Том 51, № 8 (2017)
- Год: 2017
- Статей: 32
- URL: https://journals.rcsi.science/1063-7826/issue/view/12504
XV International Conference “Thermoelectrics and Their Applications—2016”, St. Petersburg, November 15–16, 2016
Analysis of the crystal structure of alloys of the [(Ge, Sn, Pb)(Te, Se)]m[(Bi, Sb)2(Te, Se)3]n (m, n = 0, 1, 2,...) family within the theory of closely packed spheres
Аннотация
The processes of the formation of complex crystal structures–layered crystals of [(Ge, Sn, Pb)(Te, Se)]m[(Bi, Sb)2(Te,Se)3]n (m, n = 0, 1, 2,…) ternary alloys with different symmetries (space groups R\(\overline 3 \)m, P\(\overline 3 \)m1, and P21/m)—are investigated within the theory of closely packed spheres (CPS theory). The structures of layered binary alloys of the Bi2Te3 (R\(\overline 3 \)m) and PbTe (Fm\(\overline 3 \)m and R3m) types are found to be related and formed based on the s3 triplet with cubic symmetry (Fm\(\overline 3 \)m), which is constructed by chalcogen (Se or Te) atoms.
Anisotropy of the thermopower in higher silicide of transitions metals
Аннотация
A new technique for measuring the thermopower anisotropy using polycrystals on the basis of repeated measurements of an array of randomly oriented microcrystals and statistical analysis of the data obtained is proposed. The thermopower anisotropy of a series of higher silicides of transition metals (β-FeSi2, doped MnSi1.75, ReSi1.75, and CrSi2) is measured using polycrystals. It is found that, in β-FeSi2 crystals, a very high thermopower anisotropy can be observed; in ReSi1.75 crystals, the thermopower can have different signs; and, in CrSi2, the absolute value of the thermopower depends heavily on the heat-treatment mode. The temperature dependences of the electrical conductivity and thermopower of CrSi2 needles are measured. The thermopower anisotropy in CrSi2 needles, as in bulk single crystals, is retained in a wide temperature range.
Response of thermoelectric parameters of Bi0.5Sb1.5Te3 films to secondary recrystallization
Аннотация
Flash evaporation is used to grow Bi0.5Sb1.5Te3 films 1200 nm thick on mica substrates. The average lateral crystallite sizes in the as-grown films are ~800 nm. The (0001) plane in the crystallites is preferentially parallel to the substrate plane. After heat treatment in an argon atmosphere, the effective lateral size of crystallites in which the third-order axis is perpendicular to the substrate plane increased by a factor of 3–5. The crystallites were preferentially oriented in the substrate plane as well. The thermoelectric-power parameter of Bi0.5Sb1.5Te3 films after their heat treatment in an inert environment increased approximately twofold to values close to that of the corresponding single crystals.
Determination of the thermoelectric efficiency of thermoelectric materials from measurements of linear series of branches for n- and p-types of conductivity
Аннотация
The Harman approach is used to develop a method and derive formulas for simultaneous determination of the thermoelectric efficiency for materials with n- or p-type of conductivity. The data for calculation are obtained from measurements of the voltage drop arising when direct and alternating current are consecutively passed through linear assemblies consisting of an odd number of branches of both types of conductivity.
Development of a mathematical model for optimizing the design of an automotive thermoelectric generator taking into account the influence of its hydraulic resistance on the engine power
Аннотация
A complex of models for optimizing the design and operation modes of an automotive thermoelectric generator is developed within the proposed approach taking into account the influence of hydraulic resistance of the generator on the internal combustion engine. Several designs of generators for converting the thermal energy of exhaust gases (EGs) of internal combustion engines into electricity due to the Seebeck effect in semiconductor elements, which have different geometries of the continuous-flow part of the generator with different hydraulic resistances, are considered. Models for calculating the thermoelectric elements, gas heat exchanger, and automotive engine are considered jointly. Simulation is performed using the example of a VAZ-21126 engine, which demonstrated that up to 500 W of electric power can be obtained using semiconductor thermoelectric elements based on germanium and lead tellurides.
Thermoelements from antimony- and bismuth-chalcogenide alloys
Аннотация
Thermoelements for thermoelectric coolers from fine-crystalline materials based on p- and n-type antimony- and bismuth-chalcogenide alloys produced by hot pressing and extrusion from powders, prepared by melt spinning, are studied. The characteristics of the modules in the range of 100–400 K are calculated taking into account the temperature dependences of the thermoelectric parameters of legs, and the parameters of thermoelectric-leg matching are estimated. The modules are fabricated and the experimental and calculated characteristics are compared.
