bluuebunny/crossref_metadata_2025 huggingface dataset:
Integrated mass spectrometer–microreactor system for the in situ investigation of oxide semiconductor sensors
Authors: Bene R. , Hárs G. , Perczel I. V. , Réti F.
Published: July 2002

This work describes the construction of an experimental setup — a mass spectrometer coupled microreactor — which makes possible the simultaneous monitoring of the electric resistance of a solid sensor sample and the composition of the near surface gas atmosphere. The information obtained from the constant voltage resistance measurement is complemented by the information about of the near-sensor gas composition. The evaluation of both sets of results offers insight into the mechanism of the sensing process. As an example, the results obtained on an undoped, oven-heated CeO2 sensor sample is presented, in a CO and oxygen containing atmosphere between 25 and 900 °C. It was found that between 200 and 300 °C the decrease in the sensor resistance is mainly due to the CO chemisorption. In this temperature range the oxidation of CO is negligible. The CO sensitivity decreases with the temperature in the whole interval examined. However, its value is the highest in this temperature range due to the relatively high surface coverage. In the 300–400 °C range the most important process is the gas phase oxidation of the CO; about 85% of it is transformed. The transformation of CO leads to a smaller surface coverage, and so to a decreasing — but still high enough — sensitivity. A further temperature rise hinders the CO chemisorption and the lack of chemisorption results in no CO sensitivity. It is noted that even the catalytic effect of the CeO2 sample is important in the studied system; its effect on the near-sensor gas composition is easily detectable. The maximum catalytic activity of CeO2 in the CO oxidation was found to be around 400 °C.
Photoacoustic measurements for catalytic effects of perovskite electrodes in a sealed-off CO2 discharge tube
Authors: Kim Sung-Ho , Choi Joong-Gill , Cho Ung-In
Published: July 2002

To investigate catalytic activities of perovskite-type electrodes in a sealed-off CO2 laser cavity, we studied the dissociation and reformation reactions of CO2 during electric discharge by monitoring the concentration change of CO2 as a function of time. A photoacoustic spectroscopy detection was introduced to measure the amount of CO2 in a closed discharge system. The catalytic activities in the dissociation of CO2 were demonstrated and compared for both Cu and La0.7Sr0.3CoO3 cathodes. The discharge with the gas mixture of CO2:N2:He=1:1:8 was performed under nine combinations of different conditions at pressures of 10, 20, and 30 Torr and discharge currents of 3, 4, and 5 mA. Within 1 min after the discharge, the concentration of CO2 reduced drastically by as much as 60% for both cathodes. In the case of the Cu cathode, the amounts of CO2 in the tube had not recovered even after 30 min, while with the perovskite cathode the CO2 contents recovered slowly up to 90%–95% within 5 min and then remained almost unchanged. The dissociation of CO2 with the Cu cathode increased as the total pressure decreased and the discharge current increased. The La0.7Sr0.3CoO3 cathode, however, demonstrated strong suppression in the dissociation of CO2 according to the reformation reaction of CO+(1/2)O2→CO2 by its catalytic activity.
Neutron transport calculations in support of neutron diagnostics at JET
Authors: Loughlin M. J. , Watkins N. , Bertalot L. , Esposito B. , Roquemore A. L.
Published: July 2002

At JET the primary diagnostic for monitoring the neutron emission is a set of fission chambers. These are calibrated using the neutron activation technique. A second independent measurement of the neutron yield can be obtained from the neutron profile monitor. Both measurements depend on neutron transport calculations and at JET this has been done using the neutron transport code MCNP. The introduction of tritium into JET plasmas provided an opportunity to check the modeling by comparing the activation of different materials. The calculation of the neutron spectrum at a re-entrant irradiation position was found to be consistent with measurements and an accuracy of <7% in the neutron yield measurement was obtained. Neutron transport calculations of the effective collimator correction and attenuation for the profile monitor are also described.
The fusion diagnostic gamma experiment: A high-bandwidth fusion diagnostic of the National Ignition Facility
Authors: Moran Michael J.
Published: July 2002

Diagnostics for the National Ignition Facility/inertial confinement fusion program must include good characterization of the fusion source. Ideally, diagnostics would measure the spatially resolved history of the fusion reaction rate and temperature. Existing diagnostics can satisfy this goal only partially. One class of new techniques that could play a major role in high-yield diagnostics is measurements based on fusion γ rays. The fusion diagnostic gamma experiment can perform energy-resolved measurements of (D,T) fusion reaction rates. This diagnostic is based on the 16.7 MeV γ rays that are produced by (D,T) fusion. The γ rays are free of spectral dispersion and can be detected (via Compton recoil electrons) with a high bandwidth Cherenkov detector. A simple magnetic monochromator selects signals from the 16.7 MeV γ rays and reduces background signals from nonfusion γ rays.
Electron cyclotron emission imaging diagnostic for the helically symmetric experiment stellarator (abstract)
Authors: Liang C. , Domier C. W. , Luhmann N. C.
Published: July 2002

An eight channel electron cyclotron emission imaging (ECEI) diagnostic system is being developed for use on the helically symmetric experiment (HSX) stellarator. The system utilizes a linear Schottky diode mixer/receiver array, coupled with low cost conventional ECE radiometer IF boards under development at U.C. Davis, to measure the second harmonic x-mode radiation from the plasma. The array is fed with a fixed local oscillator, with single sideband operation enabled via a dichroic plate high pass filter whose cutoff frequency equals that of the local oscillator. A multithrow, high speed (<100 ns switching times) microwave switch is utilized to sequentially connect the output from each mixer channel to the broadband IF receiver system, thus making the array capable of generating a high resolution two-dimensional image of the HSX electron temperature profile at rates exceeding 50 kHz. Instrument details of the system will be described.
High speed switching arrays for moderate pulse reflectometry applications (abstract)
Authors: Jiang F. , Domier C. W. , Luhmann N. C.
Published: July 2002

