bluuebunny/crossref_metadata_2025 huggingface dataset:
The tomographic atom probe: A quantitative three-dimensional nanoanalytical instrument on an atomic scale
Authors: Blavette D. , Deconihout B. , Bostel A. , Sarrau J. M. , Bouet M. , Menand A.
Published: July 2002

The physical architecture and the performance of a quantitative three-dimensional atom probe recently constructed are described. The development of such an instrument relies on the design of a multi-impact position sensitive detector. The multidetection system that we have developed is based on the use of a 10×10 anode array placed behind a two microchannel plate assembly in a chevron arrangement. The spread of charge between the microchannel plate and the multianode is used to derive the position of ion striking the detector. Spatial coordinates can be calculated for multiple and simultaneous time-of-flight events. The procedure used for the derivation of ion positions from charge measurements is given. Specific experiments were carried out in order to determine the intrinsic spatial resolution of the multidetector. Three-dimensional reconstruction of two-phase materials are provided and illustrate the performance of this new apparatus. The reconstructed images demonstrate that atoms are positioned with a precision of a few tenths of a nanometer. The mass resolution M/ΔM (FWHM) of the apparatus is close to 200.
Solid-state bonding of accurately oriented metal/ceramic bicrystals in ultrahigh vacuum
Authors: Fischmeister H. F. , Elssner G. , Gibbesch B. , Kadow K.-H. , Kawa F. , Korn D. , Mader W. , Turwitt M.
Published: July 2002

A machine is described which allows bicrystals of metals and ceramics to be made by pressure bonding at temperatures up to 1500 °C in ultrahigh vacuum (UHV). The mutual orientation of the two component crystals can be adjusted to within 0.1°. The surfaces to be bonded can be sputter cleaned with Ar ions while monitoring by Auger electron spectroscopy, and dopants can be applied by evaporation in a separate chamber. The machine allows the production of bicrystals with interfaces of precisely defined crystallography and chemistry. The specimen size is sufficient for fracture mechanical characterization of interface strength. The performance of the machine is illustrated with data for niobium-sapphire bicrystals. The machine has been shown to produce bicrystal interfaces whose purity is not measurably impaired by atmospheric contaminants. Fracture resistant levels of Nb-Al2O3 interfaces bonded at 1700 °C in ordinary high vacuum are reached in the UHV machine already at 1000 °C. Going to temperatures around 1450 °C raises the fracture resistance to more than twice the level obtained in ordinary high vacuum at 1700 °C.
Characterization of strain in an advanced semiconductor laser structure with nanometer range resolution using a new algorithm for electron diffraction contrast imaging interpretation
Authors: Janssens Koenraad G. F. , Van der Biest Omer , Vanhellemont Jan , Maes Herman E. , Hull Robert
Published: July 2002

In the present letter electron diffraction contrast imaging (EDCI) computations for strain characterization on the nanometer scale are presented. Using newly developed simulation software for EDCI image interpretation it is demonstrated how the technique can be used in the characterization, with submicron resolution, of localized strain fields in any crystalline material. Strain fields of arbitrary geometrical symmetry can be treated. As a case study, localized strain in an advanced semiconductor laser structure is investigated.
Development of capacitance tomographic imaging systems for oil pipeline measurements
Authors: Yang W. Q. , Stott A. L. , Beck M. S. , Xie C. G.
Published: July 2002

UMIST is one of the establishments where process tomography techniques were first developed. This paper introduces the development of capacitance tomographic imaging systems at UMIST for oil pipeline measurements. The principles of electrical capacitance tomography (ECT) are briefly presented and two ECT systems based on charge/discharge capacitance measuring circuits are described. These systems, a PC-based 8-electrode system and a Transputer-based 12-electrode system, have been used to visualise oil pipelines. Future developments of ECT technology are discussed, including a low-cost PC-based ECT system, a twin-plane ECT system which can measure the velocity profile of a pipeline flow, a multi-frequency ECT system using ac capacitance measuring circuits and an ECT system with a rotating parallel excitation field.
Design and characterization of pulsed uniform supersonic expansions for chemical applications
Authors: Atkinson Dean B. , Smith Mark A.
Published: July 2002

A general formalism is presented for the production, characterization, and use of uniform supersonic flows for the study of structural and collisional molecular properties. These flows make possible the generation of thermally equilibrated gaseous environments at temperatures generally extending from near 10 to above 300 K at pressures between 0.1 and 10 Torr. In addition, the invariance of flow conditions for distances of many nozzle diameters (5–30) beyond the exit makes the flows ideal for collisional or temporal studies. A concise outline for the design of the Laval nozzles necessary to produce specified density and temperature conditions is presented. Pulsed operation of these expansions is demonstrated, and a variety of useful characterization tools providing information on flow uniformity and thermal characteristics is considered. Recent applications of these flows for the study of low-temperature chemical reactions are reviewed.
The measuring and mapping of calcium in mineralized tissues by absorption difference imaging
Authors: Buckley C. J.
Published: July 2002

This paper reviews the mapping of calcium in mineralized tissues by absorption difference imaging using scanning x-ray microscopy at the calcium L edge. In particular, the mapping of embedded and sectioned mineralized tissues is considered. Here, the technique is discussed in detail and the accuracy is evaluated with and without corrections. The technique is considered under two scenarios: the use of differences in absorption cross section at pre- and postionization energies, and the use of differences arising from near-edge x-ray-absorption features.
Digital mammography with synchrotron radiation
Authors: Arfelli F. , Bravin A. , Barbiellini G. , Cantatore G. , Castelli E. , Di Michiel M. , Poropat P. , Rosei R. , Sessa M. , Vacchi A. , Dalla Palma L. , Longo R. , Bernstorff S. , Savoia A. , Tromba G.
Published: July 2002

