Search

Herein, we have reported domain engineering of epitaxial (001) Bi2Te3 thin films by miscut (100) substrates. On a nominal flat (100) GaAs substrate, two-variant domains that were in-plane rotated by 60°, including the 60° domain boundaries, were formed in the epitaxial Bi2Te3 film, such that the symmetry elements of two-fold rotational and/or mirror symmetries of the GaAs substrate were preserved. The domain variants were successfully reduced to obtain mono-domain Bi2Te3 thin films without any domain boundaries using the 2°-miscut GaAs substrates, where a particular step-and-terrace structure on the vicinal surface macroscopically broke the intrinsic symmetry of GaAs, lowering the number of possible domains. Depending on the miscut directions, the in-plane orientations of the mono-domain Bi2Te3 films were varied with respect to the GaAs substrate. A model was proposed to explain the effect of miscut substrate on the domain structure of Bi2Te3 thin films. Low-temperature Hall measurements revealed that in the intrinsic regime (10 K) the electron concentration of the mono-domain Bi2Te3 films (~2 × 1018 cm−3) was significantly lower than that of the two-domain films (~1019 cm−3). This was attributed to the donor-like effect of the 60° domain boundaries. These results provide an opportunity not only to integrate the single-crystalline, mono-domain, layered-chalcogenides on semiconductor single crystals, but also to manipulate their electronic transport properties by domain engineering.

The ease of Te sublimation from Bi2Te3-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi2Te3-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi2Te2.7Se0.3 powders resulted in significant suppression of the generation of pores in Bi2Te2.7Se0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi2Te2.7Se0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi2Te3-based alloys without sacrificing thermoelectric performance.

A wearable thermoelectric generator (WTEG) that utilizes human body heat can be a promising candidate for the wearable power generators. The temperature difference (ΔT) between the body and the environment is a stable source driving the WTEG, but this driving force is limited by the ambient temperature itself at the same time. Here, a novel WTEG that can be operated using the dual source of body heat and light with exceptionally high driving force is fabricated. The printable solar absorbing layer attached to the bottom of the WTEG absorbs ≈95% of the light from ultraviolet to far infrared and converts it into heat. To optimize the power density of WTEGs, the fill factor of the thermoelectric (TE) leg/electrode is considered through finite-difference time-domain (FDTD) simulation. When operated by the dual sources, the WTEG exhibits a power density of 15.33 µW cm

The atomic layer deposition (ALD) of SnO 2 films has drawn interest from many researchers as a way to enhance the performance of functional materials for emerging applications such as transistors, sensors, and transparent electrodes. Among the critical issues in the exploitation of SnO 2 ALD in industrial applications is the lack of commercially available Sn precursors. Here, we demonstrate an ALD process for SnO 2 films using commercially available tetraethyltin (TET) as a precursor. H 2 O 2 is used as the oxygen source to overcome the lack of reactivity between TET and H 2 O, consequently the reaction of TET and H 2 O 2 results in the growth of SnO 2 films. The ALD process has a wide ALD window of 250–400 °C. In the range of the growth temperature, the grown films show a high density of ~6.2 g/cm 3 and an optical band gap of 3.7–3.9 eV, which is comparable to that of bulk SnO 2 . Negligible impurities remained in the films grown over the entire temperature range. The crystallization behavior and electrical properties of the SnO 2 films were examined as well. The ability of this ALD process to produce high-quality SnO 2 films with a commercially available tin precursor will allow it to be exploited in various applications.

Grafting nanotechnology on thermoelectric materials leads to significant advances in their performance. Creation of structural defects including nano-inclusion and interfaces via nanostructuring ac...

