bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental and calculational study on diamond growth by an advanced hot filament chemical vapor deposition method
Authors: Kondoh Eiichi , Ohta Tomohiro , Mitomo Tohru , Ohtsuka Kennich
Published: July 2002

An advanced hot filament chemical vapor deposition (AHF-CVD), which enables independent control of CVD parameters, was performed to investigate the effect of each CVD parameter upon diamond growth. The simple configuration of this AHF-CVD system allows modeling as a one-dimensional plug flow reactor. Gas-phase chemical reactions were modeled by taking into account the temperature distribution between the filament and the substrate, to assess the role of such chemical species as hydrogen atom and methyl radical, in diamond growth. From a comparison between the experimental and calculated results, it was strongly suggested that the methyl radical is a promising diamond precursor in a system using CH4-H2 as the source gas, and that a well-crystallized diamond would be deposited under the conditions of a large proportion of hydrogen atoms with respect to the precursor.
Calibration of the ruby R1 and R2 fluorescence shifts as a function of temperature from 0 to 600 K
Authors: Ragan Deirdre D. , Gustavsen R. , Schiferl David
Published: July 2002

Recent work by Gupta and Shen [Appl. Phys Lett. 58, 583 (1991)] has shown that in a nonhydrostatic environment, the frequency of the ruby R2 line provides a reliable measure of the mean stress or pressure. When using the frequency of either the R1 or R2 line to measure pressure at nonambient temperature, it is necessary to know the temperature dependence of the line shift. Unfortunately, the shift of the R2 line with temperature has not been reported. The ruby R1 and R2 fluorescence shifts have been determined as a function of temperature from 15 to 600 K. Both can be fitted very well to the simple cubic forms R1(T) =14 423+4.49×10−2T−4.81×10−4T2+3.71×10−7T3 cm−1 and R2(T)=14 452 +3.00×10−2T−3.88×10−4T2+2.55×10−7T3 cm−1. From 300 to 600 K the shifts fit well to linear functions of temperature. In addition, it is found that the R1-R2 splitting changes by about 3 cm−1 over the 600 K temperature range. Linewidths were found to vary both with temperature and from sample to sample.
In situ x-ray diffraction studies of YBa2Cu3Ox
Authors: Williams S. , Zheng J. Q. , Shih M. C. , Wang X. K. , Lee S. J. , Rippert E. D. , Maglic S. , Kajiyama Hiroshi , Segel D. , Dutta P. , Chang R. P. H. , Ketterson J. B. , Roberts T. , Lin Y. , Kampwirth R. T. , Gray K.
Published: July 2002

Using a specially designed off-axis faced magnetron sputtering chamber we have performed in situ x-ray diffraction studies of the growth of YBa2Cu3Ox films using a synchrotron light source. The orientation and rocking curve width were studied as a function of substrate temperature, O2/Ar partial pressures, and deposition rate. Growth rate was studied on SrTiO3, LaAlO3, and MgO.
A new method for extracting the trap energy in insulators
Authors: Fleischer S. , Lai P. T. , Cheng Y. C.
Published: July 2002

A simple procedure is reported for extracting the energy of trap levels from the I-V characteristics of insulators at room temperature. It is shown that by plotting ln(JE) vs 1/E it is possible to obtain the trap energy directly from the slope, and an estimate of the trap density from the intercept. Furthermore, it is demonstrated that our simplified, trap-assisted tunneling equation differs from the exact solution by only ∼1%–3% for ‘‘typical’’ nitridation-induced trap energies (2–3 eV), and for fields above 4 MV/cm. At lower trap energies this error is shown to be manifested only in the extracted trap densities and hence the new model can still be used to obtain the trap depth. For nitrided-oxide capacitors it is shown that this nitridation-induced trap energy is ∼2.2 eV which is in good agreement with the work of others. For transistors fabricated on the same wafers evidence is found for a distribution of shallow (≤1 eV) traps which could be due to damage introduced during the source/drain implant.
Voltage dependence of electron dynamics in high-power extended planar-anode diodes
Authors: Sanford T. W. L. , Poukey J. W. , Halbleib J. A. , Mock R. C. , McAtee W. H.
Published: July 2002

Measurements confirm the ability of a numerical model to simulate basic features of electron flow and subsequent radiation output from EPA (extended planar-anode) diodes on HERMES III. HERMES III is a 19 MV, 700 kA, 25 ns electron accelerator. These diodes simultaneously terminate a coaxial MITL (magnetically insulated transmission line), and produce intense bursts of bremsstrahlung. The measurements and modeling show that (a) the EPA diode provides an impedance match to the MITL, (b) parapotential flow theory and the range of H− ions generated at the cathode can be used to measure the time-dependent voltage pulse and peak voltage VP, respectively, and (c) that ∼95% of the electrical energy transmitted to the diode can be converted to kinetic energy of electrons incident on bremsstrahlung targets. Further, it is demonstrated that the forward radiation fluence can be described by DA/Q=(76±11)Ṽ1.61 over the VP range 13–19 MV. DA, Q, and Ṽ refer to the forward radiation fluence measured in Mrad(CaF2) cm2, the incident charge measured in C, and the median current-weighted accelerating voltage measured in MV, respectively. The dependence on VP of the beam characteristics at the targets is shown and the detailed kinetic-energy distribution there is discussed.
Vector Preisach modeling (invited)
Authors: Charap Stanley H. , Ktena Aphrodite
Published: July 2002

