bluuebunny/crossref_metadata_2025 huggingface dataset:
Use of the 28Si(n, p)28Al reaction for the measurement of 14 MeV neutrons from fusion plasmas
Authors: Sadler G. , Jarvis O. N. , van Belle P. , Pillon M.
Published: July 2002

The 28Si(n, p)28Al reaction is shown to be ideal for monitoring the emission of 14 MeV neutrons from d-t fusion plasmas. The daughter nuclide, 28Al, has a 2.246 min half-life and so is suitable for studying discharges of up to 1 min duration; the decay gamma radiation has an energy of 1.779 MeV and is easily detected with a NaI(Tl) detector. The same sample can be recycled for successive JET discharges and the data taking is fully automatic. Absolute neutron yields are obtained by calibrating with the standard 63Cu(n, 2n)62Cu reaction. The effective threshold energy of 5 MeV completely discriminates against 2.5 MeV d-d neutrons, making this reaction also highly appropriate for the study of the 14 MeV neutrons emitted during the burnup of the 1.0 MeV tritons produced in the second branch of the d-d reaction. Some results from this application will be presented.
High voltage pulse generation using current mode second breakdown in a bipolar junction transistor
Authors: Baker R. J.
Published: July 2002

The characteristics of a bipolar junction transistor operating in the avalanche region and then triggered into current mode second breakdown are formulated. If the time the BJT is subjected to secondary breakdown is limited the BJT may be used as a nanosecond, high voltage switch without sustaining damage. Several methods of fast pulse generation, electrical and optical, using this mode of operation are discussed. A 2000 V pulse generator, into 50 Ω, with a risetime of approximately 1 ns, jitter <100 ps, is then designed using these results.
Coherent hard x-ray focusing optics and applications
Authors: Yun W. B. , Viccaro P. J. , Lai B. , Chrzas J.
Published: July 2002

Coherent hard x-ray beams with a flux exceeding 109 photons/sec with a bandwidth of 0.1% will be provided by undulators at the third-generation synchrotron radiation sources such as APS, ESRF, and Spring-8. The availability of such high flux coherent x-ray beams offers excellent opportunities for extending the coherence-based techniques developed in the visible and soft x-ray part of the electromagnetic spectrum to the hard x-ray region. These x-ray techniques (e.g., diffraction-limited microfocusing, holography, interferometry, phase contrast imaging, and signal enhancement) may offer substantial advantages over noncoherence-based x-ray techniques currently used. For example, the signal-enhancement technique may be used to enhance an anomalous x-ray or magnetic x-ray scattering signal by several orders of magnitude. Coherent x rays can be focused to a very small (diffraction-limited) spot size, thus allowing construction of high spatial resolution microprobes. This paper will discuss the feasibility of extending some coherence-based techniques to the hard x-ray range and the significant progress that has been made in the development of diffraction-limited focusing optics. Specific experimental results for a transmission Fresnel phase zone plate that can focus 8.2 keV x rays to a spot size of about 2 microns will be briefly discussed. The comparison of measured focusing efficiency of the zone plate with that calculated will be made. Some specific applications of zone plates as coherent x-ray optics will be discussed.
Ellipsometrical determination of the optical constants of gold in the vacuum ultraviolet region
Authors: Kandaka Noriaki , Yuri Masatada , Fukutani Hirohito , Koide Tsuneharu , Shidara Tetsuo
Published: July 2002

An ellipsometer has been designed and constructed for use in the vacuum ultraviolet region. To test the performance of the ellipsometer, the optical constants of Au were determined in the energy region 15–28 eV. The theoretical background and the experimental procedures of our ellipsometer are described.
Structural evolution in epitaxial metalorganic chemical vapor deposition grown GaN films on sapphire
Authors: Kapolnek D. , Wu X. H. , Heying B. , Keller S. , Keller B. P. , Mishra U. K. , DenBaars S. P. , Speck J. S.
Published: July 2002

The structural evolution of epitaxial GaN layers grown on basal plane sapphire has been studied by atomic force microscopy (AFM), x-ray diffraction, and transmission electron microscopy (TEM). High-temperature growth (1050–1080 °C) on optimized nucleation layers leads to clear, specular films. AFM on the as-grown surface shows evenly spaced monatomic steps indicative of layer by layer growth. AFM measurements show a step termination density of 1.7×108 cm−2 for 5 μm films. This value is in close agreement with TEM measurements of screw and mixed screw-edge threading dislocation density. The total measured threading dislocation density in the 5 μm films is 7×108 cm−2.
Method for the calibration of atomic force microscope cantilevers
Authors: Sader John E. , Larson Ian , Mulvaney Paul , White Lee R.
Published: July 2002

The determination of the spring constants of atomic force microscope (AFM) cantilevers is of fundamental importance to users of the AFM. In this paper, a fast and nondestructive method for the evaluation of the spring constant which relies solely on the determination of the unloaded resonant frequency of the cantilever, a knowledge of its density or mass, and its dimensions is proposed. This is in contrast to the method of Cleveland et al. [Rev. Sci. Instrum. 64, 403 (1993)], which requires the attachment of masses to the cantilever in the determination of the spring constant. A number of factors which can influence the resonant frequency are examined, in particular (i) gold coating, which can result in a dramatic variation in the resonant frequency, for which a theoretical account is presented and (ii) air damping which, it is found, leads to a shift of ∼4% in the resonant frequency down on its value in a vacuum. Furthermore, the point of load on the cantilever is found to be extremely important, since a small variation in the load point can lead to a dramatic variation in the spring constant. Theoretical results that account for this variation, which, it is believed will be of great practical value to the users of the AFM, are given.
Adaptive x-ray mirror prototype: First results (invited)
Authors: Susini J. , Baker R. , Krumrey M. , Schwegle W. , Kvick Å.
Published: July 2002

