bluuebunny/crossref_metadata_2025 huggingface dataset:
Catalytic graphitization and Ohmic contact formation on 4H–SiC
Authors: Lu Weijie , Mitchel W. C. , Landis G. R. , Crenshaw T. R. , Collins W. Eugene
Published: April 2003

Electrical contact properties and graphitic structures of metal/carbon/4H–SiC structures are investigated. Metals studied include Ni, Co, Cr, NiCr, Ti, W, Mo, Al, and Au. Ohmic contacts are formed on Ni/C, Co/C, Cr/C, and NiCr/C films on 4H–SiC with n-type, C-face, and a doping concentration of 1.8×1019 cm−3 . Only Ni/C and Co/C films exhibit Ohmic contact behavior on SiC with n-type, Si-face, and a doping concentration of 1.6×1018 cm−3. Ni and Co are well known as excellent graphitization catalysts. Raman spectra show that the formation of graphitic carbon is related to the formation of Ohmic contacts in the annealed metal/carbon/SiC structures. Generally accepted catalytic graphitization mechanisms are applied to explain the scanning electron microscopy images, which demonstrate a relationship between the catalytically reacted morphology and Ohmic contact behavior. This study provides evidence that the metals with better catalytic graphitization activities form better Ohmic contacts on metal/carbon/SiC structures.
Dielectric properties of carbon fiber filled low-density polyethylene
Authors: Dang Zhimin , Shen Yang , Fan Lizhen , Cai Ning , Nan Cewen , Zhao Shujin
Published: April 2003

Carbon fiber (CF) filled low-density polyethylene (LDPE) matrix composites was prepared by means of the Hakke driver. The dielectric properties of the CF/LDPE composites as a function of the frequency and the volume fraction of CF were studied. The dielectric constants decrease slowly with increasing the frequency and rise gradually with increasing the CF contents in the composites. As the volume fraction of the CF is up to 0.20, an abrupt increase in the dielectric constant occurs, and accordingly electrical conductivity measured at low frequency increases rapidly at this volume fraction. The percolation theory and the simple concept of polarization in the capacitors are employed to explain these experimental results.
Phonon absorption by superconducting tunnel junction x-ray detectors
Authors: Wigmore J. K. , Steele A. C. , Kozorezov A. G. , Peacock A. , den Hartog R. , Verhoeve P.
Published: April 2003

Pulses of nonequilibrium phonons (heat pulses) have been used to mimic the absorption of x-ray photons in superconducting niobium tunnel junctions. For device characterization, the technique provides a valuable alternative to photoabsorption with good time resolution and continuous variability of absorbed energy. In addition, excitation is uniform across the tunnel junction, so that the effects of quasiparticle diffusion can be neglected in the analysis, and hence, the Rothwarf–Taylor equations solved exactly. Consistency is obtained between device parameters obtained from phonon measurements and those inferred from modeling of photoabsorption. In addition, the quasiparticle recombination rate can be determined directly from the nonlinearity of the energy response.
Microstructure and open-circuit voltage of n−i−p microcrystalline silicon solar cells
Authors: Bailat J. , Vallat-Sauvain E. , Feitknecht L. , Droz C. , Shah A.
Published: April 2003

A series of microcrystalline silicon n−i−p solar cells has been deposited by very high frequency plasma enhanced chemical vapor deposition at various values of silane to hydrogen source gas ratio and on two different substrate types. Relationships between microstructure and electrical characteristics of these solar cells are investigated by transmission electron microscopy, atomic force microscopy, and I(V) measurements. A mixed phase (so-called heterophase) layer consisting of amorphous plus microcrystalline material is observed at the bottom of the solar cell and identified here as one of the key microstructural features of the device: the relationship between the crystalline nuclei density and the heterophase layer thickness is presented as well as its relationship with the open-circuit voltage (Voc). The effects of substrate roughness and of silane to hydrogen gas ratio used for the fabrication of the device on the heterophase layer are evidenced. These observations underline the importance of the first stages of growth of the intrinsic (i) layer for the fabrication of high- Voc n−i−p microcrystalline solar cells.
A study of electronic and bonding properties of Sn doped Lin clusters and aluminum based binary clusters through electron localization function
Authors: Shetty Sharan , Pal Sourav , Kanhere D. G.
Published: April 2003

We have carried out a thorough investigation of bonding and energetics in two different class of heteroatomic clusters viz. single impurity based LinSn (n⩽9) clusters and mixed aluminum-based binary clusters Al4X4 (X=Be, Mg, B, Si). We employ first principle Born–Oppenheimer molecular dynamics within the framework of density functional theory using generalized gradient approximation. The bonding characteristics have been analyzed using electron localization function (ELF) along with charge density and valence molecular orbital pictures. The analysis of bonding through ELF reveals that in LinSn (n⩽9) clusters there is a transition from ionic bond to a metallic bond through an intermediate ionic–metallic bond. On the other hand, it is found that Al4X4 (X=Be, Mg, B, Si) clusters show interesting characteristics of lone pairs on Al, polar covalent bonding, and a multicenter bonding. We demonstrate that ELF can play an important role in analyzing different types of bonding characteristics in heteroatomic clusters.
Sub-picosecond shock interferometry of transparent thin films
Authors: McGrane S. D. , Moore D. S. , Funk D. J.
Published: April 2003

