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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Implementation and Testing of High Voltage System for Pulsed Low-Energy Positron Beam and Conventional Positron Beam Studies in InN

Antti Pelli

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 2nd of February, 2007, at 13 o'clock.

Overview in PDF format (ISBN 978-951-22-8594-5)   [1634 KB]
Dissertation is also available in print (ISBN 978-951-22-8593-8)

Abstract

Positron annihilation spectroscopy is one of the few methods to study atomic scale lattice imperfections. The conventional positron lifetime spectroscopy is limited to bulk samples and a lifetime beam is required for measurements in epitaxially grown thin layers. This work describes the operation of the TKK pulsed positron lifetime beam with a grounded sample. Particularly, the unique high voltage realizations and challenges of HV design are discussed in detail. Thorough high voltage withstand tests are performed for every component in the HV system of the lifetime beam. Additionally, to protect electronics against breakdown induced transients, multi-stage transient suppressors are constructed and tested.

The timing properties of the lifetime beam are determined using electrons and positrons. A fast multichannel plate is utilized as a detector for the resolution measurements. Higher detector count rate and a brighter beam are achieved with electrons to accelerate the testing procedure. The time resolution of 160 ps observed for the lifetime beam is sufficient for positron lifetime measurements in semiconductors. The peak-to-background ratio is also sufficient, more than 5000.

A conventional continuous slow positron beam is used to study indium nitride grown by metal-organic chemical vapor deposition. The observed indium vacancy concentration of ∼1017 cm−3 is almost independent on the V/III molar ratio at 4800-24000. At lower ratios, below 4000, the In droplet formation is accompanied by the formation of vacancy clusters. The In vacancy formation depends on the growth temperature. The concentration increases from 9×1016 cm−3 to 7×1017 cm−3 when the growth temperature is increased from 550°C close to the decomposition temperature of 625°C.

Finally, 2 MeV 4He+ irradiated InN grown by molecular beam epitaxy and gallium nitride grown by metal-organic chemical vapor deposition are studied. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600 cm−1. The In vacancies are introduced in InN at significantly lower rate of 100 cm−1 making them negligible in the compensation of the irradiation-induced additional n-type conductivity. On the other hand, negative non-open volume defects are introduced at a rate higher than 2000 cm−1. We propose that these defects are related to N interstitials and ultimately limit the free electron concentration at high irradiation fluences.

This thesis consists of an overview and of the following 6 publications:

  1. A. Laakso, A. Pelli, K. Rytsölä, K. Saarinen, and P. Hautojärvi. Determination of the timing properties of a pulsed positron lifetime beam by the application of an electron gun and a fast microchannel plate, Applied Surface Science 252, 3148 (2006). © 2006 Elsevier Science. By permission.
  2. A. Laakso, M. O. Hakala, A. Pelli, K. Rytsölä, K. Saarinen, and P. Hautojärvi. Scattering effects in a positron lifetime beam line, Materials Science Forum 445-446, 489 (2004). © 2004 by authors and © 2004 Trans Tech Publications. By permission.
  3. A. Pelli, A. Laakso, K. Rytsölä, R. Aavikko, M. Rummukainen, and K. Saarinen. HV design of a pulsed lifetime beam with a grounded sample, Materials Science Forum 445-446, 504 (2004). © 2004 by authors and © 2004 Trans Tech Publications. By permission.
  4. A. Pelli, A. Laakso, K. Rytsölä, and K. Saarinen. The design of the main accelerator for a pulsed positron beam, Applied Surface Science 252, 3143 (2006). © 2006 Elsevier Science. By permission.
  5. A. Pelli, K. Saarinen, F. Tuomisto, S. Ruffenach, and O. Briot. Influence of V/III molar ratio on the formation of In vacancies in InN grown by metal-organic vapor-phase epitaxy, Applied Physics Letters 89, 011911 (2006). © 2006 American Institute of Physics. By permission.
  6. A. Pelli, F. Tuomisto, K. M. Yu, and W. Walukiewicz. Compensating point defects in He-irradiated n-type InN and GaN, Helsinki University of Technology Publications in Engineering Physics, Report TKK-F-A846 (2006). © 2006 by authors.

Keywords: pulsed positron beam, high voltage, indium nitride

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© 2007 Helsinki University of Technology

Last update 2011-05-26