bluuebunny/crossref_metadata_2025 huggingface dataset:
A plasma shutter to generate a synchronized subnanosecond pulse for optical probing of laser-produced plasmas
Authors: Dhareshwar L. J. , Naik P. A. , Bhawalkar D. D.
Published: July 2002

A simple and reliable technique to temporally shorten a multinanosecond Nd:glass laser pulse to less than nanosecond duration at the second harmonic wavelength is described in this article. Using this technique a short probe pulse synchronized with the main laser was generated for optical probing of laser-produced plasmas. Experiments reported were conducted with a Nd:glass laser of wavelength 1.06 μm and of 20 ns duration to yield a temporally shortened pulse of duration less than a nanosecond at a wavelength of 0.53 μm. This technique would be particularly useful and give better results for shorter wavelength lasers (UV and VUV) for which the conventional techniques of pulse slicing are sophisticated as well as add to the expense and complexity of the laser system.
Fiber-optic detection system for capillary waves: An apparatus for studying liquid surfaces and spread monolayers
Authors: Bohanon T. M. , Mikrut J. M. , Abraham B. M. , Ketterson J. B. , Dutta P.
Published: July 2002

Generating capillary waves with an alternating electric field gradient is a noncontact way to study the mechanical properties of monolayers. By studying capillary waves the surface tension can be calculated. In addition, the surface elasticity can be deduced and film inhomogeneity can be determined. We have incorporated a fiber-optic detection system into a capillary wave apparatus. This makes the apparatus more compact, movable, versatile, and easier to use than detection systems currently in use.
Evidence of M-type oxygen octahedral rotations in the high-temperature rhombohedral ferroelectric phase region of Pb(Zr0.95Ti0.05)O3
Authors: Xu Z. , Dai Xunhu , Li Jie-Fang , Viehland Dwight
Published: July 2002

Evidence of M-type oxygen octahedral rotations has been observed by transmission electron microscopy in the high-temperature rhombohedral ferroelectric (FER(HT)) state of Pb(Zr0.95Ti0.05)O3. 1/2{110} superlattice reflections were found to develop on cooling through the dielectric maximum. With further cooling (accompanying a ferroelectric-antiferroelectric transformation), a gradual transition from 1/2{110} to 1/2{111} reflections was observed. We believe this gradual transition is due to a transformation from M-type to R-type octahedral tilts on cooling. Lattice imaging in the FER(HT) state at room temperature revealed a one dimensionality in the 〈110〉 structural modulations. The size of the ordered regions was ∼50–100 Å.
A noncontacting technique for measuring surface tension of liquids
Authors: Cinbis C. , Khuri-Yakub B. T.
Published: July 2002

We have developed a novel instrument for measuring the surface tension of liquids. It utilizes ultrasonic and optical methods to excite and detect a capillary wave packet. A focused ultrasonic transducer is used to generate a mound on the surface of a liquid. Capillary waves propagate radially outward from the mound, and a noncontacting confocal optical microscope measures the amplitude of the wave packet at any distance away from the excitation point. We invert the measured wave amplitude to obtain the dispersion relation of the capillary waves and hence the surface tension.
Development of inside launch reflectometer systems on the DIII-D tokamaka)
Authors: Doyle E. J. , Rhodes T. L. , Doane J. L. , Peebles W. A.
Published: July 2002

Inside launch (high field side) reflectometry is necessary for routine core access on DIII-D and is a very attractive option for ITER and TPX. For high temperature reactor relevant plasmas, relativistic corrections for the left-hand cutoff (inside launched) are relatively small, whereas these substantially affect the location of the normal right hand cutoff. On DIII-D, core access using the right hand cutoff is restricted to low density plasmas, while O-mode polarization is incompatible with flat H-mode density profiles. Routine core access can, however, be obtained using the left-hand cutoff and inside launch. On ITER and TPX, outside launch X-mode systems need to operate at high frequency, ∼100–250 GHz, such that the left-hand cutoff, which is at much lower frequency, ∼0–75 GHz, is an attractive option for high density operation. In addition, inside launch reflectometry can be used to investigate plasma asymmetries; on DIII-D an inboard/outboard asymmetry in turbulence response is observed at the L-H transition. An upgraded inside launch reflectometer system was recently installed on DIII-D, specifically to investigate the use of the left hand X-mode cutoff and inside launch for profile and turbulence measurements in reactor relevant plasmas. This system will also provide routine core access for reflectometry on DIII-D.
An apparatus for sputter coating the inner walls of tubes
Authors: Ensinger W.
Published: July 2002

A device for coating the inner walls of tubes by physical vapor deposition is described. An ion beam enters the tube parallel to the axis and impinges on a conical sputter target which is pushed through the tube. Material from the sputter target is thereby deposited onto the tube walls. With this device, the inner walls of steel tubes were coated with silver, and tantalum tubes were coated with platinum. The latter is effective in preventing tantalum tubes from becoming embrittled in contact with acid.
A simple low-temperature ultrahigh-vacuum scanning tunneling microscope capable of atomic manipulation
Authors: Meyer Gerhard
Published: July 2002

