bluuebunny/crossref_metadata_2025 huggingface dataset:
Retractable miniature double pass cylindrical mirror analyzers
Authors: Teodorescu C. M. , Gravel D. , Rühl E. , McAvoy T. J. , Choi Jaewu , Pugmire D. , Pribil P. , Loos J. , Dowben P. A.
Published: July 2002

The construction and performance of a double pass cylindrical mirror analyzer (CMA) is described. This electron energy analyzer is retractable and has a diameter of less than 1.5 in. (30 mm). This analyzer is superior in performance, for gas phase photoemission experiments, to the single pass CMA of similar diameter.
Description of a dust particle detection system and measurements of particulate contamination from shock, gate valve, and ion pump under ultrahigh vacuum conditions
Authors: Dorier J.-L. , Hilleret N.
Published: July 2002

Dust particle contamination is known to be responsible for reduced quality and yield in microelectronic processing. However it may also limit the operation of particle accelerators as a result of beam lifetime reduction or enhanced field emission in radio-frequency accelerating cavities. Intrinsic dust contamination from sources such as valves or ion pumps has not yet been studied due to the inability of commercial particle counters to be able to detect across large cross sections under ultrahigh vacuum (UHV) conditions. This motivated the development of the dust particle detector described here which is able to quantify, in situ, the level of contamination on a representative part of a vacuum vessel. This system operates under UHV conditions and measures flashes of scattered light from free falling dust particles as they cross a thin laser light sheet across a 100 mm diam vacuum vessel. A calibration using microspheres of known diameter has allowed estimation of the particle size from the scattered signal amplitude. Measurements of particulate contamination generated by shocks onto the vessel walls are presented and determination of the height of origin of dust particles from their transit time across the irradiation sheet is discussed. Measurements of dust particle release right to operation of an all-metal gate valve are also presented in the form of time resolved measurements of dust occurrence during the open/close cycles of the valve, as well as histograms of the particle size distribution. A partial self-cleaning effect is witnessed during the first 10 operation cycles following valve installation. The operation of an ion pump has also been investigated and revealed that, in our conditions, particles were released only at pump startup.
Optimal conditions for imaging in scanning tunneling microscopy: Experimental
Authors: Anguiano E. , Oliva A. I. , Aguilar M.
Published: July 2002

We present a study of the behavior of the texture of surfaces imaged by scanning tunneling microscopy (STM) under different control conditions in the feedback loop. The quality and the reliability of the STM images can be described quantitatively by giving the values of two adimensional parameters that describe the measurement conditions. The analysis of the main surface texture parameters such as the rms roughness and the average wavelength shows that they have a strong dependence on the values of the parameters used in the feedback loop for imaging regardless of the visual quality of the images that can be the same.
Direct observation of light-absorbing nanocrystals in glass matrices by the projection laser photoion microscopy technique
Authors: Sekatskii S. K. , Serebryakov D. V. , Letokhov V. S.
Published: July 2002

This article reports on the achievement of actual laser (spectral) selectivity in the field ion microscopy technique: when irradiating needle tips fabricated from heat-treated red light-filter glass by radiation from a copper vapor laser, light-absorbing CdSxSe1−x nanocrystallites present in the transparent glass matrix were registered in the form of bright spots against a dark background in photoion images. Mean radii of the observed nanocrystallites are equal to 10–15 nm which corresponds well to sizes being anticipated due to the heat treating conditions. Masses of the ions desorbed also indicated on the evaporation of rather heavy Cd, CdS, or analogous ions rather than light Si and analogous ions which should be typical for the laser-induced field evaporation of the transparent glass.
The detective quantum efficiency of medical x-ray image intensifiers
Authors: Thirlwall J. T.
Published: July 2002

The recognition of small features of low contrast in the image recorded by an x-ray image intensifier is usually limited by photon noise. This detail-contrast perceptibility is dependent on the detective quantum efficiency (DQE) and the modulation transfer function of the imaging tube and associated scanning system. The DQE can be calculated for all input photon energies from the statistical behavior of each stage of photon detection and amplification where quanta are lost or converted. The potential improvement to be obtained from any proposed design change can be estimated. The escape of characteristic photons from the input phosphor layer causes significant change to the performance particularly when gadolinium oxysulphide is employed for that purpose.
A removable cryogenic window for transmission of light and neutrons
Authors: Butterworth J. S. , Brome C. R. , Huffman P. R. , Mattoni C. E. H. , McKinsey D. N. , Doyle J. M.
Published: July 2002

A demountable, superfluid-tight window for the transmission of neutrons and light has been developed. A fluorocarbon polymer (Teflon) film is both the window and sealing gasket. Transmission of blue light and bursting pressure are measured for films of varying thicknesses.
On the role of neutrons in hadron calorimetry
Authors: Wigmans Richard
Published: July 2002

High-energy hadrons are absorbed in dense matter through a process called shower development. In this process, large numbers of soft neutrons are produced. These neutrons, which carry in total typically ∼10% of the shower energy, deposit this energy in ways very different from that for the charged shower components. This has important consequences for calorimetry, the particle detection technique based on total absorption of the objects to be measured. These consequences range from very beneficial (e.g., compensation) to very detrimental (e.g., the “Texas Tower effect”). In this article, the role played by neutrons in the formation of calorimeter signals is reviewed. The consequences for calorimetry in practical experiments are investigated.
An efficient method for calculating asymmetric diffraction peak profiles
Authors: Ida Takashi
Published: July 2002

