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

By combining n-type Bi2Te3 and p-type Sb2Te3 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 Bi2Te3/Sb2Te3 heterostructures with varying relative thicknesses of the top and bottom layers. With increasing thickness of the Sb2Te3 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 semiclassical 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.