bluuebunny/crossref_metadata_2025 huggingface dataset:
Cubic boron nitride films deposited by electron cyclotron resonance plasma
Authors: Shapoval S. Y. , Petrashov V. T. , Popov O. A. , Westner A. O. , Yoder M. D. , Lok C. K. C.
Published: July 2002

By employing an electron cyclotron resonance plasma-enhanced chemical vapor deposition technique, we report the successful growth of cubic boron nitride films on single-crystal (100) silicon wafer without external rf or dc substrate biasing. Ammonia and boron trifluoride gases were used for the deposition of cubic boron nitride. The substrate temperature during deposition was about 675 °C. The films were characterized by infrared spectroscopy and ellipsometry. The existence of cubic boron nitride was identified by the characteristic boron nitride infrared signal at 1110 cm−1. The film thickness was about 1000 Å, with a growth rate of 100 Å/min.
Low-frequency noise in transport through quantum point contacts
Authors: Li Yuan P. , Tsui D. C. , Heremans J. J. , Simmons J. A. , Weimann G. W.
Published: July 2002

We report the noise characteristics of quantum point contacts between 100 Hz and 100 kHz at 4.2 K. The noise consists of a 1/f component on top of a white background. The 1/f noise increases as the contact width decreases and shows peaks between the quantized resistance plateaus. The white noise background increases with current but is much lower than the full shot noise level, suggesting that shot noise is not generated in an ideal quantum point contact, where the electrons do not suffer backscattering as they enter and traverse the contact.
High-efficiency InGaAlP/GaAs visible light-emitting diodes
Authors: Sugawara H. , Ishikawa M. , Hatakoshi G.
Published: July 2002

High-efficiency InGaAlP surface emission light emitting diodes (LEDs) have been successfully fabricated. Newly designed double-heterostructure LEDs with a GaAlAs current spreading layer were employed to expand the light emission area, which is necessary to take out the light efficiently. The external quantum efficiency was 1.5% at 620 nm orange light for an In0.5 (Ga0.8Al0.2)0.5P active layer LED. This LED is five times more efficient at 620 nm than that of the GaAlAs and GaAsP LEDs. 563 nm green electroluminescence, which is the shortest wavelength ever reported for InGaAlP LEDs, was also achieved with an In0.5(Ga0.5Al0.5)0.5P active layer.
Electric field measurement in the cathode sheath of a hydrogen glow discharge
Authors: Barbeau C. , Jolly J.
Published: July 2002

Spatially resolved electric field measurements in the cathode region of a dc glow discharge in hydrogen are performed using polarization-dependent Stark broadening of plasma-induced emission of the Balmer lines. The large concentrations of excited atoms in the sheath provide an accurate, sensitive measure of discharge electric fields.
Are bare surfaces detrimental in epitaxial growth?
Authors: Copel M. , Tromp R. M.
Published: July 2002

For growth of epitaxial silicon-germanium structures by hydride chemical vapor deposition (CVD), the growth front is hydrogen-stabilized. Using medium energy ion scattering to examine the abruptness of an embedded Ge film in a Si(001) host, intermixing can be directly assessed. We have explored CVD films grown with varying hydrogen coverages, and find that adsorbed hydrogen serves a beneficial role in maintaining the abruptness of the interface. Embedded layers grown by molecular beam epitaxy are also more abrupt when the surface is stabilized, in this case by an adsorbed passivant such as Sb or As. Growth in the presence of a surface active agent (surfactant) results in greater control of constituents with no loss of epitaxial quality.
Quantum well, voltage-induced quantum well, and quantum barrier electron waveguides: Mode characteristics and maximum current
Authors: Wilson Daniel W. , Glytsis Elias N. , Gaylord Thomas K.
Published: July 2002

It is shown that finite-potential heterostructure wells, homostructure voltage-induced wells, and heterostructure barriers can act as waveguides for ballistic electrons and that waveguiding is described by a single dispersion relation and can occur at energies above all band edges. The guided mode cutoffs, electron velocity, effective mass, density of states, and ballistic current density (applicable to 2D electron gases) are presented. The maximum ballistic guided current flowing in a given direction for a 10 monolayer Ga0.75Al0.25As/GaAs/Ga0.9Al0.1As waveguide is found to be 2.3 mA per μm of waveguide width–allowing considerably greater currents than in single-mode quantum wires.
Positron annihilation study of low pressure chemical vapor deposited silicon nitride films
Authors: Hakvoort R. A. , Schut H. , van Veen A. , Bik W. M. Arnold , Habraken F. H. P. M.
Published: July 2002

Doppler S-parameter measurements have been performed on low-pressure chemical vapor deposited (LPCVD) Si3N4 samples. Annealing of the samples at 1000 °C for times up to 1 h resulted in a decrease of both the S parameter and the positron diffusion length of the silicon nitride. The diffusion length recovers after reimplantation of deuterium, but the S parameter remains low. The data are discussed using a model also applied to understand some electrical properties of the LPCVD nitride.
THz spectroscopy and source characterization by optoelectronic interferometry
Authors: Ralph Stephen E. , Grischkowsky D.
Published: July 2002

