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

3. Irradiation Facility 47the inuence of changes in the small impedance of the voltage source on the measurements,decoupling resistors have been used (gure 3.10).Most of the measurements were performed in a thermostat chamber at 5 C. Whenstored, samples were kept at stable temperature in one of the temperature stabilisedchambers. For biasing during storage Wentzel high voltage power supplies were used.Annealing and reverse annealing have been accelerated by heating in an oven stabilisedto 60 C, stable to about 2 C. Temperature history during heating 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 heating time for temperaturevariations (eq. 4.5).3.5.2 C/V MeasurementsFull depletion voltage was determined from the kink in C/V plot (see section 2.1.6). Itwas used to determine the average eective dopant concentration according to equations2.10 and 2.13.Capacitance versus voltage curves were measured by Hewlet-Packard LCR meter.Capacitance is determined from the complex impedance. It is calculated from the measuredamplitude and phase shift of the current signal in response to an AC measuringvoltage with user selected frequency. The AC voltage amplitude can be set from 5 mV to2V.High measurement voltages give theadvantage of better signal to noise ratio. Theyare however a signicant disturbance at bias voltages of few volts. Thus at the reactorthe amplitude of 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 transform measuredcomplex impedance to dierent physical parameters according to a selected model. Sincea partially depleted diode can be roughly described by an equivalent circuit as presentedin gure 3.11, an adequate RC model should be selected. HP LCR-meters oer a serialand a parallel RC model, neither of them corresponding to the equivalent circuit. Thusan approximate inuence of each component inthemodel from g. 3.11 was estimated.Let us consider the most common case in these studies, a partially depleted diodeof 1 cm 2 area and 300 m thickness, irradiated to about 10 14 n/cm 2 . Measurementtemperature of 5 C and measuring frequency of 10 kHz are assumed.23A HP4263B LCR measure unit was used at the reactor, see section 3.3.2.