bluuebunny/crossref_metadata_2025 huggingface dataset:
Improvement of ohmic contacts on GaAs with in situ cleaning
Authors: Ren F. , Emerson A. B. , Pearton S. J. , Fullowan T. R. , Brown J. M.
Published: July 2002

An in situ argon ion mill clean step prior to ohmic metal deposition has been demonstrated to improve the uniformity of the contact parameters and reduce the contact resistance. After ion mill cleaning, the native oxide regrowth of molecular beam epitaxy grown GaAs and AlGaAs layers in vacuum chamber was also studied to optimize the processing. These oxide layers were identified as the cause of problems in the formation of good ohmic contacts to the GaAs or AlGaAs.
Molecular layer etching of GaAs
Authors: Aoyagi Yoshinobu , Shinmura Kohji , Kawasaki Kiyoshi , Tanaka Tomoko , Gamo Kenji , Namba Susumu , Nakamoto Ichirou
Published: July 2002

Layer-by-layer controlled molecular layer self-limiting etching at one molecular layer of GaAs was successfully achieved for the first time by alternatively feeding an etchant of Cl and applying a low energy Ar ion beam to the GaAs substrate. The etching rate saturates exactly at one molecular layer per cycle and is independent of etchant feeding rate and the energetic ion beam flux.
Arsenic precipitates in Al0.3Ga0.7As/GaAs multiple superlattice and quantum well structures
Authors: Mahalingam K. , Otsuka N. , Melloch M. R. , Woodall J. M.
Published: July 2002

Arsenic precipitates in Al0.3Ga0.7As/GaAs multiple superlattices and quantum well structures which were grown at low substrate temperatures by molecular beam epitaxy were studied by transmission electron microscopy. Novel precipitate microstructures were observed in annealed samples, including confinement of precipitates in GaAs wells and nearly complete depletion of precipitates in a short period superlattice. It is shown that these observed microstructures can be explained as a result of the difference of precipitate/matrix interfacial energies in GaAs and Al0.3Ga0.7As.
Single-crystalline, epitaxial cubic SiC films grown on (100) Si at 750 °C by chemical vapor deposition
Authors: Golecki I. , Reidinger F. , Marti J.
Published: July 2002

Single-crystalline, epitaxial cubic (100) SiC films have been grown on (100) Si substrates at 750 °C by low-pressure chemical vapor deposition, using methylsilane, SiCH3H3, a single precursor with a Si:C ratio of 1:1, and H2. This epitaxial growth temperature is the lowest reported to date. The films were characterized by means of transmission electron microscopy, x-ray diffraction, infrared transmission, four-point probe and other methods. Based on double-crystal x-ray diffractometry, the crystalline quality of our films is equivalent to that of commercial films of similar thickness. The letter describes the novel growth apparatus used and the properties of the films.
Observation of enhanced hydrogen diffusion in solar cell silicon
Authors: Sopori Bhushan L. , Jones Kim , Deng Xiao Jun
Published: July 2002

We report the observation of higher diffusivity of hydrogen in some solar cell silicon compared to that in Czochralski and float zone wafers. SIMS profiles of hydrogen/deuterium, implanted at low energies and in a temperature range of 100–300 °C, are compared for a variety of different types of silicon substrates. In addition, a new technique that utilizes hydrogen decoration of dislocations was applied to directly verify long diffusion depths in some solar cell silicon. Higher diffusivity of hydrogen permits backside hydrogenation of solar cells to be carried out in less than 30 min with a significant improvement in the cell performance.
Fabrication and characteristics of 8-hydroxyquinoline aluminum/aromatic diamine organic multiple quantum well and its use for electroluminescent diode
Authors: Ohmori Yutaka , Fujii Akihiko , Uchida Masao , Morishima Chikayoshi , Yoshino Katsumi
Published: July 2002

Multiple quantum well structure consisting of alternating layers of organic 8-hydroxyquinoline aluminum (Alq3) and aromatic diamine (TPD) has been grown by organic molecular beam deposition. The multiple quantum well structure was determined by x-ray diffraction, optical absorption, and photoluminescence. Photoluminescence peak of Alq3 shifts to higher energy with decreasing layer thickness, suggesting a quantum size effect. An electroluminescent diode has also been fabricated by using Alq3/TPD multiple quantum well structure.
Ho3+ to Yb3+ back transfer and thermal quenching of upconversion green emission in fluoride crystals
Authors: Zhang X. X. , Hong P. , Bass M. , Chai B. H. T.
Published: July 2002

Energy transfer from Ho3+ to Yb3+ has been demonstrated in fluoride crystals such as KYF4, BaY2F8, and LiYF4. This transfer was found to occur above a certain temperature and its efficiency increases with temperature. It accounts for the thermal quenching of green emission from upconverted Ho3+ and partly explains the failure, to date, to obtain room temperature, Yb3+ sensitized Ho3+ upconversion lasers in these three hosts. Low Yb3+ concentration is suggested for future attempts to achieve room temperature Yb3+, Ho3+ upconversion green laser operation of these crystals.
Adsorbate effects on photoluminescence and electrical conductivity of porous silicon
Authors: Ben-Chorin Moshe , Kux Andreas , Schechter Israel
Published: July 2002

Porous silicon (p type) has been exposed to several chemical vapors at various partial pressures. The quenching of the photoluminescence by the adsorbates has been quantified and correlation to the electrical conductivity of the porous silicon sample has been studied. Some gases, e.g., water and benzene, have a small effect on the photoluminescence and on the conductivity, while others, e.g., methanol, reduce the photoluminescence by a factor of 2 and increase the conductivity by four orders of magnitude. This is accompanied with a qualitative change in the current-voltage characteristics. These changes have been found to be reversible and the temporal behavior of the system has been investigated.
Acoustic microscopy by atomic force microscopy
Authors: Rabe U. , Arnold W.
Published: July 2002