A topological insulator (TI), a new quantum state featured with the topologically-protected surface state (TSS) originating from its unique topology in band structure, has attracted much interest due to academic and practical importance. Nonetheless, a large contribution of the bulk conduction, induced by unintended doping by defects, has hindered the characterization of the surface state and the application of it into a device. To resolve this problem, we have investigated the transport properties of epitaxial Bi 2-δ Sn δ Te 3 thin films with varying δ. With the bulk conduction being strongly suppressed, the TSS is separately characterized, resulting in a large phase relaxation length of ∼250 nm at 1.8 K. In addition, the magnetoresistance ratio (MR) has shown a non-monotonic temperature dependence with a maximum value at an elevated temperature depending on δ. These results are associated with the compensation of carriers and, we believe, provide an important step for the application of topological insulators for developing novel functional devices based on the topological surface states.

The carrier concentration in Bi2Te3-based alloys is a decisive factor in determining their thermoelectric performance. Herein, we propose a novel approach to modulate the carrier concentration via the encapsulation of the alloy precursor powders. Atomic layer deposition (ALD) of ZnO and SnO2 was performed over the Bi2Te2.7Se0.3 powders. After spark plasma sintering at 500 &deg, C for 20 min, the carrier concentration in the ZnO-coated samples decreased, while the carrier concentration in the SnO2-coated samples increased. This trend was more pronounced as the number of ALD cycles increased. This was attributed to the intermixing of the metal ions at the interface. Zn2+ substituted for Bi3+ at the interface acted as an acceptor, while Sn4+ substituted for Bi3+ acted as a donor. This indicates that the carrier concentration can be adjusted depending on the materials deposited with ALD. The use of fine powders changes the carrier concentration more strongly, because the quantity of material deposited increases with the effective surface area. Therefore, the proposed approach would provide opportunities to precisely optimize the carrier concentration for high thermoelectric performance.

We investigated the magnetotransport properties of Bi2Te3 films grown on GaAs (001) substrate by a cost-effective metallo-organic chemical vapor deposition (MOCVD). We observed the remarkably high carrier mobility and the giant linear magnetoresistance (carrier mobility ∼ 22 000 cm(2) V(-1) s(-1), magnetoresistance ∼ 750% at 1.8 K and 9 T for a 100 nm thick film) that depends on the film thickness. In addition, the Shubnikov-de Haas oscillation was observed, from which the effective mass was calculated to be consistent with the known value. From the thickness dependence of the Shubnikov-de Haas oscillation, it was found that a two dimensional electron gas with the conventional electron nature coexists with the topological Dirac fermion states and dominates the carrier transport in the Bi2Te3 film with thickness higher than 300 nm. These results are attributed to the intrinsic nature of Bi2Te3 in the high-mobility transport regime obtained by a deliberate choice of the substrate and the growth conditions.

The initial growth behavior of ZnO films by atomic layer deposition (ALD) on layer-structured Bi2Te3 was investigated. Despite the lack of adsorption sites on the basal plane of Bi2Te3, negligible incubation in the ALD of ZnO on Bi2Te3 was found in the temperature range from 100 to 160 °C, and even the enhancement of the initial growth was observed at 200 °C. We demonstrate that a ZnTe interfacial layer was formed in the early growth stage by the interaction between diethylzinc and Bi2Te3, which improved the nucleation of ZnO on the basal plane of Bi2Te3. These results indicate that surface modification via the interaction between a precursor and layer-structured materials is an efficient way to achieve fluent and uniform nucleation on layer-structured materials such as Bi2Te3.

SnO 2 thin films were grown by atomic layer deposition (ALD) with dimethylamino-2-methyl-2-propoxy-tin(II) (Sn(dmamp) 2 ) and O 3 in a temperature range of 100–230 °C. The ALD window was found to be in the range of 100–200 °C. The growth per cycle of the films in the ALD window increased with temperature in the range from 0.018 to 0.042 nm/cycle. Above 230 °C, the self-limiting behavior which is a unique characteristic of ALD, was not observed in the growth because of the thermal decomposition of the Sn(dmamp) 2 precursor. The SnO 2 films were amorphous in the ALD window and exhibited quite a smooth surface. Sn ions in all films had a single binding state corresponding to Sn 4+ in SnO 2 . The concentration of carbon and nitrogen in the all SnO 2 films was below the detection limit of the auger electron spectroscopy technique and a very small amount of carbon, nitrogen, and hydrogen was detected by secondary ions mass spectroscopy only. The impurity contents decreased with increasing the growth temperature. This is consistent with the increase in the density of the SnO 2 films with respect to the growth temperature. The ALD process with Sn(dmamp) 2 and O 3 shows excellent conformality on a hole structure with an aspect ratio of ∼9. This demonstrates that the ALD process with Sn(dmamp) 2 and O 3 is promising for growth of robust and highly pure SnO 2 films.

