bluuebunny/crossref_metadata_2025 huggingface dataset:
Sulfur diffussion into Ga1−xAlxAs with various Al content
Authors: Nishi Hiroshi , Horikoshi Yoshiji , Ito Hiromasa
Published: July 2002

This communication describes the diffusion of sulfur into Ga1−xAlxAs layers having various amounts of Al content. GaS and As in a weight ratio of 2:1 were used as the diffusion source. The GaS diffusion source was carefully selected via x-ray diffraction analysis. Sulfur diffusion was carried out in a sealed quartz ampoule at a temperature of 820 °C and in ambient arsenic at a vapor pressure of 1 atm. Ga1−xAlxAs layers of various x values were grown by liquid phase epitaxy. The Al content indicated by x was in the range of 0–0.3. For x values of 0, 0.08, 0.2, and 0.3, the surface concentrations and diffusion coefficients were found to be 1.0×1018, 3.8×1017, 2.0×1017, and 1.4×1011 cm−3, and 3.5×10−13, 2.0×10−12, 3.5×10−12, and 5.5×10−12 cm2/s, respectively. The carrier concentration profiles from the experimental results agree well with the theoretical profiles, which were calculated with the complementary error function solution. The activation energies of the diffusion coefficient in GaAs and Ga0.7Al0.3As were found to be about 4.7 and 2.5 eV, respectively. A smooth diffusion of sulfur was observed to occur without causing any surface damage.
Transmission electron energy-loss spectroscopy measurements of the dielectric function of Si/SiO2 multilayers
Authors: Keränen J. , Lepistö T. , Ryen L. , Novikov S. V. , Olsson E.
Published: July 2002

High resolution analytical transmission electron microscopy was used to characterize the structural features of light emitting Si/SiO2 multilayers. Electron energy-loss spectra were recorded in the low-loss region 0–40 eV, across Si/SiO2 interfaces in cross-sectional specimens. The Si/SiO2 multilayer had a major volume plasmon peak at 17.5 eV and a broad feature ranging from 7 to 13 eV. This energy range could be deconvoluted into two different peaks located at ∼9 and 12 eV. These plasmon peaks could in turn be related to the silicon nanocrystallites and surface plasmon oscillations at the interface between the nanometer size silicon and SiO2 layers, respectively. Complex dielectric function was extracted from the experimental single-scattering distribution using Kramers–Kronig analysis. This showed the existence of an optical band gap at 1.5 eV. The first resonance peak in the imaginary part of the complex dielectric constant was seen at 1.9 eV. This resonance peak is generally related to the structural short range ordering and quantum confinement in a Si/SiO2 multilayers.
Mössbauer effect determination of relative recoilless fractions for iron oxides
Authors: Oh Sei J. , Cook D. C.
Published: July 2002

The relative recoilless fraction (F-value) of each of six iron oxides, defined as the ratio of the recoil-free fractions of two different materials, was experimentally determined relative to hematite at 300 K and 77 K by Mössbauer spectroscopy. Using the relative recoil-free fractions compared to that of hematite, the relative recoilless fractions between all pairs of the seven iron oxides were determined. The F-values can allow conversion of Mössbauer subspectral areas to the relative atomic, molecular, or weight fractions of each iron oxide present in a mixed oxide phase sample.
Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells
Authors: Nadenau V. , Hariskos D. , Schock H.-W. , Krejci M. , Haug F.-J. , Tiwari A. N. , Zogg H. , Kostorz G.
Published: July 2002

The microstructure of the CdS/CuGaSe2 interface region in Cu-rich CuGaSe2-based polycrystalline thin film solar cells with KCN-treated absorber layers are characterized. Two recipes for the chemical bath deposition (CBD) of CdS with different bath temperatures (60 and 80 °C) are compared. Coherent Cu–Se precipitates are observed in both cases in the grains of the absorber layer. This precipitation cannot be avoided and seems to be a principal limitation for the performance of Cu-rich CuGaSe2-based thin film solar cells. There is a significant difference between both recipes concerning the interaction with the absorber layer surface. For bath temperatures of 80 °C the interaction is much stronger and Cu–S inclusions are found in the buffer layer. These may be responsible for shunts across the pn junction. Owing to the reduced interaction of the CdS deposited at 60 °C there are no Cu–S inclusions. For the 80 °C recipe the CdS/CuGaSe2 interface region consists of a continuous transition zone with low defect density, whereas for the 60 °C recipe the interface is sharper, but the CdS layer contains a high density of stacking faults. The structure of the CdS layer depends also on the bath temperature and the growth orientation of the CuGaSe2 grains. CdS(80 °C) crystallizes predominantly in the zincblende structure and contains less linear and planar defects than CdS(60 °C) which tends to incorporate hexagonal regions in the cubic matrix. Strains due to lattice mismatch as well as mixture between wurtzite and zincblende structures were revealed in high resolution transmission electron microscopy (HRTEM) images of the CdS(60 °C) layer. For CdS(80 °C) the strain is relaxed by twinning and small-angle grain boundaries which were imaged by HRTEM. A suitable CdS buffer layer for Cu-rich absorber layers could not be obtained by CBD because of either the low crystal quality [CdS(60 °C)] or the formation of Cu–S inclusions [CdS(80 °C)]. The enhanced interaction with the Ga-rich absorber layer and improved quality of CdS(80 °C) results in an improved device performance of Ga-rich CuGaSe2-based solar cells.
Influence of anisotropic surface diffusivity on electromigration induced void migration and evolution
Authors: Fridline D. R. , Bower A. F.
Published: July 2002

