Thermodynamic restrictions of LPE of bismuth-containing А3В5 solid solutions

A heterophase equilibrium estimation procedure is proposed for systems InBiAsSb and GaInBiAsSb using the simple solution model and a ‘virtual’ GaBi compound. Thermodynamic characteristics and crystal-chemical parameters of the GaBi compound have been determined for the calculation with the linear interpolation method. Phase equilibrium in In–Bi–As–Sb and Ga–In–Bi–As–Sb systems has been analyzed for the 650–780 K temperature range. The existence limits for solid solutions have been set and the thermodynamic restrictions characterizing the capacity for synthesis have been ascertained. The data obtained may be useful in optimization of LPE methods.     

I–V characteristics and break down studies of Sb2O3 structures

I–V characteristics of sandwiched Al–Sb₂O₃–Al structures have been studied for different thicknesses. The current-voltage curves in general exhibit three regions, ohmic, non ohmic and breakdown regions. The breakdown voltage increases whereas the dielectric strength decreases with increase in Sb₂O₃ film thickness. The electrical breakdown studies have been done for dc and ac voltages and optical photomicrographs of breakdown patterns during different stages of voltage have been taken and the results are explained.     

Undoped semi-insulating GaAs crystals grown by a modified low pressure LEC method

A modified low pressure in-situ synthesis LEC method of growing undoped SI (semi-insulating) GaAs crystals has been established. The key points for controlling melt composition and As evaporation during synthesis and growth have been described. Using this novel approach, crystals are able to be grown from the nominal melt composition in the range of 0.491–0.499 As fraction with high reproducibility. Some characteristics of the undoped SI crystals grown by the present work including electrical properties, dislocation density, carbon and EL2 concentrations and thermal annealing effects have been studied.     

Electrical Properties of Large-grain Polycrystalline GaP

I–U and I–T measurements have been performed on large-grain polyerystalline p-GaP grown by the SSD method or synthesized in a high-pressure autoclave. There exists a clear correlation between these characteristics and those of corresponding bierystals from which it can be concluded on a strongly unequal division of the applied voltage on both depletion regions of the grain boundaries. On the other hand I–U characteristics of polysilicon mostly are described by a sinh-behaviour which supposes an equal division of the voltage. These differences are discussed. The I–U characteristics of the polycrystalline GaP at room temperature are governed by thermionie emission and thermionic field emission whereas at low temperatures (and high voltages) tunneling dominates. At high temperatures the influence of grain boundaries disappears.     

Rare-earth photoluminescence in sol–gel derived confined glass structures

The characteristics of rare-earth luminescence in selected sol–gel derived confined structures have been examined. Erbium and erbium/ytterbium doped photonic materials and structures have been prepared by sol–gel processing, in the form of silicate optical planar waveguides, modified with titania and hafnia, and 1-D photonic bandgap structures consisting of multilayer stacks of silica and titania. The Er³⁺ ions were found to be sensitive probes of the waveguide glass matrix structure, especially when hafnia-containing nanocrystallites were present, which narrowed and resolved different Stark components of the photoluminescence peaks. In 1-D Fabry–Perot coupled microcavities, efficient energy transfer was observed from Yb³⁺ to Er³⁺ ions when these were present simultaneously in the same defect layer, but not when the two types of ions were isolated in separate defect layers.     

Optical and X-ray studies of a four component cyano liquid crystal mixture: Comparison of properties with that of a similar mixture

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano–end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano.     

Electrical and sensory properties of zinc oxide – porous silicon nanosystems

Abstract

Zinc oxide nanostructures have been grown by electrochemical deposition on porous silicon–silicon substrate. The photoelectric and sensory properties of the obtained ZnO–porous silicon nanosystems were investigated in both DC and AC regimes. The obtained structures were characterized by photosensitivity in the 400–1100?nm wavelength range and by high sensitivity to moisture. Increase of relative humidity resulted in significant decrease of the electrical resistance and increase of the capacitance of the hybrid structures. To estimate the sensory properties of the ZnO–porous silicon nanostructures their adsorption sensitivity and dynamic characteristics were analyzed. Discovered features of the charge transport processes broaden the prospects of the semiconductor nanosystems application in gas sensors and photodetectors.     

