Sb Te System Research Articles

Mineralogical zoning of the PGE–Cu–Ni orebodies in the central part of the Oktyabr'sky Deposit, Norilsk District, Russia

AbstractMineralogical features of two orebodies, lenses (C-3 and C-4), at the central part of the Oktyabr'sky deposit have been investigated. Multidirectional mineralogical zoning in the northern and southern orebodies is shown, confirming the hypothesis of their formation from various magmatic flows, which have individual features and their own modes of formation. The southern C-3 and northern C-4 orebodies differ in their mineralogical associations: C-3 is characterised by a high-sulfur association of sulfides; and C-4 contains minerals with a sulfur deficit (talnakhite, sugakiite). Variations in Fe and Ni ratios in pentlandite are controlled by the volatility of sulfur during ore crystallisation. Direct crystallisation zoning is observed in the disseminated ores of the C-4 orebody (borehole RT-107), where the fugacity of sulfur ($f_{S_2}$) increases from bottom to top. In contrast in orebody C-3 (borehole RT-30) $f_{S_2}$ decreases in the same direction. This reverse zoning coincides with the vectors of the evolution of ore systems in various blocks of the Main Ore Body of the Oktyabr'sky deposit. The difference in typomorphic features of disseminated ores of the southern and northern orebodies is confirmed by differences in the associations of platinum-group element minerals (PGMs). Disseminated ores in picritic gabbro-dolerites and massive pyrrhotite ores in the exocontact of the intrusion within the southern orebody differ in the specialisation of PGMs: the former is characterised by minerals of the Pd–Bi–Sb–Te system, the latter by only Pt minerals. The similarity of these types of ores lies in the similar reverse mineralogical and geochemical zoning from top to bottom along the sections, caused by the evolution of the sulfide melt in this direction. The formation of reverse zoning of disseminated ores (zones of ‘marginal reversion’) is probably due to the action of a mechanism of repeated influx of a melt of an increasingly primitive composition into the upper parts of the crystallising flow. Unidirectional trends in massive and disseminated ores are more likely to be due to the action of the same type of mechanism.

Thermodynamic modeling of the Sb–Te system supported by DSC measurement and ab initio calculations

Thermodynamic modeling of the Sb–Te system supported by DSC measurement and ab initio calculations

Phase diagrams of Bi–Sb–Se–Te system

Bi–Sb–Se–Te is one of the most important material systems for thermoelectric applications. Phase diagrams of its constituent binary and ternary systems are reviewed and assessed. The Bi–Sb–Se–Te isothermal section tetrahedron at 400 °C and liquidus projection tetrahedron were proposed. Ternary compounds are only found in the Bi–Sb–Se system. There are eight three-phase regions at 400 °C and seven primary solidification phases in the Bi–Sb–Se system, including (Bi,Sb), (Bi2)m(Bi2Se3)n, Bi2Se3, Se, Sb2Se3, Bi3Sb5Se2, Bi3Sb12Se15. In the Bi–Sb–Te system, there are four three-phase regions at 400 °C. The (Bi,Sb)2Te3 and (Bi,Sb) are continuous solid solutions. There are six primary solidification phases, including (Bi,Sb)2Te3, (Te), γ, δ, (Bi,Sb), and (Bi2)m(Bi2Te3)n. In the Bi–Se–Te system, there is one three-phase region. (Bi2)m(Bi2Se3)n and (Bi2)m(Bi2Te3)n form a continuous solid solution phase at 400 °C. There are four primary solidification phases, including Bi, (Bi2)m(Bi2(Se,Te)3)n, Bi2(Se,Te)3 and (Se,Te). In the Sb–Se–Te system, there are five three-phase regions and six primary solidification phases, including (Sb), δ-(Sb2Te), γ-(SbTe), Sb2Te3, and (Se,Te). A wide range of (Bi,Sb)2(Se,Te)3 single-phase region was observed in the Bi–Sb–Se–Te quaternary system.

