Sb Te Research Articles

Structural Transformations in (Bi, Sb)2Te3 Solid Solutions Grown by Spark Plasma Sintering

We have studied the dependence of the thermoelectric properties of the bulk (Bi,Sb)2Te3 material on the temperature of spark plasma sintering (SPS). For analysis of the experimental results we took into account the regularities of structure formation in the material. The average crystallite size decreases with an increase in the SPS temperature to above 400 °C. Transmission electron microscopy showed that at above 400 °C the bulk and boundaries of the initial grains that are several micrometers in size contain a large number of nanosized grains (approx 10 – 20 nm) having the same composition. We show that the dependence of the thermoelectric properties of the material on SPS temperature correlates with changes in the fine structure of the material which is controlled by the redistribution of the intrinsic point defects. Our results suggest that, along with the well-known nanostructure formation processes, there is one more high-temperature self-organizing process of the formation of the nanostructural material based on the redistribution and change in the form of occurrence of the nonequilibrium point defects. This information can be used to broaden the possibilities of controlling the properties of the bulk thermoelectric material on the basis of (Bi,Sb)2Te3.

Scaling of Power Generation With Dopant Density in Integrated Circuit Silicon Thermoelectric Generators

Integrated circuit (IC) thermoelectric generators (TEGs) fabricated by standard “65 nm” Si processing have been shown to be potentially competitive with (Bi, Sb)2Te3 TEGs in terms of maximum power generated, ${P}_{\text {max}}$ , per conduction cross-sectional area at a given temperature difference. Optimization of ${P}_{\text {max}}$ in these Si IC TEGs with respect to design and processing parameters remains an open issue. Here the dependence of ${P}_{\text {max}}$ on dopant density in Si thermopiles is examined both experimentally and by developing a simple physics-based model using empirical material parameters. Experimentally, ${P}_{\text {max}}$ improved by $\sim 1.8\times $ with increasing dopant density from ${3} \times {10}^{{17}}$ to ${4} \times {10}^{{18}}$ cm−3. Modeling shows that ${P}_{\text {max}} \propto $ thermopile figure-of-merit ZT , which scales with dopant density approximately as (density)0.24 from 1017 to 1019 cm−3. A model extrapolation to 1020 cm−3 predicts that parasitic resistance and thermal conductance will cause ${Z}$ and ${P}_{\text {max}}$ to have a maximum at densities above 1019 cm−3 and then decline at higher densities. Without parasitics ${Z}$ and ${P}_{\text {max}}$ would continue to increase roughly as (density)0.4. The results emphasize the importance of minimizing parasitics.

Parameters of Lateral and Unsteady Cord Currents in a Cylindrical Chalcogenide Glassy Semiconductor

The heat-conduction equation describing the current cord in a semiconductor is approximately solved for a Ge–Sb–Te semiconductor system of cylindrical configuration. It is shown that the cord current at infinitely long times is proportional to the squared maximum temperature at the cord center and inversely proportional to the applied electric field. The scale of the lateral current perpendicular to the main cord current is estimated. It is found that the lateral current is low in comparison with the cord current; hence, the formation of lateral cords growing from the main cord is highly improbable.

Fast and scalable phase change materials Ti–Sb–Te deposited by plasma-enhanced atomic layer deposition

Current features of phase change memory (PCM) based on Ge2Sb2Te5 (GST) as a non-volatile memory is insufficient to meet the needs of storage class memory (SCM) such as extremely high-density, ultra-fast operating speed and long life cycle. In this study, Ti0.28Sb2Te3 (TST) device was fabricated based on atomic layer deposition (ALD) method and used as a phase change material. Due to the fast crystallization and low melting temperature of TST, the set speed can be reduced to 6 ns while the reset voltage can be decreased by 20% compared with GST-based device with the same cell structure. In addition, the ALD-deposited TST alloy showed a good gap-fill capability. These results indicate that the ALD-deposited TST film is a fast and scalable phase-change material applied to SCM.

Origin of Band Modulation in GeTe-Rich Ge–Sb–Te Thin Film

Germanium tellurides and its pseudo binary compounds offer interesting properties that are important in thermoelectric and phase-change applications. Despite being a class of materials under scrutiny since its discovery, unique properties and functionalities kept on emerging in recent years. In this work, we observed another unique property of Ge-Sb-Te (GST) thin film that can be beneficial in its development for thermoelectric application. A rapid heating and quenching process of the GST film resulted in a metastable rock-salt cubic structure, exhibiting a unique electronic-transition-like behavior. Above the transition temperature at 150 °C, we observed a temperature-induced band modulation, corroborated with changes in its effective mass and valence band position that leads to favorable electronic and thermoelectric properties. Charge transfer between Sb and Te occurred, accompanied by a distorted cubic-to-cubic structural change. The interplay of the electronic and lattice structure born out of the co...