While the scalar Preisach model has long served as a description of hysteretic processes, the response of ferromagnetic materials is essentially vector in character and scalar models are likely to fail in important respects for many advanced applications. A 2D vector Preisach model has been developed for the simulation of magnetic recording and playback for oriented media. The objectives of the model are (i) faithful reproduction of both scalar and rotational measurements made on bulk material and (ii) calculational efficiency for use in the simulations. Past work on vector hysteresis models is reviewed. The present model is a superposition of Preisach models with different orientation directions. On the basis of the properties of the single-domain uniaxial particle, the effect of the transverse field is included as the boundary between ‘‘up’’ and ‘‘down’’ magnetization on the Preisach plane is calculated; then the reversible rotation is calculated as well. The performance of the model in describing measured bulk properties is presented.
A study of complex permeability in rare earth-substituted cobalt/nonmagnetic transition metal amorphous thin films
Authors: Russat J. , Suran G. , Ouahmane H. , Rivoire M. , Sztern J.
Published: July 2002

By using radio-frequency (rf) sputtering, amorphous ferromagnetic thin films of composition (Co93Zr7)100−yREy (RE=Nd, Pr, Dy, Tb, Gd, or Sm: magnetic rare earth elements) have been prepared. This work was performed for small amounts of RE substitution (y<3). These films exhibit static soft magnetic properties similar to CoZr alloys. For all compositions studied, a low in-plane uniaxial anisotropy Hk appears, whose magnitude depends upon composition and preparation parameters. Above a critical substitution level, a biaxial anisotropy exhibiting an in-plane Hk and a perpendicular component Hp is measured for films containing Gd or Sm. The substitution by RE atoms essentially affects the frequency-response shape of complex permeability, depending on nature, concentration of RE atoms and on values of both Hk and Hp. High-frequency permeability spectra could be well fitted by a theoretical model based on magnetization changes due to rotation, using a composition-dependent Landau–Lifshitz damping term. The results on films exhibiting uniaxial or biaxial anisotropy are reported and discussed.
Temperature dependence of the Schottky barrier in Al/AlGaAs metal-semiconductor junctions
Authors: Revva P. , Langer J. M. , Missous M. , Peaker A. R.
Published: July 2002

The dependence on temperature and alloy composition of the Schottky barrier height of Al on AlxGa1−xAs metal-semiconductor junctions for n- and p-type substrates and 0<x<1 is reported. All the structures were grown by molecular beam epitaxy. The compositional dependence of the barrier heights is the same as that of the band offsets in GaAs/GaAlAs heterojunctions. The barrier height for the p-type substrates is practically independent of temperature over the whole composition range, while for the n-type substrates the temperature change of the Schottky barrier follows that of the energy gap. This observation questions validity of the class of models of the Schottky barrier formation based on the concept of a neutrality level. Such behavior can, however, be reconciled if localized defects, whose ground-state wave function is of a bonding type are the source of the Fermi-level pinning at the interface.
Interface structure and Schottky barrier height of buried CoSi2/Si(001) layers
Authors: Werner P. , Jäger W. , Schüppen A.
Published: July 2002

The atomic interface structure of implanted buried layers in (100) oriented Si substrates has been characterized by quantitative high-resolution transmission electron microscopy on cross-section specimens. The buried layers were produced by high-dose Co+ ion implantation [100 and 200 keV Co+ ions, (1–2)×1017 cm−2] at 350 °C and subsequent rapid thermal annealing at 750 and at 1150 °C. Planar interface regions of high perfection with domains of different atomic interface structures, and interface steps, frequently with {111} facets, were observed. Comparison with computer-simulated images for various interface models yields evidence for interface regions with six-fold and eight-fold coordination of the Co interface atoms. Furthermore, regions with interfaces showing a continuous transition as well as Co-rich interfaces were found. Measurements of the Schottky barrier heights have been performed and show smaller values for the upper CoSi2/n-Si(001) interfaces than for the lower ones. Possible correlations between the interface structures and the resulting electronic properties are discussed.
Arc characteristics in gas-metal arc welding of aluminum using argon as the shielding gas
Authors: Jönsson P. G. , Westhoff R. C. , Szekely J.
Published: July 2002

A mathematical model has been developed describing transport phenomena in gas-metal arc welding. In the statement of the model a cylindrical electrode was considered and attention was concentrated on representing the electrodynamic, heat-transfer, and fluid-flow phenomena in the plasma column. Solutions were generated for the axisymmetric Maxwell’s equations, Navier–Stokes equations, and thermal-energy balance equation for variable properties. The specific system considered involved the use of an aluminum electrode and argon as the shielding gas. Several current levels were explored and the theoretical predictions of temperatures were found to be in good agreement with spectroscopically measured temperatures. This appears to have been the first time that gas-metal arc-welding problems were treated in such a fundamental manner.
Nematic director-fields captured in polymer networks confined to spherical droplets
Authors: Crawford G. P. , Polak R. D. , Scharkowski A. , Chien L.-C. , Doane J. W. , Zumer S.
Published: July 2002

The director-field of a nematic liquid crystal droplet is captured by a polymer network. The anisotropic polymer network is formed by photopolymerizing a small amount of monomer (2 wt %) in a nematic solvent suspended in a glycerin matrix. The glycerin matrix enforces parallel surface anchoring and a lecithin dopant is used to promote homeotropic surface anchoring. Polymer networks formed under these surface conditions as well as under applied electric fields are characterized with optical polarizing microscopy.
A study of strain in thin epitaxial films of yttrium silicide on Si(111)
Authors: Siegal Michelle F. , Martínez-Miranda L. J. , Santiago-Avilés J. J. , Graham W. R. , Siegal M. P.
Published: July 2002

