Disentangling surface and bulk transport in topological-insulator $p$-$n$ junctions

TL;DR

This study investigates how surface and bulk transport contribute to electrical conduction in topological insulator $p$-$n$ junctions by combining experimental magnetotransport measurements with theoretical modeling.

Contribution

It provides a detailed analysis of surface and bulk transport mechanisms in topological insulator heterostructures, highlighting the role of bulk conduction channels often inaccessible to surface-sensitive techniques.

Findings

Bulk conduction channels significantly influence transport properties.

Transition from $n$- to $p$-type behavior with increasing layer thickness.

Theoretical models accurately reproduce experimental resistivity data.

Abstract

By combining $n$-type $\mathrm{Bi_2Te_3}$ and $p$-type $\mathrm{Sb_2Te_3}$ topological insulators, vertically stacked $p$-$n$ junctions can be formed, allowing to position the Fermi level into the bulk band gap and also tune between $n$- and $p$-type surface carriers. Here we use low-temperature magnetotransport measurements to probe the surface and bulk transport modes in a range of vertical $\mathrm{Bi_2Te_3/Sb_2Te_3}$ heterostructures with varying relative thicknesses of the top and bottom layers. With increasing thickness of the $\mathrm{Sb_2Te_3}$ layer we observe a change from $n$- to $p$-type behavior via a specific thickness where the Hall signal is immeasurable. Assuming that the the bulk and surface states contribute in parallel, we can calculate and reproduce the dependence of the Hall and longitudinal components of resistivity on the film thickness. This highlights the role played by the bulk conduction channels which, importantly, cannot be probed using surface sensitive spectroscopic techniques. Our calculations are then buttressed by a semi-classical Boltzmann transport theory which rigorously shows the vanishing of the Hall signal. Our results provide crucial experimental and theoretical insights into the relative roles of the surface and bulk in the vertical topological $p$-$n$ junctions.