Threefold coordinated model structure of amorphous GeS,GeSe and GeTe

The black P structure is presented as a model for the structures of amorphous GeS, GeSe and GeTe. It is shown that the short interatomic distances, low near neighbor coordinations and high covalencies of the amorphous materials, relative to the crystalline, can be rationalized with the model. When scaled to the near neighbor interatomic distances in the amorphous materials, the model yields satisfactory agreement with the observed position and area of the second neighbor X-ray radial distribution function peaks. The model predicts: (a) A first neighbor peak area for GeS which is significantly different from that predicted by the random covalent model and (b) phase separation in certain composition regions which, for the Ge-S system, should be observable by means of transmission electron microscopy. An explanation of why phase separation is not likely to be observable through transmission electron microscopy studies of amorphous GeTe and GeSe is also presented.     

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.     

The preparation and electrical properties of thin chalcogenide semiconductor films

Thin chalcogenide films from the GeTeAsSi system have been prepared using electron beam evaporation and R.F. sputtering techniques. The techniques are described in some details as it is believed that the deposition procedure has a significant on subsequent electrical switching behaviour.     

On the reliability of amorphous chalcogenide switching devices

Switching devices consisting of a thin amorphous AsTeGe film sandwiched between two molybdenum electrodes were prepared by electron-beam evaporation and investigated by subsequent switching. The number of switching events from the beginning of the test until a prefixed change in switching voltage appeared, was defined as the “lifetime”. Samples which were subjected to an ageing process showed relatively stable operation, and longer lifetime than unaged samples. In addition, the lifetime was found to depend strongly on device geometry and to be limited by the formation of crystalline regions in the amorphous film. This was explained to be due to heating effects during the preswitching region, during the transition from off-state to on-state, and during the on-state, although the underlying switching model is assumed to be non-thermal.     

X-ray diffraction studies of glasses in the systems Na2OMeOSiO2 (MeMg,Zn, Ca,Ba)

X-ray diffraction studies of glasses in the following ternary systems have been made: Na₂OMgOSiO₂, Na₂OZnOSiO₂, Na₂OCaOSiO₂ and Na₂OBaOSiO₂. The following heavy atom substitutions have been used: Ag for Na and Ge for Si. The changes in the electron radial distribution curves resulting from AgNa replacement can be explained as amplifications of relatively well-defined NaSi distances, which are nearly the same in all the glasses investigated. The GeSi substitution causes changes which can be explained on the basis of isostructural GeSi substitutions.     

A study of the crystallization of two non-crystalline CuZr alloys

Two CuZr alloys, Cu-50 at% Zr and Cu-54 at% Zr, were splat quenched to the non-crystalline state using a piston and anvil device. The glass transition and crystallization temperatures, as well as the enthalpy releases observed during crystallization were measured using differential scanning calorimetry of as-splat specimens and specimens aged below T_g. Transmission electron microscopy and X-ray diffraction experiments were utilized to monitor phase and structural changes in the alloys as they were transformed to the crystalline state.The results of the investigation indicated that the non-crystalline to crystalline transformation of these two alloys in constant heating rate experiments above T_g was a two-step process. The initial step, which is associated with a large exothermic reaction, results in the appearance of crystallites in a matrix of non-crystalline material. The final step, associated with a smaller exothermic reaction, results in the total transformation of glass to the crystalline state and the formation of the equilibium crystalline phases.The effect of aging these splat-quenched non-crystalline alloys at temperatures below T_g was also investigated. It was determined from these experiments that crystallization does occur when the non-crystalline alloys are aged ～ 15°C below T_g. However, the incubation time for crystalline nucleus formation was found to be substantially greater for the Cu₅₀Zr₅₀ glass. Finally, it was determined that the thermal stability of the aged glass relative to the spontaneous crystallization observed during the constant heating rate experiments above T_g decreases as a function of aging time.     

Compositional dependence of the thermal,structural, and electrical properties of AsTeI glasses

