Thermoelectric transport in the topological insulator Bi₂Se₃

Topological insulators (TIs) are materials that are insulators in the bulk but possess metallic topological surface states formed due to the band inversion caused by strong spin orbit coupling. Thermoelectric transport characteristics in TIs such as Bi₂Te₃ and Bi₂Se₃ have been studied theoretically [1, 2]. Thermoelectric power factor is predicted to increase at few quintuple layer thickness although this is attributed variously in different theories. In terms of experiments, thermoelectric characterization of thin film TIs is challenging due to high intrinsic carrier concentration. Gated measurements on thin film Bi₂Se₃showed a Seebeck coefficient of -150 μV/K at 240 K at a high gate voltage of 100 V [3]. However, the role of bulk vs topological states is unclear from these measurements. Separating the contributions from bulk and topological states to the Seebeck coefficient (<i>S</i>) and electrical conductivity (<i>σ</i>) can provide a more robust test of theory. Toward this end, we measured <i>S</i> and <i>σ</i> of 10nm thick Bi₂Se₃ film in the temperature range of 100 – 300 K. The film was grown on sapphire substrate using molecular beam epitaxy. The substrate temperature was set to 200⁰C and the molecular fluxes of Bi and Se were chosen at 1.4 – 1.5 x 10¹³ atom. cm^-2. s^-1 and 3.2 – 3.7 x 10¹³ atom. cm^-2. s^-1 to achieve the appropriate stoichiometric ratio of Bi and Se. We used electron beam lithography to fabricate heaters, thin film resistance temperature detectors and voltage probes on the Bi₂Se₃ film. We determined <i>S</i> by measuring the temperature gradient and voltage generated across Bi₂Se₃film and used the four-point probe method to determine <i>σ</i>. Treating the bulk and topological contributions to act in parallel, we were able to separate contributions from the 3D bulk and 2D surface states. We find that bulk and surface states contribute almost equally to S and <i>σ</i>. In comparison to the bulk, the measured <i>S</i> is lower owing to the more conducting nature of surface states. Overall, the thermoelectric power factor increases with temperature due to an increasing <i>S</i>. This work provides insight into the contributions of bulk versus topological electronic states to the Seebeck coefficient and thermoelectric power factor in Bi₂Se₃.<br/><u>References:</u><br/>1) Takahashi, R., & Murakami, S. (2012). Thermoelectric transport in topological insulators. <i>Semiconductor Science and Technology</i>, <i>27(12)</i>, 124005.<br/>2) Xu, Y., Gan, Z., & Zhang, S. C. (2014). Enhanced thermoelectric performance and anomalous Seebeck effects in topological insulators. <i>Physical review letters</i>, <i>112(22)</i>, 226801.<br/>3) Kim, D., Syers, P., Butch, N. P., Paglione, J., & Fuhrer, M. S. (2014). Ambipolar surface state thermoelectric power of topological insulator Bi2Se3. <i>Nano letters</i>, <i>14(4)</i>, 1701-1706.