Bismuth Telluride Thin Films Research Articles

FABRICATION AND CHARACTERIZATION OF BISMUTH TELLURIDE THIN FILMS BY THERMAL EVAPORATION TECHNIQUE

An interest in the study of the structural and optical properties of topological insulators, led to the selection of Bismuth telluride thin films. Among the topological insulators, Bismuth telluride is one of the promising materials that exhibit applications in the field of thermoelectric power generation. The films were prepared using the thermal evaporation method under vacuum conditions of [Formula: see text] mbar on glass substrates maintained at a temperature of 120∘C. The thickness of the film samples ranges from 400 Å to 1400 Å. Pure phases of Bi2Te3 with a rhombohedral structure are confirmed using X-ray Diffraction. The crystallite size was calculated using Debye–Scherrer’s formula. Structural morphology and compositions are examined using SEM and EDAX. The UV–Vis study reflects the optical behavior of the thin film samples. The absorption spectra gives the direct and allowed transition band gap of the Bi2Te3 thin film samples for various thicknesses. The band gap decreases with the increase in the thickness of the Bi2Te3 thin films, suggesting that the material has significant absorption towards the visible spectrum. The Photoluminescence (PL) emissions for Bi2Te3 thin film samples of various thicknesses are examined and analysed. Raman study reveals the presence of two active modes.

Flexo-photovoltaic effect in strained bismuth telluride thin films without substrate bending under light irradiation

Bismuth telluride (Bi2Te3) is gaining increasing attention as a material that exhibits flexo-photovoltaic effect in the infrared region, in addition to conventional thermoelectric materials and topological insulators. However, the flexo-photovoltaic effect is observed only when strains are applied to the Bi2Te3 thin films by bending the substrates. To enhance the versatility of the flexo-photovoltaic effect, in this study, Bi2Te3 thin films were sputtered onto a flexible substrate under a pre-curved condition, and strains were induced when the films were flattened. The crystal orientation and strains in the Bi2Te3 thin films were affected by the pre-curved conditions. The Bi2Te3 thin film deposited using a concave pre-curved condition exhibited an output voltage increase of approximately 15 % compared with the film deposited under a flat condition. This discovery suggests that strained Bi2Te3 thin films can efficiently produce an output voltage under light irradiation without requiring a p–n junction, making them suitable for various photodetector applications.

Fabrication of Bi2Te3 and Sb2Te3 Thermoelectric Thin Films using Radio Frequency Magnetron Sputtering Technique.

Through various studies on thermoelectric (TE) materials, thin film configuration gives superior advantages over conventional bulk TEs, including adaptability to curved and flexible substrates. Several different thin film deposition methods have been explored, yet magnetron sputtering is still favorable due to its high deposition efficiency and scalability. Therefore, this study aims to fabricate a bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) thin film via the radio frequency (RF) magnetron sputtering method. The thin films were deposited on soda lime glass substrates at ambient temperature. The substrates were first washed using water and soap, ultrasonically cleaned with methanol, acetone, ethanol, and deionized water for 10 min, dried with nitrogen gas and hot plate, and finally treated under UV ozone for 10 min to remove residues before the coating process. A sputter target of Bi2Te3 and Sb2Te3 with Argon gas was used, and pre-sputtering was done to clean the target's surface. Then, a few clean substrates were loaded into the sputtering chamber, and the chamber was vacuumed until the pressure reached 2 x 10-5 Torr. The thin films were deposited for 60 min with Argon flow of 4 sccm and RF power at 75 W and 30 W for Bi2Te3 and Sb2Te3, respectively. This method resulted in highly uniform n-type Bi2Te3 and p-type Sb2Te3 thin films.

Etching and Compositional Ratio Effect on the Surface Properties of Bismuth Telluride Thin Films

Bismuth telluride has garnered considerable attention owing to its versatile properties applicable in thermoelectric and antibacterial domains, as well as its intriguing topological insulating properties. In this work, our group fabricated bismuth telluride thin films with various ratios using radio frequency magnetron sputtering. The surface properties of these thin films were thoroughly analyzed by employing a diverse array of analytical techniques, including X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), four-point probe and contact angle (CA) measurements. Specifically, our XPS findings indicated that Bi is more susceptible to oxidation than Te following Ar+-ion etching. Pure Te thin films exhibited the highest Rq value of 31.2 nm based on AFM and SEM results due to their larger grain sizes. The XRD patterns revealed a peak at 27.75° for thin films with 20% Te, attributed to its rhombohedral structure. Moreover, thin films with 30% Te yielded the highest weighted average work function with a value of 4.95 eV after etching. Additionally, pristine Bi and Te thin films demonstrated the most robust hydrophobic properties compared to intermediate-composition thin films, as determined by CA measurements.

