allman D. J.
Published: July 2002

Reaction of WF6 with air-exposed 27- and 250-nm-thick Ti films has been studied using Rutherford backscattering spectroscopy, scanning and high-resolution transmission electron microscopy, electron and x-ray diffraction, and x-ray photoelectron spectroscopy. We show that W nucleates and grows rapidly at localized sites on Ti during short WF6 exposures (≈6 s) at 445 °C at low partial pressures pWF6<0.2 Torr. Large amounts of F, up to ≈2.0×1017 atoms/cm2 corresponding to an average F/Ti ratio of 1.5 in a 27-nm-thick Ti layer, penetrate the Ti film, forming a solid solution and nonvolatile TiF3. The large stresses developed due to volume expansion during fluorination of the Ti layer result in local delamination at the W/Ti and the Ti/SiO2 interfaces at low and high WF6 exposures, respectively. WF6 exposure at pWF6>0.35 results in the formation of a network of elongated microcracks in the W film which allow WF6 to diffuse through and attack the underlying Ti, consuming the 27-nm-thick Ti film through the evolution of gaseous TiF4.
Ultrafast shock-induced orientation of polycrystalline films: Applications to high explosives
Authors: Franken Jens , Hambir Selezion A. , Dlott Dana D.
Published: July 2002

Tiny laser-driven shock waves of ∼5 GPa pressure (nanoshocks) are used to study fast mechanical processes occurring in a thin layer of polycrystalline insensitive energetic material, (3-nitro-1,2,4-triazol-5-one) (NTO). Ultrafast coherent Raman spectroscopy of shocked NTO shows the existence of three distinct mechanical processes. Very fast (∼600 ps) changes in intensity and the appearance of new transitions are associated with the uniaxial nature of compression by the shock front. Frequency shifting and broadening processes which track the ∼2 ns duration nanoshock are associated with transient changes in density and temperature. A novel slower process (5–10 ns) starts as the shock begins to unload, and continues for several nanoseconds after the shock is over, resulting in changes of widths and intensities of several vibrational transitions. By comparing ultrafast spectra to static Raman spectra of single NTO crystals in various orientations, it is concluded that this process involves shock-induced partial orientation of the crystals in the NTO layer. The NTO crystals are oriented faster than the time scale for initiating chemical reactions. The sensitivity of explosive crystals to shock initiation may depend dramatically on the orientation of the crystal relative to the direction of shock propagation, so the implications of fast shock-induced orientation for energetic materials initiation are discussed briefly.
Comparative study of analytical methods for Hugoniot curves of porous materials
Authors: Boshoff-Mostert L. , Viljoen H. J.
Published: July 2002

This study compares five methods to compute Hugoniots for porous materials, with experimental data for porous copper. The methods compare well when porosities are low. Three methods model shock velocity versus particle velocity data over a range of porosities with reasonable accuracy. P–v data are much more discriminatory and only two methods reproduce experimental data at high porosities with some degree of accuracy. Comparisons are made by plotting analytical results on curves fitted through experimental data points. Above certain porosities Hugoniot curves display anomalous behavior where (dP/dv)>0. A new method is presented which contains only two parameters {KT, and [(∂KT/∂P)]T} and which models both normal and anomalous Hugoniots curves.
Determination of the Ce+3 bonding in the Ca0.5Sr0.5Ga2S4:Ce phosphor via x-ray photoelectron spectroscopy
Authors: Rack Philip D. , O’Brien Ted A. , Zerner Michael C. , Holloway Paul H.
Published: July 2002

The addition of oxygen in Ca0.5Sr0.5Ga2S4:Ce phosphor was found to nearly double the luminance and to blue shift the emission spectrum of the Ce+3 4f–5d transition. X-ray photoelectron spectroscopy (XPS) analysis was used to investigate the local chemistry of the Ce+3 luminescent center in the undoped and oxygen-doped Ca0.5Sr0.5Ga2S4 phosphor. The shake up structure in the Ce+3 3d3/2 and 3d5/2 XPS spectra is due to a ligand-to-metal charge transfer process, and the relative intensity of the charge transfer peaks increased when oxygen was introduced. Because the charge transfer probability is controlled by the local ionicity of the Ce bonds, the increase in the charge transfer peak intensity is consistent with the formation of Ce–O bonds. Furthermore, the binding energy of the charge transfer peaks and the Ce MNN Auger peak shifted, again consistent with oxygen bonding to the Ce+3 luminescent center. A self-consistent field configuration interaction model is used to calculate the positions of the 4f and 5d levels with Ce–S and Ce–O bonds. By replacing the three of eight sulfur ligands with oxygen the SCF/CI model predicted a 1035 cm−1 blue shift in the 4f–5d transition energy which was in excellent agreement with the observed 730 cm−1 blue shift.
Secondary ion mass spectroscopy study of Al diffusion in sandwich-ZnSTe structures grown by molecular beam epitaxy
Authors: Ma Z. H. , Smith T. , Sou I. K.
Published: July 2002

