bluuebunny/crossref_metadata_2025 huggingface dataset:
Amplitude modulation reflectometry system for the FTU tokamak
Authors: Buratti P. , Zerbini M. , Brodsky Y. , Kovalev N. , Shtanuk A.
Published: July 2002

Amplitude modulation (AM) reflectometry is a modification of the classical frequency sweep technique that allows to achieve unambiguous phase delay measurements. A broadband AM reflectometer has been realized for the measurement of density profiles on the FTU tokamak in the 0.34–1×1020 m−3 range. The characteristics of the instrument have been determined in extensive laboratory tests; particular attention has been devoted to the effect of interference with parasitic reflections. Possible ways to overcome this effect are indicated. An example of the first experimental data collected on FTU is discussed.
Novel cathode for field-emission applications
Authors: Garate Eusebio , McWilliams Roger D. , Voss Donald E. , Lovesee Alexander L. , Hendricks Kyle J. , Spencer Thomas A. , Clark M. Collins , Fisher Amnon
Published: July 2002

A CsI salt-based cathode which is capable of producing a modest perveance, 10 s of A/cm2 electron beam for several microseconds pulse lengths, and has little susceptibility to diode closure has been experimentally characterized. This explosive field-emission CsI-coated carbon fiber cathode has operated in modest 10−5 Torr vacuums at voltages up to 160 kV, and can easily be configured to provide space-charge-limited solid or annular electron beams in arbitrarily large diameter configurations. The CsI cathode has demonstrated negligible closure for 2 μs pulses, and has operated for 200 shots with no degradation in cathode performance. Data on the operating performance of this salt cathode, including effective gap time history and streak photographs demonstrating uniformity of the current density, are presented. A comparison of CsI cathode performance with a velvet explosive field emitting cathode used in electron-beam production is also presented.
High quality Si1−x−yGexCy epitaxial layers grown on (100) Si by rapid thermal chemical vapor deposition using methylsilane
Authors: Mi J. , Warren P. , Letourneau P. , Judelewicz M. , Gailhanou M. , Dutoit M. , Dubois C. , Dupuy J. C.
Published: July 2002

We have produced epitaxial Si1−x−yGexCy/Si heterostructures by rapid thermal chemical vapor deposition using methylsilane SiCH6). These layers were grown in the SiH4/GeH4/SiCH6/H2 system between 550 and 600 °C at 1.5 Torr. Suitable process conditions were found that allow very efficient substitutional carbon incorporation. No carbon cross contamination was observed. Crystal quality, chemical composition, and lattice strain were deduced from Nomarski microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, secondary ion mass spectrometry, and x-ray diffraction. Defect-free alloy layers with compositions of up to 20 at.% Ge and 2.2 at. % C were produced. The lattice parameter was tailored so that the strain in these layers gradually moved from compressive to tensile. A tensile strain of up to 0.35% was achieved.
Real time monitoring of the growth of transparent thin films by spectroscopic ellipsometry
Authors: Kildemo M. , Drévillon B.
Published: July 2002

Real time monitoring of the growth of plasma deposited transparent thin films by spectroscopic phase modulated ellipsometry (SPME) is presented. Two on-line methods of determination of the refractive index n and the film thickness d are evaluated. The first one is based on the fast inversion of the Fresnel equations (0.2 s with a conventional PC 486 computer). Combining the measurements simultaneously recorded at two photon energies, a 5% relative precision is obtained on 5–6000 Å thick films even deposited at high deposition rate (32 Å s−1). On the other hand, a better precision can be obtained using a least squares fit procedure based on single photon energy SPME measurements. In the latter case, it is shown that the product nd can be determined with a 1% precision, up to 6000 Å.
Fast reciprocating Langmuir probe for the DIII-D divertor
Authors: Watkins J. G. , Hunter J. , Tafoya B. , Ulrickson M. , Watson R. D. , Moyer R. A. , Cuthbertson J. W. , Gunner G. , Lehmer R. , Luong P. , Hill D. N. , Mascaro M. , Robinson J. I. , Snider R. , Stambaugh R.
Published: July 2002

A new reciprocating Langmuir probe was used to measure density and temperature profiles, ion flow, and potential fluctuation levels from the lower divertor floor up to the X point on the DIII-D Tokamak. This probe is designed to make fast (2 kHz swept, 20 kHz Mach, 500 kHz Vfloat) measurements with 2 mm spatial resolution in the region where the largest gradients on the plasma open flux tubes are found and therefore provide the best benchmarks for scrap-off layer and divertor numerical models. Profiles are constructed using the 300 ms time history of the probe measurements during the 25 cm reciprocating stroke. Both single and double null plasmas can be measured and compared with a 20 Hz divertor Thomson scattering system. The probe head is constructed of four different kinds of graphite to optimize the electrical and thermal characteristics. Electrically insulated pyrolytic graphite rings act as a heat shield to absorb the plasma heat flux on the probe shaft and are mounted on a carbon/carbon composite core for mechanical strength. The Langmuir probe sampling tips are made of a linear carbon fiber composite. The mechanical, electrical, data acquisition, and power supply systems will be described. Initial measurements will also be presented.
First results from the small tight aspect ratio tokamak multifrequency pulse radar reflectometer
Authors: Shevchenko V. F. , Walsh M. J.
Published: July 2002

