bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-beam photoacoustic phase measurement of the thermal diffusivity of a Gd-doped bulk YBCO superconductor
Authors: Aravind M. , Fung P. C. W. , Tang S. Y. , Tam H. L.
Published: July 2002

The two-beam photoacoustic phase measurement was applied to measure quantitatively the thermal diffusivity (αs) of a ceramic bulk high-Tc superconductor. Neglecting the effects of thermal dilation, and thermoelastic bending was proved valid in accordance with our composite piston model for the chosen experimental conditions. It was found that αs shows different features at the onset and offset temperatures corresponding to the normal–superconducting (NS) transition. A dip was seen at the resistivity transition onset temperature and a cusp at the offset temperature where the electrical resistance disappears. The presence of the cusp at the offset temperature is proposed to be related to weak coupling between superconducting grains. Our studies indicate that the two-beam phase measurement is a very sensitive method for superconductor characterization and NS transition detection. The experimental results also confirm the presence of a large energy gap and strong electron–phonon coupling mechanism in the YBCO superconductor.
Focused ion-beam direct deposition of metal thin film
Authors: Nagamachi Shinji , Yamakage Yasuhiro , Ueda Masahiro , Maruno Hiromasa , Ishikawa Junzo
Published: July 2002

Focused ion-beam direct deposition has been developed as a new method for fabricating patterned metal films directly on substrates. The principle of this method is to perform ion-beam deposition by low-energy focused ion beams. We designed and constructed a low-energy focused ion-beam apparatus for direct deposition. Metal ions are extracted from liquid metal ion source, accelerated to 20 keV for single charged ions, focused, mass separated, deflected, and finally, decelerated to 30–1000 eV in this system. The beam diameter estimated by the deposited linewidth can be tuned between 0.5 and 8 μm and the beam current varies from 40 pA to 10 nA corresponding to the beam diameter for the Au+ ion in the energy range from 30 to 200 eV. The sticking probabilities of ion-beam deposition were measured and the critical energies for Au+, Cu+, Al+, and Nb2+ were about 210, 230, 800, and 1300 eV, respectively. The purity of gold film was measured by Auger electron spectroscopy and secondary-ion-mass spectroscopy. The concentration of carbon and oxygen was estimated below 100 ppm and was consistent with theoretically expected amounts. Resistivities of deposited gold, copper, and aluminum line were measured 1.5–1.6 times larger than that of bulk gold, 1.2–1.5 times larger than that of bulk copper, and 2.2–2.7 times larger than that of bulk aluminum. The critical temperature of deposited niobium line was also measured and a clear relationship was obtained between the critical temperature and the concentration of contaminations.
Surface corona discharge excited rare gas halide lamps
Authors: Taylor R. S. , Leopold K. E.
Published: July 2002

A simple surface corona discharge excitation technique has been used to generate high repetition rate, high average power rare gas halide UV lamp radiation. The lamp exhibited reliable performance with very low jitter and high shot to shot reproducibility. An average power of 9 W of UV centered at 248 nm (KrF) was obtained from a small 7.0-cm-diam emitting surface. XeCl lamp lifetimes of over one billion shots were obtained on a single gas fill without the need for gas circulation or cryogenic cleanup.
Interferometric spectral imaging of liquid in laser ablation
Authors: Hirai Akiko , Tsuboi Yasuyuki , Itoh Kazuyoshi , Ichioka Yoshiki
Published: July 2002

The first experiment of time-resolved spectral imaging of liquid in laser ablation is reported. An interferometric spectral imaging system dedicated to the measurements of the laser ablation is constructed. A pair of two-dimensional arrays of images are formed by a lenslet array coupled with a polarization interferometer and superposed on a detection plane. Each image in the array is provided with a varied path difference by the wedge-shaped liquid-crystal polarization interferometer to generate the interference signals. These interference signals include all the information necessary to reconstruct a spectral image and can be collected simultaneously. The mechanism of ablation of phenanthrene/ethanol solution was investigated by making use of the time-resolved spectral images of the laser induced plume. The results suggest that the photothermal processes are dominant in the ablation of the solution.
X-ray diagnostics of hohlraum plasma flow
Authors: Back C. A. , Glenzer S. H. , Landen O. L. , MacGowan B. J. , Shepard T. D.
Published: July 2002

In this study we use spectroscopy and x-ray imaging to investigate the macroscopic plasma flow in mm-sized laser-produced hohlraum plasmas. By using multiple diagnostics to triangulate the emission on a single experiment, we can pinpoint the position of dopants placed inside the hohlraum. X-ray emission from the foil has been used in the past to measure electron temperature. Here we analyze the spatial movement of dopant plasmas for comparison to hydrodynamic calculations.
A mechanically tunable magnetron injection gun
Authors: Wang Ch. , Yeh Y. S. , Yang T. T. , Chen H. Y. , Chen S. H. , Tsai Y. C. , Barnett L. R. , Chu K. R.
Published: July 2002

The magnetron injection gun (MIG) has been the predominant choice as the electron-beam generator in gyrodevices and it can be tuned by externally installed magnetic-field trim coils. The double-anode MIG, traditionally employed in most experiments, has the intermediate anode as an internal tuning mechanism. The single-anode design, on the other hand, is simpler to construct but lacks an internal beam tuning mechanism. The current paper describes a method to allow mechanical tuning of the MIG. Since the tip of the cathode is located in the region of the highest electric-field gradient, its extension and retraction relative to the rest of the cathode assembly provide an effective means for local electric-field profile modification and, therefore, a tuning mechanism for the electron-beam property and quality. It is demonstrated, in computer simulations and experimental tests, that the tuning mechanism adds a useful degree of freedom to the single-anode MIG for beam optimization. This method may also be applied to the double-anode design.
Beam extraction in the presence of space charge forces (invited)
Authors: Spädtke P.
Published: July 2002

