The Optical Properties of Bismuth Telluride

The shape of the absorption edge in the semiconductor Bi 2 Te 3 has been determined from transmission measurements on cleavage sections. The edge is of the form expected for indirect transitions, but an interpretation in terms of phonons characterized by a single energy is not applicable. A brief study of anisotropy effects is included. The energy gap at room temperature is close to 0.13 ev with a temperature coefficient of -0.95 × 10 -4 ev deg -1 . The refractive index, determined from interference fringes, is 9.2 in the region 8-14 μ . In degenerate samples the absorption edge is displaced towards higher energies. A simple quantitative discussion of this effect is given.