bluuebunny/crossref_metadata_2025 huggingface dataset:
Uniform refractive index cladding for LiNbO3 single-crystal fibers
Authors: Sudo S. , Yokohama I. , Cordova-Plaza A. , Fejer M. M. , Byer R. L.
Published: July 2002

A uniform refractive index cladding for LiNbO3 single-crystal fibers has been achieved using a Mg ion indiffusion process. This cladding has a uniform MgO concentration of ∼20 mol %, resulting in a uniform refractive index. The refractive indices of the cladding material have been evaluated to be ne=2.08 and no=2.09 at 0.6328 μm by using the light reflection technique.
p-type doping of gallium antimonide grown by molecular beam epitaxy using silicon
Authors: Rossi T. M. , Collins D. A. , Chow D. H. , McGill T. C.
Published: July 2002

We report the first effective p-type doping of molecular beam epitaxy (MBE) grown GaSb using silicon. The samples were grown by molecular beam epitaxy and characterized by Hall-effect measurements and photoluminescence. Room-temperature hole concentrations ranging from 4.0×1015 to 4.3×1018 cm−3 were obtained. Photoluminescence (PL) spectra undergo considerable broadening with increasing doping concentration, consistent with an impurity banding picture. Furthermore, the MBE-grown samples display only one of the two PL features found in a melt-grown substrate and no other satellites, suggesting higher material purity. This may be a direct benefit from the use of an antimony cracker, ultrahigh vacuum conditions, and high-purity elemental sources. The short-period strained-layer superlattice buffering scheme employed may have also contributed to better structural quality.
Study of crystallographic orientations in the diamond film on the (100) surface of cubic boron nitride using a Raman microprobe
Authors: Yoshikawa M. , Ishida H. , Ishitani A. , Koizumi S. , Inuzuka T.
Published: July 2002

We have measured Raman spectra of a diamond film prepared on the (100) surface of cubic boron nitride(c-BN) by the dc plasma chemical vapor deposition method. It is found that the polarization property of the Raman line of diamond coincides well with that of the LO phonon for the (100) surface of c-BN. The coincidence between the polarization property of Raman lines of diamond and c-BN evidences heteroepitaxial growth of the diamond film on the (100) surface of c-BN.
Magneto-optical properties of Pd-Ni multilayers
Authors: Flevaris N. K.
Published: July 2002

The magneto-optical polar Kerr effect properties of compositionally modulated Pdm-Nin multilayers have been studied, at room temperature, for modulation wavelengths containing just a few monolayers. A perpendicular anisotropy component was observed to develop for very thin Ni layers (n=2 or 1 atomic planes). The rotation values were found to depend strongly on both m and n suggesting a modulation-induced property modification. These results were supported by magnetic studies. Also, they are discussed on the basis of extensive structural and other investigations.
Two-step debonding of hydrogen from boron acceptors in silicon
Authors: Seager C. H. , Anderson R. A.
Published: July 2002

Previous work has demonstrated that B⋅H pairs in silicon thermally dissociate obeying simple first-order kinetics if they are located in a region depleted of majority carriers. B⋅H debonding in equilibrium, however, is a slower, more complex process. We have investigated the dissociation of B⋅H pairs under a variety of equilibrium and nonequilibrium conditions and have demonstrated that the dissociation process is strongly influenced by the local concentration of majority and minority carriers. In particular, we show that injection of minority carriers can markedly accelerate the dissociation process. A model is proposed which suggests that hydrogen released from an acceptor, while initially positive, must always be neutralized before escape is possible. This picture correctly describes the time dependence of debonding in both equilibrium and forward-biased diodes.
Observation of large second order susceptibility via intersubband transitions at λ∼10 μm in asymmetric coupled AlInAs/GaInAs quantum wells
Authors: Sirtori Carlo , Capasso Federico , Sivco Deborah L. , Chu S. N. G. , Cho Alfred Y.
Published: July 2002

We report the first observation of second harmonic generation associated with intersubband transitions in AlInAs/GaInAs quantum wells. The measured second order susceptibility in our new asymmetric coupled well structure lattice matched to InP is 4.8×10−8 m/V, about 100 times larger than bulk values in InP, GaAs, and InAs.
Mechanism for diamond nucleation and growth on single crystal copper surfaces implanted with carbon
Authors: Ong T. P. , Xiong Fulin , Chang R. P. H. , White C. W.
Published: July 2002

The nucleation and growth of diamond crystals on single-crystal copper surfaces implanted with carbon ions has been studied. Microwave plasma-enhanced chemical vapor deposition was used for diamond growth. The single-crystal copper substrates were implanted either at room temperature or at elevated temperature (∼820 °C) with carbon ions prior to diamond nucleation. This procedure leads to the formation of a graphite film on the copper surface which greatly enhances diamond crystallite nucleation. From our study we construct a simple lattice model for diamond growth on graphite as 〈111〉diamond parallel to 〈0001〉graphite and 〈110〉diamond parallel to 〈112̄0〉graphite.
Ge profile from the growth of SiGe buried layers by molecular beam epitaxy
Authors: Godbey D. J. , Ancona M. G.
Published: July 2002

