Sb-Te Bonds Research Articles

Atomistic oxidation mechanism of Bi0.5Sb1.5Te3 (0001) surface

Surface oxidation can potentially influence transport properties of thermoelectric materials and service performance of thermoelectric devices. Here, the oxidation mechanism of the (0001) surface of a widely used p-type TE material, Bi0.5Sb1.5Te3 (BST), was investigated using spherical aberration corrected scanning transmission electron microscopy (STEM). Combining atomic resolution STEM and density function theory (DFT), it was revealed that oxidation occurs at room temperature in the dry air through O diffusion into the surface quintuple layer, facilitating formation of both cation and anion vacancies, weakening the Bi-Te and Sb-Te bonds, leading to oxidation product Sb2O3, amorphous Te, and Bi-rich Bi2Te3. Formation of a thin O-rich cation-deficient quintuple layer on the surface and the Bi-rich BST region can significantly reduce the interfacial reaction between BST and Ni electrode, as the reaction layer thickness is reduced by 75% after the surface oxidation. This work provides essential structural information on surface oxidation mechanism and its influence on properties and performance of TE materials and devices.

High performance antimony-rich RexSb3Te for phase-change random access memory applications

Phase-change random access memory (PCRAM) demands for high-performance phase change materials, including operation speed, data retention, resistance drift etc. In this work, a high performance PCRAM device based on antimony-rich RexSb3Te material was put forward and studied. Excellent data retention, fast operation speed, low resistance drift, low power consumption and good cycle characteristics were achieved. The lattice information, bonding properties and surface morphology of the films were characterized by transmission electron microscopy, Raman spectroscopy and atomic force microscopy. Density functional theory calculations show that the Re dopant is more inclined to replace Sb and forms chemical bonds with Te, and the Re-Te bonds are more stable than the previous Sb-Te bonds, which is conducive to the formation of stable precursors to accelerate nucleation. The research shows that the antimony-rich Re0.18Sb3Te material has good overall performance, which provides a strategy for developing high-performance memory.

Optical excitation-induced ultrafast amorphization in the Y-Sb-Te alloy system: Insights from real-time time-dependent DFT with molecular dynamics calculations

In general, shortening the laser pulse width can suppress thermal dissipation to allow for sub-picosecond phase change during amorphization, yet there is no consistent evidence for the mechanistic exploration of its ultrafast dynamics at the atomic scale. Herein, a real-time time-dependent density-functional theory molecular dynamic (rt-TDDFT-MD) in a photoexcited fashion is applied to disclose an undetected effect of the experimentally observed Y-Sb-Te amorphization process at elevated temperature. Varying the optical excitation from $2%$ to $8%$ of valence electrons during the amorphization process can result in a variety of structures ranging from entire crystalline to amorphied shapes, satisfying the amorphous state at 1.2 ps upon $8%$ excitation. We conclude that the low concentration of photoexcitation fails to conquer the potential barrier height of Y-Sb-Te for fully facilitating amorphization. The $8%$ of photoinduced Te-$p$ electrons enhances the massive occupation of Y-$d{t}_{2g}$ orbitals and creates a flatter potential energy surface, which forces Y-centered motifs to perform $d{t}_{2g}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{and}\phantom{\rule{4.pt}{0ex}}{e}_{g}$-directed transition in the signature bond angle of the rock salt lattice. Thus it can rapidly dissociate the Sb-Te bond and facilitate the ultrafast formation of the amorphous phase. The incorporation of optical interventions drives the lattice to manifest disordered states, as well as allowing for accurate manipulation of the electronic structure to further reduce the time consumed in the amorphization process. The present works pave the way for the design of PCMs-based optoelectronic and microelectronic devices with comprehensive ultrafast response.

