bluuebunny/crossref_metadata_2025 huggingface dataset:
Linear stability of relativistic space-charge flow in a magnetically insulated transmission line oscillator
Authors: Lemke Raymond W.
Published: July 2002

The magnetically insulated transmission line oscillator (MILO) is a high-power microwave device that combines the technologies of magnetically insulated electron flow and slow-wave tubes. This combination makes the MILO a unique and robust device capable of operating at the same frequency over a wide range of voltages. Microwaves are generated in the MILO via an unstable interaction between slow electromagnetic modes and space-charge waves. We present a linear analysis of this microwave instability that includes the physics of magnetically insulated electron flow. A dispersion relation is derived for transverse magnetic modes in a cylindrical MILO. The dispersion relation is solved numerically to obtain the MILO frequency spectrum and instability growth rates.
Polymer dispersed nematic liquid crystal films: The density ratio and polymer’s curing rate effects
Authors: Fuh A. , Caporaletti O.
Published: July 2002

The effects of the density ratio and the polymer’s curing rate on the characteristics of polymer dispersed nematic liquid crystal (NLC) films are reported. Measurements indicated that the slope of the electro-optical response curve, which determines the film’s multiplexing capability, is a strong function of the uniformity of the NLC droplet’s size. Both the density ratio and the polymer’s curing rate are shown to be crucial in the production of a more uniform droplet size. Polymer dispersed LC films, suitable for different applications, can be produced by varying the curing rate.
Measurement of thermal expansion coefficients of W, WSi, WN, and WSiN thin film metallizations
Authors: Lahav Alex , Grim Karen A. , Blech Ilan A.
Published: July 2002

Thermal expansion coefficients and biaxial elastic moduli of the sputtered W, WNx, WSi0.45, and WSi0.67N0.10 thin films, used as refractory gate in the self-aligned metal-semiconductor field effect transistors on GaAs, were determined by insitu stress measurements during heating and cooling of the films on GaAs and Si substrates in the temperature range of 20–450 °C. For the WNx films the average values of thermal expansion coefficients increased with the nitrogen content, and varied from 4.5×10−6 °C−1 for tungsten to 5.80×10−6 °C−1 for WN0.43. For the WSi0.45 and WSi0.67N0.10 films, the measured values of coefficients of thermal expansion (6.55×10−6 and 6.37×10−6 °C−1, respectively) were close to that of GaAs (6.40×10−6 °C−1, respectively). Thus by using these films as refractory gates, the stress-induced interdiffusion and interaction at the interface with GaAs can be substantially reduced.
Raman spectroscopic investigation of ion-beam-irradiated glassy carbon
Authors: Prawer S. , Ninio F. , Blanchonette I.
Published: July 2002

Raman spectroscopy has been used to monitor the changes induced in glassy carbon as a result of irradiation with H, He, C, N, Si, and Xe ions. The Raman spectrum of unirradiated glassy carbon consists of an intrinsic graphite peak at 1590 cm−1 (the a peak) and a disorder-induced D peak at 1350 cm−1. The G peak position and FWHM and the ratio of the D peak intensity to that of the G peak [I(D)/I(G)] are plotted as functions of the calculated damage density induced by the ion beam. The results show that at very low damage levels [ < 0.008 displacements per atom (DPA)], considerable disorder is being introduced into the system, and the average crystallite size has been reduced from 35 Å for the unirradiated material to about 25 Å. At damage levels of ∼0.21 DPA, the material starts to undergo an ion-beam-induced modification which saturates at about 5 DPA. The Raman spectra for these heavily irradiated samples are very similar to those obtained from amorphous carbons. At yet higher doses there is some evidence suggesting that ion-beam-induced partial graphitization of the irradiated glassy carbon has occurred. The results are discussed in the context of previously reported Raman studies of amorphous carbons and highly oriented pyrolytic graphite.
Influence of elastic stress on the growth kinetics of planar thin-film binary diffusion couples
Authors: Johnson William C. , Martin G.
Published: July 2002

Some possible effects of elastic stress, engendered by epitaxial strain and compositional inhomogeneity, on the kinetics of intermediate phase formation in a planar thin-film binary diffusion couple are identified. A simple quasistatic approximation to the diffusion equation is used to estimate the thickening rate of the intermediate phases when elastic effects are incorporated into both the diffusion equation and the interfacial boundary conditions for diffusion using the thermodynamics of stressed solids. The various growth regimes of the phases are visualized using a simple graphical representation of the phase growth velocities (phase plot). Growth kinetics are shown to depend on both the strain engendered by and the crystallographic orientation of the substrate. It is also proposed that elastic stresses could influence the sequence of phase formation in thin-film systems including such observations as the absence of those phases appearing in the equilibrium phase diagram and the initial disappearance and subsequent reappearance of equilibrium phases.
Ionic conductivity in single-crystal proustite Ag3AsS3
Authors: Yang S. R. , Taylor K. N. R.
Published: July 2002