The space charge is one important force acting on the charged particle beam, when a beam is extracted from the particle source. Analytical solutions describing the beam properties are available for only a few special cases. Computational methods are therefore required for the optimization of extraction systems. These optimization methods are described and examples are shown.
Multiply charged ions of heavy elements produced by an iodine laser with subnanosecond pulses
Authors: Láska L. , Krása J. , Mašek K. , Pfeifer M. , Rohlena K. , Králiková B. , Skála J. , Straka P. , Woryna E. , Parys P. , Wołowski J. , Mróz W. , Haseroth H. , Golubev A. , Sharkov B. , Korschinek G.
Published: July 2002

Multiply charged ions of Ta, W, Pt, Au, Pb, and Bi, produced by the photodissociation iodine laser system PERUN (λ=1.315 μm, EL<40 J, τL∼350 ps) are reported. The ions with maximum charge states around 50+ and with energies of several MeV were registered at a distance of about 2 m from a plasma plume. The existence of several groups of ions corresponds to the different processes that occur in the plasma. The influence of different factors on the ion production is discussed. Measured ion current densities higher than 10 mA/cm2 in the distance of 1 m from the target demonstrate the superiority of laser ion sources.
Scattering and recoiling imaging spectrometer (SARIS)
Authors: Kim C. , Höfner C. , Al-Bayati A. , Rabalais J. W.
Published: July 2002

An ultrahigh vacuum spectrometer system has been designed and constructed for obtaining spatial- and time-resolved, element-specific images of atoms that are scattered and recoiled from surfaces. A pulsed noble gas ion beam in the 1–5 keV range is used to scatter and recoil atoms from a surface. A large, position-sensitive microchannel plate detector with resistive anode encoder, that is sensitive both to ions and fast neutrals records the spatial distribution patterns of the emitted atoms. The use of time-of-flight methods allows capture of these patterns in time windows as short as 10 ns. The sensitivity of these patterns to the details of surface structure provides the basis for a scattering and recoiling imaging spectrometry (SARIS). The primary ion beam current is ∼0.1 nA/cm2, supplied in 20 ns pulses at a rate of 30 kHz, resulting in ∼5×102 ions/pulse; images with adequate statistics can be obtained in several seconds with a total ion dose of <1010 ions/cm2. The SARIS technique can provide unique, element-specific, real-space, surface structural information at a resolution of <0.1 Å on a time scale of seconds for analysis of surface composition, structure, and dynamics. Examples provided for demonstrating the capabilities of the system are 4 keV He+ and Ar+ scattering and recoiling from Pt{111} and 4 keV Kr+ scattering and recoiling from CdS{0001}.
Advances in Stark effect measurements in a molecular beam Fourier transform microwave spectrometer
Authors: Consalvo Daniela
Published: July 2002

For the first time Stark experiments in a pulsed nozzle Fourier transform microwave spectrometer are successfully performed with the pulsed molecular beam parallel to the resonator axis, together with a new geometry for the Stark electrodes which prevents line broadening due to E-field inhomogeneity. The coaxially oriented beam-resonator arrangement, introduced in the 1990s, is known to provide a considerable improvement in resolution and sensitivity in comparison to the perpendicular geometry, but Stark effect experiments performed in a microwave cavity, using the conventional cubic electrode geometry with this arrangement, suffer from an inhomogeneous field distribution. This gives rise to broadened profiles of the Stark shifted lines and a persistent zero-field line. The linewidth achieved with our new configuration is now close to the typical value found in the absence of a Stark field: less than 10 kHz at 10 GHz half width at half height, which makes the system very suitable for yielding precise values of the dipole moment, a piece of chemical information especially important in understanding both structure and charge distribution of molecular complexes. The performances of the novel design electrodes and the improvements achieved are demonstrated by several examples.
Optical fiber velocimetry: A technique for measuring velocity in two-dimensional flows
Authors: Rivera M. , Belmonte A. , Goldburg W. I. , Wu X. L. , Kellay H.
Published: July 2002

In this article we describe a new technique, optical fiber velocimetry (OFV), to instantaneously measure two components of velocity at a point in freely suspended flowing liquid films. The technique relies on the measurement of displacements of an optical fiber tip which is coupled to the flowing film. The deflection of the fiber tip is proportional to the velocity and it behaves as a simple harmonic oscillator. Thus the low frequency response of the fiber gives direct measurements of the flow velocity. A statistical test using data acquired simultaneously by the fiber and by a laser Doppler velocimeter shows good agreement between the two techniques. Velocity power spectra measured in the wake of a von Karman street and in two-dimensional (2D) grid turbulence using the OFV also compare favorably with the laser Doppler velocimeter. The OFV technique is simple and robust, allowing it to be used in a wide variety of flows that have strong 2D characteristics. New prospects of using multiple fibers to measure circulation and velocity correlations at several separated spatial points are discussed.
1 nm x-ray lithography using novel mask fabrication technique
Authors: Berry G. J. , Cairns J. A. , Davidson M. R. , Rodley D. R. G. , Thomson J. , Turcu I. C. E. , Shaikh W.
Published: July 2002

