Thermoelectric power factor of topological insulator Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Te$_{\mathrm{3-y}}$Se$_{\mathrm{y}}$

Topological insulator (TI) is a new quantum material. The surface states of TIs are protected by time-reversal symmetry which allows charge carrier to propagate on the edge of surface conducting channel without scattering. Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ is a well-known TI [1] and thermoelectric material because of its promising thermoelectric performances at room temperature. The conversion efficiency of~thermoelectric material is characterized by the dimensionless figure of merit ZT.~Decades of effort were devoted to ZT optimization either through composition alteration or nanostructure fabrication. In this study, the temperature dependence of resistance of bulk (exfoliated specimen with 140 $\mu $m thickness) shows semiconductor behavior (0.04 $\Omega $ cm at 300 K) without saturating regime in lower temperatures. In contrast, its nanoflake counterpart (100-500 nm) [2] shows a transition from semiconductor to metallic behavior near 100 -- 150 K with decreasing temperature and saturation at 10 K. Surface contribution to the total conductance of exfoliated specimens was acquired through Hall effect measurements in the magnetic field ranging from -9 to 9 Tesla. Surface contribution of BSTS samples increases from 3{\%} to 70{\%} as thickness decreases from 140 to 7~$\mu $m. In this work, we report a systematic study of thermoelectric power factor for various thicknesses of BSTS specimens to examine the thermoelectric power factor of their surfaces.\\[4pt] [1] Zhi Ren et al., Phys. Rev. B 84, 165311 (2011).\\[0pt] [2] Bin Xia et al., e-print arXiv1203.2997