bluuebunny/crossref_metadata_2025 huggingface dataset:
Picosecond excitonic absorption recovery of 100 nm GaAs/AlGaAs narrow multiple quantum-well wires
Authors: Tackeuchi Atsushi , Kitada Hideki , Arimoto Hiroshi , Sugiyama Yoshihiro , Endoh Akira , Nakata Yoshiaki , Inata Tsuguo , Muto Shunichi
Published: July 2002

We report the time-resolved absorption measurement of narrow multiple quantum-well (MQW) wires to investigate their fast recoveries from excitonic absorption bleaching. Wires down to 130 nm were fabricated from MQWs using focused ion beam lithography and electron cyclotron-resonance chlorine-plasma etching. In this structure, the photoexcited carriers diffuse toward the sidewalls and recombine on the surface of the sidewalls. We show that the strong optical nonlinearity of excitons is preserved, even in wires of 130 nm width, and having a fast recovery time in the picosecond region. We also briefly discuss the possibility of making quantum wires which have a faster recovery time and larger optical nonlinearity.
Correlation between silicon hydride species and the photoluminescence intensity of porous silicon
Authors: Tsai C. , Li K.-H. , Kinosky D. S. , Qian R.-Z. , Hsu T.-C. , Irby J. T. , Banerjee S. K. , Tasch A. F. , Campbell Joe C. , Hance B. K. , White J. M.
Published: July 2002

The role of silicon hydride species in the photoluminescence intensity behavior of porous Si has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH2) coincides with an increase in the photoluminescence intensity.
Porous silicon photoluminescence versus HF etching: No correlation with surface hydrogen species
Authors: Robinson M. B. , Dillon A. C. , George S. M.
Published: July 2002

The photoluminescence (PL) and infrared absorbance of electrochemically anodized, initially nonphotoluminescent, porous silicon samples were examined as a function of hydrofluoric acid (HF) etching time. Transmission Fourier transform infrared spectroscopy measurements revealed that the infrared absorbance from silicon hydrogen surface species immediately decreased with HF etching. In contrast, the PL did not appear until after HF etching times of 20–80 min, depending on initial sample porosity. Subsequently, the PL intensity increased, reached a maximum, and then progressively decreased versus HF etching time. These HF etching results demonstrate that there is no direct correlation between the PL and the silicon hydrogen surface species.
Enhancement and stabilization of porous silicon photoluminescence by oxygen incorporation with a remote-plasma treatment
Authors: Xiao Y. , Heben M. J. , McCullough J. M. , Tsuo Y. S. , Pankove J. I. , Deb S. K.
Published: July 2002

We report a treatment that enhances and stabilizes the photoluminescence (PL) from porous Si films. Films prepared by anodization in a 50% HF/ethanol solution were annealed at 450 °C in vacuum, exposed to air, and then exposed to a remote-hydrogen plasma. Infrared absorption spectroscopy revealed that the concentration of oxygen, rather than hydrogen, was increased by the processing steps, and that silicon dihydride species had been eliminated from the surface. The PL from a treated film was initially ∼30 times more intense than from the as-etched films. The PL intensity increased with illumination time in air until a steady-state intensity was reached.
Regulation of a microcantilever response by force feedback
Authors: Mertz J. , Marti O. , Mlynek J.
Published: July 2002

A feedback mechanism is used to control the forces incident on a mechanical microcantilever as a function of the monitored cantilever motion. The control is effected by modifying the intensity of an auxiliary laser beam that generates a thermally induced stress. The feedback is designed to reduce the effective resonance quality factor of the cantilever. The resultant regulation of the cantilever motion is shown to improve the measurement dynamics in atomic force microscopy, without significantly degrading the signal to noise ratio.
A quantum-dot refrigerator
Authors: Edwards H. L. , Niu Q. , de Lozanne A. L.
Published: July 2002

A cryogenic refrigeration scheme is presented which utilizes the discrete energy levels of quantum dots to customize the electronic Fermi–Dirac distribution, cooling a small but macroscopic reservoir to far below the ambient temperature. Several physical limitations of this scheme are discussed within the context of a model device constructed from a two-dimensional electron gas. These are explored in a simple example where, at an ambient operating temperature of 150 mK, this quantum-dot refrigerator can achieve a cooling power of 1 μW/cm2, a base temperature of 10 mK, and a cooling efficiency of 35%.
Highly efficient Cherenkov-type second harmonic generation in a Ta2O5/KTiOPO4 waveguide
Authors: Doumuki Tohru , Tamada Hitoshi , Saitoh Masaki
Published: July 2002

Highly efficient Cherenkov-type second harmonic generation is investigated. The waveguide is composed of a KTiOPO4 substrate, a Ta2O5 thin-film core and a SiO2 clad in the strip-loaded configuration. The conversion efficiency strongly depends on the waveguide geometry, and it can be a maximum for a particular Ta2O5 film thickness with a Cherenkov angle of nearly 0°. This maximum efficiency is further increased by narrowing the waveguide width down to 1.8 μm. Consequently, a normalized conversion efficiency of 210%/W cm and a blue second harmonic power of 28 mW were achieved with the 4.5-mm-long waveguide.
Giant surface acoustic waves generated by a multiple beam laser: Application to the detection of surface breaking slots
Authors: Chenu C. , Noroy M.-H. , Royer D.
Published: July 2002

