The evolution of defects in neutron transmutation-doped Czochralski-type material was studied, by using positron annihilation and electrical measurements, as a function of annealing temperatures of up to 1100C. Three annealing stages were found, depending upon the annealing behavior of the vacancy and vacancy-impurity complexes. Hall effect measurements showed that the sample-type alternated, with temperature, from n-type to p-type at about 300C and 600C and from p-type to n-type at about 500C and 650C. The change from p-type to n-type at 500C was attributed to the formation of thermal donors. Shallow donors, or thermal donors, were usually formed when Czochralski material was annealed at temperatures of between 300 and 500C. These donors were suggested to be Si-O clusters (SiO4+) that formed via the diffusion and aggregation of about 4 interstitial O atoms. At about 500C, the annealing of V3-O complexes was suggested by positron data, and the O atoms which were released by this process were expected to take part in the formation of thermal donors. These defects were not detected by positron techniques as they had a positive effective charge. The free carrier concentration went through a minimum value at 600C, and the material became p-type. It was proposed that thermal donors were annealed at this temperature. At 650C, the samples again became n-type and the donor concentration increased up to an annealing temperature of 850C. This was attributed to so-called new donors. A large-cluster model was suggested for the new donors. It was noted that there had been a previous report of the appearance of a short lifetime component of about 100ps for annealing temperatures of between 700 and 800C. This was attributed to annihilation in O clusters. No such lifetime was detected by the present measurements. At temperatures above 850C, the donor concentration decreased. This was suggested to be due to the annealing out of the less stable type of new donor.

X.T.Meng, S.Charalambous, M.Chardalas, S.Dedoussis, C.A.Eleftheriadis, A.K.Liolios: Radiation Effects and Defects in Solids, 1995, 133[1], 97-101