The electrical switching and thermal behavior of bulk Ge15Te85 − xSnx and Ge17Te83 − xSnx glasses

Bulk Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses, are found to exhibit memory type electrical switching. The switching voltages (V_t) and thermal stability of Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses are found to decrease with Sn content. The composition dependence of V_t has been understood on the basis of the decrease in the OFF state resistance and thermal stability of these glasses with tin addition. X-ray diffraction studies reveal that no elemental Sn or Sn compounds with Te or Ge are present in thermally crystallized Ge–Te–Sn samples. This indicates that Sn atoms do not interact with the host matrix and form a phase separated network of its own, which remains in the parent glass matrix as an inclusion. Consequently, there is no enhancement of network connectivity and rigidity. The thickness dependence of switching voltages of Ge₁₅Te_85 ? xSn_x and Ge₁₇Te_83 ? xSn_x glasses is found to be linear, in agreement with the memory switching behavior shown by these glasses.     

Magnetic behavior,transport properties and nanocrystallization of melt-spun YxCe50 − xCu42Al8 (0 ≤ x ≤ 50) amorphous system

Amorphous Y_xCe_50 ? xCu₄₂Al₈ (0 ≤ x ≤ 50) ribbons prepared by melt-spinning on the Cu wheel were subjected to X-ray diffractometry (XRD), differential scanning calorimetry (DSC) and to the measurements of magnetization and resistivity in the temperature range from 1.7 to 300 K. Effective activation energies, characteristic crystallization temperatures and enthalpies of as-quenched alloys have been determined. Two stages of crystallization have been observed in most samples (except shallow effects in Ce₅₀Cu₄₂Al₈). The activation energies have been calculated from the Kissinger relation to be 247 ± 18 and 570 ± 56 kJ/mol for Y₂₅Ce₂₅Cu₄₂Al₈ and Y₅₀Cu₄₂Al₈, respectively. The absence of the endothermic effect for x = 50, usually associated with a glass transition, below the primary crystallization event, indicates the presence of dispersed polyamorphous packing with a wide range of local glass transitions. The magnetization versus temperature plot for Y₂₅Ce₂₅Cu₄₂Al₈ points to a magnetic ordering at temperatures considerably below 50 K. This observation has been confirmed by the temperature dependence of resistivity, which exhibits a singularity at the same temperature. Moreover, a negative temperature coefficient of resistivity, characteristic of disordered systems, was observed. The electrical resistivity in the Y₂₅Ce₂₅Cu₄₂Al₈ amorphous alloy is governed by the weak localization of electrons.     

Optical and electrical properties of as-prepared and annealed (Se80Te20)100 − xAgx (0 ≤ x ≤ 4) ultra-thin films

Optical and electrical properties of the (Se₈₀Te₂₀)_100 ? xAg_x (0 ≤ x ≤ 4) ultra-thin films have been studied. The ultra-thin films were prepared by thermal evaporation of the bulk samples. Thin films were annealed below glass transition temperature (328 K) and in between glass transition temperature and crystallization temperature (343 K). Thin films annealed at 343 K showed crystallization peaks for Se–Te–Ag phases in the XRD spectra. The transmission and reflection of as-prepared and annealed ultra-thin films were obtained in the 300–1100 nm spectral region. The optical band gap has been calculated from the transmission and reflection data. The refractive index has been calculated by the measured reflection data. It has been found that the optical band gap increases, but the refractive index, extinction coefficient, real and imaginary dielectric constant decrease with increase in Ag content. The optical band gap and refractive index show the variation in their values with increase in the annealing temperature. The extinction coefficient increases with increasing annealing temperature. The surface morphology of ultra-thin films has been determined using a scanning electron microscope (SEM). The measured dc conductivity, under a vacuum of 10^? 5 mbar, showed thermally activated conduction with single activation energy in the measured temperature range (288–358 K) and it followed Meyer–Neldel rule. The dc activation energy decreases with increase in Ag content in pristine and annealed films. The results have been analyzed on the bases of thermal annealing effects in the chalcogenide thin films.     

Structural analysis of xCsCl(1 − x)Ga2S3 glasses

Sulphide glasses doped with rare-earth ions have been demonstrated to be suitable for photonic applications such as optical amplifiers, up-converters and fiber lasers. The substitution of metal halides into the glass network has been shown to result glasses with desirable properties in terms of quantum efficiency and fiber manufacture [J.R. Hector, J. Wang, D. Brady, M. Kluth, D.W. Hewak, W.S. Brocklesby, D.N. Payne, Journal of Non-Crystalline Solids 239 (1998) 176]. To assist in the understanding of this improvement a structural analysis of glasses with a composition xCsCl(1 ? x)Ga₂S₃ has been undertaken in order to examine the nature of the gallium environment. Information collected by high energy X-ray diffraction and neutron diffraction have been analyzed to permit the identification of the structural units as Ga centered tetrahedra. The interconnection between the tetrahedra was found to be predominantly corner sharing.     

