bluuebunny/crossref_metadata_2025 huggingface dataset:
Possible degradation mechanism in ZnS:Mn alternating current thin-film electroluminescent display
Authors: Lee Yun Hi , Chung In-Jae , Oh Myung-Hwan
Published: July 2002

To study origins of alternating current thin-film electroluminescent display (AC-TFELD) degradation phenomena, we have fabricated two different types of AC-TFELD with multilayered insulators. For an accelerated aging, high-frequency voltages were applied and brightness versus operating time characteristics were measured. Surface morphology (SEM), Auger electron spectroscopy (AES) in-depth profiles, and AES spectra were examined for the aged devices with and without Si3N4 interlayers. From the obtained results, we proposed that the formation of zinc-oxy-sulfides in ZnS:Mn-insulator at the ITO side may play a significant role in the aging phenomena and degradation of brightness (B)-operating time (T) characteristics for our devices. Therefore, we suggest that the dielectric interlayer of stable stochiometry must be inserted in ZnS:Mn-insulator at the In2O3-SnO2(ITO) side to improve the aging characteristics.
Third-order nonlinear optical properties of thin films of poly(p-phenylene benzobisthiazole) and its molecular composites with polyamides
Authors: Vanherzeele Herman , Meth Jeffrey S. , Jenekhe Samson A. , Roberts Michael F.
Published: July 2002

The nonresonant third-order nonlinear optical properties of thin films of poly(p-phenylene benzobisthiazole) (PBZT) and PBZT/nylon 66 and PBZT/poly(trimethylhexamethylene terephthalmide) (PTMHT) molecular composites have been investigated using picosecond third-harmonic generation at 1.9 μm. The measured χ(3)(−3ω,ω,ω,ω) for the pure PBZT was (1.37±0.27)×10−11 esu. The χ(3) of PBZT/nylon composites showed a linear dependence on composition. In contrast, the χ(3) of PBZT/PTMHT molecular composites significantly deviated from this linear dependence, showing enhanced values. No in-plane anisotropy in any of the films was detected. The optical damage threshold of PBZT was measured to be ≳50 GW/cm2 for 30 ps pulses at 1.9 μm.
Self-consistent model for two-dimensional accumulation layer states in resonant tunneling devices
Authors: Fiig Thomas , Jauho A. P.
Published: July 2002

We present a simple model which allows a simultaneous and self-consistent treatment of extended and size-quantized states in biased double-barrier systems. Our model preserves charge neutrality in the asymptotic regions, and the self-consistent charge density is free of unphysical cusps, found in simple screening models. We discuss the implications of the 2D emitter states to the current-voltage characteristics.
Anomalous behavior of ion-implanted GaSb
Authors: Callec R. , Favennec P. N. , Salvi M. , L’Haridon H. , Gauneau M.
Published: July 2002

Anomalous elevations up to 6 μm of the ion-implanted GaSb surface were observed. This swelling phenomenon is related to the formation of a porous layer and is dependent on the mass, energy, and dose of the implanted ions. A strong amount of oxygen was measured in the porous layers but this oxygen is likely not responsible for the swelling. The behavior of implanted GaSb is very similar to that of InSb previously described.
Rapid thermal annealing induced order-disorder transition in Ga0.52In0.48P/(Al0.35Ga0.65)0.5In0.5P heterostructures
Authors: Hämisch Y. , Steffen R. , Forchel A. , Röntgen P.
Published: July 2002

The thermally induced transition from the ordered to the disordered state of Ga0.52In0.48P/(Al0.35Ga0.65)In0.5P layers was investigated by means of rapid thermal annealing and photoluminescence spectroscopy. The annealing temperature and annealing time dependence of the luminescence was studied in the temperature range from 500 up to 1050 °C and for annealing times between 5 and 600 s. Within a very small temperature range of less than 40° we observe an emission energy blue shift of the GaInP luminescence band by about 100 meV. The shift occurs due to a complete disordering of the previously ordered layers without a simultaneous destruction of the heterostructure. The photoluminescence of the quaternary AlGaInP barrier was also observed and shows a simultaneous blue shift of about 90 meV due to an order-disorder transition.
Tunneling effective mass in hydrogenated amorphous silicon
Authors: Shannon J. M. , Nieuwesteeg K. J. B. M.
Published: July 2002

The tunneling effective mass of electrons in undoped a-Si:H has been determined from measurements on Schottky diodes operating with high reverse fields. Under these conditions, the change of current with electric field is a sensitive function of effective mass. The tunneling effective mass was measured to be 0.09±0.02 me for a range of different samples giving a tunneling constant of ≊40 Å.
Hole generation by icosahedral B12 in high-dose boron as-implanted silicon
Authors: Mizushima I. , Watanabe M. , Murakoshi A. , Hotta M. , Kashiwagi M. , Yoshiki M.
Published: July 2002

It was found that a high concentration of holes was generated without any post-annealing by boron ion implantation into silicon in the high-dose region of more than 1×1016 cm−2. X-ray photoelectron spectroscopy and Fourier transform infrared absorption spectrum revealed that B12 icosahedra were created just after implantation. The generation of holes can be explained by the model in which B12 icosahedra act as a double acceptor.
Very high mobility two-dimensional hole gas in Si/GexSi1−x/Ge structures grown by molecular beam epitaxy
Authors: Xie Y. H. , Monroe Don , Fitzgerald E. A. , Silverman P. J. , Thiel F. A. , Watson G. P.
Published: July 2002

