bluuebunny/crossref_metadata_2025 huggingface dataset:
Deposition and analysis of silicon clusters generated by laser-induced gas phase reaction
Authors: Ehbrecht M. , Ferkel H. , Huisken F. , Holz L. , Polivanov Yu. N. , Smirnov V. V. , Stelmakh O. M. , Schmidt R.
Published: July 2002

Silicon clusters have been generated by CO2-laser-induced decomposition of SiH4 in a flow reactor. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. The mass spectra show that the cluster source emits, besides small clusters, also nanosized species containing around 103 atoms. These clusters were deposited on silicon and sapphire targets at room temperature. The deposited films have been analyzed with a Raman spectrometer and with a field emission scanning electron microscope (FE-SEM). The Raman spectra reveal a broad amorphouslike band and a relatively sharp peak at 518.1 cm−1. Interpretation of the sharp Raman feature, based on the phonon confinement model, suggests the presence of silicon nanocrystallites in the deposited films with a particle size of about 3–3.6 nm in diameter. The FE-SEM micrographs show an agglomerate of spherical particles of 3–12 nm in diameter, with a pronounced maximum in the size distribution at around 3.5 nm. The various methods of characterization allow us to conclude that the size of the nanoclusters is largely preserved if they are deposited on the substrate. Therefore, the technique presented here might be an efficient means to produce silicon quantum dots of about 3 nm in diameter.
A systematic study on structural and dielectric properties of lead zirconate titanate/(Pb,La)(Zr(1−x)Ti(x))O3 thin films deposited by metallo-organic decomposition technology
Authors: Zhu W. , Liu Z. Q. , Lu W. , Tse M. S. , Tan H. S. , Yao X.
Published: July 2002

Pin-hole free ferroelectric (Pb,La)(Zr1−xTix)O3 thin films with uniform composition have been fabricated using the metallo-organic precursor compounds, which were carefully home synthesized. The structural development, spectroscopic, and dielectric properties of these films have been systematically investigated using atomic force microscopy (AFM), x-ray diffraction, Fourier transform infrared spectroscopy, Raman scattering, and dielectric measurements. It has been found from our experimental results of PZT 40/60 thin films that the overlapping of (h00) and (00l) peaks of these films in x-ray diffraction patterns, mainly due to the small grain sizes in films, makes it very difficult to distinguish individual diffraction peaks and to identify the phases. However, Raman measurements undoubtedly reveal the Raman spectra of these films in the tetragonal phase field, demonstrating that Raman spectroscopy is an effective tool to identify structures, especially in the case of thin films having very small grains. AFM results show that the PZT perovskite structure in films may grow radially by rosettes and that microcracks appear in the three-dimensional AFM pictures at grain boundaries, which may be the cause for easy dielectric breakdown.

A striking feature of the AFM observation is that three polycrystalline perovskite regions intersect symmetrically at a point with 120° to each other, and a rosette growth model for the perovskite structure in PZT films is thus proposed to explain this new phenomenon. The excellent ferroelectric properties of these films, such as the high fatigue resistance and low leakage current, are attributed to the high quality of the metallo-organic solutions and to reduce the amount of oxygen vacancies in the films by optimizing the annealing conditions and by doping a suitable amount of La ions to minimize the charge blocking of oxygen vacancy at the interface by Pt electrode. It seems that the rhombohedral PZT films with softer hysteresis loops are suitable for nonvolatile random access memory application.
Numerical modeling of a two-coil apparatus for measuring the magnetic penetration depth in superconducting films and arrays
Authors: Turneaure Stefan J. , Ulm Eric R. , Lemberger Thomas R.
Published: July 2002

We determine the magnetic penetration depth λ(T) in superconducting films by measuring the mutual inductance of two coils located on opposite sides of the films. The apparatus is designed to produce an accurate value for λ(0) without any assumptions about the dependence of λ(T/Tc)/λ(0) on T/Tc. In a typical configuration of coils and a 100±10 nm thick by 6 mm radius film, λ can be measured to better than ±12% as long as λ≳1500 Å. The noise level is typically ±2 Å. If two films are made the same way, so that they have the same thickness d, and they are measured in the same apparatus, then the relative uncertainty in λ between the two films is only ±9%, because uncertainties in d, the coil dimensions, etc., are eliminated. This article describes the apparatus and a detailed numerical model which illustrates the induced current density in the film and establishes the sources of uncertainties. The accuracy of the model is demonstrated through comparison with measurements on 0.15 mm thick circular Pb disks.
Nanoindentation investigation of the Young’s modulus of porous silicon
Authors: Bellet D. , Lamagnère P. , Vincent A. , Bréchet Y.
Published: July 2002

Young’s modulus of porous silicon samples, with porosity ranging from 36% to 90%, is measured by the nanoindentation technique. The analysis of the nanoindentation data, including the specific problem linked with porous materials, is presented. The Young’s modulus values Ep thus obtained appear to be drastically dependent on the porosity and on the doping level (p or p+ type). The dependence of Ep versus the relative density (for a series of p+ type samples) is quadratic, in good agreement with the model of Gibson and Ashby developed for cellular materials. This also shows that highly porous silicon layers exhibit very low Young’s modulus (for a porosity of 90% it is about two orders of magnitude smaller than that of the nonporous material).
Thermal depth profile reconstruction by neural network recognition of the photothermal frequency spectrum
Authors: Glorieux C. , Thoen J.
Published: July 2002

