bluuebunny/crossref_metadata_2025 huggingface dataset:
Normal mode analysis of large systems with icosahedral symmetry: Application to (Dialanine)60 in full and reduced basis set implementations
Authors: van Vlijmen Herman W. T. , Karplus Martin
Published: July 2002

A group theoretical method is described for calculating the normal modes of icosahedral systems such as viruses. The use of symmetry reduces the size of the matrices that have to be diagonalized from 60N×60N to 5N×5N (where N is the number of degrees of freedom of every subunit) while preserving complete accuracy. The method includes algorithms to calculate the normal modes, the atomic fluctuations and cross-correlations, and the projections of normal modes on vectors of interest. The correctness and accuracy of the method are verified using a model system, consisting of 60 Dialanine peptides arranged in a nonbonded icosahedral complex. The effects of using reduced basis sets in the normal mode calculations are examined. Reduced basis sets, especially those consisting of dihedral and/or bond angles, are shown to have relatively small effects on the frequencies, spatial fluctuations, and directions of the normal mode displacements. The current implementation allows accurate reduced basis normal mode calculations on icosahedral virus molecules with moderately powerful computers.
Angle-dependent photocurrent spectroscopy of oxide-apertured vertical-cavity surface-emitting lasers during aging
Authors: Jaeger A. , Petroff P. M. , Lowes T. D.
Published: July 2002

Aging properties of oxide-apertured vertical-cavity surface-emitting lasers have been analyzed by photocurrent (PC) measurements at room temperature. We demonstrate that this method allows an analysis of the aging process and provides insight into degradation mechanisms occurring during operation of these devices. By changing the angle of incidence in the PC experiment the energy positions of the top mirror features shift to higher energy while the excitonic transitions do not change. Thus, angle dependent PC spectra reveal both the appearance of defect bands located in the intrinsic region of the device as well as modifications of the top mirror transmission upon aging.
Absolute fluorine atom concentrations in fluorocarbon plasmas determined from CF2 loss kinetics
Authors: Cunge G. , Chabert P. , Booth J.-P.
Published: July 2002

We present a simple technique for determining the absolute fluorine atom concentration in pure CF4 capacitively coupled radio-frequency discharges. It is based on the measurement, by laser-induced fluorescence, of the loss rate kloss of the CF2 radical in the afterglow of a pulsed plasma. We first demonstrate that in our conditions, CF2 is lost only by gas phase recombination with F atoms (with a known rate constant krec) and by recombination at the reactor walls at a rate kwall, independent of the rf power injected. Hence, the total CF2 loss rate, kloss=kwall+krec[F], varies linearly with [F] when the rf power is increased. By recording kloss and the relative variation of the F atom concentration (by optical emission actinometry) as a function of rf power, kwall and [F] can be determined. These measurements of [F] complement previous quantitative measurements of CF and CF2 radicals [Booth et al., J. Appl. Phys. 85, 3097 (1999); and Cunge and Booth, J. Appl. Phys. 85, 3952 (1999)] made in the same reactor for the same plasma conditions.
Ultrashort InGaAsP/InP lasers with deeply etched Bragg mirrors
Authors: Kamp M. , Hofmann J. , Forchel A. , Lourdudoss S.
Published: July 2002

We have fabricated short cavity lasers with deeply etched Bragg mirrors based on 1.55 μm emitting InGaAsP/InP laser structures. Continuous-wave operation has been obtained for devices with a length of 40 μm, showing threshold currents of 12 mA. The dynamic properties of the lasers were studied by measurements of the relative intensity noise (RIN). A maximum modulation frequency of 8.4 GHz was extracted from the RIN data.
Collisional quenching of high rotational levels in A 2Σ+ OH
Authors: Hemming Brooke L. , Crosley David R. , Harrington Joel E. , Sick Volker
Published: July 2002

Collisional removal of the v′=0 level of the A 2Σ+ state of the OH radical has been studied as a function of rotational level N′ at room temperature. OH in high rotational levels of the X 2Πi state were created by 193 nm photolysis of HNO3 and excited to A 2Σ+ by a tunable dye laser. Time decays of fluorescence at varying pressures were measured. For O2 and H2, the quenching cross section σQ decreased with increasing N′ until N′∼10; for higher N′ it appears to remain approximately constant. Xe behaves the same way except that the decrease continues to N′=15. For Kr, σQ appears to decrease to within experimental error of zero at N′=10; and for N2 it was within error of zero above N′=10. These results have implications for laser-induced fluorescence atmospheric monitoring of OH and combustion temperature determinations, as well as a fundamental understanding of collisional quenching. Quenching of OH, N′∼1, by HNO3 was found to be 81±8 Å2.
Fluid flow in nanopores: An examination of hydrodynamic boundary conditions
Authors: Sokhan V. P. , Nicholson D. , Quirke N.
Published: July 2002

Steady-state Poiseuille flow of a simple fluid in carbon slit pores under a gravity-like force is simulated using a realistic empirical many-body potential model for carbon. In this work we focus on the small Knudsen number regime, where the macroscopic equations are applicable, and simulate different wetting conditions by varying the strength of fluid–wall interactions. We show that fluid flow in a carbon pore is characterized by a large slip length even in the strongly wetting case, contrary to the predictions of Tolstoi’s theory. When the surface density of wall atoms is reduced to values typical of a van der Waals solid, the streaming velocity profile vanishes at the wall, in accordance with earlier findings. From the velocity profiles we have calculated the slip length and by analyzing temporal profiles of the velocity components of particles colliding with the wall we obtained values of the Maxwell coefficient defining the fraction of molecules thermalized by the wall.
A molecular dynamics study of structural transitions in small water clusters in the presence of an external electric field
Authors: Vegiri Alice , Schevkunov Sergei V.
Published: July 2002

