bluuebunny/crossref_metadata_2025 huggingface dataset:
Many-particle diffusion in continuum: Influence of a periodic surface potential
Authors: Lahtinen J. M. , Mašı́n M. , Laurila T. , Ala-Nissila T. , Chvoj Z.
Published: July 2002

We study the diffusion of Brownian particles with a short-range repulsion on a surface with a periodic potential through molecular dynamics simulations and theoretical arguments. We concentrate on the behavior of the tracer and collective diffusion coefficients DT(θ) and DC(θ), respectively, as a function of the surface coverage θ. In the high friction regime we find that both coefficients are well approximated by the Langmuir lattice-gas results for up to θ≈0.7 in the limit of a strongly binding surface potential. In particular, the static compressibility factor within DC(θ) is very accurately given by the Langmuir formula for 0⩽θ⩽1. For higher densities, both DT(θ) and DC(θ)show an intermediate maximum which increases with the strength of the potential amplitude. In the low friction regime we find that long jumps enhance blocking and DT(θ) decreases more rapidly for submonolayer coverages. However, for higher densities DT(θ)/DT(0) is almost independent of friction as long jumps are effectively suppressed by frequent interparticle collisions. We also study the role of memory effects for many-particle diffusion.
Impact of atomic-layer-deposited TiN on the gate oxide quality of W/TiN/SiO2/Si metal–oxide–semiconductor structures
Authors: Park Dae-Gyu , Lim Kwan-Yong , Cho Heung-Jae , Cha Tae-Ho , Yeo In-Seok , Roh Jae-Sung , Park Jin Won
Published: July 2002

We demonstrate the impact of atomic-layer-deposited TiN gate on the characteristics of W/TiN/SiO2/p-Si metal–oxide–semiconductor (MOS) systems. Damage-free gate oxide quality was attained with atomic-layer-deposition (ALD)–TiN as manifested by an excellent interface trap density (Dit) as low as ∼4×1010 eV−1 cm−2 near the Si midgap. ALD–TiN improved the Dit level of MOS systems on both thin SiO2 and high-permittivity (high-k) gate dielectrics. The leakage current of a MOS capacitor gated with ALD–TiN is remarkably lower than that with sputter-deposited TiN and poly-Si gate at the similar capacitance equivalent thickness (CET). Less chlorine content in ALD–TiN films appears to be pivotal in minimizing the CET increase against postmetal anneal and improving gate oxide reliability, paving a way for the direct metal gate process.
Electron-beam-induced optical memory effects in GaN
Authors: Chang Y. C. , Cai A. L. , Johnson M. A. L. , Muth J. F. , Kolbas R. M. , Reitmeier Z. J. , Einfeldt S. , Davis R. F.
Published: July 2002

Metastable effects in unintentionally doped GaN films grown on SiC substrates have been investigated using cathodoluminescence (CL). Memory effect patterns produced optically are observed in CL images. An electron beam can also produce memory effect patterns and the resulting changes in the luminescence spectra are quite similar for either optical or electron-beam-induced patterns. CL spectra reveal that the yellow luminescence at 2.2 eV increases significantly with little change in the band-edge emission in both cases. Samples that do not exhibit optically induced memory effects are also investigated and do not exhibit electron-beam-induced patterns, either. Monochromatic CL images at 540 and 365 nm confirm the similarity of optically and electron-beam-induced memory effects based on changes in luminescence spectra.
High-resolution work function imaging of single grains of semiconductor surfaces
Authors: Sadewasser S. , Glatzel Th. , Rusu M. , Jäger-Waldau A. , Lux-Steiner M. Ch.
Published: July 2002

The size reduction of modern electronic devices creates a growing demand for characterization tools to determine material properties on a nanometer scale. The Kelvin probe force microscope is designed to obtain laterally resolved images of the sample’s work function. Using a setup in ultrahigh vacuum, we were able to distinguish work function variations for differently oriented crystal facets of single grains on a semiconductor surface. For the tetragonal solar cell material CuGaSe2 the experiments demonstrate differences as low as 30 meV between (102) and (111) oriented surfaces and up to 255 meV between (1̄1̄2̄) and (110) surfaces. This influences the band bending of solar cell heterostructures and consequently also the solar power conversion efficiency.
Vinyl radical visible spectroscopy and excited state dynamics
Authors: Shahu Milena , Yang Chun-Hui , Pibel Charles D. , McIlroy Andrew , Taatjes Craig A. , Halpern Joshua B.
Published: July 2002

The vinyl radical (C2H3)Ã 2A″←X̃ 2A′ spectrum has been measured between 530 and 385 nm using cavity-ringdown spectroscopy. The active vibrational progressions involve C–C stretching and alpha H–C–C bending vibrations. Optimal rotational constants and linewidths were determined for the first four vibrational bands by modeling the spectrum as an asymmetric top. The best-fit rotational constants obtained for the excited electronic state are consistent with the molecular geometry predicted by ab initio calculations. The lifetime of the vibrationless level in the excited electronic state is estimated to be a few picoseconds, and increasing vibrational excitation leads to a decrease in the lifetime, based upon an increasing linewidth. Various possibilities for the predissociation mechanism are discussed. The most likely is judged to be a conical intersection or seam of intersections. A preliminary CASSCF calculation has found the point on the relevant potential energy surfaces at which the ground and electronically excited states are closest. While the geometry and other properties of this crossing point are in accord with the experimental results, the calculated position of the point of closest approach of the two electronic states lies considerably (>1 eV, including zero-point energy) above the already predissociative Ã 2A″ state origin. Other mechanisms are also discussed to account for the observed rapid predissociation. Clearly there is a need for a higher level theoretical work on this problem.
Optical dephasing in doped organic glasses over a wide (0.35–100 K) temperature range: Solid toluene doped with Zn–octaethylporphine
Authors: Vainer Yu. G. , Kol’chenko M. A. , Naumov A. V. , Personov R. I. , Zilker S. J. , Haarer D.
Published: July 2002

