Characterizations of solid-state microwave-synthesized Sb2Te3-based alloys with various compositions of bismuth in Bi2xSb2(1−x)Te3

In this article, thermoelectric (TE) materials based on p-type Sb₂Te₃ samples and dispersed with x amounts of Bi (x=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) in the form Bi_2xSb_2(1−x)Te₃ were synthesized via a standard solid-state microwave route. The microstructure of the ingots was characterized by field emission scanning electron microscopy. As-synthesized ingots were formed by the assembly of micro-sheet grains. The phase composition of the powders was characterized by X-ray diffraction, revealing a rhombohedral structure. The influence of variations in Bi content (x) on the TE properties of the resulting alloy was studied in the temperature range of 303 K to 523 K. Increases in x caused a decrease in hole concentration and electrical conductivity and an increase in Seebeck coefficient. A maximum power factor of 4.96 mW/mK² was obtained at about 373 K for a Bi_2xSb_2(1−x)Te₃ sample with x=0.2.     

Preparation and Characterization of Bi2Te3/Sb2Te3 Thermoelectric Thin-Film Devices for Power Generation

A device based on a new double-layer-leg thin-film concept has been successfully fabricated by flip-chip bonding of 242 pairs of n-type Bi₂Te₃ and p-type Sb₂Te₃ thin-film legs electrodeposited on top substrates to those processed on bottom substrates. Based on the output voltage–current curve, the internal resistance of the double-layer-leg thin-film device was measured to be 3.47 kΩ at an apparent temperature difference of 25.9 K across the device. The actual temperature difference across the thin-film legs was estimated to be 3.51 K, which is ～13% of the apparent temperature difference ΔT of 25.9 K applied across the thin-film device. The double-layer-leg thin-film device exhibited an open-circuit voltage of 0.43 V and maximum output power of 13.1 μW at an apparent temperature difference ΔT of 25.9 K.     

Charge transport mechanism of Al/Bi2Te3/Al thin film devices

Results of dielectric and conduction properties of vacuum evaporated Bi₂Te₃ thin film capacitors (Al/Bi₂Te₃/Al) have been reported in the frequency range 12 Hz to 100 kHz at various temperatures (303–423 K). The variation of capacitance and dielectric constant was found to be temperature and frequency dependent. The capacitance of the film decrease with increasing temperature which may be due to the expansion of the lattice and also due to the excitation of charge carriers at the sites of imperfection. The dielectric constant decreases with frequency at all temperature. This can be attributed to an interfacial polarization caused by space charge. The prevailing ac conduction mechanism in these films is hopping type. The activation energies were evaluated for various thicknesses and it is found to be 0.016 and 0.014 eV for the frequencies 200 Hz and 1 kHz, respectively.     

Fabrication of Microstructured thermoelectric Bi2Te3 thin films by seed layer assisted electrodeposition

In this work, bismuth telluride (Bi₂Te₃) thin films have been fabricated on Bi₂Te₃/ITO substrates by constant potential electrochemical deposition at room temperature. Bi₂Te₃ seed layers with different thicknesses (2 nm, 4 nm and 6 nm) were deposited onto ITO substrates using molecular beam epitaxy (MBE) method. The SEM images show that the morphology of Bi₂Te₃ thin films can be controlled not only by the deposition potential, but also the thickness of seed layer. Moreover, the morphologies of Bi₂Te₃ thin films with different thickness of seed layers tend to be similar and contain two-layer structure along the vertical direction after prolonged deposition time. Due to the two layers structure, Bi₂Te₃ thin films have shown different electrical conductivity performances. At room temperature, Bi₂Te₃ thin films with 4 nm-thick seed layer possess the maximum electrical conductivity value of 617.9 s cm^-1.     