X-ray photoelectron spectroscopy measurements were obtained and interpreted by a kinetic simulation to determine the germanium concentration profile of thin Si/SiGe heterostructures grown at 500 °C using elemental source molecular beam epitaxy. The primary finding is that there are significant segregation effects in these commonly grown structures which affect both the ‘‘leading’’ and ‘‘trailing’’ interfaces. Upon opening of the germanium shutter, the surface monolayer must be built up to a germanium composition of greater than 96% before the composition of the deposited alloy layer is equal to the flux composition for a Ge ratio of 0.3. This buildup causes the germanium depletion at the leading interface. Upon termination of the germanium flux, the incorporation of the germanium rich monolayer into the growing silicon cap layer causes a corresponding degradation of the trailing interface.
High pressure synthesis of semiconducting Be-doped polycrystalline cubic boron nitride and its electrical properties
Authors: Taniguchi T. , Tanaka J. , Mishima O. , Ohsawa T. , Yamaoka S.
Published: July 2002

Be-doped cubic boron nitride (c-BN) powders, prepared at 5 GPa and 1700 °C by using Li3BN2 as a catalyst, were sintered at 7.7 GPa and 2100 °C without the addition of any sintering aids. The polycrystals so obtained were p-type semiconductors as determined by the electric and thermoelectric methods. Nonlinear current-voltage characteristics, showing similar behavior to that of varistors, were observed by a four-probe method. The activation energy of the resistivity in the ohmic region at temperatures from 25 to 700 °C was about 0.3 eV.
Epitaxial metallic LaNiO3 thin films grown by pulsed laser deposition
Authors: Satyalakshmi K. M. , Mallya R. M. , Ramanathan K. V. , Wu X. D. , Brainard B. , Gautier D. C. , Vasanthacharya N. Y. , Hegde M. S.
Published: July 2002

Epitaxial LaNiO3(LNO) thin films on LaAlO3(LAO), SrTiO3(STO), and YSZ are grown by pulsed laser deposition method at 350 mTorr oxygen partial pressure and 700 °C substrate temperature. As-deposited LNO films are metallic down to 10 K. c-axis oriented YBa2Cu3O7 (YBCO) films were grown on LNO/LAO as well as LNO/STO surfaces without affecting superconducting transition temperature of YBCO. Textured LNO thin films were grown on c-axis oriented YBCO/STO and YBCO/YSZ . Transport measurements of these bilayer films showed that LNO is a good metallic contact material for YBCO.
Optical properties of porous silicon superlattices
Authors: Vincent G.
Published: July 2002

Porous silicon superlattices have been fabricated electrochemically. The current was monitored periodically during the electrolysis and samples were made with several periods. They have been investigated using reflectance measurements in the infrared and visible range. We demonstrate that porous silicon superlattices act like a multilayer dielectric device.
Far-infrared generation by doubly resonant difference frequency mixing in a coupled quantum well two-dimensional electron gas system
Authors: Sirtori C. , Capasso F. , Faist J. , Pfeiffer L. N. , West K. W.
Published: July 2002

Far-infrared generation (λ=60 μm) by difference-frequency mixing in a coupled quantum well two-dimensional electron system is reported. Extremely large values of the nonlinear susceptibility are found (χω2−ω1(2)−10−6 m/V) by engineering a doubly resonant structure with narrow intersubband transition linewidths (Γ∼1–2 meV). The effect of many body interactions (depolarization shift) on the frequency dependence of the process is observed.
Considerations for capacitance DLTS measurements using a lock-in amplifier
Authors: Auret F. D.
Published: July 2002

An outline is presented of how to numerically account for the gate-off time, consisting of both the pulse width and delay times, when performing lock-in amplifier based capacitance deep level transient spectroscopy (DLTS) measurements at frequencies up to 2000 Hz. This frequency is about ten times higher than the maximum frequency normally used for these measurements when using 1-MHz capacitance meters. Further, the relationship between the frequency, pulse width, and delay time was established for which the commonly used value of the normalized thermal emission decay time constant τmax/T0 may be assumed constant (=0.424) without introducing observable errors in the defect parameters calculated from DLTS data. Because the effect of measuring at higher frequencies is to shift the DLTS peaks to higher temperatures, it was found that when using frequencies between 50 and 2000 Hz, defects such as the E2 level in radiation-damaged GaAs, which are usually observed below 77 K at frequencies below 50 Hz, may be conveniently and accurately characterized by capacitance DLTS at temperatures above that of liquid nitrogen.
New attempt for measuring thermal diffusivity of thin films by means of a laser flash method
Authors: Ohta Hiromichi , Shibata Hiroyuki , Waseda Yoshio
Published: July 2002

A new facility and its requirements have been developed for the measurement of the thermal diffusivity of thin films or ribbons which are less than about 100 μm in thickness, using a laser flash method. In this method, a pulsed laser beam which has a narrow line-shaped cross section compared with the length of the sample is flashed onto the front surface of the sample. The temperature response at the back surface at a certain distance away from the line-shaped illuminated area of the laser beam is measured by an infrared detector. The thermal diffusivity value can be estimated from the time required for the temperature to reach the half of the maximum temperature rise finally acquired. The usefulness of the present facility has been demonstrated by measuring successfully thermal diffusivities of thin foils of platinum (25 μm) and of copper (18 μm). This method can be applied to the samples having various thermal diffusivities, particularly in cases where the conventional method using a thermocouple is found to be technically difficult. For example, a thin fragile sample for which it is almost impossible to weld the temperature detecting device onto it.
Cryogenic cooling of x-ray crystals using a porous matrix
Authors: Kuzay Tuncer M.
Published: July 2002

