Thermoelectric Characterization of (Ga,In)<Subscript>2</Subscript>Te<Subscript>3</Subscript> with Self-Assembled Two-Dimensional Vacancy Planes

The key properties for the design of high-efficiency thermoelectric materials are a low thermal conductivity and a large Seebeck coefficient with moderate electrical conductivity. Recent developments in nanotechnology and nanoscience are leading to breakthroughs in the field of thermoelectrics. The goal is to create a situation where phonon pathways are disrupted due to nanostructures in “bulk” materials. Here we introduce promising materials: (Ga,In)₂Te₃ with unexpectedly low thermal conductivity, in which certain kinds of superlattice structures naturally form. Two-dimensional vacancy planes with approximately 3.5-nm intervals exist in Ga₂Te₃, scattering phonons efficiently and leading to a very low thermal conductivity.     

Superionic conductivity in TlGaTe<Subscript>2</Subscript> crystals

Temperature dependences of electrical conductivity σ(T) and permittivity ɛ(T) of one-dimensional (1D) TlGaTe₂ single crystals are investigated. At temperatures higher than 305 K, superionic conductivity of the TlGaTe₂ is observed and is related to diffusion of Tl⁺ ions via vacancies in the thallium sublattice between (Ga³⁺Te₂^{2− −} nanochains. A relaxation character of dielectric anomalies is established, which suggests the existence of electric charges weakly bound to the crystal lattice. Upon the transition to the superionic state, relaxors in the TlGaTe₂ crystals are Tl⁺ dipoles ((Ga³⁺Te₂²⁻)⁻ chains) that arise due to melting of the thallium sublattice and hops of Tl⁺ ions from one localized state to another. The effect of a field-induced transition of the TlGaTe₂ crystal to the superionic state is detected.     

Memory Switching Characteristics in Amorphous Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> Films

Ga₂Se₃ films were deposited by the thermal evaporation of the bulk material onto pyrographite substrates under vacuum. The I–V characteristic curves were found to be typical for a memory switch. They exhibited a transition from an ohmic region in the lower-field region to a non-ohmic region in the high-field region in the preswitching region, which has been explained by the Poole–Frenkel effect. The temperature dependence of the resistance in the ohmic region was found to be that of a thermally activated process. It was also found that the mean value of the switching voltage increased linearly with increasing film thickness in the range from 291 nm to 516 nm, while it decreased exponentially with increasing temperature in the range from 298 K to 393 K. The results were explained in accordance with the electrothermal model for the switching process.     

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.     

Thermoelectric Properties of Spark Plasma-Sintered In<Subscript>4</Subscript>Se<Subscript>3</Subscript>-In<Subscript>4</Subscript>Te<Subscript>3</Subscript>

We report the thermoelectric properties of spark plasma-sintered In₄Se₃-In₄Te₃ materials. For comparison, pure In₄Se₃ and In₄Se₃ (80 wt.%)/In₄Te₃ (20 wt.%) mixture samples were prepared. In₄Se₃ and In₄Te₃ powders were synthesized by a conventional melting process in evacuated quartz ampoules, and a spark plasma method was used for the sintering of the pure In₄Se₃ and mixture samples. Thermoelectric and structural characterizations were carried out, and the mixing effect of In₄Se₃ and In₄Te₃ on the thermoelectric properties was investigated.     

Effects of SiC Nanodispersion on the Thermoelectric Properties of <Emphasis Type="Italic">p</Emphasis>-Type and <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Alloys

Polycrystalline p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.7Se_0.3 thermoelectric (TE) alloys containing a small amount (vol.% ≤5) of SiC nanoparticles were fabricated by mechanical alloying and spark plasma sintering. It was revealed that the effects of SiC addition on TE properties can be different between p-type and n-type Bi₂Te₃-based alloys. SiC addition slightly increased the power factor of the p-type materials by decreasing both the electrical resistivity (ρ) and Seebeck coefficient (α), but decreased the power factor of n-type materials by increasing both ρ and α. Regardless of the conductivity type, the thermal conductivity was reduced by dispersing SiC nanoparticles in the Bi₂Te₃-based alloy matrix. As a result, a small amount (0.1 vol.%) of SiC addition increased the maximum dimensionless figure of merit (ZT _max) of the p-type Bi_0.5Sb_1.5Te₃ alloys from 0.88 for the SiC-free sample to 0.97 at 323 K, though no improvement in TE performance was obtained in the case of n-type Bi₂Te_2.7Se_0.3 alloys. Importantly, the SiC-dispersed alloys showed better mechanical properties, which can improve material machinability and device reliability.     

