bluuebunny/crossref_metadata_2025 huggingface dataset:
Different operational regimes in a helicon plasma source
Authors: Kaeppelin V. , Carrère M. , Faure J. B.
Published: July 2002

The characterization of a helicon plasma source has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around 109 cm−3), an inductive mode (known as the H mode) with higher densities (1010 cm−3), and a helicon-wave sustained mode (known as the W mode) with very high densities (1011–1012 cm−3). By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the rf power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the H–W transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.
Near-infrared electroluminescence from conjugated polymer/lanthanide porphyrin blends
Authors: Harrison Benjamin S. , Foley Timothy J. , Bouguettaya Mohamed , Boncella James M. , Reynolds John R. , Schanze Kirk S. , Shim Joonbo , Holloway Paul H. , Padmanaban G. , Ramakrishnan S.
Published: July 2002

Near-infrared-emitting polymer light-emitting diodes (PLEDs) have been fabricated using blends of conjugated polymers and lanthanide tetraphenylporphyrin complexes. Host polymers include MEH–PPV and a bis-alkoxy-substituted poly(p-phenylene) (PPP–OR11), and the lanthanide complexes include Yb(TPP)acac and Er(TPP)acac (where TPP=5,10,15,20-tetraphenylporphyrin and acac=acetylacetonate). Electroluminescence (EL) is observed at 977 nm from devices fabricated using MEH–PPV or PPP–OR11 blended with Yb(TPP)acac, and EL is observed at 1560 nm from a device fabricated using a blend of MEH–PPV and Er(TPP)acac. Visible EL from the host polymers is strongly suppressed in all of the devices, however, in the device fabricated using the PPP–OR11 polymer blue emission from the host is completely quenched. Very efficient quenching of the EL from the host in the PPP–OR11 device is believed to occur due to efficient Förster energy transfer, which is facilitated by the excellent spectral overlap between the PPP–OR11 fluorescence and the Soret absorption band of the TPP ligand.
Coherent quantum phenomena in a normal cylindrical conductor with a superconducting coating
Authors: Gogadze G. A. , Shekhter R. I. , Jonson M.
Published: July 2002

The thermodynamic properties of a mesoscopic-size, simply connected cylindrical normal metal in good metallic contact with superconducting banks are studied theoretically. It is commonly accepted that if the superconductor thickness is quite small (of the order of the coherence length), as is assumed to be the case here, a vector potential field, whose value can be varied, exists inside the normal layer. It is further assumed that the quasiparticles with energy E<Δ (2Δ is the superconducting gap) move ballistically through the normal metal and undergo Andreev scattering caused by the off-diagonal potential of the superconductor. An equation is obtained within the multidimensional quasiclassical method which permits us to determine the spectrum of the Andreev levels and to calculate the density of states of the system in question. It is shown that the Andreev levels shift as the trapped flux Φ changes inside the normal conductor. At a certain flux value they coincide with the Fermi level. A resonance spike in the density of states ν(E) appears in this case, since near E=0 there is strong degeneracy of the quasiparticle states in respect to the quantum number q characterizing their motion along the cylinder axis. As a result, a macroscopic number of q states contribute to the amplitude of the effect. As the flux is increased, the density of states v(E) behaves as a stepwise function of Φ. The distance between the steps is equal to the superconducting flux quantum hc/2e.
Three dimensional arrays of cobalt nanocrystals: Fabrication and magnetic properties
Authors: Petit C. , Legrand J. , Russier V. , Pileni M. P.
Published: July 2002

Cobalt nanocrystal three dimensional (3D) arrays were fabricated by deposition of a ferrofluid on a highly oriented pyrolitic graphite substrate and by applying a magnetic field parallel to the substrate during the deposition process. Depending on the strength of the magnetic field, regular stripes of cobalt nanocrystals are obtained. The influence of the deposition process parameters was investigated. The magnetic properties of these 3D arrays are discussed in terms of orientation of the easy axes and ferromagnetic domain formation.
Current-crowding-induced electromigration failure in flip chip solder joints
Authors: Yeh Everett C. C. , Choi W. J. , Tu K. N. , Elenius Peter , Balkan Haluk
Published: July 2002

In a flip chip solder joint, the cross-section of the solder bump is one to two orders of magnitude bigger than that of an interconnect wire. At the contact interface between the bump and the wire, a very large current crowding occurs and it causes a unique and fast electromigration failure in the bump. Simulation of the current crowding phenomenon in a flip chip solder bump is reported here. Experimental results of real flip chip solder bumps show that void formation begins near the current crowding region of the contact, and after it is nucleated, it spreads quickly across the contact area. By designing the solder bump to achieve a uniform current distribution, we can improve its electromigration resistance and increase its current carrying capacity.
Comparison of PbI2 and HgI2 for direct detection active matrix x-ray image sensors
Authors: Street R. A. , Ready S. E. , Van Schuylenbergh K. , Ho J. , Boyce J. B. , Nylen P. , Shah K. , Melekhov L. , Hermon H.
Published: July 2002