The temperature dependent ac conductivity of single-crystal proustite measured at several frequencies over the temperature range from 82 to 467 K reveals six distinct regions associated with different conduction mechanisms. A phase transition from ionic to superionic conduction has been found to occur at a temperature of approximately 420 K. The frequency dispersion spectra obtained at different temperatures from 82 to 373 K, over the frequency range 10−2 to 103 Hz show two regions of frequency dependent conductivity. The dc conductivity and ionic hopping rate as well as the carrier densities have been extracted from the ac conductivity data. The results show that the ac conductivity of proustite is characterized by low hopping rates and high carrier densities suggesting that the Ag+ ions may execute a long range hopping motion in the lattice. The relationship between the dc and ac components of conductivity have been critically examined and the results show that the ratio σdc/σac is thermally stimulated with an activation energy nΔHm, where ΔHm is the enthalpy of ion migration, and n is the slope of the conductivity dispersion curve.
Thermodynamics and magnetism in U1−xThxBe13−yBy
Authors: Heffner R. H. , Beyermann W. P. , Hundley M. F. , Thompson J. D. , Smith J. L. , Fisk Z. , Bedell K. , Birrer P. , Baines C. , Gygax F. N. , Hitti B. , Lippelt E. , Ott H. R. , Schenck A. , MacLaughlin D. E.
Published: July 2002

We report specific heat and μSR measurements on Th (x =0.019) and/or B (y=0.03) substituted UBe13. The specific heat data show that either Th or B substitution reduces the Kondo temperature TK and increases the entropy at the superconducting transition by almost 20%, indicating an enhanced density of states. However, whereas μSR shows clear evidence for magnetic correlations for Th substitutions (0.019<x<0.043), no magnetism is observed for B substitutions. The enhanced specific heat jump in the B-substituted material is associated with a change in the superconducting properties as TK is reduced.
Antiferromagnetic interlayer coupling in Fe/V and Fe/Cr
Authors: Vega A. , Rubio A. , Balbas L. C. , Dorantes-Davila J. , Bouarab S. , Demangeat C. , Mokrani A. , Dreyssé H.
Published: July 2002

Antiferromagnetic (AF) coupling between Fe and V atoms is observed experimentally at the Fe/V and Fe/Cr interfaces. In the self-consistent tight-binding derivation of the unrestricted approximation of the Hubbard Hamiltonian, we have investigated Fe3Vn (n=1–5) superlattices and Fe3Vn (n=1–5) films. In all cases one finds an AF coupling between Fe and V together with a decreasing oscillation of the induced magnetic moment on the V atoms when the distance from Fe increases. The distributions of local magnetic moments on Fe and Cr layers in Fe3Crn (n=3–5) superlattices have been determined self-consistently. Both AF and ferromagnetic (F) coupling between Fe layers separated by Cr have been studied. In all cases one finds strong AF couplings between Fe and Cr nearest neighbors with an important increase of the Cr magnetic moment at the interface as compared to the Cr bulk value.
Low-frequency ac conduction in lightly doped polypyrrole films
Authors: Singh Ramadhar , Tandon R. P. , Panwar V. S. , Chandra Subhas
Published: July 2002

The total measured ac conductivity σ(ω)m of lightly doped polypyrrole films has been measured in the frequency range 100 Hz–10 MHz and in the temperature range 77–350 K. In the low-temperature region the measured ac conductivity is almost independent of temperature but shows a strong dependence on frequency and can be described by a relation σ(ω) = Aωs where the exponent s has been observed to be less than unity. In the high-temperature region the frequency dependence becomes weak at low frequencies but remains strongly frequency dependent at high frequencies. The weak frequency dependence is due to the contribution of dc conductivity to the measured ac conductivity. A clear Debye-type loss peak is observed by substracting the contribution of dc conductivity. The frequency dependence of conductivity remains less than quadratic at low frequencies indicating thereby some distribution of relaxation times. This is confirmed by the measurement of dielectric constant as a function of frequency and temperature. The frequency and temperature dependence of ac conductivity can be qualitatively explained by the proposed model which considers the contributions from two mechanisms, one giving a linear dependence of conductivity on frequency and the other having distribution of relaxation times giving rise to broad dielectric loss peak.
Fractal-like aggregation of Au islands induced by laser irradiation
Authors: Carrière T. , Ortiz C. , Fuchs G.
Published: July 2002

Au islands deposited on Al2O3 films were irradiated with a pulsed laser beam. The result is the formation of depleted zones and chains of aggregates at the edge of the irradiated area. The fractal analysis of these Au cluster chains formed after irradiation gives a dimension of 1.50–1.55 consistent with the process of cluster-cluster aggregation which occurs under diffusion limited conditions.
Thermochemistry on the hydrogenated diamond (111) surface
Authors: Harris Stephen J. , Belton David N. , Blint Richard J.
Published: July 2002

As part of our effort to control the growth of diamond films by chemical vapor deposition, we are studying the chemical mechanism for conversion of gas phase hydrocarbons into diamond. In this work we analyze the thermochemistry of a number of structures on the hydrogenated diamond (111) surface. We use the MM2 molecular mechanics force field to calculate strain energies, which are due to crowding of adsorbed species on the surface, and we use a group additivity scheme to estimate bond enthalpies and entropies. These data allow calculation of equilibrium structures on the surface and, together with estimates for rate constants, will permit a prediction for the kinetics of diamond formation as a function of growth conditions. We find that a straightforward abstraction/addition mechanism using either CH3 or C2H2 to grow on a hydrogenated (111) surface cannot account for experimentally measured growth rates. We suggest that experimental measurements of growth rates on (111) surfaces are strongly influenced by growth at steps, kinks, and edges on those surfaces.
Monte Carlo study of phase transitions in ferromagnetic bilayers
Authors: Ferrenberg Alan M. , Landau D. P.
Published: July 2002

