Effect of vacuum annealing on charge transport and trapping in a-Si1 − xCx:H/c-Si heterostructures

The processes of charge transport and trapping in amorphous Si_1 ? xC_x:H films deposited on crystalline p-type Si wafers and annealed in vacuum in the temperature range 300–650 °C have been evaluated. Current–voltage (I–V), capacitance–voltage (C–V) and admittance–temperature (G–T) characteristics were measured in the temperature range 100–350 K. The spectrum of thermal effusion of hydrogen was measured from room temperature up to 1000 °C.C–V characteristics indicate a slight increase of the dielectric constant k and a large hysteresis after annealing at 450 °C. The hysteresis is believed to be associated with mobile hydrogen effusion from the a-SiC:H film, and it is not seen after a 650 °C anneal. From I–V data the maximum rectification ratio is observed after annealing at 450 °C. Variable-range hopping (VRH) conduction at the Fermi level is found to dominate the forward current of the as-deposited structure. After annealing at 450 °C the forward current can be described by space-charge limited (SCL) mechanisms with trapping at shallow levels with energy of about 0.12 eV. After annealing at 650 °C the process of VRH conduction appears again, but the density of hopping sites is much higher than in the as-grown sample. From admittance spectra, the energy position of respective traps in a-SiC:H is at (E_V + 0.45) eV for as-deposited material and it decreases slightly after vacuum annealing. On the basis of these results, an energy band diagram of the a-Si_1 ? xC_x:H/p-Si structure annealed at 450 °C is proposed.     

Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550 °C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650 °C. The size of anatase crystallites can reach 30–50 nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti–O–NbO groups at the interface area. Slight shift and broadening of E_g (144 cm^?1), A_1g (515 cm^?1) and E_g (630 cm^?1) modes in Raman spectra can be explained by niobium insertion into the anatase structure.     

Relaxation of polaronic charge carriers in lithium manganese spinels

Lithium manganese spinels Li_1+xMn_2−xO₄, 0 ⩽ x ⩽ 0.33, were prepared by wet chemistry technique followed by heat-treatment at 750 °C or 800 °C. Differential scanning calorimetry was used to reveal phase transitions. Electrical properties were studied by impedance spectroscopy. LiMn₂O₄ exhibited phase transition below room temperature. The transition, seen as an exothermic event in DSC and a steep decrease of conductivity upon cooling, was sharp in sample sintered at 800 °C and broadened over a range of temperature in sample sintered at 750 °C. In the low temperature phase of LiMn₂O₄, two relaxations of similar strength were observed in the frequency dependent permittivity. The low frequency process was identified as relaxation of charge carriers since the relaxation frequency followed the same temperature dependence as the dc conductivity. The high frequency process exhibited milder temperature dependence and was attributed to dipolar relaxation in the charge-ordered structure. The dipolar relaxation was barely visible in Li substituted samples, x ⩾ 0.05, which did not undergo structural phases transition. Measurements extended to liquid nitrogen temperature showed gradual lowering of the activation energy of conductivity and relaxation frequencies, behavior typical for phonon-assisted hopping of small polarons.     

Manufacturing of porous niobium phosphate glasses

Niobium phosphate glasses with composition 33P₂O₅ · 27K₂O · 40Nb₂O₅ are usually very stable with regard to crystallization resistance, with a relatively high glass transition temperature (T_g ∼ 750 °C), and are potentially suitable for nuclear waste immobilization. Porous niobium phosphate glasses were prepared by the replication method. The porous glasses were produced via the dip-coating of an aqueous slurry containing 20 wt% powdered glass into commercial polyurethane foams. The infiltrated foams were oxidized at 600 °C for 30 min to decompose the polymeric chains and to burn out the carbon, leading to a fragile glass skeleton. Subsequent heating above the glass transition temperature in the range of 780–790 °C for 1 h, finally resulted in mechanically stable glass foams, which maintained the original interconnected pore structure of the polyurethane foam. The struts showed the neck formation between particles, evidencing the initial stage of sintering. The open and interconnected porosity of the glassy foams lies in the range of 85–90 vol.%. It was concluded that porous niobium phosphate glasses are potential candidates for immobilizing liquid nuclear waste.     

