bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectral broadening measurements in poly(phenylene vinylene) polymer channel waveguides
Authors: Gabler Th. , Waldhäusl R. , Bräuer A. , Michelotti F. , Hörhold H.-H. , Bartuch U.
Published: July 2002

In a polyconjugated main chain polymer strip waveguide the nonlinear, nonresonant refractive index n2 was measured by monitoring the signal spectrum broadening due to self-phase modulation. The two photon absorption coefficient α2 was obtained by calibrating the inverse transmission measurement. The nonlinear coefficients were determined to be n2=0.85×10−14 cm−2/W andα2=0.08 cm/GW at a wavelength λ=885.6 nm. The used polymer was poly[1,4-phenylene1,2-di(phenoxyphenyl)vinylene]. The result is in good agreement with interferometer measurements. The material is suitable for all-optical switching.
Origin of direct current drift in electro-optic polymer modulator
Authors: Park Heuk , Hwang Wol-Yon , Kim Jang-Joo
Published: July 2002

Characteristics of the dc drift phenomenon in electro-optic polymer modulators have been analyzed. It is found that the dc drift originates from the difference between the dielectric relaxation times of the guiding and cladding layers. The dc drift is accelerated as the intensity of the guided light increases. Furthermore, it becomes faster when the device is exposed to visible light. The effect of the guided light and visible light on the dc drift is interpreted in terms of the photoconductivity of the guiding layer.
Strain-induced island scaling during Si1−xGex heteroepitaxy
Authors: Dorsch W. , Strunk H. P. , Wawra H. , Wagner G. , Groenen J. , Carles R.
Published: July 2002

We investigated the composition-dependent size of pseudomorphic Si1−xGex islands on Si(001). Si1−xGex layers with 0.05⩽x⩽0.54 were deposited from metallic solution. The island growth occurs near thermodynamic equilibrium and facilitates a comparison of the results with predictions based on energetics. We find pseudomorphic islands with base widths ranging from several μm to a few nm. We show that it is possible to adjust the island size by simply choosing the appropriate layer composition. Varying deposition temperatures and growth velocities do not affect the scaling behavior.
Stacked high-ε gate dielectric for gigascale integration of metal–oxide–semiconductor technologies
Authors: Roy P. K. , Kizilyalli I. C.
Published: July 2002

Advances in lithography and thinner SiO2 gate oxides have enabled the scaling of metal–oxide–semiconductor technologies to sub-0.25 μm feature size. A major hurdle in the gate dielectric scaling using conventional thermally grown SiO2 has been excessive tunneling that occurs in ultrathin (<25 Å) SiO2. High dielectric constant materials such as Ta2O5 have been suggested as a substitute for SiO2. However, these materials have high concentrations of bulk fixed charge, unacceptable levels of Si–Ta2O5 interface trap states, and low silicon interface carrier mobilities. This letter summerizes an elegant solution to these issues through synthesis of a thermally grown SiO2(15 Å)–Ta2O5(30 Å)–SiO2(5–10 Å) dielectric with improvements in leakage, tunneling, charge trapping behavior, and interface substructure. Transistors fabricated using this stacked gate dielectric exhibit excellent subthreshold, saturation, and drive currents.
Materials integration of gallium arsenide and silicon by wafer bonding
Authors: Kopperschmidt P. , Senz S. , Kästner G. , Hesse D. , Gösele U. M.
Published: July 2002

We present a technique for the fabrication of materials integration of (100) silicon and (100) gallium arsenide by direct wafer bonding. GaAs wafers 3 in. in diameter were hydrophobically bonded to commercially available 3 in. silicon-on-sapphire wafers at room temperature. After successive annealings in hydrogen and arsenic atmospheres at temperatures up to 850 °C the Si/GaAs interfacial energy was increased by the formation of strong covalent bonds. Due to the difference in the lattice constants of about 4.1%, extra Si lattice planes were observed at the interface. No threading dislocations were introduced into the GaAs.
Preparation of size-classified PbS nanoparticles in the gas phase
Authors: Kruis F. Einar , Nielsch Kornelius , Fissan Heinz , Rellinghaus Bernd , Wassermann Eberhard F.
Published: July 2002

We report on the synthesis of size-classified PbS nanocrystals by differential mobility analysis of a polydisperse aerosol formed by nucleation and aggregation processes in a furnace reactor. The sublimation temperature employed is below the stoichiometric evaporation temperature, thus enabling direct and simple synthesis by sublimation. The irregularly shaped and amorphous agglomerate particles are then sintered in a second furnace, resulting in the formation of monocrystalline and quasispherical particles with sizes adjustable between 3 and 20 nm and a standard deviation of 1.13.
Localized excitonic emissions of ZnCdSe/ZnSe quantum wells grown on a GaAs(110) cleaved surface
Authors: Ko Hyun-Chul , Park Doo-Cheol , Kawakami Yoichi , Fujita Shizuo , Fujita Shigeo , Kim Yang-Soo
Published: July 2002

