Positron annihilation spectroscopy, experimental and theoretical aspects

Positron annihilation spectroscopy, experimental and theoretical aspects

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In summary, PAS gives microscopic information about vacancy defects in semiconductors in the concentration range 10 15 -10 19 cm -3 . The positron lifetime is the fingerprint of the open volume associated with a defect, and it can be used to identify mono- and divacancies and larger vacancy clusters. Doppler broadening of the annihilation radiation, on the other hand, can be used to identify the nature of the atoms surrounding the vacancy. Consequently, vacancies on different sub lattices of a compound semiconductor can be distinguished, and impurities associated with the vacancies can be identified. The charge state of a vacancy defect can be determined by the temperature dependence of the positron -trapping coefficient, and positron localization into Rydberg states around negative centers yields information about ionic acceptors that have no open volume. Importantly, as shown in this chapter, the methods based on positron annihilation are not restricted by the nature or physical dimensions of the semiconductor. Defects can be studied in narrow- and wide-bandgap semiconductor materials in samples of any conductivity. Bulk crystals as well as thin films can be subjected to the experiments and defects identified.

Chapter Contents:

  • 6.1 Introduction
  • 6.2 Positrons in crystalline semiconductors
  • 6.2.1 Positrons obtained from radioactive sources
  • 6.2.2 Slow monoenergetic positron beams for thin-layer studies
  • 6.2.3 Positron thermalization and diffusion
  • 6.2.4 Positron trapping
  • 6.2.5 The kinetic trapping model and positron lifetime spectroscopy
  • 6.3 Doppler broadening techniques
  • 6.3.1 Doppler broadening spectroscopy
  • 6.3.2 Coincidence Doppler broadening spectroscopy
  • 6.3.3 Supporting theory for defect identification
  • 6.4 Temperature-dependent measurements in narrow bandgap semiconductors
  • 6.4.1 Vacancy annealing in Ge
  • 6.4.2 Positron traps in low temperature irradiated GaSb
  • 6.5 Acceptor-like defects in wide bandgap semiconductors
  • 6.5.1 Cation vacancies and acceptor impurities in GaN and ZnO
  • 6.5.2 Cation and oxygen vacancies in metal oxides
  • 6.6 Summary and outlook
  • References

Inspec keywords: Rydberg states; vacancies (crystal); Doppler broadening; semiconductor materials; positron annihilation

Other keywords: semiconductors; Rydberg states; vacancy defects; positron lifetime; Doppler broadening; positron annihilation spectroscopy

Subjects: Positron annihilation (condensed matter); Interstitials and vacancies

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