Preparation and study of the thermoelectric properties of Fe<Subscript>2</Subscript>TiSn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Heusler alloys

Polycrystalline alloys Fe₂TiSn_1–x Si _x (0 ≤ x ≤ 1) theoretically predicted as highly efficient thermoelectric materials are experimentally studied. Structural studies show that the partial substitution of Sn with Si results in the formation of a multiphase state in samples with x > 0. Impurity phases in general lead to a significant decrease in the Seebeck coefficient and an increase in the thermal conductivity of Fe₂TiSn_1–x Si _x samples, which does not allow consideration of these materials as promising thermoelectrics.     

Thermoelectric effects in nanoscale layers of manganese silicide

The values of the thermoelectric power, layer resistivity and thermal conductivity of a Mn _x Si_1–x nanoscale layer and Mn _x Si_1–x/Si superlattice on silicon depending on the growth temperature in the range T = 300–600 K are found experimentally. The contribution of the nanoscale film and substrate to the thermoelectric effect is discussed. The thermoelectric figure of merit of a single manganese-ssilicide layer, superlattice, and layer/substrate system is estimated. The largest figure of merit ZT = 0.59 ± 0.06 is found for Mn_0.2Si_0.8 at T = 600 K.     

Influence of Sedimentation of Atoms on Structural and Thermoelectric Properties of Bi-Sb Alloys

Functionally graded thermoelectric materials (FGTMs) have been prepared by sedimentation of atoms under a strong gravitational field. Starting samples of Bi _x Sb_1?x alloys with different composition x were synthesized by melting of metals and subsequent annealing of quenched samples. The thermoelectric properties (Seebeck coefficient, electrical conductivity) of the starting materials were characterized over the temperature range from 300 K to 525 K. Strong gravity experiments were performed in a unique ultracentrifuge apparatus under acceleration of over 0.5 × 10⁶ G at temperatures of 538 K and 623 K. Changes of the microstructure and chemical composition were analyzed using scanning electron microscopy with energy-dispersive x-ray spectroscopy analysis. The distribution of the Seebeck coefficient of the Bi-Sb alloys was characterized by scanning thermoelectric microprobe. As a result of sedimentation, large changes in chemical composition (x = 0.45 to 1) were obtained. It was found that the changes in chemical composition were correlated with alterations of the Seebeck coefficient. The obtained experimental data allowed the development of a semiempirical model for the selection of optimal processing parameters for preparation of Bi-Sb alloys with required thermoelectric properties.     

New organometallic precursors and processes for chemical vapor deposition in the technology of nanomaterials

Processes of chemical vapor deposition (CVD) of metal and dielectric (high-k and low-k) films with the help of unconventional initial reagents (volatile complex and organoelement compounds) were developed. Complex investigation of the chemical and phase composition and structure of (HfO₂)_{1 ? x} (Me ₂O₃) _x double oxides (where Me = Al, Sc), and silicon carbonitrides and oxycarbonitrides was carried out. It was shown that the resulting materials enjoy a number of unique functional properties, which makes them promising for application in micro-, nano-, and optoelectronic devices.     

Special features of radiation-defect annealing in silicon <Emphasis Type="Italic">p-n</Emphasis> structures: The role of Fe impurity atoms

Deep-level transient spectroscopy is used to study the formation of complexes that consist of a radiation defect and a residual impurity atom in silicon. It is established that heat treatment of the diffused Si p⁺-n junctions irradiated with fast electrons lead to the activation of a residual Fe impurity and the formation of the FeVO (E_0.36 trap) and FeV₂ (H_0.18 trap) complexes. The formation of these traps is accompanied by the early (100–175°C) stage of annealing of the main vacancy-related radiation defects: the A centers (VO) and divacancies (V₂). The observed complexes are electrically active and introduce new electron (E_0.36: E _t ^e =E _c -0.365 eV, σ _n =6.8×10^?15 cm²) and hole (H_0.18: E _t ^h =E _v +0.184 eV, σ _p =3.0×10^?15 cm²) levels into the silicon band gap and have a high thermal stability. It is believed that the complex FeVO corresponds to the previously observed and unidentified defects that have an ionization energy of E _t ^e =E _c ?(0.34–0.37) eV and appear as a result of heat treatment of irradiated diffused Si p⁺-n junctions.     

