bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple apparatus for determining column density and absolute photoabsorption cross sections
Authors: Fung H. S. , Chu C. C. , Hsu S. J. , Wu H. H. , Yih T. S.
Published: July 2002

This article presents a simple measuring apparatus that is capable of measuring column density and absolute photoabsorption cross sections. The apparatus includes a simple high-temperature heatpipe furnace, an MKS Instruments Baratron, a hydrogen discharge lamp, and an optical detecting system. The heatpipe furnace provides an absorbing medium with a steady total pressure in a closed system so that the column density can be obtained using the ideal gas relation. The apparatus is designed for a working temperature range below 1000 K and a pressure range up to 10 Torr. Encountered experimental difficulties and their found solutions will be discussed in detail. In this work, the absolute photoabsorption cross section of Mg is measured near the 3s threshold using this apparatus. The absolute photoabsorption cross section result at the 3s threshold, which is 2.1±0.3 Mb, is in good agreement with hook method data (2.36 Mb).
Small-displacement monochromator for microdiffraction experiments
Authors: Ice Gene E. , Chung Jin-Seok , Lowe Walter , Williams Ernest , Edelman Joel
Published: July 2002

We describe the design, construction, and performance of the MHATT-CAT microdiffraction x-ray monochromator. This monochromator is specially engineered for x-ray microdiffraction experiments with a high brilliance undulator source. The monochromator passes a small emittance beam, suitable for focusing to submicron size with submilliradian divergence. Over its energy range of 8–22 keV the absolute energy calibration is better than 2 eV and scans of ± 1 keV show no measurable hysterisis. The monochromator operates with a simple water-cooled first crystal and shows no measurable warm-up time. Horizontal linear bearings allow the monochromator crystals to be rapidly inserted or removed from the beam. Slits before and after the monochromator work to pass broad bandpass or monochromatic x-ray beams at the same vertical height. The monochromatic beam direction is adjusted so the monochromatic and broad bandpass beams are coaxial. The design and performance of the monochromator allows efficient collection of microdiffraction data when coupled to a nondispersive Kirkpatrick–Baez focusing mirror pair.
Distributed Bragg reflection in integrated waveguides of polymethylmethacrylate
Authors: Mukherjee N. , Eapen B. J. , Keicher D. M. , Luong S. Q. , Mukherjee A.
Published: July 2002

Distributed Bragg reflection from integrated polymethylmethacrylate waveguides fabricated on corrugated spin-on-glass substrate is reported. Utilizing third-order Bragg resonance, a maximum reflection of 92% from the grating filter is measured at a wavelength of 423.6 nm. A maximum bandwidth of the Bragg reflection is found to be ∼0.5 nm for a 6 mm long filter. The broad bandwidth and the significant transmission loss (∼90%) away from resonance are believed to be related to the radiation coupling by the grating. It is shown that the radiation loss can be minimized without sacrificing high reflectivity by limiting the waveguide length to 0.5 mm, the saturation length of the Bragg reflector.
Mechanochemical synthesis of ultrafine Fe powder
Authors: Ding J. , Miao W. F. , McCormick P. G. , Street R.
Published: July 2002

Ultrafine Fe powders were synthesized by mechanochemical solid-state reduction of FeCl3 by Na and subsequent removal of the reaction by-products. X-ray diffraction, Mössbauer spectroscopy, and transmission electron microscopy measurements showed that Fe particles with a relatively uniform particle size ∼10 nm were formed during the mechanical milling process. The powders exhibited coercivity values of 350–550 Oe, consistent with that expected for separated nanosized particles. This study demonstrates that mechanochemical processing has significant potential for the synthesis of ultrafine metal powders in an economic and efficient way.
Nonelectrooptic nonlocal photorefractive effect in a polymer composite
Authors: Orczyk Maciek E. , Zieba Jaroslaw , Prasad Paras N.
Published: July 2002

The grating spacing dependence of photorefractive diffraction efficiency in an organic polymer composite is presented and discussed. The analysis of the grating spacing dependence of the refractive index modulation with the standard model of photorefractive grating dynamics is shown to lead to inconsistencies in determination of the maximum attainable index change. It is shown that the effects resulting from orienting of the composite’s optically second-order nonlinear chromophores in the periodic local field affect the formation of the refractive index grating.
Magnetic properties and giant magnetoresistance in La0.67Ca0.33MnOx bulk material
Authors: Zhang X. X. , Yu R. H. , Tejada J. , Sun G. F. , Xin Y. , Wong K. W.
Published: July 2002

The magnetic and transport properties of a perovskite-type crystal sample with a nominal composition La0.67Ca 0.33MnOx have been investigated in the temperature range from 5 to 300 K with applied magnetic fields up to 5 T. Magnetization data obtained in the zero-field-cooled process exhibit the magnetic behavior with a Néel temperature, TN∼50 K and Curie temperature, TC∼300 K. The interesting feature in the temperature dependence of magnetoresistance is the appearance of a peak at T∼50 K, with value Δρ/ρ=[ρ(0)−ρ(5 T)]/ρ(5 T)=1300% at the Néel temperature, rather than at Curie temperature.
Oscillator strength dependence of cavity-polariton mode splitting in semiconductor microcavities
Authors: Kadoya Y. , Kameda K. , Yamanishi M. , Nishikawa T. , Kannari T. , Ishihara T. , Ogura I.
Published: July 2002

