Oxygen-deficient Centers Research Articles

X-ray-induced long persistent luminescence of Cu+-doped high alumina borosilicate glass

A Cu+ doped high alumina borosilicate glass with long persistent luminescence (LPL) was prepared by reduction melting and radiation induction based on glass defect engineering. The maximum time of the persistent luminescence can arrive 16 h. Spectral and EPR measurements indicate that two kinds of oxygen defects can be induced by X-ray radiation. One defect was eliminated after UV irradiation, and the other can synergistically participate in the LPL process of glass with reduction induced oxygen deficiency centers(ODCs). In addition, there is a highly linear relationship between the initial LPL intensity of the glass and the radiation dose; which implies the glass can be used as a radiation detection material. The internal mechanism of the linear relationship and LPL was discussed.

Comprehensive Review on the Impact of Chemical Composition, Plasma Treatment, and Vacuum Ultraviolet (VUV) Irradiation on the Electrical Properties of Organosilicate Films.

Organosilicate glass (OSG) films are a critical component in modern electronic devices, with their electrical properties playing a crucial role in device performance. This comprehensive review systematically examines the influence of chemical composition, vacuum ultraviolet (VUV) irradiation, and plasma treatment on the electrical properties of these films. Through an extensive survey of literature and experimental findings, we elucidate the intricate interplay between these factors and the resulting alterations in electrical conductivity, dielectric constant, and breakdown strength of OSG films. Key focus areas include the impact of diverse organic moieties incorporated into the silica matrix, the effects of VUV irradiation on film properties, and the modifications induced by various plasma treatment techniques. Furthermore, the underlying mechanisms governing these phenomena are discussed, shedding light on the complex molecular interactions and structural rearrangements occurring within OSG films under different environmental conditions. It is shown that phonon-assisted electron tunneling between adjacent neutral traps provides a more accurate description of charge transport in OSG low-k materials compared to the previously reported Fowler-Nordheim mechanism. Additionally, the quality of low-k materials significantly influences the behavior of leakage currents. Materials retaining residual porogens or adsorbed water on pore walls show electrical conductivity directly correlated with pore surface area and porosity. Conversely, porogen-free materials, developed by Urbanowicz, exhibit leakage currents that are independent of porosity. This underscores the critical importance of considering internal defects such as oxygen-deficient centers (ODC) or similar entities in understanding the electrical properties of these materials.

Improvement of Laser Damage Resistance of Fused Silica Using Oxygen-Aided Reactive Ion Etching

Reactive ion etching (RIE) with fluorocarbon plasma is a facile method to tracelessly remove the subsurface damage layer of fused silica but has the drawback of unsatisfactory improvement in laser damage resistance due to the induction of secondary defects. This work proposes to incorporate O2 into the CHF3/Ar feedstock of RIE to suppress the formation of secondary defects during the etching process. Experimental results confirm that both the chemical structural defects, such as oxygen-deficient center (ODC) and non-bridging oxygen hole center (NBOHC) defects, and the impurity element defects, such as fluorine, are significantly reduced with this method. Laser-induced damage resistance is consequently greatly improved, with the 0% probability damage threshold increasing by 121% compared to the originally polished sample and by 41% compared to the sample treated with conventional RIE.

Evolution of point defects in mechanical cracks of fused silica after CO2 laser melting

The traditional polishing method inevitably results in subsurface cracks in the fused silica, which seriously degrades their ultraviolet laser damage resistance. CO2 laser irradiation can melt these cracks and improve their laser induced damage threshold (LIDT). Photoluminescence spectrum and SEM-FIB were employed to investigate the changes in the material microstructure at the crack location with CO2 laser melting. The density of the oxygen-deficient centers of type II (ODC II) defects decreases, while the density of the non-bridging oxygen hole center (NBOHC) defects increases after high-temperature melting. The reason for this change is related to the dihydroxylation reaction and the participation of environmental oxygen in the defect type conversion. The reduction of ODC II defects is most likely the reason for the improvement of LIDT.

