bluuebunny/crossref_metadata_2025 huggingface dataset:
Accurate measurements of the intrinsic diffusivities of boron and phosphorus in silicon
Authors: Haddara Yaser M. , Folmer Brennan T. , Law Mark E. , Buyuklimanli Temel
Published: July 2002

All activity in modeling transient diffusion behavior relies on knowledge of the inert intrinsic diffusivities of dopants in Si. The measurements upon which these values are based were conducted over 15 years ago. Since then, the quality of wafers used in industrial applications has significantly changed. This will affect the effective diffusivity through changes in trap concentrations. The reliability of measurement techniques has also changed dramatically from tracer and staining methods to secondary ion mass spectrometry (SIMS) measurements that are dominant today. Finally, our understanding of diffusion behavior has changed significantly. For example, we now understand that the extraction of diffusivities from implanted samples with no pre-anneal includes a significant transient effect. We have measured the inert intrinsic diffusivities of As, B, P, and Sb in different substrates in defect-free Czochralski and float zone wafers and epitaxially grown layers. All samples underwent a 30 min anneal at 1000 °C in dry oxygen in order to grow a cap oxide and eliminate transient enhanced diffusion. We performed SIMS analysis on an initial batch of samples to evaluate the different factors that may affect the diffusivity in a nonideal manner and concluded that there are no transient effects but that surface effects are important. Hence, for the fast moving dopants (B, P) we restrict our data extraction to the deep implants. Our data show that B and P diffusivities are different than the values commonly assumed in the literature at low temperatures. We compare our results to previously published data in light of the factors mentioned here.
Effect of work function and surface microstructure on field emission of tetrahedral amorphous carbon
Authors: Ilie A. , Hart A. , Flewitt A. J. , Robertson J. , Milne W. I.
Published: July 2002

The work function of tetrahedral amorphous carbon (ta-C) has been measured by Kelvin probe to lie in the range 4–5 eV, irrespective of its sp3 content or nitrogen addition. This implies that the surface barrier to emission is dominant and that emission changes caused by sp3 bonding or nitrogen addition are not directly due to changes in work function. Hydrogen, oxygen, and argon plasma treatments are all found to increase the emission of a-C, but hydrogen and argon treatments are found to reduce the work function while oxygen treatment increases it. Detailed studies of the surface with varying plasma treatment conditions suggest that the changes in emission arise mainly from changes in the surface microstructure, such as the formation of sp2 regions within the sp3 bulk. The need for local field enhancement mechanisms to account for emission over the sizeable barrier is emphasized, which may arise from local chemical nonhomogeneity, or formation of nanometer-size sp2 clusters embedded in an sp3 matrix.
Construction of an accurate potential energy surface by interpolation with Cartesian weighting coordinates
Authors: Rhee Young Min
Published: July 2002

A modified method to construct an accurate potential energy surface by interpolation is presented. The modification is based on the use of Cartesian coordinates in the weighting function. The translational and rotational invariance of the potential is incorporated by a proper definition of the distance between two Cartesian configurations. A numerical algorithm to find the distance is developed. It is shown that the present method is more exact in describing a planar system compared to the previous methods with weightings in internal coordinates. The applicability of the method to reactive systems is also demonstrated by performing classical trajectory simulations on the surface.
Quantum wave packet dynamics with trajectories: Implementation with adaptive Lagrangian grids
Authors: Wyatt Robert E. , Bittner Eric R.
Published: July 2002

The quantum trajectory method was recently developed to solve the hydrodynamic equations of motion in the Lagrangian, moving-with-the-fluid, picture. In this approach, trajectories are integrated for fluid elements (“particles”) moving under the influence of the combined force from the potential surface and the quantum potential. To accurately compute the quantum potential and the quantum force, it is necessary to obtain the derivatives of a function given only the values on the unstructured mesh defined by the particle locations. However, in some regions of space–time, the particle mesh shows compression and inflation associated with regions of large and small density, respectively. Inflation is especially severe near nodes in the wave function. In order to circumvent problems associated with highly nonuniform grids defined by the particle locations, adaptation of moving grids is introduced in this study. By changing the representation of the wave function in these local regions (which can be identified by diagnostic tools), propagation is possible to much longer times. These grid adaptation techniques are applied to the reflected portion of a wave packet scattering from an Eckart potential.
Distributed-feedback sol-gel dye laser tunable in the near ultraviolet
Authors: Zhu Xiao-lei , Lo Dennis
Published: July 2002

Laser emission in the near ultraviolet region was observed from a distributed-feedback solid-state dye laser based on Exalite 377E dye-doped sol-gel silica slabs. The output performance of the laser was studied in detail. Pumped at 308 nm, wavelength tuning from 367 to 383 nm with the emission linewidth of 80 pm has been achieved. Exalite 377E-doped sol-gel silica distributed feedback laser showed a slope efficiency of 4.4% and a pump energy threshold of approximately 100 μJ (15 mJ/cm2 in energy density). The variation of output energy as a function of the number of pump pulse at 0.5 Hz repetition rate was also investigated.
Optical anisotropy of Langmuir–Blodgett sapphyrin films
Authors: Di Natale C. , Goletti C. , Paolesse R. , Della Sala F. , Drago M. , Chiaradia P. , Lugli P. , D'Amico A.
Published: July 2002

