Band gap and type of optical transitions at the interband absorption edge in solid solutions based on bismuth telluride

The spectra of optical absorption in multicomponent n- and p-type solid solutions based on bismuth and antimony chalcogenides with substitutions in both sublattices of Bi2Te3 have been investigated. It has been found that, in all the solid solutions studied, just as in the parent compound Bi2Te3, direct allowed transitions occur at the interband absorption edge at T = 300 K. The band gap E g in the n-Bi2 − x Sb x Te3 − y − z Se y S z solid solutions weakly increases with increasing number of substituted atoms in the Bi and Te sublattices. These atomic substitutions do not leads to an increase in E g as compared to that of the n-Bi2Te2.7Se0.3 composition. An analysis of the optical absorption spectra suggests that the solid solutions under consideration are weakly degenerate, a conclusion supported by the earlier studies of the thermoelectric and galvanomagnetic properties. It has been established that, in the conduction band of the Bi1.8Sb0.2Te2.7Se0.15S0.15 solid solution, there is an additional extremum lying above the main extremum at a distance no more than 0.1 eV.