We have performed optical photon echo measurements on amorphous toluene doped with zinc-octaethylporphine over a wide temperature interval (0.35–100 K). This broad temperature range, reaching from subkelvins up to the Debye temperature of toluene (100.7 K), enabled a clear separation between the low and high temperature contributions of line broadening processes to the total optical dephasing. The high temperature part of the measured dephasing times was analyzed using recent data from light scattering experiments which determined the spectrum of low-frequency modes of the toluene matrix. The analysis shows that two line-broadening mechanisms contribute mainly to total dephasing: the tunneling transitions of two-level systems, which lead to a power-law temperature dependence of the linewidth at low temperatures (T<3–4 K), and the interaction of the chromophore with low-frequency phonon modes of the glassy matrix, which gives rise to an exponential behavior at high temperatures. It was shown that the contribution of acoustic phonons to optical dephasing in the studied system is negligible over the whole temperature region of the measurements.
Dynamically localized wave packets as a tool to study the dynamics of the LiNC⇌LiCN isomerization reaction
Authors: Martı́n-Fierro E. , Borondo F. , Gomez Llorente J. M. , Benito R. M.
Published: July 2002

The dynamics of the isomerization reaction LiNC⇌LiCN is considered at a quantum level. This study is performed with the aid of dynamically localized wave packets, which provide a wealth of information about the relevant parts of the molecular phase space. Three cases are considered, corresponding to energies close to the threshold for isomerization. The initial position of the packet is always localized in the LiNC well (which is the most stable), and different energies and distributions among the vibrational modes of the molecule are used. The packet will then explore phase space and, as a result of these dynamics, different time scales (including those from intramolecular energy transfer processes) exist, which appear in the low resolution features of the corresponding spectra. The characteristics of these spectra are discussed and analyzed using a variety of techniques, making extensive use of the ideas of nonlinear dynamics, and the conditions for the applicability of statistical theories to evaluate reaction rates in this case are considered.
Phase-locking of tearing modes in the reversed field experiment
Authors: Fitzpatrick Richard , Zanca Paolo
Published: July 2002

In the reversed field experiment (RFX) [F. Gnesotto et al., Fusion Eng. Des. 25, 335 (1995)], the m=1 and m=0 tearing modes present in the plasma are observed to phase-lock together to form a highly peaked, strongly toroidally localized, pattern in the perturbed magnetic field. This pattern, which is commonly known as the “slinky” pattern, gives rise to severe edge loading problems which limit the maximum achievable toroidal current. A theory is presented which explains virtually all salient features of the RFX slinky pattern. The central premise of this theory is that at high ambient mode amplitude the various tearing modes occurring in the plasma phase-lock together in a configuration which minimizes the magnitudes of the electromagnetic torques exerted at the various mode rational surfaces. The theory successfully predicts the profiles of the edge radial and toroidal magnetic fields generated by the m=0 and m=1 modes, the phase relations between the various modes, the presence of a small toroidal offset between the peaks of the m=0 and m=1 contributions to the overall slinky pattern, and the response of the pattern to externally generated m=0 and m=1 magnetic perturbations.
Lower-dimensional tori in reversible systems
Authors: Sevryuk M. B.
Published: July 2002

On a (2n+d)-dimensional manifold M consider a vector field V reversible with respect to an involution G whose fixed point manifold is of dimension n+d. It is conjectured that generically for each 0≤m≤n, the phase space M contains (m+d)-parameter Cantor families of m-tori invariant under both the involution G and the flow of V. To be more precise, vector fields V with this property constitute an open set in the space of all vector fields equipped with an appropriate topology. The flow of V induces on these tori quasiperiodic motions with strongly incommensurable frequencies. Extreme cases of this conjecture (d=0, m=n, m=1, m=0) have been proven.
Monte Carlo methods for turbulent tracers with long range and fractal random velocity fields
Authors: Elliott, Jr. Frank W. , Horntrop David J. , Majda Andrew J.
Published: July 2002

Monte Carlo methods for computing various statistical aspects of turbulent diffusion with long range correlated and even fractal random velocity fields are described here. A simple explicit exactly solvable model with complex regimes of scaling behavior including trapping, subdiffusion, and superdiffusion is utilized to compare and contrast the capabilities of conventional Monte Carlo procedures such as the Fourier method and the moving average method; explicit numerical examples are presented which demonstrate the poor convergence of these conventional methods in various regimes with long range velocity correlations. A new method for computing fractal random fields involving wavelets and random plane waves developed recently by two of the authors [J. Comput. Phys. 117, 146 (1995)] is applied to compute pair dispersion over many decades for systematic families of anisotropic fractal velocity fields with the Kolmogorov spectrum. The important associated preconstant for pair dispersion in the Richardson law in these anisotropic settings is compared with the one obtained over many decades recently by two of the authors [Phys. Fluids 8, 1052 (1996)] for an isotropic fractal field with the Kolmogorov spectrum.
Refractive index dispersion in group IV and binary III-V semiconductors: Comparison of calculated and experimental values
Authors: Campi D. , Papuzza C.
Published: July 2002