Controlling photoluminescence of silicon quantum dots using pristine-nanostates formation

This study investigates the effective control of photoluminescence (PL) of silicon nanocrystals (Si–NCs) embedded in a SiO2 matrix, with a specific focus on tuning the luminescent wavelength profile and intensity for silicon-based optoelectronic applications. By manipulating the composition ratio (x) of SiOx, the well-known simultaneous changes in PL peak position and intensity were observed after annealing. To preserve the desired wavelength characteristics and enhance the luminesce intensity, we propose a novel approach to generate pristine nanostates (PNs) for Si-NC formation using proton irradiation (PI). The application of proton irradiation was found to be successful in significantly increasing PL intensity while strongly suppressing the peak position shifts. Through extensive spectroscopic analysis including PL, X-ray photoelectron spectroscopy, and absorption spectroscopy, we investigate specific defect states in the SiOx matrix, and identified as PNs. We interpret that these PNs consist of self-trapped exciton, E′ center, non-bridging oxygen hole center, and oxygen-deficiency center states. In the as-grown sample, the PNs promote appropriate phase separation and play a significant role in stabilizing the structure during the subsequent annealing processes. This study elucidates the Si-NC formation mechanism by enhancing O-diffusion-induced phase separation, a process accelerated in SiOx with controlled PNs. The proposed PI-induced PNs offer a novel pathway for developing Si-based optoelectronic devices with desired characteristics in the operating wavelength range. These findings open up promising possibilities for advanced silicon-based optoelectronic applications.

Effects of oxygen loading on the red-luminescence of non-bridging oxygen hole centers in γ-irradiated silica glasses

Ultra-high purity Corning 7980 and Corning 7979 silica glasses, respectively with high and low contents of hydroxyl and hydrogen, were specifically selected to investigate the effects of O2 loading on the 1.9 eV red luminescence of the gamma-ray induced non-bridging oxygen hole center (NBOHC), whose luminescence intensity is beneficial not only to predict the laser damage of silica but also to the targeted improvements in the preparation-treatment processes of silica. The UV trasmittance spectra, the UV excitation spectra, the visible emission spectra and the red emission lifetime decay curves were determined for these two types of the gamma-ray irradiated silica glasses before and after O2 loading. The content ratios of Si dangling bond (Si-E′) to NBOHC in silica glasses before and after O2 loading are estimated. The formation precursors and other related defects for NBOHC, such as Si-E′, Si-O-Si bond, Si-O-H bond, peroxy radical and linkage, oxygen-deficient centers ODC(I) and ODC(II), etc, are discussed. The bandwidths of the excitation and emission spectra before and after O2 loading are compared at the lower energy side of the spectra. The average emission lifetimes and especially the decay curves of the red luminescence before and after O2 loading are analyzed. There appear two types of NBOHCs with and without neighbouring hydrogen interaction at room temperature, respectively corresponding to the shorter and longer lifetime components. After O2 loading, the shorter lifetime component disappears and becomes the longer one, indicating that the hydrogen-bonded NBOHC is altered into the non-bonded. It is suggested that the analysis of the shorter and longer lifetime components of the red luminescence can be utilized in the laser damage prediction as well as the preparation and treatment processes of silica.

Defect generation mechanisms in silica under intense electronic excitation by ion beams below 100 K: Interplay between radiative emissions

Ion-beam effects on bulk silica at low temperature have been studied with the aim of understanding the routes and mechanisms leading from the initial generation of free carriers and self-trapped excitons (STEs) to the production of two stable defect structures in irradiated silica, non-bridging oxygen hole centers (NBOHCs) and oxygen deficient centers (ODCs). Ion beam induced luminescence (ionoluminescence, IL) spectra were obtained using 3 MeV H, 3.5 MeV He, 19 MeV Si, and 19 MeV Cl ions and a range of cryogenic irradiation temperatures from 30 to 100 K. The kinetic behavior of three emission bands centered at 1.9 eV (assigned to NBOHCs), 2.1 eV (assigned to the intrinsic decay of STEs), and 2.7 eV (assigned to ODCs) reveal the physical origin of these emissions under intense electronic excitation. The creation of NBOHCs is governed by a purely electronic mechanism. The kinetics curve of the NBOHC band shows two main contributions: an instantaneous (beam-on) contribution, followed by a slower fluence- and temperature-dependent process correlated with the concentration of STEs. The beam-on contribution is proportional to deposited ionization energy. The growth of the ODC band is linear in fluence up to around 2 × 1012 cm−2. The growth rate is independent of temperature but proportional to the number of radiation-induced oxygen vacancies per ion, showing, unambiguously, that the 2.7 eV emission can be associated with ODCs created in an excited state.