On the preparation and photoelectric properties of Tl1–x In1–x Sn x Se2 (x = 0.1–0.25) alloys

The technological conditions for growing single crystals of Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys are developed. The spectral distribution of the photoconductivity of the grown crystals at T = 300 K and thermally stimulated conductivity are studied. The effect of In³⁺cation substitution with Sn⁴⁺ in Tl_1–x In_1–x Sn _x Se₂ (x = 0.1–0.25) alloys on their photoelectric properties is shown.     

Effect of anion and cation substitution in tungsten disulfide and tungsten diselenide on conductivity and thermoelectric power

The temperature dependences of the conductivity and thermoelectric power for a series of samples W_1–x Nb _x S₂, W_1–x Nb _x Se₂, WS_2–y Se _y , W_1–x Nb _x S_2–y Se _y are studied at low temperatures. It is found that the cation substitution of W atoms with Nb leads to an increase in the conductivity and a decrease in the thermoelectric power. The anion substitution of S with Se atoms results in a simultaneous increase in the conductivity and thermoelectric power. The highest power factor among the samples studied is inherent to W_0.8Nb_0.2Se₂.     

Increase in the rate and discretization of the kinetics of isothermal surface generation of minority charge carriers in metal-insulator-semiconductor structures with a planar-inhomogeneous insulator

Surface generation of minority charge carriers in silicon metal-oxide-semiconductor (MOS) structures is efficient only at the initial recombinationless stage. Quasi-equilibrium between surface generation centers and the minority-carrier band is established in a time t ～ 10^?5 s. In the absence of other carrier generation channels, an equilibrium inversion state at 300 K would need t = t_∞ > 10³ years to become established. In fact, the time t ∞ is much shorter, due to excess-carrier generation via centers located at the SiO₂/Si interface over the gate periphery. This edge-related generation can easily be simulated in an MOS structure with a single gate insulated from Si by oxide layers of various thicknesses. At gate depleting voltages V _g , the role of the periphery is played by a shallow potential well under a thicker oxide, and the current-generation kinetics becomes unconventional: two discrete steps are observed in the dependences I(t), and the duration and height of these steps depend on V _g . An analysis of the I(t) curves allows determination of the electric characteristics of the Si surface in the states of initial depletion (t = 0) and equilibrium inversion (t = t_∞), as well as the parameters of surface lag centers, including their energy and spatial distributions. The functionally specialized planar inhomogeneity of a gate insulator is a promising basis for dynamic sensors with integrating and threshold properties.     

Valence-band offsets in strained SiGeSn/Si layers with different tin contents

Admittance spectroscopy is used to study hole states in Si_0.7–y Ge_0.3Sn _y /Si quantum wells in the tin content range y = 0.04–0.1. It is found that the hole binding energy increases with tin content. The hole size-quantization energies in structures containing a pseudomorphic Si_0.7–y Ge_0.3Sn _y layer in the Si matrix are determined using the 6-band kp method. The valence-band offset at the Si_0.7–y Ge_0.3Sn _y heterointerface is determined by combining the numerical calculation results and experimental data. It is found that the dependence of the experimental values of the valence-band offsets between pseudomorphic Si_0.7–y Ge_0.3Sn _y layers and Si on the tin content is described by the expression ΔE _V ^exp = (0.21 ± 0.01) + (3.35 ± 7.8 × 10^–4)y eV.     

On the band gap of Cu<Subscript>2</Subscript>ZnSn(S<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>)<Subscript>4</Subscript> alloys

The transmittance spectra of single-crystal Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ compounds and Cu₂ZnSn(S _x Se_1–x )₄ alloys grown by chemical vapor-transport reactions are studied in the region of the fundamental absorption edge. From the experimental spectra, the band gap of the compounds and their alloys is determined. The dependences of the band gap on the composition parameter x of the alloy are constructed. It is established that the band gap nonlinearly varies with x and can be described as a quadratic dependence.     

Alternating current conductivity and electron spin resonance of the Cd<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>x</Subscript>Te alloys

The results of measurements of frequency dependences of conductivity and electron spin resonance (ESR) of the Cd_{1 ? x} Fe_xTe alloys (0.01 ≤ x ≤ 0.05) at room temperature are presented. It is found that, in the composition range 0.003 ≤ x ≤ 0.05, a new asymmetric ESR absorption line with g ≈ 3.9 emerges, and its hysteresis manifests itself while the samples are magnetized. The line with g ≈ 3.9 is attributed to the charge state 3⁺ of the Fe atoms, which occurs in tetrahedra with three Fe atoms.     