The factors determining the x-ray sensitivity of HgI2 and PbI2 as direct detector materials for large area matrix addressed x-ray image sensors are described, along with a model to explain their different properties. The imaging studies are made on test arrays with 512×512 pixels of size 100 μm. The x-ray sensitivity and spatial resolution are reported, along with measurements of the various mechanisms that influence the sensitivity, such as charge collection, x-ray absorption, fill factor, and image lag. The spatial resolution of PbI2 decreases with increasing film thickness, but this effect is not observed in HgI2. The x-ray response data are used to compare the sensitivity to the theoretical values for the ionization energy and to identify the various loss mechanisms. We find that the sensitivity of HgI2 can be explained by a few small and well characterized loss factors. This material exhibits good spatial resolution, high fill factor, and high charge collection. PbI2 films exhibit lower sensitivity, principally attributable to a very large image lag. We propose that the x-ray response of the two materials is distinguished by their different depletion layer properties, and present a model that accounts for the sensitivity, image lag, and spatial resolution of PbI2.
Geometries and spectroscopic properties of silicon clusters (Si5, Si5+, Si5−, Si6, Si6+, and Si6−)
Authors: Zhao Cunyuan , Balasubramanian K.
Published: July 2002

Ground and excited electronic states of the neutral, cationic, and anionic silicon pentamer and hexamer (Si5 and Si6) are investigated. Different geometries such as trigonal bipyramid (TBP; D3h), distorted-TBP (C2v), and edge-capped tetrahedron (ECT; C2v) for Si5 and tetragonal bipyramid (TEBP; D4h, D2h), edge-capped trigonal bipyramid (ECTBP; C2v) for Si6 were studied. We have employed a number of techniques such as large scale complete active-space multiconfiguration self-consistent field (CAS-MCSCF), mutireference singles+doubles configuration interaction (MRSDCI) computations up to 12 million configurations, Møller–Plesset (MP2) and coupled cluster singles and doubles+triple excitation estimate [CCSD(T)] techniques to investigate the low-lying electronic states, their geometries and energy separations of neutral, cationic and anionic Si5 and Si6. A A1g1 TEBP structure (D4h symmetry) is computed as the ground state for Si6, in accord with the previously suggested experimental assignments, while the Si5 cluster is found to have a TBP (D3h) ground state. The excitation energy, atomization energies, ionization potentials, and vertical and adiabatic electron affinities are computed and compared with the experimental results. Our computations of the excited states of these species have facilitated assignment of the anion X, A, and B bands of the photoelectron spectra of Si5− and Si6− observed by Neumark and co-workers.
Chemical ordering at low temperatures in FePd films
Authors: Ravelosona D. , Chappert C. , Bernas H. , Halley D. , Samson Y. , Marty A.
Published: July 2002

We demonstrate that, if a high degree of short range order is present in FePd disordered films, a high value of the long range order parameter S can be obtained by using postgrowth ion irradiation at very low processing temperatures. FePd films deposited monolayer by monolayer at room temperature on MgO(001) substrates exhibit a very low degree of long range order (S∼0.1) but a high degree of short range order as demonstrated by extended x-ray absorption fine structure measurements. Irradiation with 130 keV He+ ions at low fluences (2.0×1016 ions/cm2) leads to a large increase in the long range order parameter and to a large increase in perpendicular anisotropy for irradiation at substrate temperatures lower than 200 °C. This could have a great impact on the current race toward high magnetic recording density media.
Attenuated total reflection spectroscopy for infrared analysis of thin layers on a semiconductor substrate
Authors: Rochat N. , Chabli A. , Bertin F. , Olivier M. , Vergnaud C. , Mur P.
Published: July 2002

A theoretical analysis based on a perturbation method is used to elucidate the results of attenuated total reflection (ATR) measurements performed on silicon oxide layers of different thicknesses on silicon substrates. This analysis shows that the absorbance ATR spectrum in p polarization is the image of the layer energy loss function, under specific conditions. It is pointed out that the enhanced sensitivity of ATR is controlled by the air gap thickness, the optical properties of the media involved, and the probing light polarization. An exact ATR spectrum simulation using a matrix formalism showed that straightforward interpretation in terms of the layer dielectric function is limited to a very narrow layer thickness range. The ATR spectrum fitting process is considered for layers out of this range and evaluated for the interpretation of experimental silicon oxide layer spectra.
Ab initio studies on the van der Waals complexes of polycyclic aromatic hydrocarbons. II. Naphthalene dimer and naphthalene–anthracene complex
Authors: Lee Nam Ki , Park Soonyong , Kim Seong Keun
Published: July 2002

