Deviations from square-root distributions of electronic states in hydrogenated amorphous silicon and their impact upon the resultant optical properties

We study the distributions of conduction band and valence band electronic states associated with hydrogenated amorphous silicon. We find that there are substantial deviations from square-root distributions, particularly deep within the bands and within the gap region. The impact of these deviations is assessed through a determination of the spectral dependence of both the joint density of states function and the imaginary part of the dielectric function. These deviations are found to have a considerable effect upon the determination of the corresponding Tauc optical gap, the optical gap obtained for the case of hydrogenated amorphous silicon being 220 meV lower than the energy difference between the valence band and conduction band band edges. We suggest that the standard interpretation for the Tauc optical gap, as the energy difference between these band edges, should be reconsidered in light of these results.     

Spectroscopic characterization of intrinsic losses in an organic-inorganic hybrid waveguide synthesized by the sol-gel process

A promising way of fabricating integrated optics components is based on the sol-gel synthesis and photocuring of hybrid materials. However, the presence of OH groups in these materials is a major factor in optical amplification inhibition. In particular, high losses at 1550 nm are mainly due to non-condensed OH groups originating from the sol-gel process at low temperature. Thus, improvement of the final properties of these materials is correlated with the inhibition of OH group concentration. In this study, we used ²⁹Si NMR and near infrared spectroscopy to demonstrate the catalytic effect of zirconium (IV) n-propoxide on the condensation reactions of silanol groups. ²⁹Si NMR showed the absence of silanol species at the end of the synthesis. This result is attributed to the zirconate hydrophilic effect which consumes OH groups by catalysing the polycondensation of Si-OH bonds. In parallel, near-infrared experiments showed the presence of a high proportion of OH species at the end of the synthesis showing that the remaining OH groups are only present in the zirconium species.     

Bridging, non-bridging and free (O) oxygen in Na2O-SiO2 glasses: An X-ray Photoelectron Spectroscopic (XPS) and Nuclear Magnetic Resonance (NMR) study

High resolution Na 1s, O 1s and Si 2p core level XPS spectra of six Na₂O-SiO₂ glasses ranging in composition from 100 to 45 mol % SiO₂ have been collected using the Kratos Ultra Axis instrument with its unique charge compensation system. The O 1s spectra for the glasses are well resolved so that bridging oxygen (BO, Si-O-Si) and non-bridging oxygen (NBO, Na-O-Si) signals can be accurately fitted and quantified without resorting to constraints or assumptions. The same samples were analysed by ²⁹Si MAS NMR to obtain Q-species abundances from which BO and NBO proportions were calculated. Similar BO:NBO ratios were obtained by both methods over the entire compositional range studied. They are also consistent with most previous XPS and NMR results for glasses containing more than ~ 65 mol % SiO₂. Our XPS and NMR experimental results, however, differ somewhat from previously published XPS and NMR results for glasses containing less than about 65 mol % SiO₂.Na is mobile in the X-ray beam and mobility causes BO:NBO ratios to increase with time of exposure. Na mobility here has been circumvented to yield reliable BO:NBO ratios of the glasses. The ratios are lower than previously reported in XPS studies and are similar to ratios obtained from our ²⁹Si MAS NMR results on the same glasses. The XPS and ²⁹Si MAS NMR results also indicate the presence of a third oxygen species in sodic glasses. As has been proposed for CaSiO₃ glass and for sodic and potassic glasses containing La, we suggest that O²⁻ is present in sodic glasses at small concentrations. The O²⁻ content correlates with increased soda content and may be associated with, and instrumental in development of, three dimensional percolation channels in the glasses. The XPS O 1s line width of the BO peak is broader than the NBO peak, indicating more than one contribution to the BO peak. As observed in crystalline Na metasilicate and Na disilicate, BO of Na-silicate glasses may be of two types, one arising from BO bridging two Si atoms, and the second BO signal arising from BO bonded not only to two Si atoms but also to Na.     

