Fundamental Optical Absorption in SnS2and SnSe2

Optical absorption in single-crystal $n$-type Sn${\mathrm{S}}_2$ and Sn${\mathrm{Se}}_2$ has been studied at 300°K throughout the wavelength range 0.26 to 6.5 μ. Samples suitable for optical measurements were prepared by various vapor-deposition techniques. The electrical characteristics of the samples used in the absorption measurements were as follows: for Sn${\mathrm{Se}}_2$, conductivity 3.6 ${\left(\mathrm{\Omega }\mathrm{c}\mathrm{m}\right)}^{-1\mathrm{}}$, electron concentration ${10}^{18}$ per ${\mathrm{cm}}^3$, mobility 27 ${\mathrm{cm}}^2$/V sec; and for Sn${\mathrm{S}}_2$, conductivity ${10}^{-7\mathrm{}}$ ${\left(\mathrm{\Omega }\mathrm{c}\mathrm{m}\right)}^{-1\mathrm{}}$. From transmittance and reflectance measurements, the absorption coefficient and index of refraction were determined for light polarized perpendicular to the crystallographic symmetry axis. From an analysis of the data in the high-absorption region, direct-transition band gaps of 1.62 and 2.88 eV were found for Sn${\mathrm{Se}}_2$ and Sn${\mathrm{S}}_2$, respectively. A threshold for possible indirect phonon-assisted transitions was found to occur at 0.97 eV for Sn${\mathrm{Se}}_2$ and at 2.07 eV for Sn${\mathrm{S}}_2$. Photo-conductivity data for Sn${\mathrm{S}}_2$ are also presented.