bluuebunny/crossref_metadata_2025 huggingface dataset:
The GPCOG: A New Screening Test for Dementia Designed for General Practice
Authors: Brodaty Henry , Pond Dimity , Kemp Nicola M. , Luscombe Georgina , Harding Louise , Berman Karen , Huppert Felicia A.
Published: July 2002

OBJECTIVES:

To design and test a brief, efficient dementia‐screening instrument for use by general practitioners (GPs).

DESIGN:

The General Practitioner Assessment of Cognition (GPCOG) consists of cognitive test items and historical questions asked of an informant. The validity of the measure was assessed by comparison with the criterion standard of diagnoses of dementia derived from the Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition).

SETTING:

Primary care doctors' offices.

PARTICIPANTS:

Sixty‐seven GPs administered the GPCOG to 283 community‐dwelling patients aged 50 to 74 with memory complaints or aged 75 and older.

MEASUREMENTS:

The Cambridge Mental Disorder of the Elderly Examination, the Abbreviated Mental Test (AMT), the Mini‐Mental State Examination (MMSE), the 15‐item Geriatric Depression Scale, and the 12‐item Short‐Form Health Survey.

RESULTS:

The GPCOG was reliable and superior to the AMT (and possibly to the MMSE) in detecting dementia. The two‐stage method of administering the GPCOG (cognitive testing followed by informant questions if necessary) had a sensitivity of 0.85, a specificity of 0.86, a misclassification rate of 14%, and positive predictive value of 71.4%. Patient interviews took less than 4 minutes to administer and informant interviews less than 2 minutes. The instrument was reported by GPs to be practical to administer and was acceptable to patients.

CONCLUSION:

The GPCOG is a valid, efficient, well‐accepted instrument for dementia screening in primary care. J Am Geriatr Soc 50:530–534, 2002.
Genome Size and Pollen Viability as Taxonomic Criteria: Application to the Genus Hosta
Authors: Zonneveld B. J. M. , Van Iren F.
Published: July 2002

Abstract: Genome size (C values) and pollen viability staining were applied as new criteria to investigate the taxonomy of the genus Hosta Tratt. (Hostaceae). Nearly all species of the genus Hosta have the same basic chromosome number (2n = 2x = 60). However, the nuclear DNA contents, as measured by flow cytometry with propidium iodide, could be demonstrated to range between 17.2 to 26.6 pg. This implies that the largest genome contains roughly 1010 more base pairs than the smallest. Therefore, nuclear DNA content is a very relevant taxonomic trait that can be measured simply by flow cytometry. In addition, differences in overall DNA composition were demonstrated by comparing to DAPI fluorescence. In general, genome size data confirmed the division into three subgenera. The geographical distribution of genome sizes indicates the migration pattern of Hosta throughout East Asia. The species belonging to the mainly Korean subgenus Bryocles, with a low nuclear DNA content (17.2 ‐ 19.3 pg), can now largely be distinguished from the mainly Japanese species of the subgenus Giboshi (21.3 ‐ 26.5 pg). The exception is H. longissima, that with only 19.6 pg provides a nice example of a decrease in DNA content. On the mainland, as well as on Honshu, species with increased and decreased DNA content have evolved independently. The usefulness of pollen viability to detect hybrids in Hosta was demonstrated in a large series of artificial crosses between bona fide species. Consequently, pollen viability was measured in all available Hosta described as species. Several had low pollen viability and were concluded to be hybrids. Morphology and DNA content confirmed this in most cases. The resulting 23 species approximate the number of Hosta species that follows from the combined studies by Fujita (197618) on the Japanese species and Chung (1991 a11) on the Korean species.
Inelastic tunneling in AlAs-GaAs-AlAs heterostructures
Authors: Mendez E. E. , Calleja E. , Wang W. I.
Published: July 2002

Current-voltage measurements in AlAs-GaAs-AlAs heterostructures under hydrostatic pressure show that inelastic tunneling through the XAlAs-ΓGaAs discontinuity is reduced as the thickness of the AlAs layers is decreased. This reduction makes possible large peak-to-valley current ratios in resonant tunneling devices with thin AlAs barriers.
Resonant tunneling through a Si/GexSi1−x/Si heterostructure on a GeSi buffer layer
Authors: Rhee S. S. , Park J. S. , Karunasiri R. P. G. , Ye Q. , Wang K. L.
Published: July 2002

Resonant tunneling of holes through an unstrained GeSi well between two strained Si barriers on a relaxed GeSi buffer layer has been observed for the first time. The peak-to-valley ratios of 2.1/1 at 4.2 K and 1.6/1 at 77 K in current-voltage characteristics were attained for light holes. Resonant tunneling from heavy-hole states was also observed at room temperature, as well as 77 and 4.2 K by conductance measurement. The positions of the resonance peaks are in good agreement with the light- and heavy-hole bound states in the quantum well.
Autodoping of GaAs grown by organometallic vapor phase epitaxy on silicon substrates
Authors: Azoulay R. , Draidia N. , Gao Y. , Dugrand L. , Leroux G.
Published: July 2002

We have grown GaAs on silicon substrates by organometallic chemical vapor deposition and we have studied the autodoping of the GaAs layers by silicon during the growth. The epitaxies were performed on GaAs wafers and on GaAs/Si wafers side by side during the same run. By measuring the atomic silicon concentration on the grown layers, we show that a contamination of silicon occurs during the growth process via SiH4 formation and diffusion from the substrate. We propose a model to explain the contamination of the GaAs layers by the silicon substrate.
Phosphorus ion implantation induced intermixing of InGaAs-InP quantum well structures
Authors: Tell B. , Shah J. , Thomas P. M. , Brown-Goebeler K. F. , DiGiovanni A. , Miller B. I. , Koren U.
Published: July 2002

