Multi-Mode behavior of optical phonons in II-VI ternary and quaternary alloys.

Zone-center optical phonons of tetrahedrally coordinated II-VI semiconductor ternary (ABC) and quaternary (ABCD) alloys display a variety of distinct multi-mode behavior patterns. A brief review of the modified random element isodisplacement model, which can satisfactorily account for the composition dependence of the frequencies of the zone-center optical phonons, is presented; in this model, one uses the macroscopic parameters of the binary end members and their LO-TO phonon frequencies. When a constituent is present in extreme dilution in a binary (or in a ternary), e.g., C in AB (or in AB D), one sees either a local or a gap mode associated with it. For higher concentrations, one can see two (three) LO-TO phonon pairs for the ternary (quaternary) in the first order Raman spectrum or in the infra-red deduced from an analysis of the reststrahlen bands, when the masses of B and C differ significantly but are lighter than that of A. We illustrate these features in the Raman spectra of CdMgTe and Cd MgMnTe. Also shown are the infra-red spectra of the local mode of Mg and Mn in Cd Te, where their isotopic nature is clearly manifested. [ABSTRACT FROM AUTHOR]