bluuebunny/crossref_metadata_2025 huggingface dataset:
Nutritional Deprivation of α‐Linolenic Acid Decreases but Does Not Abolish Turnover and Availability of Unacylated Docosahexaenoic Acid and Docosahexaenoyl‐CoA in Rat Brain
Authors: Contreras Miguel A. , Greiner Rebecca Sheaff , Chang Michael C. J. , Myers Carol S. , Salem Norman , Rapoport Stanley I.
Published: July 2002

Abstract: We applied our in vivo fatty acid method to examine concentrations, incorporation, and turnover rates of docosahexaenoic acid (22:6 n‐3) in brains of rats subject to a dietary deficiency of α‐linolenic acid (18:3 n‐3) for three generations. Adult deficient and adequate rats of the F3 generation were infused intravenously with [4,5‐3H]docosahexaenoic acid over 5 min, after which brain uptake and distribution of tracer were measured. Before infusion, the plasma 22:6 n‐3 level was 0.2 nmol ml‐1 in 18:3 n‐3‐deficient compared with 10.6 nmol ml‐1 in control rats. Brain unesterified 22:6 n‐3 was not detectable, whereas docosahexaenoyl‐CoA content was reduced by 95%, and 22:6 n‐3 content in different phospholipid classes was reduced by 83‐88% in deficient rats. Neither plasma or brain arachidonic acid (20:4 n‐6) level was significantly changed with diet. Docosapentaenoic acid (22:5 n‐6) reciprocally replaced 22:6 n‐3 in brain phospholipids. Calculations using operational equations from our model indicated that 22:6 n‐3 incorporation from plasma into brain was reduced 40‐fold by 18:3 n‐3 deficiency. Recycling of 22:6 n‐3 due to deacylation‐reacylation within phospholipids was reduced by 30‐70% with the deficient diet, but animals nevertheless continued to produce 22:6 n‐3 and docosahexaenoyl‐CoA for brain function. We propose that functional brain effects of n‐3 deficiency reflect altered ratios of n‐6 to n‐3 fatty acids.
Characterization of the Contents of Oil Bodies from the Liverwort Radula complanata1
Authors: Flegel M. , Becker H.
Published: July 2002

Abstract: With a combination of enzymatic treatment and micromanipulation, the contents of oil bodies of Radula complanata were obtained. A GC‐MS analysis revealed that 3‐methoxybibenzyl is the main constituent of these compartments. Its structure was elucidated by NMR spectroscopy.
Pollinators and Reproductive Success of the Wild Cucurbit Cucurbita maxima ssp. andreana (Cucurbitaceae)
Authors: Ashworth L. , Galetto L.
Published: July 2002

Abstract: We studied the reproductive success and pollinators of Cucurbita maxima ssp. andreana in different disturbed habitats where it grows naturally. Data were obtained from three populations. One grew within a soybean crop, the other within a corn crop, and the third in an abandoned crop field. Cucurbita maxima ssp. andreana is an annual vine with a flowering period from December to April. Male flowers appear first, thereafter female and male flowers appear together. Flower lifetime (9 h) was similar in male and female flowers. The pollinator guild was comparable for the three populations but some differences in the frequency of the insect species were observed. Native bees were the main pollinators in the population in the abandoned field, while beetles pollinated the populations in crop fields. These differences were not linked with the pre‐emergent reproductive success, fruit and seed set, or fruit quality. This is a self‐compatible plant. Fruit and seed set and fruit traits (total mass, width and length of fruits, number of seeds per fruit, and seed mass) did not show significant differences between hand‐cross and hand‐self pollinated flowers. This wild cucurbit is a generalist with respect to pollinator guild, and flower visitors seem to be highly efficient in pollen transference. Cucurbita maxima ssp. andreana is well adapted to disturbed habitats because plants ripened fruits successfully, regardless of the group of insects visiting flowers.
Flower Scent and Pollination in Selected Neotropical Palms
Authors: Knudsen J. T. , Tollsten L. , Ervik F.
Published: July 2002

Abstract:The flower scents of 14 palm species were collected in the field in Ecuador and Puerto Rico by head‐space adsorption and analysed by gas chromatography‐mass spectrometry. Insect visitors were recorded in seven of the species in Ecuador. The floral scent of the different species was dominated by a variety of compounds, e.g., the fatty‐acid derived 3‐pentanone and the hydrocarbon series dodecane to pentadecane, the benzenoid compound 1,4‐dimethoxybenzene, the isoprenoids (E)‐ocimene, myrcene, linalool, and (E)‐α‐farnesene and the nitrogen‐containing compound 2‐methoxy‐sec‐butylpyrazine. Rather than mirroring the systematics of the studied palm species, the chemical composition of the floral scent reflected the pollination mode. The scent of beetle‐pollinated species was characterized by large amounts of one or a few dominant compounds, whereas fly‐ and bee‐pollinated species contained a mixture of several compounds in smaller total amounts. We suggest that specific scent compounds, as found in the beetle‐pollinated species, have evolved as a response to pollinator preferences. The importance of olfactory cues in relation to visual cues is higher in beetle‐pollinated species than in species pollinated by flies and bees.
Negative differential resistance in AlAs/NiAl/AlAs heterostructures: Evidence for size quantization in metals
Authors: Tabatabaie N. , Sands T. , Harbison J. P. , Gilchrist H. L. , Keramidas V. G.
Published: July 2002