We demonstrate a new type of THz optoelectronic interferometer, by fully characterizing a recently developed THz source to beyond 6 THz, and by measuring the absorption coefficient of high-resistivity GaAs from 1 to 5 THz. The two source THz interferometer is driven with two 4 mW beams of 60 fs dye-laser pulses and produces interferograms with exceptional signal-to-noise ratios.
Improvement in the properties of high T
c films fabricated in situ by laser ablation of YBa2Cu3O7-Ag targets
Authors: Singh Rajiv K. , Bhattacharya D. , Tiwari P. , Narayan J. , Lee C. B.
Published: July 2002

We have studied the microstructural and superconducting properties of YBa2Cu3O7 films fabricated by pulsed laser evaporation of YBa2Cu3O7-Ag targets, containing approximately 30% by weight of silver. The films produced by excimer laser evaporation/ablation of YBa2Cu3O7-Ag targets were found to possess better superconducting and microstructural properties than films prepared under the same conditions from bulk YBa2Cu3O7 targets. YBa2Cu3O7 films deposited on yttria stabilized zirconia (YSZ) substrates from YBa2Cu3O7-Ag targets at 650 °C showed helium ion channeling yields of approximately 9%–12%, while films deposited from YBa2Cu3O7 targets had channeling yields in the range of 18%–25%. The critical current density of YBa2Cu3O7Agx films on YSZ substrates had values of approximately 1.8×106 A/cm2 at 77 K which corresponds to approximately an 80% improvement from the values obtained from films deposited from cold pressed YBa2Cu3O7 targets. Based on the structural and chemical information, it appears that two effects play a major role in improving the superconducting properties of laser ablated thin films made from silver composite superconductor targets (i) improved evaporation characteristics of the targets due to silver incorporation and (ii) the catalyst effect of silver in stabilizing the superconducting phase and enhancing grain growth.
Dislocation glide in {110} planes in semiconductors with diamond or zinc-blende structure
Authors: Albrecht M. , Strunk H. P. , Hull R. , Bonar J. M.
Published: July 2002

The activation of the secondary a/2〈110〉{110} glide systems as observed by transmission electron microscopy in epitaxial Ge(Si) and InGaAs layers grown on comparatively highly misfitting substrates, is rationalized in terms of a mechanical equilibrium analysis that includes a frictional force on the gliding dislocations. The conditions for occurrence of further secondary glide planes, such as {113} and {100}, are outlined.
On the structure of thin hydrocarbon films
Authors: Jacob W. , Möller W.
Published: July 2002

We compare reported compositions of a-C:H films in a ternary phase diagram. It is assumed that the films comprised three phases: sp3 hybridized carbon, sp2 hybridized carbon and hydrogen. The data are found to split into two well-separated groups. This separation depends on the method used to measure the sp3/sp2 ratio. We conclude from the comparison of NMR and infrared data that infrared analysis does not provide a quantitative measure of the sp3/sp2 ratio.
Study on excimer laser induced defects in SiO2 films on Si by variable-energy positron annihilation spectroscopy
Authors: Fujinami M. , Chilton N. B.
Published: July 2002

Using variable-energy positron annihilation spectroscopy, we have investigated the defects induced by ArF excimer laser (6.4 eV) irradiation of SiO2 films (500 nm thick) on Si(100). Films of both thermally grown oxide, and plasma enhanced chemical vapor deposited oxide were studied. It is proposed that E′ centers (≡Si⋅), nonbridging oxygen hole centers (NBOHC, ≡Si-O⋅) and their charge transfer states may trap positrons and affect the positronium annihilation modes. For the thermally grown oxide, the content of E′ centers, and hence original structural defects such as oxygen vacancies (≡Si-Si≡) at the SiO2/Si interface, is much higher than that in the SiO2 film itself. For SiO2 films deposited on Si by plasma-enhanced chemical vapor deposition, the impurity of ≡Si-OH (1.2 wt %) throughout the oxide layer acts as a precursor to the radiation-induced defects. The NBOHC formed strongly trap positrons and decrease the positronium formation fraction. The annealing behavior of the radiation-induced defects in both samples is characteristic of the original structural defects.
Drawing-enhanced defect precursors in low-OH content, oxygen-deficient synthetic silica optical fibers
Authors: Li J. , Kannan S. , Lehman R. L. , Sigel G. H.
Published: July 2002

The 74 and 13 G electron paramagnetic resonance hydrogen hyperfine doublets of the Si E′ center were observed in low-OH content, oxygen-deficient silica optical fibers exposed to hydrogen at room temperature. These spectra were also found in hydrogen treated bulk samples but at negligible intensities compared to those in fiber samples. The significant enhancement of these doublets in the fiber samples indicates that the precursors of the 74 and 13 G hydrogen hyperfine doublets of Si E′ are substantially produced in the fiber drawing process. Possible structures of the two precursors are discussed.
Laser-induced photochromic damage in potassium titanyl phosphate
Authors: Blachman R. , Bordui P. F. , Fejer M. M.
Published: July 2002

We report studies of photochromic damage induced in KTiOPO4 single crystals by ultraviolet (UV) irradiation with peak intensities from 0.3 to 4.0 MW/cm2. The photochromic damage progressed to asymptotic values which were not linear with respect to UV intensity. Following UV exposure, transparency of damaged crystals recovered completely over tens of hours at room temperature, faster at elevated temperature. UV irradiation at significantly lower peak but higher average power did not produce damage. We also studied photochromic damage occurring during 1.064-μm second-harmonic generation. Fundamental infrared intensities ranged from 1.3 to 5.5 times larger than those producing highest nonlinear frequency conversion efficiency. Damage sites produced by the two methods exhibited similar absorption spectra.
Intensity dependence of photoluminescence in GaN thin films
Authors: Singh R. , Molnar R. J. , Ünlü M. S. , Moustakas T. D.
Published: July 2002

