P-type Antimony Telluride Research Articles

Wafer-Scale Growth of Sb2Te3 Films via Low-Temperature Atomic Layer Deposition for Self-Powered Photodetectors.

In this work, we demonstrate the performance of a silicon-compatible, high-performance, and self-powered photodetector. A wide detection range from visible (405 nm) to near-infrared (1550 nm) light was enabled by the vertical p-n heterojunction between the p-type antimony telluride (Sb2Te3) thin film and the n-type silicon (Si) substrates. A Sb2Te3 film with a good crystal quality, low density of extended defects, proper stoichiometry, p-type nature, and excellent uniformity across a 4 in. wafer was achieved by atomic layer deposition at 80 °C using (Et3Si)2Te and SbCl3 as precursors. The processed photodetectors have a low dark current (∼20 pA), a high responsivity of (∼4.3 A/W at 405 nm and ∼150 mA/W at 1550 nm), a peak detectivity of ∼1.65 × 1014 Jones, and a quick rise time of ∼98 μs under zero bias voltage. Density functional theory calculations reveal a narrow, near-direct, type-II band gap at the heterointerface that supports a strong built-in electric field leading to efficient separation of the photogenerated carriers. The devices have long-term air stability and efficient switching behavior even at elevated temperatures. These high-performance and self-powered p-Sb2Te3/n-Si heterojunction photodetectors have immense potential to become reliable technological building blocks for a plethora of innovative applications in next-generation optoelectronics, silicon-photonics, chip-level sensing, and detection.

Ex Situ Photoelectron Emission Microscopy of Polycrystalline Bismuth and Antimony Telluride Surfaces Exposed to Ambient Oxidation.

The surfaces of textured polycrystalline N-type bismuth telluride and P-type antimony telluride materials were investigated using ex situ photoelectron emission microscopy (PEEM). PEEM enabled imaging of the work function for different oxidation times due to exposure to air across sample surfaces. The spatially averaged work function was also tracked as a function of air exposure time. N-type bismuth telluride showed an increase in the work function around grain boundaries relative to grain interiors during the early stages of air exposure-driven oxidation. At longer time exposure to air, the surface became homogenous after a ∼5 nm-thick oxide formed. X-ray photoemission spectroscopy was used to correlate changes in PEEM imaging in real space and work function evolution to the progressive growth of an oxide layer. The observed work function contrast is consistent with the pinning of electronic surface states due to the defects at a grain boundary.

Deposition and Fabrication of Sputtered Bismuth Telluride and Antimony Telluride for Microscale Thermoelectric Energy Harvesters

Thermoelectric (TE) n-type bismuth telluride (Bi2Te3) films and p-type antimony telluride (Sb2Te3) films are grown on SiO2/Si substrates via radio frequency magnetron sputtering. The crystal structures and TE properties are characterized for 1 µm and 10 µm films deposited using different deposition conditions and using various heat treatment conditions. Single-target sputtered deposition of n-type Bi2Te3 films resulted in a Te-deficient off stoichiometric films due to the evaporation of tellurium. Two-target co-sputtered deposition using Bi2Te3 and Te targets at room temperature and subsequent anneal at 250°C yielded a 10 µm n-type film with -102 µV/K for the Seebeck coefficient and 0.7 mW/K2.m for the power factor. Similarly, prepared p-type Sb2Te3 film but using a single sputtering target yielded +110 µV/K and 1.3 mW/K2.m for their Seebeck coefficient and power factor respectively. The fabrication of the micro-scale thermoelectric energy generator (µTEG) using separate n-type and p-type wafers is demonstrated.

Optimizing the Structural, Electrical and Thermoelectric Properties of Antimony Telluride Thin Films Deposited on Aluminum Nitride-coated Stainless Steel Foil

