bluuebunny/crossref_metadata_2025 huggingface dataset:
Tritium production in He-3 gas cells immersed in the Tokamak Fusion Test Reactor neutron field
Authors: Jassby D. L. , Gentile C. A. , Ascione G. , Kugel H. W. , Roquemore A. L. , Kumar A.
Published: July 2002

Tritium generated in an external cell by the reaction He3(n,p)T can be used as a gauge of long-term fusion neutron production, because of the 12-year half-life of T and the relative ease of measuring the T content either by sampling or from the saturation current of the cell when operated as an ionization chamber. Two high-pressure He3 gas cells enclosed in polyethylene neutron moderators were exposed to Tokamak Fusion Test Reactor (TFTR) neutrons during high-power D–T operation. The tritium produced in the cells was assayed by the Princeton Differential Atmospheric Tritium Sampler. The measured tritium generated per 1019 fusion neutrons was 510 pCi/cc at 2.3 m from the TFTR vessel and 1.3 m below the midplane, and 2020 pCi/cc at 1.0 m from the TFTR vessel in the midplane. Combining these results with previous measurements at a third location, we found 0.11 to 0.23 triton produced per neutron incident on the projected cell cross section, with an asymptotic local tritium breeding ratio of 0.32.
Neutron emission spectrum from anisotropic DT plasmas
Authors: Gorini G. , Guadagno C. , Ballabio L. , Källne J.
Published: July 2002

The diagnostic information from fusion neutron energy spectra under anisotropic plasma conditions is investigated analytically and numerically. The ion anisotropy is parametrized by a Maxwellian velocity distribution with different temperatures along (T∥) and across (T⊥) the magnetic field. The (T∥,T⊥) dependence of selected spectral moments (peak shift, width, and kurtosis) is studied for different viewing angles.
Long-term performance of neutron detectors during deuterium–tritium operation in the Tokamak Fusion Test Reactor
Authors: Jassby D. L. , Barnes Cris W. , Bitter M. , Johnson L. C. , Lemunyan G. , Roquemore A. L. , von Goeler S.
Published: July 2002

During its tritium campaign the Tokamak Fusion Test Reactor produced more than 750 deuterium–tritium (D–T) shots involving at least one tritium neutral beam, and some 20 000 shots with D beams and no T beams. A calibrated system of four fission chambers and two Si surface barrier threshold detectors (SBDs) monitored the fusion neutron production rate in all these shots, while elemental activation foils measured total yields, Y, on more than 300 D–T shots. The baseline fission-detector current and log-Campbell modes had stable detection efficiencies throughout the D–T campaign, but there were several cross-calibration corrections as large as 10% to the efficiencies of the count-rate and log-Campbell modes of some backup fission chambers and of the collimated SBD detector. For Y>3×1016 neutrons per shot, the ratio of fission-chamber and SBD yields to activation yields varied between 0.88 and 1.1, with an average value of 0.96–0.99 that was fairly constant throughout the D–T period. All detectors and electronics performed reliably in the radiation field except for the uncollimated SBD detector, which had to be replaced periodically because of radiation damage.
Quantum well intersubband heterodyne infrared detection up to 82 GHz
Authors: Liu H. C. , Li Jianmeng , Brown E. R. , McIntosh K. A. , Nichols K. B. , Manfra M. J.
Published: July 2002

We demonstrate the heterodyne detection of two CO2 laser signals offset in frequency up to 82.16 GHz using a multiple quantum well intersubband infrared photodetector. The high frequency is reached by down conversion using the detector itself as a microwave or millimeter-wave mixer.
Electron cyclotron emission diagnostic of high temperature electron cyclotron resonance heated plasmas on Frascati tokamak upgrade
Authors: Zerbini M. , Buratti P. , Tudisco O. , Giruzzi G. , Bruschi A. , Cirant S. , Granucci G. , Simonetto A. , Sozzi C. , Gandini F. , Pacella D. , Fournier K. B. , Finkenthal M.
Published: July 2002

The electron cyclotron emission (ECE) diagnostic on FTU tokamak is routinely performed with a Michelson interferometer with spectral range extending up to 1300 GHz. The diagnostic allowed accurate electron temperature measurements during the recent 140 GHz electron cyclotron resonance heating (ECRH) experiments on FTU. Very accurate measurements have been performed on a wide range of electron temperatures and profile peaking. The ECE measurements have been compared with Thomson scattering and with observations of x-ray spectra from highly stripped molybdenum ions. The suprathermal emission in these conditions has been studied.
Imaging properties of beam line optics for undulator based third generation synchrotron facilities
Authors: Sundin S. , Bässler M. , Deneche R. , Weiss M. , Väterlein P. , Nilsson A. , Svensson S.
Published: July 2002

Beam line optics for synchrotron facilities are generally designed from geometric calculations wherein the bending magnet, undulator, or wiggler photon source is approximated by a high divergence point source. In this article we will, however, show that in the case of a synchrotron radiation source with very small divergence (such a source can be encountered at an undulator based third generation facility) the imaging properties resemble those of a laser source. This implies certain criteria for the focusing properties of the optical elements in the beam line. For instance, a beam waist is found close to the back focal plane of the optical element, with its position and size dependent on the divergence. If the divergence is determined predominantly by the diffraction part, this also means that the size and position of the beam waist is photon energy dependent. The study has been carried out by analytic treatment, numerical simulations as well as beam profile measurements carried out at beam line I511 at the MAX II synchrotron in Lund, Sweden.
Cryogenic multiple reflection absorption cell and Fourier transform spectrometer system for the far infrared
Authors: Wishnow E. H. , Leung A. , Gush H. P.
Published: July 2002

