bluuebunny/crossref_metadata_2025 huggingface dataset:
Methods for optimizing and assessing diagnostic capability, demonstrated for collective Thomson scattering (invited)
Authors: Bindslev H.
Published: July 2002

Commonly diagnostic systems are sensitive to a wide range of parameters, only a subset of which are parameters of interest. Optimal analysis and assessment of diagnostic capability must take account of the sensitivity to the remaining nuisance parameters which may not be accurately known. New least squares type methods of inference in the presence of uncertain nuisance parameters have been developed, along with methods for assessing diagnostic capability and identifying critical nuisance parameter uncertainties. These methods are of a general nature and suitable for complex and computationally demanding forward models. They are demonstrated by application to collective Thomson scattering for fast ion diagnosis in fusion plasmas.
Ultrastable superconducting magnet system for a penning trap mass spectrometer
Authors: Van Dyck R. S. , Farnham D. L. , Zafonte S. L. , Schwinberg P. B.
Published: July 2002

A custom-designed magnet/cryostat system is described which has demonstrated remarkably improved field stability over previous designs. To shield from external magnetic noise, a custom-fabricated flux-gate device remotely senses the changes in magnetic field and cancels them out at the site of the magnet/cryostat via a 1.7-m-diam Helmholtz coil. To provide further shielding, the basic superconducting solenoid includes a passive flux-stabilizing coil. To stabilize internal field shifts, the temperature of the materials in the immediate vicinity of the solenoid (which have a temperature-dependent susceptibility) is stabilized via the new cryostat geometry and by controlling the pressure of the evaporating liquid helium to a few parts per million. As a result, the total system now has a composite shielding factor of approximately 104 and an overall temporal stability on the order of 17(2) parts in 1012 per hour. This instrument, the heart of our new Penning trap mass spectrometer, has recently been used to determine a preliminary value of the proton’s atomic mass to an accuracy of 1.4 parts in 1010. With the new magnet/cryostat system, this spectrometer now has a potential resolution which exceeds 2 parts in 1011 with 100 hours of data using a single carbon 4+ ion.
Band alignment at CdS/CuInS2 heterojunction
Authors: Hashimoto Yoshio , Takeuchi Kazuhiro , Ito Kentaro
Published: July 2002

Band offsets at CdS/CuInS2 heterojunctions are studied by x-ray photoemission spectroscopy. At the CdS/cyanide-treated CuInS2 junction, the conduction band offset is as small as −0.05±0.15 eV, while the valence band offset is 1.18±0.10 eV. The CdS/Cu-rich CuInS2 heterojunction without the treatment, however, possesses a conduction band offset of ∼−0.7 eV. This result suggests that the cyanide treatment improves the interface to give rise to the alignment of the conduction band minima, which is suitable to the solar cell applications.
Application of band-stop filters for the 30–200 GHz range in oversized microwave systems
Authors: van der Meiden H. J.
Published: July 2002

A simple and cheap method has been developed to prepare band-stop filters for the frequency region 30–200 GHz with a typical attenuation in the stop band of more than 27 dB. This filter consists of periodic metallic structures deposited on a thin substrate. The preparation is achieved by applying a commonly used chemical etching technique on a commercially available thin print board. These so called “capacitive meshes” are applied successfully at the Rijnhuizen Tokamak Project for protection of a microwave diagnostic against high power microwave input.
Compact electron-beam ion trap using NdFeB permanent magnets
Authors: Motohashi Kenji , Moriya Akihiko , Yamada Hiroyuki , Tsurubuchi Seiji
Published: July 2002

A compact electron-beam ion trap (EBIT) as a multiply charged ion source for medium Z, e.g., Ne10+, Ar18+, Kr26+, and so on, has been developed. A pair of NdFeB permanent magnets generates an intense magnetic field (0.64 T) along with the electron-beam axis. The maximum electron-beam energy and current were designed as 10 keV and 30 mA, respectively. Over 95% of the electron beam up to 20 mA was transmitted to an electron collector. All parts, including electrical feedthroughs, are mounted on a con-flat flange with an 8 in. diam. Portability is added to the EBIT because no cooling system for the electromagnet is needed. Hydrogen-like Ar17+ were extracted.
1.3 μm photoluminescence from InGaAs quantum dots on GaAs
Authors: Mirin R. P. , Ibbetson J. P. , Nishi K. , Gossard A. C. , Bowers J. E.
Published: July 2002

We use molecular beam epitaxy to grown coherently strained InGaAs islands on (100) GaAs substrates. The islands show room-temperature photoluminescence at 1.3 μm with a full width at half-maximum of only 28 meV. The integrated photoluminescence intensity is comparable to that of a quantum well. The islands are formed by depositing 22 monolayers of In0.3Ga0.7As with alternating beams of In, Ga, and As2. Atomic force microscopy measurements show that the islands are ellipsoidal sections with an average peak height of 24 nm. The intersection of the islands with the (100) plane is an ellipse whose major axis is along [011̄] and has a mean length of 54 nm, and whose minor axis is along [011] and has a mean length of 36 nm. The islands form a dense array with an areal coverage of about 40%.
Thermally stable poled polyquinoline thin film with very large electro-optic response
Authors: Cai Y. M. , Jen A. K-Y.
Published: July 2002