We report the intensity dependence of band-gap and midgap photoluminescence in GaN films grown by electron cyclotron resonance (ECR) microwave plasma-assisted molecular beam epitaxy. We find that the band-gap luminescence depends linearly while the midgap luminescence has a nonlinear dependence on the incident light intensity. These data were compared with a simple recombination model which assumes a density of recombination centers 2.2 eV below the conduction band edge. The concentration of these centers is higher in films grown at higher microwave power in the ECR plasma.
High resolution atomic core level spectroscopy with laser harmonics
Authors: Haight R. , Seidler P. F.
Published: July 2002

High resolution atomic core level spectroscopy is carried out on condensed matter systems using tunable harmonics generated by focusing light from an amplified 150 femtosecond dye laser system operating at 610 nm, into a pulsed source of Ar gas. We show core level spectra collected with the 15th (30.54 eV), 17th (34.61 eV), and 19th (38.68 eV) harmonics of the dye laser light. Each harmonic is separated by 4.07 eV and possesses a narrow energy bandwidth which can be used to generate high resolution core level spectra.
Thermal polarization reversal of lithium niobate
Authors: Houé M. , Townsend P. D.
Published: July 2002

An approach for polarization reversal of lithium niobate has been demonstrated. The method involves laser generated thermal pulses in the presence of a uniform electric field, on the −z faces of lithium niobate. Laser input energies in the range 40–90 mJ per pulse and a wavelength of 532 nm, for applied fields as low as 187 V/cm on samples at 400 °C, caused domain inversion. Observation of the inversion was carried out on a scanning electron microscope at a low primary electron energy of 2 keV and a probe current of under 25 pA so that no conductive layer was needed. This new poling method has the advantage of being simple to use, region specific and hence suitable for patterning, and does not require a vacuum chamber.
Quantitative analysis of growth-induced reduction of long range lattice order in ion-beam sputtered YBa2Cu3O6.9 films
Authors: Gauzzi Andrea , Pavuna Davor
Published: July 2002

We report evidence for the reduction of long range lattice order caused by slight departures from the optimal growth temperature in fully doped (x≊0.9) YBa2Cu3O6+x films deposited by ion-beam sputtering on 〈001〉 SrTiO3. We estimate the characteristic length of this disorder from the broadening Δϑ of the 〈005〉 x-ray diffraction rocking curve. The depression of superconductivity and normal conductivity scales as Δϑ and disappears when the in-plane lattice coherence length rc∼1/Δϑ is larger than ≊10 nm.
Lasing characteristics of low threshold microcavity lasers using half-wave spacer layers and lateral index confinement
Authors: Huffaker D. L. , Shin J. , Deppe D. G.
Published: July 2002

Data are presented characterizing threshold and transverse mode behavior of microcavity lasers, which use a half-wavelength cavity spacer layer surrounding a single quantum well active region. Selective conversion of AlAs into AlxOy is used to define lateral device dimensions of 2, 5, and 8 μm. Initial results demonstrate a continuous-wave room-temperature lasing threshold current of 97 μA for a 2 μm device and 220 μA for an 8 μm device. We show that lasing operation is influenced by the AlxOy located only 200 Å from the quantum well.
Apparatus for sound velocity measurements in gases up to 10 kbar: Experimental data for argon
Authors: Kortbeek P. J. , Muringer M. J. P. , Trappeniers N. J. , Biswas S. N.
Published: July 2002

An apparatus for the measurement of sound velocity in gases up to pressures of 10 kbar and down to temperatures of 77 K is described. Important features of the experimental setup include a specially designed ultrasonic cell with double reflectors, based on the phase comparison pulse-echo technique, and a vacuum type of cryostat using a cold finger system for temperature control. Detailed measurements on argon down to the melting line show that the method is capable of giving a precision of 0.02% for the sound velocity.
Light amplification in a dye-doped glass planar waveguide
Authors: Sorek Y. , Reisfeld R. , Finkelstein I. , Ruschin S.
Published: July 2002

Glass waveguiding films were prepared from titania and modified silica using the sol-gel method and doped by the laser dye rhodamine B. The guided and amplified fluorescence (pumped by a double frequency Nd-YAG laser) was coupled out either by static grating written on the film or by a prism. The gain of the emitted superradiance was determined from the amplified spontaneous emission intensity dependence on the pumped strip length. A maximum net gain of 54 dB/cm was measured.
Soft x-ray microscopes
Authors: Kirz Janos , Rarback Harvey
Published: July 2002

Recent advances in sources, optics, and detectors for soft x rays have led to rapid developments in applications to microscopy. The history of the field and the advantages of soft x rays for the examination of biological specimens are briefly described. Limitations in resolution and radiation induced damage are mentioned before embarking on a survey of the various approaches in use. Contact microscopy is the highest resolution technique available today. It is particularly powerful when used with flash sources for stop-action imaging. Imaging with Fresnel zone plates provides direct magnification, high resolution, and short exposure times. Scanning microscopy leads naturally to digitally recorded images, with minimal radiation exposure to the specimen. The focused beam for scanning can be generated with multilayer mirrors, or with zone plates. This last approach is treated in greater detail. Extensions of soft x-ray imaging to three dimensions, and to elemental mapping, are considered as important areas under active development.
Surface roughness effects in laser crystallized polycrystalline silicon
Authors: McCulloch D. J. , Brotherton S. D.
Published: July 2002

