bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of hydrogen in controlling the growth of μc-Si:H films from argon diluted SiH4 plasma
Authors: Jana Madhusudan , Das Debajyoti , Barua A. K.
Published: July 2002

Hydrogenated microcrystalline silicon thin films have been prepared by the rf glow discharge method using argon as a diluent of SiH4 to achieve a high growth rate. μc-Si:H film having conductivity ∼10−5 S cm−1 was achieved at a deposition rate of 36 Å/min at a moderate power density of 90 mW/cm2, without hydrogen dilution. Micrograins were identified with several well defined crystallographic orientations. Inhomogeneity and porosity at the grain boundary zone have a significant effect on the electrical properties of the films due to adsorption when exposed to atmosphere. However, by adding hydrogen to the Ar-diluted SiH4 plasma, a homogeneous and improved network structure without having any effect of adsorption was obtained at a reduced deposition rate. Highly conducting (σD∼10−3 S cm−1) undoped μc-Si:H film was prepared at a deposition rate of 15 Å/min having 90% crystalline volume fraction. The energy released by the de-excitation of Ar* in the plasma initiates rapid nucleation in the Si network and atomic hydrogen in the plasma helps in the defect elimination, structural reorientation, and grain growth.
Electrical and spectroscopic comparison of HfO2/Si interfaces on nitrided and un-nitrided Si(100)
Authors: Kirsch P. D. , Kang C. S. , Lozano J. , Lee J. C. , Ekerdt J. G.
Published: July 2002

The interfacial chemistry of the high-k dielectric HfO2 has been investigated on nitrided and un-nitrided Si(100) using x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). The samples are prepared by sputter depositing Hf metal and subsequently oxidizing it. A 600 °C densification anneal is critical to completing Hf oxidation. These spectroscopic data complement electrical testing of metal oxide semiconductor capacitors fabricated with ∼50 Å HfO2 on nitrided and un-nitrided Si(100). Capacitors with interfacial nitride show reduced leakage current by a factor of 100 at a −1 V bias. Concurrently, interfacial nitride increased capacitance 12% at saturation. XPS shows that an interfacial layer composed of nonstoichiometric hafnium silicate (HfSixOy), forms at both the HfO2/Si and HfO2/SiNx interfaces. Differences in the Si 2p and O 1s XP spectra suggest more silicate forms at the un-nitrided interface. HfO2 films on un-nitrided Si show more O 1s and Si 2p photoemission intensity characteristic of HfSixOy. SIMS depth profiles through the buried interface are consistent with interfacial silicate formation, as shown by a HfSiO+ ion signal, that is sandwiched between HfO2 and SiNx. SiNx is suggested to minimize interfacial HfSixOy formation by limiting the amount of Si available to interact with the HfO2 layer.
Electronic structure and magnetic properties of Gd5(Si1−xGex)4
Authors: Samolyuk G. D. , Antropov V. P.
Published: July 2002

We study the electronic structure and magnetic properties of Gd5(Si1−xGex)4 system of alloys using band structure methods. The total energy, atomic magnetic moments are compared for the monoclinic and orthorhombic crystallographic structures. We confirmed that the orthorhombic structure has lower energy compared to the monoclinic one. The total energy calculations for the transformation from orthorhombic to monoclinic structure revealed no energy barrier. The addition of spin-orbit coupling does not change this result. The exchange coupling calculations show that effective Heisenberg model parameters do not exceed 100 K and decrease with transition from orthorhombic to monoclinic structure. We found that the total energy is sensitive to the specific ordering of Ge and Si atoms. This result agrees with experimental data of occupation numbers of T and T′ positions and may be related to the anomalies observed experimentally.
Magnetic printing technique for longitudinal thin film media with high coercivity of 6000 Oe
Authors: Saito Akira , Yonezawa Eiichi , Takano Yukihiro , Watanabe Takeshi
Published: July 2002

A magnetic printing technique was satisfactorily demonstrated for longitudinal hard-disk media with coercivity of 3000 Oe, in which soft magnetic patterns formed on a master disk as servo signals were magnetically transferred onto the media. The magnetic field (the printing field) applied to the master disk during the printing process is required to be as high as the coercivity of the media, which increases as the areal density of the hard-disk drive increases. However, high printing fields have been reported to cause magnetic saturation of the patterns and extraneous subpulses due to that saturation. This article addresses the issue of whether magnetic printing for media with high coercivity can be satisfactorily carried out. We observed playback signals from printed media with different coercivities ranging up to 6450 Oe and demonstrated that the magnetic printing technique for media with coercivity of up to 6000 Oe was satisfactorily implemented.
Organometallic synthesis and magnetic properties of ferromagnetic Sm–Co nanoclusters
Authors: Ono K. , Kakefuda Y. , Okuda R. , Ishii Y. , Kamimura S. , Kitamura A. , Oshima M.
Published: July 2002

We have successfully prepared Sm–Co composite nanoclusters using liquid-phase organometallic synthesis. The chemical composition was determined by quantitative x-ray fluorescence analysis and it is found that the composition of synthesized Sm–Co nanoclusters was Sm poor while the Sm–Co nanoclusters with required composition could be obtained in starting from the excess Sm(acac)3. From the transmission electron microscopy measurements, the Sm–Co nanoclusters have the uniform size with the diameter of 9 nm. The crystalline structure was fcc which is different from that of bulk SmCo alloy with the same Sm and Co content. The magnetic property was observed by superconducting quantum interference magnetometer and shows the ferromagnetic characteristics.
Electron paramagnetic resonance linewidths and line shapes for the molecular magnets Fe8 and Mn12
Authors: Park Kyungwha , Novotny M. A. , Dalal N. S. , Hill S. , Rikvold P. A.
Published: July 2002

