Abstract
At a vacant lattice cell positron-ion repulsion is reduced leading to positron trapping. This causes observable changes in annihilation characteristics: positron lifetime increases and positron-electron momentum distribution narrows. Positron trapping in semiconductors is analogous to carrier capture. Due to long-range Coulomb interaction, the charge state of a vacancy has strong effect on positron trapping. Also the lattice relaxation due to a charge-state transition of a vacancy is well seen in positron lifetime. Thus ionization levels of vacancy defects can be determined.Positron lifetime results on vacancies in GaAs will be discussed. Positrons reveal As and Ga vacancies in as-grown bulk GaAs. The As vacanacies have negative, neutral and positive charge states in the upper part of the band gap, whereas Ga vacancies are negative. The EL2 defect is shown to have a vacancy character in its metastable state.
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