POSITRON-ANNIHILATION AND THE CHARGE STATES OF THE PHOSPHORUS VACANCY PAIR IN SILICON

After electron irradiation two positron lifetimes, 250 +/- 1 and 268 +/- 3 ps, are observed in phosphorus-doped Si ([P] = 3 x 10(16) cm-3 ) depending on the temperature and the electron fluence. The lifetimes are assigned to the negative and neutral charge states of the phosphorus-vacancy pair (P-V). The longer lifetime at the neutral charge state indicates a larger open volume of the vacancy, and implies an outward relaxation of the phosphorus-vacancy pair in the charge-state transition (P-V)- -->(P-V)0. A comparison with the theoretical lifetimes yields a breathing-mode relaxation of the nearest-neighbour atoms DELTA-r/r0 approximately 5.1% (r0 is the Si-Si bond length), or DELTA-r approximately 0.12 angstrom. The positron trapping coefficients 2 x 10(16) s-1 at 20 K and 2 x 10(15) s-1 at room temperature at the negatively charged state (P-V)- are estimated. A weakly bound Rydberg-like precursor state is invoked to explain the temperature dependence of positron trapping at the negatively charged centres (P-V)- and V- in Si. The binding energy at the precursor state is 15-30 meV.