Dispersion of Raman Cross Section in CdS and ZnO over a Wide Energy Range

The dispersion of the Raman cross sections of several phonon modes in CdS and ZnO using a wide range of laser energies has been measured. The $E_1\left(\mathrm{LO}\right)$ and multi-LO (2-LO, 3-LO and 4-LO) modes in CdS have been studied over the energy range extending from $0.45E_{\mathrm{ex}}$ (exciton energy) to $1.1E_{\mathrm{ex}}$. For the $E_1\left(\mathrm{LO}\right)$ mode, both the "forbidden"• ($\mathrm{zz}$) and allowed ($\mathrm{zx}$) geometries were employed. The dispersive behavior of the cross section in these two geometries is in agreement with theory for energies below $E_{\mathrm{ex}}$. The dispersion of multi-LO-phonon scattering is also presented. For ZnO, the dispersion of the cross section for the $E_1\left(\mathrm{LO}\right)$, $E_1\left(\mathrm{TO}\right)$, $A_1\left(\mathrm{TO}\right)$, and $E_2$ modes was measured over the energy range of $\mathrm{}(0.57-0.76)\mathrm{}{E}_{\mathrm{ex}}$. The behavior of the LO modes is expalined in terms of a cancellation between the deformation and Fröhlich contributions to the LO scattering cross section. For the TO modes, resonant cancellation was not observed, in contrast to the results of recent studies of CdS and ZnS. A simple model is used to discuss these differences between the ZnO and CdS behavior. Finally, a resonant enhancement of the scattering from the 2-LO mode in ZnO when the scattered-photon energy was coincident with $E_{\mathrm{ex}}$ is reported.