High-resolution commercial photomasks and x-ray masks are at present manufactured by a multistep process which involves the use of etching. This route imposes a limitation on the resolution that can be achieved. We describe here a new approach to the fabrication of x-ray masks involving the use of organometallic compounds which undergo direct conversion to metal under the influence of electron beam irradiation. This approach permits fabrication of both x-ray masks and extreme ultraviolet masks in essentially a single processing step. The attainable resolution is unaffected by any limitation incurred by etching processes and is dependent only on such factors as electron scattering and electron beam diameter. We describe here the production of a gold x-ray mask, which was used in conjunction with a compact laser plasma source to demonstrate the lithographic process.
Improving thermal insulation in high-pressure experiments
Authors: Sundqvist Bertil
Published: July 2002

The use of internal ovens is common in high-temperature–high-pressure experiments. Improving the thermal insulation means better efficiency and lower thermal strain on pressure vessels and feedthroughs, but is difficult when using solid media. The addition of a layer of amorphous solid (glass) in the form of a mixed powder is reported to give a significant reduction (20%–50%) in the power dissipated in a high pressure oven.
Real time interferometric measurement of the parameters of a mechanical oscillator
Authors: Olivos-Fuentes Brenda E. , Gutiérrez Luis , Morales Alejandro
Published: July 2002

By using a computer automated measurement and control interval counter and a workstation to analyze the data, a system with a Michelson interferometer is used to measure amplitude, frequency, instantaneous speed and other parameters of a mechanical oscillator.
Refractory graphite skimmers for supersonic free-jet, supersonic arc-jet, and plasma discharge applications
Authors: Jordan D. C. , Barling R. , Doak R. B.
Published: July 2002

The fabrication of molecular beam “skimmers” from electrical discharge machining (EDM) graphite is reported. EDM graphite is highly refractory and is easily machined using conventional cutting and grinding techniques. In its most fine-grained form, EDM graphite can be machined free-standing to a knife-edge lip radius of ∼1 μm, providing excellent skimmer aerodynamics. Being refractory, such EDM skimmers are of particular interest in sampling or collimating high-temperature plasma discharges. Our explicit application is in skimming an electrical discharge supersonic free-jet of molecular nitrogen, forming a molecular beam of A 3Σu+ metastable N2 to be used in the heteroepitaxial growth of III–N wide-band-gap semiconductors. In view of the their economy, ease of manufacture, and excellent aerodynamics, the skimmers may also find use in skimming conventional, nondischarge supersonic free-jets. The performance of the EDM skimmers was tested in a conventional helium supersonic free-jet expansion, measuring the time-of-flight distribution and beam intensity as a function of nozzle pressure and nozzle-skimmer separation. Direct comparison with commercial nickel and copper skimmers showed the EDM graphite skimmers to perform nearly as well as the best commercial metal skimmers. The refractory properties of the skimmers were tested in a high-temperature arc-discharge supersonic free-jet expansion of 10% nitrogen in argon. Exposure to a plasma plume of ∼7500 K for over 6 h produced only relatively minor deterioration of the EDM graphite skimmer.
Microwave-frequency alternating current scanning tunneling microscopy by difference frequency detection: Atomic resolution imaging on graphite
Authors: Schmidt J. , Rapoport D. H. , Fröhlich H.-J.
Published: July 2002

We present a detailed description of an experimental setup for alternating current scanning tunneling microscopy, in which two slightly detuned high frequency signals are mixed at the tunneling junction and the resulting difference frequency signal is amplified using conventional scanning tunneling microscope electronics. This signal is used to control the distance between the microscope tip and the sample. With graphite as a model surface atomic resolution images have been obtained. It is demonstrated that the origin of the generated signal on graphite is the nonlinearity of the static current–voltage characteristics.
Experimental developments to obtain real-time x-ray diffraction measurements in plate impact experiments
Authors: Gupta Y. M. , Zimmerman K. A. , Rigg P. A. , Zaretsky E. B. , Savage D. M. , Bellamy P. M.
Published: July 2002

An experimental facility was developed to obtain real-time, quantitative, x-ray diffraction data in laboratory plate impact experiments. A powder gun, to generate plane wave loading in samples, was designed and built specifically to permit flash x-ray diffraction measurements in shock-compression experiments. Spatial resolution and quality of the diffracted signals were improved significantly over past attempts through partial collimation of the incident beam and the use of two-dimensional detectors to record data from shocked crystals. The experimental configuration and synchronization issues are discussed, and relevant details of the x-ray system and the powder gun are described. Representative results are presented from experiments designed to determine unit cell compression in shock-compressed LiF single crystals subjected to both elastic and elastic-plastic deformation, respectively. The developments described here are expected to be useful for examining lattice deformation and structural changes in shock wave compression studies.
Construction of a variable-frequency radio-frequency quadrupole linac for the RIKEN heavy-ion linac
Authors: Kamigaito O. , Goto A. , Miyazawa Y. , Chiba T. , Hemmi M. , Kase M. , Kohara S. , Batygin Y. , Ikezawa E. , Nakagawa T. , Yano Y.
Published: July 2002

