bluuebunny/crossref_metadata_2025 huggingface dataset:
Particle motions and segregation in dielectrophoretic microfluidics
Authors: Markarian Nikolai , Yeksel Mike , Khusid Boris , Farmer Kenneth R. , Acrivos Andreas
Published: December 2003

We studied, experimentally and theoretically the motion and segregation of positively polarized particles flowing through dielectrophoretic microfluidics. The batch fabrication technique which we present enables one to construct an apparatus featuring a large number of mechanically robust and chemically inert microdevices having a very low ratio of dielectrophoretic-to-fluid residence time. Several apparatuses of this type can easily be integrated into a higher-level system able to handle flow rates several orders of magnitude larger than those in a single microdevice. Experiments were conducted on microfluidics with electrodes of different sizes arranged parallel and perpendicular to the flow. The simulations of the particle motions and their segregation required no fitting parameters because the suspension properties were measured independently. We demonstrate that the presence of the interparticle dipolar interactions imposes a lower bound on the size of the electrodes below which the miniaturization of microelectrodes adversely affects the ability to position the particles with precision in preselected locations [briefly reported by Markarian et al., Appl. Phys. Lett. 82, 4839 (2003)]. When the device characteristics meet certain requirements, the theoretical predictions for the variation of the amount of the segregated particles along the channel were found to be quantitatively consistent with experimental data. Combined with the earlier test of the predictions for the particle accumulation with time in millimeter-scale channels [J. Appl. Phys. 88, 5463 (2000); 92, 2829 (2002)], this completes the experimental validation of a theory for the kinetics of dielectrophoretic phenomena in a flowing dilute suspension. The results of our studies provide the basic characteristics of the ac-field-driven particle motions and segregation in a microchannel and validate a simulation procedure needed for the design and operation of dielectrophoretic microfluidics.
Quantized acoustic vibrations of single-wall carbon nanotube
Authors: Raichura Amit , Dutta Mitra , Stroscio Michael A.
Published: December 2003

Acoustic vibrational modes are derived for both zigzag and armchair nanotubes of finite length using the continuum theory and a variational solution of Donnell’s equation. Calculations of both even and odd modes of the displacement are performed. The dispersion relations are shown to vary with the length of the tube and the displacement field of the nanotube is used to calculate the deformation potential interaction Hamiltonian.
Microwave permeability of Y-type hexaferrites in zero field
Authors: Obol Mahmut , Vittoria Carmine
Published: December 2003

We have investigated the permeability at microwave frequencies of Y-type hexaferrites in zero-magnetic field. The permeability was measured from 0.045 to 10 GHz and it was correlated with multidomain and domain-wall resonances in zero-magnetic field. The basal plane anisotropy field, HφA, magnetic remanence, 4πM, domain-wall thickness δ, and length L of the domain wall were estimated from resonances and permeability data.
Influence of Schottky and Poole–Frenkel emission on the retention property of YMnO3-based metal/ferroelectric/insulator/semiconductor capacitors
Authors: Ito Daisuke , Fujimura Norifumi , Yoshimura Takeshi , Ito Taichiro
Published: December 2003

The relationship between the memory retention properties and the leakage current density of Pt/YMnO3/Y2O3/Si capacitors was discussed. The leakage current of the ferroelectric YMnO3 layer was varied by temperature and by annealing the capacitors. It was found that the retention time became shorter upon increasing the leakage current density at the retention voltage. The retention time was prolonged from about 103 to up to 104 s when the leakage current density was reduced to 2×10−9 A/cm2 by annealing under N2 ambient. The analysis of the leakage current revealed that the retention degradation mechanism was related to the Schottky emission. From the temperature dependence of the leakage current and the pseudo isothermal capacitance transient spectrum, it was found that applied voltage with an unnecessarily long time to polarize the ferroelectric layer generated Frenkel defects in the ferroelectric layer, and that the amount of the defects greatly affected the memory retention time. These results suggest that to improve the memory retention properties, reducing the defect density of the ferroelectric layer is important, as well as lowering the Schottky current.
Reactor design considerations in the hot filament/direct current plasma synthesis of carbon nanofibers
Authors: Cruden Brett A. , Cassell Alan M. , Ye Qi , Meyyappan M.
Published: December 2003

A combined hot filament/direct current (dc) plasma approach to chemical vapor deposition of carbon nanofibers (CNFs) using an acetylene/ammonia feedstock has been explored. As a part of the study, the impact of filament usage and substrate holder design has been examined by scanning electron microscopy imaging of deposition products and monitoring of downstream products by residual gas analysis (RGA). It is demonstrated that the filament wire is important primarily in the pretreatment of the substrate, improving CNF growth quality. However, the filament has a more minor impact when combined with the dc plasma, increasing growth rate but reducing growth quality. The substrate holder is modified by introducing a graphite spacer into the electrode. By varying the size of the spacer, the effective surface area of the cathode is modified, allowing control over the power input to the reactor while holding the voltage constant. This allows for some independent control of physicochemical processes that are typically inseparable in plasma processing, including gas phase chemistry, substrate heating and etching by ion bombardment, and growth alignment effects due to the electric field. This work demonstrates how separating these processes allows for better control over the desired growth product.
Correlation function for relaxations in disordered materials
Authors: Svanberg C.
Published: December 2003

