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  • Artikel  (15)
  • 78.70
  • Springer  (15)
  • American Association for the Advancement of Science
  • American Geophysical Union
  • American Physical Society (APS)
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  • Public Library of Science
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  • 1995-1999  (15)
  • Maschinenbau  (15)
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  • Artikel  (15)
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  • Springer  (15)
  • American Association for the Advancement of Science
  • American Geophysical Union
  • American Physical Society (APS)
  • Nature Publishing Group
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  • 1
    Digitale Medien
    Digitale Medien
    A slow positron lifetime study of the annealing behaviour of an amorphous silicon layer grown by MBE (1995)
    Springer
    Applied physics 61 (1995), S. 71-74 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 68.55 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract We have studied MBE grown amorphous silicon, which was recrystallized at different temperatures for one hour, with a pulsed positron beam. A positron lifetime of 538±10 ps in the as-grown state is attributed to microvoids containing at least 10 vacancies. An incompletely recrystallized sample annealed at 500°C shows an additional long lifetime from ortho-positronium (o-Ps) pick-off annihilation. The o-Ps component disappears for samples, recrystallized at 700°C and above, and the defect lifetime steadily decreases with higher annealing temperature until a value of 310 ps is reached for the layer annealed at 1200°C. This value is explained by positron trapping at dislocations or small vacancy defects stabilized by dislocations or impurities.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538214
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  • 2
    Digitale Medien
    Digitale Medien
    Irradiation-induced atomic defects in SiC studied by positron annihilation (1995)
    Springer
    Applied physics 61 (1995), S. 51-53 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 61.80 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The present paper reports on positron lifetime measurements on atomic defects in SiC after low-temperature (80 K) electron irradiation of low (0.47 MeV) and high (2.5 MeV) electron energies and doses from 1.8×1017 to 1.9×1019 e/cm2 as well as after subsequent isochronal annealing up to 1900 K. For these studies the single crystals of nitrogen doped (2–3×1018 cm−3) SiC grown by a modified Lely technique with hexagonal structure (6H polytype) were used. According to the positron lifetime measurements, very different types of vacancy-like positron traps are introducted after irradiation with electrons of either low or high energy. The formation of defect agglomerates and their decay at high temperatures is studied during isochronal annealing and related to earlier studies.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538210
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  • 3
    Digitale Medien
    Digitale Medien
    Positron lifetime and 2D-ACAR studies of divacancies in Si (1995)
    Springer
    Applied physics 61 (1995), S. 65-70 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 78.70 ; 61.70 ; 61.80
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract We have measured positron lifetime and Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) distributions of Floating-Zone grown (FZ) Si specimens containing divacancies (V2) with the definite charge states, V 2 0 , V 2 −1 or V 2 −2 from room temperature to about 10 K. These charge states are accomplished by an appropriate combination of dopant species, their concentration and irradiation doses of 15 MeV electrons. with reference to the currently accepted ionization level of divacancies. The positron lifetime of the negatively charged divacancy increases with temperature, while that of the neutral divacancy shows little change with temperature. The positron trapping rate, obtained from lifetime and 2D-ACAR measurements, increases markedly with decreasing temperature. This is found not only for the negative divacancies but also for the neutral divacancy. We need a model which explains this temperature dependence. The 2D-ACAR distribution from positrons trapped at divacancies shows nearly the same distribution for the different charge states, which differs considerably from the case of As vacancies in GaAs studied by Ambigapathy et al. We have observed a small but definite anisotropy in the distribution of trapped positrons in V 2 − using a specimen containing oriented divacancies.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538213
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  • 4
    Digitale Medien
    Digitale Medien
    Investigations of positron lifetimes in InP with a pulsed positron beam (1995)
    Springer
    Applied physics 61 (1995), S. 321-324 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 78.70 ; 68.55 ; 61.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Indium Phosphide layers grown by gas source Molecular Beam Epitaxy, (MBE) have been studied by positron lifetime spectroscopy using the recently modified pulsed positron beam in Munich. The as-grown samples are known to be phosphorous rich and contain a high concentration of vacancy-type defects. On annealing, phosphorous precipitates are formed and the concentration of free volume defects increases. Positron lifetime spectroscopy has identified the grown in defects to be indium vacancies at a concentration around 1018cm−3. The dominant defects after annealing exhibit a positron lifetime characteristic of divacancies and are present at concentrations in excess of 5×1019cm−3.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538197
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  • 5
    Digitale Medien
    Digitale Medien
    Microstructural evolution of ZnS during sintering monitored by optical and positron annihilation techniques (1995)
    Springer