Ultrafast spatial interferometric measurements of shock dynamics in transparent thin films exhibit phase shifts caused by both surface motion and the interference of multiple reflections off the moving shock wave interface. The interference effects are strong perturbations on the phase shift, which do not allow independent measurement of surface motion. Calculations of the time dependent phase shift that include reflective surface motion, shock wave transit through the transparent thin film, and thin film interference effects are shown to match experimental measurements in 625-nm-thick films of polymethylmethacrylate (PMMA) shocked to 19 GPa. Interferometric data obtained at two angles of incidence and two polarizations were sufficient to uniquely determine the PMMA shocked refractive index, shock speed, and particle velocity. Interferometric results as a function of shock strength, 2–20 GPa, suggest that submicron PMMA films have essentially the same material response to shock loading (Hugoniot) as macroscopic samples.
Direct measurements of magnetocaloric effect in the first-order system LaFe11.7Si1.3
Authors: Hu F. X. , Ilyn Max , Tishin A. M. , Sun J. R. , Wang G. J. , Chen Y. F. , Wang F. , Cheng Z. H. , Shen B. G.
Published: April 2003

The magnetocaloric effect was investigated in LaFe11.7Si1.3, which undergoes a first-order transition at ∼188 K from the ferromagnetic to paramagnetic state. The magnetic entropy change upon a field increase from 0 to 5 T is as large as 29 J/kg K (212 mJ/cm3 K). The adiabatic temperature change obtained via direct measurements reaches 4 K under a field change from 0 to 1.4 T. The large values of entropy change and adiabatic temperature change confirmed the large potential of present compound LaFe11.7Si1.3 as a magnetic refrigerant in the corresponding temperature range.
Structure evolution in electrorheological and magnetorheological suspensions from a continuum perspective
Authors: von Pfeil Karl , Graham Michael D. , Klingenberg Daniel J. , Morris Jeffrey F.
Published: April 2003

A two-fluid continuum model is developed to describe mass transport in electro- and magnetorheological suspensions. The particle flux is related to the field-induced and hydrodynamic stresses. Solutions of the resulting mass balance show column formation in the absence of flow and stripe formation when a suspension is subjected simultaneously to an applied electric field and shear flow.
Determination of nanostructures and mechanical properties on the surface of molybdenum dithiocarbamate and zinc dialkyl-dithiophosphate tribochemical reacted films using atomic force microscope phase imaging technique
Authors: Ye Jiping , Kano Makoto , Yasuda Yoshiteru
Published: April 2003

Nanostructures and mechanical properties on the surface of two kinds of tribofilm formed from zinc dialkyl-dithiophosphate (ZDDP) and molybdenum dithiocarbamate (MoDTC) additives, which exhibited obviously different friction coefficients in a pin-on-disc test, were determined by using an atomic force microscopy (AFM) phase imaging technique. The level of interactive force between the tip and sample was modulated for distinguishing well-defined structures and mechanical properties of individual components not only on the uppermost surface but also in the underlying area near the surface in the AFM tapping mode. It was found that the MoDTC/ZDDP tribofilm possessed a lower surface modulus than the ZDDP film in the elastic deformation range. Most importantly, nanostrips oriented in the sliding direction were found in the MoDTC/ZDDP tribofilm at a depth of around 10 nm from the surface. These nanostrips possessed lower shearing stress than the surface matrix and formed the inner skin layer, which exhibited lower friction behavior than that of the ZDDP tribofilm. These results agreed with our recent nanoindentation and nanoscratch measurements for estimating the mechanical and frictional properties of MoDTC/ZDDP and ZDDP tribofilms. These findings and previous surface analytical results suggest that the nanostrips act as a type of solid lubricant, such as MoS2 single sheets, to lower the boundary friction coefficient.
Direct calculation of the crystal–melt interfacial free energies for continuous potentials: Application to the Lennard-Jones system
Authors: Davidchack Ruslan L. , Laird Brian B.
Published: April 2003

Extending to continuous potentials a cleaving wall molecular dynamics simulation method recently developed for the hard-sphere system [Phys. Rev. Lett. 85, 4751 (2000)], we calculate the crystal–melt interfacial free energies, γ, for a Lennard-Jones system as functions of both crystal orientation and temperature. At the triple point, T*=0.617, the results are consistent with an earlier cleaving potential calculation by Broughton and Gilmer [J. Chem. Phys. 84, 5759 (1986)], however, the greater precision of the current calculation allows us to accurately determine the anisotropy of γ. From our data we find that, at all temperatures studied, γ111<γ110<γ100. A comparison is made to the results from our previous hard-sphere calculation and to recent results for Ni by Asta, Hoyt, and Karma [Phys. Rev. B 66 100101(R) (2002)].
Ultrafast dichroism spectroscopy of anthracene in solution. IV. Merging of inertial and diffusive motions in toluene
Authors: Zhang Yunhan , Jiang Jianwen , Berg Mark A.
Published: April 2003

