Preparation and properties of silica films with higher-alkyl groups

Silica films with various types of alkyl groups were grown from the liquid phase, at room temperature, using alkyl tri-alkoxy silane compounds. From Fourier transform infrared (FTIR) absorption measurements, a majority of Si-alkyl bonds in the source molecules were found to remain in the grown film, while Si-alkoxy bonds were completely removed. Electrical and thermal properties have been measured comparatively for various films having dense alkyl groups. The films with the methyl group are promising for silicon production in ultra large-scale integrated circuits (Si-ULSIs) and the film having the vinyl group for non-heat-tolerant compound-semiconductor large-scale integrated circuits (LSIs).     

As2S3 and AgI thin layers as ion sensitive membranes

The possibilities of using thin layers of As₂S₃ and AgI as ion sensitive membranes for ion selective field-effect transistors (ISFETs) are investigated. The thin films have been prepared by vacuum deposition on static and rotating substrates. The As₂S₃ layers were additionally doped with silver. The influence of the type of substrates and preparation conditions on electrochemical properties of the layers was studied. Electrochemical measurements revealed a reasonable sensitivity of chalcogenide and halide layers to silver and iodide ions, respectively. The near Nernstian behavior of sensitivity of As₂S₃ layers to Ag⁺ ions and of AgI layers to I⁻ ions is observed. The results obtained are promising for the development of ISFETs.     

Homogeneity of a ZrF4-based glass at the nano-scale

A glass of composition 53ZrF₄–20BaF₂–4LaF₃–3AlF₃–20NaF (T_g=260°C) was prepared by careful crucible melting. High-resolution atomic force microscopy of fracture surfaces displayed the presence of nano-pores with diameters of 20–50 nm, being 4–10 nm deep, in all glasses. It was further found that only glasses without annealing and glasses with an annealing step considerably below T_g showed a distinct pattern, i.e. ripples of ≈20 nm in diameter and an rms roughness of ≈0.6 nm. Glasses annealed either near T_g or at the temperatures of maximum nucleation or maximum crystal growth rates showed both regions with the ripple pattern and regions with nano-hillocks, growing in size with increasing annealing temperature and time. Thus these hillocks nearly reach micro-dimensions of ≈270 nm in diameter and ≈65 nm in height following a 90 min annealing step at 343°C, the temperature of maximum crystal growth. These findings give evidence that the glass system, which is thought to be one of the most suitable for fiber drawing, is much less stable against nucleation and crystallization than anticipated.     

Oscillator strength of the infrared absorption band near 1080 cm in SiO2 films

It has been well known that the absorption maximum of the peak near 1080 cm⁻¹ in amorphous SiO₂ films shifts continuously with variation of thickness and properties such as stress. This is a first report on the oscillator strength of the absorption against frequency at the absorption maximum. SiO₂ films on silicon wafers were prepared by thermal growth in either dry O₂ or an O₂/H₂ mixture or liquid-phase deposition in HF saturated with silica gel. The oscillator strength continuously decreased from 1×10⁻⁴ down to 1×10⁻⁵ with the frequency shift from 1099 to 1063 cm⁻¹.     

A new method for fabricating water repellent silica films having high heat-resistance using the sol–gel method

There has been a great demand in the field of kitchen appliances to develop transparent water repellent films which have high heat-resistance around 300°C. However, those films have not been obtained by conventional sol–gel methods. In this paper, we propose a new method for fabricating transparent water repellent films with high heat-resistance using the sol–gel method, in which silicon or germanium substrates were coated with a solution including tetraethoxysilane (Si(OC₂H₅)₄) and (2-perfluorooctyl)ethyltrimethoxysilane (CF₃(CF₂)₇C₂H₄Si(OCH₃)₃), followed by ‘ammonia-treatment' and annealed at 300°C. The contact angles of water on the ammonia-treated film maintained its initial value, 110° after the heat treatment at 300°C for 250 h while those on the untreated film decreased to 70°, indicating that the ammonia-treatment improves heat-resistance on the film. The mechanism of ammonia-treatment was inferred from FT-IR results; the ammonia-treatment should accelerate hydrolysis and polymerization of FAS and TEOS molecules, resulting in high density of siloxane bonds between FAS and silica glass. These bonds suppress the evaporation of FAS molecules from the film during the heat treatment at 300°C, thus the film has high heat-resistance.     

