Bismuth Telluride Thin Films Research Articles

High Seebeck coefficient in thermally evaporated Sb-In co-alloyed bismuth telluride thin film

In the present work, we have obtained high magnitudes of the Seebeck coefficient in Sb and In coalloyed bismuth telluride thin film that has been deposited by a simple and cost-efficient thermal evaporation procedure. The films display an exceptional peak Seebeck coefficient of −310 μV/K at the working temperature of 90 °C. In addition to this, a high value of −191.6 μV/K is obtained at room temperature along with appreciable conductivity (6.2 × 103 S/m). The x-ray diffraction (XRD) pattern of the film has been analyzed for probing the crystal profile that depicts a polycrystalline and nanoscale structure. Films’ surface and cross-sectional morphologies are investigated using Field Emission Scanning Electron Microscope (FESEM), where a nanocrystalline morphology of thickness 150 nm is observed. Raman analysis supports the results obtained from XRD and FESEM for nanomorphology and indicates the presence of Te segregates. Atomic composition of the film produced is probed using Energy Dispersive x-ray spectroscopy, which also indicates the presence of excess Te. The Seebeck coefficient of the films shows an enormous enhancement as compared to previously reported work for undoped samples (BST-100S). The magnitudes of the Seebeck coefficient obtained in the present work are among the highest values reported for a bismuth antimony telluride material. These enhancements are attributed to the combined effect of coalloying, the presence of highly mobile (00l) orientations, and confinement effects of a nanocrystalline profile.

EFFECT OF ALTERNATE LAYERS OF Bi2Te3 - Sb2Te3 THIN FILMS ON STRUCTURAL, OPTICAL AND THERMOELECTRIC PROPERTIES

There is an increasing demand for the development of new thermoelectric materials with high figure of merit. This is due to the fact that convectional energy generating methods causes many environmental problems. Thermoelectric materials have capacity to harvest waste energy as they convert directly any type of waste heat into electricity. Telluride based materials are known as best thermoelectric materials as they have high figure of merit. Present work is based on multilayered structure of bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin films. In this work thin film of single layered Bi2Te3, Sb2Te3, their bilayers (Bi2Te3-Sb2Te3), 5 layers and 10 layers have been deposited on clean glass substrate by e-beam evaporation method. X- Ray diffraction (XRD) has been done for the characterization of these thin films. In this paper, studies of electrical properties are also reported which have been carried out by Hall and I-V measurements while optical properties have been studied by employing by UV – Vis spectrometer. Thermoelectric characteristics have been measured by laboratory thermoelectric measurement setup.

Investigations on morphology and thermoelectric transport properties of Cu+ ion implanted bismuth telluride thin film

Present study reports the effect of copper (Cu+) ion implantation (with different Cu concentrations of 0.5 at%, 1 at%, 1.5 at%, and 2 at%) on thermoelectric transport properties of bismuth telluride thin films. The field emission-scanning electron microscopy and atomic force microscopy results reveal the coalescence of columnar grains with the formation of pinholes in the Cu+ ion implanted samples and the evolution of morphology was explained successfully in the framework of spherical thermal spike model. X-ray diffraction and Raman scattering experiments revealed the reduction in the crystalline nature of Bi2Te3 thin films upon Cu+ ion implantation. The depth profiles for the atomic concentration of Bi, Te, and Cu were determined using Rutherford backscattering spectrometry. As a function of Cu+ ion fluences, the Bi atomic concentration gradually decreases across the Bi2Te3 thin films. The Hall-effect and Seebeck coefficient measurements demonstrate the n-type to p-type charge carrier conversion upon ion implantation. The charge carrier conversion is attributed to the high density of defect complexes in addition to implanted Cu atoms. These results show the possibility of fabricating TE micro-modules that comprises of n- and p-type semiconductors using ion implantation.

