Abstract
Rare earth doped semiconductors have received much attention in the last decades because of potential applications in photonics and illumination. Because silicon is the most widely employed material in the semiconductor industry, much of this effort is devoted to obtain efficient light emitters involving silicon based alloys. The excitation cross section of rare earth ions is orders of magnitude higher in silicon-rich amorphous alloys than in ordinary oxides. Energy transfer is often very efficient in the presence of silicon nanocrystals or nanoclusters. This has led to energy transfer models involving quantum confinement and the electronic structure of silicon nanocrystals. Although these models are elegant and persuasive, they demand a highly homogeneous nanoparticle size distribution, absence of states associated with surface or strain in very small particles and the excitation of many rare earth ions by a single nanoparticle, all very unlikely in real systems.