Study of radiation damage in silicon detectors for high ... - F9

5Dose Rate Dependence5.1 MotivationIrradiation of silicon detectors in the environment of future detectors will be a slow process,taking many years to accumulate the predicted uence of the order of 10 14 n/cm 21 MeV neutrons equivalent. For obvious reasons it is unpractical, if not impossible, toperform studies of irradiation damage eects with uxes that low. Most often samplesare irradiated to those uences in a few hours or days. It is thus important tocheck for apossible ux eect. Even more so because results on ion implantation with Br, Si and Care showing considerable decrease in trap generation rates for uxes above a threshold ofabout 10 7 cm ;2 s ;1 , 10 8 cm ;2 s ;1 and few times 10 8 cm ;2 s ;1 , respectively [52]. This canbe explained by clusters overlapping at high uxes, enhancing prompt vacancy-interstitialsilicon recombination, thus reducing radiation induced damage.The reactor where the present irradiations were performed can cover a wide rangeof uxes. The maximal ux is determined by a uence of about 10 14 n/cm 2 that can beobtained in an about 20 ms long pulse, giving a ux of about 510 15 n/cm 2 s. The lowerux was limited by practical reasons (about 150 h for 10 14 n/cm 2 )toabout210 8 n/cm 2 s.Thus, a range of more than seven orders of magnitude in ux can be investigated.A set of 7 samples was used to study the ux inuence. They were all irradiated toabout 10 14 n/cm 2 with neutron uxes covering the above mentioned range. All sampleswere 11 cm 2 p + -n-n + planar Micron diodes. Except for samples SA1 and SA2 that had1 and 2 guard rings respectively, all diodes had 3 guard rings. Details about irradiationand storage are listed in table 5.1. For samples K0, K1 and K3, about a week afterirradiation, the time development was accelerated by heating to 60 C. In case of SA andP0 diodes, heating started 8 and 4 months after the irradiation. In the meanwhile they81