bluuebunny/crossref_metadata_2025 huggingface dataset:
Noninvasive picosecond ultrasonic detection of ultrathin interfacial layers: CFx at the Al/Si interface
Authors: Tas G. , Stoner R. J. , Maris H. J. , Rubloff G. W. , Oehrlein G. S. , Halbout J. M.
Published: July 2002

A picosecond ultrasonics technique has been used to detect interfacial fluorocarbon (CFx) layers as thin as 0.5 nm between aluminum and silicon. The presence of the CFx material reduces acoustic damping and heat loss from the Al film into the Si substrate. This provides a means for noninvasive identification of organic/polymeric contaminants at the buried interface and potentially for characterizing interfacial mechanical properties.
Surface modification and metallization of fluorocarbon polymers by excimer laser processing
Authors: Niino Hiroyuki , Yabe Akira
Published: July 2002

The surface chemical modification of poly(tetrafluoroethylene) and poly(tetrafluoroethylene- co-hexafluoropropylene) films was carried out in hydrazine gas photolyzed with ArF excimer laser irradiation. The contact angle of the modified surfaces with water changed from 130° to 30° due to the reaction with hydrazine. Nitrogen on the surface was detected with x-ray photoelectron spectroscopy, suggesting that amino groups were introduced onto the surface. In addition, on the basis of hydrophilic behavior, we succeeded in selective-area electroless plating of nickel metal on the chemically modified surface.
Electron intersubband transitions to 0.8 eV (1.55 μm) in InGaAs/AlAs single quantum wells
Authors: Smet J. H. , Peng L. H. , Hirayama Y. , Fonstad C. G.
Published: July 2002

We report a polarization-resolved infrared absorption study of the quantum-well-width dependence of the electron intersubband transitions in strained InGaAs/AlAs single quantum wells (SQWs) 3, 4, and 5 monolayers (ML) in width. An intersubband transition energy as high as 0.8 eV (i.e., a wavelength as short as 1.55 μm) is observed with transverse magnetic field polarization for a 3-ML-thick InGaAs SQW. This is the highest quantum-well intersubband transition energy ever reported in any materials system.
Spin coupled resistance observed in ferromagnet-superconductor-ferromagnet trilayers
Authors: Johnson Mark
Published: July 2002

Permalloy-niobium-permalloy trilayers are fabricated in the spin transistor geometry, and the results of transport measurements above and below the superconducting transition temperature Tc are reported. The data below Tc show evidence for the injection of nonequilibrium, spin-polarized quasiparticles, and their diffusion across the niobium film.
Homeotropic, rub-free liquid-crystal light shutter
Authors: Crandall Karl A. , Fisch Michael R. , Petschek Rolfe G. , Rosenblatt Charles
Published: July 2002

A liquid-crystal light shutter based upon homeotropic orientation has been fabricated, such that no rubbing is required. The device, which is dark in the voltage-off state and transmitting in the voltage-on state, exhibits contrast ratios in excess of 3000:1, with transmission efficiencies of 70% to 95%. Characterization results of one prototype are reported herein.
Antiferroelectric PbZrO3 thin films prepared by multi-ion-beam sputtering
Authors: Kanno Isaku , Hayashi Shigenori , Kitagawa Masatoshi , Takayama Ryouichi , Hirao Takashi
Published: July 2002

Antiferroelectric PbZrO3 thin films have been fabricated by a multi-ion-beam sputtering technique at a substrate temperature as low as 415 °C. Single crystal perovskite PbZrO3 films oriented along the a axis could be epitaxially grown on (100)MgO, (100)Pt/MgO substrates using a PbTiO3 buffer layer. The PbZrO3 films achieved high dielectric constants of about 400, which are almost 2.4 times larger than that of bulk PbZrO3. The measurements of D–E hysteresis loops and Curie temperature demonstrated the antiferroelectric to ferroelectric phase transition of PbZrO3 films with a thickness of 1770 Å, while for PbZrO3 films of 875 Å the phase transition could not be clearly observed.
Growth and composition of covalent carbon nitride solids
Authors: Zhang Z. John , Fan Shoushan , Lieber Charles M.
Published: July 2002

The composition, growth mechanism, and phases of carbon nitride thin films obtained from the reaction of laser ablated carbon and atomic nitrogen have been investigated. The nitrogen composition was found to increase to a limiting value of 50% as the fluence was decreased for laser ablation at both 532 nm and 248 nm. Analysis of these data shows that the overall growth rate determines the nitrogen composition, and suggests that a surface reaction between carbon and nitrogen represents a key step in the growth mechanism. Infrared spectroscopy has also been used to assess the phases present in the carbon nitride thin films. The implications of these results to the stoichiometry of covalent carbon nitride are discussed.
Pyroelectric x-ray detectors and x-ray pyrometers
Authors: Wang C. L. , Auerbach J. M. , Eckels J. D. , Koo J. C. , Kornblum H. N. , Price D. F. , Smith J. A. , Stotlar S. C.
Published: July 2002

Pyroelectric detectors are very promising x-ray detectors for intense pulsed x-ray/γ-ray measurements and can be used as x-ray pyrometers. They are fast, passive, and inherently flat in spectral response for low-energy x rays. We report our tests of LiTaO3 detectors at Nova laser with 1-ns low-energy x rays and at Zapp Z-pinch machine with 100-ns x rays. The temporal and spectral responses are discussed.
Elastic and hardness properties of Fe–Ag (001) multilayered thin films
Authors: Scanlon M. R. , Cammarata R. C. , Keavney D. J. , Freeland J. W. , Walker J. C. , Hayzelden C.
Published: July 2002

