Positron trapping at vacancies in electron-irradiated Si at low temperatures

Experimental results on positron trapping at vacancies in electron-irradiated silicon are presented. The positron lifetimes 273±3 and 248±2 ps in pure Si and heavily-phosphorus-doped Si ([P]=${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$) are assigned to a negative monovacancy $V^{\mathrm{-}}$ and a negative vacancy-phosphorus pair (V-P${)}^{\mathrm{-}}$, respectively. In pure Si, positron trapping displays a strong negative temperature dependence, and the specific trapping rate reaches very large values ${(10\mathrm{}}^{17–}$ ${10}^{18}$ ${\mathrm{s}}^{\mathrm{-}1}$) at low temperatures. In Si:P the trapping rate is independent of temperature. These different temperature behaviors are attributed to different positron-trapping mechanisms, a cascade of one-phonon transitions in pure Si, and an Auger process in Si:P.