We have used Monte Carlo computer simulations to study the behavior of an Ising model consisting of two ferromagnetic layers with different interaction constants coupled weakly together. For the range of lattice sizes studied it appears as though the system undergoes a single transition at the transition temperature of an isolated layer with the stronger coupling, but substantial changes in the thermodynamic properties also occur near the transition temperature of an isolated layer with the weaker of the two couplings.
Connections between magnetism and superconductivity in UBe13 doped with thorium or boron (invited)
Authors: Heffner R. H. , Ott H. R. , Schenck A. , Mydosh J. A. , MacLaughlin D. E.
Published: July 2002

Magnetism and superconductivity appear to be intimately connected in the heavy electron (HE) superconductors. For example, it has been conjectured but not proven that the exchange of antiferromagnetic spin fluctuations are responsible for pairing in HE superconductors. In this paper we review recent results in U1−xThxBe13, where specific heat, lower critical field, and zero-field muon-spin-resonance (μSR) measurements reveal another second-order phase transition (below the superconducting transition) to a state which possesses small-moment magnetic correlations for 0.019≤x≤0.043. We present a new phase diagram for (U,Th)Be13 which indicates that the superconducting and magnetic order parameters are closely coupled. A discussion of the nature of the lower phase is presented, including the consideration of a possible magnetic (time-reversal-violating) superconducting state. When UBe13 is doped with B (UBe12.97B0.03) the Kondo temperature is decreased and the specific heat jump at the superconducting transition temperature is significantly enhanced. However, μSR measurements reveal no magnetic signature in UBe12.97B0.03, unlike the case for Th doping. The correlation between changes in the Kondo temperature and changes in the superconducting properties induced by B doping provide evidence for the importance of magnetic excitations in the superconducting pairing interaction in UBe13.
Low-resistivity epitaxial YBa2Cu3O7 thin films with improved microstructure and reduced microwave losses
Authors: Poppe U. , Klein N. , Dähne U. , Soltner H. , Jia C. L. , Kabius B. , Urban K. , Lubig A. , Schmidt K. , Hensen S. , Orbach S. , Müller G. , Piel H.
Published: July 2002

Epitaxial thin films of YBa2Cu3O7 have been prepared on SrTiO3 and LaAlO3 substrates by a high-pressure planar dc-sputtering technique. By covering the substrate heater with a frame of polycrystalline YBa2Cu3O7 substantial improvements of the YBa2Cu3O7 film properties were achieved. These are characterized by dc-resistivity values ρ(T) of less than 50 μΩ cm at 100 K and ρ(300 K)/ρ(100 K) values of up to 3.9. Significant deviations from the usual linear ρ(T) behavior were found. Critical temperatures above 90 K, resistive transition widths down to 0.3 K, and critical current densities of about 5 × 106 A/cm2 at 77 K confirm the high quality of the films. As indicated by Rutherford backscattering and high-resolution transmission electron microscopy the films exhibit a microstructure characterized by a reduced density of lattice defects. However, lattice-coherent precipitates with a diameter of about 5–10 nm were observed. As an outstanding feature the films exhibit, besides the initial steep falloff at Tc, a further gradual decrease of the microwave surface resistance at 87 GHz below 50 K by at least one order of magnitude. These results are very promising for millimeter-wave applications of epitaxial YBa2Cu3O7 thin films.
Modeling reactive sputtering process in symmetrical planar direct current magnetron systems
Authors: Tsiogas C. D. , Avaritsiotis J. N.
Published: July 2002

A steady-state model for the reactive sputtering process in a symmetrical planar magnetron configuration is presented which takes the specific system geometry into consideration. Further to the simulation of the well known hysteresis effect and its consequences in the composition of the deposited film, the model allows for the calculation of the radial film thickness and compositional distributions of the growing film as a function of the distance between the target and the condensation surface. The proposed model introduces for the first time two realistic assumptions: a reactive gas profile across the reaction zone and a current density distribution. The case of introducing reactive gas through the center of the magnetron surface is also examined and the effects on the stoichiometry of the film as well as on the stability of the system are presented. Experimental results for film thickness profile for several values of the target diameter-to-substrate separation ratio show excellent agreement with simulated results.
X-ray scattering from interfacial roughness in multilayer structures
Authors: Stearns D. G.
Published: July 2002

A quantitative theory of the nonspecular scattering of x rays from multilayer structures having rough interfaces is presented. The results are valid for arbitrary polarization and angles of incidence (measured from the normal) less than the critical angle for total external reflection. A structural model is adopted wherein each interface is assumed to be described by a surface having statistically random roughness with a well-behaved power spectrum. In addition, the model accounts for arbitrary correlation of the roughness between different interfaces. Calculations are presented for a variety of roughness configurations to investigate the dependence of the nonspecular scattering on the fundamental structural parameters. In particular, it is shown that the scattering from correlated roughness exhibits characteristic resonance behavior (quasi-Bragg diffraction).
Study of the decomposition of SF6 under dc negative polarity corona discharges (point-to-plane geometry): Influence of the metal constituting the plane electrode
Authors: Casanovas A. M. , Casanovas J. , Lagarde F. , Belarbi A.
Published: July 2002

