Properties Of Bi2Te3 Thin Films Research Articles

Effect of Fe doping on the structural, electrical and optical properties of Bi2Te3 thin films

Effect of Fe doping on the structural, electrical and optical properties of Bi2Te3 thin films

Determining phonon transport properties of bismuth telluride thin films with extremely small grain size using nanoindentation and 3ω method

The phonon transport properties of Bi2Te3 thin films with extremely small grain size (3.4 nm) is investigated in this study using radio-frequency magnetron sputtering. The films exhibited an average group velocity of 2083 m s−1 and lattice thermal conductivity of 0.42 W (m−1∙K−1), as determined by nanoindentation and 3ω method, respectively. The phonon mean free path (MFP) was 0.49 nm, determined from the group velocity and lattice thermal conductivity. The lattice thermal conductivity decreased to 35% for single-crystal Bi2Te3 owing to the decrease in the phonon MFP, whereas the group velocity hardly changed in the region of extremely small grain size.

Role of ion irradiation induced defects in thermoelectric transport properties of Bi2Te3 thin films

Present study investigates the contribution of defects in the thermoelectric transport properties of Bi2Te3 thin films using 120 keV He+ and Ar+ ion irradiation. Charge carrier type is converted from n-type to p-type with He+ ion irradiation. Upon Ar+ ion irradiation, thin film exhibited p-type conductivity at lower ion fluence and n-type conductivity at higher ion fluences. In case of He+ ion irradiation, bismuth vacancies were suggested to increase hole density and responsible for p-type conductivity whereas in the case of Ar+ ion irradiation, Ar+ ion induce grain melting that suppresses defect density and consequently increases electron concentration. Bi2Te3 films irradiated with the He+ ion fluence of 1 × 1015ions/cm2 exhibited highest power factor value of ~6.3 μW/mK2 at room temperature.

Effects of post-annealing on crystalline and transport properties of Bi2Te3 thin films* *Project supported by the National Natural Science Foundation of China (Grant Nos. 52072030, 52071025, and 51871018), the Beijing Outstanding Young Scientists Projects (Grant No. BJJWZYJH01201910005018), Beijing Natural Science Foundation, China (Grant No. Z180014), the Science and Technology Innovation Team Program of Foshan (Grant No. FSOAA-KJ919-4402-0087), and Beijing Laboratory

A well-established method is highly desirable for growing topological insulator thin films with low carrier density on a wafer-level scale. Here, we present a simple, scalable method based on magnetron sputtering to obtain high-quality Bi2Te3 films with the carrier density down to 4.0 × 1013 cm−2. In contrast to the most-used method of high substrate temperature growth, we firstly sputtered Bi2Te3 thin films at room temperature and then applied post-annealing. It enables the growth of highly-oriented Bi2Te3 thin films with larger grain size and smoother interface. The results of electrical transport show that it has a lower carrier density as well as a larger coherent length (∼228 nm, 2 K). Our studies pave the way toward large-scale, cost-effective production of Bi2Te3 thin films to be integrated with other materials in wafer-level scale for electronic and spintronic applications.

Thermoelectric Properties of Bismuth Telluride Thin Films Electrodeposited from a Nonaqueous Solution.

We report the thermoelectric properties of Bi2Te3 thin films electrodeposited from the weakly coordinating solvent dichloromethane (CH2Cl2). It was found that the oxidation of porous films is significant, causing the degradation of its thermoelectric properties. We show that the morphology of the film can be improved drastically by applying a short initial nucleation pulse, which generates a large number of nuclei, and then growing the nuclei by pulsed electrodeposition at a much lower overpotential. This significantly reduces the oxidation of the films as smooth films have a smaller surface-to-volume ratio and are less prone to oxidation. X-ray photoelectron spectroscopy (XPS) shows that those films with Te(O) termination show a complete absence of oxygen below the surface layer. A thin film transfer process was developed using polystyrene as a carrier polymer to transfer the films from the conductive TiN to an insulating layer for thermoelectrical characterization. Temperature-dependent Seebeck measurements revealed a room-temperature coefficient of −51.7 μV/K growing to nearly −100 μV/K at 520 °C. The corresponding power factor reaches a value of 88.2 μW/mK2 at that temperature.

