Carrier lifetime of silicon wafers doped by neutron transmutation

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Abstract
Radiation damage in wafers cut from neutron transmutation doped float-zone silicon crystals has been studied by using minority carrier lifetime and conductivity measurements. The isochronal annealing behaviour of the lifetime is characterised by three regions in the temperature range from 480 to 1160 degrees C. For the samples annealed in the temperature region I where the recovery of conductivity is not complete, a minority carrier trapping effect is clearly observed. The annealing temperature to remove the trapping effect depends on the fast-neutron fluence for pure FZ-Si, and it is above 700 degrees C for the neutron fluences of phi th=5.5*1018, phi f=5.5*1017 n cm-2. In region II, the recombination lifetime begins to increase after the conductivity reaches full recovery, and a light-sensitive effect in which the lifetime increases remarkably with the illumination of an additional photo-bias is observed. The effect is tentatively explained by a defect cluster model. In region III beyond 1000 degrees C, the light-sensitive effect disappears, and the lifetime sharply increases with annealing temperature. However, the annealing temperature required to remove this effect is influenced not only by fast-neutron irradiation or contamination in the starting materials but also by the thermal-neutron fluence.