We demonstrate the continuous alteration of the exciton-polariton mode splitting in semiconductor microcavities, through the change of the exciton oscillator strength induced by quantum confined Stark effect (QCSE). This is clear evidence of the direct influence of the oscillator strength on the mode splitting. A wide range variation, from 0 to 2 nm, of the splitting is obtained by the tuning of the cavity resonance based on the in-plane dispersion of the photon mode as well as the tuning of exciton energy by QCSE. The influence of the field-induced exciton line broadening on the mode splitting is also observed for the applied electric field as high as 90 kV/cm.
Amorphous films formed by solid-state reaction in an immiscible Y–Mo system and their structural relaxation
Authors: Chen Y. G. , Liu B. X.
Published: July 2002

Amorphous films of 300 nm thick were obtained by solid state reaction in an immiscible Y–Mo system with a positive heat of mixing being +26 kJ/mol. The experimental results revealed that the Y51Mo49 multilayers, including 19% interfacial atoms, were vitrified by thermal annealing at 300 °C for one and half hours, and that the formed amorphous films recrystallized at about 600 °C. These confirmed that the excess interfacial free energy stored in the multilayered films elevated the initial energetic state of the multilayers to a level higher than that of the amorphous phase, providing a major driving force in solid state amorphization. The elastic modulus and hardness changes of the formed amorphous films upon annealing were characterized by nanoindentation, and the results were in accordance with the proposed free volume model for the amorphous structure. ]
Species characterization for a direct-current-biased hot filament growth of diamond using spatial resolved optical emission spectroscopy
Authors: Cui Jingbiao , Ma Yurong , Fang Rongchuan
Published: July 2002

Spatial resolved optical emission spectroscopy was applied for a direct-current biased hot filament diamond growth system with a spatial resolution of better than 1 mm. Emission lines from atomic hydrogen, CH, CH+, H2, and Ar were observed in the visible region. It is found that emission intensity of all the observed lines decreases rapidly as the probe distance from the filament center raises, and increases near the substrate surface. The relative concentration of atomic hydrogen has been estimated by using the emission line of Ar at 750.4 nm as an actinometer. The concentration of atomic hydrogen decreases from filament to the substrate and drops sharply at the substrate surface. The emission spectra and the distribution of hydrogen atom along the substrate surface have also been measured, which are closely correlated with the homogeneity of diamond film growth. And the low concentration of atomic hydrogen at the edge of the substrate results in a worsening of diamond films.
Real-time assessment of overlayer removal on GaN, AlN, and AlGaN surfaces using spectroscopic ellipsometry
Authors: Edwards N. V. , Bremser M. D. , Weeks T. W. , Kern R. S. , Davis R. F. , Aspnes D. E.
Published: July 2002

Spectroscopic ellipsometry was used to assess the preparation of smooth and abrupt GaN, AlN, and AlGaN surfaces by wet chemical treatments in real time. About 20–50 Å of overlayer typically can be removed from air-exposed samples.
Kinetics of epitaxial Si1−xGex growth using SiH2Cl2–GeH4–H2 mixture in reduced-pressure chemical vapor deposition
Authors: Ito S. , Nakamura T. , Nishikawa S.
Published: July 2002

The dependences of the Ge/Si ratio in epitaxial Si1−xGex layers grown using a SiH2Cl2–GeH4–H2 mixture by reduced-pressure chemical vapor deposition on the flow rate ratio of GeH4/SiH2Cl2 and on the total flow rate have been examined in the temperature range from 480 to 600 °C. As previously reported, the Ge/Si ratio does not increase linearly with the flow rate ratio; instead, the rate of increase decreases at high flow rate ratios. Additionally, it increases with increasing total flow rate when the ratio of GeH4/SiH2Cl2 is fixed. It is found that these flow rate dependences are well accounted for by assuming that the reaction order for SiH2Cl2 adsorption is twice as large as that for GeH4 adsorption.
Photocurrent multiplication in naphthalene tetracarboxylic anhydride film at room temperature
Authors: Katsume Tadashi , Hiramoto Masahiro , Yokoyama Masaaki
Published: July 2002

A large photocurrent multiplication reaching 130 000-fold at room temperature has been observed in naphthalene tetracarboxylic anhydride (NTCDA) film sandwiched between metal electrodes. This phenomenon is reasonably interpreted in terms of the tunneling injection of electrons from a metal electrode, which is triggered by the accumulation of photogenerated trapped holes near the metal/organic interface. The combination of an ultrathin NTCDA film with another photoconductive pigment film in a layered structure allowed us to fabricate a photocurrent multiplication device with the desired spectral response.
Characterizations of deep levels in SnTe-doped GaSb by admittance spectroscopy
Authors: Chen J. F. , Chen N. C. , Liu H. S.
Published: July 2002

A dominant deep level with an activation energy of 0.23–0.26 eV was observed by admittance spectroscopy for SnTe-doped GaSb layers grown directly on GaAs substrates by molecular beam epitaxy (MBE). The activation energy and capture cross section of the deep level are similar to the levels detected in S-doped and Te-doped GaSb grown by MBE, indicating that this deep level originates either from a native defect or a common impurity in n-type GaSb. The Sb4/Ga flux ratio was found to affect the Hall mobility and concentration of the deep level in a similar way, with an optimal beam equivalent pressure ratio around 7 obtained for GaSb grown at 550 °C, which should correspond to the lowest ratio to maintain a Sb-stabilized surface reconstruction. Analysis of this result suggests that the deep level seen by us is a complex defect.
Trap signatures of As precipitates and As-antisite-related defects in GaAs epilayers grown by molecular beam epitaxy at low temperatures
Authors: Goo C. H. , Lau W. S. , Chong T. C. , Tan L. S.
Published: July 2002

