bluuebunny/crossref_metadata_2025 huggingface dataset:
Ecology of the Pteridophytes on the Southern Slopes of Mt. Kilimanjaro. Part II: Habitat Selection
Authors: Hemp A.
Published: July 2002

Abstract: Based on the evalutation of 957 vegetation plots on the southern slope of Mt. Kilimanjaro, habitat preferences for 140 species of pteridophytes were evaluated. Using the average percentage cover value, and taking into account the pteridophyte flora's composition, life form spectra and its spectra of seasonal growth pattern, eight vegetation formations were recognized. Ferns contributed less than 1 % of the vegetation cover of salt marshes, ruderal vegetation, grasslands and (sub‐)alpine heathlands. In contrast, pteridophytes constituted the most important vascular plant group on rocks, where 64 species were found, forming about two‐thirds of the vegetation cover. With respect to alpha and beta diversity and fern biomass, luxuriant montane forest was the main habitat for pteridophytes on Mt. Kilimanjaro. Here 130 pteridophyte species (93 % of the whole pteridophyte flora of the study area), on average, contributed 16 % of the total vegetation cover. Epiphytic ferns, tree ferns and filmy ferns had their main distribution between 1900 and 2400 m, in a zone coinciding with the maximum rainfall on Mt. Kilimanjaro's southern slope.

Poikilohydrous species were typical of dry habitats, such as on rocks, in meadows or along roadsides, but they also occurred in the often sun‐exposed epiphyte layer in moist montane forests. Deciduous species, which were in many cases fire resistant, had a similar distribution; however, inside the forest belt they were restricted to the lower and upper parts, where fires are a common phenomenon. Evergreen species were the dominant group in swamps, forests and forest clearings.

Compared to other volcanoes in East Africa, Mt. Kilimanjaro is distinctly richer in fern species in general and in filmy ferns, tree ferns and epiphytic ferns in particular, suggesting that the forest belt of the southern slope of Mt. Kilimanjaro is wetter than those of other high mountains in East Africa.
Phillips–Tikhonov regularization of plasma image reconstruction with the generalized cross validation
Authors: Iwama Naofumi , Yoshida Hiromasa , Takimoto Hitoshi , Shen Yun , Takamura Shuichi , Tsukishima Takashige
Published: July 2002

The use of the Phillips–Tikhonov regularization is proposed for numerically stabilizing the ill-conditioned plasma image reconstructions. An objective function to be minimized leads to a linear estimator of the image intensity distribution and, with the aid of the singular value decomposition, makes it possible to use the generalized cross validation for optimizing a regularization parameter. An excellent behavior of the estimator with computational facility is obtained on the Hα emission computerized tomography of a toroidal plasma.
Thin-film deposition by a new laser ablation and plasma hybrid technique
Authors: Krishnaswamy J. , Rengan A. , Narayan J. , Vedam K. , McHargue C. J.
Published: July 2002

We have developed a new laser ablation and plasma hybrid technique for depositing thin diamond-like carbon (DLC) films on Si〈100〉 substrates at room temperature and at 110 °C with improved optical and mechanical properties. The technique involves coupling of laser energy (λ=0.308 μm, pulse duration=40 ns, and power 125 MW/cm2) to a graphite target and superimposing capacitively stored energy (2–3 J at 3 kV) to the laser ablated spot. The laser- and plasma-deposited diamond-like carbon films were analyzed by spectroscopic ellipsometry and microhardness measurements. These films showed considerable improvements in both uniformity and homogeneity. Optical properties and hardness of the films deposited by this technique closely match the DLC films. We discuss possible causes of improvements in the above properties of these films.
Thermodynamic explanation to the enhanced diffusion of base dopant in AlGaAs-GaAs n
p
n bipolar transistors
Authors: Deppe D. G.
Published: July 2002

A model is presented which accounts for the anomalous diffusion of the p-type base dopant during the growth of AlGaAs-GaAs npn bipolar transistors. The model is based upon Fermi level pinning at the crystal surface during epitaxial growth which leads to an increased concentration of column III interstitial defects in heavily n-type AlGaAs or GaAs. The excess column III interstitials generated in the n-type crystal flow into the p+ base region causing a transfer of the p-type impurity atoms from column III lattice sites to interstitial positions through a ‘‘kick-out’’ mechanism. Once in interstitial positions the impurity atoms are known to diffuse rapidly. The model is consistent with previously proposed mechanisms for both impurity diffusion and column III self-diffusion.
Microwave surface resistance of epitaxial YBa2Cu3O7 thin films on sapphire
Authors: Char K. , Newman N. , Garrison S. M. , Barton R. W. , Taber R. C. , Laderman S. S. , Jacowitz R. D.
Published: July 2002

Microwave surface resistance data were measured on pairs of YBa2Cu3O7 thin films on Al2O3 {11_02} substrates by a parallel plate resonator technique. The surface resistance Rs at 10 GHz was 65 μΩ at 10 K and 850 μΩ at 77 K. These epitaxial YBa2Cu3O7 films were grown on 500-Å-thick buffer layers of SrTiO3. X-ray diffraction data showed that the YBCO thin films with the SrTiO3 buffer layers have better in-plane epitaxy than those without such buffer layers. Critical current density of 2×106 A/cm2 at 74 K was measured by the ac mutual inductance response of the films. The improved microwave surface resistance and the higher critical current density are believed to be the results of better in-plane epitaxy.
Kinetics of surface reactions in very low-pressure chemical vapor deposition of Si from SiH4
Authors: Gates S. M. , Kulkarni S. K.
Published: July 2002

