Calculated and Measured Displacement Damage in Silicon for Monoenergetic Neutrons

Displacement damage in silicon has been calculated in 5 keV intervals from 0.1 to 20 MeV, and it has been measured with monoenergetic neutrons (accelerator produced) at a variety of energies from 0.7 to 14.2 MeV. The program to calculate damage determines the Lindhard nuclear stopping fraction corrected for multiple collisions, thus giving the fraction of the recoil energy available for displacement damage. The information available on an evaluated neutron cross section file (magnetic tape) is used, and the displacement energy is determined at many angles at each energy for each nuclear reaction provided in the file. The contributions from all reactions are then summed to give the energy available for displacements as a function of neutron energy. For the measurements, wide-base silicon diodes were exposed to accelerator-produced neutrons at each energy and typical energy spreads were about 50 keV. The diodes were exposed for accelerator runs of a few hours and, after the irradiations, the change in forward voltage at fixed current (injection level) was observed. A high injection level produces the required sensitivity and the low levels of neutron exposure (~3 × 1010 n/cm2) insure that carrier lifetime degradation is the only physical property of the diode that changes significantly. The results show, for example, that there is a factor-of-two more damage at 0.957 MeV where there is a peak in the cross section than at either 0.7 or 1.16 MeV. Also, 1.63 MeV neutrons produce more damage than 14.2 MeV neutrons.