We have constructed an atomic force microscope enabling one to image the topography of a sample, and to monitor simultaneously ultrasonic surface vibrations in the MHz range. For detection of the distribution of the ultrasonic vibration amplitude, a part of the position-sensing light beam reflected from the cantilever is directed to an external knife-edge detector. Acoustic images taken on the surface of a wafer show a lateral resolution of about 100 nm at an ultrasonic frequency of 20 MHz.
High current YBa2Cu3O7−δ thick films on flexible nickel substrates with textured buffer layers
Authors: Wu X. D. , Foltyn S. R. , Arendt P. , Townsend J. , Adams C. , Campbell I. H. , Tiwari P. , Coulter Y. , Peterson D. E.
Published: July 2002

High current YBa2Cu3O7−δ (YBCO) thick films on flexible nickel substrates with textured buffer layers were fabricated. Highly textured yttria-stabilized-zirconia buffer layers were deposited by using ion beam assisted deposition (IBAD). Pulsed laser deposited YBCO films were not only c-axis oriented with respect to the film surface but also strongly in-plane textured. The in-plane mosaic spread of YBCO films was ∼10°. A critical current density of 8×105 A/cm2 was obtained at 75 K and zero field for thin YBCO films. It was also demonstrated that thick YBCO films with a high critical current and excellent magnetic field dependence at liquid nitrogen temperature can be obtained on flexible nickel substrates by using the textured buffer layers. The result indicates that thick film technology in combination with IBAD buffer layers could be a viable method for fabricating YBCO tapes in long lengths.
Reproducible sharp-pointed tip preparation for field ion microscopy by controlled ac polishing
Authors: Morikawa Hiroshi , Goto Keisuke
Published: July 2002

Sharp-pointed tip preparation method by ac polishing has been improved by controlling the applying voltage, wave shape, phase angle, frequency, and number of waves. This was performed by designing a controller in cooperation with a function generator and an audio amplifier.
Technique for measuring the frequency-dependent complex dielectric constants of liquids up to 20 GHz
Authors: Wei Yan-Zhen , Sridhar S.
Published: July 2002

A technique for the measurement of the frequency-dependent complex dielectric constant ε=ε′−jε″ of liquids applicable to frequencies up to at least 20 GHz is described. The technique utilizes a coaxial probe dipped into the liquid. The reflection coefficient of the probe was measured using a network analyzer between 45 MHz and 20 GHz. A new de-imbedding procedure for eliminating the connector and coax mismatches is described, during which the impedance Z(ε) of the probe end was determined. The dielectric constant ε was determined from Z(ε) by modeling the coax–liquid interface as a capacitance. The de-imbedding procedure, which utilizes three calibrations, directly eliminates the (unknown) fringe-field impedance. Radiation effects were minimized by using narrow (0.047-in.) semirigid coax. The technique yields accurate results for ε′ and ε″ of liquids such as methanol and water over the entire frequency range up to 20 GHz, and can be used to determine the relaxation spectra of liquid and liquidlike samples.
A flowing solvent reactor for coal liquefaction with direct electrical resistance heating
Authors: Gibbins Jon , Kandiyoti Rafael
Published: July 2002

A flowing solvent reactor for coal liquefaction has been developed, which uses direct electrical resistance heating of the reactor tube wall to achieve rapid, controllable heating. A computerized control system previously developed for a wire-mesh pyrolysis apparatus is used for temperature control; heating rates between 0.1 and 10 K/s, peak temperatures up to 450 °C, and isothermal holding times from 5 to 1600 s have been obtained. Alternating current is used for heating, allowing a simple thyristor bridge to be used for power control. Solvent is fed from a pressurized reservoir at up to 100 bars, obviating the need for a supply pump. Using this apparatus the effects of a variety of parameters on primary coal liquefaction processes can be examined; the design of the system and typical results are presented.
Remote scanning ultraviolet detection for capillary gas chromatography
Authors: Bornhop Darryl J. , Hlousek Louis , Hackett Murray , Wang Houle , Miller Glenn C.
Published: July 2002

A remote flow cell has been interfaced to a scanning uv-visible absorbance detector for the detection of gas chromatography (GC) analytes. The unique optical bench, detection scheme, and thermal control allow low-noise operation at flow cell temperatures of 350 °C. Noise of 1×10−5 a.u. (peak to peak over a 1-min period) and drift at the 1×10−4 a.u./h level facilitate detection of coumarin in the gas phase at 40 pg/s. Flow cell volume was optimized for use with capillary GC columns. An effective band pass of <3 nm provides spectral resolution which shows solute absorbance fine structure to aid in compound identification.
High-power InGaN single-quantum-well-structure blue and violet light-emitting diodes
Authors: Nakamura Shuji , Senoh Masayuki , Iwasa Naruhito , Nagahama Shin-ichi
Published: July 2002

High-power blue and violet light-emitting diodes (LEDs) based on III–V nitrides were grown by metalorganic chemical vapor deposition on sapphire substrates. As an active layer, the InGaN single-quantum-well-structure was used. The violet LEDs produced 5.6 mW at 20 mA, with a sharp peak of light output at 405 nm, and exhibited an external quantum efficiency of 9.2%. The blue LEDs produced 4.8 mW at 20 mA and sharply peaked at 450 nm, corresponding to an external quantum efficiency of 8.7%. These values of the output power and the quantum efficiencies are the highest ever reported for violet and blue LEDs.
Surface diffusion measurements by digitized autocorrelation of field emission current fluctuations
Authors: Whitten J. E. , Gomer R.
Published: July 2002

