Sb Te Research Articles

Enhancement of mechanical properties and thermoelectric performance of spark plasma sintered P-type Bismuth Telluride by powder surface oxide reduction

In the present investigation, key mechanical design parameters and the thermoelectric performance of powder ball milled spark plasma sintered P-type Bi0·4Sb1·6Te3 material was determined. In particular, the effect of the powder preparation technique on the powder surface oxide layer as well as both the mechanical and thermoelectric properties were determined. All measurements were performed on identically prepared sintered samples in both parallel and perpendicular to the applied sintering pressure. The mechanical properties determined included relative density, uniaxial tensile strength, coefficient of thermal expansion, Young’s modulus, Poisson’s ratio, and shear modulus. The thermoelectric properties measured included the Seebeck coefficient, electrical resistivity, and thermal conductivity. A mechanically agitated reduction facility built in-house was used to reduce the powder surface oxide at 380 °C for 24 h using dry 5% H2 95% Ar. To separate between the annealing and the reduction effects, the P-type (Bi, Sb)2Te3 powder was annealed using highly purified Ar at the same temperature and time as the reduction process. The oxygen content in the sintered P-type samples was then measured using the inert gas fusion technique. The oxygen content was found to be decreased by 35 wt% after 24 h of reduction but did not change significantly after annealing with Ar only relative to the dried powders. The reduction of the surface oxides increased the relative density of the sintered parts from 97.1% to 99.2%. Consequently, the characteristic uniaxial tensile strength increased from 13.9 MPa to 26.3 MPa and from 16.3 MPa to 30.6 MPa in the parallel and the perpendicular directions to the sintering pressure, respectively, for the sintered bodies from the reduced powder. Both the Young’s modulus and the shear modulus increased from 44.3 GPa to 48.7 GPa and 17.8 GPa–19.5 GPa in the parallel direction, respectively, after the reduction treatment and sintering. The figure of merit, ZT, increased from 0.35 to 0.80 and from 0.42 to 1.13 at room temperature (25 °C) in the parallel and the perpendicular directions, respectively, after the surface oxide reduction treatment.

Chalcogenide Materials Engineering for Phase‐Change Memory and Future Electronics Applications: From Sb–Te to Bi–Te

Chalcogenide materials play essential roles in modern nonvolatile memory technology in the form of both phase‐change memory (PCM) and selector devices. Herein, Bi–Te binary alloys are explored as an alternative candidate for superlattice (SL) or interfacial PCM (iPCM). GeTe/Bi4Te3 (GT/BT) SL exhibits similar structural features to conventional GeTe/Sb2Te3 (GT/ST) SL, such as highly oriented crystal grains and intermixing. Furthermore, preliminary device measurements show that Ge–Bi–Te (GBT) SL switches in a similar manner to conventional Ge–Sb–Te (GST), suggesting that they may be a promising candidate for memory applications. In addition, Bi2Te3/Sb2Te3 (BT/ST) heterostructure films have been successfully fabricated and show clear interface stacking at the atomic level. Although the BT/ST heterostructure is ostensibly a p–n junction, rectifying behavior is not observed in current (I)–voltage (V) measurements due to the existence of a large number of carriers in both layers. Finally, density functional theory (DFT)‐based simulations suggest that an ideal BT/ST heterostructure may possess intriguing topological properties that can enable novel functional devices. The Bi–Te binary alloys offer promising potential for optimizing PCM performance as well as for the realization of novel functional electronic devices.

Progress in metasurfaces based on Ge–Sb–Te phase-change materials

Recently, metasurfaces based on phase-change materials (PCMs) have attracted increasing attention due to the dramatic optical properties contrast between amorphous and crystalline states. The chalcogenide PCMs can be reversibly switched by electrical or optical pulses, offering tunability and reconfigurability for the metasurfaces. In this Perspective, the latest achievements and ongoing development in reconfigurable metasurfaces based on chalcogenide PCMs are presented, including the applications in nonlinear optics, anapole control, beam steering, perfect absorbers, and polaritons. This Perspective ends with perspectives for the growing demands of PCMs based on metasurfaces.