We present the results of an x-ray diffraction analysis of epitaxial yttrium silicide films grown on Si(111), with thicknesses ranging from 14 to 100 Å. The macroscopic strain along the out-of-plane direction for films containing pits or pinholes follows the trend observed previously in films of thicknesses up to 510 Å. The out-of-plane lattice parameter decreases linearly with film thickness. We show preliminary evidence that pinhole-free films do not follow the above trend, and that strain in these films has the opposite sign than in films with pinholes. Finally, our results also indicate that the mode of growth, coupled to the interfacial thermal properties of the films, affects the observed value for the strain in the films.
Magnetic properties of a novel Pr-Fe-Ti phase
Authors: Li Hong-Shuo , Suharyana  , Cadogan J. M. , Bowden G. J. , Xu Jian-Min , Dou S. X. , Liu H. K.
Published: July 2002

In a systematic study of the (Pr1−xTix)Fe5 alloy series, the (Pr0.65Ti0.35)Fe5 alloy has been found to have a dominant phase with either the rhombohedral Th2Zn17 structure or the newly discovered Nd2(Fe,Ti)19 (S. J. Collocott, R. K. Day, J. B. Dunlop, and R. L. Davis, in Proceedings of the Seventh International Symposium on Magnetic Anisotropy and Coercivity in R-T Alloys, Canberra, July 1992, p. 437) structure, depending on the annealing procedure. Powder-x-ray-diffraction patterns and scanning electron microscopy show that the sample annealed at a temperature of 850 °C followed by 1000 °C has the 2:17 structure whereas annealing at 1000 °C directly leads to the new 2:19 structure. Energy-dispersive x-ray analysis yields Pr:Fe:Ti ratios of 10.7:86.2:3.1 for the Pr2(Fe,Ti)17 phase and 9.2:85.9:4.9 for the Pr2(Fe,Ti)19 phase. 57Fe Mössbauer spectroscopy (at 295 K) gives values for the average 57Fe hyperfine field of 15.7 T for the 2:17 phase and 17.5 T for the 2:19 phase, respectively.
Effects of nanocrystallization upon the soft magnetic properties of Co-based amorphous alloys
Authors: Quintana P. , Amano E. , Valenzuela R. , Irvine J. T. S.
Published: July 2002

Amorphous samples of Vitrovac(R) 6025 metallic alloys were nanocrystallized by thermal annealing at temperatures in the range 576–716 K, well below the crystallization temperature (831 K, as obtained by differential scanning calorimetry at 20 K/min) and times in the range 10–90 min. Initial magnetic permeability exhibited a maximum of ∼109 000 (from ∼20 000 for the as-quenched state) for annealing times of ∼15 min, and then a decrease toward a plateau value (30 000–40 000) for longer times. Domain wall relaxation frequency showed the opposite behavior, a minimum of ∼9 kHz (from 40 kHz for the as-quenched value) for the permeability maximum, and then a stabilization at ∼15 kHz. Transmission electron microscopy observations confirmed that for annealing times up to 60 min., crystallite size is smaller than 30 nm. Permeability and relaxation frequency results are interpreted in terms of the dependence of wall bulging on anisotropy and free-wall surface area between pinning sites.
Reversal mechanisms in Tb/Fe multilayers
Authors: O’Grady K. , Thomson T. , Greaves S. J. , Bayreuther G.
Published: July 2002

Reversal processes in Tb/Fe multilayer films are reported. The form of the hysteresis is explained as a function of the number of bilayers in terms of a two-coercivity model. Remanence measurements have been made which provide an insight into the distribution of the energy barriers for the two coercive processes associated with nucleation or domain-wall motion. The time dependence of magnetization has also been measured and this has been used to evaluate fluctuation fields and activation volumes for the series of films. The variation of the fluctuation field with applied field indicates that the two-coercivity model proposed is consistent with the time dependence. An increase is seen in the average activation volume with the number of bilayers.
Equations for electrical and electrothermal simulation of anisotropic semiconductors
Authors: Lindefelt U.
Published: July 2002

The equations for electrothermal simulation of semiconductor devices within the drift-diffusion approximation for anisotropic materials are presented. The heat generation source term in the heat conduction equation is likewise generalized to describe anisotropic materials. It is demonstrated that a simple extension of the mathematical form of the equations make them applicable to practically all semiconductor materials with a Bravais lattice.
A description of the temperature dependence of the conductivity for composite polymeric electrolytes by effective medium theory
Authors: Wieczorek W. , Siekierski M.
Published: July 2002

The temperature dependence of the conductivity of composite polymeric electrolytes based on a poly (ethylene oxide) matrix is described by a model based on effective medium theory. The influence of grain size distribution of the powder used as well as grain concentration on the conductivity of the electrolytes studied is analyzed. It will be shown that increases in the concentration of inorganic fillers influence the conductivity of the interface layer covering the filler. This layer is assumed to be responsible for an increase in the conductivity of composite polymeric electrolytes in comparison to pristine poly (ethylene oxide)-based systems. Utilization of effective medium models to study the temperature dependence of the conductivity of composite electrolytes enables us to predict the applicability of the compensation law for studies of the entropy effect in the electrolytes studied. The characteristic order-disorder temperature found from calculations based on effective medium theory is equal to that experimentally found and corresponds to the melting temperature of the crystalline polymer phase.
Schottky barrier height modification on n- and p-type GaInP with thin interfacial Si
Authors: Miller T. J. , Backes G. B. , Nathan M. I.
Published: July 2002