A multifrequency pulse radar reflectometer (PRR) has been designed, commissioned, and is operating successfully on the small tight aspect ratio tokamak (START). The data obtained with this technique allow the study of the density profile evolution during the shot, revealing aspects of the plasma behavior during such events as the internal reconnection. A simple and effective profile-reconstruction algorithm using the stepwise profile approximation permits the monitoring of the plasma density profile immediately after each shot. Cross checks between the START hydrogen cyanide (HCN) interferometer and the line integral density determined by integrating the PRR generated data shows good agreement.
Preparation of ultrathin free-standing targets for (e,2e) spectroscopy
Authors: Fang Z. , Guo X. , Utteridge S. , Canney S. A. , McCarthy I. E. , Vos M. , Weigold E.
Published: July 2002

We describe in detail the procedures used for the preparation of ultrathin (∼10 nm) free-standing membranes for (e,2e) spectroscopy. Such a thin target is needed to minimize electron multiple scattering before and after an (e,2e) event. The development of a rf plasma source which allows in situ thinning and thickness monitoring is of key importance to the success of the target preparation. Materials (C, Si, Ni, Cu, Al2O3, SiO2, CuO) with different properties and structures are usually prepared in different ways. For insulating targets it is important to have a conducting sublayer to avoid the charging problem. A well prepared target usually has a thin area larger than the (e,2e) beam size (∼0.2 mm in diameter) and yields high quality (e,2e) data from which the electron energy-momentum density in a chosen direction is determined. Efforts demonstrated in this article indicate that the preparation of ultrathin free-standing films is a challenging area where significant technical development is needed.
Novel high-temperature, high-vacuum, all-metal sample cells for microcalorimetric measurements of solids
Authors: Coker Eric N. , Karge Hellmut G.
Published: July 2002

The design of a novel microcalorimetric sample cell which allows rapid transfer of heat from the sample to the cell wall is described. The solid sample is dispersed around the internal walls of the cell. Since the thermopiles of the microcalorimeter are in contact with the outer surface of the cell, the new design of the cell allows rapid recording of the heat generated by the sample. The robust all-metal cell allows in situ sample activation under conditions of high vacuum and at temperatures up to 500 °C. The dispersion of the sample avoids any “deep bed” effects where gas may not be able to diffuse freely to the surface of all of the sample, which may be the case when a thick layer or plug of the sample is used. The cell enables faster acquisition of heat data and minimizes effects of self-heating of the sample. A simple tool, which assists the introduction of the solid sample into the cell and a compact resistor-element, which fits inside the cell for calibration, are described.
Stabilized recording and thermal fixing of holograms in photorefractive lithium niobate crystals
Authors: Breer S. , Buse K. , Peithmann K. , Vogt H. , Krätzig E.
Published: July 2002

Holograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 °C. The setup is actively stabilized during recording against movements or vibrations of the interference pattern which especially occur during long-period writing at enhanced temperatures. Two different techniques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilization. (2) Alternatively, one of the beams is periodically phase modulated and the beam-coupling signal is used for stabilization. Reproducible refractive index changes of thermally fixed holograms up to 7.5×10−4 are obtained with both stabilization techniques. However, the second method is advantageous for multiplexing experiments, because no readjustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed.
A sample cell and low-temperature accessory design for fluorescence-mode x-ray absorption spectroscopy of air-sensitive and corrosive compounds
Authors: Weber Stefan , Ostafin Agnes E. , Norris James R.
Published: July 2002

A sample cell and low-temperature cooling unit design suitable for x-ray absorption spectroscopy (XAS) of oxygen and/or water sensitive compounds is presented. The cell is a simplified and optimized modification of a previously suggested design. The low-temperature accessory is specifically designed for good thermal isolation and equilibration at liquid nitrogen temperatures, eliminating the need for a more expensive commercial cryostat. The entire construction can be directly attached to an ion chamber and either a standard Lytle detector or energy-discriminating germanium-element solid-state detector via helium gas-purged adaptor flanges. The apparatus was tested at liquid nitrogen temperatures and gave good temperature stability within ±0.1 K. The XAS signal from an air-sensitive potassium/pyrene complex at 100 K taken at the X-6B beam line at the National Synchrotron Light Source facility at Brookhaven National Laboratory is presented.
Automatic lateral calibration of tunneling microscope scanners
Authors: Lapshin Rostislav V.
Published: July 2002

A practical method is described to find automatically the calibration coefficients and residual nonorthogonality of a tunneling microscope scanner. As initial data, the coordinates of three atoms were used forming a triangle in a highly oriented pyrolytic graphite surface appearing in the form of a spatially geometrical measure. A recognition procedure is described which can be applied to determine the lateral coordinates of the atoms. Length and orientation distortions were calculated, estimates of calibration errors were given and the requirement on the nonorthogonality limit was formulated for manipulator a given that ensures measurements of the predetermined accuracy. The sensitivity of the method to a noise in atom coordinates was determined. Experimental data showing the practical suitability of the method developed are presented.
Three-dimensional time-resolved H pellet trajectory reconstruction in RFX by position sensitive detector Hα diagnostic
Authors: Innocente P. , Boscolo B. , Martini S. , Garzotti L.
Published: July 2002

