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Microstructure evolution and crystallography of the phase-change material TiSbTe films annealed in situ
There is an increasing demand for high-density memories with high stability for supercomputers in this big data era. Traditional dynamic random access memory cannot satisfy the requirement due to its limitation of volatile and power-consumable data storage. The multi-level cell phase-change memory (MLC PCM) based on phase-change material possesses a higher storage density, which is considered to be the most promising candidate. However, the detrimental resistance drift exists commonly in phase-change materials, which destroys the stability and greatly limits the development of MLC PCM. Here, we propose a completely new strategy to suppress resistance drift by exploring its microscopic mechanism via combinations of theoretical calculations and experiments. We find that, for the first time, resistance drift originates from change in electron binding energy induced by structural relaxation and is proportional to the reciprocal of dielectric coefficient according to hydrogen-like model. On this basis, we propose to reduce resistance drift by increasing thermal stability of dielectric coefficient. Two series of experiments prove the effectiveness of our new strategy. The resistance drift exponent of phase-change films is significantly reduced to 0.023 by using our strategy, which is lower by half than the best result (0.050) reported previously. Interestingly, the films also show improved storage properties. These results not only unravel that the stability and storage function of phase-change films can be simultaneously improved by modification of dielectric properties but also pave the way for future material design for stable MLC PCM.
Phase-change memory is regard as one of the most promising candidates for the next-generation non-volatile memory. In this work, we proposed a Sb7Te3/ZnSb multilayer thin films to improve the thermal stability of Sb-rich Sb3Te7. The sheet resistance ratio between amorphous and crystalline states reached up to 4 orders of magnitude. With regard to the thermal stability, the calculated temperature for 10-year data retention is about 127 °C. The threshold current and threshold voltage of a cell based on Sb7Te3/ZnSb are 6.9 μA and 1.9 V, respectively. The lower RESET power is presented in the PCM cells of Sb7Te3/ZnSb films, benefiting from its high resistivity.
Herein, the GaSb-doped Ti0.09Sb0.38Te0.53(TST) material is investigated and considered to be a potential candidate for phase change random access memory (PCM) application due to its overall good performance. With a high crystallization temperature of 220 °C, (GaSb)0.11(Ti0.09Sb0.38T0.53)0.89 ((GaSb)0.11TST) exhibits a data retention of 10 y at 136.8 °C, which is much better than that of Ge2Sb2Te5 as well as TST. For the (GaSb)0.11TST-based cell, an electric pulse as short as 5 ns can fulfill the SET operation, thus demonstrating an extremely rapid crystallization speed. Furthermore, the cell shows a significantly lower power consumption for SET/RESET reversible switching than that of the Ge2Sb2Te5-based cell. The programming cycles can reach 1 × 104 cycles with stability resistance of about a two orders of magnitude on/off ratio.
Phase-change memory based on Ti0.4Sb2Te3 material has one order of magnitude faster Set speed and as low as one-fifth of the Reset energy compared with the conventional Ge2Sb2Te5 based device. However, the phase-transition mechanism of the Ti0.4Sb2Te3 material remains inconclusive due to the lack of direct experimental evidence. Here we report a direct atom-by-atom chemical identification of titanium-centered octahedra in crystalline Ti0.4Sb2Te3 material with a state-of-the-art atomic mapping technology. Further, by using soft X-ray absorption spectroscopy and density function theory simulations, we identify in amorphous Ti0.4Sb2Te3 the titanium atoms preferably maintain the octahedral configuration. Our work may pave the way to more thorough understanding and tailoring of the nature of the Ti-Sb-Te material, for promoting the development of dynamic random access memory-like phase-change memory as an emerging storage-class memory to reform current memory hierarchy.