Al/n:GaInP and Al/p:GaInP Schottky diodes have been grown by molecular-beam epitaxy with and without thin (6 Å) interfacial Si layers. The Schottky barrier heights were measured by C-V, I-V, and I-V-T techniques. The n- and p-type barrier heights for the samples without interfacial Si were 0.86 and 0.93 eV, respectively. Interfacial Si enhanced the n-type barrier by 0.17 eV, and reduced the p-type barrier by 0.08 eV. The sum of the n- and p-type barrier heights for the samples with the Si layer was equal to the band gap of GaInP; without Si the sum was less. By comparing this data to the Al/Si/GaAs data, GaAs-GaInP conduction- and valence-band discontinuities of 0.05 and −0.41 eV, respectively, have been inferred.
Kr incorporation in sputtered amorphous Si layers
Authors: Greuter M. J. W. , Niesen L. , van Veen A. , Hakvoort R. A. , Verwerft M. , de Hosson J. Th. M. , Berntsen A. J. M. , Sloof W. G.
Published: July 2002

Amorphous Si layers were grown by krypton plasma sputter deposition at 310 °C. By pulsation of the substrate potential between 0 and 50 eV, the Kr concentration in the layers could be varied to a maximum of 5.5 at. %. A model which describes trapping of inert gas atoms in the sputtered layer in terms of implantation and trapping, diffusion, growth, resputtering, and gas sputtering is presented. High-resolution electron microscopy, electrode-probe (x-ray) microanalysis, positron annihilation, Raman spectroscopy, Mössbauer spectroscopy, and bending and hardness measurements were performed on the deposited layers. It turns out that the ion assisted growth leads to a strong reduction of open volume defects. The experiments point to the presence of very small Kr agglomerates. From the Mössbauer experiments a lower limit of 250 K for the Debye temperature of the Kr agglomerates is derived. Molecular-dynamic simulations from which the Debye temperatures of Kr mono-, di-, and trimers in amorphous Si can be derived are presented. The simulations indicate the presence of predominantly Kr monomers and dimers.
Structural characterization of oxidized titanium surfaces
Authors: Jobin M. , Taborelli M. , Descouts P.
Published: July 2002

Oxidized titanium surfaces resulting from various processes have been structurally characterized by means of scanning force microscopy, x-ray photoemission spectroscopy (XPS), x-ray diffraction, and electron energy-loss spectroscopy (EELS) with losses in the 0–100 eV range. It has been found that the surface morphology has a granular structure for electropolished titanium and for titanium evaporated on mica at low substrate temperature (570 K), but changes to flat terraces for the films evaporated at higher temperature (770 K). Angular-dependent XPS has revealed the presence of a Ti2O3 suboxide at the Ti/TiO2 interface for electropolished titanium. Dry oxidation has been performed at 770 and 970 K on both weakly and highly crystallized evaporated titanium films oriented along (0001). In the case of underlying crystallized metallic titanium, the resulting TiO2 films are crystallized with the anatase (004) orientation for oxidation at 770 K and with rutile (200) orientation for oxidation at 970 K. EELS spectra interpreted in terms of the molecular orbitals of a (TiO6)8− cluster show that the local octahedral environment of titanium atoms is preserved on native oxides, even if these oxides are not crystallized.
Two conducting spheres in a parallel electric field
Authors: van den Bosch H. F. M. , Ptasinski K. J. , Kerkhof P. J. A. M.
Published: July 2002

A method is presented to calculate the force exerted on a conducting sphere by another conducting sphere in a parallel electric field. The method allows any size ratio, distance, electric charge, and field angle. It uses a number of electric charge images and dipole images, depending on the required accuracy and the distance between the spheres.
Polycrystalline silicon formed by ultrahigh-vacuum sputtering system
Authors: Mishima Y. , Takei M. , Uematsu T. , Matsumoto N. , Kakehi T. , Wakino U. , Okabe M.
Published: July 2002

Using an ultrahigh-vacuum (UHV) sputtering system, we could grow two new methods of polycrystalline silicon films. The one is as-deposited polycrystalline silicon on glass at substrate temperatures under 500 °C. The other is solid-phase-crystallization by thermal annealing of as-deposited amorphous silicon films in a UHV. As-deposited polycrystalline silicon films were oriented to (220) and grain sizes were determined from half-width of x-ray diffraction to be about 40 nm. From the deposition temperature dependence of the x-ray diffraction peak intensity, the activation energy of the crystalline growth was calculated to be about 0.6 eV. Hydrogen atoms in the sputtering gas lower the reproducibility of as-deposited poly-Si. Polycrystalline silicon films produced by thermal annealing of as-deposited amorphous silicon films at 550 °C in UHV have a (111) orientation. Field-effect mobilities of the as-deposited polycrystalline silicon film and the polycrystalline silicon film by UHV thermal annealing were 5 and 18 cm2/V s, respectively.
Neutron diffraction analysis of Nd3Fe29−xTx (T=Ti, Cr, Mn)
Authors: Yelon W. B. , Hu Z.
Published: July 2002

