bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigation of magnetic tunneling junctions with wedge-shaped barrier
Authors: Du J. , Xiang X. H. , Landry G. , You B. , Hu A. , Zhao H. W. , Xiao John Q.
Published: July 2002

By oxidizing wedge-shaped Al films we were able to simultaneously fabricate Ni80Fe20/AlOx/Ni80Fe20 tunnel junctions with over, completely, and underoxidized barriers. We have systematically investigated TMR ratio, resistance, barrier height and thickness, and bias dependence along the wedge. The bias dependence of MR ratio is asymmetric in overoxidized regions and symmetric in underoxidized regions. The interesting bias dependences of resistance at parallel and antiparallel configurations will also be presented.
Sharp asymmetric line shapes in side-coupled waveguide-cavity systems
Authors: Fan Shanhui
Published: July 2002

We show that, for an optical microcavity side coupled with a waveguide, sharp, and asymmetric line shapes can be created in the response function by placing two partially reflecting elements into the waveguides. In such a system, the transmission coefficient varies from 0% to 100% in a frequency range narrower than the full width of the resonance itself. We numerically demonstrate this effect by simulating the propagation of electromagnetic waves in a photonic crystal.
Magnetization of semiconductor quantum dots
Authors: Schwarz M. P. , Grundler D. , Wilde M. , Heyn Ch. , Heitmann D.
Published: July 2002

We present experimental studies of the magnetization of electrons in semiconductor quantum dots. Starting from a modulation-doped AlGaAs/GaAs heterostructure an array of dots was patterned by laser-interference lithography and deep mesa etching. The quantum-dot array was integrated into a highly sensitive micromechanical cantilever magnetometer. At a temperature of 0.3 K we observe pronounced oscillations in the magnetization. With regard to their periodicity and temperature dependence they differ from the de Haas–van Alphen effect observed in a two-dimensional electron system. We find that the magnetization calculated from the single-particle Fock–Darwin energies of a quantum dot does not reproduce the experiment. From this we conclude that the electronic ground state of the dots is strongly influenced by electron–electron interaction.
Coherent control of an optically injected ballistic spin-polarized current in bulk GaAs
Authors: Stevens Martin J. , Smirl Arthur L. , Bhat R. D. R. , Sipe J. E. , van Driel H. M.
Published: July 2002

We demonstrate coherent all-optical injection and control of a ballistic spin-polarized current in bulk, low-temperature-grown GaAs at room temperature. The spin current is injected by interfering the two-photon absorption of the fundamental (1.55 μm) and the single photon absorption of the second harmonic (0.775 μm) of ∼180 fs pulses that propagate collinearly and have the same circular polarization. Adjusting the relative phase of the two pulses controls the direction of this current. The component of the electrical current transverse to the pulse propagation direction is investigated by monitoring charge collection across a pair of gold electrodes deposited on the GaAs surface. Results are in agreement with recent theoretical predictions.
Microgear laser
Authors: Fujita Masayuki , Baba Toshihiko
Published: July 2002

A microlaser, microgear, composed of a microdisk and a rotationally symmetric Bragg grating is described. A GaInAsP–InP device with μm radius was fabricated and the room-temperature continuous-wave operation was obtained by photopumping with a low threshold of 17 μW. The experiment clearly demonstrated that the Q factor in a microdisk was enhanced by the microgear due to the minimization of the radiation field. Resonant spectra showed some unique mode characteristics influenced by an elliptical cavity. These results suggest a potential of advanced engineering of whispering-gallery-mode laser with a photonic crystal, the generalized Bragg grating.
Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN
Authors: Adelmann C. , Brault J. , Jalabert D. , Gentile P. , Mariette H. , Mula Guido , Daudin B.
Published: July 2002

The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.
Study of isotope effects in the photoionization of HI and DI using phase lag spectroscopy
Authors: Khachatrian Ani , Billotto Richard , Zhu Langchi , Gordon Robert J. , Seideman Tamar
Published: July 2002

Phase lag spectroscopic measurements are performed for HI and DI in the 5d(π,δ) resonance region. Measurements of the phase lags for both molecules reveals a strong isotopic effect, especially pronounced in the ionization channel. The energy dependence of the phase lag provides clear examples of continuum and resonance coupling effects that were previously predicted theoretically but have not yet been observed. It is demonstrated that phase lag spectroscopy is a sensitive technique for studying properties of molecular continua, furnishing information that is not available from phase-insensitive observables.
Symbolic computation of energy perturbation expansions for spherically symmetric hydrogenic systems
Authors: Adams Barry G. , Arteca Gustavo A.
Published: July 2002

We show how symbolic computation using the Maple computer algebra system can be used to obtain explicit large order perturbation expansions using recurrence relations obtained directly from the differential equation and from the Hellmann–Feynman and hypervirial theorems. The approach is used to study several D-dimensional hydrogenic systems in several external spherically symmetric potentials.
Mori–Zwanzig–Daubechies decomposition of Ising-model Monte Carlo dynamics
Authors: Phillies George D. J. , Stott Jonathan
Published: July 2002