A new neural network method for reconstructing the depth profile of thermally inhomogeneous materials from a photothermal spectrum is described in detail. The possibilities and limits of this kind of photothermal depth profiling are evaluated. The influence of the network architecture and several profile and signal parameters on the reconstruction quality is discussed. Statistical predictions are given for the accuracy of the reconstructed profiles for different levels of Gaussian noise in the signal and at different depths.
Switching mechanism in Co based amorphous wire
Authors: Nderu J. N. , Yamasaki J. , Humphrey F. B.
Published: July 2002

To get a deeper insight into the mechanism involved in the flux reversal of Co based wire we have tickled the wire with an ac field of smaller amplitude than the switching field, H*, and observed the signal induced in a pancake coil moved along the wire, vibrated a small coil along the wire and observed the induced signal, and taken the temperature dependence of normalized saturation magnetization, Ms, and H*. No signal was observed in the pancake coil in the tickling experiment. A signal that has peak values close to the wire ends, but practically zero in the central region, was observed in the vibrating coil. The variations of Ms and H* with temperature only show a correlation. The results of the above experiments suggest that for Co based wire the most reasonable mechanism for the flux switching is nucleation of a free wall other than the depinning of residual reverse domains at the wire ends.
Spectra and energy levels of trivalent holmium in strontium fluorapatite
Authors: Gruber John B. , Zandi Bahram , Seltzer Michael D.
Published: July 2002

Polarized absorption and fluorescence spectra are reported for Ho3+ (4f10) ions incorporated into single crystals of strontium fluorapatite, Sr5(PO4)3F, also known as SFAP. Site-selective excitation experiments indicate that, to within instrumental resolution, the Ho3+ ions occupy similar lattice sites having Cs symmetry. Spectra were obtained between 1940 and 400 nm at temperatures from 4 K to room temperature on crystals having the hexagonal structure [P63/m(C6h2)]. Individual energy (Stark) levels for the fourteen lowest-energy multiplet manifolds of Ho3+ (4f10) were analyzed in detail. These manifolds include the I52 (ground state), the remaining I5J manifolds, F5J, S52, K38(2), G56, and the G35(2) manifolds. A Hamiltonian consisting of Coulombic, spin-orbit, and crystal-field terms was diagonalized over these manifolds in an LSJMJ basis set, which includes 146 crystal-quantum states, labeled either Γ1 or Γ2, appropriate to the crystal-field symmetry of Cs. By varying the crystal-field parameters, Bnm, and the centroid for each manifold, we obtained agreement between 101 experimental and calculated Stark levels with a rms deviation of 9 cm−1. An additional 15 Stark levels, not included in the original analyses, are predicted to within the rms deviation. The predicted symmetry label of each Stark level agrees with the experimental assignment based on the analysis of the polarized spectra.
The relationship between the distribution of electronic states and the optical absorption spectrum of an amorphous semiconductor: An empirical analysis
Authors: O’Leary Stephen K. , Johnson S. R. , Lim P. K.
Published: July 2002

An elementary empirical model for the distribution of electronic states of an amorphous semiconductor is presented. Using this model, we determine the functional form of the optical absorption spectrum, focusing our analysis on the joint density of states function, which dominates the absorption spectrum over the range of photon energies we consider. Applying our optical absorption results, we then determine how the empirical measures commonly used to characterize the absorption edge of an amorphous semiconductor, such as the Tauc gap and the absorption tail breadth, are related to the parameters that characterize the underlying distribution of electronic states. We, thus, provide the experimentalist with a quantitative means of interpreting the physical significance of their optical absorption data.
On the incorporation of Mg and the role of oxygen, silicon, and hydrogen in GaN prepared by reactive molecular beam epitaxy
Authors: Kim Wook , Botchkarev A. E. , Salvador A. , Popovici G. , Tang H. , Morkoç H.
Published: July 2002

Mg-doped GaN samples prepared by reactive molecular beam epitaxy have been investigated in an attempt to gain insight into the impurity incorporation and the origin of auto doping in otherwise undoped GaN films. The Hall and secondary ion mass spectroscopy data were utilized for the analysis of possible background impurities such as Si, O, and H in an effort to ascertain whether the background electron concentration is of impurity origin or native defect origin. The data appear to support the N vacancy as a possible cause of auto-n-type doping seen in undoped GaN. The effect of the ammonia flow rate on the incorporation of Mg atoms in GaN films and on the behavior of H were studied for layers grown on c-plane sapphire as well as 6H–SiC. Increased incorporation of Mg with larger ammonia flow rates is attributed to Ga vacancies and accompanying site selection. Moreover, p-GaN films grown under high ammonia flux are reported with a hole concentration, mobility, and resistivity of about 8×1017 cm−3, 26 cm2/V s, and 0.3 Ω cm, respectively.
The synthesis, characterization, and mechanical properties of thick, ultrahard cubic boron nitride films deposited by ion-assisted sputtering
Authors: Mirkarimi P. B. , Medlin D. L. , McCarty K. F. , Dibble D. C. , Clift W. M. , Knapp J. A. , Barbour J. C.
Published: July 2002