Comparison of thermoelectric properties of flexible bismuth telluride thin films deposited via DC and RF magnetron sputtering

Bi2Te3 thin films were deposited onto polyamide sheets with direct current (DC) or radio frequency (RF) magnetron sputtering techniques. The films were prepared using a Bi2Te3 target at a varying pre-heating temperature from 150 to 350 °C. It was observed that the type of plasma excitation and pre-heating temperature can significantly change the composition, preferred orientation, crystallinity, and thermoelectric properties of the films. The pre-heat treatment significantly affected the non-stoichiometric composition. In addition, it was shown that crystallinity and (0 0 l) planes were enhanced in the DC sputtered coatings at a high pre-heating temperature. The maximum power factor of 3.5 × 10−3 W/m K2 at 285 °C was obtained for the films deposited using DC magnetron sputtering and a pre-heating temperature of 350 °C. The carrier concentration and mobility of the film were 5.40 × 1020 cm−3 and 13.04 cm2/V s, respectively. Compared with an ordinary Bi2Te3 film, the power factor of such film has been greatly increased. The results indicated that DC magnetron sputtering can enhance the (0 0l ) plane orientation in the Bi2Te3 film.

Q-switched Thulium-doped fibre laser using Bismuth (III) Telluride-based saturable absorber

We demonstrate a passive Q-switched thulium-doped fibre laser (TDFL) pulse generation using bismuth (III) telluride (Bi2Te3) thin film as saturable absorber. The Bi2Te3 was fabricated by a simple processing technique; simply by embedding the Bi2Te3 into the polyvinyl alcohol (PVA) film. By sandwiching 1 mm x 1 mm Bi2Te3 thin film between two fibre ferrules in a TDFL ring cavity, a stable Q-switching pulses train was generated. The repetition rate and pulse width were tuneable from 10.72 kHz to 39.9 kHz and 13.08 µs to 5.98 µs, respectively, as the pump power increased from 309 mW to 454 mW. The slope efficiency was 4.51%, while, the maximum pulse energy was calculated as 0.17 µJ.

Surface Deformation Study of the Electrodeposited Nanostructured Bi2Te3 and Sb2Te3 Thin Films by Double‐Exposure Digital Holographic Interferometry

Bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin films are successfully deposited on stainless steel (SS) substrate by the potentiostatic mode of electrodeposition. Herein, the focus is on surface deformation during the electrodeposition process under the effect of deposition time. This study is carried out using double‐exposure digital holographic interferometry (DEDHI) technique and the prepared optimized films are characterized for their physico‐chemical properties. The deposition potential is found out by cyclic voltammetry; it is −170 and −260 mV for Bi2Te3 and Sb2Te3, respectively. The hologram fringes are recorded during the electrodeposition of films and the variations in the number of fringes, deposited mass, thickness of film, and stress to surface with respect to deposition time are studied.

Generation of Microsecond Ytterbium-Doped Fiber Laser Pulses using Bismuth Telluride Thin Film as Saturable Absorber

Bismuth telluride (Bi2Te3), a type of topological insulators, is currently in hot pursuit due to its unique physical properties. Therefore, this paper describes a simple Q-switched Ytterbium-doped fiber laser (YDFL) by using Bi2Te3 thin-film as saturable absorber. The few layers Bi2Te3 film was fabricated using optical deposition technique and subsequently, was used in an all-fiber, YDFL setup. As a result, a self-starting Q-switching pulses were first occurred when the laser pumping power reached 88.6 mW. As the pump power level increased, the observed pulses repetition rates had increased steadily from 17 to 29.63 kHz. Hence, this work demonstrated that Bi2Te3 thin-film can be used to successfully generate Q-switching pulses at 1-micron region and is well suited for many photonic applications operated at this wavelength region.

Electrodeposition of bismuth telluride from a weakly coordinating, non-aqueous solution

We report the electrodeposition of bismuth telluride thin films on to titanium nitride (TiN) electrodes from a weakly coordinating solvent, dichloromethane (CH2Cl2), using the halometallates, [NnBu4][BiCl4] and [NnBu4]2[TeCl6] with 0.1M [NnBu4]Cl as the supporting electrolyte. The elemental composition of the electrodeposited films was found to be independent of the deposition potential between −0.6 and −2.0V vs. Ag/AgCl but to be dependent on ratio of the concentrations of the Bi and Te precursors in the solution, with the amount of bismuth in the films increasing when the concentration of [NnBu4][BiCl4] in solution was increased. All the electrodeposited films were found to be homogenous in composition across the electrode surface and to be reproducible in composition for replicate experiments. As the deposition potential was taken less negative, the morphology of the deposits changed from uniform films to films with a compact micro/nano particle structure as seen by scanning electron microscopy (SEM). Using this system, the electrodeposition of crystalline Bi2Te3 and Bi4Te3 was confirmed by energy dispersive spectroscopy (EDX) and grazing incidence X-ray diffraction.