Microelectronic circuits often fail because of voids and cracks that cause open circuits in their interconnects. The voids form due to severe thermal stresses and high current densities present in the lines during service. Typical interconnect lines contain large numbers of voids which gradually grow and change their shape during the life of the line. It has been observed that many open circuits occur when a round void collapses into a slit perpendicular to the direction of electric current flow. O. Kraft and E. Arzt [Acta Mater. 45, 1599 (1997)] have shown that these slits may be caused by electromigration induced surface diffusion. Anisotropy in the surface diffusivity plays a key role in the formation of slit voids. In this article, our objective is to present a detailed parametric study of electromigration induced void evolution in interconnects. We idealize an interconnect as a two-dimensional electrically conducting strip which contains an initially semi-circular void. We neglect the stresses for simplicity. A finite element method is used to predict the evolution of the void after an electric field is applied to the strip. It is shown that the void may: (i) Evolve to a stable shape as it migrates down the line; (ii) break up into several smaller voids or separate from the boundary; (iii) form a slit which severs the strip. The conditions necessary for each type of behavior are explored in detail. The implications on interconnect reliability are discussed.
Magnets with random anisotropy
Authors: Chui S. T.
Published: July 2002

We discuss the spin reversal in magnets with random anisotropy in the presence of the dipolar interaction. Past work by different groups is recapitulated. We point out the deficiencies of these works and propose a model to tie these ideas together that provides for an internally consistent framework to describe nanocomposite magnets where both the dipolar interaction and the random anisotropy cannot be ignored.
Magnetic structure of deformation-induced shear bands in amorphous Fe80B16Si4 observed by magnetic force microscopy
Authors: Brown G. W. , Hawley M. E. , Markiewicz D. J. , Spaepen F. , Barth E. P.
Published: July 2002

Processing-induced magnetic structures in amorphous metallic alloys are of interest because of their impact on the performance of materials used in electric device applications. Plastic deformation associated with cutting or bending the material to the desired shape occurs through the formation of shear bands. The stress associated with these shear bands induces magnetic domains that can lead to power losses through interaction with the fields and currents involved in normal device operation. These domains have been studied previously using a variety of techniques capable of imaging magnetic domain structures. In an effort to better characterize and understand these issues, we have applied atomic and magnetic force microscopy to these materials to provide three-dimensional nanometer-scale topographic resolution and micrometer-scale magnetic resolution.
Theory of magnetic structure determination for small ferromagnets
Authors: Preda Mihai , Barnes Stewart E.
Published: July 2002

An easy plane ferromagnetic nanoparticle with a magnetic field along the easy axis in the easy plane, is considered. When the applied field is reversed the system finds itself in a metastable ground state of a double well potential. Tunneling is between this state and the various states on the other, stable, side of the double well. Unlike the much studied case of Mn12, the states into which the system tunnels have energies determined by harmonic oscillations about equilibrium. Specific expressions are obtained for the tunneling matrix elements examined between the various levels and for relaxation toward the ground state. The formalism involves auxiliary particle techniques and the results are contrasted with the existing theory based upon the functional integral approach.
Temperature and field dependence of the spin wave gap in NdCu2
Authors: Kramp S. , Pyka N. M. , Loewenhaupt M. , Rotter M.
Published: July 2002

NdCu 2 shows a complex magnetic phase diagram below its Néel temperature of TN=6.5 K and for magnetic fields applied in the easy b direction. This complicated behavior is expected to be due to Ruderman–Kittel–Kasuya–Yoshida interaction of the 4f shells of the Nd ions in the presence of crystal field splitting. In order to obtain deeper insight into the mechanism leading to such different magnetic phases we started to study the spin wave spectra in a NdCu2 single crystal. In zero field as well as in fields applied in the easy b direction of magnetization, a pronounced minimum in the excitation spectrum has been observed and it forms an energy gap. The position of this spin wave gap does not coincide with any magnetic ordering wave vector in NdCu2, indicating anisotropic magnetic coupling. The temperature and field dependence of the spin wave gap have been studied. The position of the gap remains constant at qgap=(0.35, 0, 0), whereas the character of the excitations changes with temperature and the value of the energy gap changes with the magnetic field. Applying a field of 10 T in the hard c direction of magnetization changes the position of the minimum to qgap=(0.6, 0, 0) and gives proof of magnetic anisotropy.
Redistribution of Zr and Ga in Nd2Fe14B-based magnets during the hydrogen disproportionation desorption recombination process: An x-ray absorption fine structure study
Authors: Ashfaq A. , Matsuura M. , Ikuta N. , Sakurai M.
Published: July 2002

X-ray absorption fine structure (XAFS) measurements were carried out for the Ga K edge of Nd13.0Fe68.2Co10.8Ga1.0Zr0.1B6.9 and the Zr K edge of Nd16Fe75.5Zr0.5B8 magnets during the hydrogen disproprotionation desorption recombination process. XAFS results show that Zr occupies an Fe(j2) site for the as-cast state while Ga occupies an Fe(c2) site in the Nd2Fe14B structure for the homogenized state. Small changes of the Zr XAFS spectra after disproportionation suggest that locally the Nd2Fe14B phase including Zr at an Fe(j2) site is stabilized against disproportionation. Ga changes its occupancy from an Fe(c2) site in Nd2Fe14B structure for the homogenized state to the α-Fe phase for the disproportionated stage. Ga seems to play a different role in the enhancement of magnetic anisotropy than that of Zr.
The magnetic properties of Nd–Fe–B powders produced by mechanical grinding in hydrogen atmosphere
Authors: Saito Tetsuji
Published: July 2002