The polarization dependence of the optical reflectivity for sapphyrin layers deposited by the Langmuir–Blodgett technique onto a gold substrate has been measured. The experimental results show that characteristic reflectance anisotropy spectroscopy (RAS) spectra are related to layers of different thicknesses. In order to interpret the measured spectral features, the anisotropy of the optical properties of the sapphyrin molecule has been evaluated by using a semi-empirical quantum chemistry approach. The results show clearly that the main RAS structures are related to the electronic properties of the sapphyrin molecules. In particular, two different regimes are observed. Below one (true) monolayer, the optical signal remains very low and structureless, in agreement with the sapphyrin molecules being stacked with their planes almost perpendicular to the substrate plane. Above one monolayer, instead, a strong RAS signal related to the Soret band develops, suggesting that sapphyrin molecules lay more parallel to the substrate surface, in an ordered fashion.
InGaN/GaN quantum well interconnected microdisk light emitting diodes
Authors: Jin S. X. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

Interconnected microdisk light-emitting diodes (μ-LEDs) have been fabricated from InGaN/GaN single quantum wells grown by low pressure metal organic chemical vapor deposition. These interconnected μ-disk LEDs fit into the same device area taken up by a conventional broad-area LED. The performance characteristic of these interconnected μ-disk LEDs, including I–V and L–I characteristics as well as electroluminescence spectra have been measured and compared with those of the conventional broad-area LEDs. For interconnected μ-disk LEDs, while VF, the forward biased voltage at 20 mA, was slightly increased, the emission efficiency was increased by as much as 60% over the conventional LEDs for a fixed device area. The results thus suggested that replacing a conventional broad-area LED with an interconnected μ-disk LED, the external quantum efficiency can be significantly enhanced. Finally, the fabrication processes of the interconnected μ-disk LEDs are almost the same as those of the conventional broad-area LEDs. It is thus expected the total yield of these interconnected μ-disk LEDs to be the same as the conventional broad-area LEDs. The present method of utilizing interconnected μ-disk LEDs for improving the brightness of LEDs is applicable to other semiconductor LEDs as well as polymer and organic LEDs.
Measurement of the direct energy gap of coherently strained SnxGe1−x/Ge(001) heterostructures
Authors: Ragan Regina , Atwater Harry A.
Published: July 2002

The direct energy gap has been measured for coherently strained SnxGe1−x alloys on Ge(001) substrates with 0.035<x<0.115 and film thickness 50–200 nm. The energy gap determined from infrared transmittance data for coherently strained SnxGe1−x alloys indicates a large alloy contribution and a small strain contribution to the decrease in direct energy gap with increasing Sn composition. These results are consistent with a deformation potential model for changes in the valence and conduction band density of states with coherency strain for this alloy system.
Visible photoluminescence in amorphous SiNx thin films prepared by reactive evaporation
Authors: Molinari M. , Rinnert H. , Vergnat M.
Published: July 2002

Photoluminescence in the visible domain can be observed in amorphous silicon nitride (a-SiNx) alloys prepared by evaporation of silicon under a flow of nitrogen ions. A strong improvement of the photoluminescence intensity was obtained with annealing treatments in the range 500–1150 °C. Structural investigations were performed by infrared and Raman spectrometry experiments. The optical gap was obtained from transmission measurements in the ultraviolet, visible, and near infrared range. The evolutions of the structure and the optical properties with annealing treatments are correlated to the evolution of the photoluminescence.
Correlation between the gap energy and size of single InAs quantum dots on GaAs(001) studied by scanning tunneling spectroscopy
Authors: Yamauchi T. , Matsuba Y. , Bolotov L. , Tabuchi M. , Nakamura A.
Published: July 2002

Scanning tunneling spectroscopy has been used to study quantum-size effects on the electronic structure of InAs quantum dots (QDs) in correlation with their morphologies. The measured gap energy increases with decreasing dot height in the range of 3.4–7.6 nm. Comparison between the observed height dependence and calculation based on a quantum disk model indicates that the gap energy of a single InAs QD is mainly determined by the quantum confinement in the vertical direction of the QD.
Epitaxial NiMnSb films on GaAs(001)
Authors: Van Roy W. , De Boeck J. , Brijs B. , Borghs G.
Published: July 2002

We show the growth of epitaxial NiMnSb(001) thin films, a half-metallic ferromagnet, on GaAs(001) by molecular-beam epitaxy. The Sb content of the films strongly depends on the Sb flux. Sb-deficient films show a reduced magnetization and the formation of a Mn2As interface layer. Stoichiometric films exhibit the bulk lattice constant, have a saturation magnetization close to the bulk value, and do not suffer from Mn2As formation at the interface. They show in-plane uniaxial anisotropy with nearly square hysteresis loops along the easy axis, which points either along [110] or [11̄0] depending on the growth conditions.
Spatial profiles of neutral, ion, and etch uniformity in a large-area high-density plasma reactor
Authors: Yun Seokmin , Tynan George R.
Published: July 2002

