bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple, low-cost, versatile charge-coupled device spectrometer for plasma spectroscopy
Authors: Den Hartog D. J. , Holly D. J.
Published: July 2002

We have constructed a simple, low-cost charge-coupled device (CCD) spectrometer capable of both high resolution (Δλ⩽0.015 nm) and large bandpass (110 nm with Δλ∼0.3 nm). These two modes of operation provide two broad areas of capability for plasma spectroscopy. The first major application is measurement of emission line broadening; the second is emission line surveys from the ultraviolet to the near infrared. Measurements have been made on a low-temperature plasma produced by a miniature electrostatic plasma source and the high-temperature plasma in the Madison Symmetric Torus reversed-field pinch. The spectrometer is a modified Jarrell–Ash 0.5 m Ebert–Fastie monochromator. Light is coupled into the entrance slit with a fused silica fiber optic bundle. The exposure time (2 ms minimum) is controlled by a fast electro-mechanical shutter. The exit plane detector is a compact and robust CCD detector developed for amateur astronomy by Santa Barbara Instrument Group. The CCD detector is controlled and read out by a Macintosh® computer. This spectrometer is sophisticated enough to serve well in a research laboratory, yet is simple and inexpensive enough to be affordable for instructional use.
Bench test results on a new technique for far-infrared polarimetry
Authors: Barry S. , Nieswand C. , Prunty S. L. , Mansfield H. M. , O’Leary P.
Published: July 2002

The results of bench tests performed on a new method of combined interferometry/polarimetry for the magnetic-field reconstruction of tokamak plasmas is presented. In particular, the sensitivity obtained in the polarimetric measurement shows the feasibility of Faraday rotation determination approaching a precision of ±0.2°. The method is based on an optically pumped far-infrared laser with a rotating polarization where both the interferometric and polarimetric information is determined from phase measurements. Specific sources that introduce disturbances in the optical arrangement and that can limit the attainment of the polarimetric precision, mentioned above, are discussed.
An apparatus for fabrication of optical waveguides by electromigration
Authors: Monteiro H. S. , Said J. C. , Mendes J. F. , Srivastava R.
Published: July 2002

We present an apparatus for fabricating ion exchanged glass waveguides with reproducible characteristics. It consists of a vertical furnace with a uniform temperature zone, which is flat to within 1 °C over the sample length, a silica crucible for containing molten salt, a metallic sample holder made of either aluminum or stainless steel, and a motor-driven sample lowering system. The time and temperature are accurately controlled to produce waveguides of desired index profiles.
A fast detector using stacked avalanche photodiodes for x-ray diffraction experiments with synchrotron radiation
Authors: Kishimoto S. , Ishizawa N. , Vaalsta T. P.
Published: July 2002

We have developed a fast detector using stacked avalanche photodiodes (APDs) for x-ray diffraction experiments with synchrotron radiation. This detector has four independent channels of APDs, and the detector efficiency has reached 55% in all for 16.53 keV x rays. Since the dead time of the counting system is shorter than 4 ns, output rates of up to 4.5×108 counts/s have been obtained for 16.53 keV x rays. The dynamic range is more than 1010 in the multibunch mode of a storage ring. Pulse-height measurements at output rates of up to 108 counts/s were successfully carried out by sequential single-channel discrimination.
A new optical oscilloscope
Authors: Li Shuhong , Niu Hanben , Yang Qinlao , Zhang Huanwen , Guo Baoping , Zhou Junlan
Published: July 2002

A new optical oscilloscope has been developed on the basis of the image-converter tube camera technology. Optical or electrical pulses in single or repetitive mode can be tested. When single pulse without time dependence on the sweep circuit is tested, an optical-fiber delay unit is adopted to ensure the pulse is captured. The optical oscilloscope works at a fixed frequency while repetitive pulses of any frequency less than 20 GHz were tested. A newly designed photocathode optical oscilloscope tube, control circuits, and a linear charge-coupled device readout system have been incorporated into the optical oscilloscope. Experimental results show that the temporal resolution of the optical oscilloscope is better than 5 ps, the spectral response range is 380–850 nm, and the single pulse can be captured with the probability of 100%.
A three-wave heterodyne correlation reflectometer developed in the T-10 tokamak
Authors: Vershkov V. A. , Dreval V. V. , Soldatov S. V.
Published: July 2002

A three-wave heterodyne O-mode reflectometer was developed and tested in experiments on the T-10 tokamak. Three launched waves are obtained by frequency splitting due to amplitude modulation with a p-i-n diode. Source frequencies from 36 to 78 GHz were used in the experiments. The ability of the reflectometer to measure simultaneously density profile characteristics, such as the time evolution of the relative phase and the unambiguous time delay of the reflected wave together with small-scale density fluctuation characteristics such as the radial and poloidal correlation lengths, was demonstrated with T-10 experimental data. The reflectometer can provide important information about fine changes in the density profile during sawteeth and low-m magnetohydrodynamics oscillations. Poloidal correlation measurements make it possible to measure transverse turbulence velocities and velocity shear. It was shown that both types of correlation measurements enable one to distinguish different types of turbulence according to the values of their correlation lengths, and that probing the plasma from several poloidal directions simultaneously greatly enhances the potential of reflectometry for the investigation of turbulence physics. In particular, it was found that different turbulence types may be either poloidally symmetrical or have much higher amplitude on the low field side of the discharge. Probing with several poloidal directions also enables one to make toroidal correlation measurements over long distances at certain resonant q values after one or two turns of magnetic field line around the major tokamak axis. In fact, correlations near 40% were observed experimentally. Such measurements may give additional information about turbulence characteristics on the one hand and allow radial q profile measurements on the other hand. In addition, the capabilities of reflectometry may be enhanced with the application of a proposed holography approach.
Ultrathin luminescent nanoporous silicon on n-Si: pH dependent preparation in aqueous NH4F solutions
Authors: Dittrich Th. , Rauscher S. , Timoshenko V. Yu. , Rappich J. , Sieber I. , Flietner H. , Lewerenz H. J.
Published: July 2002