Despite many separate studies of the two dominant defects, i.e., As precipitates and arsenic-antisite (AsGa)-related traps, in GaAs epilayers grown by molecular beam epitaxy at low temperatures, they are seldom examined simultaneously. In this letter, we report the detection of both defects in electron trap spectrum obtained by zero quiescent bias voltage transient current spectroscopy. The As precipitates appear as a broad continuum of states in the lower temperature region (<280 K) of the spectra whereas the AsGa-related defect appears as a discrete peak at a higher temperature. The AsGa-related trap has an activation energy of 0.65 eV and a capture cross section of 9.3×10−14 cm2. It is found that the trap characteristic of low temperature GaAs is strongly dependent on its growth temperature and the above mentioned defects may not dominate in some cases.
Light-induced electroluminescence of porous silicon layers on p-Si in persulfate solution
Authors: Peter L. M. , Wielgosz R. I.
Published: July 2002

Electroluminescence has been observed using the photostimulated reduction of persulfate ions at porous silicon layers grown on p−-Si substrates. Electrons were generated in the substrate by illumination from the ohmic contact side. The detection of visible electroluminescence shows that photogenerated electrons accumulate in the porous layer when the p− substrate is reverse biased. Voltage tuning of the electroluminescence spectrum was observed, but the tuning saturated when the reduction of persulfate became light limited. The tuning is attributed to the switching on of electroluminescence as electrons progressively populated particles of decreasing size, followed by switching off by Auger quenching when a second electron is captured before radiative decay occurs.
Time-resolved high-resolution electron microscopy of solid state direct bonding of gold and zinc oxide nanocrystallites at ambient temperature
Authors: Kizuka Tokushi , Yamada Kanji , Tanaka Nobuo
Published: July 2002

Approach, contact, and bonding processes between different kinds of materials, gold (Au) and zinc oxide (ZnO) nanocrystallites were observed at atomic scale and at time resolution of 1/60 s. The nanocrystallites contact spontaneously when they approach at a few atomic distances under an intense electron irradiation. It was found that a Au/ZnO interface forms at ambient temperature without any sintering aid after the contact.
Characterization of lateral semiconductor nanostructures by means of x-ray grazing-incidence diffraction
Authors: Paschke K. , Geue T. , Barberka T. A. , Bolm A. , Pietsch U. , Rösch M. , Batke E. , Faller F. , Kerkel K. , Oshinowo J. , Forchel A.
Published: July 2002

Free-standing wire arrays prepared by holographic exposure and wet chemical deep etching on a vertically arranged GaAs/GaInAs/GaAs[001] single quantum well structure were characterized by x-ray grazing incidence diffraction using synchrotron radiation. Using a grazing angle of αi≈0.05° the diffracted intensity stems primarily from the surface grating. It’s periodicity (D≈480 nm) was determined close to the (−220) and (220) Bragg reflection being parallel and perpendicular to the orientation of wires, respectively. The average wire width [(21.6±1.5) nm and (96.6±1.5) nm, respectively] and the coherence length of the grating (ξ≈2 μm) were obtained via Fourier transformation of the (220) shape function.
Intrinsic carrier capture time in deep-etched quantum-box (70 nm diameter) lasers at low temperature: An indication of extremely high quantum capture efficiency
Authors: Wang Jian , Griesinger Uwe A. , Schweizer Heinz
Published: July 2002

In this letter we report the measurement result of intrinsic carrier capture time in deep-etched quantum-box (70 nm diameter) lasers at low temperature (2 and 77 K). In order to investigate intrinsic carrier capture and relaxation mechanism, we have to rule out the contribution of the geometry factor and derive the local quantum capture time. For this purpose, lasers with the same box size (70 nm) but different box densities, that are arranged as two-dimensional periodic arrays for distributed feedback operations are prepared and measured. We find that the quantum capture time of the box lasers determined at 2 and 77 K is only about 2.4 ps and is smaller than the corresponding value of the quantum-well lasers. This is a direct experimental indication of the existence of an efficient channel for carrier capture and relaxation in the quantum-box system investigated.
Features of shock wave formation in a wire induced surface flashover
Authors: Lisitsyn Igor V. , Muraki Taishi , Akiyama Hidenori
Published: July 2002

Explosive wire induced surface flashover has been studied as a promising method for destruction of solid materials. This method exhibits very high efficiency of electric energy transformation into the discharge and effective shock wave generation. The framing photography method is applied in order to clarify the discharge dynamics and the mechanism of the shock wave formation and propagation. The experiments performed allow to suggest the model of the pressure wave generation in the wire explosion process.
In-plane birefringence of asymmetric (001) quantum wells
Authors: Lew Yan Voon L. C.
Published: July 2002

It is widely believed that [001]-grown semiconductor superlattices have an isotropic linear optical response in the (001) plane. In this letter, we use simple symmetry arguments to show that introduction of asymmetry in the [001] direction can induce anisotropy in the (001) plane. The latter is calculated for various AlGaAs heterostructures; it is found that the in-plane birefringence can be comparable to the birefringence of KH2PO4.
Optical superlattices—a strategy for designing phase-shift masks for photolithography at 248 and 193 nm: Application to AlN/CrN
Authors: Carcia P. F. , French R. H. , Reilly M. H. , Lemon M. F. , Jones D. J.
Published: July 2002