Ab initio calculations were carried out for the naphthalene dimer and naphthalene–anthracene complex to determine their stable geometries and binding energies. Two medium-size basis sets of 6-31G*(0.25) and 6-31+G* were employed at the MP2 level. Five local minima were found for the naphthalene dimer, three of which were parallel-displaced type and the other two T-shaped type. The global minimum geometry was a parallel-displaced structure of a two-layer graphitic type (Ci point group), not the crossed form (D2d). Its energy calculated by the 6-31G*(0.25) and 6-31+G* basis sets was −7.62 and −6.36 kcal/mol, respectively. The naphthalene–anthracene complex showed four local minima, two of which were parallel-displaced type and the other two T-shaped type. The global minimum was a twisted parallel-displaced form (C2), in which the centers of both molecules lie on the same z-axis with their two long axes skewed at an angle of ≈40°. Its energy was −11.30 and −9.52 kcal/mol with the 6-31G*(0.25) and 6-31+G* basis sets, respectively. From these results a set of general rules for the stable geometry of the polycyclic aromatic hydrocarbon clusters were derived, which turned out to be the same as those previously deduced from other systems less directly relevant to polycyclic aromatic hydrocarbons: (1) a face-to-face configuration is unstable, (2) the T-shaped structure is stable, (3) the parallel-displaced structure is also stable. We also found some additional rules: (4) the energies of the T-shaped and parallel-displaced structures are quite comparable when the molecules are small, but (5) the parallel-displaced structure becomes more stable than the T-shaped one as the molecules become larger due to the nature of the π–π interaction. The interplanar distance of stable parallel-displaced structures was about 3.3–3.4 Å, while the plane-to-center distances of T-shaped structures was about 5.0–5.1 Å. We also discovered what we would call the integer rule for the binding energy of the polycyclic aromatic hydrocarbon clusters in that the binding energy varied linearly as the number of overlapping hexagons in the parallel-displaced structures. The ratio of binding energies for the benzene dimer, benzene–naphthalene complex, naphthalene dimer, and naphthalene–anthracene complex were nearly 1:2:3:4.
Coercive fields in ferroelectrics: A case study in lithium niobate and lithium tantalate
Authors: Kim Sungwon , Gopalan Venkatraman , Gruverman Alexei
Published: July 2002

The experimentally measured coercive electric fields for domain reversal in ferroelectrics are typically many orders of magnitude lower than the estimates from phenomenological free-energy theory. This letter specifically investigates the influence of polarization gradients at pre-existing 180° domain walls in ferroelectrics on coercive fields for domain wall motion. It is shown that the ratio of theoretical coercive field without and with a preexisting domain wall is directly proportional to the ratio xo/a, where a is the lattice parameter and 2xo is the polarization wall width. This factor is 7.5–45 for a 20–120 nm wall width, the latter width determined here as the experimental upper limit for polarization wall width in lithium tantalate.
High-power 400-nm-band AlGaInN-based laser diodes with low aspect ratio
Authors: Asano T. , Takeya M. , Tojyo T. , Mizuno T. , Ikeda S. , Shibuya K. , Hino T. , Uchida S. , Ikeda M.
Published: July 2002

High-power blue-violet laser diodes with aspect ratio as low as 2.3 and threshold current down to 33 mA have been realized. The relationship between threshold current and optical confinement factor was investigated in order to minimize the beam divergence angle perpendicular to the junction plane (θ⊥). θ⊥ was found to decrease with reduction of the optical confinement factor, whereas threshold current density increased. A new layer structure, in which a p-typed cladding layer was located next to an AlGaN electron blocking layer, and a GaInN guiding layer was inserted between the active and the AlGaN electron blocking layer, was effective for obtaining small θ⊥ while maintaining low threshold current.
Suppression of chaos and other dynamical transitions induced by intercellular coupling in a model for cyclic AMP signaling in Dictyostelium cells
Authors: Li Y. X. , Halloy J. , Martiel J. L. , Goldbeter A.
Published: July 2002

The effect of intercellular coupling on the switching between periodic behavior and chaos is investigated in a model for cAMP oscillations in Dictyostelium cells. We first analyze the dynamic behavior of a homogeneous cell population which is governed by a three-variable differential system for which bifurcation diagrams are obtained as a function of two control parameters. We then consider the mixing of two populations behaving in a chaotic and periodic manner, respectively. Cells are coupled through the sharing of a common chemical intermediate, extracellular cAMP, which controls its production and release by the cells into the extracellular medium; the dynamics of the mixed suspension is governed by a five-variable differential system. When the two cell populations differ by the value of a single parameter which measures the activity of the enzyme that degrades extracellular cAMP, the bifurcation diagram established for the three-variable homogeneous population can be used to predict the dynamic behavior of the mixed suspension. The analysis shows that a small proportion of periodic cells can suppress chaos in the mixed suspension. Such a fragility of chaos originates from the relative smallness of the domain of aperiodic oscillations in parameter space. The bifurcation diagram is used to obtain the minimum fraction of periodic cells suppressing chaos. These results are related to the suppression of chaos by the small-amplitude periodic forcing of a strange attractor. Numerical simulations further show how the coupling of periodic cells with chaotic cells can produce chaos, bursting, simple periodic oscillations, or a stable steady state; the coupling between two populations at steady state can produce similar modes of dynamic behavior.
Chaos in vibrating systems with a limited power-supply
Authors: Krasnopolskaya Tatyana S. , Shvets Alexander Yu.
Published: July 2002

New models and properties of forced oscillations of the various physical systems (pendulum and piezoceramic transducer) due to the interaction with the excitation device of limited power-supply are investigated in details. Using an analysis of the largest Lyapunov exponent for a complicated system—vibrating subsystem and exciter—the regions for three steady state regimes are determined, namely: stationary, periodic and chaotic.
Secondary cycle of equilibria in a system with cosymmetry, its creation by bifurcation and impossibility of symmetric treatment of it
Authors: Yudovich V. I.
Published: July 2002

