Sb2Te3 Films Research Articles

C–N-codoped Sb2Te3 chalcogenides for reducing writing current of phase-change devices

In this work, doping C and codoping C and N into the Sb2Te3 traditional chalcogenide were investigated to reduce the writing current of the phase-change device using a chalcogenide as the active medium. No face-centered-cubic (FCC) structure was observed in the C-doped Sb2Te3 film, while it appeared after codoping C and N into Sb2Te3. The FCC crystallite size greatly reduced from 6.5 to 3.5–3.8 nm after codoping. In particular, the resistivity of FCC C–N codoped Sb2Te3 was about two orders of magnitude higher than that of Sb2Te3. The effect of the property of the chalcogenide on the writing current of the phase-change device was analyzed by the finite element method. The analysis showed that the writing current of the device using C–N-codoped Sb2Te3 as the active medium can significantly drop to about 1/8 of that of the Sb2Te3 based one.

Differences in Sb2Te3 growth by pulsed laser and sputter deposition

High quality van der Waals chalcogenides are important for phase change data storage, thermoelectrics, and spintronics. Using a combination of statistical design of experiments and density functional theory, we clarify how out-of-equilibrium van der Waals epitaxial deposition methods can improve the crystal quality of Sb2Te3 films. We compare films grown by radio frequency sputtering and pulsed laser deposition (PLD). The growth factors that influence the crystal quality for each method are different. For PLD grown films a thin amorphous Sb2Te3 seed layer most significantly influences the crystal quality. In contrast, the crystalline quality of films grown by sputtering is rather sensitive to the deposition temperature and less affected by the presence of a seed layer. This difference is somewhat surprising as both methods are out-of-thermal-equilibrium plasma-based methods. Non-adiabatic quantum molecular dynamics simulations show that this difference originates from the density of excited atoms in the plasma. The PLD plasma is more intense and with higher energy than that used in sputtering, and this increases the electronic temperature of the deposited atoms, which concomitantly increases the adatom diffusion lengths in PLD. In contrast, the adatom diffusivity is dominated by the thermal temperature for sputter grown films. These results explain the wide range of Sb2Te3 and superlattice crystal qualities observed in the literature. These results indicate that, contrary to popular belief, plasma-based deposition methods are suitable for growing high quality crystalline chalcogenides.

Growth mechanisms and related thermoelectric properties of innovative hybrid networks fabricated by direct deposition of Bi2Se3 and Sb2Te3 on multiwalled carbon nanotubes

Flexible thermoelectric generators are an emerging trend in the field of waste heat conversion, as well as wearable and autonomous devices. However, the energy conversion efficiency of the state-of-the-art flexible thermoelectric devices is too low for their wide application and commercialization. In this work, n- and p-type multiwalled carbon nanotube (MWCNT)-thermoelectric material hybrid networks that may become a promising building block for the fabrication of flexible thermoelectric devices are presented. The hybrid networks were fabricated by direct deposition of thermoelectric material (Bi2Se3, Sb2Te3) on the MWCNT networks using physical vapor deposition technique. Growth mechanisms of Bi2Se3 and Sb2Te3 on MWCNTs were investigated. The Seebeck coefficient and charge transport properties of MWCNT-Bi2Se3 and MWCNT-Sb2Te3 hybrid networks were studied as function of MWCNT wt% in the networks. Variable-range hopping models were applied for the interpretation of conductance mechanisms in the hybrid networks. The Seebeck coefficients of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with low MWCNT wt% were found to be comparable with the Seebeck coefficients of pure inorganic Sb2Te3 and Bi2Se3 thin films. At the same time, flexibility tests of the MWCNT-Sb2Te3 and MWCNT-Bi2Se3 hybrid networks with MWCNT 50% showed no significant increase in the resistance when they were bent to a radius of 5 mm. This makes hybrid networks, presented in this work, the perspective for applications in flexible thermoelectrics as thermoelectric coatings for flexible substrates and fillers for polymers-based composites.