The structures and magnetic properties of Nd–Fe–B and Nd–Fe–Zr–B powders produced by mechanical grinding in hydrogen atmosphere were studied. These powders absorbed hydrogen and the Nd2Fe14B phase disproportionated into Nd–hydride and α-Fe during the mechanical grinding in hydrogen atmosphere. Heat treatment of the hydrogenated powders in vacuum resulted in desorption of the hydrogen gas and recombined with the Nd2Fe14B phase. The annealed powders showed high coercivity values, but were magnetically isotropic.
Polarized neutron diffraction studies of exchange-coupled Fe3O4/NiO superlattices
Authors: Borchers J. A. , Ijiri Y. , Lind D. M. , Ivanov P. G. , Erwin R. W. , Lee S. H. , Majkrzak C. F.
Published: July 2002

In order to understand the interplay between exchange coupling and magnetic structure, we have examined the magnetic ordering of a series of epitaxial Fe3O4/NiO superlattices using polarized neutron diffraction techniques. As expected, the net ferrimagnetic moment of the Fe3O4 layers aligns parallel to an applied magnetic field. The antiferromagnetic NiO spins order into alternating antiparallel 〈111〉 planes as in bulk, but the direction of the spins in the planes are determined by field preparation. The NiO moments tend to align perpendicular to the field. In addition, the relative population of the NiO domains varies as the field is raised. The changes in the antiferromagnetic spin order relative to bulk seem to result from magnetic coupling with the Fe3O4 moments.
Magnetic coupling in boron-rich NeFeB nanocomposites
Authors: Lu B. , Huang M. Q. , Chen Q. , Ma B. M. , Laughlin D. E.
Published: July 2002

In this article, we report types of magnetic domain patterns caused by either dipolar coupling or exchange coupling inside a heat-treated boron-rich NdFeB melt-spun ribbon. The average grain size of the sample was about 38 nm. Single particle domains were commonly observed in Fresnel images all through the specimen, suggesting little exchange coupling among the grains in these regions. Snake-shaped long interactive domains consisting of chains of grains were found in many Foucault images at the regions where exchange coupling is weak. These domains are caused by dipolar coupling along the chains. Wider domains consisting of many grains were also observed by Fresnel images, although they were much less common than the single particle domains. The grains inside a wider domain are believed to be exchange coupled with each other. The magnetic properties of such boron-rich NdFeB composites are influenced more by the dipolar coupling than by exchange coupling between the nanocrystalline grains.
The effect of Nb additions on the thermal stability of melt-spun Nd2Fe14B
Authors: Lewis L. H. , Gallagher K. , Panchanathan V.
Published: July 2002

Elevated-temperature superconducting quantum interference device (SQUID) magnetometry was performed on two samples of melt-spun and optimally annealed Nd2Fe14B; one sample contained 2.3 wt % Nb and one was Nb-free. Continuous full hysteresis loops were measured with a SQUID magnetometer at T=630 K, above the Curie temperature of the 2–14–1 phase, as a function of field (1 T⩽H⩽−1 T) and time on powdered samples sealed in quartz tubes at a vacuum of 10−6 Torr. The measured hysteresis signals were deconstructed into a high-field linear paramagnetic portion and a low-field ferromagnetic signal of unclear origin. While the saturation magnetization of the ferromagnetic signal from both samples grows with time, the signal from the Nb-containing sample is always smaller. The coercivity data are consistent with a constant impurity particle size in the Nb-containing sample and an increasing impurity particle size in the Nb-free sample. The paramagnetic susceptibility signal from the Nd2Fe14B-type phase in the Nb-free sample increases with time, while that from the Nb-containing sample remains constant. It is suggested that the presence of Nb actively suppresses the thermally induced formation of poorly crystallized Fe-rich regions that apparently exist in samples of both compositions.
Atomic structure of the amorphous state of TiC-modified Nd2Fe14B as revealed by positron annihilation spectroscopy
Authors: Nielsen B. , Lewis L. H. , Friessnegg T. , Ghosh V. J. , Kramer M. J. , McCallum R. W. , Dennis K.
Published: July 2002

Positron annihilation spectroscopy utilizing the Doppler broadening of the annihilation radiation (DBAR) method was performed on stoichiometric Nd2Fe14B ribbons containing 0, 2, and 6 wt % Ti–C that were melt spun to the x-ray amorphous state at a wheel speed of 40 m/s. DBAR data indicate that the as-spun samples are very homogeneous. It is found that an increase in the concentration of alloying additions promotes an increase in the shape parameter (“S parameter”) of the symmetric Doppler-broadened positron–electron annihilation peak. This increase in the S parameter may be interpreted as an increase in the amount of open volume in the quenched glass. The presence of increased open volume is concurrent with an increased stability of the glass against crystallization. However, at the present time it is not clear if the increased thermal stability is a direct product of the increased open volume, or if stereochemical considerations determine the stability of the glass.
Performance of NiO spin-valve tape heads for high recording densities
Authors: Oliveira N. J. , Li S. X. , Freitas P. P.
Published: July 2002