A study is reported of the bifurcation of a cycle of equilibria of an autonomous differential equation with cosymmetry in Hilbert space, which is a simulation of the problem of planar filtrational convection of a fluid in a porous medium. The Lyapunov–Schmidt method and perturbation theory are used to find its amplitude and the damping rate of the dominant mode. It is shown that, in the abstract general model, and also in the problem of convection in a rectangular container, this damping rate varies along the cycle of equilibria. Hence, the cycle of equilibria cannot be an orbit of the action of any symmetry group of the given system.
Discontinuous and nondifferentiable functions and dimension increase induced by filtering chaotic data
Authors: Pecora Louis M. , Carroll Thomas L.
Published: July 2002

We show that one can use recently introduced statistics for continuity and differentiability to show the effect of filters of infinite extent in time on a chaotic time series. The statistics point to a discontinuous or nondifferentiable function between the unfiltered attractor and the filtered attractor as the origin of attractor dimension increase when the filtering is severe. The density of discontinuities as a function of resolution follows a scaling relation. We present direct visualization of this effect in the filtered Henon attractor where the origin of dimension increase becomes obvious.
Hysteretic transition between avalanches and continuous flow in rotated granular systems
Authors: Linz Stefan J. , Hager Wolfgang , Hänggi Peter
Published: July 2002

Experiments in drums or cylinders partly filled with a granular system and rotated constantly about their horizontally aligned axis of symmetry show a hysteretic transition from discrete avalanches to continuous flow if the rotation rate is adiabatically changed. Herein, we show that this hysteresis can be explained by the impact of global Langevin-type fluctuations in a recently proposed minimal model for surface flow along granular piles. For too large magnitudes of the fluctuations corresponding to almost elastic grains, the hysteresis vanishes. This might explain why molecular dynamical simulations were not yet able to detect the hysteretic transition.
Computer simulation of electric conductivity in a solid using classical mechanics
Authors: Bellomonte L. , Sperandeo-Mineo R. M.
Published: July 2002

This article discusses the computer simulation of a classical model of electric conductivity in metals. Its objective is mainly didactic. It allows a microscopic interpretation of the phenomenological laws studied in introductory physics courses. The model is implemented in the one- and two-dimensional copper lattice. In the latter case, the results are in partial agreement with the experimental data concerning the conduction in copper. The model considers the collisions between a free electron gas and the nuclei oscillating around the equilibrium lattice positions with average energy proportional to the room temperature and velocity distribution according to Maxwell–Boltzmann statistics. The collisions are supposed to be elastic, and a suitable lattice relaxation mechanism is considered. The results of the simulation concern the calculation of various quantities such as the electron root mean square velocity, the mean free time, the drift velocity, etc., either in the absence of an external electric field or in the presence of electric fields of various intensities. The dependence of the results on electric field intensity is reported and commented on and the validity of the model is verified. © 1996 American Institute of Physics.
Implementing an efficient collisionless N-body code on the Cray T3D
Authors: Sigurdsson Steinn , He Bohr , Melhem Rami , Hernquist Lars
Published: July 2002

We discuss a parallel implementation of a collisionless N-body code on the Cray T3D. The code was previously successfully implemented on the Connection Machine CM-5. The algorithm evolves self-gravitating systems with approximate spheroidal symmetry by solving Poisson’s equation for the global potential, given a realization of the density, and then advances the particle distribution under its own potential, solving for the new potential iteratively. The algorithm is naturally parallel and can be efficiently implemented on current massively parallel architectures. We discuss some of the issues that arise in porting efficient codes between different parallel architectures and show how changing the structure of the code can produce dramatic speedups on different architectures. We discuss some implications for automatic parallelization and optimization with portable compilers. © 1997 American Institute of Physics.
Calculation of surface generation and recombination velocities at the Si-SiO2 interface
Authors: Eades Wendell D. , Swanson Richard M.
Published: July 2002

Using deep level transient spectroscopy in the current transient mode, the interface trap density and electron and hole capture cross sections have been measured for thermally oxidized 〈100〉 silicon. We have compared oxides grown with and without HCl in the growth ambient, and also investigated the effect of the postoxidation inert ambient anneal. Values of the depleted surface generation velocity and surface recombination velocities in low- and high-level injection were then calculated from the measured interface trap parameters using Shockley–Read–Hall theory. We report here the results of the calculations and compare them with a few direct experimental measurements.
Hydrogenation of shallow-donor levels in GaAs
Authors: Pearton S. J. , Dautremont-Smith W. C. , Chevallier J. , Tu C. W. , Cummings K. D.
Published: July 2002

Shallow-donor levels due to Si, Ge, Sn, S, Se, and Te in GaAs are neutralized by association with atomic hydrogen; Si and Te donors in AlGaAs have also been shown to be neutralized. In contrast, the shallow acceptors Be, Mg, Zn, and Cd in GaAs are relatively unaffected by hydrogenation. The activation energy for recovery of the donor electrical activity is around 2.1 eV for each of the species, but varies as the strength of an isolated hydrogen-donor species bond. The neutralization depth of the donors is proportional to the inverse square root of donor concentration, and this depth is given as a function of plasma exposure temperature (100–350 °C) and bonding site density (8×1013–1.5×1018 cm−3).
Influence of process parameters on the electrical properties of plasma-sprayed silicon
Authors: Akani M. , Suryanarayanan R. , Brun G.
Published: July 2002

