bluuebunny/crossref_metadata_2025 huggingface dataset:
Direct pulse technique for spontaneous polarization dynamics and molecular reorientation processes in ferroelectric liquid crystals
Authors: Bawa S. S. , Biradar A. M. , Saxena K. , Chandra Subhas
Published: July 2002

A method has been devised for studying the spontaneous polarization, dynamics of polarization reversal, and the molecular reorientation processes that occur in the bulk and surfaces of ferroelectric liquid crystals. These studies have been carried out by analyzing the current response to triangular- and square-wave forms. The current response to a triangular wave yields the spontaneous polarization by conveniently subtracting the background contribution due to the conductive and capacitive currents. Molecular reorientation processes which take place within the bulk and the boundary surface layer can be studied by the appearance of polarization current peaks on the triangular-wave response. The dynamics of polarization reversal between bistable states and of the polarization change between the zero-field relaxed state and either of the saturation remanent polarization states have been studied by applying ‘‘symmetric’’ and ‘‘asymmetric’’ square waves. The technique can also be successfully applied to solid ferroelectrics for studying the spontaneous polarization and the domain dynamics during switching.
Novel method for accurate g measurements in electron-spin resonance
Authors: Stesmans A. , Van Gorp G.
Published: July 2002

In high-accuracy work, electron-spin-resonance (ESR) g values are generally determined by calibrating against the accurately known proton nuclear magnetic resonance (NMR). For that method—based on leakage of microwave energy out of the ESR cavity—a convenient technique is presented to obtain accurate g values without needing conscientious precalibration procedures or cumbersome constructions. As main advantages, the method allows the easy monitoring of the positioning of the ESR and NMR samples while they are mounted as close as physically realizable at all time during their simultaneous resonances. Relative accuracies on g of ≊2×10−6 are easily achieved for ESR signals of peak-to-peak width ΔBpp≲0.3 G. The method has been applied to calibrate the g value of conduction electrons of small Li particles embedded in LiF—a frequently used g marker—resulting in gLiF:Li=2.002 293±0.000 002.
A high-intensity multi-purpose piezoelectric pulsed molecular beam source
Authors: Proch D. , Trickl T.
Published: July 2002

A piezoelectric valve is described which delivers intense gas pulses limited by the nozzle for diameters of up to 1.0 mm. Minimum pulse lengths between 0.15 and 0.25 ms (fwhm) are obtained at full opening, depending on the nozzle size. The upper limit of the repetition rate is near 1 kHz. The valve and its modifications meet almost all experimental requirements. In particular, ways of operating in a wide temperature range are outlined. Reliable operation with a remote nozzle at 77 K was recently demonstrated. Nozzle temperatures of more than 1000 K should be possible with a similar setup.
Apodized pickup coils for improved spatial resolution of SQUID magnetometers
Authors: Roth Bradley J. , Wikswo John P.
Published: July 2002

The spatial resolution of a SQUID magnetometer is known to be determined by the size of the magnetometer pickup coil and the distance of this coil from the source of the magnetic field. With the advent of spatial-filtering techniques for converting magnetic field maps to current density images, it is clear that the quality and spatial resolution of the current image is also affected by the detailed characteristics of the transfer function between the current sources and the pickup coil. We show that the quality of a current image can be improved over that obtained with conventional pickup coils by using a multiturn pickup coil whose interturn spacing is adjusted to provide a source-coil transfer function with specific characteristics. This process is functionally equivalent to apodization techniques applied in optical image processing, in which the aperture function of the telescope is adjusted to improve spatial resolution. In coil design, the process of apodization involves designing a magnetometer coil whose transfer function has its first zero at a relatively large spatial frequency. Using apodization, we can decrease coil size and increase the number of turns without a large increase in coil inductance. Our model calculations demonstrate that apodization allows us to optimize the ability of a coil design to image a current distribution, subject to the constraints of signal-to-noise ratio, spatial resolution, and coil inductance.
Constant current corona triode with grid voltage control. Application to polymer foil charging
Authors: Giacometti José A. , Sinézio Carvalho Campos J.
Published: July 2002

A new version of a corona triode setup, allowing charging dielectric foils with a constant current, was developed. The charging current is kept constant by controlling the grid voltage. We show that the sample surface potential can be inferred from the grid voltage and present results of the uniformity of charge deposition during the charging procedure. Sample charging currents could be varied from 1 to 300 nA/cm2 and the foils could be charged up to ±6 kV. The method was applied to measure the potential buildup on Teflon FEP and PVDF samples and these results are discussed. They showed that the method can be successfully applied to study the polarization buildup of β-PVDF foils under corona charging.
A solid dielectric Compton diode for measuring short radiation pulse widths
Authors: Carlson G. A. , Sanford T. W. L. , Davis B. A.
Published: July 2002

