bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of ion bombardment and annealing on the electrical properties of hydrogenated amorphous silicon metal–semiconductor–metal structures
Authors: Orwa J. O. , Shannon J. M. , Gateru R. G. , Silva S. R. P.
Published: January 2005

The electrical properties of hydrogenated amorphous silicon (a-Si:H) metal–semiconductor–metal (MSM) devices are investigated as a function of Si bombardment dose prior to and after annealing. We observe that conduction in unbombarded devices is surface-barrier controlled whereas it is bulk controlled in bombarded devices. The resistance decreases with bombardment dose in a manner consistent with increased hopping conductivity in highly damaged structures. A relative permittivity of between 8 and 12, depending on dose, was calculated from experimental Poole–Frenkel plots for bombarded devices. These values compare closely with the theoretical relative permittivity for amorphous silicon of 11.7 and confirm that conduction is by Poole–Frenkel mechanism. For bulk-controlled conduction, we observe an increase in the zero-field Coulombic trap barrier height with decreasing dose, ranging from 0.53 for a Si dose of 5×1013cm−2 to 0.89 for a dose of 2×1012cm−2. We attribute this to a decrease in the concentration of charged defects with decreasing dose and find that the change in concentration of charged centers needs to be about 4×1019cm−3 to account for the change of 0.35eV from the lower to the upper dose. Activation energies obtained from Arrhenius plots of current density against temperature varied with dose and temperature in a similar way as Coulombic barrier height. We explain these results in terms of the variation in the number of charged defect centers with dose and annealing temperature and a shift in the Fermi level.
Frequency domain analysis of ultrasonic velocity: An alternative bond effect correction constraining bond properties
Authors: Yoneda A. , Song M.
Published: January 2005

Ultrasonic travel time measurements have been made in the time domain by observing the arrival time difference between the selected ultrasonic pulses. In the present work, we carried out a frequency domain analysis of ultrasonic travel time based on the phase difference between the Fourier coefficients of ultrasonic pulses. The purpose of the present analysis is to develop an easier experimental procedure for making accurate ultrasonic velocity measurements. The features of the present frequency domain analysis are to use digitized oscilloscope data of simple pulse reflection measurements, and to incorporate the bond effect correction by constraining the acoustic properties of the bond material. We examined the performance of the present method through test measurements on a fine polished glass plate (bk7 grade), and confirmed an accuracy of ∼10−4. Its accuracy is critical for utilizing velocity data for constructing the equation of state.
Morphological change of multiwalled carbon nanotubes through high-energy (MeV) ion irradiation
Authors: Kim H. M. , Kim H. S. , Park S. K. , Joo J. , Lee T. J. , Lee C. J.
Published: January 2005

Multiwalled carbon nanotubes (MWCNTs) were expanded by 2.5 times in diameter through high-energy (MeV) ion irradiation. Pristine MWCNTs were synthesized onto SiO2 substrate by chemical vapor deposition. The 4MeV Cl2+ ions with a dose of 3×1016ions∕cm2 were irradiated on MWCNTs. From high-resolution transmission electron microscopy (HR-TEM) images, the average diameter of the high-energy-ion-irradiated MWCNTs was ∼180nm, while that of the pristine MWCNTs was ∼70nm. The wall thickness of the pristine and the high-energy-ion-irradiated MWCNT samples was ∼20nm and 40–50nm, respectively. We observed the clear formation of nanocompartments with bamboolike structure inside the tubes after ion irradiation. The amorphous carbon structure in the ion-irradiated MWCNT shells was observed from Raman spectra. Based on the results of HR-TEM and Raman spectra, the expansion of the systems represents morphological transition from crystalline graphite structure to amorphous carbon or finite sized graphite structure due to the ion impact. We suggest that high-energy ion irradiation can be useful for the modification of MWCNT structures.
Time resolved evolution of structural, electrical, and thermal properties of copper irradiated by an intense ultrashort laser pulse
Authors: Sandhu Arvinder S. , Dharmadhikari A. K. , Kumar G. Ravindra
Published: January 2005

The dynamical properties of copper metal are obtained on a picosecond time scale using 100fs laser pulse at 1015Wcm−2—an intensity regime relevant to femtosecond micromachining. The dissipation mechanisms and scaling laws spanning a wide temperature range are obtained from femtosecond pump–probe reflectivity. We observe obliteration of the crystalline structure in copper within 400fs due to lattice disorder caused by the intense laser pulse. The electrical resistivity is obtained by studying the probe reflectivity evolution from 0to30ps. The “resistivity saturation” effect in an unexplored regime intermediate to hot plasma and cold solid is studied in detail. The temperature evolution and thermal conductivity values are also obtained.
Cathodoluminescence study of the radiative recombination properties of Se-doped GaSb crystals
Authors: Díaz-Guerra C. , Vincent J. , Piqueras J. , Bermúdez V. , Diéguez E.
Published: January 2005