Plasma spray has been used to prepare n- and p-type polycrystalline deposits. The influence of process parameters on the electrical properties are reported. The origin of impurities such as Fe, Ni, and Al is pointed out. Heat treatment of the deposits is shown to increase the density and the mobility.
Photoacoustic measurement of thermal diffusivity of polymer foils
Authors: Leite N. F. , Cella N. , Vargas H. , Miranda L. C. M.
Published: July 2002

The photoacoustic measurement of polymer foils, typically 170–200 μm thick, is discussed. It is shown that the measurement based upon the phase lag between the front and rear illuminations is applicable only in a limited range of frequencies from 6 to 12 Hz. The dominant mechanism responsible for the photoacoustic signal, in almost the entire frequency range 10–100 Hz, is proven to be the thermoelastic bending of the foil samples. The thermal diffusivity is then obtained from the frequency dependence of the front-phase illumination data.
Versatile computer programs for analysis of random and channeling backscattering spectra
Authors: Kido Yoshiaki , Kawamoto Junichi
Published: July 2002

Two types of computer programs have been developed to simulate random and channeling backscattering spectra with high speed. One is applicable to analysis of multielemental and multilayered structures. Depth profiles of interdiffusion and of impurities distributed inhomogeneously can be analyzed simultaneously with this program. Another allows the simulation of channeling spectrum from a perfect or partially damaged single crystal whose top surface consists of an amorphous layer with an arbitrary thickness. These simulation programs are applied to determine the thicknesses of the ten-layered structure of Ag/Al/Ag/.../Al, the mixing rates for the Al/Sb system irradiated with Xe ions, and the damage profile of GaP induced by Ar+ irradiation.
Engineered Schottky barrier diodes for the modification and control of Schottky barrier heights
Authors: Eglash S. J. , Newman N. , Pan S. , Mo D. , Shenai K. , Spicer W. E. , Ponce F. A. , Collins D. M.
Published: July 2002

A technique for fabricating controlled Schottky barrier heights to GaAs over the entire band gap is demonstrated. Thin, highly doped semiconductor layers at the metal-semiconductor interface allowed the reproducible control of the effective barrier height on n-type GaAs from near zero (i.e., ohmic behavior at 300 K) to 1.33 eV (the band gap equals 1.43 eV at 300 K) with diode ideality factors 1.02≤n≤1.21. Molecular-beam epitaxy was used to grow GaAs epitaxial layers with in situ deposited Al metal layers, resulting in diodes with nearly ideal electrical and structural characteristics. Electrical characterization by current-voltage (I-V) and capacitance-voltage (C-V) techniques, models for these I-V and C-V characteristics, and structural characterization by high resolution transmission electron microscopy lattice images are presented. Implications of this work for models of Schottky barrier formation are discussed, as well as some applications for these ‘‘engineered Schottky barrier diodes.’’
Raman scattering of SiC: Estimation of the internal stress in 3C-SiC on Si
Authors: Mukaida H. , Okumura H. , Lee J. H. , Daimon H. , Sakuma E. , Misawa S. , Endo K. , Yoshida S.
Published: July 2002

Internal stress in 3C-silicon carbide (SiC) epilayers grown on Si substrates by chemical vapor deposition was studied using Raman scattering. The stress in the epilayers grown on Si(001) at 1350 °C was found to be 5.4×109 dyn/cm2 tensile, which is comparable to that of silicon on sapphire. The magnitude of the stress is discussed in terms of the elastic deformation theory. It was found that there is not so much difference between the stress in 3C-SiC epilayers grown on Si(001) and that in 3C-SiC on Si(111) as the elastic deformation theory suggests.
Gold diffusion in InP
Authors: Parguel V. , Favennec P. N. , Gauneau M. , Rihet Y. , Chaplain R. , L’Haridon H. , Vaudry C.
Published: July 2002

InP device degradations have often been related to the presence in the active layer of gold atoms having migrated from the contacts. We have studied gold thermal diffusion in InP in the temperature range 400–700 °C, using secondary ion mass spectrometry (SIMS). We have pointed out SIMS artifacts and how to avoid them. We have found small values for the diffusion coefficient: 2×10−12 cm2/s at 550 °C. By deep-level transient spectroscopy measurements, gold appeared to behave as a shallow donor, with a level situated at 0.55 eV from the conduction band. Our conclusion is that gold thermal migration from the contact is not the mechanism responsible for the device degradation.
Electrical properties of ion-implanted p-n junction diodes in β-SiC
Authors: Edmond J. A. , Das K. , Davis R. F.
Published: July 2002

