Oxygen-deficient Centers Research Articles

Effect of the GeO2 content on the radiation resistance of Er3+ -doped silica glasses and fibers

In this work, Er/Al/Ge co-doped silica glasses with different GeO2 content (0–3 mol%), as well as Ge/Al co-doped silica glasses, are prepared by combining the sol–gel and high-temperature sintering methods. Further, the effects of the GeO2 content on the absorptions and emissions properties, and lifetimes of the glasses before and after 1KGy γ-ray irradiation are compared. The Er/Al/Ge co-doped silica fibers are prepared from a preform produced via modified chemical vapor deposition (MCVD) combined with nano sol-doping. The effects of Ge co-doping on the optical loss and amplifier gain of the Er-doped silica fibers (EDFs) before and after irradiation are also investigated. The related mechanism and species of the γ-ray radiation-induced color centers are revealed via radiation-induced-absorption (RIA) and continuous wave electron paramagnetic resonance (CW-EPR) spectroscopies. The results revealed that co-doping with GeO2 considerably improves the radiation resistance of the glass and exerts a slight effect on the spectral properties of the Er/Al/Ge co-doped silica glasses before irradiation. The RIA and CW-EPR spectra revealed that the aluminum–oxygen hole center (AlOHC) defects reduce with increasing GeO2 content because the intermediaries, the Ge-related oxygen-deficient centers (GeODC(I) and GeODC(II)), exhibit stronger abilities to trap the holes compared with the [AlO4/2]− group. This reduces the RIA level in the visible and near-infrared regions of the Er/Al/Ge co-doped silica glass. The irradiation experiment on the fiber further confirmed that the radiation resistance of EDFs can be considerably improved by Ge co-doping.

Analytical Study of Porous Organosilicate Glass Films Prepared from Mixtures of 1,3,5- and 1,3-Alkoxysilylbenzenes.

Organosilicate glass (OSG)-based porous low dielectric constant (low-k) films with different molar ratios of 1,3,5-tris(triethoxysilyl)benzene to 1,3-bis(triethoxysilyl)benzene bridging organic groups (1:3 and 1:7) were spin-on deposited, followed by a soft bake in air and N2 at 150 °C and hard bake in air and N2 at 400 °C. Non-ionic template (Brij®30) concentrations were varied from 0 to 41 wt% to control the porosity of the films. The chemical composition of the matrix of the films was evaluated and discussed with the shrinkage of the film during the curing, refractive indices, mechanical properties, k-values, porosity and pore structure. The chemical composition of the film cured in both air and N2-containing ambient were evaluated and compared. The benzene bridging groups containing films change their porosity (0 to 43%) but keep the pore size constant and equal to 0.81 nm when porosity is lower than 30%. The k-value decreases with increasing porosity, as expected. The films containing benzene bridge have higher a Young’s modulus than plasma-enhanced chemical vapor deposition (PECVD) methyl-terminated low-k films with the same porosity and show good hydrophobic properties after a hard bake and close to the values reported for 1,4-benzene-bridged films. The fabricated films show good stability after a long time of storage. However, the improvement of mechanical properties was lower than the values predicted by the published literature data. It was concluded that the concentration of 1,3,5-benzene bridges was below the stiffness threshold required for significant improvement of the mechanical properties. The films show UV-induced luminescence with a photon energy of 3.6 to 4.3 eV. The luminescence is related to the presence of oxygen-deficient-type defects or their combination with organic residues. The most intensive luminescence is observed in as-deposited and soft bake samples, then the intensity is reduced after a hard bake. It is assumed that the oxygen-deficient centers form because of the presence of Si–OC2H5 groups in the films and the concentration of these centers reduces when all these groups completely transformed into siloxane (Si–O–Si).

Investigation by Thermoluminescence of the Ionization and Annealing Processes in Irradiated Ge-Doped Silica Fiber Preform

Germano-silicate fibers, mostly employed for telecom applications, are also implemented in radiation environments for data links or sensing. In this field, the understanding of radiation-induced mechanisms leading to the degradation of the performances of silica optical fibers has an essential role. Particularly, the estimation of the activation energies of the density of trapped states (DOTSs) governing the radiation-induced attenuation (RIA) is of primary importance. To investigate these mechanisms, the thermoluminescence (TL) is a powerful technique, by which the extrapolation of DOTSs has been previously largely achieved for pure and Al-, P-, Er-, and Yb-doped silica. Despite the key applications related to Ge-doped silica, one cannot do the same for this kind of fiber, due to an “anomalous” increase of their TL response along with the heating rate. The objective of this work is primarily to elucidate the specific radiation-induced mechanisms associated with the presence of Ge in silica that notably induce the anomalous TL response. We show from the experimental study of the RIA thermal annealing during the TL readout, and its correlation with the TL glow curve, that oxygen-deficient centers (ODCs) centers, trapping electrons during irradiation, form deep states which act as recombination center (RC) in the TL process. Finally, and most important, we propose a model that explains the TL anomalous features and how they are arisen from two specific properties of the annealing processes of Ge-related radiation-induced centers.

