bluuebunny/crossref_metadata_2025 huggingface dataset:
Low resistance Ohmic contact scheme (∼μΩ cm2) to p-InP
Authors: Park Moon-Ho , Wang L. C. , Cheng J. Y. , Palmstro/m C. J.
Published: July 2002

A low resistance Pd/Sb/Zn/Pd Ohmic contact based on the solid phase regrowth principle has been investigated to p-InP. Contact resistivity as low as ∼2×10−6 Ω cm2 has been obtained for samples annealed at 500 °C for 1 min. The Ohmic behavior can be rationalized by the formation of a heavily doped surface layer or a heterojunction. The InSb phase responsible for the observed low resistivity is identified using the x-ray diffraction technique.
Low voltage organic light emitting diodes featuring doped phthalocyanine as hole transport material
Authors: Blochwitz J. , Pfeiffer M. , Fritz T. , Leo K.
Published: July 2002

We show that doping of the transport layers can strikingly improve the properties of organic light emitting diodes (OLEDs). The electroluminescence onset voltage of diodes containing an vanadyl–phthalocyanine (VOPc) hole transport layer intentionally doped with tetrafluorotetracyano-quinodimethan (F4-TCNQ) is reduced by up to an order of magnitude compared to OLED with undoped VOPc. The improved properties of our devices can be explained by the improved conductivity and better injection for a doped transport layer.
Facet roughness analysis for InGaN/GaN lasers with cleaved facets
Authors: Stocker D. A. , Schubert E. F. , Grieshaber W. , Boutros K. S. , Redwing J. M.
Published: July 2002

Atomic force microscope images reveal a root-mean-square roughness Δd=16 nm for InGaN/GaN double-heterostructure laser structures with cleaved a-plane facets. The c-plane sapphire substrate cleaves cleanly along both the a and m planes. A theoretical model is developed which shows that the power reflectivity of the facets decreases with roughness by a factor of e−16π2(nΔd/λ0)2, where n is the refractive index of the semiconductor and λ0 is the emission wavelength. Laser emission from the optically pumped cavities shows a TE/TM ratio of 100, an increase in differential quantum efficiency by a factor of 34 above threshold, and an emission line narrowing to 13.5 meV.
Spin transport in GaAs
Authors: Hägele D. , Oestreich M. , Rühle W. W. , Nestle N. , Eberl K.
Published: July 2002

We present a spectroscopic method for studying spin transport in semiconductors. Our time-resolved experiments have an important implication for spin electronics as they show that spin-polarized electron drift is possible in semiconductors over typical device lengths in high electric fields. We demonstrate an almost complete conservation of the orientation of the electron spin during transport in GaAs over a distance as long as 4 μm and fields up to 6 kV/cm.
Increasing throughput of a near-field optical fiber probe over 1000 times by the use of a triple-tapered structure
Authors: Yatsui T. , Kourogi M. , Ohtsu M.
Published: July 2002

We fabricated a new probe with extremely high throughput introducing a triple-tapered structure to reduce the loss in a tapered core, to focus the light, and to excite effectively the HE11 mode. A focused ion beam and selective chemical etching were used for fabrication. Over a 1000-fold increase in the throughput of the triple-tapered probe with the aperture diameter D<100 nm was realized in comparison with the conventional single-tapered probe. Furthermore, due to the third taper with a small cone angle, the localized optical near field on the triple-tapered apertured probe with D=60 nm has been confirmed.
Effect of temperature on Ga2O3(Gd2O3)/GaN metal–oxide–semiconductor field-effect transistors
Authors: Ren F. , Hong M. , Chu S. N. G. , Marcus M. A. , Schurman M. J. , Baca A. , Pearton S. J. , Abernathy C. R.
Published: July 2002

Ga 2 O 3 (Gd 2 O 3 ) was deposited on GaN for use as a gate dielectric in order to fabricate a depletion metal–oxide–semiconductor field-effect transistor (MOSFET). Analysis of the effect of temperature on the device shows that gate leakage is significantly reduced at elevated temperature relative to a conventional metal–semiconductor field-effect transistor fabricated on the same GaN layer. MOSFET device operation in fact improved upon heating to 400 °C. Modeling of the effect of temperature on contact resistance suggests that the improvement is due to a reduction in the parasitic resistances present in the device.
Radio frequency plasma processing effects on the emission characteristics of a MeV electron beam cathode
Authors: Rintamaki J. I. , Gilgenbach R. M. , Cohen W. E. , Jaynes R. L. , Cuneo M. E. , Menge P. R.
Published: July 2002

Experiments have proven that surface contaminants on the cathode of an electron beam diode influence electron emission current and impedance collapse. This letter reports on an investigation to reduce parasitic cathode current loss and to increase high voltage hold off capabilities by reactive sputter cleaning of contaminants. Experiments have characterized effective radio frequency (rf) plasma processing protocols for high voltage anode–cathode (A–K) gaps using a two-stage argon/oxygen and argon rf plasma discharge. Time-resolved optical emission spectroscopy measures contaminant (hydrogen) and bulk cathode (aluminum) plasma emission versus transported axial electron beam current turn on. Experiments were performed at accelerator parameters: V=−0.7 to −1.1 MV, I(diode)=3–30 kA, and pulse length=0.4–1.0 μs. Experiments using a two-stage low power (100 W) argon/oxygen rf discharge followed by a higher power (200 W) pure argon rf discharge yielded an increase in cathode turn-on voltage required for axial current emission from 662±174 kV to 981±97 kV. The turn-on time of axial current was increased from 100±22 to 175±42 ns.
Determination of nematic polar anchoring from retardation versus voltage measurements
Authors: Nastishin Yu. A. , Polak R. D. , Shiyanovskii S. V. , Lavrentovich O. D.
Published: July 2002

