Many efforts of the scientific community working in the field of nanophotonics are currently devoted towards the development of a Si-based light source able to be used for the monolithic integration of optical and electrical functions on a single Si chip. Indeed, electrical interconnections based on metal lines represent today the most important limitation on the performances of Si-based microelectronic devices. An almost definitive solution to this problem could be represented by the use of optical interconnections for the transfer of information inside a chip. To develop this strategy, materials able to generate, guide, amplify, switch, modulate and detect light are needed. The most severe requirement for such materials is represented by the compatibility with Si technology. However, because Si is intrinsically unable to efficiently emit light due to its indirect bandgap, it is evident that the main limitation to the above strategy is the lack of an efficient Si-based light source.
In this framework, Si nanostructures represent a promising candidate for the realization of efficient electrically-pumped optical sources to be employed in Si nanophotonics. The activity of the Catania Unit of IMM mainly focuses on Si nanocrystals (ncs) and Si nanowires (NWs).
Si ncs are efficient and stable light emitters at room temperature under both optical and electrical excitation due to quantum confinement effects. The typical emission range is 700-1000 nm (Fig. 1a). Si ncs can be synthesized by thermal annealing of SiOx thin films grown by PECVD or sputter deposition (Figs. 1b-1c). The embedding SiO2 matrix makes this material fully compatible with Si technology and indeed several example of room temperature operating devices fabricated in an industrial environment are available. The coupling with photonic crystals may lead to a strong increment of the external efficiency of devices based on Si ncs, through an increased efficiency of the light extraction (Figs. 1d-1g).
Metal-assisted wet etching represents a maskless, cheap and fast approach for the direct synthesis of ultra-thin Si NWs exhibiting light emission under both optical and electrical excitation. The process employs a HF + H2O2 solution and is catalyzed by a thin metal film; it is scalable up to the wafer size and is therefore compatible with Si technology. NW length can be varied within a wide range (Fig. 2a); NWs have a very high areal density (> 1012 cm-2) (Fig. 2b), and a very small diameter (less than 10 nm), which allows the observation of a strong light emission at room temperature due to quantum confinement effects (Figs. 2c-2d).