Sb Te Research Articles

Quantum Anomalous Hall Multilayers Grown by Molecular Beam Epitaxy**Supported by the National Key Research and Development Program of China under Grant No 2017YFA0303303, the National Natural Science Foundation of China under Grant No 51661135024, and the Beijing Advanced Innovation Center for Future Chip (ICFC).

Quantum anomalous Hall (QAH) effect is a quantum Hall effect that occurs without the need of external magnetic field. A system composed of multiple parallel QAH layers is an effective high Chern number QAH insulator and the key to the applications of the dissipationless chiral edge channels in low energy consumption electronics. Such a QAH multilayer can also be engineered into other exotic topological phases such as a magnetic Weyl semimetal with only one pair of Weyl points. This work reports the first experimental realization of QAH multilayers in the superlattices composed of magnetically doped (Bi,Sb)2Te3 topological insulator and CdSe normal insulator layers grown by molecular beam epitaxy. The obtained multilayer samples show quantized Hall resistance h/Ne2, where h is Planck’s constant, e is the elementary charge and N is the number of the magnetic topological insulator layers, resembling a high Chern number QAH insulator. The QAH multilayers provide an excellent platform to study various topological states of matter.

Noncontact Method of Conducting Elements “Writing” on Insulating Ge–Sb–Te Matrix Using a Laser Beam

Abstract Direct writing of low resistance wires on an amorphous Ge-Sb-Te matrix is reported. A 1342 nm continuous wave laser was used for local heating of the sample to form these wires. Mechanical contact was not needed for making the conducting elements. The properties of the samples were investigated down to 1.4 K and the laser conditions required for the writing of low resistance GST wires were found. The results are discussed with a view to possible applications, such as connectors and electrical wires made only via remote light exposure of the samples to make different resistors and non-linear elements

Impact of Stoichiometry on the Structure of van der Waals Layered GeTe/Sb2 Te3 Superlattices Used in Interfacial Phase-Change Memory (iPCM) Devices.

Van der Waals layered GeTe/Sb2 Te3 superlattices (SLs) have demonstrated outstanding performances for use in resistive memories in so-called interfacial phase-change memory (iPCM) devices. GeTe/Sb2 Te3 SLs are made by periodically stacking ultrathin GeTe and Sb2 Te3 crystalline layers. The mechanism of the resistance change in iPCM devices is still highly debated. Recent experimental studies on SLs grown by molecular beam epitaxy or pulsed laser deposition indicate that the local structure does not correspond to any of the previously proposed structural models. Here, a new insight is given into the complex structure of prototypical GeTe/Sb2 Te3 SLs deposited by magnetron sputtering, which is the used industrial technique for SL growth in iPCM devices. X-ray diffraction analysis shows that the structural quality of the SL depends critically on its stoichiometry. Moreover, high-angle annular dark-field-scanning transmission electron microscopy analysis of the local atomic order in a perfectly stoichiometric SL reveals the absence of GeTe layers, and that Ge atoms intermix with Sb atoms in, for instance, Ge2 Sb2 Te5 blocks. This result shows that an alternative structural model is required to explain the origin of the electrical contrast and the nature of the resistive switching mechanism observed in iPCM devices.

Erratum to “Metal–Insulator Transition of Ge–Sb–Te Superlattice: An Electron Counting Model Study” [Jan 18 140-146

Presents corrections to the paper, “Metal–insulator transition ofGe–Sb–Te superlattice: An electron counting model study,” (Chen, N.-K.), IEEE Trans. Nanotechnol., vol. 17, no. 1, pp. 140–146, Jan. 2018.