A simple, rugged, and inexpensive solid dielectric Compton diode (SCD) has been developed for measuring the radiation-time history of flash x-ray sources. By limiting the physical size of the Compton diode and matching its characteristic impedance to that of the signal cable, a frequency response above 1 GHz has been achieved. The SCD has been used to measure the pulse width and rise time of HERMES III, a pulsed γ-ray simulator. A second, prototype SCD has been built for use with Saturn, a pulsed x-ray simulator. The SCDs do not have high sensitivity, and are thus limited to measurements of dose rate above 109 rads/s, due to signal-to-noise limitations. At lower dose rates, more sensitive detectors such as scintillator photodiodes or p-i-n diodes are more suitable. On the other hand, for dose rates above 1010 rads/s, the more sensitive detectors tend to saturate, while the SCD continues to respond linearly to at least 1012 rads/s. Thus, the SCD is well suited for measuring radiation-time histories at high dose rates on flash x-ray sources.
Fluorescence lifetime measurements using a novel fiber-optic laser scanning confocal microscope
Authors: Ghiggino Kenneth P. , Harris Martin R. , Spizzirri Paul G.
Published: July 2002

The novel design of a fiber-optic laser scanning confocal microscope is described. The optical fiber acts as a flexible light path for both the excitation and return beams and as the confocal pinhole of the microscope. The system is capable of imaging both reflective and fluorescent objects and offers considerable adaptability in use. With a mode-locked dye laser excitation source, avalanche photodiode detector and time-correlated photon counting electronics, spatially resolved fluorescence decay profiles from fluorescent dyes in solution and polymer films have been recorded.
Reflectron time-of-flight mass spectrometer for laser photodissociation
Authors: Cornett D. S. , Peschke M. , LaiHing K. , Cheng P. Y. , Willey K. F. , Duncan M. A.
Published: July 2002

We describe a new reflectron time-of-flight mass spectrometer configuration for laser photodissociation of mass-selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed-nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are discussed.
The correction factors and their new curve for the measurement of sheet resistance of a square sample with a square four-point probe
Authors: Sun Yicai , Ehrmann Oswin , Wolf Jürgen , Reichl Herbert
Published: July 2002

In this article, a new curve of the correction factor for the measurement of sheet resistance of a square sample with a square four-point probe is presented by using the theory of conformal transformation. This curve is quite different from that obtained by using the theory of infinite dipoles. An experimental curve is recommended to use. The function of the conformal transformation used in this article is simple and can be easily used for solving the problem of boundary effect of current field. The theoretical curve is suitable to the boundary areas of wafers.
A new centrifuge pellet injector for fusion experiments
Authors: Andelfinger C. , Buchelt E. , Cierpka P. , Kollotzek H. , Lang P. T. , Lang R. S. , Prausner G. , Söldner F. X. , Ulrich M. , Weber G.
Published: July 2002

This paper reports on the new pellet injection system for refueling the ASDEX Upgrade tokamak with cubic H2 or D2 pellets having alternative side lengths of 1.5, 1.75, and 2.0 mm and optional Ne doping. The system delivers series of about 100 pellets at a maximum repetition rate of more than 40 Hz. The pellets are accelerated by means of a centrifuge with an optimized straight acceleration arm. This configuration minimizes the compulsive force acting on the pellet during the acceleration process. Since this also minimizes stresses inside the pellet, high velocities—a maximum of 1211 m/s being achieved—are possible without destroying the hydrogen cubes. A special pellet feed-in technique based on a static stop cylinder interrupting the acceleration path successfully reduced the horizontal scattering angle to values of less than ±4°; a high efficiency, with more than 90% of the pellets arriving within the acceptance angle, was thus achieved. The whole system was found to work very reliably and reproducibly during the whole test operation period, covering about 105 pellet shots, and is now being integrated into the ASDEX upgrade experiment.
Submillisecond response times of oxygen-quenched luminescent coatings
Authors: Baron Alan E. , Danielson J. D. S. , Gouterman Martin , Wan Jiang River , Callis James B. , McLachlan Blair
Published: July 2002

An apparatus and data analysis methodology is described which allows determination of response time to oxygen concentration changes of luminescent oxygen sensor coatings. Utilizing a solenoid valve, a sample chamber creates a pressure jump from 0.1 to 700 Torr in 600 μs that is followed by 15 ms of ringing. An optical detection system measures the response of porphyrin-based luminescent oxygen sensors to the pressure jump. The pressure in the chamber is measured simultaneously and independently with a piezoresistive pressure transducer. Data analysis techniques using nonlinear least-squares and numerical convolution of the luminescent response to the pressure rise allow determination of response times of the oxygen sensor. The response to pressure jumps of several luminescent oxygen sensitive coatings designed for video luminescent barometry are measured with this computer-controlled instrument. Several coatings were studied with response times of ∼2.5 s, ∼400 ms, 11 ms, 1.5 ms, and <25 μs. Studies of the system suggest that we can determine response times down to about 10 μs.
Dual mode operation of a helical resonator discharge
Authors: Bletzinger P.
Published: July 2002

Measurements of the resonance characteristics of a helical resonator plasma generator over a wide range of frequencies and gas pressures are described. It was found that, analogous to an inductive discharge, it can operate both in a capacitive and an inductive mode. However, in the helical resonator either mode can be selected for one gas pressure and at similar power simply by changing the operating frequency. This will produce an extended plasma for the capacitive mode and a plasma concentrated in the resonator for the inductive mode. The inductive mode is enhanced at higher power levels and higher pressures. The pressure range investigated ranged from 0.05 to 10 Torr. Electron densities were measured with a microwave interferometer both inside the resonator and downstream and reached 3×1010 cm−3 for 0.5 Torr argon at 0.075 W/cm−3 rf power. The resonance characteristics were affected by gases such as N2 and SF6.
Difference between the forces measured by an optical lever deflection and by an optical interferometer in an atomic force microscope
Authors: Fujisawa Satoru , Ohta Masahiro , Konishi Takefumi , Sugawara Yasuhiro , Morita Seizo
Published: July 2002

