Characterization of deposits produced by TEA CO2 pulsed laser ablation of silicon mono- and dioxide

Silicon based deposits were prepared by TEA CO₂ pulsed laser ablation (PLA) of SiO and SiO₂ targets in the atmosphere of selected gases (N₂, He, Ne, Ar, Kr). These deposits possess high specific area of several hundreds m² per gram. Owing to the high specific area, some chemical groups and hydrogen related radical were detected by means of FTIR and EPR analyses and theoretical calculations: silyl (E′ center) Si, silylen Si:, silanon SiO, POL (peroxy linkage) SiOOSi and/or NBOHC (non-bridging oxygen hole center) SiO, POR (peroxy radical) SiOO and dioxysilirane Si(O)₂. In SiO₂ deposits the concentration of silyl Si resp. POR SiOO was determined to be 5.8 × 10¹⁸/g resp. 6.2 × 10¹⁹/g. In SiO deposits the ratio [Si:]:[Si] = (3.1-5.7) × 10¹⁹/g: (5.3-9.8) × 10¹⁹/g was measured. Estimated concentration of [Si] in deposits was increased nearly five times in comparison with SiO target. After exposure of the SiO deposits to H₂ EPR doublet with hyperfine splitting of 7.7 mT was observed. The best agreement between calculated theoretical and experimental values was found for the model [(HO)₃SiO]₂HSi. FTIR measurements and calculations of the silanol theoretical model clusters enabled us to discuss the chemical surroundings of the silanol and to determine the defects in the deposits.     

Annihilation of E′ center defects in silica glass by hydrogen treatment

It was found, using electron paramagnetic resonance (EPR), that the signal of E′ centers in silica glass totally disappeared following a 1 h heat-treatment at 1000 °C under hydrogen atmosphere. However, by subsequent heat-treatment at the same temperature under a dry nitrogen atmosphere, some of the E′ centers re-appeared.     

Properties and structure of copper ultraphosphate glasses

The structures of xCuO · (1 − x)P₂O₅ glasses (0 ? x ? 0.50) prepared in vacuum sealed silica ampoules were investigated using vibrational spectroscopies. With the addition of CuO, both infrared and Raman spectra indicate a systematic transformation from a three-dimensional ultraphosphate network dominated by Q³ tetrahedra into a chain-like metaphosphate structure dominated by Q² tetrahedra. IR spectra clearly show two distinct Q³ sites with bands at 1378 and 1306 cm⁻¹, assigned to PO bonds on isolated Q³ tetrahedra and PO bonds on tetrahedra that participate in the coordination environments of the Cu-octahedra, respectively. As CuO content increases, the intensity of the PO band associated with the tetrahedra increases to a maximum x ∼ 0.33, then decreases with a concomitant increase of the intensity of the band at 1265 cm⁻¹, due to the asymmetric vibration of the PO₂ groups on Q² tetrahedra. When x > 0.33 the isolated Cu-octahedra begin to share common oxygens to form a sub-network in the phosphate matrix. The effects of glass structure on the glass properties, including density, refractive index, and glass transition temperature, are discussed.     

Surface smoothing of sputter deposited amorphous CNx films by silicon addition

Amorphous carbon nitride (CN_x) films with silicon addition up to 16 at.% are sputter deposited on Si(1 0 0) substrate, and the surface morphology is studied with scaling method based on atomic force microscopy. The surface roughness σ, the roughness exponent α, and the lateral correlation length ξ decrease with silicon content of the films, reaching 0.33 nm, 0.80 and 50 nm, respectively, for the film with [Si] = 16 at.%. The addition of silicon in the films leads to additional Si-N, Si-C-N and CN bonds revealed by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The films undergo a structural transition from columnar to smooth morphology in cross-section with silicon addition demonstrated by field emission scanning electron microscopy. Nano-sized clusters sparsely dispersed in amorphous matrix of the film with [Si] = 16 at.% are observed by high-resolution transmission microscopy. According to the surface growth mechanism in which surface diffusion and geometrical shadowing drive structural and morphological evolution of the sputter deposited films, surface smoothing of the amorphous CN_x films by silicon addition is explained by the formation of Si-N and Si-C-N bonds that impede surface diffusion of the adsorbed species during film growth, which leads to the reduced size of the columnar structures.     