Modulation-doped Si/GexSi1−x/Ge/GexSi1−x structures were fabricated in which a thin Ge layer was employed as the conduction channel for the two-dimensional hole gas. The strained heterostructure was fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The best mobility of the two-dimensional hole gas is 55 000 cm2/V s at 4.2 K with a concentration-dependent hole effective mass of ≤0.10m0. The effect of the Ge/GeSi interface roughness on the 2DHG mobility was studied.
Valence-band discontinuity between GaN and AlN measured by x-ray photoemission spectroscopy
Authors: Martin G. , Strite S. , Botchkarev A. , Agarwal A. , Rockett A. , Morkoç H. , Lambrecht W. R. L. , Segall B.
Published: July 2002

The valence-band discontinuity at a wurtzite GaN/AlN(0001) heterojunction is measured by x-ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence-band maximum in both GaN and AlN bulk films. The precise location of the valence-band maximum is determined by aligning prominent features in the valence-band spectrum with calculated densities of states. Tables of core level binding energies relative to the valence-band maximum are reported for both GaN and AlN. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on AlN and vice versa yield a valence-band discontinuity of ΔEV=0.8±0.3 eV in the standard type I heterojunction alignment.
1.3 μm to visible upconversion in Dy3+- and Er3+-codoped BaCl2 phosphor
Authors: Ohwaki Junichi , Wang Yuhu
Published: July 2002

1.3 μm to visible upconversion is observed in a BaCl2:Dy,Er phosphor system. Yellow fluorescence is visible under 1.3 μm laser diode (5 mW) irradiation. Radiative transitions from the 4S3/2 and 4F9/2 levels to the ground level 4I15/2 of the Er3+ ions cause fluorescence lines at 550 and 670 nm, respectively. This indicates that Dy3+ ions act as absorption centers at 1.3 μm (6H15/2-6H9/2 transition) and also as energy donors to the Er3+ ions. We conclude that the suppression of multiphonon relaxation from the 6H9/2 excited level to the lower levels of Dy3+ resulting from the low phonon energy chloride matrix, plays an important role in the upconversion process.
Low pressure metalorganic chemical-vapor deposition of cubic GaN over (100) GaAs substrates
Authors: Kuznia J. N. , Yang J. W. , Chen Q. C. , Krishnankutty S. , Asif Khan M. , George T. , Frietas J.
Published: July 2002

We report on the low pressure metal organic chemical-vapor deposition of single crystal cubic GaN films over (100) GaAs substrates. Using photoluminescence and direct optical absorption measurements we estimate the band gap for c-GaN at room temperature to be 3.3 eV. Reflection high energy electron diffraction, x-ray, transmission electron microscopy, optical absorption, and room-temperature photoluminescence data are presented to establish the quality of a 0.8-μm-thick cubic GaN film over (100) GaAs substrate. Preliminary measurement results for the carrier density and mobility of the as-deposited c-GaN film are also presented.
Evaluation of the scaling exponent of self-affine fractal surface from a single scanning probe microscope image
Authors: Rao M. V. H. , Mathur B. K. , Chopra K. L.
Published: July 2002

A direct method of evaluating the scaling exponent H of a self-affine fractal surface from a single topographic image recorded by using scanning probe microscopy is presented. This method uses the fact that for these surfaces the root-mean-square roughness increases with length as LH. The method is applied to study the fractal nature of silver films of different thicknesses and it is observed that the variation in the scaling exponent H is in correlation with the surface roughness and the observed growth mode.
Nonlinear absorption and refraction of quantum confined InP nanocrystals grown in porous glass
Authors: Dvorak M. D. , Justus B. L. , Gaskill D. K. , Hendershot D. G.
Published: July 2002

Single-beam Z-scan experiments have been used to measure the two-photon absorption coefficient β and bound electronic nonlinear refractive index γ of composites of InP nanocrystals grown in 40 and 150 Å Vycor porous glass. These materials were also studied with two-beam time-resolved Z scans to confirm that the nonlinearities are instantaneous with respect to the ∼100 ps pulses used. The magnitudes of the nonlinearities in the 150 Å sample are found to be similar to those of bulk InP when scaled by the volume fraction of deposited material. For the 40 Å sample the ratio γ/β is enhanced by a factor of fourteen compared to bulk InP.
Instrument to collect fogwater for chemical analysis
Authors: Jacob Daniel J. , Waldman Jed M. , Haghi Mehrdad , Hoffmann Michael R. , Flagan Richard C.
Published: July 2002

An instrument is presented which collects large samples of ambient fogwater by impaction of droplets on a screen. The collection efficiency of the instrument is determined as a function of droplet size, and it is shown that fog droplets in the range 3–100-μm diameter are efficiently collected. No significant evaporation or condensation occurs at any stage of the collection process. Field testing indicates that samples collected are representative of the ambient fogwater. The instrument may easily be automated, and is suitable for use in routine air quality monitoring programs.
Controlled manipulation of nanoparticles with an atomic force microscope
Authors: Junno T. , Deppert K. , Montelius L. , Samuelson L.
Published: July 2002

We report on the application of the atomic force microscope (AFM) to manipulate and position nanometer-sized particles with nanometer precision. The technique, which can be regarded as a nanometer-scale analogy to atomic level manipulation with the scanning tunneling microscope, allowed us to form arbitrary nanostructures, under ambient conditions, by controlled manipulation of individual 30 nm GaAs particles. A whole new set of nanodevices can be fabricated particle-by-particle for studies of quantum effects and single electron tunneling. We also demonstrate a method, based on the AFM manipulation, to determine the true lateral dimensions of nano-objects, in spite of the tip-sample convolution.
Cryogenic GaAs FET amplifiers and their use in NMR detection
Authors: Richards M. G. , Andrews A. R. , Lusher C. P. , Schratter J.
Published: July 2002

