Radiation defects in fast-neutron irradiated high-purity silicon

Radiation defects created by fast pile neutrons in high-purity n-type Si (ϱ = 2×103 to 5×103 Ωcm) and p-type Si (ϱ = 1.6×104 to 4×104 Ωcm) were investigated using Hall effect and minority carrier lifetime measurements. The oxygen concentrations in the crystals were in the range of 5×1014 to 5×1015 cm−3. It is shown that disordered regions are introduced and a level at (EV + 0.35) eV is observed. The Fermi level in both n- and p-type Si shifts to the middle of the gap. The introduced defects are effective recombination centres with a level at (EV + 0.35) eV and electron cross section σn = 5×10−14 cm2. Concerning the stability to annealing there are two types of disordered regions, the first one being slightly stable and destroyed in the temperature range 20 to 250 °C, the second one is destroyed only between 600 and 700 °C. The carrier removal rate for the defects of the first type is 0.3 neutrons−1 cm−1 and for the defects of the second type is considerably less (about 4 × 10−2 neutrons−1 cm−1). After the destruction of the disordered regions divacancies appear. It is supposed that the disordered regions of the first type are rich in divacancies and still crystalline, and the second ones have some part of amorphous structure. The defects of the first type are probably formed as a result of the sliding collisions of neutrons with the nuclei or under the action of the fission neutron having not very high energy. The disordered regions of the second type are probably formed by frontal collisions of high-energy neutrons with the silicon nuclei.