Giant surface acoustic wave (SAW) pulses have been generated by implementing on the surface of a solid an array of strip thermoelastic sources. It is shown experimentally, with a 16-beam pulsed YAG laser, that the delays introduced between the laser emission times allow the constructive summation of the SAW pulses. Mechanical displacements larger than 20 nm on a steel sample and 50 nm on aluminum were detected by a heterodyne optical interferometer. Examples of noncontact and nondestructive detection of thin surface breaking slots are presented.
Native defect related optical properties of ZnGeP2
Authors: Dietz N. , Tsveybak I. , Ruderman W. , Wood G. , Bachmann K. J.
Published: July 2002

We present photoluminescence, photoconductivity, and optical absorption spectra for ZnGeP2 crystals grown from the melt by gradient freezing and from the vapor phase by high pressure physical vapor transport (HPVT). A model of donor and acceptor related subbands in the energy gap of ZnGeP2 is introduced that explains the experimental results. The emission with peak position at 1.2 eV is attributed to residual disorder on the cation sublattice. The lower absorption upon annealing is interpreted in terms of both the reduction of the disorder on the cation sublattice and changes in the Fermi level position. The n-type conductivity of ZnGeP2 crystals grown under Ge-deficient conditions by the HPVT is related to the presence of additional donor states.
Photoinduced electron paramagnetic resonance of the phosphorus vacancy in ZnGeP2
Authors: Giles N. C. , Halliburton L. E. , Schunemann P. G. , Pollak T. M.
Published: July 2002

The electron paramagnetic resonance (EPR) spectrum of the neutral phosphorus vacancy has been observed in as-grown ZnGeP2 during illumination at liquid-helium temperatures. Without illumination, this donor is a nonparamagnetic singly ionized phosphorus vacancy (VP+ center). Either above-band-gap light (514.5 nm) or below-band-gap light (632.8 nm) can produce the paramagnetic neutral state (VP0 center) of the donor. Principal values of the g matrix for the neutral donor are 1.944, 2.046, and 2.223. The angular dependence of the EPR spectrum suggests that the unpaired spin is unequally shared by two of the zinc ions neighboring the phosphorus vacancy. These phosphorus vacancies are the dominant donor in this highly compensated material, while the previously reported zinc vacancies are the dominant acceptor.
Nanofabrication of grating and dot patterns by electron holographic lithography
Authors: Ogai Keiko , Kimura Yoshihide , Shimizu Ryuichi , Fujita Junichi , Matsui Shinji
Published: July 2002

Line and dot patterns with about a 100 nm period are exposed on poly-methyl methacrylate (PMMA) resist by electron holographic lithography with a W〈100〉 thermal field emission (TFE) gun and an electron biprism. Subsequent atomic force microscope (AFM) observation has confirmed that both patterns are successfully fabricated. This technique allows one to produce nanoscale periodic patterns simultaneously, whose spacing could be, in principle, comparable to a lattice spacing.
20-l explosibility test chamber for dusts and gases
Authors: Cashdollar Kenneth L. , Hertzberg Martin
Published: July 2002

The Bureau of Mines has designed a 20-l test chamber for the explosibility testing of dusts, gases, and their mixtures. It can be used to measure lean and rich limits of flammability, explosion pressures and rates of pressure rise, minimum ignition energies, minimum oxygen concentrations for flammability, and amounts of inhibitor necessary to prevent explosions. The 20-l chamber can be used at initial pressures that are below, at, or above atmospheric as long as the maximum explosion pressure is less than 21 bar, which is the rated pressure of the chamber. The chamber instrumentation includes a pressure transducer, optical dust probes, an oxygen sensor, and multichannel infrared pyrometers. Ignition sources used include electric sparks and electrically activated chemical ignitors. Examples of the various types of data that can be obtained for dusts and gases are shown.
Ultrahigh pressures
Authors: Jayaraman A.
Published: July 2002

This article reviews mainly the use of the diamond anvil cell (DAC) for ultrahigh-pressure generation (a few hundred GPa) and the various techniques employed in studying the high-pressure behavior of solids. A brief historical introduction to devices used in the prediamond-anvil era is presented in Sec. I. The basic principles of the modern DAC, ultrahigh-pressure generation, and pressure calibration are presented. Among the techniques used, x-ray diffraction, optical spectroscopy including Raman and Brillouin spectroscopy, microscopy, electrical resistance,and Mössbauer measurements, and positron annihilation studies with the DAC are reviewed. High-temperature–high-pressure (HT–HP) and low-temperature–high-pressure (LT–HP) generation in the DAC, and the problems associated with pressure calibration under these conditions are discussed. A brief section is devoted to the sintered diamond-tipped anvil apparatus, for it offers a very convenient way of studying resistance changes and superconductivity to 50-GPa pressure at normal and at liquid-He temperatures. In Sec. IV, Raman studies on solid H2 and solid N2 to about 150 GPa, p-v studies on Xe, CsI to 50 GPa, the metallization of CsI, and superconductivity of Si high-pressure polymorphs are presented. Present trends and future possibilities for ultrahigh-pressure research are briefly set out in Sec. V.
Single-tube three-dimensional scanner for scanning tunneling microscopy
Authors: Binnig G. , Smith D. P. E.
Published: July 2002