The noise properties of GaAs FET devices have been measured in the frequency range 0.2–20 MHz at temperatures 300, 77, and 4.2 K. Significant reduction in all contributions to noise parameters was found on cooling the devices. Cooled GaAs FETs were found to be suitable for use in the preamplifier of a pulsed NMR spectrometer. An amplifier noise temperature of 0.9 K was found at 1.16 MHz and the observed signal-to-noise ratio using a 3He gas sample was found to agree with calculation. Precise expressions for signal size are obtained using a spherical receiver coil, and employment of these facilitates the calculations and provides a basis for comparing sensitivities of different NMR spectrometers.
High-resolution duo-multichannel soft x-ray spectrometer for tokamak plasma diagnostics
Authors: Schwob J. L. , Wouters A. W. , Suckewer S. , Finkenthal M.
Published: July 2002

A high-resolution, time-resolving soft x-ray multichannel spectrometer (SOXMOS) that permits the simultaneous measurement of emission in two different spectral ranges has been developed and tested extensively for tokamak plasma diagnostics. The basic instrument is a high-resolution, interferometrically adjusted, extreme grazing incidence Schwob–Fraenkel duochromator. The instrument is equipped with two multichannel detectors that are adjusted interferometrically and scan along the Rowland circle. Each consists of an MgF2 coated, funneled microchannel plate, associated with a phosphor screen image intensifier that is coupled to a 1024-element photodiode array by a flexible fiber-optic conduit. The total wavelength coverage of the instrument is 5–340 Å with a measured resolution (FWHM) of about 0.2 Å when equipped with a 600-g/mm grating, and 5–85 Å with a resolution of about 0.06 Å using a 2400-g/mm grating. The simultaneous spectral coverage of each detector varies from 15 Å at the short wavelength limit to 70 Å at the long wavelength limit with the lower dispersion grating. The minimum readout time for a full spectral portion is 16 ms, but several individual lines can be measured with 1-ms time resolution by selected pixel readout. Higher time resolution can be achieved by replacing one multichannel detector with a single channel electron multiplier detector. Examples of data from the PLT and TFTR tokamaks are presented to illustrate the instrument’s versatility, high spectral resolution, and high signal-to-noise ratio even in the 10-Å region.
Application of FIR heterodyne detection to collective scattering measurements of tokamak microturbulence
Authors: Brower D. L. , Peebles W. A. , Kim S. K. , Luhmann N. C.
Published: July 2002

Multichannel collective scattering of far-infrared (FIR) laser radiation is employed on the TEXT tokamak to investigate small-scale turbulent density fluctuations. A twin-frequency optically pumped FIR laser operating at 1222 μm (≂20 mW) and a difference frequency ΔωIF/2π≂1 MHz is used for the source and local oscillator beams. This permits heterodyne detection of the frequency-shifted scattered radiation which is necessary to determine wave propagation direction of plasma density fluctuations. The need for heterodyne versus homodyne detection is demonstrated. Such a diagnostic capability has allowed preliminary identification of the ion pressure-gradient-driven instability (whose most notable signature is that of turbulent fluctuations propagating in the ion diamagnetic drift direction) which is potentially responsible for saturation of the global energy confinement time with increasing density in tokamak plasmas.
Dynamic method for i
n s
i
t
u measurement of the thermoelectric power of vapor quenched thin films
Authors: Compans E.
Published: July 2002

We present an improved dynamic technique for measuring the thermoelectric power and present an apparatus designed for in situ measurement of quench-condensed films in the temperature range between liquid helium temperature and some 400 K. The main advantages of the method are a high measuring speed, insensitivity to spurious EMFs and offset drifts, high accuracy, and the ease to be computer controlled. The sample holder for the examination of vapor quenched films is described in detail. It can equally well be used for the examination of ribbons or thin solids.
A low field SQUID magnetometer system for magnetic characterization of high-T
c superconducting samples
Authors: Vandervoort K. G. , Griffith G. , Claus H. , Crabtree G. W.
Published: July 2002

In this article we describe a simple SQUID magnetometer system useful for the magnetic characterization of high temperature superconductors. The system employs a commercial rf SQUID probe with associated electronics located in a 4He gas exchange cryostat. The system is equipped with a copper coil dc magnet for fields up to 50 Gauss. The cryostat is surrounded by a μ-metal shield that keeps the remanent field to less than 20 mGauss. The apparatus has been used extensively to measure both the superconducting transition temperature and width as well as the relative quantities of magnetic flux expulsion and exclusion. It has therefore served as an important first step in the evaluation of sample quality.
The Berkeley tunable far infrared laser spectrometers
Authors: Blake G. A. , Laughlin K. B. , Cohen R. C. , Busarow K. L. , Gwo D.-H. , Schmuttenmaer C. A. , Steyert D. W. , Saykally R. J.
Published: July 2002

A detailed description is presented for a tunable far infrared laser spectrometer based on frequency mixing of an optically pumped molecular gas laser with tunable microwave radiation in a Schottky point contact diode. The system has been operated on over 30 laser lines in the range 10–100 cm−1 and exhibits a maximum absorption sensitivity near one part in 106. Each laser line can be tuned by ±110 GHz with first-order sidebands. Applications of this instrument are detailed in the preceding paper.
An ultrahigh vacuum single crystal adsorption microcalorimeter
Authors: Borroni-Bird C. E. , King D. A.
Published: July 2002