A far-infrared system used to measure weak absorption spectra of gases at pressures up to a few atmospheres and at temperatures as low as 20 K is described. The principal component is a multiple reflection mirror cell, cooled either with liquid nitrogen or with the boil-off gas from liquid helium. The cell, of 35 l volume, has an f/10 optical beam and a pathlength which can be varied from 4 to 60 m. At the longest pathlength diffraction limits the lowest usable frequency to 20 cm−1. The cell is coupled to a Fourier transform spectrometer and a 1.6 K bolometer. The apparatus has been used in a detailed study of the collision-induced pure translational band of H2, and the translation–rotation bands of N2 and N2–Ar mixtures.
Magnetic detection of cracks by fatigue in mild steels using a scanning Hall-sensor microscope
Authors: Oota A. , Ito T. , Kawano K. , Sugiyama D. , Aoki H.
Published: July 2002

We fabricated a scanning Hall-sensor microscope with an active area 50 μm×50 μm that can be served as a simple and conventional tool for nondestructive evaluation of magnetic materials. Using this, we succeeded in magnetic detection of small cracks (∼10 mm long and ∼0.1 mm wide) in mild steels with a yield point of 29 kgf/mm2, caused by a plane-bending fatigue test at a stress amplitude of 28 kgf/mm2 and a frequency of 29.2 Hz.
Very low friction rotator for use at low temperatures and high magnetic fields
Authors: Palm E. C. , Murphy T. P.
Published: July 2002

We report the development of a sample rotating stage for use in a dilution refrigerator. The rotator makes use of bearings made from commercially available sapphire vee jewels and matching pivots that have extremely low coefficients of friction. This rotator allows continuous rotations through 360° at low temperatures (30 mK) and high magnetic fields (20 T) with negligible sample heating. Rotations at rates less than 0.33 deg/s resulted in less than 50 μW of dissipated power. Rotations at rates of 4.4 deg/s resulted in 383 μW of heat dissipated. In addition, the rotator is capable of making rotations with a resolution as small as 0.020°.
Uniaxial strain method for soft crystals: Application to the control of the electronic properties of organic conductors
Authors: Maesato M. , Kaga Y. , Kondo R. , Kagoshima S.
Published: July 2002

We developed the uniaxial strain method to compress a crystalline sample along any direction without involving Poisson’s effect. The uniaxial strain is realized by inserting a composite of either the sample embedded in epoxy or the sample in frozen oil into a cylinder much harder than the sample composite followed by the application of external forces to a piston put on the sample composite. We verified, by using a strain gauge embedded in the epoxy or the frozen oil, that the strain thus created has the uniaxial nature. Resistance measurements on two kinds of organic conductors, α-(BEDT–TTF)2KHg(SCN)4 and α-(BEDT–TTF)2I3 under the uniaxial strain showed novel electric properties that have never been found under hydrostatic pressures. Their properties were found to be largely dominated by the direction of the uniaxial strain even when it is applied along directions in the conducting plane having a two-dimensional nature.
Frictional properties of microchannel plates and common detector materials
Authors: Marckwordt M. , Lampton M.
Published: July 2002

Microchannel plates for spaceborne detectors need to be firmly gripped to survive vibration environments. A variety of materials commonly employed in the fabrication of microchannel plate retaining structures were studied. We present data on the frictional properties of microchannel plates and these materials to guide detector developers in choosing the proper gripping force for a given detector mass and peak acceleration.
Transparent zinc oxide films prepared by electrochemical reaction
Authors: Izaki Masanobu , Omi Takashi
Published: July 2002

Transparent zinc oxide (ZnO) films has been cathodically deposited on conductive glasses from a simple aqueous zinc nitrate electrolyte kept at 335 K. ZnO films prepared had a wurtzite structure and exhibited an optical band gap energy of 3.3 eV which is characteristic of ZnO. A 2-μm-thick ZnO film with an optical transmittance of 72% has been deposited by electrolysis only for approximately 20 minutes at the cathodic potential of −1.0 V compared with the Ag/AgCl reference electrode.
Homoepitaxy of GaN on polished bulk single crystals by metalorganic chemical vapor deposition
Authors: Ponce F. A. , Bour D. P. , Götz W. , Johnson N. M. , Helava H. I. , Grzegory I. , Jun J. , Porowski S.
Published: July 2002

Bulk single crystals of GaN were used for epitaxial growth of GaN by metalorganic chemical vapor deposition. Photoluminescence (at 2 K) from polished substrates yields a broad near-band-edge emission band centered at 3.32 eV and the commonly observed yellow luminescence band. In contrast, the epitaxial layer displays a strong, sharp bound exciton line at 3.458 eV and a weak yellow band. Transmission electron microscopy reveals a sharp, planar interface between substrate and epilayer: The substrate contains small Ga inclusions, and the epilayer consists of less than 108 dislocations per cm2, mostly in the form of dislocation loops, which originate at the interface.
Enhancement and depletion mode GaN/AlGaN heterostructure field effect transistors
Authors: Khan M. Asif , Chen Q. , Sun C. J. , Yang J. W. , Blasingame M. , Shur M. S. , Park H.
Published: July 2002

We report on the fabrication and characterization of Al0.1Ga0.9N/GaN heterojunction field effect transistors, both an enhancement mode and a depletion mode with a low pinchoff voltage, suitable for digital integrated circuit applications. For an enhancement mode device with a 1 μm gate length and 5 μm drain-to-source separation, the dc transconductance is around 23 mS/mm. Connecting the enhancement mode device as a switching transistor and a depletion mode device as a load, we demonstrate an AlGaN/GaN inverter.
Atomic layer controlled deposition of silicon nitride with self-limiting mechanism
Authors: Goto Hiroshi , Shibahara Kentaro , Yokoyama Shin
Published: July 2002