A first adaptive x-ray mirror prototype was built in 1992 and is now installed on the ESRF beamline 2 (Materials Science). This system consists of a water-cooled 1 m long Pt-coated Si mirror, supported by two rows of 11 piezoelectric actuators. The shape of the mirror is continuously monitored by an optical analyzer; its readout being used by a feedback loop in a real-time algorithm. A workstation associated to real-time electronics calculates the relevant parameters for driving the actuators as a function of both the desired shape and the possible thermal deformation due to the incident beam. Preliminary experiments carried out on the wiggler beamline showed that the system runs correctly: the feedback loop permits a control of the mirror shape in the microrad range at a frequency of 10 Hz. Various shapes (cylinder, parabola, or ellipse) can be produced at will to within the above accuracy. The different components of the device are described and the alignment and calibration procedures are discussed. The first results dealing with the properties of the reflected beam (size, divergence, and shape) versus both the calibration quality and the incoming power are presented.
X-ray undulator beamline BW1 at DORIS III
Authors: Frahm R. , Weigelt J. , Meyer G. , Materlik G.
Published: July 2002

The BW1 x-ray undulator beamline at HASYLAB offers high intensity for a wide variety of diffraction and absorption experiments. The x-ray undulator is a 4-m-long device with 127 periods. At 4.5 GeV energy of the positron storage ring DORIS III it can be tuned in its 3rd harmonic to photon energies from 9 to 15 keV. The basis for such a beamline is a high precision monochromator which has to be able to cope with high heat loads from the insertion device. There are four operating modes: (i) focused white beam using two mirrors with a cutoff energy of approximately 11 keV, (ii) focused monochromatic beam using both mirrors with a cutoff energy of ≊12 keV and a double-crystal monochromator, (iii) unfocused monochromatic beam using the monochromator without mirrors, and (iv) focused monochromatic beam using the monochromator without mirrors but with a sagittally focusing second crystal. Currently, a horizontal diffractometer for the investigation of liquid surfaces, an ultrahigh vacuum chamber for standing wave measurements and a versatile x-ray multicircle diffractometer are installed in the experiment hutch. Typical scientific investigations performed at this beamline will be discussed to demonstrate the capabilities of the whole setup.
A single-sweep, nanosecond time resolution laser temperature-jump apparatus
Authors: Ballew R. M. , Sabelko J. , Reiner C. , Gruebele M.
Published: July 2002

We describe a fast temperature-jump (T-jump) apparatus capable of acquiring kinetic relaxation transients via real-time fluorescence detection over a time interval from nanoseconds to milliseconds in a single sweep. The method is suitable for aqueous solutions, relying upon the direct absorption of laser light by the bulk water. This obviates the need for additives (serving as optical or conductive heaters) that may interact with the sample under investigation. The longitudinal temperature profile is made uniform by counterpropagating heating pulses. Dead time is limited to one period of the probe laser (16 ns). The apparatus response is tested with aqueous tryptophan and the diffusion-controlled dimerization of proflavine.
Global least-squares analysis of large, correlated spectral data sets and application to chemical kinetics and time-resolved fluorescence
Authors: Stilbs Peter , Paulsen Kim
Published: July 2002

A general, nonlinear least-squares, direct-search strategy for the analysis of the time-dependence of single or multicomponent spectral data sets is described. The generic procedure is named component resolved (CORE) spectroscopy, and has previously been successfully applied to the special case of Fourier transform pulsed field gradient spin-echo NMR data (FT-PGSE). The main purpose of the CORE processing is to improve the quality of evaluated data through its intrinsic S/N enhancement, and to confidently allow studies on multicomponent data sets characterized by either or both of (a) extensive spectral overlap and (b) only minor differences in (for example) individual component kinetic decay rates. The generality of the CORE approach is here illustrated through examples from chemical kinetics and time-resolved fluorescence. Unlike previous strategies for the same purpose, CORE can easily be adapted to a large variety of data set types, that can be of almost unlimited size. As a consequence of its direct approach to the underlying problem, the minimization approach is also perfectly stable. The generic CORE strategy also does appear to lend itself nicely to parallel processing, which would speed up the data processing by one or two orders of magnitude in future implementations.
Analysis of double exponential fluorescence decay behavior for optical temperature sensing
Authors: Sun T. , Zhang Z. Y. , Grattan K. T. V. , Palmer A. W.
Published: July 2002

Algorithms based on Prony’s method are proposed for the estimation of exponential time constants of a double exponential fluorescence–decay process. The performance of these algorithms is evaluated, using Monte Carlo simulations, the results of which are presented showing that the performance of Prony’s method can be greatly improved with the selection of an optimized observation window width and a few algorithm-related parameters. The applications of these noniterative algorithms in real-time, quasidistributed temperature sensor systems are discussed where the material decay may not approximate to a single exponential, or multimaterial sensors are used with a single, simple pump source and a single detector. Applications to quasidistributed sensor systems are considered and, thus, experimental results are presented to justify the use of the algorithms in Prony’s method in practical double exponential fluorescence decay sensor analysis.
Right-angle sample spinning electron spin resonance
Authors: Sierra Gustavo A. , Schweiger Arthur
Published: July 2002

