STUDIES OF DIVACANCY IN SI USING POSITRON LIFETIME MEASUREMENT

The charge state dependence of positron lifetime and trapping at divac ancy (V2) in Si doped with phosphorus or boron has been studied after 15 MeV electron irradiation up to a fluence of 8.0 x 10(17) e/cm2. The positron trapping cross sections for V2-, V2- and V-2(0)at 300 K were about 6 x 10(-14), 3 x 10(-14) and 0.1-3 x 10(-14) cm2, respectively. For V2+, however, no positron trapping was observed. The marked diffe rence in the cross sections comes from Coulomb interaction between the positron and the charged divacancy. The trapping rates for V-2(0) and V-2(2-) have been found to increase wi th decreasing temperature in t he temperature range of 10-300 K. These results are well interpreted b y a two-stage trapping model having shallow levels with energy of 9 me V (V-2(0)) and 21 meV (V-2(2-)). The appearance of a shallow level for V-2(0) can not be explained by a conventional ''Rydberg state'' model . The lifetime (290-300 ps) in V-2(0) is nearly constant in the temper ature range from 10 to 300 K, while that in V-2(2-) increases from 260 ps at 10 K to 320 ps at 300 K. The lifetime (260 ps) in V-2(2-) is sh orter than that in V-2(0) at low temperature. which is due to the exce ss electron density in V-2(2-). At high temperature, however, the long er lifetime of V-2(2-) than that of V-2(0) is attributed to lattice re laxation around V-2(2-).