In this study, we examined the thermoelectric (TE) properties of co-evaporated p-type antimony telluride (Sb2Te3) thin films on aluminum nitride (AlN)-coated stainless steel foil substrates. We investigated the influence of composition and substrate temperature on the thin-film microstructure and transport properties, by varying the tellurium (Te) concentration in the thin films as well as the substrate temperature during deposition (room temperature (RT) and 300 °C). Thin films prepared with an RT substrate were further annealed at 264 °C to obtain crystallized thin films with high phase purity. Columnar thin films with large grains and a standard multi-oriented crystal structure were obtained when thin films were deposited on substrates heated to 300 °C. Thin films deposited at RT and subsequently annealed at 264 °C had a dense, layered microstructure, with a preferential c-axis or (00 l) texture as the compositions approached phase stoichiometry. The temperature dependence of the thermoelectric properties was measured, and variations were interpreted in terms of the deviation from stoichiometry and the obtained microstructure. A maximum power factor (PF) of 0.87 mW/m ∙ K2 was obtained for off-stoichiometric 65.0 at% Te thin film, which was the highest among the samples deposited at high substrate temperatures. A higher PF of 1.0 mW/m ∙ K2 was found for off-stoichiometric thin films with 64.5 at% Te, which was deposited at RT and subsequently annealed. The improvement of thermoelectric power in films containing excess Te could be related to energy dependent carrier scattering at the Sb2Te3/Te interface.

High-Performance μ-Thermoelectric Device Based on Bi2Te3/Sb2Te3 p-n Junctions.

A flexible and ultralight planar thermoelectric generator based on 15 thermocouples composed of n-type bismuth telluride (Bi2Te3) and p-type antimony telluride (Sb2Te3) legs (each with 400 nm thick) connected in series, on 25 μm thick Kapton substrate, was fabricated with impressive power factor values of 2.7 and 0.8 mW K-2 m-1 (at 298 K) for Bi2Te3 and Sb2Te3 films, respectively. The p-n junction thermoelectric device can generate a maximum open-circuit voltage and output power of 210 mV and 0.7 μW (3.3 mW cm-2), respectively, for a temperature difference of 35 K, which is higher than the one observed for a conventional thermoelectric device with metallic contacts for p-n junctions. The results were combined with numerical simulations, showing a good match between the experimental and the numerical results. The current density versus voltage (J-V) characteristics of the fabricated p-n junctions revealed a diode behavior with a turn-on voltage of ≈0.3 V and an impressive rectifying ratio (I+1V/I-1V) of ≈2 × 104.

Flexible Thermoelectric Module Using Bi-Te and Sb-Te Thin Films for Temperature Sensors

Electromotive force is generated by a temperature difference in a thermocouple formed of two different metals, known as the Seebeck effect. Flexible thermoelectric (TE) temperature sensors based on the Seebeck effect can be attached to the body to provide body temperature information to health providers via wireless communications. Materials used in flexible TE temperature sensors have higher Seebeck coefficients than those of traditional thermocouples, and hence generate a high output voltage even with small temperature changes, making them highly suitable for use as body temperature sensors. In this study, a flexible TE temperature sensor was fabricated using a cheap, commercial flexible printed circuit board substrate and TE thin films of n-type bismuth telluride and p-type antimony telluride by co-evaporation. The results show that the flexible TE temperature sensor has a sensitivity of 192.70 μV K−1.

Flexible Thermoelectric Device Using Thick Films for Energy Harvesting from the Human Body

A flexible thermoelectric device using body heat has drawn attention as a power source for wearable devices. In this study, thermoelectric thick films were fabricated by cold pressing method using p-type antimony telluride and n-type bismuth telluride powders in accordance with specific loads. Thermoelectric thick films were denser and improved the electrical and thermoelectric properties while increasing the load of the cold pressing. The thickness of the specimen can be controlled by the amount of material; specimens were approximately 700 um in thickness. Flexible thermoelectric devices were manufactured by using the thermoelectric thick films on PI (Polyimide) substrate. The process is cheap, efficient, easy and scalable. Evaluation of power generation performance and flexibility on the fabricated flexible thermoelectric device was carried out. The flexible thermoelectric device has great flexibility and good performance and can be applied to wearable electronics as a power source. Key words: Thermoelectric thick films, Flexible thermoelectric device, Cold pressing, Wearable electronics

Thickness dependence of the electrical and thermoelectric properties of co-evaporated Sb2Te3 films

P-type antimony telluride (Sb2Te3) films of various thicknesses (1-, 6-, 10-, and 16-μm) were deposited on an oxidized Si (100) substrate at 250°C by effusion cell co-evaporation. Microstructural analysis using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy revealed that the grains of the films grew in a mode in which recrystallization was prevalent and grain growth subdued, in contrast to typical film growth, which is often characterized by grain growth. The resultant microstructure exhibited narrow columnar grains, the preferred orientation of which changed with film growth thickness from (1010) with the 1-μm films to (015) for the 6- and 10-μm films, and finally (110) for the 16-μm films. Carrier mobility and the overall thermoelectric properties of the Sb2Te3 films were affected significantly by changes in the film microstructure; this was attributed to the strong anisotropy of Sb2Te3 regarding electrical conductivity. The highest power factor of 3.3mW/mK2 was observed for the 1-μm-thick Sb2Te3 film.