An efficient method for evaluating asymmetric diffraction peak profile functions based on the convolution of the Lorentzian or Gaussian function with any asymmetric window function is proposed. When this method is applied to approximate the convolution with the Howard’s window function [J. Appl. Crystallogr. 15, 615 (1982)], only a few terms of numerical integration give satisfactory results, even if the asymmetry is very strong.
Streaked x-ray diagnostic for Double EAGLE z-pinch radiation sources
Authors: Failor B. H. , Coverdale C. A.
Published: July 2002

Gated-microchannel-plate cameras have primarily been used for time-resolved x-ray measurements on high current z pinches because they are compact and unaffected by magnetic fields and hard x rays. X-ray streak cameras are relatively large, and sensitive to magnetic fields, but they provide continuous time records with subnanosecond resolutions. We have demonstrated that an x-ray streak camera can be fielded successfully at the multimegampere Double EAGLE facility. We describe initial spectroscopy and imaging measurements and the vacuum housing and rf shielding developed to field the camera.
Upgrade of the RFX microwave frequency modulated reflectometer to ultrafast sweeping rate
Authors: Cavazzana R. , Moresco M. , Pomaro N. , Sardella A. , Spada E.
Published: July 2002

The swept homodyne reflectometer installed on the RFX experiment is composed of five units which cover the frequency range 26.5–110 GHz. This diagnostic system is based on solid state IMPATT sources and can be operated in the O or X mode. During the first operation, at a sweep rate of 0.1 GHz/μs, the typical density fluctuations present in the reversed field pinch configuration usually prevented correct profile reconstruction, which was possible only in some special cases. To increase the sweep speed and overcome this limit, a new IMPATT driver has been built and IF amplifiers, control, and acquisition systems have been modified accordingly. The new configuration has been extensively tested in the 34–38 GHz range with a modulation rate exceeding 4 GHz/μs on many different plasma conditions. Measurements on plasma showed that both the phase and reflected power level can be correctly recovered from the IF signals. These results indicated the criteria necessary to extend the fast sweep capability to the full 26.5–110 GHz range. In this article we present how the fast sweep capability is now being extended to the whole reflectometric diagnostic and the first successful plasma measurements obtained using a test reflectometric system.
Preliminary design of a heavy ion beam probe diagnostic for the National Spherical Torus Experiment
Authors: Mull F. H. , Crowley T. P. , Schoch P. M. , Connor K. A.
Published: July 2002

The preliminary design of a heavy ion beam probe (HIBP) for the National Spherical Torus Experiment (NSTX) is presented. The primary purpose of the diagnostic would be to measure density and electric potential fluctuations and steady state potential profiles. A secondary purpose is magnetic equilibrium and fluctuation measurements. HIBP particle trajectories have been calculated for a double null discharge case. The key HIBP measurements can be made with the injection point—detection point combination which is presented, including a radial scan line at a single energy (for potential profile measurements), poloidally aligned sample volumes (for poloidal wavenumber measurements), and coverage of a large fraction of the upper low field quadrant of the plasma. In addition, the injection point can be combined with new detectors for radial array measurements (for density fluctuation and/or magnetic measurements). Due to a large range of angles at the detector, up to 23° poloidal angle range and a 6° toroidal angle range, a secondary sweep system will be necessary for NSTX.
Long atomic imaging over a 5-μm-long region using an ultralow thermally drifted dual-tunneling-unit scanning tunneling microscope in a thermostabilized cell
Authors: Aketagawa Masato , Takada Koji , Takeshima Nobuhito , Yamada Kazutoshi , Lee Jong-Doo
Published: July 2002

This article describes the feasibility evaluation of long atomic imaging over a 5-μm-long region for the comparative length measurement using a crystalline lattice as the reference scale and a dual-tunneling-unit scanning tunneling microscope (DTU-STM) as a detector. For suppression of the thermal deformation error, ultralow thermally drifted DTU-STM was developed. The body of the DTU-STM is fabricated from ultralow linear expansion glass and Super-Invar. A thermostabilized cell was utilized to reduce the temperature fluctuation to less than 0.05 K around the DTU-STM. In the thermostabilized cell, the minimum lateral element of thermal drift rate of less than 0.2 nm/h at room temperature was achieved with the DTU-STM. In order to decrease the measurement time and thus reduce the thermal drift error, the fast scanning axis was selected as the direction of the length measurement over the μm range. After attaining thermal equilibrium, a long atomic image of highly oriented pyrolytic graphite over a 5-μm-long region along the fast scanning axis was obtained using the DTU-STM in the thermostabilized cell.
Large area and high sensitivity a-Si:H/a-SiC:H based detectors for visible and ultraviolet light
Authors: Mandracci P. , Giorgis F. , Pirri C. F. , Rastello M. L.
Published: July 2002

In this article we discuss the fabrication and performance of a-Si:H/a-SiC:H based ultraviolet sensitive devices. They were deposited over a large area (10 cm×10 cm) in p-i-n configuration using a new, multichamber, ultrahigh vacuum plasma enhanced chemical vapor deposition facility. The intrinsic layer thickness was 10–100 nm. The devices were characterized in the spectral range 365–660 nm and showed good sensitivity in the UV (365 nm). A good rejection of visible light was also measured. Responsivity as high as 0.30 A/W was measured at 365 nm for samples where the p layer and i layer were 5 and 10 nm thick, respectively. The linearity of the photogenerated current as a function of photon flux was measured. It was found that over an area of 25 cm2 the uniformity was within 15%. Using a laser (200 mW at 351 and 363 nm) the aging characteristics were measured and showed a 25% decrease in responsivity after the absorption of 105 J cm−2 under operating conditions.
A high temperature furnace for in situ and time-resolved x-ray diffraction studies
Authors: Capitán M. J. , Thouin N. , Rostaing G.
Published: July 2002

