Elevated temperature annealing behaviors of bulk resistivity and space charge density (Neff) of neutron irradiated silicon detectors and materials

The bulk resistivity of neutron irradiated detector grade silicon material has been measured under the condition of no or low electrical filed (electrical neutral bulk or ENB condition) after elevated temperature (T = 110°C) anneals (ETA). The ENB resistivity (ϱ) for as-irradiated silicon material increases with neutron fluence at low fluences (Φn < 1013 n/cm2) a starts to saturate at a value between 300–400 kΩ cm at high fluences (Φn > 1013 n/cm2). The saturation of the ENB resistivity near the intrinsic value can be explained by the near perfect compensation of all neutron induced deep donors and acceptors in the ENB. After ETA, it has been observed that ϱ increases with annealing time for silicon materials irradiated below the saturation and decreases with annealing time for those irradiated after saturation. For those irradiated near the saturation point, ϱ increases with annealing time initially and decreases thereafter. This ETA behavior of ϱ may be explained by the increase of net acceptor-like deep levels in silicon during the anneal, qualitatively consistent with the observed reverse annealing effect of the space charge density (Neff) in silicon detectors which is an increase of negative space charge density (acceptors) after long term room temperature (RTA) anneal and/or ETA. However, the amount of the increase of net hole concentration (p) of about 5 × 1011 cm−3, corresponding to 20 hours of ETA at 110°C for a fluence of 1.5 × 1014 n/cm2, is still much less than the corresponding increase of Neff of about 1.5 × 1013 cm−3. This suggests that while the ETA restores some of the free carrier concentration (namely holes), there is still a large degree of compensation. The space charge density is still dominated by the deep levels and Neff ≠ p.