Poled guest-host polyquinoline thin films are shown to demonstrate both exceptionally large electro-optic activity and long-term stability. After an initial drop from 45 to 26 pm/V in the first 100 h, the electro-optic coefficient r33 at 26 pm/V remained for more than 2000 h.
Structural properties of GaN films grown on sapphire by molecular beam epitaxy
Authors: Zhu Q. , Botchkarev A. , Kim W. , Aktas Ö. , Salvador A. , Sverdlov B. , Morkoç H. , Tsen S.-C. Y. , Smith David J.
Published: July 2002

The structural characteristics of GaN films grown on sapphire substrates by molecular beam epitaxy have been investigated using high-resolution synchrotron x-ray diffraction and electron microscopy. We find remarkable correspondence between the in-plane structural order (coherence length and mosaic spread) and the electrical and optical properties. Contrary to common belief, our observations show unequivocally that the out-of-plane structural features, which are considerably better developed than the in-plane counterparts, cannot be used for determining the material quality with respect to their optical and electrical activity. In particular, the (00l) mosaic spread is not a good indicator of film quality.
Polarization-dependent extended x-ray absorption fine structure measurements of metastable fcc Co on (100) diamond
Authors: Kemner K. M. , Elam W. T. , Idzerda Y. U. , Wolf J. A. , Prinz G. A.
Published: July 2002

Polarization-dependent extended x-ray absorption fine structure (EXAFS) measurements have been made on a single-crystal 1000 Å thick Co film deposited on (100) diamond. By comparing the EXAFS signals corresponding to the in-plane and out-of-plane structure relative to the film plane, we clearly illustrate that the film exists in the face centered cubic (fcc) phase. An analysis of the first shell Fourier transform peaks for the in-plane and out-of-plane structure surrounding the Co atoms indicates that the majority of the fcc Co crystal lattice is not distorted, demonstrating that the majority of film is relaxed and not pseudomorphic.
Analytical theory of terahertz four-wave mixing in semiconductor-laser amplifiers
Authors: Koltchanov I. , Kindt S. , Petermann K. , Diez S. , Ludwig R. , Schnabel R. , Weber H. G.
Published: July 2002

Four-wave mixing in bulk semiconductor-laser amplifiers is investigated for detunings up to 3 THz. Gain dispersion, i.e. the difference of the gain factors for pump, probe and signal waves as well as saturation effects are found to play a significant role in four-wave mixing with large detunings. To properly describe the saturation behavior, it is assumed that both differential gain and carrier density at transparency are different for different waves. Also, the nonlinear susceptibilities associated with different nonlinear effects are characterized by different dependencies on the carrier density. Generally, the carrier densities at the zero points of the nonlinear susceptibilities (if any) and those of the linear gain factors do not coincide. An analytical solution including these issues is given and a comparison with experimental data is presented.
Color changes in thin porous silicon films caused by vapor exposure
Authors: Bjorklund Robert B. , Zangooie Shahin , Arwin Hans
Published: July 2002

Thin films of porous silicon formed by electrochemically etching silicon wafers changed color when exposed to an ambient atmosphere saturated in various organic solvent vapors. The degree of the color change was related to the refractive indices of the solvents. Analysis of the data using a four-layer optical model indicated that the film refractive index increased up to 15% when solvent molecules replaced air in the pores. Solvent condensing from the saturated atmosphere filled up to 45% of the total void volume. Thermally oxidizing the films to make them hydrophilic resulted in surfaces which changed color upon exposure to water.
Terahertz measurements of resonant planar antennas coupled to low-temperature-grown GaAs photomixers
Authors: McIntosh K. A. , Brown E. R. , Nichols K. B. , McMahon O. B. , DiNatale W. F. , Lyszczarz T. M.
Published: July 2002

Resonant slot and dipole antennas coupled to low-temperature-grown GaAs photomixers have been fabricated and tested at terahertz operating frequencies. Enhanced output power is seen from the resonant structures compared to mixers coupled to broadband self-complementary spiral antennas. Driving point impedances as high as 300 Ω are attained at the resonant frequencies. These devices will be useful as fixed frequency local oscillators for submillimeter heterodyne receivers.
Transient subpicosecond Raman studies of electron velocity overshoot in an InP p-i-n nanostructure semiconductor
Authors: Tsen K. T. , Ferry D. K. , Wang Jye-Shyang , Huang Chao-Hsiung , Lin Hao-Hsiung
Published: July 2002

Transient electron transport in an InP p-i-n nanostructure semiconductor has been studied by subpicosecond Raman spectroscopy at T=300 K. Both the nonequilibrium electron distribution and electron drift velocity in the regime of electron velocity overshoot have been directly measured. It is demonstrated that electron drift velocity in an InP p-i-n nanostructure is significantly larger than that in a GaAs p-i-n nanostructure sample, as a result of the larger central to satellite valley energy separation in InP.
A simple amorphous-silicon photodetector for two-dimensional position sensing
Authors: Cárabe Julio , Gandía J. Javier , González Nieves , Galiano Enrique , Gutiérrez M. Teresa
Published: July 2002

A simple two-dimensional position-sensitive detector is proposed and initial tests are reported. The device, aimed to be high-energy-radiation resistant and reliable under magnetic fields, is based on a set of small p-i-n amorphous silicon photodiodes acting as pixels sandwiched between two perpendicular strip electrode structures. No thin-film transistors are used. A simplified prototype has been made and tested. The results show that the position of a laser spot on its surface can be deduced from the signals it generates. Further work proposed, includes tests on possible cross talk and behavior under radiation and magnetic fields, as well as the development of methods for accurate position calculation and for appropriate response calibration.
High electric field conduction mechanisms in electrode poling of electro-optic polymers
Authors: Sprave Martin , Blum Robert , Eich Manfred
Published: July 2002

