Sb2Te3 Compounds Research Articles

Growth of Textured Chalcogenide Thin Films and Their Functionalization through Confinement

A number of chalcogenides show a remarkable portfolio of properties, which enables a plethora of applications including thermoelectric power generation and phase change material (PCM)‐based information storage. PCMs exhibit fast and reversible switching between two (typically, amorphous and crystalline) states, characterized by different optical and electrical properties. In the last decade, textured chalcogenide thin films have been investigated to develop a better understanding of structure – property relationships, improve material performance and understand how these properties change upon reducing film thickness. In this review, the present knowledge concerning the textured growth is summarized, focusing on films of GeTe, Sb2Te3, and GeSbTe compounds. In particular, the impact of different deposition methods, substrate surface modifications, and methods to influence film formation is reviewed. In the second part of this review, confinement effects are discussed. Surprisingly pronounced changes in atomic arrangement, that affect film properties, and device performance are presented. These changes are attributed to the unconventional bonding mechanism in these chalcogenides coined metavalent bonding, which is trongly affected by the confinement. Bonding becomes covalent‐like in the two‐dimensional limit, whereas Metavalent Bonding emerges for thicker films, where electron delocalization is increased This explains the pronounced property changes with film thickness.

Nature of the Local Environment of Atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7 Amorphous and Crystalline Films

The valence state and local environment of atoms in Ge3Sb2Te6, Ge2Sb2Te5, GeSb2Te4, and GeSb4Te7 amorphous and crystalline films are determined by Mössbauer spectroscopy (MS) on isotopes 119Sn, 121Sb, and 125Te. In crystalline films, divalent germanium is located in octahedral positions in a rhombohedrally distorted NaCl-type of lattice, whereas in amorphous films, tetravalent germanium atoms form a tetrahedral system of chemical bonds. In all films, the nearest environment of germanium contains predominantly tellurium atoms. Trivalent antimony atoms in crystalline and amorphous films occupy two types of octahedral positions differing in the degree of distortion, and in the nearest environment of antimony there are tellurium atoms. Finally, in all films, the local structures of tellurium atoms correspond to tellurium structural units in the GeTe and Sb2Te3 compounds.

Structural Defects and Ferromagnetic Signature of V-Doped Sb2Te3 Thin Films Grown on SrTiO3(001) Produced by RF-Magnetron Sputtering.

Thin films of V-doped Sb2Te3 compounds were fabricated by co-deposition of Sb2Te3 and V using rf-magnetron sputtering onto SrTiO3(001) substrates kept at 570 K. The microstructures of the films were characterized using transmission electron microscopy (TEM) and electron diffraction. The crystal structure of the sputtered film (Sb38V2Te60) is the Bi2Te3-type structure with lattice parameters of a = 0.44 ± 0.03 nm and c = 3.02 ± 0.02 nm. A combination of cross-sectional and plan-view TEM observations revealed the preferential orientation of the c axis in the film's normal direction. A thin amorphous layer exists between the Sb2Te3 thin film and the SrTiO3 substrate. The interfacial amorphous layer relaxes the strain between the thin film and the substrate, and hence, it should promote the growth of a low-index atomic plane with a low surface free energy (i.e., (0001) of the Sb2Te3). The onset of ferromagnetic order was detected at temperatures below 70 K. A remarkable increase in magnetization was detected in the film's normal direction, which corresponds to the magnetic easy axis (i.e., c axis of the Sb2Te3). V3+ ions substituting Sb sites should contribute to ferromagnetism at low temperatures.

Theoretical investigation on the electronic and optical properties of BSTS compounds with SOC and TB_mBJ potentials

In this study, we report on electronic and optical properties of the bulk and primitive structure of A2B3 (A = Bi or Sb and B = Se or Te) compounds using FP-LAPW method within density functional theory. The potentials for exchange and correlation is treated within generalized gradient approximation-Perdew Burke Ernzerhof and Tran-Blaha modified Becke–Johnson (TB_mBJ). The inclusion of spin-orbit coupling approximation is used to calculate electronic structures and optical properties by determining the complex dielectric function, from which the other parameters are derived. The obtained results are in good agreement with the experimental results. Also, Sb2Te3 compounds have been found to possess maximum value of absorption and reflection among all the studied compounds. For all the compounds, a strong absorption coefficient (α) exists between the energy range 0 and 5 eV, and is greater than 105 cm–1, which makes them suitable for optoelectronic applications.