A high temperature furnace has been built and mounted on a six-circle goniometer for in situ diffraction studies of polycrystalline and powder samples. Temperatures up to 1400 K are controlled with a stability better than ±0.05 K in a vacuum better than 10−7 atm. The furnace is also able to work in oxidizing or reducing atmosphere and is mounted on a χ circle table using the φ circle as a driver for precise translation of the sample in the z direction. The sample can be rotated continuously via a mechanical feedthrough. The temperature is measured by means of a thermocouple placed at the sample holder (0.5 mm from the sample position). The temperature has been calibrated up to 1073 K by following the known thermal expansion of an AlCuFe quasicrystal.
Experimental adaptive optimization of mass spectrometer ion optic voltages using a genetic algorithm
Authors: Carpenter Scott D. , Schick Carolyn P. , Weber Peter M.
Published: July 2002

We use a genetic algorithm (GA) to optimize the mass resolution and detection efficiency of a multi-photon ionization, time-of-flight mass spectrometer. The algorithm uses experimental fitness functions to optimize eight voltages supplied to a set of ion optics. The GA optimized the ion detection efficiency by a factor of 10 and the mass resolution by a factor of 11 over previous settings obtained from computer simulations of the instrument. This experiment highlights the usefulness of adaptive algorithms to the experimental optimization of multidimensional search problems commonly found in modern laboratories.
Rotatable source crossed molecular beams apparatus with pulsed ultraviolet/vacuum ultraviolet photoionization detection
Authors: Willis P. A. , Stauffer H. U. , Hinrichs R. Z. , Davis H. F.
Published: July 2002

A newly constructed universal crossed molecular beams apparatus for studies of bimolecular chemical reaction dynamics is described. The apparatus employs two rotatable molecular beam sources and a fixed ultrahigh vacuum (UHV) quadrupole mass spectrometer with electron impact or pulsed photoionization of reaction products. Electronically cold neutral supersonic transition metal atomic beams are produced in one of the rotatable sources using laser vaporization. The beams are characterized by laser induced fluorescence spectroscopy, photodepletion spectroscopy, and time-of-flight analysis. Photoionization of the ZrC2H2 products from the crossed beam reaction Zr+C2H4→ZrC2H2+H2 is carried out using the pulsed 157 nm radiation from a F2 excimer laser in the UHV region of the mass spectrometer. Compared to conventional electron impact ionization, 157 nm photoionization improves signal-to-noise ratios by more than two orders of magnitude for experiments using pulsed beam sources where cross correlation methods cannot be used. We also demonstrate the use of 1+1 resonance enhanced multiphoton ionization for state selective detection of nonreactively scattered transition metal atoms from decay of long-lived collision complexes. Due to the small reaction cross sections for Y+C2H6→YC2H4+H2 and Y+CD3CDO→DYCD3+CO, these reactions could not be studied using electron impact ionization. However, photoionization detection permitted direct studies of the reactions with excellent signal-to-noise ratios. The greatly improved sensitivity of the photoionization technique facilitates studies of transition metal systems not previously amenable to the crossed beams method.
A novel bulge-testing setup for rectangular free-standing thin films
Authors: Kalkman A. J. , Verbruggen A. H. , Janssen G. C. A. M. , Groen F. H.
Published: July 2002

A newly designed bulge-testing apparatus for the mechanical testing of the tensile properties of free-standing thin films has been constructed and tested. With this instrument it is possible to measure the elastic modulus, tensile and compressive growth stress, and plain strain yield strength in thin films. The setup features a high strain resolution (4E-10), and a high stress resolution, e.g., >0.2 MPa for bulge heights larger than 10 μm. In our setup, thin films are stressed by a differential gas pressure across the film, while the deflection is measured by a scanning laser beam. The scanning laser beam measures the curvature of the bulge rather than the bulge height. This makes the setup insensitive to a possible initial nonflatness of the film. This also provides the possibility to measure the growth stress of films that were deposited under a compressive stress. We show the results of measurements of the plane strain modulus on thin tungsten films and on both flat and nonflat aluminum (alloy) film samples.
Emission optics of the Steigerwald-type electron gun
Authors: Ruan Chong-Yu , Fink Manfred
Published: July 2002

The emission properties of a Steigerwald-type electron gun are reexamined. Several optical parameters were experimentally determined, and they indicate that the original telefocus mechanism proposed by Steigerwald in 1949 needs to be revised. “Remote focusing” is found to be nonexistent in most of the telefocus electron guns deployed in high-energy electron scattering experiments. Instead, almost parallel beams with variable beam diameters were found. The physical mechanism responsible for producing a very narrow, monochromatic, and highly collimated beam without limiting apertures is explained in this article with supporting evidences from measurements. We think the new emission model proposed here can help new types of electron or ion gun designs and applications using electron gun in electron optical systems.
Optimization of spherical deflecting analyzer with finite-size effects
Authors: Ruan Chong-Yu , Nguyen Scott , Fink Manfred
Published: July 2002

