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We review in this paper our recent results on the correlation between the structural and the optoelectronic properties of Si nanocrystals (Si-nc) embedded in SiO₂. We describe as well the development of both materials and technology approaches that have allowed us to successfully produce efficient and reliable LEDs by using only CMOS processes. Si-nc were synthesised in SiO₂ by ion implantation plus annealing and display average diameters from 2.5 to 6 nm, as measured by electron microscopy. By varying the annealing time in a large scale we have been able to track the nucleation, pure growth and Ostwald ripening stages of the nanocrystal population. The most efficient structures have Si-ncs with average size of 3 nm and densities of about 10¹⁹ cm⁻³. We have estimated band-gap energies, lifetimes (20–200 μs) and absorption cross-sections (10⁻¹⁵–10⁻¹⁶ cm²) as a function of size and surface …