  For future nanoelectronic devices - such as room-temperature single electron
transistors - the site-controlled formation of single Si nanocrystals (NCs) is
a crucial prerequisite. Here, we report an approach to fabricate single Si NCs
via medium-energy Si+ or Ne+ ion beam mixing of Si into a buried SiO2 layer
followed by thermally activated phase separation. Binary collision
approximation and kinetic Monte Carlo methods are conducted to gain atomistic
insight into the influence of relevant experimental parameters on the Si NC
formation process. Energy-filtered transmission electron microscopy is
performed to obtain quantitative values on the Si NC size and distribution in
dependence of the layer stack geometry, ion fluence and thermal budget.
Employing a focused Ne+ beam from a helium ion microscope, we demonstrate
site-controlled self-assembly of single Si NCs. Line irradiation with a fluence
of 3000 Ne+/nm2 and a line width of 4 nm leads to the formation of a chain of
Si NCs, and a single NC with 2.2 nm diameter is subsequently isolated and
visualized in a few nanometer thin lamella prepared by a focused ion beam
(FIB). The Si NC is centered between the SiO2 layers and perpendicular to the
incident Ne+ beam.