First-principles study of structural and optical properties contrast for liquid (GeTe)x(x = 1,2,3)-Sb2Te3 compounds

Ge-Sb-Te compounds played an important role in present nonvolatile optical and electrical storages. In our studies, First-principles molecular dynamic methods were used to simulate the amorphization processes of (GeTe)x(x = 1,2,3)-Sb2Te3 compounds and the liquid states which consisted of crystalline and amorphous phase were obtained, then the effects of different ratio of (GeTe) on the properties of Ge-Sb-Te compounds were studied. The calculated results of pair correlation function, coordination number and compositional disorder number showed that Ge2Sb2Te5 has better stability among the three liquids. By analysis of the complex dielectric function, absorption and conductivity, Ge2Sb2Te5 has lower turn-on voltage and lower RESET current compared with Ge1Sb2Te4 and Ge3Sb2Te6. The present results are helpful to understand the multi- level data storage capabilities based on the liquid states.

Peculiarities of the Electron Structure of Pseudobinary Alloys (GeTe)m–(Sb2Te3)n

The electron band structure of GeTe, Sb2Te3, GeSb2Te4, and Ge2Sb2Te5 compounds has been calculated by the electron density functional method using the WIEN2k software package. These compounds belong to the class of (GeTe)m–(Sb2Te3)n pseudobinary alloys, which pass from the crystalline to the amorphous state and vice versa under laser irradiation or an electric pulse for extremely short times on the order of 1–100 ns. A detailed analysis of the parameters of critical points (maxima, minima, and inflection points) in the electron density distribution, located at the high-symmetry points of crystal structure, is performed. Characteristic values of the parameters of critical points in the electron density distribution are found for this class of materials.

Impact of Ultrathin Pb Films on the Topological Surface and Quantum-Well States of Bi2Se3 and Sb2Te3 Topological Insulators

The effect of an ultrathin Pb film deposited on the surface of Bi2Se3 and Sb2Te3 compounds on the electronic state structure of topological insulators is studied experimentally by the angle-resolved photoemission spectroscopy (ARPES) technique. The following features are revealed: formation of two-dimensional quantum-well states in the near-surface region, an increase in the binding energy of the Dirac cone and the core levels, and a simultaneous electronic states intensity redistribution in the system in photoemission spectra. The results obtained show that topological states may coexist at the interface between studied materials and a superconductor, which seems to be promising for application in quantum computers.

Variable-Temperature In Situ X-ray Diffraction Study of the Thermodynamic Evolution of AgSbTe2 Thermoelectric Compound

Although AgSbTe2 compound has been intensively studied as a promising medium-temperature p-type thermoelectric (TE) material, its thermodynamic stability is still a controversial issue. We have prepared selenium-doped and pristine AgSbTe2 compounds from high-purity elements by a melt-quench-spark plasma sintering (Melt-SPS) or a melt spinning-SPS technique (MS-SPS). The influences of rapid solidification and selenium impurity on the thermodynamic evolution of AgSbTe2 TE compound have been studied by variable-temperature in situ x-ray diffraction over the temperature range from 25°C to 450°C. AgSbTe2 is a high-temperature metastable phase, and ultrahigh cooling rate in melt spinning is favorable to achieve phase-pure and homogeneous AgSbTe2 compound. The maximum possible short-time working temperature for AgSbTe2 TE compound is 250°C. Above 300°C, pristine AgSbTe2 is prone to slow decomposition to Sb2Te3, Ag5Te3, and β-Ag2Te binary compounds, regardless of preparation route. The MS-SPS sample underwent a nearly reversible phase transition on cooling to room temperature, while the Melt-SPS sample decomposed irreversibly after measurement. Selenium-doped specimen showed robust thermodynamic stability below 300°C, and experienced distinct phase transition compared with pristine specimen above 330°C, evidencing a profound influence of selenium doping on the thermodynamic stability of AgSbTe2 thermoelectric compound.

