Impurity-induced phonon disordering inCd1−xZnxTe ternary alloys

A comprehensive study of impurity-induced phonon disordering in ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$Te alloys is reported for a variety of samples (with composition ranging from x=0.005–0.5, and 1) by using far-infrared reflectivity and Raman-scattering spectroscopy. Substantial differences were noted among the various published values for the optical-phonon frequencies versus x. Contrary to an earlier Raman study on molecular-beam-epitaxy grown ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$Te/GaAs films, our results yield increases in the numbers of both the CdTe- and ZnTe-like TO phonons with x. A modified random-element isodisplacement model provides excellent fits to the optical phonons. The effects of impurity-induced phonon disordering are studied, within the band mode region, by using an average-t-matrix formalism. Unlike earlier speculations in which a gap mode in ZnTe:Cd lies near ∼140–145 ${\mathrm{cm}}^{\mathrm{-}1}$, our theory predicts it to be at a higher frequency, ∼153 ${\mathrm{cm}}^{\mathrm{-}1}$. Group-theoretical analysis suggests that the gap mode exhibits a triply degenerate vibrational state and it can be detected both by IR absorption and Raman-scattering spectroscopy.