Characterization ofE′δand triplet point defects in oxygen-deficient amorphous silicon dioxide

Abstract

We report an experimental study by electron paramagnetic resonance (EPR) of γ ray irradiation induced point defects in oxygen deficient amorphous SiO 2 materials. We have found that three intrinsic (E' γ , E' δ and triplet) and one extrinsic ([AlO 4 ] 0 ) paramagnetic centers are induced. All the paramagnetic defects but E' γ center are found to reach a concentration limit value for doses above to the hyperfine structure of the E' δ center, originating from the interaction of the unpaired electron with a nucleus of 29 Si (I=1/2). Analogies between the microwave saturation properties of E' γ and E' δ centers and between those of their hyperfine structures are found and suggest that the unpaired electron wave function involves similar Si sp 3 hybrid orbitals; specifically, for the E' δ the unpaired electron is supposed to be delocalized over four such orbitals of four equivalent Si atoms.Information on the structural model of the triplet center are also obtained indicating that it could consists of the same microscopic structure as the E' δ but for a doubly ionized state.

Si29Hyperfine Structure of theE′αCenter in Amorphous Silicon Dioxide

“…Furthermore, we have found that the concentration of twofold coordinated Si defect in the KUVI sample before thermal treatment is about one order of magnitude lower than the concentration of the thermally induced E' α centers, indicating that the twofold coordinated Si does not act as precursor defect for the E' α center. Consequently, we suggest that the E' α center arises from an oxygen vacancy by trapping a hole in a process similar to that proposed for the E' γ [8,9]. Since we exclude that an isothermal treatment at T=630 K could be able to move an oxygen atom out from its regular site of a-SiO 2 , as suggested in the model proposed by Griscom [3] for the E' α , we suppose that oxygen vacancies present in the material before thermal treatment at T=630 K are involved.…”

“…After irradiation of the materials considered, E' δ and E' γ centers are induced, but virtually no EPR signal due to E' α centers is detected [7][8][9]. As discussed extensively in our previous works [7][8][9], upon thermal treatment at T>500 K the EPR signal of E' γ and E' δ centers increases as a consequence of a hole transfer process form the [AlO 4 ] 0 centers to the sites precursors of the E' centers.…”

“…As discussed extensively in our previous works [7][8][9], upon thermal treatment at T>500 K the EPR signal of E' γ and E' δ centers increases as a consequence of a hole transfer process form the [AlO 4 ] 0 centers to the sites precursors of the E' centers. After prolonged thermal treatments, a third contribution to the EPR spectrum becomes evident, suggesting that E' α centers are also induced in the same materials by hole transfer [16].…”

“…Since its first observation [6] the microscopic model of this point defect has been largely debated [9]. However, a recent experimental investigation [7] has pointed out that the unpaired electron wave function of this defect is delocalized over four nearly equivalent Si atoms, supporting a microscopic model of the E' δ center consisting in an unpaired electron delocalized over a pair of nearby oxygen vacancies or over a 5-Si cluster in aSiO 2 [7][8][9].…”

“…The E' δ center is characterized by a nearly symmetric EPR main resonance with g∼2.002 and a 29 Si hyperfine doublet split by ∼10 mT [1][2][3][6][7][8][9]. Since its first observation [6] the microscopic model of this point defect has been largely debated [9].…”