Ge Te System Research Articles

Glass formation in the Ge xTe 100−x binary system: Synthesis by twin roller quenching and co-thermal evaporation techniques

The Ge–Te system exhibits one main composition domain where glasses can be easily prepared by melt quenching technique; this domain is centered on the eutectic composition Ge 15Te 85. In this work, bulk flakes and films of composition Ge x Te 100− x with x ⩽ 50 at.% were prepared by two different quenching techniques: (i) the twin roller quenching for bulk flakes and, (ii) the co-thermal evaporation for films (with thickness comprised between 2 and 14 μm). Electron Probe Micro-Analysis was used to check the composition of the materials while X-ray diffraction allowed identifying the amorphous state and/or the crystalline phases present in the Ge x Te 100− x samples. Thermal properties for both types of materials were investigated by differential scanning calorimetry. The glass-forming regions were: 11.7–22.0 at.% Ge for bulk flakes and 10.2–35.9 at.% Ge for films. A similar thermal behavior of bulk flakes and thick films was highlighted by Differential Scanning Calorimetry.

Evaluation of charge displacement and the radii of Wigner-Seitz spheres for the components of A-Te (A = Ge, Sn, Pb) melts

Thermodynamic and structural data for A-Te (A = Ge, Sn, Pb) melts are used to evaluate the charge displacement (in Tamaki’s approach) and the radii of Wigner-Seitz spheres. The results indicate that the interaction between dissimilar components and the formation of the ATe compounds prevail in the melts. In the Ge-Te system, there is a tendency toward the formation of Ge0.85Te0.15 and Ge0.33Te0.66 associates, in which, respectively, covalent and ionic bonding prevails.

A study of the crystallization kinetics of Ge-Te amorphous systems

The glass-forming ability of binary Ge xTe 100-x ( x=15, 20, 30) alloys was investigated. The crystallization mechanism of these amorphous bulks was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential scanning calo-rimetry (DSC). The temperature of glass transition, the temperature of crystallization, the activation energy for glass transition and crystallization, and the order of the crystallization mechanism were calculated from the different heating-rates.

Static and dynamic structures of liquid GeTe mixtures

Neutron diffraction measurements of the liquid Ge–Te system and quasi- and inelastic neutron scattering experiments of the eutectic liquid Ge 15Te 85 were performed in the temperature range from 400 to 560 °C. The large temperature dependence of the static structure for liquid Ge 15Te 85 is reconfirmed; however it is found that the compositional dependence obtained at around 80 °C above the liquidus curve is rather small. The dynamic structure factors of liquid Ge 15Te 85 in the Q-region from 0.2 to 2.6 Å −1 show a normal temperature dependence. From the vibrational density of states, it is found that the Te–Te covalent bond persists in liquid Ge 15Te 85; however a bending mode was not assigned clearly.

A Family of Far‐Infrared‐Transmitting Glasses in the Ga–Ge–Te System for Space Applications

AbstractConditions of formation of bulk tellurium chalcogenide glasses, containing 70–80 at % Te associated to Ga and Ge are described. The characteristic temperatures are measured: the glass‐transition temperature, Tg, is situated in the range 140–185 °C and the difference Tx – Tg, where Tx is the onset crystallization temperature, is in the range 76–113 °C. Moreover, the optical transmission window is exceptional, ranging from 1.99 μm in the bandgap up to 28 μm in the phonon region.Developed within the framework of requirements for the Darwin mission (a search for and study of extrasolar planets), feasibility of infrared optical fibers formed from these glasses is studied. Drawing experiments are conducted with the glass Ga2‐Ge3Te15, which possesses the maximum Tx – Tg, and a first optical fiber is presented.

Thermodynamics of Structural Changes in Liquid Ge–Te Alloys Around the Eutectic Composition: Specific Heat Measurements and Thermodynamic Stability

We have obtained the specific heat, C p , of liquid Ge–Te alloys around the eutectic composition (15 at.% Ge) and alloys up to 50 at.% Ge by combining the data obtained by modulation and adiabatic calorimetries. The results cover a wide temperature range from deeply undercooled state to 800°C. A very pronounced peak of C p was observed for Te and alloys up to 20 at.% Ge. The peak position of C p takes a maximum around the eutectic composition where the peak width becomes narrowest. Using a new thermodynamic relation, we clarify a correlation between the thermodynamic stability and structural changes in the liquid Ge–Te system.

Thermodynamics of the structural changes in the liquid Ge–Te system around the Te-rich eutectic composition

To clarify the thermodynamics of the structural changes in a liquid Ge–Te alloy around the Te-rich eutectic composition (15 at.% Ge), detailed measurements of the molar volume, sound velocity and specific heat measurements were performed. The crossover temperature manifested by a sharp extremum in those thermodynamic quantities takes a maximum around the eutectic composition where the peak width becomes most narrow. The structural changes are discussed in connection with the thermodynamic fluctuations.

