Can the electric field in radiation-damaged silicon pad diodes be determined by admittance and current measurements?

Abstract

A method is proposed for determining the electric field in highly-irradiated silicon pad diodes using admittance-frequency (Y−f), and current measurements (I). The method is applied to Y−f and I data from square n+p diodes of 25 mm2 area irradiated by 24 GeV/c protons to four 1 MeV neutron equivalent fluences between 3×1015 cm−2 and 13×1015 cm−2. The measurement conditions were: Reverse voltages between 1 V and 1000 V, frequencies between 100 Hz and 2 MHz and temperatures of −20 °C and −30 °C. The position dependence of the electric field is parameterised by a linear dependence at the two sides of the diode, and a constant in the centre. The parameters as a function of voltage, temperature and irradiation fluence are determined by fits of the model to the data. For voltages below about 300 V all data are well described by the model, and the results for the electric field agrees with expectations: Depleted high-field regions towards the two faces and a constant low electric field in the centre, with values which agrees with the field in an ohmic resistor with approximately the intrinsic resistivity of silicon. For conditions at which the low field region disappears and the diode is fully depleted, the method fails. This happens around 300 V for the lowest irradiation fluence at −30 °C, and at higher voltages for higher fluences and lower temperatures. In the conclusions the successes and problems of the method are discussed.