The division of liquid dynamics into inertial (viscosity independent) and diffusive (viscosity dependent) components is followed to low viscosity. The previous papers in this series [J. Chem. Phys. 115, 4212 (2001); 115, 4223 (2001); 115, 4231 (2001)] found well distinguished inertial rotation, diffusive solvation and diffusive rotation of anthracene in benzyl alcohol over a range of moderate viscosities (2.7–14.4 cP). In this paper we extend those measurements to a lower viscosity range (0.55–0.82 cP) in toluene. Vibrational dynamics are almost entirely eliminated by the choice of solute and laser wavelength. The slow rotational decay component behaves normally for a diffusive process, i.e., the rotation time is linear in the viscosity. The shorter dynamics can be modeled as a poorly resolved combination of inertial rotation and diffusive solvation, but the fit solvation times are approximately a factor of two smaller than expected. This result is interpreted as a symptom of the breakdown of the inertial/diffusive distinction at intermediate time and low viscosity. The possibility that solvation and rotation become mixed under these conditions is discussed. In the <100 fs range, a very large signal is found. This peak is clearly too large and too broad to be explained by models including only two resonant electronic states. This system presents an example where these models are inadequate to deconvolve inertial solvation effects that are on a time scale similar to the pulse widths.
Influence of space charge on gate control of electron waveguide Y-branch switches in the coherent regime
Authors: Forsberg Erik
Published: April 2003

The influence of space charge on gate control of switching in electron waveguide Y-branch switches operating in the single-mode coherent regime is discussed. A previously reported model based on a single switch parameter to this end is reexamined. Self-consistent computer simulations are presented and the results are discussed with reference to the single-parameter model and earlier experiments.
Theory of carrier-density-fluctuation-induced transport noise in metal-oxide-semiconductor field-effect transistors
Authors: Masaoka Akira , Sumino Daijiro , Omura Yasuhisa
Published: April 2003

An advanced theoretical approach to understanding the carrier-density fluctuation noise in n-channel metal-oxide-semiconductor field-effect transistors in the nonequilibrium steady state is introduced. Starting with the current continuity condition, a partial differential equation for carrier-density fluctuation is, on the basis of the drift-diffusion current model, derived from the charge conservation equation. The simulated noise spectral density characteristics determined for two different diffusion constant values are compared. It is demonstrated that the transport noise is suppressed compared to the case in which only the drift current component is considered when deriving a theoretical formulation of transport noise. The suppression of carrier-density fluctuation power can be explained by the interference of the forward and backward propagating carrier-density fluctuation power, which is reasonable on the basis of quantum mechanical and statistical physics. Drain current noise power increases with drain voltage as expected. However, it is shown that the power is suppressed compared to the result of a past simulation that considered only the drift current component. It is also shown that the fluctuation power of drain current decreases with gate voltage; it is clearly demonstrated that this characteristic is strongly influenced by carrier mobility.
Thermal conductivity in large residual resistance ratio MgB2 wire
Authors: Pope A. L. , Tritt T. M. , Canfield P. C. , Bud’ko S. L. , Liebenberg D. H.
Published: April 2003

We measured the thermal conductivity in MgB2 wires, with Tc=39.5 K, in both the superconducting and normal state regimes between 10–70 K. As expected, the thermal conductivity is increased for this sample, which has a residual resistance ratio (RRR) of about 32, as compared with earlier measurements on bulk samples with much lower RRR values. Over the measured temperature range, this more metallic material exhibits a temperature dependence of the thermal conductivity in the superconducting state that is different from that in previous reports and can be ascribed primarily to defect scattering. Unusual results are presented for the magnetic field dependence of the thermal conductivity around Tc for fields up to 9 T.
Monitoring precipitation rates using γ rays from adsorbed radon progeny as tracers
Authors: Greenfield M. B. , Domondon A. T. , Tsuchiya S. , Tomiyama M.
Published: April 2003

A model correctly predicting the observed atmospheric γ radiation from radon progeny adsorbed onto the surface of rain and snow enables monitoring of precipitation rates to better than 0.3 mm/h. This model predicts that the surface (volume) deposition of radon progeny yields γ-ray flux rates proportional to the 2/5 (3/5) power of rain rates, respectively. The γ-ray rates observed were proportional to the 0.45 power for rain, but for snow which does not form spherical droplets a linear relationship was observed with proportionality constant dependent upon snow type. Atmospheric γ radiation was measured at one site in Mitaka, Japan since 1999 and more recently at another site a distance 220 m from the first using 12.9 cm3 NaI detectors. Correlation coefficients ranging from 0.85 to 0.95 and improving with elevation were obtained when data were fit with predominantly surface adsorption model and an age correction factor for the 30 min half-life of dominant radon progeny. Measurement of atmospheric γ radiation from radon progeny naturally adsorbed by rain at 20 m above ground (moderate building height) may serve as a reliable, live monitor of precipitation rates limited only by counting statistics. The regional applicability of this technique has been confirmed and will most likely have global and remote applicability.
Monte Carlo simulations using sampling from an approximate potential
Authors: Gelb Lev D.
Published: April 2003

A simulation algorithm is proposed in which the potential energy function used in a Monte Carlo simulation is replaced with one that is less expensive to evaluate, coupled with a correction step based on the difference between the two potentials. This can result in a substantial reduction in computational cost. A formal derivation of the appropriate sampling criteria is given, as well as estimates of the possible improvements in code performance. The method is demonstrated on the Lennard-Jones fluid at several state points, where speedups of as much as fourfold are achieved with negligible loss in precision.
Glass transition phenomena in the crystalline phase of hexa-substituted benzenes
Authors: Shahin Md. , Murthy S. S. N.
Published: April 2003

