bluuebunny/crossref_metadata_2025 huggingface dataset:
Transport and centering of high current electron beams in neutral gas filled cells
Authors: Myers M. C. , Antoniades J. A. , Meger R. A. , Murphy D. P. , Fernsler R. F. , Hubbard R. F.
Published: July 2002

Conducting tubes filled with neutral gas at pressures between 0.001 and 0.1 Torr can be used to transport, to center, and to reduce the transverse oscillations of high current (≳10 kA) electron beams. Electron impact ionization of the gas leads to partial neutralization of the beam space charge allowing self-focused beam transport and phase-mix damping of injected beam oscillations. In addition, the presence of conducting walls helps center the beam in the transport tube. High current beams, transported through a 1.3 m long tube, were centered to within one-tenth of the beam radius and input transverse oscillations were damped to submillimeter values without significant current loss or emittance growth. Beam transport properties are examined as a function of injected current, gas pressure, and cell geometry. Experimental results are compared with a theoretical model.
Erbium in oxygen-doped silicon: Optical excitation
Authors: van den Hoven G. N. , Shin Jung H. , Polman A. , Lombardo S. , Campisano S. U.
Published: July 2002

The photoluminescence of erbium-doped semi-insulating polycrystalline and amorphous silicon containing 30 at. % oxygen is studied. The films were deposited on single-crystal Si substrates by chemical vapor deposition, implanted with 500 keV Er to fluences ranging from 0.05 to 6×1015 ions/cm2, and annealed at 300–1000 °C. Upon optical pumping near 500 nm, the samples show room-temperature luminescence around 1.54 μm due to intra-4f transitions in Er3+, excited by photogenerated carriers. The strongest luminescence is obtained after 400 °C annealing. Two classes of Er3+ can be distinguished, characterized by luminescence lifetimes of 170 and 800 μs. The classes are attributed to Er3+ in Si-rich and in O-rich environments. Photoluminescence excitation spectroscopy on a sample with 1×1015 Er/cm2 shows that ∼2% of the implanted Er is optically active. No quenching of the Er luminescence efficiency is observed between 77 K and room temperature in this Si-based semiconductor. The internal quantum efficiency for the excitation of Er3+ via photogenerated carriers is 10−3 at room temperature. A model is presented which explains the luminescence data in terms of trapping of electrical carriers at localized Er-related defects, and subsequent energy transfer to Er3+ ions, which can then decay by emission of 1.5 μm photons.
Observation of 20 eV x-ray generation in a proof-of-principle laser synchrotron source experiment
Authors: Ting A. , Fischer R. , Fisher A. , Evans K. , Burris R. , Krall J. , Esarey E. , Sprangle P.
Published: July 2002

A laser synchrotron source (LSS) [P. Sprangle, A. Ting, E. Esarey, and A. Fisher, J. Appl. Phys. 72, 5032 (1992)] was proposed to generate short-pulsed, tunable x rays by Thomson scattering of laser photons from a relativistic electron beam. A proof-of-principal (p.o.p.) experiment on this LSS configuration is performed. An intense laser pulse (λ0=1.053 μm) is Thomson backscattered from a focused relativistic electron beam. Time integrated x-ray signals from a photocathode/electron multiplier, at an electron beam energy of 650 keV and an x-ray photon energy of 20 eV, indicate an increase in the x-ray signals above the baseline by an amount comparable to the theoretically predicted value.
The EX defect center in porous silicon
Authors: Carlos W. E. , Prokes S. M.
Published: July 2002

We report the observation of the EX center in porous silicon by electron spin resonance (ESR). This center has been previously observed in low temperature thin oxides on crystalline Si and is believed to comprise a delocalized hole on three or four oxygen dangling bonds at a Si vacancy. In porous Si the defect is seen only in samples which have been oxidized for a very short period of time (∼1 min). The ESR intensity of the EX center correlates with the red room-temperature photoluminescence.
Magnetic properties of nanometer-size CoPt particles
Authors: Liou S. H. , Liu Y. , Malhotra S. S. , Yu M. , Sellmyer D. J.
Published: July 2002

We have prepared nanometer-size, isolated CoPt particles. The particles in the range of 100–300 nm in diameter were formed by annealing thin films in the temperature range of 550–800 °C. Films with magnetic coercivity as high as 30 kOe were achieved. The results indicate that the high magnetic coercivity was obtained because the crystallite size approaches that of noninteracting single-domain particles.
Pulsed laser-induced damage threshold of thin aluminum films on quartz: Experimental and theoretical studies
Authors: Lee K. C. , Chan C. S. , Cheung N. H.
Published: July 2002

Pulsed 532-nm laser-induced damage thresholds of aluminum films on quartz were systematically measured for film thickness ranging from 13 to 50 nm, using simple yet reliable acoustic and optical techniques. Experimental data were found to be consistent and reproducible, and could be satisfactorily simulated by a one-dimensional heat diffusion model with a temperature-dependent absorptance. Damage thresholds measured with the acoustic probe (interpreted as evaporative events) were in the 30- to 100-mJ/cm2 range, which are about 30% higher than those measured with the optical probe (interpreted as melting events). This small difference is consistent with the temperature-dependent absorptance assumption, and helps to explain the unexpected sensitivity of the acoustic probe relative to the optical probe, especially for thicker films.
Resistance noise in uncoupled giant magnetoresistive multilayers
Authors: Hardner H. T. , Weissman M. B. , Miller B. , Loloee R. , Parkin S. S. P.
Published: July 2002