Right-angle sample spinning electron spin resonance (RAS-ESR) is introduced. In this new pulse ESR technique the sample is rotated along an axis perpendicular to the direction of the static magnetic field B0. RAS-ESR can be applied to improve sensitivity and spectral resolution of ESR, electron spin echo envelope modulation, and electron nuclear double resonance spectra of orientationally disordered systems. A detailed qualitative description of two-dimensional (2D) anisotropy-resolved ESR (AR-ESR), a technique that is based on RAS is given. It is demonstrated that the anisotropy of a spin system can be used to disentangle a field-swept powder ESR spectrum into a second dimension. This new dimension is obtained by Fourier transformation of the stimulated echo modulations caused by incrementing the spinning frequency. Details of the probe assembly for sample spinning at temperatures T⩾10 K with spinning frequencies between 0 and 500 Hz, frequency increments of minimum 0.2 Hz, a long-time frequency stability better than 1 Hz, and the possibility of sample exchange at these temperatures are given. The performance of the RAS-ESR spectrometer is demonstrated by a stimulated echo 2D AR-ESR experiment on a transition metal complex in a polycrystalline host material.
A new method for measuring thermal conductivity of thin films
Authors: Govorkov S. , Ruderman W. , Horn M. W. , Goodman R. B. , Rothschild M.
Published: July 2002

A new, relatively inexpensive, easy-to-use instrument has been developed for measuring the thermal conductivity of thin films based on a differential photoacoustic method. Measurements made on silicon dioxide and silicon nitride are consistent with those reported previously for a different technique. In addition, the room temperature thermal conductivity of conventional polymer thin films and plasma deposited thin films has been determined relative to thermally grown silicon dioxide. Knowledge of the thermal conductivity of thin films, which is critical for many applications, can now be obtained for any thin film which can be deposited on a high thermal conductivity substrate.
A new Brewster angle microscope
Authors: Lheveder C. , Hénon S. , Mercier R. , Tissot G. , Fournet P. , Meunier J.
Published: July 2002

We present a new Brewster angle microscope for the study of very thin layers as thin as monolayers, using a custom-made objective. This objective avoids the drawbacks of the models existing at the present time. Its optical axis is perpendicular to the studied layer and consequently gives an image in focus in all the plane contrary to the existing models which give images in focus along a narrow strip. The objective allows one to obtain images with a good resolution (less than 1 μm) without scanning the surface, at the video frequency, allowing for dynamic studies. A large frontal distance associated with a very large aperture is obtained by using a large lens at the entrance of the objective.
Characterization of tips for conducting atomic force microscopy in ultrahigh vacuum
Authors: Lantz M. A. , O’Shea S. J. , Welland M. E.
Published: July 2002

We have investigated the reliability of a variety of metal coated and semiconductor tips for use in conducting atomic force microscopy experiments in an ultrahigh vacuum (UHV) environment. In order to obtain reliable conduction data we find it necessary to first clean the tips using a short argon ion sputter. Scanning transmission electron microscopy is used to image tips after the conductivity experiments and found to be very useful for assessing tip wear and interpreting conductivity data. Tip reliability is found to be strongly dependent on the sample and the experimental conditions. Wear and contamination of the tip are found to be severe problems which are related to the tip-sample adhesion. We illustrate these effects and highlight some of the common reliability problems which we encountered using specific examples. In general, we find that metal coated tips are not reliable enough to obtain repeatable data, especially if lateral forces are exerted on the tip. Homogeneous semiconductor tips, once cleaned, are found to be satisfactory and a particular contrast with experiments performed in air is that Si tips can be used reliably. In addition we find that in UHV, conduction experiments may be reliably performed even at very low applied force, of order nano-Newtons. This is a clear advantage in comparison to experiments performed in air where surface contamination is present and applied forces on the order of micro-Newtons are often required to establish stable electrical contact.
Measurements and simulations of the neutron production at W7-AS
Authors: Wolle B. , Weller A. , Schill S. , Gadelmeier F. , Baloui T. , Beikert G.
Published: July 2002

The 2.5 MeV neutron diagnostic system on the W7-AS stellarator comprises a set of BF3 and He3 neutron counters, a NE-213 scintillator and activation foils. The neutron counter set provides a dynamic range of several orders of magnitude while the scintillator provides fast resolution needed for studying transient events. The counters are calibrated in situ using a Pu/B238 neutron source placed at discrete points inside the vacuum vessel. The activation samples have been used to determine the total neutron yields as well as to measure the fluence of fast and thermal neutrons at various positions in the diagnostic hall of W7-AS. Neutron transport calculations have been performed using the Monte Carlo neutron photon transport code in order to determine the neutron fluence and energy distribution at the irradiation positions and to simulate the measured counter signals for the in situ calibration. The accuracy of the neutronics simulations depends on the quality of the geometric model and the material composition of major structural components.
High intensity source for cold atomic hydrogen and deuterium beams
Authors: Koch N. , Steffens E.
Published: July 2002

A source of highly dissociated, intense, cold hydrogen and deuterium atomic beams to be used as a dissociator in polarized gas targets for storage ring experiments has been developed. This dissociator is based on a plasma source which couples a 2.45 GHz surface wave to the discharge. At the end of the discharge tube a cold atomic beam is formed by means of a cooled nozzle. The entire system has been optimized to obtain a beam with a high atomic fraction and long-term stable reproducible operation. We measured a high degree of dissociation of more than 70% at gas throughputs up to 3 mbar ℓ/s for both hydrogen and deuterium, corresponding to an atomic flow rate of 1.2×1020 s−1. Excellent long-term stability has been achieved. We also studied quantitatively the effect of recombination of atoms on the nozzle surface for various materials.
A two-axis goniometer for sensitivity enhancement in single-crystal nuclear magnetic resonance spectroscopy
Authors: Vosegaard Thomas , Hald Eigil , Daugaard Preben , Jakobsen Hans J.
Published: July 2002