Fabrication of PEDOT: PSS-PVP Nanofiber-Embedded Sb2Te3 Thermoelectric Films by Multi-Step Coating and Their Improved Thermoelectric Properties.

Antimony telluride thin films display intrinsic thermoelectric properties at room temperature, although their Seebeck coefficients and electrical conductivities may be unsatisfactory. To address these issues, we designed composite films containing upper and lower Sb2Te3 layers encasing conductive poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)- polyvinylpyrrolidone(PVP) nanowires. Thermoelectric Sb2Te3/PEDOT:PSS-PVP/Sb2Te3(ED) (STPPST) hybrid composite films were prepared by a multi-step coating process involving sputtering, electrospinning, and electrodeposition stages. The STPPST hybrid composites were characterized by field-emission scanning electron microscopy, X-ray diffraction, ultraviolet photoelectron spectroscopy, and infrared spectroscopy. The thermoelectric performance of the prepared STPPST hybrid composites, evaluated in terms of the power factor, electrical conductivity and Seebeck coefficient, demonstrated enhanced thermoelectric efficiency over a reference Sb2Te3 film. The performance of the composite Sb2Te3/PEDOT:PSS-PVP/Sb2Te3 film was greatly enhanced, with σ = 365 S/cm, S = 124 μV/K, and a power factor 563 μW/mK.

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.

Approaching high-performance of ordered structure Sb2Te3 film via unique angular intraplanar grain boundaries

In this paper, we present an innovative electric-field-assisted magnetron-sputtering deposition method for films preparation. By grain boundary-engineering, we successeful obtained the ordered Sb2Te3 film with greatly high figure of merit via controlling external electric field. It has been found that the electric field can induce the change in the angle of intraplanar grain boundaries between (0 1 5) and (0 1 5) planes, which leads to the enhanced holes mobility and maintained low thermal conductivity. The energy filtering takes place at the angular intraplanar grain boundaries. At room temperature, a high ZT value of 1.75 can be achieved in the deposited Sb2Te3 film under 30 V external electric field. This is a very promising approach that the electric field induced deposition can develop high-performance Sb2Te3-based thermoelectric films.

Pressure effects in RF and DC sputtered Sb2Te3 thin films and its applications into solar cells

In this work we developed the synthesis and characterization of Sb2Te3 thin films, which were grown by RF as well as DC sputtering as a function of the deposition pressure (Pd), 5–15 mTorr. Sb2Te3 films were characterized through X-Ray Diffraction, Energy Dispersive Spectrometry (EDS), Scanning Electron Microscopy (SEM) and electrical resistivity measurements. As a part, contact layer, of the CdTe-based Solar Cell, Current-Voltage (I–V) as well as External Quantum Efficiency (EQE) measurements were carried out. Our results indicate that the radio-frequency sputtered Sb2Te3 are polycrystalline with predominant rhombohedral crystalline structure, whereas the DC-sputtered films crystallized mainly in the monoclinical structure. Both set of samples showed a resistivity of the order of 10−4 Ω-cm. Concerning the CdTe-based solar cell, the incorporation of the Sb2Te3 as a back surface film in the back contact improves the solar cell efficiency up to 8.01%, 10 mTorr pressures into the growth chamber, as compared to the CdTe-based solar cell, 4.82% efficiency, without the Sb2Te3 layer.

