Determination of the characteristic signal for positron annihilation at divacancies in ion-irradiated silicon

Variable energy positron annihilation has been used to study simple defects created in (100) silicon by high-energy (1.0 and 2.6 MeV) proton irradiations at liquid nitrogen temperature. The damage profiles - which are uniform over depths of several micrometers - were successfully scaled to account for the different defect production rates of the two implant energies. S-parameter values were found to increase over an effective range of three orders of magnitude in the irradiation fluence before saturating at a value of 1.033 with respect to the bulk. Although determination of the exact value is complicated by the presence of impurity-based defect complexes (especially boron and oxygen), this value has been attributed as being representative of positron annihilation at divacancies within the silicon lattice.