The SYRMEP (synchrotron radiation in medical physics) Collaboration is planning to use a beam of monochromatic x-rays provided by Elettra, the synchrotron radiation facility in operation at Trieste (Italy), in conjunction with a novel silicon pixel detector to conduct research in digital mammography. A beamline dedicated to mammography is presently under construction in Trieste; it will provide, at a distance of about 20 m from an Elettra bending magnet, a monochromatic laminar-section (150×4 mm2) x-ray beam. This beam will illuminate in vitro samples and will be detected by a fixed silicon microstrip device forming a matrix of pixels. Digital images of phantoms having a size common in the diagnostic practice (150×150 mm2), can then be produced by scanning the sample itself in front of the detector. A prototype detector with a sensitive area of 24×1 mm2 and pixels of 0.5×0.5 mm2 has been built and tested. We present the current status of the SYRMEP beamline and a digital image of a mammographic phantom exposed to a radioactive x-ray source.
The elastic and piezoelectric properties of a lithium triborate single crystal
Authors: Wang Y. , Jiang Y. J. , Liu Y. L. , Cai F. Y. , Zeng L. Z.
Published: July 2002

From the velocity of sound measurements, obtained using Brillouin scattering and ultrasound techniques, the elastic and piezoelectric coefficients of lithium triborate (LiB3O5; LBO) single crystals have been determined. Based upon the sound propagation equations and above results, the slowness curves in (100), (010), and (001) crystalline planes are calculated and presented. Some properties of sound propagation and electromechanical coupling in the crystal are also discussed.
Calibration of optical lever sensitivity for atomic force microscopy
Authors: D’Costa Neill P. , Hoh Jan H.
Published: July 2002

Accurate force determinations in atomic force microscopy require the precise measurement of cantilever deflections. A limiting factor in making these measurements is the calibration of the optical lever detection system, particularly when the tip cannot be pressed against a hard surface. Here we show that, for a given instrument, a fixed displacement of the photodiode detector produces a change in the detector voltage that correlates strongly with optical lever sensitivity. This provides a simple method for optical lever sensitivity calibration not requiring contact of the tip with any surface. The data also suggest that differences in the shape of the laser spot on the photodiode are a major source of variability in optical lever sensitivity.
Calibration of frictional forces in atomic force microscopy
Authors: Ogletree D. F. , Carpick R. W. , Salmeron M.
Published: July 2002

The atomic force microscope can provide information on the atomic-level frictional properties of surfaces, but reproducible quantitative measurements are difficult to obtain. Parameters that are either unknown or difficult to precisely measure include the normal and lateral cantilever force constants (particularly with microfabricated cantilevers), the tip height, the deflection sensor response, and the tip structure and composition at the tip-surface contact. We present an in situ experimental procedure to determine the response of a cantilever to lateral forces in terms of its normal force response. This procedure is quite general. It will work with any type of deflection sensor and does not require the knowledge or direct measurement of the lever dimensions or the tip height. In addition, the shape of the tip apex can be determined. We also discuss a number of specific issues related to force and friction measurements using optical lever deflection sensing. We present experimental results on the lateral force response of commercially available V-shaped cantilevers. Our results are consistent with estimates of lever mechanical properties using continuum elasticity theory.
An endoscopic fluorescence imaging system for simultaneous visual examination and photodetection of cancers
Authors: Wagnières Georges A. , Studzinski André P. , van den Bergh Hubert E.
Published: July 2002

We describe the design and performance tested during six years of clinical trials of a fluorescence endoscope for the detection and delineation of cancers in several hollow organs. The apparatus is based on the imaging of the laser-induced fluorescence that differs between a tumor and its surrounding normal tissue. The tests are carried out in the upper aerodigestive tract, the tracheobronchial tree, the esophagus, and the colon. In the three former cases an exogenous dye is used (Photofrin II), whereas in the latter case fluorescein molecules conjugated with monoclonal antibodies directed against carcinoembryonic antigen are injected. The decrease of native tissue autofluorescence observed in early cancers is also used for detecting lesions in the tracheobronchial tree. The fluorescence contrast between the tumor and surrounding normal tissue is enhanced by real time image processing. This is done by simultaneously recording the fluorescence image in two spectral domains, after which these two images are digitized and manipulated with a mathematical operator (look-up table) at video frequency. Moreover, the device that is described below allows for an immediate observation of the endoscopic area under white light illumination during fluorescence detection in order to localize the origin of the “positive” fluorescence signals. Typical results obtained in the tracheobronchial tree and in the colon are presented and the sources of false positives and false negatives are evaluated in terms of the fluorescent dye, tissue optical properties, and illumination optics.
(111)A CdTe rotation growth on (111) Si with low growth rate by metalorganic chemical vapor deposition
Authors: Ebe H. , Nishijima Y.
Published: July 2002

