Native and radiation-induced photoluminescent defects inSiO2: Role of impurities

Abstract

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...

Intense Ultraviolet-Blue Photoluminescence from SiO2 Embedded Ge Nanocrystals Prepared by Different Techniques

“…Similar UV emission has been previously reported for Ge implanted SiO 2 film 4 and unirradiated SiO 2 film containing different impurities. 9 Oxygen deficient defects in SiO 2 with different environment are believed to result in different UV bands. 9 Tong et al observed adjustable UV emission bands from silicon rich oxide films.…”

“…9 Oxygen deficient defects in SiO 2 with different environment are believed to result in different UV bands. 9 Tong et al observed adjustable UV emission bands from silicon rich oxide films. 23 Kim et al 24 observed an UV PL band at 365 nm from silicon rich oxide layer and ascribed it to the hole trapped E' center.…”

“…7 Zacharias and Fauchet 3 argued that ∼400 nm blue luminescence must be related to the formation of Ge or GeO 2 nanocrystals in the oxide matrix and they suggested that defects at the NC/matrix interface is responsible for the emission. On the other hand, Liao et al 8 and Feinardi and Paleari 9 observed violet PL in SiO 2 samples that does not contain Ge atoms and the violet emission was attributed to oxygen deficient defects in SiO 2 . Violet luminescence in Ge nanocrystals/Ge oxide structures formed by dry oxidation of polycrystalline SiGe has been attributed to defects at the NC Ge/GeO 2 interface.…”

Photoluminescence from Pressure — Annealed Nanostructured Silicon Dioxide and Nitride Films

“…The rise of PL intensity with HP in the Si0 2 :Si/Si system can be related to stress stimulated creation of the -Si-Si-centres in the Si0 2 "bulk" [5]. Above explanation does not exclude, of course, other possible sources of HP -HT induced ultraviolet or visible PL: creation of"oxygen sufficient structures" [9] or of ODC [ 11].…”

“…This may be related to a defect associated with the OH groups. Radiation -induced PL from silica reported for about 2.7 eV (460 rim), 3,1 eV (410 nm) and 4.3 eV (290 nm) is probably related to the presence of an "oxygen deficient center" (ODC) [11].…”