The radiative recombination properties of Se-doped GaSb crystals grown by the Bridgman method have been investigated by cathodoluminescence (CL) microscopy and spectroscopy in the scanning electron microscope. A CL band centered at about 765meV, not previously observed in undoped GaSb, is generally the dominant emission. CL spectra recorded under different excitation conditions suggest that this band can be attributed to a Se-related level–to–band transition. The spatial distribution of the 765meV emission, as observed in the CL images, indicates an inhomogeneous Se distribution in the material.
Model for ferroelectric semiconductors thin films accounting for the space varying permittivity
Authors: Baudry Laurent , Tournier Jérôme
Published: January 2005

Reducing thickness of ferroelectric films typically comes with an apparent degradation of their ferroelectric and dielectric properties. The existence of low-dielectric interfacial layers is often invoked to explain such behaviors. Much work has been done on modeling ferroelectric thin films by considering a ferroelectric layer between two layers with low dielectric constant. In these models it is necessary to introduce extrinsic parameters; the dielectric constant and the polarization are step functions of the depth inside the film. We have developed a model for ferroelectric semiconductors based on the inhomogeneous Landau–Devonshire theory, including surface effects. The local electric field is determined by solving Poisson’s equation in which a differential permittivity replaces the dielectric constant. We have found that the hysteresis loops were strongly influenced by the correlation length. That point is discussed by examining the electric field, polarization, and differential permittivity profile inside the film. It is shown that the differential permittivity strongly decreases with the correlation length increases. That phenomena leads to high electric fields inside the film and especially close to the surface. As a consequence, an explanation for thin film ferroelectric properties can be given by invoking space charge density many orders of magnitude lower than those usually considered.
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Pb
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Zr
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Sn
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Ti
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antiferroelectric thin films grown on LaNiO3-buffered and Pt-buffered silicon substrates by sol-gel processing
Authors: Li Xin , Zhai Jiwei , Chen Haydn
Published: January 2005

Antiferroelectric (Pb,La)(Zr,Sn,Ti)O3 thin films have been fabricated on LaNiO3∕Pt∕Ti∕SiO2∕Si and Pt∕Ti∕SiO2∕Si substrates by a sol-gel processing technique. The films grown on LaNiO3-buffered silicon substrates showed a dense columnar microstructure and were highly (100)-oriented while those grown on Pt-buffered silicon substrates showed an equiaxed microstructure without any obvious preferred orientation. The relationship between the orientation of films and the saturation polarization was studied. The phase switching field and the dielectric constant were investigated as a function of film thickness.
Diffusion of spheres in crowded suspensions of rods
Authors: Kang Kyongok , Gapinski J. , Lettinga M. P. , Buitenhuis J. , Meier G. , Ratajczyk M. , Dhont Jan K. G. , Patkowski A.
Published: January 2005

Translational tracer diffusion of spherical macromolecules in crowded suspensions of rodlike colloids is investigated. Experiments are done using several kinds of spherical tracers in fd-virus suspensions. A wide range of size ratios L/2a of the length L of the rods and the diameter 2a of the tracer sphere is covered by combining several experimental methods: fluorescence correlation spectroscopy for small tracer spheres, dynamic light scattering for intermediate sized spheres, and video microscopy for large spheres. Fluorescence correlation spectroscopy is shown to measure long-time diffusion only for relatively small tracer spheres. Scaling of diffusion coefficients with a/ξ, predicted for static networks, is not found for our dynamical network of rods (with ξ the mesh size of the network). Self-diffusion of tracer spheres in the dynamical network of freely suspended rods is thus fundamentally different as compared to cross-linked networks. A theory is developed for the rod-concentration dependence of the translational diffusion coefficient at low rod concentrations for freely suspended rods. The proposed theory is based on a variational solution of the appropriate Smoluchowski equation without hydrodynamic interactions. The theory can, in principle, be further developed to describe diffusion through dynamical networks at higher rod concentrations with the inclusion of hydrodynamic interactions. Quantitative agreement with the experiments is found for large tracer spheres, and qualitative agreement for smaller spheres. This is probably due to the increasing importance of hydrodynamic interactions as compared to direct interactions as the size of the tracer sphere decreases.
Calculation of crystal and molecular structures of carbon disulfide CS2
Authors: Thiéry M.-M. , Rérat C.
Published: January 2005

Crystal and molecular structures of carbon disulfide CS2 were investigated by molecular packing analysis with a computed dynamical model. This model includes thermal motions, molecular deformations, and anisotropic atomic repulsive interactions. Several crystalline structures with orthorhombic symmetry Cmca have been found by the calculation. The lowest energy structure agrees with the experimental one. The temperature dependence of the crystal structure parameters reproduces the general features and the particular increase with decreasing temperature of the lattice parameter c (and orientational angle ψ) as determined by x-ray diffraction or neutron scattering experiments. The pressure behavior of the crystal structure parameters up to 12 GPa at room temperature is also correctly reproduced.
Quantum chemical study of the electronic structure of NiCH2+ in its ground state and low-lying electronic excited states
Authors: Villaume Sébastien , Daniel Chantal , Strich Alain , Perera S. Ajith , Bartlett Rodney J.
Published: January 2005