(111)A CdTe epilayers have been grown directly on (111) silicon substrates with the low growth rate of less than 0.5 μm/h by metalorganic chemical vapor deposition (MOCVD). The full width at half maximum (FWHM) of the x-ray rocking curve of a 1 μm thick epilayer is 100 arcsec, which is much better than the value for (111)B CdTe/(100)Si. The low growth rate effects are deduced from the classical capillarity theory. The nucleation stages were examined by atomic force microscopy (AFM), which showed that the nuclei were less in number when growing with the low growth rate. The geometrical advantages of (111)A CdTe/(111) Si for annihilating the dislocations are also discussed.
Investigations on liquid alloy ion sources for rare-earth elements
Authors: Machalett F. , Wesch W. , Mühle R. , Barth S.
Published: July 2002

We have developed liquid alloy ion sources for the rare-earth elements Er and Pr. They can be used in focused ion beam systems for ion implantation into optical materials and superconductors. The rare-earth elements are incorporated into ternary alloys to achieve lower melting points and lower vapor pressures as well as better wetting properties and easier handling of the sources at atmosphere. The melting properties of the alloys were determined by differential thermal analysis. The emission current stability and the mass distribution using a magnetic mass spectrometer and a time-of-flight mass analyzer have been measured. The mass spectra show a high intensity of double charged and a considerable intensity of triple charged Er and Pr ions. The ion sources exhibit a good performance even after storage for several weeks in air.
Protection circuitry for high-power diode laser arrays
Authors: Saam Brian T. , Conradi Mark S.
Published: July 2002

A comprehensive protection scheme is presented for use with high-power (∼500 W dc input) diode laser arrays. The circuitry requires no separate power, using instead the voltage from the laser’s power supply. Overcurrent and overvoltage silicon controlled rectifier crowbars are the primary protection circuits. In addition, tripping of either crowbar will turn off the main power to the laser’s power supply. This feature makes use of a main power controller that incorporates two interlock loops, for protection against overtemperature, low coolant flow, undervoltage, and other undesirable conditions.
Field emission from silicon and molybdenum tips coated with diamond powder by dielectrophoresis
Authors: Choi W. B. , Cuomo J. J. , Zhirnov V. V. , Myers A. F. , Hren J. J.
Published: July 2002

Thin diamond layers have been formed on molybdenum and single-crystal silicon field emitters using a dielectrophoresis coating method from a suspension of diamond powder in a nonconducting fluid. Transmission and scanning electron microscopy observation revealed a significant amount of diamond on the tips. The average thickness of the deposited diamond layer depended on the applied bias and the immersion time. Field emission from these diamond-coated emitters exhibited significant enhancement compared to the pure emitters.
Visible photoluminescence from porous GaAs
Authors: Schmuki P. , Lockwood D. J. , Labbé H. J. , Fraser J. W.
Published: July 2002

Porous GaAs was formed electrochemically on n-type GaAs(100) in a 0.1 M HCl electrolyte. Scanning electron microscopy revealed feature sizes of the porous structure in the micrometer to nanometer range. The optical properties of the porous material were characterized by photoluminescence (PL) measurements at 295 K. Compared with untreated GaAs, a shift down of the ‘‘infrared’’ PL maximum to ∼840 nm can be observed. An additional ‘‘green’’ PL peak occurs at ∼540 nm that in some samples is readily visible to the naked eye. The ‘‘green’’ and the ‘‘infrared’’ PL are ascribed to quantum confinement effects in GaAs nano- and microcrystallites, respectively.
Formation of phase intergrowth in the syntheses of Bi-superconducting thin films
Authors: Ota Hiroyuki , Sakai Kazuo , Mori Zon , Aoki Ryozo
Published: July 2002

Phase intergrowth among some kinds of the Bi2Sr2Can−1CunOy phases is observed in the thin film fabrication at ultralow co-deposition with multitargets by means of ion beam sputtering. The molar fraction of the Bi2212 phase in the mixed crystal of the grown films is investigated as a function of the applied ozone pressure and the substrate temperature. The activation energy for the phase transformation from the Bi2201 to the Bi2212 is estimated in terms of the Avrami equation. This study reveals that the formation of a liquid phase contributes significantly to the construction of the Bi2212 phase in the thin films, differing from the bulk synthesis.
Mg in GaN: Incorporation of a volatile species at high temperatures during molecular beam epitaxy
Authors: Guha S. , Bojarczuk N. A. , Cardone F.
Published: July 2002

We examine the epitaxial incorporation behavior of the volatile p-type dopant Mg at high growth temperatures during the molecular beam epitaxy of GaN on the [0001] surface, and report interesting doping behavior. The net Mg incorporation is independent of the arriving Mg flux over a flux variation of two decades, and dependent strongly on the growth temperature of the epilayer, limiting the net hole concentrations obtainable to the low 1017 cm−3 range. These results are explained in terms of incorporation either through a surface Mg phase, or via the availability of specific sites on the surface for Mg incorporation.
Crystalline-to-amorphous transition in chemical vapor deposition of pseudomorphic Si1−x−yGexCy films
Authors: Laursen T. , Chandrasekhar D. , Smith David J. , Mayer J. W. , Huffman J. , Westhoff R. , Robinson McD.
Published: July 2002