This letter illustrates with AlN/CrN multilayers that optical superlattices, comprised of multilayers of a uv transmitting dielectric layer and a metallic layer, offer a systematic approach to design and fabricate partially transmitting, phase-shift masks for photolithography. From the measured optical constants of sputtered AlN/CrN multilayers, it was found that films had π-phase shift and tunable optical transmission between 5% and 15% at 365, 248, and 193 nm. We compared the optical properties of sputtered AlN/CrN multilayers to “ideal” superlattices, calculated from the measured optical properties of individual thick CrN and AlN layers, and to compositionally equivalent psuedobinary alloys of (AlN)1−x(CrN)x. Although optical properties for all three systems were nearly the same, which is attractive because it implies wide process lattitude, we found systematic differences that were attributed to their individual structures. A phase shift mask with 6% transmission at 365 nm was fabricated with a 1650-Å-thick (25 Å AlN+25 Å CrN) multilayer film.
External self-gettering of nickel in float zone silicon wafers
Authors: Gay N. , Martinuzzi S.
Published: July 2002

During indiffusion of Ni atoms in silicon crystals at 950 °C from a nickel layer source, Ni–Si alloys can be formed close to the surface. Metal solubility in these alloys is higher than in silicon, which induces a marked segregation gettering of the Ni atoms which have diffused in the bulk of the wafers. Consequently, the regions of the wafers covered with the Ni layer are less contaminated than adjacent regions in which Ni atoms have also penetrated, as shown by the absence of precipitates and the higher diffusion length of minority carriers. The results suggest the existence of external self-gettering of Ni atoms by the nickel source.
Enhanced arsenic excess in low-temperature grown GaAs due to indium doping
Authors: Bert N. A. , Chaldyshev V. V. , Kunitsyn A. E. , Musikhin Yu. G. , Faleev N. N. , Tretyakov V. V. , Preobrazhenskii V. V. , Putyato M. A. , Semyagin B. R.
Published: July 2002

We have found that isovalent indium doping enhances arsenic excess in GaAs films grown by molecular beam epitaxy at low temperature. An increase in lattice expansion and near infrared optical absorption, as well as higher density of As clusters, have been observed in the indium-doped films when compared to the conventional indium-free ones.
Columnar defect formation in nanorod/Tl2Ba2Ca2Cu3Oz superconducting composites
Authors: Yang Peidong , Lieber Charles M.
Published: July 2002

Nanorod/superconductor composites were formed by depositing Tl2Ba2Ca2Cu3Oz (Tl-2223) thick films on high density MgO nanorod arrays that were grown on MgO single crystal substrates. Electron microscopy studies show that this approach creates a high density of columnar defects normal to the CuO2 planes within crystal grains of the composites. The nanorod/superconductor composites exhibited enhanced critical current densities and an upward shift in the irreversibility line compared with reference samples. These results demonstrate that a nanorod-composite approach represents an effective strategy for introducing correlated defects into high-Tc superconductors, and thus may be useful for applications.
Acoustic radiation force on micrometer-size particles
Authors: Yasuda K. , Kamakura T.
Published: July 2002

The acoustic radiation force on micrometer-sized polystyrene spheres was measured through observation of the sphere movement in a 500 kHz ultrasonic standing wave. The known spatial distribution of the force allowed verification of the correlation between the sphere velocity and the force. It was found that the linear dependency of the force on the cube of the sphere radius, as predicted by Yosioka, began to fail when the sphere radius was below 5 μm. This can be accounted for by the presence of a shell layer surrounding the sphere, which increased the effective radius of the sphere. This may point to applications of the acoustic radiation force in the handling of moicrospheres smaller than hitherto thought possible.
Manipulation of final state population in semiconductor heterostructures by pulsed laser fields
Authors: Pötz W.
Published: July 2002

Proper combination of pulsed laser fields allows optical control of electron inter(sub)band transitions in semiconductors leading to control of final state population. Based on a microscopic theory, we show that the final ratio of direct versus indirect excitons generated by a subpicosecond pump pulse in semiconductor double wells can be manipulated by simultaneous application of a resonant microwave pulse. Generalization and relevance of this result to other inter– and intersubband transitions is discussed.
A source of hyperthermal neutrals for materials processing
Authors: Goeckner M. J. , Bennett T. K. , Cohen S. A.
Published: July 2002

In this letter, we describe a unique method of producing hyperthermal neutrals for material processing. The hyperthermal neutrals are produced by accelerating ions across a sheath from a plasma onto a surface. On impact, the ions are neutralized and reflected with ∼50% of their incident energy. These neutrals then bounce off of additional surfaces prior to impacting the target. This unique multiple bounce system was developed for the following reasons: to reduce contamination from sputtered surface material, improve beam uniformity, and reduce UV radiation in the beam path. As a test of this method, we built a prototype beam source and used it to ash photoresist at rates up to 0.022 μm/min. These rates are consistent with a predicted neutral beam flux, 2×1014 cm−2 s−1. In addition, a simple model is used to indicate that this method is capable of producing economically acceptable ash rates. Comparisons with other neutral-beam production methods are made.
Fast proton-conducting P2O5–ZrO2–SiO2 glasses
Authors: Nogami Masayuki , Nagao Ritsuko , Makita Kensuke , Abe Yoshihiro
Published: July 2002

Fast proton-conducting P2O5–ZrO2–2SiO2 glasses were successfully prepared by the hydrolysis of metal alkoxides. The glasses obtained by heating at 150 to 400 °C are chemically stable and exhibit high conductivities of ∼104 S/cm at room temperature, conductivities that are higher by ∼4 order than that of glasses containing no P2O5. These high conductivities were regarded as the fast proton transfer accelerated by molecular water bonded with POH groups. These glasses have a high potential for practical applications.
Metastability modeling of compliant substrate critical thickness using experimental strain relief data
Authors: Carter-Coman Carrie , Bicknell-Tassius Robert , Brown April S. , Jokerst Nan Marie
Published: July 2002

