Fundamental Absorption Edge of Tin Telluride

Infrared transmission and reflection measurements at room temperature have been made at energies between 0.2 and 0.7 eV on single-crystal $p$-type SnTe films with carrier concentrations of ∼1×${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$. The absorption spectrum is characterized by a sharp absorption edge commencing near 0.5 eV and extending to higher energies. This rapid rise is believed to be the result of direct transitions between a valence and a conduction band. Because of the Burstein shift of the edge, the separation of the band extrema is expected to be significantly less than 0.5 eV. For energies below 0.5 eV it is shown that the absorption is primarily due to free carriers. This indicates that the observed absorption edge is associated with the smallest direct optical gap between the valence and conduction bands.