The extreme electro-optical contrast between crystalline and amorphous states in phase-change materials is routinely exploited in optical data storage and future applications include universal memories, flexible displays, reconfigurable optical circuits, and logic devices. Optical contrast is believed to arise owing to a change in crystallinity. Here we show that the connection between optical properties and structure can be broken. Using a combination of single-shot femtosecond electron diffraction and optical spectroscopy, we simultaneously follow the lattice dynamics and dielectric function in the phase-change material Ge2Sb2Te5 during an irreversible state transformation. The dielectric function changes by 30% within 100 fs owing to a rapid depletion of electrons from resonantly bonded states. This occurs without perturbing the crystallinity of the lattice, which heats with a 2-ps time constant. The optical changes are an order of magnitude larger than those achievable with silicon and present new routes to manipulate light on an ultrafast timescale without structural changes.
Phase change materials, successfully used in optical data-storage and non-volatile electronic memory, are well-known for their ultrafast crystallization speed. However, the fundamental understanding of the crystallization behavior, especially the nucleation process, is limited by present experimental techniques. Here, real-time radial distribution functions (RDFs), derived from the selected area electron diffraction, are employed as a structural probe to comprehensively study both nucleation and sequent growth stages of Ti-doped Sb2Te3 (TST) materials in the electron-irradiation crystallization process. It can be found that the incorporation of Ti atoms in Sb2Te3 formats Ti-Te, Ti-Sb, etc. wrong bonds, breaks the originally ordered atomic arrangement and diminishes the initial nuclei size of as-deposited films, which result in better thermal stability. But these nuclei hardly grow until their sizes exceed a critical value, and then a rapid growth period starts. This means that extended nucleation time is required to form the supercritical nuclei for TST alloy with higher concentration. Also, the formation of increasing four-membered rings, served as nucleation sites, after doping excessive Ti may be responsible for the change of the crystallization mechanism from growth-dominated to nucleation-dominated.
To date, slow Set operation speed and high Reset operation power remain to be important limitations for substituting dynamic random access memory by phase change memory. Here, we demonstrate phase change memory cell based on Ti0.4Sb2Te3 alloy, showing one order of magnitude faster Set operation speed and as low as one-fifth Reset operation power, compared with Ge2Sb2Te5-based phase change memory cell at the same size. The enhancements may be rooted in the common presence of titanium-centred octahedral motifs in both amorphous and crystalline Ti0.4Sb2Te3 phases. The essentially unchanged local structures around the titanium atoms may be responsible for the significantly improved performance, as these structures could act as nucleation centres to facilitate a swift, low-energy order-disorder transition for the rest of the Sb-centred octahedrons. Our study may provide an alternative to the development of high-speed, low-power dynamic random access memory-like phase change memory technology.
Compared with pure Sb2Te3, Ti0.32Sb2Te3 (TST) phase change material has larger resistance ratio, higher crystallization temperature and better thermal stability. The sharp decrease in mobility is responsible for the increasing amorphous and crystalline sheet resistance. The uniform crystalline structure of TST film is very benefit for the endurance characteristic. The Set and Reset operation voltages for TST-based phase change memory device are much lower than those of conventional Ge2Sb2Te5-based one. Remarkably, the device presents extremely rapid Set operation speed (̃6 ns). Furthermore, up to 1 × 106 programming cycles are obtained with stable Set and Reset resistances.
The study of metal-insulator transitions (MITs) in crystalline solids is a subject of paramount importance, both from the fundamental point of view and for its relevance to the transport properties of materials. Recently, a MIT governed by disorder was observed in crystalline phase-change materials. Here we report on calculations employing density functional theory, which identify the microscopic mechanism that localizes the wavefunctions and is driving this transition. We show that, in the insulating phase, the electronic states responsible for charge transport are localized inside regions having large vacancy concentrations. The transition to the metallic state is driven by the dissolution of these vacancy clusters and the formation of ordered vacancy layers. These results provide important insights on controlling the wavefunction localization, which should help to develop conceptually new devices based on multiple resistance states.
