Showing papers in "Journal of Non-crystalline Solids in 2006"

Ionic amorphous oxide semiconductors: Material design, carrier transport, and device application

Abstract: Recently we have reported the room temperature fabrication of transparent and flexible thin film transistors on a polyethylene terephthalate (PET) film substrate using an ionic amorphous oxide semiconductor (IAOS) in an In2O3–ZnO–Ga2O3 system. These transistors exhibit a field effect mobility of ∼10 cm2 (V s)−1, which is higher by an order of magnitude than those of hydrogenated amorphous Si and pentacene transistors. This article describes a chemical design concept of IAOS, and its unique electron transport properties, and electronic structure, by comparing them with those of conventional amorphous semiconductors. High potential of IAOS for flexible electronics is addressed.

Homogeneous crystal nucleation in silicate glasses: A 40 years perspective

Abstract: We review a plethora of relevant experimental results on internal homogeneous crystal nucleation in silicate glasses obtained in the last four decades, and their analyses in the framework of the classical nucleation theory (CNT). The basic assumptions and equations of CNT are outlined. Particular attention is devoted to the analysis of the properties of the critical nuclei, which, to a large extent, govern nucleation kinetics. The main methods employed to measure nucleation rates are described and the possible errors in the determination of the crystal number density (and, correspondingly, in nucleation rates) are discussed. The basic regularities of both time and temperature dependencies of nucleation rates are illustrated by numerous experimental data. Experimental evidence for a correlation between maximum nucleation rates and reduced glass transition temperatures is presented and theoretically justified. Special attention is given to serious problems that arise in the quantitative description of nucleation rates when using the CNT, for instance: the dramatic discrepancy between calculated and measured nucleation rates; the high value of the crystal nuclei/melt surface energy, σcm, if compared to the expected value estimated via Stefan’s rule; the increase of σcm with increasing temperature; and the discrepancies between the values of the surface energy and the time-lag for nucleation when independently estimated from nucleation and growth kinetics. The analysis of the above mentioned problems leads to the following conclusion: in contrast to Gibbs’ description of heterogeneous systems underlying CNT, the bulk thermodynamic properties of the critical nuclei generally differ from those of the corresponding macro-phase resulting simultaneously in significant differences of the surface properties as compared with the respective parameters of the planar interfaces. In particular, direct experimental evidence is presented for compositional changes of the crystal nuclei during formation of the critical nuclei and their growth from critical to macro-sizes. In addition, detailed examinations of crystal nucleation and growth kinetics show a decrease of both the thermodynamic driving force for nucleation and of the critical nuclei/liquid interfacial energy, as compared with the respective properties of the macro-phase. However, despite significant progress in understanding crystal nucleation in glasses in the past four decades, many problems still exist and this is likely to remain a highly interesting subject for both fundamental and applied research for a long time.

Correlation between dynamic fragility and glass transition temperature for different classes of glass forming liquids

Abstract: Here we compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers. Our analysis of the data shows that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and Tg, a correlation not previously examined. For example, for hydrogen bonding organics, polymeric and metallic glass formers, there is an approximately linear increase in m with increasing Tg. While for inorganic glass formers, m appears almost independent of Tg, remaining nearly constant over a wide range in Tg. At the same time, another important parameter, the apparent activation energy Eg at Tg has been investigated. It was found that Eg increases with Tg to the 2nd power for hydrogen bonding organics, polymeric and metallic glass forming liquids, while Eg of inorganic glasses has a linear dependence on Tg.

Thermal evolution of metakaolin geopolymers: Part 1 – Physical evolution

Abstract: The physical evolution of materials during heating is a critical factor in determining their suitability and performance for applications ranging from construction to refractories and adhesives. The effect of different cations (sodium and potassium) on the physical evolution of geopolymeric materials derived from metakaolin is investigated for a range of specimens with Si/Al ratios between 1.15 and 2.15. It is observed that the effect of potassium is to reduce the thermal shrinkage, while thermal shrinkage increases with increasing Si/Al ratio in the presence of each alkali type. The thermal shrinkage behavior of mixed-alkali specimens is observed to change from a mean of the sodium and potassium specimens at low Si/Al ratio to behave similarly to sodium specimens at high Si/Al ratios. It is clear from this investigation that alkali cations only have a significant effect on thermal shrinkage of geopolymer at low Si/Al ratios (⩽1.65), while both Si/Al ratio and alkali cation have little effect on the extent of thermal shrinkage at Si/Al ⩾ 1.65.

