Optical and morphological studies are used to investigate the effects of chemical composition and, in particular, the magnesium content of the ZnxCdyMg1-x-ySe barrier layers on the size, density and uniformity of CdSc self-assembled quantum dots (QDs). A reduction of the uncapped QD size, as well as a blue shift of the capped QD photoluminescence peak position by increasing Mg concentration in the ZnxCdyMg1-x-ySe barrier has been demonstrated by changing the Mg cell temperature during growth. In addition, a more uniform and more densely packed QD layer has been observed with an increase of the MgSe fraction in the ZnxCdyMg1-x-ySe barrier layer using three-dimensional topographic atomic force microscopy images of the surface of uncapped QDs. Results point to Mg as a chemical factor that induces QD formation, either by increasing the density of atomic steps or/and by changing the energy of the ZnxCdyMg1-x-ySe surface. (c) 2006 Elsevier B.V. All rights reserved.