We report on the first electron transport measurements in (Al,Ga)As/NiAl/(Al,Ga)As semiconductor-metal-semiconductor double heterostructures grown entirely by molecular beam epitaxy. For sufficiently thin NiAl films, a voltage-controlled negative differential resistance region is observed in the axial current-voltage characteristics of our AlAs/NiAl/AlAs double-barrier structures. Room-temperature peak-to-valley ratios as high as 2 have been obtained for a 33 Å NiAl layer. The general characteristics of this phenomenon are remarkably similar to the negative differential resistance observed in all-semiconductor resonant tunneling structures. We believe that this effect can be attributed to electron size quantization in the thin metal film.
Polarization-independent electro-optically tunable narrow-band wavelength filter
Authors: Warzanskyj W. , Heismann F. , Alferness R. C.
Published: July 2002

We propose and demonstrate the first polarization-independent electro-optically tunable wavelengh filter with single-mode waveguides. The Ti:LiNbO3 filter utilizes narrow-band electro-optic TE ↔ TM conversion and employs TE/TM polarization splitters in the input and output waveguides. The filter operates at 1.52 μm with a bandwidth of only 12 Å and can be electro-optically tuned over at least 110 Å at a tuning rate of 0.55 Å/V.
Cu valence and the formation of high T
c superconductor oxides studied by x-ray photoemission spectroscopy on 200 Å Bi-Sr-Ca-Cu oxide thin films
Authors: Yeh J.-J. , DiCenzo S. B. , Hartford E. H. , Hong M. , Felder R. J.
Published: July 2002

We have used x-ray photoemission spectroscopy to study the core level spectra of 200 Å Bi-Sr-Ca-Cu oxide thin films, in the hope that the importance of the surface sensitivity of photoemission is minimized for these samples, whose thickness is only 6–7 unit cells. The samples were annealed in oxygen at temperatures ranging from 600 to 870 °C, over which temperature range the thin films are converted from an amorphous insulating phase to an ordered superconducting phase with Tc (R=0)=80 K. Regardless of the annealing temperature, the Cu 2p spectra of all films show satellite structure indicative of Cu2+ states. Even the spectrum of an as-deposited film has a satellite similar to those observed for the annealed films, including the superconducting film. The similarity of the satellite structure in the core level spectra for all films suggests that the Cu-O interactions, such as the Cu d-O p orbital charge transfer energies and the hopping integrals, remain similar when the oxides undergo the insulator-superconductor transition through high-temperature anneal. That is, the same local Cu-O interaction exists before and after the superconducting phase sets in. This implies that the Cu valence alone does not determine the properties of high Tc superconductors.
Acoustic emission field during thermoelastic martensitic transformations
Authors: Mañosa Ll. , Planes A. , Rouby D. , Morin M. , Fleischmann P. , Macqueron J. L.
Published: July 2002

The acoustic emission generated during a thermoelastic martensitic transformation, undergone by a trained Cu-Zn-Al alloy, has been simultaneously detected on four faces of a quasi-cubic sample with well-determined crystallographic orientations. The results, interpreted using the dynamic Green’s function formalism for elastic continuum media, provide the first experimental evidence that the acoustic source for thermoelastic martensitic transformations can be described in terms of a shear and a volume change effect.
Dislocation nucleation and propagation in Si0.95Ge0.05 layers on silicon
Authors: Gibbings C. J. , Tuppen C. G. , Hockly M.
Published: July 2002

Defect reveal etching has been used to study the onset of relaxation in strained Si0.95 Ge0.05 layers grown by molecular beam epitaxy on (001) silicon substrates. Etch features corresponding to nucleation centers and to interfacial and threading segments of mismatch dislocations have been observed at thicknesses well below the expected critical thickness. From these it is deduced that mismatch dislocations take the form of half-loops on {111} planes which glide approximately symmetrically outwards from existing defects along the 〈110〉 directions parallel to the interface.
Correlation of the optical gaps and Raman spectra of hydrogenated amorphous carbon films
Authors: Tamor M. A. , Haire J. A. , Wu C. H. , Hass K. C.
Published: July 2002

The Raman and infrared absorption spectra, optical gaps, and electron spin densities of amorphous carbon films deposited from hydrocarbon plasmas have been systematically studied as a function of deposition conditions and Raman probe wavelength. Although all other probes are consistent with a monotonic increase in intermediate-range order with substrate bias voltage Vb, the optical gap decreases with increasing Vb (consistent with increasing graphitic domain size) only up to the onset of sputtering, where the gap sharply increases. We propose a simple structural model for a-C:H which is consistent with these results and requires no sp3 bonding.
Conductance considerations in the reactive ion etching of high aspect ratio features
Authors: Coburn J. W. , Winters Harold F.
Published: July 2002

Very simple vacuum conductance arguments indicate that in the reactive ion etching of high aspect ratio features, the conductance is adequate to allow etch products to flow out of the feature without building up a pressure which would allow gas phase collisions to become important. On the other hand, the conductance can be expected to limit the flow of the reactive species to the bottom of the feature where the etching is taking place, thus creating the possibility of an etch rate dependence on the aspect ratio of the etched feature.
Identification of hole transitions at the neutral interstitial manganese center in silicon
Authors: Bever T. , Emanuelsson P. , Kleverman M. , Grimmeiss H. G.
Published: July 2002

The observation of a sharp line transmission spectrum in Mn-doped silicon is reported. Isochronal annealing behavior of both the line spectrum and the interstitial Mn center as determined by electron paramagnetic resonance is shown to be in excellent agreement. The temperature dependence of the spectrum is explained by the population of a local phonon mode in the initial state of the transitions. The spectrum is tentatively assigned to bound-to-bound hole transitions at the neutral interstitial Mn center.
Evidence for a negative electron-electron correlation energy in the dominant deep trapping center in silicon nitride films
Authors: Curry Sean E. , Lenahan P. M. , Krick D. T. , Kanicki J. , Kirk C. T.
Published: July 2002

