In-situ monitoring of the growth of Bi2 Te2 and Sb2 Te3 films and Bi2 Te3-Sb2 Te3 superlattice using spectroscopic ellipsometry

In this work, we present in-situ monitoring of the growth of bismuth telluride (Bi₂Te₃) and antimony telluride (Sb₂Te₃) thin films as well as Bi₂ Te₃-Sb₂Te₃ superlattice using a spectroscopic ellipsometer (SE). Bi₂Te₃ and Sb₂ Te₃ films were grown by metalorganic chemical vapor deposition (MOCVD) at 350 C. A44-wavelength ellipsometer with spectral range from 404 nm to 740 nm was used in this work. The optical constants of Bi₂ Te₃ and Sb₂Te₃ at growth temperature were determined by fitting a model to the extracted in-situ SE data of optically thick Bi₂ Te₃ and Sb₂ Te₃ films. Compared to the optical constants of Bi₂ Te₃ and Sb₂ Te₃ at room temperature, significant temperature dependence was observed. Using their optical constants at growth temperature, the in-situ growth of Bi₂ Te₃ and Sb₂ Te₃ thin films were modeled and excellent fit between the experimental data and data generated from the best-fit model was obtained. In-situ growth of different Bi₂ Te₃-Sb₂ Te₃ superlattices was also monitored and modeled. The growth of Bi₂ Te₃ and Sb₂ Te₃ layers can be seen clearly in in-situ SE data. Modeling of in-situ superlattice growth shows perfect superlattice growth with an abrupt interface between the two constituent films.     

Solution-Chemical Syntheses of Nano-Structured Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> and PbTe Thermoelectric Materials

In this work, nano-structured Bi₂Te₃ and PbTe thermoelectric materials were synthesized separately via solvothermal, hydrothermal and low-temperature aqueous chemical routes. X-ray diffraction (XRD), field-emission scanning-electron microscopy (FESEM), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) were used to analyze the powder products. Results showed that the as-prepared Bi₂Te₃ samples were all single-phased and consisted of irregular spherical granules with diameters of ∼30 nm whereas the PbTe samples were mainly composed of well-crystallized cubic crystals with average size of approximately 100 nm. Some nanotubes and nanorods were found in Bi₂Te₃ and PbTe samples, respectively; these were identified as Bi₂Te₃ nanotubes and PbTe nanorods by EDS analysis. Possible reaction mechanisms for these syntheses are discussed in detail herein.     

Fabrication of Microstructured thermoelectric Bi2Te3 thin films by seed layer assisted electrodeposition

In this work, bismuth telluride (Bi₂Te₃) thin films have been fabricated on Bi₂Te₃/ITO substrates by constant potential electrochemical deposition at room temperature. Bi₂Te₃ seed layers with different thicknesses (2 nm, 4 nm and 6 nm) were deposited onto ITO substrates using molecular beam epitaxy (MBE) method. The SEM images show that the morphology of Bi₂Te₃ thin films can be controlled not only by the deposition potential, but also the thickness of seed layer. Moreover, the morphologies of Bi₂Te₃ thin films with different thickness of seed layers tend to be similar and contain two-layer structure along the vertical direction after prolonged deposition time. Due to the two layers structure, Bi₂Te₃ thin films have shown different electrical conductivity performances. At room temperature, Bi₂Te₃ thin films with 4 nm-thick seed layer possess the maximum electrical conductivity value of 617.9 s cm^-1.     

Characteristics of chalcogenide nonvolatile memory nano-cell-element based on Sb2Te3 material

Phase-change nonvolatile memory cell elements composed of Sb₂Te₃ chalcogenide have been fabricated by using the focused ion beam method. The contact size between the Sb₂Te₃ phase change film and electrode film in the cell element is 2826 nm² (diameter: 60 nm). The thickness of the Sb₂Te₃ chalcogenide film is 40 nm. The threshold switching current of about 0.1 mA was obtained. A RESET pulse width as short as 5 ns and the SET pulse width as short as 22 ns for Sb₂Te₃ chalcogenide can be obtained. At least 1000 cycle times with a RESET/SET resistance ratio >30 times is achieved for Sb₂Te₃ chalcogenide C-RAM cell element.     

