VUV and IR absorption spectra induced in H2-loaded and UV hyper-sensitized standard germanosilicate preform plates through exposure to ArF laser light

H₂-loaded Ge-doped preform plates have been UV hyper-sensitized and subsequently post-exposed by means of UV ArF laser pulses at 193 nm. Both Fourier Transform Infrared (FTIR) and Vacuum Ultraviolet (VUV) absorption spectroscopy has been carried out at each step of the sensitization process with a view to get a better understanding of the UV hyper-sensitization process at 193 nm. Exposing the H₂-loaded samples firstly triggers a partial bleaching of the 5 eV absorption band, followed by a new growth of the band for longer exposure time. As it has already been reported for exposure at 248 nm, the exposure at 193 nm firstly yields an increase in the absorption ascribed to hydroxyl (≈3600 cm⁻¹), hydride species (≈2140–2185 cm⁻¹) and GeE′ defects (≈6.3 eV). The evolution of the absorption related to the GeH₂ species is not monotonous but rather the absorption reaches a peak (N_pre = 2000) and then slightly decreases for further exposure time. Post-exposing the hyper-sensitized samples (N_pre = 2000) lead to a partial bleaching of the absorption ascribed to hydride species and to increases in the intensity of the GeE′ defect-related absorption and in the absorption ascribed to hydroxyl species. The accurate determination of the species concentration from the absorption spectra proved to be rather tricky due to the problem of accurately measuring the depth of the UV beam penetration at each time of the exposure. Nevertheless, a correlation could be established between the growth of the UV-induced UV excess loss ascribed to GeE′ species and the UV-induced decrease in the intensity of the IR bands related to hydride species. These observations are discussed within the frame-work of the two step model.     

Effects induced by 4.7 eV UV laser irradiation on pure silica core multimode optical fibers investigated by in situ optical absorption measurements

We investigated by in situ optical absorption measurements the effects induced by 4.7 eV UV laser irradiation on pure silica core optical fibers. Laser irradiation with 100 MW cm^? 2 laser intensity generates in the fiber E′ centers which partially decay after irradiation due to their reaction with diffusing H₂. An absorption band peaked at 5.3 eV is observed to grow in the post-irradiation stage with a kinetics anti-correlated to the decay of the 5.8 eV band of the E′ centers. The defect absorbing at 5.3 eV is proposed to be formed by trapping on pre-existing precursors of hydrogen atoms made available by breaking of H₂ on E′. We also show by repeated irradiation experiments that the 5.3 eV-absorbing center is photochemically destroyed by 4.7 eV laser light, and we estimate the cross section of this process. Possible structural models for this defect are discussed.     

Synthesis and characterization of hafnium oxide and hafnium aluminate ultra-thin films by a sol–gel spin coating process for microelectronic applications

Currently there are intense industry-wide efforts in searching for new high dielectric constant (high-k) materials for use in future generations of ultra-large scale integrated circuits (ULSI). There are number of requirements for the new high-k materials, such as high dielectric constant, thermal stability (400 °C or higher), high mechanical strength, and good adhesion to neighboring layers. Oxide spinels comprise a very large group of structurally related compounds many of which are of considerable technological significance. Spinels exhibit a wide range of electronic and magnetic properties in particular nickel, hafnium, cobalt, containing spinels. In the present investigation, crack free, dense polycrystalline monoclinic structure of pure HfO₂, and Al₂HfO₅ ultra-thin films have been prepared by a simple and cost effective sol–gel spin coating method. The formation of the monoclinic HfO₂ phase at 600 °C and complete formation of the single phase Al₂HfO₅ at 800 °C has been reported. The composition of the annealed films has been measured and found to be 70 at.% of O, 30 at.% of Hf for HfO₂ and 22 at.% of Al, 12 at.% of Hf and 66 at.% of O for Al₂HfO₅ films, which are close to the stoichiometry of the HfO₂ and Al₂HfO₅ thin films.     

Optical absorption in amorphous InN thin films

Amorphous indium nitride (a-InN) thin films were deposited onto different substrates at temperatures <325 K using RF magnetron sputtering at a rate 0.3–0.4 Å/s. X-ray diffraction patterns reveal that the films grown on the substrates are amorphous. The optical absorption edge, ‘bandgap’ energy, E_g, of a-InN has been determined by spectroscopic ellipsometry over the energy range 0.88–4.1 eV. The absorption coefficient was obtained by the analysis of the measured ellipsometric spectra with the Tauc–Lorentz model. The E_g was determined using the modified Tauc and Cody extrapolations. The corresponding Tauc and Cody optical bandgaps were found to be 1.75 and 1.72 eV, respectively. These values are in excellent agreement with the values of the bandgap energy obtained as fitting parameters in the Tauc–Lorentz model: 1.72 ± 0.006 eV as well as by using spectrophotometry (1.74 eV) and photoluminescence (1.6 eV). The spectral dependence of the polarized absorptivities was also investigated. We found that there was a higher absorptivity for wavelengths <725 nm. This wavelength, ∼725 nm, therefore indicates that the absorption edge for a-InN is about 1.70 eV. Thus, the average value of the measured optical absorption of a-InN film is approximately 1.68 ± 0.071 eV.     