Carrier free long-range magnetism in Mo doped one quintuple layer Bi2Te3 and Sb2Te3

One of the keys to the realization of quantum anomalous Hall effect is long range ferromagnetism, which is only experimentally realized in Cr or V doped (Bi,Sb)2Te3 system. Both elements are 3d transition metals, and 4d transition metals are found to be ineffective to produce long range ferromagnetism in Bi2Se3. Still, whether long range ferromagnetism can be realized by magnetic doping of 4d elements is an open question. Based on density functional theory calculations, we predict that long range ferromagnetism can be realized in Mo doped Bi2Te3 and Sb2Te3, which are semiconducting. The coupling strength is comparable with that of Cr doped Bi2Te3 and Sb2Te3. Therefore, Mo doped Bi2Te3 and Sb2Te3 or their alloys can be new systems to realize carrier free long-range ferromagnetism.

Sub-nanosecond threshold switching dynamics in GeSb2Te4 phase change memory device

Conventional Ge–Sb–Te based phase change memory (PCM) devices offer promising attributes, including fast non-volatile memory operations, higher endurance and sub-ns re-crystallization/re-amorphization capabilities for the next generation high speed non-volatile memory. The threshold switching process in the amorphous phase of the PCM device enables breakdown of electrical resistance followed by rapid flow of device current, leading to Joules heating induced crystallization, known as SET operation. The speed of SET operation is, therefore, governed by voltage dependent threshold switching dynamics and the delay time (td) characteristics at the timescale of nanoseconds. Although GeSb2Te4 offers faster crystal growth velocity, realization of fast threshold switching capabilities remains unexplored. In this work, the ultrafast threshold switching dynamics of a prototypical GeSb2Te4 PCM device are investigated. The present experimental results demonstrate an exponential reduction of td upon increasing the applied voltage (VA) above its threshold voltage (VT). The td has been reduced to a strikingly lower value of 0.5 ns for VA of 1.6 times of VT, which enables faster SET operation within 3 ns as evidenced by a permanent change in the device resistance. These experimental findings of sub-nanosecond threshold switching dynamics and faster SET operation in the GeSb2Te4 device pave a way for designing a PCM device with ns programming speed for future electronics.

Broadband transparent optical phase change materials for high-performance nonvolatile photonics

Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge2Sb2Se4Te1, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics.

Heterogeneous nucleus-induced crystallization for high-speed phase change memory applications

Phase change memory has high potential for next-generation nonvolatile memory technology. The effect of lower layer Sb2Te (ST) on the crystallization process of Ge2Sb2Te5 (GST) is investigated by observing the change of the microstructure. Compared to the GST-based device, this paper achieved a faster operation speed (20 ns) with stable SET and RESET resistances via the addition of ST under the GST. The crystallization of lower layer ST occurs prior to that in the upper layer GST due to lower crystallization temperature, and the crystal grains of ST can supply the heterogeneous nuclei to reduce the nucleation time and enhance the crystallization speed of GST, which accounts for the improved operation speed. In addition, the effect of atomic migration on the GST/ST interface and the device performance is also investigated.

Understanding the Structure and Properties of Sesqui‐Chalcogenides (i.e., V2VI3 or Pn2Ch3 (Pn = Pnictogen, Ch = Chalcogen) Compounds) from a Bonding Perspective

A number of sesqui-chalcogenides show remarkable properties, which make them attractive for applications as thermoelectrics, topological insulators, and phase-change materials. To see if these properties can be related to a special bonding mechanism, seven sesqui-chalcogenides (Bi2 Te3 , Bi2 Se3 , Bi2 S3 , Sb2 Te3 , Sb2 Se3 , Sb2 S3 , and β-As2 Te3 ) and GaSe are investigated. Atom probe tomography studies reveal that four of the seven sesqui-chalcogenides (Bi2 Te3 , Bi2 Se3 , Sb2 Te3 , and β-As2 Te3 ) show an unconventional bond-breaking mechanism. The same four compounds evidence a remarkable property portfolio in density functional theory calculations including large Born effective charges, high optical dielectric constants, low Debye temperatures and an almost metal-like electrical conductivity. These results are indicative for unconventional bonding leading to physical properties distinctively different from those caused by covalent, metallic, or ionic bonding. The experiments reveal that this bonding mechanism prevails in four sesqui-chalcogenides, characterized by rather short interlayer distances at the van der Waals like gaps, suggestive of significant interlayer coupling. These conclusions are further supported by a subsequent quantum-chemistry-based bonding analysis employing charge partitioning, which reveals that the four sesqui-chalcogenides with unconventional properties are characterized by modest levels of charge transfer and sharing of about one electron between adjacent atoms. Finally, the 3D maps for different properties reveal discernible property trends and enable material design.

