The optical properties of p-type Bi2Te3Sb2Te3 alloys between 2–15 microns

Transmittance and reflectance measurements have been made on a number of p-type Bi2Te3Sb2Te3 alloys. With the electric vector in the cleavage plane, the index of refraction, energy gap, and average effective mass have been calculated. The absorption edge indicates indirect transitions but the analysis based on phonons of a single energy is inconsistent. The reduction of the dielectric constant due to the free carrier susceptibility is observed in the alloys. In contrast to thermal energy gaps the optical energy gaps increase with increasing Sb content, a result attributed in part to the increasing degeneracy of the charge carriers. Temperature dependencies of the optical energy gaps are also reported.