  A comparative study of the properties of topological insulator Bi2Te2Se (BTS)
crystals grown by the vertical Bridgeman method is described. Two defect
mechanisms that create acceptor impurities to compensate for the native n-type
carriers are compared: Bi excess, and light Sn doping. Both methods yield low
carrier concentrations and an n-p crossover over the length of the grown
crystal boules, but lower carrier concentrations and higher resistivities are
obtained for the Sn-doped crystals, which reach carrier concentrations as low
as 8 x 1014 cm-3. Further, the temperature dependent resistivities for the
Sn-doped crystals display strongly activated behavior at high temperatures,
with a characteristic energy of half the bulk band gap. The (001) cleaved
Sn-doped BTS crystals display high quality Shubnikov de Haas (SdH) quantum
oscillations due to the topological surface state electrons. Angle resolved
photoelectron spectroscopy (ARPES) characterization shows that the Fermi energy
(EF) for the Sn-doped crystals falls cleanly in the surface states with no
interference from the bulk bands, that the Dirac point for the surface states
lies approximately 60 meV below the top of the bulk valence band maximum, and
allows for a determination of the bulk and surface state carrier concentrations
as a function of Energy near EF. Electronic structure calculations that compare
Bi excess and Sn dopants in BTS demonstrate that Sn acts as a special impurity,
with a localized impurity band that acts as a charge buffer occurring inside
the bulk band gap. We propose that the special resonant level character of Sn
in BTS gives rise to the exceptionally low carrier concentrations and activated
resistivities observed.
