[diss] Perustieteiden korkeakoulu / SCI

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Browsing [diss] Perustieteiden korkeakoulu / SCI by Degree programme/Major subject "Laskennallinen materiaalifysiikka"

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  • Modeling momentum distributions of positron annihilation radiation in solids
    (2007-09-21) Makkonen, Ilja
     Doctoral dissertation (article-based)
    Positron annihilation spectroscopy is a materials characterization method especially applicable for studying vacancy defects in solids. In typical crystal lattices positrons get trapped at vacancy-type defects. By measuring positron lifetimes and momentum distributions of positron annihilation radiation one obtains information about the open volumes and the chemical environments of the defects. Computational tools can be used in the analysis of positron annihilation experiments. Calculated lifetimes and momentum distributions of annihilating electron-positron pairs can be directly compared with experiment. Momentum spectra calculated for model defects can be used to determine, for example, characteristic effects of impurity atoms around vacancies. This information can be used when identifying the microscopic defect structures behind the measured spectra. In this thesis momentum distributions of annihilating electron-positron pairs are calculated using quantum-mechanical electronic-structure methods based on the so-called density-functional theory. A numerical implementation is created based on the so-called projector augmented-wave method which enables the construction of accurate valence electron wave functions for the calculation of momentum densities. When studying positrons localized at vacancy defects their ionic structures are determined taking into account also the forces on ions due to the localized positron. First the computational scheme is validated by comparing computational results with ones measured by Compton scattering and positron annihilation spectroscopies for well-characterized samples (defect-free samples annealed at high temperatures, electron-irradiated samples containing vacancies). The new methods are applied to the analysis of experimental positron data and resulting chemical identification of defects in different kinds of materials. Elemental (Si) and compound (GaN) semiconductors as well as metals and alloys (Al and Al-based alloys) are studied. An approach for quantitative chemical analysis of Al-based is justified using computations and the methods are also used to study the energetics of positron trapping in various solids and to show that the positron-induced lattice relaxations have an important role in the trapping process.