Anomalous optical properties of Zn1−xCdxSe/ZnSe strained-layer single quantum wells (SQWs) fabricated on cleaved GaAs(110) surfaces in ultrahigh vacuum by molecular beam epitaxy have been investigated. From the temperature-dependent photoluminescence measurement, the origins of localized excitonic emissions of two kinds of SQW structures with different Cd compositions are suggested. Localization centers of SQWs with low Cd composition (14%) are mainly originated by the well thickness fluctuation. However, SQWs with high Cd composition (27%) showed localized excitonic emissions at low temperature due to the Cd composition fluctuations rather than the lack of thickness uniformity of the well layer. Estimated averaged depths of localization of excitons by model calculations are 9 and 14 meV for the wells with 14% and 27% Cd compositions, respectively.
Gallium nitride whiskers formed by selective photoenhanced wet etching of dislocations
Authors: Youtsey C. , Romano L. T. , Adesida I.
Published: July 2002

Gallium nitride is used to fabricate high brightness blue and green light-emitting diodes in spite of high densities of extended structural defects. We describe a photoelectrochemical etching process that reveals the dislocation microstructure of n-type GaN films by selectively removing material between dislocations. The GaN whiskers formed by the etching have diameters between 10 and 50 nm and lengths of up to 1 μm. A correlation between the etched features and threading dislocations in the unetched film is confirmed through transmission electron microscopy studies. The whisker formation is believed to be indicative of electrical activity at dislocations in GaN.
Direct micropatterning of Si and GaAs using electrochemical development of focused ion beam implants
Authors: Schmuki P. , Erickson L. E.
Published: July 2002

Focused ion beam implantation of Si++ was used to write defined surface damage/implant patterns into n-type GaAs (100) and Si (100) substrates. These implant sites represent initiation sites for dissolution processes when electrochemically polarized in HCl or HF electrolytes, respectively. Selective dissolution within the patterns is achieved if anodic polarization of the n-type material is carried out in the dark at potentials below (cathodic to) the onset of dissolution potential of the unimplanted surface. Uniform etching within the implanted region takes place, when local electropolishing conditions are established. Thus, highly defined etch patterns, e.g., lines, gratings, or pits, can be produced in the submicron range. The depth of the etched patterns corresponds to the implant/damage profile created in the implantation process and etch stop occurs at less reactive crystal planes.
Single-wall carbon nanotubes synthesized by laser ablation in a nitrogen atmosphere
Authors: Zhang Y. , Gu H. , Iijima S.
Published: July 2002

The products of laser ablation of graphite in a nitrogen atmosphere have been studied by transmission electron microscopy and electron energy-loss spectroscopy. Using nickel and cobalt as catalyst, single-wall carbon nanotubes can be produced with a yield similar to that obtained in an inert gas. Nitrogen has not been found in the nanotubes and crystallized fullerenes although it has been detected in amorphous carbon. A growth model has been proposed to explain the experimental results.
Longitudinal magnetoresistance of CrO2 thin films
Authors: Suzuki Katsuhiko , Tedrow P. M.
Published: July 2002

The longitudinal magnetoresistance (LMR) of thin films of the putative half-metallic ferromagnet CrO2 deposited by chemical vapor deposition onto TiO2/Si(111) substrates has been measured in the temperature range 0.55 K<T<4.2 K in magnetic fields up to 7 T. The dependence of the LMR on small changes in the angle between the applied field and the substrate plane was also measured at T=1.3 K. X-ray diffraction measurements showed that these films have a textured rutile structure with a axis and 〈110〉 orientations normal to the substrates. The LMR of these films at about 3 T was about twice that of similar films that showed only a 〈100〉 orientation normal to their ZrO2 substrates. A change in the substrate orientation relative to the applied field direction of 10° caused a 25% decrease in the magnitude of the LMR at 3 T. The results suggest that the existence of the 〈110〉 crystallite structure strongly affects the magnetoresistance.
Photomelting of selenium at low temperature
Authors: Poborchii Vladimir V. , Kolobov Alexander V. , Tanaka Kazunobu
Published: July 2002

We report on a photoinduced phenomenon in solids, namely, photomelting at low temperature. We have found that both trigonal and amorphous selenium can be molten by illumination with light at a temperature of ∼77 K. This phenomenon is pure optical (athermal) and it is associated with light-induced breaking of the interchain (intermolecular) bonds in selenium. The photomelting is important for basic science (as an example of photoinduced phase transition in condensed matter and as a key photoinduced phenomenon in selenium and related materials) and for applications (as a tool for fine manipulation with shape of solids by light at low temperatures).
Stress effects in the oxidation of planar silicon substrates
Authors: Mihalyi A. , Jaccodine R. J. , Delph T. J.
Published: July 2002

We report here on the results of a series of experiments in which constant, controlled levels of in-plane stress were applied to oxidizing silicon substrates. The results indicate that compressive stresses tend to retard the growth of the oxide layer, while tensile stresses have an uncertain effect. These results are at variance with the predictions of a widely held model for the effect of stress upon the oxidation of silicon.
Plasma vacuum ultraviolet emission in an electron cyclotron resonance etcher
Authors: Cismaru C. , Shohet J. L.
Published: July 2002

This work investigates the vacuum ultraviolet (VUV) emission from various feed gases producing plasmas in an electron cyclotron resonance etcher. Absolute measurements of plasma VUV emission at typical pressures for processing between 0.5 and 5 mTorr, and microwave powers between 700 and 1300 W, show levels of irradiance at the wafer position of the order of tenths of mW/cm2 and integrated photon fluxes in the 1014 photons/cm2 s range. The reported level of VUV emission is sufficient to induce radiation damage in typical metal–oxide–semiconductor devices in the form of flatband voltage shift and inversion of lightly doped substrates.
Low resistance ohmic contacts on AlGaN/GaN structures using implantation and the “advancing” Al/Ti metallization
Authors: Qiao D. , Guan Z. F. , Carlton J. , Lau S. S. , Sullivan G. J.
Published: July 2002