A new type of variable-frequency radio-frequency quadrupole (RFQ) linac has been constructed as a new preinjector for the RIKEN heavy-ion linac (RILAC). The RFQ resonator, based on a folded-coaxial structure with a movable shorting plate, is compact even in a low frequency region below 20 MHz. It accelerates ions with mass-to-charge ratios of 5.3 to 26.4 in the energy range up to 450 keV per charge, by varying its resonant frequency from 17.4 to 39.0 MHz. Moreover, the power loss is small in the low frequency region; the rf power consumption in cw mode is 7 kW at 17.4 MHz, and it increases to 30 kW at 39.0 MHz at the maximum intervane voltage of 36.8 kV. We initiated this project in 1991, and installed the new preinjector consisting of an electron cyclotron resonance ion source of 18 GHz and this RFQ in 1996. Since the installation, the beam intensity has become larger by more than 1 order of magnitude than that formerly obtained with the Cockcroft–Walton preinjector. Recently, the maximum power of the beam extracted from the RILAC and that from the ring cyclotron has reached 560 W and 2 kW, respectively.
High voltage multichannel wave form generator for liquid crystal research
Authors: Matuszczyk T. , Beccherelli R.
Published: July 2002

The article describes a wave form generator designed primarily for experiments on addressing all kinds of liquid crystal displays. It can also be used in any application requiring several simultaneous sources of high-voltage arbitrary pulse trains. The instrument has eight channels capable of pulse amplitudes of ±100 V at a slew rate better than 300 V/μs. Its design differs significantly from a typical arbitrary wave form generator. First and foremost the wave forms are directly constructed from pulses with variable width as well as amplitude. Two interchangeable memory banks guarantee transient-free adjustments of generated wave forms. The generator is computer controlled with well integrated software and provides all functionality to assist in the creation of wave forms and experiments with liquid crystal addressing schemes in a straightforward and intuitive way. To point out its versatility we discuss a new mode of operation intended primarily for generation of analogue gray shades on a surface-stabilized ferroelectric liquid crystal display. The instrument creates conditions corresponding to driving a display of virtually any size with image frames changing at video rate.
Characterization of a Hadamard transform time-of-flight mass spectrometer
Authors: Brock Ansgar , Rodriguez Nestor , Zare Richard N.
Published: July 2002

A pseudorandom time-of-flight method (also called the cross-correlation method) has been used to perform time-of-flight mass spectrometry with a duty cycle of 50%. Modulation of an ion beam is accomplished by deflecting the ion beam with an interleaved comb of oppositely charged elements. Maximum-length pseudorandom sequences based on Hadamard-type difference sets are produced by feedback shift register circuitry and used for ion beam modulation. The inverse transformation of the recorded signal is carried out speedily with the help of the fast Hadamard transform, which allows real-time monitoring of the mass spectrum. The components of the instrument are described, and its performance is characterized. Trajectory simulations are found to be in good agreement with experimental findings, which aids in understanding the modulation dynamics. It is found that the wire comb modulator can be modeled as a set of ideal deflection plates of length 0.875 l, where l is the spacing between oppositely charged wires.
Enhancement of transport critical current densities at 75 K in (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes by means of fission tracks from irradiation by 0.8 GeV protons
Authors: Safar H. , Cho J. H. , Fleshler S. , Maley M. P. , Willis J. O. , Coulter J. Y. , Ullmann J. L. , Lisowski P. W. , Riley G. N. , Rupich M. W. , Thompson J. R. , Krusin-Elbaum L.
Published: July 2002

The transport critical current density Jc of oxide-powder-in-tube mono- and multifilamentary Bi-2223/Ag tapes has been determined before and after irradiation by 0.8 GeV protons at fluences up to 7.0×1016 protons/cm2. Proton-induced fission of the Bi nuclei produced up to 8.6×1013 fissions/cm3, creating long tracks at densities equivalent to matching fields up to 1.1 T. Relative to unirradiated tapes, Jc values at 75 and 64 K show no decrease in self field, indicating no breakdown of intergranular coupling, and show large, dose-dependent enhancements in magnetic fields oriented parallel to the tape normal.
Patterned self-assembly of one-dimensional arsenic particle arrays in GaAs by controlled precipitation
Authors: Kiehl R. A. , Yamaguchi M. , Ueda O. , Horiguchi N. , Yokoyama N.
Published: July 2002

A process for the patterned self-assembly of nanometer-scale particles within a solid is described. The process uses crystal strain and composition to guide the formation of arsenic precipitates in GaAs-based epitaxial layers grown at low temperature by molecular beam epitaxy. The lateral particle position is controlled by the strain produced by a surface stress structure while the vertical position is controlled by the epitaxial layer composition. Arsenic particles ∼16-nm in diameter are fabricated in one-dimensional arrays with a 23-nm edge-to-edge particle spacing at a depth of 45 nm below stressors 200 nm in width, thereby demonstrating this technique.
Local vibrational modes of the Mg–H acceptor complex in GaN
Authors: Götz W. , Johnson N. M. , Bour D. P. , McCluskey M. D. , Haller E. E.
Published: July 2002

Local vibrational modes (LVMs) are reported for Mg-doped GaN grown by metalorganic chemical vapor deposition. Hetero-epitaxial layers of GaN:Mg, either as-grown, thermally activated, or deuterated, were investigated with low-temperature, Fourier-transform infrared absorption spectroscopy. The as-grown material, which was semi-insulating, exhibits a LVM at 3125 cm−1. Thermal annealing increases the p-type conductivity, as established with Hall effect measurements, and proportionally reduces the intensity of this LVM. Deuteration of the activated material creates a LVM at 2321 cm−1. The isotopic shift establishes the presence of hydrogen in the vibrating complex. The new LVMs are assigned to the stretch modes of the Mg–H and Mg–D complexes in GaN, with the vibrational frequencies indicative of a strong N–H bond as recently proposed from total-energy calculations.
Gas-phase transport of WF6 through annular nanopipes in TiN during chemical vapor deposition of W on TiN/Ti/SiO2 structures for integrated circuit fabrication
Authors: Ramanath G. , Carlsson J. R. A. , Greene J. E. , Allen L. H. , Hornback V. C. , Allman D. J.
Published: July 2002