We investigated electrical conduction phenomena, which occurred during high electric field electrode poling of side chain χ(2) polymers. Electric current and second harmonic intensity were measured simultaneously in poling experiments performed on samples with and without an additional inorganic poly–methyl–siloxane layer. Current densities appeared to be limited by a Schottky-type potential barrier at the electrode/insulator interface. The field dependence of the current density was found to be Schottky charge emission for medium field strengths (EPOL≤100 V/μm) whereas it was dominated by Fowler–Nordheim tunneling at higher poling fields. In the presence of the siloxane layer, a significant suppression of tunneling current was observed. This leads to a reduced probability of singular dielectric breakdown events and shifts the limit of avalanche breakdown to higher internal effective poling field strengths.
Fast bistable nematic display using monostable surface switching
Authors: Dozov I. , Nobili M. , Durand G.
Published: July 2002

We present a novel surface-controlled bistable nematic liquid crystal display. This black and white device uses simple monostable planar anchorings and a dielectrically positive nematic. The two bistable textures are, respectively, quasiuniform U and almost half-turn twisted T. Short electric pulses break the surface anchorings. Hydrodynamically coupled breaking of both anchorings creates T. U is obtained by breaking only one anchoring. Write and erase times for high resolution video are achieved at voltages compatible with the usual drivers.
Fullerene and nanoparticle formation in carbon cathodic arc deposition
Authors: Chhowalla M. , Munindradasa A. I. , Amaratunga G. A. J.
Published: July 2002

We report on a method for detecting C60 and other C2n molecules in the vapor phase during their initial growth stage. The technique, which utilizes a Langmuir probe, was originally proposed by Fukuzawa et al. [T. Fukuzawa, M. Shiratani, and Y. Watanabe, Appl. Phys. Lett. 64, 3098 (1994)] for the detection of dust particles in silane plasmas. By measuring the positive and negative saturation currents in the afterglow plasma, the mass ratio of negative and positive ions can be determined. We found that a M−/M+=56±10 occurs only for discharge times greater than 0.5 s. As most of the positively charged species are expected to be singly charged carbon, a mass ratio of 56 is close enough to be attributed to the C60 molecule, in agreement with mass spectroscopy results.
Noncontact semiconductor wafer characterization with the terahertz Hall effect
Authors: Mittleman D. M. , Cunningham J. , Nuss M. C. , Geva M.
Published: July 2002

We demonstrate noncontact measurements of the Hall mobility of doped semiconductor wafers with roughly 250 μm spatial resolution, using polarization rotation of focused beams of terahertz (THz) radiation in the presence of a static magnetic field. Quantitative and independent images of both carrier density and mobility of a doped semiconductor wafer have been obtained.
Field-enhanced Si–Si bond-breakage mechanism for time-dependent dielectric breakdown in thin-film SiO2 dielectrics
Authors: McPherson J. W. , Reddy V. K. , Mogul H. C.
Published: July 2002

A field-enhanced Si–Si bond-breakage mechanism is presented which accurately describes the time-dependent dielectric breakdown behavior recently reported for thin-film SiO2 dielectrics over a wide range of fields and temperatures. The breakdown kinetics (both the field and temperature dependence) are shown to be consistent with a field-dependent dipolar energy term associated with an oxygen vacancy which serves to reduce the activation energy required for Si–Si bond breakage.
Morphology definition by disclinations and dislocations in a mesostructured silicate crystal
Authors: Feng J. , Huo Q. , Petroff P. M. , Stucky G. D.
Published: July 2002

Curved surface morphologies resulting from disclinations are unusual in inorganic phases because the relatively large lattice energies generally permit only translational dislocations. In this letter, we show, however, that disclinations can be readily observed and identified in composite organic/inorganic phases. Disclinations and dislocations in MCM-41, a two-dimensional hexagonal (p6mm) mesophase crystal synthesized using silicate and surfactant precursors, were investigated by using transmission electron microscopy. We observed two types of dislocations and two types of disclinations. All dislocations observed are of the smallest Burgers vector a (the basis vector). Dislocation dipoles and disclination quadrupoles appear more frequently than isolated ones.
Polymer light-emitting electrochemical cells with frozen p-i-n junction
Authors: Gao Jun , Yu Gang , Heeger Alan J.
Published: July 2002

The p-i-n junction in a polymer light-emitting electrochemical cell (LEC) is stabilized by cooling the device below the glass transition temperature of the ion-transport polymer. LECs with frozen p-i-n junctions exhibit typical light emitting diode (LED) behavior including diode rectification, unipolar light emission (same polarity as that used for generating the junction), and fast response. The freezeout of ion motion allows polymer LECs to be driven at bias voltages well beyond the electrochemical stability window, and thereby extends the potential applications of polymer LECs to high pixel density, column-row addressable displays.
Deposition of in-plane textured MgO on amorphous Si3N4 substrates by ion-beam-assisted deposition and comparisons with ion-beam-assisted deposited yttria-stabilized-zirconia
Authors: Wang C. P. , Do K. B. , Beasley M. R. , Geballe T. H. , Hammond R. H.
Published: July 2002