Pseudomorphic SiGeC films have been grown on (100) Si by atmospheric pressure chemical vapor deposition at 600 and 700 °C using SiH2Cl2, GeH4, and C2H4 precursors. Films with C concentrations of up to 2.5 at. % were entirely pseudomorphic and a 120-nm-thick Si66.5Ge31C2.5 film had 90% substitutional carbon. With increasing C incorporation due to increased ethylene flow, a layered structure was formed consisting of an amorphous film overlaying a buried pseudomorphic film. The crystalline-to-amorphous transition was initiated by the accumulation of C on the epitaxial growth surface. This deteriorated surface resulted in the formation of stacking faults along {111} planes and subsequent amorphization. Defect formation and amorphization could be prevented by periodically growing a thin Si epilayer.
Magnetoresistance of epitaxial thin films of ferromagnetic metallic oxide SrRuO3 with different domain structures
Authors: Kacedon D. B. , Rao R. A. , Eom C. B.
Published: July 2002

We have studied the magnetoresistive behavior of epitaxial thin films of the conductive ferromagnetic oxide SrRuO3 with different domain structures grown on both miscut (001) SrTiO3 and exact (001) LaAlO3 substrates. A strong anisotropic magnetoresistance (MR) has been observed in the single domain SrRuO3 thin film on miscut (001) SrTiO3 substrate. In contrast, the SrRuO3 thin film on (001) LaAlO3 substrate shows identical MR behavior in two orthogonal directions on the film due to the presence of 90° domains in the plane. For both the films, large negative magnetoresistance effects (∼10%) were observed when the current and the applied magnetic field are parallel. This is attributed to a reduction in spin fluctuations near Tc and to magnetization rotation leading to a change in the angle between the current and magnetization at low temperatures.
Kinetics of silicide-induced crystallization of polycrystalline thin-film transistors fabricated from amorphous chemical-vapor deposition silicon
Authors: Kim Hansuk , Couillard J. Greg , Ast Dieter G.
Published: July 2002

Self-aligned, n-channel, polycrystalline silicon thin-film transistors with 15 μm channels were fabricated by recrystallizing amorphous silicon for 6 h at 500 °C. A thin nickel silicide at the source and drain was used to seed the crystallization. The channel mobility was initially 87 cm2/V s, and improved to 170 cm2/V s after hydrogenating the devices. The recrystallization velocity in the channel was measured optically and electrically, and found to be 3.5×10−8 cm/s; this value exceeds by two orders of magnitude the solid-phase epitaxial regrowth rate of amorphous silicon. This observation, together with the low activation energy of 0.3 eV measured for the silicide-assisted regrowth velocity as compared to 2.76 eV for epitaxial regrowth, suggest that the channel recrystallization is assisted by Ni diffusing to the recrystallization front.
Large-scale and low-cost synthesis of single-walled carbon nanotubes by the catalytic pyrolysis of hydrocarbons
Authors: Cheng H. M. , Li F. , Su G. , Pan H. Y. , He L. L. , Sun X. , Dresselhaus M. S.
Published: July 2002

Rope-like bundles of single-walled carbon nanotubes (SWNTs) similar to those obtained by laser vaporization and electric-arc techniques were synthesized on a relatively large scale and at low cost by the catalytic decomposition of hydrocarbons at a temperature of about 1200 °C using an improved floating catalyst method. The SWNTs thus obtained have larger diameters and are self-organized into ropes. The addition of thiophene was found to be effective in promoting the growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes under different growth conditions.
Relaxor behavior and electromechanical properties of Pb(Mg1/3Nb2/3)O3 thin films
Authors: Kighelman Z. , Damjanovic D. , Seifert A. , Sagalowicz L. , Setter N.
Published: July 2002

Pb(Mg 1/3 Nb 2/3 ) O 3 (PMN) alkoxide precursor solutions were synthesized and used to prepare thin films by spin coating on TiO2/Pt/TiO2/SiO2/Si substrates. Many parameters like the use of homogeneous and stable precursor solutions and appropriate processing were used to greatly reduce the presence of the nonferroelectric pyrochlore phase. Transmission electron microscopy investigations, dielectric, electrostrictive, and direct current field induced piezoelectric measurements were carried out and have shown that PMN thin films exhibit a relaxor-like behavior.
Ultrafast phase dynamics of coherent carriers in GaAs
Authors: Yu X. Y. , Luo Q. , Li W. L. , Li Q. , Qiu Z. R. , Zhou J. Y.
Published: July 2002

Ultrafast phase dynamics of free-induction decay for carriers in bulk GaAs is studied with differential-phase spectroscopy. The instantaneous phase shifts of the free-induction decay with respect to the excitation pulses are extracted from simultaneously recorded laser pulse autocorrelation and free-induction decay in GaAs. Ultrafast phase dynamics during and immediately after the femtosecond pulse excitation are numerically evaluated with optical Bloch equations using pump pulse and semiconductor exciton parameters as input. Good agreement of the theory with experiment is obtained.
Artificial skin to sense mechanical stress by visible light emission
Authors: Xu C. N. , Watanabe T. , Akiyama M. , Zheng X. G.
Published: July 2002

The idea and successful practice of a stress sensor to sense mechanical stress by an artificial skin, i.e., self-diagnosis thin film, has been realized, through the fabrication of a high-luminescence thin piezoelectric film which can reproducibly emit strong visible light upon stressing. The strongest luminescent film consists of nanosized crystallites of ZnS doped with 1.5 at. % Mn, in which Mn acts as the emitting center. The intensity of the emitted luminescence responds to stress applied directly onto the film or to the underlying material reversibly and reproducibly, so it can be used as an artificial skin to sense mechanical stress.
Investigation of periodicity fluctuations in strained (GaNAs)1(GaAs)m superlattices by the kinematical simulation of x-ray diffraction
Authors: Pan Z. , Wang Y. T. , Zhuang Y. , Lin Y. W. , Zhou Z. Q. , Li L. H. , Wu R. H. , Wang Q. M.
Published: July 2002