The present work constitutes a thorough study of the response of a relatively small water cluster (N=32) to external static electric fields in the 0.5×107 to 108 V/cm range, at T=200 K. As the electric field is varied, the system undergoes a phase transition to structures resembling incomplete nanotubes consisting of stacked squares arranged perpendicularly to the field direction. For further field increase the system transforms continuously to more open structures, reminiscent of the proton ordered forms of cubic ice, found also in the liquid. Regarding the dynamic response of the cluster, this is reflected in a profound way on the nonmonotonic variation of the reorientational decay rates of the molecular intrinsic axes and of the self-diffusion coefficients along and perpendicular to the field lines. In general the external field induces a considerable increase of the reorientational decay rates of all axes, except for the strongest field where the electrofreezing effect is observed. Reorientational relaxation has been found to obey a stretched exponential behavior of the Kohlrausch–Williams–Watts-type, where a one-to-one correspondence between the β-exponent variation with the field, molecular cooperativity, and translational diffusion has been established.
Ultrafast carrier trapping in microcrystalline silicon observed in optical pump–terahertz probe measurements
Authors: Uhd Jepsen P. , Schairer W. , Libon I. H. , Lemmer U. , Hecker N. E. , Birkholz M. , Lips K. , Schall M.
Published: July 2002

We report on direct evidence of ultrafast carrier dynamics displaying features on the picosecond time scale in microcrystalline silicon (μc-Si:H). The dynamics of photogenerated carriers is studied by using above-band-gap optical excitation and probing the instantaneous carrier mobility and density with a THz pulse. Within the first picoseconds after excitation, the THz transmission transients show a fast initial decay of the photoinduced absorption followed by a slower decrease due to carrier recombination. We propose that the initial fast decay in the THz transients is due to carrier capture in the trapping states.
Detailed structure and dynamics in particle-in-cell simulations of the lunar wake
Authors: Birch Paul C. , Chapman Sandra C.
Published: July 2002

The solar wind plasma from the Sun interacts with the Moon, generating a wake structure behind it, since the Moon is to a good approximation an insulator, has no intrinsic magnetic field and a very thin atmosphere. The lunar wake in simplified geometry has been simulated via a 112D electromagnetic particle-in-cell code, with high resolution in order to resolve the full phase space dynamics of both electrons and ions. The simulation begins immediately downstream of the moon, before the solar wind has infilled the wake region, then evolves in the solar wind rest frame. An ambipolar electric field and a potential well are generated by the electrons, which subsequently create a counter-streaming beam distribution, causing a two-stream instability which confines the electrons. This also creates a number of electron phase space holes. Ion beams are accelerated into the wake by the ambipolar electric field, generating a two-stream distribution with phase space mixing that is strongly influenced by the potentials created by the electron two-stream instability. The simulations compare favorably with WIND observations.
Flow structure due to dimple depressions on a channel surface
Authors: Ligrani P. M. , Harrison J. L. , Mahmmod G. I. , Hill M. L.
Published: July 2002

Instantaneous, dynamic and time-averaged characteristics of the vortex structures which are shed from the dimples placed on one wall of a channel are described. The dimpled test surface contains 13 staggered rows of dimples in the streamwise direction, where each dimple has a print diameter of 5.08 cm, and a ratio of depth to print diameter of 0.2. Considered are Reynolds numbers (based on channel height) ReH from 600 to 11 000, and ratios of channel height to dimple print diameter H/D of 0.25, 0.50, and 1.00. For all three H/D, a primary vortex pair is periodically shed from the central portion of each dimple, including a large upwash region. This shedding occurs periodically and continuously, and is followed by inflow advection into the dimple cavity. The frequency of these events appears to scale on time-averaged bulk velocity and dimple print diameter, which gives nondimensional frequencies of 2.2–3.0 for all three H/D values considered. As H/D decreases, (i) the strength of the primary vortex pair increases, and (ii) two additional secondary vortex pairs (which form near the spanwise edges of each dimple) become significantly stronger, larger in cross section, and more apparent in flow visualization images and in surveys of time-averaged, streamwise vorticity. The locations of these primary and secondary vortex pairs near the dimpled surface coincide closely with locations where normalized Reynolds normal stress is augmented. This evidences an important connection between the vortices, Reynolds normal stress, and mixing. The large-scale unsteadiness associated with this mixing is then more pronounced, and encompasses larger portions of the vortex structure (and thus extends over larger volumes) as H/D increases from 0.25 to 1.0.
The structure and dynamics of doubly occupied Ar hydrate
Authors: Itoh Hidenosuke , Tse John S. , Kawamura Katsuyuki
Published: July 2002

It has been speculated that small guest molecules such as Ar, Kr, N2, and O2 may doubly occupy the large cages of the type II structure of clathrate hydrate. In this study we examined this possibility on Ar hydrate with molecular dynamics calculations. It was found that the total energy and pressure of the system increased significantly when the large cages were doubly occupied. Furthermore, even though Ar clathrate with double occupancy may be stabilized by external pressure, the cubic structure is severely distorted. Owing to restricted empty space, the motions of the Ar atoms in doubly occupied large cages are highly correlated and give a unique stretch vibrational band that can be used for characterization.
Sol–gel preparation of single-layer, 0.75 μm thick lead zirconate titanate films from lead nitrate-titanium and zirconium alkoxide solutions containing polyvinylpyrrolidone
Authors: Takenaka Shinsuke , Kozuka Hiromitsu
Published: July 2002