Significant ion irradiation is needed during growth to synthesize cubic boron nitride (cBN) films. This results in large film stresses, which have limited cBN film thicknesses to only a few hundred nm and represents a significant barrier in the development of cBN film technology. Using a new hybrid deposition technique, we have synthesized cubic BN films up to 700 nm (0.7 μm) thick. A compositional and structural analysis of the films using several standard characterization techniques confirms that relatively thick polycrystalline films with a high cBN content were synthesized. Thicker cBN films enable hardness measurements to be undertaken without major substrate effects. Nanoindentation measurements yield hardness values for the cubic BN films up to 60–70 GPa, which are greater than values measured for bulk cBN. The measured elastic modulus was observed to be lower than the bulk, and this can be accounted for by an elastic deformation of the silicon substrate. The mechanical properties of the cubic BN films are discussed with reference to other ultrahard thin films such as diamond and diamondlike carbon.
Dielectric dispersion of porous media as a fractal phenomenon
Authors: Thevanayagam S.
Published: July 2002

It is postulated that porous media is made up of fractal solid skeleton structure and fractal pore surface. The model thus developed satisfies measured anomalous dielectric behavior of three distinctly different porous media: kaolin, montmorillonite, and shaly sand rock. It is shown that the underlying mechanism behind dielectric dispersion in the kHz range to high MHz range is indeed Maxwell–Wagner mechanism but modified to take into account the multiphase nature of the porous media as opposed to the traditional two-phase Maxwell–Wagner charge accumulation effect. The conductivity of the surface water associated with the solid surface and charge accumulation across the surface irregularities, asperity, and bridging between particles at the micro-scale-level pores are shown to contribute to this modified Maxwell–Wagner mechanism. The latter is dominant at low frequencies. The surface water thickness is calculated to be about 2–6 nm for a variety of porous media.
Longitudinal-transverse resonance and localization related to the random anisotropy in
a
-CoTbZr films
Authors: Suran G. , Boumaiz E.
Published: July 2002

The properties of a resonance mode, the existence of which is related to the random local anisotropy of the sample, are reported. It was detected in amorphous CoZrTb thin films of low Tb content, in which there exists an in-plane uniaxial anisotropy field. Here the angular dependence of the mode is shown. In agreement with theory, it changes from a pure longitudinal to a mixed longitudinal transverse and a pure transverse state as a function of the orientation of the external field. The process is accompanied by the development of a very pronounced Anderson localization process due to the high value of the Tb related local anisotropy Kl.
Exchange biasing of permalloy films by MnxPt1−x: Role of composition and microstructure
Authors: Krishnan Kannan M. , Nelson C. , Echer C. J. , Farrow R. F. C. , Marks R. F. , Kellock A. J.
Published: July 2002

The interfacial coupling energy, Δσ for polycrystalline Ni0.8Fe0.2 (100 Å)/MnxPt1−x (t Å)/SiO2 bilayers, has been determined from wedge samples spanning a MnxPt1−x composition range 0.42<x<0.76 and t∼350 Å. Δσ exhibits a sharp peak of 0.1 erg/cm2 at 51% Mn. We compare the magnetic and microstructural properties of two bilayer films, which have the same compositions of permalloy and MnxPt1−x: [Ni0.85Fe0.15(100 Å)/Mn0.49Pt0.51(t Å)]. In one case (normal structure), the epitaxial permalloy was grown first onto Pt/MgO(001), followed by 170 Å of Mn0.49Pt0.51. In the other structure (inverted), a 350 Å thick polycrystalline film of Mn0.49Pt0.51 was grown onto a SiO2 substrate, followed by permalloy. The structures exhibited very different exchange bias fields (32 versus 86 Oe) and Δσ values (0.021 versus 0.060 erg/cm2), respectively. Significant differences in texture, grain size, and Mn0.49Pt0.51 layer thickness for the two samples are reported.
Time- and temperature-dependent wetting behavior of eutectic SnPb on Cu leadframes plated with Pd/Ni and Au/Pd/Ni thin films
Authors: Kim P. G. , Tu K. N. , Abbott D. C.
Published: July 2002

We have studied the wetting behaviors of eutectic SnPb solder on Cu leadframes plated with Pd/Ni and Au/Pd/Ni thin films. For the Pd/Ni, we used two thicknesses of Pd of 760 and 2500 Å to study the thicknesses effect. For the Au/Pd/Ni, we used either immersion Au (50 Å) or acid Au (50 Å) flashed on 1000 Å Pd to study the effect and method of adding Au to the Pd surface. In addition, we investigated the wetting on a PdSn3 compound surface made by plating Sn or eutectic SnPb on Pd/Ni samples and then stripping the Sn or SnPb layer off. In all these samples, we found the wetting behavior to be time and temperature dependent. No stable wetting angles could be observed; the diameter of the solder caps increased with wetting time. Yet rather surprisingly, the diameter decreased with increasing wetting temperature from 200 °C to 240 °C. The largest cap diameter was observed at the lower temperature of 200 °C for Pd/Ni and stripped Pd/Ni, but at the intermediate temperature of 220 °C for Au/Pd/Ni. The smallest cap diameter was obtained at the higher temperature of 240 °C for all substrates. In examining the interfaces, we found the reaction products to be a ternary Pd–Ni–Sn compound and Ni3Sn4 for all substrates. The ternary compound grains were broken off from the interface, and scattered into the molten solder as soon as the Ni3Sn4 was formed. The latter forms a rather uniform layer consisting of small scallop-type grains on the unreacted Ni. We found that the growth rate of Ni3Sn4 is much slower, by a factor of about 10, than that of Cu6Sn5 in the reaction between Cu and eutectic SnPb.
Photostimulated impurity absorption of ultrasound in semiconductor nanostructures
Authors: Nunes O. A. C. , Fanyao Q. , Santos W. , Fonseca A. L. , Agrello D.
Published: July 2002