An empirical relaxation function suitable for describing the dynamics in disordered materials is presented. The function is a generalization of the stretched exponential function and based on power-law decays. The influence of parameters on the shape of the correlation function is discussed. Numerically transformed data are compared with some of the most commonly used curve–fit functions in the frequency dependent susceptibility. It is shown that a special case of the function is an approximation of the correlation function corresponding to the Cole–Cole function [K. S. Cole and R. H. Cole, J. Chem Phys. 9, 341 (1941)]. The applicability of the correlation function to describe glass transition dynamics is also discussed. Finally, it is shown that the proposed relaxation function can provide a significant improvement in the description of experimental data.
Optimizing surface coils and the self-shielded gradiometer
Authors: Suits B. H. , Garroway A. N.
Published: December 2003

A computational method for optimizing both single- and two-layer surface coils is presented that is relatively easy to implement in practice using either a variational approach or through standard numerical matrix diagonalization techniques. The technique is applied, in particular, to develop a self-shielded gradiometer that is relatively immune to radiofrequency (rf) interference from distant sources with a minimal compromise to its ability to sense rf from nearby sources, properties necessary for nuclear quadrupole resonance detection of buried land mines. Results of the optimization procedure are evaluated using finite-element numerical techniques and measurements on prototype coils for a number of configurations. These results show that practical coils can be constructed that approximate the theoretically optimized coils quite well. In addition, the trade off between the surface coil sensitivity and noise immunity is presented for the self-shielded gradiometer configuration.
Analysis of second-harmonic generation of Lamb modes using a modal analysis approach
Authors: Deng Mingxi
Published: December 2003

An effective technique for analyzing the generation of second harmonics of Lamb modes in elastic plates is presented. The nonlinearity of the wave equation governing the wave propagation ensures that there is second-harmonic generation accompanying primary Lamb mode propagation. This nonlinearity may be treated as a second-order perturbation of the linear elastic response. Using a second-order perturbation approximation and a modal analysis approach, the complicated problems of the generation of second harmonics of Lamb modes have been investigated. The fields of the second harmonics of Lamb modes in elastic plates are considered as superpositions of the fields of a series of double-frequency Lamb modes. The solutions provide physical insight into the generation of second harmonics with a cumulative growth effect, and the corresponding second-harmonic solutions. Although Lamb modes are dispersive, the cumulative growth effect of the second harmonics does exist under some conditions. The influence of Lamb mode dispersion on second-harmonic generation is considered.
T-shaped spin filter with a ring resonator
Authors: Kiselev A. A. , Kim K. W.
Published: December 2003

A planar ballistic T-shaped structure with a ring resonator attached is shown to be highly effective in filtering electron spin from an unpolarized source into two output fluxes with opposite and practically pure spin polarizations. The operability of the proposed device relies on the peculiar spin-dependent transmission properties of the T-shaped connector in the presence of Rashba spin–orbit interaction as well as the difference in dynamic phase gain of the two alternative paths around the ring resonator through upper and lower branches for even and odd eigenmodes.
Band offset measurements of the Si3N4/GaN (0001) interface
Authors: Cook T. E. , Fulton C. C. , Mecouch W. J. , Davis R. F. , Lucovsky G. , Nemanich R. J.
Published: December 2003

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy were used to measure electronic states as Si3N4 was deposited on clean GaN (0001) surfaces. The n-type (2×1018) and p-type (1×1017) GaN surfaces were atomically cleaned in NH3 at 860 °C, and the n-and p-type surfaces showed upward band bending of ∼0.2±0.1 eV and downward band bending of 1.1±0.1 eV, respectively, both with an electron affinity of 3.1±0.1 eV. Layers of Si (∼0.2 nm) were deposited on the clean GaN and nitrided using an electron cyclotron resonance N2 plasma at 300 °C and subsequently annealed at 650 °C for densification into a Si3N4 film. Surface analysis was performed after each step in the process, and yielded a valence band offset of 0.5±0.1 eV. Both interfaces exhibited type II band alignment where the valence band maximum of GaN lies below that of the Si3N4 valence band. The conduction band offset was deduced to be 2.4±0.1 eV, and a change of the interface dipole of 1.1±0.1 eV was observed for Si3N4/GaN interface formation.
Optical spectroscopy of localized electronic states in SrS:Cu
Authors: Evans D. R. , Dennis W. M.
Published: December 2003

A combination of optical spectroscopy and thermoluminescence measurements were used to determine the mechanisms that can influence the emission in SrS:Cu+ thin film structures and increase the electroluminescence brightness by a factor of 2. In particular, the interaction of the emitting center with the trap states and the conduction band is considered. Thermal quenching, thermoluminescence, and temperature-dependent fluorescence transient results are compared to appropriate population dynamics models in order to determine the position of the Cu excited state and the trap states with respect to the conduction band.
Brillouin scattering in RbNbWO6
Authors: Ma̧czka Mirosław , Ko Jae-Hyeon , Kojima Seiji , Hanuza Jerzy , Majchrowski Andrzej
Published: December 2003