The popular “high-electric-field” technique to determine the polar anchoring coefficient W of a nematic–substrate interface requires the simultaneous measurement of the capacitance and optical phase retardation of a liquid crystal cell as a function of applied voltage. We develop a generalized model that makes it possible to eliminate the capacitance measurement. The new technique, called the RV (retardation versus voltage) technique, requires only the measurement of retardation as a function of applied voltage, and allows for the determination of W by a linear fit over a prescribed voltage window. The technique is not sensitive to uniformity of the cell thickness, does not require patterned electrodes, and allows for the local probe of the surface. The value of W obtained by the RV technique is the same as W obtained by the traditional technique.
AgGaS
2
optical parametric oscillator continuously tunable from 3.9 to 11.3 μm
Authors: Vodopyanov K. L. , Maffetone J. P. , Zwieback I. , Ruderman W.
Published: July 2002

This is the experimental realization of a silver gallium sulfide (AgGaS2) optical parametric oscillator (OPO) with a wide mid-IR tuning range. The singly resonant angle-tuned AgGaS2 type-II OPO was pumped by 1.06 μm pulses from a nanosecond Nd:YAG laser and yielded idler wave continuously tunable from 3.9 to 11.3 μm with a linewidth of 1 cm−1. The OPO threshold was 0.03 J/cm2 corresponding to sub-MW/cm2 pump intensity and sub-100 μJ pump energy. The slope and absolute quantum conversion efficiencies reached 41% and 22%, respectively.
Nanofabrication using a stencil mask
Authors: Deshmukh Mandar M. , Ralph D. C. , Thomas M. , Silcox J.
Published: July 2002

We describe tests of a technique to fabricate nanostructures by the evaporation of metal through a stencil mask etched in a suspended silicon nitride membrane. Collimated evaporation through the mask gives metal dots less than 15 nm in diameter and lines 15–20 nm wide. We have investigated the extent of hole clogging and the factors which determine the ultimate resolution of the technique.
Laser patterned arrays of interfaces in magnetoresistive La2/3Sr1/3MnO3 thin films
Authors: Bibes M. , Martı́nez B. , Fontcuberta J. , Trtik V. , Benı́tez F. , Sánchez F. , Varela M.
Published: July 2002

With the aim of developing versatile and low-cost technologies for interface engineering in manganites, we report on the fabrication of arrays of interfaces on epitaxial La2/3Sr1/3MnO3 thin films. Interfaces in the films are induced by laser patterning of monocrystalline SrTiO3 substrates. These interfaces are responsible for the low-field tunnel magnetoresistance. The magnetotransport properties, of the upper-grown La2/3Sr1/3MnO3 films, are explored as a function of the characteristics of the interface network. We show that substantial and controllable magnetoresistance values can be obtained.
Role of substrate on the dielectric and piezoelectric behavior of epitaxial lead magnesium niobate-lead titanate relaxor thin films
Authors: Nagarajan V. , Alpay S. P. , Ganpule C. S. , Nagaraj B. K. , Aggarwal S. , Williams E. D. , Roytburd A. L. , Ramesh R.
Published: July 2002

The effect of various substrates on the electrical and electromechanical properties of 100-nm-thick epitaxial 0.9[Pb(Mg1/3Nb2/3)O3]–0.1[PbTiO3](0.9PMN–0.1PT) thin films is investigated. (001) 0.9PMN–0.1PT films are grown on (001)LaAlO3(LAO), (La, Sr)(Al, Ta)O3(LSAT), SrTiO3(STO), and MgO substrates with 40-nm-thick top and bottom La0.5Sr0.5CoO3 electrodes by pulsed laser deposition. X-ray diffraction results indicate that the films on LAO, LSAT, and STO are stressed biaxially in compression in the film-substrate interface whereas the films on MgO are stressed in tension. A decrease in the temperature of dielectric maximum (Tm) together with an increase in the dielectric constant and the longitudinal piezomodulus is observed with decreasing in-plane epitaxial stresses for LAO, LSAT, and STO substrates. The films on MgO substrates have the highest dielectric constant and piezomodulus with Tm below room temperature. The variation in Tm may be attributed to the shift in the transformation temperature from the paraelectric state to the relaxor state due to internal stresses in the film-substrate interface. Electrical and electromechanical properties should depend strongly on internal stresses in the vicinity of the phase transformation, which is reflected in our experimental observations.
The reaction of benzene with a ground state carbon atom, C(3Pj)
Authors: Bettinger Holger F. , Schleyer Paul v. R. , Schaefer Henry F. , Schreiner Peter R. , Kaiser Ralf I. , Lee Yuan T.
Published: July 2002

The reaction between benzene and a single ground state carbon atom, C(3Pj), which yields a C7H5 radical without a barrier in the exit channel, has been studied using density functional theory (B3LYP), Møller–Plesset perturbation theory, and the G2(B3LYP/MP2) and complete basis set (CBS) model chemistries. Comparing the computed reaction energies for the formation of various C7H5 radicals with experimental data suggests that the 1,2-didehydrocycloheptatrienyl radical (15) is observed in crossed-beams experiments at collision energies between 2 and 12 kcal mol−1. The carbon atom attacks the π-electron density of benzene and forms without entrance barrier a Cs symmetric complex (17T) in which the carbon atom is bound to the edge of benzene. From 17T, the insertion of the C atom into a benzene CC bond to yield triplet cycloheptatrienylidene (9T) is associated with a much lower barrier than the insertion into a CH bond to give triplet phenylcarbene (7T). As both steps are strongly exothermic, high energy vinyl carbene rearrangements on the triplet C7H6 potential energy surface provide pathways between 9T and 7T below the energy of separated reactants. In addition, intersystem crossing in the vicinity of 17T and 9T might give rise to singlet cycloheptatetraene (12S). The monocyclic seven-membered ring compounds 9T or 12S are precursors of the 1,2-didehydrocycloheptatrienyl radical: the dissociation of a CH bond α to the divalent carbon atom proceeds without an exit barrier, in agreement with experiment. In contrast, a direct carbon–hydrogen exchange reaction pathway analogous to the aromatic electrophilic substitution followed by rearrangement of phenylcarbyne (13) to 15 involves high barriers (39 kcal mol−1 with respect to separated reactants) and is thus not viable under the experimental conditions.
High resolution molecular beam study of the origin bands of the Ã 2Π–X̃ 2Σ+ and Ã″ 2Π1/2–X̃ 2Σ+ systems of yttrium imide (Y14NH, Y15NH, and Y14ND)
Authors: Jakubek Zygmunt J. , Simard Benoit , Niki Hideaki , Balfour Walter J.
Published: July 2002