Using a simple model, we investigated the difference between the forces measured by an atomic force microscope (AFM) with an optical lever deflection method and that with an optical interferometer method. Then, using a mica with an atomically flat surface as a test sample, we confirmed experimentally the above difference, which says that the optical lever deflection method detects not only the surface corrugation but also the frictional force, while the optical interferometer method detects only the surface corrugation. Based on the above results, we proposed a method to measure the three-dimensional force vector by using both the optical lever AFM/LFM and the optical interferometer AFM simultaneously. Furthermore, we mention that the measurement of three-dimensional spatial distribution of the force vector will implement the computed tomography technique into AFM measurements, which yields a three-dimensional spatial distribution of the force origin.
On the use of electronic tilt sensors as angle encoders for synchrotron applications
Authors: Knapp G. S. , You H. , Holzhausen Gary R.
Published: July 2002

We have tested several electrolytic tilt sensors produced by Applied Geomechanics, Inc. A pair of sensors were tested by mounting them on the Chi circle of a Huber 4 circle diffractometer. The angles were scanned in 1° intervals over a range of ±35°. In these tests the resolution was about ±5 μrad but the repeatability depended on angle and varied from 70 μrad (one standard deviation) at large angles to 7 μrad at small angles. This type of tilt sensor may be too slow (2–10 s settling time) as a primary angle encoder for a monochromator or diffractometer, but for a system run by stepping motors, they would prove quite useful as secondary angle encoders. Other models which have a narrower angular range would be useful in setting and tuning focusing mirrors.
Study of band offsets in CdF2/CaF2/Si(111) heterostructures using x-ray photoelectron spectroscopy
Authors: Izumi A. , Hirai Y. , Tsutsui K. , Sokolov N. S.
Published: July 2002

The valence band offsets at the heterointerfaces of the CdF2/CaF2/Si(111) structure grown by molecular beam epitaxy were evaluated using x-ray photoelectron spectroscopy, and the energy band diagram of this heterostructure was proposed. It was found that the interface of CdF2/CaF2 has large conduction band offset: 2.9 eV, and the energy level of CdF2 conduction band edge is below that of Si.
An insert for single-molecule magnetic-resonance spectroscopy in an external magnetic field
Authors: van der Meer H. , Disselhorst J. A. J. M. , Köhler J. , Brouwer A. C. J. , Groenen E. J. J. , Schmidt J.
Published: July 2002

We describe an insert for optical and magnetic-resonance experiments on single molecules in a solid matrix at liquid-helium temperatures. The experimental arrangement allows in situ adjustment of the focusing lens and of the sample. A parabolic mirror serves to collect the fluorescence emission and to direct the light onto a photodetector. Microwaves can be irradiated through a coil around the sample while a superconducting magnet provides the possibility of a stationary magnetic field.
High-resolution ion mobility measurements
Authors: Dugourd Ph. , Hudgins R. R. , Clemmer D. E. , Jarrold M. F.
Published: July 2002

Gas phase ion mobility measurements can resolve structural isomers for polyatomic ions and provide information about their geometries. A new experimental apparatus for performing high-resolution ion mobility measurements is described. The apparatus consists of a pulsed laser vaporization/desorption source coupled through an ion gate to a 63-cm-long drift tube. The ion gate is a critical component that prevents the diffusion of neutral species from the source into the drift tube. Ions travel along the drift tube under the influence of a uniform electric field. At the end of the drift tube some of the ions exit through a small aperture. They are focused into a quadrupole mass spectrometer, where they are mass analyzed, and then detected by an off-axis collision dynode and by dual microchannel plates. The apparatus is operated with a drift voltage of up to 14 000 V and a helium buffer gas pressure of around 500 Torr. The resolving power for ion mobility measurements is over an order of magnitude higher than has been achieved using conventional injected-ion drift tube techniques. Examples of the application of the new apparatus in resolving isomers of laser desorbed metallofullerenes, in studying silicon clusters generated by laser vaporization, and in following the isomerization of small nanocrystalline (NaCl)nCl− clusters as a function of temperature, are presented.
Growth and characterization of carbon nitride thin films prepared by arc-plasma jet chemical vapor deposition
Authors: Yen Tyan-Ywan , Chou Chang-Pin
Published: July 2002

Carbon nitride thin films have been successfully grown on nickel substrates by a novel arc-plasma jet chemical vapor deposition. These films were characterized by Auger electron spectroscopy, transmission electron microscopy, and Raman spectroscopy. Small grains (∼0.1 μm) as well as nanocrystallites found in the films were identified to be β-C3N4. Raman spectroscopy also confirmed the existence of β-C3N4 phase in the films through five pronounced Raman bands as expected from the Hooke’s law approximation based on the vibrational frequencies obtained in analogous compound, β-Si3N4.
An experimental design for microluminescence
Authors: Monte A. F. G. , Cruz J. M. R. , Morais P. C.
Published: July 2002