The hardness and elastic behavior of Fe–Ag (001) multilayered thin films is presented. Significant enhancements in the hardness were observed that followed a Hall–Petch-type dependence on bilayer thickness. A reduction in the flexural elastic modulus of 55% relative to the large bilayer thickness value was observed when the bilayer thickness was reduced to 5 nm. Concomitant with the elastic modulus reduction was a variation in the interplanar spacing perpendicular to the plane of the film. It is shown that these behaviors are generally consistent with a model based on the effects of interface stresses. Using this model, an interface stress for Fe–Ag (001) of approximately −1.8 N/m was obtained.
A far-infrared spectrometer based on cyclotron resonance emission sources
Authors: Knap W. , Dur D. , Raymond A. , Meny C. , Leotin J. , Huant S. , Etienne B.
Published: July 2002

Practical realization of the spectrometer based on the cyclotron emission sources is presented. It is shown that it can be used for transmission measurements in the range from 35 to 110 cm−1 with resolution up to 1.3 cm−1. Performances of the spectrometer are demonstrated by its application to the studies of impurity and free-electron states in two-dimensional structures.
Bowing parameters for zinc-blende Al1−xGaxN and Ga1−xInxN
Authors: Wright A. F. , Nelson J. S.
Published: July 2002

First-principles calculations have been used to determine bowing parameters for disordered zinc-blende Al1−xGaxN and Ga1−xInxN. The direct transition at Γ is found to bow downward for both materials with parameters +0.53 and +1.02 eV, respectively, while the Γ-to-X transition bows upward for Al1−xGaxN (parameter −0.10 eV) and downward for Ga1−xInxN (parameter +0.38 eV). The similarity of the calculated bulk zinc-blende and wurtzite Γ-point transitions also allows estimates to be made of the energy gap versus composition for wurtzite alloys.
The laser ion source of multiply charged ions for the U-200 LNR JINR cyclotron
Authors: Kutner V. B. , Bykovsky Yu. A. , Gusev V. P. , Kozyrev Yu. P. , Peklenkov V. D.
Published: July 2002

Some results on the ion acceleration at the U-200 cyclotron from the laser source are presented. The normal to the target surface was situated at a 45° angle to the magnetic field lines. A possibility is considered to create an ion laser source with plasma transport from a target to emission slit along magnetic field lines.
Accurate measurement of the Q factor of an open resonator in the W-band frequency range
Authors: Mongia R. K. , Arora R. K.
Published: July 2002

It is important to be able to measure accurately the Q factor of an open resonator to compute loss tangent of low-loss dielectric materials. In this paper, we report the results of the heterodyne detection technique used to measure the Q factor of an open resonator in the W-band frequency range. A more general algorithm is used for determining the Q factor. This algorithm offers some advantages over the commonly used ‘‘half-power frequencies’’ method of measurement of the Q factor. The effect of signal/noise ratio on the accuracy of measurement of the Q factor is experimentally studied. It is shown that with the set up used, it is possible to measure the Q factor to an accuracy of better than 1%.
Graphite-diamond transition in gem anvil cells
Authors: Xu Ji-an , Huang Eugene
Published: July 2002

By replacing the anvils in a diamond cell with transparent gem materials, a gem anvil cell (GAC) has been constructed for high-pressure generation. We reached pressures of 8.9 and 7.8 GPa with cubic zirconia and sapphire anvil cell, respectively. For high-temperature studies the cell can be heated to temperatures as high as 2000 °C with resistive heating of the gasket. Simultaneously high temperature and pressure approaching 1700 °C and 6 GPa were obtained as indicated by the direct conversion of graphite to diamond in the high-temperature GAC.
Automated reed-type Kelvin probe for work function and surface photovoltage studies
Authors: Kumar C. Suresh , Subrahmanyam A. , Majhi J.
Published: July 2002

This paper reports the design of a compact, inexpensive, and ultrahigh vacuum compatible reed-type Kelvin probe for work function and surface photovoltage spectroscopy (SPS) studies. A simple method of controlling the vibration amplitude of the probe is proposed which reduces mechanical instabilities of the vibrating capacitor, leading to an autonomous and reliable probe for monitoring work function changes over a prolonged period of time. A boss-type reference gold electrode is used to simultaneously study the work function changes with adsorbents and SPS on real surfaces of semiconductors. A current preamplifier used in the measuring circuit reduces the effect of parasitic input capacitance and facilitates the application of a biasing voltage. A feedback loop system and off-null technique are used for obtaining the contact potential difference. With this improved design of the probe setup a sensitivity of better than 0.1 mV has been achieved. Some experimental results are presented to show the performance of the setup.
Tunable wavelength hot electron light emitter
Authors: Balkan N. , Teke A. , Gupta R. , Straw A. , Wolter J. H. , van der Vleuten W.
Published: July 2002

We demonstrate the operation of a surface emitting light emitting diode. The wavelength of the emitted light can be tuned with the applied voltage. The device is based on a p-GaAs and n-Ga1−xAlxAs heterojunction containing an inversion layer in the p side and, GaAs quantum wells in the n side, and, is referred to as HELLISH-II (hot electron light emitting and lasing in semiconductor heterojunction). The device utilizes hot electron longitudinal transport and, therefore, light emission is independent of the polarity of the applied voltage.
Dynamic characterization of temperature sensors by laser excitation
Authors: Castellini P. , Rossi G. L.
Published: July 2002

