Spin-Orbit Readout Using Thin Films of Topological Insulator Sb2Te3 Deposited by Industrial Magnetron Sputtering

Driving a spin-logic circuit requires the production of a large output signal by spin-charge interconversion in spin-orbit readout devices. This should be possible by using topological insulators, which are known for their high spin-charge interconversion efficiency. However, high-quality topological insulators have so far only been obtained on a small scale, or with large scale deposition techniques that are not compatible with conventional industrial deposition processes. The nanopatterning and electrical spin injection into these materials have also proven difficult due to their fragile structure and low spin conductance. The fabrication of a spin-orbit readout device from the topological insulator Sb₂Te₃ deposited by large-scale industrial magnetron sputtering on SiO₂ is presented. Despite a modification of the Sb₂Te₃ layer structural properties during the device nanofabrication, a sizeable output voltage is measured that can be unambiguously ascribed to a spin-charge interconversion process. The results pave the way for the integration of layered van der Waals materials in spin-logic 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.     

Growth of Topological Insulator Bi2Se3 Thin Films on SrTiO3 with Large Tunability in Chemical Potential

The growth of high quality, gate‐tunable topological insulator Bi₂Se₃ thin films on SrTiO₃ substrates by molecular beam epitaxy is reported in this paper. The optimized substrate preparation procedures are critical for obtaining undoped Bi₂Se₃ thin films with sufficiently low carrier densities while maintaining the strong dielectric strength of the substrates. The large tunability in chemical potential is manifested in the greatly enhanced longitudinal resistivity and the reversal of the sign of the Hall resistivity at negative back‐gate voltages. These thin films provide a convenient basis for fabrication of hybrid devices consisting of gate‐tunable topological insulators and other materials such as a superconductor and a ferromagnet.     

磁控溅射法制备Bi2Te3热电薄膜的研究

Bi2Te3薄膜是室温下热电性能最好的热电材料,利用磁控溅射在长有一薄层SiO2的n型硅样品上制备Bi/Te多层复合薄膜,经后续退火处理生成Bi2Te3。通过分析Bi2Te3薄膜的生长和退火工艺,探讨Bi/Te中Te的原子数分数对薄膜热电性能的影响。采用XRD和SEM对薄膜的结构、形貌和成分进行分析,并测量不同条件下的Seebeck系数。薄膜Seebeck系数均为负数,表明所制备样品是n型半导体薄膜,且最大值达到-76.81μV.K-1;电阻率ρ随Te的原子数分数增大而增大,其趋势先缓慢后迅速。Bi2Te3薄膜的热电性能良好,Te的原子数分数是60.52%时,功率因子最大,为1.765×10-4W.K-2.m-1。     

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.     

On the Band Structure of Bi2Te3

Semiconductors - For p-Bi2Te3 crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse...     

Effect of Initial Bulk Material Composition on Thermoelectric Properties of Bi2Te3 Thin Films

V₂VI₃ compounds and solid solutions based on them are known to be the best low-temperature thermoelectric (TE) materials. The predicted possibility of enhancement of the TE figure of merit in two-dimensional (2D) structures has stimulated studies of the properties of these materials in the thin-film state. The goal of the present work is to study the dependences of the Seebeck coefficient S, electrical conductivity σ, Hall coefficient R _H, charge carrier mobility μ _H, and TE power factor P = S ² σ of Bi₂Te₃ thin films on the composition of the initial bulk material used for preparing them. Thin films with thickness d = 200 nm to 250 nm were grown by thermal evaporation in vacuum of stoichiometric Bi₂Te₃ crystals (60.0 at.% Te) and of crystals with 62.8 at.% Te onto glass substrates at temperatures T _S of 320 K to 500 K. It was established that the conductivity type of the initial material is reproduced in films fairly well. For both materials, an increase in T _S leads to an increase in the thin-film structural perfection, better correspondence between the film composition and that of the initial material, and increase in S, R _H, μ _H, σ, and P. The room-temperature maximum values of P for the films grown from crystals with 60.0 at.% and 62.8 at.% Te are P = 7.5 × 10^?4 W/K² m and 35 × 10^?4 W/K² m, respectively. Thus, by using Bi₂Te₃ crystals with different stoichiometry as initial materials, one can control the conductivity type and TE parameters of the films, applying a simple and low-cost method of thermal evaporation from a single source.     

Thermopower Enhancement of Bi2Te3 Films by Doping I Ions

The thermoelectric properties of I-doped Bi₂Te₃ films grown by metal-organic chemical vapor deposition have been studied. I-doped epitaxial (00l) Bi₂Te₃ films were successfully grown on 4° tilted GaAs (001) substrates at 360 °C. I concentration in the Bi₂Te₃ films was easily controlled by the variation in a flow rate of H₂ carrier gas for the delivery of an isopropyliodide precursor. As I ions in the as-grown Bi₂Te₃ films were not fully activated, they did not influence the carrier concentration and thermoelectric properties. However, a post-annealing process at 400 °C activated I ions as a donor, accompanied with an increase in the carrier concentration. Interestingly, the I-doped Bi₂Te₃ films after the post-annealing process also exhibited enhancement of the Seebeck coefficient at the same electron concentration compared to un-doped Bi₂Te₃ films. Through doping I ions into Bi₂Te₃, the thermopower was also enhanced in Bi₂Te₃, and a high power factor of 5 × 10^?3 W K^?2 m^?1 was achieved.     

