Urbach rule in mixed single crystals ofZnxCd1−xSe

The dependence of the absorption coefficient on the photon energy and on temperature near the fundamental absorption edge was measured in mixed single crystals of ${\mathrm{Zn}}_{\mathrm{x}}$ ${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$Se, for x=0, 0.04, 0.3, 0.4, and 1. Exponential dependence of that coefficient according to the Urbach rule was observed. The thermal dependence of the slope parameter indicates that the absorption is influenced by the phonon-induced electric microfields and not by compositional or structural potential fluctuations. Up to about 90 K the magnitude of ħ${\omega }_0$ fits the energy of the piezoelectric LA phonon, and at higher temperatures ħ${\omega }_0$ fits the energy of the LO phonon. The energy ħ${\omega }_0$ of the LO phonons varies linearly within the experimental error with x, from 27 meV for CdSe to 31 meV for ZnSe, and is in good agreement with the Raman spectra measured for these amalgamated behavior-type crystals. The energy gap, measured at room temperature, and the value of $E_0$ change linearly with composition for the hexagonal phase crystals. The absorption process can be considered as an internal Franz-Keldysh effect, caused by the phonon-generated electric fields. It is described in the framework of the Dow-Redfield model, which ascribes the Urbach rule to the ionization of the exciton, as an extension of the Stark shift.