Sub-surface mechanical damage distributions during grinding of fused silica

Abstract: The distribution and characteristics of surface cracking (i.e., sub-surface damage or SSD) formed during standard grinding processes have been measured on fused silica glass using a surface taper polishing technique. The measured SSD depth distributions are described by a single exponential followed by an asymptotic cutoff in depth. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The length of the trailing indent is strongly correlated with a given grinding process. It is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and most likely it is the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth increased with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related to the SSD length distribution to gain insight into ‘effective’ size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth. These relationships can serve as useful rules-of-thumb for non-destructively estimating SSD depth and for identifying the process that caused the SSD.

Influence of the semiconductor thickness on the electrical properties of transparent TFTs based on indium zinc oxide

Abstract: Multicomponent amorphous oxides are starting to emerge as a class of appealing semiconductor materials for application in transparent electronics. In this work, a high performance bottom-gate n-type transparent thin-film transistors are reported, being the discussion primarily focused on the influence of the indium zinc oxide active layer thickness on the properties of the devices. For this purpose, transparent transistors with active layer thicknesses ranging from 15 nm to 60 nm were produced at room temperature using rf magnetron sputtering. Optical transmittance data in the visible range reveals average transmittance higher than 80%, including the glass substrate. The devices work in the enhancement mode and exhibit excellent saturation drain currents. On–off ratios above 107 are achieved, but this value tends to be lower for devices with thicker semiconductor films, as a result of the decrease in the resistance of the channel region with increasing thickness. Channel mobilities are also quite respectable, with some devices presenting values around 40 cm2/V s, even without any annealing or other post-deposition improvement processes. Concerning the evolution of threshold voltage with the thickness, this work shows that it increases from about 3 V in thicker films up to about 10 V in the thinnest ones. The interesting electrical properties obtained and the versatility arising from the fact that it is possible to modify them changing only the thickness of the semiconductor makes this new transparent transistors quite promising for future transparent ICs.

Bioactive glasses in the system CaO–B2O3–P2O5: Preparation, structural study and in vitro evaluation

Abstract: Glasses in the system x B2O3(1 − x) [y CaO P2O5], (x = 0, 0.1, 0.2, 0.3, y = 2, 2.6, 3, 4, 5) have been prepared by fast quenching of high temperature melts. The presence of B2O3 affected the glass forming ability, allowing the preparation of calcium phosphate glasses with y ⩾ 2.6. The structure of glasses was analyzed by μ-Raman and infrared spectroscopy. The analysis indicated that the glass network is dominated by highly charged species from phosphate tetrahedra with 3 (pyro) or 4 (ortho) NBOs, while the boron atoms are incorporated mainly in 3 coordinated sites in the form of B∅3 or B∅2O− units. A small fraction of B ∅ 4 - units was also evident from the spectra analysis of glasses with high CaO content. All calcium borophosphate glasses exhibited bioactivity after soaking in SBF solution within a few days. This was observed by μ-Raman and SEM microscopy, while XRD patterns clearly revealed growth of hydroxyapatite phase. The presence of boron in the glass network has a catalytic effect at favoring bioactivity of the otherwise bioinert calcium phosphate glasses.