We study theoretically electron paramagnetic resonance (EPR) linewidths for single crystals of the molecular magnets Fe8 and Mn12 as functions of energy eigenstates Ms, frequency, and temperature when a magnetic field along the easy axis is swept at fixed excitation frequency. This work was motivated by recent EPR experiments. To calculate the linewidths, we use density-matrix equations, including dipolar interactions and distributions of the uniaxial anisotropy parameter D and the Landé g factor. Our calculated linewidths agree well with the experimental data. We also examine the line shapes of the EPR spectra due to local rotations of the magnetic anisotropy axes caused by defects in samples. Our preliminary results predict that this effect leads to asymmetry in the EPR spectra.
Tradeoffs for magnetic printing of servo patterns
Authors: Baker Bill R.
Published: July 2002

Simple Schwarz–Christoffel mapping is used to estimate fields and flux components of a permeable master for printing reference patterns for self-servo writing. Geometry of lines and spaces govern effectiveness of the printing. Magnetic images from a spin stand illustrate adequate control of noise components.
Correlated electron-pair properties of the Be atom in position and momentum spaces
Authors: Koga Toshikatsu
Published: July 2002

Based on multiconfiguration Hartree–Fock calculations, correlated electron-pair intracule (relative motion) and extracule (center-of-mass motion) properties are reported for the Be atom in position and momentum spaces. Particularly in the latter space, the present results are more accurate and consistent than those in the literature.
Nanoindentation hardness and adhesion investigations of vapor deposited nanostructured diamond films
Authors: Catledge Shane A. , Borham James , Vohra Yogesh K. , Lacefield William R. , Lemons Jack E.
Published: July 2002

The effect of changing the N2/CH4 feedgas ratio on the structure and mechanical properties of microwave plasma chemical vapor deposited diamond films grown on Ti–6Al–4V alloy substrates was investigated. The relative concentration of CH4 and N2 (in a balance of H2) was shown to strongly influence film structure, hardness, and adhesion. For high CH4 concentration (15% by volume), nanostructured diamond films with roughness magnitudes of 15–30 nm, good adhesion and a high hardness value of 90 GPa was obtained. A distinct correlation was found between the nanoindentation hardness of the deposited film and the N2/CH4 ratio in the plasma, as well as a correlation of hardness to the ratio of the Raman peak intensities (1332 and 1555 cm−1). Scratch adhesion testing of nanostructured diamond films showed delamination at a critical force of 33 N using acoustic emission techniques. These results demonstrate that nanostructured diamond films can be tailored on metallic surfaces with hardness ranging from 10 GPa (medium-hard) to 100 GPa (superhard) and may be considered for wear resistant applications such as in the design of articulating medical implant devices.
Control of laser desorption using tunable single pulses and pulse pairs
Authors: Hess Wayne P. , Joly Alan G. , Gerrity Daniel P. , Beck Kenneth M. , Sushko Peter V. , Shluger Alexander L.
Published: July 2002

We desorb ground state Br and spin–orbit excited Br* atoms from KBr single crystals using single pulses and sequential pulse pairs of tunable nanosecond laser radiation. Irradiation of cleaved KBr crystals near the bulk absorption threshold produces hyperthermal Br emission without a significant thermal component, and with little spin–orbit excited Br* emission. The Br kinetic energy distribution may be controlled either by choice of photon energy or by excitation of transient defect centers created within the crystal. In this latter scheme, a first laser pulse generates transient centers within the bulk crystal and in the vicinity of the surface, and a second delayed laser pulse then excites the transient centers leading to atomic desorption. The Br* to Br yield ratio is significantly enhanced using two-pulse excitation as compared to resonant single-pulse desorption. Single and multiple pulse excitation of KBr produces Br and Br* in controllable quantities, velocities, and spin state distributions.
Electron-stimulated desorption of positive ions from methane and fluoromethane nanoclusters
Authors: Souda Ryutaro
Published: July 2002

Electron-stimulated positive-ion desorption has been investigated for CH4, CF4, CHF3, and CH2F2 molecules adsorbed on a solid Ar substrate. The ion yields from CH4 are enhanced markedly at the smallest coverage and decay steeply with increasing coverage. In contrast, ion yields from CF4 increase monotonically with increasing coverage. The ions from hydrofluorocarbons exhibit both behaviors. Coulomb explosion associated with the delocalization of valence holes is responsible for the enhanced ion yields from nanoclusters, whereas the monotonic increase of the ion yields with coverage is caused by the dissociative ionization of individual molecules in which valence holes are localized. The localization (delocalization) of valence holes can be ascribed to the nature of the F 2p nonbonding (C–H bonding) orbitals.
Theoretical studies of the vibrational states of Ne2SH and Ne2OH (Ã 2Σ+)
Authors: Lee Hee-Seung , McCoy Anne B.
Published: July 2002