Specific features of the transport properties of the Lu0.1Bi1.9Te3 compound
Аннотация
The temperature and electric- and magnetic-field dependences of the resistivity of the R0.1Bi1.9Te3 compound are investigated. It is shown that, in the low-temperature region, variable-range hopping conductivity is realized in this compound. In the temperature range of hopping conductivity, the electrical resistivity decreases with increasing electric-field strength in the sample, which is typical of charge-carrier tunneling from one localized state in the impurity band to another. Investigation of the transverse magnetoresistance revealed the crossover from the parabolic dependence of the magnetoresistance in low fields to the linear dependence in high fields. The established features of the transport properties of the R0.1Bi1.9Te3 compound are characteristic of inhomogeneous and disordered semiconductors.
On the band structure of Sb2Te3–xSex (0 ≤ x ≤ 0.1): Kinetic and optical data
Аннотация
The specific features of experimental data on kinetic phenomena in p-type A2V B3VI materials and the results of calculations related to these phenomena in the one- and two-band models are discussed. It is possible to correlate the calculations with the experiment in the two-band model with consideration for interband hole scattering and to estimate the parameters of their experimental spectrum. The reflectance spectra of p-Sb2Te3–xSex crystals in the range 400–7000–1 are reported. The spectra are approximated in the context of the Drude–Lorentz theory which takes into account the plasma vibrations of holes.
Contact resistance in spark plasma sintered segmented legs
Аннотация
A method for measuring the contact resistances of segmented thermoelectric legs is developed. The sensitivity of the method is 2 × 10–6 Ω cm2. The contact resistances of junctions upon switching between different segments of thermoelectric materials are measured. Both junctions and materials of the segments are produced by various methods. The determined contact resistances are in the range of 0.8–3.5 Ω cm2. Estimations show that the efficiency loss for a 4-segment leg ~1 cm high at such contact resistances is no more than 5% in the worst case.
CeB6 thin films produced on different substrates by electron-beam deposition
Аннотация
Films of cerium hexaboride, a material promising for use in thermoelectric devices at liquidhelium temperatures, are produced by electron-beam deposition. Deposition is carried out from ceramic targets onto insulator, semiconductor, and metal substrates at different temperatures. The microstructure, the elemental and phase compositions, the temperature dependences of the resistivity and the Seebeck coefficient are thoroughly studied. CaB6-structured films, for which the structure is characteristic of cerium hexaboride and the elemental composition is close to the stoichiometric composition, are obtained. At low temperatures, the resistivity of the films is somewhat higher than that of single-crystal samples, and the Seebeck coefficient is close to the corresponding coefficient for single-crystal samples. The main cause of the difference between the resistance values is a high concentration of oxygen impurity detected in the films.
Thermoelectric properties of Bi2Te2.4Se0.6 solid solutions of different particle-size composition
Аннотация
The thermoelectric properties of n-type Bi2Te2.4Se0.6 solid solution prepared by the vacuum hot pressing of powder mixtures with different particle sizes are investigated. The powders were prepared by the mechanical grinding of ingots and melt spinning. The microstructure and fracture pattern of a sample cleavage surface are analyzed using scanning electron microscopy and optical microscopy. The thermoelectric characteristics (the Seebeck coefficient, electrical conductivity, and thermal conductivity) are measured at room temperature and in the temperature range of 100–700 K.
Thermoelectric properties of the Mg2Ge0.3Sn0.7 solid solution with p-type conductivity
Аннотация
The thermoelectric properties of the Mg2Ge0.3Sn0.7 solid solution doped with Ga and Li are studied. The samples with a hole concentration as high as 5 × 1020 cm–3 are obtained. The temperature dependences of the thermopower, electrical conductivity, and thermal conductivity are measured in the range from room temperature to 800 K. A higher mobility of free charge carriers is observed in the samples doped with lithium than in the samples doped with gallium. The largest thermoelectric figure of merit in the samples under study amounts to 0.42 at 700 K.