Evaluation of vibrational properties and local structure change during phase transition in Ge2Sb2Te5 and In3SbTe2 phase change materials

Temperature-dependent local structural details of In3SbTe2 (IST) and Ge2Sb2Te5 (GST) phase change materials are explored with the aid of Raman scattering and x-ray photoelectron spectroscopy (XPS) techniques. Significant temperature-induced changes occur in the local structure of the phase change material, facilitating the amorphous to crystalline phase transformation. These two phases exhibit a large resistivity contrast, which is utilized to store data in the phase change memory application. The Raman spectra recorded for IST material suggest that the as-deposited sample (amorphous phase) first crystallized around 300 annealing temperature with two binary phases InTe and InSb formations. InTe and InSb phases were obtained at ∼85.5 cm−1 and ∼180 cm−1 Raman shift, respectively, having vibration modes associated with B1g symmetry and transversal optical phonons with Г15 symmetry. Further annealing at a higher temperature of 400 , a ternary IST phase is obtained at ∼163 cm−1 Raman shift indicating the transformation to a fully crystalline state. On the other hand, the amorphous GST material forms a metastable face-centred cubic phase and stable hexagonal (HEX) phase upon crystallization. The Raman findings demonstrate the changes in vibration modes of Ge–Te and Sb–Te bonds during phase switching without any phase separation. Crystalline GST (HEX phase) comprises rising peaks of GeTe4 and GeTe4−n Ge n (n = 1, 2) corner-sharing tetrahedra with A1 mode at ∼104.5 cm−1 and ∼137 cm−1 Raman shift, respectively, and a declining Sb2Te3 peak at ∼175 cm−1, having A1g (2) vibration mode. Furthermore, the XPS analysis displays the changes in the bonding mechanism of the elements present in the phase change material during the amorphous to crystalline phase transition, firmly supporting the Raman observations.

High optical/color contrast of Sb2Te thin film and its structural origin

This work has investigated the optical/color contrast of Sb2Te phase change material and its structural volution. It is found that Sb2Te film possesses high optical/color contrast owing to its high discrepancy of refractive index and extinction coefficient between as-deposited and crystalline states. Raman spectra reveal that the enhanced Sb–Te bonds within two-dimensional Sb2Te3 plane is favorable for the high contrast of Sb2Te. The ab initio calculations further reveal that Sb2Te as-deposited phase has a complicated local motif owing to the excess Sb atoms. Moreover, analysis of the electronic structure of Sb2Te shows that the degree of electronic delocalization may in turn result in the large optical/color contrast of Sb2Te film. This work may provide a useful reference for future color display applications.

Metalloid Chalcogen–pnictogen σ-hole bonding competition in stibanyl telluranes

In this manuscript a curious behavior observed in the solid state of several X-ray structures retrieved from the Cambridge structural database (CSD) is described and analyzed. These structures have in common the presence of a metalloid–metalloid pnictogen (Pn)–chalcogen (Ch) covalent bond (i.e., Sb–Te bond) in the structure. The Sb–Te bond presents different σ-holes at both ends of the bond in terms of size and intensity. These compounds have a tendency to form short intermolecular Sb•••Te interactions in the solid state. The aim of this work is to investigate, by using dispersion-corrected density functional theory (DFT-D3) calculations, whether the Sb•••Te contacts observed in the solid state correspond to PnB (Sb is the Lewis acid) or ChB (Te is the Lewis acid). Moreover, using two model stibanyl telluranes, the interaction energies with a series of Lewis bases and anions using both ends of the Sb–Te covalent bond have been computed to investigate which side (PnB or ChB donor) is able to establish stronger interactions with common electron rich atoms. Finally, several computational tools such as the quantum theory of atoms-in-molecules (QTAIM), noncovalent interaction plot (NCIPlot) index and electron localization (ELF) function (2D and 3D maps) have been used to further characterize the physical nature of the Sb•••Te interactions. The results reported herein suggest that in the Sb•••Te contacts the Te atom acts as σ-hole donor and the Sb as electron density donor.

First-principles study of the liquid and amorphous phases of Sb2Te phase change memory material

We have investigated the local structure of liquid and amorphous phases of Sb2Te phase change memory material by the means of density functional theory-molecular dynamics simulations. The models of liquid and amorphous states were generated by quenching from the melt. The results show that the local environment of liquid Sb2Te is a mixed bonding geometry, where the average coordination numbers (CNs) of Sb and Te atoms are 4.93 and 4.23, respectively. Compared with crystalline state, there are more Sb–Sb bonds (∼53%) and less Sb–Te bonds (∼42%) with the presence of Te–Te bonds (∼5%) in liquid Sb2Te. Therefore, the formation of homopolar bonds and the breaking of heteropolar bonds are important structural transformations in melt process. For amorphous Sb2Te, the average CNs of Sb and Te atoms are 4.54 and 3.57, respectively. They are mostly in an octahedral environment, similar to the case in crystalline phase. The fractions of Sb–Sb, Te–Te, and Sb–Te bonds are ∼52%, ∼2%, and ∼46%, respectively. Thus, the increase in the fraction of octahedron accompanied with the decrease in average CN is the major structural changes in quenching process. Furthermore, the octahedral geometry in both the crystalline and amorphous Sb2Te increases the local structural similarity, facilitating the rapid low-energy crystallization.

The coupling effect and phase transition behavior of multiple interfaces in GeTe/Sb superlattice-like films

In this paper, the phase transition mechanism and device conversion performance of the Sb/GeTe superlattice-like films were studied and showed improved amorphous thermal stability and resistance drift compared to the pure Sb films. Transmission electron microscopy showed that the Sb layer plays a leading role in the crystallization process, forming a large number of Sb grains. The metallic behavior of the Sb/GeTe thin films was confirmed by density functional theory calculations. The thermal conductivity of the crystalline film was found to be higher than that of the amorphous film, and the thermal conductivity of the Sb/GeTe film was lower than that of the other monolayer films. In the Sb/GeTe film, the Sb–Sb and Ge–Te bonds were the main bond structures, and some Sb–Te bonds were formed at the layer interfaces, enhancing the stability of the film. Laser picosecond measurements showed that Sb/GeTe film had a higher crystallization rate than the GeTe film. The phase change memory devices based on the Sb/GeTe thin films were found to have lower operational power consumption and higher transition speed. These results showed that the Sb/GeTe thin film explored in this study was a promising phase change storage thin film with excellent performance.

Ge-Rich GST Phase-Change Alloys: Thorough Investigation of Structural Evolution in Temperature

Phase-Change Memory (PCM) is considered the most mature among the emerging Non-Volatile Memory technologies. Indeed, PCM features fast programming speed, excellent scalability down to nm scale and high endurance. It relies on the reversible and fast transition between a highly resistive amorphous phase and a low resistive crystalline phase of a chalcogenide phase-change alloy. PCM’s high maturity is demonstrated by its commercialization in Storage Class Memory (SCM) market [1] and by the recent demonstration of its manufacturability and reliability in 28 nm technology node for Automotive applications [2]. Moreover, PCM is considered the best-suited device for emerging neuromorphic systems and in-memory computing applications [3].For several years, PCM has been considered not suitable to target stability at high temperature: indeed, devices based on standard material Ge2Sb2Te5 (GST225) trigger crystallization at 150°C, limiting the target applications portfolio. With the development of Ge-rich GeSbTe (Ge-rich GST) materials [4], improved PCM stability was demonstrated, fulfilling the strict requirements in terms of data retention of embedded applications, rising the questions on the origin of such improvements.Recent works on Ge-rich GST showed the effect of Ge enrichment and N doping in such alloys, in particular highlighting the increased crystallization temperature and the phase segregation at high temperature [5, 6]. However, the fine engineering of Ge-rich alloys requires a deep investigation of their structure and the understanding of its evolution in temperature.In this work, we present a thorough investigation of GeSbTe system, comparing the alloys with and without Ge enrichment, and with and without N doping. Structural analysis is performed by a wide set of complementary techniques: Raman spectroscopy, X-Ray Diffraction (XRD) and FTIR spectroscopy, TEM/EDX analyses and electrical resistivity measurements. All the analyses were performed on as-deposited amorphous samples and on samples annealed at several temperatures ranging up to more than 400°C.Comparing the Raman spectra of Ge-rich alloys with the spectra of GST225, we highlight the importance of Sb-Te structural units and their stability at all the steps of the evolution of the layer structure in temperature. The rearrangement of Ge-Te bonds, with the following crystallization of GeSbTe phase, is accompanied by the evolution of amorphous Ge phase in the layer towards the crystalline one (e.g. Fig. 1), demonstrated also in XRD spectra and TEM images. Ge single-element layer analyses allow us to evidence the important impact of the presence of Sb-Te bonds on Ge crystallization.Correlation of the Ge-N modes in N-doped Ge and Ge-rich GST samples by FTIR spectroscopy shows the effect of N on the formation of a GeN phase in GeSbTe alloy and its evolution in temperature (e.g. Fig. 2). N doping retards the rearrangement of Ge-Te bonds and the consecutive crystallization of Ge, which is delayed at higher temperature with regard to undoped sample. The sequence of phases that appear in the system with the increasing temperature is demonstrated by XRD spectra, performed on both doped and undoped layers.In summary, in this work we present a thorough investigation of the structural evolution of Ge-rich GST system in temperature thanks to the correlation of spectra and data coming from a wide set of techniques. These results help in understanding the mechanisms behind the crystallization and layer segregation, and in designing next-generation of PCM for automotive applications.[1] Huai-Yu Cheng et al., 2019 J. Phys. D: Appl. Phys. 52 473002.[2] Paolo Cappelletti et al., 2020 J. Phys. D: Appl. Phys. 53 193002.[3] T. Kim et al., IEEE TED, vol. 67, no. 4, pp. 1394-1406.[4] P. Zuliani et al., 2019 IEEE 11th International Memory Workshop, pp. 1-4.[5] V. Sousa et al., 2015 VLSI Technology, pp. T98-T99.[6] M. Agati et al., J. Mater. Chem. C, 2019, 7, 8720. Figure 1

Time-dependent density-functional theory molecular-dynamics study on amorphization of Sc-Sb-Te alloy under optical excitation

Recently, all-optical memory and optical-computation properties of phase-change materials are receiving intensive attention. Because writing/erasing information in these devices is usually achieved by laser pulses, the interaction between the laser and the phase-change materials becomes a key issue for such new applications. In this work, by a time-dependent density-functional theory molecular-dynamics study, the physics underlying the optical excitation induced amorphization of Sc-Sb-Te is revealed, which goes back to superatom-like Sc-centered structural motifs. These motifs are found to be still robust under the excitation. A selected occupation of the Sc d-t2g orbitals (as a result of optical excitation) leads to a significant change of Sc-centered bond angles. In addition, the especially weak Sb-Te bonds next to the Sc motifs are further diminished by excitations. Therefore, the Sc-centered motifs can promote breaking, switching, and reforming of the surrounding Sb-Te network and, therefore, facilitate the amorphization of Sc-Sb-Te. The study shows the unique role of Sc-centered motifs in optically induced phase transition, and displays potential applications of Sc-Sb-Te alloys in optical memory/computation.

Chemical Design Principles for Cache-Type Sc–Sb–Te Phase-Change Memory Materials

Enhanced crystal nucleation in a Sc–Sb–Te phase-change material has enabled subnanosecond switching in phase-change memory devices, making cache-type nonvolatile memory feasible. However, the microscopic mechanisms remain to be further explored. In this work, we present a systematic ab initio study of the relevant parent compounds, namely, Sc2Te3 and Sb2Te3. Despite similar bond lengths and angles in the amorphous phases of the two compounds, Sc2Te3 displays a much more ordered amorphous network without homopolar bonds. As a result, the local structural order in amorphous Sc2Te3 is dominated by square motifs, remarkably similar to those of the metastable rocksalt-like phase. Chemical bonding analysis indicates more robust Sc–Te bonds compared with Sb–Te bonds in the amorphous phase, as well as a substantial role of electrostatic interactions in Sc2Te3 but not in Sb2Te3. The robustness of Sc–Te bonds explains the enhanced nucleation in Sc–Sb–Te compounds. Finally, we discuss an alloying strategy of Sc2Te3 ...

Atomic layer deposition and tellurization of Ge-Sb film for phase-change memory applications.