It is well established that Si and SiC have very desirable thermophysical properties (principally, high thermal conductivity, and low thermal expansion) at cryogenic temperatures. Thus, cryocooled optics are a potentially good candidate for the first optical crystal of the third generation synchrotron machines, which will have very high heat flux levels. Currently, there is a great deal of interest, both experimental and analytical in such cryocooled crystals. The analytical studies involve cut micro- or capillary channel crystals. As opposed to machined channels, porous matrices provide significant advantages. Such matrices are known to effect superior heat transfer. They operate very quietly. Data available in the open literature suggest that surface heat flux levels up to ∼8 kW/cm2 are possible. For cryogens for which the boiling heat transfer heat flux is a rather low value in conventional geometries, the enhancement available with such matrices is very significant. Cryogens are poor thermal conductors themselves. At cryogenic temperatures, the Si and/or SiC matrix itself becomes highly conductive: Thus, the matrix distributes the surface heat flux into the full volume effectively offsetting the poor conductivity of the coolant. In addition, the tortuous path of the coolant through the matrix increases the dwell time resulting in better heat transfer, however, at the expense of an increased pressure drop. In this study, a first optics crystal model of Si with a Si and/or SiC porous matrix as its heat exchanger and subject to prototypic synchrotron loads is analyzed, and the feasibility limits of the cooling possible with liquid nitrogen in single phase are delineated.
Single hole quantum dot transistors in silicon
Authors: Leobandung Effendi , Guo Lingjie , Chou Stephen Y.
Published: July 2002

Novel p-channel quantum-dot transistors were fabricated in silicon-on-insulator. Strong oscillations in the drain current as a function of the gate voltage have been observed at temperatures over 81 K and drain biases over 66 mV. The oscillations are attributed to holes tunneling through the discrete single hole energy levels in the quantum dot. Measurements show that the average energy level spacing is ∼35 meV. Simple modeling indicates that about two thirds of the energy level spacing come from the Coulomb interaction between holes (i.e., hole Coulomb blockade) and one third from the quantum confinement effect. The realization of single hole quantum-dot transistors opens new possibilities for innovative circuits that utilize complementary pairs of quantum-dot transistors.
Improved NMR resonator for diamond anvil cells
Authors: Lee Sam-Hyeon , Conradi Mark S. , Norberg R. E.
Published: July 2002

A new hairpin resonator efficiently couples to nuclear spins in diamond anvil cell–nuclear magnetic resonance (DAC–NMR) experiments. The hairpin resonator is essentially a single turn coil of copper sheet, resonated with a chip capacitor. A modest tuning range is provided by trimmer capacitors. This resonator has so far been the most satisfactory of the DAC–NMR schemes tried in our laboratory. It is easy to make, robust, and reusable, and is easily cleaned to eliminate contaminant signals. The H1 homogeneity is good; the rf performance is described.
Pellet injection technology
Authors: Combs S. K.
Published: July 2002

During the last 10 to 15 years, significant progress has been made worldwide in the area of pellet injection technology. This specialized field of research originated as a possible solution to the problem of depositing atoms of fuel deep within magnetically confined, hot plasmas for refueling of fusion power reactors. Using pellet injection systems, frozen macroscopic (millimeter-size) pellets composed of the isotopes of hydrogen are formed, accelerated, and transported to the plasma for fueling. The process and benefits of plasma fueling by this approach have been demonstrated conclusively on a number of toroidal magnetic confinement configurations; consequently, pellet injection is the leading technology for deep fueling of magnetically confined plasmas for controlled thermonuclear fusion research. Hydrogen pellet injection devices operate at very low temperatures (≂10 K) at which solid hydrogen ice can be formed and sustained. Most injectors use conventional pneumatic (light gas gun) or centrifuge (mechanical) acceleration concepts to inject hydrogen or deuterium pellets at speeds of ≂1–2 km/s. Pellet injectors that can operate at quasi-steady state (pellet delivery rates of 1–40 Hz) have been developed for long-pulse fueling. The design and operation of injectors with the heaviest hydrogen isotope, tritium, offer some special problems because of tritium’s radioactivity. To address these problems, a proof-of-principle experiment was carried out in which tritium pellets were formed and accelerated to speeds of 1.4 km/s. Tritium pellet injection is scheduled on major fusion research devices within the next few years. Several advanced accelerator concepts are under development to increase the pellet velocity. One of these is the two-stage light gas gun, for which speeds of slightly over 4 km/s have already been reported in laboratory experiments with deuterium ice. A few two-stage pneumatic systems (single-shot) have recently been installed on tokamak experiments. This article reviews the equipment and instruments that have been developed for pellet injection with emphasis on recent advances. Prospects for future development are addressed, as are possible applications of this technology to other areas of research.
New fast response photofragment fluorescence hygrometer for use on the NASA ER-2 and the Perseus remotely piloted aircraft
Authors: Weinstock Elliot M. , Hintsa Eric J. , Dessler Andrew E. , Oliver James F. , Hazen Nathan L. , Demusz Joseph N. , Allen Norton T. , Lapson Larry B. , Anderson James G.
Published: July 2002