A metastability model for GaAs compliant substrates is developed using the compliant substrate partitioning formula and experimental strain relief data. The developed model agrees with compliant substrate strain relief data deduced from double crystal x-ray diffraction and indicates that, for a set of growth conditions and compliant substrate thicknesses, layers of InGaAs of any thickness can be grown free of dislocations. The model developed in this letter is also compared to other compliant substrate critical thickness models, and the authors discuss the mechanisms of partitioning in mismatched layers grown on compliant substrates.
Photothermal measurements at picowatt resolution using uncooled micro-optomechanical sensors
Authors: Varesi J. , Lai J. , Perazzo T. , Shi Z. , Majumdar A.
Published: July 2002

Deflections of bimaterial microcantilever beams were optically detected with 400 fm resolution at room temperature. This enabled photothermal radiation detection with resolutions of 40 pW for power and 10 fJ for energy. The resolution was improved by an order of magnitude by optimizing the thickness ratio of the two beam materials, as well as by modulating the incident radiation at sufficiently high frequency to be in the range of the thermal white noise limit of the cantilever vibrations. Radiative power was detected with a noise spectral density of and 250 pW/Hz and detectivity, D*, of 4.6×107 cm Hz/W.
Application of self-focusing of ps laser pulses for three-dimensional microstructuring of transparent materials
Authors: Ashkenasi D. , Varel H. , Rosenfeld A. , Henz S. , Herrmann J. , Cambell E. E. B.
Published: July 2002

Self-focusing of ps laser pulses (λ=790 nm) due to the nonlinear Kerr effect is shown to provide an easy and flexible method for producing bulk structures with dimensions on the order of 10 μm without damaging the entrance or exit surfaces of the material. The depth at which damage occurs can be controlled by adjusting either the pulse energy or pulse length. A study of the dependence of structure depth on the square root of the laser power for a given pulse length provides a straightforward method for determining the nonlinear index of refraction (Kerr coefficient), n2.
Determination of the unified surface-anchoring energy of a nematic liquid crystal on a polymer substrate
Authors: Sugimura Akihiko , Miyamoto Tetsuo , Tsuji Makoto , Kuze Michiyoshi
Published: July 2002

A saturation voltage method is described for determining the unified surface-anchoring energy, which has been proposed theoretically by Sugimura and co-workers [Phys. Rev. E, 52, 681 (1995)], at the interface of nematic (4-pentyl-4′-cyanobiphenyl) and its alignment layer (polyvinyl cinnamate). The method proposed can be employed as a standard measure of this energy without any additional numerical calculation. A conventional measurement method of an optical retardation from a liquid crystal slab is used to estimate the saturation voltage.
Optical absorption and multiphonon relaxation of Nd3+ ions in ZnCl2-based glass
Authors: Shojiya M. , Takahashi M. , Kanno R. , Kawamoto Y. , Kadono K.
Published: July 2002

Optical absorption and multiphonon relaxation have been investigated for Nd3+-doped ZnCl2-based glass. The intensity parameters for radiative transitions of Nd3+ were determined to be Ω2=4.97×10−20 cm2, Ω4=7.39×10−20 cm2 and Ω6=5.12×10−20 cm2. All of the Ωt parameters, especially Ω4, of Nd3+ in the ZnCl2-based glass were large compared with those reported for oxide and fluoride glasses. Multiphonon relaxation rates for three emission levels of Nd3+ were determined from lifetime measurements. The multiphonon relaxation rates in the ZnCl2-based glass were extremely low, because of the low-phonon-energy of the matrix. Values were found to obey the energy-gap law in the same manner as the values in Er3+-doped ZnCl2-based glasses.
Two-way switching based on turnstile operation
Authors: Tsukagoshi Kazuhito , Nakazato Kazuo
Published: July 2002

Two-way switching based on a turnstile operation was investigated. A clocking modulation carries a turnstile current through an input and one of two output multiple-tunnel junctions (MTJs). By changing side-gate voltages to the output MTJs, the output turnstile current is switched between the two output branches. The switching current consists of electron packets containing as few as 100 electrons.
Minority carrier diffusion length and lifetime in GaN
Authors: Bandić Z. Z. , Bridger P. M. , Piquette E. C. , McGill T. C.
Published: July 2002

Electron beam induced current measurements on planar Schottky diodes on undoped GaN grown by metalorganic chemical vapor deposition are reported. The minority carrier diffusion length of 0.28 μm has been measured, indicating minority carrier lifetime of 6.5 ns. The tapping mode atomic force microscopy imaging of the surfaces and scanning electron microscopy of the cross sections have been used to characterize the linear dislocations and columnar structure of the GaN. The possible influence of recombination on the extended defects in GaN on the minority carrier diffusion length and lifetime is discussed, and contrasted to other recombination mechanisms.
Asymmetric electro-optical switching of a nematic cell controlled by a corona poled ferroelectric polymer layer
Authors: Blinov L. M. , Palto S. P. , Yakovlev S. V. , Sikharulidze D. G.
Published: July 2002

A polar asymmetry of electro-optical behavior has been studied for a liquid-crystal cell, one electrode of which is covered by a thin film of a ferroelectric polymer. A well-known copolymer PVF2–TrFE (70:30) previously poled by a corona discharge has been used to modify the quadratic electro-optical response of the nematic mixture in a hybrid cell. The voltage for the onset of switching becomes polarity dependent. A simple model of an internal electric field created by a poled ferroelectric and built in the nematic layer is discussed, which is consistent with the sign and general features of the effect.
Recover the phases from intensity data of x-ray diffraction
Authors: Xu Gu
Published: July 2002