Optimization of a segmented generating leg
Аннотация
A precise method for calculating segmented legs for thermoelectric generators in a one-dimensional approximation by the method of thermal balance with effective values of the thermoelectric parameters is proposed. This method allows one to accurately take into consideration the temperature dependences of the electrical conductivity, the Seebeck coefficient, and thermal conductivity. The effect of contact resistance on the resulting power-conversion efficiency is also taken into account. The procedure of segmented leg calculation is demonstrated, and a method of transitioning from leg calculation to thermoelement calculation is proposed.
Optimum operating-temperature range and lifetime estimate for ZnSb:0.1 at % Cu thermoelectrics
Аннотация
The temperature dependences of concentration and mobility are investigated in 300–700–300 K thermal cycles at different durations of low-temperature annealing of a ZnSb:0.1 at % Cu sample before the onset of every thermal cycle. Concurrently, the material is annealed for 1500 hours at a temperature difference of 310–670 K, and, then, its thermoelectric parameters are measured. The results are analyzed taking into account the features of the crystalline structure and the covalent nature of the chemical bonds in ZnSb. The operating range and lifetime of this composition are estimated on the basis of a model of the interaction of impurity defects with a decrease in their acceptor activity and an increase in the hole-scattering cross section at low temperatures.
Superconducting properties of (Pb0.05Sn0.95)Te doped with indium under conditions of hydrostatic compression
Аннотация
The superconducting properties of Pb0.05Sn0.95Te semiconductor alloy doped with 5 at % of In are investigated at a hydrostatic pressure of P < 7 kbar. At increasing pressure P > 1.35 kbar and T ≥ 1.3 K, the resistivity step to ρ = 0 in the temperature and magnetic-field dependences ρ(T) and ρ(H), which corresponds to the transition to the superconducting state, disappears. The experimental results are indicative of a decrease in the density of states at the Fermi level with increasing pressure, which can be interpreted as an indium-level shift deep into the valence band. The obtained data refine and supplement the results of studies devoted to investigation of the baric dependences of the critical parameters of the superconducting transition in (PbzSn1–z)0.95In0.05Te with varying lead content z.
Structure of Bi2Se0.3Te2.7 alloy plates obtained by crystallization in a flat cavity by the Bridgman method
Аннотация
The property anisotropy in Bi2Se0.3Te2.7 alloy is analyzed by constructing index surfaces for the thermoelectric figure of merit and thermal expansion coefficient. Texture is an important factor forming the property anisotropy and technological applicability of an ingot for fabricating modules. The property anisotropy is analyzed based on studying the texture in ingots produced by the modified Bridgman method (thermoelectric plate growth in a flat cavity). Analysis of the texture shows that not only the crystallization rate, but also the crystallization cavity design is an important factor for the proposed crystallization method, affecting the formation of the thermoelectric-material structure. As the plate thickness is decreased by changing the heat removal conditions in a thin gap, a more perfect structure can be obtained.
Thermoelectric properties of Bi0.5Sb1.5Te3 ribbons prepared by melt spinning
Аннотация
The results of studying the thermoelectric properties of p-type Bi0.5Sb1.5Te3 alloy samples prepared by melt spinning quenching are presented. The material after melt spinning is shaped as thin ribbons and has a quasi-amorphous structure. The thermoelectric properties (thermoelectric power and electrical resistance) and crystallization processes of as-prepared melt-spun ribbons are studied at 300–800 K for the first time. The stability range of the initial state, the crystallization-onset temperature, and the effect of thermal annealing on the thermoelectric-power factor of the alloy are determined.
Nonelectronic Properties of Semiconductors (Atomic Structure, Diffusion)
Mn0.1Ag0.9In4.7S7.6 single crystals: Crystal structure, band gap, and thermal expansion
Аннотация
Mn0.1Ag0.9In4.7S7.6 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.
High-temperature diffusion of magnesium in dislocation-free silicon
Аннотация
The diffusion doping of dislocation-free single-crystal silicon with magnesium is studied in the temperature range 1000–1200°C. The conventional sandwich method is used as the doping method. It is found that the diffusion coefficient of magnesium is described by a universal Arrhenius curve in the extended temperature range 600–1200°C, with data earlier obtained for the range 600–800°C taken into consideration. The relatively low activation energy of diffusion transport (1.83 eV) is indicative of the interstitial mechanism of impurity diffusion.