Hydrogen pellets injected in reversed field pinches (RFPs) experience large poloidal and toroidal accelerations along magnetic field lines due to the rocket effect caused by asymmetric ablation processes. The 3D pellet trajectory and the global ablation rate is diagnosed in RFX by an absolutely calibrated multiple position sensitive detector (PSD) system looking at the Hα radiation coming from the pellet ablation cloud. The design of the diagnostic is presented. Then the performance is highlighted by cross-comparison between different PSD measurements and with time-integrated images from a CCD camera. Overall accuracy of the order of a few millimeters is achieved, which makes the instrument well-suited to assist transport studies (particle deposition), to estimate the radial profile of fast electron tails (derived from the modulus of pellet acceleration) and to obtain information on internal magnetic field pitch (from the direction of pellet acceleration).
High-energy 4ω probe laser for laser-plasma experiments at Nova
Authors: Glenzer S. H. , Weiland T. L. , Bower J. , MacKinnon A. J. , MacGowan B. J.
Published: July 2002

For the characterization of inertial confinement fusion plasmas, we implemented a high-energy 4ω probe laser at the Nova laser facility. A total energy of >50 J at 4ω, a focal spot size of order 100 μm, and a pointing accuracy of 100 μm was demonstrated for target shots. This laser provides intensities of up to 3×1014 W cm−2 and therefore fulfills high-power requirements for laser-plasma interaction experiments. The 4ω probe laser is now routinely used for Thomson scattering. Successful experiments were performed in gas-filled hohlraums at electron densities of ne>2×1021 cm−3 which represents the highest density plasma so far being diagnosed with Thomson scattering.
Multichannel photon counting instrument for spatially resolved near infrared spectroscopy
Authors: Ntziachristos Vasilis , Ma XuHui , Yodh A. G. , Chance Britton
Published: July 2002

We have developed a multichannel photon counting instrument for near infrared spatially resolved spectroscopy. The instrument uses two laser diodes at 780 and 830 nm. The two wavelengths are time multiplexed at a rate of 5 MHz for virtually simultaneous measurements at the two wavelengths. The photon pulses are then multiplexed with an optical switch so that the incident optical signals are directed to different source positions. Eight time-correlated channels have been employed for simultaneous acquisition of spectroscopic data at multiple locations. Photon detection is performed by multichannel plate photomultiplier tubes. Robust and accurate data analysis tools to perform deconvolution, data fitting, and absorption change quantification are presented. The instrument has been tested with phantoms simulating tissue optical properties. Absolute optical properties, namely absorption and reduced scattering coefficient, have been determined with an accuracy of ±5%. Quantification of absorption changes can be achieved with an accuracy of 10−3 cm−1 and a sensitivity of the order of 10−4 cm−1. Finally spatially resolved measurements of primary motor-cortex activation have been performed and successfully coregistered with functional magnetic resonance imaging data.
A highly sensitive quartz-crystal microbalance for sputtering investigations in slow ion–surface collisions
Authors: Hayderer G. , Schmid M. , Varga P. , Winter HP. , Aumayr F.
Published: July 2002

A quartz-crystal microbalance technique for measuring total sputter yields in ion–surface collisions is described. The electronic circuit to drive the quartz crystal ensures low noise and high frequency stability. By measuring total sputter yields for impact of singly charged ions on LiF target films a sensitivity limit of 0.5% of a monolayer per minute could be achieved.
Quantitative measurements of the chemical composition of unprepared samples, using a reflectron mass analyzer with a microchannelplate detector assembly
Authors: Mróz W. , Prokopiuk A. , Kozlov B. , Czujko T. , Józwiak S. , Krzywiński J. , Stöckli M. P. , Fehrenbach C.
Published: July 2002

This article presents measurements of the chemical composition of Al–Li samples using a reflectron mass analyzer and laser ionization of the sample. The measurements were taken of as-received samples with rough surfaces to ascertain if useful results could be obtained from samples that had not been cleaned or prepared in any way. The power density of the laser used (Nd:yttrium–aluminum–garnet λ=1.06 μm, E≅6 mJ, τ=5 ns) was I∼3×109 W/cm2. We have described the quantitative processing of our results using the measured analog particle gains of the Galileo microchannelplates. The problems associated with quantitative measurements of ion pulses using a microchannelplate detector assembly are also discussed.
A method for production of cheap, reliable Pt–Ir tips
Authors: Rogers B. L. , Shapter J. G. , Skinner W. M. , Gascoigne K.
Published: July 2002

A new method of producing Pt–Ir tips for use in scanning tunneling microscopy is described. This reproducible method is simple, cheap, fast, and avoids the use of hazardous chemicals common in many other methods. Scanning electron microscopy, time of flight–secondary ion mass spectroscopy, and x-ray photoelectron spectroscopy have been applied to understand both the chemical and morphological changes that occur as a result of the etching. The method has been demonstrated on both stock Pt–Ir wire and commercial tips and has been found to dramatically enhance image quality. It is also reusable on the same tip extending the lifetime of a single tip indefinitely.
Very low temperature drift tube mass spectrometer
Authors: Tanuma H. , Sakamoto M. , Fujimatsu H. , Kobayashi N.
Published: July 2002

A new selected ion drift tube mass spectrometer, which has been developed for ion swarm experiments at very low temperature, is presented. Gas temperature of 2 K in the drift tube as the lowest one for this apparatus is achieved by liquid helium cooling. Details of techniques in the low temperature experiment, which are concerned in the development of this apparatus, is discussed. Preliminary experiments have been carried out in measurements of drift velocity of He+ ions in He gas at 4.3 and 2 K.
Absolute detection efficiency of a microchannel plate detector for neutral atoms
Authors: Barat M. , Brenot J. C. , Fayeton J. A. , Picard Y. J.
Published: July 2002