Various uncoupled, magnetoresistive multilayer systems show quasiequilibrium resistance noise associated with ‖dR(H)/dH‖. The size of this 1/f noise, particularly in comparison with Johnson noise in various frequency ranges, is discussed for all the different systems. All the observed samples exhibit resistive Barkhausen noise, allowing the measurement of domain sizes very similar to those observed in coupled samples. In addition, the asymmetric hysteresis of the Barkhausen noise in uncoupled multilayers suggests the same type of hysteretic domain structure observed in antiferromagnetically coupled multilayers.
Optimization of x-ray sources for proximity lithography produced by a high average power Nd:glass laser
Authors: Celliers P. , Da Silva L. B. , Dane C. B. , Mrowka S. , Norton M. , Harder J. , Hackel L. , Matthews D. L. , Fiedorowicz H. , Bartnik A. , Maldonado J. R. , Abate J. A.
Published: July 2002

We measured the conversion efficiency of laser pulse energy into keV x rays from a variety of solid planar targets and a Xe gas puff target irradiated using a high average power Nd:glass slab laser capable of delivering 13 ns full width at half-maximum pulses at up to 20 J at 1.053 μm and 12 J at 0.53 μm. Targets were chosen to optimize emission in the 10–15 Å wavelength band, including L-shell emission from materials with atomic numbers in the range Z=24–30 and M-shell emission from Xe (Z=54). With 1.053 μm a maximum conversion of 11% into 2π sr was measured from solid Xe targets. At 0.527 μm efficiencies of 12%–18%/(2π sr) were measured for all of the solid targets in the same wavelength band. The x-ray conversion efficiency from the Xe gas puff target was considerably lower, at about 3%/(2π sr) when irradiated with 1.053 μm.
Electron energy distributions in silicon structures at low applied voltages and high electric fields
Authors: Abramo Antonio , Fiegna Claudio
Published: July 2002

In this article a quantitative study of the electron energy distribution in silicon devices at low applied voltages is carried out by means of Monte Carlo simulations including the main mechanisms involved in the process of carrier heating. We present a clear-cut interpretation of the build up of the electron distribution at energies higher than what is provided by the applied electric field. The influence of different boundary conditions on the simulation results is analyzed in detail. As a consequence, the hypothesis that the high energy tail simply represents the memory of the initial distribution at the injecting boundary due to ballistic transport is ruled out, even for highly inhomogeneous field profiles, such as in very short metal oxide semiconductor field effect transistors, for which a ballistic transport regime had been stated. In addition, the effect of short-range electron-electron interaction is examined and shown to be an effective process for the enhancement of the high-energy electron population.
Molecular beam deposited thin films of pentacene for organic field effect transistor applications
Authors: Dimitrakopoulos C. D. , Brown A. R. , Pomp A.
Published: July 2002

Pentacene films deposited with molecular beam deposition have been fabricated and characterized with respect to structure and morphology using x-ray diffraction and scanning electron microscopy. Metal-insulator semiconductor field-effect transistor devices based on such films were used to study their transport properties. A maximum field-effect mobility of 0.038 cm−2 V−1 s−1 is reported for devices incorporating pentacene films deposited at room temperature. The structural characterization revealed the coexistence of two phases: the thermodynamically stable single-crystal phase and the kinetically favored, metastable thin-film phase. Such mixed phase films were produced when low deposition rates were used in combination with a substrate temperature of 55 °C. Mixed phase films had transport properties inferior to films consisting solely of one phase, while amorphous films deposited at low surface mobility conditions had extremely low conductivity. Use of prepurified pentacene as source material resulted in an order of magnitude lower free-carrier concentration in the pentacene film as compared to films made with as-received pentacene.
Modeling the growth of thin silicon oxide films on silicon
Authors: Dimitrijev Sima , Harrison H. Barry
Published: July 2002

This article analyses the validity of the widely used semi-empirical oxidation models based on the Deal-Grove formulation in the light of recent advances in the understanding of the oxidation process. An extension of the Deal-Grove formulation is suggested to account for the newest experimental results. The introduced extension incorporates the effect of accelerated initial growth into the theoretical oxidation growth model. There is a direct relationship between the newly-derived theoretical term for the initial growth rate and its widely-used empirical counterpart.
Determination of densities and mobilities of heavy and light holes in p-type Si using reduced-conductivity-tensor analyses of magnetic-field-dependent Hall and resistivity measurements
Authors: Kim J. S. , Seiler D. G. , Ehrstein J. R.
Published: July 2002

The densities and mobilities of the individual heavy- and light-hole carriers have been simultaneously determined at various temperatures (40 K to 130 K) in three p-type, single-crystal Si samples. The separation of the two-hole components is achieved by multicarrier analyses of magnetic-field-dependent Hall and resistivity measurements within the two-carrier approximation of the reduced-conductivity-tensor scheme. The explicit experimental values for the densities and mobilities of the two-hole components obtained in this work should be considered as a valuable addition to the existing database for silicon material parameters. They should also be useful to silicon device physics and modeling.
The temperature dependence of the absorption coefficient of porous silicon
Authors: Kovalev D. , Polisski G. , Ben-Chorin M. , Diener J. , Koch F.
Published: July 2002

