EPR parameters ofE′centers inv−SiO2from first-principles calculations

Abstract

A first-principles investigation of E centers in vitreous silica (v-SiO 2 ) based on calculations of the electron paramagnetic resonance (EPR) parameters is presented. The EPR parameters are obtained by exploiting the gauge including projector augmented wave method as implemented in the QUANTUM-ESPRESSO package. First, we analyze the EPR parameters of a large number of Si 2 dimers. The g tensor of the Si 2 dimers is shown to possess an average rhombic symmetry and larger g principal values with respect to those observed, e.g., for the E γ center in silica. Furthermore, the g principal values clearly show a linear trend with the Si-Si dimer length. Our results suggest that the Si 2 dimers could correspond to an unidentified paramagnetic center, though occasionally the calculated g principal values of the Si 2 dimer might be compatible with those found experimentally for the E δ center. Next, we generate nondimer configurations by a procedure involving structural relaxations in the subsequent positively charged states. In particular, puckered, unpuckered, doubly puckered, and forward-oriented configurations are generated. The distributions of the calculated EPR parameters of the puckered and unpuckered configurations further support the assignment of the E γ center to an unpaired spin localized at a threefold coordinated silicon dangling bond. Moreover, by analyzing Fermi contacts and g tensors of the puckered and forward-oriented configurations, we suggest the assignment of the E α center to the latter type of configurations. This work also suggests that the differences in the EPR parameters of E α and E γ centers mainly arise from the strained geometry of the silicon dangling bond. In the forward-oriented configurations, one Si-O bond is about 0.2Å longer than the remaining two, whereas in the silicon dangling bond of the puckered and unpuckered configurations, all three bonds have a length of ∼1.6Å each.

Magnetic stability of oxygen defects on the SiO2 surface

“…[15][16][17] Calculations of the EPR of E centers remains an active field. 18 Most calculations have focused on bulk silica, but similar defects exist on silica and α-quartz surfaces. [19][20][21] Surface defects are particularly relevant in circuit processing because they can be buried at interfaces by overlaying other materials on top of the silica, and because their concentrations and properties can be controlled through the fabrication process.…”

First principles study of oxygen vacancy defects in amorphous SiO2

“…The result is very close to 2.94 Å obtained in the previous report. 23 Even though the Si atoms relax far away from the center of oxygen vacancy, none of Si atoms can relax past the plane of its three oxygen neighbors. The unpaired spin density spreads between these two Si atoms, and the average charge is 0.54 |e| with a std of 0.20 |e|.…”

“…It is obvious that the average distance d Si-Si of the puckered 5× configurations is shorter than that of the puckered 4 × configurations, which is in good accordance with the previous report. 23 For the puckered 4× and 5 × configurations, the unpaired electron strongly localizes on the threefold silicon atom and around its three oxygen neighbors. The average Fermi contact of the puckered 5× configurations is 40.73 mT, which is close to the experimental result of the E γ center.…”

“…Due to the presence of an energy barrier between the dimer configuration and the puckered ones, the outcomes are always the positively charged Si 2 dimers. 23 In order to generate other configurations of the positively charged oxygen vacancy, we shift back the position of each Si atom in the positively charged dimers past the plane defined by its three oxygen neighbors to the nearest back oxygen atom O B and then perform geometry optimization by using the intentionally deformed configurations. 39 TABLE II.…”

“…The generally recognized experimental hyperfine splittings for E γ , E α and E δ are 42, 49 and 10 mT, respectively. [21][22][23] Electronic structure theory, especially density functional theory (DFT) methods, becomes very popular for modeling and understanding results from magnetic resonance experiments. One of the most important parameters in EPR experiments on paramagnetic systems is the isotropic (Fermi contact) hyperfine coupling constant, which describes the magnetic interaction between the nuclear and electronic spins under isotropic averaging conditions.…”