The (0,0,0)–(0,0,0) bands of the Ã 2Π–X̃ 2Σ+ and Ã″ 2Π1/2–X̃ 2Σ+ systems of three isotopomers of yttrium imide (Y14NH, Y15NH, and Y14ND) have been studied by laser-induced fluorescence in a molecular beam apparatus. Rotational, fine, and nuclear magnetic hyperfine structures have been resolved and analyzed. The previously studied B̃ 2Σ+−X̃ 2Σ+ (0,0,0)–(0,0,0) bands of the three isotopomers have been reanalyzed. Global fits of all observed bands, in which the ground state has been fitted to a Hamiltonian model, while the excited states have been represented by term values, have been performed for the three isotopomers. Subsequently, the individual bands have been fitted. The ground state parameters have been fixed at the values obtained in the global fits, while the upper states have been fitted to the Hamiltonian models. The (0,0,0) Ã 2Π state of Y14NH, Y15NH, and Y14ND is severely perturbed. Even though the nature of these perturbing states can only be speculated upon, the introduction of effective perturbers made it possible to deperturb the state successfully. The Ã″ 2Π1/2 state is unperturbed. The spectra can be reproduced to better than 120 MHz (0.004 cm−1). The analyses confirm that the molecule is linear in all four states with the nuclear arrangement Y–N–H. The bond lengths (r0 structure) in the X̃ 2Σ+ ground state and the Ã″ 2Π1/2, Ã 2Π, and B̃ 2Σ+ excited states have been derived to be rYN=0.187785(17) nm, rNH=0.10039(14) nm; rYN=0.1927(1) nm, rNH=0.081(1) nm; rYN=0.19013(56) nm, rNH=0.1032(54) nm; and rYN=0.18848(52) nm, rNH=0.1236(46) nm, respectively. The electronic configurations for the X̃ 2Σ+ ground state and the Ã 2Π, Ã″ 2Π1/2, and B̃ 2Σ+ excited states are discussed and compared with ab initio calculations whenever possible.
Toward the limits of predictive electronic structure theory: Connected quadruple excitations for large basis set calculations
Authors: Kucharski Stanisl/aw A. , Kolaski Maciej , Bartlett Rodney J.
Published: July 2002

The general inclusion of the T4 operator into the coupled cluster equations requires an n10 computational procedure, and n9 in the lowest order, as in the CCSDTQ-1 (coupled cluster singles, doubles, triples, and lowest order quadruples) method. Coupled cluster methods with full inclusion of singles, doubles, triples, and an efficient noniterative inclusion of connected quadruples (CCSDT(Qf)) have been introduced in [J. Chem. Phys. 108, 9221 (1998)]. Since the connected quadruple part in the latter method scales as n7 (CCSDT itself is n8) it offers an attractive route to assess the connected quadruple contribution for larger basis sets. We present a detailed description of the Qf algorithm with explicit algebraic formulas for the spin–orbital formalism as well as for a nonorthogonal spin adapted approach. The method has been applied to obtain the equilibrium geometry and harmonic frequencies for the C2 molecule for a sequence of correlation consistent polarized (core) valence (cc-p(C)VXZ, X=D,T,Q,5) basis sets. For the largest basis sets, cc-pCVQZ and cc-pV5Z, the connected quadruple excitations lower the harmonic frequency by 10 cm−1 and raise the bond length by 0.0014 Å, providing results that agree with experiment to 3 cm−1 and 0.0003 Å.
Triplet excitation energies in the coupled cluster singles and doubles model using an explicit triplet spin coupled excitation space
Authors: Hald Kasper , Hättig Christof , Jørgensen Poul
Published: July 2002

Triplet excitation energies are calculated from the response eigenvalue equation for the coupled cluster singles and doubles (CCSD) model using an integral direct approach and an explicit spin coupled triplet excitation space. The cost of one linear transformation for the triplet excitation energy is about two times the cost of one linear transformation for the singlet excitation energy. The triplet excitation spectrum of benzene is calculated using from 147 to 432 basis functions. The calculated triplet excitation energies are compared with experimental and other theoretical values.
Quasiperiodic and periodic solutions for vector nonlinear Schrödinger equations
Authors: Eilbeck J. C. , Enolskii V. Z. , Kostov N. A.
Published: July 2002

We consider quasiperiodic and periodic (cnoidal) wave solutions of a set of n-component vector nonlinear Schrödinger equations (VNLSEs). In a biased photorefractive crystal with a drift mechanism of nonlinear response and Kerr-type nonlinearity, n-component nonlinear Schrödinger equations can be used to model self-trapped mutually incoherent wave packets. These equations also model pulse–pulse interactions in wavelength-division-multiplexed channels of optical fiber transmission systems. Quasiperiodic wave solutions for the VNLSEs in terms of n-dimensional Kleinian functions are presented. Periodic solutions in terms of Hermite polynomials and generalized Hermite polynomials for n-component nonlinear Schrödinger equations are found.
Structural distortion and ferroelectric properties of SrBi2(Ta1−xNbx)2O9
Authors: Shimakawa Y. , Kubo Y. , Tauchi Y. , Kamiyama T. , Asano H. , Izumi F.
Published: July 2002

