Energy spectrum of charge carriers in Ag<Subscript>2</Subscript>Te

On the basis of investigations of the temperature and concentration dependences of kinetic coefficients (the Hall coefficientR, the electrical conductivity σ, and thermopower α₀) in n-type Ag₂Te, it is established that Ag atoms in Ag₂Te create the shallow donor levels located at a distance of (0.002?7 × 10^?5 T) eV from the bottom of the conduction band. It is shown that silver telluride has n-type conductivity starting with the deficiency of Ag ≥ 0.01 at % in the stoichiometric composition, and it is practically impossible to achieve the stoichiometric composition in Ag₂Te.     

Thermoelectric figure of merit of Ag<Subscript>2</Subscript>Se with Ag and Se excess

In the temperature range of 100–300 K, the electric (σ) and thermoelectric (α₀) properties of Ag₂Se with an excess of Ag as high as ～0.1 at. % and Se as high as ～1.0 at. %, respectively, are investigated. From the data on σ, α₀, and χ_tot (thermal conductivities), the thermoelectric power α ₀ ² σ and the figure of merit Z are calculated. It is found that α ₀ ² σ and Z attain the peak values at room temperature and the electron concentration n ≈ 6.5 × 10¹⁸ cm^?3.     

Electron scattering by acceptor centers in <Emphasis Type=Italic>p</Emphasis>-Ag<Subscript>2</Subscript>Te at low temperatures

Resonant electron scattering in p-Ag₂Te at acceptor concentrations N _a < 4.2 × 10¹⁶ cm⁻³ has been observed in the temperature range of 50–80 K. The contribution of the resonant scattering to the temperature dependences of the conductivity σ(T) and thermopower α₀(T) has been calculated. It is shown that this contribution exceeds that of charge carrier scattering by acoustic phonons.     

Phonon drag of electrons in Ag<Subscript>2</Subscript>S

The temperature dependences of the heat-conductivity coefficient χ and the thermopower 6h of Ag₂S are investigated in the range of 4.2–300 K. It is found that the value of 6h sharply increases (6h ∞ T^-3) with decreasing T at T < 100 K and passes through a maximum at 16–18 K. The heat-conductivity coefficient passes through a maximum at ≈30 K. The sharp increase in 6h is found to be caused by the effect of long-wavelength-phonon drag of electrons. It is shown that the shift of the 6h and χ peaks, as well as the temperature dependence of the phonon thermopower 6h_ph ∞ T^-3, agrees with the Herring theory.     

The effects of defects on electrical properties of Ag<Subscript>2</Subscript>S at phase transition

The temperature dependences of electrical conductivity σ(T) and the Hall coefficient R(T) in Ag₂S at the phase transition have been studied. Qualitative discrepancies in the changes in σ(T) and R(T) in the case of phase transition have been revealed; i.e., σ increases by several orders of magnitude while R decreases by a factor of 3–4. This is interpreted in the context of a model with two types of charge carriers and with variation in the band parameters and the concentration of defects formed at the phase transition taken into account.     

Hysteresis in Ag<Subscript>2</Subscript>Te near and within the phase transition region

Electrical conductivity, the Hall coefficient, and thermoelectric power were studied and differential thermal analysis was carried out in Ag₂Te crystals near and within the range of phase transitions, in the directions of heating and cooling. A large hysteresis loop was observed. The results are discussed in terms of the theory of smeared phase transitions. Agreement between experimental data and theory is achieved in a second approximation of the inclusion function L₂(T) and its derivative with respect to temperature, dL₂/dT.     

