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  • 1
    ISSN: 1432-0630
    Keywords: 78.70.Bj ; 79.90 ; 72.20.Ht
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Positron lifetime spectroscopy measurements have been carried out for semi-insulating GaAs with applied electric fields in the samples directed towards, and away from the positron injecting contact. The lifetime spectra have been decomposed into two components, the longer of which (≈400 ps) is characteristic of open volume defects at the metal-semiconductor interface through which positrons are injected. The interesting feature of these experiments is the large increase in the intensity of this interface component as the field is directed towards the contact. We show that this increase is caused by a significant fraction of implanted positrons drifting under the influence of a strong electric field produced by a layer of space-charge formed adjacent to the positron injecting contact. The general trend of the intensity variation is well explained by the proposed model. Experiments involving the application of an ac bias to the samples strengthen the suggestion that the space charge region is largely formed from ionized EL2 donors. The results of the present work indicate that semi-insulating GaAs possesses properties that make it a suitable material for the fabrication of a high efficiency (≈10%) room-temperature field-assisted positron moderator. The extraction of positrons from the GaAs substrate into the vacuum through a thin metalization is discussed based upon available positron affinities for the GaAs and various elemental metals. These data suggest that a few monolayers of a strongly electronegative metal such as Au or Pd may allow vacuum emission through quantum tunneling.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-0630
    Keywords: 68.35.-p ; 78.70.Bj
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Recent positron lifetime studies made on the Au/GaAs interface with an applied electric field returning a significant fraction of bulk implanted positrons to the interface have revealed the presence of microvoids(≈ 1 nm diameter) at the interface. In this work an attempt has been made to study these microvoids by observing the Doppler broadening on the annihilation radiation coming from them. This is done both by observing theS-parameter as a function of applied bias and by applying the generalized least-squares method to the deconvolution of the annihilation radiation lineshape. The general conclusion is that the Doppler-broadened data are consistent with the majority of positrons trapping into microvoids, probably associated with grain boundaries. The data suggest that these open volume defects are more associated with the Au film rather than the Au-Ga alloyed interfacial region.
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  • 3
    ISSN: 1432-0630
    Keywords: PACS: 68.35.-p; 78.70.Bj
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract.  Recent positron lifetime studies made on the Au/GaAs interface with an applied electric field returning a significant fraction of bulk implanted positrons to the interface have revealed the presence of microvoids (≈ 1 nm diameter) at the interface. In this work an attempt has been made to study these microvoids by observing the Doppler broadening on the annihilation radiation coming from them. This is done both by observing the S-parameter as a function of applied bias and by applying the generalized least-squares method to the deconvolution of the annihilation radiation lineshape. The general conclusion is that the Doppler-broadened data are consistent with the majority of positrons trapping into microvoids, probably associated with grain boundaries. The data suggest that these open volume defects are more associated with the Au film rather than the Au-Ga alloyed interfacial region.
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Acta mechanica 18 (1973), S. 1-20 
    ISSN: 1619-6937
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Zusammenfassung Der Fall von kleinen Mikrostruktur-Deformationen von einer anfänglichen kugelähnlichen Form wird untersucht. Für die Ausbildung der Scherbewegung werden Lösungen angegeben.
    Notes: Summary The case of fluids which experience small microstructure deformations from an initially spherical shape is examined. Solutions are presented for transient shear flows.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    Journal of food science 39 (1974), S. 0 
    ISSN: 1750-3841
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Vegetable pieces were suspended in a vertical 3 in. glass pipe through which steam passed at known temperature, pressure and velocity, and the center temperature of the piece was measured. Thc values for center temperature vs. time were used to estimate the surface heat transfer coefficient using Gurney-Lurk charts. The volume of noncondensable gas in the vegetable affected the overall heat transfer coefficient. Reducing the volume of gas in the vegetable by vacuum degassing prior to heating increased the rate of center temperature rise, while increasing the volume of gas prior to heating by exposing the vegetable piece to nitrogen at 1500 psig decreased the rate. Steam velocities in the range of 0.1–2.6 ft/sec did not measurably affect the heating rate. However, when the velocity was increased further by using a nozzle, the rate of center temperature rise was increased.
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  • 6
    ISSN: 0008-6215
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 0040-4039
    Keywords: Cyclodextrin ; Selective Substitution ; Synthesis ; Tosylation.
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 1889-1892 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Variable energy positron beam and positron annihilation lifetime experiments have been carried out to study the DX center in Cd0.8Zn0.2Te:Cl at 50 K. A short positron effective diffusion length of 275±25 Å and a large intensity of 79.0%±0.3% for the long lifetime component indicate a strong trapping effect at DX centers. A trapping rate of κ=1.53±0.05×109 s−1 and a positron lifetime of 335±2 ps at the DX center were obtained. The concentration of DX centers is found to be 5.9±0.7×1016 cm−3, which is in good agreement with the results obtained using Hall effect and thermo-electric effect measurements. © 1998 American Institute of Physics.
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The interfacial electric field established under different reverse bias conditions in Au and Ni on semi-insulating GaAs junctions has been studied by means of a low energy positron beam. The technique used is that of monitoring the positron drift to the interface through changes in the annihilation radiation lineshape as a function of incident positron beam energy at different reverse biases. The data show a small but clear electric field drift of positrons towards the interface that increases more rapidly at low voltages (less than 50 V) which at higher biases tends towards saturation. This confirmation of electric field saturation adds further weight to the picture of an electric field enhanced electron capture cross section for the ionized EL2 defect. Electric field values extracted from the data are compared with results from other techniques and suggest that enhanced electron capture is already occurring at the relatively low built-in fields (∼1 kV cm−1) found at the unbiased junction, with a rapid increase of EL2+ neutralization occurring for biases above 10 V. At still higher fields ∼10 kV cm−1 (biases〉50 V), there appears to be an additional threshold for more complete EL2+ neutralization adjacent to the contact. The present study clearly demonstrates the often overlooked necessity of catering for built-in electric fields in positron diffusivity studies of III–V semiconductors where surface midgap Fermi-level pinning is common. © 1997 American Institute of Physics.
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  • 10
    ISSN: 0008-6215
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
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