The electronic structure of NiCH2+, representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states (2A1, A22, B12, and B22) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree–Fock and density functional theory (Kohn–Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the B12 electronic ground state to the low-lying doublet excited states. The A22 and B12 states are nearly degenerate, found to be separated by 940 cm−1 at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The B22 state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree–Fock reference is discussed.
Penning ionization electron spectroscopy of C6H6 by collision with He*(2 3S) metastable atoms and classical trajectory calculations: Optimization ofab initiomodel potentials
Authors: Yamazaki Masakazu , Maeda Satoshi , Kishimoto Naoki , Ohno Koichi
Published: January 2005

The potential energy surface of benzene (C6H6) with a He*(2 3S) atom was obtained by comparison of experimental data in collision-energy-resolved two-dimensional Penning ionization electron spectroscopy with classical trajectory calculations. The ab initio model interaction potentials for C6H6+He*(2 3S) were successfully optimized by the overlap expansion method; the model potentials were effectively modified by correction terms proportional to the overlap integrals between orbitals of the interacting system, C6H6 and He*(2 3S). Classical trajectory calculations with optimized potentials gave excellent agreement with the observed collision-energy dependence of partial ionization cross sections. Important contributions to corrections were found to be due to interactions between unoccupied molecular orbitals and the He*2s orbital. A C6H6 molecule attracts a He*(2 3S) atom widely at the region where π electrons distribute, and the interaction of −80 meV (ca. −1.8 kcal/mol) just cover the carbon hexagon. The binding energy of a C6H6 molecule and a He* atom was 107 meV at a distance of 2.40 Å on the sixfold axis from the center of a C6H6 molecule, which is similar to that of C6H6+Li and is much larger than those of the C6H6+[He,Ne,Ar] systems.
Mechanisms of the α and γ modes in radio-frequency atmospheric glow discharges
Authors: Shi J. J. , Kong M. G.
Published: January 2005

Large-volume and uniform atmospheric glow discharges are finding a vast range of processing applications, many of which have been traditionally addressed with the vacuum plasma technology. When excited at kilohertz or above, these atmospheric plasmas operate typically at low current densities below 30mA∕cm2 and often they are perceived to have very similar properties regardless of their operation conditions. Recently a radio-frequency (rf) atmospheric glow discharge was observed at high current density of up to 1A∕cm2, thus suggesting a previously overlooked and potentially different operation regime. Through a computational study of rf atmospheric glow discharges over a wide range of current density, this paper presents evidence of at least two glow modes, namely, the α mode and the γ mode. It is shown that gas ionization in the α mode is volumetric occurring throughout the electrode gap whereas in the γ mode it is dominated by localized events near the boundary between the sheath and the plasma bulk. Secondary electron emission strongly influences gas ionization in the γ mode yet matters little in the α mode. These findings suggest a wider operation range of atmospheric glow plasmas than previously believed. The contrasting dynamic behaviors of the two glow modes highlight both the potential to preferentially match the operation regime of atmospheric glow discharges to the specific requirements of their intended applications and the importance to develop diagnostics strategies appropriate for their operation regimes.
Etching of porous and solid SiO2 in Ar∕c-C4F8, O2∕c-C4F8 and Ar∕O2∕c-C4F8 plasmas
Authors: Sankaran Arvind , Kushner Mark J.
Published: January 2005

C - C 4 F 8 -based plasmas are used for selective etching of high aspect ratio (HAR) trenches in SiO2 and other dielectrics for microelectronics fabrication. Additives such as Ar and O2 are often used to optimize the process. Understanding the fundamentals of these processes is critical to extending technologies developed for solid SiO2 to porous SiO2, as used in low-dielectric constant insulators. To investigate these issues, reaction mechanisms developed for etching of solid and porous SiO2 in fluorocarbon plasmas and for etching of organic polymers in O2 plasmas have been incorporated into a feature profile model capable of addressing two-phase porous materials. The reaction mechanism was validated by comparison to experiments for blanket etching of solid and porous SiO2 in Ar∕c-C4F8 and O2∕c-C4F8 plasmas using inductively coupled plasma reactors. We found that the blanket etch rates of both solid and porous SiO2 had maxima as a function of Ar and O2 addition to c-C4F8 at mole fractions corresponding to an optimum thickness of the overlying polymer layer. Larger Ar and O2 additions were required to optimize the etch rate for porous SiO2. Whereas etch stops occurred during etching of HAR features in solid and porous SiO2 using pure c-C4F8 plasmas, Ar and O2 addition facilitated etching by reducing the polymer thickness, though with some loss of critical dimensions. Mixtures of Ar∕O2∕c-C4F8 can be used to manage this tradeoff.
Comprehensive study of the resistivity of copper wires with lateral dimensions of 100 nm and smaller
Authors: Steinhögl W. , Schindler G. , Steinlesberger G. , Traving M. , Engelhardt M.
Published: January 2005