Spherical analyzers have been widely used in electron spectroscopy due to their high energy resolution. In this article, we present a way of optimizing the performance of a 90° spherical deflecting analyzer (SDA-90) considering finite-source effects, using an imaging matrix formalism. Results are shown for both symmetrically and asymmetrically charged cases. The asymmetrical case is found to be superior both in transmission and resolution. This unexpected advantage can be explained by the rotation of the image caused by the broken symmetry along the principle trajectory in the analyzer using an asymmetrical potential configuration. The net effect is equivalent to a reduction in the lateral magnification and a gain in energy dispersion. Both factors improve the performance of the analyzer. The fringe fields, carefully controlled by two Herzog lenses, are very important in the optimization. A telefocus electron gun is used to check the numerical result, and to investigate the transverse focusing behavior.
Metal vapor vacuum arc ion source development at GSI
Authors: Reich H. , Spädtke P. , Oks E. M.
Published: July 2002

The status of experimental research, ongoing development, and upgrade of the high current metal vapor vacuum arc ion source is presented. By applying a magnetic field in the cathode region of the vacuum arc ion source it is possible to shift the charge state distribution to higher mean charge state. The combination of a magnetic field and small metal meshes in the plasma drift region is used to decrease the beam noise and to improve the ion beam stability. The results of experiments on the beam stability with different magnetic fields, cathode materials, and mesh are presented and discussed.
Construction and characterization of a heating stage for a scanning probe microscope up to 215 °C
Authors: Xie Z. , Luo E. Z. , Xu J. B. , Wilson I. H. , Zhao L. H. , Zhang X. X.
Published: July 2002

In this article, we present a study on construction and characterization of a heating stage compatible to commercially available scanning probe microscopes working in contact and tapping modes. Thermal properties of the heating stage have been characterized. With the heating stage, sample surface temperature can reach as high as 215 °C while the scanner temperature is kept below 125 °C. Below 50 °C, the stage temperature is very stable, with fluctuations less than 0.05 °C within half an hour. In both the contact and tapping mode of the force microscope, the image distortions have been calibrated, which occurs due to the decrease of piezoelectric coefficient at high temperature. It has been found that a cork wood spacer is excellent for thermal isolation to prevent the scanner from overheating. Examples of applications of the heating stage will be presented and discussed.
Resonant ultrasound spectroscopy for measurement of mechanical damping: Comparison with broadband viscoelastic spectroscopy
Authors: Lee T. , Lakes R. S. , Lal A.
Published: July 2002

This article compares resonant ultrasound spectroscopy (RUS) and other resonant methods for the determination of viscoelastic properties such as damping. RUS scans from 50 to 500 kHz were conducted on cubical specimens of several materials including brass, aluminum alloys, and polymethyl (methacrylate) (PMMA), a glassy polymer. Comparison of damping over the frequency ranges for broadband viscoelastic spectroscopy (BVS) and RUS for indium tin alloy in shear modes of deformation discloses a continuation of the tan δ power-law trend for ultrasonic frequencies up to 300 kHz. For PMMA, resonant peaks were sufficiently broad that higher modes in RUS began to overlap. Tan δ via RUS and BVS for PMMA agreed well in the frequency range where the methods overlap. RUS is capable of measuring tan δ as high as several percent at the fundamental frequency. Since higher modes are closely spaced, it is impractical to determine tan δ above 0.01–0.02 at frequencies other than the fundamental.
New impedance spectrometer for scientific and clinical studies of the respiratory system
Authors: Lopes de Melo Pedro , Werneck Marcelo Martins , Giannella-Neto Antonio
Published: July 2002

The simple techniques generally used to investigate the biomechanical properties associated with the respiratory process are able to identify only basic characteristics, just supplying an initial representation of the complex behavior of the respiratory system. A much more detailed description can be obtained by the forced oscillation technique (FOT). Requiring very little cooperation of the subject, this technique allows a noninvasive evaluation of the mechanical impedance of the respiratory system. Although the FOT has a great scientific and clinical potential, this is attended by technical difficulties and has yet to be put to widespread use. In this context, this work describes a new flexible, open architecture, virtual-instrument-based impedance spectrometer for the FOT. After the design details are described, the system accuracy is evaluated investigating mechanical models simulating normal and individuals with restrictive disease. This analysis revealed errors smaller than 4% in modulus and 7° in phase. Experimental results obtained in a normal and an asthmatic subject, presented as examples, revealed mean resistance in asthmatic (5.72 cm H2O/l/s) higher than in normal (2.47 cm H2O/l/s). The mean-reactance curve revealed higher resonance frequency in the asthmatic subject (∼29 Hz) than in the normal one (∼9 Hz). A higher influence of the upper airways impedance of the asthmatic individual was also observed. These results are in close agreement with physiological fundamentals, supplying substantial evidence that this instrument can contribute to the production of more detailed biophysical respiratory descriptions and composes an improved basis for the fast and easy implementation of complex medical research projects.
Absorption and resonant dispersion associated with normal incidence intersubband transitions in Si/SiGe quantum wells
Authors: Wu L. , Boucaud P. , Lourtioz J.-M. , Julien F. H. , Sagnes I. , Campidelli Y. , Badoz P.-A.
Published: July 2002