Sandwiched structures that consist of ZnSTe/ZnSTe:Al/ZnSTe were fabricated by the molecular beam epitaxy technique on three GaAs substrates oriented along (100), (511), and (711), respectively, to study the thermal diffusion of Al dopant in ZnS0.986Te0.014 matrix by secondary ion mass spectroscopy depth profiling. The relative sensitivity factor of Al respect to Zn was determined to be 4.5±0.5×1019 cm−3. The Al diffusion coefficients at annealing temperature of 450 and 550 °C were found to be dependent on Al concentration. The upper and lower limits of the diffusion coefficients were obtained through a data fitting program based on Fick’s second law, the results suggest that the diffusion is anisotropic possibly due to channeling effect. The results also reveal that Al in the matrix is thermally stable at temperature as high as 300 °C and thus prove that Al is a good candidate for the n-type doping of ZnSTe alloy system in terms of thermal stability.
Bond length variation in In0.25Ga0.75As/InP epitaxial layers thicker than the critical thickness
Authors: Tormen M. , De Salvador D. , Natali M. , Drigo A. , Romanato F. , Rossetto G. , Boscherini F. , Mobilio S.
Published: July 2002

We address the issue of the local structure in an epitaxial semiconductor thin film undergoing strain relaxation due to extended defects when the critical thickness for their introduction is exceeded. The nearest neighbor environment is probed by x-ray absorption fine structure spectroscopy. The particular system studied is a set of In0.25Ga0.75As films grown on InP(001) of increasing thickness; the thicknesses were chosen so as to obtain a varying degree of relaxation, ranging from pseudomorphic growth to completely relaxed state. The samples have been thoroughly characterized with complementary structural techniques and the residual strain is measured by x-ray diffraction. We find that the Ga–As bond length exhibits a linear decrease with decreasing residual strain. By comparing these results with previous studies on bond lengths in pseudomorphic InxGa1−xAs films as a function of concentration we conclude that the bond lengths have an identical behavior as a function of the mean residual strain independently from its elastic or plastic origin. This result is reproduced by an analytical model based on the transfer of the mean macroscopic deformation at a local level. The broadening of the bond length distribution induced by extended defects is also discussed, concluding that it is not experimentally dectable.
Optical and structural properties of strained InAlAs/InAsxP1−x multi-quantum wells grown by solid source molecular beam epitaxy
Authors: Kim T.-H. , Brown A. S. , Metzger R. A.
Published: July 2002

We report on the optical and structural properties of strained InAlAs/InAsxP1−x heterostructures grown by solid source molecular beam epitaxy. The incorporation of As4 into strained InAsP has been studied using x-ray diffraction and photoluminscence measurements. We find that the incorporation of As decreases as x in the InAsxP1−x increases due, presumably, to increased strain. In addition, we have examined the effect of interface growth conditions and barrier material on the quantum well properties. The 4.2 K photoluminescence spectra of InAlAs/InAsP multi-quantum wells grown with different As and P flux switching conditions at the interfaces were compared with that of higher quality InP/InAsP multi-quantum wells. This comparison shows that the interface quality of InAlAs/InAsP multi-quantum wells is highly dependent on the switching conditions at the interfaces, especially the exposure time of InAsP surface to an As4 overpressure. This sensitivity is due to the As–P exchange reaction at the interfaces of InAlAs/InAsP.
Electrolytic conduction of silver metaphosphate glass until dielectric breakdown
Authors: Doi A. , Takano H. , Takaki H.
Published: July 2002

Ac/dc bias was applied to gold- or silver-electroded AgPO3 glass samples at ∼150 °C. Dielectric breakdown took place with bias fields of ∼700 V/cm. Depth profiles of silver, gold, oxygen, and phosphorus were measured from the electrode surfaces by x-ray photoelectron spectroscopy. Electrolytic conduction of silver and others and field-induced injection of gold ions into glass from the gold anode were found.
Broad band p-Ge optical amplifier of terahertz radiation
Authors: Muravjov A. V. , Withers S. H. , Pavlov S. G. , Shastin V. N. , Peale R. E.
Published: July 2002