The spectral behavior and the temperature dependence of the absorption coefficient of microporous silicon films are studied in the energy range of 1.2–3.8 eV, between 7 and 450 K. For photon energies above the direct band gap at 3.3 eV, the spectral behavior of the absorption coefficient is similar to that of crystalline silicon, and its absolute value is comparable to that estimated using Bruggeman effective medium approach. For lower energies the absorption coefficient is much reduced with respect to the effective medium model prediction. In this energy region, the absorption depends exponentially on the photon energy. From the temperature dependence of the absorption it is shown that this ‘‘Urbach tail’’-like behavior is not due to absorption in surface states. In the range 300–400 K, the temperature dependence of the absorption coefficient of microporous and crystalline silicon is the same. This is a strong indication that phonon-assisted optical absorption takes place as expected from the indirect nature of crystalline silicon.
Surface control of luminescence in silicon nanoparticles
Authors: Seraphin A. A. , Ngiam S.-T. , Kolenbrander K. D.
Published: July 2002

The intensity of visible photoluminescence from thin films of silicon nanoparticles is shown to be dependent upon the degree of surface passivation on the nanoparticles, while the emission energy is independent of the specific chemical nature of the passivating species. Nanoparticles are deposited into films using a pulsed laser ablation supersonic expansion source. Surface chemistry is controlled with a variety of postdeposition processing steps, including acid, methanol, and iodine treatments. These steps are then correlated with the visible photoluminescence behavior of the sample and the surface chemistry as characterized by x-ray photoelectron spectroscopy. Surface passivation controls the emission intensity through the elimination of competing nonradiative carrier relaxation pathways. The chemical nature of the passivating species does not determine the emission energy and can be easily changed with no anomalous photoluminescence behavior resulting. All results are consistent with a simple quantum confinement model of emission where particle size determines emission energy and surface passivation determines emission intensity.
Structural characterization of Si0.7Ge0.3 layers grown on Si(001) substrates by molecular beam epitaxy
Authors: Obata T. , Komeda K. , Nakao T. , Ueba H. , Tatsuyama C.
Published: July 2002

Structural properties of Si0.7Ge0.3 alloy films grown on Si(001) substrate by molecular beam epitaxy have been characterized by means of several available techniques. Different types of buffer layers were predeposited on a clean Si(001) substrate in order to relax the lattice mismatch between the topmost Si0.7Ge0.3 alloy layer and Si substrate. The effect of buffer layers on the structural quality of the overgrown Si0.7Ge0.3 was investigated by x-ray diffraction, x-ray photoemission spectroscopy, photoluminescence, and cross-sectional transmission electron microscope. It is confirmed that the threading dislocation density in the alloy layer drastically decreases by using buffer layers. The samples with step buffer layers relax the strain by introducing the dislocations at the interfaces, part of which goes through the alloy layer. On the other hand, the samples with superlattice buffer layers relax the strain by introducing the dislocations in the buffer layers which terminate at the interface of the superlattice buffer layer and the topmost alloy layer. The residual strain in the alloy layers on buffer layers grown at 550 °C is relaxed upon the annealing at 700 °C, or the growth at 700 °C.
Heat transport in thin dielectric films
Authors: Lee S.-M. , Cahill David G.
Published: July 2002

Heat transport in 20–300 nm thick dielectric films is characterized in the temperature range of 78–400 K using the 3ω method. SiO2 and SiNx films are deposited on Si substrates at 300 °C using plasma enhanced chemical vapor deposition (PECVD). For films >100 nm thick, the thermal conductivity shows little dependence on film thickness: the thermal conductivity of PECVD SiO2 films is only ∼10% smaller than the conductivity of SiO2 grown by thermal oxidation. The thermal conductivity of PECVD SiNx films is approximately a factor of 2 smaller than SiNx deposited by atmospheric pressure CVD at 900 °C. For films <50 nm thick, the apparent thermal conductivity of both SiO2 and SiNx films decreases with film thickness. The thickness dependent thermal conductivity is interpreted in terms of a small interface thermal resistance RI. At room temperature, RI∼2×10−8 K m2 W−1 and is equivalent to the thermal resistance of a ∼20 nm thick layer of SiO2 .
Gate controlled Coulomb blockade effects in the conduction of a silicon quantum wire
Authors: Smith R. A. , Ahmed H.
Published: July 2002

Side gated silicon quantum wires of nominally uniform 60 nm/70 nm width, 40 nm height, and 4 μm/6 μm length have been fabricated in the silicon-on-insulator configuration. Measurements of the current–voltage characteristics at a temperature of 2.0 K show significant nonlinearities including a zero conductivity blockade region and conductance peaks suggestive of single electron behavior. The blockade size is significantly affected by application of a gate potential and oscillations of the blockade and wire conductivity with gate potential are also observed. These features are explained by a single electron tunneling model of a multiple island system.
A model for the nucleation of diamond clusters on Si(111) substrates
Authors: Mahalingam Pushpa , Liu Huimin , Dandy David S.
Published: July 2002

A theoretical study of the nucleation, size, and structure of diamond phase carbon clusters on Si(111) substrates is presented. Molecular mechanics analysis has been utilized to predict energetically and entropically feasible pathways for nucleation of the carbon clusters. Several mechanistic pathways for nucleation of carbon clusters are examined with CH3 and/or C2H2 as the nucleation precursors. A possible model for the nucleation mechanism of diamond-phase carbon clusters on the β-SiC(111) surface, which forms epitaxially on Si(111) substrates, is presented. The critical size of the carbon clusters is computed based on the atomistic theory of nucleation and the proposed nucleation mechanisms.
Time-dependent and nonlinear effects in electrorheological fluids
Authors: Davis L. C.
Published: July 2002