Thermoelectric Properties for a Suspended Microribbon of Quasi-One-Dimensional TiS<Subscript>3</Subscript>

Transition-metal trichalcogenides MX₃ (M = Ti, Zr, Nb, Ta; X = S, Se) are well-known inorganic quasi-one-dimensional conductors. Among them, we have investigated the thermoelectric properties of titanium trisulfide TiS₃ microribbon. The electrical resistivity ρ, thermal conductivity κ, and thermoelectric power S were measured using 3ω method. The weight mean values were found to be ρ = 5 mω m and κ = 10 W K^?1 m^?1 along the one-dimensional direction (b-axis) of the TiS₃ microribbon. Combined with the thermoelectric power S = ?530 μV K^?1, the figure of merit was calculated as ZT = 0.0023. This efficiency is the same as that of randomly oriented bulk TiS₃. We also estimated the anisotropy of σ and κ using the present results and those for randomly oriented bulk material. The obtained weak anisotropy for TiS₃ is attributable to strong coupling between triangular columns consisting of TiS₃ units. These experimental results are consistent with theoretical results obtained using density functional theory (DFT) calculations.     

Energy spectrum of charge carriers in TlIn<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Yb<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te<Subscript>2</Subscript> solid solutions

The temperature dependences of the electrical conductivity σ(T), the Hall coefficient R(T), and the thermoelectric coefficient α(T) are investigated in TlIn_1–xYb_xTe₂ (0 < x < 0.10) solid solutions at 80–1000К. From the kinetic parameters, the effective masses of electrons and holes are determined. The obtained experimental data on σ(T) and α(T) are interpreted within the context of a model with one and two types of charge carriers. It is established that, since x = 0.05, the TlIn_1–xYb_xTe₂ solid solutions belong to the class of narrow-gap semiconductors that have high matrix elements of interaction.     

Influence of Composition on the Thermoelectric Properties of Bi<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Thin Films

Bi_1?x Sb _x solid solutions have attracted much attention as promising thermoelectric (TE) materials for cooling devices at temperatures below ～200 K and as unique model materials for solid-state science because of a high sensitivity of their band structure to changes in composition, temperature, pressure, etc. Earlier, we revealed a non-monotonic behavior of the concentration dependences of TE properties for polycrystalline Bi_1?x Sb _x solid solutions and attributed these anomalies to percolation effects in the solid solution, transition to a gapless state, and to a semimetal–semiconductor transition. The goal of the present work is to find out whether the non-monotonic behavior of the concentration dependences of TE properties is observed in the thin film state as well. The objects of the study are Bi_1?x Sb _x thin films with thicknesses in the range d = 250–300 nm prepared by thermal evaporation of Bi_1?x Sb _x crystals (x = 0–0.09) onto mica substrates. It was shown that the anomalies in the dependence of the TE properties on Bi_1?x Sb _x crystal composition are reproduced in thin films.     

Transport coefficients of <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> in a two-band model of the electron spectrum

The Hall factor and thermoelectric properties of an n-Bi₂Te_2.7Se_0.3 solid solution with the roomtemperature Seebeck coefficient |S| = 212 μV/K have been studied in the temperature range 77–350 K. The observed temperature dependences demonstrate a number of specific features, which were earlier found in samples with a lower electron density N. The effect of these specific features on the thermoelectric figure of merit Z appears to be more favorable for the sample under study: this sample is most efficient in the temperature range 120–340 K, and the average value of ZT is 0.71. It is found that a rise in the density N enhances the factor responsible for the effective mass decreasing as the temperature increases. This effect appears when the analysis is carried out in terms of a single-band parabolic model with N = const(T). This finding suggests that the most probable reason for the unusual behavior of these properties is the complex structure of the electron spectrum. Temperature dependences obtained from calculations of the transport coefficients show good agreement with the experimental data for two samples of the mentioned composition with different electron densities. The calculations have been performed in terms of a two-band model and an acoustic scattering mechanism and take into account the anisotropy and nonparabolicity of the light-electron spectrum.     

