Effects of Single Neutron-Induced Displacement Clusters in Special Silicon Diodes

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Abstract
Radiation damage in uniform silicon avalanche diodes having an active volume of 10−11 cm3 has been investigated. Radiation-induced centers contribute to the carrier generation within the diode breakdown region, causing a corresponding increase in the avalanche pulse rate of the diode. The high sensitivity and the small volume of the diodes allow the study of permanent radiation damage resulting from single primary neutron collisions. By measuring the distribution function of the pulse rate on a statistical lot of diodes, a relationship between the pulse rate and the energy dissipated in atomic processes is derived. A superlinear increase of pulse rate with energy is observed. In general the higher pulse rates have a temperature dependence characterized by a lower activation energy than is the case for the lower pulse rates. For comparison, the effect of 2-MeV electron irradiation has been studied. The pulse rate is found to increase linearly with integrated flux. At low fluxes the pulse rate is dominated by generation centers only, while in heavily irradiated diodes trapping effects become observable. Also at low fluxes the observed activation energy for carrier generation is 0.66 eV, while at higher fluxes this activation energy is lower by as much as 0.3 eV. The dependence of the observed activation energy on the flux in the electron case and the generally lower activation energies for the higher pulse rates in the neutron case is attributed to a tunneling interaction between individual defects. The defects in the larger neutron displacement damage clusters will have a wider spread in energy levels than those in the smaller clusters. This, in connection with the interaction between the individual defects, results in a reduced activation energy and an increased pulse rate.