Enhancement of the PL Intensity of Silica Based Polymer Electret after Electro-Thermo-Polarization

In this study, the influence of electro-thermo-polarization (ETP) on the charge state and luminescence properties of polymer nanocomposites based on PP and nanoparticles of amorphous silicon dioxide nanoparticles were investigated. By comparing the results of the charge state and the intensity of the PL spectra of nanocomposites, it was determined that polarized composites with a 3% mass content of SiO2 nanoparticles have relatively high luminescence. For the same composites PP + 3% SiO2, relatively high values of the spatial space charge density, the intensity of the internal local field, and the thickness of the interphase layer are observed. Space charges create a large internal local field that stimulates the already existing oxygen deficiency centers in amorphous silica nanoparticles. This leads to the activation of the luminescence centers, which is the cause of the increase in the PL emission intensity in this nanocomposite after the ETP.

Excited states of modified oxygen-deficient centers and Si quantum dots in Gd-implanted silica glasses: emission dynamics and lifetime distributions.

The emission centers and excited state characteristics of silica glasses implanted with Gd ions were studied by time-resolved pulsed cathodoluminescence. It was found that in the process of ion implantation, two types of new emission centers associated with Gd ions as well as Si quantum dots are formed in glassy silica. The distributions of excited states over the lifetime were found for both new centers and Si quantum dots. The nature of dispersion of the emission decay time was discussed in terms of structural disorder in the matrix. Thermal annealing and an increase in the ion fluence lead to the stimulation of the formation of Gd-related new centers and Si quantum dots. The micromechanisms for the formation of new Gd-related centers and two types of Si quantum dots were proposed on the basis of two scenarios for the introduction of Gd ions into the SiO2 network: insertion of Gd into interstitial voids near oxygen-deficient centers and Gd → Si substitution with subsequent expulsion of Si atoms to the interstitial voids. New emission oxygen-deficient centers and quantum dots created by ion-beam technology in silica glasses are of interest for the development of new functional materials for photonics, and micro- and opto-electronics.

Characterization of historical minai and colorful luster glaze ceramics, using ionoluminescence technique

Some historical minai and colorful luster glaze ceramic samples were investigated by means of ionoluminescence technique. The complementary ion beam analysis techniques RBS and PIXE were employed to reveal their elemental compositions and depth profiles. The samples were discovered in the historical site of Tappeh Ghale in Iran, which dates back to 12th century. The ionoluminescence spectra of the samples reveal the strong dependence of the luminescence bands, mostly originating from oxygen-deficient centers, on the elemental compositions of the glaze samples. It is inferred that most of the luminescence centers likely have been created during the formation process of tin-opacified lead-rich glazes. The demonstrated capabilities of ionoluminescence in the current work, confirm the possibility of its application for fast and non-destructive characterization of historical glaze samples.

Multiphoton-Absorption-Excited Up-Conversion Luminescence in Optical Fibers

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation.

Amorphous Silica Np-Embedded-Polymer Nanocomposites with Enhanced Optical and Dielectric Properties

Inorganic filler and polymer matrix based PP + SiO2 nanocomposites have been prepared by the hot pressing method. The scanning electron microscopy (SEM) and X-ray diffractometer(XRD) were used for structure investigation. It was found that the average size of the nanoparticles is around 50 nm. Photoluminescence properties of PP + SiO2 nanocomposite and the effect of concentration of the SiO2 nanofiller on dielectric and electrical dispersion behavior of the nanocomposite have been investigated. Besides, it was shown that the dielectric permittivity of PP + SiO2 nanocomposite samples was gradually increased with raising filler content. Furthermore, it was observed a significant decreasing in the values of the ε′ and δ parameters of the nanocomposites in the 103-106Hs frequency range for all concentrations of filler. This decrease perhaps associated with the interphase polarization of the PP + SiO2 nanocomposites. It was also shown that PP + SiO2 nanocomposite demonstrates wide-band photoluminescence in visible region (400-700nm) when the excitation wavelength is 370 nm. This is due to the defects of SiO2 nanoparticles embedded in the polymer matrix, and more precisely, with presence of oxygen-deficiency centers.

