Intrinsic surface dipole in topological insulators

TL;DR

This study uses density functional theory to reveal an intrinsic surface dipole in topological insulators, causing band bending and suggesting their potential as intrinsic Schottky barrier solar cells.

Contribution

It demonstrates the existence of an intrinsic surface dipole in topological insulators and its effects on band structure without disorder or doping.

Findings

A 2 nm surface dipole exists in pristine topological insulators.

Band bending of about 75 meV occurs near the surface.

Topological insulators can function as intrinsic Schottky barrier solar cells.

Abstract

We calculate the local density of states of two prototypical topological insulators (Bi$_2$Se$_3$ and Bi$_2$Te$_2$Se) as a function of distance from the surface within density functional theory. We find that, in the absence of disorder or doping, there is a 2 nm thick surface dipole the origin of which is the occupation of the topological surface states above the Dirac point. As a consequence, the bottom of the conduction band is bent upward by about 75 meV near the surface, and there is a hump-like feature associated with the top of the valence band. We expect that band bending will occur in all pristine topological insulators as long as the Fermi level does not cross the Dirac point. Our results show that topological insulators are intrinsic Schottky barrier solar cells.