Divacancy acceptor levels in ion-irradiated silicon

High-purity n-type silicon samples have been irradiated with mega-electron-volt ions ${(}^1$ ${\mathrm{H}}^{+}$, ${}_{}{}^4{}_{}{}^{}\mathrm{He}_{}^{2+}{}_{}{}^{}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}_{}^{4+}{}_{}{}^{}$, ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}_{}^{7+}{}_{}{}^{}$, ${}_{}{}^{79}{}_{}{}^{}\mathrm{Br}_{}^{8+}{}_{}{}^{}$, and ${}_{}{}^{127}{}_{}{}^{}\mathrm{I}_{}^{10+}{}_{}{}^{}$), and the two divacancy-related acceptor levels ∼0.23 and ∼0.42 eV below the conduction band (${\mathit{E}}_{\mathit{c}}$), respectively, have been studied in detail using deep-level transient spectroscopy (DLTS). Depth concentration profiles show identical values for the two levels at shallow depths, while in the region close to the damage peak large deviations from a one-to-one proportionality are found. These deviations increase with ion dose and also hinge strongly on the density of energy deposited into elastic collisions per incoming ion. Evidence for a model of the two levels is presented and, in particular, the model invokes excited states caused by motional averaging and lattice strain associated with damaged regions.