Reflectivity Spectra of the Rhombohedral Crystals Bi₂Te₃, Bi₂Se₃, and Sb₂Te₃ over the Range from 0.7 to 12.5 eV

The reflectivity spectra (polarisation E ⟂ c ) of the homologous group of rhombohedral crystals Bi 2 Te 3 , Bi 2 Se 3 , and Sb 2 Te 3 have been investigated in the range 0.7 to 12.5 eV. The complex structure of the two reflectivity main maxima, lying in the range 1 to 5 and 6 to 11 eV, respectively, and its similarity in all three crystals have been established. The energy variation of the reflectivity peaks in dependence of the atomic composition shows the anomalous decrease of the interband gaps in Sb 2 Te 3 respective to Bi 2 Te 3 . The results obtained are discussed in terms of published bismuth telluride band structure.