Ferroelectric materials of the SrBi2(Ta1−xNbx)2O9 solid-solution system were synthesized, and their structural and ferroelectric properties were investigated. Atomic displacements of the ions in the (Ta,Nb)O6 octahedron significantly increase as x increases, which leads to more structural distortion of the perovskite-type unit. The Bi2O2 layer, in contrast, is less distorted in SrBi2Nb2O9 than in SrBi2Ta2O9. The contribution of the perovskite-type unit to total ferroelectric polarization is greater in the SrBi2Nb2O9 sample, while that of the Bi2O2 layer is less; consequently, the total calculated polarization slightly increases. The ferroelectric Curie temperature also increases from 300 (SrBi2Ta2O9) to 440 °C (SrBi2Nb2O9). Three short (Ta,Nb)–O bonds in the (Ta,Nb)O6 octahedron, whose lengths are less than 2 Å, have a covalent character, and the substitution of Nb for Ta makes the bonds more covalent. The strong covalent interaction of the (Ta,Nb)–O bonds increases the structural distortion, resulting in the higher ferroelectric Curie temperature and the larger contribution of the perovskite-type unit to the total spontaneous ferroelectric polarization.
Linear bundles of Lie algebras and their applications
Authors: Yanovski A. B.
Published: July 2002

We consider the algebraic properties of families of Lie brackets, also called linear bundles of Lie algebras, and especially ones that can be defined over the classical algebras. Some applications to the theory of the integrable systems are given as such bundles naturally define compatible Poisson–Lie (Kirillov) tensors.
Growth and emission characteristics of vertically well-aligned carbon nanotubes grown on glass substrate by hot filament plasma-enhanced chemical vapor deposition
Authors: Han Jae-hee , Yang Won-Suk , Yoo Ji-Beom , Park Chong-Yun
Published: July 2002

Vertically well-aligned multiwall carbon nanotubes (MWNTs) were grown on nickel-coated glass substrates by plasma-enhanced chemical vapor deposition at low temperatures, below 600 °C, with and without hot filament. Acetylene and ammonia gas were used as the carbon source and a catalyst. Effects of growth parameters, such as plasma intensity, filament current, and substrate temperature, on the growth characteristics of MWNTs were investigated. Plasma intensity was found to be the most critical parameter controlling the growth of MWNTs. Field emission from the MWNTs was obtained using a phosphor anode with an onset electric field of 1.5 V/μm.
Effect of confinement on chemical reaction equilibria: The reactions 2NO⇔(NO)2 and N2+3H2⇔2NH3 in carbon micropores
Authors: Turner C. Heath , Johnson J. Karl , Gubbins Keith E.
Published: July 2002

We report reactive Monte Carlo (RMC) simulations of reaction equilibria for both the nitric oxide dimerization and the ammonia synthesis reactions. We have applied the RMC technique to both a single bulk phase and also to a two-phase system, composed of the bulk gas and a slit-shaped pore, with pore parameters chosen to model activated carbon fibers. We achieve close agreement with the experimentally measured conversions of nitric oxide and ammonia in the bulk phase. Both reactions involve a stoichiometric decrease in mole number, which should cause the yield of each to be enhanced by the increased density within the pore phase. We show that the effect of confinement on the yield of both reactions is significant, and is particularly dramatic for the nitric oxide reaction; in addition, the ammonia synthesis reaction is affected by the selective adsorption of nitrogen over hydrogen in the pore under certain conditions.
Antiferromagnetically coupled magnetic media layers for thermally stable high-density recording
Authors: Fullerton Eric E. , Margulies D. T. , Schabes M. E. , Carey M. , Gurney B. , Moser A. , Best M. , Zeltzer G. , Rubin K. , Rosen H. , Doerner M.
Published: July 2002

We describe a magnetic recording media composed of antiferromagnetically coupled (AFC) magnetic recording layers as an approach to extend areal densities of longitudinal media beyond the predicted superparamagnetic limit. The recording medium is made up of two ferromagnetic layers separated by a nonmagnetic layer whose thickness is tuned to couple the layers antiferromagnetically. For such a structure, the effective areal moment density (Mrt) of the composite structure is the difference between the ferromagnetic layers allowing the effective magnetic thickness to scale independently of the physical thickness of the media. Experimental realizations of AFC media demonstrate that thermally stable, low-Mrt media suitable for high-density recording can be achieved.
Cathodoluminescence depth profiling of ion-implanted GaN
Authors: Kucheyev S. O. , Toth M. , Phillips M. R. , Williams J. S. , Jagadish C. , Li G.
Published: July 2002

Cathodoluminescence (CL) spectroscopy shows that even relatively low-dose keV light-ion bombardment (corresponding to the generation of ∼5×1019 vacancies/cm3) of wurtzite GaN results in a dramatic quenching of visible CL emission. Postimplantation annealing at temperatures up to 1050 °C generally causes a partial recovery of measured CL intensities. However, CL depth profiles indicate that, in most cases, such a recovery results from CL emission from virgin GaN, beyond the implanted layer due to a reduction in the extent of light absorption within the implanted layer. In this case, CL emission from the implanted layer remains completely quenched even after such an annealing. These results show that an understanding of the effects of ion bombardment and postimplantation annealing on luminescence generation and light absorption is required for a correct interpretation of luminescence spectra of GaN optically doped by keV ion implantation.
X-ray diffraction study of undercooled molten silicon
Authors: Kimura Hidekazu , Watanabe Masahito , Izumi Koichi , Hibiya Taketoshi , Holland-Moritz Dirk , Schenk Thomas , Bauchspieß Karl Rudolf , Schneider Stephan , Egry Ivan , Funakoshi Kenichi , Hanfland Michael
Published: July 2002