Study of defects in fused silica via a Franck-Condon analysis

We present a spectroscopic study of defects in fused silica (SiO2) samples, showing their signatures in photoluminescence (PL) excitation and emission spectra, PL decay rate, and Fourier-transform infrared and Raman spectra. Using the multi-transition Franck-Condon (FC) analysis, we were able to characterize phonon coupling with oxygen-deficient centres (ODC) in fused silica. Four observed FC emission bands, located at 2.8, 3.2, 4.2, and 4.4 eV were identified as singlet-singlet and singlet-triplet transitions in two different types of ODC defect. This model was confirmed by PL decay data and allowed us to describe different emission pathways of ODC defects.

UV-Excited Luminescence in Porous Organosilica Films with Various Organic Components.

UV-induced photoluminescence of organosilica films with ethylene and benzene bridging groups in their matrix and terminal methyl groups on the pore wall surface was studied to reveal optically active defects and understand their origin and nature. The careful selection of the film's precursors and conditions of deposition and curing and analysis of chemical and structural properties led to the conclusion that luminescence sources are not associated with the presence of oxygen-deficient centers, as in the case of pure SiO2. It is shown that the sources of luminescence are the carbon-containing components that are part of the low-k-matrix, as well as the carbon residues formed upon removal of the template and UV-induced destruction of organosilica samples. A good correlation between the energy of the photoluminescence peaks and the chemical composition is observed. This correlation is confirmed by the results obtained by the Density Functional theory. The photoluminescence intensity increases with porosity and internal surface area. The spectra become more complicated after annealing at 400 °C, although Fourier transform infrared spectroscopy does not show these changes. The appearance of additional bands is associated with the compaction of the low-k matrix and the segregation of template residues on the surface of the pore wall.

Ultrafast Laser Writing in Different Types of Silica Glass

AbstractIt is demonstrated that ultrafast laser writing in silica glass depends on the grade of silica glass associated with the method of its manufacture. Moreover, laser‐written modifications, in particular birefringent modifications, reveal a dependence on the geometry of writing, that is, the modification strength of voxels is smaller than that of single line structures and multi‐line scanned areas, which can be explained by free carrier diffusion and reduced electric field in scanning writing. The retardance of scanned birefringent region produced in the regime of anisotropic nanopores formation in silica glass manufactured by vapor axial deposition (VAD) is about five times higher than that in an electrically fused sample at the same laser writing parameters, while the difference in retardance of a nanograting based modification in synthetic and fused silica is only about 10%. The phenomenon is interpreted in terms of the higher concentration of oxygen deficient centers in the electrically fused silica glass, which can confine self‐trapped holes and prevent the nanopores formation. Improvement of high transmission optical elements is demonstrated in the VAD sample, and low cost multiplexed optical data storage with higher capacity and readout accuracy is realized in the electrically fused silica glass.

Franck Condon analysis of emission and excitation spectra of fused silica materials

Analysis of defects in optical materials is essential for their applicability in cutting-edge optical components. Since fused silica (FS) counts among the most used materials, deep knowledge about the defects in FS is of high importance. These defects have been routinely identified by studying photoluminescence (PL) emission and its analysis via multiple Gaussian bands. Here we present an extended approach based on the Franck-Condon model to study the defects in FS and the connected pathways of charge carrier relaxation. First, we performed the optical characterization of the FS, including optical absorption, photoluminescence (PL) emission and excitation (PLE), and Fourier-transform infrared (FTIR) and Raman spectroscopy (RS). Based on the analysis of the PLE spectra and vibrational frequencies via RS and FTIR, we created a multi-transition Franck-Condon model, which is able to fully reproduce the PL and PLE spectra. Based on the experimental data and the Franck-Condon fit, we discuss two types of oxygen-deficient centres (ODC) present in this fused silica material and their emission pathways.

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

Photocured Organofunctional Silicon-Based Polymer and Its Y2O3 Nanocomposite as the Luminescence Tracer of Thermal History