The absolute detection efficiency (ADE) of microchannel plates for neutral sodium and potassium atoms is measured in the low keV energy range. It is shown that ADE is primarily a function of the particle energy. This result is compared to measurements made by other authors for ionic particles.
Single crystal optic elements for helium atom microscopy
Authors: MacLaren D. A. , Allison W. , Holst B.
Published: July 2002

Focusing characteristics of asymmetrically bent single crystal mirrors are discussed in the context of fabricating an optic element for an helium atom microscope. We demonstrate the principle that deforming a clamped, elliptical, single crystal under electrostatic pressure can produce submicron focusing of an helium beam. We present a systematic procedure that may be used to fabricate high precision mirrors close to the Cartesian ideal of any chosen optical configuration. In particular, imaging systems with asymmetric mirror profiles are discussed. Results are independent of crystal characteristics and can be adapted to fit a range of experimental geometries. The calculations indicate that mirror-induced aberrations can be eliminated to fourth order by use of a single actuation electrode in an ideal system.
Nanoscratch tester for thin film mechanical properties characterization
Authors: Randall N. X. , Consiglio R.
Published: July 2002

Quantitative instrumentation has become a necessity for adequate evaluation of material properties, particularly scratch resistance and adhesion at the film-substrate interface. The nanoscratch tester is a new instrument that combines the features of both the classical stylus scratch test (normal force range) and the scanning force microscope (SFM) (short sliding distances), allowing scratch lengths of up to 10 mm. Tangential force and penetration depth are measured during the scratching process. For high-resolution inspection of the scratched area, a SFM is integrated into the system. Typical uses are presented for a range of applications, particularly on curved sample geometries and the use of scratch mapping.
The “Leakage Current Sentinel”: A novel plug-in socket device for online biomedical equipment electrical safety surveillance
Authors: Cappa Paolo , Marinozzi Franco , Sciuto Salvatore Andrea
Published: July 2002

The Leakage Current Sentinel (LCS) has been designed and implemented for the detection of hazardous situations caused by dangerous earth leakage current values in intensive care units and operating theaters. The device, designed and manufactured with full compliance of the high risk environment requirements, is able to monitor online the earth leakage current and detect ground wire faults. Operation utilizes a microammeter with an overall sensitivity of 2.5×104 V/A. In order to assure the reliability of the device in providing alarm signals, the simultaneous presence of absorbed power current is monitored by means of another ammeter with decreased sensitivity (3.0 V/A). The measured root mean square current values are compared with reference values in order to send signals to NAND and OR complementary metal–oxide–semiconductor gates to enable audible and visible alarms according to the possible hazardous cases examined in the article. The final LCS packaging was shaped as a wall socket adapter for common electromedical device power cord plugs, with particular attention to minimizing its dimensions and to provide analog voltage outputs for both measured leakage and power currents, in order to allow automatic data acquisition and computerized hazardous situation management. Finally, a personal computer based automatic measuring system has been configured to simultaneously monitor several LCSs installed in the same intensive care unit room and, as a consequence, to distinguish different hazardous scenarios and provide an adequate alert to the clinical personnel whose final decision is still required. The test results confirm the effectiveness and reliability of the LCS in giving an alert in case of leakage current anomalous values, either in case of a ground fault or in case of a dangerous leakage current.
Raman scattering characterization of the crystalline qualities of ZnSe films grown on S-passivated GaAs(100) substrates
Authors: Wang J. , Liu X. H. , Li Z. S. , Su R. Z. , Ling Z. , Cai W. Z. , Hou X. Y. , Wang Xun
Published: July 2002

A comparative study of the Raman spectra of ZnSe films grown by molecular beam epitaxy on GaAs(100) substrates passivated by NH4)2)Sx and S2Cl2 solutions is presented. Based on the analysis of the line shape of the first-order longitudinal-optical phonon of ZnSe with spatial correlation model of Raman scattering, it is shown that the ZnSe films grown on the GaAs substrates passivated by S2Cl2 solutions have longer coherence lengths, which indicate that their crystalline qualities are better than those passivated by NH4)2Sx solutions. In addition, the barrier heights of ZnSe/GaAs interfaces for different S passivations have been obtained from the ratios of the intensity of the coupled longitudinal-optical phonon-plasmon mode to that of the longitudinal-optical mode of GaAs Raman peak. The results show that the ZnSe/GaAs samples passivated by S2Cl2 solutions have lower density of interfacial states.
Formation of outgrowths at the initial growing stage of YBa2Cu3Ox ultrathin films on ZrO2 substrates
Authors: Gao J. , Wong W. H. , Xhie J.
Published: July 2002

Ultrathin films of YBa2Cu3Ox with good crystallinity and superconductivity were prepared by a modified off-axis sputtering. The microstructure, with emphasis on surface morphology and formation of outgrowths, was studied by using atomic force microscopy and electron microscopy. It was found that many outgrowths were formed at the initial growing stage. Therefore it is important to suppress the nucleation of outgrowths at a very early growing step to obtain a smooth film. The nucleation of outgrowths is significantly influenced by surface defects on the substrate. Discussion is also made in correlation with the occurrence of an intermediate layer commonly observed onZrO2. From our observation, YBa2Cu3Ox might be grown directly on the surface of ZrO2 without forming any intermediate layer, if the film thickness is very small.
Thermal and plasma-assisted nitridation of GaAs(100) using NH3
Authors: Jones M. E. , Shealy J. R. , Engstrom J. R.
Published: July 2002

