Radiation Damage in Ge and Si Detected by Carrier Lifetime Changes: Damage Thresholds

Minority carrier lifetime, $\tau$, in semiconductors is shown to be more sensitive, by a factor of ${10}^4$, to radiation-induced defects than the conductivity. Thus, in some cases, the introduction of as few as ${10}^{10}$ defects per ${\mathrm{cm}}^3$ can be detected by its effect on $\tau$. Both direct measurements of $\tau$, and measurements of dependent parameters (such as the photovoltaic effect and the particle voltaic effect) are described for Si and Ge. Using such parameters, and an electron accelerator, the minimum energy needed to produce a Frenkel defect was found to be 14.5±0.4 ev in Ge and 12.9±0.6 ev in Si. An analysis of the phenomenon shows how the location of the radiation-induced energy levels and the relative minority carrier capture cross sections can be determined experimentally. The shape of the curve of the displacement cross section versus the incident particle energy is compared to calculations from collision theory. Qualitative explanations for the observed "tail" on this curve are presented. Calculations based on these explanations fail to yield complete agreement with the experimental curves.