We have studied P ion implantation induced intermixing of In0.53Ga0.47As quantum wells embedded between InP barriers. Data are presented for both standard furnace anneals at 650 °C and rapid thermal anneals at 750 °C. Low-temperature photoluminescence and quantitative Auger electron spectroscopy were performed for furnace-annealed samples for which photoluminescence shifts of ∼200 meV were in excellent agreement with the calculated band gap for the lattice matched composition determined by Auger spectroscopy. Photoluminescence studies of rapid thermal annealed samples yield an energy gap shift of ∼150 meV for annealing times of at least 10 s.
Investigation of the light-induced effects in nitrogen-rich silicon nitride films
Authors: Jousse D. , Kanicki J.
Published: July 2002

We report the first observations of photoconductivity fatigue and generation of positive fixed charges under ultraviolet illumination in gate-quality nitrogen-rich silicon nitride films, using metal-nitride-silicon and metal-nitride-oxide-silicon structures. The photoconductivity fatigue is correlated with the density of neutral silicon dangling bonds measured by electron spin resonance. Spins and positive fixed charges are generated at the same time, but a different rates and in a different amount. Possible models are discussed to explain the observed results.
Disordering of GaAs/AlGaAs multiple quantum well structures by thermal annealing for monolithic integration of laser and phase modulator
Authors: Ribot H. , Lee K. W. , Simes R. J. , Yan R. H. , Coldren L. A.
Published: July 2002

Conventional thermal annealing with various caps on the surface of the sample is used to selectively disorder one section of a multiple quantum well GaAs/AlGaAs structure. In the disordered section, the shift of the photoluminescence peak is in the range where the chirp parameter (ratio of real index change over absorption change) is large at the laser wavelength in the undisordered material. Moreover, photoconductivity measurements on samples under reverse bias, as well as laser testing, prove that the electric properties are preserved by the thermal treatment. Therefore, the investigated process should be highly suitable for the monolithic integration of a laser and phase modulator.
Voltage tunable quantum well infrared detector
Authors: Parihar S. R. , Lyon S. A. , Santos M. , Shayegan M.
Published: July 2002

Measurements on a single quantum well infrared detector with its peak responsivity at 5.3 μm show a large Stark shift under bias. The detector has one graded barrier to enhance the effect of an electric field on the subband spacing. Several times greater change in the intersubband transition energy with applied field is obtained this way compared to the square well. Calculations show an even larger Stark shift possible with a stepped well structure.
Generation of picosecond electrical pulses by a pulse-forming optoelectronic device
Authors: Sano E. , Nagatsuma T. , Shibata T. , Iwata A.
Published: July 2002

Full-wave analysis demonstrates that a picosecond electrical pulse can be generated by a pulse-forming optoelectronic device, composed of a photoconductor and an impedance mismatch structure in a coplanar stripline. An electro-optic sampling system is used to measure the response of the pulse-forming optoelectronic device, fabricated on a semi-insulating GaAs substrate. Electrical pulses with 1.3 ps duration (full width at half maximum) are achieved using the device.
Molecular beam epitaxy of superconducting (Rb,Ba)BiO3
Authors: Hellman E. S. , Hartford E. H. , Fleming R. M.
Published: July 2002

The simple cubic perovskite (Rb,Ba)BiO3 can be grown at temperatures below 350 °C by molecular beam epitaxy using a rf plasma atomic oxygen source. Films with superconducting onsets in resistivity as high as 27 K are obtained without annealing. The epitaxy proceeds in the normal (100) orientation on {100} SrTiO3, despite a 10% lattice mismatch. (110) epitaxy and spotty reflection high-energy electron diffraction (RHEED) patterns are obtained on {100} MgO substrates, despite the good lattice match for (100) growth. Streaked and spotty RHEED patterns have been obtained on either substrate. Sticking coefficients for bismuth depend on the growth conditions, indicating that the epitaxy is partially controlled by desorption kinetics.
Holographic image storage in iron-doped lithium niobate fibers
Authors: Yoshinaga Hisao , Kitayama Ken-ichi , Oguri Hitoshi
Published: July 2002

Holographic recording on a real-time basis of two-dimensional images in an iron-doped lithium niobate fiber with a 200 μm diameter and 3 mm length is demonstrated for the first time. The erasure time constant as a function of readout beam power, the diffraction efficiency as a function of incident reference angle, and the results of angular-multiple recording of two images are shown. These results suggest that the LN fiber could replace bulk materials for holographic recordings in optical parallel processing.
Particle-plasma interactions in low-pressure discharges
Authors: Jellum G. M. , Graves D. B.
Published: July 2002

Spatial profiles of size and number density of aluminum particles in argon plasmas have been measured. Angular dissymmetry laser light scattering indicates that larger particles tend to segregate towards the anodic plasma-sheath boundary; however, the larger particles (≳400 nm diameter) near this boundary are present in number densities lower than those closer to the center of the discharge, where particles are smaller (∼300 nm). Nonspherical particle aggregates appear to form near the plasma-sheath interface under some conditions. Optical emission intensity and positive ion number density are reduced in discharges with particles as compared to discharges without particles.
Particle trapping phenomena in radio frequency plasmas
Authors: Selwyn Gary S. , Heidenreich John E. , Haller Kurt L.
Published: July 2002

