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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3. Ir<strong>radiation</strong> Facility 51The full depleti on voltage was determ<strong>in</strong>ed as the position <strong>of</strong> the k<strong>in</strong>k <strong>in</strong> 1/C 2 vs.voltage curve. The exact position <strong>of</strong> the k<strong>in</strong>k was determ<strong>in</strong>ed by the <strong>in</strong>tersection <strong>of</strong> twol<strong>in</strong>es, one tted to the slope at large depletion depths and the second to the plateauat full depletion (see g. 3.12). Fitted range was determ<strong>in</strong>ed manually after a visualexam<strong>in</strong>ation <strong>of</strong> the plot.To determ<strong>in</strong>e the eective dopant concentration N eff , a homogeneous distributionover the sample was assumed. In such case N eff can be determ<strong>in</strong>ed 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 <strong>for</strong> 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 ma<strong>in</strong> sources <strong>of</strong> systematic error on determ<strong>in</strong>ation <strong>of</strong> the full depletion voltageare <strong>in</strong> the selection <strong>of</strong> the measur<strong>in</strong>g frequency and temperature. While measurementswere per<strong>for</strong>med at dierent frequencies (1 kHz, 3 kHz, 10 kHz, 30 kHz, 100 kHz, 300 kHzand 1 MHz <strong>in</strong> the laboratory and 100 Hz, 1 kHz, 10 kHz and 100 kHz at the reactor), afrequency <strong>of</strong> 10 kHz was most usually used to determ<strong>in</strong>e FDV. 10 kHz is a compromisebetween a good signal to noise ratio 29 , sensitivity to traps with long trapp<strong>in</strong>g times andapplicability <strong>of</strong> the simple RC model used. S<strong>in</strong>ce it is also the most commonly usedfrequency <strong>of</strong> other groups, it also enables better comparison.In the laboratory, all measurements were per<strong>for</strong>med at 5 Ctoavoid the temperatureeect. This was however not possible at the reactor, where measurements were per<strong>for</strong>medat storage temperature.The systematic error due to the sources listed above is estimated to be below 10%.The variations <strong>of</strong> FDV due to dierent selections <strong>of</strong> the tted range <strong>for</strong> ts <strong>of</strong> the l<strong>in</strong>earpart at <strong>high</strong> depletion depths is on a percent level. That means that the largest contributionto the error on <strong>in</strong>troduction rates <strong>of</strong> dierent defects comes from the 15% systematicerror <strong>of</strong> the dosimetry.3.5.3 Bulk CurrentThe time development <strong>of</strong> reverse bulk current has been studied together with N eff . Toseparate bulk-generated current from the surface generated one, only diodes with guardr<strong>in</strong>gs were used <strong>in</strong> the analysis. While both guard r<strong>in</strong>g and the pad were connected tothe same potential, only current ow<strong>in</strong>g through the pad was measured to reduce thecontribution <strong>of</strong> the surface current.29Impedance <strong>of</strong> capacitor is <strong>in</strong>versely proportional to the frequency <strong>of</strong> the measurement voltage.