Effects of annealing on the thermoelectric and microstructural properties of deformed n-type Bi2Te3-based compounds

We studied the effects of deformation and annealing of n-type 90Bi₂Te₃-5Sb₂Te₃-5Sb₂Se₃ thermoelectric compound. Hot-extrusion was conducted to prepare the deformed compound and then this compound was annealed at 400°C for 1–24 hr. When the undoped cast-ingot was extruded, the compound was changed from p-type to n-type due to the electrons generated during the extrusion process. For the compound extruded with SbI₃-doped powders, the thermoelectric properties were also varied for the extrusion process. After annealing at 400°C more than 9 hr, the powder-extruded compound was recrystallized. This caused a decrease in carrier concentration and crystallographic anisotropy. In case of the compound extruded at the ratio of 10:1, the Seebeck coefficient α and the electrical resistivity ρ increased due to recrystallization. However, thermal conductivity κ of the compound decreased. This resulted in an increase in the figure-of-merit from 1.23 × 10⁻³ to 1.63 × 10⁻³ K⁻¹.     

Thermal stress effects of Ge2Sb2Te5 phase change material: Irreversible modification with Ti adhesion layers and segregation of Te

This paper reviews material properties of chalcogenide phase change material Ge₂Sb₂Te₅ under thermal anneal treatments. Stress evolutions of pure Ge₂Sb₂Te₅ films and stacks of Ge₂Sb₂Te₅ integrating with Ti adhesion layers are investigated. Segregation of Te atoms in the Ge₂Sb₂Te₅ film to the interface drives an interaction between Ti and Te atoms and formation of Ti-Te binary phases. The irreversible phase segregation and modification of Ge₂Sb₂Te₅ change the crystallization process, completely suppress the final transformation into otherwise stable hcp phase, and thus impact the ultimate life-cycle of such a phase change based memory cell. Since the adhesion layer is required in cell applications, the optimization of adhesion layer material and thickness may improve the life-cycles and reliability of devices.     

Sb2Te3 and Bi2Te3 Thin Films Grown by Room-Temperature MBE

Sb₂Te₃ and Bi₂Te₃ thin films were grown on SiO₂ and BaF₂ substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2?nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2?h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500?nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb₂Te₃ films at room temperature, i.e., low charge carrier density (2.6?×?10¹⁹?cm^?3), large thermopower (130???V?K^?1), large charge carrier mobility (402?cm²?V^?1?s^?1), and resulting large power factor (29???W?cm^?1?K^?2). Bi₂Te₃ films also showed low charge carrier density (2.7?×?10¹⁹?cm^?3), moderate thermopower (?153???V?K^?1), but very low charge carrier mobility (80?cm²?V^?1?s^?1), yielding low power factor (8???W?cm^?1?K^?2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM.     

The effect of Tl2Te3 on the properties of the solid solution (Sb2Te3)0.75.(Bi2Te3)0.25

The effect on transport properties of the addition of 0.5-5% Tl₂Te₃ to p-type solid solutions of antimony and bismuth tellurides was studied. It was found that the addition of Tl₂Te₃ caused a lessening of the increase of hole concentration as low temperatures were approached, resulting in a slower decrease of the Seebeck coefficient with a decrease in temperature. In partial fulfillment of M.Sc. degree, Hebrew University, Jerusalem. Permanent address, Dept. of Inorganic and Analytical Chemistry, Hebrew University, Jerusalem.     

Nanostructure, Excitations, and Thermoelectric Properties of Bi2Te3-Based Nanomaterials

The effect of dimensionality and nanostructure on thermoelectric properties in Bi₂Te₃-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi₂Te₃ nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al₂O₃ matrix, yielding transport in the basal plane. Polycrystalline, textured Sb₂Te₃ and Bi₂Te₃ thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb₂Te₃ films revealed low charge carrier density of 2.6?×?10¹⁹?cm^?3, large thermopower of 130???V?K^?1, and large charge carrier mobility of 402?cm²?V^?1?s^?1. Bi₂(Te_0.91Se_0.09)₃ and (Bi_0.26Sb_0.74)₂Te₃ nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50?nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab?initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi₂Te₃ nanomaterials are discussed.     

Lorenz Function of Bi2Te3/Sb2Te3 Superlattices

Combining first-principles density functional theory and semiclassical Boltzmann transport, the anisotropic Lorenz function was studied for thermoelectric Bi₂Te₃/Sb₂Te₃ superlattices and their bulk constituents. It was found that, already for the bulk materials Bi₂Te₃ and Sb₂Te₃, the Lorenz function is not a clear function of charge carrier concentration and temperature. For electron-doped Bi₂Te₃/Sb₂Te₃ superlattices, large oscillatory deviations of the Lorenz function from the metallic limit were found even at high charge carrier concentrations. The latter can be referred to quantum well effects, which occur at distinct superlattice periods.     