A new method is described for the calculation of the refractive index of semiconductors at energies below the lowest band gap, in which a model form of the imaginary part of the dielectric function is assigned and the refractive index is obtained, without introducing any empirical parameter. It provides good insight into the physics of the problem and a quantitative account of refractive index dispersion as well. Double-beam reflectance measurements were made for the refractive index of four InP and GaAs samples. Results are presented for Si, Ge, AlAs, GaP, GaAs, and InP and show very good agreement with this model.
Electron paramagnetic resonance monitoring of recovery of fast neutron irradiated GaAs
Authors: Goltzené A. , Meyer B. , Schwab C.
Published: July 2002

Electron paramagnetic resonance spectra of fast neutron irradiated semi-insulating GaAs, recorded at 9 GHz and 4.2 K, have been studied as a function of isochronal thermal anneals. Their decomposition into quadruplet and singlet allows one to determine the main annealing temperatures of the corresponding defects, previously identified as As4+Ga and V2−Ga, which occur respectively at 400 and 600 °C. Comparison with the behavior of implantation damage shows that electrical activation of Be+ implants is correlated with the annealing of the main defects on the cation sublattice. Finally, the linewidth variation of the quadruplet during its decay indicates a concomitant change of the local environment of the As4+Ga center.
Further evidence of chromium, manganese, iron, and zinc redistribution in indium phosphide after annealing
Authors: Gauneau M. , Chaplain R. , Rupert A. , Rao E. V. K. , Duhamel N.
Published: July 2002

We report on Cr, Mn, Fe, and Zn redistributions either in undoped and unimplanted, or in doped and implanted indium phosphide substrates. Depth profiles, obtained by secondary ion mass spectrometry, demonstrate that residual impurities, as well as dopants, redistribute during thermal processing and that several parameters govern simultaneously the observed phenomena. Results are interpreted in terms of implantation-related damage and thermally generated defects, solubility modifications induced by the position of the Fermi level, p-n junction-related electric fields, and pure diffusion mechanisms.
Photoelectronic properties of hydrogenated amorphous silicon/silicon oxide heterostructures
Authors: Carasco F. , Mort J. , Jansen F. , Grammatica S.
Published: July 2002

A glow-discharge hydrogenated amorphous silicon/insulator heterostructure has been characterized by a range of measurements including optical absorption, temperature dependence of photo- and dark conductivity, internal photoemission, xerographic discharge, and spectral dependence of photoconductivity. Efficient injection of dark and photocarriers from amorphous hydrogenated silicon into, and transport through, relatively thick SiOx:N:H has been achieved. Unlike the conventional thermal oxide on Si, no significant energy barrier to injection is found in the plasma-deposited heterostructure. The use of the structure as a potential xerographic device is demonstrated. A mobility lifetime product as high as 6×10−10 cm2/V and a transport process with an activation energy of ∼0.3 eV is found for electrons in the SiOx:N:H films.
Line broadening in optically thick gas discharges: Influence on plasma temperature determination
Authors: Karabourniotis D. , Karras C.
Published: July 2002

The study of asymmetrically self-reversed lines emitted by inhomogeneous gas discharges has been made on the basis of an extension of the multiparameter method. A new procedure has been developed which makes it possible to determine the line broadening at the arc axis, its radial variation, and the ratio shift/width. An investigation is made into the effect of these quantities on the determination of the plasma temperature. The proposed method has been applied to high-pressure mercury discharges operating on dc and ac (50 Hz) using the mercury line 5461 Å. The pressure broadening in the impact approximation has been measured to be (1.43±0.11)×104 mÅ atm−1  K0.7; the corresponding line shift is towards the blue line wing.
Time-dependent behavior of the contraction regions of high-pressure mercury arcs
Authors: Schäfer R. , Stormberg H. P.
Published: July 2002

The paper describes an investigation of the time-dependent behavior of the impedance of high-pressure mercury lamps operated with pulsed current. The lamp impedance was obtained experimentally from electric as well as spectroscopic measurements, the latter also allowing a determination of axial impedance profiles. These measurements show that during current-off the most significant part of the lamp impedance is located close to the electrodes, whereas during current-on a more homogeneous impedance profile is observed. The arc channel close to the electrodes is found to be rather narrow and therefore its high impedance during current-off can be reduced significantly by an additional current which is small compared to the average lamp current.
Density sensitive x-ray line ratios in the Be i, B i, and Ne i isoelectronic sequences
Authors: Feldman U. , Seely J. F. , Bhatia A. K.
Published: July 2002

The intensities of x-ray transitions in highly charged ions in the Be i, B i, and Ne i isoelectronic sequences have been calculated as functions of electron density. The intensities of the transitions from the 2sn 2pm 3p configuration, relative to the intensities from the 2sn 2pm 3s and 2sn pm 3d configurations, are strong functions of electron density in high-density plasmas. The density sensitivity occurs at electron densities between 1016 cm−3 (for Si ions) and 1022 cm−3 (for Kr ions). Opacity is unimportant for plasma dimensions that are characteristic of dense laser-produced plasmas. These x-ray line ratios represent a promising new density diagnostic for high-density plasmas.
Interface oxidation of epitaxial silicon deposits on (100) yttria stabilized cubic zirconia
Authors: Pribat D. , Mercandalli L. M. , Siejka J. , Perriere J.
Published: July 2002

