Vibrational Properties of MixedCdSxSe1−x

An investigation of the effects of heavy doping has been carried out in the case of the system $\mathrm{Cd}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$. For CdS containing a low concentration of Se, the dopant acts as a heavy impurity and gives a gap mode at ${\omega }_{G\mathrm{EII}c}=182.5\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and ${\omega }_{G\mathrm{E}\perp c}=187\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. An increase in the Se concentration brings into prominence two constitutents of the ${\omega }_G$ band having, respectively, the character of a longitudinal mode ${{\omega }_{\mathrm{LO}}}^G$ and a transverse mode ${{\omega }_{\mathrm{TO}}}^G$. The frequencies ${{\omega }_{\mathrm{LO}}}^G$ and ${{\omega }_{\mathrm{TO}}}^G$ are dependent on the impurity concentration and tend toward the normal-mode frequencies of CdSe in the high-concentration limit. S in CdSe gives two localized modes, ${\omega }_{L\mathrm{EII}c}=266.5\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ and ${\omega }_{L\mathrm{E}\perp c}=269\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$, whose frequencies are also concentration-dependent and at high S concentration reach the normal-mode frequencies for CdS. The localized-mode frequencies of S in CdSe and the resonance-mode frequencies of Se in CdS have been investigated by Green's-function techniques. A model assuming a random arrangement of impurities, developed by Elliott and Taylor, has been applied to the study of mixed $\mathrm{Cd}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$, using the now available CdS Green's function. Up to an impurity concentration of 20%, the calculated optical properties (such as the normal-mode and impurity-mode frequency shift, oscillator strength, and absorption coefficient) compare satisfactorily with the experimental data.