Noble-gas-related defects in Si and the origin of the 1018 meV photoluminescence line

The implantation of noble-gas ions in Si results in the appearance of photoluminescence centers that are closely associated with the intrinsic-defect luminescence at 1018 meV. We present the results of a theoretical study aimed at identifying these defects. The calculations are performed in molecular clusters at the ab initio and approximate ab initio Hartree-Fock levels. Our predictions are as follows. (i) Interstitial noble-gas impurities are not associated with the luminescence and their activation energies for diffusion are large. (ii) Noble-gas atoms do not become substitutional, but are strongly repelled by vacancies instead. This suggests an unusual vacancy-enhanced diffusion mechanism. (iii) Noble-gas-divacancy complexes are very stable and their calculated properties show them to be excellent candidates as the defects responsible for the noble-gas-related photoluminescence. (iv) Larger vacancy aggregates (up to the hexavacancy) cannot be responsible for the observed luminescence, although the formation of a hexavacancy-noble-gas complex could nicely explain the disappearance of the luminescence at higher temperatures. (v) Our results imply that the 1018-meV line is due to the neutral divacancy.