L. N. Lukyanova

Optimization of (Bi,Sb)/sub 2/(Te,Se)/sub 3/-based thermoelectrics for low-temperature applications

General situation with thermoelectric cooling at temperatures below 200 K is discussed. The problem of optimization of (Bi,Sb)/sub 2/(Te,Se)/sub 3/-based thermoelectrics for operation in this temperature range is distinguished as the actual one especially in connection with a possible practical application of high-temperature superconductors. Then detailed results of the first systematic study of n-type Bi/sub 2/Te/sub 3-x-y/Se/sub x/S/sub y/ and p-type Bi/sub 2-x/Sb/sub x/Te/sub 3-y/Se/sub y/ are presented. The composition of solid solutions and the current carrier concentration were optimized. Thermopower, electrical and thermal conductivity were measured at 80-350 K, and thermoelectric figure of merit was calculated. Figure of merit for both type materials was increased significantly for the temperatures near 100-150 K. These optimized materials were used for the fabrication of the special low-temperature thermoelectric modules. These modules were tested, and temperature 130 K and 124 K on the cold junction of 2 types of 6-stages coolers was achieved.

Thermoelectric and galvanomagnetic properties of bismuth chalcogenide nanostructured heteroepitaxial films

Hot wall technique was used to grow block single crystal films of Bi2Te3 and solid solutions of Bi0.5Sb1.5Te3 on mica (muscovite) substrates. X-ray diffraction studies demonstrated that the crystalline c-axis in the films was normal to the substrate plane. Seebeck coefficient, electrical conductivity and magnetoresistivity tensor components were measured at various orientations of magnetic and electric fields in the temperature interval 77–300 K. Scattering mechanism of charge carriers in the films was studied using temperature dependences of the degeneracy parameter and the Seebeck coefficient in terms of a many-valley model of energy spectrum. Obtained results have shown that the effective scattering parameter is considerably different from the value specific for an acoustic scattering of charge carriers in the weakly degenerate films due to an additional scattering of charge carriers on interface and interctystallite boundaries. These features of charge carrier scattering are supposed to affect electronic transport in the films and enhance figure of merit.

Galvanomagnetic properties of multicomponent solid solutions based on Bi and Sb chalcogenides

Galvanomagnetic properties of multicomponent solid solutions of n-Bi/sub 2-x/Sb/sub x/Te/sub 3-y-z/Se/sub y/S/sub z/ composition were studied in weak and intermediate magnetic fields. Components of the effective mass tensor m/sub i//m/sub j/ were determined in the framework of the many-valley energy spectrum model using the isotropic scattering mechanism for a variety of solid solution compositions and electron concentrations. The influence of constant-energy surface parameters on the solid solutions thermoelectric efficiency was analyzed.

Anisotropic Scattering in the (Bi, Sb)2 (Te, Se, S)3 Solid Solutions

Analysis of thermoelectric and galvanomagnetic properties is carried out for solid solutions (Bi, Sb)²(Te, Se, S)³ with various substitutions of atoms in sublattices of basic Bi ₂Te₃ compound. The increase in thermoelectric figure of merit Z is observed in the compositions with a weak anisotropy of charge carriers in mirror plane. In addition, relaxation time tensor component along trigonal direction (tau₃₃), which is perpendicular to a direction of crystal growth, decreases in comparison with components along binary (tau₁₁) and bisector (tau ₁₁) directions

Transport properties of heteroepitaxial films based on bismuth telluride in strong magnetic fields

The temperature and magnetic-field dependences of the galvanomagnetic properties of n-Bi2Te3 heteroepitaxial films in magnetic fields to 14 T at low temperatures are studied. It is shown that steps in the magnetic-field dependences of the quantum Hall effect and the plateau in the temperature dependences of the magnetoresistance of films are caused by topological surface states of Dirac fermions.

On the conduction-band structure of bismuth telluride: optical absorption data

The optical absorption spectra of n-Bi2Te3 in the range 40–300 meV are studied at room temperature in relation to the electron concentration and sample thickness in order to determine the parameters of the additional subband in the conduction band of Bi2Te3 and to clarify the possible effect of this subband on charge-carrier transport. It is shown that bismuth telluride is a direct-gap semiconductor with an additional subband in the conduction band. These data are consistent with the results of studies of quantum oscillations in n-Bi2Te3 in high magnetic fields at temperatures below 20 K.

On the morphology of the interlayer surface and micro-Raman spectra of layered films in topological insulators based on bismuth telluride

Resonant micro-Raman spectra and the morphology of the interlayer Van der Waals surface are studied for layered thin films of n-Bi2Te3 and solid solutions based on Bi2Te3. It is found that the composition, thickness, surface morphology, and the method of obtaining films affect the relative intensity of Raman phonons, which are sensitive to the topological surface states of Dirac fermions.

On the density-of-states effective mass and charge-carrier mobility in heteroepitaxial films of bismuth telluride and Bi0.5Sb1.5Te3 solid solution

It is shown that the Seebeck coefficient α, the power factor α2σ, and the density-of-states effective mass m/m0 in heteroepitaxial films of Bi0.5Sb1.5Te3 solid solution are higher than the corresponding characteristics of bulk thermoelectric materials. The elevated values and weak temperature dependences of these parameters lead to a rise in the parameter proportional to the effective mass, the charge-carrier mobility, and the figure of merit. The character of change in α, α2σ, and m/m0 is determined by the peculiarities of the mechanism of charge-carrier scattering, the anisotropy of the constant-energy surface, and the possible influence of topological surface states of Dirac fermions in the films.

Optimization of multicomponent thermoelectrics based on Bi 2 Te 3 in the range below and above room temperature

Thermoelectric and galvanomagnetic properties have been studied in n-Bi_2-xSb_xTe_3-y-zSe_yS_z and p-Bi_2-xSb_xTe_3-y-zSe_y multicomponent solid solutions with various atomic substitutions in both sublattices of Bi₂Te₃. Anisotropy of the constant energy surface and charge carrier scattering was determined within the framework of a many-valley-model of energy spectrum using isotropic and anisotropic scattering mechanism for compositions of solid solutions with higher figure-of-merit.

Peculiarities of the figure of merit temperature dependence in n-Bi/sub 2/(Te,Se)/sub 3/ solid solution at low charge carrier concentration

A study has been made of the temperature dependence of the thermoelectric properties of n-Bi/sub 2/(Te,Se)/sub 3/ solid solutions (x=0.3 and 0.36). An optimal charge carrier concentration in the low temperature range (80-120) K was provided with addition of excess Te. A minimum of the Z=f(T) relation was observed near 120 K; the value of Z is increased when the temperature falls to 80 K. This may be explained by the absence of interband scattering of carriers at the studied carrier concentrations.