A technique for tailor making silicon nanocrystals on 4-inch wafers has been developed. Following a standard procedure in silicon technology, a thermally unstable silicon compound in the form of an ultra-thin layer is first deposited on a substrate. A subsequent thermal treatment leads to a phase separation in this layer, in which silicon clusters and nanocrystals form depending on the temperature; these clusters and crystals are embedded in a matrix of thermally stable silicon dioxide. The size of the nanocrystals is controlled via the thickness of the deposited layer. This process makes the cost-effective manufacture of high-density arrays of silicon clusters or nanocrystals possible. The present paper summarizes our work on size controlled Si nanocrystals. The energy transfer to dopants, i.e. erbium, is presented. The results of our work are used to develop the optimization strategies for optical applications.