Structure and properties of the 70Li2S · (30 − x)P2S5 · xP2O5 oxysulfide glasses and glass–ceramics

The 70Li₂S · (30 ? x)P₂S₅ · xP₂O₅ (mol%) oxysulfide glasses were prepared by the melt quenching method. The glasses were prepared in the composition range 0 ≦ x ≦ 10. The glass–ceramics were prepared by heating the glasses over crystallization temperatures. The PO_nS_3?n (n = 1–3) oxysulfide units were produced in the glasses and glass–ceramics by partial substituting P₂O₅ for P₂S₅. In particular, the P₂OS₆^4? unit would be produced by substituting a small amount of P₂O₅ for P₂S₅. The oxygen atoms were incorporated into the Li₇P₃S₁₁ crystal structure because the diffraction peaks of the oxysulfide glass–ceramic shifted to the higher angle side. The glass–ceramic with 3 mol% of P₂O₅ exhibited the highest conductivity of 3.0 × 10^?3 S cm^?1 and the lowest activation energy for conduction of 16 kJ mol^?1. The P₂OS₆^4? dimer units in the oxygen-incorporated Li₇P₃S₁₁ crystal would improve conductive behavior of the Li₂S–P₂S₅ glass–ceramics.     

Thermally reversing window in Ge15Te85 − xInx glasses: Nanoindentation and micro-Raman studies

Nanoindentation studies on Ge₁₅Te_85 ? xIn_x glasses indicate that the hardness and elastic modulus of these glasses increase with indium concentration. While a pronounced plateau is seen in the elastic modulus in the composition range 3 ≤ x ≤ 7, the hardness exhibits a change in slope at compositions x = 3 and x = 7. Also, the density exhibits a broad maximum in this composition range. The observed changes in the mechanical properties and density are clearly associated with the thermally reversing window in Ge₁₅Te_85 ? xIn_x glasses in the composition range 3 ≤ x ≤ 7. In addition, a local minimum is seen in density and hardness around x = 9, the chemical threshold of the system. Further, micro-Raman studies reveal that as-quenched Ge₁₅Te_85 ? xIn_x samples exhibit two prominent peaks, at 123 cm^? 1 and 155 cm^? 1. In thermally annealed samples, the peaks at 120 cm^? 1 and 140 cm^? 1, which are due to crystalline Te, emerge as the strongest peaks. The Raman spectra of polished samples are similar to those of annealed samples, with strong peaks at 123 cm^? 1 and 141 cm^? 1. The spectra of lightly polished samples outside the thermally reversing window resemble those of thermally annealed samples; however, the spectra of glasses with compositions in the thermally reversing window resemble those of as-quenched samples. This observation confirms the earlier idea that compositions in the thermally reversing window are non-aging and are more stable.     

Square-wave voltammetry and impedance spectroscopy in tin doped melts with the compositions xNa2O · 15Al2O3 · (85 − x)SiO2 (x = 8.5, 11 and 16)

Glasses with the base compositions xNa₂O · 15Al₂O₃ · (85 ? x)SiO₂ (x = 8.5, 11 and 16) doped with 0.5 mol% SnO₂ were investigated by both square-wave voltammetry and impedance spectroscopy in the temperature range from 1300 to 1600 °C. Each recorded square-wave voltammogram exhibits a well pronounced peak attributed to the Sn²⁺/Sn⁴⁺-redox pair. Impedance spectra were measured in a frequency range from 0.1 to 10⁵ s^?1 as a function of the superimposed dc-potential and were simulated using an equivalent circuit taking into account the resistivity of the melt, the electrochemical double layer, a resistor attributed to a kinetically hindered electron transfer and a Warburg parameter which accounts for the diffusion process of Sn⁴⁺ and Sn²⁺ to and from the electrode. Additionally, two impedance elements, a resistor and a capacitance both attributed to adsorption processes were necessary to fit the impedance spectra.     

