  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.
