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

52 3. Irradiation FacilityFigure 3.13: I/V measurement of the diode K3. Guard rings were kept at the same potentialas the pad but their current is not included in the measurement. Measurement a) was taken 5days after irradiation (stored at 5 C) and was performed at 5 C. The full depletion voltage fromthe C/V measurement at 10 kHz is 30610 V, agreeing well with the position of the minimumin the I/V curve. Value in the minimum after the bump (45 A) is taken as the value of thebulk generation current at full depletion. b) Measurement of the same diode taken after totalof 5 days at 60 C.From equations 2.38 and 2.9 it follows that the bulk generation current is increasingas the square root of the depletion voltage until the sample is fully depleted and thevalues at the plateau could be used to determine (eq. 2.50). The I/V curve of aninverted p + -n-n + diode (g. 3.13) however deviates from the expected I / p V shape. Acharacteristic bump can be observed on all samples after inversion. The explanation hasbeen proposed in [51]. After inversion, the depleted region starts to grow from the n sidewhile guard rings are on the p side. For low depletion voltages, the surface current isthusnot fully collected by guard rings, contributing to the current measured on the centralpad. When the sample is fully depleted guard rings are fully operational again (g. 3.13).Thus throughout this work, the leakage current at the minimum after the bump has beenused as the best estimate of the bulk generation current.The main contribution to the error comes from the surface current that remainspresent at the minimum after the bump. As can be seen from data presented later(e.g. g. 4.16), this contribution is rather small for diodes with working guard rings.The variations of the alpha parameter determined by this method are at the level of few