Some hexa-substituted benzenes are known to exhibit molecular rotation in their crystalline phase, but this phenomenon is not very well understood. In this context, we have critically examined two materials, namely pentachloronitrobenzene and 2,3,4,5,6-pentabromotoluene over a wide range of temperatures, using dielectric spectroscopy and differential scanning calorimetry (DSC) to ascertain the nature of the above said relaxation. A relaxation process is found in the crystalline phase, which is slightly non-Arrhenius in its temperature dependence in the former and deviation from Debye behavior has been found in both the samples. This behavior is similar to that of the primary (or α-) relaxation found in glass forming materials. This phenomenon is found to correspond to a steplike (small) change in the specific heat in the DSC measurements indicating that it is a glass transition phenomenon. The sub-Tg relaxation data do not indicate any secondary (or β-) process (of measurable magnitude). However, the same materials when studied in small quantities in the liquid matrix of o-terphenyl (nonpolar) indicated a considerable magnitude for the β-process of molecules even below the liquid Tg of these mixtures. The results are discussed in the context of the glass transition phenomena of the above materials.
Nuclear magnetic resonance and dielectric spectroscopy of a simple supercooled liquid: 2-methyl tetrahydrofuran
Authors: Qi F. , El Goresy T. , Böhmer R. , Döß A. , Diezemann G. , Hinze G. , Sillescu H. , Blochowicz T. , Gainaru C. , Rössler E. , Zimmermann H.
Published: April 2003

The small-molecule glass former methyl tetrahydrofuran (MTHF) was investigated using dielectric spectroscopy, spin-lattice relaxometry, multidimensional stimulated-echo nuclear magnetic resonance techniques, and field gradient diffusometry. We show experimentally that MTHF nicely fits into the pattern of related small-molecule glass-forming liquids, including the existence of a high-frequency contribution to the dielectric loss, the appearance of a pronounced translational enhancement, the dominance of small average rotational jump angles, and the existence of short-lived dynamical heterogeneity.
Microscopic observation and in situ Raman scattering studies on high-pressure phase transformations of a synthetic nitrogen hydrate
Authors: Sasaki Shigeo , Hori Shinsuke , Kume Tetsuji , Shimizu Hiroyasu
Published: April 2003

Visual observations through a microscope and in situ Raman scattering measurements of a synthesized nitrogen hydrate have been performed at pressures up to 6 GPa and 296 K. High-pressure transformations have been found at 0.85 and 1.45 GPa. The cubic structure II (sII) nitrogen hydrate initially transforms to the hexagonal structure (sH) at 0.85 GPa and finally forms the orthorhombic dihydrate (sO) above 1.45 GPa. The sO phase of nitrogen hydrate exists up to at least 6 GPa. A variety of Raman spectra composed of three peaks have been sometimes observed in sII phase below 0.50 GPa, which implies that the guest nitrogen molecules doubly occupy the large hexakaidecahedron cages. Two Raman bands of the guest nitrogen vibrations with nearly equal intensities appearing in sH phase suggest that five nitrogen molecules are filling in extra large icosahedron cages.
Response to “Comment on ‘Transport equations for concentrated electrolyte solutions: Reference frame, mutual diffusion’ ” [J. Chem. Phys. 118, 8114 (2003)]
Authors: Dufrêche J.-F. , Bernard O. , Jardat M. , Turq P.
Published: April 2003

The derivation criticized by Felderhof in the preceding Comment is clarified. The result is not changed. The connection with Brownian dynamics simulation is also examined.
The water–hydroxyl radical complex: A matrix isolation study
Authors: Engdahl Anders , Karlström Gunnar , Nelander Bengt
Published: April 2003

The water–hydroxyl radical complex was prepared by irradiating peroxy radicals in hydrogen-doped argon matrices. The low water content of the matrices made it possible to observe the fundamental bands of the complexed water molecule. The experimental results are compared with the results from ab initio calculations. The complex rotates around the O–O axis in the matrix.
Tunneling currents in long-distance electron transfer reactions. V. Effective one electron approximation
Authors: Stuchebrukhov Alexei A.
Published: April 2003

In this paper we further develop the formalism of tunneling currents for the description of the tunneling transition in long-distance bridge-mediated electron transfer reactions introduced in our previous work [A. A. Stuchebrukhov, Adv. Chem. Phys. 118, 1 (2001)]. Here we consider the Hartree–Fock picture of electron tunneling in a many-electron system and, based on the corresponding orbitals analysis of the tunneling process, we introduce an effective one-electron approximation. In this picture, the (electron or hole) tunneling is described by a single pair of orbitals, as in a true one-electron theory, yet all the polarization and exchange effects characteristic for many-electron treatment are retained in it. The pair of tunneling orbitals, which is different from the usual HOMO’s of the donor and acceptor complexes, is found in a self-consistent way in a special orthogonalization procedure. This picture results in much simplified formulas for current density and for interatomic currents, as well as for the tunneling matrix element, and provides a simplified way of thinking about electron tunneling in many-electron systems.
Laser–plume dynamics during excimer laser nitriding of iron
Authors: Han M. , Lieb K.-P. , Carpene E. , Schaaf P.
Published: April 2003