We report the growth of in-plane textured (100) MgO on amorphous Si3N4 substrates by ion beam assisted deposition (IBAD). The textured MgO can be used as a structural template for subsequent epitaxial thin film deposition. The results are compared with IBAD of (100) and (111) yttria-stabilized-zirconia (YSZ). Based on transmission electron microscopy (TEM) and in situ reflection high energy-electron diffraction (RHEED), we find that MgO texturing is a nucleation-controlled process and the alignment is a function of nuclei size and density. This differs greatly from the evolutionary-type texturing process observed for IBAD (100) YSZ. Consequently, we are able to make 100 Å thick MgO films with 7° in-plane alignment, whereas IBAD (100) YSZ films need to be thicker than 5000 Å to achieve in-plane alignment better than 13°. This has important implications for the economical application of IBAD induced alignment in real manufacturing processes, including high Tc superconductor (i.e., YBCO) coated tapes, photovoltaics, magnetic thin films, and semiconductor devices.
Surface characterization of silicon on insulator material
Authors: Lin K. C. , Holland O. W. , Feldman L. C. , Weitering H. H.
Published: July 2002

The surface of ultrathin silicon on insulator (SOI) material has been characterized with surface science analysis techniques including atomic-resolution scanning tunneling microscopy. It is shown that the (100) SOI surface can be fabricated with a comparable degree of structural perfection as the (100) surface of bulk Si. Fermi level pinning by “type C” dimer defects results in a fully depleted and thus effectively insulating Si film.
Field-activated structural traps at organic pigment/metal interfaces causing photocurrent multiplication phenomena
Authors: Hiramoto Masahiro , Nakayama Ken-ichi , Katsume Tadashi , Yokoyama Masaaki
Published: July 2002

Carrier traps at the perylene pigment/metal interface, which give rise to a large photocurrent multiplication phenomenon showing a gain of more than 104-fold, were investigated. The energetic depth of these traps increased with applied electric field, contrary to the behavior of usual electronic traps. To explain this, a model of field-activated structural traps was proposed; that is, dead ends at an imperfect organic/metal contact capture holes more tightly under a stronger electric field. Estimated values of five trapped holes per 10 nm2 and dead ends of the order of 1 nm long suggest that the proposed structural traps are closely related to the molecular-sized roughness at the pigment surface.
Asymptotic decay of radius of a weakly conductive viscous jet in an external electric field
Authors: Spivak A. F. , Dzenis Y. A.
Published: July 2002

Motion of a weakly conductive viscous jet accelerated by an external electric field is considered. Nonlinear rheological constitutive equation applicable for polymer fluids (Oswald–deWaele law) is applied. A differential equation for the variation of jet radius with axial coordinate is derived. Asymptotic variation of the jet radius at large distances from the jet origin is analyzed. It is found that the well-known power-law asymptote for Newtonian fluids with the exponent 1/4 holds for more general class of fluids, i.e., pseudoplastic (shear thinning) and dilatant (shear thickening) fluids with the flow index between 0 and 2. Dilatant fluids with the flow index greater than 2 exhibit power-law asymptotes with the exponents depending on the flow index. Results can be applied for the analysis of viscous polymer jets in the electrospinning process.
Deformation of carbon nanotubes in nanotube–polymer composites
Authors: Bower C. , Rosen R. , Jin L. , Han J. , Zhou O.
Published: July 2002

Composites of uniaxially oriented multiwalled carbon nanotubes embedded in polymer matrices were fabricated and investigated by transmission electron microscopy. In strained composite films, buckling was ubiquitously observed in bent nanotubes with large curvatures. By analyses of a large number of bent nanotubes, the onset buckling strain and fracture strain were estimated to be ≈5% and ⩾18%, respectively. The buckling wavelengths are proportional to the dimensions of the nanotubes. Examination of the fracture surface showed adherence of the polymer to the nanotubes.
Determination of EL2 capture and emission coefficients in semi-insulating n-GaAs
Authors: Bonilla L. L. , Hernando P. J. , Kindelan M. , Piazza F.
Published: July 2002

We have determined the capture and emission coefficients by EL2 traps in semi-insulating n-GaAs using available experimental results. To this end, we have derived a simplified mathematical model from the complete drift–diffusion equations by singular perturbation methods. The capture and emission coefficients are adjusted so that the numerically obtained steady-state and high-field charge dipole solutions of the simplified model match the available experimental results.
Large third-order optical nonlinearity in SrBi2Ta2O9 thin films by pulsed laser deposition
Authors: Zhang W. F. , Zhang M. S. , Yin Z. , Gu Y. Z. , Du Z. L. , Yu B. L.
Published: July 2002

Well-crystallized SrBi2Ta2O9 (SBT) thin films with good surface morphology were prepared on quartz substrates by the pulsed laser deposition technique at a deposition temperature of 750 °C. The third-order nonlinear optical properties of the films were measured by the Z-scan technique. The magnitude and sign of the nonlinear refractive index n2 were determined, as was the negative sign, which indicated a self-defocusing optical nonlinearity. A nonlinear refractive index as high as 1.9×10−6 esu was displayed in the SBT thin film. These results show that SBT ferroelectric thin films have potential applications in nonlinear optics.
Quasicrystals in a partially devitrified Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass
Authors: Chen M. W. , Zhang T. , Inoue A. , Sakai A. , Sakurai T.
Published: July 2002

