bluuebunny/crossref_metadata_2025 huggingface dataset:
Textured silicon calorimetric light detector
Authors: Di Stefano P. C. F. , Frank T. , Angloher G. , Bruckmayer M. , Cozzini C. , Hauff D. , Pröbst F. , Rutzinger S. , Seidel W. , Stodolsky L. , Schmidt J.
Published: November 2003

We apply the standard photovoltaic technique of texturing to reduce the reflectivity of silicon cryogenic calorimetric light detectors. In the case of photons with random incidence angles, absorption is compatible with the increase in surface area. For the geometrically thin detectors studied, energy resolution from athermal phonons, dominated by position dependence, is proportional to the surface-to-volume ratio. With the CaWO4 scintillating crystal used as light source, the time constants of the calorimeter should be adapted to the relatively slow light-emission times.
Impact of virtual substrate growth on high performance strained Si/SiGe double quantum well metal-oxide-semiconductor field-effect transistors
Authors: Olsen S. H. , O’Neill A. G. , Chattopadhyay S. , Kwa K. S. K. , Driscoll L. S. , Zhang J. , Robbins D. J. , Higgs V.
Published: November 2003

Strained Si/SiGe n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated using a dual quantum well structure. The heterostructure is designed for maximum performance from both n- and p-channel devices using a single Si0.85Ge0.15 virtual substrate. An optimized thermal budget has been used for device fabrication, which was possible due to the strain-compensated layer structure providing increased material robustness to strain relaxation. Epitaxial growth has been carried out by low-pressure chemical vapor deposition (LPCVD) at two different temperatures. Strained Si MOSFETs fabricated on virtual substrates grown at high temperature exhibited drain current enhancements three times as large as those demonstrated by strained Si MOSFETs fabricated on material grown at low temperature, compared with control Si devices. Detailed material analysis suggests that the higher degree of surface roughness and higher defect density of the low temperature LPCVD material limits the performance enhancements achievable due to the increased carrier scattering in these devices. Nevertheless, the results demonstrate that by incorporating strain-compensated Si/SiGe layers into conventional Si MOSFETs, even degraded SiGe material can offer performance advantages over bulk Si devices. However, high temperature material growth is required to achieve both maximum performance gains and the same degree of uniformity as that achieved from bulk Si technologies.
X-ray Bragg diffraction from langasite crystal modulated by surface acoustic wave
Authors: Roshchupkin D. V. , Irzhak D. V. , Tucoulou R. , Buzanov O. A.
Published: November 2003

X-ray diffraction on the X cut of a langasite crystal (La3Ga5SiO14) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) has been studied at the ESRF synchrotron radiation source. Due to the sinusoidal modulation of the crystal lattice involved by the SAW diffraction satellites appear on the rocking curve, with their number, angular positions, and intensities depending on the amplitude and wavelength of the ultrasonic superlattice. Full extinction of a specific satellite could be performed by adjusting the acoustic amplitude. It is shown that x-ray diffraction can be used to study surface acoustic wave field distributions in crystals.
High-precision thermal conductivity measurements as a probe of polymer/nanoparticle interfaces
Authors: Putnam Shawn A. , Cahill David G. , Ash Benjamin J. , Schadler Linda S.
Published: November 2003

We use the 3ω method to study the thermal conductivity of composites of nanoscale alumina particles in polymethylmethacrylate (PMMA) matrices. Effective medium theory and data for the changes in conductivity produced by low volume fractions of particle fillers are used to estimate the thermal conductance G of PMMA/alumina interfaces in the temperature range of 40<T<280 K. Near room temperature, G≈30±10 MW m−2 K−1 and the critical particle radius, r0=Λ0/G, is extremely small, r0≈7.5±2.5 nm. (Λ0=0.205 W m−1 K−1 is the conductivity of the PMMA matrix.) Therefore, high volume fractions of typical ceramic nanoparticles with r≫r0 can be used in thermal interface materials such as adhesives, pastes, and pads.
On the difference in valence electron plasmon energy and density of states between beta- and cubic-Si3N4
Authors: Egdell R. G. , Henrich V. E. , Bowdler R. , Sekine T.
Published: November 2003

Core and valence level photoemission spectra of β- and cubic-Si3N4 have been measured under monochromatic Al Kα excitation. Strong satellites to high binding energy of the core lines are shown to arise from excitation of valence electron plasmons during the photoemission process. An increase in the plasmon energy from 23.45 eV for β-Si3N4 to 26.10 eV for cubic-Si3N4 is of the magnitude expected from the 26% increase in the valence electron density associated with the shock-induced β-to-cubic phase transition. The measured valence band density of states for cubic-Si3N4 is in agreement with theoretical calculations.
Thickness dependence of magnetization reversal in a soft cylindrical particle
Authors: Grebenshchikov Yu. B. , Usov N. A. , Pestchanyi K. S.
Published: November 2003