Two surface roughening mechanisms have been distinguished in laser crystallized polycrystalline Si: one is related to rapid release of hydrogen from hydrogen-rich plasma enhanced chemical vapor deposited amorphous Si and the other is independent of the hydrogen content of the material and is determined by the total number of pulses incident on the surface. At, or beyond, the melt threshold energy there is a positive feedback effect between a beam-induced periodic surface roughness pattern and enhancement of this pattern by interference effects in subsequent pulses.
Microwave reflectometry with the extraordinary mode on tokamaks: Determination of the electron density profile of Petula-B
Authors: Bottollier-Curtet H. , Ichtchenko G.
Published: July 2002

The use of an ordinary mode reflectometer has already proved to be a good means for determining the density profile of a plasma, but the fact that this mode reflects where its frequency equals the local plasma frequency restricts its use to the region of the density gradient. The extraordinary mode reflectometry investigated here permits the observation of the scrape-off layer as well as the near-axis region and in certain conditions also the high magnetic field side of the density profile. It is also shown that this mode requires a range of frequency sweep for measuring a given density profile smaller than the ordinary mode. On the Petula-B tokamak, the density profile has been measured in 200 μs, its evolution when applying the lower hybrid current drive could be also followed. Since the measurement is particularly sensitive near the reflection point of the wave, it could be used to determine precisely the position of magnetic islands and the fluctuation amplitudes. In addition, the access and place occupied in the vacuum chamber being very small, this diagnostic should be compatible with reactor conditions.
Fiber-linked interferometric pressure sensor
Authors: Beheim G. , Fritsch K. , Poorman R. N.
Published: July 2002

A fiber-optic pressure sensor is described which uses a diaphragm to modulate the mirror separation of a Fabry–Perot cavity (the sensing cavity). A multimode optical fiber delivers broadband light to the sensing cavity and returns the spectrally modulated light which the cavity reflects. The sensor’s output spectrum is analyzed using a tunable Fabry–Perot cavity (the reference cavity) to determine the mismatch in the mirror separations of the two cavities. An electronic servo control uses this result to cause the mirror separation of the reference cavity to equal that of the sensing cavity. The displacement of the pressure-sensing diaphragm is then obtained by measuring the capacitance of the reference cavity’s metal-coated mirrors. Relative to other fiber-optic sensors, an important advantage of this instrument is its high immunity to the effects of variations in both the transmissivity of the fiber link and the wavelength of the optical source.
Atmospheric pollution monitoring using CO2-laser photoacoustic spectroscopy and other techniques
Authors: Meyer P. L. , Sigrist M. W.
Published: July 2002

The subject of air pollution monitoring is reviewed. First, we discuss the main conventional techniques currently applied to the detection of gaseous air pollutants, as well as the state of the art of novel detection schemes. We distinguish between nonspectroscopic and spectroscopic methods. Spectroscopic techniques are of primary interest since they offer several advantages, e.g., the simultaneous monitoring of numerous substances. Photoacoustic (PA) spectroscopy represents a promising spectroscopic technique due to its intrinsically high sensitivity, the large dynamic range, and the comparatively simple experimental arrangement. Emphasis is put on detection selectivity which often may restrict the range of applications of the technique for pollution monitoring due to the lack of powerful, continuously tunable IR laser sources. The theoretical aspects of PA spectroscopy with respect to trace gas detection and multicomponent analysis are thus discussed in detail. Some characteristics of a portable PA system built by us are outlined in order to illustrate the usefulness of such systems.
A mixer based electro-optic sampling system for submillivolt signal detection
Authors: Chwalek James M. , Dykaar Douglas R.
Published: July 2002

We report on a new technique which allows electro-optic sampling to be performed in a frequency range which is above the 1/f noise regime of the pump-probe laser, while at the same time utilizing the high-sensitivity, low-noise properties of an audio-frequency lock-in amplifier. This new approach to electro-optic signal extraction has resulted in an unprecedented improvement of two orders of magnitude in signal-to-noise capability. The new technique is compared to previous, conventional approaches.
Cryogenic monocrystalline silicon Fabry–Perot cavity for the stabilization of laser frequency
Authors: Richard J.-P. , Hamilton J. J.
Published: July 2002

A 1.6 kg silicon monocrystal was used to make a Fabry–Perot optical cavity operated at cryogenic temperatures. High-resolution thermal expansion measurements were made as the silicon cooled to 4.2 K in order to characterize the cavity as a length reference standard. A helium–neon laser was then locked to a transmission resonance at liquid-helium temperatures, and the laser frequency tracked the cavity resonance with error fluctuations at the level of 10 Hz/√Hz in the bandwidth dc to 1 Hz. Implications of the combined set of data, thermal expansion plus frequency-tracking fluctuations, for using such a system as a frequency standard are discussed.
Low-energy electron diffraction with energy resolution
Authors: Claus H. , Büssenschütt A. , Henzler M.
Published: July 2002