SF6 samples (PSF6=100 or 200 kPa) were submitted to point-to-plane dc negative polarity corona discharges in the presence of water [concentration=2000 ppmv (parts per million by volume)] or without the addition of water. The stable gaseous byproducts formed, (SO2F2, SOF2, and S2F10) were assayed by gas-phase chromatography. The variation of their yields against the charge transported (up to 10 C) was studied for two metals (aluminum and stainless steel) constituting the plane electrode, at various values of the SF6 pressure, the water content, the gap spacing (2.5 and 8 mm), and the discharge current [12≤Ī (μA)≤25]. The results indicate an important effect of the metal constituting the plane electrode and of the moisture conditions, particularly on the production of SOF2 and S2F10.
Gas-phase kinetics during microwave plasma-assisted diamond deposition: Is the hydrocarbon product distribution dictated by neutral-neutral interactions?
Authors: Hsu Wen L.
Published: July 2002

The mole fractions of H, H2, CH3, CH4, and C2H2 that were generated in a microwave plasma under conditions typical for diamond deposition are reported. When the methane percentage in the feed gas was <1%, the measured concentrations were surprisingly similar to those reported for a hot-filament system operated under nearly identical conditions. However, as the methane percentage was increased beyond 1%, a noticeable deviation from the hot-filament results became apparent. In the hot-filament case, surface-induced dissociation of hydrogen on the filament surface was significantly inhibited at this high methane level and lead to a pronounced reduction of the H-atom concentration. But, in the microwave system where dissociation is a homogeneous process, no such reduction was observed. Other observations were: (i) The acetylene concentration scaled as the square of the methyl concentration; (ii) The CH4+H↔CH3+H2 reaction was found to be at the same level of nonequilibrium as in the hot-filament case. These results, plus arguments based on general principles of plasma chemistry, are taken as indications that the hydrocarbon chemistry in the microwave system is as strongly dictated by neutral-neutral reactions as in a thermal system. The primary effect of the plasma is perhaps its role in dissociating hydrogen molecules to H atoms without having to raise the gas temperature significantly.
Hydrogen activated radiative states in GaAs/GaAlAs heterostructures and InGaAs/GaAs multiquantum wells
Authors: Capizzi M. , Coluzza C. , Emiliani V. , Frankl P. , Frova A. , Sarto F. , Bonapasta A. Amore , Sobiesierski Z. , Sacks R. N.
Published: July 2002

Direct observation of optical emission from H-related complexes in molecular beam epitaxy grown bulk GaAs and GaAlAs, as well as InGaAs/GaAs strained multiquantum wells (MQWs), is obtained from liquid He photoluminescence experiments. Hydrogenation is achieved by low-energy ion irradiation from a Kaufman source. The volume incorporation of hydrogen, for equal treatment, is dependent upon the density of impurities and defects where H can bind. For moderate H treatment, in addition to passivation of shallow acceptors, in GaAs we observe novel emission bands, δ, peaking at 1.360, and γ, peaking at 1.455 eV. After heavy hydrogen treatment in GaAs of low radiative efficiency−even p type originally−there appears a deeper structure α at ∼1.20 eV, of the kind known for ‘‘internal’’ transitions in the Ga vacancy-donor complex. Equivalent bands are found in bulk GaAlAs and also in InGaAs/GaAs MQWs. The results allow an approximate estimate of the various optically active Ga-vacancy levels, as affected by the different degrees of hydrogenation of the dangling bonds, and a comparison with theoretical values. Moreover, they provide evidence for the creation of a H-related donor whose binding energy is of order 25 meV. Finally, the γ band is suggestive of a transition between localized conduction and valence states associated with the local distortion that is introduced in the lattice when H binds to impurities, defects, and lattice atoms.
Nonlinear couplers composed of different nonlinear cores
Authors: Atai Javid , Chen Yijiang
Published: July 2002

Nonlinear couplers composed of two different saturable nonlinear cores are examined. By combining one self-focusing core appositely with another core of different nonlinearity, which can be self-focusing, linear, or self-defocusing, the detrimental effect of nonlinear saturation on the coupler formed is greatly reduced compared with that on the conventional coupler composed of two identical self-focusing cores. This finding provides a way of relaxing the practical limitation to operational power of a saturable nonlinear coupler especially when high nonlinearity is involved for low-power operation.
Bounds to the conductivity of some two-component composites
Authors: Helsing Johan
Published: July 2002

Calculation of third-order bounds to the conductivity of isotropic two-component composites is discussed. Coincidence of the Beran bounds and bounds derived using trial fields based on the solution of a single-body electrostatic boundary-value problem is demonstrated for a random distribution of impenetrable ellipsoids. This extends a proof of Beasley and Torquato [J. Appl. Phys. 60, 3576 (1986)]. A structural parameter related to third-order bounds is calculated for a face-centered cubic array of cubes in a matrix. For an array of rectangular blocks an upper bound in one direction is derived. This bound, and its two-dimensional analogs, become very sharp in the limit of strong inhomogeneity. Improved third- and fourth-order bounds for the three-dimensional checkerboard are presented.
X-ray characteristics of a niobium superconducting tunnel junction with a highly transmissive tunnel barrier
Authors: Rando N. , Peacock A. , Foden C. , Dordrecht A. v. , Lumley J. , Pereira C.
Published: July 2002