Topological characterization of the Bi-Ge-Te phase diagram

Topological images of the Ge-Sb-Te phase diagram are constructed on the basis of two versions of the phase relations between the low-temperature forms of Ge1-δTe in the Ge-Te system. The images have the form of flow diagrams and graphs, with labeled nodes representing the phases existing in the system. The graph edges represent two-phase mixtures and are labeled by the numbers of the phase reactions in which these mixtures appear and disappear. The graphs help to visualize the topology of the phase diagram in a compact form and are convenient for topological characterization and identification of phase reactions at invariant points.

Molecular structure of ternary Ge–As–Te glasses studied by Mössbauer spectroscopy

Local environments in As–Ge–Te glasses are studied using 125Te and 119Sn Mössbauer spectroscopy. Complementary to previous works on the binary As–Te and Ge–Te systems, the influence of Te content is reported in the ternary As–Ge–Te system. Particular attention is made in fitting the observed lineshapes, first using a `classical' method and second using a quadrupole splitting distribution. The results show evidence of three Te local environments, a 3-fold co-ordinated site and a pair of 2-fold co-ordinated sites. The concentration of the 3-fold co-ordinated Te site is found to increase with As content of glasses, suggesting specific microscopic structures of the observed sites.

Glass formation in germanium telluride glasses containing metallic additives

Differential scanning calorimetric (DSC) studies have been carried out on germanium telluride glasses containing Cu and Ag. Both Cu x Ge 15Te 85− x (2≤ x≤10) and Ag x Ge 15Te 85− x (2.5≤ x≤21.5) glasses are found to exhibit single glass transition and single crystallization. On the basis of the devitrification behaviour of these glasses one can conclude that the network connectivity of the parent Ge–Te matrix is not improved by the addition of Cu whereas Ag improves the connectivity. Over-constraining of the structural network for x>5 is rapid in Cu added glasses and more gradual in Ag added glasses. The difference in the glass formation in the Cu–Ge–Te and Ag–Ge–Te systems is understood in light of the above differences in the structural network.

Effect of Metal Additive on the Electrical Properties of Glassy Ge-Te

Ternary compositions Ge1Te4 (In. or Cu.) with x= 0.05 and 0.1 were prepared by conventional melt-quenching. Thin films of these compositions were deposited by thermal evaporation with thickness 300 nm. Dependence of d.c conductivity on compositions has been reported. It was found that the activation energy decreases with increasing additive (In or Cu). The obtained results have been interpreted in the frame of the model proposed by Mott and Davis (1979). The ac electrical properties of adding Cu or In to amorphous GeTe system have been measured in the frequency range from 100 Hz to 100KHz and temperature range (303-208)K. It is found that, in Ge1Te4 (In. or Cu. ) compositions where x is limited to x=0.05 and x=0.1, the change of conductivitycracdeFends linearly on the frequency and temperature. The value of the exponent (S) in all this system is reasonably interpreted by the correlated barrier-hopping model (CBH). The analysis of the results reveals that electronic conduction of Ge1Te4 (Inx or Cu.) takes place via bipolaron hopping. The dielectric investigation provides a fundamental method for studying the rotational and hopping electron processes involved in conduction. Conductivity values, which depend on the glass microstructure, are observed as a term of cohesive energy, expre-ssed by the force of binding ions. An attempt has been made to evaluate this correlation. The experimental results support to some extent the above criterion in the case of Ge1Te4 (In. or Cu.) ternary alloy.

A study on the thermal stability of some alloys in the Cu–Ge–Te glassy system by differential scanning calorimetry

The glass formation and devitrification of alloys in the Cu–Ge–Te system were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of the glassy materials in the above-mentioned system is presented. All of these methods are based on characteristic temperatures such as the glass transition temperature, T g, the onset temperature of crystallization, T in, the temperature corresponding to the maximum crystallization rate, T p, or the melting temperature, T m. In this work, the parameter K r( T) is added to the stability criteria. The thermal stability of some ternary compounds of the Cu x Ge 0.20− y Te 0.80− x+ y type has been evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria.

Thermodynamic evaluation of the Germanium — Tellurium system

In order to optimize the binary Ge-Te system available experimental data were critically compiled from the literature. The cubic high-temperature β-phase and the rhomboedric room-temperature α-phase were described by a two-sublattice model with one kind of defect (vacancies on germanium sites). The room-temperature orthorhombic γ-phase was treated as a stoichiometric compound, as there is a lack of information about its solubility. The liquid phase was modelled by the the associate model with one kind of associate, namely ‘GeTe’. A set of thermodynamic parameters was obtained and the calculated phase diagram is presented.