Effect of OH-content on thermal and chemical properties of SnOP2O5 glasses

Glasses with 55–60 mol% SnO and 40–45 mol% P₂O₅ have shown extremely large differences in the chemical and thermal properties depending on the temperature at which they were melted. Glasses prepared at low melting temperature, 450–550 °C, had low T_g, 150–200 °C, and low chemical stability. Glasses prepared at high melting temperature, 800–1200 °C, had much higher T_g, 250–300 °C, and much higher chemical stability. No significant differences were found by ¹¹⁹Sn Mössbauer and ³¹P Nuclear Magnetic Resonance spectroscopy. Large differences in the OH-content could be detected as the reason by infrared absorption spectroscopy, thermal analyses, and ¹H Nuclear Magnetic Resonance spectroscopy. In samples with low T_g, a broad OH – vibration band around 3000 nm with an absorption intensity >20 cm^?1, bands at 2140 nm with intensity ～5 cm^?1, at 2038 nm with intensity ～2.7 cm^?1, and at 1564 nm with intensity ～0.4 cm^?1 were measured. These samples have shown a mass loss of 3–4 wt% by thermal gravimetric analyses under argon in the temperature range 400–1000 °C. No mass loss and only one broad OH-band with a maximum at 3150 nm and low absorption intensity <4 cm^?1 could be detected in samples melted at high temperature, 1000–1200 °C, which have much higher T_g, ～300 °C, and much higher chemical stability.     

Properties of high nitrogen content mixed alkali earth oxynitride glasses (AExCa1−x)1.2(1)SiO1.9(1)N0.86(6), AE = Mg,Sr, Ba

Mixed alkali earth element containing high nitrogen content oxynitride glasses (Ca_1?xAE_x)_1.2(1)SiO_1.9(1)N_0.86(6), with AE = Mg, Sr, Ba, x ≤ 0.30 for Mg and x ≤ 0.46 for Sr and Ba, and nominally constant (Ca/AE):Si:O:N ratios were prepared in order to investigate the compositional dependencies of physical properties on alkali earth element composition. The glasses were prepared by melting mixtures of AEH₂, CaH₂, SiO₂ and Si₃N₄ powders in nitrogen atmosphere at 1600–1700 °C and characterized by X-ray powder diffraction and scanning and transmission electron microscopy. Cation and anion glass compositions were determined by respectively energy dispersive X-ray analysis and combustion analysis. The determined physical properties were density, glass transition temperature, Vickers hardness, and refractive index. The physical properties were found to vary linearly with the degree of substitution of Ca by the AE elements. The density of the glasses increases substantially upon substitution by Sr and Ba, up to 3.99 g/cm³. Glass transition temperatures are found to be higher for Mg and Sr substituted glasses, ca. 900 °C, in comparison with Ba substituted glasses, ca. 850 °C. The hardness increases upon substitution by Mg, up to 12.2 GPa at x = 0.46, and decreases upon substitution by Sr and Ba. The refractive index increases upon substitution by Sr and Ba, up to 1.97 for Ba at x = 0.46, and decreases upon substitution by Mg. The transparency of the glasses was found to increase upon increasing substitution by Mg and completely transparent glasses were obtained for x = 0.24.     

Electrical and dielectric properties of glass system NaPO3–KHSO4

Glass samples have been prepared in the NaPO₃–KHSO₄ binary system with the classical melting, casting and annealing steps. Electrical and dielectrical properties of glass samples were studied. Measurements of DC and AC conductivity and complex electrical permittivity of xNaPO₃–(100 ? x)KHSO₄ glass system were carried out at temperatures ranging from room temperature to temperature located 15 °C below glass transition temperature T_g. Results showed that changes of NaPO₃ concentration considerably affect values of observed parameters. DC conductivity of glass increases as NaPO₃ concentration grows until concentration x = 60. However, beyond this value a sharp decrease of DC conductivity was observed. In addition relaxation times showed abrupt changes at concentration x = 60, corresponding to the lowest relaxation times at the temperature 90 °C.     