The results of an investigation into the x-ray properties of a superconducting tunnel junction (STJ) are presented. The photoabsorption of an x-ray photon by one of the thin superconducting films of the junction results in the production of quasiparticles, which may subsequently tunnel through the thin oxide barrier into the second superconducting film. The transfer of charge across the barrier is detected, and gives a measure of both the x-ray photon energy and the effective energy gap ε of the superconducting film in which the photoabsorption occurred. A charge output of 55% of the theoretical maximum has been obtained for a niobium-based STJ. Such a charge output indicates a mean energy ε of ≂4.7 meV is required to create a single charge carrier in the junction such that ε/Δ≂3, where 2Δ is the junction energy gap. This is the lowest value of ε/Δ obtained to date for x-ray photoabsorption in STJs. The energy resolution of the device is, however, still poor, with a full width half maximum of ≂200 eV for 6 keV x rays, compared with the theoretical Fano limited resolution of ≂4 eV. The principle mechanisms which are believed to degrade the resolution are discussed. These are principally quasiparticle recombination, phonon leakage out of the junction, quasiparticle diffusion into and out of the tunneling region, in addition to possible local variations in the energy gap. A Monte Carlo simulation of the nonequilibrium system has been performed. The results assist in the identification of the major charge loss mechanisms, and indicate various means by which Fano limited resolution may be obtained.
Electron mobilities and photoelectron lifetimes in AlGaAs/GaAs and InGaAs/GaAs quantum-well infrared detectors
Authors: Kane M. J. , Millidge S. , Emeny M. T. , Guy D. R. P. , Lee D. , Whitehouse C. R.
Published: July 2002

Gain and geometric magnetoresistance electron mobility measurements are presented for quantum-well infrared photoconductors. The electron mobility is found to be identical for both the photocurrent and the dark current and has a value of 2.6×103 cm2 V−1 s−1 in devices made from GaAs/AlxGa1−xAs and 9.4×103 cm2 V−1 s−1 in the InxGa1−xAs/GaAs devices at 77 K. This difference in mobility is associated with the purity of the barrier material which is much greater in GaAs than in AlxGa1−xAs. The photoelectron lifetime before recapture can be deduced from the combination of the gain and mobility measurements. This lifetime is found to be 7.2 ps in the AlxGa1−xAs/GaAs detectors and 5.5 ps in the InxGa1−xAs/GaAs devices.
Reduced quantum efficiency of a near-surface quantum well
Authors: Chang Ying-Lan , Tan I-Hsing , Zhang Yong-Hang , Bimberg D. , Merz James , Hu Evelyn
Published: July 2002

The effect of the proximity of a bare barrier surface on the quantum efficiency of underlying GaAs/Al0.3Ga0.7As and In0.13Ga0.87As/GaAs quantum wells (QWs) is studied by low-temperature photoluminescence. The quantum efficiency of the resonantly excited QWs diminishes with decreasing surface barrier thickness; the onset of the reduction in quantum efficiency of the InGaAs QW occurs for a barrier that is 50 Å thicker than for the GaAs QW. A simple model of carrier tunneling to the surface is formulated to explain the dependence of the quantum efficiency on surface barrier thickness and well width and height. This model shows good agreement with both sets of experimental data.
Irradiation nonuniformity due to imperfections of laser beams
Authors: Murakami M. , Nishihara K. , Azechi H.
Published: July 2002

Intractable low-mode nonuniformities caused by power imbalance or pointing error of beams are studied quasi-analytically based on Skupsky’s axially symmetric model [S. Skupsky and K. Lee, J. Appl. Phys. 54, 3662 (1983)]. These nonuniformities can be improved by decreasing the imperfections σPΩ, or by increasing the number of laser beams NB: deteriorated irradiation uniformity is shown to be proportional to σPΩ/√NB. Criteria of these imperfections for high irradiation uniformity [≤1% root-mean square (rms)] are then presented in terms of NB. The model is further extended to treat general asymmetric beam patterns. Optimization of beam pattern is also a central issue.
Electrical and photoconductive properties of orientation-controlled chloroaluminumphthalocyanine thin films
Authors: Yanagi Hisao , Imamura Miwa , Ashida Michio
Published: July 2002

Structure dependence of electrical properties was investigated for amorphous and epitaxial thin films of chloroaluminumphthalocyanine vacuum deposited on both glass and KCl. The extrinsic conductivity in the dark was improved by elevating substrate temperatures, depending on the morphological change from granular, discontinuous films deposited at a substrate temperature of 25 °C to uniform, continuous ones at 250 °C. The photocurrents under monochromatic light illumination increased in proportion to the amount of adsorbed O2 in the amorphous films on glass. A regular doping form of O2 in the epitaxial film on KCl could enhance photocarrier generation. The granular, discontinuous morphology in the films caused a frequent charge-carrier recombination at the grain boundaries and defect sites. In the intrinsic region the amorphous films exhibited a higher dark conductivity due to the π-electron interaction between the randomly arranged molecules and gave a moderate activation energy for conduction. The epitaxial film with a regular orientation exhibited a significantly higher activation energy for intrinsic dark conduction because of a weak molecular interaction between the standing molecular columns.
Phase transition revealed by Raman spectroscopy in screen-printed lead zirconate titanate thick films
Authors: Zhang Hongxue , Uusimäki Antti , Leppävuori Seppo , Karjalainen Pentti
Published: July 2002