Fluorine passivation of ODC defects in amorphous germanium dioxide

We present first-principle calculations on the interaction of atomic and molecular fluorine with germanium oxygen-deficient center (GODC) in amorphous germanium dioxide (a-GeO2). The interaction of atomic fluorine with GODC in a-GeO2 results in the formation of Ge-F-Ge and GE′ center. The reaction pathways and the energy barriers have been investigated to understand the formation of the fully passivated configurations which are obtained by the interaction of molecular fluorine with GODC in a-GeO2. Our results indicate that the GODC defects prefer to be passivated by the molecular fluorine. It is found that in the near-ultraviolet region, the optical absorption of the fully passivated configurations is significantly reduced by doping of fluorine. These results help to better understand the mechanism of how atomic and molecular fluorine passivate GODC defects and optimize the optical characteristics in F/Ge co-doped silica optical fiber.

Investigation of photodarkening in tandem-pumped Yb-doped fibers

Investigation of photodarkening (PD) in Yb-doped fibers tandem-pumped at 1018 nm is reported. For a homemade Yb-doped aluminosilicate double-clad fiber (YADF), the transmitted power of a 633 nm probe beam is reduced by 2.4% over 2 hours for the tandem pumping configuration at 1018 nm, which is significantly smaller than 33.3% for a laser diode (LD) pumping at 976 nm. A tandem-pumped Yb fiber amplifier also shows a much smaller decrease in the amplified output power over time than a LD-pumped Yb fiber amplifier. Based on fluorescence spectra of the YADF, we can not only associate PD of the YADF to intrinsic oxygen deficiency centers or Tm3+ impurities but also confirm the impact of the excited Yb3+ ion density on PD. The benefits of the tandem pumping in a high-power Yb fiber laser system will be discussed.

Critical properties and charge transport in ethylene bridged organosilica low-κ dielectrics

Organosilicate-glass-based low-κ films containing both terminal methyl groups and an ethylene bridge between the silicon atoms are spin-on deposited by using 1,2-bis(trimethoxysilyl)ethane and methyltrimethoxysilane, Brij30 template, and thermal curing. The chemical composition, porosity, and internal defects are studied using Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, electron energy loss spectroscopy, UV induced luminescence, and ellipsometric porosimetry. It was found that the studied films contain oxygen-deficient centers (Si—Si bonds). The high defect density of the states near the valence-band edge of the studied low-κ films leads to a relatively small bandgap value of about 6.3 eV. The current–voltage characteristics at different temperatures were analyzed using six theoretical charge transport models where the transport is limited by the traps ionization. It was found that the best qualitative and quantitative agreement between the calculations and experimental data is achieved by using the model of phonon-assisted electron tunneling between the neutral traps and is supplemented by considering the space charge and charge carrier kinetics. Since the thermal and optical energies of the traps in the studied films are 1.6 eV and 3.2 eV, respectively, it is concluded that the traps are responsible for the charge transport in the Si—Si bonds.

Intrinsic defects and their influence on optical properties of ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites prepared by spray pyrolysis

Abstract New germanate ALa9(GeO4)6O2 (A = Li, Na, K, Rb, Cs) oxyapatites, showing promise as host lattices for phosphors with emission in the visible and infrared spectral ranges, have been obtained for the first time by ultrasonic spray pyrolysis. The discussion is focused on native defectiveness and optical properties of these compounds, depending on the nature of alkaline ions. Spray pyrolysis method was consciously used to obtain the compounds with intrinsic defects as confirmed by electron paramagnetic resonance. Reviewing available EPR data, the registered EPR response was assigned to the paramagnetic centers related to germanium tetrahedra embracing the oxygen vacancies. According to quantum-chemical calculations the oxygen-deficient centers in ALa9(GeO4)6O2 are presumably stabilized by antisite defects in the cation sublattice. Diffuse reflectance and pulse cathodoluminescence spectra of the compounds were investigated in detail, establishing a distinct dependence of optical properties on the type of alkaline ion. The observed optical absorption maxima at 275 and 370 nm were attributed to the F-like centers. The inverse dependence of luminescence intensity on the degree of defectiveness was interpreted on the assumption that these centers were involved in non-radiative energy relaxation.