A steady-state kinetic model for the chemical vapor deposition (CVD) growth of Si films from SiH4 on Si(100) is presented. The only adsorbing species is SiH4 (absence of homogeneous SiH4 dissociation is presumed). Model predictions of surface hydrogen coverage and Si film growth rate as a function of growth temperature ( T ) are compared with literature values for these quantities. The rate of each reaction step is calculated at selected T. Adsorption of SiH4 and decomposition of SiH3 control the growth rate in the high T limit. In the low T limit, SiH4 adsorption is slowest but is not a simple rate determining step. The SiH4 adsorption rate is controlled by the rate of H2 desorption from two surface SiH species, producing dangling bonds.
Neutral gas temperatures in a multipolar electron cyclotron resonance plasma
Authors: Hopwood J. , Asmussen J.
Published: July 2002

Optical emission measurements of the Doppler broadening of argon (549.6 nm) and helium (501.6 nm) neutral lines in the unmagnetized regions of an electron cyclotron resonance plasma show that the gas temperature ranges from 300 to 900 K. After compensation for Zeeman splitting, Doppler widths are found to be constant across the radius of the plasma. Plasma heating of the argon gas (0.77 mTorr) is shown to increase from 300 to 500 K as microwave power absorption increases from 80 to 330 W. Long neutral residence times are observed to increase the argon gas temperature to ≊900 K. Helium and argon neutral temperatures decrease as the neutral mean free path increases indicating that the gas may be heated by ion-neutral collisions including charge exchange.
Luminescence quenching and the formation of the GaP1−xNx alloy in GaP with increasing nitrogen content
Authors: Baillargeon J. N. , Cheng K. Y. , Hofler G. E. , Pearah P. J. , Hsieh K. C.
Published: July 2002

A study of the luminescence properties of epitaxial GaP containing atomic N grown by molecular beam epitaxy using NH3 and PH3 as the column V sources was conducted. The 77 K photoluminescence spectra of the N-doped epitaxial GaP showed a continuous redshift, from 5691 Å (2.18 eV) to 6600 Å (1.88 eV), resulted when the N concentration exceeded ∼5–7×1019 cm−3. This energy shift was found to be consistent with energy gap predictions using the dielectric theory of electronegativity for the GaP1−xNx system. The data also indicate that the emission intensity was maximum for N∼1×1020 cm−3, and then monotonically decreases with increasing N content. This is consistent with the formation of an indirect band-gap semiconductor.
Giant surface magnetostriction in polycrystalline Ni and NiFe films
Authors: Song O. , Ballentine C. A. , O’Handley R. C.
Published: July 2002

We have measured the effective magnetoelastic coupling coefficients, Beff, of polycrystalline NiFe/Ag/Si, NiFe/Cu/Si, and Ni/SiO2/Si films in situ as functions of magnetic layer thickness over the range from 1.5 to 50 nm using magneto-optic Kerr effect and applied static strain. The Beff’s agree well with bulk values at large thicknesses but take on anomalously large values for thicknesses below about 5 nm. The data are well fit by a Néel model, Beff=Bbulk+Bsurf/(t−t0), where t0 may be related to intermixing at the interface with the substrate (verified by Auger depth profiling). These data suggest that the surface conditions are of enhanced importance in controlling magnetic properties, particularly anisotropy, permeability, and noise, even in films whose compositions are nominally of zero magnetostriction.
GaInO3: A new transparent conducting oxide
Authors: Cava R. J. , Phillips Julia M. , Kwo J. , Thomas G. A. , van Dover R. B. , Carter S. A. , Krajewski J. J. , Peck W. F. , Marshall J. H. , Rapkine D. H.
Published: July 2002

GaInO3, a layered material with the β Ga2O3 crystal structure, can be doped with electrons through the introduction of oxygen deficiency, Sn doping for In, or Ge doping for Ga. At atomic doping levels of 10% or less, resistivities as low as 3 mΩ cm are obtained. In contrast to polycrystalline indium tin oxide (ITO), which is distinctly green, conductive gallium indium oxide is light grey with no visible coloration. Thin films of doped GaInO3 display good transparency over the whole optical window, superior to that of ITO in the green-blue region.
Tapping mode atomic force microscopy in liquids
Authors: Hansma P. K. , Cleveland J. P. , Radmacher M. , Walters D. A. , Hillner P. E. , Bezanilla M. , Fritz M. , Vie D. , Hansma H. G. , Prater C. B. , Massie J. , Fukunaga L. , Gurley J. , Elings V.
Published: July 2002

Tapping mode atomic force microscopy in liquids gives a substantial improvement in imaging quality and stability over standard contact mode. In tapping mode the probe-sample separation is modulated as the probe scans over the sample. This modulation causes the probe to tap on the surface only at the extreme of each modulation cycle and therefore minimizes frictional forces that are present when the probe is constantly in contact with the surface. This imaging mode increases resolution and reduces sample damage on soft samples. For our initial experiments we used a tapping frequency of 17 kHz to image deoxyribonucleic acid plasmids on mica in water. When we imaged the same sample region with the same cantilever, the plasmids appeared 18 nm wide in contact mode and 5 nm in tapping mode.
Giant magnetoresistance of nanowires of multilayers
Authors: Blondel A. , Meier J. P. , Doudin B. , Ansermet J.-Ph.
Published: July 2002