An integral equation method is used to calculate particle–particle forces in electrorheological fluids. The method focuses on the gap region between particles where large electric-field concentrations occur. Effects due to time-dependent excitation and nonlinear (field-dependent) fluid conductivity are analyzed. It is found that the response to step-function changes in applied field closely follows a simple form that can be derived from the dipole approximation. Qualitatively different stress-vs-time curves are obtained for large dielectric mismatch (e.g., barium titanate/dodecane) relative to large conductivity mismatch (zeolite/silicone oil). In fluids where the conductivity is strongly field dependent, it is found that particle–particle forces scale linearly with applied field E0 at large fields. Likewise, the shear yield stress scales as E03/2.
Nonradiative decay processes and mechanisms of frequency upconversion of Er3+ in ZrF4–BaF2–LaF3 glass
Authors: Takahashi M. , Shojiya M. , Kanno R. , Kawamoto Y. , Kadono K. , Ohtsuki T. , Peyghambarian N.
Published: July 2002

We have investigated the decay processes of the Er:4S3/2 and F39/2 states and the mechanisms of frequency upconversion under 800 nm excitation of Er3+ in ternary ZrF4–BaF2–LaF3 glasses. The decay processes of the Er:4S3/2 and F49/2 states are discussed on the basis of the Er concentration dependence of the fluorescence lifetime. The quantum efficiencies of emission of the S43/2 and F49/2 states are 73% and 25%, respectively. The difference in the quantum efficiencies between these states can be explained from the difference in the multiphonon decay rates: the multiphonon decay rate of the S43/2 state is much smaller than that of the F49/2 state. The multiphonon relaxation is a dominant nonradiative decay process of the F49/2 state. A dominant nonradiative decay process of the S43/2 state is self-quenching at high Er concentrations. The upconversion mechanisms under 800 nm excitation are also investigated from the dependence of upconversion luminescence on excitation wavelengths (800 and 980 nm), excitation laser power, and Er concentration. The mechanism of upconversion from 800 to 660 nm is mainly due to the energy transfer between the I49/2→4I13/2 and F49/2←4I11/2 transitions. We also show that the upconversion from 800 to 550 nm is due to excited-state absorption.
Elastic strain relaxation and piezoeffect in GaN-AlN, GaN-AlGaN and GaN-InGaN superlattices
Authors: Bykhovski A. D. , Gelmont B. L. , Shur M. S.
Published: July 2002

We calculated the elastic strain relaxation in (GaN)n-(AlN)n, (GaN)n(AlxGa1−xN)n and (GaN)n(InxGa1−xN)n superlattices where n is the number of layers in the superlattice cell. This calculation and a similar calculation for a semiconductor–insulator–semiconductor structure allowed us to determine the lower and upper bounds for the elastic strain relaxation in (GaN)m(AlN)n superlattices with arbitrary n/m ratios, i.e., we determine a full range of the critical thicknesses for GaNm(AlN)n superlattices. The obtained theoretical results can also be applied to other superlattices based on III nitrides and their solid solutions. Our theory agrees with the experimental data for GaN-AlN superlattices. Also, we show that the piezoelectric effect may cause a large shift of the absorption edge in defect-free GaNm(AlxGa1−xN)n superlattices.
Arrays of magnetic wires created in phase-separating Fe-containing alloys by interference laser irradiation
Authors: Polushkin Nikolay I. , Gusev S. A. , Drozdov M. N. , Verevkin Yu. K. , Petryakov V. N.
Published: July 2002

Using the interference of laser beams, we have fabricated the periodic arrays of submicron wires in Fe/C and Fe/Cr thin-film alloys. The array formation is found to arise from iron agglomeration in interference maxima. The dramatic decay of the in-plane magnetic anisotropy was observed in the fabricated arrays with decrease in the interference periodicity down to 170 nm. This decay can rather be explained by transforming the wires into separate granules than a smallness of the temperature modulation at small periodicities. Such an explanation is supported by the electronmicroscopic and ferromagnetic resonance data.
Magnetic hardening in FePt nanostructured films
Authors: Liu J. P. , Liu Y. , Luo C. P. , Shan Z. S. , Sellmyer D. J.
Published: July 2002

FePt films have been prepared by sputtering Fe/Pt multilayers onto glass or silicon substrates. The thickness of the Fe and Pt layers was adjusted with the Fe:Pt atomic ratio from about 1:1 to 2:1. Magnetic hardening is observed after heat treatment at elevated temperatures, which led to coercivity values exceeding 20 kOe in samples with an Fe:Pt ratio around 1.2:1. The hardening originates from the formation of the tetragonal FePt phase with high magnetocrystalline anisotropy and a favorable microstructure. Two-phase composite films containing hard and soft phases were obtained when the Fe:Pt ratio increased. Under optimized processing conditions, composite films with energy products larger than 30 MG Oe at room temperature have been successfully produced.
High frequency impedance spectra of soft amorphous fibers
Authors: Ménard D. , Britel M. , Ciureanu P. , Yelon A. , Paramonov V. P. , Antonov A. S. , Rudkowski P. , Ström-Olsen J. O.
Published: July 2002