For many years, people believed that in conventional x-ray crystallography one can only record the scattering intensities but not the phases. In order to study structures at the atomic scale, one has to use multibeam measurement or to rely on additional knowledge such as bonding length and to fit the intensity data by trial structure models. In this letter, however, we show that the phases are in fact hidden in the intensity data and can be recovered by studying the peak shoulders. To demonstrate, the x-ray diffraction data of aluminum powder were used to recover the phases and to reconstruct the electron density map.
Growth and optical investigation of strain-induced AlGaAs/GaAs quantum dots using self-organized GaSb islands as a stressor
Authors: Wang T. , Forchel A.
Published: July 2002

The strain-induced quantum dots in a two-dimensional AlGaAs/GaAs quantum well plane are first reported to be fabricated by using self-organized GaSb islands as a stressor, which is confirmed by magnetophotoluminescence measurement. In the photoluminescence spectra, the emission peaks from this quantum dots and undisturbed quantum well are observed. In addition, the energy separation between these two peaks increases with the decrease of the thickness between the self-organized GaSb island and quantum well plane. When this thickness is 12.5 nm, the energy separation between these two peaks is about 21 meV. The calculation based on the strain distribution and parabolic potential model gives a good agreement with photoluminescence results. Furthermore, the optimum size of the self-organized GaSb islands as a stressor on the GaAs surface is about 36 nm.
Second-harmonic generation in BaTiO3 films doped with cerium
Authors: Xuan Lin-zhen , Pan Shao-hua , Chen Zheng-hao , Wang Rong-ping , Shi Wen-sheng , Li Chun-ling
Published: July 2002

Second-harmonic generation of 1.064 μm incident light was measured on BaTiO3 thin films, both a-axis and c-axis oriented, undoped and doped with cerium, prepared by pulsed laser deposition. The dependence of second-harmonic generation (SHG) coefficients of these films on the incidence angle and the polarization direction of the fundamental beam were investigated. The effective SHG coefficients was enhanced greatly when the films were doped with cerium.
Nanocrack junctions in unidirectionally twinned thin films of YBa2Cu3O7−δ on (001) NdGaO3
Authors: Koren G. , Polturak E. , Levy N. , Deutscher G. , Zakharov N. D.
Published: July 2002

We found unidirectional nanocracks in unidirectionally twinned films of YBa2Cu3O7−δ on NdGaO3 which are normal to the twin planes. In 70–250 nm thick films, transmission electron and atomic force microscopy show that these unidirectional nanocracks are found to be 1–2 μm apart and have 2–4 nm width. Microbridges that cross these nanocracks behave as natural Josephson junctions. They show Shapiro steps under microwave irradiation and suppression of the critical current Ic in magnetic fields. A very large in-plane anisotropy of the resistivities and Ic was observed and this was correlated with the nanocracks orientation, length, and film thickness. The observed anisotropy of Ic near Tc is shown to be consistent with a d+is wave symmetry of the order parameter in YBa2Cu3O7−δ.
Elastic strain in In0.18Ga0.82N layer: A combined x-ray diffraction and Rutherford backscattering/channeling study
Authors: Wu M. F. , Vantomme A. , Hogg S. M. , Langouche G. , Van der Stricht W. , Jacobs K. , Moerman I.
Published: July 2002

An InGaN layer was grown by metalorganic chemical vapor deposition on a sapphire (0001) substrate using a thick (>2.2 μm) GaN intermediate layer. The In composition, which cannot be unambiguously determined by x-ray diffraction (XRD) or by photoluminescence, was determined by Rutherford backscattering (RBS). The perpendicular and parallel elastic strain of the In0.18Ga0.82N layer, e⊥=+0.21% and e∥=−0.53%, respectively, were derived using a combination of XRD and RBS/channeling. The small ratio |e⊥/e∥|=0.40 indicates that the In0.18Ga0.82N layer is much stiffer in the c-axis direction than in the a-axis direction.
Time-dependent diffusivity of boron in silicon oxide and oxynitride
Authors: Ellis K. A. , Buhrman R. A.
Published: July 2002

The diffusivities of boron in silicon oxide and oxynitride have been determined from secondary ion mass spectroscopy measurements of annealed metal-oxide-silicon structures. The results clearly show a decrease in diffusivity with increasing anneal time which is approximately exponential in form. This effect implies a similar time dependence in the concentration of a secondary species, such as hydrogen, or a defect within the dielectric, which promotes diffusion even in a nominally pure oxide.
Stress relaxation by surface rippling and dislocation generation in mismatched channels of InGaAs/InAlAs/InP high-electron-mobility transistors
Authors: Peiró F. , Cornet A. , Beck M. , Py M. A.
Published: July 2002

We have investigated InGaAs strained channels of high-electron-mobility transistor heterostructures with In compositions of 70% and 80% and channel thicknesses covering the range of 3–14 nm. Transmission electron microscopy characterization has revealed the existence of two regimes of strain relaxation in these strained InGaAs channels: (i) an anisotropic rippling of the channel surface at low mismatch and (ii) dislocation generation as the layer thickness and mismatch increase. The correlation of structural features with electrical measurements has provided evidence of a significant reduction of the Hall mobility values measured across the rippling of the InGaAs well.
Self-waveguided blue light emission in p-sexiphenyl crystals epitaxially grown by mask-shadowing vapor deposition
Authors: Yanagi Hisao , Morikawa Takayuki
Published: July 2002

