Bi ₂Te _1.6S _1.4: A topological insulator in the tetradymite family

We describe the crystal growth, crystal structure, and basic electrical properties of Bi ₂Te _1.6S _1.4, which incorporates both S and Te in its tetradymite quintuple layers in the motif -[Te _0.8S _0.2]-Bi-S-Bi-[Te _0.8S _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 ₂Te _1.6S _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 ₂Te ₃ and similar to that seen in Bi ₂Te ₂Se. 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.