Thin (2–10 nm) silicon nitride films have been grown by repetitive plasma nitridation of Si using a NH3 remote plasma and deposition of Si by a SiH2Cl2 thermal reaction. The deposition rate is self-limited at nearly half-molecular layer (ML) per one deposition cycle. The process window for the half-ML/cycle of growth has been investigated with respect to the NH3 plasma power, SiH2Cl2 exposure time, and substrate temperature. The thickness fluctuation of the film over a 2 in. wafer is within measurement accuracy of the ellipsometer (± 1.9%) for the atomic layer controlled film while it is ± 8.5% for all the remote-plasma chemical vapor deposition film.
Macroporous silicon with a complete two-dimensional photonic band gap centered at 5 μm
Authors: Grüning U. , Lehmann V. , Ottow S. , Busch K.
Published: July 2002

We have fabricated a two-dimensional photonic band structure based on macroporous silicon with a gap common to both polarizations and centered at 5 μm. A triangular lattice of circular air rods with a lattice constant of 2.3 μm was etched 75 μm deep in an n-type silicon substrate by electrochemical pore formation in hydrofluoric acid. The porous layer was then micromechanically structured in such a way that 200 μm thick free-standing bars of porous material were left over on the silicon substrate. These bars were then used for measuring the transmission of the photonic lattice. The results showed an excellent agreement with the theoretically calculated structure.
Lateral self-limitation in the laser-induced oxidation of ultrathin metal films
Authors: Gorbunov A. A. , Eichler H. , Pompe W. , Huey B.
Published: July 2002

cw-laser-induced local oxidation of ultrathin (3–60 nm) titanium films on glass in air is studied. It is shown, that the brightening of the films upon through-oxidation forms a negative feedback to this highly nonlinear process. It offers the possibility of stable writing of oxide line structures narrower than the diffraction limited focused laser spot. The optimum metal film thickness is of the order of the light absorption length in the metal. Transparent isolated oxide lines and gratings with periods down to 250 nm and line width down to 165 nm were recorded in 6–15 nm thick Ti films on glass by using the radiation of the Ar ion laser (λ=488, 514 nm).
Nanometer-scale local oxidation of silicon using silicon nitride islands formed in the early stages of nitridation
Authors: Tabe Michiharu , Yamamoto Takeshi
Published: July 2002

We have studied the silicon nitride (SiN) nucleation on the Si(111) 7×7 surface due to thermal nitridation with scanning tunneling microscopy (STM), and applied the resultant small SiN islands to oxidation masks in local oxidation of Si (LOCOS) process for fabrication of nanometer-scale Si structures. The nitrides appear as dark regions in STM images and the average size increases (the density decreases) with increasing nitridation temperature. When the nitrided surface is successively oxidized in the etching mode with a reaction of Si+O2→SiO↑, the nitrides turn to bright regions by selective etching of the clean 7×7 regions and the brightness (height) increases with increasing etching time. Thus, the microscopic LOCOS process is demonstrated and nanometer-scale Si pillars are fabricated.
Room-temperature luminescence from erbium-doped silicon thin films prepared by laser ablation
Authors: Komuro Shuji , Maruyama Shinya , Morikawa Takitaro , Zhao Xinwei , Isshiki Hideo , Aoyagi Yoshinobu
Published: July 2002

We present a useful and simple technique to prepare controllable Er-doped Si thin films using KrF excimer laser ablation. The sharp intense photoluminescence (PL) at 1.54 μm originating from the intra-4f shell transition in Er3+ ions was observed from 18 K up to room temperature. Characteristics of PL thermal quenching and time decay of prepared Er-doped Si thin films are very similar to those of Er-doped porous Si and/or Er-doped amorphous Si. Furthermore, observation of Er3+ emission from as-prepared thin films without thermal annealing suggests that the Er doping in the form of Er atomic radical species produced by laser ablation is essential in activation of Er3+ ions. Moreover, incorporating a prescribed amount of Er in the bulk target enables us to control the Er doping level in thin films prepared by laser ablation.
A magneto-photoluminescence investigation of the band offset between InAs and arsenic-rich InAs1−xSbx alloys
Authors: Tang P. J. P. , Pullin M. J. , Li Y. B. , Phillips C. C. , Stradling R. A. , Chung S. J. , Yuen W. T. , Hart L. , Bain D. J. , Galbraith I.
Published: July 2002

InAs/InAs0.865Sb0.135 quantum wells are characterized using magneto-photoluminescence. Band- to-band transitions are found at energies lower than the band gaps of either the InAs or the InAs0.865Sb0.135 with photoluminescence emission at wavelengths up to 4.8 μm. By modeling the quantum size shifts of the photoluminescence transitions and their energy shift in a magnetic field, the valence band offset between InAs and In(As,Sb) is deduced to be type II with electron confinement in the In(As,Sb) alloy and hole confinement in InAs.
A near-field scanning optical microscopy study of the uniformity of GaAs surface passivation
Authors: Liu Jutong , Kuech T. F.
Published: July 2002

We have achieved spatially resolved photoluminescence (PL) from metalorganic vapor phase epitaxy (MOVPE) grown GaAs surfaces by near-field scanning optical microscopy (NSOM). We have performed the topography, reflection, and PL measurements by NSOM combined with the topography measurements by atomic force microscopy (AFM) on the as-grown and (NH4)2S-passivated GaAs samples. The uniformity of GaAs with a thin Al0.65Ga0.35As cap layer has also been studied and compared with the (NH4) 2S treated samples. We found the submicron scale variations in PL intensity which were not correlated to the topographic features. The PL intensity variation was related to the changes in the surface state density. Semiquantitative analyses of the resolution limits of NSOM-based PL measurements and surface state variations are presented.
Use of poly(phenyl quinoxaline) as an electron transport material in polymer light-emitting diodes
Authors: O’Brien D. , Weaver M. S. , Lidzey D. G. , Bradley D. D. C.
Published: July 2002