The design of an ultrahigh vacuum microcalorimeter enabling calorimetric heats of adsorption to be obtained on single crystal surfaces as a detailed function of coverage is discussed. The system comprises a pulsed supersonic molecular beam source, an ultrathin metal single crystal, and remote infrared temperature sensing. Sticking probabilities and coverages are determined pulsewise by the King and Wells method, and heat capacity calibrations are conducted in situ by laser beam pulsing. Results for CO and O2 on Ni{110} demonstrate excellent sensitivity to adsorption of ∼10−13 moles of gas (∼0.01 monolayer). The heat capacity of the calorimeter is 4.2 μJ K−1.
Development of large area rf ion sources for fusion applications
Authors: Wells S. B. , Takeiri Y. , Newman A. F. , McAdams R. , Holmes A. J. T.
Published: July 2002

A rf ion source with a large extraction area suitable for fusion applications is considered. The source is a metallic magnetic multipole bucket with an internal antenna to couple the rf (at a frequency of 2 MHz) to the plasma. The source is diagnosed by an array of Langmuir probes to measure uniformity, density, and temperature and a small probe accelerator to extract a negative ion beam. A two-turn antenna is found to couple more power into the plasma than a half-turn antenna. When operated as a negative ion source at low powers, the source efficiency is found to be high and not to have a strong dependence on gas pressure. An extracted negative ion current density of about 4 mA/cm2 at 7 mTorr was obtained when 3 kW of power was coupled into the plasma.
A fast scanning probe for DIII–D
Authors: Watkins J. G. , Salmonson J. , Moyer R. , Doerner R. , Lehmer R. , Schmitz L. , Hill D. N.
Published: July 2002

A fast reciprocating probe has been developed for DIII–D which can penetrate the separatrix during H mode with up to 5 MW of NBI heating. The probe has been designed to carry various sensor tips into the scrape-off layer at a velocity of 3 m/s and dwell motionless for a programmed period of time. The driving force is provided by a pneumatic cylinder charged with helium to facilitate greater mass flow. The first series of experiments have been done using a Langmuir probe head with five graphite tips to measure radial profiles of ne, Te, φf, ñe, and φ̃f. The amplitude and phase of the fluctuating quantities are measured by using specially constructed vacuum compatible 5-kV coaxial transmission lines which allow us to extend the measurements into the MHz range. TTZ ceramic bearings and fast stroke bellows were also specially designed for the DIII–D probe. Initial measurements will be presented.
A pulsed microchannel-plate-based non-neutral plasma imaging system
Authors: Peurrung A. J. , Fajans J.
Published: July 2002

The design and operation of a pure electron plasma imaging system is described. The system yields images with high resolution and a large dynamic range. A two-dimensional image of the plasma is acquired by streaming the plasma directly onto a microchannel plate and phosphor screen. Resolution of the rapidly evolving plasma requires that all the plasma strike the microchannel plate in a single charge pulse lasting less than 1 μs. The expected performance and actual operational characteristics of the system are detailed, with special attention given to the properties of a microchannel plate used in a pulsed imaging mode.
Orientation effect in electronic properties of silicon wires
Authors: Filonov A. B. , Petrov G. V. , Novikov V. A. , Borisenko V. E.
Published: July 2002

Electronic properties of (100), (110), and (111) oriented H-terminated silicon quantum-size wires have been calculated within the self-consistent LCAO method. The quantum confinement induced direct band gap only appears in the (100) wires. Surface silicon d and p electrons are found to be responsible for the bottom of the conduction band while the top of the valence band are formed by p electrons of the core atoms. Possible reconstruction of the wire surface is discussed.
Charge exchange of hydrogen atoms in carbon foils at 0.4–120 keV
Authors: Gonin M. , Kallenbach R. , Bochsler P.
Published: July 2002

Charge exchange properties of H atoms passing through thin carbon foils at incident energies from 0.5 to 120 keV/u are discussed in the context of charge transfer models. A model is presented in which the charge state equilibrium in the solid is explained by the overlap of the atomic and the solid-state electron wave functions in k space. Outside the solid, near the surface, charge exchange occurs by tunneling of electrons between the carbon surface and the exiting projectile.
Shear cell for the study of liquid-solid interfaces by neutron scattering
Authors: Baker Shenda M. , Smith Gregory , Pynn Roger , Butler Paul , Hayter John , Hamilton William , Magid Lee
Published: July 2002

A cell for examining the density profile of sheared fluids at the solid-liquid interface by neutron reflectometry is presented. This cell has also proven valuable in examining near-surface bulk structures in the plane perpendicular to the shear flow using small angle neutron scattering. The shear rates can be controlled by changing the volume flow through the cell over three orders of magnitude. All components of the cell are designed to be chemically inert. A temperature-controlled environment compatible with neutron studies is also briefly described. Preliminary neutron reflectivity and small angle neutron scattering results using this cell are presented, and potential applications are discussed.
Comparison between 3p and 2p magnetic circular dichroism in transition metals and alloys: Is the sum rule applicable to itinerant magnetic systems?
Authors: Miyahara Tsuneaki , Park Serng-Yerl , Hanyu Takaaki , Hatano Tadashi , Moto Sadatsugu , Kagoshima Yasushi
Published: July 2002

