Thermoelectric properties of 3D topological insulator: Direct observation of topological surface and its gap opened states

We report thermoelectric (TE) properties of topological surface Dirac states (TSDS) in three-dimensional topological insulators (3D-TIs) purely isolated from the bulk by employing single-crystal Bi_2-xSbₓTe_3-ySey films epitaxially grown in the ultrathin limit. Two intrinsic nontrivial topological surface states, a metallic TSDS (m-TSDS) and a gap-opened semiconducting topological state (g-TSDS), are successfully observed by electrical transport, and important TE parameters [electrical conductivity ({σ}), thermal conductivity ({κ}), and thermopower (S)] are accurately determined. Pure m-TSDS gives S=-440.16em0exμV0.16em0exK^-1, which is an order of magnitude higher than those of the conventional metals and the value is enhanced to -2120.16em0exμV0.16em0exK^-1 for g-TSDS. It is clearly shown that the semiclassical Boltzmann transport equation (SBTE) in the framework of constant relaxation time ({τ}) most frequently used for conventional analysis cannot be valid in 3D-TIs and strong energy dependent relaxation time {τ}(E) beyond the Born approximation is essential for making intrinsic interpretations. Although {σ} is protected on the m-TSDS, {κ} is greatly influenced by the disorder on the topological surface, giving a dissimilar effect between topologically protected electronic conduction and phonon transport.