Lead zirconate titanate (PZT) films were deposited on nesa silica glass substrates by spin coating using a Pb(NO3)2–Zr(OC3H7n)4–Ti(OC3H7i)4 solution containing polyvinylpyrrolidone and acetylacetone. The coating solution had stable viscosity for over 1300 h at room temperature. Single-layer PZT films as thick as 0.75 μm could be obtained via single-step spin coating with successive gel film heat treatments at 80, 300, and 700 °C. The fired films were optically transparent and crack free, exhibiting P–E hysteresis loops with remanent polarization of 24.6±1.6 μC cm−2 and coercive field of 95.6±9.8 kV cm−1.
Prediction of electron paramagnetic resonance g values using coupled perturbed Hartree–Fock and Kohn–Sham theory
Authors: Neese Frank
Published: July 2002

A method for calculating the EPR g-tensor based on coupled perturbed Hartree–Fock (HF) and density functional theory (DFT) is presented. The one-electron molecular orbitals of a spin- unrestricted Slater determinant are calculated up to first order in the applied magnetic field. The g-tensor is evaluated as a mixed second derivative property with respect to the applied field and the electron magnetic moment. Thus, spin-polarization and spin–orbit coupling are simultaneously included in the calculation. The treatment focuses on orbitally nondegenerate molecules but is valid for a general ground state spin S and, for the first time, it is possible to include hybrid density functionals in the treatment. The relativistic mass and diamagnetic gauge corrections are also considered. An implementation of the theory is described. Extensive numerical calculations for a series of small molecules are reported with the Hartree–Fock (HF) method, the local density approximation (LSD), the generalized gradient approximation (GGA) and hybrid density functionals such as B3LYP and PBE0 and large Gaussian basis sets. Detailed comparison with available ab initio and DFT calculations are made. The results indicate that the hybrid functionals offer little or no improvement over the GGA functionals for small radicals made of light atoms. For transition metal complexes the situation is different. The hybrid functionals give, on average, better results than the GGA functionals but significant disagreement between theoretical and experimental g-shifts still remain. Overall, the results indicate that the present method is among the most accurate so far developed models for the prediction of g values.
Effects of La doping on the cubic–tetragonal phase transition and short-range ordering in PbTiO3
Authors: Kim Tae-Yong , Jang Hyun M. , Cho Seong M.
Published: July 2002

Effects of cation-site vacancies on the characteristics of the cubic–tetragonal displacive transitions in La-modified PbTiO3 (PLT) were studied using in situ XRD measurements and Raman scattering. For this purpose, two distinct types of PLT series having cation-site vacancies either at the A site (PLT-A) or at the B site (PLT-B) were separately prepared. The PLT-A series exhibited normal ferroelectricity and underwent the cubic-to-tetragonal structural transition for La substitution up to 30 at. %. Contrary to this, the PLT-B became a typical relaxor with the absence of the macroscopic cubic–tetragonal transition. However, for both types of PLTs, Raman-forbidden bands appeared in the high-temperature cubic-phase field at 30 at. % La modification. These observations were attributed to the formation of short-ranged clusters with tetragonal symmetry in a globally cubic matrix and to the reduced dipolar interaction between the clusters in the PLT-B, as compared with the PLT-A.
Improved techniques for measurement of superconductivity in diamond anvil cells by magnetic susceptibility
Authors: Timofeev Yuri A. , Struzhkin Viktor V. , Hemley Russell J. , Mao Ho-kwang , Gregoryanz Eugene A.
Published: July 2002

Recent improvements in magnetic inductive measurements of superconductivity over a broad range of pressures in high-pressure diamond anvils cells are described. A significant increase in sensitivity is obtained by the application of resonant circuits with voltage increasing transformers, allowing an increase in the working frequency of the experimental setup. As a result, superconductivity in samples with linear dimensions as small as 10 μm can be detected. Recent results obtained with this resonant circuit on samples of sulfur at 230 GPa are presented. Further increase in the sensitivity of the experimental setup can be obtained by summing the results of repeated measurements obtained in the same temperature interval. An example is provided by measurements on a small sample of lead.
Behavior of atomic radicals and their effects on organic low dielectric constant film etching in high density N2/H2 and N2/NH3 plasmas
Authors: Nagai Hisao , Takashima Seigou , Hiramatsu Mineo , Hori Masaru , Goto Toshio
Published: July 2002

An organic film, FLARE™, is one of the most prospective candidates for interlayer insulating films with low dielectric constants (low k). This organic low k film was etched in inductively coupled high-density plasmas employing N2/H2 and N2/NH3 gases. By changing the mixing ratio of these gases, the anisotropic etching profile was obtained. The etching plasmas were evaluated by quadruple mass spectroscopy and the vacuum ultraviolet absorption spectroscopy employing microplasma as a light source. N and H radical densities were estimated on the order of 1011–1012 cm−3 and 1012–1013 cm−3, respectively. The behavior of etch rate corresponded well to that of H radical density. H radicals were found to be important species for organic low k film etching, while N radicals could not etch without ion bombardments. On the other hand, N radicals were found to be effective for the formation of protection layer on the sidewall against the etching by the H radicals. The ratio of H and N radical densities would be important for the etching of organic low k film employing N–H plasmas.
Raman spectroscopic studies on matrix-isolated phosphorus molecules P4 and P2
Authors: Kornath Andreas , Kaufmann Alexander , Torheyden Martin
Published: July 2002

