Dielectric capping effects on binary and ternary topological insulator surface states

TL;DR

This study investigates how different crystalline dielectric cappings, especially oxygen-terminated quartz, affect the surface states of topological insulators, revealing that passivation can restore the Dirac cone.

Contribution

It provides a detailed theoretical analysis of dielectric effects on TI surface states, highlighting the impact of oxygen termination and passivation, which was not thoroughly explored before.

Findings

BN and quartz cappings have negligible effects on Dirac surface states

Oxygen-terminated quartz distorts the Dirac cone due to dangling bonds

Hydrogen passivation restores the Dirac cone integrity

Abstract

Using a density functional based electronic structure method, we study the effect of crystalline dielectrics on the metallic surface states of Bismuth- and chalcogen-based binary and ternary three dimensional topological insulator (TI) thin films. Crystalline quartz (SiO2) and boron nitride (BN) dielectrics were considered. Crystalline approximation to the amorphous quartz allows to study the effect of oxygen coverage or environmental effects on the surface states degradation which has gained attention recently in the experimental community. We considered both symmetric and asymmetric dielectric cappings to the sufaces of TI thin films. Our studies suggest that BN and quartz cappings have negligible effects on the Dirac cone surface states of both binary and ternary TIs, except in the case of an oxygen-terminated quartz surface. Dangling bond states of oxygens in oxygen-terminated quartz dominate the region close to Fermi level, thereby distorting the TI Dirac cone feature and burying the Dirac point in the quartz valence band region. Passivating the oxygen-terminated surface with atomic hydrogen removes these dangling bond states from the Fermi surface region, and consequently the clear Dirac cone is recovered. Our results are consistent with recent experimental studies of TI surface degradation in the presence of oxygen coverage.