Giant magnetoimpedance (GMI) spectra of soft amorphous magnetic fibers, measured in the 1 kHz–1.2 GHz frequency range, and GMI responses, measured in the field range of ±120 Oe, have reinforced the assumption that linear giant magnetoimpedance and ferromagnetic resonance (FMR) have the same physical origin. The samples, NiCo-rich, CoFe-rich, and Metglas-type fibers, 30–40 μm in diameter, were cast by melt extraction. Their impedance has been measured up to 13 MHz, in the presence of a magnetic field, using an impedance analyzer. These measurements have been extended up to 1.2 GHz by using a network analyzer. The reflection coefficient of a shorted coaxial line whose inner conductor was replaced by a magnetic fiber was measured, and the input impedance per unit length of this line was then calculated. The two impedances above are equivalent and their spectra show a behavior associated with FMR: the real part of the impedance peaks at a frequency where the imaginary part passes through zero.
Neutron irradiation defects in gallium sulfide: Optical absorption measurements
Authors: Manjón F. J. , Segura A. , Muñoz V.
Published: July 2002

Gallium sulfide single crystals have been irradiated with different thermal neutron doses. Defects introduced by neutron irradiation turn out to be optically active, giving rise to absorption bands with energies ranging from 1.2 to 3.2 eV. Bands lying in the band-gap exhibit Gaussian shape. Their energies and widths are independent of the irradiation dose, but their intensities are proportional to it. Thermal annealing is completed in two stages, ending at around 500 and 720 K, respectively. Centers responsible for the absorption bands are proposed to be gallium-vacancy-galliuminterstitial complexes in which the distance between the vacancy (acceptor) and the interstitial (donor) determines the energy and intensity of the absorption band, as well as the annealing temperature.
Detonation waves in pentaerythritol tetranitrate
Authors: Tarver Craig M. , Breithaupt R. Don , Kury John W.
Published: July 2002

Fabry–Perot laser interferometry was used to obtain nanosecond time resolved particle velocity histories of the free surfaces of tantalum discs accelerated by detonating pentaerythritol tetranitrate (PETN) charges and of the interfaces between PETN detonation products and lithium fluoride crystals. The experimental records were compared to particle velocity histories calculated using very finely zoned meshes of the exact dimensions with the DYNA2D hydrodynamic code. The duration of the PETN detonation reaction zone was demonstrated to be less than the 5 ns initial resolution of the Fabry–Perot technique, because the experimental records were accurately calculated using an instantaneous chemical reaction, the Chapman–Jouguet (C-J) model of detonation, and the reaction product Jones–Wilkins–Lee (JWL) equation of state for PETN detonation products previously determined by supracompression (overdriven detonation) studies. Some of the PETN charges were pressed to densities approaching the crystal density and exhibited the phenomenon of superdetonation. An ignition and growth Zeldovich–von Neumann–Doring (ZND) reactive flow model was developed to explain these experimental records and the results of previous PETN shock initiation experiments on single crystals of PETN. Good agreement was obtained for the induction time delays preceding chemical reaction, the run distances at which the initial shock waves were overtaken by the detonation waves in the compressed PETN, and the measured particle velocity histories produced by the overdriven detonation waves before they could relax to steady state C-J velocity and pressure.
Texture and stress of Ag films in Ag/Ti, Ag/Cr bilayers, and self-encapsulated structures
Authors: Zeng Yuxiao , Zou Y. L. , Alford T. L. , Deng F. , Lau S. S. , Laursen T. , Ullrich B. Manfred
Published: July 2002

The texture of evaporated Ag films prepared on Ti or Cr underlayers before and after encapsulation process has been studied by x-ray diffraction. In addition, the stress state in self-encapsulated Ag/Ti structures has also been investigated using a “sin2 ψ” technique. Silver films deposited on Ti layers exhibit a strong 〈111〉 texture, which is in contrast to the nearly random orientation of Ag films on Cr underlayers. The minimization of interfacial energy with respect to lattice match can account for this underlayer dependence. After an encapsulation process involving Ti reactions in an ammonia ambient, the texture of Ag films in Ag/Ti bilayers is further enhanced. Highly textured Ag films may provide the basis for electromigration-resistant Ag metallization in integrated circuit devices. For the Ag/Ti bilayer structures, a low tensile stress of approximately 61 MPa arising from the nonequilibrium growth during the film deposition is present in the Ag films. This results in a lattice tension state in the film plane and a lattice compression state along the film normal. Thermal mismatch stress is produced by the encapsulation process at 600 °C. Most of this stress relaxes during the cooling stage and a residual tensile stress of ∼320 MPa in the film plane was determined.
Fabrication and characterization of nanoscale arrays of nickel columns
Authors: O’Barr R. , Yamamoto S. Y. , Schultz S. , Xu Weihua , Scherer A.
Published: July 2002

We have fabricated arrays of high aspect ratio nickel columns in both PMMA and SiO2, and we have characterized their structure using transmission electron microscopy and scanning electron microscopy. We have also characterized the magnetics of the columns with magnetic force microscopy. Such characterization of high aspect ratio perpendicular magnetic particles is only rarely found in the literature. This work lays a foundation for further studies to test the viability of lithographically defined arrays of magnetic particles as an alternative high density storage medium.
In situ characterization of the oxidative degradation of a polymeric light emitting device
Authors: Cumpston B. H. , Parker I. D. , Jensen K. F.
Published: July 2002

