TE properties of 3D TI (Bi_1−xSb_x)₂Te₃ thin films. (a) The measured Hall (left) and Seebeck coefficients (right) as a function of temperature T for the 5-quintuple-layer thin film with x = 0.9. (b) Schematic drawing shows the coexistence of N-type surface states and P-type bulk states in a TI thin film. (c) A theoretical phase diagram for TI thin films with N-type surface and P-type bulk charge carriers. (d) The experimental phase diagram of (Bi_1−xSb_x)₂Te₃ thin films summarizing the Seebeck coefficient as a function of x and T. Three different regions (I, II, III) are defined according to the signs of S and R_H in panels (c) and (d). For TIs, there is a sizable region II that gives opposite signs between S and R_H. Adapted from Ref. [66].