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

28 2. Operation and Radiation Damage of Silicon Detectorsab c de f ghFigure 2.6: Various possible defect congurations. Simple defects are: a) vacancy V, b)interstitial silicon atom I, c) interstitial impurity atom, d) substitutional impurity atom (e.g.phosphorus as donor). Examples of defect complexes are: e) close pair I-V, f) divacancy V-V,g) substitutional impurity atom and vacancy (e.g. VP complex), h) interstitial impurity atomand vacancy (e.g, VO complex)capture processes and thus aecting the leakage current. They can act as donors or acceptors, changing the eective dopant concentrationN eff thus aecting the full depletion voltage. Increased capture probability decreases charge carrier lifetime thus decreasing thecharge collection eciency.2.3.1 Leakage CurrentDefects generated due to irradiation may introduce energy levels in the band gap. Mostdefects have one energy level with two charge states as was also assumed in the derivationof the generation rate in the depleted region (section 2.1.5). Acceptors are negativelycharged when occupied by an electron and neutral when empty, while donors are positivelycharged when unoccupied but neutral otherwise. Some of the defects however have morethan one energy level and more charge states. Such an example is divacancy (V 2 ) withthree energy levels in the band gap, giving four charge states [28].As shown in section 2.1.5 the largest contribution to the leakage current comes