Variational calculations of the 25 lowest energy vibrational states of the Ne2SH and Ne2OH (Ã 2Σ+) complexes are performed using potentials that are obtained by the pairwise addition of accurate neon dimer and Ne⋅XH potentials. The Ne2XH complexes are able to sample several local minima in their respective potentials at relatively low internal energies. Consequently, both complexes display average densities of states that exceed one vibrational state per cm−1 in the energy regime covered by this study. In spite of this, all of the calculated states are assigned to specific vibrational excitations, based on a structure of the complex that corresponds to one of the minima on the potential, and all of the minima are found to support progressions of bending states. In most cases, it is found that the wave functions have most of their amplitude in one of the potential minima, but the couplings between zero-order states that are localized in different minima increase as the internal energy is increased. The extent of mixing is found to depend on the symmetry of the wave functions with respect to the interchange of the two neon atoms. Effects of the differences in the anisotropies of the Ne⋅OH and Ne⋅SH potentials on the vibrational energies and wave functions are also investigated.
Regime for a self-ionizing Raman laser amplifier
Authors: Clark Daniel S. , Fisch Nathaniel J.
Published: July 2002

Backward Raman amplification and compression at high power might occur if a long pumping laser pulse is passed through a plasma to interact resonantly with a counter-propagating short seed pulse [V. M. Malkin et al., Phys. Rev. Lett. 82, 4448–4451 (1999)]. One critical issue, however, is that the pump may be unacceptably depleted due to spontaneous Raman backscatter from intrinsic fluctuations in the amplifying plasma medium prior to its useful interaction with the seed. Premature backscatter may be avoided, however, by employing a gaseous medium with pump intensities too low to ionize the medium and using the intense seed to produce the plasma by rapid photoionization as it is being amplified [V. M. Malkin et al., Phys. Plasmas 8, 4698–4699 (2001)]. In addition to allowing that only rather low power pumps be used, photoionization introduces a damping of the short pulse which must be overcome by the Raman growth rate for net amplification to occur. The parameter space of gas densities, laser wavelengths, and laser intensities is surveyed to identify favorable regimes for this effect. Output laser intensities of 2×1017 W/cm2 for 0.5 μm radiation are found to be feasible for such a scheme using a pump of 1×1013 W/cm2 and an initial seed of 5×1014 W/cm2 over an amplification length of 5.6 cm in hydrogen gas.
A device for surface study of confined micron thin films in a total internal reflection geometry
Authors: Mamedov Sergey , Schwab Alexander D. , Dhinojwala Ali
Published: July 2002

A device to probe the molecular structure of materials next to a solid interface in a thin film geometry has been developed. The device can produce controlled thicknesses as small as 1 μm with parallelity better than 0.003°. We have shown that the thickness and the parallelity of the film produced between two optical surfaces can be quantified using white light and monochromatic light interferometry, respectively. In addition, this apparatus allows the study of these films in a static state or under shear using spectroscopic techniques involving transmission or reflection measurements.
Photo-induced intra-complex reactions in Mg+-2,2,2-trifluoroethanol
Authors: Guo Wenyue , Liu Haichuan , Yang Shihe
Published: July 2002

We induced the intra-complex reactions in Mg+-2,2,2-trifluoroethanol by photons in the spectral region of 240–410 nm. We observed the nonreactive channel product Mg+ and a number of reactive channel products throughout the whole excitation region. The reactive photoproducts originate from the scission of the C–O or C–F bond, as well as from the simultaneous rupture of both bonds. The action spectrum consists of two broad peaks between the atomic transition of Mg+(3 2P←3 2S). Ab initio calculations show a minimum-energy structure, in which Mg+ attaches to the O atom and one of the three F atoms of 2,2,2-trifluoroethanol, forming a five-membered ring. The calculated absorption spectrum corresponding to the minimum-energy structure agrees nicely with the experimental action spectrum. The branching fractions of the photoproducts are approximately constant in a given spectral range, but they change significantly across the spectral ranges. The constant branching fractions point to the ground state reactions following traversals from the initially accessed excited states through conical intersections. On the other hand, the observation of the electronic state-specific branching fractions is explained by the orbital alignments of the excited electron, which is dictated by the five-membered ring structure of the complex.
Optimal leveling of flow over one-dimensional topography by Marangoni stresses
Authors: Gramlich C. M. , Kalliadasis Serafim , Homsy G. M. , Messer C.
Published: July 2002

A thin viscous film flowing over a step down in topography exhibits a capillary ridge preceding the step. In applications, a planar liquid surface is often desired and hence there is a need to level the ridge. This paper investigates optimal leveling of the ridge by means of a Marangoni stress such as might be produced by a localized heater creating temperature variations at the film surface. The differential equation for the free surface based on lubrication theory and incorporating the effects of topography and temperature gradients is solved numerically for steps down in topography with different temperature profiles. Both rectangular “top-hat” and parabolic profiles, chosen to model physically realizable heaters, were found to be effective in reducing the height of the capillary ridge. Leveling the ridge is formulated as an optimization problem to minimize the maximum free-surface height by varying the heater strength, position, and width. With the optimized heaters, the variation in surface height is reduced by more than 50% compared to the original isothermal ridge. For more effective leveling, we consider an asymmetric n-step temperature distribution. The optimal n-step heater in this case results in (n+1) ridges of equal size; 2- and 3-step heaters reduce the variation in surface height by about 70% and 77%, respectively. Finally, we explore the potential of coolers and step temperature profiles for still more effective leveling.
Mono-halobenzenes anion fragmentation induced by atom–molecule electron-transfer collisions
Authors: Lobo R. F. M. , Vieira P. Limão , Godinho S. S. M. C. , Calhorda M. J.
Published: July 2002