We report results of electron spin resonance and capacitance versus voltage measurements of silicon nitride/silicon dioxide/silicon devices subjected to high electric field carrier injection into the nitride and (in some samples) ultraviolet illumination. Our results strongly indicate that the dominant trapping center in these silicon nitride films has a negative electron-electron correlation energy.
Lattice-matched Sc1−xErxAs/GaAs heterostructures: A demonstration of new systems for fabricating lattice-matched metallic compounds to semiconductors
Authors: Palmstro/m C. J. , Mounier S. , Finstad T. G. , Miceli P. F.
Published: July 2002

Successful growth of lattice-matched Sc1−xErxAs layers buried in GaAs with a room-temperature resistivity of ∼50 μΩ cm demonstrates the feasibility of fabricating heterostructures of lattice-matched rare-earth monopnictides and monochalcogenides in semiconductors. Reflection high-energy electron diffraction oscillations during ScAs, ErAs, and Sc1−xErxAs growth indicate monolayer-by-monolayer growth.
Giant nonlinear optical rectification at 8–12 μm in asymmetric coupled quantum wells
Authors: Rosencher E. , Bois Ph. , Vinter B. , Nagle J. , Kaplan D.
Published: July 2002

We report giant, nonlinear optical rectification in asymmetric quantum wells weakly coupled by an intermediate potential barrier. This phenomenon originates from (i) macroscopic displacements (30 nm) of carriers during optical transitions and (ii) large storage times of excited electrons because of a slow transfer mechanism between the wells (≊6 ps at 77 K). The resulting rectification coefficient is 1.62×10−3 m/V per well, more than six orders of magnitude higher than in bulk GaAs. These structures really behave as giant ‘‘quasimolecules’’ optimized for infrared optical nonlinearities and their use may be envisioned for a new class of infrared detectors.
High precision temperature- and energy-dependent refractive index of GaAs determined from excitation of optical waveguide eigenmodes
Authors: Kisting S. R. , Bohn P. W. , Andideh E. , Adesida I. , Cunningham B. T. , Stillman G. E. , Harris T. D.
Published: July 2002

A GaAs-Al0.22Ga0.78As heterostructure was prepared and used as a multimode optical waveguide. Propagation constants for individual modes were measured by exciting one mode at a time via real-space surface grating couplers, and the resulting eigenmode distributions were used to obtain the refractive index of GaAs at a matrix of temperatures and photon energies spanning 40 K<T<300 K and 1.40 eV<hν<1.50 eV. Values for dn/dT and the extrapolated refractive index at T=0 K were also obtained. The dominant error source in these measurements was uncertainty in the angular placement of the sample. These measurements agree well with the few pre-existing temperature-dependent measurements, but are an order of magnitude more precise.
Broadband magneto-optic waveguide isolator
Authors: Wolfe R. , Dillon J. F. , Lieberman R. A. , Fratello V. J.
Published: July 2002

Isolation ratios between −32 and −37 dB have been measured on a magneto-optic waveguide isolator over a wavelength range from 1.43 to 1.58 μm. This tunable broadband operation was achieved by rotating the input polarization angle by 22.5° from the waveguide surface of a 45° Faraday rotator to overcome the harmful effects of moderate linear birefringence. The output polarizer was then rotated to maximize the isolation ratio at each wavelength. The excess forward loss introduced by this procedure was 10% or less over this wavelength range. It is shown that excellent isolation and moderate excess forward loss (1 dB or less) can be achieved with triple garnet film, single-mode ridge waveguides over a wavelength range of ±0.2 μm around the etch-tuned wavelength, and over a temperature range of ±30 °C, which greatly enhances the usefulness of these waveguide isolators.
Synthesis of superconducting YBa2Cu3O7−δ thin films on nickel-based superalloy using in situ pulsed laser deposition
Authors: Kumar Ashok , Ganapathi L. , Kanetkar S. M. , Narayan J.
Published: July 2002

Good quality superconducting YBa2Cu3O7−δ thin films have been grown on nickel-based superalloy, viz., Hastelloy with yttria-stabilized zirconia buffer layers employing a single chamber in situ pulsed laser deposition process. YBa2Cu3O7−δ films were found to be textured with c axis perpendicular to the substrate. A plot of normalized resistance against temperature exhibited a smooth metallic behavior followed by an onset superconducting transition at 92 K with the zero resistance temperature (Tc0) of 86 K. Critical current density for these films at 77 K was ∼7000 A cm−2 and critical current (Ic) was observed to be proportional to (1 − T/Tc0)1.4.
Observation of multiple wavelength soliton collisions in optical systems with fiber amplifiers
Authors: Andrekson P. A. , Olsson N. A. , Becker P. C. , Simpson J. R. , Tanbun-Ek T. , Logan R. A. , Wecht K. W.
Published: July 2002

Solitons of different wavelengths are found to exhibit substantial spectral and temporal changes when collisions are centered in erbium-doped fiber amplifiers. By using two soliton pulse trains, with ∼70-ps-wide pulses, spectrally separated by 1.8 Å, and 106 km of non-dispersion-shifted fiber, we observe a spectral and temporal shift of as much as 0.35 Å and 55 ps, respectively, for each soliton. Both soliton wavelengths shift the same amount, but in opposite directions and remain undisturbed in terms of shape and amplitude after the collision. This shift may impose limitations on multiple wavelength soliton based communication systems utilizing fiber amplifiers.
Subsurface heating effects during pulsed laser evaporation of materials
Authors: Singh Rajiv K. , Bhattacharya D. , Narayan J.
Published: July 2002

