Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
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102 6. Inuence <strong>of</strong> Bias Voltageunbiased. It is thus not possible yet to determ<strong>in</strong>e the portion <strong>of</strong> the <strong>in</strong>itial dierence thatrecovers after restoration <strong>of</strong> the bias voltage.The bistable nature <strong>of</strong> the defect is also shown <strong>in</strong> the time development <strong>of</strong> thedierence <strong>of</strong> the diode D2A (g. 6.10), where reverse bias was replaced by a small (about10 V) <strong>for</strong>ward bias. One can see that the <strong>in</strong>itial dierence between the biased (D2A) andunbiased (D2B) sample was anneal<strong>in</strong>g out at the beg<strong>in</strong>n<strong>in</strong>g, but started to <strong>in</strong>crease aga<strong>in</strong>after afewdays at 20 C.Bistable behaviour <strong>of</strong> all presented samples could also be expla<strong>in</strong>ed by reaction 6.1,assum<strong>in</strong>g it can proceed both waysA + B $ C (6.5)the equilibrium balance be<strong>in</strong>g distorted by the electric eld sweep<strong>in</strong>g out the mobilereaction <strong>in</strong>gredient. Two time constants <strong>of</strong> the bias eect anneal<strong>in</strong>g <strong>in</strong>dicate two suchreactions responsible <strong>for</strong> its anneal<strong>in</strong>g. In that case results <strong>of</strong> 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 <strong>for</strong>ward bias.Bias dependent anneal<strong>in</strong>g <strong>of</strong> bistable defects has also been reported by M. Moll etal. [54]. There are however a few major dierences between the observed properties, <strong>in</strong>dicat<strong>in</strong>gdierent defects compared to those reported <strong>in</strong> this work. First, defects reported<strong>in</strong> [54] are only seen <strong>in</strong> irradiated samples, activated by heat treatment, charge <strong>in</strong>jectionby <strong>for</strong>ward bias or illum<strong>in</strong>ation. The eect reported <strong>in</strong> this work are present s<strong>in</strong>ce their<strong>radiation</strong> and no special treatment is needed to activate them. Second, though bothanneal only if no reverse bias is applied, there is a big dierence <strong>in</strong> time constants. While[54] reports simple exponential anneal<strong>in</strong>g with a time constant <strong>of</strong> about 10h at 20 C andan activation energy <strong>of</strong> 1.040.06 eV we observe anneal<strong>in</strong>g with two exponentials. Thetime constants <strong>of</strong> the fast component at 20 C is four times <strong>high</strong>er (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 <strong>in</strong>dicate that processes responsible <strong>for</strong> the reported phenomena are notidentical.6.3 Inuence on Reverse CurrentS<strong>in</strong>ce diodes, dedicated to the study <strong>of</strong> the dierence between biased and unbiased samples,had no connectable guard r<strong>in</strong>gs they could not be used to study the bias voltageeect on the reverse current. However, results presented <strong>in</strong> section 4.4.2, where time