The design of a low-temperature scanning tunneling microscope is described. The microscope can be operated in ultrahigh vacuum in the temperature range between 15 and 300 K. The main features are a scanner which is based on the Besocke ‘‘beetle’’ design principle combined with a spring suspension of the microscope and complete surrounding of the whole microscope by a 4 K radiation shield. The microscope can be extended to work as a force microscope using the optical lever detection technique for force measurement. It is demonstrated that this comparatively small and reliable setup is well suited to study the adsorption of single atoms and molecules at low temperatures. Moreover, examples of lateral and vertical manipulation of atoms and molecules are shown to demonstrate the performance of the system.
Polarization spectroscopy of x-ray transitions from beam-excited highly charged ions
Authors: Beiersdorfer P. , López-Urrutia J. Crespo , Decaux V. , Widmann K. , Neill P.
Published: July 2002

Polarization spectroscopy of x-ray lines represents a diagnostic tool to ascertain the presence of electron beams in high-temperature plasmas. Making use of the Livermore electron beam ion trap, which optimizes the linear x-ray line polarization by exciting highly charged ions with a monoenergetic electron beam, we have begun to develop polarization diagnostics and test theoretical models. Our measurement relies on the sensitivity of crystal spectrometers to the linear polarization of x-ray lines which depends on the value of the Bragg angle. We employed two spectrometers with differing analyzing crystals and simultaneously recorded the K-shell emission from heliumlike Fe24+ and lithiumlike Fe23+ ions at two different Bragg angles. A clear difference in the relative intensities of the dominant transitions is observed, which is attributed to the amount of linear polarization of the individual lines.
Investigation of violin mode Q for wires of various materials
Authors: Dawid Daryush J. , Kawamura Seiji
Published: July 2002

The Q factors of violin modes for wires of various materials have been measured in order to determine which would be most suitable for use in the suspension of test masses in the initial laser interferometer gravitational wave observatory (LIGO) interferometers. A “guitar” type apparatus was employed to measure violin mode Qs, and losses due to clamping and other practical sources were successfully suppressed below the level of intrinsic wire losses. Steel music wire was found to give the highest extrapolated Q factors under LIGO conditions among the wires we tested. This extrapolated Q sets a target for the LIGO suspension which can be attained if all the losses other than the intrinsic wire loss are successfully suppressed. The measured Qs for the steel, tungsten, and titanium wire, which were approximately frequency independent for the first two to three modes, were found to be roughly proportional to the square root of the tension in the wire. This is consistent with the theory of violin mode losses due to frequency-independent intrinsic wire losses.
Capacitive sensor for micropositioning in two dimensions
Authors: Kolb P. W. , Decca R. S. , Drew H. D.
Published: July 2002

A compact sensor for measuring position in two dimensions has been developed. The device, operating on the principle that the capacitance of parallel plate electrodes depends on their mutual area of overlap, is compatible with high magnetic fields and cryogenic temperatures. A resolution of approximately 1.2 μm has been achieved and is limited by the electronics used. The position reproducibility, which is limited by drift of the positioner used to test the sensor, has been measured to be better than 3 μm.
Numerical study of microwave reflectometry in plasmas with two-dimensional turbulent fluctuations
Authors: Mazzucato E.
Published: July 2002

This article describes a numerical study of the role played by two-dimensional turbulent fluctuations in microwave reflectometry—a radar technique for density measurements using the reflection of electromagnetic waves from a plasma cutoff. The results indicate that, if the amplitude of fluctuations is below a threshold which is set by the spectrum of poloidal wave numbers, the measured backward field appears to originate from a virtual location behind the reflecting layer, and to arise from the phase modulation of the probing wave, with an amplitude given by one-dimensional geometric optics. These results suggest a possible scheme for turbulence measurements in tokamaks, where the backward field is collected with a wide aperture antenna, and the virtual reflecting layer is imaged onto the plane of an array of detectors. Such a scheme should be capable of providing additional information on the nature of the short-scale turbulence observed in tokamaks, which still remains one of the unresolved issues in fusion research.
Radiachromic film as a detector for intense MeV proton beams
Authors: Young F. C. , Boller J. R. , Stephanakis S. J. , Jones T. G. , Neri J. M.
Published: July 2002

A simple technique has been developed to image intense pulsed proton beams with radiachromic film and to measure their spatial distribution. The optical density (OD) of 50 μm thick film, sufficient to stop 1.5 MeV protons, is measured to infer the incident beam fluence. The OD increases nearly linearly with fluence up to 0.25 cal/cm2. This film is used to measure the uniformity of large-area beams as well as the detector in a multiple-pinhole camera to determine the source uniformity and divergence of applied-magnetic field and pinched-beam ion diodes.
Production of intense Ca48 ion beam at the U-400 cyclotron
Authors: Kutner V. B. , Bogomolov S. L. , Efremov A. A. , Lebedev A. N. , Lebedev V. Ya. , Loginov V. N. , Yakushev A. B. , Yazvitsky N. Yu.
Published: July 2002