FTIR and UV–VIS optical absorption spectra of gamma-irradiated MoO3-doped lead borate glasses

Structure and optical properties of MoO₃-doped lead borate glasses which contain high PbO content (60, 70 and 80%) have been studied using Fourier transform infrared (FTIR) and ultraviolet–visible (UV–VIS) spectroscopic tools. FTIR spectra reveal absorption bands which are characteristic for various structural units of borate network, mainly BO₃ triangles and BO₄ tetrahedra, in addition to the PbO_n (where n = 3 and/or 4) structural units. UV–VIS optical absorption spectra reveal broad intense charge transfer UV bands due to Pb^2 + ions in the range 320–385 nm. Within this range, molybdenum ions, preferably Mo^3 + and Mo^5 +, can interfere at about 360–385 nm. Additionally, molybdenum ions give a weak visible band at about 850–860 nm. The optical absorption spectra of the studied glasses show marked resistance to successive gamma irradiation up to 5 Mrad. This shielding behavior can be related to the present high content of the high atomic mass Pb^2 + ions. Changes in the atomic structure before and after gamma irradiation are described and explained.     

X-ray photoelectron spectroscopy of erbium-activated-silica–hafnia waveguides

X-ray photoelectron spectroscopy (XPS) has been used in the study of sol gel-derived Er³⁺-activated xHfO₂–(100 − x)SiO₂ (x = 10, 20, 30, 40, 50 mol) planar waveguides. The analysis of Si 2p and O 1s core lines were related to the Hf/Si molar ratio to assess the role of hafnia in modifying the silica network. Increasing the HfO₂ content brings about a change of the Si 2p and O 1s binding energy respect to those from pure silica. This trend is explained with a formation of hafnium silicate in the matrix with successive phase separation between HfO₂ and SiO₂ rich phases. XPS results show that hafnia is well dispersed in the silica matrix for molar concentration below 30%. Formation of pure HfO₂ domains was detected at higher hafnia concentrations in agreement with previous spectroscopic analyses.     

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.     

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.     

Role of sol-gel networking and fluorine doping in the silica Urbach energy

We present the results of the analysis of the ultraviolet (UV) absorption edge of fluorine-modified sol-gel silica. UV transmission data, obtained by means of synchrotron radiation, have been analyzed in the spectral range 7.5–8.5 eV, with a spectral resolution of about 10 meV. Data on silica samples with different F content (from 0 to few 10^? 1 mol%) have been analyzed and compared with literature data on quartz and pure synthetic commercial silica. The analysis allows us to discriminate between the effects of the fluorine addition and those ascribable to structural peculiarities of the sol-gel networking. The estimated Urbach energy E_U(T = 0) ranges between 45 and 55 meV, higher that in crystalline quartz and lower than in commercial synthetic silica. The study of the temperature dependence of E_U(T) shows that the fluorine modification of the silica network causes the lowering of the static disorder and the widening of the energy gap. However, there is also a relevant effect of the production process, since sol-gel silica samples show lower E_U values with respect to other types of silica, quite independently of the fluorine content. The analysis of the Raman spectra however shows that the starting amount of fluorine-modified molecular precursor influences the network condensation process, independently of the final fluorine content into the matrix.     

Thermal decomposition and new luminescence bands in wet,dry, and additional oxygen implanted silica layers

Wet and dry silica oxide layers have been treated thermally up to T_a = 1300 °C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 10¹⁷ ions/cm² and high thermal treatment T > 1100 °C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.     