We have developed an in situ instrument to measure water vapor on the NASA ER-2 as a prototype for use on the Perseus remotely piloted aircraft. It utilizes photofragment fluorescence throughout the stratosphere and the upper to middle troposphere (mixing ratios from 2 to 300 ppmv) with simultaneous absorption measurements in the middle troposphere (water vapor concentrations ≳5×1014 mol/cc). The instrument flew successfully on the NASA ER-2 aircraft during the 1993 CEPEX and SPADE campaigns. The 2σ measurement precision for a 10 s integration time, limited by variation in the background from scattered solar radiation, is ±6% and the data were tightly correlated with other long-lived stratospheric tracers throughout the SPADE mission. Its accuracy is estimated to be ±10%, based on laboratory calibrations using a range of water vapor concentrations independently determined by both standard gas addition techniques and by absorption. This accuracy is confirmed by in-flight absorption measurements in the troposphere. The fast response time of the instrument, limited by the 4 Hz data sampling rate, was demonstrated when the ER-2 flew across its own exhaust plume. Improvements to the instrument planned for the Perseus aircraft are described which include the addition of an in-flight water vapor addition system, significant reduction in solar background, and weight reduction.
A wide-range ultraviolet lidar system for tropospheric ozone measurements: Development and application
Authors: Kempfer U. , Carnuth W. , Lotz R. , Trickl T.
Published: July 2002

A KrF-laser-based ozone lidar system is described which operates in the wide vertical range between 0.1 and 12 km. The wavelengths used for the measurements are generated by efficient stimulated Raman scattering which is optimized by using a KrF laser in an oscillator-amplifier configuration. Two receiving telescopes are used to divide the considerable dynamic range of the backscattered signal which covers more than eight orders of magnitude. The ozone density errors are smaller than 7.5×1016 m−3 (i.e., 3.1 ppbv near the ground) which is comparable to those of standard ozone monitors. A further improved performance is expected in the future. A first annual series with about 580 individual measurements was carried out in 1991 and is briefly discussed.
A laser tracking robot-performance calibration system using ball-seated bearing mechanisms and a spherically shaped cat’s-eye retroreflector
Authors: Nakamura Osamu , Goto Mitsuo , Toyoda Kouji , Takai Nozomi , Kurosawa Toshiro , Nakamata Tohru
Published: July 2002

A new robot-performance calibration system based on tracking multilaser trilateration has been developed. To realize 1 μm coordinate accuracy, a beam-deflecting tracking mechanism with both a ball-seated bearing and a spherically shaped cat’s eye retroreflector whose refractive index is 2.0 has been developed. The coordinate error and the repeatability of a commercial industrial robot have been verified by the system we developed in a two-dimensional plane.
A novel cryogenic scanning laser microscope tested on Josephson tunnel junctions
Authors: Holm J. , Mygind J.
Published: July 2002

A novel cryogenic scanning laser microscope with a spatial resolution of less than 5 μm has been designed for on-chip in situ investigations of the working properties of normal and superconducting circuits and devices. The instrument relies on the detection of the electrical response of the circuit to a very localized heating induced by irradiation with 675 nm wavelength light from a semiconductor laser. The hot spot is moved by a specially designed piezoelectric scanner sweeping the tip of a single-mode optical fiber a few μm above the circuit. Depending on the scanner design the scanning area can be as large as 50×500 μm2 at 4.2 K. The microscope can be operated in the temperature range 2–300 K using a standard temperature controller. The central microscope body is mounted inside the vacuum can of a dip-stick-type cryoprobe. A damped spring system is used to reduce interference from extraneous mechanical vibrations. The microscope is integrated in a personal-computer-based data acquisition and control setup.
An overview of fiber-optic sensors
Authors: Udd Eric
Published: July 2002

Fiber-optic sensor technology has experienced tremendous growth since its early beginnings in the 1970s with early laboratory demonstrations of fiber-optic gyros and acoustic sensors and the introduction of the first commercial intensity and spectrally based sensors. These early efforts were followed by a tremendous growth of interest in the 1980s when the number of workers in the field increased from perhaps a few hundred to thousands. The result was the introduction in the 1990s of the first mass produced fiber-optic sensors that are being used to support navigation and medical applications. The number of fiber-optic sensors products can be expected to grow tremendously in the years to come as rapid progress continues to be made in the related optoelectronic and communication fields. This paper provides an overview of some of the technologies being used to support fiber-optic sensor development and how they are being applied.
Ultrafast response micropipette-based submicrometer thermocouple
Authors: Fish G. , Bouevitch O. , Kokotov S. , Lieberman K. , Palanker D. , Turovets I. , Lewis Aaron
Published: July 2002

Submicrometer-size thermocouples at the tip of gold-coated glass micropipettes containing a platinum core were produced and tested. The response time of such thermocouples measured with different techniques appeared to be not bigger than a few microseconds. The calculations indicate that the spatial selectivity of this new class of thermocouple devices can be less than 2 μm along the pipette and less than 50 nm across the pipette. The suitability of this thermocouple for light intensity measurements with micrometer spatial resolution is demonstrated by measuring the focused beam of an argon-ion laser. In addition, such thermocouples are intrinsically suitable for applications in scanned probe microscopies. All these unique advantages make the pipette thermocouples a new and promising sensor in a variety of applications.
Adjustable nanofabricated atomic size contacts
Authors: van Ruitenbeek J. M. , Alvarez A. , Piñeyro I. , Grahmann C. , Joyez P. , Devoret M. H. , Esteve D. , Urbina C.
Published: July 2002

