bluuebunny/crossref_metadata_2025 huggingface dataset:
Concentration of interstitial and substitutional nitrogen in GaNxAs1−x
Authors: Ahlgren T. , Vainonen-Ahlgren E. , Likonen J. , Li W. , Pessa M.
Published: July 2002

The interstitial to substitutional nitrogen atoms as a function of concentration in GaNAs were determined by nuclear reaction analysis utilizing the N14(d,p) N15 and N14(d,α) C12 reactions using ion channeling technique. The GaNAs films with mean nitrogen concentration between 0.3% and 3%, measured with secondary ion mass spectrometry and time-of-flight elastic recoil detection analysis, were grown using gas-source molecular-beam epitaxy. The fraction of nitrogen atoms occupying substitutional sites was observed to increase linearly with increasing nitrogen amount, while the concentration of interstitial nitrogen was nearly constant at 2×1019 cm−3 throughout the concentration region. Annealing at 750 °C decreases the concentration of interstitial nitrogen.
High quality iron silicide films by simultaneous deposition of iron and silicon on Si(111)
Authors: Starke U. , Weiss W. , Kutschera M. , Bandorf R. , Heinz K.
Published: July 2002

Growth, thermal reaction, and crystalline structure of ultrathin iron silicide films on Si(111) are studied by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The structural development of silicide layers is monitored in dependence on iron coverage and annealing temperature. Below approximately 10 monolayers (ML) of iron, two film structures appear, that are not stable in bulk material, while above that limit a switch to the bulk structures is observed. The morphology of the films is strongly dependent on the growth conditions. Their homogeneity can be considerably improved by simultaneous deposition (coevaporation) of Fe and Si in the desired stoichiometry compared to annealing predeposited Fe films. This improvement is accompanied by the suppression of pinholes in the film. The Fe:Si stoichiometry of the (1×1) and (2×2) phase can be assigned 1:1 and 1:2, respectively. The crystal structure of the former was previously determined to be CsCl, so called c-FeSi. For codeposition in 1:2 stoichiometry an initially disordered (1×1) phase transforms to a well ordered (2×2) phase after annealing. For these phases, γ-FeSi2 in CaF2 structure, the tetragonal α-FeSi2 or an iron depleted variant of the CsCl structure are compatible with LEED and angle resolved AES results. In case of 1:2 stoichiometric films, the stability range of the (2×2) periodic phase can be extended to more than 60 Å (equivalent to more than 20 ML Fe) by coevaporation.
Biomimetic self-assembly of helical electrical circuits using orthogonal capillary interactions
Authors: Gracias David H. , Boncheva Mila , Omoregie Osahon , Whitesides George M.
Published: July 2002

This letter describes the biomimetic self-assembly of mm-sized polyhedra into helical aggregates. The system used two orthogonal, capillary interactions that acted in parallel. The design of the self-assembly process, and of the resulting structures, was modeled on the formation and structure of tobacco mosaic virus. The self-assembled, helical aggregates carried one, two, or four isolated, electrical circuits.
Micromechanics approach for effective magnetostriction of composite materials
Authors: Huang Jin H. , Nan Ce Wen , Li Rui-Mu
Published: July 2002

A micromechanics method is developed for the effective magnetostrictive behavior of magnetostrictive composite materials. Explicit relations for determining the effective magnetostriction of the particulate composites with cubic magnetostrictive crystallites are derived and reduce to the well-known results in the case of simple polycrystals with elastically isotropic magnetostrictive crystallites. Numerical calculations for the effective saturation magnetostriction of SmFe2/Fe or Al, and Terfenol-D/epoxy composites show that the theoretical estimates correlate well with experimental data, and they also show the interesting magnetostrictive behavior of the composites.
Electrical properties of amorphous silicon carbide films
Authors: Biswas N. , Wang X. , Gangopadhyay S.
Published: July 2002

The interfacial properties of silicon carbide films on silicon substrate, using capacitance–voltage (C–V) and conductance–voltage techniques have been studied. The C–V characteristics observed largely depended on processing conditions. An interface state density of 2.38×1010 eV−1 cm−2 at the midgap was calculated for this sample. When CF4 was added to the precursors, the C–V curves were observed to have a “ledge” at the end of the accumulation region, indicative of the presence of additional localized defect states that respond to the applied ac signal. An interface state density of 2.11×1011 eV−1 cm−2 at the midgap was observed for this sample. However fluorination in plasma caused the etching of the weak bond, and hence a reduction in the fixed charge density and hysteresis.
Determination of the order parameter of CuPt-B ordered GaInP2 films by x-ray diffraction
Authors: Li J. H. , Forrest R. L. , Moss S. C. , Zhang Y. , Mascarenhas A. , Bai J.
Published: July 2002

We present quantitative characterization of atomic ordering in semiconductor alloy films by x-ray diffractometry. In particular, we show that the order parameter of CuPt-B ordered GaInP2 films can be determined without measuring the fundamental reflections or examining structural details of the ordered domains. Our method is based on the fact that the ordering peak is modulated by statistical displacements of atom planes, which is a function of the degree of ordering. Therefore, by comparing two or more ordering peaks in an x-ray spectrum, the order parameter of an ordered film can be extracted solely for those regions that are, in fact, ordered. The method can straightforwardly be extended to other ordered alloys.
Solubility isotope effects in aqueous solutions of methane
Authors: Bacsik Z. , Lopes J. N. Canongia , Gomes M. F. Costa , Jancsó G. , Mink J. , Pádua A. A. H.
Published: July 2002

