Why is the bulk resistivity of topological insulators so small?

TL;DR

This paper explains why topological insulators have very low bulk resistivity, attributing it to band bending caused by poorly screened Coulomb potential fluctuations, supported by numerical simulations and experimental data.

Contribution

The study demonstrates that small bulk resistivity in topological insulators results from band bending due to Coulomb potential fluctuations, providing a quantitative simulation-based explanation.

Findings

Bulk resistivity has an activation energy of 0.15 times the band gap.

At low temperatures, transport shifts from activation to variable range hopping.

Simulations align well with experimental measurements.

Abstract

As-grown topological insulators (TIs) are typically heavily-doped $n$-type crystals. Compensation by acceptors is used to move the Fermi level to the middle of the band gap, but even then TIs have a frustratingly small bulk resistivity. We show that this small resistivity is the result of band bending by poorly screened fluctuations in the random Coulomb potential. Using numerical simulations of a completely compensated TI, we find that the bulk resistivity has an activation energy of just 0.15 times the band gap, in good agreement with experimental data. At lower temperatures activated transport crosses over to variable range hopping with a relatively large localization length.