Radiation-Induced Formation ofH2*in Silicon

Abstract

Two kinds of hydrogen dimers have been observed in $c$-Si: the interstitial ${\mathrm{H}}_{2}$ molecule and the ${\mathrm{H}}_{2}^{*}$ complex. Theory predicts that ${\mathrm{H}}_{2}$ and ${\mathrm{H}}_{2}^{*}$ are within a few tenths of an eV of each other, but the two centers have never been observed to coexist. Instead, the irradiation of samples rich in ${\mathrm{H}}_{2}$ induces its conversion into ${\mathrm{H}}_{2}^{*}$. No such conversion has been thermally induced, and ${\mathrm{H}}_{2}^{*}$ is seen only in irradiated material. In the present paper, ab initio molecular-dynamics simulations demonstrate in real time how these reactions occur. A vacancy $\left(V\right)$ or a self-interstitial $\left(I\right)$ break up interstitial ${\mathrm{H}}_{2}$, resulting in the ${V,\mathrm{H},\mathrm{H}}$ or ${I,\mathrm{H},\mathrm{H}}$ complex. Then $V\ensuremath{-}I$ recombination forms ${\mathrm{H}}_{2}^{*}$, as in ${I,\mathrm{H},\mathrm{H}}+V\ensuremath{\rightarrow}{H}_{2}^{*}$.