The isotope effect on the Henry’s law coefficients of methane in aqueous solution (H/D and C12/C13 substitution) are interpreted using the statistical mechanical theory of condensed phase isotope effects. The missing spectroscopic data needed for the implementation of the theory were obtained either experimentally (infrared measurements), by computer simulation (molecular dynamics technique), or estimated using the Wilson’s GF matrix method. The order of magnitude and sign of both solute isotope effects can be predicted by the theory. Even a crude estimation based on data from previous vapor pressure isotope effect studies of pure methane at low temperature can explain the inverse effect found for the solubility of deuterated methane in water.
Temperature dependence of intersublevel absorption in InAs/GaAs self-assembled quantum dots
Authors: Bras F. , Boucaud P. , Sauvage S. , Fishman G. , Gérard J.-M.
Published: July 2002

We have studied the temperature dependence of the intersublevel absorption in n-doped InAs/GaAs self-assembled quantum dots. The investigated intersublevel transition corresponds to the optical transition between the s-type conduction ground state to the p-type first excited states. These transitions, resonant between 20 and 22 μm, are in-plane polarized along the [110] and the [1̄10] directions. A redshift lower than 3 meV is observed for the transition resonance from low temperature to room temperature. While the effective barrier height from the ground state is around 150 meV, the integrated absorption amplitude decreases by a factor of 4 from low temperature to room temperature. This decrease is modeled by the thermionic emission of the carriers, taking into account the density of states of the two-dimensional wetting layer, the density of states of the three-dimensional bulk layer surrounding the dots and the existence of polaron states associated with the strong electron-phonon coupling in the dots.
Islands of accelerator modes and homoclinic tangles
Authors: Rom-Kedar Vered , Zaslavsky George
Published: July 2002

Islands are divided according to their phase space structure—resonant islands and tangle islands are considered. It is proved that in the near-integrable limit these correspond to two distinct sets, hence that in general their definitions are not trivially equivalent. It is demonstrated and proved that accelerator modes of the standard map and of the web map are necessarily of the tangle island category. These islands have an important role in determining transport—indeed it has been demonstrated in various works that stickiness to these accelerator modes may cause anomalous transport even for initial conditions starting in the ergodic component.
Coupling of radiation and hydrodynamics in a Z-pinch plasma
Authors: Bailey J. , Fisher A. , Rostoker N.
Published: July 2002

The relationship between hydrodynamics and radiative energy loss from a gas puff Z pinch was studied using helium, krypton, and a helium–krypton mixture. The results show that radiation cooling enables the magnetic field to compress the plasma to a higher electron density and a smaller final radius. The data are consistent with a simple model describing the thermalization of the plasma on the axis of the pinch. The spectrum of the krypton radiation from the mixture was harder than from the pure krypton pinch. This may be important for application of the Z pinch as a radiation source.
Active feedback-controlled dielectrophoretic levitation
Authors: Jones T. B. , Kraybill J. P.
Published: July 2002

Dielectrophoretic levitation of small metallic particles in insulating dielectric liquids is achieved using a microprocessor-controlled digital feedback system. An optical detector senses vertical particle position and the error signal generated controls the voltage applied to the electrodes of the levitator cell. Experiments with single particles confirm a theoretical expression for the axial profile of the electric field. The method of images is used to calculate the effective dipole moments for chains of conducting particles. Experimental levitation data obtained using two- and three-particle chains provide preliminary confirmation for these effective dipole moment expressions.
A fracture mechanics model of fragmentation
Authors: Glenn L. A. , Gommerstadt B. Y. , Chudnovsky A.
Published: July 2002

A model of the fragmentation process is developed, based on the theory of linear elastic fracture mechanics, which predicts the average fragment size as a function of strain rate and material properties. This approach permits a unification of previous results, yielding Griffith’s solution in the low-strain-rate limit and Grady’s solution at high strain rates.
The effect of Mn on the positive temperature coefficient of resistance characteristics of donor doped BaTiO3 ceramics
Authors: Al-Allak H. M. , Brinkman A. W. , Russell G. J. , Woods J.
Published: July 2002

BaTiO3 positive temperature coefficient of resistance (PTCR) specimens were made from commercial BaTiO3 which was mixed with 0.35 mol. % of Ho2O3 to make it semiconducting and 0.07 mol. % of MnCO3. After sintering at 1320 °C the samples were annealed in batches at 1220 °C for various periods between 0 and 5 h. The effect of Mn was studied by making direct comparison with previous results obtained from Mn free but otherwise identical specimens. Room-temperature dielectric measurements in the audio and radio frequency ranges revealed that Mn had a negligible effect on the grain bulk resistance. Mn was found to result in an increase in the minimum and maximum values of the resistivity, the temperature at which the resistivity is a maximum and the slope of the resistivity-temperature characteristic in the transition region, all of which were attributed to an enhancement of the potential barrier at the grain boundaries. The acceptor state energy of the Mn-doped samples was found to be ∼1.4 eV, while a lower value was obtained for Mn-free material (∼1.12 eV). The effective concentration of Mn acceptors was observed to increase with the annealing time. To explain this behavior a thin Mn-rich boundary layer was assumed to exist, within which there was partial compensation of the Mn ions due to the formation of ionized oxygen vacancies. During annealing in air at 1220 °C, oxidation takes place and more of these vacancies are filled resulting in an increase in the effective Mn concentration. This model is also capable of explaining other phenomena such as the disappearance of the PTCR effect in reduced samples and the effects of cooling rate and quenching on the PTCR behavior.
Electrical and magnetic properties of semiconducting ternary U compounds: UTSn and UTSb
Authors: Palstra T. T. M. , Nieuwenhuys G. J. , Vlastuin R. F. M. , Mydosh J. A. , Buschow K. H. J.
Published: July 2002

