The effects of electrode materials on the switching behaviour of the amorphous semiconductors Si12Ge10As30Te48

The switching behaviour of the amorphous chalcogenide alloy Si₁₂Ge₁₀As₃₀Te₄₈ has been systematically investigated using silver, indium, aluminum, and graphite for electrodes. The experimental results show that the stability in both the threshold voltage for the onset of switching action, and the holding current required to maintain the conducting state, depends strongly on electrode materials. The switching mechanisms related to the electrical and thermal properties of various electrode materials are discussed, and experimental evidence of the deteriorating effect of some electrode materials is given.     

New technique for crystal growth and Phase Transition in TlInS2 Crystals

New technique for crystal growth was designed in the present work. The electrical conductivity and Hall effect of the TInS₂ single crystals, grown by this technique, were measured in the temperature range 150–500 K. From the measurements, the conductivity type, the energy gap, ionization energy of the impurity level, Hall mobility and carrier concentration were determined. Also the measurements revealed a presence of phase transition in the crystal at 189 and 220 K.     

Highly Electrical Conductivity of Hybrid Langmuir-Blodgett Films of Transition Metal Dichalcogenide and Amphiphilic Compounds

Abstract

Hybrid alternate layered films of transition metal dichalcogenides and amphiphilic compounds were prepared by Langmuir-Blodgett (LB) technique. The conductivity at room temperature depended on the transition metal dichalcogenide species, showing the highest value for the hybrid LB films of MoS₂ system. This methodology was successfully applied to the fabrication of the hybrid LB films using various organic amphiphiles such as cyanine dyes, phthalocyanines, azobenzene, and ferrocene derivatives. The conductivity depended on the interlayer spacing: the conductivity decreased with increasing interlayer spacing of the film. The highest electrical conductivity of the hybrid LB films was over 100 Scm^?.     

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.     

DC conductivity studies in K3Na(SeO4)2 single crystals

DC electrical conductivity studies were carried out along the three crystallographic axes for Tripotassium sodium diselenate (K₃Na(SeO₄)₂ or KNSe). Earlier studies of phase transition in this crystal show successive phase transitions at 334 K, 346 K, 730 K, and 758 K. In this paper we report the dc electrical conductivity measurements in the temperature region 303 K – 430 K along a, b and c – axes. An anomaly in conductivity was obtained around 341 K and another one around 333 K. These can be attributed as due to phase transitions in this crystal. A strong anomaly also has been observed along the c‐axis and comparatively week one along a and b axes around 395 K for the first time. This can be due to newly observed phase transition in the crystal. DSC taken for the sample also shows endothermic peak supporting the occurrence of newly observed phase transition. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Switching in Al-As-Te glass system

The switching properties previous to the memory effects of bulk chalcogeni de semiconductor glasses in the Al-As-Te system are investigated. The values of the switching voltage are related to the glass transition temperatures as well as to glass composition and a linear relation between ln V_th and Te at% is found. A constant electrical power for switching occurrence has been found. The behaviour of the switching voltage V_th with temperatures ranging from room temperature to 100°C follows an exponential law previously reported by other authors. The delay time versus applied voltage shows a typical glass bulk behaviour. The experimental results support an electrothermal model for switching in this system for the used conditions, although a low-field dependence must be introduced for a complete agreement.     

Nonvolatile resistance switching in amorphous In–Zn–O films

In amorphous In–Zn–O thin films we found a polar reversible sharp transition between a high-resistance state (HRS) with insulating properties and a degenerated semiconducting or metallic low-resistance state (LRS). The switching with resistance ratio up to 10³ is completely similar to the colossal electrical resistance (CER) phenomenon, discovered in the highly correlated oxides with the structure of perovskite. We suggest that the polar reversible resistance switching occurs due to the electric field induced oxygen exchange between high-resistance thin interface part and low-resistance thick bulk part of In–Zn–O. The oxygen exchange changes the donor concentration (oxygen vacancies) in the transition layer which leads to the Fermi level shift. The transition between hopping and band conductivity occurs when the Fermi level crosses the mobility edge.     