The problem of attenuation of ultrasound in a semiconductor with a quantum well structure with infinite potential barriers is tackled for the case in which the acoustic lattice vibrations interact with impurity centers in the presence of an exciting laser field. The electron transitions from an acceptor-impurity “band” to the first quantized energy level of the conduction band, forbidden in the absence of the laser field for the absorption of an acoustic wave quantum, become permitted in the presence of the external high frequency field. The external laser field will then supply the energy deficit for the electron to make the transition to absorb the acoustic phonon. The total ultrasound absorption coefficient αT was calculated using second-order perturbation theory and the result was specialized for the case of a GaAs/AlGaAs quantum well sample. It was found that αT is a fairly large quantity as compared with its value in bulk as the well width decreases from values corresponding to the almost bulk situation.
Basics mechanisms for enhanced prompt charge collection in a n+p junction following single charged particle interaction
Authors: Islam N. E. , Pugh R. D. , Brothers C. P. , Shedd W. M. , Singaraju B. K. , Howard J. W. , Dussault H. , Fageeha O.
Published: July 2002

This study shows that in the early stages of a single particle transient, the presence of residual holes in the depletion region of a n+p junction affect depletion profile and terminal currents. Due to their greater mobility, electrons deposited in the depletion region move out faster than holes, leaving residual holes in the region. The resulting charge imbalance in the depletion region may cause the depletion region to briefly extend into the substrate before the beginning of recovery process. This extended depletion region influences prompt collection of carriers in the vicinity. In particular, carrier response to this field produces a conduction and/or displacement current. The sum of the conduction plus displacement current is immediately seen as an enhanced prompt conduction current at the device terminals. Enhanced prompt charge collection depends on substrate doping, junction bias, and the density of charge deposited. Low-doped p-type substrates, with longer hole residence time in the depletion region, are more susceptible to enhanced prompt collection and substrate field disturbances. Response of p-Si and n-Si substrates to single event effects and the effects of laser beams for enhanced prompt collection are also analyzed.
Experimental setup and electrical characteristics of an inductively coupled plasma
Authors: Godyak V. A. , Piejak R. B. , Alexandrovich B. M.
Published: July 2002

This article describes an experimental system that has been used to study inductively coupled plasma (ICP). The aim of this effort was to generate an experimental ICP database over a wide range of discharge conditions (power, frequency, and gas pressure) obtained in the same experimental device and using the same diagnostic means. Design of the experimental setup and diagnostics for measurement of ICP external electrical characteristics (coil voltage and current), power dissipated in plasma and radio frequency plasma potential are given here together with experimental results obtained over a wide range of discharge conditions.
Resonance modes and losses in 1-3 piezocomposites for ultrasonic transducer applications
Authors: Geng Xuecang , Zhang Q. M.
Published: July 2002

The article presents the results of a recent investigation on the ultrasonic performance of 1-3 piezocomposites. Using a guided wave approach, the electromechanical properties of the thickness resonance are modeled and the results are compared with the experimental data. The influence of various losses in a 1-3 composite on the dispersion curves and the quality factor for the thickness mode is examined. It is found that the reduction in the quality factor of a composite compared with piezoceramic is mainly due to the acoustic coupling between the two constituents. Even for a composite with the mechanical Q of the polymer higher than that of the ceramic, the mechanical Q of the composite is still lower than that of the ceramic except when the ceramic volume content is very low. Hence, in most of piezoceramic polymer composites, the mechanical Q of the ceramic phase plays a major role in determining the quality factor of a 1-3 composite transducer. For the lateral modes in a 1-3 composite, it is found experimentally that the frequencies of the two lowest lateral modes can be determined approximately by the shear wave velocity and the width of the polymer gap over a broad ceramic volume fraction range, suggesting that they correspond to the half-wave standing waves in the polymer gap.
Influence of the hole transport layer on the performance of organic light-emitting diodes
Authors: Giebeler Carsten , Antoniadis Homer , Bradley Donal D. C. , Shirota Yasuhiko
Published: July 2002