Mesa structure junction diodes prepared via high-temperature ion implantation of Al+ (100 keV, 4.8×1014 Al/cm2) in n-type or N+ (90 and 180 keV, 0.9 and 1.3×1014 N/cm2) in p-type β-SiC thin films were electrically characterized as a function of temperature using current-voltage and capacitance-voltage measurements. In either case, rectification was observed to the highest measurement temperature of 673 K. Closer examination of the device current-voltage characteristics yielded diode ideality factors greater than 2. Additionally, the log dependence of these two parameters indicated space-charge-limited current in the presence of traps as the dominant conduction mechanism. From the temperature dependence of log-log plots, trap energies and densities were determined. Two trapping levels were observed: (1) 0.22 eV with a density of 2×1018 cm−3 and (2) 0.55 eV with a density of 2×1016 cm−3. The former is believed to be ionized Al centers (in the case of Al-implanted sample) and the latter a compensating acceptor level, both of which lie within the bottom third of the band gap. Reverse currents at low biases were characteristic of generation in the depletion region. At intermediate biases an ohmic dependence was observed, whereas at high biases the current appeared to be space-charge limited. Capacitance-voltage measurements indicated both types of diodes were abrupt junctions.
Shallow donors and D-X centers neutralization by atomic hydrogen in GaAlAs doped with silicon
Authors: Mostefaoui R. , Chevallier J. , Jalil A. , Pesant J. C. , Tu C. W. , Kopf R. F.
Published: July 2002

Hydrogen plasma exposure of silicon-doped Ga1−xAlxAs epilayers with x<0.37 causes a strong reduction of the free-electron concentration in the layers. For x<0.29, this effect is accompanied by a simultaneous increase of the electron mobility. This is interpreted, as in GaAs, in terms of a neutralization of the active silicon donors by atomic hydrogen. The neutralization efficiency of the shallow donors increases as x increases. For x≂0.25, the D-X centers are very efficiently neutralized by hydrogen and, as a consequence, the conductivity mechanisms after exposure are only governed by the remaining shallow donors. For 0.29<x<0.37, most of the D-X centers are neutralized, but the electron mobility after hydrogenation is reduced.
Effect of surface tension gradient driven convection in a laser melt pool: Three-dimensional perturbation model
Authors: Chan C. L. , Mazumder J. , Chen M. M.
Published: July 2002

A three-dimensional model of the thermocapillary flow within the molten region during laser surface heating is developed. This physically corresponds to the process of a stationary laser beam irradiating on the surface of a moving workpiece. The recirculating flow due to the surface tension gradient is much faster than the scanning motion. This allows a perturbation solution. The basic solution corresponds to the stationary axisymmetric case, and the perturbation is based on a small scanning velocity. The advantage of seeking a perturbation solution is that the three-dimensional flow is modeled by two sets of two-dimensional equations which are presumably much more tractable than the original three-dimensional equations. Numerical solutions are obtained. The solid-liquid interface is determined by an iterative scheme. In the presence of the recirculating flow, the heat transfer becomes convection dominated. The absorbed laser energy is convected sideways so that a wider and shallower molten region is obtained. The molten pool shape is obtained and presented. The effect of various operation parameters (such as laser power, beam radius) on the pool shape are obtained and discussed. The cooling rate of resolidifying materials is also determined. Trajectory of a fluid particle is presented. This provides the most realistic scenario of mixing in a laser melted pool obtained to date. This also gives a semiquantitative understanding of the mechanism of solute redistribution. The effect of alloying elements, which may change the temperature dependence to an increasing function, is also considered. A reverse recirculating vortex is obtained and its effect on pool shape is presented and discussed.
Frequency dependence of core loss in rapidly quenched Fe-6.5 wt. %Si
Authors: Fish G. E. , Chang C.-F. , Bye R.
Published: July 2002

The frequency, field, and temperature dependencies of core loss in rapidly quenched Fe-6.5 wt. %Si ribbons are reported. The annealed material exhibits (001)〈uv0〉 fiber texture, resulting in isotropic properties in plane; equivalent losses and B-H loops are seen in tape-wound toroids and in annuli punched from ribbon. Hysteresis losses predominate in the present Fe-6.5%Si up to about 1 kHz, as the thin gauge and high resistivity reduce eddy current losses from those of conventional nonoriented steels. The 2-kHz/1.0-T core losses are a factor of 6–8 higher in conventional M-15 than in Fe-6.5%Si. The core losses of Fe-6.5%Si are about 40% higher at 77 than at 373 K. The loss data are not in agreement with the predictions of Bertotti’s theory.
Hydrogen passivation of acceptors in p-InP
Authors: Dautremont-Smith W. C. , Lopata J. , Pearton S. J. , Koszi L. A. , Stavola M. , Swaminathan V.
Published: July 2002

The problem of hydrogenation of InP without surface degradation has been surmounted by exposure of the InP surface to a hydrogen plasma through a thin SiNx(H) cap layer. This layer is H permeable at the hydrogenation temperature of 250 °C, but P or PH3 impermeable thus minimizing PH3 loss and the attendant In droplet formation. In contrast to our results for this type of plasma exposure of GaAs, we find that shallow acceptors in InP are heavily passivated, whereas shallow donors are only very weakly affected. For example, p+-InP(Zn) of 3×1018 cm−3 has its residual hole concentration reduced to ≤3×1014 cm−3 over a depth of 1.3 μm by a 250 °C, 0.5 h deuteration. The presence of acceptors impedes H (or D) indiffusion, as indicated by D diffusion under the same conditions occurring to depths of 18 and 35 μm in p-InP (Zn, 2×1016 cm−3) and n-InP (S or Sn), respectively. Annealing for 1 min at 350 °C causes the acceptor passivation to be lost and the hole concentration to be returned to its prehydrogenation level, indicating that the passivation has similar thermal stability to that of acceptors in GaAs, but lower than that of donors.
Photocarrier generation, injection, and trapping at the interface in a layered organic photoconductor: Metal-free phthalocyanine/molecularly doped polymer
Authors: Kanemitsu Yoshihiko , Imamura Shunji
Published: July 2002