We have theoretically and experimentally investigated the thermal effects of targets evaporated by nanosecond laser pulses. The subsurface temperatures were calculated to be higher than the surface temperatures during planar surface evaporation of the target material. While the evaporating surface is being cooled due to the latent heat of vaporization, subsurface superheating occurs due to a finite absorption depth of the laser beam. The temperature profiles of silicon targets irradiated by nanosecond laser pulses were determined by solving the one-dimensional heat flow equation using an implicit finite difference method. The subsurface superheating increased at higher energy densities, and decreased with increasing absorption coefficient of the material. This internal heating of the target during pulsed laser irradiation can be correlated with the explosive removal of material from the target. This may lead to deposition of small particles on films fabricated by the pulsed laser evaporation technique.
Photobleaching of light-induced paramagnetic defects in amorphous silicon nitride films
Authors: Crowder M. S. , Tober E. D. , Kanicki J.
Published: July 2002

We report for the first time the photobleaching of light-induced paramagnetic defects in nitrogen-rich hydrogenated amorphous-silicon nitride thin films. We have determined the photon energy dependence and the stretched exponential time dependence for photoproduction and the subsequent photobleaching of the light-induced electron spin resonance signal. Photobleaching is a reversible process that occurs at low temperature (100 K) and at room temperature. These findings are in support of an optically induced rearrangement of charges in existing defects as an important light-induced effect in a-SiN1.6:H.
Cryogenic reactive ion etching of silicon in SF6
Authors: Bestwick Tim D. , Oehrlein Gottlieb S. , Angell David
Published: July 2002

Reactive ion etching of Si and SiO2 in SF6 plasmas in which the samples are mounted on a liquid-nitrogen-cooled electrode has been studied. At this temperature SF6 condenses on the electrode surface, but it is possible to maintain a plasma. Si etch anisotropy has been demonstrated at low temperature, in agreement with previous studies. Mass spectrometry and optical emission spectroscopy indicate that fluorine is the dominant species in the plasma because SF6 and SFx species are removed from the gas phase by condensation.
THz time-domain spectroscopy of high Tc substrates
Authors: Grischkowsky D. , Keiding So/ren
Published: July 2002

Using the method of time-domain spectroscopy, we have measured the absorption and dispersion from 0.2 to 2 THz of the high Tc substrates, magnesium oxide, yttria-stabilized zirconia (YSZ), and lanthanum aluminate. Our measurements on YSZ and LaAlO3 at both room temperature and 85 K show unacceptably large absorptions for high-speed transmission line applications. At 85 K, MgO is shown to be an excellent material in terms of its low loss at THz frequencies.
Sub-100 nm lines produced by direct laser ablation in polyimide
Authors: Phillips H. M. , Callahan D. L. , Sauerbrey R. , Szabó G. , Bor Z.
Published: July 2002

Periodic line structures with a period of 167 nm and linewidths varying from 35 to 100 nm have been produced on polyimide by direct ablation with a KrF laser using an interferometric technique. Since ablation is a nonlinear process, the resolution can exceed that expected from the wavelength and numerical aperture of the system and the linewidth can be controlled by varying the laser fluence. This externally generated period of 167 nm prevents the spontaneous growth of periodic surface structures due to radiation remnants.
Characterization of boron-doped silicon epitaxial layers by x-ray diffraction
Authors: Baribeau J.-M. , Rolfe S. J.
Published: July 2002

Heavily boron-doped (Na≳1×1019 cm−3)Si epilayers grown on (100)Si by molecular beam epitaxy are investigated using double-crystal x-ray diffractometry. The variation of the Si lattice constant as a function of boron concentration is obtained from a combination of (400) rocking curve analysis and secondary-ion mass spectrometry measurements on uniformly doped epilayers. This result is then used to determine the concentration profile in nonuniformly doped structures via dynamical simulations of experimental x-ray rocking curves. The structures investigated include a pipi doping superlattice and the results are found to be in excellent agreement with secondary-ion mass spectrometry depth profiles.
Reflective filters based on single-crystal GaN/AlxGa1−xN multilayers deposited using low-pressure metalorganic chemical vapor deposition
Authors: Khan M. Asif , Kuznia J. N. , Van Hove J. M. , Olson D. T.
Published: July 2002

In this letter we report the first ever fabrication and characterization of reflective quarter wave filter stacks based on single-crystal GaN/AlxGa1−xN multilayers. The filters were designed for peak reflectivities at 400 and 450 nm. The designed epitaxial layers were deposited using a low-pressure metalorganic chemical vapor deposition system. We measured the interface abruptness and the thicknesses of individual stack layers (GaN and AlxGa1−xN) using a unique Zalar sputter Auger depth profiling procedure and x-ray analysis. The abruptness was found to be better than 40 Å, which is the instrument measurement limit. For 18-period filters the measured peak reflectivities of 80% and 95% agreed reasonably well with the estimations of 88% and 95% of our multilayer designs.
Large-area, two-sided superconducting YBa2Cu3O7−x films deposited by pulsed laser deposition
Authors: Foltyn S. R. , Muenchausen R. E. , Dye R. C. , Wu X. D. , Luo L. , Cooke D. W. , Taber R. C.
Published: July 2002

The in situ excimer-laser deposition process is demonstrated, for the first time, to be an effective technique for producing superconducting films on both sides of substrate wafers of up to 5 cm in diameter. By exploiting the directed nature of the laser produced plume and controlling its off-axis composition, thickness and stoichiometric variations of less than ±5% have been obtained over the entire surface. Coatings on 5-cm-diam LaAlO3 substrates have uniform transition temperatures of ≥ (R18)90 K on both sides, with a critical current density at 77 K and zero field of 2.5×106 A/cm2. The 10 GHz surface resistance for samples cut from a 5 cm wafer is 40–60 μΩ at 4 K.
Plasma-assisted laser deposition of YBa2Cu3O7−δ
Authors: Kwok H. S. , Kim H. S. , Witanachchi S. , Petrou E. , Zheng J. P. , Patel S. , Narumi E. , Shaw D. T.
Published: July 2002