The crossed molecular beam technique is used to study the formation of negative ions in monosubstituted halobenzenes, by electron transfer collisions, using potassium atoms as electron donor projectiles, in a collision energy range from 10 up to 500 eV. The negative ions are detected by time-of-flight mass spectrometry and a quasidiatomic dynamical behavior in the fragmentation is found. A minor negative fragmentation of the phenyl ring is also reported. The parent negative ion is not observed for the range of collision energies in use. Density functional theoretical calculations support the major abundance of the halogen negative ions and explain the completely different behavior in the negative fragmentation of these aromatic systems compared to the one previously observed with the three fluoro-iodo-benzene isomers.
A high-energy x-ray spectrometer diagnostic for the OMEGA laser
Authors: Hudson L. T. , Henins A. , Deslattes R. D. , Seely J. F. , Holland G. E. , Atkin R. , Marlin L. , Meyerhofer D. D. , Stoeckl C.
Published: July 2002

A new x-ray diagnostic has been commissioned at the OMEGA laser facility at the University of Rochester. It is a transmission curved crystal spectrometer designed primarily to characterize the hot-electron energy distribution of laser generated plasmas by registering the continuum x-ray spectrum produced by these hot plasmas from 12 to 60 keV. The diagnostic package is assembled in a linear configuration to ride in a standard instrument insertion module. The instrument consists of a nosecone with a blast shield, spectrometer, electronic imager, drive electronics, and battery. The instrument is connected to the external diagnostic processor and control unit by trigger and data fiber optic cables. Time integrated spectra from various targets have been registered with high sensitivity from single shots of the OMEGA laser.
Application of ion imaging to the electron impact ionization of oxygen using crossed beams and a custom designed fast multichannel pulse generator
Authors: Harland Peter W. , Cameron Brett R. , Bart Mark , Pearce Mike O.
Published: July 2002

In this article we introduce the application of ion imaging to the detection of product ions produced in electron–molecule collisions. Unlike photoinitiation experiments in which a short duration laser pulse is used to excite a reagent molecule or to prepare an exotic species for subsequent reaction, collisions involving electrons (or ions) involve field-sensitive charged beam pulses of longer duration than laser pulses. Such experiments require the fast switching of electric fields with particular attention to the elimination of stray fields, contact potentials, and noise pickup on cables and electrical feedthroughs. We have recorded preliminary O2+ and O+ ion images from 100 eV electron impact ionization of O2 supersonic molecular beams. The triggering of the nozzle, field plates, electron gun, and repeller/extractor elements was achieved using a low cost, versatile, fast multichannel pulse generator with fiber-optic coupling that was designed to produce low noise trigger pulses for commercial and custom made pulsed high voltage power supplies.
Vibrational spectra of halobenzene cations in the ground and B̃ 2B2 electronic states obtained by one-photon mass-analyzed threshold ionization spectrometry
Authors: Kwon Chan Ho , Kim Hong Lae , Kim Myung Soo
Published: July 2002

Vibrational spectra in the ground electronic states, X̃ 2B1, of fluoro-, chloro-, bromo-, and iodobenzene cations and in the excited electronic states, B̃ 2B2, of chloro- and bromobenzene cations have been measured by mass-analyzed threshold ionization (MATI) via one-photon excitation with vacuum ultraviolet light. The spectra consist mostly of strong 0–0 bands and fundamentals, overtones, and combinations of totally symmetric a1 modes. In particular, the observed vibrational bands in the B̃ states have been assigned by referring to the assignments for the ionic ground states and those for the neutral molecules. From the MATI spectra, the ionization energies of fluoro-, chloro-, bromo-, and iodobenzene to the ionic ground states have been determined to be 74 229±5, 73 177±5, 72 570±5, and 70 638±5 cm−1, respectively. The ionization energies to the B̃ states have also been determined to be 91 404±5 and 85 822±5 cm−1 for chloro- and bromobenzene, respectively.
Maximum x-ray energy from compact electron cyclotron resonance based source of x rays
Authors: Rouillé C. , Serebrennikov K. , Bacal M.
Published: July 2002

The x-ray spectra from an electron cyclotron resonance argon plasma produced in a compact magnetic trap, containing a tungsten target, are reported. The maximum attainable x-ray energy was recorded for different magnetic field configurations and energies higher than 300 keV were found. The maximum x-ray energy is strongly peaked near the change of the resonance surface topology from a one sheet hyperboloid to a two sheet hyperboloid. This observation is explained theoretically. Different regimes of x-ray production are described. Two distinct pressure ranges where x rays are produced were found. The characteristic lines of metals, e.g., from the target (tungsten), protection (lead), are clearly resolved. The radiography of an integrated circuit is shown as an example of possible use of the x-ray source for nondestructive control.
Time-of-flight spectrometer for absolute measurements of multiply charged ions and ionic molecular fragments in the gas phase
Authors: Santos A. C. F. , Melo W. S. , Sant’Anna M. M. , Sigaud G. M. , Montenegro E. C.
Published: July 2002

