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.     

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 effects of annealing and rate of deposition on the electrical,optical and structural properties of amorphous GeSi alloy films

The electrical, optical and structural properties of rf-sputtered amorphous GeSi alloy films have been investigated as functions of the rate of deposition between 90 and 360 Å min^?1, and various annealing conditions. The stabilization evidenced in the electrical conductivity, optical absorption and density of these films deposited at higher rates and/or annealing temperatures may be explained in terms of the reduction of imperfections in the as-deposited films.     

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.     

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.     

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.     

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.     

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.     

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.     

Thermally-induced structural and electrical effects in GeTe-based amorphous alloys

The thermal, structural electrical properties of bulk glasses based on GeTe compositions near the binary eutectic, Ge₁₅Te₈₅, are studied. Information regarding the non-crystalline state and the transformation from the non-crystalline to the crystalline state is reported. The particular alloys studied represent binary (Ge₁₇Te₈₃), ternary (Ge₁₅Te₈₀As₅) and quaternary (Ge₁₅Te₈₁Sb₂S₂) compositions. Structural information is obtained using X-ray diffraction techniques and density measurements. Thermal data are reported from differential scanning calorimetry (DSC), thermogravimetry (TGA) and mass spectrometry results. The electrical conductivity is measured as a function of temperature and, on the ‘as-prepared’ glasses, shows semi-conducting behavior with activation energies, E, of 0.43–0.48 eV. DSC, TGA and X-ray powder diffraction patterns indicate the samples crystallize as Te and GeTe in a two-step process, and melt at the binary eutectic temperature. The binary vaporizes as Te and GeTe in a two-step process. GeTeAs and GeTeSbS vaporize by essentially the same mechanism, with As evaporating (<300°C) before the Te, and Sb and S evaporating (420–480°C) after the Te but before GeTe. The results show that the properties of the bulk ‘as-prepared’ glasses are strikingly similar. Thermally-induced changes in the structural and electrical properties of bulk samples have been examined following a series of anneals (5 h, vacuum) at temperatures from 111°C to 190°C (glass transition temperature $\text{?125?133°}\text{C}$; crystallization temperature $\text{?206?228°}\text{C}$ as determined by DSC). DSC, TGA and mass spectrometry results have been correlated to electrical and structural changes. Results show that crystalline Te nucleates at the surface and forms a conductive surface layer. The conductivity of this surface layer is nearly temperature independent with E ≈ 10^?2 eV for all three alloys. Crystallization and the associated electrically conductive regions extend into the bulk material with further annealing. In these disordered alloys the additives As and Sb + S apparently do not act as electrical dopants in the sense of affecting the conductivity activation energy. The additives Sb + S however do retard crystallization of GeTe. The secondary crystallization product, GeTe, apparently changes the conduction mechanism to either a metallic or degenerate semiconductor type behavior.     

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.     

The study of transformation kinetics of the amorphous PdSi alloys

The kinetics of transformation behavior of roller-quenched amorphous Pd₈₃Si₁₇ and Pd₈₀Si₂₀ alloys after linear and isothermal heating is reported. The transformation was examined with electron microscopy, electron diffraction and electrical resistivity measurement. The crystallization of amorphous alloys with 17 at% Si begins with metastable ordered fcc solid solution. The ordered fcc solution is transformed to a ordered metastable phase with, probably, orthorhombic structure. The crystallization of amorphous alloys with 20 at% Si begins by formation of spherulites with lamellar structure. Using electron diffraction we found that spherulites consist of two phases - orthorhombic Pd₃Si silicide and Pd-rich silicide. From resistivity measurements, activation energies of 28.5 kcal/mol for Pd₈₃Si₁₇ and 80 kcal/mol for Pd₈₀Si₂₀, respectively, were calculated.     

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.     

Refractive-index dispersion of glassy semiconductors in the pseudo-binary As2Se3–SbSI system

The bulk chalcogenide glasses in the pseudo-binary system (As₂Se₃)_100?x(SbSI)_x (with x = 20, 30, 50, 70 and 80 at.%) were prepared from high-purity elemental components by melt-quenching technique. It is a system with the variable ratio of classical amorphous compound As₂Se₃ and the molecule of antimony-sulphoiodide, SbSI, which in the monocrystal form is characterized as a ferroelectric. The refractive-index behaviour (magnitude and spectral dispersion) of investigated glasses was determined by the prism method and analyzed. To calculate and discuss the parameters of dispersion in the band gap region three different approaches were used (Cauchy, Sellmeier and Wemple–DiDomenico single-oscillator model). The comparison of the results shows good agreement between all applied models. It was found that the value of the refractive-index shows normal dispersion behaviour and increases with the increase of Sb (or SbSI) content in the investigated system, whereas the optical gap of these glasses E_g slightly decreases.     

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.     

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.     

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.     

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.     

Properties and structure of AsTe glasses (II). Local order parameters and structural model

Diffraction measurements on bulk and thin films AsTe glasses are reported, throughout the whole range of stability. FNP distances and areas, average valence angles and compactness are determined from the Fourier transform. For As-rich glasses important differences are found between short-range order in both vitreous and crystalline states. An interpretation at the molecular scale is proposed. It takes account of the particular topological properties of the AsTe molecules, which are evidenced by hand-made models. The proposed structure consists of a network of AsTe_3/2 units which are interconnected through “AsAs locks”. It is shown that these “locks” play a crucial role in the stability of the As-rich glasses in favoring strongly the covalent bonds. On the other hand, in Te-rich glasses there is no similar locking mechanism. The presence of threefold-coordinated tellurium sites enhances the metallic character, and weakens the difference between crystalline and vitreous states. The structural models which are proposed enable us to explain the anomalies and the discontinuities of most thermodynamic properties which are described in paper I.     

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.