Tunable point defects in hydroxyl fused silica enabled by ultrashort laser pulses: photostimulated luminescence and functional module fabrication

Three kinds of point defects, SiE’ center (≡ Si·), unrelaxed oxygen deficiency center (ODC (II)) and non-bridging oxygen hole center (≡ Si−O·, NBOHC), have been generated in hydroxyl fused silica by ultrashort pulsed laser irradiation. Hydroxyl is proved to be a decisive component for defect producing: NBOHC originates directly from hydroxyl; Hydroxyl facilitates the generation of SiE’ in an indirect way; No obvious relevance could be built between ODC (II) and hydroxyl. By improving hydroxyl content to 1000 ppm, NBOHC becomes the dominant defect species and its red luminescence is hence discernible to naked eye. Intended for application, high hydroxyl fused silica is screened out as the desired candidate, and NBOHC becomes the final interested defect. NBOHC’s intrinsic features of lifetime and temperature stability and extrinsic properties of laser condition dependence are specifically and systematically investigated. Prospective use of defect manipulation and fabrication in one-chip exploited for anti-counterfeiting and lab-on-a-chip is also discussed.

Effect of Etching Duration on the Morphological and Opto-Electrical Properties of Silicon Nanowires Obtained by Ag-Assisted Chemical Etching

Silicon nanowires (SiNWs) were obtained on p-Si (100) substrate by Ag-assisted chemical etching method in two-step process. The influence of the etching duration on the morphological, optical and electrical properties of SiNWs samples was investigated. SEM images show clearly the presence of nanowires and the existence of porous silicon structure especially for low etching duration. Fourier Transformed Infrared spectroscopy (FTIR) measurements allow identifying several particles on the SiNWs surfaces such as oxygen and hydrogen elements. The optical properties of the SiNWs layer were investigated by Photoluminescence spectroscopy (PL). A strong broad PL band was observed at room temperature for an etching time of 30 s. The PL spectra exhibit multi-band profile in the red-green region. The luminescence of SiNWs is mainly attributed to Si-oxide interface states and oxygen deficient centers. Ag/SiNWs/Si Schottky diodes were analyzed by X-ray diffraction (XRD) and studied by current - voltage (I-V) measurements. Diode parameters such as ideality factor (n), series resistance (Rs), saturation current (Is) and energy barrier (φb) were determined by analyzing the I-V characteristics. Ag/SiNWs/Si diode for an etching duration of 30 s exhibits a rectifying behavior with a factor ideality of 2.8 and a low threshold voltage of about 0.44 V in forward bias. Space charge limited current (SCLC) model is the dominant transport mechanism through Ag/SiNWs/Si diode. The trap states, on the Ag/SiNWs interface, play in important role in the conduction phenomenon through these structures.

New optical oxygen-deficient centers in 80 keV Re-implanted amorphous silica

The formation of radiation-induced defects in amorphous SiO2 under the implantation with Re ions was studied by optical absorption and photoluminescence spectroscopies. A new type of oxygen-deficient center, denoted as ODC/Re, has been discovered. The origin of ODC/Re is associated with disturbing influence of Re ions located in the nearest environment of ODCs and causing a change in their energy structure. The absorption and emission bands of Re-modified ODC occur with participation of singlet S1 and triplet T1 excited states, adiabatic terms for which are shifted relative to those for typical Si-ODC. Spectral characteristics of absorption and photoluminescence of ODC/Re along with emission kinetic parameters allow us supposing that the structure of center is close to the ODC(II)-type. Thermal annealing leads to an increase in the emission intensities by a factor of three for both Si-ODC and ODC/Re.

Strong white light emission from SiCxOy films grown by HFCVD technique

In this work, we present the study of a strong white emission (3.35–1.72 eV) from non-stoichiometric silicon oxycarbide (SiCxOy) films deposited by hot filament chemical vapor deposition technique (HFCVD). For the growth of SiCxOy films, carbon and silicon oxide compounds in gas phase were generated from ethanol vapor and the chemical reaction of atomic hydrogen (H°) with a solid fused quartz source, respectively. To incorporate ethanol vapor to the reaction chamber, hydrogen flow was used as a carrier gas of ethanol molecules bubbled from liquid ethanol source. The analysis of composition obtained by X-ray photoelectron spectroscopy indicates that the carbon content in the SiCxOy films increases from 3.8 to 9.3 at.% as the carrier gas flow (H2) increases from 124 to 249 sccm, respectively. Fourier transform infra-red spectroscopy shows that the increase in carbon content favors the formation of both: Si–C bonds located at about 816 cm−1 and Si–O–Si vibrations located at around 1050-1070 cm−1 the last of which associates to a SiOx network. It is observed that SiCxOy films with carbon concentrations of 3.8 at.% emit a high photoluminescence (PL) intensity, however for carbon concentrations of 9.3 at.%, the PL intensity decreases. The analysis of luminescent centers indicates that high PL response could be associated to Si-related oxygen vacancies (Si-NOVs) and Si-related oxygen deficiency centers (Si-ODCs) due to low carbon concentration into the SiCxOy films.