Highly polarized and self-waveguided blue light emission was observed in oriented organic crystals of p-sexiphenyl (6P). The 6P molecules were epitaxially grown in needlelike crystals with a submillimeter length by mask-shadowing vapor deposition on the KCl (001) surface. In the crystals, the molecular axes are oriented parallel to the [110] direction of the KCl substrate and perpendicular to the needle axis of the crystal. Under ultraviolet excitation, the blue light emission was confined within the crystal and self-waveguided along the needle axis with the transverse-electric mode, then radiated from the tips of the needles. This self-waveguided emission in the crystal was based on the uniaxially oriented transition dipoles which lay on the KCl surface and were aligned perpendicular to the needle axis in the crystal.
Formation of epitaxially strained islands by controlled annealing
Authors: Zhang Y. W.
Published: July 2002

A three-dimensional continuum method is developed to simulate the formation of epitaxially strained islands during controlled annealing. In the formulation, the strain energy density, surface energy and surface energy anisotropy are taken into account. Our simulations show that no surface energy anisotropy leads to spherical caps while high anisotropy leads to elongated islands. Under the present framework, a relatively uniform and regular island array can be obtained at a certain annealing time from a random surface. An almost perfectly uniform and regular island array can be obtained at a certain annealing time from a random surface with one dominant wavelength, which is in a specific range; for long-time annealing, these island arrays are undergoing ripening.
Ultrafast modulation of semiconductor lasers through a terahertz field
Authors: Ning C. Z. , Hughes S. , Citrin D. S.
Published: July 2002

We demonstrate, by means of numerical simulation, an interesting mechanism to modulate and switch semiconductor lasers at THz and sub-THz frequency rates. A sinusoidal terahertz field applied to a semiconductor laser heats the electron-hole plasma and consequently modifies the optical susceptibility. This allows an almost linear modulation of the output power of the semiconductor laser and leads to a faithful reproduction of the terahertz-field waveform in the emitted laser intensity.
A regime of the yield point of silicon at high temperatures
Authors: Siethoff Hans , Brion Hans Georg , Schröter Wolfgang
Published: July 2002

We present measurements of the lower yield point of undoped floating-zone silicon at temperatures between 800 and 1300 °C. The knowledge of the defect structure in this temperature range is of considerable importance for the numerical simulation of dislocation generation in various solar silicon materials. Above about 1050 °C, we find marked deviations from the well-known low-temperature behavior, thus establishing a further deformation regime. It is characterized by an activation energy of 4.1 eV. Comparison to preliminary work indicates that this effect depends on the as-grown dislocation density, but not on the ambient during deformation. We tentatively assume that it may reflect the change in the mechanism of self-diffusion typical for silicon at high temperatures.
Observation of a non Fowler–Nordheim field-induced electron emission phenomenon from chemical vapor deposited diamond films
Authors: Chen J. , Deng S. Z. , Xu N. S. , Wu K. H. , Wang E. G.
Published: July 2002

A field-induced electron emission phenomenon has been observed from nitrogen-doped diamond films deposited on molybdenum substrates by microwave plasma enhanced chemical vapor deposition using N2/CH4 as feedstock. Their field-induced electron emission characteristics, i.e., current–voltage characteristics and distribution of emission sites, were studied using the transparent anode imaging technique. A repeatable abrupt change of “on” and “off” states of emission was observed at two corresponding specific fields during circling of both increasing and decreasing applied gap fields. A plausible explanation is given to this type of field-induced electron emission phenomenon, in which a two-layer structure consisting of amorphous carbon and diamond is proposed.
Switching behavior of epitaxial perovskite manganite thin films
Authors: Oshima H. , Miyano K. , Konishi Y. , Kawasaki M. , Tokura Y.
Published: July 2002

We have observed electric-field-induced and photoinduced switching from insulating to conducting state in stressed epitaxial Sm0.5Sr0.5MnO3 thin films fabricated by pulsed-laser deposition. Previously known only in bulk crystals of perovskite manganese oxides, the switching behavior in thin film form is more significant for potential practical utility of the strongly correlated electron systems including optical devices.
Patterned surface dipole layers for high-contrast electroluminescent displays
Authors: Nüesch F. , Li Y. , Rothberg L. J.
Published: July 2002

A straightforward and general method to pattern organic light-emitting devices is demonstrated that entirely obviates the need for patterning either the organic electroactive material or the electrodes. The technique takes advantage of elastomeric stamp patterning of indium tin oxide electrodes using simple surface chemistry. A base reacts with the oxide surface to form a double ionic layer that drastically inhibits the ability to inject positive charge carriers from the oxide anode. We have achieved remarkably high contrast ratios (>105) in the technologically-useful hybrid triphenyl diamine and 8-(hydroxyquinoline) aluminum light-emitting diode. Furthermore, the adsorbed double layer shows excellent stability even when the device is driven beyond the damage threshold.
Photoluminescence linewidths of piezoelectric quantum wells
Authors: Khoo E. A. , David J. P. R. , Woodhead J. , Grey R. , Rees G. J.
Published: July 2002

