Anomalous Temperature Dependence of Positron Trapping due to Divacancies in Si

The temperature dependence of positron lifetime and trapping rate due to electrically neutral divacancies in Si has been determined for temperatures from 5.5 K to 210 K. The charge state of divacancies was verified through the Hall effect, electron spin resonance and infrared absorption measurements. The positron lifetime at neutral divacancies was 280 ps at low temperatures and increased to 295 ps at around 30 K. This is explained in terms of the lattice relaxation effect accompanying the positron trapping. The positron trapping rate due to neutral divacancies was found to increase upon cooling, to decrease after reaching a maximum at around 30 K and to increase again below 10 K. The observed temperature dependences of the trapping rate could not be explained by existing models. The temperature dependence was found to be reproduced by the Breit-Wigner formula for resonance phenomena. Possible reasons were discussed.