The operating criteria of analog and digital methods for obtaining field emission fluctuation autocorrelation functions for the determination of surface diffusion coefficients are discussed and the strengths and limitations of both methods pointed out. Two digital systems, one using a spiraltron for pulse counting, the other a fast phosphor-photomultiplier combination for the same purpose, are described and test results presented. Some results obtained digitally for W self-diffusion on W(123) and for oxygen diffusion on W(110) are presented and shown to agree with previous results obtained by the analog method. To illustrate the potential of the digital method for obtaining very small D values results for the diffusion of H on Re(112̄0) at 80 K are also presented. For this case τ0=13 s, and D=1×10−14 cm2 s−1.
Europium chelate solid laser based on morphology-dependent resonances
Authors: Taniguchi Hiroshi , Kido Junji , Nishiya Masahisa , Sasaki Shinobu
Published: July 2002

We show lasing (with the threshold of μJ order) due to morphology-dependent resonances (MDRs) from solid polystyrene spheres with volume-distributed europium (Eu) chelates. Lasing spectra depending on MDRs are reported, and a practical usefulness of the Eu chelate laser with the solid spheres is described compared with the previous one with liquid spheres.
A millimeter/submillimeter spectrometer for high resolution studies of transient molecules
Authors: Ziurys L. M. , Barclay W. L. , Anderson M. A. , Fletcher D. A. , Lamb J. W.
Published: July 2002

A design is presented for a millimeter/submillimeter direct absorption spectrometer for studies of the pure rotational spectra of metal-bearing free radicals. The spectrometer operates in the frequency range of 65–550 GHz with an instrumental resolution of 200–1000 kHz and an absorption sensitivity of a few ppm. The instrument utilizes phase-locked Gunn oscillators as the tunable, coherent source of radiation from 65–140 GHz. Higher source frequencies are obtained with Schottky diode multipliers. The gas cell and optics path are designed utilizing Gaussian beam optics to achieve maximum interaction between molecules and the mm-wave radiation in the reaction region. Scalar feedhorns and a series of PTFE lenses are used to propagate the source signal. The gas cell is a cylindrical tube 0.5 m in length with a detachable Broida-type oven. The detector for the spectrometer is a helium-cooled InSb hot electron bolometer. Phase-sensitive detection is achieved by FM modulation of the Gunn oscillators and use of a lock-in amplifier. Spectra are recorded by electrical tuning of the Gunn oscillator, which is done under computer control. The millimeter and sub-mm rotational spectra of several free radicals have been observed for the first time using this instrument, including CaOH, MgOH, CaH, MgF, and BaOH.
Ultrasmall-angle x-ray scattering at the HASYLAB wiggler beamline BW4
Authors: Gehrke R. , Bark M. , Lewin D. , Cunis S.
Published: July 2002

At the x-ray wiggler beamline BW4 a new instrument for small-angle x-ray scattering is available. It uses pinhole collimation thus allowing the use of position sensitive detectors for the data acquisition. It is equipped with a doubly focusing mirror and a double-crystal monochromator. The energy of the focused beam can be tuned in the range from 4 to 12 keV. Due to the high intensity of the wiggler source a geometry could be realized which extends the attainable range of scattering vector lengths markedly towards lower values compared with the previously existing instruments. Presently with the new instrument using 8 keV photons values down to 0.0016 nm−1 can be observed which corresponds to maximum correlation lengths of about 600 nm. This resolution is demonstrated, as an example, with measurements carried out on a polymer system.
Measurement of ICRF waves in the GAMMA10 tandem mirror using reflectometers
Authors: Mase A. , Tokuzawa T. , Oyama N. , Ito Y. , Itakura A. , Hojo H. , Ichimura M. , Inutake M. , Tamano T.
Published: July 2002

Microwave reflectometry has been applied to the central cell of the GAMMA 10 tandem mirror for measurements of fluctuations in the ion cyclotron range of frequency (ICRF). The instability with frequency ω<ωci is identified as the Alfvén ion cyclotron wave driven by plasma β and ion pressure anisotropy. Internal magnetic fluctuations with δB/B∼10−4 as well as density fluctuations of the same level for this wave are determined by a combination of X- and O-mode reflectometers. The fluctuations associated with the ICRF heating wave (ω=ωci) are also observed by three separated X/O mode reflectometers to study the physics of wave excitation and propagation. The radial profile of the fluctuation level is expected to give an estimation of the deposition profile of the heating power. The results will be applied in order to achieve efficient ion heating and to help the power balance study of central-cell plasma.
Electrical properties of 1N4007 silicon diode
Authors: Nanda K. K. , Sarangi S. N.
Published: July 2002

In the case of semiconductor diodes, the temperature range over which the temperature dependence of the forward voltage is linear, can be increased by lowering the operating current along with the increase of the sensitivity (dVf/dT) which is found to vary logarithmically with I. The temperature and current dependence of forward voltage Vf can be explained by using the theory of the p-n junction. The capacitance-voltage (C−V) measurements of p-n junctions are carried out at different temperatures and are discussed in light of the theory of the p-n junction. The band gap Eg, estimated from Vf−T measurement, is found to be ∼1.17 eV, whereas it is found to be 1.189 eV from C−V measurement.
New low background crossed molecular beam apparatus: Low background detection of H2
Authors: Lin Jim J. , Hwang Dennis W. , Harich Steven , Lee Yuan T. , Yang Xueming
Published: July 2002