Micro-Brillouin scattering measurements of RbNbWO6, a defect pyrochlore, that exhibits superionic conductivity and ferroelectric properties, have been performed in the 213–573 K temperature range. In this work the presence of quasielastic scattering (central peak) has been documented. The obtained results suggest that the main contribution to the observed central peak comes from Rb+ ion hopping. The temperature dependences of the central peak intensity and bandwidth exhibit broad anomalies around 360 K. This result clearly shows that ion hopping is related to the phase transitions at 361.6 and 356.5 K.
Optical properties of CdxZn1−xTe epilayers grown by molecular-beam epitaxy
Authors: Shih Y. T. , Fan W. C. , Yang C. S. , Kuo M. C. , Chou W. C.
Published: December 2003

Cd x Zn 1−x Te epilayers were grown on GaAs(001) substrates by molecular-beam epitaxy. The optical properties of the epilayers were studied by taking reflectance (R) and photoluminescence (PL) measurements. From the R and PL spectra, the energy gaps of the epilayers were determined and shown to decrease as the Cd concentration increased. A quadratic equation that related the energy gap to Cd concentration was also derived. The dependence of the PL spectra on temperature was studied as well. A redshift of the PL peaks was observed as the temperature increased. The exciton activation energy of the epilayers was determined from the plots of integrated PL intensity versus temperature. It tends to decrease as the Cd content increased. The temperature dependence of the energy gaps, determined from the PL spectra, was fitted according to both Varshni’s and O’Donnell and Chen’s relations [Y. P. Varshni, Physica 34, 149 (1967); K. P. O’Donnell and X. Chen, Appl. Phys. Lett. 58, 2924 (1991)]. Good fits were obtained using both relations for the samples with small Cd concentration. However, the fitted O’Donnell’s curve for the Cd0.582Zn0.418Te epilayer shows a more reasonable trend as temperature increases.
Theoretical model for polarization superlattices: Energy levels and intersubband transitions
Authors: Ridley B. K. , Schaff W. J. , Eastman L. F.
Published: December 2003

A theoretical model for stress-free polarization superlattices composed of wurtzite semiconductors is described, which exploits Airy function solutions of the Schrödinger equation for a superlattice in the absence of free carriers. The theory is applied to several stress-free structures consisting of AlN barriers and GaN wells. The part played by the crystal-field splitting of the valence band and its dependence on biaxial elastic strain in determining the conduction-band offset is fully taken into account. Electric fields were determined from the spontaneous and nonlinear piezoelectric polarization. Energy bands, transition energies between subbands 1 and 2, and associated eigenfunctions are calculated, and the sensitivity to the conduction-band discontinuity explored. Intersubband radiative and nonradiative transition rates are estimated taking into account the Bloch-function overlap integrals and the role of intervalley scattering. Upper limits to technologically useful subband separations set by conduction-band offsets and the energy of conduction-band valleys are discussed. An upper limit of around 1.5 eV for the energy of the second subband is indicated.
Photoluminescence fatigue and related degradation in thin-film photovoltaics
Authors: Shvydka Diana , Verzella C. , Karpov V. G. , Compaan A. D.
Published: December 2003

We observe that junction photoluminescence (PL) intensity excited by a constant power laser beam in polycrystalline CdTe/CdS solar cells gradually decreases, which is similar to the PL fatigue in chalcogenide glasses. As a function of time, it was studied at different laser beam powers and temperatures for contact-free and metallized regions. We were able to discriminate between the fatigue per se and a concomitant short-time PL intensity drop due to the laser heating. The fatigue shows substantial variations between different spots on the sample. It is more profound at higher temperatures and laser beam powers where its value can be as large as 80% in 2 h. At low temperatures and beam powers, it saturates rather quickly not exceeding 10% of the initial PL intensity. We attribute the observed phenomenon to defect creation by the light-generated electrons and holes. The defects provide additional nonradiative recombination channels thus decreasing the PL. Simultaneously, this negative feedback makes the defect-generation rate slow down, so that the PL fatigue saturates. We propose a simple analytical model that fits the data.
Jones matrix method for predicting and optimizing the optical modulation properties of a liquid-crystal display
Authors: Moreno I. , Velásquez P. , Fernández-Pousa C. R. , Sánchez-López M. M. , Mateos F.
Published: December 2003

We present a simple technique for the calibration, prediction, and optimization of the optical modulation properties of a liquid-crystal display (LCD). The method is useful when there is no information about the internal fabrication parameters of the device (the orientation of liquid-crystal molecules, the twist angle, or the birefringence of the material). A complete determination of the LCD Jones matrix is accomplished by means of seven irradiance measurements for a single wavelength. This technique only requires two linear polarizers and one quarter-wave plate. Once the Jones matrix has been calibrated, the amplitude, phase, and polarization modulation response can be predicted. Therefore, it can be optimized through the control of the polarization configuration. The validity of the proposed method is experimentally probed. Finally, we present a particular application to produce phase-only modulation.
High anisotropy CoPtCrB magnetic recording media
Authors: Toney Michael F. , Marinero Ernesto E. , Doerner Mary F. , Rice Philip M.
Published: December 2003