Bi2Te3 Thin Films Deposited by the Combination of Bi and Te Plasmas in a PLD Process

Bismuth telluride thin films were grown by pulsed laser deposition by implementing a novel method that combines both Te and Bi plasmas resulting from the laser ablation of individual Bi and Te targets. Furthermore, the mean kinetic ion energy and density of the plasmas, as estimated by TOF curves obtained from Langmuir probe measurements, were used as control parameters for the deposition process. The obtained thin films exhibit a metallic mirror-like appearance and present good adhesion to the substrate. Morphology of the thin films was observed by SEM, yielding smooth surfaces where particulates were also observed (splashing). Chemical composition analysis obtained by EDS showed that apparently the films have a Te-rich composition (ratio of Te/Bi of 3); however, Te excess arises from the splashing as revealed by the structural characterization (XRD and Raman spectroscopy). The XRD pattern indicated that depositions have the rhombohedral (D3d5 (R3¯m)) structure of Bi2Te3. Likewise, Raman spectra exhibited the presence of signals that correspond to Eg2, A1u2 and A1g2(LO) vibrational modes of the same rhombohedral phase of Bi2Te3. Additionally, oxidation states, analyzed by XPS, resulted in signals associated to Bi3+ and Te2- that correspond to the Bi2Te3 compound. Finally, surface topology and thickness profiles were obtained from AFM measurements, confirming a combination of a smooth surface with particulates on top of it and a film thickness of 400 nm.

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.

Fabrication of Bismuth Telluride Thin Films using Thermal Evaporation Technique and its Electrical Properties

Fabrication of Bismuth Telluride Thin Films using Thermal Evaporation Technique and its Electrical Properties

Ultrafast carrier and phonon dynamics in thin films of bismuth telluride on a flexible substrate

Thin films of topological insulators (TIs) possess exotic nonlinear optical properties such as strong light-matter interaction, broadband spectral sensitivity, thickness-dependent tunable bandgap, higher harmonic generation, etc. Due to the presence of metallic surface states with Dirac fermions and an insulating bulk band in TI, they are projected to be a viable material for studying novel physics, resulting in exciting new properties and technologies. The peculiar electron-phonon interactions at the surface have been linked to various unexpected physical features of topological insulators. Although electron behaviour in topological insulators has been extensively investigated on non-flexible substrates, electron-phonon interactions at TI bulk and surfaces states are less well known on a flexible substrate. Because of its potential uses in wearable devices, communications, sensors, and other fields, there is a significant need for the manufacture of high performance flexible optoelectronic responses employing novel exotic materials. In this paper, we preformed ultrafast pump-probe method to explore TI (Bi2Te3) thin films on a flexible PET (polyethylene terephthalate) substrate. We studied the dynamics of Bi2Te3 thin films' hot carrier relaxation progression and coherent phonon behaviour using transient absorbance measurements. Thickness-dependent low-frequency coherent acoustical phonon oscillations are observed in 10 nm thick films, and the changes vanish for 25 nm thick films, and high-frequency optical phonon oscillations are absent in our work. Subpicosecond range of time constant for the photon excitation, diffusion, carrier thermalization, and relaxation are reported. Longer characteristics time was observed for 25 nm film as compared to 10 nm film, and its variation has been discussed here. The thickness-dependent coherent acoustic phonon oscillating in the terahertz frequency range has been experimentally calculated. The film's terahertz frequency response varies with its thickness, allowing it to be employed in future terahertz applications based on flexible topological insulator thin films.

