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

6. Inuence of Bias Voltage 101(about a year) is however still long enough to play an important role for the detectoroperation in the ATLAS environment. A careful planing of the operation scenario is thusnecessary to obtain optimal operation conditions for the vertex detector.6.2.2 BistabilityFigure 6.12: Time development of N eff for both diodes from the S3 set. Samples were keptat room temperature.Bistability of the defects, responsible for bias dependent annealing, was checked ondiodes S3. The test was started about 2 months (at 20 C) after the bias induced inuenceannealed out (see gure 6.12). It can also be seen that both samples have an equal reverseannealing constant after the annealing of bias dependent defects was completed. First thediode S3A (biased during, unbiased after irradiation) was biased with 200 V for about 3weeks and then the bias was switched o. Then the same procedure was repeated for thediode S3B (unbiased during, biased after irradiation). The results of the test are shownin gure 6.12. One can see that for both samples about a 15% raise in N eff appearedduring the biased time interval, that disappeared within a day after the bias had beenremoved.A similar behaviour was observed with the sample UO6B. After bias dependentdefects annealed out, reverse bias of 500 V was applied again. As shown in gure 6.9,N eff started to increase. However, after two weeks, when the raise amounted to about30% of the initial dierence, an accidental power cut-o caused the sample to remain