Recombination luminescence of oxygen-deficient centers in silica

The luminescence of silica glass, prepared by plasma chemical vapor deposition (PCVD) and quartz glass of type IV (trade mark KS-4V) methods, were studied while irradiated with pulses of ArF laser (193 nm) light in the range of sample temperatures between 10 and 300 K. The samples contain less than 0.1 ppm metallic and hydroxyl impurities. The samples synthesized by PCVD were of two kinds. The first one (amorphous) was as-deposited from plasma at a substrate tube temperature of ～1200 °C. The second one (fused) was prepared from the first by the tube collapsing with an external burner. In this process, a section of the substrate tube with the deposited glass was installed in a lathe and processed at a temperature of ～2100 °C during ～20 min until the tube was transformed to a rod. After such processing, the rod was cooled down to room temperature in air at an average rate of about 400 °C per min. The only observed luminescence possesses two broad bands, with not well defined position, one at 2.6–2.9 eV (a blue band) and another in the range of 4.4 eV (an UV band). There is a correspondence in luminescence properties between KS-4V silica and fused PCVD silica. Those bands have been attributed to oxygen deficient centers (ODC). No luminescence is observed in amorphous PCVD silica under irradiation with 193 nm laser light. So, formation of the sample by melting at least stimulates formation of ODCs at 193 nm. The blue band decays obeys to power law ～t^?1 and is detected in the range of time 10 ns to 300 μs. The UV band possesses a fast, practically repeating excitation pulse, and a slow component (～30 μs). The obtained new kinetics data are compared with known in literature for lone twofold-coordinated silicon having exponential decay for the blue band equal to 10 ms and 4.5 ns for the UV band. That shows the blue band of new studied samples under ArF laser possesses decay component faster and the UV band slower than that of the twofold-coordinated silicon center. This corresponds to the recombination process of luminescence excitation by laser. We propose a model of the processes as charge separation under excitation with creation of a nearest self-trapped hole and electron trapped on the twofold-coordinated silicon, modified by its surrounding atoms or ions. This pair is recombining then with luminescence.     

γ-ray induced GeODC(II) centers in germanium doped α-quartz crystal

Main luminescence of α-quartz crystal doped with germanium results from the luminescence of a self-trapped exciton (STE) near germanium. In as grown Ge-doped α-quartz crystal, the luminescence associated with the twofold coordinated Ge center (GeODC) in amorphous silica glass doped with germanium, was never observed. In this work, we performed experiments to investigate if a GeODC like luminescence could appear after a γ-irradiation of a Ge-doped α-quartz crystal. The answer is positive: under excitation with pulsed light of an ArF laser (193 nm): a new luminescence with two bands — a blue one associated to a time constant of about 100 μs appears and another one with faster decay of ~ 1.5 ns appears in the ultraviolet part of the spectrum under the same excitation. This last emission has similar characteristics as the GeODC luminescence of silica glass. However, clear differences exist between the radiation-induced center associated with this luminescence and the GeODC. The excitation with a KrF laser does not provide emission with a decay time constant of about 100 μs but provides blue luminescence with a faster decay of about 4 μs. The pulses of the ArF laser also excite this component of decay for the blue band. We attribute this emission to various types of γ-ray created centers in radiation damaged areas of the Ge-doped crystal. Under excitation with an F₂ excimer laser (157 nm), the luminescence of STE near Ge remains in the irradiated sample.     

Radiation processes in oxygen-deficient silica glasses: Is ODC(I) a precursor of E′-center?

