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Semi-insulating GaAs: A possible substrate for a field-assisted positron moderator (1994)

Shan, Y. Y. ; Au, H. L. ; Ling, C. C. ; [et al.]

Springer

Applied physics 59 (1994), S. 259-273

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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

URL: http://dx.doi.org/10.1007/BF00348229

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Transmission EELS of oxide superconductors with a cold field emission TEM (1995)

Wang, Y. Y. ; Zhang, H. ; Dravid, V. P.

New York, NY [u.a.] : Wiley-Blackwell

Microscopy Research and Technique 30 (1995), S. 208-217

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ISSN: 1059-910X

Keywords: Superconductors ; Electron energy loss spectrometry ; Transmission electron microscope ; Life and Medical Sciences ; Cell & Developmental Biology

Source: Wiley InterScience Backfile Collection 1832-2000

Topics: Natural Sciences in General

Notes: Electron energy loss spectrometry (EELS) with a cold field emission gun (cFEG) transmission electron microscope (TEM) is implemented to analyze the evolution of the electronic structure and dielectric function of oxide superconductors. The O-K core loss spectra of p-type doped oxide superconductors are analyzed in terms of holes formation on oxygen sites, while low loss spectra are analyzed for free carrier plasmas, other spectral excitations, and their crystallographic confinement.It is illustrated that the transmission EELS with a cFEG TEM very much complement soft X-ray absorption spectroscopy and optical spectroscopy, with the added advantages of high spatial resolution (∼1-100 nm), and is compatible with other analytical, diffraction, and imaging techniques, which are readily available in a cFEG TEM. © 1995 Wiley-Liss, Inc.

Additional Material: 10 Ill.