Thin nanoporous Si layers were formed on n-Si(100) in 0.1 M NH4F electrolyte and investigated with field emission scanning electron microscopy and photoluminescence (PL). The microstructure of the electrochemically prepared Si surfaces depended strongly on the pH of the electrolyte. A pebblelike surface structure was formed during anodization at pH 4.5, while nanoporous Si layers were formed at lower pH. The onset of macropore formation was observed after anodization at pH 3.0. The shape of the PL spectra was found to be practically independent of the nanoporous structure. The results are discussed on the basis of the etch rate of oxidized Si atoms and of the role of surface states for PL.
Laser time-of-flight mass spectrometry for space
Authors: Brinckerhoff W. B. , Managadze G. G. , McEntire R. W. , Cheng A. F. , Green W. J.
Published: July 2002

We describe a miniature reflection time-of-flight mass spectrometer for in situ planetary surface analysis. The laser ablation mass spectrometer (LAMS) measures the elemental and isotopic composition of regolith materials without any sample preparation or high-voltage source extraction. The small size (<2×103 cm3) and low mass (∼2 kg) of LAMS, due to its fully coaxial design and two-stage reflectron, satisfy the very strict resource limitations of landed science missions to solar system bodies. Microscopic surface samples are obtained with a short-pulse laser focused to a spot with a diameter ∼30–50 μm. Coupled with a microimager, LAMS can interactively select and analyze a range of compositional regions (with lateral motion) and access unweathered, subsurface materials (with repeated pulses). The mass resolution is sufficient to distinguish isotopic peaks at unit masses, and the detection limits are on the order of a few ppm. The design and calibration method of a prototype LAMS device is given, including the development of preliminary relative sensitivity coefficients for major element bulk abundance measurements.
Longitudinal and transverse magneto-impedance in amorphous Fe73.5Cu1Nb3Si13.5B9 films
Authors: Sommer R. L. , Chien C. L.
Published: July 2002

Magneto-impedance effects have been observed in thin films of amorphous Fe73.5Cu1Nb3Si13.5B9. Large magneto-impedance values have been obtained in field-annealed samples, but not in as-deposited samples. Transverse magneto-impedance effects with magnitudes comparable to the longitudinal case have been observed. These results are discussed in terms of appropriate magnetic permeability.
Growth oscillations with monolayer periodicity monitored by ellipsometry during metalorganic vapor phase epitaxy of GaAs(001)
Authors: Zettler J.-T. , Wethkamp T. , Zorn M. , Pristovsek M. , Meyne C. , Ploska K. , Richter W.
Published: July 2002

In this letter we report on the observation of growth oscillations with monolayer periodicity by ellipsometry. An oscillation amplitude of δ〈ε1〉=0.05 was measured using an optimized spectroscopic in situ ellipsometer whose wavelength was tuned to the 2.65 eV resonance energy of the arsenic dimers covering the GaAs (001) growth surface. The monolayer periodicity was verified by parallel monitoring of the growth with reflectance anisotropy spectroscopy (RAS).
Electroluminescence from a conjugated polymer microcavity structure
Authors: Fisher T. A. , Lidzey D. G. , Pate M. A. , Weaver M. S. , Whittaker D. M. , Skolnick M. S. , Bradley D. D. C.
Published: July 2002

We report the observation of electroluminescence and photoluminescence from microcavity structures containing poly(2,5-dialkoxy-p-phenylene vinylene) (PDAOPV) conjugated polymer emitting regions. Strong spectral narrowing from 112 nm full width half-maximum to 34 nm in electroluminescence and from 128 to 16 nm in photoluminescence and clear angular dependence of the peak emission wavelength are observed. These are characteristic signatures of the modification of spontaneous emission properties in microcavity structures.
Ripening-assisted asymmetric spalling of Cu-Sn compound spheroids in solder joints on Si wafers
Authors: Kim H. K. , Tu K. N. , Totta P. A.
Published: July 2002

In reacting eutectic SnPb solder with Ti/Cu and Cr/Cu/Au thin film metallization on Si wafers, we have observed spalling of Cu6Sn5 spheroids when the solder consumes the Cu. The formation of the spheroids is assisted by the ripening reaction among the compound grains. In addition we have observed an asymmetric spalling phenomenon using a sandwich structure, in which two wafers were soldered face-to-face. The spalling occurs predominantly at the interface at the bottom of the solder joint. It suggests that gravity plays a role.
Diamondlike properties in a single phase carbon nitride solid
Authors: Zhang Z. John , Fan Shoushan , Huang Jinlin , Lieber Charles M.
Published: July 2002