A time-of-flight spectrometer using an extended target gas cell was built to perform absolute measurements of the yields of ionic species in the gas phase produced by charged particles. A detailed description of the spectrometer design and of an independent, absolute calibration procedure is given. The performance of the spectrometer was verified through absolute measurements of multiple ionization of noble gases by charged particles.
Slit-wire camera, a new diagnostics method for measurement of small scale high-energy density structures in hot plasmas
Authors: Choi P. , Dumitrescu C. , Wyndham E. , Favre M. , Chuaqui H.
Published: July 2002

A new technique has been developed for the detection of small scale plasma objects, particularly in the x-ray and extreme ultraviolet region. The technique is based on a new device, called a slit-wire (S-W) camera. The S-W camera uses a combination of a slit, to provide one-dimensional imaging with large effective aperture, and a set of micron size wires across the slit to provide information about the emitting source based upon blocking of the incident light signal. A series of simultaneous slit-wire images of the same discharge, each with a different filter material, allows the characteristic size of the emitting plasma to be determined over a wide spectral range. Time resolved measurements of the plasma radius of a capillary discharge plasma and time integrated measurements of the hot-spot size in a plasma focus discharge are presented to illustrate the technique.
Increased transmission of a thin gold film by adsorbed layers of dye molecules
Authors: Fischer Ulrich C. , Bortchagovsky Eugene , Heimel Jörg , Hanke René T.
Published: July 2002

Thin multilayers of a negatively charged cyanine dye and a positively charged polyelectrolyte were deposited onto a thin, partially transparent film of gold, initially covered by a self-assembled monolayer of cystamine. Transmission spectra of the multilayers have an antisymmetric shape with a dip and a peak in the transmission. Unlike the transmission spectra of the same dye on a transparent glass support, the spectra reflect the shape of the real part of the dielectric function of the dye layer. At the peak the transmission exceeds the transmission of the uncovered gold film by a factor of up to 2.5.
On dynamical zeta function
Authors: Bogomolny Eugene
Published: July 2002

The dynamical zeta function is usually defined as an infinite (and divergent) product over all primitive periodic orbits. It is possible to show that as ℏ→0 it can be represented as det(1−T), where the operator T(q,q′) defines the semiclassical Poincaré map. Here, certain consequences of this representation for chaotic systems are discussed. In particular, it is shown that the zeta function can be expressed through a subset of specially selected orbits, the error of this approximation being small as ℏ→0. Assuming that the chosen Poincaré surface of section is divided into small cells of phase-space area of 2πℏ, these trajectories are uniquely characterized by the requirement that they never go twice through the same cell.
Complex patterns in reaction-diffusion systems: A tale of two front instabilities
Authors: Hagberg Aric , Meron Ehud
Published: July 2002

Two front instabilities in a reaction-diffusion system are shown to lead to the formation of complex patterns. The first is an instability to transverse modulations that drives the formation of labyrinthine patterns. The second is a nonequilibrium Ising–Bloch (NIB) bifurcation that renders a stationary planar front unstable and gives rise to a pair of counterpropagating fronts. Near the NIB bifurcation the relation of the front velocity to curvature is highly nonlinear and transitions between counterpropagating fronts become feasible. Nonuniformly curved fronts may undergo local front transitions that nucleate spiral-vortex pairs. These nucleation events provide the ingredient needed to initiate spot splitting and spiral turbulence. Similar spatiotemporal processes have been observed recently in the ferrocyanide–iodate–sulfite reaction.
Synchronization and control of spatiotemporal chaos using time-series data from local regions
Authors: Parekh Nita , Ravi Kumar V. , Kulkarni B. D.
Published: July 2002

In this paper we show that the analysis of the dynamics in localized regions, i.e., sub-systems can be used to characterize the chaotic dynamics and the synchronization ability of the spatiotemporal systems. Using noisy scalar time-series data for driving along with simultaneous self-adaptation of the control parameter representative control goals like suppressing spatiotemporal chaos and synchronization of spatiotemporally chaotic dynamics have been discussed.
Implementation of the Car–Parrinello algorithm for ab initio total energy calculations on a massively parallel computer
Authors: Brommer Karl D. , Larson B. E. , Needels M. , Joannopoulos J. D.
Published: July 2002

An implementation of the Car–Parrinello algorithm for total energy pseudopotential calculations is undertaken on a massively parallel supercomputer, the Connection Machine CM-2. Its large memory and throughput makes feasible computations with systems involving more atoms than previously possible on conventional vector supercomputers. Calculations of the Takayanagi 7×7 reconstruction of the Si(111) surface demonstrate that computations involving 1000 atoms at realistic cutoff energies are now possible using ab initio techniques.
Transputer-based simulations of image recording in silver-halide materials
Authors: Hailstone R. K.
Published: July 2002

Monte Carlo simulations are used to study the photophysics and photochemistry of the image-recording process in silver-halide microcrystals in a film coating. Because the microcrystals are independent of each other during exposure to light, the simulation is ideally suited to a parallel processor in which each processor represents a microcrystal in the film coating. This paper describes the hardware and software of a 64-node transputer-based parallel processor. Emphasis is placed on the occam code used to distribute the simulation software over the transputer array and on how communications and computations are balanced in a multiprocessor system. Benchmarking results and preliminary data are also included.
Temperature effects on the optical energy gap values of CdxZnyMnzTe alloys
Authors: Donofrio T. , Lamarche G. , Woolley J. C.
Published: July 2002

