bluuebunny/crossref_metadata_2025 huggingface dataset:
Coarsening and phase transition of FeSi2 precipitates in Si
Authors: Lin X. W. , Washburn J. , Liliental-Weber Z. , Bernas H.
Published: July 2002

FeSi2 precipitates were produced in Si(001) wafers by an ion-beam induced epitaxial crystallization process and subsequently annealed at temperatures in the range 650–900 °C. The resulting precipitate coarsening and phase transition were studied by transmission electron microscopy. The coarsening process basically involves the evolution of plate-shaped precipitates. The lengthening rate of the precipitates is considerably greater than the thickening rate, because the two broad faces of a plate are coherent or semicoherent, while the plate edges are incoherent. The lengthening kinetics was shown to be volume-diffusion controlled and obey a cube power law. The corresponding activation energy was determined to be 3.55 eV, in excellent agreement with the value predicted by the classical Ostwald ripening model. In contrast, we demonstrated that the thickening process is interface controlled, which involves the migration of the interfaces via a ledge mechanism. Accordingly, an apparent activation energy of 2.18 eV was obtained. The precipitate coarsening is accompanied by phase transitions. Upon annealing at 650 °C, it was observed that γ-FeSi2 precipitates tend to transform from a fully aligned (A-type) to a twinned (B-type) orientation with respect to the Si matrix. For higher temperature anneals, nearly all the precipitates transform from the γ phase into the β phase, except those having a relatively small diameter (<≊5 nm) which remain as A-type γ-FeSi2. These observations suggest that the phase transition of FeSi2 is size dependent. This can be understood, in terms of the interfacial energy versus the volume free energy of a precipitate as a function of precipitate size.
Shallow levels in n-type 6H-silicon carbide as determined by admittance spectroscopy
Authors: Evwaraye A. O. , Smith S. R. , Mitchel W. C.
Published: July 2002

Admittance spectroscopy has been used to study shallow levels in n-type 6H-SiC single crystals. A total of eight unintentionally doped n-type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n-type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n-type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).
Modeling and characterization of interface state parameters and surface recombination velocity at plasma enhanced chemical vapor deposited SiO2–Si interface
Authors: Yasutake K. , Chen Z. , Pang S. K. , Rohatgi A.
Published: July 2002

The effective surface recombination velocity (Seff) at plasma enhanced chemical vapor deposited (PECVD) SiO2/Si interface as a function of surface band bending under illumination was obtained by combining the photoconductive voltage decay measurements with indium tin oxide gate bias voltage, metal-oxide-semiconductor-capacitance voltage, measurements and theoretical calculations. The capture cross sections for electrons and holes are obtained for the first time for the PECVD SiO2/Si interface state. Theoretical calculations of Seff based on the interface parameters, including interface state density and cross sections for electron and hole, were performed to see the effects of the positive oxide charge density (Qox) on Seff. It is found that roughly a 10 times larger value of Qox compared to the midgap interface state density is required to reduce Seff below 10 cm/s for 5 Ω cm (100) p-type Si. These results prove the potential of PECVD SiO2 for effective passivation of Si surfaces for devices like solar cells.
Unique correlation of the Fermi energy with the metastable defect density in amorphous silicon
Authors: Bube Richard H. , Benatar Lisa , Redfield David
Published: July 2002

This paper reports an empirically based correlation between the metastable defect density and the value and temperature dependence of the Fermi energy in undoped hydrogenated amorphous silicon. According to this correlation, to specify two of the three quantities: Fermi energy, defect density, and temperature, is to specify the third, independent of the history of the sample. Almost 300 measurements of dark conductivity over a wide range of defect densities and temperatures, obtained in the course of measuring the kinetics of optical degradation of amorphous silicon at four temperatures, were subjected to analysis. A detailed empirical summary of these data is given, since their relevance extends beyond this present work. A model with two different types of multivalent defects whose relative density changes with total defect density is the simplest model consistent with the data.
Submillimeter wave generation and noise in InP diodes
Authors: Mitin V. , Gruzinskis V. , Starikov E. , Shiktorov P.
Published: July 2002

High-frequency (350–750 GHz) generation in submicrometer InP diodes is investigated by modified hydrodynamic and Monte Carlo particle (MCP) techniques. The noise power spectral density Pn in the diode loaded by resistor R and generation spectra Pg in a series resonant RL circuit are calculated using the MCP technique. It is shown that at the biases above the generation threshold the Pn has a peak at the frequency fmax which corresponds to the highest generation frequency at the given R. The excess noise arises in the frequency region where the real part of diode impedance Re Z has negative values. At the bias below the generation threshold (i.e., when Re Z is positive over entire frequency range) the Pn(f) has the usual Lorenzian shape. The MCP simulation of Pg for 0.25-μm-length diode shows the Gaussian shape of the spectra at frequencies 517 and 622 GHz. The Pg broadening at higher frequencies is the result of interaction between the self-oscillations at frequency fmax and circuit-driven oscillations.
Tunneling measurement of the quasiparticle lifetime in Ba1−xKxBiO3
Authors: Zasadzinski Roberta , Vandervoort K. G. , Hinks D. G. , Crabtree G. W.
Published: July 2002

