Structure and properties of defects in amorphous silica: new insights from embedded cluster calculations

Abstract

We briefly review some of the approaches which have been used to study the distributions of defect properties in amorphous silica and focus mainly on the implementation of the embedded cluster method. We illustrate some of the results obtained using this method and discuss the remaining problems using the example of oxygen vacancy defects in amorphous SiO 2 . The neutral vacancies are characterized by a wide distribution of formation energies and structural parameters. Our modelling predicts the two major structural kinds of positively charged vacancies (E centres): dimers and dangling bond centres. The local structure for both kinds of centres depends on the medium range structure of the surrounding amorphous network. We found that the majority of the dangling bond centres are unpuckered. The optical spectra and electron paramagnetic resonance parameters calculated for all defects are in good agreement with experimental data. The structural criteria which favour the formation of different kinds of centres in the original amorphous structure are formulated in terms of the average Si-O distance of oxygen ion with its two neighbouring silicon ions.

First-Principles Investigation of Low EnergyE′Center Precursors in Amorphous Silica

“…agreement with electron paramagnetic resonance (EPR) experiments [3,4]. However, significant challenges remain; the high concentration of E' centers observed experimentally in stoichiometric a-SiO 2 , see for example…”

“…Theoretical calculations have shown that an EPR-active center occurs when one of the silicon atoms retreats from the oxygen vacant site and becomes tetrahedrally coordinated with another oxygen atom in the amorphous network, known as the puckered configuration [3]. The resulting structure leaves an unpaired electron spin localized on a single silicon sp 3 hybrid orbital and has spectroscopic signals consistent with the EPR experiments on the E' γ center.…”

“…Theoretical calculations have shown that an EPR-active center occurs when one of the silicon atoms retreats from the oxygen vacant site and becomes tetrahedrally coordinated with another oxygen atom in the amorphous network, known as the puckered configuration [3]. The resulting structure leaves an unpaired electron spin localized on a single silicon sp 3 hybrid orbital and has spectroscopic signals consistent with the EPR experiments on the E' γ center. Electronic structure calculations have also shown that if the Si-Si bond in a NOV captures a hole and remains a dimer, the resulting properties agree well with the E' δ EPR signals [3,4]; this time the unpaired electron spin is delocalized over a pair of silicon sp 3 hybrid orbitals.…”

Atomic origin of high-temperature electron trapping in metal-oxide-semiconductor devices

“…Figure 1b shows the puckered structure in which one of the two Si atoms that originally bond to the missing O atom puckers back and bonds with an O atom, forming a 3-fold coordinated O (solid arrow), while the other Si atom retains a Si-dangling bond (dashed arrow). This structure plays a key role in hole-trapping-related phenomena in SiO 2 [10,[20][21][22]. Figure 1c is the new configuration of oxygen vacancy.…”

“…In Figure 1 we show the two known structures of an oxygen vacancy V O [19][20][21][22] along with the new structure that accounts for the experimental data. Figure 1a shows the dimer structure of V O that is formed when one O atom (marked by solid arrow) is removed.…”