The hysteresis loops of a soft cylindrical particle with radius R=40–100 nm have been studied by means of numerical simulation in the range of thickness Lz=10–40 nm, close to the exchange length R0. Qualitatively, three different scenarios of magnetization reversal have been obtained depending on the particle thickness and aspect ratio. For a “thick” particle (Lz≫R0) with aspect ratio Lz/R∼1, the magnetization reversal begins via the nucleation of magnetization curling similar to the case of spherical or ellipsoidal particles of a soft magnetic type. On the contrary, for a particle with a small aspect ratio (Lz/R≪1) the magnetization reversal begins via the nucleation of a bending state, whose magnetization is constrained within the particle plane. For a particle with thickness Lz>R0, the bending state can lose its stability at certain value of applied magnetic field, at which one or two vortices nucleate within the particle. The evolution of the vortex pattern in an applied magnetic field determines the shape of the hysteresis loop. On the other hand, for a thin particle (Lz<R0), the bending state tends to rotate as a whole with a decrease in the applied magnetic field. The total particle magnetization is close to the saturation magnetization, the hysteresis loop being practically rectangular. Thus, the behavior of the particle resembles that of a single-domain one, although the actual particle size exceeds the effective single-domain radius considerably.
Strong-contrast expansions and approximations for the effective conductivity of isotropic multiphase composites
Authors: Pham D. C. , Torquato S.
Published: November 2003

We extend the previous approach of one of the authors on exact strong-contrast expansions for the effective conductivity σe of d-dimensional two-phase composites to case of macroscopically isotropic composites consisting of N phases. The series consists of a principal reference part and a fluctuation part (a perturbation about a homogeneous reference or comparison material), which contains multipoint correlation functions that characterize the microstructure of the composite. The fluctuation term may be estimated exactly or approximately in particular cases. We show that appropriate choices of the reference phase conductivity, such that the fluctuation term vanishes, results in simple expressions for σe that coincide with the well-known effective-medium and Maxwell approximations for two-phase composites. We propose a simple three-point approximation for the fluctuation part, which agrees well with a number of analytical and numerical results, even when the contrast between the phases is infinite near percolation thresholds. Analytical expressions for the relevant three-point microstructural parameters for certain mixed coated and multicoated spheres assemblages (extensions of the Hashin–Shtrikman coated-spheres assemblage) are given. It is shown that the effective conductivity of the multicoated spheres model can be determined exactly.
Enhancement of the recombination rate of Br atoms by CF4 addition and resist etching in HBr/Cl2/O2 plasmas
Authors: Cunge G. , Joubert O. , Sadeghi N.
Published: November 2003

Mass spectrometry has been used to study the influence of the plasma chamber walls on the recombination rates of halogen atoms in a high density HBr/Cl2/O2/CF4 plasma used for silicon gate etching. In these plasmas, CF4 addition results in a transition from SiOx to CFx layer deposition on the chamber walls. We show that this is accompanied by an important enhancement of the surface recombination rate of halogen atoms (specially Br), whose gas phase concentration consequently drops. The same phenomenon is observed when photoresist is etched in an HBr/Cl2/O2 plasma resulting in carbon coating of the chamber walls. This demonstrates that the recombination coefficient of Br atoms on organic polymer surfaces is much larger than on SiO2-like surfaces, and that significant consequences on gate etch processes are expected.
Spinodal decomposition range of AlxGa1−xNyAs1−y alloys
Authors: Elyukhin V. A. , Sorokina L. P. , Rodriguez de Santiago M.
Published: November 2003

We have described a spinodal decomposition range of the low N-content AlxGa1−xNyAs1−y alloys grown on GaAs(001). In our analysis we take into account transformations of the bonds and strain, coherency strain, and elastic energies. The alloys are considered in the simple solution approximation. The strain energy is presented by the interaction parameters between the compounds estimated in the valence-force field model. The spinodal decomposition range of AlxGa1−xNyAs1−y alloys with compositions 0⩽x⩽0.4, y=0.01, 0.02, 0.03, 0.04, and 0.05 are demonstrated up to 1000 °C. It is shown that Al and N increase dramatically the temperature of the phase separation.
Investigations of chemical and electronic inhomogeneities in BaPb1−xBixO3 via highly spatially resolved electron energy loss spectroscopy
Authors: Gutiérrez-Sosa A. , Bangert U. , Flavell W. R.
Published: November 2003

BaPb 1−x Bi x O 3 crystals, doped to various nominal x, were investigated in a scanning transmission electron microscope equipped with energy dispersive x-ray and electron energy loss spectroscopy facilities. Large compositional variations on the nm scale were found in single crystalline regions of any nominal x value. These were accompanied by changes in the core-loss and low-loss energy loss spectra. The low-loss energy loss spectra, furthermore, demonstrate the evolution of electronic structure with doping (i.e., transitions from semiconductor to semimetal) and fluctuations with variations in the local composition.
Transport properties of ultrathin SrTiO3 barriers for high-temperature superconductor electronics applications
Authors: Morán O. , Hott R. , Schneider R. , Halbritter J.
Published: November 2003

Current transport through ultrathin SrTiO3 (STO) barriers has been studied systematically with respect to its dependence on barrier thickness d=2–30 nm, temperature T, and voltage V in state-of-the-art planar YBa2Cu3O7−x/SrTiO3/Au (YBCO/STO/Au) heterojunctions with c-axis oriented YBCO layer. We identified different transport regimes: Elastic tunneling was observed for samples with a nominal barrier thickness of 2 nm, which represents our experimental minimum for obtaining insulating transport characteristics. Already for slightly thicker STO barriers, resonant tunneling and hopping via a small number of localized states begins to dominate the transport behavior. For d>20 nm, a crossover to variable range hopping behavior is observed in the high-bias voltage regime as well as in the high temperature regime. A localization length of ∼0.46 nm indicating the spread of the localized states can be derived from these experiments. This value is close to the STO lattice constant and corresponds to a high density of localized states of nL∼6×1019(eV)−1 cm−3. In a free electron tunneling model, this corresponds to an average tunnel barrier height of ∼0.4 eV.
Investigation of the importance of interface and bulk limited transport mechanisms on the leakage current of high dielectric constant thin film capacitors
Authors: Baniecki J. D. , Shioga T. , Kurihara K. , Kamehara N.
Published: November 2003

