Positron trapping rates and their temperature dependencies in electron-irradiated silicon

Abstract

Defects created by 2-MeV electron irradiation of Czochralski-grown silicon have been investigated with use of positron-lifetime spectroscopy and results have been correlated with EPR and ir data. The trapping rate for the positrons was smallest for the neutral divacancy where a concentration of ∼${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ gave rise to a trapping rate of 1 ${\mathrm{ns}}^{\mathrm{-}1}$. For the singly negative divacancy the trapping rate was increased at 300 K by a factor of about 3.5 and for the doubly negative divacancy a further increase by a factor of 2 was found. Isochronal annealing showed that the neutral divacancy annealed at only 150 °C, while the charged states annealed in a broad temperature range starting at 230°C. The monovacancy component found in some of the samples could be identified as being due to divacancy-oxygen complexes. Measurements in thermal equilibrium between 30 K and room temperature showed that shallow positron traps became activated at low temperatures. Oxygen vacancy pairs (A centers) are concluded to be the defects acting as shallow traps and yielded a lifetime of 225 ps, very close to the bulk lifetime of silicon. The trapping cross section of the charged defects varied with temperature as $T^{\mathrm{-}n}$, with 2n