Monte Carlo dynamics of one- and two-dimensional Ising lattices were studied by computer simulation. A comparison of decompositions made with Haar and Daubechies wavelets finds that wavelet–wavelet time correlation functions 〈cn(t)cn(t+τ)〉 and their long-time decay constants Γn are virtually independent of the choice of wavelet basis. An intermediate-temperature scaling relation between Γn and 〈cn(t)cn(t+τ)〉 fails at low temperature. The temperature at which failure occurs decreases with increasing wavelet decimation level n. Mori–Zwanzig memory kernels φ(τ) are extracted from 〈cn(t)cn(t+τ)〉 without resort to Laplace transforms. Numerically, φ(τ) computed from the random force autocorrelation function 〈 fi(t)fi(t+τ)〉 is in good agreement with φ(τ) computed from the 〈cn(t)cn(t+τ)〉. Even for a system as simple as the two-dimensional periodic Ising lattice with nearest-neighbor interactions, φ(τ) is nonexponential; our results are consistent with a power-law decay of φ(τ) at large τ. © 1995 American Institute of Physics.
High-field and current-induced positive charge in thermal SiO2 layers
Authors: Nissan-Cohen Y. , Shappir J. , Frohman-Bentchkowsky D.
Published: July 2002

The generation of a bulk positive charge in SiO2 layers of silicon gate metal-oxide-silicon (MOS) devices, under the conditions of high-field and charge injection is studied. The time dependence of the positive charge and its spatial distribution as a function of the oxide thickness and electric field are all consistent with an impact ionization-recombination model which takes into account both the spatial and the field dependence of the ionization probability. The nature of the ionization, either band-to-band, or traps ionization, is still unknown. Bulk positive charge of the same nature is also formed in Al gate oxides. Nevertheless, it was not always observed in previous works since a much larger Si-SiO2 interfacial positive charge is also generated in these samples.
Candidate muon-probe sites in oxide superconductors
Authors: Dawson W. K. , Tibbs K. , Weathersby S. P. , Boekema C. , Chan K.-C. B.
Published: July 2002

Two independent search methods (potential-energy and magnetic-dipole-field calculations) are used to determine muon stop sites in the RBa2Cu3Ox (x≂7) superconductors. Possible sites, located about 1 Å away from oxygen ions, have been found and are prime candidates as muon-probe locations. The results are discussed in light of existing muon-spin-relaxation (μSR) data of these exciting oxides, and compared to H- and positron-oxide superconductor studies. Further work is in progress to establish in detail the muon-probe sites; this knowledge is an essential ingredient for a correct interpretation of μSR data of high-temperature superconducting oxides.
Structure and polarity of {111} CdTe on {100} GaAs
Authors: Glanvill S. R. , Rossouw C. J. , Kwietniak M. S. , Pain G. N. , Warmiński T. , Wieluński L. S.
Published: July 2002

Ultramicrotome techniques are used to prepare thin cross sections of a {111} epilayer of CdTe deposited by metalorganic chemical vapor deposition onto a {100} GaAs substrate. The structure of these samples is investigated by transmission electron microscopy using high-resolution (HRTEM) and diffraction contrast, and the polarity of the {111} layer by convergent beam electron diffraction (CBED) and characteristic x-ray emission under various electron channelling conditions, or ALCHEMI. Rutherford backscattering and channelling experiments on the bulk film confirm the presence of a multiply twinned lamellar structure as observed by electron beam techniques. Strong channeling confirms that the crystallinity is good, and that no significant concentration of defects occurs. HRTEM images of the {111} epilayer from the interface across the lamellar twins show few dislocations or crystal defects. Diffraction contrast indicates the presence of a periodic strain in the GaAs and parallel to the interface. CBED and ALCHEMI results confirm that the layer is B type, and that the lamellar twins do not invert phase.
A layered-shell model of isotropic composites and exact expressions for the effective properties
Authors: Milgrom Mordehai , Shtrikman S.
Published: July 2002

A family of models for multicomponent, isotropic mixtures is suggested and discussed for which the exact values of the effective, linear-response constants can be calculated: The basic unit of the model is a hollow system of contacting concentric shells, each of which is made of one of the components. Space is packed with such units of different sizes, but the same proportions; the cavity within each such shell system is then packed with similar systems, and this continues in an infinite nesting sequence. We deal with the general case of a linear-response phenomenon involving n coupled driving fields; the effective response matrix is given in a closed form as a function of the volume fractions and the response matrices of the components. Some subfamilies are treated in more detail. For example, a two-shell two-phase family spanned by q, the volume fraction of the shells within the sphere they define. The special case q=1 is the well-known coated-sphere model. Varying q between zero and one, we get exact models that cover the whole range of allowed values of the effective constant of any isotropic composite. Another special case of interest is q≪1 (very thin shells). The value of the effective response matrix L is derived for this model for a multicomponent mixture; this turns out to have a simple form. The response matrix is symmetric under the interchange of any two components. This thin-shell model is an automatically isotropized laminate.
Carrier concentration model for n-type silicon at low temperatures
Authors: Dickstein Richard M. , Titcomb Stephen L. , Anderson Richard L.
Published: July 2002