Synthesis and Evaluation of Thick Films of Electrochemically Deposited Bi₂Te₃ and Sb₂Te₃ Thermoelectric Materials.

This paper presents the results of the synthesis and evaluation of thick thermoelectric films that may be used for such applications as thermoelectric power generators. Two types of electrochemical deposition methods, constant and pulsed deposition with improved techniques for both N-type bismuth telluride (Bi2Te3) and P-type antimony telluride (Sb2Te3), are performed and compared. As a result, highly oriented Bi2Te3 and Sb2Te3 thick films with a bulk-like structure are successfully synthesized with high Seebeck coefficients and low electrical resistivities. Six hundred-micrometer-thick Bi2Te3 and 500-µm-thick Sb2Te3 films are obtained. The Seebeck coefficients for the Bi2Te3 and Sb2Te3 films are −150 ± 20 and 170 ± 20 µV/K, respectively. Additionally, the electrical resistivity for the Bi2Te3 is 15 ± 5 µΩm and is 25 ± 5 µΩm for the Sb2Te3. The power factors of each thermoelectric material can reach 15 × 10−4 W/mK2 for Bi2Te3 and 11.2 × 10−4 W/mK2 for Sb2Te3.

Effects of Cr interlayer thickness on adhesive, structural, and thermoelectric properties of antimony telluride thin films deposited by radio-frequency magnetron sputtering

We investigated the effects of Cr interlayer thickness on adhesive, structural, and thermoelectric properties of p-type antimony telluride (Sb2Te3) thin films. Prior to depositing Sb2Te3 films with a thickness of 1μm using radio-frequency magnetron sputtering, a Cr film with a thickness ranging from 0 to 216nm was deposited on a glass substrate using the same sputtering equipment. The obtained samples were thermally annealed at 300°C for 1h. Adhesiveness of the films was examined by a cross-cut method. Adhesion of Sb2Te3 thin films could be improved by incorporating the Cr interlayer, independent of the thickness of the interlayer. The surface morphology of the Sb2Te3 thin films became flat with a dense structure as the thickness of the Cr interlayer increased. Atomic composition analysis indicated that Te atoms might be diffused into the Cr interlayer and formed Cr-based alloys when the Cr interlayer thickness was increased to >100nm. X-ray diffraction analyses indicated that the crystal orientation, crystallite size, and strain of the films reached their maximum values at Cr interlayer thicknesses of 46–70nm. Thermoelectric properties of the films were improved by incorporating the Cr interlayer of appropriate thickness. In particular, a power factor of 18.7μW/(cmK2) was achieved at a Cr interlayer thickness of 46nm, which was approximately 4 times larger than that of the film with no Cr interlayer.

Effect of thermal annealing on the structural and thermoelectric properties of electrodeposited antimony telluride thin films

We investigated the effect of thermal annealing on structural and thermoelectric properties of p-type antimony telluride (Sb2Te3) thin films fabricated on a stainless steel (SUS304) substrate by electrodeposition. Antimony telluride thin films were annealed for 1 h at temperatures between 200 and 400 °C. We observed that the as-deposited thin film and thin films annealed at temperatures less than 250 °C possessed a stoichiometric atomic composition (Sb:Te = 40:60) with no impurities from the substrate. At the annealing temperature of 300 °C, we observed a certain amount of impurities (Fe, Cr, Ni) in the thin film. The Seebeck coefficient was decreased but the electrical conductivity was increased for films treated at 300 °C. As a result, the thin film exhibited a maximum power factor of 13.6 μW/(cm·K2). On further increasing the annealing temperature to 400 °C, the film structure and thermoelectric properties were drastically changed. The impurity concentration in the thin film reached approximately 50 at.%. The crystal structure of Sb2Te3 completely disappeared, and instead, other chemical compounds formed by alloying the elements of the thin film and the substrate were observed.

