Seebeck effect in 3D topological insulator thin films

Growing thin films is one of the methods to investigate the intrinsic physical properties of Dirac surface state in three-dimensional Topological insulators (3D-TIs). However, the suppression of the bulk in thin films can be recognized only at low temperatures, and no systematic study has been carried out to confirm whether surface dominant transport can be realized in a wide range of temperature. Here, we report our systematic measurements of sheet resistance (R), Hall coefficient (R_H), Shubnikov-de-Haas (SdH) quantum oscillations, Seebeck coefficient (S) as a function of both thickness (d) and temperature (T) using high-quality Bi_2-xSb_xTe_3-ySe_y single crystal thin films. R and S shift systematically from the bulk/surface coexisting regime to the surface dominant one by reducing d. S of thinner films clearly shows a linear T-dependence with a negative polarity from 300 to 2K, indicating a surface dominant transport in a wide-temperature region. Quantitative arguments are made as to how the contribution of bulk carrier can be suppressed, using both R_H and SdH measurements. We propose that Seebeck coefficient can become a convenient and powerful tool to evaluate the intrinsic carrier concentration for the topological surface in 3D-TIs even in the absence of magnetic field.