Moderate pulse reflectometry requires the efficient (low loss) gating/switching (∼100–300 ps rise times) of millimeter wave pulses with a high on/off contrast ratio. To satisfy this need, high speed switches utilizing beam control array technology are under development at U.C. Davis. A quasioptical planar grid switch array comprised of hundreds of Schottky varactor diodes embedded within an overmoded waveguide, is capable of generating the requisite 100–300 ps switching times with Watt-level power handling capabilities. A proof-of-principle, high speed V-band switch array has been constructed with excellent agreement between theory and experiment. Less than 1.6 dB insertion loss and >16 dB on/off contrast ratio are measured at 61.5 GHz, with rise and fall times of <127 ps. An instantaneous bandwidth >7 GHz (contrast ratio >15 dB) has also been measured. A discussion will also be presented on designs at frequencies up to 170 GHz.
The effects of density fluctuation on density profile measurements using a broadband frequency modulated reflectometer
Authors: Tokuzawa T. , Mase A. , Oyama N. , Kogi Y. , Goto N. , Bruskin L. G. , Kubota S. , Itakura A. , Yatsu K.
Published: July 2002

Numerical simulations of frequency modulation (FM) reflectometry are carried out for the effects of density fluctuations on phase measurements. It is found that the fluctuation amplitude, the fluctuation frequency, the sweep time of a source, and the beat frequency of a mixer output play the important roles. The reliability of density profile measurement is investigated for various sweep times and local path lengths. It is demonstrated that the reconstructed density profiles seem to be improved when the sweep time is faster than 100 μs and the local path length is shorter than 1 m. The numerical simulations are compared with the experimental results obtained from a broadband FM reflectometer applied to the GAMMA 10 tandem mirror.
Phase grating modulator
Authors: Hasan Mohammed S. , Feldman Martin
Published: July 2002

A new type of optical modulator is reported in which a phase grating is used to diffract a beam of light. Piezoelectric elements modulate the phase shift between adjacent portions of the grating, and hence control the diffraction efficiency. Efficiencies of up to about 17% have been observed, compared to the theoretical maximum of about 40% for diffraction into a single order by a pure square wave phase grating.
X-ray microdiffraction studies to measure strain fields in a metal matrix composite
Authors: Lee H.-R. , Kupperman D. , Yun W. , Cai Z. , Rodrigues W.
Published: July 2002

An x-ray diffraction microscope (XDM) has been used to map the strain field in a fiber-reinforced composite material. The monochromatic x-ray (11 keV) beam was focused by a phase zone plate to produce a focal spot of 1×4 μm2 on the specimen. The microfocusing method using a zone plate enables the XDM to produce a high spatial resolution of ∼1 μm. The change in the peak position of diffraction patterns due to interatomic spacing change, caused by stress in the sample, was measured by using a two-dimensional charge coupled device detector. The radial residual strain field in the fiber-reinforced composite (SCS–6/Ti–14Al–21Nb) was measured from diffraction patterns with a sensitivity of ∼10−4 and an average standard deviation of 9.4×10−5.
High power 170 GHz gyrotron with synthetic diamond window
Authors: Sakamoto Keishi , Kasugai Atsushi , Tsuneoka Masaki , Takahashi Koji , Imai Tsuyoshi , Kariya Tsuyoshi , Mitsunaka Yoshika
Published: July 2002

A large sized synthetic diamond window assembly was installed on a 170 GHz gyrotron. An aperture and a thickness of the window are 83 and 2.23 mm, respectively, whose edge was directly cooled by water. Gyrotron performances of 520 kW at 6.2 s and 450 kW at 8 s were attained. A temperature increase of the window stabilized after ∼5 s, whereas the power had been limited to below 170 kW with conventional windows. This drastic improvement of a deliverable power was obtained from the outstanding properties of the diamond, that is, an extremely high thermal conductivity (∼1800 W/mK) and a low value of loss tangent (tan δ<1.3×10−4). The gyrotron was inspected after the ∼1300 times operation at 520 kW (>1 s), but no damage nor trouble was found both on the gyrotron and the diamond window. These results give a prospect for a multimegawatt power output from the gyrotron, i.e., the diamond window gives a solution for the window problem which has been regarded as the most serious issue on the development of high power long pulse gyrotron.
Influence of oxygen on the formation of epitaxial erbium silicide film on 〈111〉Si
Authors: Grimaldi M. G. , Xinshui Yan , Scerra G. , Ravesi S. , Spinella C.
Published: July 2002

Epitaxial ErSi2−x thin films have been grown by the reactive deposition epitaxy technique on 〈111〉Si substrate at a pressure of ∼10−8 Torr in a controlled atmosphere. A close relation between the O partial pressure during deposition and the crystalline quality of the silicide film has been observed and good quality epitaxial ErSi2−x layers have been obtained if O is incorporated in the silicide layer at a concentration of ∼7 at. %.
Construction and performance of a figure-8 undulator
Authors: Tanaka Takashi , Hara Toru , Oura Masaki , Ohashi Haruhiko , Kimura Hiroaki , Goto Shunji , Suzuki Yoshio , Kitamura Hideo
Published: July 2002

