Construction of a pulsing system for low-energy positrons
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Doctoral thesis (article-based)
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en
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45, [app]
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Dissertations of Laboratory of Physics, Helsinki University of Technology, 133
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.Description
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- 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. [article1.pdf] © 2001 Elsevier Science. By permission.
- 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. [article2.pdf] © 2004 Trans Tech Publications. By permission.
- 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. [article3.pdf] © 2005 by authors and © 2005 Elsevier Science. By permission.
- 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. [article4.pdf] © 2004 Trans Tech Publications. By permission.
- 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. [article5.pdf] © 2005 by authors and © 2005 Elsevier Science. By permission.
- 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. [article6.pdf] © 2004 American Institute of Physics. By permission.
- 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. [article7.pdf] © 2004 Elsevier Science. By permission.