The intrinsic properties and bonding of single phase carbon nitride thin films of stoichiometry C2N have been studied. C2N exhibits high electrical resistivity and thermal conductivity that are similar to the properties shown by diamondlike carbon (DLC) films. Electron energy loss spectroscopy studies how, however, that the carbon in C2N is sp2-hybridized rather than sp3-bonded carbon as in DLC. This carbon bonding in C2N film leads to a high thermal stability that makes C2N distinct from DLC and nitrogen doped DLCs, and thus an attractive candidate for applications.
Above band gap absorption spectra of the arsenic antisite defect in low temperature grown GaAs and AlGaAs
Authors: Dankowski S. U. , Streb D. , Ruff M. , Kiesel P. , Kneissl M. , Knüpfer B. , Döhler G. H. , Keil U. D. , So/renson C. B. , Verma A. K.
Published: July 2002

Room temperature absorption spectra of low temperature molecular beam epitaxy grown GaAs (LT-GaAs) and AlGaAs (LT-AlGaAs) are reported. We performed measurements in an extended spectral range from 0.8 eV to photon energies of 2.8 eV far above the band gap. For as-grown LT-materials, the absorption coefficients at the band gap are twice as high as for high temperature grown materials. By annealing the samples, we obtained a drastic reduced absorption coefficient below as well as above the band gap. We observed absorption changes up to 17 000 cm−1 for LT-GaAs and 9000 cm−1 for LT-AlGaAs taking place in a two phase process.
Ultrafast high-intensity nonlinear absorption dynamics in low-temperature grown gallium arsenide
Authors: Siegner U. , Fluck R. , Zhang G. , Keller U.
Published: July 2002

We study carrier dynamics in GaAs thin films grown by molecular beam epitaxy at 250, 300, and 350 °C by differential transmission experiments at various carrier excitation densities. The differential transmission shows that carrier trapping in point defects is much faster than the recombination of the trapped carriers. As a consequence, the defect states can be saturated at high carrier densities. If the growth temperature is decreased, the initial trapping becomes faster while the subsequent recombination of the trapped carriers becomes slower. We show that this is due to the growth temperature dependent defect densities.
Investigation of ultrashort photocarrier relaxation times in low-temperature-grown GaAs
Authors: McIntosh K. A. , Nichols K. B. , Verghese S. , Brown E. R.
Published: July 2002

Photocarrier relaxation times τr in low-temperature-grown (LTG) GaAs have been determined with time-resolved reflectance measurements. Measured τr values are extremely sensitive to the substrate temperature during LTG GaAs growth and postgrowth anneal. Photogenerated-electron relaxation times as short as 90 fs are found for LTG GaAs grown at temperatures near 200 °C and annealed at temperatures below 580 °C. We report the results of a systematic investigation of the dependence of τr on growth temperatures between 180 and 260 °C and anneal temperatures between 480 and 620 °C.
The electrical conductivity of levitated liquids
Authors: Enderby J. E. , Ansell S. , Krishnan S. , Price D. L. , Saboungi M.-L.
Published: July 2002

The electrical conductivities of aerodynamically levitated liquid spheres have been determined by an electrodeless method. It is shown that this technique is reliable over a wide range of temperatures; results are presented for a variety of systems including metals, semiconductors at room temperature and at their melting points, and solid and liquid Al2O3.
Millimeter wave oscillator based on a quasiplanar superlattice electronic device
Authors: Schomburg E. , Hofbeck K. , Grenzer J. , Blomeier T. , Ignatov A. A. , Renk K. F. , Pavel’ev D. G. , Koschurinov Yu. , Ustinov V. , Zhukov A. , Ivanov S. , Kop’ev P. S.
Published: July 2002

We report on a millimeter wave oscillator based on a quasiplanar superlattice electronic device (SLED). The SLED, a lateral structured GaAs/AlAs superlattice, showing, at room temperature, a negative differential conductance, was provided with two terminals lying in one plane and mounted in a waveguide structure. The oscillator delivered radiation, in a relative bandwidth of 10−5, that was tunable by about 10% around 70 GHz and had a power of 100 μW; depending on the voltage across the superlattice, additional oscillation lines (up to 180 GHz) appeared. We associate the generation of radiation with a current oscillation caused by traveling dipole domains in the superlattice.
Optical properties of GaN/AlGaN multiple quantum well microdisks
Authors: Mair R. A. , Zeng K. C. , Lin J. Y. , Jiang H. X. , Zhang B. , Dai L. , Tang H. , Botchkarev A. , Kim W. , Morkoç H.
Published: July 2002

An array of microdisks with a diameter of about 9 μm and spacing of 50 μm has been fabricated by dry etching from a 50 Å/50 Å GaN/AlxGa1-xN (x∼0.07) multiple quantum well (MQW) structure grown by reactive molecular beam epitaxy. Optical properties of these microdisks have been studied by picosecond time-resolved photoluminescence (PL) spectroscopy. Photoluminescence emission spectra and decay dynamics were measured at various temperatures and pump intensities. With respect to the original MQWs, we observe strong enhancement of the transition intensity and lifetime for both the intrinsic and barrier transitions. The intrinsic transition is excitonic at low temperatures and exhibits an approximate tenfold increase in both lifetime and PL intensity upon formation of the microdisks. This implies a significant enhancement of quantum efficiency in microdisks and a bright future for III-nitride microcavity lasers.
Second-harmonic generation at λ=1.6 μm in AlGaAs/Al2O3 waveguides using birefringence phase matching
Authors: Fiore A. , Janz S. , Delobel L. , van der Meer P. , Bravetti P. , Berger V. , Rosencher E. , Nagle J.
Published: July 2002