(100) single crystal silicon films have been deposited on (100) oriented yttria stabilized zirconia (YSZ) single crystal substrates. Taking advantage of the high-temperature oxygen ion and electronic transport properties of YSZ material, we have subsequently grown buried SiO2 layers (with thicknesses in the 1000-Å range) at the Si/YSZ interface, the result being a Si(100)/a-SiO2/YSZ (100) structure. Since, similarly to silicon films on sapphire, the planar defect density in as-deposited material is highest at the Si/substrate interface, the oxidation process results in an overall increase of the crystalline quality of the epi-Si deposits. We present here our first results of thermal and anodic oxidation of the interface. The buried SiO2 layers have been characterized by Rutherford backscattering spectroscopy and scanning electron microscopy on chemically delineated cross sections. Thermal oxidation has been performed at typically 1150 °C with oxygen pressures ranging from 10−1 to 2 atm. In our operating conditions, the thickness of the buried SiO2 layers has been found to increase linearly as a function of both time and oxygen pressure. Anodic oxidations have also been carried out using either a molten salt (KNO3) solution at 500 °C as electrolyte or an oxygen plasma, the substrates being held at 550 °C. Typically, 1800-Å-thick interfacial SiO2 layers have been grown in 10 min with a current density of 1.5 mA/cm2.
Efficiency calculations of thin-film GaAs solar cells on Si substrates
Authors: Yamaguchi Masafumi , Amano Chikara
Published: July 2002

Dislocation effect upon the efficiency of single-crystal thin-film AlGaAs-GaAs heteroface solar cells on Si substrates is analyzed. Solar-cell properties are calculated based on a simple model; in the model, dislocations act as recombination centers to reduce the minority-carrier diffusion length in each layer and increase the space-charge layer recombination current. Numerical analysis is also carried out to optimize thin-film AlGaAs-GaAs heteroface solar-cell structures. The fabrication of thin-film AlGaAs-GaAs heteroface solar cells with a practical efficiency larger than 18% on Si substrates appears possible if the dislocation density in the thin-film GaAs layer is less than 106 cm−2.
Equation of state for polymer solids
Authors: Hartmann Bruce , Haque Mustafa A.
Published: July 2002

A simple, three-constant pressure-volume-temperature PVT equation of state, previously applied to polymer liquids, is applied to polymer solids, both crystalline and amorphous. In terms of reduced variables P̄=P/B0, V̄=V/V0, and T̄=T/T0, the equation P̄ V̄5=T̄3/2−ln V̄ fits experimental volume data taken from the literature for 19 polymers to within the accuracy of the measurements, about 0.1%. Using the different sets of reducing parameters for the liquid and glassy states of an amorphous polymer, a second order phase transition is predicted. At zero pressure, this transition temperature T2 lies about 20 K below the glass transition temperature Tg but has a greater pressure dependence. It was also found that the combination of reducing parameters RT0/B0V0 is approximately constant for amorphous and crystalline polymers.
Resistivity, grain size, and impurity effects in chemically vapor-deposited tungsten films
Authors: Learn Arthur J. , Foster Derrick W.
Published: July 2002

The room-temperature electrical resistivity, grain size, and impurity content of tungsten films deposited at low pressure on silicon wafers from tungsten hexafluoride and hydrogen reactants were determined. These properties were examined as functions of deposition temperature and film thickness. The resistivity is independent of thickness at a value of approximately 13.5 μΩ cm for films deposited at 300 °C. For films deposited at 400 °C, the resistivity decreases from 24 to 8.5 μΩ cm as thickness increases from 0.075 to 1 μm. The resistivity behavior is interpreted in terms of grain-boundary scattering with a zero reflection coefficient for the 300 °C films. For the 400 °C films, a reflection coefficient that decreases from 0.67 to 0.38 over the above thickness range and a linear dependence of grain size on thickness are utilized. For both deposition temperatures, the grain size exhibits a rapid initial growth to 30 nm followed by growth at a slope of 0.32 with respect to thickness. The rapid initial growth is associated with the rapid, self-limiting tungsten hexafluoride/silicon reaction. The oxygen content of the films tracks the trend of the reflection coefficient in that it increases with increased deposition temperature or decreased film thickness. Evidence suggests that the oxygen is located predominantly at grain boundaries and may be the primary determinant of boundary scattering. Fluorine is found in films at concentrations (0.03–0.05 at. %) similar to those of oxygen but does not vary with thickness and decreases with increased deposition temperature. No evidence is found for surface scattering of carriers suggesting that all are specularly scattered. Grain-boundary scattering, with a constant reflection coefficient, adequately interprets previously published data. Utilizing such results, together with the present ones, leads to the conclusions that the grain size is relatively independent of deposition temperature while the intragrain resistivity decreases with temperature and approaches the bulk value for 600 °C deposition.
Liquid-phase-epitaxial growth of In0.49Ga0.51P on (100) GaAs by a supercooling method
Authors: Wu M. C. , Su Y. K. , Cheng K. Y. , Chang C. Y.
Published: July 2002