Shielded NiO biased spin-valve tape heads with permanent magnet stabilization are described. Heads with 5 and 9 μm trackwidths, shield-to-shield spacing of 0.43 and 0.53 μm, and height of 1 μm are characterized on DLT–IV tape (0.2 μm magnetic thickness, Hc=1850 Oe, Mr.δ=6.5 memu/cm2) up to 150 kfci. Maximum head output is ∼1 mVpp/μm (3–5 mA sense current), decaying to a maximum of 23 μVpp/μm at 150 kfci (the 40–150 kfci resolution is 5%), with signal loss with linear density of 0.34 dB/kfci. Distortion level is ∼−33 dB for second and third harmonics at high linear density. Harmonic distortion above −20 dB is observed for linear density under 60 kfci due to partial head saturation. Head output is compared with a linear model. This model is used to predict head performance with smaller gaps (calculated resolution increases to 35% with 500 Å half-gap, but head efficiency decreases) and with thinner, smoother media (calculated output level at 150 kfci increases by 13 dB, and resolution increases to 14%).
Effect of annealing on anisotropy field in Fe84Zr7B8Cu1 amorphous ribbons evaluated by giant magnetoimpedance
Authors: Yoon S. S. , Yu S. C. , Ryu G. H. , Kim C. G.
Published: July 2002

The magnetoimpedance (GMI) profiles were measured in Fe84Zr7B8Cu1 amorphous ribbons annealed at various temperatures. The anisotropy field evaluated from GMI profiles changed sensitively with annealing temperature. The decreasing Hk and increasing magnitude of GMI with the annealing temperature were ascribed for by the increase of magnetic softness, while the increasing Hk and decreasing magnitude of GMI for the further increment of annealing temperature was associated with the microstructural change due to the annealing.
Magnetic and electric properties of Mn5Ge3/Ge nanostructured films
Authors: Miyoshi T. , Matsui T. , Tsuda H. , Mabuchi H. , Morii K.
Published: July 2002

We have investigated the magnetic, electric, and structural properties of Mn5Ge3/Ge nanostructured films produced by solid-state reaction of Mn/Ge multilayered films. The films composed of strongly uniaxially oriented Mn5Ge3 and randomly oriented Ge were successfully produced. The average grain size of Mn5Ge3 considerably changed according to the discharging power (E) of the ion source: 15 nm for E=25 W and 50 nm for E=50 W. The temperature dependence of the conductivity for the E=25 sample showed semiconductor-type behavior over all the temperature region. Whereas the E=50 sample, the conduction type changed from a metallic type (low temperature) to a semiconductor type (high temperature) with the transition temperature TTR=360 K. We also observed the anomalous magnetic behavior of Mn5Ge3. We discuss these behaviors in conjunction with the microstructure of the annealed films. The possibility of the carrier–spin exchange interactions has also been studied.
Mössbauer and magnetization studies of La0.5Ca0.5Mn1−xFexO3
Authors: Kallias G. , Pissas M. , Devlin E. , Simopoulos A. , Niarchos D.
Published: July 2002

Fe 57 Mössbauer measurements and magnetization data were used to study both microscopically and macroscopically the first-order antiferromagnet-to-ferromagnet (AFM-FM) transition observed in La0.5Ca0.5MnO3. The zero field cooled and field cooled Mössbauer spectra for H⩾40 kOe are extremely history dependent and can be fitted by predominantly AFM and FM components, respectively, in agreement with the magnetization data. Our data are consistent with the concept of a first-order, field-induced transition.
Pyrochlore manganites spin dynamics in the paramagnetic regime
Authors: Causa M. T. , Alejandro G. , Tovar M. , Pagliuso P. G. , Rettori C. , Oseroff S. B. , Subramanian M. A.
Published: July 2002

We report electron spin resonance (ESR) and dc magnetic susceptibility in polycrystalline pyrochlores A2Mn2O7 (A=Y, Tl) measured in the paramagnetic phase up to 600 K. It is shown that the ESR linewidth has a universal behavior given by ΔHpp(T)=ΔHpp(T→∞)[C/Tχdc(T)], where C is the single-ion Curie constant. The high-temperature limit for the linewidth, given by ΔHpp(T→∞)∝(ωp)2/J, is determined by the superexchange constant J for each material. A value of (ωp/γ)=6300 G was found for all pyrochlores, including In2Mn2O7. Different broadening mechanisms are discussed.
Pressure effect on magneto-optical properties in CdTe/(Cd, Mn)Te single quantum wells with high Mn concentration
Authors: Yokoi Hiroyuki , Tozer Stanley W. , Kim Yongmin , Rickel Dwight , Kakudate Yozo , Usuba Shu , Fujiwara Shuzo , Takeyama Shojiro , Karczewski Grzegorz , Wojtowicz Tomesz , Kossut Jacek
Published: July 2002

The pressure effect on the magnetic field induced type I–type II transition is studied in a CdTe/Cd1−xMnxTe (x=0.24) single quantum well. Photoluminescence measurements under hydrostatic pressures up to 1.07 GPa and long pulsed magnetic fields up to 60 T with a pulse duration of 2 s are reported. The pressures were generated in a plastic diamond anvil cell. A bend toward lower energies (additional red shift) is observed above 28.5 T in magnetic field dependence of the exciton energy for a 13 Å thick quantum well. We attribute this red shift to a phenomenon preceding the type I–type II transition after a comparison with a simple quantum mechanical calculation. The onset field of the additional red shift increases by 3.4 T by applying a pressure of 1.07 GPa. Spin–spin coupling between the exciton and the Mn ion in the interface region is also investigated and found to be enhanced by pressure.
A new method for the synthesis of magnetoliposomes
Authors: Sangregorio Claudio , Wiemann Joan K. , O’Connor Charles J. , Rosenzweig Zeev
Published: July 2002