Study of elastic moduli of lithium borobismuthate glasses using ultrasonic technique

Ultrasonic studies on x B₂O₃25 Li₂O(75 − x) Bi₂O₃, (0 ≤ x ≤ 75 mol%) glasses have been carried out. The elastic moduli of glasses have been investigated using ultrasonic velocity measurements at 4 MHz. The results revealed an increase of the ultrasonic velocity and Debye temperature with the increase of the B₂O₃ content which was attributed to the increase in the packing density, the local contraction of the network around the Bi and Li cations and the increase of the number of bonds per unit volume. Also, the increase of the elastic moduli with the increase in the B₂O₃ content is affected with the increase in the dissociation energy, the average cross-link density, the increase in the number of bridging oxygen atoms, and the substitution of high bond strength BO with low bond strength BiO. The optical properties such as the refractive index, the energy gap and the optical polarizability were evaluated from the values of the elastic moduli. It was observed that as the bulk modulus increases, the optical energy gap increases and both the refractive index and the optical basicity decrease.     

Thermal dissolution of Ag(Cu) in amorphous Ge(Sx Se1 − x)2 system

Thermally induced solid state reaction of Ag(Cu) into thin Ge(S_x Se_1 − x)₂ films with x = 0, 0.1, 0.4 and 1.0 was investigated using a step by step technique in order to design films with exact Ag(Cu) concentrations for applications in integrated IR optical devices. A thin film of Ag(Cu) was deposited on top of the host Ge(S_x Se_1 − x)₂ films followed by annealing in vacuum at constant temperature, which resulted in homogeneous films of good optical quality. The variation in Ag(Cu) concentration in the films ranged between 5 and 35 at.%. The kinetics of the diffusion and dissolution of metal in the host films was measured by optically monitoring the change in thickness of doped chalcogenide during consecutive thermal annealing steps. The kinetics studies revealed that the thermal dissolution rate of the Cu is greater than that of Ag. Optical UV-VIS transmission spectra of chalcogenide glass layers, undoped and thermal doped by Ag(Cu), were measured to establish the optical properties of the films. The spectra were analyzed using the technique proposed by Swanepoel and the results show that the addition of metal increases the absorption coefficient in the power-law regime and consequently the optical gap decreases and the refractive index increases. The amorphous character of the films was checked by X-ray diffraction which confirmed the amorphous structure of all Ag(Cu)GeSSe thin films.     

Photosensitive defects in silica layers implanted with germanium ions

Ge-implanted silica layers have been investigated by high-power pulsed synchrotron-photoluminescence (PL), photoluminescence excitation spectroscopy (PLE), and optically stimulated electron emission (OSEE) with respect to association of excitation and absorption bands to respective emission bands and lifetimes of excited defect states. In this way singlet-singlet (4.35 eV) and triplet-singlet (3.18 eV) radiative transitions from excited states of oxygen-deficient centers (ODC) in Ge-doped silica glass are characterized by their absorption and emission bands as well as their lifetimes. The main channel for non-radiative relaxation of photoexcitation is electron emission by the OSEE effect. The OSEE shows non-radiative transitions of surface and bulk E′-centers found with concentrations of (2.7-3.4) × 10¹² cm⁻² and (2-4) × 10¹⁶ cm⁻³, respectively.     

Stability of E′ centers induced by 4.7 eV laser radiation in SiO2

The kinetics of E′ centers ( Si) induced by 4.7 eV pulsed laser irradiation in dry fused silica was investigated by in situ optical absorption spectroscopy. The stability of the defects, conditioned by reaction with mobile hydrogen of radiolytic origin, is discussed and compared to results of similar experiments performed on wet fused silica. A portion of E′ centers and hydrogen are most likely generated by laser-induced breaking of Si–H precursors, while an additional fraction of the paramagnetic centers arise from another formation mechanism. Both typologies of E′ centers participate in the reaction with H₂ leading to the post-irradiation decay of the defects. This annealing process is slowed down on decreasing temperature and is frozen at T = 200 K, consistently with the diffusion properties of H₂ in silica.     