In1−xGaxP epitaxial layers were grown on (100) GaAs substrates by liquid-phase epitaxy using supercooling technique. The lattice mismatch normal to the wafer surface between In1−xGaxP layer and GaAs substrate varies linearly with the supercooled temperature of the growth solution. The composition-pulling phenomenon was not observed in this study. The growth rate, the intensity, and the full width at half maximum of the photoluminescent spectrum are also dependent on the supercooling temperature. It is shown that the narrowest full widths at half maximum of photoluminescent peak are 10.6 and 35 meV at 14 and 300 K, respectively, when ΔT is 6 °C, and the strongest intensity is occurred at ΔT=12–18 °C. Carrier concentrations of undoped epitaxial layers are in the range of 1016 cm−3 measured by capacitance-voltage method at 300 K and Hall method at 77 and 300 K. The optimum growth condition was then determined.
Punch-through current under diffusion-limited injection: Analysis and applications
Authors: Esener Sadik , Lee Sing H.
Published: July 2002

A simple analytical theory is developed to describe the carrier distribution and the current flow in a punch-through structure under the diffusion-limited injection condition and negligible generation recombination effects. The current expression determined is similar to the diode current expression with the diffusion length replaced by the injection length. According to the nature of the injection length, the current flow process is found to be intimately related to the doping concentration and carrier mobility in the base. Recombination generation effects are next included in the theory to account for the photogain observed in punch-through structures. Finally the model is applied to the gate-field-controlled barrier-injection transit-time transistor and the field and light injection-controlled punch-through transistor.
Theory of elastic wave scattering: Applications of the method of optimal truncation
Authors: Opsal Jon L. , Visscher William M.
Published: July 2002

The method of optimal truncation is developed and applied to a variety of rotationally symmetric defects in elastic materials. Results are presented for oblate and prolate spheroidal voids, cracks, irregularly shaped voids, and compound inclusions. Most of the examples are in the frequency domain, but samples of time-domain calculations are included. Physical interpretations of some features of the calculated amplitudes are given. Checks on accuracy of the results are emphasized and implemented.
Direct mass spectrometric diagnostics for a CO2 gas laser
Authors: Wang Yu-zhi , Liu Jian-sheng
Published: July 2002

An extremely small hole of approximately 10 μm in diameter was made on a glass plate by a special technique. Using this small hole, together with a turbomolecular pump, a pressure difference of seven orders of magnitude was achieved. Using this system, a direct analysis of the gas components in CO2 lasers of 300-mm-long waveguide type and 800-mm-long common type during their short-term lifetime has been performed. The following results were obtained: (1) In the case of filling with pure CO2, a direct mass-spectrometric analysis of CO2 decomposition process has been performed showing a decomposition of 80% in partial pressure. (2) In the case of filling with pure CO, the continuous increase of CO2 was observed, and an equilibrium CO2 peak height of about 8% of the total peak height was reached. (3) In the case of filling with a laser gas mixture, the variations of all components were observed, and an analysis of the curve shapes showed that there exists a cataphoresis effect. (4) When a procedure of ‘‘discharge-rest–discharge-rest’’ was used, the results were different for anode sampling and for cathode sampling. This proves cataphoresis action occurs. The technique presented in this paper should be valuable to the study of neutral dissociation and recombination mechanisms within gas lasers.
AlAs/n-GaAs superlattice and its application to high-quality two- dimensional electron gas systems
Authors: Baba Toshio , Mizutani Takashi , Ogawa Masaki
Published: July 2002

A novel AlAs/n-GaAs superlattice structure, in which the GaAs midlayers are selectively doped with Si, has been designed as an electron supplying layer for two-dimensional electron gas (2DEG) systems to obtain a high sheet carrier concentration, using a simple Kronig–Penney model calculation. The optimized structure is a very short period superlattice (∼40 Å), in which minibands are formed. This design realizes the elimination of DX centers and obtains a shallow donor level, in contrast with alloyed n-AlxGa1−xAs (x≂0.3), which has the same band gap as the superlattice. A 2DEG existence at the undoped GaAs and the AlAs/n-GaAs superlattice heterointerface is confirmed by Shubnikov–de Haas measurements for normal and inverted structures. High sheet carrier concentrations (8×1011 cm−2) were obtained for both structures with a one period undoped AlAs/GaAs spacer layer. Namely, total spacer layer thickness is about 60 Å. The 2DEG concentrations showed no variation with decrease in temperature from 300 to 4.2 K and with room light illumination at 4.2 K. Electron mobilities at 4.2 K are as high as 190 000 and 110 000 cm2/V s for the normal structure and the inverted structure, respectively.
Infrared thermal particle detection physics
Authors: Utterback S. , Melcher R.
Published: July 2002

Thermal particle detection is a new approach to the detection of generic particulate contaminants on products. The method takes advantage of the low thermal conductivity of a particle/substrate interface by using rapid thermal heating to raise the particle temperature over the temperature of its surroundings. The perturbed particle is then detected by its subsequent thermal emissions with a highly sensitive IR detector. The physics of particle heating and detection both in vacuum and in air is described.
Resonant modes in small inductance superconducting interferometers
Authors: Paternò G. , Cucolo A. M. , Vaglio R.
Published: July 2002

The magnetic field dependence of resonant modes in small inductance two-junction interferometers has been investigated. Both symmetric and asymmetric configurations with equal and different junction size, respectively, have been considered. The experimental data have been compared with the theoretical dependence computed by a simple model of a single junction with a current density peaked at the edges. This model also takes into account the effects due to the field coupled with the junctions. An excellent agreement has been found between theoretical and experimental data.
Acoustic waves in random discrete media via a differential scheme
Authors: Beltzer Abraham I. , Brauner Neima
Published: July 2002

The concept of differential effective media is extended to the dynamic case by imposing the condition of causality. The dispersion of acoustic waves in such media is treated and results are compared with those following from alternative models.
Illumination uniformity of laser-fusion pellets using induced spatial incoherence
Authors: Schmitt Andrew J. , Gardner John H.
Published: July 2002

