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  • 1
    Electronic Resource
    Electronic Resource
    Vacancy-type defects in crystalline and amorphous SiO2 (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 884-890 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime spectroscopy and two-dimensional angular correlation of annihilation radiation have been used to investigate grown-in vacancy structures in synthetic crystalline α-SiO2, synthetic fused quartz, and in a 60-μm-thick chemical-vapor-deposited amorphous SiO2 film. For α-SiO2 a ∼300 ps lifetime component suggests trapping by either silicon monovacancies or by oxygen divacancies (or both). The vacancies are neutral and present at a concentration level of 1017/cm3. The positron bulk lifetime for α-SiO2 is estimated to be ∼238 ps in good agreement with semiempirical predictions. In the fused quartz significant positronium formation is found (80%) and the remaining positrons annihilate in voids yielding a lifetime of ∼500 ps. The amorphous SiO2 film contains a mixture of small vacancy clusters and voids and ∼30% of the positrons form positronium. Heat treatment above 950 °C results in a substantial reduction in defect concentration, but up to 1100 °C a small vacancy cluster contribution persists. The positron data indicate that positronium formation in the fused quartz and in the amorphous film takes place in the voids.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354882
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  • 2
    Electronic Resource
    Electronic Resource
    Annealing studies of vacancies in proton irradiated silicon (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 3740-3743 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Annealing of vacancies produced by heavy proton irradiation of float-zone (Fz) and P-doped Czochralski-grown (Cz) silicon has been investigated by positron lifetime spectroscopy. In Fz-Si divacancies are retained after irradiation, and these defects are completely annealed out at 700 °C. In Cz P-doped silicon, impurities are found to enhance both the amount of retained vacancies as well as the tendency for vacancy clustering. Two annealing stages appear at 100 °C and close to 450 °C which seem to be a result of interstitial migration. Vacancy migration takes place in a wide temperature range between 100 and 1000 °C.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352905
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  • 3
    Electronic Resource
    Electronic Resource
    Deformation-induced defects in GaAs (1991)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 69 (1991), S. 4080-4091 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Semi-insulating undoped GaAs was plastically deformed and then investigated by positron lifetime spectroscopy. Strains between 0% and 40% and temperatures of deformation of 450, 500, and 600 °C were investigated, with detailed investigations carried out for the lowest temperature of deformation. Between 0% and 4% strain a reduction of the grown-in vacancy response takes place simultaneously with a slight increase in vacancy cluster size to 2 or 3 vacancies. Between 4% and 6% strain a very substantial increase in vacancy production occurs but nearly all of these vacancies are clustered into voids with a radius of about 50 A(ring) and density of the order of 1013–1014 cm−3. The total concentration of vacancies necessary to produce these voids is 1017–1018 cm−3. This clearly shows that vacancies are formed upon deformation and that they are mobile at 450 °C. The small vacancy clusters (2 or 3 vacancies) are present at a concentration of about 5×1016 cm−3, the same as for the 4% strained samples. Upon further deformation to 20% strain the overall defect concentration becomes so high that all positrons become trapped for which reason no absolute defect concentrations can be deduced. The dominant defect types can nevertheless be identified as voids (with average size of 20 A(ring)), two- or three-vacancy clusters, and shallow traps. These shallow traps have a positron binding energy of about 40 meV. Isochronal and isothermal annealing of 40% strained samples shows that heat treatment reduces the void concentration but increases the average void size, reduces the small vacancy cluster concentration, but results only in a small decrease in shallow trap concentration. The shallow traps are likely the dislocation lines themselves and the small vacancy clusters appear to be associated with the dislocation lines.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.348420
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  • 4
    Electronic Resource
    Electronic Resource
    Indium vacancy in as-grown InP: A positron annihilation study (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 4697-4699 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The positron lifetime technique has been used to investigate grown-in defects in various types of indium phosphide. A neutral monovacancy-type defect has been detected independently of the nature (Zn,Fe,Si,S) and concentration of the dopants. The defect is stable at least up to 800 °C, and is suggested to be a trapped indium vacancy.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.352769
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  • 5
    Electronic Resource
    Electronic Resource
    THE MACHAIR GRASSLANDS OF THE HEBRIDES (1959)
    Oxford, UK : Blackwell Publishing Ltd
    Grass and forage science 14 (1959), S. 0 
    Details
    ISSN: 1365-2494
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: The utilization of Scottish hill-land potential is severely limited by inadequate wintering facilities resulting from a low proportion of inbye land to hill-land. The machair lands of the Uists, in the Outer Hebrides, however, provide a more suitable ratio, which is not commonly found in the Highlands of Scotland.Investigations into the grassland problems of the Hebrides were intensified in 1955; and some of the data obtained from trials are given here, as are suggestions for improving the production and utilization of grass in these areas. The climate, soil, and agricultural system all interact to provide conditions which are rarely seen elsewhere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-2494.1959.tb01021.x
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  • 6
    Electronic Resource
    Electronic Resource
    A positron lifetime investigation of InP electron irradiated at 100 K (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 3750-3756 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime investigations have been made on variously doped InP samples irradiated at 100 K with 2.5 MeV electrons. Three irradiation-produced positron lifetimes were found: 240±10, 265±10, and 330±20 ps which are, respectively, ascribed to VP, VIn, and VP⋅VIn vacancies in close association with interstitials. Total introduction rates of these defects were in the range of 0.6–1.2 cm−1. Observation of the defects depends on the position of the Fermi level. In n-type materials no evidence could be found for VP⋅PI, while in p-type material VIn⋅InI was not observed. Annealing studies up to 300 K show that VP⋅PI anneals slightly below 300 K, while VIn⋅InI anneals in part around 125 K, but a sizable fraction remains at 300 K. Divacancies begin annealing at 125 K, but some can persist to 300 K. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363326
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  • 7
    Electronic Resource
    Electronic Resource
    A positron annihilation investigation of porous silicon (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 79 (1996), S. 9110-9117 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime experiments have been conducted on 13 different porous silicon layers formed from p+, p, and n+ substrates. Four distinct positron lifetimes could be associated with the porous layers. One lifetime was close to 0.5 ns and is claimed to arise from positrons trapped by small vacancy clusters. Positronium is formed either on the surface with a lifetime of ∼5 ns, or in the pores, from which exceptionally long lifetimes of up to 90 ns originate. Both of these lifetimes arise from the pickoff annihilation of ortho-positronium; the fourth lifetime is the usual 125 ps component associated with para-positronium. The long lifetime component gives rise to 3-γ annihilations which are readily detectable in the energy spectrum of the annihilation quanta. In situ heat treatments of two of the samples provide evidence that gases play a very important role in the passivation of defects, both in the native oxide layer and on the surface, albeit on very different time scales. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.362646
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  • 8
    Electronic Resource
    Electronic Resource
    Positron annihilation investigations of vacancies in InP produced by electron irradiation at room temperature (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 3446-3452 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime investigations were done on a series of InP samples irradiated to various doses with 2.5 MeV electrons. In n-type materials, positron lifetimes of 265±5 and 338±15 ps are attributed to indium vacancy–interstitial complexes and divacancy–interstitial complexes, respectively. In p-type materials these defects were not observed. Thermal annealing took place up to 200 °C for both defect types. Introduction rates were estimated to be 0.1 cm−1 for VIn⋅InI and ∼0.05 cm−1 for the divacancies. The divacancies showed a temperature dependence of the trapping rate, which suggests a thermally activated process. No evidence for VP vacancies could be found in neither p-type nor n-type materials. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.365041
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  • 9
    Electronic Resource
    Electronic Resource
    Positron annihilation investigation of porous silicon heat treated to 1000 °C (1998)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 84 (1998), S. 6559-6564 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime and Doppler broadening spectroscopies were applied to investigate a porous silicon film subjected to heat treatment in an argon atmosphere. Heating between 300 and 500 °C increased the mass of the film by 17% due to oxygen uptake and the concentration of open volume defects associated with the formation of an oxide layer on the silicon nanocrystallites increased by a factor of 3. Between 600 and 1000 °C their concentration decreased gradually to 1/2 the original concentration. Doppler broadening results indicate two distinct electron momentum distributions, one arising from open volume defects and one from pickoff annihilation of positronium at the pore walls caused by electrons with an unexpectedly narrow momentum distribution. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.369028
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  • 10
    Electronic Resource
    Electronic Resource
    Oxygen in silicon: A positron annihilation investigation (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 1313-1321 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron lifetime and Doppler broadening investigations of oxygen in silicon (Cz silicon) have been performed. It was found that positrons may be trapped by defects yielding a positron lifetime of only 100 ps and a momentum distribution of the annihilation quanta which is wider than that for defect-free silicon. This unequivocally shows that clusters of oxygen constitute positron traps. The trapping process was found to be thermally activated in the 25–300 K range. Isochronal and isothermal annealing showed that the clusters alternate during growth between having an interstitial character and a vacancy character. At 450 and 500 °C, growth of oxygen clusters takes place with an activation energy of 1.5 eV, while at 650 and 714 °C, these clusters disappear with an activation energy of 1.1 eV. At 714 and 790 °C, growth of new clusters takes place with an activation energy of 2.6 eV.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337303
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