Copper wires were prepared in a silicon oxide matrix using the methods of semiconductor manufacturing and were electrically characterized. The width of the smallest structure was 40 nm and of the largest, 1000 nm; the heights were 50, 155, and 230 nm. Many samples of each size have been measured in order to perform a systematic investigation. The resistivity of the sample was extracted using the temperature coefficient of resistance. A significant increase in the resistivity was found for the small structures (roughly a factor 2 for 50-nm width). A model based on physical parameters was used in the analysis of the electrical data and very good agreement was obtained. The sensitivity of the various model parameters obtained by a best-fit procedure to the experimental data has been investigated. The impact of width and height on the resistivity, the influence of electron scattering at grain boundaries compared to surface scattering, and the impact of grain sizes and impurities will be discussed in detail.
Effects of substrate treatment on the initial growth mode of indium-tin-oxide films
Authors: Han Younggun , Kim Donghwan , Cho Jun-Sik , Beag Young-Whan , Koh Seok-Keun , Chernysh Vladimir S.
Published: January 2005

The initial growth mode of indium tin oxide (ITO) on polycarbonate (PC) substrates was investigated. Some of the PC substrates were bombarded by 1-keV Ar ions in an oxygen environment to modify the substrate surface before ITO sputter deposition. The initial part of the film growth was transformed from a three-dimensional island growth to a two-dimensional like growth as a result of the surface treatment. The change of the growth mode was attributed to oxygen-bound functional groups newly formed on the PC surface. Models based on thermodynamic theory and on atomic kinetic approach are presented to explain the transition, respectively.
Interpolated potential energy surfaces: How accurate do the second derivatives have to be?
Authors: Crittenden D. L. , Jordan M. J. T.
Published: January 2005

A global potential energy surface for the water dimer is constructed using the modified Shepard interpolation scheme of Collins et al. According to this interpolation scheme, the energy at an arbitrary geometry is expressed as a weighted sum of Taylor series expansions from neighboring data points, where the energy and derivative data required are obtained from ab initio calculations. For some ab initio methods, errors are introduced into the second derivative matrix, either by numerical differencing of ab initio energies or numerical integration during the ab initio calculation. Therefore, we test the accuracy required of the second derivative data by truncation of the exact second derivatives to a series of approximate second derivatives, and assess the effect on the results of a quantum diffusion Monte Carlo (QDMC) simulation. Our results show that the calculated zero-point energy and wave function histograms converge to within the numerical uncertainty of the QDMC simulation by inclusion of either three significant figures or three decimal places in the second derivatives.
Structural comparisons of SiOx and Si∕SiOx formed by the exposure of silicon (100) to molecular oxygen and to hyperthermal atomic oxygen
Authors: Kisa Maja , Minton Timothy K. , Yang Judith C.
Published: January 2005

The SiOx layer and Si∕SiOx interface formed on Si(100) substrates, held at 493K, by oxidation from a beam of hyperthermal (5eV) atomic oxygen and upon exposure to thermal molecular oxygen were characterized by a variety of microcharacterization and surface science techniques. The amorphous silica formed by atomic oxygen was nearly twice as thick, more ordered (similar to a quartz structure), and more homogeneous in composition, than the oxide formed by molecular oxygen. The Si∕SiO2 interface formed by atomic oxygen was atomically abrupt and no suboxides were detected near the interface or throughout the oxide. In contrast, the Si∕SiOx interface that developed from molecular oxygen exposure was atomically rough, and a large fraction of suboxides was found near the interface. The differences in the oxide films grown by exposure to atomic and molecular oxygen are discussed in the context of a thermionic emission model of silicon oxidation.
Second order nonlinear optical properties of zinc oxide films deposited by low temperature dual ion beam sputtering
Authors: Larciprete M. C. , Passeri D. , Michelotti F. , Paoloni S. , Sibilia C. , Bertolotti M. , Belardini A. , Sarto F. , Somma F. , Lo Mastro S.
Published: January 2005

We investigated second order optical nonlinearity of zinc oxide thin films, grown on glass substrates by the dual ion beam sputtering technique under different deposition conditions. Linear optical characterization of the films was carried out by spectrophotometric optical transmittance and reflectance measurements, giving the complex refractive index dispersion. Resistivity of the films was determined using the four-point probe sheet resistance method. Second harmonic generation measurements were performed by means of the Maker fringes technique where the fundamental beam was originated by nanosecond laser at λ=1064nm. We found a relatively high nonlinear optical response, and evidence of a dependence of the nonlinear coefficient on the deposition parameters for each sample. Moreover, the crystalline properties of the films were investigated by x-ray diffraction measurements and correlation with second order nonlinearity were analyzed. Finally, we investigated the influence of the oxygen flow rate during the deposition process on both the second order nonlinearity and the structural properties of the samples.
Charge injection in doped organic semiconductors
Authors: Hosseini A. R. , Wong Man Hoi , Shen Yulong , Malliaras George G.
Published: January 2005

The influence of doping on the process of charge injection from a metal electrode into a model organic semiconductor is investigated. The contact resistance, which is the relevant figure-of-merit, is found to decrease dramatically upon doping beyond what is expected from theory and seen in crystalline semiconductors. This phenomenon is understood in terms of broadening of the transport manifold in the organic semiconductor, induced by the dopants.
Large surface potential of Alq3 film and its decay
Authors: Yoshizaki Kei , Manaka Takaaki , Iwamoto Mitsumasa
Published: January 2005