Thermal Stability of Zone Melting p-Type (Bi, Sb)2Te3 Ingots and Comparison with the Corresponding Powder Metallurgy Samples

During the past few decades, Bi2Te3-based alloys have been investigated extensively because of their promising application in the area of low temperature waste heat thermoelectric power generation. However, their thermal stability must be evaluated to explore the appropriate service temperature. In this work, the thermal stability of zone melting p-type (Bi, Sb)2Te3-based ingots was investigated under different annealing treatment conditions. The effect of service temperature on the thermoelectric properties and hardness of the samples was also discussed in detail. The results showed that the grain size, density, dimension size and mass remained nearly unchanged when the service temperature was below 523 K, which suggested that the geometry size of zone melting p-type (Bi, Sb)2Te3-based materials was stable below 523 K. The power factor and Vickers hardness of the ingots also changed little and maintained good thermal stability. Unfortunately, the thermal conductivity increased with increasing annealing temperature, which resulted in an obvious decrease of the zT value. In addition, the thermal stabilities of the zone melting p-type (Bi, Sb)2Te3-based materials and the corresponding powder metallurgy samples were also compared. All evidence implied that the thermal stabilities of the zone-melted (ZMed) p-type (Bi, Sb)2Te3 ingots in terms of crystal structure, geometry size, power factor (PF) and hardness were better than those of the corresponding powder metallurgy samples. However, their thermal stabilities in terms of zT values were similar under different annealing temperatures.

Effect of annealing on microstructure and thermoelectric properties of hot-extruded Bi–Sb–Te bulk materials

The effect of annealing on the microstructure, thermoelectric properties and hardness of the hot-extruded Bi–Sb–Te materials has been investigated systematically to optimize their thermoelectric and mechanical properties. The mechanically alloyed powder was consolidated by hot extrusion at either 340 or 400 °C, followed by annealing in a temperature range of 260–400 °C. The microstructure of the annealed samples contained submicron grains with preferred (001) texture. As annealing temperature increased, the small-angle grain boundaries (SAGBs) increased because the increased amount of Te-rich and Sb-rich phases inhibits the movements of dislocations and SAGBs. The submicron microstructure led to a low thermal conductivity, for example, ~ 0.9 W/mK after annealing at TA ≥ 380 °C. The Seebeck coefficient highly depended on carrier mobility in addition to carrier concentration. For the extruded samples prepared at a lower extrusion temperature of 340 °C, the mobility increased significantly after annealing, resulting in great enhancements in the Seebeck coefficient and electrical conductivity. A peak ZT value of 0.94 and high hardness were simultaneously obtained under the conditions of hot extrusion at 340 °C and annealing at 380 °C. It seems that the combination of low-temperature extrusion and high-temperature annealing is an effective route to prepare high-performance Bi2Te3-based materials.

Resistive switching characteristics of interfacial phase-change memory at elevated temperature

Interfacial phase-change memory (iPCM) devices were fabricated using W and TiN for the bottom and top contacts, respectively, and the effect of operation temperature on the resistive switching was examined over the range between room temperature and 200 °C. It was found that the high-resistance (RESET) state in an iPCM device drops sharply at around 150 °C to a low-resistance (SET) state, which differs by ∼400 Ω from the SET state obtained by electric-field-induced switching. The iPCM device SET state resistance recovered during the cooling process and remained at nearly the same value for the RESET state. These resistance characteristics greatly differ from those of the conventional Ge–Sb–Te (GST) alloy phase-change memory device, underscoring the fundamentally different switching nature of iPCM devices. From the thermal stability measurements of iPCM devices, their optimal temperature operation was concluded to be less than 100 °C.

Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi,Sb)Te based three dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow to determine the room temperature carrier mobilities in the surface states despite the presents of thermally activate residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30{\div}60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity.

Precision measurement of the quantized anomalous Hall resistance at zero magnetic field

In the quantum anomalous Hall effect, the edge states of a ferromagnetically doped topological insulator exhibit quantized Hall resistance and dissipationless transport at zero magnetic field. Up to now, however, the resistance was experimentally assessed using standard transport measurement techniques which are difficult to trace to the von-Klitzing constant RK with high precision. Here, we present a metrologically comprehensive measurement, including a full uncertainty budget, of the resistance quantization of V-doped (Bi,Sb)2Te3 devices without the external magnetic field. For the deviation of the quantized anomalous Hall resistance from RK, we determined a value of 0.17 ± 0.25 ppm, the smallest and most precise value reported to date. This is a step towards realization of a practical zero-field quantum resistance standard which in combination with the Josephson effect could provide the universal quantum units standard in the future.

