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

92 6. Inuence of Bias VoltageSample producer V 0 FD [V] T [ C] eq [n/cm 2 s] eq [10 14 n/cm 2 ] V bias [V]S3A Micron 32. 15 + 1.910 9 0.45 200/0 S3B Micron 34. 15 + 1.910 9 0.45 0/200 D2A Micron 17. 15 1.810 10 0.42 300D2B Micron 17. 15 1.810 10 0.42 0UO6B Sintef 35. 20 1.810 11 1.7 1000/600 yUO6S Sintef 35. 20 1.810 11 1.7 0BA2B Sintef 44. 15 2.110 11 # 1.0 # 500BA2S Sintef 44. 15 2.110 11 # 1.0 # 0BA4B Sintef 40. 20 2.110 11 # 1.0 # 600BA4S Sintef 36. 20 2.110 11 # 1.0 # 0Table 6.2: List of diodes used to study the eect of bias voltage on defect development.Thickness of samples was 3005 m.+ ) Diodes S3 were kept at bout 27 C after transfer to the laboratory to accelerate annealing.When annealed, they were moved to 20 C to study annealing of the dierence in N eff . )DiodeS3Awas biased during and unbiased after the irradiation. ) Diode S3B was unbiased during and biased after the irradiation.y Diode UO6A was biased to 1000V at the reactor and to 600V at the laboratory. Bias voltagecould be reduced due to annealing of FDV.# ) Due to frequent changes in the reactor core at the time of irradiation, the neutron spectrumwas not well known. This induced an error on dosimetry, larger than the usual 15%.a consequence of slow annealing of bias dependent dierence. It could also be explainedif reverse annealing was caused by reaction of charged defects and consequently sloweddown in electric eld. Available data however do not allow for unambiguous distinctionbetween these two possibilities. The dierence could not be explained by partial bias,since it develops already at an early stage of reverse annealing, when biasing voltage of200 V was still above FDV of the biased samples. This dierence in reverse annealing ofbiased and unbiased samples also contributes to the error in determination of the reverseannealing reaction constants of the partially biased samples as discussed in section 4.2.For further studies we wished to minimise the inuence of any uncontrolled parametersand to avoid errors from uence measurements. Thus diodes were irradiated andmonitored in pairs, with a biased and unbiased diode in each pair. Both diodes were takenfrom the same wafer to minimise the risk of dierence caused by material. Five pairs aslisted in table 6.2 were studied this way. Samples S3 and D2 were Micron oat-zone p + -n-n + diodes with 3 guard rings and an active area of 11 cm 2 . Samples UO6, BA2 andBA4 were p + -n-n + planar diodes processed by Sintef. They had a multiple guard-ringstructure that was not connectable and thus oating. Samples marked with last letter