The Raman spectra of P4 and P2 molecules have been studied in nitrogen, argon, krypton, and xenon matrices at 15 K. The vibrational frequencies of the P4 molecule are up to 14 cm−1 higher than the experimental gas phase data. This apparent blue-shift is not caused by matrix effects but rather due to an underestimation of the fundamentals in the gas phase as a course of the elevated temperatures. The observed frequencies confirm a recent theoretical prediction on the basis of high-level calculations. From the observed frequencies of the P4 molecule a general valence force field was calculated.
Thermal conductivity enhancement of suspensions containing nanosized alumina particles
Authors: Xie Huaqing , Wang Jinchang , Xi Tonggeng , Liu Yan , Ai Fei , Wu Qingren
Published: July 2002

Various suspensions containing Al2O3 nanoparticles with specific surface areas in a range of 5–124 m2 g−1 have been prepared and their thermal conductivities have been investigated using a transient hot-wire method. Nanoparticle suspensions, containing a small amount of Al2O3, have substantially higher thermal conductivity than the base fluid, with the enhancement increasing with the volume fraction of Al2O3. The enhanced thermal conductivity increases with an increase in the difference between the pH value of aqueous suspension and the isoelectric point of Al2O3 particle. For the suspensions using the same base fluid, the thermal conductivity enhancements are highly dependent on specific surface area (SSA) of nanoparticle, with an optimal SSA for the highest thermal conductivity. For the suspensions containing the same nanoparticles, the enhanced thermal conductivity ratio is reduced with the increasing thermal conductivity of the base fluid. The crystalline phase of the nanoparticles appears to have no obvious effect on the thermal conductivity of the suspensions. Comparison between the experiments and the theoretical model shows that the measured thermal conductivity is much higher than the values calculated using theoretical correlation, indicating new heat transport mechanisms included in nanoparticle suspensions.
Properties of InP self-assembled quantum dots embedded in In0.49(AlxGa1−x)0.51P for visible light emitting laser applications grown by metalorganic chemical vapor deposition
Authors: Ryou J. H. , Dupuis R. D. , Walter G. , Holonyak N. , Mathes D. T. , Hull R. , Reddy C. V. , Narayanamurti V.
Published: July 2002

We have studied the properties of InP self-assembled quantum dots embedded in various In0.49(AlxGa1−x)0.51P matrix layers to optimize the growth condition of the quantum dots and structures for III-phosphide quantum-dot-based lasers operating in visible spectral regions. Self-assembled quantum dot-related structures are grown by low-pressure metalogranic chemical vapor deposition and characterized by atomic-force microscopy, high-resolution transmission-electron microscopy, and photoluminescence. High density (∼1010 cm−2) and conveniently sized (∼5×20 nm) quantum dots are produced by growth condition optimization. We find that the quantum-dot heterostructure with a In0.49(AlxGa1−x)0.51P matrix layer having the largest direct band gap produces the most efficient luminescence at room temperature. Laser structures are prepared using optimized growth conditions and matrix materials. Laser operation with lasing wavelengths λ=650–680 nm are demonstrated at 77 and 300 K by optical pumping.
Visible–blind photoresponse of GaN-based surface acoustic wave oscillator
Authors: Ciplys D. , Rimeika R. , Shur M. S. , Rumyantsev S. , Gaska R. , Sereika A. , Yang J. , Khan M. Asif
Published: July 2002

We describe the photoresponse of GaN-based surface acoustic wave (SAW) delay-line oscillator operating in the 200 MHz range. The decrease in oscillator frequency under ultraviolet illumination of GaN transducer is caused by the SAW velocity decrease due to the acoustoelectronic interaction with photoconductivity electrons. The oscillator frequency shift reaches its maximum value at 365 nm and drops to zero above 400 nm with visible/ultraviolet rejection ratio more than 100. The optical quenching of the photoconductivity in GaN was observed. These results demonstrate the potential of the GaN-based SAW oscillators for applications as visible–blind remote UV sensors.
Acceptor diffusion and segregation in (AlxGa1−x)0.5In0.5P heterostructures
Authors: Grillot P. N. , Stockman S. A. , Huang J. W. , Bracht H. , Chang Y. L.
Published: July 2002

Acceptor segregation is investigated as a function of compositional difference, Δx, between adjacent layers in (AlxGa1−x)0.5In0.5P heterostructures. Magnesium, Zn, Be, and Mn acceptor species are all shown to segregate out of the high band gap Al-rich (AlxGa1−x)0.5In0.5P layers and into the low band gap Al-poor (AlxGa1−x)0.5In0.5P layers during high temperature epitaxial growth of such heterostructures. The observed acceptor segregation appears to be independent of growth method or dopant incorporation method (metalorganic chemical vapor deposition, gas source molecular-beam epitaxy, or ion implantation), and increases with increasing compositional difference between adjacent (AlxGa1−x)0.5In0.5P layers. A theoretical model is developed to describe acceptor segregation based on charge separation and the resulting electric field across the heterointerface, and the resulting acceptor segregation is shown to vary as (mh AL*/mh CL*)3/2 exp(ΔEV/kT) where mh AL* and mh CL* are the hole effective masses in the active layer and confining layer, and ΔEV is the valence band offset. Comparison between experimentally measured and theoretically predicted acceptor segregation ratios gives excellent agreement for (AlxGa1−x)0.5In0.5P heterostructures over the range of compositional differences from Δx=0.12 to Δx=0.93.
Adsorption of ammonia on the gold (111) surface
Authors: Bilić Ante , Reimers Jeffrey R. , Hush Noel S. , Hafner Jürgen
Published: July 2002

