Memory-switching effect in single crystals of thallium selenide

It has been found that thallium selenide single crystals exhibit bistable or memory switching. The specimen under test showed threshold switching with critical field of the switching being ∼ 10³ V/cm at room temperature. Memory switching effect measurements were performed with single crystals of p–TlSe. The results strongly indicated that the phenomenon in our sample is very sensitive to temperature, light intensity and sample thickness. Memory state persists if the current is decreased slowly to its zero value. However, if current was forced to decay suddenly, the specimen returned to the high resistance state. The current-voltage characteristics is symmetrical with respect to the reverse of the applied voltage and current. The switching parameters were checked under the influence of different factors of the ambient condition. An analysis of the results was done.     

Change in the electrical properties and structure during recrystallization of polycrystalline cadmium selenide films

The processes of structure transformation, including recrystallization processes upon annealing in different atmospheres, are considered using polycrystalline CdSe films doped with silver by ion implantation as an example. The relationship between the recrystallization and activation of implanted impurity is established, and it is shown that the structural transformations during film recrystallization decisively affect the activation of the implanted impurity and, as a result, the electrical and photoelectric properties of recrystallized films.     

Growth and properties of monocrystals for miniature lasers

We have considered the special features of growth for the Nd compounds in the form of monocrystals containing a high concentration of Nd ions (>10²¹ ions/cm³) with an anamolously low concentration quenching. These compounds are known popularly as “nezoites” containing isolated Nd polyhedra in the structure, which gives a clue to the technology of growth. All the known nezoite crystals, their X-ray analyses and the conditions of growth from multicomponent systems are tested. At present there are two well-known methods of growing nezoites: (1) by evaporation of the solvent at constant temperature and pressure; and (2) by flux method. The authors have discussed the advantages and disadvantages of these two methods. The problem of the presence of light extinguishing admixtures, particularly (OH) molecules in the crystals, and the influence of these admixtures on the basic properties of these crystals (nezoites) is discussed in detail.     

The preparation and electrical properties of thin chalcogenide semiconductor films

Thin chalcogenide films from the GeTeAsSi system have been prepared using electron beam evaporation and R.F. sputtering techniques. The techniques are described in some details as it is believed that the deposition procedure has a significant on subsequent electrical switching behaviour.     

Microstructure optical and electrical studies of cadmium selenide thin films

Nucleation and growth of cadmium selenide thin films are of considerable interest because of their direct effect on the optical and electrical properties of this material. Vacuum-deposited layers on amorphous and crystalline substrates showed a polycrystalline structure. The layers were deposited at a pressure of 10⁻⁴ Pa, with two deposition rates of 12 nm/s and 15 nm/s. The electrical resistivity and the optical absorption were studied for a group of layers on quartz substrate. As well the morphology and the microstructure were investigated by X-ray and electron microscopy. Few activation energies were estimated from the optical absorbance and thermo-resistance.     

Influence of the substrate temperature on the structural,optical, and electrical properties of tin selenide thin films deposited by thermal evaporation method

Thin films of tin selenide (SnSe) were deposited on sodalime glass substrates, which were held at different temperatures in the range of 350‐550 K, from the pulverized compound material using thermal evaporation method. The effect of substrate temperature (T_s) on the structural, morphological, optical, and electrical properties of the films were investigated using x‐ray diffraction analysis (XRD), scanning electron microscopy (SEM), transmission measurements, and Hall‐effect characterization techniques. The temperature dependence of the resistance of the films was also studied in the temperature range of 80‐330 K. The XRD spectra and the SEM image analyses suggest that the polycrystalline thin films having uniform distribution of grains along the (111) diffraction plane was obtained at all T_s. With the increase of T_s the intensity of the diffraction peaks increased and well‐resolved peaks at 550 K, substrate temperature, were obtained. The analysis of the data of the optical transmission spectra suggests that the films had energy band gap in the range of 1.38‐1.18 eV. Hall‐effect measurements revealed the resistivity of films in the range 112‐20 Ω cm for films deposited at different T_s. The activation energy for films deposited at different T_s was in the range of 0.14 eV‐0.28 eV as derived from the analysis of the data of low‐temperature resistivity measurements. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microwave preparation,structure and electrical properties of calcium–sodium–phosphate biosystem