Composition-Dependent Thermoelectric Properties of n-Type Bi2Te2.7Se0.3 Doped with In4Se3

We present the effects of In₄Se₃ addition on thermoelectric properties of n-type Bi₂Te_2.7Se_0.3. In this study, polycrystalline (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In₄Se₃ into Bi₂Te_2.7Se_0.3 resulted in segregation of In₄Se₃ phase within Bi₂Te_2.7Se_0.3 matrix. The Seebeck coefficient of the (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x samples exhibited lower values compared with that of pure Bi₂Te_2.7Se_0.3 phase. This reduction of Seebeck coefficient in n-type (In₄Se₃) _x -(Bi₂Te_2.7Se_0.3)_1?x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In₄Se₃) _x and (Bi₂Te_2.7Se_0.3)_1?x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In₄Se₃ leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In₄Se₃)_0.03-(Bi₂Te_2.7Se_0.3)_0.97 sample at 500 K.     

Thermoelectric Power Generation Characteristics of a Thin-Film Device Consisting of Electrodeposited n-Bi2Te3 and p-Sb2Te3 Thin-Film Legs

A thermoelectric thin-film device of the cross-plane configuration was fabricated by flip-chip bonding of the top electrodes to 242 pairs of electrodeposited n-type Bi₂Te₃ and p-type Sb₂Te₃ thin-film legs on the bottom substrate. The electrodeposited Bi₂Te₃ and Sb₂Te₃ films of 20-μm thickness exhibited Seebeck coefficients of ?59 μV/K and 485 μV/K, respectively. The internal resistance of the thin-film device was measured as 3.7 kΩ, most of which was attributed to the interfacial resistance of the flip-chip joints. The actual temperature difference ΔT _G working across the thin-film legs was estimated to be 10.4 times smaller than the apparent temperature difference ΔT applied across the thin-film device. The thin-film device exhibited an open-circuit voltage of 0.294 V and a maximum output power of 5.9 μW at an apparent temperature difference ΔT of 22.3 K applied across the thin-film device.     

Preparation and Thermoelectric Properties of Doped Bi2Te3-Bi2Se3 Solid Solutions

Since Bi₂Te₃ and Bi₂Se₃ have the same crystal structure, they form a homogeneous solid solution. Therefore, the thermal conductivity of the solid solution can be reduced by phonon scattering. The thermoelectric figure of merit can be improved by controlling the carrier concentration through doping. In this study, Bi₂Te_2.85Se_0.15:D _m (D: dopants such as I, Cu, Ag, Ni, Zn) solid solutions were prepared by encapsulated melting and hot pressing. All specimens exhibited n-type conduction in the measured temperature range (323 K to 523 K), and their electrical conductivities decreased slightly with increasing temperature. The undoped solid solution showed a carrier concentration of 7.37 × 10¹⁹ cm^?3, power factor of 2.1 mW m^?1 K^?1, and figure of merit of 0.56 at 323 K. The figure of merit (ZT) was improved due to the increased power factor by I, Cu, and Ag dopings, and maximum ZT values were obtained as 0.76 at 323 K for Bi₂Te_2.85Se_0.15:Cu_0.01 and 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005. However, the thermoelectric properties of Ni- and Zn-doped solid solutions were not enhanced.     

Fabrication of a CoSb3-based thermoelectric module

A CoSb₃-based thermoelectric module was fabricated using Ce_0.45Co_2.5Fe_1.5Sb₁₂ p-type leg and Yb_0.25Co₄Sb₁₂/Yb₂O₃ n-type leg. Ag–Cu foil was used to construct the junction of hot side legs. With two p–n couples, the module generated a maximum output power (P_max) of 140 mW and a maximum open-circuit voltage (V_o) of 210 mV under the thermal condition of hot side temperature T_h=810 K and a temperature difference ΔT=490 K. No deterioration in output power in vacuum was seen when thermal cycle of five times for the module was carried out under T_h=810 K and ΔT=490 K with natural cooling to room temperature, which shows the module has high durability.     