Particles generated in an argon plasma and suspended at the plasma/sheath boundary are localized by lateral trapping fields. In the commercial rf etching reactor used in this work, the particles and their motion in real time are observed by laser light scattering with the laser beam rapidly rastered in a plane parallel to the rf electrode. Repulsion between individual, relatively large particles is observed, verifying that there is significant negative charge on the particles. Two types of trapping regions are commonly seen: rings of particles around the outside edge of silicon wafers, and domes of particles over the centers of the wafers. It is shown that these effects are influenced by the topography of the electrode. In addition, particle densities >107 cm−3 for particles of diameter 0.2 μm are inferred from transmission studies for certain plasma conditions.
Stretched exponential illumination time dependence of positive charge and spin generation in amorphous silicon nitride
Authors: Kanicki J. , Sankaran M. , Gelatos A. , Crowder M. S. , Tober E. D.
Published: July 2002

We report for the first time on the stretched exponential time dependence of positive charge and spin generated by subband-gap ultraviolet illumination in gate-quality nitrogen-rich amorphous silicon nitride films. We have found that a stretched exponential function, which characterizes dispersive charge transport in silicon nitride, gives the best description of our experimental results. We also propose a mechanism which we believe is responsible for the creation of the positive charge and spin in the amorphous silicon nitride films.
Laser deposition of YBa2Cu3O7−δ films using a pulsed oxygen source
Authors: Gupta A. , Hussey B. W.
Published: July 2002

Thin films of YBa2Cu3O7−δ (YBCO) have been grown by pulsed laser deposition in a low-pressure background (10−4–10−3 Torr) by using a pulsed, high intensity jet of O2 or N2O for oxidation. The oxygen source is provided by a pulsed molecular beam valve, and the opening of the valve and the triggering of the laser are synchronized with appropriate delay so that the supersonic gas jet and the ablated fragments arrive at the substrate at the same time. This provides the necessary oxygen to form the YBCO phase while maintaining a low oxygen background. The YBCO phase is not formed if the oxygen pulse is provided either before or much after the arrival of the ablation fragments at the substrate. The ability to grow superconducting films at low background pressures should allow usage of in situ analysis techniques, such as reflection high-energy electron diffraction, during pulsed laser deposition.
Magnetically ordered amorphous copper ferrite
Authors: Srinivasan G. , Rao B. Uma Maheshwar , Zhao J. , Seehra M. S.
Published: July 2002

The first observation of a long-range magnetic order with planar anisotropy is reported in as-deposited films of amorphous copper ferrite prepared by rf sputtering techniques. Data on the temperature dependence of the saturation magnetization, in-plane anisotropy field, and gyromagnetic ratio were obtained on 1-μm-thick films sputtered in a 50% oxygen and 50% argon atmosphere. The films show a large magnetization and anisotropy field at room temperature. The low-temperature magnetization, Curie temperature, and gyromagnetic ratio are in agreement with the values expected for crystalline copper ferrite. A paramagnetic-to-antiferromagnetic transition at 50 K is evident in the magnetization data. We propose that the films contain magnetically ordered clusters of copper ferrite and Fe2O3.
Comparison of timing jitter in external and monolithic cavity mode-locked semiconductor lasers
Authors: Derickson D. J. , Morton P. A. , Bowers J. E. , Thornton R. L.
Published: July 2002

A comprehensive timing jitter comparison is made for mode-locked semiconductor lasers using active, passive, and hybrid mode-locking techniques in both external and monolithic cavity configurations. Active mode locking gives the lowest residual rms timing jitter of 65 fs (150 Hz-50 MHz), followed by the hybrid and passive mode-locking techniques. It is found that monolithic cavity devices with all active waveguides have higher timing jitter levels than the comparable external cavity case.
Heavily doped p-ZnSe:N grown by molecular beam epitaxy
Authors: Qiu J. , DePuydt J. M. , Cheng H. , Haase M. A.
Published: July 2002

The growth of p-ZnSe:N films by molecular-beam epitaxy, employing a free radical nitrogen source, has been investigated. Using this technique we have obtained p-type ZnSe with net acceptor concentrations up to 1.0×1018 cm−3, as measured by capacitance–voltage(C–V) profiling−this is the highest ever reported for p-type ZnSe. By adjusting the flux of active nitrogen and the substrate temperature, films with net acceptor concentrations from 1.0×1016 to 1.0×1018 cm−3 were grown. Evidence of compensation was found in the low temperature photoluminescence and C–V measurements; the degree of compensation depends on the amount of nitrogen incorporated into the film. The dependencies of nitrogen density, net acceptor concentration, and degree of compensation upon the flux of active nitrogen and the substrate temperature are discussed.
Low-temperature homoepitaxy on Si(111)
Authors: Weir B. E. , Freer B. S. , Headrick R. L. , Eaglesham D. J. , Gilmer G. H. , Bevk J. , Feldman L. C.
Published: July 2002

We have compared ion channeling results with molecular dynamics simulations to investigate low-temperature molecular beam homoepitaxy on silicon. We report the temperature dependence, rate dependence, and thickness dependence of films grown on Si(111). For 350 Å films, a transition to good crystalline quality is seen in ion channeling at growth temperatures of ≊400 °C; this is compared to ≊100 °C for (100) epitaxy. The evolution of surface microstructure leading to breakdown of epitaxial growth at low temperatures is discussed.
Influence of substrate topography on the nucleation of diamond thin films
Authors: Dennig Paul A. , Stevenson David A.
Published: July 2002