We demonstrate phase-matched second-harmonic generation from a λ=1.6 μm pump in a GaAs-based waveguide. Phase matching is obtained by using the form birefringence in an AlGaAs/Al2O3 multilayer obtained by selective wet oxidation.
Stress evolution in Mo/Si multilayers for high-reflectivity extreme ultraviolet mirrors
Authors: Freitag J. M. , Clemens B. M.
Published: July 2002

The stress evolution of sputter deposited Mo/Si multilayers of possible application as extreme ultraviolet light mirrors has been investigated by in situ substrate curvature measurements using a multiple parallel laser beam technique. Our preliminary results show well-defined stress modulation concurrent with the deposition of Mo and Si layers in the multilayer structure. Large changes in substrate curvature were measured during the early stages of deposition of the individual layers, with Mo exhibiting apparent tension and Si exhibiting apparent compression. The magnitudes of these curvature changes partially offset each other, resulting in an average compressive stress of −350 MPa in the multilayer. Possible stress generating mechanisms during growth of these multilayers as well as single layer films of Mo and Si will be discussed.
Large frequency range of negligible transmission in one-dimensional photonic quantum well structures
Authors: Zi Jian , Wan Jun , Zhang Chun
Published: July 2002

We show that it is possible to enlarge the range of low transmission in one-dimensional photonic crystals by using photonic quantum well structures. If a defect is introduced in the photonic quantum well structures, defect modes with a very high quality factor may appear. The transmission of the defect mode is due to the coupling between the eigenmodes of the defect and those at the band edges of the constituent photonic crystals.
Raman and x-ray studies of InN films grown by metalorganic vapor phase epitaxy
Authors: Lee Ming-Chih , Lin Heng-Ching , Pan Yung-Chung , Shu Chen-Ke , Ou Jehn , Chen Wen-Hsiung , Chen Wei-Kuo
Published: July 2002

Thin InN films were deposited on the (0001) sapphire substrate at various temperatures from 325 to 600 °C by metalorganic vapor phase epitaxy. We used Raman scattering and x-ray diffraction to investigate the film properties and crystalline structures. Significant line broadening, softening and intensity evolution were observed at the growth temperatures between 375 and 450 °C. This can be attributed to the formation of the mixed hexagonal and cubic structures and the related dislocation defects. As the growth temperature is further increased, the hexagonal phase is found to be dominant in the deposited InN film.
Surface and bulk mobilities of oligothiophene single crystals
Authors: Schön J. H. , Kloc Ch. , Laudise R. A. , Batlogg B.
Published: July 2002

High-quality single crystals of α-quaterthiophene (α-4T) and α-hexathiophene (α-6T) were investigated to compare intrinsic bulk and surface electrical properties. The bulk properties of these organic p-type semiconductors are derived from extended current–voltage characteristics and the surface properties from single crystal field-effect transistor measurements. Most significantly, charge carrier mobilities as high as 0.5 cm2/V s are observed in α-6T both in the bulk and at the surface. The high quality and purity of the crystals are evident from the low trap densities(<1015 cm−3) and the even lower dopant concentrations (2×1013 for α-4T and 7×1010 cm−3 for α-6T). These intrinsically high performance figures, together with the ease of processing, make these oligothiophenes attractive materials for “plastic electronic” devices.
Effect of compression on grain growth in Al films
Authors: Volkert C. A. , Lingk C.
Published: July 2002

The effect of compression on grain growth in Al and Al(Cu) films on oxidized Si substrates has been studied. Samples were compressed in a uniaxial press during annealing at 400 °C, and then examined using focused ion beam imaging. It was found that the compressed regions had a final grain size roughly a factor of 3 larger than in the uncompressed regions. This behavior is interpreted as due to a decrease in grain boundary grooving and the pinning forces associated with them, thus allowing enhanced grain growth.
Picosecond ultrasonic study of Mo/Si multilayer structures using an alternating-pump technique
Authors: Pu Nen-Wen , Bokor Jeffrey , Jeong Seongtae , Zhao Ri-An
Published: July 2002

We have used picosecond ultrasonics techniques to study the localized acoustic-phonon surface modes in Mo/Si multilayer reflectors for extreme ultraviolet lithography. Localized surface modes in the first (zone-boundary) and second (zone-center) gaps were simultaneously detected. Oscillation frequency as high as 0.873 THz was observed. An alternating-pump technique has been successfully demonstrated to enhance the signal-to-noise ratio by 10 dB. This technique can be used to improve the sensitivity for probing the surface modes in other multilayer thin-film structures.
Spatial hole burning and multimode generation threshold in quantum-dot lasers
Authors: Asryan L. V. , Suris R. A.
Published: July 2002

The multimode generation threshold in quantum-dot (QD) lasers is calculated as a function of the parameters of structure and temperature. Thermally excited escapes of carriers away from QDs are shown to control the multimode generation threshold. A decrease in the QD size dispersion is shown to increase considerably the relative multimode generation threshold. The maximum tolerable QD size dispersion and the minimum tolerable cavity length, at which the lasing is possible to attain, are shown to exist. Concurrent with the decrease of threshold current, the reduction of multimode generation threshold is shown to occur with decrease of temperature.
Electron paramagnetic resonance of a cation antisite defect in ZnGeP2
Authors: Setzler S. D. , Giles N. C. , Halliburton L. E. , Schunemann P. G. , Pollak T. M.
Published: July 2002