Previous models for use in calculating the carrier freezeout effects in n-type Si at low temperature are extended to include the sixfold degeneracy of excited states and the fine structure of the ground state resulting from considerations of group theory. The results are in good agreement with experimental Hall-effect measurements. For n-type Si with doping in the range of 8×1017–2×1019 cm−3, a method is proposed to estimate the electron concentration and mobilities in the conduction band and in the first excited impurity band at a given temperature.
Ceramic particle size dependence of dielectric and piezoelectric properties of piezoelectric ceramic-polymer composites
Authors: Lee Hyeung-Gyu , Kim Ho-Gi
Published: July 2002

Composites with 0-3 connectivity were fabricated from barium titanate and phenolic resin powders. These composites were investigated for dielectric and piezoelectric properties with the active particle size. Under the condition of the same density and ceramic/polymer volume ratio, the dielectric constants and piezoelectric coefficients of composite increase as the ceramic particle size in composite increases. The surface layer model was used to explain these phenomena in our composite system and was confirmed experimentally. The thickness and dielectric constant of the surface layer of the powder prepared by grinding the sintered barium titanate ceramics were 1.59 μm and 105. When the ceramic particle size is larger than about 100 μm, the properties of the composite were nearly independent of size.
Focusing properties of curved x-ray diffractors
Authors: Wittry D. B. , Sun Songquan
Published: July 2002

Calculations are described and results are given for the focusing properties of six curved x-ray diffractors for Bragg angles ranging from 15° to 75°. The geometries considered include cylindrical, spherical, and toroidal cases. The type of slit necessary to eliminate stray radiation in applications of doubly curved diffractors to scanning monochromators is discussed. Three of the most efficient diffractors are compared with respect to the solid angle for collection of x rays and the possible spectral resolution. A diffractor with spherically curved planes and toroidally curved surface is shown to be superior to the Johansson geometry and the spherically curved plate.
Deep level transient spectroscopy characterization of tungsten-related deep levels in silicon
Authors: Boughaba S. , Mathiot D.
Published: July 2002

Deep level transient spectroscopy is used to determine the deep levels introduced by tungsten in the silicon band gap. The experimental results indicate that tungsten creates three defect centers, with levels at Ev+0.22 eV, Ev+0.33 eV, and Ec−0.59 eV. The shape of the concentration profiles indicates that W does not diffuse by a simple mechanism in Si.
Optical and electrical characterization of β-FeSi2 epitaxial thin films on silicon substrates
Authors: Lefki K. , Muret P. , Cherief N. , Cinti R. C.
Published: July 2002

Electrical measurements have been carried out on epitaxial FeSi2 layers on silicon substrates, the silicide thickness being either 180 or 350 Å. A direct gap of 0.85 eV was measured by optical absorption. Current-voltage characteristics of mesa-structures Cr/Fe/FeSi2/Si show a p-type semiconductor behavior. Capacitance-voltage and capacitance-temperature data at different frequencies indicate a large response of deep levels or interface states. Admittance spectroscopy yields the activation energy and capture cross section of two levels. Finally an energy-band diagram is proposed.
Material and electrical characterization of polycrystalline boron-doped diamond films grown by microwave plasma chemical vapor deposition
Authors: Nishimura K. , Das K. , Glass J. T.
Published: July 2002

Crystal/material quality and electrical properties of B-doped diamond films synthesized by microwave plasma chemical vapor deposition were investigated. Raman spectroscopy verified the presence of diamond and indicated that the crystal quality increased with B doping. Secondary-ion mass spectroscopy showed that the B/C ratios in the films were larger than the B/C ratios in the gas phase, possibly due to differences in B and C sticking coefficients. Electrode patterns of Pt were fabricated on the films and electrical properties were investigated. On undoped diamond films with a residual B concentration of ∼5×1017 cm−3, these contacts were rectifying with small reverse leakage currents and on B-doped diamond films with a B concentration of 200 and 400 ppm, they yielded ohmic behavior. The temperature dependence of the resistivity showed that these doped films had activation energies, an order of magnitude smaller than that associated with the B impurity level in diamonds. The small activation energies associated with these heavily doped films suggest that an impurity band conduction plays an important role. Experimentally determined Hall mobility of the films was also interpreted qualitatively by an impurity band conduction mechanism and effects of grain boundaries and sp2 components on the transport properties are considered to be small for heavily doped films.
Ion and electron dynamics in the sheath of radio-frequency glow discharges
Authors: Wild C. , Koidl P.
Published: July 2002

The distribution of ion energies and the temporal modulation of ion and electron currents were measured at the cathode of an asymmetric, capacitively coupled rf discharge system operated at 13.56 MHz. The ion energy distributions (IEDs) were found to exhibit pronounced features, namely a characteristic series of peaks and double peaks, their position and intensity depending strongly on process parameters such as self-bias voltage and pressure. Those structured IEDs have been observed for a variety of gases including argon, oxygen, hydrogen, and benzene. The IED features are explained by the rf modulation of the sheath potential in combination with the creation of thermal ions by charge-exchange processes in the sheath. A model for the ion transport through collisional rf sheaths is presented which satisfactorily explains the observed IEDs. It will be shown that the observed features are an inherent property of capacitively coupled rf discharges. By analyzing these structures detailed information about the spatial field distribution in the sheath and the transport of ions through rf plasma sheaths is obtained. The time-resolved measurements revealed that the ion current behind the cathode is strongly modulated. This temporal modulation has been described by the same theory confirming the applicability of the present model. Finally the electron current was found to be confined to short periodic pulses of 2–3 ns width.
Atomic configurations and local order in laser-synthesized Si, Si-N, Si-C, and Si-C-N nanometric powders, as studied by x-ray-induced photoelectron spectroscopy and extended x-ray-absorption fine-structure analysis
Authors: Gheorghiu A. , Sénémaud C. , Roulet H. , Dufour G. , Moreno T. , Bodeur S. , Reynaud C. , Cauchetier M. , Luce M.
Published: July 2002