The accumulation of radiation-induced defects under non-destructive X-ray and destructive cathodoexcitation was studied in pure silica KS-4V glasses possessing an absorption band at 7.6 eV. The correspondence between the existence of this band and the creation of the E′-center by radiation was checked. Detection of induced defects was accomplished by measurement of the luminescence during irradiation, post irradiation afterglow or phosphorescence, induced optical absorption, and thermally stimulated luminescence. In all samples, these observed phenomena associated with charge trapping and recombination on the oxygen-deficient luminescence center. Others centers of luminescence were not significant contributors. In some samples, the intensity of the 7.6 eV absorption band was deliberately increased by treatment in hydrogen at 1200 C for 100 h. The intensity of luminescence in hydrogen-treated samples was smaller because of the known quenching effect of hydrogen on the luminescence of oxygen-deficient centers. The optical absorption method does not reveal an induced absorption band for the E′-center in the hydrogen-free samples with different levels of oxygen deficiency. Therefore, we did not detect the transformation of the defect responsible for the 7.6 eV absorption band or the ODC(I) defect into the E′-center. In the hydrogen-treated sample, the absorption of the E′-center was detected. The E′-centers creation in the hydrogen-treated sample was associated with precursors created by hydrogen treatment (≡Si–O–H and ≡Si–H) in the glass network. The destructive e-beam irradiation reveals an increase with dose of the ODC luminescence intensity in the sample exhibiting a small 7.6 eV band. That means that the corresponding luminescence centers are created. Optical absorption measurements in that case reveal the presence of E′-centers and a broad band at 7.6 eV. A compaction of the irradiated volume was detected. Therefore, we conclude that the E′-center is produced by heavy damage to the glass network or by the presence of precursors.     

Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

Luminescence decay in hydrogenated amorphous silicon and silicon nanostructures

The stretched exponential luminescence decay observed at temperatures lower than 20 K transits to the power law decay due to the electron-hopping at localized band tail states near 60 K in the hydrogenated amorphous silicon (a-Si:H). The luminescence decay at 4.2 K in a-Si:H is quite similar to that of Si-nanoparticles in the porous Si (p-Si). It is explained from the comparison with p-Si that the slow luminescence of the life time of ~ 1 ms is due to the recombination of excitonic electron–hole pairs at the spin triplet state quantum-confined in the hydrogen-free Si nanostructure in a-Si:H. The fast luminescence of the life time of ~ 1 μs is due to the recombination of the pairs at the spin-singlet state and the life time is explained as due to the indirect optical transition.     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Anisotropic luminescence photo-excitation in H2-loaded Ge-doped silica exposed to polarized 193 nm laser light

We have investigated the polarization dependence of the photo-luminescence VUV excitation spectra in H₂-loaded Ge-doped SiO₂ glass exposed to polarized 193 nm laser light. As for non-H₂-loaded Ge-doped silica, we show that the β band photo-luminescence excited in the VUV spectral range (6–9 eV) remains positively polarized. In our experiments, the polarization degree P is quite high (P ≈ 0.4) due to the preferential bleaching of polarized UV-exposure. As a result, we observe a highly anisotropic luminescence photo-excitation since the luminescence is mainly polarized in the writing laser polarization direction.     

Photoluminescence characterization of sol–gel prepared low density silica samples

By exciting with synchrotron radiation in the 4–10 eV range the emission properties of sol–gel synthesized porous silica at room temperature and at 8 K are investigated. The modifications of the ultraviolet emission in the range 3.7–4.2 eV excited at different energies (5.64, 6.20, 6.53, and 6.89 eV) support the hypothesis of the contribution of two emitting centers. In particular the two emissions are spectrally resolved when exciting at 5.64 eV and a blue shift of the emission peak is reported as the excitation energy increases. The analysis of the decay times indicates that the two centers are temporally resolved with the 5.64 eV excitation both at room temperature and at 8 K and allows to estimate lifetimes of about 3 and 18 ns. When exciting at 6.53 eV, the larger relative contribution of the 4.0 eV band does not allow to efficiently resolve the two decay times and the estimated life time of the 3.8 eV band is shortened.     

Optical properties of Ge-oxygen deficient centers embedded in silica films

The emission features of Ge-oxygen deficient centers in a 100 nm thick Ge-doped silica films were investigated by looking at the photoluminescence spectra and time decay under synchrotron radiation excitation in the 10–300 K temperature range. These centers exhibit two luminescence bands centered at 4.3 eV and 3.2 eV associated with the de-excitation from singlet (S₁) and triplet (T₁) states, respectively, that are linked by an intersystem crossing process. The comparison with results obtained in a bulk Ge-doped silica sample shows that the efficiency of the intersystem crossing process depends on the properties of the matrix embedding the Ge-oxygen deficient centers, being more effective in the film than in the bulk counterpart.     