Electron paramagnetic resonance (EPR) has been used to characterize a new donor center in ZnGeP2 crystals. An equally spaced triplet of lines with a 1:2:1 intensity ratio is observed at g=2.0026 when a crystal is exposed to 633 nm light (below band gap) while at temperatures below 50 K. These EPR lines are approximately 70 G wide and they exhibit only a slight angular dependence (their separation changes from 189 to 179 G as the magnetic field is rotated from the c axis to the a axis). The proposed model for the new donor is a germanium ion on a zinc site. This antisite center is paramagnetic (S=1/2) in the singly ionized state (i.e., GeZn+) and the triplet nature of its EPR spectrum is attributed to nearly equal strong hyperfine interactions with two of the nearest-neighbor phosphorus ions.
Field emission from tetrahedral amorphous carbon as a function of surface treatment and substrate material
Authors: Hart A. , Satyanarayana B. S. , Milne W. I. , Robertson J.
Published: July 2002

To understand the mechanism of electron field emission from diamond-like carbon, tetrahedral amorphous carbon (ta-C) films were subjected to Ar, H2, and O2 plasma treatments to change their surface condition and were deposited on substrates of different work function. The threshold fields and current densities for undoped ta-C are found to be significantly improved by the plasma treatments, largely due to an increase in emission site density, while little dependence was found on work function of substrate. This suggests that the main barrier to emission from ta-C is at the front surface.
Large room-temperature intergrain magnetoresistance in double perovskite SrFe1−x(Mo or Re)xO3
Authors: Kim T. H. , Uehara M. , Cheong S-W. , Lee S.
Published: July 2002

We report significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo or Re)xO3 at room temperature. Systematics of the temperature dependence of IMR indicates that the observed large room-temperature IMR in SrFe1/2Mo1/2O3 originates from the ferrimagnetic nature of insulating grain boundaries as well as the half-metallicity of this perovskite. Our results indicate that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.
Influence of curvature on impurity gettering by nanocavities in Si
Authors: Schiettekatte François , Wintgens Carl , Roorda Sjoerd
Published: July 2002

Competition for Au gettering in Si between two cavity layers of different diameter (34 and 12 nm) is examined. Au is initially contained in the large cavity layer made by He implantation. Transport of Au towards the second, small diameter cavity layer is measured by ion scattering. The true surface in both layers is determined by electron microscopy. Small cavities are found to be four times more efficient gettering sites than large cavities for the same amount of internal surface. This difference is explained by a simple model based on curvature thermodynamics, faceting, and surface reconstruction.
Controlled fabrication of metallic electrodes with atomic separation
Authors: Morpurgo A. F. , Marcus C. M. , Robinson D. B.
Published: July 2002

We report a technique for fabricating metallic electrodes on insulating substrates with separations on the 1 nm scale. The fabrication technique, which combines lithographic and electrochemical methods, provides atomic resolution without requiring sophisticated instrumentation. The process is simple, controllable, reversible, and robust, allowing rapid fabrication of electrode pairs with high yield. We expect the method to prove useful in interfacing molecular-scale structures to macroscopic probes and electronic devices.
Fermi-level-dependent defect formation in Cu-chalcopyrite semiconductors
Authors: Klein A. , Jaegermann W.
Published: July 2002

Valence-band photoelectron spectroscopy of CuInSe2, CuInS2, and CuGaSe2 surfaces and interfaces give evidence for the formation of Cu vacancies when the Fermi level moves upwards in the band gap due to contact formation. The effect might be a key issue in understanding basic properties of solar cell devices based on these materials.
Red, green, and blue laser light from a single Nd:YAl3(BO3)4 crystal based on laser oscillation at 1.3 μm
Authors: Jaque D. , Capmany J. , Garcı́a Solé J.
Published: July 2002

Laser radiation corresponding to the three fundamental colors (red, green, and blue) has been generated from the same Nd:YAl3(BO3)4 crystal, operating on a fundamental laser wavelength of 1338 nm in an end-pumping configuration. Continuous wave generation was obtained by pumping with a tunable Ti: sapphire laser. Red (669 nm) has been obtained by self-frequency doubling of the fundamental laser line. Green (505 nm) and blue (481 nm) have been obtained by self-sum-frequency mixing of the fundamental laser radiation at 1338 nm and the pump radiation (807 nm for green and 755 nm for blue).
Optical band gap dependence on composition and thickness of InxGa1−xN (0<x<0.25) grown on GaN
Authors: Parker C. A. , Roberts J. C. , Bedair S. M. , Reed M. J. , Liu S. X. , El-Masry N. A. , Robins L. H.
Published: July 2002

Band gap measurements have been carried out in strained and relaxed InxGa1−xN epilayers with x<0.25. Values of x were determined from x-ray diffraction of relaxed films. The lowest energy absorption threshold, measured by transmittance, was found to occur at the same energy as the peak of the photoluminescence spectrum. Bowing parameters for both strained and relaxed films were determined to be 3.42 and 4.11 eV, respectively. The dependence of the band gap shift, ΔEg, on strain is presented.
Ordering of island-like FePt crystallites with orientations
Authors: Bian Bo , Sato Kazuhisa , Hirotsu Yoshihiko , Makino Akihiro
Published: July 2002

