Temperature characteristics of positron trapping at defects in electron-irradiated silicon

Positron-annihilation lifetime and Doppler-broadening measurements are used to investigate defects in silicon irradiated at 373 K with 6 MeV electrons to a dose of 1×l0¹⁹e/cm². In the unirradiated silicon sample (p type) a temperature-independent behaviour of the bulk-lifetime is observed in the temperature interval 110–500 K with a constant value of 220±1 ps. The slight effect observed on the S-parameter evolution is explained taking into account the thermal expansion of the lattice. The lifetime results obtained at 80 K and at 300 K after isochronal annealing as well as the behaviour of the intensity of the second lifetime componentI ₂ during lifetime measurements below the irradiation temperature in the irradiated silicon sample (n type), clearly indicate the temperature dependent characteristics of the positron trapping cross section _t(T) T ⁿ withn= –1.905±0.016. From isochronal annealing results, an annealing stage is observed in which di-vacancies agglomerate into quadri-vacancies. The mean positron lifetime in those quadri-vacancies is 350 ps.A.B.O.S., on leave from University of Kinshasa, Zaïre