Structure, Properties, and Dynamics of Oxygen Vacancies in Amorphous ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$

Oxygen vacancies in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ have long been regarded as bistable, forming a Si-Si dimer when neutral and a puckered configuration when positively charged. We report first-principles calculations of O vacancies in amorphous ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ supercells that unveil significantly more complex behavior. We find that the vast majority of O vacancies do not pucker after capture of a hole, but are shallow traps. The remaining vacancies exhibit two distinct types of puckering. Upon capturing an electron, one type forms a metastable dipole, while the other collapses to a dimer. A statistical distribution of O vacancies is obtained, and the implications for charge transport and trapping in ${\mathrm{S}\mathrm{i}\mathrm{O}}_2$ are discussed.

Figure 1

Schematics of (a) “dimer,” (b) “fourfold puckered,” and (c) “fivefold puckered” oxygen vacancies as described in the text. Dotted lines show bonds prior to removing the O atom (${\mathrm{V}}_o$). The gray shading shows the unpaired electron density.

Figure 2

Total energies following electron capture by positively charged (a) fivefold pucker and (b) fourfold pucker. P denotes pucker position and D denotes dimer position.

Figure 3

Schematics illustrating the rules for fourfold and fivefold puckers after removing an O (${\mathrm{V}}_o$) and capturing a hole as described in the text.