Production of the intense accelerated Ca48 ion beam is the key problem in the experiments on synthesizing of new heavy nuclei. For this purpose an axial injection system with the electron cyclotron resonance (ECR)-4M ion source was created for the U400 cyclotron. The task was to achieve an accelerated beam with an intensity of 0.5 pμA of Ca5+48 at the Ca48 consumption of ∼0.5 mg/h. To solve this problem, a new method for the solid material feed into the ECR source was developed. The combination of a micro oven with a hot tantalum sheet inside the discharge chamber allowed the production of intense beams of ions of metals with relatively low melting point. The present article describes the method, technique, and experimental results on the production of Ca48 ion beam at the U-400 cyclotron from the ECR-4M ion source. The analysis of the working substance balance in the ion source including the ion beam extraction and material regeneration is performed. The analysis based on the experimental data has shown that the efficiency of Ca atom transformation into ion beam is close to that obtained for the gases such as Ar, and the intensity of Ca5+48 constitutes about 20% of the extracted ion beam.
Fuzzy logic algorithm to extract specific interaction forces from atomic force microscopy data
Authors: Kasas Sandor , Riederer Beat M. , Catsicas Stefan , Cappella Brunero , Dietler Giovanni
Published: July 2002

The atomic force microscope is not only a very convenient tool for studying the topography of different samples, but it can also be used to measure specific binding forces between molecules. For this purpose, one type of molecule is attached to the tip and the other one to the substrate. Approaching the tip to the substrate allows the molecules to bind together. Retracting the tip breaks the newly formed bond. The rupture of a specific bond appears in the force–distance curves as a spike from which the binding force can be deduced. In this article we present an algorithm to automatically process force–distance curves in order to obtain bond strength histograms. The algorithm is based on a fuzzy logic approach that permits an evaluation of “quality” for every event and makes the detection procedure much faster compared to a manual selection. In this article, the software has been applied to measure the binding strength between tubuline and microtubuline associated proteins.
Cubic boron nitride films grown by low energy B+ and N+ ion beam deposition
Authors: Hofsäss H. , Ronning C. , Griesmeier U. , Gross M. , Reinke S. , Kuhr M.
Published: July 2002

We have studied the growth and the properties of BN films prepared by exclusive deposition of mass separated 11B+ and 14N+ ions. BN films grown with ion energies of 500 eV and at substrate temperatures of 350 °C show the IR absorption peak at 1080 cm−1, characteristic for c-BN. These films are nearly stoichiometric and, with transmission electron diffraction, the presence of c-BN nanocrystals was revealed. We compare the growth conditions for ion beam deposition on BN, CN, and diamondlike carbon and propose that the nucleation of nanocrystalline c-BN is related to the ionicity of the BN bond.
Symmetric rate model for fluorocarbon plasma etching of SiO2
Authors: Ding Ji , Hershkowitz Noah
Published: July 2002

A symmetric rate model for plasma etching and plasma deposition in fluorocarbon plasmas is proposed. When there is no deposition, the symmetric rate model gives a plasma etch rate. When there is no etching, the model gives a plasma deposition rate. Electron cyclotron resonance and reactive ion etcher etch rates of SiO2 in CF4 plasma are found to be consistent with the model.
The deposition of SiOF film with low dielectric constant in a helicon plasma source
Authors: Kim Jung-Hyung , Seo Sang-Hun , Yun Seok-Min , Chang Hong-Young , Lee Kwang-Man , Choi Chi-Kyu
Published: July 2002

SiOF films deposited by a helicon wave plasma chemical vapor deposition method has been characterized using Fourier transform infrared spectroscopy and ellipsometry. High density plasma of ≳1012 cm−3 can be obtained on a substrate at low pressure (<10 mTorr) with rf power ≳400 W with a helicon plasma source. A gas mixture of SiF4, O2, and Ar was used to deposit SiOF films on 5 in. Si(100) wafers not intentionally heated. Optical emission spectroscopy was used to study the relation between the relative densities of the radicals and the deposition mechanism. It was found that the addition of Ar gas to the SiF4/O2 mixture greatly increased the F concentration in the SiOF film. Discharge conditions such as gas composition, sheath potential, and the relative densities of the radicals affect the properties of the film. The dielectric constant of the SiOF film deposited using the helicon plasma source was 3.1, a value lower than that of the oxide film by other methods.
Photopumped infrared vertical-cavity surface-emitting laser
Authors: Hadji E. , Bleuse J. , Magnea N. , Pautrat J. L.
Published: July 2002

The feasibility of a photopumped infrared vertical-cavity surface-emitting laser (VCSEL) based on CdHgTe alloys is demonstrated. The structure of the VCSEL consists of a 16.5-period Cd0.4Hg0.6Te/Cd0.75Hg0.25Te bottom Bragg reflector and a 3λ/4 thick Cd0.75Hg0.25Te cavity, containing a 100-nm-thick well, grown by molecular beam epitaxy. The top mirror is a 7-period YF3/ZnS dielectric stack. The cavity quality factor is Q=350. This heterostructure VCSEL operates at 3.06 μm with a measured power density threshold of 45 kW/cm2 at 10 K.
Voltage controlled two color light-emitting electrochemical cells
Authors: Yang Yang , Pei Qibing
Published: July 2002

A polymer light-emitting electrochemical cell (LEC) is a bipolar p-n junction device; when biased, a dynamic light-emitting p-n junction is created. In this letter, we report a two color LEC in which the color can be switched by simply reversing the bias voltage. The two color LEC consists of a polymer bilayer structure sandwiched between two electrodes. In this study, the polymer bilayer consists of a poly(1,4-phenylene vinylene) (PPV) layer and a poly[2-methoxy-5- (2′-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) layer. When biased at one polarity, the voltage induced p-n junction is completely inside the PPV layer, and the LEC emits the green light of PPV. When biased at the opposite polarity, the p-n junction is completely inside the MEH-PPV layer, and the LEC emits the orange light characteristic of MEH-PPV.
Metastable transformation of EL2 in semi-insulating GaAs: The role of the actuator level and the photoionization of EL2
Authors: Alvarez A. , Jiménez J. , González M. A.
Published: July 2002