Trapped-electron centers in pure and doped glassy silica: A review and synthesis

This paper reviews half a century of research on radiation-induced point defects in pure and doped glassy silica and its crystalline polymorph α quartz, placing emphasis on trapped-electron centers because the vast majority of all presently known point defects in various forms of SiO₂ are of the trapped-hole variety. The experimental technique most discussed here is electron spin resonance (ESR) because it provides the best means of identifying the point defects responsible for the otherwise difficult-to-attribute optical bands. It is emphasized that defects in α quartz have been unambiguously identified by exacting analyses of the angular dependencies of their ESR spectra in terms of the g matrix of the unpaired electron spin and the matrices of this spin's hyperfine interactions with non-zero-nuclear-spin ²⁹Si and ¹⁷O nuclides in pure α quartz and/or with substitutional ²⁷Al, ³¹P, or ⁷³Ge in quartz crystals respectively doped with Al, P, or Ge. Many defects in pure and doped glassy silica can be unambiguously identified by noting the virtual identities of their spin Hamiltonian parameters with those of their far better characterized doppelgangers in α quartz. In fact, the Ge(1) trapped-electron center in irradiated Ge-doped silica glass is shown here to have a crystal-like nature(!), being virtually indistinguishable from the Ge(II) center in Ge-doped α quartz [R.J. McEachern, J.A. Weil, Phys. Rev. B 49 (1994) 6698]. Still, there are other defects occurring in glassy silica that are not found in quartz, and vice versa. Nevertheless, those defects in glasses without quartz analogues can be identified by analogies with chemically similar defects found in one or both polymorphs and/or by comparison with the vast literature of ESR of paramagnetic atoms and small molecules. Oxygen “pseudo vacancies” comprising trigonally coordinated borons paired with trigonally coordinated silicons were proposed to exist in unirradiated B₂O₃–3SiO₂ glasses in order to account for observations of γ-ray-induced trapped-electron-type B- and Si-E′ centers [D.L. Griscom et al., J. Appl. Phys. 47 (1976) 960]. Analogous Al-E′ trapped-electron centers have been elucidated in silica glasses with Al impurities [K.L. Brower, Phys. Rev. B 20 (1979) 1799]). And it has been proposed [D.L. Griscom et al., J. Appl. Phys. 47 (1976) 960] that trapping of a second electron on such oxygen pseudo vacancies accounts for the predominant ESR-silent trapped-electron centers in irradiated silica glasses containing B or Al. The present paper additionally attempts to divine the identities of some of the ESR-silent radiation-induced trapped-electron centers in silica glasses free of foreign network-forming cations. This quest led to the doorstep of the most famous ESR-silent defect of all, the twofold-coordinated silicon, which is found only in silica glasses (not in quartz) and is responsible for the UV/visible optical properties of the oxygen-deficiency center known as ODC(II). The oxygen-deficiency center called ODC(I) is associated with an absorption band at 7.6 eV and, though commonly believed to be a simple oxygen mono-vacancy, is clearly more complicated than that [e.g., A.N. Trukhin, J. Non-Cryst. Solids 352 (2006) 3002]. Certain well documented but persistently enigmatic ODC(I)?ODC(II) “interconversions” [reviewed by L. Skuja, J. Non-Cryst. Solids 239 (1998) 16] have never been explained to universal satisfaction. An innovative combined ESR/thermo-stimulated-luminescence (TSL) study of a series of pure low-OH silica glasses with oxygen deficiencies of 0.000, ~ 0.015, and ~ 0.030 vol.% [A.N. Trukhin et al., J. Non-Cryst. Solids, 353 (2007) 1560] places new constraints on all future models for ODC(II). Taking this history into account, specific redefinitions of both ODC(I) and ODC(II) are proposed here. The present review also revisits a study of X-ray-induced point defects in an ultra-low-OH, high-chlorine but otherwise ultra-high-purity silica glass [D.L. Griscom, E.J. Friebele, Phys. Rev. B34 (1986) 7524], arguing that (1) most of the reported E′_γ and E′_δ centers were created via the mechanism of dissociative electron capture at chlorine-decorated oxygen vacancies, (2) the concomitantly created interstitial chloride ions serve as ESR-silent trapped-electron traps, (3) the ESR-detected “Cl⁰” centers arise from hole trapping on O₃≡ Si–Cl units without detachment of the resulting Cl atom, and (4) those chlorine atoms that are detached by homolytic bond fission are ESR-silent. Finally, in chlorine-free, low-OH, high-purity silica glasses, up to 100% of the trapped-electron centers appear to be ESR silent and are tentatively ascribed to electron trapping in pairs below the mobility edge of the conduction band. In such cases, the sum of all trapped-hole centers has been found to decay exponentially with increasing isochronal annealing temperature in the range 100 to ~ 300 K [D.L. Griscom, Nucl. Inst. & Methods B46 (1990) 12]. Overall, this review consolidates a large amount of long-existing but often underappreciated knowledge bearing on the natures of trapped-electron centers in pure and doped glassy silica, proposes new models for some of these, and raises a number of questions that cannot be fully answered without future performance of new experiments and/or ab initio calculations.     