The ohmic contact formation of Al/Ti on AlGaN/GaN heterostructure field effect transistors (HFETs) with and without Si implantation was investigated. Direct implantation and implantation through an AlN capping layer were studied. Compared to implantation through AlN, direct implantation is more effective in reducing the contact resistance. An Al(200 Å)/Ti(1500 Å) bilayer structure, called the “advancing” metallization, was used in this investigation to take advantage of consuming nearly all the top AlGaN layers for easy carrier access to the GaN layer underneath. Combining the direct implantation and the advancing metallization, low contact resistance of the order of 0.25 Ω mm (∼5.6×10−6 Ω cm2) can be readily obtained on HFET structures with an AlGaN layer about 340 Å thick and with an Al fraction of at least 22%.
Concentration-controlled phase selection of silicide formation during reactive deposition
Authors: Vantomme A. , Degroote S. , Dekoster J. , Langouche G. , Pretorius R.
Published: July 2002

Slow (low-rate) reactive deposition of a metal onto a Si substrate can result in direct formation of a metal disilicide, thereby skipping the metal-rich phases in the formation sequence. These observations have been explained thermodynamically by using the effective heat of formation model. As a result of this concentration-controlled phase selection, it is possible to form disilicides, such as CoSi2, NiSi2, or β-FeSi2 at much lower growth temperatures than possible in conventional solid-phase reaction of a metal layer deposited onto Si at room temperature (i.e., lower than the nucleation temperature). Moreover, epitaxial growth of CoSi2/Si(100), which is not possible by solid-phase reaction, becomes achievable when depositing Co atoms sufficiently slowly onto a heated Si substrate.
Low-temperature magnetoresistance in polycrystalline manganites: connectivity versus grain size
Authors: de Andrés A. , Garcı́a-Hernández M. , Martı́nez J. L. , Prieto C.
Published: July 2002

We have performed magnetic and transport measurements on La0.7Ca0.3MnO3 polycrystalline pellets and thin films. We propose a model that reproduces very well the low-field and high-field magnetoresistance at low temperature. The decrease of the resistance at high fields is modeled by the opening of new conduction channels in parallel due to the ordering of Mn spins blocked at the grain surface. We found that high-field magnetoresistance is independent of grain size, at least down to 12 nm, but depends strongly on the connectivity between grains.
Optically encoded second-harmonic generation in germanosilicate glass via a band-to-band excitation
Authors: Si Jinhai , Kitaoka Kenji , Mitsuyu Tsuneo , Hirao Kazuyuki
Published: July 2002

Optically encoded second-harmonic generation was enhanced by irradiating the germanosilicate glass with a pump beam of wavelength of 355 nm along with preparation beams at frequencies ω and 2ω. The origin of this enhancement was investigated by measuring the absorption spectra before and after preparation. The results suggest that the enhancement effect is related to the creation of an electron trapped center (GEC: germanium electron center) via a band-to-band excitation.
Inversion of wurtzite GaN(0001) by exposure to magnesium
Authors: Ramachandran V. , Feenstra R. M. , Sarney W. L. , Salamanca-Riba L. , Northrup J. E. , Romano L. T. , Greve D. W.
Published: July 2002

Magnesium incorporation during the molecular-beam epitaxy growth of wurtzite GaN is found to invert the Ga-polar (0001) face to the N-polar face. The polarity is identified based on the two different sets of reconstructions seen on the film prior to and after about 1 monolayer Mg exposure. The inversion boundary is seen to lie on the (0001) plane from transmission electron microscopy images, and a structural model is presented for the inversion. On the Ga-polar face, Mg is also seen to stabilize growth in the N-rich regime.
Single crystalline ZnO films grown on lattice-matched ScAlMgO4(0001) substrates
Authors: Ohtomo A. , Tamura K. , Saikusa K. , Takahashi K. , Makino T. , Segawa Y. , Koinuma H. , Kawasaki M.
Published: July 2002

Lattice-matched (Δa/a=0.09%) ScAlMgO4(0001) substrates were employed to grow single crystalline quality ZnO films by laser molecular-beam epitaxy. Extremely smooth surface represented by atomically flat terraces and half unit cell (0.26 nm) high steps and extremely small orientation fluctuations both in-plane (<0.02°) and out-of-plane (<0.01°) are achieved. The films have high mobility (∼100 cm2/V s) together with low residual carrier concentration (∼1015 cm−3). Excellent optical properties, including a clear doublet of A and B exciton peaks in absorption spectra, were also observed. These features could not be simultaneously achieved for ZnO films grown on sapphire(0001) having a large lattice mismatch (Δa/a=18%).
Direct experimental observation of the local electronic structure at threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films
Authors: Xin Y. , James E. M. , Arslan I. , Sivananthan S. , Browning N. D. , Pennycook S. J. , Omnès F. , Beaumont B. , Faurie J-P. , Gibart P.
Published: July 2002

