Absorption and luminescence in amorphous SixGe1-xO2 films fabricated by SPCVD

Optical absorption and photoluminescence of Ge-doped silica films fabricated by the surface-plasma chemical vapor deposition (SPCVD) are studied in the 2–8 eV spectral band. The deposited on silica substrate films of about 10 μm in thickness are composed as x·GeO₂-(1-x)·SiO₂ with x ranging from 0.02 to 1. It is found that all as‐deposited films do not luminesce under the excitation by a KrF (5 eV) excimer laser, thus indicating lack of oxygen deficient centers (ODCs) in them. After subsequent fusion of silicon containing (x < 1) films by a scanning focused CO₂ laser beam absorption band centered at 5 eV as well as two luminescence bands centered at blue (3.1 eV) and UV (4.3 eV) wavelengths arise, highlighting the formation of the ODCs. The excitation of unfused SPCVD films by an ArF (6.4 eV) excimer laser yields a luminescence spectrum with two bands typical for the ODCs, but with a faster decay kinetics. Intensities of these bands grow up with samples cooling down to a temperature of 80–60 K. Unfused films excited by the ArF laser also demonstrate luminescence due to recombination of a trapped charge resulted from the excitation of localized electron states of the glass network. In the unfused GeO₂ film luminescence related to a self-trapped exciton (STE) typical for GeO₂ crystals with α-quartz structure is observed. The observed STE luminescence can be indicative of the crystalline fraction availability in the film. Whereas GeO₂ crystals are known as not containing twofold coordinated germanium, a polycrystalline inclusion in the SPCVD GeO₂ film serves as a factor explaining the absence of any spectroscopic manifestation of this type of defects in it even after fusion. On the other hand, lack of STE luminescence in other unfused films with x < 1 testifies truly amorphous state of the matter in them.     

Silica nano-rings and nano-hollows: Preparation and UV photoluminescence emission

By annealing fused silica coated with ultra-thin Ag film, silica nano-rings and nano-hollows were prepared on the substrate. The Ag nano-particles attached on the wall of nano-hollows or embedded in silica were confirmed with energy dispersive spectroscopy and transmission electron microscopy. Besides the well-known characteristic stretching bands of silica, three novel stretching bands around 1579, 1320 and 270 cm^?1 were found in the annealed Ag-coated silica by Raman scattering spectroscopy, which have been attributed to the O₂ in ground state, O–O and metal–oxygen stretching bands, respectively. The formation mechanism of nano-rings and nano-hollows has been discussed based on the experimental results. An ultraviolet photoluminescence emission of 360–370 nm from annealed Ag-coated silica was found when the excitations were 230 nm and 280 nm or longer. The possible photoluminescence emission mechanism has been discussed, which suggests that oxygen excess defects are responsible for the photoluminescence emission, and photoexcitation occurs in the silica as well as in Ag⁺ ions.     

Fabrication of buried waveguides in planar silica films using a direct CW laser writing technique

A CW CO₂ laser ablation technique is used to form buried waveguides in planar silica films. It is shown that the refractive index of a silica thin film is reduced sufficiently adjacent to the laser processed region to allow the fabrication of low loss waveguides. The refractive index distribution of these structures is measured using the reflectance of a focussed spot from the surface of the films. The change in refractive index is measured to be of the order of the core cladding refractive index difference of typical single mode waveguides. The spatial resolution of the reflectance technique is 1.3 μm with a refractive index resolution of ±5 × 10^?4. Devices such as 1 × 2 and 1 × 4 multi-mode interference (MMI) splitters have also been demonstrated and shown to exhibit low transmission losses.     