The chemical bonding and local order in Si, Si-N, Si-C, and Si-C-N nanometric powders prepared by laser synthesis have been investigated by using two experimental methods. The local environment of Si, C, and N atoms have been studied by x-ray-induced photoelectron spectroscopy, from a detailed analysis of the Si 2p, C 1s, and N 1s core levels. Complementary information concerning the structural arrangement around Si atoms has been deduced from extended x-ray-absorption fine-structure analysis performed at the Si K edge. The results show that the Si and Si-C powders prepared with a 600 W laser power are pure, well-crystallized materials; the Si-N sample prepared with the same laser power has a disordered structure, although chemical order exists. For the composite samples, the results reveal that, in particular preparation conditions, a local C-Si-N3 structure is formed.
Polymerized complex synthesis of a pure 93 K Y2Ba4Cu7O15−d superconductor without the need of high oxygen pressure and additive catalysts
Authors: Berastegui Pedro , Kakihana Masato , Yoshimura Masahiro , Mazaki Hiromasa , Yasuoka Hiroshi , Johansson Lars-Gunnar , Eriksson Sten , Börjesson Lars , Käll Mikael
Published: July 2002

High-purity ceramic material of the superconducting phase Y2Ba4Cu7O14.82 (247) has been synthesized at 870 °C by the polymerized complex method using neither high oxygen pressure nor additive catalysts. The method is based on the formation of a polymer-metal complex precursor which is prepared through polyesterification between metal citrate complexes and ethylene glycol. Apart from the fact that high oxygen pressure is unnecessary, the present preparation technique offers more convenient and easier fabrication of highly pure 247 material compared with other wet chemical routes since it eliminates many steps such as centrifugation, filtration, aging, and pH control. X-ray diffraction and Raman scattering analyses show that the material is single-phase without any indication of secondary phases. Zero-resistance has been achieved at 88.0 K with a transition width narrower than 4 K. Complex ac magnetic susceptibility measurements confirm the presence of a single bulk superconducting 247 phase with Tc(onset)=93.0 K and ΔTc (10%–90%)=4.5 K.
Silicide formation and silicide-mediated crystallization of nickel-implanted amorphous silicon thin films
Authors: Hayzelden C. , Batstone J. L.
Published: July 2002

The nucleation and growth of isolated nickel disilicide precipitates in Ni-implanted amorphous Si thin films and the subsequent low-temperature silicide-mediated crystallization of Si was studied using in situ transmission electron microscopy. Analysis of the spatial distribution of the NiSi2 precipitates strongly suggested the occurrence of site saturation during nucleation. NiSi2 precipitates were observed in situ to migrate through the amorphous Si thin films leaving a trail of crystalline Si at temperatures as low as ∼484 °C. Initially, a thin region of epitaxial Si formed on {111} faces of the octahedral NiSi2 precipitates with a coherent interface which was shown by high-resolution electron microscopy to be Type A. Migration of the NiSi2 precipitates led to the growth of needles of Si which were parallel to 〈111〉 directions. The growth rate of the crystalline Si was limited by diffusion through the NiSi2 precipitates, and an effective diffusivity was determined at 507 and 660 °C. A mechanism for the enhanced growth rate of crystalline Si is proposed.
The defect structure of congruently melting lithium niobate
Authors: Wilkinson A. P. , Cheetham A. K. , Jarman R. H.
Published: July 2002

Six samples of congruently melting lithium niobate (∼Li0.95Nb1.01O3) have been examined by powder x-ray diffraction for the occurrence of niobium antisite defects (niobium on lithium sites). Contrary to current thinking, we have found no evidence for the occurrence of high concentrations of such defects, which also require the presence of Nb site vacancies. Rather, it was found that the number of antisite defects is close to the minimum that is required to sustain charge neutrality in a model of the type ([Li1−xNbx/5][Nb]O3), where the Nb site is fully occupied.
Optical constants of vacuum-evaporated polycrystalline cadmium selenide thin films
Authors: Pal U. , Samanta D. , Ghorai S. , Chaudhuri A. K.
Published: July 2002

The optical constants (n,K) of vacuum-evaporated polycrystalline CdSe thin films are determined over 900–3100 nm photon wavelengths. Variation of band gap and optical constants with film thickness and substrate temperature is studied. Anomalous variation of refractive index near the band gap is explained by the volume and surface imperfections. Average spin-orbit splitting of valence band (0.32) is estimated for the films deposited on mica substrates. A theoretical plot of refractive index near the band edge is done. The dispersion of refractive index in films is studied by considering a single-oscillator model.
Forced diffusion of impurities in natural diamond and polycrystalline diamond films
Authors: Popovici Galina , Sung T. , Khasawinah S. , Prelas M. A. , Wilson R. G.
Published: July 2002