Enhancing the thermoelectric properties of sputtered Sb2Te3 thick films via post-annealing treatment

Sb2Te3 films of more than 10 μm in thickness were deposited on flexible polyimide substrates by heat treatment-assisted DC magnetron sputtering. The post-annealing parameters including the temperature (150–350 °C) and time (15–60 min) were varied to investigate the microstructure, chemical composition, porosity and thermoelectric properties of the thick films. X-ray diffraction showed that both the as-deposited and post-annealed films were polycrystalline with significant preferential growth along the (015) plane. The films showed slightly off-stoichiometric compositions after post-annealing treatment. Increasing the annealing temperature and annealing time led to an increase in crystalline size and a decrease in porosity of the thick films. This was related to grain growth, agglomeration and surface improvement. The electrical transport and thermoelectric properties including carrier concentration, carrier mobility, electrical conductivity and Seebeck coefficient were investigated using Hall effect measurements and a ZEM-3 apparatus. A maximum power factor of 1.7 mW/K2m was obtained following annealing at 350 °C for 30 min.

Growth mechanism of highly oriented layered Sb2Te3 thin films on various materials

Sb2Te3 is a layered material with outstanding properties leading to applications in interfacial phase-change memories, spintronic and thermoelectric devices. For successful integration in devices, controlling the orientation of the atomic planes of Sb2Te3 deposited by sputtering on various materials used for electrodes and on dielectric layers is required. We have succeeded in depositing Sb2Te3 thin films (thickness in the range 10–100 nm) by sputtering in industrial deposition equipments on WSi, TiN, amorphous Si as well as on native and thermal silicon oxide layers. The structure and orientation of the films were studied by x-ray diffraction. The Sb and Te planes are found parallel to the substrate, whatever the nature of the bottom material, provided that the sputtering conditions avoid a Te deficiency in the deposited film. These results show that deposition of Sb2Te3 with out-of-plane orientation on silicon oxide is actually possible, in contrast with previous literature results. Scanning transmission electron microscopy images of the interface between the Sb2Te3 film and the bottom material allow to elucidate the growth mechanism. The formation of a surface layer containing a few Te planes on top of the bottom material is mandatory for the subsequent growth of an out-of-plane oriented Sb2Te3 film by van der Waals epitaxy.

ALD growth of ultra-thin Co layers on the topological insulator Sb2Te3

Taking the full advantage of the conformal growth characterizing atomic layer deposition (ALD), the possibility to grow Co thin films, with thickness from several tens down to few nanometers on top of a granular topological insulator (TI) Sb2Te3 film, exhibiting a quite high surface roughness (2–5 nm), was demonstrated. To study the Co growth on the Sb2Te3 substrate, we performed simultaneous Co depositions also on sputtered Pt substrates for comparison. We conducted a thorough chemical-structural characterization of the Co/Sb2Te3 and Co/Pt heterostructures, confirming for both cases, not only an excellent conformality, but also the structural continuity of the Co layers. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) analyses evidenced that Co on Sb2Te3 grows preferentially oriented along the [00l] direction, following the underlying rhombohedric substrate. Differently, Co crystallizes in a cubic phase oriented along the [111] direction when deposited on Pt. This work shows that, in case of deposition of crystalline materials, the ALD surface selectivity and conformality can be extended to the definition of local epitaxy, where in-plane ordering of the crystal structure and mosaicity of the developed crystallized grains are dictated by the underlying substrate. Moreover, a highly sharp and chemically-pure Co/Sb2Te3 interface was evidenced, which is promising for the application of this growth process for spintronics.

Influence of Sputtering Power Density on the Thermoelectric and Mechanical Properties of Flexible Thermoelectric Antimony Telluride Films Deposited by DC Magnetron Sputtering

Antimony telluride (Sb2Te3) films were deposited on flexible polyimide substrates by DC magnetron sputtering technique using a 99.9% alloy Sb2Te3 target. We measured structural, electrical, thermoelectric and mechanical properties with sputtering power density in the range 30–50 W. X-ray diffraction confirmed that all Sb2Te3 films have high crystallinity with a significant preferential growth along the (015) plane. Surface morphologies were verified by scanning electron microscope: deposited film grain size increased with sputtering power density. The elemental composition was determined by energy dispersive x-ray spectroscopy. Electrical transport properties, carrier concentration, was measured by Hall effect measurement at room temperature. Electrical conductivity and Seebeck coefficient were simultaneously measured by a DC four-terminal method (ZEM-3). The power factor was strongly dominated by electrical conductivity, leading to a maximum of 1.95 mW/K2m with sputtering power 45 W at 300°C. The wettability test, based on the contact angle, evaluated surface energy and hydrophilicity. Nanoindentation was measured on a NHT2 CSM Instrument with diamond Berkovich indenter (B-P 31) at room temperature. The hardness and elastic modulus of deposited Sb2Te3 films increased with the power density.