Delamination of TiN/Ti bilayers on SiO2 is a serious problem during W chemical vapor deposition (CVD) using WF6 to form vertical interconnects in integrated circuits. In order to obtain insight into the delamination mechanism, we have determined depth-distributions of W and F in sputter-deposited TiN/Ti bilayers on SiO2 as a function of WF6 exposure time tWF6 at 445 °C. Even for tWF6<6 s, significant concentrations of both W (≊3.5 at. %) and F (≊2 at. %) penetrate through the 106-nm-thick TiN film. W piles up at the TiN/Ti interface, while F rapidly saturates the TiN layer and accumulates in the Ti underlayer at concentrations up to ≊10 at. % for tWF6=60 s. Cross-sectional and scanning transmission electron microscopy analyses demonstrate that WF6 penetrates into the TiN layer through nanometer-scale intercolumnar voids spanning the entire film thickness and reacts with the Ti underlayer. We propose that the high F concentrations in the Ti layer weakens the Ti/SiO2 interface leading to adhesion failure of the TiN/Ti bilayer.
Optical properties of the epitaxial Pb1−xLaxTiO3 thin films grown by metalorganic chemical vapor deposition
Authors: Kim Y. , Erbil A.
Published: July 2002

The dispersion of the refractive indices for the epitaxial Pb1−xLaxTiO3 thin films with x=0.28 and 0.23 grown on the MgO (100) and Al2O3 (0001) substrates, respectively, has been investigated. The refractive indices in the wavelength range of 435–1523 nm were measured by a prism coupling method. At a wavelength range of 632.8 nm, the refractive index of a PLT film grown with (100) orientation was determined to be 2.609. The transmission spectrum was used as an additional measurement for the dispersion of the refractive index and showed a good agreement with the prism coupling measurements. The dispersion of refractive index fits well to a single-term Sellmeier relation.
Damage evolution and surface defect segregation in low-energy ion-implanted silicon
Authors: Bedrossian P. J. , Caturla M.-J. , Diaz de la Rubia T.
Published: July 2002

We have combined computer simulations and atomic-resolution tunneling microscopy to investigate the kinetics of defect migration during annealing of ion implanted Si(111)-7×7. Using atomically-clean and flat surfaces as sinks for bulk point defects introduced by the irradiation, we observe distinct, temperature-dependent surface arrival rates for vacancies and interstitials, and we demonstrate that the distinct kinetics of each type of bulk point defect govern their surface segregation kinetics. A combination of simulation tools provides a detailed description of the processes that underlie the observed temperature-dependence of defect migration.
Ordering of As impurities in a Si dislocation core
Authors: Maiti A. , Kaplan T. , Mostoller M. , Chisholm M. F. , Pennycook S. J. , Pantelides S. T.
Published: July 2002

We demonstrate by ab initio calculations that segregation of As in a dislocation core in Si occurs in the form of an ordered chain of As atoms running along the dislocation pipe. All As atoms in the chain achieve threefold coordination and the segregation energy is close to 1 eV per As atom.
Direct observation of sp3 bonding in tetrahedral amorphous carbon using ultraviolet Raman spectroscopy
Authors: Gilkes K. W. R. , Sands H. S. , Batchelder D. N. , Robertson J. , Milne W. I.
Published: July 2002

The vibrational modes of the sp3 sites in tetrahedral amorphous carbon (ta-C) thin films are revealed directly using ultraviolet Raman spectroscopy at 244 nm excitation and are shown to produce a Raman peak centered around 1100 cm−1. In addition, the main Raman peak associated with sp2 vibrational modes is shifted upward in frequency by 100 cm−1 relative to its position in spectra excited at 514 nm. The spectra are interpreted in terms of the bonding in ta-C.
Spontaneous emission factor of oxidized vertical-cavity surface-emitting lasers from the measured below-threshold cavity loss
Authors: Shin J. H. , Ju Y. G. , Shin H. E. , Lee Y. H.
Published: July 2002

A method to estimate the spontaneous emission factor β is proposed and applied to the 780 nm oxidized vertical-cavity surface-emitting lasers. The proportionality of the measured cavity loss multiplied by optical power to injected current is used. Our results agree better with theoretical calculations than those of conventional light-current curve fitting. The spontaneous emission factor of 0.0021 is obtained for a 2-μm-square device at room temperature. Since only the below-threshold information of cavity loss and output power are used in our method, the obtained β values are independent of any complex and unexpected above-threshold effects such as thermally induced mode-size contraction, as they should be.
High sensitivity measurement of magnetic fields using microcantilevers
Authors: Cowburn R. P. , Moulin A. M. , Welland M. E.
Published: July 2002

We have mounted magnetic γ−Fe2O3 particles on the end of a microcantilever in order to make a high sensitivity magnetic field sensor. An externally applied magnetic field exerts a torque on the acicular oxide particles which in turn deflects the cantilever. The deflection is measured using a sensitive optical detection system. We show how the effect of drift in the optical signal can be eliminated by continuously flipping the magnetization direction within the oxide by applying short, intense pulses of magnetic field. The sensor can resolve changes in applied field as small as 10−4 G (10 nT), and has good linearity over five orders of magnitude of applied field strength.
Anomalous longitudinal mode hops in GaAs/AlGaAs distributed Bragg reflector lasers
Authors: Hofstetter Daniel , Zappe Hans P.
Published: July 2002