Erbium- and ytterbium-doped sol–gel SiO2–HfO2 crack-free thick films onto silica on silicon substrate

Aware of the difficulties in applying sol–gel technology on the preparation of thin films suitable for optical devices, the present paper reports on the preparation of crack-free erbium- and ytterbium-doped silica: hafnia thick films onto silica on silicon. The film was obtained using a dispersion of silica-hafnia nanoparticles into a binder solution, spin-coating, regular thermal process and rapid thermal process. The used methodology has allowed a significant increase of the film thickness. Based on the presented results good optical-quality films with the required thickness for a fiber matching single mode waveguide were obtained using the erbium- and ytterbium-activated sol–gel silica:hafnia system. The prepared film supports two transversal electric modes at 1550 nm and the difference between the transversal electric mode and the transversal magnetic mode is very small, indicating low birefringence. Photoluminescence of the ⁴I_13/2 → ⁴I_15/2 transition of erbium ions shows a broad band centered at 1.53 μm with full width at a half maximum of 28 nm. Up-conversion emission was carried out under different pump laser powers, and just one transition at red region was observed.     

XPS study of pulsed Nd:YAG laser oxidized Si

X-ray photoelectron spectra (XPS) of thin SiO₂ layers grown by pulsed Nd:YAG laser at a substrate temperature of 748 K are presented. The peak decomposition technique combined with depth profiling is employed to identify the composition and chemical states of the film structure. It is established that the oxide is non-stoichiometric, and contains all oxidation states of Si in different amounts throughout the film. The interface Si/laser-grown oxide is not abrupt, and the coexistence of Si₂O₃ and Si₂O suboxides in a relatively wide interfacial region is found. It is concluded that post-oxidation annealing is necessary in order to improve the microstructure of both oxide and near interface region.     

Excimer laser crystallization of microcrystalline silicon for TFTs on flexible substrate

Microcrystalline silicon (μc-Si) films have been deposited on PDMS as well as on PEN substrate. Excimer laser annealing was used to improve the crystalline structure and so to obtain high mobility TFTs. The effect of the laser annealing on the crystalline structure of silicon films is studied using different characterization techniques and discussed. Mobility values of 60 cm²/V s with PDMS and 46 cm²/V s with PEN are obtained.     

Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis from zinc acetate (Zn(CH₃COO)₂ 2H₂O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min⁻¹ can be achieved at T_s=543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.     

Characteristics of ITO films fabricated on glass substrates by high intensity pulsed ion beam method

Transparent conductive oxides such as indium tin oxide (ITO) are interesting materials due to their wide-band gaps, high visible light transmittance, high infrared reflectance, high electrical conductivity, hardness and chemical inertness. ITO films were fabricated on soda lime glass substrates by using high-intensity pulsed ion beam (HIPIB) technique. The as-deposited films comprised of partially crystallized In₂O₃ and after annealing at 500 °C for 1 h the film changed to polycrystalline phase. After annealing carrier concentration and Hall mobility increased while specific resistance and sheet resistance decreased quickly; and this trend was also observed when film thickness increased up to 300 nm for the post-annealed samples. Further increase in thickness of the film changed the electrical properties slightly. Atomic force microscopy (AFM) revealed that roughness decreased after 500 °C annealing for 1 h in air, except for the film of 65 nm thick. The thickness of the film which relates to the carrier concentration and mobility, degree of crystallization, size of the grain, and connections among grains in film are main factors to determine film’s electrical properties.     

Quantum Size Effect and very localized random laser in ZnO@mesoporous silica nanocomposite following a two-photon absorption process

ZnO@mesoporous silica nanocomposite was prepared by the impregnation of a CMI-1 material in a Zn(NO₃)₂ solution followed by calcination under O₂. Intensive characterization was carried out by N₂ adsorption–desorption measurements, scanning and transmission electron microscopy. The optical properties of the ZnO@mesoporous silica nanocomposite were studied by photoluminescence spectroscopy. Quantum Size Effect was firstly demonstrated by subjecting the sample to a 254 nm excitation light, and was further confirmed by using a 680 nm excitation laser beam, which implies a two-photon absorption process. By focusing the 680 nm laser beam on different places in the sample, a very localized random laser effect, also induced by a two-photon absorption process, was detected.     