Formation and structural characteristics of Pd–Ag/SiO2 and Pd–Cu/SiO2 catalysts synthesized by cogelation

Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of either Pd and Ag or Pd and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). After an extensive study of the influences of synthesis operating variables over structural characteristics of gels, highly dispersed bimetallic Pd–Ag/SiO₂ and Pd–Cu/SiO₂ xerogel catalysts were obtained. These samples are then composed of completely accessible Pd–Ag and Pd–Cu alloy crystallites with sizes of 2–3.5 nm located inside silica particles exhibiting a monodisperse microporous distribution. It appears also that the metal complex acts as a nucleation agent in the formation of silica particles.     

Thermoelectric power of amorphous compound semiconductors

Thermoelectric power measurements as a function of temperature have been made on four V–VI amorphous chalcogenides, ten IV–VI amorphous chalcogenides of the Ge_xTe_1?x system, and five amorphous IV–V materials. In the IV–V and the intermediate composition IV–VI materials, the charge transport cannot be described on the basis of conduction at only one energy level. The former exhibit characteristics of conduction both in extended states and in localized states at the Fermi level. Transport in the V–VI materials can be formally described in terms of conduction at one level in either the chaotic band or the small polaron models, but the use of a two-channel model with transport simultaneously in both p-type extended and localized states seems the most promising.     

Photovoltaic properties of sputtered silicon films

The photovoltaic properties of several 1 μm thick films of rf-sputtered amorphous Si (a-Si) sandwiched between lower Al electrodes and upper semitransparent Mo or Al electrodes have been investigated. After fabrication, the Al–Si–Al samples were annealed in vacuum at temperatures between 100 and 450°C. The spectral variation of the photoresponse in the wavelength region between 300 nm and 2 μm was measured. The results indicate that the optical gap of a-Si is 1.5 eV and the Schottky barrier height at the Al–a-Si interface is 0.75 eV. the current-voltage characteristics of these Schottky-type devices were investigated under illumination. Photovoltaic energy conversion efficiencies of up to 0.03% were observed for the 100 mW/cm² white light of a tungsten-halogen lamp.     

Formation of nanoparticles in soda-lime glasses by single and double ion implantation

Structural characteristics and optical properties of monometallic and bimetallic Ag and Au nanoparticles in the surface region of soda-lime glass fabricated by ion implantation have been studied by transmission electron microscopy and optical spectroscopy. As a result it has been found that both, implantation dose and process temperature, strongly influence the metal nanoparticle formation governed by ion diffusion and metal precipitation as well as the involved stress generation around the particles. Thus, the mean size of metal nanoparticles and the width of the particle containing region beneath the glass surface increase with increasing temperature as well as implanted dose. Upon sequential high-dose double implantation to form bimetallic Ag–Au nanoparticles a rather complex configuration has been obtained. Particles of sizes above a threshold of 5–10 nm exhibit distinct image contrast features indicating the development of central voids whose sizes are proportional to the outer particle diameter.     

The study of Sm3+-doped low-phonon-energy chalcohalide glasses

The Sm³⁺-doped low-phonon-energy (LPE) Ge–Ga–Se–CsI glasses were studied. Upon excitation at 980 nm diode laser, intense 1.25 and 1.49 μm near-infrared fluorescence bands with broad full width at half maximum (FWHM) of 49 and 53 nm were observed, respectively. About 180–300 μs fluorescence lifetimes were obtained for the 1.49 μm emission. The thermal properties and structure of glasses were investigated by differential thermal analysis (DTA) and Raman spectra, respectively. Spectroscopic characteristics of the optical transitions have been calculated by using the Judd–Ofelt theory and evaluated for excited levels.     

Characteristics of nickel ammonium sulphate crystals precipitated from aqueous solution

The main factors affecting the characteristics of precipitated nickel ammonium sulphate hexahydrate crystals from aqueous solution have been investigated. Over the range 20–35 °C, the crystal median size increases with temperature, but decreases with supersaturation. The size distribution is relatively unaffected by temperature, but becomes narrower at high supersaturations. The yield of crystalline precipitate is greatly increased when non-stoichiometric ratios of reactants are employed due to the sharp decrease in the equilibrium solubility.     

Study of the temperature dependence of the electrical conductivity in strontium-barium niobate crystals with different dopants

The electrical characteristics of single crystals of strontium-barium niobate solid solutions have been studied. The temperature dependences of the electrical conductivity were obtained for different SBN compositions, nominally pure SBN:61 and SBN:75 and SBN doped with rare earth and transition-metal impurities. The influence of the chemical composition of a solid solution, thermal treatment conditions, dopant concentration, and electrode type on the specific features of the electrical conductivity of single crystals in the temperature range 20–450°C is shown.     

