Enhanced photogalvanic current in topological insulators via Fermi energy tuning

TL;DR

This study demonstrates that tuning the Fermi energy in topological insulator thin films significantly enhances the circular photogalvanic effect, with peak responses near the Dirac point, revealing insights into spin-polarized photocurrent generation.

Contribution

It introduces a method to enhance photogalvanic current in topological insulators by Fermi level tuning, highlighting the importance of electronic structure design.

Findings

Peak photogalvanic current occurs near the Dirac point.

Surface-Dirac and bulk state scatterings affect photocurrent generation.

Optimizing electronic structure reduces scattering and enhances effects.

Abstract

We achieve the enhancement of circular photogalvanic effect arising from the photo-injection of spins in topological insulator thin films by tuning the Fermi level ($E_{\rm F}$). A series of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ 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 $E_{\rm F}$ 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 bulkstate electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.