The importance of interface and bulk transport mechanisms on the leakage current of high dielectric constant thin film capacitors is examined by deriving an equation for the J–VA characteristic of a capacitor that includes the transport mechanisms of thermionic emission (TE), thermionic field emission (TFE), and carrier drift–diffusion (DD). The current is controlled by the slowest of three effective velocity parameters v1md, vD, and ṽ2dm characterizing electron injection into the dielectric at the cathode by TE and TFE, carrier DD in the film bulk, and electron ejection from the dielectric at the anode by TE and TFE, respectively. The effective velocity parameters are evaluated for a Pt/BST/Pt thin film capacitor that has been exposed to forming gas and it is shown that the dominant transport mechanism is interface limited TFE from the cathode with negligible influence of carrier transport by DD in the film bulk. Implications of these results on existing transport calculations for high dielectric constant thin film capacitors are discussed.
Electromechanical coupling in free-standing AlGaN/GaN planar structures
Authors: Jogai B. , Albrecht J. D. , Pan E.
Published: November 2003

The strain and electric fields present in free-standing AlGaN/GaN slabs are examined theoretically within the framework of fully coupled continuum elastic and dielectric models. Simultaneous solutions for the electric field and strain components are obtained by minimizing the electric enthalpy. We apply constraints appropriate to pseudomorphic semiconductor epitaxial layers and obtain closed-form analytic expressions that take into account the wurtzite crystal anisotropy. It is shown that in the absence of free charges, the calculated strain and electric fields are substantially different from those obtained using the standard model without electromechanical coupling. It is also shown, however, that when a two-dimensional electron gas is present at the AlGaN/GaN interface, a condition that is the basis for heterojunction field-effect transistors, the electromechanical coupling is screened and the decoupled model is once again a good approximation. Specific cases of these calculations corresponding to transistor and superlattice structures are discussed.
Production and thermal decay of radiation-induced point defects in KD2PO4 crystals
Authors: Chirila M. M. , Garces N. Y. , Halliburton L. E. , Demos S. G. , Land T. A. , Radousky H. B.
Published: November 2003

Optical absorption and electron paramagnetic resonance (EPR) techniques have been used to characterize the production and thermal decay of point defects in undoped single crystals of KD2PO4 grown at Lawrence Livermore National Laboratory. A crystal was irradiated at 77 K with x rays, and then warmed to room temperature. Immediately after irradiation, broad optical absorption bands were observed to peak near 230, 390, and 550 nm. During warming, these absorption bands thermally decayed in the 80–140 K range. Another absorption band peaking near 450 nm appeared as the three bands disappeared. This last band decayed between 140 and 240 K. Correlations with EPR data suggest that the band at 230 nm is associated with interstitial deuterium atoms, the two bands at 390 and 550 nm are associated with self-trapped holes, and the band at 450 nm is associated with holes trapped adjacent to deuterium vacancies. Additional EPR spectra from several oxygen-vacancy centers and a silicon-associated hole center were observed as the crystal was warmed. All the electron and hole traps reported in this study are expected to participate in the room-temperature response of KD2PO4 crystals to pulsed high-power ultraviolet laser beams.
Magnetic properties of asymmetric antirectangular Ni80Fe20 arrays
Authors: Wang C. C. , Adeyeye A. O. , Wu Y. H.
Published: November 2003

The magnetic properties of antirectangular arrays (2×10 μm2) embedded into 800 Å thick continuous Ni80Fe20 films with interhole spacing(s) in the range of 2–6 μm have been investigated. We observed that the presence of the rectangular holes strongly modifies the shape of the magnetic hysteresis loop compared with continuous film of the same thickness. Specifically, we observed that the coercivity increases dramatically with the reduction of interhole spacing due to magnetic domain wall pinning, in agreement with our magnetic force microscopy images. Complex magnetotransport curves were obtained, which is not readily explained using the conventional anisotropic magnetoresistance effect. We attribute our results to the contribution by magnetic domains to overall resistivity of the antirectangular structures. From field angle dependent magnetotransport measurements, we are able to deduce the pinning strength for different interhole spacing.
Cathode dependence of the open-circuit voltage of polymer:fullerene bulk heterojunction solar cells
Authors: Mihailetchi V. D. , Blom P. W. M. , Hummelen J. C. , Rispens M. T.
Published: November 2003