Magnetic circular dichroism of Ni-Pd, Co-Pd, and Fe-Pd alloy systems were measured in the 3p and 2p excitation regions of Ni, Co, and Fe and in the 4p and 3p excitation regions of Pd. It was found that the number of 3d holes and the orbital moment of 3d electrons show different values for the 3p and the 2p data. This suggests that interpretation of the sum rule for the orbital moment is not simple especially for itinerant magnetic systems. The observed difference, however, gives some additional information on the degree of localization of the magnetic moments.
Using cosmic rays to monitor large scintillator arrays
Authors: Knauer J. P. , Kremens R. L. , Russotto M. A. , Tudman S.
Published: July 2002

Large arrays of scintillator-photomultiplier detectors are becoming the technique of choice to measure neutron spectrum from ICF implosions. A 32×30 array of detectors is currently under construction at LLE (MEDUSA). This array is at an angle of 26° relative to vertical and thus cosmic rays can be used to monitor individual channel performance. We will present: an analysis of the expected count rates and expected signal levels for single scintillator-photomultiplier detectors; a comparison of the above analysis to a test string of 30 detectors mounted in the MEDUSA frame; and the triggering scheme used to acquire data for routine operation of the instrument.
Stress determination of micromembranes using laser vibrometry
Authors: Biswas Abhijit , Weller Tom , Katehi Linda P. B.
Published: July 2002

A laser vibrometry based dynamic technique for biaxial residual stress determination in micromembranes is reported. Layered, low pressure chemical vapor deposition grown, rectangular shaped micromembranes are ultrasonically excited. The resulting micromembrane displacement, as a function of frequency and position, is recorded to yield resonance frequencies and associated vibrational mode numbers. The stress is determined from this information. Vibrations forced from ultrasonic pressure (∼25 Pa) resulted in peak displacements of tens of nanometers at resonance. For the sample set studied, the (3,1) mode resonance peak displayed the least damping in atmospheric testing and was used to establish stress levels ∼108 N/m2. A trend for this stress showing an overall decrease by ∼40% as a result of thermomechanical cycling was identified.
Implementation of the continuous wavelet transform for digital time series analysis
Authors: Jordan D. , Miksad R. W. , Powers E. J.
Published: July 2002

The wavelet transform has emerged as an important tool for the analysis of intermittent and nonstationary signals. This article addresses implementation issues of a digital continuous wavelet transform that is sufficiently general that any continuous wavelet can be used. Working equations are derived for wavelet sampling, aliasing, and scale population by using the band-pass filter interpretation of wavelet functions. The implementation procedure is applied to the Morlet wavelet in detail as an example. Finally, the Morlet wavelet is applied to velocity fluctuations measured in a subsonic wake undergoing transition to turbulence.
Deposition of extremely thin (Ba,Sr)TiO3 thin films for ultra-large-scale integrated dynamic random access memory application
Authors: Hwang Cheol Seong , Park Soon Oh , Cho Hag-Ju , Kang Chang Suk , Kang Ho-Kyu , Lee Sang In , Lee Moon Yong
Published: July 2002

(Ba,Sr)TiO3 (BST) thin films with thicknesses ranging from 15 to 50 nm are prepared by a rf magnetron sputtering on Pt/SiO2/Si substrates. The dielectric constants of BST thin films increase with increasing deposition temperature and thicknesses. The leakage current increases with increasing deposition temperature and this prevents the deposition temperature of the 20 nm thick BST thin film from being increased to a value more than 640 °C. The leakage current is also critically dependent upon the postannealing temperature and atmosphere after the top electrode fabrication. The dielectric constant increases with increasing postannealing temperature which further reduces the SiO2 equivalent thicknesses of the BST thin films. A 20 nm thick BST thin film deposited at 640 °C and postannealed at 750 °C under N2 atmosphere for 30 min, shows a SiO2 equivalent thickness of 0.24 nm, dielectric dissipation factor less than 1%, and leakage current of about 40 nA/cm2 at ∓1.5 V.
Space and time resolved temperature measurements in laser pulse-produced metal melts
Authors: Otte D. , Kleinschmidt H. , Bostanjoglo O.
Published: July 2002

A fast broadband pyrometer with a spatial resolution of 2 μm and an accuracy of 10% in temperature is described plus a calibration method suitable for liquid metals. The time resolution of temperature measurements is limited by the electronics rise time (3.5 ns) to 14 ns (10%–90% rise time) of a temperature change because of the nonlinear relation between electronic signal and temperature. The pyrometer was applied to determine the temperature of melts produced by focused nanosecond laser pulses in gold and iron films. Measured heating rates (1010–1011 K/s) and peak temperatures (3000 K) are correctly reproduced by numerical simulations for gold, but theoretical peak temperatures in iron were 1000 K in excess. The low actual temperature in iron can be explained by native oxides, decomposing during the laser pulse and thus dissipating energy. This effect may cause considerable differences between computed and actual temperatures during laser processing of thin metal films with unstable impurities.
Poloidal polarimeter system for current density measurements in ITER
Authors: Donné A. J. H. , Edlington T. , Joffrin E. , Koslowski H. R. , Nieswand C. , Segre S. E. , Stott P. E. , Walker C.
Published: July 2002