Experimental Study on Flexible Bismuth Telluride Thin Films Deposited by DC Sputtering at Different Powers

Bismuth telluride (Bi2Te3) thin films have been deposited onto polyimide sheet substrates by direct-current (DC) magnetron sputtering at different powers and their microstructure, composition, and electrical and thermoelectrical properties studied. The experimental results indicated that the sputtering power was the key parameter determining their thermoelectric properties. X-ray diffraction analysis confirmed the highly (015) preferred orientation of the films. The Te content and grain size depended on the sputtering power. The power factor of Bi2Te3 deposited by DC sputtering at power of 60 W was comparable to that obtained for highly (00l) Bi2Te3 thin film.

Preparation and Characterization of The Nanostructured Bismuth Telluride Thin Films Deposited by Thermal Evaporation Technique

Bismuth telluride-based heavily doped semiconductors are known to be one of the best thermoelectric materials, which have been applied in cooling and power generation. In this paper, we combined both zone melting and thermal evaporation techniques to prepare Bi2Te3 thin films and annealing under vacuum and N2 atmospheres at 250° C. For the bulk Bi2Te3 material, the measured atomic Bi percentage incorporated was smaller than the corresponding percentage of evaporated Bi. So, the used material was adjusted to get the stoichiometric ratio. Structural, morphological characterizations and thermoelectric investigations were carried out for the thermoelectric material as-deposited and after annealing. The X-ray diffraction patterns confirmed the formation of the nano-particles Bi2Te3 structure and showed that the prepared thin films are polycrystalline in nature. Field Emission Scanning Electron Microscopy showed its granular growth. The grains were homogenous and compact. As the samples annealed, the crystallinity and the grain size were increased. Energy-dispersive X-ray spectroscopy confirmed that the samples are stoichiometric. The power factor, Seebeck coefficient, electrical resistivity, carrier concentration and Hall mobility were measured and evaluated at room temperature with the best power factor of 25.7 μWK-2cm-1

Highly-efficient thermoelectric pn-junction device based on bismuth telluride (Bi2Te3) and molybdenum disulfide (MoS2) thin films fabricated by RF magnetron sputtering technique

Layered structure bismuth telluride and molybdenum disulfide thin films were successfully deposited on different substrates using radio-frequency magnetron sputtering technique. The structural, morphological, and thermoelectric transport properties of bismuth telluride and molybdenum disulfide thin films have been investigated systematically to fabricate high-efficient thermal energy harvesting thermoelectric device. The magnitude of the Seebeck coefficient of bismuth telluride thin films decreases with increase in film thickness. Bismuth telluride grown at 350 °C for 10 min, which is approximately 120 nm, displays a maximum Seebeck coefficient of −126 μV K−1 at 435 K. The performance shows strong temperature dependence when the films were deposited at 300 °C, 350 °C, and 400 °C. The power factor increases from 0.91 × 10−3 W/mK2 at 300 K to about 1.4 × 10−3 W/mK2 at 350 K. Molybdenum disulfide films show the positive Seebeck coefficient values and their Seebeck coefficient increases with film thickness. The AFM images of bismuth telluride thin films display a root-mean-square (rms) roughness of 32.3 nm and molybdenum disulfide thin films show an rms roughness of 6.99 nm when both films were deposited at 350 °C. The open-circuit voltage of the pn-junction thermoelectric generator (TEG) device increases with increase in ΔT to about 130 mV at ΔT = 120 °C. We have demonstrated a highly efficient pn-junction TEG device for waste heat recovery applications.

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.