We investigate normal and anomalous longitudinal mode hops in GaAs/AlGaAs-based distributed Bragg reflector (DBR) lasers; anomalous mode hops are defined as those which move toward shorter wavelengths with increasing temperature, which is unexpected. The two-section DBR lasers discussed in this letter, consisting of a gain section and an unpumped Bragg reflector, typically exhibit one mode hop in a 10 K temperature range. Although the longer wavelength modes are expected to start lasing when raising device temperature, occasional mode hops to a shorter wavelength are seen. We derive a model for temperature-dependent wavelength tuning, with which the overheating of the gain section is described empirically. This model allows an accurate numerical simulation of both kinds of temperature-induced longitudinal mode hops.
Ultrafast all-optical switch using complex refractive index changes of thin films containing photochromic dye
Authors: Sasaki Kyoichi , Nagamura Toshihiko
Published: July 2002

An all-optical switch has been proposed based on photoinduced changes of an imaginary part of complex refractive index in a composite thin film made by silver and polymer thin films containing photochromic dye. Very fast switching on and switching off of a reading beam was demonstrated at the incident angle of guided wave mode using a photochromic spiropyran-doped polystyrene thin film and a ns pulsed laser as a writing beam. These responses were caused by the changes of an extinction coefficient due to photochromism. No applied power was required to hold the switched state.
Demonstration of an imide coupling reaction on a Si(100)-2×1 surface by molecular layer deposition
Authors: Bitzer T. , Richardson N. V.
Published: July 2002

In this letter, we report the successful growth of an ultrathin organic film on Si(100)-2×1 by reactive coupling of polyimide precursors. Using the molecular layer deposition technique, 1,4-phenylene diamine and pyromellitic dianhydride were sequentially dosed on clean Si(100)-2×1 under ultrahigh vacuum conditions. The interfacial imidization was initiated by thermal curing at 200 °C. High resolution electron energy loss spectroscopy was employed to identify surface species. The spectra show clearly, that an oligimide chain has been formed which stands upright on the substrate. The chain bonds to the silicon substrate via a Si–(NH)–C linkage.
Crystal stress dynamics by means of nanosecond time-resolved x-ray diffraction
Authors: Chen P. , Tomov I. V. , Rentzepis P. M.
Published: July 2002

Time-resolved transient lattice deformation in Pt (111) crystal has been directly observed from the shift of a nanosecond x-ray diffraction rocking curve. The development of a tabletop nanosecond x-ray system, which consists of an excimer laser, x-ray diode, and charge coupled device detector is described. With this system, we observed the time-dependent strain distribution during the laser heating process in the bulk of a Pt (111) crystal. This method provides a means of relative low cost for the study of fast transient properties and structures of materials.
External charge state detection of a double-dot system
Authors: Amlani Islamshah , Orlov Alexei O. , Snider Gregory L. , Lent Craig S. , Bernstein Gary H.
Published: July 2002

We report direct measurements of the charging diagram of a nanoscale series double-dot system at low temperatures. Our device consists of two metal dots in series, with each dot capacitively coupled to another single dot serving as an electrometer. This configuration allows us to externally detect all possible charge transitions within a double-dot system. In particular, we show that transfer of an electron between two dots, representing a polarization switch of the double dot, can be most prominently detected by our differential sensing scheme. We also perform theoretical calculations of the device characteristics and find excellent agreement with experiment. We discuss possible applications as an output stage for quantum-dot cellular automata architecture.
Energy resolved ultrafast relaxation dynamics close to the band edge of low-temperature grown GaAs
Authors: Segschneider G. , Dekorsy T. , Kurz H. , Hey R. , Ploog K.
Published: July 2002

We investigate the relaxation dynamics of photogenerated carriers in low-temperature grown GaAs by femtosecond pump-probe measurements. The carrier dynamics in the vicinity of the band edge is disentangled in a two-color technique. The filling of shallow bound states close beneath the band edge is resolved. A temporal delay in the occupation of these states as well as a large optical nonlinearity points towards microscopic potential fluctuations forming these states.
Transient measurements with an ultrafast scanning tunneling microscope on semiconductor surfaces
Authors: Keil Ulrich D. , Jensen Jacob R. , Hvam Jo/rn M.
Published: July 2002

We demonstrate the use of an ultrafast scanning tunneling microscope on a semiconductor surface. Laser-induced transient signals with 1.8 ps rise time are detected. The investigated sample is a low-temperature grown GaAs layer placed on a sapphire substrate with a thin gold layer that serves as a bias contact. For comparison, the measurements are performed with the tip in contact to the sample as well as in tunneling above the surface. In contact and under bias, the transient signals are identified as a transient photocurrent. An additional signal is generated by a transient voltage induced by the nonuniform carrier density created by the absorption of the light (photo Dember effect). The transient depends in sign and in shape on the direction of optical excitation. This signal is the dominating transient in tunneling mode. The signals are explained by a capacitive coupling across the tunneling gap.
Built-in voltages and asymmetric polarization switching in Pb(Zr,Ti)O3 thin film capacitors
Authors: Lee J. , Choi C. H. , Park B. H. , Noh T. W. , Lee J. K.
Published: July 2002