The metastable transformation of EL2 is modeled in terms of an additional acceptor level, whose role is the activation of the metastable transformation of EL2. This level, called the actuator of the metastability, enables the metastable transformation after capturing a hole. Thermal emission of this hole at 80–85 K renders the metastable transformation of EL2 quite ineffective above this temperature. The double acceptor 78/203 meV is proposed as a possible actuator.
Metalorganic chemical vapor deposition of monocrystalline GaN thin films on β-LiGaO2 substrates
Authors: Kung P. , Saxler A. , Zhang X. , Walker D. , Lavado R. , Razeghi M.
Published: July 2002

We report the metalorganic chemical vapor deposition growth and characterization of monocrystalline GaN thin films on β-LiGaO2 substrates. The influence of the growth temperature on the crystal quality was studied. The structural, electrical, and optical properties of the films were assessed through scanning electron microscopy, x-ray diffraction, Hall measurements, optical transmission, photoluminescence.
Microscopic structure of gold particles in a metal polymer composite film
Authors: José-Yacamán M. , Pérez R. , Santiago P. , Benaissa M. , Gonsalves K. , Carlson G.
Published: July 2002

Gold particles embedded in a polymer are studied using high resolution electron microscopy, nanodiffraction, and image processing. These particles have interesting properties such as nonlinear optical susceptibility. It is found that most of the nanoparticles are single crystals, and twins and planar defects are scarcely observed. On the other hand, it is found that nanoparticles do not show smooth facets but they are instead very rough. The effects of this extended roughness in the optical properties are discussed.
Raman spectra of indium nitride thin films grown by microwave-excited metalorganic vapor phase epitaxy on (0001) sapphire substrates
Authors: Kwon Hyuk-Joo , Lee Yong-Hyun , Miki Osamu , Yamano Hirofumi , Yoshida Akira
Published: July 2002

We report Raman scattering study on InN single crystalline films with wurtzite form. The films were grown on sapphire substrates by microwave-excited metalorganic vapor phase epitaxy and optical phonon properties of the films were investigated. Both A1(LO) and E2(2) peaks, which are related to a longitudinal optical phonon mode and a doubly degenerated mode, respectively, were observed at 596 cm−1 with the full width of 36 cm−1 at half maximum intensity and at 495 cm−1 with the full width of 20 cm−1 at half maximum intensity.
New method of carbon nanotube growth by ion beam irradiation
Authors: Yamamoto Kazuhiro , Koga Yoshinori , Fujiwara Shuzo , Kubota Masaaki
Published: July 2002

We report a new method of carbon nanotube growth by the argon ion beam irradiation of amorphous carbon under high vacuum conditions of 4×10−5 Torr. Earlier, carbon nanotubes have been produced at gas pressures above 100 Torr. The incident angle of the ion beam was normal to the target surface, and the acceleration ion energy was 3 keV. Nanotubes are produced outside the sputtering region on the target surface after ion irradiation. The tubes have multilayered walls, the distance between carbon layers is 0.34 nm, and wall thickness of tubes ranges from 10 to 15 sheets.
Phase-matched second-harmonic generation at 789.5 nm in a GaSe crystal
Authors: Kador L. , Haarer D. , Allakhverdiev K. R. , Salaev E. Yu.
Published: July 2002

Phase-matched second-harmonic generation in the highly χ(2)-active layered semiconductor GaSe is demonstrated in the near-infrared frequency region. Due to the high indices of refraction, the internal phase-matching angle of about 30° is beyond the critical angle of total reflection for a z-cut crystal. This problem is overcome by sandwiching the crystal between two half-cylindrical glass rods, which leads to a shift of the critical angle to larger values.
Photoluminescence study of p-type CdGeAs2 ordered semiconductor crystals
Authors: McCrae J. E. , Hengehold R. L. , Yeo Y. K. , Ohmer M. C. , Schunemann P. G.
Published: July 2002

Results of a photoluminescence (PL) study as a function of excitation laser power, sample temperature, crystal orientation, and polarization are reported for the nonlinear optical material CdGeAs2. One broad weak PL peak near 0.38 eV, and another somewhat narrower and often far brighter PL peak near 0.55 eV were found at 4 K. The high energy PL peak shifts first towards lower, then higher, and back to lower energies again as the temperature increases from 2.2 to 295 K. This high energy peak is attributed to donor–acceptor pair dominant transitions at low temperatures, but it is attributed to band-to-band dominant transitions at higher temperatures. Strongly polarized PL was observed with the E field of the PL parallel to the material’s c axis, and a polarization ratio as high as 6:1 was obtained.
Selective electrodeposition of nanometer scale magnetic wires
Authors: Fasol Gerhard , Runge Katharina
Published: July 2002

A selective electrodeposition method for the fabrication of extremely thin and long metallic and magnetic wires is introduced. Growth is done on the cleaved edge of a semiconductor multilayer structure incorporating a 4-nm-wide modulation doped quantum well. This conducting quantum well is connected to the negative current contact during electrodeposition. Since electrodeposition requires the neutralization of positive metal ions from the solution, deposition takes place selectively onto the edge of the quantum well, leading to the fabrication of extremely thin magnetic metal wires, which should be useful for the investigation of the limits of magnetic storage.
High-power near-resonant 1.55 μm emitting InGaAsP/InP antiguided diode laser arrays
Authors: Bhattacharya A. , Botez D.
Published: July 2002