Superionic conductivity and switching effect with memory in TlInSe<Subscript>2</Subscript> and TlInTe<Subscript>2</Subscript> crystals

The temperature dependences of the conductivity σ(T) and the switching and memory effects in one-dimensional TlInSe₂ and TlInTe₂ single crystals have been studied. A specific feature is found in the dependence σ(T) above 333 K, which is related to the transition of crystals to the state with superionic conductivity. It is suggested that the ion conductivity is caused by the diffusion of Tl⁺ ions over vacancies in the thallium sublattice between (In³⁺Te₂²⁻)⁻ and (In³⁺Se₂²⁻)⁻ nanochains (nanorods). S-type switching and memory effects are revealed in TlInSe₂ and TlInTe₂ crystals, as well as voltage oscillations in the range of negative differential resistance. It is suggested that the switching effect and voltage oscillations are related to the transition of crystals to the superionic state, which is accompanied by “melting” of the Tl sublattice. The effect of electric-field-induced transition of TlInSe₂ and TlInTe₂ crystals to the superionic state is found.     

Features of reflection spectra of single crystals of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi₂Te₃-Sb₂Te₃ solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb₂Te₃ have been studied in the range of 400–4000 cm⁻¹ at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C ₃ (E ⊥ C ₃). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm⁻¹ corresponding to the optical band gap E _{g opt}. The features become more pronounced as the content of Bi₂Te₃ is increased to 80 mol % in the composition of the Bi₂Te₃-Sb₂Te₃ solid solution. A further increase in the content of Sb₂Te₃ is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in E _{g opt}.     

Influence of Element Substitution on Corrosion Behavior of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Based Compounds

Atmospheric water may condense on the surface of Bi₂Te₃-based compounds constituting the Peltier module, depending on the operating environment used. In the stage of disposal, Bi₂Te₃-based compounds may come into contact with water in waste disposal sites. There are very few publications about the influence of condensed water on Peltier modules. Bi₂Te₃-Sb₂Te₃ or Bi₂Te₃-Bi₂Se₃ pseudo binary system compounds are used as p-type material or n-type material, respectively. The lattice distortion will be induced in the crystal of Bi₂Te₃-based compounds by element substitution due to the reduction in their thermal conductivity. However, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds remains unclear. In this study, the influence of element substitution on the corrosion behavior of Bi₂Te₃-based compounds with practical compositions has been investigated. Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 was prepared by the vertical Bridgman method. The electrochemical properties at room temperature were evaluated by cyclic voltammetry in a standard three-electrode cell. The working electrolyte was a naturally aerated 0.6 or 3.0 mass% NaCl solution. From the tendency for corrosion potential for all the samples, the corrosion sensitivity of ternary compounds was slightly higher than that of binary compounds. From the trend of current density, it was found that Bi_0.5Sb_1.5Te₃ had a corrosion resistance intermediate between Bi₂Te₃ and Sb₂Te₃. On the other hand, corrosion resistance was affected despite a small amount of Se substitution, and the corrosion resistance of Bi₂Te_2.85Se_0.15 was close to or lower than that of Bi₂Se₃. From the observation results of the corrosion products, the trends of morphology and composition of corrosion products for Bi_0.5Sb_1.5Te₃ or Bi₂Te_2.85Se_0.15 were consistent with those of Sb₂Te₃ or Bi₂Se₃, respectively. From the results of x-ray photoelectron spectroscopy for the electrolyte after testing, the possibility that a corrosion product diffuses to the environment including the salt was suggested in Bi_0.5Sb_1.5Te₃. However, the amount of dissolved corrosion product was very low, and the chemical stability of the corrosion product was not changed or improved by element substitution.     