Low-energy electron diffraction (LEED) is a well established technique for evaluation of the surface atom arrangement. The information is derived from the elastically scattered electrons. Usually the thermal diffuse background between diffraction spots is assumed to be constant and subtracted as a constant. For spot profile analysis, which provides the arrangement of units like islands or domains, the elastic intensity has to be measured throughout the Brillouin zone. The usual LEED systems (with energy resolution of some eV) cannot distinguish between elastic and thermal diffuse scattering. For that purpose a new instrument has been developed. We combined the deflection unit from a high-resolution LEED system with 127° analyzer from a high-resolution electron energy loss spectrometer (EELS) in a suitable manner. The new instrument was checked with the Si(111), Al(111), and Al(111)+O2 surface. The energy resolution (ΔE=6.7 meV) allows separation of a large fraction of phonon losses. The momentum resolution (transfer width 150 nm) is the same as other high-resolution LEED systems. As a result the first LEED system with both high momentum and high energy resolution is presented (ELS-LEED).
A new impedance spectrometer for the investigation of electrochemical systems
Authors: Popkirov G. S. , Schindler R. N.
Published: July 2002

The development of a computer-controlled electrochemical impedance spectrometer, based on a Fourier transform algorithm is described. Together with a fast potentiostat the system can be used in the frequency range 1 mHz to 100 kHz. The perturbation signal is a superposition of sine waves with properly chosen frequencies. The overall measurement time is limited only by the lowest frequency in the spectrum and by the data transfer to the computer, thus, time-resolved impedance spectra measurements can be performed. The principle of operation and technical details are presented and discussed.
Compact gas gun injection system for variable sized solid pellets
Authors: Lang P. T. , Cierpka P. , Lang R. S. , Egorov S. M. , Kuteev B. V. , Reznichenko P. V. , Sergeev V. Yu.
Published: July 2002

The design, construction, and the testbed results for a novel compact gas gun injector for solid diagnostic pellets of different sizes and materials is reported here. The injector was optimized for the diagnostic requirements of the ASDEX Upgrade tokamak, yielding the possibility of a widely varying deposition profile of ablated material inside the plasma. This allows variation of the pellet velocity and the total number of injected atoms. The use of spherical carbon pellets and different propellant gases (He,N2,H2) results in an accessible velocity range from about 150 m/s to more than 600 m/s and pellet masses from 2×1018 to 1020 atoms. Both the scattering angle (∼1°) and the maximum propellant gas throughput to the tokamak (less than 1016 gas particles) were found to be sufficiently low. The injector provided both high efficiency (≥85%) and high reliability during the whole testbed operation period and also during the first injection experiments performed on ASDEX Upgrade. The pellet velocities achieved for different propellant gas pressures and pellet diameters were analyzed. It was discovered that, although the pellet diameters range from 0.45 to 0.85 times the barrel diameter, the pellet acceleration is mostly caused by gas drag. Pellet velocities in excess of those calculated on the basis of the gas drag model were observed. Additional acceleration that increases with the pellet diameter contrary to the gas drag model may be explained by the influence of the pellet on the gas dynamics in the barrel.
Analytical model for the approximation of hysteresis loop and its application to the scanning tunneling microscope
Authors: Lapshin Rostislav V.
Published: July 2002

A new model description and type classification carried out on its base of a wide variety of practical hysteresis loops are suggested. An analysis of the loop approximating function was carried out; the parameters and characteristics of the model were defined—coersitivity, remanent polarization, value of hysteresis, spontaneous polarization, induced piezocoefficients, value of saturation, hysteresis losses of energy per cycle. It was shown that with piezomanipulators of certain hysteresis loop types, there is no difference in heat production. The harmonic linearization coefficients were calculated, and the harmonically linearized transfer function of a nonlinear hysteresis element was deduced. The hysteresis loop type was defined that possesses minimum phase shift. The average relative approximation error of the model has been evaluated as 1.5%–6% for real hysteresis loops. A procedure for definition of the model parameters by experimental data is introduced. Examples of using the results in a scan unit of a scanning tunneling microscope for compensation of raster distortion are given.
Very low threshold 1.55 μm grating coupled surface-emitting lasers for optical signal processing and interconnect
Authors: Choa F. S. , Shih M. H. , Fan J. Y. , Simonis G. J. , Liu P.-L. , Tanbun-Ek T. , Logan R. A. , Tsang W. T. , Sergent A. M.
Published: July 2002

We report in this work a record low threshold (∼4 mA, cw), 1.55 μm wavelength, grating-coupled surface-emitting laser (GCSEL) which rivals all other existing SELs in the 1.3 or/and 1.55 μm regions. Methods to improve the asymmetric far-field patterns and to obtain even lower threshold SELs are also demonstrated and/or suggested.
Long-time present tweedlike precursors and paraelectric clusters in ferroelectrics containing strong quenched randomness
Authors: Viehland Dwight , Kim Myung-Chul , Xu Z. , Li Jie-Fang
Published: July 2002

Transmission electron microscopy studies of the (1−x)Pb(Mg1/3Nb2/3)O3–(x)PbTiO3 [PMN-PT (1-x)/x] crystalline solution have been performed for x=0.1, 0.2, 0.35, and 0.60. Bright-field imaging has revealed a common sequence of domainlike states with increasing x. Normal micron-sized ferroelectric domains were found for x≳0.40. Tweedlike structures were found for x∼0.35. These tweedlike structures are similar to those previously reported in premartensitic states. Paraelectric clusters were found for x<0.30. The paraelectric cluster state was characterized by the lack of self-assembly amongst embryos and the presence of relaxor behavior in the macroscopic response characteristics. The composition PMN-PT 65/35 was then modified with La for a detailed study of the transition between the tweedlike precursor and paraelectric cluster states with increasing impurity content.
Near band-edge photoluminescence in relaxed Si1−xGex layers
Authors: Tilly L. P. , Mooney P. M. , Chu J. O. , LeGoues F. K.
Published: July 2002