Local atomic and electronic structures of equiatomic liquid alloy KSb from 923 to 1773 K

The local atomic and electronic structures of equiatomic liquid alloy KSb have been investigated using ab initio molecular dynamics simulations. There is a small peak located at about 62° in the covalent SbSb bond-angle distribution function. The height of 62° peak increases with temperature. The further analysis show that this peak attributes to the triangular structures with each Sb atom covalently bonded to the other two in the liquid KSb and their number increases with temperature. Therefore, there exist isolated Sb atoms, covalently bonded Sb dimers, triangles with three covalent SbSb bonds and short covalently bonded Sb chains in the liquid KSb. Upon temperature increasing the covalent Sb chains tend to break into short pieces so that the isolated Sb atoms, covalently bonded Sb dimers and triangles with three covalent SbSb bonds increase in numbers. The obtained total electronic densities of states at different temperatures give a reasonable explanation of the electronic conductivity increasing with temperature.     

Consolidation mechanism of materials obtained from sodium silicate solution and silica-based aggregates

A study based on the use of sodium silicate gels as binder for cold consolidation of silica-based aggregates has been investigated. The gels used as precursor of binder were synthesized by adding hydrochloric acid to a concentrated sodium silicate solution. Consolidated materials were obtained by mixing the previous solution before gelation with granular materials (fine silica powder and sand). The study of the gel-silica-sand ternary system shows that the existence domain of materials depends on the sand size distribution. The microstructure of gel-silica-sand ternary samples reveals the presence of the three components with a partial attack of grain surface. This was confirmed by FTIR experiments during the monitoring of the synthesis. Actually, the ν_asSiOSi broad band resulting from the average of the contribution of the set of Q⁴, Q³ and Q² units with a sharp peak located around 1078 cm^− 1, firstly shifts to lower wavenumber until 21 days and then to higher wavenumber characteristic of dissolution/precipitation reactions. On the other hand, the consolidation of the material is strong when the amount of fine silica in the material is high leading to efficient mechanical properties. Therefore, consolidation could be explained by the dissolution of small particles of silica and their precipitation into the grain boundary of sand.     

Influence of the deposition parameters on the microstructure and opto-electrical properties of hydrogenated nanocrystalline silicon films by HW-CVD

Hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared at high deposition rates (> 13 Å/s) from pure silane without hydrogen dilution by hot wire deposition method by varying filament-to-substrate distance (d_s-f). In this study we have systematically and carefully investigated the effect of filament-to-substrate distance on structural, optical and electrical properties of the Si:H films. A variety of characterization techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FE-SEM), UV-Visible-NIR spectroscopy and electrical dark and photoconductivity measurement were used to characterize these films. Films deposited at d_s-f > 5 cm are amorphous while those deposited at d_s-f < 5 cm are biphasic; a crystalline phase and an amorphous phase with nano-sized crystallites embedded in it. Low angle X-ray diffraction analysis showed that the crystallites in the films have preferential orientation along (111) directions. Decrease in d_s-f, the crystallinity and crystalline size increases whereas hydrogen bonding shifts from mono-hydride (SiH) to di-hydride (SiH₂) and poly-hydride (SiH₂)_n complexes. The band gaps of nc-Si:H films (~ 1.9-2.0 eV) are high compared to the a-Si:H films, while hydrogen content remains < 10 at.%. We attribute the high band gap to the quantum size effect. A correlation between electrical and structural properties has been established. Finally, from the present study it has been concluded that the filament-to-substrate distance is a key process parameter to induce the crystallinity in the films by hot wire method. The ease of depositing films with variable crystallite size and its volume fraction, and tunable band gap is useful for fabrication of tandem/micro-morph solar cells.     

Fourier transform infrared spectroscopy investigation of chemical bonding in low-k a-SiC:H thin films

Fourier Transform Infrared (FTIR) Spectroscopy has long been utilized as an analytical technique for qualitatively determining the presence of various different chemical bonds in gasses, liquids, solids, and on surfaces. Most recently, FTIR has been proven to be extremely useful for understanding the different types of bonding present in low dielectric constant “low-k” organosilicate materials. These low-k materials are predominantly utilized in the nanoelectronics industry as the interlayer dielectric material in advanced Cu interconnect structures. In this article, we utilize FTIR to perform a detailed analysis of the changes in chemical bonding that occur in Plasma Enhanced Chemically Vapor Deposited (PECVD) low-k a-SiC:H thin films. PECVD low-k a-SiC:H materials are equally important in advanced Cu interconnects and are utilized as both etch stop and Cu diffusion barrier layers. We specifically investigate the changes that occur in low-k a-SiC:H films as the dielectric constant and mass density of these films are decreased from > 7 to < 3 and from 2.5 to 1 g/cm³ respectively. We show that decreases in mass density and dielectric constant are accompanied by both an increase in terminal SiH_x and CH_x bonding and a decrease in SiC network bonding. At densities of 1.85 g/cm³, the concentration of terminal SiH_x bonding peaks and subsequent hydrogen incorporation are achieved predominantly via terminal CH₃ groups. Low-k a-SiC:H films with k < 3.5 and density < 1.3 g/cm³ can be achieved via incorporating larger organic phenyl groups but result in non-stoichiometric carbon rich films. Electron beam curing of these lower density a-SiC:H films results in volatilization of the phenyl groups leaving behind nanoporous regions and production of some CCC chain linkages in the network.     