The figure-8 undulator is a novel insertion device adopted as a light source at the soft x-ray photochemistry beamline of the Super Photon ring-8GeV. It was designed to produce linearly polarized soft x rays between 100 and 5000 eV with low on-axis power density. Magnetic fields of the constructed device were measured with compensation for the planar Hall effect using two different methods. After magnetic-field corrections, it was found that the first-harmonic intensity and the on-axis power density calculated using the measured fields were almost the same as ideal ones. After installation in the storage ring, a power-distribution pattern was observed by a fluorescent screen and it was found that the shape corresponded to expectation. The quantity of fluctuation of the photon-beam axis was estimated by measuring spectra at various observation angles and it was found to be small compared to the photon-beam size. In addition, it was also found that the measured spectrum at the gap of 77 mm agreed well with the calculated one.
Photocathode source for studying two-dimensional fluid phenomena with magnetized electron columns
Authors: Durkin D. , Fajans J.
Published: July 2002

Magnetized electron columns are a valuable experimental tool used to study two-dimensional (2D) fluid phenomena. Traditionally, the electrons have been generated with thermionic sources, typically limiting the initial electron distribution to one filled circle and thereby restricting the range of accessible fluid phenomena. Here, we describe a new electron source based on a cesium antimonide photocathode that can generate more complicated initial electron distributions. Experiments so far have focused on the stability of 2D vortex patterns.
Improved laser-heated pedestal growth system for crystal growth in medium and high isostatic pressure environment
Authors: Reyes Ardila D. , Andreeta J. P. , Ribeiro C. T. M. , Li M. Siu
Published: July 2002

A modified apparatus for single-crystal-fiber growth in an isostatic gaseous environment and for a large range of pressure (100 mbar up to 100 bar) using a floating-zone-type method known as laser-heated pedestal growth is described. The difficulties and advantages of its design are discussed through examples of crystal growth of some oxide materials.
Thermal diffusivity measurement of silicon samples by a combined piezoelectric and pyroelectric method
Authors: Aravind M. , Fung P. C. W.
Published: July 2002

In the past, when thermal diffusivity measurement of materials were carried out by photoacoustic signal detection using transducers, only the piezoelectric or the pyroelectric property of the transducers was considered. In case the transducer exhibits both piezoelectric and pyroelectric properties, one of these properties had been suppressed during the experimentation, obviously more errors are introduced this way. We use polyvinylidene difluoride (PVDF) as the detector for thermal waves. Since PVDF has both piezoelectric and pyroelectric properties, we show in this article that the signal detected by the transducer is a sum of both the piezoelectric and pyroelectric effects. Silicon semiconductor samples are considered in this article to compare the theory with experimental results. Although both the piezoelectric and pyroelectric properties are found in the resultant signal at all the frequency ranges considered, we find that when the samples are thermally thick, the piezoelectric contribution to the detected signal is slightly more than the pyroelectric contribution and vice versa when the sample is thermally thin. This behavior of the combined signal can be explained by the fact that in an optically opaque solid heat is generated very close to the surface, following absorption. This heat is communicated to the PVDF as long as the thermal diffusion length is larger than the thickness (i.e., the sample is thermally thin). At high frequencies the solid becomes thermally thick and the pyroelectric nature decreases as both the optical and thermal contact of the sample with the detector diminishes. Since both the properties are considered in our theory, we can measure the thermal diffusivity of a general sample without “artificial suppression.” Moreover, from our analysis we can deduce the physical thickness of the sample from the critical frequency, which is the frequency at which the sample changes from thermally thin to thermally thick. This transition is clearly evident in the amplitude curve as a change in slope is detected at the critical frequency.
Spatio-temporal measurements of Trichel corona discharge using capacitive probe diagnostic
Authors: Gupta Deepak K. , Ramachandran H. , John P. I.
Published: July 2002

A nonintrusive capacitive probe diagnostic is developed to estimate the spatio-temporal charge density variation of corona discharge. Tikhonov regularization is used to calculate the charge density from measured potential. A good time resolution and restricted space resolution in charge density is achieved. The axial electric field due to space charge is also estimated by considering the discharge to be of finite radius and with uniformly distributed charge density along the radial direction. Space charge wave front movement, as predicted by existing theories, is noticed. Constraints of present technique and scope for further improvements are discussed.
Status report on electron cyclotron resonance ion sources at the Heavy Ion Medical Accelerator in Chiba
Authors: Kitagawa A. , Muramatsu M. , Sekiguchi M. , Yamada S. , Jincho K. , Okada T. , Yamamoto M. , Hattori T. , Biri S. , Baskaran R. , Sakata T. , Sawada K. , Uno K.
Published: July 2002

The Heavy Ion Medical Accelerator in Chiba at the National Institute of Radiological Sciences (NIRS) is not only dedicated to cancer therapy, it is also utilized with various ion species for basic experiments of biomedical science, physics, chemistry, etc. Two electron cyclotron resonance (ECR) ion sources are installed for production of gaseous ions. One of them, the NIRS-ECR, is a 10 GHz ECR ion source, and is mainly operated to produce C4+ ions for daily clinical treatment. This source realizes good reproducibility and reliability and it is easily operated. The other source, the NIRS-HEC, is an 18 GHz ECR ion source that is expected to produce heavier ion species. The output ion currents of the NIRS-ECR and the NIRS-HEC are 430e μA for C4+ and 1.1e mA for Ar8+, respectively.
Localization of emission through interpretation of observed Zeeman pattern
Authors: Weaver J. L. , Welch B. L. , Griem H. R. , Terry J. , Lipschultz B. , Pitcher C. S. , Wolfe S. , Pappas D. A. , Boswell C.
Published: July 2002