Improvement of Thermoelectric Properties in (Bi0.5Sb0.5)2Te3 Films of Nanolayered Pillar Arrays

In this work, it is found that unique pillar arrays with nanolayered structure can favorably influence the carrier and phonon transport properties of films. p-(Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) orientation was successfully achieved by a simple ion-beam-assisted technique at deposition temperature of 400°C, owing to the enhanced mobility of deposited atoms for more sufficient growth along the in-plane direction. The pillar diameter was about 250 nm, and the layered nanostructure was clear, with each layer in the pillar array being <30 nm. The properties of the oriented (Bi_0.5Sb_0.5)₂Te₃ pillar array were greatly enhanced in comparison with those of ordinary polycrystalline films synthesized at deposition temperature of 350°C and 250°C. The (Bi_0.5Sb_0.5)₂Te₃ pillar array film with (0 1 5) preferred orientation exhibited a thermoelectric dimensionless figure of merit of ZT = 1.25 at room temperature. The unique pillar array with nanolayered structure is the main reason for the observed improvement in the properties of the (Bi_0.5Sb_0.5)₂Te₃ film.     

用于非易失性相变存储器中Si2Sb2Te5在CF4/Ar等离子体下的反应离子刻蚀

Phase change random access memory（PCRAM） is one of the best candidates for next generation nonvolatile memory,and phase change Si₂Sb₂Te₅ material is expected to be a promising material for PCRAM.In the fabrication of phase change random access memories,the etching process is a critical step.In this paper,the etching characteristics of Si₂Sb₂Te₅ films were studied with a CF₄/Ar gas mixture using a reactive ion etching system.We observed a monotonic decrease in etch rate with decreasing CF4 concentration,meanwhile,Ar concentration went up and smoother etched surfaces were obtained.It proves that CF4 determines the etch rate while Ar plays an important role in defining the smoothness of the etched surface and sidewall edge acuity.Compared with Ge₂Sb₂Te₅, it is found that Si₂Sb₂Te₅ has a greater etch rate.Etching characteristics of Si₂Sb₂Te₅ as a function of power and pressure were also studied.The smoothest surfaces and most vertical sidewalls were achieved using a CF₄/Ar gas mixture ratio of 10/40,a background pressure of 40 mTorr,and power of 200 W.     

Patterning of Ge2Sb2Te5 phase change material using UV nano-imprint lithography

DRAM is the most commonly used memory due to many advantages such as high speed and easy manufacturability owing to its simple structure, but is volatile. On the other hand, flash memory is non-volatile, but has other disadvantages such as slow speed, short lifetime, and low endurance for repetitive data writing. Compared to DRAM and flash memory, PRAM (Phase-change Random Access Memory), which is a non-volatile memory using a reversible phase change between amorphous and crystalline state, has many advantages such as high speed, high sensing margin, low operating voltage, and is being pursed as a next generation memory. Being able to pattern and etch phase change memory in nanometer scale is essential for the integration of PRAM. This study uses the Nano-Imprint Lithography (NIL) for patterning the PRAM in nanometer scale which is believed to be a future lithography technology that will replace the conventional Photo Lithography. Si wafers coated with SiO₂ were used as substrates, and Ge₂Sb₂Te₅ (GST) films with the thicknesses of 100 nm were deposited by RF sputtering. Poly-benzylmethacrylate based polymer patterns were formed using NIL on the surface of GST films, and the GST films were etched using Cl₂/Ar plasma in an Oxford ICP (inductively coupled plasma) etcher.     