Improvement of thermoelectric properties of flexible Bi2Te3 thin films in bent states during sputtering deposition and post-thermal annealing

High-quality thermoelectric thin films formed on flexible substrates are important for the development of flexible thermoelectric generators as power supplies for wireless sensors and wearable devices. However, the thermoelectric properties of the films formed on flexible substrates are inferior to those of films formed on rigid substrates because flexible substates shrink during the processes of film deposition and post-thermal annealing. To overcome this problem, we propose a modification during these processes. Bismuth telluride thin films were deposited on polyimide substrates using radio-frequency magnetron sputtering. The substrates were bent to a convex or concave shape, followed by thermal annealing under the same bending conditions. The crystallite size and crystal orientation improved when the films were set to a concave shape during the film deposition and post-thermal annealing processes. This phenomenon occurred presumably because the substrate shrinkage was counteracted by the stretching of the substrate, and the strains did not accumulate much in the films, resulting in improved crystal growth. Consequently, the electrical conductivity was increased, and the resulting power factor of 16.1 μW/(cm K2) is 1.4 times higher than that of the Bi2Te3 film formed on a flat substrate.

Enhancing thermoelectric properties of bismuth telluride and germanium telluride thin films for wearable energy harvesting

High performance semiconducting thin films enable the implementation of thermoelectric generators as energy harvesters for wearable applications. The optimization of material properties is critical in such applications, as restrictions introduced by the substrates on deposition temperature and active layer size, impose fundamental limitations on performance. Here we present the optimization of sputtered bismuth telluride, BiTe, and germanium telluride, GeTe, annealed at 300 °C allowing comfortable development on polyimide substrates. The crystal structure and material composition of the films, before and after annealing, were measured by X-ray diffraction and X-ray photoelectron spectroscopy to reveal the changes in the material that enhanced the performance. The power factor of the BiTe films increased post-anneal up to 2.2 µW/cmK2, whilst the GeTe increased over 5 orders of magnitude to 7.6 µW/cmK2. A flexible thermoelectric generator was fabricated with pairs of alternating annealed BiTe and GeTe strips, reaching 7 nW per pair of output power at a temperature difference of 20 °C. Using this micro-fabrication process of thin films, compatible to roll-to-roll technologies, the physical dimensions of the generators can be tuned to deliver the required power, for providing storage energy for on-demand devices such as periodic sensing.

High ZT and performance controllable thermoelectric devices based on electrically gated bismuth telluride thin films

High-ZT materials and high-performance devices have always been hot topics in the field of thermoelectric applications. Here we present a field effect transistor (FET) based method to optimize the thermoelectric properties of bismuth telluride thin films, by which Seebeck coefficient, electrical conductivity and carrier type of the films are continuously controllable. As a result, the maximum power factor of the sample reaches 14.9 µWcm−1 K−2 in N-type and 12.5 µWcm−1 K−2 in P-type at room temperature. As compared with the bulk, the thermal conductivity of the bismuth telluride thin film is greatly reduced, which is measured to be lower than 0.37 Wm−1 K−1. The actual ZT of the sample exceeds 1.22 in N-type and 1.02 in P-type at 303 K, respectively. A π-shaped in-plane N-P pair device was built by applying different gate voltage on the bismuth telluride thin films at the two legs, which open circuit voltage is 10.5 mV and the maximum output power is 10.3 nW at a temperature difference of 30 K. By using the Te-doped bismuth telluride thin films, the intrinsic carrier concentration of the sample decreases to 1.05 × 1017/cm3 (N-type) and the films can be tuned into N-type and P-type by the opposite gate voltage symmetrically. Using this material, a thin film thermocouple with a quasi-linearly adjustable sensitivity from 2.4 to 225.5 µVK−1 by voltage was obtained. This work provides a general method to obtain high-ZT of thermoelectric materials and new ideas for fabricating performance controllable device with the same thermoelectric material, which greatly simplifies the material growth process in device manufacturing.

Temperature and Strain Effects in Micro-Raman Thermometry for Measuring In-Plane Thermal Conductivity of Thin Films

ABSTRACT Micro-Raman thermometry is an effective method for measuring thermal conductivity of thin films. It features noncontact and nondestructive probing and convenience of sample preparation. However, there is a concern of its accuracy when using the Raman peak shift as the temperature transducer since it responds to both temperature and strain upon optical heating. In this work, a series of detailed experiments are carried out to evaluate contributions to Raman signals from temperature only vs. from thermomechanical strain. It is shown that using proper calibration, contributions to Raman signals from temperature only and from thermomechanical strain can be decoupled and thermal conductivity can be evaluated correctly. These procedures are then applied to bismuth telluride thin films to illustrate measurement of thin film thermal conductivity.

