Assembly of nanocrystalline silicon quantum dots based on a colloidal solution method

In this paper we propose and develop a new bottom-up approach to the formation of silicon nanostructures based on assembly of nanocrystalline (nc) Si dots from the colloidal solution. The nc-Si dots with a diameter of 8 ± 1 nm were fabricated by using VHF plasma decomposition of pulsed SiH₄ gas supply and deposited on the substrate randomly. For preparing the nc-Si dot colloidal solution, we first examined various kinds of solvent. The substrates on which the nc-Si dots deposited were immersed into the solvents, and ultra sonic treatment was applied for a few tens seconds. It was found that methanol works as a suitable solvent for nc-Si dots. The nc-Si dot solution was then condensed by evaporating the solvent a fraction. We dropped the nc-Si dot solution onto other substrates and evaporated it completely. We observed that the nc-Si dots were assembled in the solution via the lateral capillary meniscus force which works as an attractive force between the dots. Use of SiO₂ substrate with good surface wettability with the solution was found vital to have the maximum meniscus force and to have two-dimensional assembly of the dots. The evaporation speed was carefully controlled via temperature and evaporation pressure to achieve high dot density assembly. In addition, we examined the assembly of the nc-Si dots on the silicon-on-insulator substrates with various kinds of nanoscale patterning and succeeded in making the nc-Si dots cluster bridging between the nanoelectrodes with a gap of as small as 20 nm.