The transition from rhombohedral phase to tetragonal in screen-printed lead zirconate titanate (PZT) thick films was studied using Raman-scattering spectroscopy, and the results were compared with those of x-ray-diffractometry investigations. The unfired films were subjected to rapid firing in an air atmosphere at temperatures ranging from 960 to 1150 °C. During firing the composition of the films changed gradually as lead evaporated, which moved the composition of films to the ZrO2-PZT region and resulted in precipitation of ZrO2. This caused the original rhombohedral structure to be converted to tetragonal. Correspondingly, the Raman spectra also changed with increasing firing temperature. The variation in Raman spectra was characterized by three frequency regions, denoted as the low-, intermediate-, and high-frequency region, respectively, which are related to three cubic T1u modes. Moreover, it was also observed that the appearance of tetragonal modes was delayed from the formation of tetragonal structure during the process. This discrepancy suggested the existence of a certain ‘‘mismatch’’ in structure.
Temperature-dependent Raman-scattering studies of Li0.02Na0.98NbO3
Authors: Juang Y. D. , Hu M. L. , Tse W. S.
Published: July 2002

Raman studies of Li0.02Na0.98NbO3 ceramics were observed in the temperature range between 10 and 300 K. The spectral pattern evolves gradually at a transition temperature (130–170 K) in the cooling process and then changes suddenly near 280 K on heating. The observed thermal hysteresis of 110 K and also the transition temperatures agree well with evidence coming from the differential scanning calorimetric, the dielectric constant, and the thermal expansion measurements. The observed spectra are discussed following the separation of modes into internal stretching modes of the NbO6 octahedron and cation translation modes. The 62- and 74-cm−1 peaks are proposed due to the translational modes of Na+ cation.
Structure and performance of Si/Mo multilayer mirrors for the extreme ultraviolet
Authors: Slaughter J. M. , Schulze Dean W. , Hills C. R. , Mirone A. , Stalio R. , Watts R. N. , Tarrio C. , Lucatorto T. B. , Krumrey M. , Mueller P. , Falco Charles M.
Published: July 2002

We report the results of structural, chemical, and extreme ultraviolet (EUV) characterization of Si/Mo multilayers grown by sputtering and by UHV evaporation. This study includes mirrors designed for normal incidence with peak reflectivities Rpeak between 22 and 24 nm, and 45° mirrors having Rpeak between 16 and 19 nm. The deposition conditions were varied to produce multilayers with a wide range of interface morphologies. A variety of techniques were used to determine the structure and composition of the multilayers, including x-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and Auger depth profiling. All of the mirrors have amorphous Si layers and polycrystalline Mo layers with thin amorphous alloy interlayers. We obtain good fits to the low-angle x-ray diffraction data only when these interlayers are taken into account. The best sputter-deposited mirrors were made at the lowest Ar pressure studied, 3 mTorr. The best evaporated mirrors were produced at a substrate temperature of 200 °C. The EUV reflectivity as a function of wavelength was measured using synchrotron radiation. Both the multilayer structure and surface contamination significantly affect the EUV reflectivity, and must be considered to obtain good fits to the reflectivity curves. The best 45° mirror had a peak reflectivity of 53% at 18.6 nm for 100% S-polarized light, and the best normal-incidence mirror had a peak reflectivity of 33% at 23.6 nm.
Experimental and theoretical study of the crystallization of chemical-vapor-deposited mixed-phase silicon films
Authors: Voutsas Apostolos T. , Hatalis Miltiadis K.
Published: July 2002

The deposition of polycrystalline silicon films by the low-temperature crystallization of as-deposited, mixed-phase silicon films obtained by low-pressure chemical-vapor deposition was studied. It was found that this type of material, consisting of crystalline regions embedded in an amorphous matrix, can crystallize in threefold to fourfold shorter crystallization time than that required for the complete crystallization of as-deposited amorphous films. The faster crystallization was attributed to the combination of the growth of the preexisting crystallites and the faster nucleation and growth of random crystallites in mixed-phase films. In addition, it was shown that by utilizing composite films, consisting of an amorphous layer and a mixed-phase layer, a comparable grain size to that of crystallized amorphous films can be obtained. The key process parameters, controlling the grain size and the crystallization time of composite films, were found to be the layer sequence, the deposition rates of the layers, and the thickness ratio of the layers. A mathematical model, accounting for the crystallization of composite films, was also formulated to explain the experimental trends. The theoretically predicted values by this model, for the grain size of the crystallized films, were found to be in excellent agreement with the experimental observations.
Detection of defects at homoepitaxial interface by deep-level transient spectroscopy
Authors: Lu Fang , Gong Dawei , Sun Henghui , Wang Xun
Published: July 2002

The interfacial defects at the p-Si epitaxial layer/p-Si substrate interface have been studied by deep-level transient spectroscopy (DLTS). By solving Poisson equation, the electron concentration at the defect level varied with external voltage is derived. The emission and capture of electrons at the defect level, which are not observable in conventional DLTS, can be detected simultaneously in a single temperature scan by properly choosing the experimental parameters. The experimental results show that the energy level of the interfacial defects is located at Ec−0.30 eV.
Thermal anisotropy of polymer carbon fiber composites as revealed by photodeflection methods
Authors: Bertolotti M. , Ferrari A. , Liakhou G. L. , Li Voti R. , Marras A. , Ezquerra T. A. , Balta-Calleja F. J.
Published: July 2002

