Raman study of ZnSe/SiO. multilayers

Multilayers of ZnSe/SiOx having different ZnSe layer thicknesses (2.0, 3.5, 4.0, 5.0, 7.0 and 10 nm) have been prepared by thermal evaporation in vacuum. Raman scattering measurements have been performed at room temperature using the 442 nm line of a He-Cd laser. Two bands, which appear at about 250 and 500 cm', have been attributed to the 1L0 and 2L0 modes from 'pure' ZnSe in multilayers. Both modes show a large homogeneous broadening. The 1L0 Raman mode displays an asymmetric shape and redshift, which can be related to the phonon confinement effect, due to the nanometric size of the ZnSe layers. The size effect on the band shape has been simulated by using a one-dimensional phonon confinement model for ZnSe nanolayers. The comparison of experimental and calculated data also implies the existence of surface phonon modes. Moreover, a change in the intensity of Raman scattering with ZnSe layer thickness points to the resonant enhancement of the Raman bands when the photon energy of the exciting light (2.8 eV) approaches the energy band gap of some samples.