The construction of a single-crystal nuclear magnetic resonance (NMR) probe with a new two-axis geometry for the goniometer has improved the sensitivity of the NMR response from small crystals. This allows studies of the smallest crystals used so far in single-crystal NMR. The goniometer of the probe uses only two different mountings of the crystal, and the rotation axis of the goniometer is tilted 45° with respect to the direction of the magnetic field. By moving the goniometer outside the radio frequency coil, the filling factor and the probe sensitivity increase. The improved sensitivity for the probe is illustrated by the P-{H1}31 cross-polarization spectrum of a 0.088 mm3 crystal of (NH4)2HPO4 and a Rb87 single-pulse spectrum of a 0.048 mm3 crystal of RbZn2(HPO4)PO4. Moreover, the capability to determine precise parameters for the solid-state NMR interactions using the new probe is demonstrated by Na23 and Rb87 single-crystal studies of NaNO3 and RbClO4.
Time-resolved detection and identification of single analyte molecules in microcapillaries by time-correlated single-photon counting (TCSPC)
Authors: Becker W. , Hickl H. , Zander C. , Drexhage K. H. , Sauer M. , Siebert S. , Wolfrum J.
Published: July 2002

A PC plug-in card for on-line time resolved fluorescence detection of single dye molecules based on a new time-correlated single photon counting (TCSPC) module is described. The module contains all electronic components constant fraction discriminators (CFDs), time-to-amplitude converter (TAC), analog-to-digital converter (ADC), multichannel analyzer (MCA timers) on board required for TCSPC. A fast TAC design in combination with a fast flash ADC and an error-correcting ADC/MCA principle results in a maximum count rate of 8 MHz (dead time 125 ns). A dual memory architecture allows for unlimited recording of decay curves with collection times down to 150 μs without time gaps between subsequent recordings. Applying a short-pulse diode laser emitting at 640 nm with a repetition rate of 60 MHz in combination with a confocal microscope, we studied bursts of fluorescence photons from individual dye labeled mononucleotide molecules (Cy5-dCTP) in a cone shaped microcapillary with an inner diameter of 0.5 μm at the end of the tip. The flow of the conjugates was controlled by electrokinetic forces. The presented technique permits the counting and identification of all labeled analyte molecules present in a given sample due to their characteristic velocities, burst sizes, and fluorescence decay times.
A novel resonant beam technique to determine the elastic moduli in dependence on orientation and temperature up to 2000 °C
Authors: Lins W. , Kaindl G. , Peterlik H. , Kromp K.
Published: July 2002

A new equipment was developed to determine the elastic moduli from the resonance frequencies of the flexural vibrations of a beam up to temperatures of 2000 °C. The elastic moduli are calculated by minimizing the difference of the experimental versus the theoretical resonance frequencies. The theoretical frequencies are obtained by numerically solving Timoshenko’s equation, which takes into account the influence of the shear deformation and the rotatory inertia. From the fundamental frequency and the higher modes of vibration, one Young’s modulus and two shear moduli (e.g., E11, G12, and G13) are obtained, the latter two coincide for an elastically isotropic material. With the additional effort of cutting out specimens in specific directions, the complete elastic tensor of anisotropic materials can be measured at high temperatures. The limits and the precision of the method are discussed.
Resistance heating of the gasket in a gem-anvil high pressure cell
Authors: Balzaretti N. M. , Gonzalez E. J. , Piermarini G. J. , Russell T. P.
Published: July 2002

Resistance heating of the gasket strip in a gem-anvil high pressure cell was successful in obtaining sample temperatures up to 1100 °C, under pressures up to 4.0 GPa. The heating capabilities, as well as the mechanical and chemical stability, of several different gasket strips (two Ni-based alloys, Ta, Pt/Rh, and a Re/Mo alloy) with different design shapes, and two different single-crystal anvil materials (diamond and cubic zirconia) were investigated. Two gasket-strip designs were found to provide optimum uniform heating conditions while decreasing the required current needed to achieve 1100 °C. Two anvil systems were investigated to reduce the temperature increase of the pressure cell body. Cubic zirconia anvils reduced the cell-body temperature to 100 °C at sample temperatures up to 1100 °C. However, zirconia anvils often failed during heating and almost always failed during cooling. Diamond anvils with cubic zirconia mounting plates also permitted temperatures up to 1100 °C to be reached without anvil failure. However, the cell-body temperature increased to 300 °C. A sealed vacuum-type chamber was employed to eliminate the problem with gasket and anvil oxidation. The optimized operating parameters reported here provide a routine method for high temperature-high pressure studies. The method was used to densify and sinter nanosize amorphous silicon nitride and γ-alumina powders at high temperatures and high pressures.
Pulsed excimer laser deposition of Si1−xGex thin films
Authors: Antoni F. , Fogarassy E. , Fuchs C. , Grob J. J. , Prevot B. , Stoquert J. P.
Published: July 2002

