Search results for "Crystallographic"
showing 10 items of 202 documents
Coupled irradiation-temperature effects on induced point defects in germanosilicate optical fibers
2017
International audience; We investigated the combined effects of temperature and X-rays exposures on the nature of point defects generated in Ge-doped multimode optical fibers. Electron paramagnetic resonance (EPR) results on samples X-ray irradiated at 5 kGy(SiO2), employing different temperatures and dose rates, are reported and discussed. The data highlight the generation of the Ge(1), Ge(2), E0 Ge and E0 Si defects. For the Ge(1) and Ge(2), we observed a decrease in the induced defect concentrations for irradiation temperatures higher than *450 K, whereas the E0 defects feature an opposite tendency. The comparison with previous post-irradiation thermal treatments reveals peculiar effects…
X-ray irradiation effects on a multistep Ge-doped silica fiber produced using different drawing conditions
2011
International audience; We report an experimental study based on confocal microscopy luminescence (CML) and electron paramagnetic resonance (EPR) measurements to investigate the effects of the X-ray (from 50 krad to 200 Mrad) on three specific multistep Ge doped fibers obtained from the same preform by changing some of the drawing conditions (tension and speed). CML data show that, both before and after the irradiation, Germanium Lone Pair Center (GLPC) concentrations are similarly distributed along the diameters of the three fibers and they are partially reduced by irradiation. The irradiation induces also the Non Bridging Oxygen Hole Center (NBOHC) investigated by CML and other paramagnet…
Evolution of Photo-induced defects in Ge-doped fiber/preform: influence of the drawing
2011
International audience; We have studied the generation mechanisms of two different radiation-induced point defects, the Ge(1) and Ge(2) centers, in a germanosilicate fiber and in its original preform. The samples have been investigated before and after X-ray irradiation using the confocal microscopy luminescence and the electron paramagnetic resonance techniques. Our experimental results show the higher radiation sensitivity of the fiber as compared to the perform and suggest a relation between Ge(1) and Ge(2) generation. To explain our data we have used different models, finding that the destruction probability of the Ge(1) and Ge(2) defects is larger in fiber than in preform, whereas the …
Influence of Drawing Conditions on the Properties and Radiation Sensitivities of Pure-Silica-Core Optical Fibers
2012
International audience; The structure and radiation sensitivities of three different pure-silica-core fluorine-doped-cladding optical fibers, made from the same original preform, were investigated by several experimental techniques. The fibers were obtained by changing the drawing speed and tension in the typical ranges of values used for the radiation-tolerant waveguides. The Raman spectroscopy revealed no significant difference among the fibers before irradiation. At variance, the comparison between the fibers and their associated preform highlighted an increase in the amplitude of the D2 band that is related to the concentration of 3 member rings. Moreover, in the zones where the D2 incr…
Influence of F centres on structural and electronic properties of AlN single-walled nanotubes
2007
We analyse the influence of uncharged N vacancies (neutral F centres), created either under conditions of AlN nanotube growth or by its soft irradiation, on the atomic and electronic structure. Periodic one-dimensional (1D) density functional theory (DFT) calculations on models of defective single-walled nanotubes (SW NTs) allow us to analyse how NT chirality and concentration of F centres change their properties compared to the corresponding defect-free nanotubes. We have simulated reconstruction around periodically repeated F centres on 1 nm AlN SW NTs with armchair- and zigzag-type chiralities. To achieve the limit of an isolated vacancy for both chiralities, we have considered different…
Simplified Monte Carlo simulations of point defects during industrial silicon crystal growth
2004
Abstract The paper proposes Monte-Carlo method-based 2D and 3D models of vacancies and interstitials in a cubic crystal. The model exploits the concept of lattice gas with covalent bounds between neighbour nodes. Two lattices shifted by half-period serve as nodes for atoms of the main crystal and interstitials. Distribution of particles between both lattices characterizes the entropy of the crystal. Successfully chosen interaction energies between main and sub-lattices allows the authors to detect a phase transition solid–liquid as well as to study the production of crystal defects/their agglomeration as a function of cooling/heating rate. Although the introduced 3D modification of the mode…
Spatial inhomogeneities and defect structures in CIGS and CIS materials: An ab-initio based Monte Carlo study
2011
The chalcopyrite semiconductors CuIn 1−x Ga x Se 2 (CIGS) and CuInSe 2 (CIS) are excellent materials for high efficiency and low cost thin-film solar cells. This is due to the effective absorption of the solar spectrum and the inherent resilience to defects and composition fluctuations. Although the CIGS and CIS material in solar cells is highly inhomogeneous and exhibits a lot of different defects, the cell efficiencies are exceptionally high. If single crystalline absorbers are used, efficiencies are lower. Therefore, studying spatial inhomogeneities and defect structures is of great importance for understanding what supports and what diminishes the efficiency and robustness of the cells.…
Formation of optically active oxygen deficient centers in Ge-doped SiO2 by γ- and β-ray irradiation
2010
Abstract We report an experimental study on the comparison between the γ- or β-ray induced Ge related point defects in Ge-doped silica. Silica samples doped with ∼2.2 1017 Ge atoms/cm3 produced with the sol–gel technique have been irradiated with γ-ray or with β-ray. The effects of the irradiation have been investigated by optical absorption, photoluminescence and electron paramagnetic resonance spectroscopy in order to evaluate the generation and the dependence on dose of the Ge(1), E’Ge, GLPC (Germanium lone pair center) and H(II) point defects. No relevant differences between the concentrations of γ- or β-ray induced Ge(1) and E’Ge point defects have been observed and, in addition, it ha…
The landscape of the excitation profiles of the αE and β emission bands in silica
1999
Abstract We report data about the relevance of the conformational heterogeneity in determining the optical properties of oxygen deficiency point defects in natural silica samples. The spectral profiles of the photoluminescence emissions at about 4.2 eV (α E band) and at about 3.15 eV (β band), and the efficiency of the intersystem-crossing mechanism connecting them appear modified by a fine tuning of the excitation energy within the B 2β absorption band. Moreover, the relative excitation optically spectra indicate the presence of optically distinguishable contributions to the emission profile. The reported data are attributed to a distribution of centers that maps into a spectral inhomogene…
Optically detected magnetic resonance investigation of oxygen luminescence centres in BaF2
2002
The structure of two oxygen-related luminescence centres in oxygen-doped BaF2 was investigated by means of photoluminescence (PL) and photoluminescence-detected electron paramagnetic resonance (PL-EPR). One of the oxygen-related luminescences peaking at 2.83 eV is associated with an excited triplet state (S = 1) of an oxygen–vacancy complex with the z-axis of the fine-structure tensor parallel to the 110 direction. This complex can be described as an oxygen on a fluorine lattice site with a next-nearest fluorine vacancy along the 110 direction. The luminescence at 2.25 eV is also associated with a triplet state. Its PL-EPR spectrum is probably due to oxygen–vacancy complexes with a nearest …