A new method for the synthesis of magnetoliposomes, i.e., nanosized magnetic particles coated by a phospholipid membrane, is presented. Magnetoliposomes are prepared by directly using the phospholipid vesicles as nanoreactors for the precipitation of the magnetic particles. The magnetoliposomes have been characterized using transmission electron microscopy imaging and x-ray powder diffraction. The magnetic properties of the magnetoliposomes have been investigated with a superconducting quantum interference device magnetometer. Our results indicate that the magnetoliposomes contain approximately spherical maghemite nanoparticles averaging 25 nm in diameter. The occurrence of a phospholipid bilayer surrounding the magnetic particles is confirmed both by transmission electron micrographs of samples negatively stained with uranyl acetate and by digital fluorescence imaging microscopy measurements of magnetoliposomes labeled with fluorescein. The temperature dependence of the zero field cooled and field cooled susceptibilities of the magnetoliposomes is consistent with their expected superparamagnetic nature.
Investigation of optically active defect clusters in KH2PO4 under laser photoexcitation
Authors: Demos S. G. , Staggs M. , Yan M. , Radousky H. B. , De Yoreo J. J.
Published: July 2002

Photoexcited defect clusters in the bulk of KH2PO4 crystals are investigated using a microscopic fluorescence imaging system with 1 μm spatial resolution. The observed defect cluster concentration is approximately 104–106 per mm3 depending on the crystal growth method and sector of the crystal. The intensity of the emission clusters varies widely within the image field while a nearly uniformly distributed background is present. Spectroscopic measurements provided information on the emission characteristics of the observed defect population.
Effect of chloride on the photoluminescence of ZnS:Mn thin films
Authors: Lu X. , Chen C. , Husurianto S. , Koretsky M. D.
Published: July 2002

ZnS:Mn thin films at various substrate temperature are grown by halide transport chemical vapor deposition. These films show blue and red photoluminescence (PL) in addition to the typical yellow-orange emission. The manganese crystal environment is characterized by electron spin resonance (ESR) spectroscopy. A computer simulation of the ESR spectra is used to quantify the number of isolated manganese and the number of clustered manganese in the crystal lattice. These data reveal that the red emission occurs in films with low manganese concentration, and, therefore, occurs from a mechanism different than those previously posed. The activation energy for Mn incorporation is measured to be Ea=137 kJ/mol. From these data, a Mn–Cl defect pair is proposed as the red emission center. Self-activated blue emission in intentionally Cl-doped ZnS films is also demonstrated. Thus both red and blue PL in ZnS thin films result from chloride impurities.
Effects of optothermal nonlinearities on photothermal beam deflection
Authors: Gruss C. , Bein B. K. , Pelzl J.
Published: July 2002

The usual theory of the photothermal probe beam deflection, which is normally used for the thermal characterization of solids, is based on the assumption that the optical refractive index of the air surrounding the heated sample behaves linearly with the temperature. Here we study the effects of the nonlinear variation of the refractive index with higher air temperatures in front of the sample. These optothermal nonlinearities become important, when the photothermal probe beam deflection is applied at higher temperatures to analyze the effects of thermal ablation and erosion of heated surfaces. Examples of measurements in front of a heated vertical plate are presented and compared with theory.
Acoustic emission and normal components in an alternating current superconducting coil
Authors: Arai K. , Yamaguchi H. , Umeda M. , Kaiho K. , Fuji H. , Sadakata N. , Saitoh T.
Published: July 2002

The relationship between acoustic emission (AE) signals and normal components that appeared in a superconducting winding in an alternating current superconducting coil cooled in liquid helium was investigated around the time of quench initiation. The coil was impregnated with epoxy to prevent wire motion of the superconducting winding. The operating current of 50 Hz at the coil was ramped from zero to a level that caused quenching of the coil, where external heat energy was not applied to the superconducting winding. AE envelopes were used for the investigation because the envelopes provide higher time resolution and higher dynamic range than the method of using AE counting rates that were conventionally used in monitoring direct current superconducting magnets and coils. AE envelopes appeared immediately after the appearance of normal components in the superconducting winding before and after quench initiation. In the cases before quench initiation, normal components appeared several tens of milliseconds earlier than quench initiation. The AE envelopes appeared momentarily immediately after the normal components. When the quench initiated, AE envelopes drastically increased in accordance with the high rate of increase of the normal components in the superconducting winding.
A deep level transient spectroscopy study of electron irradiation induced deep levels in p-type 6H–SiC
Authors: Gong M. , Fung S. , Beling C. D. , You Zhipu
Published: July 2002

1.7 MeV electron irradiation-induced deep levels in p-type 6H–SiC have been studied using deep level transient spectroscopy. Two deep hole traps are observed, which are located at EV+0.55 eV and EV+0.78 eV. They have been identified as two different defects because they have different thermal behaviors. These defects at EV+0.55 eV and EV+0.78 eV are annealed out at 500–200 °C, respectively, and are different from the main defects E1/E2, Z1/Z2 observed in electron irradiated n-type 6H–SiC. This indicates that new defects have been formed in p-type 6H–SiC during electron irradiation.
Dislocation dynamics near film edges and corners in silicon
Authors: Schwarz K. W. , Chidambarrao D.
Published: July 2002