We have examined the possibility of using a realistic laser system to produce uniform illumination of a direct drive laser-fusion pellet. This laser system consists of a short wavelength laser (KrF, λ=0.25 μm) with the induced-spatial-incoherence method. A numerical analysis describes the laser beams propagating through the pellet plasma, energy deposition by inverse bremmstrahlung absorption, and energy conversion to ablation drive pressure. The results indicate that highly uniform (ΔP/〈P〉<2%) distributions can be produced while a large fraction of the laser beam energy (>75%) is simultaneously absorbed. The spatial structure of the distribution is also analyzed and found to contain primarily low spatial frequencies with maximum magnitudes less than 0.5%. Misalignment of the laser beams increases the overall nonuniformity by about 1.5% for every 1% increase in the relative misaiming error (measured as a percentage of the pellet radius). A maximum misalignment error on the order of a few microradians in the final optics is suggested by this analysis.
Two-dimensional electron gas structures with mobilities in excess of 3×106 cm2 V−1 s−1
Authors: Harris J. J. , Foxon C. T. , Barnham K. W. J. , Lacklison D. E. , Hewett J. , White C.
Published: July 2002

We have demonstrated that the mobility of two-dimensional electron gas (2DEG) structures shows a gradual improvement with the number of uninterrupted growth runs in the molecular-beam-epitaxy system. Some, but not all, of this improvement can be attributed to the cleanup of the GaAs layers in the structure, and we suggest that there is a corresponding cleanup of the (Al,Ga)As which is also influencing the mobility. Once the system has passed the cleanup phase, a systematic trend of carrier density and mobility with undoped spacer thickness was observed, with a peak mobility at 4 K of 2.12×106 cm2 V−1 s−1 for a sheet carrier density of 2×10−11 cm−2 occurring at a spacer thickness of 800 Å. A further increase in mobility was achieved by using a thicker region of doped (Al,Ga)As, thereby moving the ionized centers in the surface depletion layer further away from the 2DEG. This has enabled us to produce a sample with mobility at 4 K of 3.1×106 cm2 V−1 s−1 (at a sheet charge of 3.1×1011 cm−2)—the first time such a value, to our knowledge, has been reported.
Structural studies of low-temperature low-pressure chemical deposited polycrystalline silicon
Authors: Meakin D. , Stoemenos J. , Migliorato P. , Economou N. A.
Published: July 2002

Structural studies were performed on Si films formed by chemical-vapor deposition on SiO2 films using silane under pressures below 150 mTorr and temperatures below 640 °C. The mode of growth of these films was found pressure dependent. Films grown at pressures below 10 mTorr were found to have a columnar structure with a (001) preferred orientation ending at a curved surface. The radius of the crystallites increases and the radius of curvature of their free surface decreases as the pressure decreases, while the converse is true for the temperature dependence. Transition from this mode is associated with diminishing of the capillarity effects. For pressures above 10 mTorr the structure is striated with a 〈111〉 twin texture almost perpendicular to the substrate. At pressures above 100 mTorr the structure is similar to the previous one but with a tufty appearance. This structure is associated with compressive and dilatational strain. The size of initial crystallites was found also pressure dependent increasing with pressure.
Thermal expansion anomalies of R2(Fe1−x Mx)14B
Authors: Cheng Ben-pei , Yang Ying-chang , Fu Shou-can , James W. J.
Published: July 2002

Thermal expansion anomalies of R2(Fe1−xMx)14B (R=Nd and Y; M=Co and Al) have been observed with x-ray powder diffraction in the range from room temperature to 1000 K. All these compounds show an abnormal thermal expansion below the Curie temperature. The substitution effect of Fe by Co and Al on the thermal expansion behavior has been investigated to determine how atomic ordering of these substituent atoms influence the Fe-Fe exchange interactions so as to alter the magnetic properties of the R-Fe-B intermetallic compounds.
Magnetic phase transition in Mn0.5Te0.5−xSbx
Authors: Noor S. S. Abdul
Published: July 2002

The magnetic phase transition in the (NiAs-type) system Mn0.5Te0.5−xSbx was investigated by electrical resistivity and x-ray powder diffraction techniques. Crystallographically, the lattice parameter (a) was found to decrease smoothly with x, while (c) behaved in a steplike manner with the changeover taking place at 0.2<x<0.3. Likewise the unit cell volume and the c/a ratio followed suit, with the later changing value from (1.4) (0<x<0.2) corresponding to an insulating state to (>1.63) which typifies the metallic nature (at x>0.3) in nickel arsenides. On the other hand, the shapes of the electrical resistivity curves exhibited a gradual evolution with increasing x values, confirming the proposed phase transition. A plot of resistivity minima versus x showed a clear inversion in the range 0.2<x<0.3. Moreover, activation energies of specimens characterized by the insulating nature, determined from the ln σ vs 1/T plots, dropped sharply with x. It is all thought to be due to changes in the electronic concentration which may alter the state of hybridization of the (p-d) bands, reducing the energy gap, and causing the material to change from a nonconducting state (antiferromagnetic MnTe) to that of metalliclike ferromagnetic MnSb.
Temperature dependence of spontaneous magnetization of ultrafine Fe particles in Fe-SiO2 granular solids
Authors: Xiao Gang , Chien C. L.
Published: July 2002