Phase change memory materials and their applications

Over the past 30 years, phase change memory materials based on chalcogenide semiconductors have rapidly developed from laboratory prototypes to materials extensively used as functional layers in various devices. First of all, this concerns compounds of the Ge–Sb–Te system, which can be reasonably considered as full-fledged functional materials. The review presents a current view of the control of properties of phase change memory materials by their chemical and structural modification. Both the existing and prospective applications of these materials are highlighted. The discussion of chemical modification focuses on popular dopants such as bismuth, tin, oxygen and nitrogen and also refractory metals. In the discussion of structural modification, the use of laser radiation is considered in detail. Currently, this is a key trend in increasing the operation speed of devices based on phase change memory materials. Data on the formation of periodic surface structures in these materials are highlighted and it is emphasized that this effect could find application in nanophotonics and optoelectronics in the near future.<br> The bibliography includes 336 references

Tailoring of Multisource Deposition Conditions towards Required Chemical Composition of Thin Films.

The model to tailor the required chemical composition of thin films fabricated via multisource deposition, exploiting basic physicochemical constants of source materials, is developed. The model is experimentally verified for the two-source depositions of chalcogenide thin films from Ga–Sb–Te system (tie-lines GaSb–GaTe and GaSb–Te). The thin films are deposited by radiofrequency magnetron sputtering using GaSb, GaTe, and Te targets. Prepared thin films are characterized by means of energy dispersive X-ray analysis coupled with a scanning electron microscope to determine the chemical composition and by variable angle spectroscopic ellipsometry to establish film thickness. Good agreement between results of calculations and experimentally determined compositions of the co-deposited thin films is achieved for both the above-mentioned tie-lines. Moreover, in spite of all the applied simplifications, the proposed model is robust to be generally used for studies where the influence of thin film composition on their properties is investigated.

Unraveling the Atomic Structure of Bulk Binary Ga-Te Glasses with Surprising Nanotectonic Features for Phase-Change Memory Applications.

Binary Ge-Te and ternary Ge-Sb-Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga-Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy GaxTe1-x, 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor-metal transition above melting. We also show that a phase change in binary Ga-Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4-8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

Obtaining glasses in the extremely crystallizing Ge–Sb–Te phase change material

Using thermal co-evaporation techniques, we show that various glassy compositions can be obtained along the GexSbxTe100-2x join in the ternary Ge–Sb–Te system which is known to display dramatic crystallization tendencies. Earlier attempts to produce bulk glasses have been limited to Sb-poor compositions close to the eutectic GeTe6. Results indicate a weak variation of Tg with composition x and also a thermal stability that is weak for most systems, and especially for compositions close to the domain where Ge2Sb2Te5 can be formed. Our results are put in perspective with other network-forming chalcogenides and the Tg variation suggests the preferential formation of Te–Sb–Te, at variance with isochemical compounds such as Ge–Sb–Se. Data are discussed within the framework of topological based approaches which not only indicate that optimal glass formation is achieved for compositions satisfying the Maxwell stability criterion but also predict a flexible to rigid transition at x=8.5%. Most of our glasses could be formed around this composition.

Comments on the electronic transport mechanisms in the crystalline state of Ge—Sb—Te phase-change materials

It is known that phase-change materials, such as Ge–Sb—Te ternary system, are promising resistive non-volatile random-access memory applications with ultra-rapid reversible transformations between the crystalline and amorphous phases. This class of electronic transition is categorized to be the metal-insulator transition (MIT). The Anderson-type MIT has been discussed extensively in phase-change materials (PCMs) and isothermal annealing of amorphous PCMs (a-PCMs) which, above a certain temperature, leads to the metallic (crystalline) phase. In the insulator regime near the MI transition, Mott-type variable-range hopping (VRH) and/or Efros-Shkolvskii hopping (ESH) at low temperatures below 20 K (and down to 1 K) in Ge1Sb2Te4 (GST124) have been discussed extensively, however, we criticize the above argument through a detailed discussion of physical parameters that support the VRH mechanism. It is not clear whether or not the density-of-states (DOS) near the Fermi level is localized (like the Fermi glass) in the crystalline phase. It is also suggested that grain boundaries are expected to interfere with the electronic transport in the crystalline state. We should take into account the grain boundary effects on the electronic transport in the crystalline phase of Ge—Sb—Te system.