The effect of film edge and corner stress fields on the behavior of dislocations in silicon is investigated. Stresses arising from a silicon–nitride film pad on a silicon substrate are calculated using the finite element method, and the resulting spatially varying stress–tensor components are utilized to drive numerical simulations of dislocation behavior. The dislocation dynamics code involves a full three-dimensional implementation of the Peach–Koehler force formalism. By studying the motion of dislocations on various slip systems in various locations relative to the nitride pad, we are able to determine the stationary dislocation configurations which can be achieved in this geometry. The zero resolved-stress contours near the silicon surface are shown to be a useful tool for understanding both the nature of the dislocation propagation as well as the final dislocation configurations. We examine the nucleation of dislocations qualitatively from the critical-radius point of view, and identify the “hot spots” for nucleation near the nitride film edge. Thicker nitride films are found to have a greater number of possible nucleation sites, and a greater variety of possible stable configurations, as experimentally observed. The simulations are extended to study the effect of changing the pad edge orientation relative to the silicon lattice. Finally, we demonstrate the usefulness and power of these techniques in handling more complex situations by illustrating the types of behavior resulting from cross-slip, reconnection between dislocations nucleated on intersecting glide planes, Frank–Read spiral sources, and traveling dislocations interacting with nitride edges.
Pt/PbZr
x
Ti
1−x
O
3
interfacial reaction and Schottky barrier formation studied by x-ray photoelectron spectroscopy: Effect of H2 and O2 annealing
Authors: Takatani Shinichiro , Miki Hiroshi , Kushida-Abdelghafar Keiko , Torii Kazuyoshi
Published: July 2002

The Pt/PbZrxTi1−xO3 (PZT) interfacial reaction caused by low-temperature (320 °C) annealing and resulting change in the surface Fermi level position (Schottky barrier height) has been studied by using in vacuo x-ray photoelectron spectroscopy (XPS). A thin (2 nm) Pt layer was deposited on a polycrystalline PZT film and annealed in 0.5-Torr H2 and O2 in a chamber connected to the XPS chamber. For the PZT samples annealed prior to Pt deposition, a small amount of metallic Pb was produced after the Pt deposition. The annealing also moved the surface Fermi level (EF) at the Schottky interface toward the conduction band minimum (EC) of PZT by 0.4 eV, as evidenced by the band-bending shift of the PZT core levels. Excess metallic states appeared in the valence-band spectra of an annealed bare PZT surface. There was no appreciable annealing-atmosphere dependence in the amount of metallic Pb produced and surface EF position at the PT/PZT interface. However, a clear annealing-atmosphere dependence was observed for the samples annealed after the Pt deposition. The H2 annealing enhanced the production of metallic Pb, whereas the O2 annealing suppressed the metallic Pb formation. The H2 annealing moved the surface EF toward EC by 1.0 eV, whereas the O2 annealing caused no measurable change in the surface EF position. The catalytic effect of Pt during the annealing was postulated to explain the observed annealing-atmosphere dependence.
Characterization of vacancies in as-grown and electron irradiated α-quartz by means of positron annihilation
Authors: Dannefaer S. , Bretagnon T. , Craigen D.
Published: July 2002

Synthetic α-quartz is shown to contain a significant concentration (several ppm) of vacancies. The major concentration of vacancies is suggested to be in the form of divacancies, giving rise to a positron lifetime of 285±5 ps, but in addition, there is a much smaller concentration of large vacancy clusters that are observable only after electron irradiation, whereupon they give rise to a positron lifetime close to 425 ps. Annealing between 900 and 1000 °C causes disappearance of divacancies and formation of vacancy clusters giving rise to a positron lifetime close to 300 ps. Above ∼950 °C positronium is formed with an exceptionally long lifetime (3–5 ns) ascribable to the formation of an amorphous phase connected with the thermal instability of β-quartz. Electron irradiation (2.3 MeV at 8 °C) gave rise to a 250±5 ps lifetime component interpreted to signify formation of neutral monovacancies, V0 and/or VSi. Their introduction rate is nonlinear, decreasing abruptly by a factor of ∼5 above a dose of 1×1017 e/cm2, an effect which is ascribed to the removal of irradiation-produced vacancy-interstitial complexes by means of electron-hole recombination during the irradiation.
Field mapping with the magnetic resonance force microscope
Authors: Ruskell Todd G. , Löhndorf Markus , Moreland John
Published: July 2002

We have developed magnetic resonance force microscopy for quantitative measurements of magnetic fields. A microscopic particle attached near the end of a microcantilever serves as the field sensing probe. We have demonstrated two-dimensional field mapping with a lateral resolution of 3.2 μm and a field resolution of 0.19 mT (1.9 G). The instrument holds considerable promise for field mapping with spatial resolution better than 0.1 μm at room temperature. Applications include field mapping of magnetic recording heads.
Stability of photoluminescence in iron-passivated porous silicon
Authors: Zhu Deliang , Zheng Lei , Li Xinjian , Zhang Yuheng
Published: July 2002

Stable photoluminescence (PL) was achieved in iron-passivated porous silicon (IPS). The PL spectra of IPS and normal porous silicon were fitted by the method of Yorikawa and Muramatsu [Appl. Phys. Lett. 71, 644 (1997)], based on a statistical distribution of particle size. The fitting results indicate that the unchanged size distribution of Si nanocrystallites results in the nonblueshift PL in IPS with time. The quantum confinement model is strongly supported by our experiments and analyses.
Study of hydrogenated diamond-like carbon films using x-ray reflectivity
Authors: Zhang Qing , Yoon S. F. , Rusli  , Ahn J. , Yang H. , Bahr D.
Published: July 2002