Spatial profiles of neutral density, ion density, and etch rate have been measured in a large-area high-density plasma reactor. Blanket photoresist films on 200 mm wafers are etched by oxygen plasmas in a magnetized inductively coupled plasma reactor. Ion density and relative neutral density are measured by a scanning Langmuir probe and optical probe, respectively. Spatially resolved atomic oxygen density is then measured by optical emission spectroscopy and spatially resolved actinometry. The etch rate is calculated from film thickness measurements taken before and after the wafer is exposed to the oxygen plasma. Ion energy and wafer surface temperature were also measured. The mechanism linking neutral density, ion density, and etching rate is discussed.
Judd–Ofelt parameters and multiphonon relaxation of Ho3+ ions in ZnCl2-based glass
Authors: Shojiya M. , Kawamoto Y. , Kadono K.
Published: July 2002

Spectroscopic properties of Ho3+-doped ZnCl2-based glasses are investigated. The Ω2 value of the glass is especially large compared with those in fluoride glasses while the Ω4 and Ω6 parameters are relatively small. Decay measurements for three excited states F55, S25+F45, and F2,35+K83 were performed and multiphonon relaxation (MPR) rates of the respective states were estimated. The experimental data are approximately on a line expressed by WMPR=C exp(−αΔE) with C=5.0×107 s−1 and α=4.0×10−3 cm. The host-dependent constant C is very small, contributing to high quantum efficiencies of emissions. The Ho3+ concentration dependence of the fluorescence lifetime of the F55 and S25+F45 levels was also investigated. The concentration quenching of the S25+F45 level can be explained by the cross-relaxation mechanism: S25→I45 and I75←I85 transitions.
Single-hole transistor in a p-Si/SiGe quantum well
Authors: Dötsch U. , Gennser U. , David C. , Dehlinger G. , Grützmacher D. , Heinzel T. , Lüscher S. , Ensslin K.
Published: July 2002

A single-hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.
Si
(100)–
SiO
2
interface properties following rapid thermal processing
Authors: O’Sullivan B. J. , Hurley P. K. , Leveugle C. , Das J. H.
Published: July 2002

An experimental examination of the properties of the Si(100)–SiO2 interface measured following rapid thermal processing (RTP) is presented. The interface properties have been examined using high frequency and quasi-static capacitance-voltage (CV) analysis of metal-oxide-silicon (MOS) capacitor structures immediately following either rapid thermal oxidation (RTO) or rapid thermal annealing (RTA). The experimental results reveal a characteristic peak in the CV response measured following dry RTO and RTA (T>800 °C), as the Fermi level at the Si(100)–SiO2 interface approaches the conduction band edge. Analysis of the QSCV responses reveals a high interface state density across the energy gap following dry RTO and RTA processing, with a characteristic peak density in the range 5.5×1012 to 1.7×1013 cm−2 eV−1 located at approximately 0.85–0.88 eV above the valence band edge. When the background density of states for a hydrogen-passivated interface is subtracted, another peak of lower density (3×1012 to 7×1012 cm−2 eV−1) is observed at approximately 0.25–0.33 eV above the valence band edge. The experimental results point to a common interface state defect present after processes involving rapid cooling (101–102 °C/s) from a temperature of 800 °C or above, in a hydrogen free ambient. This work demonstrates that the interface states measured following RTP (T>800 °C) are the net contribution of the Pb0/Pb1 silicon dangling bond defects for the oxidized Si(100) orientation. An important conclusion arising from this work is that the primary effect of an RTA in nitrogen (600–1050 °C) is to cause hydrogen desorption from pre-existing Pb0/Pb1 silicon dangling bond defects. The implications of this work to the study of the Si–SiO2 interface, and the technological implications for silicon based MOS processes, are briefly discussed. The significance of these new results to thin oxide growth and optimization by RTO are also considered.
Photorefractive chiral smectic A phases
Authors: Termine R. , De Simone B. C. , Golemme A.
Published: July 2002

We present results of the photorefractive properties of a chiral smectic A liquid-crystalline phase. In our approach, the mesophase is sandwiched between two photoconducting layers. In contrast to the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to the electroclinic effect. We measured the grating spacing and the field dependence of the gain coefficient and obtained a net two-beam coupling gain of 600 cm−1 at 35 V/μm.
Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum dot lasers
Authors: Sellin R. L. , Ribbat Ch. , Grundmann M. , Ledentsov N. N. , Bimberg D.
Published: July 2002

Close-to-ideal device characteristics of high-power InGaAs/GaAs quantum-dot lasers are achieved by the application of an annealing and growth interruption step at 600 °C after the deposition of the dots. The transparency current is reduced to below 20 A/cm2 at room temperature. The internal differential quantum efficiency is increased from below 50% to above 90% by improvement of the barrier material and subsequent reduction of leakage current. A peak power of 3.7 W at 1140 nm lasing wavelength in pulsed operation at room temperature is demonstrated.
Magnetoelastic effects and magnetic anisotropy in Ni2MnGa polycrystals
Authors: Albertini F. , Morellon L. , Algarabel P. A. , Ibarra M. R. , Pareti L. , Arnold Z. , Calestani G.
Published: July 2002