Processing and characterization of a novel nonporous poly(vinilidene fluoride) films in the β phase

Abstract: Poly(vinylidene fluoride) (PVDF) has remarkable properties leading to electro-optics, electro-mechanical and biomedical applications. In particular, its piezo- and pyroelectric properties provide possibilities for many technological applications. The semicrystalline nature of PVDF, combined with the occurrence of at least four crystalline phases implies a complicated physical microstructure. The most frequently described and important phase is the β phase. The piezo- and pyroelectric properties mainly depend on this phase, so increasing the β phase content has always been a great concern. It is possible to obtain films in the β phase by solution but this material presents a high porosity leading to an opaque appearance and a decrease of the mechanical and electrical properties. In this work, porous films in the β phase were obtained directly from the solution at 60 °C. After applying pressure perpendicular to the surface of the film at elevated temperature, the pores in the original sample are eliminated. The changes on the morphology and crystallinity associated to the pressure treatment were also studied.

Studies of crystal phase formations in high-strength lithium disilicate glass–ceramics

Abstract: The objective of the study was to analyze the nucleation, primary phase formation and solid state reaction to form lithium disilicate glass–ceramics derived from the SiO 2 –Li 2 O–Al 2 O 3 –K 2 O–ZrO 2 –P 2 O 5 system. The concentration of P 2 O 5 was increased from zero up to 3.2 wt%. Thermal analysis, scanning electron microscopy and X-ray diffraction were used to characterize the microstructure formation, the nucleation process and the solid state reaction of the crystal phase precipitation in the glass–ceramics. Additives of P 2 O 5 allowed the control of bulk crystallization. Nucleation was catalyzed by nano-scaled Li 3 PO 4 phases, visualized by HR-SEM. Li 3 PO 4 reacts most probably as the heterogeneous catalyst, acting by epitaxy, of both Li 2 SiO 3 and Li 2 Si 2 O 5 crystals. Based on the discussion of the main results, the authors deduced a four-step reaction mechanism. This mechanism demonstrated that after nucleation of lithium metasilicate and lithium disilicate, the latter phase grows as agglomerated nanocrystals, but remained in a relative small amount. By contrast, lithium metasilicate grows rapidly and decomposes at 780–820 °C with the result of a drastic increase of lithium disilicate phase. This was a result of a solid state reaction with the SiO 2 -rich glassy phase. In a parallel reaction, cristobalite was formed as a preliminary phase. The final product of a glass–ceramic with 3.2 wt% P 2 O 5 shows a highly crystalline interlocking microstructure demonstrating a high-strength of 726 ± 63 MPa and translucency.

Synthesis and characterization of ilmenite NiTiO3 and CoTiO3 prepared by a modified Pechini method

Abstract: Powders of ilmenite structure NiTiO 3 and CoTiO 3 were prepared by a simple method based on the modified Pechini process. The raw compounds and citric acid (CA) were mixed in ethanol (EA) with the molar ratio Ni(Co)/Ti/CA/EA = 1/1/1/7.5. The DTA curve shows exothermic peaks only around 300–350 °C and 600 °C, which correspond to the decomposition of the organic compound and direct crystallization of the ilmenite phase. X-ray diffraction patterns indicated that the ilmenite phase was successfully synthesized as the Ni(Co)-Ti precursor calcined above 600–900 °C for 3 h, and the activation energies of NiTiO 3 and CoTiO 3 were calculated to be about 8.84 and 13.23 kJ/mol. TEM bright field images showed that the grain sizes of powders of NiTiO 3 and CoTiO 3 at 600–900 °C were estimated to be about 10–250 and 20–200 nm depending on the nature of the aggregate. The samples of NiTiO 3 calcined at 600–800 °C have the larger specific surface area of 31.51, 18.78, and 6.01 m 2 /g, respectively. The UV-Vis diffuse reflectance spectra show the optical band gaps of NiTiO 3 and CoTiO 3 as 3.02 and 2.43 eV.