Influence of Nanoinclusions on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Nanocomposites

n-Type Bi₂Te₃ nanocomposites with enhanced figure of merit, ZT, were fabricated by a simple, high-throughput method of mixing nanostructured Bi₂Te₃ particles obtained through melt spinning with micron-sized particles. Moderately high power factors were retained, while the thermal conductivity of the nanocomposites was found to decrease with increasing weight percent of nanoinclusions. The peak ZT values for all the nanocomposites were above 1.1, and the maximum shifted to higher temperature with increasing amount of nanoinclusions. A maximum ZT of 1.18 at 42°C was obtained for the 10 wt.% nanocomposite, which is a 43% increase over the bulk sample at the same temperature. This is the highest ZT reported for n-type Bi₂Te₃ binary material, and higher ZT values are expected if state-of-the-art Bi₂Te_3−x Se _x materials are used.     

Electrical properties of In<Subscript>2</Subscript>Se<Subscript>3</Subscript> single crystals and photosensitivity of Al/In<Subscript>2</Subscript>Se<Subscript>3</Subscript> Schottky barriers

In₂Se₃ single crystals ∼40 mm long and 14 mm in diameter were grown by the Bridgman method. The composition of grown single crystals and their crystal structure were determined. The conductivity (σ) and Hall constant (R) of grown single crystals were measured and the first Schottky barriers Al/n-In₂Se₃ were fabricated. Rectification and photovoltaic effect were detected in the new structures. Based on the study of the photosensitivity spectra of Al/n-In₂Se₃ structures, the nature of the interband transitions and band gap of In₂Se₃ crystals were determined. It was concluded that the new structures can be applied to develop broadband photoconverters of optical radiation.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Formation of nanostructures in a Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript>/GaAs system

The topology of GaAs(100) and GaAs(111) surfaces before and after short treatments in Se vapor is studied by atomic-force microscopy. On the basis of this study, as well as ellipsometry and electron microscopy, a mechanism for the formation and growth of Ga₂Se₃(110) nanoislands and a layer on the GaAs(100) and GaAs(111) surfaces is proposed.     

The effect of irradiation with electrons on the electrical parameters of Hg<Subscript>3</Subscript>In<Subscript>2</Subscript>Te<Subscript>6</Subscript>

The results of studying the electrical properties of Hg₃In₂Te₆ crystals irradiated with electrons with the energy E _e = 18 MeV and the dose D = 4 × 10¹⁶ cm⁻² are reported. It is shown that, irrespective of the charge-carrier concentration in the initial material, the Hg₃In₂Te₆ samples acquire the charge-carrier concentration (1.6–1.8) × 10¹³ cm⁻³ after irradiation. The phenomenon of Fermi level pinning in an irradiated material is discussed. The initial charge-carrier concentration, which remains virtually unchanged after irradiation and which ensures the high radiation resistance of Hg₃In₂Te₆ crystals, corresponds to a compensated material, similar to an intrinsic semiconductor at T > 260 K.     

Effects of Various Reductants and Surfactants on the Nanostructure of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> Synthesized by a Hydrothermal Process

Various reductants and surfactants were used to investigate their effects on the structure of Bi₂Te₃ nanopowders prepared via hydrothermal synthesis at 150°C for 24 h. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) were applied to analyze the phase distributions, microstructures, and grain sizes of the as-grown Bi₂Te₃. The results showed that the grain sizes and morphologies of the synthesized powders were mainly related to the effects of the reductants on the nucleation rate and the surface-adsorbed energy of the diverse surfactants on special crystal planes. Some surfactants could control the growth of the Bi₂Te₃ crystals and promote one-dimensional growth of nanorods during hydrothermal synthesis.     