Rietveld analysis of neutron-diffraction data from Nd3Fe29−xTx (T=Ti, Cr, and Mn) has been used to determine the location of the substitutional atoms and the magnetic moments. Reanalysis of the T=Ti data confirms that the space group A2/m is a better choice than P21/c, which had previously been used to describe the structure. The Ti atom locations and concentrations remain unaffected in the two space groups, but for the other substituents the refined concentrations are well behaved in A2/m, whereas in P21/c the refinements were unstable due to the symmetry relations between certain substituted sites. The site occupancies are analyzed in terms of steric and environment effects. A possible explanation for the high Curie temperature of the Cr compound is proposed.
Two-dimensional profiling of shallow junctions in Si metal-oxide-semiconductor structures using scanning tunneling spectroscopy and transmission electron microscopy
Authors: Yu E. T. , Barmak K. , Ronsheim P. , Johnson M. B. , McFarland P. , Halbout J.-M.
Published: July 2002

We have used cross-sectional scanning tunneling spectroscopy and cross-sectional transmission electron microscopy to perform two-dimensional profiling of shallow pn junctions in Si metal-oxide-semiconductor structures. In the tunneling spectroscopy measurements, clear differences were observed between current-voltage spectra obtained from p-type, n-type, and depleted regions of the metal-oxide-semiconductor structures; current images generated from the tunneling spectra revealed the profiles of the pn junctions with spatial resolution in both the lateral and vertical directions on the order of 10 nm. Calculated tunneling current-voltage spectra were found to be consistent with observed differences in experimental spectra obtained from p-type and n-type regions of the junctions. Junction profiles obtained by scanning tunneling spectroscopy have also been compared to transmission electron microscopy images of chemically delineated junctions and measurements of vertical dopant profiles obtained by secondary ion mass spectrometry.
Theory of microstructure and mechanics of the ...a1/a2/a1/a2... domain pattern in epitaxial ferroelectric and ferroelastic films
Authors: Romanov A. E. , Pompe W. , Speck J. S.
Published: July 2002

The micromechanics of the lamellar ...a1/a2/a1/a2... domain pattern are developed for the case of epitaxial tetragonal ferroelectric or ferroelastic films grown on (001) substrates. This problem is treated in the framework of the theory of defects. Coherency defects are necessary to maintain epitaxy as a result of the symmetry-reducing phase transition in the film (ferroelectric or ferroelastic transition). The defects include continuous distributions of edge and screw dislocations. The screw dislocation distributions are equivalent to an alternating Somigliana dislocation chain. Using this approach, fully analytic solutions are derived for the stress and strain in the film and substrate. These calculations include all the effects of the free surface. An integral expression is derived for the elastic energy. Using dipole and quadrupole approximations, analytic expressions are derived for the rotations of the crystal axes for individual domains, the elastic energy, and thickness dependence of the domain periodicity. We find, in agreement with previous work, that there is no critical thickness to form the ...a1/a2/a1/a2... domain pattern. For thick films the domain periodicity scales with the square root of the film thickness.
Domain structure of iron single crystals grown on Si(111) investigated by magnetic force microscopy (abstract)
Authors: Foss S. , Proksch R. , Moloni K. , Dahlberg E. D. , Cheng Y.
Published: July 2002

Single-crystal iron films grown epitaxially on (111) silicon were studied using a magnetic force microscope (MFM). The crystalline anisotropy easy axes (〈100〉), directed 35° out of the film plane, result in a significant magnetic charge density at the film surface and a sixfold symmetry in the energy minima of the system. To reduce the magnetostatic energy of this configuration, an alternating, stripe domain pattern formed in samples of thickness ranging from 1240 to 315 nm. Because of the wavelike form of the MFM response across the stripes, two-dimensional Fourier transforms of the MFM images were used to give measures at the domain period, i.e., the wavelength of the stripes, as well as the complexity of the patterns. The stripe domain period was found to be approximately equal to the film thickness which is consistent with previous theoretical predictions. The domain patterns of the films in various remanent states and progressive stages of the magnetization process were investigated using the MFM and an in situ, variable magnetic field. A MFM tip that is magnetically soft compared to the iron is most suitable for this. Measurements of the domain period and pattern complexity as a funciton of applied field were correlated with bulk hysteresis measurements.
Magnetic viscosity and microstructure: Particle size dependence of the activation volume
Authors: González J. M. , de Julián C. , Giri Anit K. , Castro S. , Gayoso M. , Rivas J.
Published: July 2002

Ba hexaferrite samples having average particle sizes in the range from the tens up to the thousands of nm have been prepared by using conventional ceramic techniques and the exothermic reaction between Ba and Fe nitrates and oxalic dihydrized acid. The magnetic characterization of these samples included the measurement of the temperature dependence of the high field magnetization, the coercive force, and the evaluation, from thermally activated demagnetization measurements, of the activation volume at different temperatures. From the results of this characterization it was possible to reach conclusions about the occurrence of remarkable differences between the average particle size dependence of the coercive force and that corresponding to the activation volume: whereas the coercive force value increased monotonically with the decrease of the particle size, an activation volume minimum was observed for particle diameters of the order of hundreds of nm. We suggest that this minimum reflects a change of the mechanism ruling the magnetization reversal of the particles forming our samples.
Phase transformation of sputter deposited tungsten thin films with A-15 structure
Authors: O’Keefe M. J. , Grant J. T.
Published: July 2002