We have modeled the adsorption of ammonia on the Au(111) surface at coverages of 1/4 and 1/9 of a monolayer using density-functional theory employing the pseudopotential method, periodic imaging, a plane-wave basis set, and the PW91 density functional. The geometries of the adsorbate and the surface are fully optimized. The adsorption is found to be highly favored on top of a surface atom. Adsorption energies of 26 and 32 kJ mol−1 are obtained for the 1/4 and 1/9 of a monolayer coverage, respectively, extrapolating to 34 kJ mol−1 at zero coverage; the experimental estimate is 32–42 kJ mol−1. Small changes in the work function are predicted and interpreted as arising from a surface layer whose effective dipole moment is 2.15 D, 0.77 D larger than the calculated value of isolated ammonia. Examination of the calculated charge density and the local electric field strengths indicate that the change in dipole moment is due to polarization effects and that ammonia to gold charge transfer is minimal, at most 0.01 e in magnitude. Qualitatively, the local densities of states and the charge distribution provide little indication of covalent bonding between the gold and ammonia, and quantitatively the adsorption is interpreted as arising from dispersive interactions with some contribution from polarization. This picture is in contrast with common notions of gold to ammonia binding which depict weak chemisorption rather than physisorption, but the usefulness of PW91 in distinguishing between these processes is questioned through examination of the calculated potential energy surface of Ne2. PW91 is shown only to mimic dispersive interactions using modified covalent terms.
Control of the polarity of molecular-beam-epitaxy-grown GaN thin films by the surface nitridation of Al2O3 (0001) substrates
Authors: Mikroulis S. , Georgakilas A. , Kostopoulos A. , Cimalla V. , Dimakis E. , Komninou Ph.
Published: July 2002

The nitridation of Al2O3 (0001) substrate surfaces by radio-frequency nitrogen plasma has been investigated. A 1.5-nm-thick surface nitride layer occurred for 100 min nitridation at high substrate temperature, while the nitridation appeared to be limited to a surface atomic plane at low temperature. In-plane lattice constant relaxation was observed in both cases. A high nitridation temperature resulted into a Ga face and a low temperature to N-face polarity of overgrown GaN films. However, low temperature nitridation and an AlN buffer layer also produced a Ga-face polarity. The results are consistent with low formation energy of AlN/sapphire films with Ga-face polarity.
LiInSe
2
:
A biaxial ternary chalcogenide crystal for nonlinear optical applications in the midinfrared
Authors: Isaenko L. , Yelisseyev A. , Lobanov S. , Petrov V. , Rotermund F. , Slekys G. , Zondy J.-J.
Published: July 2002

Single crystals of lithium selenoindate (LiInSe2) are grown by the Bridgman–Stockbarger technique up to sizes of 10 mm in diameter and 20 mm in length. The different phases have color from yellow (as grown) to dark red (after annealing) but all have the same wurtzite type structure mm2 with slightly differing cell parameters. The band gap for the yellow phase at 300 K is at 2.86–2.87 eV. The red color is attributed to point defects and can be removed by proper illumination. Sellmeier equations are constructed for the 0.5–11 μm range and their validity is checked with second harmonic generation which provides first estimations of the nonlinear coefficients of LiInSe2. The potential of LiInSe2 is compared to that of the widely spread and technologically mature AgGaS2.
Complexity of precipitation patterns: Comparison of simulation with experiment
Authors: Polezhaev A. A. , Müller S. C.
Published: July 2002

Numerical simulations show that a simple model for the formation of Liesegang precipitation patterns, which takes into account the dependence of nucleation and particle growth kinetics on supersaturation, can explain not only simple patterns like parallel bands in a test tube or concentric rings in a petri dish, but also more complex structural features, such as dislocations, helices, ‘‘Saturn rings,’’ or patterns formed in the case of equal initial concentrations of the source substances. The limits of application of the model are discussed.
Average exit time for volume-preserving maps
Authors: Meiss J. D.
Published: July 2002

For a volume-preserving map, we show that the exit time averaged over the entry set of a region is given by the ratio of the measure of the accessible subset of the region to that of the entry set. This result is primarily of interest to show two things: First, it gives a simple bound on the algebraic decay exponent of the survival probability. Second, it gives a tool for computing the measure of the accessible set. We use this to compute the measure of the bounded orbits for the Hénon quadratic map.
High-voltage solar-cell chip
Authors: Kapoor V. J. , Valco G. J. , Skebe G. G. , Evans J. C.
Published: July 2002