The ability to grow Si1−xGex thin films on single-crystal silicon or fused silica substrates by the pulsed excimer laser ablation from a sintered SiGe target is investigated. The stoichiometry of the deposits was found to depend essentially on the Ge enrichment of the ablated target surface. Crystalline layers were obtained for substrate temperatures higher than a threshold value depending on its nature. Finally, surfaces, nearly free from droplets, were achieved by optimizing the laser irradiation conditions.
Atomistic simulation of point defects in silicon at high temperature
Authors: Sinno Talid , Jiang Z. Kurt , Brown Robert A.
Published: July 2002

The Stillinger–Weber interatomic potential is used in molecular dynamics simulations to compute estimates of the equilibrium and transport properties of self-interstitials and vacancies in crystalline silicon at high temperature. Equilibrium configurations are predicted as a 〈110〉 dumbbell for a self-interstitial, and as an inwardly relaxed configuration for a vacancy. Both structures show considerable delocalization with increasing temperature, which leads to a strong temperature dependence of the entropy of formation, as suggested by diffusion experiments. Diffusion coefficients and mechanisms are predicted as a function of temperature. The predictions are discussed in the context of experiments and first-principle calculations.
AlGaN ultraviolet photoconductors grown on sapphire
Authors: Walker D. , Zhang X. , Kung P. , Saxler A. , Javadpour S. , Xu J. , Razeghi M.
Published: July 2002

AlxGa1−xN (0≤x≤0.50) ultraviolet photoconductors with a minimum cutoff wavelength shorter than 260 nm have been fabricated and characterized. The AlGaN active layers were grown on (00⋅1) sapphire substrates by metalorganic chemical vapor deposition (MOCVD). The spectral responsivity of the GaN detector at 360 nm is about 1 A/W biased at 8 V at room temperature. The carrier lifetime derived from the voltage-dependent responsivity is 0.13–0.36 ms.
Induced magnetic anisotropy in Fe80B20 metallic glass by mechanical milling
Authors: Fan G. J. , Quan M. X. , Hu Z. Q.
Published: July 2002

By means of transmission Mössbauer spectroscopy, the effect of mechanical milling on the magnetic anisotropy of Fe80B20 metallic glass has been studied. On the basis of variation of the areal ratio R21 of the second to the first Mössbauer line with milling time, it was found that a rotation of the average magnetic-moment directions from the in-plane to an out-of-plane orientation may take place by mechanical milling without onset of bulk crystallization, and that the distribution of the magnetic-moment directions becomes random after the bulk crystallization has occurred. The induced out-of-plane magnetic anisotropy by mechanical milling can be attributed to the occurrence of the chemical short-range order and surface crystallization in the Fe80B20 metallic glass. The latter effect may cause the formation of compressive stresses in the bulk and therefore a rotation of magnetic-moment directions in the Fe80B20 metallic glass.
Homoepitaxial diamond films with large terraces
Authors: Hayashi Kazushi , Yamanaka Sadanori , Okushi Hideyo , Kajimura Koji
Published: July 2002

High-quality homoepitaxial diamond films having large terrace width have been successfully grown on (001) diamond substrates. Schottky contact properties as well as the surface morphology and the film crystallinity of the deposited films were characterized in order to examine a potential for electronic applications. It is found that the diamond films are covered with macroscopic steps running parallel to [110] direction and have atomically flat surfaces with 2×1 and 1×2 double-domain structure. Detailed analysis of the atomic force microscope image for the atomically flat region indicates that the terrace width for double atomic steps is around 50 nm, suggesting that the migration length of precursors under present condition is much longer than that previously reported. The current–voltage characteristics of Al–Schottky contacts to these diamond films show excellent rectification properties.
Ion-beam synthesis and stability of GaAs nanocrystals in silicon
Authors: White C. W. , Budai J. D. , Zhu J. G. , Withrow S. P. , Aziz M. J.
Published: July 2002

Sequential implantation of Ga and As into silicon followed by thermal annealing has been used to synthesize GaAs buried inside silicon. The GaAs exists in the form of nanocrystals which are three-dimensionally oriented with respect to the silicon matrix. Thermodynamic criteria which are important in determining whether the desired compound will form are discussed.
Preparation of cubic boron nitride films by radio frequency magnetron sputtering and radio frequency ion plating
Authors: Ulrich S. , Scherer J. , Schwan J. , Barzen I. , Jung K. , Scheib M. , Ehrhardt H.
Published: July 2002

Cubic boron nitride (c-BN) thin films have been deposited by unbalanced rf (13.56 MHz) magnetron sputtering of a hexagonal boron nitride target in a pure argon discharge. Deposition parameters have been 300 W rf target power, 8×10−4 mbar argon pressure, 3.5 cm target substrate distance, and 800 K substrate temperature. Under these conditions the ion current density is 2.25 mA/cm2 and the growth rate is ∼1.1 Å/s. By applying a rf substrate bias the ion plating energy is varied from plasma potential of 37 eV up to 127 eV. The films have been characterized by infrared (IR) and Auger electron spectroscopy (AES), x-ray diffraction (XRD), x-ray reflectivity, elastic recoil detection (ERD), Rutherford backscattering (RBS), nuclear resonance analysis (NRA), and stress measurements. The subplantation model proposed by Lifshitz and Robertson can be applied to the c-BN formation. An energy of about 85±5 eV is found where the stress (25 GPa, 200 nm film thickness) and the c-BN content (≳90%) have a maximum. The grain size of the crystalline c-BN phase was estimated to be in the range of 5 nm. Below an energy of 67±5 eV no c-BN could be detected. An excellent adhesion has been obtained by a special interface treatment.
Fabrication of boron-carbide/boron heterojunction devices
Authors: Hwang Seong-Don , Byun Dongjin , Ianno N. J. , Dowben P. A. , Kim H. R.
Published: July 2002