The thermal and plasma-assisted nitridation of GaAs(100) using NH3 has been examined employing x-ray diffraction, Auger electron spectroscopy, and atomic force microscopy to characterize the nitrided films. All thermally nitrided films were composed of a mixture of hexagonal and cubic GaN, whereas the addition of plasma excitation produced films purely of the cubic structure. Thicknesses of the thermally nitrided films, up to 7000 Å, increased with both increasing temperature and nitridation time. The plasma-assisted process holds promise for the formation of templates for homoepitaxial growth of cubic GaN.
Strain relaxation in Fe3(Al,Si)/GaAs: An x-ray scattering study
Authors: Noh D. Y. , Hwu Y. , Je J. H. , Hong M. , Mannáerts J. P.
Published: July 2002

Lattice strain of epitaxially grown single crystal Fe3(Al,Si)/GaAs films was measured in a synchrotron x-ray scattering experiment. The Fe3Al film (2.5% lattice mismatch) was partially strained and tetragonally distorted at room temperature. As the sample was annealed to 500 °C, the internal strain was mostly relaxed while the tegragonal distortion was greatly reduced. We believe that the strain relaxation was caused by the interdiffusion of atoms through domain boundaries at elevated temperatures. In comparison, the Fe3Si film with much less lattice mismatch (0.17%) was completely strained up to 600 °C without being relaxed.
High responsitivity intrinsic photoconductors based on AlxGa1−xN
Authors: Lim B. W. , Chen Q. C. , Yang J. Y. , Asif Khan M.
Published: July 2002

This letter reports on the fabrication and characterization of visible-blind ultraviolet photoconductors using single-crystal AlxGa1−xN layers deposited on basal plane sapphire substrates. With aluminum mole fractions ranging from 5% to 61%, the long-wavelength cutoff can be varied from 350 to 240 nm. Photoresponsitivities as high as several hundred amperes per watt were measured with 10 μm interelectrode spacing.
Ultrafast dynamics of nonlinear absorption in low-temperature-grown GaAs
Authors: Benjamin S. D. , Loka H. S. , Othonos A. , Smith P. W. E.
Published: July 2002

We present the results of a study of the subpicosecond dynamic behavior of optically induced absorption changes in low-temperature-grown GaAs. We show that the observed behavior is dominated by mid-gap trap states, and can be accurately modeled by the rate equations previously developed to describe quasi-cw results. Our data give the first approximate values for trap emptying times in this material.
Electron capture in GaAs quantum wells via electron-electron and optic phonon scattering
Authors: Kálna K. , Moško M. , Peeters F. M.
Published: July 2002

Electron capture times in a quantum well (QW) structure with finite electron density are calculated for electron-electron (e-e) and electron-polar optic phonon (e-pop) scattering. We find that the capture time oscillates as function of the QW width for both processes with the same period, but with very different amplitudes. For an electron density of 1011 cm−2 the e-e capture time is 101−103 times larger than the e-pop capture time except for QW widths near the resonance minima, where it is only 2−3 times larger. With increasing density the e-e capture time decreases and near the resonance becomes smaller than the e-pop capture time. Our e-e capture times are three orders larger than the results of Blom et al. [Appl. Phys. Lett. 62, 1490 (1993)]. The role of the e-e capture in QW lasers is therefore readdressed.
Dynamic range of an electro-optic field sensor and its imaging applications
Authors: Wu Q. , Sun F. G. , Campbell P. , Zhang X.-C.
Published: July 2002

We report the measurement of the dynamics of free-space electro-optic field sensors for pulsed electromagnetic wave radiation. With an optical probe power spanning six decades of linearity and excellent signal-to-noise ratio (SNR), it is feasible to convert a far-infrared 2D image into an optical 2D image. A simple estimation indicates that 100 mW of optical probe power can achieve an image of 256×256 pixels with a 50 pA signal current per pixel and a SNR ≳200. We also present a comparison measurement of an ultrafast photoconductive antenna and an electro-optic sensor crystal.
Characterization of diffusion length degradation in Czochralski silicon solar cells
Authors: Reiss J. H. , King R. R. , Mitchell K. W.
Published: July 2002

Commercially produced, unencapsulated, Czochralski (CZ) silicon solar cells can lose 3%–4% (relative) of their initial efficiency after exposure to light, after minority-carrier injection during dark forward bias, or after thermal treatment at 100–400 °C. All three degradation methods reduce the minority-carrier diffusion length in the cell substrate. Under light, the decrease in efficiency is rapid (<30 min at 1 sun), but the cell power remains stable thereafter. The effect is completely reversible and the cell performance recovers in <12 h in the dark at room temperature. The various conditions under which CZ silicon cells degrade, and the reverse process, annealing, are characterized by spectral response and current–voltage (I–V) measurements. Iron impurities are a possible cause of this effect.
Ultrafast magneto-optic sampling of picosecond current pulses
Authors: Elezzabi A. Y. , Freeman M. R.
Published: July 2002