We present high peak-pulsed coherent power results from large-aperture, 1.55 μm antiguided laser arrays. The InP-based devices have a compressively strained InGaAsP double-quantum-well active region and are fabricated by two-step self-aligned metalorganic chemical vapor deposition. We have obtained 2.5 W front-facet peak power in a 2.6° wide beam [6×diffraction limit (D.L.)], with 1.2 W in the central lobe, from 40-element, 250 μm aperture devices. The width of the central lobe remains constant from 4 to 15×threshold. Devices with improved temperature characteristics and of geometry closer to the resonance condition provide 1 W peak power in a beam 1.2° wide (2.7×D.L.), with 61% of the energy in the central lobe; and 1.75 W peak power in a 1.5° wide beam (3.5×D.L.) at heatsink temperatures between 15 and 45 °C.
Stress relaxation and stability in thick amorphous carbon films deposited in layer structure
Authors: Gioti M. , Logothetidis S. , Charitidis C.
Published: July 2002

We have developed amorphous carbon films in sequential layers and studied their intrinsic stress, composition, and stress relief mechanisms. The films were deposited by sputtering either thin, with fixed bias voltage Vb or thick, with alternative (positive-layer A/negative-layer B) Vb. In situ spectroscopic ellipsometry and stress studies were used and supported by nanoindentation and x-ray reflectivity measurements. The films deposited with fixed negative (positive) Vb exhibit a thickness-dependent increase (decrease) of compressive stresses up to 200 Å and saturate at 6(1) GPa. In thick films deposited with alternative Vb the results demonstrate that: (1) the same modulation in stress values and sp3 content versus film thickness exists, supporting their direct interrelation; (2) the A layers (rich in sp2 sites) promote the stress relaxation of the films during a compositional rearrangement when a B layer is deposited; and (3) this process develops thick and stable films with lower stresses.
Temperature dependence of electromigration dynamics in Al interconnects by real-time microscopy
Authors: Prybyla J. A. , Riege S. P. , Grabowski S. P. , Hunt A. W.
Published: July 2002

Real-time transmission electron microscopy (TEM) was used to directly examine the temperature dependence of electromigration-induced void and failure dynamics over the range 200–350 °C. The studies were done using submicron Al interconnects and a special sample stage, which allowed TEM observations to be recorded while heating and passing current through the sample. A novel sample design dramatically minimized any Joule heating in the runners. Our experiments directly reveal that the mechanism responsible for void and failure dynamics at temperatures <250 °C is distinctly different from that at higher temperatures. These results have implications regarding methods used for predicting electromigration reliability at use conditions from accelerated test data.
Field emission properties of GaN films on Si(111)
Authors: Berishev I. , Bensaoula A. , Rusakova I. , Karabutov A. , Ugarov M. , Ageev V. P.
Published: July 2002

GaN thin films were grown by electron cyclotron resonance molecular beam epitaxy on Si(111) wafers. X-ray diffraction and transmission electron microscopy revealed that the thin films were single crystals with a hexagonal symmetry and a clear textured structure. The average column size was determined to be close to 100 nm in diameter. Despite the large defect density, a strong room temperature photoluminescence signal with a full width at half maximum of 138 meV was observed from these samples. The surface exhibited random array of sharp tips at the microscopic level with about 5×109 tips/cm2 density. The field emission characteristics of the as-grown thin films were measured, and a threshold electric field as low as 30–40 V/μm and an emission current density of more than 100 mA/cm2 were obtained.
Identification of structural changes in carbon–nitrogen alloys by studying the dependence of the plasmon energy on nitrogen concentration
Authors: Alvarez F. , dos Santos M. C. , Hammer P.
Published: July 2002

The effect of nitrogen on the valence-electron plasmon energy in amorphous carbon–nitrogen alloys (a-C1−xNx) is used to identify structural changes in the material. The samples were prepared by dual-ion-beam-assisted deposition and studied in situ by x-ray photoemission spectroscopy. The plasmon energy of the alloy goes through a maximum above 15–20 at.% nitrogen concentration. This behavior is correlated with structural changes obtained in a semiempirical quantum chemical calculation on graphite-like carbon clusters, randomly substituted by nitrogen. At that concentration, the geometry optimization shows that the graphite-like conformation is unstable against the buckling of the structure.
Sub-50 nm planar magnetic nanostructures fabricated by ion irradiation
Authors: Devolder T. , Chappert C. , Chen Y. , Cambril E. , Bernas H. , Jamet J. P. , Ferré J.
Published: July 2002

He + ion irradiation of Co–Pt multilayers through a silica mask obtained by a combination of high resolution lithography and reactive ion etching can produce an optical contrast-free, entirely planar, sub-50 nm magnetically patterned array. Furthermore, the specificity of magnetization reversal in such arrays leads to a weak dispersion of coercive forces. The technique holds promise for both present hard disk technology and future near field magneto-optical recording.
Observation of terahertz radiation from higher-order two-dimensional plasmon modes in GaAs/AlGaAs single quantum wells
Authors: Sekine N. , Yamanaka K. , Hirakawa K. , Voßebürger M. , Haring-Bolivar P. , Kurz H.
Published: July 2002