A study of the fracture mechanisms in pristine silica fibres utilising high speed imaging techniques

The 40 kg test masses of the Advanced LIGO interferometric gravitational wave detector will each be suspended on four fibres fabricated from Heraeus Suprasil synthetic silica glass. The ultimate tensile breaking stress and fracturing process of loaded silica fibres was investigated in order to develop the technology required to suspend the test mass in a robust and safe manner. The majority of the fibres in this study were pulled by hand in a H₂ ? O₂ flame; for comparison, fibres were also pulled on a CO₂ laser machine. Carefully prepared fibres were shown to be pristine, i.e. free of surface cracks. Such fibres exhibited breaking strengths as high as 5 ? 6 GPa. To analyse the mechanisms of fracture a high speed photographic setup was developed in addition to the use of a high speed video camera. The pristine fibres break in the zone of maximal stress as expected in the model of flawless fibres. Some fibres break at a lower stress and these fractures occur at the fibre ends. This type of fracture is related to the thermal stress induced by local heating which was used to align the fibre in the test structure. The most likely fracture mechanism is based on a thermo-kinetic approach.     

Fe3+ doped SiO2 nanostructured gel-glasses: Structural,optical and magnetic properties

Transparent crack-free and bubble-free Fe³⁺ doped SiO₂ nanostructured gel-glasses were obtained by the sol–gel process. The process involves the hydrolysis and condensation of an appropriate molar ratio of tetraethoxysilane (TEOS), absolute ethanol, nitric acid and ferric nitrate, followed by stepwise annealing at temperatures ranging from 110 °C to 1000 °C. The structural variation of the gel-glasses and their influence on physical properties during annealing has been studied. It has been observed that monolithicity and chemical environment around Fe³⁺ in the gel-glasses are strongly dependent on the annealing temperatures. The colour of gel-glass samples is different for different annealing temperatures, mainly due to the different co-ordination state of Fe³⁺ and the generation of Fe₂O₃ colloids of size 20–60 nm in the silica matrix. The annealing process facilitates the tuning of the UV–visible transmission cut-off edge in high optical quality Fe³⁺ doped silica gel-glasses. A marked difference in the magnetic properties of these glasses is also observed with annealing temperatures.     

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.     

Crucial dependence of excimer laser toughness of “wet” silica on excess oxygen

Creation of point defects by ArF (6.4 eV) and F₂ laser (7.9 eV) irradiation in synthetic “wet” silica glass thermally loaded with interstitial O₂ molecules was studied by optical absorption, electron paramagnetic resonance and infrared absorption. The presence of excess oxygen caused a significant increase of laser-induced ultraviolet (UV) absorption, which was 4 times (7.9 eV-irradiation) and > 20 times stronger (ArF irradiation) as compared to O₂-free samples. The spectral shape of photoinduced absorption nearly completely coincided with the spectral shape of oxygen dangling bonds (NBOHC) in 3 to 6.5 eV regions. The contribution of Si dangling bonds (E' centers) was less than few % and was not dependent on oxygen content. Peroxy radical defects were not detected. The photoinduced NBOHCs thermally decayed at 400...500 C. However, a subsequent brief 7.9 eV irradiation restored their concentration up to 70%. This sensitization can be in part attributed to generation of interstitial Cl₂ and HCl. These data show that oxygen stoichiometry is an important factor for maximizing laser toughness of wet silica.     

Optical properties of PbSe quantum dots embedded in oxide glass

The controlled synthesis of PbSe quantum dots in Se-doped glass matrix (SiO₂–Na₂CO₃–Al₂O₃–PbO₂–B₂O₃) with narrow size distributions was achieved. Quantum dot size can be manipulated by tuning annealing time in the process of thermal treatment. The PbSe QD sizes estimated by 4 × 4 k · p theory were in very good agreement with the measurements of atomic force microscopy.     

Synthesis of bulky mesoporous silica (FSM) by hydrothermal hot-pressing method

In this study, we successfully synthesized the bulk of dense mesoporous silica (FSM) bodies by a short-term hydrothermal hot-pressing (HHP) method. It was thought that these dense bodies were achieved by solution/precipitation mechanism during HHP. Bulk pieces with the dimension in the range of centimeters in diameter possessed a high surface area of over 1000 m²/g and sharp distribution of mesopores with an average diameter of approximately 2.4 nm. This successful synthesis of bulky mesoporous silica will expand the capabilities of application to various fields, for example, gas separation (CO₂ and H₂ etc.) and catalysis.     

