Vacancy defects in semiconductor materials for opto and spin electronics

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Dissertations / Laboratory of Physics, Helsinki University of Technology, 134
The vacancy defects in GaN, ZnO and (Ga,Mn)As have been studied by positron annihilation spectroscopy. We show that both the thermodynamical quantities and the kinetics of the growth have a significant impact on impurity incorporation and point defect formation in all three materials. In addition the incorporation and stability of point defects in electron irradiated ZnO is studied. In GaN the Ga vacancies and vacancy clusters are more abundant at the N polar side. The concentrations of oxygen and of acceptor-type impurities are similarly correlated with the polarity. The vacancy concentrations are similar in both HVPE and high-pressure grown GaN in spite of the much higher growth temperature of the latter. This suggests that the thermal stability of the point defect complexes is an important factor determining which defects survive the cooling down from the growth temperature. The Zn vacancy is shown to be the dominant intrinsic acceptor in undoped ZnO. Vacancies on both sublattices and negative ion type defects are produced in electron irradiation, and their concentrations are determined. In addition, the irradiation-induced Zn vacancies have an ionization level close to 2.3 eV, hence it is likely that they are involved in the transition responsible for the green luminescence in ZnO. The irradiation-induced point defects fully recover after the annealing at 600 K. The Zn vacancies anneal out of the material in two stages, indicating that the Zn vacancies are parts of two different defect complexes. The O vacancies anneal simultaneously with the Zn vacancies at the later stage. The negative ion type defects anneal out between the two annealing stages of the Zn vacancies. The Zn vacancies are the dominant defects observed by positrons in thin heteroepitaxial ZnO layers. Their concentration depends on the surface plane of sapphire over which the ZnO layer has been grown. There is a correlation between the misorientation of the sapphire surface planes and the concentration of the vacancies for layers with thickness at most 500 nm. In addition to the misorientation, the defect content in the layer depends on the layer thickness. The concentrations of both native As antisites (donors) and Ga vacancies (acceptors) vary as a function of the Mn content in (Ga,Mn)As, following thermodynamic trends, but the absolute concentrations are determined by the growth kinetics and stoichiometry.
positron annihilation spectroscopy, zinc oxide, gallium nitride, gallium manganese arsenide
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  • F. Tuomisto, T. Suski, H. Teisseyre, M. Kryśko, M. Leszczyński, B. Łucznik, I. Grzegory, S. Porowski, D. Wasik, A. Witowski, W. Gębicki, P. Hageman, and K. Saarinen, Polarity dependent properties of GaN layers grown by hydride vapor phase epitaxy on GaN bulk crystals, physica status solidi (b) 240, 289-292 (2003). [article1.pdf] © 2003 Wiley-VCH Verlag. By permission.
  • F. Tuomisto, K. Saarinen, B. Łucznik, I. Grzegory, H. Teisseyre, T. Suski, S. Porowski, P. Hageman, and J. Likonen, Effect of growth polarity on vacancy defect and impurity incorporation in dislocation-free GaN, Applied Physics Letters 86, 031915: 1-3 (2005). [article2.pdf] © 2005 American Institute of Physics. By permission.
  • F. Tuomisto, V. Ranki, K. Saarinen, and D. C. Look, Evidence of the Zn vacancy acting as the dominant acceptor in n-type ZnO, Physical Review Letters 91, 205502: 1-4 (2003). [article3.pdf] © 2003 American Physical Society. By permission.
  • F. Tuomisto, K. Saarinen, D. C. Look, and G. C. Farlow, Introduction and recovery of point defects in electron-irradiated ZnO, Physical Review B 72, 085206: 1-11 (2005). [article4.pdf] © 2005 American Physical Society. By permission.
  • A. Zubiaga, F. Tuomisto, F. Plazaola, K. Saarinen, J. A. Garcia, J. F. Rommeluere, J. Zuñiga-Pérez, and V. Muñoz-Sanjosé, Zinc vacancies in the heteroepitaxy of ZnO on sapphire: influence of the substrate orientation and layer thickness, Applied Physics Letters 86, 042103: 1-3 (2005). [article5.pdf] © 2005 American Institute of Physics. By permission.
  • F. Tuomisto, K. Pennanen, K. Saarinen, and J. Sadowski, Ga sublattice defects in (Ga,Mn)As: thermodynamical and kinetic trends, Physical Review Letters 93, 055505: 1-4 (2004). [article6.pdf] © 2004 American Physical Society. By permission.
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