Observations of spectral line profiles commonly represent the integration of emission along the line of sight. Depending on the number of views and the symmetries involved, one can use techniques ranging from simple Abel inversion to complex tomographic reconstruction to find the spatial distribution emitters. In tokamak experiments, the spatial dependence of the magnetic field is typically available and can be used to gain important insights into the absence of other spatial information. The Zeeman patterns of spectral lines from neutral atoms and low-Z ions in tokamak plasmas can contain enough information to restrict the location of emission to well defined positions along a given line of sight. Simple modeling of observations with high spectral resolution from Alcator C-Mod plasmas demonstrates the application of this technique to the interpretation of experimental data. This localization of emission is not only of interest to spectroscopists and modelers of tokamak edge and divertor regions, it could be used to aid the operation of other visible-light diagnostics that seek spatial resolution.
Design and optimization of a corona discharge ionization source for ion mobility spectrometry
Authors: Tabrizchi M. , Khayamian T. , Taj N.
Published: July 2002

In this work the possibility of using corona discharge as an ionization source in ion mobility spectrometry has been investigated. The results for both positive and negative polarity in air are consistent with the Townsend formula which states that I/V is a linear function of V. The distribution profile of the corona ions has also been investigated and compared with that of the Ni63 ionization source. Generally, the total ion current obtained from the corona ionization source was greater than that of the Ni63 source by about an order of magnitude, which results in a better sensitivity and a higher signal-to-noise ratio. The positive and negative spectra of air were recorded and the positive spectra of acetone and dimethylmethyl-phosphonate were compared with that of Ni63. In the negative mode, a number of new peaks, apart from those of reactant ions, were observed which are mainly due to nitrogen oxides, formed by corona discharge. This problem was minimized by increasing the interdistance of the electrodes.
Characterization of a microfocused circularly polarized x-ray probe
Authors: Pollmann J. , Srajer G. , Maser J. , Lang J. C. , Nelson C. S. , Venkataraman C. T. , Isaacs E. D.
Published: July 2002

We report on the development of a circularly polarized x-ray microprobe in the intermediate energy range from 5 to 10 keV. In this experiment linearly polarized synchrotron radiation was circularly polarized by means of a Bragg-diffracting diamond phase retarder and subsequently focused down to a spot size of about 4×2 μm2 by a Fresnel zone plate. The properties of the microprobe were characterized, and the technique was applied to the two-dimensional mapping of magnetic domains in HoFe2.
Positive-intrinsic-negative diode-based duplexer for microcoil nuclear magnetic resonance
Authors: Seeber D. A. , Hoftiezer J. H. , Pennington C. H.
Published: July 2002

Microcoil nuclear magnetic resonance (NMR), using receiver coils of diameters of order 100 μm, is increasingly employed to observe very small (∼0.3 nl) samples with high sensitivity. However, many experimental aspects of microcoil NMR differ greatly from conventional NMR. In particular, the duplexer is a device used to switch between the transmit and receive phases of the experiment. The conventional duplexer is a passive device employing crossed diodes, that switch automatically to transmit mode when high rf power is present. In microcoil NMR, however, the transmitter power is necessarily quite low, with voltages that do not greatly exceed characteristic diode voltage drops. Here we present the complete design and construction methods for a duplexer well suited to the special demands of microcoil NMR.
Examination of Si(100) surfaces treated by ultrapure water with 5 ppb dissolved oxygen concentration
Authors: Kanaya H. , Usuda K. , Yamada K.
Published: July 2002

Si(100) surfaces treated by ultrapure water with 5 ppb dissolved oxygen concentration after dipping in HF solution were examined by attenuated total reflection (ATR)–Fourier-transform infrared (FT-IR) spectroscopy, transmittance FT-IR, and reflection high energy electron diffraction (RHEED). FT-IR spectra and RHEED patterns depended on the rinsing time in the ultrapure water. The (111) and (110) facets appeared after rinsing for a long period of time (20–45 h) in 5 ppb dissolved oxygen concentration ultrapure water. It was suggested that the surface morphology depended on not only pH value but also the amount of etching.
Thin layer thickness measurements based on the acousto-optic technique
Authors: Devolder S. , Wevers M. , De Meester P. , Leroy O.
Published: July 2002

The acousto-optic technique presented by Wevers, Devolder, Leroy, and De Meester [Appl. Phys. Lett. 66, 1466 (1995)] was based on the information concerning the phase shift between the incident and the reflected ultrasound at critical angles. Two laser beams were used, one being diffracted by the incident ultrasound, the other being diffracted by the reflected ultrasound. Information concerning the geometrical shape of the sample as well as information concerning the thickness of a thin layer could be obtained from measurements in the near-field of the light diffraction. In this letter it is shown that using only one laser beam, traveling through the intersection region of both ultrasounds, exact thickness information of the layer can be obtained. Theoretical calculations are derived and compared with practical measurements for a 3 μm copper layer on a stainless-steel plate.
An approach to electrical studies of single nanocrystals
Authors: Klein David L. , McEuen Paul L. , Katari Janet E. Bowen , Roth Richard , Alivisatos A. Paul
Published: July 2002