Structural and Dielectric Study of Thin Amorphous Layers of the Ge–Sb–Te System Prepared by RF Magnetron Sputtering

The results of studying the structure and processes of dielectric relaxation in thin layers of Ge–Sb–Te are presented. The found permittivity dispersion and occurrence of dielectric-loss maxima in the low-frequency region are explained by the structural features of the compounds under study.

Boundaries of the X Phases in Sb–Te and Bi–Te Binary Alloy Systems

Sb–Te and Bi–Te compounds are key components of thermoelectric or phase change recording devices. These two binary systems form commensurately/incommensurately modulated long-period layer stacking structures known as homologous phases that comprise discrete intermetallic compounds and X phases. In the latter, the homologous structures are not discrete but rather appear continuously with varying stacking periods that depend on the binary composition. However, the regions over which these X phases exist have not yet been clarified. In this study, precise synchrotron X-ray diffraction analyses of various specimens were conducted. The results demonstrate that the X phase regions are located between Sb20Te3 and Sb5Te6 in the Sb–Te system and between Bi8Te3 and Bi4Te5 in the Bi–Te system.

Thermal stability and microstructure of germanium antimony telluride thin films under interdiffusion conditions

This paper investigates the interdiffusion behaviors of alloy components of the Ge–Sb–Te system and its influence on the thermal and electrical instability of Ge–Sb–Te alloy films. When SbxTe1−x/Ge bilayer films are annealed at 400 °C, the Ge–Sb–Te intermediate layers are formed at the interface between SbxTe1−x and Ge. This is caused by the interdiffusion of Ge, Sb, and Te. The highest degree of interdiffusion is observed in the Ge–Te system, while the existence of Sb retards the interdiffusion of Ge and Te in the SbxTe1−x/Ge bilayer films. The outdiffusion and sublimation of low-melting-point Sb and Te occur at the annealing temperature of 500 °C, which leads to the instability of the Ge–Sb–Te intermediate layers. The phase-change memory (PCM) device employing a Ge2Sb2Te5 intermediate layer exhibit stable endurance, because of the robustness of Ge2Sb2Te5 compound with respect to the phase separation during the repetitive set–reset cycles.

MOVPE deposition of Sb2Te3 and other phases of Sb-Te system on sapphire substrate

The films of Sb-Te system have been deposited by MOVPE on (0001) Al2O3 substrates with thin ZnTe buffer layers at different temperatures and Te/Sb ratios in the vapor phase. X-ray diffractometry, SEM microscopy, Raman and EDX spectroscopy were used to study as-grown films. The surface morphology and stoichiometry of Sb-Te films strongly depend on Te/Sb ratio in vapor phase. We have deposited the phases of homologous series nSb2·mSb2Te3 with following stoichiometries: Sb2Te3, Sb4Te5, Sb8Te9, Sb10Te9, Sb4Te3, Sb2Te, Sb8Te3, Sb10Te3, Sb16Te3, Sb18Te3 and Sb. Transport properties of Sb2Te3, Sb4Te5, Sb8Te9, Sb4Te3, Sb2Te were evaluated using Van der Pauw technique at 300K.

Electrophysical Properties of Ge–Sb–Te Thin Films for Phase Change Memory Devices

In this work, we studied temperature dependences of the resistivity and current-voltage characteristics of amorphous thin films based on the materials of a Ge–Sb–Te system of compositions GeSb4Te7 (GST147), GeSb2Te4 (GST124), and Ge2Sb2Te5 (GST225) applied in the phase change memory devices. The effect of changes in the composition of thin films on the crystallization temperature, resistivity of films in amorphous and crystalline states, and on the activation energy of conductivity is determined. It is found that the peculiarity of these materials is the mechanism of two-channel conductivity where the contribution to the conductivity is made by charge carriers excited into localized states in the band tails and by carriers of the delocalized states in the valence band.

