Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.     

Crucial dependence of excimer laser toughness of “wet” silica on excess oxygen

Creation of point defects by ArF (6.4 eV) and F₂ laser (7.9 eV) irradiation in synthetic “wet” silica glass thermally loaded with interstitial O₂ molecules was studied by optical absorption, electron paramagnetic resonance and infrared absorption. The presence of excess oxygen caused a significant increase of laser-induced ultraviolet (UV) absorption, which was 4 times (7.9 eV-irradiation) and > 20 times stronger (ArF irradiation) as compared to O₂-free samples. The spectral shape of photoinduced absorption nearly completely coincided with the spectral shape of oxygen dangling bonds (NBOHC) in 3 to 6.5 eV regions. The contribution of Si dangling bonds (E' centers) was less than few % and was not dependent on oxygen content. Peroxy radical defects were not detected. The photoinduced NBOHCs thermally decayed at 400...500 C. However, a subsequent brief 7.9 eV irradiation restored their concentration up to 70%. This sensitization can be in part attributed to generation of interstitial Cl₂ and HCl. These data show that oxygen stoichiometry is an important factor for maximizing laser toughness of wet silica.     

Electrochemical deposition of ZnO film and its photoluminescence properties

ZnO thin films were prepared by the electrochemical deposition method on conductive substrates. The as-deposited film was ZnO crystallites of the wurzite structure highly oriented along the (0 0 2) plane. The specific crystalline morphology may be attributed to the growth mechanism through the orientation attachment mode, which is one of the characteristics peculiar for the present process, because the terrace has been clearly observed in high-resolution AFM images. The film shows high transmittance and an optical band gap energy of 3.3 eV. After annealing in N₂ or Ar, strong green emission was observed, which should be related to the generation of singly ionized oxygen defects. Improving emission intensity further by optimizing the annealing conditions, this method may be promising to replace the traditional method for preparation of ZnO green phosphor.     

Structural and luminescence properties of amorphous SiO2 nanoparticles

We report an experimental study on the photoluminescence band peaked at 2.7 eV (blue band) induced by thermal treatments in nanometric amorphous SiO₂. In particular the emission dependence on the nanometric particles size as a function of their mean diameter from 7 nm up to 40 nm is investigated. We found that the emission amplitude increases on decreasing the particle diameter, showing a strong correlation between the blue band and the nanometric nature of the particles. By Raman spectroscopy measurements it is evidenced that the SiO₂ nanoparticles matrix is significantly affected by the reduction of size. Basing on the shell-like model, these findings are interpreted assuming that the defects responsible for the photoluminescence are localized on a surface shell of the particles and not simply on their surface. In addition it is found that the generation efficiency of these defects depends on the structural properties of the SiO₂ matrix in the surface shell.     

Photosensitivity of SiO2–Al and SiO2–Na glasses under ArF (193 nm) laser

Photosensitivity of SiO₂–Al and SiO₂–Na glass samples was probed by means of the induced optical absorption and luminescence as well as by electron spin-resonance (ESR) after irradiation with excimer-laser photons (ArF, 193 nm). Permanent visible darkening in the case of SiO₂–Al and transient, life time about one hour, visible darkening in the case of SiO₂–Na was found under irradiation at 290 K. No darkening was observed at 80 K for either kind of material. This investigation is dedicated to revealing the electronic processes responsible for photosensitivity at 290 and 80 K. The photosensitivity of both materials is related to impurity defects excited directly in the case of SiO₂–Na and/or by recapture of self-trapped holes, which become mobile at high temperature in the case of SiO₂–Al. Electrons remain trapped on the localized states formed by oxygen deficient defects.     

Influence of Ce codoping and H2 pre-loading on Er/Yb-doped fiber: Radiation response characterized by Confocal Micro-Luminescence

Confocal microscopy luminescence measurements were applied to study the X-ray radiation response of Er/Yb-doped optical fibers in connection with H₂ pre-loading and with the addition of another lanthanide element (Cerium) in the core composition. Laser excitations at 488 nm and 325 nm allow deriving the emission and absorption pattern of Er³⁺, the latter derived from the dips appearing in a wide luminescence band related to defects in silica. We found that the luminescence spectrum of the X-irradiated Er/Yb-doped core fiber evidences an increase in the emission intensity around 520 and 660 nm; in contrast, no changes are induced by radiation neither after H₂ pre-loading nor when the Cerium is added to the core composition. Both treatments reduce the generation of defects in the Er-doped fibers thus providing hardness in the radiative environment.     

