Band Structure and Polarization Effects in Photothermoelectric Spectroscopy of a Bi2Se3 Device

TL;DR

This study investigates the optical absorption and polarization effects in a Bi2Se3 topological insulator device using photothermoelectric spectroscopy, revealing band transitions and contact-induced polarization dependence.

Contribution

It demonstrates the use of photothermoelectric spectroscopy to probe energy-dependent optical transitions and polarization effects in Bi2Se3, highlighting surface and bulk band features.

Findings

Identification of spectral signatures for valence to conduction band transitions

Observation of a broad peak at 1.5 eV related to bulk and surface states

Detection of linear polarization dependence influenced by metal contacts

Abstract

Bi2Se3 is a prototypical topological insulator which has a small band gap (~0.3 eV) and topologically protected conducting surface states. This material exhibits quite strong thermoelectric effects. Here we show in a mechanically exfoliated thick (~100 nm) nanoflake device that we can measure the energy dependent optical absorption through the photothermoelectric effect. Spectral signatures are seen for a number of optical transitions between the valence and conduction bands, including a broad peak at 1.5 eV which is likely dominated by bulk band-to-band optical transitions but is at the same energy as the well known optical transition between the two topologically protected conducting surface states. We also observe a surprising linear polarization dependence in the response of the device that reflects the influence of the metal contacts.