Thin films of oriented island-like crystallites of L10 FePt separated by amorphous (a-)Al2O3 were fabricated. The process took advantage of the overgrowth of α-Fe on Pt “seed” particles epitaxially grown on (100) NaCl and MgO substrates and the ordering reaction between Fe and Pt upon annealing at temperatures higher than 500 °C. The coercivities of the annealed (600 °C for 6 h) a-Al2O3/Fe(1 nm)/Pt (1.5 nm) films on both the NaCl (100) and MgO (100) substrates are higher than 3.3 kOe. Transmission electron microscopy observation showed that the film consisted of oriented L10 FePt island-like crystals with an average size of 12 nm and a separation of 4 nm. It was found that any one of the three 〈100〉 axes of the fcc Pt parent particles could act as the tetragonal c axis of the L10 FePt superstructure and all three-variant ordered crystalline domains of the tetragonal L10 phase could exist in the small 10-nm-size FePt crystallites. The high magnetic coercivity of the film, with well-separated FePt particles, can be attributed to the ordering (L10) as well as the oriented nature of the FePt crystallites.
Highly nonlinear photoluminescence threshold in porous silicon
Authors: Nayfeh M. , Akcakir O. , Therrien J. , Yamani Z. , Barry N. , Yu W. , Gratton E.
Published: July 2002

Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ∼106 W/cm2 (8×1010 W/cm2 peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si–Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption.
High-speed tapping mode imaging with active Q control for atomic force microscopy
Authors: Sulchek T. , Hsieh R. , Adams J. D. , Yaralioglu G. G. , Minne S. C. , Quate C. F. , Cleveland J. P. , Atalar A. , Adderton D. M.
Published: July 2002

The speed of tapping mode imaging with the atomic force microscope (AFM) has been increased by over an order of magnitude. The enhanced operation is achieved by (1) increasing the instrument’s mechanical bandwidth and (2) actively controlling the cantilever’s dynamics. The instrument’s mechanical bandwidth is increased by an order of magnitude by replacing the piezotube z-axis actuator with an integrated zinc oxide (ZnO) piezoelectric cantilever. The cantilever’s dynamics are optimized for high-speed operation by actively damping the quality factor (Q) of the cantilever. Active damping allows the amplitude of the oscillating cantilever to respond to topography changes more quickly. With these two advancements, 80μm×80 μm high-speed tapping mode images have been obtained with a scan frequency of 15 Hz. This corresponds to a tip velocity of 2.4 mm/s.
p-type doping with N and Li acceptors of ZnS grown by metalorganic vapor phase epitaxy
Authors: Svob L. , Thiandoume C. , Lusson A. , Bouanani M. , Marfaing Y. , Gorochov O.
Published: July 2002

ZnS layers were grown by metalorganic vapor phase epitaxy on GaAs substrates using diethylzinc, ditertiarybutyl sulphide, and triallylamine as organometallic sources. After postgrowth rapid thermal annealing, the ZnS layers showed p-type conductivity with hole concentrations up to 1018 cm−3. Photoluminescence measurements gave additional indications of the presence of electrically active nitrogen acceptors. In separate experiments, lithium was diffused from a LiH solid source into ZnS layers grown without the nitrogen precursor. High-conductivity p-type material was directly obtained with no need of thermal anneal.
Metal-to-insulator transition in sputter deposited 3Ni/Al thin films
Authors: Ng H. P. , Ngan A. H. W.
Published: July 2002

Thin films with a 3Ni/1Al atomic ratio were synthesized using low-power dc planar magnetron sputtering from a nickel–aluminum alloy target. Chemical analysis revealed that a significant amount of oxygen was incorporated into the Ni–Al thin films which were prepared under a typical vacuum of ∼1×10−5 mbar. These thin films were found to exhibit a prominent temperature dependence of electrical resistance, the magnitude of which diminishes upon increasing the film thickness or the in situ deposition temperature. Cross-sectional transmission electron microscopy revealed a nanocrystalline structure of the films which changes as a function of deposition temperature. Electron diffraction indicates the existence of a single-phase face-centered cubic structure in the nanocrystallites, yet with an enormous expansion of the crystal lattice for the low temperature deposited films when compared with the intermetallic Ni3Al lattice. On raising the deposition temperature or increasing the film thickness, however, the lattice constant gradually declines toward the lattice constant of bulk Ni3Al. An attempt is made to correlate the lattice structures of the crystallites and the electrical properties of the films with the potential influence of the dissolved oxygen in the Ni–Al lattice.
Incubation: Subthreshold ablation of poly-(methyl methacrylate) and the nature of the decomposition pathways
Authors: Blanchet Graciela B. , Cotts Patricia , Fincher Curtis R.
Published: July 2002

A study of the decomposition of poly-(methyl methacrylate) (PMMA) by pulsed UV laser ablation in the incubation regime is presented here. We propose that incubation proceeds via the photoinduced formation of defects centers; while ablation following incubation, at subthreshold fluences, is a thermally driven phenomena. The light scattering data, in the incubation regime, is consistent with molecular weight reduction via backbone cleavage. The defect centers, C=C at chain ends following backbone cleavage, increase the UV absorption coefficient thereby lowering the ablation threshold. Similarities between the mass spectra of ablative PMMA following incubation and pyrolysis suggest that ablation following incubation is thermally driven. The implications of these results in an extensive and contradictory body of available data are discussed.
Fourier transform infrared study of the pressure and laser induced polymerization of solid acetylene
Authors: Ceppatelli Matteo , Santoro Mario , Bini Roberto , Schettino Vincenzo
Published: July 2002

