Fig. 3: Doping-dependent photocurrent profiles.

a, b Photocurrent and optical images, where vertical yellow lines indicate the contacts. Colour scales are current in nanoampere. Laser power is 723 nW. c, d Gate-dependent conductance measured in the dark at 7 K. Insets: band diagrams showing E_F positions. a, c are for pure Bi₂Se₃, where E_F is close to the conduction band. Field-effect mobility and electron concentration are estimated to be μ = 329 cm² V⁻¹ s⁻¹, n = 3.25 × 10¹⁸ cm⁻³. The photocurrent is only observed when excitation is close to the contacts. b, d Sb doping lowers E_F as evidenced by ambipolar conduction. μ = 371 cm² V⁻¹ s⁻¹, n = 9.3 × 10¹⁶ cm⁻³ for electrons. The scale bars correspond to 3 μm in a and 30 μm in b.