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

102 6. Inuence of Bias Voltageunbiased. It is thus not possible yet to determine the portion of the initial dierence thatrecovers after restoration of the bias voltage.The bistable nature of the defect is also shown in the time development of thedierence of the diode D2A (g. 6.10), where reverse bias was replaced by a small (about10 V) forward bias. One can see that the initial dierence between the biased (D2A) andunbiased (D2B) sample was annealing out at the beginning, but started to increase againafter afewdays at 20 C.Bistable behaviour of all presented samples could also be explained by reaction 6.1,assuming it can proceed both waysA + B $ C (6.5)the equilibrium balance being distorted by the electric eld sweeping out the mobilereaction ingredient. Two time constants of the bias eect annealing indicate two suchreactions responsible for its annealing. In that case results of the sample D2 are consistentwith an assumption that the fast reaction can proceed both ways (6.5) while the slow onecan not be reversed (reaction 6.1) and is thus, contrary to the fast one, not signicantlyaected by a weak forward bias.Bias dependent annealing of bistable defects has also been reported by M. Moll etal. [54]. There are however a few major dierences between the observed properties, indicatingdierent defects compared to those reported in this work. First, defects reportedin [54] are only seen in irradiated samples, activated by heat treatment, charge injectionby forward bias or illumination. The eect reported in this work are present since theirradiation and no special treatment is needed to activate them. Second, though bothanneal only if no reverse bias is applied, there is a big dierence in time constants. While[54] reports simple exponential annealing with a time constant of about 10h at 20 C andan activation energy of 1.040.06 eV we observe annealing with two exponentials. Thetime constants of the fast component at 20 C is four times higher (40 h) and its activationenergy about two times lower (0.5 eV). The slow component is even further o(about 1000 h at 20 C) with about the same activation energy as the fast component. Allthose dierences indicate that processes responsible for the reported phenomena are notidentical.6.3 Inuence on Reverse CurrentSince diodes, dedicated to the study of the dierence between biased and unbiased samples,had no connectable guard rings they could not be used to study the bias voltageeect on the reverse current. However, results presented in section 4.4.2, where time