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

6. Inuence of Bias Voltage 95Figure 6.5: a) 1/C 2 vs. V plot and b) N eff distribution for the sample I3 after the end of heattreatment. Two regions with dierent N eff , corresponding to dierent biasing conditions,, canbe clearly seen.this time it was biased to 200 V. This means that at least after the second irradiation thesample was only partially biased during all of the annealing stage, causing part of it toanneal depleted and part undepleted. 1/C 2 vs. V plot after this procedure is shown ingure 6.5, together with the distribution of N eff through the sample width as determinedfrom the C/V measurement according to equations 2.44 and 2.45. At 200 V a kink inthe slope of the 1/C 2 vs. V curve can be clearly seen. Since the sample was kept at 200V during all the time, the position of the kink agrees well with the border between thedepleted and non-depleted regions. The corresponding dierence in N eff is about a factorof two (gure 6.5b).To conrm the correlation between the dierence in N eff and biasing conditions,the bias was switched o. About one third of the dierence annealed out in the rst dayat 20 C (g. 6.6 red points) but no additional change developed in the following twoweeks. To accelerate the processes, the sample was heated to 60 C. Measurements after7 h (green points), one day (blue points) and 4.5 days (magenta points) are also shownin gure 6.6. One can see that N eff concentration in the region that used to be depletedis slowly decreasing towards the value of the non-depleted region where no signicantchange in N eff is observed. This shows that defects, whose annealing is inhibited by theelectric eld, remain in silicon also during long term annealing. They start annealing out,however, upon switching o the bias.