The change in superconducting properties due to the presence of a bias electrode in in situ laser deposition was explored. It was found that the bias ring allowed a 50–70 °C reduction in the deposition temperature. This reduction is correlated to an increase in ion current impinging on the substrate.
Focused ion beam high T
c superconductor dc SQUIDs
Authors: Zani M. J. , Luine J. A. , Simon R. W. , Davidheiser R. A.
Published: July 2002

Light-ion irradiation can controllably reduce the critical current density of high-temperature superconductor thin films. Reported here is the behavior of ErBa2Cu3O7 films patterned into microbridge dc superconducting quantum interference device (SQUID) structures and irradiated with a rastered high-energy focused ion beam of 70 nm diameter. The resultant SQUIDs have demonstrated up to 51% critical current modulation at 46 K. Microbridges with appropriate critical currents manifest Josephson-like behavior in the form of well-defined Shapiro steps appearing with microwave radiation. The IcRn products of these microbridges are typically a few millivolts and the temperature dependence of their normal-state resistance is consistent with conduction through metallic filaments.
Oscillations in giant magnetoresistance and antiferromagnetic coupling in [Ni81Fe19/Cu]N multilayers
Authors: Parkin S. S. P.
Published: July 2002

We report giant magnetoresistance in [Ni81Fe19/Cu]N multilayers. Saturation magnetoresistance values exceeding 16% for saturation fields of only 600 Oe are found at 300 K. In addition, we show evidence for well-defined oscillations in saturation magnetoresistance as a function of Cu spacer layer thickness at 4.2 K, with an oscillation period and phase similar to that in Co/Cu multilayers. However, the temperature dependence of the magnetoresistance, while weak for thin Cu layers, is much stronger for thicker Cu layers. Consequently at 300 K only a single oscillation in magnetoresistance for thin Cu layers is found. We show that the properties of the Ni81Fe19/Cu multilayers are very sensitive to annealing at moderate temperatures, which may limit the possible technological applications of such structures.
Thermal stability of GaAs (C)/InAs superlattices grown by metalorganic molecular beam epitaxy
Authors: Abernathy C. R. , Wisk P. W. , Pearton S. J. , Hobson W. S. , Fuoss P. H. , Lamelas F. J. , Chu S. N. G. , Ren F.
Published: July 2002

The thermal stability of GaAs(C)/InAs superlattices grown by metalorganic molecular beam epitaxy on InP substrates has been examined by Hall measurements, transmission electron microscopy, and high resolution x-ray diffraction. These structures provide an ordered counterpart to a random In0.53Ga0.47As alloy, in which high concentration carbon doping is generally difficult to achieve. In a 43 period (23 Å GaAs/26 Å InAs) superlattice in which the GaAs was C-doped and the InAs undoped an average hole concentration of 7×1019 cm−3 and hole mobility of 20 cm2 V−1 s−1 was achieved. Such structures are stable against rapid thermal annealing (10 s) up to 750 °C. An 850 °C/10 s anneal reduced the hole concentration to 1.5×1019 cm−3, accompanied by the onset of intermixing of the superlattice. The surface morphology of all but very thick (36 Å GaAs/40 Å InAs) period superlattice structures remained specular, even after 850 °C, 10 s annealing. These superlattices show properties suitable for use in a range of electronic and photonic devices, particularly InP-based lasers and heterojunction bipolar transistors.
Isoelectronic oxygen in II-VI semiconductors
Authors: Akimoto K. , Okuyama H. , Ikeda M. , Mori Y.
Published: July 2002

The properties of isoelectronic oxygen in II-VI semiconductors were studied by photoluminescence measurements. It was found that oxygen in CdTe, CdS, and ZnS can act as an acceptor as well as in ZnSe, and that the acceptor levels of oxygen in CdTe, CdSe, and ZnS are shallower than those of typical acceptors such as Na. Charge transfer from the host lattice to the oxygen atom may play an important role in oxygen acting as an acceptor. Based on the charge-transfer model, it can be qualitatively interpreted that there are two roles of oxygen in II-VI compounds: acting as an acceptor or as a trap, and that they are classified by the ionicity of the compound. We also now understand better the chemical trend of the oxygen-acceptor levels becoming more shallow compared to the typical acceptors.
Existence of a photonic band gap in two dimensions
Authors: Meade Robert D. , Brommer Karl D. , Rappe Andrew M. , Joannopoulos J. D.
Published: July 2002

A systematic theoretical investigation is undertaken in order to identify a two-dimensional periodic dielectric structure that has a complete in-plane photonic band gap for both polarizations. Of the various structures studied, only a triangular lattice of air columns is found to have the desired band-gap properties. Microwave transmission experiments are performed to test the theoretical predictions.
Defects in low-temperature-grown GaAs annealed at 800 °C
Authors: Ohbu I. , Takahama M. , Hiruma K.
Published: July 2002

GaAs grown by molecular beam epitaxy (MBE) at 300 °C is annealed at 800 °C and optical properties are studied using photoluminescence spectroscopy (PL) and infrared-absorption spectroscopy. Three kinds of defects are observed. One of them is attributed to gallium vacancies with an energy level at 0.3 eV above the valence-band edge. The concentration of gallium vacancies is increased by the high-temperature annealing. The GaAs can render inactive free electrons of 1.3×1018 cm−3 even after annealing at 800 °C for 10 min.
Excitation of high frequency surface acoustic waves by phase velocity scanning of a laser interference fringe
Authors: Nishino Hideo , Tsukahara Yusuke , Nagata Yoshihiko , Koda Toshio , Yamanaka Kazushi
Published: July 2002