Periodicity fluctuations of layer thickness and composition in a superlattice not only decrease the intensity, they also broaden the width of the satellite peaks in the x-ray diffraction pattern. In this letter, we develop a method that is dependent on the width of satellite peaks to assess periodicity fluctuations of a superlattice quickly. A linear relation of the magnitude of fluctuations, peak width and peak order has been derived from x-ray diffraction kinematical theory. By means of this method, periodicity fluctuations in strained (GaNAs)1(GaAs)m superlattices grown on GaAs substrates by molecular beam epitaxy have been studied. Distinct satellite peaks indicate that the superlattices are of high quality. The N composition of 0.25 and its fluctuation of 20% in a strained GaNxAs1−x monolayer are obtained from simulations of the measured diffraction pattern. The x-ray simulations and in situ observation results of reflection high-energy electron diffraction are in good agreement.
Epitaxial growth of Y2O3:Eu thin films on LaAlO3
Authors: Gao H-J. , Kumar D. , Cho K. G. , Holloway P. H. , Singh R. K. , Fan X. D. , Yan Y. , Pennycook S. J.
Published: July 2002

We report the epitaxial growth of europium-activated yttrium oxide (Y2O3:Eu) (001) thin films on LaAlO3 (001) using laser ablation deposition at a substrate temperature of 775 °C and 10 Hz pulse repetition rate. The orientation relationship between the films and the substrates is [110]Y2O3∥[100]LaAlO3 and [−110]Y2O3∥[010]LaAlO3 which results in a lattice mismatch of only 0.8%. Transmission electron microscopy (TEM) of the films reveals the single crystalline Y2O3:Eu thin film to contain small pores. Scanning transmission electron microscopy (STEM) imaging of the films shows the substrate always terminates with the Al sublattice. Moreover, the STEM reveals that no precipitates of Eu had formed in the films.
Effect of indium doping on the transient optical properties of GaN films
Authors: Kumano Hidekazu , Hoshi Ken-ichi , Tanaka Satoru , Suemune Ikuo , Shen Xu-Qiang , Riblet Philippe , Ramvall Peter , Aoyagi Yoshinobu
Published: July 2002

We have investigated the effects of In doping on the optical properties of GaN films grown by gas-source molecular-beam epitaxy. Time-resolved photoluminescence was carried out to study the transient optical properties of the epitaxial films. In comparison to the undoped GaN film, the spontaneous emission lifetime was prolonged from below 20 to 70 ps by doping with In. Under high-excitation density, stimulated emission was observed from both samples. The threshold excitation density was found to be reduced in the In-doped sample. These significant improvements of the optical properties are attributed to the effective suppression of the formation of the nonradiative recombination centers caused by a change of the growth kinetics induced by a small amount of In supplied during growth of the GaN films.
Effect of mechanical constraint on the dielectric and piezoelectric behavior of epitaxial Pb(Mg1/3Nb2/3)O3(90%)–PbTiO3(10%) relaxor thin films
Authors: Nagarajan V. , Ganpule C. S. , Nagaraj B. , Aggarwal S. , Alpay S. P. , Roytburd A. L. , Williams E. D. , Ramesh R.
Published: July 2002

The effect of heteroepitaxy-induced constraint on the structure and piezoelectric properties of the relaxor ferroelectric lead magnesium niobate–lead titanate (PMN–PT) were investigated. Relaxor PMN–PT epitaxial thin films with oxide electrodes were grown by pulsed-laser deposition on (100) LaAlO3 substrates. We observe a systematic decrease in the phase transition temperature (temperature at which a maximum in dielectric response occurs), from around 250 to around 60 °C as the relaxor thickness is increased from 100 to 400 nm. This is accompanied by an increase in the relative dielectric constant (εr), measured at room temperature and 10 kHz, from 300 to 2000. The piezoelectric coefficient d33 measured using a scanned probe microscope, increase by almost an order of magnitude with increasing film thickness.
Surface polarity dependence of Mg doping in GaN grown by molecular-beam epitaxy
Authors: Li L. K. , Jurkovic M. J. , Wang W. I. , Van Hove J. M. , Chow P. P.
Published: July 2002

The effect of surface polarity on the growth of Mg-doped GaN thin films on c-plane sapphire substrates by molecular-beam epitaxy has been investigated. The doping behavior of Mg and resulting conductivity of the doped layers were found to strongly depend on the surface polarity of the growing GaN planes. The samples grown on the Ga-polar face (A face) exhibited a p-type conductivity with a free-hole concentration up to 5×1017 cm−3, while the samples grown on the N-polar face (B face) were highly resistive or semi-insulating. The incorporation of residual impurities (O, Si, and C) in the two different polar surfaces was studied by secondary ion mass spectrometry analysis and its effect on the Mg doping was discussed. Our results suggest that the A face (Ga face) is the favored surface polarity for achieving p-type conductivity during the growth of Mg-doped GaN.
Thermoelectric properties of Bi2Sr2Co2Ox polycrystalline materials
Authors: Funahashi Ryoji , Matsubara Ichiro , Sodeoka Satoshi
Published: July 2002