We report the use of a poly(phenyl quinoxaline) (PPQ) as an electron transporting conjugated polymer for electroluminescence (EL) applications. Single-layer PPQ devices with ITO anode and aluminium cathode show unipolar electron transport with current densities up to 60 mA/cm2 but no emission. Two-layer structures combining PPQ as electron transport material with the hole transporting poly(2,5-dialkoxy-p-phenylene vinylene) (PDAOPV) show strong emission from the PDAOPV with brightnesses up to 250 cd/m2. These two-layer structures have a maximum EL quantum efficiency of 0.35% which is ten-fold enhanced compared with the corresponding single-layer PDAOPV devices.
Anisotropy of boron nitride thin-film reflectivity spectra by generalized ellipsometry
Authors: Schubert M. , Rheinländer B. , Franke E. , Neumann H. , Hahn J. , Röder M. , Richter F.
Published: July 2002

Generalized variable angle spectroscopic ellipsometry (gVASE) over the photon energy range from 1.5 to 3.5 eV has been used to study and distinguish the hexagonal and cubic phases of boron nitride in thin films (50–500 nm) deposited by magnetron sputtering onto (100) silicon. Furthermore, gVASE is used to characterize the anisotropic reflectivity of the hexagonal phase films with different microstructures.
Synthesis and characterization of hard magnetic materials: PrFe10.5V1.5Nx
Authors: Mao Weihua , Cheng Benpei , Yang Jinbo , Pei Xiedi , Yang Yingchang
Published: July 2002

The PrFe10.5V1.5 intermetallics and their nitrides were successfully synthesized. In terms of magnetocrystalline anisotropy, PrFe10.5V1.5Nx are characteristics of an easy axis from 0 K to Curie temperature, with an anisotropy field up to 152.9 kOe at 1.5 K and 108.4 kOe at room temperature. In combination with a high Curie temperature of 820 K and a large saturation magnetization of 157.46 emu/g at 1.5 K and 142.77 emu/g at room temperature, these nitrides are favorable for permanent magnet applications. As a preliminary attempt, magnetic powders based on PrFe10.5V1.5Nx were obtained with a maximum energy product of 16.0 and 28.8 MGOe at room temperature and 1.5 K, respectively.
Evanescent-light guiding of atoms through hollow optical fiber for optically controlled atomic deposition
Authors: Ito H. , Sakaki K. , Ohtsu M. , Jhe W.
Published: July 2002

This work demonstrates that the technique of guiding atoms through hollow optical fiber escorted by evanescent light is useful as a novel scheme of optical atom deposition. To show the feasibility of fabricating micron-sized structures with nanometric depth on a substrate, we measure the spatial distribution of the guided atom flux with a hot-wire detector. Moreover, precise control of the deposition rate is illustrated with a 1.4-μm-hollow fiber via photoionization spectroscopy. The ratio of the guided atom flux relative to the background transmission is enhanced up to 80-fold with a slightly tilted hollow fiber.
Resonant-tunneling transmission-line relaxation oscillator
Authors: Brown E. R. , Parker C. D. , Verghese S. , Geis M. W. , Harvey J. F.
Published: July 2002

Experimental and numerical results are presented for a high-frequency oscillator consisting of a resonant-tunneling diode (RTD) series-embedded in a transmission line, one end of which is short circuited and the other end terminated with a load resistor. Like relaxation oscillators, the ac voltage across the RTD is a square wave. However, the current wave form (and hence the load wave forms) consists of a sequence of sharp pulses that are essentially locked to the fundamental mode of the transmission line.
Micro-Raman characterization of the electronic properties of Si-doped GaAs layers grown on patterned substrates
Authors: Gerster J. , Schneider J. M. , Ehret C. , Limmer W. , Sauer R. , Heinecke H.
Published: July 2002

The coupled modes of plasmons and LO phonons appearing in the Raman spectra of highly doped GaAs layers have been investigated in order to study the local incorporation character of Si on different crystal facets. The Si-doped GaAs layers were grown by molecular beam epitaxy on patterned GaAs (100) substrates with etched ridges forming (111)A and (111)B facets with a lateral extension of a few micrometers. The local type and concentration of free charge carriers have been determined from an analysis of the coupled-mode Raman spectra. It is shown that Si acts as a donor in the material grown on the (111)B facets and as an acceptor in the material grown on the (111)A facets.
Growth of epitaxial GaN films by pulsed laser deposition
Authors: Vispute R. D. , Talyansky V. , Sharma R. P. , Choopun S. , Downes M. , Venkatesan T. , Jones K. A. , Iliadis A. A. , Asif Khan M. , Yang J. W.
Published: July 2002

High crystalline quality epitaxial GaN films with thicknesses 0.5–1.5 μm have been successfully grown directly on Al2O3(0001) substrate by pulsed laser deposition (PLD). For films grown at 950 °C, we obtained an x-ray diffraction rocking curve linewidth of 7 arc min. The ion channeling minimum yield in the near-surface region (∼2000 Å) for a 0.5 μm thick film was ∼3%–4% indicating a high degree of crystallinity. The optical absorption edge measured by UV-visible spectroscopy was sharp, and the band gap was found to be 3.4 eV. The crystalline properties of these PLD GaN films are comparable to those grown by metalorganic chemical vapor deposition and molecular beam epitaxy.
Magnetoresistance in the oxygen deficient LnBaCo2O5.4 (Ln=Eu, Gd) phases
Authors: Martin C. , Maignan A. , Pelloquin D. , Nguyen N. , Raveau B.
Published: July 2002