We present a novel method for generating 100 MHz band surface acoustic wave (SAW) by using a scanning interference fringe at the phase velocity of the SAW. The scanning interference fringe is obtained by intersecting two laser beams with different frequencies, and used as a thermoelastic source. The principle of this method is described, and experimentally demonstrated in the 110 MHz Rayleigh waves on an aluminum specimen generated by a long-pulse (140 ns) Q-switched Nd:YAG laser.
Raman scattering analysis of relaxed GexSi1−x alloy layers
Authors: Mooney P. M. , Dacol F. H. , Tsang J. C. , Chu J. O.
Published: July 2002

We have used Raman scattering to evaluate thick epitaxial GexSi1−x layers with 0.20≤x≤0.43 grown on Si (100) substrates. We show that a detailed consideration of the composition dependencies of the relative intensities of the various phonon modes can enhance the sensitivity of Raman scattering to variations in composition and strain. We find that samples are uniform on a scale of ≂1 μm laterally and <1000 Å in the growth direction.
Phase and amplitude modulation based on intersubband transitions in electron transfer double quantum wells
Authors: Dupont E. , Delacourt D. , Berger V. , Vodjdani N. , Papuchon M.
Published: July 2002

We present an experimental study of the intersubband electroabsorption and electrorefraction of a coupled asymmetrical quantum well structure. The field induced carrier transfer from a wide well to a narrow well yields interesting electro-optical properties. For instance, we found a particular wavelength region where a strong phase modulation is associated with a low absorption modulation. We verified that the measurements of the correlated phase and amplitude modulation were consistent with Kramers–Kronig transformations of experimental differential absorption spectra.
Successive paraelectric-antiferroelectric-ferroelectric phase transitions in highly ordered perovskite lead ytterbium tantalate
Authors: Yasuda Naohiko , Konda Junichi
Published: July 2002

Dielectric properties such as the permittivity, dielectric loss tangent and spontaneous polarization of highly ordered perovskite Pb(Yb1/2Ta1/2)O3 (PYT) compound which were prepared through the wolframite phase oxide were investigated. A dielectric anomaly which is characterized by a slight dielectric dispersion was found at a temperature Tm (e.g., Tm; 177 and 183 °C at 1 and 100 kHz, respectively). At temperatures below Tm, polarization P-electric field E hysteresis loops were observed, and then ferroelectricity in antiferroelectric ordered perovskite PYT was confirmed. Highly ordered PYT has two successive paraelectric-antiferroelectric-ferroelectric phase transitions, similar to highly ordered perovskite lead cobalt tungstate (PCW).
Surface recombination velocities on processed InGaP p-n junctions
Authors: Pearton S. J. , Ren F. , Hobson W. S. , Abernathy C. R. , Masaitis R. L. , Chakrabarti U. K.
Published: July 2002

InGaP p-n junction mesa diodes were fabricated by wet or dry etching, and the surface recombination velocities (Sv) measured from the size dependence of the current density-voltage characteristics. Within experimental error, the dry etching does not increase the S values, which are in the range 4.4–5.2×104 cm s−1. Subsequent low temperature annealing or plasma exposure did not degrade the surface properties of the mesa diodes, and some improvement was observed with (NH4)2Sx treatment. While the InGaP diodes are relatively insensitive to typical processing steps, comparable AlGaAs p-n junction show much larger changes in surface recombination velocities.
Spatial solitons in photorefractive Bi12TiO20 with drift mechanism of nonlinearity
Authors: Castillo M. D. Iturbe , Aguilar P. A. Marquez , Sanchez-Mondragon J. J. , Stepanov S. , Vysloukh V.
Published: July 2002

The analysis performed shows that in cubic photorefractive crystals (of Bi12SiO20 and GaAs type in particular) with drift (‘‘quasilocal’’) mechanism of optical nonlinearity spatial soliton propagation can be observed for reasonable external dc electric fields 10–15 kV/cm. These photorefractive solitons which can be excited at very low cw laser power are stable in a broad region of signal(soliton)/incoherent(spatially uniform) intensity ratio, and allow easy switching from bright to dark type. Original experimental results of self-channeling of submicrowatt HeNe laser beam in a cubic Bi12TiO20 sample under external dc field are presented.
Method for imaging sidewalls by atomic force microscopy
Authors: Martin Yves , Wickramasinghe H. Kumar
Published: July 2002

We demonstrate a new method for imaging vertical and near vertical surface features by atomic force microscopy (AFM). It is based on an attractive force mode AFM, equipped with a special boot-shaped tip, coupled with a measurement of slope and with a special tracking technique. Surface profiling is achieved through a novel servo and scanning system. Mapping sidewall profile opens the door to measurement of critical dimensions (width and wall angles) of lines and trenches in integrated circuits, with high accuracy.
Electronic domain pinning in Pb(Zr,Ti)O3 thin films and its role in fatigue
Authors: Warren W. L. , Dimos D. , Tuttle B. A. , Nasby R. D. , Pike G. E.
Published: July 2002

Switchable polarization can be significantly suppressed in Pb(Zr,Ti)O3 thin films by optical, thermal, and electrical processes. The optical (thermal) suppression effects occur by biasing the ferroelectric near the switching threshold and illuminating the material with band-gap light (heating the material to ≊100 °C). The electrical suppression effect, commonly known as electrical fatigue, occurs by subjecting the ferroelectric to repeated polarization reversals. It is found that the suppressed polarization in all three cases can be restored to essentially its initial polarization value by injecting electronic charge carriers into the ferroelectric. This strongly suggests that all three forms of degradation involve locking domains by electronic charge trapping centers.
Low-temperature Si(001) epitaxy using low-energy (〈E 〉≂18 eV) Si atoms
Authors: Lee N.-E. , Tomasch G. A. , Greene J. E.
Published: July 2002

