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

110 8. ConclusionsReverse annealing of N eff A study of the initial phase of reverse annealing of N eff ona set of dedicated samples gave clear evidence for a linear dependence of reverse annealingtime constant kYlin on irradiation uence. This means that at least at the initial stage,relevant for the LHC experiments, the reverse annealing rate is governed by a rst orderprocess.Long term annealing of reverse current Study of the long term annealing of reversecurrent showed no saturation up toafewmonths at 60 C. An eective ansatz(t) = E e ;t= E ; L ln(t= L ) (8.1)was found to t the results. Lack of a natural explanation of the logarithmic behaviourhowever indicates the need for better understanding of the processes responsible.Eect of neutron ux Irradiation ux was varied from a few times 10 8 n/cm 2 s to afew times 10 15 n/cm 2 s to check for the eect of neutron ux on defect generation. Nosignicant dierence among samples irradiated with dierent uxes was observed neitherin radiation induced change of N eff nor in generated reverse current [56]. The investigatedux range covers all presently available neutron sources and it seems safe to extrapolateresults from neutron irradiation studies to the expected ux at LHC of about 10 6 n/cm 2 s.Eect of bias voltage Strict control of the biasing condition both during and afterirradiation lead to the discovery of bias inuence on defect development. Its origin wasfound not to be in the creation of defects but in their annealing. Fully biased samplesgave about a factor of two higher N eff values at the plateau after benecial annealingas compared to unbiased ones. The dierence, however, annealed out if the bias voltagewas removed. Decrease of the dierence can be described by two exponentials with timeconstants at 20 C of about 40 h for the fast component ( 45%) and six weeks for theslow one ( 40%). Activation energy of both reactions is around 0.5 eV. The remainingdierence anneals out in less than 4 hours at all measured temperatures (-7, 5 and 20 C).Few ten percent of the dierence however recovers if the sample is biased again. No eectof the bias voltage on time development of reverse current was observed.Comparison with previous results In all parameters, previously studied by othergroups (reverse annealing of N eff and long term annealing of reverse current), a goodagreement of our results with reported values was found. The rst systematic search foran eect of neutron ux on defect creation was performed in this work. Finally, in thescope of this work the eect of bias voltage on annealing of eective dopant concentrationwas discovered.