Electron paramagnetic resonance on iron-related centers in silicon

The behavior of interstitial iron in high-resistivity dislocation-free silicon has been studied by annealing and by electron irradiation and subsequent annealing. Annealing of iron-doped samples at temperatures above 120°C yielded one, new electron paramagnetic resonance (EPR) spectrum labeled Si-$\mathrm{NL}22\mathrm{}$. For the corresponding center we suggest a cluster of four iron atoms in a trigonal arrangement. Irradiation at about 20°C yielded many new EPR spectra, part of which are related with iron. One center with only one iron atom in trigonal symmetry was identified. Its spectrum is labeled Si-$\mathrm{NL}19\mathrm{}$. As a model we propose a $〈111〉$-distorted substitutional iron atom. Four centers involving two equivalent iron atoms are formed. The spectra and tentative models are Si-$\mathrm{NL}20\mathrm{}\triangleq {(2\mathrm{}{\mathrm{Fe}}_i+V)}^{-}$, Si-$\mathrm{NL}21\mathrm{}\triangleq {(2\mathrm{}{\mathrm{Fe}}_i+2V)}^{+/-}$, Si-$\mathrm{NL}24\mathrm{}\triangleq {\left(2\mathrm{}{\mathrm{Fe}}_i\right)}^{+}$, and Si-$\mathrm{NL}25\mathrm{}\triangleq {(2\mathrm{}{\mathrm{Fe}}_i+V)}^{+}$. Many spectra without resolved hyperfine interactions with iron were observed. Only one of these spectra was analyzed. This spectrum, labeled Si-$\mathrm{NL}23\mathrm{}$, has only triclinic symmetry. The formation of iron-iron pairs and the disappearance of isolated interstitial iron during irradiation at only 20°C shows that iron is subject to radiation-induced diffusion. From our study we conclude that in the absence of dislocations or acceptors as precipitation centers, isolated interstitial iron does not become substitutional during annealing. Instead it forms pairs and eventually larger clusters.