Diamond film growth on the Mo–Re alloy foil

Nanocrystalline diamond film was deposited on the substrate of Mo–Re alloy foil by using a hot filament chemical vapor deposition (HFCVD) method. The morphology, band structures and crystalline structure of the film were analysed by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffractometer (XRD), respectively. The results show that the thickness of the diamond film is about 300 nm after 1 h deposition. There is a 2H-Mo₂C layer between the diamond film and the Mo–Re substrate. The values of a and the ratio c/a of Mo₂C are 3.003 and 1.579 Å, respectively. This Mo₂C layer might be formed due to carbon atoms in the gas phase diffusing into the Mo–Re alloy.     

A study of Roman glass from Mala Barutana/Belgrade Fortress irradiated with pulsed CO2, Nd:YAG and ruby laser — Comparison

Application of non-contact and rapid laser technique, which is minimally invasive, non-contaminant and efficient method, for ancient glass investigation and cleaning is highly desirable for restoration purposes. Irradiation of Roman glass dated from 1st to 4th/5th century AD with TEA CO₂ (wavelength 10.6 μm; pulse duration t_p = 100 ns), Nd:YAG (wavelength 1064 nm and 532 nm; t_p = 150 ps) and ruby laser (wavelength 694 nm; t_p = 30 ns) in air ambience was studied. For all three lasers, moderate energy densities (15–30 J/cm²) induced significant changes of morphology — from superficial exfoliation and occurrence of mosaic structure after few pulses to deep damages and hydrodynamic features after higher number of accumulated shots. Irradiation with moderate energy density, accompanied with plasma appearance in front of the samples, is convenient for numerous potential applications, particularly surface elemental analysis such as laser induced breakdown spectroscopy. On the other hand, lower densities are more suitable for Roman glass cleaning. Calculations of Roman glass surface temperature have shown that pulsed CO₂ laser is favorable for surface cleaning and optimal fluence is ~ 2 J/cm². This was confirmed by additional experiments for fluences 1.5 and 3 J/cm². Morphological changes on the Roman glass surface induced by lasers were studied by optical microscopy (OM) and scanning electron microscopy (SEM). The composition of Roman glass was determined by energy dispersive X-ray analysis (EDX) and inductively coupled plasma (ICP) method. Chemical analysis confirmed that the investigated glass dates from the Roman period.     

Band gap energy of PbS quantum dots in oxide glasses as a function of concentration

PbS nanocrystals embedded in glass matrix (SiO₂–Na₂CO₃–Al₂O₃–B₂O₃) were synthesized by means of the fusion method using four different PbS concentrations (0.05, 0.1, 1.5, and 2.0 wt%). Thermal treatment was performed at 550 °C with annealing time of 6 h. Measurements of optical absorption and photoluminescence were carried out as a function of PbS concentration. It is argued that, with the same thermal treatment and annealing time, the formation of large nanocrystals becomes easier as the PbS concentration increases. Optical absorption spectra showed that the band-gap energy increases as the PbS concentration decreases, making this relationship important in the obtainment of a desired band-gap in PbS-doped glasses.     

Kinetics of the crystallization in amorphous NiTi thin films

The crystallization kinetics in Ni_50.54Ti_49.46 film was studied by differential scanning calorimetry through continuous heating and isothermal annealing. The activation energy for crystallization was determined to be 411 and 315 kJ/mol by Kissinger and Augis & Bennett method, respectively. In the isothermal annealing study, The Avrami exponents were in the range of 2.63–3.12 between 793 and 823 K, suggesting that the isothermal annealing was governed by diffusion-controlled three-dimensional growth for Ni_50.54Ti_49.46 thin films.     

Correlation between the luminescence properties and the surface structures of submicron silica particles

We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Stöber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar⁺ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing.