Glass formation and properties of chalcogenide systems XVIII. On glass formation in the system GePbSeTe

The glass formation range of the system GePbSeTe has been investigated. For selected series properties such as the mole volume, glass transition temperature and the electrical direct current conductivity are discussed with respect to their dependence on composition. For some series the results can be compared with the properties of glasses containing Sn instead of Pb. The change in the energy gap for the crystalline compounds SnSe, SnTe, PbSe and PbTe is reflected by the data of the Sn or Pb containing glasses.     

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.     

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.     

The amorphous SbSe system

Essentially the complete compositional range of the amorphous SbSe system has been prepared in thin-film form by a simple evaporation technique. Optical and transport properties have been measured as a function of composition.     

Optical absorption of glassy semiconductors of the GeSbSe system

The optical transmittance of chalcogenide glasses Ge₂SbSe₇ (I), Ge₃SbSe₆ (II), GeSb₂Se₇ (III) and GeSbSe₃ (IV) was studied in the near infrared spectral region, 0.7–25 μm. The longwavelength tail of the absorption edge can be described by Urbach's rule. At higher absorption levels the absorption coefficient K depends quadratically on the energy of incident radiation. The temperature dependence of the absorption edge is discussed and the optical gaps 1.77 eV (I), 1.67 eV (II), 1.66 eV (III), 1.57 eV (IV) together with the corresponding temperature coefficients are also determined. The studied glasses are quite transparent in the 600–5000 cm^?1 wavenumber range.     

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.     

A reversible optical change in the AsSeGe glass

A novel reversible change in the optical property, transmission and refractive index, due to light irradiation has been found in the AsSeGe glass film. The change is not related with a usual amorphous-crystalline type transformation; a weak lightr causes an edge shift to longer shorter wavelengths also occur upon heating the sample, which suggests that the shift du to the strong light irradiation is originated from the “heating effect” of light. This was confirmed by an experiment with a sample coated with metal-smoke. The change of optical transmission during the weak light irradiation was measured; it is decreased exponentially with time, and the rate of decrease became larger as the power of irradiationg light increased. Good reversibility of the optical transmission was obtained, which makes the material suitable for erasable (optical) memory medium.     

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.     

Thermodynamic and thermokinetic characteristics of the glass transition in a GeSe2GeTeSb2Te3 alloy

Glassy alloys of (GeSe₂)₇₀ (GeTe)₁₅ (Sb₂Te₃)₁₅ were prepared by water-quenching and subjected to several thermal treatments through the glass transition region. The thermodynamic and thermokinetic characteristics of the glass were inferred from heat capacity measurements by differential scanning calorimetry. It was demonstrated that the undercooled liquid obtained by heating the glass is in equilibrium, and what is more, that not only each particular cooling process through the glass transition produces a given glass, but also that any trance of the glass may be suppressed by reheating above glass transition. The enthalpy and entropy differences between each glass and the undercooled liquid used to obtain that particular glass were taken as properties sensitive to the relaxation inherent to the formation of the glass. The activation energy spectrum characterizing the relaxation processes on cooling through the glass transition has been obtained. It has a peak energy of 1.43 eV which may be related to the bonds between the constituent atoms of the sample with weaker interaction energy. Therefore, the relaxation may be due to a breaking and rearrangement of these bonds in the glassy structure.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.     

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.     

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.     

Glass formation in the systems (K or Cs)2O(Nb or Ta)2O5Al2O3

The glass-forming ability of melts in the systems K₂O(Nb and/or Ta)₂O₅Al₂O₃ as well as those in which K₂O was replaced with Li₂O, Na₂O, Cs₂O, BaO or PbO was investigated. Some melts in the systems (K or Cs)₂O(Nb and/or Ta)₂O₅Al₂O₃ could be made into glasses by cooling, yielding practically useful amounts. The structures of these glasses were discussed on the basis of their infrared spectroscopic and X-ray emission spectroscopic analyses.     

Glass formation and properties of chalcogenide systems (XXII) the phase diagram of the system PbSe?GeSe2 and on the compound Pb2GeSe4

The phase diagram of the system PbSe GeSe₂ is studied by X-ray analysis and DTA. Pb₂GeSe₄ is the only compound which is formed in the system. It melts incongruently at 590 ± 4°C. The formation of Pb₂GeSe₄ as a definite compound is supported by studying the phase relations in the corresponding area of the ternary system PbSe GeSe GeSe₂.     

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.     

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T_G. The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T_X. Values of T_g and T_X have been tabulated for all alloys investigated and the compositional dependence of T_g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge₂₀Se₄₀Te₄₀ in which a GeSe₂-rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe₂-rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.     

Ionic conductvity and NMR investigation of quaternary glasses in the ZrF4BaF2ThF4LiF system

The existence of glasses involving large amounts of LiF (up to 60%) within the ZrF₄BaF₂ThF₄LiF quaternary systems has allowed the authors to study the evolution of transport properties with varying LiF content.A minimum of ionic conductivity bound to a maximum of activation energy has been detected when the atomic Li/F ratio is equal to ≅ 0.07. In the Li-low concentration domain, σ increases regularly and ΔE decreases simultaneously when the BaF₂ concentration increases; on the contrary in the Li-high concentration region log σ and ΔE are quasi-linear functions, increasing and decreasing respectively, of the LiF rate.A ⁷Li and ¹⁹F NMR investigation has shown that Li⁺ and F⁻ ions are simultaneously mobile and the temperature dependence of the number of mobile F⁻ ions has been determined. In the Li-low concentration domain transport properties result from mixed contributions of mobile Li⁺ and F⁻ ions, for high Li concentrations they depends only on the Li⁺ rate.Glasses with high Li-content have good electrical performnces (e.g. σ_175°C ≅ 2.10⁻⁴ω⁻¹ cm⁻¹ for Zr_0.20Ba_0.10Li_0.60Th_0.10F₂).     

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.