We report a new type of three-dimensional mechanical scanner fabricated from a single piezoelectric tube. It has a typical response of 5 nm/V in each orthogonal direction and mechanical resonances at 8 kHz (bending perpendicular to the tube axis) and 40 kHz (motion parallel to the tube axis). When used in a scanning tunneling microscope it is the higher frequency mode which is most critical since it corresponds to motion perpendicular to the sample surface. We show an image of the atomic surface of graphite taken in air using a tube scanner incorporated into a scanning tunneling microscope. The tube scanner allows the development of smaller, simpler, and faster scanning tunneling microscopes.
Measurements of radial profiles of the ion temperature and the plasma rotation velocity with the TFTR vertical x-ray crystal spectrometer
Authors: Bitter M. , Hsuan H. , Rice J. E. , Hill K. W. , Diesso M. , Grek B. , Hulse R. , Johnson D. W. , Johnson L. C. , von Goeler S.
Published: July 2002

The TFTR vertical x-ray crystal spectrometer has now been operating with three crystals and position-sensitive detectors according to the original design specifications. The observed spectra of heliumlike Ti xxi, Cr xxiii, Fe xxv, and Ni xxvii have permitted a detailed comparison with the predictions from atomic theories, and they have provided data on the radial profiles of the ion temperature and toroidal rotation velocity, as well as the radial ion charge-state distribution in TFTR discharges. Central ion temperatures of 20 keV and central plasma rotation velocities of 5×105 m/s have been recorded from plasmas with auxiliary neutral beam heating. These experimental results are presented. Also discussed are further instrumental improvements, such as the installation of two additional crystals and detectors and the installation of γ and neutron shielding, which will make it possible to measure under full DD and DT operation with 27 MW of neutral beam injection where neutron production rates of 1019 neutrons/s are expected.
An instrument for measuring circular dichroism simultaneously at all wavelengths in a limited spectral range
Authors: Andert K. , Schälike W. , Nölting B. , Pittelkow R. , Wetzel R. , Snatzke G.
Published: July 2002

An instrument is described for the simultaneous measurement of circular dichroism at all wavelengths in a limited spectral range. A polychromator and a charge-coupled device (CCD), serving as multichannel sensor, are arranged behind the sample cell, which is located close to the entrance slit, in contrast to the arrangement of a monochromator before the cell and using a photomultiplier as radiation detector, as usual until now. The CCD with low-noise electronics is driven by the system clock of a microprocessing unit controlled by a quartz oscillator and works fully synchronously with modulation and acquisition cycles. This leads to a high suppression of noise and systematic deviations. An electro-optic modulator with approximately rectangular excitation voltage is used. Partial CD spectra over the range of 80 nm each down to 200 nm have been recorded. The detection of a smaller amount of substance is possible than with other modern commercial instruments such as a JASCO J-600, with the same signal-to-noise ratio.
Optimization of signal-to-noise ratios in pump-probe spectroscopy
Authors: Moon John A.
Published: July 2002

Analytic expressions for the signal-to-noise ratio in time resolved pump-probe spectroscopy experiments are presented for two common signal averaging schemes in the presence of strongly correlated laser noise. The case of modulated pump/synchronous probe demodulation (‘‘slow scanning’’) is compared to the case of repetitively averaging many scans together (‘‘fast scanning’’) without pump modulation. Data are presented from a pump-probe experiment on an amorphous semiconductor thin film which agrees with the simple model developed in this article. The model can be used to predict the optimal scan frequency if the laser noise power spectrum, the magnitude of the signal, and the noise subtraction efficiency are known.
Two wire thermocouple: Frequency response in constant flow
Authors: Forney L. J. , Fralick G. C.
Published: July 2002

Experimental measurements are made with a novel two wire thermocouple. Signals from two wires of unequal diameters are recorded from the thermocouple suspended in constant flow with a periodic temperature fluctuation. It is demonstrated that the reconstructed signal from the two wire thermocouple requires no compensation for ω≤2ω1 where ω1 is the natural frequency of the smaller wire. A compensation factor is recommended for larger frequencies ω≳2ω1.
Mechanical strength of sapphire windows under pressure
Authors: Chervin J. C. , Syfosse G. , Besson J. M.
Published: July 2002

Systematic tests of failure have been performed up to 700 MPa on 75 cylindrical sapphire windows of various thicknesses in the Poulter unsupported area geometry. Three distinct failure patterns are found to occur. The influence on the bursting pressure of the c-axis orientation, annealing, and fatigue are reported and discussed. Maximum working pressures are determined, as a function of the ratios of thickness and unsupported area diameter, to the window’s diameter.
High power gain-switched diode laser master oscillator and amplifier
Authors: Poelker M.
Published: July 2002

A tapered-stripe, traveling-wave semiconductor optical amplifier was seeded with 3.3 mW of gain-switched diode laser light to obtain over 200 mW average power with pulse widths≊105 ps full width at half-maximum (FWHM) and a pulse repetition rate of 499 MHz corresponding to a peak power of 3.8 W. Shorter pulse widths were obtained when the amplifier was driven with less current at the expense of reduced output power. Pulse widths as short as 31 ps FWHM and an average power of 98 mW corresponding to a peak power of 6.3 W were obtained when a different, lower power seed laser was used.
Elimination of loading reverberation in the split Hopkinson torsional bar
Authors: Xue Q. , Shen L. T. , Bai Y. L.
Published: July 2002