We describe the synthesis, magnetism, and structure of CoPtCrB alloys with Pt concentrations from 10%–43%. The Cr concentration in the alloys was 15%–17% and the B concentration was 9%–11%. The magnetic anisotropy and coercivity increase with increasing Pt up to ≈30%, plateau at ≈35 000 Oe and ≈6000 Oe, respectively, and then decrease. Transmission electron microscopy results show the media form fine isolated grains for all Pt concentrations. X-ray diffraction measurements show that with increasing Pt a face-centered-cubic (fcc) Co-alloy phase is progressively formed at the expense of the hexagonal-close-packed Co-alloy and that this fraction becomes significant for >35% Pt. The formation of the fcc phase likely causes the behavior in the anisotropy. No Pt concentration dependence is observed for the stacking fault density. The x-ray data show that with increasing Pt, the CoPtCrB-alloy lattice parameters exhibit two distinct regions with the slope changing at 16% Pt. The presence of these two regions is discussed.
Molecular dynamics study of structural, mechanical, and vibrational properties of crystalline and amorphous Ga1−xInxAs alloys
Authors: Branicio Paulo S. , Rino Jose P. , Shimojo Fuyuki , Kalia Rajiv K. , Nakano Aiichiro , Vashishta Priya
Published: December 2003

Using an interaction potential scheme, molecular dynamics (MD) simulations are performed to investigate structural, mechanical, and vibrational properties of Ga1−xInxAs alloys in the crystalline and amorphous phases. For the crystalline phase we find that: (i) Ga–As and In–As bond lengths vary only slightly for different compositions; (ii) the nearest-neighbor cation–cation distribution has a broad peak; and (iii) there are two nearest-neighbor As–As distances in the As (anion) sublattice. These MD results are in excellent agreement with extended x-ray absorption fine structure and high-energy x-ray diffraction data and also with ab initio MD simulation results. The calculated lattice constant deviates less than 0.18% from Vegard’s law. The calculated phonon density of states exhibits a two-mode behavior for high-frequency optical phonons with peaks close to those in binary alloys (GaAs and InAs), which agrees well with a recent Raman study. Calculated elastic constants show a significant nonlinear dependence on the composition. For the amorphous phase, MD results show that: (i) the nearest-neighbor cation–anion distribution splits into well-defined As–Ga and As–In peaks as in the crystal phase; (ii) the cation–cation distribution is similar to that in the crystal phase; and (iii) the As–As distribution is quite different from that in the crystal, having only one nearest-neighbor distance.
Molecular dynamics simulations of mechanical deformation of amorphous silicon dioxide during chemical–mechanical polishing
Authors: Chagarov Evgueni , Adams James B.
Published: December 2003

We present molecular dynamics simulations of the mechanical deformation which occurs during chemical–mechanical polishing (CMP) of amorphous silicon dioxide for different geometries and relative velocities. The simulations clarify asperity shape evolution during the process of shear and reveal temperature distributions as a function of time. It was found that the ratio of radii of a particle and asperity strongly affects the amount of the material removed whereas the relative velocity has a weaker affect on it. During shear, the local temperature inside the deformed asperity is raised to 700 K at 0.5 Å/ps and up to 2200 K at 5.0 Å/ps. This temperature increase lasts for only a short time, but it can have a major impact on the amount of material removed. It was found that there could be significant deposition of the material from the particle to the slab, which can fill surface trenches and thereby make the surface smoother. An analytic model was developed for describing the amount of material removed as a function of asperity and particle radii and relative velocity. It was fit to the simulated data, and used to predict the amount of material removed at typical CMP velocities (∼1 m/s).
Mechanically induced strain enhancement of metal–oxide–semiconductor field effect transistors
Authors: Haugerud B. M. , Bosworth L. A. , Belford R. E.
Published: December 2003

Device characteristics and analysis are reported for strained silicon n- and p- channel partially depleted metal oxide semiconductor field effect transistors (MOSFETs) at 300 K. The devices were fabricated commercially on standard silicon-based silicon-on-insulator substrates and strain was applied mechanically after fabrication. Uniaxial tensile strain was applied within the elastic region using a back-end process and the relaxed structures were characterized under steady state conditions. Characterization was performed before and after straining. At ultralow strain levels (0.031%), pMOSFETs showed an increase in effective mobility μeff of 14.35% and an enhanced saturation current, Isat of 14.56%. An improvement in μeff of 15.19% and in Isat of 15.34% was observed for nMOSFETs strained by 0.039%. The latter die was debonded, released, and restressed at an elevated level of 0.052%. We observed an increased effective mobility μeff of 18.49% and Isat of 18.05%. Elastic uniaxial strain was fixed and characterization was performed at each strain level. The greatest mobility enhancement was observed for holes with strain applied at right angles to the channel length and applied field.
Boron and phosphorus diffusion in strained and relaxed Si and SiGe
Authors: Zangenberg N. R. , Fage-Pedersen J. , Hansen J. Lundsgaard , Larsen A. Nylandsted
Published: December 2003