Thermal diffusivity for carbon fiber composites was measured using different fiber types and polymer matrices. Photothermal testing was performed in the various directions parallel and perpendicular to the carbon fiber axis by different photothermal configurations. By focusing the laser beam with a spherical lens, local inhomogeneities of the composite surface in the range of 10 μm are distinguished. When focusing is done with the aid of a cylindrical lens an averaging over larger scales of the photothermal deflected signal takes place. The results for various carbon fiber materials are discussed in terms of thermal diffusion lengths and thermal diffusivity values. It is shown that the thermal photodeflection method is suitable for measuring anisotropy in oriented carbon fiber composites.
Thermal diffusivity measurement of thin wires using photothermal deflection
Authors: Barkyoumb J. H. , Land D. J.
Published: July 2002

The Transverse Photothermal Beam Deflection method for measuring thermal diffusivity (or thermal conductivity) is developed for and applied to samples with cylindrical geometry. The basic theoretical equations governing the absorption of the heating (pump) laser beam, the diffusion of the heat, and subsequent deflection of the probe laser beam in the refractive index gradient of the surrounding medium are derived. To demonstrate the theory, thermal diffusivities of selected metal wires are measured and compared to values of the diffusivity as derived from electrical resistance measurements. The pump beam is scanned along the length of the wire samples to measure the photothermal deflection signal as a function of distance along the long axis of the samples. From either the decrease in magnitude of the photothermal signal with distance along the wire, or the phase lag of the measured deflection with respect to the pump beam, the thermal diffusivity of the sample can be obtained. The advantages and disadvantages of determining the thermal diffusivity of thin cylindrical samples by this method are discussed.
Effective interatomic potential for body-centered-cubic metals
Authors: Wang Y. R. , Boercker D. B.
Published: July 2002

An effective interatomic potential suitable for all body-centered-cubic (bcc) metals is developed based on the embedded atom method. The potential predicts all major anomalies displayed in the phonon spectra of the bcc transition metals, as well as the large relaxation of the (100) surface of V, Nb, and Ta.
Electron beam deposited lead-lanthanum-zirconate-titanate thin films for silicon based device applications
Authors: Panda B. , Ray S. K. , Dhar A. , Sarkar A. , Bhattacharya D. , Chopra K. L.
Published: July 2002

A simple single-source electron-beam evaporation technique has been used for the deposition of lead-lanthanum-zirconate-titanate (PLZT) thin films for silicon-based device applications. An optimized annealing condition has been established for the formation of crystalline perovskite phases. The effect of the bottom electrodes and the barrier layer on the growth of the films has been studied. Films with good dielectric and optical properties have been obtained under opt- imized conditions. Electrical properties of the films have been evaluated using metal–insulator–semiconductor and metal–insulator–metal structures. A moderately low interface trap density and very low leakage current density demonstrate the potential of the deposited films for device applications.
Electromigration transport mechanisms in Al thin-film conductors
Authors: Oates A. S.
Published: July 2002

Electromigration occurs along grain boundaries in polycrystalline thin-film conductors when the grain size is significantly larger than the conductor width. As circuit feature sizes have been reduced, microstructures have become near bamboo and alternate diffusion mechanisms must operate within the bamboo grains. In this article we examine drift velocity data to determine electromigration mechanisms in bamboo grains. We present measurements of the temperature and width dependence of the drift velocity of thin-film conductors with bamboo microstructures. The activation energy for drift is consistent with lattice diffusion in Al. There is no width dependence of the drift velocity in the range 0.6–2.7 μm, indicating a negligible flux of atoms along the conductor edges. We compare these measurements with data available for a variety of conductor microstructures, and with drift data for bulk Al. The drift velocities of conductors with bamboo microstructures obtained from a variety of sources show very good agreement, and are similar to drift velocities observed in bulk Al. We conclude that the dominant mode of electromigration in thin-film conductors with bamboo microstructures and bulk Al is the same, and occurs by lattice vacancy diffusion.
Hydrogen induced corrosion mechanism in NdFeB magnets
Authors: Kim A. S. , Camp F. E. , Lizzi T.
Published: July 2002

An investigation was conducted comparing the corrosion behavior of NdFeB magnets in flowing hydrogen and in the heat and humidity of an autoclave. The results show that corrosion is both macroscopically and microscopically similar in both environments. In both cases, the corrosion progressed most rapidly in those areas where the magnetic orientation of the Nd2Fe14B matrix grains was perpendicular to the outer surface. A corrosion mechanism involving the reaction of hydrogen—either as a pure gas or as a by-product of the decomposition of water vapor—with the neodymium-rich grain boundary phase is proposed.
Low temperature spin dynamics of geometrically frustrated antiferromagnets Y2Mo2O7 and Y2Mo1.6Ti0.4O7 studied by muon spin relaxation
Authors: Dunsiger S. R. , Kiefl R. F. , Chow K. H. , Gaulin B. D. , Gingras M. J. P. , Greedan J. E. , Keren A. , Kojima K. , Luke G. M. , MacFarlane W. A. , Raju N. P. , Sonier J. E. , Uemura Y. J. , Wu W. D.
Published: July 2002