We have succeeded in the fabrication of a boron–carbide/boron diode on an aluminum substrate, and a boron–carbide/boron junction field effect transistor. Our results suggest that with respect to the approximately 2 eV band gap pure boron material, 0.9 eV band gap boron–carbide (B5C) acts as a p-type material. Both boron and boron–carbide (B5C) thin films were fabricated from single source borane cage molecules using plasma enhanced chemical vapor deposition (PECVD). Epitaxial growth does not appear to be a requirement.
Optical switching of coherent VO2 precipitates formed in sapphire by ion implantation and annealing
Authors: Gea Laurence A. , Boatner L. A.
Published: July 2002

Coherent precipitates of vanadium dioxide have been formed in the near-surface region of sapphire by the stoichiometric coimplantation of vanadium and oxygen combined with subsequent thermal processing at temperatures ranging from 700 to 1000 °C. The embedded VO2 precipitates, which are three-dimensionally oriented with respect to the Al2O3 host lattice, undergo a first-order monoclinic-to-tetragonal (and also semiconducting-to-metallic) phase transition at ∼77 °C. This transformation is accompanied by a significant variation in the optical transmission of the implanted region and results in the formation of an optically active, thermally ‘‘switchable’’ surface region on Al2O3.
Si/SiO2 resonant cavity photodetector
Authors: Diaz D. C. , Schow C. L. , Qi Jieming , Campbell J. C. , Bean J. C. , Peticolas L. J.
Published: July 2002

It has been shown earlier that GeSi/Si resonant-cavity photodiodes can achieve high speed without sacrificing quantum efficiency. In this letter, we report a Si-based resonant-cavity photodiode that utilizes a Si/SiO2 Bragg reflector. This structure is more compatible with standard Si processing technology than the GeSi/Si resonant-cavity photodiodes. The absorbing region is a 1-μm-thick polysilicon layer that has been annealed to enhance secondary grain growth and the bottom mirror consists of three quarter-wavelength pairs of Si and SiO2. After annealing the dark current was 9 μA at 1 V, the peak quantum efficiency was 44%, and the bandwidth was ≳1.4 GHz.
A critical thickness condition for a strained compliant substrate/epitaxial film system
Authors: Freund L. B. , Nix W. D.
Published: July 2002

The physical system under study is a single crystal film grown epitaxially on a substrate of comparable thickness which is constrained to remain flat. In general, the layers are strained due to a mismatch in lattice parameter between the film and substrate materials. The free energy change of the system due to formation of strain-relaxing interface misfit dislocations is estimated, and the discriminating case of zero energy change leads to a critical thickness condition on mismatch strain, film thickness, substrate thickness, and crystallographic slip orientation which is necessary for the spontaneous formation of such dislocations. The condition obtained generalizes the Matthews–Blakeslee (MB) criterion for a thin film on a thick substrate to the case of a complaint substrate/epitaxial film system, and it reduces to the MB criterion when either the film or substrate is relatively thick.
An increase of structural order parameter in Fe–Co–V soft magnetic alloy after thermal aging
Authors: Zhu Q. , Li L. , Masteller M. S. , Del Corso G. J.
Published: July 2002

Alloys of Fe49Co49V2 (Hiperco Alloy 50) (Hiperco is a registered trademark of CRS Holdings, Inc.), both annealed and thermally aged, were studied using anomalous synchrotron x-ray and neutron powder diffraction. Rietveld and diffraction profile analysis indicated both an increase in the structural order parameter and a small lattice expansion (∼0.0004 Å) after aging at 450 °C for 200 h. In addition, a cubic minority phase (<0.3%) was identified in the ‘‘annealed’’ sample, which increased noticeably (0.3%→0.8%) as a result of aging. The presence of antiphase domain boundaries in the alloys was also revealed. These results directly correlate with the observed changes in the magnetization behavior and challenge the notion that a ‘‘fully’’ ordered Fe–Co alloy demonstrates optimum soft magnetic properties.
Hardness measurement of thin films: Separation from composite hardness
Authors: He J. L. , Li W. Z. , Li H. D.
Published: July 2002

A composite hardness model based on the function of depth weight factor is presented. The model can be applied to determine the characteristic hardness of surface coatings which are too thin for the hardness to be directly measured. Its application requires only composite hardness data obtained from coated specimens by conventional microhardness measurement. DLC, TiN, and Cu films on substrates of glass, silicon, and AISI 52100 steel were used to verify this composite hardness model. It proved valid for a variety of cases.
Step-free surface grown on GaAs (111)B substrate by selective area metalorganic vapor phase epitaxy
Authors: Nishida Toshio , Kobayashi Naoki
Published: July 2002

Selective metalorganic vapor phase epitaxy of GaAs within a finite area was investigated on (111)B substrate. The surface stoichiometry is determined based on surface photo-absorption measurement. High-temperature growth at 800 °C on stable GaAs (111)B and a cooling procedure with the (2×2)-like surface to improve flatness result in an intentionally step-free surface with device dimensions as wide as 8 μm on a selectively grown GaAs mesa.
Wafer bonding of 50-mm diameter GaP to AlGaInP-GaP light-emitting diode wafers
Authors: Höfler G. E. , Vanderwater D. A. , DeFevere D. C. , Kish F. A. , Camras M. D. , Steranka F. M. , Tan I.-H.
Published: July 2002