A low background and almost hydrocarbon free (∼1×10−14 Torr) molecular beam apparatus with an improved universal detector, based on electron bombardment ionization, has been constructed for crossed molecular beam research. Extremely high vacuum (∼1×10−12 Torr) for the detector’s ionization region is achieved using multiple ultrahigh vacuum pumps. In addition to a home-made liquid nitrogen cryopump and a turbomolecular pump, a two stage cryogenic He cold head (∼10 K) is used to pump the detector’s ionization region. Using this arrangement, the H2 background in the detector can be reduced by about two orders of magnitude in comparison with previously built similar instruments. Therefore, the signal-to-noise for detecting H2 product detection sensitivity is substantially enhanced, making experimental studies of H2 elimination channels in photodissociation processes much easier. Backgrounds at m/e=28 (CO+), 16 (CH4+,O+), 15 (CH3+), 14 (CH2+), and 13 (CH+) in the ionization detection region are also significantly reduced. In this article, we will describe the new apparatus in detail and present some preliminary results of the unimolecular decomposition studies of CH3OH at 157 nm excitation to demonstrate the capabilities of low background detection H2 using this new instrument.
Absolute detection efficiency of a microchannel plate detector for kilo-electron volt energy ions
Authors: Straub H. C. , Mangan M. A. , Lindsay B. G. , Smith K. A. , Stebbings R. F.
Published: July 2002

Measurements of the absolute detection efficiency of a commercial microchannel plate detector for kilo-electron volt energy ions are presented. The detector comprises two microchannel plates mounted in front of a resistive anode. It is found that when the detector is appropriately biased, and the ion impact energy is sufficiently high, ions with masses up to 131 amu are detected with equal efficiency and that this efficiency may remain constant over a period of years.
Broad bandwidth frequency domain instrument for quantitative tissue optical spectroscopy
Authors: Pham Tuan H. , Coquoz Olivier , Fishkin Joshua B. , Anderson Eric , Tromberg Bruce J.
Published: July 2002

Near-infrared (NIR) optical properties of turbid media, e.g., tissue, can be accurately quantified noninvasively using methods based on diffuse reflectance or transmittance, such as frequency domain photon migration (FDPM). Factors which govern the accuracy and sensitivity of FDPM-measured optical properties include instrument performance, the light propagation model, and fitting algorithms used to calculate optical properties from measured data. In this article, we characterize instrument, model, and fitting uncertaintics of an FDPM system designed for clinical use and investigate how each of these factors affects the quantification of NIR absorption (μa) and reduced scattering (μs′) parameters in tissue phantoms. The instrument is based on a 500 MHz, multiwavelength platform that sweeps through 201 discrete frequencies in as little as 675 ms. Phase and amplitude of intensity modulated light launched into tissue, i.e., diffuse photon density waves (PDW), are measured with an accuracy of ±0.30° and ±3.5%, while phase and amplitude precision are ±0.025° and ±0.20%, respectively. At this level of instrument uncertainty, simultaneous fitting of frequency-dependent phase and amplitude nonlinear model functions derived from a photon diffusion approximation provides an accurate and robust strategy for determining optical properties from FDPM data, especially for media with high absorption. In an optical property range that is characteristic of most human tissues in the NIR (5×10−3<μa<5×10−2 mm−1, 0.5<μs′<2 mm−1), we theoretically and experimentally demonstrate that the multifrequency, simultaneous-fit approach allows μa and μs′ to be quantified with an accuracy of ±5% and ±3%, respectively. Although exceptionally high levels of precision can be obtained using this approach (<1% of the estimated absorption and scattering values), we show that the absolute accuracy of optical property measurements is highly dependent on specific factors associated with instrument performance, model function relevance, and details of the fitting strategy used to calculate μa and μs′.
Scanning tip microwave near-field microscope
Authors: Wei T. , Xiang X.-D. , Wallace-Freedman W. G. , Schultz P. G.
Published: July 2002

A near-field microscope which operates in the rf/microwave frequency range is described. In this microscope, a scanning tunneling microscope (STM)-like tip rather than an aperture is used as a point-like evanescent field emitter. A spatial resolution of ∼5 μm (∼λ/100 000) is achieved in the current version. The design of the microscope as well as the principal factors which affect its performance are discussed.
Measurement of AlN/GaN (0001) heterojunction band offsets by x-ray photoemission spectroscopy
Authors: Waldrop J. R. , Grant R. W.
Published: July 2002

X-ray photoemission spectroscopy has been used to measure the valence band offset ΔEv for the AlN/GaN (0001) heterojunction interface. The heterojunction samples were grown by reactive molecular beam epitaxy on 6H–SiC (0001) substrates. A nested interface band alignment with ΔEv=1.36±0.07 eV is obtained (ΔEc/ΔEv=52/48).
Study of yielding mechanics in nanometer-sized Au contacts
Authors: Stalder A. , Dürig U.
Published: July 2002

Yielding properties of Au point contacts of nanometer-scale dimensions have been studied using a scanning tunneling microscope supplemented by a force sensor for measuring tip–sample forces. The contacts are made by indenting the tip typically 10 nm into the substrate, whereby an adhesion neck is formed. Three consecutive deformation phases of the neck can be identified during retraction of the tip: (1) buildup of tensile stress, (2) incomplete fracture, and (3) quasicontinuous plastic flow. Finally the neck breaks when a maximum of three to four atoms are left in the contact. In the plastic flow regime, the conductance and thus the contact area shrink exponentially with elongation of the neck, suggesting that plastic deformation occurs locally within 5 to 6 atomic layers. The stress applied during plastic flow is initially of the order of 10 GPa and gradually increases to ≂ 20 GPa shortly before the neck breaks. Accounting for a surface force contribution, an intrinsic yield strength of the order of 5 to 8 GPa is obtained, which is more than one order of magnitude larger than the macroscopic yield strength of Au.
Blue luminescence from Si+-implanted SiO2 films thermally grown on crystalline silicon
Authors: Liao Liang-Sheng , Bao Xi-Mao , Zheng Xiang-Qin , Li Ning-Sheng , Min Nai-Ben
Published: July 2002