New “112” phases, LnBaCo2O5.4, with an ordered oxygen deficient perovskite structure, derived from the YBaFeCuO5-type were studied for Ln=Eu, Gd. The appearance of giant negative magnetoresistance in this structural type is demonstrated. Resistance ratio R0/R7 T reaches at least 10 at 10 K, i.e., is significantly larger than those observed in the other cobalt perovskites, such as La1−xSrxCoO3. These properties are linked to an original magnetic behavior of these materials that exhibit two types of transition—antiferromagnetic to ferromagnetic, and ferromagnetic to paramagnetic—as T increases. This magnetic behavior may be related to a possible I.S. and L.S. spin ordering of trivalent cobalt in pyramidal and octahedral coordinations, respectively.
Triggering a radial multichannel pseudospark switch using electrons emitted from material with high dielectric constant
Authors: Bergmann K. , Lebert R. , Kiefer J. , Neff W.
Published: July 2002

The electron emission from a field emission trigger unit was matched to the requirements for the simultaneous ignition of all channels in a radial multichannel pseudospark switch for a large parameter range of the pseudospark discharge. In such trigger unit the electrons are extracted from the surface of a high-ε material by field emission after applying a pulsed voltage. Electron emission occurs in a pressure range from 1–50 Pa, where the pseudospark discharge is operated. The access to low working gas pressures (<10 Pa) using such trigger unit allows for achieving high hold-off voltages. The ignition of the pseudospark discharge was investigated by means of high speed photography and by a parallel observation of the electron beam of a single discharge channel, which occurs in hollow-cathode phase.
Absorption coefficient, energy gap, exciton binding energy, and recombination lifetime of GaN obtained from transmission measurements
Authors: Muth J. F. , Lee J. H. , Shmagin I. K. , Kolbas R. M. , Casey H. C. , Keller B. P. , Mishra U. K. , DenBaars S. P.
Published: July 2002

The absorption coefficient for a 0.4-μm-thick GaN layer grown on a polished sapphire substrate was determined from transmission measurements at room temperature. A strong, well defined exciton peak for the A and B excitons was obtained. The A, B, and C excitonic features are clearly defined at 77 K. At room temperature, an energy gap Eg=3.452±0.001 eV and an exciton binding energy ExA,B=20.4±0.5 meV for the A and B excitons and ExC=23.5±0.5 meV for the C exciton were determined by analysis of the absorption coefficient. From this measured absorption coefficient, together with the detailed balance approach of van Roosbroek and Shockley, the radiative constant B=1.1×10−8 cm3/s was obtained.
Study of Ge bonding and distribution in plasma oxides of Si1−xGex alloys
Authors: Seck M. , Devine R. A. B. , Hernandez C. , Campidelli Y. , Dupuy J.-C.
Published: July 2002

Secondary ion mass spectroscopy (SIMS) and Fourier transform infrared (FTIR) absorption have been studied in thin oxides of Si1−xGex grown by plasma oxidation. SIMS analysis reveals that Ge can migrate to the oxide film surface leaving the oxide in the SiGe interface region Ge-depleted. This is in contrast to thermally grown oxides. Water selectively attacks the Ge-rich part of the oxide. In the FTIR spectra of the SiGe oxides, specific peaks identified with the vibration of O in Si–O–Ge and Ge–O–Ge bonds have been observed for the first time. These latter observations confirm that for the plasma oxidized films, the Ge is chemically bonded in the oxide network.
Chemical interactions at Ta/fluorinated polymer buried interfaces
Authors: Yang G.-R. , Zhao Y.-P. , Wang B. , Barnat E. , McDonald J. , Lu T.-M.
Published: July 2002

In this letter, we study Ta/parylene-F (PA–F) buried interfaces using x-ray photoelectron spectroscopy. We found that the Ta–F bond was formed at the Ta/PA–F interface after depositing a layer of thin Ta film (<50 Å). For the Ar+ or O2 plasma pretreated PA–F surface, in addition to the Ta–F bond, a Ta–C bond was observed at the buried interface after Ta metallization. The Ta–C bond may be responsible for the enhancement of Ta/PA–F adhesion.
High-resolution x-ray spectra measured using tantalum superconducting tunnel junctions
Authors: Verhoeve P. , Rando N. , Peacock A. , van Dordrecht A. , Taylor B. G. , Goldie D. J.
Published: July 2002

The spectral response of a 100×100 μm2 tantalum based superconducting tunnel junction to 5.9 keV x-ray photons from a Fe55 source has been studied. In full illumination the energy resolution for the Mn Kα line complex is 56 eV, dominated by spatial nonuniformity in the response of the detector. When illuminating selectively a 5–10 μm diam spot in the center of the detector, the energy resolution improves to 22 eV, corresponding to 15.7 eV for the individual Mn Kα1 and Mn Kα2 lines. This exceeds the predicted theoretical energy resolution of 7.3 eV for this type of device by only a factor of ∼2.
Synchrotron x-ray microbeam diagnostics of combinatorial synthesis
Authors: Isaacs E. D. , Marcus M. , Aeppli G. , Xiang X.-D. , Sun X.-D. , Schultz P. , Kao H.-K. , Cargill G. S. , Haushalter R.
Published: July 2002

X-ray microbeam techniques (spot size=3×20 μm2) have been applied to characterize the composition and structure of rare earth activated Gd(La, Sr)AlO3 phosphor thin films grown by combinatorial synthesis. Using x-ray fluorescence, x-ray diffraction and near-edge x-ray absorption spectroscopy, we have measured the chemical composition, crystallographic structure, and valence state of the rare earth activator atom Eu. These measurements represent the direct application of x-ray techniques to solid-state materials prepared by combinatorial synthesis and demonstrate the power of x-ray microbeam analysis to nondestructively characterize as-grown combinatorial libraries.
Intense short-wavelength photoluminescence from thermal SiO2 films co-implanted with Si and C ions
Authors: Zhao J. , Mao D. S. , Lin Z. X. , Jiang B. Y. , Yu Y. H. , Liu X. H. , Wang H. Z. , Yang G. Q.
Published: July 2002