Calorimetric investigations of liquid Ge-Te and Si-Te alloys

The heats of solution of Ge and Te in liquid Ge-Te alloys and of Si in liquid Si-Te alloys were determined in a heat flow calorimeter at different temperatures. The excess enthalpies were calculated from the measured heat effects. In addition the heats of formation of liquid Ge-Te alloys were measured at 1203 K. The associate model was used to describe the thermodynamic functions. Minima of the enthalpy curves in the Ge-Te and Si-Te systems at 1213 K were found at 57 mol.% Te and −11.2 kJ mol−1 and at 62 mol.% Te and −22.3 kJ mol−1 respectively. A temperature dependence of the excess enthalpies was only observed in the Ge-Te system below about 1093 K. The enthalpies of the Si-Te system exhibited an almost linear temperature dependence.

Indium quasi-local level and superconductivity in GezSn1−zTe:In

Starting with the idea that indium creates a narrow impurity band of quasi-local resonant states in the valence band of the SnTeGeTe system, we have noticed that superconductivity with Tc>0.4 K in these materials requires not only partial electron occupation of the resonant states pinning the Fermi level, but also the following additional condition: on the energy scale the band of resonant impurity states should overlap additional valence band states with a high density of states. An analysis of a correlation between the experimental data of the resistivity, the Hall concentration and superconducting properties was made for solid solutions of GezSn1−zTe:In (0 ⩽ z ⩽ 0.4; In: 5 and 16 at.%). It was found that the superconducting parameters (Tc, Hc2(0)) have a bell-shaped function of the Ge content, revealing a maximum near the edge of the heavy-hole valence band.

Etude des verres du système Ag-Ge-Te

A middle-sized vitreous domain resulting from binary glasses of the Ge-Te system has been found. DTA measurements of the glass-transition (Tg) and crystallization (Tc) temperatures have been performed which show that, for a constant value of the Ge/Te ratio, the values of these parameters do not depend on the silver content. Electrical properties of the vitreous Ag x GeTe 4,7 system (with 0.00≤x≤1.40) have been investigated by means of the complex impedance method. At 25 o C, electrical conductivity and activation energies vary within 10 -5 and 10 -4 Ω -1 -cm -1 and within 0.47 and 0.42 eV, respectively. Optical gap and e.m.f. measurements as well as the detemination of the electronic transference number by means of electrochemical semipermeability flux measurements reveal a semiconductor behaviour in spite of the rather high silver content. In order to investigate the local structure around the silver and germanium atoms, an EXAFS structural appraoch has been undertaken. The obtained reults show that in ternary glasses the arrangement if the tellurium atoms around a germanium atom is appreciably the same as that found in binary glasses. It can be noticed that Fourier transform moduli are higher for glassy phases than for crystalline ones; such a features has previously been observed in the As-Te system. Besides, Ag + ions appear to be tetracoordinated

Description du système ternaire AgGeTe

The phase diagram of the AgGeTe system is described using differential thermal analysis, X-ray diffraction and metallography experiments. Only one ternary compound was observed: Ag 8GeTe 6, which has a binary type peritectic decomposition at 644 °C. Seven ternary invariants were found: four ternary eutectics and three transistory ternay peritectics. A liquid-liquid miscibility gap starts from the AgAg 2Te binary system and extends a long way into the ternary system. A glassy domain in the ternary AgGeTe system is established.

The Anomalous Negative Thermal Expansion and the Compressibility Maximum of Molten Ge-Te Alloys

The high energy γ-ray attenuation and sound velocity have been measured to investigate the bonding nature and structural changes in liquid Ge-Te system. The results have been used to deduce the molar volume and adiabatic compressibility. The volume for Te rich alloys around the eutectic composition shows anomalous increase as the liquidus temperature is approached. Correspondingly the thermal expansion coefficient and adiabatic compressibility show very sharp extrema at around 400°C, which indicates that rapid structural changes occur in the liquid. At high temperatures, the composition dependences of the volume and compressibility show small positive deviation from the linear interpolations where anomalies in the magnetic susceptibility and electrical properties have been found.

Formation et comportement thermique des verres dans le systeme AsGeTe relations avec le diagramme de phases - mise en évidence d'une séparation sub-liquidus de phases vitreuses

Extent of the glass former domain in the ternary AsGeTe system. Influence of the cooling speed on the glass formation. Correlation of the glass former ability with the phase diagram. Variation of Tg and crystallization temperatures as a function of the composition. 4 regions of glasses are characterized by their relations with the phase diagram and their thermal behaviour. A region of sub-liquidus separation of vitreous phases was observed, by annealing the glasses of lower content in tellurium; it is related to a similar phenomenon observed in the As-Te system.

High-pressure studies on Ge-Te glasses. Evidence for a critical composition in IV-VI chalcogenide glassy systems

Abstract The electrical resistivity of bulk GexTe100-x glasses has been measured as a function of temperature and pressure. Under high pressure, all the glasses were found to undergo sharp discontinuous transitions from glassy semiconductors to crystalline metal. Several of the observed properties such as the transition pressure, conductivity activation energy and pre-exponential factor, exhibit anomalous trends at a composition x = 20. These results suggest that the x = 20 composition in the Ge-Te system should possess salient structural features. A model based on the unusual stability of structural units is proposed for explaining the anomaly at 20 at.% Ge concentration.