We present electrical measurements of single Au and CdSe nanocrystals. The devices are fabricated using a hybrid scheme which combines electron beam lithography and wet chemistry to bind nanocrystals in tunneling contact between two closely spaced metallic leads. The current–voltage characteristics of these devices exhibit a Coulomb staircase with a charging energy of ∼50 meV. This technique is readily adapted to the study of a host of nanocrystals made by solution chemistry.
Fluorinated amorphous carbon thin films grown by helicon plasma enhanced chemical vapor deposition for low dielectric constant interlayer dielectrics
Authors: Endo Kazuhiko , Tatsumi Toru
Published: July 2002

Fluorinated amorphous carbon thin films (a-C:F) for interlayer dielectrics are grown by helicon plasma enhanced chemical vapor deposition. The source gases are CH4, CF4, C2F 6 and their H2mixtures. a-C:F films can be fabricated without adding hydrogen using the helicon reactor, while in the previously reported parallel-plate reactor, no film grows unless a hydrogen source is added. The films grown in the helicon reactor have no hydrogen content. The growth rate of the films reaches 0.3 μm/min (C2F 6) and 0.15 μm/min (CF4). The thickness of the films deposited with C2F6 does not decrease on heating to 300 °C, while the films with CF4 shrink. The dielectric constants of the films deposited from C2F6 and CF4 are 2.4 and 2.3 respectively at 1 MHz. The dielectric loss tangent of these films is 0.01 at 1 MHz.
Multipurpose sensor tips for scanning near-field microscopy
Authors: Mihalcea C. , Scholz W. , Werner S. , Münster S. , Oesterschulze E. , Kassing R.
Published: July 2002

The reproducible micromachining of hollow metal tips on Si cantilevers and their applicability to scanning probe microscopy techniques are described. Provided with apertures below 130 nm and hollow pyramidal tips proved to be highly suited probes for scanning near-field optical microscopy (SNOM). First results of combined SFM/SNOM measurements together with scanning electron microscopy (SEM) photographs of the new sensors are presented. The SNOM images show a resolution of about 100 nm demonstrating the usefulness of these probes.
Key trade-offs for second harmonic generation in poled polymers
Authors: Otomo Akira , Jäger Matthias , Stegeman George I. , Flipse Marinus C. , Diemeer Mart
Published: July 2002

We discuss the critical trade-off between the spectral dispersion in the second order nonlinearity d(2) and the linear absorption coefficient for second harmonic generation (SHG) by the mixing of copropagating or counterpropagating fundamental beams in poled polymers. The 4-dimethylamino–4′-nitrostilbene side-chain polymer is used as a specific example. The predictive power of the classical two-level model for SHG is also assessed.
Fabrication of n-doped magnetic semiconductor heterostructures
Authors: Smorchkova I. , Samarth N.
Published: July 2002

We demonstrate the introduction of high carrier densities into two and three-dimensional magnetic semiconductor heterostructures that contain large local concentrations of magnetic moments. The use of (ZnSe)m−f(MnSe)f digital alloys readily allows the fabrication of epilayer samples of moderate Mn composition with carrier concentrations up to ∼1019 cm−3. Modulation doping of ZnSe/Zn1−x−yCdyMnxSe quantum wells enables the fabrication of a 2-D electron gas confined within the magnetic region with sheet concentrations up to ∼6×1011 cm−2.
Nondestructive measurements of stoichiometry in undoped semi-insulating gallium arsenide by x-ray bond method
Authors: Chen NuoFu , Wang Yutian , He Hongjia , Wang Zhanguo , Lin Lanying , Oda Osamu
Published: July 2002

The influences of microdefects and dislocations on the lattice parameters of undoped semi-insulating GaAs single crystals were analyzed, and a novel nondestructive method for measuring stoichiometry in undoped semi-insulating GaAs was established in this letter. The comparison of this method with coulometric titration indicates that the method of nondestructive measurements is indeed convenient and reliable.
Magnetic field dependence of exciton oscillator strength by measurements of magnetoexciton-polariton mode splitting in quantum wells with a microcavity
Authors: Tanaka Takuji , Zhang Zhenlong , Nishioka Masao , Arakawa Yasuhiko
Published: July 2002

Exciton-polariton mode splitting of quantum wells (QWs) in a microcavity is experimentally and theoretically investigated under a high magnetic field up to 14.5 T. The magnetic field dependence of the splitting interval has been measured for the precisely tuned photon and exciton mode. It is found that the splitting interval increases with an increase of the magnetic field, which shows enhancement of magnetoexciton-photon interaction. Theoretical analysis using a transfer matrix method and a variational method demonstrates that these properties are mainly caused by the increase of exciton oscillator strength due to the magnetic field. The effect of the number of QWs on the mode splitting is also discussed.
Thermal fixing of volume holograms in potassium niobate
Authors: Tong Xiaolin , Zhang Min , Yariv Amnon , Agranat Aharon
Published: July 2002

Volume holograms are thermally fixed in a pure and an iron doped potassium niobate crystal through the screening of a photorefractive space-charge field by a nonphotoactive species at elevated temperature (110 °C). The nonphotoactive species is identified as the hydrogen ion. Fixed holograms are reconstructed at different temperatures. A diffraction efficiency of 16% in a 6 mm thick KNbO3:Fe sample is measured. The time constant and activation energy associated with the thermal induced decay of the hologram are determined to be 2.9×10−7 s and 0.81 eV for the KNbO3:Fe sample, and 1.35×10−7 s and 0.79 eV for the pure KNbO3 sample.
Nucleation of cubic GaN/GaAs (001) grown by gas source molecular beam epitaxy with hydrazine
Authors: Nikishin S. A. , Antipov V. G. , Ruvimov S. S. , Seryogin G. A. , Temkin H.
Published: July 2002