The polarimeter system for ITER features 10 viewing chords covering almost the full poloidal cross section of the plasma in a fan-like arrangement. The 10 laser beams all enter the plasma via 150 mm diam penetrations through the diagnostic plug at the low-field side. Small retroreflectors at the high-field side mirror the laser beams back towards the penetrations at the low-field side. Extensive feasibility calculations have led to the conclusion that the optimum wavelength for the polarimeter is around 100 μm. At this wavelength counterproductive effects like refraction and electron cyclotron absorption are negligibly small. However, the anticipated Faraday rotation angles are still very large, such that several important plasma parameters can be measured within the requirements set by the ITER team. In this article the status of the conceptual design of the polarimeter system for ITER will be presented.
Ultrahigh vacuum/high pressure chamber for surface x-ray diffraction experiments
Authors: Bernard P. , Peters K. , Alvarez J. , Ferrer S.
Published: July 2002

We describe an ultrahigh vacuum chamber that can be internally pressurized to several bars and that is designed to perform surface x-ray diffraction experiments on solid-gas interfaces. The chamber has a cylindrical beryllium window that serves as the entrance and exit for the x rays. The sample surface can be ion bombarded with an ancillary ion gun and annealed to 1200 K.
Time-to-digital converter of very high pulse stretching ratio for digital storage oscilloscopes
Authors: Park Keunoh , Park Jaehong
Published: July 2002

A time-to-digital converter (TDC) for digital storage oscilloscopes has been developed. The TDC utilizes an analog interpolation scheme named a pulse stretcher of very high stretching ratio of about 5000 to achieve fine timing resolution. With the high stretching ratio, high single-shot accuracy can be achieved with a relatively low counting clock frequency. The start and stop inputs of the TDC are the trigger pulse and the 250 MHz sampling clock respectively, so they have at most a 4 ns time interval. The time interval between two inputs is stretched by a pulse stretcher with a high discharging–charging ratio, and the stretched interval is counted with a coarse counter. The resulting count is the output of the TDC which is proportional to the time interval between the two inputs. The least-significant-bit step and the root-mean-squared deviation of the TDC output were measured to be 7.80 and 14.1 ps. The TDC was used in a 250 MS/s data acquisition system for interpolating the 4 ns sampling clock period and its interpolation capability was verified.
Applications of heavy-negative-ion sources for materials science (invited)
Authors: Ishikawa Junzo
Published: July 2002

Applications of heavy negative ions produced by sputter-type negative-ion sources for materials science are reviewed. Submilliampere and milliampere heavy-negative-ion beams can be produced by a neutral- and ionized-alkaline–metal-bombardment-type heavy-negative-ion source and rf plasma sputter-type negative-ion sources, respectively. These negative-ion beams can be applied for materials processing such as ion implantation, ion beam etching, and ion beam deposition. In negative-ion implantation the charge-up of implanted material surfaces is greatly reduced, and thus ion implantation without target charging is possible. The etching rate due to fluorine-negative ion is mainly determined by its kinetic energy. Pure diamondlike carbon films with high sp3 structure have been prepared by C− and C2− ion beam deposition, and CN films by CN− ion beam deposition. Negative ions provide an excellent tool for materials processing applications.
Grating coupled multicolor quantum well infrared photodetectors
Authors: Tidrow M. Z. , Choi K. K. , DeAnni A. J. , Chang W. H. , Svensson S. P.
Published: July 2002

Gratings of different period spacings are fabricated on quantum well infrared photodetectors having two-stack multiple quantum wells (MQW) to determine the grating coupling efficiency in multicolor detection. The peak wavelength of each MQW is 4.8 and 9.4 μm, respectively, covering the two atmospheric windows. 2D gratings of box-shaped cavities with grating period either in 3 or 4.6 μm spacing are used to coupled normal incident light into the detector. Comparing detectors with and without gratings at either normal or 45° facet incidence, the detectors with an optimized grating show efficient light coupling for both wavelengths due to the fact that the longer wavelength in the present case is an integral multiple of the shorter wavelength. The optimized long wavelength grating couples the shorter wavelength through additional higher orders of diffraction.
Zinc-blende MnTe(111) on BaF2(111) substrates for optical measurements
Authors: Janik E. , Dynowska E. , Ba̧k-Misiuk J. , Domagal/a J. , Kutrowski M. , Wojtowicz T. , Stachow A.
Published: July 2002

Thin films of MnTe having zinc-blende structure were grown on BaF2(111) substrates under various growth conditions by molecular beam epitaxy. The structural properties of the deposited films were characterized by x-ray diffraction which evidenced sometimes the presence of other crystallographic phases apart from a dominating zinc-blende-like phase. Detailed measurements of symmetrical and asymmetrical reflections showed that the unit cell of (111) MnTe is not cubic, as is the case of epitaxial (001) MnTe, but it is distorted to an orthorhombic one. The transparent BaF2 substrates enabled optical measurements in transmission, thus, permitting a study of the band structure of the material in the vicinity of its fundamental band gap.
Lateral wet oxidation of AlxGa1−xAs-GaAs depending on its structures
Authors: Kim J.-H. , Lim D. H. , Kim K. S. , Yang G. M. , Lim K. Y. , Lee H. J.
Published: July 2002

Data are presented demonstrating that the lateral wet oxidation of Al(Ga)As layer is strongly influenced by its thicknesses and heterointerface structures as well as Al compositions. The oxidation length decreases rapidly with decreasing AlAs thickness in the range of <80 nm and oxidation nearly stops at a thickness of ∼11 nm. Also, the oxidation rate of AlxGa1−xAs decreases quickly with decreasing Al composition, providing a high degree of oxidation selectivity. AlGaAs layers on both sides of AlAs layer reduce the lateral oxidation rate which is enhanced by the stress induced by oxidized AlAs.
Low-stiffness silicon cantilevers for thermal writing and piezoresistive readback with the atomic force microscope
Authors: Chui B. W. , Stowe T. D. , Kenny T. W. , Mamin H. J. , Terris B. D. , Rugar D.
Published: July 2002