The open-circuit voltage (VOC) of bulk-heterojunction solar cells based on [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor and poly[2-methoxy-5(3′,7′-dimethyloctyloxy)-p-phenylene vinylene] (OC1C10-PPV) as an electron donor has been investigated. In contrast to the present understanding, it is now demonstrated that for non-ohmic contacts the experimental VOC is determined by the work function difference of the electrodes. A total variation of more than 0.5 V of the VOC was observed by variation of the negative electrode (cathode) work function. For ohmic contacts the VOC is governed by the LUMO and HOMO levels of the acceptor and donor, respectively, which pin the Fermi levels of the cathode and anode. The band bending created by accumulated charges at an ohmic contact produce a considerable loss in VOC of 0.2 V at room temperature. The experimentally observed voltage loss in VOC of 0.38 V due to the presence of ohmic contacts at both interfaces strongly limits the maximum open-circuit voltage of OC1C10-PPV/PCBM solar cells.
Strain-induced modifications of the electronic states of InGaAs quantum dots embedded in bowed airbridge structures
Authors: Nakaoka T. , Kakitsuka T. , Saito T. , Kako S. , Ishida S. , Nishioka M. , Yoshikuni Y. , Arakawa Y.
Published: November 2003

We have fabricated bowed airbridges in which self-assembled InGaAs quantum dots are embedded. Strong strain distribution induced in the bowed airbridge and the effect on the electronic states of the quantum dots are investigated through the measurement of the photoluminescence from the individual dots and the theoretical calculation. A finite element calculation shows the strain in the bowed airbridge to distribute from tensile to compressive along the growth direction. The strain effect on the electronic states of the dots is probed through the photoluminescence peak shift following the deformation of the GaAs matrix of the dots from a wall-shaped structure to the bowed airbridge. The magnitude of the peak shift varies systematically with the position of the quantum dot along the growth direction, clearly reflecting the strain distribution in the bridge. The energy level shift following the deformation is calculated by solving the three-dimensional Schrödinger equation taking into account the strain distribution around the dots embedded in the bridge. The calculation, which agrees well with the experiment, demonstrates that the characteristic strain distribution around the dot embedded in the bowed airbridge modifies not only the energy levels, but also the wave functions. The electron and hole wave functions are modified differently, mainly due to the opposite contribution of the biaxial strain to the hydrostatic ones.
Super hardening and deformability in epitaxially grown W/NbN nanolayers under shallow and deep nanoindentations
Authors: Ennis B. M. , Madan A. , Slaughter W. S. , Barnett S. A. , Mao S. X.
Published: November 2003

Epitaxially grown W/NbN nanolayers are superlattice materials that exhibit a hardness at small bilayer repeat periods which exceed the hardness predicted by the rule of mixtures for normal composites. Interfaces in the bilayered superlattice play a critical role in the superhardening process. The objective of this investigation was to examine the behavior of the superlattice material, W/NbN. Nanoindentations and in situ surface imaging were conducted over a range of applied loads on samples of W/NbN with two different bilayer periods (Λ=5.6 and 10.4 nm), and monolithic samples of the niobium nitride (NbN) ceramic and the tungsten (W) metal which comprise the superlattice material. Shallow nanoindentations were made to a depth equal to the individual layer thicknesses of the epitaxially grown W/NbN nanolayers in order to investigate the individual interface effect. The mechanical properties were determined using the Oliver and Pharr method and compared for all the samples. The energies of indentation are calculated. The characteristics of the material pileup resulting from the nanoindentations are determined from the scanned surface images. An increase in hardness is observed in the superlattice materials at deeper indentation depths. The results indicate that this increase in hardness is related to the nature of the interface between the layers in the superlattice materials.
Effect of nanoscale surface roughness on the bonding energy of direct-bonded silicon wafers
Authors: Miki N. , Spearing S. M.
Published: November 2003

Direct wafer bonding of silicon wafers is a promising technology for manufacturing three-dimensional complex microelectromechanical systems as well as silicon-on-insulator substrates. Previous work has reported that the bond quality declines with increasing surface roughness, however, this relationship has not been quantified. This article explicitly correlates the bond quality, which is quantified by the apparent bonding energy, and the surface morphology via the bearing ratio, which describes the area of surface lying above a given depth. The apparent bonding energy is considered to be proportional to the real area of contact. The effective area of contact is defined as the area sufficiently close to contribute to the attractive force between the two bonding wafers. Experiments were conducted with silicon wafers whose surfaces were roughened by a buffered oxide etch solution (BOE, HF:NH4F=1:7) and/or a potassium hydroxide solution. The surface roughness was measured by atomic force microscopy. The wafers were direct bonded to polished “monitor” wafers following a standard RCA cleaning and the resulting bonding energy was measured by the crack-opening method. The experimental results revealed a clear correlation between the bonding energy and the bearing ratio. A bearing depth of ∼1.4 nm was found to be appropriate for the characterization of direct-bonded silicon at room temperature, which is consistent with the thickness of the water layer at the interface responsible for the hydrogen bonds that link the mating wafers.
Modeling of indium diffusion and end-of-range defects in silicon using a kinetic Monte Carlo simulation
Authors: Noda Taiji
Published: November 2003

