The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Modeling Momentum Distributions of Positron Annihilation Radiation in Solids

Ilja Makkonen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 21st of September, 2007, at 13 o'clock.

Overview in PDF format (ISBN 978-951-22-8920-2)   [901 KB]
Dissertation is also available in print (ISBN 978-951-22-8919-6)

Abstract

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.

This thesis consists of an overview and of the following 7 publications:

  1. I. Makkonen, M. Hakala, and M. J. Puska, Calculation of valence electron momentum densities using the projector augmented-wave method, Journal of Physics and Chemistry of Solids 66, 1128-1135 (2005). © 2005 Elsevier Science. By permission.
  2. I. Makkonen, M. Hakala, and M. J. Puska, Modeling the momentum distributions of annihilating electron-positron pairs in solids, Physical Review B 73, 035103: 1-12 (2006). © 2006 American Physical Society. By permission.
  3. M. Rummukainen, I. Makkonen, V. Ranki, M. J. Puska, K. Saarinen, and H.-J. L. Gossmann, Vacancy-impurity complexes in highly Sb-doped Si grown by molecular beam epitaxy, Physical Review Letters 94, 165501: 1-4 (2005). © 2005 American Physical Society. By permission.
  4. S. Hautakangas, I. Makkonen, V. Ranki, M. J. Puska, K. Saarinen, X. Xu, and D. C. Look, Direct evidence of impurity decoration of Ga vacancies in GaN from positron annihilation spectroscopy, Physical Review B 73, 193301: 1-4 (2006). © 2006 American Physical Society. By permission.
  5. A. Calloni, A. Dupasquier, R. Ferragut, P. Folegati, M. M. Iglesias, I. Makkonen, and M. J. Puska, Positron localization effects on the Doppler broadening of the annihilation line: Aluminum as a case study, Physical Review B 72, 054112: 1-6 (2005). © 2005 American Physical Society. By permission.
  6. P. Folegati, I. Makkonen, R. Ferragut, and M. J. Puska, Analysis of electron-positron momentum spectra of metallic alloys as supported by first-principles calculations, Physical Review B 75, 054201: 1-10 (2007). © 2007 American Physical Society. By permission.
  7. I. Makkonen and M. J. Puska, Energetics of positron states trapped at vacancies in solids, Physical Review B 76, 054119: 1-10 (2007). © 2007 American Physical Society. By permission.

Keywords: positron annihilation, electron momentum spectroscopies, density-functional theory

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© 2007 Helsinki University of Technology


Last update 2014-01-16