Anomalous Hall effect in a magnetically extended topological insulator heterostructure

Constructing heterostructures of a topological insulator (TI) with an undoped magnetic insulator (MI) is a clean and versatile approach to break the time-reversible symmetry in the TI surface states. Despite a lot of efforts, the strength of interfacial magnetic proximity effect (MPE) is still too weak to achieve the quantum anomalous Hall effect and many other topological quantum phenomena. Recently, a new approach based on intercalation of atomic layers of MI, referred to as magnetic extension, was proposed to realize strong MPE [2D Mater. 4, 025082(2017)]. Motivated by this proposal, here, we study a magnetic extension system prepared by molecular beam epitaxy growth of MnSe thin films on topological insulator (Bi,Sb)2Te3. Direct evidence is obtained for intercalation of MnSe atomic layer into a few quintuple layers of (Bi,Sb)2Te3, forming either a double magnetic septuple layer (SL) or an isolated single SL at the interface, where one SL denotes a van der Waals building block consisting of B-A-B-Mn-B-A-B (A=Bi1-xSbx, B= Te1-ySey). The two types of interfaces (namely TI/mono-SL and TI/bi-SL) have different MPE, which is manifested as distinctively different transport behaviors. Specifically, the mono-SL induces a spinflip transition with a sharp change at small magnetic field in the anomalous Hall effect of TI layers, while the bi-SL induces a spin-flop transition with a slow change at large field. Our work demonstrates a useful platform to realize the full potential of the magnetic extension approach for pursuing novel topological physics and related device applications.

Leveraging Deep Levels in Narrow Bandgap Bi0.5Sb1.5Te3 for Record‐High zTave Near Room Temperature

AbstractDeep levels in a narrow bandgap semiconductors are considered detrimental to their electrical performance. Here the constructive role of Indium‐induced deep levels in regulating the majority and minority carriers for state‐of‐the‐art average thermoelectric figure‐of‐merit zTave between 300 and 500 K in narrow bandgap p‐type (Bi,Sb)2Te3 is reported. Two compositional series in the pseudo‐ternary Bi2Te3‐Sb2Te3‐In2Te3 phase diagram: Bi0.475−x Sb1.525InxTe3 (0 ≤ x ≤ 0.15) and Bi0.475Sb1.525−yInyTe3 (0 ≤ y ≤ 0.10), namely, the x‐series and y‐series are explored. In the x‐series, the combined experimental and theoretical study shows that Indium doping induced donor‐like and acceptor‐like deep levels, enlarges the band gap, and flattens the conduction band edge, thereby weakening the temperature dependence of Seebeck coefficient and the bipolar heat conduction. Further doping the x‐series with copper (aka shallow acceptors) to optimize the majority carrier concentration leads to a state‐of‐the‐art zT ≈ 1.61 at 390 K and record‐high average zTave ≈ 1.47 between 300 and 500 K in p‐type Bi0.396Sb1.525In0.075Cu0.004Te3. These results attest to the efficacy of deep levels in narrow bandgap thermoelectrics for both power generation and solid‐state refrigeration near room temperature.

Quantum Anomalous Hall Effect in Magnetic Doped Topological Insulators and Ferromagnetic Spin-Gapless Semiconductors-A Perspective Review.

Quantum anomalous Hall effect, with a trademark of dissipationless chiral edge states for electronics/spintronics transport applications, can be realized in materials with large spin-orbit coupling and strong intrinsic magnetization. After Haldane's seminal proposal, several models have been presented to control/enhance the spin-orbit coupling and intrinsic magnetic exchange interaction. After brief introduction of Haldane model for spineless fermions, following three fundamental quantum anomalous Hall models are discussed in this perspective review: i) low-energy effective four band model for magnetic-doped topological insulator (Bi,Sb)2 Te3 thin films, ii) four band tight-binding model for graphene with magnetic adatoms, and iii) two (three) band spinful tight-binding model for ferromagnetic spin-gapless semiconductors with honeycomb (kagome) lattice where ground state is intrinsically ferromagnetic. These models cover 2D Dirac materials hosting spinless, spinful, and spin-degenerate Dirac points where various mass terms open bandgap and lead to quantum anomalous Hall effect. With emphasis on the topological phase transition driven by ferromagnetic exchange interaction and its interplay with spin-orbit-coupling, various symmetry constraints on the nature of mass term and the materialization of these models are discussed. This study will shed light on the fundamental theoretical perspectives of quantum anomalous Hall materials.