Precise sound velocity measurement for liquid Se50Te50 under high pressure

We have measured the sound velocity in liquid Se₅₀Te₅₀ at 19.5, 32.6, 45.6, and 58.2 MHz simultaneously by means of an ultrasonic pulse transmission/echo method. By using a phase-sensitive-detection technique the relative error was reduced to less than 0.1%. The temperature dependence of sound velocity at 100 MPa exhibits a minimum at 600 ^oC and a maximum at 1035 ^oC, which are related to the large structural change accompanied by the semiconductor–metal (S–M) transition. In addition, sound dispersion has been observed at temperatures from 500 °C to 900 °C for the first time, where anomalous sound attenuation was previously reported. This result implies that a structural relaxation on nanosecond timescale takes place in the S–M transition region.     

Substrate temperature dependence of microcrystalline silicon growth by PECVD technique

Undoped hydrogenated silicon films have been prepared from a gas mixture of silane and hydrogen, varying substrate temperature from 180–380 °C in an ultrahigh vacuum system using RFPECVD technique. XRD and Raman measurements enable us to know that the films are microcrystalline throughout the substrate temperature range. Bond formation of the SiH films at different substrate temperature is studied through different characterisation techniques like Fourier transform infrared spectroscopy and hydrogen evolution study. The infrared absorption spectroscopy and hydrogen evolution study reveal two types of growth: the formation of a void rich material at low T_s (∼180 °C) and a compact material at comparatively higher T_s.     

Pyrolytic conversion studies of blended polycarbosilane and polyaluminasilazane

Multinuclear solid-state NMR spectroscopy, FTIR and Raman experiments are employed to investigate the pyrolytic conversion of blended polycarbosilane and polyaluminasilazane (denoted CA) up to 800 °C, with the aim of studying structural evolutions and interactions between polycarbosilane and polyaluminasilazane during the pyrolysis process. Vinyl and SiCH₃ units can react with Si–H, SiCH₃ and Si–CH₂–Si groups below 400 °C. These crosslinking reactions can increase the ceramic yield of the blended precursors. At 500 °C aromatic carbon is formed, and N–H and Si–H groups vanish at 600 °C and 700 °C, respectively. At 600 °C, SiCH₃ and Si–H units can further react with SiCN₃, SiC₂N₂, N–H and C–H units. An amount of amorphous carbon and CSi₄ and CSi₃H groups are detectable at 800 °C. Even at this temperature there are still many aromatic protons. In addition, there are also SiC₄, SiC₃N, SiCN₃ and SiN₄ units. Silicon forms SiN₄ more readily than SiC₄. Many AlN₅ groups transform into AlN₆ groups. The D and G bands of graphite are observed in CA pyrolyzed at 1400 °C. According to the XRD patterns, the reflection of crystalline β-Si₃N₄ vanishes at 1700 °C, and the residue pyrolyzed at 1800 °C mainly contains a large number of 2H-SiC/AlN solid solution crystals and a few β-SiC crystals.     

Microstructure and properties of spark plasma sintered Fe–Cr–Mo–Y–B–C bulk metallic glass

Fabrication of Fe-based amorphous alloy using spark plasma sintering (SPS) process has been reported. Fully amorphous compacts with ～95% relative density were successfully sintered at temperature about 100 °C lower than glass transition temperature (T_g: 575 °C). Formation of crystalline Fe₂₃(C, B)₆ phases within near-fully dense (～99%) amorphous matrix is observed at sintering temperatures (>550 °C) close to glass transition temperature. Microstructure evolution in sintered compacts indicated that density, degree of crystallinity, and mechanical properties can be effectively controlled by optimizing SPS parameters.     

Characterization of thermo-optical and mechanical properties of calcium aluminosilicate glasses

In this work several different compositions of CaO:Al₂O₃:SiO₂ were prepared under vacuum atmosphere to study the glass forming ability of this system as a function of the SiO₂ content. Samples containing 25–45 wt% of Al₂O₃, 31–44 wt% of CaO, 14–39 wt% of SiO₂ and 4.1 wt% of MgO were prepared in graphite crucibles, for approximately 2 h at ∼ 1600 °C. The influence of silica content is discussed in terms of the mechanical properties, glass transition temperature, crystallization temperature, transmittance spectrum, refractive index, mass density, specific heat, thermal diffusivity, thermal conductivity and the temperature coefficient of optical path length change. The results reinforce the idea that these glasses are strong materials, having useful working-temperature range, good combination of thermal, mechanical and optical properties that could be exploited in many optical applications, in particular, as glass laser materials.     