Integrated circuit technology has been successfully applied to the design and fabrication of 0.5×0.5-cm planar multijunction solar-cell chips. Each of these solar cells consisted of six voltage-generating unit cells monolithically connected in series and fabricated on a 75-μm-thick, p-type, single crystal, silicon substrate. A contact photolithic process employing five photomask levels together with a standard microelectronics batch-processing technique were used to construct the solar-cell chip. The open-circuit voltage increased rapidly with increasing illumination up to 5 AM1 suns where it began to saturate at the sum of the individual unit-cell voltages at a maximum of 3.0 V. A short-circuit current density per unit cell of 240 mA/cm2 was observed at 10 AM1 suns.
Diffusion of radiolytic molecular hydrogen as a mechanism for the post-irradiation buildup of interface states in SiO2-on-Si structures
Authors: Griscom David L.
Published: July 2002

A review is made of recent literature dealing with radiation-induced point defects distributed volumetrically in thermally grown SiO2-on-Si or superficially at the silicon interface, with particular emphasis on the results of electron-spin-resonance experimentation. The observed defect types and their anneal kinetics are then compared with recent advances in the understanding of similar species and processes in irradiated bulk fused silica. It is concluded that radiolytic molecular hydrogen is formed in thermally grown SiO2 layers, just as it is in bulk fused silica, and that the diffusion of this hydrogen determines the temperature and time dependencies of the post-irradiation interface state buildups.
A model for the stationary cesium coverage on a converter surface in a cesium seeded hydrogen discharge
Authors: van Amersfoort P. W. , Tong Ying Chun , Granneman E. H. A.
Published: July 2002

A model is presented for the mechanism which establishes the dynamic equilibrium cesium coverage on a W(110) converter surface in the discharge chamber of a negative hydrogen ion source. The charge state of cesium particles arriving at the converter surface is found to be a crucial parameter. A coverage greater than 0.26 monolayers cannot be maintained if the cesium component is highly ionized. The corresponding negative hydrogen ion formation probability is far from optimum. This situation is probably present in all high-density surface conversion negative ion sources employed nowadays.
Optical properties of transparent and heat-reflecting indium tin oxide films: Refinements of a model for ionized impurity scattering
Authors: Hamberg I. , Granqvist C. G.
Published: July 2002

We compute the dynamic resistivity ρ≡ρ1+iρ2 for a model of heavily n-doped semiconductors with special regard to transparent and heat-reflecting In2O3:Sn films. The Sn impurities are ionized and the ensuing electrons occupy part of the In2O3 conduction band as a free-electron gas (with plasma energy ℏωp). We use pseudopotential arguments to show that the Sn ions act as Coulomb scatterers up to the largest electron densities of practical interest. By representing screening in the electron gas with different models, it was found that exchange and correlation could decrease ρ1 by ≲20% below ℏωp and alter the exponent s in the relation ρ1∝ωs, applying at ℏω>ℏωp, from −3/2 to ∼ −2.
Theoretically predicted and experimentally determined effects of the Si/(Si+C) gas phase ratio on the growth and character of monocrystalline beta silicon carbide films
Authors: Kim H. J. , Davis R. F.
Published: July 2002

The effects of the Si/(Si+C) ratio in the reaction gas stream on the growth and properties of monocrystalline β-SiC films grown on Si(100) substrates via chemical vapor deposition have been theoretically and experimentally studied. The amounts of condensed phases of β-SiC and Si, and the partial pressures of the remaining Si and C-containing gases as a function of the Si/(Si+C) ratio in the source gases have been initially obtained from thermodynamic calculations using the ‘‘solgasmix-pv’’ computer program. Complementary and comparative experimental growth studies have shown that inclusion-free films having maximum values in growth rate and carrier concentration and a minimum value of resistivity were obtained near Si/(Si+C)=0.5.
Co/GaAs interfacial reactions
Authors: Palmstro/m C. J. , Chang C. C. , Yu A. , Galvin G. J. , Mayer J. W.
Published: July 2002

The reactions of metals with GaAs are substantially more complex than comparable metal-silicon systems. In this paper, phase formation in the Co-GaAs system is examined in detail and contrasted with other metal-GaAs and metal-silicon systems. Both elemental and microstructural characterization techniques are used, and the limitations of each are discussed. Cobalt begins to react with GaAs at 375 °C and initially forms a ternary phase of approximate composition, Co2GaAs. Subsequent reactions or higher-temperature annealing results in the formation of binary phases CoGa and CoAs. The observed phase formation is controlled by the species with the largest diffusion coefficient, and this species is determined by the temperature. At sufficiently high annealing temperatures, arsenic is lost from the surface, and an epitaxial CoGa phase is formed on the GaAs.
Binding energies of shallow donors in semi-insulating GaAs
Authors: Wagner J. , Ramsteiner M.
Published: July 2002

We have used photoluminescence from two electron satellites of donor-bound excitons as well as selective pair luminescence spectroscopy to study shallow donor levels in semi-insulating bulk GaAs. Bound exciton spectroscopy gives a donor binding energy equal to the effective mass value (5.8 meV) for all samples investigated. Selective pair luminescence, in contrast, yields a sample-dependent value for the average donor binding energy, which varies between 5.8 and ∼7 meV. This discrepancy is attributed to the fact that bound exciton spectroscopy probes all donor levels whereas selective pair luminescence is sensitive to donors close to acceptors which are therefore more strongly disturbed.
A new secondary ion mass spectrometry technique for III-V semiconductor compounds using the molecular ions CsM+
Authors: Gao Y.
Published: July 2002

