Microstructure of bismuth centers in silicon before and after irradiation with 15 MeV protons.

A decrease of two-gamma annihilation rate of a positron in a strong spin-orbit field of the annihilation site of bismuth impurity centerBi (= 9/2) in silicon with natural isotope composition was revealed (is the nuclear spin). This decrease was observed along with increasing occupancy of Bi donor states (binding energy{Bi} ≈ 69 meV). Atoms ofSi (= 1/2) isotope are involved in spin interactions of positron with Bi impurity centers. The growth of occupancy of Bi donor states inhibits two-gamma annihilation rate. The estimated cross-section of positron trapping by the Bi impurity center is≈ (1.23-1.5) × 10 cm. Together with this surprisingly large value, the integral rate of two-gamma annihilation in a hypothetical polyelectron system of the Bi impurity center is by a factor of just Δ ∼ 2.18 higher compared to the magnitude ≈2.09 × 10 sknown for elemental isolated polyelectron, (eee). Possible formation of the positron-containing exciton-like states, (e)(= Bi, P) is also discussed. Irradiation of material with 15 MeV protons results in decreasing the factor Δ by ∼11% due to forming the radiation complex in which Bi atom is in an open volume ambient it. Such complex is suggested to havesymmetry and be the deep donor. Low-temperature measurements of both the positron annihilation rate and Hall effect have been applied for studying the isochronal annealing of these point radiation defects which were found to be thermally stable up to ∼370 °C; they can be annealed at ∼430 °C - 470 °C. According to available data ofcluster calculations, the complex of Bi atom with a simulated vacancy hassymmetry with the energy gain ∼0.92 eV, thus indicating qualitative agreement between experimental and theoretical data.