Metallic point contacts and tunnel junctions with a small and adjustable number of conduction channels have been obtained in the last few years using scanning tunneling microscope and break junction techniques. For conventional break junctions, the reported drift of the interelectrode spacing in the tunnel regime is typically of the order of 0.5 pm/min (1 pm=10−12 m). We have nanofabricated break junctions which display a drift smaller than 0.2 pm/h. The improvement results from the scaling down by two orders of magnitude of the device dimensions. We describe the nanofabrication process, which can be adapted to most metals. We have performed measurements on Al, Cu, and Nb devices. The results illustrate the ability of the technique to explore phenomenalike conductance quantization and two level fluctuations. These new adjustable atomic size contacts and tunnel junctions can be integrated in complex circuits.
Vibrations of free and surface-coupled atomic force microscope cantilevers: Theory and experiment
Authors: Rabe U. , Janser K. , Arnold W.
Published: July 2002

With an optical interferometer, the free vibration spectra and the local vibration amplitude of four rectangular atomic force microscope cantilevers made of silicon have been examined experimentally in a spectral range of 100 kHz to 10 MHz. A good agreement with the flexural wave theory of elastic beams was found. Coupling to torsional vibrations was also observed. When the sensor tip of the cantilever is in contact with a sample surface the resonances are shifted in frequency and the vibration amplitudes along the cantilever change. A method is presented to calculate this frequency shift using a linear approximation for the tip–sample interaction forces, and the results are compared with the frequency shift calculated from the point-mass model. The measured resonance frequencies of a surface-coupled cantilever do not correspond as well to the theoretical ones as in the free case even if the elastic-beam model is used. The reason for the disagreement is found to be the geometry of the commercial cantilevers and the nonlinearity of the tip-sample interaction force.
Development of a fast solid-state high-resolution density profile reflectometer system on the DIII-D tokamak
Authors: Kim K. W. , Doyle E. J. , Rhodes T. L. , Peebles W. A. , Rettig C. L. , Luhmann, Jr. N. C.
Published: July 2002

A new fast-sweep, broadband frequency-modulated reflectometer on the DIII-D tokamak has produced routine, reliable density profiles with high spatial (⩽1 cm) and temporal resolution (∼100 μs). The system utilizes a solid-state microwave source and an active quadrupler, covering the full Q-band (33–50 GHz) and providing high output power (20–60 mW). The rf source frequency is linearized using an arbitrary function generator and the temperature of the source is actively controlled to reduce rf frequency drifts. The system hardware allows the rf frequency to be swept fullband in 10 μs, but, due to digitization limits, the sweep time used currently is 75–100 μs. The reliability of the reconstructed profiles was improved by a combination of fast frequency sweep, which reduces density fluctuation effects on the measurements, and advanced signal analysis based on digital complex demodulation, which improves phase accuracy. The fast-sweep system has resolved fast-changing edge density profiles during edge localized modes with unprecedented resolution.
A high-frequency modulated tunable diode laser absorption spectrometer for measurements of CO2, CH4, N2O, and CO in air samples of a few cm3
Authors: Güllük T. , Wagner H. E. , Slemr F.
Published: July 2002

Investigations of the relationship between changes in climate and the variation of composition of the atmosphere can be performed most directly and with the highest resolution possible by analyzing the air sampled and preserved in the polar ice sheets for hundreds of thousands of years by nature itself. For the determination of CO2, CH4, N2O, and CO in air samples of 1–3 cm3 extracted from ice cores, a high-frequency modulated tunable diode laser absorption spectrometer has been developed. The instrument can measure CO2, CH4, N2O, and CO at ambient mixing ratios of 300 ppmv (1 ppmv=10−6 volume mixing ratio), 1700, 300, and 100 ppbv (1 ppbv=10−9 volume mixing ratio), respectively, with a precision of 1%–2%. The measurement of high CO2 mixing ratios is not constrained by the detection limit. For other gases mixing ratios of 20 ppbv of CH4, 0.9 ppbv of N2O, and 1.6 ppbv of CO are detectable with the instrument in 2 cm3 standard temperature and pressure. These detection limits and the measurement precision are sufficient for the determination of past changes in atmospheric composition. The technique is also suitable for other applications in which several infrared active trace gases have to be determined in the low ppbv and possibly even in the upper pptv range in air samples of a few cm3 or small samples of other matrices. The sensitivity or the reproducibility of the instrument can be improved by another order of magnitude by using more powerful lasers and by averaging over longer periods. Such improvement could result in a capability to detect sub-ppbv mixing ratios in samples of the present size or ppbv measurements in even smaller air samples. The higher reproducibility would also make the technique interesting for measurements of isotope ratios.
Shot-noise-limited radio-frequency lock-in photodetection with a continuous wave mode-locked laser
Authors: Paraschuk D. Yu. , Kulakov T. A. , Nedopekin O. Yu. , Chigarev N. V. , Pan X. Y.
Published: July 2002

We have developed quite a simple and original lock-in photodetection system that works at a fixed radio frequency. The system includes a photodetector, a frequency down converter and a standard low-frequency lock-in amplifier. The resonant implementation of the radio-frequency attachment provides high gain and high selectivity, which have enabled us to attain the shot-noise-limited sensitivity to relative optical power change δP/P≃4×10−8 Hz−1/2 at a detection frequency of 6.2 MHz in measurements with a cw mode-locked Nd:yttrium aluminum garnet laser. We have realized the precise position-sensitive photodetection for laser pointing noise measurements and picosecond photoacoustic spectroscopy of solids.
In situ stress measurement apparatus for liquid applied coatings
Authors: Payne Jason A. , McCormick Alon V. , Francis Lorraine F.
Published: July 2002