Linear thermal expansion and magnetostriction measurements under a magnetic field up to 20 kOe, applied parallel and perpendicular to the measuring direction and in the temperature range of 10–300 K, have been performed on a Ni2MnGa polycrystal. The effects of zero-field cooling and field cooling on both the self-strain (H=0) and the magnetic-field-induced strain (MFIS) have been studied. We have found that the MFIS strongly depends on whether a magnetic field was applied during the cooling process. The applied magnetic field facilitates the growth of specific orientation variants along the field direction as the sample is cooled down through martensitic transformation. However, the application of a field in the martensitic phase induces a negligible motion of twin boundaries. On this basis, the singular point detection technique performed on polycrystalline specimens results in being the easiest and most direct way by which to determine the anisotropy field in this class of materials. The temperature behavior of the anisotropy constant and field has been determined for Ni2MnGa.
Fast particle generation and energy transport in laser-solid interactions
Authors: Zepf M. , Clark E. L. , Krushelnick K. , Beg F. N. , Escoda C. , Dangor A. E. , Santala M. I. K. , Tatarakis M. , Watts I. F. , Norreys P. A. , Clarke R. J. , Davies J. R. , Sinclair M. A. , Edwards R. D. , Goldsack T. J. , Spencer I. , Ledingham K. W. D.
Published: July 2002

The generation of MeV electron and ion beams using lasers with intensities of up to 1020 W cm−2 is reported. Intense ion beams with high energies (up to 40 MeV and to 3×1012 protons >5 MeV) are observed. The properties of these particle beams were measured in considerable detail and the results are compared to current theoretical explanations for their generation.
Thermal resistance and temperature characteristics of GaAs/Al0.33Ga0.67As quantum-cascade lasers
Authors: Spagnolo Vincenzo , Troccoli Mariano , Scamarcio Gaetano , Becker Cyrille , Glastre Geneviève , Sirtori Carlo
Published: July 2002

We report on the determination of thermal resistance, facet temperature profile, and heat flux of GaAs/Al0.33Ga0.67As quantum-cascade lasers operating in pulsed mode, using a microprobe band-to-band photoluminescence technique. The thermal resistance of epilayer-side mounted lasers is ∼30% smaller than that of substrate-side mounted ones. The dependence of the thermal resistance on the injection conditions and its correlation with the output power is also reported.
88 °C, continuous-wave operation of apertured, intracavity contacted, 1.55 μm vertical-cavity surface-emitting lasers
Authors: Nakagawa S. , Hall E. , Almuneau G. , Kim J. K. , Buell D. A. , Kroemer H. , Coldren L. A.
Published: July 2002

We demonstrate a lattice-matched 1.55 μm vertical-cavity surface-emitting laser operating continuous wave up to 88 °C. The laser employs AlAsSb-based mirrors, which provide high reflectivity and lattice matching to InP. The poor electrical and thermal conductivity of these mirrors is circumvented by utilizing an InP double-intracavity contacted structure. Benefits of the intracavity contacts are addressed by comparing the characteristics with the alternative contact scheme where current is injected through the Sb-based mirrors. Current and optical confinement is provided by an undercut aperture. The device shows a threshold current of 800 μA, a differential efficiency of 23%, and a maximum output power of over 1 mW at 20 °C.
3.6 mW blue light by direct frequency doubling of a diode laser using an aperiodically poled lithium niobate crystal
Authors: Birkin D. J. L. , Rafailov E. U. , Sokolovskii G. S. , Sibbett W. , Ross G. W. , Smith P. G. R. , Hanna D. C.
Published: July 2002

3.6 mW blue (490 nm) light has been generated from a frequency-doubled, gain-switched InGaAs/GaAs diode laser using an aperiodically poled lithium niobate crystal. The matching of the crystal acceptance bandwidth to the laser spectral profile resulted in significant improvements to the conversion efficiency.
Pulse electrical discharges in water and their applications
Authors: Šunka Pavel
Published: July 2002

Partial electrical discharges in a water solution as a function of conductivity have been studied experimentally. Using needle-plate electrodes it has been demonstrated that the discharge evolves in two phases. During the first streamer-like phase, the discharge propagated with a velocity of 106 cm/s, while during the second arc-like phase the length of the discharge remained almost constant although the current still increased. Higher solution conductivity resulted in the generation of shorter channels, in larger discharge current, and in a higher plasma electron density. Degradation of phenol by the discharge has also been demonstrated. A special metallic electrode covered by a thin layer of porous ceramic has been developed and used for generation of a multichannel discharge. At comparable solution conductivity the ceramic-coated electrode produced plasma with very similar parameters as the needle-plate electrode configuration. Generation of strong focused shock waves by the multichannel discharge in a highly conductive solution has also been demonstrated.
Transient absorption anisotropy study of ultrafast energy transfer in porphyrin monomer, its direct meso–meso coupled dimer and trimer
Authors: Min Chang-Ki , Joo Taiha , Yoon Min-Chul , Kim Cheon Min , Hwang Young Nam , Kim Dongho , Aratani Naoki , Yoshida Naoya , Osuka Atsuhiro
Published: July 2002