A new technique is required which enables tailoring of the morphology of a metallic nanostructured material down to the 10 nm length scale. Using nanoporous nuclear track etched membranes as templates for electrodeposition, an assembly of wires with diameters as low as 30 nm could be obtained. Alternating the electrodeposition of two metals resulted in multilayers grown perpendicular to the wire axis. Layer thicknesses as low as 2 nm could be reached. Application is demonstrated by making wires 6 μm long, 80 nm in diameter, having a succession of either Co and Cu layers or of (Ni,Fe) and Cu layers. Wires containing layers of 5–10 nm in thickness exhibited a giant magnetoresistance. The current was naturally perpendicular to the layers. At ambient temperature, a magnetoresistance of 14% for Co/Cu and of 10% for (Fe,Ni)/Cu was observed.
High-efficiency silicon solar cells by rapid thermal processing
Authors: Rohatgi A. , Chen Z. , Doshi P. , Pham T. , Ruby D.
Published: July 2002

Silicon solar cell efficiencies of 16.9% have been achieved on 0.2 Ω cm float zone silicon, using a simplified cost effective rapid thermal process (RTP). Although the individual processing steps are not fully optimized yet, this represents the highest reported efficiency for solar cells processed with simultaneous front and back diffusion with no conventional high-temperature furnace steps. A diffusion temperature schedule coupled with an added short in situ slow cooling during RTP resulted in greater than 200 μm diffusion length and appropriate diffusion profiles for high efficiency cells. Plasma enhanced chemical vapor deposition (PECVD) of SiN/SiO2 was used for surface passivation and antireflection coating. Conventional cells fabricated by furnace diffusions and oxidations gave an efficiency of 18.8%. Process optimization can further reduce the gap between the conventional and RTP/PECVD cells.
Efficient electro-optic modulator for optical pumping of Na beams
Authors: Kelly J. F. , Gallagher A.
Published: July 2002

An electro-optic modulator using LiTaO3 is described which yields 34% of the carrier intensity in each of the first-order sidebands with rf phase modulation frequencies fm∼1.0 GHz and ≲1.0 W input power. The modulator makes use of a lumped resonator with Q∼200 to obtain efficient production of the sidebands. It is shown that the device can be scaled for operation at ∼2 GHz. Applications of this modulator include optical pumping of the lighter alkali atoms, FM sideband spectroscopy, and laser phase/frequency stabilization using rf modulation techniques.
Magnetic deflection analysis of supersonic metastable atom beams
Authors: Weiser C. , Siska P. E.
Published: July 2002

Deflection of 3S1 and 3P2 metastable rare-gas atoms in an electron-impact-excited thermal supersonic expansion by an inhomogeneous magnetic field produces well-defined daughter beams for each M state. The deflection patterns are spatially resolved with a translatable detector and used to derive fine-structure populations in the 3P0 and 3P2 states, and to measure absolute efficiency of state selection by optical depletion. The fine-structure population ratio 3P2/3P0 is measured for electron excitation energies from 100–400 eV, showing a 20%–40% decline with increasing energy for all the rare gases. Radiative decay due to perturbation by the magnetic field appears to be negligible, but the M-state-resolved populations are unequal and asymmetric, perhaps due to Majorana transitions at the entrance to the field. The method may also be useful in polarized angular momentum studies of metastable atom–molecule collisions.
Soft x-ray imaging diagnostic on the TdeV tokamak
Authors: Janicki C. , Decoste R. , Noël P.
Published: July 2002

A high-resolution soft x-ray diagnostic for tomographic imaging of the plasma has been developed for the TdeV tokamak. Five arrays of sixteen surface barrier detectors located at one toroidal location yield chord averaged emission profiles from five different poloidal viewing angles with a time resolution of 10 μs. The Fourier–Bessel reconstruction algorithm with radial and poloidal harmonics up to L=9 and M=4 is used to yield high-resolution 2D emissivity maps of the plasma cross section. Each detector array can be energy filtered by means of various gaseous and solid filters which can be selected separately for specific impurity transport studies and electron temperature measurements. This diagnostic is routinely used on TdeV for the study of plasma position and shape, MHD phenomena, impurity distribution, and heat-pulse transport.
X-ray standing waves and x-ray photoemission measurements in the energy range 2.7–7 keV
Authors: Drube W. , Lessmann A. , Materlik G.
Published: July 2002

A versatile experimental setup is presented for both x-ray standing waves (XSW) and x-ray photoemission spectroscopy (XPS) studies in an energy range up to 7 keV. The compact apparatus operates under ultrahigh vacuum and includes in situ sample preparation and characterization techniques. For the XSW scans the incident photon energy is tuned through the Bragg reflection from the sample for ΘB ≊ 90° while measuring the intensity and energy distributions of both the emitted electrons and fluorescence radiation simultaneously. Alternatively, desorbing positive ions can be analyzed as a function of the standing wave phase shift using a time-of-flight spectrometer. In addition, energy dependent high-energy XPS measurements with medium overall energy resolution (ΔE=1.5 eV at 3 keV) can be performed. Selected results obtained at the focused EXAFS II beamline of HASYLAB are shown.
A noncontacting magnetostrictive strain sensor
Authors: Kleinke Darrell K. , Mehmet Uras H.
Published: July 2002

The operation of a noncontacting magnetostrictive strain sensor (NMASS) is verified experimentally. The sensor has a single excitation and a single detection coil, and utilizes the magnetostrictive effect. The magnetostrictive sensor offers noninvasive strain measurements on ferromagnetic materials with an accuracy comparable to conventional strain gages. The NMASS produces significantly more output compared to strain gages and holds promise for development into a generic strain sensor.
A magnetostrictive force sensor
Authors: Kleinke Darrell K. , Uras H. Mehmet
Published: July 2002

