Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 42

Fig. 9. (a) A fabricated ensemble (cell) of 4 tunnel-coupled DBs, or artificial molecule,
calibrated to result in an average net filling of 2 extra electrons. A corresponding dimer
lattice diagram is shown below. (b) The result of a statistical mechanical description
of average occupation versus distance of separation in such a square cell at different
temperatures (300 K top graph, 100 K bottom graph). The occupation probabilities
with 1, 2, 3, 4 extra electrons are plotted for each case.
adding a fourth dot the previously darker sites become relatively light in appear-
ance. This is due to the electrons attaining a lower energy configuration along
a newly available longer diagonal. In a symmetric square or rectangular cell the
freely tunneling electrons equally occupy the degenerate diagonal configurations.
On the slow time scale of the STM measurement no instantaneous asymmetry
can be seen.
In order to embody the QCA architecture it must be possible to break that
symmetry electrostatically and thereby to polarize electrons within a cell. This
capacity is illustrated first by referral to a 2 dot cell. Figure 11 shows the sequen-
tial building of a 2 dot cell occupied by one extra electron and the polarization
of that cell by one perturbing charge [ 14 ]. Figure 11 a shows a small area, 3 nm
across, of H-terminated silicon at room temperature. Figure 11 b shows the cre-
ation of one ASiQD, while Fig. 11 c shows the creation of a second ASiQD and
the concomitant reduction in charge and darkness as seen by the STM. Upon
charge removal, rapid tunnel exchange ensues. The coupled entity resulting may
be described as an artificial homonuclear diatomic molecule. Like in an ordi-
nary molecule, the Born-Oppenheimer approximation is valid. In other words,
the electron resides so very briefly on one atom that nuclear relaxation does not
have time to occur. On the electronic time scale, the nuclei are frozen. Finally in
Fig. 11 d another charged DB is created. Using the knowledge displayed in Fig. 6 ,
the last DB is placed near enough to the molecule to affect it electrostatically,