Raman scattering characterization of ZnSe/Zn_0.6Cd_0.4Se multilayers prepared by thermal vacuum evaporation

Abstract

Nanocrystalline multilayers from ZnSe/Zn 0.6 Cd 0.4 Se with three different layer thicknesses (3.5, 5.0, 10.0 nm) and, for comparison, 400 nm thick Zn 0.6 Cd 0.4 Se nanocrystalline single layers have been prepared by thermal evaporation of ZnSe and CdSe in vacuum. Raman scattering spectra have been measured in the range 100-1000 cm À1 under excitation with the 488 and 514.5 nm lines of an Ar þ laser. Series of four bands have been seen in the spectra of all samples. They have been related to replicas of the longitudinal optical LO-phonon in the Zn x Cd 1Àx Se layers and interpreted as an indication for crystallinity of both kinds of samples. A blue shift of the 1 LO maximum has been observed, which has been connected with an increase of the compressive stress in the ternary layers when the layer thickness decreases. It has also been obtained that the ratio of the integrated intensity of the 1 LO band in the spectra taken with the 488 and 514.5 nm lines decreases with decreasing Zn 0.6 Cd 0.4 Se layer thickness. The result has been assigned to size-induced increase of the optical band gap of the layers. This conclusion has been confirmed by spectral photocurrent and optical transmission measurements.