On the time scale of tens to hundreds of nanoseconds, high intensity pulsed excimer laser irradiation of iron in nitrogen atmosphere produces thin iron nitride layers with high nitrogen concentration. The laser plasma, or laser plume, which plays a crucial role in the complicated interactions within the laser–plasma–metal system, depends strongly on the ambient nitrogen gas pressure. Its influence was investigated in the nitrogen gas pressure range from 0.05 bar to 10 bar. The nitrogen depth profiles were measured via the nuclear resonance reaction N15(p,αγ)12C, while the phases formed in the surface layer were analyzed by conversion electron Mössbauer spectroscopy and x-ray diffraction. Utilizing sequentially N15-enriched and natural nitrogen atmospheres, the evolution of the nitrogen depth profiles during the laser nitriding process was traced. The experimental results suggest that the one-dimensional laser-supported combustion wave model reasonably describes the laser–plume dynamics and the nitriding effect.
Formation of highly conductive polycrystalline GaAs from annealed amorphous (Ga,As)
Authors: Epple J. H. , Chang K. L. , Xu C. F. , Pickrell G. W. , Cheng K. Y. , Hsieh K. C.
Published: April 2003

Polycrystalline (Ga,As) grown by molecular-beam epitaxy (MBE) is compared to polycrystalline (Ga,As) formed by annealing amorphous (Ga,As) also grown by MBE. Both amorphous and polycrystalline materials were grown at 100 °C and crystallinity is controlled by the As overpressure. As the amorphous material was annealed at varying temperatures from 300 to 500 °C several properties of the material changed such as levels of excess As in the material, grain structure of annealed material, strength and adhesion of crystallized layer, and conductivity of the material. After annealing at temperatures around 400 °C, the material is specular, polycrystalline, has good adhesion, and is very conductive. Resistivity values of less than 2 mΩ cm and acceptor concentrations near 5×1020 cm−3 were detected according to Hall/van der Pauw measurements. Conduction is believed to be due to the large amount of excess As in the material forming an As conduction path when the material is annealed. Material grown in a polycrystalline form by MBE differs in both grain structure and conductivity from material crystallized from an amorphous form.
Particle-in-cell/Monte Carlo simulations of a low-pressure capacitively coupled radio-frequency discharge: Effect of adding H2 to an Ar discharge
Authors: Neyts E. , Yan M. , Bogaerts A. , Gijbels R.
Published: April 2003

A one-dimensional particle-in-cell/Monte Carlo code with three velocity components is developed to simulate a capacitively coupled radio-frequency Ar/H2 discharge at low pressure, and to investigate the effect of adding hydrogen to an argon discharge. This self-consistent kinetic simulation technique allows one to study fundamental processes in the discharge at the molecular level. It is shown that the addition of small amounts of H2 to an Ar discharge has profound effects on the discharge behavior, i.e., a change in the electron energy probability function, an increase in the electron density at low H2 content and a decrease at higher H2 content, as well as a dip in the Ar+ ion density in the center of the discharge at higher pressure. These effects can be explained by the collision processes taking place in the discharge. The simulations were carried out in the pressure range 50–250 mTorr, at voltages of 300 and 800 V, while the H2 content was varied between 0% and 10%, at a constant driving frequency of 13.56 MHz.
Investigation of accumulated carrier mechanism on sulfurated GaN layers
Authors: Lin Yow-Jon , Lee Chi-Sen , Lee Ching-Ting
Published: April 2003

We study the induced electron concentration accumulated on the sulfurated layer of the (NH4)2Sx- treated n-type GaN layers using a simple resistance model. The electron concentration within the sulfurated layer increased from its original value of 6.9×1017 cm−3 to 8.2×1019 cm−3. The induced electrons are attributable to the sulfur atoms occupying nitrogen vacancies. An increase in the induced electron concentration could play an important role in lowering the specific contact resistance of Ti/Al contacts with the (NH4)2Sx-treated n-type GaN layer.
Effects of presheath dynamics on radio-frequency sheaths
Authors: Xiang N. , Waelbroeck F. L.
Published: April 2003

A common approach to the study of radio frequency (rf) sheaths is to separate the description of the sheaths from that of the bulk plasma. In order to solve the resulting set of equations, an appropriate boundary condition for the sheath model has to be specified at the sheath–plasma boundary. In the existing sheath models, this boundary condition is assumed to be stationary. In present article, we investigate the ion dynamics in both presheath region and sheath region by using numerical as well as analytical methods. It is found that the presheath introduces an additional time scale ωpre=VB/l (here, ℓ is the characteristic scale of the presheath and VB is the Bohm velocity). If the rf frequency ω is low enough so that ω⩽ωpre, the boundary condition for the sheath depends on the ion dynamics in the presheath. In the very low-frequency regime ω≪VB/l, an analytical expression for the ion current is obtained which agrees well with the numerical result.
Study of vibrational interactions in DCO+ by millimeter-wave spectroscopy and determination of the equilibrium structure of the formyl ion
Authors: Dore Luca , Beninati Sabina , Puzzarini Cristina , Cazzoli Gabriele
Published: April 2003