The temperature dependence of the energy gap of bulk polycrystalline Ba1−xKxBiO3 (BKBO) was measured with a low-temperature scanning tunneling microscope in the point contact mode. At 4.2 K, the tunneling conductance shows nearly ideal BCS behavior, however at higher temperatures, the lifetime broadening term Γ was required to fit the data. Γ(T) displays a surprising T3 temperature dependence and the magnitude is significantly larger than the calculated theoretical recombination times. Comparisons with other conductance data obtained from grain boundary and sandwich-type BKBO junctions show similar results. The large scattering rates for BKBO may be important for the utility of this material in SIS mixers.
Fundamentals of two-step etching techniques for ideal silicon-hydrogen termination of silicon(111)
Authors: Yang S.-K. , Peter S. , Takoudis C. G.
Published: July 2002

White deposits are seen to form in highly basic etching solutions which contain NH4F such as 40% NH4F or pH-enhanced buffered HF (BHF) (a mixture of BHF and NH4OH). Surface infrared spectroscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy are used to characterize a hydrogen terminated Si(111) surface and the chemistry of these white deposits; such analyses show significant amounts of nitrogen, nonbonded to silicon, and fluorine in the deposits. The morphology of the flat surface is examined with optical microscopy, scanning tunneling microscopy, and atomic force microscopy. A two-step etching, a 7 min dipping in 1% HF followed by a short dipping (∼3 s) in pH-enhanced 1% HF solution (a mixture of 1% HF and NH4OH, pH=9.25), is proposed to obtain an atomically smooth and impurity-free surface, based on studies with all spectroscopies and microscopies used. The second etching step converts a microscopically rough surface with all defects on Si(111) into an atomically smooth, ideally monohydride-terminated surface within 3 s. Similar results are obtained with a 3 s dipping in pH-enhanced BHF solution (a mixture of BHF and NH4OH, pH=9.25); however, studies with the spectroscopies and microscopies used show that longer treatments with this solution result in the formation of white surface deposits. Possible ways of inhibiting the formation of white surface deposits from cleaning solutions are found to be the use of purer chemicals and shorter last-step cleaning times. For example, an increase of the last-step cleaning duration from 10 to 20 min is observed to increase the substrate area covered by the white deposits from about 1% to 5% for purer 40% NH4F, and from about 20% to 30% for less pure 40% NH4F solutions.
Semiclassical model for the inelastic scattering probability of electrons traveling parallel to four dielectric layers
Authors: Turowski Marek A. , Kelly Thomas F. , Batson Philip E.
Published: July 2002

Weak features in the electron energy-loss spectra of Al/SiO2/Si heterostructures in cross section have been found below the bulk plasmon energy when measured in the vicinity of either interface. A four-layer model for the electric potential distribution in this metal/oxide/semiconductor heterostructure was developed. The inelastic scattering probability curves are calculated for the electron beam moving through the oxide and are compared with the corresponding experimental loss function. A small peak around 3.5–4.0 eV is found to be due to interband transitions in Si. An 8 eV peak is attributed to the relaxed surface plasmon from Al or Si depending on which side of the oxide an electron beam is moving. The lowest intensity of this feature occurs at the oxide center. These and other smaller features are attributed to interface or bulk contributions to the electron energy-loss probability.
Characterization of carbon films microstructure by atomic force microscopy and Raman spectroscopy
Authors: Yáñez-Limón J. M. , Ruiz F. , González-Hernández J. , Vázquez-López C. , López-Cruz E.
Published: July 2002

Atomic force microscopy has been used to study the surface irregularities of hydrogenated and unhydrogenated carbon films grown by rf-powered glow discharge and dc-magnetron sputtering, respectively. In general films produced with the latter technique have rougher surfaces. In glow discharge produced samples, roughness can be reduced by increasing rf power during deposition. In sputtered films surface roughness is reduced by either lowering substrate temperature or reducing the power density during deposition. The relation between the microstructure and the crystalline state was done using Raman scattering.
Analysis of free-electron-laser-saturated behavior using one- and three-dimensional models
Authors: Dattoli G. , Faatz B. , Giannessi L. , Ottaviani P. L.
Published: July 2002

An analysis of free-electron-laser-saturated behavior is developed using one- and three-dimensional codes. The dependence of the gain and of the induced energy spread on the input power is studied. It is shown that these quantities are reproduced by simple relations which can be exploited to study the laser dynamics, with a negligible computational effort.
Picosecond optical studies of amorphous diamond and diamondlike carbon: Thermal conductivity and longitudinal sound velocity
Authors: Morath Christopher J. , Maris Humphrey J. , Cuomo Jerome J. , Pappas David L. , Grill Alfred , Patel Vishnubhai V. , Doyle James P. , Saenger Katherine L.
Published: July 2002

A picosecond pump-probe technique is used to measure the room-temperature thermal conductivity κ and longitudinal sound velocity cl of amorphous diamond (a-D) and diamondlike carbon (DLC) thin films. Both κ and cl were found to decrease with film hydrogen content. Depending on the film deposition technique, κ is in the range 5–10×10−2 W cm−1 K−1 for a-D, and 3–10×10−3 W cm−1 K−1 for DLC. Values of cl were found to be in the range 14–18×105 cm s−1 for a-D, and 6–9×105 cm s−1 for DLC.
Distribution of current in spin valves (abstract)
Authors: Gurney Bruce A. , Speriosu Virgil S. , Lefakis Harry , Wilhoit Dennis R.
Published: July 2002

