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

2. Operation and Radiation Damage of Silicon Detectors 23Figure 2.4: C/V measurement of a diode, irradiated to 5 10 13 n/cm 2 and annealed for vemonths at 20 C. The dierence in signals obtained by dierent measurement frequencies isshown. The measurement was performed at 5 C.giving the N eff value in terms of applied voltage. To determine it in terms of depth onecan use the relation [8]w(V )= Si 0 SC(V ) : (2.45)The situation is more complicated in case of deep energy levels. For those theemission time, dened as the inverse of emission probability T = 1e T N CV(eq. 2.31), canbe large compared to the oscillation time of the measuring signal. In such a case deep trapsdo not contribute to the measured signal. As can be seen from eq. 2.31 the emission timedepends exponentially on the temperature. Therefore a linear decrease in temperaturecorresponds to a logarithmic decrease in frequency. Achange in the measuring frequencyor temperature thus aects the signal from deep energy levels and consequently the shapeof the C/V curve (gure 2.4). It also results in somewhat higher full depletion voltage asdetermined from the kink in C/V plot at lower frequencies. This eect was systematicallystudied by [16, 17] who report about 10% increase of FDV asfrequency changes from 10kHz to 1 kHz. It is suggested the reason may be the presence of a transition region betweenthe space charge region and the neutral bulk in heavily irradiated silicon. Measurementsignals of dierent frequencies might thus probe dierent parts of this transition region.To avoid the inuence of this eect when comparing dierent samples, all measure-