We have measured the electrical-transport and magnetic properties of several intermetallic compounds UTSn and UTSb, where T is a transition metal. The electrical resistivity is up to three orders of magnitude larger than usually found for U-based compounds. This is ascribed to the occurrence of a spin polarized energy gap related to the MgAgAs-type crystal structure. For UNiSn at least one spin band closes at low temperature, resulting in half-metallic behavior. Interestingly, the magnetic properties exhibit Kondo-lattice character and weak-moment ordering.
Mössbauer study of the permanent-magnet material Nd2(Fe1−xNix)14B
Authors: Dai S. , Morrish A. H. , Zhou X. Z. , Hu B. P. , Zhang S. G.
Published: July 2002

Samples of Nd2(Fe1−xNix)14B, 0≤x≤0.20, with good phase homogeneity have been made by heating under vacuum for 300 h. 57Fe Mössbauer parameters have been determined for all six different iron crystallographic sites at room temperature. The nickel atoms preferentially substitute for iron atoms on the 16k2 and 8j2 sites. The nickel atoms seem to alter the hyperfine parameters of the 57Fe nuclei via localized interactions.
Magnetic rare-earth superlattices (invited)
Authors: Majkrzak C. F. , Gibbs Doon , Böni P. , Goldman Alan I. , Kwo J. , Hong M. , Hsieh T. C. , Fleming R. M. , McWhan D. B. , Yafet Y. , Cable J. W. , Bohr J. , Grimm H. , Chien C. L.
Published: July 2002

The magnetic structures of several single-crystal, magnetic rare-earth superlattice systems grown by molecular-beam epitaxy are reviewed. In particular, the results of recent neutron diffraction investigations of long-range magnetic order in Gd-Y, Dy-Y, Gd-Dy, and Ho-Y periodic superlattices are presented. In the Gd-Y system, an antiphase domain structure develops for certain Y layer spacings, whereas modified helical moment configurations are found to occur in the other systems, some of which are commensurate with the chemical superlattice wavelength. References are made to theoretical interaction mechanisms recently proposed to account for the magnetic states of these novel materials.
n-ZnO/p-CdSiAs2 heterojunction solar cells
Authors: Kimmel M. , Lux-Steiner M. , Vögt M. , Bucher E.
Published: July 2002

Heterojunction solar cells were prepared by magnetron sputtering of n-ZnO onto p-CdSiAs2 single crystals. These crystals were grown by chemical vapor-phase transport and have a resistivity of 2 Ω cm, a Hall mobility of 300 cm2/V s, and a net carrier density of 1016 cm−3 at room temperature. The heterojunctions exhibit short-circuit current densities up to 12 mA/cm2 and open-circuit voltages between 180 and 250 mV with power conversion efficiencies up to 1% under 72 mW/cm2 halogen lamp illumination. Open-circuit voltage values could be increased by heat treatment at 80 °C for 15 min in an argon atmosphere. By measuring the reflection and the quantum yield versus wavelength the efficiency losses in the short-wavelength region could be attributed to the corresponding high reflection.
Substrate and size effects on the coalescence of small particles
Authors: Iijima Sumio , Ajayan P. M.
Published: July 2002

Here we present experimental results, using high-resolution electron microscopy and real time video recording, on the liquid-like coalescence of small gold particles supported on silica. We report that particles that are few nanometers in size, and decoupled from substrate interactions, coalesce extremely fast, in fractions of a second. When the particles are properly supported on a substrate, coalescence is orders of magnitude slower, depending on the particle size, and influenced by the diffusion of atoms on the substrate surface. Necks are seen to develop distinctly during coalescence only for particles above certain size.
Uniform deposition of YBa2Cu3O7−δ films over large areas using ion-beam sputtering
Authors: Lichtenwalner D. J. , Soble C. N. , Woolcott R. R. , Auciello O. , Kingon A. I.
Published: July 2002

Uniform films of YBaCuO have been deposited over 4-in.- (100-mm) diam, unheated Si wafers using ion-beam sputtering of a stoichiometric YBa2Cu3O7−δ target. Rutherford backscattering spectrometry reveals that within a 3-in. diam, the thickness variation is ±2.9%, and the compositional uniformity of Y, Ba, and Cu is ±0.9%, ±5.2%, and ±3.4%, respectively. The average film composition is Y1.03Ba1.88Cu3.09O7−δ. Deposition onto unheated substrates virtually eliminates the problem of the low sticking coefficients of Ba and Cu, and is much simpler for film synthesis and processing. The observed difference from the 1:2:3 target composition is attributed mainly to angular differences in the sputtered-flux distribution of each element. These effects should be minimized by substrate rotation. In order to obtain useful superconducting films, a post-deposition anneal on a suitable substrate or buffer layer is required. Using a relatively low temperature (∼810 °C), low O2 pressure (1.2 Torr) post-anneal, we show that oriented, superconducting films can be obtained on single-crystal MgO and SrTiO3 substrates. Films annealed under these conditions on MgO are predominantly c-axis oriented, whereas those on SrTiO3 are predominantly a-axis oriented.
Reflection high-energy electron diffraction studies of InSe and GaSe layered compounds grown by molecular beam epitaxy
Authors: Emery J.-Y. , Brahim-Ostmane L. , Hirlimann C. , Chevy A.
Published: July 2002