アンチモン・テルル熱電薄膜の斜め蒸着法による配向性制御

We prepared p-type antimony telluride thin films by an oblique deposition at the incident angle ranging from 0º to 80º. We investigated the relationship between the structural and electrical properties of the thin films. As the structural properties, we analyzed cross-section morphology by scanning electron microscope (SEM), and the crystal orientation and crystallite size by x-ray diffraction (XRD) analysis. As the electrical properties, we measured in-plane electrical conductivity, Seebeck coefficient and power factor at room temperature. As a result, we found the thin film at the incident angle of 40° obtained the highest crystal orientation. As the incident angle increased, the crystallite size were enhanced but the power factor decreased. The power factor of thin film at the incident angle of 0° was 1.26 μW / ( cm·K2 ) and thin film at the incident angle of 80° was 0.38 μW / ( cm·K2 ).

Investigation of the effects of compressive and tensile strain on n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films for use in flexible thermoelectric generators

To assess the potential performances of flexible thin film thermoelectric generators, we prepared n-type bismuth telluride and p-type antimony telluride nanocrystalline thin films on flexible polyimide substrates using a RF magnetron sputtering method. We applied various compressive and tensile strains to the films by changing the bending radii of the flexible substrates prior to deposition. The structural and thermoelectric properties of the completed samples were analyzed. The bismuth telluride and antimony telluride thin films had dense granular and a chaff-like surface morphologies, respectively. The shapes of both types of films did not greatly change when strains were applied. Both types of thin films demonstrated higher electrical conductivities when under compressive strain than when under tensile strain. The absolute Seebeck coefficients of the bismuth telluride thin films were slightly higher when under tensile strain than when under compressive strain. In contrast, the antimony telluride thin films demonstrated the opposite trend, having higher values when under compressive strain than when under tensile strain. The power factors of both types of thin films were found to decrease with increasing tensile stain. Therefore, we expect that the thermoelectric performances of flexible thin film generators are not largely changed under compressive strain but they decrease under tensile strain.

30am2-PN-52 めっき法を用いたビスマステルライド系薄膜及び熱電モジュールに及ぼす熱処理の影響

N-type bismuth telluride (Bi-Te) and p-type antimony telluride (Sb-Te) thin films have been prepared by electrodeposition, followed thermal annealing. The electrodeposition was performed by changing the mole ratios of Te/(Bi+Te) and Sb/(Sb+Te). The thermal annealing was performed at 300 ℃ for 1 hour under Ar/H_2 (95/5%). We measured the in-plane Seebeck coefficient and electrical conductivity at room temperature, and then the power factor was estimated using both the measured properties. For n-type films, with the Te/(Bi+Te) of 0.5, the power factor of the Bi-Te thin film reached 4.5 μW/(cm・K^2). For p-type films, with the Sb/(Sb+Te) of 0.35, the power factor of the Sb-Te thin film reached 8.5 μW/(cm・K^2). The XRD peaks of both types of thin films exhibited well crystalized regardless of the mole ratios. Finally, we fabricated thermoelectric generators on flexible substrates by electrodeposition and transfer method, and observed that the generators worked.

21pm3-PM022 n型ビスマステルル及びp型アンチモンテルル薄膜熱電素子の応力負荷時の熱電特性

In order to investigate the effect of strain on p-type antimony telluride (Sb-Te) and bismuth telluride (Bi-Te) thin films, we applied various compressive and tensile strains to the thin films by changing the bending radius of a flexible substrate, wherein the strain ranged from -0.3% (compressive) to +0.3% (tensile). We analyzed the structural properties by XRD, and measured the thermoelectric properties at room temperature. It was found that the thermoelectric properties of p-type Sb-Te thin films were influenced by induced strains more than that of the n-type Bi-Te thin films. As a result, the power factor of p-type Sb-Te thin films at the strain of -0.15% (compressive strain) reached 10 μW/cm/K^2.

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb2Te3) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb2Te3 materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb2Te3 thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb2Te3 was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb2Te3 structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K−1 to 161 μV K−1 and 0.81 × 103 S cm−1 to 3.91 × 103 S cm−1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10−3 W m−1 K−2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb2Te3 thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.

Unique hierarchical structure and high thermoelectric properties of antimony telluride pillar arrays

The p-type antimony telluride (Sb2Te3) pillar arrays with unique hierarchical architecture have been self-assembled in large scale by a simple vacuum thermal evaporation technique. The composition and the microstructure of the films are studied by X-ray diffraction, scanning electron microscopy with energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. The results show that the hierarchical film with multi-scale and multi-dimensional structure is well-oriented pillar arrays perpendicular to the substrate. A large number of nanowires (NWs) are assembled into a submicro/micro-scaled pillar, while antisite defects and dislocations as well as other defects are found in the one-dimensional NWs. The growth mechanism of such nanostructure is proposed and investigated. The thermoelectric (TE) properties, i.e., electrical conductivity (σ), Seebeck coefficient (S), and thermal conductivity (κ) of the films are measured. The properties of the hierarchical Sb2Te3 film have been greatly enhanced in comparison with those of the ordinary Sb2Te3 films. A TE dimensionless figure-of-merit ZT = 0.88 in the novel hierarchical Sb2Te3 film is obtained at room temperature.