Light-emitting devices with polymeric emissive layers have great promise for the production of large-area, lightweight, flexible color displays, but short lifetimes currently limit applications. We address mechanisms of bulk polymer degradation in these devices and show through in situ Fourier transform infrared characterization of working light-emitting devices with active layers of poly[2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene] that oxygen is responsible for the degradation of the polymer film. A mechanism is given based on the formation of singlet oxygen from oxygen impurities in the film via energy transfer from a nonradiative exciton. Fourier transform infrared and x-ray photoelectron spectroscopy results are consistent with the mechanism, involving singlet oxygen attack followed by free radical processes. We further show that oxygen readily diffuses into the active polymer layer, changing the electrical characteristics of the film even at low concentrations. Thus, polyphenylene-vinylene-based light-emitting devices will self-destruct during operation if fabricated without special attention to eliminating oxygen contamination during fabrication and device operation.
Modeling and characterizing the propagation velocity of exothermic reactions in multilayer foils
Authors: Mann A. B. , Gavens A. J. , Reiss M. E. , Van Heerden D. , Bao G. , Weihs T. P.
Published: July 2002

Combustible multilayer foils can be fabricated by sputter depositing alternate layers of materials which react exothermically during thermally induced intermixing. Current models for these reactions consider pure materials which only intermix during the self-propagating stage of the reaction, though in reality during fabrication the materials undergo partial intermixing. An analytical model dealing with the premixing is presented and compared with experimental results for Al/Ni and Al/(Ni:Cu) multilayers. The model and the results indicate that premixing lowers the propagation velocity both by slowing the rate of atomic diffusion between layers and by lowering the temperature of the reaction. The lower temperature can cause solid/liquid phase changes to dominate the reaction path. It is concluded that to use these foils in commercial and engineering applications, the method of fabrication and the phase changes occurring during the reaction must be controlled to give the desired characteristics.
Simultaneous observation of strong and weak quantum confinement effect in chemically deposited CdSe thin films: A spectro-structural study
Authors: Rai Brajesh K. , Bist H. D. , Katiyar R. S. , Nair M. T. S. , Nair P. K. , Mannivannan A.
Published: July 2002

CdSe thin films deposited chemically on glass substrates for 4, 8, and 16 h, and subsequently annealed at 400 °C for 1 h, have been studied by a combination of spectroscopic (photoluminescence and Raman scattering) and structure-determining (x-ray diffraction and atomic force microscopy) techniques. Due to a size distribution of constituent grains, photoluminescence spectra of the as-deposited films show weak but broad bands at ∼2.2 eV (strongly confined band) and ∼1.73 eV (weakly confined band). On annealing, intensity of the weakly confined band, at ∼1.7 eV increases as a result of an improvement in the crystalline quality of CdSe nanoclusters. A surface-optic Raman mode at ∼250 cm−1 in as-deposited samples has been observed for the first time. The x-ray diffraction studies of annealed samples show a diffraction peak at 2θ=13° from the (001) plane. The improvement in crystallinity of the films as observed by atomic force microscopy and photoluminescence techniques, the appearance of (001) reflection in the x-ray diffraction pattern, the disappearance of surface-optic Raman mode, and the enhancement of weakly confined band—all as a consequence of annealing—have been discussed and correlated with each other. A film deposition mechanism has been described, which explains the origin of the simultaneous existence of strong and weak quantum confinement effects; the significance this observation in the development of high efficiency photovoltaic solar cells has been emphasized.
Electromodulation reflectance of low temperature grown GaAs
Authors: Hsu T. M. , Sung J. W. , Lee W. C.
Published: July 2002

We have studied low temperature grown GaAs by electromodulation reflectance spectroscopy for growth temperatures between 200 and 320 °C. The reflectance spectra can be observed only when the concentration of the arsenic antisite defect is lower than 3.4×1019 cm−3. The absence of the signal for a rich defect sample is related to the short carrier lifetime and surface depletion width. The transition energies of E0 and E1 are found to increase with the decrease of the growth temperatures. The increase of the transition energies is attributed to lattice mismatch and deviation of the stoichiometry.
Discreteness-induced resonances and ac voltage amplitudes in long one-dimensional Josephson junction arrays
Authors: Duwel A. E. , Watanabe Shinya , Triás E. , Orlando T. P. , van der Zant Herre S. J. , Strogatz Steven H.
Published: July 2002

New resonance steps are found in the experimental current-voltage characteristics of long, discrete, one-dimensional Josephson junction arrays with open boundaries and in an external magnetic field. The junctions are underdamped, connected in parallel, and dc biased. Numerical simulations based on the discrete sine-Gordon model are carried out, and show that the solutions on the steps are periodic trains of fluxons, phase locked by a finite amplitude radiation. Power spectra of the voltages consist of a small number of harmonic peaks, which may be exploited for possible oscillator applications. The steps form a family that can be numbered by the harmonic content of the radiation, the first member corresponding to the Eck step. Discreteness of the arrays is shown to be essential for appearance of the higher order steps. We use a multimode extension of the harmonic balance analysis, and estimate the resonance frequencies, the ac voltage amplitudes, and the theoretical limit on the output power on the first two steps.
Optical and electrical characterization of nitrogen ion implanted ZnSSe/p-GaAs (100)
Authors: Hong H. , Anderson W. A. , Haetty J. , Petrou A. , Lee E. H. , Chang H. C. , Na M. H. , Luo H. , Peck J. , Mountziaris T. J.
Published: July 2002