We present a model describing the distribution of spin up and spin down currents within the layers of spin valve structures. With this model and experimentally determined bulk mean free paths we successfully describe a variety of experimental results, including the variation of resistance and magnetoresistance with both ferromagnetic and nonferromagnetic layer thicknesses, and the current-induced field acting on the ferromagnetic layers versus overall current. Our model is based on an approximate path integral solution of the Boltzmann equation for in-plane transport in a multiply layered structure. For a given orientation of the magnetizations it calculates the current density jσ(r) at each point r for each spin σ throughout the structure. From jσ(r) it is straightforward to obtain the current in each layer for parallel versus antiparallel magnetizations, or of the current density near interfaces relevant to electromigration. Included in our model are both spin dependent bulk scattering as well as scattering at interfaces, which are treated as thin layers. For example, in order to obtain quantitative agreement with experiments with permalloy based structures it is essential to include spin independent scattering arising from Fe and Ni atoms rendered nonferromagnetic next to the spacer due to interfacial intermixing. Our results bear directly on the fundamentals of GMR by successfully describing transport in spin valves using the measured bulk spin dependent mean free paths of the individual layers. Our results are also of technological interest because they predict how current affects the MR response to an external field.
Photoemission study of potassium on Si-based semiconductors: a-Si:H, a-Si, and c-Si(001)
Authors: Pi Tun-Wen , Wu Rong-Tzong , Cheng Chiu-Ping
Published: July 2002

A synchrotron-radiation photoemission study of K evaporated on rf-sputtered a-Si:H, a-Si, and c-Si(001) semiconductor surfaces at room temperature is reported. The a-Si substrate was obtained from sputtering of the c-Si(001) surface. It is found that the change in work function upon K adsorption is smallest on the most disordered surface, a-Si:H. A strong covalent K–Si interface state at about 5.9 eV binding energy appears only in disordered surfaces. Surface structure plays a major role in its occurrence. Further, the amorphous surfaces stay semiconducting despite large doses of K atoms, while the K/c-Si(001) surface becomes metallic at the instance of a work-function minimum.
Crystalline fibers in chemically polymerized ultrathin polypyrrole films
Authors: Chu X. , Chan V. , Schmidt L. D. , Smyrl W. H.
Published: July 2002

The microstructures of chemically polymerized polypyrrole films (100–800 Å thick) were studied by transmission electron microscopy, electron diffraction, and x-ray photoelectron spectroscopy. In contrast to previous work where only amorphous polypyrrole was found, crystalline fiber structures were observed in the chemically polymerized thin films. The fibers are embedded in an amorphous matrix which forms a self-reinforced composite. The shape of the fibers ranged from thin rods to ellipsoids depending on the preparation conditions. The density and size of the fibers were affected by the polymerization time and the concentration ratio of pyrrole and oxidants. Polypyrrole fibers were aligned along the thin-film plane and were randomly oriented in the plane. The two-dimensional orientation of the crystalline fibers produced strongly anisotropic electrical properties in the thin films.
Asymmetrical source-drain characteristics in in-plane-gated transistors written by focused ion beam
Authors: de Vries D. K. , Wieck A. D. , Ploog K. H.
Published: July 2002

We observe electrically asymmetrical source-drain characteristics in in-plane-gated transistors with shaped channels, and investigate them for different geometries. We give a qualitative explanation of the asymmetry, which is supported by voltage contrast scanning electron micrographs and the measured interaction between two neighboring channels. These shaped channels show a much higher dc voltage gain than conventional in-plane-gated transistors.
Hot-electron-induced electroluminescence and avalanche multiplication in hydrogenated amorphous silicon
Authors: Toyama Toshihiko , Hiratsuka Kazuhiro , Okamoto Hiroaki , Hamakawa Yoshihiro
Published: July 2002

High-electric-field phenomena above 1 MV/cm in a-Si:H have been investigated using an ac-driven double-insulating electroluminescent device structure. An emission tailing into energies above the optical energy gap has been observed, giving direct evidence for the existence of hot electrons in a-Si:H generated by an electric field. An analysis of the emission spectrum indicates an average energy of 0.13 eV and a mean free path of 1.0 nm for hot electrons. Electroabsorption reveals that the internal electric field in a-Si:H saturates at about 1.2–1.5 MV/cm, which implies multiplication of charges transferred across the a-Si:H layer at this field strength. Experimental data suggest avalanche multiplication due to field-induced hot electrons.
Electron density measurements in a magnetically insulated ion diode
Authors: Tuszewski M. , Waganaar W. J. , Desjarlais M. P.
Published: July 2002

Electron densities between the electrodes of a flashover anode intense ion beam diode are studied with laser interferometry. Visible and infrared data show no measurable density near the anode and at mid-gap during ion beam emission. Hence, anode or cathode plasma closures do not cause impedance collapse. However, a two-phase density build-up observed near the cathode during beam emission appears associated with impedance collapse and with the termination of the ion beam. Substantial density is measured after the end of the ion beam, consistent with ionization of the neutrals produced by the flashover process.
Occurrence of multiple dielectric-loss curves for silver halide emulsions: Difficulties of interpretation
Authors: Callens F. , Vandenbroucke D. , Soens L. , Cardon F.
Published: July 2002