Intense short-wavelength photoluminescence (PL) observed at room temperature from thermal SiO2 films co-implanted with Si and C is reported. A flat Si profile was first implanted, followed by 1100 °C annealing for 60 min. C ions were subsequently used to be implanted into the same depth region. PL was observed from the as-implanted samples with and without annealing. The PL intensity increases with annealing temperature. Comparing the PL spectra and the PL dynamics of the C-implanted, annealed, Si-implanted (CIASI) SiO2 films with those from Si- and C-implanted SiO2 films suggests that the interaction of Si and C in SiO2 films plays an important role in the luminescence in CIASI SiO2 films.
Epitaxial growth of (001)-oriented and (110)-oriented SrBi2Ta2O9 thin films
Authors: Lettieri J. , Jia Y. , Urbanik M. , Weber C. I. , Maria J-P. , Schlom D. G. , Li H. , Ramesh R. , Uecker R. , Reiche P.
Published: July 2002

Epitaxial SrBi2Ta2O9 thin films have been grown with (001) and (110) orientations by pulsed laser deposition on (001) LaAlO3–Sr2AlTaO6 and (100) LaSrAlO4 substrates, respectively. Four-circle x-ray diffraction and transmission electron microscopy reveal nearly phase pure epitaxial films. Minimization of surface mesh mismatch between the film and substrate (i.e., choice of appropriate substrate material and orientation) was used to stabilize the desired orientations and achieve epitaxial growth.
Metal–semiconductor–metal photodiodes fabricated from thin-film diamond
Authors: Looi Hui Jin , Whitfield Michael D. , Jackman Richard B.
Published: July 2002

A metal–semiconductor–metal Schottky barrier photodetector has been fabricated on a “hydrogen-doped” surface-conducting chemical vapor deposition (CVD) diamond. The device is fabricated in one step by forming two back-to-back aluminum Schottky diodes on the p-type surface. This simple process is compatible with previously reported metal–semiconductor field-effect transistor fabrication on this type of CVD diamond and offers the prospect of the monolithic integration of a ultraviolet detector and active circuitry. Preliminary electrical and optical characteristics of the device have been measured, including the spectral response over the range 180–800 nm. The device exhibits a linear response with the applied optical power at 220 nm, operates at a bias of only 2 V, and shows visible blind characteristics, with a spectral discrimination of three orders of magnitude as determined from the ratio of 200–550 nm responses.
Theoretical transport studies of p-type GaN/AlGaN modulation-doped heterostructures
Authors: Hsu L. , Walukiewicz W.
Published: July 2002

We have calculated hole transfer and low-temperature mobilities in p-type GaN/AlGaN modulation-doped heterostructures. Although substantial p-type conduction is difficult to achieve in bulk nitrides, the strain-induced polarization field can greatly enhance the transfer of holes from relatively deep Mg acceptors in the AlGaN barrier into the GaN well. The calculations predict formation of a two-dimensional hole gas with densities greater than 1012 cm−2 and with low-temperature mobilities in excess of 104 cm2/V s.
Nanocrystalline composites with high strength obtained in Zr–Ti–Ni–Cu–Al bulk amorphous alloys
Authors: Fan Cang , Louzguine Dmitri V. , Li Chunfei , Inoue Akihisa
Published: July 2002

Nanocrystalline composites with the grain size less than 10 nm were produced by annealing of Cu-mold cast Zr70−x−yTixNi10Cu20Aly (X=5–7.5 and Y=10–15 at %) bulk amorphous alloys. The nanostructured alloys show increased tensile strength at the volume fraction of nanoparticles less than 30%. The microstructure of the amorphous alloys was found to contain medium range order (MRO) domains, which uniformly distributed in the amorphous matrix. We suggest that MRO domains provide nucleation sites for precipitation of the primary crystals and lead to the formation of nanocrystalline composites.
1.55 μm reflection-type optical waveguide switch based on SiGe/Si plasma dispersion effect
Authors: Li Baojun , Li Guozheng , Liu Enke , Jiang Zuimin , Pei Chengwen , Wang Xun
Published: July 2002

Based on total internal reflection and plasma dispersion effect, a SiGe/Si asymmetric optical waveguide switch with transverse injection structure has been proposed and fabricated. The switch performance is measured at the wavelength of 1.55 μm. A modulation depth of 90% at an injection current of 110 mA is obtained, and the switching time is about 0.2 μs. The device reaches a maximum optical switching at the injection current of 120 mA. The extinction ratio is larger than 34 dB and the crosstalk and insertion loss are less than −18.5 and 2.86 dB, respectively.
Local charge trapping and detection of trapped charge by scanning capacitance microscope in the SiO2/Si system
Authors: Hong J. W. , Shin S. M. , Kang C. J. , Kuk Y. , Khim Z. G. , Park Sang-il
Published: July 2002

Charge trapping induced by hot carriers in a metal–oxide–semiconductor capacitor was studied with a scanning capacitance microscope (SCM). The local charge trapping/detrapping and the readout of the trapped charge in SiO2 was performed using a SCM combined with an atomic force microscope. When applying a voltage between the conductive probe tip and the p-type silicon substrate, hot carriers injected from silicon substrate into the SiO2 generated a positive trapped charge in the 10-nm-thick SiO2 layer. The resulting shift in capacitance–voltage (C–V) curves due to the locally trapped charge was measured by the high-frequency C–V measurement. For an application to the ultrahigh density data storage field, we investigated how the charge trapping and detrapping characteristics depend on the writing speed and bias voltage.
Thermal and electrical transport properties of the single-phase quasicrystalline material: Al70.8Pd20.9Mn8.3
Authors: Pope A. L. , Tritt Terry M. , Chernikov M. A. , Feuerbacher M.
Published: July 2002