Low-stiffness silicon cantilevers have been developed for proposed data storage devices based on the atomic force microscope, in particular thermomechanical recording. The cantilevers combine a sharp tip with an integrated piezoresistive sensor for data readback from a rotating polycarbonate disk. A novel process was developed to make shallow piezoresistors in cantilevers 1 μm thick, significantly thinner and therefore softer than previously possible. Readback was demonstrated at linear velocities up to 120 mm/s. Separate cantilevers with resistively heated tips were fabricated for writing data marks on polycarbonate, with measured thermal time constants of 30 μs.
Nanoscale investigation of fatigue effects in Pb(Zr,Ti)O3 films
Authors: Gruverman A. , Auciello O. , Tokumoto H.
Published: July 2002

Scanning force microscopy has been used to perform a comparative nanoscale study of domain structures and switching behavior of Pb(ZrxTi1−x)O3 (PZT) thin films integrated into heterostructures with different electrodes. The study revealed a significant difference between polarization state of as-deposited PZT films on RuO2 and Pt electrodes. The PZT/RuO2 films exhibit polydomain crystallites and show almost symmetric switching behavior, while the PZT/Pt films are mainly in a single polarity state and exhibit highly asymmetric piezoelectric hysteresis loops. Formation of unswitchable polarization within the grains of submicron size as a result of fatigue process was directly observed.
Laser operation of an Nd:Gd3Ga5O12 thin-film optical waveguide fabricated by pulsed laser deposition
Authors: Gill Devinder S. , Anderson Andrew A. , Eason Robert W. , Warburton T. J. , Shepherd D. P.
Published: July 2002

We report the laser operation of a thin-film waveguide structure grown by the pulsed laser deposition technique. A 2.7-μm-thick crystalline film of neodymium doped Gd3Ga5O12 (Nd:GGG) lases at a wavelength centered at 1.06 μm when pumped by a Ti:sapphire laser at 808 nm.
Electron distribution and capacitance-voltage profiles of multiple quantum well structure from self-consistent simulations
Authors: Moon C. R. , Choe Byung-Doo , Kwon S. D. , Lim H.
Published: July 2002

Carrier profiles of multiple quantum wells are studied using self-consistent simulations. The free carrier density of the well is found to be distributed nonuniformly and symmetrically, although the doping level in barriers is uniform. The calculated apparent carrier density obtainable from the capacitance-voltage profile is found to be distributed asymmetrically. Simulation results show that, even if electrons are confined in quantum wells, the apparent electron distribution can be flattened if barrier thickness or doping level in barriers are reduced to such an extent that the Debye length is comparable to the barrier thickness.
The reversal of light-induced degradation in amorphous silicon solar cells by an electric field
Authors: Carlson D. E. , Rajan K.
Published: July 2002

A strong electric field has been shown to reverse the light-induced degradation of amorphous silicon solar cells while exposed to intense illumination at moderate temperatures. The rate of reversal increases with temperature, illumination intensity, and with the strength of the reverse bias field. The reversal process exhibits an activation energy on the order of 0.9 eV and can be increased by the trapping of either electrons or holes in the presence of a strong electric field.
Tunneling induced transparency: Fano interference in intersubband transitions
Authors: Schmidt H. , Campman K. L. , Gossard A. C. , Imamoǧlu A.
Published: July 2002

We report the observation of tunneling induced transparency in asymmetric double quantum well structures. Resonant tunneling through a thin barrier is used to coherently couple the two upper states in a three level system of electronic subbands in a GaAs/AlGaAs structure. This creates Fano-type interferences for the collective intersubband excitations in the absorption from the ground state, analogous to electromagnetically induced transparency (EIT) in atomic systems. We observe a 50% reduction in absorption between the subband resonances which can be explained by taking into account the coherent coupling of the upper states. We analyze the bias dependent absorption spectra and determine the relevant lifetime broadening and dephasing rates for the transitions.
Observation of multiple Er3+ sites in Er-implanted GaN by site-selective photoluminescence excitation spectroscopy
Authors: Kim S. , Rhee S. J. , Turnbull D. A. , Reuter E. E. , Li X. , Coleman J. J. , Bishop S. G.
Published: July 2002

Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the 1540 nm I13/24→I15/24 emission of Er3+ in Er-implanted films of GaN grown by metalorganic chemical vapor deposition. The PLE spectra exhibit several broad below-gap absorption bands, which excite three distinct site-selective Er3+ PL spectra. The excitation of two of the site-selective Er PL bands involves optical absorption by defects or background impurities, rather than direct intra-f shell absorption, with subsequent nonradiative transfer of the energy to nearby Er3+ luminescence centers. The characteristics of the PLE spectrum of the third site-selective PL band suggest that an exciton bound at an Er-related trap is involved in the excitation mechanism.
Free excitons with n=2 in bulk GaN
Authors: Steube M. , Reimann K. , Fröhlich D. , Clarke S. J.
Published: July 2002

The direct observation of free A, B, and C excitons with n=2 in hexagonal bulk gallium nitride (GaN) by two-photon spectroscopy is reported. From these data, the band gaps, exciton binding energies, and hole masses for the three uppermost valence bands are calculated.
Junction properties of aluminum/polypyrrole (polypyrrole derivatives) Schottky diodes
Authors: Singh Ramadhar , Narula Amarjeet K.
Published: July 2002