X-ray reflectivity under grazing incidence conditions is a powerful technique to study thin film density, surface roughness, and the multilayer structure without destroying the samples. In this article, we, using x-ray reflectivity technique, have studied the effects of bias voltage and deposition pressure on the mass density, surface and interface roughness of hydrogenated diamond-like carbon (DLC) films. It is observed that the surface roughness increases with increasing the bias voltage and with decreasing the deposition pressure over a wide range. The mass density, hardness and sp3/sp2 ratio deduced from the Raman spectra pass through their maximal values with the bias voltage, while they are increased monotonously with the pressure. A thin SiC layer is found to be formed between the DLC films and silicon substrate. According to our results, the influences of the bias voltage and deposition pressure on the film properties are clearly reflected through hydrocarbon ion energy impinging on the growing surface. Based on the film surface feature as a function of the ion energy, we suggest that the shallow ion implantation (subplantation) process could be a dominant deposition mechanism.
Precise determination of indium composition and channel thickness in pseudomorphic high electron mobility transistors using room temperature photoluminescence
Authors: Brierley Steven K. , Torabi Abbas , Lyman Peter S.
Published: July 2002

Proper composition and thickness of the InGaAs channel in pseudomorphic high electron mobility transistors (PHEMTs) is critical to assuring good device performance. Typically these characteristics have been measured by high-resolution x-ray diffraction. The results presented in this work show that the subband energy levels obtained from line shape analysis of room temperature photoluminescence spectra on these structures can be correlated very well with thickness and composition obtained from x-ray diffraction. Since the photoluminescence measurement and analysis is quite fast, this technique is suitable for rapid, nondestructive screening of PHEMT epitaxial material.
Arrays of ultrathin monocrystalline submicrometer-sized Fe dots: Néel–Brown relaxation and activation volume
Authors: Wegrowe J.-E. , Fruchart O. , Nozières J.-P. , Givord D. , Rousseaux F. , Decanini D. , Ansermet J. Ph.
Published: July 2002

Magnetic after-effect measurements were performed on an array of etched monocrystalline disk-shaped Fe dots of thickness 1 nm and diameter 200 nm. From room temperature down to 5 K, the dependence on applied field and temperature of the relaxation rate was well described on the basis of the Néel–Brown model of magnetization reversal by a rotational Brownian motion. The distribution of anisotropy fields of the dots was deduced from the nonlogarithmic relaxation rate SH,T(t) at room temperature by using a scaling procedure. From the analysis of the field dependence of SH,T(t), the activation volume was estimated at approximately 1500 nm3, with a tendency to increase slightly for temperatures below 100 K. In terms of micromagnetic reversal mechanisms, such a small value indicates that a noncoherent reversal mode occurs in the dots.
Characterization of p-dopant interdiffusion in 1.3 μm InGaAsP/InP laser structures using modulation spectroscopy
Authors: Jaeger A. , Sun W. D. , Pollak Fred H. , Reynolds C. L. , Geva M.
Published: July 2002

We have investigated three In1−xGaxAsyP1−y/InP p-i-n multiple quantum well (MQW) laser structures with different p-doping profiles using contactless electroreflectance (CER) and piezoreflectance (PZR). From the observed Franz–Keldysh oscillations originating in the i-InGaAsP regions, we have evaluated the electric field and hence the amount of p-dopant interdiffusion, which is in agreement with secondary ion mass spectrometry measurements. The CER/PZR spectra from the MQWs makes it possible to evaluate the parameters of these regions of the samples.
Temperature dependence of structure and magnetic properties in MnBi-based alloy films
Authors: Fang Qingqing , Fang Ruiyi , Zhang Sheng , Dai Daosheng
Published: July 2002

MnBi alloy films, which have large magnetocrystalline anisotropy and high remanence, are synthesized with small dopants of rare-earth elements (RE=Dy, Sm, or Tb) by vacuum evaporation method. In contrast with most ferromagnetic materials, the low temperature phase (LTP) of MnBi film shows a large coercivity, Hc, over an extended temperature range. The hexagonal crystal structure of MnBi LTP thus has good thermal stability. The experiments found that doping further extends the temperature range through which the LTP is stable. Moreover, MnBi-doped films still exhibit square hysteresis loops at high temperatures. We attribute the unusual temperature dependence of the coercivity to an increased perpendicular anisotropy field, Ha, with temperature. For this reason, MnBi-based alloy films have great potential as permanent magnets at high temperatures.
Lanthanide doping in ZnS and SrS thin-film electroluminescent devices
Authors: Keir P. D. , Maddix C. , Baukol B. A. , Wager J. F. , Clark B. L. , Keszler D. A.
Published: July 2002

The relative high field transport efficiency and short wavelength electroluminescence (EL) potential of the phosphors ZnS and SrS for alternating-current thin-film electroluminescent (ACTFEL) device flat-panel display applications are assessed via a comparison of the EL spectra of ZnS and SrS ACTFEL devices prepared in a very similar manner and doped with the same lanthanide luminescent impurities: Dy, Er, Ho, Tb, and Tm. For all of the lanthanide luminescent impurities studied, it is found that the the higher energy EL peaks are much more intense for SrS than for ZnS, even though the average phosphor field in SrS is smaller than in ZnS. These observations show SrS to be a superior high-field electron transport material compared to ZnS. All of the ZnS EL spectra show a dramatic cut off in their EL intensities at about 440–460 nm; this suggests that ZnS is not an appropriate phosphor for blue light emission since its electron distribution does not appear to be adequately heated to efficiently excite blue luminescent impurities.
Investigation of nanoscale Ge segregation in p-channel SiGe/Si field effect transistor structures by electron energy loss imaging
Authors: Norris D. J. , Cullis A. G. , Grasby T. J. , Parker E. H. C.
Published: July 2002