In this article, a new dynamic calibration technique for sensors used to measure flow temperature is developed. The technique is based on laser heating of the sensor and allows one to operate in situ when optical access to the sensor is provided and the flow is transparent to laser light. It is possible to obtain both the time constant of the sensor and the convective heat exchange coefficient. Laboratory tests performed inside a wind tunnel, have shown the reliability and the advantage of such a technique. It is also possible to reduce the uncertainty on thermal exchange coefficient estimation (and therefore on the time constant) in order to obtain a more accurate model of the dynamic characteristics of the probes which are used for flow temperature measurements.
Improved spectral properties of diode lasers
Authors: Milic Dragana , Lu Weijian , Hoogerland Maarten D. , Blacksell Michael , Baldwin Kenneth G. H. , Buckman Stephen J.
Published: July 2002

We report on several complementary methods for improving the stability of a free running diode laser. We were primarily concerned with: (a) the construction of a stable power supply to reduce the main frequency and rf noise, and (b) long term drift of the laser frequency, for which we have developed a lock-in amplifier to be combined with a simple proportional-integrating circuit. While we have kept everything simple and inexpensive to construct, the stability is comparable with that of far more expensive, commercially available systems. Development and testing of our stabilization scheme was done on the distributed Bragg reflector diode lasers for use in a range of atomic collision and atom optics experiments, but it is applicable to all diode lasers, and can be used in a large range of atomic physics experiments.
Electrostatic containerless processing system
Authors: Rulison Aaron J. , Watkins John L. , Zambrano Brian
Published: July 2002

We introduce a materials science tool for investigating refractory solids and melts: the electrostatic containerless processing system (ESCAPES). ESCAPES maintains refractory specimens of materials in a pristine state by levitating and heating them in a vacuum chamber, thereby avoiding the contaminating influences of container walls and ambient gases. ESCAPES is designed for the investigation of thermophysical properties, phase equilibria, metastable phase formation, undercooling and nucleation, time–temperature–transformation diagrams, and other aspects of materials processing. ESCAPES incorporates several design improvements over prior electrostatic levitation technology. It has an informative and responsive computer control system. It has separate light sources for heating and charging, which prevents runaway discharging. Both the heating and charging light sources are narrow band, which allows the use of optical pyrometry and other diagnostics at all times throughout processing. Heat is provided to the levitated specimens by a 50 W Nd:YAG laser operating at 1.064 μm. A deuterium arc lamp charges the specimen through photoelectric emission. ESCAPES can heat metals, ceramics, and semiconductors to temperatures exceeding 2300 K; specimens range in size from 1 to 3 mm diam. This article describes the design, capabilities, and applications of ESCAPES, focusing on improvements over prior electrostatic levitation technology.
An in situ x-ray absorption spectroscopic cell for high temperature gas-flow measurements
Authors: Meitzner George , Bare Simon R. , Parker Deborah , Woo Hyung , Fischer Daniel A.
Published: July 2002

The problem of controlling the temperature, pressure, and atmosphere around a solid sample for x-ray absorption spectroscopy, while accurately positioning it in the x-ray beam, has given rise to a wide range of designs. We describe here a new versatile design that provides excellent temperature control, and good hydrodynamics and corrosion resistance. Briefly, the cell comprises a stainless steel tube beam path, with a wafer sample positioned across the tube. Heat is supplied from an aluminum block that surrounds the tube. Gases for in situ sample treatment are flowed between fittings at the ends of the tube, and the dead volume is less than 0.3 cm3.
An integrated Couette system for in situ shearing of polymer and surfactant solutions and gels with simultaneous small angle x-ray scattering
Authors: Pople J. A. , Hamley I. W. , Diakun G. P.
Published: July 2002

A Couette cell designed for in situ small angle x-ray scattering (SAXS) studies of polymeric systems under shear is described. Representative results are presented for the effect of shear on the lamellar phase of a concentrated poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution. It is shown that the application of shear resulted in a reduction in the lamellar spacing and of the defect density. The former was observed as a shift in the principal peak position in the SAXS pattern, and the latter as an increase in the associated correlation length. It was further shown that the cessation of shear led to a relaxation of both shear induced effects, over a time period of hours.
Photoelectron–multiple-photofragment coincidence spectrometer
Authors: Hanold K. A. , Luong A. K. , Clements T. G. , Continetti R. E.
Published: July 2002

A new photoelectron–photofragment-coincidence spectrometer is described. Using a multiparticle time- and position-sensitive detector, this apparatus allows the study of dissociation processes of negative ions yielding three photofragments in coincidence with a photoelectron. The photoelectron spectrometer uses two detectors and works in time of flight mode, detecting 10% of the photoelectrons with an energy resolution of 5% at 1.3 eV as shown in studies of the photodetachment of O2−. A third detector is used for collection of multiple photofragments (up to 8) in coincidence. This multiparticle detector uses a crossed-delay-line anode and fast timing signals to encode the time- and position-of-arrival of multiple photofragments. The detector was demonstrated to record all three particles produced in a single three-body dissociation event, yielding an energy resolution of ≈15% ΔE/E at 0.7 eV in experiments on the three-body dissociative photodetachment of O6−.
Determination of the activation energy ε3 for impurity conduction in n-type 4H–SiC
Authors: Evwaraye A. O. , Smith S. R. , Mitchel W. C. , Roth M. D.
Published: July 2002