The current–voltage characteristics of Schottky junctions formed by using aluminum on polypyrrole, poly(N-methyl pyrrole) and the copolymer poly(N-methyl pyrrole–pyrrole) have been investigated. The formation of the junctions has been confirmed by capacitance–voltage characteristics and Chot plots. The results have been explained on the basis of thermionic emission theory.
High resolution polarization gratings in liquid crystals
Authors: Slussarenko S. , Francescangeli O. , Simoni F. , Reznikov Y.
Published: July 2002

Efficient recording of polarization gratings in dye-doped liquid crystals is reported. By exploiting the effect of light-induced anchoring of the molecular director, it has been possible to write stable holographic gratings with a diffraction efficiency of 8% at a resolution of 1000 lines/mm. The required surface energy density of 0.3 J/cm2 sets these materials among the most sensitive for optical storage.
Use of nonstoichiometry to form GaAs tunnel junctions
Authors: Ahmed S. , Melloch M. R. , Harmon E. S. , McInturff D. T. , Woodall J. M.
Published: July 2002

A tunnel diode was formed from GaAs containing excess arsenic incorporated by molecular beam epitaxy at reduced substrate temperatures. The incorporation of excess arsenic during growth results in a more efficient incorporation of silicon on donor sites and beryllium on acceptor sites. The better dopant incorporation, along with trap assisted tunneling through deep levels associated with the excess arsenic, results in a tunnel junction with record peak current density of over 1800 A/cm2, zero-bias specific resistance of under 1×10−4 Ω cm, and a room-temperature peak-to-valley current ratio of 28.
Two-dimensional electron gas formed in a back-gated undoped heterostructure
Authors: Hirayama Y. , Muraki K. , Saku T.
Published: July 2002

By using a back-gate operation, a high-quality two-dimensional electron gas (2DEG) is formed in an undoped GaAs/AlGaAs inverted heterostructure. A high mobility of around 3×106 cm2/V s at 1.6 K is obtained for the structure without any compensating surface doping. The electron density is controllable down to 7×109 cm−2. The relation between electron density and mobility is studied for samples both with and without a surface gate. The obtained results indicate that background impurities and an inhomogeneity of the electric field coming from the surface govern the mobility in a low-electron-density region and that the interface inhomogeneity becomes important at a high electron density.
Photoluminescence of pseudomorphic SiGe formed by G57e+ ion implantation in the overlayer of silicon-on-insulator material
Authors: Patel C. J. , Zhao Q. X. , Nur O. , Willander M.
Published: July 2002

A metastable SiGe-on-insulator structure is realized by a high-dose G57e+ ion implantation in the overlayer of silicon-on-insulator followed by solid phase epitaxial regrowth. Studies of the optical properties of the germanium-implanted and post-implantation annealed layers with 18% peak germanium concentration were carried out using photoluminescence (PL) spectroscopy. The electrical integrity of the strained layer was qualitatively inferred from the pseudo-mosfet characterization technique. The PL results show that the broadband (BB) emission related to germanium implantation damage can be completely eliminated by post-implantation thermal treatment. PL spectra and measured transconductance of the sample heat-treated at 500 °C indicate conclusively that a defect-free strained SiGe layer has been formed. However, samples heat-treated at higher temperatures show degradation in the charge carrier lifetime, a new BB emission with 0.816 eV peak energy and an emergence of defect related emission at 0.870 eV for samples annealed at and above 900 °C.
Electrical characterization of alternating-current thin-film electroluminescent devices
Authors: Krasnov A. N. , Bajcar R. C. , Hofstra P. G.
Published: July 2002

We suggest a formalism built on the analogy between the pressure-volume (P–Ṽ) diagram of a gas system and the charge-voltage (Q–V) characteristics of the alternating-current thin-film electroluminescent (ACTFEL) device. The formalism extends the understanding of the processes in ACTFEL devices and the complete characterization of their internal parameters based on the Q–V diagram.
Observation of confinement-dependent exciton binding energy of GaN quantum dots
Authors: Ramvall Peter , Tanaka Satoru , Nomura Shintaro , Riblet Philippe , Aoyagi Yoshinobu
Published: July 2002

The photoluminescence emission peak energy of GaN quantum dots was observed to shift to higher energy with decreasing quantum dot size. This effect was found to be a combination of a blueshift from the confinement-induced shift of the electronic levels and a redshift from the increased Coulomb energy induced by a compression of the exciton Bohr radius. From this observation, absolute values of the exciton binding energy as a function of quantum dot size are determined.
Magnetotransport investigations of ultrafine single-crystalline bismuth nanowire arrays
Authors: Zhang Zhibo , Sun Xiangzhong , Dresselhaus M. S. , Ying Jackie Y. , Heremans Joseph P.
Published: July 2002

We have measured the magnetotransport properties of ultrafine single-crystalline Bi nanowire arrays embedded in a dielectric matrix. At low temperatures (T⩽50 K), the wire boundary scattering is shown to be the dominant scattering process for carriers. A reversal in the temperature dependence of the magnetoresistance was observed for wires with 65 nm average diameter relative to those with 109 nm average diameter when T⩽100 K. We attribute this difference to effects due to the quantization of the transverse momentum of the carriers, which results in a semimetal-semiconductor transition for Bi nanowires as the wire diameter becomes sufficiently small.