Characterization of a-B5C:H prepared by PECVD of orthocarborane: Results of preliminary FTIR and nuclear reaction analysis studies

The electronic properties of a-Si:H vary with hydrogen passivation of dangling bond defects. It appears this effect is also operative in semiconducting amorphous hydrogenated boron carbide (a-B₅C:H). Therefore, the ability to quantify the amount of hydrogen will be key to development of the materials science of a-B₅C:H. The results of an initial investigation probing the ability to quickly correlate hydrogen concentration in a-B₅C:H films with infrared spectroscopy are reported. a-B₅C:H thin films were growth on Si (1 1 1) substrates by plasma-enhanced chemical vapor deposition (PECVD) using sublimed orthocarborane and argon as the precursor gas. Nuclear reaction analysis (NRA) was performed to quantify the atomic concentration of H in the a-B₅C:H films. While the observed vibronic structure does not show stretches due to terminal C–H or bridging B–H–B, analysis of the terminal B–H stretch at ～2570 cm^?1 gives a proportionality constant of A = 2 × 10²² cm^?2. We conclude that the methods previously developed for correlating H concentration to infrared data in a-Si:H are similarly viable for a-B₅C:H films.     

Catalysis and proton-conduction of novel phosphate glasses

The performance of phosphate glasses as a catalyst for water decomposition and a proton conductor was investigated. Glasses with a composition of 30Na₂O–10BaO–30P₂O₅–(30?x)WO₃–xNb₂O₅ (5 < x < 25) decompose water vapor and generate hydrogen at 500 °C. The best decomposition performance was observed on a specimen with the Nb₂O₅ composition of x = 15. A part of hydrogen produced on the glass surface changes to protons by reducing W⁶⁺ ions and penetrates into the glass. The electron is the dominant charge carrier in the electric conduction of W-rich glasses, whereas proton conduction is predominant in Nb-rich glasses in hydrogen atmosphere. A Raman scattering experiment revealed that Nb contributes to depolymerize the –P–O–P– chains in the phosphate glass producing non-bridging oxygen. A possible model was proposed for the water decomposition and proton conduction processes.     

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.     

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.     

An increased F2-laser damage in ‘wet’ silica glass due to atomic hydrogen: A new hydrogen-related E′-center

A dramatic increase of F₂-laser induced room temperature-stable point defects in ‘wet’ synthetic silica glass occurs when irradiation temperature is lowered to 80 K. Contrary to the predictions based on the established models of defect processes, a large part of defects induced at 80 K remains stable also at the room temperature. The intensities of the laser-induced optical absorption bands of the non-bridging oxygen hole centers (2.0 and 4.8 eV) and E′-centers (5.8 eV) are comparable to those created by neutron irradiation (10¹⁸ n/cm²). A growth of infrared absorption peak at 2237 cm⁻¹ indicates creation of silicon hydride (SiH) groups. A study of irradiation dose dependences and irradiation efficiency at intermediate temperatures (160 K) suggests a novel radiation damage mechanism by insertion of atomic hydrogen in electronically excited Si–O bond. EPR spectra show ${\text{E}}_{\mathrm{\gamma }}^{\prime }$-like centers overlapped by a new type of E′-centers, characterized by a hyperfine splitting of 0.08 mT of the low-field peak in the derivative spectrum. The new E′-center is assigned to a silicon dangling bond, with the Si atom bonded by two bridging oxygens and an OH group (‘E′(OH)’). Similar centers have been previously observed on SiO₂ surfaces.     

