Nanometer Structured Epitaxial Films and Foliated Layers Based on Bismuth and Antimony Chalcogenides with Topological Surface States

The thermoelectric and galvanomagnetic properties of nanometer structured epitaxial films and foliated layers based on bismuth and antimony chalcogenides were investigated, and an increase in the figure of merit Z up to 3.85 × 10-3 K-1 was observed in the Bi0.5Sb1.5Te3 films over the temperature range of 180–200 K. It is shown that an increase in the Seebeck coefficient and the change in the slope on temperature, associated with changes in the effective scattering parameter of charge carriers and strong anisotropy of scattering in the films, lead to enhance power factor due to the growth of the effective mass of the density of states. These features are consistent with the results of research of oscillation effects in strong magnetic fields at low temperatures and research of Raman scattering at normal and high pressures in the foliated layers of solid solutions (Bi, Sb)2(Te, Se)3, in which the topological Dirac surface states were observed. The unique properties of topological surface states in the investigated films and layers make topological insulators promising material for innovation nanostructured thermoelectrics

Improved Thermoelectrics Based on Bismuth and Antimony Chalcogenides for Temperatures below 240 K

Thermoelectric and galvanomagnetic properties for solid solution based on bismuth and antimony chalcogenides were studied for the optimal compositions and carrier concentrations in the temperature interval 100-240 K. Galvanomagnetic properties were analyzed in the framework of the many–valley energy spectrum model with isotropic and anisotropic scattering of charge carries. The figure-of-merit is shown to be determined with optimal relations between the values of the density-of-states effective mass, the carrier mobility taking into account degeneration of charge carriers, and the lattice thermal conductivity. The figure-of-merit also depends on anisotropy of the constant energy surface and scattering mechanism. Average values of the figure-of-merit &lt;Z&gt; through the temperature interval 100-240 K are equal to (2.5-2.65) 10-3 K1 for optimal compositions and carrier concentrations. </jats:p

Thermal Conductivity for p–(Bi, Sb)₂Te₃ Films of Topological Insulators

In this study, we investigated the temperature dependencies of the total, crystal lattice, and electronic thermal conductivities in films of topological insulators p–Bi0.5Sb1.5Te3 and p–Bi2Te3 formed by discrete and thermal evaporation methods. The largest decrease in the lattice thermal conductivity because of the scattering of long-wavelength phonons on the grain interfaces was observed in the films of the solid-solution p–Bi0.5Sb1.5Te3 deposited by discrete evaporation on the amorphous substrates of polyimide without thermal treatment. It was shown that in the p–Bi0.5Sb1.5Te3 films with low thermal conductivity, the energy dependence of the relaxation time is enhanced, which is specific to the topological insulators. The electronic thermal conductivity was determined by taking into account the effective scattering parameter in the relaxation time approximation versus energy in the Lorentz number calculations. A correlation was established between the thermal conductivity and the peculiarities of the morphology of the interlayer surface (0001) in the studied films. Additionally, the total κ and the lattice κL thermal conductivities decrease, while the number of grains and the roughness of the surface (0001) increase in unannealed films compared to annealed ones. It was demonstrated that increasing the thermoelectric figure of merit ZT in the p–Bi0.5Sb1.5Te3 films formed by discrete evaporation on a polyimide substrate is determined by an increase in the effective scattering parameter in topological insulators due to enhancement in the energy dependence of the relaxation time