The use of energetic (average energy ≂18 eV), rather than thermal (≂0.2 eV), Si beams during deposition at R=1 Å s−1 was found to increase the Si(001) epitaxial thickness te (100 Å–1.2 μm) by up to an order of magnitude over the growth temperature range Ts=80–300 °C. The overall increase in te is attributed primarily to a more effective filling of interisland trenches which form during growth in the low adatom mobility two-dimensional multilayer mode and provide preferential sites for the nucleation of the terminal amorphous phase. In addition, the behavior of te(Ts) at constant R and te(R) at constant Ts is quite different than that reported for films grown by molecular-beam epitaxy. A decrease in the slope of ln(te) versus −1/Ts at Ts<225 °C indicates an additional increase in the epitaxial thickness at very low growth temperatures while at constant Ts, 150 °C, te increases with decreasing R, reaches a maximum, and then decreases. These latter effects are explained in terms of changes in average island sizes giving rise to corresponding changes in interlayer mass transport.
Pulsed laser deposition of high-quality NbN thin films
Authors: Treece Randolph E. , Horwitz James S. , Claassen John H. , Chrisey Douglas B.
Published: July 2002

Pulsed laser deposition has been used to grow superconducting NbN thin films from niobium targets in a reactive gas atmosphere of N2 (10% H2). The structural and electrical properties of the deposited films have been determined as a function of substrate temperature and crystallographic orientation. Highly textured NbN was deposited on MgO (100). Films deposited on MgO at 600 °C in 60 mTorr gas pressure were characterized by Tc=16.6 K, Jc (4.2 K)=7.1 MA/cm2, and λ(0)=3200 Å. Films grown on amorphous fused silica, under the same conditions, were polycrystalline and characterized by Tc=11.3 K and Jc (4.2 K)=1.8 MA/cm2.
Reaction and regrowth control of CeO2 on Si(111) surface for the silicon-on-insulator structure
Authors: Chikyow T. , Bedair S. M. , Tye L. , El-Masry N. A.
Published: July 2002

The interface structure of CeO2/Si(111) grown by laser ablation in ultrahigh vacuum was investigated by, reflection high-energy electron diffraction (RHEED), high resolution transmission electron microscopy (HRTEM), and Auger electron spectroscopy (AES). The deposited film was single-crystalline CeO2. However, during the deposition, a reaction between CeO2 and Si occurred at the interface, that resulted in the formation of an oxygen deficient amorphous CeO2−x and SiO2 layer. Post annealing in oxygen atmosphere caused the regrowth of crystalline CeO2 from CeO2−x and increased of the SiO2 thickness. The modified structure of CeO2/SiO2/Si is expected to be useful as a silicon-on-insulator structure since it maintains the desirable SiO2/Si interface followed by a single-crystal insulating film lattice-matched to Si.
Delta-doped Schottky diode nonlinear transmission lines for 480-fs, 3.5-V transients
Authors: van der Weide D. W.
Published: July 2002

Waveforms having 480-fs, 3.5-V transients were generated with a delta-doped GaAs Schottky diode nonlinear transmission line and measured with an on-chip diode sampling bridge at room temperature. Integration of delta-doped diodes with high C(V) nonlinearity and uniform-doped diodes with high speed made possible a transient waveform two times shorter than any previously measured.
1.54 μm wavelength emission of erbium-doped silicon films grown by ion beam epitaxy using sputtering-type metal ion source
Authors: Matsuoka Morito , Tohno Shun-ichi
Published: July 2002

Erbium-doped silicon films are grown by ion beam epitaxy using a newly developed electric-mirror sputtering-type metal ion source in an ultrahigh vacuum. A precise and steep profile of the erbium concentration, ranging from 1×1016 to 6×1020 cm−3, is achieved in situ by sputtering the erbium metal pellet with ions extracted from the silicon ion source. The oxygen concentration in the films, which is important to effective luminescence of erbium in silicon, is controlled in situ in the range from below 1×1018 to 2×1020 cm−3 by using argon gases containing oxygen impurities ranging from 1 ppb to 100 ppm. The oxygen concentration trapped in the silicon films strongly depends on the erbium concentration doped in the films. The erbium atoms are selectively oxidized in the host silicon film. As a result, the photoluminescence of 1.54 μm wavelength light is clearly observed in as-deposited films.
Paradoxical predictions and a minimum failure time in electromigration
Authors: Filippi R. G. , Biery G. A. , Wachnik R. A.
Published: July 2002

A paradox arises when the two-parameter log-normal distribution is used to predict early electromigration lifetimes of a two-level structure with Ti–AlCu–Ti stripes and interlevel W stud-vias. The paradox is a direct consequence of the observed increase in the log-normal sigma (σ) as the current density decreases and/or as the maximum allowed resistance change increases. The increase in σ implies that the first failures from equal and large sample sizes are expected to occur at low current densities rather than at high current densities. Similarly, for example, samples at relatively low cumulative failure are expected to fail at high levels of resistance change before failing at low levels of resistance change. This apparent paradox is resolved by testing a large set of samples and fitting the failure data to the three-parameter log-normal distribution. The third parameter, an incubation time or a minimum time required before failure can occur, is shown to increase as the maximum allowed resistance change increases.
Measurement of internal quantum efficiency and losses in vertical cavity surface emitting lasers
Authors: Kuksenkov D. V. , Temkin H. , Swirhun S.
Published: July 2002

A new experimental technique for the measurement of intrinsic parameters of vertical cavity surface emitting lasers is suggested. Using an external cavity with variable reflectivity we measure the internal quantum efficiency of ηi=0.6 and internal losses of αi=10 cm−1 in a gain guided AlGaAs/GaAs laser operating at 850 nm. In addition, transparency current of I0∼0.46×Ith is obtained, in excellent agreement with a direct measurement.
SiO2 hole traps with small cross section
Authors: Afanas’ev V. V. , de Nijs J. M. M. , Balk P.
Published: July 2002