Alkaline earth zinc borate glasses doped with Cu2+ ions studied by EPR,optical and IR techniques

Copper ions incorporated into alkaline earth zinc borate glasses 10RO + 30ZnO + 60B₂O₃ (R = Mg, Ca and Sr) and 10SrO + (30 ? x)ZnO + 60B₂O₃ + xCuO (x = 0, 0.1, 0.3, 0.5, and 0.7 wt.%) were characterized by electron paramagnetic resonance (EPR), optical absorption and FTIR techniques. The EPR spectra of all the glass samples exhibit resonance signals characteristic of Cu²⁺ ions. The values of spin-Hamiltonian parameters indicate that the Cu²⁺ ions in alkaline earth zinc borate glasses were present in octahedral sites with tetragonal distortion. The spin concentration (N) participating in resonance was calculated as a function of temperature for strontium zinc borate (SrZB) glass sample containing 0.7 wt.% of Cu²⁺ ions and the activation energy was calculated. From the EPR data, the paramagnetic susceptibility (χ) was calculated at different temperatures and the Curie constant was evaluated from the 1/χ-T graph. The optical absorption spectra of these samples show only one absorption band. The optical band gap energies (E_g) and Urbach energy (ΔE) are calculated from their ultraviolet edges. The FTIR studies show different stretching and bending vibrations of alkaline earth zinc borate glasses.     

Photoinduced changes in UV absorption spectra of nitrogen-doped silica caused by exposure to ArF excimer laser

Absorption spectra of high-purity silica, in which 1.2% or 3.0% oxygen atoms are replaced by nitrogen, are measured in the spectral interval of 3.0–6.5 eV. Photoinduced changes of these spectra when exposed to 193 nm wavelength excimer laser radiation, pulse intensity being ～50 mJ/cm², are examined. Absorption spectra relaxation under subsequent annealing as well as changes brought about by saturation of glass with molecular hydrogen are studied. Parts of graded-index fiber ～1 mm in length and 200 μm thick transverse slices of a fiber preform served as samples. It is found that exposure of fiber samples to laser irradiation brings about a significant decrease of initially more intense absorption band of Si-ODC in the region of 5.0 eV and an increase of initially less intense band centered at 5.8 eV at a time. In bulk samples correlation of these bands is opposite, photoinduced changes are less expressed against structureless absorption tail increase in the spectral interval of 3.0–6.5 eV. It is shown that subsequent 20 min 700 °С annealing leads to the relaxation of photoinduced changes in absorption spectra in bulk and fiber samples. By placing irradiated samples into molecular hydrogen atmosphere at room temperature absorption bands are suppressed and transparency at shorter wavelengths of UV region is increased. Data obtained is discussed in the context of photosensitivity of nitrogen-doped silica-core fibers, which serve a technological platform for thermoresistant in-fiber Bragg gratings fabrication.     

Structural investigation of photonic materials at the nanolevel using XPS

This work deals with X-ray photoemitted spectra (XPS) from materials which are of interest for photonic applications. In particular xHfO₂ ? (100 ? x) SiO₂ (x = 10, 20, 30 mol%) glass–ceramics planar waveguides and silver ion-exchanged (0.5, 1.5, 5 mol%) sodalime glasses are investigated. The aim of the work is to explore the material structural changes occurring at the nanometric scale which are produced during the fabrication process in order to enlighten the formation of the nanostructures. The results show that XPS is sufficiently sensitive to detect the formation of nanostructures in the analyzed materials providing at the same time also chemical information. Both these inputs are important to tune the production processes to increase the efficiency of the optical devices.     

Investigation of the conductivity of the lithium borosilicate glass system

The lithium borosilicate system (Li₂O)_0.4(B₂O)_(0.6x)(Si₂O₄)_0.6(1−x) with x = 0, 0.2, 0.3, 0.4, 0.6, and 0.8 was investigated using impedance spectroscopy. Impedance spectra were taken in the frequency range from 50 Hz to 1 MHz and in the temperature range from 100 to 280 °C. The ac- and dc-conductivity, relaxation frequency and activation energy of the dc-conductivity were extracted from the impedance spectra. The dc-conductivity of the investigated glass samples increases almost linearly from silica rich (x = 0) to the boron rich (x = 0.8) samples. Activation energy (E_a) was found to be 0.65 eV for high conducting sample and 0.8 eV for low conducting sample, respectively. The mixed glass-former effect was not observed on the samples studied. The effect of temperature scaling of ac-conductivity was observed, which indicates, that ionic conductivity relaxation mechanism is temperature independent for samples with x = 0, 0.2, 0.3. However, some deviations from scaling were found for the samples with higher x (x = 0.4, 0.6, 0.8).     

