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 E1(LO) and multi-LO (2-LO, 3-LO and 4-LO) modes in CdS have been studied over the energy range extending from 0.45Eex (exciton energy) to 1.1Eex. For the E1(LO) mode, both the "forbidden" (zz) and allowed (zx) geometries were employed. The dispersive behavior of the cross section in these two geometries is in agreement with theory for energies below Eex. The dispersion of multi-LO-phonon scattering is also presented. For ZnO, the dispersion of the cross section for the E1(LO), E1(TO), A1(TO), and E2 modes was measured over the energy range of (0.57-0.76)Eex. 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 Eex is reported.
ASJC Scopus subject areas
- Condensed Matter Physics