The exponential optical absorption band tail of Hg_1-xCd _xTe

The optical absorption edges of Hg_0.71Cd_0.29Te and CdTe were measured over the temperature range 80≤T≤300 K. The refractive indices of these materials are determined over a large energy range at T=300 K and their dispersion relation is given. The absorption band tail of Hg _1-xCd_xTe has an exponential shape. Its slope increases with decreasing temperature. It is suggested that this tail is not caused by permanent lattice disorder but is a material property. An empirical expression for the absorption coefficient as a function of temperature and composition is derived. Using this expression, the zero-intercept cut-on energy of the infrared transmission spectrum at room temperature is used to define the composition for any desired thickness of Hg_1-xCd_xTe sample.