Optical and electrical properties of Si-nanocrystals ion beam synthesized in SiO2

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 passivation. Based on these results, we propose a mechanism for exciton recombination based on the strong coupling of excitons with the heterointerfaces. From highly luminescent Si-nc, LEDs consisting of MOS capacitors were fabricated. Stable red electroluminescence has been obtained at room temperature and the I–V characteristics prove that the current is related to a pure tunnelling process. Fowler–Nordheim injection is not observed during light emission for electric fields below 5 MV/cm. Thus, hot carrier injection is avoided and efficient and reliable devices are obtained.