  Combining high resolution scanning tunneling microscopy and first principle
calculations, we identified the major native defects, in particular the Se
vacancies and Se interstitial defects that are responsible for the bulk
conduction and nanoscale potential fluctuation in single crystals of archetypal
topological insulator Bi2Se3. Here it is established that the defect
concentrations in Bi2Se3 are far above the thermodynamic limit, and that the
growth kinetics dominate the observed defect concentrations. Furthermore,
through careful control of the synthesis, our tunneling spectroscopy suggests
that our best samples are approaching the intrinsic limit with the Fermi level
inside the band gap without introducing extrinsic dopants.
