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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Construction of a Pulsing System for Low-Energy Positrons

Antti Laakso

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, 2005, at 13 o'clock.

Overview in PDF format (ISBN 951-22-7746-8)   [1485 KB]
Dissertation is also available in print (ISBN 951-22-7745-X)

Abstract

Positron lifetime spectroscopy is a powerful tool to identify vacancy type defects in semiconductor materials. Conventionally, applicability of the positron lifetime measurement has been limited to bulk samples. However, many interesting materials are nowadays available only as thin epitaxial samples. In order to measure these samples a positron lifetime beam is needed.

This work describes the concept and the design of a pulsed low-energy positron lifetime beam facility. The theoretical background of the beam pulsing is reviewed. The construction of the pulsing system is presented in detail and results from electron and positron tests with this beam facility are introduced. As an application of the pulsed lifetime beam the results of the measurements on InN layers done at Universität der Bundeswehr are also presented.

The pulsing system in this work is composed of three different components, two bunchers and a chopper. The bunchers modulate the beam velocity to create a time varying positron beam intensity and to result a very narrow positron bunch at the target. The chopper is used to get rid of the background between the pulses and to improve the peak to background ratio. When this pulsing system is tested with electrons, a full width at half maximum of 160 ps and a peak to background ratio of 5000:1 is obtained. For positrons, the values are 270 ps and 4000:1 respectively.

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

  1. A. Laakso, K. Saarinen, and P. Hautojärvi. 2001. Positron lifetime beam for defect studies in thin epitaxial semiconductor structures. Physica B, 308-310: 1157-1160. © 2001 Elsevier Science. By permission.
  2. A. Laakso, M. Hakala, A. Pelli, K. Rytsölä, K. Saarinen, and P. Hautojärvi. 2004. Scattering effects in a positron lifetime beam line. Materials Science Forum, 445-446: 489-491. © 2004 Trans Tech Publications. By permission.
  3. F. Reurings, A. Laakso, K. Rytsölä, A. Pelli, and K. Saarinen. 2005. Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources. Applied Surface Science, accepted for publication. © 2005 by authors and © 2005 Elsevier Science. By permission.
  4. A. Pelli, A. Laakso, K. Rytsölä, R. Aavikko, M. Rummukainen, and K. Saarinen. 2004. HV design of a pulsed lifetime beam with a grounded sample. Materials Science Forum, 445-446: 504-506. © 2004 Trans Tech Publications. By permission.
  5. A. Laakso, A. Pelli, K. Rytsölä, K. Saarinen, and P. Hautojärvi. 2005. Determination of the timing properties of the pulsed positron lifetime beam by the application of an electron gun and a fast microchannel plate. Applied Surface Science, accepted for publication. © 2005 by authors and © 2005 Elsevier Science. By permission.
  6. J. Oila, A. Kemppinen, A. Laakso, K. Saarinen, W. Egger, L. Liszkay, P. Sperr, H. Lu, and W. J. Schaff. 2004. Influence of layer thickness on the formation of In vacancies in InN grown by molecular beam epitaxy. Applied Physics Letters, 84: 1486-1489. © 2004 American Institute of Physics. By permission.
  7. A. Laakso, J. Oila, A. Kemppinen, K. Saarinen, W. Egger, L. Liszkay, P. Sperr, H. Lu, and W. J. Schaff. 2004. Vacancy defects in epitaxial InN: identification and electrical properties. Journal of Crystal Growth, 269: 41-49. © 2004 Elsevier Science. By permission.

Keywords: pulsed positron beam, positron spectroscopy, pulsing systems, low-energy positrons

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© 2005 Helsinki University of Technology

Last update 2011-05-26