The surface potential across Alq3 [tris(8-quinolinolato)aluminum] evaporated films on a metal (Au, Al) electrode was measured by the Kelvin probe method and the results were discussed taking into account the orientational ordering of Alq3 molecules, the presence of surface charges on Alq3 films and the displacement of excess charge (electrons and holes) from the electrode into Alq3 films. The very large surface potential established in as-deposited Alq3 films was mainly due to the alignment of dipoles of Alq3 molecules, whereas the small surface potential that remained in the films after photoirradiation was due to excess charge. In order to clarify the drastic decrease of surface potential by photoirradiation, the surface potential decay was examined.
Spin reversal in small magnetic elements as a function of orientation in external magnetic field
Authors: Oriade A. A. , Chui S. T.
Published: January 2005

Monte Carlo simulations are used to investigate magnetization reversal mechanisms and temperature dependence of Hc,ψ. Hc,ψ is the switching field when the external field is directed at an angle ψ to the easy axis. ψ is defined as the acute angle the external field makes with the easy axis, positive ψ corresponding to a counter-clockwise turn from the easy axis. We find different magnetization reversal mechanisms for different ψ and show the role of edge domains. For ψ=−π∕4 and ψ=0 switching occurs by the nucleation of vortices while for ψ=+π∕4 switching occurs by a skewed stretching of the edge domains. Differences in the location and growth of the vortices are reported. Vortices can facilitate switching as evidenced by the low values of Hc,−π∕4. Comparison with known temperature dependencies of Hc,+π∕4 and Hc,0 obtained via theory, simulation and experiment shows that (i) values of Hc,−π∕4 are consistently lower than Hc,+π∕4 and Hc,0 at all temperatures considered and (ii) Hc,−π∕4 has the same linear temperature dependence as Hc,0 but different from the T2 dependence Hc,+π∕4.
Temperature dependence of polarized absorption bands in p-type CdGeAs2
Authors: Bai Lihua , Giles N. C. , Schunemann P. G.
Published: January 2005

The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2→V1 intervalence band transition, and its intensity taken with E‖c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E‖c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E‖c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems.
Lattice-dynamical calculation of phonon scattering at ideal Si–Ge interfaces
Authors: Zhao H. , Freund J. B.
Published: January 2005

Detailed phonon scattering at an ideal Si–Ge interface is studied with a linear lattice dynamics model. Frequency dependent transmission coefficients indicate the significance of acoustic-optical phonon mode conversion at the interface. Applied to multiple interfaces, the method shows how the overall thermal resistance approaches a finite (Bloch mode) limit with the increasing number of interfaces in absence of other scattering mechanisms. The dependence of thermal resistance on the superlattice layer thickness is not significant even in the interface-scattering-only limit we study. We also assess errors incurred by the finite domain size and classical statistics in molecular dynamics simulations of interface thermal resistance. Results suggest that using 6×6 unit cells in the transverse directions, a tractable size for such simulations, will incur only a 5% error in the predicted thermal resistance. Similarly, the error due to the classical (Boltzmann) phonon distribution in molecular dynamics simulations is predicted to be less than 10% for temperatures above 300 K.
Room temperature oxidation kinetics of Si nanoparticles in air, determined by x-ray photoelectron spectroscopy
Authors: Yang D.-Q. , Gillet Jean-Numa , Meunier M. , Sacher E.
Published: January 2005

The air oxidation kinetics of low coverages of ∼5nm Si nanoparticles, deposited by pulsed excimer laser ablation (KrF, 248 nm) in He, have been characterized by x-ray photoelectron spectroscopy. A simple model, based on the evolution of the Si 2p spectral components during oxidation, has been developed to determine the nanoparticle oxide thickness. It is found that the short-term oxide thickness is greater, and the long-term room-temperature air oxidization rate of these nanoparticles is less, than those reported for bulk a-Si and c-Si. The results are also consistent with an earlier transmission electron microscope observation of the oxidation of larger Si particles at higher temperatures. The greater short-term oxide thickness may be attributed to surface defects on the prepared Si nanoparticles, and lower long-term oxidation rate is due to the nonlinear decrease of oxygen diffusion in spherical systems.
Direct observation of deep electron traps in aged organic light emitting diodes
Authors: Kondakov D. Y.
Published: January 2005