A study of the dielectric-loss curves of cubic and octahedral AgBr emulsions is presented. Special attention is paid to the occurrence of multiple loss peaks. Several analytical Debye-like forms were used to fit the experimental curves. The effect of several experimental parameters such as crystal habit, edge length, temperature, pAg, triazaindolizine addition, etc. were examined. The meaning of the calculated Fmax frequencies and resulting activation energies is critically discussed.
New AlxGa1−xAs related deep luminescence observed in modulation doped quantum wells
Authors: Plentz F. , Meneses E. A. , Meseguer F. , Sánchez-Dehesa J.
Published: July 2002

An emission band that appears in the high energy side of the photoluminescence spectra of a modulation doped multiple quantum well structure was investigated by means of magneto-photoluminescence and photoluminescence excitation spectroscopy. We show that this emission band is related to the Al0.36Ga0.64As layers of the structure. We analyze the data within the framework of the configuration coordinate model and attribute this photoluminescence band to the recombination of electrons trapped to a new deep center in the alloy and the photogenerated holes.
The general Green’s-function solutions for the two carrier concentrations under any small signal nonuniform carrier generation
Authors: Drory A. , Balberg I.
Published: July 2002

Thus far the general theory of the spatial distribution of photocarriers under the presence of a small signal inhomogeneous photocarrier generation has been limited to the one carrier case. In this work the more general and more complicated case of the spatial distribution of the two types of excess carriers is addressed. This distribution is pertinent in particular to the newly developed photocarrier grating (PCG) technique. The above two carrier problem and the PCG technique are discussed in a much broader context than previously given in the literature. A Green’s-function approach, applied here for the first time, provides a general recipe for the problem of inhomogeneous carrier generation and it gives a better physical insight into the corresponding system by showing how the local carrier concentration is determined by the carrier generation in the entire sample. Hence the mathematical and physical basis for the PCG technique is provided here. The generality and the usefulness of the present approach for the nontrivial carrier generations of the ‘‘single square pulse,’’ the ‘‘square wave,’’ and the ‘‘one period sinusoidal pulse,’’ which have not been considered previously, are demonstrated.
Response to ‘‘Comment on ‘Anisotropy studies of molecular-beam epitaxy-grown Co(111) thin films by ferromagnetic resonance’ ’’ [J. Appl. Phys. 77, 5484 (1995)]
Authors: Schreiber F. , Soliman A. , Bödeker P. , Meckenstock R. , Bröhl K. , Pelzl J. , Garifullin I. A.
Published: July 2002

It is explained why the analysis our FMR data from Co/Cu(111) single layers and superlattices is appropriate. This is supported by the results of structural investigations.
A method of detecting the number of electrons in a square-well quantum dot using far-infrared absorption
Authors: Ugajin Ryuichi
Published: July 2002

We calculate the intensity of far-infrared absorption of a single electron and two electrons in a square-well quantum dot under an external electric field. The intensity is effectively modulated by an applied electric field in different ways, according to the number of electrons in the quantum dot. The difference is due to the effect of Coulomb interaction between electrons. The Stark effect on optical transition of interacting electrons confined in a quantum dot is discussed. When a quantum dot is selected out of an array of quantum dots by a local electric field, we can detect the number of electrons in it using electric-field-induced far-infrared absorption spectroscopy.
Bias induced diamond nucleation studies on refractory metal substrates
Authors: Wolter S. D. , Glass J. T. , Stoner B. R.
Published: July 2002

The carbide forming nature of the substrate appears to be an important property when performing bias-enhanced nucleation (BEN); therefore, various refractory metals were studied since they are known carbide formers. Nucleation densities approaching 1×1010/cm2 were observed on both hafnium and titanium. The nucleation density on tantalum, niobium, and tungsten was enhanced to a lesser extent in descending order of influence, respectively. An induction time prior to the onset of significant diamond nucleation was observed on the refractory metals as well as on silicon and may be reliant upon the formation of a critical carbide thickness. Shorter induction times were observed for silicon which may be explained since this material forms a carbide of typically only several nanometers in thickness as opposed to the refractory metals which may form carbides on the order of several microns in thickness. Also, a strong correlation was observed between the carbide heat of formation and the nucleation densities at 60 min of BEN. These findings verify the relevance of a carbide formation to diamond nucleation via BEN and also provides a clue as to the mechanism(s) by which diamond is nucleating.
Site-selective spectroscopy and crystal-field analysis for Nd3+ in strontium fluorovanadate
Authors: Peale R. E. , Summers P. L. , Weidner H. , Chai B. H. T. , Morrison C. A.
Published: July 2002