Optimal Design and Simulation of a Cross-Plane Micro-Thermoelectric Generator

This paper presents a new way to design a low-cost micro-thermoelectric generator (μ-TEG) which can be fabricated by using electrochemical and MEMS technology. The overall dimension of the μ-TEG is about 13mm × 13mm × 0.4mm, which contains 128 p- and n-type pairs of semiconductors connected electrically in series and thermally in parallel. The p-type antimony telluride (Sb2Te3) and n-type bismuth telluride (Bi2Te3) with an optimal thickness of 20μm were designed to deposit in a flexible polymer mold formed by photolithographic patterning of Polyimide (PI) with a three electrode configuration. Simulations of the thermocouple with PI mold were carried on, using finite element analysis. The analysis shows the possibility to achieve 3.5 mV while the difference in temperature is 10K and the thickness of the silicon substrate is 400μm, which reveals that the output power of the thermocouple without releasing process is only 4% lower than the one with the releasing process. Therefore the PI mold is not removed, considering the potential for easier fabrication and lower cost. The deposition parameters were also studied and optimized for the best thermoelectric performance. In our experiments, the n- and p-type semiconductors could be obtained when the voltage and current are around 50mV versus saturated calomel electrode (SCE) and 40 mA, respectively.

Digitally-controlled array of solid-state microcoolers for use in surgery

This paper presents an approach for generating a well-defined cooling pattern over an area of tissue. An array of solid-state microcoolers is used, which could be included in a probe that provides local cooling. This medical instrument can be used for removal of scar tissue in the eye or for the rapid stopping of bleeding due to micro-cuts, which makes it a useful tool to medical doctors and could make surgery more secure to the patient. The array of microcoolers is composed of 64 independent thermo-electric elements, each controlled using an integrated circuit designed in CMOS. The independent control allows the flexible programming of the surface temperature profile. This type of control is very suitable in case abrupt temperature steps should be avoided. Cooling by lateral heat flow was selected in order to minimize the influence of heat by dissipation from the electronic circuits. Moreover, a thermo-electric component with lateral heat allows fabrication of the cooling elements using planar thin-film technology, lithography and wet etching on top of the silicon wafer. This approach is potentially CMOS compatible, which would allow for the fabrication of the thermo-electric elements on top of a pre-fabricated CMOS wafer as a post-process step. Each pixel is composed of thin-films of n-type bismuth telluride, Bi2Te3 and p-type antimony telluride, Sb2Te3, which are electrically interconnected as thermocouple. These materials have excellent thermoelectric characteristics, such as thermoelectric figures-of-merit, ZT, at room temperatures of 0.84 and 0.5, respectively, which is equivalent to power-factors, PF, of 3.62 × 10−3 W K−1 m−2 and 2.81 × 10−3 W K−1 m−2, respectively. The theoretical study presented here demonstrates a cooling capability of 15°C at room temperature (300 K ≈ 27°C). This cooling performance is sufficient to maintain a local tissue temperature at 25°C, which makes it suitable for the intended application. A first prototype was successfully fabricated to demonstrate the concept.

Enhanced Thermopower via Carrier Energy Filtering in Solution-Processable Pt–Sb2Te3 Nanocomposites

This work demonstrates the first solution-processable metal-semiconductor nanocomposites with enhanced thermoelectric properties via carrier energy filtering. Platinum nanocrystals are embedded in a p-type antimony(III) telluride (Sb(2)Te(3)) semiconductor matrix, thus introducing band-bending potentials for holes. By scattering low energy holes, an increase in thermopower is observed. Introduction of Pt nanocrystals also increases carrier concentration thereby partially compensating for reduced electrical conductivity due to the decreased mobility. At room temperature, an improvement in thermoelectric power factor was achieved compared to that of the Sb(2)Te(3) films. This work highlights the possibility of combining a diverse set of n- and p-type semiconductor matrices with nanocrystals to engineer and optimize energy-dependent carrier scattering with the ease of materials processing.