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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2. Operation and Radiation Damage <strong>of</strong> Silicon Detectors 23Figure 2.4: C/V measurement <strong>of</strong> a diode, irradiated to 5 10 13 n/cm 2 and annealed <strong>for</strong> vemonths at 20 C. The dierence <strong>in</strong> signals obta<strong>in</strong>ed by dierent measurement frequencies isshown. The measurement was per<strong>for</strong>med at 5 C.giv<strong>in</strong>g the N eff value <strong>in</strong> terms <strong>of</strong> applied voltage. To determ<strong>in</strong>e it <strong>in</strong> terms <strong>of</strong> depth onecan use the relation [8]w(V )= Si 0 SC(V ) : (2.45)The situation is more complicated <strong>in</strong> case <strong>of</strong> deep energy levels. For those theemission time, dened as the <strong>in</strong>verse <strong>of</strong> emission probability T = 1e T N CV(eq. 2.31), canbe large compared to the oscillation time <strong>of</strong> the measur<strong>in</strong>g 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. There<strong>for</strong>e a l<strong>in</strong>ear decrease <strong>in</strong> temperaturecorresponds to a logarithmic decrease <strong>in</strong> frequency. Achange <strong>in</strong> the measur<strong>in</strong>g frequencyor temperature thus aects the signal from deep energy levels and consequently the shape<strong>of</strong> the C/V curve (gure 2.4). It also results <strong>in</strong> somewhat <strong>high</strong>er full depletion voltage asdeterm<strong>in</strong>ed from the k<strong>in</strong>k <strong>in</strong> C/V plot at lower frequencies. This eect was systematicallystudied by [16, 17] who report about 10% <strong>in</strong>crease <strong>of</strong> FDV asfrequency changes from 10kHz to 1 kHz. It is suggested the reason may be the presence <strong>of</strong> a transition region betweenthe space charge region and the neutral bulk <strong>in</strong> heavily irradiated <strong>silicon</strong>. Measurementsignals <strong>of</strong> dierent frequencies might thus probe dierent parts <strong>of</strong> this transition region.To avoid the <strong>in</strong>uence <strong>of</strong> this eect when compar<strong>in</strong>g dierent samples, all measure-