Drift mobilities in amorphous AsSeTe

The electron and hole drift mobilities in As_xSe_100?x?yTe_y (0 ? x ? 7.1, 0 ? y ? 20.9 at.%) evaporated thin amorphous films and their temperature dependence were measured by means of the time-of-flight technique. The electron mobility decreases with increasing As content, while the hole mobility decreases upon addition of 6.4 at.% Te, then remains almost similar on further addition of Te. These effects of As and Te on the drift mobilities of carriers were shown to be independent of each other.     

Thermoelectric power of AsSeTe glasses

The result of the measurement of the thermoelectric power of glasses in the As₂Se₃As₂Te₃ system are reported. The results indicate that towards the As₂Te₃ end of the composition, there is an anomalous increase in thermoelectric power, the origin of which is not clear. Polaron hopping seems to contribute to conductivity in tellurium rich glasses. Structural restrictions on polaron hopping, such as the availability of AsTeAsTe chains seems to be important in discussing transport behaviour.     

Glass transition and specific heats in the systems PS,PSe,AsS and AsSe

The glass transition temperatures were measured in the systems AsS, As_0.5P_0.5S, PSe, AsSe and PAsSe. Heat capacities of the glasses in the selenium systems were obtained by differential scanning calorimetry. As shown by the residual entropies departures from ideality are high in the chalcogen glasses. The results are discussed in terms of the structure of glasses in these systems. The thermodynamic data of glasses and liquids in these systems indicate a balance of intra- and intermolecular saturation of bonds. The amount of polymerization increases with increasing average molecular weight in the glass and with increasing temperature in some of the investigated liquids.     

Mechanical damping of silver-containing SiAsChalcogenide glasses

Mechanical damping measurements were used to study the structural conditions responsible for the increase in thermal stability that has been observed when Ag and Se are substituted simultaneously in Si₃₅As_25?xAg_xTe_40?ySe_y glasses. Glasses containing Ag exhibited a mechanical damping peak whose magnitude was approximately proportional to the Ag content and which was absent in the more completely cross-linked Si₃₅As₂₅Te₄₀ base glass. This peak shifted to lower temperatures and split into two overlapping peaks with increasing Se content. The splitting of the peak has been tentatively attributed to phase separation which has been postulated to occur at higher Se contents.     

Self-diffusion in the chalcogenide glass system SeGeAs

Se, As and Ge self-diffusion were investigated in three different glasses of the chalcogenide system SeGeAs by means of the radioactive tracers ⁷⁵Se, ⁷³As and ⁷¹Ge. All D values (Se between 200 and 290°C, As between 240 and 290°C and Ge between 280 and 295°C) lay between 10^?14 and 5 × 10^?16 cm² s^?1. The diffusion profiles were analyzed using a chemical micro-etching technique. Roles of glass structure and possible diffusion mechanism are discussed.     

Sensitivity of the photo-darkening effect in CuAsSe glasses

When CuAsSe glasses are irradiated, they exhibit higher concentrations of darkening than AsSe glasses. Since darkening depends on the composition, the darkening centers in CuAsSe glasses to be of the same kind as those in AsSe glasses, i.e. arsenic clusters. Concerning the kinetics of erasing, it was found that the activation energy and the rate constant of erasing in CuAsSe are almost equal to those in AsSe glasses, but for the kinetics of darkening, it was found that the activation energy of darkening is equal to that of AsSe but α₀, which is proportional to the number of latent darkening centers, and the darkening rate constant k₁ are about twice as high as the corresponding constants of AsSe glasses. This may be the reason for the greater darkening in CuAsSe glasses. The high value of α₀ was attributed to the generation of more AsAs bonds on the addition of Cu to the AsSe glass network. The high value of k₁ was attributed to the increase in efficiency of photo-decomposition because of the many impurity levels in the band gap and also because of the narrow optical energy gap in the CuAsSe glasses.     

Electrical and optical properties of the AsTeIn and GeSeIn chalcogenide systems

The influence of indium on the optical and properties of As_2−xTe_3−xxIn_2x, As_20−xTe_80−xIn_2x and Ge_20−xSe_80−xIn_2x is described. In Te-containing glasses the Fermi level is shifted by 0.05 eV and in Se-containing glasses by 0.2 eV towards the valence band.     