Static and dynamic structures of liquid GeTe mixtures

Neutron diffraction measurements of the liquid Ge–Te system and quasi- and inelastic neutron scattering experiments of the eutectic liquid Ge₁₅Te₈₅ were performed in the temperature range from 400 to 560 °C. The large temperature dependence of the static structure for liquid Ge₁₅Te₈₅ is reconfirmed; however it is found that the compositional dependence obtained at around 80 °C above the liquidus curve is rather small. The dynamic structure factors of liquid Ge₁₅Te₈₅ in the Q-region from 0.2 to 2.6 Å⁻¹ show a normal temperature dependence. From the vibrational density of states, it is found that the Te–Te covalent bond persists in liquid Ge₁₅Te₈₅; however a bending mode was not assigned clearly.     

Growth and characterization of Hexakis(thiourea)nickel(II) nitrate crystals

Single crystals of hexakis(thiourea)nickel(II) nitrate [Ni(SC(NH₂)₂)₆](NO₃)₂ are grown by slow evaporation of methanolic solution at room temperature. Structural analysis by single crystal X-ray diffraction analysis reveals that the crystal belongs to monoclinic system with space group C2/c and the cell parameters are a=22.046(2) Å, b=9.3325(4) Å, c=16.221(2) Å, Z=8. The metal is coordinated by six thiourea groups with Ni–S–C bond angles ranging from 114.81° to 116.85° and Ni–S bond lengths lying in the range 2.35 to 2.61 Å in a distorted octahedral geometry. The interesting feature observed in this study is that although it crystallizes in centrosymmetric structure, contrary to expectations, it exhibits a positive second harmonic generation (SHG) result, quite likely due to the change in stereochemical arrangement. An Nd:YAG laser with a modulated radiation of 1064 nm directed on the powdered sample leads to local noncentrosymmetry and this could be due to the loss of thiourea ligands resulting in tetrakis(thiourea)nickel(II) complex causing green light emission. The powder X-ray diffraction study reveals the crystallinity of the grown material. The vibrational patterns in FT-IR clearly evidence the complex formation. Thermogravimetric analysis (TG) reveals the purity of the sample and no decomposition is observed up to the melting point. The crystal is further characterized by diffused reflectance spectroscopy, dielectric studies and microhardness analysis.     

Systematic study of spectroscopic properties and thermal stability of lead germanate glass doped with rare-earth ions

Glasses with composition 50GeO₂–(50?x)PbO–5PbF₂–xLnF₃ (Ln = Pr³⁺–Yb³⁺) were prepared from commercial raw materials. The content of PbF₂ was constant and amounted to 5 mol% whereas the concentration of luminescent ions was diverse (0.2 and 2 mol%). Thermal stability of the glasses were monitored by differential thermal analysis (DTA). It has been found that increase of rare-earth fluoride content from 0.2 mol% to 2 mol% brings about a shift of the glass crystallization onset from 450 °C to 487 °C for Nd-doped samples and from 466 °C to 482 °C for Tm-doped samples. Optical absorption and emission spectra of Ln active ions in oxyfluoride glass have been investigated at room temperature in the ultraviolet (UV), visible (VIS) and near-infrared (NIR) region. Oscillator strengths, phenomenological Judd–Ofelt intensity parameters Ω_2,4,6, radiative transition probabilities, branching ratios and radiative lifetimes of luminescent levels have been estimated. Analysis of decay curves of luminescence revealed that the increase of rare-earth fluoride content from 0.2 mol% to 2 mol% shortens the lifetime of the ⁴F_3/2 level of Nd³⁺ from 224 μs to 90 μs.     