Site-selective spectroscopy reveals that Nd3+ ions occupy more than 40 different crystal-field environments in Sr5(VO4)3F. Preferential energy transfer to the site responsible for 1 μm lasing occurs but becomes less complete with increasing temperature. The 4I and 4F3/2 Stark levels of the lasing site have been determined and an analysis of the crystal field performed. From the crystal-field fitting parameters Bkq, a calculated energy-level spectrum is determined up to 17 500 cm−1 with a rms deviation from the available experimental levels of 6 cm−1.
Accurate calculation of capacitances of grain boundary Josephson junctions in high critical temperature superconductors
Authors: Töpfer H. , Mäder G. , Uhlmann H.
Published: July 2002

We report a quantitative calculation of the capacitances of high critical temperature (high-Tc) Josephson junctions. The matter is of practical interest, because the usual junction realizations cause the conventional plate capacitor formula sufficient in low critical temperature (low-Tc) technique to fail. In our work we used a three-dimensional algorithm for computing the electrostatic field. We show for two planar junction types, how the capacitive contributions are composed of direct and stray components and that the latter for the wide used SrTiO3 substrates lead to capacitance values which cannot be predicted by simple analytical approximations. The derived data can be used for simulations of high-Tc Josephson devices in the framework of the resistively and capacitively shunted junction model.
‘‘Clean’’ processing of polymers and smoothing of ceramics by pulsed laser melting
Authors: Tokarev V. N. , Marine W. , Prat C. , Sentis M.
Published: July 2002

Surface stability during laser pulsed melting of polymers and ceramics is studied theoretically. Irradiation conditions and material parameters are found giving rise to the suppression of surface wavy relief of a nonresonant type (with period Λ≫λ, where λ is the radiation wavelength) and thus to the smooth flat irradiation spots. For example, for the polymers considered this process takes place for wavelengths where the absorption coefficient is sufficiently high: α(λ)≳105 cm−1. Thus, it is shown that the formation of such spots, previously referred to as ‘‘clean ablation,’’ can be explained using only a thermal mechanism without reference to the concept of ‘‘photodecomposition.’’ Moreover, laser smoothing and polishing of a surface, if it had roughness before irradiation, can be achieved by appropriate matching of the characteristic size of this roughness along the surface with the values of α(λ) and laser fluence. Methods are proposed to decrease the parasitic influence of droplets on the deposition of thin films by laser ablation of massive ceramic pellets. The results of theoretical modeling are shown to be in good agreement with experiments on smoothing of rough alumina ceramics and ‘‘clean’’ processing of polymers by excimer laser radiation.
Thermogravimetric measurements of thin iron films magnetization near their Curie temperature
Authors: Cabrera A. L. , Pino M. , Volkmann U. G.
Published: July 2002

One of the important problems in bulk ferromagnetism is still the determination of magnetization as a function of temperature. The problem becomes more complicated when one tries to explain the magnetization of thin films near Tc. The experimental situation is far from clear, mainly because the experiments are difficult to perform. We have implemented a thermogravimetric technique using a modern microbalance to determine the magnetization behavior of thin films of pure Fe near Tc. We are able to obtain very precisely the films’ magnetization, by measuring the force exerted over the films when they are imbedded in a fixed external magnetic field, with this technique. We prepared several samples of pure Fe films, evaporated in ultrahigh vacuum conditions onto Cu foils ranging in thickness between 50 and 300 nm and measured their magnetization near the critical point. The value of the Tc of the films is systematically lowered from the bulk Tc, being 342 K lower for the 50 nm films. The magnetization curves show a dependence on the temperature as a series containing terms in (T/Tc)3/2, (T/Tc)5/2, and (T/Tc)7/2 at temperatures low compared with Tc. Near Tc, the magnetization decreases linearly on temperature for films of all thicknesses.
Structural properties of a-C:H deposited using saddle-field glow-discharge decomposition of methane
Authors: Lim P. K. , Gaspari F. , Zukotynski S.
Published: July 2002

Diamondlike hydrogenated carbon films were deposited using saddle-field glow discharge in pure methane. The structure of the films was studied using x-ray photoelectron spectroscopy, x-ray stimulated Auger electron spectroscopy, and Raman spectroscopy. It was found that for appropriate conditions of pressure and substrate bias a very high percentage of sp3 bonding could be achieved.
Deposition and hydrogen content of carbon films grown by CH+3 ion-beam bombardment
Authors: Plank H. , Wang W. , Eckstein W. , Schwörer R. , Steffen H. J. , Roth J.
Published: July 2002

Carbon deposition and hydrogen codeposition is investigated as a function of ion energy, fluence, and target temperature at normal incidence by bombardment of silicon and pyrolitic graphite substrates with mass selected CH+3 molecules. An amorphous hydrogenated carbon layer (a-C:H) is formed in a thickness range of 40–130 nm at a fluence of 3×1018/cm2. The deposition process, the re-erosion phenomenon, the hydrogen content, and the H/C ratios of the carbon films are studied between 300 and 1000 K in the ion energy range from 150 eV to 3 keV. The experimental results are compared with those of TRIDYN computer simulations and previous experimental results of carbon sputtering by atomic H+ and C+ beams in order to obtain a better understanding of the interaction between hydrocarbon ions and the carbon-based wall materials in fusion devices.
Ferromagnetic resonance studies of Fe/Ni and Fe/CoNbZr multilayers: Model and experiments
Authors: Acharya B. Ramamurthy , Prasad Shiva , Venkataramani N. , Kaabouchi M. , Krishnan R. , Sella C.
Published: July 2002