The diffusion of boron and phosphorus has been investigated in SiGe grown by molecular beam epitaxy. The analysis was done in relaxed Si1−xGex for x=0, 0.01, 0.12, and 0.24 for B and x=0, 0.07, 0.12, 0.24, and 0.40 for P. B diffusion in relaxed samples shows little effect of changing the Ge content while for P diffusion, increasing Ge content increases the diffusion coefficient from Si up to Si0.76Ge0.24. This is explained by a pairing of B and Ge atoms retarding the diffusion. B diffusion in compressively strained Si0.88Ge0.12 and Si0.76Ge0.24 and tensile strained Si and Si0.88Ge0.12 was also investigated. Compressive strain was found to decrease the diffusion coefficient of B and tensile strain to increase it.
Study on atomic coordination around Er doped into anatase– and rutile– TiO2: Er–O clustering dependent on the host crystal phase
Authors: Ishii Masashi , Komuro Shuji , Morikawa Takitaro
Published: December 2003

Alternative Er doping into anatase (A–) or rutile (R−)TiO2 can be achieved by O2 pressure control during Er-doped TiO2 deposition using laser ablation. An x-ray absorption fine structure analysis of the atomic coordination around Er revealed the dynamics of Er–O clustering in A– and R−TiO2. The Er local structure in A−TiO2 can be determined by a Ti–O arrangement of the host crystal surrounding Er, while that in R−TiO2 can be decided by the Er–O chemical properties rather than the Ti–O arrangement. Moreover, a substitution of Er for Ti induced O deficit in A−TiO2 and induced Ti removal in R−TiO2. These differences can be explained by the ErO6 stabilization mechanism based on the spontaneous symmetry reduction (SSR) theory. The Ti–O arrangement in A−TiO2 prevents the SSR, resulting in ErO6 instability. However, ErO6 is stabilized in the host crystal because the SSR is smoothly induced in R−TiO2.
Effects of dry etching processes on effective refractive index of ZnTe surface layers in terahertz region
Authors: Wu S. , Ren Z. Q. , Shen W. Z. , Ogawa H. , Guo Q. X.
Published: December 2003

We report on the far-infrared (FIR) reflection spectra of high-quality ZnTe crystals exposed to CH4/H2 gases under different rf plasma powers. The effects of reactive ion etching on the effective refractive index of perturbed ZnTe surface layers have been investigated in the terahertz (THz) frequency range of 1.5–10.0 THz. The FIR refractive index has been found to decrease with the increase of plasma power, which has been well described by Sellmeier formulas. Furthermore, the obtained experimental data have been employed for THz electro-optic applications by predicting the effects of RIE on the second-order nonlinear susceptibility and the THz pulse intensity after dispersive propagation/absorption in the ZnTe surface layers.
Arsenic pressure dependence of carrier lifetime and annealing dynamics for low-temperature grown GaAs studied by pump–probe spectroscopy
Authors: Yano R. , Hirayama Y. , Miyashita S. , Uesugi N. , Uehara S.
Published: December 2003

Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 °C. We determined the annealing dynamics of LT-GaAs based on a model in which AsGa antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA that was different from the true activation energy. The annealing time dependences of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPAs of our data and other works, which proved that the annealing dynamics of AsGa antisite defects was dominated by VGa vacancy assisted diffusion.
Target optimization of a water-window liquid-jet laser–plasma source
Authors: de Groot J. , Hemberg O. , Holmberg A. , Hertz H. M.
Published: December 2003

We optimize the water-window x-ray flux and debris deposition for a liquid-jet laser plasma source by varying both the target diameter and the jet material. For two target liquids, methanol and ethanol, measurements of the λ=3.37 nm C VI x-ray flux and the debris deposition rates are presented as function of the jet diameter. It is shown that the effective carbon debris deposition is more than 1 order of magnitude smaller for methanol, while the x-ray flux is reduced only ∼40%. The reduction in carbon debris deposition may be explained by reactive ion etching by oxygen from the plasma. Thus, the methanol water-window source may be operated at a 5–10× higher flux without increasing the debris deposition. The optimization potentially allows a reduction of the exposure time of compact soft x-ray microscopy or other water-window applications based on such sources without increasing damage to sensitive x-ray optics.
Structure of electroless deposited Co0.9W0.02P0.08 thin films and their evolution with thermal annealing
Authors: Kohn A. , Eizenberg M. , Shacham-Diamand Y.
Published: December 2003

Electroless deposited Co0.9W0.02P0.08 thin films have been proposed as diffusion barriers and encapsulation layers for Cu metallization in ultralarge-scale integrated microelectronic devices. In this article, we present a study of the structure of these films and their evolution with thermal anneal up to 700 °C. The as-deposited microstructure is comprised of an amorphous CoWP component and nanocrystallites of hexagonal-close-packed (hcp) Co, approximately 5 nm in size. The amorphous CoWP component crystallizes to hcp Co at approximately 290 °C with an apparent activation energy of 1.6±0.1 eV, according to the Kissinger analysis. Isothermal anneals show that the rate of nucleation with time of the hcp Co grains is constant, and grain growth is controlled by diffusion. This diffusion is most probably of the P and W elements, which enrich the grain boundaries. At approximately 420 °C, the orthorhombic (o-) Co2P phase nucleates. The apparent activation energy of this phase transformation is 4.6±0.1 eV, according to the Kissinger analysis. We suggest that the nucleation of the o-Co2P phase occurs when the grain boundaries are saturated with P. At higher temperatures, the main structural changes observed in the films are the grain growth of the predominately hcp Co grains and a delayed transformation to face-centered-cubic Co compared to bulk Co. The chemical binding state of P after high temperature anneals is covalent as in the o-Co2P phase. In the as-deposited film, the chemical binding state of P is significantly different, attributed to the amorphous CoWP component and the P in a solid solution in the hcp Co grains.
Simulation of plasma molding over a ring on a flat surface
Authors: Kim Doosik , Economou Demetre J.
Published: December 2003