Near band-gap photoluminescence was observed at low temperatures from relaxed Si1−xGex layers with 0.17<x<0.32 grown on Si(001) by ultrahigh vacuum chemical vapor deposition. The luminescence from undoped samples was dominated at low temperature and low excitation densities by recombination of excitons bound to alloy fluctuations exhibiting the smallest full width at half-maximum, 2.44 meV, reported for relaxed epitaxial Si1−xGex layers. Excitons bound to phosphorous and boron were also observed as was free exciton recombination.
Measurement of loss of DT fusion products using scintillator detectors in TFTR (invited)
Authors: Darrow D. S. , Herrmann H. W. , Johnson D. W. , Marsala R. J. , Palladino R. W. , Zweben S. J. , Tuszewski M.
Published: July 2002

A poloidal array of MeV ion loss probes previously used to measure DD fusion product loss has been upgraded to measure the loss of alpha particles from DT plasmas in the Tokamak Fusion Test Reactor (TFTR). The following improvements to the system have been made in preparation for the use of tritium in TFTR: (1) relocation of detectors to a neutron-shielded enclosure in the basement to reduce neutron-induced background signals; (2) replacement of ZnS:Cu (P31) scintillators in the probes with the Y3Al5O12:Ce (P46) variety to minimize damage and assure linearity at the fluxes anticipated from DT plasmas; and (3) shielding of the fiber optic bundles which carry the light from the probes to the detectors to reduce neutron- and gamma-induced light within them. In addition to the above preparations, the probes have been absolutely calibrated for alpha particles by using the Van de Graaf accelerator at Los Alamos National Laboratory. Alpha particle losses from DT plasmas have been observed, and losses at the detector 90° below the midplane are consistent with first orbit loss.
High resolution x-ray tomography in an electron microprobe
Authors: de Paiva Rogério Ferreira , Bisiaux Michel , Lynch John , Rosenberg Elisabeth
Published: July 2002

Conversion of an electron microprobe for use as an x-ray microtomograph is described. Only minor modifications are required, indeed several of the utilities of the microprobe can be used to ensure high resolution radiography and tomography. System performance was evaluated by analysis of glass fiber reinforced polymer composites and of a carbonate reservoir rock. Individual glass fibers of 10 and 20 μm diameter are clearly visible in the former. In the latter, comparison with scanning electron micrographs shows reliable reconstruction of external morphology. In addition, internal pore structure can be studied a posteriori using the tomography data. Spatial resolutions in radiography of better than 3 μm and in tomography of 10 μm have been demonstrated. Further improvements to the detector system and increased data handling capacity are possible to achieve 1 μm resolution tomographic images.
Liquid crystal variable retarders in atomic scattering
Authors: Furst J. E. , Yu D. H. , Hayes P. A. , D’Souza C. M. , Williams J. F.
Published: July 2002

The applications of liquid crystal variable retarders in the production of spin-polarized electrons and in the determination of the polarization of optical radiation from atoms excited by polarized electrons are discussed. The advantages of using liquid crystal variable retarders in the measurement of Stokes parameters are insensitivity to the incident photon direction, large transmission diameter, variable retardation over large wavelength range without mechanical movement, and in situ calibration.
Time-series analysis of nonstationary plasma fluctuations using wavelet transforms
Authors: Santoso S. , Powers E. J. , Bengtson Roger D. , Ouroua A.
Published: July 2002

A wavelet or time-scale approach to analyzing a single time series and two time series, in which the fluctuating quantities are statistically nonstationary, is presented. The time scale and scale “power spectra” are introduced and utilized to analyze transient potential fluctuations measured at the core of sawtoothing TEXT-U plasmas. The results show features that have not been previously observed using any Fourier techniques. In addition, the linear time-scale “coherence spectrum” is developed to quantify the degree of linear relationship between two nonstationary fluctuating quantities in the time-scale domain. Such a spectrum is also useful in tracking the time-varying phase difference. A numerical example is provided to demonstrate the efficacy of the time-scale spectra.
A new detection system for x-ray microanalysis based on a silicon drift detector with Peltier cooling
Authors: Fiorini C. , Kemmer J. , Lechner P. , Kromer K. , Rohde M. , Schülein T.
Published: July 2002

A detection system made with a silicon drift detector cooled by a Peltier element has been designed and tested. This system has been conceived to be implemented in electron probe microanalyzers for the x-ray microanalysis of the specimen. The good energy resolution of the detector, 155 eV full width at half-maximum at the Mn–Kα line, obtained at moderately low temperature (−15 °C) by a small single-stage Peltier cooler, let this module be used in many applications in place of a standard Si(Li) detector, without the need of a liquid-nitrogen cryostat and with a good geometrical collection efficiency. Also, the particularly low values of shaping time (<2 μs) used for optimum energy resolution suggest the use of this system in operations with high counting rates (>30 000 counts/s) like, for example, fast scannings for elemental mapping. The module has been implemented in an electron microscope and the test results are reported in the article.
A method to improve the nonrepetitive acquisition of transient absorption spectra with an intensified charge-coupled device camera
Authors: Baldacchino G. , Hickel B.
Published: July 2002