Bi 2 Sr 2 Co 2 O 9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). All three samples are p-type conductors. The electric properties, namely, the Seebeck coefficient (S) and electric resistivity (ρ), of the samples are dependent on chemical composition. The S values increase with temperature at T>673 K and, at 973 K, reach 100, 110, and 150 μV K−1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (κ) for all samples is lower than for ordinary conducting oxides. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77×10−4, 0.61×10−4, and 2.0×10−4 K−1 for the 2202, Bi-1.8, and Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its high performance at high temperature (∼1000 K).
Annealing of ultrashallow p+/n junction by 248 nm excimer laser and rapid thermal processing with different preamorphization depths
Authors: Chong Y. F. , Pey K. L. , Wee A. T. S. , See A. , Chan L. , Lu Y. F. , Song W. D. , Chua L. H.
Published: July 2002

Ultrashallow p+/n junctions formed by B+-ion implantation and annealed by spike rapid thermal annealing (RTA) or laser annealing were studied. The effect of the preamorphizing depth on the redistribution of boron atoms after annealing has also been investigated. Our results show that for ultrashallow junctions formed by ultra-low-energy ion implantation and spike RTA, the depth of the preamorphizing implant has very little impact on the junction depth. By optimizing the laser fluence and preamorphization depth, a highly activated, ultrashallow, and abrupt junction can be obtained using a 248 nm excimer laser. The secondary-ion-mass spectrometry results clearly indicate that a step-like profile with a junction depth of 370 Å (for a B+ implant at 1 keV) can be formed with a single-pulse laser irradiation at 0.5 J/cm2.
Oxygen-vacancy ordering as a fatigue mechanism in perovskite ferroelectrics
Authors: Scott J. F. , Dawber Matthew
Published: July 2002

We present a paradigm for fatigue in ferroelectric perovskite oxides: That of a structural phase transition in which oxygen vacancies order into two-dimensional planar arrays capable of pinning domain wall motion.
Origin of hexagonal-shaped etch pits formed in (0001) GaN films
Authors: Hong S. K. , Yao T. , Kim B. J. , Yoon S. Y. , Kim T. I.
Published: July 2002

We report the origin of hexagonal-shaped etch pits generally observed by conventional wet etching processes on (0001) GaN-based films based on the investigation with transmission electron microscopy on GaN films after being etched with molten KOH. The origin of hexagonal-shaped etch pits is identified as nanopipes through careful characterization of abnormal contrast of nanopipe (open-core screw dislocation), “lobe contrast” of end-on edge and screw (full-core) dislocations, visible and invisible conditions of edge and screw dislocations. Consideration of energetics of these defects also suggests preferential etch pit formation at nanopipes because of much higher energy.
Order–disorder behavior in Ba(Zn1/3Ta2/3)O3
Authors: Reaney I. M. , Qazi I. , Lee W. E.
Published: July 2002

Sintered (1475 °C/2 h) pellets of Ba(Zn1/3Ta2/3)O3 were annealed at temperatures ranging from 1550 to 1625 °C/15 h and subsequently quenched into water. The degree of B-site order between the Zn and Ta cations was investigated using x-ray diffraction (XRD) and transmission electron microscopy (TEM). Samples quenched from 1575 to 1600 °C exhibit the highest degree of order by XRD. Dark-field TEM images from these samples revealed 1:2, Zn:Ta, trigonal, ordered domains approximately 0.4 μm diam near grain boundaries but 100 nm in the grain interior. Samples annealed and quenched from 1625 °C exhibited no ordered superlattice reflections by XRD. However, diffuse scatter along 〈111〉 directions and fcc rather than trigonal ordered reflections were observed in electron diffraction patterns. Disordered samples were subsequently annealed at 1500 °C/15 h (below the order–disorder phase transition ∼1625 °C) and ±13{hkl} superlattice reflections reappeared in XRD patterns demonstrating the reversibility of the phase transition.
Faceted inversion domain boundary in GaN films doped with Mg
Authors: Romano L. T. , Northrup J. E. , Ptak A. J. , Myers T. H.
Published: July 2002

Homoepitaxial GaN films, doped with Mg, were grown by rf-plasma molecular-beam epitaxy on Ga-polarity (0001) templates. Convergent-beam electron diffraction analysis establishes that the film polarity changes from [0001] to [0001_] when the Mg flux during growth is approximately 1 ML/s. Secondary ion mass spectrometry indicates a doping concentration of ∼1020 cm−3 in the film where the inversion occurs, and a reduced Mg incorporation in the [0001_] material. Transmission electron microscopy shows that the inversion domain boundary is faceted predominantly along the {0001} and {h,h,−2h,l} planes, with l/h approximately equal to 3. Using first-principles total energy calculations, we show that the {h,h,−2h,l} segments of the boundary are stabilized by the incorporation of Mg in threefold coordinated lattice sites.
Excitation energy transfer in condensed media
Authors: Hsu Chao-Ping , Fleming Graham R. , Head-Gordon Martin , Head-Gordon Teresa
Published: July 2002