Mesoscopic fluctuations in the conductance of silicon-based MOS structures with a doped surface layer

In n-Si-based metal-oxide-semiconductor structures with a boron-doped surface layer, at the temperature T = 77 K, mesoscopic fluctuations in nondiagonal resistance-tensor component R _xy are discovered under hole depletion of the Si: B layer caused by the field effect. A sharp increase in the fluctuation in R _xy is discovered during the study of a shift from the 3D weakly perturbed hole transport to the quasi-2D percolation conduction on the back boundary of the Si: B layer. The fluctuations in R _xy are due to the restructuring that occurs in the percolation cluster when the hole shielding of the fluctuation potential of ionized acceptors is weakened. Correlation length L _c of the percolation cluster, which determines the boundary of the transition from the macroscopic to the mesoscopic system, has been estimated experimentally. Good agreement between the estimates and the computational results is observed for L _c ranging from ≈ 10 nm to ≈ 1 μm.     

High-efficiency dual-junction GaInP/GaAs tandem solar cells obtained by the method of MOCVD

Monolithic dual-junction GaInP/GaAs solar cells grown by the MOCVD method were studied. The conditions of the growth of ternary Ga _x In_1?x P and Al _x In_1?x P alloys lattice-matched to GaAs are optimized. Technology for fabrication of a tunneling diode with a high peak current density of 207 A/cm² on the basis of heavily doped n ⁺⁺-GaAs:Si and p ⁺⁺-AlGaAs:C layers is developed. Cascade GaInP/GaAs solar cells obtained as a result of relevant studies featuring a good efficiency of the solar-energy conversion both for space and terrestrial applications. The maximum value of the GaInP/GaAs solar-cell efficiency was 30.03% (at AM1.5D, 40 suns).     

Transport properties of cobalt monosilicide and its alloys at low temperatures

The electrical resistivity and thermopower of cobalt monosilicide (CoSi) and dilute alloys of CoSi with iron at temperatures from 2 to 370 K are experimentally investigated. CoSi is a semimetal and considered to be a promising thermoelectric material. It crystallizes into the cubic structure without an inversion center. This feature suggests the existence of topologically nontrivial electronic states and makes CoSi a candidate for the Weyl semimetal class. The main goal of the study is to find experimental confirmation of this assignment. It is shown that the experimental temperature dependences of the electrical resistivity and thermopower of CoSi and Co_1–xFe_xSi (x = 0.04) at low temperatures cannot be interpreted within the standard theory of conductivity in metals and may be related to topological features of the electronic structure of this compound.     

Electrical and thermoelectric properties of <Emphasis Type="Italic">p</Emphasis>-Ag<Subscript>2</Subscript>Te in the β phase

The conductivity σ, Hall coefficient R, and thermoelectric power α₀ of p-Ag₂Te were studied in the temperature range of 300–550 K. Inconsistency between the signs of R and α₀ was observed at 420–550 K. These results are interpreted within the two-phase model with spherical constant energy surfaces. It is established that the inconsistency between the R and α₀ signs is due to the emergence of the scattering mechanisms with the parameters r_0ac, r₀₀, and r_0d an increase of about 50% in the ratio of the effective electron and hole masses as a result of the transition α → β.     

Effect of Rare Earth Elements on Electromagnetic and Microwave Absorption Properties of Fe-Based Alloys

The RE₂Fe₁₇ (RE = Ce, Pr, Nd, Sm) and La_xPr_2?xFe₁₇ (x = 0.0, 0.1, 0.2, 0.3, 0.4) alloys were synthesized by arc smelting and high-energy ball milling methods. The phase structure, morphology, magnetic properties and electromagnetic parameters of the powders were characterized by x-ray diffraction, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer, respectively. The results reveal that the lattice parameters a and c and unit-cell volume V of the La_xPr_2?xFe₁₇ alloys increase linearly upon the La content. The minimum absorption peak frequency shifts towards a lower-frequency region upon the La content. And the minimum reflection loss and saturation magnetization of the La_xPr_2?xFe₁₇ alloys decrease upon the La content, while the minimum reflection loss of Pr₂Fe₁₇ and La_0.4Pr_1.6Fe₁₇ alloy of the 2.0 mm coating thickness reaches about ?13.65 dB and ?7.15 dB at 5.92 GHz and 3.6 GHz, respectively.