Rotational transitions of DCO+ in vibrationally excited states were measured in the region 360–440 GHz. The states investigated are: 0 2 0 (with l=0,2), 0 3 0 (with l=1,3), 0 4 0 (with l=0,2,4), 0 11 1, and 1 00 0. This study allowed us to carefully analyze the vibrational interaction affecting the first C–D stretching state 1 00 0, which was confirmed to be due to the 0 11 1 state. Taking account of the perturbation effect on the rotational constants of the involved states allowed to derive an accurate equilibrium structure of HCO+, which resulted to be rCH=1.092 04 Å and rCO=1.105 58 Å.
Pure rotational spectrum of the NCCS radical studied by Fourier-transform microwave spectroscopy
Authors: Nakajima Masakazu , Sumiyoshi Yoshihiro , Endo Yasuki
Published: April 2003

Pure rotational transitions of the NCCS radical, showing resolved fine and hyperfine splittings, have been observed by Fourier-transform microwave spectroscopy in a discharged supersonic jet of acetonitrile and carbon disulfide. Since the transitions have been observed at frequencies corresponding to the even multiples of the rotational constant, it is concluded that the NCCS radical has a bent structure in the ground electronic state, X̃ 2A′, and the Ka=0 ladder of the radical has been observed under the jet-cooled condition. Precise molecular constants, including the hyperfine constants of the nitrogen nucleus, are determined by a least-squares fit for the observed transition frequencies using a standard asymmetric top Hamiltonian. The determined rotational constant is compared with results of high-level ab initio calculations in order to confirm the spectral carrier to be the bent NCCS radical.
Effects of confinement on protein folding and protein stability
Authors: Ping G. , Yuan J. M. , Vallieres M. , Dong H. , Sun Z. , Wei Y. , Li F. Y. , Lin S. H.
Published: April 2003

In a cell, proteins exist in crowded environments; these environments influence their stability and dynamics. Similarly, for an enzyme molecule encapsulated in an inorganic cavity as in biosensors or biocatalysts, confinement and even surface effects play important roles in its stability and dynamics. Using a minimalist model (two-dimensional HP lattice model), we have carried out Monte Carlo simulations to study confinement effects on protein stability. We have calculated heat capacity as a function of temperature using the histogram method and results obtained show that confinement tends to stabilize the folded conformations, consistent with experimental results (some reported here) and previous theoretical analyses. Furthermore, for a protein molecule tethered to a solid surface the stabilization effect can be even greater. We have also investigated the effects of confinement on the kinetics of the refolding and unfolding processes as functions of temperature and box size. As expected, unfolding time increases as box size decreases, however, confinement affects folding times in a more complicated way. Our theoretical results agree with our experimentally observed trends that thermal stability of horseradish peroxidase and acid phosphatase, encapsulated in mesoporous silica, increases as the pore size of the silica matrix decreases.
Blue-shifting hydrogen bonding in N2⋯HKrF
Authors: McDowell Sean A. C.
Published: April 2003

An ab initio computational study of the properties of the weakly bound linear complex N2⋯HKrF was undertaken at three different levels of theory using a 6-311G** basis set. The dimer was found to have a zero-point vibrational energy corrected binding energy of −2.8 kJ mol−1 with respect to the monomer subunits, at the highest level of theory used in this study. This complex exhibits a large harmonic vibrational frequency blue-shift of about 60 cm−1 for the H–Kr stretching mode with a decrease in the infrared intensity for this mode on formation of the complex. This unusual result was rationalized mainly by consideration of the effect of the electrostatic interaction and charge transfer between the HKrF and N2 molecules.
Geometry optimizations with the coupled-cluster model CC2 using the resolution-of-the-identity approximation
Authors: Hättig Christof
Published: April 2003

An implementation of the gradient for the second-order coupled-cluster singles-and-doubles model CC2 is reported, which employs the resolution-of-the-identity (RI) approximation for electron repulsion integrals. The performance of the CC2 model for ground state equilibrium geometries and harmonic frequencies is investigated and compared with experiment and other ab initio methods. It is found that CC2 equilibrium geometries have a similar accuracy to those calculated with second-order Møller–Plesset perturbation theory (MP2), but the bond lengths are larger. In particular, double and triple bonds and bonds in electron-rich compounds are elongated by 0.5–1.5 pm. Thereby CC2 slightly outperforms MP2 for single bonds, in particular in electron-rich compounds, but for strong double and triple bonds CC2 is somewhat inferior to MP2. The results for harmonic frequencies go in parallel with the results for equilibrium structures. The error introduced by the RI approximation is found to be negligible compared to the remaining one-electron basis set error, if optimized auxiliary basis sets are used. Typically, the RI error in bond lengths is of the order of 10−3 pm and the error in angles 10−3–10−2 deg. Applications are reported for the geometry of trans-azobenzene and for the geometry and harmonic frequencies of cis,trans-1,4-difluorobutadiene.
Detection of strongly and weakly ionizing radiation by triode structure based on SiC films
Authors: Strokan N. B. , Ivanov A. M. , Savkina N. S. , Strelchuk A. M. , Lebedev A. A. , Syväjärvi M. , Yakimova R.
Published: April 2003

