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

3. Irradiation Facility 51The full depleti on voltage was determined as the position of the kink in 1/C 2 vs.voltage curve. The exact position of the kink was determined by the intersection of twolines, one tted to the slope at large depletion depths and the second to the plateauat full depletion (see g. 3.12). Fitted range was determined manually after a visualexamination of the plot.To determine the eective dopant concentration N eff , a homogeneous distributionover the sample was assumed. In such case N eff can be determined from the full depletionvoltage by the equationN eff = 2 Si 0e 0 W 2 V FD : (3.15)The conversion factor depends on sample thickness and for 300 m thick samples equation3.15 can be written with a xed numerical factor N eff =1:44 10 10 V ;1 cm ;3 V FD .The main sources of systematic error on determination of the full depletion voltageare in the selection of the measuring frequency and temperature. While measurementswere performed at dierent frequencies (1 kHz, 3 kHz, 10 kHz, 30 kHz, 100 kHz, 300 kHzand 1 MHz in the laboratory and 100 Hz, 1 kHz, 10 kHz and 100 kHz at the reactor), afrequency of 10 kHz was most usually used to determine FDV. 10 kHz is a compromisebetween a good signal to noise ratio 29 , sensitivity to traps with long trapping times andapplicability of the simple RC model used. Since it is also the most commonly usedfrequency of other groups, it also enables better comparison.In the laboratory, all measurements were performed at 5 Ctoavoid the temperatureeect. This was however not possible at the reactor, where measurements were performedat storage temperature.The systematic error due to the sources listed above is estimated to be below 10%.The variations of FDV due to dierent selections of the tted range for ts of the linearpart at high depletion depths is on a percent level. That means that the largest contributionto the error on introduction rates of dierent defects comes from the 15% systematicerror of the dosimetry.3.5.3 Bulk CurrentThe time development of reverse bulk current has been studied together with N eff . Toseparate bulk-generated current from the surface generated one, only diodes with guardrings were used in the analysis. While both guard ring and the pad were connected tothe same potential, only current owing through the pad was measured to reduce thecontribution of the surface current.29Impedance of capacitor is inversely proportional to the frequency of the measurement voltage.