Temperature shift of the absorption edge in mixed single crystals ofZnxCd1−xSe

The thermal dependence of the absorption-edge shift has been measured in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$ ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$Se, in the temperature range 13–300 K. The experimental results support the argument that the energy gap shift is caused by an internal Franz-Keldysh effect which results from electric fields induced by phonons. However, the exact dependences of the energy gap shift and the increase in the absorption coefficient on the electric fields differ from those calculated by Franz and Keldysh, because of final-state interaction correction. It is shown that in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$ ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$Se, as well as in other single crystals of pure II-VI compounds, the electric fields which determine the thermal shift of the absorption edge are dominated by the LO-phonon-induced electric fields.