We have investigated the molecular beam homoepitaxial and heteroepitaxial growth of InSe and GaSe III–VI compounds semiconductors. In situ reflection high-electron energy diffraction measurements reveal that two-dimensional epitaxial growth occurs on (00.1) oriented substrates, at substrate temperatures ranging from 300 to 350 °C. A scanning electron microscopy study of the films surfaces has been performed to correlate their morphology to the growth conditions.
Exchange anisotropy in oxide passivated Co fine particles
Authors: Gangopadhyay S. , Hadjipanayis G. C. , Sorensen C. M. , Klabunde K. J.
Published: July 2002

Oxide passivated Co particles have been prepared by vapor deposition in the size range of 50–350 Å. A strong exchange anisotropy was observed due to the core-shell structure of the Co particles, where the core consists of metallic Co and the shell of Co oxides. The exchange anisotropy of the particles was studied using shifted hysteresis loops after the sample was field cooled (FC). The shift in the FC loop exhibited a maximum at a particle size of 80 Å (shift=10.7 kOe) and is related to the amount of surface oxidation. The shift in the loops disappeared around 150 K in all the samples irrespective of their particle size, and this is attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell.
Formation of ‘‘intrinsic’’ surface defects during 248 nm photoablation of polyimide
Authors: Krajnovich Douglas J. , Vázquez José E.
Published: July 2002

Photoablation of polymers by pulsed excimer laser radiation is commonly believed to be a controlled layer-by-layer removal process. A mass spectrometer was used to monitor neutral species ejected from polyimide samples in vacuum by 248 nm laser radiation. For fluences close to threshold, the ablation rate starts to drop after the first 100–200 pulses and eventually falls almost to zero. The falloff in etch rate is accompanied by a dramatic slowdown in the product translational energy distributions and by the appearance of conical defects on the sample surface. The number of cones is approximately the same for samples irradiated in air or vacuum for the same number of pulses, proving that ablation debris is not the initiating factor. It is argued that carbon enrichment at the sample surface initiates cone formation by locally shifting the ablation threshold to higher values. In effect, the polymer surface becomes ‘‘radiation hardened.’’
In-vacuum undulator installed in a 6.5 GeV storage ring for producing brilliant 14.4 keV Mössbauer photons
Authors: Yamamoto Shigeru , Zhang Xiaowei , Kitamura Hideo , Shioya Tatsuro , Mochizuki Tetsuro , Sugiyama Hiroshi , Ando Masami , Yoda Yoshitaka , Kikuta Seishi , Takei Humihiko
Published: July 2002

Using a newly developed in-vacuum type undulator installed in the 6.5 GeV TRISTAN Accumulation Ring at the National Laboratory for High Energy Physics, a beam comprising of more than 2.4×104 Mössbauer photons/s at 14.4 keV has been achieved from an enriched α-57Fe2O3 single crystal. The spectrum in terms of brilliance in the region from 5 to 25 keV agrees well with the calculation.
Characterization of highly Sb-doped Si using high-resolution x-ray diffraction and transmission electron microscopy
Authors: Radamson H. H. , Sardela M. R. , Hultman L. , Hansson G. V.
Published: July 2002

Two-dimensional reciprocal space mapping with high-resolution x-ray diffraction has been used to characterize the strain in as-grown and annealed Sb-doped Si. Si(100) layers with Sb concentrations 1×1019–3×1021 cm−3 were grown by molecular-beam epitaxy at a growth temperature of 310 °C. High-resolution transmission electron microscopy has been applied to determine the critical thickness for epitaxial growth, growth morphology, and defect structure. The critical thickness for low-temperature epitaxial growth decreases with increasing Sb concentration from ∼1000 Å for CSb≊2×1020 cm−3 to ∼25 Å for CSb≊2×1021 cm−3. Following the defect-free epitaxial layer was a (100)-oriented layer with stacking faults and facets to the final amorphous phase. The strain in as-grown Si layers increased with increasing Sb concentration CSb up to 1×1021 cm−3. The thermal stability for concentrations above 2×1020 cm−3 was poor, resulting in Sb precipitation. The lattice expansion obtained due to Sb-doping in Si was β=5.4×10−24 cm3 atom−1. Electrical characterization of the samples showed that doping concentrations ≤4×1020 cm−3 could be obtained.
Giant magnetoresistance effects in intermetallic compounds (invited)
Authors: Sechovský V. , Havela L. , Prokeš K. , Nakotte H. , de Boer F. R. , Brück E.
Published: July 2002

Giant magnetoresistance (GMR) effects are observed in several classes of bulk magnetic materials. The resistance changes at metamagnetic transitions connected with reorientation of 4f moments are only moderate due to the relatively weak coupling of the 4f and conduction electrons. Much larger GMR effects can be achieved by mechanisms involving the d states (RhFe, RCo2), though the most spectacular resistance variations are connected with metamagnetic transitions in U-intermetallic antiferromagnets. This phenomenon can be interpreted as due to Fermi surface gapping (due to magnetic superzones) and/or due to spin-dependent scattering in analogy with magnetic multilayers.
Synthesis, identification, and growth mechanism of Fe, Ni, and Co crystals encapsulated in multiwalled carbon nanocages
Authors: Yosida Y. , Shida S. , Ohsuna T. , Shiraga N.
Published: July 2002

