Ultrathin films of three-dimensional topological insulators by vapor-phase epitaxy: Surface dominant transport in a wide temperature range as revealed by measurements of the Seebeck effect

Abstract

Realization of intrinsic surface dominant transport in a wide temperature region for a topological insulators (TIs) is an important frontier research in order to promote the progresses of TIs towards the future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov-de-Haas (SdH) quantum oscillations, Hall coefficient (R_H^2D), and Seebeck coefficient as a function of film thickness (d) and temperature using high quality Bi2-xSbxTe3-ySey (BSTS) single crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet conductance and Seebeck coefficient clearly show the suppression of semiconducting hole carriers of bulk states by reducing film thickness, reaching to the surface dominant transport at below dc=14 nm. Quantitative arguments are made as to how the contribution of itinerant carrier number (n) can be suppressed, using both R_H^2D (n_Hall^2D) and SdH (n_SdH). Intriguingly, the value of n_Hall^2D approaches to be twice of n_SdH below dc. While R_H^2D shows a negative sign in whole temperature region, a change from negative to positive polarity is clearly observed for S at high temperatures when d is thick. We point out that this inconsistency observed between R_H^2D and S is intrinsic in 3D-TIs and its origin is the large difference in carrier mobility between the bulk and the topological surface. 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.