Modulation of surface states in Sb2Te3/Bi2Te3 topological insulator heterostructures: The crucial role of the first adlayers

Abstract

The hunt for an ideal topological insulator, where the Dirac point is situated in a desirable energetic position and the bulk remains insulating, has motivated experiments on band structure engineering in these materials. To achieve this, ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are commonly combined in ternary compounds or, less frequently, in heterostructures. Here we report on the growth of ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}/{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ heterostructures by means of molecular-beam epitaxy. Using angle-resolved photoelectron spectroscopy, we are able to differentiate between the shift of the chemical potential and the changes in the electronic structure, causing the lift-off of the Dirac point away from the bulk valence band when varying the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ adlayer thickness. Our paper demonstrates that the important modulation of the surface states takes place for the very first ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ layers, while thicker adlayers only cause a gradual change of the bulk states and a rigid shift of the chemical potential. Furthermore, we observe the occurrence of diffusion between the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ layers and conclude that a growth at room temperature, followed by annealing, maintains an acceptable crystalline quality while substantially reducing the interdiffusion.