A fluid/Monte Carlo simulation model was developed to study plasma molding over an axisymmetric feature (a ring) resting on an otherwise planar surface in contact with a high-density rf plasma. The two-dimensional (r,z) time-dependent sheath potential, and ion density and flux profiles were predicted with a self-consistent fluid simulation. The trajectories of ions and energetic neutrals (resulting mainly by ion neutralization on the cylindrical sidewalls of the ring) were then followed with a Monte Carlo simulation, in an effort to obtain their energy and angular distributions on the substrate surface. When the sheath thickness was comparable to the size of the ring, strong radial electric fields deflected oncoming ions toward the sidewalls of the ring. The ion density was lower in the cylindrical well formed by the ring, compared to outside, resulting in a locally thicker sheath and a smaller spread in the double-peaked ion energy distributions at the bottom of the well. The ion impact angle increased progressively as the sidewalls were approached. The angular distribution of energetic (fast) neutrals at the bottom of the well was bimodal. The energy distribution of fast neutrals at the bottom of the well was broader compared to the parent ion energy distributions.
Properties of Cu/Au Schottky contacts on InGaP layer
Authors: Liu Day-Shan , Lee Ching-Ting , Wang Ching-Wu
Published: December 2003

We use Auger electron spectroscopy (AES) measurements to obtain elemental distributions for Cu/Au Schottky contacts to an InGaP layer in as-deposited and thermally annealed samples. The AES depth profile shows an obvious Cu layer with no apparent interdiffusion between the Cu and the InGaP layers in either the as-deposited samples or in the samples annealed at 450 °C. However, when the annealing temperature reached 500 °C, the metallic Cu layer was released, and a distinct interdiffusion between the Cu and the InGaP layers is observed. Metallic Cu and Au intermixed to form a stable intermetallic region. Below this intermetallic region, an interaction region, composed mainly of Cu and P, is observed. The formation of this region is attributable to the diffusion of Cu into the InGaP layer, and is responsible for the thermal degradation of Cu/Au Schottky contacts to the InGaP layer. In the deep-level transient spectroscopy spectra, we find an obvious trap at 150 K. This was determined to be a native trap that exists in the InGaP materials with activation energies of 0.35–0.42eV. After annealing at 500 °C, another trap appeared. According to a thermal degradation investigation, this trap at Ec−0.70 eV can be attributed to the diffusion of Cu into the InGaP layer.
Spin-polarized electron tunneling across a Si delta-doped GaMnAs/n-GaAs interface
Authors: Andresen S. E. , So/rensen B. S. , Rasmussen F. B. , Lindelof P. E. , Sadowski J. , Guertler C. M. , Bland J. A. C.
Published: December 2003

We study the spin-polarized tunneling of electrons from the valence band of GaMnAs into the conduction band of n-type GaAs with Si delta-doping at the interface. The injection of spin-polarized electrons is detected as circular polarized emission from a GaInAs/GaAs quantum well light emitting diode, corresponding to magneto-optical Kerr effect loops. The angular momentum selection rules are simplified by the strain-induced heavy-hole/light-hole splitting, allowing a direct relation between circular polarization and spin-polarization. Comparison with the influence of Zeeman splitting allow us to conclude a spin-injection from the majority spin-band.
Averaging of laterally inhomogeneous lifetimes for one-dimensional modeling of solar cells
Authors: Isenberg Joerg , Dicker Jochen , Warta Wilhelm
Published: December 2003

This paper investigates the influence of laterally inhomogeneous cell parameters on solar cell efficiency. A universal procedure is proposed which allows the reduction of a frequency distribution of an arbitrary cell parameter to just one single value for one-dimensional (1D) device modeling. This procedure is in detail investigated for the case of lifetime and diffusion length distributions showing that the frequency distribution of local minority carrier densities (Δn) is the appropriate parameter for solar cell modeling. The actual position of high and low lifetime areas on the cell is found to be of minor influence if Δn is used as an input parameter. Therefore, using the lifetime frequency distribution allows us to include the effect of spatial inhomogeneities in recombination activity in 1D solar cell modeling. Both simulated and experimental examples for the modeling procedure and the quantitative influence of spatial inhomogeneities on solar cell performance are discussed. This article presents a possibility to include laterally resolved measurements of material and process quality data in routine predictive modeling of solar cell performance.
Stray grain formation in single crystal Ni-base superalloy welds
Authors: Park J.-W. , Babu S. S. , Vitek J. M. , Kenik E. A. , David S. A.
Published: December 2003