Electronic Properties of Semiconductors
Complex structure of optical transitions from the core d-levels of InAs and InSb crystals
Аннотация
Using an improved parameterless unified Argand diagram method, the permittivity spectra of InAs and InSb crystals in the range of 15–40 eV are decomposed into thirteen and twelve separate components, respectively, with the three main parameters, specifically, the maximum energies, half-width, and oscillator strength determined for each of them. The oscillator strengths are in the ranges of 0.006–0.10 for InAs and 0.014–0.076 for InSb. The permittivity spectra were preliminary calculated on the basis of experimental reflectance and absorption spectra using Kramers–Kronig integral relations. On the basis of a model of interband and exciton transitions, the nature of the obtained transition bands is suggested.
Impurity levels in Hg3In2Te6 crystals
Аннотация
The parameters of impurity levels in Hg3In2Te6 samples are studied using the temperature dependences of the electron concentration n(T) and the Fermi-level energy EF(T). The dependences n(T) and EF(T) are obtained from data on the Hall coefficient R(T) and the thermopower α(T). Differential analysis of the dependences n(T) shows that, under variations in the degree of compensation by heat treatment of the samples, a wider spectrum of impurity levels in the band gap of Hg3In2Te6 can be analyzed.
Spectroscopy, Interaction with Radiation
Effects of irradiation with 8-MeV protons on n-3C-SiC heteroepitaxial layers
Аннотация
The effects of 8-MeV proton irradiation on n-3C-SiC epitaxial layers grown by sublimation on semi-insulating 4H-SiC substrates are studied. Changes in the sample parameters were recorded by the Hall-effect method and judged from photoluminescence spectra. The Hall method was employed to distinguish between the effects of irradiation on the charge-carrier concentration and mobility. It is found that the charge-carrier removal rate (Vd) is ~110 cm–1. Full compensation of the samples with an initial charge-carrier concentration of ~6.5 × 1017 cm–3 is observed at irradiation doses of ~6 × 1015 cm–2. It is found that the mobility at these doses decreased by only a factor of 2.5. Compared with 4H and 6H silicon carbide, no significant increase in the intensity of so-called defect-related photoluminescence is observed.
Surfaces, Interfaces, and Thin Films
Effect of free charge carriers on birefringence and dichroism in anisotropic porous silicon layers
Аннотация
The infrared optical properties of anisotropic mesoporous silicon films containing free charge carriers (holes) are studied experimentally and theoretically. The results of simulation of the optical properties of the produced samples in the effective-medium approximation show a heavy dependence of the birefringence, reflection anisotropy, and dichroism on the concentration of free charge carriers. The unsteady behavior of the differential transmittance recorded for the samples at mutually perpendicular polarization directions of light are attributed to the effect of charge carriers with a concentration on the order of 1019 cm–3. The results of the study suggest that anisotropic silicon nanostructures are promising materials for infrared photonics and terahertz engineering.
Semiconductor Structures, Low-Dimensional Systems, and Quantum Phenomena
On the propagation of charge carriers fluxes in the thin layer of a plane-parallel solid-state structure with scattering at the boundaries of the layer taken into account
Аннотация
The composition of the thin layer of a plane-parallel solid-state structure affects the propagation of differential charge carriers fluxes in the thickness of the layer and through it. Scattering of these fluxes at the boundaries of the thin layer affects also the distribution function of charge carriers. This scattering is correctly taken into account in integrated boundary conditions for the kinetic equation. For a plane-parallel layer, the kinetic equation (in the approximation of the relaxation time) is reduced to a convenient form, which describes the propagation of differential fluxes in the thickness of the layer. The solution for thick and thin layers of different types (metal, insulator, semiconductor) is analyzed. Practically important problems, in which the obtained solutions can be effectively used, are specified.
Influence of traps in silicon dioxide on the breakdown of MOS structures
Аннотация
A model for numerical calculation of the voltage and delay time of the breakdown of MOS structures based on the anode-hole-injection mechanism, which takes into account the depth distributions of hole and electron traps in the dielectric layer, is developed. It is shown that the breakdown voltage is determined by charge accumulation at hole traps in the gate dielectric near the cathode and depends also on the presence of hole traps near the anode and electron traps. The calculated breakdown delay time follows the exponential law 1/E in a wide range of field strengths and is in agreement with the experimental data. At short impact times (t < 10–5 s), breakdown is determined by charge accumulation at free holes.