We have observed terahertz (THz) radiation from higher-order two-dimensional (2D) plasmon modes in GaAs/AlGaAs single quantum wells by time-resolved THz emission spectroscopy. Up to fifth-order plasmon modes are clearly resolved and the observed mode frequencies are in excellent agreement with theory. Relaxation times of the 2D plasmon modes are found to be almost independent of the plasmon wave vector accessible in the present experiment.
Phonon-assisted capture and intradot Auger relaxation in quantum dots
Authors: Ferreira R. , Bastard G.
Published: July 2002

We report on calculations of capture and relaxation of carriers in quantum dots, specifically, InAs/GaAs self-assembled dots. We point out that the phonon-assisted carrier capture presents strong resonances versus the dot size and that the intradot Auger relaxation is extremely fast in these structures. This shows that energy relaxation in InAs/GaAs self-organized quantum dots is dominated by capture effects.
Influence of (001) vicinal GaAs substrates on the optical properties of defects in low-temperature grown GaAs/AlGaAs multiple quantum wells
Authors: Zhang M. H. , Li Q. , Zhang Y. F. , Huang Q. , Zhou J. M. , Xu Z. Y.
Published: July 2002

Photoluminescence (PL) spectroscopy and carrier lifetime measurement has been used to characterize optical properties of defects in the low-temperature (LT) grown GaAs/AlGaAs multiple quantum well structures. Two sets of samples were grown at 400 °C by molecular beam epitaxy on nominal (001) and miscut [4° off (001) towards (111) A] GaAs substrates, respectively. After growth, samples were subjected to 30 s rapid thermal annealing at 600–800 °C. It is found that after annealing, two defect-related PL features appear in the samples grown on nominal (001) GaAs substrates, but not in those grown on miscut (001) GaAs substrates. The carrier lifetimes are about 31 and 5 ps in as-grown samples grown on nominal and miscut (001) GaAs substrates, respectively. The different PL spectra and carrier lifetimes in two sets of samples are attributed to different structures of the AsGa-like defects formed during LT growth.
Thickness dependence of magnetic properties of granular thin films with interacting particles
Authors: Malkinski Leszek M. , Wang Jian-Qing , Dai Jianbiao , Tang Jinke , O’Connor Charles J.
Published: July 2002

The effect of film thickness on magnetic properties of Cu80Co20 granular alloy was studied. It was observed that the susceptibility peak temperature, TM, strongly increases with the film thickness, t, for t<100 nm. The long-range nature of this effect points to magnetic dipole interaction as responsible mechanism. This dependence of TM can be explained within the framework of Dormann’s theory of dipolar interaction between magnetic particles. The coercive field has different thickness dependence and it is related to formation of magnetic domain structure of Co particles in the granular alloy.
SiGe-on-insulator substrate using SiGe alloy grown Si(001)
Authors: Ishikawa Yukari , Shibata N. , Fukatsu S.
Published: July 2002

Low-energy oxygen ion (25 keV O+) implantation was performed on a pseudomorphic Si1−xGex/Si(001) of uniform composition in an attempt to create a SiGe-on-insulator (SiGe-OI) substrate using the separation-by-implanted-oxygen technique. Choosing a small Ge composition (<0.3) was found to be essential to achieving a SiGe-OI geometry of structural integrity.
Nonlinear optical properties of self-organized complex oxide Ce:BaTiO3 quantum dots grown by pulsed laser deposition
Authors: Shi Wensheng , Chen Zhenghao , Liu Ningning , Lu Huibin , Zhou Yueliang , Cui Dafu , Yang Guozhen
Published: July 2002

Ordered self-organized complex oxide Ce:BaTiO3 quantum dots shaped as a pyramid with a square base were fabricated by pulsed laser deposition. The density of the quantum dots was about 140 μm−2. The nonlinear optical properties were determined by the single beam Z scan method at a wavelength of 0.532 μm with a laser duration of 10 ns. The nonlinear refractive indices of the wetting layer and the self-organized ordered quantum dots were 8.87×10−8 and 6.39×10−7 esu, respectively. The mechanisms of the nonlinear effect enhancement for these low-dimensional structures are discussed.
The controlled formation of ordered, sinusoidal structures by plasma oxidation of an elastomeric polymer
Authors: Bowden Ned , Huck Wilhelm T. S. , Paul Kateri E. , Whitesides George M.
Published: July 2002

This letter describes a technique for generating waves on polydimethylsiloxane (PDMS) patterned in bas-relief. The PDMS is heated, and its surface oxidized in an oxygen plasma; this oxidation forms a thin, stiff silicate layer on the surface. When the PDMS cools, it contracts and places the silicate layer under compressive stress. This stress is relieved by buckling to form patterns of waves with wavelengths from 0.5 to 10 μm. The waves are locally ordered near a step or edge in the PDMS. The wavelength, amplitude, and pattern of the waves can be controlled by controlling the temperature of the PDMS and the duration of the oxidation. The mechanism for the formation and order of the waves is described.
Midinfrared emission from near-infrared quantum-dot lasers
Authors: Grundmann M. , Weber A. , Goede K. , Ustinov V. M. , Zhukov A. E. , Ledentsov N. N. , Kop’ev P. S. , Alferov Zh. I.
Published: July 2002