This article describes in detail a new method of data acquisition using an intensified charge coupled device (CCD) camera coupled with a monochromator as a time-resolved absorption spectrometer. The method is based on a full image transfer taking full advantage of the dynamic range of the CCD. The accumulation of several lines of the matrix is then limited by the 32-bit range of the computer calculation. The absorbance characteristics are a good signal-to-noise ratio and a large linearity domain (absorbances between 0.05 and 0.6) in the visible range (350–700 nm) with temporal gates less than 100 ns. The time-resolved absorption spectra obtained for two transient species in aqueous solution [CO3− and (SCN)2−] are presented and are in good agreement with those considered as reference and obtained with a multistep method with a classical monochromator-photomultiplier device.
Recursive algorithm for fast evaluation of the Abel inversion integral in broadband reflectometry
Authors: Nunes F. D. , Santos J. , Manso M. E.
Published: July 2002

The application of Kalman filtering is proposed for the numerical evaluation of plasma density profiles using noisy group delay data obtained from O-mode frequency-modulated broadband reflectometry. This method allows faster computation of the Abel inversion integral than the usual numerical methods (e.g., the trapezoidal rule) while maintaining a good accuracy in the presence of noise. In addition, density profiles with different degrees of smoothing can be produced by adjusting a single parameter. The algorithm seems promising in applications to plasma control in real time.
Abel inversions: Error propagation and inversion reliability
Authors: Ramsey A. T. , Diesso M.
Published: July 2002

Inversion of chordally integrated data to infer the profile of a plasma parameter (plasma density, radiated power, or, in our case, the visible continuum from bremsstrahlung) propagates errors from outer shells inward as the inversion progresses. If the inversion is done by a matrix technique, error propagation can likewise be determined by a matrix technique. Where other considerations make the matrix method undesirable, there is no clearly defined analytical method to test the reliability of the inversion. We have solved this problem by taking real data from the Tokamak Fusion Test Reactor (TFTR) visible bremsstrahlung (VB) diagnostic, applying normally distributed random noise of a known mean value, and inverting the signal. When we have done this hundreds of times at all points in the profile, we can clearly track the error propagation. At the plasma center, for example, the error is several times the average error in the relative (chord to chord) calibration of the system. This error multiplier is a powerful generalization; it depends little on the details of the inversion, and almost entirely on the profile shape. For the TFTR VB profile it ranges from 2 to 4 for most types of plasma. We have also looked at the damping of the inversion error from an error in a single chord; this damping can be rather fast in some cases.
Current induced second harmonic generation in semiconductors
Authors: Khurgin Jacob B.
Published: July 2002

Direct current in semiconductor is theoretically shown to be capable of doubling the frequency of the incoming optical radiation. The second order susceptibility, proportional to the current is calculated to be in the 10−14−10−13 m/V range. Applications of the novel phenomenon in probing and mapping of the current in semiconductor devices are considered.
Liquid stub tuner for ion cyclotron heating
Authors: Kumazawa R. , Mutoh T. , Seki T. , Sinpo F. , Nomura G. , Ido T. , Watari T. , Noterdaeme Jean-Marie , Zhao Yangping
Published: July 2002

Ion cyclotron range of frequency (ICRF) heating on the large helical device (LHD) is characterized by high power (up to 12 MW) and steady state operation (30 min). The LHD is a helical device (with a major radius of 3.9 m and a minor radius of 0.6 m) with superconducting coil windings (l=2, m=10). The main purpose of physical research is to investigate currentless and disruption-free plasma. Research and development for steady state ICRF heating has been carried out in recent years: A high rf power transmission system consisting of stub tuners, a ceramic feedthrough, and an ion cyclotron heating loop antenna has been developed. In addition, steady state operation of a rf oscillator has been achieved at a power higher than 1 MW. A liquid stub tuner has been proposed as an innovation. The liquid stub tuner makes use of the difference between the rf wavelengths in liquid and in gas due to the different relative dielectric constants. The liquid stub tuner has been experimentally proved to be a reliable rf component for high power transmission systems. Test results have quantitatively demonstrated that it can be used at high rf voltage: 61 kV for 10 s and 50 kV for 30 min. Furthermore, the liquid surface can be shifted under high rf voltage without breakdown, which suggests that it can be employed as a feedback control impedance matching tool to keep reflected rf power at a low level with regard to a temporal variation of plasma loading resistance.
Sapphire filter thickness optimization in neutron scattering instruments
Authors: Stamatelatos I. E. , Messoloras S.
Published: July 2002

The present work is concerned with the optimization of the sapphire fast neutron filter thickness used in neutron diffraction instruments. The optimization is based on maximization of the slow neutron transmission, minimization of the fast neutron transmission, and also taking into consideration the neutron background at the vicinity of an instrument. Scattering properties of the sapphire in the fast and slow neutron regions are discussed.
Microwave ion sources for industrial applications (invited)
Authors: Sakudo N.
Published: July 2002