Impurity conduction (or hopping conduction) has been observed in the more heavily n-type 4H–SiC samples by both temperature dependent resistivity measurements and thermal admittance spectroscopy. The measured activation energies ε3 for hopping were 4–5 meV and 2.3–3.0 meV, respectively. No evidence of hopping conduction was seen by either method in the sample where ND–NA<1018 cm−3. The thermal admittance spectrum of the lightly n-type sample showed the two nitrogen levels at 53 and 100 meV.
Electrical properties of Ta2O5 films obtained by plasma enhanced chemical vapor deposition using a TaF5 source
Authors: Devine R. A. B. , Vallier L. , Autran J. L. , Paillet P. , Leray J. L.
Published: July 2002

High quality Ta2O5 thin films have been obtained from TaF5 and O2 using a microwave excited electron cyclotron resonance plasma at low pressure (∼2 mTorr). Physical and electrical measurements reveal that the as-deposited amorphous films have excellent properties: refractive indices ∼2.16, dielectric constants ∼25, and leakage currents <10−10 A cm−2 at 2.5 V (0.3 MV cm−1, 85 nm thick, 13 nm SiO2 equivalent). Trapping and conduction properties of these layers have also been investigated, showing a reversible electron trapping and a trap-limited Poole–Frenkel effect.
Second harmonic generation from trinitro-substituted subphthalocyanines films: Evidence of noncentrosymmetric molecular organization
Authors: Rojo G. , Hierro A. , Díaz-Garcia M. A. , Agulló-Lopez F. , del Rey B. , Sastre A. , Torres T.
Published: July 2002

Second-harmonic generation from λ=1.06 μm fundamental light has been measured in both evaporated and spin-coated films of trinitro-substituted subphthalocyanines. Ordering of these noncentrosymmetric molecules with the dipole moment perpendicular to the film face has been achieved either by the deposition process (evaporated samples) or by corona poling (spin-coated samples). The second-order susceptibilities χ31(2)=2.36×10−9 esu (for vacuum evaporated samples) and χ31(2)=9.62×10−10–1.14×10−9 esu (for poled spin-coated samples) have been determined by comparison with an X-cut LiNbO3 crystal used as reference.
Time-reversal in an ultrasonic waveguide
Authors: Roux Philippe , Roman Benoit , Fink Mathias
Published: July 2002

The aim of this letter is to study, by using a time-reversal mirror (TRM), ways to overcome the distortions induced by multipaths in an acoustic transmission. A set of experiments is performed with a TRM made of 96 reversible transducers and results will be presented. One result is related to the high focusing property obtained with a TRM working in a bounded channel. It will be shown that the time-reversed beam is much thinner than the one measured in an unbounded medium. The second result concerns the time compression observed for the time-reversed acoustic pulse. The influence of the number of TRM elements on the time compression is discussed.
High-Tc super conducting quantum interference devices with slots or holes: Low 1/f noise in ambient magnetic fields
Authors: Dantsker E. , Tanaka S. , Clarke John
Published: July 2002

High-transition temperature dc superconducting quantum interference devices (SQUIDs) have been fabricated with arrays of slots or holes patterned in the 186×204 μm2 washers. In two designs, with seven slots or 248 holes, the linewidth of the remaining YBa2Cu3O7−x film was 4 μm. These devices exhibited virtually no increase in their low-frequency noise when they were cooled in static magnetic fields up to a certain threshold, above 100 μT in the best case. Furthermore, the mutual inductance between these devices and a seven-turn, spiral coil was at least 85% of that for a solid SQUID with the same outer dimensions.
Ion implantation induced swelling in 6H-SiC
Authors: Nipoti R. , Albertazzi E. , Bianconi M. , Lotti R. , Lulli G. , Cervera M. , Carnera A.
Published: July 2002

Ion implantation induced surface expansion (swelling) of 6H-SiC was investigated through the measurement of the step height between implanted and unimplanted areas. The samples were irradiated at room temperature with 500 keV Al+ ions in the dose range 1.25×1014–3×1015 ions cm−2. Swelling was related to dose and the area density of ion-induced damage measured by Rutherford backscattering channeling technique. The observed trend is consistent with the hypothesis that the volume expansion of the ion damaged crystal is proportional to the area density of displaced atoms, plus an additional relaxation occurring at the onset of the crystalline to amorphous transition.
Thermal oxide growth at chemical vapor deposited SiO2/Si interface during annealing evaluated by difference x-ray reflectivity
Authors: Awaji Naoki , Ohkubo Satoshi , Nakanishi Toshiro , Aoyama Takayuki , Sugita Yoshihiro , Takasaki Kanetake , Komiya Satoshi
Published: July 2002

The x-ray interference technique has been applied to evaluate the structural changes of high temperature grown chemical vapor deposited (CVD) SiO2 film under several post annealing conditions. In annealing above 800 °C in O2 ambient, a thermal oxide growth has been found at the CVD SiO2/Si interface, and its precise thicknesses have been determined. The estimated diffusion coefficient of the oxidant in CVD film was about three times larger compared to that of thermal oxide. A threshold voltage shift in the oxide was found to strongly correlate to the thickness of the thermal oxide rather than to thermal modifications of the CVD SiO2 itself.
Ultrafast intersubband photocurrent response in quantum-well infrared photodetectors
Authors: Ehret S. , Schneider H. , Fleissner J. , Koidl P. , Böhm G.
Published: July 2002

