Self-Trapped Excitons in Silicon Dioxide: Mechanism and Properties

Irradiating silica produces self-trapped excitons (STEs) that spontaneously create atomic-scale distortions on which they localize themselves. Despite enduring interest in STEs and subsequent defects in this key technological material, the trapping mechanism and geometry remain a mystery. Our ab initio study of STEs in $\alpha$-quartz using a many-electron Green’s function approach answers both questions. The STE comprises a broken O-Si bond with the hole localized on the defected oxygen and the electron on the defected silicon atom in a planar $s{p}^2$ conformation. The results further explain quantitatively the measured STE spectra.

Figure 1

Imaginary part of the frequency-dependent macroscopic dielectric function for $\alpha$-quartz. Red squares are calculated results neglecting electron-hole interactions, blue dashes are calculated including electron-hole interactions, and the solid black is experimental data [32].

Figure 2

Left: total ground-state energies (open circles) and triplet excited-state energies (solid dots) vs the $\mathrm{Si}\mathrm{\text{−}}\mathrm{O}$ bond length sampled along the relaxation path going from the delocalized exciton to the STE. The bond refers to the $\mathrm{Si}\mathrm{\text{−}}\mathrm{O}$ pair whose bond ruptures during formation of the STE. Right: excitation energy (or equivalently the luminescence energy at that configuration) for the triplet exciton vs the length of the ruptured bond.

Figure 3

Top: isosurfaces of probability for finding the hole in the self-trapped configuration. The view is along the $c$ axis. Smaller red and larger blue spheres represent Si and O atoms, respectively. Bottom: isosurfaces of the conditional electron probability when the hole is located at the position indicated by the square in the self-trapped configuration. Isosurfaces are at 80%, 60%, 40%, and 20% of the maximum.