Memory effect in CuAsSe glasses containing small amounts of Au

The Cu, As and Se contents of Au-containing CuAsSe glasses significantly affected both the resistivity in the memory state and the time required to reach the memory state, whereas the Au content affects only the latter. The main crystalline species formed in these glasses was a CuAsSe compound, and its formation was enhanced by the addition of Au. The role of Au in the memory effect was discussed on the basis of crystallization behavior of glasses.     

Relation between electrical and optical gaps of amorphous semiconductors in the SiAsTe system

Electrical and optical properties of semiconducting SiAsTe glasses have been investigated. Compositional dependences of the properties in the Si_xAs_yTe_z system are examined as a function of atomic percentage x (or y, z) of one element with parameters of constant atomic ratio y/z (or x/z, x/y) of the other two elements. A pre-exponential factor σ₀ in the dc conductivity formula is estimated to be $\text{(2.1 ± 0.6) × 10}{}^4\text{(Ω ·}\text{cm}\text{)}{}^{\mathrm{?1}}$, inependently of the compositions. A systematic relationship between the compositional changes in the electrical gap E_g(el) and optical gap E_g(op) has been found. The energy gaps increase linearly with increasing Si content and decreasing Te content, but are almost independent of As content. The relation between E_g(el) and E_g(op) is expressed by E_g(el) = 1.60 E_g(op) ? 0.15 in eV. On the other hand, the optical absorption coefficient α(Ω) near the band edge follows the empirical formula, $\text{α(Ω) = α}{}_0$ exp ($\text{h}\text{?}\text{Ω/E}{}_{\mathrm{<mtext>s</mtext>}}$). The experimentally determined factor E_s increases linearly with E_g(op) and is closely related to the energy difference between the two gaps. A tentative model to explain these experimental results is proposed by taking into account of the effect of the potential fluctuations in such disordered materials.     

Local atomic arrangement and bonding studies in amorphous As2Se3As4Se4

The local atomic arrangement in amorphous As₂Se₃As₄Se₄ glasses was investigated with ESCA (electron spectroscopy for chemical analysis) and RDF (radial-distribution function) analysis. Measured changes of RDF peak areas and ESCA chemical shifts give direct evidence for the preservation of the local bonding schemes of both crystalline As₂Se₃ and As₄Se₄ in their amorphous states. The valence-band structures of amorphous As, Se, As₂Se₃ and As₄Se₄ are also measured with ESCA and discussed in conjunction with the bonding schemes of these glasses.     

Electronic properties of vitreous and liquid AsSe alloys (I). Solid alloys

Measurements of the temperature dependence of the d.c. conductivity, therm?opower and optical absorption (near the fundamental edge) for five vitreous AsSe alloys containing from 30% to 50% As are presented. A conductivity maximum at the stoichiometric composition, As₂Se₃, corresponding to minima in the activation energies for conduction and thermopower, is accompanied by a minimum in the optical gap at this composition. These data suggest a minimum mobility gap at As₂Se₃, and a Fermi level independent of composition. Chemical bonding and bond edge state localization arguments are put forward to justify this proposal, and also to indicate why other amorphous semiconducting binary alloys behave differently at stoichiometric compositions.     

Thermal expansion and density of glassy PdNiP and PtNiP alloys

Thermal expansion and density of (Pd_1?xNi_x)_0.80P_0.20 and (Pt_1?yNi_y)_0.75P_0.25 alloys in their various states have been measured from room temperature to the glass transition temperature T_g. The thermal expansion of the glassy alloys at room temperature varies linearly with x and y and is 10 to 20% higher than that of corresponding pure metals. The thermal expansion of the undercooled alloy liquids near T_g as well as the molar volume $\text{v}\text{?}$ deduced from the density of glasses in contrast exhibits a negative deviation with composition x and y.This behavior is in line with the previously reported negative deviation of the glass transition temperature of these glassy alloys with metal content and may be explained in terms of excess volume associated with a mixture of hard spheres.     

