  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.