The effect of implanting nitrogen on the optical absorption and electron paramagnetic resonance spectra of silica

Silica samples (type III, Corning 7940) were implanted with N using multiple energies to produce a layer ∼600 nm thick in which the concentration of N was constant to within ±5%. The optical absorption spectra of the samples were measured from 1.8 to 6.5 eV. Electron paramagnetic resonance (EPR) measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 K to 100 K for most measurements. The components identified in the EPR spectra, based on comparison with reported parameters, were due to E′ centers and peroxy radicals. By comparing the changes in the optical absorption at 5.85 eV with the changes in the concentrations of the various EPR components and with the reports in the literature, we conclude that there is an additional band at 5.7-5.9 eV other than the E′ center band. We conclude that the bands between 2 and 6.5 eV and the EPR spectral components produced by implantation of N are due to radiation damage processes; neither optical bands nor EPR components related to N are detected.     

Generation and annealing of defects in virgin fused silica (type III) upon ArF laser irradiation: Transmission measurements and kinetic model

The transmission of ArF laser pulses in virgin fused silica (type III) samples changes during N = 10⁶ pulses at an incoming fluence H_in = 5 mJ cm⁻² pulse⁻¹. The related absorption is determined by the pulse energy absorption coefficient α(N, H_in) using a modified Beer’s law, yielding initial values α_ini around 0.005 cm⁻¹, a maximum α_max ? 0.02 cm⁻¹ at N = 10³-10⁴ and stationary values 0.0045 cm⁻¹ ? α_end ? 0.0094 cm⁻¹ after N ≈ 6 × 10⁵ pulses. The development α(N, H_in = const.) is simulated by a rate equation model assuming a pulse number dependent E′ center density E′(N). E′(N) is determined by a dynamic equilibrium between E′ center generation and annealing. Generation occurs photolytically from the precursors ODC II and unstable SiH structures upon single photon absorption and from strained SiO bonds via two-photon excitation. Annealing in the dark periods between the laser pulses is dominated by the reaction of E′ with H₂ present in the SiO₂ network. The development α(N, H_in = const.) is observed for the very first sample irradiation only (virgin state). The values α_end are not accessible by simple spectrophotometer measurements.     

A sharp thermally reversing window in As45Te55−xIx chalcohalide glasses revealed by alternating differential scanning calorimetry and photo thermal deflection studies

The composition dependence of different thermal parameters such as glass transition temperature, non-reversing enthalpy, thermal diffusivity etc., of bulk As₄₅Te_55−xI_x chalcohalide glasses (3 ? x ? 10), has been evaluated using the temperature modulated Alternating Differential Scanning Calorimetry (ADSC) and Photo Thermal Deflection (PTD) studies. It is found that there is not much variation in the glass transition temperature of As₄₅Te_55−xI_x glasses, even though there is a wide variation in the average coordination number . This observation has been understood on the basis that the variation in glass transition temperature of network glasses is dictated by the variation in average bond energy rather than . Further, it is found that both the non-reversing enthalpy (ΔH_nr) and the thermal diffusivity (α) exhibit a sharp minimum at a composition x = 6. A broad hump is also seen in glass transition and crystallization temperatures in the composition range 5 ? x ? 7. The results obtained clearly indicate a sharp thermally reversing window in As₄₅Te_55−xI_x chalcohalide glasses around the composition x = 6.     