Nanometer size crystalline particles of α-Fe, γ-Fe, Ni, β-Co, and Co3C encapsulated in multiwalled carbon cages are synthesized by a standard dc arc evaporation of composite anodes containing 10–15 wt. % each of the metals or their compounds. The typical particle size is ∼10 nm. Elemental analysis is made by x-ray energy dispersive spectroscopy. The spectra show only the peaks associated with the respective metal and carbon. The stoichiometry and phase of the crystals are assigned from the characteristic lattice spacings obtained by high-resolution transmission electron microscopy and x-ray diffractometry. Particle size distribution is obtained from a statistical survey of electron micrographs. A statistical growth model is adequate for explaining the crystalline particle size distribution of the encapsulated crystals consisting of α-Fe and γ-Fe smaller than 15 nm. Significant deviation from the model is found for the encapsulated crystals consisting of α-Fe and γ-Fe larger than 15 nm, those of Ni larger than 4 nm, and those consisting of β-Co and Co3C larger than 6 nm. The deviation is discussed in terms of encapsulation effect on the particle growth mechanism.
Conduction type control from n to p type for organic pigment films purified by reactive sublimation
Authors: Hiramoto Masahiro , Ihara Kiyoaki , Fukusumi Hiroyuki , Yokoyama Masaaki
Published: July 2002

The effects of purification by reactive sublimation technique and bromine doping on the Fermi level and the photovoltaic properties of n-type perylene pigment films were investigated. Photovoltage arisen from the Schottky junction between n-type perylene pigment film and Au increased significantly by repeating the train sublimation under methylamine gas atmosphere. This phenomenon was revealed to be due to the negative shift of the Fermi level resulting from the effective removal of unknown but specific impurity acting as an acceptor by reactive sublimation. On the other hand, by bromine doping, Fermi level of the pigment film shifted largely to a positive direction and reached the nearby valence band, while the direction of photocurrent flow arising from the Schottky junction with Au was reversed. This result is a clear demonstration of alternating the type of conduction from n type to p type. This means that the pn control of organic semiconductors is possible.
Investigations on the transition between column and micropinch mode of plasma focus operation
Authors: Lebert R. , Engel A. , Neff W.
Published: July 2002

X-ray emission from pinch plasma devices with pinch currents ranging from 200 to 400 kA operated with pure high-Z gases are investigated with temporal spatial and spectral resolution. If operated using elements Z<18 (e.g., nitrogen or neon) K-shell emission is observed from column-like volumes several 100 μm in diameter and several mm in length (bulk or column mode). For Z≳18 (e.g., krypton or xenon) emission with hν≳1 keV is only observed from micropinches. For argon (Z=18) both modes of operation can be observed. The occurrence of a specific mode depends on the initial gas pressure. In this paper the transition regime between column and micropinches is investigated with particular regard to argon. A criterion is proposed to decide whether column mode or micropinch mode is expected.
Aging behavior of the microstructure and soft magnetic properties in amorphous Fe–Si–B–Nb alloys
Authors: Naohara T.
Published: July 2002

The crystallization behaviors of amorphous Fe82Si6B10Nb2 and Fe79Si6B10Nb5 alloys were investigated using differential scanning calorimetry (DSC), transmission electron microscopy, and soft magnetic measurements. The peak morphology of the DSC curve was significantly different between the two alloys, which suggests that their amorphous structures are essentially dissimilar. The formation of a large amount of the dendritic α-Fe phase with a grain size of 0.2–0.3 μm was caused by aging the former alloy for 0.6 Ms at 573 K, which leads to a marked increase in the coercivity as well as a decrease in the effective permeability μe. However, the notable enhancement of μe was achieved upon aging below 623 K for the latter alloy, despite the fact that no crystallization occurs during the prolonged aging for 0.6 Ms. Whereas the μe at 1 MHz attained a maximum value of 2700 at 573 K, the aging at the first crystallization temperature for 3.6 ks resulted in a decrease in μe to 1650, which indicates the formation of nanoscale α-Fe particles. Such a difference in the crystallization behaviors between the two alloys was reasonably interpreted within the framework of the structural model for amorphous Fe–Si–B alloys.
Optical properties of GaSb alloys and photodiodes grown by liquid-phase epitaxy
Authors: Sun Yuh-Maoh , Jiang Wen-Jang , Wu Meng-Chyi
Published: July 2002

The carrier concentration of GaSb epitaxial layers grown from Sb-rich solutions by liquid-phase epitaxy is controlled by replacing nominally undoped GaSb by Te-doped polycrystalline GaSb to suppress or compensate the background hole concentration. The dependence of the 10 K photoluminescence peak wavelength and intensity on the doping level of the Te-compensated GaSb layers has been investigated. The absorption spectra are examined as a function of carrier concentration. Low-dopant-concentration p-type GaSb samples exhibit the band gap shrinkage with increasing hole concentration, while the lightly Te-doped n-type GaSb samples exhibit a band gap increase due to the Burstein–Moss shift with increasing electron concentration. By using Te-doped GaSb layers with an electron concentration of 5.6×1015 cm−3, the fabricated GaSb photodiodes exhibit a low dark current of 2 μA at −5 V, a high breakdown voltage of 28.7 V at 20 μA, and a maximum photoresponsivity of 0.55 A/W with an external quantum efficiency of 40% at 1.72 μm wavelength.
The effect of density and microstructure on the performance of TiN barrier films in Cu metallization
Authors: Park Ki-Chul , Kim Ki-Bum , Raaijmakers Ivo J. M. M. , Ngan Ken
Published: July 2002