High-Performance μ-Thermoelectric Device Based on Bi2Te3/Sb2Te3 p-n Junctions.

A flexible and ultralight planar thermoelectric generator based on 15 thermocouples composed of n-type bismuth telluride (Bi2Te3) and p-type antimony telluride (Sb2Te3) legs (each with 400 nm thick) connected in series, on 25 μm thick Kapton substrate, was fabricated with impressive power factor values of 2.7 and 0.8 mW K-2 m-1 (at 298 K) for Bi2Te3 and Sb2Te3 films, respectively. The p-n junction thermoelectric device can generate a maximum open-circuit voltage and output power of 210 mV and 0.7 μW (3.3 mW cm-2), respectively, for a temperature difference of 35 K, which is higher than the one observed for a conventional thermoelectric device with metallic contacts for p-n junctions. The results were combined with numerical simulations, showing a good match between the experimental and the numerical results. The current density versus voltage (J-V) characteristics of the fabricated p-n junctions revealed a diode behavior with a turn-on voltage of ≈0.3 V and an impressive rectifying ratio (I+1V/I-1V) of ≈2 × 104.

Effect of Thermoelectric Leg Thickness in a Planar Thin Film TEC Device on Different Substrates

Recently, mobile application processors (APs) have suffered from thermal issues such as local hot spot generation. Several approaches for chip cooling, such as dynamic thermal management, and heat pipe cooling, have been attempted so far, but, these solutions cannot completely eliminate increasing thermal issues. Therefore, in this study, we fabricated a planar type of thin film thermoelectric cooler (TEC) as an active cooling device for a mobile AP chip. We studied the effect of thickness on a planar thin film TEC device related to Joule heating and demonstrated the Peltier cooling effect on polyimide (PI) and Si substrates. The optimal thicknesses of n-type Bi2Te3 and p-type Sb2Te3 films were evaluated by ANSYS® simulation, and are 5.05 μm and 5.45 μm, respectively. It was shown that heat moves to the TE leg on the PI substrate, while the Si substrate serves as a heat sink according to the IR thermography analysis. The optimal thickness of the TE showed a temperature difference between the cold junction and hot junction up to 1.3 °C.

Cr2Te3 Thin Films for Integration in Magnetic Topological Insulator Heterostructures

Chromium telluride compounds are promising ferromagnets for proximity coupling to magnetic topological insulators (MTIs) of the Cr-doped (Bi,Sb)2(Se,Te)3 class of materials as they share the same elements, thus simplifying thin film growth, as well as due to their compatible crystal structure. Recently, it has been demonstrated that high quality (001)-oriented Cr2Te3 thin films with perpendicular magnetic anisotropy can be grown on c-plane sapphire substrate. Here, we present a magnetic and soft x-ray absorption spectroscopy study of the chemical and magnetic properties of Cr2Te3 thin films. X-ray magnetic circular dichroism (XMCD) measured at the Cr L2,3 edges gives information about the local electronic and magnetic structure of the Cr ions. We further demonstrate the overgrowth of Cr2Te3 (001) thin films by high-quality Cr-doped Sb2Te3 films. The magnetic properties of the layers have been characterized and our results provide a starting point for refining the physical models of the complex magnetic ordering in Cr2Te3 thin films, and their integration into advanced MTI heterostructures for quantum device applications.

