Infrared Optical Properties of Single Crystals of Tellurium

The fundamental optical absorption and photoconductivity of single crystals of tellurium has been investigated. Tellurium crystals exhibit dichroism in the infrared, i.e., the absorption constant depends on the polarization of the incident radiation. At 300\ifmmode^\circ\else\textdegree\fi{}K for radiation polarized perpendicular to the $C$ axis the absorption edge, located by means of an arbitrary criterion, is at 3.82 microns (0.324 ev); for the case of polarization parallel to the $C$ axis the same criterion locates the second edge at 3.29 microns (0.374 ev). The difference between absorption constants survives at wavelengths beyond the edges and is ascribable to the effective masses of the carriers, which depend on their direction of motion relative to the crystal axis. The variation of the energy gaps with temperature has been observed to be -2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ ev/\ifmmode^\circ\else\textdegree\fi{}C, i.e., the energy gaps decrease with increasing temperature. Comparison is made with existing theories. The photoconductivity which is barely detectable at room temperature is considerably enhanced by a reduction of the temperature to 90\ifmmode^\circ\else\textdegree\fi{}K. The spectral response indicates the presence of two peaks corresponding in position to the absorption edges. Measurements of the dependence of photoresponse on voltage, light intensity, and shuttering speed indicate that the photoresponse is at least partially in conformity with a monomolecular model. Selenium-tellurium alloys containing 2.7 percent and 11 percent selenium by weight and therefore possessing a considerably reduced lattice volume show a shift of photoconductivity and transmission toward shorter wavelengths, i.e., larger energy gaps.