Microwave ion sources for industrial use are usually driven by 2.45 GHz microwaves and operated in a very wide range of magnetic field from zero to over the electron cyclotron resonance magnetic field. They are used mostly under off-resonant conditions. For ion implantation into usual semiconductor devices, the weak points of the microwave ion source against the conventional implanter sources, the Freeman and the Bernas, had been the lower B+ ion current and the slightly narrower dynamic range of the current. However, the optimization of the discharge-chamber shape and volume resolved the problems. Consequently, the microwave sources exceed the conventional sources in most principal performances for implantation into semiconductor devices. For the sophisticated separation by implanted oxygen devices, the microwave ion source is very suitable for stable production of high-current O+ ion beams. 100 mA class O+ ion implanters dedicated to silicon on insulator technology were developed. On the other hand, for application to surface modification of materials, mass separation is completely eliminated in some cases. Recently, a new ion source for the purpose was developed, in which 2.45 GHz microwaves are absorbed by 13.56 MHz inductively coupled plasma without static magnetic field. The alternate magnetic field induced by 13.56 MHz rf power is considered to help microwaves penetrate into the plasma. Since the volume of the source is not restricted by solenoids as a usual microwave source, it can be applied to three dimensional implantation or plasma source ion implantation.
Calculated differential reflectance of the (110) surface of cubic silicon carbide
Authors: Gavrilenko V. I.
Published: July 2002

Surface band structure and differential reflectance spectra of the (110)(1×1) surface of cubic silicon carbide crystals are investigated within a self-consistent tight binding theory. The surface electronic band structure is characterized by the pronounced surface states in the gap and show metallic character. Optical transitions between the surface states cause an enormous reflectivity within the range of 1.0 to 1.5 eV and reveal strong polarization anisotropy. Influence of the atomic geometry of the (110)(1×1) surface on the differential reflectance of 3C SiC is studied.
Effect of LaNiO3/Pt double layers on the characteristics of (PbxLa1−x)(ZryTi1−y)O3 thin films
Authors: Tseng Tzu-Feng , Liu Kuo-Shung , Wu Tai-Bar , Lin I.-Nan
Published: July 2002

Characteristics of LaNiO3 (LNO) thin films deposited by rf sputtering process were compared with that prepared by pulsed laser deposition (PLD) process. rf sputtered LNO films were [200] preferentially oriented with a low surface resistivity (ρs=0.55 mΩ cm) and the PLD deposited films were [110] predominated with a slightly larger surface resistivity (ρs=6.38 mΩ cm). Using Pt coating as underneath layer markedly reduced the surface resistivity (ρs=0.05 mΩ cm) without modifying the texture characteristics of the LNO layers. PLZT films subsequently deposited on LNO layers inherited the texture characteristics of the underlying LNO layers. Ferroelectric properties of PLZT films were optimized when using LNO–Pt double layers as bottom electrodes. The remanent polarization and coercive force obtained were Pr=14.9 μC/cm2 and Ec=3.5 kV/cm, respectively.
Tailoring a high-transmission fiber probe for photon scanning tunneling microscope
Authors: Saiki T. , Mononobe S. , Ohtsu M. , Saito N. , Kusano J.
Published: July 2002

Transmission efficiency of a fiber probe used in photon scanning tunneling microscope is evaluated as a function of aperture diameter. The apertured probe has been fabricated by chemical etching technique and metal coating. By comparing two types of probes with different cone angles, we determine the most influential factor in the transmission property of the metal-cladding tapered waveguide. A long tip with high efficiency is developed by a multistep etching method so as to be suitable during actual scanning operation. Photoluminescence imaging of lateral p-n junctions on the GaAs substrate is demonstrated in the illumination-collection hybrid mode operation of photon scanning tunneling microscope.
Air-stable Cl-terminated Ge(111)
Authors: Lu Z. H.
Published: July 2002

It is shown that an air-stable Ge(111) surface can be made by wet chemical HCl treatment in ambient atmosphere. X-ray photoelectron spectroscopy (XPS) and photon electrical field polarization-dependent Cl K-edge x-ray absorption spectroscopy (XANES) were used to study the chemical structure and the physical orientation of chemisorbed Cl. It is found that Cl forms a monovalent bond with the Ge(111) dangling bond (DB) along 〈111〉 surface normal. The stability of HCl-treated Ge(111) is attributed to the surface DBs being terminated by monochloride. Thermodynamics are found to be the governing force in the Ge–HCl reaction processes.
Penetration depth measurements of single-crystal NbN films at millimeter-wave region
Authors: Komiyama Bokuji , Wang Zhen , Tonouchi Masayoshi
Published: July 2002

The zero temperature penetration depth λ(0) of single-crystal NbN thin films has been measured at 82 GHz using an open resonator. The NbN films used in this study have a (200) single-crystal structure and Tc and ρ20 (normal state resistivity at 20 K) are 16 K and 62 mΩ cm, respectively. λ(0) was obtained by applying a least-squares fit to the change of penetration depth using the BCS theory. We obtain λ(0)=194 nm, which shows good agreement with the calculated value from Tc,ρ20 and Δ(0).
Optical quadratic nonlinearity in multilayer corona-poled glass films
Authors: Horinouchi S. , Imai H. , Zhang G. J. , Mito K. , Sasaki K.
Published: July 2002

Second order nonlinearity via corona poling at room temperature in a Corning 7059 glass film on a Pyrex glass substrate was studied. It was clearly pointed out that the origin of the induced nonlinearity in this waveguide was located at the interface region between a top layer glass film and a glass substrate. This region is formed to be a depletion layer in the consequence of a drift of positive ions towards a negative electrode during corona poling. X-ray photoelectron spectroscopy measurement on the corona-poled Pyrex glass substrate gave the evidence that the main mobile ions were sodium ions.