A new magnetostrictive force sensor is presented and its operation is verified experimentally. The sensor has a single excitation coil and a single detection coil, which utilize the magnetostrictive effect. The cores of the coils function as ‘‘spring elements’’ of the sensor, and are linked together by rigid end plates. The magnetostrictive force sensor is compensated for asymmetric magnetostrictive behavior, and measures both tensile and compressive forces. Force sensitivity increases with excitation current and coil turns, but is less influenced by excitation frequency.
Bicolor pyroreflectometer using an optical fiber probe
Authors: Hernandez D. , Olalde G. , Beck A. , Milcent E.
Published: July 2002

A method and a device to measure temperature using bicolor optical pyrometry are presented. The device is equipped with an optical fiber probe designed for in situ measurements in severe conditions (high temperatures, difficult accessibility, and electromagnetic fields). The directional monochromatic emissivities, at two wavelengths 1.3 and 1.55 μm, were deduced from reflection measurements. The device can be used for opaque Lambertian surfaces in the temperature range from 500 to 2500 °C. The method was put in practice on rare-earth metal oxide powders at temperatures up to 1000 °C.
A novel probe design for pulsed nitrogen-14 nuclear quadrupole resonance spectrometer
Authors: Li Geng-Ying , Xia Xiao-Jian , Xie Hai-Bin , Liu Yi
Published: July 2002

A fast recovery probe circuit, which involves an active damping unit, is proposed in this paper. Because of its simplicity and high efficiency in converting radio-frequency power into an oscillating magnetic field, the suggested probe configuration is suitable for operation at low frequency (below 10 MHz), such as 14N nuclear quadrupole resonance where the ringing of a conventional probe usually lasts quite long.
Remote optical detection using microcantilevers
Authors: Wachter E. A. , Thundat T. , Oden P. I. , Warmack R. J. , Datskos P. G. , Sharp S. L.
Published: July 2002

The feasibility of microcantilever-based optical detection is demonstrated. Microcantilevers may provide a simple means for developing single-element and multielement infrared sensors that are smaller, more sensitive, and lower in cost than quantum well, thermoelectric, or bolometric sensors. Here we specifically report here on an evaluation of laboratory prototypes that are based on commercially available microcantilevers, such as those used in atomic force microscopy. In this work, optical transduction techniques were used to measure microcantilever response to remote sources of thermal energy. The noise equivalent power at 20 Hz for room temperature microcantilevers was found to be approximately 3.5 nW/√Hz, with a specific detectivity of 3.6×107 cm Hz1/2/W, when an uncoated microcantilever was irradiated by a low-power diode laser operating at 786 nm. Operation is shown to be possible from dc to kHz frequencies, and the effect of cantilever shape and the role of absorptive coatings are discussed. Finally, spectral response in the midinfrared is evaluated using both coated and uncoated microcantilevers.
Dynamic light scattering system with a novel scattering cell for the measurement of particle diffusion coefficients
Authors: Will Stefan , Leipertz Alfred
Published: July 2002

We present a novel optical scattering cell for the determination of the particle diffusion coefficient by dynamic light scattering, which can be utilized for particle size analysis and viscosity measurements. A major advantage of the light scattering setup introduced is the possibility of performing measurements without having to know the refractive index of the liquid used. Test results are given for the diffusion coefficient of silica particles in water, which show a total standard deviation including measurements at different angles of 0.5%.
X1A: Second-generation undulator beamlines serving soft x-ray spectromicroscopy experiments at the NSLS
Authors: Winn B. , Ade H. , Buckley C. , Howells M. , Hulbert S. , Jacobsen C. , Kirz J. , McNulty I. , Miao J. , Oversluizen T. , Pogorelski I. , Wirick S.
Published: July 2002

The X1A undulator beamline is being rebuilt with two separate monochromators on its two branches. The new arrangement will deliver spatially coherent beams to imaging experiments, with spectral resolving power of up to 5000, and the capability to optimize the resolving power versus flux. The beamlines will operate simultaneously, and each will use 15 percent of the undulator beam, yet deliver high coherent flux. Because of the small beam divergence, the spherical grating monochromators can operate with fixed exit arms throughout the 250–800 eV range.
Fiber optic sensors for use in monitoring flow front in vacuum resin transfer molding processes
Authors: Bernstein J. R. , Wagner J. W.
Published: July 2002

A distributed fiber optic sensor array developed for use in vacuum resin transfer molding systems is described. The sensor is placed in the mold and can be used to monitor resin infiltration even in complicated parts because of its small size and flexibility. The sensor components themselves are low cost and become incorporated in the part during the molding process. Careful design of the sensor elements is necessary to avoid sensitivity to mechanical stresses associated with the resin infiltration process.
Design study of a visible/infrared periscope for intense radiation applications using reflective optics
Authors: Medley S. S.
Published: July 2002

In magnetically confined fusion devices employing deuterium–tritium (D–T) operation, refractive optical components exposed to neutron and gamma radiation can be subject to degradation of the transmission characteristics, induced luminescence, and altered mechanical properties including dimensional changes. Although radiation resistant refractive optics functioned well for the Tokamak Fusion Test Reactor periscope system during D–T operation, this design approach is unpromising in the much more hostile radiation environment of future D–T devices such as International Thermonuclear Experimental Reactor (ITER). Under contract to the Princeton Plasma Physics Laboratory, Ball Aerospace of Colorado carried out a periscope design study based on the use of reflective optics. In this design, beryllium reflective input optics supported by a fused silica optical bench were interfaced to a Cassegrain relay system to transfer plasma images to remotely located cameras. This system is also capable of measuring first-wall surface temperatures in the range of 300–2000 °C even under projected heating of the reflective optics themselves to several hundred degrees Celsius. Tests of beryllium mirror samples, however, revealed that operation at temperatures above 700 °C leads to a loss of specular reflectivity, thus placing an upper limit on the acceptable thermal environment. The main results of this periscope study are presented in this article.
Wet chemical etching of AlN
Authors: Mileham J. R. , Pearton S. J. , Abernathy C. R. , MacKenzie J. D. , Shul R. J. , Kilcoyne S. P.
Published: July 2002

