Energy dependence ofE’ spin production in x-irradiated vitreousSiO2

Abstract

We have irradiated fused silica with monochromatic x rays from a synchrotron radiation source, and have shown for the first time that the efficiency of producing E' spins increases nonlinearly with photon energy for silica containing a large concentration of preexisting defects, and almost linearly for silica containing very few preexisting defects. It is concluded that £" defects are created or activated with efficiencies proportional to approximately the first or second powers, respectively, of the initial energy of the energetic electrons produced by x-ray absorption. PACS numbers: 6l.80.Cb, 61.42.+h, 76.30.Mi There are three distinct point defects which have been observed at room temperature by electron paramagnetic resonance (EPR) in irradiated high purity silica: the £" center, the nonbridging oxygen hole center (NBOHC), and the peroxyl radical oxygen hole center (PROHC). The nature of these dominant broken bond defects in vitreous (r-) SiC>2 has been reviewed by Griscom [1], The E' center was first studied by Weeks [2], and has been assigned by Feigl, Fowler, and Yip [3] to an electron trapped on a three-bonded Si atom which is part of an asymmetrically relaxed oxygen vacancy. The NBOHC has been assigned by Stapelbroek et al. [4] to a hole trapped on the unbonded p 2 orbital of a nonbridging oxygen, while the PROHC has been attributed by Friebele et al, [5] to a hole trapped on the unbonded p 2 orbital of the singly bonded oxygen atom in a dangling peroxyl radical. The growth of the E' signal with x irradiation in fused silica depends strongly on the concentration of hydroxyl groups, as well as the annealing history of the sample. Figure 1 displays data from Galeener and co-workers [6,7], showing the total number of E' spins (TV) produced as a function of dose (D) for identically annealed fused silica samples that had been irradiated with x rays from a 40-kV Cu-target x-ray tube. The N induced in Suprasil-1 [8] (a "wet" silica which contains a high concentration of hydroxyl groups, [OH] -1200 ppm) is linear with D over the entire dose regime investigated, while the "dry" Suprasil-Wl ([OH]-4 ppm) exhibits a nonlinear behavior. Galeener and co-workers [6,7] have shown that the new spins appearing in the high dose linear regions of Fig. 1 in both Suprasil-1 and Suprasil-Wl are due to the creation of new E' defect centers, while the nonlinear growth at lower doses in Suprasil-Wl is due to the activation of preexisting defects. Creation by x irradiation involves the breaking of one or more bonds at a previously completely bonded site, while activation merely involves the transfer of an electron or hole to or from a previously existing broken bond [7]. Quantitative analysis of the data in Fig. 1 shows that at doses below about 3 Mrad (SiOa) both curves are again linear, and that at least 80% of the E' spins observed in Suprasil-1 have been created, while in the Suprasil-Wl at least 80% have been activated from preexisting defects [7]. Thus creation dominates in wet Suprasil-1 and activation dominates in dry Supr...

Si ion implantation-induced damage in fused silica probed by variable-energy positrons

“…8 The Ps formation probability for synthesized vitreous SiO 2 specimens with OH concentrations Ͻ5 ppm was found to be lower than that for one with an OH concentration of Ͻ200 ppm. Kerwin and Galeener 22 have shown that the creation of EЈ centers due to x-ray irradiation increases linearly with radiation dose for OHϷ1200 ppm and nonlinearly for OHϷ4 ppm. The increased relative growth of the EЈ center in the low OH concentration sample was attributed to the abundance of preexisting EЈ centers which merely require activating by irradiation.…”