A magnetic resonance study on the structure of amorphous networks in the Si–B–N(–C) system

The amorphous networks Si₃B₃N₇ and ‘SiBN₃C' are studied by solid-state nuclear magnetic resonance (NMR), continuous-wave and pulse electron paramagnetic resonance (EPR), and by one- and two-dimensional electron nuclear double resonance spectroscopy. In both compounds, boron is found to be coordinated exclusively by nitrogen with close to trigonal planar geometry and close to equal bond lengths. Silicon is four-coordinated by nitrogen with the coordination tetrahedra being distorted to accommodate the coordination preferences of boron. REDOR measurements demonstrate that boron resides in the second coordination sphere of silicon. Carbon incorporation into the Si–B–N network does not lead to any observable changes in NMR parameters including the average dipolar coupling between ¹¹B nuclei which depends on the average distance of the boron atoms. Only spin–lattice relaxation of the nuclei is accelerated due to the generation of paramagnetic centers. The unpaired electrons appear to be delocalized over several carbon atoms and exhibit significant hyperfine couplings to boron, silicon, nitrogen, and some residual protons. In contrast to electron spectroscopic imaging experiments, the magnetic resonance results suggest formation of carbon clusters.     

Lateral photovoltage measurements in hydrogenated amorphous silicon and silicon-oxygen thin films

Hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon-oxide alloy films (a-SiO_x:H) were investigated by temperature dependence of lateral photovoltage (LPV) measurements. The suboxide sample with [O] = 27 at.%, was found to exhibit larger LPV compared to the unalloyed sample. It is difficult to simply correlate LPV measurements to related diffusion length measurements, only. On the other hand, the observed magnitude of LPV in a-Si:H and its decrease with temperature, could be explained based on an internal electric field induced by diffusion electron and hole currents, and multiple trapping of the photocarriers.     

Surface removal of a weathered organic glass

A tubular, stratified glass–epoxy material composite has been subjected to either natural or artificial aging. After aging, the degradation of this material is characterised by the ablation of the organic matrix. For the first time, we have measured the ablation using a three-dimensional surface microanalysis system equipped with a sensitive topographical probe. In a previous study involving the measurement of the ablation during artificial photo-aging, an innovative hypothesis concerning the creation of a thin photo-oxidation layer in close proximity to the irradiated surface of the organic matrix was proposed. During weathering, from time to time, rain leaches the thin photo-oxidation layer and causes a significant ablation via a complementary mechanism. Given that the material surface always presents hemi-cylindrical concavity imprints, we studied the surface changes, due to the ablation, by measuring the cross-section half-circle radii of the concavities after different photo-aging times. The surface erosion is characterized by growth of the circle radii, whose changes progressively lead towards surface flatness. A 2-D model of the concavities’ growth has been validated: the simulation results lead to calculated circle radii growth values near the measured ones. Comparison between photo-aged organic matrix removal and erosion of a leached mineral glass shows that the morphological removal of this isotropic organic matrix evolves in a manner analogous to that of an amorphous inorganic material.     

Amorphous silicon formation by shearing at ultrahigh pressure

Transmission X-ray diffraction pattern taken from silicon sheared at 10 GPa revealed 8 halo rings. Their densest radii correspond to spacings of 308.4, 264.7, 187.2, 160.5, 152.8, 124.8, 104.9, and 91.5 pm, respectively. The ratios of these values correspond to those of interplanar spacings (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1) and (4 2 0) of face-centered cubic (fcc) lattice. These facts suggest the formation of amorphous state based on fcc structure. This state remains at atmospheric pressure.     