We studied the atomic layer deposition (ALD) and the tellurization of Ge–Sb films to prepare conformal crystalline Ge–Sb–Te (GST) films and to achieve void-free gap filling for emerging phase-change memory applications. ALD Ge–Sb film was prepared by alternating exposures to GeCl2-dioxane and Sb(SiEt3)3 precursors at 100 °C. The growth rate was 0.021 nm per cycle, and the composition ratio of Ge to Sb was approximately 2.2. We annealed the ALD Ge–Sb films with a pulsed feeding of di(tert-butyl)tellurium. The ALD Ge–Sb films turned into GST films by the tellurization annealing. When the tellurization temperature was raised to 190 °C or higher temperatures, the Raman peaks corresponding to Ge–Sb bond and amorphous Ge–Ge bond disappeared. The Raman peaks corresponding to Ge–Te and Sb–Te bonds were evolved at 200 °C or higher temperatures, resulting in the phase transition temperature of 123 °C. At 230 °C or higher temperatures, the entire film was fully tellurized to form a GST film having a relatively uniform composition of Ge3Sb2Te6, and the carbon impurities in the as-deposited ALD Ge–Sb film were eliminated. As the tellurization temperature increases, the volume of the ALD film is expanded owing to the incorporation of tellurium, resulting in complete filling of a trench pattern by GST film after the tellurization at 230 °C.

Local structure of amorphous Ag5In5Sb60Te30 and In3SbTe2 phase change materials revealed by X-ray photoelectron and Raman spectroscopic studies

Reversible switching between highly resistive (binary “0”) amorphous phase and low resistive (binary “1”) crystalline phase of chalcogenide-based Phase Change Materials is accredited for the development of next generation high-speed, non-volatile, data storage applications. The doped Sb-Te based materials have shown enhanced electrical/optical properties, compared to Ge-Sb-Te family for high-speed memory devices. We report here the local atomic structure of as-deposited amorphous Ag5In5Sb60Te30 (AIST) and In3SbTe2 (IST) phase change materials using X-ray photoelectron and Raman spectroscopic studies. Although AIST and IST materials show identical crystallization behavior, they differ distinctly in their crystallization temperatures. Our experimental results demonstrate that the local environment of In remains identical in the amorphous phase of both AIST and IST material, irrespective of its atomic fraction. In bonds with Sb (∼44%) and Te (∼56%), thereby forming the primary matrix in IST with a very few Sb-Te bonds. Sb2Te constructs the base matrix for AIST (∼63%) along with few Sb-Sb bonds. Furthermore, an interesting assimilation of the role of small-scale dopants such as Ag and In in AIST, reveals rare bonds between themselves, while showing selective substitution in the vicinity of Sb and Te. This results in increased electronegativity difference, and consequently, the bond strength is recognized as the factor rendering stability in amorphous AIST.

Local structural characteristics of Sb2Te3 films studied by reverse Monte Carlo modeling

Atomic configuration and connectivity of Sb2Te3 thin film are investigated using high-energy X-ray diffraction and reverse Monte Carlo simulation. Atomic model details of Sb2Te3 thin film are compared with liquid and amorphous Sb2Te3 reported in other article. Simulations show that both Sb–Sb and Te–Te homopolar bonds are present in the models. In phase transition process, atomic configuration of the sample rearranges gradually through the forming of Sb–Te bonds and the breaking of Sb–Sb and Te–Te bonds.

First-principles study of amorphousGa4Sb6Te3phase-change alloys

First-principles molecular dynamics simulations within the density functional theory framework were performed to generate amorphous models of the ${\mathrm{Ga}}_{4}{\mathrm{Sb}}_{6}{\mathrm{Te}}_{3}$ phase change alloy by quenching from the melt. We find that Ga-Sb and Ga-Te are the most abundant bonds with only a minor amount of Sb-Te bonds participating to the alloy network. Ga and four-coordinated Sb atoms present a tetrahedral-like geometry, whereas three-coordinated Sb atoms are in a pyramidal configuration. The tetrahedral-like geometries are similar to those of the crystalline phase of the two binary compounds GaTe and GaSb. A sizable fraction of Sb-Sb bonds is also present, indicating a partial nanoscale segregation of Sb. Despite the fact that the composition ${\mathrm{Ga}}_{4}{\mathrm{Sb}}_{6}{\mathrm{Te}}_{3}$ lies on the pseudobinary $\mathrm{Ga}\mathrm{Sb}\ensuremath{-}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ tie line, the amorphous network can be seen as a mixture of the two binary compounds GaTe and GaSb with intertwined elemental Sb.

Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding

Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3–4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.

First-principles study of structural, elastic, electronic and thermodynamic properties of topological insulator Sb2Te3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Sb2Te3 (α-Sb2Te3) are investigated by the generalized gradient approximation (GGA) with the PBESOL exchange–correlation functional in the framework of density-functional theory. The calculated lattice parameters of Sb2Te3 at the ground state are in good agreement with the available experimental data and better than other calculated results. The pressure dependence of the elastic constants Cij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Sb2Te3 are also obtained successfully. The obtained density of states, charge density difference and total charge density show that Sb–Te bonds have some covalent features at the ground state. Finally, we investigate the shear sound velocity VS, longitudinal sound velocity VL, and Debye temperature ΘE from our elastic constants of Sb2Te3, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity Cv and the thermal expansion coefficient α of Sb2Te3 at 0GPa and 300K are 120.79Jmol−1K−1 and 5.19×10−5K−1, respectively.

TixSb2Te Thin Films for Phase Change Memory Applications

Sb2Te films with different Ti contents (TixSb2Te) are derived via the target-attachment method by using the magnetron sputtering technique. The effects of the Ti content on the phase change characteristics and the microstructures are investigated by x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy and atom force microcopy. Resistance-temperature measurements are carried out to reveal the enhanced crystallization temperature of TixSb2Te films, indicating a better thermal stability in such films. Both the activation energy and the temperature for 10 y data retention increase with increasing the concentration of Ti. It indicates that the crystallization of the amorphous Sb2Te film could be suppressed by the introduction of Ti. The improvement of crystallization temperature and the thermal stability of the amorphous Sb2Te film results from the introduction of Ti in Sb-Te bond that decreases the binding energy of Sb 4d and Te 4d.

Theoretical and experimental investigations of the properties of Ge2Sb2Te5 and indium-doped Ge2Sb2Te5 phase change material

We have carried out comprehensive computational and experimental study on the face-centered cubic Ge2Sb2Te5 (GST) and indium (In)-doped GST phase change materials. Structural calculations, total density of states and crystal orbital Hamilton population have been calculated using first-principle calculation. 5 at.% doping of In weakens the Ge–Te, Sb–Te and Te–Te bond lengths. In element substitutes Sb to form In–Te-like structure in the GST system. In–Te has a weaker bond strength compared with the Sb–Te bond. However, both GST and doped alloy remain in rock salt structure. It is more favorable to replace Sb with In than with any other atomic position. X-ray diffraction (XRD) analysis has been carried out on thin film of In-doped GST phase change materials. XRD graph reveals that In-doped phase change materials have rock salt structure with the formation of In2Te3 crystallites in the material. Temperature dependence of impedance spectra has been calculated for thin films of GST and doped material. Thickness of the as-deposited films is calculated from Swanepoel method. Absorption coefficient (α) has been calculated for amorphous and crystalline thin films of the alloys. The optical gap (indirect band gap) energy of the amorphous and crystalline thin films has also been calculated by the equation \( \alpha h\nu = \beta (h\nu - E_{\text{g }} )^{2} \). Optical contrast (C) of pure and doped phase change materials have also been calculated. Sufficient optical contrast has been found for pure and doped phase change materials.

First-principles study of the amorphous In3SbTe2phase change compound

Ab initio molecular dynamics simulations based on density functional theory were performed to generate amorphous models of the phase change compound In${}_{3}$SbTe${}_{2}$ by quenching from the melt. In-Sb and In-Te are the most abundant bonds with only a minor fraction of Sb-Te bonds. The bonding geometry in the amorphous phase is, however, strongly dependent on the density in the range 6.448--5.75 g/cm${}^{3}$ that we investigated. While at high density the bonding geometry of In atoms is mostly octahedral-like as in the cubic crystalline phase of the ternary compound In${}_{3}$SbTe${}_{2}$, at low density we observed a sizable fraction of tetrahedral-like geometries similar to those present in the crystalline phase of the two binary compounds InTe and InSb that the ternary system can be thought to be made of. We show that the different ratio between octahedral-like and tetrahedral-like bonding geometries has fingerprints in the optical and vibrational spectra.