We investigate the influence of the hole-transporting layer (HTL) on the performance of bilayer vapor-deposited organic light-emitting diodes. Three different HTL materials were used: m-MTDATA, triphenyl-diamine, and naphthyl-phenyl-diamine. In all cases, Alq3 was the electron-transporting layer (ETL). We measure and compare the current density-voltage (J–V) and luminance–voltage (L–V) characteristics of these devices and we conclude that the operating voltage is controlled by the type of HTL used and the nature of the hole-injecting indium tin oxide/HTL interface. We found that the device quantum efficiency depends not only on the electron transport characteristics of the ETL but also on the energetics of the HTL/ETL interface. Analysis of the J–V characteristics suggests that current flow in bilayer devices cannot be described sufficiently by a single carrier theory; both hole and electron currents should be considered.
On the cation distribution in Ni1−xCuxFe2−yAlyO4 spinels
Authors: Fayek M. K. , Ata-Allah S. S. , Refai H. S.
Published: July 2002

Two spinel solid solution series, Ni1−xCuxFe2−yAlyO4 with y=0.6 and 1.0, where 0.0⩽x⩽1.0 have been investigated by x-ray diffractometry and Mössbauer spectroscopy. Lattice, hyperfine parameters, and cation distribution among the two distinct crystallographic sites as a function of Cu2+ and Al3+ substitution are determined. Mössbauer effect (ME) spectra at 293 K for compositions with y=0.6 consist of one sextet for A sites, and three sextets for B sites due to different fields at Fe nuclei. On the other hand, the ME spectra for the compounds with y=1.0 in the ferrimagnetic state at (293 and 77 K) show only one sextet for Fe3+ at A site giving clear evidence for a completely inverse spinel with Ni, Cu, and Al ions occupying B sites. ME parameters are deduced and analyzed for all studied compounds, where the observed hyperfine fields are explained on the basis of the superexchange and supertransferred hyperfine interactions.
Process-dependent thermal transport properties of silicon-dioxide films deposited using low-pressure chemical vapor deposition
Authors: Ju Y. S. , Goodson K. E.
Published: July 2002

The volumetric heat capacity and thermal conductivity of silicon-dioxide films prepared using low-pressure chemical vapor deposition (LPCVD) are measured. The measurements employ the 3ω technique, which is extended to determine the thermal conductivity anisotropy and volumetric heat capacity of thin dielectric films. The thermal conductivity of the silicon-dioxide films exhibits a significant process dependence, which cannot be attributed to highly oriented microvoids or impurities. The volumetric heat capacity, in contrast, is largely independent of processing history provided that appropriate corrections are made to account for porosity and impurity contributions. This study provides evidence that process-dependent structural disorder strongly influences the thermal conductivity of amorphous films.
Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2
Authors: Schlaf R. , Schroeder P. G. , Nelson M. W. , Parkinson B. A. , Lee P. A. , Nebesny K. W. , Armstrong N. R.
Published: July 2002

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) allow the measurement of band bending with much higher accuracy than could be achieved in experiments carried out by UPS alone. These experiments also show that, due to the different depth sensitivity of high binding energy cutoff (secondary edge) and XPS core levels (or UPS valence bands), it is very important to grow relatively thick overlayers in order to measure orbital alignment and interface dipole correctly.
Photoinduced modification of photoluminescent and electroluminescent properties in poly(p-phenylene vinylene) derivative
Authors: Tada Kazuya , Onoda Mitsuyoshi
Published: July 2002

Modification of photoluminescent and electroluminescent properties due to photoirradiation in air of a poly(p-phenylene vinylene) derivative, poly(2-methoxy-5-dodecyloxy-p-phenylene vinylene), and their application to the patterning of the emission from polymer light-emitting device (PLED) are mentioned and discussed. The observed change of the photoluminescence spectrum upon irradiation can be divided into two stages. At the first stage, the photoluminescence intensity rapidly drops to about 40% of initial intensity while the spectrum shape is unchanged. On the other hand, the intensity is unchanged while the spectrum is considerably blue shifted by further irradiation. This result is discussed in terms of photoinduced defect creation and scission of main chain due to photo-oxidation. The emission from the PLED with semitransparent Al cathode drops upon photoirradiation of the device in air and eventually disappears. This indicates that the Al electrode is so thin and the atmospheric oxygen can pass through the Al film and react with the polymer during photoirradiation. The importance of thickness of the Al electrode to the patternability of the device is also mentioned.
Perturbed-angular-correlation studies of In-vacancy pairs in Hg1−xCdxTe
Authors: Austin J. C. , Hughes Wm. C. , Patnaik B. K. , Triboulet R. , Swanson M. L.
Published: July 2002