Transient absorption anisotropies of the Zn(II)porphyrin monomer, its direct meso–meso coupled dimer, and trimer are measured in the B (Soret) band region by sub-30 fs laser pulses. It is shown that detailed information on the electronic structures and energy transfer dynamics can be obtained from the anisotropy and the magic angle data. The anisotropies of all three molecules exhibit remarkable behaviors in the first 200 fs region. Experimental observations can be accounted for adequately by treating the transient absorption signal as an explicit sum of ground state bleach and excited state stimulated emission contributions. In the monomer, the anisotropy decay denotes an oscillatory feature followed by a 100 fs time constant exponential decay. It is argued that the Bx and By transitions of the porphyrin monomer are nondegenerate with an energy splitting of ∼170 cm−1. Furthermore, equilibration of the excitation energy within the B states is slightly underdamped. Excitation of the monomerlike B band of the dimer (trimer) leads to ultrafast ∼30 fs (60 fs) anisotropy decay and a subsequent rise with ∼60 fs (70 fs) time constant. It is concluded that the anisotropy decay is due to the ultrafast energy transfer to the low-energy exciton split B band, while the rise is due to the redshift of the excited state stimulated emission spectrum thereby decreasing the contribution of the excited state stimulated emission component in the overall transient absorption signal.
Luminescence spectra of a quantum-dot cascade laser
Authors: Apalkov V. M. , Chakraborty Tapash
Published: July 2002

A quantum cascade laser in which the quantum wells in the active regions are replaced by quantum dots with their atom-like discrete energy levels is an interesting system with which to study novel features in optical spectroscopy. We study structures suitable for diagonal lasing transitions in coupled dots, and vertical transitions in a single dot. The luminescence spectra as a function of electron number and dot size show that for diagonal transitions a significant amount of blueshift in the emission spectra can be achieved by increasing the electron population in the quantum dots as well as by decreasing the size of the dots.
Variational calculations of fermion second-order reduced density matrices by semidefinite programming algorithm
Authors: Nakata Maho , Nakatsuji Hiroshi , Ehara Masahiro , Fukuda Mitsuhiro , Nakata Kazuhide , Fujisawa Katsuki
Published: July 2002

The ground-state fermion second-order reduced density matrix (2-RDM) is determined variationally using itself as a basic variable. As necessary conditions of the N-representability, we used the positive semidefiniteness conditions, P, Q, and G conditions that are described in terms of the 2-RDM. The variational calculations are performed by using recently developed semidefinite programming algorithm (SDPA). The calculated energies of various closed- and open-shell atoms and molecules are excellent, overshooting only slightly the full-CI energies. There was no case where convergence was not achieved. The calculated properties also reproduce well the full-CI results.
Atomic shell structure in Hartree theory
Authors: Sahni Viraht , Qian Zhixin , Sen K. D.
Published: July 2002

In this paper we show that atomic shell structure is exhibited throughout the periodic table, and accurate core–valence separations thereby obtained, via the radial probability density determined from the uncorrelated wave functions of Hartree theory. Further, essentially equivalent results are obtained via Hartree-theory-level quantal density functional theory in an approximation in which the correlation contributions to the kinetic energy are also neglected. Thus, accurate atomic shell structure can be obtained solely via electrostatic fields determined from charge distributions that are derived from wave functions which neither obey the Pauli exclusion principle nor incorporate Coulomb correlations.
Critical current density of YBa2Cu3O7−δ low-angle grain boundaries in self-field
Authors: Verebelyi D. T. , Cantoni C. , Budai J. D. , Christen D. K. , Kim H. J. , Thompson J. R.
Published: July 2002

A study has been perfomed on the superconducting critical current density Jc flowing across low-angle grain boundaries in epitaxial thin films of YBa2Cu3O7−δ. The materials studied were dual grain boundary rings deposited on SrTiO3 and containing 2°, 3°, 5°, and 7° tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to Tc. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving Jc by reducing the grain boundary angle below ∼3°.
Coexistence of intrinsic and extrinsic magnetoresistance in the double-perovskite Sr2Fe(Mo1−xWx)O6−w system
Authors: Lindén J. , Yamamoto T. , Nakamura J. , Karppinen M. , Yamauchi H.
Published: July 2002

In a recent study, it was shown that by partially substituting Mo with W in the double-perovskite Sr2FeMoO6−w system, the magnetoresistivity can be enhanced. [K.-I. Kobayashi, T. Okuda, Y. Tomioka, T. Kimura, and Y. Tokura, J. Magn. Magn. Mater. 218, 17 (2000).] In order to explain the increase in the magnetoresistivity a series of W-substituted Sr2Fe(Mo1−xWx)O6−w samples with 0⩽x⩽1 was synthesized. Upon increasing the W content, the samples began to exhibit coexistence of paramagnetism and ferrimagnetism at 300 K. Signatures of antiferromagnetic ordering appeared around TN≈50 K for x⩾0.6. In samples with 0.7⩽x⩽0.8 a broad peak was observed in the magnetoresistance data at temperatures corresponding to the Néel temperature. The peak was found to have its origin in the colossal magnetoresistance effect. The W-substituted samples were partially ferrimagnetic and therefore also exhibited the tunneling-type magnetoresistance, which is characteristic of pure Sr2FeMoO6−w. The coexistence of the two types of magnetoresistance effect is responsible for the enhancement of the overall magnetoresistance value. A slight enhancement in the magnetoresistance values around 300 K for the strongly W-substituted samples was found to be related to a second colossal magnetoresistance peak related to the para- to ferrimagnetic transition at Tc.
Superlow friction behavior of diamond-like carbon coatings: Time and speed effects
Authors: Heimberg J. A. , Wahl K. J. , Singer I. L. , Erdemir A.
Published: July 2002