Special features of charge transport in PbGa<Subscript>2</Subscript>Se<Subscript>4</Subscript> crystals

The current-voltage (I-V) characteristics of PbGa₂Se₄ single crystals grown by the Bridgman-Stockbarger method with a resistivity of 10¹⁰–10¹² Ω cm were measured. The value of the majority carrier mobility μ=14 cm² V^?1 s^?1, calculated by the differential method of analysis of I-V characteristics, makes it possible to evaluate a number of parameters: the carrier concentration at the cathode (n_c0=2.48 cm^?3), the width of the contact barrier d_c=5.4×10^?8 cm, the cathode transparency D _c ^* =10^?5–10^?4 eV, and the quasi-Fermi level E_F=0.38 eV. It is found that a high electric field provides the charge transport through PbGa₂Se₄ crystals in accordance with the Pool-Frenkel effect. The value of the dielectric constant calculated from the Frenkel factor is found to be equal to 8.4.     

Effect of a phase transition on the electron energy spectrum in Ag<Subscript>2</Subscript>S

Temperature dependences of electrical conductivity σ, Hall coefficient R, and thermopower α₀ in Ag₂S are reported. It is established that at T ≈ 435 ± 5 K, all kinetic parameters vary drastically, which is associated with a change in parameters of the conduction band. It is shown that the dispersion law of electron energy in β-Ag₂S corresponds to the Kane model.     

Thermoelectric Properties of the Pseudobinary Alloy (Ag<Subscript>0.365</Subscript>Sb<Subscript>0.558</Subscript>Te)<Subscript>0.975</Subscript>(GeTe)<Subscript>0.025</Subscript> Prepared by Spark Plasma Sintering

Ag-Sb-Te-Ge-based alloys have received great attention in recent years. In the present work we prepared the pseudobinary alloy (Ag_0.365Sb_0.558Te)_0.975 (GeTe)_0.025 using spark plasma sintering and evaluated its thermoelectric (TE) properties over the temperature range from 318 K to 551 K. Rietveld analysis revealed that about 1.3 at.% Ge atoms occupy the Sb sites and that the alloy exhibits the same crystal structure as AgSbTe₂. By using back-scattered electron imaging, we observed two instead of one phase in the sample. The small white AgSbTe₂ chunks embedded in the matrix can substantially scatter phonons. Compared with the transport properties of Ag_0.365Sb_0.558Te, we obtained a slightly increased Seebeck coefficient and reduced thermal conductivity without sacrificing electrical conductivity. The highest TE figure of merit, ZT, was 0.69 at 551 K, whereas that of the ternary alloy Ag_0.365Sb_0.558Te was 0.61 at the corresponding temperature, suggesting that (Ag_0.365Sb_0.558Te)_0.975(GeTe)_0.025 has the potential to improve TE performance with optimization of its chemical composition.     

On the photoconductivity of TlInSe<Subscript>2</Subscript>

The current–voltage (I–V) and photocurrent–light intensity (I _pc –Φ) characteristics and the photoconductivity relaxation kinetics of TlInSe₂ single crystals are investigated. Anomalously long relaxation times (τ ≈ 10³ s) and some other specific features of the photoconductivity are observed, which are explained within the barrier theory of inhomogeneous semiconductors. The heights of the drift and recombination barriers are found to be, respectively, E _dr ≈ 0.1 eV and E _r ≈ 0.45 eV.     

Effect of the magnetic phase transition on the charge transport in layered semiconductor ferromagnets TlCrS<Subscript>2</Subscript> and TlCrSe<Subscript>2</Subscript>

TlCrS₂ and TlCrSe₂ crystals were synthesized by solid-state reaction. X-ray diffraction analysis showed that TlCrS₂ and TlCrSe₂ compounds crystallize in the hexagonal crystal system with lattice parameters a = 3.538 Å, c = 21.962 Å, c/a ≈ 6.207, z = 3; a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3; and X-ray densities ρ _x = 6.705 and 6.209 g/cm³, respectively. Magnetic and electric studies in a temperature range of 77–400 K showed that TlCrS₂ and TlCrSe₂ are semiconductor ferromagnets. Rather large deviations of the experimental effective magnetic moment of TlCrS₂ (3.26 μ_B) and TlCrSe₂ (3.05 μ_B) from the theoretical one (3.85 μ_B) are attributed to two-dimensional magnetic ordering in the paramagnetic region of strongly layered ferromagnets TlCrS₂ and TlCrSe₂. The effect of the magnetic phase’s transition on the charge transport in TlCrS₂ and TlCrSe₂ is detected.     