The polymerization of solid acetylene under pressure has been studied by Fourier transform infrared (FTIR) spectroscopy. Controlled laser irradiation cycles and the employment of infrared sensors to measure the sample pressure, allowed to separate the photochemical and the pressure effect on the injection and on the evolution of the reaction. The careful assignment of all the spectral features and analysis of their relative intensities and frequencies gave evidence to the specific effect of pressure and laser irradiation on the reaction products. Pressure induces an ordered growth of trans-polyenic species, while irradiation produces the opening of the double bonds and a consequent branching of the chains. Constant pressure measurements allowed to obtain precise information on the kinetics of the reaction. A monodimensional growth geometry, involving the molecules on the bc plane, agrees with the parameters extracted by the kinetic curves. Comparison between experiments at different temperatures suggests an activation of the reaction essentially due to the translational lattice modes.
Crossed beam reaction of cyano radicals with hydrocarbon molecules. III. Chemical dynamics of vinylcyanide (C2H3CN;X 1A′) formation from reaction of CN(X 2Σ+) with ethylene, C2H4(X 1Ag)
Authors: Balucani N. , Asvany O. , Chang A. H. H. , Lin S. H. , Lee Y. T. , Kaiser R. I. , Osamura Y.
Published: July 2002

The neutral–neutral reaction of the cyano radical, CN(X 2Σ+), with ethylene, C2H4(X 1Ag), has been performed in a crossed molecular beams setup at two collision energies of 15.3 and 21.0 kJ mol−1 to investigate the chemical reaction dynamics to form vinylcyanide, C2H3CN(X 1A′) under single collision conditions. Time-of-flight spectra and the laboratory angular distributions of the C3H3N products have been recorded at mass-to-charge ratios 53−50. Forward-convolution fitting of the data combined with ab initio calculations show that the reaction has no entrance barrier, is indirect (complex forming reaction dynamics), and initiated by addition of CN(X 2Σ+) to the π electron density of the olefin to give a long-lived CH2CH2CN intermediate. This collision complex fragments through a tight exit transition state located 16 kJ mol−1 above the products via H atom elimination to vinylcyanide. In a second microchannel, CH2CH2CN undergoes a 1,2 H shift to form a CH3CHCN intermediate prior to a H atom emission via a loose exit transition state located only 3 kJ mol−1 above the separated products. The experimentally observed mild “sideways scattering” at lower collision energy verifies the electronic structure calculations depicting a hydrogen atom loss in both exit transition states almost parallel to the total angular momentum vector J and nearly perpendicular to the C2H3CN molecular plane. Since the reaction has no entrance barrier, is exothermic, and all the involved transition states are located well below the energy of the separated reactants, the assignment of the vinylcyanide reaction product soundly implies that the title reaction can form vinylcyanide, C2H3CN, as observed in the atmosphere of Saturn’s moon Titan and toward dark, molecular clouds holding temperatures as low as 10 K. In strong agreement with our theoretical calculations, the formation of the C2H3NC isomer was not observed.
Very narrow spectral filters with multilayered grating-waveguide structures
Authors: Levy-Yurista Guy , Friesem Asher A.
Published: July 2002

Promising configurations for grating-waveguide structures are presented. In these structures, the grating layer, which is normally adjacent to the waveguide layer, is displaced by means of an intermediate layer, resulting in significant reduction of losses and weaker coupling. This leads to very narrow spectral bandwidths and high contrast ratios. Experimental results reveal that the spectral bandwidths can be as low as 0.1 nm with contrast ratios greater than 1000, suggesting that these grating-waveguide structures could be useful for optical communication networks.
Observations of the formation of streamwise vortices by rotation of arch vortices
Authors: Heist D. K. , Hanratty T. J. , Na Y.
Published: July 2002

Vortices, oriented in the streamwise direction, play an important role in generating Reynolds shear stresses (and turbulence) for flow over a smooth wall. Many of these have been observed to originate from tiny streamwise vortices, located in the immediate vicinity of the wall. In this paper we identify a different process that forms about 30 percent of the streamwise vortices at locations, away from the wall, in the middle of the viscous wall layer. This is accomplished by examining the changes, with time, of the turbulent field obtained from a direct numerical simulation of turbulent flow in a channel. Streamwise vortices create a shear layer by pumping low momentum fluid from the wall. One or more small spanwise vortices are formed at the top of this layer. These grow in size and rotate in the direction of flow. Previous investigators have suggested that spanwise vortices could have a direct role in the formation of streamwise vortices. This paper describes, in detail, a process by which this is accomplished. Of particular interest is the need to recognize that the shear layer is asymmetric since it is formed by a single streamwise vortex rather than a pair of counter-rotating vortices.
Hydrogen adsorption and phase transitions in fullerite
Authors: Ye Y. , Ahn C. C. , Fultz B. , Vajo J. J. , Zinck J. J.
Published: July 2002