The short-range order of molten silicon was investigated in a wide temperature range from 1893 K down to 1403 K, corresponding to an undercooling of 290 K. Energy-dispersive x-ray diffraction was used in combination with electromagnetic levitation. The structure factor and the pair correlation function were determined as a function of temperature from the experimental data. A small hump on the higher wave vector side of the first peak in the structure factor was observed at all temperatures. The position of the first peak in the pair distribution function shifted to shorter distances and its height increased gradually with decreasing temperature. No discontinuous behavior was observed in the entire temperature range investigated.
A closer look at crystallization of parallel hard cubes
Authors: Groh Benito , Mulder Bela
Published: July 2002

The model system of parallel hard cubes is studied by using fundamental measure theory (FMT) and extensive Monte Carlo simulations. A continuous freezing transition occurs in this system to which finite-size scaling analysis is applied. Significant deviations from a previous simulation study are found for the position of the critical point and for the critical exponents. Our results are compatible with the Heisenberg universality class. Moreover, both theory and simulation show that also at high densities the solid phase is thermodynamically more stable than a possible columnar phase. FMT appears quantitatively more reliable at high densities than near the critical density, which is substantially underestimated.
Effect of buffer layers and stacking faults on the reduction of threading dislocation density in GaN overlayers grown by metalorganic chemical vapor deposition
Authors: Cho Hyung Koun , Lee Jeong Yong , Kim Ki Soo , Yang Gye Mo , Song Jae Ho , Yu Phil Won
Published: July 2002

We have studied the effect of the trimethylgallium (TMGa) flow rate in the GaN buffer layer on the optical and structural quality. From low temperature photoluminescence measurements, a GaN overlayer grown on a buffer layer with the TMGa flow rate of 80 μmol/min shows the intense donor-acceptor pair transition peak at 3.27 eV and the weak yellow band emission at 2.2 eV, which are related with stacking faults and threading dislocations from transmission electron microscopy images, respectively. As the TMGa flow rate of the GaN buffer increases, the threading dislocation density rapidly decreased and stacking faults increased in the GaN overlayers. Also, a total threading dislocation density at the optimum condition of the buffer layer is the very low 1×108 cm−2, which is due to the interaction of stacking faults with the vertical threading dislocations and the bending of threading dislocations near the stacking faults. High-resolution x-ray diffraction results show that a high density of stacking faults is correlated with the compressive strain of a GaN overlayer at the growth temperature.
Work function at the tips of multiwalled carbon nanotubes
Authors: Gao Ruiping , Pan Zhengwei , Wang Zhong L.
Published: July 2002

The work function at the tips of individual multiwalled carbon nanotubes has been measured by an in situ transmission electron microscopy technique. The tip work function shows no significant dependence on the diameter of the nanotubes in the range of 14–55 nm. Majority of the nanotubes have a work function of 4.6–4.8 eV at the tips, which is 0.2–0.4 eV lower than that of carbon. A small fraction of the nanotubes have a work function of ∼5.6 eV, about 0.6 eV higher than that of carbon. This discrepancy is suggested due to the metallic and semiconductive characteristics of the nanotube.
Large remanent polarization of vanadium-doped Bi4Ti3O12
Authors: Noguchi Yuji , Miyayama Masaru
Published: July 2002

Effects of vanadium doping on the ferroelectric properties of Bi4Ti3O12 were investigated using dense ceramics. The incorporation of vanadium resulted in a large remanent polarization (2Pr) of over 40 μC/cm2 without sacrificing other physical properties, and the polarization characteristics were shown to be superior to SrBi2Ta2O9 and Sr0.8Bi2.2Ta2O9. In addition, dense ceramics of vanadium-doped Bi4Ti3O12 could be obtained by sintering at temperatures 100–200 °C lower than those for the SrBi2Ta2O9 system.
Chirality induction from chiral molecules to adsorbed monolayers
Authors: Yoneya Makoto , Yokoyama Hiroshi
Published: July 2002

Monolayer structures of a chiral mesogen, (R)- or (S)-[4′-(1-methylheptyloxy)-3′-nitrobiphenyl-4yl] 4-(trans-5-decenyloxy) benzoate, adsorbed on graphite were investigated by molecular dynamics simulations to clarify the mechanism of chirality inductions from chiral mesogens to chiral monolayers. We propose a molecular model that is based on coupling between the up-down symmetry breaking and the molecular tilt direction symmetry breaking of the monolayer. From the proposed model, we further predict alternative reversal of the two dimensional chirality of the monolayer when the chiral carbon is shifted along alkyl chain. This is a two-dimensional analogue of the “odd-even” effect on twist sense reversal in bulk chiral nematic systems.
Surface potential measurements on GaN and AlGaN/GaN heterostructures by scanning Kelvin probe microscopy
Authors: Koley G. , Spencer M. G.
Published: July 2002