The electronic structure of pure edge threading dislocations in metalorganic vapor phase epitaxy grown wurtzite GaN thin films has been studied directly by atomic resolution Z-contrast imaging and electron energy loss spectroscopy in a scanning transmission electron microscope. Dislocation cores in n-type samples grown in N-rich conditions show no evidence for the high concentration of Ga vacancies predicted by previous theoretical calculations. Nitrogen K-edge spectra collected from edge dislocation cores show a sudden and significant increase in the intensity of the first fine-structure peak immediately above the edge onset compared to the bulk spectra. The origin of this increase is discussed.
Midinfrared emission from InGaN/GaN-based light-emitting diodes
Authors: Hofstetter Daniel , Faist Jérôme , Bour David P.
Published: July 2002

Midinfrared emission on violet, blue, and green InGaN light-emitting diodes has been measured between 85 and 300 K for various injection current densities. We found that the diode with the highest In composition in the active region had the shortest midinfrared emission wavelength and vice versa. With increasing In content, a significantly decreasing amount of TM polarization was observed in the midinfrared emission spectrum. This result suggests that the density of states in the higher-In content devices corresponds to a zero-dimensional electronic system rather than a two-dimensional electron gas. In contrast to this, the violet light-emitting diode exhibited a higher degree of TM polarization; similar to a red InGaP-based quantum-well device.
Generation of narrow-band terahertz radiation via optical rectification of femtosecond pulses in periodically poled lithium niobate
Authors: Lee Y.-S. , Meade T. , Perlin V. , Winful H. , Norris T. B. , Galvanauskas A.
Published: July 2002

We demonstrate a promising technique for generating narrow-band terahertz electromagnetic radiation. Femtosecond optical pulses are propagated through a periodically poled lithium-niobate crystal, where the domain length is matched to the walk-off length between the optical and THz pulses. The bandwidth of the THz wave forms is 0.11 at 1.7 THz. Optical rectification gives rise to a THz wave form which corresponds to the domain structure of the periodically poled lithium niobate.
X-ray photoelectron spectroscopy study of (NH4)2Sx-treated Mg-doped GaN layers
Authors: Lin Yow-Jon , Tsai Chang-Da , Lyu Yen-Tang , Lee Ching-Ting
Published: July 2002

We present here the passivation mechanism and the chemistry of an (NH4)2Sx-treated Mg-doped GaN surface examined by using x-ray photoelectron spectroscopy. The native oxide on the GaN surface can be removed by the (NH4)2Sx treatment process. The S atoms not only bond as elemental sulfur and disulfides, but occupy nitrogen-related vacancies.
Inelastic light scattering in B2O3 glasses with different thermal histories
Authors: Surovtsev N. V. , Wiedersich J. , Batalov A. E. , Novikov V. N. , Ramos M. A. , Rössler E.
Published: July 2002

This contribution presents a systematic light scattering study of a series of boron oxide glasses which are characterized by different thermal histories. The thermal treatment was obtained by annealing the samples close to the glass transition temperature for times of several hours. Both low-frequency (0.1–30 cm−1) and high-frequency (5–1600 cm−1) spectra were monitored by using a tandem Fabry–Perot interferometer and a Raman spectrometer, respectively. The low-frequency spectra include quasielastic contributions and the boson peak. It was found that different thermal histories lead to pronounced changes in the low-frequency spectrum. The position of the boson peak shifts to higher frequencies and the magnitude of the quasielastic contribution decreases as a function of annealing time. Both quantities correlate linearly with the density of the samples (ρ=1.804–1.866 g/cm3). On the other hand, the high-frequency modes do not show discernible changes. In particular, no alteration of the modes which correspond to the boroxol ring is found, indicating that the fraction of boroxol rings is constant within 2% accuracy. Taking the boson peak as a manifestation of medium-range order, we conclude that annealing the glass influences the intermediate-range order rather than the short-range order.
Near-field zone analysis of the Faraday rotation of magneto-optical thin films
Authors: Richard N. , Dereux A. , Bourillot E. , David T. , Goudonnet J. P. , Scheurer F. , Beaurepaire E.
Published: July 2002

This article studies, in the near-field zone, the magneto-optical Faraday effect of metallic multilayers illuminated under the condition of total internal reflection. We present numerical results obtained by the Green’s dyadic technique which depicts the distribution associated with each Cartesian component of the electric field inside, as well as outside, the magneto-optical structure. The Faraday rotation is found to increase significantly around the resonances associated either with plasmons or to interferences inside thin films.
Dipyrazolopyridine derivatives as bright blue electroluminescent materials
Authors: Tao Y. T. , Balasubramaniam E. , Danel A. , Tomasik P.
Published: July 2002

Very bright blue organic light emitting diodes were fabricated using highly fluorescent dipyrazolopyridine derivatives, 4-(4-substituted phenyl)-1,7-diphenyl-3,5-dimethyl-1,7dihydrodipyrazolo[3,4-b,4′,3′-e]pyridine (PAP–X, X=CN, Ph, and OMe), as emitter by doping the dye in an electron-transporting host, 2,2′,2″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI). Two hole-transporting layers, 4,4′-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl (NPB) and 4,4′-dicarbazolyl-1,1′-biphenyl (CBP) were used to achieve the emission from PAP–X. The devices with a general configuration of indium tin oxide/NPB/CBP/TPBI:PAP(2%)/Mg:Ag showed a bright blue emission. The PAP–CN-based device is exceptionally good, with a brightness of 11 200 cd/m2 at 14.2 V and the peak external quantum efficiency of 3.2%. The efficiency is the highest for the blue emission.
Effect of heterobarrier leakage on the performance of high-power 1.5 μm InGaAsP multiple-quantum-well lasers
Authors: Shterengas L. , Menna R. , Trussell W. , Donetsky D. , Belenky G. , Connolly J. , Garbuzov D.
Published: July 2002