Materials with irreversibly changing properties after thermal heating are of great interest as thermal history trackers and are in demand as part of almost any device. Here, a promising thermoreporting material based on an organosilicone nanocomposite is suggested. Samples of photocured poly(1-trimethoxysilylpropyloxymethyl)ethylenoxide (polyGPTMS) and its nanocomposite with a filler of Y2O3 nanoparticles (polyGPTMS–Y2O3) were obtained with UV laser polymerization. The subsequent heat treatment of the samples revealed luminescence, the spectral position of which demonstrated a red shift with temperature increase. Comparative analysis of luminescence and Fourier-transform infrared spectroscopy spectra as well as theoretical modeling were used to characterize the structural peculiarities of the polymer and the nanocomposite. It was found that the luminescence behavior correlates with the color centers formation in the course of thermal treatment. The red shift of luminescence with temperature is associated with the redistribution of the chemical network and changes in the contribution of different color centers. The main effect is connected with an E′ center and oxygen-deficient centers, where the predominant contribution of the latter at high temperatures determines the observed luminescence red shift. The different dynamics of the color centers formation for polyGPTMS and polyGPTMS–Y2O3 results in a correspondingly stepwise and gradual luminescence red shift. The uncovered gradual luminescence red shift with thermal treatment of the polyGPTMS–Y2O3 makes the nanocomposite a promising tracer of thermal history. Using polyGPTMS–Y2O3 nanocomposite as a reporting material allows establishing not only the fact of heating but also the heating temperature.

High damage threshold birefringent elements produced by ultrafast laser nanostructuring in silica glass.

Birefringent patterning by ultrafast laser nanostructuring in silica glass has been used for space-variant birefringent optics with high durability and high optical damage threshold. We demonstrate that the oblate-shaped birefringent modification (type X) with ultrahigh optical transmission has higher optical damage resistance, comparable to pristine silica glass. The lower damage threshold of nanogratings based modification (type 2) following thermal annealing at 900°C for an hour is improved from 0.96 J/cm2 to 1.62 J/cm2 for 300 fs laser pulses and approaches the optical damage threshold of type X (1.56 J/cm2). This opens the door to utilize these optical elements for high power laser applications where optical transmission and damage threshold are the key parameters. The lower damage threshold of type 2 modification is related to the relatively high concentration of defects, such as E' centers and oxygen-deficiency centers (ODCs).

EPR study of intrinsic defects within NaYGeO4 olivine

An intense electron paramagnetic resonance (EPR) spectrum has been unexpectedly observed at room temperature in polycrystalline samples of NaYGeO4 with olivine crystal structure. Signals were registered in a wide range of fields up to 6000 ​G. Along with strongly broadened unresolved resonance lines, the relatively narrow spectral components were found. It is shown that the spectrum profile depends on synthesis conditions and mechanical treatment of samples, all suggesting a partial orientational order of microcrystallites in the powders. The spectra are interpreted under the assumption of strong superhyperfine interaction between the oxygen-deficient centers like [GeO3]3- and the nuclei of Na+ cations. In addition, a sharp anisotropic resonance at g-values exceeding free-electron value is attributed to oxygenic-hole centers. The density functional theory (DFT) calculations are employed for the estimation of the thermodynamic stability of various intrinsic defects.

Two-photon absorption induced dangling bonds in amorphous silica revealed by molecular dynamics simulations

The amorphous silica (a-silica) has been troubled by dangling bonds induced strong optical absorption of intense ultraviolet (UV) rays, where the two-photon absorption (TPA) plays a critical role in the formation of dangling bonds. Through the combination of Keldysh theory and molecular dynamics, we studied the effects of TPA on the evolution of dangling bonds in a-silica, where the ab initial calculations need a larger size of amorphous structure. The damage precursor of (Si-O)3 rings experimentally observed was reproduced, a process where the strained Si-O bonds acted as the TPA site. We showed that TPA played a critical role in initiating dangling bonds and the following densification process depressed it. Formation channels for oxygen deficiency center I (ODC(I)) and ODC(II) were shown, as well as the interconversion with Frenkel-type defects. Our research describes the static and dynamic properties of dangling bonds and gives an interpretation of experimental observations.