Composition effect on the structure and optical parameters of Ge–Se–Te thin films

The present work reported the influence of Ge content variation on the optical properties of Ge_xSe₅₀Te_50-x (x=0, 5, 15, 20, 35 at%). Vacuum thermal evaporation technique was employed to prepare amorphous Ge_xSe₅₀Te_50−x thin films. The stoichiometry of the chemical composition was checked by energy dispersive X-ray spectroscopy (EDX), whereas the thin films structure was determined by an X-ray diffraction and a scanning electron microscope (SEM). The optical absorption measurements were performed at room temperature in the wavelength range of 200–900 nm. Many optical constants were calculated for the studied thin films utilizing the optical absorption data. It was observed that the optical absorption mechanism follows the rule of the allowed direct transition. The optical band gap was found to increase from 2.31 to 2.60 eV as the Ge content increases from 0 to 35 at%. This result was explained in terms of the chemical bond approach.     

Thermoelectric Properties of n-Type Bi2Te3/PbSe0.5Te0.5 Segmented Thermoelectric Material

To investigate the effects of segmentation of thermoelectric materials on performance levels, n-type segmented Bi₂Te₃/PbSe_0.5Te_0.5 thermoelectric material was fabricated, and its output power was measured and compared with those of Bi₂Te₃ and PbSe_0.5Te_0.5. The two materials were bonded by diffusion bonding with a diffusion layer that was ～18 μm thick. The electrical conductivity, Seebeck coefficient, and power factor of the segmented Bi₂Te₃/PbSe_0.5Te_0.5 sample were close to the average of the values for Bi₂Te₃ and PbSe_0.5Te_0.5. The output power of Bi₂Te₃ was higher than those of PbSe_0.5Te_0.5 and the segmented sample for small ΔT (300 K to 400 K and 300 K to 500 K), but that of the segmented sample was higher than those of Bi₂Te₃ and PbSe_0.5Te_0.5 when ΔT exceeded 300 K (300 K to 600 K and 300 K to 700 K). The output power of the segmented sample was about 15% and 73% higher than those of the Bi₂Te₃ and PbSe_0.5Te_0.5 samples, respectively, when ΔT was 400 K (300 K to 700 K). The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges.     

Investigation on the thermal behavior of 0.15 μm gate-length In0.4Al0.6As/In0.4Ga0.6As MHEMT

In this study, we have successfully investigated the electrical performances of In_0.4Al_0.6As/In_0.4Ga_0.6As metamorphic high-electron-mobility transistor (MHEMT) at temperatures range from 275 K to 500 K comprehensively. By extracting the device S-parameters, the temperature dependent small signal model has been established. At room temperature, 0.15 μm T-gate device with double δ-doping design exhibits f_T and f_MAX values of 103 GHz and 204 GHz at V_ds = 1 V, an extrinsic transconductance of 678 mS/mm, and a current density of 578 mA/mm associated with a high breakdown voltage of ?13 V. Power measurements were evaluated at 40 GHz and the measured output power, linear power gain, and maximum power-added efficiency, were 7.12 dBm, 10.15 dB, and 23.1%, respectively. The activation energy (E_a) extracted from Arrhenius plots is = 0.34 eV at 150 ≦ T ≦ 350 K. The proposed device is promisingly suitable for millimeter-wave power application.     

Effects of Bi2Se3 Nanoparticle Inclusions on the Microstructure and Thermoelectric Properties of Bi2Te3-Based Nanocomposites

A series of thermoelectric nanocomposite samples were prepared by integrating Bi₂Se₃ nanoparticles into a bulk Bi₂Te₃ matrix. Primarily, spherical Bi₂Se₃ nanoparticles with diameter of ～30 nm were synthesized by combining bismuth acetate with elemental Te in oleic acid solution. Bi₂Te₃-based nanocomposite samples were prepared by consolidating the appropriate quantity of Bi₂Se₃ nanoparticles with the starting elements (Bi and Te) using typical solid-state synthetic reactions. The microstructure and composition of the Bi₂Te₃-based nanocomposites, as well as the effects of the Bi₂Se₃ nanoparticles on their thermoelectric properties, are investigated. Transmission electron microscopy observation of the Bi₂Te₃-based nanocomposites reveals two types of interface between the constituent materials, i.e., coherent and incoherent, depending on the Bi₂Se₃ concentration. The Bi₂Se₃ nanoparticles in the Bi₂Te₃ matrix act as scattering centers for a wider range of phonon frequencies, thereby reducing the thermal conductivity. As a result, the maximum ZT value of 0.75 is obtained for the Bi₂Te₃ nanocomposite with 10 wt.% Bi₂Se₃ nanoparticles at room temperature. It is clear that the reduction in the thermal conductivity plays a central role in the enhancement of the ZT value.     