The interaction of vacancies with In111 atoms is studied in Hg1−xCdxTe compounds via perturbed-angular correlation (PAC) experiments, for x=0.065, 0.21, 0.44, and 0.95. In the low-x (Hg-rich) compounds, Hg vacancies are created by heating in vacuum. For the x=0.21 alloy, we have previously shown that InC-vacC pairs (A centers consisting of an In atom on a cation site and a vacancy at a neighboring cation site) are abundant after quenching from elevated temperatures. These defects are characterized by two PAC signals with quadrupole interaction frequencies νQ1=83 MHz and νQ2=92 MHz, and asymmetry parameters η1=η2=0.08. For the x=0.065–0.44 alloys, the data presented in this article show that the fractions f1 and f2 of In atoms associated with these two frequencies vary with x according to whether one or two Hg atoms are nearest neighbors to the Te atom that is bound to the In atom and the vacancy. The data are explained by the polarizable point-ion model. For the x=0.95 compound, PAC signals are observed only when stable In is added to the compound, indicating that the presence of In creates vacancies, and that self-compensation via A centers is dominant. In this case, the well-known frequencies νQ4=100 MHz and νQ5=112 MHz are seen for samples quenched from several different temperatures between 325 and 525 °C, or slow cooled from 475 °C or below. In contrast, for a sample slowly cooled from 525 °C, the frequency νQ6=60 MHz was dominant. This signal could be due to InC−vacC pairs in which the vacancy is singly charged, or to In-group I pairs. We attribute the frequencies νQ4=100 MHz and νQ5=112 MHz, like νQ1 and νQ2, to InC-vacC pairs having doubly charged vacancies.
Rapidly convergent expansion method for calculating the effective conductivity of three-dimensional lattices of symmetric inclusions
Authors: Simovski Constantin , He Sailing
Published: July 2002

An exact method is introduced to determine the electric potential in an infinite rectangular lattice of particles described by curvilinear coordinates in which Laplace's equation separates. The potential is expanded in harmonic functions, and suitable auxiliary functions are used to obtain an infinite system of linear algebraic equations for the expansion coefficients. Special attention is paid to lattices of spheres and prolate spheroids. For these cases, the truncated system converges very rapidly as the number of terms in the truncation series increases. The method works well for calculating the effective conductivity for dense or sparse inclusions, and for highly conducting lattices or lattices of cavities. Numerical results for the effective conductivity are given and compared with data obtained by other methods.
Transmission electron microscope study of the dry oxidation kinetics of WSi2 on (001)Si and polycrystalline silicon
Authors: Hung S. F. , Chen L. J.
Published: July 2002

Cross-section transmission electron microscopy has been applied to study the growth kinetics of oxide of WSi2 on (001)Si and polycrystalline silicon (poly Si) in the dry oxidation process. The linear activation energy and the parabolic activation energy of WSi2 on (001)Si were found to be 0.8±0.2 and 1.0±0.2 eV for samples dry oxidized at 800–890 °C for 10–60 min, respectively. On the other hand, the linear activation energy and parabolic activation energy of WSi2 on polycrystalline silicon for samples dry oxidized at 800–890 °C for 10–60 min were 1.0±0.2 and 1.8±0.2 eV. The linear activation energy is attributed to the diffusion of Si atoms from the substrate to the reaction interface. The different parabolic activation energies of 1.0 and 1.8 eV for WSi2 on (001)Si and poly Si, respectively, indicate significant difference in diffusion of O2 through grown SiO2. It is conceivable that stress generated during oxidation and the amount of fluorine atoms, introduced during the low pressure chemical vapor deposition process, present in the silicide layer and grown oxide, can influence the oxide quality and oxidation kinetics.
Photoluminescence study of deep levels in CuGaTe2 crystals
Authors: Krustok J. , Raudoja J. , Yakushev M. , Pilkington R. D. , Collan H.
Published: July 2002

A deep photoluminscence band at 0.95 eV was studied in CuGaTe2 crystals. The shape of this band did not change with laser power and no j shift was detected. This band shifts towards higher energy with increasing temperature and its shape becomes more asymmetric. The activation energy of this band was ET=0.20 eV as measured with thermal quenching. The possible association of the 0.95 eV band with grain boundaries or dislocations is discussed.
Low temperature dependence of photoluminescence band near 2.0 eV in silica glass
Authors: Sakurai Yuryo
Published: July 2002

Previous studies have reported the existence of a 1.9 eV photoluminescence (PL) band in silica glass exposed to visible and ultraviolet light. Nonbridging oxygen hole centers (O3≡Si–O↑,↑: unpaired electron) were thought to be the origin of this PL band. In this article, we describe the characteristics of a 1.9 eV PL band in silica glass (excited by visible and ultraviolet light). At low temperature (∼20 K), we observed broadening of the full width at half maximum and a peak shift for this PL band. We propose that the change of peak energy and full width at half maximum for the 1.9 eV PL band at low temperature was really caused by the existence of another peak rather than by structural changes in the local network structure.
Comment on “Assignment of the Raman active vibration modes of β-Si3N4 using micro-Raman scattering” [J. Appl. Phys. 85, 7380 (1999)]
Authors: Dong Jianjun , Sankey Otto F.
Published: July 2002

In this comment to a recent experimental paper by Honda et al. [J. Appl. Phys. 85, 7380 (1999)], we report first-principles calculations of phonon modes of β-Si3N4, which theoretically confirm the experimental micro-Raman assignments. In addition, the theory is able to locate the “missing” Ag mode, and we show that this mode is a bond-stretching mode. Hence, it is not in the low-frequency band as previously believed, but rather at intermediate frequency. We discuss the possible reasons this mode is “missing” in the spectra, and show that the application of high pressure will separate this mode from others.
Generation of second harmonics in Ge-doped SiO2 thin films by ultraviolet irradiation under poling electric field
Authors: Khaled J. , Fujiwara T. , Ohama M. , Ikushima A. J.
Published: July 2002

