Crystal growth and thermoelectric properties in nonstoichiometric bismuth chalcogenides

• Main Authors: Kuei-Kuan Wu, 吳奎寬
• Other Authors: Yung-Kang Kuo
• Format: Others
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/51507750338434396895

博士 === 國立東華大學 === 物理學系 === 104 === 　Topological insulators (TIs) are new electronic materials (which can be used in the topological quantum computation) that have a bulk band-gap like a normal insulator as well as the conducting surface-states protected by time reversal symmetry. At present, the existing TI materials can be divided into two groups, i.e., HgTe and Bi2Se3 families. The electronic band structures of some of TIs also make them as good thermoelectric systems such as Bi2Se3, Bi2Te3, and Bi2Te2Se. Besides, the topological states can be a new quantity to optimize the thermoelectric figure of merit (ZT) for the realization of high performance thermoelectric devices. With these understandings, the compositional effects on electrical and thermal transport properties of Bi2+xSe3-δ- and Bi2TexSey-based single crystals were investigated and presented in the present thesis. 　The Bi2+xSe3-based single crystals using the self-fluxes with Bi-Se ratios of 2 : 3 (sample A) and 2.2 : 3 (sample B) were grown by a vertical Bridgman technique. Structural and compositional studies on the grown crystals were performed using x-ray diffraction and electron probe microanalyzer techniques. Sample A is found to grow perpendicular to the c-axis (along the ab basal plane) with a homogenous composition of Bi2.0Se2.53, while sample B was grown along the c-axis with four naturally cleaved disc-shaped crystal pieces of Bi2.0Se2.99, Bi2.04Se2.99, Bi2.12Se2.99, and Bi2.37Se2.99. Additionally, the lattice constant (c) of the crystal B specimens increases gradually with increasing Bi content, essentially due to the increase in Bi-intercalant content in the van der Waals gap of these layered structure samples. The distinct growth mechanism for the crystals A and B were proposed phenomenologically based on their growth habits and structural properties. 　Thermoelectric measurements such as the electrical resistivity, Seebeck coefficient, and thermal conductivity on the Bi2Se3-based single crystals were done, in the temperature range of 10-300 K. All grown crystals are weak metals with a resistivity ratio ρ300K/ρ10K of < 2.2. Besides, the crystals show n-type thermoelectric transport in the entire temperature range, and their Fermi energy is in the range of 0.20-0.43 eV. Thermal conductivity study showed that the lattice thermal conductivity of the Bi2Se3-based crystals below 150 K can be reproduced rather well by a Debye-Callaway phonon model, which reveals the boundary and point-defect scattering are mainly contributing to the low-T phonon transport in these crystals. The near-stoichiometric crystals (Bi2.0Se2.99 and Bi2.04Se2.99) have a room-temperature ZT of about ~0.1; however it decreases for the non-stoichiometric crystals due to the introduction of defects such as Bi intercalants, Se vacancies, and antisite defects. Here, it is understood that the physical properties of the Bi2Se3-based crystals can be greatly controlled by the induced native defects. 　The crystal growth of ternary alloy compound Bi2TexSey was done using the direct solidification method with a slow cooling process. Six samples with x + y ~ 3 in the mixed phase region (i.e., 0.4 < y < 1.4), namely Bi2Te1.51Se1.38, Bi2Te1.63Se1.23, Bi2Te1.71Se1.10, Bi2Te1.89Se0.98, Bi2Te2.16Se0.71, and Bi2Te2.10Se0.66, were selected using the electron probe microanalysis for the transport studies. The grown crystals were found to crystallize in the rhombohedral structure (R3m ) along the (00l) plane, which demonstrates the single-crystalline nature of the crystals. The estimated values of lattice constant (c) of the crystals do not vary systematically with the Se or Te content. 　Temperature-dependent electrical resistivity, Seebeck coefficient, Hall coefficient, and thermal conductivity of the chosen Bi2TexSey crystals with x + y ~ 3 were carried out to probe the effects of Se/Te content on the electrical and thermal transport properties. The crystals with high Se content (y > 1.0) show a weak metallic nature, whereas the crystals with low Se content (y  1.0) have an unconventional semiconductor (insulator-like) behavior with the surface-states conduction at low-temperature. This can be understood by the modification in the electronic band structure and the Fermi surface as a result of the change in the Se/Te content in the Bi2TexSey-based systems, which results in the tuning of band-gap of the materials. Besides, the observed complex transport in the crystals with y  1.0 can be realized by the impurity band conduction along with the hoping conduction. Here, the maximum room-temperature ZT of about 0.46 was obtained for the Bi2Te1.63Se1.23 crystal, owing to its high power factor and low thermal conductivity. Finally, two high resistive compounds with low bulk carrier density, namely the n-type Bi2Te1.89Se0.98 and the p-type Bi2Te2.16Se0.66 crystals, were identified as suitable compounds to investigate the topological insulating characteristics in them.