Use of H2-Ar gas mixtures in radio-frequency magnetron sputtering to produce high-performance nanocrystalline bismuth telluride thin films

Nanocrystalline bismuth telluride (Bi2Te3) thin films with good thermoelectric performance were prepared using mixtures of hydrogen and argon gas by radio-frequency magnetron sputtering. The effects of hydrogen addition on the surface morphology, crystal structure, elemental composition, and thermoelectric properties of the Bi2Te3 thin films were investigated. The mixing ratio, H2/(H2 + Ar), was varied from 0 to 15%. The thin films were deposited on glass substrates heated at 200 °C. It was observed that the surface morphologies of the thin films were greatly affected by the mixing ratio. The deposition rate and composition ratio Te/(Bi+Te) decreased with increasing mixing ratio, indicating that tellurium atoms evaporated from the film surface by a chemical reaction between hydrogen and tellurium. The oxygen concentration inside the films decreased as the mixing ratio increased, resulting in an increased power factor. A maximum power factor of 9.0 μW/(cm·K2) was observed at a mixing ratio of 10%, as this thin film showed a relatively high electrical conductivity and high Seebeck coefficient. However, at a higher mixing ratio of 15%, the power factor of the thin film drastically decreased, possibly due to the appearance of the hexagonal BiTe phase, which exhibits metallic characteristics. Therefore, we conclude that the addition of a moderate amount of hydrogen (10%) during sputtering can improve the thermoelectric performance of Bi2Te3 thin films.

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.

Continuous Machine Health Monitoring Enabled Through Self-Powered Embedded Intelligence and Communication

This paper presents a fully self-powered machine health monitoring wireless node. A bismuth telluride thin film thermoelectric generator (TEG) is used to convert the temperature difference between the hot machine and the cooler ambient into electrical energy. By providing an autonomous power supply to the wireless condition-based monitoring (CBM) system, the cost of ownership is significantly reduced and this technology can become readily accessible to a much wider variety of applications. The paper discusses the system operation, component choices, and thermal transport issues that must be addressed to make an autonomous system viable. The system power consumption can range from 12 – 200 µW depending on the refresh rate which spans the range of 30 s to 30 min. A temperature difference of 2.4 – 10 °C is sufficient to cover this operating span, considering the performance of typical thin-film thermoelectric harvesters.

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.

Thermoelectrical devices based on bismuth-telluride thin films deposited by direct current magnetron sputtering process

Thermoelectrical thin films based on doped bismuth telluride materials have been developed using direct current magnetron sputtering process.Devices based on multiple thermocouples of n-type and p-type semiconducting Bi2Te3 materials processed by photolithographic patterning are presented. Configurations parallel to the substrate have been investigated. A maximum generated voltage of 0.5 V has been obtained using a thermal difference of 36 K for a device made of 35 n-p junctions. Eventhough the output power with this in plane configuration is too low to envisage cooling or thermogeneration application, the feasibility of thermoelectrical devices based on thin films deposited by direct current magnetron sputtering is proved. The contact resistance of several metals has been studied. Ti as contact electrode allows more than 200 times contact resistivity reduction after an annealing at 473 K. This value leads to a calculated power generated by the 5 n-p device close to 0.7 μW under a temperature difference of 36 K. Such maximum value has to be considered for the development of autonomous systems based on thermal harvesting.

Structure and thermoelectric properties of electrodeposited bismuth telluride thin films by controlling electrolyte temperature

Structure and thermoelectric properties of electrodeposited bismuth telluride thin films by controlling electrolyte temperature

Characteristics of nanostructured bismuth telluride thin films fabricated by oblique deposition

Oblique angle deposition can reduce ion bombardment and facilitate precursor migration on surfaces, which can affect the crystallographic properties of thin films. We prepared nanostructured Bi2Te3 thin films by oblique deposition at room temperature, and analyzed their structural and thermoelectric properties. The thin films were deposited by radio-frequency magnetron sputtering on a glass substrate, which was tilted at angles of 0–80° to the target. Cross-sectional SEM images showed that the samples consisted of columnar nanostructures, with the voids between the columnar grains increasing as the oblique angle was increased owing to the shadowing effect. XRD analysis indicated that crystallite size increased as the oblique angle was increased, although highly oriented thin films were not obtained. The highest mobility and electrical conductivity were observed at an oblique angle of 20° due to the relatively large crystallite size of the thin film obtained at this angle, as well as the small number of voids between the columnar grains. As a result, this film exhibited the highest power factor (3.1 μW/(cm·K2)), approximately six times higher than that obtained by normal sputtering deposition. Therefore, we conclude that using a moderate oblique angle can improve the thermoelectric properties of thin films.