Si ions were implanted into thermally grown SiO2 films on crystalline Si at an energy of 120 keV and with a dose of 1016 cm−2. Under an ultraviolet excitation of ∼5.0 eV, the implanted films exhibit blue luminescence with a peak of ∼2.7 eV at room temperature. The blue emission is caused by oxygen vacancies in the films.
Characterization of 3C–SiC crystals grown by thermal decomposition of methyltrichlorosilane
Authors: Steckl A. J. , Devrajan J. , Tlali S. , Jackson H. E. , Tran C. , Gorin S. N. , Ivanova L. M.
Published: July 2002

Single crystal 3C–SiC platelets, formed by thermal decomposition of methyltrichlorosilane at 1650–1750 °C, have been characterized in terms of structure and morphology. The platelets are ∼3–5 mm in length and 1–1.5 mm in thickness. The (111) C face of the crystal, which has an effective zero growth rate, presents a large, mirrorlike surface in the as-grown 3C crystals. Atomic force microscopy indicates that these as-grown surfaces are extraordinarily flat and uniform, with a mean surface roughness of 1–2 Å. This value is comparable with the roughness of state-of-art polished Si wafers. X-ray rocking curves of the 〈111〉 peak were obtained with a linewidth of 12.3 arcsec. This is the smallest value reported to date for any polytype of SiC. Raman spectroscopy at 300 K reveals a very sharp TO–phonon peak at 797.8 cm−1, with a linewidth of 2.1 cm−1.
Vibrational states of tetrahedral amorphous carbon
Authors: Lopinski G. P. , Merkulov V. I. , Lannin J. S.
Published: July 2002

High resolution electron energy loss spectroscopy was used to observe variations in the vibrational states of amorphous carbon films with differences in preparation methods. Diamondlike films made by pulsed laser deposition (PLD) exhibit differences in the weighted phonon density of states relative to sputtered films. These differences are due to an increase of fourfold bonded atoms in the PLD films and are in qualitative agreement with theoretical calculations for amorphous carbon networks.
Observation of large low-field magnetoresistance in trilayer perpendicular transport devices made using doped manganate perovskites
Authors: Sun J. Z. , Gallagher W. J. , Duncombe P. R. , Krusin-Elbaum L. , Altman R. A. , Gupta A. , Lu Yu , Gong G. Q. , Xiao Gang
Published: July 2002

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current-perpendicular-to-plane transport. These results show that low-field spin-dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.
Characterization of Cu–Na ion-exchanged glass waveguides
Authors: Gonella Francesco
Published: July 2002

Copper–sodium ion-exchanged glass waveguides are fabricated and characterized. Optical absorption and m-lines spectroscopy analyses are presented. No stress-induced birefringence is observed from refractive index profiles reconstruction, and the Cu+ oxidation state is shown to mostly determine the refractive index change. Comparison between compositional in-depth profiles and index profiles is allowed to obtain the variation in the electronic polarizability due to the Cu+–Na+ exchange. The diffusion coefficient of Cu+ in soda-lime glass is also calculated.
Tip-surface interactions studied using a force controlled atomic force microscope in ultrahigh vacuum
Authors: Jarvis S. P. , Yamamoto S.-I. , Yamada H. , Tokumoto H. , Pethica J. B.
Published: July 2002

A magnetic force has been used to control a partially magnetized atomic force microscope cantilever in ultrahigh vacuum in order to investigate tip-surface interactions. It is shown that forces applied by an oscillating tip can drastically affect the measured response of the lever. In particular the oscillating force influences the measured tip-surface adhesion and can be treated as a simple addition to the static applied force in breaking adhesive bonds. Qualitative differences in force spectroscopy measurements due to the amplitude of the driving oscillation of the lever are presented and an alternative nonintrusive technique introduced.
Lateral growth kinetics of α-alumina accompanying the formation of a protective scale on (111) NiAl during oxidation at 1100 °C
Authors: Lipkin D. M. , Schaffer H. , Adar F. , Clarke D. R.
Published: July 2002

During the early stages of oxidation of (111) NiAl at 1100 °C, an alumina scale forms and undergoes a series of allotropic phase transformations. In the final stages of phase development, the metastable Θ-alumina transforms to the equilibrium α-alumina phase. Images formed using the photoluminescence from trace Cr3+ impurities in the scale, in conjunction with scanning electron microscopy, indicate that the α-alumina forms by a nucleation and growth process within the Θ-alumina matrix. Lateral impingement of the growing α-alumina islands coincides with the reported drop in oxidation kinetics to those characteristic of long-term oxidation of NiAl. Unlike most interface-controlled transformations, the kinetics of the θ–to–α transformation in the lateral growth are found to be logarithmic. Possible causes of these unusual kinetics are proposed.
Enhancement of the photonic gap of opal-based three-dimensional gratings
Authors: Romanov S. G. , Johnson N. P. , Fokin A. V. , Butko V. Y. , Yates H. M. , Pemble M. E. , Sotomayor Torres C. M.
Published: July 2002

The “semimetallic” photonic band gap formed in a synthetic opal has been increased by depositing a layer (InP or TiO2) with high refractive index on the inner surface of opal voids. Reflectance spectra of the composites (nanolayers assembled within grating voids) are correlated with both the photonic structure of the opal and electronic structure of the semiconductor.
Field-assisted photoemission from InP/InGaAsP photocathode with p/n junction
Authors: Niigaki M. , Hirohata T. , Suzuki T. , Kan H. , Hiruma T.
Published: July 2002