We report on a high bandwidth measurement of the transient intersubband photocurrent of a GaAs/AlGaAs multiple quantum-well infrared photodetector (QWIP). The photocurrent is excited via tunable subpicosecond infrared pulses. The response time of the detector has a full width at half-maximum of 18.5 ps and a rise time of 14.5 ps, which is limited by the electrical circuit. The decay time of the photocurrent response exhibits a significant dependence on the applied voltage, with increasing decay times for increasing bias voltages. From the experimental data, we conclude that the intrinsic response time of a QWIP is less than 7 ps.
Energy efficient plasma processing of gaseous emission using a short pulse discharge
Authors: Puchkarev Victor , Gundersen Martin
Published: July 2002

This letter reports efficient treatment of NO emission with a transient, nonequilibrium plasma created by pulsed corona discharge. The transient plasma (≈50 ns) is found to reduce NO emission by 50% in a flow of 2–25 l/s with energy cost ≈10–20 eV/molecule, corresponding to a fraction of source power of ≈5%. The efficiency of NOx reduction is a complex function of parameters that include pulse width, pulse polarity, current density, repetition rate, and reactor design. It was found that the best efficiencies are correlated with a low current density (0.2 A/cm2)and high repetition rate (1 kHz) under high flow rate. Careful optimization of all these parameters is required to reach cost effective NOx reduction.
Radiation-enhanced network formation in copolymer galvanoforms for diffractive nickel x-ray optics with high aspect ratios
Authors: Weiss D. , Peuker M. , Schneider G.
Published: July 2002

High-resolution x-ray microscopy makes use of nanostructured diffractive optics such as Fresnel zone plates made of nickel. These micro zone plates are manufactured using microscopic galvanoforms and electrodeposition techniques. Copolymer galvanoforms for nickel micro zone plates were irradiated with high doses of x-ray radiation to increase the degree of cross-linking of the copolymer network; in this way, exact pattern replication in the galvanoform was obtained for zone aspect ratios of up to 8:1. Using these galvanoforms, nickel micro zone plates were produced for the soft x-ray wavelength λ=2.4 nm, with smallest zone widths of 30 nm and 40 nm, achieving first-order diffraction efficiencies of up to 13% and 20%, respectively.
Spin dependent electron absorption in Fe(001)-p(1×1)O: A new candidate for a stable and efficient electron polarization analyzer
Authors: Bertacco Riccardo , Merano Michele , Ciccacci Franco
Published: July 2002

The low energy electron absorption by a p(1×1) ordered oxygen overlayer on magnetized Fe(001) depends on the spin polarization of the primary beam. The peak value of the spin asymmetry is 12.5% at 6 eV electron kinetic energy, where the percentage absorption is 65%. The presence of the surface oxide prevents the system from contamination, so that an almost negligible asymmetry deterioration is observed after prolonged operation in vacuum. After exposure to atmosphere followed by heat-cleaning a peak value still larger than 7% is achieved. These results suggest spin dependent current absorption from Fe(001)-p(1×1)O as a very promising candidate for an efficient and extremely stable electron polarization analyzer.
X-ray absorption spectroscopy and atomic force microscopy study of bias-enhanced nucleation of diamond films
Authors: Garcı́a M. M. , Jiménez I. , Vázquez L. , Gómez-Aleixandre C. , Albella J. M. , Sánchez O. , Terminello L. J. , Himpsel F. J.
Published: July 2002

The bias-enhanced nucleation of diamond on Si(100) has been studied by x-ray absorption near-edge spectroscopy (XANES) and atomic force microscopy, two techniques well suited to characterize nanometric crystallites. Diamond nuclei of ∼15 nm are formed after 5 min of bias-enhanced treatment. The number of nuclei and its size increases with the time of application of the bias voltage. A nanocrystalline diamond film is attained after 20 min of bias-enhanced nucleation. At the initial nucleation stages, the Si substrate appears covered with diamond crystallites and graphite, without SiC being detected by XANES.
Laser action in GaN pyramids grown on (111) silicon by selective lateral overgrowth
Authors: Bidnyk S. , Little B. D. , Cho Y. H. , Krasinski J. , Song J. J. , Yang W. , McPherson S. A.
Published: July 2002

Laser action was observed in GaN pyramids under strong optical pumping at room temperature. The pyramids were laterally overgrown on a patterned GaN/AlN seeding layer grown on a (111) silicon substrate by metal–organic chemical vapor deposition. Each pyramid had a 15-μm-wide hexagonal base and was on average 15 μm in height. The pyramids were individually pumped, imaged, and spectrally analyzed through a high-magnification telescope system using a high-density pulsed excitation source. Under high levels of optical pumping, multimode laser at room temperature was observed. The integrated emission intensity for both spontaneous and lasing peaks was studied as a function of excitation power density. The effects of pyramid geometry and short-pulse excitation on the multimode nature of laser oscillations inside of the pyramids is discussed. This study suggests that GaN microstructures could potentially be used as pixel elements and high-density two-dimensional laser arrays.
Surface near-edge x-ray adsorption fine structure of hydrogenated diamond films and Di(100) surfaces studied by H+ and H− ion desorption
Authors: Hoffman A. , Comtet G. , Hellner L. , Dujardin G. , Petravic M.
Published: July 2002

The near-edge x-ray absorption fine structures (NEXAFS) of hydrogenated diamond films and single-crystal diamond surfaces have been studied by recording the partial electron yield and the H+ and H− ion desorption yields as a function of photon energies around the C(1s) core level. It has been found that ion desorption is much more surface sensitive than electron emission, especially for the C(1s)–σ*(C–H) surface resonance which is enhanced in the H+ ion yield. This enhanced surface sensitivity of ion desorption has enabled us to compare in detail the surface NEXAFS structure of both hydrogenated surfaces and to ascertain the quality of the diamond film.
Single-mode picosecond blue laser emission from a solid conjugated polymer
Authors: Stagira S. , Zavelani-Rossi M. , Nisoli M. , DeSilvestri S. , Lanzani G. , Zenz C. , Mataloni P. , Leising G.
Published: July 2002