Surface potentials on GaN epilayers and Al0.35Ga0.65N/GaN heterostructures have been studied by scanning Kelvin probe microscopy (SKPM) in conjunction with noncontact atomic force microscopy. The dependence of the surface potential on doping in GaN films, as well as the variation of surface potential with Al0.35Ga0.65N barrier layer thickness has been investigated. The bare surface barrier height (BSBH), as measured by SKPM, is observed to decrease from ∼1. 40±0.1 eV to ∼0.60±0.1 eV with increasing doping in the GaN epilayers. Schottky barrier height calculated from the measurements of BSBH on n-GaN agrees very well with results from previous studies. We have also estimated the surface state density for GaN based on the measured values of BSBH. The semiconductor “work function” at the Al0.35Ga0.65N surface (in heterostructure samples) is observed to decrease by ∼0.60 eV with increase in barrier layer thickness from ∼50 to ∼440 Å. A simple model considering the presence of a uniform density of charged acceptors in the Al0.35Ga0.65N layer is proposed to explain the observed decreasing trend in work function.
On the combination of a linear field free trap with a time-of-flight mass spectrometer
Authors: Luca Alfonz , Schlemmer Stephan , Čermák Ivo , Gerlich Dieter
Published: July 2002

A new instrument has been developed which combines a rf ring electrode trap and a time-of-flight mass spectrometer (TOF-MS). The wide field free storage volume of such a trap enables the study of low temperature ion-molecule collisions; however it is not straightforward to match the nonlocalized ion cloud to the TOF-MS. For obtaining sufficient mass resolution, a special pulse sequence has been developed to transfer the ions from the whole trap volume to a small region in the vicinity of the exit electrode. Additional compression is achieved via buffer gas relaxation prior to extracting the ions. Using a linear flight path of 57 cm, a mass resolution of about 50 is routinely achieved. The mass range of the whole instrument, which is determined by the operating conditions both of the trap and the TOF-MS, has been estimated to be 3–700 u. The actual characteristics of the instrument such as mass range, resolution, and dynamical range have been determined and the results have been analyzed. As a typical application of the new instrument, the growth of (CO)n+ cluster ions is investigated at 80 K. The simultaneous detection of all masses of interest as a function of storage time allows one to follow in detail the kinetics of the reaction and loss processes involved. Limitations of the method are discussed together with ways to overcome them in an improved setup.
Static and dynamic properties of stretched water
Authors: Netz Paulo A. , Starr Francis W. , Stanley H. Eugene , Barbosa Marcia C.
Published: July 2002

We present the results of molecular dynamics simulations of the extended simple point charge model of water to investigate the thermodynamic and dynamic properties of stretched and supercooled water. We locate the liquid–gas spinodal, and confirm that the spinodal pressure increases monotonically with T, supporting thermodynamic scenarios for the phase behavior of supercooled water involving a “non-reentrant” spinodal. The dynamics at negative pressure show a minimum in the diffusion constant D when the density is decreased at constant temperature, complementary to the known maximum of D at higher pressures. We locate the loci of minima of D relative to the spinodal, showing that the locus is inside the thermodynamically metastable regions of the phase diagram. These dynamical results reflect the initial enhancement and subsequent breakdown of the tetrahedral structure and of the hydrogen bond network as the density decreases.
Noncontact monitoring of surface-wave nonlinearity for predicting the remaining life of fatigued steels
Authors: Ogi Hirotsugu , Hirao Masahiko , Aoki Shinji
Published: July 2002

A nonlinear acoustic measurement is studied for fatigue damage monitoring. An electromagnetic acoustic transducer (EMAT) magnetostrictively couples to a surface-shear-wave resonance along the circumference of a rod specimen during rotating bending fatigue of carbon steels. Excitation of the EMAT at half of the resonance frequency caused the standing wave to contain only the second-harmonic component, which was received by the same EMAT to determine the second-harmonic amplitude. Thus measured surface-wave nonlinearity always showed two distinct peaks at 60% and 85% of the total life. We attribute the earlier peak to crack nucleation and growth, and the later peak to an increase of free dislocations associated with crack extension in the final stage. This noncontact resonance-EMAT measurement can monitor the evolution of the surface-shear-wave nonlinearity throughout the metal’s fatigue life and detect the pertinent precursors of the eventual failure.
Low-threshold optically pumped λ=4.4 μm vertical-cavity surface-emitting laser with a PbSe quantum-well active region
Authors: Felix C. L. , Bewley W. W. , Vurgaftman I. , Lindle J. R. , Meyer J. R. , Wu H. Z. , Xu G. , Khosravani S. , Shi Z.
Published: July 2002

We report pulsed emission from an optically pumped lead-salt vertical-cavity surface-emitting laser with a PbSe/PbSrSe quantum-well active region. The lasing wavelength of λ=4.44 μm is nearly constant over the temperature range 200–280 K, and the threshold is only 10.5 kW/cm2 at 260 K, where the gain peak and cavity mode are in resonance. Over 330 mW of peak power is emitted at 260 K, into a circular beam whose divergence angle increases with pump intensity.
A robust approach to protein foldability measures based on spin-glass models
Authors: Roy Tapon
Published: July 2002

Spin-glass models and related methods have been applied to protein folding problems, often by assuming an underlying Gaussian distribution for the energy level distribution. In this paper, we derive robust foldability measures that relax the Gaussian distribution assumption implicit in current foldability and energy gap measures.
Magnetic and electric properties of transition-metal-doped ZnO films
Authors: Ueda Kenji , Tabata Hitoshi , Kawai Tomoji
Published: July 2002

3d-transition-metal-doped ZnO films (n-type Zn1−xMxO (x=0.05–0.25): M=Co, Mn, Cr, Ni) are formed on sapphire substrates using a pulsed-laser deposition technique, and their magnetic and electric properties are examined. The Co-doped ZnO films showed the maximum solubility limit. Some of the Co-doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than room temperature. The magnetic properties of Co-doped ZnO films depend on the concentration of Co ions and carriers.
Ultrathin (<4 nm) SiO2 and Si–O–N gate dielectric layers for silicon microelectronics: Understanding the processing, structure, and physical and electrical limits
Authors: Green M. L. , Gusev E. P. , Degraeve R. , Garfunkel E. L.
Published: July 2002