SiO2 hole traps with small cross section (σ≤10−15 cm2) in Al gated metal–oxide–silicon capacitors have been studied using vacuum ultraviolet hole injection in combination with capacitance voltage measurements. The data show that small σ hole traps are related to H/hole pairs or protons trapped in the oxide. Since accumulated positive charge and H vanish simultaneously from the oxide in times of the order of 102 s, it is proposed that they migrate together through the oxide as a proton. The small σ hole traps are not associated with defects in the as-grown oxide; they are generated by radiation induced release of atomic H. Their number is governed by the rates of release and of removal (dimerization) of atomic H.
Optical second harmonic images of 90° domain structure in BaTiO3 and periodically inverted antiparallel domains in LiTaO3
Authors: Uesu Y. , Kurimura S. , Yamamoto Y.
Published: July 2002

By using the second harmonic generation (SHG) microscope, which we have recently developed, two-dimensional images of ferroelectric domain structure were successfully obtained by exploiting the inhomogeneous distribution of the quadratic nonlinear optical constant in specimens. 90° domain structure of BaTiO3 was observed with intensity contrast of the second harmonic wave. A periodically inverted antiparallel domain in LiTaO3 was also observed. This fact indicates that the SHG microscope provides a new method of observing antiparallel ferroelectric domain structure in a nondestructive way, which is especially important for characterizing the quasiphase matched devices for compact blue light lasers.
Structural and electrical properties of Ba0.5Sr0.5TiO3 thin films with conductive SrRuO3 bottom electrodes
Authors: Jia Q. X. , Wu X. D. , Foltyn S. R. , Tiwari P.
Published: July 2002

Epitaxial Ba0.5Sr0.5TiO3 (BST) thin films were deposited on LaAlO3 substrates with the conductive metallic oxide SrRuO3 (SRO) as a bottom electrode by pulsed laser deposition. The BST and SRO films were (𝒽00) and (00ℓ) oriented normal to the substrate surface, respectively. The epitaxial nature of both BST and SRO layers was determined by the measurement of in-plane orientation with respect to the major axes of the substrate. Ion beam channeling with a minimum yield of around 10% from Rutherford backscattering spectrometry demonstrated the films to be of high crystallinity. A dielectric constant around 500 and dielectric loss less than 0.01 at a frequency of 10 kHz were measured on the capacitors with a configuration of Ag/BST/SRO. Electrical measurements on such epitaxial BST films showed a breakdown voltage above 106 V/cm and a leakage current density of less than 5×10−8 A/cm at a field intensity of 2×105 V/cm. These results prove the BST/SRO heterostructure to be a good combination for microelectronic device applications.
Slit pulsed valve for generation of long-path-length supersonic expansions
Authors: Lovejoy Christopher M. , Nesbitt David J.
Published: July 2002

We describe a valve for production of jet-cooled species in a pulsed, long-path-length (1.2-cm) supersonic expansion. The valve produces 150–600-μs-duration pulses at repetition rates up to 60 Hz from a nozzle with variable slit width, and is suitable for use with corrosive gases and vapors.
Electronic subband studies in In0.52Al0.48As/InxGa1−xAs one-side-modulation-doped asymmetric coupled double quantum wells
Authors: Kim T. W. , Jung M. , Park T. H. , Yoo Keon-Ho , Yoo Kyung-Hwa , Ihm G.
Published: July 2002

Shubnikov–de Haas (SdH) and van der Pauw Hall effect measurements on In0.52Al0.48As/InxGa1−xAs coupled double quantum wells grown by metalorganic chemical vapor deposition have been carried out to investigate the magnetotransport properties of an electron gas and to determine the subband energies and wave functions in the coupled quantum wells. Transmission electron microscopy measurements showed that In0.8Ga0.2As and In0.53Ga0.47As quantum wells were separated by an In0.25Ga0.75As potential barrier in an active region. The SdH measurements at 1.5 K demonstrated clearly the existence of a quasi-two-dimensional electron gas in the quantum wells. The fast Fourier transformation results for the SdH data clearly indicate the occupation of three subbands in the In0.52Al0.48As/InxGa1−xAs coupled quantum wells. Electron subband energies and wave functions in the quantum wells were calculated by a self-consistent method taking into account exchange-correlation effects. The first and second excited subband wave functions in the asymmetric quantum well are strongly coupled over both In0.8Ga0.2As and In0.53Ga0.47As wells.
Thermally stable, low specific resistance (1.30×10−5 Ω cm2) TiC Ohmic contacts to n-type 6Hα-SiC
Authors: Chaddha A. K. , Parsons J. D. , Kruaval G. B.
Published: July 2002

An array of transfer length measurement (TLM) structures was formed on an electrically isolated (0001) n+6Hα-SiC epilayer. The n+6Hα-SiC epilayer contained an in situ incorporated nitrogen concentration of 4×1019 cm−3. The specific contact resistance (ρc), sheet resistance (Rs), contact resistance (Rc), and transfer length (LT) were calculated from resistance (RT) versus contact spacing (d) measurements obtained from 17 TLM structures. The linear curves used for these calculations were fit to the RT versus d data by calculating the standard error of linear regression of RT on d; where, the average correlation coefficient with a straight line was 1.0000 and the average standard error of linear regression of RT on d was 0.08 Ω. The resulting average values were: ρc=1.30×10−5 Ω cm2, Rs=14.4 Ω/square, Rc=1.6 Ω, and LT=9.5 μm. The (111) TiC contacts, epitaxially grown by chemical vapor deposition, were thermally and chemically stable at 1400 °C. The TiC contacts could not be scratched with a tungsten carbide scriber, nor delaminated from the 6Hα-SiC substrate.