Aalto University Schools of Technology - electronic academic dissertations - http://otalib.aalto.fi/fi/kokoelmat_tiedonhaku/e-julkaisut/vaitoskirjat/
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Engineering Physics and Mathematics, Helsinki University of Technology, for public examination and debate in Auditorium E at Helsinki University of Technology (Espoo, Finland) on the 1st of June, 2001, at 13 o'clock.
Overview in PDF format (ISBN 951-22-5468-9) [453 KB]
Dissertation is also available in print (ISBN 951-22-5467-0)
Instrumentation and methods for positron annihilation spectroscopy of point defects in semiconductors have been developed. In particular, techniques to enhance the stability of positron lifetime spectrometers have been investigated. The ageing of the photomultiplier tubes (PMT) of the scintillation detectors can be slowed down by lowering the operating voltages over the PMTs and by compensating the lower gain with fast preamplifiers. The timing characteristics of the apparatus are preserved if the voltages in the input electron optics of the PMTs are high enough and the pulse amplitudes above some tens of millivolts. A positron lifetime spectrometer stabilized against fast inherent drifts in time zero is constructed. An artificial time reference peak in the lifetime spectrum is produced by feeding light pulses from a light-emitting diode onto the photomultipliers via optical fibers of different lengths. The reference peak serves as a basis of stabilization in a digitally stabilized multichannel analyzer.
Positron thermalization in Si and GaAs at low temperatures (8-100 K) is investigated both by experiments and theoretical calculations. Thermalization in GaAs is observed to be noticeably slower than in Si. The mass density of a material is found to play an important role in thermalization since the positron scattering rate off longitudinal-acoustic phonons is inversely proportional to it.
Point defects have been investigated by positron annihilation spectroscopy in Si and CdF2. V-As and V-P pairs are observed in electron-irradiated silicon. Native V-As3 complexes are found to be formed when the As-concentration exceeds 1020cm–3. The ionization level V2–2/– of the silicon divacancy is detected at Ec–0.40 eV by measurements under illumination with monochromatic light. An open-volume defect is observed to be a constituent of the deep-state atomic configurations of the bistable donors In and Ga in CdF2. The size of the open volume is at least half of a Cd monovacancy.
This thesis consists of an overview and of the following 6 publications:
Keywords: positron lifetime spectrometer, fast photomultiplier, positron thermalization, point defects in semiconductors, highly n-type Si, defect bistability
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© 2001 Helsinki University of Technology