Phase change memory based on Ta–Sb–Te alloy –Towards a universal memory

To date, high Reset energy and the contradiction between crystallization speed and amorphous thermal stability are still bottlenecks for phase change memory (PCM) to achieve universal memory. Here, we demonstrate PCM based on Ta0.3Sb2Te3 alloy, showing 3 ns Set speed, 165 °C ten-years data retention and 95% reduced power consumption compared to Ge2Sb2Te5-based device. The Ta exists in the grain boundary and partially enters Sb2Te3 grains, resulting in defects and extremely small grain size, which is responsible for the superior all-round performance. The excellent thermal property rooted in the strong Ta–Te bonds strengthens the robustness of atomic matrices. The ultra-low Reset energy is attributed to the low melting temperature, low thermal conductivity and easier melting of the cubic phase. The ultrafast crystallization originates from fine grains and defects inducing rapid atomic structure change under external electric field. These findings provide a feasible solution for implementing a universal memory.

RETRACTED: Achievement of Gradual Conductance Characteristics Based on Interfacial Phase-Change Memory for Artificial Synapse Applications

In this paper, gradual and symmetrical long-term potentiation (LTP) and long-term depression (LTD) were achieved by applying the optimal electrical pulse condition of the interfacial phase-change memory (iPCM) based on a superlattice (SL) structure fabricated by stacking GeTe/Sb2Te3 alternately to implement an artificial synapse in neuromorphic computing. Furthermore, conventional phase-change random access memory (PCRAM) based on a Ge–Sb–Te (GST) alloy with an identical bottom electrode contact size was fabricated to compare the electrical characteristics. The results showed a reduction in the reset energy consumption of the GeTe/Sb2Te3 (GT/ST) iPCM by more than 69% of the GST alloy for each bottom electrode contact size. Additionally, the GT/ST iPCM achieved gradual conductance tuning and 90.6% symmetry between LTP and LTD with a relatively unsophisticated pulse scheme. Based on the above results, GT/ST iPCM is anticipated to be exploitable as a synaptic device used for brain-inspired computing and to be utilized for next-generation non-volatile memory.

Liquid-State Interfacial Reactions of Sn and Sn-Ag-Cu Solders with p-Type (Bi,Sb)2Te3 Thermoelectric Material

Liquid-state interfacial reactions of p-type (Bi,Sb)2Te3 thermoelectric (TE) material with Sn and Sn-3.0 wt.%Ag-0.5 wt.%Cu (SAC305) solders, respectively, were examined at 250°C on a commercial highly orientated (Bi,Sb)2Te3 substrate with a (110) soldering plane. For the reactions with Sn, the initial reaction phase was the porous SnTe intermetallic compound (IMC). Then, a dense SnTe layer with many tiny cracks formed between the porous phase and TE substrate. With longer aging time, the dense SnTe gradually changed to the porous phase due to Sb dissolution. In the subsequent stage, the reaction phase zone included porous SnTe, an alternating layer microstructure of SnTe and liquid solder, and SnTe/Sn3Sb2 alternating layers from the solder to TE. The IMC growth was extremely fast, being approximately 15 μm/min, and nearly linear with aging time, suggesting reaction control. For the reactions with SAC305, a similar microstructure of porous SnTe and the dense layer was observed. Compared with the reactions with Sn, however, the IMC growth rate was greatly suppressed by ~ 80%. This can be attributed to the Ag-rich phase layer between the dense SnTe and (Bi,Sb)2Te3. Ag was verified to be the key element for dramatically changing the interfacial reaction behavior.

Large non-reciprocal charge transport mediated by quantum anomalous Hall edge states.