Wavelength modulated reflectance measurements were made on a wide range of CdxZnyMnzTe alloys to determine the variation of the energy gap E0 as a function of temperature in the range 10–300 K. It was shown that in the vicinity of the spin-glass transition temperature Tg, there is a magnetic contribution ΔE which increases E0, and ΔE was determined as the difference between the experimental values and a simple Manoogian–Leclerc extrapolation from higher temperatures. These values of ΔE were fitted to the relation (dt/dt)(ΔE)=−Pt−μ where t=‖T−Tg‖/Tg and good agreement found with the theoretical predictions of Alexander et al. for the effect on the band gap of a critical point such as Tg. As predicted, different behavior was observed close to Tg and at temperatures outside the critical region, μ having a value close to zero in the first range and close to 0.5 in the second. The values of P were very similar for both ranges, extrapolating to zero at the nearest-neighbor percolation limit of z=0.18 and showing a discontinuity in slope in the vicinity of z∼0.6 where a transition from spin-glass to antiferromagnetic behavior occurs.
Domain behavior in sintered Nd-Fe-B magnets during field-induced and thermal magnetization change
Authors: Li D. , Strnat K. J.
Published: July 2002

On sintered Nd-Fe-B permanent magnets, magnetic domain pattern changes in applied fields and at elevated temperatures were observed by the Kerr effect. Patterns on pole and side faces were recorded for different magnetization states in fields up to 17 kOe at 20 °C, and remanent patterns at temperatures up to TC. They are qualitatively interpreted. On pole faces, most grains are multidomain even at remanence. But the surface domains seen are not characteristic of the magnet interior. Side faces show mostly single-domain grains at remanence. These images appear to reflect bulk behavior. Walls are strongly pinned at grain boundaries but move easily in the main magnetic phase, a behavior analogous to that of sintered SmCo5.
High brightness electrostatically focused field emission electron gun for free electron laser applications
Authors: Kirkpatrick D. A. , Shefer R. E. , Bekefi G.
Published: July 2002

An electrostatically focused, multistage electron gun using a cold (field emission) cathode has been developed for use in a free electron laser. This gun produces a 1.1-kA, 2.0-MV electron beam with a normalized emittance εn≂38×10−3 (π cm rad), and a brightness of 74 kA/cm2 rad2. The measured brightness is almost two orders of magnitude higher than that found in most rf linacs using conventional thermionic cathodes, and is comparable to that achieved using electron guns immersed in strong guiding magnetic fields. The emittance and brightness have been studied as a function of cathode material, surface finish, and the electric field strength at the cathode surface.
Hole transport in gallium antimonide
Authors: Heller M. W. , Hamerly R. G.
Published: July 2002

The Hall coefficients R, Seebeck coefficients S, and electrical resistivities ρ of two undoped samples of p-type GaSb were measured over the temperature ranges 20–300 K for R and ρ and 40–300 K for S. The theoretical transport coefficients R, S, and ρ were fitted in a self-consistent way to the experimental data by adjusting the material parameters. The temperature range of the analysis (80–300 K) was found to be in the transition region between a multiellipsoidal valence band structure at low temperatures and a warped sphere structure at higher temperatures. The value deduced for the heavy hole conductivity effective mass was found to be about one third of the heavy hole density-of-states effective mass md1≊0.9 m0 which was used from the theory of Kolodziejczak.
Hg1−xCdxTe-Hg1−yCdyTe (0≤x, y≤1) heterostructures: Properties, epitaxy, and applications
Authors: Herman M. A. , Pessa M.
Published: July 2002

This article presents a review of the state of the art of research and development on Hg1−xCdxTe -Hg1−yCdyTe (0≤x,y≤1) heterostructures important for applications in the modern infrared detection technique. It deals with the fundamental physical properties, epitaxial growth methods, and applications of these structures. The most important experimental results relevant to this subject are described and discussed. Following a short survey of the physical properties of Hg1−xCdxTe, the travelling heater method for growing bulk crystals of Hg1−xCdxTe has been described and compared with the epitaxial growth techniques used to prepare thin films and layered structures of this compound. Some important aspects of substrate preparation procedures related to CdTe wafers have been discussed. Then the most important problems regarding the liquid-phase, vapor-phase, and molecular-beam-epitaxy methods of Hg1−xCdxTe -Hg1−yCdyTe (0≤x,y≤1) heterostructures have been studied. A comprehensive discussion of technology and the parameters of different heterostructure photodiodes made of Hg1−xCdxTe with electrically passive and electrically active heterointerfaces has been presented. The review is concluded with an overview of research problems relevant to HgTe-CdTe superlattices and the surfaces and heterointerfaces of Hg1−xCdxTe.
Structure and morphology of polycrystalline silicon-single crystal silicon interfaces
Authors: Bravman John C. , Patton Gary L. , Plummer James D.
Published: July 2002

Using high resolution transmission electron microscopy, morphological aspects of the polycrystalline silicon (polysilicon)-single crystal silicon interface have been correlated to the surface treatment used prior to polysilicon deposition, and to high-temperature annealing. Specimens which were chemically oxidized prior to the deposition exhibited a continuous layer of amorphous oxide ∼15 Å thick; high-temperature annealing results in the formation of small discontinuities in this oxide, and thus small regions of epitaxial realignment within the polysilicon layer. Specimens which were etched in HF prior to deposition were characterized by nearly oxide-free interfaces, and, following high-temperature annealing, exhibited regions of epitaxial realignment an order of magnitude larger than those found in the chemically oxidized samples.
Molecular model for cooperative propagation of shock-induced detonations in energetic solids and its application to nitromethane
Authors: Peyrard M. , Odiot S. , Lavenir E. , Schnur J. M.
Published: July 2002

