Reflectivity of Tin Telluride in the Infrared

The reflectivity of tin telluride at near-normal incidence and room temperature was measured at wavelengths from 1 to 200 μ. Thirteen single-crystal samples with free-hole concentrations ranging from 4.8×${10}^{19}$ ${\mathrm{cm}}^{-3\mathrm{}}$ to 4.8×${10}^{20}$ ${\mathrm{cm}}^{-3\mathrm{}}$ were studied. In the wavelength range from 3 to 200 μ, the results are well described by the classical theory of free-carrier dispersion. The free-carrier effective mass $m_s$, free-carrier optical mobility ${\mu }_{\mathrm{opt}}$, and optical dielectric constant ${\epsilon }_{\infty }$, which are parameters of the classical theory, were determined from curve fitting. These parameters are shown to be carrier-concentration-dependent. The dependence of $m_s$ is indicative of a complex electronic band structure. The corresponding variation of ${\epsilon }_{\infty }$ is shown by a Kramers-Kronig-type analysis to be attributable to the Burstein shift of the fundamental absorption edge. For wavelengths less than $3\mu$, experimental reflectivities vary appreciably from those expected from free-carrier dispersion theory. It is shown that the deviations are the result of bound-carrier absorptions associated with the fundamental absorption edge.