We have investigated the effect of substrate topography on nucleation behavior by producing a variety of substrate features on molybdenum and silicon substrates. The initial stages of nucleation on these substrates were studied by exposing them to similar conditions in a hot filament chemical vapor deposition reactor. For these materials, nucleation is favored on prominent features of the substrate; that is, features that protrude with sharp edges or apexes, as opposed to sharp valleys or flat regions. Several possible explanations are given for this behavior.
Electrical tunability of infrared detectors using compositionally asymmetric GaAs/AlGaAs multiquantum wells
Authors: Martinet E. , Luc F. , Rosencher E. , Bois Ph. , Delaitre S.
Published: July 2002

For the first time, an electrically tunable infrared photoconductor is made with GaAs/AlGaAs asymmetric step multiquantum wells, using the linear Stark shift of the intersubband transition. An applied electric field excursion of ±40 kV/cm is sufficient to shift the peak responsivity wavelength from 8.5 to 13.5 μm. The photoresponse tunability is studied by comparing photocurrent and absorption spectra for different applied electric fields.
Electrochromic properties of vanadium pentoxide thin films prepared by new wet process
Authors: Nagase Katsumi , Shimizu Youichi , Miura Norio , Yamazoe Noboru
Published: July 2002

Electrochromic properties of the V2O5 thin films prepared from a spinnable organic vanadium solution were investigated. The films heat-treated at 400 °C showed clear two-step electrochromism, deep blue⇄green⇄yellow, with its response time for obtaining ΔOD=0.5 being ∼2 s. The yellow and green colors showed good memory effects for more than 20 h, while the blue one was considerably degraded within 10 h.
Oriented crystal particles of semiconductor PbS on Langmuir monolayer surfaces
Authors: Zhao Xiao Kang , McCormick Larry D.
Published: July 2002

Oriented PbS crystal particles have been prepared by the infusion of H2S onto Langmuir monolayers of arachidic acid (AA), [CH3(CH2)18COOH]. The morphology and crystallographic orientation of PbS crystals have been determined by transmission electron microscopy/electron diffraction (TEM/ED) and atomic force microscopy (AFM). Two related morphological types of platelike crystals with narrow size distributions were observed: right-angle and equilateral triangular particles with their [100] and [111] axes perpendicular to the monolayer surface, respectively. The crystallographic relations between PbS and their organic templates were elucidated. Optical spectra and AFM images provided evidence for the formation of one-dimensional quantum-confined semiconductor crystals at the early stages of growth.
Boron redistribution in arsenic-implanted silicon and short-channel effects in metal–oxide–semiconductor field effect transistors
Authors: Sadana D. K. , Acovic A. , Davari B. , Grutzmacher D. , Hanafi H. , Cardone F.
Published: July 2002

When a high dose of As is implanted (e.g., 25 keV, 3×1015 cm−2) into B-doped Si and the sample is subsequently annealed at 900 °C/5 min, pronounced segregation of the B into the implanted region occurs. This creates a B-depleted region beyond the As profile. It is demonstrated that the B segregation is driven primarily by the implantation induced damage rather than by As-B chemical and/or by electric field effects. The B segregation is nearly complete after a relatively low temperature (≲600 °C/30 min) anneal. Two-dimensional device simulations show that the B depletion observed here can account for ≂50 mV threshold voltage roll off (at a drain bias of 0.1 V) in a Si metal–oxide–semiconductor field effect transistor of 0.2 μm gate length.
On the mechanism of dimensional change of neutron irradiated graphite
Authors: Tanabe T. , Muto S. , Niwase K.
Published: July 2002

In order to investigate the mechanism of large volume expansion of graphite due to neutron irradiation we have made in situ observation of damage structure of graphite with a high resolution transmission electron microscope (HRTEM). Both bending and randomizing the orientation of broken basal planes are proposed to be the origin for the large volume expansion of the irradiated graphite. It is also found that graphite easily loses its lattice ordering in the basal planes as demonstrated by halo rings in the (101̄0) diffraction pattern, while hardly losing its layered structure, i.e., (002) spots are retained.
X-ray emission from high-Z mixture plasmas generated with intense blue laser light
Authors: Nishimura H. , Endo T. , Shiraga H. , Kato Y. , Nakai S.
Published: July 2002

A mixture plasma composed of high-Z elements is proposed as an efficient converter of laser light to x rays. It is shown theoretically that an energy band of small opacity for one element, corresponding to a partial cutout on its emission spectrum, can be enhanced with a large opacity band of another element so that the mean opacity for the mixture with an optimum mixing ratio becomes larger than the opacities of the individual elements. As a result, higher re-emission and efficient conversion to x rays can be expected for the mixture. Absolute x-ray emission from Au-Sm and Au-Tb mixture plasmas irradiated with a blue-laser pulse was measured for the first time. The results show a trend that the mixtures at the optimum mixing ratio give higher x-ray conversion efficiencies than their constituent materials.
On the estimation of depth resolution during sputter profiling
Authors: Petravić M. , Svensson B. G. , Williams J. S.
Published: July 2002

We have estimated broadening of sputter profiles for several elements, either buried or implanted in Si, under Ar+, Cs+, and O2+ bombardment using a semiempirical model developed recently by P. C. Zalm and C. J. Vriezema [Nucl. Instrum. Methods B 67, 495 (1992)]. Excellent agreement has been found between this model and experimental data for all three types of primary ions. In the case of oxygen bombardment, however, good agreement is only achieved by taking into account beam-induced changes at the surface, including swelling and the formation of a surface oxide.
Metal contacts to gallium nitride
Authors: Foresi J. S. , Moustakas T. D.
Published: July 2002