We demonstrate single-mode laser emission at 490 nm from a stable cavity based on a methyl-substituted poly-(para-phenylene)-type ladder polymer. The laser emission is characterized by a well-defined excitation fluence threshold, a high directionality, and a drastic spectral narrowing. The laser generates picosecond pulses of excellent amplitude stability.
Effect of the hydrogen on the intrinsic stress in hydrogenated amorphous carbon films deposited from an electron cyclotron resonance plasma
Authors: Racine B. , Benlahsen M. , Zellama K. , Goudeau P. , Zarrabian M. , Turban G.
Published: July 2002

The intrinsic stresses have been investigated in detail in particular diamondlike carbon films prepared by chemical vapor deposition assisted by electron cyclotron resonance plasma, as a function of the substrate bias and sample thickness in relation with the H content and bonding. Combined infrared absorption, elastic recoil detection analysis, and residual stress measurements are used to fully characterize the films in their as deposited state. The results indicate clearly that both the low and high biased samples exhibit compressive stresses. The stresses are found to be higher in the high biased films and are affected not only by the [H]/[C] ratio but also by the C–H and C–C volumetric distortions.
Optical properties of GaN pyramids
Authors: Zeng K. C. , Lin J. Y. , Jiang H. X. , Yang Wei
Published: July 2002

Picosecond time-resolved photoluminescence (PL) spectroscopy has been used to investigate the optical properties of GaN pyramids overgrown on hexagonal-patterned GaN(0001) epilayers on sapphire and silicon substrates with AlN buffer layers. We found that: (i) the release of the biaxial compressive strain in GaN pyramids on GaN/AlN/sapphire substrate led to a 7 meV redshift of the spectral peak position with respect to the strained GaN epilayer grown under identical conditions; (ii) in the GaN pyramids on GaN/AlN/sapphire substrate, strong band edge transitions with much narrower linewidths than those in the GaN epilayer have been observed, indicating the improved crystalline quality of the overgrown pyramids; (iii) PL spectra taken from different parts of the pyramids revealed that the top of the pyramid had the highest crystalline quality; and (iv) the presence of strong band-to-impurity transitions in the pyramids were primarily due to the incorporation of the oxygen and silicon impurities from the SiO2 mask.
Low-resistance ohmic contacts to p-type GaN
Authors: Ho Jin-Kuo , Jong Charng-Shyang , Chiu Chien C. , Huang Chao-Nien , Chen Chin-Yuen , Shih Kwang-Kuo
Published: July 2002

Low-resistance ohmic contacts with high transparency to p-type GaN have been developed by oxidizing Ni/Au thin films. Compared to the metallic Ni/Au contacts, the oxidized Ni/Au contacts exhibited lower specific contact resistance and much improved transparency. The transparency was from 65% to 80% in the wavelength of 450–550 nm. A specific contact resistance below 1.0×10−4 Ω cm2 was obtained by oxidizing Ni(10 nm)/Au(5 nm) on p-type GaN. The mechanism of low-resistance ohmic contact could be related to the formation of NiO.
Electrochemical texturization of ZnTe surfaces
Authors: Zenia F. , Lévy-Clément C. , Triboulet R. , Könenkamp R. , Ernst K. , Saad M. , Lux-Steiner M. C.
Published: July 2002

We report the results of electrochemical etching on ZnTe crystals using the acidic etchant HNO3:HCl:H2O. Under optimized conditions we are able to etch several micrometer deep patterns into 〈111〉 and 〈110〉 surfaces. A comparison between etched and polished crystals shows that a surface enlargement of approximately 50 times can be obtained. The etched surfaces have strongly reduced optical reflectance, high photoluminescence efficiency, and excellent photosensitivity. Needlelike structures can be prepared, which exhibit a blueshift of the excitonic transition energies, indicating that parts of the etch pattern are in the quantum confinement regime.
Electrical properties of (001)- and (116)-oriented epitaxial SrBi2Ta2O9 thin films prepared by metalorganic chemical vapor deposition
Authors: Ishikawa Katsuyuki , Funakubo Hiroshi
Published: July 2002

(001)- and (116)-oriented epitaxial SrBi2Ta2O9 (SBT) thin films were deposited on (100)SrRuO3∥(100)SrTiO3 substrates at 750 °C and (110)SrRuO3∥(110)SrTiO3 substrates at 820 °C by metalorganic chemical vapor deposition, respectively. The remanent polarization and the coercive field of the 200-nm-thick (116)-oriented SBT films normal to the substrate were 11.4 μC/cm2 and 80 kV/cm, respectively. The dielectric constant of this film was 140 at 1 kHz. On the other hand, the 200-nm-thick (001)-oriented SrBi2Ta2O9 films normal to the substrate showed no ferroelectricity and the dielectric constant was 70 at 1 kHz.
Interface trap profile near the band edges at the 4H-SiC/SiO2 interface
Authors: Saks N. S. , Mani S. S. , Agarwal A. K.
Published: July 2002

