| Summary: | Displacement damage from individual heavy ions results in discrete, measurable electrical degradation in <sup>252</sup>Cf-irradiated silicon diodes. This work presents measurements of discrete increases in diode reverse current, or current steps, associated with damage from fission fragments emitted by a <sup>252</sup>Cf radiation source. A current-to-voltage circuit has been constructed to measure femtoampere-regime current steps following displacement damage as well as picoampere-regime current pulses caused by ionizing energy deposition. Because <sup>252</sup>Cf is a source of fission fragments, alpha particles and neutrons, current pulse size is used to differentiate pulses associated with heavy ions from those associated with alpha particles and secondary ionization from neutrons. Measurable current steps are only observed in tandem with current pulses associated with heavy ions. In the 3 to 5 minutes following a current step, reverse current relaxes to a new stable value that higher than the magnitude of reverse current before the step. This relaxation period is associated with short term annealing.
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The magnitude distribution of heavy ion-induced current can be calculated with Shockley-Read-Hall (SRH) theory when the expression for generation lifetime incorporates the effects of electric fields in depletion regions. A priori knowledge of experimental damage factors is not required to calculate the magnitude distribution of current steps. Radiation-induced defect density is obtained with Monte Carlo simulations of atomic displacements. Electric field effects are incorporated by modeling midgap defects as 1-D Coulomb potentials in the presence of electric field strengths obtained from TCAD simulations.
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The maximum magnitudes of heavy ion-induced current steps obtained from the expression for SRH generation are consistent with the largest current steps measured in <sup>252</sup>Cf-irradiated JFET diodes when electric field enhancement of defect emission rates, radiation-induced defect density, and the proximity of multiple depletion regions are taken into account.
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