Thermal, structural, and electrical properties of semiconducting AsTeI (and AsTe) glasses have been examined as a function of concentration. Analytical techniques have been developed for quantitative chemical analysis of all three components. Differential scanning calorimetry data indicate a broad endothermic reaction, T_min ∼ 145 °C, above the glass transition (T_g = 120 ± 8 °C) for iodine compositions of 0 to ∼ 20 at%. Above this reaction temperature the thermal data are composition dependent. For glasses with I ? 35 at %, T_g is much lower (65–70 °C) and the endothermic reaction is much sharper with a minimum at ∼ 133 °C. The wide variation in thermal properties with composition suggests that electrical effects associated with high temperatures (e.g. switching and memory phenomena) may also be composition dependent, as well as being dependent upon kinetics. Structural studies show that phase segregation above T_g is dependent upon kinetics as well as upon temperature. Thermal, structural, and electrical data give evidence that As₂Te₃ exists as a unit in the non-crystalline state. This structural unit, stable at high temperatures, is present in the molten material and is thought to be present as the supercooled liquid is quenched. The short-range order extends to long-range order upon devitrification and the first crystalline phase detected is monoclinic As₂Te₃. Apparently metastable under the conditions of formation, this phase converts to a previously unreported fcc phase upon further heat treatment. Similar crystalline structures are known to be associated with thermally- and electrically-induced memory phenomena in AsTeGe and AsTeI glasses. Density measurements at room temperature show that (1) there are no phase miscibility gaps in the glass substructure or different crystalline phases segregating as the I content is varied, and (2) there is a large change in molar volume with increasing I concentration, whereas the molar weight does not change significantly. Electrical conductivity, σ, data in the region around room temperature, show that the σ₀ values, determined from σ = σ₀e^−E/kt, range from 10² to 10³ (Ω-cm)⁻¹. A possible dependence of σ₀ on I content may be due to changes in molar volume. The more homogeneous glasses appear to show breakpoints in the σ versus 1/T data; the corresponding changes in E are only 0.02–0.06 eV, with E = 0.04 eV being most frequently observed. For As₅₀Te_50−xI_x glasses, σ at a given temperature increases as the iodine concentration is increased to about 5 at % and then decreases with further increase in I content. Correspondingly there is a minimum in the E versus I concentration data at I ∼ 5 at %. Results suggest that dependence of the σ breakpoints on glass homogeneity and the variation of E with I concentration may be due to trapping effects.     

Structure and morphology of oxide deposition on SiO2 (I) correlations between parameters of layer formation and structure

The oxides of Ge, Sn and Ti were produced by reaction of the adequate metal tetrachlorides in dependence on reaction parameters (pressure, temperature, time) on crystalline and non-crystalline SiO₂ substrates. Structure and morphology of the formed oxides were investigated by electron diffraction and electron microscopy. In dependence on the reaction conditions results on the degree of crystallisation and on the particle size distribution could be received. On the basis of the obtained results of investigations conceptions for the mechanism of layer formation were derived.     

Polynary silicon arsenic chalcogenide glasses with high softening temperatures

The introduction of Ag in SiAsTe glasses permits the incorporation of Se, otherwise volatile and/or degradable as a constituent in Si-containing chalcogenide glasses. SiAsAgTeSe glasses exhibit much higher softening ranges and glass transition temperatures than encountered in known chalgogenide systems. A glass Si₃₅As₁₅Ag₁₀Te₂₀Se₂₀ had the viscosity log ν = 13 at about 500°C, as compared to 370°C for the base glass Si₃₅As₂₅Te₄₀, the viscosity of log ν = 9.8 at about 560°C, as compared to 442°C for the base glass. Phase separation occurs in the system SiAsAgTeSe and becomes manifest in two glass transitions indicated by changes in the slopes of the expansion curves and breaks in the softening point-composition relations. The existence and behavior SiAsAgTeSe glasses suggests the possible development of higher T_g i.r. transparencies and higher T_g semiconductor glasses than described so far.     

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.     

Dielectric breakdown in metal-anodic oxide film-metal sandwiches

This paper presents measured data on glass transition temperature, thermal expansion, density, hardness, refractive index and electron microscopy of GeAsSe glasses and bring together some literature data on viscosity and mechanical properties. Interpretation of this information to identify suitable compositions for bulk synthesis is discussed.     

Strain sensitive properties of threshold switch devices

A blend of integrated circuit manufacturing techniques and rf sputtering techniques has resulted in TeAs-XXX threshold devices sustaining 100 million switchings. The fabrication and properties of these devices are outlined in this paper. Observations of space charge effects and the influence of strain on the switching properties of these devices are also reported.     

Hydrothermal synthesis of Bi2Te3 nanowires through the solid-state interdiffusion of Bi and Te atoms on the surface of Te nanowires

Polycrystalline Bi₂Te₃ nanowires were prepared by a hydrothermal method that involved inducing the nucleation of Bi atoms reduced from BiCl₃ on the surface of Te nanowires, which served as sacrificial templates. A Bi–Te alloy is formed by the interdiffusion of Bi and Te atoms at the boundary between the two metals. The Bi₂Te₃ nanowires synthesized in this study had a length of 3–5 μm, which is the same length as that of the Te nanowires, and a diameter of 300–500 nm, which is greater than that of the Te nanowires. The experimental results indicated that volume expansion of the Bi₂Te₃ nanowires was a result of the interdiffusion of Bi and Te atoms when Bi was alloyed on the surface of the Te nanowires. The morphologies of Bi₂Te₃ are strongly dependent on the reaction conditions such as the temperature and the type and concentration of the reducing agent. The morphologies, crystalline structure and physical properties of the product were analyzed by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS).     

