Dielectric capping effects on binary and ternary topological insulator surface states

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 one to study the effect of oxygen coverage or environmental effects on the surface-state degradation, which has gained attention recently in the experimental community. We considered both symmetric and asymmetric dielectric cappings to the surfaces 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.