We have studied the temperature dependence of the spontaneous magnetization of ultrafine Fe particles by measuring the initial susceptibility χ∼M2s (T)/kBT in the superparamagnetic state. It was found that Bloch’s law Ms=M0(1−BT3/2) gives good description at temperatures much higher than the blocking temperature of the Fe particles. The value B is substantially larger than that of the bulk Fe, due to magnetic surface effect and finite size effect. Size distribution and magnetostatic interaction between particles have also been studied.
Crystallization of Fe-B amorphous alloys: A NMR and x-ray study
Authors: Zhang Y. D. , Budnick J. I. , Ford J. C. , Hines W. A. , Sanchez F. H. , Hasegawa R.
Published: July 2002

Based on a knowledge of the NMR spectra for FeB, Fe2B, orthorhombic Fe3B (o-Fe3B), and body-centered-tetragonal Fe3B (bct-Fe3B), the phases produced during the annealing of Fe-B amorphous alloys can be identified with greater sensitivity by NMR techniques than by x-ray diffraction. In the present work, a combination of x-ray diffraction experiments and spin-echo NMR measurements of the hyperfine field distributions for the 10B, 11B, and 57 Fe nuclei has been performed on both as-quenched and annealed Fe100−xBx (14≤x≤25) amorphous alloys. In general terms, it was found that annealing at temperatures near 400 °C resulted in crystallization products which included bct-Fe3B, while annealing at approximately 800 °C leads to the formation of o-Fe3B. The behavior of the B hyperfine field indicates that, for the Fe100−xBx amorphous alloys, an o-Fe3B-like local order is favored in the low B concentration regime, and a bct-Fe3B-like local order is favored in the high B concentration regime.
Selective porous silicon formation in buried p+ layers
Authors: Tsao S. S. , Myers D. R. , Guilinger T. R. , Kelly M. J. , Datye A. K.
Published: July 2002

We report a systematic microstructural study of enhanced lateral porous silicon formation in the buried p+ layers of n/p+/p− and p−/p+/p− structures. We find, surprisingly, extremely selective porous silicon formation due to the thin p+ layer in both structures, despite the absence of a p-n junction in the p−/p+/p− structure. The interface between the isolated island and the buried porous silicon layer was always located at the depth where the net p-type dopant concentration was 1–8×1015/cm3. The observed microstructure can largely be understood in terms of a recent model for porous Si formation in uniformly doped Si, proposed by Beale et al. [J. Cryst. Growth 73, 622 (1985)]. However, we also observe, for the first time, important effects unique to a nonuniform dopant concentration.
CuInSe2 thin films produced by rf sputtering in Ar/H2 atmospheres
Authors: Martil I. , Santamaria J. , Iborra E. , Gonzalez-Diaz G. , Sanchez-Quesada F.
Published: July 2002

Structural, compositional, optical, and electrical properties of CuInSe2 thin films grown by rf reactive sputtering from a Se excess target in Ar/H2 atmospheres are presented. The addition of H2 to the sputtering atmospheres allows the control of stoichiometry of films giving rise to remarkable changes in the film properties. Variation of substrate temperature causes changes in film composition because of the variation of hydrogen reactivity at the substrate. Measurements of resistivity at variable temperatures indicate a hopping conduction mechanism through gap states for films grown at low temperature (100–250 °C), the existence of three acceptor levels at about 0.046, 0.098, and 0.144 eV above valence band for films grown at intermediate temperature (250–350 °C), and a pseudometallic behavior for film grown at high temperatures (350–450 °C). Chalcopyrite polycrystalline thin films of CuInSe2 with an average grain size of 1 μm, an optical gap of 1.01 eV, and resistivities from 10−1 to 103 Ω cm can be obtained by adding 1.5% of H2 to the sputtering atmosphere and by varying the substrate temperature from 300 to 400 °C.
Characterization of the silicon-on-insulator material formed by high-dose oxygen implantation using spectroscopic ellipsometry
Authors: Ferrieu F. , Vu D. P. , D’Anterroches C. , Oberlin J. C. , Maillet S. , Grob J. J.
Published: July 2002

Nondestructive characterization of high-dose oxygen implanted and 1350 °C annealed silicon-on-insulator structures has been performed by spectroscopic ellipsometry. This method provides a fully in-depth profiling (thickness and nature) of the structure including interfaces. Results have been confirmed by other techniques such as cross-sectional transmission electron microscopy and high-resolution Rutherford backscattering spectroscopy.
On mechanical stresses at cracks in dielectrics with application to dielectric breakdown
Authors: McMeeking Robert M.
Published: July 2002

The problem of an electric field applied to an isotropic dielectric containing a crack is considered as far as induced distortions and mechanical stresses are concerned. Near-tip distortions occur when the crack contains a conducting fluid or the surfaces are lined by a conducting layer. Previous treatments of the problem failed to distinguish between material and Maxwell stresses and an unphysical singularity in stress, which was divergent in elastic strain energy, was permitted. In this work, the material stress is identified, and a model is developed in which only physically realistic stress singularities are present. These results are obtained by treating the crack as a narrow but open ellipse rather than a slit crack as in previous models. A conventional crack-tip stress intensity factor KI characterizes the near-tip mechanical fields in the results obtained in this paper. The magnitude of KI due to an electrostatic field is deduced for a conducting central through crack in plane strain. The results suggest a model for dielectric breakdown caused by mechanical propagation of flaws. Data for dielectric breakdown and fracture of MgO and Al2O3 correlate in a way that supports this concept.
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n s
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u resistivity measurement of cobalt silicide formation
Authors: Ottaviani G. , Tu K. N. , Psaras P. , Nobili C.
Published: July 2002

