Microscopic mechanisms of electron trapping by self-trapped holes and protons in amorphous SiO2

The electronic structure mechanisms underlying the electron capture characteristics of oxide-trapped holes (OTHs) and stable protons in amorphous SiO₂ are presented from first-principles Hartree-Fock calculations on model atomic clusters. The positive charge of the two species is found to have different spatial distribution. In the case of the OTHs, identified here with self-trapped holes (STHs), the positive charge is calculated to be localized on a bridging oxygen atom, creating a strong Coulombic attractive center for an electron. In contrast, the positive charge of a stable proton, is calculated to he delocalized over a large region of space, creating a weak Coulombic attractive center for an electron. Furthermore, the electron affinity for an STH is calculated to be much higher, by a factor of 35, than that for the protonated oxide complex. These results suggest that the probability of electron capture and recombination is much higher for an OTH than that for stable protons, explaining the recently observed difference in the electron-injected anneal characteristics of the two species in a-SiO₂