An apparatus for measuring stress development in polymer and ceramic coatings is described. The device combines the coating, processing (chilling, curing, drying), and stress measurement steps necessary to accurately monitor stress development in situ. The measurement system was developed to study the effects of processing variables (e.g., temperature, air flow, relative humidity, etc.) on stress development in coatings ranging from solvent-cast polymers and particulate films to radiation-cured materials. The combination of coating, processing, and stress measurement allows us to mimic conditions used in an industrial setting. Results are reported for polyisobutyl methacrylate cast in toluene and aqueous gelatin coatings that demonstrate the utility of the apparatus in studying how stress evolution and magnitude relate to different process controls.
Detection of shear wave in ultrasonic range by using a laser Doppler vibrometer
Authors: Nishizawa O. , Satoh T. , Lei X.
Published: July 2002

A laser Doppler vibrometer is used to measure shear waves propagating through solids in the ultrasonic frequency range. The laser Doppler vibrometer detects translational motion of small spherical lenses mounted on a solid surface. Stacking of wave-form signals increases the signal-to-noise ratio. Wave motion on a solid surface can be obtained from the surface vibrations measured at different incident angles of a laser beam. The observed longitudinal and shear wave amplitudes from a thickness-dilation-mode transducer agreed fairly well with the calculated amplitudes modeled by a piston force source.
Microfabricated cantilever-based detector for molecular beam experiments
Authors: Bachels T. , Schäfer R.
Published: July 2002

A low cost detector for particles in molecular beam experiments is presented which can easily be mounted in a molecular beam apparatus. The detector is based on microfabricated cantilevers, which can be employed either as single sensors or as sensor arrays. The single cantilever technique has been used to measure the absolute number of atoms coming out of a pulsed laser vaporization cluster source. The particles are detected by the shift of the thermally excited resonance frequency of the cantilever due to the cluster deposition. We have determined with the single cantilever the ratio of neutral to ionized clusters and we have investigated the cluster generation at different source conditions. In addition to this, a microfabricated cantilever array has been used to measure molecular beam profiles, which opens new possibilities for molecular beam deflection experiments.
Exciton lifetimes in GaN and GaInN
Authors: Harris C. I. , Monemar B. , Amano H. , Akasaki I.
Published: July 2002

Results from temperature dependent photoluminescence (PL) transient measurements on metalorganic vapor phase epitaxy grown epitaxial layers of GaN and GaInN are reported. In sufficiently pure GaN layers the free-exciton PL dominates even at the lowest temperatures (2 K), and the intrinsic excitonic lifetimes can be obtained. We report a value of about 125 ps for the radiative lifetime of the free exciton in GaN at 2 K, as obtained from the PL transients of a 3 μm buried undoped GaN layer sandwiched between AlN and GaInN. The PL decay time in the ternary alloy GaInN, which is dominated by localized excitons at low temperatures, is much longer, about 500 ps.
Highly efficient electroluminescence of new wide band gap ladder-type poly(para-phenylenes)
Authors: Tasch S. , Niko A. , Leising G. , Scherf U.
Published: July 2002

We present the optical and electronic properties of a polymeric semiconductor, a new soluble stable ladder-type poly(para-phenylene) (LPPP, band gap=2.69 eV) and demonstrate its excellent applicability in blue-green electroluminescence (EL) devices. The high intrachain order and the low defect concentration of the polymer lead to a remarkably low absorption coefficient in the subgap region (αmin<12 cm−1 at 1.5 eV) and also to a high photoluminescence quantum efficiency (30% in the solid state). The current/electric field characteristics of the high efficiency (up to 0.04 photons/electron) EL devices reveal a polymer film thickness dependence on the required onset field due to ordering effects in the active polymer layer. The shape of the recorded EL spectra can be controlled by the applied electric field in the blue-green spectral range.
A ferroelectric transparent thin-film transistor
Authors: Prins M. W. J. , Grosse-Holz K.-O. , Müller G. , Cillessen J. F. M. , Giesbers J. B. , Weening R. P. , Wolf R. M.
Published: July 2002

Operation is demonstrated of a field-effect transistor made of transparant oxidic thin films, showing an intrinsic memory function due to the usage of a ferroelectric insulator. The device consists of a high mobility Sb-doped n-type SnO2 semiconductor layer, PbZr0.2Ti0.8O3 as a ferroelectric insulator, and SrRuO3 as a gate electrode, each layer prepared by pulsed laser deposition. The hysteresis behavior of the channel conductance is studied. Using gate voltage pulses of 100 μs duration and a pulse height of ±3 V, a change of a factor of two in the remnant conductance is achieved. The dependence of the conductance on the polarity of the gate pulse proves that the memory effect is driven by the ferroelectric polarization. The influence of charge trapping is also observed and discussed.
Effect of textured LaNiO3 electrode on the fatigue improvement of Pb(Zr0.53Ti0.47)O3 thin films
Authors: Chen Ming-Sen , Wu Tai-Bor , Wu Jenn-Ming
Published: July 2002