I n situ resistivity measurements have been utilized to study the reaction and silicide formation between cobalt and amorphous silicon thin films from room temperature to 800 °C. In conjunction, structure and composition changes were analyzed by x-ray diffraction and Rutherford backscattering spectrometry. Formation of Co2Si, CoSi, and CoSi2 were observed. Interfacial reaction to form Co2Si occurs at approximately 400 °C. In bilayers of excess silicon, CoSi forms at approximately 520 °C and, if free silicon is still present, CoSi2 forms at about 550 °C. In the case of excess cobalt, Co2Si forms first and is followed by a cobalt-rich solid solution. Co3Si silicide was not observed.
Sphere-resonance method to determine elastic constants of crystal
Authors: Mochizuki E.
Published: July 2002

Theoretical basis is developed for the sphere-resonance method to determine elastic constants of a very small crystal. In this method, a sphere of crystal is formed, and frequencies of its free oscillations are measured by the resonance method. To calculate frequencies of the sphere, we use the Rayleigh–Ritz (variational) method as in the rectangular parallelepiped resonance method developed by Ohno [J. Phys. Earth 24, 355 (1976)]. It is necessary in the Rayleigh–Ritz method to evaluate matrix elements which are quadratic forms in the strain tensor with coupling by the elastic tensor. To do this the elastic tensor is transformed to spherical coordinates, and the contravariant canonical components of the elastic tensor are expanded by the generalized spherical harmonics YNml with nonzero coefficients only for l=0, 2, 4. For seven crystal systems, properties of the matrix elements are examined, and free oscillations are classified. We also obtain selection rules for excitation which are useful for the mode identification.
Solid phase reactions in free-standing layered M-Si (M=Ti, V, Cr, Co) films
Authors: Nathan Menachem
Published: July 2002

The reactions induced by rapid thermal annealing in free-standing amorphous (a) Si/M/a-Si (M=Ti, V, Cr, and Co) layered films deposited directly on electron microscope grids were studied with transmission electron diffraction. Stacks with ten different Si/M thickness ratios were simultaneously pulse heated by cw radiation from tungsten halogen lamps, and the sequence of phase formation determined from diffraction patterns after various annealing times. Solid-phase amorphization or glass formation occurs in Ti-Si and V-Si reactions but not in Co-Si and Cr-Si. Following amorphization in Ti-Si and V-Si, two new chemical environments are formed at the glass/Si and glass/metal interfaces and found to facilitate nucleation of Si-rich and metal-rich silicides, respectively. These silicides have different nucleation kinetics. For example, in films with Ti/glass/Si, i.e., where excess Ti and Si are still available, the first silicide detected is Ti-rich, probably Ti5Si3, followed by C49 TiSi2. In the nonglass forming Co-Si and Cr-Si systems, a single, unique silicide (CoSi and CrSi2, respectively), nucleates at the M/Si interface, independent of stoichiometry.
Temperature dependence of Faraday rotation in Bi-substituted terbium iron garnet films
Authors: Umezawa H. , Yokoyama Y. , Koshizuka N.
Published: July 2002

The role of Tb3+ in the Faraday rotation θF of (Bi,Lu)-substituted terbium iron garnet Tb3−x−yLuyBixIG (x=0–1.1, y=0–0.9) films is studied. The temperature dependence of θF at λ=633 nm is measured in the temperature range of 10–600 K and the saturation magnetization 4πMs is measured in the temperature range of 4.2–450 K to clarify the contribution of the dodecahedral Tb3+. The temperature dependence of θF is analyzed by using two different models based on the three sublattice magnetizations. It is found that in the model where the Fe3+ contributions to θF in Bi-substituted iron garnet are assumed to be the same in all rare-earth iron garnets, the contribution of the Tb3+ site to θF is increased by Bi substitution. The variations of the Tb3+ coefficients with Bi content and temperature are also presented and discussed.
Photoluminescence from annealed semi-insulating GaAs crystals: The 1.360-eV band
Authors: Swaminathan V. , Caruso R. , Pearton S. J.
Published: July 2002

We investigated the near-band-edge photoluminescence from semi-insulating GaAs crystals after they were annealed either in the wafer form or in the bulk form. Our results, with respect to the uniformity achieved after annealing, are in agreement with previous studies. The 1.360-eV emission band which is seen in annealed crystals and which has been taken to imply a VAs related rapid diffusion process as the mechanism responsible for the annealing-induced uniformity, is shown not to be connected with it. We also cite evidence which questions the involvement of VAs in the band. From data in the literature we estimate the diffusion coefficient of VAs and find it to be too low to bring about bulk equilibrium and uniformity via vacancy diffusion from the surface at the annealing temperatures. We conclude that the local rearrangement of defects is a viable mechanism for producing uniformity during post-growth annealing.
Extremely high hole concentration in GaAs by dual Mg/As implants
Authors: Landgren G. , van Berlo W. H.
Published: July 2002

The activation of magnesium implants in GaAs has been found to increase drastically with coimplantation of arsenic and to depend strongly on the As implantation parameters. p-type carrier concentrations as high as 4×1020 cm−3 have been achieved for 5×1015 cm−2, 190-keV Mg+/1×1016 cm−2, 540-keV As2+ implants. The role of the As has been shown to block the Mg diffusion during the silicon-nitride-capped 5-min anneals at 750 °C.