Structure and electric conductivity of reduced lead–germanate,bismuth–germanate and bismuth–silicate glasses modified with potassium

Lead–germanate, bismuth–germanate and bismuth–silicate glasses, as potential materials for detectors production, have been modified with potassium with the purpose of improvement of mechanical properties. Studies on the influence of potassium modification on the structure and electric properties have been conducted with AFM, DSC and conductivity measurements. It has been observed that potassium modification improves the mechanical and technological properties of glasses and simultaneously changes slightly their electric properties. The studies have confirmed that bismuth–silicate and bismuth–germanate glasses are good materials for microchannel plates or channeltrons.     

DC current transport in rf sputtered SiO2 films in the temperature range 77–357 K

The dc current conduction in rf sputtered SiO₂ films has been studied in the temperature range 77–357 K with electric fields up to 9 × 10⁷V m^?1. Nonlinear current voltage characteristics have been observed. Two activation energies of about 0.3 eV and 0.03 eV appear in the high and low temperature ranges respectively. Different transport mechanisms are discussed. The Poole-Frenkel effect would not be applicable. The most probable mechanism is electron hopping between defect states but with deformation of the surroundings.     

Crystallization of the amorphous phase and martensitic transformations in multicomponent (Ti,Hf,Zr)(Ni,Cu)-based alloys

Shape memory materials with specific microstructure can be obtained from amorphous precursors. Rapidly solidified amorphous and crystalline–amorphous ribbons have been produced by planar flow casting for a number of multicomponent TiNi-based alloys of pseudobinary 50:50 and 55:45 compositions with substitutions of Zr, Hf, Nb for Ti and Cu, Co, Pd, Ag, Al for Ni. The glass transition and crystallization behavior of the amorphous phase as well as the martensitic transformations in the crystallized materials have been studied by differential scanning calorimetry. To control the crystal structure at different stages of crystallization, X-ray diffractometry and transmission electron microscopy were used. The effects of crystallization conditions and the resulting microstructural state on the martensitic transformation characteristics have been investigated. The transformation temperature intervals are shown to depend on the mean size of the parent phase crystals.     

Controlled crystallization of GeS2–Sb2S3–CsCl glass for fabricating infrared transmitting glass-ceramics

The crystallization of selected glasses from the GeS₂–Sb₂S₃–CsCl ternary system has been studied under non-isothermal condition. The nucleation and crystal growth mechanisms have been investigated and proved to be dependant on the glass composition. It has been found that the 80GeS₂–10Sb₂S₃–10CsCl is a good candidate for controlled crystallization. The best nucleation temperature and time have been determined. Crystals of about 20–30 nm have been uniformly generated in the glass and the obtained glass-ceramics have the same transmission in the mid and far infrared transmission.     

Thermal expansion and transition temperature of glasses in the systems BeO–Al2O3–B2O3 and MgO–Al2O3–B2O3

Researches were conducted for glasses in the systems BeO–Al₂O₃–B₂O₃ and MgO–Al₂O₃–B₂O. The following characteristics have been determined: thermal expansion coefficient within 20–300 °C, structural thermal expansion coefficient (STEC) and glass transition temperature. The discussed properties of Be-aluminoborate and Mg-aluminoborate glasses have been compared with those of Ca-, Sr- and Ba-aluminoborate glasses. TEC of studied glasses gets higher going from Be-aluminoborate glasses to Ba-aluminoborate ones, but T_g decreases under the same succession. The dependence of STEC on the nature of a given cation is more complicated. The pattern of dependence of the properties on composition is due to changing the boron coordination number with respect to oxygen, which is the main network-former, and to competition between the aluminium and boron for the oxygen brought by an ion-modifier. The difference between the effect of one ion-modifier and that of another is determined by decreasing the radius of an ion and increasing its electric field strength in the succession from Ba²⁺ to Be²⁺. The ions of Be and Mg can also act as network-former to some extent.     

Optical properties of lanthanum-gallium tantalate at different growth and post-growth treatment conditions

Optical properties of lanthanum-gallium tantalate crystals grown in different atmospheres (argon, argon-oxygen mixture, air) have been investigated. Optical transmission spectra in the ranges 0.185–0.800 and 2.0–10.0 μm have been recorded. The range of lanthanum gallium tantalate transparency is established to be 0.26–7.6 μm. The main refractive indices n _o and n _e have been measured and their inhomogeneity and dispersion have been investigated in the range 0.40–0.70 μm. It is shown that the growth atmosphere significantly affects the optical properties of the crystals studied.