Transformation of sputter deposited A-15 crystal structure tungsten thin films by rapid thermal annealing and in situ heat treatments in a transmission electron microscope was investigated. Phase transformation of A-15 W into body-centered-cubic (bcc) W did not occur at a specific annealing condition but over a range of annealing times and temperatures. However, complete A-15 to bcc W transformation occurred in all samples rapid thermal annealed at 650 °C for 1 min. Variations in the transformation time and temperature for each sample were attributed to differences in the oxygen concentration in the films. The higher the oxygen concentration in the film, the higher the temperature or longer the time required to transform A-15 W into bcc W. No measurable difference in the oxygen concentration before and after transformation was found and the O and W atom’s binding energies were the same in both the A-15 and bcc crystal structure. The resistivity of A-15 crystal structure films decreased significantly after transformation into the body-centered-cubic phase, on the order of 80%. The decrease in resistivity as a result of the transformation was attributed to the elimination of the A-15 defect structure and an increase in particle size by coalescence during transformation, from ∼5 nm in diameter in the as-deposited A-15 crystal structure to ∼25–40 nm in diameter after transformation into bcc W.
Solid solubility and diffusion of boron in single-crystalline cobalt disilicide
Authors: Zaring C. , Pisch A. , Cardenas J. , Gas P. , Svensson B. G.
Published: July 2002

The temperature dependence of the solid solubility [CsCoSi2(B)] and the lattice diffusion coefficient [DCoSi2(B)] of boron in single-crystalline cobalt disilicide (CoSi2) has been investigated between 450 and 1000 °C. Both CsMCoSi2(B) and DCoSi2(B) are found to be considerably higher than the corresponding quantities in silicon. Using a thermodynamical interpretation, the experimental data show that boron-doped CoSi2 can be described as a regular solution in the dilute limit with an enthalpy of solution of ∼−0.4 eV. The experimental data and theoretical estimates of the excess enthalpy of solution indicate a weak interaction of boron with the silicon and cobalt atoms in CoSi2 suggesting that the boron atoms occupy sites in the CoSi2 lattice with a small contribution to the Gibbs energy of the solution phase. The diffusion data yield a high mobility of the boron atoms with an activation energy of ∼2.0 eV for the lattice diffusion coefficient which is ∼1.0 eV lower than that reported for the self-diffusion of Si(Ge) and Co in CoSi2. Boron is a small atom which can occupy interstitial sites in the relatively opened CaF2 structure of CoSi2, and it is argued that the diffusion of boron may occur via a mixed process where interstitial/substitutional interchange takes place, including trapping and detrapping of the boron atoms.
Efficient method for calculating the transmission coefficient of two-dimensional quantum wire structures
Authors: Yiu Chinghong , Wang Jian
Published: July 2002

We present a very simple and efficient method for calculating the transmission coefficient of two-dimensional quantum wire structures based on the time-dependent solution of the Schrödinger equation. We apply the new method to a specific two-dimensional quantum wire structure. The new method is much faster than the finite element method and can be used to study electron transport in the presence of electron–phonon interaction and nonlinear interactions in the Schrödinger equation.
Organic films deposited on Si p-n junctions: Accurate measurements of fluorescence internal efficiency, and application to luminescent antireflection coatings
Authors: Garbuzov D. Z. , Forrest S. R. , Tsekoun A. G. , Burrows P. E. , Bulović V. , Thompson M. E.
Published: July 2002

Ultraviolet (UV) optical pumping followed by fluorescence wavelength downconversion of thin film organic light emitting materials deposited directly on the surface of Si p-n junction diodes is found to be an accurate and rapid means to determine the film internal fluorescence efficiency. By measuring the photoresponse of the Si detectors in the UV, we find that the organic light emitting films of aluminum tris-(8-hydroxyquinoline) (Alq3), N,N′-diphenyl-N,N′-bis-(3-methylphenyl)- 1,1′-biphenyl-4,4′-diamine, and bis-(8-hydroxyquinaldine)-chlorogallium (Gaq2′Cl), have internal fluorescence efficiencies of 0.30±0.05, 0.35±0.03, and 0.36±0.03, respectively. It has also been found that the organic films can be grown to a thickness which optimizes UV light conversion and can, at the same time, serve as antireflection coatings in the visible spectral region, thereby resulting in enhanced Si photodiode sensitivity extending from the UV to the infrared.
Characterization of a porous silicon diode with efficient and tunable electroluminescence
Authors: Lalic Nenad , Linnros Jan
Published: July 2002

Electrical and optical properties of electroluminescent porous silicon pn diodes have been investigated. The measurements were performed under pulsed conditions to maintain high quantum efficiency (∼0.2%) operation. Diodes with different peak wavelengths, obtained by varying the etching parameters, were used. A high correlation between electroluminescence (EL) and photoluminescence (PL) of porous silicon (PSi) structures was observed: similar luminescence peak position, spectral width, quantum efficiencies, decay time constants, and luminescence quenching at increasing temperature, suggesting the same recombination mechanism for both EL and PL. The temperature dependence of the forward current and EL revealed a thermally activated conduction mechanism with an activation energy of ∼0.1–0.2 eV which is attributed to potential barriers in the undulating silicon wires. EL quenching and a redshift of the EL peak at an increased temperature were also observed. This is attributed to shorter nonradiative lifetimes and a thermally enhanced escaping of carriers from ‘‘active’’ crystallites. Finally, exhausting of the EL during long pulse durations has been characterized and we propose a charging mechanism in the PSi network as a possible origin.
Surface conduction on insulators: Analysis and interpretation of the Faraday cage experiment
Authors: Wintle H. J.
Published: July 2002

We provide a quantitative analysis of the Faraday cage method for determining the leakage of electric charge across the surface of insulators, and correct some inconsistencies in earlier work. We show that it is possible to distinguish between the mechanisms of surface conduction and surface charge driven flow, since in the latter case the time constant depends on the amount of the initial charge and the logarithmic decay rate slows down at long times.
Scanning tunneling microscopy of epitaxial YBa2Cu3O7−x films prepared by thermal plasma flash evaporation method
Authors: Hayasaki Kei , Takamura Yuzuru , Yamaguchi Norio , Terashima Kazuo , Yoshida Toyonobu
Published: July 2002