The transconductance of SiC metal–oxide–semiconductor field-effect transistors (MOSFETs) is typically much lower in devices fabricated on the 4H-SiC polytype compared to 6H. It is believed that this behavior is caused by extreme trapping of inversion electrons due to a higher density of traps Dit at the SiC/SiO2 interface in 4H-SiC. Here we present an approach for profiling Dit versus energy in the band gap using a modified capacitance–voltage technique on large-area MOSFETs. We find that Dit increases towards the conduction band edge Ec in both polytypes, and that Dit is much higher in 4H- compared to 6H-SiC for devices fabricated in the same process lot.
Confinement of phonons in InGaAs/InP superlattices
Authors: Shin H. K. , Lockwood D. J. , Poole P. J.
Published: July 2002

A comparison is presented of optical mode Raman spectra of an In0.48Ga0.52As/InP superlattice, an In0.48Ga0.52As strained epilayer on InP, and bulk InP. The sharpness of the interfaces between the alloy and InP layers in the superlattice sample is confirmed through the observation of folded acoustic modes up to folding index m=6. Mixed interface modes were correspondingly not observed in the superlattice optic mode Raman spectra. The GaAs-like longitudinal optic (LO) and transverse optic and InP LO modes in the superlattice are found to decrease in frequency with respect to the corresponding strained-epilayer and bulk-InP modes. These frequency shifts arise from phonon confinement effects in the superlattice.
Local poloidal and toroidal plasma rotation velocities and ion temperature in a tokamak plasma obtained with a matrix inversion method considering asymmetries
Authors: Condrea I. , Haddad E. , Gregory B. C. , Abel G.
Published: July 2002

An inversion technique is presented for the local poloidal and toroidal rotation velocities and for the ion temperature from line integrated measurements performed on Tokamak de Varennes (TdeV) [R. Decoste and TdeV Team, Proceedings of the 15th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Seville, 1994 (International Atomic Energy Agency, Vienna, 1995) IAEA-CN-60/A4-11]. The velocity is obtained using two matrix inversions; the first for the emissivity and the second with the velocity weighted emissivity. The temperature is obtained with three matrix inversions: emissivity, temperature weighted emissivity and rotation velocity squared. The effect of the rotation velocity represents up to 16% in the ion temperature for TdeV plasmas. The local values obtained using the lengths matrix with the magnetic flux lines from the equilibrium code are compared with those obtained by a standard Abel inversion with circular flux lines. Differences up to 20% are observed between the emissivities deduced with circular and real flux lines, whereas the rotation velocity and the ion temperature are very similar. The technique was applied for the poloidal and toroidal geometry to determine the poloidal and toroidal velocities and the emission asymmetries. Top poloidal and toroidal emissivities present strong asymmetries due to the divertor plates and the X point whereas bottom poloidal and toroidal emissivities show an inner–outer symmetry, making the inversion more reliable in this region. A first approach to model the strong asymmetry was made assuming that the emissivity has both a radial and a poloidal dependence. The best result was obtained using a radial dependence and a peaked function of the poloidal angle for the poloidal asymmetric part of the emissivity. Both emissivity and velocity asymmetries are present in the upper part of the plasma implying that the X point behaves as a source. Examples of emissivities, rotation velocities and ion temperatures observed in TdeV plasmas in H and L (high and low confinement) modes with different bottom plasma triangularity are shown.
Dielectric properties in heteroepitaxial Ba0.6Sr0.4TiO3 thin films: Effect of internal stresses and dislocation-type defects
Authors: Canedy C. L. , Li Hao , Alpay S. P. , Salamanca-Riba L. , Roytburd A. L. , Ramesh R.
Published: July 2002

A series of heteroepitaxial Ba0.6Sr0.4TiO3 were grown on 0.29(LaAlO3):0.35(Sr2TaAlO6) substrates using pulsed-laser deposition. X-ray characterization revealed compressive in-plane stresses in the thinnest films, which were relaxed in a continuous fashion with increasing thickness. A theoretical treatment of the misfit strain was in good agreement with the measured out-of-plane lattice parameter. The low-frequency dielectric constant was measured to be significantly less than the bulk value and found to decrease rapidly for films less than 100 nm. A thermodynamic model was developed to understand the reduction in dielectric constant. By observing the microstructure using plan-view and cross-section transmission electron microscopy, we identified local strain associated with a threading dislocation density on the order of 1011 cm−2 as a possible mechanism for dielectric degradation in these films.
Variation in the fixed charge density of SiOx/ZrO2 gate dielectric stacks during postdeposition oxidation
Authors: Houssa M. , Afanas’ev V. V. , Stesmans A. , Heyns M. M.
Published: July 2002

The effect of postdeposition oxidation of SiOx/ZrO2 gate dielectric stacks at different temperatures (500–700 °C) on the density of fixed charge and interface states is investigated. It is shown that with increasing oxidation temperature the density of negative fixed charge is reduced, but the density of interface states increases. The net positive charge observed after oxidation at T>500 °C resembles the charge generated at the Si/SiO2 interface by hydrogen in the same temperatures range. This association is supported by the resistance of both types of charge against molecular hydrogen anneal but their fast removal in the presence of atomic hydrogen at 400 °C. Therefore, we propose that the observed oxidation-induced positive charge in the SiOx/ZrO2 gate stack may be related to overcoordinated oxygen centers induced by hydrogen.
Asymmetric peak line shape of infrared dielectric function spectra for thermally grown silicon dioxide films
Authors: Ishikawa Kenji , Suzuki Kunihiro , Okamura Shigeru
Published: July 2002