Sputtered p-Type Sb2Te3/(Bi,Sb)2Te3 Soft Superlattices Created by Nanoalloying

In this work, p-type nanoscale ??soft superlattices?? consisting of multilayer stacks of 25?nm Sb₂Te₃ on 25?nm (Bi_0.2Sb_0.8)₂Te₃ were fabricated by nanoalloying. With this technique, nanoscale layers of the elements Bi, Sb, and Te are deposited by sputtering onto a Si/SiO₂ substrate and subsequently annealed to induce interdiffusion and a solid-state reaction to form the final superlattices. Different combinations of annealing temperatures were used in the annealing process. The in-plane electronic properties (Seebeck coefficient, electrical conductivity, charge carrier concentration, and carrier mobility) of these soft superlattices were examined. The cross-plane thermal conductivity was determined using time-domain thermal reflectance (TDTR). Secondary-ion mass spectrometry (SIMS) depth profiles reveal that the nanostructured thin films exhibit high stability against thermal interdiffusion during the annealing process. X-ray patterns of the samples display very strong texture with preferred c-orientation of the crystallites after the heat treatment. Scanning electron microscopy (SEM) cross-section images of the films show distinctly polycrystalline structure with increasing grain size for higher annealing temperatures, as confirmed by x-ray diffraction (XRD) analysis. Very high power factors exceeding 40???W/cm?K², similar to values for bulk single crystals with comparable compositions, are observed for the soft superlattices. The nanostructure appears to be stable up to 300°C. For a sample annealed at 150°C, a thermal conductivity as low as 0.45?W/mK was determined. Based on different assumptions concerning the degree of anisotropy of the transport properties, a cross-plane figure of merit ZT of 0.6 to 1.9 can be estimated for the thin films annealed at 300°C.     

Effect of Se content on the structural,morphological and optical properties of Bi2Te3−ySey thin films electrodeposited by under potential deposition technique

Bi₂Te_3−ySe_y thin films with different Se contents ranging from 0.3 to 2.5 were successfully electrodeposited by under potential deposition (UPD) technique onto gold foil substrates from an electrolyte consist of Bi(NO₃)₃, TeO₂, and SeO₂ at ambient conditions. The effects of Se content on structural, morphological and optical properties of the products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and FT-IR spectroscopy, respectively. The XRD analysis revealed that the diffraction peaks positions of Bi₂Te_3−ySe_y thin films shifts gradually towards the higher angle side due to replacement of Te by Se atoms in the crystal structure with increasing Se content. The SEM results showed that the particle size of Bi₂Te_3−ySe_y thin films decreased as the Se content increased. The optical constants of ternary Bi₂Te_3−ySe_y thin films such as refractive index, extinction coefficient, and dielectric constant were obtained from the transmission spectra in the range of 2500–10.000 nm. The direct allowed band gap energies were estimated using Tauc equation and found to increase from 0.210 to 0.282 eV with increasing Se content from 0.3 to 2.5. The dispersion behavior of refractive index was studied by the single oscillator Wemple-DiDomenico model.     

Complex Band Structures and Lattice Dynamics of Bi2Te3‐Based Compounds and Solid Solutions

Bi₂Te₃‐based compounds and derivatives are milestone materials in the fields of thermoelectrics (TEs) and topological insulators (TIs). They have highly complex band structures and interesting lattice dynamics, which are favorable for high TE performance as well as strong spin orbit and band inversion underlying topological physics. This review presents rational calculations of properties related to TEs and provides theoretical guidance for improving the TE performance of Bi₂Te₃‐based materials. Although the band structures of these TE materials have been studied theoretically and experimentally for many years, there remain many controversies on band characteristics, especially the locations of band extrema and the exact values of bandgaps. Here, the key factors in the theoretical investigations of Bi₂Te₃, Bi₂Se₃, Sb₂Te₃, and their solid solutions are reviewed. The phonon spectra and lattice thermal conductivities of Bi₂Te₃‐based materials are discussed. Electronic and phonon structures and TE transport calculations are discussed and reported in the context of better establishing computational parameters for these V₂VI₃‐based materials. This review provides a useful guidance for analyzing and improving TE performance of Bi₂Te₃‐based materials.     

Confinement-Induced Phonon Softening and Hardening in Sb2Te3 Thin Films

Scaling effects in Sesqui-chalcogenides are of major interest to understand and optimize their performance in heavily scaled applications, including topological insulators and phase-change devices. A combined experimental and theoretical study is presented for molecular beam epitaxy-grown films of antimony-telluride  (Sb₂Te₃). Structural,vibrational, optical, and bonding properties upon varying confinement are studied for thicknesses ranging from 1.3 to 56 nm. In ultrathin films, the low-frequency coherent phonons of A_1g¹ symmetry are softened compared to the bulk (64.5 cm⁻¹ at 1.3 nm compared to 68 cm⁻¹ at 55.8 nm). A concomitant increase of the high-frequency A_1g² Raman mode is seen. X-ray diffraction analyses unravel an accompanying out of plane stretch by 5%, mainly stemming from an increase in the Te-Te gap. This conclusion is supported by density functional theory slab models, which reveal a significant dependency of chemical bonding on film thickness. Changes in atomic arrangement, vibrational frequencies, and bonding extend over a thickness range much larger than observed for other material classes. The finding of these unexpectedly pronounced thickness-dependent effects in quasi-2D material Sb₂Te₃ allows tuning of the film properties with thickness. The results are discussed in the context of a novel bond-type, characterized by a competition between electron localization and delocalization.     