Drastic increase in thermoelectric power factor of mixed Sb2Te3-In2Te3 thin films

Thermoelectric power factor is an indicator of the performance of a thermoelectric material. Attempts have been made by various techniques, like doping, to improve the thermoelectric conversion efficiency of materials. In the present study, a layer structured thermoelectric material Sb2Te3 is alloyed to In2Te3 using vacuum deposition method at 423 K to significantly enhance the power factor of ∼118 μWm-1K−2 (at 450 K). Structurally, all films were polycrystalline in nature as clearly reflected in XRD patterns. All films were showing p-type conductivity, and electrical conductivity of In2Te3 films increased with increasing Sb2Te3 content. The seebeck coefficient is found to be higher for un-doped In2Te3 than that of Sb2Te3-In2Te3 and pure Sb2Te3 films. However, the thermoelectric power factor of 25% Sb2Te3 alloyed In2Te3 films is enhanced by 11.9 times that of In2Te3 films and 4 times that of Sb2Te3 films at 320 K. It is interesting to note that efficiency of the mixed films is higher than that of the individual films.

Effect of real working environment/formation of oxide phase on thermoelectric properties of flexible Sb2Te3 films

Abstract Flexible Sb2Te3 thin films, for thermoelectric generator applications, were deposited by DC magnetron sputtering. As-deposited films were annealed in air to simulated a realistic operating environment. The oxidation behavior of the films was studied by monitoring their phase change on exposure to air at different temperatures between 50 and 300 °C for annealing times from 1 to 15 h. Oxidation of Sb and Te formed Sb2Te4 and TeO2 phases when annealing above 100 °C and Sb2Te3 decomposed into oxide phases at an annealing temperature of 250 °C for 15 h. The thermoelectric performance decreased as the content of Sb2O4 and TeO2 phases increased. These findings show the limitations of Sb2Te3 films operating in air without vacuum or a protective environment. We propose that the kinetic growth of oxide formation on the Sb2Te3 thin films depend on chemical activation energy and oxygen diffusion through the oxide barrier by the variation of annealing temperature and annealing time, respectively.

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.

Surface-tensile-stress induced polishing-voids in cross-point phase-change-memory cells: corrosion mechanism and solution

In the fabrication of cross-point phase-change-memory-cells through atomic-layer-deposition of Ge-doped SbTe (Ge-ST) film followed by chemical-mechanical-polarization (CMP), a remarkable surface tensile stress was generated, originating from the surface stress induced by the pad down force and the surface structure tensile stress in a confined memory-cell structure. It was maximized at the position of the Si3N4-film spacer where the curvature becomes zero. The maximized surface tensile stress produced polishing induced voids via the generation mechanism of the stress corrosion cracking. The generation frequency and nanoscale size of the polishing induced voids rapidly increased at the maximum surface tensile stress during CMP. Then, they slightly decreased and saturated when the surface tensile stress reduced during further CMP. A design of a spacer using a SiO2-film spacer rather a Si3N4-film spacer could prevent the generation of the polishing induced voids, confirming that the generation mechanism of the polishing induced voids is associated with the stress corrosion cracking.

Characterization of commercial thermoelectric organic composite thin films

A series traditional thermoelectric films, including n-type Bi, Bi2Te3, and p-type Sb, Sb2Te3, were synthesized with CH3NH3I by combining magnetron sputtering and thermal evaporation method. It is found that the Seebeck coefficients for all the composite films have enhanced although the electrical conductivities reduced due to the decreased carrier concentration. This leads to an obvious improvement of the power factor, especially for p-type Sb2Te3-based organic–inorganic hybrid films. As expected, a maximum power factor value around 2.3 mWm−1K−2 has been achieved for the composition of Sb2Te3/CH3NH3I in range of the whole measured temperature, which is nearly seven times higher than that of pristine Sb2Te3 film. By combination with the reduced thermal conductivity, a ZT value nearly 1.0 has been obtained at 380 K for the composition of Sb2Te3/CH3NH3I.