We report measurements on the nature of aluminum and gold contacts to GaN. The GaN films were deposited onto the R-plane of sapphire substrates by molecular beam epitaxy and are autodoped n-type. Metal contacts were deposited by evaporation and were patterned photolithographically. Current-voltage characterization shows that the as-deposited aluminum contacts are ohmic while the as-deposited gold contacts are rectifying. The gold contacts become ohmic after annealing at 575 °C, a result attributed to gold diffusion. The specific contact resistivity of the ohmic aluminum and gold contacts were found by transfer length measurements to be of device quality (10−7–10−8 Ω m2). The results of these studies suggest a direct correlation between barrier height and work function of the metal, consistent with the strong ionic character of GaN.
Detailed analysis of the in situ magneto-optic Kerr signal of gadolinium films near the Curie temperature
Authors: Farle M. , Lewis W. A. , Baberschke K.
Published: July 2002

The temperature and magnetic-field-dependent Kerr signal of layer-by-layer grown 300 Å Gd(0001) on W(110) is analyzed below and above the Curie temperature TC. The changes in the signal shape when passing through TC are discussed. We introduce a new and independent method for accurately determining TC in thin magnetic films in ultrahigh vacuum. As a consequence, discussions of the remanence, the field induced magnetization, and the critical exponent β become more transparent.
InAIP/InAlGaP distributed Bragg reflectors for visible vertical cavity surface-emitting lasers
Authors: Schneider R. P. , Lott J. A.
Published: July 2002

Distributed Bragg reflectors (DBRs) composed of In0.5Al0.5P/In0.5(AlyGa1−y)0.5P quarter-wave layers have been prepared using metalorganic vapor phase epitaxy. The structures were grown over a wide range of high-index layer composition (0≤y≤0.6) and peak reflectivity wavelength (720 nm≤λ≤565 nm, covering the spectrum from deep red to green). In all cases observed and calculated reflectance spectra were in excellent agreement. Using these DBRs, an undoped all-phosphide visible vertical cavity surface-emitting laser structure was grown. Under pulsed optical excitation at room temperature, lasing was obtained at a wavelength of λ∼670 nm, with a threshold power density comparable to that observed from similar structures prepared using AlAs/AlGaAs DBRs.
Heterogeneous formation of atomic hydrogen in hot-filament diamond deposition
Authors: Wolden C. , Gleason K. K.
Published: July 2002

A two-dimensional finite element model has been developed for the reactor of W. L. Hsu [Appl. Phys. Lett. 59, 1427 (1991)], focusing on the relative importance of heterogeneous and homogeneous chemistry in the production of atomic hydrogen. Although hot-filament reactors have been the subject of extensive modeling efforts, most previous work has neglected the impact of the filament on both chemistry and fluid flow. Our calculations show that catalytic activity is responsible for at least 95% of the atomic hydrogen production. In addition, the concentration of atomic hydrogen at the filament is far below thermal equilibrium values. An analysis of experimental data reveals an activation energy of 150 kJ/mol for the filament-catalyzed reaction. From these measurements, a kinetic expression is developed to explain catalytic production of atomic hydrogen at the surface. Finally, the effect of recombination of hydrogen atoms at the growing surface was examined.
Novel fabrication method for nanometer-scale silicon dots and wires
Authors: Chen G. S. , Boothroyd C. B. , Humphreys C. J.
Published: July 2002

We have discovered that a thin film of SiO2 can be directly reduced to Si under electron beam irradiation. The application of this effect to the fabrication of nanometer-sized Si dots and wires is demonstrated. In particular, if SiO2 is irradiated with a high intensity 100 keV electron beam of nanometer scale, then a column of Si is formed which can be as small as 2 nm in diameter. If the beam is moved in a straight line, then a very thin wire of Si is formed. These columns and wires are formed directly under electron irradiation with a dose of ≥3×109 C m−2 and no resists or chemical development are required.
Measurement of the intersubband scattering rate in semiconductor quantum wells by excited state differential absorption spectroscopy
Authors: Faist Jerome , Capasso Federico , Sirtori Carlo , Sivco Deborah L. , Hutchinson Albert L. , Chu Sung Nee G. , Cho Alfred Y.
Published: July 2002

A new technique for the measurement of the intersubband lifetime in semiconductor quantum wells is demonstrated. Electrons are optically excited from the ground state to the first excited state of a doped quantum well. From measurements of the absorption cross section between excited states, we find a lifetime equal to τs=0.65±0.15 ps for a 85 Å GaAs quantum well and τs=0.8±0.2 ps for a 100 Å Ga0.47In0.53As quantum well, in good agreement with the theoretical predictions of Ferreira and Bastard [Phys. Rev. B 40, 1074 (1989)]. In addition, our experiments unambiguously show that the intersubband absorption line is homogeneously broadened at cryogenic temperatures (≤100 K).
Resonant-cavity GaAs/InGaAs/AlAs photodiodes with a periodic absorber structure
Authors: Huang F. Y. , Salvador A. , Gui X. , Teraguchi N. , Morkoç H.
Published: July 2002

Resonant-cavity photodiodes with a periodic absorber structure are demonstrated. The photodiode consists of three thin InGaAs absorbing layers located at the antinodes of standing wave optical field in a resonant cavity. The bottom mirror of the resonant cavity is formed by a GaAs/AlAs quarter-wave stack, while the top mirror is formed by the GaAs/air interface. Theoretical estimation based on the standing wave effect gives 70% peak quantum efficiency at the resonant wavelength. The peak quantum efficiency of this photodiode, as measured by a calibrated Si photodetector, enhanced by 30% as compared to a conventional resonant-cavity photodiode with a single absorbing layer of the same overall thickness. The observed enhancement in quantum efficiency is consistent with theory.
Optical frequency mixing through cascaded second-order processes in β-barium borate
Authors: Tan H. , Banfi G. P. , Tomaselli A.
Published: July 2002

