We describe the crystal growth, crystal structure, and basic electrical properties of Bi${}_2$Te${}_{1.6}$S${}_{1.4}$, which incorporates both S and Te in its tetradymite quintuple layers in the motif -[Te${}_{0.8}$S${}_{0.2}$]-Bi-S-Bi-[Te${}_{0.8}$S${}_{0.2}$]-. This material differs from other tetradymites studied as topological insulators due to the increased ionic character that arises from its significant S content. Bi${}_2$Te${}_{1.6}$S${}_{1.4}$ forms high quality crystals from the melt and is the S-rich limit of the ternary Bi-Te-S γ-tetradymite phase at the melting point. The native material is $n$ type with a low resistivity; Sb substitution, and with adjustment of the Te to S ratio, results in a crossover to $p$ type and resistive behavior at low temperatures. An angle-resolved photoemission study shows that topological surface states are present, with the Dirac point more exposed than it is in Bi${}_2$Te${}_3$ and similar to that seen in Bi${}_2$Te${}_2$Se. Single crystal structure determination indicates that the S in the outer chalcogen layers is closer to the Bi than the Te, and therefore that the layers supporting the surface states are corrugated on the atomic scale.

• Received 26 March 2012

DOI:https://doi.org/10.1103/PhysRevB.85.201103