Comparative study of ITO layers deposited by DC and RF magnetron sputtering at room temperature

Abstract: Two different magnetron sputtering techniques with RF and DC plasma discharge modes were tested for room temperature deposition of ITO layers from In2O3:SnO2 targets (10 wt% SnO2). The influence of sputtering conditions (mainly the sputtering power and oxygen content in the Ar–O2 gas mixture) on the transparency and conductivity of the formed layers was investigated. The ITO films obtained both by RF and DC magnetron sputtering at room temperature have a high transparency in the visible wavelength range (80–85%) and a low surface resistance (20–25 X/sq for � 300 nm thickness) but their optimized deposition conditions and structural properties are significantly different. The DC sputtering can be performed in pure Ar with a lower discharge power density (1.0 W/cm 2 ) and relatively high deposition rate (� 60 nm/min) while the RF sputtering requires Ar–O2 (3.0 vol.% O2) gas mixture, higher discharge power density (1.5 W/cm 2 ) and yields a lower deposition rate (� 20 nm/min). The RF sputtered ITO layers show a crystalline structure with strong (2 2 2), (4 0 0), (4 4 0), (6 2 2) X-ray diffraction peak intensities while the DC layers are amorphous with surface morphology formed by nano-scale grains.

Structural analysis and devitrification of glasses based on the CaO–MgO–SiO2 system with B2O3, Na2O, CaF2 and P2O5 additives

Abstract: Glasses, whose basic composition was based on the CaO–MgO–SiO 2 system and doped with B 2 O 3 , P 2 O 5 , Na 2 O, and CaF 2 , were prepared by melting at 1400 °C for 1 h. Raman and infrared (IR) spectroscopy revealed that the main structural units in the glass network were predominantly Q 1 and Q 2 silicate species. The presence of phosphate and borate units in the structure of the glasses was also evident in these spectra. X-ray analysis showed that the investigated glasses devitrified at 750 °C and higher temperatures. The crystalline phases of diopside and wollastonite dominated, but weak peaks, assigned to akermanite and fluorapatite, were also registered in the diffractograms. The presence of B 2 O 3 , Na 2 O, and CaF 2 had a negligible influence on the assemblage of the crystallized phases, but it caused a reduction of crystallization temperature, comparing to similar glasses of the CaO–MgO–SiO 2 system.

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

Abstract: The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.

Influence of the melting conditions of heavy metal oxide glasses containing bismuth oxide on their optical absorption

Abstract: Glasses of the systems Bi2O3–SiO2, Bi2O3–PbO–Ga2O3, Bi2O3–PbO–Ga2O3–GeO2 and Bi2O3–GeO2–Li2O have been prepared and the interaction of their melts with crucibles of different materials has been analytically determined. Silica and porcelain crucibles were very strongly corroded and the glass composition was noticeably altered. Instead platinum crucibles are not affected if the Bi2O3 content is not too high. The color of the glasses changes in all cases from pale yellow to deep brown when the melting temperature reaches approximately 1000 °C. The higher the temperature and the Bi2O3 content the darker the brown color, independently of the nature of the employed crucible. The addition of oxidizing ions (Sb5+, As5+ or Ce4+) to the glass batch prevents darkening. Nanoparticles of elementary bismuth Bi0 are observed by transmission electron microscopy in the glasses melted above 1000 °C. The partial thermoreduction of the Bi2O3 during the heating of the glass melt is proposed as the mechanism responsible for the observed darkening.

Thermal lens and Z-scan measurements: Thermal and optical properties of laser glasses – A review

Abstract: In this paper, the application of thermal lens and Z-scan techniques to the study of the thermo-optical and spectroscopic properties of solid-state laser glasses is described. The theoretical basis for quantitative measurements of thermal diffusivity and conductivity, temperature coefficient of the optical path length change, heat efficiency, fluorescence quantum efficiency, losses mechanisms (Auger upconversion and concentration quenching) and the line shape of the nonlinear refractive index are presented and discussed. The electronic contribution to the nonlinearity was investigated using the Z-scan technique in the time-resolved mode. The measurements were performed spectroscopically, allowing the determination of the line shapes of real and imaginary parts of the nonlinear refractive index, n2, in resonance with laser transitions. The results were interpreted by considering resonant and nonresonant contribution to n2. The magnitude of electronic and thermal contributions to the refractive index changes in solid-state laser glasses were also compared, and the thermal properties as a function of the temperature in the range of 20 up to 600 K are presented.

