Enhanced photogalvanic current in topological insulators via Fermi energy tuning

We achieve the enhancement of the circular photogalvanic effect arising from the photoinjection of spins in topological insulator thin films by tuning the Fermi level (EF). A series of (Bi_1-xSbₓ)₂Te₃ thin films were tailored so that the Fermi energy ranges above 0.34 eV to below 0.29 eV of the Dirac point, i.e., from the bulk conduction band bottom to the valence band top through the bulk in-gap surface-Dirac cone. The circular photogalvanic current, indicating a flow of spin-polarized surface-Dirac electrons, shows a pronounced peak when the EF is set near the Dirac point and is also correlated with the carrier mobility. Our observation reveals that there are substantial scatterings between the surface-Dirac and bulk state electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.