We describe in this article atomistic modeling of transient enhanced diffusion of indium and end-of-range defects in silicon using a kinetic Monte Carlo (KMC) simulation technique. All types of defects, including small point defect clusters, {311} defects, dislocation loops, and voids are taken into account during the random walk of indium in silicon. Neutral point defect-mediated indium migration, which includes both indium-interstitial and indium-vacancy pairs, is implemented in the KMC diffusion simulator. Indium diffusion simulation after a subamorphous implant dose (In 200 keV, 1×1013/cm2) could be fully explained by indium-interstitial pair migration with reasonable activation energy and the Frank–Turnbull mechanism was not dominant. In the case of an amorphous implant dose (In 200 keV, 1×1014/cm2), the KMC simulation shows small {311} defects at the initial stage of annealing that nucleate into dislocation loops. Atomistic KMC simulation also confirms that it is primarily interstitial clusters, {311} defects, and loops that play the most important roles in indium diffusion above the amorphous implant dose.
Magnetostrictive bending of cantilever beams and plates
Authors: Guerrero Victor H. , Wetherhold Robert C.
Published: November 2003

The models currently used to determine strains, stresses, and deflections in beams and plates made of magnetostrictive films deposited on nonmagnetic substrates are based on the assumption that the film is relatively thin compared to the substrate. Despite the lack of self-consistency and the limitations of these models, they can be used to calculate approximate strains and deflections when the ratio of the thickness of the film to the thickness of the substrate is smaller than about 0.001; even then they do not consistently predict stresses or energies. Unfortunately, the large deflections required in modern applications are only achievable with films that do not satisfy this assumption of relative film thinness, and the results obtained with the traditional models show large errors. In these circumstances it is necessary to introduce robust methods that can be applied regardless of the relative magnitude of the thickness of the film. In this article, one such method is presented. The method represents a self-consistent approach based on the minimization of the total internal energy of a film-substrate system, performed based on the assumption that the magnetostrictive strains can be modeled as anisotropic expansional strains. The expressions obtained using this approach can be used to calculate strains, stresses, deflections, and energies in an accurate way. The method is suitable for generalization to the cases in which the deformation of beams and plates is produced not only due to magnetostriction in the films, but may also include thermal, piezoelectric, or hygroscopic strains.
p-n junctions in (In,Se)/Cu(In,Ga)(Se,S)2 photovoltaic systems
Authors: Massé G. , Djessas K.
Published: November 2003

The formation of the p-n junction in (In,Se)/Cu(In,Ga)(Se,S)2/SnO2 structures is studied. The space charge region can be shifted from the (In,Se)/CIGSeS interface to the CIGSeS/SnO2 interface by annealing at different temperatures. In this structure, SnO2 is a simple electrode. When the p-n junction is near SnO2, photovoltages up to 650 mV can be reached when lighting through SnO2. The formation of this p-n junction, in other words the formation of an n-type absorber layer, is related to copper and indium diffusions, resulting in an increase of VCu and InCu defect concentrations. We believe n-type conductivity of the junction is due to a high concentration of (2VCu+InCu) complex defects, which exists in the absorber layer driving the diffusion processes.
Femtosecond laser-induced nanofabrication in the near-field of atomic force microscope tip
Authors: Kirsanov A. , Kiselev A. , Stepanov A. , Polushkin N.
Published: November 2003

The formation of nanocraters on the surface of metallic films under the tip of an atomic force microscope by femtosecond laser pulses is demonstrated. The influence of laser polarization, pulse duration, and tip–sample distance on threshold fluence for nanoprocessing is investigated. Analysis of experiments shows that heating of the tip by laser radiation and its lengthening is the predominant mechanism for laser-induced nanoprocessing.
Submicron scale hybrid structures of diluted magnetic semiconductor quantum wells with ferromagnetic Co wires
Authors: Sakuma M. , Hyomi K. , Souma I. , Murayama A. , Oka Y.
Published: November 2003

Submicron scale hybrid structures of diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co wires have been developed for the purpose of applying microscopic magnetic fields to the DMS quantum structures. The Zn1−x−yCdxMnySe QW was made into wires with the width down to 0.1 μm and each wire was sandwiched between the Co wires. The magneto-optical properties of the hybrid structures were studied by spin–flip light scattering of paramagnetic Mn ions in the DMS QW in addition to ferromagnetic surface spin-wave scattering of the Co wires, where the spin–flip energy is highly sensitive to the magnetic field HCo flux applied from the Co wires. The application of uniform magnetic fields HCo flux⩾0.25 T from the Co wires to the DMS QW has been attained with the Co-wire spacing of 0.2 μm. In addition, the distribution of HCo flux in the DMS QW is quantitatively evaluated from the spectral shape of the spin–flip scattering. Moreover, the application of HCo flux to the DMS QW is switched on and off by weak external fields less than 0.02 T depending on the magnetic shape anisotropy of the collective Co wires, which can realize the switching behavior of the spin alignment of magnetic ions in the DMS QW.
Orientational dynamics in dye-doped organic electro-optic materials
Authors: Apitz D. , Svanberg C. , Jespersen K. G. , Pedersen T. G. , Johansen P. M.
Published: November 2003

The time dependent birefringence of polymer-based electro-optic materials is investigated using ellipsometry. We show that the birefringence after switching off the poling field does not depend only on the induced refractive index, but also on how that level was reached. The role of the poling voltage and poling time is discussed in turn-on and turn-off experiments and an original curve-fit function is introduced. We also propose a schematic model of the polymer dynamics in the system, which is consistent with complementary dielectric measurements.
Polarized emission from self-organized low-molecular-weight liquid crystals with the aid of hole-injecting materials
Authors: Kawamoto Masuki , Mochizuki Hiroyuki , Ikeda Tomiki , Lee Bong , Shirota Yasuhiko
Published: November 2003

