Positron Annihilation in Neutron-Irradiated p -Type Silicon

Effects of neutron irradiation and subsequent thermal annealing on positron-lifetime spectra in $p$-type silicon have been studied. In unirradiated samples, two annihilation rates (4.13 \ifmmode\times\else\texttimes\fi{} ${10}^{9}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ with an intensity of 98% and 7.14 \ifmmode\times\else\texttimes\fi{} ${10}^{8}$ ${\mathrm{sec}}^{\ensuremath{-}1}$ with an intensity of 2%) are observed. The annihilation rate of the dominant component becomes smaller upon irradiation. The effect saturates at high integrated neutron flux, \ensuremath{\sim} 1 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ n/${\mathrm{cm}}^{2}$, and disappears completely upon thermal annealing at 400-500 \ifmmode^\circ\else\textdegree\fi{}C. The annealing behavior is dependent on the integrated neutron flux. These results show that some neutron-induced defects act as positron traps at room temperature. Our study also gives evidence that high-order vacancy defects are formed during annealing near 300 \ifmmode^\circ\else\textdegree\fi{}C. The mean electron density of the positron-sensitive defects (positron traps) is estimated from the saturation value of the annihilation rate to be about 35% less than that in the perfect crystal. The diffusion coefficient and the mobility of thermalized positrons in silicon are estimated to be about 0.16 ${\mathrm{cm}}^{2}$/sec and 6.4 ${\mathrm{cm}}^{2}$/sec V, respectively.