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

2. Operation and Radiation Damage of Silicon Detectors 29~0.17 eV(- , 0) ~0.23 eV(= , - )~0.1 eV(- , 0)~0.17 eV ~0.09 eV(- , 0)(- , 0)Ec~0.4 eV(-, 0)~0.41 eV(- , 0)~0.25 eV(+ , 0)~0.5 eV (- , 0)~0.41 eV(- , 0)~0.28 eV(+ , 0)~0.38 eV(+ , 0)Ci - CsEiVPVOV2V2OV30CiCi-Oi~0.11 eV(- , 0)~0.05 eV (+ , 0)EvFigure 2.7: Energy levels of some vacancy and carbon related defects. The defect charge statesare shown in brackets, e.g. minus and neutral (-,0). Above the dotted line the energy of thetrap is written in the lower half E V and in the upper half E C . Sign = means the doublecharged state. In the gure, data from [14] were used.from defects with energy levels close to the middle of the bandgap. While standarddoping materials (P , B) have energy levels close to the conduction (donors) or valenceband (acceptors) this is not the case with the irradiation induced defects (gure 2.7). Thismeans that radiation induced defects can contribute a major part of the leakage currentgenerated in the depleted region.Since the defect density is proportional to the total uence (N def = g def withg def the defect generation rate) and the generation current proportional to mid-gap defectconcentration (eq. 2.35, 2.38), also the irradiation induced leakage current is proportionalto the equivalent uence eqIV = eq (2.50)where is the leakage current damage constant and V the volume.2.3.2 Eective Dopant ConcentrationThe eective dopant concentration N eff is composed of ionised shallow levels (donorsand acceptors) and deep levels. Before irradiation, the concentration of deep levels isnegligible and the eective dopant concentration is determined by the dierence of donorand acceptor concentrations Neff 0 = N D 0 ; N A 0 . During the irradiation two types ofprocesses take place. One is the introduction of new defects that may act either asdonors or acceptors and the second is the removal of initial shallow levels by creation of