Modulation of midinfrared beam by photoinduced intersubband absorption in undoped Si/SiGe quantum wells is investigated at normal incidence. Optical pumping of interband transitions is used to photocreate carriers in the wells. The modulated transmission of the sample is measured by Fourier transform spectroscopy. Resonant dispersion associated with intersubband transitions as well as interference effects in the quantum well stack are clearly evidenced. It is shown that the phase modulation due to refractive index variations may compensate for amplitude modulation due to absorption. Measurements are well fitted by a phenomenological model. It is also shown that the modulation depth and the wavelength modulation profile both depend on the interband excitation wavelength. This result is attributed to the different nature of photoinduced transitions according to the excitation wavelength.
High critical current densities in Nb47%Ti multilayers with a planar copper flux pinning nanostructure
Authors: Kadyrov E. , Gurevich A. , Larbalestier D. C.
Published: July 2002

The critical current densities (Jc) of Nb47 wt% Ti–Cu multilayers were investigated for the Nb–Ti layer thickness ranging from ds=10 to 100 nm and for fixed Cu thickness dn=10 nm. Jc showed a strong enhancement with decreasing ds, reaching new very high values in this system [0.91 MA/cm2 (5 T) and 0.38 MA/cm2 (7 T)] for ds=30 nm, declining at smaller ds. The nonmonotonic dependence of Jc on ds arises from competition between the enhanced pinning force produced by the increased pin density at smaller ds which is opposed by a proximity-effect-induced depression of Tc at smaller superconductor thickness.
Soft x-ray emission from plasmas produced by ultraintense KrF-laser pulses in colloidal Al
Authors: Wülker C. , Theobald W. , Gnass D. R. , Schäfer F. P. , Bakos J. S. , Sauerbrey R. , Gordon S. P. , Falcone R. W.
Published: July 2002

The interaction of subpicosecond KrF-laser pulses with polished and colloidal metal surfaces at an intensity of 8×1015 W/cm2 was investigated experimentally. The measurements with spectral and temporal resolution for the colloidal targets showed a higher energy conversion efficiency into soft x rays and an increase in x-ray pulse lengths.
Microwave intermodulation products and excess critical current in YBa2Cu3O7−x Josephson junctions
Authors: Sollner T. C. L. Gerhard , Sage Jay P. , Oates Daniel E.
Published: July 2002

Third-order intermodulation power generated by a Josephson junction in an YBa2Cu3O7−x microwave resonator is measured and compared to a calculation based on the resistively shunted junction model. The results agree with junction parameters determined by power-dependent loss measurements on the same resonator at temperatures near the transition temperature TC, but measurements below TC/2 suggest that excess critical current, which does not contribute to the nonlinear properties of the junction, occurs at lower temperatures. Local heating is suggested to explain this feature. These results are relevant to the coupled-grain model used to describe microwave properties of high-TC superconductors and to the understanding of Josephson junctions in these materials.
Molecular beam epitaxy of low defect density (⩽1×104 cm−2) ZnSSe on GaAs
Authors: Wu B. J. , Haugen G. M. , DePuydt J. M. , Kuo L. H. , Salamanca-Riba L.
Published: July 2002

We study the growth of pseudomorphic ZnSSe layers on GaAs. The dependence of the epilayer quality on Zn exposure to the GaAs surface is investigated. Zn treatment prior to the ZnSe buffer growth on the As-rich GaAs surface results in the lowest defect density. Transmission electron microscopy studies show that an atomic scale smooth interface is formed. Based on the etch pit density and photoluminescence image analysis, ZnSSe layers with defect density ≤1×104 cm−2 can be reproducibly obtained.
Incoherently coupled soliton pairs in biased photorefractive crystals
Authors: Christodoulides D. N. , Singh S. R. , Carvalho M. I. , Segev M.
Published: July 2002

We show that incoherently coupled soliton pairs are possible in biased photorefractive crystals, under steady-state conditions. These solitons can propagate in bright-bright, dark-dark, as well as in bright-dark configurations. Such soliton pairs can be established provided that the carrier beams share the same polarization, wavelength, and are mutually incoherent. Relevant examples are provided where the photorefractive crystal is of the strontium barium niobate type. The characteristics and stability properties of these soliton states are also discussed in detail.
Morphology of luminescent GaN films grown by molecular beam epitaxy
Authors: Trager-Cowan C. , O’Donnell K. P. , Hooper S. E. , Foxon C. T.
Published: July 2002

GaN thin films were grown by molecular beam epitaxy on sapphire substrates. Scanning electron (SE) and atomic force microscopies reveal that on a typical film an assembly of oriented hexagonal microcrystallites rises above a background of polycrystalline or amorphous material. Cathodoluminescence (CL) spectra of the films feature bright UV exciton peaks and a broad green emission band. We identify the exciton peaks as those of the wurtzite form of GaN. A comparison of SE and CL micrographs of the same sample area shows that the luminescence emanates almost entirely from the hexagonal crystallites.
Intersubband relaxation time in the valence band of Si/Si1−xGex quantum wells
Authors: Boucaud P. , Julien F. H. , Prazeres R. , Ortega J.-M. , Sagnes I. , Campidelli Y.
Published: July 2002