The loading reverberation is a multiple wave effect on the specimen in the split Hopkinson torsional bar (SHTB). Its existence intensively destroys the microstructure pattern in the tested material, and therefore, interferes with the study correlating the deformed microstructure to the macroscopic stress–strain response. This paper discusses the problem of the loading reverberation and its effects on the post-mortem observations in the SHTB experiment. The cause of the loading reverberation is illustrated by a stress wave analysis. The modification of the standard SHTB is introduced, which involves attaching two unloading bars at the two ends of the original main bar system and adopting a new loading head and a couple of specially designed clutches. The clutches are placed between the main bar system and the unloading bars in order to lead the secondary loading wave out of the main bar system and to cut off the connection in a timely manner. The loading head of the standard torsional bar was redesigned by using a tube-type loading device associated with a ratchet system to ensure the exclusion of the reflected wave. Thus, the secondary loading waves were wholly trapped in the two unloading bars. The wave recording results and the contrasting experiments for examining the post-mortem microstructure during shear banding both before and after the modification highly support the effectiveness of the modified version. The modified SHTB realizes a single wave pulse loading process and will become a useful tool for investigating the relation between the deformed microstructure and the macroscopic stress–strain response. It will play an important role especially in the study of the evolution of the microstructure during the shear banding process.
Inversion technique to obtain an emissivity profile from tangential line-integrated hard x-ray measurements
Authors: Bell Ronald E.
Published: July 2002

A matrix inversion technique has been developed to calculate local x-ray emissivity profiles from line-integrated x-ray measurements from an anisotropic source. A 2D hard x-ray camera has been used to measure tangential line-integrated bremsstrahlung emission from suprathermal electrons generated by lower hybrid current drive in the PBX-M tokamak. Local emissivities, E, on the midplane of the tokamak are useful to localize the lower hybrid-driven current. The anisotropy is assumed to be uniform over the plasma with a form E(R,θ)=E(R)cosn(θ), where E(R) is the forward emissivity at the major radius R, θ is the angle to the camera from tangency, and the exponent n defines the degree of anisotropy. For isotropic emission, n=0. The measured brightness B(R) on the midplane is then related to the forward emissivity E(R) by a matrix M=L(R,φ)cosn[θ(R,φ)], where φ is the toroidal angle, L is a length matrix containing the path lengths through the plasma. By inverting the matrix M, the forward emissivity is obtained from the measured brightness, E=M−1B. The anisotropy exponent n is adjusted to match the emission at the inner and outer radii of the plasma.
An endoscopic system for the early detection of cancers of the gastrointestinal tract
Authors: Mckechnie Tracy , Jahan Ali , Tait Iain , Cuschieri Alfred , Sibbett Wilson , Padgett Miles
Published: July 2002

We report a compact endoscopic imaging system suitable for the early detection of cancers throughout the gastrointestinal tract. The system incorporates a three-color camera to provide a conventional color image, a mercury lamp filtered between 360 and 450 nm to excite the photosensitizer, and an intensified camera to image the resulting fluorescence at 630 nm. Our results on over 30 resected specimens show a high optical contrast between the cancerous and healthy tissue.
Electron cyclotron emission diagnostics on the large helical device
Authors: Nagayama Y. , Kawahata K. , England A. , Ito Y. , Bretz N. , McCarthy M. , Taylor G. , Doane J. , Ikezi H. , Edlington T. , Tomas J.
Published: July 2002

The electron cyclotron emission (ECE) diagnostic system is installed on the large helical device (LHD). The system includes the following instruments: a heterodyne radiometer, a Michelson spectrometer, and a grating polychromator. A 63.5 mm corrugated waveguide system is fully utilized. Large collection optics and notch filters at the frequency of the LHD electron cyclotron heating (ECH) were developed for this system. In addition to these filters, the rectangular waveguide notch filters, the ECE measurement with the radiometer has been successfully performed during the ECH.
Measurement of the complex dielectric constant down to helium temperatures. II. Quasioptical technique from 0.03 to 1 THz
Authors: Reedijk J. A. , Martens H. C. F. , Smits B. J. G. , Brom H. B.
Published: July 2002

A quasioptical method is described that allows the determination of the complex dielectric constant almost continuously in the millimeter wave regime without the use of electrical contacts. The technique allows the dielectric properties of bulk samples (solids, powders, and liquids) and thin films (free standing or deposited on a substrate) to be measured with excellent absolute accuracy down to 2 K.
White light emission from a polymer blend light emitting diode
Authors: Granström Magnus , Inganäs Olle
Published: July 2002

A new type of polymer light emitting diodes that emit white light is reported. In these diodes, several electroluminescent substituted polythiophenes have been combined to give the necessary components of the visible spectrum. These emitting polymers are then mixed with an insulating polymer to diminish the energy transfer from high-band-gap polymers to low-band-gap polymers. The resulting emission at 20 V is shown to be close to the equienergy white point as defined by the CIE (Commission Internationale de l’Eclairage).
Minority-carrier-enhanced reactivation of hydrogen-passivated Mg in GaN
Authors: Pearton S. J. , Lee J. W. , Yuan C.
Published: July 2002