We present a simple molecular model for the propagation of shock-induced detonation waves in energetic solids. The explosive propagation of molecular dissociation is explained by energy transmission from one molecule to the next one. A numerical study shows that the model can explain the main experimental features of detonations in energetic compounds and particularly the fact that the detonation speed is characteristic of the material and not of the initial excitation, the existence of two unequal dissociation and detonation thresholds, and the possibility of ‘‘superdetonations’’ for large initial excitations. This model allows us to point out microscopic conditions that a solid must fulfill to sustain detonation waves. It demonstrates that the intramolecular potential plays a significant role in determining the characteristics of a detonation. The model predicts that a materials sensitivity to shocks and the velocity of the detonation wave are largely independent. It is then applied to solid nitromethane: The parameters that it involves are determined for this compound and we show that nitromethane meets the requirements to sustain detonation waves with a propagation speed which is in agreement with the experimental value estimated for this compound in solid phase.
Monte Carlo simulation of wetting transitions in the ferromagnetic Ising model
Authors: Binder K. , Landau D. P.
Published: July 2002

We present a Monte Carlo study of both critical wetting and first-order wetting transitions in an Ising model with a surface field opposite to the magnetization direction in the bulk. ‘‘Profiles’’ of magnetization and energy as well as the surface layer magnetization m1 and susceptibility χ1,1 are obtained and discussed in the light of current theory.
An analysis of dislocation reduction by impurity hardening in the liquid-encapsulated Czochralski growth of 〈111〉 InP
Authors: Jordan A. S. , Brown G. T. , Cockayne B. , Brasen D. , Bonner W. A.
Published: July 2002

Excessive impurity additions have been widely used to suppress dislocation generation in the liquid-encapsulated Czochralski (LEC) growth of InP. We have analyzed this approach by means of the quasi-steady-state heat transfer/thermal stress model. A strong motivation for the investigation was provided by the recent measurement of the critical resolved shear stress σCRS of InP as a function of temperature in the range 748–948 °K for several Ge and S concentrations. The experimental data were analyzed by the method of least squares via the usually accepted logarithmic dependence of σCRS on reciprocal temperature. The extrapolated values of σCRS exhibit a monotonic increase with impurity addition at temperatures near the melting point. Introducing the σCRS and realistic estimates of other physical properties (thermal diffusivity, thermal expansion coefficient, elastic constants, etc.) in the thermal stress model, the dislocation distribution pattern in a {111} substrate cut from a 〈111〉 boule was constructed. This necessitated a suitable recasting of the formalism that was previously applicable only to the {100} orientation. The computed dislocation contour maps on {111} wafers display sixfold symmetry resembling the ‘‘Star of David,’’ in overall agreement with etch-pit patterns. InP crystals 2.5 cm in diameter grown in a standard high ambient temperature gradient but containing a large amount of Ge (≂1019 cm−3) are predicted and observed to be dislocation-free. On the other hand, in nominally undoped material a large density of defects is forecast, especially at the periphery, in line with the etchpit configuration. Intermediate doping levels (∼1017 cm−3 Ge, ∼1018 cm−3 S) reduce the density in the core but leave the outer edge essentially unaltered.
Hydrogen adsorption states at the external and internal palladium surfaces of a palladium-silicon dioxide-silicon structure
Authors: Petersson L.-G. , Dannetun H. M. , Fogelberg J. , Lundström I.
Published: July 2002

The hydrogen adsorption states at the external and internal Pd surfaces of a hydrogen sensitive Pd-SiO2-Si (Pd-MOS) structure have been studied in high and ultrahigh vacuum. The steady-state response of the Pd-MOS structure (due to the hydrogen present at the Pd-SiO2 interface) shows a logarithmic dependence on hydrogen pressure, independent of external surface condition. The heat of adsorption of the internal surface is coverage dependent and described by ΔHi =ΔH0(1-aθi) giving ΔH0≊1.4 eV/molecule with a=1. Thus, there are always hydrogen adsorption states available at the internal surface independent of the applied pressure. This is why the Pd-MOS structure works as a hydrogen sensor over a large pressure range. The importance of a surface cleaning procedure in order to obtain an atomically clean Pd surface after Ar sputtering is pointed out. It is observed that a, probably carbon, contaminated Pd surface has hydrogen adsorption properties similar to those of the palladium-oxide interface. A clean Pd surface has adsorption sites with smaller heat of adsorptions than those at the interface. Furthermore, these adsorption sites give an increase in work function, whereas a decrease in work function is observed at the interface. It is pointed out that the difference in the heat of adsorption between surface and interface hydrogen adsorption sites means that hydrogen atoms are detected at the interface even when the hydrogen coverage at the clean surface is very small.
Direct measurement of solid-phase epitaxial growth kinetics in GaAs by time-resolved reflectivity
Authors: Licoppe C. , Nissim Y. I. , Meriadec C.
Published: July 2002

