Contribution of both bulk and surface states on photothermoelectric transport in epitaxial Bi2Se3 thin films

Abstract

Probing the surface states in Bi2Se3 via electronic transport measurements is difficult due to the significantly larger numbers of bulk carriers. Herein, we report the contribution of both bulk and surface states on the measured photocurrent at room temperature and the electrical transport measurements at low temperatures in an epitaxial thin film of intrinsic Bi2Se3 capped with a high K dielectric, Al2O3. The measured photocurrent continuously increases when the gate voltage is swept toward the negative side from 10 to −10 V, indicating the movement of the Fermi level below the conduction band edge. Moreover, the contribution of the surface state conduction increases when the Fermi level moves inside the bandgap toward the Dirac point. Furthermore, the measured Seebeck coefficient (S) continuously increases with sweeping gate voltage from 10 to −10 V. Similar correlations of the photocurrent and S value with the gate voltage illustrate the contribution of the photothermoelectric effect due to the conduction of both bulk and surface states. Additionally, the ambipolar charge transport was observed in the electrical measurement at 32 K for the four-probe configuration. The ambipolar charge transport is possibly indicative of surface state transport.