The study of oxygen-deficient centers in Al-doped amorphous germanium oxides

Electronic and optical properties of Germanium oxygen-deficient center (GeODC) defect and Al-doped GeODC defect in 96 atoms amorphous GeO2 (a-GeO2) supercells have been investigated. All the 64 oxygen sites in the a-GeO2 configuration have been orchestrated as possible candidates for the construction of 64 different GeODCs. A Ge atom has been substituted by an Al atom in GeODCs to form the Al-GeODCs. It is discovered that the refractive index of a-GeO2 is reduced by GeODC defect, contrary to the situation when Al is doped in GeODC defect. The absorption peak at 5.15 eV is related to GeODC defect. After Al-doping, a new defect state is generated near the top of valence band (TVB), resulting in an absorption peak at 4 eV. The variations of refractive index and optical properties exhibited offer a significant and profound guidance of Al and Ge co-doping in silica optical fiber.

Photoluminescence of Yb3+/Ce3+ co-doped aluminosilicate glasses under ultraviolet irradiation

The photoluminescence and decay properties of Yb3+/Ce3+ co-doped aluminosilicate glasses were investigated under ultraviolet irradiation. A strong excitation band was ascribed to 4f → 5d transitions of Ce3+ ions. A broad ultraviolet-visible emission band belonged to 5d → 4f transitions of Ce3+ ions and oxygen-deficient centers (ODCs) in the host matrix, and the visible-near infrared emission band was attributed to non-bridging oxygen hole centers. Comparative tests of the core composition were performed. The results showed Yb2O3 and the ODCs played a crucial role on ultraviolet-visible emission band. ODCs rather than Ce3+ ions were responsible for the 1–35 μs lifetime decay of the ultraviolet-visible emission, but had no contribution to the longer decay lifetimes (77.58 μs) for Ce-free Yb3+-doped glasses. A complex schematic energy-level diagram including the electron transfer mechanism such as Ce3+-Yb3+ → Ce4+-Yb2+, the cooperative up- and down-conversion processes was proposed to describe different luminescence mechanisms UV-expose.

Investigations of interstitial Bi0 interacting with intrinsic defects in bismuth-doped silica optical fiber

The interaction mechanisms of interstitial Bi0 with intrinsic defects in bismuth-doped silica optical fiber are investigated by first-principles calculations. The results reveal that interstitial Bi0 easily interact with peroxygen defects and the reaction products are more stable. By analyzing the effective charges of the reaction products of interstitial Bi0 interacting with peroxy linkage (POL), non-bridging oxygen hole center with silicon dangling bond (NBOHC-Eʹ), and peroxy radicals with silicon dangling bond (POR-Eʹ), it finds that Bi atom is nearly divalent. For interstitial Bi0 interacting with oxygen deficiency center (ODC(I)), it forms a complex with two Si atoms in the defect. The method will be effective to understand interaction mechanism of interstitial Bi0 with intrinsic defects in bismuth-doped silica optical fiber.

Energy band gaps and excited states in Si QD/SiOx/Ry Oz (R = Si, Al, Zr) suboxide superlattices

X-ray and optical spectroscopies were applied in order to study the band structure and electronic excitations of the SiOx/Ry Oz (R = Si, Al, Zr) suboxide superlattices. The complementary x-ray emission and absorption measurements allow for the band gap values for the SiOx layers to be established, which are found to have almost no dependency on the cation type R. It is determined that, after annealing, the stoichiometric factor x remains near 1.8 in all the systems under study, implying that the silicon quantum dot synthesis reaction is not fully completed. It is shown that the SiOx/Al2O3 multilayer contains octahedral structural motifs (SiO6) usually found in stishovite, whereas SiOx/SiO2 and SiOx/ZrO2 demonstrate an electronic structure similar to conventional silica. The intrinsic electronic excited states are examined by means of synchrotron-excited photoluminescence spectroscopy. Low-energy UV-excited luminescence of SiOx layers is found to have the same spectrum in all of the studied structures, while VUV-excited spectra strongly depend on the cation R. In these measurements, manifestations of ‘slow’ exciton-mediated and ‘fast’ defect-related luminescence are distinguished using nanosecond time resolution. It is shown that both mobile and bounded excitons appear in the suboxide layer under 6.2 eV and 5.8 eV irradiation and then relax radiatively through the triplet-singlet transition of the neighbouring oxygen-deficient centers. The complete picture of the optical excitation and relaxation processes in these materials is illustrated in a general diagram depicting electronic states.