Optical Absorption Related to Interband and Intersubband Electron Transitions in Bismuth Telluride

The spectral dependences of the absorption coefficient alpha in the submicron samples of the n-Bi2Te3 + CdBr2 topological insulator, which have the optimal thermoelectric properties at high values of the Seebeck coefficient at temperatures below room temperature, are studied at temperatures of 205 and 300 K. The components of the optical absorption spectra related to interband and intersubband electron transitions are revealed and analyzed. It is found that in bismuth telluride, as the temperature decreases, the optical transitions of electrons at the threshold of interband absorption remain direct and permitted. The energy gaps between the absolute extremums of the conduction band and the valence band, as well as between the main and additional subbands of the conduction band, are estimated. It is shown that the rates at which they change with temperature are opposite in sign. The energy band diagram of Bi2Te3, corresponding to the experimental data, is constructed.