Glass formation in and structural investigation of Li2S + GeS2 + GeO2 composition using Raman and IR spectroscopy

Li₂S + GeS₂ + GeO₂ ternary glasses have been prepared and a wide glass-forming range was obtained. The glass transition temperatures increase with the GeO₂ concentration in the glasses. The vibrational modes of both bridging (Ge–S–Ge) and non-bridging (Ge–S⁻) sulfurs are observed in Raman and IR spectra of binary Li₂S + GeS₂ glasses. Additions of GeO₂ to this binary glass increase the bridging oxygen band (Ge–O–Ge) at the expense of decreasing the bridging sulfur band (Ge–S–Ge), whereas the bands associated with the non-bridging sulfurs (Ge–S⁻) remain constant in intensity up to high GeO₂ concentrations. At higher concentrations of GeO₂ (⩾60%), the non-bridging oxygen band, which is not observed at low and intermediate GeO₂ concentrations, appears and grows stronger. From these observations, it is suggested that the added lithium ions favor the non-bridging sulfur sites over the oxygen sites to form non-bridging sulfurs, whereas the added oxygen prefers the higher field strength Ge⁴⁺ cation to form bridging Ge–O–Ge bonds. The structural groups in the Li₂S + GeS₂ + GeO₂ glasses that are consistent with results of Raman and IR spectra are described and are used to develop a structural model of these glasses.     

Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review

This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO₂ (a-SiO₂). The most common form of interstitial oxygen species is oxygen molecule (O₂), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272 nm. The PL observation of interstitial O₂ provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O₂ in long-range oxygen transport in a-SiO₂; formation of the Frenkel defect pair (Si–Si bond and interstitial oxygen atom, O⁰) by dense electronic excitation; efficient photolysis of interstitial O₂ into O⁰ with F₂ laser light (λ = 157 nm, hν = 7.9 eV); and creation of interstitial ozone molecule via reaction of interstitial O₂ with photogenerated O⁰. The efficient formation of interstitial O⁰ by F₂ laser photolysis makes it possible to investigate the mobility, optical absorption, and chemical reactions of interstitial O⁰. The observed properties of O⁰ are consistent with the model that O⁰ takes the configuration of Si–O–O–Si bond. Interstitial O₂ and O⁰ react with dangling bonds, oxygen vacancies, and chloride groups in a-SiO₂. Reactions of interstitial O₂ and O⁰ with mobile interstitial hydrogen species produce interstitial water molecules and hydroperoxy radicals. Interstitial hydroxyl radicals are formed by F₂ laser photolysis of interstitial water molecules.     

The Eγ′ center as a probe of structural properties of nanometer-sized silica particles

A Q-band electron spin resonance (ESR) study is reported of E′ type point defects observed in ～7 nm-sized fumed silica nanoparticles following 10-eV irradiation to photodissociate H from passivated defects. In a comparative study with bulk silica (suprasil), the E′ center is used as an atomic probe to get more in depth information on the network structure of the nm-sized particles. The nanoparticles were brought into contact with ‘bulk’ Si/SiO₂ entities at an elevated temperature in vacuum (T_an = 1105 °C), i.e., the presence of an Si/SiO₂ interface. As a result of this post manufacture treatment, the E′ density increased drastically (>order of magnitude), enabling us to resolve hyperfine (hf) structure of the E′ centers located in the core region of the nanoparticles. Two doublet structures are observed, one each assigned to O₂Si–H entities and the primary ²⁹Si hf structure of the E′ centers. Analysis of these hf spectra reveals interesting information on the network structure of the core region of the nanoparticles: (1) Fumed silica is found to contain relatively less hydrogen than suprasil. (2) An increased ²⁹Si hf splitting (439 ± 2 G) is observed compared to bulk silica (418 ± 2 G), indicating that the E′ centers located in the core of the nanoparticles exhibit on average a slightly more pyramidal defect structure, and moreover, providing evidence that the fumed silica particles are densified compared to standard bulk silica, possibly originating from the presence of more low-membered rings (n < 5) in the nm-sized silica network.     

Photoluminescence in silicon rich oxide thin films under different thermal treatments

Photoluminescence (PL) was studied in silicon rich oxide (with the atomic percentage ranges of Si from 35% to 75%) thin film samples, fabricated by the plasma assisted CVD technique. A broad PL peak, blue-shifted from the bulk silicon band edge of ～1.1 eV, was observed. In one typical sample, the PL peak intensity shows a non-monotonic temperature dependence. This non-monotonic dependence was also observed in previous work by others and attributed to an energy splitting between the excitonic singlet and triplet levels in silicon nanocrystals, a consequence of quantum confinement effect. Finally, in more than 20 samples under different thermal treatments (with the annealing temperature range from 800 °C to 1100 °C), the wavelength of PL peak was observed to be pinned between ～900 and ～1000 nm, independent of thermal budget. This pinning effect, we believe, is probably due to the formation of oxygen-related interface states.     

