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 47the <strong>in</strong>uence <strong>of</strong> changes <strong>in</strong> the small impedance <strong>of</strong> the voltage source on the measurements,decoupl<strong>in</strong>g resistors have been used (gure 3.10).Most <strong>of</strong> the measurements were per<strong>for</strong>med <strong>in</strong> a thermostat chamber at 5 C. Whenstored, samples were kept at stable temperature <strong>in</strong> one <strong>of</strong> the temperature stabilisedchambers. For bias<strong>in</strong>g dur<strong>in</strong>g storage Wentzel <strong>high</strong> voltage power supplies were used.Anneal<strong>in</strong>g and reverse anneal<strong>in</strong>g have been accelerated by heat<strong>in</strong>g <strong>in</strong> an oven stabilisedto 60 C, stable to about 2 C. Temperature history dur<strong>in</strong>g heat<strong>in</strong>g was measured by aPt100 sensor, placed close to the sample and connected to a Yokogawa controller. Thetemperature was read from the controller by computer and stored to a le. Measuredtemperature history was used to correct the equivalent heat<strong>in</strong>g time <strong>for</strong> temperaturevariations (eq. 4.5).3.5.2 C/V MeasurementsFull depletion voltage was determ<strong>in</strong>ed from the k<strong>in</strong>k <strong>in</strong> C/V plot (see section 2.1.6). Itwas used to determ<strong>in</strong>e the average eective dopant concentration accord<strong>in</strong>g to equations2.10 and 2.13.Capacitance versus voltage curves were measured by Hewlet-Packard LCR meter.Capacitance is determ<strong>in</strong>ed from the complex impedance. It is calculated from the measuredamplitude and phase shift <strong>of</strong> the current signal <strong>in</strong> response to an AC measur<strong>in</strong>gvoltage with user selected frequency. The AC voltage amplitude can be set from 5 mV to2V.High measurement voltages give theadvantage <strong>of</strong> better signal to noise ratio. Theyare however a signicant disturbance at bias voltages <strong>of</strong> few volts. Thus at the reactorthe amplitude <strong>of</strong> the measurement voltage was kept at 1 V due to harsh measurementconditions (long partially unshielded cables). At the laboratory, it was usually kept atlower values, typically 10 mV.Both HP4284A and HP4263B 23 LCR meters provide means to trans<strong>for</strong>m measuredcomplex impedance to dierent physical parameters accord<strong>in</strong>g to a selected model. S<strong>in</strong>cea partially depleted diode can be roughly described by an equivalent circuit as presented<strong>in</strong> gure 3.11, an adequate RC model should be selected. HP LCR-meters oer a serialand a parallel RC model, neither <strong>of</strong> them correspond<strong>in</strong>g to the equivalent circuit. Thusan approximate <strong>in</strong>uence <strong>of</strong> each component <strong>in</strong>themodel from g. 3.11 was estimated.Let us consider the most common case <strong>in</strong> these studies, a partially depleted diode<strong>of</strong> 1 cm 2 area and 300 m thickness, irradiated to about 10 14 n/cm 2 . Measurementtemperature <strong>of</strong> 5 C and measur<strong>in</strong>g frequency <strong>of</strong> 10 kHz are assumed.23A HP4263B LCR measure unit was used at the reactor, see section 3.3.2.