The spin dynamics of geometrically frustrated pyrochlore antiferromagnets Y2Mo2O7 and Y2Mo1.6Ti0.4O7 have been investigated using muon spin relaxation. In Y2Mo2O7 a dramatic slowing down of the Mo4+ spin fluctuations occurs as one approaches the spin freezing temperature TF of 22 K from above. At lower temperatures the spins freeze into a disordered magnetic state similar to that found in a spin glass but with a small residual muon spin relaxation rate at low temperatures. This residual relaxation rate is larger in Y2Mo1.6Ti0.4O7  where TF=15 K. These results suggest that there is a nonzero density of states for magnetic excitations in these systems near zero energy.
Effect of interstrip gap on the sensitivity of high sensitivity magnetoresistive transducers
Authors: Pant Bharat B.
Published: July 2002

For high sensitivity magnetoresistive (MR) transducers, the role of interstrip gap is examined as a design parameter. This is modeled using analytic and numerical approximations. As the gap is narrowed the range of linearity of the MR transducer decreases and the sensitivity increases. For gap/width ratio exceeding 1.8 the strips behave essentially as isolated strips. An example using typical design parameters shows that the minimum detectable field is reduced from 6.3 μOe/√Hz to 3.6 μOe/√Hz as gap/width ratio is reduced from ∞ to 0.1.
Optical absorption spectra of C70 thin films
Authors: Zhou Wei-ya , Xie Si-shen , Qian Sheng-fa , Zhou Tang , Zhao Ri-an , Wang Gang , Qian Lu-xi , Li Wen-zhi
Published: July 2002

Both optical transmission spectroscopy and photothermal deflection spectroscopy are used to determine the spectra of C70 thin films over a wide energy range (0.6–6.5 eV). Based on a molecular orbital model, the optical transitions for the C70 thin film are analyzed. The weak absorption spectra of C70 thin films are similar to that of an amorphous semiconductor. The optical energy gap is derived by a Tauc plot as 1.66 eV. The gap region can be described in terms used for amorphous semiconductors, having features such as an Urbach edge and subgap defect absorption, which are interpreted as a broadening due to disorder or impurities. The effects of the deflection medium on the weak absorption spectra of C70 films are discussed.
Structural and electrical properties of sputtered vanadium oxide thin films for applications as gas sensing material
Authors: Manno D. , Serra A. , Di Giulio M. , Micocci G. , Taurino A. , Tepore A. , Berti D.
Published: July 2002

A detailed structural and morphological investigation has been carried out by conventional transmission electron microscopy, high resolution electron microscopy and nanodiffraction methods on vanadium oxide films obtained by reactive rf sputter at a high power discharge (1000 W) and different O2/Ar ratio. Electrical characterization has been also performed in controlled atmosphere in order to investigate the influence of NO2 oxidizing gas on the material conductance as a function of deposition parameters. A strict relation between structure, morphology and resistance variation in controlled atmosphere has been observed.
Space charge effects on the current-voltage characteristics of gated field emitter arrays
Authors: Jensen K. L. , Kodis M. A. , Murphy R. A. , Zaidman E. G.
Published: July 2002

Microfabricated field emitter arrays (FEAs) can provide the very high electron current densities required for rf amplifier applications, typically on the order of 100 A/cm2. Determining the dependence of emission current on gate voltage is important for the prediction of emitter performance for device applications. Field emitters use high applied fields to extract current, and therefore, unlike thermionic emitters, the current densities can exceed 103 A/cm2 when averaged over an array. At such high current densities, space charge effects (i.e., the influence of charge between cathode and collector on emission) affect the emission process or initiate conditions which can lead to failure mechanisms for field emitters. A simple model of a field emitter will be used to calculate the one-dimensional space charge effects on the emission characteristics by examining two components: charge between the gate and anode, which leads to Child’s law, and charge within the FEA unit cell, which gives rise to a field suppression effect which can exist for a single field emitter. The predictions of the analytical model are compared with recent experimental measurements designed to assess space charge effects and predict the onset of gate current. It is shown that negative convexity on a Fowler–Nordheim plot of Ianode(Vgate) data can be explained in terms of field depression at the emitter tip in addition to reflection of electrons by a virtual cathode created when the anode field is insufficient to extract all of the current; in particular, the effects present within the unit cell constitute a newly described effect.
The effects of the stress dependence of atomic diffusivity on stress evolution due to electromigration
Authors: Park Young Joon , Thompson Carl V.
Published: July 2002

The evolution of mechanical stress resulting from electromigration in a polycrystalline thin film Al interconnect is simulated as a function of current density assuming zero stress boundary conditions. A polygranular cluster region is introduced into lines with otherwise bamboo microstructures, and results in a pair of atomic flux divergence sites and a tensile-compressive stress dipole. In the tensile stress region, the atomic diffusivity is higher than that in the compressive stress region, leading to more rapid electromigration, so that the tensile stress is initially slightly larger than the compressive stress in the dipole. Eventually though, the tensile stress drops to a small value while the compressive peak in the cluster increases above its initial values due to a stress buildup at the end of the cluster. For a given line, the maximum compressive stress resulting from electromigration is larger than the maximum tensile stress. Increasing the current density leads to an increasing difference in the maximum compressive and tensile stresses.