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.
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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 14th of October, 2005, at 13 o'clock.
Overview in PDF format (ISBN 951-22-7824-3) [496 KB]
Dissertation is also available in print (ISBN 951-22-7823-5)
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.
This thesis consists of an overview and of the following 6 publications:
Keywords: positron annihilation spectroscopy, zinc oxide, gallium nitride, gallium manganese arsenide
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© 2005 Helsinki University of Technology