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

104 6. Inuence of Bias Voltagebiasing schemes are consistent with the assumption that electric eld inuences timedevelopment of defects created during the irradiation and not their introduction rates.When the bias voltage is switched o, the dierence starts to anneal. The timedependence is consistent with two exponentially decaying defects. Contributions of eachdefect type agree for samples annealed at -7 C and 5 C (about 20% for the fast andabout 70% for the slow component). They however dier for the sample annealed at 20 C(about 40% of each component). More systematic studies are therefore necessary beforerm conclusions on this eect can be made. Decay times at 20 C are about 40 h for thefast component and six weeks for the slow one. Time development of the dierence canbe scaled to other temperatures using activation energies. First measurements of thosewere performed giving activation energies of E 1 = 0:54 0:15 and E 2 = 0:47 0:15 forthe fast and slow component, respectively.As detectors in the LHC environment can be left unbiased for a signicant portionof time, a proper biasing and temperature scheme could reduce the observed eect. Sinceit takes more than a month at 20 C to anneal out 90% of the dierence, the planned14 days at 17 C will not be sucient. And since the activation energy for the slowdecrease of the bias induced dierence is about twice lower than for the reverse annealing,prolonged warm periods would inuence reverse annealing stronger than annealing of thebias dependent dierence. The remaining 8 monthsat-7 C when the detector will not beoperating and can be kept without bias, amount onlyto about half of the lifetime of theslow component. Due to long operation time and low irradiation ux this can howeversuce to remove most of the eect. The bistable component may, on the other hand,amount to a signicant increase of the full depletion voltage during the operation periods.A detailed study of the bias voltage eect is thus necessary. The data it will provideshould be used to develop a scheme that will minimise the inuence of bias eect andreverse annealing.