A new understanding of near-threshold damage for 200 keV irradiation in silicon

Abstract

Recently we reported room temperature point defect creation and subsequent extended defect nucleation in nitrogen-doped silicon during 200 kV electron irradiation, while identical irradiation of nitrogen-free silicon produced no effect. In this paper, first principles calculations are combined with new transmission electron microscope (TEM) observations to support a new model for elastic electron-silicon interactions in the TEM, which encompasses both nitrogen doped and nitrogen free silicon. Specifically, the nudged elastic band method was used to study the energetics along the diffusion path during an electron collision event in the vicinity of a nitrogen pair. It was found that the 0 K estimate for the energy barrier of a knock-on event is lowered from ∼ 12 to 6.2 eV. However, this is still inadequate to explain the observations. We therefore propose an increase in the energy barrier for Frenkel pair recombination associated with N₂-V bonding. Concerning pure silicon, stacking fault formation near irradiation-induced holes demonstrates the participation of bulk processes. In low oxygen float zone material, 2–5 nm voids were formed, while oxygen precipitation in Czochralski Si has been verified by electron energy-loss spectroscopy. Models of irradiation-induced point defect aggregation are presented and it is concluded that these must be bulk and not surface mediated phenomena.