We report the demonstration of a magneto-optic sampling technique that allows for the detection of picosecond electrical pulses propagating down a microstrip transmission line. A unique feature of the method is that it directly measures the current in ultrashort electrical pulses. The technique is based on time-resolved detection of the magneto-optic Faraday effect in a Bi-substituted yttrium-iron-garnet (Bi-YIG) film. The detailed magnetic response is found to be in excellent agreement with the Bloch-Bloembergen equations. The bandwidth of the sampling system has been demonstrated to scale with the ferromagnetic resonance frequency up to 82.3 GHz, with a potential for increase towards 1 THz.
Low threshold PbEuSeTe/PbTe separate confinement buried heterostructure diode lasers
Authors: Feit Z. , McDonald M. , Woods R. J. , Archambault V. , Mak P.
Published: July 2002

Continuous wave (cw) operating temperature of 223 K was achieved with molecular beam epitaxy grown separate confinement buried heterostructure (SCBH) PbTe diode lasers with PbEuSeTe electrical and optical confinement layers. This is the highest cw operating temperature reported for midinfrared diode lasers. The active region of the SCBH diode lasers varies laterally to form a crescent-shaped waveguide with a maximum thickness of 0.15 μm and a lateral width of 2 μm. Exceptionally low threshold currents of 102 mA at 200 K, 166 mA at 210 K, and 249 mA at 215 K were measured.
Competitive gettering of copper in Czochralski silicon by implantation-induced cavities and internal gettering sites
Authors: McHugo Scott A. , Weber E. R. , Myers S. M. , Petersen G. A.
Published: July 2002

The effectiveness of copper gettering by implantation-induced cavities in competition with internal gettering sites in silicon was demonstrated. The cavities were formed in the near surface region by He implantation and annealing while the internal gettering sites were created in the material’s bulk by a ramped hi–lo–hi oxygen precipitation heat treatment. Ion implantation was used to controllably introduce the copper. The quantity of implanted copper was below that corresponding to saturation of solution throughout the wafer at the gettering temperatures of 700 and 800 °C. The cavities were found to be an effective gettering site in the presence of internal gettering sites with only a small amount of copper being gettered at the internal gettering sites. These results have important implications for optimal gettering of metallic impurities from integrated circuit device regions.
Long wavelength infrared (λ≂11 μm) quantum cascade lasers
Authors: Sirtori C. , Faist J. , Capasso F. , Sivco D. L. , Hutchinson A. L. , Cho A. Y.
Published: July 2002

AlInAs/GaInAs quantum cascade (QC) lasers operating at λ=11.2 μm wavelength are reported. In pulsed operation the peak power is in excess of 50 mW at 110 K heat sink temperature and the devices have been operated at temperatures as high as 200 K. Continuous wave single-mode operation with powers ≂7 mW at 10 K has been achieved. This work, combined with our previous reports on QC lasers, demonstrates that these new light sources can be tailored, by suitable quantum design, over a wide wavelength range (4–11 μm) using the same heterostructure material.
Mapping electrically active dopant profiles by field-emission scanning electron microscopy
Authors: Turan R. , Perovic D. D. , Houghton D. C.
Published: July 2002

Secondary electron (SE) image contrast from p-type silicon has been studied using field-emission scanning electron microscopy (FE-SEM). Cross-sectional FE-SEM images of boron-doped silicon heterostructures have been compared with atomic concentration and free carrier profiles measured by secondary-ion mass spectroscopy and electrochemical capacitance-voltage profiling, respectively. FE-SEM image contrast due to dopants has been shown to be electronic in origin. Since electrically active dopant species contribute solely to SE image contrast, FE-SEM can be effectively used to map electrically active dopant profiles in two dimensions with a sensitivity as low as 1016 cm−3.
Laser induced stimulated emission for hydrogen atom density measurements in a hydrogen pulsed microwave discharge
Authors: Tomasini L. , Rousseau A. , Baravian G. , Gousset G. , Leprince P.
Published: July 2002

The laser induced stimulated emission (LISE) of the Balmer α line is used to measure the ground state hydrogen atom density in an efficient source of hydrogen atoms generated by a low-pressure (1 Torr), pulsed microwave discharge at 2.45 GHz. The H atom kinetics in pure hydrogen discharges and in hydrogen–air mixture discharges are compared. It is shown that, for high atomic density and long plasma length crossing, the LISE Balmer α line is partially absorbed by the plasma. This provides an estimation of the H(n=2) atom density.
Enhanced formation of the C54 phase of TiSi2 by an interposed layer of molybdenum
Authors: Mouroux A. , Zhang S. -L. , Kaplan W. , Nygren S. , Östling M. , Petersson C. S.
Published: July 2002

The phase formation during rapid thermal annealing of a Ti/Mo bilayer sequentially deposited on Si substrates has been studied. The Mo layer varied from 0.5 to 2 nm and the Ti layer was always 60 nm thick. The presence of the Mo interposing layer enhances the formation of the C54 of TiSi2 by first forming a Mo-bearing silicide phase of hexagonal structure. The desired C54 phase then nucleates and grows on top of this Mo-bearing silicide phase at a temperature as low as 650 °C via Si diffusion through the growing silicide layers. This is about 100 °C lower than what is usually needed for the C49–C54 transformation. The significance of this finding is that the usual route for the formation of TiSi2, i.e., the C49 phase forms as a result of the Ti–Si interaction and the C54 phase forms as the product of phase transformation, is altered by the interposition of a thin refractory metal (here Mo) layer between Ti and Si.
Transient photoconductivity in quantum well infrared photodetectors
Authors: Ershov M.
Published: July 2002