Measured infrared dielectric function spectra ranging from 300 to 1400 cm−1 for thermally grown silicon dioxide films were studied. The dielectric function spectra were calculated by requiring the calculated spectra to fit to the actual spectra. The peak line shape of the dielectric function can be described by a Gaussian distribution, but not by a Lorentzian line shape. In detail, frequencies of half height for the imaginary part of the dielectric function of high- and low-frequency edges are not actually symmetrical. According to interpretations based on a central and noncentral force network model, the asymmetric distribution of the dielectric function arises from the symmetrical distribution of the bond angles of a random network of the SiO4 tetrahedra. For that reason, we developed a dielectric function model which can describe an asymmetrical Gaussian line shape. This asymmetric Gaussian model minimizes the number of parameters needed, which are the one center position and low- and high-Gaussian distribution widths, compared with the assignment method for multiple purely Gaussian peaks.
Perylene: A promising organic field-effect transistor material
Authors: Schön J. H. , Kloc Ch. , Batlogg B.
Published: July 2002

Field-effect transistors based on single crystalline perylene have been prepared and analyzed in the temperature range from 50 to 300 K. Room temperature electron mobilities as high as 5.5 cm2/V s have been achieved. In addition, ambipolar device operation, i.e., n- and p-channel activity, is observed. The temperature dependence of the electron and hole mobilities is discussed in the limits of hopping and band-like transport mechanisms.

At the request of the authors and editor, this article is being retracted 24 February 2003.
The ice/water interface: Molecular dynamics simulations of the basal, prism, {202̄1}, and {21̄1̄0} interfaces of ice Ih
Authors: Hayward Jennifer A. , Haymet A. D. J.
Published: July 2002

The structure and dynamics of the {0001} (basal), {101̄0} (prism), {202̄1}, and {21̄1̄0} ice Ih/water interfaces have been investigated using molecular dynamics and the flexible CF1 central force model of water. The translational order profile, the average density profile, and the diffusion profile have been calculated for all four interfaces as a function of distance normal to the interface. Dynamical molecular trajectories have been used to explore the loss of translational order from within the crystal region, through the interface, and into the liquid region. The thickness of the interfaces has been determined from each order parameter and compared with results from rigid models of water and experiment. The high index faces have thinner interfacial regions than the basal and prism interfaces. All interfacial regions contain molecules that are neither ice-like nor water-like.
Effect of the basicity of the support on the properties of deposited metal atoms
Authors: Lopez Nuria
Published: July 2002

The interaction energy of a single palladium or platinum atom on different alkaline–earth oxides has been determined by means of ab initio calculations on cluster models representing the nonpolar, nondefective (100) surface. The variation of the Madelung potential along the series implies a change on the polarization degree of the deposited metal atom but also an increase of the basicity of the oxide surface that allows a larger charge transfer at the interface. In addition, the presence of the oxide surface modifies the energetic position of the levels of the metal and depending on the metal, even the occupation of the metallic d-orbitals. As a consequence, a rather strong modification on the supported metal properties is found through the analysis of the adsorption of a CO molecule.
Strain relaxation in AlGaN/GaN superlattices grown on GaN
Authors: Einfeldt S. , Heinke H. , Kirchner V. , Hommel D.
Published: July 2002

Lattice relaxation of strained AlxGa1−xN/GaN superlattices grown on thick GaN buffer layers is investigated using optical microscopy, x-ray diffraction, and photoluminescence spectroscopy. The results are compared to strained bulk AlxGa1−xN layers particularly with regard to the impact of the superlattice period and the Al content. A relaxation process which keeps the coherency between AlxGa1−xN barriers and GaN wells in the superlattice is found and it is attributed to misfit dislocations at the buffer/superlattice interface. Additionally, the AlxGa1−xN barriers relax via crack channels which form beyond a critical Al content and limit the additional strain energy compared to a free-standing superlattice to a maximum value. Cracks relieve tensile plane stress to an extent similar as in bulk layers, i.e., they do not put the GaN wells of the superlattice under additional plane compression. This is explained by misfit dislocations which nucleate at crack faces and glide into the superlattice at the well/barrier interfaces. The onset of cracking is found to shift to higher tensile stresses in the AlxGa1−xN barriers when increasing the superlattice period which is discussed in view of edge cracks being the starting point of crack channels.
Sharp band edge photoluminescence of high-purity CuInS2 single crystals
Authors: Yoshino Kenji , Ikari Tetsuo , Shirakata Sho , Miyake Hideto , Hiramatsu Kazumasa
Published: July 2002

Temperature-dependent photoluminescence measurements were carried out between 8 and 300 K on CuInS2 single crystals grown by a traveling heater method. Ten distinct peaks were present in the near-band edge region. Four unknown peaks, observed at 8 K, were found to be due to bound exciton emission. Moreover, the luminescence remained stable up to room temperature.
Optical and plasma smoothing of laser imprinting in targets driven by lasers with SSD bandwidths up to 1 THz
Authors: Boehly T. R. , Goncharov V. N. , Gotchev O. , Knauer J. P. , Meyerhofer D. D. , Oron D. , Regan S. P. , Srebro Y. , Seka W. , Shvarts D. , Skupsky S. , Smalyuk V. A.
Published: July 2002

The creation of a plasma atmosphere in laser-target interactions increases the distance between the regions of laser absorption and hydrodynamic instability (ablation front), thus allowing thermal smoothing and a reduction of laser-imprinted modulations that reach the unstable ablation region. The total laser imprinting is reduced with pulse shapes that produce a plasma atmosphere more rapidly and by the implementation of temporal beam smoothing. These effects are measured and found to be consistent with models for the hydrodynamics and optical smoothing by spectral dispersion (SSD). Imprinting is reduced as the laser bandwidth is increased from 0.2 to 1.0 THz.