Structural and Optical Properties of Phase Change Material in Sb2Te3 Thin Film

Chalcogenide based compound, especially Sb2Te3 and related compounds, exhibit pronounced structural and optical contrast with rapid phase transition from amorphous and crystalline phase. This makes them suitable candidates for rewritable optical storage media and phase change random access memory. Simultaneously, Sb2Te3 have reported that one of the best p-type thermoelectric materials characteristics at room temperature, and topological insulators property. As electron and phonon plays a crucial role in determining the performances of any real devices, a better understanding of the amorphous phase and crystal phase, and the transition mechanism between them, is imperative. Recent, Optical pump THz probe spectroscopy (OPTP) is a powerful tool to study the ultrafast carrier dynamics of structural phase transition occurring on ultrafast time scales, as has been applied to a variety of materials, such as semimetal and semiconductor, and Mott insulators examined by using a femtosecond pump-probe technique and found that the appearance of the coherent vibrational modes was significantly modified upon the phase change.In this study, we investigate ultrafast carrier dynamics in {Sb(3)Te(9)}n thin film that the phase change from amorphous into crystalline states can be explained by THz-TDS and OPTP, which is the relationship between structural phase transition and optical properties transition in THz range. We observed that OPTP spectroscopy was closely dependent on phase of the Sb2Te3 film, indicating that decay time is a crucial carrier dynamics mechanism to the phase transformation process.

Low thermal conductivity and high thermoelectric figure of merit in p-type Sb2Te3/poly(3,4-ethylenedioxythiophene) thermoelectric composites

p-type Sb2Te3/poly(3,4-ethylenedioxythiophene) (PEDOT) thermoelectric composites are fabricated by embedding PEDOT into Sb2Te3 matrix. The grains of Sb2Te3 in the composites are found to be in micron degree and keep plate-like shapes. The measurements of thermoelectric properties show that the thermal conductivity κ of the composites is about 0.14 W m−1 K−1 in the temperature range of 300–523 K, much lower than that of Sb2Te3 compounds. The maximum of dimensionless figure of merit of the composites reaches to 1.18 at 523 K, which is the highest value for the reported Sb2Te3/organic composites. It is suggested that the plate-like Sb2Te3 grains and the embedded PEDOTs may play a significant role in decreasing the thermal conductivity. Furthermore, results of the thermal cycling between the room temperature and 523 K for 50 cycles show that the composites are stable with κ remaining a low value.

Crystal structures of X‐phase in the Sb–Te binary alloy system

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.

Effect of ball milling time on the thermoelectric properties of p-type (Bi,Sb)2Te3

P-type (Bi,Sb)2Te3 compounds were fabricated with high energy ball-milled powder and their thermoelectric properties were investigated as a function of the ball milling time. The fine grain sizes were confirmed with the X-ray analysis and the microscopy images in the samples fabricated with powders from long ball milling time. The values of the lattice thermal conductivity of the samples at high temperature show smaller values for long milling time (>24h) than those for short time, resulted from the finer grain sizes of the samples. As well as the microstructural change, the Seebeck coefficient and the electrical resistivity of the compounds were significantly altered with the ball milling time. The carrier concentration was dramatically increased in the samples after 24h milling, which was attributed to the formation of the antisite defects introduced by the accumulated thermal energy. The formation of antisite defects may be also promoted by the unintentional Fe doping from jars and balls. The highest value of ZT=1.14 was achieved at 323K for the samples with 10h. The temperature where the value of ZT was maximized and the values of ZT was varied with the ball milling time, which implies that the ball milling time should be carefully optimized for the suitable application of this compounds.

Optical Properties and Electronic Band Structure of Topological Insulators (on A5 2B6 3 Compound Based)

We have performed a first principles study of structural, electronic, and optical properties of rhombohedral Sb2Te3 and Bi2Te3 compounds using the density functional theory within the local density approximation. The lattice parameters, bulk modulus, and its pressure derivatives of these compounds have been obtained. The linear photon-energy dependent dielectric functions and some optical properties such as the energy-loss function, the effective number of valance electrons and the effective optical dielectric constant are calculated and presented in the study.