A solid state broad band amplifier of terahertz radiation (1.5–4 THz), based on intersubband transitions of hot holes in p-Ge is demonstrated. The gain is investigated as a function of applied magnetic and electric fields by transmission measurements using a laser system with two p-Ge active crystals, when one operates as an oscillator and one as an amplifier. A peak gain higher than usually reported for p-Ge lasers has been achieved using time separated excitation of the oscillator and amplifier. Distinct differences in gain dependence on applied fields are noted between low- and high-frequency modes of p-Ge laser operation.
Leakage currents in high-quality pulsed-laser deposited aluminum nitride on 6H silicon carbide from 25 to 450 °C
Authors: Scozzie C. J. , Lelis A. J. , McLean F. B. , Vispute R. D. , Choopun S. , Patel A. , Sharma R. P. , Venkatesan T.
Published: July 2002

The high-temperature dielectric properties of thin-film AlN that were pulsed-laser deposited on a heavily doped n-type 6H–SiC substrate are investigated from 25 to 450 °C. Capacitor leakage current densities of low 10−8 A/cm2 at 25 °C and mid 10−3 A/cm2 at 450 °C are reported for a 1.7 MV/cm dielectric field. The primary high-temperature leakage mechanism appears to be Schottky emission with a zero-field barrier height of 1.76 eV. A dielectric strength in excess of 1.7 MV/cm at 450 °C is reported.
Scanning tunneling microscope assisted nanostructure formation: Two excitation mechanisms for precursor molecules
Authors: Lyubinetsky I. , Mezhenny S. , Choyke W. J. , Yates J. T.
Published: July 2002

The scanning tunneling microscope in a near-field emission mode has been employed to create nanostructures using the hexafluoroacetylacetonate Cu (I) vinyltrimethylsilane precursor molecule on the Si(111) surface at 300 K. Two distinctive mechanisms controlling the nanostructure formation have been delineated. The first process involves excitation of the molecule by the applied electric field, and the field induced surface diffusion acts to supply molecules to the nanostructure growth region under the tip. The second mechanism involves the dissociation of the molecule by an electron attachment process. The generated nanostructure topology is quite different for each excitation mechanism. Narrow cone-like structures are produced by the electric field while broad structures of lower height are produced by the electron attachment process. Both mechanisms operate simultaneously in the low bias voltage regime (V<8 V), with the field activated process dominating. The electron induced process becomes the governing process at higher voltages.
Enhanced tunability in infrared photodetectors through optimization of the quantum-confined Stark effect
Authors: Gug R. K. , Hagston W. E.
Published: July 2002

A method is described which optimizes the quantum well (QW) shape in an AlxGa1−xAs/GaAs structure so that the quantum-confined Stark effect between the n=1–2 intersubband electron transition is maximized. The resulting structure, which is double QW like and asymmetric, will be referred to as an asymmetric coupled QW. Such structures maximize the tunability of a photodetector by utilizing the associated intersubband transition. However this is at the expense of the radiative transition rate. A systematic means of changing the optimized profile which simultaneously maximizes the photocurrent while increasing the radiative transition rate to acceptable values is then described.
Study on the piezoresistive effect of crystalline and polycrystalline diamond under uniaxial strains
Authors: Fang L. , Wang W. L. , Ding P. D. , Liao K. J. , Wang J.
Published: July 2002

The piezoresistive effect of p-type crystalline materials under uniaxial stress was analyzed by the deformation-potential theory and the valence-bands split-off model. The calculation formula of the gauge factor under the condition of the strain-induced separation of the heavy- and light-hole bands was obtained. It is found that the great difference of the effective mass between the heavy and light hole in diamond is one of the main factors which are responsible for the piezoresistive effect in the p-type diamond. The piezoresistive effect of the p-type polycrystalline material was also discussed from the Mayadas–Shatzkes model. The results showed that the piezoresistive effect of polycrystalline materials is related to both the valence-bands split off and the grain-boundary scattering. The grain-boundary scattering played a role of decreasing the piezoresistive effect, which made the polycrystalline materials have a smaller gauge factor than the crystalline materials.
Femtosecond studies of coherent acoustic phonons in gold nanoparticles embedded in TiO2 thin films
Authors: Qian W. , Lin L. , Deng Y. J. , Xia Z. J. , Zou Y. H. , Wong G. K. L.
Published: July 2002