Tailoring Interfacial Charge Transfer for Optimizing Thermoelectric Performances of MnTe-Sb2Te3 Superlattice-Like Films

Interfacial charge transfer has a vital role in tailoring the thermoelectric performance of superlattices (SLs), which, however, is rarely clarified by experiments. Herein, based on epitaxially grown p-type (MnTe)_x(Sb₂Te₃)_y superlattice-like films, synergistically optimized thermoelectric parameters of carrier density, carrier mobility, and Seebeck coefficient are achieved by introducing interfacial charge transfer, in which effects of hole injection, modulation doping, and energy filtering are involved. Carrier transport measurements and angle-resolved photoemission spectroscopy (ARPES) characterizations reveal a strong hole injection from the MnTe layer to the Sb₂Te₃ layer in the SLs, originating from the work function difference between MnTe and Sb₂Te₃. By reducing the thickness of MnTe less than one monolayer, all electronic transport parameters are synergistically optimized in the quantum-dots (MnTe)_x(Sb₂Te₃)₁₂ superlattice-like films, leading to much improved thermoelectric power factors (PFs). The (MnTe)_0.1(Sb₂Te₃)₁₂ obtains the highest room-temperature PF of 2.50 mWm⁻¹K⁻², while the (MnTe)_0.25(Sb₂Te₃)₁₂ possesses the highest PF of 2.79 mWm⁻¹K⁻² at 381 K, remarkably superior to the values acquired in binary MnTe and Sb₂Te₃ films. This research provides valuable guidance on understanding and rationally tailoring the interfacial charge transfer of thermoelectric SLs to further enhance thermoelectric performances.     

Three-Dimensional Bi2Te3 Nanocrystallites Embedded in 2D Bi2Te3 Films Grown by MOCVD

Two- (2D) and three-dimensional (3D) growth of nanostructured Bi₂Te₃ films was performed on 4° tilt (100) GaAs substrates using a metalorganic chemical vapor deposition system. To obtain 3D Bi₂Te₃ crystallites embedded in 2D planar film, we alternately changed the gas flow rate in the reactor. By repeating two steps, 3D Bi₂Te₃ crystallites embedded in 2D planar Bi₂Te₃ film were obtained. The thermoelectric properties in terms of the thermal conductivity, electrical conductivity, and Seebeck coefficient were investigated at room temperature. The thermal conductivities of the nanostructured Bi₂Te₃ films were from 0.63?W/(m?K) to 0.94?W/(m?K) at room temperature, which are low compared with that of film without nanostructure [1.62?W/(m?K)]. The thermal conductivity of the film was effectively decreased with the decrease of size and increase of density of 3D crystallites. The results of this study open up a new method to fabricate nanostructured thermoelectric films with high thermoelectric figure of merit.     

Multi-Functional and Stretchable Thermoelectric Bi2Te3 Fabric for Strain,Pressure, and Temperature-Sensing

Fiber-based electronics are essential components for human-friendly wearable devices due to their flexibility, stretchability, and wearing comfort. Many thermoelectric (TE) fabrics are investigated with diverse materials and manufacturing methods to meet these potential demands. Despite such advancements, applying inorganic TE materials to stretchable platforms remains challenging, constraining their broad adoption in wearable electronics. Herein, a multi-functional and stretchable bismuth telluride (Bi₂Te₃) TE fabric is fabricated by in situ reduction to optimize the formation of Bi₂Te₃ nanoparticles (NPs) inside and outside of cotton fabric. Due to the high durability of Bi₂Te₃ NP networks, the Bi₂Te₃ TE fabric exhibits excellent electrical reliability under 10,000 cycles of both stretching and compression. Interestingly, intrinsic negative piezoresistance of Bi₂Te₃ NPs under lateral strain is found, which is caused by the band gap change. Furthermore, the TE unit achieves a power factor of 25.77 µWm⁻¹K⁻² with electrical conductivity of 36.7 Scm⁻¹ and a Seebeck coefficient of −83.79 µVK⁻¹ at room temperature. The Bi₂Te₃ TE fabric is applied to a system that can detect both normal pressure and temperature difference. Balance weight and a finger put on top of the 3 × 3 Bi₂Te₃ fabric assembly are differentiated through the sensing system in real time.