Defect creation by subthreshold irradiation in semiconductors

Defects are created in a crystal lattice by irradiation when the incident particles displace atoms from their substitutional positions. When the mechanism is elastic scattering, displacement occurs when the energy transmitted by the incident particle, $T$, is greater than $T_d$, the threshold energy. It has been argued, however, that defects can also be created in semiconductors for $T<\mathrm{}{T}_d$ through mechanisms involving electron excitation. The occurrence of such mechanisms has not been demonstrated conclusively because effects related to surface states or impurity diffusion cannot be discounted in the experiments which have been reported. To obtain further evidence of intrinsic subthreshold defect production, we have used a transient-capacitance technique which is appropriate because it can detect low defect densities, measure the distribution of defects below the surface of the crystal, and distinguish individual defect species. Because we do not see any defects, which implies defect introduction rates lower than 2×${10}^{-8\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$, in both silicon and germanium irradiated with electrons of below-threshold energy, we conclude that intrinsic point defects are not created by subthreshold irradiation.