Growth nucleation and evolution of morphology of GaN on (001) GaAs is investigated as a function of the N2H4/Ga flux ratio. The use of hydrazine allows us to reach high flux ratios without causing any damage to the epitaxial layer. Epitaxial GaN is purely cubic but shows growth anisotropy dependent on the flux ratio. GaN layers grown at low flux ratios show three-dimensional nucleation and no preferential island orientation. With higher flux ratios, the nucleation rate increases, the surface becomes smoother, and the growth anisotropy markedly increases. The growth morphology reflects the surface anisotropy of the underlying GaAs substrate.
Subband gap carrier dynamics in low-temperature-grown GaAs
Authors: Grenier P. , Whitaker J. F.
Published: July 2002

Measurements of the carrier relaxation dynamics in low-temperature-grown GaAs have been made with a femtosecond-resolution, time-resolved pump-probe technique using a subband-gap probe-beam wavelength. The transient absorption and index of refraction changes have been analyzed using a relaxation model with up to four different excited state populations. The carrier recombination time within the midgap trap states is found to be longer than the subpicosecond free-carrier trapping time. Two time scales are observed for the recombination rate, one of a few picoseconds and one of hundreds of picoseconds, indicating the presence of at least two different trap states for the free-carriers in this material.
Spatial correlations across n+n semiconductor junctions
Authors: Bulashenko O. M. , Gomila G. , Rubı́ J. M. , Kochelap V. A.
Published: July 2002

An analytical model for the spatial correlations and noise across an abrupt n+n junction is presented. The model is able to treat the junction as a whole in strongly inhomogeneous conditions self-consistently, by taking into account both the drift and diffusion contributions to the current. It is shown that within the analytical approach, the voltage noise across the junction can be decomposed into the sample and the contact contributions and the term representing sample-contact cross correlation. It is argued that the proposed analytical method is quite universal and could be effectively applied to different devices, operating under strongly inhomogeneous distributions of the electric field and charge concentration.
High-repetition-rate femtosecond optical parametric oscillator based on periodically poled lithium niobate
Authors: Burr Kent C. , Tang C. L. , Arbore Mark A. , Fejer Martin M.
Published: July 2002

A high-repetition-rate, femtosecond optical parametric oscillator based on bulk periodically poled lithium niobate is described. This optical parametric oscillator is continuously tunable from 1.12 to 1.50 μm in the signal branch, and signal pulses as short as 60 fs have been observed. The corresponding turning range for the idler branch is from 1.68 to 2.72 μm. Modifications which should result in a femtosecond optical parametric oscillator with a pumping threshold of less than 50 mW are discussed.
Effect of indirect Γ-L and Γ-X transfer on the carrier dynamics of InGaP/InAlP multiple quantum wells
Authors: Menoni C. S. , Buccafusca O. , Marconi M. C. , Patel D. , Rocca J. J. , Robinson G. Y. , Goodnick S. M.
Published: July 2002

Indirect Γ-L scattering within the well, and real space carrier transfer to the barrier X1c states are shown to significantly affect the carrier dynamics in In0.48Ga0.52P/In0.5Al0.5P multiple quantum wells. When carriers transfer to the indirect states occurs, the carrier dynamics is modified by the slow return of the carriers from the low mobility states to the well. As a result, the absorption recovery time increases by almost an order of magnitude. Carrier transfer to the indirect states also increases the carrier lifetime to values characteristic of indirect recombination.
Soft x-ray pulse generation from femtosecond laser-produced plasma with reduced debris using a metal-doped glass target
Authors: Nakano H. , Nishikawa T. , Uesugi N.
Published: July 2002

We have demonstrated soft x-ray short pulse generation from femtosecond laser-produced plasma with much lower debris using metal-doped glass as a target. Soft x-ray emission (50–200 Å) from a gold-doped glass target was about 40% of that from a solid gold target while the density of gold in the doped-glass was less than 0.001 vol % and the target was transparent at the wavelength of the laser light. Due to the low metal density, the particulate deposition, which is one of the serious obstacles to putting laser-produced plasma x-ray into practical use, was greatly reduced by the use of metal-doped glass as a target.
Preconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridation
Authors: Heinlein Christian , Grepstad Jostein , Berge Torunn , Riechert Henning
Published: July 2002

The crystalline quality of molecular beam epitaxy grown layers of GaN on sapphire is generally improved by nitridation of the substrate. In this study, we use x-ray photoelectron spectroscopy, low energy electron diffraction, and atomic force microscopy to examine the case for nitridation of c-plane sapphire upon exposure to rf plasma generated nitrogen radicals. We find that a monolayer of surface nitride is formed after ∼300 min exposure with the substrate at 400 °C. Extended exposure causes growth of protrusions from the c-plane sapphire and thus leads to a rough surface morphology. Moreover, we report removal of adventitious surface carbon upon heat treatment at 300 °C in nitrogen plasma, albeit with reduced efficiency compared with hydrogen plasma cleaning.
Structural and electrical properties of crystalline (1−x)Ta2O5–xAl2O3 thin films fabricated by metalorganic solution deposition technique
Authors: Joshi P. C. , Stowell S. , Desu S. B.
Published: July 2002