Compositional dependence of the first sharp diffraction peaks in alkali silicate glasses: A molecular dynamics study

Abstract: The compositional dependence of the first sharp diffraction peaks (FSDPs) of lithium, sodium and potassium silicate glasses were studied by calculating the neutron static structure factors. The advantage of using molecular dynamics simulations is that the contributions of partial structure factors to the FSDPs can be easily determined and provides the basis for detailed analysis. Examination of the correlations of the FSDP with the short and medium range structure reveal that the position and the shape of FSDP strongly depend on the type and concentration of alkali oxide in alkali silicate glasses. The characteristic repeating distances and the characteristic correlation lengths both decrease with increasing alkali oxide concentration indicating a decrease in the intermediate range order. In the lithium silicate glasses, the characteristic correlation length increases with lithium oxide concentration that is anomalous in that the trend is opposite to the other alkali silicate glasses. This anomaly is explained by the high field strength of lithium ions that increases the intermediate range order of the silicon oxygen network.

Thin chalcogenide films prepared by pulsed laser deposition – new amorphous materials applicable in optoelectronics and chemical sensors

Abstract: Principles, advantages, applications and drawbacks of pulsed laser deposition (PLD) technique for thin films preparation are reviewed. The PLD method is promising for preparation of thin films of complex composition, including rare-earth and Ag-doped chalcogenide films; all components of the target can be evaporated at once. Low volatility and refractory materials can be also deposited. The deposition in vacuum, inert or reactive atmosphere is possible. Results obtained in a study of chalcogenide films are discussed and the current state-of-the-art is reviewed. The composition and structure of PLD films can be different from thermally evaporated films and new materials or materials with new properties applicable in optics and optoelectronics can be prepared. The method can be used for fabrication of different chalcogenide-based sensors and memory materials of complex composition. Photoinduced changes of structure and properties of PLD films and chalcogenides exposed to intense laser pulses are also discussed. The intense laser pulses can change the properties of the materials prepared and can be used for fabrication of chalcogenide-based waveguides, diffractive elements and high-density optical data storage media.

Mass transport in chalcogenide electrolyte films - materials and applications

Abstract: Certain metals can be added to thin films of chalcogenide glasses by photodissolution to create materials with unique morphology and properties. When Ag is combined in this fashion with Ge–Se or Ge–S glasses, the resulting ternary contains a dispersed nanocrystalline Ag2S(e) phase that has large quantities of mobile metal ions. The presence of these ions allows the ternaries to act as solid electrolytes. If an anode which has an oxidizable form of the ionic metal and an inert cathode are applied in contact with such a phase-separated electrolyte, an ion current can flow under an applied bias in excess of a few hundred millivolt. Electrons from the cathode reduce the excess metal due to the ion flux and an electrodeposit forms on or in the electrolyte. Utilizing this effect, we developed programmable metallization cell (PMC) technology which offers new functionality for such materials. Based on mass transport driven by electrochemical processes, PMC technology may be applied in solid state electronics, integrated optics, microelectromechanical systems (MEMS), and microfluidics. This paper is a review of the unique materials aspects of thin film solid electrolytes formed by photodissolution of metal into a chalcogenide base glass and the demonstrated applications of this technology.

Ternary chalcogenides LiBC2 (B = In, Ga; C = S, Se, Te) for mid-IR nonlinear optics

Abstract: We review on the growth improvement and optical characterization of a new family of ternary lithium-based chalcogenide crystals of generic formula LiB III C 2 VI (B = In, Ga; C = S, Se, Te) which displays improved thermo-mechanical properties for mid-IR nonlinear optical applications. Some of these compounds are now produced in sufficiently large size, single-domain quality to allow their implementation in optical parametric oscillators.

