Divacancy clustering in neutron-irradiated and annealed<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>n</mml:mi></mml:math>-type germanium

We have studied the annealing of vacancy defects in neutron-irradiated germanium. After irradiation, the Sb-doped samples $[(\text{Sb})=1.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }{\text{cm}}^{\ensuremath{-}3}]$ were annealed at 473, 673, and 773 K for 30 min. The positron lifetime was measured as a function of temperature (30--295 K). A lifetime component of 330 ps with no temperature dependence is observed in as-irradiated samples, identified as the positron lifetime in a neutral divacancy and indicating that the divacancy is stable at room temperature (RT). Annealing at 673 K resulted in an increase in the average positron lifetime, and in addition, the annealed samples further showed a larger lifetime component of 430 ps at RT, which is due to larger vacancy clusters. The average positron lifetime in the samples annealed at 473 K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upwards in the band gap. Annealing at 673 K, reduces the average lifetime and intensity of the defect component ${\ensuremath{\tau}}_{2}$ at RT, indicating that the vacancy clusters have started to anneal. Negative divacancies are still present in the samples after this anneal. Annealing at 773 K is enough to remove all observable vacancy defects.

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