The influence of doping upon the phase change characteristics of Ge2Sb2Te5 (GST) has been determined with a variety of techniques including four-point-probe electrical resistance measurements, grazing incidence X-ray diffraction (XRD), X-ray reflectometry (XRR) and a variable incident angle spectroscopic ellipsometer (VASE) and a static tester. Doping with Bi, Sn or In maintains the NaCl-type crystalline structure of GST but expands the lattice due to the larger atomic radii. Sufficient optical contrast is exhibited and can be presumably correlated with the pronounced density change upon crystallization. In the Bi and Sn doped case transition temperatures are reduced with regard to the undoped case. Ultra-fast crystallization within 10ns is demonstrated, which is correlated with a single NaCl-structure phase and a lower transition temperature arising from the weaker bonds. In the In doped case, however, crystallization is retarded, which can be correlated with the observed phase separation and the increased transition temperature.
The crystallization of a sputtered Sb8Te3 film was examined in an X-ray powder diffraction experiment. An as-sputtered, amorphous Sb8Te3 film crystallized during heating into a structure of Sb–Te homologous series modulated along the stacking direction. During heating the lattice parameters and the modulation period γ were found to change significantly and continuously; this observation suggests a continuous change in the stacking sequence. A superspace analysis revealed that with heating the modulation period γ increased to a value that seemed to be determined by the atomic composition. Once γ reached this value it remained unchanged with cooling. A three-dimensional projection of the converged four-dimensional superspace structure corresponded to the homologous Sb8Te3 structure.
Phase-change memory technology relies on the electrical and optical properties of certain materials changing substantially when the atomic structure of the material is altered by heating or some other excitation process. For example, switching the composite Ge(2)Sb(2)Te(5) (GST) alloy from its covalently bonded amorphous phase to its resonantly bonded metastable cubic crystalline phase decreases the resistivity by three orders of magnitude, and also increases reflectivity across the visible spectrum. Moreover, phase-change memory based on GST is scalable, and is therefore a candidate to replace Flash memory for non-volatile data storage applications. The energy needed to switch between the two phases depends on the intrinsic properties of the phase-change material and the device architecture; this energy is usually supplied by laser or electrical pulses. The switching energy for GST can be reduced by limiting the movement of the atoms to a single dimension, thus substantially reducing the entropic losses associated with the phase-change process. In particular, aligning the c-axis of a hexagonal Sb(2)Te(3) layer and the 〈111〉 direction of a cubic GeTe layer in a superlattice structure creates a material in which Ge atoms can switch between octahedral sites and lower-coordination sites at the interface of the superlattice layers. Here we demonstrate GeTe/Sb(2)Te(3) interfacial phase-change memory (IPCM) data storage devices with reduced switching energies, improved write-erase cycle lifetimes and faster switching speeds.
Present-day multimedia strongly rely on rewritable phase-change optical memories. We demonstrate that, different from the current consensus, Ge(2)Sb(2)Te(5), the material of choice in DVD-RAM, does not possess the rocksalt structure but more likely consists of well-defined rigid building blocks that are randomly oriented in space consistent with cubic symmetry. Laser-induced amorphization results in drastic shortening of covalent bonds and a decrease in the mean-square relative displacement, demonstrating a substantial increase in the degree of short-range ordering, in sharp contrast to the amorphization of typical covalently bonded solids. This novel order-disorder transition is due to an umbrella-flip of Ge atoms from an octahedral position into a tetrahedral position without rupture of strong covalent bonds. It is this unique two-state nature of the transformation that ensures fast DVD performance and repeatable switching over ten million cycles.
The crystal structure of the δ-phase in the Sb–Te binary system has been determined by synchrotron powder diffraction. It is clearly shown that many intermetallic compounds, which have different stacking periods depending on compound composition, exist in this phase. These structures are based on the cubic ABC stacking structure, and two kinds of fundamental structural units form an intergrowth along the stacking direction at the atomic level. The chemical formulae of these compounds are expressed as Sb2nTe3, where n is an integer and the number of stacking layers is 2n + 3. There is a relationship of inverse proportionality between the stacking period and the Te concentration.