Broad stripe 1.5 μm InGaAsP/InP multiple-quantum-well graded-index separate-confinement heterostructure lasers with different waveguide widths and doping profiles were designed, fabricated, and characterized. A record value of more than 16 W of pulsed optical power was obtained from lasers with a broadened waveguide design. Studies of the characteristics of lasers with different p-doping profiles as well as modeling data show that the heterobarrier electron leakage is responsible for the effect of the device optical power saturation with current.
Deuterium-induced degradation of (Ba, Sr)TiO3 films
Authors: Ahn Joon-Hyung , McIntyre Paul C. , Mirkarimi Laura Wills , Gilbert Stephen R. , Amano Jun , Schulberg Michelle
Published: July 2002

Electrical degradation of (Ba, Sr)TiO3 (BST) thin films was investigated by annealing Pt/BST/Pt structures in D2/N2 and D2O-containing furnace ambients. Deuterium depth profiles were correlated to the current–voltage characteristics of the BST thin films. The dependence of the D distribution and leakage current density on the D incorporation method indicates that mobile, donor-type deuterium defects dissolve in large concentrations within BST thin films, and that their effects on leakage properties depend on the nature of their charge compensation. A mechanism is proposed for the leakage current increase after D2/N2 anneals, and good quantitative agreement between the theoretical results and experimental data is demonstrated.
Experimental and theoretical study of acceptor activation and transport properties in p-type AlxGa1−xN/GaN superlattices
Authors: Goepfert I. D. , Schubert E. F. , Osinsky A. , Norris P. E. , Faleev N. N.
Published: July 2002

Experimental and theoretical results of Mg-doped superlattices consisting of uniformly doped AlxGa1−xN, and GaN layers are presented. Acceptor activation energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2×1018 cm−3 and 4×1018 cm−3 for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curve data indicate excellent structural properties of the superlattices. We discuss the origin of the enhanced doping, including the role of the superlattice and piezoelectric effects. The transport properties of the superlattice normal and parallel to the superlattice planes are analyzed. In particular, the transition from a nonuniform to a uniform current distribution (current crowding) occurring in the vicinity of contacts is presented. This analysis provides a transition length of a few microns required to obtain a uniform current distribution within the superlattice structure.
Nanocrystalline gallium nitride thin films
Authors: Preschilla A. Nisha , Major S. , Kumar Nigvendra , Samajdar I. , Srinivasa R. S.
Published: July 2002

Nanocrystalline gallium nitride (GaN) thin films were deposited on quartz substrates by reactive rf sputtering of GaAs target with nitrogen as the reactive cum sputtering gas. X-ray diffraction and transmission electron microscopy confirmed the presence of GaN crystallites with particle size increasing from 3 to 16 nm, as the substrate temperature was increased from 400 to 550 °C. The particle size in films grown at temperatures below 550 °C were less than the exciton Bohr radius of GaN. The band gap of these films obtained from absorption and photoluminescence measurements showed a blueshift with respect to bulk GaN.
A test of the dependence of an optimal control field on the number of molecular degrees of freedom: HCN isomerization
Authors: Shah Suhail P. , Rice Stuart A.
Published: July 2002

This paper reports the results of a study of the robustness of the field required to generate the isomerization reaction HCN→CNH as a function of number of degrees of freedom and the complexity of the description of the dynamics. The particular reduced state representation of the control process that is tested in this paper is the reaction path method proposed by Zhao and Rice. We show that if the description of the system dynamics includes vibrational motions perpendicular to the one-dimensional reaction path and both the interactions between those vibrations and between them and the reaction path, the fields found by the conventional computational scheme represent local optima, and none of these correspond to generating a transfer of 100% of the population from the ground vibrational state of HCN to a mixture of vibrational states of CNH. Moreover, it is very difficult to find fields that will efficiently transfer population from the ground vibrational state of HCN to particular vibrational states of CNH. Comparing the optimized control fields reported in this paper with those previously obtained using simplified versions of the reaction path reduction, one finds that the complexity (measured by the power spectra) of optimal control fields increases as the dynamical description includes more degrees of freedom and then the interactions between all of the degrees of freedom. The optimal control field generated using a simpler dynamical description is not a good guide to the optimal control field associated with a more complex dynamical description. We conclude that the reaction path method of reduction of the complexity of calculation of the optimal field required to drive a particular reaction is not likely to be useful for the design of fields with which to actively control reactions of polyatomic molecules.
GaN
0.011
P
0.989
red light-emitting diodes directly grown on GaP substrates
Authors: Xin H. P. , Welty R. J. , Tu C. W.
Published: July 2002