Effect of H atoms and UV wideband radiation on cured low-k OSG films

Effects of hydrogen atoms and UV radiation (λ > 210 nm) on nanoporous organosilicate glass (OSG) low-k films are studied in the temperature range from 20 °C to 300 °C. The purpose of the study is to understand the mechanisms of low-k films modification that can happen during the cleaning from carbon containing residues formed from sacrificial porogen and accumulated during the air storage. It is shown that exposure of low-k films to hydrogen atoms at low temperature leads to slight modification of hydrocarbon bonds in hydrocarbon residues not bonded to Si. At high temperature (T ⩾ 300 °C), the relative concentration of –CH x bonds changes in a complex way and depends on the amount and structure of the carbon-containing compounds. The general trend is relatively rapid decrease of –CH2 bonds concentration, while the terminal –CH3 groups are more stable. Temperature also initiates the reaction of hydrogen atoms with low-k with partial modification of low-k matrix breaking Si–O bonds. The destruction of Si–O and Si–CH2 groups leads to the formation of oxygen-deficient centers, followed by the formation of Si–(CH3)2 groups due to their interaction with methyl groups. At 300 °C, the total number of Si–CH3 + Si–(CH3)2 groups starts to decrease indicating on partial removal of the methyl groups bonded to silicon. Besides with increasing temperature a slight modification of the structure of matrix under exposure to H atoms is also observed. UV radiation has almost no effect on these processes in the studied conditions. Thus, there exist the ‘optimal’ conditions for H atom impact on OSG low-k films which allows improving film performance by removing porogen residue without damage.

Effect of carbon concentration on optical and structural properties in the transition from Silicon Rich Oxide to SiCxOy films formation

The role of carbon atomic concentration on the transition from Silicon Rich Oxide (SRO) to Silicon Oxicarbide films (SiCxOy) is reported. For this, optical and structural properties were analyzed by Photoluminescence (PL), Micro-Raman, Fourier Transform Infrared (FTIR) and X-Ray Photoelectron Spectroscopy (XPS) measurements. An SRO film with 16.6 at.% of Si-excess and SiCxOy films with carbon concentration from 1.3 to 7.2 at.% were grown by Hot Filament Chemical Vapor Deposition Technique (HFCVD). The SRO film (C = 0 at.%) and SiCxOy films with low carbon atomic concentration (C = 1.3 at.%) exhibited a very low PL emission in the red-infrared region (1.5–1.7 eV) manly attributed to SiO defects which are reduced or annihilated by incorporation of carbon. An increase in carbon atomic concentration greater than 1.3 produces a strong PL emission at white region (3.35–1.72 eV) characteristic of SiCxOy films attributed to radiative defects such as defects associated to Si-related oxygen vacancies (Si-NOVs), Si-related oxygen deficiency centers (Si-ODC) and C-related oxygen vacancies (C-NOVs). In this work it was found that a carbon atomic concentration greater that 1.3 is required to have a high density of these radiative defects. Also, due to the high growth temperature (900 °C) and Si-excess (from 16.6 to 3.9 at.%), we propose the presence of Si nanocrystals (Si–NCs) embedded in both SRO and SiCxOy films with low carbon concentration (SiO2-Like characteristics). Whereas an increase in carbon concentration of 7.2 at. %, Si–NCs precipitation was inhibited in the SiCxOy films.

Electroluminescence of SiO2 films grown on Si by thermal oxidation and plasma-enhanced chemical vapor deposition

Emission of the silicon oxide films grown on Si by wet thermal oxidation at 900 °С and by plasma-enhanced chemical vapor deposition from the SiH4 + N2O mixture at 350 °С has been compared using electroluminescence. The electroluminescence spectra were recorded in electrolyte – insulator – semiconductor system. The intense band in the red range with a maximum at 1.9 eV dominates the electroluminescence spectrum of the thermal oxide film. It was concluded that this band is related with the existence of silanol groups (Si — OH) in the oxide matrix. Multiband emission in the UV range is observed in the electroluminescence spectrum of the oxide film formed by plasma-enhanced chemical vapor deposition. Additional investigations using IR and RS spectroscopy revealed that observed spectrum modulation is of an oscillatory nature and is not the result of interference. Presumably, the luminescence in the UV region is due to the presence of oxygen deficiency centers containing bonds with hydrogen atoms.

Interconversion mechanisms of Ge-related oxygen deficient defects in germanium dioxide optical fiber

The interconversion mechanisms of Ge-related oxygen deficient defects including Ge-related oxygen deficient center (GeODC), Ge-related E' center (Ge-E') and germanium lone pair center (GLPC) have been investigated in germanium dioxide optical fiber. Our results revealed that the conversion paths from GeODC through Ge-E' centers, GLPC and then back to GeODC were similar in four- to six-membered rings. Furthermore, the transformation from Ge-E' to GLPC is spontaneous, indicating that it is the most likely to take place. We employed first-principles calculations to calculate optical properties and electronic properties of Ge-related oxygen deficient defects as well. All these models have a series of absorption peaks at 4-6 eV in the optical absorption spectrum, which may be related to the defect levels generated in the band gap. Our work provides a better understanding of the interaction among the three defects in germanium dioxide optical fiber.