Self-trapped holes in pure-silica glass: A history of their discovery and characterization and an example of their critical significance to industry

Abstract: It has long been assumed that hole self-trapping should take place in amorphous silicon dioxide (a-SiO 2 ). However, no spectroscopic evidence for this was claimed before 1989, when the author used electron spin resonance (ESR) to identify self-trapped holes (STHs) in bulk samples of low-OH pure fused silica X irradiated ⩽100 K, and Chernov et al. reported a low-temperature infrared absorption near 1600 nm in irradiated pure-silica-core fibers, which they ascribed to STHs. Based on g values and 29 Si and 17 O hyperfine coupling constants measured by ESR, Griscom [D.L. Griscom, Phys. Rev. B 40 (1989) 4224; D.L. Griscom, J. Non-Cryst. Solids 149 (1992) 137] deduced the existence of two types of STHs: STH 1 (a hole trapped on a single bridging oxygen) and STH 2 (a hole delocalized over two equivalent bridging oxygens of the same SiO 4 tetrahedron). The validity of Griscom’s models for STH 1 and STH 2 are supported by the ab initio calculations of Pacchioni and Basile [G. Pacchioni, A. Basile, Phys. Rev. B 60 (1999) 9990] and Gabriel [M.A. Gabriel, PhD dissertation, Department of Chemistry, University of Washington, Seattle, WA, in preparation]. In 1984, Nagasawa et al. reported that low-OH-pure-silica-core optical fibers γ irradiated at ∼300 K exhibit metastable optical absorption bands at 660 and 760 nm. Griscom [D.L. Griscom, J. Non-Cryst. Solids 349 (2004) 139] recorded these same bands in low-OH-pure-silica-core fibers γ irradiated at 77 K, showing their isochronal annealing behaviors to correlate with his earlier ESR data for STHs in bulk silica and also with Harari et al.’s data [E. Harari, S. Wang, B.S.H. Royce, J. Appl. Phys. 46 (1975) 1310] for trapped positive charges in silica thin films following X irradiation at 77 K. Sasajima and Tanimura [Y. Sasajima, K. Tanimura, Phys. Rev. B 68 (2003) 014204] established direct correlations of an induced band at 574 nm with STH 2 by performing both ESR and a variety of optical measurements on three types of high-purity bulk silicas following pulsed electron irradiations at 77 K; however, these authors did not detect the 660 or 760 nm bands. Yamaguchi et al. [M. Yamaguchi, K. Saito, A.J. Ikushima, Phys. Rev. B 68 (2003) 153204] demonstrated that the yield of ESR-detected STHs photoinduced in bulk pure-silica samples at 77 K depends exponentially on fictive temperature ( T f ). The present paper recounts the forgoing history in greater detail, while attempting to reconcile some seemingly disparate findings into a unified picture of STHs in silica. In the fall of 1998, a number of satellites in orbit were tumbling out of control due to failure of their HeNe ring-laser-gyro (RLG) attitude control systems. The author, acting in the capacity of pro-bono US government consultant, proposed the correct solution to this problem (replace Al-contaminated silica mirror coatings with high-purity ones). However, accelerated tests (short operation times at much-higher-than-normal laser powers) of the corrected devices failed to corroborate this fix. Thus, all further launches of satellites employing these RLGs remained grounded until the author was able to convince industry troubleshooters that (1) the accelerated test failures were due to the 660-nm STH band (which is induced even in the highest-purity silica coatings by 20-eV photons emitted by the laser plasma), (2) the strength of this band is initially proportional to ionizing dose rate ( inevitably giving false positives in accelerated tests ), and (3) this band eventually disappears after several months of irradiation, even at dose rates as low as 0.15 Gy/s. All of these insights derived from curiosity-driven components of the author’s research.

Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses

Abstract: The morphology and microstructure induced in high quality fused silica by UV (355 nm) laser pulses at high fluence (10–45 J/cm 2 ) have been investigated using a suite of microscopic and spectroscopic tools. The laser beam has a near-Gaussian profile with a 1/e 2 diameter of� 0.98 mm at the sample plane and a pulse length FWHM (full width at half maximum) of 7.5 ns. The damage craters consist of a molten core region (thermal explosion), surrounded by a near concentric region of fractured material. The latter arises from propagation of lateral cracks induced by the laser-generated shock waves, which also compact the crater wall,� 10 lm thick and� 20% higher in density. The size of the damage crater varies with laser fluence, number of pulses, and laser irradiation history. In the compaction layer, there is no detectable change in the Si/O stoichiometry to within ±1.6% and no crystalline nano-particles of Si were observed. Micro(1–10 lm) and nano- (20–200 nm) cracks are found, however. A lower valence Si 3+ species on the top 2–3 nm of the compaction layer is evident from the Si 2p XPS. The results are used to construct a physical model of the damage crater and to gain critical insight into laser damage process. � 2005 Elsevier B.V. All rights reserved. PACS: 61.80.Ba; 61.72.Ji; 78.60.Hk; 76.30.Mi

Electrical, structural and optical properties of SnO2 thin films prepared by spray pyrolysis

Abstract: This study investigated the effect of the substrate temperature on the structural, optical, morphological, and electrical properties of undoped SnO 2 films prepared by a spray deposition method. The films were deposited at various substrate temperatures ranging from 300–500 °C in steps of 50 °C and characterized by different optical and structural techniques. X-ray diffraction studies showed that the crystallite size and preferential growth directions of the films were dependent on the substrate temperature. These studies also indicated that the films were amorphous at 300 °C and polycrystalline at the other substrate temperatures used. Infrared and visible spectroscopic studies revealed that a strong vibration band, characteristic of the SnO 2 stretching mode, was present around 630 cm −1 and that the optical transmittance in the visible region varied over the range 75–95% with substrate temperature, respectively. The films deposited at 400 °C exhibited the highest electrical conductivity property.

Non-Langevin bimolecular recombination in low-mobility materials

Abstract: We have investigated the bimolecular recombination coefficient ( B ) of charge carriers in low-mobility materials, in which the Langevin recombination is reduced: inorganic a-Si:H, μc-Si:H and the organic regioregular poly(3-hexylthiophene)/1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene (RR-PHT/PCBM) blend. We have been using a multitude of experimental techniques, namely space-charge-limited current (SCLC), photogenerated charge carrier extraction by linearly increasing voltage (photo-CELIV) and double injection (DoI) current transient techniques for investigation of temperature and electric field dependencies of B . For RR-PHT/PCBM blends, we observed a weak dependence of B on electric field and the most significant reduction of Langevin recombination.

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol-formaldehyde gels

Abstract: Resorcinol–formaldehyde gels were produced at 50, 70 and 90 °C and with three different R / C ratios (500, 1000 and 2000). The effect of these variables combined with that of aging time was studied in order to optimize the synthesis conditions. The convective air-drying process was used, and the drying duration was studied with regard to the synthesis conditions. The aging time has no effect on the pore texture after 24 h at 90 °C or 48 h at 70 °C, whatever the R / C value. The synthesis-aging step can be shortened by increasing the temperature. Nevertheless, the pore size tends then to decrease, especially when R / C is high, but this can be counterbalanced by increasing R / C . Moreover, bubbles often appear in the gel at high synthesis temperature, which limits the temperature to about 70 °C in the case of monolithic parts. At 70 °C and with an air velocity of 2 m/s, the elimination of 90% of the solvent requires 1 h drying when the pore size reaches 400–600 nm, 2.5 h for 50 nm wide pores and 3 h when the pore size decreases to 15–20 nm. The drying duration does not exceed 8 h in all cases and could be shortened by increasing the temperature at the end of the process.

Optical properties of Tm3+ ions in alkali germanate glass

Abstract: Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 m diodes because of low phonon energies. Spectroscopic analysis indicates low nonradiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.