Phase-change storage is widely used in optical information technologies (DVD, CD-ROM and so on), and recently it has also been considered for non-volatile memory applications. This work reports advances in thermal data recording of phase-change materials. Specifically, we show erasable thermal phase-change recording at a storage density of 3.3 Tb inch(-2), which is three orders of magnitude denser than that currently achievable with commercial optical storage technologies. We demonstrate the concept of a thin-film nanoheater to realize ultra-small heat spots with dimensions of less than 50 nm. Finally, we show in a proof-of-concept demonstration that an individual thin-film heater can write, erase and read the phase of these storage materials at competitive speeds. This work provides important stepping stones for a very-high-density storage or memory technology based on phase-change materials.
Phase-change materials are of tremendous technological importance ranging from optical data storage to electronic memories. Despite this interest, many fundamental properties of phase-change materials, such as the role of vacancies, remain poorly understood. 'GeSbTe'-based phase-change materials contain vacancy concentrations around 10% in their metastable crystalline structure. By using density-functional theory, the origin of these vacancies has been clarified and we show that the most stable crystalline phases with rocksalt-like structures are characterized by large vacancy concentrations and local distortions. The ease by which vacancies are formed is explained by the need to annihilate energetically unfavourable antibonding Ge-Te and Sb-Te interactions in the highest occupied bands. Understanding how the interplay between vacancies and local distortions lowers the total energy helps to design novel phase-change materials as evidenced by new experimental data.
Phase-change materials are some of the most promising materials for data-storage applications. They are already used in rewriteable optical data storage and offer great potential as an emerging non-volatile electronic memory. This review looks at the unique property combination that characterizes phase-change materials. The crystalline state often shows an octahedral-like atomic arrangement, frequently accompanied by pronounced lattice distortions and huge vacancy concentrations. This can be attributed to the chemical bonding in phase-change alloys, which is promoted by p-orbitals. From this insight, phase-change alloys with desired properties can be designed. This is demonstrated for the optical properties of phase-change alloys, in particular the contrast between the amorphous and crystalline states. The origin of the fast crystallization kinetics is also discussed.
Implementing on-chip non-volatile photonic memories has been a long-term, yet elusive goal. Photonic data storage would dramatically improve performance in existing computing architectures by reducing the latencies associated with electrical memories and potentially eliminating optoelectronic conversions. Furthermore, multi-level photonic memories with random access would allow for leveraging even greater computational capability. However, photonic memories have thus far been volatile. Here, we demonstrate a robust, non-volatile, all-photonic memory based on phase-change materials. By using optical near-field effects, we realize bit storage of up to eight levels in a single device that readily switches between intermediate states. Our on-chip memory cells feature single-shot readout and switching energies as low as 13.4 pJ at speeds approaching 1 GHz. We show that individual memory elements can be addressed using a wavelength multiplexing scheme. Our multi-level, multi-bit devices provide a pathway towards eliminating the von Neumann bottleneck and portend a new paradigm in all-photonic memory and non-conventional computing.
Crystallization process and amorphous state stability of pseudobinary ZnSb–Sb2Te3 materials have been studied for application in phase change memory. The effects of Zn concentration and Sb content on crystalline resistance, crystallization temperature, crystallization activation energy and amorphous state stability of films have been studied. The microstructures of Sb-rich Zn–Sb–Te films were analyzed through X-ray diffraction. Different crystalline phases have been observed in annealed Sb-rich Zn–Sb–Te films. Low Zn-doping concentration Zn–Sb–Te films crystallized into rhombohedral Sb2Te3 phase while high Zn-doping concentration Zn–Sb–Te films crystallized into rhombohedral Sb phase. The crystallization activation energy (Ea) of Zn1.1Sb45.7Te53.2 and Zn5.2Sb46.3Te48.5 films were confirmed to be 2.0 and 2.93 eV, while Ea of Zn16.0Sb47.3Te36.7 film increased to 3.2 eV and further reached to 3.3 eV for Zn19.7Sb48.1Te32.2 film. Zn addition increased the crystallization temperature and crystalline resistance of Zn–Sb–Te films largely, and enhanced the amorphous thermal stability and data retention ability of the films, while high Sb/Te ratio reveals the improvement in crystallization speed and good cycle ability. Therefore, Sb-rich Zn–Sb–Te film seems to be a good way to solve the contradiction between thermal stability and fast crystallization speed.
