  Electron-electron interactions in topological p-n junctions consisting of
vertically stacked topological insulators are investigated. n-type Bi2Te3 and
p-type Sb2Te3 of varying relative thicknesses are deposited using molecular
beam epitaxy and their electronic properties measured using low-temperature
transport. The screening factor is observed to decrease with increasing sample
thickness, a finding which is corroborated by semi-classical Boltzmann theory.
The number of two-dimensional states determined from electron-electron
interactions is larger compared to the number obtained from
weak-antilocalization, in line with earlier experiments using single layers.