The description of a new experimental design for microluminescence is presented in this article. The design is based on the optical analysis of a magnified luminescent region and has been proven to be useful for studying the photon transport mechanism in a solid luminescent material. Lock-in detection and a liquid-nitrogen-cooled detector are used to obtain signal-to-noise ratio as good as 102. The performance of the system is discussed by using the measurements taken from a natural ruby crystal. Using transport theory to fit our data, we found the photon diffusion length in the ruby crystal to be on the order of 46 μm.
Performance of the vacuum ultraviolet high-resolution and high-flux beamline for chemical dynamics studies at the Advanced Light Source
Authors: Heimann P. A. , Koike M. , Hsu C. W. , Blank D. , Yang X. M. , Suits A. G. , Lee Y. T. , Evans M. , Ng C. Y. , Flaim C. , Padmore H. A.
Published: July 2002

At the Advanced Light Source an undulator beamline, with an energy range from 6 to 30 eV, has been constructed for chemical dynamics experiments. The higher harmonics of the undulator are suppressed by a novel, windowless gas filter. In one branchline high-flux, 2% bandwidth radiation is directed toward an end station for photodissociation and crossed molecular-beam experiments. A photon flux of 1016 photon/s has been measured at this end station. In a second branchline a 6.65 m off-plane Eagle monochromator delivers narrow bandwidth radiation to an end station for photoionoization studies. At this second end station a peak flux of 3×1011 was observed for 25 000 resolving power. This monochromator has achieved a resolving power of 70 000 using a 4800 grooves/mm grating, one of the highest resolving powers obtained by a vacuum ultraviolet monochromator.
Remote thermometry with thermographic phosphors: Instrumentation and applications
Authors: Allison S. W. , Gillies G. T.
Published: July 2002

The temperature-dependent characteristics of fluorescence of several rare-earth-doped ceramic phosphors has made these materials the focus of a major effort in the field of noncontact thermometry over the past few decades. These “thermographic phosphors,” e.g., Y2O3:Eu, have been used for remote measurements of the temperatures of both static and moving surfaces, and have performed many other tasks that standard sensors (thermocouples, thermistors, etc.) cannot. The range of usefulness of this class of materials extends from cryogenic temperatures to those approaching 2000 °C. The instrumentation needed for this type of thermometry has followed many different lines of development, and this evolution has produced a wide variety of both field- and laboratory-grade systems that are now described in the literature. In general, the technique offers high sensitivity (≈0.05 °C), robustness (e.g., stability of the sensor sample in harsh environments), and NIST traceability. In addition, such systems have been successfully adapted to make remotely sensed measurements of pressure, heat flux, shear stress, and strain. In this review, we summarize the physical mechanisms that form the basis for the technique, and then catalog and discuss the instrumentation-related aspects of several different remote thermometry systems that employ thermographic phosphors as the sensors.
Low-pressure hollow-cathode glow discharge plasma for broad beam gaseous ion source
Authors: Oks Efim M. , Vizir Alexey V. , Yushkov George Yu.
Published: July 2002

We have developed a hollow-cathode glow discharge plasma for a dc broad beam ion source. For a broad beam ion source, it is hard to obtain adequate pressure drop between the discharge space and the extraction region. The high-current low-voltage mode of discharge is limited to a pressure about 10−3 Torr, but for stable extraction of ions without breakdown the pressure needs to be at least one order of magnitude lower. To decrease the limited operation pressure for the high-current mode of the hollow-cathode glow, an external electron beam was used. These electrons were generated in a “keep-alive” discharge and are accelerated in the cathode layer of the primary discharge. In this way we successfully decreased the operation pressure to 10−4–3×10−5 Torr, as well as reaching a value of the discharge voltage as low as 80 V. The characteristics of this discharge system are presented, and the influence of external electrons on the discharge parameters is discussed.
MEPHISTO spectromicroscope reaches 20 nm lateral resolution
Authors: De Stasio Gelsomina , Perfetti Luca , Gilbert B. , Fauchoux O. , Capozi M. , Perfetti P. , Margaritondo G. , Tonner B. P.
Published: July 2002

The recently described tests of the synchrotron imaging photoelectron spectromicroscope MEPHISTO (Microscope à Emission de PHotoélectrons par Illumination Synchrotronique de Type Onduleur) were complemented by further resolution improvements and tests, which brought the lateral resolution down to 20 nm. Images and line plot profiles demonstrate such performance.
Very high sensitivity ac capacitance bridge for the dielectric study of molecular solids at low temperatures
Authors: Pilla S. , Hamida J. A. , Sullivan N. S.
Published: July 2002

An ac capacitance bridge with excellent stability and two parts per billion resolution in the real part of the dielectric constant at temperatures ranging from 4.2 to 80 K and in the audio frequency range (0.2–20 kHz) is reported. The method uses commercially available components as well as few homemade cables and capacitors. Construction details and data on the resolution are presented.
Measurement of the absolute penetration depth and surface resistance of superconductors and normal metals with the variable spacing parallel plate resonator
Authors: Talanov Vladimir V. , Mercaldo Lucia V. , Anlage Steven M. , Claassen John H.
Published: July 2002

