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wires of lengths between 10 and 20 cells long. It is indicated that the number of
nanomagnets that can be clocked with a single phase is limited and that a
multiphase clocking scheme similar to that used in electronic QCA will be required
for a large-scale implementation of MQCA circuits.
4.12.3. Metallic-Island QCA
The metallic-island implementation was the first experimental realization of QCA
[6, 7, 9-14, 16-26, 55]. In all these prototypical devices, the metal islands are used
to replace the semiconductor quantum dots. These experiments demonstrated that
single-electron tunneling between two Al metallic islands coupled with tunnel
junctions would encourage single-electron tunneling in the opposite direction in
two adjacent coupled metal islands, realizing this bistable behavior. This idea has
been extended to majority gates and short shift registers. The advantage of this
implementation is that quantized electron tunneling between metal islands could
be observed for large device sizes that are relatively easily fabricated using
established electron beam lithography (EBL) techniques. The schematic repre-
sentation of a metal island QCA cell is shown in Figure 4.35. The cell consists of
four metallic islands grouped into two sets of double-dots. The two metallic
islands that make up a single double-dot are coupled to each other through
tunneling junctions. The two double-dots are then coupled to each other with
capacitors increasing the interactions between the two double-dots in order to
enhance the bistable behavior of the cell.
The devices operate in the so called Coulomb Blockade regime. The island
capacitance C is a parameter that determines the device operating temperature
and is dominated by the capacitance of the tunnel junctions connecting the
metallic island to the external electrodes and adjacent islands. Due to the relatively
+
+
Coupling
capacitor
Tunneling
junction
Previous
cell
Next
cell
+
−
+
−
Metal island
Figure
4.35.
Schematic representation of a metal-island QCA cell. The two
vertically stacked metal islands are coupled through tunnel junctions, which
enable the quantum mechanical switching of the cell.
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