The effect of Sn doping on the thermoelectric properties of p-type Bi0.4Sb1.6Te3 (BST) thin films was studied. Sn-doped BST films were grown on 4° tilted GaAs (001) substrates by metal–organic chemical vapor deposition. To control the Sn ion concentration in the films, we systematically controlled the dose of the Sn precursor by varying the H2 flow rate from 0 sccm to 100 sccm. The hole carrier concentration increased as the H2 flow rate was increased. Interestingly, the Seebeck coefficient of the films simultaneously increased with the carrier concentration when the H2 flow rate was increased up to 60 sccm. This might be attributed to the formation of virtual bound states in the valence band by Sn doping. Consequently, the Sn ion doping contributed to the thermopower enhancement of the BST films.

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.

Interfaces, such as grain boundaries in a solid material, are excellent regions to explore novel properties that emerge as the result of local symmetry-breaking. For instance, at the interface of a layered-chalcogenide material, the potential reconfiguration of the atoms at the boundaries can lead to a significant modification of the electronic properties because of their complex atomic bonding structure. Here, we report the experimental observation of an electron source at 60° twin boundaries in Bi2Te3, a representative layered-chalcogenide material. First-principles calculations reveal that the modification of the interatomic distance at the 60° twin boundary to accommodate structural misfits can alter the electronic structure of Bi2Te3. The change in the electronic structure generates occupied states within the original bandgap in a favourable condition to create carriers and enlarges the density-of-states near the conduction band minimum. The present work provides insight into the various transport behaviours of thermoelectrics and topological insulators., Grain boundaries in polycrystalline materials may offer the opportunity to explore physical phenomena that do not normally occur within the crystal grains. Here, the authors show that twin boundaries in Bi2Te3 works as an electron supply for the whole bulk material.

Bismuth telluride films were prepared by a modified metal organic chemical vapor deposition (MOCVD) on (001) GaAs substrates, and their thermoelectric properties were investigated. In the modified MOCVD system used in this study, metal organic sources of bismuth and tellurium were mixed with hydrogen gas in a graphite mixing room which can be heated by radio frequency induction and transferred to the substrate through a planar gap of mixing room. The effect of deposition parameters such as Te/Bi ratio in the reactor and mixing room temperature on the surface morphologies and the thermoelectric properties of the films was investigated. It was found that the grain size and the growth rate of the films can be controlled by adjusting the mixing room temperatures. Growth rates 2–3 times faster than that of the conventional MOCVD were obtained. A maximum growth rate of 7 μm/h was achieved at the mixing room temperature of 300 °C. The highest Seebeck coefficient was about −225 μV/K. Reduction of thermal conductivity can be expected when the size of grain can be controlled to nano-scale. The results in this study suggest that the fabrication of Bi 2 Te 3 films with high thermoelectric performance using a high throughput process can be achieved by the modified MOCVD system used in this work.

Bi2Te3-In2Se3 films were prepared by co-sputtering followed by annealing, and their structural and thermoelectric properties were investigated. The immiscible nature of the two alloys results in precipitation of the second phase, thus leading to structures with self-assembled dots that are a few nanometers in scale. HAADF-STEM and HRTEM were used to confirm that In2Se3 nanodots that were a few nanometers in size did indeed form in the Bi2Te3 thin film. It was found that the incorporation of these nanodots can reduce the thermal conductivity of the thin film.

