Crystal structure and elementary electronic properties of Bi-stabilized α-In₂Se₃

The introduction of Bi as a substitution for In at ∼12% in In ₂Se₃ stabilizes the α-polymorph and facilitates the crystal growth by the modified Bridgeman method. The crystal structure (R-3m, Z = 3, a = 3.9978(8), c = 28.276(6)) and composition, (In_0.88Bi _0.12)₂Se₃, of the crystals were determined by single crystal X-ray diffraction. The structure, of the tetradymite type, displays positional disorder within the middle Se layer. Optical measurements indicate that (In_0.88Bi_0.12)₂Se₃ has an indirect band gap of about 1.19 eV, shown by electronic structure calculations to be from valence band states near the Γ point to conduction band states at the L point. Resistivity and Hall effect measurements on Sn-doped crystals of composition (In_0.88Bi_0.115Sn _0.005)₂Se₃ show it to have a relatively high semiconducting resistivity, about 6 × 10⁴ Ω cm at 300 K, with an n-type carrier concentration varying from 10¹²/cm³ at 300 K to 10¹⁵/cm³ at 400 K.