The outstanding properties of SiO2, which include high resistivity, excellent dielectric strength, a large band gap, a high melting point, and a native, low defect density interface with Si, are in large part responsible for enabling the microelectronics revolution. The Si/SiO2 interface, which forms the heart of the modern metal–oxide–semiconductor field effect transistor, the building block of the integrated circuit, is arguably the worlds most economically and technologically important materials interface. This article summarizes recent progress and current scientific understanding of ultrathin (<4 nm) SiO2 and Si–O–N (silicon oxynitride) gate dielectrics on Si based devices. We will emphasize an understanding of the limits of these gate dielectrics, i.e., how their continuously shrinking thickness, dictated by integrated circuit device scaling, results in physical and electrical property changes that impose limits on their usefulness. We observe, in conclusion, that although Si microelectronic devices will be manufactured with SiO2 and Si–O–N for the foreseeable future, continued scaling of integrated circuit devices, essentially the continued adherence to Moore’s law, will necessitate the introduction of an alternate gate dielectric once the SiO2 gate dielectric thickness approaches ∼1.2 nm. It is hoped that this article will prove useful to members of the silicon microelectronics community, newcomers to the gate dielectrics field, practitioners in allied fields, and graduate students. Parts of this article have been adapted from earlier articles by the authors [L. Feldman, E. P. Gusev, and E. Garfunkel, in Fundamental Aspects of Ultrathin Dielectrics on Si-based Devices, edited by E. Garfunkel, E. P. Gusev, and A. Y. Vul’ (Kluwer, Dordrecht, 1998), p. 1 [Ref. 1]; E. P. Gusev, H. C. Lu, E. Garfunkel, T. Gustafsson, and M. Green, IBM J. Res. Dev. 43, 265 (1999) [Ref. 2]; R. Degraeve, B. Kaczer, and G. Groeseneken, Microelectron. Reliab. 39, 1445 (1999) [Ref. 3].
Temperature-dependent measurement of Auger recombination in self-organized In0.4Ga0.6As/GaAs quantum dots
Authors: Ghosh S. , Bhattacharya P. , Stoner E. , Singh J. , Jiang H. , Nuttinck S. , Laskar J.
Published: July 2002

We report experimental studies of temperature-dependent Auger recombination coefficients in self-assembled quantum dots. The results are based on a study of temperature-dependent large signal modulation experiments made on self-organized In0.4Ga0.6As/GaAs quantum dot lasers. The Auger coefficient decreases from ∼8×10−29 cm6/s at 100 K to ∼4×10−29 cm6/s at 300 K. This behavior, which is different from results in other higher-dimensional systems, is explained in terms of the temperature dependence of electron-hole scattering in the dots and contribution from higher lying states in the dot and adjoining layers.
Two-phase modeling of deflagration-to-detonation transition in granular materials: Reduced equations
Authors: Kapila A. K. , Menikoff R. , Bdzil J. B. , Son S. F. , Stewart D. S.
Published: July 2002

Of the two-phase mixture models used to study deflagration-to-detonation transition in granular explosives, the Baer–Nunziato model is the most highly developed. It allows for unequal phase velocities and phase pressures, and includes source terms for drag and compaction that strive to erase velocity and pressure disequilibria. Since typical time scales associated with the equilibrating processes are small, source terms are stiff. This stiffness motivates the present work where we derive two reduced models in sequence, one with a single velocity and the other with both a single velocity and a single pressure. These reductions constitute outer solutions in the sense of matched asymptotic expansions, with the corresponding inner layers being just the partly dispersed shocks of the full model. The reduced models are hyperbolic and are mechanically as well as thermodynamically consistent with the parent model. However, they cannot be expressed in conservation form and hence require a regularization in order to fully specify the jump conditions across shock waves. Analysis of the inner layers of the full model provides one such regularization [Kapila et al., Phys. Fluids 9, 3885 (1997)], although other choices are also possible. Dissipation associated with degrees of freedom that have been eliminated is restricted to the thin layers and is accounted for by the jump conditions.
Reconstruction of frozen-core all-electron orbitals from pseudo-orbitals
Authors: Hetényi Balázs , De Angelis Filippo , Giannozzi Paolo , Car Roberto
Published: July 2002

We investigate the numerical feasibility of reconstructing frozen-core all-electron molecular orbitals from corresponding pseudo-orbitals. We perform density-functional calculations on simple atomic and molecular model systems using ultrasoft pseudopotentials to represent the atomic cores. We apply a transformation due to Blöchl [Phys. Rev. B 50, 17953 (1994)] to each calculated pseudo-orbital to obtain a corresponding frozen-core all-electron molecular orbital. Our model systems include the reconstruction of the 5d orbital of a gold atom, and the occupied valence states of the TiO2 molecule. Comparison of the resulting all-electron orbitals to corresponding ones that were obtained from calculations in which the core electrons were explicitly included indicates that all-electron molecular orbital reconstruction is a feasible and useful operation in reproducing the correct behavior of molecular orbitals in the nuclear core regions.
Variance method for the evaluation of particle size and dislocation density from x-ray Bragg peaks
Authors: Borbély A. , Groma I.
Published: July 2002

A method is presented to determine the particle size and internal strain from broadened x-ray peaks obtained on small coherent domains containing dislocations. In contrast with the evaluation techniques applied earlier, the proposed procedure requires only the measurement of a single line profile. It is based on the asymptotic behavior of the second-and fourth-order restricted moments. At the asymptotic part of the second-order restricted moment, the finite grain size causes a linear diffraction vector dependence, while that of the dislocation-induced strain is logarithmic. On the fourth-order moment, the two sources of broadening result in cubic and parabolic behavior. The method is illustrated by analyzing x-ray peaks measured on a cold-and hot-deformed AlMg alloy and on a nanocrystalline iron powder.
Statistical analysis of the sizes and velocities of laser hot spots of smoothed beams
Authors: Garnier J. , Videau L.
Published: July 2002