The feasibility of wafer bonding 50-nm diameter wafers consisting of GaP-AlGaInP light-emitting diode epitaxial films to GaP substrates is demonstrated. Wafer bonding over the entire wafer area is achieved while maintaining optical transparency and low-resistance electrical conduction at the wafer-bonded interface. Using this technique, visible-spectrum transparent-substrate GaP-AlGaInP/GaP light emitting diodes (LEDs) are fabricated across an entire 50-mm wafer with typical operating voltages <2.1 V at 20 mA and twice the flux of absorbing-substrate GaP-AlGaInP/GaAs LEDs. This large-area wafer-bonding method is further shown to be capable of producing very high efficiency emitters, with an external quantum efficiency of 23.7% (300 K, 20 mA, dc) at 635.6 nm.
Si
29
nuclear magnetic resonance of luminescent silicon
Authors: Brandt M. S. , Ready S. E. , Boyce J. B.
Published: July 2002

Nuclear magnetic resonance of Si29 is used to study structural properties of porous silicon and siloxene. Evidence for changes in the chemical shift of porous silicon due to quantum confinement could not be observed. A hydride phase exhibiting a chemical shift of −75 ppm versus tetramethyl silane (TMS) is found in both porous silicon and siloxene. In as-prepared Wöhler siloxene, a chemical shift of −83 ppm versus TMS is assigned to threefold coordinated Si atoms forming a planar polysilane.
Thermally stable PtSi Schottky contact on n-GaN
Authors: Liu Q. Z. , Yu L. S. , Lau S. S. , Redwing J. M. , Perkins N. R. , Kuech T. F.
Published: July 2002

Platinum silicide (PtSi) and Pt Schottky contacts on n-GaN have been investigated and compared. The PtSi contacts were formed on n-GaN by annealing a multilayer structure of Pt/Si with the appropriate thickness ratio at 400 °C for 1 h in forming gas. The barrier height of the as-formed PtSi contacts was found to be 0.87 eV capacitance–voltage (C–V), and remained unchanged after further annealing at 400 and 500 °C. Upon annealing at 600 °C for 1 h, the barrier height decreased to 0.74 eV (C–V), but the diodes remained well-behaved. The as-deposited Pt yielded a barrier height of 1.0 eV (C–V). Upon annealing at 400 °C for 1 h, the Pt diodes degraded and most of the diodes did not survive additional annealing at 400 °C for longer times. The electrical measurements and the Rutherford backscattering spectrometry results indicated that PtSi contacts are thermally much more stable than Pt contacts on GaN.
Electro-optic detection of THz radiation in LiTaO3, LiNbO3 and ZnTe
Authors: Winnewisser C. , Jepsen P. Uhd , Schall M. , Schyja V. , Helm H.
Published: July 2002

Freely propagating THz pulses are detected in electro-optic (eo) crystals by monitoring the phase retardation (PR) of an infrared femtosecond probe pulse. This technique permits the determination of the temporal shape of the THz pulse in the subpicosecond time domain. We present measurements in LiTaO3, LiNbO3, and ZnTe and compare their signal performance as eo crystals with theoretical calculations for the PR signal. ZnTe shows the best performance for eo detection.
Optical absorption edge of semi-insulating GaAs and InP at high temperatures
Authors: Beaudoin M. , DeVries A. J. G. , Johnson S. R. , Laman H. , Tiedje T.
Published: July 2002

The temperature dependences of the optical absorption edges of semi-insulating GaAs and InP have been measured from room temperature to 905 °C and 748 °C, respectively, with accuracies of ±1 °C at room temperature and ±5 °C at 900 °C. The temperature dependence of the optical absorption edge is adequately reproduced by an Einstein model although the Varshni model gives an improved fit to the band gap. Finally, the widths of the absorption edges are correlated with ionicity.
Pressure-induced blueshift of Nd3+ fluorescence emission in YAlO3: Near infrared pressure sensor
Authors: Hua Hong , Vohra Yogesh K.
Published: July 2002

Pressure-induced shift of fluorescence emission in garnets and other oxides is investigated for applications as optical pressure sensors. We report high pressure studies on the fluorescence emission from neodymium-doped yttrium aluminum perovskite (Nd3+:YAlO3) to 80 GPa at room temperature. Unusual nonlinear blueshift of wavelength with pressure was found for several fluores cence peaks in the wavelength region of 850–900 nm. These peaks are identified as the lines associated with transition Nd3+:4F3/2→4I9/2 in YAlO3. The blueshift of these emissions is compared with the redshift of the same system in neodymium-doped yttrium aluminum garnet (Nd3+:Y3Al5O12). The shift with pressure is related to the change in the Stark level splitting with pressure. In view of the strong red emission from diamond anvils at ultrahigh pressures, this near infrared emission of Nd3+:YAlO3 holds promise as an ultrahigh pressure sensor.
Ion-induced effective surface diffusion in ion sputtering
Authors: Makeev Maxim A. , Barabási Albert-László
Published: July 2002

Ion bombardment is known to enhance surface diffusion and affect the surface morphology. Here we demonstrate that preferential erosion during ion sputtering can lead to a physical phenomenon reminiscent of surface diffusion, what we call effective surface diffusion (ESD), that does not imply mass transport along the surface and is independent of the temperature. We calculate the ion-induced ESD constant and its dependence on the ion energy, flux and angle of incidence, showing that sputtering can both enhance and suppress surface diffusion. The influence of ion-induced ESD on ripple formation and roughening of ion-sputtered surfaces is discussed and summarized in a morphological phase diagram.
Persistent photoconductivity and defect levels in n-type AlGaN/GaN heterostructures
Authors: Dang X. Z. , Wang C. D. , Yu E. T. , Boutros K. S. , Redwing J. M.
Published: July 2002