The topological nature of the quantum anomalous Hall effect (QAHE) causes a dissipationless chiral edge current at the sample boundary1,2. Of fundamental interest is whether the chirality of the band structure manifests itself in charge transport properties. Here we report the observation of large non-reciprocal charge transport3 in a magnetic topological insulator, Cr-doped (Bi,Sb)2Te3. When the surface massive Dirac band is slightly carrier doped by a gate voltage, the edge state starts to dissipate and exhibits a current-direction-dependent resistance with a directional difference as large as 26%. The polarity of this diode effect depends on the magnetization direction as well as on the carrier type, electrons or holes. The correlation between the non-reciprocal resistance and the Hall resistance indicates that the non-reciprocity originates from the interplay between the chiral edge state and the Dirac surface state.

Structural, vibrational and electronic properties in the glass-crystal transition of thin films Sb70Te30 doped with Sn

Antimony-telluride based phase-change materials doped with Sn have been proposed to be ideal materials for improving the performance of phase-change memories. It is well known that Sb70Te30 thin films show a sharp fall in the electrical resistance in a narrow temperature range when heating. Therefore, it is interesting to study the effect of adding Sn into this composition. In this work, undoped and Sn-doped Sb–Te thin films of composition Snx[Sb0.70Te0.30]100-x, with x = 0.0, 2.5, 5.0 and 7.5 at. %, have been obtained by pulsed laser deposition. Their electrical resistance has been measured while heating from room temperature to 650 K. A sharp fall in the electrical resistance, associated to the glass-crystal transition, has been detected in all the samples within a narrow temperature range. The onset temperature of this transformation increases with the Sn content. Both as-obtained and thermally-treated films have been structurally characterized by X-ray and by Raman spectroscopy. We have compared the results among these compositions in terms of the identified crystallization products, transformation onset temperatures, transformation temperature ranges and amorphous/crystallized electrical resistance ratio. We have found that the frequency of the Raman modes decreases with Sn-doping. Finally, in order to study the electronic structure and to determine the band gap, the frequencies of the allowed Raman modes and the vibration directions of the Sb70Te30 compound, Density Functional Theory based ab initio calculations have been performed as a function of the Sn concentration.

Important role of Cu in suppressing bipolar conduction in Bi-rich (Bi,Sb)2Te3

Cu doping has been known to enhance thermoelectric performance of p-type (Bi,Sb)2Te3 alloys by suppressing bipolar conduction at higher temperatures. However, detailed mechanisms are not provided yet. Herein, we investigate the unchallenged role of Cu doping in (Bi,Sb)2Te3 alloys by means of Bi-rich compositions of Bi0.6Sb1.4Te3 and Bi0.7Sb1.3Te3, which were chosen since they exhibit much inferior thermoelectric performance owing to severe bipolar conduction. It was found that Cu doping increases non-degenerate mobility and density-of-states effective mass of valence band as well as hole concentration. Small amount of Cu doping enhanced maximum thermoelectric figure-of-merit of Bi0.6Sb1.4Te3 composition from 0.25 to 0.98.

Direct observation of amorphous to crystalline phase transitions in Ge–Sb–Te thin films by grazing incidence X-ray diffraction method

Ge–Sb–Te (GST)-based PCM alloys are currently used in optical data storage. The crystallization of GST materials is the rate-limiting step for these devices, hence a deeper knowledge of the crystallization mechanism is crucial for insightful development of faster devices. In the present work, the diffraction patterns for GST-225 thin films are studied using the in situ Grazing Incidence X-ray Diffraction method upon heating. It is shown that initial amorphous film in the temperature range from 120 to 140 °C is crystallized into two phases-cubic GST-225 (Fm$${\bar{3}}$$m), and trigonal GST-147 (P$${\bar{3}}$$m1). The crystallized film is stressed and highly textured, and should be characterized by the value of the lattice parameters averaged over all crystallographic planes. The structural transition of GST-225 from cubic to trigonal phase begins at T > 180 °C. The appearance of large-scale inhomogeneities in GST-225 film at T ≥ 100 °C indicates that the process of rearrangement of atoms Ge, Sb, and Te in the as-deposited amorphous film begins long before the onset of crystallization.