Thin films of Ge-doped silica were prepared by radio-frequency sputtering in a controlled atmosphere of Ar and O2 and then subjected to ultraviolet irradiation under poling electric field (UV poling). Effects of oxygen mass flow content during the sputtering on the second-order nonlinearity of the films were investigated. Second harmonic generation (SHG) was observed and its intensity was found to depend greatly on the oxygen mass flow content with an optimum at 1 cm3/min. The annealing in a vacuum enhanced SHG intensity and values as high as 12.5±0.6 pm/V have been achieved with films containing 50 mole % of GeO2. This is on one hand, and we also found that loading thin films with hydrogen prior to UV poling largely improved the decay of SHG. A decay time as long as 7 years has been obtained. The change in second-order optical nonlinear properties of the films is discussed based on structural defects.
Electroluminescence of composite channel InAlAs/InGaAs/InP/InAlAs high electron mobility transistor
Authors: Cavassilas N. , Aniel F. , Boucaud P. , Adde R. , Maher H. , Décobert J. , Scavennec A.
Published: July 2002

An experimental investigation of impact ionization by electroluminescence in composite channel lattice-matched InAlAs/InGaAs/InP/InAlAs high electron mobility transistors (HEMTs) is presented. In these transistors, an InP subchannel layer is added to the InGaAs channel. Radiative recombinations at two different energies are observed, characteristic of recombinations in the InGaAs channel and at the InP/AlInAs interface. The bias-dependent electroluminescence line intensities are used to analyze the role played by the InP layer for relaxing the hot carriers. A large fraction of electrons in the InGaAs channel of the composite HEMT is transferred to the InP subchannel in the high field gate-drain region.
High magnetoresistance permalloy films deposited on a thin NiFeCr or NiCr underlayer
Authors: Lee W.-Y. , Toney M. F. , Tameerug P. , Allen E. , Mauri D.
Published: July 2002

Significantly enhanced anisotropic magnetoresistance (MR) in permalloy (Ni0.81Fe0.19) films deposited on a thin (Ni0.81Fe0.19)1−xCrx or Ni1−xCrx underlayer is reported. The maximum ΔR/R enhancement was observed using the underlayer with x≈0.44 at an optimum thickness of ≈30–45 Å, depending on the deposition technique. An enhancement of 75%–150% was observed for 45–430 Å thick permalloy films, compared to the films deposited without this underlayer. The ΔR/R enhancement is attributed to the decrease in the resistivity ρ and the increase in Δρ of the permalloy film due to the formation of large (111) textured crystal grains in the permalloy films deposited on this underlayer, as revealed by the x-ray diffraction results obtained using synchrotron radiation.
Micromagnetic design of spin dependent tunnel junctions for optimized sensing performance
Authors: Tondra Mark , Daughton James M. , Nordman Catherine , Wang Dexin , Taylor John
Published: July 2002

Pinned spin dependent tunneling devices have been fabricated into high sensitivity magnetic field sensors with many favorable properties including high sensitivity (∼10 μOe/Hz at 1 Hz and ∼100 nOe/Hz at >10 kHz), a linear bipolar output versus applied field, high processing yields, and high temperature stability and operability (over 200 °C). However, the performance of fabricated sensors has not yet approached the theoretical limit one calculates assuming ideal behavior of the sensors’ ferromagnetic layers’ magnetizations. Given a total magnetoresistive signal of 30%, and typical anisotropy fields and hard axis biasing conditions, there should be a region of linear nonhysteretic response at zero field with a slope of greater than 20%/Oe. Measured responses are 1%–3%/Oe, and exhibit some hysteresis. These less than desirable effects are the result of several factors including: (1) Self-demagnetizing fields of the soft (sensing) layer; (2) stray fields from the hard (pinned) layer; (3) imperfect pinning of the hard layer; and (4) interlayer magnetic coupling across the tunnel barrier. This paper describes, in detail, the extent to which these factors affect sensor performance, and specific steps to be taken in order to minimize their deleterious influence. Specifically, the simple pinned layer is replaced by an exchange coupled synthetic antiferromagnet (CoFe/Ru/CoFe), the soft layer is made to be significantly larger in the plane than the pinned layer, and the soft layer is made as thin as possible.
Magnetic irreversibility in ultrafine ZnFe2O4 particles
Authors: Goya G. F. , Rechenberg H. R. , Chen M. , Yelon W. B.
Published: July 2002

Pure ultrafine ZnFe2O4 particles have been obtained from mechanosynthesis of the ZnO and Fe2O3 oxides. The average grain diameter was estimated from x-ray diffraction to be 〈d〉=36(6) nm. Refinement of neutron diffraction data showed that the resulting cubic spinel structure is oxygen deficient, with ∼7% of Fe3+ ions occupying the tetrahedral A sites. Magnetization curves taken at 4.2 K showed the absence of saturation in fields up to H=9 T, associated with a spin-canting produced by the milling process. Field-cooled (FC) and zero-field cooled (ZFC) curves showed irreversible behavior extending well above room temperature, which is associated with spin disorder. Annealing samples at 300 °C yields an average grain size 〈d〉=50(6) nm, and ∼16% of Fe3+ ions at A sites. Partial oxygen recovery is also deduced from neutron data refinement in annealed samples. Concurrently, decrease of magnetic irreversibility is noticed, and assigned to partial recovery of the collinear spin structure. Complex Mössbauer spectra were observed at room temperature and 80 K, with broad hyperfine field distributions spanning from ∼10 to ∼40 T. At T=4.2 K, hyperfine field distributions indicate high disorder in Fe local environments. The above data suggest the existence of Fe-rich clusters, yielding strong superexchange interactions between Fe ions at A and B sites of the spinel structure.
Characterization of wire x pinches driven by a microsecond-long capacitive discharge
Authors: Christou C. , Dangor A. E. , Hammer D. A.
Published: July 2002