    Applied physics 61 (1995), S. 217-220 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Positron lifetime and optical absorption techniques were employed to track the microstructural evolution of polycrystalline ZnS grown by Chemical Vapor Deposition (CVD). As grown material and material treated with Hot Isostatic Pressure (HIP) was sintered at temperatures ranging from 400 to 1000°C for 2–18 h. A 290 ps defect lifetime could be resolved in all samples, while an additional longer lifetime (τ=430 ps) was found only in samples annealed at low temperatures. This component gradually disappeared during annealing at 800°C. Associated with the disappearance of the long-lived component, the apparent bulk lifetime of the material changed from 235 to 215 ps. A 215±2 ps bulk parameter was also found for HIP-treated material annealed at temperatures greater than 400°C and hence is taken to represent the delocalized state of the positrons in ZnS. Optical absorption measurements showed that annealing at 800°C also caused the absorption profiles of the CVD and HIP samples to converge. The rate of the bulk lifetime transition correlates with the absorption changes. The observed sharpening of the absorption profile is attributed to a decrease in scattering from grain boundaries and voids, and a decrease in absorption from point defects. The 430 ps lifetime is believed to be due to trapping at voids and grain boundaries, while the 290 ps lifetime likely is due to a monovacancy stabilized as a small complex.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538393
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  • 6
    Digitale Medien
    Digitale Medien
    On the interpretation of positron-annihilation data in powders and fine-grained materials (1995)
    Springer
    Applied physics 61 (1995), S. 203-206 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Monte-Carlo simulations of positron diffusion are carried out for powders consisting of spherical and ellipsoidal particles with and without defects. Following Bergersen et al. [1], elastic positron-phonon interaction is considered to be dominant for scattering processes in positron diffusion. The central question is which fraction of the positrons would be able to reach the particle boundaries. Hence, we calculate the Fraction of Positrons reaching particle Surface (FPS). The presence of defects in the particles can drastically reduce FPS depending on the defect concentration and capture rate. We demonstrate that for small-grained materials the grain surface can influence the lifetime signal significantly.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538390
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  • 7
    Digitale Medien
    Digitale Medien
    The bonding characteristics between rare-earth elements and fluorine in fluorozirconate glasses (1996)
    Springer
    Applied physics 62 (1996), S. 163-167 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.40 ; 78.70 ; 61.10
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The bonding characteristics between Rare-Earth (RE) elements and fluorine in fluorozirconate glasses doped with RE fluorides are studied by X-ray absorption near edge structure. The RE elements include La, Pr, Nd, Eu and Er. The intensities of the “White Lines” (WLs) at RE LII, and LIII absorption edges in these glasses increase as those in the corresponding fluorides. On the other hand, the WL intensity ratios of LIII to LII, in the studied materials deviate from the statistical value of two to one, but this deviation in the glasses is also larger than that in the corresponding fluorides. Besides, the changes of the WLs at RE LII, III edges between the glasses containing different RE fluorides and the corresponding fluorides are different for various REs. The reason for these differences is discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01575714
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  • 8
    Digitale Medien
    Digitale Medien
    Positron diffusion in solids and the reconstruction of inhomogeneous defect distributions from lifetime measurements (1996)
    Springer
    Applied physics 63 (1996), S. 227-235 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 78.70 ; 66.30 ; 61.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The time dependent diffusion trapping equations for positrons implanted into inhomogeneous solids are analyzed. This problem is of central importance in the study of polycrystalline materials and for the application of pulsed positron beams to defect studies in materials research. The main problem in previous investigations was the necessity to solve the time-dependent diffusion equation. It prevented analytical treatment in all but the simplest applications. For the first time this difficulty is eliminated by invoking a new concept, the observable local annihilation characteristics for local implantation of positrons into the thermalized ensemble. It will be shown that the local annihilation characteristics are governed by field equations which reduce to the well known quantities of the standard trapping model in the case of homogeneous defect distributions. Furthermore, inhomogeneous defect distributions are uniquely determined from the field equations provided the local annihilation characteristics are known. Analytical solutions are derived and applied successfully to recent experimental results for a selection of simple, but realistic problems. The formal procedure includes internal drift fields and could be extended to cover also the epithermal period of positron thermalization, if necessary.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01567874
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  • 9
    Digitale Medien
    Digitale Medien
    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
    Schlagwort(e): 61.70 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01571688
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  • 10
    Digitale Medien
    Digitale Medien
    Positron lifetimes for neutron-irradiated C60 (1995)
    Springer
    Applied physics 61 (1995), S. 335-337 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.80 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The neutron-irradiation effects on C60 fullerite powder were studied by positron lifetime spectroscopy. Below 0.5 ns, a single lifetime of 382±1 ps was found for the unirradiated annealed sample and two components were resolved after neutron irradiation with a dose of 2×1016 n/cm2. Possible origins of these components are briefly discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538200
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  • 11