Mn<Subscript>0.1</Subscript>Ag<Subscript>0.9</Subscript>In<Subscript>4.7</Subscript>S<Subscript>7.6</Subscript> single crystals: Crystal structure,band gap,and thermal expansion

Mn_0.1Ag_0.9In_4.7S_7.6 single crystals are for the first time grown by the Bridgman method (vertical variant). The single crystals crystallize in the cubic spinel structure. The band gap of the single crystals is determined from the transmittance spectra in the region of the fundamental absorption edge at temperatures of T = 295 and 80 K. Thermal expansion is studied by the dilatometric method in the temperature range 80–500 K. The coefficients of thermal expansion, the Debye temperatures, and the rms (root mean square) dynamic displacements of atoms are calculated.     

Structure of the Cu<Subscript>2</Subscript>Se compound produced by different methods

Copper-selenide (Cu₂Se) samples are produced by mechanochemical synthesis and compaction by spark plasma sintering and hot pressing. The structure and phase composition of the samples before and after heat treatment are studied by the X-ray diffraction technique and electron microscopy. The character of changes in the shape and size of structural elements of the samples is shown. Variations in the phase composition of copper selenide in the temperature range from 25 to 500°C are studied in situ.     

Charge carrier transport in Ge<Subscript>20</Subscript>As<Subscript>20</Subscript>S<Subscript>60</Subscript> chalcogenide semiconductor films

Charge-carrier transport in Ge₂₀As₂₀S₆₀ films has been studied using the transit time method under low-injection conditions at room temperature. It was found that drift mobilities of electrons and holes in Ge₂₀As₂₀S₆₀ films are close to each other, i.e., μ _e ≈ μ _h ≈ 2 × 10⁻³ cm² V⁻¹ s⁻¹ at T = 295 K and F = 5 × 10⁴ V/cm. It was shown that the time dependence of the photocurrent during carrier drift and the voltage dependence of the drift mobility allowed the use of the concept of anomalous dispersive transport. Experimental data were explained using the model of transport controlled by carrier trapping by localized states with energy distribution near conduction and valence band edges described by the exponential law with a characteristic energy of ∼0.05 eV.     

On the magnetic phase transition in strongly chained TlFeS<Subscript>2</Subscript> and TlFeSe<Subscript>2</Subscript> semiconductors

The published data on the magnetic properties of strongly chained TlFeS₂ and TlFeSe₂ semiconductors have been analyzed. Based on this analysis, it is suggested that these semiconductors are quasi-one-dimensional antiferromagnets. Each of them has two Néel temperatures, which characterize the long-range (T _N3D) and short-range (T _N1D) magnetic orders.     

High-Performance (Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>SbTe<Subscript><Emphasis Type="Italic">x</Emphasis>/2+1.5</Subscript>)<Subscript>15</Subscript>(GeTe)<Subscript>85</Subscript> Thermoelectric Materials Prepared by Melt Spinning

A new preparation process combining melt spinning and hot pressing has been developed for the (Ag _x SbTe _x/2+1.5)₁₅(GeTe)₈₅ (TAGS-85) system. Compared with samples prepared by the traditional air-quenching and hot-pressing method, electrical conductivity and thermal conductivity are lowered. The thermoelectric performance of the TAGS-85 samples varied with changing Ag content and reached the highest ZT of 1.48 when x was 0.8 for the melt-spun sample, compared with the maximum ZT of 1.36 for the air-quenched sample. The Seebeck coefficient of the melt-spun TAGS-85 alloys was improved, while both the electrical conductivity and thermal conductivity were decreased. The net result of this process is to effectively enlarge the temperature span of ZT > 1, which will benefit industrial application.     

Switching effects in the films based on Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>

Experimental results on the switching effects related to the phase transitions in Ge₂Sb₂Te₅ in the presence of external voltage or laser irradiation are presented. An electron model of the reversible switching is discussed.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

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.