Despite the importance of the operational stability of organic light emitting diodes (OLEDs) for commercial application, relatively little is known about the intrinsic mechanism of OLED “aging”—gradual loss of electroluminescence efficiency in devices operated for extended periods of time. We have previously found that OLED aging results in the formation of deep hole traps at or near the electron transport layer∣hole transport layer interface. We now report that the aging of an archetypal anode ∣NPB∣Alq∣ cathode OLED device also creates deep electron traps in the vicinity of the recombination zone, which can be selectively populated via short irradiation of an aged device with white light. These traps appear to be weakly emissive, with the estimated luminescence efficiency being 100–150× smaller than the efficiency of Alq. Short irradiation with white light results in the charging of these electron traps, placing the aged OLED device into a state with nearly zero internal charge. The subsequent driving of an electric current results in (i) an “early” electroluminescence pulse, (ii) discharging of the electron traps, and (iii) placing the aged OLED into a state with substantial positive internal charge. Although the chemical nature of these traps remains elusive, the redshifted, broad emission spectrum is consistent with the formation of Alq degradation products as a result of electrical aging. An accumulation of such electron traps can be envisioned to result in the luminance efficiency loss via two principal mechanisms: (i) the direct recombination of a mobile hole and trapped electron yielding a nearly nonemissive excited state, and (ii) long-range energy transfer from the OLED emitter molecule to the unfilled electron trap molecule, yielding the same nearly nonemissive excited state.
Spin filtering in a two-dimensional electron gas device with asymmetric spatially spread magnetic-electric barriers
Authors: Jalil M. B. A.
Published: January 2005

We calculate, based on the effective mass Hamiltonian, the spin dependence of ballistic transport through a two-dimensional electron gas (2DEG) under the influence of an electric U and a pair of magnetic B barriers with finite spatial width. The spin polarization of electron transmission P is strongly dependent on the degree of asymmetry in the magnetic barriers. An asymmetry in the B barrier widths and heights which conserves the magnetic vector potential A along the conduction path leads to a low P of less than 1% in the high transmission region. If conservation of A is broken, much higher P values of up to 30% is achievable but at the cost of increasing the energy threshold Eth. Finally, we suggest a scheme which utilizes a thin break in the 2DEG conduction path to achieve a comparable modulation of P of 25%, while conserving A and maintaining Eth at moderate electron energies.
Exciton dynamics of GaSe nanoparticle aggregates
Authors: Tu H. , Mogyorosi K. , Kelley D. F.
Published: January 2005

Time-resolved and static spectroscopic results on GaSe nanoparticle aggregates are presented to elucidate the exciton relaxation and diffusion dynamics. These results are obtained in room-temperature TOP/TOPO solutions at various concentrations. The aggregate absorption spectra are interpreted in terms of electrostatic coupling and covalent interactions between particles. The spectra at various concentrations may then be interpreted in terms of aggregate distributions calculated from a simple equilibrium model. These distributions are used to interpret concentration-dependent emission anisotropy kinetics and time-dependent emission spectral shifts. The emission spectra are reconstructed from the static emission spectra and decay kinetics obtained at a range of wavelengths. The results indicate that the aggregate z axis persistence length is about 9 particles. The results also show that the one-dimensional exciton diffusion coefficient is excitation wavelength dependent and has a value of about 2×10−5 cm2/s following 406 nm excitation. Although exciton diffusion results in very little energy relaxation, subsequent hopping of trapped electron/hole pairs occurs by a Forster mechanism and strongly red shifts the emission spectrum.
Pore formation in fluctuating membranes
Authors: Farago Oded , Santangelo Christian D.
Published: January 2005

We study the nucleation of a single pore in a fluctuating lipid membrane, specifically taking into account the membrane fluctuations, as well as the shape fluctuations of the pore. For large enough pores, the nucleation free energy is well-described by shifts in the effective membrane surface tension and the pore line tension. Using our framework, we derive the stability criteria for the various pore formation regimes. In addition to the well-known large-tension regime from the classical nucleation theory of pores, we also find a low-tension regime in which the effective line and surface tensions can change sign from their bare values. The latter scenario takes place at sufficiently high temperatures, where the opening of a stable pore of finite size is entropically favorable.
Isotropic periodic sum: A method for the calculation of long-range interactions
Authors: Wu Xiongwu , Brooks Bernard R.
Published: January 2005

This work presents an accurate and efficient approach to the calculation of long-range interactions for molecular modeling and simulation. This method defines a local region for each particle and describes the remaining region as images of the local region statistically distributed in an isotropic and periodic way, which we call isotropic periodic images. Different from lattice sum methods that sum over discrete lattice images generated by periodic boundary conditions, this method sums over the isotropic periodic images to calculate long-range interactions, and is referred to as the isotropic periodic sum (IPS) method. The IPS method is not a lattice sum method and eliminates the need for a reciprocal space sum. Several analytic solutions of IPS for commonly used potentials are presented. It is demonstrated that the IPS method produces results very similar to that of Ewald summation, but with three major advantages, (1) it eliminates unwanted symmetry artifacts raised from periodic boundary conditions, (2) it can be applied to potentials of any functional form and to fully and partially homogenous systems as well as finite systems, and (3) it is more computationally efficient and can be easily parallelized for multiprocessor computers. Therefore, this method provides a general approach to an efficient calculation of long-range interactions for various kinds of molecular systems.
Electron tunneling dynamics in anharmonic bath
Authors: Dahnovsky Yuri
Published: January 2005

Tunneling transition probability for a particle interacting with an anharmonic bath is found in a time-dependent Hartree approximation. The general expression is presented in terms of medium Keldysh functions that are assumed to be known. Furthermore, the transition probability is calculated in the noninteracting-blip approximation where the rate constant does not exhibit an activation dependence at high temperatures. The reorganization energy Er and the renormalized reaction heat ε̃ are experessed in terms of the correlation matrix for a solvent and internal modes in both quantum and classical regimes. It is shown that Er and ε̃ are temperature dependent.
Extended-range order, diverging static length scales, and local structure formation in cold Lennard-Jones fluids
Authors: Whitford Paul C. , Phillies George D. J.
Published: January 2005