Single-crystal AlN grown on Al2O3 is found to be wet etched by AZ400K photoresist developer solution, in which the active component is KOH. The etching is thermally activated with an activation energy of 15.5±0.4 kcal mol−1, and the etch rate is found to be strongly dependent on the crystalline quality of the AlN. There was no dependence of etch rate on solution agitation or any crystallographic dependence noted, and the etching is selective over other binary group III nitrides (GaN, InN) and substrate materials such as Al2O3 and GaAs.
J
c
enhancement of electrophoretically deposited YBa2Cu3O7−δ superconducting wire by BaF2 addition
Authors: Cho S. , Yao Y. T. , Ketterson J. B. , Telschow K. L.
Published: July 2002

We have prepared YBa2Cu3O7−δ (YBCO) superconducting wires by electrophoretic deposition. Ag wire was used as a substrate. Adding a small amount of BaF2 tends to remove cracks in the YBCO layer which develop during the heating process. The suppression of cracks produced a dramatic improvement in the critical current density (Jc). At 77 K, Jc increased from 18 A/cm2 for BaF2-free YBCO wire to 646 A/cm2 for a critical concentration of 2.7 wt% BaF2 after which it decreased monotonically with increasing BaF2 content.
Valence-band discontinuities of wurtzite GaN, AlN, and InN heterojunctions measured by x-ray photoemission spectroscopy
Authors: Martin G. , Botchkarev A. , Rockett A. , Morkoç H.
Published: July 2002

The valence-band discontinuities at various wurtzite GaN, AlN, and InN heterojunctions were measured by means of x-ray photoemission spectroscopy. A significant forward–backward asymmetry was observed in the InN/GaN–GaN/InN and InN/AlN–AlN/InN heterojunctions. The asymmetry was understood as a piezoelectric strain effect. We report the valence band discontinuities for InN/GaN=1.05±0.25 eV, GaN/AlN=0.70±0.24 eV, and InN/AlN=1.81±0.20 eV, all in the standard type I lineup. These values obey transitivity to within the experimental accuracy. Tables of photoemission core level binding energies are reported for wurtzite GaN, AlN, and InN.
Structure and magnetic properties of epitaxial spinel ferrite thin films
Authors: Suzuki Y. , van Dover R. B. , Gyorgy E. M. , Phillips Julia M. , Korenivski V. , Werder D. J. , Chen C. H. , Cava R. J. , Krajewski J. J. , Peck W. F. , Do K. B.
Published: July 2002

We have grown epitaxial spinel ferrite thin films of (Mn,Zn) Fe2O4 and CoFe2O4 on (100) and (110) SrTiO3 and MgAl2O4 buffered by spinel structure buffer layers. High quality spinel ferrite films were grown at 400 °C on buffer layers that were grown at 600 °C and postannealed at 1000 °C. Although (Mn,Zn) Fe2O4 grown directly on SrTiO3 and MgAl2O4 shows mediocre structural and magnetic properties, ferrite films grown on (100) and (110) SrTiO3 and MgAl2O4 buffered with CoCr2O4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the immediately underlying layer.
Size effects on the phonon spectra of quantum dots in CdTe-doped glasses
Authors: de Paula A. M. , Barbosa L. C. , Cruz C. H. B. , Alves O. L. , Sanjurjo J. A. , Cesar C. L.
Published: July 2002

We studied the confinement effects on the phonon spectra of CdTe quantum dots by means of resonant Raman scattering measurements. The spectra show clearly longitudinal optical phonons, surface phonons and some of their overtone combinations. We show that the scattering due to surface phonons increases as the quantum dot size decreases. The results are obtained by tuning the laser excitation energy to resonance for quantum dots of different sizes inside the broad size distribution in CdTe-doped glasses.
Excitonic luminescence and absorption in dilute GaAs1−xNx alloy (x<0.3%)
Authors: Makimoto Toshiki , Saito Hisao , Nishida Toshio , Kobayashi Naoki
Published: July 2002

Dilute GaAs1−xNx alloys (x<0.3%) were grown by metalorganic vapor phase epitaxy to investigate their photoluminescence and photoluminescence excitation characteristics. Photoluminescence excitation spectra show clear excitonic absorption peaks at low temperatures and their peak energy drastically decreases with increasing nitrogen concentration due to the band-gap bowing in the GaAsN system. This result indicates that the band-gap bowing starts at a nitrogen concentration as low as 1018 cm−3, and its bowing parameter is −22 eV. According to this band-gap bowing, the GaAsN alloys show two photoluminescence lines whose peak energy decreases with increasing nitrogen concentration. Their dependence on the nitrogen concentration suggests that these lines correspond to excitonic and carbon-related transitions in the GaAsN alloy.
Effect of external stress on polarization in ferroelectric thin films
Authors: Kumazawa Tetsuo , Kumagai Yukihiro , Miura Hideo , Kitano Makoto , Kushida Keiko
Published: July 2002