A method is proposed for the determination of the state of an impurity (donor, acceptor, or deep level) in semiconductor lattice. To demonstrate the method boron was diffused into type Ia natural diamond under a dc electric field. The concentration and diffusion profiles of boron were affected by the applied field. Boron diffuses as a negative ion since it is an acceptor shallow enough to be partially ionized at the temperature of diffusion. The drift velocity of boron ions at the temperature of diffusion was also estimated. The diffusion of lithium and oxygen from a Li2CO3 source in chemical vapor deposited diamond films was performed under bias at 1000 °C in an argon atmosphere. After diffusion, the concentrations of Li, O, and H in the diamond films were found to be around (3–4)×1019 cm−3. No dependence of these concentrations on the applied bias was observed. It was found that the diffusion of Li goes primarily through grain boundaries, which may explain why it does not depend on the applied voltage. Fluorine was present as an impurity in the dopant source. Its concentration in the films was around (1–2)×1017 cm−3 and did depend on the applied bias, indicating that fluorine may have formed a shallow level in the diamond band gap.
Epitaxial nucleation and growth of chemically derived Ba2YCu3O7−x thin films on (001) SrTiO3
Authors: McIntyre P. C. , Cima M. J. , Roshko A.
Published: July 2002

The nucleation and growth kinetics of epitaxial Ba2YCu3O7−x thin films prepared on (001) SrTiO3 by postdeposition annealing of a chemically derived intermediate layer were investigated in specimens quenched from the growth anneal. The films were produced by spin-on deposition of a metal-organic precursor solution at room temperature and ambient pressure, and subsequent postdeposition annealing to form Ba2YCu3O7−x (BYC). Integrated x-ray intensities of reflections from the majority, c-axis-normal epitaxial BYC in the films were analyzed as a function of time at annealing temperature. Data collected from specimens annealed at 730 and 750 °C were consistent with upward growth of the BYC from nuclei at the substrate surface with a constant growth rate. The linear transformation kinetics suggest the growth process is controlled by molecular processes at the BYC/intermediate interface, and is not rate limited by diffusion. The activation energy for film growth along its [001] axis was estimated to be ∼5 eV. The microstructure of fully annealed films grown on (001) SrTiO3 was, in general, similar to that of films prepared on (001) LaAlO3 using the same metal-organic deposition technique. The volume fraction of the c-axis in-plane epitaxial orientation in films on (001) SrTiO3 was, however, much smaller than in films prepared identically on (001) LaAlO3. Vicinal polishing of the SrTiO3 by as much as 2.5° off [001] in the direction of [101] increased the volume fraction of c-axis in-plane domains in the films somewhat. These domains were preferentially oriented so that their (001) planes were parallel to the steps in the SrTiO3 surface. The fraction of the films on vicinal SrTiO3 occupied by c-axis in-plane domains was, however, still significantly less than that present in BYC films prepared on nominally on-axis (001) LaAlO3.
Helium reemission, desorption and microstructure evolution of graphites under helium ion implantation
Authors: Alimov V. Kh. , Scherzer B. M. U. , Chernikov V. N. , Ullmaier H.
Published: July 2002

Helium reemission, trapping, and thermal desorption from highly ordered pyrolytic graphite (HOPG and HPG) of different orientation, polycrystalline graphite (EK98), and titanium doped graphite (RG-Ti-91) have been measured at irradiation temperatures of 300 K and 800 K. The implantation was performed with a 40 keV 4He ion beam. Detailed transmission electron microscopy (TEM) investigation of the microstructure evolution was made on the implanted specimens. He reemission from basal oriented (BO) highly oriented pyrolytic graphite is accompanied by blistering and flaking leading to repetitive gas bursts. On edge oriented (EO) pyrolytic graphite three reemission peaks are observed during room temperature implantation, the first and smallest one being assigned to He release from intrinsic lenticular cavities, the second one occurs during early bubble formation when a small amount of implanted gas still escapes accumulation, and the third and largest peak being due to He release by bubble coalescence. The He reemission rate grows very slowly at room temperature and does not reach 100% up to the highest implanted fluence of 3.5⋅1018 He/cm2. At 800 K the He reemission rate from EO pyrolytic graphite reaches 100% immediately after starting implantation due to the high diffusive mobility of He.

EK98 and RG-Ti-91 show similar reemission behaviour. No gas bursts due to blistering are observed. The initial reemission rate at 300 K is higher than in EO pyrolytic graphite due to release of He via a network of intergranular channels. At 800 K reemission is rather similar to that from EO pyrolytic graphite. No thermal desorption of He from BO HOPG up to 1200 K is observed for implanted fluences ≤1016 He/cm2. At higher fluences the onset temperature of desorption decreases from 750 K at 2⋅1016 He/cm2 to 380 K at 1017 He/cm2 caused by thermal flaking due to pressure increase of He in submicroscopic cracks. In the other materials two desorption peaks are observed, the first one being related to He release from ‘‘solid solution,’’ while the second is attributed to gas escape from He filled bubbles. In contrast to the conclusions of Niwase et al. we find from the reemission kinetics and from selected area electron diffraction patterns (SADP) that graphite implanted at 300 K with He up to a damage of 10 dpa and more cannot be regarded as amorphous and shows a distorted turbostratic structure. The c parameter increases to (3.6..3.7)Å. Radiation effects in graphite implanted at 800 K are less pronounced up to damage levels of 200 dpa but may decrease the He diffusion coefficient.
Structural and electronic transport properties of ReSi2−δ single crystals
Authors: Gottlieb U. , Lambert-Andron B. , Nava F. , Affronte M. , Laborde O. , Rouault A. , Madar R.
Published: July 2002