The photoluminescence linewidths of strained InxGa1−xAs/GaAs quantum wells grown on (111)B GaAs substrates are shown to differ radically from those grown on (001) substrates. In (111)B structures, the linewidth in wide wells is broadened significantly compared with (001) structures. This broadening cannot be explained by well width or alloy fluctuations alone, and suggests the presence of another mechanism which we believe is due to the piezoelectric field and its sensitivity to screening under illumination.
Surface micro-roughness and transport properties of Si delta-doped GaAs/InxGa1−xAs/GaAs (0.1⩽×⩽0.25) quantum wells grown by molecular-beam epitaxy on GaAs (001) and GaAs (111)B
Authors: Zervos M. , Elliott M. , Westwood D. I.
Published: July 2002

We investigated the surface quality and electron transport properties of 200 Å GaAs/InxGa1−xAs/GaAs, 0.1⩽x⩽0.25, quantum well structures grown by molecular-beam epitaxy on GaAs (001) and (111)B, center delta-doped with Si to 1×1013 cm−2. For GaAs barriers grown above 600 °C, room temperature atomic force microscopy of the (111)B samples showed a surface micro roughness below 2 Å for x⩽0.15 but reaching 60 Å for x=0.25. In addition, analysis of Shubnikov–de Haas and Hall effects at 4.2 K showed a strong deterioration of the electrical properties. In contrast, the (001) structures had surface micro roughness below 2 Å and superior transport properties even at x=0.25. At lower growth temperatures the surface morphology roughened, but without a commensurate deterioration of the electrical properties. For the (001) layers self-consistent Poisson–Schrödinger calculations give quantum well subband densities in good agreement with those measured, assuming full incorporation of Si substitutional donors. However, a 20% loss of carriers was observed in the (111) layers, and the excited state subband densities were a smaller fraction of the ground subband occupancy compared to the (001). Amphoteric (Si-on-As sites) behavior is believed responsible for this behavior.
Strong and stable visible luminescence from Au-passivated porous silicon
Authors: Chen C. H. , Chen Y. F.
Published: July 2002

We report on porous silicon (PS) samples with strong and stable red photoluminescence (PL) prepared by chemical anodization of gold-plated substrate. We demonstrate that the structural stability of Au-passivated porous silicon is much better than that of normal PS. It is also found that the PL intensity of Au-passivated PS can be enhanced by a factor of 3 when prepared under the same condition as that for normal PS. The infrared absorption spectra reveal that the photoluminescence stability can be attributed to the formation of stable Au–Si bonds on the surface of porous silicon. In addition, our study provides further evidence to support the quantum confinement model of the red emission of porous silicon.
Studies on the reaction of the 5′-phosphorimidazolide of adenosine with Cu(II)-exchanged hectorite
Authors: Porter T. L. , Whitehorse R. , Eastman M. P. , Bain E. D.
Published: July 2002

The role of clay minerals in the prebiotic synthesis of nucleotide oligomers has received considerable attention in recent years. Scanning force microscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry are used to identify oligomers of adenylic acid formed on the clay mineral Cu(II)-exchanged hectorite in simulated prebiotic cycling experiments. Electron-spin resonance and x-ray diffraction data indicate that the monomer (5′-phosphorimidazolide of adenosine, or ImpA) penetrates into the intergallery regions of the mineral substrate, and complexes the gallery Cu(II) cations. It is postulated that polymerization of the monomer is initiated in the clay intergallery regions, producing oligomers of adenylic acid up to 8 units in length or more.
Mechanism for acoustic leakage in surface-acoustic wave resonators on rotated Y-cut lithium tantalate substrate
Authors: Koskela J. , Knuuttila J. V. , Tikka P. T. , Hartmann C. S. , Plessky V. P. , Salomaa M. M.
Published: July 2002

We discuss an acoustic loss mechanism in surface-acoustic wave resonators on 36° YX-cut lithium tantalate substrate. Recent acoustic field scans performed with an optical Michelson interferometer reveal a spatially asymmetric acoustic field atop the busbars of a resonator, giving rise to acoustic beams which escape the resonator area and lead to undesired losses. Here, we link the phenomenon with the inherent crystalline anisotropy of the substrate: the shape of the slowness curves and the asymmetry of the polarization for leaky surface-acoustic waves, propagating at an angle with respect to the crystal X-axis.
Controlled arrangement of self-organized Ge islands on patterned Si (001) substrates
Authors: Jin G. , Liu J. L. , Thomas S. G. , Luo Y. H. , Wang K. L. , Nguyen Bich-Yen
Published: July 2002

We report the ability to arrange self-organized Ge islands on patterned Si (001) substrates. Selective epitaxial growth of Si is carried out with gas-source molecular beam epitaxy to form Si mesas followed by subsequent Ge growth. Self-aligned and regularly spaced Ge islands are formed on the 〈110〉-oriented ridges of the Si stripe mesas. A mono-modal size distribution of the islands has been observed on the ridges. Using preferential nucleation sites allows us to place Ge islands at predetermined positions. The controlled arrangement of self-organized nanostructures offers the potential applications of island arrays for the implementation in nanoelectronics and quantum computation.
Anomaly of Hall effect in magnetoresistive La0.67Ca0.33MnO3
Authors: Cao X. W. , Fang J. , Wang Z. H. , Li K. B.
Published: July 2002

We have simultaneously measured the longitudinal resistivity and the Hall resistivity for the magnetoresistive La0.67Ca0.33MnO3 thin films as a function of temperature in temperature region of colossal magnetoresistance at several magnetic fields. Two reversions of the Hall sign with reducting temperature have been observed. The temperature at which the reversion of the Hall sign occurs shifts to the higher temperature with the increase of applied magnetic field. It was argued that the coexistence of two types of conduction mechanisms, the itinerant electron and small polaron, competing with each other, might be responsible for the two reversions of the Hall sign.