Density functional theory of structural transformations of oxygen-deficient centers in amorphous silica during hole trapping: Structure and formation mechanism of theEγ′center

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We investigate the hole trapping process of a neutral oxygen vacancy in amorphous silicon dioxide $(a\text{\ensuremath{-}}\mathrm{Si}{\mathrm{O}}_{2})$ using cluster calculations based on the density functional theory (DFT) method. We show that trapping a hole at a neutral oxygen vacancy leads to the formation of several types of positively charged defects. One immediate consequence of the hole trapping process at the oxygen vacancy site is the creation of the positively charged dimer, in which a unpaired electron is almost equally distributed over the two Si aoms in the defect. Our calculations further demonstrate that the dimer configuration can be transformed into other minimum energy structures. Three possible relaxation channels are likely to exist, leading to the three distinctive defect configurations called the puckered, forward-oriented, and bridged hole-trapping oxygen-deficiency center (BHODC) configurations. To evaluate the stability of these positively charged defects against discharging, we then calculate the electrical levels for all the positively charged clusters investigated here. It is shown that the BHODC configuration has the highest electrical level, implying that this type of positively charged defect is the most stable configuration against electron trapping. We also calculate the hyperfine parameters and $g$ values of the BHODC using the DFT method. The calculated hyperfine parameters and $g$ values are in good agreement with those observed for the ${E}_{\ensuremath{\gamma}}^{\ensuremath{'}}$ center. These results corroborate our previous attribution of the ${E}_{\ensuremath{\gamma}}^{\ensuremath{'}}$ center [Uchino and Yoko, Phys. Rev. Lett. 86, 5522 (2001)].