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×{}10¹⁹ cm^-3 to 4.8×{}10²⁰ cm^-3 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 mₛ, free-carrier optical mobility μₒₚₜ, and optical dielectric constant ε_∞, which are parameters of the classical theory, were determined from curve fitting. These parameters are shown to be carrier-concentration-dependent. The dependence of mₛ is indicative of a complex electronic band structure. The corresponding variation of ε_∞ is shown by a Kramers-Kronig-type analysis to be attributable to the Burstein shift of the fundamental absorption edge. For wavelengths less than 3μ, 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.