A quasicrystalline phase was discovered in a Zr65Al7.5Ni10Cu12.5Ag5 bulk metallic glass annealed in the supercooled liquid range. Through systematical transmission electron microscopy rotation analyses, the selected area electron diffraction patterns are obtained corresponding to fivefold, threefold, and twofold rotational symmetry, demonstrating that the precipitated phase is an icosahedral quasicrystalline phase with good rotational symmetry. It is concluded that the addition of Ag in the Zr-based metallic glass promotes the formation of the quasicrystalline phase. The present experimental result implies that there is a natural relationship between the Zr-based bulk metallic glasses and icosahedral quasicrystalline.
Air-bridge-structured silicon nanowire and anomalous conductivity
Authors: Fujii Hideo , Kanemaru Seigo , Matsukawa Takashi , Itoh Junji
Published: July 2002

An air-bridge-structured silicon nanowire was made by micromachining a silicon-on-insulator (SOI) substrate and electrically characterized. The nanowire was isolated from the substrate by an air gap and typically 20–100 nm in diameter and 300–600 nm in length. Current–voltage characteristics of these wires were anomalous electric conductivity such as negative resistance and hysteresis at room temperature. Charge accumulation into surface states is considered a dominant characteristic.
Temperature and magnetic field dependent transport anisotropies in La0.7Ca0.3MnO3 films
Authors: O’Donnell J. , Eckstein J. N. , Rzchowski M. S.
Published: July 2002

We report the temperature and field dependence of anisotropic magnetoresistance (AMR) in fully strained tetragonal thin films of the colossal magnetoresistance manganite La0.7Ca0.3MnO3. The behavior of the AMR is very different from that found in conventional metallic alloys. Its magnitude is peaked near the Curie temperature TC and becomes small at low temperature. It has two distinct symmetry components: one twofold symmetric about the transport current direction dominant near TC, the other fourfold symmetric with symmetry axes of the crystal, appearing at low temperature.
Parallel nanolithography in carbon layers with conductive imprint stamps
Authors: Mühl Thomas , Kretz Johannes , Mönch Ingolf , Schneider Claus M. , Brückl Hubert , Reiss Günter
Published: July 2002

Nanometer-scale lithography in amorphous carbon layers was carried out by locally oxidizing the carbon under the tip of a scanning probe microscope. Although this patterning technique is able to yield very small structures, its speed is severely limited due to the serial character of the writing process. We exploit the potential of local carbon oxidation to give a parallel lithography approach which uses prepatterned stamps for electron-induced parallel structuring of the carbon film. This technique allows the transfer of complex, three-dimensional patterns into a carbon resist layer within a single process step.
High anisotropic conductivity in organic insulator/semiconductor monolayer heterostructure
Authors: Collet J. , Lenfant S. , Vuillaume D. , Bouloussa O. , Rondelez F. , Gay J. M. , Kham K. , Chevrot C.
Published: July 2002

We demonstrate a highly anisotropic conductivity, with a 109 ratio, between the in-plane and perpendicular electrical transport in organic insulator/semiconductor heterostructures of monolayer thickness. These heterostructures are self-assembled monolayers made of alkyl chains and functionalized by various conjugated moieties at their ends. The high anisotropic conductivity is due to the close packing of the conjugated end groups. These structures might be the building blocks of molecular-scale devices.
Influence of strain relaxation of the AlxGa1−xN barrier on transport properties of the two-dimensional electron gas in modulation-doped AlxGa1−xN/GaN heterostructures
Authors: Shen B. , Someya T. , Arakawa Y.
Published: July 2002

Influences of the thickness of the Si-doped n-type Al0.22Ga0.78N barrier and the thickness of the Al0.22Ga0.78N spacer on mobility and density of the two dimensional electron gas (2DEG) in modulation-doped Al0.22Ga0.78N/GaN heterostructures were investigated. 2DEG mobilities of 1274 cm2/V s at 300 K and 4495 cm2/V s at 77 K were reached. Both 2DEG mobility and density decrease dramatically when the Al0.22Ga0.78N barrier becomes partially relaxed, indicating that transport properties of the 2DEG are influenced significantly by the piezoelectric polarization of the Al0.22Ga0.78N layer. From our results, the critical thickness of an Al0.22Ga0.78N layer on GaN is estimated to be between 65 and 75 nm, which is much higher than that predicted by theoretical calculation. This may be attributed to the interaction of misfit dislocations and the presence of a high density of extended defects in the Al0.22Ga0.78N layer.
Ultrahard carbon nanocomposite films
Authors: Siegal M. P. , Tallant D. R. , Provencio P. N. , Overmyer D. L. , Simpson R. L. , Martinez-Miranda L. J.
Published: July 2002

Modest thermal annealing to 600 °C of diamondlike amorphous-carbon (a-C) films grown at room temperature results in the formation of carbon nanocomposites with hardness similar to diamond. These nanocomposite films consist of nanometer-sized regions of high density a-C embedded in an a-C matrix with a reduced density of 5%–10%. We report on the evolution of density and bonding topologies as a function of annealing temperature. Despite a decrease in density, film hardness actually increases ∼15% due to the development of the nanocomposite structure.
Polarization reversal and domain grating in flux-grown KTiOPO4 crystals with variable potassium stoichiometry
Authors: Rosenman G. , Urenski P. , Arie A. , Roth M. , Angert N. , Skliar A. , Tseitlin M.
Published: July 2002

A significant decrease is observed in the dielectric dispersion, conductivity, coercive and internal fields of KTiOPO4 crystals grown with relatively high potassium concentration in the flux. A periodic domain structure with a period of 24.7 μm was fabricated on a 10 mm long crystal by electric field poling. Whereas standard flux-grown KTiOPO4 crystals can be periodically poled only at low temperatures (near 170 K), the periodic poling in this case was successfully performed near room temperature at a much lower electric field. Quasiphase-matched frequency doubling of 1550 nm radiation was achieved with the poled crystal.
Photo- and cathodoluminescence characteristics of blue-light-emitting epitaxial Sr2CeO4 thin-film phosphors
Authors: Lee Yong Eui , Norton David P. , Budai John D. , Rack Philip D. , Potter Michael D.
Published: July 2002