Electroluminescent (EL) properties of a low-molecular-weight liquid crystal (LC), which is composed of an oxadiazole moiety as an electron-transporting unit and an amine moiety as a hole-transporting unit, were investigated. The most significant characteristic of the LC is to possess bipolar carrier-transporting abilities. The bipolar LC makes it possible to fabricate a simple EL device. We fabricated an EL device (cell gap: 2 μm) with configuration of indium tin oxide (ITO)/LC/MgAg. The device emitted visible light at above 10 V. In particular, the values of luminance and current density of an EL device containing poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT) as a hole-injecting layer were ten and six times higher than those of the ITO/LC/MgAg device without the PEDOT layer, respectively. Furthermore, surfaces of the glass substrate with an ITO electrode and a PEDOT layer were rubbed to align mesogens. Dichroic ratio of the emission from an ITO/rubbed PEDOT/LC/MgAg device was estimated to be 2.8. These results indicate that the origin of polarized emission in the LC phase is the anisotropic arrangement of the mesogenic units.
Effect of electron flow direction on model parameters of electromigration-induced failure of copper interconnects
Authors: Padhi Deenesh , Dixit Girish
Published: November 2003

This investigation studies the effects of the direction of electron flow on the activation energy and current exponent of electromigration failure of copper interconnects using conductors terminated by vias at both ends. The activation energy of a downstream case (0.91 eV) was found to be similar to that of an upstream case (0.86 eV), suggesting that failure was primarily caused by diffusion along the Cu/SiNx interface for both cases. Mean time to fail with the upstream flow condition exceeded that with the downstream condition by a factor of ∼2 at 300 °C and 1(106) A/cm2. The current exponent (with a link current density in the range of 1×106 to ∼5×106 A/cm2) of a 0.22 μm via/link structure was determined to be 1.44 and 1.87 with upstream and downstream electron flow, respectively. These differences have been correlated to the locations of void nucleation and their physical size for the two electron flow conditions. Furthermore, increasing the via/link size resulted in a slight increase in the current exponent, consistent with the model proposed by Lloyd [J. R. Lloyd, J. Appl. Phys. 69, 7601 (1991).]
Physics of thermionic dispenser cathode aging
Authors: Longo R. T.
Published: November 2003

A dispenser cathode life model (DCLM) was originally published in 1984. More recent life test data have substantiated the basic physics used in the DCLM. However, re-evaluation of the model with this latest data alters the numerical parameters used in the model. The most important modification is the incorporation of the shape factor, α, in the emission equation, so that accurate descriptions of the cathode activity curves (i.e., current versus temperature) can be made as a function of cathode age. The original model was fit only to the cathode current as a function of time for a fixed operating temperature. This revision fits the cathode activity curves as a function of both time and cathode temperature. Variation in cathode current as a function of temperature is quite dependent upon the underlying physics, and gives a better measure of how the internal parameters, such as work function and knee position and knee shape change with time. Knowing these details provides a more accurate measure of how the cathode current at the operating point will change over time. The modification made to the emission equation in this revision incorporates the shape factor, α, which is a single number that describes the shape of the cathode activity curves. The shape factor is found to be dependent on time: The knee softens and rounds with age. Even though the shape factor was originally introduced as an empirical factor, I will present a theoretical model for the shape factor that provides some insight into its physical interpretation. This theory will show that it can be related to the thermodynamics of the emitter surface. The re-evaluation of the DCLM, based on the latest life data, and including the theory for the shape factor yields a longer life expectancy for the M-type dispenser cathode then was predicted by the original more conservative life model. The DCLM matches the observed life data more accurately.
Production of highly polarized 3He using spectrally narrowed diode laser array bars
Authors: Chann B. , Babcock E. , Anderson L. W. , Walker T. G. , Chen W. C. , Smith T. B. , Thompson A. K. , Gentile T. R.
Published: November 2003

We have produced 70%–75% 3He polarization by spin-exchange optical pumping in cells ≈100 cm3 in volume. The polarization achieved is consistent with known spin-exchange and spin-relaxation rates, but only when the recently discovered temperature dependence of 3He relaxation is included. Absolute 3He polarization measurements were performed using two different methods in two different laboratories. The results were obtained with either a spectrally narrowed laser or one type of broadband laser. Based on tests of several larger cells at pressures near 1 bar, we find that the power required to reach the same polarization is typically three times lower for the spectrally narrowed laser. This last result indicates that spectrally narrowed lasers will be important for obtaining the highest polarization in large volume neutron spin filters. Polarization in excess of 55% as obtained in cells up to 640 cm3 in volume and 70% polarization is anticipated with available increases in spectrally narrowed laser power.
Improved dispersion relations for GaAs and applications to nonlinear optics
Authors: Skauli T. , Kuo P. S. , Vodopyanov K. L. , Pinguet T. J. , Levi O. , Eyres L. A. , Harris J. S. , Fejer M. M. , Gerard B. , Becouarn L. , Lallier E.
Published: November 2003