Phase change material rules the basic scientific issues in research of phase change memory. As an important member in phase change material system, GeTe is competitive for both technological applications and fundamental studies. However, relatively poor thermal stability in amorphous state and serious grain clustering are needed to be overcome for the application of GeTe. Here we proposed Ge-Te-Ti (GTT) as a novel phase change material. For GTT, just with a small Ti fraction, the temperature for 10-yr’s data retention reaches 175℃, and the grain size decreases one order of magnitude. Results of Raman scatting measurements indicate that the basic structure unit distribution of GTT deviates from the normal distribution to the Ge-rich direction with Ti fraction increasing. The Ti-induced amorphous structure adjustment in GTT is the physical origin for the thermal stability enhancement, which makes GTT more extensively applicable in high temperature field through appropriate disordering adjustment.
It is well known that the Sb–Te binary system has a large number of incommensurately or commensurately modulated structures between Sb and Sb2Te3 compounds. These structures, which are long-period trigonal stacking structures, possess their own modulation period γ, according to their composition in the thermal equilibrium. However, the structure of sputtered Sb–Te films with various compositions between the two compounds at both ends formed in a non-thermal equilibrium showed smaller γ values, than those expected from their compositions without exception. A smaller γ value implies that its structure is closer to that of Sb with the shortest period in all Sb–Te modulated structures. With increase in temperature, all these transient structures with smaller γ, however, became stable, accompanying an increase of γ to acquire their original modulated structures.
Electron-diffraction radial distribution function (RDF) was used as a structural probe to study the process of crystallization of Ge2Sb2Te5 (GST) films annealed in situ. The GST thin film began to crystallize after a characteristic peak of 0.52 nm appeared in the RDF, indicating the formation of third nearest neighbour ordering. The GST films preferentially form uniform nanosized grains. The similarities and differences in the structures of the amorphous phases and the polycrystalline phases are described.
GeSbTe (GST) thin films were deposited on quartz substrates using electron beam evaporation system and then annealed in nitrogen atmosphere at different temperatures, ranging from 20 °C to 300 °C. X-ray diffraction (XRD) and Atomic Force microscope (AFM) measurements were used to characterize the as-deposited and post-annealed thin films. Annealing treatment was found to induce changes on microstructure, surface roughness and grain size, indicating that with the increase of annealing temperature, the amorphous GST films first changed to face-centered-cubic (fcc) phase and then the stable hexagonal (hex) phase.Meanwhile, conductive-AFM (C-AFM) was used to produce crystallized GST dots on thin films. I–V spectroscopy results show that GST films can switch from amorphous state to crystalline state at threshold voltage. After switching, I–V curve exhibits ohmic characteristic, which is usually observed in crystallized GST films. By applying repeated I–V spectroscopies on the thin films, crystallized nuclei were observed. As the times of I–V spectroscopies increases, the area of written dots increases, and the center of the mark begin to ablate. The AFM images show that the shape of marks is an ablated center with a raised ring surrounding it.
A procedure is described for optimizing the extraction of information from the diffraction data of a glass. A least-squares technique that minimizes the spurious detail in the radial distribution function (RDF) at small and large interatomic distances is employed both to isolate that portion of the total intensity function which contains the interatomic distance information and to remove from this function contributions from the shortest distances, thus eliminating the major source of termination errors. X-ray diffraction data for silica glass is utilized to illustrate the procedure. It is apparent that the procedure would be also applicable to the liquid state.