Polycrystalline (1−x)Ta2O5–xAl2O3 thin films were fabricated by metalorganic solution deposition technique on Pt-coated Si substrate at a temperature of 750 °C. Thin films with 0.9Ta2O5–0.1Al2O3 composition exhibited improved dielectric and insulating properties compared to Ta2O5 thin films. The measured small signal dielectric constant and dissipation factor at 100 kHz were 42.8 and 0.005, respectively. The temperature coefficient of capacitance was 20 ppm/°C in the measured temperature range of 25–125 °C. The leakage current density was lower than 6×10−8 A/cm2 up to an applied electric field of 1 MV/cm. A charge storage density of 18.9 fC/μm2 was obtained at an applied electric field of 0.5 MV/cm. The high dielectric constant, low dielectric loss, low leakage current density, and good temperature and bias stability suggest (1−x)Ta2O5–xAl2O3 thin films to be a suitable dielectric layer in integrated electronic devices in place of conventional dielectrics such as SiO2 or Si3N4.
Frictional imaging in a scanning near-field optical/atomic-force microscope by a thin step etched optical fiber probe
Authors: Muramatsu Hiroshi , Chiba Norio , Fujihira Masamichi
Published: July 2002

Use of a thin step etched optical fiber probe in a scanning near-field optical/atomic-force microscope (SNOM/AFM) produced frictional imaging. The probe was fabricated by the etching of an optical fiber to decrease its diameter and sharpen the tip end with a HF solution and by irradiating a CO2 laser beam to bend the tip. The spring constant of the thin probe is 100 times smaller than that of a conventional optical fiber probe, which allows the probe to be used as a contact AFM mode and in frictional imaging.
Pit formation in GaInN quantum wells
Authors: Chen Y. , Takeuchi T. , Amano H. , Akasaki I. , Yamada N. , Kaneko Y. , Wang S. Y.
Published: July 2002

The formation of pits in GaInN quantum wells (QWs) has been studied by atomic force microscopy and transmission electron microscopy. It is found that the pits have a hexahedron cone morphology with six sidewalls on 〈11̄01〉 planes and dislocations connected to their vertexes. The dislocations may induce the formation of pits during the growth of GaInN QWs.
Fabrication of a low-operating voltage diamond thin film metal–semiconductor–metal photodetector by laser writing lithography
Authors: Jiang Wei , Ahn Jaeshin , Xu Feng-Lan , Liaw Chin-Yi , Chan Yuen-Chuen , Zhou Yan , Lam Yee-Loy
Published: July 2002

An ultraviolet-sensitive photodetector based on the metal-semiconductor-metal structure has been fabricated on a chemical-vapor-deposited diamond thin film grown on a silicon substrate. Device processing techniques employed include maskless laser writing lithography, image reversal processing and sacrificial layer inclusion, which resulted in devices with an electrode finger separation of only 2.5 μm. This allows for a low operating voltage of only 5 V, which is very much lower than what has been achieved so far.
Inhomogeneous strain in individual quantum dots probed by transport measurements
Authors: Akyüz C. D. , Zaslavsky A. , Freund L. B. , Syphers D. A. , Sedgwick T. O.
Published: July 2002

Resonant tunneling measurements are used to probe the inhomogeneous strain in individual SiGe quantum dots. Current–voltage characteristics of strained Si/SiGe resonant tunneling diodes of diameter D⩽0.25 μm exhibit additional fine quasi-periodic structure in the resonant peaks. The fine structure is consistent with lateral quantization in the SiGe quantum well due to in-plane confining potentials arising from inhomogeneous strain, which we calculate by finite element techniques for various D. Quenching of the fine structure by a magnetic field is consistent with the effective length scale of the strain-induced potential.
A comparison of spectroscopic and microscopic observations of ion-induced intermixing in InGaAs/InP quantum wells
Authors: Piva P. G. , Goldberg R. D. , Mitchell I. V. , Chen Huajie , Feenstra R. M. , Weatherly G. C. , McComb D. W. , Aers G. C. , Poole P. J. , Charbonneau S.
Published: July 2002

Using n-doped InGaAs/InP multi-quantum-well samples we compare measurements of ion-beam-induced quantum well broadening made by cross-sectional scanning tunneling microscopy and cross-sectional scanning transmission electron microscopy with the broadening calculated from the blueshift of the low temperature photoluminescence peak using different models of the intermixing process. Results are consistent with a simple square well model used to interpret photoluminescence shifts, but disagree with a model of separate Fick’s law diffusion on group III and V sublattices.
Native point defects in Si with a CoSi2 film: Single crystal versus polycrystalline
Authors: Herner S. B. , Gossmann H.-J. , Tung R. T.
Published: July 2002

We have determined the native point defects concentrations in silicon with single and polycrystalline CoSi2 films by annealing Sb and B doping superlattices at 850 °C/60 min in N2. The polycrystalline film results in enhanced Sb diffusion in Si, indicating a vacancy supersaturation (CV/CV*)∼2.4±0.8, while the single crystal film maintains a vacancy concentration near equilibrium. Boron diffusion is retarded by the same amount by both films, indicating an interstitial undersaturation (CI/CI*∼0.3±0.1). This directly implies that CoSi2 grain boundaries are the cause of the higher vacancy supersaturation that is well known to occur during standard Co silicidation.
Spatially resolved photoluminescence in GaAs surface acoustic wave structures
Authors: Santos P. V. , Ramsteiner M. , Jungnickel F.
Published: July 2002