We show that the large effective optical nonlinearity, which is produced by cascaded second-order processes, allows us to accomplish the frequency conversion of a signal pulse with a pump pulse of moderate intensity. In a 1 cm long β-barium borate crystal, the cascaded effect enhances by 5 orders of magnitude the efficiency of the frequency mixing process. The enhancement is larger than 104 even in non-phase-matched condition for the intermediate second-harmonic field, when the imaginary part of the induced nonlinear susceptibility is negligible.
Phase transformation mechanisms involved in excimer laser crystallization of amorphous silicon films
Authors: Im James S. , Kim H. J. , Thompson Michael O.
Published: July 2002

We have investigated the phase transformation mechanisms and the resulting microstructures of excimer laser-induced crystallization of amorphous Si films on SiO2. It is shown that the process can be characterized into two major regimes, based on the dependence of the grain size and the melt duration as a function of the incident energy density. It is found that at the transition between the two regimes, exceedingly large grain-sized polycrystalline films can be obtained. We call this the super lateral growth phenomenon, and propose a model based on liquid-phase regrowth from the residual solid seeds when near-complete melting of the Si film occurs.
Nonlinear optical side-chain polymer with high thermal stability and its pyroelectric thermal analysis
Authors: Bauer S. , Ren W. , Yilmaz Ş. , Wirges W. , Molzow W.-D. , Gerhard-Multhaupt R. , Oertel U. , Hänel B. , Häussler L. , Komber H. , Lunkwitz K.
Published: July 2002

Stable polymers with nonlinear optical side groups were synthesized from maleic anhydride copolymers and the azo dye Disperse Red 1 via esterification. After electrode poling under 135 V/μm at 185 °C, thermal stabilities were measured by pyroelectric and electro-optic thermal analysis and compared to a guest/host polymer of polymethylmethacrylate and the same dye. The pyroelectrically detected dipole orientation and the electro-optic activity of the side-chain polymers decreased only above 150 °C, and electro-optic r33 coefficients of up to 6 pm/V were achieved at 780 nm even without optimizing the dye content.
Structure and stability of sputter deposited beta-tungsten thin films
Authors: Weerasekera I. A. , Shah S. Ismat , Baxter David V. , Unruh K. M.
Published: July 2002

The structure and stability of thin tungsten films prepared by radio frequency magnetron sputter deposition have been studied by x-ray diffraction and x-ray photoelectron spectroscopy. The structure of these films has been found to systematically evolve from the metastable A15 β-W phase to the equilibrium A2 α-W phase with decreasing oxygen impurity concentration. Within the β-W phase a decrease in the concentration of incorporated oxygen results in a monotonic decrease in the lattice parameter of the unit cell until the β-W phase eventually becomes unstable, and the α-W phase is formed.
Links between oxide, interface, and border traps in high-temperature annealed Si/SiO2 systems
Authors: Warren W. L. , Fleetwood D. M. , Shaneyfelt M. R. , Schwank J. R. , Winokur P. S. , Devine R. A. B. , Mathiot D.
Published: July 2002

Evidence is provided to show that enhanced hole-, interface-, and border-trap generation in irradiated high-temperature annealed Si/SiO2/Si systems are all related either directly, or indirectly, to the presence of oxygen vacancies. We find that the calculated oxygen vacancy due to high-temperature anneals from 800 to 950 °C in metal-oxide-semiconductor capacitors closely matches the radiation-induced oxide-trapped charge. This strongly suggests that oxygen vacancies (or vacancy-related complexes) are the dominant hole trapping sites in this particular case. Along with the increase in radiation-induced oxide-trap charge, we observe a concomitant increase in the interface- and border-trap densities. This suggests that in devices that receive high-temperature anneals, all these phenomena are linked to the existence of oxygen vacancies either directly, or indirectly, perhaps via hole trapping at these vacancies.
Deep level defects in n-type GaN
Authors: Götz W. , Johnson N. M. , Amano H. , Akasaki I.
Published: July 2002

In n-type GaN grown by metalorganic chemical vapor deposition two new electronic defects were detected and characterized by deep level transient spectroscopy (DLTS). Schottky-barrier diodes with Ohmic back contacts and low series resistance were fabricated in GaN layers grown on sapphire. The diodes display well behaved current-voltage and capacitance-voltage characteristics and permit unambiguous DLTS evaluation. The new deep levels display thermal activation energies for electron emission of 0.49 and 0.18 eV.
Effect of gas composition on texture of diamond films
Authors: Marinelli M. , Milani E. , Montuori M. , Paoletti A. , Paroli P. , Thomas J.
Published: July 2002

Textured diamond films have been deposited on single-crystal silicon substrates by microwave plasma-enhanced chemical vapor deposition. Several unconventional gas mixtures (CO2–CH4, CO2–C2H6, and CO2–C2H4) were used in order to study the texturing formation within the diamond stability region in the C–O–H phase diagram. Optical emission spectroscopy was used to monitor the compositional dependence of the plasma emission. The obtained diamond films were characterized by scanning electron microscopy and x-ray diffraction. Diamond films deposited with all the gas mixtures showed very similar surface morphologies and (100) texturing was achieved only within a very narrow compositional range. Finally, a correlation was found between texturing conditions and CH and C2 line intensities in the plasma emission spectra.
Simple electrochemical method for the preparation of a highly oriented and highly photoactive α-PbO film
Authors: Veluchamy P. , Minoura H.
Published: July 2002