Quantum anomalous Hall effect driven by magnetic proximity coupling in all-telluride based heterostructure

The quantum anomalous Hall effect (QAHE) is an exotic quantum phenomenon originating from dissipationless chiral channels at the sample edge. While the QAHE has been observed in magnetically doped topological insulators (TIs), exploiting the magnetic proximity effect on the TI surface from adjacent ferromagnetic layers may provide an alternative approach to the QAHE by opening an exchange gap with less disorder than that in the doped system. Nevertheless, the engineering of a favorable heterointerface that realizes the QAHE based on the magnetic proximity effect remains to be achieved. Here, we report on the observation of the QAHE in a proximity coupled system of a nonmagnetic TI and a ferromagnetic insulator (FMI). We have designed sandwich heterostructures of (Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te that fulfills two prerequisites for the emergence of the QAHE: the formation of a sizable exchange gap at the TI surface state and the tuning of the Fermi energy into the exchange gap. The efficient proximity coupling in the all-telluride based heterostructure as demonstrated here will enable a realistic design of versatile tailor-made topological materials coupled with ferromagnetism, ferroelectricity, superconductivity, and so on.

Electron Microscopic Study of the Influence of Annealing on Ge–Sb–Te Thin Films Obtained by Vacuum Thermal Evaporation

Using transmission electron microscopy it is demonstrated that the annealing of amorphous Ge2Sb2Te5 films deposited by vacuum thermal evaporation at 250°C leads to the formation of a hexagonal phase with a peculiar block structure. Herewith, island defects are formed on their surface representing the cubic modification of Sb2O3. The sizes of these defects and portion of the surface area occupied by them are estimated using images of scanning electron microscopy. The formation of antimony-oxide crystallites can be attributed to enrichment of the surface area of the initial film with antimony which is oxidized during annealing. Due to the formation of defects, the film composition in adjacent local areas varies and becomes close to the stoichiometric values for Ge3Sb2Te6.

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.

Selective area growth and stencil lithography for in situ fabricated quantum devices.

The interplay of Dirac physics and induced superconductivity at the interface of a 3D topological insulator (TI) with an s-wave superconductor (S) provides a new platform for topologically protected quantum computation based on elusive Majorana modes. To employ such S-TI hybrid devices in future topological quantum computation architectures, a process is required that allows for device fabrication under ultrahigh vacuum conditions. Here, we report on the selective area growth of (Bi,Sb)2Te3 TI thin films and stencil lithography of superconductive Nb for a full in situ fabrication of S-TI hybrid devices via molecular-beam epitaxy. A dielectric capping layer was deposited as a final step to protect the delicate surfaces of the S-TI hybrids at ambient conditions. Transport experiments in as-prepared Josephson junctions show highly transparent S-TI interfaces and a missing first Shapiro step, which indicates the presence of Majorana bound states. To move from single junctions towards complex circuitry for future topological quantum computation architectures, we monolithically integrated two aligned hardmasks to the substrate prior to growth. The presented process provides new possibilities to deliberately combine delicate quantum materials in situ at the nanoscale.

Large Anomalous Hall Effect in Topological Insulators with Proximitized Ferromagnetic Insulators.

We report a proximity-driven large anomalous Hall effect in all-telluride heterostructures consisting of the ferromagnetic insulator Cr_{2}Ge_{2}Te_{6} and topological insulator (Bi,Sb)_{2}Te_{3}. Despite small magnetization in the (Bi,Sb)_{2}Te_{3} layer, the anomalous Hall conductivity reaches a large value of 0.2e^{2}/h in accord with a ferromagnetic response of the Cr_{2}Ge_{2}Te_{6}. The results show that the exchange coupling between the surface state of the topological insulator and the proximitized Cr_{2}Ge_{2}Te_{6} layer is effective and strong enough to open the sizable exchange gap in the surface state.

High-Density Sb2 Te3 Nanopillars Arrays by Templated, Bottom-Up MOCVD Growth.

Sb2 Te3 exhibits several technologically relevant properties, such as high thermoelectric efficiency, topological insulator character, and phase change memory behavior. Improved performances are observed and novel effects are predicted for this and other chalcogenide alloys when synthetized in the form of high-aspect-ratio nanostructures. The ability to grow chalcogenide nanowires and nanopillars (NPs) with high crystal quality in a controlled fashion, in terms of their size and position, can boost the realization of novel thermoelectric, spintronic, and memory devices. Here, it is shown that highly dense arrays of ultrascaled Sb2 Te3 NPs can be grown by metal organic chemical vapor deposition (MOCVD) on patterned substrates. In particular, crystalline Sb2 Te3 NPs with a diameter of 20 nm and a height of 200 nm are obtained in Au-functionalized, anodized aluminum oxide (AAO) templates with a pore density of ≈5 × 1010 cm-2 . Also, MOCVD growth of Sb2 Te3 can be followed either by mechanical polishing and chemical etching to produce Sb2 Te3 NPs arrays with planar surfaces or by chemical dissolution of the AAO templates to obtain freestanding Sb2 Te3 NPs forests. The illustrated growth method can be further scaled to smaller pore sizes and employed for other MOCVD-grown chalcogenide alloys and patterned substrates.

