Vibrational Properties of MixedCdSxSe1−x

Abstract

An investigation of the effects of heavy doping has been carried out in the case of the system $\mathrm{Cd}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$. For CdS containing a low concentration of Se, the dopant acts as a heavy impurity and gives a gap mode at ${\ensuremath{\omega}}_{G\mathrm{EII}c}=182.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{G\mathrm{E}\ensuremath{\perp}c}=187$ ${\mathrm{cm}}^{\ensuremath{-}1}$. An increase in the Se concentration brings into prominence two constitutents of the ${\ensuremath{\omega}}_{G}$ band having, respectively, the character of a longitudinal mode ${{\ensuremath{\omega}}_{\mathrm{LO}}}^{G}$ and a transverse mode ${{\ensuremath{\omega}}_{\mathrm{TO}}}^{G}$. The frequencies ${{\ensuremath{\omega}}_{\mathrm{LO}}}^{G}$ and ${{\ensuremath{\omega}}_{\mathrm{TO}}}^{G}$ 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, ${\ensuremath{\omega}}_{L\mathrm{EII}c}=266.5$ ${\mathrm{cm}}^{\ensuremath{-}1}$ and ${\ensuremath{\omega}}_{L\mathrm{E}\ensuremath{\perp}c}=269$ ${\mathrm{cm}}^{\ensuremath{-}1}$, 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 $\mathrm{Cd}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$, 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.