Phase Stability and Anisotropic Sublimation of Cubic Ge–Sb–Te Alloy Observed by In Situ Transmission Electron Microscopy

Phase stability and anisotropic sublimation dynamics of the cubic Ge–Sb–Te alloy have been investigated by in situ transmission electron microscopy (TEM). The starting point of the phase-transition study is an epitaxially aligned Ge1Sb2Te4 grain on a Si(111) substrate. Upon in situ heating, the cubic phase remains stable up to the sublimation point without a transition to the thermodynamically stable trigonal crystal structure which is attributed to Si diffusion into the Ge1Sb2Te4 grain. The sublimation process is made visible with atomic resolution. The anisotropic process leads to the formation of stable {111} facets via kink nucleation on stable steps and subsequent sublimation from those kink sites as predicated by the terrace-step-kink model. Kink nucleation sites are identified and are in accordance with the broken-bond model approach.

Understanding the fast crystallization kinetics of In–Sb–Te by using ultrafast calorimetry

The crystal growth rates of supercooled liquids In–Sb–Te can be well characterized by ultrafast DSC, and it benefits to the optimization for phase-change materials.

Can surface reactivity of mixed crystals be predicted from their counterparts? A case study of (Bi1−xSbx)2Te3 topological insulators

The behavior of mixed crystals (Bi,Sb)2Te3 and its correlation with the properties of their constituents is comprehensively explored.

Density of State Calculations for Tl3SbS3 and SbTeI

Based on the full-potential linearized augmented plane waves method (FL-LAPW) with local density approximation (LDA), the partials and totals densities of state of Tl3SbS3 and SbT eI are calculated in order to find the semiconductor character via direct or indirect gap. Tl3SbS3 and SbTeI present the most important candidates of the antimony chalcogenides family. Their densities of states curves bring out characteristic features in the valence band a core like peak, at environ 13.00 eV below the valence band maximum, originating mainly from S 3s and I 5s states respectively, and a three-peak structure at the top of the valence band from S 3p and I 5p states hybridized with Sb 5p and Te 5p states. Our results give a good agreement with other theoretical calculations and experimental data.

Ultralow-fluence single-shot optical crystalline-to-amorphous phase transition in Ge-Sb-Te nanoparticles.

Here we demonstrate that the 0-dimensional confinement of Ge2Sb2Te5 results in a drastic reduction of the minimum critical fluence required for optical-induced amorphization when compared to the thin-film cases. We show that by using single-shot laser pulses, the investigated nanoparticles display a crystalline-to-amorphous transition, satisfying a mandatory requirement of a bit-memory element. These unprecedented results open a viable route to boost energy efficient phase-change processes.

Ultrafast interfacial transformation from 2D- to 3D-bonded structures in layered Ge-Sb-Te thin films and heterostructures.

Two-dimensional van-der-Waals-bonded chalcogenide heterostructures have recently received a lot of attention due to promising applications in the fields of photonics, plasmonics and data storage. Of particular interest is the interfacial switching process inherent in these structures, which is assumed to occur locally at the van-der-Waals interfaces and thus represents an intracrystalline transition. However, detailed experimental studies on the underlying mechanism are still lacking. In this work, epitaxially grown thin films consisting of van-der-Waals-bonded Ge-Sb-Te and GeTe/Sb2Te3 based heterostructures are employed as a model system to investigate structural changes induced by a single ns-laser pulse. A combined approach using X-ray diffraction and advanced transmission electron microscopy is applied to study phase transitions within the Ge-Sb-Te-based thin films in detail. The results reveal ultrafast transitions from 2D-bonded layered structures to 3D-bonded structures via a transient molten phase. Moreover, the interface between the 2D- and 3D-bonded structures is well defined by a single van-der-Waals gap, suggesting that the transition can be controlled very precisely in its spatial extent by an appropriate choice of the laser fluence. Overall, the results of this work offer a new perspective on the switching mechanism in Ge-Sb-Te-based materials and demonstrate the potential of van-der-Waals-bonded Ge-Sb-Te compounds to be applied for novel phase-change memory concepts.