Evolution of quasiperiodic microstructure of Ge–Sb–Te-based films irradiated by multi-pulsed picosecond laser

In this paper, the crystal morphology and grain size evolution of Ge–Sb–Te (GST) irradiated by multi-pulsed picosecond (ps) laser were characterized by transmission electron microscopy and selected area electron diffraction. Morphologies induced by multi-pulsed laser were radially quasiperiodic structures, and the crystallization area induced by the first laser pulse acted as the periodic repeated unit. Single laser pulse irradiation produced three different conterminal annular crystallization zones, and their formation mechanisms were discussed. GST films accumulated thermal and structural change induced by per laser pulse, reducing crystallization threshold and extending crystallization fluence range. Sequentially, every subsequent laser pulse created a new crystallization area, and then accumulated to be a pulse number dependence radially quasiperiodic structure. Meanwhile, the laser pulse train dramatically refined grains. The grain boundaries are presented as an alternating-layered structure or atomic ordering domain. The present study revealed microstructure evolution of intermediate states for multi-level storage and realized multi-phase changes and is critical to enhance storage density.

Correlating thermoelectric (Bi,Sb)2Te3 film electric transport properties with microstructure

The room temperature electronic transport properties of 1 μm thick Bi0.4Sb1.6Te3 (BST) films correlate with overall microstructural quality. Films with homogeneous composition are deposited onto fused silica substrates, capped with SiN to prevent both oxidation and Te loss, and postannealed to temperatures ranging from 200 to 450 °C. BST grain sizes and (00l) orientations improve dramatically with annealing to 375 °C, with smaller increases to 450 °C. Tiny few-nanometer-sized voids in the as-deposited film grain boundaries coalesce into larger void sizes up to 300 nm with annealing to 350 °C; the smallest voids continue coalescing with annealing to 450 °C. These voids are decorated with few-nanometer-sized Sb clusters that increase in number with increasing annealing temperatures, reducing the Sb content of the remaining BST film matrix. Resistivity decreases linearly with increasing temperature over the entire range studied, consistent with improving crystalline quality. The Seebeck coefficient also improves with crystalline quality to 350 °C, above which void coalescence and reduced Sb content from the BST matrix correlate with a decrease in the Seebeck coefficient. Nevertheless, a plateau exists for an optimal power factor between 350 and 450 °C, implying thermal stability to higher temperatures than previously reported.

Ge-Sb-Te Chalcogenide Thin Films Deposited by Nanosecond, Picosecond, and Femtosecond Laser Ablation.

Ge-Sb-Te thin films were obtained by ns-, ps-, and fs-pulsed laser deposition (PLD) in various experimental conditions. The thickness of the samples was influenced by the Nd-YAG laser wavelength, fluence, target-to-substrate distance, and deposition time. The topography and chemical analysis results showed that the films deposited by ns-PLD revealed droplets on the surface together with a decreased Te concentration and Sb over-stoichiometry. Thin films with improved surface roughness and chemical compositions close to nominal values were deposited by ps- and fs-PLD. The X-ray diffraction and Raman spectroscopy results showed that the samples obtained with ns pulses were partially crystallized while the lower fluences used in ps- and fs-PLD led to amorphous depositions. The optical parameters of the ns-PLD samples were correlated to their structural properties.

Reduction of thermal conductivity in Al2O3 dispersed p-type bismuth antimony telluride composites

In this work, BiSbTe/Al2O3 composites were prepared by incorporation of alumina (with 2, 4, and 6 wt%) particles into the p-type Bi–Sb–Te gas atomized powders via mechanical milling and, subsequently consolidated by spark plasma sintering (SPS). The Seebeck coefficient was significantly improved (over 41%) at 325 K, while the electrical conductivity was abruptly decreased for 6 wt% Al2O3 added BiSbTe/Al2O3 composites, which was mainly due to the decreases in carrier concentration, and energy filtering of carriers at interfaces. The thermal conductivity was eminently decreased over 27.5% for 6 wt% Al2O3 added BiSbTe/Al2O3 composites, was mainly attributed to the scattering of carriers and phonons at enormous grain boundaries as well as newly formed interfaces. The Vickers hardness of the BiSbTe/Al2O3 composites was improved compared to that of base matrix, and this was attributed to refinement of grains, and to a dispersion strengthening mechanism.