Complete solid-phase epitaxial regrowth of ion-implanted layers in GaAs has been obtained in the temperature range 150–400 °C. Implantation of tellurium at an energy and dose slightly greater than the amorphization threshold was used to produce an amorphous layer in the near-surface region of the GaAs samples. Complete crystallization was achieved over the entire temperature range using a resistively heated sample holder and cw laser irradiation. In situ time-resolved optical reflectivity measurements were used to observe and measure the epitaxial growth rate of the process. It has been found that the solid-phase epitaxy process follows an activation law whose activation energy is 1.6 eV.
High-pressure and molecular weight variation of electrical relaxation in poly(ethylene oxide)
Authors: Wintersgill Mary C. , Fontanella John J. , Welcher Peter J. , Andeen Carl G.
Published: July 2002

Vacuum audio frequency complex admittance measurements have been performed on poly(ethylene oxide) using a fully automated dielectric spectrometer. Measurements have been made over the temperature range 5.5–320 K for molecular weights from 1.85×104 to 4×106. The γ relaxation is essentially unchanged over the range of molecular weights studied. This represents evidence against assigning the γ relaxation to the motion of end groups. Next, previously reported data for the γ relaxation are reanalyzed using a reduced plot. The activation volume is found to be 3–4 cm3/mol in good agreement with the previous work and the small value implies that the relaxation is associated with the motion of very small segments of the polymer chain. Next, the relaxation associated with the glass transition αa is only observed in temperature for some samples but is observed in both frequency and temperature for others. The strength of αa decreases monotonically as molecular weight decreases. Finally, αa was studied up to pressures of about 0.2 GPa for 5×106 molecular weight material. The results for the shift of αa with pressure are in qualitative agreement with recent theories.
Semi-insulating properties of Fe-implanted InP. I. Current-limiting properties of n+-semi-insulating-n+ structures
Authors: Cheng Julian , Forrest S. R. , Tell B. , Wilt D. , Schwartz B. , Wright P. D.
Published: July 2002

Semiconductor-insulator-semiconductor n+-SI-n+ InP structures have been used effectively for current confinement in channeled substrate buried heterostructure lasers. The semi-insulating (SI) layer is produced by the implantation of Fe into a n+-InP substrate. The properties of this implanted layer and the role of the deep acceptor levels of Fe in controlling the transport properties of SI-InP are studied in this and the following paper (Part II). The principle of compensation is discussed and applied to the implanted layer, and current injection phenomena are described, emphasizing the important role of space-charge-limited (SCL) current. The electrical characteristics of the n+-SI-n+ structures reveal the classic trap-controlled SCL current behavior at low temperatures with a trap-filled limit achieved at 1.2 V, and an approximately trap-free SCL current at room temperature.
Annealing behavior of ion-implanted Fe in InP
Authors: Schwarz S. A. , Schwartz B. , Sheng T. T. , Singh S. , Tell B.
Published: July 2002

The annealing behavior of implanted Fe+ in InP is studied using secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM). Dual implants (275 keV, 1.25×1014 cm−2 and 400 keV, 1.25×1014 cm2) were performed at room temperature (RT) and at 200 °C and then annealed at 725 °C for one hour. TEM reveals a 3100-Å amorphous region in the unannealed RT implant. Significant defect production is observed in this sample at the amorphous-crystalline interface following the anneal. SIMS reveals an Fe pileup at this interface. No such pileup is observed in the samples implanted at 200 °C. The data also suggest an Fe diffusion constant which is lower than typically reported in the literature. The results are contrasted with the SIMS study by M. Gauneau, H. L’Haridon, A. Rupert, and M. Salvi [J. Appl. Phys. 53, 6823 (1982)].
Electrohydrodynamically driven large-area liquid-metal ion sources
Authors: Pregenzer Arian L.
Published: July 2002

Application of electrohydrodynamic instability theory to a planar liquid-lithium surface suggests that liquid lithium may provide a large-area ion source for intense ion-beam diodes. Such a source could uniformly produce ions on a nanosecond time scale if the applied electric field exceeds 10 MV/cm. Wavelengths and growth times of the most rapidly growing instability have been calculated to first order as a function of applied electric field. Results have been interpreted in the context of an ion source for a light ion-beam driver for inertial confinement fusion.
Alignment of liquid crystals which exhibit cholesteric to smectic C*phase transitions
Authors: Patel J. S. , Goodby J. W.
Published: July 2002

An alignment method for materials with cholesteric to smectic C* phase transitions is described. Ordering of the molecules, confined between glass plates which have been coated with a polymer and buffed, is achieved using an electric field. While the buffing direction defines the orientation of the long axis of the molecules, the electric field defines the direction of the layer tilt. In cells with only one buffed surface, polar surface interactions are thought to be responsible for producing monodomain specimens.
The effect of interface roughness on the intensity profiles of Bragg peaks from superlattices
Authors: Chrzan D. , Dutta P.
Published: July 2002

We have derived an expression for the structure factor of a superlattice with ‘‘rough’’ interfaces, i.e., with random variations in the thicknesses of the layers. Our expression allows the determination of x-ray diffraction profiles in both radial (θ-2θ) and rocking scans. We have shown that the rocking curve of the central peak is essentially unaffected by this kind of disorder; and we have calculated the rocking curve widths of the satellites as a function of roughness. The implications of our model for the use of satellite/central peak intensity ratios as a measure of superlattice perfection are discussed.