The effects of processing conditions and crystallographic orientation of growing dendrites on stray grain formation have been investigated in welds of single crystal Ni-base superalloys. The degree of constitutional supercooling (CS) at the solidification front has been used as a metric to quantify the effects. Numerical calculation results showed an appreciable effect of dendrite orientation on CS. For experimental verification, welds were made on a single crystal but in an asymmetric crystallographic orientation under three different processing conditions. Stray grain formation was observed to increase and become extensive on one side of the weld as power and welding speed increased. This result is in agreement with the calculated predictions and confirms that the degree of CS is a good metric.
Cu gettering in ion implanted and annealed silicon in regions before and beyond the mean projected ion range
Authors: Kögler R. , Peeva A. , Lebedev A. , Posselt M. , Skorupa W. , Özelt G. , Hutter H. , Behar M.
Published: December 2003

The strong gettering of Cu atoms in single-crystal Si implanted with 3.5 MeV P+ ions is studied after thermal treatment and Cu contamination. Cu decorates the remaining implantation damage. Three separate Cu gettering layers are detected by secondary ion mass spectrometry: at the main projected ion range RP below RP (RP/2 effect) and beyond RP (trans-RP effect). The defects acting as gettering centers at RP/2 and RP are implantation induced excess vacancies and excess interstitials, respectively. Cu profiles fit very well with depth distributions of excess vacancies and excess interstitials determined by binary collision simulations for random and channeled ion incidence. The RP/2 effect for P+ implantation is found to be significantly reduced in comparison with Si+ implantation. It disappears completely for higher P+ ion fluences. The trans-RP gettering layer is formed by thermal treatment. The Cu accumulation in the trans-RP region increases with increasing temperature and/or with increasing annealing time. These results are in qualitative agreement with the assumption that interstitials carried by P diffusion are the origin of Cu gettering in the trans-RP region. The P diffusion may inject interstitials into the bulk and also into the RP/2 region thus decreasing the RP/2 effect.
Arrays of ferromagnetic FeCo and FeCr binary nanocluster wires
Authors: Lee G. H. , Huh S. H. , Jeong J. W. , Kim S. H. , Choi B. J. , Ri H.-C. , Kim Bongsoo , Park J. H.
Published: December 2003

We report fabrication of the arrays of ferromagnetic FeCo and FeCr binary nanocluster wires (NCWs) by thermally decomposing the Fe(CO)5/Co2(CO)8 and Fe(CO)5/Cr(CO)6 metal carbonyl vapors, respectively, with a resistive heater placed in the middle of a pair of permanent disc magnets. The NCWs are produced through the pileup of binary nanoclusters along the lines of magnetic flux, perpendicular to the substrates attached to a pair of permanent disc magnet surfaces. For both arrays of FeCo and FeCr binary NCWs, the diameter of individual NCWs ranges from 8 to 20 nm and the NCWs exist as bundles with their length up to 2–3 mm. We observed compositional dependence of structure.
Noise power spectrum of a long-channel current line with electron traps: Slave-boson mean field theory
Authors: Tanamoto Tetsufumi , Ohba Ryuji , Uchida Ken , Fujita Shinobu
Published: December 2003

We calculated the noise power spectrum of a one-dimensional free electron system corresponding to a long-channel current line when there are several electron traps near the current line. The calculation was performed starting from the Anderson Hamiltonian in the framework of the slave-boson mean field theory. When there is a single trap, the noise power spectrum shows a peak structure. When there are two traps and the distance between the two traps is less than the Fermi momentum, the peak of the noise power is enhanced because of interference between the two traps. When there are multiple traps, the noise power is analytically found to have an ω−2 dependence in the low-frequency limit and an ω−1/2 dependence in the high-frequency limit. These results are applicable to the noise analysis of nanodevices such as a single-electron transistor (SET) if the traps are regarded as the SET islands.
Hydrogen plasma passivation effects on properties of p-GaN
Authors: Polyakov A. Y. , Smirnov N. B. , Govorkov A. V. , Baik K. H. , Pearton S. J. , Luo B. , Ren F. , Zavada J. M.
Published: December 2003

The effects of hydrogen on the electrical and optical properties of p-GaN were investigated. Hydrogen is readily incorporated into the material at temperatures of 250–350 °C, which is consistent with the low activation energy for diffusion reported by Seager et al. [J. Appl. Phys. 92, 7246 (2002)] in GaN p-n junctions. From comparison with the results of earlier experiments, hydrogen diffusivity appears to be a strong function of the hydrogen concentration incorporated into the material during growth. More than an order of magnitude decrease in hole concentration was observed after the hydrogen plasma treatment and from the measurements of the temperature dependence of conductivity. This is the result of hydrogen passivation of acceptors rather than of increased compensation by donor centers. Hydrogen treatment was also shown to lead to a strong suppression of 0.3 eV and 0.6 eV traps and to a strong increase in the magnitude of the photocurrent which are the results of passivation of deep-level defects by hydrogen. At the same time, hydrogen plasma treatment led to a strong increase in the concentration of 0.4 eV hole traps.
T-junction waveguide experiment to characterize left-handed properties of metamaterials
Authors: Chen Hongsheng , Ran Lixin , Huangfu Jiangtao , Zhang Xianmin , Chen Kangsheng , Grzegorczyk Tomasz M. , Kong Jin Au
Published: December 2003