Crystallisation kinetics,glass forming ability and thermal stability in glassy Se100 − xInx chalcogenide alloys

Differential scanning calorimetry (DSC) studies have been done under non-isothermal conditions at different heating rates for glassy Se_100 ? xIn_x (5 ≤ x ≤ 20) alloys. DSC traces with well-defined endothermic and exothermic troughs and peaks at glass transition (T_g), crystallisation (T_c) and melting (T_m) temperatures were observed. The crystallisation kinetics parameters, Avrami index (n), activation energy for crystallisation (E_c) and frequency factor (K_o), have been calculated on the basis of the classical Johnson–Mehl–Avrami (JMA) model and related methods derived by Kissinger, Augis–Bennett and Mahedevan. Activation energy for glass transformation (E_t) has been evaluated on the usual two different non-isothermal methods developed by Moynihan and Kissinger. An extension of the Augis–Bennett method well known for evaluating E_c to calculate E_t has been explored with satisfactory results. Results obtained from these methods are in close agreement with each other. Close correlation between E_t, E_c and heating rate (β) was observed. The glass forming ability (GFA) and thermal stability parameters have been calculated for each glass system. It was found that the proportion of indium additive changed significantly the values of glass/crystal transformation, GFA and thermal stability of the studied system.     

Topology and chemical order in AsxGexSe1 − 2x glasses: A high-resolution X-ray photoelectron spectroscopy study

The evolution of topology and chemical order along the As_xGe_xSe_1 ? 2x composition-line within the As–Ge–Se glass-forming region is studied by high-resolution X-ray photoelectron spectroscopy. It is shown that cation–cation bond formation becomes a dominant process for the compositions with x > 0.09. The results explain the peculiarities observed around this composition recently in the temperature-modulated differential scanning calorimetry data. Substitution of two selenium atoms within constituent structural units (pyramids and/or tetrahedra) by corresponding cations explains a second peculiarity point at x > 0.16 compositions observed recently with the above technique. The present observations show segregation of As and then Ge at high concentrations of cations in the system (x > 0.20).     

Crystallization kinetics of a-Se75Te25 − xGax (x = 0, 5, 10 and 15 at wt %) glassy system

Bulk glasses of a-Se₇₅Te_25 ? xGa_x (x = 0, 5, 10 and 15 at wt %) have been prepared by melt quenching technique. These samples were structurally characterized by using X-ray diffraction. Kinetic of crystallization in these glasses was studied under non-isothermal conditions using differential thermal analysis (DTA). DTA is performed at different heating rates of 5, 10, 15, 20 and 30 °C/min. The values of glass transition (T_g) and crystallization peak temperature (T_p) are found to be composition and heating-rate dependent. The obtained results have been analyzed in terms of activation energy of glass transition (E_g) using Kissinger's and Mahadevan et al. relations. Values of E_g obtained by the two relations are in agreement with each other. The results indicate that the crystallization process is a three-dimensional growth.     

The glass transition temperature of mixed glass former 0.35Na2O + 0.65[xB2O3 + (1 − x)P2O5] glasses

The mixed glass former effect (MGFE) is defined as the non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass former compositions. In this study, sodium borophosphate glasses, 0.35Na₂O + 0.65[xB₂O₃ + (1 ? x)P₂O₅] with 0 ≤ x ≤ 1, have been prepared and their glass transition temperatures (T_g) have been examined as an alternative indicator of the MGFE and as an indicator of changes in the short range order (SRO) structural network units that could cause or contribute to the MGFE. The changes in T_g show a positive non-additive and non-linear trend over the changing glass former fraction, x. The increase in T_g is related to the increasing number of bridging oxygens (BO) in the glass samples, which is caused by the increase in the number of tetrahedral boron, B⁴, units in the SRO structure.     

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.     

Synthesis and structure–property relations in xCuO–(1 − x)Bi2O3 (0.5 ⩽ x ⩽ 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses

Synthesis of the xCuO–(1 ? x)Bi₂O₃ (0.5 ? x ? 0.9) (C1–C5: x = 0.5, 0.6, 0.7, 0.8, 0.9) glasses was done via nitrate–citrate gel route. Glassy phase is ascertained by XRD studies. Magnetic susceptibility results in the range 4.2–400 K show weak paramagnetic nature with exchange integrals ～0.024–0.13 eV in the glasses. The electron paramagnetic resonance (EPR) in the range 4.2–363 K shows g ≈ 2.0 and the trend of the g-matrix elements g_|| > g_⊥ > g_e for the glasses C1–C5 at 4.2 K are due to the Cu²⁺ (3d⁹) paramagnetic site in the glasses which is in a tetragonally elongated octahedron [O_1/2–CuO_4/2–O_1/2] having D_4h symmetry. IR spectroscopic results show the presence of octahedron [BiO_6/2]^3? and [CuO_6/2]^4? units and pyramidal [BiO_2/2O]^? unit in the glasses.     

