Nature of intrinsic and extrinsic electron trapping in SiO2

Using classical and ab initio calculations we demonstrate that extra electrons can be trapped in pure crystalline and amorphous SiO${}_{2}$ (a-SiO${}_{2}$) in deep band gap states. The structure of trapped electron sites in pure a-SiO${}_{2}$ is similar to that of Ge electron centers and so-called [SiO${}_{4}$/Li]${}^{0}$ centers in $\ensuremath{\alpha}$ quartz. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons in crystalline and amorphous silica. The calculations demonstrate that an extra electron can be trapped at a Ge impurity in $\ensuremath{\alpha}$ quartz in six different configurations. An electron in the [SiO${}_{4}$/Li]${}^{0}$ center is trapped on a regular Si ion with the Li ion residing nearby. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO${}_{2}$, while the electron self-trapping in $\ensuremath{\alpha}$ quartz requires overcoming a barrier of about 0.6 eV. The precursors for electron trapping in amorphous SiO${}_{2}$ comprise wide ($\ensuremath{\ge}$132${}^{\ensuremath{\circ}}$) O--Si--O angles and elongated Si--O bonds at the tails of corresponding distributions. Using this criterion, we estimate the concentration of these electron trapping sites at $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{19}$ cm${}^{\ensuremath{-}3}$.