Conformational heterogeneity of the point defects in silica: The lifetime of the phosphorescence band at 2.7 eV

We have measured the excitation and emission energy dependence of the lifetimes of the 2.7 eV photoluminescence band associated to oxygen deficient centers in silica glasses. The non-exponential behavior of this time decay is consistent with intrinsic conformational heterogeneity of these point defects in the amorphous matrix. Accordingly, we have analyzed the data in terms of a radiative rates distribution. Moreover, both surface and bulk typologies of these point defects have been studied. The mean value of the lifetime distribution of the surface defects increases from 12 to 15 ms varying the excitation energy from 4.6 to 5.2 eV, and it increases from 14 to 15 ms in the emission energy interval between 2.6 and 3.0 eV. As well similar variations of the lifetime are observed for interior defects, when measured at different excitation and emission energies. We can also estimate the width of the lifetime distribution of this ensemble of point defects in silica glass.     

Optical properties of Tm3+ ions in alkali germanate glass

Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 μm diodes because of low phonon energies. Spectroscopic analysis indicates low non-radiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.     

Correlation between the luminescence properties and the surface structures of submicron silica particles

We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Stöber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar⁺ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing.     

Strong green emission from Er3+ in a fluorine containing (Pb,La)–tellurite glass without using up-conversion route

Absorption, emission, excitation spectra and the lifetime of the ⁴S_3/2 excited luminescent state of Er³⁺ ions in a fluorine containing (lead, lanthanum)–tellurite glass have been studied. The glass exhibits a strong green luminescence upon excitation through 380 nm (⁴I_15/2 → ⁴G_11/2) absorption band of its Er³⁺ ions. The spectrum consists of a strong green component in the wavelength range 534–553 nm due to luminescence transitions ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 and a very weak red component in the range 650–710 nm due to ⁴F_9/2 → ⁴I_15/2 transition. The Stark split components of the ⁴S_3/2 state are not very clear in the spectrum, but the biexponential luminescence decay of the ⁴S_3/2 state confirms the presence of the Stark levels. A rapid conversion of the upper Stark level to the lower level is also evident from the decay kinetics which helps greater number of ions to populate in the lower stark level of the ⁴S_3/2 state. Thus, the present study indicates that the glass may be a suitable candidate for use as a laser medium in making a solid state green laser by pumping the later by normal route.     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