The properties of 100-nm-thick reactively sputter-deposited TiN films with different deposition conditions and one chemical-vapor-deposited film have been studied as diffusion barriers between Cu and Si by using sheet resistance measurements, x-ray diffractometry, etch-pit test, and Auger electron spectroscopy (AES). Based on Rutherford backscattering spectrometry (RBS), AES, and transmission electron microscopy (TEM) analyses, the relationship between the density, oxygen content, and microstructure of the as-deposited TiN film has been established. As the density of the TiN film decreases, the microstructure of the film becomes porous and the oxygen content in the film increases. The result of the etch-pit test has shown that the failure temperature of the TiN diffusion barrier varies from 500 to 750 °C, depending upon the microstructure of the film. It is concluded that high density of the TiN film is of primary importance in achieving a good diffusion barrier performance between Cu and Si.
The role of an electric field applied during pulsed laser deposition of LiNbO3 and LiTaO3 on the film orientation
Authors: Hu W. S. , Liu Z. G. , Feng D.
Published: July 2002

A low electric field applied during film deposition has a significant influence on the orientation of ferroelectric thin films prepared by pulsed laser deposition. C-axis oriented growth of LiNbO3 films was demonstrated on fused silica with the aid of a low bias voltage Vb=0–120 V, and complete c-axis orientation was achieved at Vb=110 V. In contrast, the electric field cannot induce c-axis orientation of LiTaO3 films. Theoretical analysis suggests that the electrostatic energy provides an extra driving force for the c-axis oriented growth of the ferroelectric films if the permittivity components satisfy ε33/ε11≤2, as in LiNbO3 film, but not if ε33/ε11>2 at the deposition temperature, as in LiTaO3 film.
Fermi level determination in organic thin films by the Kelvin probe method
Authors: Pfeiffer M. , Leo K. , Karl N.
Published: July 2002

We show that the Kelvin probe method (KPM) is well suited for in situ measurements of the Fermi level of organic vapor-deposited dye layers. The method works well even for high resistivity materials, is nondestructive, and does not need any top contacts. First results for zinc-phthalocyanine (ZnPc) and perfluorinated ZnPc (ZnPc–F16) are presented. The KPM results show that a 2.5% admixture of ZnPc–F16 to a ZnPc matrix leads to a 0.2 eV lowering of the Fermi level of ZnPc, i.e., ZnPc–F16 has acceptorlike properties relative to ZnPc.
Transient stimulated Brillouin scattering and damage of optical glass
Authors: Yu Haiwu , Meng Shaoxian
Published: July 2002

A theory of the excitation of ultrasound waves arising from photoelastic coupling with light is presented in this article. Using several reasonable approximations, a numerical calculation of the transient stimulated Brillouin scattering process shows that a large amplitude of acoustic wave can be built up during the pulse of pump light, and it may result in damage to optical materials. This model gives a damage threshold value of ∼30–50 J/cm2 when using Gaussian or square nanosecond pulses on optical materials.
Mössbauer study of La0.75Ca0.25Mn0.98Fe0.02O3 compound
Authors: Pissas M. , Kallias G. , Devlin E. , Simopoulos A. , Niarchos D.
Published: July 2002

A thorough study of the Mössbauer spectra (MS) of a lightly iron doped (2%) polycrystalline La0.75Ca0.25MnO3 compound has been carried out from T=4.2 K up to 300 K. The MS for T>Tc and at T=4.2 K show one component. The corresponding hyperfine parameters are compatible with S=5/2 Fe3+ in octahedral coordination. As temperature increases from 4.2 K the spectra consist of several broad sextets and doublets (especially the Tc). This first reported behavior of MS resembles that of ferromagnetic clusters.
Shape-selective Raman scattering from surface phonon modes in aggregates of amorphous SiO2 nanoparticles
Authors: Glinka Yu. D. , Jaroniec M.
Published: July 2002

A spontaneous Raman scattering from the surface phonon modes (SPMs) of amorphous silica (SiO2) nanoparticles which is small compared with the excitation wavelength (514.5 nm) and aggregated in micron size clusters (globules) is utilized to determine the shape of particles. The excitation dynamics for these modes were studied for silica samples pretreated at different temperatures (30–1000 °C). The SPMs are located between Raman peaks corresponding to the bulk traverse-optical (TO) and longitudinal-optical (LO) asymmetrical and bending phonon modes. An anomalously large TO-LO splitting (∼185 cm−1) of bending modes was observed. The shape examination was based on the fact that three depolarization factors are equal in value for spherical nanoparticles, but they are different for an elliptical one. According to this, one can observe one or three SPMs, respectively. The position of corresponding Raman peaks allows one to determine the depolarization factors and subsequently the shape of nanoparticles. The single SPM is located between Raman peaks corresponding to the TO and LO asymmetrical phonon modes and can be attributed to small spherical particles. Six different SPMs, which are located between Raman peaks corresponding to the TO and LO bending phonon modes, were observed. These modes can be assigned to elliptical nanoparticles in two surrounding mediums of different effective dielectric constants. The intensity of these SPMs is decreased drastically by heat treatment at a temperature around 950 °C, suggesting that aggregates of nanoparticles form the bulk α-quartz structure. The bulk structure formation from small silica particles was supported by thermogravimetric measurements. The values of the fraction of the total sample volume occupied by particles were calculated theoretically. A good agreement with the theory was obtained by assuming the anomalously large TO-LO splitting of bending phonon modes in nanoparticles of the amorphous silica.
Photoluminescence and electroluminescence of new lanthanide-(methyoxybenzoyl)benzoate complexes
Authors: Edwards A. , Claude C. , Sokolik I. , Chu T. Y. , Okamoto Y. , Dorsinville R.
Published: July 2002