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1? 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1? 1? 2 0] || Si[1 1? 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.     

The densities of mixed glass former 0.35 Na2O + 0.65 [xB2O3 + (1 − x)P2O5] glasses related to the atomic fractions and volumes of short range structures

The mixed glass former effect (MGFE) is defined as a non-linear and non-additive change in the ionic conductivity with changing glass former fraction at constant modifier composition between two binary glass forming compositions. In this study, mixed glass former (MGF) sodium borophosphate glasses, 0.35 Na₂O + 0.65 [xB₂O₃ + (1 ? x)P₂O₅], 0 ≤ x ≤ 1, which have been shown to have a strong positive MGFE, have been prepared and their physical properties, density and molar volume, have been examined as predictors of structural change. The density exhibits a strong positive non-linear and non-additive change in the density with x and a corresponding negative non-linear and non-additive change in the molar volume. In order to understand the structural origins of these changes, a model of the molar volume was created and best-fit to the experimentally determined molar volumes in order to determine the volumes of the short range order (SRO) structural units in these glasses, how these volume change from the molar volumes of the binary glasses, and how these volumes change across the range of x in the ternary glasses. The best-fit model was defined as the model that required the smallest changes in the volumes of the ternary phosphate and borate SRO structural groups from their values determined by the densities of the binary sodium phosphate and sodium borate glasses. In this best-fit molar volume model, it was found that the volumes of the various phosphate and borate SRO structural groups decreased by values ranging from a minimum value of ~ 1% for x = 0.1 and 0.9 to a maximum value of ~ 6% for the phosphate and ~ 9% for the borate SRO groups at the minimum in molar volume at x = 0.4. The free volume was found to have a negative deviation from linear which is unexpected given the positive deviation in ionic conductivity.     

Bulk glassy (Fe0.474Co0.474Nb0.052)100 − x(B0.8Si0.2)x alloys prepared using commercial raw materials

The alloy compositions have been optimized by modifying the B and Si contents in (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x alloy system with commercial materials. The thermal stability of the supercooled liquid improves with the increased B and Si contents from x = 22 to 28. The composition of the alloy with x = 26 is close to a eutectic point. By copper mold casting, (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x bulk glassy alloys with diameters up to 5 mm were synthesized for the composition range of x from 22 to 28. In addition to high glass-forming ability (GFA), the (Fe_0.474Co_0.474Nb_0.052)_100 ? x(B_0.8Si_0.2)_x glassy alloys exhibit good soft magnetic properties as well, i.e., rather high saturation magnetization of 0.84–1.07 T, low coercive field of 1.8–3.2 A/m, high initial permeability of 10100–24100 at 1 kHz under a field of 1 A/m and Curie temperature of 620–730 K.     

Growth of Pb on Si(3 3 5)–Au surface

The growth, crystallographic structure and electronic properties of ultrathin Pb films grown on a vicinal silicon surface are investigated with reflection high energy electron diffraction (RHEED) and specific resistivity measurement techniques. A Si(3 3 5) surface with a perfect distribution of monoatomic steps separated with (1 1 1) terraces induced by a submonolayer amount of Au is used as a substrate. In the early stage, Pb growth is anisotropic. Apparently, the presence of steps forces the growth of short crystalline Pb chains along the steps. The layer is amorphous in the perpendicular direction. With the increasing thickness, a phase transition takes place between 3 and 4 monolayers (ML) that makes crystalline order also across the terraces. A further increase in thickness causes the layer surface to repeat the substrate morphology. It consists of regularly distributed monoatomic steps and narrow (1 1 1) terraces.     

The preparation and structural studies in the (80 − x)TeO2–20ZnO–(x)Er2O3 glass system

Er³⁺-doped tellurite glasses of the (80 ? x)TeO₂–20ZnO–(x)Er₂O₃ system (0.5 mol% ? x ? 2.5 mol%) have successfully been made by melt-quenching technique and their structure has been investigated by means of DTA and Raman spectroscopy. The DTA results show the thermal parameters; such as the glass transition temperature (T_g) and crystallization temperature (T_c) were determined. It is found that this system provides a stable and wide glass formation range in which the glass stability around 99–140 °C may be obtained. The Raman spectroscopy used the structural studies in the glass system. Two Raman shift peaks were observed around 640–670 cm^?1 and 720–740 cm^?1, which correspond to the stretching vibration mode of TeO₄ tbp and TeO₃ tp, respectively. It is found that the spectral shift in Raman spectra is depending on the Er₂O₃ content. This evolution is an indication of the changes in the basic unit of the glass structure.