Nitrogen ions were implanted into ZnSxSe1−x epilayers grown on p-GaAs (100) substrates by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD). Dopant activation and annealing out the implant damage were achieved by a postannealing process in a N2 ambient. Schottky structures employing the implanted p-type ZnSxSe1−x were fabricated and device efficacy was examined by photoluminescence (PL) spectroscopy, current–voltage (I–V), current–voltage temperature (I–V–T), and high frequency capacitance–voltage (C–V) measurements. PL spectra showed a clear donor–acceptor pair (DAP) recombination at an energy of 2.735 and 2.72 eV, in both MBE and MOCVD ZnSSe epilayers, respectively, regardless of the postannealing temperatures. The diode conduction in forward bias proceeds by the combination of thermionic and tunneling emission. C–V measurement proved the maximum doping concentration to be around 1017 cm−3 after ion implantation.
Effects of lead vacancies on the spontaneous relaxor to ferroelectric phase transition in Pb[(Mg1/3Nb2/3)0.9Ti0.1]O3
Authors: Bidault O. , Husson E. , Morell A.
Published: July 2002

We report electrical measurements in different Pb[(Mg1/3Nb2/3)0.9Ti0.1]O3 ceramics exhibiting a spontaneous relaxor to ferroelectric transition. Both the relaxor behavior and the phase transition are found to depend strongly on the lead vacancies content. An explanation based on an increase of the ordered domains and lesser interactions between polar clusters accounts for the experimental observations.
Tunneling transmission in two quantum wires coupled by a magnetically defined barrier
Authors: Sheng Wei-Dong , Gu Ben-Yuan , Wang Jian , Xia Jian-Bai
Published: July 2002

A numerical analysis of an electron waveguide coupler based on two quantum wires coupled by a magnetically defined barrier is presented with the use of the scattering-matrix method. For different geometry parameters and magnetic fields, tunneling transmission spectrum is obtained as a function of the electron energy. Different from that of conventional electron waveguide couplers, the transmission spectrum of the magnetically coupled quantum wires does not have the symmetry with regard to those geometrically symmetrical ports. It was found that the magnetic field in the coupling region drastically enhances the coupling between the two quantum wires for one specific input port while it weakens the coupling for the other input port. The results can be well understood by the formation of the edge states in the magnetically defined barrier region. Thus, whether these edge states couple or decouple to the electronic propagation modes in the two quantum wires, strongly depend on the relative moving directions of electrons in the propagating mode in the input port and the edge states in the magnetic region. This leads to a big difference in transmission coefficients between two quantum wires when injecting electrons via different input ports. Two important coupler specifications, the directivity and uniformity, are calculated which show that the system we considered behaves as a good quantum directional coupler.
Temperature-dependence studies of photorefractive effect in a low glass-transition-temperature polymer composite
Authors: Swedek Bogdan , Cheng Ning , Cui Yiping , Zieba Jaroslaw , Winiarz Jeffrey , Prasad Paras N.
Published: July 2002

The temperature dependence of the photorefractive effect in a polymer composite containing poly(9-vinycarbazole), tricresyl phosphate, buckminsterfullerene, and 4-(N,N- diethylamino)-β-nitrostyrene is presented. The photoconductive, electro-optic and photorefractive properties of the material have been studied in the temperature range of 22–61 °C. An apparent increase of electro-optic modulation with temperature and its eventual saturation is observed. This behavior is attributed to the temperature activated orientational mobility of the second-order nonlinear chromophores. The polarization anisotropy between the p- and s-polarized readouts is consistent with what would be expected on the basis of directly measured effective electro-optic coefficients. By correlating the electro-optic value with the diffraction efficiency, the temperature dependence of the space-charge field is obtained and explained by temperature dependencies of the dark conductivity and the photoconductivity of the material.
Comparison of a consistent theory of radio frequency sheaths with step models
Authors: Gierling J. , Riemann K.-U.
Published: July 2002

In order to find analytically tractable models for the electrode sheath of a capacitively coupled rf discharge, step models are used in the literature, describing the electron density by a step function oscillating with the radio frequency ω (ωpi≪ω≪ωpe). Comparison of the results of these step models with the results of numerical calculations shows similar current–voltage characteristics for symmetric discharges. On the other hand, distinct deviations in the spatial behavior of field and potential for a single sheath are observed. We discuss the deviations of the step models and show that they arise from systematic errors in the boundary conditions which are due to misinterpretations of the asymptotic two scale formalism. On the basis of a systematic two scale theory, we develop an improved step model with correct boundary conditions.
Effects of high vacancy concentrations on the magnetic properties of La1−xMn1−yO3 (0.02⩽x, y⩽0.13)
Authors: de Silva P. S. I. P. N. , Richards F. M. , Cohen L. F. , Alonso J. A. , Martínez-Lope M. J. , Casais M. T. , Thomas K. A. , MacManus-Driscoll J. L.
Published: July 2002

