Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 49

Our first approach to making suciently fine lithographic features to con-
trollably interact with atom scale structures began nearly 20 years ago. Tita-
nium silicide contacts were prepared using a normal optical lithography and
lift-off approach [ 27 , 28 ]. When examined at the atomic scale, lithographic fea-
tures prepared in this way are unacceptably rough and crudely defined for our
purposes. The transition from pure silicide to pure silicon is not abrupt, but
spans 10s of nm, and is of unknown composition and of uncontrolled electronic
character. However, it is possible to grow crystalline Titanium Silicide features
on a silicon substrate with atomically-precise boundaries by simply evaporat-
ing titanium and annealing in ultra high vacuum. Figure 16 shows a highly
crystalline Titanium Silicide island on a silicon substrate. The boundary is atom-
ically abrupt. The silicon nearby is well ordered as required. No spatial pattern-
ing is imposed. Refinements of this technique will enable the type and quality
of contacts required. Ultra fine extensions from such contacts with linear wire
structures composed of closely spaced ASiQDs will enable precise electrostatic
addressing of atomic structures.
Fig. 17. (a) A schematic diagram of a silicide contact and nearby DBs. The spatially
varying potential built-in at the silicide-silicon interface causes DBs nearby to take
position dependent charge states. (b) An experimental verification of the scheme.
The potential to controllably alter the charge state of individual DBs has been
demonstrated [ 29 ]. Figure 17 shows a schematic diagram of a silicide contact and
nearby DBs. Because of the spatially varying potential built-in at the silicide-
silicon interface (and because the H-terminated surface is not pinned) the DBs
nearby take position dependent charge states. Figure 17 b shows an experimental
verification of the scheme. Going forward, combined lithographic approaches
will allow for suitably small and high quality lithographic features. Multiple