Microcrystalline, Nanocrystalline, Porous, and Composite Semiconductors
Effect of electrolyte temperature on the cathodic deposition of Ge nanowires on in and Sn particles in aqueous solutions
Аннотация
The results of studying the growth of filamentous Ge structures in aqueous electrolytes at various temperatures using In and Sn nanoparticle arrays as nucleation sites are given. The temperature of Ge cathodic deposition process from aqueous solutions has a significant effect on the layer structure deposited onto the surface. In the presence of metal particles in the molten state, filamentous Ge structures grow due to the cathodic reduction of Ge-containing ions on the electrode surface, followed by dissolution and crystallization in the melt at the substrate interface. The results obtained show the crucial role of liquid metal particles during the electrochemical formation of germanium nanowires.
Carbon Systems
Study of the crystal and electronic structure of graphene films grown on 6H-SiC (0001)
Аннотация
The structural, chemical, and electronic properties of epitaxial graphene films grown by thermal decomposition of the Si-face of a semi-insulating 6H-SiC substrate in an argon environment are studied by Raman spectroscopy, atomic-force microscopy, the low-energy electron diffraction method, X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy and X-ray absorption spectroscopy at the carbon K edge. It is shown that the results of a systematic integrated study make it possible to optimize the growth parameters and develop a reliable technology for the growth of high-quality single-layer graphene films with a small fraction of bilayer graphene inclusions.
Physics of Semiconductor Devices
Current–voltage characteristics of Schottky diodes at high current densities under the injection of minority carriers
Аннотация
An analytical expression is derived for the current–voltage characteristic of a Schottky diode at a high injection level of minority carriers. It is shown that, even at very high current densities, the higher the base doping level, the larger the voltage drop across the diode. The physical mechanism responsible for this “paradoxical” result is analyzed. The validity of the analytical result is confirmed by a numerical calculation with software that takes into account the whole set of nonlinear effects caused by a high injection level in the base layer and by heavy doping of the emitter region.
Fabrication, Treatment, and Testing of Materials and Structures
Experimental studies of the effects of atomic ordering in epitaxial GaxIn1–xP alloys on their structural and morphological properties
Аннотация
The properties of epitaxial GaxIn1–xP alloys with an ordered arrangement of atoms in the crystal lattice are studied by a number of structural and microscopic methods. The alloys are grown by metal-organic chemical vapor deposition onto single-crystal GaAs(100) substrates. It is shown that, under conditions for the coherent growth of an ordered GaxIn1–xP alloy on a GaAs(100) substrate, atomic ordering results in radical modifications of the structural properties of the semiconductor compared to the properties of disordered alloys. Among these modifications are a change in the crystal-lattice parameter and, as a consequence, lowering of the crystal symmetry and the formation of two different types of surface nanorelief. With consideration for elastic strains, the parameters of GaxIn1–xP alloys with ordering as functions of the order parameter are calculated for the first time. It is shown that all experimental data are in good agreement with the developed theoretical concepts.
Effect of ammonium-sulfide solvent on the surface passivation of GaSb (100)
Аннотация
X-ray photoemission spectroscopy and scanning electron microscopy are used to study the effect of a solvent on the chemical composition of a sulfide passivating layer formed on GaSb (100) surface. Treatment of the GaSb (100) surface with ammonium-sulfide solutions results in the formation of a thick (several tens of nanometers) coating composed of a mixture of various binary and ternary gallium and antimony sulfides. The chemical compositions of the passivating sulfide layers formed in aqueous and alcoholic ammonium-sulfide solutions are different. Annealing of the sulfide layer at 360°C in ultrahigh vacuum leads to the partial desorption of antimony from the surface, the decomposition of ternary sulfides into their binary constituents, and change in the surface band bending of this layer.
InSb quantum dots produced by liquid-phase epitaxy on InGaAsSb/GaSb substrates
Аннотация
Indium-antimonide quantum dots are for the first time formed on the surface of an epitaxial In0.25GaAsSb layer isoperiodic to a GaSb(001) substrate by liquid-phase epitaxy in the range of temperatures T = 450–467°C. Transmission electron microscopy shows that, the shape of quantum dots is close to a truncated cone and their distribution in terms of height and base size in the ensemble is monomodal. Large-sized quantum dots (with a base size of 30–50 nm and height of 3 nm) exhibit specific contrast in the plane-view diffraction-mode image, which is indicative of the presence of misfit defects. Modification of the chemical composition of the working surface of the substrate by the deposition of an epitaxial In0.25GaAsSb layer makes possible a threefold increase in the density of the ensemble of InSb quantum dots (1 × 1010 cm–2) compared to the density in the case of deposition directly onto the GaSb binary compound.