An approach to modelling radial distribution functions (RDFs) of nanoparticle samples over a wide range of interatomic distances is presented. Two different types of contribution to the model RDF are calculated. The first explicitly reflects the structure of the nanoparticle parts with more or less crystalline atomic structure. It can be calculated precisely and contains comparatively sharp peaks, which are produced by the set of discrete interatomic distances. The second includes RDF contributions from distances between weakly correlated atoms positioned within different nanoparticles or within different parts of a nanoparticle model. The calculation is performed using the approximation of a uniform distribution of atoms and utilizes the ideas of the characteristic functions of the particle shape known in small-angle scattering theory. This second RDF contribution is represented by slowly varying functions of interatomic distance r. The relative magnitude of this essential part of the model RDF increases with increasing r compared with the part that represents the ordered structure. The method is applied to test several spherical and core/shell models of semiconductor nanoparticles stabilized with organic ligands. The experimental RDFs of ZnSe and CdSe/ZnS nanoparticle samples were obtained by high-energy X-ray diffraction at beamline BW5, HASYLAB, DESY. The ZnSe nanoparticles have a spherical core with approximately 26 Å diameter and zincblende structure. The RDF of the CdSe/ZnS nanoparticle sample shows resolved peaks of the first- and the second-neighbour distances characteristic for CdSe (2.62 and 4.27 Å) and for ZnS (2.33 and 3.86 Å) and for the first time clearly confirms the presence of CdSe and ZnS nanophases in such objects. The diameters of the CdSe and ZnS spherical cores are estimated as 27 and 15 Å. CdSe and ZnS are present in the sample for the most part as independent nanoparticles. A smaller amount of ZnS forms an irregularly shaped shell around the CdSe cores, which consists of small independently oriented ZnS particles.
We present a direct atom-by-atom chemical identification of the nanostructures and defects of topological insulators (TIs) with a state-of-the-art atomic mapping technology. Combining this technique and density function theory calculations, we identify and explain the layer-chemistry evolution of Bi2Te3-xSex ternary TIs. We also reveal a long neglected but crucially important extended defect found to be universally present in Bi2Te3 films, the seven-layer Bi3Te4 nano-lamella acceptors. Intriguingly, this defect is found to locally pull down the conduction band, leading to local n-type conductivity, despite being an acceptor which pins the Fermi energy near the valence band maximum. This nano-lamella may explain inconsistencies in measured conduction type as well as open up a new route to manipulate bulk carrier concentration. Our work may pave the way to more thoroughly understand and tailor the nature of the bulk, as well as secure controllable bulk states for future applications in quantum computing and dissipationless devices.
Based on ab initio molecular-dynamics simulations, we generated models of liquid and amorphous Sb2Te3 of interest for applications as phase change material in optical and electronic data storage. The local geometries of Sb and Te atoms in a-Sb2Te3 are similar to that found in the extensively studied Ge2Sb2Te5 and GeTe phase change materials already exploited for nonvolatile memory applications. Analysis of the vibrational properties and electronic structure of a-Sb2Te3 is presented and compared to the crystalline counterparts.
The amorphous-to-crystal transition has been studied through in situ resistance measurements in Ge2Sb2Te5 thin films doped by ion implantation with nitrogen or oxygen. The dependence of the electrical resistivity and structure on the annealing temperature and time has been investigated in samples with different dopant concentrations. Enhancement of the thermal stability and increase of the mobility gap for conduction have been observed in O- and N-doped amorphous Ge2Sb2Te5. Larger effects have been found in the case of nitrogen doping.
Medium-range structural order consisting of polyhedral connections,
rings, and cluster structures in various covalent amorphous solids is
discussed. The experimental structural probes used to investigate the
structure of amorphous solids are described, as is the use of the first
sharp diffraction peak in diffraction data of such solids. The need for
new experimental techniques and theoretical descriptions of the
disordered state of matter is addressed.
Many of the structural elements of importance in materials applications (e.g., thin films, barrier layers, intergranular films in ceramics) are small in volume and amorphous. Although the characterization of the structure of amorphous materials by X-ray and neutron diffraction methods is well established, these techniques are not suitable for studies of nanovolumes of materials because of the relatively small scattering cross sections. This chapter reviews recent developments in electron techniques, and particularly electron diffraction, for overcoming this problem.