New luminescent thermally stable lanthanide-organic complexes, namely europium tris(4-methoxy benzoylbenzoate), Eu(MeOBB)3, and terbium tris(4-methoxy benzoylbenzoate), Tb(MeOBB)3, were synthesized and characterized. The UV absorption bands in these compounds are attributed to the ligand while the emission originates from the lanthanide ions’ electronic transitions. The quantum efficiency of photoluminescence, Φ, measured in 5×10−4 M methylene chloride solutions is relatively large in both compounds: Φ=0.27 for Tb(MeOBB)3 and Φ=0.16 for Eu(MeOBB)3. The narrow-band electroluminescence from these complexes is studied by fabricating and characterizing single- and multi-layer organic light emitting diodes. Energy transfer between the organic components of these devices is also discussed.
Reactive crossed beam scattering of a Ti plasma and a N2 pulse in a novel laser ablation method
Authors: Willmott P. R. , Timm R. , Huber J. R.
Published: July 2002

The interaction and energy transfer of a laser ablation plasma of Ti with a pulsed N2 supersonic expansion are investigated using time-of-flight quadrupole mass spectroscopy and Langmuir probe techniques. The Ti ablation target and the exit nozzle of the pulsed gas source are positioned so that the plasma plume and gas pulse interact near to their respective origins, where the number density is still high, which hence results in strong coupling of the nascent plasma with the gas pulse. The timing between the gas pulse and ablation plume is shown to be critical in determining the scattering processes and the chemical nature of the films grown by this method, an example of which is presented. The degree of ionization of the plasma when crossed with the gas pulse compared to that for expansion into vacuum increases from less than 10−3 to 0.28±0.11, which is attributed to collision-induced ionization of Ti atoms. Further increasing the N2 number density quenches the ion signal. The effective bimolecular cross section for scattering of Ti with the high density N2 pulse is about 4 times larger than that with a static background of low pressure N2, while the fractional depletion of the N2 pulse by the Ti plume depends on the N2 number density in the pulse, indicating that at these high local pressures, collective effects prevail. We propose a simple model for the resulting evolution of the plasma based on electrostatic considerations.
Uncertainty in Gaussian noise generalized for cross-correlation spectra
Authors: Briaire J. , Vandamme L. K. J.
Published: July 2002

The ensemble average and variance of Gaussian noise measured as cross-correlation spectra are calculated as a function of the number of time averages. The calculations are based on the Carson–Campbell theorem which treats noise spectra as a collection of individual deterministic pulses transformed into the frequency domain. We have compared our calculations with experimental results on metal film resistors. There is an excellent agreement between the predicted and the measured statistical behavior.
Core loss reduction in electrical steels through materials processing
Authors: Verbrugge B. , Jiles D. C.
Published: July 2002

Research has been conducted into the effects of laser scribing on the core losses of soft magnetic materials. Various types of lamination steels for use in electric motors and transformers were studied including iron based alloys containing carbon, silicon, nickel, and cobalt. The results show that the change in core losses for carbon, nickel, and cobalt alloys was small for the scribing conditions examined, but core loss reductions of up to 18% were achieved with iron–silicon alloys.
Microstructure evolution of GaSe thin films grown on GaAs(100) by molecular beam epitaxy
Authors: Dai Z. R. , Chegwidden S. R. , Rumaner L. E. , Ohuchi F. S.
Published: July 2002

GaSe thin films were grown on a GaAs(100) substrate by molecular beam epitaxy. Microstructures of the thin films and interface were characterized by transmission electron microscopy. The dominant polytype formed in the GaSe thin films was a γ type, which has a 3R-rhombohedral structure with R3m space group. Predominant crystallographic orientation between the GaSe thin films and the GaAs substrate was characterized as: [1̄100]GaSe‖[011]GaAs/(0001)GaSe‖(100)GaAs. In addition, GaSe thin films with orientation of [1̄21̄0]GaSe‖[011]GaAs/(0001)GaSe‖(100)GaAs can also grow in some local areas. The interface between GaSe thin films and GaAs substrate constitutes thin intermediate layers of a vacancy ordered β-Ga2Se3, the structure of which inherits the crystallographic features of the GaAs(100) surface. Mechanisms responsible for formation of the preferable crystallographic orientation in the GaSe thin films in the initial growth stage are suggested.
Influence of nanostructure (layers and particles) on the magnetism of rare-earth materials
Authors: O’Shea M. J. , Perera P.
Published: July 2002

Elemental rare-earths R (Gd, Dy, Tb) are prepared in layer and particle form to look at the effects of geometry (layers or particles) and size (layer thickness or particle diameter) on their magnetism. All of the rare-earth systems show a maximum in coercivity (measured at 4.5 K) at about 10 nm (average particle diameter or layer thickness). Magnetic ordering temperature is reduced significantly compared to the bulk when size is reduced below 10 nm for both the layer and the particle geometry. Geometry has a significant effect on the maximum coercivity with particles tending to have larger coercivity than layers.
Random telegraph signals and 1/f noise in a silicon quantum dot
Authors: Peters M. G. , Dijkhuis J. I. , Molenkamp L. W.
Published: July 2002