We investigated some structural and transport properties of semiconducting ReSi2−δ . In the literature this silicides is reported to crystallize in an orthorhombic structure and to be stoichiometric ReSi2. Our investigations clearly show that the stable composition is ReSi1.75 crystallizing in the space group P1. Transport measurements show thermally activated behavior at high temperatures with one (or two) energy gap Eg=0.16 (0.30 eV). We also report Hall-effect measurements on this material: we found that RH is positive between 30 and 660 K and at room temperature the Hall number nH=1/eRH is equal to 3.7×1018 cm−3. The Hall mobility at room temperature is relatively high (μH=370 cm2/V s) for a single crystal.
Dusty plasma formation: Physics and critical phenomena. Theoretical approach
Authors: Fridman A. A. , Boufendi L. , Hbid T. , Potapkin B. V. , Bouchoule A.
Published: July 2002

Kinetics of the many-stage process of particle nucleation and growth in low-pressure rf discharge in silane SiH4–Ar is considered. The particle growth is considered in an analytical model as a chain of negative-ion molecular reactions, stimulated by vibrational excitation. In the framework of this model, a limitation of first generation particle size is explained as well as the strong temperature effect on cluster growth. A theory of critical phenomena of cluster trapping in discharge area has been elaborated to describe the neutral particle selection by size, and the particle concentration increases during a period exceeding the residence time in plasma. Finally, an analytical model of critical phenomena of particle coagulation and its influence on plasma parameters is developed to explain the latest experimental results on supersmall 2–10 nm cluster kinetics. All theoretical results are presented in comparison with corresponding new experimental data and with results of an especially made computer simulation.
Aluminum-induced crystallization and counter-doping of phosphorous-doped hydrogenated amorphous silicon at low temperatures
Authors: Haque M. S. , Naseem H. A. , Brown W. D.
Published: July 2002

Aluminum metal-induced crystallization and doping of hydrogenated amorphous silicon (a-Si:H) have been investigated. Aluminum was evaporated onto device quality a-Si:H films deposited in an ultrahigh vacuum plasma-enhanced chemical vapor deposition system. These Al/a-Si:H structures were annealed in the 100–300 °C range. Electrical, surface morphological, and chemical characterizations of the material were performed. The transmission line model technique was used for electrical characterization. Raman spectroscopy showed that crystallization of the interacted a-Si:H film underneath Al pads initiates at temperatures as low as 180 °C. X-ray diffraction analysis showed very good polycrystallinity of the interacted film. Electrical measurement, Hall measurement and x-ray photoelectron spectroscopy analysis results revealed that a-Si:H film in contact with Al becomes heavily doped by Al during crystallization as a result of annealing at relatively low temperatures.
Frame decoupling at low frequency in thin porous layers saturated by air
Authors: Bardot Anne , Brouard Bruno , Allard Jean-François
Published: July 2002

According to the Biot theory and other models of propagation, two compressional waves can propagate in a porous medium saturated by air. At low frequencies each wave creates displacements of the same order of magnitude in the fluid and the frame. It is shown that for the case of thin materials bonded onto a rigid impervious backing, at low frequencies the acoustic field can nevertheless be described by only one wave which propagates in the air. This simple description is related to a mixing of both compressional waves which leaves the frame motionless.
Electron irradiation-induced changes in the surface topography of silicon dioxide
Authors: Stevens Kalceff M. A. , Phillips M. R. , Moon A. R.
Published: July 2002

The irradiation of crystalline (α-SiO2) and amorphous (a-SiO2) silicon dioxide with a stationary electron beam produces characteristic changes in the surface topography. The development of these changes has been investigated using cathodoluminescence spectroscopy and microscopy, scanning probe (atomic force) microscopy, and scanning electron microscopy. Electron irradiation produces a permanent volume increase on (crystalline) α-SiO2, while in (amorphous) a-SiO2 an initial small volume increase is followed by volume loss as irradiation continues. The observed changes are consistent with electromigration of oxygen under the influence of the electric field induced by charge trapping at preexisting or irradiation-induced defects. Oxygen enrichment may produce expansion of the surface region due to the formation of peroxy linkage defects. In a-SiO2, charges trapped by defects at grain boundaries produce enhanced electric fields which may result in volume reduction at the surface, when critical field strengths are exceeded. The observed volume reductions may be attributed to electron stimulated desorption of constituents, in particular oxygen mass loss, and densification of the surface region associated with the formation of oxygen-deficient defect centers.
Magnetic properties of iron-polyethylene nanocomposites prepared by high energy ball milling
Authors: Giri Anit Kumar
Published: July 2002