Third-order elastic constants determination in soda–lime–silica glass by Brillouin scattering

Brillouin light scattering allows the measurement of sound velocity and elastic moduli in transparent materials. The ability to select a small scattering volume and to use specific scattering configurations gives important information about the gradient and anisotropy of mechanical properties. Brillouin experiments are often used to measure the second-order elastic constants. When a high accuracy in frequency measurements is achieved, Brillouin scattering may allow the determination of third-order elastic constants in pre-stressed media. Hence, Brillouin scattering provides in principle, a method for the analysis of stress fields in tempered glasses. In order to validate the technique, samples of float soda-lime–silica glass submitted to controlled stresses by four-point flexion were investigated. The results show the expected profile for the velocity of sound waves propagating in directions parallel and perpendicular to the surface of the samples, respectively. They allow the determination of several third-order elastic constants of the investigated glass. This technique was applied to several samples of tempered glass corresponding to different values of the surface stress. The main result is the observation of the expected general trend, namely, through the thickness of the sample, a parabolic variation of the sound velocity whose amplitude increases with the magnitude of the surface stress.     

Defect levels in the band gap of amorphous selenium

Energy locations in the band gap have been determined for the thermally accessible levels of the negative-U defect centers in amorphous selenium. Both the temperature dependence of the steady-state photocurrents, and an analysis of emission currents in the post-transit regime of a time-of-flight transient photoconductivity experiment on the same samples, agree on the presence of defect levels at (0.42 ± 0.04) eV above the valence band mobility edge and (0.53 ± 0.06) eV below the conduction band. Both measured current levels and the resolved energy positions of the defects are subject to the Poole-Frenkel effect.     

Ionic transport crossover in mixed conducting chalcogenide glasses detected by Te-Mössbauer spectroscopy

¹²⁵Te-Mössbauer spectroscopy has been used to verify whether the observed ionic transport crossover for the Ag–As–Se–Te glasses with a silver content of 15 at.% at approximately equimolar Se/Te ratio is accompanied by variations in the hyperfine interaction parameters produced by corresponding changes in the microstructural organization of the glass network. We found that an unusual decrease of the quadrupole splitting with decreasing tellurium fraction, r=Te/(Se+Te)<0.5, is caused by the combined effect of increasing Te–Ag(As) interatomic distances and a non-random substitution of multiple Se sites by Te with a preferential occupation of those with at least one Ag nearest neighbour. The latter process leads also to a step-like decrease in the isomer shift at r0.35. The assumed expansion of the glass network at smaller r and the contrasting network contraction for the Te-rich glasses are consistent with the ionic transport crossover.     

Boron-doped a-SiOx:H films prepared by photo-CVD technique

The optoelectronic and structural properties of p-type a-SiO_x:H films have been studied. The deposition parameters e.g. chamber pressure and diborane to silane ratio are optimized to get a film with dark conductivity (σ_d) 7.9×10⁻⁶ S cm⁻¹ and photoconductivity 9.3×10⁻⁶ S cm⁻¹ for an optical gap (E₀₄) of 1.94 eV. The decrease of optical gap accompanied by the increase of conductivity is due to less oxygen incorporation in the film, which is substantiated by the decrease of the intensity of SiO absorption spectra. The properties are very much effected by the chamber pressure and diborane to silane ratio.     

The glass transition of Pd40Ni10Cu30P20 studied by temperature-modulated calorimetry

The frequency dependence of the heat capacity in the glass-transition region of Pd₄₀Ni₁₀Cu₃₀P₂₀ was studied by temperature-modulated differential scanning calorimetry (TMDSC) during slow heating and cooling. Such data for low frequencies between 0.1 and 0.01 Hz are not available, especially for metallic glasses. A crossover between mixed static/dynamic and purely dynamic response signals was observed for the lowest frequencies between 1/80 and 1/100 s⁻¹, which allows a direct determination of the average relaxation time at a given cooling rate during the static glass transition. Further, these results were used to evaluate the experimental parameters necessary to truly separate the static and dynamic response in low-frequency modulation calorimetry experiments to obtain the moduli of the dynamic specific heat.     