The friction behavior of a diamond-like carbon coating was studied in reciprocating sliding contact at speeds from 0.01 to 5 mm/s, in dry nitrogen. “Superlow” friction coefficients of 0.003–0.008 were obtained in continuous sliding at the higher speeds (>1 mm/s). However, friction coefficients rose to values typical of diamond-like carbon in dry and ambient air (0.01–0.1) at lower speeds (<0.5 mm/s) as well as in time-delayed, higher speed tests. The rise of the friction coefficients in both speed and time-delay tests was in good quantitative agreement with gas adsorption kinetics predicted by the Elovich equation for adsorption onto carbon. More generally, superlow friction could be sustained, suppressed, and recovered as a function of exposure time, demonstrating that duty cycle cannot be ignored when predicting performance of superlow friction coatings in devices.
Kinetics of buckling of a compressed film on a viscous substrate
Authors: Sridhar N. , Srolovitz D. J. , Suo Z.
Published: July 2002

Compressively-stressed elastic films on finite-thickness viscous substrates can undergo a buckling instability that relieves stresses but destroys the planarity of the film. A linear-stability analysis is performed to determine the onset and maximally unstable mode of this buckling instability as a function of misfit strain, viscous layer thickness, and viscosity. We find that the onset of the buckling instability of the film on a glass layer is the same as that for a compressively stressed free-standing film. However, the maximally unstable wavelength increases as the glass layer thickness increases. Comparisons with experimental data are provided.
Improved approximation of the Brinkman equation using a lattice Boltzmann method
Authors: Martys Nicos S.
Published: July 2002

In this Brief Communication, a new approach to generalize the lattice Boltzmann method to produce fluid flow consistent with the Brinkman equation is presented. The method described in this communication both eliminates second order errors in velocity and improves stability over that of a previously proposed lattice Boltzmann-based model.
Characterizing and calibrating a large Helmholtz coil at low ac magnetic field levels with peak magnitudes below the earth’s magnetic field
Authors: Schill Robert A. , Hoff Karin
Published: July 2002

Characterizing and calibrating a low impedance large Helmholtz coil generating 60 Hz magnetic fields with amplitudes well below the earth’s magnetic field is difficult and imprecise when coil shielding is not available and noise is an issue. Parameters influencing the calibration process such as temperature and coil impedance need to be figured in the calibration process. A simple and reliable calibration technique is developed and used to measure low amplitude fields over a spatial grid using a standard Hall effect probe gaussmeter. These low amplitude fields are typically hard or impossible to detect in the presence of background fields when using the gaussmeter in the conventional manner. Standard deviations of two milligauss and less have been achieved over a spatial grid in a uniform field region. Theoretical and measured fields are compared yielding reasonable agreement for a large coil system designed and built for bioelectromagnetic experiments at the University of Nevada at Las Vegas using simple tools. Theoretical results need to be compared with and adjusted in accord with measurements taken over a large parameter space within the design constraints of the coil. Magnetic field measurements made over a four year period are shown to be consistent. Characterizing and calibrating large Helmholtz coils can be performed with rulers, levels, plumb lines, and inexpensive gaussmeters.
End effects of strongly charged polyelectrolytes: A molecular dynamics study
Authors: Limbach Hans Jörg , Holm Christian
Published: July 2002

We investigate end effects in the ion distribution around strongly charged, flexible polyelectrolytes with a quenched charge distribution by molecular dynamics simulations of dilute polyelectrolyte solutions. We take the counterions explicitly into account and calculate the full Coulomb interaction via a Ewald summation method. We find that the free counterions of the solution are distributed in such a way that a fraction of the chain charges is effectively neutralized. This in turn leads to an effective charge distribution which is similar to those found for weakly charged titrating polyelectrolytes that have an annealed charge distribution. The delicate interplay between the electrostatic interactions, the chain conformation and the counterion distribution is studied in detail as a function of different system parameters such as the chain length Nm, the charge fraction f, the charged particle density ρ, the ionic strength and the solvent quality. Comparisons are made with predictions from a scaling theory.
Field emission from GaN surfaces roughened by hydrogen plasma treatment
Authors: Sugino Takashi , Hori Takamitsu , Kimura Chiharu , Yamamoto Tomohide
Published: July 2002

GaN layers are grown on sapphire substrates with AlN buffer layers by the metalorganic chemical vapor deposition method. GaN layers are doped with Si. The electron density of the n-type GaN is 2×1017 cm−3. It is found that the GaN surface is etched with hydrogen (H2) plasma produced by supplying microwave power leading to the formation of the roughened surface of GaN. A variation in the surface morphology occurs due to microwave power and gas pressure. Field emission measurements are carried out for GaN with various surface morphologies. It is observed that the turn-on electric field decreases with increasing surface roughness of the GaN. A turn-on electric field of the electron emission is estimated to be as low as 12.4 V/μm.
Dielectric properties of Mg-doped Ba0.96Ca0.04Ti0.84Zr0.16O3 thin films fabricated by metalorganic decomposition method
Authors: Yi Woo-Chul , Kalkur T. S. , Philofsky Elliott , Kammerdiner Lee , Rywak Anthony A.
Published: July 2002

