OPTICAL DAMPING CONSTANT DUE TO FREE-CARRIERS IN NARROW GAP SEMICONDUCTORS

Far infrared reflection spectra of narrow gap semiconductors (HgSe, Zn//xHg//1// minus //xSe and Cd//xHg//1// minus //xSe) are measured by a Fourier transform system in the range of 20 to 500 cm** minus **1 at 5 K. The spectra of these materials are analyzed by using the dynamic dielectric function as a sum of interband and intraband transitions and phonon contributions. Damping constants of free carriers are estimated from best fitting with experimental reflectivity curves. The frequency dependence of the damping constant for HgSe shows the Drude type in lower frequency than the plasma frequency. That in higher than the plasma frequency is explained well by the impurity scattering including single and collective excitations and the effect of the electron-LO phonon polar coupling. The dependence of the damping constant on the frequency for Zn//xHg//1// minus //xSe and Cd//xHg//1// minus //xSe shows the similar tendency for HgSe. The compensation in our samples is about 30% for doubly ionized impurities (donors and acceptors).