Metal-polymer nanocomposites in the system iron-polyethylene have been prepared by high energy ball milling. Minimum average grain size obtained is of the order of 9 nm. Enhanced coercivity of 230 Oe at room temperature and 510 Oe at 5 K have been obtained for the sample containing 0.1 volume fraction of iron in polyethylene after milling for 200 h. This high value of the coercivity could be ascribed to the presence of a fraction of single-domain particles in the sample as evidenced from the thermal dependence of the magnetization and ac susceptibility measurements.
Anomalous index contrast due to point source illumination in scanning optical microscopy
Authors: McDaniel Eric B. , Hsu J. W. P.
Published: July 2002

Using a point source formed by the sub-wavelength aperture of a near-field scanning optical microscope tip, we examine anomalies in the image contrast arising from the use of point source illumination as opposed to the more common collimated light geometry. For samples with predominately spatial index variations, the image contrast obtained using a point source is the reverse of what is normally expected from collimated light illumination at normal incidence. The point source image contrast is unusually large and has a strong dependence on the numerical aperture of the collection optics. We compare the measured contrast across two adjoining glasses of slightly different indices with the result of a two-dimensional model, and find that refraction at the index change boundary is the primary reason for the anomalous contrast.
Relativistic band structure calculation of cubic and hexagonal SiC polytypes
Authors: Persson C. , Lindefelt U.
Published: July 2002

A full-potential band structure calculation, within the local density approximation to the density functional theory, has been performed for the polytypes 3C, 2H, 4H, and 6H of SiC. The calculated effective electron masses are found to be in very good agreement with experimental values. The electron-optical phonon coupling has been estimated and the polaron masses are calculated to be 3%–13% larger than the corresponding bare masses. The effective electron masses of the second lowest conduction band minima are also presented and the calculated energy difference between the two lowest minima in 4H–SiC is only 0.12 eV. The lowest conduction band in 6H–SiC is found to be very flat and to have a double-well-like minimum along the ML line. The top of the valence bands has been parametrized according to the k⋅p approximation, whereupon the effective hole masses have been determined. The spin-orbit interaction was found to have a strong influence on the value of the effective hole masses. Furthermore, total and partial densities of states are presented.
Growth anisotropy and self-shadowing: A model for the development of in-plane texture during polycrystalline thin-film growth
Authors: Karpenko O. P. , Bilello J. C. , Yalisove S. M.
Published: July 2002

The development of a preferred crystallographic orientation in the plane of growth, an in-plane texture, is addressed in a model that incorporates anisotropic growth rates of a material and self-shadowing. Most crystalline materials exhibit fast growth along certain crystallographic directions and slow growth along others. This crystallographic growth anisotropy, which may be due to differences in surface free energy and surface diffusion, leads to the evolution of specific grain shapes in a material. In addition, self-shadowing due to an obliquely incident deposition flux leads to a variation in in-plane grain growth rates, where the “fast” growth direction is normal to the plane defined by the substrate normal and the incident flux direction. This geometric growth anisotropy leads to the formation of elongated grains in the plane of growth. Neither growth anisotropy alone can explain the development of an in-plane texture during polycrystalline thin-film growth. However, whenever both are present (i.e., oblique incidence deposition of anisotropic materials), an in-plane texture will develop. Grains that have “fast” crystallographic growth directions aligned with the “fast” geometric growth direction overgrow grains that do not exhibit this alignment. Furthermore, the rate of texturing increases with the degree of each anisotropy. This model was used to simulate in-plane texturing during thin-film deposition. The simulation results are in excellent quantitative agreement with recent experimental results concerning the development of in-plane texture in sputter deposited Mo films.
High-frequency response of atomic-force microscope cantilevers
Authors: Turner Joseph A. , Hirsekorn Sigrun , Rabe Ute , Arnold Walter
Published: July 2002

Recent advances in atomic-force microscopy have moved beyond the original quasistatic implementation into a fully dynamic regime in which the atomic-force microscope cantilever is in contact with an insonified sample. The resulting dynamical system is complex and highly nonlinear. Simplification of this problem is often realized by modeling the cantilever as a one degree of freedom system. This type of first-mode approximation (FMA), or point-mass model, has been successful in advancing material property measurement techniques. The limits and validity of such an approximation have not, however, been fully addressed. In this article, the complete flexural beam equation is examined and compared directly with the FMA using both linear and nonlinear examples. These comparisons are made using analytical and finite difference numerical techniques. The two systems are shown to have differences in drive-point impedance and are influenced differently by the interaction damping. It is shown that the higher modes must be included for excitations above the first resonance if both the low and high frequency dynamics are to be modeled accurately.
Effects of barrier height distribution on the behavior of a Schottky diode
Authors: Chand Subhash , Kumar Jitendra
Published: July 2002

