Polarization Effects in the Electroreflectance of Bismuth Telluride at Oblique Incidence

The optical transitions above the threshold of the uniaxial crystal bismuth telluride have been investigated by means of electroreflectance over the energy range 1-5.5 eV using an electrolyte to produce electric fields at the surface. Both near-normal and oblique incidence measurements were made using light polarized under different orientations of the electric vector with respect to the $c$ axis. The combination of these techniques permitted an unambiguous identification of the symmetry of the critical points and yielded the precise polarization dependence of the transitions. The broad electroreflectance spectra of bismuth telluride have been understood and analyzed in terms of strongly lifetime-broadened Franz-Keldysh tunneling. A correlation, although preliminary, has been attempted with the band structure of this material.