Orthorhombic epitaxial Sr2CeO4 thin-film phosphors were grown on (100) SrTiO3 and yttria-stabilized-zirconia (YSZ) single crystal substrates with epitaxial relationships of (100) [010]Sr2CeO4//(100)[010]SrTiO3 and (100)[010]Sr2CeO4//(100)[011]YSZ, respectively. As-deposited films exhibit a broadband photoluminescence (PL) emission, peaking at about 477–481 nm with a maximum cathodoluminescence (CL) intensity at about 465 nm. Enhanced PL intensity was observed for the films deposited on (100) YSZ and SrTiO3 substrates as compared to deposits on c-plane sapphire substrates, correlating with improved intragranular crystallinity and reducing defects via epitaxy. Postannealing in air significantly enhanced both of the PL and CL intensities. Luminous efficiency of 0.14 lm/W at 1 kV and 22 μA/cm2 was observed for a 2-μm-thick film annealed at 1000 °C in air.
Gyrokinetic theory in the White–Chance–Boozer coordinates
Authors: Chiu S. C. , Chan V. S. , Lin-Liu Y. R. , Omelchenko Y.
Published: July 2002

The noncanonical Lagrangian theory of guiding center orbits is applied to the coordinate system of White–Chance–Boozer. The result is a generalized set of equations of motion which eliminates certain approximations of the static field and includes the effects of large rotations and high frequency waves. A quasilinear gyrokinetic theory is shown to readily follow from these equations.
Optical evidence of intrinsic quantum wells in the transparent conducting oxide β-Ga2O3
Authors: Binet Laurent , Gourier Didier
Published: July 2002

A peculiar peak structure is observed in the optical absorption edge of β-Ga2O3 at low temperature. These peaks appear in an energy range corresponding to the excitation of the acceptor defects. It is suggested that some of these acceptors be assembled in low dimensional clusters with size about 30–40 Å and forming potential wells with depth about 0.5 eV. The extra peaks are interpreted as transitions between the discrete energy levels of these potential wells and the conduction band.
Scanning near-field infrared microscopy and spectroscopy with a broadband laser source
Authors: Michaels Chris A. , Stranick Stephan J. , Richter Lee J. , Cavanagh Richard R.
Published: July 2002

A scanning near-field microscope that allows the fast acquisition of midinfrared absorption spectra is described. The microscope couples the nanoscale spatial resolution of a scanning probe microscope with the chemical specificity of vibrational spectroscopy. Key design elements of the microscope include a tunable broadband infrared light source; an infrared focal plane array-based spectrometer which allows parallel detection of the entire pulse bandwidth (200 cm−1); and a single mode, fluoride glass, near-field probe fabricated with a chemical etching protocol. Infrared transmission images of a micropatterned thin gold film are presented that demonstrate spatial resolution conservatively estimated to be λ/7.5 at 3.4 μm, in the absence of optical artifacts due to topography. Constant height mode images of a polymer nanocomposite demonstrate instrumental sensitivity to fractional transmission changes of 1×10−3. Near-field absorption spectra (λ=3.4 μm) of a 2 μm thick polystyrene film are presented which demonstrate the instrumental sensitivity required for high spatial resolution, near-field absorption imaging. These spectra are acquired in 2 s and indicate a film thickness detection limit for polystyrene of 200 nm. Measurements exploring the coupling between the infrared absorption magnitude and changes in tip–sample separation suggest that near-field absorption imaging is relatively insensitive to topographic artifacts.
Reliability of photopumped AlxGa1−xAs–GaAs quantum well heterostructure lasers with top and bottom distributed native-oxide reflectors
Authors: Kellogg D. A. , Holonyak N. , Dupuis R. D.
Published: July 2002

Data on an AlxGa1−xAs–GaAs quantum well heterostructure (QWH) designed to be oxidized and used as a photopumped vertical cavity surface emitting laser (VCSEL) are presented showing the full range in reliability (5+ years) from atmospheric decay of the as-grown crystal by hydrolyzation to protection of the QWH VCSEL by means of the Al-based native oxide (“wet” oxidation, 425 °C). In contrast to fully oxidized crystals in a VCSEL configuration that remain stable (AlAs layers converted to oxide), broad area p-n diodes made from the as-grown crystal, with buried AlAs layers exposed at edges, hydrolyze.
Direct x-ray synchrotron imaging of strains at 180° domain walls in congruent LiNbO3 and LiTaO3 crystals
Authors: Kim Sungwon , Gopalan Venkatraman , Steiner B.
Published: July 2002

We present direct evidence for wide regions of strain on length scales of many micrometers associated with 180° domain walls in congruent LiNbO3 and LiTaO3 crystals. Strain contrast in symmetric high-resolution diffraction images of these crystals in Bragg geometry arises in part from curvature in the basal planes (0001) across a domain wall as well as from lateral variation in the lattice spacing of the basal planes extending over many micrometers in going across a wall.
Dielectric function of amorphous tantalum oxide from the far infrared to the deep ultraviolet spectral region measured by spectroscopic ellipsometry
Authors: Franke Eva , Trimble C. L. , DeVries M. J. , Woollam J. A. , Schubert M. , Frost F.
Published: July 2002