By changing the electrode combination of Pt and LaNiO3 (LNO), four capacitor types of Pt/PZT/Pt/Si, Pt/LNO/PZT/Pt/Si, Pt/LNO/PZT/LNO/Pt/Si, and Pt/PZT/LNO/Pt/Si, were prepared to investigate the fatigue and hysteresis characteristics of the sol-gel-derived Pb(Zr0.53Ti0.47)O3 (PZT) ferroelectric thin films. Among them, the (100)- and (001)-oriented PZT films were grown on the (100)-textured LNO electrode, but randomly oriented films were obtained on the Pt electrode. It was found that the use of LNO bottom electrode would improve the fatigue property quite significantly, but only the capacitor with LNO as both top and bottom electrodes is shown to be fatigue-free up to 1011 cycles, and the shapes of the hysteresis loop almost unchanged after the fatigue test.
Noise temperature limit of a superconducting hot-electron bolometer mixer
Authors: Karasik B. S. , Elantiev A. I.
Published: July 2002

A theoretical analysis for the noise temperature of a low-temperature hot-electron superconducting mixer has been presented. The contribution of both Johnson noise and electron temperature fluctuations has been evaluated. The theoretical limit of the single-side band noise temperature of the device due to the intrinsic noise mechanisms has been estimated to be as low as 20–40 K, depending on the film material. The corresponding noise bandwidth can be as large as 2–3 GHz, for a device with the electron-phonon cooling mechanism. An improvement of the sensitivity is not necessarily followed by a decrease of the mixer conversion gain bandwidth.
Nitrogen induced increase of growth rate in chemical vapor deposition of diamond
Authors: Müller-Sebert W. , Wörner E. , Fuchs F. , Wild C. , Koidl P.
Published: July 2002

Polycrystalline diamond films have been grown by microwave assisted chemical vapor deposition from methane/hydrogen gas mixtures. The addition of small amounts of nitrogen with concentrations below 50 ppm to the process gas was found to drastically increase the deposition rate depending on the microwave power. At 4.2 kW microwave power a five times higher growth rate compared to nitrogen-free depositions was achieved. The optical transmission and thermal conductivity have been measured. The incorporation of small amounts of nitrogen does not degrade the infrared transmission of the samples; the thermal conductivity measured at room temperature decreased only slightly from 20.5 to 18 W/(cm K).
Bulk periodically poled MgO-LiNbO3 by corona discharge method
Authors: Harada Akinori , Nihei Yasukazu
Published: July 2002

We propose and demonstrate a novel corona discharge method for the fabrication of bulk periodically poled MgO-LiNbO3 substrates. The bulk periodic domain inversion has been successfully formed with a first-order modulation period of 5.2 μm for a wavelength of 980 nm over a 6.5 mm interaction length in a Z-cut 0.4-mm-thick MgO-LiNbO3 substrate for the first time. It was found that MgO-LiNbO3 crystal is capable of forming domain inversion under a low applied voltage of no more than 6 kV/mm, which is lower than the case of LiNbO3 and LiTaO3. A 490 nm blue light power of 6.7 mW was achieved in a single pass through periodically poled MgO-LiNbO3, by using a high-power monolithically integrated flared master oscillator power amplifier of 790 mW as a fundamental source.
Raman, optical-absorption, and transmission electron microscopy study of size effects in germanium quantum dots
Authors: Bottani C. E. , Mantini C. , Milani P. , Manfredini M. , Stella A. , Tognini P. , Cheyssac P. , Kofman R.
Published: July 2002

Quantum confinement effects in Ge nanocrystals in the size range 26–130 Å have been investigated both on the electronic joint density of states and Raman spectra. The high degree of crystallinity of the particles as well as the minimized interaction with the matrix allow detection of shift and broadening of the TO Raman peak, which can be compared with theoretical expectations based on phonon confinement. The evolution of Raman and absorption spectra and their dependence on the nanocrystal dimensions can be related.
Atomic layer controlled growth of SiO2 films using binary reaction sequence chemistry
Authors: Klaus J. W. , Ott A. W. , Johnson J. M. , George S. M.
Published: July 2002

SiO 2 thin films were deposited with atomic layer control using binary reaction sequence chemistry. The SiO2 growth was accomplished by separating the binary reaction SiCl4+2H2O→SiO2+4HCl into two half-reactions. Successive application of the half-reactions in an ABAB… sequence produced SiO2 deposition at temperatures between 600 and 800 K and reactant pressures of 1–10 Torr. The SiO2 growth was monitored using ellipsometry versus substrate temperature and reactant exposure time. The maximum SiO2 deposition per AB cycle was 1.1 Å/AB cycle at 600 K. The surface topography measured using atomic force microscopy was extremely flat with a roughness nearly identical to the initial substrate.
Aspect ratio independent etching of dielectrics
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

Monte Carlo simulations of pattern-dependent charging during oxide etching predict that the etch rate scaling with aspect ratio breaks down when surface discharge currents are significant. Under conditions of ion-limited etching and no inhibitor deposition, the etch depth depends on the maximum incident ion energy, reaction threshold, and surface discharge threshold, and is the same irrespective of the trench width (⩽0.5 μm).
Exciton saturation in room temperature GaAs/AlGaAs multiple quantum wells
Authors: Holden T. M. , Kennedy G. T. , Cameron A. R. , Riblet P. , Miller A.
Published: July 2002