Properties and structure of AsTe glasses: (I). Glass-forming ability and related properties

AsTe glasses have been investigated for determining the molecular mechanisms of glass formation and of crystallization. Stable and homogeneous bulk glasses are prepared throughout the range of concentration from 20 to 65 at % As, by using an improved quenching method. GFA (glass-forming ability) is determined with respect to the rate of cooling. Density, microhardness, T_g and ΔC_p at T_g are measured in the whole range of glass formation. Finally a quantitative analysis of the kinetics of crystallization is carried out. It appears that most properties which involve molecular motions exhibit anomalies for As₄₀ Te₆₀ composition (As₂Te₃ is the unique definite compound of the system). So GFA shows a sharp depression for this critical composition; similarly enthalpy and kinetics of crystallization are drastically modified at As₄₀Te₆₀. For Te-rich glasses, crystallization occurs at low temperature by homogeneous nucleation with a small energy of activation. For As-rich glasses, crystallization occurs at much higher temperatures. Above 200°C it is controlled by growth with an high activation energy which indicates deep rearrangements of the relative positions of the atoms by chemical diffusion. A coherent analysis of all the data is developed which shows that important differences of local order exist between amorphous and crystalline states and also between the two well-distinct types of glasses we are dealing with. The aim of paper II is to determine these differences from diffraction data.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Transformation behavior of PdAuSi metallic glasses

The transformation behavior of roller-quenched amorphous Pd_0.82?xAu_xSi_0.18 and Pd_0.835?xAu_xSi_0.165 alloys, where x ? 0.10, after rapid heating to temperatures near to or above T_g, is reported. The calorimetrically determined glass (T_g) and kinetic crystallization (T_c) temperatures both increased with x up to x ≈ 0.04. With increasing x, at x ? 0.04, T_c decreased rapidly while T_g varied little. Binary Pd_0.82Si_0.18 alloys crystallized to an fcc phase without apparent composition segregation. The tendency to phase separate at T near T_g, as manifested by small- (SAXS) and large-angle X-ray scattering and calorimetry, increased with increasing Au substitution. Pd_0.8Au_0.035Si_0.165 alloy apparently phase separated by a nucleation and growth mechanism, with a growth rate limited by the melt viscosity, to form an fcc phase dispersed in an amorphous phase which later crystallized. Pd_0.74Au_0.08Si_0.18 alloy phase separated initially to two melts, each of which later crystallized in turn. The initial separation behavior was generally consistent with the predictions of the spinodal theory but with some deviation from Cahn's linear relation.     

Formation of glass in the GeSiS system

The formation of glass in the GeSiS system was investigated. After synthesis of material with the general formula Ge_1?xSi_xS_y, where x was chosen to be 0.05, 0.1, 0.2, 0.3 and y was in the range 1.28–3.6, cylindrical samples were prepared and used for the characterization of glass by means of DTA. It was found that the substitution of germanium with silicon does not lead to any expressive change of the glass transition temperature, crystallization and the onset of melting.     

ESR study of Mn2+ in GeTe and GeTeSi glasses

The electron spin resonance spectra of Mn²⁺ ions have been studied in Ge_xTe_100?x with x = 15, 17.5 and 20, and Ge_20?xTe₈₀Si_x with 0 ?x? 20. All samples are found to exhibit six hyperfine lines centered at g = 4.3 with hyperfine interaction constant A = 56 × 10^?4cm^?1. The g = 4.3 line is interpreted as being caused by Mn²⁺ ions incorporated in the amorphous network and surrounded by four Te atoms in an arrangement of orthorhombic symmetry. Some of the samples of GeTe show a g = 2.0 line. This line also appears after heat treatment in air at temperatures above the glass transition temperature. It is concluded that the g = 2.0 line is caused by Mn²⁺ ions in phase separated microcrystalline or concentrated regions of MnO in the glass.     

The kinetics of formation of A AuGeSi metallic glass

The glass-forming ability of a Au_0.778Ge_0.138Si_0.084 alloy is analyzed theoretically by computing a time-temperature-transformation curve which describes the time required to produce a barely detectable fraction of crystallization, at various temperatures. The calculation is based on an interpolated viscosity-temperature curve, this alloy being exceptional among metallic glass formers in that experimental viscous-flow data are available at both high and low temperatures. Allowing for uncertainties, the critical cooling rate for glass formation lies within the range 10⁶?10⁸ K/sec which is in satisfactory agreement with experimental estimates of cooling rates in splat quenching. This and previous comparisons for Ni and PdSi alloys suggest that the approach may have useful and general applicability to metallic glasses.