Pair distribution function analysis (PDF) of X-ray diffraction data, collected at 11-IDC and 1-ID at the Advanced Photon Source, provides the atomic structure and primary crystallite size of FeS both freshly precipitated (FeSfresh) and aged (FeSaged). The short- to medium-range structure of both FeSfresh and FeSaged are nearly identical with that of highly crystalline (bulk) mackinawite. Attenuation in the observed range of structural coherence of the PDF for FeSfresh indicates an average crystallite size on the order of 2 nm. This range of structural coherence increased with aging of the sample under hydrothermal conditions due to growth of the individual crystallites, although the mechanism by which this growth occurs is not clear at present. Electron microscopic imaging confirms the presence of individual nanoscale crystallites and provides some insight into their aggregation behavior as larger clusters. The initial, fresh precipitate does not exhibit long-range atomic structure because it is nanocrystalline. The so-called X-ray amorphous nature of FeSfresh is the result of the limited range of structural coherence imposed by the size of the individual crystallites rather than the result of a lack of medium- and long-range atomic order. We propose that the discrepancies in the literature over crystallite size and the atomic structure of FeSfresh are due primarily to the varying degrees of aggregation of uniformly distributed and nanocrystalline FeS particles.
The atomic structure of chalcogenide glasses As3Se7−xTex (0 ≤ x ≤ 3) and As2Se3−xTex (0 ≤ x ≤ 2.5) has been investigated by different methods. Short-range order has been studied by Wide-Angle X-ray Scattering (WAXS). 77Se NMR as well as Raman and infrared measurements were also performed on the different compositions. We show that the progressive introduction of tellurium in As3Se7−xTex or As2Se3−xTex induces breaking of Se–Se bonds and the formation of AsSe3−xTex pyramidal units. Experimental data also reveal the absence of Te–Te bonds even in the tellurium richest composition which let suppose a homogeneous repartition of tellurium atoms in the glassy network.Research highlights▶ First-neighbor Te–Te bonds are unlikely. ▶ Te is homogeneously distributed in the glassy network. ▶ The formation of mixed structural units AsSe3−xTex is confirmed.
First-principles study of the structural and magnetic properties of cubic and amorphous phase-change materials doped with 3d impurities. We find that Co- and Ni-doped Ge(2) Sb(2) Te(5) is non-magnetic, whereas Cr- and Mn-doped Ge(2) Sb(2) Te(5) is magnetic and exhibits a significant magnetic contrast between the two phases in the ferromagnetic configuration. These results are explained in terms of differences in local structure and hybridization of the impurity d-orbitals with the host states.
The precipitation of Sb2Te3 in Sb-rich AgSbTe2 is studied by X-ray diffraction and electron microscopy. The results indicate that Sb2Te3 does not form directly, but rather through the precipitation of an intermediate metastable phase. Diffraction, energy-dispersive
spectroscopy, and high-resolution transmission electron microscopy indicate that this intermediate phase has a nominal composition
(Ag,Sb)3Te4 and a structure with a seven-layer stacking sequence rather than a five-layer one as in Sb2Te3. Two mechanisms based on experimental observations are proposed for the conversion of (Sb,Ag)3Te4 to Sb2Te3: evaporation–condensation and individual step motion. The microstructural evolution and mechanisms of the transformation
are discussed in detail.
A software package for computing radial distribution functions and other pair correlation functions from electron diffraction patterns of disordered solids is presented. The package, called RDFTools, is freely available via the internet and allows rapid in situ measurements of such quantities as interatomic nearest neighbor distances, average bond angles and coordination numbers. The software runs under DigitalMicrograph™ (Pleasanton, California, Gatan), a very widely used program in transmission electron microscopy. All implemented algorithms have been designed to compute diffraction integrals and data-processing averages in a fast and efficient manner to enable quick processing of publication ready, quantitative pair distribution function information. In the development of RDFTools, significant attention was paid to provide a robust and intuitive user-interface for deriving reliable semiquantitative information. For example, RDFTools enables accurate pair separation distances to be revealed upon immediate interrogation at the microscope; even for potentially thick specimens and/or regions of unknown elemental composition.
Nitrogen doping effect on phase change optical disks
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Aspects of the modelling of the radial distribution function for small nanoparticles Structural characterizations of AseSeeTe glasses
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