Thermoelectric properties in nonstoichiometric crystal growing Bismuth chalcogenide Bi2TexSey
K.K. Wu1,2*, F.C. Chou2, Y.K. Kuo1
1Department of Physics, National Dong-Hwa University, Hualien, Taiwan
2Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan
* presenting author:Kuei-Kuan Wu,
The nonstoichiometric crystals of ternary compounds Bi2TexSey (x + y = 3) were grown by the direct solidification with a very slow cooling rate (6 K/h). We have selected six of the twelve obtained crystals with a gradual variation in the composition, as determined by electron probe microanalysis. The electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) of these selected crystals were measured to probe their thermoelectric performance. It is found that the estimated value of the Fermi energy increases with increase in Te content (x value) from 1.51 to 2.16, while the corresponding Se content (y value) decreases from 1.38 to 0.66. This finding is mainly due to the modification of the electronic band structure and the Fermi surface of the Bi2TexSey-based crystals. Besides, the increase in the Te/Se ratio could lead to the formation of the p-type material (Bi2Te2.10Se0.66) from an originally n-type compound (Bi2Te1.89Se0.98). Analysis of thermal conductivity of the studied compounds revealed that the phonon-grain boundary scattering and phonon-point defect scattering govern the low-temperature thermal transport, whereas phonon-phonon scattering contributes significantly to high-temperature thermal conduction. Finally, the highest room-temperature thermoelectric figure-of-merit, ZT of about 0.45 was achieved for the n-type compound Bi2Te1.63Se1.23.

Keywords: topological insulator, thermoelectric properties, nonstoichiometric