The variable spacing parallel plate resonator (VSPPR) is a microwave transmission line resonator with a continuously variable thickness of the dielectric spacer between the superconducting or metallic plates, filled by cryogenic liquid or vacuum. We measure the dielectric spacer thickness dependencies of the resonator frequency and quality factor, and fit them to theoretical forms, in order to extract the absolute values of penetration depth, λ, and surface resistance, Rs. A cryogenic micropositioning setup is developed to vary the spacer thickness from 0 to 100 μm with a resolution of 8.5 nm, and to maintain parallelism of the resonator plates. Measurement of ac capacitance between the plates is utilized to directly determine the separation between the resonator plates and to reduce the effect of their tilt and nonflatness on the accuracy of the measured Rs and λ. Because the operating temperature is fixed (77 K), the result for a superconductor is independent of an a priori model for the penetration depth versus temperature. This technique can also be employed as a surface impedance standard for characterization of high temperature superconducting films for microwave applications.
Simple microwave preionization source for ohmic plasmas
Authors: Choe W. , Kwon Gi-Chung , Kim Junghee , Kim Jayhyun , Jeon Sang-Jean , Huh Songwhe
Published: July 2002

A simple economical 2.45 GHz microwave system has been developed and utilized for preionization on the Korea Advanced Institute of Science and Technology (KAIST)-TOKAMAK. The magnetron microwave source was obtained from a widely used, household microwave oven. Since ac operation of the magnetron is not suitable for tokamak application, the magnetron cathode bias circuit was modified to obtain continuous and stable operation of the magnetron for several hundred milliseconds. Application of the developed microwave system to KAIST-TOKAMAK resulted in a reduction of ohmic flux consumption.
Lattice damage in III/V compound semiconductors caused by dry etching
Authors: Heinbach M. , Kaindl J. , Franz G.
Published: July 2002

The crystal damage in optoelectronic devices caused by dry etching methods [ion beam etching, reactive ion etching, etching with plasmas excited by electron cyclotron resonance (ECR)] was evaluated. The analytics applied are photoluminescence and Fabry-Perot damping measurement which were applied to waveguides. A significant improvement is observed using ECR etching as low damage combined with high etching rates is concerned. To evaluate a method as soft or hard etching, Fabry-Perot damping measurement has emerged to be the most discriminate and decisive tool.
Subwavelength amorphous silicon transmission gratings and applications in polarizers and waveplates
Authors: Chou Stephen Y. , Deng Wenyong
Published: July 2002

Subwavelength amorphous silicon transmission gratings with a period from 100 to 800 nm and a thickness of 180 nm on silica substrates have been fabricated and studied. For a normal incident light (λ=633 nm), in addition to strong birefringence (as large as 170° in phase difference), it was observed that the transmission of TE (the electric field parallel to the grating finger) wave is significantly different from that of TM (the electric field perpendicular to the grating fingers) wave (as large as 12 dB), and that both polarization and birefringence effects strongly depend on the ratio of grating period to wavelength, having the largest variations at the grating periods around a half of the wavelength. This observation deviates significantly from simple form birefringence theory and effective medium theory. Applications such as polarization selection mirrors for vertical cavity surface emitting lasers and waveplates are discussed.
Highly efficient blue electroluminescence from a distyrylarylene emitting layer with a new dopant
Authors: Hosokawa Chishio , Higashi Hisahiro , Nakamura Hiroaki , Kusumoto Tadashi
Published: July 2002

We report that organic electroluminescence devices with a distyrylarylene (DSA) emitting layer including a new dopant realized highly efficient and bright emission in blue region. This dopant was amino-substituted DSA. The luminous efficiency was obtained to be 1.5 lm/W. The external quantum efficiency was estimated to be 2.4%, which is one of the highest efficiencies ever reported in blue emitting organic EL devices.
Correlation of anisotropic strain relaxation with substrate misorientation direction at InGaAs/GaAs(001) interfaces
Authors: Goldman R. S. , Wieder H. H. , Kavanagh K. L.
Published: July 2002

We have investigated the effects of substrate misorientation towards (111)A, (111)B, and (011) on asymmetries in the strain relaxation of InxGa1−xAs, grown on (001) GaAs substrates by molecular beam epitaxy. For epilayers grown under conditions of two-dimensional growth, we find large anisotropies in bulk strain relaxation and epilayer rotation about an in-plane axis (epilayer tilt) in proportion to the degree of (111)A misorientation. The residual strain asymmetry is largest for the (111)A misoriented substrate (≳50%) and smallest for the (111)B misoriented substrate (<15%). At higher growth temperatures, the bulk strain relaxation becomes isotropic while the epilayer tilt remains sensitive to the offcut direction. At all temperatures, a preference for epilayer tilt toward the [110] direction for (011) misorientations is observed.
Measurement of large electro-optic coefficients in thin films of strontium barium niobate (Sr0.6Ba0.4Nb2O6)
Authors: Trivedi Dhrupad , Tayebati Parviz , Tabat M.
Published: July 2002

A reflection-grating measurement technique is reported for measurement of the electro-optic coefficient in crystalline strontium barium niobate thin films grown using pulsed laser deposition. The measurement yields a value of about 350 pm/V for the r33 coefficient of the Sr0.6Ba0.4Nb2O6(SBN:60) film. The measurement technique is based on a modulated diffraction pattern measurement using coplanar electrodes and can be extended to a range of cases where it is not possible to deposit electrodes on both sides of a film.
Room-temperature electron spin dynamics in GaAs/AlGaAs quantum wells
Authors: Tackeuchi Atsushi , Nishikawa Yuji , Wada Osamu
Published: July 2002