Static and Dynamic Postannealing Strategies for Roll-to-Roll Fabrication of DC Magnetron Sputtered Bismuth Telluride Thin Films onto Polymer Webs.

High-throughput roll-to-roll processes are desirable to scale up the manufacture of flexible thermoelectric generators. While vacuum deposition onto a heated dynamic substrate presents a considerable engineering challenge, viable postdeposition in-line annealing processes are considered as an alternative to improve the functional performance of as-deposited films. The effect of infrared and electron-beam irradiations of 1 μm thick bismuth telluride thin films, produced by a vacuum roll-to-roll process for use as thermoelectric materials, was examined. A static vacuum oven and pulsed high-energy electron beam were also studied as control groups. All annealing strategies increased the crystallite size and decreased the Te content. Only the static vacuum oven treatment was shown to significantly improve the film's crystallinity. After 1 h annealing, the power factor improved by 400% (from 2.8 to 14 × 10-4 W/mK2), which, to the knowledge of the authors, is the highest reported thermoelectric performance of postannealed or hot-deposited Bi-Te films. As for in-line annealing, infrared and electron-beam post treatments improved the power factor by 146% (from 2.8 to 6.9 × 10-4 W/mK2) and 64% (from 2.8 to 4.6 × 10-4 W/mK2), respectively.

Fabrication, micro-structure characteristics and transport properties of co-evaporated thin films of Bi2Te3 on AlN coated stainless steel foils

N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)-coated stainless steel foil substrate to obtain optimal thermoelectric performance. The thermal co-evaporation method was adopted so that we could vary the thin film composition, enabling us to investigate the relationship between the film composition, microstructure, crystal preferred orientation and thermoelectric properties. The influence of the substrate temperature was also investigated by synthesizing two sets of thin film samples; in one set the substrate was kept at room temperature (RT) while in the other set the substrate was maintained at a high temperature, of 300 °C, during deposition. The samples deposited at RT were amorphous in the as-deposited state and therefore were annealed at 280 °C to promote crystallization and phase development. The electrical resistivity and Seebeck coefficient were measured and the results were interpreted. Both the transport properties and crystal structure were observed to be strongly affected by non-stoichiometry and the choice of substrate temperature. We observed columnar microstructures with hexagonal grains and a multi-oriented crystal structure for the thin films deposited at high substrate temperatures, whereas highly (00 l) textured thin films with columns consisting of in-plane layers were fabricated from the stoichiometric annealed thin film samples originally synthesized at RT. Special emphasis was placed on examining the nature of tellurium (Te) atom based structural defects and their influence on thin film properties. We report maximum power factor (PF) of 1.35 mW/m K2 for near-stoichiometric film deposited at high substrate temperature, which was the highest among all studied cases.

Identifying Optimal Strain in Bismuth Telluride Thermoelectric Film by Combinatorial Gradient Thermal Annealing and Machine Learning.

The thermoelectric properties of bismuth telluride thin film (BTTF) was tuned by inducing internal strain through a combination of combinatorial gradient thermal annealing (COGTAN) and machine learning. BTTFs were synthesized via magnetron sputter coating and then treated by COGTAN. The crystal structure and thermoelectric properties, namely Seebeck coefficient and thermal conductivity, of the treated samples were analyzed via micropoint X-ray diffraction and scanning thermal probe microimaging, respectively. The obtained combinatorial data reveals the correlation between internal strain and the thermoelectric properties. The Seebeck coefficient of BTTF exhibits largest sensitivity, where the value ranges from 7.9 to -108 μV/K. To further explore the possibility to enhance Seebeck coefficient, the combinatorial data were subjected to machine learning. The trained model predicts that optimal strains of 3-4% and 1-2% along the a- and c-axis, respectively, significantly improve Seebeck coefficient. The technique demonstrated herein can be used to predict and enhance the performance of thermoelectric materials by inducing internal strain.