Asymmetric polarization switching of Pb(Zr,Ti)O3 (PZT) thin films with different electrode configuration has been studied in (La,Sr)CoO3/Pb(Zr,Ti)O3/(La,Sr)CoO3 (LSCO) heterostructures in which the conducting oxide (La,Sr)CoO3 and/or LaCoO3 (LCO) have been used as an electrode. Polarization-voltage (P-V) hysteresis loop of LSCO/PZT/LSCO was symmetric. However, LCO/PZT/LSCO showed a largely asymmetric P-V hysteresis loop and large relaxation of the remanent polarization at the negatively poled state, eventually leading to an imprint failure. On the other hand, LSCO/PZT/LCO exhibited large relaxation of the positively poled state. The asymmetric behavior of the polarized states implies the presence of an internal electric field inside the PZT layer. It is suggested that the internal electric field is caused by built-in voltages at LCO/PZT and LSCO/PZT interfaces. The built-in voltages at LCO/PZT and LSCO/PZT interfaces were 0.6 V and 0.12 V, respectively.
Two-dimensional photorefractive spatial solitons in centrosymmetric paraelectric potassium–lithium–tantalate–niobate
Authors: DelRe Eugenio , Tamburrini Mario , Segev Mordechai , Refaeli Eli , Agranat Aharon J.
Published: July 2002

We report the observation of steady-state two-dimensional photorefractive self-trapping and screening spatial soliton formation in a sample of potassium–lithium–tantalate–niobate in the centrosymmetric paraelectric phase.
Spectra analysis of annealed Hg1−xCdxTe molecular beam epitaxial films
Authors: Li Biao , Wu Yan , Gui Yongsheng , Ye Hongjuan , Chang Yong , He Li , Chu Junhao
Published: July 2002

This letter discusses the analysis of infrared and visible spectra of Hg1−xCdxTe thin films deposited by molecular beam epitaxy (MBE) onto a CdTe buffer layer on a GaAs substrate. The spectra were obtained by infrared transmission and spectroscopic ellipsometry. Two mathematical techniques, fast Fourier transform (FFT) of the multiple reflectance spectrum associated with a multilayer system and fractional-derivatives spectra (FDS) were employed. Compared to the conventional fitting procedure, the FFT method directly offers the thickness of individual layers. It can also provide insight into the interfaces. The FDS method, however, gives information of composition and lattice perfection, which is useful in in situ real-time monitoring during the MBE run. The results show that annealing increases the compositional grading of Hg1−xCdxTe MBE films. Furthermore, the crystal microstructure deteriorates due to the irregular arrangement of diffusing atoms in the lattice sites.
Initial carrier relaxation dynamics in ion-implanted Si nanocrystals: Femtosecond transient absorption study
Authors: Klimov V. I. , Schwarz Ch. J. , McBranch D. W. , White C. W.
Published: July 2002

Transient absorption spectra of ion-implanted Si nanocrystals (NCs) exhibit two picosecond photoinduced absorption features, attributed to carriers in NC quantized states (high-energy band) and Si/SiO2 interface states (low-energy band). Fast relaxation of the high-energy band indicates that populations of quantized states are short lived and decay on the sub-10-ps time scale due to efficient surface trapping. This shows that the red emission in our samples is not due to carriers in quantized states but rather is a result of deactivation of surface traps.
A study of electron field emission as a function of film thickness from amorphous carbon films
Authors: Forrest R. D. , Burden A. P. , Silva S. R. P. , Cheah L. K. , Shi X.
Published: July 2002

The electron field-emission properties of hydrogenated amorphous carbon and nitrogenated tetrahedral amorphous carbon thin films are examined by measuring the field-emission current as a function of the applied macroscopic electric field. The experimental results indicate the existence of an optimum film thickness for low-threshold electron field emission. The predictions of various emission models are compared to the experimental results.
Electrical characterization of electroluminescent polymer/nanoparticle composite devices
Authors: Blom P. W. M. , Schoo H. F. M. , Matters M.
Published: July 2002

The current–voltage characteristics of light-emitting devices containing thin films of poly(dialkoxy-p-phenylene vinylene) (PPV) incorporated with silicon dioxide nanoparticles have been investigated. It is demonstrated that the current enhancement of the devices containing composite layers can be modeled by assuming that the effective thickness of the composite layers is about half of their actual thickness. Field-effect measurements reveal that the mobility of the charge carriers in PPV is not significantly changed by the incorporation of nanoparticles.
Generation of dark pulse trains from continuous-wave light using cross-phase modulation in optical fibers
Authors: Cao Wen-hua , Li Shenping , Chan Kam-tai
Published: July 2002

A technique for the generation of dark pulse trains from continuous-wave (cw) light is presented. It consists of co-propagating a cw signal with intense pump pulses in an optical fiber where both the signal and the pump experience normal group-velocity dispersion. The pump imposes a positive frequency chirp on the signal through cross-phase modulation while normal dispersion tends to chip out the signal energy in the chirped region, thus leading to the generation of a dark pulse train with a repetition rate identical to that of the pump. A train of 30.3 ps dark pulses has been observed when using 31.7 ps pump pulses from an actively mode-locked fiber ring laser. The experimental results agree well with numerical simulations.
Near-field second harmonic imaging of lead zirconate titanate piezoceramic
Authors: Smolyaninov Igor I. , Lee Chi H. , Davis Christopher C.
Published: July 2002

