Raman spectra of semiconductor nanoparticles: Disorder-activated phonons

Abstract

We present Raman spectra of four semiconductor doped glasses and a single crystal of ${\mathrm{CdS}}_{0.55}{\mathrm{Se}}_{0.45}$ in the range 30--800 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the backscattering geometry. This includes the first-order Raman scattering from the disorder-activated zone-edge phonons and the LO phonons. TO phonon modes are not observed, as in bulk CdS, for the excitation well above the lowest gap. We show that the asymmetric line profile of the LO phonon structure can be understood as a composite of two phonon modes: the zone center and the zone edge phonons. Disorder-activated modes in the (30--${130)\ensuremath{-}\mathrm{c}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}$ range and the higher-order Raman spectra are also observed and found to be consistent with this assignment.