Reflectivity of Tin Telluride in the Infrared

Abstract

The reflectivity of tin telluride at near-normal incidence and room temperature was measured at wavelengths from 1 to 200 \ensuremath{\mu}. Thirteen single-crystal samples with free-hole concentrations ranging from 4.8\ifmmode\times\else\texttimes\fi{}${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ to 4.8\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ were studied. In the wavelength range from 3 to 200 \ensuremath{\mu}, the results are well described by the classical theory of free-carrier dispersion. The free-carrier effective mass ${m}_{s}$, free-carrier optical mobility ${\ensuremath{\mu}}_{\mathrm{opt}}$, and optical dielectric constant ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$, 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 ${\ensuremath{\epsilon}}_{\ensuremath{\infty}}$ is shown by a Kramers-Kronig-type analysis to be attributable to the Burstein shift of the fundamental absorption edge. For wavelengths less than $3\ensuremath{\mu}$, 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.