Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
Study of radiation damage in silicon detectors for high ... - F9
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6. Inuence <strong>of</strong> 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 determ<strong>in</strong>e time constants <strong>of</strong> the decrease<strong>of</strong> bias <strong>in</strong>duced dierence at the operation (-7 C) and ma<strong>in</strong>tenance (20 C) temperaturescorrespond<strong>in</strong>g to the ATLAS operation scenario [6]. The measurement at 5 C providesan additional po<strong>in</strong>t <strong>for</strong> determ<strong>in</strong>ation <strong>of</strong> the activation energy. Bias voltage was switchedo on the biased samples, except <strong>for</strong> the diode D2A, where 350V <strong>of</strong> reverse bias has beenreplaced by approximately 10V <strong>of</strong> <strong>for</strong>ward bias 39 . Time development <strong>of</strong> the dierencebetween the <strong>for</strong>merly biased and unbiased samples was observed. To reduce the error<strong>in</strong> determ<strong>in</strong>ation <strong>of</strong> the dierence, values from a l<strong>in</strong>ear t to the <strong>in</strong>itial stage <strong>of</strong> reverseanneal<strong>in</strong>g were used <strong>for</strong> the time development <strong>of</strong>unbiased samples. In gures 6.7a to 6.9atime development <strong>of</strong>N eff <strong>for</strong> each pair from the end <strong>of</strong> ir<strong>radiation</strong> is shown. Figures 6.7bto 6.9b show time development <strong>of</strong> the dierence normalised to the value be<strong>for</strong>e the biaswas switched o.One can see that <strong>in</strong> samples, where the bias voltage was switched o, the dierencetends to anneal out completely. Time constants <strong>for</strong> the anneal<strong>in</strong>g are <strong>in</strong>creas<strong>in</strong>g with thedecreas<strong>in</strong>g temperature (table 6.3). Dierence <strong>for</strong> the <strong>for</strong>ward biased sample (D2) is atthe beg<strong>in</strong>n<strong>in</strong>g show<strong>in</strong>g the same behaviour as <strong>in</strong> the unbiased sample kept at the sametemperature (UO6). However, after about four days at 20 C, when approximately a half<strong>of</strong> the dierence has annealed out, it starts <strong>in</strong>creas<strong>in</strong>g aga<strong>in</strong>. This eect will be furtherdiscussed <strong>in</strong> 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 <strong>of</strong> the two exponential ts (eq. 6.2) <strong>for</strong> samples BA2, BA4 and UO6.All samples show a fast 40 anneal<strong>in</strong>g <strong>of</strong> approximately 10% <strong>of</strong> the <strong>in</strong>itial dierence.Anneal<strong>in</strong>g <strong>of</strong> the rema<strong>in</strong><strong>in</strong>g 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)391mA<strong>for</strong>ward current limit was used.40With<strong>in</strong> the rst 4 hours after switch<strong>in</strong>g the bias o, that be<strong>in</strong>g the time <strong>in</strong>terval between the rstmeasurements.