Giant and anisotropic magnetoresistances in p-type Bi-doped Sb2Te3 bulk single crystals

Antimony telluride (Sb2Te3) compounds are well known as excellent thermoelectric materials and have been recently confirmed as three-dimensional topological insulators. In this letter, we have investigated the anisotropic magneto-transport properties of p-type Bi-doped Sb2Te3 bulk single crystals over a broad range of temperatures, degrees and magnetic fields. Giant magnetoresistance (MR) of up to 230% was observed, which exhibits quadratic field dependences in low fields and becomes linear at high fields without any trend towards saturation. The giant MR also shows strong anisotropy with anisotropy ratio up to 210% in angle-dependent measurements. The giant MR might result from strong intervalley and intravalley scattering of holes and the strong anisotropy is attributable to the anisotropy of hole mobility, relaxation time and effective mass in the Fermi surface. The observed giant anisotropic MR could find applications in Sb2Te3-based anisotropic magneto-electronic devices.

Tellurium-evaporation-annealing for p-type bismuth–antimony–telluride thermoelectric materials

A tellurium evaporation annealing method has been investigated to control the carrier concentration of sintered (Bi,Sb)2Te3 compounds. Hot-pressed (Bi,Sb)2Te3 bulk alloys and tellurium powders located in an evacuated ampoule, were heated to 673K and held for 3, 12 and 48h. The crystal structure and chemical composition in the annealed specimens were preserved, while the carrier concentrations were varied between 1.53×1019 and 2.57×1019cm−3, and the thermal conductivity at 300K ranged between 1.20 and 1.25Wm−1K−1. The figure of merit at 300K was enhanced from 0.86 to 1.06 when the specimens were annealed for 3h. To identify the underlying mechanism, we utilized ab initio density functional theory calculations. These computations indicated that a Te ad-layer on top of the Bi2Te3 energetically favors bulk Bi atoms to migrate to the surface. Our experimental measurements and the first-principles validations consistently indicate that the tellurium evaporation annealing method is a novel process for enhancing the thermoelectric performance of Bi–Te compounds by controlling their carrier concentrations, which is particularly useful in dealing with nano-scale composites.

Investigation of the sintering pressure and thermal conductivity anisotropy of melt-spun spark-plasma-sintered (Bi,Sb)2Te3 thermoelectric materials

Abstract

First-principles studies of the three-dimensional strong topological insulators Bi2Te3, Bi2Se3 and Sb2Te3

Bi2Se3, Bi2Te3 and Sb2Te3 compounds have recently been predicted to be three-dimensional (3D) strong topological insulators. In this paper, based on ab initio calculations, we study in detail the topological nature and the surface states of this family of compounds. The penetration depth and the spin-resolved Fermi surfaces of the surface states are analyzed. We also present a procedure from which a highly accurate effective Hamiltonian can be constructed based on projected atomic Wannier functions (which keep the symmetries of the systems). Such a Hamiltonian can be used to study the semi-infinite systems or slab-type supercells efficiently. Finally, we discuss the 3D topological phase transition in the Sb2(Te1−xSex)3 alloy system.

Magnetic properties of Mn‐doped Bi2Te3 and Sb2Te3

AbstractWe have fabricated Mn‐doped Bi2Te3 and Sb2Te3 single crystals by the vertical gradient solidification method. The compositions and crystal structures of Bi2−xMnxTe3 and Sb2−xMnxTe3 were determined using Electron Probe Micro‐Analyzer (EPMA) and powder X‐ray diffraction (XRD) patterns, respectively. Both crystal structures were rhombohedral with smaller lattice constants because of the smaller atomic radius of Mn than those of Bi and Sb. Based on the magnetization measurements, Mn‐doped Bi2Te3 and Sb2Te3 compounds have ferromagnetic ordering at TC = 10 and 17 K, respectively. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)