Persistent photoconductivity effects have been characterized in n-type Al0.15Ga0.85N/GaN heterostructures using both monochromatic light and room light illumination. Time constants of ∼1×104 s have been observed, and measurements of photocurrent specta performed using various illumination geometries and techniques have shown that defect levels exist in both the Al0.15Ga0.85N and GaN layers. Broad distributions of defect levels with excitation energies lower than the bandgap energies are found in both Al0.15Ga0.85N and GaN, and evidence is observed that these levels contribute significantly to the aforementioned persistent photoconductivity effects. The photocurrent spectra also reveal the presence of a level with an excitation energy of 3.36 eV that contributes to the persistent photoconductivity in the heterostructure.
Energy level alignment at Alq/metal interfaces
Authors: Lee S. T. , Hou X. Y. , Mason M. G. , Tang C. W.
Published: July 2002

The energy level alignment for both Mg/8-hydroxyquinoline aluminum (Alq) and Au/Alq interfaces has been determined by the ultraviolet photoemission measurements. For both interfaces, the difference between the Fermi level and the low-energy edge of the highest occupied molecular orbital (HOMO) is around 1.7 eV. This implies that the Fermi level with respect to the HOMO edge of Alq is independent of the work function of Mg and Au despite a large difference in the metal work function. A Fermi level alignment model is proposed, invoking a charge transfer between the metal and Alq and the formation of a dipolar layer at the metal/Alq interface.
Removal of 90° domain pinning in (100) Pb(Zr0.15Ti0.85)O3 thin films by pulsed operation
Authors: Kohli Markus , Muralt Paul , Setter Nava
Published: July 2002

X-ray diffraction showed that the volume fraction of 90° domains in tetragonal Pb(Zr,Ti)O3 thin films could be substantially reduced by either hot dc poling or by a bipolar pulsed-field process. In both cases, the (001) Bragg peak increased and the (100) peak decreased. However, ferroelectric hysteresis loops look quite different. Hot dc poling leads to large internal fields and high frozen-in polarisation, whereas pulse treatment removes the voltage shifts. This relaxation of the loop and x-ray diffraction results indicate a liberation of defect-pinned domain walls by removing, reorienting, or randomizing charged defects or defect dipoles. Alignment of defects during hot dc poling contributes to piezoelectric and pyroelectric activities.
Electrochemical determination of the ionization potential and electron affinity of poly(9,9-dioctylfluorene)
Authors: Janietz S. , Bradley D. D. C. , Grell M. , Giebeler C. , Inbasekaran M. , Woo E. P.
Published: July 2002

We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential Ip and electron affinity Ea of the polymer are obtained for the same sample under the same experimental conditions. We estimate Ip=5.80 eV and Ea=2.12 eV. These results disagree with the common assumption that Ea is, to good approximation, given by the difference between Ip and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.
Comparison of terahertz waveforms measured by electro-optic and photoconductive sampling
Authors: Park Sang-Gyu , Melloch M. R. , Weiner A. M.
Published: July 2002

Terahertz waveforms measured by free-space electro-optic sampling and a photoconductive dipole antenna were carefully compared. We show that the difference between the waveforms could be explained quantitatively in terms of carrier lifetime and frequency dependent response of the photoconductive receiver antenna.
The band-gap bowing of AlxGa1−xN alloys
Authors: Lee S. R. , Wright A. F. , Crawford M. H. , Petersen G. A. , Han J. , Biefeld R. M.
Published: July 2002

The band gap of AlxGa1−xN is measured for the composition range 0⩽x<0.45; the resulting bowing parameter, b=+0.69 eV, is compared to 20 previous works. A correlation is found between the measured band gaps and the methods used for epitaxial growth of the AlxGa1−xN: directly nucleated or buffered growths of AlxGa1−xN initiated on sapphire at temperatures T>800 °C usually lead to stronger apparent bowing (b>+1.3 eV); while growths initiated using low-temperature buffers on sapphire, followed by high-temperature growth, lead to weaker bowing (b<+1.3 eV). Extant data suggest that the intrinsic band-gap bowing parameter for AlGaN alloys is b=+0.62(±0.45) eV.
Emission mechanisms of bulk GaN and InGaN quantum wells prepared by lateral epitaxial overgrowth
Authors: Chichibu S. F. , Marchand H. , Minsky M. S. , Keller S. , Fini P. T. , Ibbetson J. P. , Fleischer S. B. , Speck J. S. , Bowers J. E. , Hu E. , Mishra U. K. , DenBaars S. P. , Deguchi T. , Sota T. , Nakamura S.
Published: July 2002

The emission mechanisms of bulk GaN and InGaN quantum wells (QWs) were studied by comparing their optical properties as a function of threading dislocation (TD) density, which was controlled by lateral epitaxial overgrowth. Slightly improved excitonic photoluminescence (PL) intensity was recognized by reducing TD density from 1010 cm−2 to less than 106 cm−2. However, the major PL decay time was independent of the TD density, but was rather sensitive to the interface quality or material purity. These results suggest that TDs simply reduce the net volume of light-emitting area. This effect is less pronounced in InGaN QWs where carriers are effectively localized at certain quantum disk size potential minima to form quantized excitons before being trapped in nonradiative pathways, resulting in a slow decay time. The absence of any change in the optical properties due to reduction of TD density suggested that the effective band gap fluctuation in InGaN QWs is not related to TDs.