This paper presents a precise description of the characteristics of the hot spots of a partially coherent laser pulse. The average values of the sizes and velocities of the hot spots are computed, as well as the corresponding probability density functions. Applications to the speckle patterns generated by optical smoothing techniques for uniform irradiation in plasma physics are discussed.
Transport by intermittent convection in the boundary of the DIII-D tokamak
Authors: Boedo J. A. , Rudakov D. , Moyer R. , Krasheninnikov S. , Whyte D. , McKee G. , Tynan G. , Schaffer M. , Stangeby P. , West P. , Allen S. , Evans T. , Fonck R. , Hollmann E. , Leonard A. , Mahdavi A. , Porter G. , Tillack M. , Antar G.
Published: July 2002

Intermittent plasma objects (IPOs) featuring higher pressure than the surrounding plasma, and responsible for ∼50% of the E×BT radial transport, are observed in the scrape off layer (SOL) and edge of the DIII-D tokamak [J. Watkins et al., Rev. Sci. Instrum. 63, 4728 (1992)]. Conditional averaging reveals that the IPOs, produced at a rate of ∼3×103 s−1, are positively charged and also polarized, featuring poloidal electric fields of up to 4000 V/m. The IPOs move poloidally at speeds of up to 5000 m/s and radially with E×BT/B2 velocities of ∼2600 m/s near the last closed flux surface (LCFS), and ∼330 m/s near the wall. The IPOs slow down as they shrink in radial size from 4 cm at the LCFS to 0.5 cm near the wall. The IPOs appear in the SOL of both L and H mode discharges and are responsible for nearly 50% of the SOL radial E×B transport at all radii; however, they are highly reduced in absolute amplitude in H-mode conditions.
Thermally enhanced frequency upconversion in Nd3+-doped fluoroindate glass
Authors: Menezes L. de S. , Maciel G. S. , Araújo Cid B. de , Messaddeq Y.
Published: July 2002

Multiphonon assisted frequency upconversion was observed in a Nd3+-doped fluoroindate glass pumped at 866 nm. A near-infrared upconverted emission at 750 nm with a peculiar linear dependence with the laser intensity was observed and explained. The intensity of the upconverted emission experienced a 40-fold enhancement when the sample’s temperature was varied from 298 to 498 K. A rate equation model that includes light pumping and multiphonon absorption via thermally coupled electronic excited states of Nd3+ was used, describing quite well the experimental results.
Three-stack, three-color quantum-well infrared photodetector for mid-, long-, and very long-wavelength infrared detection
Authors: Jiang Lin , Li Sheng S. , Tidrow M. Z. , Dyer W. R. , Liu W. K. , Fastenau J. M. , Yurasits T. R.
Published: July 2002

A high-performance three-stack, three-color quantum-well infrared photodetector for the mid-, long-, and very long-wavelength infrared detection has been developed in this work. The detection bandwidths with full-width at half-maximum are 5.9–7.0, 9.1–11.2, and 12.2–16.9 μm, which cover the major portion or entire range of the three atmospheric blocking bands: the water band (5.5–7.5 μm), the ozone band (9.4–9.9 μm), and the carbon dioxide band (14–16 μm). Values of the dark-current detectivity were found to be D*=8.5×1012 cm Hz1/2/W at T=40 K, Vb=−1.3 V, and λp=6.5 μm for the mid-wavelength infrared stack; D*=1.5×1010 cm Hz1/2/W at T=40 K, Vb=−1.4 V, and λp=10.1 μm for the long-wavelength infrared stack, and D*=1.2×1011 cm Hz1/2/W at T=30 K, Vb=−1.1 V, and λp=15.1 μm for the very long-wavelength infrared stack.
Generalized analysis of quasi-steady-state and transient decay open circuit voltage measurements
Authors: Kerr Mark J. , Cuevas Andres , Sinton Ronald A.
Published: July 2002

The current–voltage characteristics of solar cells and photodiodes can be determined by measuring the open-circuit voltage as a function of a slowly varying light intensity. This article presents a detailed theoretical analysis and interpretation of such quasi-steady-state Voc measurements (QssVoc). The ability of this analysis to accurately obtain the true steady-state device characteristics even in the case of high lifetime, high resistivity silicon devices is demonstrated experimentally. The QssVoc technique can be used to determine the minority carrier lifetime, and the new generalized analysis is required to do this accurately. An important outcome is that solar cell and diode device characteristics can be obtained from measurements of either the photoconductance or the open-circuit voltage, even using transient techniques.
Magnetic studies of polymer-coated Fe nanoparticles synthesized by microwave plasma polymerization
Authors: Srikanth H. , Hajndl R. , Chirinos C. , Sanders J. , Sampath A. , Sudarshan T. S.
Published: July 2002

Static and dynamic magnetic experiments were conducted on polymer-coated Fe nanoparticles synthesized by a microwave plasma method. Transmission electron microscopy indicates the formation of 15–20 nm particles and agglomerated clusters. The field-dependent magnetization (M–H) data show the presence of sharp switching at low fields (∼60 Oe) followed by a gradual approach to saturation. A saturation magnetization of Ms∼80 emu/g is obtained for the nanopowder. Dynamic transverse susceptibility using a radio-frequency resonant method provides evidence for sharp switching fields and a broad distribution in anisotropy fields. These results indicate that the polymer coating plays an effective role in the underlying magnetic response of the particles.