Synthesis and Properties of a New Kind of One-Dimensional Conductors 17. Studies on monomeric and polymeric phthalocyaninato-iron(II) and -cobalt(II) complexes

A new approach for a one-dimensional organic conductor with a chemically bonded backbone is presented. The proposed compounds consist of a linear arrangement of transition metal atoms, e.g. Fe, Co, bridged by linear bidentate π-electron containing ligands, e.g. pyrazine, 4,4'-bipyridine, and 1,4-diisocyanobenzene. This structure is stabilized by the tetradentate planar phthalocyanine molecule complexing each octahedrally coordinated central atom in its equatorial plane. The synthesis and characterization of the monomeric, dimeric and polymeric compounds PcML/PcML₂, PcM-L-PcM and [PcML]_n are described and a comparison of their electrical conductivity as a function of their structural features is given in detail. The electrical conduction of the monomeric units increases after polymerisation by a factor of 10⁷ without doping. In the second part soluble tetrasubstituted phthalocyaninato(IVB)-metalloxanes are described and the effect of homogeneous and heterogeneous iodination on conductivity is discussed.     

Aspects of Take Temperature Dependence of Electrical Conductivity in Polymeric Materials

The temperature dependence of the electrical conductivity of a number of polymeric organic and inorganic materials is described by a function, G(E,T), that is the derivative of the Fermi function. The mathematical justification for the use of this function in place of the Fermi function is described. Use of the G(E,T) function in the appropriate conductivity equation allows one to reproduce the temperature dependence of electrical conductivity in materials as diverse as (SN) and germanium single crystals, The function is found to be applicable to experimental conductivity data that collectively span a range of about 20 orders of magnitude, and a total temperature range of approximately 1000 K for the materials cited. The G(E,T) function adequately simulates the commensurate-incommensurate transition that is observed in materials such as (TMTSF)₂PF₆ and TTF-TCNQ. The importance of lattice order and the degree of single crystal perfection on derived properties of the conductivity curve are discussed. The G(E,T) function is applicable to other electron transport properties. The temperature dependence of electrical capacitance of SrTiO₃ is cited as an example,     

Electrical conductivity and thermoelectric power of liquid AgSb Te2

The electrical conductivity and thermoelectric power of liquid AgSb Te₂ have been investigated as a function of temperature. Experimental data are analyzed in terms of a recent model proposed by Mott. The activation energy for electrical conductivity and thermoelectric power is found to be approximately 0.50 eV with a large temperature coefficient γ ～ 7 × 10^?4 eV/deg K. The gradual transition from a semiconducting to a metallic behaviour has been observed at high temperature.     

Electrical conductivity of NbN-SiO2 films obtained by ammonolysis of Nb2O5-SiO2 sol-gel derived coatings

The studies of electrical conductivity of NbN-SiO₂ films are reported. To obtain these films, sol-gel derived xNb₂O₅-(100 − x)SiO₂ (where x = 100, 90, 80, 70, 60, 50 mol%) coatings were nitrided at 1200 °C. The nitridation process leads to the formation of some disordered structures, with NbN metallic grains dispersed in insulating SiO₂ matrix. The structure of the samples was studied using X-ray diffraction (XRD) and atomic force microscopy (AFM). The electrical conductivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The superconducting transition was not observed even for the sample that does not contain silica. All the samples exhibit negative temperature coefficient of resistivity. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system.     

Electrical and dielectric properties of MnF2–ZnF2–NaPO3 glasses

Direct electrical conductivity and dependencies of complex electrical modulus vs. temperature and frequency have been measured on glasses from the MnF₂–ZnF₂–NaPO₃ system. These glasses are sensitive to atmospheric humidity and as a consequence, the electrical conductivity increases up to temperature of 50 °C. A hydrated layer is created by the effect of water and leads to the significant increase of the electrical conductivity in the case of 0MnF₂–20ZnF₂–80NaPO₃ glass. This behavior is governed by Arrhenius relation where the values of activation energy are increasing and values of the electrical conductivity are decreasing with the amount of MnF₂. Dielectric measurements show that a heterogeneous phase is formed in the bulk of glasses. This may be seen when plotting complex electrical modulus in the complex plane. The records made by the light microscope confirmed the occurrence of the other phase in the bulk of glasses.     