The interaction between a surface acoustic wave (SAW) and the excitonic photoluminescence (PL) in GaAs SAW structures is investigated. The dependence of the PL on the SAW amplitude and illumination intensity is explained by a simple model based on the field-induced ionization of the excitons and on the screening of the SAW electric field by photogenerated carriers. Microscopic PL constitutes a powerful technique for spatially resolving electric-field distributions in SAW structures.
Femtosecond tunneling response of surface plasmon polaritons
Authors: Keil Ulrich D. , Ha Taekjip , Jensen Jacob R. , Hvam Jo/rn M.
Published: July 2002

We obtain femtosecond (200 fs) time resolution using a scanning tunneling microscope on surface plasmon polaritons (SPPs) generated by two 100 fs laser beams in total internal reflection geometry. The tunneling gap dependence of the signal clearly indicates the tunneling origin of the signal and suggests that nanometer spatial resolution can be obtained together with femtosecond temporal resolution. This fast response, in contrast to the picosecond decay time of SPPs revealed by differential reflectivity measurements, can be attributed to a coherent superposition of SPPs rectified at the tunneling junction.
Determination of the absolute CH3 radical flux emanating from a methane electron cyclotron resonance plasma
Authors: Pecher P. , Jacob W.
Published: July 2002

Methyl radicals from a methane electron cyclotron resonance plasma are measured quantitatively at the sample position by ionization-threshold mass spectrometry (ITMS). The absolute fluxes are determined by calibrating the CH3 ITMS results with those of methane, taking into account the published energy-dependent cross sections for the ionization of CH3 and CH4, respectively. The measured CH3 radical fluxes are on the order of some 1015 cm−2 s−1, which is in accordance with recent modeling results.
0.45 W diffraction-limited beam and single-frequency operation from antiguided phase-locked laser array with distributed feedback grating
Authors: Nesnidal M. P. , Earles T. , Mawst L. J. , Botez D. , Buus J.
Published: July 2002

A second-order diffraction grating placed below the active region of a phase-locked resonant antiguided array selects the in-phase array mode in addition to its role as a single-longitudinal-mode selector. This type of array-mode discrimination relies on the fact that the resonant in-phase array mode has significantly better field overlap with the grating region than nonresonant array modes. Furthermore, it eliminates the need for a conventional array-mode discriminator: interelement loss; which can cause self-pulsations. Diffraction-limited beam and single-frequency operation is obtained to at least 0.45 W peak pulsed power from 20 element, InGaAs/InGaP/GaAs structures (λ=0.97 μm) of 120-μm-wide aperture. Distributed-feedback operation is confirmed over the 20–40 °C temperature range. The results are in good agreement with theory.
Modeling the optical properties of hexagonal GaN
Authors: Djurišić Aleksandra B. , Li E. Herbert
Published: July 2002

An adjustable broadening function instead of the conventional Lorentzian one is incorporated in the dielectric function model for hexagonal GaN. One-electron contributions at E1 critical points and higher-state (m>1) exciton terms, which were incorrectly disregarded in the previous study, are taken into account. Model parameters were determined using the acceptance-probability-controlled simulated annealing. As a result, excellent agreement with experimental data for both real and imaginary parts in the range from 1.5 to 10 eV is obtained. Average discrepancy between experimental and calculated data for the real part of the index of refraction equals 2.75×10−4, and for the imaginary part is 1.66×10−3.
Temperature and bias dependence of magnetoresistance in doped manganite thin film trilayer junctions
Authors: Sun J. Z. , Abraham D. W. , Roche K. , Parkin S. S. P.
Published: July 2002

Thin film trilayer junction of La0.67Sr0.33MnO3-SrTiO3-La0.67Sr0.33MnO3 shows a factor of 9.7 change in resistance, in a magnetic field around 100 Oe at 14 K. The junction magnetoresistance is bias and temperature dependent. The energy scales associated with bias and temperature dependence are an order of magnitude apart. The same set of energies also determine the bias and temperature dependence of the differential conductance of the junction. We discuss these results in terms of metallic cluster inclusions at the junction-barrier interface.
Dynamics of electron beam ablation of silicon dioxide measured by dye laser resonance absorption photography
Authors: Kovaleski S. D. , Gilgenbach R. M. , Ang L. K. , Lau Y. Y.
Published: July 2002

The dynamics of electron beam ablation plumes have been characterized through the application of dye laser resonance absorption photography. The ablation of fused silica by a channelspark electron beam was studied by probing the near-ground state, 3p2 1D−4s 1P0 neutral Si transition at 288.158 nm. Necessary background gases (Ar or N2) were tested at pressures of 15 or 30 mTorr. A two-lobed, Si atom plume shape was discovered that is hydrodynamically more complex than laser ablation plumes. These plumes merge into a single-lobed plume at about 400 ns after the e-beam current pulse rise. Plume front expansion velocities of Si atoms were measured at nearly 1 cm/μs, and are comparable to the expansion of laser ablated metal atom plumes with laser fluences of a few J/cm2.
Thermal effects in widely tunable germanium terahertz lasers
Authors: Bründermann E. , Chamberlin D. R. , Haller E. E.
Published: July 2002

We report laser emission from Be-doped Ge lasers at high repetition rates of up to 45 kHz which is one order of magnitude higher than previously reported. Laser radiation was detected from Ge:Be crystals with volumes as small as 0.5 mm3 and with intercontact distances down to 1 mm. We present a thermal analysis and derive the design parameters for continuous wave Ge lasers.