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A comparative study of the properties of topological insulator Bi2Te2Se (BTS)crystals grown by the vertical Bridgeman method is described. Two defectmechanisms that create acceptor impurities to compensate for the native n-typecarriers are compared: Bi excess, and light Sn doping. Both methods yield lowcarrier concentrations and an n-p crossover over the length of the growncrystal boules, but lower carrier concentrations and higher resistivities areobtained for the Sn-doped crystals, which reach carrier concentrations as lowas 8 x 1014 cm-3. Further, the temperature dependent resistivities for theSn-doped crystals display strongly activated behavior at high temperatures,with a characteristic energy of half the bulk band gap. The (001) cleavedSn-doped BTS crystals display high quality Shubnikov de Haas (SdH) quantumoscillations due to the topological surface state electrons. Angle resolvedphotoelectron spectroscopy (ARPES) characterization shows that the Fermi energy(EF) for the Sn-doped crystals falls cleanly in the surface states with nointerference from the bulk bands, that the Dirac point for the surface stateslies approximately 60 meV below the top of the bulk valence band maximum, andallows for a determination of the bulk and surface state carrier concentrationsas a function of Energy near EF. Electronic structure calculations that compareBi 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 insidethe bulk band gap. We propose that the special resonant level character of Snin BTS gives rise to the exceptionally low carrier concentrations and activatedresistivities observed.

Comparison of Sn-doped and nonstoichiometric vertical-Bridgman-grown crystals of the topological insulator Bi2Te2Se