The hydroxyapatite biomaterial (Ca₁₀(PO₄)₆(OH)₂ – HAP) is the main mineral constituent of teeth and bones, with excellent biocompatibility with hard and muscle tissues. These materials exhibit several problems of handling and fabrication, which can be overcome by mixing them with a suitable binder. In this study, a microwave process was used to produce hydroxyapatite using the starting materials CaCl₂ and Na₂HPO₄ with previous precipitation. The mixture was exposed to microwave radiation for 5, 10, 15 and 20 min through domestic microwave ovens with an output power of 1 kW and frequency oscillation of 2.45 GHz. The samples were analyzed by X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray, dc conductivity and impedance spectroscopy. X-ray powder diffraction revealed the formation of HAP after 5 min of exposure. The presence of a low crystallinity state and the transition, with the rise of radiation exposure time, to a crystalline phase was related to the Ca/P mass and Ca/P atomic ratio. The presence of this low crystalline phase, detected by X-ray powder diffraction and scanning electron microscopy, seems to have a direct influence on electrical and dielectrical characteristics.     

Preparation and some properties of thallium orthothioarsenate single crystals

Growing of large (Ø 25–30 mm, L = 50–60 mm) optically homogenous single crystals of Tl₃AsS₄ using the Bridgman-Stockbarger method is described. Tl₃AsS₄ is orthorhombic (Pnma), a = 8.85 ± 0.03, b = 10.86 ± 0.03, c = 9.18 ± 0.03 Å; z = 4; ϱ_x = 6.15 g · cm⁻³; ϱ_p = 6.15 ± 0,03 g · cm⁻³, T_m = 424 ± 3°C, Moh's hardness = 3, microhardness and ultrasonics velocity along x, y, z are 65–85 kg · mm⁻² and 2.16 – 2.37 · 10⁵ cm · sec⁻¹, respectively. The crystals possess perfect cleavage plane (010). Their transparency range is 0.6–12 microns. – Unsuccessful attempts to obtain Tl₃AsS₄ and its alloys in the vitreous state were taken. The possibilities of glass formation and boundaries of the vitreous region in the system Tl–As–S based on the characteristic features of the melt forming structural units are analyzed.     

The effect of copper and silver on the conductivity and photoelectric properties of glassy arsenic selenide

A study was made of the effect of copper and silver on the electric properties, photoconductivity and its kinetics in glassy arsenic selenide.     

Dielectric properties of thallium gallium diselenide layered crystal in the incommensurate phase

The dielectric measurements of the layered crystal were studied in temperature range of successive phase transitions. The measurements revealed that the phase transition occurred in 242 K is an incommensurate phase transition. When the sample is annealed at a stabilized temperature in the incommensurate phase, a remarkable memory effect has been observed on cooling run. The mechanism of the memory effect in the incommensurate phase of the semiconducting ferroelectric TlGaSe₂ can be interpreted in the frame of the theory of defect density waves. This theory claims that the memory effect is the result of pinning of the incommensurate structure by the lattice inhomogeneities. With decreasing the annealing temperature the phase transition temperature shifts to lower temperatures gradually. Moreover, the peak intensities also increase gradually. In addition to these effects, the phase transition temperature shifts to lower temperatures with increasing annealing time. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of substrate temperature on the properties of vacuum evaporated indium selenide thin films

Thin films of InSe were obtained by thermal evaporation techniques on glass substrates maintained at various temperatures (T_sb = 30°, 400°C). X‐ray diffraction analysis showed the occurrence of amorphous to polycrystalline transformation in the films deposited at higher substrate temperature (400°C). The polycrystalline films were found to have a hexagonal lattice. Compositions of these films have been characterized by EDAX and the surface analysis by scanning electron microscopy. Optical properties of the films, investigated by using spectrophotometer transmittance spectra in the wavelength range (300 – 1100 nm), were explained in terms of substrate temperatures. Films formed at room temperature showed an optical band gap (E_g^opt) 1.56 eV; where as the films formed at 400°C were found to have a E_g^opt of 1.92 eV. The increase in the value of E_g^opt with T_sb treatment is interpreted in terms of the density of states model as proposed by Mott and Davis. The analysis of current ‐Voltage characteristics, based on space charge limited currents (SCLC) measurements, confirms the exponential decrease of density of states from the conduction band edge towards the Fermi level for both the amorphous and polycrystalline films. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the formation and growth processes of silicon carbide monocrystals

The formation and growth processes of SiC monocrystals from vapour phase by sublimation method have been investigated. It is shown that the peaks of the evaporation and growth whiskers are the centres of SiC monocrystals formation. One of the mechanisms of development of the whisker into SiC platelet is established. It is shown that on the naturally mirror like crystal face either the edge of the crystal or the screw dislocation, situated on the crystal edge, is the source of the steps. On the stepped crystal face the root of the crystal is the source of the steps.     

