  The observation of helical surface states in Bi-based three-dimensional
topological insulators has been a challenge since their theoretical prediction.
The main issue raises when the Fermi level shifts deep into the bulk conduction
band due to the unintentional doping. This results in a metallic conduction of
the bulk which dominates the transport measurements and hinders the probing of
the surface states in these experiments. In this study, we investigate various
strategies to reduce the residual doping in Bi-based topological insulators.
Flakes of Bi$_2$Se$_3$ and Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ are grown
by physical vapor deposition and their structural and electronic properties are
compared to mechanically exfoliated flakes. Using Raman spectroscopy, we
explore the role of the substrate in this process and present the optimal
conditions for the fabrication of high quality crystals. Despite of this
improvement, we show that the vapor phase deposited flakes still suffer from
structural disorder which leads to the residual n-type doping of the bulk.
Using magneto-measurements we show that exfoliated flakes have better
electrical properties and are thus more promising for the probing of surface
states.