The influence of minority-carrier injection on the reactivation of hydrogen passivated Mg in GaN at 175 °C has been investigated in p-n junction diodes. The dissociation of the neutral MgH complexes is greatly enhanced in the presence of minority carrier and the reactivation process follows second-order kinetics. Conventional annealing under zero-bias conditions does not produce Mg-H dissociation until temperatures ≥450 °C. These results provide an explanation for the e-beam-induced reactivation of Mg acceptors in hydrogenated GaN.
Nanotubes in boron nitride laser heated at high pressure
Authors: Golberg D. , Bando Y. , Eremets M. , Takemura K. , Kurashima K. , Yusa H.
Published: July 2002

Here we report on the finding of pure boron nitride (BN) nanotubes that do not contain any additional inclusions and on a new method for their growth: laser heating of boron nitrides at high nitrogen pressure (5–15 GPa). The multiwalled nanotubes were observed using high resolution electron microscopy and were chemically characterized by electron energy loss spectroscopy. The circular or polygonal cross-sectional nanotubes, which have 3–8 shells and a characteristic outer dimension cross section of 3–15 nm, were found to have grown either in melted cubic BN or in hexagonal+amorphous BN that had recrystallized on the specimen’s surface from the fluid phase.
Second harmonic generation in a photonic crystal
Authors: Martorell Jordi , Vilaseca R. , Corbalán R.
Published: July 2002

Phase matched second harmonic generation is observed experimentally in a centrosymmetric crystalline lattice of dielectric spheres of optical dimensions. The inversion symmetry is broken locally at the surface of each sphere in such a way that the scattered second harmonic light interferes constructively leading to a nonvanishing macroscopic field. Phase matching of the fundamental and second harmonic waves in such periodic lattice is observed to be naturally provided by the bending of the photon dispersion curve at the edge of the Bragg reflection band of a given set of lattice planes.
Density and structural changes in SiC after amorphization and annealing
Authors: Heera V. , Prokert F. , Schell N. , Seifarth H. , Fukarek W. , Voelskow M. , Skorupa W.
Published: July 2002

The density of amorphous SiC layers formed by 2 MeV Si+ implantation into single-crystalline 6H–SiC was measured by x-ray reflectometry and compared with the results of step height measurements. Reactive ion etching was used to investigate the density as a function of depth. The density of the as-amorphized SiC is about 12% less than that of the crystalline material. Within experimental accuracy, the density reduction is homogeneous across the whole layer thickness. Low-temperature annealing leads to the formation of relaxed amorphous SiC with a density about 7% below the crystalline one. These large density changes are in contrast to results in amorphous Si. They can be explained by the high atomic density of SiC and the chemical disorder in the amorphous state of SiC.
Synthesis and control of conductivity of ultraviolet transmitting β-Ga2O3 single crystals
Authors: Ueda Naoyuki , Hosono Hideo , Waseda Ryuta , Kawazoe Hiroshi
Published: July 2002

β- Ga 2 O 3 single crystals were grown by the floating zone method and their conductivity along the b axis was controlled from <10−9 to 38 Ω−1 cm−1 by changing the growth atmosphere. By using feed rods doped with Sn, the grown crystal became highly conductive even under oxidative atmosphere. The optical transmission spectra showed that the β-Ga2O3 single crystal with 0.32 mm was transparent in the visible and ultraviolet region, with 20% transmittance at the fourth-harmonic wave of the Nd:YAG laser (266 nm). The band-gap widening was observed with the increasing of the carrier concentration. It is expected that the light of the KrF laser can be transmitted in the heavily doped β-Ga2O3.
Formation and laser-induced-fluorescence study of SiO+ ions produced by laser ablation of Si in oxygen gas
Authors: Matsuo Yukari , Nakajima Takashi , Kobayashi Tohru , Takami Michio
Published: July 2002

We have studied laser-induced-fluorescence (LIF) spectra of SiO+ produced by laser ablation of a Si wafer in oxygen ambient gas. Emission from neutral and ionic species of Si atoms and SiO molecules is also studied in the laser-induced plasma plume. The optimum oxygen pressure for the formation of molecular ions is found to be ∼120 mTorr. The rotational temperature of SiO+ ions shows rapid thermalization toward room temperature within 20 μs.
Domain–wall contribution to the piezoelectric response of epitaxial ferroelectric thin films
Authors: Pertsev N. A. , Emelyanov A. Yu.
Published: July 2002

Domain–wall contribution to the longitudinal piezoelectric coefficient d33 is calculated theoretically for prepolarized tetragonal ferroelectric thin films epitaxially grown on cubic substrates. To that end, translational vibrations of ferroelastic 90° domain walls induced by a weak measuring electric field are analyzed in the typical case of a laminar c/a/c/a domain structure. The average orthogonal displacement of the film free surface caused by collective domain–wall movements is calculated by the method of effective dislocations. On this base the domain–wall contribution Δd33 to the converse piezoelectric effect is evaluated and shown to be substantial in common heterostructures.
Melting point depression of Al clusters generated during the early stages of film growth: Nanocalorimetry measurements
Authors: Lai S. L. , Carlsson J. R. A. , Allen L. H.
Published: July 2002