Laser co-ablation of bismuth antimony telluride and diamond-like carbon nanocomposites for enhanced thermoelectric performance

A series of innovative heterogeneous nanocomposites comprising diamond-like carbon (DLC) clusters and well-aligned Bi–Sb–Te based nanoassemblies were realized for thermoelectric enhancement.

Metal–Insulator Transition of Ge–Sb–Te Superlattice: An Electron Counting Model Study

Ge–Sb–Te superlattice (GST-SL) is a newly emerging electronic material for nonvolatile phase-change memory with ultralow energy cost. However, its switching mechanism is still unclear with intensive debates. In this work, by first-principles calculations and an electron counting model study, we study the possible mechanism of phase change and the accompanying property transition of GST-SL. GST-SL are separated into individual layers by van der Waals gaps. We demonstrate that both the global chemical stoichiometry of the material and the local chemical stoichiometry of individual layer block are required to have an insulating band gap according to an electron counting model analysis. The electrical property can be adjusted by changing the local stoichiometry, such as producing defects around van der Waals gaps. Inspired by a previous experiment, we propose that a stacking-fault motion can spontaneously alter the band gap and results in a metal–insulator transition. This transition may provide a significant change of carrier concentration and indicate an ultralow energy-consumption process with a low energy barrier. The present investigations reveal a picture of electrical transition in GST-SL and may guide us to improve its device performances.

Temperature dependence of ferromagnetic resonance spectra of permalloy on (Bi1−xSbx)2Te3

We investigate the temperature dependence of ferromagnetic resonance spectra of permalloy (Py: Ni0.80Fe0.20) on (Bi1−xSbx)2Te3 with x = 0–1. Smaller resonance fields and broader linewidths are observed for Py/(Bi,Sb)2Te3 than those for Py/substrate below ∼40 K. The result indicates the presence of the coupling as well as spin pumping at the interface between Py and (Bi,Sb)2Te3.

Stress-controlled thermoelectric module for energy harvesting and its application for the significant enhancement of the power factor of Bi2Te3-based thermoelectrics

We propose a model of a thermoelectric module in which the performance parameters can be controlled by applied tuneable stress. This model includes a miniature high-pressure anvil-type cell and a specially designed thermoelectric module that is compressed between two opposite anvils. High thermally conductive high-pressure anvils that can be made, for instance, of sintered technical diamonds with enhanced thermal conductivity, would enable efficient heat absorption or rejection from a thermoelectric module. Using a high-pressure cell as a prototype of a stress-controlled thermoelectric converter, we investigated the effect of applied high pressure on the power factors of several single-crystalline thermoelectrics, including binary p-type Bi2Te3, and multi-component (Bi,Sb)2Te3 and Bi2(Te,Se,S)3 solid solutions. We found that a moderate applied pressure of a few GPa significantly enhances the power factors of some of these thermoelectrics. Thus, they might be more efficiently utilized in stress-controlled thermoelectric modules. In the example of one of these thermoelectrics crystallizing in the same rhombohedral structure, we examined the crystal lattice stability under moderate high pressures. We uncovered an abnormal compression of the rhombohedral lattice of (Bi0.25,Sb0.75)2Te3 along the c-axis in a hexagonal unit cell, and detected two phase transitions to the C2/m and C2/c monoclinic structures above 9.5 and 18 GPa, respectively.

Chiral transport along magnetic domain walls in the quantum anomalous Hall effect

The quantum anomalous Hall effect in thin film magnetic topological insulators (MTIs) is characterized by chiral, one-dimensional conduction along the film edges when the sample is uniformly magnetized. This has been experimentally confirmed by measurements of quantized Hall resistance and near-vanishing longitudinal resistivity in magnetically doped (Bi,Sb)2Te3. Similar chiral conduction is expected along magnetic domain walls, but clear detection of these modes in MTIs has proven challenging. Here, we intentionally create a magnetic domain wall in an MTI, and study electrical transport along the domain wall. In agreement with theoretical predictions, we observe chiral transport along a domain wall. We present further evidence that two modes equilibrate while co-propagating along the length of the domain wall.