The effect of addition of 5 at% sulphur or tellurium to selenium on its electrical properties and on its rate of crystallization

The characteristic properties of selenium: density d, logarithm of electrical conductivity logσ, and the activation energy ΔE_g with addition of 5 at% sulphur and tellurium was measured for the glassy state and for the crystalline state. The process of transition of the amorphous into crystalline selenium was followed by measuring these quantities for all intermediate stages. Both S and Te decrease the rate of crystallization of selenium. In the crystalline state, the presence of S and Te increases the energy of activation of the conductivity process.     

Experimental evidence of filament “forming” in non-crystalline chalcogenide alloy threshold switches

The results of quantitative electrical measurements on threshold switching devices fabricated by photoengraving techniques from non-crystalline chalcogenide alloy thin films are presented. An electrical forming process is observed prior to the establishment of a stable switching characteristic, and the results are used to demonstrate that the actual threshold switching occurs in a filament produced during this forming process, with a much higher conductivity than the as-deposited thin film. The implications of these observations for theories of switching are discussed.     

Experimental evidence of energy-controlled switching in amorphous semiconductors

The switching delay time and transition time, and threshold voltage for the onset of switching in the amorphous semiconductor Si₁₂Ge₁₀As₃₀Te₄₈ have been measured under various conditions using rectangular voltage pulses. The results show that both the threshold voltage and the delay time decrease, but the transition time increases with increasing temperature; and that these switching properties are strongly dependent on the width and the repetition rate of applied pulses. It is proposed that the delay time is associated with the time required for the formation of a filament to cause switching, and that the transition time is associated with the transit time of a carrier across the switching filament. All the experimental phenomena indicate clearly that the switching process is energy-controlled.     

The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO4 glass–ceramic

A Li_1.5[Al_0.5Ge_1.5(PO₄)₃] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (T_g), crystallization onset (T_x) and melting (T_m) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature T_gr = T_g/T_m = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between T_g and T_x. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.     

On the conduction mechanism and thermoelectric phenomena in In6S7 layer crystals

In the present paper, measurements of the electrical conductivity and Hall coefficient on single crystal of In₆S₇, grown by a new crystal growth technique, were done. The crystal was found to be of n-type conductivity. The low conductivity sample showed as the most striking feature an exponential increase of the Hall mobility with temperature. This effect was explained by assuming a mixed conduction and different scattering mechanisms for electrons and holes in the same temperature range. Also we have made thermoelectric power measurements to support this assumption. An energy gap of 0.64 eV was found.     

Electrical properties of copper phosphate glasses II. Dielectric properties

The ac and dc electrical properties of the P₂O₅BaOCuO glass system have been measured.TSPC and TSDC experiments and dc conductivity as a function of time indicate the predominantly electronic character of these glasses. The conduction process can be basically explained by a polaron hopping model in an adiabatic regime. Conductivity values which depend on the glass microstructure and switching phenomena are observed. The filamentary-feature of this process suggests a Poole-Frenkel mechanism.A Debye dielectric relaxation non-simple process is deduced from the frequency and temperature dependence of loss tangent and dielectric constant. The activation energies agree with those determinated from dc measurements, suggesting a unique electronic hopping conduction mechanism in both regimes.The ac and dc electrical properties are strongly affected by the glass composition and essentially by the redox Cu⁺/Cu_t ratio.A conduction model accounting for ac and dc behaviour is finally proposed.     

Electrical properties of copper phosphate glasses I. DC electrical behaviour

The ac and dc electrical properties of the P₂O₅BaOCuO glass system have been measured.TSPC and TSDC experiments and dc conductivity as a function of time indicate the predominantly electronic character of these glasses. The conduction process can be basically explained by a polaron hopping model in an adiabatic regime. Conductivity values which depend on the glass microstructure and switching phenomena are observed. The filamentary-feature of this process suggests a Poole-Frenkel mechanism.A Debye dielectric relaxation non-simple process is deduced from the frequency and temperature dependence of loss tangent and dielectric constant. The activation energies agree with those determined from dc measurements, suggesting a unique electronic hopping conduction mechanism in both regimes.The ac and dc electrical properties are strongly affected by the glass composition and essentially by the redox Cu⁺/Cu_t ratio.A conduction model accounting for ac and dc behaviour is finally proposed.