A high quantum efficiency of photoemission to a 1.35 μm threshold has been achieved from an externally biased InP/InGaAsP photocathode which has a p/n junction instead of a Schottky contact. The quantum efficiency at 1.3 μm is 5% (electron per incident photon) with the photocathode cooled to −80 °C. The photocathode consists of a n+InP contact layer, a p−InP photoelectron-emitting layer and a p−InGaAsP photon-absorbing layer on a p+InP substrate.
Enhancement of microbeam x-ray fluorescence analysis using monolithic polycapillary focusing optics
Authors: Gao N. , Ponomarev I. Yu. , Xiao Q. F. , Gibson W. M. , Carpenter D. A.
Published: July 2002

A monolithic polycapillary focusing optic was tested in the microbeam x-ray fluorescence system at the Oak Ridge Center for Manufacturing Technology. The optic was designed to cover a wide energy range from 4 keV to 20 keV. The focal spot size of the output beam at 17.4 keV (Mo Kα) was measured to be 21 μm full width of half maximum. An average beam intensity of 1.5×105 photon/s/μm2 was obtained at the focus for Mo Kα line using a 12 W microfocus x-ray source. This intensity is about 2400 times over that of a direct beam at 100 mm away from the x-ray source. The small, intense x-ray beam obtained was used to analyze and map the compositions of different elements in industrial samples.
Sizable photocurrent and emission from solid state devices based on CdS nanoparticles
Authors: Nanda J. , Narayan K. S. , Kuruvilla Beena Annie , Murthy G. L. , Sarma D. D.
Published: July 2002

CdS nanoparticles exhibit size dependent optical and electrical properties. We report here the photocurrent and I–V characteristic studies of CdS nanoparticle devices. A sizable short circuit photocurrent was observed in the detection range governed by the size of the clusters. We speculate on the mechanisms leading to the photocurrent and emission in these nanometer scale systems.
Domain structure of epitaxial SrRuO3 thin films on miscut (001) SrTiO3 substrates
Authors: Jiang J. C. , Tian W. , Pan X. Q. , Gan Q. , Eom C. B.
Published: July 2002

The microstructure of epitaxial SrRuO3 thin films grown on vicinal (001) SrTiO3 substrates with miscut angle of 1.9° and miscut direction of 12° away from [100] direction was studied using transmission electron microscopy (TEM). Cross-section as well as plan-view TEM studies revealed that these films are single domain with the in-plane epitaxial orientation relationship of SrRuO3[001]//SrTiO3[010] and SrRuO3[1̄10]//SrTiO3[100]. This result is in contrast to the previous studies of the SrRuO3 thin films grown on exactly (001) SrTiO3, which are composed of two types of [110] domains with nearly the same volume fraction. The occurrence of these different domain structures is attributed to the step-flow growth of the film on the substrate surface due to the miscut.
Dominant defect levels in diamond thin films: A photocurrent and electron paramagnetic resonance study
Authors: Nesládek M. , Stals L. M. , Stesmans A. , Iakoubovskij K. , Adriaenssens G. J , Rosa J. , Vaněček M.
Published: July 2002

Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on “as-grown” samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.
Ionization energy of acceptors in As-doped HgCdTe grown by molecular beam epitaxy
Authors: Shi X. H. , Rujirawat S. , Ashokan R. , Grein C. H. , Sivananthan S.
Published: July 2002

The p-type activation of arsenic (As) in (211)B mercury cadmium telluride (HgCdTe) grown by molecular beam epitaxy (MBE), with different compositions covering the 3–5 and 8–14 μm atmospheric transmission windows and after annealing at 300 °C is reported. The composition and thickness of the MBE layers were determined from Fourier transform infrared transmission measurements at room temperature. The ionization energies of shallow acceptors related to As in MBE HgCdTe layers with different Cd compositions have been obtained by fitting variable temperature Hall measurement results to a two-band nonparabolic Kane model. The results indicate that As incorporated during the MBE growth can be activated to provide a shallow acceptor level in MBE HgCdTe and the ionization energy of the As acceptor decreases with decreasing Cd composition, in agreement with the theory.
Independent manipulation of density and size of stress-driven self-assembled quantum dots
Authors: Mukhametzhanov I. , Heitz R. , Zeng J. , Chen P. , Madhukar A.
Published: July 2002

A method to independently manipulate the density and size of stress-driven self-assembled quantum dots (QDs) is demonstrated in the InAs/GaAs material system. In bilayer stacks, different InAs deposition amounts in the initial (seed) and second layer are shown to enable independent control, respectively, of the density and the size distribution of the second layer QDs. The approach allows enhancing the average volume and improving the uniformity of InAs QDs, resulting in, respectively, low and room temperature photoluminescence at 1.028 eV (∼1.2 μm) and 0.955 eV (∼1.3 μm) with remarkably narrow linewidths of 25 and 29 meV for 1.74 ML (seed)/3.00 ML InAs stacking.
Silicon vacancy in SiC: A high-spin state defect
Authors: Torpo L. , Nieminen R. M. , Laasonen K. E. , Pöykkö S.
Published: July 2002