    Digitale Medien
    Digitale Medien
    High-resolution electron energy-loss spectroscopy at epitaxially grown GaAs(100) (1995)
    Springer
    Applied physics 60 (1995), S. 567-572 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 68.55 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract High-Resolution Electron Energy-Loss Spectroscopy (HREELS) is shown to be a very sensitive tool to investigate the space-charge regime of n-respectively p-type semiconductors. The most simple model we applied to fit experimental spectra is based on a step-like distribution of free carriers with the Drude dielectric response function. In this case, the dispersion of surface plasmon excitations is neglected, but it is considered in the Thomas-Fermi and the Debye-Hückel models. We use these models to fit HREELS-spectra, obtained from heavily Si-doped GaAs(100), which was grown by Molecular Beam Epitaxy (MBE). A comparison shown that the Drude model overestimates both the free-carrier concentration and the plasmon damping factor. The use of a more realistic smooth free-carrier profile, obtained by the self-consistent solution of the Schrödinger and Poisson equations, leads to plasmon excitations with lower frequencies. Besides Ohmic damping, the calculations show that Landau damping should be incorporated in order to obtain a better fit, particularly at intermediate frequencies.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538530
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  • 12
    Digitale Medien
    Digitale Medien
    The coupling of Cr to Fe studied by circular magnetic X-ray dichroism (1995)
    Springer
    Applied physics 61 (1995), S. 119-122 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 78.70 ; 73.40 ; 75.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The coupling of Cr to Fe in stacked Cr/Fe/Cr trilayers is studied by circular magnetic X-ray dichroism using fluorescence yield to monitor the absorption. From the dichroic spectra, we determine the thickness dependent average magnetic coupling of Cr to Fe for Cr layer thicknesses of 1, 2 and 3 ML. We compare our results to new calculated spectra. Additionally, all Cr spectra show significant contributions due to photon scattering processes at theL 2, 3 absorption edges depending on the degree of circular polarization.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538376
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  • 13
    Digitale Medien
    Digitale Medien
    Transmission of positrons with aβ + energy distribution through thin films (1996)
    Springer
    Applied physics 63 (1996), S. 299-301 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 68.55 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract The transmission through Al foils of isotropically implanted positrons from a22Naβ + source has been measured. It is shown that the transmission is reasonably well-described using an exponential profile once backscattering is accounted for, except for thicknesses below approximately 7 mg/cm2. Below this thickness, the measured transmission is slightly less than that predicted by the exponential profile. Such a deviation has previously been observed for collimated positrons, suggesting that the implantation profile has no significant dependence on the spatial distribution of the incident positrons. This deviation is critical for the proper interpretation of positron lifetime experiments on thin films using a conventional positron lifetime spectrometer.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01567885
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  • 14
    Digitale Medien
    Digitale Medien
    An investigation of point defects in silicon carbide (1995)
    Springer
    Applied physics 61 (1995), S. 55-58 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 61.70 ; 68.55 ; 78.70
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Positron lifetime measurement on bulk samples of single crystal SiC wafers have shown that both 6H and 4H polytypes exhibit a bulk lifetime of 150±2ps. All samples contained a second, defect-related lifetime component, ranging in value from 250 to about 300 ps with rather low intensities. The defect structure exhibited by the nanocrystalline samples is, not unexpectedly, much more complex. Positron beam experiments on Electron Cyclotron Resonance Chemical Vapour Deposited (ECR-CVD) SiC thin films showed that the positrons are very sensitive to changes in important film parameters as a function of the deposition conditions. It was found that the film density is lower than expected, probably due to hydrogen incorporation; variations in compositions among different films were detected through variations in theS parameters, and differences were observed in the electric field at the film-substrate interface due to hydrogen passivation of dangling bonds and different substrate resistivities.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538211
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  • 15
    Digitale Medien
    Digitale Medien
    Beam optics studies for the planned hybrid electrostatic-magnetic guided slow positron beam in Hong Kong (1995)
    Springer
    Applied physics 61 (1995), S. 325-329 
    Details
    ISSN: 1432-0630
    Schlagwort(e): 78.70 ; 41.85 ; 07.77
    Quelle: Springer Online Journal Archives 1860-2000
    Thema: Maschinenbau , Physik
    Notizen: Abstract Design simulations are presented of a variable energy (0–40 keV) slow-positron beam that incorporates the convenience of magnetic guidance with the desirability of some electrostatic focusing produced by a lens structure operating in a low but increasing magnetic field region. Filtering of unmoderated positrons and gamma rays is to be accomplished using two conventionalE×B filters. In a novel way it is proposed that the final beam energy be adjusted using a simple linear de-acceleration stage and that the optimal electrostatic element potentials for efficient extraction, focusing and filtering of positrons be floated relative to the de-acceleration potential. The resulting intense and easily controllable sub-millimetre diameter beam should be suitable for both lateral and depth profiling of defects in semiconductor materials.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1007/BF01538198
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