We report the electrically pumped emission of midinfrared (MIR) radiation from bipolar quantum-dot lasers during near-infrared lasing. The MIR spectrum exhibits a peak at 16 μm and is dominantly TM polarized. The MIR intensity exhibits a superlinear dependence on the injection; a maximum MIR power of 0.1 μW per facet was realized. Such a device is also modeled theoretically, and conditions for MIR lasing are predicted.
Surface electronic phase transition in colossal magnetoresistive manganese perovskites: La0.65Sr0.35MnO3
Authors: Dulli Hani , Plummer E. W. , Dowben P. A. , Choi Jaewu , Liou S.-H.
Published: July 2002

We have observed a distinct surface phase transition for an important class of giant magnetoresistance materials [La1−xSrxMnO3(x=0.35)]. The surface phase transition occurs at 240 K compared to 370 K for the bulk and is fundamentally different. In the bulk, a ferromagnetic metal to paramagnetic bad-metal transition occurs, while the lower-temperature surface transition is from an insulator to a semimetal. The surface of this manganese perovskite is electronically and compositionally quite different from the bulk with important implications for the behavior of artificially grown layered transition-metal oxides and for the use of surface sensitive techniques to probe the bulk.
Permittivity of a multiphase and isotropic lattice of spheres at low frequency
Authors: Whites Keith W.
Published: July 2002

A solution methodology is presented in this article to compute the effective permittivity for a multiphase lattice of dielectric and/or conducting spheres at low frequencies. It is assumed that the lattice is effectively isotropic. This methodology relies on two central developments. The first is a T-matrix solution for a multiphase lattice of spheres immersed in a uniform electric field. This solution is presented in a succinct matrix-vector notation and is valid for any lattice type. The second development is a simple and accurate equation for the effective permittivity that incorporates all mutual coupling between the spheres. Results are shown in this article for three situations. The first is a two-phase system of conducting spheres (used for verification) and the second is a dielectric-conductor (cermet composite) lattice of spheres. The third and final result is from a lattice containing a cluster of conducting spheres. It is suggested that this last material type displays a behavior in between that of random materials and two-phase lattices due to “permittivity enhancement” at low volume fraction. It is also shown that the Maxwell Garnett formula is not nearly as accurate for this cluster lattice, also because of this enhancement effect.
The temperature and polymerization effects on the relaxation time and conductivity, and the evolution of the localized motions
Authors: Tombari E. , Salvetti G. , Johari G. P.
Published: July 2002

To examine the manner in which molecular dynamics of a polymerizing liquid (stoichiometric amounts of 4,4′-diaminodicyclohexylamine and diglycidyl ether of bisphenol-A) evolves during thermal cycling from its (molecular) vitreous state to its fully polymerized vitreous state, calorimetry, and dielectric spectrometry were performed simultaneously in real time. The half-width of the relaxation spectrum of the liquid was relatively narrow and became narrower on heating. This was followed by an increase in the characteristic relaxation time and the spectrum became broader as polymerization occurred and reached completion. The dc conductivity initially increased and then decreased. The faster dynamics of the Johari–Goldstein relaxation in the fully polymerized state evolved as polymerization reached completion and the temperature increased. The dielectric polarization associated with this relaxation had a broad spectrum, whose half-width increased with decrease in the temperature. Its relaxation rate followed the Arrhenius equation with an activation energy of 63.4 kJ/mol. The temperature dependence of the faster relaxation did not change with the change in the overall configurational entropy of the liquid, a feature that substantiates the dynamic heterogeneity theories for the structure of the liquid and for the origin of the relaxation.
Design and performance of the lattice-cooled hot-electron terahertz mixer
Authors: Semenov A. D. , Hübers H.-W. , Schubert J. , Gol’tsman G. N. , Elantiev A. I. , Voronov B. M. , Gershenzon E. M.
Published: July 2002

We present the measurements and the theoretical model of the frequency-dependent noise temperature of a superconductor lattice-cooled hot-electron bolometer mixer in the terahertz frequency range. The increase of the noise temperature with frequency is a cumulative effect of the nonuniform distribution of the high-frequency current in the bolometer and the charge imbalance, which occurs at the edges of the normal domain and at the contacts with normal metal. We show that under optimal operation the fluctuation sensitivity of the mixer is determined by thermodynamic fluctuations of the noise power, whereas at small biases there appears additional noise, which is probably due to the flux flow. We propose the prescription of how to minimize the influence of the current distribution on the mixer performance.
Transient thermal loading induced microbending loss in carbon-coated optical fibers
Authors: Yang Yu-Ching , Chang Win-Jin , Lee Haw-Long
Published: July 2002

The transient microbending loss in a carbon-coated optical fiber, subjected to time-dependent thermal loading, is analyzed. First, the temperature distributions in the fiber and coating, which are varying with time, are derived. Then, the thermally induced, time-dependent compressive lateral pressure at the interface of the fiber and the coating is obtained. Finally, the transient microbending loss of the carbon-coated fiber is presented. It has been found that the transient microbending loss in the carbon-coated optical fiber increases with increasing Young’s modulus, Poisson’s ratio, and thermal expansion coefficient of the carbon coating. Therefore, in order to minimize the transient microbending loss of the carbon-coated optical fiber, the Young’s modulus, Poisson’s ratio, and thermal expansion coefficient of the carbon coating should be decreased. Nevertheless, there exists an optimum value for the thickness of the carbon coating to reduce the transient microbending loss and sustain the mechanical force at the same time.
Electrical and reliability characteristics of ZrO2 deposited directly on Si for gate dielectric application
Authors: Qi Wen-Jie , Nieh Renee , Lee Byoung Hun , Kang Laegu , Jeon Yongjoo , Lee Jack C.
Published: July 2002

