Bulk Conducting States of Intrinsically Doped Bi2Se3

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 have, however, been hindered by inherent structural defects that donate electrons and make the bulk conductive. Consequently, controlling these defects is of great importance for future technological applications, and while past literature has focused on adding external doping elements to the mixture, a complete study on undoped Bi2Se3 is 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 intrinsic defects on the bulk transport are investigated in samples with electron densities tuned in the range of 10(17)-10(19) cm(-3) (from Se-rich to Bi-rich mixtures, respectively), encompassing the transition into a degenerate semiconductor regime. We find that electron-donor defects, likely Se vacancies, unavoidably shift the Fermi level to up to 200 meV inside the conduction band. Other defects, such as interstitial Bi and Se, are shown to play a significant role as scattering centers, especially at low temperatures and in the decoherence of the SdH oscillations. Open questions on Bi2Se3, such as the unusual transport behavior below 30 K, the different scattering times between transport and quantum oscillation experiments, and the presence of additional low mobility bands, are addressed. The results outlined herein provide a concise picture on the bulk conducting states of flux-grown Bi2Se3 single crystals, enabling precise control of the structural defects and electronic properties, which is crucial for the material's value both as a prototype system and as a functional material.