  Artificially ordered Ge quantum dot (QD) arrays, where confined carriers can
interact via exchange coupling, may create unique functionalities such as
cluster qubits and spintronic bandgap systems. Development of such arrays for
quantum computing requires fine control over QD size and spatial arrangement on
the sub-35 nm length scale. We employ fine-probe electron-beam irradiation to
locally decompose ambient hydrocarbons onto a bare Si (001) surface. These
carbonaceous patterns are annealed in UHV, forming ordered arrays of nanoscale
SiC precipitates that serve as templates for subsequent Ge quantum dot
self-assembly during heteroepitaxy. This templating approach has so far
produced interdot spacings down to 22.5 nm, and smaller spacings should be
possible. We investigate the templated feature evolution during UHV processing
to identify key mechanisms that must be controlled in order to preserve pattern
fidelity and reduce broadening of the quantum dot size distribution. A key
finding is that the presence of a small background of excess carbon reduces Ge
surface diffusion, thereby suppressing coarsening to relatively high
temperatures. In fact, coarsening of the carbonaceous nanodot template prior to
conversion to SiC can be a more important contributor to size dispersion, and
must be avoided through control of thermal budget.
