Recombination Lifetimes in Gamma-Irradiated Silicon

Abstract

Photoconductivity decay measurements on a wide variety of silicon crystals (boron- and phosphorusdoped, Czochralski and float-zone grown, 20≤ρ≤200 Ω·cm) irradiated with 60Co gamma rays indicate that a separate system of levels controls the radiation-induced lifetime in n- and p-type specimens. In n-type material the introduction rates for two recombination active defects (the A center and a deeper level) were seen to be nearly independent of oxygen and donor concentrations in the range encountered in the experiment, while the introduction rate of a second deep level, the E center (vacancy-phosphorus pair), was seen to be inhibited by the presence of oxygen and to increase with increasing donor concentrations. The behavior of the lifetime was studied after the samples were annealed and it was found that a shallow level 0.23 eV from the conduction band edge was introduced. Hole-capture cross sections for the two deep levels were obtained from Hall and lifetime measurements. In p-type material recombination levels at Eν+0.18 eV and Ec−0.3 eV were introduced by irradiation. The introduction rate of the level nearest the conduction band was strongly inhibited by the presence of oxygen and dominated the lifetime only in float-zone material. Low-temperature annealing was seen to occur in the defects having levels at Ec−0.3 eV with the subsequent creation of defects with levels at Eν+0.18 eV in some materials. In other materials a second type of defects with levels at Ec−0.3 eV was created, while in some materials a simple annealing with no creation of new defects was found.