Red light-emitting diodes (LEDs) emitting at 670 nm and employing GaN0.011P0.989 p–n homojunction grown on a (100) GaP substrate by gas-source molecular beam epitaxy with a rf plasma nitrogen source have been obtained. The integrated photoluminescence intensity of GaNP p–n homojunction LED is 5 times stronger than that of Ga0.51In0.49P bulk layer, but the peak width is much broader. Compared to conventional high-brightness AlGaInP red LEDs, our LED structure saves two process steps of etch removing of the GaAs absorbing substrate and wafer bonding to a GaP transparent substrate.
On the origin of planarity in Al5− and Al5 clusters: The importance of a four-center peripheral bond
Authors: Geske Grant D. , Boldyrev Alexander I. , Li Xi , Wang Lai-Sheng
Published: July 2002

Ab initio calculations were combined with anion photoelectron spectroscopy to unravel the structural origin of Al5 and Al5−. Well-resolved photoelectron spectra of Al5− were obtained and compared to theoretical calculations performed at various levels of theory. It was shown that the best agreement between the experimental and theoretical data is for a planar C2v structure. Analyses of the electronic structure and molecular orbitals revealed that the planarity in Al5 and Al5− are due to the presence of a four-center peripheral bond that is common in a whole family of planar pentaatomic species recently uncovered.
Probing the new bond in the vibrationally controlled bimolecular reaction of O with HOD(4νOH)
Authors: Pfeiffer Joann M. , Woods Ephraim , Metz Ricardo B. , Crim F. Fleming
Published: July 2002

Previous studies of the hydrogen abstraction from vibrationally excited H2O and HCN by various atoms have probed the vibrational and rotational energy of the product containing the surviving bond to assess the energy disposal and determine the mechanism of the reaction. Estimating the relative translational energy of the products from the Doppler broadening of the probe transitions has allowed the inference of the internal energy of the unobserved product containing the new bond using conservation of energy. The experiments presented here directly measure the vibrational and rotational energy of both the OH product (containing the new bond) and OD product (containing the old bond) from the reaction of O atoms with HOD having four quanta of O–H stretching excitation (4νOH). All of the OH products are vibrationally excited, being formed almost exclusively in ν=2. Nearly all of the OD products are vibrationally unexcited, with 93% in v=0 and only 7% in v=1. The results are consistent with a spectator picture of the reaction in which the new bond receives most of the available energy.
Microscopic treatment of substrate effects on linear and quadratic optical response of model Langmuir–Blodgett multilayers
Authors: in het Panhuis M. , Munn R. W.
Published: July 2002

Microscopic calculations are reported of linear and quadratic nonlinear optical response in model Langmuir–Blodgett films on a substrate, which is treated by the method of images. The effect of the substrate is significant in the first two layers, and is greatest for tilted molecules with their head groups adjacent to the substrate. The main qualitative effect is to lower the symmetry relative to a free-standing film. Calculations for stearic acid films show that the substrate effect is most important for molecules with nonuniform response on a metallic substrate.
Water structure changes induced by hydrophobic and polar solutes revealed by simulations and infrared spectroscopy
Authors: Sharp Kim A. , Madan Bhupinder , Manas Eric , Vanderkooi Jane M.
Published: July 2002

A combination of simulations and Fourier transform infrared spectroscopy was used to examine the effect of three ionic solutes (KCl, NaCl, and KSCN), the polar solute urea, and the osmolyte trimethylamine-N-oxide (TMAO) on a water structure. The ionic solutes increase the mean water–water H-bond angle in their first hydration shell concomitantly shifting the OH stretching mode to higher frequency, and shifting the HOH bending mode to lower frequency. TMAO decreases the mean water–water H-bond angle in its first hydration shell, shifts the OH stretching mode frequency down, and shifting the HOH bending mode frequency up. Urea has no effect on the mean H-bond angle, OH stretch, and HOH bend frequencies. These results can be explained in terms of changes in the relative proportions of two H-bond angle populations: Ionic solutes increase the population of more distorted (larger angle) H bonds relative to the less distorted population, TMAO has the reverse effect, while urea does not affect the H-bond angle probability distribution. The negligible effect of urea on water structure supports the direct binding model for urea-induced protein denaturation.
Mass spectrometric studies of a CH4/H2 microwave plasma under diamond deposition conditions
Authors: Fujii Toshihiro , Kareev Michael
Published: July 2002

We report on mass spectroscopic measurements of species originating from a microwave (MW) discharge plasma under simulated diamond deposition conditions. The plasma is produced in a 30 W MW flow tube through which flows a gas mixture of about 1% methane admixed in hydrogen. Plasma composition was investigated as a function of gas component ratio by Li+-ion attachment mass spectrometry. C, C2, C2H, C2H2, and C2H4, together with the free-radical species of C2H3 and C2H5, were observed on the mass spectra as Li+-ion adducts. Atomic carbon was the most abundant species, suggesting an important role for atomic carbons in diamond film growth.
Quantitative field measurements from magnetic force microscope tips and comparison with point and extended charge models
Authors: McVitie S. , Ferrier R. P. , Scott J. , White G. S. , Gallagher A.
Published: July 2002