We have investigated the spin relaxation process of electron at room temperature in nominally undoped GaAs/AlGaAs multiple-quantum wells. The spin relaxation times are measured for different well thicknesses using time-resolved polarization absorption measurement. The spin relaxation time τs, is found to depend on the electron confined energy E1e, according to τs∝E1e−2.2, showing that the main spin relaxation mechanism at room temperature is the D’yakonov–Perel’ interaction.
Formation of copper and silver nanometer dimension clusters in silica by the sol-gel process
Authors: De G. , Tapfer L. , Catalano M. , Battaglin G. , Caccavale F. , Gonella F. , Mazzoldi P. , Haglund R. F.
Published: July 2002

Ag and Cu (pure and/or mixture) nanoclusters doped silica films were prepared by the sol-gel process. In the case of Ag and Cu codoped silica films, Cu/Ag molar ratio was 1, 2, and 3 at constant (Ag+Cu)/SiO2 molar ratio of 0.175. Separated Ag and Cu nanoclusters are formed in the silica matrix. The size of the clusters and their distribution are dependent on the film composition. Optical absorption was measured from 185 to 800 nm. Intensity-dependent nonlinear refractive index was measured for pure Cu and Ag–Cu (1:1 molar ratio) samples using z-scan technique in the wavelength range from 570 to 596 nm. The measured nonlinear refractive index is of the order of 10−13 m2/W at a pulse repetition rate of 15.2 MHz.
Dislocation formation mechanism in strained InxGa1−xAs islands grown on GaAs(001) substrates
Authors: Chen Y. , Lin X. W. , Liliental-Weber Z. , Washburn J. , Klem J. F. , Tsao J. Y.
Published: July 2002

The formation mechanism of misfit dislocations in lattice-mismatched InxGa1−xAs epilayers (0.2≤x≤1) grown on GaAs substrates has been investigated experimentally. The results suggest that 1/3〈111〉 Frank partial dislocations are grown-in at island edges in highly lattice-mismatched epilayers (x≥0.4). Then after further island growth 90° Shockley partial dislocations are nucleated to remove the stacking faults, reacting with the Frank partials to form complete 90° dislocations. An atomic model is proposed to explain the formation mechanism of the Frank partial dislocation. This model could explain the observed change in the dominant type of dislocation from the 60° at small mismatches to 90° edge dislocations at large lattice mismatches.
Heterodyne interferometer with two-wave mixing in photorefractive crystals for ultrasound detection on rough surfaces
Authors: Pouet Bruno F. , Ing R. K. , Krishnaswamy Sridhar , Royer D.
Published: July 2002

Heterodyne interferometers using two-wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces are demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency-shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. The signal to noise ratio for the two commonly used crystallographic configurations with cubic crystals, G∥〈110〉∥ and G∥〈001〉, where G is the grating wave vector, are investigated. Very good sensitivity is demonstrated for the detection of small amplitude ultrasonic surface displacements.
Combined magnetron sputtering and pulsed laser deposition of carbides and diamond-like carbon films
Authors: Voevodin A. A. , Capano M. A. , Safriet A. J. , Donley M. S. , Zabinski J. S.
Published: July 2002

A hybrid technique is reported, which combines magnetron sputtering and pulsed laser ablation to produce plasma fluxes intersected on a substrate surface to form metal, ceramic and diamond-like materials. Deposition of crystalline Ti, TixCy, TiCN, and amorphous diamond-like carbon films at low temperatures by the new technique is discussed. The variation of laser pulse frequency is found to be a simple way to control film chemical composition. The technique can be used to prepare materials with transitional structure, as, for example, between metal carbides and diamond-like carbon.
Near-field scanning optical microscopy of polarization bistable laser diodes
Authors: Lienau Ch. , Richter A. , Klehr A. , Elsaesser T.
Published: July 2002

TE/TM polarization bistability in a λ=1.3 μm ridge-waveguide InGaAsP/InP bulk laser is studied by near-field scanning optical microscopy with an optical resolution of better than λ/8. The near-field mode profiles of TE and TM emission show different lateral widths and distinctly different mode center positions. This lateral shift is related to a nonuniform strain distribution along the active layer. Based on this strain gradient, we present a model that accounts for the hysteresislike current dependence of the polarization resolved laser output.
Uniform deposition of YBa2Cu3O7 thin films over an 8 inch diameter area by a 90° off-axis sputtering technique
Authors: Rao R. A. , Gan Q. , Eom C. B. , Suzuki Y. , McDaniel A. A. , Hsu J. W. P.
Published: July 2002

The uniform deposition of YBa2Cu3O7 (YBCO) thin films over an 8-in.-diam. area, using a 3-in.-diam. sputtering target and optimized substrate rotation in a single target 90° off-axis sputtering technique, is reported. Two dimensional maps of the thickness profile of YBCO films deposited on a stationary substrate have been obtained using surface profilometry. These thicknesses were used in a computer simulation to predict which distance of the target from the center of the substrate rotation will produce the maximum area with uniform thickness. The films deposited on substrates mounted on a rotating arm displayed uniform thickness (<±5% variation) and composition (<2.3% deviation from the target stoichiometry) and a consistently high transition temperature (Tc>87.5° K) and critical current density (Jc 4.2 K>2×107 A/cm2) over an 8-in.-diam area.
A general approach to the epitaxial growth of rare-earth-transition-metal films
Authors: Fullerton Eric E. , Sowers C. H. , Pearson J. P. , Bader S. D. , Wu X. Z. , Lederman D.
Published: July 2002