SiGe/Si p-channel heteroepitaxial field effect transistor test structures in Si were fabricated by molecular beam epitaxy. Combined high-resolution transmission electron microscopy and energy-loss filtered imaging have been used to quantitatively determine the nanoscale Ge distribution across the SiGe alloy channel. The alloy grading at the edges of the channel has been found to be asymmetrical due to Ge segregation, with an exponential-like extended distribution directed towards the surface. The results agree well with the predictions of segregation theory and indicate that the concentration of Ge in the extended distribution lay in the range 10%–1% over a distance of several nanometers from the body of the channel. Secondary ion mass spectrometry measurements upon the same samples were insensitive to this short range extended Ge distribution.
Influence of indium composition on the surface morphology of self-organized InxGa1−xAs quantum dots on GaAs substrates
Authors: Li Hanxuan , Zhuang Qiandong , Wang Zhanguo , Daniels-Race Theda
Published: July 2002

In x Ga 1−x As self-organized quantum dots with x=1.0, 0.5, and 0.35 have been grown by molecular beam epitaxy. The areal density, distribution, and shapes have been found to be dependent on x. The dot shape changes from a round shape for x=1.0 to an elliptical shape for x⩽0.5. The major axis and minor axis of the elliptical InxGa1−xAs dots are along the [1̄10] and [110] directions, respectively. The ordering phenomenon is also discussed. It is suggested that the dot–dot interaction may play important roles in the self-organization process.
Quantum-confinement effects on the optical and dielectric properties for mesocrystals of BaTiO3 and SrBi2Ta2O9
Authors: Kohiki Shigemi , Takada Syozo , Shimizu Akihiko , Yamada Kenichi , Higashijima Hirotaka , Mitome Masanori
Published: July 2002

We present optical and dielectric quantum-confinement effects for mesocrystals smaller than 30 Å of BaTiO3 and SrBi2Ta2O9 in uniform mesopores of the MCM-41 molecular sieve. For BaTiO3 mesocrystals we observed a blueshift in optical absorption edge from 3.02 to 3.35 eV, and a decrease in dielectric constant maximum temperature from 130 to 55 °C. For SrBi2Ta2O9 mesocrystals we also observed an increase in the optical absorption edge from 2.70 to 4.18 eV, and a decrease in the dielectric constant maximum temperature from 320 to 180 °C. The observed optical and dielectric quantum-confinement effects generally agree with the recognized consequence of reduced size.
Photorefractive holographic contrast enhancement via increased birefringence in polymer composites containing electro-optic chromophores with different alkyl substituents
Authors: Rahn Mark D. , West David P. , Shakos John D.
Published: July 2002

The birefringence of a photorefractive polymer composite in response to an applied electric field is used to predict the holographic index contrast attainable. Predictions have been obtained for three different polymer composites based on the charge transfer complex matrix poly(N-vinyl carbazole)/2,4,7-trinitro-9-fluorenone, doped with electro-optic azo chromophores differing only in their alkyl substitutions. A comparison with experimental data indicates that unexpected large variations in the holographic contrast obtainable from different chromophores are, in most cases, accounted for by a corresponding variation in electric field-induced birefringence. This variation may be due to a combination of the chromophore number density and the composite viscosity. Where the birefringence does not correlate with holographic index contrast, a prediction based on a trap density limited space-charge field fits the data well.
Growth-temperature-dependent role of In(4×1) surface phase for the heteroepitaxy of InSb on Si(111)
Authors: Rao B. V. , Gruznev D. , Tambo T. , Tatsuyama C.
Published: July 2002

Heteroepitaxial growth of InSb was performed on Si(111)–(7×7) and Si(111)–In(4×1) surface phases over a wide temperature range, by optimizing the growth rate and substrate temperature. When the heteroepitaxy was performed on the Si(111)–In(4×1) surface, the In(4×1) reconstruction modified the growth process depending on the growth temperature. At low temperatures, the In(4×1) reconstruction contributes mildly to the growth, and as the growth temperature increases, it starts degrading the quality of the films. For temperatures above 300 °C, the In(4×1) reconstruction virtually destroys the growth. In the present article, we illustrate this behavior using the growth of InSb on both Si(111)–(7×7) and Si(111)–In(4×1) surfaces at 210, 250, and 300 °C. Based on reflection high-energy electron diffraction observations, we discuss the initial stages of growth. A model for the interface formation is proposed based on our earlier results suggesting the temperature-dependent modification of In-induced surface phases on Si(001) and Si(111) surfaces during Sb deposition.
Large magnetoresistance of the polycrystalline metamagnetic layered manganite La1.2Nd0.2Sr1.6Mn2O7
Authors: Dörr K. , Müller K.-H. , Schultz L. , Ruck K. , Krabbes G.
Published: July 2002

Magnetization and magnetotransport have been investigated for polycrystalline samples of the bilayer Ruddlesden–Popper compound La1.2Nd0.2Sr1.6Mn2O7 that shows the colossal magnetoresistance effect connected with ferromagnetic ordering at TC=74 K. At T2=38 K the compound reveals a transition from the ferromagnetic to an antiferromagnetic spin arrangement of alternating ferromagnetic bilayers, which undergoes a first order metamagnetic transition towards ferromagnetism in a magnetic field of ∼3.5 kOe at 20 K. In measurements of resistance vs magnetic field, this transition is accompanied by a large resistance drop of [R(5 kOe)−R(0)]/R(0)=−60% at 5 K. The tunneling-like magnetoresistance (MR) known for current transport perpendicular to the layers in (La, Nd)1.4Sr1.6Mn2O7 single crystals contributes to this large MR observed in a polycrystalline sample. While current is expected to mainly follow the bilayers within the grains due to the anisotropy of conductivity, the high value of resistivity of our sample (∼8 Ω cm at 5 K) suggests that current paths are partially directed perpendicular to the layers.