A technique to characterize the local nonlinearity of lead zirconate titanate piezoceramic (PZT) has been developed. Near-field optical microscopy has been used to image variations in local optical second harmonic generation from the PZT surface. Individual crystalline grains and grain boundaries are the main features visible in the images. The technique allows us to determine the local poling direction of individual submicrometer size crystalline grains of ceramic by near-field second harmonic imaging at different angles of incidence and polarization states of fundamental excitation light.
Kinetics of surface-state laser annealing in Si by frequency-swept infrared photothermal radiometry
Authors: Rodriguez Mario E. , Garcia J. A. , Mandelis A. , Jean C. , Riopel Y.
Published: July 2002

Frequency-swept (“chirped”) infrared photothermal radiometry was combined with conventional single-frequency modulation of an Ar ion laser beam to yield a quantitative study of the surface-state annealing processes induced by the low-fluence laser beam on n- and p-type Si wafers. The appearance of a signal transient was found to be strongly dependent on the electronic quality of the wafer surface and was absent in the thermally oxidized p-Si wafer. The low-injection minority-carrier lifetimes and diffusion coefficients were not affected by the laser-surface interaction, but the surface recombination velocity strongly decreased with time of exposure. A two-trap rate model was advanced to explain the transient behavior in terms of surface-state annealing and carrier ejection.
High-quality indium oxide films at low substrate temperature
Authors: Adurodija Frederick O. , Izumi Hirokazu , Ishihara Tsuguo , Yoshioka Hideki , Matsui Hiroshi , Motoyama Muneyuki
Published: July 2002

Low-resistivity (ρ) and highly transparent pure indium oxide (In2O3) thin films grown on glass substrates by pulsed laser deposition at substrate temperature (Ts) between room temperature and 200 °C are reported. As-deposited films with resistivity (ρ) of ∼3×10−4 Ω cm and transmittance (visible), above 87% were obtained within a narrow range of PO2 (1×10−2–1.5×10−2 Torr). Hall effect measurements showed that the low ρ resulted from the high carrier concentration (n)∼7×1020 cm−3, whereas modest Hall mobility (μ<45 cm2 V−1 s−1) was measured. X-ray diffraction indicated that the films prepared at Ts⩽100 °C were amorphous, while at Ts⩾150 °C polycrystalline films were obtained.
Memory window of highly c-axis oriented ferroelectric YMnO3 thin films
Authors: Lee Ho Nyung , Kim Yong Tae , Park Young K.
Published: July 2002

YMnO 3 thin films have been sputtered with different O2 partial pressures from 0% to 20%. Only the YMnO3 sputtered without O2 can be crystallized along (001) c-axis orientation after annealing at 870 °C. Mixing 10%–20% O2 partial pressure in the sputtering ambient, the excess Y2O3 in the YMnO3 films suppresses the c-axis oriented crystallization. Effects of crystallization on the ferroelectric properties of Pt/YMnO3/Y2O3/Si (MEFIS) and Pt/YMnO3/Si (MFS) gate capacitors have been investigated. The memory window of the MEFIS capacitor is enhanced by the c-axis oriented crystallization of the YMnO3.
High-dose oxygen ion implantation into 6H-SiC
Authors: Ishimaru Manabu , Dickerson Robert M. , Sickafus Kurt E.
Published: July 2002

Microstructures of oxygen ion implanted SiC have been examined using transmission electron microscopy (TEM) and scanning transmission electron microscopy equipped with an energy-dispersive x-ray spectrometer. 6H-SiC (0001) substrates were implanted with 180 keV oxygen ions at 650 °C to fluences of 0.7×1018 and 1.4×1018/cm2. A continuous buried oxide layer was formed in both samples, while the surrounding 6H-SiC contained minimal damage. These results suggest that oxygen implantation into SiC is a useful technique to establish SiC-on-insulator structures. In bright-field TEM images, the amorphous layer possessed uniform contrast in the low-dose sample, while it consisted of three distinct layers in the high-dose sample: (1) a bubbled or mottled layer; (2) a dark contrast layer; and (3) a light contrast layer. Chemical measurements revealed that the bubbled and light contrast regions have low silicon and oxygen contents, while carbon enrichment was found in these layers.
Role of implantation-induced defects on the response time of semiconductor saturable absorbers
Authors: Tan H. H. , Jagadish C. , Lederer M. J. , Luther-Davies B. , Zou J. , Cockayne D. J. H. , Haiml M. , Siegner U. , Keller U.
Published: July 2002

Arsenic ion implantation with thermal annealing was used to shorten the response times of GaAs-based saturable absorber structures. Ultrafast absorption bleaching measurements indicated that the recovery time was decreased with increasing the implantation dose. However, above a certain dose the recovery time increased again. This behavior was correlated with the microstructure of the residual implantation defects.
Efficient light-emitting diodes based on a binaphthalene-containing polymer
Authors: Jen Alex K-Y. , Liu Yunqi , Hu Qiao-Sheng , Pu Lin
Published: July 2002

We report an efficient electroluminescent device using a binaphthalene-containing polymer as the emissive layer and a fluorinated copper phthalocyanine as the electron injecting/transporting layer. The structural feature of this polymer is a twisted, nonplanar backbone with two hexyloxy side groups. The energy levels for the highest occupied molecular orbital and the lowest unoccupied molecular orbital of this polymer were −5.71 and −2.38 eV as determined by cyclic voltammetry and ultraviolet-visible measurements. A light-emitting diode with aluminum electrode exhibited high brightness (9400 cd/m2 at 24.3 mA/cm2), excellent luminous efficiency (4.9 lm/W), and a large external quantum efficiency (2.0%).