0.15 THz coherent acoustic phonons have been observed in gold nanoparticles embedded in TiO2 thin films by virtue of femtosecond time-resolved transmissivity measurements at room temperature. The generation of these phonons is attributed to the resonant excitation of localized surface plasmon in gold nanoparticles. Their damping time is 55 ps which is acquired by fitting the oscillatory part of time-resolved transmissivity to single exponentially damped cosine function.
Magneto-optical properties of Al-substituted erbium iron garnet single crystals
Authors: Ostoréro J.
Published: July 2002

The magneto-optical (MO) properties of Er3Fe5−zAlzO12 (z=0.35 and 0.55) single crystals, where Al3+ ions show a large preferentially tetrahedral Fe3+ replacement, are presented and compared to those of pure ErIG and octahedrally substituted ErIG:Sc compounds. Faraday rotation (FR) Φ measurements at 1152 nm wavelength were performed in the 6–600 K temperature range under a magnetic field up to 20 kOe applied parallel to the [100] and [111] crystallographic axes of the “as-grown” crystals. For both Al concentrations, within experimental accuracy, the measured anisotropy of the spontaneous FR is weak (less than 10%) or absent. Sample with z=0.35 is nearly isotropic. In contrast to ErIG:Sc, the FR susceptibility χF(T)=dΦ/dH deduced from the saturated part of the different FR curves shows no anisotropy. The analysis of the MO properties of the compounds is performed using the one ion model and assuming the total Fe3+ MO contribution is represented by the magnetooptical measurements performed on YIG:Al single crystals of same composition. The temperature variation of the Er3+ FR contribution ΦEr shows that the FR due to magnetic dipole transitions compensates the electric dipole contribution when T/TNéel∼0.13, and is not affected by diamagnetic tetrahedral (Al3+) or octahedral (Sc3+) Fe3+ substitutions. The magneto-optical electric dipole coefficient Ce of the Er sublattice is also shown to be a function of temperature.
Magnetic properties of as-deposited Fe–Al–O alloy films
Authors: Kim S. R. , Han S. H. , Kim H. J. , Kim K. H. , Kim J.
Published: July 2002

The magnetic and structural properties of as-sputtered nanocrystalline Fe–Al–O films, fabricated by a magnetron sputtering apparatus, are investigated as a function of the sputtering input power and the contents of Al and oxygen. A nanocrystalline Fe88.5Al3.5O8.0 film is found to have 4πMs of 18.2 kG, Hc of 0.6 Oe, μeff of 4600 up to 100 MHz. These excellent soft magnetic properties and high frequency characteristics result from nanocrystalline structure, high electrical resistivity, and moderate anisotropy field. These values are sufficient to apply to a high-density recording head.
Proton spin-lattice relaxation at low temperature in the ferromagnetic spin ring Cu6
Authors: Furukawa Y. , Lascialfari A. , Jang Z. H. , Borsa F.
Published: July 2002

We report H1 nuclear magnetic resonance spin-lattice relaxation rate (NSLR) measurements as a function of temperature (1.5–4.2 K) and as a function of applied magnetic field (0.2–8.2 T) in the molecular magnet [(PhSiO2)6Cu6(O2SiPh)6] in short Cu6. The results are explained in terms of a simple model whereby the NSLR is driven by the fluctuations of the local hyperfine field due to the reorientation of the total spin of the molecule in its ground state. From the analysis of the data, we infer the temperature and field dependence of the characteristic rate of the fluctuations of the total magnetization of the Cu6 ring in its ground state.
Design optimization of axially laminated rotors for synchronous reluctance motors
Authors: Wakao S. , Nishimura Y. , Ando H. , Onuki T.
Published: July 2002

This article describes a design optimization of an axially laminated rotor of the synchronous reluctance motor, using the optimization method combined with the magnetic field analysis. The magnetic flux distribution in the synchronous reluctance motor is distorted due to the complicated rotor configuration and the influence of magnetic saturation, which results in a large number of local minimum solutions. Therefore, taking account of the magnetic saturation by the finite element method, the authors propose a novel optimization approach by coupling the stochastic search method with the deterministic one. In the optimization process a suitable objective function for the stability of machine performances is also proposed. Finally some numerical results that demonstrate the validity of the proposed approach are presented.
Magnetically transduced surface acoustic wave devices
Authors: Ellis K. A. , van Dover R. B. , Klemmer T. J. , Alers G. B.
Published: July 2002

