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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Positron Lifetime Spectroscopy: Digital Spectrometer and Experiments in SiC

Reino Aavikko

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 K at Helsinki University of Technology (Espoo, Finland) on the 29th of June, 2006, at 13 o'clock.

Overview in PDF format (ISBN 951-22-8243-7)   [1222 KB]
Dissertation is also available in print (ISBN 951-22-8242-9)

Abstract

A digital positron lifetime spectrometer has been designed, set up and tested comprehensively. The system consists of a fast commercial digitizer connected to a computer, a simple coincidence circuit and software to extract the timing from the collected detector pulses. The system has the same time resolution as a conventional analog apparatus using the same detectors. The pulse processing part of the spectrometer is able to analyze and store in real time several thousands of events per second, which is an order of magnitude more than the count rates in typical positron lifetime experiments. The data acquisition can handle small pulses, down to a few tens of millivolts, and its time scale linearity and stability are very good.

Positron spectroscopy has been used to study native vacancy defects in High Temperature Chemical Vapor Deposition grown semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of 4-5 missing atoms and (ii) Si vacancy related negatively charged defects. The growth of the clusters due to the annealing of the samples was observed. The total open volume bound to the clusters anticorrelates with the electrical resistivity both in as-grown and annealed material. It is concluded that Si vacancy complexes compensate electrically the as-grown material, but are suggested to migrate to increase the size of the clusters during annealing, leading to loss of resistivity.

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

  1. K. Rytsölä, J. Nissilä, J. Kokkonen, A. Laakso, R. Aavikko and K. Saarinen, Digital measurement of positron lifetime, Applied Surface Science 194 (1-4) (2002) pp. 260-263. © 2002 Elsevier Science. By permission.
  2. R. Aavikko, K. Rytsölä and J. Nissilä, Linearity tests of a Digital Positron Lifetime Spectrometer, Materials Science Forum 445-446 (2004) pp. 462-464. © 2004 Trans Tech Publications. By permission.
  3. J. Nissilä, K. Rytsölä, R. Aavikko, A. Laakso, K. Saarinen and P. Hautojärvi, Performance analysis of a digital positron lifetime spectrometer, Nuclear Instruments and Methods in Physics Research A 538 (2005) pp. 778-789. © 2005 Elsevier Science. By permission.
  4. R. Aavikko, K. Rytsölä, J. Nissilä and K. Saarinen, Stability and performance characteristics of a Digital Positron Lifetime Spectrometer, Acta Physica Polonica A 107 (2005) pp. 592-597.
  5. R. Aavikko, K. Saarinen, B. Magnusson and E. Janzén, Clustering of vacancies in semi-insulating SiC observed with positron spectroscopy, Materials Science Forum, in print. © 2006 Trans Tech Publications. By permission.
  6. R. Aavikko, K. Saarinen, F. Tuomisto, B. Magnusson, N. T. Son and E. Janzén, Clustering of vacancy defects in high-purity semi-insulating SiC, arXiv:cond-mat/0603849.
  7. B. Magnusson, R. Aavikko, K. Saarinen, N. T. Son and E. Janzén, Optical studies of deep centers in semi-insulating SiC, Materials Science Forum, in print. © 2006 Trans Tech Publications. By permission.

Keywords: positron lifetime spectrometer, digital, timing, SiC, HTCVD, semi-insulation, vacancy clusters

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© 2006 Helsinki University of Technology

Last update 2011-05-26