We report results from spin-polarized ab initio local spin-density calculations for the silicon vacancy (VSi) in 3C– and 2H–SiC in all its possible charge states. The calculated electronic structure for SiC reveals the presence of a stable spin-aligned electron-state t2 near the midgap. The neutral and doubly negative charge states of VSi in 3C–SiC are stabilized in a high-spin configuration with S=1 giving rise to a ground state, which is a many-electron orbital singlet T13. For the singly negative VSi, we find a high-spin ground-state A24 with S=3/2. In the high-spin configuration, VSi preserves the Td symmetry. These results indicate that in neutral, singly, and doubly negative charge states a strong exchange coupling, which prefers parallel electron spins, overcomes the Jahn–Teller energy. In other charge states, the ground state of VSi has a low-spin configuration.
Multiphoton detection using visible light photon counter
Authors: Kim Jungsang , Takeuchi Shigeki , Yamamoto Yoshihisa , Hogue Henry H.
Published: July 2002

Visible light photon counters feature noise-free avalanche multiplication and narrow pulse height distribution for single photon detection events. Such a well-defined pulse height distribution for a single photon detection event, combined with the fact that the avalanche multiplication is confined to a small area of the whole detector, opens up the possibility for the simultaneous detection of two photons. In this letter, we investigated this capability using twin photons generated by parametric down conversion, and present a high quantum efficiency (∼47%) detection of two photons with good time resolution (∼2 ns), which can be distinguished from a single-photon incidence with a small bit-error rate (∼0.63%).
Optical properties of InAs quantum dots in a Si matrix
Authors: Heitz R. , Ledentsov N. N. , Bimberg D. , Egorov A. Yu. , Maximov M. V. , Ustinov V. M. , Zhukov A. E. , Alferov Zh. I. , Cirlin G. E. , Soshnikov I. P. , Zakharov N. D. , Werner P. , Gösele U.
Published: July 2002

We investigate the optical properties of nanoscale InAs quantum dots (QDs) in a Si matrix. At a growth temperature of 400 °C, the deposition of 7 ML InAs leads to the formation of coherent islands with dimensions in the 2–4 nm range. A luminescence band in the 1.3 μm region found exclusively for samples with such InAs QDs exhibits a pronounced excitation density dependence of the peak position and a decay time of 440 ns. The optical properties suggest an indirect type II transition for InAs/Si QDs. The electronic structure of InAs/Si QDs is discussed in view of available band offset information.
Large current density from carbon nanotube field emitters
Authors: Zhu W. , Bower C. , Zhou O. , Kochanski G. , Jin S.
Published: July 2002

We observe that field emitters made from carbon nanotubes exhibit excellent macroscopic emission properties; they can operate at a very large current density, as high as 4 A/cm2. At electric fields as low as 4–7 V/μm, they emit technologically useful current densities of 10 mA/cm2. We show that the emission originates from nanotube ends with a characteristic structured ring pattern. The emission characteristics and durability of the carbon nanotube cold cathodes offer promising applications for vacuum microelectronic devices.
High collection efficiency in fluorescence microscopy with a solid immersion lens
Authors: Koyama Kazuko , Yoshita Masahiro , Baba Motoyoshi , Suemoto Tohru , Akiyama Hidefumi
Published: July 2002

High efficiency of light collection was demonstrated by applying solid immersion lenses (SILs) with refractive indices n=1.845 and 1.687 to fluorescence microscopy of 0.11-μm-radius dye-doped polystyrene sphere beads. This was analyzed with theories on the radiation of dipoles near the surface of the dielectric medium. The estimated collection efficiency with the NA=0.8 objective and the n=1.845 SIL was about 60%–70%, which well exceeds the diffraction-limit value of 50% for 2π-solid angle collection in conventional methods.
Highly ordered two-dimensional carbon nanotube arrays
Authors: Suh Jung Sang , Lee Jin Seung
Published: July 2002

Highly ordered multiwalled carbon nanotube arrays have been fabricated by using porous anodic alumina templates prepared by a two-step anodization process. Nanotubes were very uniform in diameter. Two-dimensional tube arrays were highly ordered within domain boundaries, and their vertical alignment was nearly perfect. The density of nanotubes, 1.1×1010 tubes/cm2, was very high. Our fabrication technique enabled us to easily control the tube diameter, length, and density. There was no practical limitation to the size of the alumina template, so that very large panels of well-aligned carbon nanotubes could be made. Our results offer a potentially elegant technique for fabricating cold-cathode flat panel displays.
High-power truncated-inverted-pyramid (AlxGa1−x)0.5In0.5P/GaP light-emitting diodes exhibiting >50% external quantum efficiency
Authors: Krames M. R. , Ochiai-Holcomb M. , Höfler G. E. , Carter-Coman C. , Chen E. I. , Tan I.-H. , Grillot P. , Gardner N. F. , Chui H. C. , Huang J.-W. , Stockman S. A. , Kish F. A. , Craford M. G. , Tan T. S. , Kocot C. P. , Hueschen M. , Posselt J. , Loh B. , Sasser G. , Collins D.
Published: July 2002

A truncated-inverted-pyramid (TIP) chip geometry provides substantial improvement in light extraction efficiency over conventional AlGaInP/GaP chips of the same active junction area (∼0.25 mm2). The TIP geometry decreases the mean photon path-length within the crystal, and thus reduces the effects of internal loss mechanisms. By combining this improved device geometry with high-efficiency multiwell active layers, record-level performance for visible-spectrum light-emitting diodes is achieved. Peak efficiencies exceeding 100 lm/W are demonstrated (100 mA dc, 300 K) for orange-emitting (λp∼610 nm) devices, with a peak luminous flux of 60 lumens (350 mA dc, 300 K). In the red wavelength regime (λp∼650 nm), peak external quantum efficiencies of 55% and 60.9% are measured under direct current and pulsed operation, respectively (100 mA, 300 K).
Recovery phenomenon of mechanoluminescence from Ca2Al2SiO7:Ce by irradiation with ultraviolet light
Authors: Akiyama Morito , Xu Chao-Nan , Matsui Hiroaki , Nonaka Kazuhiro , Watanabe Tadahiko
Published: July 2002