Non-isothermal crystallization kinetic studies on MgO–Al2O3–SiO2–TiO2 glass

Thermal stability and crystallization kinetics of the glass 21% MgO, 21.36% Al₂O₃, 53.32% SiO₂ and 4.11% TiO₂ (mol%) has been studied using differential thermal analysis (DTA), dilatometry and X-ray diffraction (XRD). Glass in both bulk and frit forms were produced by melting in platinum crucible at 1600 °C for 1–2 h. From variation of DTA peak maximum temperature with heating rate, the activation energies of crystallization were calculated to be 340 kJ mol⁻¹ and 498 kJ mol⁻¹ for first and second crystallization exotherms, respectively. Crystallization of bulk glass was carried out at various temperatures and for different time durations in the range of 850–1000 °C. The influence of the addition of TiO₂ on the crystallization sequence of the glass was experimentally determined and discussed.     

Kinetic fragility of hydrous soda-lime-silica glasses

The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na₂O 10CaO 74SiO₂ in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na₂O 10B₂O₃ 74SiO₂ in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content C_W up to ≈ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m, was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses (C_W > 1 wt%) were surveyed together with literature data on the (H₂O)–Na₂O–CaO–SiO₂, (H₂O)–Na₂O–SiO₂ and (H₂O)–SiO₂ systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a ? b logC_W where a, b are fitting parameters, enables m to be predicted, for each glass series as function of the water content C_W. The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.     

Enhancement of photoluminescence in SiCxNy nanoparticle films by addition of a Ni buffer layer

This work reports the effect of the presence of a Ni buffer layer on the photoluminescence (PL) of SiC_xN_y nanoparticle films prepared by RF plasma magnetron sputtering process in a reactive N₂ + Ar + H₂ gas mixture. An introduction of a Ni buffer of 80 nm or thicker remarkably improves the PL of the films. Annealing in a temperature range of 400–1100 °C is found to significantly affect the PL intensity. Optimal PL is achievable at 600 °C. X-ray photoelectron and Fourier-transform infrared spectroscopy suggest that the strong PL is directly related to the composition of the SiC_xN_y nanoparticle and the concentration of Si–O, and Si–N bonds. The results are relevant to the development of wide bandgap optoelectronic devices.     

Structure and electrical properties of nitrided NbN–TiN sol–gel derived films

The results of investigations of electrical conductivity and the structure of NbN–TiN thin films in a different NbN/TiN molar ratio are presented in this work. Sol–gel derived xNb₂O₅?(100?x)TiO₂ coatings (where x = 100, 90, 80, 70, 60, 50, 40, 0 mol%) were nitrided at 1200 °C to obtain NbN–TiN films. The structural transformations occurring in the films as a result of ammonolysis were studied using X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The electrical conductivity was measured with a conventional four-terminal method in the temperature range of 5–280 K. The NbN–TiN samples exhibited a negative temperature coefficient of resistivity. The positive temperature coefficient of resistivity was observed only for the x = 0 sample. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system. The film thickness effect on the superconducting properties was studied for x = 80 and x = 100 samples. The superconducting transition was not observed in all samples, the exception was x = 80 sample, 1050 nm in thickness. It is not clear, why all x = 100 samples do not exhibit superconducting transition in resistivity measurements. It seems to be possible, that the Josephson junction formation between NbN grains could be blocked by non-superconducting phases present in these samples.     

Influence of TiO2 on proton conductivity in fuel cell electrolytes based on sol–gel derived P2O5–SiO2 glasses

P₂O₅–TiO₂–SiO₂ based glasses have been prepared by a sol–gel process. The glasses were characterized by structural, thermal, nitrogen adsorption–desorption and conductivity measurements. The structural formation has been confirmed by the FTIR and NMR analysis. The proton conductivity of the glasses increased linearly with increase in temperature. Glasses with an average pore size less than 2 nm showed higher values of proton conductivity in humid atmosphere. The conductivity value increased from 6.47 × 10⁻⁴ S/cm to 3.04 × 10⁻² S/cm at 70% RH in the temperature range 30–90 °C. We observed in fuel cell measurements that the performance of the E1 electrode is superior to that of the other electrodes at the same operating condition. The power density shows a similar pattern to current density.