Characteristics of electrodeposited bismuth telluride thin films with different crystal growth by adjusting electrolyte temperature and concentration

Bismuth telluride (Bi2Te3) thin films were prepared with various electrolyte temperatures (10°C–70 °C) and concentrations [Bi(NO3)3 and TeO2: 1.25–5.0 mM] in this study. The surface morphologies differed significantly between the experiments in which these two electrodeposition conditions were separately adjusted even though the applied current density was in the same range in both cases. At higher electrolyte temperatures, a dendrite crystal structure appeared on the film surface. However, the surface morphology did not change significantly as the electrolyte concentration increased. The dendrite crystal structure formation in the former case may have been caused by the diffusion lengths of the ions increasing with increasing electrolyte temperature. In such a state, the reactive points primarily occur at the tops of spiked areas, leading to dendrite crystal structure formation. In addition, the in-plane thermoelectric properties of Bi2Te3 thin films were measured at approximately 300 K. The power factor decreased drastically as the electrolyte temperature increased because of the decrease in electrical conductivity due to the dendrite crystal structure. However, the power factor did not strongly depend on the electrolyte concentration. The highest power factor [1.08 μW/(cm·K2)] was obtained at 3.75 mM. Therefore, to produce electrodeposited Bi2Te3 films with improved thermoelectric performances and relatively high deposition rates, the electrolyte temperature should be relatively low (30 °C) and the electrolyte concentration should be set at 3.75 mM.

Highly oriented nanocrystalline bismuth telluride thin films obtained by radio-frequency magnetron sputtering with a magnetic field applied to the substrate via an affixed permanent magnet

We deposited nanostructured bismuth telluride (Bi2Te3) thin films on glass substrates with NdFeB permanent magnets of varying magnetic flux densities attached to the backside, and then thermally annealed them in an electric furnace. Surface scanning electron microscopy (SEM) revealed that the thin films deposited without the additional magnetic field and in the presence of an additional magnetic field with a magnetic flux density of 189 mT were both composed of nano-sized crystal grains with flat surfaces. XRD patterns indicated that the thin film deposited at 189 mT exhibited very high crystal orientation along the c-axis. As a result, this thin film exhibited high mobility and a power factor of 14.1 μW/(cm·K2), which was 65% higher than that of the thin film deposited in the absence of an additional magnetic field. When the magnetic flux density was increased to 378 mT or more, the thin film surface was covered with irregularly shaped powder depositions, resulting in a decrease in the crystal orientation, and by extension, the desirable thermoelectric properties. Therefore, we conclude that application of an additional magnetic field with a moderate magnetic flux density can improve the structural and thermoelectric properties of Bi2Te3 thin films deposited by RF magnetron sputtering.

Theoretical and experimental analyses to determine the effects of crystal orientation and grain size on the thermoelectric properties of oblique deposited bismuth telluride thin films

The effects of crystal orientation and grain size on the thermoelectric properties of Bi2Te3 thin films were investigated by conducting experimental and theoretical analyses. To vary the crystal orientation and grain size, we performed oblique deposition, followed by thermal annealing treatment. The crystal orientation decreased as the oblique angle was increased, while the grain size was not changed significantly. The thermoelectric properties were measured at room temperature. A theoretical analysis was performed using a first principles method based on density functional theory. Then the semi-classical Boltzmann transport equation was used in the relaxation time approximation, with the effect of grain size included. Furthermore, the effect of crystal orientation was included in the calculation based on a simple semi-experimental model. A maximum power factor of 11.6 µW/(cm·K2) was obtained at an oblique angle of 40°. The calculated thermoelectric properties were in very good agreement with the experimentally measured values.

Nanocrystalline Bi2Te3 thin films synthesized by electrodeposition method for photoelectrochemical application

Bismuth Telluride (Bi2Te3) thin films are prepared onto the stainless steel substrates by electrodeposition technique. Effects of varying deposition time on physico-chemical properties of Bi2Te3 thin films are studied. X-ray diffraction (XRD) analysis shows that all the Bi2Te3 films are polycrystalline in nature and it has Rhombohedral crystal structure. The results analyzed from FT-Raman and XRD analysis are well matched with each other and confirms the rhombohedral crystal structure. Scanning electron microscopy (SEM) images show that the variation in surface microstructure with respect to deposition time. Compositional elements (Bi and Te) distributions on the surface of Bi2Te3 film are estimated by EDX spectroscopy. Bi2Te3 thin films show the water contact angle in hydrophilic range. Photoconversion efficiency and fill factor of photoelectrochemical (PEC) cell formed with Bi2Te3 film are found to be 0.0987% and 0.3979 respectively.