We have investigated the mechanoluminescence (ML) from Ca2Al2SiO7:Ce. The ML is clearly visible to the naked eye in the atmosphere. The luminescence integrated intensity is about 400 times as high as that of crystal sugar. The ML center has been identified as the Ce3+ ion from spectra of the ML and also from the photoluminescence studies of Ca2Al2SiO7:Ce. The ML intensity decreases on repetitive application of stress but is completely recovered by irradiation with ultraviolet light. It is suggested that the ML mechanism arises from the movement of dislocations and recombination between electrodes and holes released from these traps which are associated with the Ce3+ centers.
Poling-induced crystallization of tetragonal BaTiO3 and enhancement of optical second-harmonic intensity in BaO–TiO2–TeO2 glass system
Authors: Narazaki Aiko , Tanaka Katsuhisa , Hirao Kazuyuki
Published: July 2002

Effect of poling on surface crystallization behavior of BaTiO3 in 15BaO–15TiO2–70TeO2 glass and on second-harmonic generation of the resultant glass ceramics has been examined. A direct current voltage in a range of 0.3–1 kV was applied to the original glass at around 400 °C, which is above its glass transition temperature, followed by cooling of the sample to room temperature with the voltage kept constant. X-ray diffraction patterns indicate that the poling increased the fraction of crystalline phases precipitated at the glass surface and that BaTiO3 crystallites were precipitated preferentially with (101)- or (110)-orientation. It is also observable that the x-ray diffraction line assigned to BaTiO3 shifted from the position corresponding to bulk cubic crystal to that of ferroelectric tetragonal one when the applied voltage was increased. In other words, the poling treatment stabilizes the tetragonal BaTiO3 phase, leading to a larger optical second-order nonlinear susceptibility. In fact, the second-harmonic intensity for the glass ceramic poled at 420 °C was enhanced by a factor of ten compared with the glass ceramic heat treated without an external electric field.
Room-temperature blue-green emission from InGaN/GaN quantum dots made by strain-induced islanding growth
Authors: Damilano B. , Grandjean N. , Dalmasso S. , Massies J.
Published: July 2002

InGaN/GaN self-assembled quantum dots (QDs) were obtained by molecular beam epitaxy making use of the Stranski–Krastanov growth mode. Room-temperature photoluminescence (PL) energy of QDs was observed from 2.6 to 3.1 eV depending on the dot size. PL linewidths as low as 40–70 meV at 10 K and 90–110 meV at 300 K indicate low dot size dispersion. The comparison of PL intensity versus temperature of an InGaN epilayer and InGaN/GaN QDs demonstrates the higher radiative efficiency of the latter.
Triggering and guiding leader discharges using a plasma channel created by an ultrashort laser pulse
Authors: Comtois D. , Chien C. Y. , Desparois A. , Génin F. , Jarry G. , Johnston T. W. , Kieffer J.-C. , La Fontaine B. , Martin F. , Mawassi R. , Pépin H. , Rizk F. A. M. , Vidal F. , Couture P. , Mercure H. P. , Potvin C. , Bondiou-Clergerie A. , Gallimberti I.
Published: July 2002

In a 2.8 m positive rod–plane air gap, we have studied how a plasma channel produced by focusing a 200 mJ ultrashort laser beam is able to trigger and guide a leader discharge. We have observed that the plasma channel allowed the lowering of the leader inception voltage by 50% and the guiding of the leader propagation on a distance of up to 2.3 m, with a tenfold increase of its speed. This led to an effective 40% reduction of the breakdown voltage. For the conditions studied here, the laser energy per unit length required to guide a leader is between 60 and 100 mJ/m.
Efficient GaAs light-emitting diodes by photon recycling
Authors: Dupont E. , Liu H. C. , Buchanan M. , Chiu S. , Gao M.
Published: July 2002

Heterostructure AlGaAs/GaAs light-emitting diodes (LEDs) with a thick active region have shown high external efficiencies, thanks to reabsorption in the active region. For high injection currents and low temperature, we report a 22% efficiency which corresponds to a 98% efficiency internally. We discuss the application of such LED when integrated with a quantum-well infrared photodetector for pixelless thermal imaging systems.
1.54 μm emission dynamics of erbium-doped zinc-oxide thin films
Authors: Komuro Shuji , Katsumata Tooru , Morikawa Takitaro , Zhao Xinwei , Isshiki Hideo , Aoyagi Yoshinobu
Published: July 2002

Erbium-related 1.54 μm emission dynamics of Er-doped ZnO thin films has been investigated for the different excitation conditions. The excitation was achieved either by exciting indirectly Er3+ ions due to an electron–hole-mediated process or exciting directly discrete energy levels of Er3+ ions. There is no change in the 1.54 μm emission spectrum feature in spite of the different excitation conditions, whereas dramatic change can be seen in the rise time of 1.54 μm emission. The shorter rise time of 1.54 μm emission observed for indirect excitation implies an excitation efficiency superior to direct excitation of Er3+ ions.
Effects of high repetition rate and beam size on hard tissue damage due to subpicosecond laser pulses
Authors: Kim Beop-Min , Feit Michael D. , Rubenchik Alexander M. , Joslin Elizabeth J. , Eichler Jürgen , Stoller Patrick C. , Da Silva Luiz B.
Published: July 2002

We report the effects of the repetition rate and the beam size on the threshold for ultrashort laser pulse induced damage in dentin. The observed results are explained as cumulative thermal effects. Our model is consistent with the experimental results and explains the dependence of the threshold on repetition rate, beam size, and exposure time.