This work investigates the thermodynamic properties of small structures of Al using an ultrasensitive thin-film differential scanning calorimeter. Al thin films were deposited onto a Si3N4 surface via thermal evaporation over a range of thicknesses from 6 to 50 Å. The Al films were discontinuous and formed nanometer-sized clusters. Calorimetry measurements demonstrated that the melting point of the clusters is lower than the value for bulk Al. We show that the melting point of the clusters is size dependent, decreasing by as much as 140 °C for 2 nm clusters. The results have relevance in several key areas for Al metallization in micro-electronics including the early stages of film growth and texture formation, the Al reflow process, and the dimensional stability of high aspect ratio Al lines.
Alkanethiol self-assembled monolayers as the dielectric of capacitors with nanoscale thickness
Authors: Rampi Maria A. , Schueller Olivier J. A. , Whitesides George M.
Published: July 2002

Alkanethiol self-assembled monolayers (SAMs) on a mercury surface are used to build a junction consisting of two opposing mercury surfaces with interposed SAMs: Hg-SAM/SAM-Hg. The liquid mercury surface provides a support for the SAM that is smooth, compliant, free of defects, and without the incommensurate lattice properties that characterize solid metal surfaces. The thickness of the dielectric (∼30–90 Å) in this junction can be easily changed by using alkanethiols with different lengths. From capacitance measurements, a dielectric constant of 2.7±0.3 is calculated for the SAMs. The conductivity of SAMs on the Hg surface is σ=6±2×10−15 Ω−1 cm−1, a value close to that of bulk polyethylene. The junction sustains an electric field of 6 MV/cm.
Noncontact scanning probe microscope potentiometry of surface charge patches: Origin and interpretation of time-dependent signals
Authors: Cunningham Samia , Larkin Ivan A. , Davis John H.
Published: July 2002

We have modeled the generation of a voltage signal in noncontact potentiometric scanning force microscopy for a locally enhanced potential due to an isolated patch of electrostatic charge on an insulating surface. Both the distribution of the charge and the dielectric nature of the insulator is taken into account. When the charge is assumed to disperse ohmically on the surface, a complex time-dependent decay in the signal occurs. The profile of the decay predicted by this assumption has a shape similar to that observed experimentally in submicron scale contact charging experiments.
Effect of surface roughness on the secondary ion yield in ion sputtering
Authors: Makeev Maxim A. , Barabási Albert-László
Published: July 2002

There is extensive experimental evidence that, at low temperatures, surface erosion by ion bombardment roughens the sputtered substrate, leading to a self-affine surface. These changes in the surface morphology also modify the secondary ion yield. Here, we calculate analytically the secondary ion yield in terms of parameters characterizing the sputtering process and the interface roughness.
Near-field photolithography with a solid immersion lens
Authors: Ghislain L. P. , Elings V. B. , Crozier K. B. , Manalis S. R. , Minne S. C. , Wilder K. , Kino G. S. , Quate C. F.
Published: July 2002

We have exposed 190 nm lines in photoresist by focusing a laser beam (λ=442 nm) in a solid immersion lens (SIL) that is mounted on a flexible cantilever and scanned by a modified commercial atomic force microscope. The scan rate was 1 cm/s, which is several orders of magnitude faster than typical reports of near-field lithography using tapered optical fibers. The enhanced speed is a result of the high optical efficiency (about 10−1) of the SIL. Once exposed with the SIL, the photoresist was developed and the pattern was transferred to the silicon substrate by plasma etching.
Phonon scattering in silicon films with thickness of order 100 nm
Authors: Ju Y. S. , Goodson K. E.
Published: July 2002

Although progress has been made in the ab initio simulation of lattice dynamics in semiconducting crystals, information about the relaxation of nonequilibrium lattice vibrations remains incomplete. This work studies the relaxation times of room-temperature thermal phonons through measurements of thermal conduction along monocrystalline silicon films of thickness down to 74 nm. A repetitive oxidation and etching process ensures that the purity and crystalline quality of the films are comparable with those of bulk samples. Phonon-interface scattering reduces the thermal conductivity by up to 50% at room temperature. The data indicate that the effective mean-free path of the dominant phonons at room temperature is close to 300 nm and thus much longer than the value of 43 nm predicted when phonon dispersion is neglected. This study indicates that a broad variety of lattice transport characteristics for bulk silicon can be obtained through measurements on carefully prepared silicon nanostructures. The present data are also valuable for the thermal simulation of silicon-on-insulator (SOI) transistors.
Depth and thermal stability of dry etch damage in GaN Schottky diodes
Authors: Cao X. A. , Cho H. , Pearton S. J. , Dang G. T. , Zhang A. P. , Ren F. , Shul R. J. , Zhang L. , Hickman R. , Van Hove J. M.
Published: July 2002

GaN Schottky diodes were exposed to N2 or H2 inductively coupled plasmas prior to deposition of the rectifying contact. Subsequent annealing, wet photochemical etching, or (NH4)2S surface passivation treatments were examined for their effect on diode current–voltage (I–V) characteristics. We found that either annealing at 750 °C under N2, or removal of ∼500–600 Å of the surface essentially restored the initial I–V characteristics. There was no measurable improvement in the plasma-exposed diode behavior with (NH4)2S treatments.
Influence of columnar defects on surface resistance of YBa2Cu3Ox superconducting thin films; nonlinear effects
Authors: Wosik J. , Xie L.-M. , Mazierska J. , Grabovickic R.
Published: July 2002