We have studied photocarrier generation and injection at the interface in a double-layered organic photoconductor consisting of a charge generation layer (CGL) of vacuum-deposited phthalocyanine film and a charge transport layer (CTL) of p-diethylaminobenzaldehyde-1,1-diphenyl hydrazone doped polymer film. The photocarrier generation efficiency in the CGL was measured by the photoacoustic method. The transport and trapping of holes in the CTL and at the CGL/CTL interface were studied by xerographic discharge measurements and time-of-flight photoconductivity measurements. The photogeneration efficiency in the CGL is affected by the hydrazone concentration in the CTL. At high hydrazone concentrations, the photogeneration efficiency means the quantum efficiency of photocarrier generations controlled by geminate and nongeminate recombination in the CGL. At low hydrazone concentrations, the photogeneration efficiency is affected by the injection and trapping of holes at the CGL/CTL interface. Xerographic and time-of-flight photoconductivity measurements show that the lifetime of holes at the CGL/CTL interface is short compared with that in the CTL and no trapping of holes occurs in the CTL. The lifetime of holes at the CGL/CTL interface decreases with increasing hydrazone concentration in the CTL. The injection efficiency of holes is determined by the competition between the trapping of holes at the CGL/CTL interface and the hopping transport of holes across the CGL/CTL interface. We discuss the influence of the hydrazone concentration in the CTL on the photocarrier generation in the CGL and the injection of holes at the CGL/CTL interface.
Efficient numerical simulation of electron states in quantum wires
Authors: Kerkhoven Thomas , Galick Albert T. , Ravaioli Umberto , Arends John H. , Saad Youcef
Published: July 2002

We present a new algorithm for the numerical simulation of electrons in a quantum wire as described by a two-dimensional eigenvalue problem for Schrödinger’s equation coupled with Poisson’s equation. Initially, the algorithm employs an underrelaxed fixed point iteration to generate an approximation which is reasonably close to the solution. Subsequently, this approximate solution is employed as an initial guess for a Jacobian-free implementation of an approximate Newton method. In this manner the nonlinearity in the model is dealt with effectively. We demonstrate the effectiveness of our approach in a set of numerical experiments which study the electron states on the cross section of a quantum wire structure based on III-V semiconductors at 4.2 and 77 K.
Plasma waveguide: A concept to transfer electromagnetic energy in space
Authors: Shen Hao-Ming
Published: July 2002

This paper studies the possibility of using a laser-generated ‘‘plasma waveguide’’ to transfer electromagnetic (EM) energy. The plasma waveguide is a cylindrical vacuum core surrounded by a plasma cladding. The analysis shows that guided-mode fields do exist inside the core. Like a general dielectric waveguide, the plasma waveguide is characterized by a ‘‘normalized frequency parameter’’ (also called the V number). Although the permittivity of the plasma varies strongly with frequency, the V number surprisingly remains constant over the entire frequency range. Because of this property, the frequency dependence of the plasma waveguide is different; it has a wider high-frequency response than the general dielectric waveguide. The EM pulse can propagate in the plasma waveguide at close to the speed of light and keep its profile and shape unchanged. When the V number is smaller than 2.4048 (the first root of the zero-order Bessel function), only the single HE11 mode exists in the plasma waveguide. Unlike the dielectric waveguide, however, there is no high-frequency limitation for single-mode propagation. The EM fields outside the core in the plasma decrease exponentially with increasing radius. Thus, practically, a plasma cladding of sufficient thickness is all that is required to confine the EM wave. Such a plasma waveguide can be generated by a hollow laser beam in upper space and used for guiding EM pulses. A brief survey on laser-generated plasmas is given.
Peculiarities of pulse laser alloying: Influence of spatial distribution of the beam
Authors: Smurov I. , Covelli L. , Tagirov K. , Aksenov L.
Published: July 2002

This paper deals with the influence of the spatial distribution of laser beam intensity on the dynamic of laser melting, the surface tension driven convection, and the distribution of alloying elements in the molten pool. The shape of the melting front boundary, the flow pattern, as well as the concentration fields of alloying elements are evaluated for different pulse laser actions operated by different types of nonmonotonic spatial distributions of energy density flow. The relationships between the shape of the laser beam and the structures of the alloyed zones are determined.
Fundamental properties of intrinsic gettering of iron in a silicon wafer
Authors: Aoki Masaki , Hara Akito , Ohsawa Akira
Published: July 2002