The current–voltage characteristics of a Schottky diode are simulated numerically using the thermionic emission-diffusion mechanism and considering a Gaussian distribution of barrier heights, with a linear bias dependence of both the mean and standard deviation. The resulting data are analyzed to get insight into the effects of distribution parameters on the barrier height, activation energy plots and the ideality factor over a temperature range 50–300 K. It is shown that with a Gaussian distribution of the barrier heights the system continues to behave like a single Schottky diode of apparently low zero-bias barrier height and a high ideality factor. Its barrier height decreases, activation energy plot becomes non-linear and ideality factor increases with a decrease in temperature. While the distribution parameters are responsible for the abnormal decrease of barrier height, their bias dependences account for the higher ideality factor at low temperatures. Also, the pivotal role played by series resistance in influencing the linearity of the ln(I)–V plots of Schottky diodes with a Gaussian distribution of barrier heights is discussed.
Monte Carlo simulation of x-ray emission by kilovolt electron bombardment
Authors: Acosta E. , Llovet X. , Coleoni E. , Riveros J. A. , Salvat F.
Published: July 2002

A physical model for the simulation of x-ray emission spectra from samples irradiated with kilovolt electron beams is proposed. Inner shell ionization by electron impact is described by means of total cross sections evaluated from an optical-data model. A double differential cross section is proposed for bremsstrahlung emission, which reproduces the radiative stopping powers derived from the partial wave calculations of Kissel, Quarles and Pratt [At. Data Nucl. Data Tables 28, 381 (1983)]. These ionization and radiative cross sections have been introduced into a general-purpose Monte Carlo code, which performs simulation of coupled electron and photon transport for arbitrary materials. To improve the efficiency of the simulation, interaction forcing, a variance reduction technique, has been applied for both ionizing collisions and radiative events. The reliability of simulated x-ray spectra is analyzed by comparing simulation results with electron probe measurements.
Persistent photoconductivity in SiGe/Si quantum wells
Authors: Tsai L. C. , Huang C. F. , Fan J. C. , Chang Y. H. , Chen Y. F. , Tsai W. C. , Chang C. Y.
Published: July 2002

Persistent photoconductivity (PPC) has been observed in boron-doped Si1−xGex/Si quantum wells. The decay kinetics of the PPC effect can be well described by a stretched-exponential function, Ippc(t)=Ippc(0)exp[−(t/τ)β](0<β<1), which is usually observed in many disorder materials. Through the studies of the PPC effect under various conditions, such as different temperature, different photon energy of photoexcitation, and different Ge content, we identify that the alloy potential fluctuations induced by compositional disorder are the origin of the PPC effect in Si1−xGex/Si quantum wells.
Cathodic arc carbon plasma/gas interaction in fullerenes synthesis study
Authors: Meunier J.-L.
Published: July 2002

Cathodic arc plasmas show characteristics similar to the fullerene producing laser ablation plasmas. Experimental and theoretical results on cathodic arc systems are applied to study the possible zones of formation of the large carbon molecules. These are believed to form in a relatively localized region whose position is governed by basic gas and expanding plasma parameters. A snowplow plasma expansion model gives a probable zone for the nucleation of larger carbon molecule located at 3 mm from the cathode for a 120 A arc in 55 Torr of helium. This correlates with preliminary spectroscopic measurements of C2 molecular radiation emission observed in pulsed cathodic arc plasma discharges in helium.
Electrical and defect thermodynamic properties of nanocrystalline titanium dioxide
Authors: Knauth P. , Tuller H. L.
Published: July 2002

The electrical properties of dense nanocrystalline TiO2 ceramics with an average grain size of 35 nm were investigated by impedance spectroscopy and compared with a coarsened material with micron size grains. The nanocrystalline ceramics show an uncommon domain of ionic conductivity at high oxygen pressures and a steep increase of electronic conductivity at low oxygen pressures. These results are discussed in terms of defect chemical models. The enthalpy of reduction of the nanocrystalline TiO2 was deduced to be significantly lower than that of the coarsened material. This may be related to lower defect formation energies at interface sites.
Chemical trends for acceptor impurities in GaN
Authors: Neugebauer Jörg , Van de Walle Chris G.
Published: July 2002

We present a comprehensive investigation of acceptor impurities in GaN, based on first-principles total-energy calculations. Two main factors are identified that determine acceptor incorporation: the strength of chemical bonding between the acceptor and its neighbors (which can be assessed by comparison with existing compounds) and the atomic size match between the acceptor and the host atom for which it substitutes. None of the candidates (Li, Na, K, Be, Zn, and Ca) exhibits characteristics which surpass those of Mg in all respects. Only Be emerges as a potential alternative dopant, although it may suffer from compensation by Be interstitial donors.
Effects of ion implantation and thermal annealing on the photoluminescence in amorphous silicon nitride
Authors: Seol Kwang Soo , Futami Tsuyoshi , Watanabe Takashi , Ohki Yoshimichi , Takiyama Makoto
Published: July 2002

When amorphous silicon nitride films are irradiated by a KrF excimer laser, they exhibit broad photoluminescence (PL) centered around 2.4 eV. The PL intensity gradually decreases and the PL peak energy shifts to a lower energy with an increase of the implanted dose of Ar+ ions. This means that the PL consists of two components with peak energies at 2.66 and 2.15 eV and that implantation-induced defects such as vacancies are not the PL centers. The PL intensity is found to decrease if the film was thermally annealed, while the decreased PL intensity of the ion-implanted film recovers by the thermal annealing. Based on these results, it is concluded that the defects generated by hydrogen release or bond breaking act as nonradiative recombination centers that quench the PL.