The polarization changes caused by applying mechanical stresses to a lead zirconate titanate (PZT) thin film were investigated. Both the remnant and spontaneous polarizations decreased when the PZT film was loaded with tensile stress. For compressive stresses, the remnant polarization increased, but spontaneous polarization did not change. In fatigue with tensile stress state, the polarization decreased earlier than when there was no stress, which depend on whether or not the initial polarization value was high. Conversely, in fatigue with compressive stress, the initial higher remnant polarization value was maintained compared with the polarization in the unstress condition.
Local oxidation of silicon surfaces by dynamic force microscopy: Nanofabrication and water bridge formation
Authors: Garcı́a Ricardo , Calleja Montserrat , Pérez-Murano Francesc
Published: July 2002

Local oxidation of silicon surfaces by atomic force microscopy is a very promising lithographic approach at nanometer scale. Here, we study the reproducibility, voltage dependence, and kinetics when the oxidation is performed by dynamic force microscopy modes. It is demonstrated that during the oxidation, tip and sample are separated by a gap of a few nanometers. The existence of a gap increases considerably the effective tip lifetime for performing lithography. A threshold voltage between the tip and the sample must be applied in order to begin the oxidation. The existence of a threshold voltage is attributed to the formation of a water bridge between tip and sample. It is also found that the oxidation kinetics is independent of the force microscopy mode used (contact or noncontact).
Time-resolved imaging of gas phase nanoparticle synthesis by laser ablation
Authors: Geohegan David B. , Puretzky Alex A. , Duscher Gerd , Pennycook Stephen J.
Published: July 2002

The dynamics of nanoparticle formation, transport, and deposition by pulsed laser ablation of c-Si into 1–10 Torr He and Ar gases are revealed by imaging laser-induced photoluminescence and Rayleigh-scattered light from gas-suspended 1–10 nm SiOx particles. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles, while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. Nanoparticles unambiguously formed in the gas phase were collected on transmission electron microscope grids for Z-contrast imaging and electron energy loss spectroscopy analysis. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described.
Structural and electronic properties of low dielectric constant fluorinated amorphous carbon films
Authors: Ma Yanjun , Yang Hongning , Guo J. , Sathe C. , Agui A. , Nordgren J.
Published: July 2002

Fluorinated amorphous carbon (a-CFx) films were studied by high-resolution x-ray absorption, emission, and photoelectron spectroscopy. The composition and local bonding information were obtained and correlated with substrate temperature during deposition. The data suggest that the structure of the a-CFx is mostly of carbon rings connected by CF2 groups. The cross linking increases with substrate temperature.
GaAs/AlGaAs self-sensing cantilevers for low temperature scanning probe microscopy
Authors: Beck R. G. , Eriksson M. A. , Topinka M. A. , Westervelt R. M. , Maranowski K. D. , Gossard A. C.
Published: July 2002

We have fabricated scanning probe microscope cantilevers with dimensions 65×11.4×0.25 μm3 and 3×2×0.129 μm3 from GaAs/Al0.3Ga0.7As heterostructures containing two-dimensional electron gases. Deflection is measured by an integrated field-effect transistor (FET) that senses strain via the piezoelectric effect and provides a low noise, low power displacement readout. We present images of a 200 nm mica grating taken with the large cantilever having a deflection (force) noise 10 Å/√Hz (19 pN/√Hz) at T=2.2 K. The small cantilever has a resonant frequency of 11 MHz, a FET gate charge noise of 0.001 e/√Hz, and is projected to have a deflection (force) noise of 0.002 Å/√Hz (1 pN/√Hz) at T=4.2 K.
Effect of heating rate on positive-temperature-coefficient-of-resistivity behavior of conductive composite thin films
Authors: Hirano S. , Kishimoto A.
Published: July 2002

A phenomenon was discovered that leads to the selective detection of abrupt increases in the temperature of conductive composite thin films consisting of conductive ceramic fillers and an insulating polymer matrix. Examining the heating rate dependence of the positive-temperature–coefficient-of-resistivity (PTCR) effect provided information about this intelligent phenomenon. The anomalous PTCR effect was observed above 0.3 °C min−1 for all the prepared films. However, the magnitude of the anomaly decreased when the heating rate decreased below 0.1 °C min−1, and when the heating rate further decreased below 0.04 °C min−1, the anomalous resistivity–temperature relationship disappeared. The results suggest that these thin films can selectively detect abrupt increases in temperature, which could lead to an intelligent mechanism. Our results also suggest a PTCR mechanism, in which the expansion of crosslinked polymers in the thermodynamic nonequilibrium state essentially produces the anomaly.
Pulsed-laser-deposited epitaxial Sr2FeMoO6−y thin films: Positive and negative magnetoresistance regimes
Authors: Asano H. , Ogale S. B. , Garrison J. , Orozco A. , Li Y. H. , Li E. , Smolyaninova V. , Galley C. , Downes M. , Rajeswari M. , Ramesh R. , Venkatesan T.
Published: July 2002

Epitaxial thin films of ordered double-perovskite Sr2FeMoO6−y are deposited on (001) SrTiO3 substrates by pulsed-laser deposition using a two step growth process. Selection of growth conditions is found to lead to either highly conductive metallic thin films (residual resistivity of about 1 μΩ cm) or semiconducting films. The metallic films show a positive magnetoresistance (MR) as high as 35%, while the semiconducting films show a negative MR of −3%, at a temperature of 5 K and a field of 8 T.
Magnetic field biasing in Faraday effect sensors
Authors: Guerrero H. , Pérez del Real R. , Fernández de Caleya R. , Rosa G.
Published: July 2002