Device structures based on thick and thin SiC epitaxial films have been studied as detectors of alpha particles and weak ionization radiation (x ray and UV quanta), respectively. In the first case relatively no transistor effect is observed and the signal is formed similarly to that in a diode structure. The possibility of alpha particle spectrometry in spite of slow carrier transport via diffusion has been demonstrated. In the second case, the signal value of the transistor-like detector on applied voltage is investigated. Different modes are used: single alpha-particle detection and induced-current recording from fluxes of x ray and optical (UV) quanta. A superlinear rise in the resulting signal is observed with increasing voltage. The signal is amplified by a factor of several tens with respect to the value chosen for normalization. A description in terms of the phototriode model gives acceptable values for the main parameters: base width, diffusion length of electrons, and space charge of ionized impurities.
Characterization of traps related to InAs quantum-dot growth-induced defects in GaAs by low-frequency noise measurements in reverse-biased Schottky diodes
Authors: Hastas N. A. , Dimitriadis C. A. , Dozsa L. , Gombia E. , Mosca R.
Published: April 2003

The trap properties of Au/n-GaAs Schottky diodes, with self-organized InAs quantum dots (QDs) confined between buffer and capping GaAs layers on n+-GaAs substrates, are investigated by low-frequency noise measurements. The noise data indicate that the trap density at the interface of the buffer/capping GaAs layers is higher by about one order of magnitude than the trap density at the Au/GaAs interface. In samples with the QD-structure, deep traps are created within the buffer GaAs layer due to QD formation, with a trap density higher by about three orders of magnitude than the trap density at the Au/GaAs interface. Conduction measurements show that in Schottky diodes without QDs, the increase in the leakage current is due to an interfacial layer contributing to the barrier height lowering. In Schottky diodes comprising QDs, an anomalous leakage current is observed, attributed to local generation of deep traps due to QD formation.
Nanoparticle formation in microchannel glass by plasma enhanced chemical vapor deposition
Authors: McIlroy D. N. , Huso J. , Kranov Y. , Marchinek J. , Ebert C. , Moore S. , Marji E. , Gandy R. , Hong Y.-K. , Grant Norton M. , Cavalieri E. , Benz Rudy , Justus B. L. , Rosenberg A.
Published: April 2003

Iron nanoparticles were synthesized in situ within the channels of a microchannel glass by differentially pumping across the plate in conjunction with plasma enhanced chemical vapor deposition. The microchannel glass was mounted on a custom designed differentially pumped sample holder, which produced 10 Torr of differential pressure across the 5 μm channels. The iron precursor was ferrocene [(C5H5)2–Fe]. The composition and structure of the nanoparticles was determined by electron diffraction to be Fe3O4, where oxidation of the nanoparticles occurred upon exposure to air. Transmission electron microscopy revealed the formation of nanoparticles ranging in size from 150 to 960 nm, which are formed from smaller nanoparticles on the order of 5–10 nm. The large nanoparticles (150–960 nm) grow independently of one another and are largely freestanding within the channels, i.e., they are not highly attached to the channel walls or agglomerated. The magnetic signature of the large nanoparticles is consistent with ferrimagnetic Fe3O4. The magnetic hysteresis loops of the particles are reminiscent of superparamagnetic behavior, which would suggest that the magnetic coupling between the 5 and 10 nm nanoparticles is weak. A model for nanoparticle formation based on plasma suspension of the Fe nanoparticles inside the glass channels is proposed.
Polymer depletion interaction of small mesoscopic particles: Effects beyond leading order and anisotropic particles
Authors: Eisenriegler E. , Bringer A. , Maassen R.
Published: April 2003

We discuss the depletion interaction between a wall and a mesoscopic particle of ellipsoidal shape induced by long, flexible, nonadsorbing polymer chains. Both a force and a torque are exerted on the particle. We concentrate on the case in which the particle size is much smaller than typical polymer lengths, such as the radius of gyration Rg, where a rigid polymer approximation of the Asakura–Oosawa-type cannot be applied. Explicit analytical results are obtained for ideal polymers. For particle–wall distances z large compared to Rg an orientation of the ellipsoid perpendicular to the wall is favored. For z small compared to Rg (but z still large compared to the particle size), parallel orientation is favored. The perturbation of the polymer system due to the small particle is represented by a series of point-operators in the corresponding field theory, with next-to-next-to-leading anisotropic derivative-operators characterizing the particle orientation. For the interaction between a spherical particle and a wall the simple analytical results predicted by the proposed small particle expansion beyond leading order display an interesting structure which is confirmed by direct numerical computation.
Defects in Eu- and Tb-doped GaN probed using a monoenergetic positron beam
Authors: Uedono A. , Bang H. , Horibe K. , Morishima S. , Akimoto K.
Published: April 2003

We probed defects in Eu- and Tb-doped GaN films grown on sapphire substrates by gas-source molecular-beam epitaxy with a monoenergetic positron beam. In both Eu- and Tb-doped samples, we observed vacancy clusters consisting of two or more vacancies. These defects were introduced by replacing Ga with rare-earth elements, and resulting in distortion of the host matrix. We studied the correlation between luminescence originating from the intra-4f-transitions of Eu3+ and the crystal quality of the GaN film. In film doped at 2-at. % Eu, the mean open volume of the vacancies near the interface between the GaN film and the sapphire substrate was found to be larger than that in the subsurface region. The increase in the open volume of the defects correlated with the lowering coordination symmetry of Eu3+ and the increase in the transition rate of its 4f-electrons.
A model for instability growth in accelerated solid metals
Authors: Colvin J. D. , Legrand M. , Remington B. A. , Schurtz G. , Weber S. V.
Published: April 2003

