Modelling Techniques for Simulating Large QCA Circuits - Field-Coupled Nanocomputing: Paradigms, Progress, and Perspectives - page 256

Modelling Techniques
for Simulating Large QCA Circuits

B

Faizal Karim (

) and Konrad Walus

The University of British Columbia, Vancouver, BC, Canada
[e-mail]

1 Introduction

In the past several years, incredible advances in the availability of nano fabri-
cation processes have been witnessed, and have demonstrated molecular-scale
production beyond the usable limit for CMOS process technology [ 1 - 3 ]. This
has led to the research and early development of a wide-range of novel comput-
ing paradigms at the nanoscale; amongst them, quantum dot cellular automata
(QCA) [ 4 ]. QCA is a nanoelectronic computing paradigm in which an array of
cells, each electrostatically interacting with its neighbors, is employed in a locally
interconnected manner to implement general purpose digital circuits [ 4 ]. Several
proof-of-concept QCA devices have been fabricated using silicon-on-insulator
(SOI) [ 5 ], metallic island devices operating in the Coulomb blockade regime
[ 6 - 12 ], and nano-magnetics [ 13 - 18 ]. In recent years, research into implementing
these devices using single molecules has also begun to generate significant inter-
est [ 1 , 19 - 25 ], and most recently, it was demonstrated that silicon atom dangling
bonds (DBs), on an otherwise hydrogen terminated silicon crystal surface, can
serve as quantum dots [ 1 ].
Research into QCA has been motivated by the potential of reaching the
limits to physical scaling, as well as the predictions of ultra low power consump-
tion, a major problem in the industry [ 4 , 11 , 20 , 26 - 28 ]. It has been reported
that molecular and atomic implementations can achieve device densities on the
order of 10 14 /cm 2 (for 1 nm 2 devices) [ 29 ]. The power dissipated from current-
switched devices such as FETs operating at GHz speeds could melt the chip
at those densities [ 30 ], however, molecular QCA has been predicted to reduce
the power dissipation by several orders of magnitude [ 26 , 31 ]. Additionally, as
the size of the cells gets smaller, the interaction energies between cells increases.
At the molecular (and atomic) scale, these energies are expected to be in the
0.2-0.5 eV range [ 1 , 4 , 32 , 33 ] which allow for room temperature operation since
these energies are greater than the thermal ambient energy (i.e, thermal noise),
k B T ( 25 meV at room temperature), where k B is Boltzmann's constant and T

is the temperature in kelvin ( T = 293 K at room temperature). However, while
the development of such devices is promising, a number of technical challenges,
including choice of molecules [ 19 , 25 ], the design of proper interfacing mecha-
nisms [ 34 ], and clocking technology [ 35 , 36 ] - amongst other things - remain to

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