Highly oriented (110 plane parallel to the substrate) and highly photoactive α-PbO films are prepared by electrochemical anodic oxidation of Pb electrode in alkaline solution at 80 °C by polarizing at +0.3 V versus saturated calomel electrode. Within an hour of anodization, nearly 10–12 μm thick oxide film is produced which is found to be exclusively α-PbO with a preferred orientation along the 110 plane. The films show about 80% quantum efficiency and 1.55% solar to electrical energy conversion efficiency for white-light irradiation in a liquid junction solar cell containing 0.1 M Fe(CN)3−/4−6 redox solution. These values are the highest reported so far for lead oxide thin-film based solar cells.
Low temperature limits to molecular beam epitaxy of GaAs
Authors: Mirin Richard P. , Ibbetson James P. , Mishra Umesh K. , Gossard Arthur C.
Published: July 2002

We report reflection high-energy electron diffraction oscillations during molecular beam epitaxy of GaAs at temperatures as low as 125 °C using As2 as the arsenic source. At a substrate temperature of 200 °C, the As:Ga ratio is shown to be the critical parameter which determines the amplitude of these oscillations. The largest amplitude oscillations are obtained when the As:Ga ratio is about 1:1. In order for these oscillations to occur, the arriving atoms must have some surface mobility. Monte Carlo simulations show that step density oscillations with a period corresponding to one monolayer are expected if the incident atoms are allowed to move as little as one lattice site upon arrival at the surface.
Microfluorometric technique for the determination of localized heating in organic particles
Authors: Liu Y. , Cheng D. K. , Sonek G. J. , Berns M. W. , Tromberg B. J.
Published: July 2002

We describe a novel microfluorometric technique, based on the temperature-dependent fluorescence emission from single dye-labeled phospholipid vesicles, for the determination of localized heating effects. An increase in sample temperature results in a red shifting of the probe fluorescence spectrum. As individually calibrated microthermometers, fluorescent liposomes exhibit a temperature sensitivity of ∼0.1 °C in the vicinity of the bilayer phase transition temperature. Through modification of the bilayer components, both the sensitivity and operating temperature range of these microthermometers can be controlled. Micron spatial resolution is achieved at a signal-to-noise ratio in excess of 103:1. We use the above technique, for the first time, to determine localized heating effects induced by a laser beam focused to its near-diffraction limited spot size. At the laser wavelength of λ=1.064 μm, a temperature change of 1.1 °C/100mW in 10-μm-diam organic liposomes is reported. Implications for the real-time optical monitoring of temperature in biological systems are discussed.
Violet-blue GaN homojunction light emitting diodes with rapid thermal annealed p-type layers
Authors: Khan M. Asif , Chen Q. , Skogman R. A. , Kuznia J. N.
Published: July 2002

In this letter we report the fabrication and optical-electrical characterization of violet-blue GaN homojunction light emitting diodes. Rapid thermal annealing at 1150 °C (for 30 s) was used to activate the p-dopant species (Mg), which resulted in p-type GaN whose photoluminescence response centered around 438 nm is much stronger than that obtained from material annealed in the growth chamber at lower temperatures (700–800 °C) and a longer time (20 min).
Demonstration of a silicon field-effect transistor using AlN as the gate dielectric
Authors: Stevens K. S. , Kinniburgh M. , Schwartzman A. F. , Ohtani A. , Beresford R.
Published: July 2002

As a precursor to cointegration of blue and ultraviolet light emitting nitrides with silicon microelectronics, an AlN on Si(111) metal-insulator-semiconductor field effect transistor has been fabricated and characterized. Current–voltage data show the threshold voltage and transconductance to be −0.8 V and 6 ms/mm, respectively, for a 50 nm insulator thickness. The ideal threshold voltage is 0.09 V. From the transconductance, a channel mobility of 45 cm2/V s is inferred. Capacitance versus voltage (C–V) measurements on a 210 nm film showed a flatband voltage of −4.2 V, corresponding to 8×1011 cm−2 trapped charges at the interface. Comparison of the experimental high-frequency (1 MHz) C–V plot to the theoretical plot indicates the presence of fast interface states. High-resolution transmission electron microscopy studies showed the AlN/Si(111) interface to be surprisingly well ordered in places, with the large lattice mismatch accommodated via a quasiperiodic array of misfit dislocations.
Parametric study of electron density profile evolution following injection in TFTR
Authors: Park H. K. , Mansfield D. K. , Johnson L. C.
Published: July 2002

Recent results from tokamak fusion test reactor (TFTR)1 pellet experiments have extended previous results to a larger volume and longer time scales. In this paper, the detailed study of electron density profile response following pellet injection is discussed. In particular, hollow density profiles with relatively long relaxation times (10–100 ms) from inverted gradients are obtained by injection of 2.67-mm-diam pellets, which penetrate to r≂a/2. Strongly peaked profiles [ne(0)/n̄e∼3] and central electron density ∼3.9×1020 m−3 are obtained by injection of three pellets, which penetrate to or beyond the magnetic axis. In both cases, the pellet mass is well accounted for by the measured density perturbation. This work was supported by U.S. DOE Contract No. DE-AC02-CHO-3073.
X-ray spectroscopic diagnostics of a copper plasma produced by a laser line focused onto a thin foil
Authors: Lee T. N. , Molander W. A. , Ford J. L. , Elton R. C.
Published: July 2002