A type of surface acoustic wave (SAW) device is introduced in which coupling between the transducer and the acoustic medium is achieved using a magnetostrictive thin film. These magnetically transduced SAW devices have several advantages over conventional piezoelectric SAWs, and may expand the functionality of SAW devices in general. Several possible designs are presented, along with data from two prototypes. Results from these devices are promising, and they indicate that several acoustic modes, both surface and bulk, can be stimulated.
Microstructure and noise characteristics of Co-Cr-Ta films on ultraflat glass substrates for longitudinal recording disks
Authors: Noda Kohki , Kadokura Sadao , Naoe Masahiko
Published: July 2002

Co–Cr–Ta/Cr bilayered films for longitudinal recording disks were deposited by facing targets sputtering (FTS) on 2.5 in. and ultraflat disk substrates of durable single-crystal silicon at temperature Ts of 100 °C and Ar pressure PAr of 0.2 mTorr. TEM observation of the films revealed ultrafine microstructures, and grain boundaries were unclear. The noise and recording characteristics of those disks were comparable with those of very high-performance disks with Co–Cr–Pt films, with coercivity Hc of 2.4 kOe, employed as a reference, even though the Co–Cr–Ta films exhibited macroscopic Hc of only 800 Oe. In this study, Co85Cr13Ta2/Cr bilayered films with the same composition as the above-mentioned disks were deposited by plasma-enhanced (PEFTS) on 2.5 in. and glass–ceramic substrates with ultraflat surface and tough durability at Ts of 100 °C to obtain better microstructure, signal-to-noise ratio (SNR), and surface flatness, with consequent lower flying height, than those on Si substrates, thus making it possible to fabricate ultrahigh-density recording disks. The PEFTS apparatus has several outstanding advantages such as plasma-free substrates, low background pressure of 5×10−7 Torr, low PAr of 0.1 mTorr, and higher mobility of the adatoms than in conventional FTS because of the proximity of the substrate to the central axis of the targets, indicating uniform plasma distribution, with up to six target units in a vacuum chamber. The film’s surface roughness Ra of 0.7 nm and the high mobility of the adatoms result in narrow grain boundaries without pores and inclusions, which were observed by TEM, and better overwrite performance than that of Si substrates and the same SNR as Si substrates, which were measured with a read/write test stand. These results will allow practical application in ultrahigh-density recording systems, including 1 in. microdrives.
Ferromagnet–semiconductor device with tunable tunnel characteristics
Authors: Heide C.
Published: July 2002

A device is proposed consisting of a GaAs/AlxGa1−xAs heterostructure with a ferromagnetic strip placed on top that has an easy axis of magnetization perpendicular to the underlying two-dimensional electron gas. In addition, the strip is gated to form a combined magnetostatic and electrostatic barrier for the conduction electrons. On the basis of a simple model, which is of the Landauer–Büttiker type, such a structure is shown to have three different regimes of operation. Whereas for a certain regime, electrons can move according to the classical diamagnetic motion, there is also the possibility of tuning the device between nonresonant and resonant tunneling behavior. In the latter case the combined magnetostatic and electrostatic barrier acts as an energy and momentum filter. The proposed device could find application in digital logic circuits as an electromagnetic field-effect transistor.
Domain flexibility and pseudospin valve memory cell switching thresholds
Authors: Pohm A. V. , Anderon J. M. , Beech R. S. , Daughton J. M.
Published: July 2002

An analysis shows that for pseudospin valve elements with smooth edges, the sense field becomes ineffective in reducing the switching threshold for the thin layer in the body of the element if the element is wider than about 0.35 μm because of domain flexibility. Experimental results at 1.2 μm element width agree with the analysis. The sense field can lower the thresholds in wider elements if the switching is initiated in the tapered ends or in rough edges.
Corrosion-free dry etch patterning of magnetic random access memory stacks: Effects of ultraviolet illumination
Authors: Cho H. , Lee K.-P. , Jung K. B. , Pearton S. J. , Marburger J. , Sharifi F. , Hahn Y. B. , Childress J. R.
Published: July 2002

