Bidirectional surface photovoltage on a topological insulator

Abstract

Controlled extraction of spin-polarized currents from the surface of topological insulators (TIs) would be an important step to use TIs as spin-electronic device materials. One way is to utilize the surface photovoltage (SPV) effect, by which the surface current may flow upon irradiation of light. To date, unipolar SPV has been observed on TIs, while the realization of ambipolar SPV is crucial for taking control over the direction of the flow. By using time-resolved photoemission, we demonstrate the ambipolar SPV realized on the TI ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The topological surface states showed downward and upward photovoltaic shifts for the $n$- and $p$-type samples, respectively. We also discerned the photogenerated carriers accumulated in the surface states for $g4\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. We provide the keys besides the in-gap Fermi level to engineer the SPV on TIs.