Two-Dimensional Transition-Metal Oxides Mn2O3 Realized the Quantum Anomalous Hall Effect

The quantum anomalous Hall effect is a intriguing topological nontrivial phase arising from spontaneous magnetization and spin-orbit coupling. However, the tremendously harsh realizing requirements of the quantum anomalous Hall effects in magnetic topological insulators of Cr or V-doped (Bi,Sb)2Te3 film, hinder its practical applications. Here, we use first principles calculations to predict that the three Mn2O3 structure is an intrinsic ferromagnetic Chern insulator. Remarkably, a quantum anomalous Hall phase of Chern number C = -2 is found, and there are two corresponding gapless chiral edge states appearing inside the bulk gap. More interestingly, only a small tensile strain is needed to induce the phase transition from Cmm2 and C222 phase to P6/mmm phase. Meanwhile, a topological quantum phase transition between a quantum anomalous Hall phase and a trivial insulating phase can be realize. The combination of these novel properties renders the two-dimensional ferromagnet a promising platform for high effciency electronic and spintronic applications.

High-quality sputter-grown layered chalcogenide films for phase change memory applications and beyond

This paper summarizes recent progress on thin film growth of chalcogenides by sputtering. The materials discussed include Sb–Te, Bi–Te, Ge–Te, and their superlattices, materials that are technologically important particularly for non-volatile phase change memory. In this work, the sputter-growth behavior of high-quality layered chalcogenide films is discussed. Sputtering is one of the most commonly used thin-film growth techniques in the semiconductor industry, however, the complex interrelationship between growth parameters can lead to difficulty in fabricating high-quality films although the deposition method itself is relatively simple. Here, we successfully demonstrate the fabrication of highly-oriented layered chalcogenide materials by sputtering. The selection of the appropriate sputtering target is important. In particular, it was found that a Te-rich Sb–Te alloy target such as Sb33Te67 is necessary in order to obtain a stoichiometric Sb2Te3 film. Moreover, the growth temperature is also a key factor in obtaining a highly-oriented film, namely the ideal growth temperature for an Sb2Te3 film is between 230 °C and 250 °C after the growth of an amorphous seed layer at room temperature. Furthermore, it was found that this technique is also useful to grow films epitaxially on Al2O3 or Si(111) substrates even though there are some misoriented grains as well as twins present. Finally, we demonstrate the growth of highly-oriented Sb2Te3 films on a flexible substrate. The versatility of sputtering will become technologically more and more important for the various applications represented by phase change memory.

Concomitance of superconducting spin–orbit scattering length and normal state spin diffusion length in W on (Bi,Sb)2Te3

We report the observation of an anomalously large in-plane upper critical field, exceeding at least 2.5 times the Pauli paramagnetic limit, in a thin superconducting W film grown on a topological insulator (Bi,Sb)2Te3. This can be accounted for by setting the spin–orbit scattering length of superconducting W to a value ranging from 1 to 2 nm, which is comparable to the spin diffusion length of normal state W. The coupling between the topological surface states of (Bi,Sb)2Te3 and the wave functions of superconducting W may also contribute to the observed giant critical field. Our results suggest the universality of the spin–orbit scattering formalism for describing the transport involving the diffusive carriers as well as the Cooper pairs in systems with strong spin–orbit coupling.

Room temperature Bi2Te3-based thermoelectric materials with high performance

Several off-stoichiometric compositions Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) were deliberately synthesized to produce in-situ composites based on compositional engineering approach. The structural characterization of these materials employing XRD, SEM, and HR-TEM reveals the formation of in-situ-composites containing Bi0.5Sb1.5Te3 as matrix phase and minor phases of either Sb rich or Te rich in different compositions. Thermoelectric properties of these Bi0.5Sb1.5+xTe3+δ (x = 0.2; δ = 0, 0.12, 0.14) composites were studied in a wide range of temperatures extending from room temperature to 500 K. The electronic transport of these composites exhibits p-type semiconducting materials. The lowest thermal conductivity of ~ 0.69 W/m K @310 K was observed for Bi0.5Sb1.7Te3.12 composite, which was noted to be 14% reduced thermal conductivity when compared with that of the state-of-the-art Bi0.5Sb1.5Te3 (κ$$=$$ 0.82 W/m K) material. In addition to this, an enhanced power factor was also observed in Bi0.5Sb1.7Te3.12 which is primarily due to increased electrical conductivity of these materials. This enhanced power factor of the composition of Bi0.5Sb1.7Te3.12 coupled with reduced thermal conductivity yields to high ZT ~ 1.13 at nearly room temperature, making these materials viable for large scale applications.