We report molecular-dynamics simulations on a three-dimensional, two-component Lennard-Jones fluid. We used 125 000 particles (equal numbers of A and B) at density N/V=1.29 and 34 temperatures T covering 5×104⩾T⩾0.56. The pair potential was 4ε[(σij/r)12−(σij/r)6] with σAA=1, σAB=11/12, and σBB=5/6. We computed specific and generic radial distribution functions gij(r), and several density-momentum dynamic correlation functions whose static (t=0) parts vanish by symmetry. Evidence is presented that our systems were adequately annealed to eliminate remnant initial order and were adequately equilibrated at each temperature. Static spatial correlations in cold Lennard-Jones liquids have longer ranges than are often reported: g(r)−1≠0 is found out to r⩾7 at T=2 and out to r⩾10 at T=0.56. |g(r)−1| has an envelope function that simultaneously fits both crests and troughs of g(r). The envelope function implies a temperature-dependent static length scale l1; over (0.56⩽T⩽100), l1∼T−0.3, contrary to suggestions that g(r) is temperature independent as the glass is approached. The highest-melting-point crystal that we identified melts at Tm≈1.08. In the fluid phase, we observe short-range noncrystalline local structure formation in g(r) as the glass is approached. Local structure is only found below a local structure melting temperature Tmc=2.0. Local structure vanishes above T=2. Local structure becomes more pronounced as temperature is reduced. However, at all temperatures at which there is local structure in g(r), the local structure is confined to r⩽4. Within the region r⩽4, the amplitude of the local structure diminishes with distance r from the central atom approximately as exp(−r/l2), thereby defining a second distance scale in the fluid. l2, while more difficult to measure, appears to scale with temperature as l2∼T−0.6; l2 is not the same as l1. The static and dynamic properties of the local structure match properties assigned by Kivelson’s glass model [S. A. Kivelson et al., J. Chem. Phys. 101, 2391 (1994)] to that model’s frustration-limited local clusters.
On the microscopic theory of polar solvation dynamics
Authors: Matyushov Dmitry V.
Published: January 2005

A microscopic model of the time-resolved Stokes shift is developed. The model calculates the solvation dynamics by combining the atomic resolution of the solute structure with dipolar dynamics from the polarization structure factors of the homogeneous solvent. Calculations are made for coumarin 153 and quinoxaline optical dyes with atomic geometries and charge distributions taken from quantum calculations. Stokes shift dynamics is calculated and compared to experiment in high-temperature acetonitrile and methanol and in low-temperature 2-methyl-tetrahydrofurane using dielectric relaxation data from experiment.
Phase separation of saturated and mono-unsaturated lipids as determined from a microscopic model
Authors: Elliott R. , Katsov K. , Schick M. , Szleifer I.
Published: January 2005

A molecular model is proposed of a bilayer consisting of fully saturated dipalmitoylphosphatidylcholine (DPPC) and mono-unsaturated dioleoylphosphatidylcholine (DOPC). The model not only encompasses the constant density within the hydrophobic core of the bilayer, but also the tendency of chain segments to align. It is solved within self-consistent field theory. A model bilayer of DPPC undergoes a main-chain transition to a gel phase, while a bilayer of DOPC does not do so above zero degrees centigrade because of the double bond which disrupts order. We examine structural and thermodynamic properties of these membranes and find our results in reasonable accord with experiment. In particular, order-parameter profiles are in good agreement with NMR experiments. A phase diagram is obtained for mixtures of these lipids in a membrane at zero tension. The system undergoes phase separation below the main-chain transition temperature of the saturated lipid. Extensions to the ternary DPPC, DOPC, and cholesterol system are outlined.
The proper structure of cubic ice confined in mesopores
Authors: Morishige Kunimitsu , Uematsu Hiroaki
Published: January 2005

To examine the reason for the formation and the structure of cubic ice in a restricted space, we measured the powder x-ray diffraction patterns of cubic ice formed within the mesopores of porous silicas as a function of pore size (4–70 nm). The results strongly suggest that cubic ice formed in the mesopores does not take a cubic structure as envisaged by König. It may be actually composed of very small crystallites of hexagonal ice that contains a large amount of growth faults depending on the crystallite size, that is, ice with disordered stacking sequence. Suppression of crystal growth of ice in the mesopores seems to be a vital factor for the formation and the stability of cubic ice.
Calculations of near-threshold cross sections for photodissociation of CH+ using the Lanczos algorithm
Authors: Bouakline F. , Grozdanov T. P. , Andric L. , McCarroll R.
Published: January 2005