Dielectric relaxation study of chalcogenide glasses

The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (?′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (?′). The curves: log ?′ versus temperature, ?′ versus log ω, ?″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times.Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys.Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys.It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.     

Phase separation in GeGeO2 glasses

De Neufville prepared homogeneous glasses ranging in composition from pure GeO₂ to GeO by quenching bulk samples from the melt and by vapor deposition. For compositions in the range of 10–20 mol % excess Ge dissolved in GeO₂, he found that phase separation into amorphous Ge rich and amorphous GeO₂ phases occurred. The results reported here on a 7.5 mol % excess Ge composition using differential scanning calorimetry have shown that a two-step phase separation mechanism is operative. A homogeneous GeGeO₂ glass phase separates at 450°C into amorphous GeO₂ and amorphous GeO. The GeO phase separates at 570°C into crystalline Ge and amorphous GeO₂. The heat measured at 570°C is equal to the sum of the heats of phase separation of GeO and crystallization of Ge. The amorphous GeO₂ crystallizes at 670°C with a heat of crystallization of 4.65 kcal/mol (± 0.5). Additional support for a two-step phase separation mechanism is provided by kinetic arguments based on the viscosity dependence on composition and on the structure of the amorphous GeO phase and its stability relative to the homogeneous GeGeO₂ glass.     

Radial distribution studies of glassy tellurium-silicon alloys

X-ray investigations of glassy tellurium-silicon alloys (Si content 10–40 at%) are described. Alloys with 13 to 27 at% Si were obtained in bulk form, non-crystalline samples with 10, 30, 33, 36 and 40 at% Si were obtained by rapid quenching from the melt by the splat-cooling technique. The interference functions and the radial distribution functions of all alloys show a great similarity. However, the positions of the maxima of the pair distribution function decrease continuously with growing Si content and lead to values which are similar to atomic distances in crystalline Te and Si₂Te₃. The areas of the first two peaks of the RDF, F₁ (rougnly 2) and F₂ (roughly 12), are compared with calculated areas derived from different structural models which are based on the short-range order in crystalline Te with a chain structure and in Si₂Te₃ with a tetrahedron configuration. An interstitial model with Si atoms in holes of Te chains and a random substitution model, where the Te atoms and the Si atoms have the same short-range order, both show a concentration dependence of the areas F₁ contrary to the experimental values. A structural model which assumes the tetrahedron configuration of Si₂Te₃ for Si atoms (coordination number N_Si = 4) and a coordination number N_Te = 2 for Te atoms can describe the short-range order of glassy TeSi alloys in the whole investigated concentration range with 10–40 at% Si.     

Phase formation and physical properties of mechanically alloyed amorphous 55Mg35Ni10Si

Amorphous 55Mg35Ni10Si alloy powder has been synthesized by mechanical alloying technique using pure Mg, Ni and Si elemental powders. The transformation of the crystalline powders into an amorphous one has been investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The new material produced has a higher thermal stability than reported results, which is beneficial to the fabrication of Mg–Ni–Si bulk amorphous components through powder metallurgy.     

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.     

On the compositional dependence of the optical gap in amorphous semiconducting alloys

It is found that the optical gap E_AB for amorphous A-B alloys can be determined by the energy gap E_A for element A and E_AB for the element B in the equation E_AB(Y) = YE_A + (1?Y) E_B where Y is the volume fraction of element A. Calculations based on a random bond network agree with experiments for SiGe, SbSe, and AsTe films (class A). Calculations based on a chemically ordered bond network which tends to form microscopic molecular species gree with experimental results for the AsSe, AsS, GeTe and Sb₂Se₃As₂Se₃ systems (class B). In contrast to the above systems, agreement with experiment is not obtained for the TeSe, As₂Te₃As₂Se₃ and GeTe₂GeSe₂ systems which contain atoms of both Te and Se (class C). The classification into three types (classes A, B and C) is consistent with the calculation based on effective medium percolation theory which interprets the compositional dependence of the conductivity of chalcogenide glasses.     

The origin of some ideas on non-crystalline materials

The glass-forming region of the ternary GeSIn system is determined by differential thermal analysis (DTA), X-ray diffraction and electron microscope studies. The glass phases of the system have a region of maximum In content of 15 atomic percent. The experimental studies determined the transition temperature T_g, the density d, the microhardness H and the refraction index n. An interpretation of the results from a chemical bond point of view has been made. The resistivity ? (2.0 + 7.5 × 10¹⁴) ω cm has also been measured and a switching effect has been observed. These physical and electrical studies have been carried out with regard to the practical applications of the formed glasses of the GeSIn system.