Phase dependence of Schottky barrier heights for Ge–Sb–Te and related phase-change materials

The large difference of dielectric functions between the amorphous and crystalline phases of Ge–Sb–Te based phase-change materials (PCMs) used in memory storage devices also affects their Schottky barrier heights (SBHs) and thus their electrical device properties. Here, the SBHs of each phase of Ge2Sb2Te5, GeTe, GeSe, and SnTe are found by density functional supercell calculations. The Fermi level pinning factor S calculated for the crystalline phases (with a larger dielectric constant) is smaller than their amorphous phases, agreeing well with the empirical relationship linking SBH to a dielectric constant. The relatively large dielectric constant of crystalline PCMs arises from their resonant bonding (metavalent bonding), but their pinning factor is not always as small as empirically expected. The results are useful for optimizing the design of metal contacts for Ge–Sb–Te type phase-change memory devices.

Effects of the Ni electrodeposit on microstructure evolution and electrical resistance of the P-type Bi2Te3 solder joint

Abstract A systematic investigation was carried out to evaluate the effect of Ni electrodeposit on the microstructure evolution and electrical resistance of the P-type Bi2Te3-based thermoelectric (TE) joint. In this study, two systems (SAC305/(Bi,Sb)2Te3 and SAC305/Ni/(Bi,Sb)2Te3) were set up. The intermetallic compounds (IMCs) were mainly detected by using scanning electron microscope (SEM), X-ray diffractometer (XRD) and transmission electron microscope (TEM). A model of overlapping joint was set up for electrical resistance testing. Results indicated that the SnTe phase was the major and thermodynamically stable product in the SAC305/(Bi,Sb)2Te3 system, and its porous structure determined the presence of micro-tunnels, which facilitated the diffusion process of Sn and Te atoms, therefore the SnTe layer continuously grew in the whole reflowing duration. In this system, the time exponent was analyzed to be 1.2741, indicating that the growth of the IMC layer followed the linear law, and was reaction-controlled. In the SAC305/Ni/(Bi,Sb)2Te3 system, it was remarkable that the Ni barrier inhibited the rapid growth of the IMC layer, and the dominated product was detected to be (Ni,Cu)3Sn4 whose dense distribution could effectively suppress the diffusion of Sn atoms. The time exponent in this system was 0.5010, indicating that the growth of the Ni–Cu–Sn ternary IMC followed the parabolic laws and was mainly volume diffusion-controlled. After reflowing treatment, the electrical resistance remained at 2.203 mΩ for NiTL (the TE leg with the Ni electrodeposit) system while that of TL (the TE leg without the Ni electrodeposit) system rapidly increased to a higher value about 4.879 mΩ. Owing to the lower growth rate of the IMC layer in NiTL system, it could reasonably conclude that the Ni electrodeposit was of great importance in depressing the electrical resistance.

Practical High-Performance (Bi,Sb)2Te3-Based Thermoelectric Nanocomposites Fabricated by Nanoparticle Mixing and Scrap Recycling.

Bi2Te3-based compounds are the most mature and widely used thermoelectric materials. However, industrial device fabrication will inevitably produce a lot of Bi2Te3 scraps, which results in wastes of expensive material resources. In this work, we recycled p-type (Bi,Sb)2Te3 scraps and reprocessed them by making nanocomposites with nano-SiC. The thermoelectric performance was enhanced, and a high ZT value of 1.07 was achieved, which is a significant improvement compared with commercial p-type (Bi,Sb)2Te3 ingots. Also, the hardness showed a notable increase, which is beneficial for device fabrication. In addition, we adjusted the proportion of Bi/Te of the commercial p-type (Bi,Sb)2Te3 scraps, thereby improving the thermoelectric performance and obtaining a higher ZT value of 1.2.