Mg-doped Ba0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) thin films have been proposed as a promising material for microelectronic device applications based on high dielectric materials. Perovskite polycrystalline Mg-doped BCTZ thin films were fabricated on a Pt/Ti/SiO2/Si substrate by metalorganic decomposition method. Dielectric properties were improved after a Pt/Mg-doped BCTZ thin film/Pt capacitor was post-annealed at 700 °C in O2 atmosphere for 30 min. A high dielectric constant of 460 at 1 MHz, a low dissipation factor less than 4.5%, and a low leakage current density of 4×10−7 A/cm2 at 3 V were obtained. Improved dielectric properties were discussed in conjunction with reduction of oxygen vacancies and electrons due to the post-annealing and Mg dopants.
Decay of stimulated and spontaneous emission in highly excited homoepitaxial GaN
Authors: Juršėnas S. , Kurilčik N. , Kurilčik G. , Žukauskas A. , Prystawko P. , Leszcynski M. , Suski T. , Perlin P. , Grzegory I. , Porowski S.
Published: July 2002

The high-density effects in the recombination of electron–hole plasma in photoexcited homoepitaxial GaN epilayers were studied by means of transient photoluminescence at room temperature. Owing to the “backward” and “lateral” photoluminescence measurement geometries employed, the influence of stimulated transitions on the decay of degenerate nonthermalized plasma was revealed. The lateral stimulated emission was demonstrated to cause a remarkable increase in the recombination rate on the early stage of the luminescence transient. A delayed enhancement of the stimulated emission due to the cooling of plasma from the initial temperature of 1100 K was observed. After completion of the thermalization process and exhaustion of the stimulated emission, the spontaneous-luminescence decay exhibited an exponential slope that relates to the nonradiative recombination of the carriers. The homoepitaxially grown GaN layer featured a luminescence decay time of 445 ps that implies a room-temperature free-carrier lifetime of 890 ps (considered to be extremely high for undoped hexagonal GaN).
Crystalline to amorphous transition and band structure evolution in ion-damaged silicon studied by spectroscopic ellipsometry
Authors: Giri P. K. , Tripurasundari S. , Raghavan G. , Panigrahi B. K. , Magudapathy P. , Nair K. G. M. , Tyagi A. K.
Published: July 2002

Crystalline to amorphous transition and subsequent microstructural evolution in silicon induced by Ar+-ion implantation over a wide range of ion fluences (6×1013–1×1017 cm−2) have been investigated by spectroscopic ellipsometry. In the evaluation of the optical and microstructural properties of the damaged layer, the contribution of the surface overlayer to the measured dielectric spectra was separated by fitting a multilayer model with an effective medium approximation. The best fit to the dielectric spectra for disordered silicon could be obtained by taking our highest-fluence implanted (fluence=1×1017 ions/cm2) amorphous silicon (a-Si) data as reference data instead of a-Si data available in the handbook. The derivative spectra as a function of fluence show a distinct and sharp transition from the crystalline to amorphous phase. The threshold fluence for this transition is derived from fitting. Evaluation of standard sum rules and optical moments for imaginary part of the pseudodielectric function reveals no substantial change in various physical parameters below the transition indicating their insensitivity to point defects, while it shows a large change with fluence above the threshold for amorphization. The disorder induced changes in the effective dielectric constant, number of valence electrons per atom participating in optical transition, Penn gap energy, average bond length, coordination number, effective dispersion oscillator energy, an average strength of the interband optical transition with fluence is discussed on the basis of microstructural evolution and corresponding band structure modification. It is also shown that the dielectric functions of damaged silicon are well represented by a sum of six classical Lorentz oscillators. With increasing fluences, each of the oscillator amplitude decreases and linewidth increases except for the 3.3 eV transition which shows increasing amplitude with fluence. These results are discussed in the context of short-range order/disorder and effective band gap reduction along with flattening of the bands with increasing fluence above the amorphization threshold.
Wavelength tunable diffractive transmission lens for hard x rays
Authors: David C. , Nöhammer B. , Ziegler E.
Published: July 2002

We report on the fabrication and testing of linear transmission Fresnel zone plates for hard x rays. The diffractive elements are generated by electron beam lithography and chemical wet etching of 〈110〉 oriented silicon substrates. By tilting the cylindrical lenses with respect to the x-ray beam, the effective path through the phase shifting zones can be varied. This makes it possible to optimize the diffraction efficiency over a wide range of photon energies, and to obtain effective aspect ratios not accessible with untilted optics. The diffraction efficiency of such a lens was measured as a function of the tilt angles for various energies between 8 and 29 keV. Values close to the theoretical limit were obtained for all energies. Because of the coherence preserving properties of diffractive optics, the method opens up opportunities for experiments using coherent hard x rays.
False vacuum decay: Effective one-loop action for pair creation of domain walls
Authors: Dias Óscar J. C. , Lemos José P. S.
Published: July 2002