A 500-J, 2–6-ns, Nd laser was focused to a 100-μm-wide line on a thin (375–1500 Å) copper film of 4–23 mm length, supported by a 1500-Å thick Formvar substrate. Space-resolved 8–16-Å wavelength x-ray spectroscopy and pinhole photography were utilized. The main spectral features include intense neon-like Cu xx lines, and somewhat less intense Cu xix and xxi lines. Two types of axial structure are observed, one characterized by a finely (≲100 μm) corrugated structure, and the other by a mm scale irregular nonuniformity. The former could well be due to breakups caused by a filamentation instability. The latter appears to be associated with a mismatch between the laser pulse rise time and a characteristic (thickness-dependent) foil heating time, which makes it very sensitive to both laser-beam and copper-coating nonuniformities. The electron temperature in the Cu xx plasma, as estimated from intensity ratios of the 2p-nd (n≥4) transition lines, increases with copper thickness.
Instrument for the measurement of heat flux from a surface with uniform temperature
Authors: Baughn J. W. , Cooper D. , Iacovides H. , Jackson D.
Published: July 2002

An instrument for the measurement of heat flux from a surface with a nearly uniform temperature is described. This instrument contains a thin-film electrical resistance heater embedded in a copper cone which is thermally isolated from the surrounding walls. A differential thermocouple between the copper cone and the wall is nulled such that the electrical power becomes a direct measure of the surface heat flux. The advantage of this design over earlier sensors is its modular characteristic and its ability to be flush mounted in an external surface or mounted in the wall of a duct. It has been used to measure the local time-average heat transfer coefficient inside a circular duct. The time constant in this application was 43 s. For these measurements an uncertainty analysis is presented which shows that this instrument has an uncertainty of ±3.6% for a convective heat flux of 342 W/sq m. The major source of uncertainty was the surface area.
Ultrasonic velocity and attenuation measurements with a computer-controlled phase-sensitive detection technique
Authors: Wallace P. W. , Garland C. W.
Published: July 2002

A new ultrasonic measurement system which combines phase-sensitive detection with a computer-driven, four-channel sample-and-hold and analog-to-digital converter device is described. A microcomputer acquires the in-phase and quadrature signals from two acoustic echoes and displays these in the fashion of an x-y recorder. The ultrasonic velocity and attenuation can be calculated and plotted point by point as the run proceeds, and all data are stored for later analysis.
Some properties of microwave resonant cavities relevant to pulse-compression power amplification
Authors: Alvarez R. A.
Published: July 2002

One method of generating short, high-power microwave pulses is to store rf energy in a resonant cavity over a relatively long fill time and extract it rapidly. With this pulse compression technique power gain roughly equal to the ratio of fill time to extraction time can be obtained. During the filling of a resonant cavity some of the energy is lost in heating the cavity walls, and some will generally be reflected at the input coupling of the cavity. In this paper, we discuss the time dependence of the stored energy and related quantities and the way in which it depends on the coupling of the source to the cavity.
Coupling of an optical multichannel analyzer to a vacuum spectrometer using a photon convertor
Authors: Carolan P. G. , Manley A. M.
Published: July 2002

Multichannel spectral devices that operate in the vacuum ultraviolet typically employ microchannel plates (MCP) detectors at the instrument focal plane. An alternative is to replace the MCP with a scintillator that converts the short wavelength light to the visible. A multichannel normal incidence spectrometer that incorporates a tetraphenyl-butadiene (TPB) scintillator has been coupled to an optical multichannel analyzer (OMA) by the use of high-quality camera lenses. These allow for magnification, or reduction, of the spectral image for high-resolution or spectral survey applications.
Method for accurate resonant frequency measurements with a phase-modulated feedback loop
Authors: Bruschi L. , Torzo G.
Published: July 2002

A new method for accurate measurements of resonant frequency is described here, which uses a phase-modulated locked loop to avoid errors due to unknown phase shifts or to synchronous pickup noise.
Modified Angström’s method for measurement of thermal diffusivity of materials with low conductivity
Authors: Belling Jeffrey M. , Unsworth Joe
Published: July 2002

Semiconducting Peltier devices are used as heat sources and sinks to apply sinusoidally varying temperature versus time profiles through square sheets of material. The thermal diffusivity may be determined by recording the temperature as a function of time at the interfaces of a stack of two or three sample sheets. The design of the experimental rig is described and its use is illustrated. The thermal diffusivity of a variety of poorly conducting materials such as ceramics and polymers, having a wide range of values of thermal diffusivity, may be measured using this approach by merely selecting the frequency and amplitude to allow for the thickness and material so as to be able to resolve the temperature phase differences and amplitudes at the junctions of the sheets. In this case, the technique is applied to the determination of thermal diffusivity of polyethylene by way of illustration of the method. Reproducibility and precision achieved throughout the temperature range 4–46 °C used was found to be no better than ±5%, but precision could be improved by interfacing the equipment with a computer. Good agreement was obtained with published values of thermal diffusivity.
Cell for absorption and fluorescence spectroscopy of high-pressure and high-temperature sodium vapor
Authors: Schlejen J. , Post J. , Korving J. , Woerdman J. P.
Published: July 2002

We have constructed a 23-cm-long cell for spectroscopy of high-pressure sodium (and sodium–mercury) vapors at high temperatures. This cell operates in contrast to a heat pipe, without buffer gas. Sodium pressures up to 100 Torr (and mercury pressures up to 3000 Torr) have been obtained at temperatures up to 1500 K.