A method for improving the performances of Faraday effect sensors, which is based in the passive magnetic field biasing of a Faraday rotator, is described. It allows obtaining reflective optical fiber sensor architectures, based on a single polarizer and a mirror-coated Faraday rotator, which exhibit linear response and maximum sensitivity. It can be achieved by means of a permanent magnet providing a 45° rotation of the polarization of the optical beam propagating through a Faraday rotator. Operation of this method in both a point magnetic field sensor and a current sensor, with a ferromagnetic ring as concentrator, has been demonstrated.
Nanocrystal size dependence of the third-order nonlinear optical response of Cu:Al2O3 thin films
Authors: Ballesteros J. M. , Solis J. , Serna R. , Afonso C. N.
Published: July 2002

Metal nanocomposite thin films formed by Cu nanocrystals embedded in an amorphous Al2O3 host have been synthesized by pulsed laser deposition. The mean nanocrystal diameter d was varied in the range 3.0±0.6 to 6±1 nm. The linear and nonlinear optical properties of the films were studied in the vicinity of the surface plasmon resonance and the size dependence of the third-order nonlinear optical susceptibility of the metal nanocrystals has been determined. The observed dependence (1/d3) indicates that in the studied diameter interval, the nonlinear response is due to quantum confinement effects in which the major contribution is associated with electronic intraband transitions.
Increase of charge carriers density and reduction of Hall mobilities in oxygen-plasma treated indium–tin–oxide anodes
Authors: Kim J. S. , Cacialli F. , Cola A. , Gigli G. , Cingolani R.
Published: July 2002

We report investigations of the electronic transport properties carried out by means of the Hall technique for indium–tin–oxide thin films on glass after a variety of surface treatments. We find that oxygen-plasma treatments induce a significant increase in the carrier concentration, and a less significant decrease of mobilities with respect to “as-received” or aquaregia treated substrates. We consider that this is indicative of an increased concentration of defects, as a result of the plasma exposure.
Confinement induced decrease of the exciton-longitudinal optical phonon coupling in GaN quantum dots
Authors: Ramvall Peter , Tanaka Satoru , Nomura Shintaro , Riblet Philippe , Aoyagi Yoshinobu
Published: July 2002

We report on an investigation of the coupling between excitons and longitudinal optical phonons as a function of GaN quantum dot size. Photoluminescence measurements of the quantum dots from cryogenic temperatures up to above room temperature are presented. The experiments were performed on ensembles of AlN capped GaN quantum dots grown on an Al0.15Ga0.85N surface by means of metalorganic vapor phase epitaxy. The results are analyzed on the basis of a Bose–Einstein-type expression describing the exciton to longitudinal optical phonon coupling of the dots as a function of the lattice temperature. A reduction of the exciton to LO-phonon coupling with decreasing quantum dot size was found.
Extensional piezoelectric coefficients of gallium nitride and aluminum nitride
Authors: Guy I. L. , Muensit S. , Goldys E. M.
Published: July 2002

Measurements of piezoelectric coefficients d33 and d31 in wurtzite GaN and AlN using an interferometric technique are presented. We report on the clamped values, d33c of these coefficients found in GaN and AlN thin films, and we derive the respective bulk values, d33b. The clamped value of d33c in GaN single crystal films is 2.8±0.1 pm V−1 which is 30% higher than in polycrystalline films grown by laser assisted chemical vapor deposition. The value of d33b in bulk single crystal GaN is found to be 3.7±0.1 pm V−1. The value of d33c in plasma assisted and laser assisted chemical vapor deposited AlN films was 3.2±0.3 and 4.0±0.1 pm V−1, respectively. The bulk value estimate of d33b in AlN of 5.6±0.2 pm V−1 was deduced. The values of d31, both clamped and bulk, were calculated for wurtzite GaN and AlN. We have also calculated the values of d14 in cubic phase film and bulk GaN and AlN. Interferometric measurements of the inverse piezoelectric effect provide a simple method of identifying the positive direction of the c axis, which was found to be pointing away from the substrate for all films.
Microstructure-dependent coercivity in monodispersed hematite particles
Authors: Rath C. , Sahu K. K. , Kulkarni S. D. , Anand S. , Date S. K. , Das R. P. , Mishra N. C.
Published: July 2002

Microstructure and magnetic properties of monodispersed pseudocubic and trapezoidal particles with varying sizes prepared through the hydrothermal precipitation route are reported. The coercivity for trapezoidal particles was similar to that of reported values. For pseudocubic particles, however, the coercivity is unusually high (∼6 kOe) as compared to the maximum value (3 kOe) reported in the literature. Detailed microstructural analysis revealed that particles with a well-defined shape are, in fact, polycrystalline. The high coercivity and its variation with particle shape and size are correlated to the internal nanostructure of the particles.
Perforated tips for high-resolution in-plane magnetic force microscopy
Authors: Folks L. , Best M. E. , Rice P. M. , Terris B. D. , Weller D. , Chapman J. N.
Published: July 2002

We describe a technique to modify batch-fabricated magnetic force microscopy (MFM) tips to allow high resolution imaging of the in-plane components of stray field. A hole with a diameter as small as 20 nm was milled through the magnetic layer at the apex of each tip using a focused ion beam. The tips were magnetized in the direction parallel to the sample plane. The hole at the apex forms a small pole gap, and the MFM signal arises from interaction of the stray field leakage from this gap with magnetic charge distribution of the sample. Data tracks written in recording media have been used to characterize tip performance.
Straight β-SiC nanorods synthesized by using C–Si–SiO2
Authors: Lai H. L. , Wong N. B. , Zhou X. T. , Peng H. Y. , Au Frederick C. K. , Wang N. , Bello I. , Lee C. S. , Lee S. T. , Duan X. F.
Published: July 2002