Optical properties of Pr3+ in selenide glasses modified with CsI

Effects of CsI content on the optical properties of Ge₃₀Ga₅Sb₅Se₆₀ glasses were evaluated. Linear and non-linear absorption properties of the glasses without Pr³⁺ were examined in addition to 1.6 μm emission properties of the Pr-doped glasses. Blueshift of the UV-side absorption edge was accompanied with increasing CsI concentration, while non-linear absorption coefficients measured at 1.06 μm by the Z-scan method remained unaffected. Measured lifetimes of the 1.6 μm emission from modified glasses were comparable to those of the unmodified glass. These experimental observations are discussed in connection with a pronounced weak absorption tail appeared in selenide glasses with the addition of CsI.     

Structural,compositional and optical properties of gallium selenide thin films doped with cadmium

Polycrystalline cadmium doped gallium selenide thin films were obtained by the thermal co‐evaporation of GaSe crystals and Cd grains onto glass substrates. The structural, compositional and optical properties of these films have been investigated by means of X‐ray diffraction, energy dispersive X‐ray analysis and UV‐visible spectroscopy techniques, respectively. Particularly, the elemental analysis, the crystalline nature, the energy band gap, the refractive index, the dispersion energy and static dielectric constant have been identified. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct forbidden energy band gap of 1.22 eV. The cadmium doping has caused a significant decrease in the values of the energy band gap and in all the dispersive optical parameters, as well. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Optical and electrical properties of Tl-S glasses

The frequency and temperature dependence of ac conductivity and optical absorption have been measured for four Tl-S glasses, TlS, Tl₂S₃,TlS₂ and Tl₂S₅, prepared by a melt-quenching method. The ac conductivity has been measured over a frequency range 0.1 Hz to 1.8 GHz and a temperature range 190-273 K. The optical absorption was measured at room temperature over a wavelength range 200-2600 nm. We have determined the electrical and optical band gaps from the experimental results. For each glass, the electrical band gap is larger than the optical band gap and the difference increases with increasing sulfur concentration. The frequency dependence of ac conductivity varies with composition of the glasses. We suggest that these results are due to an increase of localized states in the band gap with increasing sulfur concentration.     

The preparation and electrical switching properties of nickel sulphides in As2S3 glass

A preliminary investigation into the formation of ‘glasses’ from mixtures of arsenic and nickel sulphides is reported. Memory- and threshold-switching devices have been produced from glasses containing particles of nickel sulphide. Threshold switches show a high resistance in the off-state ($\text{> 10}{}^6\text{μ}$), and a linear on-state.     

Polymorphic transformation in TlSe and the electrical properties of phases

Alloys in the TlSe-Se system on the side of the TlSe compound in the phase diagram have been investigated using differential thermal, X-ray powder diffraction, and microstructural analyses. The phase diagram of the system has been constructed, and the temperature dependences of the electrical conductivity of the phases obtained have been examined. An analysis of the thermograms of alloys in the (TlSe)_0.96-Se_0.04 system has revealed a structural phase transition at a temperature of 470 ± 1 K. Investigations into the temperature dependences of the electrical conductivity in the range 120–450 K have demonstrated that the temperature dependence of the electrical conductivity for the (TlSe)_0.96-Se_0.04 alloy exhibits metallic behavior.     

The fracture behaviour of Chromium- and Titanium-doped sapphire monocrystals

The influence of chromium and titanium on the fracture behaviour of sapphire crystals was analysed by means of crack indentation pattern, and the critical stress intensity factors for the easiest spalling planes of the sapphire were measured from the crack lengths at the indents. Together with information on the room temperature plasticity of sapphire the results allow conclusions on the influence of the dopants on crystallographic and glass-like fracture features, respectively.