Amorphous tantalum oxide thin films were deposited by reactive rf magnetron sputtering onto [001] silicon substrates. Growth temperature, oxygen partial pressure, and total gas pressure have been varied to obtain thin films with different densities. The thin films were analyzed by glancing angle-of-incidence x-ray diffraction, atomic force microscopy, and variable angle-of-incidence spectroscopic ellipsometry in the near infrared to vacuum ultraviolet spectral region for photon energies from E=1 to 8.5 eV, and in the infrared region from E=0.03 to 1 eV. We present the dielectric function of amorphous tantalum oxide obtained by line shape analysis of the experimental ellipsometric data over the range from E=0.03 to 8.5 eV (40 μm–145 nm). In the infrared spectral region the ellipsometric data were analyzed using Lorentzian line shapes for each absorption mode observed in the spectra. Amorphous tantalum oxide optical properties in the near infrared to vacuum ultraviolet spectral region were extracted by using a Kim and Garland parameter algorithm [C. C. Kim et al., Phys. Rev. B 45, 11 749 (1992)] in order to model the absorption due to the fundamental band gap of the material. We consider thin film porosity, and therefore analyzed the experimental ellipsometric data by an effective medium approach. We obtain information on the tantalum oxide optical properties, a percentage of void fraction, and film thickness. The “optical” percentage of void fractions corresponds to surface roughness measured by atomic force microscopy and depends on deposition parameters.
Domain mapping of periodically poled lithium niobate via terahertz wave form analysis
Authors: Lee Y.-S. , Meade T. , Naudeau M. L. , Norris T. B. , Galvanauskas A.
Published: July 2002

We demonstrate a nondestructive probing technique to investigate the inner crystal domain structure of ferroelectric crystals by analyzing the terahertz wave forms generated by optical rectification. Quantitative analysis of the domain structure has been performed for the domain width in periodically poled lithium niobate. Simulation results show that the terahertz wave form analysis reproduces root-mean-square domain width fluctuations.
Scanning nanolithography using a material-filled nanopipette
Authors: Hong Mun-Heon , Kim Ki Hyun , Bae Joonho , Jhe Wonho
Published: July 2002

A scanning nanolithography is demonstrated by employing near-field optical microscopy with a pulled micropipette which is used for material transport as well as distance regulation. Delivering the photoresist through the small aperture (300 nm diameter) of the pulled pipette with the shear-force distance control, we have fabricated nanometric dots (300 nm diameter) on the gold-sputtered substrate. This scheme may be also useful in nanometric control of chemical reaction and repair of nanometric structures.
Surface diffusion in mixed overlayers with superlattice ordering: Percolative transport around obstacles and along domain boundaries
Authors: Liu Da-Jiang , Evans J. W.
Published: July 2002

To elucidate surface diffusion in the presence of a coadsorbate with superlattice ordering, we consider particle hopping on a square lattice with some fraction, θB, of quenched blocking sites arranged with checkerboard or c(2×2) ordering. Behavior for low θB corresponds to diffusion around isolated obstacles, and can be described by exact density expansions. Behavior for high θB corresponds to percolative diffusion along (or sometimes away from) domain boundaries. The connectivity of these domain boundaries is closely related to the existence of symmetry breaking [i.e., long-range c(2×2) order] in the distribution of blocking sites. In some cases, symmetry breaking induces critical behavior for diffusive transport which is fundamentally different from that for the conventional “ant in the labyrinth” problem. Our results apply to recently developed models for CO oxidation, where CO(ads) diffuses rapidly through coadsorbed relatively immobile c(2×2)-O(ads). The characterization of CO diffusion in these systems is key to describing spatial pattern formation.
Ab initio molecular dynamics studies of the photodissociation of formaldehyde, H2CO→H2+CO: Direct classical trajectory calculations by MP2 and density functional theory
Authors: Li Xiaosong , Millam John M. , Schlegel H. Bernhard
Published: July 2002

The dynamics of H2CO→H2+CO photodissociation have been studied by classical trajectory calculations at the MP2/6-311G(d,p), B3LYP/6-311G(d,p), and VSXC/6-311G(d,p) levels of theory. The trajectories were calculated directly from the electronic structure computations without first fitting a global potential energy surface. A Hessian based method with updating was used to integrate the trajectories. The translational energy distribution of the products is in better agreement with experiment than the previous Hartree–Fock direct trajectory calculations, since the MP2 and density functional methods reproduce the barrier height better. The MP2 and density functional calculations give very good descriptions of the product rotational state distributions and the CO vibrational state populations. The MP2 method yields a very good representation of the H2 vibrational state populations, whereas the density functional methods give too little H2 vibrational excitation and Hartree–Fock produces too much. This can be attributed to the difference in the potential energy release that accompanies the formation of the H2 bond.
Band gap and lattice constant of GaxIn1−xAsySb1−y
Authors: Shim Kyurhee , Rabitz Herschel , Dutta Partha
Published: July 2002

The energy band gap and lattice constant of the quaternary alloy GaxIn1−xAsySb1−y were determined over the entire composition space (x, y) using a correlated function expansion (CFE) technique to interpolate from observed ternary compound data. Considerable anomalous band gap behavior (i.e., deep bowing) was found, which produced band gap minima as a function of composition. This deep bowing effect was experimentally observed in a quasibinary alloy (GaSb)1−z(InAs)z at low values of z. In addition, the CFE lattice matching relations of the alloy grown on GaSb and InAs were obtained as a function of composition, and the corresponding band gaps were estimated. The CFE estimates were in good agreement with existing experimental data.
Polymer waveguides useful over a very wide wavelength range from the ultraviolet to infrared
Authors: Zhao Y.-G. , Lu W.-K. , Ma Y. , Kim S.-S. , Ho S. T. , Marks T. J.
Published: July 2002