Ferromagnetic resonance (FMR) studies were carried out on Fe/Ni and Fe/CoNbZr multilayers. A simple model taking into consideration the multilayered nature of the film was used to calculate the field positions and mode intensities of the experimental spectra. The complicated intensity patterns of the modes observed in the FMR spectra of the multilayers were explained in terms of model calculations. The analysis of the spectra showed that the interface in case of these multilayers is not sharp and suggested the formation of interfacial alloy.
Low-pressure Cs-Ba discharge. II. Probe measurements
Authors: Luke James R. , El-Genk Mohamed S.
Published: July 2002

Experiments are performed using Langmuir probes to measure the plasma parameters in both steady-state and I-V modes in a low-pressure Cs-Ba diode. The data show that at low discharge currents the electron energy distribution function is nonMaxwellian due to the presence of an electron beam from the emitter. Also, the plasma properties were nonhomogeneous across the 1 mm diode gap; the electron temperature was 10%–15% higher and the plasma potential ∼1 V higher near the emitter, and the plasma density was a factor of three higher near the collector. The good agreement between calculations and experiment, in spite of the presence of the electron beam from the emitter, suggested that the beam electrons do not undergo collisions with Cs atoms in the gap. Experimental I-V curves showed the presence of two different thermal emission currents, each corresponding to a different emitter work function, at the beginning and the end of the I-V pulse.
New carbon nitride phase by high-dose N ion implantation in glassy carbon
Authors: Withrow S. P. , Williams J. M. , Prawer S. , Barbara D.
Published: July 2002

The possibility for the synthesis of CN compounds by high-dose N implantation of glassy carbon is investigated. The changes in volume and surface morphology and the retained N concentration as a function of ion dose are reported. For both N and C (used as a control), implantation initially induces compaction in the surface region that saturates at a density above 2.6 g/cm3. After formation of this dense surface layer, additional implantation causes the material to expand to accommodate the implanted ions and vacancies formed during the implantation process. For N the swelling is initially linear in fluence up to 5×1017/cm2 with a volume increase of 16 Å3 per added N atom; this is twice the volume per atom in the compacted substrate. Above a dose of 5×1017 N/cm2 the swelling behavior is more complex. A phase change is observed to occur at a dose between 5×1017 and 1×1018/cm2, which is concomitant with at least some of the implanted N being driven to the surface. In addition, micron-sized surface features are seen by scanning electron microscopy, and a significant surface roughening occurs. Ion backscattering spectra have been used to characterize the implanted N profile. The maximum retained concentration was found to be 30 at. %.
Step bunching on {111} facets in the selective growth of GaAs by metalorganic vapor phase epitaxy
Authors: Nishida Toshio , Shinohara Masanori , Inoue Naohisa
Published: July 2002

Step bunching on the {111}A and {111}B side facets formed in the selective growth of GaAs on (001) GaAs substrates by metalorganic vapor phase epitaxy is investigated by atomic force microscopy. It is shown that the bunching characteristics are determined by the misalignment angle of the selective growth mask from [1̄10] and [110] directions of the substrate: Bunched steps are clearly observed when the misalignment angle is more than 0.25°, but their density decreases below this angle, and they completely disappear at 0.008°. Bunching behavior on the {111} facets is compared with that on (001) surfaces and the bunching mechanism is discussed.
Spraying fine fluid particles in insulating fluid systems by electrostatic polarization forces
Authors: Feng James Q. , DePaoli David W. , Tsouris Costas , Scott Timothy C.
Published: July 2002

It is experimentally demonstrated that electrostatic polarization forces can in principle be utilized in generating fine bubbles and droplets, despite the lack of charge carriers that have traditionally been thought to be necessary for successful electrostatic spraying. Under the condition that the permittivity of the dispersed phase is lower than that of the continuous phase, such as when gas bubbles are sprayed into insulating liquids, the spraying behavior is regular and easy to control. If the permittivity of the dispersed phase is higher than that of the continuous phase, such as when insulating liquids are sprayed into gases, the spraying behavior lacks regularity and further research is needed before pure polarization forces can find significant applications in practical processes.
Focusing and wiggling intense electron beams by a superconducting tube
Authors: Roth Peter
Published: July 2002

An intense electron beam travelling axially through a superconducting tube is studied. We show numerically that it is necessary to compensate the space-charge effect of the electron beam nearly completely in order to focus the beam by means of the superconductor tube. The tube functions as a lens not only for electrons injected parallel to the tube axis but also for electrons having a small initial radial velocity component. Additionally, it is suggested that the simple straight superconducting tube may function as a wiggler for free-electron lasers.
Flux penetration of an aluminum liner during working fluid compression
Authors: Bell David E. , Hussey Thomas W.
Published: July 2002