We combine the Lanczos algorithm with the absorbing-potential method, implemented in a discrete variable representation to calculate the near-threshold photodissociation cross sections of CH+. The method is iterative, based on a continued fraction representation of the Green function and avoids any explicit matrix diagonalization. A very good agreement is found with experiment and close-coupling calculations.
Nickel solubility in intrinsic and doped silicon
Authors: Istratov A. A. , Zhang P. , McDonald R. J. , Smith A. R. , Seacrist M. , Moreland J. , Shen J. , Wahlich R. , Weber E. R.
Published: January 2005

Solubility of nickel in intrinsic, moderately, and heavily doped n-type and p-type silicon was determined using instrumental neutron activation analysis. The solubility data for intrinsic silicon were found to be in good agreement with the literature data. In heavily doped p-type silicon the enhancement of nickel solubility, if present, was close to the error margins of the experiment, indicating that interstitial nickel is predominantly neutral in silicon and suggesting that its donor level lies close to the valence band edge, if not within the valence band itself. No dependence of nickel solubility on doping level of the samples was observed in n-type silicon. This is consistent with the model reported in the literature of two acceptor levels of substitutional nickel located in the upper half of the band gap, one of the levels close to the conduction band edge. Consequently, unlike copper or iron, nickel does not segregate in heavily p-type or n-type doped areas of silicon wafers.
Measurement accuracy analysis of photocarrier radiometric determination of electronic transport parameters of silicon wafers
Authors: Li Bincheng , Shaughnessy Derrick , Mandelis Andreas
Published: January 2005

Simulations are performed to investigate the accuracy of the simultaneous determination of the electronic transport properties (the carrier lifetime, the carrier diffusion coefficient, and the front and rear surface recombination velocities) of silicon wafers by means of the photocarrier radiometry (PCR) technique through fitting frequency-scan data to a rigorous model via a multi-parameter fitting process. The uncertainties of the fitted parameter values are analyzed by calculating the dependence of the square variance including both amplitude and phase variances on the electronic transport properties. Simulation results show that the ability of the PCR to accurately determine carrier lifetimes gradually decreases for lifetimes longer than roughly 100 microseconds. In case the carrier diffusion coefficient is previously known, the carrier lifetime and front surface recombination velocity can be determined with uncertainties approximately ±20% or less. Experiments with an ion-implanted silicon wafer were performed and the carrier lifetime and front surface recombination velocity were determined with estimated uncertainties approximately ±30% and ±15%, respectively.
GaAs surface cleaning by low-energy hydrogen ion bombardment at moderate temperatures
Authors: Schubert E. , Razek N. , Frost F. , Schindler A. , Rauschenbach B.
Published: January 2005

Low-energy hydrogen ion beam treatment is used to remove contaminations of carbon and oxygen from gallium arsenide (GaAs) surfaces. Real time spectroscopic ellipsometry is utilized to monitor the time evolution of the in situ ellipsometric parameters ⟨ε1⟩ and ⟨ε2⟩ during exposure of GaAs surfaces to inert H2 molecules, hydrogen plasma radicals, and hydrogen ions with energies up to Eion=400eV. Model analysis of the ellipsometric data allows the determination of both, the oxide layer degradation and the hydrogen-induced dielectric function alteration of GaAs. The degradation of the oxide layer as well as the changes of the dielectric function of GaAs depends on the surface temperature and hydrogen ion energy. Ellipsometry reveals oxygen clean surfaces for hydrogen ion bombardment with an energy of Eion=300eV at temperatures as low as 150°C, which is confirmed by in situ x-ray photoelectron spectroscopy measurements. The incorporation behavior of hydrogen and therefore the dielectric function alteration depends on the doping type of GaAs. In hydrogen-modified semi-insulating GaAs the observed energy redshift of the critical points E1 und E1+Δ1 is mainly caused by the increase of the intrinsic stress level, whereas in p-type GaAs additionally surface band bending effects are responsible for the change of the dielectric function of GaAs.
Short range interaction in tunneling near-field optical microscopy
Authors: Flaxer Eli , Palachi Eldad
Published: January 2005

We use a modulation technique to detect a short-range interaction between a probe and a sample of a tunneling near-field optical microscope. This interaction might explain the resolution enhancement of the apertureless near-field techniques compared with conventional aperture near-field ones. To detect this interaction we measure approach curves of the modulating probe towards the sample and fit the experimental data with a simulation of the intensity, where a coupled dipoles interaction is added to the conventional near-field interaction. While the effect of the coupled dipoles interaction is negligible in conventional near-field microscopes, it dominates the modulated signal at scanning distances. Thus better resolution can be achieved, even in an aperture near-field microscope, by vibrating the tip vertically over the sample.
Magnetic metal-base transistor with organic emitter
Authors: Meruvia M. S. , Munford M. L. , Hümmelgen I. A. , da Rocha A. S. , Sartorelli M. L. , Pasa A. A. , Schwarzacher W. , Bonfim M.
Published: January 2005

We have fabricated a magnetic metal-base transistor using particularly simple processes. The emitter is organic, consisting of evaporated C60 or tris(8-hydroxiquinoline) aluminum, the base is an electrochemically deposited Co∕Cu∕Co multilayer, and the collector is the n-Si substrate. The collector current measured in the common-base configuration increases significantly in the presence of an applied field.