Direct measurement of the thermoelectric properties of electrochemically deposited Bi2Te3 thin films

The best known thermoelectric material for near room temperature heat-to-electricity conversion is bismuth telluride. Amongst the possible fabrication techniques, electrodeposition has attracted attention due to its simplicity and low cost. However, the measurement of the thermoelectric properties of electrodeposited films is challenging because of the conducting seed layer underneath the film. Here, we develop a method to directly measure the thermoelectric properties of electrodeposited bismuth telluride thin films, grown on indium tin oxide. Using this technique, the temperature dependent thermoelectric properties (Seebeck coefficient and electrical conductivity) of electrodeposited thin films have been measured down to 100 K. A parallel resistor model is employed to discern the signal of the film from the signal of the seed layer and the data are carefully analysed and contextualized with literature. Our analysis demonstrates that the thermoelectric properties of electrodeposited films can be accurately evaluated without inflicting any damage to the films.

Atomic composition changes in bismuth telluride thin films by thermal annealing and estimation of their thermoelectric properties using experimental analyses and first-principles calculations

Abstract We investigated the structural and thermoelectric properties of bismuth telluride films when the atomic composition is changed by evaporation of Te atoms from the films during thermal annealing. The bismuth telluride films were deposited using a magnetron radio frequency sputtering method without a substrate heating. Thereafter, the films were thermally annealed up to 450 °C in an electric furnace with Ar(95%)–H2(5%) gas mixture. The atomic composition of the as-deposited film was approximately that of Bi2Te3. By increasing the annealing temperature, the Te atoms were evaporated from the films and the composition approached that of Bi1Te1, as discovered from electron probe microanalysis. Furthermore, a change in crystalline phase was observed through X-ray diffraction patterns. When the thermoelectric properties of the films were measured at room temperature, the carrier concentration was significantly affected by the change in atomic composition. We confirmed this finding using energy band structures using first-principles calculations. The highest power factor of 16.9 μW/(cm∙K2) was achieved at an annealing temperature of 300 °C because the film achieved a high crystallinity and crystal orientation with relatively low defects owing to less evaporation of Te atoms.

Study on thermoelectric property optimization of mixed-phase bismuth telluride thin films deposited by co-evaporation process

The development of Bi2Te3 thin films has huge potential in the pursuit of efficient thermoelectric micro/nanodevices due to their high Seebeck coefficient, high electrical conductivity and low thermal conductivity. The optimization of experimental parameters of Bi-Te thin films produced by co-evaporation will be investigated in this study. Co-evaporation is a low cost, easy-to-control process which can be used for high throughput and is scalable. We found that an optimal Te/Bi ratio of 1.5 with good thermoelectric properties can be directly synthesized by Bi and Bi2Te3 co-evaporation. Compared to the conventional Bi/Te co-evaporation process, high temperature annealing or substrate heating is not necessary for the process mentioned in this paper, which is a desirable feature when using polymer-based substrates, organic/inorganic hybrid thermoelectric generators, and flexible devices since they have relatively low tolerance to heat. The optimized Bi2Te3 thin films, which are mixed phases of Bi2Te3, Bi3Te4 and Te, possess high carrier concentration (6.65 × 1020 cm−3), low electrical resistivity (3.17 × 10−3 Ωcm), and extremely low thermal conductivity (0.59 W/mK) at room temperature on a smooth surface (roughness <5.5 nm) and are achieved by adjusting the deposition rate of Bi and Bi2Te3. The correlation between the structures of mixed phases, electrical and thermal properties will be discussed in detail.

Transport properties of the bismuth telluride thin films with different stoichiometry in the temperature range 77-300 K

Transport properties of the bismuth telluride thin films with different stoichiometry in the temperature range 77-300 K

Effect of an Organic Additive, N-Dimethyldithiocarbamic acid Ester Sodium salt, on Properties of Bismuth Telluride thin Films Fabricated by Galvanic Displacement Reaction of Electroplated Cobalt thin Films

Effect of an Organic Additive, N-Dimethyldithiocarbamic acid Ester Sodium salt, on Properties of Bismuth Telluride thin Films Fabricated by Galvanic Displacement Reaction of Electroplated Cobalt thin Films