The growth of epitaxial rare-earth-transition-metal thin films is reported by magnetron sputtering on single-crystal MgO substrates. The use of epitaxial W buffer layers demonstrates a general approach to control the phase and orientation of the films. Structure and magnetism results for SmFe12(001) on W(100) and magnetically hard Sm2Co7 (110) and (001) on W(100) and (110), respectively, are highlighted to illustrate the utility of the approach.
On the electronic interaction between additives and semiconducting oxide gas sensors
Authors: Vlachos D. S. , Papadopoulos C. A. , Avaritsiotis J. N.
Published: July 2002

A model for the electronic interaction between additives and semiconducting oxide gas sensors is presented. The model is based on the depletion layer that is created due to the metal–semiconductor contact forced by the presence of a metallic additive. This depletion layer corresponds to an active grain size that is smaller than the geometrical one. Thus, the work function of the metal is connected to the change of sensor characteristics. Moreover, the dependence of the sensitivity on the amount of the deposited additive is explained on the same basis. Experimental results are presented in the case of tin oxide thin film gas sensors with Pd, Pt, and Ni in the presence of zero grade air and carbon monoxide. The experimental results are in excellent qualitative agreement with the proposed model.
Amphoteric properties of substitutional carbon impurity in GaN and AlN
Authors: Bogusl/awski P. , Briggs E. L. , Bernholc J.
Published: July 2002

Electronic structure and formation energies of substitutional carbon impurities in hexagonal GaN and AlN were studied by quantum molecular dynamics. Substitutions on both cation and anion sites were considered. Ccation is a shallow donor, while CN is a shallow acceptor. A DX-like configuration is metastable for CGa and CAl0, and stable for CAl−. The solubility of carbon is excellent, but it is accompanied by efficient self-compensation of carbon acceptors by carbon donors. This is due to two factors: (i) the large energy gain induced by electron transfer from Ccation to CN, which is close to the band gap energy, and (ii) the large binding energy (∼1 eV) of nearest neighbor Ccation+-CN− pairs.
Electronic structure of the ferroelectric layered perovskite SrBi2Ta2O9
Authors: Robertson J. , Chen C. W. , Warren W. L. , Gutleben C. D.
Published: July 2002

The band structure of the layered perovskite SrBi2Ta2O9 (SBT) was calculated by tight binding and the valence band density of states was measured by x-ray photoemission spectroscopy. We find both the valence and conduction band edges to consist of states primarily derived from the Bi–O layer rather than the perovskite Sr–Ta–O blocks. The valence band maximum arises from O p and some Bi s states, while the conduction band minimum consists of Bi p states, with a wide band gap of 5.1 eV. It is argued that the Bi–O layers largely control the electronic response whereas the ferroelectric response originates mainly from the perovskite Sr–Ta–O block. Bi and Ta centered traps are calculated to be shallow, which may account in part for its excellent fatigue properties.
Excess carrier lifetime of 3C–SiC measured by the microwave photoconductivity decay method
Authors: Ichimura M. , Tajiri H. , Morita Y. , Yamada N. , Usami A.
Published: July 2002

Excess carrier lifetime of 3C–SiC grown on a Si substrate by chemical vapor deposition is measured at room temperature by the noncontact microwave photoconductivity decay method. A N2 laser is used to excite carriers in the SiC layer. The measured decay curves of the excess carrier concentration have fast (τ ≈3 μs) and slow (τ >200 μs) components. The origin of the slow decay is discussed on the basis of the numerical simulation of the recombination process, and the presence of traps with a very small electron capture cross section (<1×10−21 cm2) is predicted.
Violet/blue emission from epitaxial cerium oxide films on silicon substrates
Authors: Morshed A. H. , Moussa M. E. , Bedair S. M. , Leonard R. , Liu S. X. , El-Masry N.
Published: July 2002

Violet/blue photoluminescence was observed from epitaxial cerium oxide films on silicon substrates. The films were deposited on silicon (111) substrates under ultrahigh vacuum conditions using pulsed laser ablation of a cerium oxide target and treated by rapid thermal annealing in argon. High resolution transmission electron microscopy and x-ray diffraction measurements indicated the formation of a single crystal cerium oxide phase Ce6O11 different from CeO2 in the annealed films. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide.
Large third-order optical nonlinearity in Au:SiO2 composite films near the percolation threshold
Authors: Liao H. B. , Xiao R. F. , Fu J. S. , Yu P. , Wong G. K. L. , Sheng Ping
Published: July 2002

Very large third-order optical nonlinearity, χ(3)∼2.5×10−6 esu, measured by a degenerate four wave mixing method using a short pulse (70 picosecond) laser, has been found in the rapid-thermal annealed Au:SiO2 composite films at concentrations below the Au percolation threshold. The dependence of the χ(3) on Au concentration, p, follows a cubic power law. The maximum figure of merit, χ(3)/α (with α being the absorption coefficient) is about 10−11 esu cm. We explain this result as due to local field enhancement arising from the Mie resonance of the Au nanoclusters, with strong interaction between the nanoclusters further promoting the effect.
Roughness improvement and hardness enhancement in nanoscale Al/AlN multilayered thin films
Authors: Wang X. , Kolitsch A. , Möller W.
Published: July 2002

