  SmB6 is a proposed topological Kondo insulator where the presence of
topological nontriviality can be tuned by variations in the Sm valence.
Experimentally, Sm valence can be changed by tuning stoichiometry of SmB6. We
show that Raman scattering can detect vacancies lower than 1% of Sm sites in
SmB6 crystal by probing the intensity of defect-induced scattering of the
acoustic phonon branch at 10~meV. In the electronic Raman spectra of SmB6 at
temperatures below 130~K, we observe features developing in A$_{1g}$ and E$_g$
symmetries at 100 and 41~meV which we assign to excitations between hybridized
bands, and depressed spectral weight below 20~meV associated with the
hybridization gap. With the increased number of Sm vacancies up to 1% we
observe an increase of spectral weight below 20~meV showing that the gap is
filling in with electronic states. For the sample with the lowest number of
vacancies the in-gap exciton excitations with long lifetimes protected by
hybridization gap are observed at 16-18~meV in E$_g$ and T$_{2g}$ symmetries.
These excitations broaden as a decrease in the lifetime with increasing number
of vacancies and are quenched by the presence of in-gap states at concentration
of Sm vacancies of about 1%. Based on this study we suggest that only the most
stoichiometric SmB6 samples have a bulk gap necessary for topological Kondo
insulators.