Properties of intrinsic gettering of Fe were studied by measuring Fe-B complex concentration and interstitial Fe concentration in a denuded zone after isochronal or isothermal annealing followed by quenching using deep level transient spectroscopy. We calculated the Fe concentration as the Fe-B complex concentration plus the interstitial Fe concentration. Silicon wafers were contaminated with a surface Fe concentration of 4.2×1011 to 3.2×1013 cm−2 to show the relation between Fe concentration in the wafer and the temperature at which gettering occurs. Supersaturation of Fe impurities was found necessary for intrinsic gettering of Fe in the contamination range of 4.0×1012 to 3.5×1014 cm−3. Therefore, the gettering temperature is lower for low-level Fe contamination than for high-level contamination. The reduction of Fe concentration saturated with annealing time, which shows that the oxygen precipitates in the bulk defect region do not work as an infinite gettering sink. We found that the saturated Fe concentration follows a simple Arrhenius relationship, so that gettering stops at the thermal equilibrium concentration. We think that in intrinsic gettering, Fe precipitates preferentially in the bulk defect region when the Fe impurities supersaturate with decreasing temperature.
Multiple internal reflection infrared spectroscopy of silicon surface structure and oxidation process at room temperature
Authors: Ling L. , Kuwabara S. , Abe T. , Shimura F.
Published: July 2002

It is demonstrated that the multiple internal reflection infrared (IR) spectroscopy using a germanium prism is a very powerful nondestructive diagnostic technique for the study of silicon wafer surfaces in a wide range of IR irradiation region. The technique limits neither the shape of samples nor the IR range due to the absorption by silicon itself. With this technique, it is demonstrated that; (i) dangling bonds of a silicon surface treated with HF solution and de-ionized (DI) water are terminated mostly with H atoms, (ii) native oxide growth is enhanced by DI water rinsing, and the interstitial oxygen concentration in the silicon surface region increases during native oxide growth process, and (iii) DI water rinsing after HF etching replaces Si—F bonds with Si—H and Si—OH bonds on a silicon surface.
Post-irradiation cracking of H2 and formation of interface states in irradiated metal-oxide-semiconductor field-effect transistors
Authors: Stahlbush R. E. , Edwards A. H. , Griscom D. L. , Mrstik B. J.
Published: July 2002

Molecular hydrogen is alternately introduced into and removed from the gate oxide of irradiated metal-oxide-semiconductor field-effect transistors at room temperature by changing the ambient between forming gas (10/90% H2/N2) and nitrogen. Using charge pumping, it is observed that H2 causes a simultaneous buildup of interface states and decrease of trapped positive charge. The results are explained by a reaction sequence in which H2 is cracked to form mobile H+, which under positive bias drifts to the Si/SiO2 interface, and reacts to produce a dangling-bond defect. The rate limiting step over most of the time domain studied is the cracking process. Two types of cracking sites are modeled by molecular orbital calculations: oxygen vacancies (E′ centers) and broken bond hole traps (BBHTs). Initial- and final-state energies, as well as the activation energies, are calculated. The calculations indicate that the latter is the more likely H2 cracking site. The combined experimental and theoretical results suggest that at least 15% of the trapped positive charge is at sites similar to the BBHT sites. Implications of the model and similarities between interface-state formation by cracked H2 and irradiation are discussed.
Stress generated in polyimide by excimer-laser irradiation
Authors: Zweig A. D. , Venugopalan V. , Deutsch T. F.
Published: July 2002

Stress transients are generated in polyimide by irradiation with excimer-laser pulses at radiant exposures between 3×10−3 and 102 J/cm2. The duration and peak stress of these transients are measured using piezoelectric film transducers. For all the wavelengths tested (193, 248, 308, 351 nm) we determine three ranges of radiant exposure within which different physical mechanisms govern the stress generation. The scaling of stress with radiant exposure depends on wavelength only in the low fluence regime. In this regime the stresses observed are attributed to subsurface thermal decomposition at 351 and 308 nm and to photodecomposition at 248 and 193 nm. At higher radiant exposure the stress generation is governed either by the thermal expansion of the gaseous ablation products or by the formation and expansion of a dense plasma. The boundary between these two regimes is identified from the variation of the mechanical coupling coefficient with radiant exposure. The results also indicate that heat conduction contributes significantly to the coupling of energy into the target, leading to a reduction in the stresses generated and to an increase in etch depth when ablation is achieved.
Direct writing of gold nanostructures using a gold-cluster compound and a focused-ion beam
Authors: Hoffmann P. , Ben Assayag G. , Gierak J. , Flicstein J. , Maar-Stumm M. , van den Bergh H.
Published: July 2002

Spin-coated solid films of the gold-cluster compound Dodeca-(triphenylphosphine), hexa(chloro)pentapentacontagold Au55(PPh3)12Cl6 are irradiated with a focused 20-keV Ga+ focused-ion beam. The writing speeds on the substrate were ranging from 50 up to 2000 μm/s. This treatment locally decreases the solubility of the metalorganic precursor layer in CH2Cl2. Removal of the nonirradiated part of the surface layer with this solvent, followed by thermal decomposition of the remaining metalorganic nanostructures, leads to conducting gold lines. The width (150–360 nm) and height (20–80 nm) of these metallic lines depend on the ion dose and the original film height of the gold compound. The possible interactions leading to the fixation are briefly discussed.