We investigated random telegraph signals and 1/f noise in a submicron metal–oxide–semiconductor field-effect transistor at low temperatures in the Coulomb-blockade regime. The rich noise characteristics were studied as a function of gate voltage, drain current, and temperature, both in and beyond the Ohmic regime. The results can be understood within a simple model assuming a uniform potential fluctuation of constant magnitude at the location of the dot. Clear signatures of electron heating are found from the noise at higher currents.
Debye temperature of erbium-doped yttrium aluminum garnet from luminescence and Brillouin scattering data
Authors: Beghi M. G. , Bottani C. E. , Russo V.
Published: July 2002

Erbium-doped yttrium aluminum garnet (YAG) has been investigated by Brillouin scattering and photoluminescence (PL) spectroscopy in wide temperature (77–500 K) and erbium concentration (0.1%–50%) ranges. A characteristic temperature Θ, connected to the Debye temperature, has been evaluated by two different methods: from the thermal shifts and broadening of PL spectral lines, and from the elastic constants, estimated by Brillouin scattering. Both methods show that the Debye temperature weakly depends on the doping level (up to 50 at.%) of the erbium ions. This means that YAG is an ideal host for erbium ions; this result is relevant for application of rare earth elements in YAG crystal in the fields of telecommunications and optical devices.
Long-range atomic ordering and magnetic properties of Co1−xPtx/SiO2 granular films
Authors: Chen Chen , Kitakami Osamu , Okamoto Satoshi , Shimada Yutaka
Published: July 2002

The structure and magnetic properties of Co1−xPtx/SiO2 granular films with the Pt atomic content x varying from 0.42 to 0.71 have been investigated. It was found that the CoPt grains transform from disordered fcc to the ideal ordered L10 structure after annealing at 680 °C. In the range of x=0.42–0.60, the lattice parameter ratio a/c slightly varies from 0.965 to 0.979. For the Pt content x∼0.7, the superstructure exists but the lattice constant ratio is equal to 0.998. The coercivity dramatically increases from 1.8 to 9.1 kOe and then decreases to several hundred Oe while x increases from 0.42 to 0.71 for the grain size d∼10 nm. We also found that the coercivity increases with the grain size. Our results show that the highly ordered fct structure, accompanied by the high crystalline anisotropy, is a prerequisite for the high coercivity, and the coercivity can be tailored by controlling either the composition or the grain size.
Monte Carlo simulation of ferroelectric domain structure and applied field response in two dimensions
Authors: Potter B. G. , Tikare V. , Tuttle B. A.
Published: July 2002

A two-dimensional, lattice-Monte Carlo approach, based upon the energy minimization of an ensemble of electric dipoles, was developed to simulate ferroelectric domain behavior. The model utilizes a Hamiltonian for the total energy based upon electrostatic terms involving dipole–dipole interactions, local polarization gradients, and the influence of applied electric fields. The impact of boundary conditions on the domain configurations obtained was also examined. In general, the model exhibits domain structure characteristics consistent with those observed in a tetragonally distorted ferroelectric. The model was also extended to enable the simulation of ferroelectric hysteresis behavior. Simulated hysteresis loops were found to be very similar in appearance to those observed experimentally in actual materials. This qualitative agreement between the simulated hysteresis loop characteristics and real ferroelectric behavior was also confirmed in simulations run over a range of simulation temperatures and applied field frequencies.
Properties of the short-lived triplet states of pyridazine and 3,6-dichloropyridazine studied by a time-resolved EPR method
Authors: Terazima Masahide , Yamauchi Seigo , Hirota Noboru
Published: July 2002

The magnetic and decay properties of the short-lived triplet (T1) states of pyridazine and 3,6-dichloropyridazine (DCP) were studied by means of time-resolved EPR with laser excitation. The zfs of DCP in the p-dibromobenzene (DBB) host are X=0.094 cm−1, Y=−0.164 cm−1, and Z=0.070 cm−1, and the nitrogen hfcc are Ax=27 G, Ay=18 G, and Az=25 G, which confirms the nπ* character of the T1 state. Similar zfs were obtained for DCP and pyridazine in other environments. The sublevel decay rate constants and the relative populating rates were determined from the analysis of the transient EPR signals measured at 3.0 K where the effect of the spin lattice relaxation can be neglected. It was found in both systems that ky is much greater than kx, kz as predicted by a simple group theoretical consideration. ky of DCP and pyridazine were determined to be on the order of 106 s−1, being larger than that of pyrimidine by a factor of ∼104. The reasons for the large ky are discussed, particularly, in terms of the distorted potential of the T1 state.
Liquid drops of polar molecules
Authors: Shreve A. P. , Walton J. P. R. B. , Gubbins K. E.
Published: July 2002

We report a molecular dynamics study of liquid drops composed of Stockmayer molecules. We calculate density profiles, the density-orientation profiles, liquid and vapor pressures, the Irving–Kirkwood (IK) pressure tensor, surface tensions, the positions of surfaces of tension, and Tolman’s length δ. In the process of performing these calculations we present a novel computational method for the IK pressure tensor for inhomogeneous molecular fluids that is derived from stress arguments. The effects of curvature on the vapor–liquid interface of a Stockmayer fluid are discussed at a qualitative level.