Hydrogen desorption and adsorption properties of the fullerene materials C60, C70, and fullerite (a mixture of C60 and C70) were measured volumetrically using a Sievert’s apparatus. Over several cycles of isotherm measurements at 77 K, the hydrogen storage capacities of one of the fullerite samples increased from an initial value of 0.4 wt % for the first cycle to a capacity of 4.4 wt % for the fourth cycle. Correspondingly, the surface area of this sample increased from 0.9 to 11 m2/g, and there were changes in its x-ray powder diffraction pattern. In comparison, two other fullerite samples, prepared by a different procedure showed no such behavior. Pure C60 and pure C70 were also cycled and exhibited small and constant capacities of 0.7 and 0.33 wt %, respectively, as a function of number of cycles. The enhanced storage capacity of fullerite material is tentatively attributed to the presence of C60 oxide.
Preparation of crack-free antiferroelectric PbZrO3 thin films by a two-step annealing process
Authors: Kong L. B. , Ma J.
Published: July 2002

Crack-free antiferroelectric PbZrO3(PZ) thin films were prepared by a two-step annealing technique via a modified sol-gel process. Although x-ray diffraction results showed that single phase of perovskite PbZrO3 was obtained in the thin films annealed at 550, 700, and 550 °C/700 °C, double P-E hysteresis loop indicating phase transformation from antiferroelectric to ferroelectric phase was only observed for the two-step annealed PZ thin film at room temperature, with a forward switching field (EAFE-FE) of 151 kV/cm, a backward switching field (EFE-AFE) of about 77 kV/cm, and saturated polarization of 54 μC/cm2. The dielectric constant and dielectric loss of the PZ film annealed at 700 °C are 260 and 0.04 at 1 kHz, respectively. The prevention of the PZ film from being cracking by the two-step annealing procedure is believed to be a result of the more stable property of the film caused by the 550 °C pre-annealing.
Effective-mass enhancement and nonparabolicity in thin GaAs quantum wells
Authors: Städele M. , Hess K.
Published: July 2002

We study size quantization effects on the conduction band dispersion in GaAs/AlAs quantum wells using a semiempirical tight-binding method. For GaAs well thicknesses between 3 and 11 nm, we find a significant increase of the conduction band mass of up to 50% compared with bulk GaAs. Concomitantly, the confinement reduces the highest achievable group velocities for electrons in the Γ conduction valley of the well by up to 30%. We discuss some of the consequences for quantum-well-based devices.
Structural and electrical properties of In2O3–SeO2 mixed oxide thin films for gas sensing applications
Authors: Manno D. , Micocci G. , Serra A. , Di Giulio M. , Tepore A.
Published: July 2002

In 2 O 3 – SeO 2 mixed oxide thin films have been obtained by thermal evaporation of a stoichiometric In–Se starting charge and subsequent thermal annealing in an oxygen flow. High-resolution transmission electron microscopy, small area electron diffraction, and digital image processing have been employed in order to investigate the structure and the morphology of the films obtained. Hall effect and resistivity measurements in a vacuum and in a controlled atmosphere have been performed to obtain information about physical properties of these films. The experimental data show evidence that the electrical conductivity of these films undergoes a remarkable variation due to exposure to small concentrations of NO in dry synthetic air or argon. To interpret the behavior of such films, an adsorption kinetic model has been developed and the conductivity variation as a function of gas concentrations, time, and temperature has been derived. The fitting of the theoretical and experimental behavior allows us to determine the adsorption kinetic parameters.
Explosion implosion duality and the laboratory simulation of astrophysical systems
Authors: O’C. Drury L. , Mendonça J. T.
Published: July 2002

The Euler equations of ideal gas dynamics possess a remarkable nonlinear involutional symmetry which allows one to factor out an arbitrary uniform expansion or contraction of the system. The nature of this symmetry (called by cosmologists the transformation to supercomoving variables) is discussed and its origin clarified. It is pointed out that this symmetry allows one to map an explosion problem to a dual implosion problem and vice versa. The application to laboratory simulations of supernova remnants is considered; in principle this duality allows the complete three-dimensional evolution of highly structured explosion ejecta to be modeled using a static target in an implosion facility.
An experimental research on the flow field of water entry by pressure measurements
Authors: Shi Hong-Hui , Kume Makoto
Published: July 2002

The underwater acoustic field in water entry of a 352 m/s blunt body has been investigated by pressure measurements. The experimental results show that from the initial stage to the later stage of water entry, there are: (1) a high peak pressure caused by the compressible behavior in high speed solid/liquid impact; (2) a negative pressure region caused by the expansion waves which are generated when the initial primary shock wave detaches from the solid body and the high pressure region is released; and (3) a significant motion of the fluid particles driven by the shock wave. It is found that at the entry velocity of this experiment, the shock wave behaves like an acoustic wave in water. The wave has a stronger energy distribution on the forward direction and a weaker energy distribution on the lateral direction.
High temperature growth of ZnS films on bare Si and transformation of ZnS to ZnO by thermal oxidation
Authors: Yoo Y.-Z. , Osaka Y. , Fukumura T. , Jin Zhengwu , Kawasaki M. , Koinuma H. , Chikyow T. , Ahmet P. , Setoguchi A. , Chichibu S. F.
Published: July 2002

ZnS films were grown on Si (100) at high temperatures by pulsed laser deposition using a KrF excimer laser. The growth temperature was varied from 200 to 700 °C and all films were found to have a specific preferential orientation. With increasing Ts, growth rate decreased but the quality of the film improved. The highest quality ZnS film was obtained at 700 °C. The presence of ZnS+ ions among the ablation products of a ZnS target was verified by laser desorption time of flight mass spectroscopy measurements. ZnO was formed by thermal oxidation of ZnS and the films showed strong near band-edge emission at 3.26 eV.