Annealing effect on structural and optical properties of Se87.5Te10Sn2.5 thin films

Thin films of Se_87.5Te₁₀Sn_2.5 were prepared by vacuum thermal evaporation technique. Various optical constants were calculated for the studied composition. The mechanism of the optical absorption follows the rule of direct transition. It was found that the optical energy gap (E_g) decreases from 2.26 to 1.79 eV with increasing the annealing temperature from 340 to 450 K. This result can be interpreted by the Davis and Mott model. On the other hand, the maximum value of the refractive index (n) is shifted towards the long wavelength by increasing the annealing temperature. In addition, the high frequency dielectric constant (ε_L) increased from 31.26 to 48.11 whereas the ratio of the free carriers concentration to its effective of mass N/m^⁎ decreased from 4.3 to 2.09 (×10⁵⁷ (m⁻³ Kg⁻¹)). The influence of annealed temperature on the structure was studied by using the X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies show that the as-deposited films are amorphous in nature, but the crystallinity improved with increasing the annealing temperature. Furthermore the particle size and crystallinity increased whereas the dislocation and strains decreased with increasing the annealing temperature. SEM examination showed that the annealing temperature induced changes in the morphology of the as-deposited film.     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

Raman scattering in the Bi2(Te0.9Se0.1)3 solid solution films

The Bi₂(Te_0.9Se_0.1)₃ solid solutions thin films are produced by ??hot wall?? thermal evaporation in vacuum. From the data of X-ray diffraction studies, atomic-force microscopy of the surface relief, and Raman spectroscopy, it is established that vacuum thermal annealing at a temperature of 200°C for 1 h substantially increases the degree of film crystallization. The laser radiation excitation power optimal for studies of the Raman spectra of the Bi₂(Te_0.9Se_0.1)₃ films is determined.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.     

High-Efficiency Thermoelectric Module Based on High-Performance Bi0.42Sb1.58Te3 Materials

Bismuth-telluride-based alloy is the sole thermoelectric candidate for commercial thermoelectric application in low-grade waste heat harvest near room temperature, but the sharp drop of thermoelectric properties at higher temperature and weak mechanical strength in zone-melted material are the main obstacles to its wide development for power generation. Herein, an effective approach is reported to improve the thermoelectric performance of p-type Bi_0.42Sb_1.58Te₃ hot-pressed sample by incorporating Ag₅SbSe₄. A peak ZT of 1.40 at 375 K and a high average ZT of 1.25 between 300 and 500 K are achieved. Such outstanding thermoelectric performance originates from the synergistic effects of improved density-of-states effective mass, reduced bipolar thermal conductivity by the boosted carrier concentration, and suppressed lattice thermal conductivity by the induced phonon scattering centers including substitute point defects, dislocations, stress–strain clusters, and grain boundaries. Comprised of the p-type Bi_0.42Sb_1.58Te₃ + 0.10 wt% Ag₅SbSe₄ and zone-melted n-type Bi₂Te_2.7Se_0.3, the thermoelectric module exhibits a high conversion efficiency of 6.5% at a temperature gradient of 200 K, indicating promising applications for low-grade heat harvest near room temperature.     