Transient photoconductivity in quantum well infrared photodetectors (QWIPs) is studied using numerical modeling. Transient photocurrent in QWIPs illuminated by steplike infrared radiation consists of two components. A fast component, representing the primary photocurrent, is determined by the processes of the carrier capture to the QWs and transit through the QW structure. Its amplitude can be lower than the steady-state photocurrent due to the sweep-out of the photoexcited carriers. A slow component, comprising the multiplied photocurrent, is governed by the recharging of the QWs. The QWIPs can exhibit high photocurrent gain only at frequencies of harmonic infrared radiation lower than the inverse characteristic time of establishing equilibrium at the injecting contact.
An approach to threading dislocation ‘‘reaction kinetics’’
Authors: Romanov A. E. , Pompe W. , Beltz G. E. , Speck J. S.
Published: July 2002

An approach is developed to describe the evolution of threading dislocation (TD) densities in lattice-mismatched epitaxial films. TD ensembles are treated in close correspondence to chemical species in chemical reaction kinetics. ‘‘Reaction rate’’ equations are derived for changing TD density with increasing film thickness for first- and second-order reactions. Selective area growth is an example of a first-order reaction. TD annihilation, fusion, and scattering are examples of second-order reactions. Analytic models are derived for TD behavior in relaxed homogeneous buffer layers, selective area growth, and strained layers.
Carrier capture in quantum well embedded quantum wire structures
Authors: Mansour N. S. , Sirenko Yu. M. , Kim K. W. , Littlejohn M. A. , Stroscio M. A.
Published: July 2002

We propose a novel quantum wire (QWR) laser structure with improved carrier capture characteristics, where the carriers are injected into a quantum well (QWL) and subsequently recombine within an embedded QWR. The corresponding electron capture rates via polar optical phonon scattering are calculated for this system. An oscillatory behavior of the electron capture rate is observed as a function of the QWR thickness at the temperatures considered (77 K and 300 K). The amplitude of these oscillations also increases as the QWR width decreases. Our calculations show that the electron capture rate in the QWL embedded QWR structure can be improved by more than 30% when compared to single QWLs. Therefore, the proposed QWR laser design provides improved modulation bandwidth and optical gain over conventional QWL and QWR structures.
In situ investigation of degradation in polymeric electroluminescent devices using time-resolved confocal laser scanning microscope
Authors: Do Lee-Mi , Kim Kwangjoon , Zyung Taehyoung , Shim Hong-Ku , Kim Jang-Joo
Published: July 2002

Degradation progress in polymeric electroluminescent devices during device operation was clarified by in situ measurement of photoluminescence image change using time resolved confocal laser scanning microscope. It is found that the degradation stems from two kinds of dark spots, which have the dented circle shape and the dome shape. Dented circle shapes grow during the operation. Dome shapes, where the polymer layers are detached from the indium-tin-oxide electrode, appear suddenly and do not change in size. Comparison of the Auger analysis depth profiles of the dented and the normal sites indicates that the polymer layer at the dented sites disappears. Time-resolved confocal laser scanning microscopy is found to be a powerful technique to analyze the degradation process.
Diameter dependence of current–voltage characteristics of ultrasmall area AlSb–InAs resonant tunneling diodes with diameters down to 20 nm
Authors: Nomoto K. , Taira K. , Suzuki T. , Hase I. , Hiroshima H. , Komuro M.
Published: July 2002

We have studied the current–voltage characteristics I(V) of ultrasmall area AlSb–InAs resonant tunneling diodes (RTDs) with diameters down to 20 nm. Resonant tunneling peaks were observed for all the diodes at room temperature. The peak-to-valley ratio reduces with the decreasing diameter of the RTD. We found from the diameter dependence of the valley current that the reduction is due to a contribution of the thermally activated surface current to the valley current. For RTDs with diameters less than 100 nm, we observed fine structures around zero bias at 4 K. They can be attributed to tunneling through zero-dimensional states confined by a RTD sidewall.
Strong pyroelectric response in semiconducting Y-Ba-Cu-O and its application to uncooled infrared detection
Authors: Jahanzeb A. , Travers C. M. , Butler D. P. , Çelik-Butler Z. , Gray J. E.
Published: July 2002

A pyroelectric infrared detector operating at room temperature and based on semiconducting Y-Ba-Cu-O was fabricated. The capacitor structure consisting of semiconducting Y-Ba-Cu-O layer sandwiched between two Nb electrodes was fabricated on top of a thermally isolated SiO2 bridge. The polycrystalline Y-Ba-Cu-O layer was deposited by ambient-temperature sputtering while standard Si micromachining techniques were used to etch the silicon under the oxide bridge. The pyroelectric response of the material is characterized by the temperature coefficient of polarization (p=dP/dT) which was measured to be as high as 65 nC/K cm2 without an externally applied electric field. The pyroelectric figure of merit Fd for the material was estimated to be 0.032 (cm3/J)1/2. Employing a radiometric infrared source, the room-temperature voltage responsivity RV and specific detectivity D* of the unbiased detector were measured to be over 103 V/W and 108 cm Hz1/2/W, respectively. This letter discusses the characteristics of this complementary metal-oxide semiconductor-compatible pyroelectric detector and suggests methods for the improvement for its figures of performance.
Quantitative analysis of magnetization reversal based on time-dependent domain patterns
Authors: Choe Sug-Bong , Shin Sung-Chul
Published: July 2002