We describe a new secondary ion mass spectrometry technique for compositional analysis of matrix elements. It consists of detecting the molecular ions CsM+ rather than M± ions (M is the matrix element to be analyzed) under Cs+ primary ion bombardment. The linear behavior of the CsM+ ion yield makes the analysis independent of the matrix effect. An application to in-depth quantitative compositional analysis for AlGaAs/GaAs multilayer structures is given. The compositions were determined with an absolute accuracy better than 2% and the depth resolution was smaller than 100 Å.
Comparative study of low-pressure rare-gas fluoride/chloride lasers excited by a short-pulse electron beam
Authors: Suda Akira , Kumagai Hiroshi , Izawa Shingo , Obara Minoru
Published: July 2002

Output characteristics of the short-pulse electron-beam excited ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), and XeCl (308 nm) lasers were experimentally investigated at the same time on the same device for various mixtures all pumped at a high excitation rate of 2.3 MW/cm3. The optimum pressure of rare gas forming its rare-gas halide excimer increased for the sequence of ArF, KrCl, KrF, XeCl, and XeF, indicating that rare-gas halides were more quenched by the heavier rare gases, such as Xe, even at near-atmospheric pressures. At nearly atmospheric pressures the Ar-buffered mixtures gave higher laser energy than Ne-buffered mixtures except for the XeF laser. Through a series of measurements, the ArF laser showed the best result on the output energy of 96 J (5 J/ℓ ) with an intrinsic efficiency of 3.4%.
Reversible phase-change optical data storage in InSbTe alloy films
Authors: Maeda Yoshihito , Andoh Hisashi , Ikuta Isao , Minemura Hiroyuki
Published: July 2002

Some characteristics of reversible phase-change optical data storage based on an amorphous-crystalline transformation in InSbTe alloys are given. The reversible phase change was observed in a wide region of composition. The laser amorphized spot of a ternary compound In3SbTe2 film could be crystallized using a diode laser pulse of less than 100 ns with an incident laser power of more than 10 mW. The crystallization temperature of the amorphized spot was 280 °C and the activation energy was about 1.8 eV which shows that long-term data retention at room temperature is possible. The repetition number of static write and erase using the pulse of 50 ns reached above 105. These data show that the ternary compound film has potential for reversible optical data storage media with high-speed erasing and long-term data retention.
Photoluminescence investigation of the 1.356 eV band and stoichiometry in undoped GaAs
Authors: Shin K. C. , Kwark M. H. , Choi M. H. , Oh M. H. , Tak Y. B.
Published: July 2002

The origin of the 1.356 eV emission band measured by photoluminescence (PL) is investigated by correlating the Hall measurement data for the same materials. The crystals grown by the horizontal-Bridgman technique were As or Ga rich by controlling As-zone temperatures. Type conversion from n to p type with the thermal activation energy ΔE=0.133 eV and ΔE=0.05 eV were achieved by heat treatment of the As-rich crystals. The Ev +0.133 eV level and the 1.356 eV band may not be attributed to the same defect because the Ev +0.133 eV level is electrically active with a nonradiative center and the 1.356 eV band is radiative with an electrically inactive (neutral) center. Therefore, both Ev +0.133 eV level and the 1.356 eV band were not attributed to copper impurity because copper is electrically and optically an active center in GaAs. The 1.356 eV band measured by PL may be due to a gallium-vacancy related complex, but differs from VGa -donor complex.
Quantitative description of a high J
c Nb-Ti superconductor during its final optimization strain. I. Microstructure, T
c, H
c2, and resistivity
Authors: Meingast C. , Lee P. J. , Larbalestier D. C.
Published: July 2002

A most important step in the critical current density (Jc) optimization of Nb-Ti is the large final drawing strain, in which α-Ti precipitates, initially approximately equiaxed and 100–200 nm in diameter, are drawn into ribbons, whose thickness (1–2 nm) is less than the superconducting coherence length [ξ (4.2 K)∼5 nm]. Using transmission electron microscopy, the precipitate thickness, spacing, cross-sectional area, and circumference were measured over the whole final drawing strain range. Each of these parameters was found to have a simple power dependence on the wire diameter. Tc, Hc2, and the resistivity (ρn) were also change considerably during the refinement of the precipitates. Directly after precipitation, Tc increased, and (dHc2/dT)Tc and ρn were reduced from the single-phase values. Drawing the wire returned these parameters to their single-phase values, as the precipitate thickness was reduced to less than ξ. This observation explains a long-standing peculiarity in this system, namely that the optimum Hc2 of high Jc conductors occurs for a composition close to Nb 46 wt.% Ti, even when the precipitation of 18 vol % of α-Ti shifts the matrix composition to a Nb-rich composition of theoretically lower Hc2.
Response times and energy partitioning in electron-beam-excited plasmas
Authors: Kushner Mark J.
Published: July 2002

Excimer lasers are typically excited by electron beams (e beams) with initial energies of 100’s of keV to a few MeV. The e-beam response time is the interval required for beam electrons and their energetic secondary electrons to slow below the first inelastic thresholds of the buffer gas, below which the electrons thermalize by elastic momentum transfer collisions. In this paper, e-beam response times for rare gases and for gas mixtures typically used for excimer lasers are discussed using results from a Monte Carlo simulation. Issues pertaining to energy partitioning (W values in mixtures and effective electron temperatures) are also discussed. We find that e-beam response times may be >10’s of ns in gas mixtures of a few atm. As these times are commensurate with the rise time of e-beam pulses or the width of shorter pulses, beam slowing effects must be considered when modeling these phases of e-beam pumping.
Flicker noise in metal-oxide-semiconductor transistors from liquid helium to room temperature
Authors: Hafez I. M. , Ghibaudo G. , Balestra F.
Published: July 2002