Epitaxial CdTe thin films were grown on GaAs/Si(001) substrates by metalorganic chemical vapor deposition using thin GaAs as a buffer layer. The interfaces were investigated using high-resolution transmission electron microscopy and geometric phase analysis strain mapping. It was observed that dislocation cores exist at the CdTe/GaAs interface with periodic distribution. The spacing of the misfit dislocation was measured to be about 2 nm, corresponding to the calculated spacing of a misfit dislocation (2.6 nm) in CdTe/Si with Burgers vector of a[110]/2. From these results, it is suggested that the GaAs buffer layer effectively absorbs the strain originating from the large lattice mismatch between the CdTe thin film and Si substrate with the formation of periodic structural defects.

Thermoelectric bismuth telluride () films were deposited on off oriented (001) GaAs substrates using a modified metal organic chemical vapor deposition (MOCVD) system. The effects of substrate temperature on surface morphologies, crystallinity, electrical properties and thermoelctric properties were investigated. Two dimensional growth mode (2D) was observed at substrate temperature lower than . However, three dimensional growth mode (3D) was observed at substrate temperature higher than . Change of growth mechanism from 2D to 3D was confirmed with environmental scanning electron microscope (E-SEM) and X-ray diffraction analysis. Seebeck coefficients of all samples have negative values. This result indicates that films grown by modified MOCVD are n-type. The maximum value of Seebeck coefficient was -225 and the power factor was at the substrate temperature of . films deposited using modified MOCVD can be used to fabricate high-performance thermoelectric devices.

The liquid phase epitaxy(LPE) method was widely used to growth of mercury cadmium telluride(MCT) thin films. However, this method lead to Hg-vacancies in MCT thin film, because Hg has high vapor pressure at this temperature range. This is a well known defect in HgCdTe grown by LPE method. In this study, we report the development of techniques for improving the crystalline quality and controlling the composite uniformity of HgCdTe thin films using high- pressure Hg-ambient annealing method. As a result, we achieved the improvement of the composite uniformity of HgCdTe thin films. It was observed by the high angle annular dark field scanning TEM(HAADF-STEM) analysis. Moreover, new HgTe phase and a shrinking of lattice fringe were observed.

Nano structure films were deposited on (100) GaAs substrates using a modified MOCVD system and the effect of growth parameters on the structural properties were investigated. Different from conventional MOCVD systems, our reactor consist of pressure control unit and two heating zones ; one for formation of nano-sized particles and the other for the growth of nano particles on substrates. By using this instrument we successfully grow films with nano-grain size. The film grown at high reactor pressure has large grain size. On the contrast, the grain size decreases with a decrease in pressure of the reactor. Here, we introduce new growth methods of nano-grain structured films for high thermoelectric figure of merit.

In order to design nano structured materials with enhanced thermoelectric properties, the alloys in the pseudo-binary Bi2Te3-PbTe system are investigated for their micro structure properties. For this synthesis, the liquid alloys are cooled by the water quenching method. Micro structure images are obtained by using an electron probe micro analyzer(EPMA). Dendritic and lamellar structures are clearly observed with the variation in the composition ratio between Bi2Te3 and PbTe. The increase in the Bi2Te3 composition ratio causes to change of the structure from dendritic to lamellar. The Seebeck coefficient of sample 5, in which the mixture rate of Bi2Te3 is 83%, is measured as the highest value. In contrast, the others decrease with the increase of the Bi2Te3 composition ratio. Meanwhile, p-type characteristics are observed in sample 6, at 91%-Bi2Te3 mixture rate. The power factors of the all samples are calculated with the Seebeck coefficient and resistivity.

