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Electron paramagnetic resonance data on x-ray irradiated Sn-doped SiO 2 samples produced by the sol-gel method are presented. Two variants of Sn-related sites were identified, an orthorhombic one with g 1 ϭ1.994, g 2 ϭ1.986, and g 3 ϭ1.975 and an axial one with g ʈ ϭ1.994 and g Ќ ϭ1.977. A relation between g anisotropy and the spin-orbit coupling constant along the isoelectronic Si-Ge-Sn series strongly supports the attribution to EЈ-Sn centers consisting of unpaired spins in sp 3 orbitals of three-coordinated Sn sites. An asymmetric 215 mT doublet due to hyperfine interaction with Sn isotopes with non-null nuclear spin confirms this assignment.

We report photoluminescence ͑PL͒ data on amorphous SiO 2 in the spectral range of the ␣ ͑4.3 eV͒, ␤ ͑3.1 eV͒, and ␥ ͑2.7 eV͒ emissions excited with synchrotron radiation. Differently prepared and treated samples are compared. Neutron irradiation gives rise to PL features distinct from those observed both in unirradiated Ge-doped silica and in samples with other impurities. Our data suggest that the defect-formation processes determine the different PL patterns observed in SiO 2 , while the usually proposed distinction between Si-and Ge-like centers is not relevant. ͓S0163-1829͑98͒02231-0͔Silica-based materials are widely employed both in microelectronics and fiberoptics. Point defects play an important role in this regard owing to their effects on the physical properties of the material. In fact, impurities and radiationinduced defects are not only related to undesirable degradation of electrical or optical characteristics, since defects can induce specific features for particular applications. For example, substitutional doping and irradiation treatments make silica-based optical fibers suitable for the production of modulated refraction index patterns by defect photoconversion ͑Hill gratings 1,2 ͒. Therefore, defects are currently investigated to identify their local structure and the relation with the impurities introduced in the SiO 2 network. In particular, photoluminescence ͑PL͒ from radiative decay of photoexcited defects may give useful insight on the defect structures.Several PL bands are observed in silica. Among these, three emissions peaked at about 4.3 eV ͑␣ band͒, 3.1 eV ͑␤ band͒, and 2.7 eV ͑␥ band͒ are excited at 5 and above 7 eV. 3-12 Distinct components within the ␣ band ͑at 4.4 and 4.3 eV, we will refer to as ␣ a and ␣ b , respectively͒ were resolved. 3-8 Also the ␤ band seems to possess a double structure, 9,10 although less pronounced, while the ␥ band appears as a broad emission centered at 2.7 eV with an excitation-dependent peak energy. 11 The involved electronic transitions are usually attributed to one type of defectprobably an ''oxygen deficient center'' ͑ODC͒-but with two slightly different variants. 13,14 In fact, two types of PL patterns are observed-the ''␣ a -␥'' and the ''␣ b -␤'' patterns-since the ␣ a band is generally accompanied by the ␥ PL while the ␤ band is always observed together with the ␣ b band. The ␣-like emissions show decay times of a few ns, while ␤ and ␥ PL's are characterized by slower decay times of the order of 10 2 s. 13 Defect models were elaborated to account for the presence of two correlated emissions ͑the ␣ a and ␥ bands or the ␣ b and ␤ bands͒ and the observed decay kinetics. In particular, ODC like Si-Si or -Si-sites ͑the neutral oxygen vacancy and the divalent cation site͒ can adequately describe the phenomenology. 13,14 In the Si-Si model, a doubly occupied bonding and an empty * antibonding state appear from the combination of the sp 3 hybrids on each Si atom. The ground state is the singlet S 0 () 2 (*) 0 , the first excited state is the triplet T 1...

Temperature dependent photoluminescence from porous silicon nanostructures: Quantum confinement and oxide related transitions J. Appl. Phys. 110, 094309 (2011) Micropipe absorption mechanism of pore growth at foreign polytype boundaries in SiC crystalsVapor-phase silanization of oxidized porous silicon for stabilizing composition and photoluminescence J. Appl. Phys. 105, 114307 (2009) Highly efficient and air-stable near infrared emission in erbium/bismuth codoped zeolites

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