The refractive index of GaAs has been measured in the wavelength range from 0.97 to 17 μm, which covers nearly the entire transmission range of the material. Linear and quadratic temperature coefficients of the refractive index have been fitted to data measured between room temperature and 95 °C. In the midinfrared, the refractive index and temperature dependence are obtained from analysis of etalon fringes measured by Fourier-transform spectroscopy in undoped GaAs wafers. In the near infrared, the refractive index is deduced from the quasiphasematching (QPM) wavelengths of second-harmonic generation in orientation-patterned GaAs crystals. Two alternative empirical expressions are fitted to the data to give the refractive index as a function of wavelength and temperature. These dispersion relations agree with observed QPM conditions for midinfrared difference-frequency generation and second-harmonic generation. Predictions for various nonlinear optical interactions are presented, including tuning curves for optical parametric oscillators and amplifiers. Also, accurate values are predicted for QPM conditions in which extremely large parametric gain bandwidths can be obtained.
Nondegenerate pump and probe spectroscopy in InGaN thin films
Authors: Nomura Masahiro , Arita Munetaka , Arakawa Yasuhiko , Ashihara Satoshi , Kako Satoshi , Nishioka Masao , Shimura Tsutomu , Kuroda Kazuo
Published: November 2003

Carrier dynamics in In0.10Ga0.90N/In0.03Ga0.97N multiple quantum wells and epilayers were studied by nondegenerate femtosecond pump and probe spectroscopy and by using monochromatic continuous wave light at room temperature. Structurally dependent transient absorption spectra were observed after pump pulse excitation. These spectral changes are caused by excitonic absorption quenching and screening of internal piezoelectric fields by photoinduced carriers. Both rapid (∼400 ps) and slow (∼90 μs) temporal behavior was observed in the transient absorption spectra. The results of time-resolved photoluminescence measurements and the existence of saturation pump fluences indicate that trapped carriers in potential minima originating from In-rich regions and/or midgap carrier traps are responsible for the lasting screening of the internal electric field.
Praseodymium-modified SrBi2Ta2O9 with improved polarization properties at low electric field
Authors: Noguchi Yuji , Kitamura Atsushi , Woo Lee-C. , Miyayama Masaru , Oikawa Kenichi , Kamiyama Takashi
Published: November 2003

Effects of praseodymium modification on the crystal structure and polarization properties were investigated for layered ferroelectric SrBi2Ta2O9 (SBT). Neutron powder diffraction and x-ray photoelectron analyses showed that praseodymium ions are substituted at the Sr site as Pr3+ and that Sr vacancies are created for compensating the charge difference between Sr2+ and Pr3+. Polarization measurements using dense ceramics indicated that low-field polarization properties were significantly improved by the praseodymium substitution. It is demonstrated that Pr–SBT has an advantage over SBT and Bi-modified SBT for a low-voltage operating ferroelectric-memory material.
High-temperature thermoelectric properties of single-crystal Ca3Co2O6
Authors: Mikami M. , Funahashi R. , Yoshimura M. , Mori Y. , Sasaki T.
Published: November 2003

We measured the high-temperature thermoelectric properties of single-crystal Ca3Co2O6, which consists of parallel one-dimensional Co2O66− chains separated by Ca2+ ions. The resistivity (ρ) along the c-axis decreases rapidly upon increasing temperature from 2×104 to 8 mΩ cm in a temperature region of 300 to 1073 K. The Seebeck coefficient (S) is positive and is 230 and 160 μV/K at 373 and 1073 K, respectively. The thermal conductivity (κ) is as low as 8 W/mK at room temperature and decreases linearly with the increase of temperature to 4.6 W/mK at 773 K. Thus, the estimated dimensionless figure-of-merit [ZT=(S2/ρκ)T (T: absolute temperature)], is about 0.15 at 1073 K. As the thermoelectric figure-of-merit (Z), shows a sharp rise in the measured temperature range, Ca3Co2O6 is expected to possess high thermoelectric efficiency at higher temperatures, and is considered to be a potential candidate for use as a thermoelectric material at high temperatures by virtue of being chemically stable up to 1300 K.
Role of carbon atoms in the remote plasma deposition of hydrogenated amorphous carbon
Authors: Benedikt J. , Wisse M. , Woen R. V. , Engeln R. , van de Sanden M. C. M.
Published: November 2003

The aim of this article is to determine the role of carbon atoms in the growth of hydrogenated amorphous carbon (a-C:H) films by means of an argon/acetylene expanding thermal plasma. Cavity ring down absorption spectroscopy is used to detect metastable carbon atoms by probing the 1s2 2s2 2p 3s 1P1←1s2 2s2 2p2 1S0 electronic transition. In addition to absorption measurements, the emission of the same transition is monitored by means of optical emission spectroscopy. These two measurements provide information about the local production of the C atoms and about their reactivity in the gas phase. It will be shown that under growth conditions in an Ar/C2H2 expanding thermal plasma, the metastable carbon density is also representative for the ground state carbon density. From obtained results it is concluded that the carbon atoms react rapidly with acetylene in the gas phase and therefore their contribution to the growth of hard diamond-like a-C:H films can be neglected. Only at low acetylene flows, the condition when soft polymer-like films are deposited, carbon atoms are detected close to the substrate and can possibly contribute to the film growth.
Picosecond photocarrier-lifetime in ErAs:InGaAs at 1.55 μm
Authors: Sukhotin M. , Brown E. R. , Driscoll D. , Hanson M. , Gossard A. C.
Published: November 2003