Generation of oxygen deficient point defects in silica by γ and β irradiation

We report an experimental study of the effects of γ and β irradiation on the generation of a point defect known as ODC(II) in various types of commercial silica (a-SiO₂). The ODC(II) has been detected by means of photoluminescence (PL) spectroscopy measuring the PL band centered at 4.4 eV and excited at 5.0 eV associated to this defect. Our experiments show that ODC(II) are induced in all the investigated materials after irradiation at doses higher than 5 × 10² kGy. A good agreement is observed between the efficiencies of generation of ODC(II) under γ and β irradiation, enabling a comprehensive study up to the dose of 5 × 10⁶ kGy. Two different growth rates, one in the low and one in the high dose range, can be distinguished in all the samples examined, suggesting that the efficiency of generation of the ODC(II) depends on the dose but not on the kind of irradiation and on the dose rate. Furthermore a nonlinear dependence of the photoluminescence band amplitude on the dose D, through a power law of the kind D^α with α < 1, has been observed in the low dose range in all the materials examined.     

Optical activity of Sn-variants of oxygen deficient centers in fluorine-modified silica

Photoluminescence in fluorine-modified Sn-doped silica has been analyzed by means of synchrotron radiation in the UV and vacuum-UV, from 120 to 330 nm, looking at the optical activity of oxygen-deficient-centers ODC(II) in Sn-substituted cationic sites. The comparison between F-modified Sn-doped samples and previous data on F-free Sn-doped material evidences differences in the intensity of the 3.2 eV emission band excited at 3.7 eV, and in the thermal dependence of the intensity of this emission excited via intersystem crossing. The role of fluorine in modifying the optical activity of ODC(II) and in the SnO₂ clustering is discussed, showing that an efficient excitation transfer may be activated from SnO₂ to the Sn-variant of ODC(II).     

Stability of E′ centers induced by 4.7 eV laser radiation in SiO2

The kinetics of E′ centers ( Si) induced by 4.7 eV pulsed laser irradiation in dry fused silica was investigated by in situ optical absorption spectroscopy. The stability of the defects, conditioned by reaction with mobile hydrogen of radiolytic origin, is discussed and compared to results of similar experiments performed on wet fused silica. A portion of E′ centers and hydrogen are most likely generated by laser-induced breaking of Si–H precursors, while an additional fraction of the paramagnetic centers arise from another formation mechanism. Both typologies of E′ centers participate in the reaction with H₂ leading to the post-irradiation decay of the defects. This annealing process is slowed down on decreasing temperature and is frozen at T = 200 K, consistently with the diffusion properties of H₂ in silica.     

Fluorescence properties and laser demonstrations of Nd-doped high silica glasses prepared by sintering nanoporous glass

Porous glass with high-SiO₂ content was impregnated with Nd ions, and subsequently sintered at 1100 °C into a compact non-porous glass in air or reducing atmosphere. Sintering in a reducing atmosphere produced an intense violet–blue fluorescence at 394 nm. However, the sintering atmospheres almost did not affect the fluorescence properties in the infrared range. A good performance Nd³⁺-doped silica microchip laser operating at 1064 nm was demonstrated. The Nd-doped sintering glasses with high-SiO₂ content are potential host materials for high power solid-state lasers and new transparent fluorescence materials.     

Energy transfer and surface plasmon resonance in luminescent materials based on Tb(III) and Ag or Au nanoparticles in silica xerogel

Two-component material consisting of terbium(III) ions doping silica xerogel prepared by sol–gel procedure shows enhanced Tb(III) photoluminescence. We postulate that in this material the enhancement is owing to energy transfer from one of the defect states in silica to emitting states ³D₃ and ⁵D₄ of Tb(III).Surface plasmon resonance effect causes two contrary phenomena. Thus, if in the vicinity of the Tb(III) emission centers are present nanoparticles of Ag, observed is additional improvement of the Tb(III) emission. While, presence of Au nanostructures in the silica doped with Tb(III) causes quenching of the luminescence. In general, both the three-component materials exhibit enhancement of the component silica emission band in the resultant 380 nm band and relatively high thermal stability, especially above 600 °C.