We report time resolved studies of optically induced circular dichroism in room temperature GaAs/AlGaAs multiple quantum wells to resolve spin-dependent and spin-independent contributions to exciton saturation. Phase-space filling and Coulomb contributions were separated by spin dependence while the effects of broadening and screening were distinguished using different pulse bandwidths. Bound and free carrier contributions were compared by observing the temporal dependence of the circular dichroism on femtosecond timescales. The spin independent contribution to exciton saturation was found to be independent of whether the carriers were bound or free.
Adsorption-controlled growth of Bi4Ti3O12 by reactive MBE
Authors: Theis C. D. , Yeh J. , Schlom D. G. , Hawley M. E. , Brown G. W. , Jiang J. C. , Pan X. Q.
Published: July 2002

Adsorption-controlled conditions have been identified and utilized to grow epitaxial bismuth titanate thin films by reactive molecular beam epitaxy. Growth of stoichiometric, phase pure, c-axis oriented, epitaxial films is achieved by supplying a large overabundance of bismuth and ozone continuously to the surface of the depositing film. Titanium is supplied to the film in the form of shuttered bursts each containing a three monolayer dose of titanium to grow one formula unit of Bi4Ti3O12. It is seen from measured film thickness, Rutherford backscattering spectrometry composition measurements, monitoring of reflection high-energy electron diffraction half-order intensity oscillations during growth, and in situ flux measurements using atomic absorption spectroscopy that at suitable temperature and ozone background pressure, the titanium sticking coefficient approaches one and the excess bismuth desorbs from the surface. Film growth proceeds by the formation of mounds whose step heights are predominantly integral multiples of a half-unit cell.
Negative magnetoresistance in GaAs with magnetic MnAs nanoclusters
Authors: Akinaga H. , De Boeck J. , Borghs G. , Miyanishi S. , Asamitsu A. , Van Roy W. , Tomioka Y. , Kuo L. H.
Published: July 2002

We show a negative magnetoresistance (MR) in GaAs with magnetic MnAs nanoclusters (about 1.5% at 30 K in 1 T). The clusters were formed in a two step process consisting of the molecular beam epitaxy of (Ga,Mn)As layer and the subsequent annealing. The origin of the negative MR is attributed to the presence of the MnAs clusters. The mechanism is considered to be a spin-dependent scattering of carriers by MnAs clusters which decreases when the direction of the magnetization between the clusters aligns with the magnetic field.
Soft breakdown fluctuation events in ultrathin SiO2 layers
Authors: Miranda E. , Suñé J. , Rodríguez R. , Nafría M. , Aymerich X.
Published: July 2002

When an ultrathin (<5 nm) oxide is subjected to electrical stress, several soft-breakdown events can occur prior to the final dielectric breakdown. After the occurrence of such failure events, the current–voltage (I–V) characteristic corresponds to the superposition of highly conductive spots and background conduction through the undegraded capacitor area. In this conduction regime, the application of a low constant voltage gives rise to large leakage current fluctuations in the form of random telegraph signal. Some of these fluctuations have been identified with ON/OFF switching events of one or more local conduction spots, and not with a modulation of their conductance. The experimental soft-breakdown I–V characteristics are shown to be better understood if the spot conduction is considered to be locally limited by the silicon electrodes and not by the oxide.
Evaluation of damage induced by inductively coupled plasma etching of 6H–SiC using Au Schottky barrier diodes
Authors: Li Binghui , Cao Lihui , Zhao Jian H.
Published: July 2002

Surface damage of 6H–SiC induced by inductively coupled plasma (ICP) etching with a CF4/O2 gas mixture has been evaluated by Au Schottky barrier diodes formed on the etched surfaces. The influence of substrate dc bias has been studied. It is found that there is an optimum dc bias for ICP etching. Under the optimum dc bias voltage, Schottky barrier diodes on the etched surface are of high quality and are comparable with diodes formed on the control sample, indicating that a very low damage and low contamination surface is obtained after etching. A deterioration of etched surface has been observed for both smaller and larger dc biases compared to the optimum bias in term of characteristics of Schottky diodes. Explanations are provided for the observed dependence of Schottky barrier diode characteristics on the substrate dc bias.
Ion induced disordering and dissolution of Ni3Al precipitates
Authors: Ewert J. C. , Schmitz G. , Harbsmeier F. , Uhrmacher M. , Haider F.
Published: July 2002

The disordering of homogeneous Ni3Al and decomposed Ni(Al) alloys under irradiation with 300 keV Ni58 ions is studied at room temperature. In order to determine the development of the long-range order parameter, electron diffraction patterns are quantitatively evaluated taking into account dynamical diffraction effects. Although postulated by a recent model based on the thermal spike concept, no finite rest order is observed in the high-dose limit. The disordering rate of Ni3Al precipitates is much higher than that of homogeneous Ni3Al which is explained by the additional chemical dissolution of the particles due to ballistic mixing.
Newtonian to non-Newtonian master flow curves of a bulk glass alloy Pd40Ni10Cu30P20
Authors: Kato Hidemi , Kawamura Yoshihito , Inoue Akihisa , Chen H. S.
Published: July 2002

The viscosity and flow stress of a bulk Pd40Ni10Cu30P20 alloy glass near the glass transition were measured as a function of temperature and strain rate under compression. The steady-state viscosity of the glass for a given temperature remains constant at low strain rate, then decreases by many orders of magnitude above a critical strain rate. A master curve in term of the viscosity ratio η/ηN and the product ηNε̇ has been constructed, where η and ηN are, respectively, the steady-state viscosity and Newtonian viscosity, and ε̇ is the strain rate. The flow stress also can be represented in terms of the product ηNε̇. The master curves are fitted with a simple stress relaxation of the form 1−exp[−t/λ] with t=ε̇−1.