A study of the normalized drain current spectral density SId/I2d as a function of drain current has been carried out on n channel metal-oxide-semiconductor transistors at very low (4 K), low (77 K), and room temperatures. It is demonstrated experimentally that the drain current dependence of SId/I2d is strongly correlated to that of the transconductance-current ratio squared (gm/Id)2. This result clearly shows that the carrier-number fluctuation model is applicable not only at room temperature but also at low and very low temperatures. Therefore, it is proved that the mobility fluctuation model is inadequate for the interpretation of our flicker noise data.
Nucleation-limited phase selection during reactions in nickel/amorphous-silicon multilayer thin films
Authors: Clevenger L. A. , Thompson C. V.
Published: July 2002

Isothermal and constant-heating-rate differential scanning calorimetry, cross-sectional transmission electron microscopy, thin-film x-ray diffraction, and thermodynamic and kinetic analyses have been used to investigate silicide phase selection in nickel/amorphous-silicon multilayer thin-film reactions. The atomic concentration ratio of the films was two Ni atoms to one Si atom and the layer thickness ratio was one to one. During deposition, a thin layer of amorphous nickel silicide formed between the nickel and amorphous-silicon layers. Upon heating, this amorphous nickel silicide thickened slightly, until crystalline Ni2Si formed at the nickel/amorphous-nickel-silicide interface. Further heating caused the simultaneous growth of both the amorphous nickel silicide and crystalline Ni2Si. Comparison of thermodynamic data to kinetic models for silicide formation and the analysis of calorimetry data suggests that nucleation barriers are responsible for the initial formation of the amorphous nickel silicide and limit subsequent formation of crystalline Ni2Si.
Reactive-ion- and plasma-etching-induced extended defects in silicon studied with photoluminescence
Authors: Weman H. , Lindström J. L. , Oehrlein G. S. , Svensson B. G.
Published: July 2002

Defects introduced by reactive-ion etching and plasma etching using deuterium have been studied in boron-doped Si with the photoluminescence (PL) technique. We have observed a set of broad luminescence bands in the below-band-gap range between 1.05 and 0.8 eV. These bands change in intensity as well as in photon energy with annealing. This has been studied by isochronal annealing treatments from 75 to 800 °C in steps of 50 °C, each for 30 min. Directly after the plasma treatment we observe overlapping broad bands at liquid-He temperature, with a peak around 0.9 eV and a half-width of about 100 meV. There is a large shift of these bands to higher photon energy after the annealing step at 325 °C, peaking at about 0.925 eV with a half-width of about 60 meV. The intensities of the broad PL bands increase with increasing annealing temperature up to about 375 °C, while they decrease in intensity at higher temperatures. The changes in PL intensity of the broad bands after annealing are shown to be related to the difference in deuterium concentration near the surface, as determined by secondary-ion mass spectrometry, due to the passivation effect the deuterium has on other competing recombination channels. The samples have not completely recovered after annealing at 800 °C, where a broad PL band at 0.96 eV still remains. PL bands observed in hydrogenated samples containing ‘‘bubbles’’ will also be reported. We attribute all these PL bands to electron-hole recombination in heavily damaged regions, where electrons and holes can be localized in potential wells caused by the strain from the hydrogen-induced microscopic defects. This ‘‘strain-induced intrinsic quantum well’’ model is supported by the temperature and excitation intensity dependence of the broad PL bands.
Evaporation of small drops
Authors: Kuz Victor A.
Published: July 2002

A theoretical approach to the phenomena of small drop evaporation is presented here. By considering the momentum balance and the energy balance surface equations, it is found that the evaporated mass rate is a parabolic function of the drop’s radius. This theory also predicts that the evaporation proceeds until the drop radius gets equal to the ‘‘Tolman’s length.’’ At this limit the liquid and the vapor become indistinguishable phases. The results are compared and discussed at the light of recent experimental work.
Stability of nonlinear optical characteristics and dielectric relaxations of poled amorphous polymers with main-chain chromophores
Authors: Teraoka I. , Jungbauer D. , Reck B. , Yoon D. Y. , Twieg R. , Willson C. G.
Published: July 2002

We have studied second-order nonlinear optical properties and dielectric relaxation characteristics for two amorphous epoxy polymers based on bisphenol-A with nitroaniline-type nonlinear optical moieties covalently attached as part of the main chain. The nonlinearities, rather large immediately after corona poling, undergo a slow decay at ambient temperature (e.g., ∼29% reduction in the nonlinear coefficient d33 in 28 days), even though the nominal glass transition temperature Tg is ∼80 °C. In dielectric relaxation measurements, both the unpoled and poled samples exhibit two relaxation modes: an α relaxation attributed to glass transition and a β relaxation due to local segmental motions of the main chain. Surprisingly, the critical temperature T∞ associated with the glass transition, obtained by a WLF [M. L. Williams, R. F. Landel, and J. P. Ferry, J. Am. Chem. Soc. 77, 3701 (1955)] fit of the α relaxation characteristics, shows a significant decrease (as much as 40 °C) with increasing poling field. Moreover, the relaxation times of temporal decay of field-induced alignment at ambient temperature, as determined from the stretched exponential function fit of birefringence changes, are found to be consistent with extrapolations from the the dielectric α relaxation characteristics of poled samples.