We have investigated the saturation of intersubband absorption and the intersubband relaxation time in the valence band of Si/SiGe quantum wells. The quantum wells consist of 3 nm thick p-doped Si0.8Ge0.2 layers separated by 20 nm thick silicon barriers. At room temperature, these quantum wells exhibit intersubband absorption in p polarization (component of the electric field parallel to the growth axis), which peaks around 130 meV (≊10 μm). The saturation of intersubband absorption has been investigated using a picosecond free electron laser by two different techniques. The intersubband absorption is first analyzed as a function of the pump intensity. The saturation is found to occur around 90 MW/cm2, which yields a characteristic lifetime T1≊0.7 ps. In a second set of experiments, the lifetime is determined by time-resolved pump and probe experiments in cross polarization. The intersubband transition is pumped in p polarization and the saturated absorption is probed with a delayed beam in s polarization. An intersubband lifetime ≊0.4 ps is deduced by this technique. The saturation behavior is analyzed in terms of an inhomogeneously broadened two-level system.
Hydrogen-free amorphous carbon films approaching diamond prepared by magnetron sputtering
Authors: Logothetidis S.
Published: July 2002

Unhydrogenated amorphous carbon films were deposited by rf magnetron sputtering at room temperature. In situ spectroscopic ellipsometry and real time ellipsometry were used to monitor and characterize the films during deposition as well as to define the region of the preparation conditions where carbon films rich in sp3 C–C bonds are deposited. Evidence that the amorphous films exhibit the diamond character as well as their metastability were also provided and discussed by Raman spectroscopy and other techniques.
Inductively coupled plasma etching of GaN
Authors: Shul R. J. , McClellan G. B. , Casalnuovo S. A. , Rieger D. J. , Pearton S. J. , Constantine C. , Barratt C. , Karlicek R. F. , Tran C. , Schurman M.
Published: July 2002

Inductively coupled plasma (ICP) etch rates for GaN are reported as a function of plasma pressure, plasma chemistry, rf power, and ICP power. Using a Cl2/H2/Ar plasma chemistry, GaN etch rates as high as 6875 Å/min are reported. The GaN surface morphology remains smooth over a wide range of plasma conditions as quantified using atomic force microscopy. Several etch conditions yield highly anisotropic profiles with smooth sidewalls. These results have direct application to the fabrication of group-III nitride etched laser facets.
Formation of self-assembling CdSe quantum dots on ZnSe by molecular beam epitaxy
Authors: Xin S. H. , Wang P. D. , Yin Aie , Kim C. , Dobrowolska M. , Merz J. L. , Furdyna J. K.
Published: July 2002

We report the formation of self-assembling CdSe quantum dots during molecular beam epitaxial growth on ZnSe and ZnMnSe. Atomic force microscopy measurements on specimens with uncapped dots show relatively narrow dot size distributions, with typical dot diameters of 40±5 nm, and with a diameter-to-height ratio consistently very close to 4:1. Uncapped CdSe dots are unstable with time: their density was observed to drop by an order of magnitude in 10 days, with clear evidence of ripening observed for some dots. Photoluminescence from capped dots indicates exciton localization much stronger than in ZnCdSe/ZnSe quantum wells, due to the additional lateral confinement.
Growth and characterization of magnetoresistive La-Ca-Mn-O films on Si(100) and Si(111) substrates
Authors: Kung Pang-Jen , Fenner D. B. , Potrepka Daniel M. , Budnick Joseph I.
Published: July 2002

In situ grown La-Ca-Mn-O films with (100), (110), and mixed orientations have been prepared by pulsed-laser deposition on YSZ-buffered Si(100) and Si(111) substrates. X-ray diffractometry, magnetoresistance (MR), and magnetization of these films were measured. The best magnetoresistance was observed in the single-phase, epitaxial (110), and this structure was grown with the best quality on the YSZ(111)-buffered Si(111) substrates. These films showed higher temperatures for the peak resistivity than those in the (100) films. In a magnetic field of 5 T, the maximum magnetoresistance of 250% and 164% occurred at 195 and 140 K, respectively, in the as-deposited (110) and (100) films. The MR behavior of these two orientations, as a function of the substrate temperature during La-Ca-Mn-O film growth and the orientation of the YSZ buffer layer, is discussed.
Depth-resolved and excitation power dependent cathodoluminescence study of GaN films grown by metalorganic chemical vapor deposition
Authors: Li X. , Coleman J. J.
Published: July 2002

We have studied the luminescence properties of GaN films grown by metalorganic chemical vapor deposition using depth-resolved and excitation power dependent cathodoluminescence spectroscopy. In the thickness range we studied (∼3 μm), the luminescence properties, measured by the ratio of band-edge emission to the yellow band intensity, improve as the layer thickness increases.
Pencil-like mm-size electron beams produced with linear inductive voltage adders
Authors: Mazarakis M. G. , Poukey J. W. , Rovang D. C. , Maenchen J. E. , Cordova S. R. , Menge P. R. , Pepping R. , Bennett L. , Mikkelson K. , Smith D. L. , Halbleib J. , Stygar W. A. , Welch D. R.
Published: July 2002

We present the design, analysis, and results of the high brightness electron beam experiments currently under investigation at Sandia National Laboratories. The anticipated beam parameters are the following: energy 12 MeV, current 35–40 kA, rms radius 0.5 mm, and pulse duration 40 ns full width at half-maximum. The accelerator is SABRE, a pulsed linear inductive voltage adder modified to higher impedance, and the electron source is a magnetically immersed foilless electron diode. 20–30 T solenoidal magnets are required to insulate the diode and contain the beam to its extremely small-sized (1 mm) envelope. These experiments are designed to push the technology to produce the highest possible electron current in a submillimeter radius beam. Design, numerical simulations, and experimental results are presented.
Wiggler enhanced cyclotron autoresonance maser amplifiers
Authors: Lin A. T. , Lin Chih-Chien
Published: July 2002