An effective one-loop action built from the soliton field itself for the two-dimensional (2D) problem of soliton pair creation is proposed. The action consists of the usual mass term and a kinetic term in which the simple derivative of the soliton field is replaced by a covariant derivative. In this effective action the soliton charge is treated no longer as a topological charge but as a Noether charge. Using this effective one-loop action, the soliton–antisoliton pair production rate Γ/L=A  exp[−S0] is calculated and one recovers Stone’s exponential factor S0 and the prefactor A of Kiselev, Selivanov, and Voloshin. The results are also valid straightforwardly to the problem of pair creation rate of domain walls in dimensions D⩾3.
Properties of highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures prepared by chemical solution routes
Authors: Wang G. S. , Cheng J. G. , Meng X. J. , Yu J. , Lai Z. Q. , Tang J. , Guo S. L. , Chu J. H. , Li G. , Lu Q. H.
Published: July 2002

Highly (100) oriented Ba0.9Sr0.1TiO3/LaNiO3 heterostructures have been grown on Si(100) using chemical solution routes. X-ray diffraction analysis shows that Ba0.9Sr0.1TiO3 thin films are high (100) orientation (α100=0.92). Atomic force microscopy investigation shows that they have large grains about 80–200 nm. A Pt/Ba0.9Sr0.1TiO3/LaNiO3 capacitor has been fabricated and showed excellent ferroelectricity, the remnant polarization and coercive field are 10.8 μC/cm2 and 96 kV/cm, respectively. The electric field dependence of capacitance measurement shows that the capacitor has large capacitance tuning ([Cmax−Cmin]/Cmax×100%) of 63%. The Ba0.9Sr0.1TiO3 thin films have high dielectric constant (ε) of 200 at 1 MHz.
Quantitative surface stress measurements using a microcantilever
Authors: Godin Michel , Tabard-Cossa Vincent , Grütter Peter , Williams Peter
Published: July 2002

A method for calculating the surface stress associated with the deflection of a micromechanical cantilever is presented. This method overcomes some of the limitations associated with Stoney’s formula by circumventing the need to know the cantilever’s Young’s modulus, which can have a high level of uncertainty, especially for silicon nitride cantilevers. The surface stress is calculated using readily measurable cantilever properties, such as its geometry, spring constant, and deflection. The method is applicable to both rectangular and triangular cantilevers. A calibration of the deflection measurement is also presented. The surface stress measurement is accurate to within 4%–7%.
Visible luminescence from a-SiN films doped with Er and Sm
Authors: Zanatta A. R. , Ribeiro C. T. M. , Jahn U.
Published: July 2002

Relatively strong and narrow red and green light emission has been achieved from amorphous (a-) SiN films independently doped with Er3+ and Sm3+ ions. The films were deposited by cosputtering a Si target partially covered with small pieces of metallic Er (and Sm) in an atmosphere of pure nitrogen. As a consequence of the deposition method and conditions, the films have an amorphous structure, and contents of Er (and Sm) in the low 0.5 at. %. All characterizations were accomplished on as-deposited samples and at room temperature and included: ion-beam analysis (Rutherford backscattering spectrometry and nuclear reaction analysis) and optical techniques (light absorption, Raman scattering, and photoluminescence and cathodoluminescence). A detailed examination of the experimental results allowed the identification of all luminescence features existing in the films.
Minority carrier lifetime degradation in boron-doped Czochralski silicon
Authors: Glunz S. W. , Rein S. , Lee J. Y. , Warta W.
Published: July 2002

The minority carrier lifetime in boron-doped oxygen-contaminated Czochralski (Cz) silicon is strongly reduced under illumination or carrier injection. This process can be fully reversed by a 200 °C anneal step. In several recent studies it was shown that boron and oxygen are the major components of the underlying metastable Cz-specific defect. The energy level of the defect in its active state A was determined to be around midgap [Schmidt et al., J. Appl. Phys. 86, 3175 (1999)] while the energy level of the defect in its passive state P is very shallow. The Cz-specific defect in its passive state can be identified with the shallow thermal donor. The kinetics of the excess carrier-induced transformation from state P to state A can be described using recombination-enhanced defect reaction theory. On the basis of these experimental facts different solutions for the reduction or elimination of the metastable defect are suggested. Two promising solutions are discussed in more detail: the use of gallium-doped Cz silicon and the introduction of high-temperature anneals into the process sequence. Gallium-doped Cz silicon shows no degradation and excellent lifetimes over a wide resistivity range, although the concentration of interstitial oxygen is in the same range as in standard Cz silicon. Stable solar cell efficiencies comparable to FZ silicon have been achieved. If standard boron-doped Cz silicon is used, the defect concentration can be reduced permanently by a high-temperature anneal using conventional tube or rapid thermal processing. This leads to an improvement of the carrier lifetime by a factor of 2–3. Nevertheless, it is always necessary to use an optimized set of process parameters because otherwise the lifetime of all oxygen-contaminated materials (including gallium-doped Cz silicon) is severely reduced
Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots
Authors: Moon Yong-Tae , Kim Dong-Joon , Park Jin-Sub , Oh Jeong-Tak , Lee Ji-Myon , Ok Young-Woo , Kim Hyunsoo , Park Seong-Ju
Published: July 2002

The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions.