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  • 1
    Electronic Resource
    Electronic Resource
    Defects in GaAs grown by molecular-beam epitaxy at low temperatures: stoichiometry, doping, and deactivation of n-type conductivity (1999)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 86 (1999), S. 1888-1897 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We use a low-energy positron beam to study the influence of doping and stoichiometry on the native defects in GaAs grown by molecular-beam epitaxy at 250 °C. Ga vacancies are identified in all samples by measuring the momentum distribution of annihilating core electrons. The charge of VGa is negative in Si-doped samples but neutral in undoped and Be-doped material. We propose that the Ga vacancies are complexed with As antisites in undoped and Be-doped samples and with Si impurities in n-type material. The concentration of Ga vacancies depends on the doping and stoichiometry of growth conditions. It follows generally the trends in the VGa formation energy as a function of the Fermi level position and stoichiometry. The strong loss of free carriers in the As-rich Si-doped samples is attributed to the formation of Ga vacancy complexes, negative ion defects and inactive clusters of Si atoms. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.370984
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  • 2
    Electronic Resource
    Electronic Resource
    Observation of Ga vacancies in silicon δ-doping superlattices in (001) GaAs (1997)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 71 (1997), S. 1843-1845 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Positron annihilation experiments have been performed to investigate the compensating defects in silicon δ-doping superlattices in (001) GaAs. The results reveal vacancies and ion-type defects, which are located between the delta planes in undoped GaAs. The vacancy defect is identified as the Ga vacancy and the negative ion is attributed to the Ga antisite. The concentrations of these defects increase strongly, when the areal concentrations of free carriers are reduced at the delta planes. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.119417
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  • 3
    Electronic Resource
    Electronic Resource
    Near-band-edge absorption and positron trapping under illumination in semi-insulating GaAs: Role of As vacancies (1995)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 66 (1995), S. 2534-2536 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We show that positron trapping at negative As vacancies revealed under illumination in bulk semi-insulating GaAs correlates with a form of near-band-edge absorption known as reverse contrast (RC). We conclude that it is the ionization of As vacancies to their negative charge state that gives rise to RC absorption. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.113158
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  • 4
    Electronic Resource
    Electronic Resource
    The influence of Mg doping on the formation of Ga vacancies and negative ions in GaN bulk crystals (1999)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 2441-2443 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Gallium vacancies and negative ions are observed in GaN bulk crystals by applying positron lifetime spectroscopy. The concentration of Ga vacancies decreases with increasing Mg doping, as expected from the behavior of the VGa formation energy as a function of the Fermi level. The concentration of negative ions correlates with that of Mg impurities determined by secondary ion mass spectrometry. We thus attribute the negative ions to MgGa〈sup ARRANGE="STAGGER"〉−. The negative charge of Mg suggests that Mg doping converts n-type GaN to semi-insulating mainly due to the electrical compensation of ON〈sup ARRANGE="STAGGER"〉+ donors by MgGa〈sup ARRANGE="STAGGER"〉− acceptors. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.125041
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  • 5
    Electronic Resource
    Electronic Resource
    Gallium vacancies and the growth stoichiometry of GaN studied by positron annihilation spectroscopy (1998)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 73 (1998), S. 3253-3255 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have applied positron spectroscopy to study the formation of vacancy defects in undoped n-type metal organic chemical vapor deposition grown GaN, where the stoichiometry was varied. Ga vacancies are found in all samples. Their concentration increases from 1016 to 1019 cm−3 when the V/III molar ratio increases from 1000 to 10 000. In nitrogen rich conditions Ga lattice sites are thus left empty and Ga vacancies are abundantly formed. The creation of Ga vacancies is accompanied by the decrease of free electron concentration from 1020 to 1016 cm−3, demonstrating their role as compensating centers. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.122735
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  • 6
    Electronic Resource
    Electronic Resource
    Microscopic nature of thermally stimulated current and electrical compensation in semi-insulating GaAs (1997)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 81 (1997), S. 3512-3521 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this work undoped semi-insulating (SI) GaAs grown by vertical gradient freeze and liquid encapsulated Czochralski methods was studied by near-infrared absorption (NIRA), thermally stimulated current (TSC) and positron annihilation techniques. The positron experiments reveal both gallium and arsenic vacancies, as well as gallium and arsenic antisites, in the samples. By comparing the results from the TSC and positron measurements, the following relations are found in the defect concentrations: trap T2 correlates with the arsenic antisite and trap T5 with the arsenic vacancy. The ionized fraction of the arsenic-antisite-related EL2 defect is obtained from NIRA measurements. The positive charge of these ionized EL2 defects correlates with the net negative charge, 3[VGa3−]+2[GaAs2−]−[VAs+], related to the gallium vacancies and antisites and arsenic vacancies detected in positron measurements. The intrinsic defects may thus contribute significantly to the electrical compensation in SI GaAs. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.364705
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  • 7
    Electronic Resource
    Electronic Resource
    Using positron 2D-ACAR as a probe of point defects in GaAs: The As vacancy as a case study (1996)
    Springer
    Applied physics 62 (1996), S. 529-532 
    Details
    ISSN: 1432-0630
    Keywords: PACS: 61.70; 78.70
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract.  Two-Dimensional Angular Correlation of positron Annihilation Radiation (2D-ACAR) experiments have been performed on n-type GaAs. By combining these results with those from positron lifetime experiments, the momentum distribution of the arsenic vacancy in its neutral (V 0 As) and negative (V - As) charge states have been extracted. These distributions were all normalized to the respective positron lifetime that characterizes them. The first thing to be noticed is that the momentum distributions of the vacancies, as seen by the positron, are fairly isotropic and structureless. The distribution for V 0 As is more peaked than that of V - As, while the latter is more intense in the large momentum regions of the spectra. From this, it can be inferred that V - As has a smaller open volume than V 0 As. A closer look at the momentum distribution of the vacancies reveals that they are not entirely isotropic, but, in fact, have a bulk-like component. Finally, the experimental results for bulk GaAs and V - As compare well in a qualitative manner with the momentum distributions that result from an ab-initio molecular dynamics calculation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01571688
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  • 8
    Electronic Resource
    Electronic Resource
    Using positron 2D-ACAR as a probe of point defects in GaAs: The As vacancy as a case study (1996)
    Springer
    Applied physics 62 (1996), S. 529-532 
    Details
    ISSN: 1432-0630
    Keywords: 61.70 ; 78.70
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Two-Dimensional Angular Correlation of positron Annihilation Radiation (2D-ACAR) experiments have been performed on n-type GaAs. By combining these results with those from positron lifetime experiments, the momentum distribution of the arsenic vacancy in its neutral (V aAs 0 ) and negative (V As − ) charge states have been extracted. These distributions were all normalized to the respective positron lifetime that characterizes them. The first thing to be noticed is that the momentum distributions of the vacancies, as seen by the positron, are fairly isotropic and structureless. The distribution forV As 0 is more peaked than that ofV As − while the latter is more intense in the large momentum regions of the spectra. From this, it can be inferred that VA. has a smaller open volume thanV As 0 A closer look at the momentum distribution of the vacancies reveals that they are not entirely isotropic, but, in fact, have a bulk-like component. Finally, the experimental results for bulk GaAs andV As − compare well in a qualitative manner with the momentum distributions that result from an ab-initio molecular dynamics calculation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01571688
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  • 9
    Electronic Resource
    Electronic Resource
    A positron lifetime and Doppler-broadening study of positron states and free volume in polyethylene (1996)
    Springer
    Journal of radioanalytical and nuclear chemistry 210 (1996), S. 495-503 
    Details
    ISSN: 1588-2780
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract Positron lifetime (LT) and Doppler-broadening (DB) studies of polyethylene have been performed simultaneously in the temperature range between 80 and 300 K. The LT spectra have been analysed assuming four exponential components. Two long-lived components appear, which were attributed too-Ps pick-off annihilation in crystalline regions (τ3 = 0.9 to 1.2 ns) and at free-volume holes in the amorphous phase ( $$\tau _{\Delta _3 } = 1.5$$ to 2.8), The variation in τ4 correponds to an increase of the mean hole size from 0.053 nm3 at 80 K to 0.188 nm3 at 300 K. From the data the glass transition temperature (T g=195 K), the coefficient of thermal expansion of holes in the glassy and rubbery phase (α h, g = 14.5 · 10−4 K−1 and α h, r = 189 · 10−4 K−1) and the fractional free volume (2.8% to 10.4%) were estimated. The DB curves were fitted by a sum of three Gaussians, the narrowest of which is assumed to represent the self-annihilation ofp-Ps localised at holes. The intensity of the narrow component,I n, varies between 0 and 7.3% in a similar way as the LT intensityI 4/3 varies. From this it was concluded that other Ps reactions beside pick-off are not important. Further, it was shown that the average positron lifetime is dominated by theo-Ps component,T 4 τ g, while the behaviour of the DB peak height is mainly affected by thep-Ps narrow componentI n .
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02056389
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  • 10
    Electronic Resource
    Electronic Resource
    The temperature dependence of the local free volume in polyethylene and polytetrafluoroethylene: A positron lifetime study (1998)
    Bognor Regis [u.a.] : Wiley-Blackwell
    Journal of Polymer Science Part B: Polymer Physics 36 (1998), S. 1513-1528 
    Details
    ISSN: 0887-6266
    Keywords: free volume ; thermal expansion ; positron lifetime measurements ; semicrystalline polymers ; polyethylene ; polytetrafluoroethylene ; Physics ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: Positron lifetime measurements, performed in the temperature range 80-300 K, are reported for polyethylene (PE) and polytetrafluoroethylene (PTFE). The lifetime spectra have been analyzed using the data processing routines LIFSPECFIT and MELT. Two long-lived components appear, which are attributed to pick-off annihilation of ortho-positronium in crystalline regions and at holes in the amorphous phase. The ortho-positronium lifetimes, τ3 and τ4, are used to estimate the crystalline packing density and the size of local free volumes in the crystalline and amorphous phases. The interstitial free volume in the crystals exhibits a weak linear increase with the temperature which is attributed to thermal expansion of the crystal unit cell. In the amorphous phase, the hole volume varies between 0.053 and 0.188 nm3 (PE) and between 0.152 and 0.372 nm3 (PTFE). Its temperature variation may be fitted by two straight lines, the intersection of which is used to estimate a glass transition temperature of Tg = 195 K for both PE and PTFE. The slopes of the free volume in the glassy and crystalline phases with the temperature correlate well with each other. The coefficients of thermal expansion of the hole volume are compared with the macroscopic volume change below and above the glass transition. From this comparison a fractional hole volume at Tg of 4.5 (PE) and 5.7% (PTFE) and a number of 0.73 (PE) and 0.36 (PTFE) × 1027 holes/m3 is estimated. Finally, it is found that the intensity of o-Ps annihilation in crystals shows a different temperature dependence to that in the amorphous phase. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1513-1528, 1998
    Additional Material: 11 Ill.
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