In this article we present results and analysis from measurements of the field distributions from standard magnetic force microscope tips. These measurements are made using Lorentz microscopy and tomographic reconstruction techniques with the field reconstructed in a plane situated about 50 nm from the end of the magnetic force microscope (MFM) tip; this corresponds approximately to the sample plane in the MFM. By examination of the experimental results and comparison with simulated field distributions from point and extended charge distributions, we conclude that the magnetization configuration of the tip is best represented by an extended charge distribution and that the point pole approximation represents only, at best, the central part of the field distribution.
Temperature dependence of the structure of a 0.06 mole fraction tertiary butanol-water solution
Authors: Bowron D. T. , Soper A. K. , Finney J. L.
Published: July 2002

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute–solute, solute–solvent and solvent–solvent structural correlation functions in a 0.06 mole fraction solution of tertiary butanol in water. The measurements were made at two temperatures, 25 °C and 65 °C. Data analysis to the level of intermolecular orientational correlation functions was performed by the Empirical Potential Structure Refinement (EPSR) technique in combination with a spherical harmonic coefficient analysis. The results reveal in particular a subtle temperature dependence of the intermolecular structure of the solvent water. This structural perturbation is suggestive of a hydrophobic hydration induced, local density enhancement in the solvent water that could relate to the entropic driving force underlying the hydrophobic interaction mechanism.
Quantum toroidal algebra Uq(sl2,tor) and R matrices
Authors: Miki Kei
Published: July 2002

We show the existence of R matrices acting on the tensor product of a certain class of representations of the quantum toroidal algebra Uq(sl2,tor). In particular, the explicit expressions of R matrices acting on the tensor product of level 1 integrable highest weight representations of Uq(sl2∧) are obtained. Our approach is based on the work of Chari and Pressley.
Time evolution of colliding laser produced magnesium plasmas investigated using a pinhole camera
Authors: Harilal S. S. , Bindhu C. V. , Kunze H.-J.
Published: July 2002

Time resolved studies of colliding laser produced magnesium plasmas are performed using a pinhole camera. A ruby laser pulse is split into two beams using a movable glass wedge and focused onto a planar target or targets placed at 90° to each other. A gated pinhole camera provides an orthogonal view of the collision. Measurements with an aluminum filter allowed identification of soft x-ray production zones. A good interpenetration of the two plasmas was observed in the 90° target geometry, because of higher relative velocities, than in the case of laterally colliding plasmas. The appearance of the collision region depended strongly on the power density and on the separation of the foci on the target surfaces.
Phase field microelasticity theory and modeling of multiple dislocation dynamics
Authors: Wang Y. U. , Jin Y. M. , Cuitiño A. M. , Khachaturyan A. G.
Published: July 2002

The phase field theory and model of a multidislocation dynamics in an elastically anisotropic crystal under applied stress is developed. The proposed three-dimensional (3D) model is a particular case of our phase field microelasticity model of the stress-induced martensitic transformations. A spontaneous self-organization of dislocations in the evolving ensemble, which involves multiplication/annihilation and movement of dislocations controlled by their elastic interaction, is described by the Ginzburg–Landau kinetic equation. Examples of 3D computer simulation of dislocation dynamics are considered.
Configurational entropy and diffusion in a hard disc fluid
Authors: Speedy Robin J.
Published: July 2002

A fluid mixture of hard discs is made glassy in a molecular dynamics experiment by permanently constraining each disc center to stay within a cage of three near neighbors. The constraints can be applied at any density and they cause a thermodynamic glass transition where the compressibility, heat capacity, and entropy decrease. The decrease in entropy provides a direct measure of the configurational entropy of the fluid. A relation between diffusivity and the configurational entropy suggested by Adam and Gibbs [J. Chem. Phys. 43, 139 (1965)] is shown to be accurate from the ideal gas to the kinetic glass transition.
Oxide thickness mapping of ultrathin Al2O3 at nanometer scale with conducting atomic force microscopy
Authors: Olbrich Alexander , Ebersberger Bernd , Boit Christian , Vancea Johann , Hoffmann Horst , Altmann Hans , Gieres Guenther , Wecker Joachim
Published: July 2002

In this work, we introduce conducting atomic force microscopy (C-AFM) for the quantitative electrical characterization of ultrathin Al2O3 films on a nanometer scale length. By applying a voltage between the AFM tip and the conductive Co substrate direct tunneling currents in the sub pA range are measured simultaneously to the oxide surface topography. From the microscopic I–V characteristics the local oxide thickness can be obtained with an accuracy of 0.03 nm. A conversion scheme was developed, which allows the calculation of three-dimensional maps of the local electrical oxide thickness with sub-angstrom thickness resolution and nanometer lateral resolution from the tunneling current images. Local tunneling current variations of up to three decades are correlated with the topography and local variations of the electrical oxide thickness of only a few angstroms.
Mass transport regrowth of GaN for ohmic contacts to AlGaN/GaN
Authors: Heikman Sten , Keller Stacia , DenBaars Steven P. , Mishra Umesh K.
Published: July 2002

Ohmic contacts were fabricated for AlGaN/GaN high-electron-mobility transistors by selective-area mass transport regrowth of GaN. The contact resistance ranged from 0.23 to 1.26 Ω mm for different contact areas and geometries. The average resistivity of the autodoped regrown GaN was measured to 4×10−3 Ω cm. Devices with regrown contacts were fabricated, achieving a transconductance of 210 mS/mm. The technique provides a low-cost regrowth process, with applications in particular for high Al-composition AlGaN/GaN devices.
Switchable diffraction gratings based on inversion of the dielectric anisotropy in nematic liquid crystals
Authors: Kang S. W. , Sprunt S. , Chien L. C.
Published: July 2002

