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

6. Inuence of Bias Voltage 99that they were stored at -7 C (samples BA2), 5 C (samples BA4) and 20 C (samples UO6and D2). Storage temperatures were chosen to determine time constants of the decreaseof bias induced dierence at the operation (-7 C) and maintenance (20 C) temperaturescorresponding to the ATLAS operation scenario [6]. The measurement at 5 C providesan additional point for determination of the activation energy. Bias voltage was switchedo on the biased samples, except for the diode D2A, where 350V of reverse bias has beenreplaced by approximately 10V of forward bias 39 . Time development of the dierencebetween the formerly biased and unbiased samples was observed. To reduce the errorin determination of the dierence, values from a linear t to the initial stage of reverseannealing were used for the time development ofunbiased samples. In gures 6.7a to 6.9atime development ofN eff for each pair from the end of irradiation is shown. Figures 6.7bto 6.9b show time development of the dierence normalised to the value before the biaswas switched o.One can see that in samples, where the bias voltage was switched o, the dierencetends to anneal out completely. Time constants for the annealing are increasing with thedecreasing temperature (table 6.3). Dierence for the forward biased sample (D2) is atthe beginning showing the same behaviour as in the unbiased sample kept at the sametemperature (UO6). However, after about four days at 20 C, when approximately a halfof the dierence has annealed out, it starts increasing again. This eect will be furtherdiscussed in section 6.2.2.6.2.1 Time Constants and Activation EnergySample T [ C] C 1 1 [h] C 2 2 [h]BA2 -7 0.200.04 22020 0.710.05 90001500BA4 5 0.220.04 605 0.650.05 2100200UO6 20 0.440.04 4210 0.410.05 1050100Table 6.3: Results of the two exponential ts (eq. 6.2) for samples BA2, BA4 and UO6.All samples show a fast 40 annealing of approximately 10% of the initial dierence.Annealing of the remaining dierence can be described by atwo-exponential tN eff (t)N eff (t 0 ) = C 1e ; t;t 0 1 + C 2 e ; t;t 0 2 (6.2)391mAforward current limit was used.40Within the rst 4 hours after switching the bias o, that being the time interval between the rstmeasurements.