We have designed and fabricated polymer waveguides using the glassy polymers Cytop™ (a fluorinated polyether), PMMA C6 [poly(methyl methacrylate)], and Cyclotene™ 3022-35 (bisbenzocyclobutane). Since these materials exhibit excellent transparency over a wide wavelength range, and since the refractive index difference of Cytop™ and Cyclotene™ or Cytop™ and PMMA is greater than 0.19, both Cytop™/Cyclotene™/Cytop™ and Cytop™/PMMA/Cytop™ waveguide structures can be employed over a very wide wavelength range from the ultraviolet to the infrared. Efficient waveguiding is achieved for different light sources with 390, 633, 1064, 1310, and 1550 nm wavelengths.
Dynamic plowing nanolithography on polymethylmethacrylate using an atomic force microscope
Authors: Heyde M. , Rademann K. , Cappella B. , Geuss M. , Sturm H. , Spangenberg T. , Niehus H.
Published: July 2002

We present dynamic plowing nanolithography on polymethylmethacrylate films, performed with a scan-linearized atomic force microscope able to scan up to 250 μm with high resolution. Modifications of the surface are obtained by plastically indenting the film surface with a vibrating tip. By changing the oscillation amplitude of the cantilever, i.e., the indentation depth, surfaces can be either imaged or modified. A program devoted to the control of the scanning process is also presented. The software basically converts the gray scale of pixel images into voltages used to control the dither piezo driving cantilever oscillations. The advantages of our experimental setup and the dependence of lithography efficiency on scanning parameters are discussed. Some insights into the process of surface modifications are presented.
Growth and characterization of excimer laser-ablated BaBi2Nb2O9 thin films
Authors: Laha A. , Krupanidhi S. B.
Published: July 2002

The pulsed-laser ablation technique has been employed to deposit polycrystalline thin films of layered-structure ferroelectric BaBi2Nb2O9 (BBN). Low-substrate-temperature growth (Ts=400 °C) followed by ex situ annealing at 800 °C for 30 min was performed to obtain a preferred orientation. Ferroelectricity in the films was verified by examining the polarization with the applied electric field and was also confirmed from the capacitance–voltage characteristics. The films exhibited well-defined hysteresis loops, and the values of saturation (Ps) and remanent (Pr) polarization were 4.0 and 1.2 μC/cm2, respectively. The room-temperature dielectric constant and dissipation factor were 214 and 0.04, respectively, at a frequency of 100 kHz. A phase transition from a ferroelectric to paraelectric state of the BBN thin film was observed at 220 °C. The dissipation factor of the film was observed to increase after the phase transition due to a probable influence of dc conduction at high temperatures. The real and imaginary part of the dielectric constant also exhibited strong frequency dispersion at high temperatures.
Magnetic exchange interaction in a pair of orbitally degenerate ions: Magnetic anisotropy of [Ti2Cl9]−3
Authors: Borrás-Almenar J. J. , Clemente-Juan J. M. , Coronado E. , Palii A. V. , Tsukerblat B. S.
Published: July 2002

The theory of the kinetic exchange in a pair of orbitally degenerate ions developed by the authors [J. Phys. Chem. A 102, 200 (1998)] is applied to the case of face-shared bioctahedral dimer (overall D3h-symmetry). The effective kinetic exchange Hamiltonian is found for a T22–T22 system taking into account all relevant transfer pathways and charge-transfer crystal field states. The influence of different transfer integrals involved in the kinetic exchange on the energy pattern and magnetic properties of the system is examined. The role of other related interactions (trigonal crystal field, spin–orbit coupling) is also discussed in detail. Using the pseudoangular momentum representation and the technique of the irreducible tensor operators of R3-group we give a general outlook on the nontrivial symmetry properties of the effective Hamiltonian for the D3h-pair, and on the magnetic anisotropy arising from the orbital interactions specific for the case of orbital degeneracy. The magnetic properties of the binuclear unit [Ti2Cl9]−3 in Cs3Ti2Cl9 are discussed with a special emphasis on the magnetic anisotropy experimentally observed in this system. The existing exchange models for [Ti2Cl9]−3 and the concept of the effective Hamiltonian are discussed in the context of the present study.
Examining the rheology of 9-octylheptadecane to giga-pascal pressures
Authors: McCabe Clare , Cui Shengting , Cummings Peter T. , Gordon Peter A. , Saeger Roland B.
Published: July 2002

The properties of alkanes in the C20–C40 mass range are of fundamental importance in industrial applications as they are important constituents of synthetic lubricant base stocks. In an extension to earlier work on alkanes in the C20–C40 carbon number range we present the results of molecular simulations for 9-octylheptadecane, a starlike isomer of C25. Both equilibrium (EMD) and nonequilibrium molecular dynamics (NEMD) simulations have been performed under ambient state conditions and to pressures in the gigapascal range. The EMD simulations focus on calculations of the rotational relaxation times, while the NEMD simulations reveal the dependence of the viscosity on strain rate. Additionally, we calculate the viscosity number and pressure–viscosity coefficient for 9-octylheptadecane and compare the results with those obtained experimentally.