We present an approximate analytical dispersion relationship for elastic–plastic acceleration-driven instability growth. In this model, the accelerated solid behaves like a viscoplastic after its elastic yield strength is exceeded, with the viscosity inversely proportional to the strain rate. We have applied this model, or a 1993 model of shock-driven viscous instability growth, where applicable, to perturbation growth measurements made in three separate types of experiments: High-explosive (HE)-driven planar Al plates, HE-driven implosions of steel cylinders, and planar Al foils driven indirectly by Lawrence Livermore National Laboratory’s Nova laser. We have also compared the analytical modeling of these experiments with simulations done with a two-dimensional Lagrangian radiation-hydrodynamics computer code containing an elastic–plastic constitutive model. We find that for the moderate strain rates of the HE experiments, the simulations and analytical modeling of perturbation growth agree with each other and with the data, using an equivalent plastic viscosity consistent with the von Mises plasticity criterion. For the high strain rates of the Nova experiments, on the other hand, the early-time growth data is consistent with viscoplastic growth, with viscosity ten to a hundred times less than the von Mises plastic viscosity for nominal strength. This observed initial material weakening is followed by a transition to a strengthened state to match the late-time growth data, which we show to be consistent with a “relaxation” hypothesis in which plastic flow at high strain rate is confined to discrete shear bands. We also show under what conditions the perturbation growth is independent of initial amplitude.
Transparent Ohmic contacts of oxidized Ru and Ir on p-type GaN
Authors: Jang Ho Won , Lee Jong-Lam
Published: April 2003

Low-resistance, high-transparency, and thermally stable Ohmic contacts on p-type GaN were achieved using Ru and Ir. Oxidation annealing under O2 atmosphere led to the reduction of contact resistivity by greater than one order of magnitude, compared to annealing under N2. The bilayer contacts of Ru (50 Å)/Ni (50 Å) and Ir (50 Å)/Ni (50 Å) exhibited a low contact resistivity of ∼4×10−5 Ω cm2 and high light transmittance of ∼85% after annealing at 500 °C for 1 min under O2. The barrier height for hole injection could decrease via contact formation of RuO2 (or IrO2) on p-type GaN. The Au-free contact structure of NiO/RuO2 (IrO2)/GaN led to high light transmittance and good thermal stability.
Thermodynamic model of low-temperature molecular beam epitaxy of GaN with hydrazine
Authors: Elyukhin V. A. , Garcı́a-Salgado G. , Peña-Sierra R. , Nikishin S. A.
Published: April 2003

A thermodynamic model is developed to describe the low-temperature molecular beam epitaxy of GaN with hydrazine. We consider an epitaxial process which takes into account the physisorbed states of the Ga atoms and hydrazine molecules on the (0001) A and B planes of GaN. The incident and desorbed fluxes are considered as a one-dimensional ideal gas. The thermodynamic affinity is used in order to estimate the deviation from thermodynamic equilibrium at the growth reactions. The thermodynamic characteristics of the growth reactions are estimated and they are in excellent agreement with experimental results.
Growth of uniformly a-axis-oriented ferroelectric lanthanum-substituted bismuth titanate films on silicon substrates
Authors: Lee Ho Nyung , Hesse Dietrich , Zakharov Nikolai , Lee Sung Kyun , Gösele Ulrich
Published: April 2003

Uniformly a-axis-oriented, epitaxially twinned La-substituted Bi4Ti3O12 (BLT) thin films having the major spontaneous polarization entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates covered with very thin SrRuO3 bottom electrodes. Using SrRuO3 bottom electrodes of a specific low thickness in combination with a relatively high growth rate and a high oxygen pressure, the volume fraction of the BLT (100) orientation, which is competing with the BLT (118) orientation, was increased up to 99%. In this way the growth of fully a-axis-oriented BLT films was achieved, attaining a remanent polarization of 32 μC/cm2. Initial fatigue experiments indicated hardly any fatigue after 109 switching cycles.
Method to extract diffusion length from solar cell parameters—Application to polycrystalline silicon
Authors: Taretto K. , Rau U. , Werner J. H.
Published: April 2003

A closed form, analytical expression for the interdependence of the effective diffusion length Leff and the open-circuit voltage of solar cells is derived for the parallel connection of recombination in the space-charge region and in the neutral base region. This expression allows for the calculation of Leff from the open-circuit voltage, the short-circuit current, and the base doping of the solar cell as the only quantities that need to be determined experimentally. Values of Leff calculated with our method match with an accuracy of 35% values that are determined experimentally by quantum-efficiency measurements of silicon solar cells. The agreement holds in a range 0.3 μm <Leff<300 μm. Alternatively, Leff is calculated from the short-circuit current and from the overall light absorption in the solar cell via an analytical expression for the Leff -dependence of carrier collection. Again, we find a good match between the measured Leff and the values calculated by this method. We further analyze photovoltaic output parameters from literature data of polycrystalline silicon solar cells covering grain sizes from 10−2 to 104 μm. We calculate Leff for these solar cells with our method and interpret the results in terms of grain-boundary recombination velocity SGB. We find that the data points split into two distinct groups, one with 105 cm/s<SGB<107 cm/s, and one showing 101 cm/s<SGB<103 cm/s. Since all cells with low SGB have a {220} surface texture, we ascribe the low-recombination velocity of grain boundaries to the low-electronic activity of [110]-tilt grain boundaries in these films.