A switchable liquid crystal diffraction grating, responsive to both the frequency and magnitude of an applied voltage, is demonstrated. The grating is based on polymer stabilization of modulated states of the liquid crystal optic axis induced at frequencies near a sign inversion of the dielectric anisotropy. In a particular case of frequency-based switching, 70% of the transmitted intensity of a 633 nm laser beam can be shifted into or out of a ±12° forward arc by changing the frequency of an applied voltage between 10 and 16 kHz at a fixed amplitude of 27 V.
Surface-emitting, single-lobe operation from second-order distributed-reflector lasers with central grating phaseshift
Authors: Witjaksono Gunawan , Botez Dan
Published: July 2002

Surface-emitting diode lasers containing a second-order Bragg grating with a central phaseshift, Δφ, of values around π are found to fundamentally favor lasing in a mode of symmetric near-field amplitude profile, which in turn provides emission in a single-lobed beam orthonormal to the surface. Devices of 500 μm long distributed-feedback (DFB) active region (λ=0.98 μm) and 500 μm long distributed feedback reflector passive regions provide, for Δφ=π, a surface-emitted beam pattern with 88% central-lobe power content, and external differential quantum efficiency, ηD, of 51%. Since the guided field is antisymmetric to start with, and a central π phaseshift causes two grating-outcoupled beams to be out-of-phase with each other, the net result is a single-lobed far-field pattern. The guided-field peak-to-valley (intensity) ratio, R, in the active (i.e., DFB) region is only 2, which insures single-mode operation to high powers, since the intermodal discrimination is high (⩾100 cm−1). Over a wide range in Δφ:60°; ηD remains high (>50%) and the degree of guided-field uniformity remains low (R<2).
Vibrations of nitrous oxide: Matrix isolation Fourier transform infrared spectroscopy of twelve N2O isotopomers
Authors: Łapiński Andrzej , Spanget-Larsen Jens , Waluk Jacek , Radziszewski J. George
Published: July 2002

Isotopically labeled nitrous oxide has been produced in solid nitrogen matrices using mixtures of nitrogen and water containing N14, N15, O16, O17, and O18. All twelve possible N2O isotopomers have been obtained, and their fundamental, overtone and combination frequencies were assigned by the joint use of infrared spectroscopy and quantum chemical calculations (B3LYP/AUG-cc-pVTZ). Specific influence of the nitrogen matrix upon frequency and anharmonicity of the vibrations has been discussed.
Miscible droplets in a porous medium and the effects of Korteweg stresses
Authors: Chen Ching-Yao , Wang Lilin , Meiburg Eckart
Published: July 2002

Numerical simulation results are presented for the displacement of a drop in a porous medium. The drop is surrounded by a more viscous fluid with which it is fully miscible. The simulations are based on a set of augmented Hele–Shaw equations that account for nonconventional, so-called Korteweg stresses resulting from locally steep concentration gradients. Globally, these stresses tend to stabilize the displacement. However, there are important distinctions between their action and the effects of surface tension in an immiscible flow. Since the Korteweg stresses depend on the concentration gradient field, the effective net force across the miscible interface region is not just a function of the drop’s geometry, but also of the velocity gradient tensor. Locally high strain at the leading edge of the drop generates steep concentration gradients and large Korteweg stresses. Around the rear of the drop, the diffusion layer is much thicker and the related stresses smaller. The drop is seen to form a tail, which can be explained based on a pressure balance argument similar to the one invoked to explain tail formation in Hele–Shaw flows with surfactant. The dependence of such flows on the Peclet number is complex, as steeper concentration gradients amplify the growth of the viscous fingering instability, while simultaneously generating larger stabilizing Korteweg forces.
Spherical bubble motion in a turbulent boundary layer
Authors: Felton Keith , Loth Eric
Published: July 2002

Monodisperse dilute suspensions of spherical air bubbles in a tap-water turbulent vertical boundary layer were experimentally studied to note their motion and distribution. Bubbles with diameters of 0.37–1.2 mm were injected at various transverse wall-positions for free-stream velocities between 0.4 and 0.9 m/s. The bubbles were released from a single injector at very low frequencies such that two-way coupling and bubble–bubble interaction were negligible. The experimental diagnostics included ensemble-averaged planar laser intensity profiles for bubble concentration distribution, as well as Cinematic Particle Image Velocimetry with bubble tracking for bubble hydrodynamic forces. A variety of void distributions within the boundary layer were found. For example, there was a tendency for bubbles to collect along the wall for higher Stokes number conditions, while the lower Stokes number conditions produced Gaussian-type profiles throughout the boundary layer. In addition, three types of bubble trajectories were observed—sliding bubbles, bouncing bubbles, and free-dispersion bubbles. Instantaneous liquid forces acting on individual bubbles in the turbulent flow were also obtained to provide the drag and lift coefficients (with notable experimental uncertainty). These results indicate that drag coefficient decreases with increasing Reynolds number as is conventionally expected but variations were observed. In general, the instantaneous drag coefficient (for constant bubble Reynolds number) tended to be reduced as the turbulence intensity increased. The averaged lift coefficient is higher than that given by inviscid theory (and sometimes even that of creeping flow theory) and tends to decrease with increasing bubble Reynolds number.