High performance co-sputtered Bi2Te3 thin films with preferred orientation induced by MgO substrates

Bi2Te3 thin films were fabricated on the (00l)-oriented MgO substrates by the magnetron co-sputtering method. The films microstructure of highly preferred orientation along the c-axis had been obtained through the inducing of single crystal MgO substrates. And some abundant Te was introduced into the Bi2Te3 films through sputtering tellurium target, which affected the scattering process. The (00l) preferred orientation is beneficial to the carriers transport. In this case, the higher performance of Bi2Te3 thin films can be obtained. And microstructure and thermoelectric properties of Bi2Te3 thin films were investigated systematically.

Quantum Size Effects in Transport Properties of Bi2Te3 Topological Insulator Thin Films

Bi2Te3 compound and Bi2Te3-based solid solutions have attracted much attention as promising thermoelectric materials for refrigerating devices. The possibility of enhancing the thermoelectric efficiency in low-dimensional structures has stimulated studies of Bi2Te3 thin films. Now, interest in studying the transport properties of Bi2Te3 has grown sharply due to the observation of special properties characteristic of three-dimensional (3D) topological insulators in Bi2Te3. One of the possible manifestations of quantum size effects in two-dimensional structures is an oscillatory behavior of the dependences of transport properties on film thickness, d. The goal of this work is to summarize our earlier experimental results on the d-dependences of transport properties of Bi2Te3 thin films obtained by thermal evaporation in a vacuum on glass substrates, and to present our new results of theoretical calculations of the oscillations periods within the framework of the model of an infinitely deep potential well, which takes into account the dependence of the Fermi energy on d and the contribution of all energy subbands below the Fermi level to the conductivity. On the basis of the data obtained, some general regularities and specificity of the quantum size effects manifestation in 3D topological insulators are established.

Nanomechanical and wettability properties of Bi2Te3 thin films: Effects of post-annealing

In this study, Bi2Te3 thin films were deposited on SiO2/Si(100) substrates by pulsed laser deposition (PLD) at 250 °C. The films were then annealed in-situ in the deposition chamber at various annealing temperatures (Ta) ranging from 200 to 300 °C. The microstructural, morphological, and nanomechanical properties of the Bi2Te3 thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy, and nanoindentation techniques, respectively. The XRD results indicated that all the Bi2Te3 thin films have high crystalline quality with predominant (0015) texture. Nano-indentation measurements performed with a Berkovich nanoindenter operating under the continuous contact stiffness measurement mode revealed that both the hardness and Young's modulus of the Bi2Te3 films decreased with increasing Ta. In addition, the water contact angle measurements were carried out to delineate the effects of annealing on the changes in the surface energy and wettability of the films.

Gate‐Tunable Transport Properties of In Situ Capped Bi2Te3 Topological Insulator Thin Films

Combining of the ability to prepare high‐quality, intrinsic Bi2Te3 topological insulator thin films of low carrier density with in situ protective capping, a pronounced, gate‐tunable change in transport properties of Bi2Te3 thin films is demonstrated. Using a back gate, the carrier density is tuned by a factor of ≈7 in an Al2O3 capped Bi2Te3 sample and by a factor of ≈2 in Te capped Bi2Te3 films. Full depletion of bulk carriers is achieved, which allows access to the topological transport regime dominated by surface state conduction. When the Fermi level is placed in the bulk band gap, the presence of two coherent conduction channels associated with the two decoupled surfaces is observed. The magnetotransport results show that the combination of capping layers and electrostatic tuning of the Fermi level provide a technological platform to investigate the topological properties of surface states in transport experiments and pave the way toward the implementation of a variety of topological quantum devices.

Influence of doping on structural and optical properties of Bi2Te3 thin films

The structural and optical properties of Se doped Bi2Te3−xSex (0≤x≤0.3) and Fe doped Bi2−yFeyTe3 (0≤y≤0.3) films, deposited by electron beam deposition technique at moderate substrate temperature, have been analyzed for two film thicknesses. The structural properties, analyzed by X-ray diffraction data, indicate the polycrystalline nature of the films with rhombohedral structure. Changes in the structural parameters have been estimated in terms of the lattice constant as function of Se and Fe concentration. The grain size is found to be in the range of 76–130nm. The optical properties have been characterized by transmittance and reflectance measurements in the wavelength range of 400–1000nm. The optical constants, refractive index (n) and extinction coefficient (k), have been calculated. Moreover, the variations in refractive index and extinction coefficient have been analyzed and reported for different Se and Fe doping concentration of the film of 100nm and 150nm thickness along with the variation in the real (εr) and imaginary (εi) parts of the dielectric constants, in the visible to ultra-violet range of wavelengths.