This letter reports the measurement of photocarrier lifetime in ErAs:InGaAs epitaxial material grown on InP substrates. The measurement technique is ultrafast time-resolved phototransmission using a 1.55 μm mode-locked erbium-doped fiber laser. A lifetime of 3.56 ps is found in a sample containing an InAlAs smoothing layer, compared to 0.96 ps in a sample without any InAlAs. The difference is explained using a model of ambipolar diffusion of photocarriers.
Electrical properties of single-wall carbon nanotube and epoxy composites
Authors: Kim Bumsuk , Lee Jongjin , Yu Insuk
Published: November 2003

The homogeneous composites of single-wall carbon nanotube (SWNT) and Stycast 1266 epoxy were made with the nanotube concentrations of 0.01–0.21 wt % by using a high frequency sonication method. Direct current conductivities of various mass fractions show typical percolation behavior and the percolation threshold pc is found to be 0.074 wt % and the conductivity exponent t is estimated as 1.3±0.2. The anomalously small pc is attributed to the large aspect ratio of the nanotubes. Dielectric measurement was performed at temperatures 220–300 K with the frequency range of 20 Hz–1 MHz. There appeared different dielectric aspects according to the mass fractions p≪pc, p∼pc, or p>pc. The dielectric relaxation of epoxy dominates at low concentrations (p≪pc), while at high concentrations (p>pc) we found a scaling property in the frequency dependence of loss tangent. The scaled loss tangent spectra show a Debye type relaxation. Near the percolation threshold (p∼pc), the composites show peaks in the frequency dependence of the imaginary part of their electric moduli. Log–log plots of ac conductivity versus frequency for the midrange (p∼pc) composites exhibit the scaling and universal behavior regardless of temperature and mass fraction of SWNT.
Efficient room temperature carrier trapping in quantum dots by tailoring the wetting layer
Authors: Colombo D. , Sanguinetti S. , Grilli E. , Guzzi M. , Martinelli Lucio , Gurioli M. , Frigeri P. , Trevisi G. , Franchi S.
Published: November 2003

The temperature dependence of carrier confinement in states of self-assembled In0.5Ga0.5As quantum dots (QDs) embedded in AlyGa1−yAs barriers has been investigated by means of photoluminescence (PL) measurements. We show that photoexcited carriers above the AlGaAs barriers have two recombination channels that contribute to the temperature quenching of the PL from QDs: (a) carrier losses in the AlGaAs layers during the relaxation process and (b) thermal evaporation of captured carriers out of QDs. The interplay between these two mechanisms determines the behavior of the nonresonantly excited photoluminescence as a function of temperature. Eliminating the first contribution by using resonant excitation of the QD PL, we demonstrate a definite enhancement of the carrier confinement at room temperature in InGaAs/AlGaAs QDs by increasing the Al content. We show that this effect is related to the increase in the energy separation between the electronic states in the QD and the wetting layer.
Influence of deposition conditions on mechanical properties of low-pressure chemical vapor deposited low-stress silicon nitride films
Authors: Toivola Yvete , Thurn Jeremy , Cook Robert F. , Cibuzar Greg , Roberts Kevin
Published: November 2003

The effect of deposition temperature, deposition pressure, or input gas ratio (SiH2Cl2:NH3) on film stress was determined for low-pressure chemical vapor deposited silicon nitride films. Wafer curvature measurements were performed for films deposited on single crystal silicon and amorphous silica wafer substrates to determine film stress σdep, biaxial modulus Ef+, and coefficient of thermal expansion αf. Apparent plane strain film modulus Ēf′ and hardness H were measured using depth-sensing indentation. Ellipsometry was used to measure film thickness tf and refractive index n. Infrared spectroscopy, x-ray photoelectron spectroscopy (XPS), forward recoil energy spectroscopy (FReS), and Rutherford backscattering spectroscopy (RBS) experiments were performed to determine film composition. Although film deposition stress varied from −135 MPa (compressive) to 235 MPa (tensile) Ef+, Ēf′, H, and αf remained nearly constant. Infrared spectroscopy resolved only Si-N species for all films, and results from FReS on three films confirmed that the hydrogen content was negligible. RBS and XPS indicated that Si/N increased with increased compressive σdep. Ellipsometry and RBS indicated that all films were silicon-rich, to a greater extent with increased compressive σdep. As RBS indicated that atomic density decreased with increased compressive deposition stress, it was concluded that the deposition conditions changed both thermal and intrinsic deposition stress for all films. In particular, intrinsic stress was tensile, and became increasingly tensile for increased Si/N and decreased atomic density. Assuming thermal stress was similar for all films examined here, the intrinsic stress must have varied from changes dependent on the deposition conditions.
Charge transfer and doping at organic/organic interfaces
Authors: Peisert H. , Knupfer M. , Zhang F. , Petr A. , Dunsch L. , Fink J.
Published: November 2003

We studied the electronic properties of technically relevant organic/organic interfaces using photoemission spectroscopy. Representatives of organic semiconductors from the family of the phthalocyanines were evaporated onto PEDOT:PSS [mixture of poly-3,4-ethylenedioxy-thiophene and polystyrenesulfonate] thin films, which are often applied as electrode material in organic semiconductor devices. Besides the formation of interface dipoles we observe energetic shifts of the electronic levels of the semiconductors, which are explained by a doping of the organic semiconductor near the interface due to a mixing of the two organic materials within the interface region.