Towards the origin of the memory effect in oxide glasses

Glass samples with same composition and properties but different thermal histories can exhibit different behavior upon a subsequent heat-treatment, a phenomenon known as the memory effect. Generally, it is considered that the memory effect is due to nanoscale density fluctuation, which exists in all glasses and causes non-exponential relaxation with more than one relaxation time. Earlier, we studied the memory effect of various silica glasses and found that some pure silica glasses did not exhibit the memory effect while some silica glasses with higher impurity contents exhibited the memory effect. Based upon this finding, we suggested that the phenomenon originated from concentration fluctuation rather than density fluctuation. In this study, the memory effect in 6 mol% K₂O-94 mol% SiO₂ glass was investigated. The K₂O-SiO₂ glass system has a metastable immiscibility below the glass transition temperature and the chosen glass composition is the critical composition corresponding to the estimated critical temperature of the immiscibility boundary. Thus, it is expected that this glass composition would exhibit large super-critical concentration fluctuation, which increases with decreasing heat-treatment temperature. Density fluctuation, on the other hand, increases with increasing heat-treatment temperature. A much larger memory effect was observed at the lower heat-treatment temperature for the present glass. This result supports our earlier contention that the origin of the memory effect is composition fluctuation rather than density fluctuation.     

The dielectric behavior of a thermoelectric treated B2O3-Li2O-Nb2O5 glass

A transparent glass with the composition 60B₂O₃-30Li₂O-10Nb₂O₅ (mol%) was prepared by the melt-quenching technique. Glass-ceramics, containing LiNbO₃ ferroelectric crystallites, were obtained by heat-treatment (HT) above 500 °C, with and without the presence of an external electric field. The dielectric properties of the glass and glass-ceramic were investigated, as a function of temperature (270-315 K), in the 10 mHz-32 MHz frequency range. The presence of an external electric field, during the heating process, improves the formation of LiNbO₃ crystallites. The rise of the treatment temperature and the applied field, during the heat-treatment, leads to a decrease in the dc electric conductivity (σ_dc), indicating a decrease of the charge carriers number. The dielectric permittivity (ε′) values (300 K;1 kHz) are between 16.25 and 18.83, with the exception of the 550 °C HT sample that presents a ε′ value of 11.25. An electric equivalent circuit composed by an R in parallel with a CPE element was used to adjust the dielectric data. The results reflect the important role carried out by the heat-treatment and the electric field during the HT in the electric properties of glass-ceramics.     

Synthesis of silica aerogel blanket by ambient drying method using water glass based precursor and glass wool modified by alumina sol

Silica aerogel blankets have been synthesized by ambient drying technique using cheap water glass as the silica source and glass wool modified by alumina sol. One step solvent exchange and surface modification were simultaneously conducted by immersing the wet hydrogel blanket in EtOH/TMCS/hexane solution. The synthesized silica aerogel blanket was light with the density of 0.143-0.104 g/cm³ and 89.4-95% porosity. The microstructure of silica aerogel blanket exhibits a porous structure consisting of glass fibers of diameter ∼2.5 μm interconnected with solid silica clusters (5-20 μm).     

Interfacial reactions between tellurite melts and silica during the production of microstructured optical devices

Interfacial reactions between silica glass and tellurite melts were studied under confined conditions in the temperature regime of 400-700 °C, applying two different sampling techniques: isothermal heat-treatment of a several micrometer thick tellurite film, confined in a silica/tellurite/silica sandwich, and capillary filling of tellurite melts into silica microcapillaries. The sandwich technique provides detailed ex situ insights on the interface chemistry, microstructure and diffusion after given treatment times and temperatures. Data on dynamic viscosity, surface tension, wetting behaviour and eventual scaling effects was obtained from the capillary filling technique. For temperatures > 500 °C, silica is completely wet by the considered tellurite melts. At T > 600 °C and for a treatment time of 20 min or longer, cationic diffusion of Na⁺ and Te⁴⁺ into the silica substrate occurs to a depth of several micrometers. At the same time, the tellurite melt attacks the silica surface, leading to the formation of a stationary silica-tellurite reaction layer and silica dissolution. Dissolved silica was observed to re-precipitate from the tellurite melt by liquid-liquid phase separation. In the early reaction stages, as a result of alkali diffusion into the silica substrate, β-quartz crystallizes at the interface (what can be avoided by using alkali-free filling glasses). Obtained data set the boundary conditions for the generation of tellurite-silica all-solid fiber waveguides by melt infiltration of silica photonic crystal fibers or microcapillaries.