Temperature dependent blue second harmonic generation in Ba5Li2Ti2Nb8O30 microcrystals embedded in TeO2 glass–matrix

The as-quenched samples in the system (100 ? x) TeO₂-(x) Ba₅Li₂Ti₂Nb₈O₃₀ (2 ? x ? 8) were found to be embedded with 10–20 μm sized crystallites of the polar phase Ba₅Li₂Ti₂Nb₈O₃₀ (BLTN). Blue (400 nm) second harmonic generation (SHG) was observed in transmission mode when 800 nm laser light was allowed to pass through the individual crystallites. The blue SHG signal was temperature dependent and its intensity was maximum at ～175 °C which was tentatively attributed to the concomitant changes associated with the refractive indices of the BLTN crystallites. The SHG intensity attained a minimum value around the Curie temperature of BLTN crystals.     

Effect of OH-content on thermal and chemical properties of SnOP2O5 glasses

Glasses with 55–60 mol% SnO and 40–45 mol% P₂O₅ have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550 °C, had low T_g, 150–200 °C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200 °C, had much higher T_g, 250–300 °C, and much higher chemical stability. No significant differences were found by ¹¹⁹Sn Mössbauer and ³¹P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and ¹H Nuclear Magnetic Resonance spectroscopy. In samples with low T_g, a broad OH – vibration band around 3000 nm with an absorption intensity >20 cm^?1, bands at 2140 nm with intensity ～5 cm^?1, at 2038 nm with intensity ～2.7 cm^?1, and at 1564 nm with intensity ～0.4 cm^?1 were measured. These samples have shown a mass loss of 3–4 wt% by thermal gravimetric analyses under argon in the temperature range 400–1000 °C. No mass loss and only one broad OH-band with a maximum at 3150 nm and low absorption intensity <4 cm^?1 could be detected in samples melted at high temperature, 1000–1200 °C, which have much higher T_g, ～300 °C, and much higher chemical stability.     

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.     

Junction capacitance study of an oxygen impurity defect exhibiting configuration relaxation in amorphous silicon–germanium alloys deposited by hot-wire CVD

We report the observation of light induced electron capture in oxygen contaminated (～5 × 10²⁰ cm^?3) hydrogenated amorphous silicon–germanium alloys grown by hot-wire chemical vapor deposition (HWCVD). By examining the time evolution of dark capacitance after 1.2 eV photoexcitation, we are able to estimate the free energy barrier (?0.8 eV) for the release of electrons into the conduction band. Such a large thermal barrier, for a defect whose optical threshold is centered (～1.35 eV) so close to the band-gap (1.5 eV), indicates significant configurational relaxation once the oxygen impurity state is occupied with photoexcited electrons.     

Self-assembling of supramolecular erbium–oxygen clusters in magnetron plasma,and the optimization of erbium luminescence in a-SiOx:H〈Er, O〉 films

Films of erbium-doped amorphous hydrogenated silicon a-SiO_x:H〈Er, O〉 were fabricated by dc-magnetron sputtering at different concentrations of oxygen in the magnetron plasma and different areas of erbium metallic target. It was demonstrated that the increase of oxygen concentration in the plasma gaseous phase above ∼5 mol% leads to a sharp rise in the amount of oxygen bound to erbium in the a-SiO_x:H〈Er, O〉 films. Simultaneously, a smooth increase in the concentration of oxygen bound to matrix-forming elements (silicon, hydrogen) is observed. The increase of the area of erbium target, corresponding to the rise of concentration of erbium ions in the plasma, also favors the binding of erbium with oxygen. However, the content of erbium in the a-SiO_x:H〈Er, O〉 film (in atomic percents) significantly drops with intense binding of erbium with oxygen. These facts point to the formation of erbium–oxygen clusters, with a large number of oxygen atoms, which are probably formed in the magnetron plasma but are deposited as a separate species on the substrate in the reaction chamber. The intensity of erbium photoluminescence rises significantly in the region of formation of these large erbium–oxygen clusters. A ‘phase-transition’ model is formulated, describing the properties of a-SiO_x:H〈Er, O〉 films, based on the assumption of the formation of large erbium–oxygen clusters in the magnetron plasma. The size and composition of these clusters are determined. The model is semi-quantitatively consistent with all the experimental findings.