The left-handed property of a metamaterial is demonstrated by measuring the power at the output of a T-junction waveguide loaded with a metamaterial having one edge cut at 45° with respect to the waveguide axis. Both experimental scattering data and numerical results are presented. The metamaterial sample is realized by using a periodic arrangement of wires and split-ring resonators, and the operating frequency is chosen to correspond to a refraction index of about −1. The results show that when the T junction is empty, most of the power is received at the output port directly facing the input port, whereas when it is loaded with the sample of metamaterial, most of the power is received at the output port perpendicular to the input axis. This indicates that the energy is bent by the oblique edge of the sample by an angle of −45° with respect to the normal, which suggests a negative index of refraction.
Atomic force microscopy-induced electric field in ferroelectric thin films
Authors: Wang Biao , Woo C. H.
Published: December 2003

The use of atomic force microscopy (AFM) to tailor and image ferroelectric domains in the submicron and nanometer ranges is gaining increasing attention. Many applications have been developed that make use of the superhigh electric field generated by the sharp AFM tip in a local area. In this article, we derive an explicit expression for the AFM-induced electric field in a ferroelectric thin film. Based on a similar approach, we also obtain the depolarization field created by polarization charges using the Green function technique. Based on the expressions derived, the effects of the substrate are discussed.
Determination of band offsets in semiconductor quantum well structures using surface photovoltage
Authors: Dumitras Gh. , Riechert H.
Published: December 2003

Surface photovoltage in semiconductor single quantum well structures is studied. The surface photovoltage spectra of such structures contain the essential information to enable the determination of band offsets. To estimate the band offsets only one sample is necessary, which is an advantage over other methods. The cases of type I- and type II-band alignment are discussed separately. Two particular single quantum well samples are studied by this measurement method. GaAs/In0.65Ga0.35As0.983N0.017/GaAs and GaAs/Ga0.7As0.3Sb/GaAs which are of type I and type II, respectively. The values 79/21 (type I) and 12/88 (type II) are obtained for the conduction/valence-band ratio ΔEC/ΔEV in the InGaAsN and GaAsSb quantum well structures, respectively.
Role of excess Mg and heat treatments on microstructure and critical current of MgB2 wires
Authors: Serquis A. , Civale L. , Hammon D. L. , Liao X. Z. , Coulter J. Y. , Zhu Y. T. , Peterson D. E. , Mueller F. M.
Published: December 2003

We present a detailed analysis of the effect of heat treatments on the microstructure, magnetization, and transport properties of MgB2 wires produced by the powder-in-tube method. We have used commercial MgB2 powder with 5 at. % Mg powder added as an additional source of magnesium and stainless steel as sheath material. We measure the dc transport critical current that can be increased or decreased by more than one order of magnitude as compared with the as-drawn wire, depending on the annealing parameters. We correlate the changes in the critical current with changes in the microstructure, as determined from scanning and transmission electron microscopy analysis. We show through magnetization measurements of short annealed wires that inappropriate annealing conditions result in a deterioration of the connectivity due to the loss of Mg and in inhomogeneous weak-link limited current flow, rendering the critical state model inapplicable. We discuss the optimization of the annealing conditions that strongly improve the connectivity by eliminating most of the microcracks present in the unannealed wires, where excess Mg promotes the recrystallization. The loss of Mg during the heat treatment may be precluded by annealing long wire lengths with a high heating rate.
Numerical study of Ar/CF4/N2 discharges in single- and dual-frequency capacitively coupled plasma reactors
Authors: Georgieva V. , Bogaerts A. , Gijbels R.
Published: December 2003

A one-dimensional particle-in-cell/Monte Carlo model is developed to study capacitively coupled (cc) radio-frequency discharges in a gas mixture of Ar, CF4, and N2. The charged species, which are followed in the model, are: Electrons and Ar+, CF3+, N2+, F−, and CF3− ions. The simulation considers electron – neutral (Ar, CF4, and N2) collisions, various kinds of collisions of ions with neutrals, positive–negative ion recombination, and electron–ion recombination. The model yields results for electron and ion densities, fluxes and energy distributions, collision rates and electric field, and potential distributions. The simulations are performed for a 0.8/0.1/0.1 ratio of Ar/CF4/N2 mixture at a pressure of 30 mTorr in single (13.56 MHz) and dual frequency (2+27 MHz) cc reactors and a comparison between the two frequency regimes is made. Results show that the structure of the discharges is electronegative in both cases. F− and CF3− ions are the main negative charge carriers in the single and dual frequency regime, respectively. In the presence of low-frequency (2 MHz) and a strong electric field, the light F− ions are no longer confined in the bulk plasma and they partially respond to the instantaneous electric field. The calculated electron energy probability function profiles can be approximated to a Druyvesteyn and bi-Maxwellian distribution with high-energy tails in the single- and dual-frequency regime, respectively. The ion energy distribution is narrow with one outstanding peak in the single-frequency scheme, whereas it is wide and bimodal in the dual-frequency scheme.