Single-crystalline CdTe(133) films have been grown by metalorganic chemical vapor deposition on Si(211) substrates. We studied the effect of various growth parameters on the surface morphology and structural quality of CdTe films. Proper oxide removal from the Si substrate is considered to be the principal factor that influences both the morphology and epitaxial quality of the CdTe films. In order to obtain single-crystalline CdTe(133) films, a two-stage growth method was used, i.e., a low-temperature buffer layer step and a high- temperature growth step. Even when the low-temperature buffer layer shows polycrystalline structure, the overgrown layer shows single-crystalline structure. During the subsequent high-temperature growth, two-dimensional crystallites grow faster than other, randomly distributed crystallites in the buffer layer. This is because the capturing of adatoms by steps occurs more easily due to increased adatom mobility. From the viewpoint of crystallographic orientation, it is assumed that the surface structure of Si(211) consists of (111) terrace and (100) step planes with an interplanar angle of 54.8°. This surface structure may provide many preferable nucleation sites for adatoms compared with nominally flat Si(100) or (111) surfaces. The surface morphology of the resulting films shows macroscopic rectangular-shaped terrace—step structures that are considered to be a (111) terrace with two {112} step planes directed toward 〈110〉.

We have investigated growth of CdTe thin films by using (As, GaAs) buffer layers for application of large scale IR focal plane arrays(IFPAs). Buffer layers were grown by molecular beam epitaxy(MBE), which reduced the lattice mismatch of CdTe/Si and prevented native oxide on Si substrates. CdTe thin films were grown by metal organic chemical deposition system(MOCVD). As a result, polycrystalline CdTe films were grown on Si(100) and arsenic coated-Si(100) substrate. In other case, single crystalline CdTe(400) thin film was grown on GaAs coated–Si(100) substrate. Moreover, we observed hillock structure and mirror like surface on the (400) orientated epitaxial CdTe thin film.

Epitaxial (001) CdTe thin film was successfully grown on a (001) Si substrate using an atomically smooth, thin (2 nm) GaAs buffer layer deposited using the metal organic chemical vapor deposition method. High-resolution transmission electron microscopy studies revealed a periodic distribution of dislocations due to lattice mismatch along the interfaces between CdTe film, GaAs buffer layer, and Si substrate. Most of the dislocation cores were located inside the GaAs buffer layer or at the interface between the CdTe and GaAs layers, indicating that GaAs buffer layer originally coherent to Si, was fully relaxed during the CdTe layer growth. The thin GaAs buffer layer effectively absorbed any strain coming from the large lattice mismatch between a CdTe layer and a Si substrate by forming an array of structural defects in the GaAs layer, which permitted epitaxial growth of CdTe on the Si substrates.

Bi2Te3-In2Se3 films were prepared by co-sputtering followed by annealing, and their structural and thermoelectric properties were investigated. The immiscible nature of the two alloys results in precipitation of the second phase, thus leading to structures with self-assembled dots that are a few nanometers in scale. HAADF-STEM and HRTEM were used to confirm that In2Se3 nanodots that were a few nanometers in size did indeed form in the Bi2Te3 thin film. It was found that the incorporation of these nanodots can reduce the thermal conductivity of the thin film.

A bismuth telluride (BT)/indium selenide (IS) multilayer film was deposited at room temperature by rf magnetron sputtering on a sapphire substrate in order to investigate how the multilayered structure affects the microstructure and thermoelectric properties. The effect of annealing at different temperatures was also studied. The results were compared with those from a BT film with the same thickness. A TEM study showed that the interface between the BT and IS layers became vague as the annealing temperature increased, and the BT layer crystallized while the IS layer did not. The presence of thin IS layers can help to limit the evaporation of Te from the BT/IS multilayer film, thus increasing the amount of Bi2Te3 phase in the multilayer film as compared with that of the BT film. An abrupt increase in the Seebeck coefficient of the multilayer film was observed when annealed at 300 degrees C, and the resistivity of the annealed multilayer film was high compared to that of the BT film. This result can also be explained by the proposed role of the IS layer, which limits the evaporation of Te at high temperature. The highest power factor of -3.9 x 10(-6) W/K2 cm was obtained at room temperature from the multilayer film annealed at 300 degrees C.