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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Unintentionally doped and silicon doped GaN films prepared by molecular beam epitaxy using ammonia are investigated. Hall, secondary ion mass spectroscopy (SIMS), photoluminescence, and x-ray data are utilized for analysis of sources of autodoping of GaN epitaxial films in an effort to identify whether the n-type background electron concentration is of impurity origin or native defect origin. We identify and quantify an anomalous relationship between the Si doping concentration and free carrier concentration and mobility using temperature dependent Hall measurements on a series of 2.0-μm-thick GaN(0001) films grown on sapphire with various Si doping concentrations. SIMS is used to identify oxygen as the origin of the excess free carriers in lightly doped and undoped GaN films. Further, the source of the oxygen is positively identified to be dissociation of the sapphire substrate at the nitride-sapphire interface. Dissociation of SiC at the nitride-carbide interface is also observed. Finally, SIMS is again utilized to show how Si doping can be utilized to suppress the diffusion of the oxygen into the GaN layer from the sapphire substrate. The mechanism of suppression is believed to be formation of a Si–O bond and a greatly reduced diffusion coefficient of the subsequent Si–O complex in GaN. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A UHV compatible compact double-crystal monochromator was designed and constructed for BEPC diffraction and the scattering beamline 4B9A. It is a compact device with a 325 ×250×250 mm3 vacuum chamber. A T-structure mechanism has been designed for allowing the monochromator to have a Bragg angle range from 5° to 70° and keep the exit beam position and direction fixed during wavelength tuning. The first crystal is cooled by a heat-pipe cooling system and a pair of PZT drivers provide a two-dimensional adjustment to optimize the alignment between two crystals. The design and operating principles are described in this paper.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 60 (1989), S. 2085-2088 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The BEPC beamline 4B9B is intended for VUV-SX photoelectron spectroscopy and other soft x-ray applications. The beamline optical system consists of a focusing mirror system and a VUV-SX spherical grating monochromator with four gratings, which cover a large spectral range 10–1100 eV. Three laminar gratings (1100, 500, and 200 lines/mm) with a 57–m radius are used at a 174° including angle, and the other grating (600 lines/mm) with a 6-m radius is used at a 142° including angle to be suited for low-energy use. Scanning is achieved by the rotation of the grating, and a pair of stepping motor-driven slits follow the Rowland circle to improve the monochromator's performance. The optical design and ray tracing results about this monochromator are described
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 2727-2729 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The substrate temperature and arsenic pressure dependence of the density of InAs three-dimensional (3D) islands formed on GaAs(100) is found to exhibit a behavior that cannot be reconciled within the currently popular view of MBE growth. Rather, either an arsenic coverage induced strain enhanced In migration or strain dependent arsenic incorporation at islands, or both, appear to be operative. Plan-view and cross-sectional transmission electron microscopy, including the nature of the Moiré fringes, are used to obtain cluster size distribution and demarcation between size regime for coherent versus incoherent islands. The results point to the possibility of realizing a regular array of quantum dots made of coherently strained 3D islands of uniform size via growth on prepatterned substrates.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of clinical periodontology 24 (1997), S. 0 
    ISSN: 1600-051X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Abstract The aim of this 12-month parallel design controlled clinical trial was to assess the effect of the Braun Oral-B Plak Control electric toothbrush on supra-gingival plaque and gingival health, and to compare it with a conventional soft manual toothbrush (Jordan). A total of 111 patients aged between 20 and 63 years, from a general population, with bleeding on probing at 30% or more of all sites examined were entered into the study. At baseline, immediately after periodontal examination, all volunteers received a thorough scaling of their teeth. Volunteers in both groups were told to brush their teeth for 2 min 2× a day. Oral hygiene instruction was given at the start of the study and was not repeated. At 3, 6 and 12 months, assessments were carried out by a single clinician who was not aware which group the volunteers belonged to. Analysis of results demonstrated that over the 12 months of the study, the Braun Oral-B Plak Control was significantly more effective in improving gingival health than the manual toothbrush. There was, however, no difference between the 2 groups in terms of plaque removal, with the number of sites with visible plaque decreasing by a similar amount in both groups. In conclusion, results indicate that the Braun Oral-B Plak Control toothbrush is safe and more effective than a manual toothbrush in improving gingival health.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 57 (1990), S. 2803-2805 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The distribution of microtwins in GaAs epilayer grown on Si (001) substrates tilted towards the [1¯11] direction by molecular beam epitaxy has been studied by transmission electron microscopy. An asymmetric distribution of microtwins attributed to substrate misorientation and two-dimensional (2D) growth mode has been found. Orthogonal [1¯10] and [110] cross sections are identified by the angle of tilt in large-angle convergent beam electron diffraction Tanaka patterns [J. Electron. Microsc. 29, 408 (1980)] taken across the GaAs/Si interface. It is found that (11¯1) microtwins are preferentially grown in GaAs epilayers on a tilted Si (001) substrate where the growth mode is 2D, while symmetrical (1¯1¯1) and (111) twins are observed when there is a reversal of twin distribution and the growth mode is 3D.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The intensities of sidebands in convergent beam electron diffraction reflections from plan-view specimens of strain modulated InGaAs/GaAs superlattices are dependent on the natural lattice mismatch, the ratio of the thicknesses of the superlattice layers, the period (sum of layer thicknesses) of the superlattice, and the g vectors of the reflections. The intensities of kinematic higher-order Laue zone (HOLZ) reflections have been calculated from a simple model based on alternate contraction and expansion of lattice spacings of the superlattice layers. An accurate estimate of the In content of InGaAs can be deduced from the elastic strains in the superlattice layers so obtained.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Geostandards and geoanalytical research 18 (1994), S. 0 
    ISSN: 1751-908X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: The concentrations of 27 trace elements in the basalt BIR-1, as well as in other mafic rock reference materials, have been determined by ICP-MS, using pure elemental solutions for external calibration and standard additions. Concentrations of rare earth elements in BIR-1 for the new ICP-MS data, agree with compiled values, and previous ID-SSMS results, whereas ICP-MS data on HFSE's are systematically higher by about 40% to 70% than ID-SSMS, but are consistent with data generated at other ICP-MS laboratories.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of oral rehabilitation 23 (1996), S. 0 
    ISSN: 1365-2842
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: SUMMARY Alveolar atrophy and its relation to osteo-penia of the mandible were studied in 124 edentulous 76-, and 81- and 86-year-old subjects (32 males and 92 females). Both variables were assessed visually from panoramic radiographs. Mandibular alveolar atrophy was found to be more severe in women (P 〈 0.001). No statistically significant differences were found between the three age-groups in mandibular alveolar atrophy, nor between the sexes or agegroups in maxillary alveolar atrophy. The structure of the mandibular bone was osteopenic in 38 patients (30.7%), a situation that was more frequent in women (P 〈 0.001). The relationship between the observed osteopenia and the severity of the mandibular alveolar atrophy in women was statistically significant (P 〈 0.05).
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Analytica Chimica Acta 276 (1993), S. 411-417 
    ISSN: 0003-2670
    Keywords: Iron ; Spectroelectrochemistry ; UV-Visible spectrophotometry ; Voltammetry ; Waters
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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