Direct observation of metastable face-centered cubic Sb2Te3 crystal

Although phase change memory technology has developed drastically in the past two decades, the cognition of the key switching materials still ignores an important member, the face-centered cubic Sb2Te3. Apart from the well-known equilibrium hexagonal Sb2Te3 crystal, we prove the metastable face-centered cubic Sb2Te3 phase does exist. Such a metastable crystal contains a large concentration of vacancies randomly occupying the cationic lattice sites. The face-centered cubic to hexagonal phase transformation of Sb2Te3, accompanied by vacancy aggregation, occurs at a quite lower temperature compared to that of Ge2Sb2Te5 alloy. We prove that the covalent-like bonds prevail in the metastable Sb2Te3 crystal, deviating from the ideal resonant features. If a proper doping technique is adopted, the metastable Sb2Te3 phase could be promising for realizing reversibly swift and low-energy phase change memory applications. Our study may offer a new insight into commercialized Ge–Sb–Te systems and help in the design of novel phase change materials to boost the performances of the phase change memory device.

Voltage oscillations in the case of the switching effect in thin layers of Ge–Sb–Te chalcogenides in the current mode

The I–V characteristics obtained for layers of chalcogenide vitreous semiconductor of the Ge–Sb–Te system in the current-generator mode are investigated. The instability region, i.e., the region of conductivity oscillations, observed in the case of the switching effect under conditions of a set current is revealed. The key parameters describing these oscillations and the conditions of their occurrence are investigated in detail. Analysis of the obtained data shows that, to explain the oscillations in the instability region, it is necessary to take into account an increase in the current density and the process of heat exchange between the current filament that arises in the film upon switching and the environment.

Effect of doping on the crystallization kinetics of phase change memory materials on the basis of Ge–Sb–Te system

The influence of different amounts of Bi, Ti and In on the thermal properties and crystallization kinetics of Ge2Sb2Te5 thin films for phase change memory devices was investigated. Temperatures and heat effects of crystallization were evaluated for all investigated compositions. Joint utilization of model-free Ozawa–Flynn–Wall and model-fitting Coates-Redfern methods allowed to estimate effective activation energies and pre-exponential factors for crystallization processes of amorphous films as functions of conversion, and determine reaction models. It was found that crystallization process most adequately can be described by the second- or third-order reaction. Storage and data processing times of the phase change memory cells on the basis of investigated materials were calculated with using of determined kinetic triplets of crystallization processes. Calculations showed that crystallization time decreases nearly on the order of magnitude for Ge2Sb2Te5+1 mass% In in comparison with undoped Ge2Sb2Te5. On the other hand, compositions with 0.5 and 3 mass% In allow to increase sufficiently storage time. Introduction of Ti does not significantly affect data processing time of phase change memory cell; however, it decreases storage time. Ge2Sb2Te5+0.5 mass% Bi composition have the most suitable kinetic parameters for phase change memory among the studied thin films.

Thermodynamic study of the Ag–Sb–Te system with an advanced EMF method

In this work, thermodynamic properties of antimony saturated AgSbTe2 (Ag0.22Sb0.28Te0.50) and silver saturated SbxTe1−x (Ag0.08Sb0.49Te0.43) have been determined by solid state EMF technique with Ag+ ion conducting β″-alumina electrolyte, for the first time. The measurements were done by electrochemical cells of type:(−)Pt, Ag|β″-alumina|Sb2Te3+Ag2Te+SbxTe1−x, Re, Pt(+)Over the temperature range 566⩽T/K⩽620.5 and(−)Pt, Ag|β″-alumina|AgSbTe2+Ag2Te+SbxTe1−x, Re, Pt(+)Over the temperature range 620.5⩽T/K⩽710.The phase transition temperature, where Sb2Te3 and Ag2Te form AgSbTe2 at 101.3kPa pressure, was found to be T=620.5K. Standard thermodynamic quantities of SbxTe1−x and AgSbTe2 were calculated from the results obtained as well as Gibbs energies of formation as linear functions of temperature. Each test electrode, containing 3 phases in equilibrium, was synthetized separately inside an evacuated quartz glass ampoule and quenched into ice water. The contents of the test electrodes were confirmed by SEM–EDS analysis. Also, the average solubility of substances in the test electrodes was calculated from the SEM–EDS data obtained. The parasitic thermovoltage involved with the measured EMF was determined by symmetrical cell arrangement and compensated from the experimental values.