ZrO 2 thin film has been studied as an alternative gate dielectric. It was deposited directly on a Si substrate by reactive sputtering. An equivalent oxide thickness of less than 11 Å with a leakage current of 1.9×10−3 A/cm2 at −1.5 V relative to the flat band voltage has been obtained. Well-behaved capacitance–voltage characteristics with an interface state density of less than 1011 cm−2 eV−1 and no significant frequency dispersion have been achieved. Excellent reliability properties (e.g., low charge trapping rate, good time-dependent dielectric breakdown, low stress-induced leakage current, and high dielectric breakdown) have also been obtained.
Error reduction study employing a pseudo-random binary sequence for use in acoustic pyrometry of gases
Authors: Ewan B. C. R. , Ireland S. N.
Published: July 2002

Acoustic pyrometry uses the temperature dependence of sound speed in materials to measure temperature. This is normally achieved by measuring the transit time for a sound signal over a known path length and applying the material relation between temperature and velocity to extract an “average” temperature. Sources of error associated with the measurement of mean transit time are discussed in implementing the technique in gases, one of the principal causes being background noise in typical industrial environments. A number of transmitted signal and processing strategies which can be used in the area are examined and the expected error in mean transit time associated with each technique is quantified. Transmitted signals included pulses, pure frequencies, chirps, and pseudorandom binary sequences (prbs), while processing involves edge detection and correlation. Errors arise through the misinterpretation of the positions of edge arrival or correlation peaks due to instantaneous deviations associated with background noise and these become more severe as signal to noise amplitude ratios decrease. Population errors in the mean transit time are estimated for the different measurement strategies and it is concluded that PRBS combined with correlation can provide the lowest errors when operating in high noise environments. The operation of an instrument based on PRBS transmitted signals is described and test results under controlled noise conditions are presented. These confirm the value of the strategy and demonstrate that measurements can be made with signal to noise amplitude ratios down to 0.5.
Rigorous Franck–Condon absorption and emission spectra of conjugated oligomers from quantum chemistry
Authors: Karabunarliev Stoyan , Baumgarten Martin , Bittner Eric R. , Müllen Klaus
Published: July 2002

A harmonic Condon approach is used to calculate excitation and emission band shapes for the lowest dipole-allowed electronic transitions in conjugated oligomers: polyenes, oligorylenes, and para-phenylenevinylenes. Ground- and excited-state adiabatic energies, equilibrium structures, and vibrational modes are obtained within standard all-valence-electron molecular Hamiltonian incorporating extended configuration interaction. The interstate distortion is cast in normal coordinates and used to calculate transition probabilities from the zero-phonon initial state to the vibrational manifold of the final state. Spectral profiles are obtained as a superposition of Lorentzian line shapes. Theoretical band shapes reproduce prominent features in the absorption and fluorescence spectra of the oligomers in question. The strength of the bond-stretching vibronic progression increases with oligomeric length in polyenes, but decreases in para-phenylenevinylenes. In line with experiment, absorption and emission band shapes of para-phenylenevinylenes are obtained intrinsically nonsymmetric due to stiffening of the accepting vibrational modes in the excited state. The Stokes shifts of the apparent 0-0 features in the latter are reproduced and traced back to relaxations in slow, ring-torsional motions.
Quantum dimensionality, entropy, and the modulation response of quantum dot lasers
Authors: Deppe D. G. , Huffaker D. L.
Published: July 2002

Rate equations are used to show that the modulation response of quantum dot lasers can be limited by entropy change due to carrier relaxation from a wetting layer. The entropy change is due to the difference in the spectral density of quantum states between the two-dimensional wetting layer and the zero-dimensional quantum dot. When limited by this effect, the laser’s 3 dB modulation response becomes strongly temperature dependent.
Mechanical strain and damage in Si implanted with O and N ions at elevated temperatures: Evidence of ion beam induced annealing
Authors: de Souza J. P. , Suprun-Belevich Yu. , Boudinov H. , Cima C. A.
Published: July 2002

The accumulation of damage and the development of mechanical strain in crystalline Si (c-Si) by O and N ion implantation to doses up to 4×1017 cm−2 at elevated temperatures have been studied using Rutherford backscattering spectrometry and high resolution x-ray diffraction. The implantation of O or N ions at high temperatures produces two distinct layers in the implanted c-Si: (i) a practically damage-free layer extending from the surface up to ≃ half of the depth of the mean projected range (Rp) and presenting negative strain (of contraction); and (ii) a heavily damaged layer located around and ahead of the Rp with no significant strain. Both the damage distribution and the magnitude of the strain were found to be dependent on the ion species implanted. We proposed that besides the spatial separation of Frenkel pair defects due to the mechanics of the collision processes and the intensive dynamic annealing, an ion beam induced annealing process also participate in the formation of the near-surface damage-free layer during high temperature implantation of c-Si.