Short-range order in non-stoichiometric amorphous silicon oxynitride and silicon-rich nitride

By de-convoluting the Si 2p X-ray photoelectronic spectra, it was found that the short-range order in amorphous silicon oxynitride (SiO_xN_y) films with different compositions can be quantitatively described by the random bonding model. In this model the SiO_xN_y consists of five types of randomly distributed tetrahedra and it indicates that metal-oxide-semiconductor transistor with this gate dielectric will not result in any gigantic potential fluctuation in the conduction channel. On the contrary, the structure of silicon-rich silicon nitride SiN_x can only be described by the random mixture model where the local composition fluctuations in this film will result in gigantic potential contra-variant fluctuation.     

Complex permeability and electromagnetic wave absorption properties of amorphous alloy-epoxy composites

Electromagnetic wave absorption properties of amorphous alloy-epoxy composites have been investigated with various amorphous alloy particle sizes and fractions in the 45 MHz to 10 GHz frequency range. The fraction of amorphous alloy in amorphous alloy-epoxy composites varied from 30 to 60 vol.% at a fixed amorphous alloy particle size and the size of amorphous alloy particles was varied from several μm to 125 μm at a fixed amorphous alloy particle fraction. Complex permeability (μ), permittivity (ε) of composites and reflection loss were measured by the reflection/transmission technique. The composites with small sized amorphous alloy particles (<26 μm) and a small amount of particles (<50%) had good reflection loss values less than −20 dB with thin thickness (<1 cm). The decreasing amorphous alloy particle size and fraction in amorphous alloy-epoxy composites resulted in a high minimum reflection loss frequency and small minimum reflection loss thickness. Variations of minimum reflection loss frequency and thickness of composites resulted from variations of materials constants such complex permeability, permittivity and resonance frequency. Changes in materials variables were due to the demagnetization effect and the eddy current effect, which operate differently in composites according to amorphous alloy particle size and fraction.     

Er-doped hydrogenated amorphous silicon: structural and optical properties

The effect of erbium-doping on the structural and optical properties of hydrogenated amorphous silicon (a-Si:H) is investigated. Optical absorption and Raman spectra indicate that erbium doping introduces defect states, and that above a concentration of 0.27 at.%, induces strong structural disorder. The photoluminescence measurements show that erbium doping introduces non-radiative decay paths for carriers in a-Si:H, leading to decrease in both the Er³⁺ and intrinsic a-Si:H luminescence intensity when the Er concentration is increased to more than 0.04 at.%. The results are compared to that of Er-doped crystalline Si, and the possible excitation mechanisms of Er in a-Si:H are discussed.     

Environmental stabilization of a-Si:H photoconductors via reaction with an alkylsilane

Reactively sputtered a-Si:H photoconductive layers approximately one μm thick on flexible conductive substrates were treated with a monomeric octadecylsilane derivative and cured briefly at 120 °C to form and bond an alkylsiloxane surface layer. Whether the a-Si:H layer was bare or first topcoated with 10 nm of Si₃N₄, treated photoconductors had spatially uniform dark surface potentials greater than 22 V at 105 s after charging by negative corona in ambient relative humidity (ARH) of 65%. The improved environmental stability enabled stable, high-quality electrophotographic performance in ARH exceeding 70% with electrophoretic toners optimized for CdS photoreceptors. The simple chemical stabilization may be of broader utility in silicon-based technology.     

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

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

Polymorphic forms of o-benzylphenol and slow molecular motions in the amorphous state

The slow molecular motions in o-benzylphenol were studied by thermally stimulated depolarization currents (TSDC). No dipolar reorientational motions were found in the crystal, in the temperature region from −165 °C up to the melting temperature. Oppositely, a strong α-relaxation was detected in the amorphous solid state, but no local (secondary) relaxations were observed. The glass transition relaxation was characterized by TSDC, by conventional differential scanning calorimetry (DSC) and by modulated temperature differential scanning calorimetry (MTDSC). The fragility index of this glass forming system was determined based on data from the different techniques. The obtained values were m = 61 (DSC), m = 83 (MTDSC), and m = 65 (TSDC). Conventional DSC was used to identify the polymorphs of o-benzylphenol and their most favorable formation conditions. Two metastable polymorphs were observed, with melting temperatures near 15 and 21 °C.