Improvement of Thermoelectric Power Factor of Hydrothermally Prepared Bi<Subscript>0.5</Subscript>Sb<Subscript>1.5</Subscript>Te<Subscript>3</Subscript> Compared with its Solvothermally Prepared Counterpart

We report on the successful hydrothermal synthesis of Bi_0.5Sb_1.5Te₃, using water as the solvent. The products of the hydrothermally prepared Bi_0.5 Sb_1.5Te₃ were hexagonal platelets with edges of 200–1500 nm and thicknesses of 30–50 nm. Both the Seebeck coefficient and electrical conductivity of the hydrothermally prepared Bi_0.5Sb_1.5Te₃ were larger than those of the solvothermally prepared counterpart. Hall measurements of Bi_0.5Sb_1.5Te₃ at room temperature indicated that the charge carrier was p-type, with a carrier concentration of 9.47 × 10¹⁸ cm⁻³ and 1.42 × 10¹⁹ cm⁻³ for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively. The thermoelectric power factor at 290 K was 10.4 μW/cm K² and 2.9 μW/cm K² for the hydrothermally prepared Bi_0.5Sb_1.5Te₃ and solvothermally prepared sample, respectively.     

Optical Study of Filled Tetrahedral Compounds Li<Subscript>3</Subscript>AlN<Subscript>2</Subscript> and Li<Subscript>3</Subscript>GaN<Subscript>2</Subscript>

A detailed analysis of the optical properties of filled tetrahedral semiconductors Li₃AlN₂ and Li₃GaN₂ has been performed, using the full potential linearized augmented plane wave method within the density functional theory. The real and imaginary parts of the dielectric function ε(ω), the optical absorption coefficient I(ω), the reflectivity R(ω), and the electron energy loss function are calculated within the random phase approximation. The interband transitions responsible for the structures in the spectra are specified. Looking at optical matrix element, we note that the major peaks are dominated by transition from metal s, N 2p states to N 2p, Ga 3d states. The theoretical calculated optical properties and electron energy loss spectrum yield a static dielectric constant of 5.34 and a plasmon energy of 19.47 eV for Li₃GaN₂. In the Li₃AlN₂ compound, the static dielectric constant decreases to 4.75 and yields a plasmon energy of 18.5 eV. The effect of spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low-energy region. In order to check the reliability of our calculations, analogous results obtained for Be₃N₂ in the same structure [space group Ia3(206)] are included in this work.     

Features of the charge-transport mechanism in layered Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> single crystals doped with chlorine and terbium

The temperature dependences (T = 5−300 K) of the resistivity in the plane of layers and in the direction perpendicular to the layers, as well as the Hall effect and the magnetoresistance (H < 80 kOe, T = 0.5−4.2 K) in Bi₂Te₃ single crystals doped with chlorine and terbium, are investigated. It is shown that the doping of Bi₂Te₃ with terbium atoms results in p-type conductivity and in increasing hole concentration. The doping of Bi₂Te₃ with chlorine atoms modifies also the character of its conductivity instead of changing only the type from p to n. In the temperature dependence of the resistivity in the direction perpendicular to layers, a portion arises with the activation conductivity caused by the hopping between localized states. The charge-transport mechanism in Bi₂Te₃ single crystals doped with chlorine is proposed.     

Effect of copper doping on kinetic coefficients and their anisotropy in PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

In anisotropic PbSb₂Te₄ and PbSb₂Te₄:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb₂Te₄ single crystals exhibit a high hole concentration (p ≈ 3 × 10²⁰ cm⁻³). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb₂Te₄. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R ₁₂₃/R ₃₂₁ ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb₂Te₄. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c ₃; as a result, two components of the Nernst-Ettingshausen tensor Q ₃₂₁ and Q ₁₃₂ in the PbSb₂Te₄:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q ₃₂₁ component is positive.     

Current-voltage characteristics of ZnGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> compound polycrystals

Current-voltage characteristics of the In-ZnGa₂Se₄-In structure have been studied in the temperature range of 90–335 K. Based on the data calculated for the concentration of three trap types in ZnGa₂Se₄, the values N _t = 1.4 × 10¹³, 8.2 × 10¹², and 2.6 × 10¹² cm⁻³ are obtained. The contact region transparency D _k ^*= 10⁻⁵, surface recombination velocity S _k = 0.65 m/s, and carrier lifetime τ = 1.5 × 10⁻⁴ s were determined. It was found that the current transmission mechanism in electric fields weaker than 10³ V/cm is caused by monopolar carrier injection.