Optical Properties of Tellurium and Selenium

The optical properties of tellurium and of trigonal and amorphous selenium have been investigated at wavelengths extending from the intrinsic absorption edge to about 152 microns using polarized radiation. The refractive indices of tellurium and trigonal selenium have been determined from 4 to 14 microns and from 9 to 23 microns, respectively. For amorphous selenium, the refractive index estimated from measured reflectivity shows no appreciable variation from 30 to 152 microns. The temperature shift and the pressure shift of the intrinsic absorption edge in amorphous selenium are found to be -1.45×${10}^{-3\mathrm{}}$ ev/°K and -2.0×${10}^{-5\mathrm{}}$ ev/atmos, respectively. Lattice absorption bands have been observed in tellurium and selenium; they are attributed to the excitation of combination modes. In tellurium, a strong absorption band has been observed at 11 microns, which is present only for $E\parallel c$ radiation. The band indicates that there are overlapping branches in the valence band which are separated by about 0.11 ev. The effective mass in the lower branch is estimated to be about four times smaller than that in the upper branch. It is possible that the structure of the valence band is responsible for the high-temperature reversal of the Hall effect in tellurium. The usual carrier absorption increasing smoothly with wavelength has been studied for tellurium using polarized radiations. The effective mass of holes in tellurium has been determined from reflectivity measurements: $m_{\perp }\sim m_{\mathrm{II}}=0.45\mathrm{} m$ at 300°K, $m_{\perp }=0.30\mathrm{} m$ and $m_{\mathrm{II}}=0.45\mathrm{} m$ at 100°K.