Oscillatory Thermopower Waves Based on Bi2Te3 Films

Exothermic chemical reactions that are coupled to Bi₂Te₃ porous layers, which are deposited onto terracotta or alumina (Al₂O₃) substrates, are used to produce self‐propagating thermal waves that are guided along the surface. Nitrocellulose is used as the highly reactive chemical. Bi₂Te₃ is employed because of its large Seebeck coefficient and high electrical conductivity. For the Al₂O₃ based structures, the thermal conduction of the substrate results in strong oscillations of the output signals. Such thermopower waves produce a power as large as 10 mW and voltages as high as 150 mV. The power per mass ratio of the developed system is quite remarkable, namely, on the order of 1 kW kg^?1. Depending on the thermal conductivity of the substrate used, the wave front average propagation velocity is either slow (ca. 0.009 m s^?1 for terracotta) or much faster (on the order of 0.4 m s^?1 for Al₂O₃). We have used a mathematical model based on two coupled heat transport equations, in conjunction with the chemical reaction equation, to predict the behavior of the system, which describes the average propagation velocity and the time between oscillation peaks.     

Electrodeposition of MWNT/Bi2Te3 Composite Thermoelectric Films

The effect of multiwalled carbon nanotubes (MWNTs) on the electrochemical behavior of the Bi-Te binary system in nitric acid baths was investigated by means of cyclic voltammetry and electrochemical impedance spectroscopy. Based on the results, MWNT/Bi₂Te₃ composite thermoelectric films were prepared by potentiostatic electrodeposition at room temperature. The morphology, composition, and structure of the MWNT/Bi₂Te₃ composite films were analyzed by environmental scanning electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. The results show that addition of MWNTs to the electrolyte did not change the electrochemical reduction mechanisms of Bi³⁺, HTeO ₂ ⁺ or their mixture, but the reduction processes of Bi³⁺, HTeO ₂ ⁺ , and their mixture become easier. MWNT/Bi₂Te₃ composite thermoelectric films can be obtained by potentiostatic electrodeposition at a wide range of potentials with subsequent annealing. The MWNTs in the films act as nucleation sites for Bi₂Te₃ compound and thereby elevate the film deposition rate. The content of Bi element and MWNTs in the films increased as the potential was shifted negatively. In addition, the MWNTs can enhance the crystallization of Bi₂Te₃ film.     

Assessing Antisite Defect and Impurity Concentrations in Bi2Te3 Based Thin Films by High‐Accuracy Chemical Analysis

In Bi₂Te₃‐based materials charge‐carrier densities are determined by antisite defects and controlling these defects is a key issue for thermoelectric and topological insulator materials. Bi‐Te thin films with high‐quality thermoelectric properties are deposited using a nano‐alloying approach by molecular beam epitaxy (MBE) and sputtering. The in‐plane transport properties are measured at room temperature as a function of charge‐carrier density. High‐accuracy chemical analysis by wavelength‐dispersive X‐ray spectrometry (WDX) is applied for the first time to these Bi₂Te₃‐based thin films. The acquisition conditions for WDX spectrometry are established using Monte Carlo simulations for the electron trajectories, which guarantees a high lateral resolution and rules out stray radiation generated in the substrate of the films. In contrast to energy‐dispersive X‐ray spectrometry (EDX), which is usually applied, WDX offers unprecedented accuracy for measuring antisite defect concentrations and thus has a high impact on improving the quality of thin films. The charge‐carrier densities are calculated from the WDX results according to the point‐defect model of Miller and Li and the thermopower and electrical conductivity are calculated for different charge‐carrier densities by solving the linearized Boltzmann transport equation. A good quantitative agreement is found for the dependence of the thermopower on stoichiometry, whereas the electrical conductivity is sensitively affected by contaminants.     

Post-annealing Effect on Microstructures and Thermoelectric Properties of Bi0.45Sb1.55Te3 Thin Films Deposited by Co-sputtering

p-Type Bi_0.45Sb_1.55Te₃ thermoelectric (TE) thin films have been prepared at room temperature by a magnetron cosputtering process. The effect of postannealing on the microstructure and TE properties of Bi_0.45Sb_1.55Te₃ films has been investigated in the temperature range from room temperature to 350°C. x-Ray diffraction analysis shows that the annealed films have polycrystalline rhombohedral crystal structure, and the average grain size increases from 36?nm to 64?nm with increasing annealing temperature from room temperature to 350°C. Electron probe microanalysis shows that annealing above 250°C can cause Te reevaporation, which induces porous thin films and dramatically affects electrical transport properties of the thin films. TE properties of the films have been investigated at room temperature. The hole concentration shows a trend from descent to ascent and has a minimum value at the annealing temperature of 200°C, while the Seebeck coefficient shows an opposite trend and a maximum value of 245?μV?K^?1. The electrical resistivity monotonically decreases from 19.8?mΩ?cm to 1.4?mΩ?cm with increasing annealing temperature. Correspondingly, a maximum value of power factor, 27.4?μW?K^?2?cm^?1, was obtained at the annealing temperature of 250°C.     

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.     

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.     

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.     

Multilevel Recording in Ge2Sb2Te5 Thin Films

Semiconductors - The current–voltage characteristics measured on Ge2Sb2Te5 thin films in the current mode are studied. The effect of multilevel recording is established when applying...