Parasitic conduction channels in topological insulator thin films

TL;DR

This paper presents a method to estimate the charge carrier concentration in topological insulator thin films, accounting for parasitic conduction channels, to better isolate the contribution of topologically protected surface states.

Contribution

It introduces a calculation approach for the interior charge carrier concentration in TI films using surface measurements, independent of dopant levels in the thin-film limit.

Findings

Band bending is largely independent of dopant concentration in thin films.

The method allows quantification of parasitic channels' contributions to total conductance.

Total charge carrier concentration can be reliably estimated despite unknown dopant levels.

Abstract

Thin films of topological insulators (TI) usually exhibit multiple parallel conduction channels for the transport of electrical current. Beside the topologically protected surface states (TSS), parallel channels may exist, namely the interior of the not-ideally insulating TI film, the interface layer to the substrate, and the substrate itself. To be able to take advantage of the auspicious transport properties of the TSS, the influence of the parasitic parallel channels on the total current transport has to be minimized. Because the conductivity of the interior (bulk) of the thin TI film is difficult to access by measurements, we propose here an approach for calculating the mobile charge carrier concentration in the TI film. To this end, we calculate the near-surface band bending using parameters obtained experimentally from surface-sensitive measurements, namely (gate-dependent) four-point resistance measurements and angle-resolved photoelectron spectroscopy (ARPES). While in most cases another parameter in the calculations, i.e. the concentration of unintentional dopants inside the thin TI film, is unknown, it turns out that in the thin-film limit the band bending is largely independent of the dopant concentration in the film. Thus, a well-founded estimate of the total mobile charge carrier concentration and the conductivity of the interior of the thin TI film proves possible. Since the interface and substrate conductivities can be measured by a four-probe conductance measurement prior to the deposition of the TI film, the total contribution of all parasitic channels, and therefore also the contribution of the vitally important TSS, can be determined reliably.