The use of a simple capacitive discharge as a driver for an x-pinch soft x-ray source is demonstrated. The 30 kV, 4 kJ capacitive discharge had a quarter period of 1.2 μs, peak current of 320 kA, and current rise of 2.5×1011 A s−1. X-pinch x-ray emission was characterized by pinhole photography and solid-state detectors. Soft x-ray emission (800 eV–4 keV) was observed in both single and multiple bursts, with yields from 180 mJ for aluminum to 1.5 J for tungsten wire x pinches. X-ray emission from x pinches was higher than z-pinch emission from the same materials using the same power source. Hard x-ray emission (>8 eV) from the x pinch was lower with the long pulse capacitive discharge than with a 360 kV pulsed power driver delivering 100 kA peak with a rate of current rise of 2.2×1012 A s−1. Visible photography and laser-based schlieren photography showed that the x pinch was asymmetric about the crossing point of the wires. This asymmetry is due to the influence of electron beam generation at this point.
Composition, residual stress, and structural properties of thin tungsten nitride films deposited by reactive magnetron sputtering
Authors: Shen Y. G. , Mai Y. W. , McKenzie D. R. , Zhang Q. C. , McFall W. D. , McBride W. E.
Published: July 2002

Thin tungsten nitride (WNx) films were produced by reactive dc magnetron sputtering of tungsten in an Ar–N2 gas mixture. The effects of the variation of nitrogen partial pressure on the composition, residual stress, and structural properties of these films as well as the influence of postdeposition annealing have been studied. The films were analyzed in situ by a cantilever beam technique, and ex situ by x-ray photoelectron spectroscopy, electron energy-loss spectroscopy, x-ray diffraction, and transmission electron microscopy (TEM). It was found that at N concentrations below 8 at. %, the films (typical 150 nm in thickness) were essentially bcc α-W. An amorphous phase was observed in the range of about 12–28 at. % N. When N concentrations reached ∼32 at. % or above, a single-phase structure of W2N was formed. Annealing of the as-deposited films resulted in crystallization of the amorphous or an improved crystallinity of the W2N structure, which was related to the N concentration. Stresses of all W and WNx films were compressive. As the N concentration was increased, the stress decreased and reached its lowest value for amorphous samples near 20 at. % N. Past this point, the compression of films rose again. These results can be ascribed to structural changes induced by the pressure-dependent variation in the average energy of particles bombarding the film during deposition. Cross-sectional TEM studies showed that all crystalline WNx films had columnar microstructures. The average column width near stoichiometry of W2N was ∼20±5 nm near the film surface.
Heat, work, and the thermodynamic temperature at nonequilibrium steady states
Authors: Keizer Joel
Published: July 2002

We continue our investigation of the properties of matter at nonequilibrium steady states using a thermodynamic formalism derived from molecular fluctuations. Based on the generalized Clausius inequality, we extend the definition of ‘‘reversible’’ process to include transformations between steady states and relate reversible work and heat to changes in state functions. The variable which is thermodynamically conjugate to the internal energy, that is, the inverse of the generalized thermodynamic temperature, is an integrating factor for the reversible heat. We discuss the relationship of the generalized temperature to the Kelvin temperature, how the generalized intensive variables can be measured, define generalized heat capacities, and obtain the Gibbs–Duhem relationship satisfied by the intensive variables. These ideas are illustrated for two simple steady state systems, a fluid under homogeneous shear and a two level gas which is pumped by radiation. Finally we analyze under appropriate conditions the electromotive force of a chemical reaction system held at a nonequilibrium state. We predict corrections to the Nernst equation which depend on how far the chemical reactions are removed from equilibrium.
The X̃ 1A1 → Ã 1B1 transition moment of H2O using state-specific configuration-interaction wave functions
Authors: Theodorakopoulos Giannoula , Petsalakis Ioannis D. , Nicolaides Cleanthes A. , Buenker Robert J.
Published: July 2002

We report the results on the oscillator strength of the H2O X̃ 1A1 → Ã 1B1 transition from two types of calculations. One from using correlated state-specific wave functions for each state and one from using large MRD-CI wave functions with a common basis set. In the first case nonorthonormality is considered explicitly via the use of corresponding orbitals. Both types of calculation converge to a value of f=0.065. This result agrees with one experimental value (f=0.060) and disagrees with the most recent theoretical calculation which employed the Stiltjes imaging method (f=0.021). As is the case with previous atomic calculations, the state-specific approach improves the quality of the wave functions of excited states while reducing their size considerably.