Individual layers of Ni0.8Fe0.2, Ni0.8Fe0.13Co0.07, TaN, and CrSi, and a full magnetic random access memory stack consisting of NiFeCo/CoFe/Cu/CoFe/NiFeCo/TaN/CrSi multilayers, were etched in high density Cl2/Ar plasmas either with or without concurrent ultraviolet (UV) illumination. Under optimized conditions, etch rate enhancement up to a factor of 5 was obtained for NiFeCo, TaN, and CrSi with UV illumination; whereas the etch rate of NiFe was retarded. Post-etch cleaning with H2 or SF6 plasmas or H2O rinsing was necessary in all cases in order to prevent corrosion of the metal layers from chlorinated etch residues.
d=3 random field behavior near percolation
Authors: Montenegro F. C. , Belanger D. P. , Slanič Z. , Fernandez-Baca J. A.
Published: July 2002

The highly diluted antiferromagnet Mn0.35Zn0.65F2 has been investigated by neutron scattering for H>0. A low-temperature (T<11 K), low-field (H<1 T) pseudophase transition boundary separates a partially antiferromagnetically ordered phase from the paramagnetic one. For 1<H<7 T at low temperatures, a region of antiferromagnetic order is field induced but is not enclosed within a transition boundary.
Strong unidirectional anisotropy in spin-density-wave (Cr+6.5%Co)1−xVx alloys
Authors: Galkin V. Yu. , Ortiz W. A. , Fawcett E.
Published: July 2002

The magnetization, M(T), and magnetic susceptibility, χ(T) = M(T)/H, of the (Cr+6.5% Co)1−xVx (0⩽×⩽5% V) spin-density-wave (SDW) alloy system, have been measured in the presence of a magnetic field, H=100 Oe–5 T, in the field-cooled and zero field-cooled states, over the temperature range, 2⩽T⩽400 K. The thermal hysteresis corresponds to an irreversibility temperature Ti above the Néel temperature TN. M(H) measured at T=2 K in the FC (H=5 T) state exhibits magnetic hysteresis corresponding to a strong unidirectional anisotropy. The remanent magnetization MRem(T) decreases rapidly with increasing temperature, disappearing at Ti. The behavior is explained in terms of spin frustration of Cr moments close to the Co impurity atoms, with a component also due to Co clusters, whose moments are frozen in the SDW phase.
Acceleration of LaBonte’s iteration by multigrid method
Authors: Hirano Satoshi , Hayashi Nobuo
Published: July 2002

A fast numerical method is developed in which the LaBonte method is accelerated using one of the multigrid methods, the V-cycle method. The developed method is based on the previously derived equilibrium equation which has a form amenable to implementation in the multigrid schemes. The basic property of the derived method is presented together with a summary of derivation and the results of calculations which demonstrate the effectiveness of the method.
Structural and magnetic properties of the exchange-coupled PrFeB alloys
Authors: Murakami R. K. , Villas-Boas V. , Chen M. , Yelon W. B.
Published: July 2002

Ribbons of the Pr4.5Fe77B18.5 alloys were produced by melt spinning and then annealed by flash or conventional annealing to develop enhanced-remanence nanocrystalline magnetic material. These materials were studied by magnetic measurements, x-ray, and neutron diffraction. Curie temperature measurements show that the phases present are Pr2Fe14,B Fe3B, α-Fe, and Pr2Fe23B3. Rietveld analyses of the neutron and the x-ray diffraction spectra showed that the main phases of these materials were Pr2Fe14B and Fe3B, Pr2Fe14B being the major phase (≈60%). Differences between the values of the lattice parameters of Pr2Fe14B in these materials may indicate that this phase is not stoichiometric.
Spin wave excitations in Fe/Cu multilayers as a function of its parameters
Authors: Kuprin A. P. , Cheng L. , Altounian Z. , Ryan D. H.
Published: July 2002

Correlation of low-temperature magnetic properties with morphology of (Cu(111)/Fe)×36/Cu multilayers prepared by dc-magnetron sputtering with tCu=38.5 Å, 21.0 Å, and tFe=7.3 Å, 8.5 Å, respectively, was investigated. The primary methods used were Fe57 low-temperature conversion electron Mössbauer spectroscopy (LT–CEMS) from RT down to 50 K and measurements of magnetization from RT down to 5 K. The Fe layers were found to be discontinuous at tFe=7.3 Å, but continuous at tFe=8.5 Å. Transition to the continuity of the Fe layers enhances both in-plane shape and perpendicular surface magnetic anisotropies. This is reflected in the change of the behavior of the temperature dependencies of spontaneous magnetization σs measured with external field oriented parallel and perpendicular to the multilayer plane.