A method to simultaneously determine both the wall-motion speed and the nucleation probability of magnetic films is presented, where the domain reversal patterns are modelized by time-dependent circular domains formed by the nucleation process and then, expanded by the wall-motion process. The present method has been applied to understand the contrasting reversal behaviors in the Co/Pd multilayered system and the difference in the nucleation probabilities of the samples was found to be a major origin for the observed contrasting behaviors in this system.
Nanoscale silicon field effect transistors fabricated using imprint lithography
Authors: Guo Lingjie , Krauss Peter R. , Chou Stephen Y.
Published: July 2002

We report the fabrication and characterization of nanoscale silicon field effect transistors using nanoimprint lithography. With this lithographic technique and dry etching, we have patterned a variety of nanoscale transistor features in silicon, including 100 nm wire channels, 250-nm-diam quantum dots, and ring structures with 100 nm ring width, over a 2×2 cm lithography field with good uniformity. Compared with devices fabricated by the conventional electron-beam lithography, we did not observe any degradation in the device characteristics. The successful fabrication of the semiconductor nanodevices represents a step forward to make nanoimprint lithography a viable technique for the mass production of semiconductor devices.
Si single electron tunneling transistor with nanoscale floating dot stacked on a Coulomb island by self-aligned process
Authors: Nakajima Anri , Futatsugi Toshiro , Kosemura Kinjiro , Fukano Tetsu , Yokoyama Naoki
Published: July 2002

We fabricated a Si single electron tunneling transistor which has a nanoscale floating dot gate stacked on a Coulomb island by a self-aligned process. This device exhibits drain current (Id) oscillations due to the Coulomb blockade effect and quantized threshold voltage (Vth) shifts resulting from a single electron tunneling from the channel to the floating dot gate. The high on/off current ratio of the Id oscillation combined with the quantized Vth shifts leads to the possibility of developing ultralow power consumption memory.
Polarization responses in lead magnesium niobate based relaxor ferroelectrics
Authors: Zhang Q. M. , Zhao J.
Published: July 2002

Changes in the electrostrictive coefficients Qij, especially the volumetric coefficient, with temperature and bias field provides important information regarding the nature of the polarization in lead magnesium niobate based relaxor ferroelectrics. We show that the polarization response at temperatures near the dielectric constant maximum is mainly through the polar-vector reorientation of the nanopolar regions, as suggested by the polar glass model. As the temperature is lowered through the freezing transition, the polarization response is governed by the phase switching and intrinsic contributions rather than by the domain wall motions found in normal ferroelectrics.
Transport properties and colossal magnetoresistance in epitaxial La0.67Cd0.33MnO3 thin film
Authors: Sahana M. , Hegde M. S. , Vasanthacharya N. Y. , Prasad V. , Subramanyam S. V.
Published: July 2002

An optimal composition of La0.67Cd0.33MnO3 was synthesized by ceramic route. The compound crystallized in a rhombohedral structure with lattice parameters a=5.473(4) Å and α=60°37′. Resistivity measurement showed an insulator-to-metal transition coupled with a ferromagnetic transition of around 255 K. Epitaxial thin films were fabricated on the LaAlO3 (100) substrate by a pulsed laser deposition technique. The psuedocubic lattice parameter a of the film is 3.873(4) Å. The insulator-to-metal transition of the film was observed at 250 K which is comparable with the bulk value. The film was ferromagnetic below this temperature. Magnetoresistance defined as ΔR/R0=(RH−R0)/R0 was over −86% near the insulator-to-metal transition temperature of 240 K at 6 T magnetic field and over-30% at relatively low fields of 1 T. No magnetoresistance was observed at low temperatures in the film unlike in the polycrystalline sample, where about a 40% decrease in resistance was observed on applying 6 T magnetic field due to the spin dependent scattering at the grain boundaries.
A “smarter-cut” approach to low temperature silicon layer transfer
Authors: Tong Q.-Y. , Scholz R. , Gösele U. , Lee T.-H. , Huang L.-J. , Chao Y.-L. , Tan T. Y.
Published: July 2002

Silicon wafers were first implanted at room temperature by B+ with 5.0×1012 to 5.0×1015 ions/ cm2 at 180 keV, and subsequently implanted by H2+ with 5.0×1016 ions/cm2 at an energy which locates the H-peak concentration in the silicon wafers at the same position as that of the implanted boron peak. Compared to the H-only implanted samples, the temperature for a B+H coimplanted silicon layer to split from its substrate after wafer bonding during a heat treatment for a given time is reduced significantly. Further reduction of the splitting temperature is accomplished by appropriate prebonding annealing of the B+H coimplanted wafers. Combination of these two effects allows the transfer of a silicon layer from a silicon wafer onto a severely thermally mismatched substrate such as quartz at a temperature as low as 200 °C.
Mechanism for reducing recovery time of optical nonlinearity in semiconductor laser amplifier
Authors: Usami Masashi , Tsurusawa Munefumi , Matsushima Yuichi
Published: July 2002

We propose and demonstrate a novel scheme to reduce the recovery time of optical nonlinearity in a semiconductor laser optical amplifier driven under a loss condition for the signal light pulse. Additional light, which is set at a transparency wavelength in the active layer, promotes stimulated recombination of excess carriers induced by the absorption of the signal light. This scheme excludes any additional carrier transport mechanism and nonradiative recombination and hence generation of heat. The principle of operation is experimentally verified by measuring time-domain transmission variance using a pump–probe method. A drastic reduction of the excess-carrier lifetime to less than 70 ps was confirmed.