Straight beta-silicon carbide nanorods have been grown on silicon wafers using hot filament chemical vapor deposition with iron particles as catalyst. A plate made of a C–Si–SiO2 powder mixture was used as carbon and silicon sources. Hydrogen, which was the only gas fed into the deposition system, acts both as a reactant and as a mass transporting medium. The diameter of the β-SiC nanorod ranged from 20 to 70 nm, while its length was approximately 1 μm. A growth mechanism of beta-silicon carbide nanorods was proposed. The field emission properties of the beta-silicon carbide nanorods grown on the silicon substrate are also reported.
Enhancement of silicon oxidation rate due to tensile mechanical stress
Authors: Yen Jui-Yuan , Hwu Jenn-Gwo
Published: July 2002

Oxidation of silicon wafers under external mechanical stress was studied in this work. From the oxide thickness profile measured by an automatic ellipsometer, it was found that the oxidation kinetics of silicon was affected by the mechanical stress. The tensile stress strongly enhances the oxidation rate of silicon. A concept was proposed to explain this phenomenon by using a well-known physical Si–SiO2 lattice model. The tensile stress in the silicon will enlarge the atom spacing of silicon and make the oxidation to be easier and faster. A simulated deformation of silicon substrate under tensile stress was also given to support this concept. This work is a direct evidence of the effect of mechanical stress on silicon oxidation.
Observed insensitivity of stimulated Raman scattering on electron density
Authors: Fernández Juan C. , Cobble J. A. , Montgomery D. S. , Wilke M. D. , Afeyan B. B.
Published: July 2002

The results from experiments in quasihomogeneous plasmas to evaluate the potential threat of high laser reflectivity from stimulated Raman scattering (SRS) to inertial-confinement fusion (ICF) are presented. The SRS laser reflectivity is observed to be sizable (up to 50%) and very weakly dependent on electron density (and kλD), over a large range of density that corresponds to a large variation in Landau damping of plasma waves. In contrast, the reflectivity increases monotonically over time, along with ion temperature, until gross hydrodynamic activity interferes with SRS. This is consistent with previous observations of SRS reflectivity scaling with the damping rate of ion acoustic waves [Fernández et al., Phys. Rev. Lett. 77, 2702 (1996); Kirkwood et al., ibid. 77, 2706 (1996)]. The data from plasmas with the highest kλD values indicate anomalously low damping rates for the SRS plasma wave.
Triboluminescence of alkaline earth aluminate polycrystals doped with Dy3+
Authors: Tanaka Katsuhisa , Fujita Koji , Taniguchi Tomohiro , Hirao Kazuyuki , Ishihara Tsuguo
Published: July 2002

Triboluminescence spectra have been measured for polycrystalline (Sr,Ba)Al2O4 and (Sr,Ca)Al2O4 doped with Dy3+. Sr1−xBaxAl2O4:Dy3+ with x=0, 0.1, 0.2, and 0.4 and Sr0.9Ca0.1Al2O4:Dy3+ clearly exhibit triboluminescence caused by the 4f–4f transitions of Dy3+. In contrast, triboluminescence is barely observed in Sr0.8Ca0.2Al2O4:Dy3+ and Sr0.6Ca0.4Al2O4:Dy3+ although both of them show photoluminescence due to the 4f–4f transitions of Dy3+. For the Sr1−xBaxAl2O4:Dy3+ with x=0, 0.1, 0.2, and 0.4 and Sr0.9Ca0.1Al2O4:Dy3+, the compositional dependence of the relative integrated intensity of the emission lines is different between triboluminescence and photoluminescence spectra. We suggest two possibilities to explain this phenomenon; one of them is the self-absorption by Dy3+ in the case of triboluminescence, and the other is a situation that a strain is imposed on Dy3+, which brings about the triboluminescence due to the 4f–4f transitions. We speculate that the latter is the main cause for the difference between triboluminescence and photoluminescence spectra.
Magnetic easy axis development in steel—the influence of manufacturing
Authors: Clapham L. , White S. , Lee J. , Atherton D. L.
Published: July 2002

Oil and gas pipelines often display a bulk magnetic easy magnetization direction, and this may influence results obtained during magnetic nondestructive evaluation. The origin of this magnetic easy axis is not well known. Earlier work [L. Clapham, C. Heald, T. Krause, and D. L. Atherton, J. Appl. Phys. 86, 1574 (1999)] speculated that it was a result of residual stresses introduced during pipeline manufacturing, specifically cold bending. This article reports a study in which a magnetic Barkhausen noise technique is used to follow the development of a magnetic easy axis in a steel plate sample subjected to progressive amounts of cold bending.
6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni–Mn–Ga
Authors: Murray S. J. , Marioni M. , Allen S. M. , O’Handley R. C. , Lograsso T. A.
Published: July 2002

Field-induced strains of 6% are reported in ferromagnetic Ni–Mn–Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependence of strain on magnetic field and external stress using as input parameters only measured quantities.
Effect of interfacial specular electron reflection on the anisotropic magnetoresistance of magnetic thin films
Authors: Dieny B. , Li M. , Liao S. H. , Horng C. , Ju K.
Published: July 2002

We investigated the effect of specular reflection on the anisotropic magnetoresistance (AMR) of magnetic thin films. The sheet conductance is calculated as a function of the angle between magnetization and current from the microscopic transport parameters by using an extension of the Fuchs–Sondheimer theory. The calculation combines specular reflection on the film interfaces with mean-free paths which depend on the angle between the local magnetization and the electron velocity. The theoretical results are compared with experimental ones. Specular reflection can explain the quite large AMR amplitude observed in thin NiFe films used in the last generation of AMR heads.