Effect of UV and visible light radiation on the electrical performances of transparent TFTs based on amorphous indium zinc oxide

Abstract: Insensitivity to light irradiation is desirable for conventional applications of thin-film transistors, i.e., the active matrices of displays. However, if one produces a device presenting controlled sensitivity to light, many other applications can benefit or can even be created. In this work it is shown the influence of the photon energy on the optoelectronic properties presented by n-type bottom-gate thin-film transistors based on indium zinc oxide. In the dark, the devices present very good electrical properties, working in the enhancement mode, exhibiting on–off ratios higher than 10 7 and channel mobility above 30 cm 2 /V s. Remarkable results were achieved when the devices were exposed to light radiation, the most striking one is the possibility to switch between enhancement (in the dark) and depletion (illuminated) modes, with different threshold voltages and on/off ratios, function of the light power density and wavelength used. This type of behavior permits to design circuits where one can have the same transistor working either in the enhancement or depletion modes, function of the light beam and intensity impinging on it, highly important for short wavelength detector applications.

Hydrogen–sodium interdiffusion in borosilicate glasses investigated from first principles

Abstract: Density functional calculations are performed to investigate the effect of hydronium–sodium interdiffusion in a sodium borosilicate glass. It is found that the substitution of the Na+ network modifiers does not alter the glass covalent network, whereas a bridging bond is systematically broken when the substitution involves Na+ ions bonded to BO4 tetrahedra. The systematic dissociation of hydronium into water and proton suggests that this chemical species probably has a short lifetime in the glass, due to the capability of anionic sites to trap the protons very strongly. The relative basicities of the sites involved are compared, and a nice correlation is found between hydroxyl bond lengths and hydrogen bond strengths present in the hydrated glass.

Elastic models for the non-Arrhenius viscosity of glass-forming liquids

Abstract: This paper first reviews the shoving model for the non-Arrhenius viscosity of viscous liquids. According to this model the main contribution to the activation energy of a flow event is the energy needed for molecules to shove aside the surrounding, an energy which is proportional to the instantaneous shear modulus of the liquid. Data are presented supporting the model. It is shown that the fractional Debye–Stokes–Einstein relation, which quantitatively expresses the frequently observed decoupling of, e.g., conductivity from viscous flow, may be understood within the model. The paper goes on to review several related explanations for the non-Arrhenius viscosity. Most of these are also ‘elastic models’, i.e., they express the viscosity activation energy in terms of short-time elastic properties of the liquid. Finally, two alternative arguments for elastic models are given, a general solid-state defect argument and an Occam’s razor type argument.

Materials for high-power fiber lasers

Abstract: The absorption and emission properties of silica based ytterbium doped preforms, made by Modified Chemical Vapor Deposition, and of the drawn fibers were investigated as a function of the atmosphere during the preform collapsing. Under increasingly reducing conditions for the collapsing atmosphere, a strong UV/VIS absorption is built up, accompanied with an UV excited emission in the visible region, obviously induced by the formation of Yb2+ ions. Both the fluorescence lifetime of the Yb3+ lasing level and the cooperative fluorescence in the visible are remarkably diminished. Laser power experiments with the different fibers show a strong deterioration of the laser efficiency parallel to the degree of reduction of the doped glasses.

Cooperative and non-cooperative dynamics in ultra-thin films of polystyrene studied by dielectric spectroscopy and capacitive dilatometry

Abstract: The effect of thickness reductions on the glass transition dynamics in ultrathin films of polystyrene has been studied by dielectric spectroscopy (DS) and capacitive dilatometry (CD). Upon reduction of the film thickness, a systematic decrease in the dilatometric glass transition temperatures, T g (dil), was observed via CD, while DS revealed a continuous speed-up and broadening of the α-process, accompanied by only minor reductions in the fragility index. A good agreement between ‘spectroscopic’ and the dilatometric glass transition temperatures was found for films thicker than 20 nm, while for thinner films both quantities diverge increasingly. A likely explanation for this discrepancy is the presence of another dynamic process showing Arrhenius-behavior ( E a ∼ 72 kJ/mol) with a pre-exponential factor of 10 −12 s being indicative for non-cooperative dynamics. Such a new process might be assigned to distinct surface dynamics in polystyrene films as suggested in recent papers.