The surface morphology of epitaxial YBa2Cu3O7−x films prepared by thermal plasma flash evaporation was extensively investigated by scanning tunneling microscopy. Under epitaxial film growth conditions with the deposition rate up to 0.42 μm/min, two-dimensional nucleus growth and spiral growth were observed. The main deposition species in this process was found to be the cluster ranging from 0.3 to 9 nm and the size of the cluster influenced the growth mode strongly. Theoretical analysis based on the two-dimensional critical radius revealed that smaller clusters became weakly bonded nuclei resulting in spiral growth and larger clusters became stable nuclei resulting in two-dimensional nucleus growth, which we named two-dimensional cluster nucleus growth. The clusters generated in the plasma boundary layer undoubtedly involve sufficient energy necessary for crystallization and show quite different characteristics from those of the clusters generated in vacuum by adiabatic expansion process. Hence, this process must be named “hot cluster epitaxy.’’
First-principles atomic cluster study of boron interactions in Ni3Al
Authors: Painter Gayle S. , Fu C. L. , Averill F. W.
Published: July 2002

First-principles atomic cluster calculations have been carried out in the local density approximation to understand the segregation behavior and strengthening effects of boron in Ni3Al. The binding energy of boron is calculated in lattice fragment clusters representing the perfect crystal, as well as various defect sites. The agreement between trends in energetics determined for small clusters and periodic supercells indicates the dominant role of boron’s interaction with nearest-neighbors of the host. The stereochemical factor underlying boron’s preferential bonding to nickel atoms in four-fold planar coordination (i.e., sp3 hybridization) suggests a mechanism for the boron-effect in Ni3Al: increased cohesion provides a driving force for B segregation to open sites, such as at Ni-enriched grain boundary sites, and the strengthening is a result of strong localized Ni–B covalent bond formation.
Zero-crossing steps in intrinsic Josephson junctions of Bi2Sr2CaCu2O8+δ
Authors: Johnson H. L. , Hechtfischer G. , Götz G. , Kleiner R. , Müller P.
Published: July 2002

We have investigated the response to microwave irradiation of small mesas fabricated on single crystals of Bi2Sr2CaCu2O8+δ. We observe zero-crossing steps in the current-voltage characteristic which are consistent with the intrinsic Josephson junctions in the mesa stack phase-locking to the external microwave field.
Experimental analysis of the phase dynamics in small parallel arrays of Josephson junctions
Authors: Gambardella Umberto , Grimaldi Gaia , Caputo Pasqualina , Pace Sandro
Published: July 2002

We analyze Fiske resonances of one-dimensional parallel arrays of underdamped Josephson tunnel junctions. They appear in the current voltage (I–V) characteristics as resonant current singularities (steps) at finite voltages Vm when a magnetic field H is applied perpendicular to the array cells. We present measurements of current step amplitudes Icm, and of the maximum Josephson current Ic0 as a function of H,for arrays made of four, six, and ten small Josephson junctions. The I–V characteristics of the arrays exhibit three, five, and eight resonant current steps, respectively, at increasing voltages. In all devices we find that the current amplitude of the highest order step has just one maximum occurring at H≈1/2H*, being H* the first field value where Ic0(H*)≈Ic0(0). Numerical simulations of the phase dynamics in small parallel arrays as a function of the applied magnetic flux are performed. The results of the simulation reproduce the experimentally observed features.
Characteristics of InGaN/AlGaN light-emitting diodes on sapphire substrates
Authors: Egawa T. , Jimbo T. , Umeno M.
Published: July 2002

We report characteristics and degradation of an InGaN/AlGaN double-heterostructure light-emitting diode (LED) grown by metalorganic chemical vapor deposition on a sapphire substrate. The InGaN/AlGaN LED exhibited an optical output power of 0.17 mW, an external quantum efficiency of 0.2%, a peak emission wavelength at 440 nm with a full width at half-maximum of 63 nm, and a stable operation up to 3000 h under 30 mA dc operation at 30 °C. However, the InGaN/AlGaN LED showed electrical and optical degradations under high injected current density and high ambient temperature. Electroluminescence, electron-beam-induced current and cathodoluminescence observations showed that the degraded InGaN/AlGaN LED exhibited formation and propagation of dark regions, which act as nonradiative recombination centers. The values of the degradation rate were determined to be 1.1×10−3, 1.9×10−3, and 3.9×10−3 h−1 under the injected current density of 100 A/cm2, and 1.6×10−2, 3.6×10−2, and 8×10−2 h−1 under 200 A/cm2 at ambient temperatures of 30, 50, and 80 °C, respectively. The activation energy of degradation was also determined to be 0.23–0.25 eV. The degradation of electrical and optical characteristics was caused by the growth of dark regions. It was also observed that GaN-based LEDs on sapphire substrates have longer lifetime than the ZnSe-based LED, but shorter than the AlGaAs and InGaAsP LEDs.
Interfacial reaction in the poly-Si/Ta2O5/TiN capacitor system
Authors: Lee Hoo-Jeong , Sinclair Robert , Lee Myeong-Bum , Lee Hyeon-Deok
Published: July 2002

