Optical conductivity of a gated Topological Insulator with a THz-transparent gate

THz time-domain spectroscopy (THz-TDS) is a powerful tool to investigate topological insulator systems as the optical conductivity is sensitive to more scattering events than traditional transport measurements. As such, this technique is well suited for capturing the behaviour not just of the surface carriers, but also of the bulk carriers that dominate the transport of most topological insulator systems. However, due to the sensitivity of time-domain spectroscopy measurements, there is some ambiguity over the role the bulk and surface carriers play in the THz conductivity response.

A 2D carrier gas will only ever partially screen an electric field from a gate, applied perpendicular to the 2D gas. Therefore, the presence of any 2DEG on the surface of a topological insulator will cause the bulk carriers to experience a smaller applied electric field than would otherwise be the case. This remains true even in the case where the carriers within the 2DEG do not significantly contribute to the DC transport. We have grown a Bi₂Se₃ topological insulator by molecular beam epitaxy and subsequently developed and deposited a THz-transparent top-gate stack in order to facilitate electrostatic gating of this topological insulator material. We will perform THz-TDS and DC transport measurements on this topological insulator field effect device in order to investigate the effect that an applied gate bias has on the carrier dynamics, with an aim to deconvolute the bulk and surface response in the optical conductivity spectrum.