MULTIMODE BEHAVIOR OF OPTICAL PHONONS IN II-VI TERNARY AND QUATERNARYALLOYS

Zone-center optical phonons of tetrahedrally coordinated II-VI semicon ductor ternary (AB1-xCx) and quaternary (AB1-x-yCxDy) alloys display a variety of distinct multi-mode behavior patterns. A brief review of t he modified random element isodisplacement model, which can satisfacto rily 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 phon on frequencies. When a constituent is present in extreme dilution in a binary (or in a ternary), e.g., C in AB (or in AB1-yDy), one sees eit her a local or a gap mode associated with it. For higher concentration s, one can see two (three) LO-TO phonon pairs for the ternary (quatern ary) in the first order Raman spectrum or in the infra-red deduced fro m an analysis of the reststrahlen bands, when the masses of B and C di ffer significantly but are lighter than that of A. We illustrate these features in the Raman spectra of Cd1-xMgxTe and Cd1-xMgxMnyTe. Also s hown are the infra-red spectra of the local mode of Mg2+ and Mn2+ in C dTe, where their isotopic nature is clearly manifested.