The Phillips Laboratory working fluid experiment is a research effort to study the compression of a hot hydrogen gas using an electromagnetically imploded solid liner. In our experiments, the solid liner is driven by a 5 MJ discharge which Joule heats the aluminum, melting and eventually vaporizing it. This numerical study explores the vaporization and flux penetration of a solid aluminum liner during its implosion. In particular, it considers the effect that flux which has penetrated the liner has on the hot hydrogen working fluid. A study of the dynamics of the solid liner was performed with one-dimensional radiation magnetohydrodynamic simulations, which included a careful treatment of the electrical resistivity near the phase transitions. An analytic snowplow model is developed in order to estimate the minimum working fluid density required to ignore flux penetration through the liner.
Dissociation levels in fast-axial-flow CO2 lasers: A quantitative model
Authors: Leys C. , van Egmond C. , Desoppere E.
Published: July 2002

A plasma chemistry model is presented that explains the observed CO2 dissociation levels in a closed-cycle fast-axial-flow CO2 laser. The model includes reactions between the neutral species CO2, CO, O, O2, O3, H2O, and OH, and the negative ions O−, O−2, CO−3, and CO−4. Dissociation rates are computed by solving the electron Boltzmann equation for experimental values of the reduced field E/N. It is found that gas replenishment and the neutral recombination reaction between CO and the OH radical are the most effective mechanisms to suppress the CO concentration in the gas circuit. The influence of CO2 dissociation on the laser output power level is discussed.
Catastrophic quenching of superconductivity in melt-textured YBa2Cu3O7−δ with second-phase additions
Authors: Blackstead H. A. , Pulling D. B. , McGinn P. J.
Published: July 2002

Using rf (microwave) currents, the quenching of superconductivity in melt-textured YBa2Cu3O7−δ with second-phase additions at temperatures near the superconducting onset temperature has been observed. The current density was varied from roughly 104 to 105 A/cm2. Quenching is indicated by dramatic changes in the surface resistance, induced by a combination of the rf power level (current density), and an applied magnetic field. The catastrophic response is interpreted as the failure of a percolative network of junctions. The observed ‘‘failure’’ is attributed to variations in the critical current of the junctions. The junctions which fail are thought to be intragranular, not ‘‘weak links’’ associated with grain boundaries.
Distribution of relaxation times in perovskite-type relaxor ferroelectrics
Authors: Gui Hong , Gu Binglin , Zhang Xiaowen
Published: July 2002

The distribution of relaxation times in perovskite-type relaxor ferroelectrics is studied by computer simulation. It has been proved that the spectra move to longer relaxation times with the decreasing of temperature and the increasing of the external field. The frequency dispersion of relaxors is explained by a gradual freezing process of polar microregions. The relationship between the time dependence of the polarization and the relaxation time distribution at various temperatures and external fields has been investigated. The macroscopic polarization is suggested to be sustained mainly by frozen dipoles during the observation time.
Electrical switching in AgI based fast ion conducting glasses: Possibility for newer applications
Authors: Vaidhyanathan B. , Rao K. J. , Prakash S. , Murugavel S. , Asokan S.
Published: July 2002

A current controlled, high speed, near ideal electrical switching has been observed for the first time in microwave prepared AgI based fast ion conducting glasses. The switching is found to be memory type, which is not reverted to OFF state by the application of a resetting current pulse. The threshold fields exhibit a linear dependence on the sample thickness, with the fields decreasing with the increasing thickness. Also, the switching fields are almost independent of the temperature, clearly indicating that the memory switching in these samples is nonthermal in nature. The above results are satisfactorily explained on the basis of a chemical model.
Optical matrix elements in [hhk]-oriented quantum wires
Authors: Yamaguchi A. Atsushi , Usui Akira
Published: July 2002

Optical matrix elements in quantum wires (QWIs) are theoretically investigated as functions of wire crystallographic direction taking the valence-band anisotropy into account. Calculations are performed analytically for GaAs cylindrical QWIs with infinite barriers. It is shown that the optical matrix element for light polarized to the wire direction shows weak dependence on the wire direction. On the other hand, the valence-band anisotropy causes strong dependence on the wire direction for light polarized to the perpendicular directions, and large in-plane optical anisotropy appears for [110]- and [112]-oriented QWIs. It is found that consideration of the valence-band anisotropy is important for estimation of optical polarization properties in QWIs.
Modulation of the polarization state in an optical fiber coated with Langmuir–Blodgett films
Authors: Zhu Rong , Wei Yu , Scholl Bernhard , Schmitt Hans J.
Published: July 2002

The optical fiber is bent in a slot and polished until one side of the fiber coating is only several micrometers thick. The hemicyanine Langmuir–Blodgett (LB) film is then deposited on the polished optical fiber surface. By applying an electric field between the LB film, the polarization state in the optical fiber has been modulated. The preparation details of the in-line optical fiber modulator and the measurement setup are described.
A model for cw laser induced mode-mismatched dual-beam thermal lens spectrometry based on probe beam profile image detection
Authors: Shen Jun , Soroka Andrew J. , Snook Richard D.
Published: July 2002

A model for cw laser induced mode-mismatched dual-beam thermal lens spectrometry using probe beam profile detection has been developed. This model exhibits the following behavior: after passing through the thermal lens the TEM00 Gaussian probe beam becomes the superposition of several Gaussian beams of different parameters. This model has a compact mathematical expression, for which experimental proofs are given.
B incorporation in Ge(001) grown by gas-source molecular-beam epitaxy from Ge2H6 and B2H6
Authors: Lu Q. , Bramblett T. R. , Hasan M.-A. , Lee N.-E. , Greene J. E.
Published: July 2002