The electrical and thermal transport properties of a single-phase quasicrystalline material of composition Al70.8Pd20.9Mn8.3 have been measured as a function of temperature (T) over a range of 10 K<T<320 K. The following room-temperature values were found: electrical conductivity [640 (Ω cm)−1], Seebeck coefficient (85 μV/K), and thermal conductivity (1.6 W/m K). As the temperature was decreased from room temperature, the electrical conductivity decreased undergoing a minimum at T≈50 K after which it increased slightly and the Seebeck coefficient decreased monotonically. The thermal conductivity decreased with temperature and exhibited a broad plateau region between 50 K<T<150 K. This class of materials is being investigated for potential thermoelectric applications.
Nanomachining of (110)-oriented silicon by scanning probe lithography and anisotropic wet etching
Authors: Chien F. S.-S. , Wu C.-L. , Chou Y.-C. , Chen T. T. , Gwo S. , Hsieh W.-F.
Published: July 2002

We have demonstrated that silicon nanostructures with high aspect ratios, having ∼400 nm structural height and ∼55 nm lateral dimension, may be fabricated by scanning probe lithography and aqueous KOH orientation-dependent etching on the H-passivated (110) Si wafer. The high spatial resolution of fabricated features is achieved by using the atomic force microscope based nano-oxidation process in ambient. Due to the large (110)/(111) anisotropic ratio of etch rate and the large Si/SiO2 etch selectivity at a relatively low etching temperature and an optimal KOH concentration, high-aspect-ratio gratings with (111)-oriented structural sidewalls as well as hexagonal etch pit structures determined by the terminal etch geometry can be obtained.
Efficient Auger-excitation of erbium electroluminescence in reversely-biased silicon structures
Authors: Bresler M. S. , Gusev O. B. , Pak P. E. , Yassievich I. N.
Published: July 2002

In p–n junctions based on c-Si:Er, we have observed strongly efficient excitation of erbium electroluminescence at 1.54 μm. Excitation of erbium ions is accompanied by strong recombination of free carriers indicating a participation of an Auger mechanism. A possible excitation mechanism is proposed which is the Auger recombination of electrons occupying the upper subband of the conduction band with free holes in the valence band, whereas the energy of the recombination process is transferred by Coulomb interaction to 4f electrons of an erbium ion transmitting it to the second excited state I411/2 (excitation energy 1.26 eV). The observed three-level excitation of erbium ions is promising for the development of a Si:Er laser.
Nanotube electronic states observed with thermal field emission electron spectroscopy
Authors: Dean Kenneth A. , Groening Oliver , Küttel Olivier M. , Schlapbach L.
Published: July 2002

We observe nonmetallic electronic states above the Fermi level in single-walled carbon nanotubes by measuring the energy distribution of thermal-field-emitted electrons. This measurement method examines electronic states associated with the nanotube cap or end termination, and with it, we resolve electronic states greater than 3 eV above the Fermi level. The observed emitting states are broad at high temperatures (0.7–1.5 eV full width at half maximum), and the peak positions shift linearly with applied voltage. We present possible mechanisms responsible for these states.
Electron-stimulated conversion of chemisorbed O to Al2O3 on Al(111)
Authors: Popova I. , Zhukov V. , Yates J. T.
Published: July 2002

The transformation of a chemisorbed O layer on the Al(111) surface into Al2O3 under electron impact has been monitored by high-resolution electron energy-loss and x-ray photoelectron spectroscopies. Electron irradiation (100 eV) of chemisorbed O was observed to cause spectroscopic changes in the surface layer indicative of oxide formation. The energy transfer from the electrons to the chemisorbed O atoms through electronic excitations, leading to vibrationally excited adsorbed particles, is proposed to be responsible for the observed conversion of chemisorbed oxygen to Al2O3. The cross section for this electron-induced process is estimated to be 2×10−19 cm2 for 100 eV electrons.
Field emission study of gated GaN and Al0.1Ga0.9N/GaN pyramidal field emitter arrays
Authors: Kozawa T. , Ohwaki T. , Taga Y. , Sawaki N.
Published: July 2002

A self-aligned process has been proposed for fabrications of gated GaN and Al0.1Ga0.9N/GaN pyramidal field emitters with a small and precise distance between the gate electrode and the emitter tip. The sharp pyramidal emitters were obtained by a selective area growth technique. SiO2 and polyimide were used as the insulating layers between the emitters and the gate electrode. The gate-tip spacing and the tip protrusion through the gate openings were precisely controlled by adjusting the thickness of the SiO2 and polyimide layers. The turn-on voltage was reduced by narrowing the gate-tip spacing and by using Al0.1Ga0.9N/GaN emitter instead of GaN; the low onset gate voltage of 42 V was obtained for a gate-tip spacing of 0.4 μm.
Piezoelectric field-enhanced second-order nonlinear optical susceptibilities in wurtzite GaN/AlGaN quantum wells
Authors: Liu Ansheng , Chuang S.-L. , Ning C. Z.
Published: July 2002

Second-order nonlinear optical processes including second-harmonic generation, optical rectification, and difference-frequency generation associated with intersubband transitions in wurtzite GaN/AlGaN quantum well (QW) are investigated theoretically. Taking into account the strain-induced piezoelectric (PZ) effects, we solve the electronic structure of the QW from coupled effective-mass Schrödinger equation and Poisson equation including the exchange-correlation effect under the local-density approximation. We show that the large PZ field in the QW breaks the symmetry of the confinement potential profile and leads to large second-order susceptibilities. We also show that the interband optical pump-induced electron-hole plasma results in an enhancement in the maximum value of the nonlinear coefficients and a redshift of the peak position in the nonlinear optical spectrum. By use of the difference-frequency generation, THz radiation can be generated from a GaN/Al0.75Ga0.25N with a pump laser of 1.55 μm.
Coulomb-blockade effect observed at room temperature in Ge nanocrystalline films deposited by the cluster-beam evaporation technique
Authors: Banerjee Souri , Nozaki S. , Morisaki H.
Published: July 2002

