  With a large band gap and a single Dirac cone responsible for the topological
surface states, Bi2Se3 is widely regarded as a prototypical 3D topological
insulator. Further applications of the bulk material has, however, been
hindered by inherent structural defects that donate electrons and make the bulk
conductive. Consequently, controlling these defects is of great interest for
future technological applications, and while past literature has focused on
adding external doping elements to the mixture, a complete study on undoped
Bi2Se3 was still lacking. In this work, we use the self-flux method to obtain
high-quality Bi2Se3 single-crystals in the entire concentration range available
on the phase-diagram for the technique. By combining basic structural
characterization with measurements of the resistivity, Hall effect and
Shubnikov-de Haas (SdH) quantum oscillations, the effects of these impurities
on the bulk transport are investigated in samples with electron densities
ranging from 10^17 cm^-3 to 10^19 cm^-3, from Se-rich to Bi-rich mixtures,
respectively, evidencing the transition into a degenerate semiconductor regime.
We find that electron-donor impurities, likely Se vacancies, unavoidably shift
the Fermi level up to 200 meV inside the conduction band. Other impurities,
like interstitial Bi and Se, are shown to play a significant role as scattering
centres, specially at low temperatures and in the decoherence of the SdH
oscillations. Previous open questions on Bi2Se3, such as the upturn in
resistivity below 30 K, the different scattering times in transport and quantum
oscillations, and the presence of additional low mobility bands, are addressed.
The results outlined here provide a concise picture on the bulk conducting
states in flux-grown Bi2Se3 single crystals, enabling better control of the
structural defects and electronic properties.