Origins of optical absorption between 4.8 and 4.9 eV in silica implanted with Si and with O ions

We have determined some of the sources of the optical absorption bands between 4.8 and 4.9 eV in Si and O-implanted silica using several ion energies to produce layers of implanted ions with constant concentrations. The concentrations of implanted ions in the implanted layers ranged from >0.015 at.% to <3 at.%. Optical absorption was measured from 2.0 to 6.5 eV. Electron paramagnetic resonance measurements were made at ∼20.3 and 33 GHz for sample temperatures ranging from 77 to 100 K for most measurements. The components identified in the spectra, based on comparison with reported parameters, were, in the Si case, due to E′ centers and peroxy radicals. In the O case they were due to E′ centers, non-bridging oxygen hole centers, peroxy radical centers, and a newly appearing state which we labeled the OS center. By comparing the changes in the absorption at 4.83 eV with the changes in the concentrations of the various electron paramagnetic resonance components and with the reports in the literature, we conclude that there are at least four oxygen related centers and one Si related center absorbing between 4.8 and 4.9 eV.     

Fluorine laser-induced silicon hydride Si–H groups in silica

Formation and destruction of silicon hydride (Si–H) groups in silica by F₂ laser irradiation and their vacuum ultraviolet (VUV) optical absorption was examined by infrared (IR) and VUV spectroscopy. Photoinduced creation of Si–H groups in H₂-impregnated oxygen deficient silica is accompanied by a growth of infrared absorption band at 2250 cm⁻¹ and by a strong increase of VUV transmission at 7.9 eV. Photolysis of Si–H groups by 7.9 eV photons in this glass was not detected when the irradiation was performed at temperature 80 K. However, a slight destruction of Si–H groups under 7.9 eV irradiation was observed at the room temperature. This finding gives a tentative estimate of VUV absorption cross section of Si–H groups at 7.9 eV as 4 × 10⁻²¹ cm², showing that Si–H groups do not strongly contribute to the absorption at the VUV fundamental absorption edge of silica glass.     

Optical and kinetic properties of cathodically deposited amorphous tungsten oxide films

Optical properties and the coloration–decoloration kinetics of electrochromic films of amorphous tungsten oxide (a-WO₃), produced by cathodic deposition from a sodium tungstate based aqueous peroxide electrolyte, have been investigated. As films color in 1 N H₂SO₄, sequential appearance of bands with maxima at ∼1 eV, 1.6 eV, 2 eV, and 2.4 eV is observed in their optical absorption and electrosorption spectra, is the same as in the case of reduction of nanosized hydrated-WO₃ colloids with a gradual decrease in their size to that of 12-tungsten polyanions with Keggin structure, indicating the presence of such polytungstates in cathodically deposited a-WO₃, too. When polytungstate is reduced by one electron, an absorption band with a maximum at ∼1.6 eV appears in the optical spectrum of the film. This band corresponds to the optical excitation of charge transfer of the W⁵⁺ → W⁶⁺ type between two adjacent tungsten atoms. The reduction of polytungstate by a second electron with potential shift towards more negative values is accompanied by the appearance of an analogous band with a maximum at ∼2 eV. The reduction of such polytungstates involves participation of the bulk of injected electrons, indicating their dominate role in the nanostructure of the films investigated. The effective co-diffusion coefficient of electrons and protons in cathodically deposited a-WO₃ exhibits a potential dependence with a maximum at 0.1 V against a silver-chloride electrode, where its value is ∼10⁻⁸ cm²/s. It has been shown that the decrease in this coefficient at potential values of over 0.1 V is caused by a decrease in electron mobility.     

Incipient amorphous-to-crystalline transition in HfO2 as a function of thickness scaling and anneal temperature

A series of 1.4, 1.8, and 4.0 nm thick HfO₂ films deposited on Si(1 0 0) substrates have been measured by extended X-ray absorption fine-structure prior to anneal processing, following a standard post deposition anneal of 700 °C for 60 s in NH₃ ambient, and following an additional rapid thermal anneal cycle of 1000 °C for 10 s in N₂ ambient. Analysis of the second coordination shell gives clear evidence of increased ordering with increasing film thickness at each temperature. Similarly, increased ordering with increasing anneal temperature is evident for each film thickness. Although X-ray diffraction and high resolution transmission electron microscopy indicated the 1.4 nm HfO₂ samples to be amorphous, EXAFS has distinguished nanocrystalline from amorphous states for these films.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

Temperature dependence of the generation and decay of E′ centers induced in silica by 4.7 eV laser radiation

We report a study of the generation of silicon dangling bonds (E′ centers) induced in fused silica by 4.7 eV laser irradiation in the 10 < T < 475 K temperature range, carried out by in situ optical absorption spectroscopy. The generation of the defects, occurring by transformation of pre-existing precursors, results to be a thermally activated process, quenched below 150 K and with a 0.044 eV activation energy. At T > 200 K the induced defects undergo a post-irradiation decay due to their reaction with mobile H₂. The interplay between generation and annealing gives rise to a bell-shaped temperature dependence of the concentration of induced E′ centers, peaking at 250 K.