We derive an expression for resonance energy transfer between a pair of chromophores embedded in a condensed medium by considering the energy splitting of the chromophores from their resonant excited states. We employ time-dependent density functional response theory in our derivation. The linear response theory treatment is rigorous within the framework of time-dependent density functional theory, while in obtaining the energy transfer coupling, the standard first-order approximation is used. The density response function for the medium, which can be replaced by the macroscopic dielectric susceptibility, enables the inclusion of the medium influence on the energy transfer coupling between the donor and acceptor. We consider the Coulomb coupling, and determine that our result is isomorphic to the Coulomb interaction between two charge densities inside a dielectric medium. The isomorphism we found not only provides a general and useful expression for applications, but additionally offers a basis for the extension of the dielectric response model to energy transfer coupling, which has been implicitly used earlier. An illustrative model shows that for two separated molecules, the medium adds a dielectric screening effect to the Coulomb coupling of their transitions. However, if the two molecules are so closely spaced that they effectively reside in a single cavity, the medium can enhance or reduce the strength of the coupling depending on the orientation and the alignment of the two chromophores.
Julia sets for the super-Newton method, Cauchy’s method, and Halley’s method
Authors: Kneisl Kyle
Published: July 2002

We study numerically and dynamically three cubically convergent iterative root-finding algorithms, namely Cauchy’s method, the super-Newton method, and Halley’s method. Using the concept of a universal Julia set (motivated by the results of McMullen), we establish that these algorithms converge when applied to any quadratic with distinct roots. We give examples showing the existence of attracting periodic orbits not associated to a root for the super-Newton method and Halley’s method applied to cubic polynomials. We include computer plots showing the dynamic structure for each algorithm applied to a variety of polynomials.
The role of polarization current in magnetic island evolution
Authors: Connor J. W. , Waelbroeck F. L. , Wilson H. R.
Published: July 2002

The polarization current plays an important role in the evolution of magnetic islands with a width comparable to the characteristic ion orbit width. Understanding the evolution of such small magnetic islands is important for two reasons: (1) to investigate the threshold mechanisms for growth of large-scale islands (e.g., neoclassical tearing modes), and (2) to describe the drive mechanisms for small-scale magnetic turbulence and consequent transport. This article presents a two-fluid, cold ion, collisional analysis of the role of the polarization current in magnetic island evolution in slab geometry. It focuses on the role played by the conjunction of parallel electron dynamics and perpendicular transport (particle diffusion and viscosity) in determining the island rotation frequency and the distribution of the polarization current within the island.
Photoreflectance line shape of excitonic transitions analyzed with a redefined set of fitting parameters
Authors: Ghosh Sandip , Grahn H. T.
Published: July 2002

We analyze experimental and simulated photoreflectance spectra of excitonic transitions in semiconductors with a redefined set of fitting parameters in Aspnes’ line shape function. The redefinition, based on a comparison with the first derivative of a Gaussian, allows one to directly relate these parameters to the observed experimental spectrum, making them independent of the exponent m. None of the original information, including the underlying physics involved as determined by m, is lost in this process. The physical significance of the fitting parameters is explained. Their usefulness lie in making line shape fitting easier and the possibility of comparing different sets of samples exhibiting inhomogeneities with regard to their broadening, oscillator strengths, and hence, quality.
Observation of ion-acoustic solitary waves in a dusty plasma
Authors: Nakamura Y. , Sarma A.
Published: July 2002

An experiment on ion-acoustic solitary waves in dusty plasma is performed in a dusty double-plasma device. The leading part of a sinusoidal pulse develops to an oscillation. The troughs of the oscillation become shallow when dust particles are mixed into the plasma. The Mach velocity of the first peak in the oscillation decreases due to the presence of negatively charged dust grains for a fixed height of the peak. The width of the peak normalized with Debye length increases when the dust particles are introduced into the plasma. The experimental results are compared with the numerical integrations of a modified Korteweg–de Vries–Burgers equation which contains the terms of kinematic viscosity and of collisions of ions with the dust particles.
Rectified photocurrent of the protein-based bio-photodiode
Authors: Choi Jeong-Woo , Nam Yun-Suk , Lee Won-Hong , Kim Dongho , Fujihira Masamichi
Published: July 2002

A bio-photodiode was fabricated using an array of electronically functional molecular layers, which consisted of a thin film of the green fluorescent protein and organic hetero-Langmuir–Blodgett layers between metal electrodes. With repeated step light illumination, the photocurrent was generated accordingly, which indicates a photoswitching function. Under illumination, the rectified photocurrent of the bio-photodiode was observed from current–voltage measurements. To verify the switching function and charge transfer through the functional molecular layers, transient photocurrents were measured. In transient photocurrent measurements, unidirectional charge flows generated by the redox potential difference through the molecular layers were observed.
Interaction of neoclassical tearing modes with trapped fast ions
Authors: Marchenko V. S. , Lutsenko V. V.
Published: July 2002

Resonance between rotating neoclassical tearing mode (NTM) and precessing trapped energetic ions has been considered. Resonant ions experience large radial excursions of their banana guiding centers leading to the loss of a considerable fraction of fast ions deposited in the resonance region. The resulting toroidal torque accelerates the magnetic island rotating in the direction of the ion diamagnetic drift. The torque balance equation has been coupled with the modified Rutherford equation, retaining the contributions from the bootstrap current and the ion polarization current. It has been shown that, due to the resonant interaction with energetic ions, this autonomous system of equations exhibits a Hopf bifurcation for some critical ratio of the momentum damping rate to the resistive diffusion rate. The resulting stable limit cycle resembles the frequency jump events observed during NTM activity in the Axisymmetric Divertor Experiment Upgrade (ASDEX Upgrade) tokamak [S. Sesnic et al., Phys. Plasmas 7, 935 (2000)].
Millimeter-wave scanning near-field microscope using a resonant waveguide probe
Authors: Park Wonkyun , Kim Jooyoung , Lee Kiejin
Published: July 2002

We demonstrate a millimeter-wave surface imaging technique using a near-field scanning millimeter-wave microscope with a resonant standard waveguide probe. The metallic probe tip in the resonant waveguide was designed to couple energy into and out of the resonant waveguide. By measuring the shift of the resonant frequency and the change of the quality factor in the near-field zone, we obtained millimeter-wave near-field images of YBa2Cu3Oy thin films on MgO substrates with a spatial resolution better than 2 μm.
High-resolution spectroscopy of HCl and DCl isolated in solid parahydrogen: Direct, induced, and cooperative infrared transitions in a molecular quantum solid
Authors: Anderson David T. , Hinde Robert J. , Tam Simon , Fajardo Mario E.
Published: July 2002

The infrared spectroscopy and rovibrational dynamics of HCl and DCl dopants in solid parahydrogen (pH2) were investigated using high-resolution spectroscopic methods. The absorption spectra of HCl and DCl monomers in solid pH2 closely resemble the corresponding low temperature gas phase spectra, indicating that the gas phase vibrational and rotational quantum numbers of the dopant are conserved within the pH2 solid. Small deviations from gas phase behavior are observed, however, such as a reduced HCl rotational energy level spacing and splitting of the fivefold orientational degeneracy of HCl rotational states with J=2. In addition, the pure vibrational Q1(0) (v=1←0, J=0←0) H2 transition, which is infrared inactive in pure solid pH2, is detected in the HCl doped sample. We propose that this transition is induced in pH2 molecules by neighboring HCl molecules through a weak “overlap induction” mechanism that is the only induction mechanism operative for J=0 impurities in pH2. Rovibrational transitions are also detected near the induced Q1(0) H2 absorption; these are attributed to cooperative transitions involving single photon excitation of pH2–HCl pairs. Detailed isotopic analysis reveals that these cooperative transitions involve pure vibrational excitation of the pH2 and pure rotational excitation of the HCl. Two-molecule transitions have long been studied for isotopic and rotational dopants (e.g., D2, HD, orthohydrogen) in solid pH2, but this is the first time such cooperative transitions have been attributed to a chemical impurity in pH2 matrices.
Space-selective valence state manipulation of transition metal ions inside glasses by a femtosecond laser
Authors: Qiu Jianrong , Zhu Congshan , Nakaya Takayuki , Si Jinhai , Kojima Kazuo , Ogura Fumito , Hirao Kazuyuki
Published: July 2002

We report the observation of space-selective oxidation of Mn2+ to Mn3+ in a transparent and colorless Mn and Fe ions codoped silicate glass at room temperature by using an 800-nm-femtosecond laser. Difference absorption spectrum of the glass after and before the irradiation of the focused infrared femtosecond pulsed laser showed that a portion of Mn2+ ions near the focused part of the laser beam inside the glass were oxidized to Mn3+ ions after the laser irradiation. Mn3+ ions were stable at the temperatures below 300 °C. It is suggested that Mn2+ ions act as hole trapping centers while Fe3+ ions as well as active sites in the glass matrix act as electron trapping centers. A promising application was demonstrated for the fabrication of three-dimensional colored image inside a transparent material.
Logarithmic moments of relaxation time distributions
Authors: Zorn Reiner
Published: July 2002

In this paper a novel way to quantify “nonexponential” relaxations is described. So far, this has been done in two ways: (1) by fitting empirical functions with a small number of parameters, (2) by calculation of the underlying distribution function g(ln τ) of (exponential) relaxations using regularization methods. The method described here is intermediate, it does not assume a specific functional form but also does not aim at the complete distribution g(ln τ) but only its logarithmic moments 〈(ln τ)k〉. It is shown that these exist (in contrast to the linear moments) and can be calculated analytically for all currently used empirical descriptions of nonexponential relaxations. Therefore, the logarithmic moments represent a common basis for comparing literature data from authors which prefer different empirical formulas (e.g., those of Kohlrausch and Havriliak-Negami). The logarithmic moments are also shown to be related in a simple way to the (linear) moments of an underlying distribution of activation energies giving them a physical significance.
Tuning exchange bias and coercive fields in ferromagnet/antiferromagnet bilayers with ion irradiation
Authors: Juraszek J. , Fassbender J. , Poppe S. , Mewes T. , Hillebrands B. , Engel D. , Kronenberger A. , Ehresmann A. , Schmoranzer H.
Published: July 2002

The effect of He ion irradiation on the magnetic properties of NiFe exchange coupled to different antiferromagnetic alloys (FeMn, CrMn, and PtMn) with the same layer thickness is investigated. All systems exhibit an enhanced coercivity prior to irradiation. An exchange bias field is only observed for FeMn and PtMn. Upon ion irradiation the FeMn-based system shows with increasing ion dose an enhancement followed by a decrease and finally a full suppression of the exchange bias field. For systems exchange coupled to PtMn only a decrease and suppression of the bias field is found. This can be attributed to the ion induced chemical disordering of the antiferromagnetic phase in the latter case. In the case of CrMn the antiferromagnetic layer thickness is too small to induce an exchange bias field, but an enhanced coercivity is observed which is caused by the exchange coupling between the antiferromagnetic and ferromagnetic layers. For all systems, this enhanced coercivity of the exchange coupled bilayer system is modified by ion irradiation.