We report on measurements of surface resistance (Rs) of YBa2Cu3Ox thin films irradiated with 0.9 GeV Pb ions. Measurements of Rs versus microwave-frequency (rf) magnetic field and temperature were carried out using a dielectric resonator technique. At low temperatures, no significant difference was observed between surface resistance of irradiated and nonirradiated films. However, at higher temperatures the irradiated films exhibited not only higher Rs but also a nonlinear dependence of Rs on rf magnetic field. A simple model for microwave losses in thin superconducting films induced by columnar defects is proposed. Using this model, the observed changes in Rs have been attributed to additional losses and/or heating effects in the insulator-superconductor boundary between an amorphous core of the columnar defects (several nanometers in diameter) and the surrounding superconducting material.
Infrared extinction properties of gold nanoshells
Authors: Oldenburg S. J. , Jackson J. B. , Westcott S. L. , Halas N. J.
Published: July 2002

Gold nanoshells, nanoparticles consisting of a silica core coated with a thin gold shell, exhibit a strong optical resonance that depends sensitively on their core radius and shell thickness. Gold nanoshells have been fabricated with a peak optical extinction that can be varied across the near-infrared region of the spectrum (800 nm–2.2 μm). Multipolar plasmon resonances are clearly resolvable in the extinction spectra and agree well with electromagnetic theory. Additional resonances due to particle aggregation are also observed. The frequency agile infrared properties of these nanoparticles make them particularly attractive for a range of technologically important applications.
Effects of surface resonance state on the plasmon resonance absorption of Ag nanoparticles embedded in partially oxidized amorphous Si matrix
Authors: Yang L. , Li G. H. , Zhang L. D.
Published: July 2002

Nanocomposite films consisting of nanosized Ag particles embedded in partially oxidized amorphous Si matrices were prepared by radio frequency cosputtering deposition. Subsequent heat treatment at different temperatures ranging from 200 to 500 °C and its effects on the optical absorption of the films were investigated by spectrometry in the wavelength ranging from 200 to 1200 nm. “Surface resonance state” is introduced to discuss the broadening, blue-shift, and intensity decrease of the plasmon resonance absorption peak with decreasing Ag particle size.
High-resolution micromachined interferometric accelerometer
Authors: Cooper E. B. , Post E. R. , Griffith S. , Levitan J. , Manalis S. R. , Schmidt M. A. , Quate C. F.
Published: July 2002

We demonstrate a promising type of microfabricated accelerometer that is based on the optical interferometer. The interferometer consists of surface-micromachined interdigital fingers that are alternately attached to a proof mass and support substrate. Illuminating the fingers with coherent light generates a series of diffracted optical beams. Subangstrom displacements between the proof mass and frame are detected by measuring the intensity of a diffracted beam. The structure is fabricated with a two-mask silicon process and detected with a standard laser diode and photodetector. We estimate that the minimum detectable acceleration is six orders of magnitude below the acceleration of gravity, i.e., 2 μg/Hz in a 1 Hz bandwidth centered at 650 Hz.
An analytical solution of the stochastic master equation for reversible bimolecular reaction kinetics
Authors: Laurenzi Ian J.
Published: July 2002

The kinetics of the binding reaction A+B⇄C are solved exactly via the stochastic master equation, in which molecular populations are considered as time-dependent, integer-valued random variables. This transient solution extends previous work on the irreversible bimolecular reaction and the equilibrium state of the reversible bimolecular reaction. For small ensembles of reactants, comparisons are made between the results of the deterministic and stochastic approaches to the problem, and methods are presented to numerically evaluate the solution.
Ultraviolet mini-Raman lidar for stand-off, in situ identification of chemical surface contaminants
Authors: Ray Mark D. , Sedlacek Arthur J. , Wu Ming
Published: July 2002

The Mini-Raman Lidar System (MRLS) is a portable chemical sensor that combines the spectral fingerprinting of Raman spectroscopy with the principles of solar-blind ultraviolet lidar for short-range, noncontact detection and identification of unknown substances on surfaces. The MRLS has the potential to detect contaminant films several microns thick at distances of meters and bulk quantities of substances at distances of tens of meters. The signal acquisition time is less than 1 min. The device has application to those involved in emergency response, environmental remediation, and military reconnaissance who respond initially at the site of a chemical spill or attack.
Effect of preferential concentration on turbulent collision rates
Authors: Reade Walter C. , Collins Lance R.
Published: July 2002

The effect of particle inertia on the interparticle collision rates of a turbulent aerosol was investigated recently by Sundaram and Collins (1997) using direct numerical simulation (DNS). They observed that for values of the particle Stokes number (here defined as the ratio of the particle response time to Kolmogorov time scale) near unity, the collision frequency was enhanced by between one and two orders of magnitude. This enhancement was attributed in part to the local enrichment of the particle concentration in low-vorticity regions of the flow due to the centrifuge effect commonly referred to as preferential concentration (Eaton and Fessler 1994). Sundaram and Collins (1997) showed that the correction factor for the collision kernel in a preferentially concentrated system is g(σ), where g(r) is the particle radial distribution function and σ is the collision diameter. This paper uses DNS, in combination with statistical analysis, to study the dependence of the radial distribution function on the turbulence and particle parameters. A curve fit of the results over a broad range of the relevant dimensionless parameters enables easy estimation of g(σ). The effect of system Reynolds number over the limited range accessible by DNS is also presented. In general, the degree of preferential concentration increases with increasing Reynolds number.
Localization of nonlinear waves in layered media
Authors: Gerasimchuk I. V. , Kovalev A. S.
Published: July 2002