Ferromagnetism of magnetically doped topological insulators in CrxBi2−xTe3 thin films

We investigated the effect of magnetic doping on magnetic and transport properties of Bi2Te3 thin films. CrxBi2−xTe3 thin films with x = 0.03, 0.14, and 0.29 were grown epitaxially on mica substrate with low surface roughness (∼0.4 nm). It is found that Cr is an electron acceptor in Bi2Te3 and increases the magnetization of CrxBi2−xTe3. When x = 0.14 and 0.29, ferromagnetism appears in CrxBi2−xTe3 thin films, where anomalous Hall effect and weak localization of magnetoconductance were observed. The Curie temperature, coercivity, and remnant Hall resistance of thin films increase with increasing Cr concentration. The Arrott-Noakes plot demonstrates that the critical mechanism of the ferromagnetism can be described better with 3D-Heisenberg model than with mean field model. Our work may benefit for the practical applications of magnetic topological insulators in spintronics and magnetoelectric devices.

Synthesis and Characterization of Bi2Te3 Nanostructured Thin Films.

Thin films of Bi2Te3 were obtained using vacuum evaporation and inert gas evaporation techniques. To study the effect of nanocrystallite size on thermal and electrical properties, deposition temperature and gas pressure were varied and thin films of Bi2Te3 having different crystallite sizes ranging from 7-20 nm were obtained. X-ray Diffraction and scanning electron microscopic studies were carried out to determine phase, crystallite size, strain and surface morphology of nanocrystalline films. Effect of nanocrystallite size on electron transport and thermal properties of Bi2Te3 thin films was studied using Hall effect and Harman's four probe methods. Calculated ZT values were correlated with the carrier concentration, carrier mobility and electrical conductivity of Bi2Te3 thin films. This study shows that strain may influence the electron transport and thermoelectric properties of Bi2Te3 films along with nanocrystallite size.

Nanomechanical properties of Bi2Te3 thin films by nanoindentation

In this study, the microstructural, morphological and nanomechanical properties of Bi2Te3 thin films are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and nanoindentation techniques. The Bi2Te3 thin films were deposited on c-plane sapphire substrates using pulsed laser deposition (PLD) under the various helium gas pressures. The XRD results indicated that the Bi2Te3 thin films are textured with the c-axis preferentially oriented normal to the films surface. Both the grain size and surface roughness of the Bi2Te3 thin films exhibit an increasing trend with increasing the helium gas pressure. Furthermore, the hardness and Young’s modulus of the Bi2Te3 thin films are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) mode were found to range from 2.92±0.12 to 4.02±0.14GPa and from 106.31±0.63 to 127.46±9.21GPa, respectively, when the helium gas pressure was increased from 2×10−5 to 2×10−3Torr.

Effect of Thickness and Annealing on Structural and Optical Properties of Bi2Te3 Thin Films Prepared from Bi2Te3 Nanoparticels

Thin films Bi2Te3 with the thickness range of 37-110 nm have been deposite d on clean glass substrates by the thermally evaporated technique in a vacuum of 1 10 -5 Torr. The influence of thick ness and annealing on the structural and optical properties has been studied. The XRD patterns showed after thermal annealing the Bi2Te3 thin films became polycrystalline in structure. The lattice parameters of the samples were calculated and the structural parameters were discussed on the basis of annealing effect. The AFM images revealed a homogeneous Bi2Te3 dispersion within the layer in higher annealing temperature. Absorption spectra indicate that films were having considerable absorption throughout visible region. Op tical band gap energy decreased with increasing film thickness and annealing temperature.

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 RH, charge carrier mobility μH, and TE power factor P = S2σ 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 TS 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 TS 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, RH, μH, 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.

Annealing Effect on the Thermoelectric Properties of Bi2Te3 Thin Films Prepared by Thermal Evaporation Method

Bismuth telluride‐based compounds are known to be the best thermoelectric materials within room temperature region, which exhibit potential applications in cooler or power generation. In this paper, thermal evaporation processes were adopted to fabricate the n‐type Bi2Te3 thin films on SiO2/Si substrates. The influence of thermal annealing on the microstructures and thermoelectric properties of Bi2Te3 thin films was investigated in temperature range 100–250°C. The crystalline structures and morphologies were characterized by X‐ray diffraction and field emission scanning electron microscope analyses. The Seebeck coefficients, electrical conductivity, and power factor were measured at room temperature. The experimental results showed that both the Seebeck coefficient and power factor were enhanced as the annealing temperature increased. When the annealing temperature increased to 250°C for 30 min, the Seebeck coefficient and power factor of n‐type Bi2Te3‐based thin films were found to be about −132.02 μV/K and 6.05 μW/cm·K2, respectively.