Separating the bulk and surface n- to p-type transition in the topological insulator GeBi(4-x)SbxTe7

TL;DR

This study investigates the electronic properties of GeBi(4-x)SbxTe7, revealing a surface-bulk transition from n- to p-type doping driven by Sb content, with implications for topological surface states and band bending effects.

Contribution

It demonstrates the separation of surface and bulk n- to p-type transitions in a topological insulator using ARPES and Seebeck measurements, highlighting the role of band bending.

Findings

Topological insulator GeBi4Te7 has a Dirac point above the valence band maximum.

Sb doping induces a transition from n- to p-type in bulk and surface regions.

Band bending causes different n- to p-type transitions in surface and bulk.

Abstract

We identify the multi-layered compound GeBi4Te7 to be a topological insulator with a freestanding Dirac point, slightly above the valence band maximum, using angle-resolved photoemission spectroscopy (ARPES) measurements. The spin polarization satisffies the time reversal symmetry of the surface states, visible in spin-resolved ARPES. For increasing Sb content in GeBi(4-x)SbxTe7 we observe a transition from n- to p-type in bulk sensitive Seebeck coefficient measurements at a doping of x = 0.6. In surface sensitive ARPES measurements a rigid band shift is observed with Sb doping, accompanied by a movement of the Dirac point towards the Fermi level. Between x = 0.8 and x = 1 the Fermi level crosses the band gap, changing the surface transport regime. This difference of the n- to p-type transition between the surface region and the bulk is caused by band bending effects which are also responsible for a non-coexistence of insulating phases in the bulk and in the near surface region.