The performance of cyclotron autoresonance maser amplifiers can be significantly improved by using a properly spatially profiled helical wiggler. The proposed configuration is able to alleviate the severe problem of performance degradation by beam velocity spread. Simulation results show that more than 20% output efficiency can still be attained at 94 GHz, even with 5% beam velocity spread.
Effect of stress on exchange coupling field, coercivity, and uniaxial anisotropy field of NiFe/NiO bilayer thin films
Authors: Han De-Hua , Zhu Jian-Gang , Judy Jack H. , Sivertsen John M.
Published: July 2002

The effects of stress on the exchange coupling field, Hex, and coercivity, Hc, in NiFe/NiO bilayers deposited onto a Si wafer using rf-reactive sputtering were experimentally studied. Samples of the bilayers were externally and constantly uniaxial strained (either tensile or compressive) using a specially designed sample holder. The hysteresis loops of the stressed NiFe/NiO bilayer samples were measured in situ along the easy and hard axis of the NiFe films using a vibrating sample magnetometer. The composition of the NiFe film was characterized to be Ni80.2Fe19.8 using x-ray photoelectron spectroscopy. The Hc of the bilayers were significantly affected by the stress, while the Hex was not apparently changed by the same stress. The large changes in the coercivity in the stressed NiFe/NiO bilayer were produced by the large change of the effective uniaxial anisotropy field, HK, of the bilayer. The control and reduction of both intrinsic and external stresses are important in the fabrication of spin-valve giant magnetoresistance heads and sensors.
Interface composition and stacking fault density in II-VI/III-V heterostructures
Authors: Heun S. , Paggel J. J. , Sorba L. , Rubini S. , Franciosi A. , Bonard J.-M. , Ganière J.-D.
Published: July 2002

The Zn/Se flux ratio employed during the early stages of molecular beam epitaxy of pseudomorphic ZnSe/GaAs(001) as well as lattice-matched ZnSe/In0.04Ga0.96As(001) heterostructures controls the density of the native stacking faults, which have been associated with the early degradation of blue-green lasers. In particular, the density of Shockley stacking fault pairs decreases by three to four orders of magnitude and that of Frank stacking faults by one order of magnitude in going from Zn-rich to Se-rich interfaces.
Mesoscopic magnetoquenched superconducting valve
Authors: Clinton T. W. , Johnson Mark
Published: July 2002

A superconducting switch has been developed using the magnetic fringe field of a ferromagnetic film to control the critical current in an underlying superconducting Sn film. The magnetization of the ferromagnet is rotated in the plane of the film to vary the magnitude of the fringe field applied locally to the superconductor from negligible to substantial values. A large suppression of the critical current is observed. Applications as an amplifier and a nonvolatile storage cell are possible, and a device based on high Tc materials has promise.
Separation of plasma implantation of oxygen to form silicon on insulator
Authors: Lu Xiang , Iyer S. Sundar Kumar , Liu Jingbao , Hu Chenming , Cheung Nathan W. , Min Jing , Chu Paul K.
Published: July 2002

Separation by plasma implantation of oxygen (SPIMOX) is an economical method for silicon-on-insulator (SOI) wafer fabrication. This process employs the plasma immersion ion implantation (PIII) for the implantation of oxygen ions. The implantation rate, which is independent of the wafer size, is considerably higher than that of conventional implantation. The simpler implanter set-up is lower in cost and easier to maintain. The feasibility of SPIMOX has been demonstrated with successful fabrication of SOI structures implementing this process. The operational phase space on implantation condition, oxygen dose, and annealing requirement are identified. Secondary ion mass spectrometry analysis and cross-sectional transmission electron microscopy micrographs of the SPIMOX sample showed continuous buried oxide under single crystal overlayer with sharp silicon/oxide interfaces.
Evolution of the local environment around Er upon thermal annealing in Er and O co-implanted Si
Authors: Terrasi A. , Franzò G. , Coffa S. , Priolo F. , D’Acapito F. , Mobilio S.
Published: July 2002

We present extended x-ray absorption fine structure (EXAFS) analyses of the Er LIII edge in Er-doped (100) Si samples. The samples were prepared by multiple implants of Er and O resulting in the incorporation of 1×1019 Er/cm3 and 1×1020 O/cm3 in a 2.3-μm-thick amorphous layer. It has been found that the local environment around the Er sites, which consists of six Si first neighbors in the amorphous layer, evolves towards a mixed coordination with O and Si atoms after epitaxial regrowth of the layer at 620 °C for 3 h. A further thermal treatment at 900 °C removes the residual Er–Si coordination and produces a full oxygen coordinated first shell with an average of 5 O neighbors. The effects of the different thermal processes on the high resolution spectra of the 1.54 μm Er photoluminescence were also measured and related to the EXAFS results.
Spectroscopic ellipsometric studies of InAs monolayers embedded in GaAs
Authors: Rheinländer B. , Schmidt Heidemarie , Gottschalch V.
Published: July 2002

Ellipsometric measurements of metalorganic vapor phase epitaxially grown InAs monolayers (0.5–2.0 ML) in GaAs were made at room temperature in the spectral range of the GaAs fundamental energy gap (hν=1.3–1.5 eV). Due to the introduction of InAs single-, double-, and multilayers, the optical properties of the GaAs host material are strongly modified, depending on the number and thickness of the InAs layers.
Measuring voltage transients with an ultrafast scanning tunneling microscope
Authors: Keil Ulrich D. , Jensen Jacob R. , Hvam Jo/rn M.
Published: July 2002