Boundaries of the homologous phases in Sb–Te and Bi–Te binary alloy systems

Sb–Te and Bi–Te binary systems have long-period stacking structures called homologous phases. Within these structures, two types of fundamental structural units change their numbers according to their composition, and the stacking periods also change systematically. X-ray powder diffraction data on bulk specimens with different compositions reveal both the phase boundaries of the homologous phases and the structures of the boundary phases. The boundary phases are Sb20Te3 in the Sb–Te system and Bi8Te3 in the Bi–Te system.

Maximizing thermoelectric properties by nanoinclusion of γ-SbTe in Sb2Te3 film via solid-state phase transition from amorphous Sb–Te electrodeposits

In this work, we demonstrate an enhancement of thermoelectric properties by creating γ-SbTe/Sb2Te3 nanocomposite film, where γ-SbTe nanoinclusions are embedded in a nanocrystalline Sb2Te3 matrix. The two-phase nanocomposite was formed via solid-state phase transition using an amorphous Sb2Te3 electrodeposits as the starting materials. The crystallinity and crystal structure of intermediate states during the amorphous-crystalline solid-state transformation were characterized by sequentially annealing the sample. The formation of the γ-SbTe is attributed to the different enthalpy of mixing for the bonding structures available in the Sb–Te system. Room temperature measurement of electrical and thermoelectrical properties as a function of annealing temperature revealed that the two-phase system provided the carrier energy filtering effect at the interfaces between two phases, leading to enhanced Seebeck coefficient without affecting its electrical transport properties. The band bending at the two-phase interfaces was indirectly manifested by measuring their difference in the valence band, advocating the possibility of a strong energy-dependent charge scattering to the enhanced Seebeck coefficient.

Thermodynamic Modeling of the Pt-Te and Pt-Sb-Te Systems

The Pt-Te and the Pt-Sb-Te systems are modeled using the calculation of phase diagram (CALPHAD) technique. In the Pt-Te system, the liquid phase is modeled as (Pt, PtTe2, Te) using the associate model, and four intermediates, PtTe2, Pt2Te3, Pt3Te4 and PtTe, are treated as stoichiometric compounds and their enthalpies of formation are obtained by means of first-principles calculations. The solution phases, fcc(Pt) and hex(Te), are described as substitutional solutions. Combined with the thermodynamic models of the liquid phase in the Pt-Sb and Sb-Te systems in the literature, the liquid phase of the Pt-Sb-Te ternary system is modeled as (Pt, Sb, Te, Sb2Te3, PtTe2) also using the associate model. The compounds, PtTe2, Pt2Te3, Pt3Te4 and PtTe in the Pt-Te system and PtSb2, PtSb, Pt3Sb2 and Pt7Sb in the Pt-Sb system are treated as line compounds Ptm(Sb,Te)n in the Pt-Sb-Te system, and the compound Pt5Sb is treated as (Pt,Sb)5(Pt,Sb,Te). A set of self-consistent thermodynamic parameters is obtained. Using these thermodynamic parameters, the experimental Pt-Te phase diagram, the experimental heat capacities of PtTe and PtTe2, the enthalpies of formation from first-principles calculations for PtTe2, Pt2Te3, Pt3Te4, and PtTe, and the ternary isothermal sections at 873 K, 923 K, 1073 K and 1273 K are well reproduced.