It has been found that the synthesis conditions and subsequent annealing treatments can significantly change the magnetic and transport behavior of the colossal magnetoresistive materials of the general formula La1−zAzMnO3, where A is a divalent ion. In order to clarify the role of vacancies within this structure, resistivity and magnetization measurements have been carried out on a series of samples derived from the parent compound LaMnO3, with La and Mn vacancies introduced by systematically varying the oxygen annealing conditions. Previous studies have shown that for a given carrier concentration, the Curie temperature of the paramagnetic to ferromagnetic transition (Tc) increases as the tolerance factor of the perovskite structure increases and then begins to decrease slowly for even higher tolerance factors. Generally, Tc also increases with the Mn4+ content, consistent with a double exchange mechanism responsible for ferromagnetism. In this study Tc was found to decrease as the vacancy concentration (and therefore the Mn4+ concentration) and tolerance factor both increase. The magnetic and transport data are discussed in the light of structural information obtained from neutron diffraction studies. In the present study it is found that La and Mn vacancies play a significant role in determining the physical properties of these materials: for high vacancy concentrations (x,y⩾0.080) the magnetic properties are analogous to those of spin glasses associated with disorder and frustration. Semiconductorlike resistivity was observed for all samples at all temperatures.
Influence of diffraction on low thermal diffusivity measurements with infrared photothermal microscopy
Authors: Bisson J. F. , Fournier D.
Published: July 2002

We consider a photothermal microscope using infrared detection for local thermal diffusivity measurements. In order to extract a reliable thermal diffusivity estimate from the recorded temperature profile, we show that diffraction arising from the Cassegrain objective of the microscope must be taken into account. Indeed, the apparent temperature profile is deformed significantly when the thermal diffusion length approaches the size of the diffraction pattern. This is especially a problem for low diffusivity samples because avoiding high-noise figures at very low modulation frequencies implies small thermal diffusion length measurements. The measurement of the impulse response of the microscope allows us to correct the recorded profile for diffraction and detector size effects. In particular, whereas the amplitude profile is smoothed only in the excitation beam area, the phase profile is globally distorted, yielding up to a 60% thermal diffusivity overestimate if neglected. Furthermore, nonlinear infrared emission contributes to enhance the impact of diffraction. Validation of our model is done with a Y2O3-doped ZrO2 sample by comparing the estimation at different modulation frequencies and also with the well-established photothermal beam deflection technique.
A study of electron energy distributions in an inductively coupled plasma by laser Thomson scattering
Authors: Hori T. , Kogano M. , Bowden M. D. , Uchino K. , Muraoka K.
Published: July 2002

The electron energy distribution function (eedf) in an inductively coupled plasma was studied using the method of laser Thomson scattering. eedfs were measured for various plasma conditions, at different gas pressures, and at different input rf powers. In high electron density plasmas, the eedf was observed to be Maxwellian, while in low electron density plasmas, a non-Maxwellian eedf was observed. The transition between Maxwellian and non-Maxwellian eedfs was attributed to the thermalization of the electron population in higher density plasmas. In order to completely characterize the eedf, spatial and temporal dependencies were measured and measurements were also made in the afterglow period of a pulsed discharge. Discussions are made of the electron behavior and the shape of the eedf.
Coulomb blockade-based nanothermometry in strong magnetic fields
Authors: Pekola J. P. , Toppari J. J. , Kauppinen J. P. , Kinnunen K. M. , Manninen A. J. , Jansen A. G. M.
Published: July 2002

We have performed experiments to test for the susceptibility to strong magnetic fields of electron tunneling in normal metal -based nanostructures for Coulomb blockade primary thermometry. We have confirmed that, to within our accuracy of about ±1%, the single electron charging -induced zero bias differential resistance maximum is unaffected by the field up to 23 T at temperatures of 0.4–4.2 K. We discuss the simple theoretical basis of this immunity. We also report on the practical limitation at low temperatures imposed by superconductivity of aluminium in small magnetic fields.
Hall effect and magnetic properties of III–V based (Ga1−xMnx)As/AlAs magnetic semiconductor superlattices
Authors: Hayashi T. , Tanaka M. , Seto K. , Nishinaga T. , Shimada H. , Ando K.
Published: July 2002

We present magnetotransport properties, with emphasis on Hall effect, of a new class of III–V based magnetic (GaMnAs)/nonmagnetic (AlAs) semiconductor superlattices (SLs) grown by low-temperature molecular beam epitaxy. The SLs having relatively wide (GaMn)As layers (⩾70 Å) are ferromagnetic at low temperatures, and their hole concentrations and Curie temperatures are estimated through the analysis of Hall measurements. The dependence of the magnetic and transport properties on the GaMnAs well width is discussed.
Magnetic, dielectric, and transport properties of La1−xSrxMnO3 at submillimeter wavelengths
Authors: Ivanov V. Yu. , Travkin V. D. , Mukhin A. A. , Lebedev S. P. , Volkov A. A. , Pimenov A. , Loidl A. , Balbashov A. M. , Mozhaev A. V.
Published: July 2002

Antiferromagnetic resonance (AFMR), dynamic conductivity, and dielectric permittivity have been studied in the La1−xSrxMnO3 perovskites at a frequency range of ν=3–33 cm−1 by the use of a quasi-optical backward-wave-oscillator technique. Two AFMR modes have been observed in LaMnO3 and La0.95Sr0.05MnO3. Temperature dependencies of their resonance frequencies, linewidths, and mode contributions to a static magnetic permeability have been obtained and corresponding parameters of magnetic interactions have been extracted. A jump in the dielectric permittivity ε′(T) was found at T=100–120 K in La0.9Sr0.1MnO3, which indicates the existence of an additional phase transition (probably, a polaron ordering) besides the ferromagnetic transition at 170 K. A sharp increase of the dynamic conductivity σ′(ν,T) was observed near Curie temperature (285 K) at a metal–semiconductor transition in La0.825Sr0.175MnO3, which was slightly smaller than its static value.