Al/AlN multilayered thin films with periodic thickness λ less than 24 nm were developed by ion beam assisted deposition. A considerably small surface roughness comparable to that of the silicon substrate and much smaller than those of both monolithic Al and AlN films was obtained. Over the investigated range of λ, all the multilayers are harder than the homogeneous AlN film, and a significant hardness enhancement by a factor of ∼2 over that of the AlN film was observed in the multilayer with λ of 6 nm. Moreover, the hardness enhancement is not at the expense of the multilayer toughness, with the multilayer Al/AlN films showing improved plasticity as compared with the AlN film.
Surface resistance imaging with a scanning near-field microwave microscope
Authors: Steinhauer D. E. , Vlahacos C. P. , Dutta S. K. , Wellstood F. C. , Anlage Steven M.
Published: July 2002

We describe near-field imaging of sample sheet resistance via frequency shifts in a resonant coaxial scanning microwave microscope. The frequency shifts are related to local sample properties, such as surface resistance and dielectric constant. We use a feedback circuit to track a given resonant frequency, allowing measurements with a sensitivity to frequency shifts as small as two parts in 106 for a 30 ms sampling time. The frequency shifts can be converted to sheet resistance based on a simple model of the system.
Piezoelectrical shear-force control on soft biological samples in aqueous solution
Authors: Brunner R. , Hering O. , Marti O. , Hollricher O.
Published: July 2002

In order to apply scanning near-field optical microscopy to life science, it is essential to have an accurate distance feedback that also works on soft biological samples in liquids. In this letter, we report measurements of neuron cells in aqueous solution using an advanced piezoelectrical shear-force detection setup. Simultaneously obtained topographical and fluorescence images are presented, demonstrating a resolution below 100 nm in the optical image. The influence of the water level on the shear-force signal and the interaction between near-field probe and soft organic samples are discussed. Stable feedback in fluids is obtained with tip–sample interaction forces below 100 pN.
Electron transport properties of double delta-doped GaAs structures grown by low-pressure metalorganic chemical vapor deposition
Authors: Gurtovoi V. L. , Valyaev V. V. , Shapoval S. Yu. , Pustovit A. N.
Published: July 2002

Electron transport properties of double delta-doped GaAs structures based on high electron mobility single delta-doped layers with a sheet concentration of 3×1012 cm−2 have been investigated for spacer thicknesses up to 1100 Å at 77 and 300 K. At an optimized spacer thickness of about 200 Å, a maximum in conductivity is observed, which exceeds the conductivity of a single delta-doped layer with the same total concentration by 30% and 20% at 300 and 77 K. The mobility and concentration as a function of spacer thickness are also presented and analyzed.
Surface morphology and surface p-channel field effect transistor on the heteroepitaxial diamond deposited on inclined β-SiC(001) surfaces
Authors: Kawarada H. , Wild C. , Herres N. , Koidl P. , Mizuochi Y. , Hokazono A. , Nagasawa H.
Published: July 2002

The effect of an inclined substrate on heteroepitaxial diamond has been investigated on 4° off β-SiC(001) tilted around the [1̄10] axis. Homogeneous macro steps with (001) terraces are observed in the [1̄10] direction forming a vicinal angle of 3°–4° from the (001) surface reflecting the substrate inclination. The selective growth is effective even if the dominant orientation is inclined by several degrees from the surface normal. The tilt deviation is less than 1° in the heteroepitaxial film. p-channel field effect transistors have been fabricated on these heteroepitaxial films. The device performance is as good as that on homoepitaxial films.
Spontaneous emission from fluorescent molecules embedded in photonic crystals consisting of polystyrene microspheres
Authors: Yamasaki Takashi , Tsutsui Tetsuo
Published: July 2002

Spontaneous emission from dye molecules embedded in periodic dielectric structures was investigated. The structures consist of close-packed arrays of dye-doped polystyrene microspheres. Periodic arrays of polystyrene spheres with a submicrometer diameter were shown to provide a nonoverlapping gap or pseudogap in the visible spectral region. The modification of the spontaneous emission from inside the pseudogap structure was observed as a deep dip in the photoluminescence spectra. The alteration in the emission spectra is explained in terms of the partial suppression of radiation modes due to the periodic dielectric structures.
Electrical properties of semiconductive Nb-doped BaTiO3 thin films prepared by metal–organic chemical-vapor deposition
Authors: Nagano Daisuke , Funakubo Hiroshi , Shinozaki Kazuo , Mizutani Nobuyasu
Published: July 2002

Epitaxially grown semiconductive Nb-doped BaTiO3 thin films with low electrical resistivity similar to that of the bulk single crystal were prepared by metal–organic chemical-vapor deposition. Thin films with 1.5–7.5 at. % Nb content showed n-type semiconductor character. The films with 5.7 at. % Nb content showed the lowest resistivity, 2.8×10−2 Ω cm. This value is three orders of magnitude lower than those reported for sintered of Nb-doped BaTiO3, and similar to that of Nb-doped BaTiO3 single crystals.