Defect studies in Cu2ZnSnSe4 and Cu2ZnSn(Se0.75S0.25)4 by admittance and photoluminescence spectroscopy

To achieve higher record efficiencies for solar cells containing Cu₂ZnSnSe₄ (CZTSe), Cu₂ZnSnS₄ (CZTS) or their solid solution Cu₂ZnSn(Se_xS_1?x)₄ (CZTSSe) as an absorber, it is necessary to obtain more knowledge about defect structure of these materials. In this work, admittance spectroscopy (AS) and low temperature photoluminescence spectroscopy (PL) were used for defect studies. Admittance spectroscopy in the frequency range from 20 Hz to 10 MHz was used for studies of CZTSe/CdS and CZTSSe/CdS monograin layer heterojunctions. The measurement temperature varied from 140 K to 245 K. Two defect states (labelled E_A1 and E_A2) were found in Cu₂ZnSnSe₄ and Cu₂ZnSn(Se_0.75S_0.25)₄. In different CZTSe/CdS heterojunctions the E_A2 state was present at 74 meV, but the second E_A1 defect state changed from 87 meV to 100 meV during time and had varying properties. In Cu₂ZnSn(Se_0.75S_0.25)₄ the E_A2 state was found at 25 meV. The E_A1 state at 154 meV showed the same properties as the two defect levels in CZTSe. In both cases the E_A2 defect state was attributed to an acceptor defect and the E_A1 state with changing properties to interface states. The detected PL bands were at 0.946 eV in CZTSe and at 1.028 eV in Cu₂ZnSn(Se_0.75S_0.25)₄. Obtained by PL measurements, defect states at 69 meV in CZTSe and at 39 meV in Cu₂ZnSn(Se_0.75S_0.25)₄ were attributed to the same acceptor defect that was found from the AS measurements.     

Thermoelectric Properties of Bi2Te2Se Compensated by Native Defects and Sn Doping

In Bi₂Te₂Se the defect chemistry involves native defects that compete such that they can either exchange dominance or else significantly compensate each other. Here we show how the net carrier concentration, n ? p, which depends on the relative amounts of these defects and is readily obtained from Hall data, can be used as a fundamental materials parameter to describe the varied behavior of the thermoelectric properties as a function of compensation. We report the effects of tuning this parameter over multiple orders of magnitude by hole-doping the n-type material Bi₂Te₂Se_0.995, which is already significantly compensated because of its Se deficiency. Crystals with different levels of hole doping were achieved by two separate approaches, namely by selecting pieces from different locations in an undoped crystal in which a systematic carrier concentration gradient had been induced by its growth conditions, and alternatively by doping with Sn for Bi. The thermoelectric power factors for Bi_2?x Sn _x Te₂Se_0.995 for x = 0, 0.002, 0.005, 0.010, and 0.040 are reported, and the dependence of the transport properties on the extent of compensation is discussed.     

Fabrication and characterization of Pb1−xYbxTe-based alloy thin-film thermoelectric generators grown by thermal evaporation technique

In this study p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te powders synthesized by solid-state microwave technique were used to fabricate thermally evaporated thin films. The nanostructure and composition of the films were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energy dispersive X-ray spectroscopy (EDX). Electrical characterizations of the as-deposited films in terms of the Seebeck coefficient and electrical conductivity and power factor were conducted at a range of 298 K to 523 K. The microthermoelectric devices were composed of 20-pair and 10-pair p-Pb_0.925Yb_0.075Te:Te and n-Pb_0.94Yb_0.06Te thin films on glass substrates. The dimensions of the thin-film thermoelectric generators, which consisted of 20-pair and 10-pair legs connected by aluminum electrodes, were 23 mm×20 mm and 12 mm×10 mm, respectively. The 20-pair p–n thermocouples in series generated a maximum open-circuit voltage output (V_oc) of 0.581 V and a maximum output power of 25.87×10^?8 W at a temperature difference ΔT=164 K, whereas the 10-pair p–n thermocouples generated 0.311 V and 13.71×10^?8 W maximum V_oc and maximum output power, respectively, at ΔT=164 K.