The localization of nonlinear waves propagating in an anharmonic medium along a system of two identical plane-parallel defects (waveguides) is investigated in a simple model describing the nonlinear dynamics of layered media (magnetically ordered, elastic, and optical). A method of analytical investigation of this problem is proposed which reduces to a model of coupled anharmonic oscillators whose parameters are all determined on microscopic considerations. The results yield an adequate description of the nonlinear dynamics of layered media in the framework of well-studied discrete models of nonlinear mechanics. It is shown that when the total energy of the wave exceeds a threshold value, the state with equal energy fluxes localized near the waveguides becomes unstable, and an inhomogeneous regime is realized in which the wave flux propagates mainly along one of the plane layers.
Observation of magnetization reversal of thin-film permalloy nanostructures using ballistic electron magnetic microscopy
Authors: Rippard W. H. , Perrella A. C. , Chalsani P. , Albert F. J. , Katine J. A. , Buhrman R. A.
Published: July 2002

We have imaged the magnetization reversal process of thin-film permalloy (Ni80Fe20) nanostructures using ballistic electron magnetic microscopy. Their switching behavior is often influenced by the formation of end domains and the subsequent domain-wall propagation under application of a magnetic field. Occasionally, this process leads to the formation of a 360° domain wall that is stable in fields which would otherwise switch the structure. The resulting state of the nanostructure in zero-applied field is very different from the near-single-domain state typically observed. The magnetization of the structure can show abrupt changes in a fixed magnetic field.
Sulfur: A potential donor in diamond
Authors: Saada D. , Adler Joan , Kalish R.
Published: July 2002

We performed first-principle calculations of substitutional sulfur in diamond, in the neutral (S0) and charged states. The energy levels induced by sulfur in diamond are calculated to be 0.15 and 0.5 eV from the bottom of the conduction band, for S0 and the singly ionized state S+, respectively. The formation energy for the neutral state of sulfur is found to be 7.2 eV, lower than that of phosphorus in diamond. The most likely state of sulfur in diamond is found to be the doubly ionized state S++, which cannot act as a donor. However, a small fraction of sulfur can be found in the singly ionized state S+, which can donate an electron for conduction at reasonable temperatures.
New Equation of State for Ethylene Covering the Fluid Region for Temperatures From the Melting Line to 450 K at Pressures up to 300 MPa
Authors: Smukala J. , Span Roland , Wagner Wolfgang
Published: July 2002

This work reviews the available data on thermodynamic properties of ethylene and presents a new equation of state in the form of a fundamental equation explicit in the Helmholtz energy. The functional form of the residual part of the Helmholtz energy was developed by using state-of-the-art linear optimization strategies. The new equation of state contains 35 coefficients which were fitted to selected data of the following properties: (a) thermal properties of the single phase (pρT) and (b) of the liquid–vapor saturation curve (ps,ρ′,ρ″) including the Maxwell criterion, (c) speed of sound w of the single-phase region and the saturated vapor and liquid, (d) isochoric heat capacity cv, (e) specific isobaric heat capacity cp of the single-phase region and of the saturated liquid, and (f) second and third thermal virial coefficients B and C. For the density, the estimated uncertainty of the new equation of state is less than ±0.02% for pressures up to 12 MPa and temperatures up to 340 K with the exception of the critical region. Outside the range mentioned above, the estimated uncertainty is less than ±0.03% for pressures up to 30 MPa and temperatures between 235 and 350 K. The new formulation shows reasonable extrapolation behavior up to very high pressures and temperatures. Independent equations for the vapor pressure, for the pressure on the sublimation and melting curve, and for the saturated-liquid and saturated-vapor densities are also included. Tables for the thermodynamic properties of ethylene from 104 to 450 K for pressures up to 300 MPa are given in the appendix.
Cross-sectional electrostatic force microscopy of thin-film solar cells
Authors: Ballif C. , Moutinho H. R. , Al-Jassim M. M.
Published: July 2002

In a recent work, we showed that atomic force microscopy (AFM) is a powerful technique to image cross sections of polycrystalline thin films. In this work, we apply a modification of AFM, namely, electrostatic force microscopy (EFM), to investigate the electronic properties of cleaved II–VI and multijunction thin-film solar cells. We cleave the devices in such a way that they are still working with their nominal photovoltaic efficiencies and can be polarized for the measurements. This allows us to differentiate between surface effects (work function and surface band bending) and bulk device properties. In the case of polycrystalline CdTe/CdS/SnO2/glass solar cells, we find a drop of the EFM signal in the area of the CdTe/CdS interface (±50 nm). This drop varies in amplitude and sign according to the applied external bias and is compatible with an n-CdS/p-CdTe heterojunction model, thereby invalidating the possibility of a deeply buried n-p CdTe homojunction. In the case of a triple-junction GaInP/GaAs/Ge device, we observe a variation of the EFM signal linked to both the material work-function differences and to the voltage bias applied to the cell. We attempt a qualitative explanation of the results and discuss the implications and difficulties of the EFM technique for the study of such thin-film devices.