The temperature-dependent current–voltage (I–V) characteristics have been studied across the thickness of Ge nanocrystalline films prepared by the cluster beam evaporation technique. It is found that a film with a thickness of 30 nm, deposited on a substrate kept at 77 K, does not exhibit any distinct step-like feature in the I–V characteristics at room temperature. However, with the lowering of the temperature, a “Coulomb gap” is observed and a pronounced step-like feature appear in the I–V characteristics suggesting the Coulomb blockade (CB) effect. It is hypothesized that the observed CB effect in these nanocrystalline thin films results from a very highly selective electron transport process where the electron transport is dominated by one local well-defined current path with the largest conductance. The result of similar measurements on a photo-oxidized sample, which shows a signature of a step-like feature in the I–V characteristics, even at room temperature, supports this hypothesis.
Pinhole analysis in magnetic tunnel junctions
Authors: Schad R. , Allen D. , Zangari Giovanni , Zana Iulica , Yang D. , Tondra Mark , Wang Dexin
Published: July 2002

Pinholes in the insulating layer of magnetic tunnel junctions are local shortcuts and cause malfunction of such devices. The need for reduction of the tunnel resistance by reduction of the insulator thickness will make this problem even more severe. Therefore, the development of low-resistance magnetic tunnel junctions requires analyzing the pinhole density. We developed a method for pinhole imaging using electrodeposition of copper. Selective nucleation at pinholes produces characteristic structures that can be visualized by conventional microscopy techniques. The experimental conditions were carefully chosen in order to avoid uncontrolled damage of the insulator layer.
Compositionally step-varied (Pb, Ca)TiO3 thin films with enhanced dielectric and ferroelectric properties
Authors: Bao Dinghua , Zhang Liangying , Yao Xi
Published: July 2002

Compositionally step-varied (Pb, Ca)TiO3 thin films were prepared on platinum-coated silicon substrates by a monoethanolamine-modified sol-gel technique. The dielectric constant and dissipation factor were found to be 342 and 0.019, respectively, for thin films of 0.6μm in thickness annealed at 550 °C for 60 min. The remanent polarization and coercive field were 37.7 μC/cm2 and 60.4 kV/cm, respectively. The leakage current was 1.31×10−7 A/cm2 at the voltage of 5 V. The compositionally step-varied (Pb, Ca)TiO3 thin films had enhanced dielectric and ferroelectric properties which were suitable for various device applications.
Laser direct writing of phosphor screens for high-definition displays
Authors: Fitz-Gerald J. M. , Piqué A. , Chrisey D. B. , Rack P. D. , Zeleznik M. , Auyeung R. C. Y. , Lakeou S.
Published: July 2002

A laser-based forward transfer direct writing technique was used to deposit phosphor powder screens for high-resolution display applications. With this technique, called matrix-assisted pulsed-laser evaporation direct write, dense oxide phosphor powders of Y2O3:Eu (red) and Zn2SiO4:Mn (green) were deposited on alumina and polymer substrates. All processing was performed in air at room temperature. Cathodoluminescent measurements showed that the luminous efficiency of the phosphor powders was not degraded by the deposition process. A 6×6 red and green matrix with pixel sizes of 100 μm (250 lines per inch) with a 100 μm spot size is demonstrated; however, with smaller spot sizes this technique is easily scalable to pixel sizes <10 μm (>2500 lines per inch).
The kinetics and mechanism of scanned probe oxidation of Si
Authors: Snow E. S. , Jernigan G. G. , Campbell P. M.
Published: July 2002

We report measurements of the kinetics of scanned probe oxidation under conditions of high humidity and pulsed bias. For a hydrophobic Si surface the oxidation rate for short pulse times (∼10 ms) is controlled by the density of H2O molecules in the ambient humidity surrounding the tip-sample interface. At longer pulse times (∼0.1 s) liquid H2O bridges this interface and the maximum oxidation rate increases by a factor of ∼104 because of the increased density of H2O molecules. We propose that the rate-limiting step of the oxidation process is the production of O anions from the ambient humidity.
Thermal stability and electrical characteristics of ultrathin hafnium oxide gate dielectric reoxidized with rapid thermal annealing
Authors: Lee Byoung Hun , Kang Laegu , Nieh Renee , Qi Wen-Jie , Lee Jack C.
Published: July 2002

Dielectric properties of ultrathin hafnium oxide reoxidized with rapid thermal annealing (RTA) have been investigated. Capacitance equivalent oxide thickness (CET) of 45 Å hafnium oxide was scaled down to ∼10 Å with a leakage current less than 3×10−2 A/cm2 at −1.5 V (i.e., ∼2 V below VFB). Leakage current increase due to crystallization was not observed even after 900 °C rapid thermal annealing (RTA), but CET did increase after high temperature RTA due to the interfacial layer growth and possible silicate formation in the HfO2 film.
Dependence of impurity incorporation on the polar direction of GaN film growth
Authors: Sumiya M. , Yoshimura K. , Ohtsuka K. , Fuke S.
Published: July 2002

We have investigated the dependence of impurity incorporation on the polar direction of GaN growth by using secondary ion mass spectroscopy (SIMS). GaN films were deposited under conditions used for growing device-quality materials on sapphire substrates while controlling their polar direction. It was found that the polarity of the GaN film influences the incorporation of impurities. SIMS analysis has revealed that the impurities related to carbon, oxygen, and aluminum are more readily incorporated into N-face GaN films.