Secondary-ion-mass spectrometry (SIMS) was used to determine the concentration and depth distribution of B incorporated into Ge(001)2×1 films grown on Ge(001) substrates by gas-source molecular-beam epitaxy using Ge2H6 and B2H6. B concentrations CB (3×1016–4×1019 cm−3) were found to increase linearly with increasing flux ratio JB2H6/JGe2H6 (8.2×10−3–1.7) at constant film growth temperature Ts (300–400 °C) and to increase exponentially with 1/Ts at constant JB2H6/JGe2H6 ratio. The difference in the overall activation energies for B and Ge incorporation over this growth temperature range is ≂0.22 eV while B2H6 reactive sticking probabilities ranged from 8×10−4 at 300 °C to 2×10−5 at 400 °C. SIMS depth profiles from B modulation-doped samples and two-dimensional δ -doped samples grown at Ts<350 °C were abrupt to within instrumental resolution with no indication of surface segregation. Structural analysis by in situ reflection high-energy electron diffraction combined with postdeposition high-resolution plan-view and cross-sectional transmission electron microscopy showed that all films were high-quality single crystals with no evidence of dislocations or other extended defects. B doping had no measurable affect on Ge deposition rates.
Dielectric spectroscopic determination of temperature behavior of electroclinic parameters in the liquid crystal W317
Authors: Williams P. A. , Clark Noel A.
Published: July 2002

We report measurements of the temperature behavior of the electroclinic coupling coefficient c(T) and the inverse tilt susceptibility A(T), the principal phenomenological parameters determining the magnitude of the electroclinic effect in the chiral smectic A phase of the liquid crystal W317, a material which exhibits an anomalously large electroclinic effect with unusual thermal behavior. We find that c(T) decreases by ∼30% in response to a 40 °C increase in temperature. A(T) exhibits a mean field behavior at high temperature, increasing by a factor of 3 as temperature is increased over the range 40 °C<T<65 °C. However, below ∼40 °C, A(T) becomes nearly independent of temperature over a ∼10 °C range around room temperature. This A(T) behavior has not been previously found in electroclinic materials.
Extremely long capacitance transients in 6H-SiC metal-oxide-semiconductor capacitors
Authors: Pan J. N. , Cooper J. A. , Melloch M. R.
Published: July 2002

We have measured capacitance-time (C-t) transients on n-type 6H-SiC MOS capacitors to obtain information on the generation lifetime near the SiO2/SiC interface. At temperatures between 260 and 370 °C, the capacitance recovery transient is thermally activated with an activation energy of about 2.0 eV. Because of the wide band gap of SiC, these transients are extremely long. As a figure of merit, extrapolation of the high-temperature C-t data indicates a room-temperature recovery time of over 1010 yr.
Effects of Penning reactions and excitation rate on the pulsed transverse-discharge neon laser for photodynamic therapy
Authors: Yang C.-H. , Itoh K. , Tomita H. , Obara M.
Published: July 2002

Theoretical analysis of the output performance of a transverse discharge pumped neon Penning laser (585.3 nm) using a mixture of Ne/H2 is described. The validity of the kinetic model is confirmed by comparing the results to the experimental discharge and laser performance. It is theoretically shown that the optimum mixing ratio of the Ne/H2 mixture is 1:2.5, and the optimum operating pressure is about 56 Torr. The model also predicts that the intrinsic efficiency reaches a peak of 8.5×10−6 at an excitation rate of 0.5 MW/cm3 under the optimum mixing ratio and operating pressure conditions. At excitation rates in excess of 0.5 MW/cm3 the laser output power is slowly increasing and then saturates due to electron collisional quenching of the upper laser level. The laser power extraction is increased by laser injection seeding in order to rapidly build up the lasing. The improved intrinsic efficiency is about two times higher than without the injection seeding. The improved specific laser output is 8 W/cm3, therefore, a discharge volume of 125 cm3 will be able to generate the peak laser power reaching 1 kW. This power value is sufficient to obtain the same treatment effect as the gold vapor laser used in photodynamic therapy. Moreover, by fitting this model to the experimental results of the laser output energy with a Ne/D2 mixture, it is shown that the Penning ionization rate constant of H2 is larger than that of D2.
Spectroscopic analysis of plasma during rf magnetron sputtering of a YBCO target: Evidence for oxidation on the surface of the cathode
Authors: Lecœur Ph. , Mercey B. , Murray H.
Published: July 2002

In this article, we present spectroscopic analysis of plasma during reactive rf magnetron sputtering of a YBa2Cu3O7 target. Particular attention is paid to the influence of oxygen pressure on the emission lines of cationic species. From the optical response of barium and copper resonance lines, we have deduced that the oxidation of barium into barium oxide BaO (or BaO2) is complete for an oxygen content up to 10% whereas the oxidation of copper into copper oxide CuO needs an oxygen content of up to 30% which corresponds to the value of the gas composition generally used by different groups. We have shown that these oxidation reactions are represented by a mass law in the vapor phase. These results are confirmed by x-ray photon spectroscopy analysis and give the experimental process for mixed valence copper oxide sputtering deposition.