For tantalum pentoxide capacitors in 1 Gbit dynamic random access memory, titanium nitride is adopted as the top electrode. After postannealing at temperatures higher than 750 °C, the capacitance of Ta2O5 reduces due to an interfacial reaction between TiN and Ta2O5. This was studied by high resolution electron microscopy and energy dispersive spectroscopy with a 1 nm electron probe. It is found that voids form along the interface between TiN and Ta2O5 and a large proportion of Ta dissolves into the TiN film. The origin of the reaction is concluded to be a large solid solubility of Ta in TiN. After Ta outdiffusion into TiN, vacancies agglomerate and form voids in Ta2O5 and thereby reduce its capacitance. Since the driving force of the reaction is the solid solubility of Ta in TiN, the amount of the reaction is affected by the thickness of the TiN films.
Magnetoresistance, Hall effect, and thermoelectric power in spin valves
Authors: Sato H. , Miya S. , Kobayashi Y. , Aoki Y. , Yamamoto H. , Nakada M.
Published: July 2002

Comparison of the magnetoresistance (MR) on two groups of spin-valve multilayers, NiO/NiFe/Cu/NiFe/Cu/NiO and NiO/NiFe/Cu/NiFe/Cu, has been made in order to investigate the possibility of the enhanced specular scattering at NiO/metal interface. No clear difference in MR between the two systems has been found, suggesting that the enhancement of specular scattering at interfaces is not the origin of the large MR. For the field direction almost perpendicular to the plane, we found a sensitive angular dependence of MR along with a large unidirectional anisotropy. To sort out any specific characteristics of the spin-valve system compared to the multilayers, thermoelectric power and Hall effect have been investigated for the first time.
Ni and Ni silicide Schottky contacts on n-GaN
Authors: Liu Q. Z. , Yu L. S. , Deng F. , Lau S. S. , Redwing J. M.
Published: July 2002

The electrical characteristics of Ni and NiSi Schottky diodes on n-GaN have been investigated as a function of annealing. Ni diodes were found to be stable up to 500 °C for 1 h in sequential annealing, with a barrier height φ (I–V) of 0.8–0.9 eV and an n factor of ∼1.1. The barrier height deduced from C–V measurements, φ (C–V), was typically 0.15 eV higher than φ (I–V). At 600 °C the diodes failed, and Ga was found to migrate into the Ni layer. NiSi diodes were stable up to 600 °C for 1 h, φ (I–V) was found to be about 0.8–1 eV with an n-factor of about 1.15. The value of φ (C–V) was between 0.3 to 0.6 eV higher than φ (I–V), consistent with the notion of the presence of a thin insulating layer at the NiSi/GaN interface. The electrical characteristics obtained in this study are also compared with those obtained for Pt and PtSi Schottky diodes on n-GaN.
Critical current in high Tc grain boundary junctions
Authors: Luine Jerome A. , Kresin Vladimir Z.
Published: July 2002

The temperature dependence of YBa2Cu3O7−δ (YBCO) grain boundary junction (GBJ) maximum critical current IM is measured and successfully described by a theoretical model that incorporates both the proximity effect and tunneling within a s_uperconductor-n_ormal-i_nsulator-n_ormals_uperconductor (SNINS) structure. The correspondence between experiment and theory shows that that GBJs can be described by an Sα-Nβ-I-Nβ-Sα structure where Sα, Nβ, and I are superconducting, normal conducting, and insulating states of YBCO. The different YBCO electrical properties within a GBJ are the result of oxygen loss at the grain boundary. Correspondence between experimental data and theoretical predictions is obtained in most cases with the adjustment of just one parameter: α=σ−1Lβ/ξ, where σ is the tunneling probability through the S–N interface, Lβ is the width of the normal barrier, and ξ is the coherence length in the superconductor Sα. The theory also predicts that grain boundary junction critical current density Jc is related to reduced normal resistance ρn by Jc∼ρnγ with −2⩽γ⩽−1 as widely observed. Thus, we postulate that many natural and artificial grain boundaries have an Sα-Nβ-I-Nβ-Sα structure and, therefore, the previously developed theory of Sα-Nβ-I-Nβ-Sα structures applied here has wide applicability to the study and analysis of YBCO grain boundary physics.
Kinetic aspects of the solid hydrogenation– disproportionation–desorption–recombination process in Nd13.67Co15.74Al0.77Ga0.27Zr0.03Fe62.2B7.33 alloys
Authors: Rodrı́guez Torres C. E. , Fernández van Raap M. B. , Sánchez F. H. , Mendoza Zélis L. , Actis F.
Published: July 2002

The hydrogenation–disproportionation–desorption–recombination process is known as a novel method for producing permanent magnets. The hydrogen driven reactions involved in this process are carefully studied here using Mössbauer effect, x-ray diffraction and scanning microscopy, in the Nd13.67Co15.74Al0.77Ga0.27Zr0.03Fe62.2B7.33 alloy. The temperature dependence of the hydrogenation disproportionation in the 119 kPa isochore line was obtained. Hydrogen absorption seems to occur in two steps: absorption from Nd at the grain boundaries followed by absorption from the Nd2Fe14B type phase. In order to determine the recombination kinetics, a typical mass of the alloy was submitted to disproportionation reaction at 800 °C under a hydrogen pressure of 119 kPa, followed by recombination, which was achieved by vacuum desorption at 800 °C and subsequent quenching to room temperature after various time intervals. It was found that the relative fraction of the Nd2(Fe,Co)14B phase, as obtained from the Mössbauer effect, increases as f=1−exp(−ktn) with n=0.99±0.05. From this growing law it is inferred that recombination occurs without long range diffusion and starts at the grain boundaries with zero nucleation rate. An activation energy of 0.64±0.13 eV/atom was obtained for this process.