Preparation and optical properties of titania-doped hybrid polymer via anhydrous sol-gel process

Incorporation of metal alkoxides (Ti, Zr, etc.) for tuning the optical properties of silica glasses by the sol-gel process is of significant interest for optical applications. In this paper, we report an anhydrous sol-gel process for preparation of photosensitive titania-doped hybrid glassy polymer with good homogeneity and high doping concentration (TiO₂ up to 40 mol%). The process consists of two steps: in the first step methacryloxypropyltrimethoxysilane (MPS) is hydrolyzed by boric acid through ligand exchange reaction (OH↔OR) under anhydrous conditions; and in the second step dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPhDMS) and titanium ethoxide (TET) were added to condense with the silanols formed in the first step. The optical properties of the synthesized hybrid polymer were studied, and results showed that the hybrid material has low OH absorption, low optical losses (0.45 dB/cm at 1550 nm and 0.16 dB/cm at 1310 nm respectively), and good thermo-optical linearity with tuneable refractive index. The effect of TiO₂ doping in reducing the OH concentration of the hybrid material was observed, and the mechanism for this effect is discussed.     

A new class of infrared transmitting glass-ceramics based on controlled nucleation and growth of alkali halide in a sulphide based glass matrix

Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 μm region. In this paper we describe a new type of glass-ceramics which are transparent up to 11 μm and based on the controlled nucleation and crystallization of cesium chloride sub-micron particles inside a Ge-Sb-S glass matrix. The evolution of the optical transmission versus annealing time and temperature has been investigated. Observations under scanning electronic microscopy as well as X-ray diffraction indicate that the crystalline phase is a primitive cubic cell with a parameter slightly inferior to that of pure CsCl and that the grain sizes are about 100 nm. A preliminary test on fracture propagation shows a much better resistance of glass-ceramics to cracks than the corresponding pure glass matrix.     

X-ray photoelectron spectroscopy studies of thin GexSb40−xS60 chalcogenide films

Thin Ge_xSb_40−xS₆₀ (x=5, 15, 20, 25 and 27) chalcogenide films have been investigated by X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra show that there is a peculiarity in the relative intensity ratio of the Sb 4d photoelectron peak associated with Sb₂S₃ to the Sb 4d photoelectron peak associated Sb₂S₅ at an average co-ordination number Z of 2.65-2.67. After contamination and photo-oxidation layers were removed from the surface of the films, X-ray photoelectron spectra were measured again. It has been found that binding energies of the Ge 2p and Sb 3d_3/2 photoelectron peaks, which reflect the electronic structure at lower core energy levels, are independent of Z. However, the binding energies of the Ge 3d and Sb 4d photoelectron peaks are more sensitive to Z and have a discontinuity at Z=2.65.     

Structural investigations of copper doped B2O3-Bi2O3 glasses with high bismuth oxide content

Raman and infrared spectroscopy have been employed to investigate the 99.5%[xB₂O₃(1−x)Bi₂O₃]0.5%CuO glasses with different Bi/B nominal ratios (0.07?x?0.625) in order to obtain information about the competitive role of B₂O₃ and Bi₂O₃ in the formation of the glass network. The glass samples have been prepared by melting at 1100 °C and rapidly cooling at room temperature. In order to relax the structure, to improve the local order and to develop crystalline phases the glass samples were kept at 575 °C for 10 h. The influence of both Bi₂O₃ and CuO on the vitreous B₂O₃ network as well as the local order changes around bismuth and boron atoms in as prepared and heat treated samples was studied. Structural modifications occurring in heat treated samples compared to the untreated glasses have been observed.     

Equilibrium concentration of oxygen rich and deficient defect centers in germanosilicate glasses

Equilibrium concentrations of oxygen rich and deficient defect centers are calculated as a function of temperature and glass redox condition for germanosilicate glasses. We have here used the approach of Silin and Lace [J. Non-Cryst. Solids 149 (1992) 54-61] but extended it to include the case of binary system of germanosilicate glasses. A set of 23 reactions is identified as the possible pathway for formation of different defect centers. Each of these reactions are represented by forward and backward steps with Arrhenius kinetics and their activation energies are estimated based on the relevant bond energies of the involved species. Equilibrium concentrations are determined by setting rates of each of these reactions equal to zero. Typical results are presented for glasses which are: (i) stoichiometric, (ii) glasses with excess oxygen levels of 10¹⁶ to 10²⁰ cm⁻³, and (iii) glasses which are oxygen deficient by 10¹⁶ to 10¹⁸ cm⁻³.