We use an ultrafast scanning tunneling microscope to resolve propagating voltage transients in space and time. We demonstrate that the previously observed dependence of the transient signal amplitude on the tunneling resistance was only caused by the electrical sampling circuit. With a modified circuit, where the tunneling tip is directly connected to the current amplifier of the scanning tunneling microscope, this dependence is eliminated. All results can be explained with coupling through the geometrical capacitance of the tip-electrode junction. By illuminating the current-gating photoconductive switch with a rigidly attached fiber, the probe is scanned without changing the probe characteristics.
The effects of high pressure on carbon nitride—in situ measurements of micro photoluminescence and infrared spectra
Authors: Zhao J. , Che R. Z. , Xu J. R. , Kang N.
Published: July 2002

The effects of high pressure up to 24 GPa on carbon nitride were studied using in situ micro photoluminescence (PL) and infrared (IR) absorption measurements at room temperature. The experiments indicate pressure-induced PL quenching with pressure increasing from 0 to 7 GPa, but the PL remains unchanged from 7 to 24 GPa. The PL results display the pressure effect of PL enhancement after release of pressure to ambient atmosphere. The IR absorption bands broaden toward low frequency after release of pressure. These may be explained by using the PL model of undistorted sp2 clusters, which, as PL centers, undergo distortion under pressure and pressure-induced local atomic rearrangement in the sample.
Influence of growth conditions on electrical characteristics of AlN on SiC
Authors: Zetterling C.-M. , Östling M. , Nordell N. , Schön O. , Deschler M.
Published: July 2002

Undoped aluminum nitride films were grown on 4H or 6H silicon carbide substrates, using metal–organic chemical-vapor deposition at 1150 °C. Different growth conditions were used, and two different V/III ratios were tested. Metal–insulator–semiconductor capacitors were made for high-frequency capacitance–voltage measurements at room temperature, from which the film thickness was determined. Accumulation, depletion, deep depletion, and inversion were seen for the best films, which also displayed peaks in x-ray diffraction rocking curves. Although large flatband voltage shifts occurred, indicating a fixed charge and interface trap problem, low conductance was observed. A flow of ammonia during ramp-up was found to improve the AlN films.
High voltage GaInP/GaAs dual-material Schottky rectifiers
Authors: Schoen K. J. , Harmon E. S. , Woodall J. M. , Chin T. P.
Published: July 2002

A dual-material structure of lattice-matched GaInP on GaAs has a calculated figure of merit which is approximately 60 times better than Si and 5 times better than GaAs. In this work, the theoretical performance of the GaInP/GaAs structure is presented and experimental data for Ni on GaInP/GaAs Schottky rectifiers is presented. The Ni on GaInP/GaAs Schottky rectifiers have a breakdown voltage of ∼80 V and low reverse leakage current. Comparable Ni on GaAs Schottky rectifiers have a breakdown voltage of ∼20 V and significantly higher reverse leakage current. The GaInP/GaAs rectifiers’ forward characteristics have a current–voltage extracted φBn of 1.0 eV with an ideality factor of 1.06. This dual-material structure of GaInP/GaAs appears to be a promising candidate for improving power device performance.
Surface-reconstruction-enhanced solubility of N, P, As, and Sb in III-V semiconductors
Authors: Zhang S. B. , Zunger Alex
Published: July 2002

We show that surface reconstructions may play an essential role in determining the equilibrium solubilities of N, P, As, and Sb in various III-V compounds. In particular, anion–anion dimerization of the (001)-β2(2×4) surface can enhance the solubility of N near the surface in GaAs, GaP, and InP by five, three, and two orders of magnitudes, respectively, at 1000 K. With certain assumptions on the growth kinetics, this high concentration of N may be frozen in as the crystal grows.
Extremely high-mobility two dimensional electron gas: Evaluation of scattering mechanisms
Authors: Umansky V. , de-Picciotto R. , Heiblum M.
Published: July 2002

We report on the characterization of selectively doped GaAs/AlGaAs heterostructures, grown by an extremely clean molecular beam epitaxy system, which exhibit a Hall mobility of a two dimensional electron gas exceeding 10×106 cm2/Vs for a wide range of undoped spacer layer thickness (50–100 nm). A maximum electron mobility of 14.4×106 cm2/Vs was measured at 0.1 K in a structure with a 68 nm spacer thickness and an areal carrier density of 2.4×1011 cm−2. This is the highest electron mobility ever reported, leading to a momentum relaxation mean-free path of ∼120 μm. We present experiments that enable us to distinguish between the main scattering mechanisms. We find that scattering due to background impurities limits electron mobility in our best samples, suggesting that further improvement in structure quality is possible.
Properties of InGaN quantum-well heterostructures grown on sapphire by metalorganic chemical vapor deposition
Authors: Grudowski P. A. , Eiting C. J. , Park J. , Shelton B. S. , Lambert D. J. H. , Dupuis R. D.
Published: July 2002

We report the growth and characterization of InGaN heteroepitaxial thin films and quantum-well heterostructures on (0001) sapphire substrates. The III-N heteroepitaxial layers are grown by metalorganic chemical vapor deposition on sapphire substrates using various growth conditions. A comparison of the 300 K photoluminescence (PL) spectra of the samples indicates that a higher PL intensity is measured for the quantum-well structures having an intentional n-type Si-doping concentration. Furthermore, three-, five-, and eight-period InGaN quantum-well structures exhibit similar narrow PL spectra.