Positron annihilation studies on vacancy defects in group IV semiconductors

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Aalto-yliopiston teknillinen korkeakoulu | Doctoral thesis (article-based)
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Dissertations of Department of Applied Physics, 159
Electrical properties of semiconductor materials are greatly influenced by point defects such as vacancies and interstitials. These defects are common and form during the growth and processing of the material. Positron annihilation spectroscopy is a method suitable for detecting and studying vacancy-type lattice defects. In this work, the formation, properties, and annealing of vacancy defects is studied in silicon, silicon-germanium, and germanium. Defects consisting of a vacancy and one or several donor atoms are one of the most common defects causing electron trapping and deactivation of n-type doping in silicon and silicon-germanium. In this work, the studies in silicon-germanium show that several germanium atoms accumulate around the vacancy-phosphorus (V-P) pair during the annealing of the samples. The increased Ge-decoration pulls the energy level (-/- -) down into the band-gap and makes the V-P pair decorated by several Ge atoms an especially effective trap for conduction electrons. The positron trapping in a vacancy surrounded by three As atoms (V-As3) is studied in highly As-doped Si. The positron detrapping from the V-As3 defect at high temperatures is observed and a binding energy of 0.27(3) eV of a positron to the V-As3 complex is determined. Defects can also be introduced deliberately by neutron-irradiation and ion-implantation. These techniques offer possibilities for studying the generation and annealing of vacancy defects. In this work, neutron-irradiated germanium is studied. Irradiation induced divacancy defects that are stable at room temperature are observed. A negative charge state of a divacancy is found to stabilize the defect even at 400°C. The divacancy is shown to form bigger clusters before the final recovery at 500°C. Finally, B-doping related problems are studied. The results show that He-implantation produces nanovoids that trap interstitials formed during the B-implantation, reducing the implantation related damage. The positron studies on the excimer laser annealed Si support theoretical calculations, which suggest vacancy formation at the maximum melt depth.
Thesis advisor
Tuomisto, Filip, Dr.
semiconductors, silicon, germanium, vacancy defects, silicon-germanium, annealing, electrical properties, V-P pair, arsenium atoms, positron annihilation spectroscopy, group IV semiconductors, neutron-irradiated germanium
  • [Publication 1]: K. Kuitunen, K. Saarinen, and F. Tuomisto. 2007. Positron trapping kinetics in thermally generated vacancy donor complexes in highly As-doped silicon. Physical Review B, volume 75, number 4, 045210, pages 1-5. © 2007 American Physical Society (APS). By permission.
  • [Publication 2]: K. Kuitunen, F. Tuomisto, and J. Slotte. 2007. Evidence of a second acceptor state of the E center in Si1−xGex. Physical Review B (BR), volume 76, number 23, 233202, pages 1-4. © 2007 American Physical Society (APS). By permission.
  • [Publication 3]: K. Kuitunen, F. Tuomisto, J. Slotte, and I. Capan. 2008. Divacancy clustering in neutron-irradiated and annealed n-type germanium. Physical Review B, volume 78, number 3, 033202, pages 1-4. © 2008 American Physical Society (APS). By permission.
  • [Publication 4]: S. Kilpeläinen, K. Kuitunen, F. Tuomisto, J. Slotte, E. Bruno, S. Mirabella, and F. Priolo. 2009. Vacancy engineering by He induced nanovoids in crystalline Si. Semiconductor Science and Technology, volume 24, number 1, 015005, pages 1-4. © 2009 Institute of Physics Publishing (IOPP). By permission.
  • [Publication 5]: A. La Magna, V. Privitera, Guglielmo Fortunato, Massimo Cuscunà, B. G. Svensson, E. Monakhov, K. Kuitunen, J. Slotte, and F. Tuomisto. 2007. Vacancy generation in liquid phase epitaxy of Si. Physical Review B, volume 75, number 23, 235201, pages 1-6. © 2007 American Physical Society (APS). By permission.